ST2115
Scientech Technologies Pvt. Ltd.
22
Direct-Sequence Spread-Spectrum Systems
Let us consider the transmission of a binary information sequence by means of binary
PSK. The information rate is
R
bits/sec and the bit interval is
T
b
= 1/
R
sec. The
available channel bandwidth is
B
c
Hz, where
c
B
R
.
At the modulator, the
bandwidth of the information signal is expanded to
W
=
B
c
Hz by shifting the phase of
the carrier pseudo randomly at a rate of
W
times/sec according to the pattern of the PN
generator. The basic method for accomplishing the spreading is shown in figure 7.
The information-bearing base band signal is denoted as
u
(
t
) and is expressed as
( )
(
)
n
b
n
t
a gr t
nT
υ
∞
=−∞
=
−
∑
where {
a
n
= ±1, –¥ <
n
< ¥} and
g
T
(
t
) is a rectangular pulse of duration
T
b
. This
signal is multiplied by the signal from the PN sequence generator, which may be
expressed as
( )
(
)
n
c
n
c t
C P t
nT
∞
=−∞
=
−
∑
(a)
PN signal
(b)
Data signal
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