Principles of Spread Spectrum

1
Principles of Spread Spectrum

• Lecture 4

2
Objectives

• List and describe the wireless modulation schemes
  used in IEEE WLANs
• Tell the difference between frequency hopping
  spread spectrum and direct sequence spread
  spectrum
• Explain how orthogonal frequency division
  multiplexing is used to increase network
  throughput

3
Introduction
Figure 4-2 OSI data flow
4
Introduction (continued)
Table 4-1 OSI layers and functions
5
OSI model
6
WLan technologies
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Infrared
8
Narrowband Transmission

• Narrowband transmission used primarily by radio
  stations
• Radio signals by nature transmit on only one
  radio frequency or a narrow portion of
  frequencies
• Require more power for the signal to be
  transmitted
• Signal must exceed noise level
• Total amount of outside interference
• Vulnerable to interference from another radio
  signal at or near same frequency
• IEEE 802.11 standards do not use narrowband
  transmissions

9
Narrowband Transmission (continued)
Figure 4-3 Narrowband transmission
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Spread spectrum
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Spread Spectrum Transmission
Figure 4-4 Spread spectrum transmission
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Spread Spectrum
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Spread Spectrum Transmission (continued)

• Advantages over narrowband
• Resistance to narrowband interference
• Resistance to spread spectrum interference
• Lower power requirements
• Less interference on other systems
• More information transmitted
• Increased security
• Resistance to multipath distortion

14
Two approaches for SS
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Frequency Hopping Spread Spectrum (FHSS)

• Uses range of frequencies
• Change during transmission
• Hopping code Sequence of changing frequencies
• If interference encountered on particular
  frequency then that part of signal will be
  retransmitted on next frequency of hopping code
• FCC has established restrictions on FHSS to
  reduce interference
• Due to speed limitations FHSS not widely
  implemented in todays WLAN systems
• Bluetooth does use FHSS

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Frequency Hopping Spread Spectrum (continued)
Figure 4-6 FHSS error correction
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Frequency hopping
18
Direct Sequence
19
Direct Sequence Spread Spectrum (DSSS)

• Uses expanded redundant code to transmit data
  bits
• Chipping code Bit pattern substituted for
  original transmission bits
• Advantages of using DSSS with a chipping code
• Error correction
• Less interference on other systems
• Shared frequency bandwidth
• Co-location Each device assigned unique chipping
  code
• Security

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Direct Sequence Spread Spectrum (continued)
Figure 4-7 Direct sequence spread spectrum
(DSSS) transmission
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DSS frequency change plan
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Radio Modulation
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Orthogonal Frequency Division Multiplexing (OFDM)

• With multipath distortion, receiving device must
  wait until all reflections received before
  transmitting
• Puts ceiling limit on overall speed of WLAN
• OFDM Send multiple signals at same time
• Split high-speed digital signal into several
  slower signals running in parallel
• OFDM increases throughput by sending data more
  slowly
• Avoids problems caused by multipath distortion
• Used in 802.11a networks

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Orthogonal Frequency Division Multiplexing
(continued)
Figure 4-8 Multiple channels
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Orthogonal Frequency Division Multiplexing
(continued)
Figure 4-9 Orthogonal frequency division
multiplexing (OFDM) vs. single-channel
transmissions
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Comparison of Wireless Modulation Schemes

• FHSS transmissions less prone to interference
  from outside signals than DSSS
• WLAN systems that use FHSS have potential for
  higher number of co-location units than DSSS
• DSSS has potential for greater transmission
  speeds over FHSS
• Throughput much greater for DSSS than FHSS
• Amount of data a channel can send and receive

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Comparison of Wireless Modulation Schemes
(continued)

• DSSS preferred over FHSS for 802.11b WLANs
• OFDM is currently most popular modulation scheme
• High throughput
• Supports speeds over 100 Mbps for 802.11a WLANs
• Supports speeds over 54 Mbps for 802.11g WLANs

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Key modulation techniques
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Modulation techniques used by 802.11a

• Modulation techniques used to encode 802.11a data
  vary depending upon speed
• Speeds higher than 54 Mbps may be achieved using
  2X modes

Table 4-7 802.11a characteristics
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Physical Layer Standards (continued)
Figure 4-19 Phase shift keying (PSK)
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Physical Layer Standards (continued)
Figure 4-20 Quadrature phase shift keying (QPSK)
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Physical Layer Standards (continued)
Figure 4-21 16-level quadrature amplitude
modulation (16-QAM)
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Physical Layer Standards (continued)
Figure 4-22 64-level quadrature amplitude
modulation (64-QAM)
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Summary

• Three modulation schemes are used in IEEE 802.11
  wireless LANs frequency hopping spread spectrum
  (FHSS), direct sequence spread spectrum (DSSS),
  and orthogonal frequency division multiplexing
  (OFDM)
• Spread spectrum is a technique that takes a
  narrow, weaker signal and spreads it over a
  broader portion of the radio frequency band
• Spread spectrum transmission uses two different
  methods to spread the signal over a wider area
  FHSS and DSSS

35
Summary (continued)

• OFDM splits a single high-speed digital signal
  into several slower signals running in parallel

36
Lab

• 3-3
• 4-1 and 4-3 from text book