A seminar report on Spread spectrum systems

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A seminar report on Spread spectrum systems

• Prepared by Guided by-
• Sanjay. M. Vaishnav Prof.K.R.Parmar
• M.E.(CSE) Sem.I EC Department
• Roll No.25
• Year 2008-09

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CONTENTS

• Introduction.
• How Spread Spectrum Works
• Spread spectrum principles
• Spread spectrum techniques
• Advantages over TDMA and FDMA
• Disadvantages
• Applications of spread spectrum
• Advantages of spread spectrum
• Conclusion
• Reference

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Introduction to Spread Spectrum

• "Spread-spectrum radio
  communications is a favorite technology of the
  military because it resists jamming and is hard
  for an enemy to intercept, Just as they are
  unlikely to be intercepted by a military
  opponent,
• "The demonstration is intended to show
  that spread-spectrum users can share a frequency
  band with conventional microwave radio
  users--without one group interfering with the
  other -- thereby increasing the efficiency with
  which that band is used. . . . "

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How Spread Spectrum Works

• Spread Spectrum uses
  wide band, noise-like signals. Because Spread
  Spectrum signals are noise-like, they are hard to
  detect. Spread Spectrum signals are also hard to
  Intercept or demodulate. Further, Spread Spectrum
  signals are harder to jam (interfere with) than
  narrowband signals. These Low Probability of
  Intercept (LPI) and anti-jam (AJ) features are
  why the military has used Spread Spectrum for so
  many years.
• Spread Spectrum signals use fast codes
  that run many times the information bandwidth or
  data rate. These special "Spreading" codes are
  called "Pseudo Random" or "Pseudo Noise" codes.
  They are called "Pseudo" because they are not
  real Gaussian noise.

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The principles of Spread Spectrum communication

• In Code Division Multiple Access
  (CDMA) systems all users transmit in the same
  bandwidth simultaneously. Communication systems
  following this concept are "spread spectrum
  systems''.
• The codes used for spreading have low
  cross-correlation values and are unique to every
  user. This is the reason that a receiver which
  has knowledge about the code of the intended
  transmitter, is capable of selecting the desired
  signal.

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• The spread spectrum refers to any system that
  satisfies the following conditions
• (1)The spread spectrum may be viewed as a kind of
  modulation scheme in which the modulated signal
  (spread spectrum) bandwidth is much greater than
  the message (base band) signal bandwidth.
• (2)The spectral spreading is performed by a code
  that is independent of the message signal.
• This same code is also used at the
  receiver to dispread the received signal .
• In secure comm. This code is known only to the
  persons for whom the message is intended.

NBss/B
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• Although we use
  much higher bw for a spectrum signal we can also
  multiplex large numbers of such signals over the
  same band by assigning a different code to each
  signal
• The codes
  are so chosen as to achieve near orthogonality of
  the waveforms. This orthogonality ,it strict
  would allow multiple users to co-exist in a given
  freq.range without mutual interference, providing
  multiple access through what is know as code
  division multiple acess(CDMA)
• 
  

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Direct sequence spread spectrum (DS/SS)

• In ss system the signal spreading code is the
  so-called pseudonoise(PN) sequence, which is
  generally periodic and consists of a periodic
  coded sequence correlation properties.
• These signals are pseudorandom as they appear
  to be unpredictable to an outsider, though they
  can be generated by deterministic means by the
  person for whom they are intended.
• The polar signal C(t) representing this
  binary sequence is the pseudo-random carrier that
  is used to multiply the message signalm(t)

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• Signal C(t) is a pseudorandom signal as it
  appears to be unpredictable though it can be
  generated by deterministic means (hence
  pseudorandom)
• The bit rate of C(t) is chosen to be much
  higher than the bit rate of m(t). The pulse in
  C(t) is called the chip.
• The bit rate C(t) is known
  as the chip rate.
• if the chip width is Tc
• than the chip rate is Rc1/Tc

Rc1/Tc
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• DETECTION
• At the recicver we generate a synchronous
  version of pseudorandom sequence C(t) used of the
  transmitter.
• The receiver DS/SS signal Y(t) when
  multiplied by C(t) ,yields the desired signal
  m(t) because
• Y(t)C(t)m(t)cc(t)m(t)
• because cc(t)1
• Thus the process of detection is identical to
  the process of spectral spreading.

DETECTION
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• HOW DOES DS/SS REALIZE ITS SPECIAL FEATURES?
• (1)Secure comm.jamming Resistance-
• The secure comm. feature of the DS/SS is due
  to the fact that this signal can be detected only
  by the authorized person who know the
  pseudorandom code used at the transmitter.
• Also because the DS/SS signal spectrum is
  spread over a very wide band, the signal PSD is
  very small, Which makes it easier to hide the
  signal within the noise floor.
• Moreover, because of the distribution of the
  signal power over a band spread spectrum signals
  are difficult to jam.

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• Jamming is effective only if the signal to be
  jammed occupies a smaller band.
• The interfering signal i(t) is the jamming
  signal that is added to the DS/SS signal The
  received signal Y(t)i(t) is now multiplied by
  c(t) at the detector to yield
• Y(t)I(t)C(t)m(t)c(t)I(t)C(t)
• 
  m(t)cC(t)I(t)C(t)
• m(t)I(t)C(t)
• Observe that the detector
  despreads the signal Y(t) to yield M(t).

