Two-way relaying in wireless cellular communications systems

Abstract

Cooperative relaying has been widely studied to improve the performance of wireless communication systems for extended coverage and enhanced power efficiency. However, conventional one-way relaying involves additional time phases to complete bi-directional signal transmission, which reduces spectral efficiency. Also, such additional phases are not consistent with the existing signaling structure, which causes difficulty in implementation. It remains a challenging issue to fully exploit the advantages of relaying in wireless communication systems. Two-way relaying is a recently emerged concept. Compared with conventional one-way relaying, two-way relaying is more spectrally efficient. So far, however, most discussions on two-way relaying are for single-end to single-end systems. There still lacks efficient methods to apply two-way relaying in multi-user cellular environment. This thesis presents a comprehensive study on the application of two-way relaying in multi-user cellular systems. The motivations of the work are as follows. First, we want to explore new two-way relaying schemes under realistic assumptions to achieve improved performance related to power efficiency, spectral efficiency and fairness. Second, the new schemes should be consistent with the existing systems. Third, it should also be easy to design the system and analyze its performance. The thesis is organized as follows. In Chapter 1, we briefly discuss the background of relay systems, including both one-way and two-way settings. The advantages and limitations of existing methods are outlined, which provides the motivations of the work presented in this thesis. In Chapter 2, we consider a single-user two-way relaying orthogonal frequencydivision multiplexing (OFDM) system, in which the base station (BS) and a user node exchange information via a relay node. At the relay node, signals from different subcarriers are interleaved so that the achievable rate of the overall system can be improved. We develop a low-cost partitioning and mapping method to solve the interleaver design problem. In this scheme, the OFDM subcarriers are first partitioned into different subsets. Each subset of subcarriers are then mapped recursively. Although the scheme is suboptimal, simulation results demonstrate that it significantly improves system performance and is near-optimal as confirmed using a performance upper bound. Starting from Chapter 3, we extend our discussions to multi-user cellular systems. In Chapter 3, we consider an orthogonal frequency-division multiple access (OFDMA) system where the BS serves different users over different subcarriers in single-input single-output (SISO) scenario. A parallel two-way relaying OFDMA (PTWR-OFDMA) scheme is proposed to exploit the advantage of PTWR. PTWROFDMA has a standard two-phase signaling structure and is thus compatible with conventional OFDMA. The users are divided into near-nodes (NNs) and far-nodes (FNs) based on their fading conditions. Each NN has two functions: communicating with the BS directly and relaying the transmission for FNs. We derive optimized ratio for NNs and FNs in the asymptotic case of K → ∞(K being the number of users) and show that this ratio works also well even when K is small. We also derive the asymptotic system performance and show that the achievable rate of PTWR-OFDMA scales like lnK, which is significantly higher than ln lnK offered by conventional OFDMA systems. In addition, PTWR-OFDMA also demonstrates better fairness. It is complicated to analyze fairness when path loss, lognormal fading and Rayleigh fading are involved. Thus the discussions on fairness comparisons in this chapter are based on simulation only. In Chapter 4, we consider multiple-input multiple-output (MIMO) scenario. Precoding and beamforming techniques are adopted to improve system performance. Denote by MMT and MBS (MMT ≥ MBS), respectively, the antenna numbers of the user and the BS. We show that the achievable rate of PTWR-OFDMA increases like MMT lnK when K → ∞, much higher than that of MBS ln lnK for conventional OFDMA. It is well-known that the effect of Rayleigh fading can be approximately neglected in a relatively large MIMO system. This greatly simplifies the analysis problem for PTWR-OFDMA. In a conventional cellular system, users with poor large-scale fading are usually allocated with low throughputs and thus suffer most from the fairness problem. In PTWR-OFDMA, such users are likely assigned as FNs and can benefit most from the beamforming effect provided by NNs. We will show that when K is small, FNs have relatively lower achievable rates than NNs as the latter usually have better channel conditions. We will also show that when K increases, the throughputs of FNs increase faster than those of NNs. This reduces the difference among user rates and so improves the fairness of PTWR-OFDMA significantly. In Chapter 5, we investigate the performance of PTWR-OFDMA in a multicell scenario. Simulation results confirm that PTWR-OFDMA can provide significantly increased achievable rate than conventional OFDMA, especially for a large number of users. Finally, in Chapter 6, we conclude the thesis and outline several possible research directions for the future work. In summary, this thesis presents a comprehensive study on the application of two-way relaying in cellular systems. Several novel schemes are proposed, which demonstrate significantly enhanced power and spectral efficiencies and better fairness, as confirmed by both analytical and simulation results. In addition, the proposed schemes are consistent with existing system, which allows smooth upgrading from the existing systems. Overall, the findings of this thesis provide useful guidelines for the evolution of future wireless communications systems.

Date of Award	2 Oct 2015
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Ping LI (Supervisor)