Statistical Distribution of Intensity Fluctuations for Underwater Wireless Optical Channels in the Presence of Air Bubbles

In this paper, we experimentally investigate the statistical distribution of intensity fluctuations for underwater wireless optical channels under different channel conditions, namely fresh and salty underwater channels with and without air bubbles. To do so, we first measure the received optical signal with a large number of samples. Based on the normalized acquired data the channel coherence time and the fluctuations probability density function (PDF) are obtained for different channel scenarios. Our experimental results show that salt attenuates the received signal while air bubbles mainly introduce severe intensity fluctuations. Moreover, we observe that log-normal distribution precisely fits the acquired data PDF for scintillation index ($σ^2_I$) values less than $0.1$, while Gamma-Gamma and K distributions aptly predict the intensity fluctuations for $σ^2_I>1$. Since neither of these distributions are capable of predicting the received irradiance for $0.1<σ^2_I<1$, we propose a combination of an exponential and a log-normal distributions to perfectly describe the acquired data PDF for such regimes of scintillation index.