"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 times.
"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.
A'ndrea Elyse Messer | Quelle: EurekAlert!
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