Multi-rate, ultra-short laser pulses--featuring specially-shaped waveforms--could help optical wireless signals better penetrate clouds, fog and other kinds of adverse weather conditions. The new approach, which is being developed by Penn State engineers, could eventually bring ample optical bandwidth to a wide range of wireless applications, including communication between battlefield air and ground vehicles, short links between office buildings and metropolitan area networks that cover an entire city.

"The multi-rate approach offers many advantages," says Mohsen Kavehrad, the project's lead researcher and a Penn State professor of electrical engineering. "For example, lower rate signals can get through clouds or fog when high rate signals can't," he says. "By sending the same message at several different rates, one of them can probably get through." Kavehrad is also director of Penn State's Center for Information and Communications Technology Research.

Rather than slowing down communication, the multi-rate approach has been shown in tests to achieve an average bit rate higher than conventional optical wireless links operating at 2.5 Gbps. The technique also provides an increased level of communication reliability by maintaining a minimum of one active link throughout channel conditions.

In optical wireless systems, also known as free-space optics (FSO), voice, video and/or data information is carried on line-of-sight, point-to-point laser beams. Outdoor FSO systems have been in use for over 30 years, but are hampered by weather and other obstructions that can prevent the transmitter and receiver from "seeing" each other. Kavehrad explains that clouds and fog often clear abruptly, providing brief windows for transmission and making pulsed delivery useful for FSO. The new approach embeds data in ultra-short pulses of laser light, shaped via fractal modulation as wavelets, and then transmits the wavelets at various rates.

The wavelets are easy to generate, says Belal Hamzeh, a project researcher and a Penn State doctoral candidate in electrical engineering. "We use holography to generate and separate the wavelets," he notes. "You just generate the mother wavelet and then the others can be generated as a fraction of the transmission bit rate of the mother. They can all co-exist in the channel without interference."

The wavelets minimize bandwidth waste and the ultra-short pulses are less likely to interact with rain or fog that could degrade the signal. The researchers note that their proposed system ensures on-the-fly operation without the need for significant electronic processing. The project is being supported by the Air Force Research Laboratory.

Copyright © 2004 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.