Performance Analysis of Wireless Communication Systems Assisted by Fluid Reconfigurable Intelligent Surfaces

This letter investigates the performance of emerging wireless communication systems assisted by a fluid reconfigurable intelligent surface (FRIS). Different from the conventional reconfigurable intelligent surfaces (RISs), an FRIS consists of ‘fluid’-inspired metamaterials arranged in a densely packed matrix of sub-elements over a surface. It dynamically activates particular elements for signal reflection and modulation based on real-time channel conditions. Considering a downlink scenario in which a base station (BS) transmits to a mobile user (MU) terminal via an FRIS, we first characterize the statistical behavior of the equivalent end-to-end channel by deriving closed-form approximations for its cumulative distribution and probability density functions. Using these expressions, an analytical approximation for the outage probability and a tight upper bound on the ergodic capacity, including their asymptotic behaviors for high signal-to-noise ratio values, are derived. Our findings reveal key performance trends demonstrating that FRIS can substantially improve link reliability and spectral efficiency compared to conventional RISs, owing to its capability to dynamically select optimal elements from a dense preconfigured grid.