A New Analytical Approximation of the Fluid Antenna System Channel

TL;DR

This paper introduces a new analytical approximation for the fluid antenna system channel, enabling more accurate and tractable performance analysis under realistic correlation models, and compares its gains to traditional multiple antenna systems.

Contribution

It develops a two-stage approximation method for the FAS channel, improving accuracy over previous models and facilitating performance evaluation under realistic conditions.

Findings

The approximation reduces complex integrals to a single integral form.

Numerical results validate the accuracy of the proposed channel model.

Limited performance gain of FAS observed under realistic correlation assumptions.

Abstract

Fluid antenna systems (FAS) are an emerging technology that promises a significant diversity gain even in the smallest spaces. Motivated by the groundbreaking potentials of liquid antennas, researchers in the wireless communication community are investigating a novel antenna system where a single antenna can freely switch positions along a small linear space to pick the strongest received signal. However, the FAS positions do not necessarily follow the ever-existing rule separating them by at least half the radiation wavelength. Previous work in the literature parameterized the channels of the FAS ports simply enough to provide a single-integral expression of the probability of outage and various insights on the achievable performance. Nevertheless, this channel model may not accurately capture the correlation between the ports, given by Jake's model. This work builds on the state-of-the-art and accurately approximates the FAS channel while maintaining analytical tractability. The approximation is performed in two stages. The first stage approximation considerably reduces the number of multi-fold integrals in the probability of outage expression, while the second stage approximation provides a single integral representation of the FAS probability of outage. Further, the performance of such innovative technology is investigated under a less-idealized correlation model. Numerical results validate our approximations of the FAS channel model and demonstrate a limited performance gain under realistic assumptions. Further, our work opens the door for future research to investigate scenarios in which the FAS provides a performance gain compared to the current multiple antennas solutions.