W-CDMA for UMTS Principles

Transcription

1 W-CDMA for UMTS Principles Introduction CDMA Background/ History Code Division Multiple Access (CDMA) Why CDMA? CDMA Principles / Spreading Codes Multi-path Radio Channel and Rake Receiver Problems to Solve Macro Diversity and Soft Handover Near-Far Problem and Power Control UMTS General Requirements FDD vs. TDD Key Parameters Spectrum Allocation

2 CDMA History Pioneer Era (Spread Spectrum) 40s and 50s: Spread Spectrum technique for military anti-jam applications 1949: Claude 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 Qualcomm) : 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 multi-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 2

3 Spread Spectrum Technology Problem of radio transmission: frequency dependent fading can wipe out narrow band signals for duration of the interference Solution: spread the narrow band signal into a broad band signal using a special code protection against narrow band interference power interference spread signal power detection at receiver signal (despreaded) spread interference Side effects: f coexistence of several signals without dynamic coordination signal can only be detected if the spreading process is known f Alternatives: Direct Sequence (UMTS) Frequency Hopping (slow FH: GSM, fast FH: Bluetooth) 3

4 Spreading and Frequency Selective Fading FDMA: Relatively small bandwidth on each channel Guard bands to avoid interference between the users Channels maybe (temporary) unavailable due to channel selective fading channel quality 1 small bandwidth guard band 5 6 frequency CDMA: relatively large bandwidth of the spread signal Frequency selective fading causes only some reduction in the level of the received signal Users are separated by the spreading sequence channel quality spread signals frequency 4

5 CDMA Multiple Access CDMA (Code Division Multiple Access) all terminals send on the same frequency probably at the same time and can use the whole bandwidth of the transmission channel each sender has a unique random number (spreading sequence), the sender modulates the signal with this random number the receiver can tune into this signal if it knows the pseudo random number, tuning is done via a correlation function Advantages: all terminals can use the same frequency, less planning needed huge code space (e.g ) compared to frequency space interference is not coded (acts like white noise) forward error correction and encryption can be easily integrated Disadvantages: higher complexity of a receiver (receiver cannot just listen into the medium and start receiving if there is a signal) all signals should have the same strength at a receiver (power control) 5

6 CDMA Multiple Access (contd.) Principle of CDMA Communication 6

7 DSSS (Direct Sequence Spread Spectrum) I Modulation of the signal with pseudorandom number (code sequence) Many chips per bit (e.g. 128) result in higher bandwidth of the signal T s 1 0 user data (data rate) Spreading factor SF: ratio between chip rate R C and data symbol rate R S R C = R S SF T C = T S / SF Processing Gain G S = 10 log 10 (SF) T c code sequence (chip rate) = resulting signal (chip rate) 7

8 DSSS (Direct Sequence Spread Spectrum) II user data X spread spectrum signal modulator transmit signal code sequence radio carrier transmitter correlator received signal demodulator baseband signal products X sums integrator decision data radio carrier code sequence receiver 8

9 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 9

10 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 10

11 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 unique, quasi-orthogonal scrambling codes Simple code management: each station can reuse the same orthogonal channelization codes No need for precise synchronization as the scrambling codes remain quasi-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 11

12 Functionality 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 users within one cell UL: Separation of terminals DL: Separation of sectors/cells Length chips ( µs) UL+DL: 10ms = chips Number of codes Number of codes under 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 12

13 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 used: 4 x SF4, 8 x SF8 256 x SF256, 512 x SF512 13

14 Downlink Dedicated Channel Symbol and Bit Rates Spreading factor Channel symbol rate (kbps) Channel bit rate (kbps) DPDCH channel bit rate range (kbps) Maximum user data rate with 1/2-rate coding (approx.) kbps kbps kbps kbps kbps 4, with 3 parallel codes Mbps 14

15 CDMA in Theory Sender A sends A d = 1, code sequence A c = sending signal A s = A d A c = (+1, 1, +1, 1, 1, +1, +1) Sender B sends B d = 1, code sequence B c = sending signal B s = B d B c = (+1, 1, 1, +1, 1, +1, 1) Both signals superimpose 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 sequence A C chipwise (inner product) A r = (+2, 2, 0, 0, 2, +2, 0) A c = = 8 result greater than 0, therefore, original bit was 1 receiving B B r = (+2, 2, 0, 0, 2, +2, 0) B c = = 8, i.e. 1 wrong sequence C C = C r = (+2, 2, 0, 0, 2, +2, 0) C c = 0, decision impossible 15

16 CDMA on signal level I data A A d code A A c signal A A s Real systems use much longer keys resulting in a larger distance between single code words in code space 16

