Compensation Of Channel Non-Reciprocity In TDD-MIMO Systems With RF Impairments

As an effective way to achieve high data rates, MIMO technique has been widely studied and employed in new-generation of wireless communications systems. By using channel reciprocity in time division duplex(TDD) MIMO systems, the base station(BS) can implement data transmitting pretreatment according to the channel state information (CSI) obtained by uplink measurement without additional feedback overhead. But radio frequency(RF) front-end imperfections can lead to the loss of channel reciprocity, which will invalidate the advantage of TDD systems.This thesis analyzes the impact of RF gain mismatch and I/Q imbalance on channel reciprocity due to RF imperfections in TDD-MIMO systems. On this base, compensation algorithms are proposed and simulated in a 2×2 TDD-MIMO system. The main contributions are as follows:1. The deleterious effects of RF gain mismatch on channel reciprocity are discussed, and the capacity of systems with nonreciprocal channels is given. Simulation results show that the bit error rate(BER) performance and system capacity are severely degraded by RF gain mismatch in TDD-MIMO systems using precoding schemes.2. Two compensation methods, limited feedback and calibration matrices, are applied to deal with the loss of channel reciprocity caused by RF gain mismatch. Simulation results show that these methods have good performance in TDD-MIMO systems with RF gain mismatch.3. The equivalent model of TDD-MIMO systems with I/Q imbalance is derived and the corresponding system capacity is given. Simulation results show that I/Q imbalance destroys the channel reciprocity and degrades the BER performance and system capacity of TDD-MIMO systems using precoding schemes.4. A calibration algorithm is proposed to compensate for the loss of channel reciprocity due to I/Q imbalance and the capacity calibrated system is given. Simulation results show that the proposed algorithm significantly improves the BER performance and the system capacity.