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Enormous Fluid Antenna Systems (E-FAS) for Multiuser MIMO: Channel Modeling and Analysis

Farshad Rostami Ghadi, Kai-Kit Wong, Masoud Kaveh, Wee Kiat New, Chan-Byoung Chae, Lajos Hanzo

TL;DR

The paper addresses analytical performance characterization of Enormous Fluid Antenna Systems (E-FAS) for MU-MIMO downlinks by integrating surface-wave propagation along engineered surfaces with conventional small-scale fading. It develops a physics-consistent end-to-end channel model and proves the effective BS-UE channel is Gaussian with enhanced variance $\\Omega_{\\text{eq}}$ that captures cylindrical surface-wave attenuation. It provides closed-form outage probability and ergodic capacity for the single-user case and analyzes ZF precoding for the multiuser case, deriving a Gamma-distributed post-ZF SNR and a tractable ergodic sum-rate approximation. Monte-Carlo simulations validate the model and show substantial gains over direct-space-wave propagation, establishing E-FAS as a power-efficient, controllable propagation framework with practical benchmarking implications.

Abstract

Enormous fluid antenna systems (E-FAS), the system concept that utilizes position reconfigurability in the large scale, have emerged as a new architectural paradigm where intelligent surfaces are repurposed from passive smart reflectors into multi-functional electromagnetic (EM) interfaces that can route guided surface waves over walls, ceilings, and building facades, as well as emit space waves to target receivers. This expanded functionality introduces a new mode of signal propagation, enabling new forms of wireless communication. In this paper, we provide an analytical performance characterization of an E-FAS-enabled wireless link. We first develop a physics-consistent end-to-end channel model that couples a surface-impedance wave formulation with small-scale fading on both the base station (BS)-surface and launcher-user segments. We illustrate that the resulting effective BS-user channel remains circularly symmetric complex Gaussian, with an enhanced average power that explicitly captures surface-wave attenuation and junction losses. For single-user cases with linear precoding, we derive the outage probability and ergodic capacity in closed forms, together with high signal-to-noise ratio (SNR) asymptotics that quantify the gain of E-FAS over purely space-wave propagation. For the multiuser case with zero-forcing (ZF) precoding, we derive the distribution of the signal-to-interference-plus-noise ratio (SINR) and obtain tractable approximations for the ergodic sum-rate, explicitly revealing how the E-FAS macro-gain interacts with the BS spatial degrees of freedom (DoF). In summary, our analysis shows that E-FAS preserves the diversity order dictated by small-scale fading while improving the coding gain enabled by cylindrical surface-wave propagation.

Enormous Fluid Antenna Systems (E-FAS) for Multiuser MIMO: Channel Modeling and Analysis

TL;DR

The paper addresses analytical performance characterization of Enormous Fluid Antenna Systems (E-FAS) for MU-MIMO downlinks by integrating surface-wave propagation along engineered surfaces with conventional small-scale fading. It develops a physics-consistent end-to-end channel model and proves the effective BS-UE channel is Gaussian with enhanced variance that captures cylindrical surface-wave attenuation. It provides closed-form outage probability and ergodic capacity for the single-user case and analyzes ZF precoding for the multiuser case, deriving a Gamma-distributed post-ZF SNR and a tractable ergodic sum-rate approximation. Monte-Carlo simulations validate the model and show substantial gains over direct-space-wave propagation, establishing E-FAS as a power-efficient, controllable propagation framework with practical benchmarking implications.

Abstract

Enormous fluid antenna systems (E-FAS), the system concept that utilizes position reconfigurability in the large scale, have emerged as a new architectural paradigm where intelligent surfaces are repurposed from passive smart reflectors into multi-functional electromagnetic (EM) interfaces that can route guided surface waves over walls, ceilings, and building facades, as well as emit space waves to target receivers. This expanded functionality introduces a new mode of signal propagation, enabling new forms of wireless communication. In this paper, we provide an analytical performance characterization of an E-FAS-enabled wireless link. We first develop a physics-consistent end-to-end channel model that couples a surface-impedance wave formulation with small-scale fading on both the base station (BS)-surface and launcher-user segments. We illustrate that the resulting effective BS-user channel remains circularly symmetric complex Gaussian, with an enhanced average power that explicitly captures surface-wave attenuation and junction losses. For single-user cases with linear precoding, we derive the outage probability and ergodic capacity in closed forms, together with high signal-to-noise ratio (SNR) asymptotics that quantify the gain of E-FAS over purely space-wave propagation. For the multiuser case with zero-forcing (ZF) precoding, we derive the distribution of the signal-to-interference-plus-noise ratio (SINR) and obtain tractable approximations for the ergodic sum-rate, explicitly revealing how the E-FAS macro-gain interacts with the BS spatial degrees of freedom (DoF). In summary, our analysis shows that E-FAS preserves the diversity order dictated by small-scale fading while improving the coding gain enabled by cylindrical surface-wave propagation.
Paper Structure (29 sections, 5 theorems, 52 equations, 7 figures, 2 tables)

This paper contains 29 sections, 5 theorems, 52 equations, 7 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Background
  3. Reconfigurable Antenna-aided Communications
  4. Motivation and Contributions
  5. Organization and Notations
  6. System and Channel Model
  7. Transmit Signal Model
  8. BS-to-Surface Excitation
  9. Surface Wave Propagation
  10. Launcher Processing and Noise
  11. Launcher-to-UE and Direct BS-to-UE Channels
  12. End-to-End Received Signal Model
  13. End-to-End Channel Statistics
  14. Scalar Effective Channel
  15. Single-User Case and Precoder Model
  16. ...and 14 more sections

Key Result

Lemma 1

Define $\mathbf{H}_{\mathrm{BS\textnormal{-}sur}}=\sqrt{\beta_\mathrm{BS}}\,\mathbf{G}_{\mathrm{BS\textnormal{-}sur}}$ with independent $\mathcal{CN}(0,1)$ entries, and let $\mathbf{h}_{\mathrm{relay\textnormal{-}UE},u}=\sqrt{\beta_{{\rm LU},u}}\,\mathbf{g}_{\mathrm{relay\textnormal{-}UE},u}$ with i where

Figures (7)

  • Figure 1: A downlink E-FAS-aided environment model with an $M$-antenna BS communicating to $K$ single-antenna UEs.
  • Figure 2: Single-user outage probability versus SNR with different surface wave gains $\Omega_{\mathrm{sw}}$.
  • Figure 3: Single-user ergodic capacity versus SNR with different surface wave gains $\Omega_{\mathrm{sw}}$.
  • Figure 4: Statistical characterization of the per-user ZF SINR in the E-FAS-assisted multiuser downlink: (a) PDF and (b) CDF.
  • Figure 5: ZF sum-rate versus SNR for the multiuser downlink with $M=16$ and $K=4$, with different surface wave gains $\Omega_{\mathrm{sw}}$.
  • ...and 2 more figures

Theorems & Definitions (5)

  • Lemma 1
  • Theorem 1: Equivalent Channel Distribution
  • Theorem 2: Outage Probability
  • Theorem 3: Ergodic Capacity
  • Theorem 4: ZF SNR Distribution