## 3 Related Works: An Overview of Wireless Power Transfer
In [5], a mobile energy charger, moving along a pre-planned path, is deployed to wirelessly recharge the energy-hungry sensor-nodes located along its predetermined route using RF-WPT. The mobile energy charger periodically travels along the predetermined route to recharge the sensors. The proposed scheme consists of an off-the-shelf RF transmitter installed on a robotic vehicle and used as a mobile energy charger. Authors in [26] discuss some of the challenges in rectifier design confronting the implementation of WPT and EH solutions and present how the amount of harvested energy is affected by multiband and broadband rectifiers. Safety and security issues surrounding RF-WPT are robustly discussed in [25; 27; 28; 29] where authors propose a simplified safe charger scheduling scheme whereby power chargers are scheduled to recharge the energy-hungry devices without accommodating the risks inherent in RF radiation. The goal is to maximize charging efficiency so that more charging energy can be harvested at a normal threshold value of EM radiation. In [25], authors discussed the vulnerabilities associated with RF-based WPT channel (as there is no encryption or authentication of the wirelessly transmitted energy), thus raising users' safety and security concerns. Application of the green cell-less design to RF-WPT network is presented in [6; 30]. Green energy is harvested by connecting the access points (APs) with outdoor energy harvesters such as solar panel and wind turbine to harvest and the harvested energy is later used to wirelessly energize the energy-hungry multiple devices via RF signals [6; 30]. Cell-less implies that devices will not associate with any base stations (BSs) before commencing information transmission. In this system, also, devices can be fed from multiple sources, thus making it beneficial for energy-constrained devices. However, the performance of this system is limited by the fact that tight restrictions are placed on the extent to which the total transmit power may be increased owing to the dangerous effects of intensity of RF radiation [6; 30]. In [5], an off-the-shelf empirical model of RF energy transfer hardware was established, and the platform was used to test the performance of the RF-based WSN. The empirical model focused on joint optimization of path planning and mobile charging scheduling for wireless-powered sensor network (WPSNs). The design of an RF-WPT opportunistic harvesting (OH) scheme was proposed in [31], which is an improvement in two energy harvesting schemes known as store-then-cooperate (STC) and energy-harvest (EH). This model, which is meant for multiple access relay systems with various signal transmission techniques, assumes that sources and relays are self-powered devices with fitted rechargeable batteries. Essentially, the nodes are powered by an RF-WPT that relies on a dedicated energy beam. Authors in [32] propose an energy-efficient scheduling scheme in software-defined wireless sensor networks (SDWSN) to reduce the energy consumption of the energy transmitters used for charging sensor devices. The basic idea is to minimize the energy consumption of the chargers while adequately feeding the energy-hungry sensor-nodes. SDWSN mechanism is used to determine the optimal placement of the energy transmitters, minimum required number of power transmitters and the scheduling operations of the power chargers. An opportunistic energy scheduling scheme is proposed in [8], which enables an energy gateway with limited power supply to wirelessly transfer energy to sensor-nodes at some favorable channel conditions. Some of the existing design challenges of WPT and EH solutions using rectifier systems are presented in [26]. With these challenges, it is difficult to achieve maximum RF-DC conversion efficiency. In [33], authors addressed the scheduling problem of data collection and WPT in a rechargeable WSN with the objective of minimizing the packet loss.