Enhancing Covertness in Hybrid Active-Passive STAR-RIS-Based RSMA Systems

This study proposes an efficient approach for the robust covertness of hybrid active-passive simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS)-based rate splitting multiple access (RSMA) systems. Under Rician fading channels, we first derive the closed-form expression for the detection error probability (DEP) that an eavesdropper monitors proximate users' covert information, then establish the approximated optimal DEP for the worst-case scenario, and finally treat it as a strict constraint for system security design. Specifically, the non-convex optimization problem of determining the power allocation (PA) resource to maximize the covert rate is tackled by transforming it into two tractable sub-optimization problems. We exploit the analyses of the developed DEP to derive closed-form solutions for the PA coefficients in each sub-problem, which are solved using an iterative algorithm with relatively low computational complexity. Extending this for the general case of double covert signal transmission, we also formulate and address the problem of jointly optimizing the resource power and rate allocation to maximize the minimal transmission rates among users. Besides the efficacy of the proposed approaches, some key findings can be achieved as follows: 1) RSMA achieves a higher DEP than NOMA; 2) Optimizing the PA resource boosts the achievable covert rate up to 1.3 bits/s/Hz for all STAR-RIS modes when the transmit power is larger than 20 dBm; and 3) Using hybrid mode helps flexibly adjust the number of STAR-RIS elements to gain the best performance with active mode at high transmission power and passive mode at lower power.