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%)
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