Experimental Investigation and Path Loss Modeling for 868-MHz ISM Band Communication Between Pipe Monitoring Sensors and Above Ground Receivers

Low power, long range, wireless communication technologies enable a range of monitoring applications in difficult to reach locations. In this paper, we investigate one such application for monitoring an underground pipeline infrastructure in the 868MHz Industrial, Scientific and Medical (ISM) band. We deployed five underground LoRa/LoRaWAN communication modules on district heating pipes, sealed off below an asphalted road segment. Several specific measurements have been conducted to characterize the propagation conditions from below to above ground, and from which we propose models decomposing propagation into separate contributions from soil, ground to free-space, and above ground propagation. The main novelty of the work lies in the application to covered surfaces and the combined above ground/aerial model framework for communication from below ground. It was found that the Peplinski soil loss model is applicable for estimating the effect of the ground, which in combination with Fresnel refraction and a log-distance model for above ground propagation leads to a consistent modeling framework for propagation to lamp post height gateway infrastructure. For aerial-based gateways, the main finding is that the propagation from below to above ground changes the propagation behavior with distance above ground, despite the otherwise unobstructed free space propagation path. This led to a path loss model with loss increasing exponentially with height and according to cylindrical wave propagation with lateral (2D) distance, aligning with the estimations based on the ground-based model. The results suggest overall that the volumetric water content (VWC) of a compacted and covered soil mixture tends to stabilize to high values.