Propagation Measurements and Models of 915 MHz LoRa Radio in a Block Cave Gold Mine
Philip Branch
Abstract
This paper reports on measurements of the propagation characteristics of LoRa radio in the extraction drive of a modern block cave underground gold mine. Block cave mines extract ore from long-lived tunnels lined with steel mesh and steel plates, constructed beneath the ore body. We measured signal propagation with both line of sight and non-line of sight. With line of sight we observed path loss was quite low characterised by a path loss index of 1.25. We also explored non-line-of-sight propagation and found similarly good propagation. We also analysed the effect of LoRa spreading factor (SF) on propagation. SF has a small but noticeable effect on received signal strength and a noticeably stronger effect on Signal to Noise Ratio (SNR). Increasing the spreading factor from 7 to 9 results in a consistent increase of 3 dB in SNR. We also measured RSSI and SNR without line of sight. Again we found very good propagation characteristics. The propagation behaviour might be the consequence of waveguide behaviour of the steel lined tunnel.