A General Receiver for Backscatter Signals: A Step-by-Step Tutorial

In conventional (Marconi) wireless transmission, information-containing symbols are conveyed by modulating the magnitude and phase of an ac voltage source connected to the terminals of an antenna, at a fixed source impedance. However, a backscatter communication system instead conveys symbols via modulating the load impedance presented to the antenna terminals. This paper demonstrates that these are electromagnetically different problems from the perspective of the receiver, once three or more symbols (e.g., QPSK, QAM or OFDM) are employed. We treat the backscatter reception problem from a general theoretical perspective of estimating the reflection coefficient during each symbol interval, and present a universal technique able to mitigate perturbations in both structural and load-dependent scattering. We present a novel method employing three pilot symbols to estimate the parameters for a Möbius transformation enabling correction of impairments due to the interaction between the structural and load-dependent scattering modes of the antenna, thus mitigating constellation distortion due to the presence of objects in the near field of the backscattering antenna. We additionally generalize the effects of carrier leakage from the carrier source to the receiver to include impairment due to the structural mode scattering from the antenna. Finally, the proposed procedure allows one to remotely estimate the impedance, the antenna mode and the structural mode of the tag antenna in its environment.