Enhanced limited rate implicit CSI feedback and its usage in covariance matrix based MU-MIMO

For downlink multi-user multiple-input multiple-output (MU-MIMO) system, the precoding scheme of maximizing signal-to-leakage-and-noise (SLNR) at base station (BS) relies on channel covariance matrix. However, the CSI feedback defined in LTE/LTE-A does not directly correspond to channel covariance. Instead, precoding matrix indicator (PMI) and channel quality indicator (CQI) are fed back from user equipment (UE) to the base station. It is possible to reconstruct channel covariance matrix based on PMI/CQI. However such reconstruction is quite rough and may cause MU-MIMO performance degradation.