Optical Wireless Signals
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, said 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
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 said, "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
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 added.
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 also will describe the system at the
IEEE MILCOM conference in Monterey, Calif., on Nov. 1. His co-author
is Belal Hamzeh, doctoral candidate in electrical
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 more than 30 years but are hampered by
weather and other obstructions that prevent the transmitter and
receiver from "seeing" each other.
Kavehrad explained 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 said 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,"
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.
researchers note that their proposed system ensures on-the-fly
operation without the need for significant electronic
The project is supported by the Air Force
Giving state and local government the information
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