Evaluation of LoRa technology in 433-MHz and 868-MHz for underground to aboveground data transmission

Abstract

• Development of nodes suited for UG2AG data transmission based on LoRa technology. • Evaluation of 433 MHz and 868 MHz radio modules in the regard of European regulations. • Experiments in real conditions on an open field at 15 and 30 cm burial depths. • Improved performance by placing the emitting antenna directly in contact with the soil. • Interest of the inclination and orientation of the receiving antenna. The development of Wireless Underground Sensor Networks (WUSNs) is currently receiving significant attention to collect data underground all along the year without impacting aboveground activities. Although the opportunities are promising for sectors as agriculture and environment monitoring, the task is particularly challenging as the radio waves are significantly more attenuated in the soil in comparison with in the air. In addition, the communication ranges are highly impacted by some operating and environmental conditions as the soil moisture, its composition and compaction as well as the burial depth of the nodes. In this paper, we developed two sets of nodes operating at 433 MHz and 868 MHz based on the LoRa technology which is the physical layer of the Low Power Wide Area Network LoRaWAN and initially developed for aboveground IoT applications. We successively tested these nodes in real conditions on underground to aboveground (UG2AG) data transmissions and with various operating conditions and radio parameters. First results highlighted the interest of the 868 MHz radio modules tuned at the maximal allowed transmit power in Europe (+14 dBm/25 mW), in comparison with the 433 MHz radio modules (+10 dBm/10 mW). Next results enabled to point out the importance of the inclination of the receiving antenna but also the impact of the burial depth of the emitting node, as well as the interest to place the emitting antenna directly in contact with the soil. The best configuration enabled to reach UG2AG ranges of more than 275 meters long with low depth buried nodes (15 to 30 cm), that clearly enables to envision agriculture and environment monitoring applications based on such radio modules.