Impact of imperfect channel state information on TDD downlink Multiuser MIMO system

In downlink Multiuser Multiple-Input Multiple-Output (MU-MIMO) systems, the zero-forcing (ZF) transmission 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 characterizing CSI inaccuracies due to channel delay and channel estimation error, 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 where each user is equipped with one antenna. By simulation, 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.