Sammendrag
Underwater communication (UWC) is important for more accessible measurements in the ocean, enabling sustainable ocean management. Acoustic communication is the only wireless option for distances above about 100 meters. Underwater sensor networks for coastal regions, will typically operate with distances of tens of meters vertically and some hundred meters horizontally. For clear water, optical UWC will be an alternative providing considerably higher data rates than acoustic (Mb/s to GB/s vs. kb/s). However, for turbid waters the light signal will be strongly absorbed and scattered, resulting in a severe reduction in communication distance. Particles and algae can give a factor of one million more scattering in the forward direction compared to the backward direction, and for most research on optical UWC, all scattered light is considered a loss in signal. For moderate modulation frequencies, however, distortion of the pulses can be avoided in such waters, preventing inter-symbol-interference. The work presented here, characterizes the contribution from forward scattered light to the measured signal. Analytical approximation of the signal to noise ratio is based on common phase functions and absorption properties in typical coastal waters, together with radiative transfer simulation of ambient sunlight. Monte Carlo simulations of light pulses provide estimates of the signal contribution from directly transmitted and scattered light. The methodology is used for optimization of optical UWC for inclusion of scattered light, with varying water turbidity and type, detector geometry, and ambient light. Optimization of modulation techniques is also investigated.
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