Fluid Antenna-Aided Robust Secure Transmission for RSMA-ISAC Systems

TL;DR

This paper tackles securing downlink RSMA-ISAC transmissions against eavesdropping by exploiting fluid antennas (FAs) to create movable spatial degrees of freedom. It formulates a secrecy-sum-rate maximization problem under perfect and imperfect CSI, and proposes an alternating-optimization framework that jointly optimizes beamformers and FA positions using SDP and SCA (perfect CSI) as well as the S-procedure for robust design (imperfect CSI). The key contributions are a novel secure RSMA-ISAC design with dual-use common streams, convexified AO algorithms, and comprehensive simulations showing substantial gains over fixed-position antennas (FPA) and SDMA baselines, plus robustness to CSI errors. The work demonstrates that FA flexibility combined with RSMA’s interference-management and secure-rate structure can significantly enhance both communication security and sensing performance in practical ISAC deployments.

Abstract

This paper leverages fluid antenna (FA) and rate-splitting multiple access (RSMA) to enhance the physical layer security (PLS) of an integrated sensing and communication (ISAC) system. We consider a practical multi-user multi-input single-output (MU-MISO) system, where a base station (BS) equipped with fixed position antennas (FPAs) employs RSMA to communicate with multiple single-FA users, while an eavesdropping target may potentially wiretap the signals. The system adopts a novel rate splitting (RS) scheme, where the common layer stream serves a dual purpose: it conveys valid data to legitimate users (LUs) while simultaneously generating jamming signals to confuse potential eavesdroppers. We establish the problem and propose the optimization algorithm under two conditions: perfect and imperfect channel state information (CSI) conditions. Specifically, under perfect the CSI condition, we address the non-convex optimization problem by proposing an alternating optimization (AO) algorithm, which decomposes the problem into two subproblems: beamforming matrix optimization and the adjustment of FA positions. For beamforming optimization, we utilize semidefinite programming (SDP) and successive convex approximation (SCA) to convert the problem into a more tractable convex form. Given a fixed beamforming matrix, SCA is applied to handle the surrogate upper bound of the constraints. In the case of imperfect CSI, the continuous nature of CSI errors leads to an infinite number of constraints. To overcome this challenge, we propose an AO-based algorithm that incorporates the S-Procedure and SCA to obtain a high-quality beamforming matrix and effective FA positions. Extensive simulation results demonstrate that the proposed FA-aided RSMA-ISAC system significantly enhances security compared to traditional FPA-based and SDMA-based systems.

Figures (7)

• Figure 1: System model of the FA-aided RSMA-ISAC for secure transmission.
• Figure 2: The convergence behavior under perfect and imperfect cases. $A_0=3\lambda$.
• Figure 3: Secure sum-rate versus the normalized region size with perfect CSI. $P_0=30\text{dBm}$, $N_T=K=4$.
• Figure 4: Secure sum-rate versus the power with perfect CSI. $A_0=3\lambda$, $N_T=K=4$.
• Figure 5: Secure sum-rate versus normalized region size with imperfect CSI. $P_0=30\text{dBm}$, $\hat{\epsilon}_k=0.01$.