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396 AM, FM, and Digital Modulated Systems Chap. 5 Frequency Hopping As indicated previously, a frequency-hopped (FH) SS signal uses a g c (t) that is of the FM type, where there are M = 2 k hop frequencies controlled by the spreading code, in which k chip words are taken to determine each hop frequency. An FH-SS transmitter is shown in Fig. 5–42a. The m(t) Modulator FSK or BPSK FH spreader Bandpass filter (about sum frequency) FSK or BPSK FH spread spectrum signal s(t) FSK or BPSK signal A c cos( i t) Frequency synthesizer k chips Spreading code generator c(t) Serial to parallel converter (a) Transmitter r(t)=s(t)+n(t) FH despreader Bandpass filter (about difference frequency) FSK or BPSK demodulator Detected data ~ m(t) 2 cos( i t) FSK or BPSK signal Frequency synthesizer k chips Spreading code generator c(t) Serial to parallel converter (b) Receiver Figure 5–42 Frequency-hopped spread spectrum system (FH-SS).
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396<br />
AM, FM, and Digital Modulated Systems Chap. 5<br />
Frequency Hopping<br />
As indicated previously, a frequency-hopped (FH) SS signal uses a g c (t) that is of the FM type,<br />
where there are M = 2 k hop frequencies controlled by the spreading code, in which k chip words<br />
are taken to determine each hop frequency. An FH-SS transmitter is shown in Fig. 5–42a. The<br />
m(t)<br />
Modulator<br />
FSK or BPSK<br />
FH spreader<br />
Bandpass filter<br />
(about sum<br />
frequency)<br />
FSK or BPSK<br />
FH spread<br />
spectrum signal<br />
s(t)<br />
FSK or BPSK<br />
signal<br />
A c cos( i t)<br />
Frequency<br />
synthesizer<br />
k chips<br />
Spreading code<br />
generator<br />
c(t)<br />
Serial to parallel<br />
converter<br />
(a) Transmitter<br />
r(t)=s(t)+n(t)<br />
FH despreader<br />
Bandpass filter<br />
(about difference<br />
frequency)<br />
FSK or<br />
BPSK<br />
demodulator<br />
Detected<br />
data<br />
~<br />
m(t)<br />
2 cos( i t)<br />
FSK or BPSK<br />
signal<br />
Frequency<br />
synthesizer<br />
k chips<br />
Spreading code<br />
generator<br />
c(t)<br />
Serial to parallel<br />
converter<br />
(b) Receiver<br />
Figure 5–42<br />
Frequency-hopped spread spectrum system (FH-SS).
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