Ecology :: Cloudy day won't rain on laser = communications

Ecology

Ecology :: Cloudy day won't rain on laser = communications

Ecology :: 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=20 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=20 W. L. Weiss professor of electrical engineering and director, Penn = State=20 Center 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=20 systems can carry large amounts of information, but, the dust, = dirt, water=20 vapor and gases in a fluffy cumulus cloud, scatter the light and = create=20 echoes. The loss of some light to scattering is less important = than those=20 parts of the beam that are deflected and yet reach their target, = because=20 then, various parts of the beam reach the endpoint at different=20 times.

"All of the laser beam photons travel at the speed = of light,=20 but different paths make them arrive at different times," says = Kavehrad.=20 "The 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=20 arriving data of hundreds of symbols, which causes echoes. The = information=20 arrives, but then it arrives again because the signal is = distributed=20 throughout the laser beam. In essence, the message is continuously = being=20 stepped on.

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

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

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

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

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

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

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

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Ecology :: Cloudy day won't rain on laser = communications

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Ecology :: Cloudy day won't rain on laser = communications

Ecology :: Cloudy day won't rain on laser communications

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