Hybrid Distributed MRC With Imperfect CSI for MU-MIMO Systems

Abstract

We present new results for uplink distributed MIMO with two-stage beamforming (BF). Two- and four-cluster base station layouts are considered with four BF strategies. Stage one considers analog (A) or digital (D) maximal ratio combining (MRC), with digital MRC or digital zero forcing (ZF) in stage two. We denote the resulting BF schemes as A-D, D-D, A-ZF and D-ZF. Using new analytical results for the expected signal-to-interference-plus-noise-ratio (SINR) of A-D and D-D, we show that the performance gap between these schemes increases with the number of clusters. Using novel simplifications for the A-ZF and D-ZF SINR, we show that the interference reduction of the first stage processing causes a delay of the MRC/ZF performance cross-over relative to a single-stage system. We analyze the asymptotics of A-D and D-D strategies, extending existing results for single-stage co-located processing to a distributed two-stage system. We show how the relative performance of A-D and D-D varies significantly depending on the distribution of interference among the base stations and that A-D can outperform D-D.