Chapter 13. Spread Spectrum

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Chapter 13. Spread Spectrum

• Park Dong-Hyun
• Department of Information and
• Communications Engineering
• The Graduate School of
• Sejong University

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Contents

• Spread Spectrum Principles
• Direct Sequence Spread Spectrum (DSSS)
• DSSS System Model
• Spreading Codes for ISI Rejection
• Synchronization
• Rake receivers
• Frequency-Hopping Spread Spectrum
• Multiuser System

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Spread Spectrum Principles (1)

• The signal occupies a bandwidth much larger than
  is needed for the information signal.
• The spread spectrum modulation is done using a
  spreading code, which is independent of the data
  in the signal
• Despreading at the receiver is done by
  correlating the received signal with a
  synchronized copy of the spreading code.
• Developed initially for military application
• Types
• Frequency hopping
• Direct sequence
• Basis for CDMA(Code Division Multiple Access)

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Spread Spectrum Principles (2)

• 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

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Spread Spectrum Principles (3)

• Spread Spectrum advantages
• Anti-jamming
• Interference Rejection
• Message Security Privacy
• Low Probability of Intercept
• Rake receivers

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Spread Spectrum Principles (4)

• Frequency Hopping Spread Spectrum
• To combat frequency-selective fading
• To combat narrow-band interference
• To protect against intentional jamming and
  hostile surveillance

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DSSS System Model (1)

• Each bit in the original signal is represented by
  multiple bits(chip code) in the transmitted
  signal
• The chipping code spreads the signal across a
  wider frequency band in direct proportion to the
  number of bits used

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DSSS System Model (2)
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DSSS System Model (3)

• Message Data (random binary wave)
• bit period (sec) Tb
• bit rate (bps)
• Power spectral density
• Spreading Code
• chip period (sec) Tc
• chip rate (cps)
• Power spectral density

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DSSS System Model (4)
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DSSS System Model (5)
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Spreading Code property

• Randomness property
• Balance property
• Have an equal number of ones and zeros.
• Run property
• ?? run length(?? type? digit sequence)?
• half length 1,
• 1/4 length 2,
• 1/8 length 3.
• Correlation property
• Random sequence? shift ??? ?? sequence? ????
  (modulo-2), agreement? disagreement? ??? ??? 1???
  ???.

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Synchronization

• The Synchronizer
• Must align the timing of the spreading code
  generator in the receiver with the spreading code
  associated with one of the multipath components
  arriving over the channel.
• Feedback control loop
• Adjust the delay of the spreading code
  generator until the function reaches
  its peak value.
• Coarse Synchronizer (Acquisition)
• is within a chip time of perfect
• synchronization.
• Fine Synchronization (tracking)

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RAKE receivers

• IS-95 transmitter ??? ??
• Qualcom patent
• Multipath ??? multipath diversity
• ? path??? ?? signal? ?? decoding (demodulation -gt
  despreading) ? ?, attenuation factor? ??? ??.
• gt ? ? signal strength? ?? ? ??.

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Frequency-Hopping Spread Spectrum(FHSS) (1)

• Rapidly change the transmission frequency
• Pseudorandom pattern in a predetermined (Fig.
  11.1)
• Timing the hops accurately is the key to success
• Synchronization between transmitter and receiver
• Frequency allocation
• FDMA Fixed allocation
• FH time dependent

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Frequency-Hopping Spread Spectrum(FHSS) (2)

• Avoid interference with primary users
• Primary users are assigned narrow frequency bands
• Transmit at a power high enough to override the
  WLAN
• Any interference caused by the secondary user
• Affect the primary user is transient
• Because the hopping sequence spreads the energy
  out over a wide band
• Primary user only looks like transient noise

Fig. 11-2. Avoiding interference with frequency
hopping
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Frequency-Hopping Spread Spectrum(FHSS) (3)

• Two FH system need to share same band
• Configure with different hopping sequences
• Do not interfere with each other
• During each time slot
• Two hopping sequences must be on different
  frequency slots
• Orthogonal hopping sequence
• EXgtFigure 11-3
• Sequence 1 2, 8, 4, 7
• Sequence 2 6, 3, 7, 2

Fig. 11-3. Orthogonal hopping sequences
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Multiuser DSSS (ex.CDMA)

• Pilot channel
• Unmodulated Direct Sequence Spread Spectrum ??
• ?Cell? ?? Cell? ??
• ?? channel? coherent demodulation? ?? reference
  ??
• Sync channel
• -1200 bps data rate
• ???? ????? ??? ?? ????
• Paging channel
• 4800 or 9600 bps flexible data rate
• ???parameter, access parameter?? ????
• Page ????? ?? ??
• Traffic channel
• 1200, 2400, 4800, or 9600 bps variable rate
  vocoding
• ????? ???? call processing? ?? channel

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Multiuser DSSS (ex.CDMA)

• CDMA Forward Link

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Multiuser DSSS (ex.CDMA)

• Forward Link Channel Signaling

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Multiuser DSSS (ex.CDMA)
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Multiuser DSSS (ex.CDMA)
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Walsh Covering/Modulation

• Walsh Function

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64-ary Walsh Function
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Multiuser DSSS

• Reverse Link

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Multiuser DSSS (ex.CDMA)

• Reverse Link

Fig. DSSS uplink system
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Q A

• Thank you for giving your attention!

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