Exact Sum-Rate Analysis of MIMO Broadcast Channels with Random Unitary Beamforming Based on Quantized SINR Feedback

There is a continuing interest in low-complexity multiuser MIMO transmission techniques based on random unitary beamforming (RUB). Different RUB schemes mandate different amount of feedback to exploit the multiuser diversity gain for sum-rate capacity benefits. In this paper, we investigate the tradeoff of sum-rate capacity versus feedback load between different RUB schemes through accurate statistical analysis. Specifically, we derive the exact sum-rate expressions for two RUB schemes based on quantized signal-to-interference-plus- noise ratio (SINR) feedback. These analysis are accompanied by the development of the complete statistical characterizations of ordered SINRs for a user, which can find their application in many other related problems. We show through selected numerical examples that the additional feedback of the quantized values of best beam SINR from each user can help considerably improve the sum-rate performance of RUB systems.