Covert Beamforming Design for Active RIS-Assisted NOMA-ISAC Systems

In this paper, an active reconfigurable intelligent surface (RIS) is deployed to assist covert communications in a non-orthogonal multiple access (NOMA) inspired integrated sensing and communication (ISAC) system. With the aid of the active RIS, a dual-function base-station (BS) serves a public user and a covert user under the NOMA principles while sensing multiple targets by using the superimposed sensing and communication signal. To maximize the covert rate of the NOMA-inspired ISAC (NOMA-ISAC) system, transmission beamforming at BS and reflection beamforming at active RIS are jointly optimized subject to QoS requirements of the NOMA public user, sensing beampattern similarity constraint, and covertness level against the warden. To tackle the optimization variables coupled in the objective function and constraints, an alternating optimization algorithm is proposed, while the resulted non-convex fractional programming subproblems of transmission and reflection beamforming optimizations are solved by exploiting a penalized Dinkelbach transformation to obtain a high-quality rank-one solution. Numerical results demonstrate that the active RIS-assisted NOMA-ISAC scheme outperforms the passive RIS-assisted counterpart in terms of the covert rate and sensing beampattern similarity.