CDMA Technology. Pr. S.Flament Pr. Dr. W.Skupin On line Course on CDMA Technology

Transcription

1 CDMA Technology Pr. Dr. W.Skupin Pr. S.Flament On line Course on CDMA Technology

2 CDMA Technology : Introduction to spread spectrum technology CDMA / DS : Principle of operation Generation of PN Spreading Codes Advanced Spreading codes Principles of CDMA/DS decoding Radio Cells & System Capacity Basics of Global Navigation Satellite Systems Galileo / European GNSS 2

3 Outline CDMA / DS : Principle Of Operation Introduction : one Channel with a single data stream Real case : one channel with many data Signal to noise Ratio and Power control Conclusion 3

4 Part 1 : One channel one data Data : Binary Random Signal +1-1 T D Bit value : Coding Sequence : Binary pseudo Random Signal +1-1 T PN Bit Value :

5 Part 1 : One channel one data Real time signals T D Data : s D Coding Sequence : PN Coding Sequence Frame (Period) 5

6 Part 1 : One channel one data Periodicity PSEUDO random signal or Pseudo Noise signal : pn(t) sequence 6

7 Part 1 : One channel one data Data : s D (t) Time waveform +1-1 T D (in W/Hz) S S D(f) First Lobe : 90% of the power Data : Power spectrum density S S D(f) 0 1/T D f 7

8 Part 1 : One channel one data Coding Sequence : PN Time waveform +1-1 T PN (in W/Hz) S pn (f) Coding PN sequence : Ideal Power spectrum density S pn (f) 0 1/T PN f 8

9 Part 1 : One channel one data Data T D Real coding Sequence : (in W/Hz) S pn (f) Coding PN sequence : Real Power spectrum density 0 1/T PN f 9

10 Part 1 : One channel one data Data T D Real coding Sequence : (in W/Hz) S pn (f) Discrete Power spectrum density due to the periodicity of the PN sequence 0 1/T PN f 10

11 Part 1 : One channel one data Data T D Real coding Sequence : PN Period PN Period PN Period 1/T D 0 1/T PN f 11

12 Part 1 : One channel one data Data T D Real coding Sequence : PN Period PN Period PN Period If T D >> T PN Discrete spectrum quasi ideal spectrum 12

13 Part 1 : One channel one data Data T D Real coding Sequence : T D = 7 T PN 13 1/T PN = 77 khz

14 Part 1 : One channel one data Data T D Real coding Sequence : T D = 31 T PN 14 1/T PN = 77 khz

15 Part 1 : One channel one data Data T D Real coding Sequence : T D = 127 T PN 15 1/T PN = 77 khz

16 Part 1 : One channel one data In conclusion : S S D(f) S pn (f) 0 1/T D 1/T PN f 16

17 Part 1 : One channel one data Data : s D (t) pn(t) Tx : s S (t) Transmitted (coded) signal Tx in CDMA/DS ss (t) = sd(t) pn(t) s D (t) negative --> s S (t) opposite to pn(t) 17

18 Part 1 : One channel one data Transmitted (coded) signal Tx in CDMA/DS Data : s D (t) Tx : s S (t)

19 Part 1 : One channel one data Transmitted (coded) signal Tx in CDMA/DS Time domain ss (t) = sd (t) pn(t) S s Frequency domain S ( f ) = S s D ( f ) S pn ( f ) S S (f) S Pn (f) S S S(f) Bandwith : 1/T PN 0 1/T D 1/T PN f 0 1/T PN f 19

20 Part 1 : One channel one data Transmitted (coded) signal Tx in CDMA/DS Time domain ss (t) = sd (t) pn(t) S s Frequency domain S ( f ) = S s D ( f ) S pn ( f ) Coding in time domain Spreading in frequency domain S Ss (f) Bandwith : 1/T PN Coding sequence = Spreading sequence 0 1/T PN f 20

21 Part 1 : One channel one data Tx : s S (t) T D Data : s D (t) Data : S D (f) T D = 127 T PN Emitted signal : S Ss (f) 21

22 Part 1 : One channel one data R Signal decoding at receiver (t) s X = s pn(t) Tx : S s (t) X R x pn(t) local code generator 22

23 Part 1 : One channel one data Data : s D (t) pn(t) at emitter Tx : s S (t) pn(t) at receiver Rx(t) R Signal decoding at receiver (t) s X = s pn(t) 23

24 Part 1 : One channel one data Signal decoding at receiver R X (t) = = s s s (t) pn(t) D (t) pn(t) pn(t) = 1 24

25 Part 1 : One channel one data Signal decoding at receiver Tx : S s (t) correlator/ despread. pn(t) local code generator R x synchron. control loop Requirement for spreading code synchronisation (see next chapters) 25

26 Part 2 : One channel many data Data : s D1 s D2 s D3. s Dn bit duration T D Coding sequence : pn 1 pn 2 pn 3. pn n bit duration T PN 26 26

