  Multiuser beamforming is considered for partially-connected millimeter wave
massive MIMO systems. Based on perfect channel state information (CSI), a
low-complexity hybrid beamforming scheme that decouples the analog beamformer
and the digital beamformer is proposed to maximize the sum-rate. The analog
beamformer design is modeled as a phase alignment problem to harvest the array
gain. Given the analog beamformer, the digital beamformer is designed by
solving a weighted minimum mean squared error problem. Then based on imperfect
CSI, an analog-only beamformer design scheme is proposed, where the design
problem aims at maximizing the desired signal power on the current user and
minimizing the power on the other users to mitigate the multiuser interference.
The original problem is then transformed into a series of independent beam
nulling subproblems, where an efficient iterative algorithm using the
majorization-minimization framework is proposed to solve the subproblems.
Simulation results show that, under perfect CSI, the proposed scheme achieves
almost the same sum-rate performance as the existing schemes but with lower
computational complexity; and under imperfect CSI, the proposed analog-only
beamforming design scheme can effectively mitigate the multiuser interference.
