Impact of RF mismatches on the performance of massive MIMO systems with ZF precoding

Thanks to the channel reciprocity, the time division duplex(TDD) operation is more preferred in massive multiple-input multiple-output(MIMO) systems. Avoiding the heavy feedback of downlink channel state information(CSI) from the user equipment(UE) to the base station(BS), the uplink CSI can be exploited for the downlink precoding. However, due to the mismatches of the radio frequency(RF) circuits at both sides of the link, the whole communication channels are usually not symmetric in practical systems. This paper is focused on the RF mismatches at the UEs and the BS for the multi-user massive MIMO systems with zero forcing(ZF) precoding. The closed-form expressions of the ergodic sum-rates are derived for evaluating the impact of RF mismatches on the system performance. Theoretical analysis and simulation results show that the RF mismatches at the UEs only lead to a negligible performance loss. However, it is imperative to perform reciprocity calibration at the BS, because the RF mismatches at the BS contribute to the inter-user interference(IUI) and result in a severe system performance degradation.