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Random unitary beamforming (RUB) is an attractive low-complexity transmission scheme for multiple-input-multiple-output (MIMO) broadcast channels. In this paper, we study the sum-rate performance of RUB-based multiuser MIMO systems with receive diversity. Two combining techniques, i.e., selection combining (SC) and optimum combining (OC), are employed to reduce feedback load and improve sum-rate performance. With these combining techniques, each user only needs to feed back one beam index and the corresponding output signal-to-interference-plus-noise ratio (SINR). For the SC scheme, we derive the statistics of the output SINR and the exact analytical expression of the sum rate. For the OC scheme, we develop an upper bound of the sum rate in the high-signal-to-noise-ratio (SNR) region. The bound is shown to be tight for the case of a moderate number of users, but the number of users must be at least five times the number of transmit antennas. Selected numerical examples show that the proposed design greatly enhances the sum-rate performance while maintaining low feedback load.
Date of Publication: Jan. 2010