Reflector-aided underwater optical channel modeling.

Underwater visible light communication (UVLC) has been proposed as a high-speed alternative to acoustic signaling. While most UVLC systems are configured to work in line-of-sight (LOS) conditions, it is also possible to exploit reflected signals for performance enhancements. In this Letter, we propose a closed-form expression for the underwater path loss assuming non-LOS (NLOS) transmission through the water surface and man-made reflector (e.g., mirror) in addition to the LOS link. Utilizing the derived expression, we quantify the achievable NLOS gain defined as the ratio between the maximum achievable channel coefficient from reflection and the overall channel coefficient. We validate our findings experimentally by utilizing the water surface and the mirror as the reflecting surfaces in an aquarium. Our results reveal that achievable gains up to around 3 dB can be observed due to reflections.

Spatial power spectrum of natural water turbulence with any average temperature, salinity concentration, and light wavelength.

The power spectrum of water optical turbulence is shown to vary with its average temperature ⟨T⟩ and average salinity concentration ⟨S⟩, as well as with light wavelength λ. This study explores such variations for ⟨T⟩∈[0∘C,30∘C], ⟨S⟩∈[0ppt,40ppt] covering most of the possible natural water conditions within the Earth's boundary layer and for visible electromagnetic spectrum, λ∈[400nm,700nm]. For illustration of the effects of these parameters on propagating light, we apply the developed power spectrum model for estimation of the scintillation index of a plane wave (the Rytov variance) and the threshold between weak and strong turbulence regimes.

Effect of eddy diffusivity ratio on underwater optical scintillation index.

The performance of underwater optical wireless communication systems is severely affected by the turbulence that occurs due to the fluctuations in the index of refraction. Most previous studies assume a simplifying, yet inaccurate, assumption in the turbulence spectrum model that the eddy diffusivity ratio is equal to unity. It is, however, well known that the eddy diffusivities of temperature and salt are different from each other in most underwater environments. In this paper, we obtain a simplified spatial power spectrum model of turbulent fluctuations of the seawater refraction index as an explicit function of eddy diffusivity ratio. Using the derived model, we obtain the scintillation index of optical plane and spherical waves and investigate the effect of the eddy diffusivity ratio.