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CDMA Technologies for Cellular Phone System

CDMA Technologies for Cellular Phone System. Prof. Li Pingan. Contents. Introduction Spread Spectrum Technology DS-CDMA Spreading Codes Features of CDMA RAKE Receiver Power Control Frequency Allocation Soft Handoff Conclusion. Introduction: Overview of Cellular systems.

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CDMA Technologies for Cellular Phone System

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  1. CDMA Technologies for Cellular Phone System Prof. Li Pingan

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

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

  6. 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

  7. Spread Spectrum Technology

  8. Spread Spectrum System

  9. Example: Spreading by using Walsh plus PN

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

  11. Power Density TIME 01001011 10110100 10110100 Radio Frequency 10110100 10110100 10110100 Base-band Frequency Demodulating DS Signals (1/2) received signal spreading sequence (spreading code) gathering energy ! 0+0=0 1+0=1 0+1=1 1+1=0 Accumulate for one bit duration 11111111 00000000 00000000 Demodulated data 0 0 1 If you know the correct spreading sequence (code) ,

  12. TIME 10101010 10101010 10101010 Base-band Frequency Demodulating DS Signals (2/2) Power Density received signal 01001011 10110100 10110100 spreading sequence (spreading code) Radio Frequency Accumulate for one bit duration 10110100 10110100 10110100 No data can be detected Demodulated data - - - If you don’t know the correct spreading sequence (code) •••

  13. Bandwidth Expansion

  14. 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 would be lower than the noise density. •••••• •••••• de-modulator transmitted SS-signal received signal Noise Other system cannot recognize the existence of communication, because of signal behind the noise.

  15. Feature of SS (Anti-interference) In base-band; Inpass-band, [SIRo/SIRi]dB increases 3dB due to coherent detection

  16. Feature of SS (CDMA)

  17. Detection of DSSS Signals • Let bk denote k-th BPSK bit • c(t) is the normalized spreading waveform (code) • Transmission in AWGN (mean 0 and variance ) without interference • The output of the de-spreader is identical to BPSK without spreading

  18. Detection of DSSS Signals • Let bk denote k-th BPSK bit • c(t) is the normalized spreading waveform (code) • A narrowband interference with averaged power J is added to the signal in the channel • The interfering part is assume Gaussian with mean 0 and variance

  19. Detection of DSSS Signals • SIR at the input of the de-spreader • SIR at the output of the de-spread • SIR gain • BER

  20. DS-CDMA

  21. Freq. Freq. DS-CDMA System Overview (Forward link) Freq. Freq. BPF Data A BPF Despreader Data A MS-A Code A Code A Freq. Freq. 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

  22. Freq. Freq. DS-CDMA System Overview (Reverse Link) Freq. Freq. BPF Data A BPF Despreader Data A Code A Code A MS-A Freq. Freq. 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

  23. Frequency Hopping Systems

  24. Spreading Code

  25. Pseudo-noise Sequence Modulo-2 sum A flip-flop

  26. PN Sequence

  27. PN Sequence • An (n, k) cyclic code • Block length : n=2m-1 (7) • # of message bits: k=m (3) • Minimun distance dmin=2m-1

  28. Primitive Polynomial • A irreducible polynomial (a factor) of xn+1 • With degree m • Satisfy: n=2m-1 Example: Hamming code

  29. 1 2 3 LFSR ExampleSimple Style RiGister (SSRG) Configuration 10111001011100

  30. 1 2 3 LFSR ExampleMulti-module Style (MSRG) …100 1100100…

  31. Properties of PN Sequence

  32. Properties of PN Sequence

  33. Properties of PN Sequence • Relatively poor cross-correlation property • # of codes for a fixed length is relatively small

  34. Preferred Pair of Primitive Polynomials

  35. Gold Sequences

  36. Gold Sequences

  37. 0 1 1 0 1 1 0 0 0 0 0 0 1 1 1 0 1 0 0 0 0 0 0 0 1 1 0 1 1 0 1 0 1 1 1 0 0 0 0 0 0 0 1 1 0 0 0 0 0 1 0 0 1 0 0 1 0 1 1 1 0 1 1 1 0 1 0 0 0 0 0 0 1 1 0 1 0 1 0 0 0 0 0 0 1 1 0 0 0 0 1 0 1 1 1 0 Example of Correlation Spreading Code A Spreading Code A one data bit duration one data bit duration Spreading Code A Spreading Code B 0+0=0 1+0=1 0+1=1 1+1=0 Self-Correlation for each code is 1. Cross-Correlation between Code A and Code B = 6/16

  38. 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

  39. Wash codes

  40. Orthogonal Variable Spreading Factor Codes

  41. 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

  42. Ａ Ｂ A A Less Interference for A station Signal for B Station (after de-spreading) 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. Forward Link (Down Link) Synchronous Chip Timing

  43. Asynchronous DS-CDMA 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. In asynchronous CDMA system, orthogonal codes produce bad cross-correlation.

  44. Features of CDMA

  45. 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.

  46. path-1 Power path-2 path-3 Path Delay path-1 Power path-3 Path Delay path-2 Power path-1 Power path-2 Path Delay Fading in CDMA System ... RAKE receiver. interference produced by path-2 and path-3 CDMA Receiver CODE A with timing of path-1 Synchronization Adder CDMA Receiver ••• CODE A with timing of path-2 •••

  47. Near-Far Problem Lp-a A CDMA Receiver Demodulated DATA Lp-b B CODE A • Desired Signal Power = P/Lp-a • Interfered Signal Power = P/Lp-b/G G: processing gain • 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.

  48. Ａ Ｂ Power Control... from A from B Detected Power Time

  49. （（（ 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

  50. switching Cell A Cell B Handover (1/2) • Handover : • 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 Handover : • In FDMA or TDMA cellular system, a new communication link is established after breaking the current communication link by hard handover. • Communication between MS and BS instantaneously breaks by switching a frequency or a time slot. Hard handover: make connection (new cell B) after break (old cell A)

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