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Spread spectrum


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spread spectrum

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Spread spectrum

  1. 1. 15 April 2015 1 Submitted to:-sir. Dinesh dayal Submitted by:- Sonu jaiswal Branch :- ECE Roll no. :- 1209031172 SEMINAR TOPIC SPREAD SPECTRUM
  2. 2. May 28-June1, 2001R. Z. Ziemer, Colorado Springs, CO 2 Spreadspectrum
  3. 3. I. History of Spread Spectrum II. Spread Spectrum System Model III. Spread Spectrum Classification IV. Spread Spectrum Techniques Outline
  4. 4. History of Spread Spectrum Spread Spectrum was actually invented by 1940s Hollywood actress Hedy Lamarr(1913-2000). 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. She was granted a patent in 1942 (US pat. 2292387) but considered it her contribution to the war effort and never profited. Techniques known since 1940s and used in military communication systems since 1950s.
  5. 5. “Spread” radio signal over a wide frequency range Several magnitudes higher than minimum requirement Gained popularity by the needs of military communication Proved resistant against hostile jammers Ratio of information bandwidth and spreading bandwidth is identified as spreading gain or processing gain Processing gain does not combat white Noise Introduction to Spread Spectrum
  6. 6. Offers the following applications: 􀂉 able to deal with multi-path 􀂉 multiple access due to different spreading sequences 􀂉 spreading sequence design is very important for performance 􀂉 low probability of interception 􀂉 privacy 􀂉 anti-jam capabilities
  7. 7. Spread Spectrum Applications Interference ̶ Prevents interference at specific frequencies ̶ E.g. other radio users, electrical systems Military ̶ Prevents signal jamming ̶ Scrambling of ‘secret’ messages Wireless LAN security ̶ Prevents ‘eavesdropping’ of wireless links ̶ Prevents ‘hacking’ into wireless LANs CDMA (Code Division Multiple Access) ̶ Multiple separate channels in same medium using different spreading codes
  8. 8. System Model: Spread Spectrum Transmission
  9. 9. Spread Spectrum Criteria A communication system is considered a spread spectrum system if it satisfies the following two criteria: Bandwidth of the spread spectrum signal has to be greater than the information bandwidth. (This is also true for frequency and pulse code modulation!) 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.
  10. 10. Spread Spectrum Classification
  11. 11. Direct Sequence Spread Spectrum Information signal is directly modulated (multiplicated) by a spreading sequences (see next slide) Spreading sequence consists of chips each with a duration of tchip A set of chips represent a bit; the exact number of chips per bit equals the spreading gain Near far effect Require continuous bandwidth
  12. 12. May 28-June1, 2001R. Z. Ziemer, Colorado Springs, CO 12
  13. 13. May 28-June1, 2001R. Z. Ziemer, Colorado Springs, CO 13
  14. 14. May 28-June1, 2001R. Z. Ziemer, Colorado Springs, CO 14
  15. 15. May 28-June1, 2001R. Z. Ziemer, Colorado Springs, CO 15
  16. 16. The information signal is transmitted on different frequencies Time is divided in slots Each slot the frequency is changed The change of the frequency is referred to as slow if more than one bit is transmitted on one frequency, and as fast if one bit is transmitted over multiple frequencies The frequencies are chosen based on the spreading sequences Frequency Hopping Spread Spectrum
  17. 17. May 28-June1, 2001R. Z. Ziemer, Colorado Springs, CO 17
  18. 18. May 28-June1, 2001R. Z. Ziemer, Colorado Springs, CO 18
  19. 19. Time Hopping Spread Spectrum Time divided into frames; each TF long Each frame is divided in slots Each wireless terminal send in exactly one of these slots per frame regarding the spreading sequence No near far effect
  20. 20. 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