Statistical eigenmode SDMA transmission for a two-user downlink

This paper proposes a statistical-eigenmode spacedivision multiple-access (SE-SDMA) transmission for a two-user downlink system where two transmit antennas are equipped at the base station and each mobile user has one receive antenna, assuming that only statistical channel state information (CSI) is available at the transmitter. By maximizing a lower bound of the ergodic signal-to-leakage-and-noise ratio, the proposed SE-SDMA approach selects two users with orthogonal principal statistical eigen-directions and transmits to each user along the corresponding eigenmode. We derive an exact expression of the ergodic achievable rate, and compare it with the zero-forcing beamforming (ZFBF) system exploiting instantaneous CSI. It is shown that SE-SDMA can achieve the maximum ergodic sum-rate of the two selected users, and provide significant user selection gain. Analytical and simulation results show that the rate gap between SE-SDMA and ZFBF with perfect CSI at the transmitter can tend to zero in highly correlated channels, which indicates that statistical precoding can be used instead of instantaneous precoding in certain environments.