27 Part 2 : One channel many data Data : s D1 s D2 s D3. s Dn Coding sequence : pn 1 pn 2 pn 3. pn n Spread signal : s (t) s (t) pn (t) s (t) pn S = D1 1 + D Dn (t) s (t) pn n (t) Bit rate : 1/T PN 27 27

28 Part 2 : One channel many data Data : s D1 s D2 s D3. s Dn Coding sequence : pn 1 pn 2 pn 3. pn n Spread signal : s (t) s (t) pn (t) s (t) pn S = D1 1 + D Dn (t) s (t) pn n (t) +/- 1 +/- 1 +/

29 Part 2 : One channel many data Data : s D1 s D2 s D3. s Dn Coding sequence : pn 1 pn 2 pn 3. pn n Spread signal : s (t) s (t) pn (t) s (t) pn S = D1 1 + D Dn (t) s (t) pn n (t) non binary signal / Bit rate : 1/ T PN 29 29

30 Part 2 : One channel many data Data : s D1 s D2 s D3. s Dn Coding sequence : pn 1 pn 2 pn 3. pn n Spread signal for 2 data streams

31 Part 2 : One channel many data Signal decoding at receiver : how to recover data 1? Tx : S s (t) X R x pn 1 local code generator RX (t) = sd1(t) pn1(t) pn1(t) + sd2(t) pn2(t) pn1(t) + + sdn (t) pnn (t) pn1(t) = 1 31

32 Part 2 : One channel many data Signal decoding at receiver : how to recover data 1? Tx : S s (t) X R x pn 1 local code generator RX (t) = sd1(t) + sd2(t) pn2(t) pn1(t) + + sdn (t) pnn (t) pn1(t) T D T PN T PN Time signals 32

33 Part 2 : One channel many data Signal decoding at receiver : how to recover data 1? Tx : S s (t) X R x Band- Filter [0 ; 1/T D ] R x pn 1 local code generator RX (t) = sd1(t) + sd2(t) pn2(t) pn1(t) + + sdn (t) pnn (t) pn1(t) Power Spectrum Density 33

34 Part 2 : One channel many data Signal decoding at receiver : how to recover data 1? Tx : S s (t) X R x Band- Filter [0 ; 1/T D ] R x pn 1 local code generator No effect of filtering on unspread data streams s Di (t) RX (t) = sd1(t) + sd2(t) pn2(t) pn1(t) + + sdn (t) pnn (t) pn1(t) 34

35 Part 2 : One channel many data Signal decoding at receiver : how to recover data 1? Tx : S s (t) X R x Band- Filter [0 ; 1/T D ] R x pn 1 local code generator s (t) Di = s Di (t) RX (t) = sd1(t) + sd2(t) pn2(t) pn1(t) + + sdn (t) pnn (t) pn1(t) 35

36 Part 2 : One channel many data Signal decoding at receiver : how to recover data 1? Tx : S s (t) X R x Band- Filter [0 ; 1/T D ] R x pn 1 local code generator RX (t) = sd1(t) + sd2(t) pn2(t) pn1(t) + + sdn (t) pnn (t) pn1(t) - Data streams s Di and PN sequences statistically independent sdi (t) pni(t) pn1(t) = sdi(t) pni(t) pn1(t) 36

37 Part 2 : One channel many data Signal decoding at receiver : how to recover data 1? Tx : S s (t) X R x Band- Filter [0 ; 1/T D ] R x pn 1 local code generator RX (t) = sd1(t) + sd2(t) pn2(t) pn1(t) + + sdn (t) pnn (t) pn1(t) Cross-correlation of PN sequences 37

38 Part 2 : One channel many data Signal decoding at receiver : how to recover data 1? Tx : S s (t) X R x Band- Filter [0 ; 1/T D ] R x pn 1 local code generator RX (t) = sd1(t) + sd2(t) pn2(t) pn1(t) + + sdn (t) pnn (t) pn1(t) Proper selection of uncorrelated PN sequences enhances effect of low pass filtering 38

39 Part 2 : One channel many data R x Data 1 R x s s Tx : S s (t) X R x Band- Filter [0 ; 1/T D ] R x pn 1 39

40 Part 2 : One channel many data R x Data 1 R x s s Tx : S s (t) X R x Band- Filter [0 ; 1/T D ] R x Threshold detector Data 1 recovered pn 1 40

41 Part 3 : Signal to noise ratio / Power control How many datas on one single channel? RX (t) = sd1(t) + sd2(t) pn2(t) pn1(t) + + sdn (t) pnn (t) pn1(t) Assuming a constant positive value α for PN sequences cross-correlation : RX (t) = sd1(t) + sd2(t) α + + sdn (t) α 41

42 Part 3 : Signal to noise ratio / Power control How many datas on one single channel? Assuming a constant positive value α for PN sequences cross-correlation : RX (t) = sd1(t) + sd2(t) α + + sdn (t) α Worst case : s D1 = +1 and s D2 = s D3 =. = s Dn = - 1 R X (t) = 1 (n 1) α 42

