Downlink Distributed Beamforming through Relay Networks

Beamforming and relaying are two methods of improving wireless system performance. Until now, these techniques have been considered separately. In this paper, a single source (basestation) with multiple antennas broadcasts to multiple single-antenna destinations through a network of single-antenna relays. A transmit precoder is first proposed to optimize the system for a set of fixed relay gains and phases (beamforming weights). Next, by straightforwardly generalizing results from the recent literature, these distributed relay beam-forming weights are optimized for this transmit precoder. The process is performed iteratively, and both problems are solved using semidefinite relaxation. For the special case of a single destination, we also provide a closed-form solution for relay optimization using a fixed linear precoder at the transmitter. Simulation results quantify the tradeoffs between the number of antennas at the basestation versus the number of relays, in terms of total relay network transmit power savings.