Investigation of Underwater Optical Wireless Communications with Turbulence

Turbulence is due to the random variations of the refractive index of the medium (in this case water), which leads to fluctuation or fading of the received light intensity. In wireless communications including underwater optical wireless communications the link performance is greatly affected. In this paper, we investigate the effect of turbulence on the probability density function (PDF) of the received light intensity. We show that lognormal and negative exponential distributions are fitted well with the PDFs of the received light intensity in weak-to-strong and saturated turbulence regimes. The goodness of fit test is performed to validate the conformity of these two distributions with the simulation results. Furthermore, we investigate the effect of the divergence angle of the Gaussian beam transmitter, the receiver’s aperture diameter and field of view on the scintillation index

Experimental study of the turbulence effect on underwater optical wireless communications

Underwater optical wireless communications (UOWC) performance is affected by turbulence. However, not much research has been carried out to estimate the probability density function (PDF) of the received optical power. In this paper, we investigate the effect of turbulence on the UOWC system using a new experimental setup with a variable link span in a water pool. Different turbulence levels are created by changing the temperature and the rate of an injected water flow in the pool water to obtain the PDF. Results show that lognormal distribution closely matches the measured PDF for a range of link spans. In UOWC systems, the link span is one of the main factors influencing fluctuations of the received optical power, and it has not been thoroughly investigated. In this work, the scintillation index and turbulence-induced power loss are obtained for a range of turbulence strengths and transmission link spans. Finally, we show that there is a good agreement between the experimental and simulated results

Receiver Parameters Effect on Underwater Optical Wireless Communication Performance in the Presence of Transmitted Gaussian Beam

Using divergent Gaussian beams mitigates the transmitter (Tx) and the receiver (Rx) misalignment in underwater optical wireless communications (UOWC) point-to-point links and increases the maximum acceptable lateral offset (MALO) at the Rx. However, extra spatial dispersion, temporal dispersion and angular spreading do occur compared to multiple scattering. Therefore, it is important to optimize the Rx for power loss, channel bandwidth and angle of arrival (AOA) distribution. Here both the field of view (FOV) and the aperture size of the Rx are investigated. We show how the required FOV of the Rx can be obtained from the AOA distribution, and also investigate how FOV and the aperture size of the Rx can be selected to minimize the power loss and the channel bandwidth degradation. To illustrate the effect of multiple scattering, results are presented for both clear and harbor waters

Use of Gaussian beam divergence to compensate for misalignment of underwater wireless optical communication links

The vast majority of underwater wireless optical communication systems use collimated blue/green laser beams to deliver high-speed data over a transmission range of a few metres to tens of metres. However, such systems are extremely susceptible to misalignment of the transmitter and the receiver. The mitigation techniques for misalignment reported in the literature are complex and costly at times. In this study, the authors consider the simple approach of increasing the divergence angle of the transmitted Gaussian beam to mitigate misalignment. Both plane and spherical beams are considered as the limitation cases. Using Monte Carlo simulations, the authors show that an optimum divergence angle for the maximum acceptable lateral offset exists with respect to the receiver sensitivity in clear waters while this is not an efficient method in harbour waters. Results demonstrate that there is a design trade-off between acceptable lateral offset, power loss and channel bandwidth. Furthermore, the authors show how the proposed scheme of beam divergence affects the maximum allowed link span as well as the channel bandwidth for a given distance

Modeling turbulence in underwater wireless optical communications based on Monte Carlo simulation

Turbulence affects the performance of underwater wireless optical communications (UWOC). Although multiple scattering and absorption have been previously investigated by means of physical simulation models, still a physical simulation model is needed for UWOC with turbulence. In this paper, we propose a Monte Carlo simulation model for UWOC in turbulent oceanic clear water, which is far less computationally intensive than approaches based on computational fluid dynamics. The model is based on the variation of refractive index in a horizontal link. Results show that the proposed simulation model correctly reproduces lognormal probability density function of the received intensity for weak and moderate turbulence regimes. Results presented match well with experimental data reported for weak turbulence. Furthermore, scintillation index and turbulence-induced power loss versus link span are exhibited for different refractive index variations