Influence of sea surface waves and bubbles on the performance of underwater-to-air optical wireless communication system

Underwater-to-air Optical Wireless Communication (OWC) systems have emerged as a viable solution for transmitting data from Unmanned Underwater Vehicles (UUVs) to Unmanned Aerial Vehicles (UAVs) in order to meet the increasing demands of marine technology applications. However, the field of Underwater-to-air OWC poses significant challenges due to harsh marine environmental conditions and significant signal attenuation caused by particulates and bubbles in the water medium. Implementing an underwater-to-air OWC system in a marine environment is significantly challenged by the turbulent nature of gravity waves. In this paper, we developed an underwater-to-air optical model using the Monte Carlo (MC) numerical method to investigate the effects of calm sea and rough sea with wind-driven waves and bubbles on the OWC system. MC simulations were performed using the seawater optical properties obtained from field measurements in the Bay of Bengal and Southern Ocean waters. The optical properties of bubbles were derived from a popular Hall-Novarini (HN) model. A three-dimensional theoretical ECKV (Elfouhaily, Chapron, Katsaros, and Vandemark) wave model that employs the Fast Fourier Transform (FFT) technique was used. The accuracy of this model was verified by comparison of the modelled and publicly available measured data obtained through stereo imaging techniques under various sea and wind conditions. The results showed that the ECKV model accurately captured spatial and temporal distributions of waves at the water-air interface, which is crucial for understanding the laser beam propagation in the real oceanic medium. The analysis of received power at the detector was done for the above scenarios at different wind speeds and time intervals and compared with another set of results obtained for the calm sea and rough sea with gravity waves, statistically approximated by the Cox and Munk (CM) model. The surface waves significantly reduced the received power as compared to the calm sea. The received power showed variations caused by surface waves using the ECKV method and remains nearly constant for the calm surface and CM model. Further analysis of the impacts of bubbles and water-air interface on an underwater-to-air OWC system showed that the received power decreased with bubble populations. These results provide significant guidance for developing and optimising underwater-to-air optical communication systems in challenging marine environment conditions.