Variance In Fade-time Of A Gamma-gamma Distributed Irradiance Signal

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

Buffer Requirements Of An Optical Communication System In Atmospheric Turbulence

Expressions related to the buffer requirements of an optical communication system in atmospheric turbulence are developed from the channel signal fade time statistics. Laser irradiance data were recorded over the course of one day by a receiving aperture of variable diameter at the Townes Institute Science and Technology Experimentation Facility (TISTEF) 1km laser range located within the Kennedy Space Center at Cape Canaveral, FL. Fade statistics of collected data and scintillometer measurements were compared to the derived model gamma-gamma fade model. Parallel to the laser instrumentation was a commercial scintillometer unit which reported the refractive index structure coefficient, Cn 2 and the inner-scale of atmospheric turbulence, l0. The atmospheric parameters inferred from the collected laser data and the commercial instruments were compared. Mean and variance of the fade times were found to agree well with theory 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 free space optical communication system fade statistics. © 2013 SPIE

Prediction Of The Ground-Level Refractive Index Structure Parameter From The Measurement Of Atmospheric Conditions

Evaluation of the methods developed by Bendersky, Kopeika, and Blaunstein1 to predict the refractive index structure parameter from the direct measurement of macroscopic atmospheric conditions were investigated. Measurements of ground-level temperature, relative humidity, wind speed, solar flux, and aerosol loading taken by the University of Central Florida weather station were compared against concurrent measurements of the refractive index structure parameter made by Scintec SLS-20 scintillometers positioned near the weather station. Wind measurements were obtained by three, three-axis sonic anemometers (capable of resolving a three-dimensional wind vector) positioned at heights of 1, 1.5, and 2.5 meters above the ground. Temperature measurements were taken at ground level, and at heights of 1 and 1.5 meters. Data were collected for two days atop Antelope Peak, NV. Collection times covered both daytime and nighttime measurements. © 2010 Copyright SPIE - The International Society for Optical Engineering

Observation And Analysis Of Aero-Optic Effects On The Orca Laser Communication System

In this paper we show evidence of aero-optic effects on the measured beacon beam as the gimbal angle of a nose-mounted turret changes from 0 to 90 degrees and greater with respect to the line of flight. Data from the beacon beam was collected with a new technology 3-aperture scintillometer over a 82km to 104km air-to-ground downlink during field testing of the ORCA system in Nevada in May 2009. In this paper we present data analysis on the impact of an aero-optic boundary layer on a laser link between an aircraft and a ground-based stationary node. Particularly we look at the impact of an aero-optic boundary layer on the mean, variance, scintillation, probability density function (PDF), power spectral density (PSD), and fading of the received irradiance. We find that the most compelling argument for the presence of strong aero-optic effects comes from calculating the PSD of the received beacon intensity. We also find the cumulative effect of the aero-optic boundary layer differs depending on the transmitted beam parameters, i.e. collimated or divergent. © 2011 SPIE

Comparing The Log-Normal And Gamma-Gamma Model To Experimental Probability Density Functions Of Aperture Averaging Data

Irradiance data were collected over a 1km horizontal terrestrial path using several different sized receiving apertures. The data were collected under moderate-to-strong turbulence conditions. The receiver system consisted of a 154mm (6 ) refracting telescope outfitted with several removable apertures. The path was instrumented with three 3-axis anemometers and three scintillometers, two of which were capable of measuring the inner scale of turbulence in addition to Cn2. Histograms were formed with the data and compared to the Log-Normal and Gamma-Gamma PDF models. As expected, neither PDF model was applicable under all conditions of aperture averaging. Hypotheses are made as to why the models were unable to completely capture the effects of aperture averaging on received irradiance data. © 2010 SPIE

Analysis Of Fading In The Propagation Channel For The Orca Laser Communication System

Irradiance data were collected over an air-to-ground path using several different sized receiving apertures. The data were collected from the Optical RF Communications Adjunct (ORCA) tracking beacon. The receiver system consisted of three telescopes of sizes 51 mm, 137 mm, and 272 mm. Probability of fade, number of fades per second, and mean fade time was computed for various intensity levels for irradiance data collected on all three telescopes. These measured statistics are compared to fading models derived from lognormal and gamma-gamma probability density function (PDF) models. Discussion is centered on the viability of these models under various conditions and on the presence of aero-optic effects. The gamma-gamma and lognormal model are found to be insufficient to model all fading statistics. © 2011 SPIE

Multiplex epithelium dysfunction due to CLDN10 mutation: the HELIX syndrome

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