The desired input Doppler spread f m of the channel, in hertz (Hz).
This parameter denotes the measure of the spectral broadening caused by the time rate of change of the channel. Doppler spread is defined as the range of frequencies over which the received Doppler spectrum is essentially nonzero. When a pure sine tone at frequency f c is transmitted, the received signal spectrum, called the Doppler spectrum, has components in the range (f c - f m ) to (f c + f m ). The Doppler spread is related to the mobile velocity v, carrier frequency f c , and the speed of light c by the relation f m = v f c /c.
The inverse of the Doppler spread T c = 1 / 16
f m (known as the coherence time) is the time duration over which the channel impulse response is essentially invariant.