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May 28-June1, 2001 R. Z. Ziemer, Colorado Springs, CO Spread spectrum
I .  History of Spread Spectrum II.  Spread Spectrum System Model III.  Spread Spectrum Classification IV.  Spread Spectru...
History of Spread Spectrum <ul><li>Spread Spectrum was actually invented by 1940s Hollywood actress  Hedy Lamarr ( 1913-20...
<ul><li>“ Spread” radio signal over a wide frequency range  </li></ul><ul><li>Several magnitudes higher than minimum requi...
<ul><li>Offers the following applications: </li></ul><ul><ul><li>􀂉  able to deal with multi-path </li></ul></ul><ul><ul><l...
Spread Spectrum Applications <ul><li>Interference </li></ul><ul><ul><li>Prevents interference at specific frequencies </li...
System Model: Spread Spectrum Transmission
Spread Spectrum Criteria A communication system is considered a spread spectrum system if it satisfies the following two c...
Spread Spectrum Classification
Direct Sequence Spread Spectrum <ul><li>Information signal is directly modulated (multiplicated) by </li></ul><ul><li>a sp...
Direct Sequence Spread Spectrum Example May 28-June1, 2001 R. Z. Ziemer, Colorado Springs, CO
Direct Sequence Spread Spectrum: Transmission Technique
Direct Sequence Spread Spectrum Transmitter May 28-June1, 2001 R. Z. Ziemer, Colorado Springs, CO
Direct Sequence Spread Spectrum Receiver May 28-June1, 2001 R. Z. Ziemer, Colorado Springs, CO
Direct Sequence Spread Spectrum Using BPSK Example May 28-June1, 2001 R. Z. Ziemer, Colorado Springs, CO
Approximate Spectrum of  DSSS Signal May 28-June1, 2001 R. Z. Ziemer, Colorado Springs, CO
<ul><li>The information signal is transmitted on different frequencies </li></ul><ul><li>Time is divided in slots </li></u...
Frequency Hopping Spread Spectrum (FHSS) May 28-June1, 2001 R. Z. Ziemer, Colorado Springs, CO
Frequency selection in FHSS May 28-June1, 2001 R. Z. Ziemer, Colorado Springs, CO
FHSS cycles May 28-June1, 2001 R. Z. Ziemer, Colorado Springs, CO
Frequency Hopping Spread Spectrum
Bandwidth sharing May 28-June1, 2001 R. Z. Ziemer, Colorado Springs, CO
Time Hopping Spread Spectrum <ul><li>Time divided into frames; each TF long </li></ul><ul><li>Each frame is divided in slo...
Comparison of different Spread Spectrum Techniques SS Technique   advantage   disadvantage Direct Sequence 􀂉 best behavior...
The Idea In MCM , we split the data in to different streams and transmit using separate Sub Carriers.
DS-CDMA Figure: The Principle of DS-CDMA
 
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Spread Spectrum

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Transcript of "Spread Spectrum"

