## 2 Seawater Channel Modelling Elements
### UOWC System Layout
#### 2.1.2 Seawater Propagation Channel  
The seawater channel characteristics encompass the following: the seawater type, associated impairments, link configuration, and geometric dimensions. The seawater channel convolutional impact on the propagation of optical pulse \(h_{p}(t)\) is represented by its CIR, which is represented by \(h_{c}(t)\). This paper will denote the water channel as the channel, except when necessary. The seawater types are listed in Table 3. Each seawater type has its attributes, which we will discuss later in Section 2. To define the associated impairments, we must state the set of photon events while propagating through the channel. The number of events depends on the target set of impairments under consideration. The most straightforward set could include just the absorption, or on the other extreme, and the set covers the absorption, scattering, scintillation, and turbulence. It is beneficial to remember the association of each channel impairment with a named propagating photon event. The link configuration could be one of the following types: (1) The line-of-sight (LOS) link, which is energy-efficient because the power is concentrated in a defined beam's limits--LD can be utilised to deploy an LOS link configuration. (2) The non-line-of-sight (NLOS) link--The NLOS can be fulfilled by diverging the transmitted light component  
Figure 3: (**A**) Typical UOWC system; (**B**) UOWC link configurations. or using multiple sources of high divergence, like LED. Moreover, the Rx and Tx link can be organised in many other topologies based on the system's requirement, such as a point-to-point-line-of-sight (PTP-LOS) arrangement (Figure 3B(a)) and a diffuse line-of-sight (D-LOS) arrangement (Figure 3B(b)) [39]. D-LOS arrangements utilise broad sources, like LED, with great power and efficiency. These light sources avail huge divergent angles that allow UOWC transmission between Rx and Tx nodes [35]. The D-LOS design seems highly susceptible towards water attenuation due to the sizeable light-water interaction volume [29]. PTP-LOS configuration is the common connection option deployed in UOWCs. Such an arrangement features the placement of the Rx so that it detects the beam focused in the direction specified via the Tx [40; 41]. These systems require precise pointing between Tx and Rx when using a light source with a narrow divergence angle [42]. The D-LOS configuration works best in short-range transmission between moving objects. PTP- LOS configuration is the best choice for longer transmission distances, and so when perfect, pointing between Rx and Tx is feasible [43]. The perfect implementation of these configurations relies on using a suitable projection system through a lens system. Recent studies of UOWC have attempted to investigate the efficiency of communication networks, employing various modulation modes and algorithms to improve the data transmission rate and increase transmission distance. More information can be found here: [44; 45; 46]. To get the most out of the UOWC systems, one needs to understand how light behaves as it travels through water due to the potential impact of the water IOP and AOP. The most critical issues are the overall power loss and the spatial and temporal distribution of light, as they are closely related to communication performances such as link budgets, the optical signal-to-noise ratio (OSNR), a multipath defect, temporal optical dispersion, and dispersion induced due to ISI [32]. The optical properties of water, objects, and suspended particles dictate the characteristics of aquatic channels. The variations in the water properties, such as the pressure, velocity, temperature, and salinity, directly influence the optical properties of the water medium, such as its refractive index, attenuation, scattering, and absorption characteristics. Changes in the refractive index, which affects the speed of light in water, can cause variations in optical path length, affecting overall power loss and light's spatial and temporal distribution.