1. Korean Intellectual Property Office – ICU seminar Code Division Multiple Access Systems Prof. Jeongseok Ha March 07 2007 Information and Communications University

2. 2 / 74 Contents 1.Introduction  CDMA Systems  Duplex Methods of Radio Links  Multiple Access Techniques 2.Spread Spectrum  Spread Spectrum Systems  Frequency Hopping Spread Spectrum  Direct Sequence Spread Spectrum  Pseudo Noise Sequences  Walsh Codes 3.Direct Sequence (DS)-CDMA  DS-CDMA System Overview  Spreading Codes 4.Features of CDMA  Multi-path Fading  Received Power Fluctuation  Power Control  Frequency Allocation  Handoff

3. 3 / 74 Introduction CDMA SystemsMultiple Access TechniquesDuplex Methods of Radio Links

4. 4 / 74 Code Division Multiple Access (CMDA) Systems  CDMA Standards  Interim Standard (IS) 95 (TIA-EIA-95): the first CDMA-based digital cellular standard pioneered by QualcommCDMAQualcomm  The brand name for IS-95 is cdmaOne  cdmaOne is a 2G system  CDMA2000 1xRTT (2.5G):  1XRTT (1 times Radio Trans. Tech.): operates in a pair of 1.25MHz spectrum  Doubles voice capacity, 144Kbit/s peak data rate  Qualified as 3G but deployed in 2G spectrum in some countries  64 more channels in downlink  Improvements in link layer for greater use of data service such as 1.media and link access control protocol 2.QoS control

5. 5 / 74 Code Division Multiple Access (CMDA) Systems  CDMA2000 3x  operates in a pair of 3.75(=3x1.25)MHz channels  Never deployed  CDMA2000 EV-DO (Evolution-Data Optimized): Evolution of CDMA 1xRTT with HDR (High Data Rate) capability  Categorized as a 3G system  Originally called 1x Evolution-Data Only  Data Rates: 4.9xN Mbit/s for downlink and 1.8N Mbit/s for uplink where N is # of 1.25MHz  Services in Korea: SKTelecom “june” and KTF “fimm”  CDMA2000 EV-DV (Evolution-Data/Voice)  Data rates: 3.1Mbit/s for downlink and 1.8Bbit/s for uplink  Supports 1xvoice users, 1x data users and EV-DV data users within the same radio channel

6. 6 / 74 Code Division Multiple Access (CMDA) Systems  Wideband-CDMA  Developed by NTT DoCoMo  Selected as the air interface for UMTS  Uses 5MHz spectrum  Supports inter-cell asynchronous operation

7. 7 / 74 Duplex Methods of Radio Links Mobile Station Base Station Forward link Reverse link

8. 8 / 74 Duplex Methods of Radio Links  Frequency Division Duplex (FDD)  Forward link and reverse link frequencies are different  In each link, signals are continuously transmitted in parallel Mobile Station Base Station Forward link (F1) Reverse link (F2)

9. 9 / 74 Duplex Methods of Radio Links  Example of FDD Systems Transmitter Receiver BPF: Band Pass Filter BPF Transmitter Receiver BPF F1 F2 F1 F2 Mobile Station Base Station

10. 10 / 74 Duplex Methods of Radio Links  Time Division Duplex (TDD)  Forward link and reverse link frequencies are the same  In each link, signals are not continuously transmitted by turns just like a ping-pong Mobile Station Base Station Forward link (F1) Reverse link (F1)

11. 11 / 74 Duplex Methods of Radio Links  Example of TDD Systems Transmitter Receiver BPF: Band Pass Filter BPF Transmitter Receiver BPF F1 Mobile Station Base Station Synchronous Switches

12. 12 / 74 Multiple Access Techniques  Resource allocation to multiple P-to-P channels Mobile Station Base Station Mobile Station Forward link Reverse link

13. 13 / 74 Multiple Access Techniques  Multiple Access Techniques 1.FDMA (Frequency Division Multiple Access) 2.TDMA (Time Division Multiple Access) 3.CDMA (Code Division Multiple Access) 4.OFDMA (Orthogonal Frequency Division Multiple Access)

14. 14 / 74 Multiple Access Techniques  FDMA Overview A A B B C C Frequency Time f 2 f 1 f 0

15. 15 / 74 Multiple Access Techniques  TDMA Overview CBACBACBACBA C A B Time f 0 Frequency

16. 16 / 74 Multiple Access Techniques  CDMA Overview SenderReceiver Code A A Code B B A B A B C B C A Code A A B C Time Frequency B C B A Base-band Spectrum Radio Spectrum spread spectrum

17. 17 / 74 Multiple Access Methods  OFDM

18. 18 / 74 Multiple Access Methods  OFDMA Overview

19. 19 / 74 Multiple Access Methods  Summary of Multiple Access FDMA TDMA CDMA time power frequency

