## 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.

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#### 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.