Statistical distribution of intensity fluctuations for underwater wireless optical channels in the presence of air bubbles

TLDR

The study experimentally investigates intensity fluctuation statistics in underwater optical channels under fresh/salty and bubble/no‑bubble conditions and proposes a hybrid exponential–log‑normal model for intermediate scintillation indices. The authors measured received optical power with a large sample set, normalized the data, and derived channel coherence times and probability density functions for each channel scenario. Experiments show salt attenuates signal, air bubbles cause severe fluctuations, log‑normal fits low scintillation indices, while Gamma‑Gamma and K distributions model high indices, and a hybrid exponential–log‑normal model is required for intermediate values.

Abstract

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 (σ2I) values less than 0.1, while Gamma-Gamma and K distributions aptly predict the intensity fluctuations for σ2I > 1. Since neither of these distributions are capable of predicting the received irradiance for 0.1 < σ2I < 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.