An IoT radio coverage survey in subway tunnels

It is becoming increasingly relevant to use wireless communications in subway tunnels to gather sensors data for advanced maintenance and security purposes. In this work, we present a propagation survey over IoT radio bands using LoRa modulation in Lisbon's Subway. Several scenarios such as straight and curved tunnels, bifurcations, and partial tunnel obstructions were considered. Measurements were taken with LoRa modulation with a transmitter and receiver set to 868 MHz with spreading factors from 7 to 12. The transmit power was 14 dBm, and half-wave whip antennas with vertical polarization were used. Based on the measurement results and the COST-231 two-slope propagation model, a loss factor of 58.5 dB/km and 80.5 dB/km, were obtained respectively, for unobstructed curved and bifurcation tunnels. An extra attenuation of 10 dB was observed for partial tunnel obstruction by one loaded convoy. The results of this study indicate that using a legal transmission power and for straight and curved tunnels, communication can be maintained for distances exceeding one kilometre.