  1. 1. May 28-June1, 2001 R. Z. Ziemer, Colorado Springs, CO Spread spectrum
  2. 2. I . History of Spread Spectrum II. Spread Spectrum System Model III. Spread Spectrum Classification IV. Spread Spectrum Techniques Outline
  3. 3. History of Spread Spectrum <ul><li>Spread Spectrum was actually invented by 1940s Hollywood actress Hedy Lamarr ( 1913-2000 ). </li></ul><ul><li>An Austrian refugee, in 1940 at the age of 26, she devised together with music composer George Antheil a system to stop enemy detection and jamming of radio controlled torpedoes by hopping around a set of frequencies in a random fashion . </li></ul><ul><li>She was granted a patent in 1942 (US pat. 2292387) but considered it her contribution to the war effort and never profited. </li></ul><ul><li>Techniques known since 1940s and used in military communication systems since 1950s. </li></ul>
  4. 4. <ul><li>“ Spread” radio signal over a wide frequency range </li></ul><ul><li>Several magnitudes higher than minimum requirement </li></ul><ul><li>Gained popularity by the needs of military communication </li></ul><ul><li>Proved resistant against hostile jammers </li></ul><ul><li>Ratio of information bandwidth and spreading bandwidth is identified as spreading gain or processing gain </li></ul><ul><li>Processing gain does not combat white Noise </li></ul>Introduction to Spread Spectrum
  5. 5. <ul><li>Offers the following applications: </li></ul><ul><ul><li>􀂉 able to deal with multi-path </li></ul></ul><ul><ul><li>􀂉 multiple access due to different spreading sequences </li></ul></ul><ul><ul><li>􀂉 spreading sequence design is very important for performance </li></ul></ul><ul><ul><li>􀂉 low probability of interception </li></ul></ul><ul><ul><li>􀂉 privacy </li></ul></ul><ul><ul><li>􀂉 anti-jam capabilities </li></ul></ul>
  6. 6. Spread Spectrum Applications <ul><li>Interference </li></ul><ul><ul><li>Prevents interference at specific frequencies </li></ul></ul><ul><ul><li>E.g. other radio users, electrical systems </li></ul></ul><ul><li>Military </li></ul><ul><ul><li>Prevents signal jamming </li></ul></ul><ul><ul><li>Scrambling of ‘secret’ messages </li></ul></ul><ul><li>Wireless LAN security </li></ul><ul><ul><li>Prevents ‘eavesdropping’ of wireless links </li></ul></ul><ul><ul><li>Prevents ‘hacking’ into wireless LANs </li></ul></ul><ul><li>CDMA ( C ode D ivision M ultiple A ccess) </li></ul><ul><ul><li>Multiple separate channels in same medium using different spreading codes </li></ul></ul>
  7. 7. System Model: Spread Spectrum Transmission
  8. 8. Spread Spectrum Criteria A communication system is considered a spread spectrum system if it satisfies the following two criteria: <ul><li>Bandwidth of the spread spectrum signal has to be greater than the information bandwidth. (This is also true for frequency and pulse code modulation!) </li></ul><ul><li>The spreading sequence has to be independent from the information. Thus, no possibility to calculate the information if the sequence is known and vice versa. </li></ul>
  9. 9. Spread Spectrum Classification
  10. 10. Direct Sequence Spread Spectrum <ul><li>Information signal is directly modulated (multiplicated) by </li></ul><ul><li>a spreading sequences (see next slide) </li></ul><ul><li>Spreading sequence consists of chips each with a </li></ul><ul><li>duration of tchip </li></ul><ul><li>A set of chips represent a bit; the exact number of chips </li></ul><ul><li>per bit equals the spreading gain </li></ul><ul><li>Near far effect </li></ul><ul><li>Require continuous bandwidth </li></ul>
  11. 11. Direct Sequence Spread Spectrum Example May 28-June1, 2001 R. Z. Ziemer, Colorado Springs, CO
  12. 12. Direct Sequence Spread Spectrum: Transmission Technique
  13. 13. Direct Sequence Spread Spectrum Transmitter May 28-June1, 2001 R. Z. Ziemer, Colorado Springs, CO
  14. 14. Direct Sequence Spread Spectrum Receiver May 28-June1, 2001 R. Z. Ziemer, Colorado Springs, CO
  15. 15. Direct Sequence Spread Spectrum Using BPSK Example May 28-June1, 2001 R. Z. Ziemer, Colorado Springs, CO
  16. 16. Approximate Spectrum of DSSS Signal May 28-June1, 2001 R. Z. Ziemer, Colorado Springs, CO
  17. 17. <ul><li>The information signal is transmitted on different frequencies </li></ul><ul><li>Time is divided in slots </li></ul><ul><li>Each slot the frequency is changed </li></ul><ul><li>The change of the frequency is referred to as slow if more than </li></ul><ul><li>one bit is transmitted on one frequency, and as fast if one bit is </li></ul><ul><li>transmitted over multiple frequencies </li></ul><ul><li>The frequencies are chosen based on the spreading sequences </li></ul>Frequency Hopping Spread Spectrum
  18. 18. Frequency Hopping Spread Spectrum (FHSS) May 28-June1, 2001 R. Z. Ziemer, Colorado Springs, CO
  19. 19. Frequency selection in FHSS May 28-June1, 2001 R. Z. Ziemer, Colorado Springs, CO
  20. 20. FHSS cycles May 28-June1, 2001 R. Z. Ziemer, Colorado Springs, CO
  21. 21. Frequency Hopping Spread Spectrum
  22. 22. Bandwidth sharing May 28-June1, 2001 R. Z. Ziemer, Colorado Springs, CO
  23. 23. Time Hopping Spread Spectrum <ul><li>Time divided into frames; each TF long </li></ul><ul><li>Each frame is divided in slots </li></ul><ul><li>Each wireless terminal send in exactly one of these slots per </li></ul><ul><li>frame regarding the spreading sequence </li></ul><ul><li>No near far effect </li></ul>
  24. 24. Comparison of different Spread Spectrum Techniques SS Technique advantage disadvantage Direct Sequence 􀂉 best behavior in multi path rejection 􀂉 no synchronization 􀂉 simple implementation 􀂉 difficult to detect 􀂉 near far effect 􀂉 coherent bandwidth Frequency Hopper 􀂉 no need for coherent bandwidth 􀂉 less affected by the near far effect 􀂉 complex hardware 􀂉 error correction needed Time Hopper 􀂉 high bandwidth efficiency 􀂉 less complex hardware 􀂉 less affected by the near far effect 􀂉 error correction needed
  25. 25. The Idea In MCM , we split the data in to different streams and transmit using separate Sub Carriers.
  26. 26. DS-CDMA Figure: The Principle of DS-CDMA
  27. 28. Thank You
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