# A Comprehensive Review of Micro UAV Charging Techniques
## 6 Simultaneous Wireless Information and Power Transfer (SWIPT)  
Gathering energy from the environment and converting this energy to electrical power has become an interesting research area. This technique is useful in delivering required power to wireless networks from renewable and clean energy resources. Generally, there are two types of energy harvesting methods: environmental and radio frequency. RF-based energy harvesting techniques include a promising method known as wireless power transfer. This technique has been further developed to transmit both energy and information which is known as \"simultaneous wireless information and power transfer\" (SWIPT). SWIPT plays a crucially important role in devices with limited energy capacity. SWIPT has become attractive through supporting sustainable and ubiquitous energy resources as well as information delivery services. SWIPT-enabled emerging technologies are presented in Figure 14. Early works reported on SWIPT assume that the same signal is used to transfer both energy and information without incurring any losses. To enable SWIPT, we need to split the received signal into two components: one for energy harvesting and other one for information decoding. This splitting can be achieved in different domains such as power, time and antenna [104]. (1) Time switching (TS)  
When time switching is used, the receiver switches according to time domain. In this method, the transmitted signals are split in terms of their time domain. As the signal arrives at the first time slot, the signal transmission or decoding information is performed. Simple hardware is used for time switching. However, the TS technique also requires accurate time synchronization and energy/information scheduling [105]. (2) Power splitting (PS)  
When power splitting is performed, the receiver is divided into two distinct power levels. The first component is passed to the rectenna circuit for energy harvesting, while sec  
Figure 14: Emerging SWIPT technologies. ond component is used for information decoding. The PS technique is different from TS as it uses a complex receiver and requires the power-splitting factor to be optimized. However, it uses only one time slot to perform energy harvesting and information decoding [106]. (3) Antenna switching (AS)  
In the antenna switching technique, antenna elements are used for rectifying and decoding purposes to realize SWIPT. In this technique, the receiver is divided into two components, one is used for energy harvesting while the other is used for information decoding. However, AS techniques require possible solutions to be optimized and suitable for all communications. This problem can be mitigated through MIMO decoding and forward (DF) relaying and dynamic algorithms [107]. The use of UAVs as aerial wireless communication platforms has gained significant attention due to their high flexibility and adjustable altitude. Therefore, UAV-aided SWIPT can support IoT and future wireless networks. However, there will be several issues involved in implementing multi-UAV-aided SWIPT, as shown in Figure 15. For instance, considering the spatial flexibility of UAVs, the location problem shifts from 2D to 3D deployment [108]. Furthermore, user resource allocation, rate-energy trade-offs and power splitting for each UAV cannot be ignored [109]. Although several studies in the literature have reported on UAV-aided SWIPT, only a few studies are dedicated to UAV-aided SWIPT for IoT. In [110], the authors propose a novel time division multiple access mechanism to reduce energy consumption of the UAV which performs computation, communication and WPT. In [111], the authors established UAV-aided SWIPT for the IoT network for emergency communication in three defined areas: emergency area, wide area and dense area. In the proposed system, a phased antenna array is embedded in a UAV to charge IoT devices. Moreover, an energy harvesting optimization strategy was studied for coverage, charging time and flight altitude of UAV.