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11/10/06

"Free-space optical communications" can improve laser = communications and resist environmental interference

Categories: Co= mmunication,=20 Op= tical=20 Electronics 12:11:16 pm

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

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

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

"All of the laser beam photons travel at the speed of light, but = different=20 paths make them arrive at different times," says Kavehrad. "The Air = Force, which=20 is funding this project through the Defense Advanced Research Projects = Agency,=20 would like us to deliver close to 3 gigabytes per second of data over a = distance=20 of 6 to 8 miles through the 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 = then it=20 arrives again because the signal is distributed throughout the laser = beam. In=20 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=20 Communications Conference in Wash., D.C.

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

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

Using a computer simulation called the atmospheric channel model = developed by=20 Penn State's CICTR, the researchers first process the signal to shorten = the=20 overlapping data and reduce the number of overlaps. Then the system = processes=20 the remaining signal, picking out parts of the signal to make a whole = and=20 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, the dense fluffy ones, = because=20 they cause the most scattering and the largest echo," says Kavehrad. = "Our model=20 is also being used by Army contractors to investigate communications = through=20 smoke and gases and it does a very good job with those as well."

The computer 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 = distance=20 and time. Using a two-step process provides the most reliable, = high-quality data=20 transfer.

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

Source: Penn State

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What's Next In Science & Technology

Today's Research, Tomorrow's Reality. No gadgets and gizmos in this = science=20 and technology news blog. Only important scientific innovations which = will=20 drastically change our lives.