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IR
Wireless Signals High Bandwidth at Low Power
Wireless
infrared local-area networking is a flexible and economical alternative to
hardwired interconnections. Unlike radio frequency transmission, both
narrow- and wide-angle infrared communications can support high data
rates, but at a cost. Narrow-angle transmission requires precise alignment
of transmitter and receiver, while wide-angle transmission demands high
power.
Mohsen Kavehrad and Svetla Jivkova,
researchers at Pennsylvania State University in University Park, have
illustrated a model that combines elements of both narrow- and wide-angle
systems to deliver high data rates with low power. Prototype components
have been constructed, and Kavehrad plans to demonstrate a system
prototype. Results described are drawn from computer simulation of room,
transmitter and receiver.
Rather than illuminating an
entire room, the transmitter integrates an eight-level computer-generated
hologram coupled to an IR diode to create a 10 x 10 array of 5-cm spots on
the ceiling. The spots are small, allowing their intensity to be kept
relatively low; the receiver's narrow 7° field of view helps filter
background illumination.
As long as the angle of
incidence does not exceed 60°, each spot acts as a secondary lambertian
source, reflecting at a constant intensity, regardless of the angle from
which it is viewed. The spot-to-spot intensity variation is less than 1.5
percent.
The receiver uses a multibranch
configuration with several adjacent small fields. It is constructed from a
holographic curved mirror with a 6-mm-diameter silicon photodiode
positioned at its focus. The mirror functions as a 20-nm bandwidth
spectral filter.
The transmission spot array and the
receiver field of view are designed to ensure that only one transmission
spot is contained within the field, which eliminates the problem of
multiple signal paths. Multiple reflections still can limit the bandwidth
of the system, but with appropriate signal encoding and modulation, data
rates of hundreds of megabits per second are achievable with a transmitter
that consumes well under 1 W.
Richard
Gaughan
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