On physical-layer concepts and metrics in secure signal transmission

Communication secrecy in the wireless systems has unique challenges due to broadcasting nature of the radio waves, as compared to its wire-line counterpart. At the same time, different and independent perceptions of the transmitted signal by the legitimate receiver and the eavesdropper provide new opportunities for secure communication. The distinctness in the physical propagation environment, e.g., in received power, wireless channel, and location of the legitimate and illegitimate nodes, when coupled with random and unique signatures, can be exploited for secure communication without using secret keys. In this paper, fundamental stages as well as requirements of the physical layer (PHY) security in information transmission are reviewed from a novel perspective. Then, main performance metrics in secure communication are surveyed including from information theoretic measures to practical considerations along with associated generalizations. The presented comprehensive viewpoint of PHY security stages and metrics is helpful to better understand the techniques exploiting the physics to secure the information in the lowest layer of the communication system.