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Difference between Doppler spread and Coherence time

This page defines Doppler spread vs Coherence time and mentions difference between Doppler spread and Coherence time.

These terms are related to characteristics of wireless channel.

Doppler spread

• Doppler shift is the random changes in a channel introduced as a result of a mobile user's mobility.
• Doppler spread has the effect of shifting or spreading the frequency components of a signal.
• It is caused by time selective fading. i.e. for a particular instance of time channel behaves as a fading channel and for rest it behaves as FLAT channel.
• Types of fading on the basis of doppler spread are fast fading and slow fading.
• Fast fading : Channel impulse response changes rapidly within the symbol duration.
• Slow fading : Channel impulse response changes at a rate much slower than the transmitted symbol bandwidth.
• Doppler spread is expressed in the following formula. As mentioned, doppler spread is defined as maximum doppler shift (fm).

Doppler spread formula

Coherence time

• The coherence time of the channel is the inverse of the Doppler spread.
• It is the measure of the speed at which channel characteristics change.
• This determines the rate at which fading occurs. When channel changes are higher than modulated symbol rate, fast fading occurs. When channel changes are slower than the symbol rate, slow fading occurs.
• Coherence time is expressed in the following formula.

Coherence time formula

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