Doppler Shift
When a wave source and a receiver are moving relative to one another the frequency of
the received signal will not be the same as the source. When they are moving toward
each other the frequency of the received signal is higher then the source, and when they
are approaching each other the frequency decreases. This is called the Doppler effect.
An example of this is the change of pitch in a car’s horn as it approaches then passes
by. This effect becomes important when developing mobile radio systems.
The amount the frequency changes due to the Doppler effect depends on the relative
motion between the source and receiver and on the speed of propagation of the wave.
The Doppler shift in frequency can be written:
Where △f is the change in frequency of the source seen at the receiver, fo is the
frequency of the source, v is the speed difference between the source and transmitter,
and c is the speed of light.
For example: Let fo = 1GHz, and v = 60km/hr (16.7m/s) then the Doppler shift will be:
This shift of 55Hz in the carrier will generally not effect the transmission. However,
Doppler shift can cause significant problems if the transmission technique is sensitive
to carrier frequency offsets (for example COFDM) or the relative speed is higher (for
example in low earth orbiting satellites).