# Railway Wireless Communications Channel Characterization
## 1 Railway Communications and Evolution  
Nowadays, more and more people use the train as an option for transport such as subways, tramways, or high-speed trains. Because of the importance of the increasing amount of people that choose the train, the transportation system has to obtain certain safety guarantees in order not to put the passengers' lives at risk, as well as offering a number of services to the passengers. To reach this main goal, railway applications are in continuous evolution. These applications are more sophisticated and restrictive in terms of Quality of Service (QoS), such as real-time applications that need low delay and jitter. In addition, not every application needs the same requirements: the safety-related applications are more restrictive than the non-safety or passenger-comfort-related applications. A large amount of these applications uses the communication channel between the onboard and trackside to exchange information about the train. Moreover, depending on the application, the technology of the communication channel is not the same. For example, Communications-Based Train Control (CBTC), a signaling system for subway trains, based on IEEE 802.11 [1], makes use of different communication technologies, in constant evolution depending on the manufacturer. By contrast, the selected signaling system for the mainline and high-speedrail in Europe, the European Rail Traffic Management System (ERTMS), uses Global System for Mobile communications-Railway (GSM-R). Some of the limitations of GSM-R are [2]:  
1. Low link rate: its maximum transmission rate per connection (9.6 kbps) is sufficient only for applications with low demands (ETCS is around 4 kbps). Moreover, its delay, which is the range of 400 ms, is too high for critical video applications whose acceptable delay is 100 ms.
2. Interferences with the GSM-R due to different sources: 1. Electromagnetic (EM) transient interferences due to the sliding contact between the pantograph and the catenary [3]. 2. Public GSM communications employ the channels adjacent to the GSM-R frequency [4]. 3. Small system capacity: GSM-R system with the available 4 MHz bandwidth can only support 19 channels of 0.2 MHz bandwidth per channel, which cannot accommodate the rapidly developing railway communications [5]. Taking into account these limitations it can be stated that GSM-R technology is becoming obsolete, with an end of support planned by 2030 onwards. Since it does not accomplish the requirements for the current applications, some other technologies shall be used as its substitute. One of the candidate technologies is Long-Term Evolution (LTE), which is supposed to be the natural evolution of GSM-R as it is the current evolution in the public operators [2,6]. However, there are also other suitable technologies either deployed or in progress, such as satellite communications or 5G, respectively. Nevertheless, the deployment and maintenance of a railway-dedicated network are really expensive; therefore, the decision of deploying a new network has to fulfill some guarantees. As the Public Land Mobile Network (PLMN) operators have already deployed a number of technologies, they could shed light on railways in their environment to know how the communication channel is. Particularly, the railway environment could affect the communication channel because of EM disturbances or the environment where the train is passing through (tunnels, rural areas, urban areas, etc.). Then, a system capable of measuring the current technologies already deployed arises from the need of knowing how the communication channel is and how it will behave in the railway environment, and with these results, finally, one can decide if it is worth deploying a new dedicated network or not. As a solution to this issue, a novel tool called the Channel Characterization Tool (CCT) is described in this paper to characterize the performance of a wireless communication link to check specifications and bring the real environment to the laboratory.