# Designing an Adaptive Underwater Visible Light Communication System +
## 3 UVLC Channel Modeling  
The UVLC channel can be modeled with its impulse response, which describes the response of the channel and dispersion of the received signal in the time domain (also known as temporal dispersion of the received signal). Figure 2 shows the underwater optical channel model. The UVLC system model in Figure 2 can be expressed as  
\[y(t)=h(t)*x(t)+n(t) \tag{1}\] \begin{table}
\begin{tabular}{l l l} \hline
**Study** & **Focus** & **Research Gap Addressed** \\ \hline [PERSON] et al. [14] & 1. Relay-based approach. & 1. SDR-based approach. \\  & 2. Expanding the optical receiver’s reception & 2. Fixed reception area but adaptive \\  & area. & receiver position, thus making the system \\  & & more cost-efficient. \\ \hline [PERSON] et al. [15] & 1. Relay-based implementation. & 1. SDR-based. \\  & 2. OOK modulation. & 2. PPM modulation. \\ \hline [PERSON] et al. [16] & Simulation-based multi-hop UWOC network. & Real-time hardware setup with dynamic \\  & & receiver position. \\ \hline [PERSON] and [PERSON] [17] & 1. SDR-based. & 1. SDR-based. \\  & 2. OFDM modulation. & 2. PPM modulation. \\  & & 3. Addressing the misalignment using \\  & & servo motor assembly. \\ \hline [PERSON] et al. [4] & Designing a smart transceiver by changing & Adaptive SDR-based transceiver model \\  & the receiver position manually to find the & proposed for robust and efficient \\  & BER and power curves. & communication. \\ \hline \end{tabular}
\end{table} 
Table 1: _Cont._  
Figure 2: Additive noise channel model [18]. where \(y(t)\) is the received signal after passing through a UVLC channel with impulse response \(h(t)\), \(n(t)\) represents different forms of noise, and \(x(t)\) is the modulated transmitted signal. Water is characterized by its inherent optical properties (IOPs) and apparent optical properties (AOPs). The IOPs depend on the aquatic medium whereas the AOPs depend on the water as well as properties of the light field [19]. Absorption and scattering fall under the category of IOPs, whereas irradiance reflectance is an important AOP. Since the latter property mentioned mostly varies with the depth of water, it is not of our concern as, in this project, the depth of water is constant. Here, we only focus on IOPs. Whenever light travels in an aquatic medium, it suffers from absorption and scattering. Absorption is an irreversible process where photons lose energy thermally, whereas, in scattering, they change their direction upon interaction with water molecules and suspended particles [20]. Both are wavelength-dependent. Figure 3 shows the system model for the UVLC link, where the light from the source encounters water molecules and hence suffers from absorption and scattering. The first step to analyzing any UVLC system is to study its channel characteristics and determine the channel impulse response (CIR). The CIR is used to analyze the effects of channel distortions [2]. [PERSON]'s law is the simplest one, and considers the linear motion of photons. The optical power (\(I\)) of a photon after a distance (\(z\)), based on the transmitted power (\(I_{o}\)), is given by  
\[I=I_{o}e^{-cz} \tag{2}\]  
where \(c\) is the extinction coefficient (sum of absorption and scattering coefficients) and \(z\) is the link range.