Because the strength, number and delay of the echoes vary with time as
the user moves, this channel sounding must be repeated frequently and a new inverse
channel transfer function calculated each time. A modern example of this approach to
combatting multipath is contained within the GSM digital cellular system.
The modulation bandwidth for GSM is 200 kHz which means that
the fading is frequency selective and equalization is necessary. A sounding data sequence
is sent embedded in the centre of each GSM data packet (frame) which is repeated every 4.615 ms.
In other words, the cellular phone is working out the echo response of its
surroundings at a rate of approximately 200 times per second.
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