Cooperative MIMO multiple-relay system with optimised beamforming and power allocation

In this paper we investigate optimised power allocation over two-hop multiple-input multiple-output (MIMO) fixed multiple relays for a given power budget. Optimum beamforming weights under the total sum power constraint for all relays, as well as maximum per-relay power constraint, are found to maximise the received SNR at destination. Results show that optimising the allocation of power improves system performance, especially foe highly unbalanced links. The system with optimised power allocation can outperform a two-hop multiple relay system using uniform power allocation and distributed beamforming at the expense of increased computational complexity. We also study the threshold decode-and-forward fixed relay network with beamforming, which is more reliable than conventional decode-and-forward relaying. The impact of multiple antennas on the outage probability of cooperating fixed relays is considered. It is determined that increasing the number of relays and antennas at each relay increases capacity. The outage probability of threshold maximal-ratio combining and threshold selection combining for multiple-antenna multiple fixed relays is also derived. It is observed that the performance of the relay network with selection combining is close to that of the network with maximal-ratio combining, but the former is less complex and less expensive to implement.