## 1 Introduction  
Wireless communication has never been more popular [1]. The 5G rates for data and availability are today's standard, with 6G right around the corner [2]. Wireless communication is crucial for performing many of our daily tasks. The reliability of the communication link is a critical issue and in some cases there is no room for error. One of the primary limitations of wireless communication in tunnels is constructive and destructive interference caused by reflections from walls [3]. However, the influence of each of the components that produce this phenomenon is insufficiently clear. These components may include: the transmission frequency, length and height of the tunnel, the location and polarization of the antennas, the type of tunnel and the roughness of its walls. One of the main parameters with which the performance of a communication link is measured is root mean square (RMS) delay spread, \(\sigma_{\tau}\). RMS delay spread describes the duration it takes for all the reflections from a broadcast at a given moment to reach the receiver; in other words, it provides us with the data rate that can be transmitted with the link. Table 1 shows typical RMS delay spread times in different environments. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multicolumn{1}{c}{**Environment**} & \multicolumn{1}{c}{**Center Frequency**} & \multicolumn{1}{c}{**Bandwidth**} & \multirow{2}{*}{\(\sigma_{\tau}\) **(ns)**} \\  & **(MHz)** & & **(MHz)** & \\ \hline Urban—New York City [4] & 910 & 10 & 1300 \\ Suburban [4] & 910 & 10 & 200–310 \\ Indoor—Office Building [5] & 1500 & 500 & 25 \\ Underground Mine & & & \\ Channel [6] & 3500 & 3000 & 11.8 \\ Uneven Underground Mine Channel [7] & 2400 & 200 & 9.92 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: RMS Delay Spread typical values. A common approach is to perform an experiment on a specific site without basing the research on any calculations and theoretical model [7; 8; 9; 10; 11; 12; 13; 14]. In this form of work, one can learn about a single and specific case, without the ability to generalize to different cases. Similarly, the opposite case also exists [15; 16]: Performing only simulated results, without any experimental verification. This may lead to a situation where a variety of variables are not taken into account while performing the sterile calculations in the simulation, which will lead to invalid results. Therefore, the simulation should be validated with an experiment. In many studies the bandwidth that was used was very limited [7; 12; 13; 17; 18]. This is contrary to the desire to get as high quality results as possible. However, in order to obtain results that accurately reflect the limitations of the tunnel, it is necessary to perform high-resolution measurements. Thus, in order to obtain a higher resolution for measurements in time domain it is necessary to increase the bandwidth in the frequency domain when making the measurements. Other studies have based their research on experiments with very low measuring resolution [19]: That is, a very limited number of points along the tunnel. Alternatively, the whole length of their experiment was very short [18; 20]. In such a situation, it is not possible to get an accurate snapshot, as the signal behavior in the tunnel is not linear or predictable and there are frequent changes along the tunnel. A common issue in wireless communication experiments is the additional phase added to the results by the measuring equipment. To avoid this phenomenon, fiber optics can be used, since they do not change the phase. Some studies on this subject were aided by fiber optics [19; 21], but no in-depth campaign was carried out. Although similar studies have been previously conducted by other authors, no in-depth and sufficiently accurate investigation has been conducted. In this study, a comprehensive study is conducted on the limitations of wireless communication in tunnels. A frequency range of 1-6 GHz was used to provide an extra large bandwidth of 5 GHz.