I/Q imbalance aware widely-linear precoding for downlink 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 analog parts of direct conversion radio frequency (RF) transceivers and can cause large performance losses. In this paper, the impact of I/Q imbalance on the sum rate performance of downlink massive multiple-input multiple-output (MIMO) systems is analyzed and I/Q imbalance aware widely-linear precoding is considered. The proposed precoder is a widely-linear (WL) extension of the regularized zero-forcing (RZF) precoder and jointly mitigates multi-user interference and I/Q imbalance by processing the real and the imaginary parts of the transmit signal separately. Results from random matrix theory are used to derive analytical expressions for the achievable sum rate of the proposed precoder. Our analytical results show that if the number of base station antennas is much larger than the number of users, the proposed I/Q imbalance aware WL-RZF precoder achieves the same sum rate as the RZF precoder in an ideal system without I/Q imbalance. We also provide an analytical expression for the sum rate loss of the I/Q imbalance unaware RZF precoder compared to the ideal case without I/Q imbalance.