If the multipath time delay spread equals D seconds, then the coherence bandwidth
Frequencies within a coherence bandwidth of one another tend to all fade in a similar or correlated fashion.
One reason for designing the CDMA IS-95 waveform with a bandwidth of approximately 1.25 MHz is because in many urban signaling environments the coherence bandwidth Bc is significantly less than 1.25 MHz.
This is the bandwidth over which the channel transfer function remains virtually constant.
If the delay spread D over a particular cellular communication path in an urban environment is 1.9 μs, then using equation above, the coherence bandwidth is approximately 0.53 MHz, which results in frequency selective fading over the IS-95 bandwidth.