I/Q Imbalance Compensation in Cell-Free Massive MIMO During Uplink Transmission

This article considers compensation issues within cell-free massive multiple-input–multiple-output (MIMO) communication systems, under the in-phase and quadrature-phase imbalance (IQI). Both access points (APs) and users are equipped with multiple antennas. We conduct an analysis into the impact of IQI and propose an efficient IQI compensation scheme to overcome the effects of IQI. Analytical expressions for the minimum mean-square error (MMSE) estimation and the achievable spectral efficiency (SE) of each user are derived, both with and without IQI compensation. In addition, to characterize the IQI effect in the massive MIMO regime, we analyze the asymptotic performance of cell-free massive MIMO when the number of APs goes to infinity. The results of our analysis demonstrate that when the number of APs grows large, a cell-free system with perfect I/Q matching will allow the SE to increase without bound. However, if IQI is present, the system performance will saturate even if the number of APs becomes very large. The introduction of a compensation technique at the APs, that requires only an estimation of the IQI coefficients, is successful in removing this performance limit, hence significantly enhancing the system performance.