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CDMA Technologies for Cellular Phone System

CDMA Technologies for Cellular Phone System. July 7th, 2004 Takashi INOUE KDDI R&D Laboratories Inc. Contents. Introduction Spread Spectrum Technology DS-CDMA Spreading Codes Features of CDMA RAKE Receiver Power Control Frequency Allocation Soft Handoff Conclusion.

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CDMA Technologies for Cellular Phone System

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  1. CDMA Technologies for Cellular Phone System July 7th, 2004 Takashi INOUE KDDI R&D Laboratories Inc.

  2. Contents • Introduction • Spread Spectrum Technology • DS-CDMA • Spreading Codes • Features of CDMA • RAKE Receiver • Power Control • Frequency Allocation • Soft Handoff • Conclusion

  3. Introduction: Overview of Cellular systems

  4. 3rd. Generation (2000s) 2nd. Generation (1990s) Digital 1st.Generation (1980s) GSM DECT DCS1800 CT2 PDC PHS IS-54 IS-95 IS-136 UP-PCS Analog NMT CT0 TACS CT1 AMPS Evolution of Cellular Systems IMT-2000 CDMA2000 W-CDMA

  5. Major Operators of Cellular Phone Services in Japan Operator Frequency Remarks for 3G handset 2G 3G KDDI/au 800 MHz (1.5GHz For Tu-Ka) 800MHz 2GHz backward compatibility with 2G (cdmaOne) NTT DoCoMo 800 MHz 1.5GHz 2GHz W-CDMA single W-CDMA/PDC Dual J-Phone (vodaphone) 1.5 GHz 2GHz W-CDMA single W-CDMA/GSM Dual

  6. Japan’s Cellular Market Growth History 80,000,000 CDMA2000 1x , W-CDMA / 70,000,000 end of Mar. 2003 Total No. of Subscribers: 74,368K 6,093K 60,000,000 cdmaOne / 7,757K 50,000,000 PDC/60,517K 40,000,000 (NTT DoCoMo, KDDI, Tu-Ka, 30,000,000 J-phone) Analog / 0(end 20,000,000 of Srv.) 10,000,000 0 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 (end of each fiscal year)

  7. 8,000 7,000 6,000 The number of subscribers surpassed 7 million in March, 2003 5,000 4,000 3,000 J-phone 2,000 FOMA CDMA2000 1x 1,000 0 Growth of 3G Mobile Subscribers in Japan No. of Subs. (x1,000) CDMA2000 1x FOMA J-phone

  8. Requirements for 3G mobile systems • High Capacity • Tolerance for interference • Privacy • Tolerance for fading • Ability to various data rate transmission • Flexible QoS

  9. IMT-2000 systems approved by ITU-R IMT-DS IMT-FT IMT-MC IMT-TC IMT-SC (Multi Carrier) (Single Carrier) (Direct Sequence) (Time Code) (Frequency Time) UTRA-TDD Popular name W-CDMA DECT CDMA2000 TD-CDMA UWC-136 TD-SCDMA Access method CDMA-FDD CDMA-FDD CDMA-TDD TDMA-FDD TDMA-TDD ARIB/TTC ARIB/TTC CWTS CWTS ESTI CWTS Organization Partners TIA ESTI ESTI T1 TIA T1 TTA TTA TTA Body of Technical Spec production 3GPP(FDD） 3GPP2 3GPP(TDD) CWTS IS-136 DECT Approved in 2000 as ITU-R M.1457

  10. Duplex & Multiple Access Methods

  11. Duplex Methods of Radio Links Base Station Forward link Reverse link Mobile Station

  12. Frequency Division Duplex (FDD) • Forward link frequency and reverse link frequency is different • In each link, signals are continuously transmitted in parallel. Forward link (F1) Reverse link (F2) Base Station Mobile Station

  13. Time Division Duplex (TDD) • Forward link frequency and reverse link frequency is the same. • In each link, signals are incontinuously transmitted by turns just like a ping-pong. Forward link (F1) Reverse link (F1) Base Station Mobile Station

  14. Example of FDD systems Mobile Station Base Station Transmitter BPF BPF Transmitter F1 F2 Receiver BPF BPF Receiver F1 F2 BPF: Band Pass Filter

  15. Example of TDD Systems Mobile Station Base Station Transmitter Transmitter BPF BPF F1 F1 Receiver Receiver Synchronous Switches BPF: Band Pass Filter

  16. Multiple Access Methods Base Station Forward link Reverse link Mobile Station Mobile Station Mobile Station Mobile Station

  17. Frequency C C f 2 B B f 1 A A f 0 Time FDMA Overview

  18. Frequency f 0 C B A C B A C B A C B A A Time B C TDMA Overview

  19. Radio Spectrum Base-band Spectrum B Code A B Frequency Code A Code B A C C B C B B A B A A C A A B Time What is CDMA ? spread spectrum Sender Receiver

  20. Summary of Multiple Access FDMA power TDMA frequency time power CDMA frequency time power frequency time

  21. Spread Spectrum Technology

  22. Power Density TIME spreading sequence (spreading code) Base-band Frequency Power Density 10110100 Radio Frequency How to spread spectrum... Direct Sequence (DS) user data data rate Modulation (primary modulation) Spreading (secondary modulation) Tx

  23. Power Density TIME spreading sequence (spreading code) 10110100 Radio Frequency you can find the spreading timing which gives the maximum detected power, and 10110100 10110100 10110100 10110100 10110100 01001011 gathering energy ! Accumulate for one bit duration 00000000 00000000 11111111 Demodulated data 0 0 1 Base-band Frequency Demodulating DS Signals (1/2) If you know the correct spreading sequence (code) , received signal

