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Spread Spectrum Communication Systems

Spread Spectrum Communication Systems. Shemi / mesmarampally / sep 2012. Spread Spectrum Communications. A means of transmission in which the data sequence occupies a bandwidth in excess of the minimum bandwidth necessary to send it.

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Spread Spectrum Communication Systems

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  1. Spread Spectrum Communication Systems Shemi /mesmarampally/ sep 2012

  2. Spread Spectrum Communications • A means of transmission in which the data sequence occupies a bandwidth in excess of the minimum bandwidth necessary to send it. • Increases the bandwidth of the signal compared to narrowband by spreading it. • Signal spreading is done before transmission by using a spreading sequence. • The same sequence is used at the receiver to retrieve the signal • Most effective against interference with fixed energy. • Main commercial applications in wireless and GPS.

  3. Why Spread Spectrum ? • Advantages: • Resists intentional and non-intentional interference • Has the ability to eliminate or alleviate the effect of • multipath interference • Can share the same frequency band (overlay) with other • users • Privacy due to the pseudo random code sequence (code division multiplexing) • Disadvantages: • Bandwidth inefficient • Implementation is somewhat more complex

  4. Narrow Band • Uses only enough frequency spectrum to carry the signal • High peak power • Easily jammed • Spread Spectrum • The bandwidth is much wider than required to send to the signal. • Low peak power • Hard to detect • Hard to intercept • Difficult to jam

  5. SPREAD SPECTRUM USES • Cordless Telephones • Global Positioning Systems (GPS) • Cell Phones • Personal Communication Systems • Wireless video cameras

  6. Spread Spectrum Techniques • Frequency Hopping Spread Spectrum (FHSS) • FHSS spreads the signal by hopping from one frequency to another • Direct Sequence spread Spectrum (DSSS) • DSSS spreads the signal by adding redundant bits to the signal prior to transmission which spreads the signal • The redundant information bits are called Pseudorandom Numbers (PN). • Chipping Code

  7. General Model of Spread Spectrum System

  8. Spread Spectrum Concept • Input fed into channel encoder • Produces narrow bandwidth analog signal around central frequency • Signal modulated using sequence of digits • Spreading code/sequence • Typically generated by pseudo noise/pseudorandom number generator • Increases bandwidth significantly • Spreads spectrum • Receiver uses same sequence to demodulate signal • Demodulated signal fed into channel decoder

  9. GAINS • Immunity from various noise and multipath distortion • Including jamming • Can hide/encrypt signals • Only receiver who knows spreading code can retrieve signal • Several users can share same higher bandwidth with little interference • Cellular telephones • Code division multiplexing (CDM) • Code division multiple access (CDMA)

  10. Generating the Spreading (Pseudo-Noise) Sequence • A periodic binary sequence with a noise like waveform • Generated by means of a feedback shift register • The feedback shift register consists of • m-stage shift registers • a logic circuit • perform modulo-2 (X-OR) arithmetic. • A sequence with period 2m-1 is called Maximal-Length sequence

  11. PN Sequence : Example s1 s2 s3 1 0 0 1 1 0 1 1 1 0 1 1 1 0 1 0 1 0 0 0 1 1 0 0 Spreading code  0 0 1 1 1 0 1 0 . . .

  12. Pseudorandom Numbers • A sequence of numbers that approximates the properties of random numbers • Generated by algorithm using initial seed (PRNG) • Deterministic algorithm • Not actually random • Determined by a relatively small set of initial values, called the PRNG's state, which includes a truly random seed • Need to know algorithm and seed to predict sequence

  13. Direct Sequence Spread Spectrum (DSSS) • Each bit represented by multiple bits using spreading code • Spreading code spreads signal across wider frequency band • In proportion to number of bits used • 10 bit spreading code spreads signal across 10 times bandwidth of 1 bit code • One method: • Combine input with spreading code using XOR • Input bit 1 inverts spreading code bit • Input zero bit doesn’t alter spreading code bit • Data rate equal to original spreading code

  14. Direct Sequence Spread Spectrum- Example

  15. DSSS • It uses a locally generated pseudo noise code to encode digital data to be transmitted • The speed of the code sequence is called the chipping rate which is measured in cps • The amount of spreading is dependent upon the ratio of chips per bit of information (which is the processing gain Gp for DSSS) • A direct sequence modulator double sideband suppressed carrier modulate the carrier frequency to be transmitted. • The resultant DSB suppressed carrier AM modulation can also be thought of as binary phase shift keying (BPSK) • At the receiver, the information is recovered by multiplying the signal with a locally generated replica of the code sequence.

