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τ-dither code tracking loop
 Code tracking loops can be categorized in coherent and
noncoherent loops.
 A former case is called coherent delay lock loop. The drawback
is that the generation of the coherent reference at low SNR is
difficult.
 The noncoherent code tracking loops can be classified as
(1) delay lock loop (full-time code loop),
(2) time-shared code tracking loop (the early and late gates are
on alternately), and
(3) τ-dither code tracking loop, which is quite similar to the timeshared loop but can be implemented with one correlator instead
of two.
 In this course, we only consider the τ-dither code tracking loop.
教育部網路通訊人才培育先導型計畫
Wireless Communication Technologies 2.4.6
1
τ-dither code tracking loop
 The τ-dither code tracking loop is only useful in moderately
high loop SNRs; however, its prime advantage is that only one
correlator is needed to provide both the code tracking function
and the dispreading function.
Despread signal
(to carrier demodulator )
y (t )
BPF
(BW=B)
( )2
Dither
generator
PN
gen.
教育部網路通訊人才培育先導型計畫
VCO
Loop
filter
Wireless Communication Technologies 2.4.6
2
Dithering to obtain error signal
 To obtain a feel of how the error signal is developed from this
loop, consider the noiseless case illustrated in Figure.
Correlated
envelope
RPN ( )
v2
v1

 Tc
t
t
Tc
Dither wipeoff
output filtered
v1  v2  0
t
g (t ) (Dither signal)
教育部網路通訊人才培育先導型計畫
Wireless Communication Technologies 2.4.6
3
Dithering to obtain error signal
 The dither signal, a square-wave signal, is used to time-share the
early and late PN codes, as shown at the lower left of the figure.

In Figure it was assumed that the code generator lags the input
code.
 After correlating (multiplying and bandpass filtering) the
resultant signal envelope is as shown in the upperright segment
of Figure .
 The envelope has an amplitude modulation impressed upon it as
the dither rate with an amplitude variation that depends upon the
magnitude of the error   T  Tˆ .
 After multiplying by the dithering signal g (t ) and filtering, the
error signal emerges to control the VCO (code generator clock)
through the loop filter as seen in the lower right portion of the
figure.
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Wireless Communication Technologies 2.4.6
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Dithering to obtain error signal
 If, for example, the loop code generator were in time with the
received code, then the dither signal would produce no envelope
variation and the filtered control voltage, the error voltage,
would be zero.
 Alternately, if the loop code generator was ahead of the received
code, it is clear that the resulting error control voltage would
now be positive.
 The tracking error curve generated by this loop is shown for
various dither amplitudes in Figure. (shown for envelope
detector)
 For the τ-dither loop typically a value of Δ equal to about (0.1
0.1Tc
chips) is used since the despread signal power drops off as
(1  ) 2 , which is about 0.92 dB for the ideal unfiltered PN
correlation curve.
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Wireless Communication Technologies 2.4.6
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τ-dither code tracking loop
 This relative loss becomes considerable less if the code
modulated carrier is prefiltered prior to the code loop as is the
normal case. However, the tracking error curve degrades also.
S ( )
1.0
  Tc
0.5
  0.2Tc
  0.1Tc
0.2

Tc
0.1
0.5
1.0
1.5
-0.5
-1.0
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Wireless Communication Technologies 2.4.6
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