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 - 2 -

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 application Types Frequency hopping Direct sequence Basis for CDMA(Code Division Multiple Access) - 3 -

Spread Spectrum Principles (2) Input fed into channel encoder Produces 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 significantly Spreads spectrum Receiver uses same sequence to demodulate signal Demodulated signal fed into channel decoder - 4 -

Spread Spectrum Principles (3) Spread Spectrum advantages Anti-jamming Interference Rejection Message Security & Privacy Low Probability of Intercept Rake receivers - 5 -

Spread Spectrum Principles (4) Frequency Hopping Spread Spectrum To combat frequency-selective fading To combat narrow-band interference To protect against intentional jamming and hostile surveillance 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 - 7 -

DSSS System Model (2) - 8 -

DSSS System Model (3) Message Data (random binary wave) bit period (sec): T b bit rate (bps) : Power spectral density : Spreading Code: chip period (sec): Tc chip rate (cps) : Power spectral density : - 9 -

DSSS System Model (4)

DSSS System Model (5)

Spreading Code property Randomness property Balance property Have an equal number of ones and zeros. Run property 모든 run length( 같은 type 의 digit sequence) 의 half : length 1, 1/4 : length 2, 1/8 : length 3…. Correlation property Random sequence 를 shift 시켜서 원래 sequence 와 비료하면 (modulo- 2), agreement 와 disagreement 의 숫자가 최대로 1 까지만 차이남

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 loop Adjust 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)

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

Frequency-Hopping Spread Spectrum(FHSS) (1) Rapidly change the transmission frequency Pseudorandom pattern in a predetermined (Fig. 11.1) Timing the hops accurately is the key to success Synchronization Synchronization between transmitter and receiver Frequency allocation FDMA : Fixed allocation FH : time dependent Fig Frequency hopping

Frequency-Hopping Spread Spectrum(FHSS) (2) Avoid interference with primary users Primary 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 Fig Avoiding interference with frequency hopping

Frequency-Hopping Spread Spectrum(FHSS) (3) Two FH system need to share same band Configure with different hopping sequences Do not interfere with each other During each time slot Two hopping sequences must be on different frequency slots Orthogonal hopping sequence EX>Figure 11-3 Sequence 1 : { 2, 8, 4, 7} Sequence 2 : { 6, 3, 7, 2} Fig Orthogonal hopping sequences

Pilot channel : Unmodulated Direct Sequence Spread Spectrum 신호 한 Cell 과 다른 Cell 을 구별 다른 channel 의 coherent demodulation 을 위한 reference 신호 Sync channel : bps data rate 전화기의 시간동기를 맞추기 위한 정보제공 Paging channel : 4800 or 9600 bps flexible data rate 시스템 parameter, access parameter 등의 제어정보 Page 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)

Multiuser DSSS (ex.CDMA) CDMA Forward Link

Multiuser DSSS (ex.CDMA) Forward Link Channel Signaling

Multiuser DSSS (ex.CDMA)

Multiuser DSSS (ex.CDMA)

Walsh Covering/Modulation Walsh Function

64-ary Walsh Function

Multiuser DSSS Reverse Link

Multiuser DSSS (ex.CDMA) Reverse Link Fig. DSSS uplink system

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