Characterization of Angular Spread in Underground Tunnels Based on the Multimode Waveguide Model

One of the biggest challenges facing wireless designers is determining how to configure multiple-input-multiple-output (MIMO) antennas in underground-environments, such as mines, so that they deliver best performance. Here, we showed that angular dispersion of the signal, a phenomenon that greatly impacts channel capacity, can be accurately predicted in both near-field and far-field of underground tunnels using a multimode waveguide. Not only this analytical model is much faster than other methods, but also it does not have other shortcomings of measurement-based modeling (e.g., poor resolution for far distances in the tunnel). Further, we characterized angular-spread for different tunnel sizes and showed that the power-azimuth-spectrum can be modeled by a zero-mean Gaussian distribution whose standard-deviation (angular-spread) is dependent on the tunnel dimension and the transmitter-receiver distance. Angular-spread is found to be a decreasing function of axial distance. After a certain distance, it becomes very small (about 4 degrees) and independent of tunnel transverse dimension. Our results are useful for design of MIMO systems in underground tunnels and sufficient to permit the correlation-matrix required to extend the IEEE 802.11n MIMO channel model to underground environments.