Traditional passive UHF RFID tags employ either ASK or PSK backscatter modulation to communicate data from memory or sensors on the tag to a remotely-located reader. These simple modulation schemes transfer data at a rate of one bit per symbol period, which for an integrated CMOS tag IC requires an on-chip oscillator with a frequency at least equal to the bit rate. Motivated by the fact that most modern UHF RFID readers already employ I/Q demodulation of the backscattered signal to account for backscatter phase rotation as the tag moves with respect to the reader, we propose a QAM backscatter method using no on-chip inductors that is compatible with a single-chip CMOS tag implementation.With QAM backscatter, tags transmit more than one data bit per symbol period, permitting tag designers to employ a lower power on-chip oscillator operating at a frequency equal to the (lower) symbol rate while maintaining the same data throughput as ASK or PSK, or alternatively to send data at a higher rate for a given on-chip oscillator frequency. We present the fundamental design equations required for arbitrary QAM backscatter modulators and present simulated bit error rate (BER) and error vector magnitude (EVM) curves for the operation of an inductor-free 4-QAM and 8-QAM modulator centered at 915MHz and evaluated over the 860MHz-950MHz worldwide UHF band.
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