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					 What is Spread Spectrum?
     Definition of Spread Spectrum:
Spread spectrum modulation refers to any
    modulation scheme that produces a
  spectrum for the transmitted signal much
wider than the bandwidth of the information
 being transmitted independently from the
         information-bearing signal.
    Spread Spectrum
-important encoding method for
  wireless communications
-analog & digital data with analog
  signal
-spreads data over wide bandwidth
-makes jamming and interception
  harder
-two approaches, both in use:
  Frequency Hopping
  Direct Sequence
          Spread Spectrum
In spread spectrum (SS), we combine
  signals from different sources to fit into
  a larger bandwidth, but our goals are to
  prevent eavesdropping and jamming.
  To achieve these goals, spread
  spectrum techniques add redundancy.
    Spread Spectrum Concept
• Input fed into channel encoder
  – Produces narrow bandwidth analog signal around
    central frequency
• Signal modulated using sequence of digits
  – Spreading code/sequence
  – Typically generated by pseudonoise/pseudorandom
    number generator
• Increases bandwidth significantly
  – Spreads spectrum
• Receiver uses same sequence to demodulate
  signal
• Demodulated signal fed into channel decoder
Spread Spectrum
General Model of Spread
   Spectrum System
 Spread Spectrum Advantages
• Immunity from various noise and multipath
  distortion
  – Including jamming
• Can hide/encrypt signals
  – Only receiver who knows spreading code can retrieve
    signal
• Several users can share same higher bandwidth
  with little interference
  – Cellular telephones
  – Code division multiplexing (CDM)
  – Code division multiple access (CDMA)
     Pseudorandom Numbers
• generated by a deterministic algorithm
  – not actually random
  – but if algorithm good, results pass reasonable
    tests of randomness
• starting from an initial seed
• need to know algorithm and seed to
  predict sequence
• hence only receiver can decode signal
   Frequency Hopping Spread
       Spectrum (FHSS)
• signal is broadcast over seemingly random
  series of frequencies
• receiver hops between frequencies in sync
  with transmitter
• eavesdroppers hear unintelligible blips
• jamming on one frequency affects only a
  few bits
       Frequency Hoping Spread
           Spectrum (FHSS)
• Signal is broadcast over seemingly random series of radio
  frequencies
   – A number of channels allocated for the FH signal
   – Width of each channel corresponds to bandwidth of
     input signal
• Signal hops from frequency to frequency at fixed intervals
   – Transmitter operates in one channel at a time
   – Bits are transmitted using some encoding scheme
   – At each successive interval, a new carrier frequency is
     selected
    Frequency Hoping Spread
           Spectrum
• Channel sequence dictated by spreading
  code
• Receiver, hopping between frequencies in
  synchronization with transmitter, picks up
  message
• Advantages
  – Eavesdroppers hear only unintelligible blips
  – Attempts to jam signal on one frequency
    succeed only at knocking out a few bits
           Basic Operation
• Typically 2k carriers frequencies forming 2k
  channels
• Channel spacing corresponds with
  bandwidth of input
• Each channel used for fixed interval
  – 300 ms in IEEE 802.11
Frequency hopping spread
spectrum (FHSS)
Frequency selection in FHSS
FHSS cycles
FHSS (Transmitter)
Frequency Hopping Spread
Spectrum System (Receiver)
          Slow and Fast FHSS
•   commonly use multiple FSK (MFSK)
•   have frequency shifted every Tc seconds
•   duration of signal element is Ts seconds
•   Slow FHSS has Tc  Ts
•   Fast FHSS has Tc < Ts
•   FHSS quite resistant to noise or jamming
    – with fast FHSS giving better performance
Bandwidth sharing
         FHSS Performance
           Considerations
• Large number of frequencies used
• Results in a system that is quite resistant
  to jamming
  – Jammer must jam all frequencies
  – With fixed power, this reduces the jamming
    power in any one frequency band
     Direct Sequence Spread
        Spectrum (DSSS)
• each bit is represented by multiple bits
  using a spreading code
• this spreads signal across a wider
  frequency band
• has performance similar to FHSS
DSSS
Direct Sequence Spread
   Spectrum System
Direct Sequence Spread
   Spectrum Example
DSSS example
Code Division Multiple Access
                • Patent holder:
                • Hedy Lamarr
           CDMA
In CDMA, one channel carries all
  transmissions simultaneously.
Chip sequences
Encoding rules
CDMA multiplexer
CDMA demultiplexer
W1 and W2N
Sequence generation
Check to see if the second property about orthogonal
codes holds for our CDMA example.

• The inner product of each code by itself is N. This is
  shown for code C; you can prove for yourself that it
  holds true for the other codes.

•       C . C = [+1, +1, -1, -1] . [+1, +1, -1, -1] = 1 + 1 +
    1+1=4

• If two sequences are different, the inner product is 0.

•       B . C = [+1, -1, +1, -1] . [+1, +1, -1, -1] = 1 - 1 - 1
    +1=0
• The inner product of each code by its complement is
  -N. This is shown for code C; you can prove for
  yourself that it holds true for the other codes.
•
•        C . (-C ) = [+1, +1, -1, -1] . [-1, -1, +1, +1] = - 1 - 1
  - 1 - 1 = -4

• The inner product of a code with the complement of
  another code is 0.

•      B . (-C ) = [+1, -1, +1, -1] . [-1, -1, +1, +1] = -1 + 1
    +1-1=0

				
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posted:2/16/2012
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