Chapter 6Chapter 6SPREAD SPECTRUM SPREAD SPECTRUM COMMUNICATIONSCOMMUNICATIONS

Chapter overviewChapter overview

Spread spectrum systemsSpread spectrum systemsSpread spectrum direct sequencesSpread spectrum direct sequencesPseudo-noise sequencesPseudo-noise sequencesFrequency hopping spread spectrumFrequency hopping spread spectrumCDMA ApplicationsCDMA Applications

SPREAD SPECTRUM SPREAD SPECTRUM SYSTEMSYSTEM

Spread spectrum system requirementsSpread spectrum system requirementsa.a. The signals occupies a bandwidth much in excess The signals occupies a bandwidth much in excess

of the minimum bandwidth necessary to send the of the minimum bandwidth necessary to send the information.information.

b.b. Spreading is accomplished by means of a Spreading is accomplished by means of a spreading signal, often called a code signal, which spreading signal, often called a code signal, which is independent of the data.is independent of the data.

c.c. At Rx, dispreading is accomplished by the At Rx, dispreading is accomplished by the correlation of the received spread signal with a correlation of the received spread signal with a synchronized replica of the spreading signal used to synchronized replica of the spreading signal used to spread the information.spread the information.

Cont’d...Cont’d...

Principles of spread spectrumPrinciples of spread spectrum Direct sequence techniqueDirect sequence technique

Two stages of modulationTwo stages of modulation Incoming data sequence is used to modulate a wideband Incoming data sequence is used to modulate a wideband

code. The code transform the narrowband data code. The code transform the narrowband data sequence into a sequence into a noise-like wideband signalnoise-like wideband signal..

Resulting wideband signal undergoes the second Resulting wideband signal undergoes the second modulation using a PSK techniquemodulation using a PSK technique

Frequency hopping techniqueFrequency hopping technique The spectrum of a data-modulated carrier is The spectrum of a data-modulated carrier is

widened by changing the carrier frequency in a widened by changing the carrier frequency in a pseudorandom manner.pseudorandom manner.

Cont’d...Cont’d...Effect of spread spreadingEffect of spread spreading

Cont’d...Cont’d... Fine time resolutionFine time resolution Spread spectrum signals can be used for Spread spectrum signals can be used for

ranging or determination of position.ranging or determination of position.Distance Distance

Measuring the time delay of a pulse as it traverses the Measuring the time delay of a pulse as it traverses the channel.channel.

Delay measurement is inversely proportional to the BW of Delay measurement is inversely proportional to the BW of the signal pulse.the signal pulse.

Larger bandwidth, precise the measurement rangeLarger bandwidth, precise the measurement range Over Gaussian channel, a one-shot measurement on a Over Gaussian channel, a one-shot measurement on a

single pulse is not very reliable.single pulse is not very reliable.

Cont’d...Cont’d...

Multiple accessMultiple access Spread spectrum can be used as MAT in Spread spectrum can be used as MAT in

order to share a communication resources order to share a communication resources among numerous users in a coordinated among numerous users in a coordinated manner – CDMAmanner – CDMA

DIRECT-SEQUENCE SPREAD-SPECTRUMDIRECT-SEQUENCE SPREAD-SPECTRUM

Basic Spread Spectrum TechniqueBasic Spread Spectrum Technique

Cont’d...Cont’d...

Direct sequenceDirect sequence The spectrum spreading technique whereby The spectrum spreading technique whereby

a carrier wave is modulated with a data signal a carrier wave is modulated with a data signal x(t).x(t).

the data modulated signal is again modulated with the data modulated signal is again modulated with a high speed spreading signal a high speed spreading signal g(t).g(t).

Cont’d...Cont’d...

Cont’d...Cont’d...Example of Direct SequencingExample of Direct Sequencing

Direct Sequence Pseudo NoiseDirect Sequence Pseudo Noise

PSEUDO NOISE PSEUDO NOISE (PN) SEQUENCES(PN) SEQUENCES

Transmitted Reference (TR) can utilize a truly Transmitted Reference (TR) can utilize a truly random code signal for spreading and random code signal for spreading and dispreading.dispreading.Stored Reference (SR) used Stored Reference (SR) used pseudonoisepseudonoise or or pseudorandompseudorandom code signal. code signal. Random signal – cannot predicted, future Random signal – cannot predicted, future variations can only be described in a statistical variations can only be described in a statistical sense.sense.Pseudorandom – deterministic, periodic signal Pseudorandom – deterministic, periodic signal that is known to both Tx & Rx. that is known to both Tx & Rx.

