11

Spread-Spread-Spectrum Spectrum

TechniquesTechniques

CHAPTER CHAPTER 66

EKT 431: Digital EKT 431: Digital Communications Communications

School of Computer and Communication School of Computer and Communication Engineering, Amir Razif B. Jamil AbdullahEngineering, Amir Razif B. Jamil Abdullah

22

Chapter 12 text.Chapter 12 text. Spread-Spectrum OverviewSpread-Spectrum Overview Pseudonoise SequencePseudonoise Sequence Direct-Sequence Spread-Spectrum Direct-Sequence Spread-Spectrum

SystemSystem Frequency Hopping SystemsFrequency Hopping Systems SynchronizationSynchronization Jamming ConsiderationsJamming Considerations Commercial ApplicationCommercial Application Cellular System Cellular System

Spread-Spectrum Spread-Spectrum TechniquesTechniques

33

Spread-Spectrum Spread-Spectrum OverviewOverview

Initial application of spread-spectrum (SS) was in Initial application of spread-spectrum (SS) was in military guidance and communication system; military guidance and communication system; jamming resistance ~ radar.jamming resistance ~ radar.The transmission The transmission bandwidth bandwidth employed in the employed in the SSSS is is mush greater than the minimum bandwidth mush greater than the minimum bandwidth required to transmit the information.required to transmit the information.The system is defined as The system is defined as spread spectrumspread spectrum if it if it fulfill the following requirement;fulfill the following requirement;(1)(1) The signal occupies a bandwidth much in The signal occupies a bandwidth much in excess of the minimum bandwidth necessary to excess of the minimum bandwidth necessary to send the information.send the information.

(2) Spreading is accomplished by means of (2) Spreading is accomplished by means of spreading signal, often called code signal, which spreading signal, often called code signal, which is independent of the data.is independent of the data.

(3) At the receiver, despreading (recovering the (3) At the receiver, despreading (recovering the original data) is accomplished by the correlation original data) is accomplished by the correlation of the received spread signal with a synchronized of the received spread signal with a synchronized replica of the spreading signal used to spread the replica of the spreading signal used to spread the informationinformation

44

Benefit Attributes of Benefit Attributes of Spread-SpectrumSpread-Spectrum

Interference Suppression Benefits;Interference Suppression Benefits;Presence of intentional jammerPresence of intentional jammerSignal BW is spread, the jammer can make one Signal BW is spread, the jammer can make one of the two choices of the two choices (1) Jam all the signal coordinates of the system (1) Jam all the signal coordinates of the system with an equal amount of power in each one, with an equal amount of power in each one, with the result that little power is available for with the result that little power is available for each coordinate.each coordinate.

- Results in a reduction in jammer noise - Results in a reduction in jammer noise spectral density by a factor (W/Wspectral density by a factor (W/Wssss) across the ) across the spread spectrum.spread spectrum.

Noise spectral density - Broadband Noise spectral density - Broadband jammer noise spectral density.jammer noise spectral density.(2) Jam a few signal coordinates, with (2) Jam a few signal coordinates, with increased power in each of the jammed increased power in each of the jammed coordinates.coordinates.- Results in a reduction in the number of signal - Results in a reduction in the number of signal coordinates that the jammer occupies.coordinates that the jammer occupies.

55

Spread spectrum Spread spectrum advantagesadvantages

Prevent jammingPrevent jamming (destroying the signal by another (destroying the signal by another party)party)Covert communicationsCovert communications (prevents eavesdropping or (prevents eavesdropping or unauthorized listening), as signal is below noise unauthorized listening), as signal is below noise floor!floor!

