Hyungsik Ju

Improving efficiency of resource usage in two-hop full duplex relay systems based on resource sharing and interference cancellation

This letter presents a new full duplex relay (FDR) system that features more efficient use of time resource and antennas. The proposed FDR system is constructed based on time and antenna-sharing and elimination of underlying interference. The underlying interference caused by time- and antenna-sharing is eliminated using precoding. We propose a design for the precoder and decoder, and investigate the achievable rate of the proposed FDR system when the precoder and decoder are applied. Simulation results show improved achievable rate and BER performances with the proposed FDR system compared to half duplex and conventional full duplex relay systems.

User cooperation in wireless powered communication networks

This paper studies user cooperation in the emerging wireless powered communication network (WPCN) for throughput optimization. For the purpose of exposition, we consider a two-user WPCN, in which one hybrid access point (H-AP) broadcasts wireless energy to two distributed users in the downlink (DL) and the users transmit their independent information using their individually harvested energy to the H-AP in the uplink (UL) through time-division-multiple-access (TDMA). We propose user cooperation in the WPCN where the user that is nearer to the H-AP and in general has a better channel for DL energy harvesting as well as UL information transmission uses part of its allocated UL time and DL harvested energy to help relay the far users information to the H-AP, in order to achieve more balanced throughput. We maximize the weighted sum-rate (WSR) of the two users by jointly optimizing the time and power allocations in the network for both wireless energy transfer in the DL and wireless information transmission and relaying in the UL. Simulation results show that the proposed user cooperation scheme can effectively improve the achievable throughput in the WPCN with desired user fairness.

Catching resource-devouring worms in next-generation wireless relay systems: Two-way relay and full-duplex relay

In spite of their potential to improve the performance of wireless communication systems, relay-based communication systems face significant challenges, resource wastage. In this article we discuss resource efficiency in relay systems, particularly in two-way and full-duplex relay systems that utilize multiple antennas and improved duplexing to enhance system efficiency. We compare in detail the features, frame structures, performances, advantages and disadvantages, and technical challenges associated with TWRs and FDRs. Using illustrations of quantitative performance, we demonstrate that both of these approaches can efficiently reduce resource wastage in relay-based communication systems.

A Novel Mode Switching Scheme Utilizing Random Beamforming for Opportunistic Energy Harvesting

Since radio signals carry both energy and information at the same time, a unified study on simultaneous wireless information and power transfer (SWIPT) has recently drawn a significant attention for achieving wireless powered communication networks. In this paper, we study a multiple-input single-output (MISO) multicast SWIPT network with one multi-antenna transmitter sending common information to multiple single-antenna receivers simultaneously along with opportunistic wireless energy harvesting at each receiver. From the practical consideration, we assume that the channel state information (CSI) is only known at each respective receiver but is unavailable at the transmitter. We propose a novel receiver mode switching scheme for SWIPT based on a new application of the conventional random beamforming technique at the multi-antenna transmitter, which generates artificial channel fading to enable more efficient energy harvesting at each receiver when the received power exceeds a certain threshold. For the proposed scheme, we investigate the achievable information rate, harvested average power and power outage probability, as well as their various trade-offs in quasi-static fading channels. Compared to a reference scheme of periodic receiver mode switching without random transmit beamforming, the proposed scheme is shown to be able to achieve better rate-energy trade-offs when the harvested power target is sufficiently large. Particularly, it is revealed that employing one single random beam for the proposed scheme is asymptotically optimal as the transmit power increases to infinity, and also performs the best with finite transmit power for the high harvested power regime of most practical interests, thus leading to an appealing low-complexity implementation. Finally, we compare the rate-energy performances of the proposed scheme with different random beam designs.

Effects of Channel Estimation Error on Full-Duplex Two-Way Networks

Bidirectional communication systems based on full-duplex transmission have been proposed as a way to increase the ergodic capacity of multiantenna two-way networks. This type of system enables simultaneous exchange of data between two nodes via bidirectional use of spatial resources. However, when channel estimation error is present, each node experiences both desired-channel interference (DI) and echo-channel interference (EI). This paper investigates the effect of channel estimation errors on the ergodic capacities for bidirectional full-duplex transmission (BFD) using one of two combining schemes: maximal-ratio combining (MRC) or optimum combining (OC). We derive the ergodic capacities as closed-form expressions and quantify the effect of channel estimation errors on ergodic capacities for BFD with MRC (BFD-MRC) or OC (BFD-OC). Numerical results demonstrate that full-duplex transmission in two-way networks is an attractive option when channel estimation error is present.

