## Multipath fading – the time domain problem

#### If the multipath delays, typically a few microseconds for outdoor communications, are significant with respect to the symbol period, then intersymbol interference will occur. Note: this effect of multipath propagation is not an additional form of distortion to frequency selective fading but rather the same effect viewed in the time domain. To overcome the problem, channel equalizers are often employed which need to respond (adapt) to the changing nature of the channel. Alternatively, the symbol rate must be reduced by using M-ary signaling or many parallel channels so that the delay becomes a less significant proportion of the symbol period (typically <10%) |
| #### It should be noted that propagation delay itself is not a major problem, but rather the spread of delays over the different propagation paths. Shown here is a typical plot of the amplitude and delay profile for a cellular telephone signal as it propagates within a city centre. The spread of time delays is about 15 ms for the significant components.

####