Spread spectrum modulation


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Spread spectrum modulation

  1. 1. 29-03-2013 A.SANYASI RAO AMIE, M.Tech, MISTE, MIETE Assoc. Prof, Dept. of ECE Balaji Institute of Engineering & Sciences Allanki Sanyasi Rao MULTIPLEXING channels ki• Multiplexing in 4 dimensions o space (si) k1 k2 k3 k4 k5 k6 o time (t) c o frequency (f) t c o code (c) t s1 f• Goal: multiple use s2 f of a shared medium c t• Important: guard spaces needed! s3 f 1
  2. 2. 29-03-2013 Allanki Sanyasi Rao FREQUENCY MULTIPLEX Separation of spectrum into smaller frequency bands Channel gets band of the spectrum for the whole time Advantages: • no dynamic coordination needed k3 k4 k5 k6 • works also for analog signals c f Disadvantages: • waste of bandwidth if traffic distributed unevenly • inflexible • guard spaces t Allanki Sanyasi Rao TIME MULTIPLEX Channel gets the whole spectrum for a certain amount of time Advantages: • only one carrier in the medium at any time • throughput high even for many users Disadvantages: • Precise synchronization necessary k1 k2 k3 k4 k5 k6 c f t 2
  3. 3. 29-03-2013 Allanki Sanyasi Rao TIME & FREQUENCY MULTIPLEX A channel gets a certain frequency band for a certain amount of time (e.g. GSM) Advantages: • better protection against tapping • protection against frequency selective interference • higher data rates compared to code multiplex Precise coordination required k1 k2 k3 k4 k5 k6 c f t Allanki Sanyasi Rao CODE MULTIPLEX k1 k2 k3 k4 k5 k6 Each channel has unique code c All channels use same spectrum at same time Advantages: • bandwidth efficient • no coordination and synchronization • good protection against interference Disadvantages: f • lower user data rates • more complex signal regeneration Implemented using spread spectrum technology t 3
  4. 4. 29-03-2013 Allanki Sanyasi Rao Spread SpectrumProblem of radio transmission: frequency dependent fading can wipe out narrow band signals for duration of the interferenceSolution: spread the narrow band signal into a broad band signal using a special codeThe spread spectrum techniques was developed initially formilitary and intelligence requirements.The essential idea is to spread the information signal over a widerbandwidth to make jamming and interception more difficult. Allanki Sanyasi Rao Spread Spectrum (Contd.)• Spread spectrum is a communication technique that spreads anarrowband communication signal over a wide range of frequencies fortransmission then de-spreads it into the original data bandwidth at thereceiver.• Spread spectrum is characterized by: - wide bandwidth and - low power• Jamming and interference have less effect on Spread spectrumbecause it is: - Resembles noise - Hard to detect - Hard to intercept 4
  5. 5. 29-03-2013 Allanki Sanyasi Rao Narrowband vs Spread SpectrumPower Narrowband (High Peak Power) Spread Spectrum (Low Peak Power) Frequency Allanki Sanyasi Rao Signal Spreading 5
  6. 6. 29-03-2013 Allanki Sanyasi RaoSPREADING AND FREQUENCY SELECTIVE FADING channel quality 1 2 5 6 narrowband 3 4 channels Narrowband guard space frequency signal channel quality 2 2 2 2 2 1 spread spectrum channels spread frequency spectrum Allanki Sanyasi RaoSPREAD SPECTRUM- MAIN OPERATION b(t) m(t) x c(t) mt   bt  ct  M  f   B f * C  f  B(f) M(f) 6
  7. 7. 29-03-2013 Allanki Sanyasi Rao Why Spread Spectrum..?Advantages:•Spread spectrum signals are distributed over a wide range of frequenciesand then collected back at the receiver•These wideband signals are noise-like and hence difficult to detect orinterfere with•Initially adopted in military applications, for its resistance to jamming anddifficulty of interception•More recently, adopted in commercial wireless communications•Has the ability to eliminate the effect of multipath interference•Can share the same frequency band with other users•Privacy due to the pseudo random code sequence (code divisionmultiplexing)Disadvantages: • Bandwidth inefficient • Implementation is somewhat more complex. Allanki Sanyasi Rao Spread Spectrum Technique• To apply an SS technique, simply inject the corresponding SS code somewhere in the transmitting chain before the antenna. The effect is to diffuse the information in a larger bandwidth. Conversely, you can remove the SS code, at a point in the receive chain before data retrieval. The effect of a de spreading operation is to reconstitute the information in its original bandwidth. Obviously, the same code must be known in advance at both ends of the transmission channel. 7
