###### Abstract
In terms of experimental channel measurement, researchers in [13] established an experimental testbed to study the LD-UOWC channel characteristics by measuring the intensity of the light transmitted through different types of water. To overcome these limitations, the Monte Carlo numerical simulation method was developed to solve RTE and model the underwater channel; several works were reported in [14; 15; 16; 17; 18; 19]. In [14], a Monte Carlo (MC) based approach was proposed to investigate the light attenuation and time-domain broadening effects in different types of seawater channels, the impact of the  
Figure 1: Architecture of an underwater wireless communication sensor network. water quality, communication distance, and receiver aperture were also analyzed. In [15; 16], authors used the MC method and single-parameter chlorophyll concentration model to study the LED-UOWC scattering channel, the IOP, and path loss in different waters and different ranges were evaluated under various conditions. Authors in [17] adopted a double gamma function to fit the numerical simulation results of channel impulse response and investigated the fitting performance in coastal and harbor water. Using the Monte Carlo statistical method, authors in [18] simulated and analyzed the laser spot-expansion and time-domain broadening characteristics of the UOWC link. The most recent work was presented in [19], here they proposed a novel UOWC channel modeling method based on the mixture of two Gamma functions, the results were validated through Monte-Carlo simulations and exhibited reasonable agreement. The aforementioned works on UOWC channel modeling are summarized in Table 1. However, most works have selected to neglect solar radiation or just give a fixed value in shallow waters. There is also a lack of an efficient and intuitional model to take this effect into account. Besides, no existing works have presented a scalable simulation platform specially designed for UOWC, combining channel modeling and estimation. The design considerations of a practical shallow UOWC transceiver face several trade-offs between system specifications. While a larger aperture and wide FOV can capture more optical signals, they also bring more ambient light noises and thus it is hard to determine the optimal combinations. To overcome such limitations, we established a scalable photon tracing-based simulation platform to characterize the underwater optical channel and investigate the UOWC system performance with strong solar irradiance. A more efficient and intuitional method is proposed to quantify the amount of solar noise level at the certain depths and its impact on the system performance is evaluated. The detailed algorithm procedure and framework of this platform are presented, which is convenient for users to define the transceiver specifications and environmental conditions without required previous experiences in the UOWC channel model or MATLAB programming. Utilizing this platform, we give a detailed study of the channel features in terms of time and spatial dispersion, and path loss. The temporal expansion and spatial attenuation with different system configurations (FOV, aperture) and oceanic types are quantified based on the channel impulse response and the statistical results derived from photon tracing simulation. The light illumination distribution pattern over the receiver plane is displayed to study the tracking and alignment difficulty for different optical sources (LED or LD). Different noise sources are analyzed and the ambient noise intensity is estimated based on the sea level downwelling solar irradiance. Eventually, the corresponding BER performance is presented to explore the effect of various FOV and apertures at different operation depths.