Atmospheric turbulence modeling and aperture analysis for optimizing receiver design and system performance on free space optical communication links
Author(s)
Advisor
Altıntaş, AyhanDate
2012Publisher
Bilkent University
Language
English
Type
ThesisItem Usage Stats
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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.