  8. 8. 29-03-2013 Allanki Sanyasi Rao Narrow Band vs Spread Spectrum• 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 Allanki Sanyasi Rao Spread Spectrum Use• In the 1980s FCC implemented a set of rules making Spread Spectrumavailable to the public. - Cordless Telephones - Global Positioning Systems (GPS) - Cell Phones - Personal Communication Systems - Wireless video cameras• Local Area Networks - Wireless Local Area Networks (WLAN) - Wireless Personal Area Network (WPAN) - Wireless Metropolitan Area Network (WMAN) - Wireless Wide Area Network (WWAN) 8
  9. 9. 29-03-2013 Allanki Sanyasi Rao FCC Specifications• The Code of Federal Regulations (CFR) Part 15 originally only described twospread spectrum techniques to be used in the licensed free Industrial, Scientific,Medical (ISM) band, 2.4 GHz, thus 802.11 and 802.11b. - Frequency Hopping Spread Spectrum (FHSS) and - Direct Sequence spread Spectrum (DSSS)• Orthogonal Frequency Division Multiplexing (OFDM) was not covered by theCFR and would have required licensing. - 802.11a, employing OFDM, was created to work in the 5GHz Unlicensed National Information Infrastructure (UNII)• In May, 2001 CFR, Part 15 was modified to allow alternative "digital modulationtechniques". - This resulted in 802.11g which employs OFDM in the 2.4 GHz range Allanki Sanyasi Rao •Direct Sequence (DS) - A carrier is modulated by a digital code sequence in which bit rate is much higher than the information signal bandwidth. • Frequency Hopping (FH) - A carrier frequency is shifted in discrete increments in a pattern dictated by a code sequence. 9
  10. 10. 29-03-2013 Allanki Sanyasi Rao What is pseudorandom number sequences? is a sequence of numbers that has been computed by some defined arithmetic process but is effectively a random number sequence for the purpose for which it is required. Allanki Sanyasi Rao• Input is fed into a channel encoder • Produce an analog signal with a relatively narrow bandwidth around some center frequency.• Further modulated using a sequence of digits known as a spreading code or spreading sequence. • The spreading code is generated by a pseudo noise, or pseudorandom number generator. • The effect of this modulation is to increase significantly the bandwidth (spread the spectrum) of the signal to be transmitted.• At the receiver, the same digit sequence is used to demodulate the spread spectrum signal.• The signal is fed into a channel decoder to recover the data.•Spread Spectrum signals use fast codes that run many times the informationbandwidth or data rate.•These special "Spreading" codes are called "Pseudo Random" or "PseudoNoise" codes. They are called "Pseudo" because they are not real Gaussiannoise. 10
  11. 11. 29-03-2013 Allanki Sanyasi Rao What can we gain from this apparent waste of spectrum?  Gain immunity from various kinds of noise and multipath distortion  Can be used for hiding and encrypting signals. Only a recipient who knows the spreading code can recover the encoded information.  Several users can independently use the same higher bandwidth with very little interference (CDMA). Allanki Sanyasi RaoGENERATING THE SPREADING (PSEUDO-NOISE) SEQUENCE • The pseudo-noise (PN) sequence is a periodic binary sequence with a noise like waveform that is generated by means of a feedback shift register. • The feedback shift register consists of m-stage shift registers and a logic circuit that perform modulo-2 (X-OR) arithmetic. • A sequence with period 2m-1 is called Maximal-Length sequence 11
