Reducing The Effect of Channel Time Variations in MIMO Broadcast Systems

A simple signaling method for multiple input multiple output (MIMO) broadcast channels with zero-forcing beamforming (ZFBF) in the presence of outdated channel state information (CSI) is proposed. We address the problem of channel variation for the case of fixed-rate users using block coded modulation (BCM). It is assumed that the perfect CSI is known at the beginning of each block, while the channel varies over time. The impact of the outdated CSI is evaluated both analytically, and experimentally through Monte-Carlo simulation. The effect of the 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. Simulation results show that the proposed method results in a significant improvement in the error performance with a negligible increase in the complexity.