Joint Beamforming and Reflection Design for Secure RIS-ISAC Systems

This correspondence investigates a secure reconfigurable intelligent surface (RIS) aided integrated sensing and communication (ISAC) system where a base station (BS) simultaneously performs multi-user multi-input single-output (MU-MISO) secure communication and detects a malicious radar target. An RIS is leveraged to assist legitimate wireless communication. Dedicated sensing signal is transmitted with communication signal to enhance the sensing performance while preventing information leakage. The radar output signal-to-noise ratio (SNR) can be improved by jointly optimizing the transmit and reflection beamforming, and radar receive filter, while satisfying the communication performance requirement of each user, the secure transmission constraint, the power budget constraint, as well as the RIS reflecting coefficients restrict. An alternating optimization method is applied to decompose the non-convex multivariate coupling problem into three sub-problems. We then utilize semi-definite relaxation (SDR), fractional programming (FP), and majorization-minimization (MM) algorithms to solve the sub-problems iteratively. Simulation results reveal that the proposed secure RIS-ISAC scheme can offer 2 dB radar performance gain compared to the scheme without RIS.