Hybrid Beamforming Optimization for Near-Field Terahertz Multi-User System With FTTDs

Abstract

In this paper, we investigate beamforming for a wideband terahertz (THz) communication system serving multiple users in the near-field region. A hybrid beamforming architecture employing fixed true time delays (FTTDs) and phase shifters (PSs) is introduced to reduce the spatial wideband effect. Both fully-connected (FC) and sub-connected (SC) architectures are considered. To maximize spectral efficiency, we introduce a penalty-based weighted minimum mean square error (PWMMSE) approach, in which the spectral efficiency maximization problem is reformulated through the weighted minimum mean square error (WMMSE) and penalty method and then solved in a block coordinate descent (BCD) manner. To reduce the computational complexity and avoid the requirement for full channel state information (CSI) in PWMMSE, a low-complexity heuristic beamforming approach is further proposed, in which the analog beamformer is first designed based on line-of-sight (LoS) channels and the digital beamformer is subsequently optimized using low-dimensional equivalent channels. Numerical results demonstrate that: 1) the proposed approaches can effectively mitigate the spatial wideband effect; 2) the proposed architectures achieve higher energy efficiency compared with true time delay (TTD)-based architectures; 3) the SC architecture achieves higher energy efficiency and exhibits superior robustness against both antenna array scaling and bandwidth expansion under a limited FTTD budget.