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• (2) MULTIPLE ACCESS- several users on the same
  band
• In spread spectrum sys. Several users can
  utilize the same band . We can view the multiple
  access problem much the same way we view the case
  of jamming.
• If individual users have independent,
  uncorrelated spreading codes, undesired signals(
  co-channel interference) will not be dispread in
  the receiving process.

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• (3) RESISTANCE TO MULTIPATH FADING-
• To Understand the
  resistance of DS/SS to multipath fading ,we note
  that the signal received from any undesired path
  is a delayed version of the DS/SS signal.
• But the DS/SS signal has a property of low
  auto correlation(small similarity) with its
  delayed version.Hence the delayed signal, looking
  more like an interfering will not be dispread by
  C(t),this effectively minimizes the effect of the
  multipath signals.

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• ADVANTAGES OF CDMA OVER TDMA AND FDMA
• The DS/SS allows greater capacity by allowing
  multiple access comm. Freq. division multiple
  access (FDMA) and Time Division Multiple
  Access(TDMA) have fixed no. of freq. or time
  slots available to the user.
• In spread spectrum ,several users can occupy
  the same freq. spectrum simultaneously and freq.
  bands can be reused without regard to the
  separation distance of the user. This is because
  all users have unique spreading codes, which are
  ideally mutually orthogonal, giving rise to the
  term code-division-multiple access(CDMA).
• there are no hard limits on the no. of users
  allowed in the system at one time. As the no. of
  users increases, the signal quality of all users
  degrades gracefully, placing a soft limit on the
  no. of users.

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• DISADVANTAGES-
• Near far problem-
• The DS/SS form of spread spectrum has the
  best performance in terms of jamming rejection
  and multipath immunity .But it does suffer from
  the near far problem.
• The discussion so far assumes that the
  signals from all users are received with the same
  signal power. when this is not true, we may
  encounter the near far problem.
• if an unwanted signal strength is strong
  due to proximity of its transmitter to the
  receiver, and the strength of the desired signal
  is weak due to remoteness of its transmitter from
  the receiver , the undesired signal may still
  drown out the desired signal.

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• If all the codes were strictly orthogonal,
  this problem would not arise, unfortunately, it
  is difficult to find a large no. of codes that
  are strictly orthogonal , and we have to use many
  codes that are nearly orthogonal.

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Spread Spectrum Technology

• Direct or Hopping
• Direct sequence and frequency hopping are the
  most commonly used methods for the spread
  spectrum technology.
• The carrier of the direct-sequence radio stays
  at a fixed frequency. Narrowband information is
  spread out into a much larger bandwidth by using
  a pseudo-random chip sequence. The signal is
  shown in Figure

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DS-concept, before and after despreading
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the narrowband signal and the
spread-spectrum signal both use the same amount
of transmit power and carry the same information.
However, the power density of the spread-spectrum
signal is much lower than the narrowband signal.
As a result, it is more difficult to detect the
presence of the spread spectrum signal.
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The Hopping Approach

• Frequency-hopping systems achieve the same
  results provided by direct-sequence systems by
  using different carrier frequency at different
  time. The frequency-hop system's carrier will hop
  around within the band so that hopefully it will
  avoid the jammer at some frequencies.

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• The frequency-hopping technique does not spread
  the signal, as a result, there is no processing
  gain. The processing gain is the increase in
  power density when the signal is de-spread and it
  will improve the received signal's
  Signal-to-noise ratio (SNR). In other words, the
  frequency hopper needs to put out more power in
  order to have the same SNR as a direct-sequence
  radio.
• The frequency hopper, however, is more
  difficult to synchronize..
• The frequency hopper also needs more time to
  search the signal and lock to it.

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APPLICATION OF SPREAD SPECTRUM

• 1) WLAN IEEE 802.11
• 2) Global positioning system (GPS)

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Advantages of Spread Spectrum

• Reduced crosstalk interference
• Better voice quality/data integrity and less
  static noise
• Lowered susceptibility to multipath fading
• Inherent security
• Co-existence
• Longer operating distances
• Hard to detect
• Hard to intercept or demodulate
• Harder to jam

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References

• Books
• (1) Digital Communication by Simon Hykin.
• Chapter 9-Spread spectrum modulation
  (page-445)
• (2) Modern Digital and analog communication
  systems Third Edition
• By B.P.Lathi, Chapter 9-Some recent
  developments and miscellaneous topics
• (Point 9.2, page-406)
• (3) Satellite Communications, Fourth Edition by
  Dennis Roddy
• (Chapter 14, Point 14-10, page-472)
• (4) Satellite Communications by Timothy Pratt,
  Charles W.Bostian
• (Chapter 6-Multiple Access, point 6.3,
  page 251)
• (5) www.spread spectrum.com
• (6) www.spread application.com
• (7) www.google.com

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Conclusion

• Spread spectrum promises
  several benefits such as higher capacity and
  ability to resist multipath propagation. Spread
  spectrum signals are difficult to intercept for
  an unauthorized person, they are easily hidden.
  For an unauthorized person, it is difficult to
  even detect their presence in many cases. They
  are resistant to jamming. They provide a measure
  of immunity to distortion due to multipath
  propagation. They have multiple access capability
• .
• Spread spectrum is now
  finding widespread civilian and commercial
  applications such as cellular telephones,
  personal communications and position location.
  For example, DS/SS is used in electronic
  Industries Associations Interim Standard IS-95
  for cellular telephones, as well as wide range of
  position location systems such as the global
  position location and other vehicle location and
  messaging systems.

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Thanks