When do we experience frequency selective fading?
When is frequency diversity effective?
Definition of Coherence Bandwidth:
Coh. BW is the frequency separation for which the correlation coefficient is down from 1 to 0.5
Insert: 1 = 4p(f1 - f2) TRMS,
So Coh BW = 1 (4p TRMS)
We derived this for an exponential delay profile
Another rule of the thumb
ISI occurs if Tb > 0.1TRMS
Conclusion: keep transmission BW much less than Coh BW
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