RIS-Assisted Wideband Beamforming for Near-Field Terahertz Systems

Reconfigurable intelligent surface (RIS)-assisted wideband terahertz (THz) communications are essential for achieving ultra-high data rates in sixth-generation (6G) networks. By adjusting the phase shifts of reflecting elements, reconfigurable intelligent surface (RIS) can effectively reshape wireless channels to enhance overall performance. However, two major challenges arise in RIS-assisted THz systems: 1) the dual beam split effect, where the large bandwidth causes subcarrier beam directions to diverge at both base station (BS) and RIS; and 2) the near-field effect, where the channel becomes a nonlinear function of both angle and distance. In this paper, we propose a novel beamforming technique, termed RIS-assisted wideband beamforming (RWB), to address these challenges and maximize data rates in RIS-assisted wideband THz systems. The RWB scheme leverages partially-connected true time delays (TTDs) and phase shifters (PSs) to generate frequency-dependent BS transmit beamforming vectors, while utilizing passive reflecting elements to control the frequency-invariant RIS reflect beamforming vector. By jointly optimizing the transmit and reflect beamforming vectors on the Riemannian manifold of unit-modulus phase shifts, RWB effectively mitigates both beam split and near-field effects. Numerical evaluations demonstrate that RWB achieves substantial data rate improvements over conventional wideband beamforming schemes.