FAS-RIS for V2X: Unlocking Realistic Performance Analysis With Finite Elements

The synergy of fluid antenna systems (FAS) and reconfigurable intelligent surfaces (RIS) promises robust vehicle-to-everything (V2X) links, yet most analyses invoke the central limit theorem (CLT) and thus fail to capture practical, finite-size deployments. This paper develops a realistic and tractable framework for FAS–RIS systems with a finite number of elements. We approximate the cascaded end-to-end gain via a Gamma distribution using moment matching, model spatial dependence across FAS ports with a block-correlation structure, and derive an accurate, closed-form approximation for the outage probability using Gauss–Chebyshev quadrature. Extensive simulations show that the proposed Gamma-based analysis markedly outperforms CLT-based baselines-especially for small numbers of RIS elements and ports-while converging to them as the array grows. The results provide actionable guidance for V2X design and dimensioning under practical constraints on RIS size, FAS aperture, and channel correlation.