Hybrid Precoding for Wideband Multi-user MIMO Millimeter Wave System

Hybrid precoding design for a wideband multiuser multiple input multiple output (MIMO) millimeter wave (mmWave) system is more challenging than the narrowband design. This is due to the fact that the radio frequency (RF) precoder and combiner are frequency non-selective and they should be optimized with respect to all scheduled users on all allocated subcarriers. In this work, we adopt the separate stage hybrid precoding strategy and propose two algorithms with good performance and complexity trade-off. Based on the proposed strategies of minimal equivalent channel gain maximization or average wideband sum rate maximization, the non-convex RF beamforming can be solved first by a semi-definite relaxation and codebook projection. In the second stage, conventional sum rate maximization digital precoder/combiner can be obtained based on smaller dimension equivalent channel feedback. Simulations show that the proposed algorithms outperform the state of art algorithms in multiple receive antennas setting.