W-CDMA for UMTS – Principles - Startseite TU Ilmenau · PDF fileW-CDMA for UMTS –...

W-CDMA for UMTS – Principles

IntroductionCDMA Background/ HistoryKey Parameters

Code Division Multiple Access (CDMA)Why CDMA ?CDMA Principles / Spreading CodesMulti-path Radio Channel and Rake Receiver

Problems to SolveMacro Diversity and Soft HandoverNear-Far Problem and Power Control

UMTS General RequirementsFDD vs. TDDSpectrum Allocation

UMTS Networks 2Andreas Mitschele-Thiel, Jens Mueckenheim Nov. 2011

References

H. Holma, A. Toskala (Ed.), “WCDMA for UMTS”, 5th edition, Wiley, 2010.T. Benkner, C. Stepping, UMTS – Universal Mobile Telecommunications System,

J. Schlembach Fachverlag, 2002.A.J. Viterbi, “CDMA, Principles of Spread Spectrum Communication”, Addison-

Wesley, 1995.R.L. Peterson, R.E. Ziemer, D.E. Borth, “Introduction to Spread Spectrum

Communications”, Prencice-Hall, 1995.T. Ojanperä, R. Prasad, “Wideband CDMA for Third Generation Mobile

Communication”, Artech House, 1998.R. Prasad, W. Mohr, W. Konhäuser, “Third Generation Mobile Communications

Systems”, Artech House, March 2000.


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)

1992–1995: RACE project CODIT (UMTS Code Division Testbed, PKI, Ericsson, Telia, etc.)

Wide-band CDMA Era

1995–1999: 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


W-CDMA for UMTS – Summary of Key Parameters

Multiple-Access DS-CDMA (TD-CDMA)Duplex scheme FDD (TDD)

Chip rate 3.84 MChip/s (TDD: 1.28/ 3.84/ 7.68 MChip/s)

Carrier spacing Flexible in the range 4.6 – 5.0 MHz (200 kHz carrier raster)

Frequency bands 1920 – 1980 / 2110 – 2170 paired (FDD)1900 – 1920 and 2010 – 2025 unpaired (TDD)

Frame length 10 ms / (15 time slots) Inter-BS synchronization

FDD mode: No accurate synchronization neededTDD mode: Synchronization needed

Multi-rate/Variable-rate scheme

Variable-spreading factor + Multi-codeSpreading factor: 4 – 256 (FDD) and 1 – 16 (TDD)

Channel coding scheme

Convolutional coding (rate 1/2 – 1/3)Turbo coding


CDMA Key Characteristics

Based upon spread spectrum technique developed for military anti-jam applicationsWide bandwidth needed to support high bit rates and to combat fading in multi-path radio channelsMany users share the same RF carrierEach user is assigned a unique random code different to and approximately orthogonal to other codesInterference limited systems; quality degrades as number of users on a channel (carrier) increasesSpreading codes keep channels apart such that the same carrier can be used in the next cell (frequency re-use is 1)


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 XORs the signal with this random numberthe 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 neededhuge code space (e.g. 232) compared to frequency spaceinterference (e.g. white noise) is not codedforward 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)


Spread Spectrum Technology

Problem of radio transmission: frequency dependent fading can wipe out narrow band signals for duration of the interferenceSolution: spread the narrow band signal into a broad band signal using a special code

protection against narrow band interference

Side effects:coexistence of several signals without dynamic coordinationtap-proof

Alternatives: Direct Sequence (UMTS)Frequency Hopping (slow FH: GSM, fast FH: Bluetooth)

detection atreceiver

interferencespread signal

signal (despreaded)

spreadinterference

f f

power power


Spreading and Frequency Selective Fading

FDMA: Relatively small bandwidth on each channel

Guard bands to avoid interference between the usersChannels maybe (temporary) unavailable due to channel selective fading

CDMA: relatively large bandwidth of the spread signal

Frequency selective fading causes only some reduction in the level of the received signalUsers are separated by the spreading sequence

