Thus, the frequency content
of the signal has changed in order to take advantage of the wireless propagation channel. Let us consider
a bit stream with Rb 10 kbps and say each bit is coded with 100 chips; thus the chip rate is
Rc 1 Mcps. A simple transmitter block diagram is shown in Fig. 7.3.
344 CHAPTER SEVEN
Spread
Spectrum
Direct
Sequence
Frequency
Hopping
Slow
Hop
Fast
Hop
FIGURE 7.1 Spread spectrum categories.
X
PN
Sequence
NCO
...
b(t)
c(t)
y(t)
y(t) = b(t) cos[wjt + q]
. . .
f1 f2 f3 fN
[Hz]
|Y( f )|
|B( f )|
[Hz]
User Signal
FIGURE 7.2 Frequency hopping spread spectrum transmitter block diagram.
3G WIDEBAND CDMA 345
In the block diagram of the DS transmitter in Fig. 7.3, the narrowband user information signal b(t)
is spectrally spread to become y(t), using the PN code sequence c(t). This spread spectrum technique
is measured by what is called the processing gain or PG, which is defined as1
(7.1)
For the example given above, we have the following value:
(7.2)
When we address the RAKE receiver, along with the frequency-selective fading channel, we will
discuss how the PG quantity is used as a measurement of interference suppression capability. It is
also common to refer to the number of chips used to encode a modulation symbol as the spreading
factor (SF).
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