43 Part 3 : Signal to noise ratio / Power control How many datas on one single channel? Assuming a constant positive value α for PN sequences cross-correlation : RX (t) = sd1(t) + sd2(t) α + + sdn (t) α Worst case : s D1 = +1 and s D2 = s D3 =. = s Dn = - 1 R X (t) = 1 (n 1) α If (n-1) x α > 1 then the sign of R X (t) is different from the sign of s D1 Error : system capacity is fixed by the sequences cross-correlation value 43

44 Part 3 : Signal to noise ratio / Power control How many datas on one single channel? Assuming a constant positive value α for PN sequences cross-correlation : RX (t) = sd1(t) + sd2(t) α + + sdn (t) α Worst case : s D1 = +1 and s D2 = s D3 =. = s Dn = - 1 R X (t) = 1 (n 1) α the smaller is α the larger n can be 44

45 Part 3 : Signal to noise ratio / Power control Signal to noise ratio Assuming a constant positive value α for PN sequences cross-correlation : RX (t) = sd1(t) + sd2(t) α + + sdn (t) α Power P Power P Power P 45

46 Part 3 : Signal to noise ratio / Power control Signal to noise ratio Assuming a constant positive value α for PN sequences cross-correlation : RX (t) = sd1(t) + sd2(t) α + + sdn (t) α Signal Noise 46

47 Part 3 : Signal to noise ratio / Power control Signal to noise ratio Assuming a constant positive value α for PN sequences cross-correlation : R X (t) = s (t) + s (t) α + + s (t) α D1 D2 Dn + Johnson Noise Signal Johnson White noise N o B eq White noise power spectrum density Noise equivalent bandwidth of bandfilter 47

48 Part 3 : Signal to noise ratio / Power control Signal to noise ratio Assuming a constant positive value α for PN sequences cross-correlation : R X (t) = s (t) + s (t) α + + s (t) α D1 D2 Dn + Johnson Noise Signal Noise Signal to Noise ratio including white noise : S N = (n 1) P P α + N o B eq 48

49 Part 3 : Signal to noise ratio / Power control What happens in case of non-uniform power of data streams? Situation 1 : -n max datas of power P --> minimum S/N leading to error free transmission S N min = (n max P 1) P α (White noise neglected) Situation 2 : - n max datas - Data 1 to (n-1) of power P and n th Data of power (β.p) > P RX (t) = sd1(t) + sd2(t) α + + sdn (t) α Power P Power P Power β P 49

50 Part 3 : Signal to noise ratio / Power control What happens in case of non-uniform power of data streams? Situation 1 : -n max datas of power P --> minimum S/N leading to error free transmission S N min = (n max P 1) P α (White noise neglected) Situation 2 : - n max datas - Data 1 to (n-1) of power P and n th Data of power (β.p) > P S N = (n P 1+ β ) P Data max N 1 to ( n 1) α < S Min 50

51 Part 3 : Signal to noise ratio / Power control What happens in case of non-uniform power of data streams? Situation 1 : -n max datas of power P --> minimum S/N leading to error free transmission S N min = (n max P 1) P α (White noise neglected) Situation 2 : - n max datas - Data 1 to (n-1) of power P and n th Data of power (β.p) > P S N = (n P 1+ β ) P Data max N 1 to ( n 1) α < S Min Data 1 to (n-1) lost 51

52 Part 3 : Signal to noise ratio / Power control What happens in case of non-uniform power of data streams? Situation 1 : -n max datas of power P --> minimum S/N leading to error free transmission S N min = (n max P 1) P α (White noise neglected) Situation 2 : - n max datas - Data 1 to (n-1) of power P and n th Data of power (β.p) > P S N n th Data = β P (n β S N > max 1+ β ) P α Min S N Min Only data n is detected 52

53 Part 4 : Conclusion -CDMA/DS allow transmission of many data on one single channel -Data are coded using binary pseudo random sequences with a bit rate much larger than the bit rate of Data -Coding causes the spreading of all data over the same band -Recovering of data at receiver requires : -pseudo random sequences at receiver identical to pseudo sequences at emitter -pseudo sequences at receiver synchronized with pseudo sequences at emitter -pseudo sequences as low correlated as possible -all coded Data to be transmitted with the same power 53

54 Part 4 : Conclusion Blockdiagram of a CDMA/DS System baseband processing transmission channel baseband processing s D spectral spreading Tx rf rf Tx s S modulator s S demodul. s S correlator/ despread. Rx bandfilter s D pn spreading code generator f O carrier frequency oscillator f O carrier frq. local oscillator pn local code generator synchron. control loop baseband transmitter baseband receiver signals: s D data signal s S spread signal (baseband) s S spread signal (rf carrier) bandwidths: B D 1/T D B S 1/T PN (baseband) B S transmission bandwidth (spread rf bandwidth) 54

55 Bibliography - If needed, a few related to spread spectrum systems : Spread Spectrum Systems, R. C. Dixon, Ed. John Wiley & Sons ISBN