Performance Analysis of MMSE Digital Beamforming Techniques in Multi-User MIMO Systems Under CSI Errors

Overview & Synopsis

This paper deals with the minimum mean square error (MMSE) digital beamforming techniques at both transmitter (precoding) and receiver (combining) sides in multi-user multiple input multiple output (MU-MIMO) systems, under errors of channel state information (CSI). We consider very general system models in both downlink and uplink cases, where the number of transmit and receive antennas can take any values, and the CSI errors are random samples. Based on this model, we analyze the performance of the MMSE beamforming methods considering imperfect CSI in the construction of the beamforming matrices, reflecting possible errors of channel estimation. We theoretically show that the additional inter-user interference power due to the CSI errors is quadratically proportional to the square of a parameter α quantifying the CSI errors. This analytic result is supported by simulations, and we also show the effect of the CSI errors on the achievable bit error rate (BER), considering multipath and free space line-of-sight channel models. In both cases, simulations and theoretical results match, and highlight that the MMSE beamforming methods experience only up to 1 dB loss in presence of potentially high CSI errors compared to perfect CSI.