Direct Sequence Spread Spectrum and You'

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Direct Sequence Spread Spectrum and You.

• Brian Kessler
• EE-194 SDR
• Spring 2006

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Spread Spectrum

• Spread Spectrum is a modulation technique that
  takes a signal centered around one frequency and
  spreads it out over others.
• This is designed to lower the effects of noise,
  make it difficult to jam, and/or make it
  difficult to intercept.
• Spread spectrum techniques use pseudo noise to
  transmit the signal, making the received signal
  sound like noise or momentary flares to the
  untrained receiver.

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Spread Spectrum
F
F
Normal Signal
Signal with Spread Spectrum
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Spread Spectrum

• For a spread spectrum transmitter-receiver system
  to work, both points need the ability to generate
  identical pseudo noise signals
• Besides Direct Sequence Spread Spectrum (DSSS),
  the other popular technique is Frequency Hopping
  Spread Spectrum (FHSS).
• This modulation does exactly what the name
  implies, hopping from carrier to carrier in a
  pseudo random order.

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DSSSDirect Sequence Spread Spectrum

• Each data bit is combined with a n-bit spreading
  code
• Logically the combination is an XOR, but using
  Bipolar notation instead of Unipolar designations
  for the bits, the system can be modeled with only
  a multiplier.
• This means instead of a 0 and 1 data signal, -1
  and 1 are used

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DSSS
A B C
0 is high
C A XOR B
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DSSS Transmission (Theoretical)

• The Data signal is generated and enters a
  spreader
• Inside the spreader, a pseudo noise signal is
  generated, and then XORed with the data signal
• D(t) XOR N(t) Sd(t)
• The new signal is then modulated with an analog
  technique such as BPSK

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DSSS Transmitter
Spreader
Sd(t)
Modulator BPSK
S(t)
Digital Data d(t)
N(t)
PN Source
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DSSS Reception (Theoretical)

• Once the data is received, it is again
  demodulated and then sent into a
  de-spreader to produce the original signal.
• Sd(t) XOR N(t)
• D(t) XOR N(t) XOR N(t)
• D(t)

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DSSS Receiver
Digital Data d(t)
De-spreader
Sd(t)
S(t)
Demodulator BPSK
C1(t)
PN Source
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Modeling DSSS

• Working in the binary (0,1) domain turned out to
  be quite complicated, with simulinks data type
  compatibility issues.
• By using the Unipolar to Bipolar Converter
  block on both the data signal and pseudo noise
  signal, then multiplying and converting back,
  binary XOR was simulated.
• -1 X -11
• -1 X 1-1
• 1 X -1-1
• 1 X 1 1
• Note in this case, 0 is high.

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Spreader
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Modeling DSSS

• The Signal was then modulated using the BPSK
  block to complete the Transmission Block

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Receiver

• On the other end, the reverse was done.
• The received signal was demodulated and sent into
  the de-spreader

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De-spreader

• The De-spreader works just like the opposite of
  the spreader
• The original data was then sent out of the
  de-spreader

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The Real World

• The real world has something that the simulated
  world doesnt NOISE
• To Test its efficiency, the signal was
  transmitted along a path with White Gaussian Noise

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Noise - Analog
Perfect World Gaussian World
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Noise - Digital
Original Data Perfect World Gaussian
World
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Noise

• With the Signal to Noise Ratio set to 10dB, the
  Noise was wreaking havoc on the signal.
• The system was doing well to compensate, but
  still was having a 7 error rate.
• More correction was needed at the receiver end.

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Integration

• By integrating the received signal over the 10
  samples data signal, an averaging system was
  created.
• Outputted would be a number higher than 0 if the
  signal was more 1 than 0, and vice versa.
• Then, after being sent through a trigger, the
  signal would be restored to its 1,-1 form

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New Receiver
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New Receiver

• This method would produce an bit error rate (BER)
  at a 10dB SNR of .09
• For a digital sound signal, this is more than
  acceptable
• For wifi, which is a major application of this
  modulation technique, this is acceptable too, due
  to the built in error correction techniques which
  are capable of fixing more than 1 bit errors.

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Final Form
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Questions?
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Work Cited

• http//www.sss-mag.com/ss.html
• http//www.bee.net/mhendry/vrml/library/cdma/Chapt
  er1.htm
• http//en.wikipedia.org/wiki/Direct-sequence_sprea
  d_spectrum
• www.raylink.com/whitepaper/fhss_dsss.pdf