Optimization of Active Reflecting Elements and Relay Gain of Double Active RIS-Aided Communication with AF Relay

The Reconfigurable Intelligent Surface (RIS) has emerged as a new technology to control the transmission media and enhance the system performance. By controlling its induced phase shifts, the RIS can mitigate interference in multi-user communication scenarios. This paper theoretically evaluates the performance of an uplink scenario where two active RISs and an intermediate amplify-and-forward (AF) relay are deployed to aid the multi-user communication. We derive the optimal solution of the relay gain and the active reflection coefficients of the two RISs to maximize the sum-rate. Simulation results show that only the RIS close to the users has significant impact on the sum-rate maximization meanwhile the relay and the RIS close to the base station can only reduce noise and can not suppress interference.