Doppler shift
The Doppler shift introduced into a signal between
transmitter and receiver units, or moving reflectors, is a function of their relative
motion, the angle of arrival of the signal, and the operating frequency or wavelength.
These parameters are related as follows:
Doppler shift (Hz) = v.f.cos(x) / c
where v is the relative speed of transmitter and
receiver units in m/s, f is the carrier frequency in Hz, c = 3 x 108 m/s
and x is the relative angle of arrival in degrees.
For example, the Doppler shift experienced by a
cellular phone within a car travelling at 70 mph directly away from a base-station
transmitter, operating on the DCS1800 system (carrier frequency 1.8 GHz) is 189 Hz.
The Doppler shift for a person walking along the street at 4 mph is, however, much smaller
at only 11 Hz.
A typical spectrum of a signal received at a mobile
terminal when moving within a multipath environment with several reflectors each giving
different angles of arrival and hence different Doppler shift is shown below. The source
was a single tone at the carrier frequency.
