Hybrid Beamforming With Finite-Resolution Phase Shifters for Multiuser Millimeter-Wave Downlink

This letter considers hybrid analog-digital beamforming with discrete-phase shifters for multiuser multiple-input and single-output (MISO) downlink transmission in millimeter-wave (mm-wave) channels. A max-min signal-to-interference-plus-noise ratio (SINR) fairness problem is formulated. Owing to the discrete-phase constraints, this problem is a mixed integer nonlinear program (MINP) and is generally NP-hard. To handle it, an alternating optimization approach is employed. Specifically, for the digital beamformer, zero-forcing (ZF) beamforming is utilized. For the discrete-phase analog beamformer, we first derive an equivalent continuous reformulation and then solve it by leveraging an inexact majorization-minimization (MM) algorithm, which is built upon the first-order method and MM update. Simulation results show that the proposed design can almost achieve the optimal performance of the exhaustive search method, and can approach the optimal full-digital performance with increasing resolution of the discrete-phase shifters.