Uploaded byDr Naim R Kidwai
spread spectrum communication
This document discusses various spread spectrum communication techniques including frequency hopping spread spectrum (FHSS), direct sequence spread spectrum (DSSS), code division multiple access (CDMA), multiuser detection, and orthogonal frequency division multiplexing (OFDM). It provides examples and explanations of how each technique works, along with their advantages such as interference suppression, low power operation, multiple access capability, and information security provided by codes. Hedy Lamarr is credited with inventing spread spectrum technology during World War II to prevent jamming of radio controlled torpedoes.
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- 1. Spread Spectrum Communications UnitIV : Spread Spectrum Communications: Frequency Hopping Spread Spectrum (FHSS) systems, Direct Sequence Spread Spectrum, Code Division Multiple Access of DSSS, Multiuser Detection, OFDM Communications 4/3/2018 1 NEC 602 by Dr Naim R Kidwai, Professor, F/o Engineering, JETGI, (JIT Jahangirabad)
- 2. Spread Spectrum Communications 4/3/20182 NEC 602 by Dr Naim R Kidwai, Professor, F/o Engineering, JETGI, (JIT Jahangirabad) • Spread Spectrum was actually invented by 1940’s Hollywood actress Hedy Lamarr (1913-2000), who was an Austrian refugee • In 1940 she along with music composer George Antheil devised a system to stop enemy detection and jamming of radio controlled torpedoes by randomly hopping around a set of frequencies. • Got patent in 1942 but never profited, considering it as war effort contribution. • It is used in military communication systems since 1950s.
- 3. Beneficial attributes ofSpread Spectrum Systems 4/3/2018 3 NEC 602 by Dr Naim R Kidwai, Professor, F/o Engineering, JETGI, (JIT Jahangirabad) • Interference suppression capability: reduces effect of interference, jamming, multipath interference. • Energy density reduction (Low power): to anyone who does not know the spreading signal, spread spectrum signal appear as buried in noise, But the intended receiver can detect the signal • Fine time resolution: spread spectrum signal can be used for ranging or location determination. Due to spreading (high frequency), more precise range measurement can be done. • Multiple access with information security: To share communication resource among numerous users, code division multiple access (CDMA) is used. Spreading signal (code) also provides encryption of information.
- 4. Spread Spectrum Communication 4/3/20184 NEC 602 by Dr Naim R Kidwai, Professor, F/o Engineering, JETGI, (JIT Jahangirabad) Digital communication focus on efficient utilization of bandwidth and power with accepted reliability. Also data security is a concern. Spread spectrum systems targets all. Spread spectrum communication • Spread spectrum is a means of transmission in which the data of interest occupies a bandwidth in excess of the minimum necessary bandwidth • Spectrum spreading is accomplished before transmission through the code that is independent of the data sequence. Same code is used in receiver to de-spread the received signal in order to recover the original sequence. Spread Spectrum Techniques Direct Sequencing Spectrum (DS) Frequency Hopping (FH) Time Hopping (TH) Hybrid DS/FH FH/THDS/FH DSS/FH/TH Spread Spectrum Classification
- 5. Spread Spectrum CommunicationModel: DSSS 4/3/2018 5 NEC 602 by Dr Naim R Kidwai, Professor, F/o Engineering, JETGI, (JIT Jahangirabad) Interference rejection capability of spread spectrum system can be explained as • Multiplication by Spreading signal once spreads bandwidth • Multiplication by Spreading signal twice followed by filter recovers original signal • Desired signal is multiplied twice while jamming signal is multiplied only once Data Signal x(t) Data rate R Spreading code (Pseudo Noise) sequence Chip rate RCh Recovered Data signal Spreading code (Pseudo Noise) sequence Chip rate RCh filter Bandwidth R Spread signal of bandwidth RCh Jamming/ interference Basic Spread Spectrum Technique
- 6. Pseudo Noise Sequence(PN Sequence) 4/3/2018 6 NEC 602 by Dr Naim R Kidwai, Professor, F/o Engineering, JETGI, (JIT Jahangirabad) • PN sequence is a coded binary sequence with certain autocorrelation properties. • PN sequence is periodic and can be generated using linear feedback shift register. • A maximum length PN sequence generated using m bit linear feedback shift register is periodic with 2m-1 period Shift Register output 1 0 0 1 1 1 0 2 1 1 1 3 0 1 1 4 1 0 1 5 0 1 0 6 0 0 1 7 1 0 0 FFFFFF x1 x2 x3 Maximum length sequence generator
- 7. Direct Sequence SpreadSpectrum Example 4/3/2018 7 NEC 602 by Dr Naim R Kidwai, Professor, F/o Engineering, JETGI, (JIT Jahangirabad) Data input 1 0 1 1 0 1 0 T TC TC T Spreading PN Sequence Transmitted Signal Transmitted Signal Spreading PN Sequence Data output Transmitter Receiver
- 8. Spread Spectrum CommunicationModel: DSSS 4/3/2018 8 NEC 602 by Dr Naim R Kidwai, Professor, F/o Engineering, JETGI, (JIT Jahangirabad) f f f 1 T 1 T 1 CT 1 CT 1 1 CT T 1 1 CT T Spectrum of data signal Spectrum of pseudo noise signal Spectrum of DSSS signal
