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					Principles of Spread
     Spectrum


       Lecture 4
                   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




                                                    2
                      Introduction




Figure 4-2: OSI data flow
                                     3
             Introduction (continued)




Table 4-1: OSI layers and functions
                                        4
OSI model




            5
WLan technologies




                    6
Infrared




           7
        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
                                                     8
 Narrowband Transmission (continued)




Figure 4-3: Narrowband transmission
                                      9
Spread spectrum




                  10
      Spread Spectrum Transmission




Figure 4-4: Spread spectrum transmission
                                           11
Spread Spectrum




                  12
      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



                                                   13
Two approaches for SS




                        14
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 today’s WLAN systems
  – Bluetooth does use FHSS

                                                             15
  Frequency Hopping Spread Spectrum
             (continued)




Figure 4-6: FHSS error correction
                                    16
Frequency hopping




                    17
Direct Sequence




                  18
  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
                                                        19
    Direct Sequence Spread Spectrum
               (continued)




Figure 4-7: Direct sequence spread spectrum (DSSS) transmission
                                                           20
DSS frequency change plan




                            21
Radio Modulation




                   22
     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
                                                           23
       Orthogonal Frequency Division
          Multiplexing (continued)




Figure 4-8: Multiple channels
                                       24
       Orthogonal Frequency Division
          Multiplexing (continued)




Figure 4-9: Orthogonal frequency division multiplexing (OFDM)
vs. single-channel transmissions
                                                                25
  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



                                                    26
  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




                                                 27
Key modulation techniques




                            28
       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
                                                  29
  Physical Layer Standards (continued)




Figure 4-19: Phase shift keying (PSK)
                                        30
  Physical Layer Standards (continued)




Figure 4-20: Quadrature phase shift keying (QPSK)
                                                    31
  Physical Layer Standards (continued)




Figure 4-21: 16-level quadrature amplitude modulation (16-QAM)
                                                            32
  Physical Layer Standards (continued)




Figure 4-22: 64-level quadrature amplitude modulation (64-QAM)
                                                            33
                   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

                                                    34
            Summary (continued)

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




                                                        35
                       Lab

• 3-3
• 4-1 and 4-3 from text book




                               36

				
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