1. EEE 461 1 Chapter 5 Digital Modulation Systems Huseyin Bilgekul EEE 461 Communication Systems II Department of Electrical and Electronic Engineering Eastern Mediterranean University  Spread Spectrum Systems

2. EEE 461 2 Introduction to Spread Spectrum Problems such as capacity limits, propagation effects, synchronization occur with wireless systems Spread spectrum modulation spreads out the modulated signal bandwidth so it is much greater than the message bandwidth Independent code spreads signal at transmitter and despread the signal at receiver

3. EEE 461 3 Spread Spectrum Systems Multiple access capability Anti-jam capability Interference rejection Secret operation Low probability of intercept Simultaneous use of wideband frequency Code division multiple access (CDMA)

4. EEE 461 4 Multiplexing in 4 dimensions –space (s i ) –time (t) –frequency (f) –code (c) Goal: Multiple use of a shared medium Important: guard spaces needed! s2s2 s3s3 s1s1 Multiplexing f t c k2k2 k3k3 k4k4 k5k5 k6k6 k1k1 f t c f t c Channels k i

5. EEE 461 5 Frequency Division Multiplex Separation of spectrum into smaller frequency bands Channel gets band of the spectrum for the whole time Advantages: –no dynamic coordination needed –works also for analog signals Disadvantages: –waste of bandwidth if traffic distributed unevenly –inflexible –guard spaces k3k3 k4k4 k5k5 k6k6 f t c Channels k i

6. EEE 461 6 f t c k2k2 k3k3 k4k4 k5k5 k6k6 k1k1 Time Division Multiplex Channel gets the whole spectrum for a certain amount of time Advantages: –only one carrier in the medium at any time –throughput high even for many users Disadvantages: –precise synchronization necessary Channels k i

7. EEE 461 7 f Time and Frequency Division Multiplex A channel gets a certain frequency band for a certain amount of time (e.g. GSM) Advantages: –better protection against tapping –protection against frequency selective interference –higher data rates compared to code multiplex Precise coordination required t c k2k2 k3k3 k4k4 k5k5 k6k6 k1k1 Channels k i

8. EEE 461 8 Code Division Multiplex Each channel has unique code All channels use same spectrum at same time Advantages: –bandwidth efficient –no coordination and synchronization –good protection against interference Disadvantages: –lower user data rates –more complex signal regeneration Implemented using spread spectrum technology k2k2 k3k3 k4k4 k5k5 k6k6 k1k1 f t c Channels k i

9. EEE 461 9 DS/SS PSK Signals Direct-sequence spread coherent phase-shift keying. (a) Transmitter. (b) Receiver.

10. EEE 461 10 Waveforms at the transmitter T b Bit interval T c Chip interval PG= T b /T c

11. EEE 461 11 Spread Spectrum Technology Problem of radio transmission: frequency dependent fading can wipe out narrow band signals for duration of the interference Solution: spread the narrow band signal into a broad band signal using a special code detection at receiver interference spread signal signal spread interference f f power

12. EEE 461 12 Spread Spectrum Technology Side effects: –coexistence of several signals without dynamic coordination –tap-proof Alternatives: Direct Sequence (DS/SS), Frequency Hopping (FH/SS) Spread spectrum increases BW of message signal by a factor N, Processing Gain

13. EEE 461 13 Effects of spreading and interference P f i) P f ii) User signal Broadband interference Narrowband interference Sender P f iii) P f iv) Receiver f v) P The narrowband interference at the receiver is spread out so that the detected narrowband signal power is much lower.

14. EEE 461 14 Spreading and frequency selective fading Narrowband signal 2 2 2 2 2 frequency channel quality 1 spread spectrum frequency channel quality 1 2 3 4 56 guard space narrowband channels spread spectrum channels Wideband signals are less affected by frequency selective multipath channels

15. EEE 461 15 Direct Sequence (DS) CDMA m(t) is polar from a digital source ± 1. For BPSK modulation, g m (t) = A c m(t). The spreading waveform complex envelope g c (t) = c(t) c(t) is a polar spreading signal). The resulting complex envelope of the SS signal becomes g(t) = A c m(t)c(t). The spreading waveform is generated by using PN code generator. The pulse width of T c is called the chip interval. When a PN sequence has the maximum period of N chips, where N = 2 r -1, it is called a maximum length sequence (m-sequence). There are certain very important properties of m-sequences: Direct Sequence Spread Spectrum (DSSS) I

16. EEE 461 16 Balance Property: In each period of maximum-length sequence, the number of 1s is always one more than the number of 0s. Run Property: Here, the 'run' represents a subsequence of identical symbols(1's or 0's) within one period of the sequence. One-half the run of each kind are of length one, one-fourth are length two, one-eighth are of length three, etc. Correlation Property: The autocorrelation function of a maximum-length sequence is periodic, binary valued and has a period T=NTc where Tc is chip duration. The autocorrelation function is Properties of Maximum Length Sequences

17. EEE 461 17 (a) Waveform of maximal-length sequence for length m  3 or period N  7. (b) Autocorrelation function. (c) Power spectral density. Maximum Length Sequences

18. EEE 461 18 Feedback shift register. Two different configurations of feedback shift register of length m  5. (a) Feedback connections [5, 2]. (b) Feedback connections [5, 4, 2, 1]. Maximum Length Sequences

19. EEE 461 19 Codes are periodic and generated by a shift register and XOR Maximum-length (ML) shift register sequences, m-stage shift register, length: n = 2 m – 1 bits R(  ) -1/n TcTc  -nT c nT c + Output Maximum Length Sequences

20. EEE 461 20 Generating PN Sequences Take m=2 =>L=3 c n =[1,1,0,1,1,0,...], usually written as bipolar c n =[1,1,-1,1,1,- 1,...] mStages connected to modulo-2 adder 21,2 31,3 41,4 5 61,6 81,5,6,7 + Output

21. EEE 461 21 Problems with m-sequences Cross-correlations with other m-sequences generated by different input sequences can be quite high. Easy to guess connection setup in 2m samples so not too secure. In practice, Gold codes or Kasami sequences which combine the output of m-sequences are used.