17 CDMA on signal level II signal A A s data B B d code B B c signal B A s + B s B s

18 CDMA on signal level III data A A s + B s A d A c (A s + B s ) A c integrator output comparator output

19 CDMA on signal level IV data B A s + B s B d B c 1 (A s + B s ) B c integrator output comparator output

20 CDMA on signal level V A s + B s wrong code C (A s + B s ) C integrator output comparator output (1) (1)? Assumptions orthogonality of keys negligance of noise no differences in signal level => precise power control 20

21 Properties of Spreading Sequences Code sequence #1 Auto correlation function (ACF) Code sequence #2 Required properties of spreading (properties of the transmitted signals): Cross correlation function (CCF) High ACF peak Low ACF sidelobe inter-symbol interference (ISI) Low CCF multi-user interference (MUI) 21

22 Multi-path Transmission Multi-path components can be resolved due to ACF of codes Spreader Despreader (Correlator) Spreading Sequence c(t) Spreading Sequence c(t Td) Receiver synchronizes to each multi-path component for de-spreading Td 22

23 RAKE Receiver Correlate and track each multi-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 multi-path components time resolution in UMTS approx. 260 ns Optimal coherent combining 23

24 RAKE Receiver Practical Realization 24

25 Macro-Diversity & Soft Handover NodeB 1 NodeB 2 UE Optimal coherent combining in the RAKE receiver (at MS) 25

26 Multi-user CDMA Conventional CDMA Receiver (Base Station): Despreading (Correlator) RAKE 1 RAKE 2 Spreading Sequence c 1 (t-t d1 ) Spreading Sequence c 2 (t-t d2 ) coherent (amplitude and phase) RF demodulation at base station separate despreading and demodulation of each signal at base station one Rake receiver with K fingers per user unsynchronized transmission between the mobiles RAKE n Spreading Sequence c n (t-t dn ) 26

27 Near-Far Problem Power Control UE 1 Near-Far Problem: Spreading sequences are not orthogonal (multi-user interference) Near mobile dominate Signal to interference ratio is lower for far mobiles and performance degrades NodeB The problem can be resolved through dynamic power control to equalize all received power levels AND/OR UE 2 By means of joint multi-user detection 27

28 Interference Cancellation Multi-user Interference Cancellation (Joint Detection): Despreading (Correlator) RAKE 1 RAKE 2 c 1 (t T d1 ) c 2 (t T d2 ) Multi-user Detector (Joint Detection/ Interference Cancellation) Detection mechanism takes into account interference from other users as all signals are known in the receiver (known interference can be canceled) RAKE n c n (t T dn ) 28

29 Interference Cancellation Realization Subtractive interference cancellation 29

30 FDD vs. TDD Mode UMTS supports FDD and TDD FDD mode: Multiple access scheme: DS-CDMA (Direct Sequence-CDMA) Symmetric capacity of up- and down-link Better suited for low bit rate transmission in larger cells (no timing advance, no synchronization from MS required) TDD mode: Multiple access scheme: TD-CDMA (JD-CDMA) Asymmetric capacity allocation for up- and down-link Strict synchronization required for MS (timing advance) Relaxed power control and near-far resistance by the use of intra-cell multi-user interference cancellation (spreading factor 1 16) 30

31 FDD vs. TDD Mode (contd.) FDD-Mode (one direction) TDD-Mode 31

32 TDD Mode Switching multiple switching points, symmetric DL/UL allocation multiple 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 32

33 W-CDMA for UMTS Summary of Key Parameters Multiple-Access Duplex scheme Chip rate Carrier spacing Frequency bands Frame length Inter-BS synchronization Multi-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 unpaired (TDD) 10 ms / (15 time slots) FDD mode: No accurate synchronization needed TDD mode: Synchronization needed Variable-spreading factor + Multi-code Spreading factor: (FDD) and 1 16 (TDD) Convolutional coding (rate 1/2 1/3) Turbo coding 33

34 Global Spectrum Allocations for IMT-2000 ITU IMT-2000 MSS MSS* IMT-2000 MSS* MSS *Region MHz Europe 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 Auxilary A D B E F C A D B E F C Reserve MSS MHz MSS: Mobile Satellite Services 34

35 UMTS Spectrum 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) 35

36 References H. Holma, A. Toskala (Ed.), WCDMA for UMTS, 5th edition, Wiley, A.J. Viterbi, CDMA, Principles of Spread Spectrum Communication, Addison- Wesley, R.L. Peterson, R.E. Ziemer, D.E. Borth, Introduction to Spread Spectrum Communications, Prentice-Hall, T. Ojanperä, R. Prasad, Wideband CDMA for Third Generation Mobile Communication, Artech House, R. Prasad, W. Mohr, W. Konhäuser, Third Generation Mobile Communications Systems, Artech House, March