User Selection and Signaling Over Time-Varying MIMO Broadcast Channels

Communication over time-varying multiple input multiple output (MIMO) broadcast channels with zero-forcing beamforming (ZFBF) is considered. Different kinds of user selection algorithms are studied and the impact of channel time variation in the presence of outdated channel state information (CSI) is evaluated both analytically and experimentally through Monte-Carlo simulation. In each user selection algorithm, M beamforming directions are selected, where M indicates the number of transmit antennas. Each active beamforming direction utilizes fixed-rate block coded modulation (BCM) scheme. It is assumed that the perfect CSI is known at the beginning of each block, while the channel varies over time. The effect of channel aging is modeled as an additive noise with time-varying power. We introduce a new method based on Hadamard transformation in order to equalize the noise power across the coding block. The performance of BCM scheme is studied using the pairwise error probability, showing that the proposed method results in a significant improvement in the error performance with a negligible increase in the complexity