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Variance in Fade-Time of a Gamma-Gamma Distributed Irradiance Signal

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Date Issued:
2012
Abstract/Description:
Free-space optical communications are predominantly hindered by optical turbulence, an effect caused by temperature and pressure variations within the atmosphere. The result is an optical wave interfering with itself due to multipath propagation via tiny refractive-index fluctuations across the wave-front. Optical communication systems are affected when the channel conditions induce fading in the irradiance signal that is received at the detector. The nature of optical interference imparted by the atmosphere is a random process and therefore the received irradiance signal is often characterized by an appropriate probability density function (PDF). Data collected during past free-space optical experiments in the atmosphere support the gamma-gamma distribution as a practical PDF model for received irradiance fluctuations, although the irradiance fluctuations do occasionally tend towards a lognormal distribution.Utilization of the gamma-gamma irradiance PDF allows for calculation of statistical moments of the irradiance threshold level-crossing distribution. Presented analysis focuses on the results of the gamma-gamma irradiance PDF. Previously, expressions were developed for the expected number of gamma-gamma distributed irradiance threshold level-crossings. Expressions for the mean square number of gamma-gamma distributed irradiance threshold level-crossings are derived and presented. The derived expressions lead to the mean and variance of signal fade time. Outcomes of the derived expressions are presented in relation to free-space optical communication system performance.Comparisons are made between the theoretical analysis and experimental data taken at the Innovative Science and Technology Facility (ISTEF) located at the Kennedy Space Center in Cape Canaveral, Florida. The strength of the atmospheric turbulence is often characterized by three measurable parameters: the refractive index structure constant Cn2, the inner scale l0, and the outer scale L0. The optical path (L~1km) was instrumented such that direct comparisons could be drawn between the measured atmospheric turbulence parameters and the parameters of the gamma-gamma irradiance model. Variance of fade time data were found to agree well for smaller apertures where effects of aperture averaging are not present and in cases where scintillation is weak to moderate. It is suggested that a more appropriate PDF, with a heavier focus on aperture averaging, may be applied in future studies of these fade statistics.
Title: Variance in Fade-Time of a Gamma-Gamma Distributed Irradiance Signal.
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Name(s): Leclerc, Troy, Author
Phillips, Ronald, Committee Chair
Weeks, Arthur, Committee Member
Richardson, Martin, Committee Member
Marinescu, Dan, Committee Member
Andrews, Larry, Committee Member
University of Central Florida, Degree Grantor
Type of Resource: text
Date Issued: 2012
Publisher: University of Central Florida
Language(s): English
Abstract/Description: Free-space optical communications are predominantly hindered by optical turbulence, an effect caused by temperature and pressure variations within the atmosphere. The result is an optical wave interfering with itself due to multipath propagation via tiny refractive-index fluctuations across the wave-front. Optical communication systems are affected when the channel conditions induce fading in the irradiance signal that is received at the detector. The nature of optical interference imparted by the atmosphere is a random process and therefore the received irradiance signal is often characterized by an appropriate probability density function (PDF). Data collected during past free-space optical experiments in the atmosphere support the gamma-gamma distribution as a practical PDF model for received irradiance fluctuations, although the irradiance fluctuations do occasionally tend towards a lognormal distribution.Utilization of the gamma-gamma irradiance PDF allows for calculation of statistical moments of the irradiance threshold level-crossing distribution. Presented analysis focuses on the results of the gamma-gamma irradiance PDF. Previously, expressions were developed for the expected number of gamma-gamma distributed irradiance threshold level-crossings. Expressions for the mean square number of gamma-gamma distributed irradiance threshold level-crossings are derived and presented. The derived expressions lead to the mean and variance of signal fade time. Outcomes of the derived expressions are presented in relation to free-space optical communication system performance.Comparisons are made between the theoretical analysis and experimental data taken at the Innovative Science and Technology Facility (ISTEF) located at the Kennedy Space Center in Cape Canaveral, Florida. The strength of the atmospheric turbulence is often characterized by three measurable parameters: the refractive index structure constant Cn2, the inner scale l0, and the outer scale L0. The optical path (L~1km) was instrumented such that direct comparisons could be drawn between the measured atmospheric turbulence parameters and the parameters of the gamma-gamma irradiance model. Variance of fade time data were found to agree well for smaller apertures where effects of aperture averaging are not present and in cases where scintillation is weak to moderate. It is suggested that a more appropriate PDF, with a heavier focus on aperture averaging, may be applied in future studies of these fade statistics.
Identifier: CFE0004397 (IID), ucf:53153 (fedora)
Note(s): 2012-08-01
Ph.D.
Engineering and Computer Science, Electrical Engineering and Computer Science
Doctoral
This record was generated from author submitted information.
Persistent Link to This Record: http://purl.flvc.org/ucf/fd/CFE0004397
Restrictions on Access: campus 2015-08-15
Host Institution: UCF

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