Coherence time is the minimum time required for magnitude or phase change of the channel to become uncorrelated from the previous value.

If delay time is lesser than the symbol time, frequency non selective or flat fading occurs.

Fast fading occurs when the channel coherence time is less than the time duration of a transmission symbol.

When a user’s moving, the user’s velocity causes shift in frequency of the transmitted signal along each signal path. This is called as Doppler shift.

When delay time is greater than symbol time, frequency selective fading occurs.

Block fading can occur in both frequency domain and time domain. This is called as doubly block fading.

Frequency non selective or flat fading occurs if channel coherence bandwidth is greater than the transmitted signal bandwidth.

The received signal usually consists of several discrete multipath components causing multipath intensity profile to exhibit multiple isolated peaks also called as fingers or returns.

Coherence time is inversely proportional to Doppler spread.

Signal dispersion and coherence bandwidth characterizes the channel’s time spreading properties in a local area.