Fluid antenna (FA) has become a highly promising technology and has recently been used to enhance the integrated sensing and communication (ISAC) system. However, the scenario where sensing targets act as eavesdroppers in ISAC and how to maximize the sum secrecy rate has not been addressed. This letter investigates secure transmission in FA-aided ISAC systems, where the spatial agility of FAs enables enhanced physical layer security. We jointly optimize antenna position vector (APV) and beamforming to maximize the multiuser sum secrecy rate, which complicates the solution process. To solve the resulting non-convex problem, we use a block successive upper-bound minimization (BSUM) algorithm, which incorporates the proximal distance algorithm (PDA) for closed-form beamformer updates and extrapolated projected gradient (EPG) for APV optimization. Simulation results show that the proposed FA-ISAC scheme achieves over 20$\%$ sum secrecy rate gain compared to fixed-position antenna (FPA) systems.

翻译：流体天线已成为一项极具前景的技术，近期已被用于增强集成感知与通信系统。然而，在ISAC中感知目标充当窃听者的场景以及如何最大化总保密速率的问题尚未得到解决。本文研究了流体天线辅助的ISAC系统中的安全传输，其中流体天线的空间灵活性能够增强物理层安全性。我们联合优化天线位置向量与波束成形以最大化多用户总保密速率，这使得求解过程变得复杂。为解决由此产生的非凸问题，我们采用块连续上界最小化算法，该算法结合了用于闭式波束成形器更新的近端距离算法以及用于APV优化的外推投影梯度法。仿真结果表明，与固定位置天线系统相比，所提出的FA-ISAC方案实现了超过20%的总保密速率增益。