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; i) 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的安全传输框架以增强物理层安全性。在功率约束下，设计了连续电流分布（即波束赋形器）以最大化保密传输速率，并为达成特定目标保密速率而最小化所需传输功率。在此基础上，通过推导最大保密速率和最小需求功率的闭式表达式及其对应的最优电流分布，分析了连续孔径阵列所能达到的基本保密性能极限。为进一步揭示其内在机理，对最大保密速率和最小需求功率进行了渐近分析，结果表明：i) 对于最大保密速率，最优电流分布在低信噪比区域简化为最大比传输波束赋形，在高信噪比区域简化为迫零波束赋形；ii) 对于最小需求功率，最优电流分布在高信噪比区域简化为迫零波束赋形。所得结论进一步具体化至典型阵列结构，如平面连续孔径阵列与平面空间离散阵列。通过假设无限大连续孔径阵列，进一步分析了速率与功率的缩放规律。数值结果表明：i) 所提出的安全连续波束赋形设计在可达保密速率和功率效率方面均优于最大比传输与迫零波束赋形；ii) 与传统空间离散阵列相比，连续孔径阵列能实现更优的保密性能。