Analytical Insights Into the Average Block Error Rate of Fluid Antenna Systems Under Short-Packet Constraints

Abstract

Fluid antenna systems (FAS) offer immense spatial diversity within compact spaces. However, existing performance analyses predominantly rely on Shannon\'s capacity bounds, assuming infinite blocklength transmissions. To address the strict requirements of emerging Internet-of-Things (IoT) and ultra-reliable low-latency communications (URLLC), this paper investigates a point-to-point FAS under short-packet communications (SPC) constraints. We rigorously derive closed-form analytical expressions for the average block error rate (BLER), effective system throughput, and reliability. Furthermore, we conduct an asymptotic analysis in the high signal-to-noise ratio (SNR) regime, mathematically revealing that the system\'s diversity order is strictly determined by the rank of the spatial correlation matrix. Extensive Monte Carlo simulations validate the high accuracy of our proposed analytical models, demonstrating that increasing the number of fluid ports drastically reduces the BLER.