Example 5.4
A digital radio system uses binary FSK for data
transmission, with the two symbol frequencies at +1200 Hz and 1200 Hz with respect
to the channel centre. The received signal is subject to a Doppler shift of +100 Hz due to
the receiver motion. Sketch the output of a PLL FSK detector for a 1, 0, 1, 0, 1, 0, .... data
stream assuming there is no pulse shaping. How can the problem of Doppler shift be
overcome in a digital FSK radio system?
Solution
The effect of the Doppler shift on the receiver
signal is to make the symbols appear at frequencies of +1300 Hz and 1100 Hz with
respect to the notional centre frequency of the PLL detector. The result is that the PLL
output will have a dc-bias voltage superimposed on the recovered data signal as shown
below, proportional to the Doppler offset.

To eliminate the Doppler shift the output of the PLL
detector can be ac-coupled, however, this would also affect any low frequency content in
the data signal itself. A coding scheme such as Manchester encoding (see Section 4.4),
could be used here to remove any low frequency content in the data signal.