# Energy Efficiency and Coverage Trade-Off in 5G for Eco-Friendly and Sustainable Cellular Networks
## 1 Introduction  
Cellular networks are one of the most important technological innovations witnessed in recent times, improving the quality of human life and contributing to economic growth. Recently, cellular networks have developed tremendously to offer video streaming data, which explains the rapid growth in mobile subscribers [1]. Mobile subscriptions grew approximately 3% between 2017 and 2018 to reach 7.9 billion in Q3 2018, while mobile data traffic grew by approximately 79% in the same period to reach approximately 20.7 ExaBytes (EB) per month in Q3 2018 [2]. Moreover, the number of mobile subscribers will increase exponentially due to continuous development of cellular networks, the advent of the fifth generation (5G) of cellular networks, and the Internet of Things (IoT), which is considered the backbone of emerging applications, moving the telecommunications forward to contribute to quality of life and grow the world's economy [3]. By 2024, mobile subscriptions are expected to reach 8.9 billion; meanwhile, cellular IoT connections will reach 4.1 billion, with mobile data traffic using approximately up to 136 EB per month, wherein 74% will be utilized for mobile video traffic, and 5G networks will carry 25% of global mobile data traffic [2]. Figure 1 presents the growth of global mobile subscribers and mobile data traffic based on cellular generations.  
The 5G technology is expected to address some of the critical requirements of network issues because this technology will not only provide personal mobile service but also massive machine communications [4]. However, 5G is not intended to replace the legacy of cellular networks but rather to enhance and support it to provide superior performance of cellular networks (CNs) [5]. Thus, the number of base stations, considered the main source of cellular networks energy utilization, will increase considerably [6]. Judging from the cellular network operators (CNOs) context, energy efficiency (EE) is of paramount interest to CNOs based on its significant economic and ecological influence in the coming years. Therefore, a novel research discipline termed \"green communication\" has emerged [7]. What actually constitutes \"green communication\" is the first question that needs attention. Secondly, is it measurable, and if so, how do we define the degree of \"greenness\" in CNs? It is plausible to use carbon footprint or CO\({}_{2}\) emissions as a measure of \"greenness,\" but CNs' carbon emissions are negligible. Apart from the carbon emissions issue, there are other drivers of \"green\" wireless technology that include financial gains (lower energy costs). With the adoption of economic gains in the \"green\" concept, energy savings or energy efficiency appears to be a more appropriate way to measure \"greenness.\" Hence, the concept of \"green communication\" technology in wireless  
Figure 1: Expected growth of global mobile subscribers and data traffic [2]. (**a**) Expected growth of global mobile subscribers and (**b**) expected traffic growth and the percentage of each type of data.  
systems can be made meaningful with a comprehensive evaluation of energy savings and performance in a practical system [8]. Thus, the goal of green communication is to enhance the EE of BSs, decrease OPEX, and eradicate greenhouse gases. For further information on green wireless communications network, see [9; 10; 11; 12].