The introduction of low-cost chip transceivers has driven the exploration of applications extending wireless sensing technology below the ground. The fundamental challenge with wireless underground sensor networks (WUSNs) however is that the high dielectric permittivity of the soil leads to significant absorption losses during electromagnetic (EM) wave propagation. The underground wave is also subject to attenuation due to phenomena such as multi-path fading, reflection and refraction, effectively limiting the node-to-node communication distance to just a few meters. Although some analytical and empirical models have been proposed to characterize the underground RF channel, validation of these models using commercial off-the-shelf wireless sensing hardware has been limited. This research aims to better characterize the performance of WUSNs in different soils, under varying soil conditions, and with different transceiver configurations. Parametric analysis of the influence of carrier frequency, data rate and modulation format on underground EM wave propagation is included. The experiment includes controlled laboratory tests in uniform soil with prepared moisture content and density as well as an in-situ field deployment. Presented are laboratory and field testing data conducted with 2nd generation wireless sensor networks, commonly used in terrestrial applications, buried in underground environments. An extensive matrix of tests is conducted to examine the influence of both radio characteristics, including modulation format, carrier frequency, and data rate, as well as locational characteristics, including transmission distance and depth of burial, on the received signal strength of the transmitted packets. Statistical analysis of the experimental data acquired in the laboratory and field is found to be consistent and indicate that carrier frequency, data rate, and transmission distance most strongly influence the received signal strength. In this study, the maximum permissible node-to-node transmission distance is not reached in either the laboratory or field testing, and thus the results indicate that node-to-node transmission distances of several meters can be reliably achieved in granular soils.
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