STAR-RIS-Aided Covert Communications in MISO-RSMA Systems

In this paper, a simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) is deployed to aid covert communications in a multiple-input single-output (MISO) rate-splitting multiple access (RSMA) system. To maximize the covert communication rate, which is characterized by the shared common rate and private rate of the covert user, the transmit beamforming, reflection/refraction beamforming, and common rate allocation need to be jointly optimized. By decoupling the original covert communication rate maximization problem into three sub-problems, an alternating optimization (AO) algorithm is designed to obtain the optimized solution to achieve the maximum covert communication rate. For the sub-problem of optimizing the common rate allocation, a closed-form expression is derived for the optimal common rate allocation. For the multiple-ratio fractional programming sub-problems of optimizing the transmit beamforming and reflection/refraction beamforming, Lagrangian dual formulation and quadratic transformation are utilized to reconstruct the objective function in the form difference of convex functions. Then, a penalized successive convex approximation is utilized to tackle the rank-one constrained beamforming optimization. Simulation results clarify the effectiveness of the proposed AO algorithm to achieve the maximum covert communication rate. Compared to the benchmark scheme in which the common rate allocation is missing, it is verified by simulation results that the STAR-RIS-aided MISO-RSMA scheme effectively increases the covert communication rate.