Empirical Study of the Underwater Turbulence Effect on Non-Coherent Light

Abstract

This letter presents an experimental study comparing the relative impact of turbulence induced scattering on coherent and non-coherent light propagating through water. It is shown that the scintillation index increases with increasing temperature inhomogeneity in the underwater channel. Our results indicate that a light beam from a non-coherent source has a greater resilience to temperature inhomogeneity induced turbulence effect in an underwater channel. These results will help researchers in simulating realistic channel conditions when modelling a light emitting diode (LED) based underwater optical wireless communication (UOWC) link.

An Empirical Comparison of Modulation Schemes in Turbulent Underwater Optical Wireless Communications

“…The UCE consists of a large aquarium style water tank with dimensions 1.5 × 0.5 × 0.5 m 3 and is filled with 225 L of tap water. In this work, a temperature gradient is applied by controlling the heat source to generate turbulence within the link, as in our earlier work [5]. The UCE is set up with a heater positioned in the centre of the tank, perpendicular to the propagation path of the collimated laser beam.…”

“…2 shows the normalised frequency response of the system. The -3 dB bandwidth extends beyond 1 GHz but there is a drop to just above -3 dB between approximately 750 and 950 MHz before The turbulence generated in the UCE is measured and characterised prior to data transmission, using the process described in [5]. The received intensity distributions are fitted with Gaussian and Log-Normal functions.…”

“…Prior works have examined channel characterisation through empirical studies. These investigations can be categorised as those that study the still water channel, as in references [2]- [4], and those that focus on the turbulent channel, as in references [5]- [9]. This knowledge of the channel characteristics can be used in system design to overcome the performance limitations imposed by the channel.…”

The BER Performance of the LDPC-Coded MPPM over Turbulence UWOC Channels

“…Various studies have investigated the scintillation index, describing turbulence strength, which is affected by the eddy diffusivity ratio, aperture diameter, wave model and so on [8][9][10]. Experimental studies also characterized underwater turbulence caused by different phenomena and proposed statistical distributions to fit the optical intensity fluctuations [7,[11][12][13][14][15][16]. A combination of exponential and log-normal distributions was proposed in [11] to describe the fluctuations of the probability density function (PDF) in the presence of air bubbles.…”

“…Furthermore, to obtain closed-form and analytically tractable expressions for crucial system performance metrics, the mixture exponential-gamma distribution is adopted to characterize channel irradiance fluctuations resulting from air bubbles [12]. In [13,14], a generalized-gamma (GG) distribution was used to model the fading of coherent and non-coherent light induced by temperature inhomogeneity. For salinity variation induced turbulence, Weibull distribution can efficiently fit the acquired scintillation data [15].…”