Enabling ZigBee Backscatter Communication in a Crowded Spectrum

To piggyback information, the instantaneous-phase shift (IPS) modulation toggles discrete phases on ambient RF carriers, which is popular with advanced backscatter systems. However, IPS has poor spectrum efficiency. It produces serious spectrum sidelobes and prevents the formation of large networks. In this paper, we propose frequency-phase shift (FPS) modulation, a fine-grained RF switches toggling that modulates carriers with a continuous phase shift. The phase continuity suppresses spectrum sidelobes without disturbing the demodulation results. We first apply FPS to optimize a ZigBee backscatter tag. ZigBee signals, consisting of a non-single-tone header and a single-tone payload, are transmitted as RF carriers. The backscatter tag leverages FPS to modulate the single-tone for phase-continuity data transmission. Further, the tag recycles the non-single-tone header using sub-symbol codeword translation to improve carrier utilization. Through extensive experiments and field studies, we demonstrate that FPS enables ZigBee transmissions with a bandwidth of 2.4 MHz, which is 3x lower than that of Interscatter [1] and much closer to active radios. The system prototype consists of a microchip transmitter, a backscatter tag, and a commodity receiver. Specifically, when the transmitter-to-tag distance is within 5 centimeters, the system enables a goodput of 16.6 kbps at a channel capacity of 16.8 kbps, and the communication distance can be extended to 17 meters.