I’m currently on a high speed train between France and Switzerland and wondering how my cell phone is able to keep up with my very rapidly changing position and generally keep data connectivity?
In: Technology
This was actually a problem with GSM. Each cell have a list of neighbouring cells and will give this information to the cell phones. So the cell phone knows the exact frequencies and timing for the other cells in the area and will listen for these. There is also a dedicated handoff protocol between the cell towers. So the phones can actually switch quite fast if set up correctly. With GSM you might notice a slight half second gap when switching but not enough to cut the call. With Edge and 3G they increased the speeds so there is now no gaps.
A bigger issue with GSM was that when you are moving on a high speed train you get a noticeable doppler shift. So the frequencies your phone sends is not the same frequencies the stationary cell towers receive. With the narrow band cell phone channels you can actually dropp out of the range of a channel. 3G came with doppler shift compensation. So the phone and tower would notice the doppler shifts and change the frequencies used in the phone. This allows cell phones to be used on high speed trains.
Mobile cells are quite large in rural areas. Up to 70km diameter. So even if you travel at 300km/h in the TGV you’re still inside the same cell for many minutes, and sending data packages happens in a matter of seconds. The “logging into” a new cell happens in the background while you’re still in the old cell because the areas overlap slightly. These all happens fast enough that you don’t notice unless you’re moving at rocket-speed
In rural areas, cell phone companies use towers that span rather large ranges (25+km radius). They will also typically be overlaid so that one tower failure will not cause an outage. Imagine a whole bunch of triangles with 20km sides (approximately, because terrain).
So somewhere in that 20km, your cell phone has to decide to jump from one tower to another. It’ll monitor the signal strength of the towers and do a handoff from the one getting weaker to the one getting stronger. If you’re on a train going 200km/h, then it only has to do this once every six minutes or so (depending on how exactly you’re traveling from tower A to tower B, but they tend to be built along infrastructure), which is more than enough time to handle it. The process itself takes only seconds.
As fast as you’re moving, you aren’t moving at light speed. The signal between your phone and the tower, however, is, and as long as the network hand-off protocol is working properly your speed relative to the tower(s) doesn’t really matter. It *really* doesn’t matter unless you’re in an active call at the time, anyway.
Of course, that’s assuming there isn’t any cellular infrastructure built into the train system, itself, which isn’t at all uncommon these days. You’ll find cellular relays built into trains, subways, large population venues, etc. all over the world.
> my very rapidly changing position
The radio signal moves at ~300,000 kilometers per second, the radio can go from phone to cell tower and back while you are in almost the same place.
People telling you that your speed is inconsequential compared to light are apparently unaware that Doppler shift compensation is part of the modern cell phone network system.
The cell radio on your phone will automatically attach to the tower radio with the strongest signal. In some cases, that means gathering signal from both until one is clearly stronger than another. The phone doesn’t know, or care, what or where the radio is so we use SIM cards to identify the traffic for when there are multiple providers (that’s why 911/119/emergency) works on all providers in most countries). On top of it all, most countries have spectrum regulators to prevent too many radios in any given site in any given radio frequency. These are known as Spectrum Licences.
Among towers of the same provider, we call this process “handoff”. Among towers of a different provider, we call this “roaming”. The system works well because all radios (both on the user handset and the tower) follow the same global standard. Each telecommunications service provider must support the global handoff protocol as if it were a law (i.e as a condition of licence). These are technical updates, like any other software updates, that must be applied on a regular basis. It also requires TSPs to continually upgrade their equipment to remain competitive. I could get into the historical reasons why telecoms (and other common carriage infrastructure like trains &ships ) usually work this way, but it’s ELI5 so I’ll say “not playing nice in the playground makes all the expensive toys your mom bought you useless”.
It seems easy to do, but it gets really challenging to do it with a 99% uptime at a national scale. Like driving a car, it’s dangerous if used improperly because it could cause harmful radiation, drop critical communications, or interfere with other devices like radio altimeters. It requires a great deal of trust among everyone to make sure they are operating safely. It also relies on the global telecoms vendors (of which there’s 3: Huawei, Nokia and Ericsson) to engineer, licence, and build in a spirit of interconnection through global standards making bodies such as 3gpp and the ITU.
Right now, the way that we handle this is fairly primitive because it’s based on the power level of the phone; which is usually taken to mean how close it is to the tower. But next generation technologies could be able to handle this function in a more directed manner depending on which device it is serving. We call the basic form of this idea “Network Shaping” within 5G, but in the future it could use predictive AI to sense people, objects, and vehicles in real time. This could enable you to touch (or stand near, or look at, or shout at) objects and “log in” seamlessly just the same as when a cell phone switches towers.
Travelling very fast is a very relative concept.
You are travelling at a mere 300 kilometer per hour. The radio waves that your phone uses to keep up communications, go 300 000 km. Per second that is. The difference in so huge that you going 300kph has no real impact on the actual radio communication.
What IS a problem is that you will be switching cell towers a lot going that fast, but even that is not a big problem at just a mere 300kph.
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