Cloudy day won’t rain on laser
communicationsNovember 13, 2006 -
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
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.
Penn State |
Print this
page
E-mail this page to a
friend
