통신망연구실 | Networking Research Lab.

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Publications | 연구실논문

Preprints (submitted or in preparation)

Y. Cho, J. Won, D.-Y. Kim, and J.-W. Lee, "Optimal placement of aerial base station utilizing topographic features," submitted to IEEE Internet of Things Journal.
J. Won, H.-S. Lee, and J.-W. Lee, "A review on multi-fidelity hyperparameter optimization in machine learning," submitted to ICT Express.
S.-M. Park, D.-Y. Kim, K.-W. Kim, J. Won, and J.-W. Lee, "Dedicated AP-based interference cancellation in cell-free massive MIMO systems: Architecture and algorithm," submitted to IEEE Transactions on Vehicular Technology.

International Journals

J. Shin, J. Won, H.-S. Lee, and J.-W. Lee, "A review on label cleaning techniques for learning with noisy labels," to appear in ICT Express (10.1016/j.icte.2024.09.007). | |

A review on label cleaning techniques for learning with noisy labels

Classification models categorize objects into given classes, guided by training samples with input features and labels. In practice, however, labels can be corrupted by human error or mistakes, known as label noise, which degrades classification accuracy. To address this issue, recently, various works propose the algorithms to clean datasets with label noise. We categorize the algorithms in granular ways, and review the algorithms, such as sample selection, label correction, and select-and-correct algorithms, based on the categorization. In addition, we provide future research directions for cleaning datasets, considering practical challenges, such as class imbalance, class incremental learning, and corrupted input features.
J. Won, D.-Y. Kim, and J.-W. Lee, "Joint optimization of location, beam, and radio resource for an aerial base station with controllable directional antennas," IEEE Internet of Things Journal, vol. 11, no. 16, pp. 27571-27583, Aug. 2024 (10.1109/JIOT.2024.3399225). | |

Joint optimization of location, beam, and radio resource for an aerial base station with controllable directional antennas

Recent advancements in an unmanned aerial vehicle (UAV)-enabled network have demonstrated potential of a directional antenna to enhance network performance by utilizing limited resources more efficiently. In the UAV-enabled network where a directional antenna is utilized, controlling both its beam direction and beamwidth appropriately is an important issue in order to maximize its efficiency. Existing studies on the UAV-enabled network with a directional antenna, however, have primarily concentrated on adjusting antenna’s beamwidth with a fixed beam direction for simplicity. In this paper, we explore joint optimization of both beam direction and beamwidth of a UAV equipped with controllable directional antennas. To this end, we consider a UAV-enabled network where the UAV functions as an aerial base station (ABS), relaying data from a ground base station (GBS) to multiple ground users (GUs), aiming at maximizing the sum rate for all GUs by controlling the location, beam direction, and beamwidth of the UAV and resource allocation. To address this complex problem, we develop an algorithm called Joint optimization of location, beam direction, beamwidth, and resource allocation (Joint-LDWR). Through comprehensive simulations, we show the outstanding performance of Joint-LDWR, focusing on its efficiency for enhancing network performance. The results highlight a significant benefit of simultaneously controlling beam direction and beamwidth of the ABS together with its location in the UAV-enabled network.
Y.-I. Park, D.-Y. Kim, and J.-W. Lee, "Hybrid offline-online UAV trajectory design and sub-channel allocation in UAV relaying OFDMA networks," IEEE Internet of Things Journal, vol. 11, no. 15, pp. 26173-26190, Aug. 2024 (10.1109/JIOT.2024.3393289). | |

Hybrid offline-online UAV trajectory design and sub-channel allocation in UAV relaying OFDMA networks

In this paper, we study an unmanned aerial vehicle (UAV) relaying orthogonal frequency division multiple access (OFDMA) network with multiple user equipment (UE) pairs, each having explicitly given quality-of-service (QoS) requirements. Our goal is to maximize the system throughput while satisfying the QoS requirements for UE pairs, despite the air-to-ground (A2G) channel randomness. To this end, we develop a hybrid offline-online algorithm that designs the UAV’s 3D trajectory in an offline manner and then allocates sub-channels during the UAV flight in an online manner. Under the proposed algorithm, the UAV’s 3D trajectory is elaborately designed with statistical channel state information (CSI), and sub-channels are opportunistically allocated based on instantaneous CSI. In practice, the QoS requirements might not be guaranteed since perfect CSI cannot be obtained a priori. Nevertheless, the proposed algorithm significantly improves the QoS satisfaction of UE pairs to the fullest extent possible by leveraging available CSI. Through simulation, we validate the performance of the proposed algorithm in enhancing the QoS satisfaction of UE pairs and the system throughput.
J. Won, D.-Y. Kim, Y.-I. Park, and J.-W. Lee, "A survey on UAV placement and trajectory optimization in communication networks: From the perspective of air-to-ground channel models," ICT Express, vol. 9, no. 3, pp. 385-397, Jun. 2023 (10.1016/j.icte.2022.01.015). | |

A survey on UAV placement and trajectory optimization in communication networks: From the perspective of air-to-ground channel models

Unmanned aerial vehicles (UAVs) have been utilized extensively in communication networks due to their versatility and maneuverability. It is also necessary to optimize their locations or trajectories. Moreover, it is important to adopt an appropriate air-to-ground (A2G) channel model that can reflect the practical environment well. In this paper, we first introduce the representative A2G channel models widely adopted in the literature and then review recent works on UAV placement and trajectory optimization from the perspective of A2G channel models. Finally, we present future research directions based on trends and lessons derived from our review of the existing studies.
Md. Shahjalal, W. Kim, W. Khalid, S. Moon, M. Khan, S. Liu, S. Lim, E. Kim, D.-W. Yun, J. Lee, W.-C. Lee, S.-H. Hwang, D. Kim, J.-W. Lee, H. Yu, Y. Sung, and Y. M. Jang, "Enabling technologies for AI empowered 6G massive radio access networks," ICT Express, vol. 9, no. 3, pp. 341-355, Jun. 2023 (10.1016/j.icte.2022.07.002). | |

Enabling technologies for AI empowered 6G massive radio access networks

Predictably, the upcoming six generation (6G) networks demand ultra-massive interconnectivity comprising densely congested sustainable small-to-tiny networks. The conventional radio access network (RAN) will be redesigned to provide the necessary intelligence in all areas to meet required network flexibility, full coverage, and massive access. In this respect, this paper focuses on intelligent massive RAN (mRAN) architecture and key technologies fulfilling the requirements. Particularly, we investigate potential artificial intelligence algorithms for network and resource management issues in 6G mRAN. Furthermore, we summarize the research issues in edge technologies and physical layer intelligence on 6G network architecture.
H.-S. Lee, S. Moon, D.-Y. Kim, and J.-W. Lee, "Packet-based fronthauling in 5G networks: Network slicing-aware packetization," IEEE Communications Standards Magazine, vol. 7, no. 2, pp. 56-63, Jun. 2023 (10.1109/MCOMSTD.0007.200062). | |

Packet-based fronthauling in 5G networks: Network slicing-aware packetization

Network slicing is one of the most important enablers of an evolved network architecture for 5G networks that can support the challenging quality-of-service (QoS) requirements of future communication services. Network slicing is realized by technologies, such as software-defined networks, network function virtualization, and cloud radio access network (C-RAN) with packet-based fronthaul (P-FH) interfaces. However, it is still challenging to effectively support the diverse QoS requirements of network slices with limited FH link resources. To this end, FH payloads should be packetized in an appropriate way to apply transport differentiation provided in the P-FH interfaces since they are transported in a form of packets. In this article, we first comprehensively investigate the characteristics of the components of the FH payloads. Based on the investigation, we then design a packetization method that enables the P-FH interfaces to address the diverse QoS requirements by effectively applying their transport differentiation to the generated packets. Finally, we discuss open issues on realizing the P-FH interface in practice.
D.-Y. Kim, W. Saad, and J.-W. Lee, "On the use of high-rise topographic features for optimal aerial base station placement," IEEE Transactions on Wireless Communications, vol. 22, no. 3, pp. 1868-1884, Mar. 2023 (10.1109/TWC.2022.3207427). | |

On the use of high-rise topographic features for optimal aerial base station placement

The use of unmanned aerial vehicles as aerial base stations (ABSs) can significantly enhance the capacity and coverage of wireless systems. In this paper, the problem of optimal ABS placement is studied while exploiting high-rise topographic features to maximize wireless coverage. In contrast to prior art that relies on simplified full line-of-sight (LoS) channel models or impractical probabilistic LoS channel models, this paper presents a novel feature-aware channel model that decisively discerns whether an air-to-ground (A2G) link is in LoS or non-LoS (NLoS) based on the topographical environment data for the target area. To resolve the challenges created by the dependence between the channel gain and the topographical environment in this feature-aware channel model, the LoS and NLoS zones in the target area are analyzed from a geometrical point of view. Then, based on the analysis results, the coverage area is derived in a tractable form and then used to develop a feature-aware ABS placement algorithm, called ABS-FA, based on particle swarm optimization (PSO). The effectiveness of the proposed approach is compared with two other baseline algorithms based on the full LoS and probabilistic LoS channel models, called ABS-LoS and ABS-Prob, respectively. Simulation results show that, depending on the topographical environment, ABS-LoS may outperform ABS-Prob, or vice versa, and even both may be very limited in some cases, because the full LoS and probabilistic LoS channel models cannot properly capture whether an A2G link is in LoS or NLoS. The results also show that the proposed ABS-FA scheme always outperforms these baseline algorithms and that, for instance, it can provide approximately 25% and 50% higher coverage performance compared to ABS-Prob and ABS-LoS, respectively. These results verify that considering a feature-aware channel model can be a very effective approach for determining the ABS location.
S.-M. Park, D.-Y. Kim, K.-W. Kim, and J.-W. Lee, "Joint antenna and device scheduling in full-duplex MIMO wireless powered communication networks," IEEE Internet of Things Journal, vol. 9, no. 19, pp. 18908-18923, Oct. 2022 (10.1109/JIOT.2022.3163421). | |

Joint antenna and device scheduling in full-duplex MIMO wireless powered communication networks

In this paper, we study a joint antenna and internet-of-things device (ID) scheduling problem in the full-duplex (FD) multiple-input multiple-output (MIMO) wireless powered communication network (WPCN) over time-varying fading channels. We first formulate an optimization problem to maximize the average sum rate of IDs while satisfying their minimum average data rate requirements by jointly scheduling ID selection for uplink data transmission, antenna switching, and beamforming. To deal with the problem, we propose a scheduling algorithm based on Lagrangian duality and the stochastic optimization theory. The proposed scheduling algorithm necessitates solving per-time-slot problems, each of which aims at maximizing the weighted sum of the selected ID’s uplink data rate and the non-selected IDs’ harvested power from the downlink by jointly optimizing ID selection, antenna switching between uplink and downlink, and beamforming at that time slot. To solve the per-time slot problem, we develop a joint ID selection, antenna switching, and beamforming (Joint-IAB) algorithm based on the block coordinate descent (BCD) and successive convex approximation (SCA) methods. Through simulation, we demonstrate that our scheduling algorithm with the proposed Joint-IAB algorithm provides better performance than the other scheduling algorithms while well satisfying the given minimum average data rate requirements of IDs.
K.-W. Kim, H.-S. Lee, R. Zhang, and J.-W. Lee, "Contextual learning-based waveform scheduling for wireless power transfer with limited feedback," IEEE Internet of Things Journal, vol. 9, no. 17, pp. 15578-15592, Sep. 2022 (10.1109/JIOT.2022.3150798). | |

Contextual learning-based waveform scheduling for wireless power transfer with limited feedback

In this paper, we study the waveform scheduling problem for a wireless power transfer (WPT) system consisting of a power beacon (PB) and multiple energy harvesting empowered Internet of Things (EH-IoT) devices. In each time slot, each device requests power to the PB if it needs power, and the PB transmits a WPT signal for which the waveform is designed based on the harvested power satisfaction rate of the power requesting devices. Under this setup, we formulate an optimization problem that maximizes the average number of EH-IoT devices whose power requests are satisfied. We first solve this problem assuming that the perfect channel state information (CSI) of all devices is known at the PB. Since the problem is difficult to solve even with perfect CSI, we transform it into a more tractable problem via proper approximations and propose an efficient algorithm to solve it. Next, to tackle the issue that it is practically difficult for the PB to acquire the perfect CSI of each device, we propose a contextual learning-based WPT waveform scheduling algorithm requiring only one-bit feedback from each device at one time. Numerical results show that our proposed waveform scheduling algorithm provides a higher satisfaction rate than existing algorithms under perfect CSI, and that with limited CSI feedback achieves performance close to the case with perfect CSI.
C. Im, J.-W. Lee, and C. Lee, "Dynamic energy beamforming for multiple IoT devices with frequency diverse array," IEEE Internet of Things Journal, vol. 9, no. 15, pp. 3995-14004, Aug. 2022 (10.1109/JIOT.2022.3143228). | |

Dynamic energy beamforming for multiple IoT devices with frequency diverse array

We propose a beamforming scheme called dynamic energy beamforming with a frequency diverse array (DEB-FDA) for enhancing the energy harvesting performance of multiple Internet of Things (IoT) devices in radio frequency-based wireless power transfer systems. We adopt the frequency diverse array (FDA) architecture, where the steering vector is a function of the distance and angle, to synthesize beam patterns that depend on both these parameters. We formulate an optimization problem for designing the transmit-beamforming vector and the frequency components of the FDA so that constructive interference occurs simultaneously at the locations of multiple IoT devices. We then develop an algorithm for dynamic energy beamforming based on alternating optimization using the minorize-maximization algorithm. Simulation results show that the proposed DEB-FDA scheme improves the energy harvesting performance of IoT devices in multiple locations.
H.-S. Lee, D.-Y. Kim, and J.-W. Lee, "Radio and energy resource management in renewable energy-powered wireless networks with deep reinforcement learning," IEEE Transactions on Wireless Communications, vol. 21, no. 7, pp. 5435-5449, Jul. 2022 (10.1109/TWC.2022.3140731). | |

Radio and energy resource management in renewable energy-powered wireless networks with deep reinforcement learning

In this paper, we study radio and energy resource management in renewable energy-powered wireless networks, where base stations (BSs) are powered by both on-grid and renewable energy sources and can share their harvested energy with each other. To efficiently manage those resources, we propose a hierarchical and distributed resource management framework based on deep reinforcement learning. The proposed framework minimizes the on-grid energy consumption while satisfying the data rate requirement of each user. It is composed of three different policies in a distributed and hierarchical way. An intercell interference coordination policy constrains the transmission power at each BS to coordinate the intercell interference among the BSs. Under the power constraints, a distributed radio resource allocation policy of each BS determines its own user scheduling and power control. Lastly, an energy sharing policy manages the energy resources of the BSs by sharing the harvested energy via power lines between them. Through the simulation, we demonstrate that the proposed framework can effectively reduce the on-grid energy consumption while satisfying the data rate requirements.
D.-Y. Kim, H. Jafarkhani, and J.-W. Lee, "Low-complexity dynamic resource scheduling for downlink MC-NOMA over fading channels," IEEE Transactions on Wireless Communications, vol. 21, no. 5, pp. 3536-3550, May 2022 (10.1109/TWC.2021.3123298). | |

Low-complexity dynamic resource scheduling for downlink MC-NOMA over fading channels

In this paper, we investigate dynamic resource scheduling (i.e., joint user, subchannel, and power scheduling) for downlink multi-channel non-orthogonal multiple access (MC-NOMA) systems over time-varying fading channels. Specifically, we address the weighted average sum rate maximization problem with quality-of-service (QoS) constraints. In particular, to facilitate fast resource scheduling, we focus on developing a very low-complexity algorithm. To this end, by leveraging Lagrangian duality and the stochastic optimization theory, we first develop an opportunistic MC-NOMA scheduling algorithm whereby the original problem is decomposed into a series of subproblems, one for each time slot. Accordingly, resource scheduling works in an online manner by solving one subproblem per time slot, making it more applicable to practical systems. Then, we further develop a heuristic joint subchannel assignment and power allocation (Joint-SAPA) algorithm with very low computational complexity, called Joint-SAPA-LCC, that solves each subproblem. Finally, through simulation, we show that our Joint-SAPA-LCC algorithm provides good performance comparable to the existing Joint-SAPA algorithms despite requiring much lower computational complexity. We also demonstrate that our opportunistic MC-NOMA scheduling algorithm in which the Joint-SAPA-LCC algorithm is embedded works well while satisfying given QoS requirements.
H.-S. Lee and J.-W. Lee, "Adaptive transmission scheduling in wireless networks for asynchronous federated learning," IEEE Journal on Selected Areas in Communications, vol. 39, no. 12, pp. 3673-3687, Dec. 2021 (10.1109/JSAC.2021.3118353). | |

Adaptive transmission scheduling in wireless networks for asynchronous federated learning

In this paper, we study asynchronous federated learning (FL) in a wireless distributed learning network (WDLN). To allow each edge device to use its local data more efficiently via asynchronous FL, transmission scheduling in the WDLN for asynchronous FL should be carefully determined considering system uncertainties, such as time-varying channel and stochastic data arrivals, and the scarce radio resources in the WDLN. To address this, we propose a metric, called an effectivity score, which represents the amount of learning from asynchronous FL. We then formulate an Asynchronous Learning-aware transmission Scheduling (ALS) problem to maximize the effectivity score and develop three ALS algorithms, called ALSA-PI, BALSA, and BALSA-PO, to solve it. If the statistical information about the uncertainties is known, the problem can be optimally and efficiently solved by ALSA-PI. Even if not, it can be still optimally solved by BALSA that learns the uncertainties based on a Bayesian approach using the state information reported from devices. BALSA-PO suboptimally solves the problem, but it addresses a more restrictive WDLN in practice, where the AP can observe a limited state information compared with the information used in BALSA. We show via simulations that the models trained by our ALS algorithms achieve performances close to that by an ideal benchmark and outperform those by other state-of-the-art baseline scheduling algorithms in terms of model accuracy, training loss, learning speed, and robustness of learning. These results demonstrate that the adaptive scheduling strategy in our ALS algorithms is effective to asynchronous FL.
S. Jung, J.-W. Lee, and C. Lee, "RSS-based channel estimation for IRS-aided wireless energy transfer system," IEEE Internet of Things Journal, vol. 8, no. 19, pp. 14860-14873, Oct. 2021 (10.1109/JIOT.2021.3071378). | |

RSS-based channel estimation for IRS-aided wireless energy transfer system

Although intelligent reflecting surface (IRS) is regarded as a promising solution to enhance the efficiency of wireless energy transfer (WET), the acquiring of channel state information (CSI) is a crucial challenge for the system in which a training sequence for channel estimation is sent by low-power Internet of Things (IoT) devices. In this paper, an IRS-aided multi-device WET system is considered. To overcome the limitation in channel estimation, we propose a received power-based channel estimation scheme that can be easily implemented and scalable in wirelessly empowered IoT devices. Specifically, at every single time slot, each device measures the received power of a randomly generated radio frequency (RF) signal and feeds it back to the transmitter. We formulate a channel estimation problem to use the history of received power measurements based on the maximum likelihood estimation using the phase retrieval framework and temporal channel evolution model. Moreover, we propose an algorithm that can be employed to obtain the stationary solution for the channel estimation problem, which is based on the inexact block coordinate descent method. We also perform algorithm modification to deal with the special case in which the transmitter-IRS channel is available. The simulation results show that the performance of the proposed algorithm approaches the upper bound as the channel slowly changes, although the proposed channel estimation protocol requires only one scalar value feedback.
Y.-X. Zhu, D.-Y. Kim, and J.-W. Lee, "Joint antenna and user scheduling in the massive MIMO system over time-varying fading channels," IEEE Access, vol. 9, pp. 92431-92445, Jun. 2021 (10.1109/ACCESS.2021.3092754). | |

Joint antenna and user scheduling in the massive MIMO system over time-varying fading channels

Massive multiple-input multiple-output (MIMO) technology, mainly equipped with dozens or even hundreds of antennas at transmitter and/or receiver, is one of the most important technologies in 5G/6G era due to its capability of achieving high transmission rate. However, since each transmit antenna typically needs a complete radio frequency (RF) chain, a large number of RF chains need to be installed accordingly, resulting in high economic cost, high hardware complexity, and high power consumption. To resolve these problems, architectures where a small number of RF chains are installed have been proposed in recent years, and an antenna selection technique that activates antennas only as many as the number of RF chains has been envisioned as one of solutions. In this paper, we study the joint antenna and user scheduling problem for the downlink massive MIMO system over time-varying fading channels to maximize the weighted average sum rate while ensuring users’ minimum average data rate requirements. To solve the problem, we first develop an opportunistic joint antenna and user scheduling algorithm (OJAUS) using the dual and stochastic subgradient methods, which makes it possible to schedule antennas and users without any underlying distributions of the fading channels. However, it requires solving a joint antenna and user selection (JAUS) problem to maximize the instantaneous weighted sum rate in every time slot. Thus, we additionally develop a simple heuristic JAUS algorithm with low computational complexity, called JAUS-LCC, which is executed in every time slot within OJAUS. Finally, through simulation results, we first show that our JAUS-LCC provides near-optimal performance despite requiring very low computational complexity, and then show that our OJAUS with JAUS-LCC well guarantees given minimum average data rate requirements.
B. Lee, H.-S. Lee, S. Moon, and J.-W. Lee, "Enhanced random access for massive-machine-type communications," IEEE Internet of Things Journal, vol. 8, no. 8, pp. 7046-7064, Apr. 2021 (10.1109/JIOT.2020.3038148). | |

Enhanced random access for massive-machine-type communications

In this article, we study a random access (RA) scheme to alleviate the RA channel (RACH) overload problem in the massive-machine-type communication (mMTC) environment. We first propose a timing advance-based preamble resource expansion (TAPRE) scheme which effectively increases preamble resources and reduces the preamble collision probability by adjusting preamble transmission timing with timing advance (TA) information. We also propose a resource allocation wait (RAW) scheme which efficiently reduces the number of RA failures due to the lack of physical uplink shared channel (PUSCH) resources. We then propose an overall procedure for enhanced RA with TAPRE and RAW (ERATAR). In addition, we provide the analysis of RA performance by applying more practical assumptions than the existing analysis. We validate our analysis with the system level simulation based on NS-3, and compare the various performances of our ERATAR to those of existing works. Numerical results show that our analysis provides more accurate results than the existing work and our ERATAR provides significantly improved performances compared with those of existing works.
H.-S. Lee and J.-W. Lee, "Adaptive wireless power transfer beam scheduling for non-static IoT devices using deep reinforcement learning," IEEE Access, vol. 8, pp. 206659-206673, Nov. 2020 (10.1109/ACCESS.2020.3037323). | |

Adaptive wireless power transfer beam scheduling for non-static IoT devices using deep reinforcement learning

In this paper, we study wireless power transfer (WPT) beam scheduling for a system which consists of IoT devices and a power beacon (PB) using switched beamforming. In such a system, the IoT devices have a non-static behavior (e.g., their location and power requests keep changing) in general, which conventional WPT beam scheduling algorithms are not capable of adaptively dealing with. To address the non-static behavior, we propose a procedure of deep neural network (DNN)-based WPT beam scheduling. In the procedure, the power-deficient IoT devices transmit a common pilot signal simultaneously. Then, the PB effectively provides power to them with a DNN-based WPT beam scheduling policy. In the DNN-based policy, an estimation of the non-static behavior from the received pilot signals and an adaptive beam generation considering the estimated non-static behavior are integrated thanks to the powerful representational capability of DNNs. To allow the DNN-based policy to learn the optimal policy, we propose a Deep WPT Beam scheduling policy Gradient (DWBG) algorithm using deep reinforcement learning. Through the simulation, we show that DWBG achieves a close performance to the optimal policy. This demonstrates that our algorithm can be applied for practical WPT IoT systems with non-static IoT devices.
K.-W. Kim, H.-S. Lee, and J.-W. Lee, "Opportunistic waveform scheduling for wireless power transfer with multiple devices," IEEE Transactions on Wireless Communications, vol. 19, no. 9, pp. 5651-5665, Sep. 2020 (10.1109/TWC.2020.2994161). | |

Opportunistic waveform scheduling for wireless power transfer with multiple devices

In this paper, we study a waveform scheduling problem for a multi-receiver wireless power transfer (WPT) system considering time-varying channel conditions and minimum average output direct-current (DC) voltage requirement of each receiver. To this end, we formulate a stochastic optimization problem that aims at maximizing the average of the sum of output DC voltages of receivers while satisfying the minimum average output DC voltage requirements of all receivers, and by solving it, we develop a waveform scheduling algorithm. In the waveform scheduling algorithm, we need to solve a problem for maximizing the weighted-sum of output DC voltages of receivers, which is a non-convex optimization problem. To cope with this difficulty, we develop a low-complexity approximated algorithm with which the waveform for the multi-receiver WPT system is optimized to maximize the weighted sum of output DC voltages of receivers. Numerical results show that our waveform design algorithm provides the higher performance of the weighted-sum of the output DC voltages than the existing algorithms, and our opportunistic waveform scheduling provides good performance while well satisfying the minimum average output DC voltage requirement of each receiver.
H.-S. Lee, J.-Y. Kim, and J.-W. Lee, "Resource allocation in wireless networks with deep reinforcement learning: A circumstance-independent approach," IEEE Systems Journal, vol. 14, no. 2, pp. 2589-2592, Jun. 2020 (10.1109/JSYST.2019.2933536). | |

Resource allocation in wireless networks with deep reinforcement learning: A circumstance-independent approach

