On BER of TDD Multiuser MIMO System with Channel Estimation Error and Delay

In Multiuser Multiple-Input Multiple-Output (MU-MIMO) downlink systems, the zero-forcing algorithm is a simple and effective technique for separating users and data streams of each user at the transmitter side, but its performance depends greatly on the accuracy of the available channel state information (CSI) at the transmitter. Due to the existence of the CSI delay and channel estimation error in practical systems, CSI is always imperfect, which degrades system performance significantly. In this paper, by using the correlation between the actual channel and the estimated one as well as the channel's time-correlation, we develop a novel bit error rate (BER) expression for M-QAM (quadrature amplitude modulation) signal in time division duplex (TDD) downlink MU-MIMO system with channel estimation error and CSI delay. We find that channel estimation error causes array gain loss while CSI delay causes diversity gain loss. Moreover, CSI delay causes more performance degradation than channel estimation error at high signal to noise ratio (SNR) for time varying channel. Especially our research is valuable for adaptive modulation and coding scheme and the optimization of MU-MIMO system. Numerical simulations show accurate agreement with the analytical expressions.