## 3 Types of Underwater Communication Systems
### Underwater Wireless Communication
Furthermore, turbidity of the water exacerbates the adverse effects of scintillation [85]. Turbulence can stretch or compress the signal wavelength from transmission to reception [85]. As a result, monochromatic transmission systems are deemed impractical in underwater environments characterised by turbulence, particularly in seawater [85]. Table 7 lists the typical attenuation coefficients for different types of water. Both the absorption and scattering coefficients are indicators of signal attenuation: a reduction in signal power in the medium. While absorption refers to the signal energy being absorbed by the molecules in the medium, scattering refers to the spread of the signal across the medium due to reflection and refraction. In addition to the inherent properties of water, the absorption of light in underwater channels is also influenced by the frequency band range, as different wavelengths are absorbed in varying rates [18]. For example, the photons in the light signal can be absorbed by water molecules and converted into thermal energy, causing heat [90]. In contrast, scattering refers to the deviation of photons from their original path, reducing the signal strength along the desired trajectory [90]. Moreover, the absorption coefficient is lowest for wavelengths in the blue and violet spectrum, making this the optimal optical band for underwater communication [18]. Light attenuation is particularly low between wavelengths of 300 nm and 650 nm, further supporting the suitability of this range for optical communication [61]. To overcome the limitations of UWOC, techniques such as innovative encoding schemes, modulation, and detailed channel analysis have been developed [79]. Although optical waves are less affected by attenuation, they are more vulnerable to scattering [60]. Considering the broad range of the RF band, its performance is influenced by factors such as the frequency sub-band, required bandwidth, and propagation range require \begin{table}
\begin{tabular}{c c c} \hline \hline
**Water Types** & **Absorption (m\({}^{-1}\))** & **Scattering (m\({}^{-1}\))** \\ \hline Pure Sea Water & 0.053 & 0.003 \\ \hline Clear Ocean & 0.114 & 0.037 \\ \hline Coastal Ocean & 0.179 & 0.219 \\ \hline Turbid Harbor & 0.295 & 1.875 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Typical attenuation coefficients for different water types [61; 76; 89]. ments [10]. The adaptability of RF frequencies allows greater communication flexibility [10]. Underwater radio communication can achieve bandwidths of up to 100 Mbps over short distances [45], resulting in low latency [10]. Furthermore, RF signals are highly tolerant to turbulence, water depth variations, bubbles, marine fouling, and water turbidity, providing reliability for underwater communication [10; 45]. However, [PERSON] et al. stated that between the frequency range of 30 Hz and 300 Hz, both RF and MI are heavily impacted by absorption in such a way that transmission distance is limited to nearly 100 m [5]. Despite the high energy consumption associated with bulky RF transceivers [45], several advantages are offered by RF technologies. RF signals can traverse various media, including air, water, ice, and solid objects and do not require a direct line of sight [10]. Thus, RF can navigate around obstacles, ensuring communication links even when direct visibility is obstructed [10]. Unlike sound waves, an RF signal is not affected by multipath interference, which enhances its reliability [10]. However, ref. [18] notes that RF does not propagate well in underwater environments, particularly in saltwater. Additionally, refs. [5; 10; 18; 60] agreed that RF requires high power consumption. In contrast, UWOC is reported to be a low-energy consumer with extremely high security standards [85]. Despite the challenges in RF technologies, [PERSON] et al.