SPREAD SPECTRUM

SPREAD SPECTRUM

1. M cGraw-Hill ©The M cGraw-Hill C ompanies, I nc., 2000
1
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SPREAD SPECTRUM
UNIT 3
PART 2

2. 6.2
3-2 SPREAD SPECTRUM3-2 SPREAD SPECTRUM
In spread spectrum (SS), we combine signals from In spread spectrum (SS), we combine signals from
different sources to fit into a larger bandwidth, different sources to fit into a larger bandwidth,
but our goals are to prevent eavesdropping and but our goals are to prevent eavesdropping and
jamming. To achieve these goals, spread spectrum jamming. To achieve these goals, spread spectrum
techniques add redundancy.techniques add redundancy.
Frequency Hopping Spread Spectrum (FHSS)
Direct Sequence Spread Spectrum (DSSS)
Topics discussed in this section:Topics discussed in this section:

3. 6.3
Spread Spectrum
A signal that occupies a bandwidth of B,
is spread out to occupy a bandwidth of B
ss
All signals are spread to occupy the same
bandwidth B
ss
Signals are spread with different codes so
that they can be separated at the receivers.
Signals can be spread in the frequency
domain or in the time domain.

4. 6.4
Figure 3.21 Spread spectrum

5. 6.5
Figure 3.22 Frequency hopping spread spectrum (FHSS)

6. 6.6
Figure 3.23 Frequency selection in FHSS

7. 6.7
Figure 3.24 FHSS cycles

8. 6.8
Figure 3.25 Bandwidth sharing

9. 6.9
Figure 3.26 DSSS

10. 6.10
DSSS MODULATION

11. 6.11
DSSS DEMODULATION

12. 6.12
Figure 3.27 DSSS example

13. M cGraw-Hill ©The M cGraw-Hill C ompanies, I nc., 2000
DS-SS Vs FH-SS
PERFORMANCE
ISSUES
6.13

14. 14
FHSS vs DSSS
l DSSS is more susceptible to narrow band noise.
4 DSSS channel is 22 Mhz wide whereas
4 FHSS is 79 Mhz wide.
l The FCC regulated that DSSS use a maximum of 1 watt
of transmitter power in Pt-to-Multipoint system.
l DSSS costs less then FHSS
l FHSS can have more systems co-located than
DSSS.
4 DSSS systems have the advantage in throughput
l The Wi-Fi alliance tests for DSSS compatibility
4 No such testing alliance exists for FHSS.

15. 15
FHSS vs DSSS contd
l DSSS generally has a throughput of 5-6 Mbps
while FHSS is generally between 1-2 Mbps.
l Both FHSS and DHSS are equally insecure.
lDSSS has gained much wider acceptance due to
its low cost, high speed and interoperability.
4This market acceptance is expected to
accelerate.
4 FHSS advancement includes HomeRF and 802.15
(WPAN) (Bluetooth), however, it is expected to not
advance into the enterprise.

16. Copyright
2005 All Rights
Reserved
16February
2005
Co-location Comparison
1 5 10 15 20
10
20
30
40
Number of Co-located Systems
11 Mbps DSSS
3 Mbps FHSS (sync.)
3 Mbps FHSS (no sync.)
Date Rate in Mbps

17. M cGraw-Hill ©The M cGraw-Hill C ompanies, I nc., 2000
UNIT - 3
THE END
6.17