  A unified statistical model is proposed to characterize turbulence-induced
fading in underwater wireless optical communication (UWOC) channels in the
presence of air bubbles and temperature gradient for fresh and salty waters,
based on experimental data. In this model, the channel irradiance fluctuations
are characterized by the mixture Exponential-Generalized Gamma (EGG)
distribution. We use the expectation maximization (EM) algorithm to obtain the
maximum likelihood parameter estimation of the new model. Interestingly, the
proposed model is shown to provide a perfect fit with the measured data under
all channel conditions for both types of water. The major advantage of the new
model is that it has a simple mathematical form making it attractive from a
performance analysis point of view. Indeed, we show that the application of the
EGG model leads to closed-form and analytically tractable expressions for key
UWOC system performance metrics such as the outage probability, the average
bit-error rate, and the ergodic capacity. To the best of our knowledge, this is
the first-ever comprehensive channel model addressing the statistics of optical
beam irradiance fluctuations in underwater wireless optical channels due to
both air bubbles and temperature gradient.