MilitaryMilitaryLess multi-path fading Less multi-path fading (fading is frequency (fading is frequency dependent)dependent)Multiple accessMultiple access in wireless applications in wireless applications

Cordless, mobile phones sharing a small geographical areaCordless, mobile phones sharing a small geographical area

Code Division Multiple AccessCode Division Multiple Access (CDMA) : modern (CDMA) : modern mobile telephonymobile telephony

66

Receive much Receive much widerwider range of frequencies now range of frequencies now Low-noise design imperativeLow-noise design imperative

How to generate the same random string at two How to generate the same random string at two different locations?different locations? Agree on a protocol for exchange of the “key” (or Agree on a protocol for exchange of the “key” (or

seed)seed) Ex: MATLAB: RAND('state',sum(100*clock)) Ex: MATLAB: RAND('state',sum(100*clock))

resets RAND to a different state each timeresets RAND to a different state each time Ex: BluetoothEx: Bluetooth

AlgorithmsAlgorithms: Barker, M-Sequence, Gold, Hadamard-: Barker, M-Sequence, Gold, Hadamard-WalshWalsh

Complex sequence Complex sequence more robust SS link more robust SS link Higher Higher “wasted” BW“wasted” BW offset by the fact that more offset by the fact that more

than one user can transmit in the same BW than one user can transmit in the same BW CDMACDMA

Spread spectrum IssuesSpread spectrum Issues

77

Figure 6.1; effect of spectrum spreading in the presence of white noise with spreading in the presence of an international jammer.

Figure 6.1: Effect of spectrum spreading. (a) Spectrum spreading in the presence of white noise.

(b) Spectrum spreading in the presence of international jammer.

Cont’d…Cont’d…

88

The The larger the dimensionality of the signal setlarger the dimensionality of the signal set or or the more signal coordinates the communicator the more signal coordinates the communicator can choose from the can choose from the greater greater is the is the jammer’s jammer’s uncertaintyuncertainty regarding the effectiveness of the regarding the effectiveness of the jamming technique; jamming technique; better protectionbetter protection against against jamming.jamming.

Energy Density ReductionEnergy Density ReductionIn spread spectrum system, the signal are In spread spectrum system, the signal are spreadspread over many signaling coordinate, the resulting over many signaling coordinate, the resulting signal power in average spread signal power in average spread thinlythinly and and uniformlyuniformly in the spread domain. in the spread domain. Without Without synchronous replicasynchronous replica of the spreading of the spreading signal, the spread spectrum seems to be buried signal, the spread spectrum seems to be buried in the noise (difficult to perceived).in the noise (difficult to perceived).SS systems exhibit LPI may also exhibit SS systems exhibit LPI may also exhibit low low probability of position fixprobability of position fix ( (LPPFLPPF); the signal is ); the signal is perceived but the direction of transmitter cannot perceived but the direction of transmitter cannot be identified.be identified.

Cont’d…Cont’d…

99

Fine Time ResolutionFine Time Resolution SS signal can be used to determine the SS signal can be used to determine the position position

locationlocation, through time , through time delay of pulsesdelay of pulses it it traverse the channel.traverse the channel.

Uncertainty in Uncertainty in delaydelay measurement is measurement is inversely inversely proportionalproportional to the to the bandwidthbandwidth of the pulse. of the pulse.

The The largerlarger the the bandwidthbandwidth, the more , the more preciselyprecisely can measure the range. can measure the range.

Multiple AccessMultiple Access SS can be used as a SS can be used as a multiple excessmultiple excess techniques, techniques,

share communication resource among share communication resource among numerous users in coordinate manner. numerous users in coordinate manner.

Code-division multiple accessCode-division multiple access (CDMA)~ provide (CDMA)~ provide communication privacy between users with communication privacy between users with different spreading signal. different spreading signal.

Cont’d…Cont’d…

1010

Catalog of Spreading Catalog of Spreading TechniquesTechniques

For a signal of bandwidth W and duration T, the For a signal of bandwidth W and duration T, the dimensionality dimensionality of signal space is approximately of signal space is approximately 2WT2WT..

To increase the To increase the dimensionalitydimensionality;;

(1) increase (1) increase WW by spectrum spreading or, by spectrum spreading or,

~ the signal is spread in frequency domain. ~ the signal is spread in frequency domain.