In the conventional approaches using reinforcement learning (RL) for resource allocation in wireless networks, the structure of the policy depends on network circumstances such as the number of users and quality-of-service requirements. Due to this dependence, the policy is hard to be used in a practical system where the network circumstance is dynamically changing. To resolve this issue, we propose a circumstance-independent policy that can effectively address the different network circumstances even with a single policy. Thus, contrary to the conventional RL approaches, the proposed policy can be easily applied in the practical system. We then develop a deep RL algorithm to learn it. Through simulation results, we show that a single proposed policy can be used over different circumstances, and it achieves a close performance to the circumstance-dependent policy for each circumstance, which learns the optimal policy for the corresponding circumstance.
H.-S. Lee and J.-W. Lee, "EHLinQ: Distributed scheduler for D2D communication with RF energy harvesting," IEEE Systems Journal, vol. 14, no. 2, pp. 2281-2292, Jun. 2020 (10.1109/JSYST.2019.2918806). | |

EHLinQ: Distributed scheduler for D2D communication with RF energy harvesting

In this paper, we study distributed scheduling for a device-to-device (D2D) communication system with radio frequency energy harvesting (RF-EH) in which D2D devices can harvest energy using the ambient RF signals from other transmitting D2D devices. In the system, due to RF-EH, the effects of link scheduling for each D2D link on the system performance, i.e., average sum rate and average total harvested energy, become much more complicated compared with conventional D2D communication without RF-EH. To address such complicated effects in a distributed manner, we study a centralized optimal link scheduling algorithm, which aims at maximizing the weighted sum of average sum rate and average total harvested energy, while satisfying the quality of service (QoS) requirements of D2D links. We then abstract the scheduling principles in the optimal algorithm, and propose a distributed scheduling algorithm for D2D communication with RF-EH, EHLinQ, which realizes the principles in a distributed manner. Moreover, we extend EHLinQ to a power beacon (PB) aided D2D communication system in which a PB transmits RF signals dedicated for RF-EH. The extended EHLinQ allows the PB to determine its own PB scheduling considering the QoS satisfaction of D2D links. Through simulation results, we show that EHLinQ outperforms conventional algorithms, while closely meeting the QoS requirements.
D.-Y. Kim and J.-W. Lee, "Joint mission assignment and topology management in the mission-critical FANET," IEEE Internet of Things Journal, vol. 7, no. 3, pp. 2368-2385, Mar. 2020 (10.1109/JIOT.2019.2958130). | |

Joint mission assignment and topology management in the mission-critical FANET

In recent years, the emergence of flying ad hoc networks (FANETs) with multiple unmanned aerial vehicles (UAVs) can make it possible to effectively perform not only far-off missions but also assorted complex missions. In this paper, we consider a mission-critical FANET to perform given missions using multiple UAVs, taking into account a dynamic environment with a time-varying network topology. To effectively operate the mission-critical FANET, we study joint mission assignment and topology management problem aiming at maximizing the weighted sum of mission and network performances, while guaranteeing end-to-end communications between mission-performing UAVs and their corresponding ground control stations (GCSs), inter-UAV safety distance maintenance, and other mission-related constraints. To address this problem, we first develop three algorithms: One is to construct a mission-critical FANET from scratch, and the others are to manage the network topology and to switch UAV roles between mission-performing and data-relaying in response to the changes in the network topology. Then, we develop a dynamic mission-critical FANET operation algorithm incorporating the three algorithms with a few rules, by which the mission-critical FANET can be effectively managed and operated with reasonable computational complexity in the dynamic environment. Through simulation results, we show that our proposed algorithm works well in the dynamic environment while satisfying the constraints, and that its performance is not only superior to the existing algorithms, but also close to the optimal performance.
H.-S. Lee and J.-W. Lee, "Adaptive traffic management and energy cooperation in renewable-energy-powered cellular networks," IEEE Systems Journal, vol. 14, no. 1, pp. 132-143, Mar. 2020 (10.1109/JSYST.2018.2890281). | |

Adaptive traffic management and energy cooperation in renewable-energy-powered cellular networks

In this paper, we consider a cellular system, in which base stations (BSs) are powered by both on-grid and renewable energy sources. To efficiently utilize the harvested energy of the BSs, we study adaptive traffic management (TM) and energy cooperation (EC) that aim at minimizing the on-grid energy consumption, while guaranteeing minimum average throughputs. To achieve this, we develop an adaptive TM and EC algorithm that jointly decides the energy sharing among BSs, the user association to BSs, and the sub-channel and power allocation in BSs.Within the algorithm, a network scheduling problem, which is mixed-integer non-linear programming (MINLP), should be solved in each timeslot. To efficiently solve it, we develop a network scheduling algorithm applying generalized Benders decomposition (GBD) that optimally solves the MINLP problem. In addition, we also develop a heuristic network scheduling algorithm that has a much lower computational complexity than the GBD algorithm, while providing comparable performance. Through the numerical results, we show that our algorithms always outperform the algorithms that use only one of TM or EC regardless of the system conditions.
C. Im, J.-W. Lee, and C. Lee, "A multi-tone amplitude modulation scheme for wireless information and power transfer," IEEE Transactions on Vehicular Technology, vol. 69, no. 1, pp. 1147-1151, Jan. 2020 (10.1109/TVT.2019.2954860). | |

A multi-tone amplitude modulation scheme for wireless information and power transfer

We propose a multi-tone amplitude modulation (MAM) scheme and its receiver architecture for simultaneous wireless information and power transfer (SWIPT). The MAM symbol consists of two-dimensional signaling of the amplitude component and the subcarrier-number component. The two components are decoded through the current intensity of rectifier output and peak to average power ratio (PAPR) value, respectively. Harvested energy and symbol error rate (SER) of proposed scheme are analyzed and evaluated for various bit assignments of two components. The simulation results show that proposed scheme achieves improved SER performance while achieving high energy harvesting.
H.-S. Lee and J.-W. Lee, "Contextual learning-based wireless power transfer beam scheduling for IoT devices," IEEE Internet of Things Journal, vol. 6, no. 6, pp. 9606-9620, Dec. 2019 (10.1109/JIOT.2019.2930061). | |

Contextual learning-based wireless power transfer beam scheduling for IoT devices

In this paper, we consider IoT systems in which IoT devices request power to a power beacon (PB) when their available power is deficient and the PB provides power to the IoT devices using switched beamforming. We study WPT beam scheduling for the IoT systems under one-bit feedback which aims at maximizing the time-average number of the IoT devices whose power requests are satisfied. To achieve this, we propose a Contextual learning based WPT Beam scheduling algorithm with One-bit feedback (CWBO) that learns the channel information using only one-bit feedback information and exploits it for the beam scheduling. Within CWBO, a beam pattern generation (BPG) problem should be solved in each time-slot. To efficiently solve it, we develop a BPG algorithm based on monotonic optimization that can optimally solve the BPG problem. In addition, we also develop a heuristic BPG algorithm that has a lower computational complexity than the monotonic optimization-based BPG algorithm, while providing comparable performance. For CWBO in single-device WPT, we prove an analytical performance bound, which shows its optimality in terms of the long-term average performance even with one-bit feedback. In addition, through the simulation results, we show that our algorithms achieve performances close to that of the optimal beam scheduling policy in multi-device WPT as well. This demonstrates that our algorithms can be used for WPT IoT systems with IoT devices having only limited capabilities for feedback and estimation of the channel information due to their limited power.
D.-Y. Kim, H.-S. Lee, K.-W. Kim, and J.-W. Lee, "Dual amplitude shift keying with double half-wave rectifier for SWIPT," IEEE Wireless Communications Letters, vol. 8, no. 4, pp. 1020-1023, Aug. 2019 (10.1109/LWC.2019.2904482). | |

Dual amplitude shift keying with double half-wave rectifier for SWIPT

We propose a receiver architecture and two modulation schemes for simultaneous wireless information and power transfer (SWIPT), in which two differently rectified signals are used for information decoding (ID) and energy harvesting (EH). The first scheme is called amplitude difference shift keying (ADSK), which gives a low bit error rate (BER) by transferring information through the amplitude difference between the two rectified signals. The second is called amplitude ratio shift keying (ARSK), which offers considerable harvested energy by transferring information through the amplitude ratio between the two rectified signals. We demonstrate how they work and provide their performance through simulations.
H.-S. Lee and J.-W. Lee, "Resource and task scheduling for SWIPT IoT systems with renewable energy sources," IEEE Internet of Things Journal, vol. 6, no. 2, pp. 2729-2748, Apr. 2019 (10.1109/JIOT.2018.2873658). | |

Resource and task scheduling for SWIPT IoT systems with renewable energy sources

In this paper, we consider IoT systems that can be applied to various applications with low-mobility or static IoT devices such as wireless sensor networks and charging systems for low-power devices with communication. The IoT systems consist of IoT devices and a hybrid access point (H-AP) powered by both on-grid and renewable energy sources. The IoT devices have a capability to harvest energy from the H-AP's RF signal, and they perform their tasks by using only their harvested energy. We consider the tasks do not have a real-time requirement which can be stored in the task queues of the IoT devices and performed later. We study resource and task scheduling for the IoT systems which aims at minimizing the on-grid energy consumption at the H-AP while guaranteeing the minimum average data rate and minimum task performing rates of IoT devices. To achieve the goal, we first propose a centralized resource and task scheduling algorithm. However, its computational complexity and signaling overhead are too large due to the task scheduling for each IoT device. Thus, to resolve these issues, we propose a hybrid resource and task scheduling algorithm in which each IoT device determines its own task scheduling in a distributed manner and the H-AP determines the resource scheduling. We then provide performance analyses showing that our proposed algorithms are asymptotically optimal and well satisfy the QoS requirements of IoT devices even with distributed task scheduling. Through the simulation results, we verify the analyses and show the performance of our algorithms.
S. Jung, J.-W. Lee, and C. Lee, "Transmit beamforming and integer matrix design for MISO SWIPT systems with integer forcing," IEEE Wireless Communications Letters, vol. 8, no. 1, pp. 229-232, Feb. 2019 (10.1109/LWC.2018.2867863). | |

Transmit beamforming and integer matrix design for MISO SWIPT systems with integer forcing

This letter considers simultaneous wireless information and power transfer (SWIPT) in multiple-input single-output (MISO) broadcasting systems based on the Integer Forcing (IF) technique where multiple receivers are equipped with a power splitting circuit and able to harvest energy and decode information at the same time. The optimization problem that incorporates the beamforming matrix and IF coefficients matrix designs is formulated to minimize the transmit power at transmitter, while guaranteeing the achievable rate and harvested energy threshold of each receiver. To tackle the NP-hardness and non-convexity of the optimization problem, we propose an iterative algorithm consisting of two sub-problems. Simulation results provide the comparison of the performance of SWIPT systems with and without IF technique.
K.-W. Kim, H.-S. Lee, and J.-W. Lee, "Waveform design for fair wireless power transfer with multiple energy harvesting devices," IEEE Journal on Selected Areas in Communications, vol. 37, no. 1, pp. 34-47, Jan. 2019 (10.1109/JSAC.2018.2872311). | |

Waveform design for fair wireless power transfer with multiple energy harvesting devices

In this paper, we study the waveform design in the wireless power transfer (WPT) system with multiple receivers. In the multi-receiver WPT system, due to the severe power attenuation of RF signals according to the distance and the heterogeneity in the energy harvesting capability, there exists severe unfairness in energy harvesting among receivers at different distances from the transmitter and/or different energy harvesting capabilities. Hence, alleviating unfairness in energy harvesting among receivers is one of the critical challenges in the multi-receiver WPT system. To tackle this challenge in a systematic way, we consider the fairness in our waveform design applying the α-proportional fairness. With the analysis of the rectenna circuit, we derive the output dc voltage and power of the rectifier in closed forms. Thereby, we formulate an optimization problem to design the waveform for fair WPT. The problem is shown to be a non-convex optimization problem, which is hard to solve in general. However, with proper approximations, we convert it into a convex optimization problem that can be solved easily and obtain the waveform for fair WPT. Numerical results show that our designed waveform makes receivers harvest energy fairly and can control the degree of the fairness easily.
D.-Y. Kim and J.-W. Lee, "Integrated topology management in flying ad hoc networks: Topology construction and adjustment," IEEE Access, vol. 6, pp. 61196-61211, Oct. 2018 (10.1109/ACCESS.2018.2875679). | |

Integrated topology management in flying ad hoc networks: Topology construction and adjustment

Flying ad hoc networks (FANETs) that consist of multiple unmanned aerial vehicles (UAVs) are promising technologies for future networked systems due to the versatility of UAVs. One of the most distinguishing features of FANET is frequent and rapid topological fluctuations due to the high-mobility of UAVs. Hence, the topology management adapting to the movements of UAVs is one of the most critical issues in FANET. In this paper, we study a FANET topology management problem that optimizes the locations and movements of UAVs to maximize the network performance, adapting to the topological changes while UAVs carry out their missions. When formulating the problem, we take into account the routing protocol as an arbitrary function since the network performance is inseparably linked with the routing protocol in use. We first develop two algorithms. One is the topology construction algorithm, which constructs a FANET topology from the scratch without any given initial topology, based on particle swarm optimization. The other is the topology adjustment algorithm, which incrementally adjusts the FANET topology adapting to the movements of UAVs with low-computational costs, based on gradient descent. Then, by defining a logical distance (the so-called topology edit distance) that measures the degree of changes in FANET topology, we develop an integrated topology management algorithm that contains the topology construction and adjustment algorithms. The simulation results show that our algorithm achieves a good network performance with low computational overhead, which is one of the most essential virtues in FANETs with rapidly varying topology.
S. Moon, H.-S. Lee, and J.-W. Lee, "SARA: Sparse Code Multiple Access-Applied Random Access for IoT Devices," IEEE Internet of Things Journal, vol. 5, no. 4, pp. 3160-3174, Aug. 2018 (10.1109/JIOT.2018.2835828). | |

SARA: Sparse Code Multiple Access-Applied Random Access for IoT Devices

In this paper, we study a random access (RA) procedure to support the massive connectivity of the Internet of Things (IoT) devices, also known as the IoT connectivity. Compared with the previous RA procedures that have limitations to support the IoT connectivity due to the exponentially increased access delay, we develop an RA procedure by applying the sparse code multiple access (SCMA) to reduce the access delay and increase the ratio of the IoT devices that successfully complete their RA procedures. We provide the theoretical performance analysis of the proposed RA procedure with the performance metrics such as the RA success probability, the average access delay, the RA throughput, and the average number of preamble transmissions. Then, we provide the numerical results to evaluate the performance of the proposed RA procedure based on our analysis and the ns-3 simulator. Numerical results show that our proposed RA procedure is able to support the massive connectivity requirement with improved RA performance metrics compared with the conventional RA procedures.
H.-S. Lee, C. Tekin, M. van der Schaar, and J.-W. Lee, "Adaptive contextual learning for unit commitment in microgrids with renewable energy sources," IEEE Journal of Selected Topics in Signal Processing, vol. 12, no. 4, pp. 688-702, Aug. 2018 (10.1109/JSTSP.2018.2849855). | |

Adaptive contextual learning for unit commitment in microgrids with renewable energy sources

In this paper, we study a unit commitment (UC) problem where the goal is to minimize the operating costs of a microgrid that involves renewable energy sources. Since traditional UC algorithms use a priori information about uncertainties such as the load demand and the renewable power outputs, their performances highly depend on the accuracy of the a priori information, especially in microgrids due to their limited scale and size. This makes the algorithms impractical in settings where the past data is not sufficient to construct an accurate prior of the uncertainties. To resolve this issue, we develop an adaptively partitioned contextual learning algorithm for UC (APCLUC) that learns the best UC schedule and minimizes the total cost over time in an online manner without requiring any a priori information. AP-CLUC effectively learns the effects of the uncertainties on the cost by adaptively considering context information strongly correlated with the uncertainties, such as the past load demand and weather condition. For AP-CLUC, we first prove an analytical bound on the performance, which shows that its average total cost converges to that of the optimal policy with perfect a priori information. Then, we show via simulations that AP-CLUC achieves competitive performance with respect to the traditional UC algorithms with perfect a priori information, and it achieves better performance than them even with small errors on the information. These results demonstrate the effectiveness of utilizing the context information and the adaptive management of the past data for the UC problem.
S. Moon and J.-W. Lee, "Multi-residential demand response scheduling with multi-class appliances in smart grid," IEEE Transactions on Smart Grid, vol. 9, no. 4, pp. 2518-2528, Jul. 2018 (10.1109/TSG.2016.2614546). | |

Multi-residential demand response scheduling with multi-class appliances in smart grid

In this paper, we study a multi-residential electricity load scheduling problem with multi-class appliances in smart grid. Compared with the previous works in which only limited types of appliances are considered or only single residence grids are considered, we model the grid system more practically with jointly considering multi-residence and multi-class appliance. We formulate an optimization problem to maximize the sum of the overall satisfaction levels of residences which is defined as the sum of utilities of the residential customers minus the total cost for energy consumption. Then, we provide an electricity load scheduling algorithm by using a PL-Generalized Benders Algorithm which operates in a distributed manner while protecting the private information of the residences. By applying the algorithm, we can obtain the near-optimal load scheduling for each residence, which is shown to be very close to the optimal scheduling, and also obtain the lower and upper bounds on the optimal sum of the overall satisfaction levels of all residences, which are shown to be very tight.
H.-S. Lee and J.-W. Lee, "Task offloading in heterogeneous mobile cloud computing: Modeling, analysis, and cloudlet deployment," IEEE Access, vol. 6, pp. 14908-14925, Mar. 2018 (10.1109/ACCESS.2018.2812144). | |

Task offloading in heterogeneous mobile cloud computing: Modeling, analysis, and cloudlet deployment

In this paper, we consider a heterogeneous mobile cloud computing (HMCC) system that consists of remote cloud servers, local cloudlets, task offloading mobile devices (TMDs), non-task offloading MDs (NTMDs), and radio access networks such as cellular networks and WLANs. TMDs have the capability of task offloading to remote cloud servers or cloudlets, whereas NTMDs are conventional cellular users that do not have such capability. By using stochastic geometry, we analyze the outage probability of task offloading in the MCC system with only remote cloud servers and that in the HMCC with both remote cloud servers and cloudlets. The analysis provides useful information, i.e., how the varying system parameters affect the outage probability. From the analysis, we show that there is an intrinsic limitation in reducing the outage probability in the MCC system due to the outage when accessing remote cloud servers. In addition, we show that the use of cloudlets is a promising solution to overcome this limitation. However, a tradeoff exists in using cloudlets due to their deployment and operation costs. Thus, to address this tradeoff, we also study the optimal cloudlet deployment to maximize the cloud service provider’s profit while guaranteeing maximum outage probability requirements.
J.-H. Kim, S.-C. Choi, J. Yun, and J.-W. Lee, "Towards the oneM2M standards for building IoT ecosystem: Analysis, implementation and lessons," Peer-to-Peer Networking and Applications, vol. 11, no. 1, pp. 139-151, Jan. 2018 (10.1007/s12083-016-0505-9). | |

Towards the oneM2M standards for building IoT ecosystem: Analysis, implementation and lessons

As the Internet of Things (IoT) revolution presents an enormous opportunity for all industry verticals ranging from startups to large enterprises to create new types of services, standard bodies and global alliances have been working on establishing common standards for IoT systems. The oneM2M is the global partnership developing standards for Machine-to-Machine (M2M) communications and the Internet of Things. It develops technical specifications for the globally-applicable, interoperable common M2M/IoT service layer platforms, which play a pivotal role in building the ecosystem driven by key players, including developers and consumers. In this paper, we analyze the oneM2M standards, and introduce Mobius and &Cube, which are oneM2M-compliant M2M/IoT software platforms for servers and devices, respectively. We also present four pilot services using the platforms and several prototype IoT devices. Finally, we discuss three aspects, advanced discovery, open API, and peer-to-peer that are required for the oneM2M to build IoT ecosystem by attracting developers and consumers into the emerging IoT ecosystem.
J.-H. Kim and J.-W. Lee, "Energy adaptive MAC for wireless sensor networks with RF energy transfer: algorithm, analysis, and implementation," Telecommunication Systems, vol. 64, no. 2, pp. 293-307, Feb. 2017 (10.1007/s11235-016-0176-0). | |

Energy adaptive MAC for wireless sensor networks with RF energy transfer: algorithm, analysis, and implementation

Radio frequency energy transfer (RET) has been proposed as a promising solution to power sensor nodes in wireless sensor networks (WSNs). However, RET has a significant drawback to be directly applied to WSNs, i.e., unfairness in the achieved throughput among sensor nodes due to the difference of their energy harvesting rates that strongly depend on the distance between the energy emitting node and the energy harvesting nodes. The unfairness problem should be properly taken into account to mitigate the drawback caused from the features of RET. To resolve this issue, in this paper, we propose a medium access control (MAC) protocol for WSNs based on RET with two distinguishing features: energy adaptive (EA) duty cycle management that adaptively manages the duty cycle of sensor nodes according to their energy harvesting rates and EA contention algorithm that adaptively manages contentions among sensor nodes considering fairness. Through analysis and simulation, we show that our MAC protocol works well under the RET environment. Finally, to show the feasibility of WSNs with RET, we test our MAC protocol with a prototype system in a real environment.
H.-S. Lee and J.-W. Lee, "QC²LinQ: QoS and channel-aware distributed link scheduler for D2D communication," IEEE Transactions on Wireless Communications, vol. 15, no. 12, pp. 8565-8579, Dec. 2016 (10.1109/TWC.2016.2616340). | |

QC²LinQ: QoS and channel-aware distributed link scheduler for D2D communication

We study a distributed link scheduling problem for device-to-device (D2D) communication considering the quality-of-service (QoS) requirements and time-varying channel conditions of D2D links. To this end, we first study an optimal centralized link scheduling problem maximizing the total average sum-rate while satisfying the QoS requirements of D2D links. We then abstract the important scheduling principles of the optimal link scheduling, i.e., giving more chance to be scheduled to the links which have a good channel condition and do not satisfy the QoS requirement, in order to utilize them to develop distributed link scheduling algorithms. With the scheduling principles, we develop a procedure with which D2D links can share their degree of QoS unsatisfaction and channel condition with each other and generate their scheduling priorities according to the shared information in a distributed manner. We also develop a novel distributed link scheduling criterion with which D2D links determine their link scheduling. By using them, we propose distributed link scheduling algorithms, QCLinQ and QC²LinQ, which have significantly smaller signaling overhead and low computational complexity compared with the centralized optimal link scheduling algorithm. Moreover, they closely meet the QoS requirements of D2D links while achieving significant sum-rate improvement over conventional distributed algorithms.
H.-T. Roh and J.-W. Lee, "Channel assignment, link scheduling, routing, and rate control for multi-channel wireless mesh networks with directional antennas," Journal of Communications and Networks, vol. 18, no. 6, pp. 884-891, Dec. 2016 (10.1109/JCN.2016.000123). | |

Channel assignment, link scheduling, routing, and rate control for multi-channel wireless mesh networks with directional antennas

The wireless mesh network (WMN) has attracted significant interests as a broadband wireless network to provide ubiquitous wireless access for broadband services. Especially with incorporating multiple orthogonal channels and multiple directional antennas into the WMN, each node can communicate with its neighbor nodes simultaneously without interference between them. However, as we allow more freedom, we need a more sophisticated algorithm to fully utilize it and developing such an algorithm is not easy in general. In this paper, we study a joint channel assignment, link scheduling, routing, and rate control problem for the WMN with multiple orthogonal channels and multiple directional antennas. This problem is inherently hard to solve, since the problem is formulated as a mixed integer nonlinear problem (MINLP). However, despite of its inherent difficulty, we develop an algorithm to solve the problem by using the generalized Benders decomposition approach [2]. The simulation results show the proposed algorithm provides the optimal solution to maximize the network utility, which is defined as the sum of utilities of all sessions.
B.-G. Kim, Y. Zhang, M. van der Schaar, and J.-W. Lee, "Dynamic pricing and energy consumption scheduling with reinforcement learning," IEEE Transactions on Smart Grid, vol. 7, no. 5, pp. 2187-2198, Sep. 2016 (10.1109/TSG.2015.2495145). | |

Dynamic pricing and energy consumption scheduling with reinforcement learning

In this paper, we study a dynamic pricing and energy consumption scheduling problem in the microgrid where the service provider acts as a broker between the utility company and customers by purchasing electric energy from the utility company and selling it to the customers. For the service provider, even though dynamic pricing is an efficient tool to manage the microgrid, the implementation of dynamic pricing is highly challenging due to the lack of the customer-side information and the various types of uncertainties in the microgrid. Similarly, the customers also face challenges in scheduling their energy consumption due to the uncertainty of the retail electricity price. In order to overcome the challenges of implementing dynamic pricing and energy consumption scheduling, we develop reinforcement learning algorithms that allow each of the service provider and the customers to learn its strategy without a priori information about the microgrid. Through numerical results, we show that the proposed reinforcement learning-based dynamic pricing algorithm can effectively work without a priori information about the system dynamics and the proposed energy consumption scheduling algorithm further reduces the system cost thanks to the learning capability of each customer.
H.-T. Roh and J.-W. Lee, "Residential demand response scheduling for multiclass appliances in the smart grid," IEEE Transactions on Smart Grid, vol. 7, no. 1, pp. 94-104, Jan. 2016 (10.1109/TSG.2015.2445491). | |

Residential demand response scheduling for multiclass appliances in the smart grid

