Multi-Hop LoRa-based underground network for monitoring soil moisture in agriculture

Wireless underground sensor networks (WUSN) have gained attention due to the benefits they can bring to many application areas, in particular, to agriculture. However, designing and evaluating WUSNs is more complex than conventional over-the-air wireless networks, especially when the WUSNs have buried nodes. The study of the possibilities and limits of these networks is an active area of research. In this paper we describe a LoRa-based multi-hop WUSN for monitoring soil moisture for an application in agriculture being developed to investigate the behaviour of different species of mamona (Ricinus communis L.) under different soil moisture levels. We first evaluate the use of LoRa for underground-to-underground (UG2UG) communication links and show how different values of the main LoRa parameters affect the quality of these links. Based on the results, we designed a network whose topology is a set of lines of buried sensor nodes covering the whole application area. In this paper we describe the behaviour of one of these lines in a real setting in terms of packet delivery ratio and delay and we estimate the energy consumed for communication. Our protocol provides an inherent level of fault-tolerance by exploring the linear topology. In our experiments, a 100% message delivery ratio was achieved. Additionally, the maximum round-trip delay was less than 200 s. The network satisfies the application message transmission requirement of one message per hour per node by scheduling communication over the six sensor lines needed to cover the whole experiment area in a round-robin fashion. Our main contributions lie in the evaluation of different parameters of LoRa in underground communication and in the development and analysis of a multi-hop routing protocol for a network of buried nodes in a real setting. We are not aware of any other work that addresses these specific issues.