22

22

2

frequency

channelquality

1

spreadsignals

frequency

channelquality

1 23

4

5 6

small bandwidth guard band


DSSS (Direct Sequence Spread Spectrum) I

XOR of the signal with pseudo-random number (code sequence)

Many chips per bit (e.g., 128) result in higher bandwidth of the signal

Spreading factor SF: ratio between chip rate RC and data rate Rb

RC = Rb · SFtb = tC · SF

Processing GainGS = 10 · log10(SF)

user data

codesequence

resultingsignal

0 1

0 1 1 0 1 0 1 01 0 0 1 11

XOR

0 1 1 0 0 1 0 11 0 1 0 01

=

tb

tc

tb: bit durationtc: chip duration

(data rate)

(chip rate)

(chip rate)


DSSS (Direct Sequence Spread Spectrum) II

Xuser data

codesequence

modulator

radiocarrier

spreadspectrumsignal

transmitsignal

transmitter

demodulator

receivedsignal

radiocarrier

X

codesequence

basebandsignal

receiver

integrator

products

decisiondata

sums

correlator


CDMA Principle (Downlink)

Code 0

Code 1

Code 2

data 0

data 1

data 2

Code 0

Code 1

Code 2

data 0

data 1

data 2

sender (base station) receiver (terminal)

Transmission overair interface


CDMA Principle (Uplink)

Code 0

Code 1

Code 2

data 0

data 1

data 2

Code 0

Code 1

Code 2

data 0

data 1

data 2

sender (terminal) receiver (base station)

transmission overair interface


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 codesNo need for precise synchronization as the scrambling codes remain quasi-orthogonal

data1 data2 data3

scramblingcode1

chan.code3

chan.code2

chan.code1

data4 data5

chan.code4

chan.code1

sender1 sender2

scramblingcode2


Functionality of Channelization and Scrambling Codes

Channelization Code Scrambling CodeUsage UL: Separation of physical data

(DPDCH) and control channels (DPCCH) from same terminalDL: Separation of DL connections to different users within one cell

UL: Separation of terminals

DL: Separation of sectors/cells

Length 4 – 256 chips (1.0 – 66.7 us) UL+DL: 10ms = 38400 chips

Number of codes Number of codes under 1 scrambling code = spreading factor (SF)

UL: several millionsDL: 256

Code Family Orthogonal Variable Spreading Factor

Long 10 ms code: Gold code

Spreading Yes, increases transmission bandwidth

No, does not affect transmission bandwidth


OVSF-Coding Tree

1

1,1

1,-1

1,1,1,1

1,1,-1,-1

X

X,X

X,-X 1,-1,1,-1

1,-1,-1,11,-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,-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

SF=n SF=2n

...

...

...

...

In UMTS, spreading factors (SF) from 4 – 512 (DL) / 4 – 256 (UL) are used:

4 x SF4, 8 x SF8 …………………… 256 x SF256, 512 x SF512


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.)

512 7.5 15 3-6 1-3 kbps

256 15 30 12-24 6-12 kbps

...

16 240 480 432 215 kbps8 480 960 912 456 kbps

4 960 1920 1872 936 kbps

4, with 3 parallel codes

2880 5760 5616 2.3 Mbps


CDMA in Theory

Sender A sends Ad = 1, code sequence Ac = 1010011 (assign: “0”= –1, “1”= +1)sending signal As = Ad Ac = (+1, –1, +1, –1, –1, +1, +1)

Sender Bsends Bd = 0, code sequence Bc = 0110101sending signal Bs = Bd Bc = (+1, –1, –1, +1, –1, +1, –1)

Both signals superimpose in space interference neglected (noise etc.)As + Bs = (+2, –2, 0, 0, –2, +2, 0)

Receiver wants to receive signal from sender Aapply sequence AC chipwise (inner product)