Cont’d...Cont’d...

PN sequence is definedPN sequence is defined as a coded as a coded sequence of ones & zeros with sequence of ones & zeros with certain auto correlation properties.certain auto correlation properties.

Cont’d...Cont’d...Randomness propertiesRandomness properties

1.1. Balance propertyBalance propertyIn each period of a maximum-length sequence, the number of In each period of a maximum-length sequence, the number of 1s & is always one more than the number of 0s.1s & is always one more than the number of 0s.

2.2. Run property Run property Among the runs of 1s & 0s in each period of a maximum –Among the runs of 1s & 0s in each period of a maximum –length sequence, ½ the runs of each kind are of length 1, ¼ length sequence, ½ the runs of each kind are of length 1, ¼ are of length 2, ⅛ are of length 3 and so on as long as these are of length 2, ⅛ are of length 3 and so on as long as these fractions represent meaningful numbers of runs.fractions represent meaningful numbers of runs.

3.3. Correlation propertyCorrelation propertyIf the period of the sequence is compared term by term with If the period of the sequence is compared term by term with any cyclic shift of itself, it is best if the number of agreements any cyclic shift of itself, it is best if the number of agreements differs from the number of disagreements by not more than differs from the number of disagreements by not more than one count.one count.The autocorrelation function is periodic & binary valued.The autocorrelation function is periodic & binary valued.

FREQUENCY HOPPING FREQUENCY HOPPING SYSTEMSSYSTEMS

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

the position of the M-ary signal set is the position of the M-ary signal set is shifted pseudorandomly by the shifted pseudorandomly by the

frequency frequency synthesizer over a hopping synthesizer over a hopping bandwidth.bandwidth.

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

ExampleExample

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?

Example - solutionExample - solution

Number of tones contained in WNumber of tones contained in Wssss = =

WWssss

ΔΔff

Minimum number of chips = logMinimum number of chips = log22 M M

Cont’d...Cont’d...Frequency hopping using 8-ary FSK Frequency hopping using 8-ary FSK modulation modulation

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

TimeFrequency

Frequency Hopping Spread Frequency Hopping Spread SpectrumSpectrum

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

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

Cont’d...Cont’d...

RobustnessRobustness Characterize a signal’s ability to withstand Characterize a signal’s ability to withstand

impairments from the channel. 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 than frequency – greater likelihood of survival than does a single such signal with equal total does a single such signal with equal total power.power.

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

Cont’d...Cont’d...Fast Hopping versus Slow HoppingFast Hopping versus Slow Hopping

Fast HoppingFast Hopping Fast-frequency hopping.Fast-frequency hopping. Several frequency hops Several frequency hops

per modulation symbol.per modulation symbol. The shortest The shortest

uninterrupted waveform : uninterrupted waveform : the hop.the hop.

Slow HoppingSlow Hopping Slow-frequency Slow-frequency

hopping.hopping. Several modulation Several modulation

symbols per hop.symbols per hop. The shortest The shortest

uninterrupted waveform: uninterrupted waveform: the data symbol.the data symbol.

Cont’d...Cont’d...In generalIn general SFH SFH in which the symbol rate, R in which the symbol rate, Rss of the of the

MFSK signal is an integer multiple of the hop MFSK signal is an integer multiple of the hop rate, Rrate, Rhh. Several symbols are transmitted on . Several symbols are transmitted on

each frequency hop.each frequency hop. FFHFFH 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 several carrier frequency will change or hop several times during the transmission of one symbol.times during the transmission of one symbol.

Cont’d...Cont’d...DS versus FHDS versus FH

Direct-SequenceDirect-Sequence For mobile applications – For mobile applications –

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

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

Frequency HoppingFrequency Hopping For mobile applications – For mobile applications –

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

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

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

Spread spectrum advantagesSpread spectrum advantages

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 floor!unauthorized listening), as signal is below noise floor!

