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2004-10-26 | |
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Multi-rate Laser Pulses Could Boost Outdoor Optical Wireless
Performance
Multi-rate, ultra-short laser pulses -- with wave
forms shaped like dolphin chirps -- offer a new approach to help
optical wireless signals penetrate clouds, fog and other adverse
weather conditions, say Penn State engineers.
The new approach could help bring optical bandwidth, capable of
carrying huge amounts of information, to applications ranging from
wireless communication between air and ground vehicles on the
battlefield to short links between college campus buildings to
metropolitan area networks that connect all the buildings in a
city.
Dr. Mohsen Kavehrad, the W. L. Weiss professor of electrical
engineering and director of the Center for Information and
Communications Technology Research, leads the study. He says, "The
multi-rate approach offers many advantages. For example, lower rate
signals can get through clouds or fog when high rate signals can't.
By sending the same message at several different rates, one of them
can probably get through."
Rather than slowing communication down, 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 as well as
providing an increased level of communication reliability by
maintaining a minimum of one active link throughout channel
conditions, he adds.
Kavehrad outlined his team's new approach at the Optics East 2004
Conference in Philadelphia, Oct. 27, in a paper, "Ultra-short Pulsed
FSO Communications System with Wavelet Fractal Modulation." He will
also describe the system at the IEEE MILCOM conference in Monterey,
California, on Nov. 1. His co-author is Belal Hamzeh, doctoral
candidate in electrical engineering.
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 prevent the transmitter and receiver from "seeing"
each other.
Kavehrad explains that clouds and fog often clear abruptly
providing brief windows for transmission, making pulsed delivery
better suited to FSO. The new Penn State approach embeds data in
ultra-short pulses of laser light, shaped via fractal modulation as
wavelets, and then transmits the wavelets at various rates.
Belal says the wavelets are easy to generate. "We use holography
to generate and separate the wavelets. 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," he notes.
The wavelets used by the Penn State team are Meyer's Type which
look like dolphin chirps. 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 supported by the Air Force Research
Laboratory.
Editor's Note: The original news release can be found here.
This story has been adapted from a news release issued by
Penn State.
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