Coherence Bandwidth and Coherence Time Explained

This page describes the difference between Coherence Bandwidth and Coherence Time.

Coherence Bandwidth

The bandwidth (range of frequencies) over which a channel frequency response remains constant is called the ‘Coherence Bandwidth’. Channel passes all frequency components of signal with approx. equal gain and linear phase.

Coherence Bandwidth ≈ (1 / Maximum Delay Spread)

A more accurate way to calculate coherence bandwidth is by using the RMS Delay Spread. This is because different channels will experience varying signal intensities over different delay spans, even with the same delay spread.

Key points

• If signal bandwidth is less than coherence bandwidth, flat fading occurs.
• If signal bandwidth is greater than coherence bandwidth, frequency selective fading occurs.

Coherence Time

It is the time duration over which the channel impulse response is approximately invariant or doesn not change significantly.

Key points

• If the symbol duration is less than the coherence time, slow fading occurs.
• If the symbol duration is greater than the coherence time, fast fading occurs.

Summary

• Coherence bandwidth and coherence time define the stability of a wireless channel in frequency and time domains, respectively; critical for determining whether a system experiences flat or frequency selective fading, and slow or fast fading.