## 2 Wireless Communication Technologies in Harsh Underground Mining Operations
### Harsh Underground Environment  
Underground mining operations themselves create and take place in confined spaces. Surrounding masses of varying material, often supported by steel or wood installations, are less penetrative for the electromagnetic waves of wireless communication technologies than open spaces. These hinder the transmission of signals significantly and result in a far different signal attenuation when considering different scenarios. Alongside these installed signal hindrances, the signal-based wave modulations and wavelengths that do not penetrate rock masses to a sufficient extent need to propagate along the mining tunnels and drifts. This paper examines the propagation of signals through various scenarios, including straight drift, reflection around corners, and penetration into parallel drifts through dense materials in underground mines. A key distinction in communication systems is the transmission of signals between antennas under line-of-sight (LOS) and non-line-of-sight (NLOS) conditions. Line-of-sight (LOS) refers to the condition where a direct unobstructed path exists between the transmitting and receiving antenna, allowing for the wireless signal to travel directly from the transmitter to the receiver. Non-line-of-sight (NLOS) refers to the condition where there is no direct unobstructed path between the transmitting and receiving antenna, resulting in the wireless signal being blocked or reflected by obstacles such as walls, rocks, and other underground materials before the receiver antenna can be reached. When we talk about NLOS, we consider two subset conditions. We call the first NLOS subset condition 'Near LOS'. In Near LOS, there is no unobstructed path, but the extent of the obstructions between the antennas is relatively low. The signal can reach the receiver antenna through propagation, such as reflection, diffraction or scattering. The second subset NLOS condition describes situations where signals need to pass obstacles and propagate significantly or even penetrate (rock) material to reach the receiver; we would call it NLOS (as a clear NLOS situation). Both described conditions can lead to weak or no signal reception and increased interference, making it difficult for wireless systems to establish or maintain a stable and reliable connection. Drift, tunnels and working areas in underground mining offer several distinctive geometric features on a macroscopic scale that are different from other tunnel environments and have a significant effect on the propagation characteristics of wireless radio waves along drifts [12; 13]. The propagation along the mining tunnels and drifts creates refraction loss in vertical and horizontal direction, roughness loss of the tunnel surface as well as tilt losses. These are dependent on the following parameters [14]:  
* Wavelength,
* Tunnel width, tunnel height,
* Relative permittivity of the rectangular tunnel sidewalls, floor, and ceiling,
* Mine wall roughness,  
Long range tilt of the mineral body [15],
* Distance to sidewalls, tilt angle (in radians) about a vertical axis, and transversal positions of the antenna. * Fortifications in the form of pillars, metal and wooden installation [13]. * Parameter correlations following tilt-variations and tunnel dimensions at a larger scale. These points lead to increased difficulty in estimating the signal behavior reliably. Experiments to understand the electromagnetic wave propagation have found that wave propagation varies even within the same mine [16]. Underground mine areas are known for their often as harsh defined conditions. This is also connected to the harsh operational characteristics that, beyond the geometric nature of the mine, have an influence on the transmission of signals when using wireless communication technologies.