Random beamforming for spatial multiplexing in downlink multiuser MIMO systems

Optimization of beamforming in broadcast MIMO channels is severely hampered by the amount of feedback. In this paper, we propose to reduce the feedback when employing a random beamforming where multiple streams can be linearly precoded before being transmitted on the antenna array and the users with channel/interference matched to the beamforming configurations are scheduled at each time. In MIMO system, the users can further improve the overall throughput by minimizing the inter-stream interference. The feedback necessary to employ the spatial multiplexing can be limited to the signal to interference ratios for a sub-set of users that are compatible for the random beamforming configuration. In spite of the simplicity, this scheduling strategy can take advantage of the multiuser diversity for a large number of independent channels (or users) as in OFDMA system. The numerical analysis validates the strategy for the selection of compatible users and the spectral efficiency for this opportunistic spatial multiplexing scheme