## 2 Preliminaries  
A generic UOWC involves a transmitter generating photons that traverse through water and convert into electrical signals at the receiver. The data generated by a UUV are modulated onto an optical carrier of a specific wavelength, amplified, beam-shaped, and transmitted. At the receiver, the photons from the transmitted light are collected by lenses into a photodetector that converts them into electrical signals, which are then amplified, noise-filtered, and demodulated to recover the data (Figure 2) [19]. Photon transmission underwater depends on the inherent and apparent optical properties (IOP and AOP) of water [20]. While IOPs relate to the absorption and scattering of light by solutes and particulates, AOPs relate to physical parameters defining the optical beam's structure. AOPs are tied to engineering choices like beam divergence angle and those defined by medium composition, such as the reflectance. The combined effects of absorption and scattering give rise to signal attenuation in which a portion of signal energy is either converted to another energy form (chemical or heat) or is redirected away from the intended path. The overall wavelength-dependent signal attenuation c(\(\lambda\)), as expressed in (1), is the sum of absorption a(\(\lambda\)) and scattering b(\(\lambda\)) coefficients at a vacuum wavelength of light \(\lambda\) in nm. \[c(\lambda)=a(\lambda)+b(\lambda) \tag{1}\]  
The nominal values for \(c\): 0.056, 0.15, 0.305, and 2.17 m\({}^{-1}\) are used for modeling pure seawater, clear ocean water, coastal ocean water, and turbid water in harbors, respectively [19; 35]. The classification by [PERSON] [36], which has been widely used in parametric simulations of UOWC, identifies ten types of seawater. Types I, IA, IB, II, and III are clear-oceanic waters, and types 1C, 3C, 5C, 7C, and 9C are coastal waters of increasing turbidity [36]. Nevertheless, [PERSON] types are only accurate up to about 10-20 m in water depth, also assuming homogeneity in this layer [37; 38]. However, this could be an issue for studying mobile UOWC scenarios where the parameters that influence the signal could change, should the link traverse through depths where the water quality has changed. This is evident in the derivation of absorption and scattering coefficients given in (2) and  
Figure 2: The key components of an underwater wireless optical communication system. (3), which are the linear sum of its subcomponents, where \(w\), \(CDOM\), \(phy\), and \(det\) denote absorption and scattering by pure seawater; colored dissolved organic matter (such as that released from decaying organisms); phytoplankton (photosynthetic organisms); and detritus (organic and inorganic suspended particulates), respectively. The weightage of each is determined by its concentration in seawater. It is generally accepted that blue-green light is optimal for transmission in clear ocean water with a shift toward green-amber wavelengths in more turbid waters, as seen in Figure 3.  
\[a(\lambda)=a_{w}(\lambda)+a_{CDOM}(\lambda)+a_{phy}(\lambda)+a_{det}(\lambda) \tag{2}\]  
\[b(\lambda)=b_{w}(\lambda)+b_{phy}(\lambda)+b_{det}(\lambda) \tag{3}\]  
[PERSON]'s law may then be used to model the loss of signal propagation for a distance \(d\), given as Equation (4), where \(I_{t}\) is the irradiance at the location of interest and \(I_{0}\) is the initial irradiance at the point of emission. \[I_{t}=I_{0}e^{-c(\lambda)d} \tag{4}\]  
However, the concentrations of these subcomponents in (2) and (3) are subject to variation if the UUV moves from one water body to another or if the quality of the seawater around a stationary UUV changes because of a change in the direction of currents (e.g., tidal currents) or diel and/or seasonal changes in physical seawater properties or biological activity.