Transmit and Cooperative Beamforming in Multi-Relay Systems

Cooperative communication has been receiving significant attention in recent years because of the great potential for performance improvement. In this paper, we will consider the joint design of transmit and cooperative beamforming vectors in cooperative systems with a multi-antenna source and multiple single-antenna relays. The cooperative beamforming, performed by distributed relay nodes, is different from the conventional transmit beamforming in multiple-input and multiple-output (MIMO) systems because each node only has access to the channel information of its own links. With instantaneous channel state information (CSI), the optimal beamforming vectors (transmit and cooperative) will maximize the received SNR by allocating power among different relay links, and each relay can determine the optimal operation in a distributed manner. If only statistical CSI is available, it is shown that the optimal transmit scheme degenerates to a relay selection algorithm where only one relay link is active.