Joint Beamforming and Reflection Design for Maximizing Energy Efficiency of RIS-Assisted ISAC Systems

Energy efficiency (EE) is an important metric of both the communication and sensing systems, which has not been studied in the reconfigurable intelligent surface (RIS) assisted integrated sensing and communication (ISAC) system with partial blocked direct links. In this paper, we consider the EE optimization of the RIS assisted ISAC system with multiple users and targets in a new and common scenario where the direct links between the base station (BS) and targets are blocked. Through analyzing the expression of the EE and signal relation, the EE maximization is modeled as an optimization problem with the constraints of sensing mutual information, transmit power of the BS, and the reflecting coefficients restrict of the passive RIS. The problem can be solved by finding out the optimal beamforming matrix and RIS phase shift. As the problem is a complex non-convex optimization problem and optimization variables are coupled, an effective approximation algorithm based on the alternating optimization is proposed to maximize the EE of the communication part. The original problem is first decomposed into two subproblems. And then, by using the sequential convex approximation and Dinkelbach algorithm, these two subproblems are alternately solved, and the beamforming matrix and the RIS phase shift are alternately optimized accordingly. The simulation results show that the proposed algorithm can converge to the optimized value rapidly and significantly improve the EE performance.