Improving wireless communications in underground mines using reconfigurable antennas

Abstract

This thesis evaluates the effect of a using reconfigurable antenna (RA) on the performance of wireless communication in underground mines. Multipath levels in the underground mine propagation environment vary from moderate to high. The severity of multipath fading is controlled by the tunnel dimension, operating frequency, wave polarisation, electrical properties, and antenna position. Antenna design considers the last three controlling factors. Thus, this thesis investigates the usefulness of an RA in mitigating multipath effects in an underground mine at the physical layer. Existing multipath mitigation techniques feature bulky antenna systems or computationally intensive processes and are therefore unsuitable for the underground mine environment. Therefore a symmetrical pattern RA was selected for investigation. The circular planar antenna consists of four radiating elements separated by a 0.1 m single-layer substrate to provide spatial and polarisation diversity. Complex channel response from three experiments was recorded and analysed to evaluate the effects of antenna placement, changing tunnel dimension, and wave polarisation. The antenna placement was conducted in an emulated tunnel while the effect of changing tunnel dimension was conducted in a hard rock underground mine. The channel sounder is a portable vector network analyser (VNA) transmitting from port 1 and receiving on port 2. The wide stationary channel was swept from 1 to 4 GHz in 15MHz steps. The S21 data was post-processed to determine the path loss exponent, time dispersion, and capacity for the third experiment. A dipole antenna was characterised in the underground mine with the RA to provide reference measurements. Results suggest that the spatial diversity and polarisation diversity gains improved the path loss exponent in the tunnel with changing dimension. The directed radiation pattern of the RA filters out multipath components to significantly reduce the mean RMS delay in the non-line-of sight (NLOS). The RA offers 22% and 21% path loss improvement over the dipole in the line-of-sight (LOS) NLOS, respectively. Results also suggest that the path loss is less correlated to the distance. Shadowing in the LOS environment is lower than that of the NLOS for both antenna types. However, the RA show 23% and 28% higher deviation of the signal power in the LOS and NLOS environment, respectively, when compared to the dipole. The time dispersion of the channel is considered in the coherence bandwidth. The coherence bandwidth in the LOS is greater than that of the NLOS. However, the RA and dipole in a narrow tunnel dimension experience a 33% decrease in coherence bandwidth. Compared with previously published data, the published directional and dipole results reported a decrease of 75%, while the dipole experienced a 54% decrease. This suggests that time dispersion is a function of the antenna radiation pattern and not the tunnel dimension. The RA placement investigation showed that placing the transmitting RA under the ceiling is optimum. Although placing the antenna on the walls induces some loss, it is not significant. The best optimum transmit-receive pair can be realised every time the channel changes. Thus, an RA offers flexibility in antenna placement with reliability in the wireless link. Spatial diversity improves the path loss exponent as well as antenna insertion loss leading to low signal attenuation. Polarisation diversity improves polarisation loss due to tunnel wall reflections and EM wave propagation around the bend. The combined effect of spatial polarisation diversity ensures that a reliable link is available with 0.7 probability. A pattern and polarisation RA is recommended as an effective physical layer multipath mitigation technique for wireless communications in an underground mine.