Wireless communication networks have become a dominant force driving the development of the future world. With the large-scale commercialization of 5G, the global academia and industry have started envisioning the next-generation wireless networks (6G), which expect to implement the efficient and intelligent interconnection of human-machine-things in the physical world and create a ubiquitous and refined digital world, hence leading human life to a new era of deep integration of virtuality and reality. To realize this vision, all-around support for sophisticated functions, applications, and services is imminent, posing great challenges to the innovative improvements in the underlying (especially the physical layer) communication technologies. Energy consumption will become a huge burden on the way to the success of 6G.

On one hand, most IoE (Internet of Everything) devices will be either battery-powered or batteryless, so their communication performance is greatly constrained by intermittent energy supplies. On the other hand, the infrastructure will consume a huge amount of energy for powering enormous radio-frequency chains, maintaining satisfactory coverage, tracking mobile devices with super-narrow beams, etc. Therefore, renovating physical layer technologies and methodologies to enable communication sustainability constitutes the first challenge. Another main feature of 6G networks is to support multi-access environments where different types of wireless spectrum, including Sub-6 GHz, Millimeter-wave, and Terahertz, are utilized. Once wireless signals are not properly used and controlled, it will cause not only a waste of spectrum resources but also severe interference among concurrent transmissions, leading to a sharp decline in communication efficiency. Thus, how to develop advanced signal processing and interference management techniques to realize enhanced communication efficiency becomes the second challenge. Moreover, devices will seamlessly integrate with people's senses in the 6G era. Mountains of data transmitted over open wireless media contain human biometric information, home appliance control information, and even vehicle automatic driving control information. Malicious cyber activity may result in the loss of property or even human life. Cryptographic techniques and associated protocols are costly for small IoE devices and become less effective with the advances in quantum computing. Therefore, how to develop lightweight physical layer solutions to protect communication security is the third challenge.

This Special Issue is dedicated to a broad range of advanced physical-layer techniques and methods designed for improving communication sustainability, efficiency, and security in future wireless networks. We welcome original research and review articles.

Potential topics include but are not limited to the following:

• Physical layer approaches for green communications and computing
• Cognitive radio and spectrum sensing for green communications
• Age of information analysis and control in communications
• Artificial intelligence-based sustainable communications
• Computation offloading and service caching
• Signal processing for green communications, energy harvesting, and wireless power transmission
• Intelligent signal processing and interference management for next-generation wireless networks
• Intelligent reflecting surface-enhanced wireless communications
• Novel secure transmitting techniques and receiving design in interfering environments
• Physical layer security design for next-generation wireless networks
• Game theory for wireless physical layer security
• Cross-layer security mechanisms incorporating cryptography and physical layer aspects
• Secret-key generation and agreement over wireless channels
• Physical layer authentication
• Practical implementations of physical layer security techniques