Cloudy day won't rain on laser communications
12th November
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. |
