  Physical layer security is a promising approach that can benefit traditional
encryption methods. The idea of physical layer security is to take advantage of
the features of the propagation medium and its impairments to ensure secure
communication in the physical layer. This work introduces a comprehensive
review of the main information-theoretic metrics used to measure the secrecy
performance in physical layer security. Furthermore, a theoretical framework
related to the most commonly used physical layer security techniques to improve
the secrecy performance is provided. Finally, our work surveys physical layer
security research over several enabling 5G technologies, such as massive
multiple-input multiple-output, millimeter-wave communications, heterogeneous
networks, non-orthogonal multiple access, and full-duplex. Also, we include the
key concepts of each of the aforementioned technologies. Future fields of
research and technical challenges of physical layer security are also
identified.
