Changwon Nam

Joint Subcarrier Assignment and Power Allocation in Full-Duplex OFDMA Networks

Recent advances in the physical layer have demonstrated the feasibility of in-band wireless full-duplex which enables a node to transmit and receive simultaneously on the same frequency band. While the full-duplex operation can ideally double the spectral efficiency, the network-level gain of full-duplex in large-scale networks remains unclear due to the complicated resource allocation in multi-carrier and multi-user environments. In this paper, we consider a single-cell full-duplex OFDMA network which consists of one full-duplex base station (BS) and multiple full-duplex mobile nodes. Our goal is to maximize the sum-rate performance by jointly optimizing subcarrier assignment and power allocation considering the characteristics of full-duplex transmissions. We develop an iterative solution that achieves local Pareto optimality in typical scenarios. Through extensive simulations, we demonstrate that our solution empirically achieves near-optimal performance and outperforms other resource allocation schemes designed for half-duplex networks. Also, we reveal the impact of various factors such as the channel correlation, the residual self-interference, and the distance between the BS and nodes on the full-duplex gain.

Radio resource allocation with inter-node interference in full-duplex OFDMA networks

In-band wireless full-duplex is a promising technology that enables a wireless node to transmit and receive at the same time on the same frequency spectrum. In OFDMA networks, the full-duplex transmission makes the resource allocation problem more challenging, in particular when user devices are not full-duplex capable. In this paper, we investigate the joint problem of subcarrier assignment and power allocation to maximize the sum-rate performance in full-duplex OFDMA networks. To achieve high throughput in the considered network, we propose to use a practical subcarrier assignment condition which allows a subcarrier to be allocated to a pair of uplink and downlink nodes when its inter-node channel gain is lower than its uplink channel gain. Considering this condition and the inter-node interference, we design three resource allocation algorithms which run for; i) uplink first, ii) downlink first, and iii) uplink and downlink in pair. Through simulation, we evaluate our solutions in comparison with conventional schemes with respect to performance gain.

Group-based contention in IEEE 802.11ah networks

IEEE 802.11ah which mainly aims for vast range sensor networks offers a transmission range of up to 1 km and about 8,000 nodes are handled by a single access point (AP). As a result, an 802.11ah network has more hidden pairs than conventional 802.11a/b/g/n/ac networks. Therefore, packets frequently collide resulting in network performance degradation. To solve the problem, the 802.11ah uses a group based contention. In this paper, we propose a guideline for choosing the number of groups. Through simulations, we also show how severely degraded the throughput performance is in a randomly deployed network.

Priority Inversion Prevention Scheme for PLC Vehicle-to-Grid Communications under the Hidden Station Problem

Power line communication (PLC) has been selected by ISO/IEC 15118 as standard technology for vehicle-to-grid communication. Communication traffic to charge an electric vehicle should have a higher priority than normal traffic in order to improve the reliability and delay performance. To this end, HomePlug Green PHY, which is a standard PLC defined in ISO/IEC 15118, provides such a priority resolution scheme. However, when there is a hidden station with respect to the high-priority station, the priority resolution scheme does not operate properly, resulting in a higher delay or losses of high-priority traffic. This paper briefly analyzes the priority inversion probability under the hidden station problem and proposes a priority inversion prevention scheme. The adverse effect of the priority inversion problem is demonstrated through a numerical analysis and extensive simulations. We also show that our proposed scheme not only solves the problem but also improves the performance of the other stations.

A near-optimal randomized algorithm for uplink resource allocation in OFDMA systems

OFDMA has been selected as the multiple access scheme for emerging broadband wireless communication systems. However, designing efficient resource allocation algorithms for OFDMA systems is a challenging task, especially in the uplink, due to the combinatorial nature of subcarrier assignment and the distributed power budget for different users. Inspired by Glauber dynamics, in this paper, we propose a randomized iteration-based uplink OFDMA resource allocation algorithm. We show that our algorithm is near-optimal in the sense that by increasing the number of iterations (which scales up the complexity), with arbitrarily large probability, the algorithm can converge to the subcarrier/power allocation pattern with the maximum sum-utility. We also show that this algorithm can be generalized to solve a joint uplink-downlink allocation problem in full-duplex OFDMA systems. Simulations are conducted to compare the performance of our algorithm with existing ones.

