In 5G networks, the number of connected devices, data rate and data volume per area, as well as the variety of QoS requirements, will attain unprecedented scales. The achievement of these goals will rely on new technologies and disruptive changes in network architecture and node design. Energy efficiency is believed to play a key role in complementing the 5G technologies and optimizing their deployment, dynamic configuration and management [1]. Within the framework of green communications and networks, especially for next-generation green cellular radio access networks, the GREAT (Green Cognitive Radio for Energy-Aware wireless communication Technologies evolution) initiative, a CominLabs Excellence Center (Laboratoire d'Excellence) and Université Européenne de Bretagne (UEB)project, has mainly addressed the fundamental issues of energy efficiency from various perspectives and angles, leveraging on cognitive techniques, at networking level as well as at thephysical layer level.