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

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