W-CDMA for UMTS Principles

Transcription

1 W-CDMA for UMTS Principles Introdction CDMA Backgrond/ History Code Division Mltiple Access (CDMA) Why CDMA? CDMA Principles / Spreading Codes Mlti-path Radio Channel and Rake Receiver Problems to Solve Macro Diversity and Soft Handover Near-Far Problem and Power Control UMTS General Reqirements FDD vs. TDD Key Parameters Spectrm Allocation

2 References H. Holma, A. Toskala (Ed.), WCDMA for UMTS, 5th edition, Wiley, A.J. Viterbi, CDMA, Principles of Spread Spectrm Commnication, Addison- Wesley, R.L. Peterson, R.E. Ziemer, D.E. Borth, Introdction to Spread Spectrm Commnications, Prentice-Hall, T. Ojanperä, R. Prasad, Wideband CDMA for Third Generation Mobile Commnication, Artech Hose, R. Prasad, W. Mohr, W. Konhäser, Third Generation Mobile Commnications Systems, Artech Hose, March

3 CDMA History Pioneer Era (Spread Spectrm) 40s and 50s: Spread Spectrm techniqe for military anti-jam applications 1949: Clade Shannon and Robert Pierce develop basic ideas of CDMA 1970s: Several developments for military systems (e.g. GPS) Narrow-band CDMA Era 1993: IS-95 standard (mainly driven by Qalcomm) : RACE project CODIT (UMTS Code Division Testbed, PKI, Ericsson, Telia, etc.) Wide-band CDMA Era : ACTS project FRAMES: FMA Mode 1 (TD/CDMA), FMA Mode 2 (W-CDMA) 1995: cdma2000 1x/ 3x (USA) 1998: UMTS (Rel.-99): FDD and TDD mode 1999: Harmonization: W-CDMA, TD-CDMA and mlti-carrier CDMA (chip rate: 3.84 Mchip/sec) 1999: Narrowband TDD mode (TD-SCDMA), chip rate: 1.28 Mchip/sec High-Speed CDMA Era since 2000: HSDPA (Rel.-5/ 2000), E-DCH (Rel.-6/ 2002), HSPA+ (Rel.-7/ 2005) cdma2000 1x EV-DO/DV 3

4 Spread Spectrm Technology Problem of radio transmission: freqency dependent fading can wipe ot narrow band signals for dration of the interference Soltion: spread the narrow band signal into a broad band signal sing a special code Þ protection against narrow band interference power interference spread signal power detection at receiver signal (despreaded) spread interference Side effects: f coexistence of several signals withot dynamic coordination tap-proof f Alternatives: Direct Seqence (UMTS) Freqency Hopping (slow FH: GSM, fast FH: Bletooth) 4

5 Spreading and Freqency Selective Fading FDMA: Relatively small bandwidth on each channel Gard bands to avoid interference between the sers Channels maybe (temporary) navailable de to channel selective fading channel qality 1 small bandwidth gard band 5 6 freqency CDMA: relatively large bandwidth of the spread signal Freqency selective fading cases only some redction in the level of the received signal Users are separated by the spreading seqence channel qality spread signals freqency 5

6 CDMA Mltiple Access CDMA (Code Division Mltiple Access) all terminals send on the same freqency probably at the same time and can se the whole bandwidth of the transmission channel each sender has a niqe random nmber (spreading seqence), the sender XORs the signal with this random nmber the receiver can tne into this signal if it knows the psedo random nmber, tning is done via a correlation fnction Advantages: all terminals can se the same freqency, less planning needed hge code space (e.g ) compared to freqency space interference (e.g. white noise) is not coded forward error correction and encryption can be easily integrated Disadvantages: higher complexity of a receiver (receiver cannot jst listen into the medim and start receiving if there is a signal) all signals shold have the same strength at a receiver (power control) 6

7 CDMA Mltiple Access (contd.) Principle of CDMA Commnication 7

8 DSSS (Direct Seqence Spread Spectrm) I XOR of the signal with psedo-random nmber (code seqence) Many chips per bit (e.g., 128) reslt in higher bandwidth of the signal Spreading factor SF: ratio between chip rate R C and data rate R b R C = R b SF t b = t C SF Processing Gain G S = 10 log 10 (SF) t b 0 1 t c t b : bit dration t c : chip dration ser data (data rate) XOR code seqence (chip rate) = reslting signal (chip rate) 8

9 DSSS (Direct Seqence Spread Spectrm) II ser data X spread spectrm signal modlator transmit signal code seqence radio carrier transmitter correlator received signal demodlator baseband signal prodcts X sms integrator decision data radio carrier code seqence receiver 9

