This paper studies an extremely large-scale reconfigurable intelligent surface (XL-RIS) empowered covert communication system in the near-field region. Alice covertly transmits messages to Bob with the assistance of the XL-RIS, while evading detection by Willie. To enhance the covert communication performance, we maximize the achievable covert rate by jointly optimizing the hybrid analog and digital beamformers at Alice, as well as the reflection coefficient matrix at the XL-RIS. An alternating optimization algorithm is proposed to solve the joint beamforming design problem. For the hybrid beamformer design, a semi-closed-form solution for fully digital beamformer is first obtained by a weighted minimum mean-square error based algorithm, then the baseband digital and analog beamformers at Alice are designed by approximating the fully digital beamformer via manifold optimization. For the XL-RIS's reflection coefficient matrix design, a low-complexity alternating direction method of multipliers based algorithm is proposed to address the challenge of large-scale variables and unit-modulus constraints. Numerical results unveil that i) the near-field communications can achieve a higher covert rate than the far-field covert communications in general, and still realize covert transmission even if Willie is located at the same direction as Bob and closer to the XL-RIS; ii) the proposed algorithm can enhance the covert rate significantly compared to the benchmark schemes; iii) the proposed algorithm leads to a beam diffraction pattern that can bypass Willie and achieve high-rate covert transmission to Bob.

翻译：本文研究了超大规模可重构智能表面（XL-RIS）在近场区域辅助的隐蔽通信系统。在XL-RIS的协助下，Alice向Bob隐蔽传输信息，同时避免被Willie检测到。为提升隐蔽通信性能，我们通过联合优化Alice处的混合模数波束赋形器以及XL-RIS处的反射系数矩阵，最大化可达隐蔽速率。提出了一种交替优化算法来求解联合波束赋形设计问题。对于混合波束赋形器设计，首先通过基于加权最小均方误差的算法获得全数字波束赋形器的半闭式解，然后通过流形优化近似全数字波束赋形器来设计Alice处的基带数字和模拟波束赋形器。对于XL-RIS的反射系数矩阵设计，提出了一种基于低复杂度交替方向乘子法的算法，以应对大变量规模和单位模约束的挑战。数值结果表明：（i）近场通信通常能实现比远场隐蔽通信更高的隐蔽速率，且即使Willie与Bob位于相同方向且更靠近XL-RIS，仍能实现隐蔽传输；（ii）所提算法相比基准方案能显著提升隐蔽速率；（iii）所提算法产生的波束衍射模式能绕过Willie，并向Bob实现高速隐蔽传输。