One method that we have already encountered is to send a reference signal alongside the data signal from which the
source carrier frequency and phase can be measured. Here we shall consider an alternative
technique that recovers the carrier from the modulated data signal itself. By locking an
oscillator to the phase of the incoming carrier when a carrier-on symbol is sent, and
holding this oscillator phase when the carrier is off, it is possible to produce the
required coherent reference. A technique that is well suited to this task is the phase-locked loop (PLL). Unfortunately the carrier reference is not
perfect, as the carrier-on symbols are corrupted by incoming noise. However, by averaging
the carrier over several on-symbols (equivalent to a narrow bandwidth in the PLL system),
a cleaner reference can be obtained. |
 |