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Sec. 5–13 Spread Spectrum Systems 395
y 2 (t) = A c m(t) + n 2 (t)
(5–133)
where
n 2 (t) = A J c(t)
(5–134)
and the terms about f = 2f c have been neglected because they do not pass through the LPF.
Referring to Fig. 5–39c, we note that the jammer power at the receiver output is
n3
=
L
R b
-R b
n2 (f) df =
L
R b
-R b
A J
2 1
2R c
df =
A J
2
R c R b
(5–135)
and the jammer power at the input to the LPF is A 2 J .[ n2 (f) = A 2 J y(2R c ), as seen from Fig.
5–41b and Eq. (5–134).] For a conventional BPSK system (i.e., one without a spread spectrum),
c(t) would be unity and Eq. (5–134) would become n 2 (t) = A J , so the jamming power
out of the LPF would be AJ
2 instead of A 2 J >(R c >R b ) for the case of an SS system. [The output
signal should be A c m(t) for both cases.] Thus, the SS receiver has reduced the effect of
narrowband jamming by a factor of R c R b . This factor, R c R b , is called the processing gain of
the SS receiver. † For our example of R c = 9.6 Mchipss and R b = 9.6 kbitss, the processing
gain is 30 dB, which means that the narrowband jammer would have to have 30 dB more
power to have the same jamming effect on this SS system, compared with the conventional
BPSK system (without SS). Thus, this SS technique provides 30 dB of antijam capability for
the R c R b ratio cited in the example.
SS techniques can also be used to provide multiple access, called code division multiple
access (CDMA). Here, each user is assigned a spreading code such that the signals are
orthogonal. The technique is used in CDMA cellular telephone systems. Thus, multiple SS
signals can be transmitted simultaneously in the same frequency band, and yet the data on a
particular SS signal can be decoded by a receiver, provided that the receiver uses a PN code
that is identical to, and synchronized with, the particular SS signal that is to be decoded.
CDMA links are designed to operate either in the synchronous mode or the asynchronous
mode. For the synchronous mode, the symbol transition times of all users are aligned. This
mode is often used for the forward link from the base station (BS) to the mobile station (MS).
For the asynchronous mode, no effort is made to align the sequences. This mode is used for
the reverse link from the MS to the BS. The asynchronous mode has more multiple access
interference, but fewer design constraints.
To accommodate more users in frequency bands that are now saturated with conventional
narrowband users (such as the two-way radio bands), it is possible to assign new SS
stations. This is called spread spectrum overlay. The SS stations would operate with such a
wide bandwidth that their PSD would appear to be negligible to narrowband receivers located
sufficiently distant from the SS transmitters. On the other hand, to the SS receiver, the
narrowband signals would have a minimal jamming effect because of the large coding gain of
the SS receiver.
† The processing gain is defined as the ratio of the noise power out without SS divided by the noise power out
with SS. This is equivalent to the ratio (SN) out (SN) in when (SN) in is the signal-to-noise power into the receiver
and (SN) out is the signal-to-noise power out of the LPF.