Doppler spread is linked to variation of the phase, but we don't talk about the variation of the amplitude. The reason is that the change of amplitude takes place slowly. The sum for a channel tap includes all the paths within a bin. But if the path changes, they can shift from one delay bin (1/W duration) to another delay bin. This variation of the channel is not considered as this change takes place in a time scale on the order of 1/W. On the other hand, phase change takes place on the order of 1/f_c which is very small usually. So, we consider this. However, this is not true for wideband consideration. Fast fading/Slow Fading Channel: Fast fading: Coherence time
Doppler spread is linked to variation of the phase, but we don't talk about the variation of the amplitude. The reason is that the change of amplitude takes place slowly.
The sum for a channel tap includes all the paths within a bin. But if the path changes, they can shift from one delay bin (1/W duration) to another delay bin. This variation of the channel is not considered as this change takes place in a time scale on the order of 1/W. On the other hand, phase change takes place on the order of 1/f_c which is very small usually. So, we consider this. However, this is not true for wideband consideration.
Fast fading/Slow Fading Channel:
Fast fading: Coherence time
These lectures are goldmines. Thanks Prof. Tse!