PricewaterhouseCoopers Telecom Direct

Free Registration

Hardware, Software & Technology
Special Reports - Hardware

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.

Print This Article