Hybrid beamforming for millimetre wave massive MU-MIMO systems with IQ imbalance

The downlink communication of a single-cell millimetre wave (mmWave) massive multi-user multiple-input multiple-output (MU-MIMO) system with hybrid beamforming (HBF) is studied. The authors consider the system with in-phase and quadrature-phase imbalance (IQI) at the radio-frequency (RF) chains. Firstly, they provide the HBF designs based on the complex-valued effective channel and augmented real-valued equivalent channel, respectively. Then, they study the impact of IQI on the achievable downlink sum rate and derive the approximation of the achievable sum rate on various HBF schemes. Results show that the sum rate of the receiver designed based on real-valued equivalent channel increases without bound, but the performance of the receiver designed based on complex-valued presentation exhibits a finite ceiling, as the number of BS antennas increases. Furthermore, both receivers limit to finite ceiling rates due to IQI as the transmit power goes to infinity. Moreover, the HBF system reaches a peak sum rate at a certain value of the number of users, and IQI has an ignorable impact on the value. Meanwhile, the impact of the amplitude imbalance is more significant on the performance degradation of the HBF system than that of the phase imbalance.