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Data and Computer Communications

Data and Computer Communications. Chapter 9 – Spread Spectrum. Eighth Edition by William Stallings Lecture slides by Lawrie Brown. Spread Spectrum. All creative people want to do the unexpected. —Ecstasy and Me: My Life as a Woman , Hedy Lamarr. Spread Spectrum.

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Data and Computer Communications

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  1. Data and Computer Communications Chapter 9 – Spread Spectrum Eighth Edition by William Stallings Lecture slides by Lawrie Brown

  2. Spread Spectrum All creative people want to do the unexpected. —Ecstasy and Me: My Life as a Woman, Hedy Lamarr

  3. Spread Spectrum • important encoding method for wireless communications • transmit either analog or digital data using analog signal • spreads data over a wider bandwidth to make jamming and interception harder • two approaches, both in use: • frequency hopping (first type) • direct sequence (more recent)

  4. General Model of Spread Spectrum System

  5. 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

  6. 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

  7. 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

  8. Frequency Hopping Example

  9. Frequency Hopping • A spreading code dictates the sequence of channels used. • Transmitter and receiver use the same code same sequence. • Typically, there are 2k carrier frequencies forming 2k channels. • The width of each channel usually corresponds to the bandwidth of the input signal. • Transmitter operates in one channel at a time for a fixed interval (e.g., IEEE 802.11 standard uses a 300-ms interval) • During an interval, a number of bit (or a fraction of a bit) is transmitted using some encoding scheme.

  10. FHSS (Transmitter)

  11. FHSS (Receiver)

  12. Slow and Fast FHSS • commonly use multiple FSK (MFSK) in conjunction with FHSS • have frequency shifted every Tc seconds • duration of signal element is Ts seconds • Slow FHSS has Tc Ts • Fast FHSS has Tc < Ts • FHSS more resistant to noise or jamming • 3 more more frequencies (chips) are used for each signal element: receiver can decide which signal element was sent on the basis of a majority of the chips being correct.

  13. Slow MFSK FHSS

  14. Fast MFSK FHSS

  15. 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

  16. DSSS Example

  17. DSSS Implementations • Assume a BPSK modulation scheme • Use +1 and –1 to represent the two binary digits. • To produce the DSSS signal, multiply the BPSK signal by c(t), which is the PN sequence taking on values of +1 and –1: s(t) = A d(t)c(t) cos(2πfct) • At the receiver, the incoming signal is multiplied again by c(t). c(t) c(t) = 1, the original signal is recovered. • Implementation 1: multiply d(t) and c(t) together and then perform the BPSK modulation • Implementation 2: next slide

  18. DSSS System

  19. Implementation • s(t) = A d(t) cos(2πfct) • d(t) = 1 or -1 for BPSK • Chipping signal c(t) = 1 or -1 for 1 or 0 • Encoding: • s'(t) = A d(t)c(t) cos(2πfct) • Decoding: • s(t) = A d(t)c(t) cos(2πfct) x c(t) • s(t) = A d(t) cos(2πfct)

  20. DSSS Example Using BPSK

  21. ApproximateSpectrum of DSSS Signal

  22. 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

  23. CDMA Example

  24. Example: Simultaneous Transmissions

  25. Simultaneous Transmissions: Decoding

  26. Single Transmission

  27. CDMA for DSSS

  28. Benefits of Code Division Multiple Access (CDMA) Technology • CDMA radio technology offers the following benefits to wireless carriers: • Increase of about 6 to 18 times the capacity compared to the original legacy analog AMPS • Simplified RF engineering, due to the N = 1 reuse pattern in CDMA systems • Increased performance over the weakest link in the wireless system, the air interface by using rake receivers to resolve multipath fading

  29. Benefits of Code Division Multiple Access (CDMA) Technology (2) • Lower transmitted power levels. Lower power bills at the base station level and longer battery life for CDMA handsets • Greater security due to the encoding of CDMA signals • Enhanced performance and voice quality due to soft-handoff operations

  30. Summary • Use of spread spectrum techniques: • FHSS • DSSS • CDMA

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