I/Q Imbalance Aware Widely-Linear Receiver for Uplink Multi-Cell Massive MIMO Systems

In-phase/quadrature-phase (I/Q) imbalance is one of the most important hardware impairments in communication systems. It arises in the analogue parts of direct conversion radio frequency (RF) transceivers and can cause severe performance losses. In this paper, I/Q imbalance (IQI) aware widely-linear (WL) channel estimation and data detection schemes for uplink multi-cell massive multiple-input multiple-output (MIMO) systems are proposed. The resulting receiver is a WL extension of the minimum mean square error (MMSE) receiver and jointly mitigates multi-user interference and IQI by processing the real and the imaginary parts of the received signal separately. The IQI arising at both the base station (BS) and the user terminals (UTs) is then taken into account. The considered channel state information (CSI) acquisition model includes the effects of both estimation errors and pilot contamination, which is caused by the reuse of the same training sequences in neighboring cells. We apply results from random matrix theory to derive analytical expressions for the achievable sum rates of the proposed IQI aware and conventional IQI unaware receivers. Our simulation results show that the performance of the proposed IQI aware WLMMSE receiver in a system with IQI is close to that of the MMSE receiver in an ideal system without IQI. Moreover, our results for the sum rate of the IQI unaware MMSE receiver reveal that the performance loss due to IQI can be large and, if left unattended, does not vanish for large numbers of BS antennas.