Active-RIS-Aided Covert Communications in NOMA-Inspired ISAC Systems

Non-orthogonal multiple access (NOMA)-inspired integrated sensing and communication (ISAC) facilitates spectrum sharing for radar sensing and NOMA communications, whereas facing privacy and security challenges due to open wireless propagation. In this paper, active reconfigurable intelligent surface (RIS) is employed to aid covert communications in NOMA-inspired ISAC wireless system with the aim of maximizing the covert rate. Specifically, a dual-function base-station (BS) transmits the superposition signal to sense multiple targets, while achieving covert and reliable communications for a pair of NOMA covert and public users, respectively, in the presence of a warden. To maximize the covert rate, the transmission beamforming at BS and reflection beamforming at active-RIS are jointly designed, subject to quality-of-service (QoS) requirements of NOMA public user, constraint of Cramér-Rao bound for multi-target estimations, and covertness level against warden. To this end, we develop the efficient methods involving the penalized Dinkelbach transformation to convert the non-concave covert rate maximization problem into the feasible one and propose an alternating optimization algorithm to obtain stationary solutions for the original problem. Numerical results demonstrate that active-RIS-aided NOMA-ISAC system outperforms the passive-RIS-aided and without- RIS counterparts in terms of covert rate and trade-off between covert communication and sensing performance metrics.