Exploiting RIS in Secure Beamforming Design for NOMA-Assisted Integrated Sensing and Communication

The integration of non-orthogonal multiple access (NOMA) with integrated sensing and communication (ISAC) heralds a novel and promising paradigm, advancing the frontier of wireless communication technologies. Nevertheless, this synergistic integration may confront security challenges attributable to intrinsic vulnerabilities within ISAC frameworks, i.e., sensing targets assume the role of potential eavesdroppers. In this paper, we exploit the additional degrees of freedom afforded by reconfigurable intelligent surfaces (RIS) to enhance secure communication and achieve target detection within the NOMA-assisted ISAC system. Specifically, the coexisting radar and NOMA signals are concurrently propagated through both direct and reflected links. Subsequently, we formulate a secure optimization problem by collaboratively designing the beamforming vectors of the base station and the phase shifts of RIS under the constraints of total transmit power, communication quality of service, and sensing quality. Due to nonconvexity, the optimization problem is decomposed into two sub-problems and addressed separately, employing the successive convex approximation approach based on the first-order Taylor expansion and the second-order cone. Finally, we obtain the solution to the original problem utilizing alternating optimization. Simulation results demonstrate that our proposed approach exhibits superior capabilities in secure communication and precise target detection.