# Railway Wireless Communications Channel Characterization
## 5 Characterization Scenarios  
The desired signal sent through the communication channel could be affected by different situations given along a track. Some of these different situations are the following ones:  
* Railway environment;
* PLMN;
* Handovers;
* EM disturbances. The environment that the train is passing through is an important element to take into account. Because of the environment, the desired signal can be perturbed by different effects as Figure 4 shows, such as multipath or scatters. Some possible scenarios that cause these effects are [19]:  
* Open Space:
* Urban area: normally many antennas are placed in this area because of the high amount of population, and consequently, more coverage. However, there are many obstacles which are characteristic of this area, the most often seen ones being buildings, barriers, ground, tracks, tractions, and trains [20]. * Rural area: fewer obstacles than the urban area; therefore, better line-of-sight (LoS), but less coverage. The transmit antenna is much higher than the surroundings which are light forests adjacent to the track (crops less than 2m) and a few buildings with an average height of less than 10 m. The link between the transmitter and receiver generally has a strong LoS component. However, after a certain distance, if there is no line-of-sight (NLoS), scatterers will be noticed at the receiver [21; 22]. Suburban area: where the buildings are typically low residential or townhouses with one or few floors [22]. Occasional uncluttered areas make the environment rather open, and the vegetation is modest. In fact, for High-Speed Railway (HSR) lines, most vegetation and buildings are 300 m away from the rail tracks. * Tunnels: normally no coverage inside the tunnel because there are no available antennas inside. * Viaduct: one of the most common scenarios for railways, especially for HSR [22; 23]. The viaduct raises the antennas and therefore creates a relatively \"clear\" LoS channel,  
Figure 4: Fishbone of the effects that affect the performance of the communication channel. thus reducing the number of the scatterers that can affect the fading and having as result an LoS propagation and few received reflection and scattering components. * Cutting (also called U-shape cutting): a semi-enclosed construction usually with two slopes along the track, constituting a common scenario in HSR environments [23; 24]. The propagation is significantly affected by the steep walls on both sides [25], generating typically scatters. * Station: classified according to their size or architecture [25]: open station, semi-closed station, and closed station. The huge steel awnings covering the platform and track, steel frames beside and above the track, and metallic pylons at the front side and the broadside of the station can block parts of LoS between the onboard receiver and the transmitting antenna outside the station. Consequently, the propagation suffers extra loss when the train is inside or near the station [26]. The CCT uses, among other possibilities, the PLMN as they have already deployed some technologies such as LTE, which is a possibility for railways. In this way, the behavior of these technologies in railways can be researched. Therefore, there is another important factor to take into account. Because of using the PLMN, different impairments can affect the signal. On one hand, each operator can offer a specific coverage, which is different from other operators due to the location and the power of the managed antennas. On the other hand, the number of users wanting to connect to the network at the same time indirectly affects the communication channel. This could be the reason that the network redirects one user to a further antenna and not the one closest to the track, which causes a worse communication channel, even no coverage. In addition, some problems related to the connection can appear due to handovers, mostly in HSR.