Integrated sensing and communication (ISAC) has been receiving a notable interest as an energy- and spectrum-efficient enabler for simultaneous communication and sensing. Notably, reconfigurable intelligent surfaces (RIS) is among the key technologies enabling robust communication and sensing, particularly in environments without a line-of-sight (LoS). Recently, a new type of RIS, called beyond-diagonal RIS (BD-RIS), has drawn attention, offering additional degrees of freedom in controlling the propagation medium. In this paper, a novel secure BD-RIS-aided ISAC scheme is proposed and evaluated. The scheme is applicable to a multi-user multi-target ISAC network, where a dual-functional radar-communication (DFRC) base station (BS) simultaneously serves multiple downlink users and senses various targets that aim to eavesdrop on the legitimate signal transmitted to the users. The presence of a BD-RIS enables circumventing the absence of the LoS link and ensures secure transmission and sensing. To this end, an optimization problem is formulated aiming at maximizing a weighted sum of per-target reflected powers, subject to secrecy and transmit power constraints. Thus, by virtue of an alternating optimization (AO)- and Riemannian conjugate gradient-based approach, local optima for the BD-RIS scattering matrix, transmit signal beamforming matrices, and artificial noise covariance matrix are obtained. Numerical results highlight (i) the notable sensing gains of the BD-RIS-aided design with respect to its diagonal RIS (D-RIS)-based baseline and (ii) the improved secrecy-sensing trade-off, whereby the BD-RIS can ensure an increasing system secrecy without degrading the per-target reflected power.

翻译：集成感知与通信（ISAC）作为一种节能且频谱高效的方案，在同时实现通信与感知方面引起了广泛关注。值得注意的是，可重构智能表面（RIS）是实现稳健通信与感知的关键技术之一，尤其是在无直射径（LoS）的环境中。近年来，一种新型RIS——超越对角RIS（BD-RIS）因其在调控传播介质方面提供额外自由度而受到关注。本文提出并评估了一种新型安全BD-RIS辅助的ISAC方案。该方案适用于多用户多目标ISAC网络，其中双功能雷达通信（DFRC）基站（BS）同时服务多个下行链路用户，并感知各种意图窃听发送给用户合法信号的目标。BD-RIS的存在使得规避LoS链路的缺失成为可能，并确保安全传输与感知。为此，构建了一个优化问题，旨在在满足保密性和发射功率约束的条件下，最大化目标反射功率的加权和。因此，基于交替优化（AO）和黎曼共轭梯度方法，获得了BD-RIS散射矩阵、发射信号波束成形矩阵以及人工噪声协方差矩阵的局部最优解。数值结果强调了（i）BD-RIS辅助设计相对于其对角RIS（D-RIS）基准方案带来的显著感知增益，以及（ii）改进的保密性-感知权衡，即BD-RIS能够在不降低每个目标反射功率的情况下确保系统保密性的提升。