In this paper, we investigate covert communications in a simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS)-aided rate-splitting multiple access (RSMA) system. Under the RSMA principles, the messages for the covert user (Bob) and public user (Grace) are converted to the common and private streams at the legitimate transmitter (Alice) to realize downlink transmissions, while the STAR-RIS is deployed not only to aid the public transmissions from Alice to Grace, but also to shield the covert transmissions from Alice to Bob against the warden (Willie). To characterize the covert performance of the considered STAR-RIS-aided RSMA (STAR-RIS-RSMA) system, we derive analytical expression for the minimum average detection error probability of Willie, based on which a covert rate maximization problem is formulated. To maximize Bob's covert rate while confusing Willie's monitoring, the transmit power allocation, common rate allocation, and STAR-RIS reflection/transmission beamforming are jointly optimized subject to Grace's quality of service (QoS) requirements. The non-convex covert rate maximization problem, consisting of highly coupled system parameters are decoupled into three sub-problems of transmit power allocation, common rate allocation, and STAR-RIS reflection/transmission beamforming, respectively. To obtain the rank-one constrained optimal solution for the sub-problem of optimizing the STAR-RIS reflection/transmission beamforming, a penalty-based successive convex approximation scheme is developed. Moreover, an alternative optimization (AO) algorithm is designed to determine the optimal solution for the sub-problem of optimizing the transmit power allocation, while the original problem is overall solved by a new AO algorithm.

翻译：本文研究了同时透射和反射可重构智能表面（STAR-RIS）辅助的速率分裂多址（RSMA）系统中的隐蔽通信问题。在RSMA原理下，隐蔽用户（Bob）和公共用户（Grace）的消息在合法发射机（Alice）处被转换为公共流和私有流以实现下行传输，而STAR-RIS不仅用于辅助Alice到Grace的公共传输，还用于屏蔽Alice到Bob的隐蔽传输，以防止被监管者（Willie）检测。为表征所研究的STAR-RIS辅助RSMA（STAR-RIS-RSMA）系统的隐蔽性能，我们推导了Willie的最小平均检测错误概率的解析表达式，并据此构建了一个隐蔽速率最大化问题。为了在混淆Willie监控的同时最大化Bob的隐蔽速率，我们联合优化了发射功率分配、公共速率分配以及STAR-RIS的反射/透射波束赋形，同时满足Grace的服务质量（QoS）要求。该非凸的隐蔽速率最大化问题包含高度耦合的系统参数，被分解为三个子问题，分别涉及发射功率分配、公共速率分配和STAR-RIS反射/透射波束赋形。针对优化STAR-RIS反射/透射波束赋形的子问题，我们提出了一种基于惩罚的逐次凸逼近方案，以获取秩一约束下的最优解。此外，设计了一种交替优化（AO）算法来确定优化发射功率分配子问题的最优解，而原始问题最终通过一种新的AO算法得到整体求解。