Doppler Model, how does it affect the fading impulse response?
This is not a full answer, but I'll try to provide some intuition. First, imagine a static channel -- neither the antennas nor the reflectors around them move. In such a channel, the impulse response is always the same -- it's time invariant. The channel characteristics are fixed.

Now, introduce movement of the antennas and/or the reflectors. The channel impulse response now varies with time. The channel coherence time indicates how fast the channel impulse response changes, and for how long can we consider the channel to remain relatively static.

If there is movement of the antennas and/or the reflectors, we also get a Doppler shift in the transmitted signal. The more movement there is, the higher the shift.

The channel coherence time and the Doppler shift are, then, two manifestations of the same phenomenon -- movement. If one increases, the other decreases. The easiest way to introduce this into the channel model is to assume that the channel is fixed for a certain amount of time, after which it changes to a completely different, independent channel.