C. Digital delay correction in polar transmitter
As previously indicated, polar transmitter is very sensitive
to delay mismatch between envelope and phase signals. An
adaptive solution is proposed in [12] where the delays are
adjusted by comparison between the output signal and the reference
one. A point here is that instead of adopting very high
sampling frequencies in order to get the required precision,
digital interpolation is implemented. This is a more effective
solution as it keeps reasonable sampling frequencies and save
consumption. Interpolator filters are well suited for digital
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implementation using the Farrow structure. The effective performances
of the algorithm for a 3G system are illustrated on
Fig. 6 for an initial desynchronisation of 0.5 symbol period.
The output spectrum ACPR is lowered to a residual noise floor
corresponding to the interpolation error which is a function of
both interpolator order and oversampling frequency.