(2) increase (2) increase TT by time spreading or time hopping by time spreading or time hopping (TH)(TH)

~ a message with data rate R is allocated a ~ a message with data rate R is allocated a longer transmission-longer transmission- time duration compare to the time duration compare to the conventional. conventional.

~ signal is spread in time domain.~ signal is spread in time domain. Direct sequencingDirect sequencing ( (DSDS) and ) and frequency hoppingfrequency hopping

((FHFH) are the most commonly used techniques for ) are the most commonly used techniques for SSSS

1111

Model for Direct Sequence Model for Direct Sequence SS Interference RejectionSS Interference RejectionFigure 12.5; model for Direct Sequence spread-Figure 12.5; model for Direct Sequence spread-spectrum (spectrum (DS/SSDS/SS) interference rejection.) interference rejection.

At the At the modulatormodulator signal signal x(t);x(t); data rate of data rate of RR bits/s, is multiplied by spreading code signalbits/s, is multiplied by spreading code signal g(t)g(t) having a chip rate having a chip rate RRchch chip/s. chip/s.

Multiplication in time domain is equivalent to Multiplication in time domain is equivalent to convolution in frequency domain.convolution in frequency domain.

At the At the demodulatordemodulator the received signal is the received signal is multiplied by a multiplied by a synchronized replica of synchronized replica of spreading codespreading code signal signal g(tg(t) results in ) results in despreading of signal.despreading of signal.

)(*)()()( GXtgtx

1212

Model for Direct Sequence Model for Direct Sequence SS Interference RejectionSS Interference RejectionFilter with bandwidth Filter with bandwidth RR will attenuate higher will attenuate higher frequency components.frequency components.

Interference rejection of a spread–spectrum Interference rejection of a spread–spectrum system:system:

(1) multiplication by spreading signal once (1) multiplication by spreading signal once spreads the signal bandwidth.spreads the signal bandwidth.

(2) multiplication by the spreading signal (2) multiplication by the spreading signal twice, follow by filtering, recover the original twice, follow by filtering, recover the original signal.signal.

1313

Modulation used is M-ary frequency shift keying Modulation used is M-ary frequency shift keying (MFSK).(MFSK).

K=logK=log22M information bits are used to determine M information bits are used to determine which one of M frequencies is to be transmitted.which one of M frequencies is to be transmitted.

At the signal is first At the signal is first FH demodulatedFH demodulated (dehopped) by (dehopped) by mixing with the same sequence of pseudo-randomly mixing with the same sequence of pseudo-randomly selected frequency tones that was used for hoping.selected frequency tones that was used for hoping.

The dehopped signal is applied to a conventional The dehopped signal is applied to a conventional bank of M noncoherent energy detector to select the bank of M noncoherent energy detector to select the most likely symbol.most likely symbol.

Frequency Hopping Frequency Hopping SystemSystem

1414

Frequency Hopping as a spread spectrum technique Frequency Hopping as a spread spectrum technique used by used by M-ary Frequency Shift Keying (MFSK).M-ary Frequency Shift Keying (MFSK).

the position of the M-ary signal set is the position of the M-ary signal set is shifted pseudo-randomly by the frequency shifted pseudo-randomly by the frequency synthesizer over a hopping bandwidth.synthesizer over a hopping bandwidth.

Information bits used to determine which one of M frequencies is to be transmitted

Frequency HoppingFrequency Hopping

1515

A hopping bandwidth WA hopping bandwidth Wssss of 4 MHz and a of 4 MHz and a frequency step size frequency step size ΔΔf of 100 Hz are f of 100 Hz are specified. What is the minimum number of specified. What is the minimum number of PN chips that are required for each PN chips that are required for each frequency word?frequency word?

