Novel Insight Into Multi-User Channels with Multi-Antenna Users

Multi-user channels with single-antenna users have proven to be favorable in massive multiple-input multiple-output regime as the asymptotic orthogonality of different channel vectors. However, the orthogonality of the channel maybe no longer holds for more general case. In this letter, we consider the scenario, where a base station equipped with M antennas that simultaneously serves K independent multi-antenna users. The orthogonality of channel vectors is examined in both spatially correlated Rayleigh channels and geometry-based channels. The theoretical analysis shows that the correlation matrix of the generalized channel in both cases will not become diagonal, which means that the favorable propagation decays when M grows without limit. A novel insight is then derived in the case of multi-antenna users for the more general channels that the inner-user interference still exists when M -> infinity. However, the analysis of the geometry-based channel indicates that the inter-user interference can still be eliminated by the large array effect.