## Coping with multipath fading

### Parallel transmission

#### Where the time delay spread is such that the fading is not flat with frequency, reference sounding techniques will not work unless several parallel data channels are used, each of which occupies a narrow bandwidth over which the fading is 'flat' with each channel having a separate sounding reference.

#### If the concept of parallel data channels is taken to an extreme, and coding redundancy is added to the data source such that the data integrity is not compromised when some of the parallel channels fall within the fading notches (remember these notches change position with time as the user moves), then it is possible to do away with the reference signals and simply rely on sufficient of the parallel channels being decoded correctly to allow the receiver to correct for any data errors in the few faded sub-channels. |

#### This very technique is being adopted for digital audio broadcast (DAB) and digital video broadcast (DVB) in Europe where 1024 parallel data channels are used to convey the digitized music, voice and images to the radio or television screen. The modulation format is termed Orthogonal Frequency Division Multiplexing (OFDM)

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