  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.
