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, the rate asymptotics, and also the capacity of ambient BC in important special cases. We then propose a capacity-approaching finite constellation design: a tailored amplitude-and-phase-shift keying on the reflection coefficient. Furthermore, we demonstrate high rates for 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.

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。受激标签天线的馈电电流通过调节无源终端负载实现调制。本文研究采用自由可调无源负载时的可达信息速率。作为前提，我们基于电路模型统一了单站、双站和环境反向散射通信系统。关键洞见在于：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正交高斯信道的现有结论描述。据此，我们推导了外部场未调制条件下，通用无源负载、纯电抗负载或纯电阻负载的信道容量。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。本文讨论了容量可达的负载统计特性、速率渐近特性，以及重要特殊场景下环境反向散射通信的信道容量。随后提出一种逼近容量的有限星座设计——基于反射系数定制的幅度-相位键控方案。此外，我们验证了仅采用少量开关电阻和电容的简单负载即可实现高传输速率。最后，针对负载取值区间受限导致的速率损失展开研究，结果表明适度约束下该损失较小。