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09-SpreadSpectrum.ppt
- 1. Data and Computer Communications Eighth Edition by William Stallings Chapter 9 – Spread Spectrum
- 2. Spread Spectrum important encoding method for wireless communications analog & digital data with analog signal spreads data over wide bandwidth makes jamming and interception harder two approaches, both in use: Frequency Hopping Direct Sequence
- 3. General Model of Spread Spectrum System
- 4. Spread Spectrum Advantages immunity from noise and multipath distortion can hide / encrypt signals several users can share same higher bandwidth with little interference CDM/CDMA Mobile telephones
- 5. Pseudorandom Numbers generated by a deterministic algorithm not actually random but if algorithm good, results pass reasonable tests of randomness starting from an initial seed need to know algorithm and seed to predict sequence hence only receiver can decode signal
- 6. Frequency Hopping Spread Spectrum (FHSS) signal is broadcast over seemingly random series of frequencies receiver hops between frequencies in sync with transmitter eavesdroppers hear unintelligible blips jamming on one frequency affects only a few bits
- 9. Frequency Hopping Spread Spectrum System (Receiver)
- 10. Slow and Fast FHSS commonly use multiple FSK (MFSK) have frequency shifted every Tc seconds duration of signal element is Ts seconds Slow FHSS has Tc Ts Fast FHSS has Tc < Ts FHSS quite resistant to noise or jamming with fast FHSS giving better performance
- 11. Slow MFSK FHSS
- 12. Fast MFSK FHSS
- 13. Direct Sequence Spread Spectrum (DSSS) each bit is represented by multiple bits using a spreading code this spreads signal across a wider frequency band has performance similar to FHSS
- 14. Direct Sequence Spread Spectrum Example
- 15. Direct Sequence Spread Spectrum System
- 16. DSSS Example Using BPSK
- 18. Code Division Multiple Access (CDMA) a multiplexing technique used with spread spectrum given a data signal rate D break each bit into k chips according to a fixed chipping code specific to each user resulting new channel has chip data rate kD chips per second can have multiple channels superimposed
- 19. CDMA Example
- 20. CDMA for DSSS
- 21. Summary looked at use of spread spectrum techniques: FHSS DSSS CDMA