Introduction

The rapid growth of mobile communications comes with the prominent energyconsumption challenge. It has become so critical that, without being dealt with in advance, it will eventually prevent the sustainable growth of the mobile industry [1]. Conventional treatments on the energy-efficiency study largely focus on the component and equipment level.

It is shown that novel architecture and advanced methods allow for significant improvement of the energy efficiency (EE) of wireless systems [2]. For this it is necessary to extend the study to the system/network level. Network energy-saving techniques tune the parameters and protocols of networks for interference mitigation, resource optimization, and energy saving. It is a prerequisite to understand key energy-consumption problems in a network. Cellular wireless access networks have been identified as the main consumer of energy in the wireless industry, while statistics show that radio base stations (RBS) in such a network consume most of the energy [2]. Various approaches have been proposed to reduce the energy consumption of an RBS, for instance, passive cooling techniques, energy-efficient backhaul solutions, and distributed base station design by using a remote radio head (RRH). The most promising approaches target the energy-consumption reduction of the power amplifier (PA) in an RBS since the PA dominates the energy consumption of a cellular RBS [3]. Due to limitations on cost and technology, the power efficiency of a PA used in recently developed RBS is less than 50%.