## 3 State-of-the-Art Review on Energy Optimization Techniques in UIoT
### Wireless Power Transfer Techniques for UIoT
#### 3.1.3 Underwater Inductive Wireless Power Transfer (UI-WPT)  
The concept of UI-WPT technologies was discussed in [173, 174, 175, 176, 177, 178, 179, 180, 181, 182, 183, 184]. The UI-WPT has begun to receive attention in UIoT networks for recharging AUVs, UUVs, ROVs, and UW-SNodes. The UI-WPT system comprises of components such as a transmitter, receiver, and coupler, through which the UIoT devices can transmit and receive energy. The UI-WPT comprises of two coils, the primary and secondary coil; the primary coil acts as the transmitter end with a high-power source, and the secondary coil acts as the receiver, and it uses magnetic field induction as the medium to transfer energy between two coils in underwater communication. Recently many researchers have enclosed dynamic wireless power transfer technology for supporting multiple power transmissions in terrestrial area networks [174], these methods can be adapted to UIoT networks.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Year** & **Reference** & **Type** & **Application** & **Frequency [kHz]** &
\begin{tabular}{c} **Analysis and** \\ **Performance Level** \\ \end{tabular} \\ \hline
2016 & \begin{tabular}{c} [PERSON] \\ et al. [185] \\ \end{tabular} & Two-coiled U-WPT & \begin{tabular}{c} Sea water and \\ fresh water \\ \end{tabular} & \
\begin{tabular}{c} 78 kHz and \\ 114 kHz \\ \end{tabular} &
\begin{tabular}{c} Good performance in both \\ sea and saltwater \\ \end{tabular} \\ \hline
2016 & \begin{tabular}{c} [PERSON] \\ et al. [186] \\ \end{tabular} & Electric Coupling & \
\begin{tabular}{c} Study on electric \\ coupling in \\ U-WPT \\ \end{tabular} & 10 kHz to 1 MHz &
\begin{tabular}{c} U-WPT system needs \\ high-speed and \\ high-voltage \\ switching devices. \\ \end{tabular} \\ \hline
2017 & \begin{tabular}{c} [PERSON] et al. [187] \\ \end{tabular} & Load modulation & \begin{tabular}{c} Analysis of the \\ voltage-mode \\ power driver with \\ magnetic \\ resonance in \\ U-WPT \\ \end{tabular} & \
\begin{tabular}{c} 104 kHz and \\ 111 kHz \\ \end{tabular} &
\begin{tabular}{c} Utilized for \\ understanding resistive \\ load modulation \\ in U-WPT. \\ \end{tabular} \\ \hline
2018 & \begin{tabular}{c} [PERSON] et al. [188] \\ \end{tabular} & Eddy current loss & \begin{tabular}{c} Analysis of eddy \\ current loss in \\ U-WPT using \\ different \\ frequencies \\ \end{tabular} & \
\begin{tabular}{c} 215.5 kHz to \\ 248.4 kHz \\ \end{tabular} &
\begin{tabular}{c} Efficiency depends on an \\ increase and decrease \\ in misalignment. \\ \end{tabular} \\ \hline
2018 & \begin{tabular}{c} [PERSON] et al. [189] \\ \end{tabular} & \begin{tabular}{c} Power efficiency \\ tracking \\ \end{tabular} & \
\begin{tabular}{c} Maximizing \\ U-WPT system \\ efficiency \\ \end{tabular} & 178 kHz &
\begin{tabular}{c} Tracking efficiency is \\ above 85\% \\ \end{tabular} \\ \hline
2018 & \begin{tabular}{c} [PERSON] \\ et al. [190] \\ \end{tabular} & \begin{tabular}{c} A capacitive \\ wireless power \\ transfer system \\ \end{tabular} & \
\begin{tabular}{c} U-WPT system \\ for freshwater \\ \end{tabular} & \(\approx\)200 kHz &
\begin{tabular}{c} Achieved efficiency \\ of 91.3\% \\ \end{tabular} \\ \hline
2018 & \begin{tabular}{c} [PERSON] et al. [174] \\ \end{tabular} & \begin{tabular}{c} Wireless charging \\ system \\ \end{tabular} & \
\begin{tabular}{c} Three-phase \\ charging system for \\ lightweight AUV \\ \end{tabular} & 465 kHz &
\begin{tabular}{c} Achieved efficiency \\ of 92.41\% \\ \end{tabular} \\ \hline
2019 & \begin{tabular}{c} [PERSON] \\ et al. [191] \\ \end{tabular} & \
\begin{tabular}{c} A curly coil \\ structure is used to \\ adapt the \\ cylindrical \\ symmetric hull \\ \end{tabular} & U-WPT system & 85 kHz &
\begin{tabular}{c} Achieved \\ efficiency of \(\approx\)95\% \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table}
Table 3: Analysis on recent wireless power transfer techniques of UIoT networks since 2015.