20. 20 / 74 Spread Spectrum Spread Spectrum SystemsFrequency Hopping Spread SpectrumDirect Sequence Spread SpectrumPseudo Noise SequencesWalsh Codes

21. 21 / 74 Spread Spectrum Systems  Brief History  1900, Nikola Tesla patented freq. hopping SS to control radio controlled submersible boat without interference  In WWII, the US Army Signal Corp invented a communication system, SIGSALY based on SS for communication between Roosevelt and Churchill  1980s, SS was begun to used in commercial systems 1.Very Small Aperture (VSAT) satellite terminal system in Equatorial Communications Systems 2.Radio-navigation system by Del Norte Technology 3.OmniTRACS by Qualcomm (communication to trucks)

22. 22 / 74 Spread Spectrum Systems  Applications 1.Anti-jamming (AJ): high tolerance to jamming 2.Position location and velocity estimation 3.Low detectability of transmitted signals by unintended receiver 4.Multiple access (for mobile communications, 1990s)  SSMA (Spread Spectrum Multiple Access): SS Modulation used in conjunction with code division multiple access (CDMA) 5.Wireless LAN 802.11 (FHSS, DSSS) Undetectable by unintended receiver

23. 23 / 74 Spread Spectrum Systems  The transmitted signal occupies a BW larger than the minimum BW required to transmit the data P watts BsBs B ss BsBs P watts/Hz f Spectra of signal before and after spreading * Processing gain: B ss /B s Narrow band signal (data) Wideband signal (transmitted SS signal)

24. 24 / 74 Spread Spectrum Systems  BW spread is accomplished by means of a code independent of the data  Synchronized reception with the code at the receiver for despreading and subsequent data recovery

25. 25 / 74  Frequency Hopping Spread Spectrum  Signal broadcasts over seemingly random series of frequencies  Receiver hops between frequencies in sync with transmitter  Eavesdroppers hear unintelligible blips  Jamming on one frequency affects only a few bits Frequency Hopping Spread Spectrum

26. 26 / 74  Basic Operation  Typically 2 k carriers frequencies forming 2 k channels  Channel spacing corresponds with bandwidth of input  Each channel used for fixed interval  300 ms in IEEE 802.11  Sequence dictated by spreading code Frequency Hopping Spread Spectrum

27. 27 / 74  Transmitter Frequency Hopping Spread Spectrum

28. 28 / 74  Receiver Frequency Hopping Spread Spectrum System

29. 29 / 74 Frequency Hopping Spread Spectrum System  Slow Frequency Hopping Spread Spectrum Using MFSK (M=4, k=2)

30. 30 / 74 Frequency Hopping Spread Spectrum System  Fast Frequency Hopping Spread Spectrum Using MFSK (M=4, k=2)

31. 31 / 74  Transmitter Direct Sequence Spread Spectrum

32. 32 / 74 Direct Sequence Spread Spectrum  Example Modulation (primary modulation) Modulation (primary modulation) user data Spreading (secondary modulation) Spreading (secondary modulation) Tx Base-band Frequency Power Density Radio Frequency Power Density TIME data rate 10110100 spreading sequence (spreading code)

33. 33 / 74  Receiver Direct Sequence Spread Spectrum

34. 34 / 74 Direct Sequence Spread Spectrum  Example If you know the correct spreading sequence (code), Radio Frequency Power Density received signal spreading sequence (spreading code) you can find the spreading timing which gives the maximum detected power, and Accumulate for one bit duration Accumulate for one bit duration Demodulated data Base-band Frequency gathering energy ! 10110100 TIME 01001011 10110100 00 1 11111111 00000000

35. 35 / 74 Direct Sequence Spread Spectrum  Example If you don’t know the correct spreading sequence (code) … Base-band Frequency received signal spreading sequence (spreading code) you cannot find the spreading timing without correct spreading code, and Accumulate for one bit duration Accumulate for one bit duration Demodulated data Radio Frequency Power Density 01010101 10101010 TIME 01001011 10110100 No data can be detected -- - 10110100

36. 36 / 74  Properties of PN sequences 1.Deterministic, being easily generated by feedback shift registers 2.They have a auto-correlation function that is highly peaked for zero delay and approximately zero for other delays 3.Low cross correlation between other sequences 4.Sequence is not statistically random but will pass many test of randomness 1.R1: relative frequencies of ‘0’ and ‘1’ are each ½ 2.R2: run length of 0’s and 1’s expected in a coin flipping experiment. That is, 1) ½ of all run lengths are 1, ¼ are of length 2, 1/8 are of lengths 3 and 2) a fraction 1/2 n of all runs are of length n for all finite n 3.If the random sequence is shifted, the resulted sequence will have an equal number of agreements and disagreements with the original sequence 5.Unless algorithm and seed are known, the sequence is impractical to predict Pseudo-Noise (PN) Sequences

37. 37 / 74 Pseudo-Noise (PN) Sequences  Maximum Length Shift Register Seq. Gen.

