Atmospheric turbulence modeling and aperture analysis for optimizing receiver design and system performance on free space optical communication links

Abstract

Strong turbulence measurements that are taken using real time optical wireless experimental setups are valuable when studying the effects of turbulence regimes on a propagating optical beam. In any kind of Free Space Optical (FSO) system, knowing the strength of the turbulence thus the refractive index structure constant (C 2 n ), is beneficial for having an optimum bandwidth of communication. Even if the FSO Link is placed very well-high-above the ground just to have weak enough turbulence effects, there can be severe atmospheric conditions that can change the turbulence regime. Having a successful theory that will cover all regimes will give us the chance of directly processing the image in existing or using an additional hardware thus deciding on the optimum bandwidth of the communication line at firsthand.In literature, simulation of beam propagation through turbulent media has always been a tricky subject when it comes to moderate-to-strong turbulent regimes. Creating a well controlled turbulent environment is beneficial as a fast and a practical approach when it comes to testing the optical wireless communication systems in diverse atmospheric conditions. For all of these purposes, strong turbulence data have been collected using an outdoor optical wireless setup placed about 85 centimeters above the ground with an acceptable declination and a path length of about 250 meters inducing strong turbulence to the propagating beam. Variety of turbulence strength estimation methods as well as frame image analysis techniques are then been applied to the experimental data in order to study the effects of different parameters on the result. Such strong turbulence data is compared with existing weak and intermediate turbulence data. The Aperture Averaging (AA) Factor for different turbulence regimes as well as the inner and outer scales of atmospheric turbulence are also investigated. A new method for calculating the Aperture Averaging Factor is demonstrated deducing spatial features at the receiver plane. Controlled turbulent media is created using multiple phase screens each having supervised random variations in its frequency and power while the propagated beam is calculated using Fresnel diffraction method. The effect of the turbulent media is added to the propagated beam using the modified Von Karman spectrum. Created scintillation screens are tested and compared with the experimental data which are gathered in different turbulence regimes within various atmospheric conditions. We believe that the general drawback of the beam propagation simulation is the difference in terms of spatial distribution and sequential phase textures. To overcome these two challenges we calculate the Aperture Averaging Factors to create more realistic results. In this manner, it is possible to create more viable turbulent like scintillations thus the relationship between the turbulence strength and the simulated turbulence parameters are distinctly available. Our simulation gives us an elusive insight on the real atmospheric turbulent media. It improves our understanding on parameters that are involved in real time intensity fluctuations that occur in every optical wireless communication system.