Coherence Bandwidth and Coherence Time
This page on Coherence Badwidth vs Coherence time describes difference between Coherence Bandwidth and Coherence Time.
Bandwidth (range of frequencies) over which channel
is constant is called coherence Bandwidth.
Coherence Bandwidth ~ (1/Maximum Delay Spread)
The better way to calculate coherence bandwidth is using RMS Delay Spread, as different channel will experience different signal intensity over different delay span with same delay spread.
Time over which channel is constant is called coherence time.