Cooperative amplify-and-forward beamforming with multi-antenna source and relays

In this paper, we propose a beamforming (BF) scheme for cooperative networks with multiple antennas at the source node and the amplify-and-forward (AF) relay nodes. Adopting the instantaneous signal-to-noise ratio (SNR) as optimality criterion, we obtain closed-form expressions for the maximum SNR and the optimal AF-BF matrices at the relays for a given BF vector at the source. Furthermore, we propose a numerical algorithm to find a locally optimal BF vector at the source. Simulation results for Rayleigh fading show that AF-BF with multi-antenna relays can achieve significant performance gains compared to direct transmission without relaying.