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Physical Layer Security for FAS-Aided Short-Packet Systems: A Variable Block-Correlation Approach

Jianchao Zheng, Tuo Wu, Kai-Kit Wong, Baiyang Liu, Runyu Pan, Maged Elkashlan, Kin-Fai Tong, Sumei Sun

Abstract

This paper presents a comprehensive physical layer security (PLS) framework for fluid antenna system (FAS)-aided short-packet communications under the variable block-correlation model (VBCM). We consider a downlink wiretap scenario in which a base station transmits confidential short packets to a legitimate receiver user (RU) in the presence of an eavesdropper user (EU), where both the RU and EU are equipped with fluid antennas. Unlike existing FAS security analyses that rely on constant block-correlation models or infinite-blocklength assumptions, we incorporate the VBCM to accurately capture the non-uniform spatial correlation structure inherent in practical FAS deployments. By employing a piecewise linear approximation of the decoding error probability and Gauss-Chebyshev quadrature, we derive closed-form and asymptotic expressions for the average achievable secrecy throughput (AAST). We further prove that the AAST is monotonically non-decreasing in the number of RU ports, which reduces the three-dimensional joint optimization of transmit power, blocklength, and port number to a two-dimensional grid search (GS). Numerical results demonstrate that the FAS-aided system achieves up to an order-of-magnitude secrecy throughput improvement over conventional fixed-position antenna systems, and reveal that blocklength selection is the most critical design parameter in the joint optimization.

Physical Layer Security for FAS-Aided Short-Packet Systems: A Variable Block-Correlation Approach

Abstract

This paper presents a comprehensive physical layer security (PLS) framework for fluid antenna system (FAS)-aided short-packet communications under the variable block-correlation model (VBCM). We consider a downlink wiretap scenario in which a base station transmits confidential short packets to a legitimate receiver user (RU) in the presence of an eavesdropper user (EU), where both the RU and EU are equipped with fluid antennas. Unlike existing FAS security analyses that rely on constant block-correlation models or infinite-blocklength assumptions, we incorporate the VBCM to accurately capture the non-uniform spatial correlation structure inherent in practical FAS deployments. By employing a piecewise linear approximation of the decoding error probability and Gauss-Chebyshev quadrature, we derive closed-form and asymptotic expressions for the average achievable secrecy throughput (AAST). We further prove that the AAST is monotonically non-decreasing in the number of RU ports, which reduces the three-dimensional joint optimization of transmit power, blocklength, and port number to a two-dimensional grid search (GS). Numerical results demonstrate that the FAS-aided system achieves up to an order-of-magnitude secrecy throughput improvement over conventional fixed-position antenna systems, and reveal that blocklength selection is the most critical design parameter in the joint optimization.
Paper Structure (16 sections, 3 theorems, 62 equations, 7 figures, 1 algorithm)

This paper contains 16 sections, 3 theorems, 62 equations, 7 figures, 1 algorithm.

Table of Contents

  1. Introduction
  2. System Model
  3. Signal Model
  4. FAS Channel Correlation Model
  5. Security Throughput of FAS Communication
  6. Achievable Secrecy Rate and Performance Metrics Under Finite Blocklength
  7. Variable Block-Correlation Model and Channel Statistics Distribution
  8. SNR Statistics Derivation
  9. Derivation of Secrecy Throughput
  10. Asymptotic AAST
  11. Optimization Problem Formulation and Algorithm Design
  12. FAS Security Optimization Problem
  13. Grid Search Optimization
  14. Theoretical Convergence Analysis
  15. Numerical Results and Analysis
  16. ...and 1 more sections

Key Result

Proposition 1

For fixed $P$ and $M$, the asymptotic AAST $\hat{T}(N_R, P, M)$ in q46 is monotonically non-decreasing in $N_R$.

Figures (7)

  • Figure 1: AAST versus SNR, where $m= 200$ bits, $N_R=20$, $N_E=10$, and $m= 300$ bits, $N_R=40$, $N_E=20$, respectively.
  • Figure 2: AAST vs Blocklength $M$, where $N_R=40$, $N_E=20$, and SNR =20 dB.
  • Figure 3: AAST vs transmit bits $m$, where SNR=20 dB, and $N_R=20$, $N_E=10$, and $N_R=40$, $N_E=20$, respectively.
  • Figure 4: AAST vs $N_R$, where $m= 300$ bits, and $N_E=20$.
  • Figure 5: FAS Security Optimization Under Varying Threat Scenarios: 3D Secrecy Throughput Surface Analysis vs $P$ and $N_R$ for Different Eavesdropper Capabilities ($N_E = 5, 15, 25$).
  • ...and 2 more figures

Theorems & Definitions (12)

  • Remark 1: FAS Diversity and Security Implications
  • Remark 2: Blocklength-Rate-Security Trade-off
  • Remark 3: High-SNR Behavior and Design Guidelines
  • Proposition 1: Monotonicity of $\hat{T}$ with respect to $N_R$
  • Proof 1
  • Corollary 1: Dimensionality Reduction
  • Remark 4: Problem Structure
  • Theorem 1: Grid Search Optimality
  • Proof 2
  • Remark 5: Practical Implementation Guidelines
  • ...and 2 more