Improving the performance of MIMO Relay Networks using Parallel Relays

Recently, multiple-input multiple-output (MIMO) relay communication systems have attracted much research interest, and provided significant improvement in terms of both spectral efficiency and link reliability. This thesis aims at improving the performance of MIMO relay communication systems using parallel relays and successive interference cancellation (SIC) technique. In the first part of the thesis, we propose the optimal transmit beamforming vector and the relay amplifying factors for a MIMO relay communication system with distributed single-antenna relay nodes when a single data stream is transmitted from the source to destination. The proposed joint source and relay beamforming algorithm provides an improved bit-error-rate (BER) performance. The second part of the thesis focuses on parallel MIMO relay communication systems where each relay has multiple antennas and multiple data streams are transmitted from the source to the destination. We derive the optimal structure of relay amplifying matrices when a linear minimal mean-squared error (MSE) receiver is used at the destination node and each relay node is subject to power constraint. Simulation results demonstrate the effectiveness of the proposed source and relay matrices design which minimizes the MSE of the signal waveform estimation using the projected gradient (PG) approach. To reduce the complexity of the algorithm in the second part of the thesis, we propose a simplified source and relay matrices design by first relaxing the power constraint at each relay node to the power constraint at the output of the second-hop channel. After solving the relaxed optimisation problem, the relay matrices are then scaled to satisfy the individual power constraint at each relay node. Simulation results show a good performance-complexity tradeoff of the simplified algorithm. In the fourth part of the thesis, we investigate the optimal structure of the source precoding matrix and the relay amplifying matrices for MIMO relay communication systems with parallel relay nodes when a nonlinear MMSE-DFE receiver is used at the destination node. The MMSE criterion is used to detect the transmitted signal at each stream. We show that the optimal source precoding matrix and the optimal relay amplifying matrices have a beamforming structure. By using a DFE receiver, we can remove the effect of interferences of data stream we have already recovered from the subsequent streams. Therefore, introducing a nonlinear MMSE-DFE receiver at the destination node yields further improvement in the system BER performance compared with the parallel MIMO relay communication systems using a linear MMSE receiver.