Random Beamforming with Systematic Beam Selection

In this paper, we propose a new random beamforming scheme for multiuser multiple-input single-output block fading channels, e.g., a wireless downlink in a cellular communication system. Unlike previous methods it achieves spatial multiplexing gain over the complete range of realistic signal-to-noise ratios (SNRs) even for moderate user numbers. The main shortcoming of the random beamforming schemes for multiple datastreams proposed so far is their interference limitation, especially under medium and high SNR. The key to mitigate this problem in our scheme is to allow feedback of each user's channel gain in each random beam. This information suffices to determine the sum-rate maximizing subset of beams and users at the base station. In this sense, our scheme, referred to as orthonormal random beamforming with systematic beam selection, can be considered as optimal among all random beamforming schemes that do not make use of additional preprocessing at the transmitter (apart from power normalization). It achieves a significant performance gain compared to previous schemes particularly for realistic SNR values