The exponential growth in both mobile data traffic and the number of terminals connected to cellular networks calls for a future-ultra dense deployment of cellular networks. This growth will inevitably be limited by energy consumption of mobile terminals and, predominantly, the radio access networks.

While semiconductor processing speed has been increasing exponentially, doubling almost every two years according to the Moore's law, processor power consumption also continues to grow by 150% every two years [114]. By contrast, advances in battery technology have not kept pace, with capacity increasing at a modest rate of 10% every two years [114]. This leads to an increasingly large gap between power thirst and battery capacity. On the other hand, information and communication technology (ICT) plays an important role in global greenhouse gas emissions since the amount of energy consumed by ICT increases dramatically to meet the explosive growing service requirements. It is shown that nowadays the total energy used by the infrastructure of cellular networks, wired networks, and Internet takes up more than 3% of worldwide electric energy consumption [80]. Its CO2 emissions have increased to about 2% of worldwide CO2 emissions over the past decade [74]. More importantly, a large portion of energy consumption originates from the operation of wireless access networks. Indeed, base station energy consumption accounts for about 70% of the total energy used by mobile operators, which is still growing fast to accommodate the increasing need for mobile communication [91].