# Energy Efficiency and Coverage Trade-Off in 5G for Eco-Friendly and Sustainable Cellular Networks
## 1 Introduction
### Motivation  
Environmental and energy concerns are core drivers of green technology.  
1. According to [13], the world's yearly electricity usage for the telecommunications sector is increasing and expected to reach 51% of global electricity in 2030 unless the electricity efficiency of wireless access networks and access/data center networks is sufficiently improved. CNs are considered the cardinal contributor to the appreciable rise in energy utilization in the telecommunication sector [14]. Figure 2 shows the expected global electricity usage of cellular networks based on different cellular generations to 2030. BSs are the major consumer of energy in CNs and account for 57% of the total energy consumed [15]. The number of BSs worldwide is increasing, which led OPEX to rise conspicuously because a greater percentage of the OPEX entails electricity bills [16].
2. CNOs contribute significantly to greenhouse gas (GHG) emissions. According to [17], the carbon dioxide (CO\({}_{2}\)) quantity emitted by the mobile sector is envisaged to increase to 179 MtCO\({}_{2}\) by 2020, which translates to 51% of the carbon footprint of the information and communication technology (ICT) sector.  
Many interesting studies [9; 10; 18; 19; 20; 21] have been conducted to find a \"greener\" cellular network and reduce OPEX. These studies can be categorized into two main approaches: first, enhance the energy efficiency of hardware [18]; and second, intelligent management of core network elements driven by dynamic traffic load fluctuations, such as switching BSs and cell zooming [22]. However, BS switching on/off is agreed to be a justifiable approach to improve the energy efficiency for the following reasons. (i) The approach of the improving energy efficiency of the hardware entails high implementation costs. Thus, network owners must meticulously consider the operational and economic dimensions of this approach before initiating hardware replacement [9; 18]. (ii) BS switching off/on schemes can be easily executed using the current network topology without hardware replacement. (iii) Out of the total energy consumption of CNs, BSs are responsible for 57% [9]. (iv) The infrastructures of CNs are fashioned to support daytime traffic. Diurnal traffic loads differ from nocturnal traffic loads. As a result, energy wastage arising because of inefficient use of resources cannot be overemphasized, especially for low traffic loads [23; 24]. (v) The BS switch-off approach is a system-level approach that operates in an area covered by multiple cells, where those cells may use different radio access technologies that offer various services.  
Figure 2: Expected case electricity usage of wireless access networks.  
In the philosophy of the BS switching on/off technique, the BSs monitor and exploit the traffic load of the cellular network to create a trade-off between network performance and EE. This can be implemented by a dynamic BSs switching on/off strategy motivated by the traffic demand conditions. In areas where there is low traffic, it is advisable that the BSs be switched off. However, with a minimum number of BSs switched on for service provisioning, users' coverage and the basic operations of the network can be supported.  
Operations of BSs switching on/off should adopt certain operational guidelines, which should be considered for execution and evaluation along CN. The typical questions that arise are as follows: (i) Which cells should be switched off and which cells should remain active? (ii) How can we manage the cooperation between BSs via a dynamic BSs switching off/on technique? (iii) What policies do we adopt for the BSs switching implementation? (iv) Parameter-wise, which are the key ones to be considered when executing the switch-off session?