Beamforming Design for Solar-Energy-Harvesting Active RIS-Aided MIMO Systems

The integration of active reconfigurable intelligent surfaces (RIS) into wireless communication systems offers the potential for significant improvements in spectral efficiency and resource allocation through amplified reflection. However, their grid dependence introduces non-negligible hardware overhead, which compromises deployment flexibility. To address these challenges, this paper investigates an active RIS-aided multi-user transmission system powered by solar-harvested energy using rechargeable batteries. A joint design of transmit beamforming and active RIS matrix is developed to maximize the average sum rate while considering energy causality and battery storage limitations. To address the inherent non-convexity of the objective function coupled with bilateral battery power limitations, we recast the primal problem as a quadratically constrained quadratic problem and solve it using the alternating direction method of multipliers approach. Numerical results demonstrate that the proposed self-sustainable active RIS design outperforms conventional active RIS designs in terms of spectral efficiency, and energy sustainability, highlighting the potential of solar-energy-harvesting active RIS implementation.