Chapter 13. Spread Spectrum

Park Dong-Hyun

Department of Information and

Communications Engineering

The Graduate School of

Sejong University

Contents

Spread Spectrum Principles

Direct Sequence Spread Spectrum (DSSS)DSSS System Model

Spreading Codes for ISI Rejection

Synchronization

Rake receivers

Frequency-Hopping Spread Spectrum

Multiuser System

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Spread Spectrum Principles (1)

The signal occupies a bandwidth much larger than is needed for the information signal.

The spread spectrum modulation is done using a spreading code, which is independent of the data in the signal

Despreading at the receiver is done by correlating the received signal with a synchronized copy of the spreading code.

Developed initially for military applicationTypes

Frequency hoppingDirect sequence

Basis for CDMA(Code Division Multiple Access)

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Spread Spectrum Principles (2)

Input fed into channel encoderProduces narrow bandwidth analog signal around central frequency

Signal modulated using sequence of digits Spreading code/sequence

Typically generated by pseudonoise/pseudorandom number generator

Increases bandwidth significantlySpreads spectrum

Receiver uses same sequence to demodulate signal

Demodulated signal fed into channel decoder

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Spread Spectrum Principles (3)

Spread Spectrum advantagesAnti-jamming

Interference Rejection

Message Security & Privacy

Low Probability of Intercept

Rake receivers

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Spread Spectrum Principles (4)

Frequency Hopping Spread SpectrumTo combat frequency-selective fading

To combat narrow-band interference

To protect against intentional jamming and hostile surveillance

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•f

•P•Narrow-

band interference

•Fading minimum

DSSS System Model (1)

Each bit in the original signal is represented by multiple bits(chip code) in the transmitted signal

The chipping code spreads the signal across a wider frequency band in direct proportion to the number of bits used

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DSSS System Model (2)

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DataSource

Channel

c(t)PN code

)cos( tA c )cos( tA cI(t)

Interference

c(t)

kT

Tk

dt)1(

(.)

t = kT

Data signal

0 0 11+1

- 1

+1

- 1

+1

- 1

Spreading code

Baseband spread spectrum signal

DSSS System Model (3)

Message Data (random binary wave)

bit period (sec): Tb

bit rate (bps) :

Power spectral density :

Spreading Code:

chip period (sec): Tc

chip rate (cps) :

Power spectral density :

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}1,1{)( td

1b

b

RT

2

( ) sinc ( )b b bS f T fT

}1,1{)( tc

cc

TR

1

2 ( ) sinc ( )c c cS f T fT

DSSS System Model (4)

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01T

frequency 1Tc

BasebandSpread spectrum signal

Data signal

DSSS System Model (5)

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Spreading Code property

Randomness propertyBalance property

Have an equal number of ones and zeros.

Run property모든 run length( 같은 type 의 digit sequence) 의

half : length 1,

1/4 : length 2,

1/8 : length 3….

Correlation propertyRandom sequence 를 shift 시켜서 원래 sequence 와 비료하면 (modulo-2), agreement 와 disagreement 의 숫자가 최대로 1 까지만 차이남 .

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Synchronization

The Synchronizer Must align the timing of the spreading code generator in the receiver with the spreading code associated with one of the multipath components arriving over the channel.

Feedback control loopAdjust the delay of the spreading code generator until the function reaches its peak value.

Coarse Synchronizer (Acquisition)is within a chip time of perfect

synchronization.

Fine Synchronization (tracking)

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( )w

RAKE receivers

IS-95 : transmitter 부분만 규정 Qualcom patent

Multipath 역이용 : multipath diversity

각 path 로부터 오는 signal 을 각각 decoding (demodulation -> despreading) 한 후 , attenuation factor 를 곱하여 합침 .

=> 더 큰 signal strength 를 얻을 수 있음 .

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Frequency-Hopping Spread Spectrum(FHSS) (1)

Rapidly change the transmission frequencyPseudorandom pattern in a predetermined (Fig. 11.1)

Timing the hops accurately is the key to successSynchronizationSynchronization between transmitter and receiver

Frequency allocationFDMA : Fixed allocation

FH : time dependent

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Fig. 11-1. Frequency hopping

Frequency-Hopping Spread Spectrum(FHSS) (2)

Avoid interference with primary usersPrimary users are assigned narrow frequency bands

Transmit at a power high enough to override the WLAN

Any interference caused by the secondary user

Affect the primary user is transient

Because the hopping sequence spreads the energy out over a wide band

Primary user only looks like transient noise

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Fig. 11-2. Avoiding interference with frequency hopping

Frequency-Hopping Spread Spectrum(FHSS) (3)

Two FH system need to share same bandConfigure with different hopping sequences

Do not interfere with each other

During each time slotTwo hopping sequences must be on different frequency slots

Orthogonal hopping sequence

EX>Figure 11-3Sequence 1 : { 2, 8, 4, 7}

Sequence 2 : { 6, 3, 7, 2}

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Fig. 11-3. Orthogonal hopping sequences

Pilot channel :Unmodulated Direct Sequence Spread Spectrum 신호한 Cell 과 다른 Cell 을 구별다른 channel 의 coherent demodulation 을 위한 reference 신호

Sync channel :-1200 bps data rate

전화기의 시간동기를 맞추기 위한 정보제공

Paging channel :4800 or 9600 bps flexible data rate

시스템 parameter, access parameter 등의 제어정보Page 메시지등을 위한 신호

Traffic channel : 1200, 2400, 4800, or 9600 bps variable rate vocoding

음성통화와 통화중의 call processing 을 위한 channel

Multiuser DSSS (ex.CDMA)

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Multiuser DSSS (ex.CDMA)

CDMA Forward Link

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Multiuser DSSS (ex.CDMA)

Forward Link Channel Signaling

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Multiuser DSSS (ex.CDMA)

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Multiuser DSSS (ex.CDMA)

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Walsh Covering/Modulation

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Walsh Function

64-ary Walsh Function

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Multiuser DSSS

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Reverse Link

Multiuser DSSS (ex.CDMA)

Reverse Link

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Fig. DSSS uplink system

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Q & A

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