Semi-Orthogonal Relay Selection and Beamforming for Amplify-and-Forward MIMO Relay Channels

In this paper we consider a two-hop MIMO parallel relay channel where a source-destination pair both equipped with M antennas, communicate via K N-antenna half-duplex amplify-and-forward relays (K,N ges M). Multi-user beamforming techniques are jointly applied at the source and destination to respectively multiplex and recover the data streams. In order to further exploit the multi-relay diversity, we propose new relay selection criteria based on semi-orthogonality among spatial eigenmode and antenna pairs of the relays. The proposed algorithms have very low complexity, decrease the amount of feedback, and consume less power across the relays compared to previously proposed MIMO relaying strategies. Furthermore, for small to medium number of relay nodes, the proposed algorithms´ capacity outperform the other strategies and in the asymptote of large number of relay nodes (K rarr infin), the capacity of the proposed algorithms scales as M/2 log log K.