- 9. Frequency Hopping SpreadSpectrum (FHSS) systems 4/3/2018 9 NEC 602 by Dr Naim R Kidwai, Professor, F/o Engineering, JETGI, (JIT Jahangirabad) • Data signal modulates a carrier whose frequency is pseudo randomly determined. • A frequency synthesiser generates a carrier frequency based on dictates of the PN code. • At each frequency hop time, PN generator feeds the frequency synthesiser a frequency word (a sequence of l chips which dictates one of the 2l symbol set). • Frequency hopping bandwidth WSS and frequency step size ∆f determine minimum number of chips in frequency word. Data Signal x(t) Data rate R PN sequence generator Chip rate RCh Recovered Data signal filter Bandwidth R Spread signal of bandwidth RCh Jamming/ interference Basic FHSS Technique FH modulator FH modulator Frequency synthesizer PN sequence generator Chip rate RCh Frequency synthesizer
- 10. Frequency Hopping SpreadSpectrum (FHSS) systems 4/3/2018 10 NEC 602 by Dr Naim R Kidwai, Professor, F/o Engineering, JETGI, (JIT Jahangirabad) • One hop consists of set of frequencies corresponding to chips of frequency word. • In a hop, occupied bandwidth may be less than WSS but over several hops, it occupies entire WSS. Ex. Frequency hopping bandwidth WSS of 400 MHz and frequency step size ∆f of 100 Hz are specified. What are minimum number of chips required for frequency word. 6SS SS 6 2 W 400 MHz Number of frequencies in W = 4 x 10 100 Hz Minimum number of chips = log (4 10 ) 22 gives smallest integer not less than f x x x
- 11. FHSS cycles 4/3/2018 11 NEC602 by Dr Naim R Kidwai, Professor, F/o Engineering, JETGI, (JIT Jahangirabad) f1 f8 f7 f6 f5 f4 f3 f2 Carrier frequencies Hop periods 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Hop1 Hop2
- 12. FHSS Example: Bluetooth 4/3/201812 NEC 602 by Dr Naim R Kidwai, Professor, F/o Engineering, JETGI, (JIT Jahangirabad) • Bluetooth is a wireless technology standard for exchanging data over short distances. • Invented by Ericsson in 1994 and named after 10-century Norway king Harald Bluetooth • operates in 2402-2480 MHz, with guard bands of 2 MHz at bottom and 3.5 MHz at top end. It is part of globally unlicensed industrial, scientific and medical (ISM) 2.4 GHz band. • Bluetooth uses a radio technology called FHSS and has low power consumption. • Bluetooth divides data into packets, and transmits each packet on one of 79 designated Bluetooth channels, each of 1 MHz bandwidth. • It usually performs 800 hops per second. Class Maximum Power (mW) Typical range (m) 1 100 100 2 2.5 10 3 1 1 4 0.5 0.5
- 13. Code Division MultipleAccess of DSSS 4/3/2018 13 NEC 602 by Dr Naim R Kidwai, Professor, F/o Engineering, JETGI, (JIT Jahangirabad) • CDMA allows all users to transmit at the same time using all available bandwidth separable on the basis of code, using which either frequency hop or direct sequence spread spectrum modulation is performed • Design of codes ensures that each code is approximately orthogonal • CDMA system operates asynchronously. Advantages: • Does not require synchronization like in TDMA • Gradual degradation in performance as the number of users increase • Offers external interference rejection capability Time Freq FDMA Time Freq TDMA Freq Time PN Code CDMA
- 14. Multiuser Detection (MUD) 4/3/201814 NEC 602 by Dr Naim R Kidwai, Professor, F/o Engineering, JETGI, (JIT Jahangirabad) • MUD development started in 1980s. MUD deals with detection of the mutually interfering digital signals like in wireless, satellite communication, digital TV etc. • Multiuser detection aims for both joint detection or single signal detection against multiuser interference. • Mutual interference originates from channel distortion and from out-of-cell interference • MUD exploits the interfering signals structure, to increase spectral efficiency, receiver sensitivity, and system capacity. • MUD can overcome near-far problem suffered by CDMA by using an optimum receiver that demodulates all users simultaneously. It consists of a matched filters bank followed by a Viterbi algorithm. MF 1 MF 2 MF 3 Viterbi Algorithm searches for ML bit sequence S1(t)+S1(t)+S1(t) b1+I1 b2+I2 b3+I3
- 15. Orthogonal frequency-division multiplexing(OFDM) 4/3/2018 15 NEC 602 by Dr Naim R Kidwai, Professor, F/o Engineering, JETGI, (JIT Jahangirabad) OFDM is a frequency-division multiplexing (FDM) scheme used as a digital multi-carrier modulation method. • A large number of closely-spaced orthogonal sub-carriers are used to carry data. • data is divided into several parallel data streams (channels), one for each sub-carrier. • Each sub-carrier is modulated with a modulation scheme (ex. QAM/PSK) at a low symbol rate, maintaining data rates similar to single-carrier modulations over same bandwidth. • OFDM finds applications in digital broadcasting, wireless networking and WiMAX. Advantages • Can easily adapt to severe channel conditions so robust against co-channel interference, inter-symbol interference (ISI) and fading caused by multipath propagation. • High spectral efficiency as compared to modulation schemes, spread spectrum, etc. • Efficient implementation using Fast Fourier Transform (FFT). • Low sensitivity to time synchronization errors.
- 16. OFDM Communications 4/3/2018 16 NEC602 by Dr Naim R Kidwai, Professor, F/o Engineering, JETGI, (JIT Jahangirabad) Serial to parallel converter IFFT DAC DAC -900 X0(t) X1(t) X2(t) XN-1(t) Re Im fc S(t) S(n) Serial to parallel converter IFFT DAC DAC -900 fc S(t) S(n) X0(t) X1(t) X2(t) XN-1(t)
- 17. FDM 4/3/2018 17 NEC 602by Dr Naim R Kidwai, Professor, F/o Engineering, JETGI, (JIT Jahangirabad) OFDM vs FDM OFDM Bandwidth saving Guard band