Asymptotic Performance Analysis of FAS over Rayleigh Fading Channels

Fluid antenna technology dynamically optimizes the received signal by adjusting the antenna's position in a preset space, thereby achieving high spatial diversity gain and significantly enhancing the reliability of the communication system. This paper first investigates the relationship between the spatial diversity of the fluid antenna system (FAS) and their port configurations by deriving both approximate and asymptotic expressions for the outage probability and average bit error rate (ABER) in the high signal-to-noise ratio (SNR) regime. Furthermore, based on the optimal port selection mechanism, the impact of increasing the number of ports on the reliability of FAS is analyzed. To overcome the computational complexity of traditional high-dimensional integration methods, this paper introduces extreme value theory (EVT) to obtain the asymptotic distribution of the maximum channel gain. Then, the asymptotic expressions for the outage probability and ABER are derived as the number of ports tends to infinity. The results demonstrate that as the number of ports increases, both the outage probability and ABER exhibit an exponential decline, highlighting the significant advantages of FAS in improving reliability. In addition, based on the asymptotic result of the outage probability, the improvement in capacity resulting from the increased number of fluid antenna ports can be measured. Finally, numerical results validate our analytical findings.