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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
Thank you for giving your attentionThank you for giving your attention!!