In Fig. 7.81, we present an oversampled autocorrelation
function to demonstrate the principle.
The signal is oversampled by a factor of 4; alternatively
stated, the time separation between time
samples is Tc/4. This first figure shows an example
where the correct receiver time sampling is
performed. We have separated the early and late
correlators by Tc chips apart.
In Fig. 7.82, we show a time offset of Tc/4
between the received signal and the locally generated
chip sequence.
In this example, the late correlator output
energy will be larger than that of the early output,
and this will indicate that we are sampling too
early or too fast. So a time correction must be
applied to move the on-time correlator to the peak
position. The same line of reasoning can be
applied to the opposite case. In either case, this
technique does require an oversampled chip signal
to operate properly.
A block diagram of the noncoherent-based
DLL time-tracking synchronizer is shown in
Fig. 7.83. The received signal is despread by two
parallel correlators. The length of the despreading
is dependent on the signal that is used for tracking.
If the DPCCH was used, then depiloting would
also be required to further coherent accumulation.
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