In this paper, we study an electricity load scheduling problem in a residence. Compared with previous works in which only limited sets of appliances are considered, we classify various appliances into five sets considering their different energy consumption and operation characteristics, and provide mathematical models for them. With these appliance models, we propose an electricity load scheduling algorithm that controls the operation time and energy consumption level of each appliance adapting to time-of-use pricing in order to maximize the overall net utility of the residence while satisfying its budget limit. The optimization problem is formulated as a mixed integer nonlinear programming (MINLP) problem, which is in general, difficult to solve. In order to solve the problem, we use the generalized Benders decomposition approach with which we can solve the MINLP problem easily with low computational complexity. By solving the problem, we provide an algorithm to obtain the optimal electricity load scheduling of various appliances with different energy consumption and operation characteristics in a unified way.
J.-H. Song, H.-T. Roh, and J.-W. Lee, "Opportunistic scheduling and incentive mechanism for OFDMA networks with D2D relaying," Computer Networks, vol. 91, pp. 772-787, Nov. 2015 (10.1016/j.comnet.2015.08.045). | |

Opportunistic scheduling and incentive mechanism for OFDMA networks with D2D relaying

The device-to-device (D2D) relaying is considered one of promising technologies to improve the spectral efficiency and extend the coverage of the cellular system with low additional costs. In the system with D2D relaying, some of user equipments (UEs) can act as relay stations (RSs) that forward other UEs' data from/to the base station (BS). Compared with the RS, the D2D relaying has several advantages such as low deployment costs and high flexibility. We study an opportunistic subchannel scheduling problem in the OFDMA cellular network with D2D relaying in this paper. We formulate a stochastic optimization problem to maximize the sum-rate of the system with D2D relaying while satisfying the minimum average data rate requirement for each UE, and then develop an opportunistic scheduling algorithm by solving it. Due to a high computational complexity of the optimal scheduling algorithm, we also propose a heuristic algorithm with a lower computational complexity. In addition, since UEs that participate in D2D relaying sacrifice their resources to relay other UEs' data, we also study incentive mechanisms to compensate their sacrifices. Through simulation results, we show the performance of our algorithms and the effects of our incentive mechanisms.
S.-Y. Kim, J.-A. Kwon, and J.-W. Lee, "Sum-rate maximization for multi-cell OFDMA systems," IEEE Transactions on Vehicular Technology, vol. 64, no. 9, pp. 4158-4169, Sep. 2015 (10.1109/TVT.2014.2363476). | |

Sum-rate maximization for multi-cell OFDMA systems

Recently, joint subchannel allocation and (transmission) power control problems for multicell orthogonal frequencydivision multiple access (OFDMA) systems have been actively studied. However, since the problems are notoriously difficult and complex, only heuristic approaches are mainly used to study them, instead of the optimal approach for achieving maximum system capacity. In this paper, we study this problem from the viewpoint of optimal subchannel allocation and power control, aiming at maximizing the sum rate of the multicellOFDMA system. By using a monotonic optimization approach, we develop an algorithm for the optimal subchannel allocation and power control that achieves the maximum sum rate of the system. In addition, we also develop an algorithm that provides both upper and lower bounds on the maximum sum rate of the system with lower computational complexity. To evaluate the tightness of the upper and lower bounds, we also study the conditions when the two bounds are close to each other so that they can be good approximations to the maximum sum rate of the system. Through numerical results, we show that the bounds provide good approximations to the maximum sum rate of the multicell OFDMA system in most cases.
M.-H. Han, B.-G. Kim, and J.-W. Lee, "Opportunistic resource scheduling for D2D communication in OFDMA networks," Computer Networks, vol. 73, pp. 319-334, Nov. 2014 (10.1016/j.comnet.2014.08.011). | |

Opportunistic resource scheduling for D2D communication in OFDMA networks

In this paper, we study a resource scheduling problem for D2D communication in OFDMA cellular networks. In addition to opportunistically scheduling power and subchannels to users as in the conventional OFDMA network, we also consider two distinguishing features for D2D communication to fully utilize the advantages of D2D communication: transmission mode selection and channel reuse. First, we allow each D2D user to opportunistically select its transmission mode between direct one-hop transmission and indirect two-hop transmission through the BS. Second, we allow channel reuse within a cell such that users can share the same subchannel within a cell. We formulate an optimization problem that aims at maximizing the average weighted sum-rate of users, while satisfying the QoS requirement of each user. Our problem is a non-convex and coupled optimization problem, which is inherently difficult to solve, especially due to channel reuse within a cell. Despite of it, we develop an optimal opportunistic resource scheduling algorithm that solves the problem. In addition, we also propose two simple heuristic algorithms to reduce the computational complexity of the optimal algorithm. Through simulations, we show our opportunistic scheduling algorithms achieve a high performance with appropriately utilizing the advantages of D2D communication.
H.-T. Roh and J.-W. Lee, "Distributed node placement algorithm utilizing controllable mobility in mobile ad hoc networks," Ad Hoc Networks, vol. 15, pp. 67-77, Apr. 2014 (10.1016/j.adhoc.2013.06.010). | |

Distributed node placement algorithm utilizing controllable mobility in mobile ad hoc networks

In this paper, we study a node placement problem in mobile ad hoc networks with controllable mobility. Especially, we consider mission-critical networks in which nodes have their own specific mission whose degree of satisfaction depends on their locations. In addition to accomplish their mission, nodes want to maintain a good communication quality with their neighbor nodes that also depends on their locations. In general, the best location of a node for its mission is not coincident with that for its communication quality, and thus it is important to control the mobility of a node to find its appropriate location jointly considering both its mission and communication quality. Hence, in this paper, we study a joint mission and communication aware node placement problem. We formulate the problem as a potential game and develop a distributed algorithm that converges to the Nash equilibrium. In addition, we also show that if some minor conditions are satisfied, our algorithm provides a global optimal solution that minimizes the weighted sum of costs for mission and communication.
B.-G. Kim, J.-A. Kwon, and J.-W. Lee, "Subchannel allocation for the OFDMA-based femtocell system," Computer Networks, vol. 57, no. 17, pp. 3617-3629, Dec. 2013 (10.1016/j.comnet.2013.08.008). | |

Subchannel allocation for the OFDMA-based femtocell system

In femtocell networks, due to their small cell size, we can achieve higher spatial diversity from the channel reuse between multiple femtocells. In addition, if femtocells operate on OFDMA systems, each subchannel can be reused separately among femtocells, improving system efficiency more significantly. However, due to a large number of femtocells and their uncoordinated and irregular deployment, we need to treat intercell interferences very carefully in OFDMA-based femtocell networks, which makes developing efficient resource allocation schemes more difficult.
In this paper, we study a subchannel allocation problem that aims at maximizing the sum utility of the OFDMA-based femtocell network, which can be applied to both the dedicated channel and shared channel femtocell deployments. Since the problem is formulated as a nonlinear integer program, which is inherently difficult to solve, we propose a suboptimal subchannel allocation algorithm. The proposed subchannel allocation algorithm consists of two steps: calculating the number of subchannels that should be granted to each cell to maximize the sum utility and finding actual subchannel allocation that achieves the granted number of subchannels for each cell. Numerical results show that the proposed subchannel allocation algorithm provides near-optimal performance.
B.-G. Kim, S. Ren, M. van der Schaar, and J.-W. Lee, "Bidirectional energy trading and residential load scheduling with electric vehicles in the smart grid," IEEE Journal on Selected Areas in Communications, vol. 31, no. 7, pp. 1219-1234, Jul. 2013 (10.1109/JSAC.2013.130706). | |

Bidirectional energy trading and residential load scheduling with electric vehicles in the smart grid

Electric vehicles (EVs) will play an important role in the future smart grid because of their capabilities of storing electrical energy in their batteries during off-peak hours and supplying the stored energy to the power grid during peak hours. In this paper, we consider a power system with an aggregator and multiple customers with EVs and propose novel electricity load scheduling algorithms which, unlike previous works, jointly consider the load scheduling for appliances and the energy trading using EVs. Specifically, we allow customers to determine how much energy to purchase from or to sell to the aggregator while taking into consideration the load demands of their residential appliances and the associated electricity bill. We propose two different approaches: a collaborative and a non-collaborative approach. In the collaborative approach, we develop an optimal distributed load scheduling algorithm that maximizes the social welfare of the power system. In the noncollaborative approach, we model the energy scheduling problem as a non-cooperative game among self-interested customers, where each customer determines its own load scheduling and energy trading to maximize its own profit. In order to resolve the unfairness between heavy and light customers in the noncollaborative approach, we propose a tiered billing scheme that can control the electricity rates for customers according to their different energy consumption levels. In both approaches, we also consider the uncertainty in the load demands, with which customers' actual energy consumption may vary from the scheduled energy consumption. To study the impact of the uncertainty, we use the worst-case-uncertainty approach and develop distributed load scheduling algorithms that provide the guaranteed minimum performances in uncertain environments. Subsequently, we show when energy trading leads to an increase in the social welfare and we determine what are the customers' incentives to participate in the energy trading in various usage scenarios including practical environments with uncertain load demands.
H.-T. Roh and J.-W. Lee, "Network coding-aware flow control in wireless ad-hoc networks with multi-path routing," Wireless Networks, vol. 19, no. 5, pp. 785-797, Jul. 2013 (10.1007/s11276-012-0501-9). | |

Network coding-aware flow control in wireless ad-hoc networks with multi-path routing

In this paper, we study a flow control problem considering network coding in wireless ad-hoc networks with multi-path routing. As a network coding scheme, we use XOR network coding, in which each node bitwise-XORs some packets received from different sessions, and then broadcasts this coded packet to multiple nodes in a single transmission. This process can reduce the number of required transmissions, and thus can improve network utilization, especially if it is used with appropriate network coding-aware protocols. Considering this XOR network coding, we formulate an optimization problem for flow control that aims at maximizing network utility. By solving the optimization problem in a distributed manner, we implement a distributed flow control algorithm that provides the optimal transmitting rate on each of multiple paths of each session. The simulation results show that our flow control algorithm performs well exploiting the advantages of network coding and provides significant performance improvement.
H.-T. Roh and J.-W. Lee, "Distributed algorithm for end-to-end rate control with user-level utility in communication networks," IEICE Transactions on Communications, vol. E96-B, no. 03, pp. 896-899, Mar. 2013 (10.1587/transcom.E96.B.896). | |

Distributed algorithm for end-to-end rate control with user-level utility in communication networks

In our previous work [2], we proposed a new concept of utility functions for rate control in communication networks. Unlike conventional utility-based rate control in which the utility function of each user is defined as a function of its transmitting data rate, in [2], we defined the utility function of each user as a function of not only its transmitting data rate but also it receiving data rate. The former is called a session-level utility function and the latter is called a user-level utility function. The user-level utility function reflects the satisfaction with the service of a user with two-way communication, which consists of transmitting and receiving sessions, better than the session-level utility function, since user's satisfaction depends on not only the satisfaction with its transmitting session but also that for its receiving session. In [2], an algorithm that required each user to know the exact utility function of its correspondent was developed. However, in some cases, this information might not be available due to some reasons such as security and privacy issues, and in such cases, the algorithm developed in [2] cannot be used. Hence, in this paper, we develop a new distributed algorithm that does not require each user to know the utility function of its correspondent. Numerical results show that our new algorithm, which does not require the utility information of the correspondent, converges to the same solution to that with the algorithm that requires the utility information of the correspondent.
S.-L. Gong, H.-T. Roh, and J.-W. Lee, "Cross-layer and end-to-end optimization for the integrated wireless and wireline network," Journal of Communications and Networks, vol. 14, no. 5, pp. 554-565, Oct. 2012 (10.1109/JCN.2012.00014). | |

Cross-layer and end-to-end optimization for the integrated wireless and wireline network

In this paper, we study a cross-layer and end-to-end optimization problem for the integrated wireless and wireline network that consists of one wireline core network and multiple wireless access networks. We consider joint end-to-end flow control/distribution at the transport and network layers and opportunistic scheduling at the data link and physical layers. We formulate a single stochastic optimization problem and solve it by using a dual approach and a stochastic sub-gradient algorithm. The developed algorithm can be implemented in a distributed way, vertically among communication layers and horizontally among all entities in the network, clearly showing what should be done at each layer and each entity and what parameters should be exchanged between layers and between entities. Numerical results show that our crosslayer and end-to-end optimization approach provides more efficient resource allocation than the conventional layered and separated optimization approach.
S.-Y. Kim and J.-W. Lee, "To cooperate or not to cooperate: system throughput and fairness perspective," IEEE Journal on Selected Areas in Communications, vol. 30, no. 9, pp. 1649-1657, Oct. 2012 (10.1109/JSAC.2012.121008). | |

To cooperate or not to cooperate: system throughput and fairness perspective

The cooperative transmission, in which some nodes help the transmission of other nodes, has been actively studied to overcome the channel fading effects that deteriorate the communication quality. Thus far, most researches on the cooperative transmission have been studied from the reliability point of view focusing on a single transmission and showing that the cooperative transmission can increase transmission reliability. In this paper, we study the effects of the cooperative transmission from the system throughput and fairness point of view, considering the following fundamental questions: Is the cooperative transmission always helpful to increase the system throughput and improve the degree of fairness among nodes? If not, when is it helpful to increase the system throughput and improve the degree of fairness among nodes? We provide the answers to the above questions with a simple system in which two source nodes are capable of being cooperative with each other by using the decode-and-forward cooperative scheme to transmit their data to a single destination node.
J.-A. Kwon and J.-W. Lee, "Opportunistic scheduling for an OFDMA system with multi-class services," Wireless Communications and Mobile Computing, vol. 12, no. 12, pp. 1104-1114, Aug. 2012 (10.1002/wcm.1039). | |

Opportunistic scheduling for an OFDMA system with multi-class services

In this paper, we study an opportunistic scheduling problem in an OFDMA system, in which sub-carriers of the system are allocated to each user in each time slot considering the time-varying channel condition and QoS requirement of each user. We consider two different classes of services that are represented with different types of utility functions. The utility function for a user in one class is defined as a function of its average data rate, which can be applicable to best-effort services and the utility function for a user in the other class is defined as a function of its instantaneous data rate, which can be applicable to rate-sensitive services. Those two types of utility functions have been extensively considered in opportunistic scheduling in wireless networks. However, in most of the previous work, they are considered separately in different problems. In this paper, we formulate a stochastic optimization problem that can treat those two types of utility functions in a single problem, which enables us to implement an opportunistic scheduling algorithm that can consider those two classes of services in a single system in a unified way. Through simulations, we first show that our algorithm provides a good approximation to the optimal solution. In addition, we also verify the appropriateness of our utility models.
H.-T. Roh and J.-W. Lee, "Joint relay node placement and node scheduling in wireless networks with a relay node with controllable mobility," Wireless Communications and Mobile Computing, vol. 12, no. 8, pp. 699-712, Jun. 2012 (10.1002/wcm.1007). | |

Joint relay node placement and node scheduling in wireless networks with a relay node with controllable mobility

In this paper, we propose an algorithm for joint relay node placement and node scheduling in wireless networks.We consider a system that consists of a relay node with controllable mobility and multiple nodes that communicate with each other via the relay node. Each node communicates with the relay node according to its schedule. The objective of our algorithm is to maximize the lowest weighted throughput among those of all nodes. To this end, we consider a problem to optimally place the relay node and optimally schedule all nodes in the network.We develop three algorithms for relay node placement and node scheduling considering three different cases: when the location of the relay node is controllable while the node scheduling is fixed, when the node scheduling is controllable while the location of the relay node is fixed, and when both the location of the relay node and node scheduling are controllable. The simulation results show that by jointly optimizing relay node placement and node scheduling, we can improve system performance significantly in many cases.
B.-G. Kim and J.-W. Lee, "Stochastic utility-based flow control algorithm for services with time-varying rate requirements," Computer Networks, vol. 56, no. 4, pp. 1329-1342, Mar. 2012. | |

Stochastic utility-based flow control algorithm for services with time-varying rate requirements

In this paper, we study a utility based flow control problem for a communication network. In most previous works on utility based flow control, the utility function of each user, which represents its satisfaction to the allocated data rate, is assumed to be fixed. This implies that the degree of the rate requirement of each user is assumed to be fixed over the entire duration of its session. However, in communication networks, many services are variable rate services, i.e., the degree of their rate requirement varies over time, which cannot be modeled with traditional static utility functions. To resolve this issue and appropriately model services with variable rate requirements, we propose a stochastic utility function that varies stochastically according to the variation of the degree of the rate requirement of a service. We formulate a flow control problem as a stochastic optimization problem with stochastic utility functions that aims at maximizing the average network utility while satisfying the constraint on link capacity and QoS requirement. By solving the stochastic optimization problem, we develop a distributed flow control algorithm that converges to the optimal rate allocation.
B.-G. Kim and J.-W. Lee, "Opportunistic resource scheduling for OFDMA networks with network coding at relay station," IEEE Transactions on Wireless Communications, vol. 11, no. 1, pp. 210-221, Jan. 2012. | |

Opportunistic resource scheduling for OFDMA networks with network coding at relay station

In this paper, we study an opportunistic resource scheduling problem for the relay-based OFDMA cellular network where relay stations (RSs) perform opportunistic network coding with downlink and uplink sessions of a mobile station (MS). To this end, we consider time-division duplexing (TDD) where each time-slot is divided into three phases according to the type of transmitter nodes, i.e., the base station (BS), MSs, and RSs. Moreover, to improve the flexibility for resource allocation, we allow dynamic TDD, in which the time duration of each phase in each time-slot can be adjusted. For opportunistic network coding, we introduce a novel model for network coding aware RSs with which an opportunistic network coding problem can be reduced to an opportunistic subchannel scheduling problem. We formulate an optimization problem that aims at maximizing the average weighted-sum rate for both downlink and uplink sessions of all MSs, while satisfying the quality-of-service (QoS) requirements of each MS. By solving it, we develop a resource scheduling algorithm that optimally and opportunistically schedule subchannel, transmission power, network coding, and time duration of each phase in each time-slot. Through the numerical results, we study how each of network coding strategy and dynamic TDD affects the network performance with various network environments.
S.-C. Choi and J.-W. Lee, "An energy-efficient MAC protocol with probabilistic scheduled listen-sleep cycles for wireless sensor networks," IEICE Transactions on Communications, vol. E94-B, no. 11, pp. 3001-3008, Nov. 2011. | |

An energy-efficient MAC protocol with probabilistic scheduled listen-sleep cycles for wireless sensor networks

In this paper, we study an end-to-end rate control problem in communication networks with the network utility maximization (NUM) framework. In many cases, a communication of a user consists of two sessions: transmitting and receiving sessions, and its overall satisfaction to the communication service depends on the satisfaction to both sessions. However, in most previous approaches with the NUM framework, the utility function of a user, which represents its satisfaction to the service, is modeled considering only the satisfaction to its transmitting session through its transmitting rate. The receiving session of a user is treated independently and indirectly through the utility function of its correspondent. Hence, it is not possible to jointly consider the satisfaction of transmitting and receiving sessions of one user. To resolve this issue, in this paper, we first propose a new concept of the utility function, which is called a user-level utility function. The user-level utility function is modeled considering both transmitting and receiving sessions of a user. We then formulate an optimization problem for end-to-end rate control with user-level utility functions. Even though two users in a communication are coupled with each other through their utility functions, we developed a distributed algorithm with appropriate message exchanges. The numerical results show that our user-level utility function and algorithm can control the transmitting rate of each user more appropriately according to the type of its communication service.
H.-T. Roh and J.-W. Lee, "User-level satisfaction aware end-to-end rate control in communication networks," Computer Networks, vol. 55, no. 8, pp. 1700-1710, Jun. 2011. | |

User-level satisfaction aware end-to-end rate control in communication networks

In this paper, we study an end-to-end rate control problem in communication networks with the network utility maximization (NUM) framework. In many cases, a communication of a user consists of two sessions: transmitting and receiving sessions, and its overall satisfaction to the communication service depends on the satisfaction to both sessions. However, in most previous approaches with the NUM framework, the utility function of a user, which represents its satisfaction to the service, is modeled considering only the satisfaction to its transmitting session through its transmitting rate. The receiving session of a user is treated independently and indirectly through the utility function of its correspondent. Hence, it is not possible to jointly consider the satisfaction of transmitting and receiving sessions of one user. To resolve this issue, in this paper, we first propose a new concept of the utility function, which is called a user-level utility function. The user-level utility function is modeled considering both transmitting and receiving sessions of a user. We then formulate an optimization problem for end-to-end rate control with user-level utility functions. Even though two users in a communication are coupled with each other through their utility functions, we developed a distributed algorithm with appropriate message exchanges. The numerical results show that our user-level utility function and algorithm can control the transmitting rate of each user more appropriately according to the type of its communication service.
H.-T. Roh and J.-W. Lee, "Communication-aware position control for mobile nodes in vehicular networks," IEEE Journal on Selected Areas in Communications, vol. 29, no. 1, pp. 173-186, Jan. 2011. | |

Communication-aware position control for mobile nodes in vehicular networks

In this paper, we study a communication-aware position control problem for mobile nodes in vehicular networks, in which the positions of some nodes can be controlled considering the network performance. We model the average achievable data rate of a link between two nodes as a function of the distance between the two nodes, i.e., as a function of positions of the two nodes. We then try to find the positions of some nodes whose positions can be controlled so as to maximize the minimum weighted average data rate among those of all links in the network. To tackle this problem, we take two approaches: optimization and game theoretic approaches. In the optimization theoretic approach, even though the optimization problem is formulated as non-convex optimization, we can develop algorithms for the optimal solution. However, since those algorithms are centralized algorithms, which may not be applicable to some cases such as vehicular ad-hoc networks (VANETs), we also use the game theoretic approach to develop distributed algorithms. In addition to developing algorithms, we also analyze and compare the performances of our algorithms, showing that the game theoretic approach could provide not only distributed algorithms but also efficient algorithms in our problem.
J.-A. Kwon, B.-G. Kim, and J.-W. Lee, "A unified framework for opportunistic fair scheduling in wireless networks: a dual approach," Wireless Networks, vol. 16, no. 7, pp. 1975-1986, Oct. 2010. | |

A unified framework for opportunistic fair scheduling in wireless networks: a dual approach

In this paper, we propose a unified framework for opportunistic fair scheduling in wireless systems. We consider a TDMA type of multiple access scheme, in which only one user can be scheduled in each time-slot. For opportunistic fair scheduling in such a system, some nice frameworks have been developed in the previous works, such as Agrawal and Subramanian (Allerton conference on communication, control and computing, 2002), Liu et al. (IEEE Journal of Selected Areas in Communications 19(10): 2053–2065, 2001) and Liu et al. (Computer Networks 41(4): 451–474, 2003). However, in this paper, we consider a more general problem that can accommodate more general types of fairness, and more general types of utility functions than those in the previous works. In addition to those generalizations, we develop a new framework for opportunistic fair scheduling based on the duality theory, which is different from those in the previous works. The duality theory is a well-defined theory in the mathematical optimization area. Hence, it can provide a unified framework for many different types of problems. In fact, we show that two different frameworks in Agrawal and Subramanian (Allerton conference on communication, control and computing, 2002), Liu et al. (IEEE Journal of Selected Areas in Communications 19(10): 2053–2065, 2001) and Liu et al. (Computer Networks 41(4): 451–474, 2003) are special cases of ours. In addition, by using the unified framework developed in this paper, we can not only develop various opportunistic fair scheduling schemes but also analyze the developed algorithm more rigorously and systematically.
J.-A. Kwon and J.-W. Lee, "Opportunistic resource scheduling for a wireless network with relay stations," IEICE Transactions on Communications, vol. E93-B, no. 8, pp. 2097-2103, Aug. 2010. | |

Opportunistic resource scheduling for a wireless network with relay stations

In this paper, we study an opportunistic scheduling scheme for the TDMA wireless network with relay stations. We model the time-varying channel condition of a wireless link as a stochastic process. Based on this model, we formulate an optimization problem for the opportunistic scheduling scheme that maximizes the expected system throughput while satisfying the QoS constraint of each user. In the opportunistic scheduling scheme for the system without relay stations, each user has only one communication path between the base station and itself, and thus only user selection is considered. However, in our opportunistic scheduling scheme for the system with relay stations, since there may exist multiple paths between the base station and a user, not only user selection but also path selection for the scheduled user is considered. In addition, we also propose an opportunistic time-sharing method for time-slot sharing between base station and relay stations. With the opportunistic time-sharing method, our opportunistic scheduling provides opportunistic resource sharing in three places in the system: user selection in a time-slot, path selection for the selected user, and time-slot sharing between base station and relay stations. Simulation results show that as the number of places that opportunistic resource sharing is applied to increases, the performance improvement also increases.
B.-G. Kim and J.-W. Lee, "Joint opportunistic subchannel and power scheduling for relay-based OFDMA networks with scheduling at relay stations," IEEE Transactions on Vehicular Technology, vol. 59, no. 5, pp. 2138-2148, Jun. 2010. | |

Joint opportunistic subchannel and power scheduling for relay-based OFDMA networks with scheduling at relay stations

In this paper, we study a joint opportunistic subchannel and power-scheduling problem in relay-based orthogonal frequency-division multiple-access (OFDMA) networks. In most previous works on relay-based networks, immediate relaying at relay stations (RSs) without allowing opportunistic scheduling at RSs was considered. Under this strategy, each RS should transmit the received data from the base station (BS) to the corresponding mobile stations (MSs) immediately within a single time slot, and thus, the effective data rate of the two-hop transmission (BS–RS and RS–MS links) is limited by the achievable data rate of the link with a worse channel state between the two links, resulting in a waste of radio resources. However, if opportunistic scheduling is allowed not only at the BS but at each RS as well, then more efficient radio resource allocation could be possible. Considering time-varying wireless channels, we formulate a stochastic optimization problem that aims at maximizing the average sum rate of the system while satisfying the quality-of-service (QoS) requirement of each MS. By solving the problem, we develop a joint opportunistic subchannel and power-scheduling algorithm for transmission at both the BS and the RSs. Numerical results show that the proposed scheduling algorithm can significantly improve system performance by allowing opportunistic scheduling at both the BS and the RSs.
S.-Y. Kim and J.-W. Lee, "On the user-level satisfactions with user-level utility functions: a case study with scheduling in TDMA wireless networks," IEICE Transactions on Communications, vol. E93-B, no. 4, pp. 1037-1040, Apr. 2010. | |

