Doppler Spread and Coherence Time
Doppler spread and coherence time are parameters which describe the time varying nature of the channel in a small-scale region.
- Doppler spread BD is a measure of the spectral broadening caused by the time rate of change of the mobile radio channel
- It is defined as the range of frequencies over which the received Doppler spectrum is essentially non-zero.
- When a pure sinusoidal tone of frequency fc is transmitted, the received signal spectrum, called the Doppler spectrum, will have components in the range —fd tofc fd where fd is the Doppler shift.
- The amount of spectral broadening depends onfd which is a function of the relative velocity of the mobile, and the angle Θ between the direction of motion of the mobile and direction of arrival of the scattered waves.
- If the baseband signal bandwidth is much greater than BD, the effects of Doppler spread are negligible at the receiver.
- Coherence tine is actually a statistical measure of the time duration overwhich the channel impulse response is essentially invariant, and quantifies thesimilarity of the channel response at different times.
- Coherence time is the time duration over which two received signals have a strong potentialfor amplitude correlation.
- Coherence time T, is the time domain dual of Doppler spread
- The Doppler spread and coherence time areinversely proportional to one another. That is,
- If the coherence time is defined as the time over which the time correlationfunction is above 0.5, then the coherence time is approximately
Where Tm is the maximum Doppler shift given by Tm = v/λ
- Coherence bandwidth is a statistical measure of the range of frequencies over which the channel can be considered “flat” (i.e., a channel which passes all spectral components with approximately equal gain and linear phase).
- In other words, coherence bandwidth is the range of frequencies over which two frequency components have a strong potential for amplitude correlation. Two sinusoids with frequency separation greater than Bc are affected quite differently by the channel.