## 3 Related Works: An Overview of Wireless Power Transfer
### Near-Field and Far-Field WPT Technologies  
WPT technologies may be classified as near-field and far-field WPT [18; 23; 34]. A technique is near-field if the wavelength of the electromagnetic signal is longer than the transfer distance [23]. For example, if the resonant frequency is relatively low (less than 5 MHz) with a short transfer distance of about 5 cm. In this case, power is delivered to only close-by devices. Examples of near-field WPT include: (i) magnetic inductive coupling [12; 15; 16; 18], (ii) magnetic resonant coupling [13; 15; 16; 18], (iii) capacitive coupling [23], and (iv) magneto dynamic coupling [23]. By contrast, a technique is considered far-field if the wavelength of the electromagnetic signal is smaller than the transfer distance [16], and power is delivered to far-reaching or remotely located devices [23]. Examples include radio waves [18; 19; 23; 44] and microwaves [19; 23; 44]. However, the use of far-field RF energy transfer is associated with path loss which consequently reduces the RF power on the rectifying antenna (rectenna). [PERSON] [19] proposes a method to optimize the propagation channel and subsystems of the rectenna to improve the RF energy transport efficiency. In [45], authors presented the design, implementation and evaluation of In-N-Out scheme, a software-hardware solution for far-field WPT. Hinging on the potential drawbacks of the near-field system which include reduction of the coil size leading to a significant drop in the charging efficiency and low flexibility, the authors present a flexible far-field WPT system that does not require users to wear cumbersome charging devices and can also constantly charge medical implant residing in deep tissues of human body. Many discussions on the recent trends in WPT technology for energy harvesting, millimeter-wave/THz rectennas, MIMO-WPT, and advances in near-field WPT applications are noted in [41]. From the discussion on WPT techniques, inductive coupling WPT is considered the best approach, followed by WPT via radio waves [41]. In terms of commercialization, near-field WPT is well commercialized compared to far-field. For instance, in Europe and China, many public buses have used near-field wireless chargers with frequencies of 20 kHz/85 kHz and power from 60-200 kW [41; 42]. Additionally, 2017 marked the promotion of a cordless wireless charger for a mobile phone known as Qi standard in Apple Inc.  
Table 2 shows the different modes of IoT energy harvesting for WPT technology while Table 3 shows the characteristics of wireless power transmission.  
An exciting application of WPT as noted in [42; 43] is the wireless charging of an electric vehicle. The various WPT technologies in Table 3 can be applied to charge electric vehicles. Discussions on regulation and safety issues reveal that inductive coupling WPT is considered the best approach, followed by WPT via radio waves [41]. However, wireless magnetic resonant power transfer has shown many advantages over other WPT methods due to its safety, lack of interference and efficiency at medium ranges [15; 42]. The advantage of MPT charging is that it can be a multiuser and multipurpose system. Both power and information can be transmitted via the microwave connection [42].