Joint beamforming and power control for MIMO relay broadcast channel with QoS constraints

This paper considers the two-hop relay broadcast channel (BC) in a wireless cellular network where the downlink transmission from the base station (BS) to mobile users is assisted by a fixed relay station (RS). Assuming both the BS and RS equipped with multi-antennas, we consider joint optimization of beamforming weights and power allocations both the BS and RS so as to minimize their weighted sum-power consumption under the minimum signal-to-interference-noise-ratio (SINR) constraints at user terminals. We extend the two well-known criteria for design of linear precoding in the conventional MIMO-BC without relay, namely, the SINR balancing and the channel-inversion, to the MIMO relay BC. First, an iterative SINR-balancing algorithm is proposed that in each iteration optimizes transmit parameters at one station (BS or RS) with those at the other fixed. Secondly, a joint BS and RS channel-inversion algorithm is proposed together with a novel technique termed "eigenmode switching" at the RS. Simulation results show that optimized joint beamforming and power control are able to achieve substantial power savings for the MIMO relay BC.