  12. 12. 29-03-2013 Allanki Sanyasi RaoPN 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 . . . Here N=2m – 1=7, length of the sequence Allanki Sanyasi RaoPROPERTIES OF MAXIMAL-LENGTH SEQUENCES• In each period of a maximal-length sequence, the number of 1’s and the number of 0’s in the sequence always differ by 1.• The autocorrelation function of a maximal-length sequence is periodic and binary valued. Tb Rc    ct ct   dt 1 Tb  2 T  b 2  N 1  1    Tc  Rc     NT c  1 for the rest of the p eriod  N  12
  13. 13. 29-03-2013 Allanki Sanyasi Rao Chip rate, Rc is the rate at which the no. of bits of the PN sequence occur. The duration of each bit is TC= 1/Rc Therefore, the period of the waveform is Tb = NTc Allanki Sanyasi RaoW1 and W2N 13
  14. 14. 29-03-2013 Allanki Sanyasi RaoSequence Generation Allanki Sanyasi RaoChip SequencesEncoding Rules 14
  15. 15. 29-03-2013 Allanki Sanyasi RaoCDMA Multiplexer Allanki Sanyasi RaoCDMA Demultiplexer 15
  16. 16. 29-03-2013 Allanki Sanyasi Rao DIRECT SEQUENCE SPREAD SPECTRUM (DSSS) Direct Sequence (DS) - A carrier is modulated by a digital code sequence in which bit rate is much higher than the information signal bandwidth. • Most widely recognized technology for spread spectrum. • This method generates a redundant bit pattern for each bit to be transmitted. This bit pattern is called a chip. • The longer the chip, the greater the probability that the Original data can be recovered, and the more bandwidth required. In a spread spectrum system, the process gain (or ‘processing gain) is the ratio of the spread bandwidth to the unspread bandwidth. It is usually expressed in decibels (dB). Allanki Sanyasi Rao• 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 is then used to carrier modulate the carrier using 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.• 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 16
  17. 17. 29-03-2013 Allanki Sanyasi Rao Processing GainThe process gain (or ‘processing gain) is the ratio of the spreadbandwidth to the unspread bandwidth. It is usually expressed indecibels (dB). Allanki Sanyasi Rao DSSS USING BPSK 17
  18. 18. 29-03-2013Allanki Sanyasi RaoAllanki Sanyasi Rao 18
  19. 19. 29-03-2013 Allanki Sanyasi Rao RANGING USING DSSS Allanki Sanyasi Rao Frequency Hopping Spread Spectrum (FHSS) • When using FHSS, the frequency spectrum is divided into channels. Data packets are split up and transmitted on these channels in a random pattern known only to the transmitter and receiver.At the transmitter, the original signal is broadcastedover a series of radio frequencies, hopping fromdifferent frequencies in a fixed pattern.The receiver should use the same hopping patternsimultaneously with the transmitter in order toreceive the data correctly.The spreading code specifies the sequence ofchannels and the receiver should use the samecode to tune into the sequence of channels that areused by the sender. 19
  20. 20. 29-03-2013 Allanki Sanyasi Rao• The information signal is transmitted on different frequencies• Time is divided in slots• In each slot the frequency is changed• The change of the frequency is referred to as slow if more than one bit is transmitted on one frequency, and as fast if one bit is transmitted over multiple frequencies• The frequencies are chosen based on the spreading sequences  Each channel used for fixed interval • Eg: 300 ms in IEEE 802.11 • Sequence dictated by spreading cod Allanki Sanyasi RaoFREQUENCY HOPPING SPREAD SPECTRUM (FHSS) 20
  21. 21. 29-03-2013 Allanki Sanyasi RaoFREQUENCY SELECTION IN FHSS (EXAMPLE 1)Normally ‘K’ successive bits of input data sequence represents 2k = Msymbols. Those distinct M symbols are transmitted with the help of M-ary FSK modulation system. When spread spectrum modulation is tobe used, then the M-ary FSK signal is further modulated to generatewideband signal. Allanki Sanyasi RaoFHSS CYCLES 21
