Alternatively, a coherent-detection DLL can be used to perform fine time tracking [33]. In this
case, the modified error signal is given in Fig. 7.84.
eNC(t) Zh(k t) Z2 # ePRccaTc
2 bP2
PRcca
Tc
2 bP2 f
e(t) h(k t) # eRccaTc
2 b Rcca
Tc
2 bf e(t) h(k t) # RccaTc
2 b h(k t) # Rcca
Tc
2 b e(t) rL(k) rE(k)
3G WIDEBAND CDMA 411
FIGURE 7.84 Coherent DLL block diagram.
X
Finger
Input
X
PN
Generator
+
LPF
??“
E L
e(t)
C(t ??“ t ??“ Tc/2)
C(t ??“ t + Tc/2)
rL(t)
rE(t)
X
Re{ }
h*(t ??“ t)
??‘
??‘
Here the estimate of the channel, obtained possibly from the on-time correlator output, is used to
derotate the error signal. Lastly, the projection onto the real axis is used as the error signal. The
coherent-based DLL error signal is given as follows:
(7.124) ec(t) Re e Zh(t t) Z2 # cRccaTc
2 b Rcca
Tc
2 bd f
Let us mention a few parameters that affect the performance of the DLL:
a. Coherent DLL versus noncoherent DLL.
b. Early-late separation. A larger separation would have a less??“steep error characteristics curve than
a smaller separation.
c. Pulse shaping used.
d. Update rate. How often does one update the time phase adjustment?
e. Oversampling factor used in the fine time tracking.
Pages:
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679