B. P. Lathi, Zhi Ding - Modern Digital and Analog Communication Systems-Oxford University Press (2009)

630 SPREAD SPECTRUM COMMUNICATIONS
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Figure 11. 11
Equivalent Data Baseband
baseband
QAM
diagram of
generator c( t)
DSSS system.
r------'-' O
PN chip
generator
uuuu
LO n n 0
Ts
Baseband
channel
PN chip
generator
t = kTs
QAM
symbol
detector
signal can also be extracted with little interference from the DSSS signal by replacing the
despreader with a narrow bandpass filter. In this case, the roles of signal and interference are
in fact reversed.
DSSS Analysis against Broadband Jammers
In many cases, interferences come from broadband sources that are not generated from the
DSSS spreading approach. Against such interferences, the despreading operation only mildly
broadens and weakens the interference spectrum.
Let the interference be broadband with the same bandwidth LB s as the spread signal.
Based on Eq. (11.24), the interference after despreading would be i a (t), which has bandwidth
of 2LB s . In other words, broadband interference i(t) will in fact be expanded to a spectrum
nearly twice as wide and half as strong in intensity. From this discussion, we can see that a
DSSS signal is most effective against narrowband interferences and not as effective against
broadband interferences.
11
.6 CODE DIVISION MULTIPLE-ACCESS (CDMA)
OF DSSS
The RF diagram of a DSSS system can be equivalently represented by the baseband diagram of
Fig. 11.11, which provides a new perspective on the DSSS system that is amenable to analysis.
Let the (complex-valued) QAM data symbol be
(k - 1) T s S t < kT s
(11 .26)
c(t) (k - l)Ts S t < kT s
Then it is clear from the PN chip sequence that the baseband signal after spreading is
(k - l)Ts S t < kT.1 ( 11.27)
In other words, the symbol Sk is using
as its pulse shape for transmission. Consequently, at the receiver, the optimum receiver would
require c(t) to be used as a correlator receiver (or, equivalently, a matched filter). As evident
from the diagram of Fig. 11.11, the despreader serves precisely the function of the optimum
matched filter (or correlator receiver). Such a receiver is known as a conventional single-user
optimum receiver.