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. We present the crucial insight 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 for the case of an unmodulated external field, for general passive, purely reactive, or purely resistive tag loads. We find that modulating both resistance and reactance is important for very high rates. We discuss the capacity-achieving load statistics, rate asymptotics, technical conclusions, and rate losses from value-range-constrained loads (which are found to be small for moderate constraints). We then demonstrate that near-capacity rates can be attained by more practical schemes: (i) amplitude-and-phase-shift keying on the reflection coefficient and (ii) simple load circuits of a few switched resistors and capacitors. Finally, we draw conclusions for the ambient BC channel capacity in important special cases.

翻译：在反向散射通信（BC）中，无源标签通过负载调制影响外部电磁场来传输信息。由此，受激标签天线的馈电电流通过调整无源终端负载进行调制。本文研究了采用可自由适配无源负载时的可达信息速率。作为前提条件，我们基于电路系统建模统一了单站、双站和环境反向散射通信。我们提出了关键性见解：由于稳态标签电流相量位于圆盘上，信道容量可由峰值功率受限正交高斯信道的现有结果描述。据此，我们推导了未调制外部场情形下的信道容量，涵盖一般无源、纯电抗或纯电阻标签负载。研究发现，同时调制电阻和电抗对于实现极高传输速率至关重要。我们讨论了容量可达的负载统计特性、速率渐近性、技术结论，以及值域受限负载引起的速率损失（在适度约束下该损失较小）。随后证明，通过更实际的方案即可实现近容量速率：（i）反射系数的幅相键控调制，以及（ii）由少量开关电阻和电容构成的简单负载电路。最后，我们在重要特例下得出环境反向散射信道容量的相关结论。