## 3 Types of Underwater Communication Systems
### Underwater Wireless Communication
As listed in Table 6, some of the EM frequency ranges have specific applications, while other segments of the spectrum have no current use. In regard to its physical properties, saltwater is electrically conductive [88]. Liquid saltwater is an electrolyte primarily composed of sodium and chlorine, making it a good electrical conductor [88]. The conductivity of saltwater imposes limitations on UWC using EM waves, resulting in attenuation [61; 85; 88]. Based on the four Maxwell-Heaviside equations, EM waves possess both magnetic and electrical fields [58]. These fields interact with the electrical and magnetic fields in water, leading to destructive interactions in the form of signal absorption [85; 88]. Optical and RF communications offer greater bandwidth and lower latency compared to UWAC [7; 62]. Light, like other high-frequency EM waves, is quickly attenuated underwater due to factors such as absorption, scattering (scattering is a change in the direction of motion of a particle because of a collision with another particle), fading, clustering, and fluctuating environmental conditions (e.g., perturbation, low visibility, and conductive ocean currents) [79; 85; 86]. Time jitters (time jitter refers to the variation \begin{table}
\begin{tabular}{l l l l} \hline \hline
**Frequency Range** & **Wavelength (m)** & **Attenuation** & **Applications** \\ \hline Extremely Low Frequency (ELF) & \(10^{3}\)–\(10^{6}\) & Low & \begin{tabular}{l} Submarine communication, \\ geophysical exploration \\ \end{tabular} \\ \hline Very Low Frequency (VLF) & \(10^{2}\)–\(10^{5}\) & Low & \begin{tabular}{l} Submarine communication, \\ navigation \\ \end{tabular} \\ \hline Low Frequency (LF) & \(10\)–\(10^{4}\) & Moderate & \begin{tabular}{l} Submarine communication, \\ navigation \\ \end{tabular} \\ \hline Medium Frequency (MF) & \(1\)–\(10^{3}\) & High & \begin{tabular}{l} Underwater communication \\ (limited range) \\ \end{tabular} \\ \hline High Frequency (HF) & \(10^{-1}\)–\(10^{2}\) & Very High & \
\begin{tabular}{l} Short-range underwater \\ communication \\ \end{tabular} \\ \hline Very High Frequency (VHF) & \(10^{-2}\)–\(10^{-1}\) & Extremely High &
\begin{tabular}{l} Limited use due to high \\ attenuation \\ \end{tabular} \\ \hline Ultra High Frequency (UHF) & \(10^{-3}\)–\(10^{-2}\) & Extremely High & Essentially unusable underwater \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Underwater electromagnetic spectrum and signal attenuation [58; 87]. in the arrival time of data bits or symbols in a digital system. It is a common phenomenon in communication systems, particularly those that transmit data over long distances or through noisy channels), chromatic dispersion (chromatic dispersion in optics refers to the phenomenon where different wavelengths of light travel at different speeds through a medium. This causes white light to be separated into its constituent colours, similar to what happens in a rainbow), water quality, and turbulence all impact wireless optical links, as highlighted in [85], which examines the probability of link fade (link fade is the reduction in signal strength or quality during transmission due to interference, absorption, scattering, and environmental conditions affecting the communication medium) associated with the propagation of chirped longitudinal Gaussian pulses used as information carriers. Although UWC has been successfully demonstrated in freshwater using EM waves, in most cases it required close sensor proximity to achieve high bandwidth [25]. Ref. [61] states that even though UWOC enables longer propagation ranges than underwater HF RF communication, long-distance communication underwater continues to rely on UWAC systems. The types of water, salt or fresh, vary in concentration, density, chemistry, and geology [61]. These factors contribute to differences in absorption and scattering coefficients, as well as attenuation [61; 88]. Time jitter affects the estimated arrival time at the receiver, negatively affecting signal synchronisation.