Physical layer security techniques for grant-free massive Machine-Type Communications in 5G and beyond: A survey, challenges, and future directions

The future of smart cities, industrial automation, and connected vehicles is heavily reliant on advanced communication technologies. These technologies, particularly massive Machine-Type Communication (mMTC), are the backbone of the many connected devices required for these applications. Grant -free access in 5G and beyond, while enhancing transmission efficiency by eliminating the need for permission requests, also introduces significant security risks. These risks, such as unauthorised access, data interception, and interference due to the absence of centralised control, are of paramount importance. Physical layer security (PLS) techniques, with their ability to exploit the unique properties of wireless channels to bolster communication security, offer a promising solution. This paper provides a comprehensive review of PLS techniques for securing grant-free mMTC, comparing different approaches and exploring the challenges of their integration. Our findings lay the groundwork for future research and the practical implementation of advanced security solutions in grant-free mMTC, a development that will also enhance the security of advanced 5G and 6G networks.