# Radio Wave Propagation and WSN Deployment in Complex Utility Tunnel Environments +
## 3 Radio Channel Analysis  
### 3D-RL Calibration and Validation  
The underground tunnel scenario has distinctive characteristics that convert it in a particular scenario for radio wave propagation purposes. These peculiar characteristics are that it is an area that is considerably long (80-m-long, in this specific case) and extremely narrow (2-m width and 3-m height). It can be found in the literature [36; 37], the convergence analysis of the in-house RL algorithm to obtain the optimal input parameters for the best performance of the algorithm in typical indoor complex scenarios. However, for this specific environment, those parameters are not the optimal, because the RL algorithm diverges due to the extremely narrow width of the scenario. The divergence of the algorithm means that there are a lot of cuboids of the scenario that do not have any ray if the same simulation parameters are used for this specific scenario. Due to that, the 3D-RL algorithm needs to be calibrated according to the experimental measurements to validate the results accuracy with  
Figure 4: Schematic representation of the simulation scenario implemented to analyze the radio propagation losses within the underground utility tunnels. the optimal input parameters. Therefore, a convergence analysis of the algorithm was done for this underground tunnel environment in order to characterize the 3D-RL convergence as a function of multiple parameters, such as cuboid dimensions, horizontal angular resolution (\(\Delta\rho\)), vertical angular resolution (\(\Delta\theta\)) or the maximum number of reflections. Due to the specific characteristics of the utility tunnel environment (i.e., strong reflections due to the partial conductor material presence and large obstacle density), the maximum number of reflections and the angular resolution of the launching rays was analyzed. For that purpose, the different number of multipath reflections and different number of launching rays were considered. Measurement results were compared with the different simulation results to find which simulation results achieved the best trade-off between precision and simulation computational cost, thus obtaining a calibrated 3D-RL technique that could be used for a radio-planning analysis in this type of complex underground environment. The considered frequency of operation was 868 MHz, emulating the operation of a typical LoRaWAN or ZigBee wireless communication system. The different cases for the convergence analysis, which was considered for the simulations of the underground tunnel scenario, are shown in Table 1.  
Once the simulation results for the different cases were obtained, the validation and the corresponding calibration was obtained by performing radio channel measurements inside the tunnel section. The scenario in which the measurements were performed is a section of the utility underground tunnels in the city of Pamplona, which is depicted in Figure 2(b). Measurements were carried out with the aid of an Agilent CSA N1996A to generate the radiofrequency (RF) signal and an Agilent FieldFox N9912A portable spectrum analyzer, both from Keysight Technologies. The transmitter was connected to a 5-dBi monopole antenna, whereas the receiver was connected to small vertical 0-dBi monopole antenna. The considered frequency of the operation was 868 MHz, with a transmitted power of \(-10\) dBm. The transmitter antenna was placed at the beginning of the tunnel, located at the center with a height of 2 m, whereas the receiver points were placed at different distances in front of the transmitter along the tunnel. Measurements were performed with no users crossing the tunnels or other potential time-varying scatterers within the scenario. Results, compared with the different cases performed (shown in Table 1) in the 3D-RL simulations from the implemented scenario, are depicted in Figure 5. It can be seen from the picture that Cases 1, 3, 4 and 5 did not have good agreement with the measurements after 50-m distance approximately.