Joint Active and Passive Beamforming Design for Double-RIS-Assisted ISAC Systems

Abstract

Integrated sensing and communication (ISAC) enables wireless systems to support data transmission and target sensing over shared resources. However, the performance of both sensing and communication can be severely degraded by path loss and line-of-sight blockage. This letter investigates a functionally separated double-reconfigurable intelligent surface (RIS)-assisted ISAC system and formulates a joint active and passive beamforming problem to maximize the minimum radar output signal-to-interference-plus-noise ratio (SINR), subject to the communication rate constraint for each user, the transmit power constraint of the base station, and the phase shift constraints of each RIS. An alternating optimization framework is developed, where the transmit beamforming subproblem is solved via successive convex approximation, and the phase-shift subproblems are transformed into lifted semi-definite forms. Furthermore, the rank-one recovery issue arising from semi-definite relaxation is analyzed. Simulation results demonstrate that the proposed double-RIS scheme achieves a higher minimum radar output SINR than conventional schemes.