I. Introduction
As a complement to higher-layer security measures, physical layer security has emerged as a significant technique for security in the lowest layer of communication, i.e., the physical layer. Founded on information-theoretic security, which is built on classical Shannon’s notion of perfect secrecy, physical layer security can offer unbreakable security, unlike conventional secret-key-based cryptosystems. Physical layer security was laid in the 1970s in Wyner’s seminal work on the wiretap channel [2] where the idea of secure communication based on the communication channel itself without using encryption keys was first introduced. In this work, Wyner proved that in a wiretap channel (a channel in which a transmitter conveys information to a legitimate receiver in the presence of an eavesdropper) communication can be both robust to transmission errors (reliable) and confidential (secure), to a certain degree, provided that the legitimate user’s channel is better than the eavesdropper’s channel. He established the capacity of the degraded wiretap channel. Later, Csiszar and Korner [4] generalized this result to arbitrary, not necessarily degraded, wiretap channels.
Later in the 1990s, Maurer proved that secret key generation through public communication over an insecure yet authenticated channel is possible even when a legitimate user has a worse channel than an eavesdropper [3].