#### In-depth

## Coping with multipath fading

### Reference sounding

#### Multipath fading is both a blessing and a curse for wireless digital communications systems. Were it not for the echoes of the transmitted signal caused by reflections from buildings, signals would not reach the user nestled within the crowded shopping street where the line-of-sight path to the transmitter is totally obscured. On the other hand, mitigating against distortion caused by multiple signals arriving at a receiver is a non-trivial task. If the fading is 'flat' with frequency, then a common method of combating the multipath-induced amplitude and phase variations is to use a 'reference sounding signal'. By sending a known frequency tone or periodic known data symbols alongside the message data, these references can be used to measure in real time the instantaneous amplitude and phase variations imposed by the channel (see Bateman (1990)). For flat fading, it can be inferred that similar gain and phase distortions are imposed on all other frequency components in the message signal. The information gleaned from the reference can thus be used to subtract out the distortion from the message data. |

#### It is generally assumed that the multipath fading will be 'flat' with frequency over a bandwidth of less than 25 kHz for most mobile radio applications.