## Coping with multipath fading

### Channel equalizers

#### For high-speed wireless data applications where pilot sounding or parallel transmission is not appropriate or there is not sufficient bandwidth available to spread the signal effectively, then it is necessary to use wideband channel equalization techniques. These in effect involve sending a sounding pulse into the channel and measuring the level, phase and time delay of each significant echo received from the various transmission paths. The receiver then has to work out the inverse channel transfer function with which to correct the subsequent message data.

#### 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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