On the user-level satisfactions with user-level utility functions: a case study with scheduling in TDMA wireless networks

To reduce energy consumption, in most MAC protocols for wireless sensor networks, listen-sleep cycles are adopted. However, even though it is a good solution for energy efficiency, it may introduce a large end-to-end delay due to sleep delay, since a node with a packet to transmit should wait until the next-hop node of the packet awakes. To resolve this issue, in this paper, we propose the Average Velocity-Based Routing (AVR) protocol for wireless sensor networks that aims at reducing the end-toend delay. The AVR protocol is a kind of a geographic routing protocol that considers both location of a node and waiting time of a packet at the MAC layer. When a node can use information of n-hop away neighbor nodes, it calculates the n-hop average velocity for each of its one-hop neighbor nodes and forwards a packet to the neighbor node that has the highest n-hop average velocity. Simulation results show that as the knowledge range, n, increases, the average end-to-end delay decreases.
S.-L. Gong, S.-Y. Kim, J.-W. Lee, S. I. Lee, and M. K. Ahn, "Link weight optimization for routing in communication networks," IEICE Electronics Express, vol. 7, no. 1, pp. 33-39, Jan. 2010. | |

Link weight optimization for routing in communication networks

To reduce energy consumption, in most MAC protocols for wireless sensor networks, listen-sleep cycles are adopted. However, even though it is a good solution for energy efficiency, it may introduce a large end-to-end delay due to sleep delay, since a node with a packet to transmit should wait until the next-hop node of the packet awakes. To resolve this issue, in this paper, we propose the Average Velocity-Based Routing (AVR) protocol for wireless sensor networks that aims at reducing the end-toend delay. The AVR protocol is a kind of a geographic routing protocol that considers both location of a node and waiting time of a packet at the MAC layer. When a node can use information of n-hop away neighbor nodes, it calculates the n-hop average velocity for each of its one-hop neighbor nodes and forwards a packet to the neighbor node that has the highest n-hop average velocity. Simulation results show that as the knowledge range, n, increases, the average end-to-end delay decreases.
J.-W. Lee and J.-A. Kwon, "Utility-based power allocation for multi-class wireless systems," IEEE Transactions on Vehicular Technology, vol. 58, no. 7, pp. 3813-3819, Sep. 2009. | |

Utility-based power allocation for multi-class wireless systems

We study power-allocation problems in wireless systems through a network utility maximization framework. In this framework, the type of utility function represents the characteristics of each service. Hence, to deal with multiclass services in the system, it is important to accommodate various types of utility functions. In this paper, we consider four types of utility functions that can represent most of the services in wireless networks.We first develop a simple algorithm that solves a general optimization problem with those four types of utility function. We show that, even though the proposed algorithm might not provide the optimal solution, it provides an asymptotically optimal solution, which could be a good approximation to the optimal solution. The proposed algorithm can be applied to various resource-allocation problems in wireless systems. Among them, in this paper, we show that the proposed algorithm can be applied to both uplink and downlink power-allocation problems.
S.-L. Gong, B.-G. Kim, and J.-W. Lee, "Optimal oppportunistic scheduling and adaptive modulation policies in wireless ad-hoc networks with network coding," IEICE Transactions on Communications, vol. E92-B, no. 9, pp. 2954-2957, Sep. 2009. | |

Optimal oppportunistic scheduling and adaptive modulation policies in wireless ad-hoc networks with network coding

To reduce energy consumption, in most MAC protocols for wireless sensor networks, listen-sleep cycles are adopted. However, even though it is a good solution for energy efficiency, it may introduce a large end-to-end delay due to sleep delay, since a node with a packet to transmit should wait until the next-hop node of the packet awakes. To resolve this issue, in this paper, we propose the Average Velocity-Based Routing (AVR) protocol for wireless sensor networks that aims at reducing the end-toend delay. The AVR protocol is a kind of a geographic routing protocol that considers both location of a node and waiting time of a packet at the MAC layer. When a node can use information of n-hop away neighbor nodes, it calculates the n-hop average velocity for each of its one-hop neighbor nodes and forwards a packet to the neighbor node that has the highest n-hop average velocity. Simulation results show that as the knowledge range, n, increases, the average end-to-end delay decreases.
S.-C. Choi, S.-L. Gong, and J.-W. Lee, "Average velocity-based routing with low end-to-end delay for wireless sensor networks," IEEE Communications Letters, vol. 13, no. 8, pp. 621-623, Aug. 2009. | |

Average velocity-based routing with low end-to-end delay for wireless sensor networks

To reduce energy consumption, in most MAC protocols for wireless sensor networks, listen-sleep cycles are adopted. However, even though it is a good solution for energy efficiency, it may introduce a large end-to-end delay due to sleep delay, since a node with a packet to transmit should wait until the next-hop node of the packet awakes. To resolve this issue, in this paper, we propose the Average Velocity-Based Routing (AVR) protocol for wireless sensor networks that aims at reducing the end-toend delay. The AVR protocol is a kind of a geographic routing protocol that considers both location of a node and waiting time of a packet at the MAC layer. When a node can use information of n-hop away neighbor nodes, it calculates the n-hop average velocity for each of its one-hop neighbor nodes and forwards a packet to the neighbor node that has the highest n-hop average velocity. Simulation results show that as the knowledge range, n, increases, the average end-to-end delay decreases.
S.-C. Choi and J.-W. Lee, "An energy-efficient mobility-supporting MAC protocol for mobile sensor networks," IEICE Transactions on Communications, vol. E91-B, no. 8, pp. 2720-2723, Aug. 2008. | |

An energy-efficient mobility-supporting MAC protocol for mobile sensor networks

This paper reverse-engineers backoff-based random-access MAC protocols in ad-hoc networks. We show that the contention resolution algorithm in such protocols is implicitly participating in a non-cooperative game. Each link attempts to maximize a selfish local utility function, whose exact shape is reverse-engineered from the protocol description, through a stochastic subgradient method in which the link updates its persistence probability based on its transmission success or failure. We prove that existence of a Nash equilibrium is guaranteed in general. Then we establish the minimum amount of backoff aggressiveness needed, as a function of density of active users, for uniqueness of Nash equilibrium and convergence of the best response strategy. Convergence properties and connection with the best response strategy are also proved for variants of the stochastic-subgradient-based dynamics of the game. Together with known results in reverse-engineering TCP and BGP, this paper further advances the recent efforts in reverse-engineering layers 2-4 protocols. In contrast to the TCP reverse-engineering results in earlier literature, MAC reverse-engineering highlights the non-cooperative nature of random access.
J.-W. Lee, M. Chiang, and A. R. Calderbank, "On the avhievable efficiency-fairness tradeoff in utility-optimal MAC protocols," IEICE Transactions on Communications, vol. E91-B, no. 4, pp. 1231-1234, Apr. 2008. | |

On the avhievable efficiency-fairness tradeoff in utility-optimal MAC protocols

This paper reverse-engineers backoff-based random-access MAC protocols in ad-hoc networks. We show that the contention resolution algorithm in such protocols is implicitly participating in a non-cooperative game. Each link attempts to maximize a selfish local utility function, whose exact shape is reverse-engineered from the protocol description, through a stochastic subgradient method in which the link updates its persistence probability based on its transmission success or failure. We prove that existence of a Nash equilibrium is guaranteed in general. Then we establish the minimum amount of backoff aggressiveness needed, as a function of density of active users, for uniqueness of Nash equilibrium and convergence of the best response strategy. Convergence properties and connection with the best response strategy are also proved for variants of the stochastic-subgradient-based dynamics of the game. Together with known results in reverse-engineering TCP and BGP, this paper further advances the recent efforts in reverse-engineering layers 2-4 protocols. In contrast to the TCP reverse-engineering results in earlier literature, MAC reverse-engineering highlights the non-cooperative nature of random access.
J.-W. Lee, A. Tang, J. Huang, M. Chiang, and A. R. Calderbank, "Reverse engineering MAC: a non-cooperative game model," IEEE Journal on Selected Areas in Communications, vol. 25, no. 6, pp. 1135-1147, Aug. 2007. | |

Reverse engineering MAC: a non-cooperative game model

This paper reverse-engineers backoff-based random-access MAC protocols in ad-hoc networks. We show that the contention resolution algorithm in such protocols is implicitly participating in a non-cooperative game. Each link attempts to maximize a selfish local utility function, whose exact shape is reverse-engineered from the protocol description, through a stochastic subgradient method in which the link updates its persistence probability based on its transmission success or failure. We prove that existence of a Nash equilibrium is guaranteed in general. Then we establish the minimum amount of backoff aggressiveness needed, as a function of density of active users, for uniqueness of Nash equilibrium and convergence of the best response strategy. Convergence properties and connection with the best response strategy are also proved for variants of the stochastic-subgradient-based dynamics of the game. Together with known results in reverse-engineering TCP and BGP, this paper further advances the recent efforts in reverse-engineering layers 2-4 protocols. In contrast to the TCP reverse-engineering results in earlier literature, MAC reverse-engineering highlights the non-cooperative nature of random access.
J.-W. Lee, M. Chiang, and R. Calderbank, "Utility-optimal random-access control," IEEE Transactions on Wireless Communications, vol. 6, no. 7, pp. 2741-2751, Jul. 2007. | |

Utility-optimal random-access control

This paper designs medium access control (MAC) protocols for wireless networks through the network utility maximization (NUM) framework. A network-wide utility maximization problem is formulated, using a collision/persistenceprobabilistic model and aligning selfish utility with total social welfare. By adjusting the parameters in the utility objective functions of the NUM problem, we can also control the tradeoff between efficiency and fairness of radio resource allocation. We develop two distributed algorithms to solve the utility-optimal random-access control problem, which lead to random access protocols that have slightly more message passing overhead than the current exponential-backoff protocols, but significant potential for efficiency and fairness improvement. We provide readily-verifiable sufficient conditions under which convergence of the proposed algorithms to a global optimality of network utility can be guaranteed, and numerical experiments that illustrate the value of the NUM approach to the complexityperformance tradeoff in MAC design.
J.-W. Lee, R. R. Mazumdar, and N. B. Shroff, "Joint opportunistic power scheduling and rate control for wireless ad-hoc networks," IEEE Transactions on Vehicular Technology, vol. 56, no. 2, pp. 801-809, Mar. 2007. | |

Joint opportunistic power scheduling and rate control for wireless ad-hoc networks

It is known that opportunistic scheduling that accounts for channel variations due to mobility and fading can give substantial improvement over nonopportunistic schemes. However, most work on this subject has focused on single-hop cellular types of architectures. The situation is quite different in ad hoc networks due to the inherent multihop nature of transmissions. In this paper, we present a joint opportunistic power scheduling and end-to-end rate control scheme for wireless ad hoc networks. We model the time-varying wireless channel as a stochastic process and formulate a stochastic optimization problem, which aims at maximizing system efficiency by controlling the power allocation of each link and the data rate of each user in the system. The joint power scheduling and rate control algorithm is obtained by using stochastic duality and implemented via stochastic subgradient techniques. We illustrate the efficacy of our approach via numerical examples.
J.-W. Lee, R. R. Mazumdar, and N. B. Shroff, "Opportunistic power scheduling for dynamic multi-server wireless systems," IEEE Transactions on Wireless Communications, vol. 5, no. 6, pp. 1506-1515, Jun. 2006. | |

Opportunistic power scheduling for dynamic multi-server wireless systems

In this paper, we present an opportunistic power scheduling scheme, i.e., a joint time-slot and power allocation scheme for downlink communication in wireless systems. Unlike past works, we allow multiple transmissions in a time-slot that could potentially interfere with each other. These multiple transmissions are allowed to achieve high system efficiency. Hence, it is important to not only select the mobiles to be scheduled in a timeslot, but also to allocate an appropriate transmission power level to these scheduled mobiles. We model the time-varying wireless channel as a stochastic process and formulate a stochastic optimization problem that attempts to maximize the expected total system utility with general constraints on performance or fairness. The power scheduling algorithm is obtained by using stochastic duality and implemented via stochastic subgradient techniques.
J.-W. Lee, M. Chiang, and A. R. Calderbank, "Price-based distributed algorithms for optimal rate-reliability tradeoff in network utility maximization," IEEE Journal on Selected Areas in Communications, vol. 24, no. 5, pp. 962-976, May 2006. | |

Price-based distributed algorithms for optimal rate-reliability tradeoff in network utility maximization

The current framework of network utility maximization for rate allocation and its price-based algorithms assumes that each link provides a fixed-size transmission “pipe” and each user’s utility is a function of transmission rate only. These assumptions break down in many practical systems, where, by adapting the physical layer channel coding or transmission diversity, different tradeoffs between rate and reliability can be achieved. In network utility maximization problems formulated in this paper, the utility for each user depends on both transmission rate and signal quality, with an intrinsic tradeoff between the two. Each link may also provide a higher (or lower) rate on the transmission “pipes” by allowing a higher (or lower) decoding error probability. Despite nonseparability and nonconvexity of these optimization problems, we propose new price-based distributed algorithms and prove their convergence to the globally optimal rate-reliability tradeoff under readily-verifiable sufficient conditions. We first consider networks in which the rate-reliability tradeoff is controlled by adapting channel code rates in each link’s physical layer error correction codes, and propose two distributed algorithms based on pricing, which respectively implement the “integrated” and “differentiated” policies of dynamic rate-reliability adjustment. In contrast to the classical price-based rate control algorithms, in our algorithms, each user provides an offered price for its own reliability to the network, while the network provides congestion prices to users. The proposed algorithms converge to a tradeoff point between rate and reliability, which we prove to be a globally optimal one for channel codes with sufficiently large coding length and utilities whose curvatures are sufficiently negative. Under these conditions, the proposed algorithms can thus generate the Pareto optimal tradeoff curves between rate and reliability for all the users. In addition, the distributed algorithms and convergence proofs are extended for wireless multiple-inpit–multiple-output multihop networks, in which diversity and multiplexing gains of each link are controlled to achieve the optimal rate-reliability tradeoff. Numerical examples confirm that there can be significant enhancement of the network utility by distributively trading-off rate and reliability, even when only some of the links can implement dynamic reliability.
J.-W. Lee, M. Chiang, and A. R. Calderbank, "Jointly optimal congestion and medium access control based on network utility maximization," IEEE Communications Letters, vol. 10, no. 3, pp. 216-218, Mar. 2006. | |

Jointly optimal congestion and medium access control based on network utility maximization

We study joint end-to-end congestion control and per-link medium access control (MAC) in ad-hoc networks. We use a network utility maximization formulation, in which by adjusting the types of utility functions, we can accommodate multiclass services as well as exploit the tradeoff between efficiency and fairness of resource allocation. Despite the inherent difficulties of non-convexity and non-separability of the optimization problem, we show that, with readily-verifiable sufficient conditions, we can develop a distributed algorithm that converges to the globally and jointly optimal rate allocation and persistence probabilities.
J.-W. Lee, R. R. Mazumdar, and N. B. Shroff, "Joint resource allocation and base station assignment for the downlink in CDMA netwroks," IEEE/ACM Transactions on Networking, vol. 14, no. 1, pp. 1-14, Feb. 2006. | |

Joint resource allocation and base station assignment for the downlink in CDMA netwroks

In this paper, we jointly consider the resource allocation and base-station assignment problems for the downlink in CDMA networks that could carry heterogeneous data services. We first study a joint power and rate allocation problem that attempts to maximize the expected throughput of the system. This problem is inherently difficult because it is in fact a nonconvex optimization problem. To solve this problem, we develop a distributed algorithm based on dynamic pricing. This algorithm provides a power and rate allocation that is asymptotically optimal in the number of mobiles.We also study the effect of various factors on the development of efficient resource allocation strategies. Finally, using the outcome of the power and rate allocation algorithm, we develop a pricing-based base-station assignment algorithm that results in an overall joint resource allocation and base-station assignment. In this algorithm, a base-station is assigned to each mobile taking into account the congestion level of the base-station as well as the transmission environment of the mobile.
J.-W. Lee, R. R. Mazumdar, and N. B. Shroff, "Non-convex optimization and rate control for multi-class services in the Internet," IEEE/ACM Transactions on Networking, vol. 13, no. 4, pp. 827-840, Aug. 2005. | |

Non-convex optimization and rate control for multi-class services in the Internet

In this paper, we investigate the problem of distributively allocating transmission data rates to users in the Internet. We allow users to have concave as well as sigmoidal utility functions as appropriate for different applications. In the literature, for simplicity, most works have dealt only with the concave utility function. However, we show that applying rate control algorithms developed for concave utility functions in a more realistic setting(with both concave and sigmoidal types of utility functions) could lead to instability and high network congestion. We show that a pricing-based mechanism that solves the dual formulation can be developed based on the theory of subdifferentials with the property that the prices “self-regulate” the users to access the resources based on the net utility. We discuss convergence issues and show that an algorithm can be developed that is efficient in the sense of achieving the global optimum when there are many users.
J.-W. Lee, R. R. Mazumdar, and N. B. Shroff, "Downlink power allocation for multi-class wireless systems," IEEE/ACM Transactions on Networking, vol. 13, no. 4, pp. 854-867, Aug. 2005. | |

Downlink power allocation for multi-class wireless systems

In this paper we consider a power allocation problem in multi-class wireless systems. We focus on the downlink of the system. Each mobile has a utility function that characterizes its degree of satisfaction for the received service. The objective is to obtain a power allocation that maximizes the total system utility. Typically, natural utility functions for each mobile are nonconcave. Hence, we cannot use existing convex optimization techniques to derive a global optimal solution.We develop a simple (distributed) algorithm to obtain a power allocation that is asymptotically optimal in the number of mobiles. The algorithm is based on dynamic pricing and consists of two stages. At the mobile selection stage, the base station selects mobiles to which power is allocated. At the power allocation stage, the base station allocates power to the selected mobiles. We provide numerical results that illustrate the performance of our scheme. In particular, we show that our algorithm results in system performance that is close to the performance of a global optimal solution in most cases.
J. S. Choi, J.-W. Lee, and M. Kang, "Design and performance analysis of a contention-based reservation protocol for local area optical Internet," ETRI Journal, vol. 24, no. 2, pp. 142-152, Apr. 2002. | |

Design and performance analysis of a contention-based reservation protocol for local area optical Internet

In this paper, we propose and analyze a new multiple access protocol for a local area optical Internet based on a wavelength division multiplexing technique which uses a passive star coupler. The proposed contention-based reservation protocol can support variable length as well as fixed length messages for transporting Internet packets with one reservation of a minislot at the beginning of a packet transmission. The minislot is used to reserve the data channel on the basis of the slotted ALOHA protocol and the control node ensures subsequent message transmission on the same wavelength. Thus, all messages need not be broken down to many fixed-length packets, and consecutive messages are transmitted through the same wavelength. Moreover, the proposed protocol reduces the collision probability of minislots and improves wavelength utilization. We determine the maximum throughput and verify the results with simulation.
J.-W. Lee, J. S. Choi, and C. K. Un, "Performance analysis of an input queueing ATM switch with two priority classes," Performance Evaluation, vol. 32, pp. 137-149, 1998. | |

Performance analysis of an input queueing ATM switch with two priority classes

In this paper, we study an input queueing asynchronous transfer mode (ATM) switch with the preemptive priority scheme. Up to now, many researchers have studied this switch, but their models do not yield accurate results. Therefore, we propose another model and analyze the performance of the switch for two different arrival processes, that is, the independent priority arrival process (IPAP) and the dependent priority arrival process (DPAP). In this paper, we model a virtual head of line (HOL) queue like the previous works, and analyze it using a three-dimensional Markov chain. As for each input queue, we model it as a two-class Geomx/Geom/1 queue with preemptive priority. By using this model, we obtain the mean queueing delay of the low priority class and the maximum throughput of the switch. In case of the IPAP, we compare our result with simulation and those of the previous works to show that our model yields more accurate results. In addition, we compare the performances of those two arrival processes to illustrate their difference.

International Conferences and Workshops

S.-M. Park, D.-Y. Kim, and J.-W. Lee, "Beamforming for O-RAN-based cell-free mMIMO systems with multiple CPUs," in Proc. ICTC Workshop on Access Technology, Softwarization, and Intelligence in Open RAN (IWORAN), Jeju, Korea, Oct. 2024, pp. 1742-1743. | |

Beamforming for O-RAN-based cell-free mMIMO systems with multiple CPUs

In this paper, we consider an Open RAN (O-RAN)-based cell-free massive multiple-input multiple-output (mMIMO) system with multiple central processing units (CPUs). In this system, to achieve high network performance, suppressing the inter-cluster interference (ICI) is important. To this end, we study the problem of maximizing the sum of data rates for user equipments (UEs) served by each CPU by optimizing the beamforming of O-RAN radio units (O-RUs). To solve this problem, we develop an effective algorithm, and show that our proposed algorithm significantly enhances performance in O-RAN-based cell-free mMIMO systems with multiple CPUs.
J. Shin, J. Won, H.-S. Lee, and J.-W. Lee, "Utilizing unclean samples with label correction in a neural network," in Proc. ICEIC 2024, Taipei, Taiwan, Jan. 2024, pp. 796-797. | |

Utilizing unclean samples with label correction in a neural network

To solve the problem that label noises downgrade the performance of a neural network, we propose an algorithm on clean sample selection and label correction.With the algorithm, we can detect unclean samples before training the neural network and correct their labels, which are finally used to train the neural network together with the clean samples. Through an experiment, we verify the performance of the proposed algorithm.
S.-M. Park, D.-Y. Kim, K.-W. Kim, and J.-W. Lee, "Cell-free massive MIMO system with dedicated interference cancellation access points," in Proc. IEEE VTC 2023-Spring, Florence, Italy, Jun. 2023, pp. 1-5 (10.1109/VTC2023-Spring57618.2023.10199613). | |

Cell-free massive MIMO system with dedicated interference cancellation access points

A cell-free massive multiple-input multiple-output (mMIMO) system employs a large number of access points (APs). Since the APs collaboratively serve user equipments (UEs), its energy and spectral efficiency can be much higher than that of a conventional cellular system. Typically, the inter-user interference (IUI) is suppressed appropriately to improve performance, which usually results in high computational complexity. To reduce this computational complexity, we propose a new cell-free mMIMO architecture, called a dedicated AP-based interference cancellation (DAP-IC) architecture with two types of APs: DS-APs that transmit the data signal (DS) to UEs and IUI-APs that transmit the signal to eliminate the IUI. To this end, we first formulate an optimization problem of maximizing the spectral efficiency while ensuring the fairness of UEs by optimizing the precoding vectors of the APs. We then develop DAP-IC algorithm that solves this problem with a low computational complexity. The simulation results show that the DAP-IC algorithm provides good performance with much lower computational complexity, compared with the weighted minimum mean square error (WMMSE) algorithm for the conventional cell-free mMIMO architecture.
Y.-I. Park, D.-Y. Kim, and J.-W. Lee, "Joint trajectory design and sub-channel allocation in the UAV relaying OFDMA network," in Proc. IEEE VTC 2023-Spring, Florence, Italy, Jun. 2023, pp. 1-5 (10.1109/VTC2023-Spring57618.2023.10200381). | |

Joint trajectory design and sub-channel allocation in the UAV relaying OFDMA network

In this paper, we consider an orthogonal frequency division multiple access (OFDMA)-based unmanned aerial vehicle (UAV) relaying system with multiple user equipment (UE) pairs, which has not yet been well studied despite its potential for promising use cases (e.g., a UAV relay-enabled standalone private 5G network). We study a joint optimization problem for UAV trajectory and sub-channel allocation to maximize the total average end-to-end throughput while satisfying the quality-of-serivce (QoS) requirements of UE pairs. To address this problem, we develop a block coordinate descent (BCD)-based algorithm that iteratively solves two sub-problems: 1) a sub-channel allocation optimization problem with a given UAV trajectory, and 2) a UAV trajectory optimization problem with a given sub-channel allocation. Through simulation, we demonstrate the effectiveness of the proposed algorithm.
J. Won, D.-Y. Kim, and J.-W. Lee, "Joint optimization of placement, beamwidth, and power allocation in the UAV-enabled network," in Proc. ICTC 2022, Jeju, Korea, Oct. 2022, pp. 236-238 (10.1109/ICTC55196.2022.9952708). | |

Joint optimization of placement, beamwidth, and power allocation in the UAV-enabled network

In this paper, we consider an unmanned aerial vehicle (UAV)-enabled network in which a UAV equipped with a directional antenna relays data from a ground base station (GBS) to ground users (GUs). Under this system, we study a problem minimizing the weighted sum power consumption of the GBS and the UAV by jointly optimizing the location of the UAV, beamwidth of the directional antenna, and power allocation. To solve the problem, we develop a simple heuristic algorithm, and verify its effectiveness through simulation results.
Y.-I. Park, D.-Y. Kim, and J.-W. Lee, "Joint trajectory and charging power optimization for laser-charged UAV relaying networks," in Proc. ICTC 2022, Jeju, Korea, Oct. 2022, pp. 224-229 (10.1109/ICTC55196.2022.9952456). | |

Joint trajectory and charging power optimization for laser-charged UAV relaying networks

