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

628 SPREAD SPECTRUM COMMUNICATIONS
and
Sx (f) = L Sn (f - k/LTc)
k
( 1 1.20)
In other words, as long as the chip sequence is approximately orthogonal, the noise at the QAM
detector remains a white Gaussian with zero mean. For practical reasons, the white noise n(t)
is filtered at the receiver to be band-limited to 1 /2T,. As a result, the noise spectrum after the
despreader sti 11 is
N
Sx (f) = 2
(11.21)
In other words, the spectral level also remains unchanged. Thus, the petiormance analysis
carried out for coherent QAM and PSK detections in Chapter 10 can be applied directly.
In Sec. 10.6 we showed that for a channel with (white) noise of PSD N/2, the error
probability of optimum receiver for polar signaling is given by
(11.22)
where E b is the energy per bit (energy of one pulse). This result demonstrates that the error
probability of an optimum receiver is unchanged regardless of whether or not we use DSSS.
While this result appears to be somewhat surprising, in fact, it is quite consistent with theAWGN
analysis. For single user, the only change in DSSS lies in the spreading of transmissions over a
broader spectrum by effectively using a new pulse shape c(t). Hence, the modulation remains
QAM whereas the channel remains AWGN. Consequently, the coherent detection analysis of
Sec. 10.6 is fully applicable to DSSS signals.
11
.5 RESILIENT FEATURES OF DSSS
As in FHSS, DSSS systems provide better security against potential jamming or interception
by spreading the overall signal energy over a bandwidth L times broader. First, its low power
level is difficult for interceptors to detect. Furthermore, without the precise knowledge of the
user spreading code [or c(t)], adversaries cannot despread and recover the baseband QAM
signal effectively. In addition, partial band jamming signals interfere with only a portion of
the signal energy. They do not block out the entire signal spectrum and are hence not effective
against DSSS signals.
To analyze the effect of partial band jamming, consider an interference i(t) that impinges
on the receiver to yield
y(t) = SQAM (t)c(t) + i(t)
Let the interference bandwidth be B; . After despreading, the output signal plus intetierence
becomes
y(t)c(t) = SQAM (t) + i(t)c(t) (11.23)