Analysis of Channel Non-Reciprocity Due to Transceiver and Antenna Coupling Mismatches in TDD Precoded Multi-User MIMO-OFDM Downlink

This paper studies the impact of two implementation imperfections, namely, transceiver frequency-response non-reciprocity and antenna mutual coupling mismatches at the base-station and user equipment on the channel reciprocity assumption in TDD systems. Comprehensive signal models are first developed to analyze the joint effects of these two imperfections in the TDD multi-user MIMO-OFDM downlink transmission system context, covering both zero-forcing (ZF) and eigenbeamforming based transmitter processing. The corresponding performance degradation is then evaluated in terms of SINRs and maximum achievable sum-rate. The analysis shows that during downlink transmission, the transceiver non-reciprocity and antenna mutual coupling mismatches at the base-station introduce inter-user interference (IUI) and are the major causes for the resulting performance degradation. Implementation imperfections at user equipment side, in turn, only introduce inter-stream interference (ISI) that can be fairly easily suppressed in detector processing as part of effective precoded channels. In order to achieve throughputs close to the ideal case, transceiver frequency-response mismatches and antenna mutual coupling at the base-station side need to be extremely well calibrated, generally below 35-40dB in terms of relative mismatch levels.