Impacts of Soil and Antenna Characteristics on LoRa in Internet of Underground Things

Long-range (LoRa) is a suitable candidate for underground wireless communications due to its capability of communicating over a long range. However, due to the uniqueness of soil properties at a given geographical location, and the varying nature of soil moisture, it is challenging to apply a universal approach to characterize LoRa in wireless underground channels. In this paper, the performance of LoRa in underground channels is studied both theoretically and empirically. The range and bit error rate (BER) formulation of LoRa is derived as a function of soil parameters based on statistical underground channel models. To validate the model, path loss measurements are conducted under different moisture levels in two soil types (sandy and silty clay loam soil). In addition, as underground communication is also dependent on the return loss of buried antennas, the path loss measurements are performed using two different types of underground antennas. Results show that the underground channel models agree well with empirical LoRa measurements, resulting in R-squared values of 0.87-0.89. The results suggest that the performance of LoRa in underground channels can be predicted using the models developed in this paper.