Hybrid Beamforming With Modular Array for Near-Field Multi-User Communication

This letter addresses the downlink near-field multi-user communication scenario and introduces a modular array (MA) based hybrid beamforming architecture, establishing the first system model for MA-enabled near-field multi-user communication. For the purpose of maximizing multi-user sum-rate, the near-field MA alternating optimization (NFMA-AO) algorithm is proposed, which alternately optimizes the digital and analog beamformers. Specifically, we reformulate the sum-rate maximization problem as a weighted minimum mean-square error (WMMSE) problem to derive the closed-form solution for the digital beamformer. For the analog beamformer, we leverage the Riemannian conjugate gradient descent (RCGD) algorithm. To mitigate computational complexity, we further develop a low-complexity beam focusing subspace compression (BF-SC) algorithm that exploits the high spatial resolution of MA. Simulation results demonstrate that the proposed algorithms outperform existing schemes in sum-rate performance.