Frequency-Tunable Ris for Beam Split Mitigation in Wideband Thz Massive MIMO Systems

The beam split effect, caused by frequencyindependent phase shifts in conventional hybrid beamforming, poses a significant challenge for wideband THz communication, undermining array gain and system performance. Despite the promise of THz frequencies for ultra-high data rates and abundant spectrum, their wideband nature exacerbates beamforming difficulties, necessitating innovative solutions to address these limitations. This paper introduces a novel frequency-tunable reconfigurable intelligent surface (FRIS) architecture that addresses beam split in angular-based hybrid beamforming THz systems. Drawing inspiration from graphene-based metasurfaces, the proposed FRIS is capable of shaping the propagation paths of different subcarriers independently, thereby ensuring consistent beam alignment over an ultra-wide bandwidth. Our design further leverages a reduced-complexity hybrid beamforming strategy at the transmitter, mitigating the hardware burden typically associated with fully digital solutions. Numerical evaluations reveal that the proposed architecture effectively mitigates beam split and achieves high spectral efficiency, providing a scalable and energy-efficient solution for future wideband THz communication systems.