Enormous Fluid Antenna Systems (E-FAS)--Part II: Channel Estimation

TL;DR

This paper analyzes the impact of practical channel estimation on enormous fluid antenna systems (E-FAS), revealing SNR saturation and interference limitations, while demonstrating that E-FAS maintains significant advantages despite CSI imperfections.

Contribution

It provides the first comprehensive analysis of E-FAS with pilot-based channel estimation, deriving closed-form expressions and characterizing system performance under realistic conditions.

Findings

E-FAS exhibits SNR saturation due to residual self-interference.

Multiuser E-FAS becomes interference-limited at high SNR.

E-FAS maintains substantial performance gains despite CSI imperfections.

Abstract

Enormous fluid antenna systems (E-FAS) have recently emerged as a new wireless architecture in which intelligent metasurfaces act as guided electromagnetic interfaces, enabling surface-wave (SW) propagation with much lower attenuation and more control than conventional space-wave transmission. While prior work has reported substantial power gains under perfect channel state information (CSI), the impact of practical channel acquisition on E-FAS performance remains largely unexplored. This paper presents the first comprehensive analysis of E-FAS-assisted downlink transmission under pilot-based channel estimation. We develop an estimation framework for the equivalent end-to-end channel and derive closed-form expressions for the statistics of the minimum mean-square-error (MMSE) channel estimate and its estimation error. Building on these results, we analyze both single-user and multiuser operation while explicitly accounting for the training overhead. For the single-user case, we characterize the outage probability and achievable rate with imperfect CSI, and reveal an inherent signal-to-noise ratio (SNR) saturation phenomenon caused by residual self-interference. For the multiuser case, we study zero-forcing (ZF) precoding based on imperfect channel estimates and show that the system becomes interference-limited in the high SNR regime because of residual inter-user interference. Furthermore, we quantify the trade-off between spatial multiplexing gains and pilot overhead when the number of users increases. Analytical findings are validated via Monte Carlo simulations and benchmarked against least-squares (LS) estimation and conventional non-E-FAS transmission. The results reveal that despite CSI imperfections and training costs, E-FAS retains substantial performance advantages and provides robustness enabled by its amplified large-scale channel gain.