MilitaryMilitaryLess multi-path fading Less multi-path fading (fading is frequency dependent)(fading is frequency 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 mobile (CDMA) : modern mobile telephonytelephony

Spread Spectrum issuesSpread Spectrum issuesReceive much wider range of frequencies now Receive much wider range of frequencies now Low-noise design imperativeLow-noise design imperativeHow 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 seed)Agree on a protocol for exchange of the “key” (or seed) Ex: MATLAB: RAND('state',sum(100*clock)) resets RAND Ex: MATLAB: RAND('state',sum(100*clock)) resets RAND

to a different state each timeto a different state each time Ex: BluetoothEx: BluetoothAlgorithms: Barker, M-Sequence, Gold, Hadamard-Algorithms: Barker, M-Sequence, Gold, Hadamard-WalshWalshComplex sequence Complex sequence more robust SS link more robust SS linkHigher “wasted” BW offset by the fact that more than Higher “wasted” BW offset by the fact that more than one user can transmit in the same BW one user can transmit in the same BW CDMA CDMA

Processing Gain (PG)Processing Gain (PG)

PG: figure of merit of spread spectrum PG: figure of merit of spread spectrum systemssystems

PG=[SNR]PG=[SNR]outout/[SNR]/[SNR]inin

FH systemsFH systems: PG = number of FH channels : PG = number of FH channels (typical PG ~ 100)(typical PG ~ 100) Slow FHSS Slow FHSS one hop every few bits one hop every few bits Fast FHSS Fast FHSS many hops during each bit many hops during each bitDS systemsDS systems:: PG = number of bits in a chip PG = number of bits in a chip sequence (also called spreading factor: sequence (also called spreading factor: typical PG ~ few thousands)typical PG ~ few thousands)

Hybrid SchemesHybrid Schemes

ApplicationsApplications

Satellite-positioning systems (GPS)Satellite-positioning systems (GPS)

3G mobile telecommunications3G mobile telecommunications

W-LANW-LAN IEEE802.11aIEEE802.11a IEEE802.11bIEEE802.11b IEEE802.11gIEEE802.11g

BluetoothBluetooth

Reducing EMI in digital electronics (SS clock)!Reducing EMI in digital electronics (SS clock)! Clock is “dithered” by 2 ~ 4% Clock is “dithered” by 2 ~ 4% no more peaks in no more peaks in

PSDPSD

CDMACDMACode Division Multiple AccessCode Division Multiple Access

All users occupy the same bandwidth at the same time!All users occupy the same bandwidth at the same time!

Users are differentiated by the spreading (PN) code Users are differentiated by the spreading (PN) code usedused

Each data bit pair (0,1) is represented by a unique Each data bit pair (0,1) is represented by a unique code pair (Pcode pair (Pn0n0, P, Pn1n1) for each specific (transmitter, ) for each specific (transmitter,

receiver) pairreceiver) pair

To each pair, the signal emanating from other pairs will To each pair, the signal emanating from other pairs will look like noiselook like noise

Ex: IS-95, IS-98, WLANs,…Ex: IS-95, IS-98, WLANs,…

CDMA APPLICATIONSCDMA APPLICATIONS

SSMA techniques allow multiple signals SSMA techniques allow multiple signals occupying the same RF BW to be occupying the same RF BW to be transmitted simultaneously without transmitted simultaneously without interfering with one another.interfering with one another.

CDMA using direct sequence – CDMA using direct sequence – DS/CDMADS/CDMA

The use of CDMA provides resistance to The use of CDMA provides resistance to external interference and requires no external interference and requires no synchronizing mechanism.synchronizing mechanism.

CDMA: Code Division Multiple AccessCDMA: Code Division Multiple Access• Allow each station to transmit over entire frequency Allow each station to transmit over entire frequency

spectrum all the time.spectrum all the time.• Multiple simultaneous transmissions are separated using Multiple simultaneous transmissions are separated using

coding theory.coding theory.• Colliding frames may not be totally garbled.Colliding frames may not be totally garbled.

