Wideband Active RIS Design for Beam Squint Mitigation Under Hardware Impairments

Abstract

Wideband terahertz (THz) communication systems assisted by active reconfigurable intelligent surfaces (RIS) are susceptible to frequency dependent beam squint, which degrades the array gain at band edge subcarriers and reduces the achievable rate. In active RIS architectures, mitigating this effect requires the insertion of true time delay (TTD) units, along with the joint optimization of their delays and the amplification and phase of all active RIS elements, while accounting for practical hardware constraints including amplifier noise, output power limitations, and finite resolution delay elements. In this work, we propose a hardware aware wideband active RIS design that jointly optimizes the per element amplitude, phase, and TTD responses to mitigate beam squint under realistic hardware impairments. The resulting non convex optimization problem captures the coupling among amplification, hardware induced noise, and frequency dependent beamforming, and is efficiently solved using a projected gradient ascent algorithm with element wise feasibility projections. Numerical results demonstrate that the proposed design significantly improves the signal to interference plus noise ratio (SINR) uniformity across subcarriers and consistently achieves the highest achievable rate among the considered benchmark schemes, including a frequency flat max min array gain active RIS design that optimizes the worst case array gain without employing TTD elements. Furthermore, the proposed scheme consistently outperforms phase only passive RIS, active RIS without TTD elements, and quantized delay active RIS baselines across different transmit power levels and active RIS noise conditions, demonstrating its effectiveness and robustness for practical wideband THz communications.