Relay Selection And Beamforming Design For Two-Way Relay Systems

In a cooperative communication system, a virtual multiple-antennas environment is constructed by sharing antennas of different nodes, and the diversity gain can be achieved without increasing the hardware complexity. Applying the cooperative techniques to two-way multi-relay system can provide high capacity, low symbol error rate and low outage probability. Amplify-and-forward (AF) and decode-and-forward (DF) are two main re-laying protocols in a two-way multi-relay system. In this paper, we investigate the S-ER performance of a two-way relay system firstly, and then propose two relay selection schemes for the DF based two-way relay system. Moreover, in order to improve the sum-rate and the security of an AF based two-way relay system, the relay beamforming design is explored.Firstly, in order to discuss the SER of a DF based two-way relay system, we extend the nearest neighbor approximation method. By this method, the SER performance with additive white Gaussian noise channel for BPSK, QPSK and8PSK modulation is derived. For fading channel, the constellation at relay is complex due to scaling and rotation of channel fading. We discuss the instantaneous SER at first, and then by calculating the distribution of the shortest distance between the constellation points, the average SER for BPSK and QPSK modulation over Rayleigh fading channels is derived.Secondly, a relay selection scheme is proposed to improve the SER performance of a two-way relay system. Since the shortest distance between points in the constellation is an important factor for the end-to-end SER of a two-way relay system, we propose another relay selection scheme, in which the relay with the maximal minimum distance between points in the constellation will be selected. When BPSK, QPSK and8PSK modulation are applied, full diversity gain can be achieved. Moreover, a look-up table based method is also designed for high order modulation.Thirdly, the sum-rate maximization with transmit power and energy harvesting con-straints are considered in the two-way AF based multi-relay system. We design an ideal scheme, three practical schemes and one robust scheme. The optimization problems are solved by semidefinite relaxation and successive convex approximation method. We also prove that the proposed algorithms at least achieve the local optimal solution.Finally, the problem of secure information exchange between two sources via mul- tiple relays in the presence of an eavesdropper is discussed. To this end, we consider both cases with and without artificial noise and formulate two optimization problems for beamforming design. The goal is to seek the optimal beamforming vectors to minimize the total power consumed by relay nodes such that the secrecy of the information ex-change between the two sources can be protected. Since both optimization problems are nonconvex, we solve them by semidefinite relaxation method, and prove that the beam-forming with artificial noise scheme outperforms the relay beamforming without artificial noise in terms of both low power consumption and low infeasibility rate.