Downlink beamforming through relays: imperfect CSI and coordinated transmission

Future wireless systems face challenges in supporting high-rate multimedia streaming with wide coverage area and at low power. Cooperative relaying is investigated in the following context: a single base station with multiple antennas simultaneously transmits different data streams to single-antenna destinations through a set of single-antenna fixed relays, with no direct link transmission between base stations and destinations. Transmission is via space-division multiple access with per-user quality-of-service constraints. The base station performs transmit beamforming (precoding) and relays cooperatively perform distributed beamforming. To address minimum-power source precoding and relay beamforming, the source precoder design with a fixed relay beamformer is first considered. Precoding is also generalized to multiple cooperating base stations. Next, the relay beamformer is optimized for a given source precoder and the process is iterated. The formulation is generalized to account for CSI estimates obtained from pilot symbol training. Simulation results quantify tradeoffs, including numbers of base station antennas and relays, effect of CSI quality on performance, as well as the impact of cooperating base stations.