Null Space Projection-Based Hybrid Beamforming for Multi-User Massive MIMO

This study employs an array-of-subarrays (AoSA) hybrid beamforming (HBF) architecture in ultra-massive multiple-input multiple-output (UM-MIMO) systems to enhance the total achievable rate. Our primary objective is to mitigate the strong multi-user interference (MUI) through the design of null-space projection (NSP)-based HBF scheme, which involves two stages: (i) RF beamforming based on introducing beam perturbations to steer beams in the null space of interfering users, and (ii) baseband MU precoding stage based on the instantaneous effective channel to mitigate the residual MUI by a regularized zero-forcing (RZF) technique. To solve this challenging non-convex optimization problem, we propose a swarm intelligence-based sequential optimization solution that finds the optimal beam perturbations while adhering to the directivity degradation constraints for the beams in each user direction. The illustrative results depict the high achievable rate by using the proposed NSP scheme over maximum-directivity beamforming (MBF) irrespective of the users' angular locations, which can be a promising beamforming solution in future sub-Terahertz (sub- THz) UM-MIMO systems.