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：在反向散射通信中，无源标签通过调整外接负载来改变外部电磁场，从而传输信息。具体而言，通过改变无源终端的阻抗来调制受激标签天线的馈电电流。本文研究了采用自由可调无源负载时的可达信息速率。作为前提，我们采用基于电路的系统建模统一了单站、双站与环境反向散射通信。我们提出关键见解：由于稳态标签电流相量位于圆盘内，信道容量可由峰值功率受限的正交高斯信道的现有结果描述。由此，推导了无调制外部场下的信道容量，适用于一般无源、纯电抗或纯电阻性标签负载。研究发现，同时调制电阻和电抗对实现极高传输速率至关重要。我们讨论了容量最优的负载统计特性、速率渐近性能、技术结论以及值域受限负载导致的速率损失（在适度约束下该损失较小）。随后，通过以下实用方案证明了近容量速率的可实现性：（i）反射系数上的幅相键控调制及（ii）由少量开关电阻和电容构成的简单负载电路。最后，针对环境反向散射信道的重要特例得出相关结论。