Vector Backscattered Signals Improve Piggyback Modulation for Sensing With Passive UHF RFID Tags

We expand our “piggyback modulation” technique to integrate sensing functions with existing standard passive UHF RFID tags through wireless coupling. Instead of focusing on the scalar backscattered power that is proportional to the radar cross section (RCS), we exploit the vector backscattered signal with piggyback modulation. Measuring both amplitude and phase of the backscattered wave makes advanced demodulation possible. The measurement setup, experimental results, demodulation analysis, and numerical simulation for the conventional amplitude-shift keying (ASK) and quasi-4-quadrature amplitude modulation (quasi-4-QAM) are presented. Quasi-4-QAM demodulation reduces the interference between two independent data streams and improves the bit error rate for the same RF power. While we do not show explicit data from any given sensor, we do show that a generic sensor signal will be improved with our technique.