# Railway Wireless Communications Channel Characterization
## 4 Communication Channel Measurement Methodology and CCT Description
* The antennas to test the communication channel can be switched as the connection with the antenna consists of an SMA connector. Consequently, different antennas can be plugged into the system, changing, e.g., the gain of it (dB). Thus, CCT is a specific railway tool which allows accurate geolocated communication channel measurements adaptable to any track or technology. The CCT architecture consists of three different parts split into on-board and trackside, shown in Figure 1.  
In the CCT, the user is responsible for taking the control of the whole system by CCT CI, configuring the tests and visualizing their results. The CCT agent and server are the  
Figure 1: CCT architecture. parts of the system that allow the testing of the already configured communication channel. Moreover, the CCT Agent is the part installed on the train, highlighting its easy installation and start-up and being space-saving due to the small size of the device. The functions of each part of the system are explained below. * CCT_Agent: the on-board part located on the train. It is the part responsible for obtaining the train position and monitoring the parameters of the communications configured from CCT_CI. It can operate from trains in operation, being a non-intrusive system, to dedicated vehicles. Moreover, it is a scalable system with regards to the number of trains on which it is installed. In addition, it adapts to wireless communications technologies to be monitored: 2G, 3G, and 4G (current ones); 5G (in development); Tetra; WIFI; SATCOM; ITS-G5; and so on. Figure 2 shows the connections of the CCT agent. Concerning the technology to be monitored, SubMiniature version A (SMA) connection is available to plug the antenna for the desired technology to be measured (e.g., it could be a multiband antenna for 3G and 4G) and as well the Ethernet connection that allows the user to plug the tool into a specific network. * CCT_Server: it manages the information of CCT_Agent and CCT_CI, acting as an interface between both systems. It is responsible for processing and analyzing the information from the test to obtain the desired parameters that the user configured. The location of this part of the CCT differs depending on the use; if the philosophy to follow is being tested in a specific area in a country, the server could be installed locally. However, the CCT can be performed in multiple tracks around the world with the need to connect to the same server; therefore, the cloud is the other option that can be chosen due to its elasticity property which allows the cloud platforms to efficiently add or remove the cloud infrastructures automatically according to the number of users for supporting the rapid fluctuation of loads to serve better [17]. For this specific case, the CCT_Server is located in the cloud. * CCT_CI (Control Interface): it is the configuration part done by the final user regarding the test that is going to be executed. In addition, it owns a database where routes, trains, monitored parameters, etc., are included. These data will be needed for the understanding of the final results. Another task for this CCT part is the visualization of the test results, which are shown in different formats such as maps or graphics:  
* Configuration of each of the CCT_Agent(s) to activate/deactivate them to monitor the parameters defined in the time interval that the user wants. * Visualization: The user can visualize the results in different display modes:
* Maps: the web interface shows in a map the different values along the track where the train was passing through during the test. * Graphics/charts: the CI shows charts comparing time vs. value. Figure 2: On-board installation of the CCT Agent. KPI chart: the quality of service of each application to work properly in chart form. In this way, this chart can be compared with the other charts from the test, letting the user know which applications will work or not (comparison with one impairment) [18].