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.
翻译:在后散射通信(BC)中,一个被动标签传递信息的方式只是通过负载调制来影响外部电磁场。 因此, 振动标签天线的反馈流通过调整被动终止负荷来调节。 本文用自由调整被动负载来研究可实现的信息率。 作为先决条件, 我们将单静态、 双静态和环态的BC 与基于电路的系统模型统一起来。 一个至关重要的洞见是, 频道能力通过峰值- 受限制的四面形高空频道的现有结果来描述, 因为稳定状态标记当前散射器位于磁盘上。 因此, 我们从一个未调整的外部域中获取频道能力, 用于一般被动或纯被动或抵抗性标签负载。 我们发现, 调和反应对高比率都是至关重要的。 我们讨论能力- 实现的负载统计、 受控率 以及环境 BC 在重要特殊情况下的能力配置定型星座设计: 一种适应性小相位和相位移动的定置器。 我们发现, 一个简单的反射电容器的压压率是高的压压。