22. EEE 461 22DSSS XOR the signal with pseudonoise (PN) sequence (chipping sequence) Advantages –reduces frequency selective fading –in cellular networks base stations can use the same frequency range several base stations can detect and recover the signal But, needs precise power control user data chipping sequence Resulting Signal 01 01101010100111 XOR 01100101101001 = TbTb TcTc

23. EEE 461 23 DSSS Transmitter and Receiver X user data m(t) chipping sequence, c(t) modulator radio carrier Spread spectrum Signal y(t)=m(t)c(t) Transmit signal TRANSMITTER demodulator Received signal radio carrier X Chipping sequence, c(t) RECIVER integratordecision data sampled sums Correlator

24. EEE 461 24 DS/SS Comments Pseudonoise (PN) sequence chosen so that its autocorrelation is very narrow => PSD is very wide –Concentrated around  < T c –Cross-correlation between two user’s codes is very small Secure and Jamming Resistant –Both receiver and transmitter must know c(t) –Since PSD is low, hard to tell if signal present –Since wide response, tough to jam everything Multiple access –If c i (t) is orthogonal to c j (t), then users do not interfere Near/Far problem: Users must be received with the same power

25. EEE 461 25 Frequency Hopping Spread Spectrum (FH/SS) A frequency-hopped SS (FH/SS) signal uses a g c (t) that is of FM type. There are M=2 k hop frequencies controlled by the spreading code. Discrete changes of carrier frequency –sequence of frequency changes determined via PN sequence Two versions –Fast Hopping: several frequencies per user bit (FFH) –Slow Hopping: several user bits per frequency (SFH) Advantages –frequency selective fading and interference limited to short period –uses only small portion of spectrum at any time Disadvantages –not as robust as DS/SS –simpler to detect

26. EEE 461 26 Illustrating slow- frequency hopping. (a) Frequency variation for one complete period of the PN sequence. (b) Variation of the dehopped frequency with time. Slow Frequency Hopping

27. EEE 461 27 Fast Frequency Hopping Illustrating fast- frequency hopping. (a) Variation of the transmitter frequency with time. (b) Variation of the dehopped frequency with time.

28. EEE 461 28 FHSS (Frequency Hopping Spread Spectrum) II user data slow hopping (3 bits/hop) fast hopping (3 hops/bit) 01 TbTb 011t f f1f1 f2f2 f3f3 t TdTd f f1f1 f2f2 f3f3 t TdTd T b : bit periodT d : dwell time

29. EEE 461 29 FHSS Transmitter and Receiver modulator user data hopping sequence modulator narrowband signal Spread transmit signal Transmitter frequency synthesizer received signal Receiver demodulator data hopping sequence demodulator frequency synthesizer

30. EEE 461 30 Applications of Spread Spectrum In 1985 FCC opened 902-928 Mhz, 2400-2483Mhz and 5725-5850 Mhz bands for commercial SS use with unlicensed transmitters. Cell phones –IS-95 (DS/SS) –GSM Global Positioning System (GPS) Wireless LANs –802.11b

31. EEE 461 31 Performance of DS/SS Systems Pseudonoise (PN) codes –Spread signal at the transmitter –Despread signal at the receiver Ideal PN sequences should be –Orthogonal (no interference) –Random (security) –Autocorrelation similar to white noise (high at  =0 and low for  not equal 0)

32. EEE 461 32 Detecting DS/SS PSK Signals X Bipolar, NRZ m(t) PN sequence, c(t) X sqrt(2)cos  (  c t +  ) Spread spectrum Signal y(t)=m(t)c(t) transmit signal transmitter X received signal X c(t) receiver integrator z(t) decision data sqrt(2)cos  (  c t +  ) LPF w(t) x(t)

33. EEE 461 33 Optimum Detection of DS/SS PSK Recall, bipolar signaling (PSK) and white noise give the optimum error probability Not effected by spreading –Wideband noise not affected by spreading –Narrowband noise reduced by spreading

34. EEE 461 34 Signal Spectra Effective noise power is channel noise power plus jamming (NB) signal power divided by N TbTb TcTc

35. EEE 461 35 Multiple Access Performance Assume K users in the same frequency band, Interested in user 1, other users interfere 4 1 3 5 2 6

36. EEE 461 36 Signal Model Interested in signal 1, but we also get signals from other K-1 users: At receiver,

37. EEE 461 37 Interfering Signal After mixing and despreading (assume  1 =0) After LPF After the integrator-sampler

38. EEE 461 38 At Receiver m(t) =+/-1 (PSK), bit duration T b Interfering signal may change amplitude at  k At User 1: Ideally, spreading codes are Orthogonal:

39. EEE 461 39 Example of Performance Degradation N=8N=32 Multiple Access Interference (MAI) If the users are assumed to be equal power interferers, can be analyzed using the central limit theorem (sum of IID RV’s)

40. EEE 461 40 Near/Far Problem Performance estimates derived using assumption that all users have same power level Reverse link (mobile to base) makes this unrealistic since mobiles are moving Adjust power levels constantly to keep equal 1 k K interferers, one strong interfering signal dominates performance Can result in capacity losses of 10-30%