Cloudy day won't rain on laser communications=20

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

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

Laser light used in communications systems can = carry=20 large amounts of information, but, the dust, dirt, water vapor and gases = in a=20 fluffy cumulus cloud, scatter the light and create echoes. The loss of = some=20 light 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 = beam=20 reach the endpoint at different times.

"All of the laser beam = photons=20 travel at the speed of light, but different paths make them arrive at = different=20 times," says Kavehrad. "The Air Force, which is funding this project = through the=20 Defense Advanced Research Projects Agency, would like us to deliver = close to 3=20 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=20 arriving data of hundreds of symbols, which causes echoes. The = information=20 arrives, but then it arrives again because the signal is distributed = throughout=20 the laser beam. In essence, the message is continuously being stepped = on.=20

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

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The researchers developed an approach = called=20 free-space optical communications that not only can improve air-to-air=20 communications, but also ground-to-air links. Because their approach = provides=20 fiber optic quality signals, it is also a solution for extending fiber = optic=20 systems to rural areas without laying cable and may eventually expand = the=20 Internet 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 to=20 shorten the overlapping data and reduce the number of overlaps. Then the = system=20 processes the remaining signal, picking out parts of the signal to make = a whole=20 and eliminate the remaining echoes. This process must be continuous with = overlap=20 shortening and then filtering so that a high-quality, fiber optic = caliber=20 message arrives at the destination. All this, while one or both of the = sender=20 and receiver are moving.

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

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

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

Source: Penn State

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