  In this paper, we consider a dual-hop Multiple Input Multiple Output (MIMO)
wireless multi-relay network, in which a source-destination pair both equipped
with multiple antennas communicates through multiple half-duplex
amplify-and-forward (AF) relay terminals which are also with multiple antennas.
Since perfect channel state information (CSI) is difficult to obtain in
practical multi-relay network, we consider imperfect CSI for all channels. We
focus on maximizing the signal-to-interference-plus-noise ratio (SINR) at the
destination. We propose a novel robust linear beamforming at the relays, based
on the QR decomposition filter at the destination node which performs
successive interference cancellation (SIC). Using Law of Large Number, we
obtain the asymptotic rate in the presence of imperfect CSI, upon which, the
proposed relay beamforming is optimized. Simulation results show that the
asymptotic rate matches with the ergodic rate well. Analysis and simulation
results demonstrate that the proposed beamforming outperforms the conventional
beamforming schemes for any power of CSI errors and SNR regions.
