4.5: RFID Receivers

4.5 RFID Receivers

RFID receivers face a unique set of requirements, quite distinct from those encountered in most other radio systems, particularly when passive tags are used.

In some respects, RFID receivers are easy to build. Because passive tags require so much forward-link power, there is usually not much point in constructing a receiver at the theoretical sensitivity limit, because the tags will have lost power to their IC's by the time they get that far away. The limitations of passive tags also mean that the return link modulation is always some variant of frequency-shift keying, so the demodulation and decoding problems are always relatively simple, compared to more sophisticated radio systems using phase-and-amplitude-keyed signals with error-correcting codes.

On the other hand, RFID receivers face a huge interfering signal or blocker in the form of the transmitted signal leaking into the receiver, either internally within the radio, or from the antenna reflections or leakage, as well as external reflections from the environment. Since this blocker is at the same RF channel as the tag signal it can't be filtered out in the RF part of the radio. Further, the interfering signal swings wildly in amplitude during the time when the transmitted signal is modulated to talk to the tags; the receiver needs to recover from the effects of these changes before it can hope to decipher the small tag response. Finally, because (absent Doppler shifts) the received signal and the local oscillator signal are at exactly the same frequency,...