Describe the consequence of the Doppler Effect on the receiver in an isotropic scattering environment. Based on your description, speculate on the meaning of the term “Doppler Spread”.
(a) Is the term “Doppler Spread” more appropriate in describing the channel than “Doppler Shift” in a scattering environment? Why?
(b) Observe the inverse relationship that exists between “Coherence Bandwidth” and “Delay Spread” in a wireless channel. Attempt to similarly define a term “Coherence Time” that has an inverse relationship with the “Doppler Spread”. What information does this term give about the channel?
The multiple reflections of signals cause a dynamic change in propagation time. Each reflection received is liable to be suffering a different Doppler shift. This means that a carrier transmitted on a single frequency will receive multiple reflections and each reflection will arrive at the receiver shifted in frequency by a different amount. The difference between the highest shift and the lowest shift gives the Doppler spread.
(a) Since the term Doppler spread determines the difference between highest and lowest Doppler shifts, it does not inform about the maximum Doppler shift experienced. Therefore, Doppler spread is more appropriate in describing the channel than “Doppler shift” in a scattering environment.
(b) Delay spread and coherence bandwidths are parameters which describe the time dispersive nature of the channel in a local area. However, information about the time varying nature of the channel caused by either relative motion between the mobile and base station, or by movement of objects in the channel is not given. Coherence time is the time domain dual of Doppler spread and is used to characterize the time varying nature of the frequency dispersiveness of the channel in the time domain.
Coherence time is the time duration over which two received signals have a strong potential for amplitude correlation. If the reciprocal bandwidth of the baseband signal is greater than the coherence time of the channel, then the channel will change during the transmission of the baseband message, thus causing distortion at the receiver.
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