Fluid Antenna and RSMA Synergy: Boosting ISAC Network Security
The deployment of integrated sensing and communication (ISAC) systems poses new challenges to physical-layer security (PLS), as sensing waveforms directed toward targets may be exploited by eavesdroppers (Eves), causing information leakage. Meanwhile, security enhancement must jointly consider multi-user quality-of-service (QoS) and radar sensing performance, resulting in coupled and conflicting objectives. The intrinsic limitation arises from the rigid spatial constraints of conventional fixed-position antennas (FPAs) and the restricted stream-level interference management of non-rate-splitting frameworks, which collectively provide insufficient degrees of freedom (DoFs) to reconcile these competing requirements, especially under imperfect channel state information (CSI). To address these gaps, we propose a robust secure transmission framework that synergistically integrates the reconfigurable spatial DoFs enabled by fluid antennas (FAs) and the flexible stream-level DoFs provided by rate-splitting multiple access (RSMA). In this architecture, the RSMA common stream is repurposed to serve a dual role: it is decoded by legitimate users (LUs) for information delivery while also acting as a controlled jamming component to impair an Eve. We formulate a secure sum-rate maximization problem by jointly optimizing the base station beamformers and the FA positions at the LUs, subject to power budget, RSMA decoding, and minimum sensing requirements. An alternating-optimization (AO) framework is developed to address the coupled design under both perfect and imperfect CSI scenarios. In the imperfect-CSI case, semi-infinite constraints induced by bounded estimation errors are converted into linear matrix inequalities (LMIs) via the S-procedure. Simulation results show that the proposed FA-RSMA design achieves significant secure sum-rate gains and improved robustness over FPA and non-rate-splitting baselines.