Just as clouds block the sun, they interfere with laser
communications systems, but Penn State researchers are using a
combination of computational methods to find the silver lining and
punch through the clouds.
"Radio frequency communications are generally reliable and well
understood, but cannot support emerging data rate needs unless they
use a large portion of the radio spectrum," says Mohsen Kavehrad,
the W. L. Weiss professor of electrical engineering and director,
Penn State Center for Information and Communications Technology
Research. "Free space optical communications offer enormous data
rates but operate much more at the mercy of the environment."
Laser light used in communications systems can carry large
amounts of information, but, the dust, dirt, water vapor and gases
in a fluffy cumulus cloud, scatter the light and create echoes. The
loss of some light to scattering is less important than those parts
of the beam that are deflected and yet reach their target, because
then, various parts of the beam reach the endpoint at different
"All of the laser beam photons travel at the speed of light, but
different paths make them arrive at different times," says Kavehrad.
"The Air Force, which is funding this project through the Defense
Advanced Research Projects Agency, would like us to deliver close to
3 gigabytes per second of data over a distance of 6 to 8 miles
through the atmosphere."
That 6 to 8 miles is sufficient to cause an overlap of arriving
data of hundreds of symbols, which causes echoes. The information
arrives, but then it arrives again because the signal is distributed
throughout the laser beam. In essence, the message is continuously
being stepped on.
Kavehrad and Sangwoo Lee, graduate student in electrical
engineering, presented their solutions to the echo problem at the
recent IEEE Military Communications Conference in Wash., D.C.
"In the past, laser communications systems have been designed to
depend on optical signal processing and optical apparatus," says
Kavehrad. "We coupled state-of-the-art digital signal processing
methods to a wireless laser communications system to obtain a
reliable, high capacity optical link through the clouds."
The researchers developed an approach called free-space optical
communications that not only can improve air-to-air communications,
but also ground-to-air links. Because their approach provides fiber
optic quality signals, it is also a solution for extending fiber
optic systems to rural areas without laying cable and may eventually
expand the Internet in a third dimension allowing airplane
passengers a clear, continuous signal.
Using a computer simulation called the atmospheric channel model
developed by Penn State's CICTR, the researchers first process the
signal to shorten the overlapping data and reduce the number of
overlaps. Then the system processes the remaining signal, picking
out parts of the signal to make a whole and eliminate the remaining
echoes. This process must be continuous with overlap shortening and
then filtering so that a high-quality, fiber optic caliber message
arrives at the destination. All this, while one or both of the
sender and receiver are moving.
"We modeled the system using cumulus clouds, the dense fluffy
ones, because they cause the most scattering and the largest echo,"
says Kavehrad. "Our model is also being used by Army contractors to
investigate communications through smoke and gases and it does a
very good job with those as well."
The computer modeled about a half-mile traverse of a cumulus
cloud. While the researchers admit that they could simply process
the signal to remove all echoes, the trade-offs would degrade the
system in other ways, such as distance and time. Using a two-step
process provides the most reliable, high-quality data transfer.
The system also uses commercially available off-the-shelf
equipment and proven digital signal processing techniques.
SOURCE: Penn State