doppler spread

This is a measure of how much the channel spreads in frequency domain.

We are basically asking at what rate does any frequency component change. Unit is Hz.

If the receiver is moving (non perpendicular to the transmitter), then by the doppler phase shift, there will be a shift in frequency. Different multipaths can have different doppler shifts, and the doppler spread is the difference in doppler shifts between these signal components.

Large doppler spread => signal paths that are changing independently over time.

Inversely related to coherence time.