In single-antenna fluid antenna systems (FASs), the transceiver dynamically selects the antenna port with the strongest instantaneous channel to enhance link reliability. However, deriving accurate yet tractable performance expressions under fully correlated fading remains challenging, primarily due to the absence of a closed-form distribution for the FAS channel. To address this gap, this paper develops a novel performance evaluation framework for FAS operating under fully correlated Rayleigh fading, by modeling the FAS channel through extreme value distributions (EVDs). We first justify the suitability of EVD modeling and approximate the FAS channel through the Gumbel distribution, with parameters expressed as functions of the number of ports and the antenna aperture size via the maximum likelihood (ML) criterion. Closed-form expressions for the outage probability (OP) and ergodic capacity (EC) are then derived. While the Gumbel model provides an excellent fit, minor deviations arise in the extreme-probability regions. To further improve accuracy, we extend the framework using the generalized extreme value (GEV) distribution and obtain closed-form OP and EC approximations based on ML-derived parameters. Simulation results confirm that the proposed GEV-based framework achieves superior accuracy over the Gumbel-based model, while both EVD-based approaches offer computationally efficient and analytically tractable tools for evaluating the performance of FAS under realistic correlated fading conditions.

翻译：在单天线流体天线系统（FAS）中，收发器动态选择具有最强瞬时信道的天线端口，以增强链路可靠性。然而，在完全相关衰落条件下推导精确且易处理的性能表达式仍具挑战性，这主要源于FAS信道缺乏闭合形式的分布。为弥补这一不足，本文通过极值分布（EVD）对FAS信道进行建模，提出了一种适用于完全相关瑞利衰落环境下FAS的创新型性能评估框架。我们首先论证了EVD建模的适用性，并利用Gumbel分布近似FAS信道，通过最大似然（ML）准则将分布参数表示为端口数和天线孔径尺寸的函数，进而推导出中断概率（OP）和遍历容量（EC）的闭合表达式。虽然Gumbel模型具有优良的拟合度，但在极端概率区域会出现微小偏差。为进一步提升精度，本文将该框架扩展至广义极值（GEV）分布，并基于ML估计参数获得了OP和EC的闭合近似表达式。仿真结果表明，所提出的基于GEV的框架在精度上优于Gumbel模型，而两种基于EVD的方法均为评估实际相关衰落条件下FAS性能提供了计算高效且分析易处理的工具。