Full Duplexity in Beamforming-Based Multi-Hop Relay Networks

This work investigates the effects of full duplexity on delay and throughput in interference-limited multi-hop networks. It is assumed that channel state information is not available at transmitters and beamforming is employed as the transmission scheme. A closed-form expression is developed for hop success probability in networks of this type. The delay and throughput of multi-hop networks employing full duplex and half duplex beamforming are than compared. The delay and throughput are studied in two cases: when full duplexity is used for full duplex relay (FDR), and bi-directional beamforming (BBF). For FDR, employing full duplex beamforming is more beneficial if a low rate constraint is required or if interferers are rare. However, this benefit diminishes as the required data rate or the interferer density increases. For BBF, the use of full duplex beamforming is always more beneficial because it leads to a reduction in the required rate constraint. Specifically, this benefit becomes more significant as the required rate rises and interferers become rarer.

Bi-Directional Use of Spatial Resources and Effects of Spatial Correlation

In multiple antenna systems, spatial fading correlation causes a significant degradation in performance. In this work, a method for reducing the sensitivity to spatial fading correlation through the bi-directional use of spatial resources is investigated. Firstly, the concept of bi-directional use of spatial resources is introduced and the fading characteristics of the proposed bi-directional multiple-input multiple-output (B-MIMO) system are investigated. The achievable rates of the B-MIMO system are then investigated and compared with those of conventional MIMO systems for beamforming and full rank transmission. Based on those results, conditions needed for the B-MIMO system to demonstrate a larger average achievable rate are evaluated. B-MIMO systems are found to efficiently reduce the sensitivity to spatial fading correlation, while supporting comparable achievable rates in spatially white environments.

A New Duplex without Loss of Data Rate and Utilizing Selection Diversity

This paper presents a new duplex approach which does not require guard resources suitable for indoor 2times2 MIMO environments. In the proposed duplex, two virtual channels in the spatial domain are generated by precoding and postcoding MIMO channels, not to use guard resources in either the time or frequency domain. In addition, this system can achieve selection diversity since the generation of virtual channels is not unique for the given channels information. We will show the generation of the virtual channel, and that the capacity and reliability of the communication links improves when the problem of the guard resource is addressed and selection diversity is utilized.

Transmission Capacity of Full-Duplex-Based Two-Way Ad Hoc Networks With ARQ Protocol

This paper investigates the network capacity of bidirectional communication systems based on full-duplex transmission in ad hoc networks while satisfying both the delay and target outage constraints. Bidirectional full-duplex transmission (BFD) has been proposed as a way to increase the system performance of two-way communications with multiple antennas. This type of system enables simultaneous exchange of data between two nodes via the bidirectional use of spatial resources. As the node density is increased to improve network capacity, however, the network performance decreases because of the simultaneous transmissions of adjacent pairwise interferers. To overcome this capacity degradation, we apply an automatic repeat request (ARQ) protocol to two-way communications. We then extend the analysis of the transmission capacity (TC) to account for packet retransmissions within the maximum allowed delay while satisfying the target outage constraint for two extreme cases (high mobility and static). After determining the optimized maximum number of packet retransmissions, the network capacity of the BFD system is then investigated as a function of the effective node density, node mobility, and required delay and target outage constraints. We can conclude that the benefits for TC obtained through full-duplex transmission and packet retransmission by an ARQ protocol outweigh the increase in interference caused by the bidirectional use of spatial resources and packet retransmissions.

A novel mode switching scheme utilizing random beamforming for opportunistic energy harvesting

This paper proposes a novel receiver mode switching scheme for simultaneous wireless information and power transfer (SWIPT) and the enabling transmitter design for a point-to-point multiple-input single-output (MISO) channel when the channel state information (CSI) is not available at the transmitter. The proposed scheme employs random beamforming at the transmitter to generate artificial channel fading to enable opportunistic energy harvesting at the receiver when the channel power exceeds a certain threshold. For the proposed scheme, we investigate the achievable average information rate and average harvested energy, as well as trade-offs between them in MISO fading channels. Compared to a reference scheme of periodic receiver mode switching without random transmit beamforming, the proposed scheme is shown to be able to achieve better rate-energy trade-offs when the harvested energy target is sufficiently large. Particularly, it is revealed that employing one single random beam for the proposed scheme is asymptotically optimal as the transmit power increases to infinity, and also performs the best with finite transmit power for high energy harvesting requirements of practical interests, thus leading to an appealing low-complexity implementation.