  22. 22. 29-03-2013 Allanki Sanyasi RaoTwo versions Fast Hopping: several frequencies per user bit (FFH) --- symbol rate is lesser than the hop rate i.e., hop rate is faster. Slow Hopping: several user bits per frequency (SFH) --- symbol rate is higher than hop rate i.e., hop rate is slower. Tb user data 0 1 0 1 1 t f Td f3 slow f2 hopping f1 (3 bits/hop) Td t f f3 fast f2 hopping f1 (3 hops/bit) t Allanki Sanyasi Rao (EXAMPLE 2) • FHSS uses the 2.402 – 2.480 GHz frequency range in the ISM band. • It splits the band into 79 non-overlapping channels with each channel 1 MHz wide.Transmission Frequency (GHz) 2.479 1 MHz Channels Divided into 79 2.401 200 400 600 800 1000 1200 1400 1600 Elapsed Time in Milliseconds (ms) Channel 1 Channel 2 Channel 78 22
  23. 23. 29-03-2013 Allanki Sanyasi RaoBANDWIDTH SHARING (EXAMPLE 3) Allanki Sanyasi Rao (EXAMPLE 4) • A number of channels are allocated for the FH signal. • Typically, there are 2k carrier frequencies forming 2k channels. • The width of each channel corresponds to the bandwidth of the input signal. 23
  24. 24. 29-03-2013 Allanki Sanyasi Rao Allanki Sanyasi RaoTransmitter of FH/MFSK FSK FH/MFSK signal signal M-ary FSK Mixer ModulatorBinary data Sequence Frequency hops Frequency Synthesizer ‘t’ successive bits of PN sequence 1 2 t generator PN Sequence Generator 24
  25. 25. 29-03-2013 Allanki Sanyasi RaoReceiver of FH/MFSK Received FSKFH/MFSK signal signal Non coherent Mixer M-ary FSK detector Binary Sequence output Frequency hops Frequency Synthesizer PN Sequence Generator Allanki Sanyasi RaoApplication of FHSS – Multipath SuppressionMultipath Interference ProblemIn mobile communication, the signal reach to the receiver from differentpaths.There is one direct path and many indirect paths due to reflections fromnearby objects.The signal due to indirect paths interfere with the required signal inamplitude as well as phase. It is called multipath fading.How FHSS overcomes multipath effect?The carrier frequency of the transmitted signal hops faster than thedifferential time delay between the direct signal and reflected signals.Therefore the reflected signal energy will fall in different frequency slots.This signal energy will be treated as interference by the matched filter of thereceiver. It is then filtered out and only signal from direct path is available atthe output.The hopping rate must be fast enough to eliminate interference due to smalltime delays between direct and reflected paths. 25
  26. 26. 29-03-2013 Allanki Sanyasi Rao Synchronization in Spread Spectrum SystemsSpread spectrum systems are essentially synchronous. The PN sequencesgenerated at the receiver and the transmitter must be same and locked toeach other so that the transmitted signal can be extracted.The synchronization of the spread spectrum systems can be considered intwo parts: •Acquisition •TrackingThe acquisition means initial synchronization of the spread spectrum signal.The tracking starts after acquisition is complete. The tracking maintains thePN generator at the receiver in synchronism with the transmitter. Allanki Sanyasi Rao Acquisition of DS signal using Serial search 26
  27. 27. 29-03-2013 Allanki Sanyasi RaoThe received signal is correlated with the generated PN sequence. Thiscross correlation is performed over the time interval of NTC.The output of the Correlator is compared with a threshold. If it exceedsthe threshold, then the required signal is obtained. If the threshold isnot exceeded, then the PN generator output is advanced by half chipduration ( 1 T ) and the correlation is performed. C 2The output of the Correlator is again compared with the threshold andthe procedure is repeated. Allanki Sanyasi RaoAcquisition of FH signal using Serial search 27
  28. 28. 29-03-2013 Allanki Sanyasi RaoThe VCO consists of frequency synthesizer, PN generator and clockgenerator. The received signal is correlated with the output of VCO.The tuned filter passes only the intermediate frequency f0.The envelop detector generates the output which is compared with thethreshold voltage.When the input frequency and frequency of VCO are same, then outputof threshold detector is high and the clock generator starts runningcontinuously. Then the signal is said to have acquired and trackingstarts. 28