## 1 Introduction  
In the last two decades, there has been a growing demand for Base Transceiver Stations (BTSs) due to the development of mobile communication networks with smaller cells and BTSs closer to the users. From the network operator (NO) point of view, BTSs are the main source of energy consumption. A decade ago, virtually 60% of the energy consumption of mobile phone operators was directly attributed to the equipment installed in the BTSs [1]. However, with the advent of the fourth (4G) or Long-Term Evolution (LTE) and fifth generation (5G) networks, the amount of traffic volume in the mobile networks has considerably grown, which has led to an increase in the total energy consumption, and therefore, in the carbon footprint generated. Moreover, 5G networks require between two and three times the number of BTSs compared to those installed for the legacy mobile generations [2]. Although 5G has been designed to be more energy efficient than the previous generations [3], the deployment of BTSs for 5G will increase the energy consumption by 5% [4]. As a result, the costs in terms of capital expenditure (CapEx) and operational expenditure (OpEx) will steadily rise. Enhancing the energy efficiency of telecommunication networks becomes a significant contribution when it comes to fighting global warming. However, in the context of rapidly rising energy prices, it is also creating economic opportunities [5]. Currently, energy consumption is regarded as an important performance indicator of the equipment comprising the BTSs [6]. This is stated in the European Telecommunications Standards Institute (ETSI) standard [7] and in 3 GPP (3 rd Generation Partnership Project) specifications [8]. For many years in the design of mobile networks, the focus has always been on increasing the user throughput and the service provider capacity, without taking energy efficiency and environmental impact into consideration. However, the last decade has seen a change of emphasis and improving energy efficiency within BTSs has become strategically vital, not only for financial reasons, reducing costs, but also for developing a more self-sustainable network of BTSs, which clearly has a positive impact on the corporate brand image of the network operators (NOs). Several projects, such as EARTH [9], ECONET [10], co-funded by the European Commission under the Framework Programme 7 (FP7), and GreenTouch [11] and 5 GrEEn [12], have been carried out, which shows the growing public and private concern about this field. Finally, OPERA-Net and OPERA-Net 2 [13] are other projects which stand out in the field of energy efficiency and low-cost implementation for Mobile Radio Access Networks. In the literature, two approaches have mainly been put forward to address this problem: (i) energy saving strategies, also called power saving [14], which take into account the traffic load, which is based on real-time data traffic and advocates the use of energy-efficient components to improve the energy consumption of the BTS at the hardware level; and (ii) the use of renewable sources of energy as main power sources for BTSs to reduce the electricity bill and the negative environmental impact of conventional fossil fuel-based energy resources due to their carbon footprint [15]. As for the use of renewable energy, sustainable BTSs have become a real solution to the problem. BTS configurations range from standalone solar powered BTSs with storage batteries, to grid-connected ones, in which the grid provides power to the BTS when the photovoltaic system (PVS) and the battery energy storage system (BESS) do not provide enough power. Hybrid configurations can also be found, which integrate different renewable sources such as wind and solar energy along with conventional energy sources such as diesel generators [16; 17; 18]. The authors in [19; 20; 21] carried out a detailed analysis of the technical and economic feasibility for different configurations used to power BTSs.