Load Modulation for Backscatter Communication: Channel Capacity and Capacity-Approaching Finite Constellations

In backscatter communication (BC), a passive tag transmits information by just affecting an external electromagnetic field through load modulation. Thereby, the feed current of the excited tag antenna is modulated by adapting the passive termination load. This paper studies the achievable information rates with a freely adaptable passive load. As a prerequisite, we unify monostatic, bistatic, and ambient BC with circuit-based system modeling. A crucial insight is that channel capacity is described by existing results on peak-power-limited quadrature Gaussian channels, because the steady-state tag current phasor lies on a disk. Consequently, we derive the channel capacity in the case of an unmodulated external field, for a general passive or purely reactive or resistive tag load. We find that modulating both resistance and reactance is crucial for high rates. We discuss the capacity-achieving load statistics, the rate asymptotics, and the capacity of ambient BC in important special cases. Furthermore, we propose a capacity-approaching finite constellation design: a tailored amplitude-and-phase-shift keying on the reflection coefficient. We also demonstrate high rates for very simple loads of just a few switched resistors and capacitors. Finally, we investigate the rate loss from a value-range-constrained load, which is found to be small for moderate constraints.