Harmonic Suppression in Frequency Shifted Backscatter Communications

This paper provides a comprehensive study of the harmonics generated by a frequency shifted backscatter communication system. The suppression and the manipulability of different harmonics are of importance to avoid detrimental inter-user interference when a number of backscattering nodes (and perhaps also other active wireless users) operate simultaneously in a network. In this paper the harmonics generated by a widely adopted open-short backscatter tag architecture is firstly presented. Then the ideal backscatter system which generates no unwanted harmonics is discussed, which inspires various harmonic suppression strategies. In particular, practical constraints of the backscatter tag hardware capabilities are applied, e.g. the number of discrete reflection coefficients that can be synthesized, and the dimension of the reflection coefficients (real-valued or complex-valued). Furthermore, the dual-transistor based IQ backscatter modulator is found useful to suppress all mirror harmonics and any specified higher order harmonics. The applicability of these proposed harmonic suppression approaches are demonstrated by an exemplar backscatter network consisting of multiple nodes performing binary frequency shifted keying (2FSK) modulated backscatter communications simultaneously.