38. 38 / 74 Pseudo-Noise (PN) Sequences

39. 39 / 74  Auto-correlation of PN sequences Auto-correlation for zero delay a PN code: Pseudo-Noise (PN) Sequences

40. 40 / 74  Auto-correlation of PN sequences (cont.) Auto-correlation for 1 bit delayed PN sequences a PN code: 1 bit delayed PN code: Pseudo-Noise (PN) Sequences

41. 41 / 74  Cross-correlation of PN sequences Cross-correlation for zero delay a PN code generated by primitive polynomial : Pseudo-Noise (PN) Sequences

42. 42 / 74  Cross-correlation of PN sequences Cross-correlation for delayed PN sequences a PN code generated by primitive polynomial : 1 bit delayed PN code generated by primitive polynomial : Pseudo-Noise (PN) Sequences

43. 43 / 74 Pseudo-Noise (PN) Sequences  The IS-95 Long PN Code Generator and Mask

44. 44 / 74  Used in synchronous systems, also in WCDMA downlink  Set of Walsh codes of length n consists of the n rows of an n n Walsh matrix  Generation recursively:  Every row and column is orthogonal to every other row and column, respectively  Requires tight synchronization  Cross correlation between different shifts of Walsh sequences is not zero Walsh Codes

45. 45 / 74  Example for zero delay Auto-correlation Cross-correlation Real Value Walsh Codes

46. 46 / 74  Example for delayed codes Auto-correlation Real Value Walsh Codes

47. 47 / 74 Direct Sequence (DS)-CDMA DS-CDMA System Overview Spreading Codes

48. 48 / 74 Freq. BPF Despreader Code B Freq. BPF Despreader Code A Data B Code B BPF Freq. Data A Code A BPF Freq. MS-A MS-B BS Data A Data B  Forward Link DS-CDMA System Overview

49. 49 / 74  Reverse Link Freq. BPF Despreader Code B Freq. BPF Despreader Code A Data B Code B BPF Freq. Data A Code A BPF Freq. MS-B MS-A BS Data A Data B DS-CDMA System Overview

50. 50 / 74 In order to minimize mutual interference in DS-CDMA, the spreading codes with less cross-correlation should be chosen. Synchronous DS-CDMA : Orthogonal Codes are appropriate. (Walsh code etc.) Asynchronous DS-CDMA : Pseudo-random Noise (PN) codes / Maximum sequence Gold codes  Preferable Codes Spreading Codes

51. 51 / 74 Forward Link (Down Link) Synchronous Chip Timing A A Signal for B Station (after re-spreading) Less Interference for A station Synchronous CDMA Systems realized in Point to Multi-point System. e.g., Forward Link (Base Station to Mobile Station) in Mobile Phone.  Synchronous DS-CDMA Spreading Codes

52. 52 / 74 In asynchronous CDMA system, orthogonal codes have bad cross-correlation. Reverse Link (Up Link) B A Signal for B Station (after re-spreading) Big Interference from A station Asynchronous Chip Timing Signals from A and B are interfering each other. ABAB  Asynchronous DS-CDMA Spreading Codes

53. 53 / 74 channel-> detecting A... -> Example of DS multiple access waveforms

54. 54 / 74 Features of CDMA Multi-path Fading Received Power Fluctuation Power Control Frequency Allocation Handoff

55. 55 / 74 The peaks and bottoms of received power appear, in proportion to Doppler frequency. Base Station (BS) Mobile Station (MS) multi-path propagation Path Delay Power path-2 path-3 path-1 Time Power Multi-path Fading

56. 56 / 74  Fading in non-CDMA System  With low time-resolution, different signal paths cannot be discriminated  These signals sometimes strengthen, and sometimes cancel out each other, depending on their phase relation This is “ fading ” Path Delay Power path-1 path-2 path-3 Multi-path Fading

57. 57 / 74  Fading in non-CDMA System (cont.)  At the fading channel, signal quality is damaged when signals cancel out each other  In other words, signal quality is dominated by the probability for detected power to be weaker than minimum required level  This probability exists with less than two paths In non-CDMA system, “fading” damages signal quality. Detected Power Time Power Multi-path Fading

58. 58 / 74  Fading in CDMA System  Because CDMA has high time-resolution, different path delay of CDMA signals can be discriminated  Therefore, energy from all paths can be summed by adjusting their phases and path delays This is a principle of RAKE receiver Path Delay Power path-1 path-2 path-3 Multi-path Fading

