Transmission Design for Double Cooperative Active RIS-Aided Communication

Reconfigurable intelligent surfaces (RISs) have emerged as a disruptive technology that can reconfigure wireless communication environments cost-effectively. In order to fully unveil the potential of RIS-aided wireless communications, some existing contributions considered the double cooperative passive RISs to achieve a higher capacity scaling orders. However, due to the multiplicative fading effect, the double cooperative passive RISs performs poorly when deployed far from the BS and user respectively. To address this issue, we investigate double cooperative active RISs which are equipped with amplifiers and can overcome the severe path loss caused by the multiplicative fading. Specifically, we aim to maximize the downlink achievable rate subject to the transmit power constraints of the base station (BS) and the double active RISs. The formulated problem is solved by using an alternating optimization (AO) algorithm based on the majorization-minimization (MM) algorithm and the fractional programming (FP) method. Simulation results demonstrate that much better rate performance can be achieved by adopting active RIS compared to passive RIS in the double RIS-aided systems. Meanwhile, deploying them appropriately far away from the BS and user and more elements assigned to the active RIS near the user will achieve better performance.