Analysis of Fluid Antenna Systems with Continuous Positioning and Spatial Correlation

TL;DR

This paper develops a level-crossing-rate framework to analyze the performance of multi-user fluid antenna systems with continuous positioning, providing asymptotically exact approximations and insights into outage reduction and interference mitigation.

Contribution

It introduces a novel LCR-based analytical approach for fluid antenna systems, extending to various channel models and receiver layouts, with practical insights for system design.

Findings

High-threshold tail probabilities scale linearly with antenna length L.

Approximately one wavelength of movement can reduce outage probability by three orders of magnitude.

Derived new LCR results for coupled correlation scenarios in multi-antenna layouts.

Abstract

We analyze multi-user fluid antenna systems with continuous positioning over a track of length L under a spatial correlation model, where exact performance distributions become analytically intractable. We develop a level-crossing-rate (LCR) framework that yields asymptotically exact approximations and tight bounds for the cumulative distribution function (cdf) of the optimized metric S* = sup_{0 <= l <= L}, where S(l) denotes the performance metric at antenna position l. For a single fluid antenna, we characterize the cdfs of signal-to-noise ratio (SNR), signal-to interference ratio (SIR) and signal-to-interference-plus-noise ratio (SINR) under Rayleigh fading and extend the approach to Ricean desired channels. We further treat two multi-antenna receiver layouts with maximum-ratio combining: (i) a fluid antenna with a fixed antenna and (ii) a two-element moving array, deriving new LCR results for the practically important case where array-element correlation and positional correlation are inherently coupled. The analysis provides actionable insights: high-threshold tail probabilities scale linearly with L, we derive the required L to neutralize a co-channel interferer, and we show that about one wavelength of movement can reduce outage by three orders of magnitude. Monte Carlo results validate the accuracy across the considered scenarios and regimes.