  24. Power Density TIME spreading sequence (spreading code) Radio Frequency you cannot find the spreading timing without correct spreading code, and 10101010 10101010 10101010 10110100 10110100 10110100 10110100 10110100 01001011 Accumulate for one bit duration No data can be detected - - - Demodulated data Base-band Frequency Demodulating DS Signals (2/2) If you don’t know the correct spreading sequence (code) ••• received signal 01010101 01010101 01010101

  25. Power Density Power Density Power Density Noise Radio Frequency Radio Frequency Base-band Frequency With correct code (and carrier frequency), data can be detected. With incorrect code (or carrier frequency), SS-signal itself cannot be detected. Power Density Base-band Frequency Feature of SS Privacy, Security Power density of SS-signals could be lower than the noise density. •••••• •••••• de-modulator transmitted SS-signal received signal Noise They cannot perceive the existence of communication, because of signal behind the noise.

  26. DS-CDMA

  27. Freq. Freq. Freq. Freq. DS-CDMA System Overview (Forward link) CDMA is a multiple spread spectrum. Freq. Freq. BPF Data A BPF Despreader Data A MS-A Code A Code A Freq. Freq. BPF Data B BPF Despreader Data B MS-B Code B Code B ••• BS ••• Difference between each communication path is only the spreading code

  28. Freq. Freq. Freq. Freq. DS-CDMA System Overview (Reverse Link) CDMA is a multiple spread spectrum. Freq. Freq. BPF Data A BPF Despreader Data A Code A Code A MS-A Freq. Freq. BPF Data B BPF Despreader Data B Code B Code B MS-B ••• ••• BS Difference between each communication path is only the spreading code

  29. Spreading Code

  30. Spreading Code A Spreading Code A 1 1 1 0 0 1 0 0 0 0 0 0 0 0 1 1 0 1 0 0 0 0 1 0 1 1 1 0 0 1 1 0 0 1 1 1 0 0 0 0 0 0 1 0 0 0 1 0 1 1 0 0 0 0 0 0 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 1 0 1 0 1 1 0 0 0 0 0 0 0 1 0 0 0 1 0 0 1 1 1 1 one data bit duration one data bit duration Spreading Code A Spreading Code B Cross-Correlation Self-Correlation for each code is 1. Cross-Correlation between Code A and Code B = 6/16

  31. Preferable Codes 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

  32. M o d u l a t o r C o d e f o r 0 0 C o d e f o r 0 1 D a t a C o d e f o r 1 0 D e m o d u l a t o r C o d e f o r 1 1 C o d e f o r 0 0 ò T d t 0 S e l e c t m a x i m u m C o d e f o r 0 1 v a l u e ò T d t 0 C o d e f o r 1 0 ò T d t 0 C o d e f o r 1 1 ò T d t 0 Multiplexing using Walsh Code

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

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

  35. Features of CDMA

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

  37. path-1 Power path-2 path-3 Path Delay Power Detected Power Time 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”. ••• In this case, 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.

  38. path-1 Power path-2 path-3 Path Delay path-1 Power path-3 CODE A with timing of path-1 Path Delay path-2 Power path-1 Power path-2 CODE A with timing of path-2 Path Delay 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. interference from path-2 and path-3 CDMA Receiver Synchronization Adder CDMA Receiver ••• •••

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

  40. Near-Far Problem P Lp-a CDMA Transmitter DATA A CDMA Receiver Demodulated DATA CODE A P Lp-b CODE A CDMA Transmitter DATA B • Desired Signal Power = P/Lp-a • Interfered Signal Power = • P/Lp-b/(processing gain ) CODE B When user B is close to the receiver and user A is far from the receiver, Lp-a could be much bigger than Lp-b. In this case, desired signal power is smaller than the interfered power.

  41. Ａ Ｂ Power Control... When all mobile stations transmit the signals at the same power (MS), the received levels at the base station are different from each other, which depend on the distances between BS and MSs. Moreover, the received level fluctuates quickly due to fading. In order to maintain the received level at BS, power control technique must be employed in CDMA systems. from A from B Detected Power Time

  42. （（（ measuring received power decide transmission power power control command estimating path loss measuring received power calculating transmission power transmit receive Power Control (continued) Open Loop Power Control Closed Loop Power Control ① ② ② transmit about 1000 times per second transmit ①

  43. Effect of Power Control • 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. from MS B from MS A closed loop power control for MS B. Detected Power for MS A. Time Ａ Ｂ

  44. f3 f2 f4 f1 f7 f5 f6 cell : a “cell” means covered area by one base station. Frequency Allocation (1/2) In FDMA or TDMA, radio resource is allocated not to interfere among neighbor cells. • Neighbor cells cannot use the same (identical) frequency band (or time slot). • The left figure shows the simple cell allocation with seven bands of frequency. • In actual situation, because of complicated radio propagation and irregular cell allocation, it is not easy to allocate frequency (or time slot) appropriately.

  45. Frequency Allocation (2/2) In 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.

  46. switching Cell A Cell B Soft Handoff (1/2) • 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. • Hard Handoff : • In FDMA or TDMA cellular system, new communication establishes after breaking current communication at the moment doing handoff. Communication between MS and BS breaks at the moment switching frequency or time slot. Hard handoff : connect (new cell B) after break (old cell A)

  47. Soft Handoff (2/2) • 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. transmitting same signal from both BS A and BS B simultaneously to the MS Σ Cell B Cell A Soft handoff : break (old cell A) after connect (new cell B)

  48. Conclusion • CDMA is based on the spread spectrum technique which has been used at military field. • CDMA cellular system is deemed superior to the FDMA and TDMA cellular systems for the time being. • Therefore, CDMA technique becomes more important in radio communication systems.

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