There are techniques to separate signals sent by different There are techniques to separate signals sent by different senders.senders.

• Similar to a party where different conversations use Similar to a party where different conversations use different language.different language.Extract desired signal and reject others as random noises.Extract desired signal and reject others as random noises.

Simple Analysis of CDMASimple Analysis of CDMA• Assume 1 MHz band for 100 stationsAssume 1 MHz band for 100 stations

• Use FDM, one station has 10kHz and 10 Use FDM, one station has 10kHz and 10 kbps (assume 1 bit per Hz)kbps (assume 1 bit per Hz)

• Use CDMA, one station has 1MHz, and Use CDMA, one station has 1MHz, and 1Mchips per seconds.1Mchips per seconds.

• If CDMA uses less than 100 chips per bit If CDMA uses less than 100 chips per bit then CDMA will be more efficient.then CDMA will be more efficient.

CDMA – cont’d…CDMA – cont’d…

• Each bit time is subdivided into m short intervals called Each bit time is subdivided into m short intervals called chips, typically 64-128 chips per bit.chips, typically 64-128 chips per bit.

• Each station is assigned a unique m-bit code or Each station is assigned a unique m-bit code or chip chip sequencesequence..

• To send a bit 1, a station sends its chip sequence.To send a bit 1, a station sends its chip sequence.• To send a bit 0, a station sends the complement of its To send a bit 0, a station sends the complement of its

chip sequence.chip sequence.• For m=8, A is assigned 00011011. For m=8, A is assigned 00011011.

A sends 00011011 as bit 1, and 11100100 as bit 0.A sends 00011011 as bit 1, and 11100100 as bit 0.

CDMA – Code Division Multiple CDMA – Code Division Multiple AccessAccess

(a)(a) Binary chip Binary chip sequences for sequences for four stationsfour stations

(b)(b) Bipolar chip Bipolar chip sequences sequences

(c)(c) Six examples Six examples of transmissionsof transmissions

(d)(d) Recovery of Recovery of station C’s station C’s signalsignal

CDMA Code ProductionCDMA Code Production

• Walsh functionWalsh function

• 2 codes:2 codes: 1 11 1• 1 01 0• 4 codes:4 codes:• 1 1 1 1 1 1 1 1 1 0 1 01 0 1 0• • 1 1 0 0 1 1 0 0 • 1 0 0 11 0 0 1

Root Copy

Copy Inv

CDMA – Code Division Multiple CDMA – Code Division Multiple AccessAccess

• 1 1 1 11 1 1 1 8 codes 8 codes 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1• 1 0 1 01 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0• 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 • 1 0 0 11 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1• 1 1 1 1 0 0 0 01 1 1 1 0 0 0 0• 1 0 1 0 0 1 0 1 1 0 1 0 0 1 0 1 • 1 1 0 0 0 0 1 11 1 0 0 0 0 1 1• 1 0 0 1 0 1 1 01 0 0 1 0 1 1 0

CDMACDMA

Differentiation by pseudo-random code used only

Cont’d...Cont’d...A typical DS/CDMA block diagram

ExerciseExercise

A Code Division Multiple Access (CDMA) uses direct-A Code Division Multiple Access (CDMA) uses direct-sequence modulation with a data bandwidth of a 10 sequence modulation with a data bandwidth of a 10 kHz and a spread bandwidth of a 10 MHz. With only kHz and a spread bandwidth of a 10 MHz. With only one signal being transmitted, the received Eone signal being transmitted, the received Ebb/N/Noo is 16 is 16 dB.dB.

a)a) If the required (EIf the required (Ebb/N/Noo + I + Ioo) is 10 dB, calculate the equal power ) is 10 dB, calculate the equal power users that can share the band.users that can share the band.

b)b) If each user’s transmitted power is reduced by 3 dB, If each user’s transmitted power is reduced by 3 dB, calculate the equal power users that can share the band.calculate the equal power users that can share the band.

c)c) If the received EIf the received Ebb/N/Noo for each receiver, calculate the for each receiver, calculate the maximum number of users that can share the band.maximum number of users that can share the band.

End of Chapter 6End of Chapter 6