The filtering that is applied to the twice carrier term to reduce the effects of channel
noise is, however, very important in reducing the amount of phase
jitter on the recovered carrier reference. There are some applications of digital
communications, such as cellular radio, where it is not possible to employ a very narrow
filter in the carrier recovery circuit. This is owing to the frequency uncertainty in the
location of the twice carrier term caused by local
oscillator error and, more directly, variable amounts of Doppler shift resulting from the user's motion. In these
cases, the carrier recovery process can often have significant residual 'phase jitter'
which degrades the performance for the expected ideal coherent PSK detector. |
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