Joint Beamforming and Deployment Optimization for Active STAR-RIS Enabled Covert Communications

This paper introduces a novel system architecture that integrates an active simultaneous transmitting and reflecting reconfigurable intelligent surface (aSTAR-RIS) with rate-splitting multiple access (RSMA) to achieve wireless covert communication in the presence of a multi-antenna warden. Departing from conventional approaches that deploy STAR-RIS at fixed positions, we treat the aSTAR-RIS location as a design variable to be optimized for maximizing the covert transmission rate. Our objective is to jointly optimize the aSTAR-RIS placement, transmit beamforming, transmission/reflection coefficients (TRCs), and common rate allocation so as to maximize both the common and private rates for the covert user. The resulting multi-variable optimization problem is addressed via an alternating optimization (AO) framework, which decomposes the problem into four tractable subproblems. For the multi-ratio fractional programming subproblem in the context of transmit beamforming and TRCs optimization, we adopt a Lagrangian dual formulation and quadratic transformation to reformulate the objective into a difference of convex (DC) form. The rank-one constrained beamforming design is then resolved using a penalized successive convex approximation (SCA) method combined with Gaussian randomization. A closed-form solution is derived for the common rate allocation subproblem, while the aSTAR-RIS location optimization is efficiently tackled via the SCA technique. Simulation results validate that the proposed scheme substantially enhances covert transmission rates while preserving communication covertness and guaranteeing reliable common signal reception. Importantly, the results highlight that optimizing the aSTAR-RIS location plays a critical role in improving the overall covert performance of the system.