This paper investigates the utilization of simultaneously transmitting and reflecting RIS (STAR-RIS) in supporting joint physical layer security (PLS) and covert communications (CCs) in a multi-antenna millimeter wave (mmWave) system, where the base station (BS) communicates with both covert and security users while defeating eavesdropping by wardens with the help of a STAR-RIS. Specifically, analytical derivations are performed to obtain the closed-form expression of warden's minimum detection error probability (DEP). Furthermore, the asymptotic result of the minimum DEP and the lower bound of the secure rates are derived, considering the practical assumption that BS only knows the statistical channel state information (CSI) between STAR-RIS and the wardens. Subsequently, an optimization problem is formulated with the aim of maximizing the average sum of the covert rate and the minimum secure rate while ensuring the covert requirement and quality of service (QoS) for legal users by jointly optimizing the active and passive beamformers. Due to the strong coupling among variables, an iterative algorithm based on the alternating strategy and the semi-definite relaxation (SDR) method is proposed to solve the non-convex optimization problem. Simulation results indicate that the performance of the proposed STAR-RIS-assisted scheme greatly surpasses that of the conventional RIS scheme, which validates the superiority of STAR-RIS in simultaneously implementing PLS and CCs.

翻译：本文研究了一种同时发射和反射的RIS（STAR-RIS）在多天线毫米波（mmWave）系统中支持联合物理层安全（PLS）与隐蔽通信（CCs）的应用场景。在该场景中，基站（BS）借助STAR-RIS与隐蔽用户和安全用户进行通信，同时抵御监护者的窃听。具体而言，通过解析推导获得了监护者最小检测错误概率（DEP）的闭式表达式。进一步，在仅假设基站已知STAR-RIS与监护者之间统计信道状态信息（CSI）的实用条件下，推导了最小DEP的渐近结果以及安全速率的下界。随后，建立了以最大化隐蔽速率与最小安全速率平均和为目标的优化问题，通过联合优化主动与被动波束成形器，在确保隐蔽约束与合法用户服务质量（QoS）的同时求解。针对变量间的高度耦合性，提出了一种基于交替策略与半定松弛（SDR）方法的迭代算法以求解该非凸优化问题。仿真结果表明，所提出的STAR-RIS辅助方案性能显著优于传统RIS方案，验证了STAR-RIS在同时实现PLS与CCs方面的优越性。