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Cloudy day won't rain on laser communications |
| TheAllINeed.com |
(NC&T/PSU) "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 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. |
| About the
Author |
©2006 All rights
reserved
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| Quotes |
Figures wont lie, but
liars will figure.
General Charles H. Grosvenor.
He
thought the formula for water was H-I-J-K-L-M-N-O (H-to-O).
He uses statistics as a drunken man uses lampposts — for
support rather than illumination. — Andrew Lang.
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