Deployment Optimization of Reconfigurable Intelligent Surface for Relay Systems

To harness the benefits of both relay and reconfigurable intelligent surface (RIS), we consider in this paper an RIS-aided wireless relaying system, in which a decode-and-forward relay (R) forwards data from a source (S) to a destination (D) with the assistance of an RIS. We first derive the closed-form of the upper bound of the achievable rate of the system. Under the homogeneous transmit power and noise, we derive the optimal RIS deployment in closed-form expressions to draw insights for practical design. Then, for general system setups, we formulate the achievable rate maximization problem by optimizing the RIS deployment. Though the problem is non-convex, we propose an solution to find the optimal RIS deployment and provide some mathematical and theoretical insights. It is unveiled that the optimal strategy is to deploy the RIS near the relay, rather than near S or D in the traditional RIS-aided systems without relay. The numerical results validate our theoretical analysis and show that the RIS and relay can complement each other, and the rate performance is significantly improved over the benchmark schemes.