Interference type based channel management using adaptive bandwidth in wireless LANs

In IEEE 802.11 based Wireless LANs, the channel bandwidth is considered as a fixed parameter. Recently, the concept of adaptive bandwidth has been newly introduced, making it possible to allocate the channel bandwidth adaptively according to the interference type of users. Such a capability enables to enhance the previous way of channel usage where each Access Point (AP) is restricted to use a fixed bandwidth channel to serve all users. In this paper, we propose a scheme where each AP is allowed to use an adaptive bandwidth channel that is adjusted by a central controller. The channel bandwidth is determined according to whether a user to be served experiences interference or not. If the user is vulnerable to interference from other APs, not its serving AP, it is served through a channel assigned to the serving AP. On the contrary, when serving interference-free users, the AP can exploit more bandwidth which is available at the moment. In this way, our proposed scheme can enhance the spectrum utilization of interference-free users without harming the other users. Simulation results show that our scheme significantly improves the average spectrum utilization of each AP.

Joint subcarrier assignment and power allocation in full-duplex OFDMA networks

Recent advances in the physical layer have demonstrated the feasibility of in-band wireless full-duplex which enables a node to transmit and receive simultaneously on the same frequency band. While the full-duplex operation can ideally double the spectral efficiency, the network-level gain of full-duplex in large-scale networks remains unclear due to the complicated resource allocation in multi-carrier and multi-user environments. In this paper, we consider a single-cell full-duplex OFDMA network which consists of one full-duplex base station (BS) and multiple full-duplex mobile nodes. Our goal is to maximize the sum-rate performance by jointly optimizing subcarrier assignment and power allocation considering the characteristics of full-duplex transmissions. We develop an iterative solution that achieves local Pareto optimality in typical scenarios. Through extensive simulations, we demonstrate that our solution empirically achieves near-optimal performance and outperforms other resource allocation schemes designed for half-duplex networks. Also, we reveal the impact of various factors such as the channel correlation, the residual self-interference, and the distance between the BS and nodes on the full-duplex gain.

ΔSNR-MAC

The performance of uplink multiuser MIMO (MU-MIMO) transmissions heavily depends on which users to transmit together. In WLANs where each user independently determines when to transmit by random access, the performance degradation occurs when a set of users for concurrent transmissions are not chosen properly. In this paper, we address this problem and propose ΔSNR-MAC protocol to enhance the uplink throughput in MU-MIMO WLANs. In ΔSNR-MAC, a set of users transmitting together are determined one after another through a multi-round contention where the number of rounds equals the number of antennas at the AP. In each round, given winning users that are already transmitting, each user calculates its SNR reduction amount due to the winning users. ΔSNR-MAC gives a higher priority to users with less SNR reduction amounts. To achieve this, each round consists of multiple stages where earlier stages are reserved for users with less SNR reduction amounts. In this way, users with the strong channel orthogonality can transmit together in a fully distributed manner. We theoretically analyze the throughput of ΔSNR-MAC and propose a parameter selection method to maximize the throughput. Our evaluation results confirm that ΔSNR-MAC improves the uplink throughput over existing schemes both in two- and three-antenna AP cases and achieves temporal fairness in mobile environments.

Channel Prediction Based Rate Adaptation Scheme in Wireless USB Networks

Ultra-wideband (UWB) communication is a promising technology to provide high data rates with low power consumption in wireless personal area networks. Among the standards adopting the UWB technology, Wireless USB (WUSB) running on the platform of WiMedia aims to provide high-speed wireless connectivity between mobile devices. To support reliable connectivity, data rates should be adapted according to UWB channel states that are time- varying. In this paper, we propose a novel rate adaptation scheme which considers the characteristics of WUSB protocol as well as the time-varying UWB channel. Our proposed scheme uses the channel prediction in a short time scale. Specifically, the channel state of the previous time window is used to determine the data rate of the next time window, assuming the channel similarity between the two successive time windows. Through extensive trace-driven simulations, we show that the proposed rate adaptation scheme outperforms the conventional schemes.

On the tradeoff between opportunistic gain and Coordination Delay of opportunistic routing in wireless networks

Opportunistic routing has gained much attention in recent years as a means of leveraging the broadcast nature of wireless medium. Most of the previous studies focused only on improving throughput, assuming that Best Effort traffic which is delay insensitive is delivered through opportunistic routing. However, when the delay sensitive traffic is involved, it is necessary to consider both the opportunistic gain and the delay incurred by opportunistic routing at the same time. In this paper, we analyze the relationship between these two factors and through extensive simulations confirm that our analyses are very accurate.