The turbulent atmosphere causes wavefront distortion, beam wander, and beam broadening of a laser beam.
These effects result in average power loss and instantaneous power fading at the receiver aperture and thus
degrade performance of a free-space optical (FSO) communication system. In addition to the atmospheric turbulence,
a FSO communication system may also suffer from laser beam pointing error. The pointing error causes
excessive power loss and power fading. This paper proposes and studies an analytical method for calculating
the FSO channel fading probability density function (pdf) induced by both atmospheric turbulence and pointing
error. This method is based on the fast-tracked laser beam fading profile and the joint effects of beam wander
and pointing error. In order to evaluate the proposed analytical method, large-scale numerical wave-optics
simulations are conducted. Three types of pointing errors are studied , namely, the Gaussian random pointing
error, the residual tracking error, and the sinusoidal sway pointing error. The FSO system employs a collimated
Gaussian laser beam propagating along a horizontal path. The propagation distances range from 0.25 miles to 2.5
miles. The refractive index structure parameter is chosen to be Cn2
= 5×10-15m-2/3 and Cn2 = 5×10-13m-2/3.
The studied cases cover from weak to strong fluctuations. The fading pdf curves of channels with pointing
error calculated using the analytical method match accurately the corresponding pdf curves obtained directly
from large-scale wave-optics simulations. They also give accurate average bit-error-rate (BER) curves and outage
probabilities. Both the lognormal and the best-fit gamma-gamma fading pdf curves deviate from those of
corresponding simulation curves, and they produce overoptimistic average BER curves and outage probabilities.
Free-space optical (FSO) communication systems suffer from average power loss and instantaneous power fading
due to the atmospheric turbulence. The channel fading probability density function (pdf) is of critical importance
for FSO communication system design and evaluation. The performance and reliability of FSO communication
systems can be greatly enhanced if fast-tacking devices are employed at the transmitter in order to compensate
laser beam wander at the receiver aperture. The fast-tracking method is especially effective when communication
distance is long. This paper studies the fading probability density functions of both fast-tracked and untracked
FSO communication channels. Large-scale wave-optics simulations are conducted for both tracked and untracked
lasers. In the simulations, the Kolmogorov spectrum is adopted, and it is assumed that the outer scale is infinitely
large and the inner scale is negligibly small. The fading pdfs of both fast-tracked and untracked FSO channels are
obtained from the simulations. Results show that the fast-tracked channel fading can be accurately modeled as
gamma-distributed if receiver aperture size is smaller than the coherence radius. An analytical method is given
for calculating the untracked fading pdfs of both point-like and finite-size receiver apertures from the fast-tracked
fading pdf. For point-like apertures, the analytical method gives pdfs close to the well-known gamma-gamma pdfs
if off-axis effects are omitted in the formulation. When off-axis effects are taken into consideration, the untracked
pdfs obtained using the analytical method fit the simulation pdfs better than gamma-gamma distributions for
point-like apertures, and closely fit the simulation pdfs for finite-size apertures where gamma-gamma pdfs deviate
from those of the simulations significantly.
Effects of beam wander on uncoded bit-error-rate (BER) of
direct-detection OOK modulated FSO communication
systems using collimated and focused Gaussian beams are studied. Channel fading statistics are obtained from
large-scale wave optics simulations and compared with the closed-form log-normal and gamma-gamma models.
The avalanche photodiode (APD) is chosen for photodetection. The accurate McIntyre-Conradi APD model is
adopted for performance evaluation. Results show that large performance gain (more than 15dB) can be achieved
with fast-tracked focused beams. The upper bound of higher-order adaptive optics gain beyond tracking gain is
also studied.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.