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 gammagamma 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. iii 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 �� 2 , the inner scale �0 , and the outer scale �0 . The optical path (�~1��) 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.


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Graduation Date





Phillips, Ronald


Doctor of Philosophy (Ph.D.)


College of Engineering and Computer Science


Electrical Engineering and Computer Science

Degree Program

Electrical Engineering









Release Date

August 2015

Length of Campus-only Access

1 year

Access Status

Doctoral Dissertation (Open Access)


Dissertations, Academic -- Engineering and Computer Science, Engineering and Computer Science -- Dissertations, Academic