Beamforming in Nonorthogonal Amplify-and-Forward Relay Networks

This paper considers wireless amplify-and-forward (AF) relay networks in which the source communicates with the relays and destination in the first phase and the relays simultaneously forward signals to the destination in the second phase over uncorrelated Rayleigh fading channels. We examined one scenario in which each relay only knows the perfect information of its source-relay channel, whereas the destination knows the exact information of the relay-destination channels and the statistics of the source-relay channels. Based on a combiner that was developed at the destination, we propose an efficient beamforming scheme at the relays and develop its quantized version using Lloyd's algorithm to work with a limited-rate feedback channel. Simulation results show that the nonorthogonal relaying with the proposed beamforming scheme outperforms the orthogonal relaying with power allocation in terms of the ergodic capacity. In terms of the signal-to-noise ratio (SNR), the nonorthogonal scheme also becomes superior to the orthogonal scheme when the number of quantization regions increases.