The most commonly used altitude-dependent model for refractive index fluctuations, over long high-altitude slant paths
or ground/space links, is the Hufnagel-Valley model. For the
near-ground turbulence portion of the path, this model
uses an exponential decay term suggested by Valley to connect ground level turbulence with the original Hufnagel
model which was constructed for turbulence above 3 km. However, it has long been observed that refractive-index
fluctuations in the first few hundred meters near the ground decrease with altitude raised to the -4/3 power rather than
exponentially. Recent and some earlier measurements of
refractive-index fluctuations are presented in this paper along
with a theoretical modification of the Hufnagel model to account for low-altitude turbulence exhibiting this power-law
B-PPM formatting for trans-atmospheric optical communication is compared experimentally to OOK (NRZ) at a single
channel data rate of 1.25Gbps in deep fading conditions. Unlike low data rate transmission using M-ary PPM
formatting, high-speed B-PPM formatting does not benefit from the theoretical improvement that has been realized at
low data rate. Although B-PPM can indeed benefit from a threshold set to near-zero, the high speed transmission
precludes the implementation of a traditional Maximum Likelihood Detection circuit that compares the integrated power
of each slot. At high speed, one has to rely on signal strength alone within the bit period which degrades the contrast
between a "one" and a "zero." Moreover, the need for twice the bandwidth for B-PPM significantly limits available
components such as APDs. More important, however, is the fact that during deep fades clock resynchronization
dominates at high data rate. The primary question to be addressed is: Does B-PPM formatting really provide sufficient
margin compared to NRZ to merit its use in deep fading atmospheric conditions? By building a special dual transceiver
system, we have been able to propagate both B-PPM and NRZ formatted signals co-linearly on two C-band wavelengths
centered close to 1550nm. Under field testing we measured the BER, including signal resynchronization, using special
InGaAs, high-speed, multimode pigtailed, APD-based detectors in the receiver. The data were collected on fully
instrumented horizontal paths of 1km and 500m with Cn2 [m-2/3] ranging from 10-15 to 10-13.
We report on a set of measurements made in December 2005 by researchers from the University of Central Florida, SPAWAR's Innovative Science and Technology Experiment Facility (ISTEF), Harris Corporation, NASA Kennedy Space Center, and Northrop Grumman. The experiments were conducted on the Shuttle Landing Facility (SLF) at Kennedy Space Center (KSC) over terrestrial paths of 1, 2, and 5 km. The purpose of the experiments was to determine the atmospheric-induced beam spreading and beam wander at various ranges. Two lasers were used in the experiments. Both were a pulsed 1.06 μm laser; however, one was single-mode and the other was multi-mode. Beam profiles were recorded near the target position. Simultaneous measurements of Cn2, wind speed and direction, humidity, visibility, temperature, and surface temperature profiles were all recorded.
Atmospheric scintillation causes fading to a free-space optical communications link. Optical communication links can be improved by the correct application of coding schemes customized to meet the atmospheric conditions. For this paper, we model atmospheric scintillation using a gamma-gamma probability distribution. From the scintillation model, the equations are derived for probability of fade along with mean fade time and the duration of fade. Parameters for the gamma-gamma model, directly related to atmospheric conditions, are used to compute theoretical cases of fading in weak and strong atmospheric turbulence. With the models for atmospheric fading, different coding techniques on a pulse-position modulation optical link are examined.
Atmospheric scintillation negatively affects point-to-point laser communication requiring either an increase in power to maintain link throughput rate or decreasing the data rate at a constant power. The effect of atmospheric scintillation on irradiance can be modeled using a gamma-gamma probability distribution. With the model of irradiance known, the Poisson transform of the irradiance is determined. Using the Poisson transform of the gamma-gamma distribution, the probability of word error is calculated for a pulse-position modulation (PPM) receiver. The word error probability for M-ary PPM decoding is then plotted with respect to varying turbulence parameters.