An alternative or supplementary approach named as physical layer security has been proposed to afford an extra security layer on top of the conventional cryptography technique. In this paper, an overview of secrecy performance investigations over the classic Alice-Bob-Eve wiretap fading channels is conducted. On the basis of the classic wiretap channel model, we have comprehensively listed and thereafter compared the existing works on physical layer secrecy analysis considering the small-scale, large-scale, composite, and cascaded fading channel models. Exact secrecy metrics expressions, including secrecy outage probability (SOP), the probability of non-zero secrecy capacity (PNZ), average secrecy capacity (ASC), and secrecy bounds, including the lower bound of SOP and ergodic secrecy capacity, are presented. In order to encompass the aforementioned four kinds of fading channel models with a more generic and flexible distribution, the mixture gamma (MG), mixture of Gaussian (MoG), and Fox's $H$-function distributions are three useful candidates to largely include the above-mentioned four kinds of fading channel models. It is shown that they are flexible and general when assisting the secrecy analysis to obtain closed-form expressions. Their advantages and limitations are also highlighted. Conclusively, these three approaches are proven to provide a unified secrecy analysis framework and can cover all types of independent wiretap fading channel models. Apart from those, revisiting the existing secrecy enhancement techniques based on our system configuration, the on-off transmission scheme, jamming approach (including artificial noise (AN) & artificial fast fading (AFF)), antenna selection, and security region are presented.

中文翻译：

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用于通过窃听衰落信道进行信息理论保密性能分析的通用工具概述

已经提出了一种称为物理层安全的替代或补充方法，以在传统密码技术之上提供额外的安全层。在本文中，对经典 Alice-Bob-Eve 窃听衰落信道的保密性能调查进行了概述。在经典的窃听信道模型的基础上，我们综合列出并比较了现有的物理层保密分析工作，考虑了小规模、大规模、复合和级联衰落信道模型。给出了精确的保密度量表达式，包括保密中断概率 (SOP)、非零保密容量 (PNZ) 的概率、平均保密容量 (ASC) 和保密界限，包括 SOP 和遍历保密容量的下限。为了以更通用和更灵活的分布包含上述四种衰落信道模型，混合伽马 (MG)、混合高斯 (MoG) 和 Fox 的 $H$ 函数分布是三个有用的候选者，主要包括上述四种衰落信道模型。结果表明，它们在辅助保密分析以获得闭式表达式时具有灵活性和通用性。它们的优点和局限性也得到了强调。最后，这三种方法被证明提供了一个统一的保密分析框架，可以覆盖所有类型的独立窃听衰落信道模型。除此之外，根据我们的系统配置、开关传输方案、干扰方法（包括人工噪声 (AN) 和