In this paper, we investigate a laser-charged unmanned aerial vehicle (UAV) relaying network where a rotarywing UAV relays data from a ground base station (GBS) to a user equipment (UE), and a charging station (CS) transmits a laser beam to charge the UAV. Instead of using a simplified propulsion power consumption (PPC) model for UAVs, which was widely used in the previous works on the laser-charged UAV relaying networks, we leverage an intractable but more accurate PPC model for UAVs.We study an optimization problem of minimizing the power consumption of the CS while satisfying the UE’s minimum data requirement, and we develop an algorithm to solve this problem by jointly optimizing UAV trajectory and charging power. Through simulation, we demonstrate the effectiveness of our trajectory and charging power control algorithm and validate the importance of leveraging the accurate PPC model for UAVs rather than using the simplified one.
K.-W. Kim and J.-W. Lee, "Waveform design for wireless power transfer with a double half-wave rectenna circuit," in Proc. ICEIC 2022, Virtual, Feb. 2022, pp. 1-2 (10.1109/ICEIC54506.2022.9748432). | |

Waveform design for wireless power transfer with a double half-wave rectenna circuit

In this paper, we study a waveform design problem for a wireless power transfer (WPT) system where a device harvests power by using a double half-wave rectenna circuit. To this end, we formulate the waveform design problem that maximizes the output DC voltage of the device, and solve it with proper approximations based on the property of WPT. Through the simulation results, we show the performance of the proposed waveform design.
D.-Y. Kim, H. Jafarkhani, and J.-W. Lee, "Low-complexity joint user and power scheduling for downlink NOMA over fading channels," in Proc. IEEE VTC 2021-Spring, Virtual, Apr. 2021, pp. 1-5 (10.1109/VTC2021-Spring51267.2021.9448865). | |

Low-complexity joint user and power scheduling for downlink NOMA over fading channels

In this paper, we study the joint user and power scheduling for downlink NOMA over fading channels. Specifically, we focus on a stochastic optimization problem to maximize the weighted average sum rate while ensuring given minimum average data rates of users. To address this problem, we first develop an opportunistic user and power scheduling algorithm (OUPS) based on the duality and stochastic optimization theories. By OUPS, the stochastic problem is transformed into a series of deterministic ones for the instantaneous weighted sum rate maximization for each slot. Thus, we additionally develop a heuristic algorithm with very low computational complexity, called user selection and power allocation algorithm (USPA), for the instantaneous weighted sum rate maximization problem. Via simulation results, we demonstrate that USPA provides near-optimal performance with very low computational complexity, and OUPS well guarantees given minimum average data rates.
H.-S. Lee, C. Shen, W. Zame, J.-W. Lee, and M. van der Schaar, "SDF-Bayes: Cautious optimism in safe dose-finding clinical trials with drug combinations and heterogeneous patient groups," in Proc. AISTATS, vol. 130, Virtual, Apr. 2021, pp. 2980-2988 (pmlr-v130-lee21c). | |

SDF-Bayes: Cautious optimism in safe dose-finding clinical trials with drug combinations and heterogeneous patient groups

Phase I clinical trials are designed to test the safety (non-toxicity) of drugs and find the maximum tolerated dose (MTD). This task becomes significantly more challenging when multiple-drug dose-combinations (DC) are involved, due to the inherent conflict between the exponentially increasing DC candidates and the limited patient budget. This paper proposes a novel Bayesian design, SDF-Bayes, for finding the MTD for drug combinations in the presence of safety constraints. Rather than the conventional principle of escalating or de-escalating the current dose of one drug (perhaps alternating between drugs), SDF-Bayes proceeds by cautious optimism: it chooses the next DC that, on the basis of current information, is most likely to be the MTD (optimism), subject to the constraint that it only chooses DCs that have a high probability of being safe (caution). We also propose an extension, SDF-Bayes-AR, that accounts for patient heterogeneity and enables heterogeneous patient recruitment. Extensive experiments based on both synthetic and real-world datasets demonstrate the advantages of SDF-Bayes over state of the art DC trial designs in terms of accuracy and safety.
H.-S. Lee, Y. Zhang, W. Zame, C. Shen, J.-W. Lee, and M. van der Schaar, "Robust recursive partitioning for heterogeneous treatment effects with uncertainty quantification," in Proc. NeurIPS, Virtual, Dec. 2020 (arXiv:2006.07917). | |

Robust recursive partitioning for heterogeneous treatment effects with uncertainty quantification

Subgroup analysis of treatment effects plays an important role in applications from medicine to public policy to recommender systems. It allows physicians (for example) to identify groups of patients for whom a given drug or treatment is likely to be effective and groups of patients for which it is not. Most of the current methods of subgroup analysis begin with a particular algorithm for estimating individualized treatment effects (ITE) and identify subgroups by maximizing the difference across subgroups of the average treatment effect in each subgroup. These approaches have several weaknesses: they rely on a particular algorithm for estimating ITE, they ignore (in)homogeneity within identified subgroups, and they do not produce good confidence estimates. This paper develops a new method for subgroup analysis, R2P, that addresses all these weaknesses. R2P uses an arbitrary, exogenously prescribed algorithm for estimating ITE and quantifies the uncertainty of the ITE estimation, using a construction that is more robust than other methods. Experiments using synthetic and semi-synthetic datasets (based on real data) demonstrate that R2P constructs partitions that are simultaneously more homogeneous within groups and more heterogeneous across groups than the partitions produced by other methods. Moreover, because R2P can employ any ITE estimator, it also produces much narrower confidence intervals with a prescribed coverage guarantee than other methods.
S. Moon and J.-W. Lee, "Integrated grant-free scheme for URLLC and mMTC," in Proc. IEEE 5G World Forum, Sep. 2020, pp. 98-102 (10.1109/5GWF49715.2020.9221204). | |

Integrated grant-free scheme for URLLC and mMTC

In this paper, we study a grant-free (GF) scheme to efficiently support the IoT user equipments (UEs). Compared with the existing studies regarding the GF schemes where only one type of the IoT UEs is considered, we develop an integrated GF scheme to support both the ultra-reliable low-latency communications (URLLC) and massive machine type communications (mMTC) UEs. Specifically, the proposed integrated GF scheme provides high reliability and low latency for the URLLC UEs by adopting both the contention-based (CB) and contention-free (CF) transmissions, while also supporting the mMTC UEs by adopting the CB transmission. We evaluate the performance of the proposed integrated GF scheme with the ns-3 simulator. Simulation results show that our integrated GF scheme is able to support the both types of the UEs effectively, and especially, the URLLC UEs can achieve high success rate and low latency compared with other GF schemes.
S. Moon and J.-W. Lee, "DMRS-applied repetition transmission (DART): Grant-free scheme for mMTC," in Proc. IEEE 5G World Forum, Sep. 2020, pp. 286-290 (10.1109/5GWF49715.2020.9221278). | |

DMRS-applied repetition transmission (DART): Grant-free scheme for mMTC

In this paper, we develop a repetition-based grantfree scheme, called DMRS-Applied Repetition Transmission (DART), to support mMTC user equipments (UEs). In DART, the demodulation reference signals (DMRSs) are exploited to estimate the channels as well as to implicitly inform the repetition patterns to the gNB. Based on the detected DMRSs, the gNB performs successive interference cancellation (SIC) to further decode the data of mMTC UEs that transmit their data on the same resource block. DART achieves high success rate, low latency, and low SIC overhead by adopting DMRS-based SIC ordering. Simulation results show that DART is able to support mMTC UEs effectively.
S. Moon and J.-W. Lee, "Performance study of repetition-based grant-free schemes in the mMTC scenario," in Proc. ITC-CSCC 2019, Jeju, Korea, Jun. 2019 (10.1109/ITC-CSCC.2019.8793363). | |

Performance study of repetition-based grant-free schemes in the mMTC scenario

In this paper, we study the performance of repetition-based grant-free (GF) schemes to efficiently in the massive machine type communication (mMTC) scenario. In specific, we investigate and compare the $K$ repetition proposed by the fifth generation (5G) standardization meeting and the irregular repetition slotted ALOHA (IRSA) which uses successive interference cancellation (SIC) to achieve better decoding performance.
D.-Y. Kim and J.-W. Lee, "Joint mission assignment and location management for UAVs in mission-critical flying ad hoc networks," in Proc. ICTC 2018, Jeju, Korea, Oct. 2018, pp. 323-328 (10.1109/ICTC.2018.8539355). | |

Joint mission assignment and location management for UAVs in mission-critical flying ad hoc networks

In recent years, the emergence of a flying ad hoc network (FANET) with multiple unmanned aerial vehicles (UAVs) can make it possible to effectively accomplish not only far-off missions but also assorted complex missions. In particular, with a limited number of UAVs, it is important to determine both mission assignment of UAVs to enhance mission performance and locations of UAVs to maintain good communication with the ground control station (GCS). In this paper, we study a problem of joint mission assignment and location management for UAVs, with jointly considering communication and mission performance. To solve the problem, employing particle swarm optimization (PSO) algorithm in part, we develop a heuristic algorithm that assigns multiple UAVs to multiple missions and finds locations of all UAVs together to maximize the weighted sum of the communication and mission performance. The simulation results demonstrate that our proposed algorithm provides a good solution.
D.-Y. Kim and J.-W. Lee, "Topology construction for flying ad hoc networks (FANETs)," in Proc. ICTC 2017, Jeju, Korea, Oct. 2017, pp. 153-157 (10.1109/ICTC.2017.8190963). | |

Topology construction for flying ad hoc networks (FANETs)

A flying ad hoc network (FANET) that consists of multiple unmanned aerial vehicles (UAVs) is a promising technology for future networked systems. In this paper, we study a way to construct a topology for FANET that guarantees end-to-end communication between the ground control station (GCS) and each mission UAV that performs its given task via optimizing the locations of the relay UAVs. To this end, we propose a topology construction algorithm based on a particle swarm optimization (PSO) algorithm. Numerical results show that the proposed algorithm achieves a significantly higher performance gain over the random-based topology construction scheme.
D.-H. Bae, H.-S. Lee, and J.-W. Lee, "Low Latency Uplink Transmission," in Proc. ICEIC 2017, Phuket, Thailand, Jan. 2017 (http://www.dbpia.co.kr/Journal/ArticleDetail/NODE07110610). | |

Low Latency Uplink Transmission

For emerging future services, such as virtual reality and remote control, low latency for uplink transmission in a few milliseconds is required. However, with a conventional uplink transmission scheme in LTE/LTE-A, such low latency is hard to be achieved since the scheduling request and grant procedure in the conventional scheme causes a fixed minimum delay. In this paper, we propose a novel uplink transmission scheme which achieves lower latency than the conventional uplink transmission scheme by reducing such delay. Through the simulation results, we show that the performance of the proposed uplink transmission scheme.
H.-S. Lee, C. Tekin, M. van der Schaar, and J.-W. Lee, "Contextual learning for unit commitment with renewable energy sources," in Proc. IEEE GlobalSIP 2016, Washington D.C., USA, Dec. 2016 (https://doi.org/10.1109/GlobalSIP.2016.7905966). | |

Contextual learning for unit commitment with renewable energy sources

In this paper, we study a unit commitment (UC) problem minimizing operating costs of the power system with renewable energy sources. We develop a contextual learning algorithm for UC (CLUC) which learns which UC schedule to choose based on the context information such as past load demand and weather condition. CLUC does not require any prior knowledge on the uncertainties such as the load demand and the renewable power outputs, and learns them over time using the context information. We characterize the performance of CLUC analytically, and prove its optimality in terms of the longterm average cost. Through the simulation results, we show the performance of CLUC and the effectiveness of utilizing the context information in the UC problem.
H.-S. Lee and J.-W. Lee, "Energy cooperation and traffic management in cellular networks with renewable energy," in Proc. IEEE Globecom 2016, Washington DC, USA, Dec. 2016 (https://doi.org/10.1109/GLOCOM.2016.7841789). | |

Energy cooperation and traffic management in cellular networks with renewable energy

In this paper, we study joint energy cooperation and traffic management in renewable energy powered cellular system where a centralized unit manages the traffic and the energy cooperation among BSs. We first formulate a stochastic optimization problem which aims at minimizing the total on-grid energy consumption while satisfying the quality-of-service (QoS) requirement of classes of services, i.e., the minimum average data rates. By using the Lyapunov optimization framework, we develop a joint adaptive energy cooperation and traffic management algorithm which does not need the statistical information of the system. Then, we provide the performance analysis which shows our proposed algorithm is asymptotically optimal. Through the simulation results, we verify the theoretical analysis and show that the performance of our algorithm.
J.-A. Kwon and J.-W. Lee, "Joint cell association and resource scheduling for OFDMA multi-cell HetNets," in Proc. URSI AP-RASC 2016, Seoul, Korea, Aug. 2016 (https://doi.org/10.1109/URSIAP-RASC.2016.7883541). | |

Joint cell association and resource scheduling for OFDMA multi-cell HetNets

In this paper, we study a joint cell association and opportunistic resource scheduling scheme for an OFDMA multicell heterogeneous network (HetNet) in an integrated and unified framework. To this end, we formulate the optimization problem and solve it with a dual approach to develop a scheduling scheme.
H.-S. Lee and J.-W. Lee, "QoS and channel-aware distributed link scheduling for D2D communication," in Proc. WiOpt 2016, Arizona, USA, May 2016 (https://doi.org/10.1109/WIOPT.2016.7492914). | |

QoS and channel-aware distributed link scheduling for D2D communication

In this paper, we study a distributed link scheduling problem for device-to-device (D2D) communication with considering the quality-of-service (QoS) requirement and time-varying channel condition of each D2D link. To this end, we first study an optimal centralized link scheduling algorithm maximizing the total average sum-rate. We then abstract the important scheduling principles of the optimal algorithm in order to use them to develop a distributed link scheduling algorithm. In our distributed link scheduling algorithm, we develop a procedure with which D2D links are able to share their degree of QoS satisfaction and channel condition with each other in a distributed manner. By utilizing those information for link scheduling, our link scheduling algorithm is able to satisfy the QoS requirement of each D2D link while achieving throughput improvement by exploiting time-varying channel condition of each D2D link in a distributed manner.
S.-Y. Kim and J.-W. Lee, "Content allocation in information-centric networks," in Proc. ICOIN 2016, Kota Kinabalu, Malaysia, Jan. 2016 (https://doi.org/10.1109/ICOIN.2016.7427102). | |

Content allocation in information-centric networks

The information-centric network (ICN) is a new paradigm shift from the traditional IP-based network architecture. In the ICN architecture, any node can have a storage capacity to store contents. Hence, the content allocation algorithm has been actively studied. However, many researches have proposed heuristic content allocation algorithms without considering the system-wise performance. In this paper, we study a content allocation algorithm to minimize the sum of the average end-to-end service times of client nodes. Numerical results show that the proposed content allocation algorithm significantly decreases the sum of the average end-to-end service times of client nodes.
S. Moon, H.-S. Jung, and J.-W. Lee, "On the usages of the battery in the smart grid: reducing cost and enhancing privacy," in Proc. ICTC 2015, Jeju, Korea, Oct. 2015 (https://doi.org/10.1109/ICTC.2015.7354489). | |

On the usages of the battery in the smart grid: reducing cost and enhancing privacy

In practice, there are two main usages of the battery in home: reducing electricity energy cost by utilizing time-varying electricity energy price and enhancing privacy by concealing the electricity energy usage pattern of appliances. By operating the battery such that it is charged/discharged during periods with low/high prices, we can reduce the electricity energy cost. On the other hand, by operating the battery such that the electricity usage pattern can be significantly changed from the original usage pattern of appliances, we can protect user’s privacy. However, those two operations are not complement with each other and there exists a trade-off between reducing cost and enhancing privacy. In this paper, we study this issue. To consider both cost and privacy in a unified way, we first define cost metric and privacy metric that should be minimized. Then, we define an overall metric as a weighted sum of the two metrics to study the trade-off between them. We formulate and solve an optimization problem that aims at minimizing the overall metric. Finally, through numerical results, we study and discuss the usage of the battery for reducing cost and enhancing privacy.
H.-S. Lee, D.-H. Bae, and J.-W. Lee, "Energy or traffic: which one to transfer," in Proc. IEEE VTC 2015-Fall, Boston, USA, Sep. 2015 (https://doi.org/10.1109/VTCFall.2015.7390919). | |

Energy or traffic: which one to transfer

In this paper, we study joint topology management and energy cooperation in cellular networks. The topology management scheme transfers users (traffic) from one base station (BS) to another BS by adjusting the cell-size of the BSs, and the energy cooperation scheme transfers harvested energy from one BS to another BS. We first formulate a joint topology management and energy cooperation problem which aims at minimizing the total on-grid energy consumption while satisfying the quality-of-service (QoS) requirement of each user. Then, by solving the problem, we develop a joint topology management and energy cooperation scheme. Topology management and energy cooperation are different approaches to save the on-grid energy consumption, and each scheme is effective to save the on-grid energy consumption in different environments such as different traffic and weather conditions. Through the simulation results, we show which scheme is more effective to save the on-grid energy consumption considering various environments. In addition, we also show that our joint scheme is most effective to deal with the environment which changes dynamically.
S. Weon, S. Lee, S. Choi, J.-W. Lee, and D. Hong, "Impacts of sub hub on connectivity of IEEE 802.15.4 in healthcare environments," in Proc. IEEE WCNC 2015, New Orleans, LA, USA, Jun. 2015 (doi.org/10.1109/WCNC.2015.7127613). | |

Impacts of sub hub on connectivity of IEEE 802.15.4 in healthcare environments

In this paper, we consider IEEE 802.15.4 based wireless body area sensor networks(WBASN) in the healthcare environment. Connectivity is a critical constraint in the healthcare environment. In particular, disconnection from a sensor hub is a significant problem that leads to sensor node orphaning. To address this, we propose to use a sub hub designed to enhance WBASN connectivity. The sub hub understudies the original (main) sensor hub and acts to resolve the problems associated with disconnection. We also propose two protocols for use in operating the sub hub. Each protocol prevents the sensor nodes from being orphaned, and forwards data from the sensor nodes to the original sensor hub. In addition, we evaluate the connectivity performance of our proposed protocols and analyze the disconnection tolerance and the number of packet drops. To demonstrate the performances of the proposed protocols, we developed an IEEE 802.15.4 compliant system level simulator with an OPNET Modeler. Simulation results show the individual merits and the demerits of each of the protocols in terms of connectivity. In addition, the energy efficiency of our proposed protocol is evaluated.
H.-S. Lee and J.-W. Lee, "QoS and channel-aware subchannel scheduling for D2D multicast communication," in Proc. ITC-CSCC 2015, Seoul, Korea, Jun. 2015 (http://www.dbpia.co.kr/Journal/ArticleDetail/NODE06386079). | |

QoS and channel-aware subchannel scheduling for D2D multicast communication

In this paper, we study a subchannel scheduling problem for device-to-device (D2D) multicast communication with considering the quality-ofservice (QoS) requirement and time-varying channel condition of each D2D link. By solving the problem, we first develop an optimal opportunistic subchannel scheduling algorithm maximizing the average sumrate, while guaranteeing the QoS requirement of each D2D link. We then develop a heuristic subchannel scheduling algorithm which has low signaling overhead and complexity. Through simulations, we show that both algorithms guarantee the QoS requirements of D2D links.
J.-A. Kwon, J.-H. Song, D.-H. Bae, and J.-W. Lee, "Joint cell association and resource scheduling for multi-cell HetNets," in Proc. CFI 2015, Seoul, Korea, Jun. 2015.
J.-H. Song, H.-T. Roh, and J.-W. Lee, "Opportunistic scheduling for OFDMA networks with mobile relay stations," in Proc. ICTC 2014, Busan, Korea, Oct. 2014 (invited).
H.-T. Roh and J.-W. Lee, "Cross-Layer Optimization for Wireless Sensor Networks with RF Energy Transfer," in Proc. ICTC 2014, Busan, Korea, Oct. 2014.
S.-Y. Kim, J.-A. Kwon, and J.-W. Lee, "Optimal subchannel and power allocation for multi-cell OFDMA systems," in Proc. ICUFN 2014, Shanghai, China, Jul. 2014 (invited).
B.-G. Kim, Y. Zhang, M. van der Schaar, and J.-W. Lee, "Dynamic pricing for smart grid with reinforcement learning," in Proc. IEEE CCSES 2014 (in conjunction with IEEE INFOCOM 2014), May 2014, pp. Toronto, Canada. | |

Dynamic pricing for smart grid with reinforcement learning

In the smart grid system, dynamic pricing can be an efficient tool for the service provider which enables efficient and automated management of the grid. However, in practice, the lack of information about the customers’ time-varying load demand and energy consumption patterns and the volatility of electricity price in the wholesale market make the implementation of dynamic pricing highly challenging. In this paper, we study a dynamic pricing problem in the smart grid system where the service provider decides the electricity price in the retail market. In order to overcome the challenges in implementing dynamic pricing, we develop a reinforcement learning algorithm. To resolve the drawbacks of the conventional reinforcement learning algorithm such as high computational complexity and low convergence speed, we propose an approximate state definition and adopt virtual experience. Numerical results show that the proposed reinforcement learning algorithm can effectively work without a priori information of the system dynamics.
J.-H. Kim and J.-W. Lee, "OpenIoT: an open service framework for the Internet of Things," in Proc. IEEE WF-IoT 2014, Seoul, Korea, May 2014. | |

OpenIoT: an open service framework for the Internet of Things

The Internet of Things (IoT) has been a hot topic for the future of computing and communication. It will not only have a broad impact on our everyday life in the near future, but also create a new ecosystem involving a wide array of players such as device developers, service providers, software developers, network operators, and service users. In this paper, we present an open service framework for the Internet of Things, facilitating entrance into the IoTrelated mass market, and establishing a global IoT ecosystem with the worldwide use of IoT devices and softwares. We expect that the open IoT service framework we proposed will play an important role in the widespread adoption of the Internet of Things in our everyday life, enhancing our quality of life with a large number of innovative applications and services, but also offering endless opportunities to all of the stakeholders in the world of information and communication technologies.
H.-T. Roh and J.-W. Lee, "Joint channel and flow assignment algorithm for wireless mesh networks," in Proc. ICOIN 2014, Phuket, Thailand, Feb. 2014. | |

Joint channel and flow assignment algorithm for wireless mesh networks

The wireless mesh networks (WMN) has attracted significant interests as a next-generation broadband wireless network to provide ubiquitous wireless access for broadband services. Especially with incorporating multiple orthogonal channels and multiple directional antennas into the WMN, each node can communicate with its neighbor nodes simultaneously without interference between them. In addition, due to directional antennas, the interference range of each transmission can be significantly reduced and we can further utilize the frequency separation. Hence, we can improve the throughput of the WMN significantly with multiple orthogonal channels and multiple directional antennas. In this paper, we study a joint channel and flow assignment problem for the WMN with multiple channels and multiple directional antennas. We first formulate an optimization problem as a mixed integer optimization problem that aims at maximizing the network utility. Since it is inherently hard to solve the mixed integer optimization problem, we propose a heuristic algorithm to solve the problem. The simulation results show our heuristic algorithm provides a reasonable performance in terms of the network utility compared to its upper bound.
J.-W. Lee, S.-Y. Jung, and H.-T. Roh, "Mobility-aware adaptive transmission in the mobile ad-hoc network," in Proc. IFAC TA 2013, Seoul, Korea, Nov. 2013. | |

Mobility-aware adaptive transmission in the mobile ad-hoc network

In this paper, we study a mobility-aware adaptive transmission between two mobile nodes in the mobile ad-hoc network. We model the amount of data that can be transmitted between two mobile nodes in a single encounter as a function of the start time of the transmission. From this model, we show that there exists a tradeoff between the data rate and the time duration for the transmission through the start time of the transmission. Hence, it is important to have the optimal start time of the transmission to maximize the amount of data that can be transmitted between two mobile nodes. In this paper, we obtain the optimal start time and show that in general, the optimal start time depends on the mobilities (i.e., velocities) of two mobile nodes.
D.-H. Lee, H.-T. Roh, B.-G. Kim, and J.-W. Lee, "Performance analysis of the IEEE 802.15.4 MAC protocol," in Proc. ICTC 2013, Jeju, Korea, Oct. 2013 (Invited). | |

Performance analysis of the IEEE 802.15.4 MAC protocol

In this paper, we analyze the throughput of the IEEE 802.15.4 medium access control (MAC) protocol with considering both contention access period (CAP) and contention free period (CFP). We first analyze the CSMA/CA algorithm in the CAP using a Markov chain considering the guaranteed time slot (GTS) request packets for the CFP. And we provide an analytical model of the GTS allocation algorithm in the CFP using a Markov chain and the results of the CAP. Numerical results show that the ratio of the length of the CAP to that of the CFP affects the throughput of the system.
Y. R. Jang, Y. Kim, J.-W. Lee, D. Hong, and S. Choi, "Adaptive queue management scheme for body area network with energy harvesting," in Proc. BodyNets 2013, Boston, USA, Sep. 2013.
S. Lee, S. Weon, S. Choi, J.-W. Lee, C. Park, Y. Kim, Y.-J. Hong, and D. Hong, "Increasing the life-time of 802.15.4-based wireless sensor networks," in Proc. BodyNets 2013, Boston, USA, Sep. 2013.
S.-Y. Kim, J.-A. Kwon, and J.-W. Lee, "Resource allocation for the multi-cell OFDMA system and its capacity bounds," in Proc. WiOpt 2013, Tsukuba Science City, Japan, May 2013. | |

