Hybrid Beamforming for Wideband Terahertz Massive MIMO Communications with Low-Resolution Phase Shifters and True-Time-Delay

Hybrid beamforming has attracted considerable interest in the development of next-generation wireless communications operating in the terahertz (THz) band. The short wavelength of THz wave has enabled thousands of antennas to be tightly packed inside a small area to provide sharp and accurate beamforming. Unfortunately, the beam squint effect, which changes the direction of beams as the frequency shifts, can only be circumvented by introducing true-time-delay (TTD) lines in the hybrid beamformers. However, most previous works neglected the resolution limitation of the precoding hardware and assumed phase shifters (PSs) and TTDs with infinite resolution. This paper proposes two quantized hybrid beamforming algorithms for the PS-only hybrid architecture and the TTD-assisted hybrid beamformer. We use a gradient descent approach with differentiable soft quantization techniques to overcome quantizer discontinuity. The RF precoder and the baseband precoder are alternately updated to maximize the similarity with the optimal precoder. Simulation results demonstrate the effectiveness of the proposed algorithms. The proposed 3-bit PS-only beamformer can achieve near-optimal performance and outperform other low-resolution precoding methods in a moderate squint scenario. The proposed low-resolution TTD-assisted architecture can effectively compensate severe squint effect and achieves near-optimal performance.