Secure Transmission Design for Active RIS-Aided ISAC System With Discrete Phase Shifts and Colluding Eavesdroppers

Abstract

This letter investigates secure transmission in active reconfigurable intelligent surface (RIS)-assisted integrated sensing and communication (ISAC) systems. We consider a scenario with multiple colluding eavesdroppers (Eves), where a dual-functional ISAC base station (BS) simultaneously serves multiple legitimate users and tracks an aerial target—which may cooperate with the malicious Eves to intercept confidential information. To safeguard user communications, we formulate a joint design problem optimizing the BS transmit beamforming alongside the amplification factors and discrete phase shifts of the active RIS, aiming to maximize the worst-case secrecy rate among all users. To tackle the resulting non-convex optimization problem, an alternating optimization (AO) framework is proposed, which successfully decomposes the problem by leveraging semidefinite relaxation (SDR) and successive convex approximation (SCA) techniques. Simulation results validate the superiority of our proposed algorithm over existing benchmark schemes.