LOW COST ASYMMETRIC
RADIOS
by
James Evans

One can imagine Infostations deployed as ubiquitously as pay phones, and portable terminals ranging in complexity from a badge with a display (or even integrated into a watch) to powerful multimedia computers. To implement a badge, a radio is needed that is small, inexpensive and consumes little power. The more powerful terminals require a radio to receive high data rates from the infostation and send data at lower rates in the reverse direction.

A radio that meets these needs could be implemented using a tuned antenna and a diode detector to receive amplitude shift keyed data on the downlink from the Infostation. The reverse (uplink) transmission could be implemented using modulated back scatter. This can be achieved at negligible cost by turning the diode detector on and off and thereby modulating the back scatter or reflection from the tuned antenna. This integration of technologies produces a two way digital radio at the extremes of miniaturization, cost and power consumption. The penalty paid for these desirable attributes is a small Infostation coverage area, controlled primarily by the two way path loss of the back scatter communication. The cell radius could be up to several hundred feet with downlink data rates of hundreds of kilobits per second, and tens of feet with mega bps downlink data rates. The commensurate uplink data rate is approximately a factor of one thousand lower because of the two way path loss mentioned earlier.


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