Driven by the applications of mobile internet and internet of things, the fifth-generation (5G) mobile communication system and network strive to meet the forthcoming challenging requirements under a diverse set of extreme use cases in the 2020s, characterized by extremely high data rates, high mobility, low latency, massive number of connections, and high traffic density, among others. Compared with the previous generations, 5G demands tremendously much higher energy efficiency and spectrum efficiency to provide immersed and tactile service to its users.

In order to fulfill the purpose of energy efficient and flexible spectrum utilization in 5G, innovative network architecture, spectrum access regimes, and physical technologies have been proposed in various aspects where evolutionary or even revolutionary solutions for 5G are under research and development. For example, large scale MIMO, full duplex, and ultradense small cells may ameliorate spectrum efficiency through refining usage of the limited spectrum resource.

To obtain further spectrum efficiency gain, cognitive radio and license assisted access concepts contribute a lot in bringing more spectrum opportunities in underutilized licensed/unlicensed frequency bands. With an aim to explore the much higher frequency bands, mmWave and THz communications benefit 5G with unprecedented potentials in extremely large bandwidth. In other words, refining the spectrum utilization with novel technologies, cultivating new access mechanism over the licensed or unlicensed frequency bands, and exploring the uncolonized radio spectrum play critical roles in paving the way to 5G era with effective energy efficiency, effective spectrum efficiency, and effective cost attributes.

The purpose of this special issue is to publish high-quality research papers as well as review articles addressing recent advances on energy efficient and flexible spectrum utilization in 5G system and networks. Original contributions that are not yet published or that are not currently under review by other journals or peer-reviewed conferences are sought.

Potential topics include, but are not limited to:

• Network or system level architecture design
• Ultradense network based energy efficient spectrum access
• Physical layer waveform and unified air interface design
• Nonorthogonal multiple access technologies
• Full duplex spectrum access technologies
• Energy efficient massive MIMO technologies
• Spatial spectrum usage technologies
• Flexible spectrum access mechanism and technologies
• Cognitive radio and LAA based spectrum access regime and technologies
• Licensed/unlicensed spectrum aggregation technologies
• Device to device communication technologies
• mmWave and THz communication
• Visible light communications