Energy efficiency in cellular networks has received significant attention from both academia and industry because of the importance of reducing the operational expenditures and maintaining the profitability of cellular networks, in addition to making these networks “greener.” Because the base station is the primary energy consumer in the network, efforts have been made to study base station energy consumption and to find ways to improve energy efficiency. In this paper, we present a brief review of the techniques that have been used recently to improve energy efficiency, such as energy-efficient power amplifier techniques, time-domain techniques, cell switching, management of the physical layer through multiple-input multiple-output (MIMO) management, heterogeneous network architectures based on Micro-Pico-Femtocells, cell zooming, and relay techniques. In addition, this paper discusses the advantages and disadvantages of each technique to contribute to a better understanding of each of the techniques and thereby offer clear insights to researchers about how to choose the best ways to reduce energy consumption in future green radio networks. 1. Introduction In the past few years, the cellular network sector has developed rapidly. This rapid growth is due to the increases in the numbers of mobile subscribers, multimedia applications, and data rates. According to [1], the data transmission rate doubles by a factor of approximately ten every five years. Figure 1 shows how the number of subscribers in cellular networks have increased [2]. Figure 1: Growth forecasts for global HSPA and LTE subscribers, 2012–2017 [ 2]. The increase in the number of mobile subscribers has led to an increase in data traffic; as a result, the number of base stations (BSs) has increased to meet the needs of customers. Reference [3] describes the growth in the number of BSs in developing regions between 2007 and 2012, and forecasted that the total number of BSs would increase by over 2 million within this period. Most of the previous studies on this subject have focused on improving both system capacity and data rates, while neglecting the increasing demand of cellular networks for energy. This increasing energy demand has prompted considerable research on the subject of “green communications.” This paper discusses the issue of energy efficiency in communications networks. Perhaps the two most important reasons to pursue the development of green communications networks are increases in carbon dioxide emissions (CO2) and increases in operational expenditures (OPEX). CO2 emissions
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