Multi-rate laser pulses could boost outdoor optical wireless performance
October 29, 2004, University Park, PA--Multi-rate,
ultra-short laser pulses -- with waveforms shaped like dolphin chirps
-- offer a new approach to help free-space optical wireless signals
penetrate clouds, fog and other adverse weather conditions, said Penn
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.
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 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,
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.
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
Co-author Belal Hamzeh 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," he noted. 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.
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