## 1 Introduction  
Mobile communication is one of the most successful technological innovations in modern history. However, the Long Term Evolution (LTE, also known as pre-4G, i.e., 3.9G) of a cellular network is considered to be the fastest developing technology in mobile communication [1]. In addition, South Korea has witnessed tremendous development in the LTE cellular network in the last five years [2]. Today, South Korea has an LTE cellular network unrivalled in both technology and reliability, with the best global coverage (97% of the time on LTE) [3]. The LTE network offers data-oriented services that include, but are not limited to, multimedia communications, online gaming, and high-quality video streaming. As a result, the number of mobile subscribers has increased exponentially, prompting the mobile operators to significantly increase the number of LTE BSs to meet the needs of mobile subscribers. In 2013, the number of LTE BSs was 35,255 units; that number increased by 4.7-fold in 2015 to 165,193 LTE BSs [2]. This increase of LTE BSs led to increases in both the energy consumption and operational expenditure (OPEX) as a result of the LTE BS being the primary source of energy consumption in cellular networks, accounting for 57% of the total energy used [4, 5]. Moreover, cellular network operators are continually seeking to increase their network coverage areas, provide services to potential customers in rural areas, and open new markets to increase profitability. Diesel generators (DGs) are typically used for power at off-grid BS sites because extending the grid connection to power off-grid BS sites is not economically attractive or existing grid electricity is not available because of geographical limitations [6]. Today, the idea of using diesel generators as a primary power supply has become less favourable because of the challenges linked to their reliability, availability, high operational and maintenance (O&M) costs, and significant environmental impacts [7]. It is now acknowledged that the LTE cellular communication network in South Korea will have greater economic and ecological impact in the coming years. The key features for power sources, such as economic, environmental, and social sustainability, of BS sites are a critically important issue. Power shortages are not allowed in the cellular mobile sector or during service outages. The specific power supply requirements for off-grid BSs, such as cost-effectiveness, efficiency, sustainability and reliability, can be met by utilising the technological advances in renewable energy [6]. The Korean government continually seeks to increase energy independence by expanding renewable energy to enable more efficient development and ensure sustainability with better planning to provide cleaner energy [8]. This study aims to give a holistic view of the sustainable power supply solution for an off-grid LTE-macro BS based on the characteristics of South Korea average solar radiation exposure and wind speed. The key contributions in this paper are summarised as follows:  
1. To determine the optimal size and technical criteria of the hybrid SPV/WTG system to feed LTE-macro BS deployment at off-grid sites of South Korea. The optimum criteria, including economic, technical, and environmental feasibility parameters, were analysed by the _Hybrid Optimisation Model for Electric Renewable_ (HOMER). 2. To analyse and evaluate the feasibility of using a hybrid SPV/WTG system in terms of the energy yield and economic feasibility over the project lifetime. 3. To analyse and compare the implications of choosing a hybrid SPV/WTG system with respect to a classical DG powered solution in terms of the (i) OPEX savings to maintain profitability for cellular network operators and (ii) GHG emissions that have a bad effect on the environment. The rest of this paper is organised as follows: Section 2 presents the related work. In Section 3, the system architecture for the hybrid SPV/WTG system to supply LTE-macro BS is described, and Section 4 presents the mathematical model.