Iterative nonlinear precoding and link adaptation for MU-MIMO with CSI impairments

An iterative nonlinear precoding technique based on the Tomlinson-Harashima precoder is proposed for MU-MIMO to alleviate the performance degradation due to mismatches between actual Channel State Information (CSI) and impaired CSI available at the transmitter. A robust design is presented to feedback the effective channel rather than the nonprecoded channel for relay backhaul channel. A Channel Quality Indicator (CQI) generation is proposed for the base station (eNB) to estimate the receivers´ signal-to-interference-plus-noise ratio taking into account impact of quantization errors. Numerical studies verify that the combination of the proposed iterative precoding and effective channel feedback significantly reduces the high sensitivity to quantization errors and improves the quality of radio link between the eNB and receiver. The proposed CQI feedback method enables the transmitter to accurately derive the effective channel quality seen by the receiver. Therefore, a tight link adaptation loop can be achieved to fully utilize the improved radio link.