Low-complexity downlink beamforming for maximum sum capacity

The problem of simultaneous multiuser downlink beamforming has recently attracted significant interest in both the information theory and signal processing communities. The idea is to employ a transmit antenna array to create multiple ´beams´ directed towards the individual users, and the aim is to increase throughput, measured by sum capacity. Optimal solutions to this problem require convex optimization and so-called dirty paper (DP) precoding for known interference; these are prohibitively complex for actual online implementation at the base station. Motivated by recent results by H. Viswanathan et al. (see IEEE J. Sel. Areas in Commun., vol.21, no.5, p.802-11, 2003) and G. Caire and S. Shamai (see IEEE Trans. Inf. Theory, vol.49, no.7, p.1691-706, 2003), we propose a computationally simple user selection method coupled with zero-forcing beamforming. Our results indicate that the proposed method attains a significant fraction of sum capacity, and thus offers an attractive alternative to DP-based schemes.