## 3 Types of Underwater Communication Systems
### Underwater Wireless Communication
The light wavelength ranges from 380 to 750 nm, corresponding to a frequency range of 400 to 790 THz [72]. Since bandwidth is directly proportional to frequency, this high-frequency range allows significant bandwidth and low latency [45]. Although light is a EM wave and susceptible to absorption (absorption happens when a wave loses energy as it transmits into a material. This can happen in a medium or at a boundary between two materials) [86], its frequency band makes it suitable for UWOC [45]. UWOC provides high bandwidth in the range of Mbps to Gbps, high reliability over short distances, a minimal environmental footprint, and low latency [45; 61; 85]. Over short distances (approximately tens of metres), optical communication can complement more advanced acoustic connections. In addition, optical communication enables the transmission of images and videos, real-time streaming, and high-performance sensor networks. However, due to extreme degradation caused by absorption, dispersion, and turbulence, UWOC is only suitable for short-range applications [45; 86]. A UWC system, although cheaper, either has limited signal propagation range or travelling distance, such as the UWOC, or low bandwidth, such as the UWAC [60; 61; 62; 80]. Underwater RF communication is characterised by the application of the EM frequency band between 3 KHz and 300 GHz [73]. However, some sources define RF to be between 30 kHz and 300 GHz [73]. The International Radio Regulations body defines RF between 9 kHz and 3000 GHz as suitable for wireless communication [77]. Furthermore, [PERSON] et al. [60] refer to RF as low as 30 Hz. RF signals travel most effectively through salty water at LF, specifically between 30 Hz and 300 Hz, but require large antennae for efficient transmission [60] due to high wavelength, since wavelength is inversely proportional to frequency. At the frequency range 30-300 Hz, [PERSON] et al. [5] reported a transmission distance of 100 m. [PERSON] et al. [7] and [PERSON] et al. [8] have described RF as impractical for underwater applications due to significant attenuation, favouring acoustic waves for signal transmission. [PERSON] et al. reported [10] that despite being part of the electromagnetic spectrum, lower-frequency RF waves experience less attenuation in water. This is because the absorption coefficient of water is inversely proportional to frequency [10]. Challenges in underwater signal propagation include signal reflection and refraction at the ocean surface and floor, which lead to multipath propagation [5; 6]. Multipath propagation causes signal pulses to scatter and spread out, reducing data rates and increasing the error rate [6]. The broad EM spectrum is divided into smaller frequency bands, such as RF, microwave, visible light (optics), infrared, ultraviolet, X-rays, gamma rays, and others [72]. The application of each band depends on factors like system design, usage, and environmental conditions [5]. For instance, while both visible light and RF bands belong to the EM spectrum, they behave differently in UWC systems due to their varied responses to water properties and conditions, leading to variations in data rates and transmission range capabilities [5; 9]. The high permittivity (approximately 80) and high electrical conductivity of water are significant factors that contribute to the attenuation of EM signals [10]. The conduction effect causes a loss primarily due to the electrical field component in the EM field [10]. The implementation of EM waves is restricted to ranges on the order of tens of metres [45; 85]. However, the behaviour of EM wave propagation varies depending on the frequency band within the EM spectrum [10]. The RF range is located at the lower end of the EM spectrum, which has a lower frequency than light and, consequently, a lower bandwidth [45]. As a result, RF is considered intermediate in terms of data rate [45]. Table 6 lists the underwater EM spectrum, signal attenuation and applications.