Adaptive Beamformer in Hybrid Massive MIMO Using Reconstructed Spatial Covariance Matrix

In order to compensate for high path loss, massive multiple-input multiple-output (MIMO) has been adopted in wireless communication systems operating at millimeter-wave (mmWave) bands. In particular, analog beamformer has been employed in hybrid massive MIMO systems to reduce the cost caused by radio frequency (RF) chains. However, due to hardware limitation, only conventional beamformer can be used at base station (BS) in existing hybrid massive MIMO systems, and as a consequence, inter-user-interference (IUI) cannot be removed completely in the presence of multiple users because of the sidelobe of conventional beamformer. In this letter, we will show that by utilizing the reconstructed spatial covariance matrix (SCM), adaptive beamformer can be also used at the BS in hybrid massive MIMO systems. Thanks to the reconstructed SCM, nulling directions of the adaptive beamformer can be adjusted adaptively to the directions of non-desired users, and therefore the residual IUI can be effectively removed. Simulation results show that the adaptive beamformer can outperform the conventional beamformer significantly in hybrid massive MIMO systems.