I. Introduction
Cognitive radio networks (CRNs) have been acknowledged as a paradigm for alleviating spectrum scarcity and improving spectrum utilization in next-generation wireless networks [1], [2], [3], [4], [5]. Consequently, the security and privacy of CRNs due to the broadcast nature of wireless media has emerged as an important issue. Conventional security techniques focus on the upper-level encryption, designed to construct and analyze protocols based on the information itself to prevent eavesdropping, such as cryptography approaches [6], [7], [8], [9]. However, the conventional cryptography approaches will be decrypted as the computing power of eavesdroppers improves, resulting in the inability to guarantee information security. Physical layer security (PLS) technology emerges as a promising one to prevent malicious eavesdropping attacks, and it has been widely studied as an attractive alternative to complement shortcomings of conventional cryptography approaches [10], [11], [12], [13].