  In Zigbee backscatter systems, tags piggyback information by adding phase
shifts to the RF carriers. The instantaneous-phase shift (IPS) modulation adds
phase shifts by toggling between discrete phases, which is easy to realize and
is widely used in previous systems. However, as the spectrum efficiency of IPS
is poor, it is not suitable for large networks. Thus, frequency-phase shift
(FPS) modulation was proposed to make up this drawback. It adds continuous
phase shifts to the non-productive carriers by toggling between square waves of
different frequencies and has higher spectrum efficiency. In this paper, we
realized IPS modulation and FPS modulation on Zigbee single-tone signals,
respectively. In addition, we creatively proposed to apply FPS modulation to
codeword translation.
  We prototype the system using a microchip transmitter, an off-the-shelf FPGA,
and a commodity Zigbee receiver. Through extensive experiments, we proved that
FPS modulated Zigbee transmissions have a bandwidth of 1.8 MHz, Which is 2x
lower than that of IPS. In addtion, we conducted simulation experiments in
MATLAB and demonstrated that FPS modulation can be used in Zigbee codeword
translation.
