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.
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