# Research on Underwater Wireless Optical Communication Channel Model and Its Application
## 3 Underwater Optical Wireless Communication Experiment
### Experimental Setup  
The water quality attenuation tester utilized in the trials employs an LED light source that allows for the measurement of the attenuation coefficient of light in water across a wide range of wavelengths, specifically, from 360 nm to 720 nm. The measurement accuracy of the tester is less than 0.2%, and this ensures precise and reliable measurements of the attenuation coefficient. The specific measurement range is provided in Table 1. This device offers optical path lengths of 10 mm, 50 mm, 100 mm, 150 mm, and 250 mm, allowing for accurate attenuation measurements across diverse water quality scenarios and application contexts. The term \"Range (1/m) @400 nm\" refers to the range of attenuation coefficients that can be measured for light with a 400 nm wavelength. These values represent the attenuation experienced by the light as it travels through a 1 m thick water sample. The experimental parameters of the optical communication devices used in the trials are provided in Table 2. In our study, we opted for LEDs as the light source for underwater optical communication because of their practical advantages. LEDs are cost-effective and consume less power compared to lasers, making them a more efficient choice for this application. Additionally, LEDs are less alignment-sensitive, offering advantages in dynamic underwater environments where maintaining precise transmitter-receiver alignment is challenging due to factors like water currents and vehicle movements. Their wider beam spread compensated for alignment variability, aligning with our goal of robust, efficient communication in such settings. The communication signal was in the On-Off Keying (OOK) format, which typically represents a voltage signal. In OOK modulation, high and low signal levels correspond to logic '1' and '0', respectively. In the context of underwater communication, the light intensity of an LED is contingent on its drive current, and therefore, high and low levels correspond to the presence and absence of light output, respectively. OOK modulation was employed due to its effective channel adaptation underwater and its extensive utilization in previous trials [20]. As presented in Table 2, an LEDs were employed as the light source, and its radiation intensity distribution was assessed using an angular photometer. Due to the three-dimensional nature of the light field, half-angle intensities were recorded at 0 degrees/180 degrees, 45 degrees/225 degrees, 90 degrees/270 degrees, and 135 degrees/315 degrees in the cross-section to illustrate its light intensity distribution. In Figure 3, the dotted line represents the relative radiant intensity distribution of the light emitted by the optical communication device. Observations of the measured data revealed that the radiati \begin{table}
\begin{tabular}{c c c c} \hline
**Optical Distance (mm)** & **Range (1/m) @400 nm** & **Range (1/m) @550 nm** & **Range (1/m) @700 nm** \\ \hline
50 & 0.2–46 & 0.2–50 & 0.2–50 \\
100 & 0.1–23 & 0.1–25 & 0.1–25 \\
150 & 0.07–15 & 0.07–17 & 0.07–17 \\
250 & 0.04–9.2 & 0.04–10 & 0.04–10 \\ \hline \end{tabular}
\end{table} 
Table 1: Parameters of water quality attenuation tester. used in our system is more concentrated and less diffuse than that described by the Lambertian model. While the Lambertian model is a good general representation for many optical sources, it assumes a perfectly diffusive emitting surface, which was not the case for our specific LED setup. Consequently, to accurately represent the light intensity distribution of the LEDs, we adopted a Gaussian function for fitting the measured data. This approach provided a superior fit, capturing the nuances of the LEDs' emission characteristics more effectively. The results of this Gaussian fitting are illustrated by the black line in Figure 3. The Gaussian function is represented by Equation (10), which describes the relationship between the divergence angle (\(X\)-axis) in degrees and the relative intensity (\(Y\)-axis) as a percentage.