10 CDMA Principle (Downlink) sender (base station) receiver (terminal) Code 0 Code 0 data 0 Code 1 Code 1 data 0 data 1 S Transmission over air interface data 1 Code 2 Code 2 data 2 data 2 10

11 CDMA Principle (Uplink) sender (terminal) receiver (base station) data 0 Code 0 Code 1 transmission over air interface Code 0 Code 1 data 0 data 1 Code 2 S Code 2 data 1 data 2 data 2 11

12 UMTS Spreading Constant chip-rate of 3.84 Mchip/s (FDD) Variable data rates are realized by different spreading factors of the orthogonal channelization codes Higher data rates: less chips per bit (and vice-versa) Senders are separated by niqe, qasi-orthogonal scrambling codes Simple code management: each station can rese the same orthogonal channelization codes No need for precise synchronization as the scrambling codes remain qasi-orthogonal data 1 data 2 data 3 data 4 data 5 chan. code 1 chan. code 2 chan. code 3 chan. code 1 chan. code 4 scrambling code 1 scrambling code 2 sender 1 sender 2 12

13 Fnctionality of Channelization and Scrambling Codes Channelization Code Scrambling Code Usage UL: Separation of physical data (DPDCH) and control channels (DPCCH) from same terminal DL: Separation of DL connections to different sers within one cell UL: Separation of terminals DL: Separation of sectors/cells Length chips ( µs) UL+DL: 10ms = chips Nmber of codes Nmber of codes nder 1 scrambling code = spreading factor (SF) UL: several millions DL: 256 Code Family Spreading Orthogonal Variable Spreading Factor Yes, increases transmission bandwidth Long 10 ms code: Gold code No, does not affect transmission bandwidth 13

14 OVSF-Coding Tree X 1,1,1,1 1,1 1,1,-1,-1 X,X 1 X,-X 1,-1,1,-1 1,1,1,1,1,1,1,1 1,1,1,1,-1,-1,-1,-1 1,1,-1,-1,1,1,-1,-1 1,1,-1,-1,-1,-1,1,1 1,-1,1,-1,1,-1,1, SF=n SF=2n 1,-1 1,-1,-1,1 1,-1,1,-1,-1,1,-1,1 1,-1,-1,1,1,-1,-1,1 1,-1,-1,1,-1,1,1,-1... SF=1 SF=2 SF=4 SF=8 In UMTS, spreading factors (SF) from (DL) / (UL) are sed: 4 x SF4, 8 x SF8 256 x SF256, 512 x SF512 14

15 Downlink Dedicated Channel Symbol and Bit Rates Spreading factor Channel symbol rate (kbps) Channel bit rate (kbps) DPDCH channel bit rate range (kbps) Maximm ser data rate with 1/2-rate coding (approx.) kbps kbps kbps kbps kbps 4, with 3 parallel codes Mbps 15

16 CDMA in Theory Sender A sends A d = 1, code seqence A c = (assign: 0 = 1, 1 = +1) sending signal A s = A d A c = (+1, 1, +1, 1, 1, +1, +1) Sender B sends B d = 0, code seqence B c = sending signal B s = B d B c = (+1, 1, 1, +1, 1, +1, 1) Both signals sperimpose in space interference neglected (noise etc.) A s + B s = (+2, 2, 0, 0, 2, +2, 0) Receiver wants to receive signal from sender A apply seqence A C chipwise (inner prodct) A r = (+2, 2, 0, 0, 2, +2, 0) A c = = 8 reslt greater than 0, therefore, original bit was 1 receiving B B e = (+2, 2, 0, 0, 2, +2, 0) B c = = 8, i.e. 0 wrong seqence C C = C r = (+2, 2, 0, 0, 2, +2, 0) C c = 0, decision impossible 16

17 CDMA on signal level I data A key A key seqence A data Å key A d A k signal A A s Real systems se mch longer keys reslting in a larger distance between single code words in code space 17

18 CDMA on signal level II signal A A s data B B d key B key seqence B data Å key B k signal B A s + B s B s

19 CDMA on signal level III data A A s + B s A d A k (A s + B s ) * A k integrator otpt comparator otpt

20 CDMA on signal level IV data B A s + B s B d B k (A s + B s ) * B k integrator otpt comparator otpt

21 CDMA on signal level V A s + B s wrong key K (A s + B s ) * K integrator otpt comparator otpt (0) (0)? Assmptions orthogonality of keys neglectance of noise no differences in signal level => precise power control 21

22 Properties of Spreading Seqences Code seqence #1 Ato correlation fnction (ACF) Code seqence #2 Reqired properties of spreading (properties of the transmitted signals): Cross correlation fnction (CCF) High ACF peak Low ACF sidelobe inter-symbol interference (ISI) Low CCF mlti-ser interference (MUI) 22

