Making Things Talk

306 MAKING THINGS TALK

transceiver and a small amount of nonvolatile memory.

They are powered by the current that the reader’s signal

induces in their antennas. The received energy is just

enough to power the tag to transmit its data once, and the

signal is relatively weak. Most passive readers can only

read tags a few inches to a few feet away.

In an active RFID system, the tag has its own power supply

and radio transceiver, and transmits a signal in response

to a received message from a reader. Active systems can

transmit for a much longer range than passive systems,

and are less error-prone. They are also much more

expensive. If you’re a regular automobile commuter and

you have to pass through a toll gate in your commute,

you’re probably an active RFID user. Systems like E-ZPass

use active RFID tags so that the reader can be placed

several meters away from the tag.

You might think that because RFID is radio-based, you

could use it to do radio distance ranging as well, but that’s

not the case. Neither passive nor active RFID systems are

typically designed to report the signal strength received

from the tag. Without this information, it’s impossible

to use RFID systems to determine the actual location of

a tag. All the reader can tell you is that the tag is within

reading range. Although some high-end systems can

report the tag signal strength, the vast majority of readers

are ...