# Renewable Energy-Based Energy-Efficient Off-Grid Base Stations for Heterogeneous Network
## 1 Introduction
A standalone solar photovoltaic (PV) with sufficient storage device powered cellular BS has been introduced in [1; 9; 23; 24], considering real-time dynamic solar radiation profile. Although in these studies energy efficiency (EE) is critically justified with both technical criteria and financial viability, in the issue of reliability it has some drawbacks. Therefore, authors in [25; 26] have suggested the incorporation of diesel generator (DG) as a non-renewable energy source with solar PV for fueling the off-grid cellular BSs. But this system increases the carbon footprint. Moreover, transportation of fuel for DG in remote areas is not economically efficient. An integration of multiple RESs has been proposed in [27; 28; 29]. In [27], a solar PV and BG based hybrid supply model has been proposed for an off-grid homogeneous cellular network. In [28], proposed a hybrid solar PV/wind turbine (WT)/biomass generator (BG) powered energy supply system has been proposed for macro BSs in Bangladesh. The works in [29; 30] use a solar PV and WT coupled supply system framework with and without an energy sharing policy for a homogeneous BSs setup. It is clear from [31], an integration of hybrid RESs with storage devices can assure reliability and minimize the gap between consumption and generation of energy. For example, in an urban area in Germany, Nokia Siemens has established a mobile BS setup powered by hybrid solar PV/WT [32]. Energy sharing between the adjacent cells through resistive power lines or smart grid has been used in [33; 34; 35]. Agglomerative and divisive hierarchical clustering algorithms are established with average energy affinity (AEA) and stochastic energy affinity (SEA) metrics to propose an energy sharing model among BSs in [33]. A RE-based community scale energy planning has been proposed by [36] to get a reasonable reduction in energy use and GHG emissions. In [37], the authors proposed an optimized community-shared solar PV framework. An optimal energy cooperation framework among cellular BSs has been proposed in [34], where both RE and grid have been incorporated using physically connected resistive power lines. An energy cooperation model between macro BSs has been demonstrated in [35]. From the aforementioned research articles, it is clear that the on-site green energy sharing between BS's is affordable and it makes the systems reliable and cost-effective. An efficient resource scheduling algorithm can optimize the tradeoff between different QoS requirements for specific applications. A minimum guaranteed throughput scheduling algorithm for NRT applications has been introduced in [38]. One of the most popular solutions for the resource scheduling of NRT applications is the proportional fair (PF) scheduler, which successfully incorporates the cell throughput and fairness. However, it ignores the queue size and does not attempt to improve the power saving from the DRX operation. A channel-adapted and buffer aware (CABA) packet scheduling algorithm has been proposed [39], which considers the queue size. However, this stand-alone proposal cannot take all the advantages of the popular PF scheduler. The work in [40] considers energy metrics at the base station, with a certain rate guarantee for each mobile station. A radio resource allocation framework is proposed in [41] to ensure proportional fairness among the user equipment (UE). A few solutions that make such resource allocators capable of supporting both guaranteed bit-rate and best-effort traffic are proposed in [42]. A new scheduling scheme, named the proportional equal throughput (PET) [43], which offers better fairness among users without reducing average user throughput, in comparison with other scheduling algorithms. In [44], a context-conscious sleep and wake-up cycle of the fog nodes have been proposed in the target of energy minimization at the fog layer. Authors in [45; 46] analyzed the effects of the DRX parameter on UE's energy saving and latency. Nokia has an exploration on industrial DRX, which is illustrated in [47].