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Date:	November 12, 2006

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Cloudy Day Won't Rain On Laser = Communications

Just as clouds block the sun, they interfere with = laser=20 communications systems, but Penn State researchers are using a = combination=20 of computational methods to find the silver lining and punch = through the=20 clouds.

"Radio frequency communications are generally reliable and well = understood, but cannot support emerging data rate needs unless = they use a=20 large portion of the radio spectrum," says Mohsen Kavehrad, the W. = L.=20 Weiss professor of electrical engineering and director, Penn State = Center=20 for Information and Communications Technology Research. "Free = space=20 optical communications offer enormous data rates but operate much = more at=20 the mercy of the environment."

Laser light used in communications systems can carry large = amounts of=20 information, but, the dust, dirt, water vapor and gases in a = fluffy=20 cumulus cloud, scatter the light and create echoes. The loss of = some light=20 to scattering is less important than those parts of the beam that = are=20 deflected and yet reach their target, because then, various parts = of the=20 beam reach the endpoint at different times.

"All of the laser beam photons travel at the speed of light, = but=20 different paths make them arrive at different times," says = Kavehrad. "The=20 Air Force, which is funding this project through the Defense = Advanced=20 Research Projects Agency, would like us to deliver close to 3 = gigabytes=20 per second of data over a distance of 6 to 8 miles through the=20 atmosphere."

That 6 to 8 miles is sufficient to cause an overlap of arriving = data of=20 hundreds of symbols, which causes echoes. The information arrives, = but=20 then it arrives again because the signal is distributed throughout = the=20 laser beam. In essence, the message is continuously being stepped = on.

Kavehrad and Sangwoo Lee, graduate student in electrical = engineering,=20 presented their solutions to the echo problem at the recent IEEE = Military=20 Communications Conference in Wash., D.C.

"In the past, laser communications systems have been designed = to depend=20 on optical signal processing and optical apparatus," says = Kavehrad. "We=20 coupled state-of-the-art digital signal processing methods to a = wireless=20 laser communications system to obtain a reliable, high capacity = optical=20 link through the clouds."

The researchers developed an approach called free-space optical = communications that not only can improve air-to-air = communications, but=20 also ground-to-air links. Because their approach provides fiber = optic=20 quality signals, it is also a solution for extending fiber optic = systems=20 to rural areas without laying cable and may eventually expand the = Internet=20 in a third dimension allowing airplane passengers a clear, = continuous=20 signal.

Using a computer simulation called the atmospheric channel = model=20 developed by Penn State's CICTR, the researchers first process the = signal=20 to shorten the overlapping data and reduce the number of overlaps. = Then=20 the system processes the remaining signal, picking out parts of = the signal=20 to make a whole and eliminate the remaining echoes. This process = must be=20 continuous with overlap shortening and then filtering so that a=20 high-quality, fiber optic caliber message arrives at the = destination. All=20 this, while one or both of the sender and receiver are moving. =

"We modeled the system using cumulus clouds, the dense fluffy = ones,=20 because they cause the most scattering and the largest echo," says = Kavehrad. "Our model is also being used by Army contractors to = investigate=20 communications through smoke and gases and it does a very good job = with=20 those as well."

The computer modeled about a half-mile traverse of a cumulus = cloud.=20 While the researchers admit that they could simply process the = signal to=20 remove all echoes, the trade-offs would degrade the system in = other ways,=20 such as distance and time. Using a two-step process provides the = most=20 reliable, high-quality data transfer.

The system also uses commercially available off-the-shelf = equipment and=20 proven digital signal processing techniques.

Can't find it? Try searching = ScienceDaily or the=20 entire web with:

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