6.4.4 Delay Locked Loop (DLL)
Next we will discuss a technique called DLL which is typically used in spread spectrum applications
for chip-time synchronization. More details will be provided in the next chapter where
we address CDMA; however, we felt it was important to provide a preview in this related section.
The block diagram shows there are two parallel correlators, one for the early timing and one
for the late timing (see Fig. 6.32). We used c(t ) to denote the chip sequence appropriately
delayed. This technique assumes the received eye diagram is reasonably symmetrical thus
forcing us to say the ???on time??? sample timing instance is directly in the center of the ???early??? and
???late??? timing instances.
In the spread spectrum application, the early and late correlators exploit the autocorrelation
properties of the PN sequence used. The time separation between the early and late correlators is
given by 2. The autocorrelation is evaluated at early and late lags and then squared to remove the
t^
1
2p
# tan1Ea a (m1)LN1
kmLN
z[k] # sin c2pk
N db
a a (m1)LN1
kmLN
z[k] # cos c2pk
N dbU
influence of the channel response. Lastly, the squared values are subtracted from each other to create
an error signal.
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