Solution:Solution:

Example: Frequency HoppingExample: Frequency Hopping

1616

Frequency hopping using 8-ary FSK modulation Frequency hopping using 8-ary FSK modulation

Frequency Hopping Frequency Hopping SystemSystem

1717

FHSS: change carrier FHSS: change carrier frequency at “random”frequency at “random”

TimeFrequency

1818

Frequency Hopping Frequency Hopping Spread SpectrumSpread Spectrum

M odulator

F requenc yS ynthes izer

Channel T ableP s eudorandom

N um berG enerator

Bas eband dataBandpas s F ilter

S pread S pec trumS ignal

M ixer

Channel

S preader

Transmitter

1919

FHSSFHSS

Receiver

D em odulator

F requenc yS ynthes izer

Channel T ableP s eudorandom

N um berG enerator

S pread s pec trum

S ignalBandpas s F ilter

M ixer

Channel

D es preader

Bas eband D ata

2020

RobustnessRobustnessCharacterize a signal’s ability to Characterize a signal’s ability to

withstand impairments from the channel. withstand impairments from the channel. A signal configured with multiple replicate A signal configured with multiple replicate

copies, each transmitted on a different copies, each transmitted on a different frequency – greater likelihood of survival frequency – greater likelihood of survival than does a single such signal with equal than does a single such signal with equal total power.total power.

Greater diversity – the more robust the Greater diversity – the more robust the signal against random interferencesignal against random interference

Frequency HoppingFrequency Hopping

2121

Fast Frequency Hopping versus Slow Fast Frequency Hopping versus Slow Frequency HoppingFrequency Hopping

Fast Frequency Fast Frequency Hopping (FFH)Hopping (FFH)Fast-frequency hopping.Fast-frequency hopping.

~ Several frequency ~ Several frequency hops per modulation hops per modulation symbol.symbol.

~ The shortest ~ The shortest uninterrupted uninterrupted waveform in the waveform in the system is that of the system is that of the hop.hop...

Slow Frequency Slow Frequency Hopping (SFH)Hopping (SFH) Slow-frequency hopping.Slow-frequency hopping.

~ Several modulation ~ Several modulation symbols per hop.symbols per hop.

~ The shortest ~ The shortest uninterrupted uninterrupted waveform in the waveform in the system is that of data system is that of data symbol.symbol.

Frequency HoppingFrequency Hopping

2222

In generalIn general SFH SFH

~ in which the symbol rate, R~ in which the symbol rate, Rss of the MFSK of the MFSK signal is an integer multiple of the hop signal is an integer multiple of the hop rate, Rrate, Rhh. Several symbols are transmitted . Several symbols are transmitted on each frequency hop.on each frequency hop.

FFH FFH

~ in which the hope rate, R~ in which the hope rate, Rhh is an integer is an integer multiple of the MFSK symbol rate, Rmultiple of the MFSK symbol rate, Rss. The . The carrier frequency will change or hop carrier frequency will change or hop several times during the transmission of several times during the transmission of one symbol.one symbol.

Frequency HoppingFrequency Hopping

2323

DS versus FHDS versus FH Direct-SequenceDirect-Sequence

For mobile For mobile applications – DS applications – DS represents a reliable represents a reliable mitigation method, mitigation method, signaling renders all signaling renders all multipath signal multipath signal copies that delayed copies that delayed by more than one by more than one chip time.chip time.

DS radios encounter DS radios encounter more randomly more randomly distributed errors that distributed errors that are continuous & are continuous & lower level.lower level.

Frequency HoppingFrequency Hopping For mobile applications For mobile applications

– FH provide the same – FH provide the same mitigation if the mitigation if the hopping rate faster hopping rate faster than the symbol rate, than the symbol rate, hopping BW is large.hopping BW is large.

SFH radios typically SFH radios typically suffer occasional strong suffer occasional strong burst errors.burst errors.

Used for providing Used for providing diversity in fixed diversity in fixed wireless access wireless access applicationsapplications

Frequency HoppingFrequency Hopping