Answer: Fading effects due to Doppler spread:-The fading effects due to Doppler spread can also be classified as fast fading and slow fading.

Depending on how rapidly the transmitted baseband signal changes as compared to the rate of change of the channel, a wireless channel is classified as a fast-fading or slow-fading channel.

In fast-fading conditions, as the mobile unit moves down a street in an urban environment, rapid fluctuations in received signal strength occur over distances of about one-half a wavelength. The channel Impulse response changes rapidly within the symbol duration. That is, the coherence time of the channel is smaller than the symbol period of the transmitted signal. This causes frequency dispersion, also called time-selective fading, due to Doppler spreading. This results into signal distortion which increases with increasing Doppler spread relative to the bandwidth of the transmitted signal. Therefore, a signal undergoes fast fading if the symbol period (\mathrm{T_s \ > T_c}), the time between fades. Fast fading occurs when the rate of change of the channel characteristics is faster than the rate of change of the information data signal, and results in distortion. 

Fast fading:- In a fast-fading channel, the channel impulse response changes rapidly within the symbol duration of the signal. Due to Doppler spreading, signal undergoes frequency dispersion leading to distortion. Therefore, a signal undergoes fast fading if 

(\mathrm{T_s \ >> T_c})

where $T_c$ is the coherence time and

(\mathrm{B_s \ >> B_D})

where $B_D$ is the doppler spread. Transmission involving very low data rates suffers from fast  fading.

Slow fading:- In slow fading the rate of the change of the channel impulse response is much less than the transmitted signal. We can consider a slow-faded channel as in which Is almost constant over atleast one symbol duration. Hence, 

(\mathrm{T_s \ << T_c})

where$T_c$is the coherence time and

(\mathrm{B_s \ >> B_D})

We observe that the velocity of the user plays an important role in deciding whether the signal experiences fast or slow fading.