Multi-user MIMO using ZF-based Multiplexing Coordinated with User-wise Spatial Diversity

In this paper, a multi-user MIMO using zero-forcing (ZF)-based multiplexing coordinated with user-wise spatial diversity is proposed assuming time-division duplex (TDD). The downlink and uplink signal-to-noise ratios (SNRs) are analyzed for the given multi-user MIMO channel. It is shown that the downlink capacity is the same for all users while the uplink capacity is not necessarily the same. The closed-form expressions for the downlink and uplink SNRs conditioned on the 2-user virtual MIMO channel are derived. It is shown that the uplink sum capacity is higher than the downlink sum capacity. The downlink and uplink capacities are numerically evaluated by Monte Carlo simulation in a Rayleigh fading environment. It is shown that the link capacity can be improved by increasing the number of user antennas while keeping the ZF matrix size in the same level.