Enormous Fluid Antenna Systems (E-FAS) under Correlated Surface-Wave Leakage: Physical Layer Security

TL;DR

This paper analyzes the physical layer security of Enormous Fluid Antenna Systems (E-FAS) considering correlated surface-wave leakage, deriving closed-form expressions for secrecy outage probability and ergodic secrecy rate.

Contribution

It introduces a correlated surface-wave leakage model for E-FAS, providing a secrecy analysis framework with closed-form expressions and key insights on power allocation and routing gain effects.

Findings

Secrecy collapses at high power without artificial noise, but positive AN prevents this.

Optimal power split between data and AN is within the interior of feasible values.

Routing gain enhances signal strength and estimation, increasing the SNR ceiling.

Abstract

Enormous fluid antenna systems (E-FAS) have recently emerged as a surface-wave (SW)-enabled architecture that can induce controllable large-scale channel gains through guided electromagnetic routing. This paper develops a secrecy analysis framework for E-FAS-assisted downlink transmission with practical pilot-based channel estimation. We consider a multiple-input single-output (MISO) wiretap setting in which the base station (BS) performs minimum mean-square-error (MMSE) channel estimation and adopts maximum-ratio transmission (MRT) with artificial noise (AN). To capture the leakage of SW routing in EFAS, we introduce a correlated SW-leakage model that accounts for statistical coupling between the legitimate and eavesdropper channels caused by partially overlapping SW propagation paths. Exploiting the two-timescale nature-with slowly varying routing gain and small-scale block fading, we then derive a closed-form conditional expression for the secrecy outage probability (SOP) and a tractable characterization of the ergodic secrecy rate (ESR) in the presence of correlated quadratic forms. Our analysis yields three key insights: (i) secrecy collapses at high transmit power if and only if AN is not present, whereas any strictly positive AN can prevent asymptotic collapse; (ii) the optimal data-AN power split is achieved by a strictly interior solution; and (iii) routing gain improves both the received signal strength and the channelestimation quality, creating a nonlinear coupling that raises the signal-to-interference plus noise ratio (SINR) ceiling in the high signal-to-noise ratio (SNR) regime, and disperses secrecy across routing states. Numerical results indicate that E-FAS markedly enlarges the secure operating region significantly when compared with conventional space-wave transmission.