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London Underground is promising to let cellphone users carry on talking at
stations far below the surface where signals can’t now penetrate. And in WO
98/35511, Nokia of Finland explains how this could work on moving, underground
trains also. When you are on the move, cellphone transmitters hand you over from
one local cell to the next, on a different frequency. But this handover doesn’t
work with the ceiling-mounted antennas that are usually used in road tunnels, as
the phone signals from one cell can get cut out by moving trains. Nokia’s answer
is to reserve fixed channels down the whole length of a tunnel, so there is no
handover to worry about.