Statistical Studies of Fading in Underwater Wireless Optical Channels in the Presence of Air Bubble, Temperature, and Salinity Random Variations (Long Version) (1801.07402v2)

Abstract: Optical signal propagation through underwater channels is affected by three main degrading phenomena, namely absorption, scattering, and fading. In this paper, we experimentally study the statistical distribution of intensity fluctuations in underwater wireless optical channels with random temperature and salinity variations as well as the presence of air bubbles. In particular, we define different scenarios to produce random fluctuations on the water refractive index across the propagation path, and then examine the accuracy of various statistical distributions in terms of their goodness of fit to the experimental data. We also obtain the channel coherence time to address the average period of fading temporal variations. The scenarios under consideration cover a wide range of scintillation index from weak to strong turbulence. Moreover, the effects of beam-collimator at the transmitter side and aperture averaging lens at the receiver side are experimentally investigated. We show that the use of a transmitter beam-collimator and/or a receiver aperture averaging lens suits single-lobe distributions such that the generalized Gamma and exponential Weibull distributions can excellently match the histograms of the acquired data. Our experimental results further reveal that the channel coherence time is on the order of $10^{-3}$ seconds and larger which implies to the slow fading turbulent channels.

• The paper presents an experimental study analyzing statistical fading in underwater optical channels caused by variations in air bubbles, temperature, and salinity.
• The study found air bubbles cause significant fading, which may require two-lobe statistical models for accurate representation in some underwater conditions.
• Generalized Gamma and Exponentiated Weibull distributions effectively model temperature/salinity fading, and the channel exhibits slow fading (coherence time over 10 -3 s).

Analysis of Statistical Fading in Underwater Wireless Optical Channels

The paper presents a comprehensive experimental paper on the statistical modeling of fading in Underwater Wireless Optical Communication (UWOC) channels, with a focus on the effects of random variations in temperature, salinity, and the presence of air bubbles. The research specifically addresses the distribution of intensity fluctuations due to environmental turbulence, a crucial challenge in optimizing UWOC systems.

The paper outlines extensive experimental setups to simulate various underwater conditions, employing elements like air blowers for bubble formation, heaters and droppers for inducing temperature gradients, and highly saline water flows to introduce salinity variations. The authors measure the effects of these conditions on the optical signal's propagation to inform the design of robust UWOC systems.

Key findings indicate that the fading induced by air bubbles is significant, leading to temporal intensity fluctuations that suggest a two-lobe statistical model may be more appropriate than conventional single-lobe models in certain conditions. The integration of a beam-collimator at the transmitter and an aperture averaging lens at the receiver helps mitigate the impact of such scattering, supporting single-lobe models such as the lognormal and K distributions.

For conditions induced by temperature and salinity variations, the paper provides evidence that the generalized Gamma and the exponentiated Weibull distributions are particularly effective at fitting empirical data across a broad range of scintillation index values. The authors demonstrate that while these models increase computational complexity, they offer superior predictive capabilities in variable undersea environments compared to simpler models such as the lognormal distribution, which may falter in accuracy under specific conditions.

Furthermore, the paper calculates the channel coherence time, finding it typically exceeds $10^{-3}$ seconds, thus confirming slow fading as a characteristic of UWOC scenarios. This is significant for system design, as it implies that the optical signal’s fading remains relatively stable over time spans considerable in digital data transmission, allowing for better error correction and signal modulation strategies.

The paper's outcomes hold significant implications for both theoretical modeling and practical design of UWOC systems. By understanding the statistical characteristics of underwater channel fading, system architects can formulate strategies to counteract signal degradation, thereby extending viable communication ranges and enhancing data throughput. From a theoretical perspective, the insights could guide the development of new models and simulation tools to further advance the domain of UWOC.

The paper sets the stage for future investigations into mathematically modeling these complex environments to derive more generalized solutions that account for the variable and often extreme conditions found in natural waters. Field validation in real-world aqueous environments remains a critical step to cement the laboratory insights and adapt them into future UWOC system designs. The intersection of theoretical exploration and empirical observation highlighted in this paper underscores the intricate balance required to progress the field towards reliable and efficient underwater communication systems.

Overall, the research presented offers valuable empirical insights and establishes a foundation for continued investigation into statistically quantifying and mitigating the impact of environmental factors on underwater optical communications.