mmWave on a Farm: Channel Modeling for Wireless Agricultural Networks at Broadband Millimeter-Wave Frequency

Millimeter-wave (mmWave) spectrum with wide bandwidth provides a promising solution to enable high throughput in next-generation wireless agricultural networks, characterized by swarms of autonomous ground vehicles, unmanned aerial vehicles (UAVs), and connected agricultural machinery. However, channel models at mmWave frequencies in agricultural environments remain elusive. Moreover, agricultural field channels bear notable distinctions from urban and rural macrocellular network channels due to the dynamic crop growth behavior. In this work, a channel model is developed to characterize the large-scale path loss, coherence bandwidth, and link quality under the effect of various environmental factors based on data collected from extensive field experiments. In particular, the wind effect on signal-to-noise ratio is investigated, and the diffuse scattering of electromagnetic waves due to near-canopy propagation at different crop growth stages. Our analysis results demonstrate that (1) during the growing season, the crop canopy surface acts as a ``new ground'' that creates multipath components that result in a higher path loss exponent, which is correlated with the relative height between the crop canopy surface and the radio, (2) the wind results in a half-power drop (3-dB SNR degradation) for an increase of 4~m/s in gust speed due to beam misalignment and increasing scattering, (3) the channel coherence bandwidth increases as the water content in the crop decreases, and (4) the beam-level spatial consistency observed allows for micro-mobility support for agricultural robotic applications. It is also shown that the impacts of humidity and water vapor on the mmWave channel are insignificant in the absence of rain and irrigation. Such characteristics are fundamental for designing advanced channel estimation and signal processing algorithms in advanced agricultural Internet-of-Things solutions.

毫米波频谱（mmWave）的宽频带为下一代无线农业网络实现高吞吐量提供了有前景的解决方案，该网络以地面自主车辆集群、无人机及联网农业机械为特征。然而，毫米波频率在农业环境中的信道模型仍然难以捉摸。此外，由于作物生长行为的动态性，农业田野信道与城市和农村宏蜂窝网络信道存在显著差异。在本研究中，基于广泛实地实验收集的数据，开发了一种信道模型，以描述在大规模路径损耗、相干带宽和链路质量方面的特性，并考虑了各种环境因素的影响。特别是，研究了风速对信噪比的影响，以及不同作物生长阶段由于近冠层传播导致的电磁波散射。我们的分析结果表明：（1）在生长季节，作物冠层表面充当“新地面”，产生多径分量，导致路径损耗指数升高，这与作物冠层表面与无线电的相对高度相关；（2）由于波束失配和散射增加，风速每增加4~m/s，会导致信噪比下降一半（3-dB SNR恶化）；（3）随着作物水分含量降低，信道相干带宽增加；（4）观察到的波束级空间一致性允许为农业机器人应用提供微移动支持。此外，还表明，在无雨和灌溉的情况下，湿度和水蒸气对毫米波信道的影响微乎其微。这些特性对于设计先进信道估计和信号处理算法，以解决高级农业物联网解决方案至关重要。