## 3 Types of Underwater Communication Systems
### Underwater Wireless Communication  
[PERSON] et al. and [PERSON] et al. [45; 79] defined UWC as the process of transmitting data wirelessly through an unguided underwater medium. Despite the existence of underwater wireless technologies for several decades, these systems still struggle to achieve high data rates over long distances [5; 80]. Optical, RF, and acoustic waves are the common forms of wireless communication employed in water, collectively referred to as UWC systems. The EM spectrum encompasses all EM radiation, ranging from extremely Low Frequency (LF) in the radio band to extremely High Frequency (HF) in the gamma-ray region [72]. Figure 4 illustrates the different regions of the EM spectrum. [PERSON] [81] reported that the first UWAC system was an underwater telephone developed in the United States in 1945. [PERSON] [81] stated that the UWAC system, operating in the 8-11 kHz frequency band, used single-sideband suppressed carrier (a carrier wave is a periodic wave (usually sinusoidal) that carries information. This information is superimposed onto the carrier wave through a process called modulation. The frequency of this carrier wave is known as the carrier frequency) modulation to communicate with submarines over several kilometers. With the advent of Very Large Scale Integration (VLSI) technology and programmable Digital Signal Processing (DSP)s, UWAC systems have experienced rapid growth and diversification [81]. Initially primarily used for military purposes, UWAC systems are now finding applications in various commercial sectors [5; 45; 81]. For instance, they are being employed in environmental monitoring, such as pollution detection, and in the offshore oil industry [5; 45; 81]. Historically, UWAC has been the dominant UWC system due to its comparatively low attenuation and long range, despite its lower bandwidth and higher latency [5; 45; 81]. Low-latency systems are desired since high volumes of information or data can easily be transferred across nodes (sources and destinations). Figure 4: The EM Spectrum [72]. Figure 3: Wet-mate connector [63]. Acoustics is the scientific study of sound, which includes its generation, manipulation, propagation, perception, and effects [82]. Unlike EM waves (such as RF and optics), which are transverse [83], acoustic waves are longitudinal [84]. The difference in how these two types of waves travel through a medium significantly impacts their propagation range and attenuation characteristics [58; 60]. Acoustic communication stands out in terms of range, mainly due to its resiliency to the physical properties of water, imparted by compression and rarefaction [84]. Although water is highly conducive to propagation of acoustic waves, allowing communication over several kilometers [21; 62], current acoustic communication systems suffer from low bandwidth (typically ranging from 5 to 10 Kbps) [21; 62]. UWAC systems also face higher latency compared to UWOC and RF technologies and are susceptible to fading and time-varying multipath interference [21; 60; 61; 62]. In contrast, UWOC is affected by dispersion caused by turbidity, which reduces both transmission distance and reliability [5]. [PERSON] et al. [5] reported that in USWN implementations, many underwater nodes are battery-powered and must operate for extended periods, adding to the need to devise energy-efficient communication strategies to prolong the operational lifetime of systems. UWOC is both cost-effective and energy-efficient [61]. Light is known for its dominance in bandwidth and low latency [79; 85]. With the increasing demand for higher bandwidth and transmission speeds, optical wireless communication is gaining attention as a viable alternative [68; 79]. UWOC, utilising laser beams and other EM wave groups, is widely implemented as a wireless communication system in underwater environments, demonstrating success across various domains [85]. When communication occurs over short distances and requires high data rates, optical communication is a highly recommended option [45; 68].