On the asymptotic optimality of beam-forming in multi-antenna Gaussian multiple access channels

In this paper, transmit schemes for multi-antenna Gaussian multiple access channels are considered. For the case of a large number of users, a short term power constraint, and slowly fading channels, asymptotic optimality of beamforming is shown under the sum rate maximization criterion. This is an interesting result because the optimal transmit scheme for each user is to beamform to one direction even if each user is equipped with a large number of transmit antennas. Also, this result extends to ISI channels assuming OFDM modulation. A suboptimal transmit scheme based on this result follows for a system with partial channel side information