In the middle of 2024, and accustomed to having a good Internet connection from practically anywhere, Traveling on a high-speed train can be exasperating. Whether with the company’s WiFi or our own mobile data, coverage failures are usually constant. Of course, there are many factors that influence whether the experience is better or worse, starting with the journey we take. But the truth is that it is an issue that has really stuck with telecommunications operators, and that it is much more complex than it may initially seem.
“The main problem is speed, because When there is movement at high speeds, communications have more failures“summarizes Ana García Armada, professor of Signal Theory and Communications at the Carlos III University of Madrid, who explains that this, to a large extent, is due to the Doppler effect.
In physics, this name refers to the change in the frequency of a wave as a result of the relative movement between the sender and the receiver. It sounds complex, but the expert gives a very clear example: “When we were children, they explained it to us by teaching us that as an ambulance approaches we notice that the tone of the siren changes from high to low. But It is something that also happens with the radio waves that provide the data connection“.
According to García Armada, This problem has not been solved despite all efforts, not even with the arrival of 5G, that it has managed to improve the situation somewhat. “Technology has not yet managed to resolve this issue in high-speed trains, which move at 200 or 300 km/h. It is an open issue for 6G, which does not even exist yet,” he says.
“The Doppler effect is a golden rule, it always happens. And it is very destructive, that’s why you have to compensate for it even when you move at slow speeds. As you move, the signal level you receive constantly changes, and communications systems are always trying to compensate for these variations. But if you move at higher speeds, these variations are more complicated to correct,” says Ángela Hernández, I3A researcher at the University of Zaragoza.
To all this is added another addition in the case of WiFi. And, as is usually the case in public networks, the fact that many people connect can cause the system to end up collapsing due to pure saturation. “My opinion as a traveler is that mobile data is better than Wi-Fialthough the problems that both have are closely related. But it’s just an opinion,” García Armada slips.
Train materials
Beyond these issues, Hernández points out that Another key aspect is the material from which the trains are made.because “the windows or the bodywork can attenuate the arrival of the signals.” Likewise, the environment through which the roads run can modify the coverage. Mountains and trees, among other things, can interfere, and any traveler knows that in tunnels, except in exceptional cases, the signal is completely cut off.
Rocío Chueca, Hernández’s colleague at I3A, has been researching for some time how to improve the transmission of mobile phone signals through the windows of trains and buildings. In fact, in just fifteen days he will defend his doctoral thesis on this topic. As detailed, The use of low-e glass in trains is increasingly common to improve their thermal efficiency (prevent heat from entering in summer or leaving in winter). These glasses have a nanometer-thick metal layer that gives them these thermal properties, but causes them to lose transparency to the electromagnetic waves used in wireless communication technologies.
“This problem can be solved by recording periodic patterns, such as grids, called frequency selective surfaces (FSS) on the metal layer, usually using a high-power laser. In this way, transmission bands can be created for the frequencies of interest in communications, while maintaining thermal properties, since the lines are very thin and very little metal layer is removed,” says Chueca.
The researcher clarifies that this type of patterns have been used for a long time in various applications, but only very recently. have begun to be used to improve the transmission of mobile signals in windows. The Photonic Technologies Group of the University of Zaragoza has worked with the company Ariño Duglass, dedicated to glass treatment, to develop products in this line, and “significant improvements in coverage” have been achieved that are already applied in trains of countries like Germany or Denmark.
The German plan to have a good connection on trains
At the end of October, the German Government signed a declaration of intent with the country’s main mobile telephone companies (1&1, Deutsche Telekom, O2Telefónica and Vodafone) to develop “seamless 5G”, with which train passengers will be able to make calls and browse the Internet with the highest quality. The technology will be deployed on the Hamburg-Berlin railway line, one of Germany’s busiest routes, and promises gigabit data speeds.
This is an ambitious initiative. But The union of the operators and the Government is, precisely, the way to move forward, According to García Armada: “Any country could be the first to have a good connection on trains, even Spain. We are talking about using technologies that are already available. But they must be deployed and an investment made. And we need the push of governments, as happened with the rural Internet.
The professor considers that for the German project to work, they have to build many telephone antennas in the places where the trains circulate, in order to make 5G more robust at high speeds. It is something, he maintains, that is already possible on a technical level, “not to have many megabits per second, but to achieve a reasonable connection.” Although she is convinced that, in the short term, the connection will not be as good as that in our homes: “5G has been an improvement, but for that we would need 6G, which will arrive within a decade.”
“We are making progress little by little, but this specific issue is a challenge. If users wonder if we already have the tools to make it work well… The truth is that I have doubts about whether they are getting the maximum possible performance out of them, because it is true that operators and telephone companies normally develop robust products that work correctly, but with the most basic features possible, almost a standard. Although it is true that improving this is not as easy as it may seem,” concludes Hernández.
