## 3 State-of-the-Art Review on Energy Optimization Techniques in UIoT
### Wireless Power Transfer Techniques for UIoT  
[PERSON] et al. [24] designed and developed the first battery-less technology for underwater wireless communication that can be recharged through acoustic waves from a long distance. In [PERSON] et al. [28], the authors present a reliable and energy-efficient power supply system for UIoT system based on multi-source energy harvesting methods which manage energies from piezoelectric harvesting and microbial fuel cell. [PERSON] et al. [29] proposed a highly resonant wireless power transfer (HR-WPT) technique to wirelessly transfer several kilowatts of energy to a remotely operated underwater vehicle. [PERSON] et al. [30] demonstrated a long-distance and efficient charging system for underwater sensor networks. The antenna was designed with a size of 24 cm \(\times\) 24 cm \(\times\) 1.5 cm. With this charging antenna, the power is transferred over 10 cm underwater with more than 60% efficiency. [PERSON] et al. [31] developed wireless power transfer in seawater  
Figure 4: Analysis of techniques developed for battery challenges vs. other challenges in UIoT networks [58].  
by considering two different magnetic inductors. The system was designed and enhanced using electromagnetic 3D simulation. [PERSON] et al. [32] proposed a contactless power supply system (CLPS) for transferring 500 W power to underwater vehicles, with 96% efficiency using 48 mm of the coil. The performance can be improved by expanding the coil size. [PERSON] et al. [33] proposed underwater inductive coupling power transfer (ICPT) techniques to evaluate the wireless power transfer system for AUV docking applications. [PERSON] et al. [34] designed and developed a contactless power transmission (CLPT) system for deep-sea underwater application; using this, they achieved 400 W of power with a 2 mm gap in the electromagnetic coupler, with an efficiency of 90% in salt water. [PERSON] et al. [45] designed and developed a rotational free wireless energy transfer system for the unmanned underwater vehicle (UUV), and the result shows that the system can deliver 664 watts at an efficiency of approximately 92%. In [121], due to the constrained environment and the unstructured, dynamic behavior of UIoT networks, the surface water docking system encounters limitations when installed in the underwater environment. These limitations include high cost, biofouling, less sensing capacity, and difficulty to move, etc. Furthermore, the design of the underwater docking system allows for pollutants, dirt, and biological waste to be collected and settle in the charging repository, which can damage the functions of the underwater docking system. One of the most promising technologies concerning underwater docking systems was proposed by the US Navy for wireless power transmission in underwater networks. The design of a self-charging AUV station and different underwater power transfer mechanisms are shown in Figure 5. Types of underwater wireless power transfer techniques (U-WPT) are discussed below. A summary of recent underwater wireless power transfer techniques since 2015 is presented in Table 3.