Chapter 13. Spread Spectrum

1. Chapter 13. Spread Spectrum Park Dong-Hyun Department of Information and Communications Engineering The Graduate School of Sejong University

2. 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

3. 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)

4. 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

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

6. P • Narrow-band interference • Fading minimum • f 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

7. 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

8. DSSS System Model (2)

9. 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 :

10. DSSS System Model (4)

11. DSSS System Model (5)

12. Spreading Code property • Randomness property • Balance property • Have an equal number of ones and zeros. • Run property • 모든 run length(같은 type의 digit sequence)의 • half : length 1, • 1/4 : length 2, • 1/8 : length 3…. • Correlation property • Random sequence를 shift 시켜서 원래 sequence와 비료하면 (modulo-2), agreement와 disagreement의 숫자가 최대로 1까지만 차이남.

13. 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)

14. RAKE receivers • IS-95 : transmitter 부분만 규정 • Qualcom patent • Multipath 역이용: multipath diversity • 각 path로부터 오는 signal을 각각 decoding (demodulation -> despreading) 한 후, attenuation factor를 곱하여 합침. => 더 큰 signal strength를 얻을 수 있음.

15. 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 between transmitter and receiver • Frequency allocation • FDMA : Fixed allocation • FH : time dependent Fig. 11-1. Frequency hopping

16. 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. 11-2. Avoiding interference with frequency hopping

17. 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. 11-3. Orthogonal hopping sequences

18. Multiuser DSSS (ex.CDMA) • Pilot channel : • Unmodulated Direct Sequence Spread Spectrum 신호 • 한Cell과 다른 Cell을 구별 • 다른 channel의 coherent demodulation을 위한 reference 신호 • Sync channel : • -1200 bps data rate • 전화기의 시간동기를 맞추기 위한 정보제공 • Paging channel : • 4800 or 9600 bps flexible data rate • 시스템parameter, access parameter등의 제어정보 • Page 메시지등을 위한 신호 • Traffic channel : • 1200, 2400, 4800, or 9600 bps variable rate vocoding • 음성통화와 통화중의 call processing을 위한 channel

19. Multiuser DSSS (ex.CDMA) • CDMA Forward Link

20. Multiuser DSSS (ex.CDMA) • Forward Link Channel Signaling

21. Multiuser DSSS (ex.CDMA)

22. Multiuser DSSS (ex.CDMA)

23. Walsh Covering/Modulation • Walsh Function

24. 64-ary Walsh Function

25. Multiuser DSSS • Reverse Link

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

27. Q & A Thank you for giving your attention! ? ?