A survey on physical layer security for 5G/6G communications over different fading channels: Approaches, challenges, and future directions

The surge in wireless network attacks has intensified the focus on physical layer security (PLS) within academia and industry. As PLS provides security solutions by leveraging the randomness of wireless channels without the need for encryption/decryption keys, fading channels play a major role in PLS solutions. This survey aims to understand the effect of fading on PLS for 5G/6G communications by utilizing various PLS techniques such as beamforming, artificial noise injection, cooperative and opportunistic relaying, physical authentication, and intelligent reflective surface-based PLS over various fading channels. Initially, the role of PLS in 5G/6G communications, its fundamentals, and various techniques available for 5G/6G communications are examined. Since PLS for 5G communications has been extensively studied in the literature, we categorize it into two cases, direct and indirect communications, and provide a comprehensive survey on PLS for 5G communications over various fading channels. Thereafter, we survey the PLS for 6G communications over various fading channels, noting that the work available for PLS in 6G communications is limited and in its early stages. Given the increasing attention on artificial intelligence and machine learning (AI/ML) for wireless communications, this survey also explores PLS based on AI/ML techniques over various fading channels. Finally, the survey concludes with observations on challenges and future directions.