Resource allocation for the multi-cell OFDMA system and its capacity bounds

Recently, joint subchannel allocation and transmission power control problems for multi-cell orthogonal frequencydivision multiple access (OFDMA) systems have been actively studied. However, since the problems are notoriously difficult and complex, only heuristic approaches are mainly used to study the problems without trying to achieve the optimal resource allocation and the maximum system capacity. In this paper, we study the joint subchannel allocation and transmission power control problem for multi-cell OFDMA systems from the point of the optimal resource allocation and the maximum system capacity with a monotonic optimization approach. Even though we do not obtain the exact optimal resource allocation that achieves the maximum system capacity, we develop an algorithm for resource allocations that provide both upper and lower bounds on the maximum system capacity. Through numerical results, we evaluate our algorithm showing that it provides good approximations to the maximum system capacity in multi-cell OFDMA systems in most cases.
B.-G. Kim, S. Ren, M. van der Schaar, and J.-W. Lee, "Tiered billing scheme for residential load scheduling with bidirectional energy trading," in Proc. IEEE SDP (in conjunction with IEEE INFOCOM 2013), Turin, Italy, Apr. 2013. | |

Tiered billing scheme for residential load scheduling with bidirectional energy trading

Future generation smart grids will allow customers to trade energy bidirectionally. Specifically, each customer will be able to not only buy energy from the aggregator during its peak hours but also sell its surplus energy during its off-peak hours. In these emerging energy trading markets, a key component will be the deployment of effective energy billing schemes which consider the customers residential load scheduling. In this paper, we consider a residential load scheduling problem with bidirectional energy trading. Compared with the previous work, in which customers are assumed to be obedient and agree to maximize the social welfare of the smart grid system, in this paper, we consider a non-collaborative approach, where consumers are self-interested. We model the energy scheduling problem as a non-cooperative game, where each customer determines its load scheduling and energy trading to maximize its own profit. In order to resolve the unfairness between heavy and light customers, we propose a novel tiered billing scheme that can control the electricity rates for customers according to their different energy consumption levels. We also propose a distributed energy scheduling algorithm that converges to the unique Nash equilibrium of the studied non-cooperative game. Through the numerical results, we study the impact of the proposed tiered billing scheme on the selfish customers’ behavior and on their incentives to participate in the energy trading market.
B.-G. Kim, S. Ren, M. van der Schaar, and J.-W. Lee, "Bidirectional energy trading for residential load scheduling and electric vehicles," in Proc. IEEE INFOCOM 2013 mini-conference, Turin, Italy, Apr. 2013. | |

Bidirectional energy trading for residential load scheduling and electric vehicles

Electric vehicles (EVs) will play an important role in the future smart grid because of their capabilities of storing electrical energy in their batteries during off-peak hours and supplying the stored energy to the power grid during peak hours. In this paper, we consider a power system with an aggregator and multiple customers with EVs and propose a novel electricity load scheduling which, unlike previous works, jointly considers the load scheduling for appliances and the energy trading using EVs. Specifically, we allow customers to determine how much energy to purchase from or to sell to the aggregator while taking into consideration the load demands of their residential appliances and the associated electricity bill. Under the assumption of the collaborative system where the customers agree to maximize the social welfare of the power system, we develop an optimal distributed load scheduling algorithm that maximizes the social welfare. Through numerical results, we show when the energy trading leads to an increase in the social welfare in various usage scenarios.
H.-S. Jung, H.-T. Roh, and J.-W. Lee, "Energy and traffic aware dynamic topology management for wireless cellular networks," in Proc. IEEE ICCS 2012, Singapore, Nov. 2012 (Invited). | |

Energy and traffic aware dynamic topology management for wireless cellular networks

In this paper, we study a dynamic topology management problem in the wireless cellular network, in which base stations can be switched on or off to save energy consumption of the network. To model non-uniform traffic demand over the network, we divide the entire network area into subareas, assuming that each of subareas has its own traffic demand, which can be satisfied by allocating enough bandwidth and power from a base station. With this network model, we propose a two-step algorithm for dynamic topology management. In the first step, we obtain the maximum radius of the service area for each base station by solving an optimization problem. In the second step, we first construct a graph based on the maximum radii obtained in the first step. We then identify the minimal set of base stations that should be switched on to satisfy the demand of each subarea in the network based on the shortest path algorithm on the constructed graph. From numerical results, we show that our algorithm provides a significant amount of energy saving, while adaptively considering the traffic demand of each subarea in the network.
S.-Y. Kim, J.-A. Kwon, and J.-W. Lee, "Study of optimal topology and routing in the BAN," in Proc. ICTC 2012, Jeju, Korea, Oct. 2012. | |

Study of optimal topology and routing in the BAN

In this paper, we study an optimal topology and routing problem in the body area network (BAN). Aiming at maximizing the network lifetime, we consider the guarantee of two important quality-of-service (QoS) requirements for BAN applications: the minimum bit error rate and the minimum data rate. Especially, we study the effect of the maximum number of hops that is allowed in routing to the system performance.
M.-H. Han, B.-G. Kim, and J.-W. Lee, "Subchannel and transmission mode scheduling for device-to-device communication in OFDMA networks," in Proc. IEEE VTC 2012-Fall, Quebec City, Canada, Sep. 2012. | |

Subchannel and transmission mode scheduling for device-to-device communication in OFDMA networks

We study an opportunistic subchannel scheduling and transmission mode selection problem for the OFDMA system with device-to-device (D2D) communication. We allow D2D users to opportunistically select its transmission mode between two transmission modes: direct transmission between D2D users (direct one- hop transmission) and indirect transmission through the BS (indirect two-hop transmission). We develop a framework with which opportunistic transmission mode selection can be modeled as opportunistic subchannel scheduling, which enables our problem to be reduced to an opportunistic subchannel scheduling problem. We formulate a stochastic optimization problem that aims to maximize the average sum-rate of the system, while satisfying the quality-of-service (QoS) requirement of each user. By solving the problem, we develop an optimal opportunistic subchannel scheduling algorithm, which enables us to perform both subchannel scheduling and transmission mode selection opportunistically.
J.-W. Lee and H.-T. Roh, "Mobility control as network resource management," in Proc. UKC 2012, Los Angeles, USA, Aug. 2012.
S.-Y. Kim, J.-A. Kwon, J.-W. Lee, C. Park, Y. Kim, and H. Hwang, "Network lifetime improvement by relaying node re-selection in the IEEE 802.15.6 BAN," in Proc. ICUFN 2012, Phuket, Thailand, Jul. 2012. | |

Network lifetime improvement by relaying node re-selection in the IEEE 802.15.6 BAN

In this paper, we study a relaying node re-selection problem in the IEEE 802.15.6 body area network (BAN) protocol, which requires to alleviate energy consumption burden of a relaying node and enhance the network life time. To this end, we develop a procedure for wake-up schedule control for a node that can be accommodated in the IEEE 802.15.6 standard. Numerical results show that by allowing a node to re-select its relaying node with our procedure, we can improve the network lifetime.
H.-T. Roh and J.-W. Lee, "Joint mission and communication aware mobility control in mobile ad-hoc networks," in Proc. WiOpt 2012, Paderborn, Germany, May 2012. | |

Joint mission and communication aware mobility control in mobile ad-hoc networks

In this paper, we study a mobility control problem in mobile ad-hoc networks with controllable mobility. Especially, we consider mission-critical networks in which nodes have their own specific missions whose degree of satisfaction depends on the their locations as well as want to maintain a good communication quality with their neighbor nodes that also depends on their locations. Hence, in this paper, we study a joint mission and communication aware mobility control problem. We formulate the problem as a potential game and develop a distributed algorithm that converges to the Nash equilibrium. In addition, we also show that if some minor conditions are satisfied, our algorithm provides a global optimal solution that minimizes the weighted sum of costs for mission and communication.
J.-W. Lee, H.-T. Roh, and D.-H. Lee, "Utility maximizing residential electricity load scheduling for multi-class appliances in the smart grid," in Proc. CMC2012, Guilin, China, May 2012 (Invited).
H.-T. Roh and J.-W. Lee, "Group based mobility management of MTC devices in the LTE system," in Proc. ICEIC 2012, Jeongseon, Korea, Feb. 2012. | |

Group based mobility management of MTC devices in the LTE system

In machine-type communications (MTC), since there are a large number of devices, the signaling congestion is an important issue. In this paper, we try to reduce the signaling overhead, which is generated by MTC devices for the mobility management. It is possible that multiple MTC devices have the same mobility. For example, the MTC devices which are attached to a vehicle or loaded in a container have the same mobility. In this paper, we propose a method of group based mobility management for MTC devices, which have the same mobility, in order to reduce the signaling overhead for the tracking area update procedure in the LTE system.
J.-W. Lee and D.-H. Lee, "Residential electricity load scheduling for multi-class applications with time-of-use pricing," in Proc. IEEE SG-ComNeTs 2011 (in conjunction with IEEE Globecom 2011), Houston, USA, Dec. 2011. | |

Residential electricity load scheduling for multi-class applications with time-of-use pricing

In this paper, we study an electricity load scheduling problem in a residence. Compared with previous works in which only limited classes of appliances are considered, to consider various appliances with different operation and energy consumption characteristics with each other, we classify appliances into four classes and provide mathematical models to describe them. With the developed appliance models, we also propose an electricity load scheduling algorithm that controls the operation time and energy consumption of each appliance adapting to Time of Use (TOU) pricing and its operation and energy consumption characteristics to minimize the total electricity bill.
J.-H. Kim and J.-W. Lee, "Performance analysis of the energy adaptive MAC protocol for wireless sensor networks with RF energy transfer," in Proc. ICTC 2011, Seoul, Korea, Sep. 2011 (Invited).
B.-G. Kim, J.-A. Kwon, and J.-W. Lee, "Utility-based subchannel allocation for OFDMA femtocell networks," in Proc. coHetNets 2011 (in conjunction with IEEE ICCCN 2011), Hawaii, USA, Aug. 2011 (Invited). | |

Utility-based subchannel allocation for OFDMA femtocell networks

In femtocell networks, due to their small cell size, we can achieve higher spatial diversity from the channel reuse between multiple femtocells. In addition, if femtocells are operated on OFDMA systems, each subchannel can be reused separately among femtocells by carefully treating inter-cell interferences. However, due to a large number of femtocells and their uncoordinated and irregular deployment, we need to treat intercell interferences very carefully in OFDMA-based fmetocell networks, which make developing efficient resource allocation schemes more difficult. In this paper, we study a subchannel allocation problem that aims at maximizing the sum utility of the OFDMA-based femtocell network. Since the problem is formulated as a nonlinear integer programming, which is inherently difficult to solve, we propose a suboptimal subchannel allocation algorithm. The proposed subchannel allocation algorithm consists of two steps: calculating the number of subchannels that should be granted to each femtocell to maximize the sum utility and finding actual subchannel allocation that achieves the granted number of subchannels for each femtocell. Numerical results show that the proposed subchannel allocation algorithm provides near-optimal performance.
J.-H. Kim and J.-W. Lee, "Energy adaptive MAC protocol for wireless sensor networks with RF energy transfer," in Proc. ICUFN 2011, Dalian, China, Jun. 2011 (Invited). | |

Energy adaptive MAC protocol for wireless sensor networks with RF energy transfer

Recently, various energy harvesting techniques from ambient environments were proposed as alternative methods for powering sensor nodes, which convert the ambient energy from environments into electricity to power sensor nodes. However, those techniques are not applicable to the wireless sensor networks (WSNs) in the environment with no ambient energy source. To overcome this problem, an RF energy transfer method was proposed to power wireless sensor nodes. However, the RF energy transfer method also has a problem of unfairness among sensor nodes due to the significant difference between their energy harvesting rates according to their positions. In this paper, we propose a medium access control (MAC) protocol for WSNs based on RF energy transfer. The proposed MAC protocol adaptively manages the duty cycle of sensor nodes according to their the amount of harvested energy as well as the contention time of sensor nodes considering fairness among them. Through simulations, we show that our protocol can achieve a high degree of fairness, while maintaining duty cycle of sensor nodes appropriately according to the amount of their harvested energy.
S.-Y. Kim and J.-W. Lee, "Study of system throughput and fairness issues in cooperative transmissions," in Proc. CNSI 2011, Jeju, Korea, 2011. | |

Study of system throughput and fairness issues in cooperative transmissions

Thus far, most researches on the cooperative transmission focused on the reliability point of view, showing that we can significantly improve the reliability of data transmissions by the cooperative transmission. On contrary, in this paper, we study the performance of the cooperative transmission from different perspectives such as system throughput and fairness.We consider a three-node system that consists of two source nodes, which can choose to either cooperate with each other or not to transmit their data, and one destination node. Based on this system, we study the conditions under which the cooperative transmission is beneficial to improve system throughput and satisfy the fairness requirement between source nodes.
B.-G. Kim and J.-W. Lee, "Opportunistic subchannel scheduling for OFDMA networks with network coding at relay stations," in Proc. IEEE Globecom 2010, Miami, USA, Dec. 2010. | |

Opportunistic subchannel scheduling for OFDMA networks with network coding at relay stations

In this paper, we study an opportunistic subchannel scheduling problem for the relay-based OFDMA cellular network where relay stations (RSs) can perform opportunistic network coding with downlink and uplink sessions of a mobile station (MS). In wireless networks, network coding is one of promising techniques to improve network performance in various aspects. However, previous studies did not provide optimal opportunistic resource allocation solutions for the OFDMA networks with network coding-aware RSs as well as they did not consider practical duplexing schemes for the transmission of network coded data. In this paper, to allow network coding with downlink and uplink sessions at RSs, we consider a time-division duplexing (TDD) based time-slot structure. We formulate a stochastic optimization problem to maximize the average weighted-sum rate for both downlink and uplink sessions. Then, by solving the problem, we provide an optimal opportunistic subchannel scheduling algorithm that can opportunistically use network coding with considering time-varying channel states.
S.-Y. Kim, S.-L. Gong, and J.-W. Lee, "Joint optimization of link weight and link capacity expansion in communication networks," in Proc. ICTC 2010, Jeju, Korea, Nov. 2010 (Invited). | |

Joint optimization of link weight and link capacity expansion in communication networks

To satisfy a target utilization of each link in com· munication networks, re.routing of sessions and expansion of the link capacity could be considered. In this paper, we study a problem that finds joint optimal link weights for routing and capacity expansion so as to alleviate over·utilization of links in communication networks with minimum costs. We compare the performance of our joint algorithm with that of the algorithm that considers only link weight optimization [1], which shows that the joint optimization provides better performance than the link weight optimization only.
and J.-W. Lee, "To cooperate or not to cooperate," in Proc. AFIWINTER-2010, Seoul, Korea, Feb. 2010.
H.-T. Roh and J.-W. Lee, "End-to-end rate control in communication networks considering user-level satisfactions," in Proc. IEEE CCNC 2010, Las Vegas, USA, Jan. 2010. | |

End-to-end rate control in communication networks considering user-level satisfactions

In this paper, we consider a rate control problem in communication networks based on the network utility maximization framework. To accurately model the satisfaction of a user to its service, we propose a new concept of the utility function, which is called a user-level utility function, considering both transmitting and receiving sessions of a user together. We then formulate an optimization problem for rate control with user-level utility functions and develop a distributed rate control algorithm.
H.-T. Roh and J.-W. Lee, "Optimal placement of a relay node with controllable mobility in wireless networks considering fairness," in Proc. IEEE CCNC 2010, Las Vegas, USA, Jan. 2010. | |

Optimal placement of a relay node with controllable mobility in wireless networks considering fairness

Since the capacity of a wireless link depends on the distance between its transmitter and receiver nodes, the optimal placement of nodes in the wireless network is important to improve the system efficiency. We study this issue in this paper considering a wireless network that consists of multiple nodes that do not have controllable mobility and a relay node that has controllable mobility. We model the capacity of a wireless link between node and relay node as a function of the distance between them. We then formulate the optimization problem that aims at maximizing the weighted throughput of a node that achieves the lowest weighted throughput among all nodes. Unfortunately, the problem is inherently non-convex, which is in general difficult to solve. However, in this paper, we develop the algorithm for the optimal placement of the relay node based on duality theories in optimization. We also show that the optimal position of the relay node obtained by our algorithm is in fact the weighted max-min fair position.
B.-G. Kim, J.-W. Lee, S. I. Lee, and B. I. Cho, "Opportunistic subchannel scheduling for OFDMA cellular networks with scheduling at relay stations," in Proc. IEEE Tencon 2009, Singapore, Nov. 2009. | |

Opportunistic subchannel scheduling for OFDMA cellular networks with scheduling at relay stations

We address an opportunistic subchannel scheduling problem in relay-based OFDMA cellular networks. In most previous works on relay-based cellular networks, immediate relaying at relay stations (RSs) was assumed, without allowing the opportunistic scheduling at RSs. Hence, each RS should transmit the received data from the base station (BS) to the corresponding mobile stations (MSs) immediately within a single time-slot and the effective data rate of the two-hop transmission (BS-RS and RS-MS links) is limited by the achievable data rate of the link with worse channel state among two links, resulting in wastes of radio resources. However, if the opportunistic scheduling is allowed at each RS, the time-varying channel state of each link can be fully exploited, and thus more efficient radio resource allocation is possible. Hence, in this paper, we consider an opportunistic subchannel scheduling at both BS and RSs. We formulate a stochastic optimization problem that aims at maximizing the average sum-rate of the relay-based network where the time-varying wireless channel is modeled as a stochastic process. By solving the problem, we develop an optimal opportunistic subchannel scheduling algorithm for both BS and RSs. Numerical results show that we can significantly improve system performance by allowing the opportunistic scheduling at RSs.
S.-L. Gong, H.-T. Roh, J.-W. Lee, M. K. Ahn, and B. I. Cho, "Joint optimization for integrated wireless and wireline networks," in Proc. ISCIT 2009, Incheon, Korea, Sep. 2009. | |

Joint optimization for integrated wireless and wireline networks

In this paper, we study a cross-layer optimization problem for a network that consists of one wireline core network and multiple wireless access networks. We consider four layers among five layers in the network layering architecture: application characteristics through the utility function and quality of service requirements in the application layer; end-to-end flow control in the transport layer; opportunistic scheduling in the data link layer; adaptive modulation and coding in the physical layer. We formulate a stochastic optimization problem considering above four layers and both wireline and wireless parts of the network jointly that results in a utility-based joint end-to-end flow control and opportunistic scheduling problem for the integrated wireless and wireline network. We solve the problem by using a dual approach and a stochastic sub-gradient algorithm. The developed algorithm can be implemented in a distributed way: vertically among four layers and horizontally among all entities in the network, clearly showing what should be done in each layer and each entity and what parameters should be exchanged between layers vertically and between network entities horizontally.
S.-L. Gong, S.-Y. Kim, J.-W. Lee, S. I. Lee, and M. K. Ahn, "Link weight optimization for routing considering link utilization in communication networks," in Proc. ISCIT 2009, Incheon, Korea, Sep. 2009. | |

Link weight optimization for routing considering link utilization in communication networks

Abstract—In this paper, we study the problem that finds optimal link weights for routing in communication networks with a target utilization for each link. Since the problem is known as NP-hard, we take a heuristic approach based on the simulated annealing method. However, to make the algorithm to be more robust and converge fast, we modify the basic simulated annealing method. Numerical results show that our algorithm provides a better performance than the basic simulated annealing method.
S.-Y. Kim and J.-W. Lee, "Joint resource allocation for uplink and downlink in wireless networks: a case study with user-level utility functions," in Proc. IEEE VTC 2009-Spring, Barcelona, Spain, 2009. | |

Joint resource allocation for uplink and downlink in wireless networks: a case study with user-level utility functions

In most of researches in resource allocation for wireless networks, uplink and downlink problems are considered separately, especially when resources for uplink and downlink are statically partitioned, as in FDD and static TDD systems. However, even in those systems, joint resource allocation for uplink and downlink can improve system efficiency and we study this issue in this paper with the concept of the user-level utility function. In most cases, a user has a two-way communication that consists of two sessions: uplink and downlink sessions and its overall satisfaction to its communication depends on its satisfaction to each of its sessions. To model user’s overall satisfaction to its communication, we define a user-level utility function, which is defined as a function of its session-level utility functions. We then formulate and solve the optimization problem with user-level utility functions for cell-level resource scheduling that jointly considers uplink and downlink resource allocation. Simulation results show that our cell-level scheduling in which resource allocation in both uplink and downlink is done jointly outperforms link-level scheduling, in which resource allocation in each of uplink and downlink is done separately in most cases, especially when the asymmetry between uplink and downlink is large.
S.-L. Gong, B.-G. Kim, and J.-W. Lee, "Opportunistic scheduling and adaptive modulation in wireless networks with network coding," in Proc. IEEE VTC 2009-Spring, Barcelona, Spain, Apr. 2009. | |

Opportunistic scheduling and adaptive modulation in wireless networks with network coding

So far, many researches on network coding are performed with higher layer protocols such as MAC, routing, and flow control protocols without consideration of physical layer issues such as channel conditions of links. However, in wireless networks, the consideration of properties at the physical layer is important to improve system performance. Hence, in this paper, we study an opportunistic scheduling and adaptive modulation problem for wireless networks with network coding, which is a joint problem for MAC and physical layers. A similar problem was studied in [1] considering an idealized system in which the data rate of each link is modeled with the Shannon capacity. They showed that to maximize the throughput of a transmission, the optimal subset of native packets that are encoded within a coded packet should be selected based on the channel condition at the destination for each native packet. Moreover, they also showed that it may not be the optimal selection to encode all possible native packets within a coded packet. In this paper, we consider a more realistic model than that of [1] with practical modulation schemes such as M-PSK and MQAM. We show that the optimal policy that maximizes the throughput of a transmission is to encode all available native packets within a coded packet regardless of the channel condition at the destination for each native packet, which is a different conclusion from that of [1]. However, we show that adaptive modulation, in which its constellation size in a coded packet is adjusted based on the channel condition of each destination node, provides a higher throughput than the scheme with fixed modulation, in which its constellation size is always fixed regardless of the channel condition at each destination node.
H.-T. Roh and J.-W. Lee, "Network coding-aware flow control in wireless ad-hoc networks," in Proc. IEEE WCNC 2009, Budapest, Hungary, Apr. 2009. | |

Network coding-aware flow control in wireless ad-hoc networks

In this paper, we study an end-to-end flow control algorithm considering network coding in wireless ad-hoc networks. As a network coding scheme, we use XOR network coding, in which each node bitwise-XORs some packets from different sessions, and then broadcasts the XORed packets. This process can reduce the number of required transmissions and, thus improve network utilization, especially if it is used with appropriate network protocols. Considering this XOR network coding, we formulate an optimization problem for end-to-end flow control in wireless ad-hoc networks that aims at maximizing network utility. We then develop a distributed flow control algorithm by solving the optimization problem. The simulation results show that our distributed flow control algorithm performs well exploiting the advantages of network coding and provides higher network utility than the flow control algorithm without considering network coding.
B.-G. Kim and J.-W. Lee, "Opportunistic power scheduling for OFDMA cellular networks with scheduling at relay stations," in Proc. IEEE WCNC 2009, Budapest, Hungary, Apr. 2009. | |

Opportunistic power scheduling for OFDMA cellular networks with scheduling at relay stations

Abstract—In this paper, we study an opportunistic power scheduling problem in relay-based OFDMA cellular networks. In most previous works on relay-based cellular networks, scheduling at relay stations (RSs) is not allowed. Hence, each RS should immediately transmit all the received data from the base station (BS) in the first phase in a time-slot to mobile stations (MSs) in the second phase in the same time-slot. In this strategy, the effective data rate of the two-hop transmission (BS-RS and RS-MS) is limited to the rate of the link with worse channel state among two links. Hence, in those systems, even though opportunistic scheduling is allowed in the BS, time-varying channel state of each link cannot be fully exploited, which may result in wastes of radio resources. However, if opportunistic scheduling is allowed not only at the BS but also at each RS, the BS and each RS can fully exploit time-varying channel state of each link, so that more efficient radio resource allocation is possible. We formulate a stochastic optimization problem that aims at maximizing the average sum-rate of relay-based networks where the time-varying wireless channel is modeled as a stochastic process. By solving the problem, we develop an optimal power scheduling algorithm which can be implemented in a distributed manner at the BS and each RS. Simulation results show that by allowing opportunistic scheduling at both the BS and RSs, we can improve system performance more significantly.
B.-G. Kim and J.-W. Lee, "Utility-based end-to-end flow control for services with time-varying rate requirements," in Proc. IEEE CCNC 2009, Las Vegas, USA, Jan. 2009. | |

Utility-based end-to-end flow control for services with time-varying rate requirements

In this paper, we study a utility based flow control problem for a communication network. In most previous works on utility based flow control, the utility function of each user, which represents its satisfaction to the allocated data rate, is assumed to be fixed. This implies that the degree of the rate requirement of each user is assumed to be fixed over the entire duration of its session. However, in the communication network, many services are variable rate services, i.e., the degree of their rate requirement varies over time. Hence, in this paper, we allow the degree of the rate requirement of each user to change over time and model it by using a stochastic utility function that varies stochastically according to the variation of the degree of its rate requirement. We formulate a stochastic optimization problem with stochastic utility functions that aims at maximizing the average network utility while satisfying the constraint on the link capacity. By solving the stochastic optimization problem, we develop a distributed flow control algorithm that converges to the optimal rate allocation.
J.-W. Lee and J.-A. Kwon, "A new framework for opportunistic scheduling in wireless networks," in Proc. ISITA 2008, Auckland, New Zealand, Dec. 2008. | |

