On the Performance of Physical Layer Security for Continuous-Aperture Array (CAPA) Systems

TL;DR

This paper introduces a CAPA-based framework for enhancing physical layer security, deriving fundamental limits and analyzing optimal beamforming strategies for different SNR regimes.

Contribution

It provides a novel continuous beamforming design for CAPA systems, deriving closed-form secrecy performance limits and analyzing asymptotic behaviors.

Findings

CAPA-based beamforming outperforms traditional methods in secrecy rate and power efficiency.

Optimal current distributions simplify to MRT and ZF beamforming in different SNR regimes.

CAPAs outperform SPDAs in secrecy performance.

Abstract

A continuous-aperture array (CAPA)-based secure transmission framework is proposed to enhance physical layer security. Continuous current distributions, or beamformers, are designed to maximize the secrecy transmission rate under a power constraint and to minimize the required transmission power for achieving a specific target secrecy rate. On this basis, the fundamental secrecy performance limits achieved by CAPAs are analyzed by deriving closed-form expressions for the maximum secrecy rate (MSR) and minimum required power (MRP), along with the corresponding optimal current distributions. To provide further insights, asymptotic analyses are performed for the MSR and MRP, which reveals that i) for the MSR, the optimal current distribution simplifies to maximal ratio transmission (MRT) beamforming in the low-SNR regime and to zero-forcing (ZF) beamforming in the high-SNR regime; ii) for the MRP, the optimal current distribution simplifies to ZF beamforming in the high-SNR regime. The derived results are specialized to the typical array structures, e.g., planar CAPAs and planar spatially discrete arrays (SPDAs). The rate and power scaling laws are further analyzed by assuming an infinitely large CAPA. Numerical results demonstrate that: i) the proposed secure continuous beamforming design outperforms MRT and ZF beamforming in terms of both achievable secrecy rate and power efficiency; ii) CAPAs achieve superior secrecy performance compared to conventional SPDAs.