  16. Direct Sequence Spread Spectrum Transmitter

  17. Direct Sequence Spread Spectrum receiver

  18. Direct Sequence Spread Spectrum Using BPSK Example

  19. ApproximateSpectrum of DSSS Signal

  20. Direct Sequence Spread Spectrum with Coherent BPSK (DS/BPSK)

  21. DSSS • ADVANTAGES • Simple hard ware implementation • Best noise and anti jam performance • Best discrimination against multi path • Do not require a high speed fast setting frequency synthesizer • DISADVANTAGES • Requires wide band channel with little phase distortion • Long acquisition time. • Fast code generator needed. • Near –far problem

  22. Frequency Hopping Spread Spectrum (FHSS) • Signal broadcast over seemingly random series of frequencies • Receiver hops between frequencies in sync with transmitter • FHSS is usually used with binary FSK or M-ary FSK • The carrier frequency is determined by the output sequence from a PN generator

  23. Hopping pattern is known to both transmitter & receiver. • In order to properly receive the signal, the receiver must be set to the same hopping code and listen to the incoming signal at the right time and correct frequency. • The net effect is to maintain a single logical channel if synchronizing sender and receiver properly. • Unintended receiver see FHSS to be short time impulse noise

  24. FHSS cont… • Slow hopping system - hopping rate is lower than the information rate (symbol rate) • Several information symbols are transmitted by the same carrier frequency • Advantage: • Coherent data detection is possible. • Disadvantage: • If one frequency hop channel is jammed, one or more data bits are lost. So we are forced to use error correcting codes.

  25. FHSS cont… • Fast hopping system - hopping rate is higher than the information rate • One information symbol is transmitted by different carrier frequencies. • Advantage: • Now error correcting codes are not needed. • Diversity can be applied. Every frequency hop a decision is made whether a -1 or a 1 is transmitted, at the end of each data bit a majority decision is made. • Disadvantage: • Coherent data detection is not possible because of phase • discontinuities. The applied modulation technique should be FSK or MFSK

  26. Frequency Hopping Example

  27. Frequency Hopping Spread Spectrum System (Transmitter)

  28. Frequency Hopping Spread Spectrum System (Receiver)

  29. Slow and Fast FHSS • Frequency shifted every Tc seconds • Duration of signal element is Ts seconds • Slow FHSS has Tc Ts • Fast FHSS has Tc < Ts • Generally fast FHSS gives improved performance in noise (or jamming)

  30. Slow Frequency Hop Spread Spectrum Using MFSK (M=4, k=2)

  31. Fast Frequency Hop Spread Spectrum Using MFSK (M=4, k=2)

  32. Frequency-Hopped (FH) Spread Spectrum Information Sequence • FH/SS is usually used with Binary FSK or M-ary FSK • The carrier frequency is determined by the output sequence from a PN generator • Slow hopping system has a hopping rate that is lower than the information rate (symbol rate) • Several information symbols are transmitted by the same carrier frequency • Fast hopping system has a hopping rate that is higher than the information rate • One information symbol is transmitted by different carrier frequencies. Mixer Encoder Mixer Channel FSK Modulator Decoder FSK Modulator Frequency Synthesizer Time Sync PN Sequence Generator Frequency Synthesizer PN Sequence Generator

  33. Slow Frequency Hopping Example Number of bits per MFSK symbol = 2  M = 4 Rs = Rb/2 Rc = max(Rh, Rs) = Rs Length of PN segment per hop = 3 Total number of frequency hops = 23 = 8

  34. Fast Frequency Hopping Example Number of bits per MFSK symbol = 2  M = 4 Rs = Rb/2 Rc = max(Rh, Rs) = Rh Length of PN segment per hop = 3 Total number of frequency hops = 23 = 8

  35. Code Division Multiple Access (CDMA) • Multiple users can use the same channel as long as different users are assigned different PN sequence (code) • Several users can transmit simultaneously on the same channel • The transmissions from other users will look like interference. • CDMA main application is wireless communication. • CDMA is the wireless standard for North America

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