A new framework for opportunistic scheduling in wireless networks

In this paper, we propose a new framework for opportunistic scheduling in wireless systems. We consider a TDMA type of multiple access scheme, in which only one user can be scheduled in each time-slot. For opportunistic scheduling for such a system, a nice framework has been developed in. However, in this paper, we consider a more general system that has a probabilistic scheduling policy and more general fairness constraints. Moreover, in addition to this generalization, we develop a new framework for opportunistic scheduling based on the duality theory. The duality theory is a well-defined theory in the mathematical optimization area. Hence, by using the framework developed in this paper, we can analyze the developed algorithm more rigorously and systematically.
J.-W. Lee and J.-A. Kwon, "Downlink power allocation for wireless systems through network utility maximiztion," in Proc. APCC 2008, Tokyo, Japan, Oct. 2008. | |

Downlink power allocation for wireless systems through network utility maximiztion

We study power allocation problems in wireless systems through network utility maximization framework. In this framework, the type of utility function represents the characteristics of each service. Hence, to deal with multiclass services in the system, it is important to accommodate various types of utility functions. In this paper, we develop a simple algorithm that can accommodate four types of utility functions in a unified way, while providing asymptotically optimal power allocation. Those types of utility functions can represent most of services in wireless networks. We first develop an algorithm that solves a general optimization problem and, then, show that our algorithm can be applied to the downlink power allocation problem.
S.-C. Choi, J.-W. Lee, and Y. Kim, "An adaptive mobility-supporting MAC protocol for mobile sensor networks," in Proc. IEEE VTC-Spring, Singapore, May 2008. | |

An adaptive mobility-supporting MAC protocol for mobile sensor networks

In this paper, we design an energy efficient MAC protocol for mobile sensor networks. In our MAC protocol, to reduce idle listening, each node has its listen-sleep schedule. A subset of nodes that follow the same schedule forms a virtual cluster. The network consists of multiple virtual clusters and nodes in one virtual cluster may follow a different listensleep schedule from that of nodes in another virtual cluster. Hence, to support a mobile sensor node that moves into a new virtual cluster that follows a different listen-sleep schedule, it is necessary to have a fast and energy efficient listen-sleep schedule adaptation protocol. To this end, in our MAC protocol, we utilize schedule information on border nodes between virtual clusters. This allows us to implement fast and energy efficient listensleep schedule adaptation, which consists of two main functions: energy efficient secondary listen period and smart scheduling adaptation. Simulation results show that our protocol can provide fast scheduling adaptation while achieving energy efficiency.
S.-C. Choi, J.-W. Lee, Y. Kim, and H. Chong, "An energy-efficient MAC protocol with random listen-sleep schedule for wireless sensor networks," in Proc. IEEE Tencon 2007, Oct. 2007. | |

An energy-efficient MAC protocol with random listen-sleep schedule for wireless sensor networks

In this paper, we propose an energy efficient MAC protocol for wireless sensor networks. Since sensor nodes are equipped with a limited energy of batteries, reducing energy consumption is a critical issue for extending network lifetime. To resolve this issue, we adopt a listen-sleep cycle as in S-MAC [1], allowing sensor nodes to turn their transceiver off during a sleep period. In S-MAC, sensor nodes have a fixed duty cycle and a synchronized scheduling in a virtual cluster. Hence, in S-MAC, it is not easy to adapt to the variation of network environment. Moreover, due to the synchronized scheduling, transmission collisions will increase resulting in energy waste and low throughput. To cope with such inefficiencies in S-MAC, we propose probability sensor MAC (PS-MAC), in which each node determines ‘listen’ or ‘sleep’ pseudo-randomly based on its own pre-wakeup probability and pre-wakeup probabilities of its neighbor nodes in each time slot. This allows the listen-sleep schedule of each transmitter and receiver pair to be synchronized while that of rest of nodes can be asynchronous. Therefore, collisions can be reduced even under heavy traffic conditions resulting in reducing energy waste and achieving high throughput. In addition, since pre-wakeup probability of each node can be adjusted adapting to the change of the network environment, by dynamically adjusting pre-wakeup probabilities of sensor nodes, system throughput can be further improved.
J.-W. Lee, M. Chiang, and R. Calderbank, "Joint congestion control and MAC design based on network utility maximization," in Proc. IEEE VTC-Spring, May 2006.
A. Tang, J.-W. Lee, J. Huang, M. Chiang, and R. Calderbank, "Reverse engineering for MAC protocol: a non-cooperative game model," in Proc. WiOpt 2006, Apr. 2006. | |

Reverse engineering for MAC protocol: a non-cooperative game model

This paper reverse engineers backoff-based random-access MAC protocols in ad-hoc networks. We show that the contention resolution algorithm in such protocols is implicitly participating in a non-cooperative game. Each link attempts to maximize a selfish local utility function, whose exact shape is reverse engineered from the protocol description, through a stochastic subgradient method in which the link updates its persistence probability based on its transmission success or failure. We prove that existence of a Nash equilibrium is guaranteed in general. The minimum amount of backoff aggressiveness needed for uniqueness of Nash equilibrium and convergence of the best response strategy are established as a function of user density. Convergence properties and connection with the best response strategy are also proved for variants of the stochastic-subgradient-based dynamics of the game. Together with known results in reverse engineering TCP and BGP, this paper completes the recent efforts in reverse engineering the main protocols in layers 2-4.
J.-W. Lee, M. Chiang, and R. Calderbank, "Network utility maximization and pricing-based distributed algorithms for rate-reliability tradeoff," in Proc. IEEE Infocom 2006, Apr. 2006. | |

Network utility maximization and pricing-based distributed algorithms for rate-reliability tradeoff

The current framework of network utility maximization for rate allocation and its price-based algorithms assumes that each link provides a fixed-size transmission ‘pipe’ and each user’s utility is a function of transmission rate only. These assumptions break down in many practical systems, where, by adapting the physical layer channel coding or transmission diversity, different tradeoffs between rate and reliability can be achieved. In network utility maximization problems formulated in this paper, the utility for each user depends on both transmission rate and signal quality, with an intrinsic tradeoff between the two. Each link may also provide a higher (lower) rate on the transmission ‘pipes’ by allowing a higher (lower) decoding error probability. Despite non-separability and nonconvexity of these optimization problems, we propose new pricebased distributed algorithms and prove their convergence to the globally optimal rate-reliability tradeoff under readily-verifiable sufficient conditions. We first consider networks in which the rate-reliability tradeoff is controlled by adapting channel code rates in each link’s physical layer error correction codes, and propose two distributed algorithms based on pricing, which respectively implement the ‘integrated’ and ‘differentiated’ policies of dynamic ratereliability adjustment. In contrast to the classical price-based rate control algorithms, in our algorithms each user provides an offered price for its own reliability to the network while the network provides congestion prices to users. The proposed algorithms converge to a tradeoff point between rate and reliability, which we prove to be a globally optimal one for channel codes with sufficiently large coding length and utilities whose curvatures are sufficiently negative. Under these conditions, the proposed algorithms can thus generate the Pareto optimal tradeoff curves between rate and reliability for all the users. The distributed algorithms and convergence proofs are extended for wireless MIMO multi-hop networks, in which diversity and multiplexing gains of each link are controlled to achieve the optimal ratereliability tradeoff.
J.-W. Lee, M. Chiang, and R. Calderbank, "Optimal MAC design based on utility maximization: reverse and forward engineering," in Proc. IEEE Infocom 2006, Apr. 2006. | |

Optimal MAC design based on utility maximization: reverse and forward engineering

This paper analyzes and designs medium access control (MAC) protocols for wireless ad-hoc networks through the network utility maximization (NUM) framework. We first reverse-engineer the current exponential backoff (EB) type of MAC protocols such as the BEB (binary exponential backoff) in the IEEE 802.11 standard through a non-cooperative gametheoretic model. This MAC protocol is shown to be implicitly maximizing, using a stochastic subgradient, a selfish local utility at each link in the form of expected net reward for successful transmission. While the existence of a Nash equilibrium can be established, neither convergence nor social welfare optimality is guaranteed due to the inadequate feedback mechanism in the EB protocol. This motivates the forward-engineering part of the paper, where a network-wide utility maximization problem is formulated, using a collision and persistence probability model and aligning selfish utility with total social welfare. By adjusting the parameters in the utility objective functions of the NUM problem, we can also control the tradeoff between efficiency and fairness of radio resource allocation through a rigorous and systematic design. We develop two distributed algorithms to solve the MAC design NUM problem, which lead to random access protocols that have slightly more message passing overhead than the current EB protocol, but significant potential for efficiency and fairness improvement. We provide readily-verifiable sufficient conditions under which convergence of the proposed algorithms to a global optimality of network utility can be guaranteed, and through numerical examples illustrate the value of the NUM approach to the complexity-performance tradeoff in MAC design.
J.-W. Lee, M. Chiang, and R. Calderbank, "Distributed algorithms for optimal rate-reliability tradeoff in networks," in Proc. IEEE ISIT'05, Sep. 2005.
J.-W. Lee, M. Chiang, and R. Calderbank, "Layering as optimizaion decomposition: a TCP/PHY case study," in Proc. Mathematical Modeling and Analysis of Computer Networks Workshop, Networking 2005, May 2005 (Invited).
J.-W. Lee, R. R. Mazumdar, and N. B. Shroff, "Opportunistic resource scheduling for wireless ad-hoc networks," in Proc. BroadWISE 2004, Oct. 2004.
J.-W. Lee, R. R. Mazumdar, and N. B. Shroff, "Opportunistic power scheduling for multi-server wireless systems with minimum performance constraints," in Proc. IEEE Infocom 2004, Apr. 2004.
J.-W. Lee, R. R. Mazumdar, and N. B. Shroff, "Non-convexity issues for Internet rate control with multi-class services: stability and optimality," in Proc. IEEE Infocom 2004, Mar. 2004.
J.-W. Lee, R. R. Mazumdar, and N. B. Shroff, "Joint power allocation and base station assignment based on pricing for the downlink in multi-class CDMA networks," in Proc. IEEE Vehicular Technology Conference 2003-Spring, Apr. 2003.
J.-W. Lee, R. R. Mazumdar, and N. B. Shroff, "Non-convex optimization and distributed pricing-based algorithms for optimal resource allocation in high speed networks," in Proc. 17th IEEE Annual Computer Communication Workshop, Oct. 2002 (Invited).
J.-W. Lee, R. R. Mazumdar, and N. B. Shroff, "Joint power and data rate allocation for the downlink in multi-class CDMA wireless networks," in Proc. 40th Annual Allerton Conference on Communications, Control, and Computing, Oct. 2002.
J.-W. Lee, R. R. Mazumdar, and N. B. Shroff, "Downlink power allocation for multi-class CDMA wireless networks," in Proc. IEEE Infocom 2002, New York, NY, USA, Jun. 2002, pp. 1480-1489.

Domestic Journals

원종현, 신종민, 김재호, 이장원, "기계학습의 하이퍼파라미터 최적화 연구 동향," 한국통신학회논문지, 48권, 6호, pp. 733-747, Jun. 2023 (10.7840/kics.2023.48.6.733). | |

기계학습의 하이퍼파라미터 최적화 연구 동향

최근 다양한 분야에서 주목받고 있는 기계학습의 성능은 기계학습 모델의 하이퍼파라미터에 의존한다. 이에 따라 기계학습의 성능을 향상시키기 위해서는 최적의 하이퍼파라미터를 찾는 것이 중요하다. 기계학습의 하이퍼파라미터 최적화는 최적화 문제의 목적 함수와 결정 변수들의 특징 때문에 풀이에 어려움이 많으며, 하이퍼파라미터 최적화의 알고리즘들은 이러한 어려움을 해결하는 방향으로 연구되고 있다. 본 논문에서는 기계학습의 하이퍼파라미터 최적화의 난점을 분석하고, 이를 해결하기 위해 제안된 하이퍼파라미터 최적화 연구들의 동향을 파악한다. 또한 이를 바탕으로 기계학습의 성능을 더 향상시키기 위해 앞으로 기계학습의 하이퍼파라미터 최적화 연구가 나아갈 방향을 제시한다.
김경원, 김다윗, 박성민, 박영익, 원종현, 이장원, "O-RAN 표준 동향 및 미래 방향," 한국통신학회논문지, 47권, 7호, pp. 934-943, Jul. 2022 (10.7840/kics.2022.47.7.934). | |

O-RAN 표준 동향 및 미래 방향

Open radio access network (O-RAN)은 특정 통신 장비 업체의 장비를 독점적으로 사용하는 폐쇄형 구조였던 기존 RAN 기술에서 탈피하여, 개방형 hardware/software (HW/SW) 및 개방형 인터페이스를 사용하여 네트워크 구축의 유연성을 증가시킬 것으로 기대되는 차세대 RAN 기술이다. 또한 O-RAN은 artificial intelligence/machine learning (AI/ML)을 사용하여 RAN 노드의 제어 및 무선 자원 관리 등의 효율성 증대를 목표로 하고 있다. 현재 O-RAN에서는 기존 3rd generation partnership project (3GPP)에서 정의한 컴포넌트와 인터페이스 사용하지만, 이외에도 독자적인 컴포넌트와 인터페이스를 정의하고 있다. 본 논문에서는 O-RAN에서 정의하고 있는 컴포넌트와 인터페이스에 대한 설명을 포함하는 전반적인 O-RAN 표준화 동향을 설명하고, 향후 O-RAN의 미래 연구 방향을 제시한다.
신종민, 김도엽, 이장원, "무선 전력 전송을 통한 IoT 기기 에너지 공급을 위한 UAV Trajectory 및 Beamwidth 최적화," 한국통신학회논문지, 47권, 5호, pp. 760-769, May 2022 (10.7840/kics.2022.47.5.760). | |

무선 전력 전송을 통한 IoT 기기 에너지 공급을 위한 UAV Trajectory 및 Beamwidth 최적화

본 논문에서는 다수의 임무 지역 내에 많은 Internet-of-Things (IoT) 기기가 배치되어 있고 자체 임무를 수행하기 위해 에너지가 필요한 환경에서, unmanned aerial vehicle (UAV)의 무선 전력 전송 기술을 통해 IoT 기기에 필요한 에너지를 공급하는 알고리즘을 개발한다. 확률적 line-of-sight (LoS) 기반 air-to-ground (A2G) 무선 채널 모델 하에서 모든 IoT 기기에 필요한 에너지를 공급하는데 필요한 총 소요 시간이 최소화될 수 있도록 UAV의 beamwidth와 비행경로 (trajectory)를 최적화하는 것을 목표로 한다. 이를 위해, 먼저 각 임무 지역에서의 에너지 공급을 위한 UAV의 최적의 에너지 공급 위치를 구하고, 이어서 이를 이용하여 최적의 UAV 비행경로를 찾는 dynamic programming (DP) 기반의 알고리즘을 제안한다.
김도우, 문석재, 이장원, "Massive MIMO 시스템에서 mMTC를 위한 반직교 랜덤 액세스," 한국통신학회논문지, 46권, 7호, pp. 1164-1172, Jul. 2021 (10.7840/kics.2021.46.7.1164). | |

Massive MIMO 시스템에서 mMTC를 위한 반직교 랜덤 액세스

Fifth Generation (5G)과 Sixth Generation (6G) 시스템에서는 Machine Type Communication Device (MTCD)의 수가 급속도로 증가함에 따라 많은 수의 MTCD가 동시다발적으로 next generation Node B (gNB)와의 접속을 시도할 것으로 예상된다. 이와 같은 mMTC 환경에서는 두 대 이상의 MTCD가 같은 프리앰블을 선택하여 접속요청을 시도하는 프리앰블 충돌 상황이 증가하게 되므로 접속 성공률은 감소하고 접속 지연시간이 증가하는 문제가 발생한다. 본 논문에서는 프리앰블 충돌 확률을 줄이기 위해 massive Multiple-Input Multiple-Output (MIMO) 시스템의 특성을 이용하여 MTCD가 Semi-Orthogonal Multiple Access (SOMA) 기술을 통해 Radio Resource Control (RRC) 연결 요청을 하는 Semi-Orthogonal Random Access (SORA) 기법을 제시한다. MTCD와 gNB사이의 각 단계 별 랜덤 액세스 절차의 세부 시그널링을 제시하며 시뮬레이션 결과를 통해 SORA 기법이 기존 기법들보다 mMTC 환경에서 접속 성공률이 높은 것을 보인다.
김도엽, 이장원, "환경 변화에 적응적인 FANET 토폴로지 재구성," 한국통신학회논문지, 44권, 1호, pp. 117-120, Jan. 2019 (10.7840/kics.2019.44.1.117). | |

환경 변화에 적응적인 FANET 토폴로지 재구성

본 논문에서는 주어진 임무를 수행하기 위해 지상 관제탑(GCS)과 다수의 무인항공기(UAV)로 구성된 공중 애드 혹 네트워크(FANET)가 운용되는 상황에서 환경 변화에 맞춰 적응적으로 토폴로지를 재구성하는 문제를 다룬다. 본 논문에서는 체계적인 시스템 모델과 최적화 문제를 세우고, 알고리즘을 제시한다.
김도엽, 이장원, "무인비행체 네트워크에서 토폴로지 운용기술 연구 동향," 한국통신학회논문지, 43권, 2호, pp. 333-343, Feb. 2018 (10.7840/kics.2018.43.2.333). | |

무인비행체 네트워크에서 토폴로지 운용기술 연구 동향

무인비행체 시스템은 다양한 분야에서 무궁무진한 활용 가능성을 가지고 있다. 특히, 다수의 무인비행체를 이용한 애드 혹 네트워크 시스템은 통신 기반시설에 대한 제약이 없고, 확장성이 뛰어나며, 또한 다양한 성능 이득도 얻을 수 있다. 하지만 무인비행체의 이동성에 따른 네트워크 토폴로지의 변화 때문에 시스템 안정성에 대한 고려가 필요하다. 또한 네트워크 성능 이득은 네트워크 토폴로지에 따라 매우 다를 것이며, 특히 무인비행체를 어디에 배치하고 어떻게 이동시킬 것인지를 결정하는 무인비행체 네트워크의 토폴로지 운용 기술에 대한 연구가 무인비행체 네트워크 시스템에서 매우 중요하다. 본 논문에서는 단일 무인비행체를 운용하는 시스템과 각 시스템의 목적에 따라 다수의 무인비행체가 운용되는 시스템에서의 토폴로지 운용에 관한 선행 연구들을 분석하고, 무인비행체 네트워크 토폴로지 운용 기법의 미래 연구 방향을 제시한다.
배덕현, 이현석, 이장원, "이동통신망에서 저지연 상향링크 전송 기법," 한국통신학회논문지, 42권, 1호, pp. 77-87, Jan. 2017 (10.7840/kics.2017.42.1.77). | |

이동통신망에서 저지연 상향링크 전송 기법

현재 사용되고 있는 LTE/LTE-A 이동통신망의 성능은 일반적인 무선 통신 서비스를 제공하는데 충분히 높은 대역폭과 낮은 지연시간을 제공하고 있지만, 차세대 이동통신망의 주요 서비스가 될 가상현실, 원격 제어 등의 초저지연 서비스를 지원하기 위해서는 수 ms 수준의 낮은 지연시간이 필요하다. 그러나 LTE/LTE-A 시스템에서의 상향링크 전송은 단말이 전송에 필요한 무선자원을 획득하기 위해 기지국으로부터 자원을 할당받는 스케줄링 승인 과정이 선행되어야 한다. 이 과정은 고정적인 지연시간을 가져오고, 상향링크 전송에서 낮은 지연시간을 달성하는데 걸림돌이 된다. 따라서 본 논문에서는 이러한 스케줄링 승인 과정으로 인해 발생하는 지연시간을 줄이기 위해 새로운 상향링크 전송 방법인 Cut-in 상향링크 전송방법을 제안한다. 제안하는 상향링크 전송방법의 검증을 위해 모의실험을 수행하였으며, 이 결과를 통하여 제안하는 상향링크 전송방법이 기존 LTE/LTE-A 상향링크 전송 방법보다 낮은 지연시간을 발생시킴을 보인다.
문석재, 노희태, 이장원, "무선 에너지 전송 센서망에서 공평성을 고려한 라우팅, 스케줄링, 전력 제어," 한국통신학회논문지, 41권, 2호, pp. 206-214, Feb. 2016 (10.7840/kics.2016.41.2.206). | |

무선 에너지 전송 센서망에서 공평성을 고려한 라우팅, 스케줄링, 전력 제어

최근들어 무선 센서망에서 센서 노드에 대한 전력 공급을 위한 무선 에너지 전송(radio frequency energy transfer: RFET)에 대한 관심이 높아지고 있다. 기존의 센서망에서는 센서 노드들의 에너지 소비를 줄이는 것이 중요한 연구주제 중의 하나였지만 무선 에너지 전송 센서망에서는 센서 노드들이 계속해서 에너지를 공급받을 수 있기 때문에 에너지 소모를 줄이는 것이 상대적으로 그리 중요한 이슈는 아니다. 하지만 센서 노드들 사이에 가용 에너지양의 차가 발생하게 되기 때문에 무선 에너지 전송 센서망의 성능 향상을 위해서는 이와 같은 성질을 고려 하여 프로토콜을 설계하는 것이 중요하게 된다. 이에 본 논문에서는 이를 고려하여 무선 에너지 전송 무선 센서망에서 라우팅, 스케줄링, 전력제어 기법을 ‘Max-min’과 ‘Max-min fairness’ 두가지의 관점에서 제안을 한다. 또한 본 논문에서 제안한 기법들이 무선 에너지 전송 센서망의 성능을 크게 향상시킴을 보이며 이와 더불어 ‘Max-min’과 ‘Max-min fairness’ 사이의 차이에 대해서도 논의를 한다.
김재호, 윤재석, 이장원, "사물인터넷 융합을 통한 스마트 기기의 발전 동향," 한국정보통신학회지, 14권, 2호, pp. 71-23, Dec. 2013. |
노희태, 이장원, "LTE 시스템에서 그룹 기반 M2M 단말 이동성 관리," 한국통신학회논문지, 37B권, 12호, pp. 1119-1127, Dec. 2012 (10.7840/kics.2012.37B.12.1119). | |

LTE 시스템에서 그룹 기반 M2M 단말 이동성 관리

최근 이동통신의 새로운 미래 사업으로 주목받고 있는 M2M 통신의 특성 중 하나는 기존의 통신에 비해 단말 의 수가 상대적으로 많다는 점이다. 따라서 다수의 단말로부터 발생하는 제어 신호가 네트워크에 혼잡을 발생시 킬 수 있기 때문에 3GPP 표준화단체에서는 이를 해결하고자 여러 단말을 하나의 그룹으로 관리하여 불필요한 시 그널링 오버헤드를 줄이는 방법에 대해 표준화를 진행 중에 있다. 본 논문에서는 이동성이 같은 다수의 M2M 단 말들을 하나의 그룹으로 관리하는 그룹 기반 이동성 관리 방법에 대해 제안한다. 본 논문에서 제안하는 방법은 이동성이 같은 다수의 M2M 단말을 Mobility Management Entity (MME)가 동적으로 그룹핑하고, 그룹을 대표 할 그룹 헤더를 선출하여 그룹 헤더가 그룹을 대표하여 Tracking Area Update (TAU)를 수행하게 함으로써 M2M 단말이 개별적으로 TAU를 수행할 때 발생하는 시그널링 오버헤드를 줄이는 것이다. 제안하는 방법이 기 존의 개별적으로 TAU를 수행했을 때보다 최대 80% 정도의 시그널링 오버헤드 감소효과를 보인다.
노희태, 김병국, 변일무, 전기준, 김광순, 이장원, "차세대 네트워크를 위한 네트워크 부호화 기반의 계층별 프로토콜 분석," Telecommunications Review, 21권, 5호, pp. 781-798, Oct. 2011 (G704-000446.2011.21.5.006 ). | |

차세대 네트워크를 위한 네트워크 부호화 기반의 계층별 프로토콜 분석

네트워크 부호화 기술은 통신 노드들이 전달해야 할 다수의 데이터를 하나의 데이터로 부호화하여 전송하는 기술 을 일컫는다. 이러한 네트워크 부호화 기술을 활용하면 네트워크상에서의 데이터 전송 효율을 향상시킬 수 있다. 따 라서 WiMAX와 LTE-Advanced와 같이 사용자에게 대용량의 무선 멀티미디어 서비스를 제공하기 위한 차세대 네트 워크의 성능을 보다 향상시키기 위하여 네트워크 부호화 기술이 주요 후보 기술로 여겨지고 있다. 본 논문에서는 앞으 로 출현할 4G 통신망에서 네트워크 부호화의 활용방안을 알아보고자 무선 네트워크에서 네트워크 부호화를 고려한 기 존 연구들을 네트워크 프로토콜의 각 계층별로 정리하여 소개한다.
권정안, 김병국, 이장원, 임재원, 김병훈, "OFDMA 셀룰러 시스템에서 펨토셀 Subchannel 할당 기법," 한국통신학회논문지, 35권, 4호, pp. 350-359, Apr. 2010. | |

OFDMA 셀룰러 시스템에서 펨토셀 Subchannel 할당 기법

In this paper, we provide a subchannel allocation algorithm for a femtocell system with OFDMA. This algorithm aims to maximize the minimum number of allocated subchannels among all femtocells and in addition, to maximize the total usage of subchannels in all femtocells. The subchannel allocation algorithm consists of three steps: constructing an interference graph, coloring algorithm, and mapping subchannels to colors. In the first step, the femtocell system is modelled by an interference graph, in which each femtocell is modeled as a node and two nodes that interfere with each other are connected by an edge. Based on this interference graph, by using a coloring scheme and mapping subchannels to each color, we can allocate subchannels to each femtocell. Finally, the performance of this algorithm is provided by simulation.
권정안, 김병국, 이장원, "펨토셀 접근 제어," 대한전자공학회지, 37권, 4호, pp. 62-70, Apr. 2010. | |

