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SYSC 4607 – Lecture 23 Outline

SYSC 4607 – Lecture 23 Outline. Overview of Spread Spectrum Modulation Direct Sequence Spread Spectrum (DSSS) ISI and Interference Rejection Properties of DSSS Basics of Frequency Hopping spread spectrum Maximum-Length Sequences (m-Sequences) Rake Receivers. Spread Spectrum.

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SYSC 4607 – Lecture 23 Outline

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  1. SYSC 4607 – Lecture 23 Outline • Overview of Spread Spectrum Modulation • Direct Sequence Spread Spectrum (DSSS) • ISI and Interference Rejection Properties of DSSS • Basics of Frequency Hopping spread spectrum • Maximum-Length Sequences (m-Sequences) • Rake Receivers

  2. Spread Spectrum • Transmitted signal occupies a bandwidth much larger than the BW of the message signal - Hides a signal below the noise floor (hard to detect) - Mitigates narrowband interference and jamming - Provides multipath (ISI) rejection - Provides diversity benefit by coherent combination of resolved multipath components - Not spectrally efficient for single user. Allows many users to share the same bandwidth • Long history of military use. Commercial applications relatively new. Forms the basis of multiuser CDMA in wireless, the dominant access method for 3G

  3. Spread Spectrum Principles

  4. Direct Sequence Spread Spectrum(DSSS)

  5. Principles of DSSS

  6. DSSS- Transceiver Structure Modulation/demodulation is performed normally.

  7. DSSS Encoder/Decoder

  8. Access Techniques in MultiuserWireless Systems

  9. Narrowband Interference Rejection

  10. Multipath (ISI) Rejection

  11. Multipath (ISI) Rejection

  12. DSSS - Combined Advantages

  13. Frequency Hopping

  14. DSSS- Transceiver Structure

  15. DSSS Performance

  16. DSSS - No ISI, No Interference

  17. DSSS - Interference Rejection

  18. DSSS - Multipath (ISI) Rejection

  19. DSSS - Multipath (ISI) Rejection

  20. DSSS - Multipath (ISI) Rejection

  21. Optimum Spreading Codes

  22. Maximum-Length Sequences(m-sequences)

  23. Autocorrelation of m-sequences

  24. RAKE Receiver • DSSS removes most of the energy from multipath. • The received signal components typically experience fading. The system normally synchronizes to the strongest multipath component. • A RAKE receiver has N branches that synchronize to N different multipath components. • Different multipath components are combined using Selection, Equal Gain, or Maximal Ratio Combining. • RAKE is a diversity combining technique, with diversity branches provided by the environment.

  25. RAKE Receiver

  26. RAKE Receiver Structure

  27. Performance in Fading Channels

  28. Main Points • Spread spectrum spreads signal over wide bandwidth for ISI/interference rejection • Direct Sequence and Frequency Hopping are major spread spectrum techniques • DSSS rejects interference by spreading gain • DSSS rejects multipath ISI by code autocorrelation • Maximal-length (m) sequence codes have good autocorrelation, poor cross-correlation properties • RAKE receivers coherently combine multipath components to improve performance

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