IR Wireless Signals High Bandwidth at Low Power
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.
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