Cell-Free Massive MIMO with Multiple-Antenna Users under I/Q Imbalance

Cell-free massive multiple-input multiple-output (MIMO) is a core technology for future wireless communication systems, since it harnesses benefits from massive MIMO and distributed MIMO systems. In order to be practically viable, cell-free massive MIMO systems are expected to make use of many low cost, low quality components. As a consequence, this makes cell-free massive MIMO susceptible to hardware imperfections, such as in-phase and quadrature-phase imbalance (IQI). Motivated by this, we provide a derivation for the achievable spectral efficiency of each user in a cell-free massive MIMO system with IQI and multiple-antenna users. Our analysis demonstrates that the performance loss from IQI is noticeable under high IQ mismatch. Furthermore, our analytical and numerical results showcase that the system performance improves without bound for a perfect IQ matching scenario, however, if IQI is present the performance saturates even for an increasing number of access points.