PG 10 log c 1MHz
10 kHz d 20 dB
PG 10 log cRc
Rb d 10 log cTb
Tc d
X
PN
Sequence
b(t)
c(t)
y(t)
y(t) = b(t) c(t)
[Hz]
|Y( f )|
|B( f )|
[Hz]
User Signal
FIGURE 7.3 Direct sequence spread spectrum transmitter block diagram.
1 Chip rate Rc 1/Tc and the bit rate Rb 1/Tb.
In the above example, we assumed ideal coherent detection and perfect time synchronization of
the PN sequences. More specifically, we assumed the locally generated oscillator used in the spectral
down-conversion operation was exactly in phase and of the same frequency with the received signal.
A more realistic analysis is as follows: Assuming the channel introduces a propagation time delay of
sec. Figure 7.6 can be used to express PN code timing offsets.
346 CHAPTER SEVEN
X
PN
Sequence
b(t)
c(t)
y(t)
X
2P ?· cos[2pfct]
S(t) =
Spreading Spectral Up Conversion
2P ?· b(t) ?· c(t) cos[2pfct]
FIGURE 7.4 BPSK-based CDMA transmitter block diagram.
FIGURE 7.5 BPSK-based CDMA receiver block diagram.
X
PN
Sequence
b(t)
c(t)
y(t)
X LPF
Received
Signal
Despreading Spectral Down Conversion
Tb
0
??«dt
2P ?· cos[2pfct]
FIGURE 7.6 BPSK-based CDMA receiver.
X
z(t)
X
2P . cos[2pfct ??“ f]
LPF
x(t)
c(t ??“ t ) ?†
I(t) = b(t ??“ t) ?† ?† PG.
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