Integrated sensing and communication (ISAC) systems have the issue of secrecy leakage when using the ISAC waveforms for sensing, thus posing a potential risk for eavesdropping. To address this problem, we propose to employ movable antennas (MAs) and reconfigurable intelligent surface (RIS) to enhance the physical layer security (PLS) performance of ISAC systems, where an eavesdropping target potentially wiretaps the signals transmitted by the base station (BS). To evaluate the synergistic performance gain provided by MAs and RIS, we formulate an optimization problem for maximizing the sum-rate of the users by jointly optimizing the transmit/receive beamformers of the BS, the reflection coefficients of the RIS, and the positions of MAs at communication users, subject to a minimum communication rate requirement for each user, a minimum radar sensing requirement, and a maximum secrecy leakage to the eavesdropping target. To solve this non-convex problem with highly coupled variables, a two-layer penalty-based algorithm is developed by updating the penalty parameter in the outer-layer iterations to achieve a trade-off between the optimality and feasibility of the solution. In the inner-layer iterations, the auxiliary variables are first obtained with semi-closed-form solutions using Lagrange duality. Then, the receive beamformer filter at the BS is optimized by solving a Rayleigh-quotient subproblem. Subsequently, the transmit beamformer matrix is obtained by solving a convex subproblem. Finally, the majorization-minimization (MM) algorithm is employed to optimize the RIS reflection coefficients and the positions of MAs. Extensive simulation results validate the considerable benefits of the proposed MAs-aided RIS-ISAC systems in enhancing security performance compared to traditional fixed position antenna (FPA)-based systems.

翻译：集成感知与通信（ISAC）系统在使用ISAC波形进行感知时存在信息泄露问题，从而带来潜在的窃听风险。为解决此问题，本文提出利用可移动天线（MAs）和可重构智能表面（RIS）来增强ISAC系统的物理层安全（PLS）性能，其中窃听目标可能截获基站（BS）发射的信号。为评估MAs与RIS提供的协同性能增益，我们构建了一个优化问题，通过联合优化基站的发射/接收波束成形器、RIS的反射系数以及通信用户处MAs的位置，在满足每个用户最低通信速率要求、最低雷达感知要求和对窃听目标最大保密泄露约束的前提下，最大化用户的和速率。为解决这一变量高度耦合的非凸问题，我们开发了一种基于惩罚的双层算法：外层迭代通过更新惩罚参数以实现解的最优性与可行性之间的权衡；内层迭代首先利用拉格朗日对偶性获得半闭式解的辅助变量，随后通过求解瑞利商子问题优化基站的接收波束成形滤波器，进而通过求解凸子问题获得发射波束成形矩阵，最后采用Majorization-Minimization（MM）算法优化RIS反射系数与MAs位置。大量仿真结果表明，与传统基于固定位置天线（FPA）的系统相比，所提出的MAs辅助RIS-ISAC系统在安全性能提升方面具有显著优势。