Unblocking Laser
Signals December 4, 2006
Laser signals can be
blocked by many different types of objects, including trees,
buildings and mountains. But it's the little things, like fog, rain,
dirt and dust, that unpredictably interfere with laser
communications and tend to vex users the most. Penn State University
researchers are now working on a solution to this problem.
Particles suspended
in the atmosphere scatter signals into separate paths, causing
echoes and dropping data rates to unacceptable levels. "All of the
laser beam photons travel at the speed of light, but different paths
make them arrive at different times," says Mohsen Kavehrad, director
of the Penn State Center for Information and Communications
Technology Research and a professor of electrical
engineering.
Current laser
communications systems are designed to depend on optical signal
processing to help guarantee connectivity. Kavehrad's researchers
are taking a different approach. "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," Kavehrad says.
The researchers have
developed a technique called free-space optical communications that
can not only improve air-to-air communications, but also
ground-to-air links. Since the approach provides fiber optic quality
signals, it is also a potential solution for extending fiber optic
systems to rural areas without laying cable. It may also eventually
be used to bring airplane passengers a clear, continuous wireless
Internet signal.
Using a computer
simulation called the Atmospheric Channel Model, the researchers
process the signal to shorten the overlapping data and reduce the
number of overlaps. The system then processes the remaining signals,
picking out discernable parts to make a whole and eliminate the
remaining echoes. To create a quality fiber optic caliber final
signal, the process must be continuous.
"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."
Using a two-step
process provides the most reliable, high-quality data transfer, says
Kavehrad. Anything less would degrade the system, such as by
shortening the usable transmission distance and increasing the
length of time needed to create the final signal. Kavehrad notes
that the system uses commercially available off-the-shelf equipment
and proven digital signal processing techniques.
The research team is
aiming at a specific goal. "The Air Force, which is funding this
project through the Defense Advanced Research Projects Agency
(DARPA), would like us to deliver close to 3 gigabytes per second of
data over a distance of 6 to 8 miles through the atmosphere," says
Kavehrad.
Copyright 2006 PricewaterhouseCoopers. PricewaterhouseCoopers
refers to the network of member firms of PricewaterhouseCoopers
International Limited, each of which is a separate and independent
legal entity. All rights reserved. The preceding article was written
by John Edwards, a freelance technology writer based in Gilbert,
Arizona. He can be reached by phone at +1-480-854-0011. |