## 3 Related Works: An Overview of Wireless Power Transfer  
The technology behind transferring power wirelessly dated back to the early 20 th century and is credited to [PERSON] [4; 13; 16]. Back then, not much progress was witnessed until the early 1990s which saw the need for the re-emergence of WPT with the proliferation of portable electronic devices including laptops, cell phones, and PDAs. Electromagnetic (EM) radiation or radioactive transfer [13], which became suitable for information transmission faced several challenges in terms of power transfer applications, with low efficiency for omnidirectional radiation while unidirectional radiation required no hinderance in the LOS propagation and some costly tracking mechanisms. Recent advances in research present magnetic resonance as a suitable \"candidate\" for such regular applications ostensibly because of the limited interactions of the communications materials with magnetic fields [13; 15]. WPT can be grouped into three [23], namely magnetic resonant coupling, inductive coupling, and electromagnetic (EM) radiation. The traditional magnetic induction coupling has shown good efficiency and ability to transfer power over short ranges (e.g., electric toothbrush [4; 15]) but shows decrease in performance over increasing distances [12; 15; 16]. Due to some strict requirements such as close-range transmission [15], accurate alignment mechanism [16] between the transmitter and the receiver, and uninterrupted LOS [4], most of the WPT technologies at that time that depended on inductive coupling were limited in applications. A current study conducted by [23] shows that EM radiation technology is limited for WSN applications. Chiefly among the key limitations is that omnidirectional radiation is characterized by low efficiency in WPT and requires LOS between the transmitter and the receiver (i.e., there should be no obstruction or obstacles b/w Tx and Rx). The other limitation is the issue of human safety. For inductive coupling, the study shows that it can only work efficiently for short-range transmissions while it presents magnetic resonant coupling as more appropriate for WSN applications. The basic idea behind resonant-coupled WPT technology is to effectively exchange energy by means of creating magnetic coupling between two devices that are tuned to the same resonance frequency [16]. Table 1 shows a comparative summary of the WPT technologies, their applications and drawbacks. Based on the comparative analysis shown in Table 1, the most suitable WPT methods are inductive coupling and magnetic resonant coupling [23]. However, these techniques of WPT are only suitable for short and midrange transmissions respectively. Wireless sensor network has become part and parcel of the Internet-of-Things (IoT) currently witnessing a wide area of research advancements especially in the area of energy harvesting and consumption. WPT--a product of this research advancement--is one of the major contributory factors for the future mobile networks comprising IoT and smart cities in overcoming the problem of energy-constrained battery-powered sensor-nodes. Typically, WPT is an emerging technology that enables recharging of battery-driven IoT-nodes without the use of plugs and/or wires but perhaps with the use of wireless charging stations (WCS) or WCVs. Sensor-nodes are typically energy-constrained owing to the limited availability of energy sources and recharging, and battery-replacements are no options due to the associated cost implications and/or mode of deployments of the sensor-nodes. One of the major breakthroughs in the emergence of WPT technology is that battery-powered devices can now harness energy from EM waves in the air and be able to recharge their batteries cordlessly even while on the move [24]. With WPT,thousands of sensor-nodes can be deployed across a network at a low cost. Some of the notable applications of WPT as indicated in [23] include mobile phones, medical implants, wireless sensor networks, electric vehicles, and unmanned aerial vehicles.