Fluid Reconfigurable Intelligent Surface (FRIS) Enabling Secure Wireless Communications

The fluid reconfigurable intelligent surface (FRIS) advances the conventional RIS paradigm by enabling spatial reconfigurability of its reflecting elements. This letter investigates FRIS-assisted physical-layer security in a MISO system where a multi-antenna access point (AP) communicates with a legitimate user in the presence of an eavesdropper. Unlike RIS with fixed elements, FRIS dynamically selects an optimal subset of elements from a set of candidate locations to exploit spatial diversity. We aim to maximize the secrecy rate by jointly optimizing the AP’s transmit beamforming, FRIS element selection, and discrete phase shifts. The resulting problem is a mixed-integer nonlinear program (MINLP), which is NP-hard. We propose an alternating optimization (AO) framework where the beamforming is solved via the generalized eigenvalue method and the combinatorial subproblem of joint selection and phase design is addressed through cross-entropy optimization (CEO). Numerical results demonstrate that FRIS significantly outperforms conventional RIS and other baselines, highlighting the substantial secrecy gains provided by element positioning as a pivotal new degree of freedom (DoF).