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

656 SPREAD SPECTRUM COMMUNICATIONS
Figure 11.23
(a) Power
spectral densities
of DSSS signal
using Barker
code of length
11 for
spreading:
(a) without
narrowband
jamming;
(b) with
narrowband
jamming at
SIR= lOdB.
Q 10 1
Vl
P-,
oJJ
·u;
Q
10 2
P-,
.. 10 1
10- 2 ---
-4 -3 -2 -1 0 2 3 4 5 U -5 -4 -3 -2 -1 0 2 3 4 5
Frequency, 1/T s
(a)
Frequency, 1/T,
(b)
Figure 11.24
Bit error
probabilities of
DSSS with QPSK
modulation
under
narrowband
jamming.
DSSS (CDMA) with spreading gain =11
10 ° ,,-----,-----,---,-----,------,-------,,-----,-----,------,
10-l
"-' 10-2
e
iiS 10- 3
10-4
-- No jamming (analysis)
-e--- SIR = 5 dB
-El- SIR = 8 dB
SIR = 10 dB
SIR =20dB
10 5--------------
0 2 3 4 5 6 7 8 9
E b //v, dB
we can witness power spectral densities before and after the addition of the jamming signal when
SIR = JO dB. Despreading at the receiver enables us to find the resulting BER of the QPSK signal
under different jamming levels (Fig. 11.24). As the jamming signal becomes stronger and stronger, we
will need to apply larger spreading factors to mitigate the degrading effect on the BER.
COMPUTER EXERCISE 11. 3: MULTIUSER DS-CDMA SYSTEM
To implement OS-CDMA systems, we must select multiple spreading codes with good cross-correlation
and autocorrelation properties. Gold sequences are a very well-known class of such good spreading
codes. Note that the Gold sequences are not mutually orthogonal. They have some nonzero but small
cross-correlations that can degrade the multiuser detection performance. We select four Gold sequences
to spread four QPSK users of equal transmission power. No near-far effect is considered in this example.