Secrecy Performance Analysis of RIS-Aided FAS in Satellite-Terrestrial RSMA Networks

To counteract channel uncertainties and eavesdropping threats in satellite-terrestrial systems, this letter investigates the physical-layer security in emerging fluid antenna systems (FAS) within reconfigurable intelligent surface (RIS)-aided satellite-terrestrial networks. The system employs rate-splitting multiple access (RSMA) and considers the presence of randomly located non-colluding eavesdroppers (Eves). We analyze the spatial correlation structure among ports in a planar FAS and derive the closed-form expressions for the end-to-end channel statistics of both legitimate users and Eves using the moment-matching method. Furthermore, compact analytical and asymptotic expressions for the secrecy outage probability (SOP) are derived leveraging numerical Gauss-Laguerre quadrature (GLQ) integration. Numerical results demonstrate that the integration of FAS and RIS significantly enhances secrecy performance. The proposed RSMA scheme is shown to outperform conventional multiple access schemes in mitigating the threat of randomly distributed Eves under imperfect channel state information (CSI) at the transmitter (CSIT).