## 2 Literature Review  
In what follows, a literature review for the most related work in the field of RF-based wireless charging and practical implementation and evaluation is presented. The authors in [16] designed such a framework to the point that comprises of a system of sensors outfitted with remote power beneficiaries or receivers, a portable wireless power charger, as well as an energy station that is responsible for observing the energy levels of sensors and consequently choosing the power charging successions at the mobile charger. To be specific, the authors have assembled a proof-of-idea model of the framework and probed the model to assess its plausibility and performance on a small-scale system. Moreover, they have performed broad simulations to consider their proposed framework in large-scale systems and experimentally exhibited that the proposed framework can use the mobile charging innovation successfully to extend the network lifetime. Additionally, they evaluated the impacts of the mobile charging in terms of productivity through simulation by considering different simulation parameters, as well as routing concept, which they believed will become valuable rules at the time the proposed framework is implemented. It is a well-known fact that solar energy harvesting is a widely accepted technique that eases out the deployment of WSNs. However, nodes in WSNs that use batteries employing this technology may suffer at night times or during cloudy periods due to energy shortages arising from the depletion of energy and insufficient solar energy to charge the batteries. In order to overcome this shortcoming, in [17], the authors have considered the TV broadcast airwaves to power the nodes wirelessly. To be specific, in their contribution, the authors have concentrated on radio frequency as an effective source of ambient energy. Despite the fact that the energy duplicated by encompassing the ambient RF fields is constrained, sensors can be operated over an extensive period of time when broadcast airwaves are utilized. Towards this goal, as a first step, the authors have prototyped a wireless sensor model and realized an adaptive duty cycle assurance technique. With the use of RF energy harvesting, which requires just a rectifier and an antenna as hardware, this model has been found to be highly successful in the detection and transmission of temperature data every 5 s. Besides, the aforementioned energy harvester comprises of only fewer components thereby resulting in low cost and hence, more sensors can be deployed densely. In addition, this harvester is useful in applications that do not call for high power requirement. Recently, due to the fact that RF signal conveys energy and data at the same time, RF energy harvesting is viewed to further advance wireless charging as it is aware of the Simultaneous Wireless Information and Power Transfer (SWIPT). The authors in [18] studied the SWIPT in two-hop Orthogonal Frequency-Division Multiplexing (OFDM) relay systems as it has gained importance as a well-known scheme for wideband digital communication and has been utilized in, for example, 4G mobile communications, DSL Internet access, audio broadcasting, and digital television. In order to address the limits of the OFDM decode-and-forward relay system performance, the authors detailed a Power Allocation (PA) optimization problem to expand its feasible data rate. Not quite same as the related works, in their design of the Resource Allocation (RA) scheme, the Subcarrier Pairing (SP), Power Splitting (PS) factor, and PA at the source and relay are together considered, and the overall energy harvesting at the relay is carried out by taking into account all possible subcarriers, which is then re-designated for the next hop. In addition, since the problem is non-curved and does not have direct solutions, the authors designed an RA approach by treating it as two separate sub-problems.