펨토셀 접근 제어

본 논문에서는 OFDMA를 이용하는 dedicated channel 펨토셀 시스템에서 각 펨토셀이 사용할 subchannel을 할당하는 방법에 대하여 연구한다. 본 subchannel 할당 기법은 가장 적은 수의 subchannel을 할당받는 펨토셀의 subchannel 수를 최대화 하면서 시스템 내의 펨토셀들이 사용하는 subchannel 수의 합을 최대화하는 것을 목표로 한다. 제안된 알고리즘은 interference graph 생성, 컬러링 수행, 각 컬러별 subchannel 연결의 세 단계로 구성된 다. 첫 번째 단계에서는 펨토셀 시스템을 각 펨토셀을 node로, 서로 간섭을 겪는 펨토셀들을 edge로 한 interference graph로 모델링 한다. 이후 interfrence graph에 graph theory의 컬러링 기법을 이용하여 각 펨토셀에 컬러를 할당하고, 각 컬러에 subchannel들을 매핑하여, 각 펨토셀에 subchannel을 할당한다. 마지막으로 본 알고리 즘의 성능을 시뮬레이션을 통하여 검증한다.
권정안, 이장원, "OFDMA 시스템에서 elastic 서비스를 위한 opportunistic 스케줄링 기법," 한국통신학회논문지, 34권, 1호, pp. 76-83, Jan. 2009. | |

OFDMA 시스템에서 elastic 서비스를 위한 opportunistic 스케줄링 기법

In this paper, we provide opportunistic scheduling schemes for elastic services in OFDMA systems with fairness constraints for each user. We adopt the network utility maximization framework in which a utility function is defined for each user to represent its level of satisfaction to the service. Since we consider elastic services whose degree of satisfaction depends on its average data rate, we define the utility function of each user as a function of its average data rate. In addition, for fair resource allocation among users, we define fairness requirements of each user by using utility functions. We first formulate an optimization problem for each fairness requirement that aim at maximizing network utility, which is defined as the sum of utilities of users. We then develop an opportunistic scheduling scheme for each fairness requirement by solving the problem using a dual approach and a stochastic sub-gradient algorithm.
공성룡, 이장원, "다중클래스 통신망을 위한 동적 과금 기반의 호수락 제어 알고리즘," 한국통신학회논문지, 33권, 8호, pp. 681-688, Aug. 2008. | |

다중클래스 통신망을 위한 동적 과금 기반의 호수락 제어 알고리즘

본 논문에서는 다중클래스 통신망을 위한 새로운 과금 (pricing) 기반의 호수락 제어 (call admission control) 알고리즘 제안한다. 본 알고리즘에서는 통신망에 새롭게 호가 도착하게 되면, 요청 회선의 수와 필요로 하는 최소 시간을 통신망에 알려준다. 통신망은 기대 수익 (expected revenue)을 최대화하는 최적의 과금을 계산하여 호에 제안한다. 이때, 최적 과금은 도착한 호의 정보와 현재 그리고 미래에 추정되는 호 예약시간 동안의 망내 정체 (congestion) 정도를 기반으로 동적으로 조정된다. 호가 제안된 과금을 수락하면 호는 연결이 되고 그렇지 않으면 연결이 되지 않는다. 본 논문에서는 제안된 동적 과금 기반 호수락 제어 알고리즘의 성능을 Courcoubetis와 Reiman[1] 그리고 Paschalidis와 Tsitsiklis [2]의 정적 과금 기반 호수락 제어 알고리즘의 성능과 비교한다. 비록 두 알고리즘은 거의 같은 수익 결과를 보이지만, 동적 알고리즘이 정적 알고리즘에 비해 더 높은 호 수락율과 각 호에 낮은 과금을 제안하는 측면에서 더 뛰어난 성능을 나타냄을 보인다. 이는 경쟁 상황에서 정적 가격 알고리 즘에 비해 동적 가격 알고리즘이 더 많은 사용자를 끌어들여 수익을 더 낼 수 있음을 의미하며 더 나아가 접속 요금이 과금에 고려된다면, 동적 과금 호수락 제어 알고리즘이 더 많은 수익을 낼 수 있음을 보인다.
이장원, 최진식, "입출력 큐를 갖는 ATM 스위치에서의 우선순위에 관한 성능 분석," 대한전자공학회논문지, 35권, 2호, pp. 149-157, Feb. 1998. | |

입출력 큐를 갖는 ATM 스위치에서의 우선순위에 관한 성능 분석

서로 다른 특정을 갖는 광대역의 서비스들을 하나의 망으후 전송할 것을 목표로 하는 BISDN의 다중화 및 스위칭 방법으로 ATM이 채택되였으며 이러한 ATM 망의 구축을 위해서는 고속 스위칭이 가능한 대용량 스 위치의 개발이 필수적이다. 따라서 본 논문에서는 ATM 망에 가장 적합한 구조라고 할 수가 있는 선매 방식 (preemptive) 우선 순위를 갖는 입출력 큐잉 스위치의 성능을 분석하였다. 입력 큐의 분석을 위해서는 각 입력 큐를 우선 순위 클래스에 따라 두 개의 가상 입력 큐로 모델링하여 각각을 독립적인 Geom/Geom/1 큐로 가정을 하였으며 가상 입력 큐의 평균 서비스 시간을 구하기 위해서 각 가상 입력 큐의 HOL 셀 중 같은 출력 포트로 향하는 셀로 구성된 가상 HOL 큐를 따로 모델링하였다. 출력 큐의 분석을 위해서는 출력 큐로의 도착 프로세스를 가상 HOL 큐의 상태로부터 근사적으로 구하여 분석하였다. 분석 방법은 Markov chain 분 석 방법을 사용하여 셀지연과 최대 수율 (throughput)을 구하였으며 시율레이션을 통하여 분석 결과를 검증하 였다.

Domestic Conferences and Workshops

원종현, 신종민, 김재호, 이장원, "하이퍼파라미터 최적화의 난점과 연구 방향," 통신정보 합동학술대회 (JCCI 2023), pp. 403, Apr. 2023. | |
원종현, 김도엽, 이장원, "백홀 용량이 제한된 네트워크에서 무인항공기 배치 및 자원 할당에 관한 연구," 한국통신학회 하계종합학술발표회, pp. 1287, Jun. 2021. | |

백홀 용량이 제한된 네트워크에서 무인항공기 배치 및 자원 할당에 관한 연구

본 논문은 기지국(BS) 역할을 하는 무인항공기(UAV)가 지상기지국(GBS)으로부터 데이터를 받아서 다수의 지상 사용자들에게 전송하는 네트워크에서 백홀 링크의 용량 제한을 고려하여 최적의 UAV 위치와 자원 할당을 결정하는 문제를 세우고 최적해를 구하는 알고리즘을 제시한다.
김도엽, 이장원, "IoT 기기의 안정적 에너지 공급을 위한 지형지물 정보를 활용한 UAV 배치 및 자원 할당에 관한 연구," 한국통신학회 하계종합학술발표회, pp. 786-787, Jun. 2021. | |

IoT 기기의 안정적 에너지 공급을 위한 지형지물 정보를 활용한 UAV 배치 및 자원 할당에 관한 연구

본 논문에서는 고층 빌딩과 같은 지형지물이 다수 존재하는 지역에 특정 최소 전력 수확량을 요구하는 internet-of-things(IoT) 기기가 무수히 많이 배치된 환경에서, UAV의 소모 전력을 최소화하기 위해 UAV의 위치와 통신 자원을 공동으로 최적화하는 문제를 다룬다. 특히, 대다수 기존 연구에서 계산 편의를 위해 채택된 불확실성이 높은 확률적 line-of-sight(LoS) 채널 모델이 아닌 지형지물 정보를 이용한 확정적 LoS 채널 모델을 제시하고, 이에 기반하여 문제를 해결한다.
김다윗, 김도엽, 김경원, 이장원, "다중 안테나 네트워크에서의 강화학습 기반 무선 자원 할당에 관한 연구," 한국통신학회 하계종합학술발표회, pp. 699-700, Jun. 2021. | |

다중 안테나 네트워크에서의 강화학습 기반 무선 자원 할당에 관한 연구

본 논문은 다중 안테나 네트워크에서 사용자의 서비스 품질을 만족시키며 높은 네트워크 성능을 제공하기 위해 시간 및 주파수 자원 할당에 관한 문제를 다루고, 해당 문제를 강화학습 기반 알고리즘을 사용하여 해결한다. 제안 알고리즘은 네트워크 환경이 바뀌어도 사용자의 서비스 품질을 보장하면서 효율적으로 자원 할당 정책을 학습함을 모의실험을 통해 보인다.
박성민, 김도엽, 김경원, 이장원, "Full-duplex MISO wireless powered communication network에서 빔포밍 및 IoT device 선택에 관한 연구," 통신정보 합동학술대회 (JCCI 2021), Apr. 2021. | |

Full-duplex MISO wireless powered communication network에서 빔포밍 및 IoT device 선택에 관한 연구

본 논문에서는 full-duplex로 동작하는 다중 안테나 base station과 다수의 half-duplex로 동작하는 단일 안테나 IoT device가 존재하는 wireless powered communication network에서 선택된 ID의 상향링크 전송률과 나머지 ID의 전력 수확의 가중합을 최대화하기 위한 문제를 다룬다. 이를 위해, ID 선택 벡터와 BS의 빔포밍 벡터를 최적화하는 방법을 개발하고, 성능 분석을 통해 제안 방법이 효과적임을 보인다.
김도엽, 이장원, "지형지물 정보를 고려한 UAV-BS 배치 및 자원 할당에 관한 연구," 한국통신학회 동계종합학술발표회, Feb. 2021. | |

지형지물 정보를 고려한 UAV-BS 배치 및 자원 할당에 관한 연구

본 논문에서는 지상 기지국(GBS)과 유저 사이에 안정적인 통신을 보장하면서 총 소모 전력을 최소화하기 위한 UAV 탑재형 기지국(UAV-BS)의 배치 및 자원 할당 문제를 다룬다. 최적화 이론과 particle swarm optimization (PSO) 알고리즘을 이용하여 해당 문제를 해결하는 기법을 제안한다. 또한, 대다수의 기존 연구에서 고려하는 확률적 line-of-sight (LoS) 채널 모델이 아닌 지형지물 정보를 이용한 확정적 LoS 채널 모델을 제시하고, 제안 채널 모델을 고려할 때 더 높은 성능이 얻어짐을 모의실험을 통해 보인다.
주원흠, 김도엽, 이장원, "Joint antenna and user selection in downlink massive MIMO systems," 한국통신학회 추계종합학술발표회, Nov. 2020. | |

Joint antenna and user selection in downlink massive MIMO systems

In massive multiple-input multiple-output (MIMO) systems, a large number of radio frequency (RF) chains is a big problem, and antenna selection technology has been attracted as one of promising solutions. In this paper, we develop a joint antenna and user selection algorithm to maximize the weighted sum rate. Then, we show that our proposed algorithm provides near-optimal performance through the simulation.
김도우, 문석재, 이장원, "Massive MIMO 시스템에서 mMTC를 위한 랜덤 액세스 연구," 한국통신학회 추계종합학술발표회, Nov. 2020. | |

Massive MIMO 시스템에서 mMTC를 위한 랜덤 액세스 연구

본 논문에서는 massive Multiple-Input and Multiple-Output (MIMO) 시스템에서 대규모의 Machine Type Communication Device (MTCD)가 접속할 때 Timing Advance (TA)를 추정한 후 MTCD가 Radio Resource Control (RRC) 연결 요청을 하는 효율적 랜덤 액세스 기법은 연구하였다.
성준혁, 이동헌, 김도엽, 이장원, 최수용, "비동기 비직교 다중 액세스를 적용한 uplink symbiotic backscatter system 분석," 대한전자공학회 하계종합학술대회, Aug. 2020. | |

비동기 비직교 다중 액세스를 적용한 uplink symbiotic backscatter system 분석

In this paper, we analyze an uplink symbiotic backscatter system with two users. The proposed analysis method improves sum-rate performance by analyzing the system using the asynchronous NOMA (A-NOMA) technique considering the time delay due to the physical distance between the receiver and transmitters. The numerical results show that A-NOMA can improve 38% in terms of sum rate than that of existing NOMA in symbiotic backscatter system.
문석재, 이장원, "대규모 IoT 접속을 위한 SCMA 기반 connectionless 접속 환경에서 spreading sequnece에 따른 성능 분석," 한국통신학회 추계종합학술대회, Nov. 2019. | |

대규모 IoT 접속을 위한 SCMA 기반 connectionless 접속 환경에서 spreading sequnece에 따른 성능 분석

본 논문에서는 대규모 IoT 기기들이 SCMA 기반의 connectionless 접속을 하는 환경에서 ns-3 기반의 네트워크 시뮬레이션을 통하여 spreading sequence 에 따른 전송 성공률 성능을 분석하였다.
이병현, 문석재, 이장원, "5세대 통신 시스템에서 대규모 접속을 위한 저복잡도 랜덤 액세스 기법 동향," 통신정보 합동학술대회 (JCCI 2019), May 2019. |
문석재, 이장원, "mMTC 서비스 지원을 위한 저지연 및 저복잡도 connectionless 접속 기술 동향," 통신정보 합동학술대회 (JCCI 2019), May 2019. |
김도엽, 이장원, "FANET에서 중계 UAV 배치 문제에 대한 메타휴리스틱 알고리즘 성능 비교," 한국통신학회 동계종합학술대회, Jan. 2019. | |

FANET에서 중계 UAV 배치 문제에 대한 메타휴리스틱 알고리즘 성능 비교

본 논문에서는 무인항공기(UAV)를 통신 중계기로 운용하여 지상 관제탑(GCS)과 타깃 노드 사이에 안정적인 통신이 가능하도록 공중 애드 혹 네트워크(FANET)를 만드는 문제에 대해서, 대표적인 메타휴리스틱 알고리즘인 simulated annealing (SA), differential evolution (DE), particle swarm optimization (PSO) 알고리즘 간의 성능을 비교 분석한다. 본 논문에서는 일반적인 중계 UAV 배치 문제를 수학적으로 모델링하고, 모의실험을 통해 PSO, DE, SA 순으로 좋은 성능을 제공함을 보인다.
문석재, 이장원, "5세대 통신 시스템에서의 grant-free 전송 기법에 관한 최신 연구 동향," 한국통신학회 추계종합학술대회, Nov. 2018. | |

5세대 통신 시스템에서의 grant-free 전송 기법에 관한 최신 연구 동향

5 세대 통신 시스템에서는 저지연/고신뢰 전송 및 대규모 단말 접속과 같이 다양한 전송 시나리오를 지원한다. 본 논문에서는 이러한 다양한 시나리오들을 효율적으로 지원하기 위해 단말이 제어 시그널링 없이 상향링크 전송을 수행하는 grant-free 전송 기법에 관한 최신 연구 동향을 살펴본다.
이병현, 이장원, "시스템 레벨 시뮬레이션을 통한 mMTC 환경에서 5G 랜덤 액세스 채널 성능 평가," 한국통신학회 추계종합학술대회, Nov. 2018. | |

시스템 레벨 시뮬레이션을 통한 mMTC 환경에서 5G 랜덤 액세스 채널 성능 평가

5G 이동 통신 시스템에서 대규모의 MTC단말이 셀 내에 존재하게 될 것으로 예상됨에 따라, 이러한 환경을 지원하기 위한 랜덤 액세스 기법의 연구가 활발하게 진행되고 있다. 본 논문에서는 3GPP 표준 기반의 시스템 레벨 시뮬레이션을 통하여 대규모 MTC단말 환경(mMTC)에서 랜덤 액세스 채널의 성능을 분석하고자 하였다.
김도엽, 이장원, "공중 애드 혹 네트워크 환경에서 안정적인 통신을 위한 무인항공기 배치에 관한 연구," 한국통신학회 하계종합학술대회, Jun. 2018. | |

공중 애드 혹 네트워크 환경에서 안정적인 통신을 위한 무인항공기 배치에 관한 연구

본 논문에서는 임무를 수행하는 무인항공기(UAV)와 임무 수행 결과 데이터를 지속적으로 수신하는 지상관제탑(GCS) 간 안정적인 애드 혹 통신을 지원하는 공중 애드 혹 네트워크(FANET)가 되도록 중계용 UAV의 최적의 위치를 찾는 연구를 하였다. 무선 링크의 필요한 데이터 전송률과 채널 용량을 고려하여 최적화 문제를 세웠고, 메타 휴리스틱 기법 중 하나인 입자 군집 최적화(PSO) 알고리즘을 이용하여 중계용 UAV의 최적 근사해 위치를 찾아서 FANET 토폴로지를 구성하였다.
이병현, 문석재, 이장원, "IoT 단말들의 signaling overhead 감소를 위한 그룹 기반 접속 방식," 통신정보 합동학술대회 (JCCI 2018), May 2018. |
문석재, 이현석, 이장원, "제어신호 오버헤드 감소를 통한 저지연 grant-free 전송 기법," 통신정보 합동학술대회 (JCCI 2018), May 2018. |
김진영, 이현석, 이장원, "Q-learning을 이용한 무선 네트워크에서 패킷 스케줄링 기법," 한국통신학회 추계종합학술대회, Nov. 2017. | |

Q-learning을 이용한 무선 네트워크에서 패킷 스케줄링 기법

본 논문에서는 최근 많은 분야에서 활용되고 있는 기계 학습의 일종인 강화 학습을 이용하여 패킷 스케줄링 policy 를 찾는 스케줄러에 대한 연구를 한다. 이를 위해 무선 채널 환경을 모델링하고 state 와 action 을 정의한 후 cell 전체의 평균 delay 최소화 및 각 사용자 별 delay 요구 조건을 만족시키는 패킷 스케줄링 policy 를 학습하는 스케줄러를 제안한다.
이현석, 이장원, "5G 네트워크를 위한 C-RAN에서의 Function Split," 통신정보 합동학술대회 (JCCI 2016), Apr. 2016.
송지훈, 이현석, 이장원, "Time-correlated Rayleigh fading 채널에서 멀티셀 OFDMA 네트워크의 opportunistic 스케쥴링에 관한 연구," 한국통신학회 동계종합학술대회, Feb. 2015.
정현식, 김성연, 이장원, "IEEE 802.15.4e LLDN에서 센서 노드 에너지 사용량 감소 기법," 통신정보 합동학술대회 (JCCI 2014), Apr. 2014. | |

IEEE 802.15.4e LLDN에서 센서 노드 에너지 사용량 감소 기법

IEEE 802.15.4 는 WPAN (Wireless Personal Area Network)에서 많이 이용되는 표준이다. 여기에 LLDN (Low-Latency Deterministic Network)을 포함한 MAC 동작이 추가되어 IEEE 802.15.4e 표준이 정립되었다. 본 논문에서는 LLDN 의 에너지 사용의 비효율성을 개선할 방법을 제안한다. 성능 분석을 통해 본 논문에서 제안한 방법이 LLDN 보다 에너지 사용량을 감소시키는 효율적인 방법임을 보인다.
이현석, 이장원, "경매를 통한 데이터 정액제의 잔여 데이터 활용에 대한 연구," 한국통신학회 추계종합학술대회, Nov. 2013. |

경매를 통한 데이터 정액제의 잔여 데이터 활용에 대한 연구

본 논문은 데이터 정액제를 사용하는 다수의 사용자가 데이터 정액제를 통해 제공받은 데이터를 경매를 통해 다른 사용자에게 제공할 수 있는 시나리오를 수학적으로 모형화한다. 또한 본 논문에서는 시나리오의 경매방식으로 최고가격 봉인경매 (First-price sealed-bid auction)와 차점가격 봉인경매 (Second-price sealed-bid auction)를 이용할 경우 데이터 제공자의 최적 입찰 가격에 대하여 분석한다. 마지막으로 최고가격 봉인경매 방식과 차점가격 봉인경매 방식에서 데이터 제공자의 최적 입찰 가격이 각각 어떻게 나타나는지 모의경매를 통해 보인다.
정현식, 김성연, 이장원, 홍대식, 최수용, 박창순, 김영수, 황효선, "IEEE 802.15.4e RIT에서 수신노드의 에너지 소비 감소를 위한 방법," 한국통신학회 추계종합학술대회, Nov. 2013. | |

IEEE 802.15.4e RIT에서 수신노드의 에너지 소비 감소를 위한 방법

IEEE 802.15.4e 에서는 비콘 비활성 모드에서 저전력으로 통신할 수 있는 방식 중의 하나로 RIT (Receiver Initiated Transmission)를 정의하였다. 본 논문은 RIT 에서 수신노드가 불필요하게 수신 모드로 동작하는 시간을 줄이고 대기 모드로 동작하게 함으로써 수신노드의 에너지 소모량을 줄이는 방법을 제안한다. 성능을 분석하여 본 논문에서 제안한 방법을 적용한 경우 IEEE 802.15.4e 에 정의된 RIT 를 따를 때보다 수신노드의 에너지 소모량이 더 작아짐을 보인다.
정신영, 김병국, 이장원, 정정수, "핸드오버시 duplicate ACK을 이용한 TCP 타임아웃 회피 기법," 한국통신학회 동계종합학술대회, Feb. 2011.
김병국, 권정안, 이장원, 임재원, 김병훈, "OFDMA 셀룰러 시스템에서 펨토셀 subchannel 할당 기법," 한국통신학회 동계종합학술대회, Feb. 2010. | |

OFDMA 셀룰러 시스템에서 펨토셀 subchannel 할당 기법

본 논문에서는 OFDMA 기반 dedicated channel 펨토셀 시스템에서 각 펨토셀의 subchannel을 할당 기법에 대하여 연구한다. 본 자원 할당 기법은 최소 subchannel 할당 수를 최대화 하면서 시스템 내 펨토셀들에 할당된 subchannel 수의 합을 최대화하 는 것을 목표로 한다. 제안된 알고리즘은 interference graph 생성, 컬러링, 컬러별 subchannel 대응의 세 단계로 구성된다.. 첫 번째 단계에서는 펨토셀 시스템을 각 펨토셀을 node로, 서로 간섭을 겪는 펨토셀들을 edge로 한 interference graph로 모델 링 한다. 이후 graph theory의 컬러링 기법을 이용하여 각 펨토셀에 컬러를 할당하고, 각 컬러에 subchannel들을 대응하여, 각 펨토셀에 subchannel을 할당한다. 마지막으로 본 알고리즘의 성능을 시뮬레이션을 통하여 검증한다
노희태, 이장원, "통신망에서 송수신세션을 동시에 고려한 흐름제어," 대한전자공학회 추계종합학술대회, Nov. 2008.
김성연, 이장원, "무선망에서 유저 수준의 유틸리티 함수를 고려한 자원할당," 대한전자공학회 추계종합학술대회, Nov. 2008.
권정안, 이장원, "OFDMA 시스템에서 best-effort 서비스를 위한 opportunistic 스케줄링 기법," 한국통신학회 하계종합학술대회, Jul. 2008. |

OFDMA 시스템에서 best-effort 서비스를 위한 opportunistic 스케줄링 기법

본 논문에서는 OFDMA 다중접속기법을 사용하는 시스템에서 best-effort 서비스 유저들에게 opportunistic 스케줄링을 적용하는 방업에 대한 연구를 수행하며 네트워크 유틸리티 최대화 기법을 이용하여 진행한다. 특히 best-effort 서비스 유저의 경우 시스템을 이용하는 전체 시간을 통하여 얻는 유틸리티가 높아야 한다. 이를 반영하기 위하여 각 유저의 유틸리티를 각 유저가 시스템 전체를 통하여 얻는 전송률의 평균에 대한 함수로 정의하며 할당 받아야 하는 유틸리티의 최소값을 QoS로 정의한다. 이에 따른 최적화 문제는 dual 기법과 stochastic sub-gradient 기법을 통하여 해를 구한다.
권정안, 이장원, "OFDMA 시스템에서 streaming 서비스를 위한 opportunistic 스케줄링 기법," 대한전자공학회 하계종합학술대회, Jun. 2008. |

OFDMA 시스템에서 streaming 서비스를 위한 opportunistic 스케줄링 기법

In this paper, we study an opportunistic scheduling scheme for the OFDMA system with streaming services. The service is modeled by using the appropriate utility function. We formulate a stochastic optimization problem that aims at maximizing network utility while satisfying the QoS requirement of each user. The problem is solved by using the dual approach and the stochastic sub-gradient algorithm.
공성룡, 이장원, "통신망에서의 동적 과금 기반의 호수락 제어 알고리즘," 대한전자공학회 하계종합학술대회, Jun. 2008.
권정안, 이장원, "무선 중계기를 가진 셀룰러 시스템에서의 opportunistic 스케줄링 기법에 관한 연구," 한국통신학회 추계종합학술대회, Nov. 2007. |

무선 중계기를 가진 셀룰러 시스템에서의 opportunistic 스케줄링 기법에 관한 연구

본 논문에서는 opportunistic 스케줄링 기법을 적용하는 시스템에 중계기가 추가됐을 경우 성능의 향상에 관한 분석과 함께 이를 효율적으로 사용하는 방안에 관한 연구를 진행한다. 본 연구는 시간에 따라 변하는 채널 환경에서, 시분할 다중 접속 기법을 사용하는 시스템을 고려한다. 이때 중계기를 이용한다면 시간 슬롯을 둘로 나누어 사용하도록 하며 이를 기지국과 중계기, 중계기와 유저 사이에서 각각 사용한다. 이에 따라 시간 슬롯을 나누는 방법에 대한 연구와 함께 무선 중계기와 opportunistic 스케줄링을 함꼐 적용하여 성능을 향상시키면서 fairness 요구조건을 만족시켜주는 적용 방법에 대하여 논한다.