# Renewable Energy-Based Energy-Efficient Off-Grid Base Stations for Heterogeneous Network
## 2 System Architecture and Mathematical Modeling  
To serve a large number of users and to run the massive volume of data traffic with a diverse quality of service (QoS) requirements, mobile operators have introduced the simultaneous operations of macro, micro, pico, and femtocells, which together form heterogeneous networks (HetNet). The typical coverage area radius for macro, micro, pico and femtocells is 5-30 km, 1-2 km, 200 m and 10 m, respectively. Large-scale deployment and supervision of IoT devices are becoming a crucial topic in the present day. To facilitate internet connectivity for the enormous amount of low throughput devices, the 3 rd Generation Partnership Project (3 GPP) has proposed a new radio technology standard well-known as the narrowband IoT (NB-IoT). This NB-IoT specification can implement the power saving mode (PSM) and extended discontinuous reception (eDRX) mechanisms to attain a long battery life. This eDRX is an extended form of the discontinuous reception (DRX) operation. DRX operation was included in 4G to achieve the power saving and prolonged battery life. With the intelligent and optimum use of eDRX, PSM, and DRX mechanisms, a resource scheduling algorithm can be proposed for IoT and NRT applications with an optimal tradeoff between latency and power saving. A schematic diagram of the proposed hybrid solar PV/BG powered rural area-based off-grid HetNet is illustrated in Figure 1.  
The HetNet consists of macro, micro, pico, femto type BSs, where each type of BS has its specific size, power consumption, and data rate requirements according to the required QoS. Generally, a small size of BS requires small radiated power due to its small coverage area. Hence, pico and femto BSs require low power consumption; they are supplied with solar PV alone. In contrast, macro and micro BSs require high power consumption. As such,  
Figure 1: A schematic diagram of the proposed system. they are connected to both solar PV and BG, with the BG only activated if PV cannot supply enough power. To ensure a continuous power supply, a battery bank is connected for each BS to overcome the shortage or outage of the generation from RES. Here, the power supply to each base station is controlled via an energy management unit (EMU). The EMU is also connecting renewable energy sources and energy storage devices. EMU primarily collects the generated power from renewable energy sources and distributes the generated energy according to the respective base station requirements. EMU also has the built-in capacity to reduce overvoltage and overcurrent problems. It consequently improves battery life by minimizing the intense discharge from the storage unit. A BS is a DC load. However, there are some AC loads such as a lamp and air cooler that need to be powered. Converters are used to convert DC into AC and vice versa, as needed. This section presents the proposed system model along with the renewable energy generation model, BS power consumption model, and energy sharing algorithm in the context of off-grid LTE cellular networks. Symbols and notations that are used in the subsequent sections are summarised in Table 1.  
An overview of the energy-efficient techniques from the prospective of both energy generation and consumption are shown in Figure 2. We illustrate an overall architecture of the proposed system and methods to establish our proposed energy-efficient techniques