As the era of 5G networks has started to arise, several pertinent issues associated with the ambitious improvements that have to be achieved in future communications are attracting increasing research attention, including scalability, throughput, capacity, end-to-end delay, heterogeneity of requirements, and security. Nonetheless, 5G technologies should also become key enablers for the environmental sustainability of modern societies, as is happening in the case of other frameworks and initiatives such as Smart Cities, Smart Grids, and the Internet-of-Things. Hence, energy efficiency should be treated as an integral part of all proposed 5G solutions and technologies.

The goal of energy efficiency, apart from its ecological value, is also associated with the reduction of operational expenses for mobile network operators, as well as with greater customer satisfaction thanks to increased battery life. At present, however, it is not clear how 5G technologies, as they focus on higher bandwidths and are expected to be deployed as an overlay (rather than a replacement) on top of preexisting network equipment, could result in a reduction in power consumption.

In light of the above, this special issue is looking for original research papers as well as literature reviews that focus on energy efficiency and environmental sustainability in the context of 5G technologies in either of two ways: (i) technologies and solutions that may vastly improve or enhance the energy efficiency of 5G communications and networks; (ii) 5G concepts and technologies enabling significant improvements in energy efficiency in vertical markets, particularly in the context of Smart Cities.

Potential topics include but are not limited to the following:

• 5G system architecture
• Software-defined networking applications in 5G
• Network function virtualization techniques in 5G
• Flexible spectrum utilization and sharing technologies in 5G
• Cross-layer optimization techniques for 5G communications
• Massive MIMO techniques for 5G
• 5G terminal power consumption reduction
• End-to-end delay optimization techniques
• Energy efficient device-to-device communications
• Big data analytics for (Internet-of-Things) IoT applications
• Efficient machine learning and reasoning techniques in 5G
• Energy efficient communications for IoT
• Green communications
• 5G as an enabler for energy efficiency in Smart Cities