Draft copy (presented at IFAC MMM Workshop on Automation in Mining, Mineral and Metal Industry, 2009) 1/6 Wireless SISO Channel Propagation Model for Underground Mines Walter Grote Electronic Engineering Department, Universidad Técnica Federico Santa María, Valparaíso, Chile (e-mail: walter.grote@usm.cl) Abstract: Modern mining procedures aim to replace human resources directly involved in the mineral extraction process by automatic or remote control systems. Data may be collected from sensors, or data and/or video may be transmitted by telecommunication equipment. The actual technology trend is to do these transmissions using wireless technology (WSN, Wi-Fi, among others). This work summarizes the experience that has been collected from characterizing a Single-Input-Single-Output (SISO) large scale 870 MHz and 2.45 GHz wireless propagation channel in an underground copper mine. Also the Rician K factor and channel coherence time are calculated for a 2.45 GHz mobile link. Keywords: telecommunication, wireless propagation, underground copper mines 1. INTRODUCTION Open or underground mine working conditions provide hazardous working conditions and therefore there has been an increasing trend to replace humans by locally or remotely controlled intelligent machines. In both cases remote data collection and control are essential. Wireless technology is an attractive solution to perform this task, since it provides mobility, communications and reduces installation costs. In spite of the increasing trend of using wireless communications, electromagnetic (EM) wave propagation in tunnels is still not well understood. In part, this can be traced to the fact that wireless technologies are aimed to achieve higher transmission bandwidths, which calls for higher carrier frequencies. EM propagation, radiated by antennas or leaky conductors can be traced to the 40’s, Shanklin (1947). Significant contributions were made by James Wait and have been very well summarized by Tripp (2000). In this publication, we present the wireless propagation channel characteristics derived from measurements performed in the Chilean copper mine El Teniente, said to be the largest underground excavation of its kind in the world, located 50 kms east of Rancagua, 3269 m above sea level, in the Andean mountain range. This mine, which began activities in 1904, has 2400 kilometers of underground drifts. El Teniente produces 404,738 metric tons of fine copper per year using panel caving methods. Different layers of tunnels are named Teniente N, being N=4 at a higher level than N=8. 2. MINE TUNNEL LARGE SCALE PATH LOSS MODELS Tunnels can be treated as waveguides and therefore transverse electric and magnetic modes may propagate above a certain cutoff frequency. This effect was described by Mahmoud (1974), who stated that “frequencies above about 500 MHz propagate through the tunnel via successive reflections from the tunnel walls. Due to the large dimensions of the cross section relative to the wavelength, the waves fall at almost grazing angles on the walls and hence suffer low loss. At lower frequencies (than 500 MHz) the losses in the walls obviously increase while at frequencies higher than 2 GHz, the excess loss due to the wall roughness becomes increasingly important as has been observed by Emslie et. al. (1975). Thus the working range of radio frequencies for free propagation is considered to lie roughly between 500 MHz – 2 GHz”. Underground mine tunnels are imperfect waveguides, since its walls are not perfect conductors, the electrical parameters are not known and vary according to rock composition within the mine. Values of the cutoff frequencies depend on the shape and the transverse dimensions of the tunnel and the given propagation mode, which vary for different carrier frequency values, as it was experimentally confirmed, Deryck (1978). 2.1 Reported Attenuation Values Dudley and Mahmoud (2006) found that the attenuation for the lower order modes for the TE 0m modes are much less severe than for the TM 0m case at a 1GHz frequency in a 2m radius tunnel. For ε r = 12, when excited by a linear current source, it is 1.098 dB/100m for the TE 01 mode. Changsen (2006) summarized EM propagation characteristics of UHF radio wave in circular, rectangular, arched, ellipse, trapezium and semi-circle tunnels. In his publication the attenuation rate α can be predicted by (1), when the transmission frequency exceeds the cutoff frequency. ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ + − = 3 3 2 1 1 h w x r r ε ε λ α (1) λ is the carrier’s wavelength, the internal free space electricity parameter is (ε 0 , μ 0 ), the exterior tunnel magnetic conductivity rate is μ 0 , ε r is relative dielectric constant, w is the maximum width and h is the height of the tunnel. In general, for rectangular, oval, circular, trapezoid semicircle arch tunnels 4.34 < x < 5.13. In the case of a circular tunnel, x