# Renewable Energy-Based Energy-Efficient Off-Grid Base Stations for Heterogeneous Network
## 1 Introduction  
The world is heading into an era of highly dense wireless networks which demand specific requirements, such as tens of Gbps data rates and around 10 years of battery life for low powered IoT devices. Cellular base stations are the main consumers of the energy used by mobile operators, e.g., around 57%, as mentioned in [1]. A recent estimate of energy consumption and greenhouse gas (GHG) emissions by mobile networks are around 130 TWh and 110 \(MtCO_{2}e\) per year, respectively. The total annual carbon footprint is about 200 \(MtCO_{2}e\), when the mobile phone emissions are included [2], and this amount has been increasing at a yearly rate of 10% [3]. According to [4], 11% of the world's energy generation source are renewable energy sources (RESs). At this rate, annually at least 12.1 \(MtCO_{2}e\) GHG emissions have been reduced due to the use of RESs in the mobile industry. Furthermore, according to the International Energy Agency, worldwide energy demand is estimated to increase by around 9% from 2019 to 2030, which is equivalent to 1870 milliontons of oil [5]. However, the reservations of global coal, oil, and natural gas remaining, warrant the needs for alternative sources of energy [6], e.g., renewable energies. High QoS at low costs is a challenge for cellular industry services because the pricing of cellular services has been reducing gradually [7]. The use of RES for powering the BSs can be a solution to this challenge. As such, efficient energy usage is one of the crucial foci of mobile industries aiming at the reduction of costs and emission-intensive carbon footprints while maintaining a guaranteed QoS. Therefore, bringing in renewable energy (RE) from the locally available sources has become an important aspect [8; 9; 10]. Banglades is a developing country, located between 33\({}^{\circ}\) N and 39\({}^{\circ}\) N latitude and between 124\({}^{\circ}\) E and 130\({}^{\circ}\) E longitude, where about 63.37% of the population live in the rural areas but only 59% of them have access to electricity [11; 12]. At present, Bangladesh has 181.32 million mobile connections [13] and around 90 million unique subscriptions, where the 4G network coverage reaches 95 percent of the population [14]. Mobile operators in Bangladesh have invested 898.2 million US$ for the combined 4G spectrum in March 2021 [15]. According to [16], Bangladesh had more than 36679 GSM BSs until 2017, which consumed around 642 million kWh of energy. A total of 14% of the total BSs are off-grid and 81% of those off-grid BSs are powered by a diesel generator (DG) plus battery system [17]. The country has immense prospects of RESs such as solar, wind, biomass (agriculture residue), and geothermal [11; 18]. As a result, it can be an ideal area to propose a RES based power generation infrastructure. Bangladesh has an average sunlight intensity within the range of 4-5 kWh/m\({}^{2}\) per day as well as more than 350 prospective oil-bearing crops that harvest biodiesel, for example, J. curcas, sunflower, sesame, castor, cottonseed, and groundnut oils [19]. According to [20], potential biomass sources in Bangladesh are 213.81 million tons, where energy content is 1344.99 PJ and possible electricity generation from the source is 373.71 TWh, which is equivalent to the consumption of 45.91 million tons coal or 34.01 billion cubic meters of gas. With the proper modeling and help from modern technologies, Bangladesh can harvest around 70 PWh and 7682 GWh electrical energy per year from the solar and biomass resources, respectively, by 2030 [21; 22]. Therefore, by integrating solar and biomass energy sources with adequate energy storage devices can be used to ensure a reliable RE-based supply system. The use of renewable energy efficiently in the field of mobile cellular communication is one of the trending research topics. Moreover, research on large scale deployment of energy-efficient IoT devices will highly influence the future mobile industry.