23 Mlti-path Transmission Mlti-path components can be resolved de to ACF of codes Spreader Despreader (Correlator) Spreading Seqence c(t) Spreading Seqence c(t-td) Receiver synchronizes to each mlti-path component for de-spreading 23

24 RAKE Receiver Correlate and track each mlti-path component separately RAKE receiver with K fingers trackers: independent tracking of dominant paths searchers: scan a time window to search (the pilot channel) for dominant mlti-path components time resoltion in UMTS approx. 200 ns Optimal coherent combining 24

25 RAKE Receiver Practical Realization 25

26 Macro-Diversity & Soft Handover NodeB 1 NodeB 2 UE Optimal coherent combining in the RAKE receiver (at MS) 26

27 Mlti-ser CDMA Conventional CDMA Receiver (Base Station): Despreading (Correlator) RAKE 1 RAKE 2 Spreading Seqence c 1 (t-t d1 ) Spreading Seqence c 2 (t-t d2 ) coherent (amplitde and phase) RF demodlation at base station separate despreading and demodlation of each signal at base station one Rake receiver with K fingers per ser nsynchronized transmission between the mobiles RAKE n Spreading Seqence c n (t-t dn ) 27

28 Near-Far Problem Power Control UE 1 Near-Far Problem: Spreading seqences are not orthogonal (mlti-ser interference) Near mobile dominate Signal to interference ratio is lower for far mobiles and performance degrades NodeB The problem can be resolved throgh dynamic power control to eqalize all received power levels AND/OR UE 2 By means of joint mlti-ser detection 28

29 Interference Cancellation Mlti-ser Interference Cancellation (Joint Detection): Despreading (Correlator) RAKE 1 RAKE 2 c 1 (t-t d1 ) c 2 (t-t d2 ) Mlti-ser Detector (Joint Detection/ Interference Cancellation) Detection mechanism takes into accont interference from other sers as all signals are known in the receiver (known interference can be canceled) RAKE n c n (t-t dn ) 29

30 Interference Cancellation Realization Sbtractive interference cancellation 30

31 FDD vs. TDD Mode UMTS spports FDD and TDD FDD mode: Mltiple access scheme: DS-CDMA (Direct Seqence-CDMA) Symmetric capacity of p- and down-link Better sited for low bit rate transmission in larger cells (no timing advance, no synchronization from MS reqired) TDD mode: Mltiple access scheme: TD-CDMA (JD-CDMA) Asymmetric capacity allocation for p- and down-link Strict synchronization reqired for MS (timing advance) Relaxed power control and near-far resistance by the se of intra-cell mlti-ser interference cancellation (spreading factor 1-16) 31

32 FDD vs. TDD Mode (contd.) FDD-Mode (one direction) TDD-Mode 32

33 TDD Mode Switching mltiple switching points, symmetric DL/UL allocation mltiple switching points, asymmetric DL/UL allocation single switching point, symmetric DL/UL allocation single switching point, asymmetric DL / UL allocation 1 Frame (10ms) of 15 Slots 33

34 W-CDMA for UMTS Smmary of Key Parameters Mltiple-Access Dplex scheme Chip rate Carrier spacing Freqency bands Frame length Inter-BS synchronization Mlti-rate/ Variable-rate scheme Channel coding scheme DS-CDMA (TD-CDMA) FDD (TDD) 3.84 MChip/s (TDD: 1.28/ 3.84/ 7.68 MChip/s) Flexible in the range MHz (200 khz carrier raster) / paired (FDD) and npaired (TDD) 10 ms / (15 time slots) FDD mode: No accrate synchronization needed TDD mode: Synchronization needed Variable-spreading factor + Mlti-code Spreading factor: (FDD) and 1 16 (TDD) Convoltional coding (rate 1/2 1/3) Trbo coding 34

35 Global Spectrm Allocations for IMT-2000 ITU IMT-2000 MSS MSS* IMT-2000 MSS* MSS *Region MHz Erope 1880 DECT 1900 IMT-2000 IMT-2000 MSS MSS MHz Japan PHS IMT-2000 MSS IMT-2000 MSS MHz China FDD- CDMA WLL TDD- WLL CDMA FDD- WLL MSS MSS MHz USA PCS PCS* PCS MSS Broadcast Axilary A D B EFC A D B EFC Reserve MSS MHz MSS: Mobile Satellite Services 35

36 UMTS Spectrm 1900 MHz Up-link 2000 MHz 2100 MHz Down-link 2200 MHz Unpaired Band: MHz ( and MHz) for TDD Paired Band: 2 x 60MHz ( and MHz) for FDD Details: Uplink 1920 MHz 1980 MHz Downlink 2110 MHz 2170 MHz MHz Satellite Band: 2 x 30MHz ( and MHz) 36