###### Abstract  
The performance of the underwater optical wireless communication (UOWC) system is highly affected by seawater's inherent optical properties and the solar radiation from sunlight, especially for a shallow environment. The multipath effect and degradations in signal-to-noise ratio (SNR) due to absorption, scattering, and ambient noises can significantly limit the viable communication range, which poses key challenges to its large-scale commercial applications. To this end, this paper proposes a unified model for underwater channel characterization and system performance analysis in the presence of solar noises utilizing a photon tracing algorithm. Besides, we developed a generic simulation platform with configurable parameters and self-defined scenarios via MATLAB. Based on this platform, a comprehensive investigation of underwater channel impairments was conducted including temporal and spatial dispersion, illumination distribution pattern, and statistical attenuation with various oceanic types. The impact of ambient noise at different operation depths on the bit error rate (BER) performance of the shallow UOWC system was evaluated under typical specifications. Simulation results revealed that the multipath dispersion is tied closely to the multiple scattering phenomenon. The delay spread and ambient noise effect can be mitigated by considering a narrow field of view (FOV) and it also enables the system to exhibit optimal performance on combining with a wide aperture. underwater optical wireless communication; photon tracing; solar irradiance; channel modeling; performance analysis; simulation platform Article  
1  
[1] 2021  
basic architecture of a typical UWSN, which is composed of sensor nodes, solar-powered communication buoys, autonomous underwater vehicles (AUV), and underwater remotely operated vehicles (ROV). The optical base station (OBS) with seabed fixed or anchored sensors is capable of obtaining ocean monitoring data and communicating with ROVs, AUVs, or divers via optical/acoustic links. Then the ROVs or AUVs deliver the transmitted data to submarines, communication buoys, ships or other devices [6]. Above the sea level, the information exchange is accomplished through radio frequency (RF) or free space optical (FSO) communication links between ships and the onshore data stations. Despite all the aforementioned advantages, UOWC links suffer from severe channel impairments due to the seawater inherent optical properties (IOP), namely absorption and scattering effect. The absorption effect refers to an irreversible process in which water molecules and particles absorb photons and convert them into thermal or chemical potential energy. The scattering process alters photon propagation directions and causes time-varying multipath effects, i.e., spatial and temporal dispersion [7]. In addition, multiple scattering also leads to energy loss due to the reduction of arrived photons at the receiver plane, especially in poor water quality environment. Many research efforts have been carried out for underwater channel characterization and performance analysis in order to cope with these issues. Previous related works have been mainly based on analytical, experimental measurement and numerical simulation approaches. Several researchers in [8; 9; 10] utilized the analytical method to solve the classical radiative transfer equation (RTE), which characterizes the light field passing through the scattering medium. However, this method involves a large number of assumptions and approximations to simplify the derivation process and results, and it is extremely difficult to derive accurate analytical solutions. Authors in [11; 12] adopted Beer-Lambert's law to model the line-of-sight (LOS) underwater optical wireless channels due to its simplicity. However, this way can only express spatial path attenuation rather than temporal dispersion.