Two-Stage Harmonics Rejection in FDMA Backscatter Communication System Using FFT Filterbank

This paper presents a harmonics rejection method for the sensor-to-interrogator link of frequency division multiple access (FDMA) backscatter communication system exploiting an FFT filterbank. It is well known that backscatter signals produce undesired outband emissions and harmonics because of the absence of a pulse shaping filter. Harmonics, produced by introducing subcarrier coding, are particularly problematic because they interfere with particular frequency channels. The proposed method features the generation of harmonics replicas from the baseband signal in an allocated channel produced by a FFT filterbank. The generation of a harmonics replica from the baseband signal demands a delicate handling of the carrier and the subcarrier phases. This paper presents this problem and proposes a two-stage harmonics replica generation exploiting the statistical independence among the interfering and interfered baseband signals. The proposal was implemented in a software defined interrogator, and was evaluated with a wired experiment with various carrier-to-interference ratios and two real-world wireless propagation experiments using prototype battery-free backscatter sensors. These evaluations revealed that the proposed harmonic rejection method could deliver about 6 dB of processing gain.