QoS Guarantees in AF Relay Networks with Multiple Source-Destination Pairs in the Presence of Imperfect CSI

A relay network approach is proposed that allows multiple source-destination pairs to communicate simultaneously. The communication is enabled by a multi-antenna amplify-and-forward relay and is implemented in two slots; in the first slot, the sources transmit, and in the second slot, the relay forwards the received signals after processing them through a Zeroforcing Beamforming (ZFBF) matrix. The relay determines the ZFBF matrix so that the transmit power is minimized while the SINR of each destination meets a predefined constraint. Initially, the design is based on perfect channel state information (CSI). Imperfect CSI results in interference at the destinations. This effect can be controlled by increasing the number of relay antennas, or by optimally selecting the antennas to be used. Antenna selection schemes are proposed, which minimize the outage probability, or the destination interference. Also, an iterative scheme is proposed, whereby the amplitudes of the beamforming weights of all source-destination pairs are iteratively increased so that the worst case SINRs meet the requirements of the destinations. It is shown that this approach, coupled with antenna selection, greatly reduces the outage probability.