Quadrature amplitude modulated (QAM) communication link for near and mid-range RFID systems

We present the theory and design equations for leveraging vector modulation in the form of M-ary quadrature amplitude modulation (QAM) for LF near and mid-range RFID systems, which can allow for the design of high bandwidth passive near and mid-range devices. The theory is developed through the impedance transformations that occur in coupled coil antennas. Using the developed theory along with system simulations, we determine the necessary load impedances for 4-QAM constellations at various distances for a carrier frequency of approximately 2.4 MHz and present 3 prototype boards that yield the desired reflections. The achievable data rates are investigated through a custom IQ demodulator, and we show that for communication distances of 17 cm, 29 cm, and 45 cm, which represent scaled coil diameter distances, data rates of 409.6 kbps, 307.2 kbps, and 153.6 kbps can be achieved. This corresponds to bandwidth percentages of 17%, 12%, and 6.5%, which surpasses the bandwidth percentage of typical NFC communication systems of about 3% for a carrier of 13.56 MHz (424 kbps).