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.

翻译：本文提出了一种基于连续孔径阵列（CAPA）的安全传输框架以增强物理层安全。通过设计连续电流分布（即波束形成器），在功率约束下最大化保密传输速率，并在达到特定目标保密速率时最小化所需传输功率。基于此，通过推导最大保密速率（MSR）与最小所需功率（MRP）的闭式表达式及相应最优电流分布，分析了CAPA所能实现的基本保密性能极限。为提供更深层次见解，对MSR与MRP进行了渐近分析，揭示：i) 对于MSR，最优电流分布在低信噪比区域简化为最大比传输（MRT）波束成形，在高信噪比区域简化为迫零（ZF）波束成形；ii) 对于MRP，最优电流分布在高信噪比区域简化为ZF波束成形。将所得结果特化至典型阵列结构，例如平面CAPA与平面空间离散阵列（SPDA）。进一步通过假设无限大CAPA分析了速率与功率缩放规律。数值结果表明：i) 所提出的安全连续波束成形设计在可达保密速率与功率效率方面均优于MRT与ZF波束成形；ii) CAPA相比传统SPDA可获得更优的保密性能。