Beam Splitting Mitigation through Active RIS Design in Wideband THz Communications

Beam splitting (or beam squint) is a critical impairment in wideband reconfigurable intelligent surface (RIS)-aided terahertz (THz) systems, where subcarriers focus in different spatial directions, causing substantial loss in beamforming gain. Conventional passive RIS (PRIS) not only suffers from this effect but also experiences severe double-hop path loss due to multiplicative fading. Active RIS (ARIS), equipped with joint amplitude and phase control, can overcome the path loss limitation and further mitigate beam splitting. While true time delay (TTD) units have been studied as a potential remedy to mitigate beam splitting effect, their large insertion loss and hardware complexity hinder practical deployment. In this letter, we propose a joint amplitude-phase optimization framework for ARIS that maximizes the minimum array gain across all subcarriers under a transmit power constraint, without requiring TTD hardware. Numerical results demonstrate that the proposed ARIS design achieves a significantly flatter array-gain profile and effectively suppresses beam splitting. Moreover, it doubles the array gain compared to the conventional passive-RIS baseline, which co-phases the cascaded channel at the central frequency. A passive-RIS based setup is also implemented with the same objective of maximizing the minimum array gain across all subcarriers; however, it achieves 33% lower array gain at the extreme subcarriers than our ARIS design. Both setups operate under the same total power budget to ensure a fair comparison. Finally, the role of the power allocation factor, which governs the allocation of power between the BS and active-RIS is also analyzed. Simulation results further show that the proposed ARIS consistently achieves higher achievable rates than both the passive-RIS and baseline schemes under the same power budget.