59. 59 / 74 CDMA Receiver CDMA Receiver Path Delay Power CODE A with timing of path-1 path-1 Power path-1 path-2 path-3 Path Delay Power CODE A with timing of path-2 path-2 interference from path-2 and path-3 Synchronization Adder  Fading in CDMA System (cont.) Multi-path Fading

60. 60 / 74 In CDMA system, multi-path propagation improves RAKE receiver the signal quality by use of RAKE receiver. Less fluctuation of detected power, because of adding all energy.  Fading in CDMA System (cont.) Time Power Detected Power Power path-1 path-2 path-3 RAKE receiver Multi-path Fading

61. 61 / 74  Near-Far Problem  When B is closer than A to the receiver, desired signal power is smaller than the interfered power  Serious problem in CDMA CODE B CDMA Transmitter DATA B CODE A CDMA Receiver CODE A CDMA Transmitter DATA A P Desired Signal Power = P/Lp-a Interfered Signal Power = P/Lp-b/( processing gain ) Demodulated DATA P Lp-a Lp-b Received Signal Power Fluctuation

62. 62 / 74  Slow Fading Ａ Ｂ Time Detected Power from A from B Power Control

63. 63 / 74  Open Loop Power Control Estimating path loss Calculating transmission power Transmit Measuring received power TransmitReceive BS MS （（（（（（ BS MS Power Control

64. 64 / 74  Closed Loop Power Control (Inner Loop Power Control) ② ① BS MS Power control command Decide transmission power BS MS Transmit Measuring received power about 1000 times per second ② ① Power Control

65. 65 / 74  Outer Loop Power Control ② ③ BS MS ① RNC *RNC: Radio Network Controller Is signal quality (BER) OK? RNC ① ② Power control command Decide transmission power BS MS Transmit Measuring received power about 1000 times per second ③ Closed Loop Power Control Target SIR Power Control

66. 66 / 74  Effect of Power Control  Power control is capable of compensating the fading fluctuation  Received power from all MS are controlled to be equal  Near-Far problem is mitigated by the power control Ｂ closed loop power control for MS B. for MS A. Ａ Time Detected Power from MS B from MS A Power Control

67. 67 / 74 Cell : a ‘cell’ means covered area by one BS f1 f2 f3 f4 f5 f6 f7 f1 f2 f3 f4 f5 f6 f7 f1 f2 f3 f4 f5 f6 f7 f1 f2 f3 f4 f5 f6 f7 f1 f2 f3 f4 f5 f6 f7 f1 f2 f3 f4 f5 f6 f7 f1 f2 f3 f4 f5 f6 f7  TDMA or FDMA  Radio resource is allocated not to interfere among neighbor cells  Neighbor cells cannot use the same frequency band (or time slot)  The right figure shows the simple cell allocation with seven frequency bands  In actual situation, because of complicated radio propagation and irregular cell allocation, it is not easy to allocate frequency (or time slot) appropriately Frequency Allocation

68. 68 / 74  CDMA  Identical radio resource can be used among all cells, because CDMA channels use same frequency simultaneously  Frequency allocation in CDMA is not necessary  In this sense, CDMA cellular system is easy to be designed Frequency Allocation

69. 69 / 74  Handoff  Cellular system tracks mobile stations in order to maintain their communication links  When mobile station goes to neighbor cell, communication link switches from current cell to the neighbor cell Handoff

70. 70 / 74  Hard Handoff  In FDMA or TDMA cellular system, new communication establishes after brea king current communication at the moment doing handoff. Communication be tween MS and BS breaks at the moment switching frequency or time slot Hard Handoff: Connect (new cell B) after break (old cell A) switching Cell B Cell A Handoff

71. 71 / 74 Soft Handoff: break (old cell A) after connect (new cell B)  Soft Handoff  In CDMA cellular system, communication does not break even at the moment doing handoff, because switching frequency or time slot is not required Σ Cell B Cell A Transmitting same signal from both BS A and BS B simultaneously to the MS Handoff

72. 72 / 74 Softer Handoff: Soft handoff between different sectors in a cell  Softer Handoff  A mobile station is in the overlapping cell coverage area of two adjacent sectors of a base station Sector 1 Sector 2 Sector 3 Handoff

73. 73 / 74 Questions and Comments

74. 74 / 74  Takashi Inoue, “CDMA Technologies for Cellular Phone System,” KDDI R&D Lab. Inc., July 2004  William Stallings, “Data and Computer Communications,” 7 th edition, Prentice Hall, May 2003  Alex W. Lam and Sawasd Tantaratana, “Theory and Applications of Spread- spectrum Systems,” IEEE/EAB, May 1994 Reference