In communications systems, a communication channel may change with time. Coherence time is the time duration over which the channel impulse response is considered to be not varying. Such channel variation is much more significant in wireless communications systems, due to Doppler effects.
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Simple model
In a simple model, a signal
where
Now, if
Coherence time (
Relation with Doppler frequency
Using Clarke's model, from the maximum Doppler frequency
Usually, we use the following relation