Channel Capacity Enhancement for Terahertz Massive MIMO Systems Relying on Line-of-Sight RIS

This paper investigates channel capacity in the reconfigurable intelligent surfaces (RIS)-assisted multiple-input multiple-output (MIMO) systems operating in the terahertz (THz) band. Specifically, we conceive an evolved version of RIS, namely line-of-sight (LOS)-RIS, where the RIS element spacing is increased to embrace the near-field effect. With the LOS-RIS, the channel capacity can be significantly enhanced even under LOS-only channels. First, we analyze the channel capacity associated with the LOS-RIS as the RIS element spacing increases under single-user scenario, elucidating the underlying reasons for the enlarged spacing. Then, the analysis is extended to multi-user scenario, which unleashes the potential of utilizing a single LOS-RIS to serve multiple users under LOS-only channels. Furthermore, to accommodate multiple users, we propose a passive beamforming scheme, where the large LOS-RIS is partitioned into multiple subarrays and the phase shifts of each subarray are tailored to steer the reflected beams towards the corresponding user. Simulation results verify the effectiveness of the proposed paradigm, which paves the way for supporting multi-user multi-stream THz communications under LOS-only channels.