## 2 Q1: What Are the Recent Trends of UIoT System?
### Communication Technologies of UIoT  
Based on the research highlighted in Section 2.1, the recent communication trends in UIoT are described underneath and the essential channel attributes are briefed in Table 2.  
From the physics perception, unlike satellite, TV, mobile and radio communication frequency ranges, the conductivity of radiofrequency in seawater is very high. Thus, Radiofrequency (RF) wave propagation is affected strongly. For this reason, it is not easy to establish links using ultra-high frequency (UHF) and very high frequency (VHF) more than 10 m away from the sea surface. As for lower frequencies, RM attenuation can be considered short enough for reliable communication to occur over a few kilometers. However, the frequencies from 3 kHz to 30 kHz and from 3 Hz to 3 kHz are not enough to transmit at high data rates. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Attributes** & **Acoustic** & **RF** & **Optical** & **MI** \\ \hline Channel speed & \(\approx\)1500 m/s & \(\approx\)3.33 \(\times\) 108 m/s & \(\approx\)3.33 \(\times\) 108 m/s & \(\approx\)3.33 \(\times\) 108 m/s \\ \hline Communication range & \(\approx\)kilometer (km) & \(\approx\)10 m & \(\approx\)10–100 m & \(\approx\)10–100 m \\ \hline Data rate & \(\approx\)kbps & \(\approx\)Mbps & \(\approx\)Gbps & \(\approx\)Mbps \\ \hline Signal operation & Audible & Non-visible and non-audible & Visible & Non-visible and non-audible \\ \hline Frequency band & 10–15 kHz & 30–300 Hz & \(\approx\)5 \(\times\) 1014 Hz & - \\ \hline Size of the Antenna & \(\approx\)0.1 s & \(\approx\)0.5 s & \(\approx\)0.1 s & \(\approx\)0.1 s \\ \hline Channel characteristics & \begin{tabular}{c} Undersea noise, \\ temperature, pressure, \\ Doppler spread, \\ salinity, etc. \\ \end{tabular} & Conductivity & \begin{tabular}{c} Undersea noise, \\ attenuation, turbidity, \\ scattering, etc. \\ \end{tabular} & Conductivity \\ \hline Bandwidth & \(\approx\)1–100 Kilohertz (kHz) & \(\approx\)Megahertz (MHz) & \(\leq\)150 Megahertz (MHz) & \(\approx\)Megahertz (MHz) \\ \hline \begin{tabular}{c} Purpose of each \\ channel \\ \end{tabular} & \begin{tabular}{c} Long-range \\ communication \\ \end{tabular} & \begin{tabular}{c} Surface water \\ communication \\ \end{tabular} & \begin{tabular}{c} Short-range \\ communication \\ in deep sea \\ \end{tabular} \\ \hline Transmission power & \(>\)10 watts (W) & \begin{tabular}{c} megawatts \\ (MW)\(-\)watts(W) \\ \end{tabular} & \begin{tabular}{c} megawatts \\ (MW)\(-\)watts(W) \\ \end{tabular} & 10\({}^{-8}\) watts (W) \\ \hline Power loss dependency & \(\approx\)0.1 dB per meter (m) or per hertz (Hz) & \begin{tabular}{c} \(\approx\)28 dB per kilometer \\ (km) or one million \\ hertz (HZ) \\ \end{tabular} & \
\begin{tabular}{c} Depending on the \\ turbulence of water \\ \end{tabular} &
\begin{tabular}{c} Depending on the \\ permeability of \\ undersea soils \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Communication technologies of UIoT. Figure 2: Existing applications of UIoT. The channel performance and behavior are the main difference between optical and RF channels in the UIoT environment. There is an insulating material named dielectric utilized for optical channel propagation in UIoT. This mechanism is explained by the plasma frequency, operating as either a dielectric or conductor, following the frequency range. There are changes from a conductor to a dielectric at around 250 GHz in seawater. Attenuation and scattering are minor in the case of short-distance communication. Furthermore, the speed is up to 3 \(\times\) 10\({}^{8}\) m/s. Therefore, the optical signal is more reliable in short-range communication up to 10 m and suitable up to 100 m. Visible light communication (VLC) is the communication technology derived from an optical signal in UIoT. The ranges are from 450 nanometers to 550 nanometers at 500 Mbps and a distance of 100 m. Moreover, the speed is very high, up to 5 m. Therefore, VLC is very effective in short-range and one-to-one communication.