Ar = (+2, –2, 0, 0, –2, +2, 0) Ac = 2 + 2 + 0 + 0 + 2 + 2 + 0 = 8result greater than 0, therefore, original bit was „1“

receiving BBe = (+2, –2, 0, 0, –2, +2, 0) Bc = –2 –2 + 0 + 0 – 2 – 2 + 0 = –8, i.e. „0“

wrong sequence CC = 1100110Cr = (+2, –2, 0, 0, –2, +2, 0) Cc = 0, decision impossible


CDMA on signal level I

data A

key A

signal A

data key

keysequence A

Real systems use much longer keys resulting in a larger distance between single code words in code space

1 0 1

10 0 1 0 0 1 0 0 0 1 0 1 1 0 0 1 101 1 0 1 1 1 0 0 0 1 0 0 0 1 1 0 0

Ad

Ak

As


CDMA on signal level II

signal A

data B

key Bkey

sequence B

signal B

As + Bs

data key

1 0 0

00 0 1 1 0 1 0 1 0 0 0 0 1 0 1 1 111 1 0 0 1 1 0 1 0 0 0 0 1 0 1 1 1

Bd

Bk

Bs

As

10-1


CDMA on signal level III

Ak

(As + Bs) * Ak

integratoroutput

comparatoroutput

As + Bs

data A

1 0 1

1 0 1 Ad

10-1

1

-1

10-1


CDMA on signal level IV

integratoroutput

comparatoroutput

Bk

(As + Bs) * Bk

As + Bs

data B

1 0 0

1 0 0 Bd

10-11

-110-1


comparatoroutput

CDMA on signal level V

wrongkey K

integratoroutput

(As + Bs) * K

As + Bs

(0) (0) ?

Assumptionsorthogonality of keysneglectance of noiseno differences in signal level => precise power control

10-11

-1

10-1


Properties of Spreading Sequences

Cross correlation function (CCF)

Auto correlation function (ACF)

Code sequence #1

Code sequence #2

Required properties of spreading(properties of the transmitted signals):

• High ACF peak• Low ACF sidelobe

inter-symbol interference (ISI)• Low CCF

multi-user interference (MUI)


Multi-path Transmission

Multi-path components can be resolved due to ACF of codes

Spreader

SpreadingSequence c(t)

Despreader(Correlator)

SpreadingSequence c(t-Td)

Receiver synchronizes to each multi-path component for de-spreading


RAKE Receiver

Correlate and track each multi-path component separately

Optimal coherent combining

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. 200 ns


RAKE Receiver – Practical Realization


Macro-Diversity & Soft Handover

Optimal coherent combiningin the RAKE receiver (at MS)

NodeB 1NodeB 2

UE


Multi-user CDMA

Conventional CDMA Receiver (Base Station):

Despreader(Correlator)

SpreadingSequence c2(t-Td2)

• coherent (amplitude and phase) RF demodulation at base station

• separate despreading and demodulation ofeach signal at base station

• one Rake receiver with K fingers per user• unsynchronized transmission between the

mobiles

SpreadingSequence c1(t-Td1)

SpreadingSequence cn(t-Tdn)


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

The problem can be resolved through dynamic power control to equalize all received power levels

AND/OR

By means of joint multi-user detection

Near-Far Problem – Power Control

NodeB

UE 1

UE 2


Interference Cancellation

Multi-user Interference Cancellation (Joint Detection):

Matched Filter toSequence c1(t)

Detection mechanism takes into account interference from other users as all signalsare known in the receiver(known interference can becanceled)

Matched Filter toSequence c2(t)

Matched Filter toSequence cn(t)

MF1

MF2

MFn

Multi-userDetector

(Joint Detection

Interference Cancellation)


Interference Cancellation – Realization

Subtractive interference cancellation


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-linkBetter 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-linkStrict 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)


FDD vs. TDD Mode (contd.)

TDD-Mode

FDD-Mode(one direction)


TDD Mode Switching

1 Frame (10ms) of 15 Slots

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


Global Spectrum Allocations for IMT-2000

ITU2010 20251980

MSS MSS*1930

IMT-2000 MSSMSS*IMT-2000

2160 2170 2200 MHz

*Region2

1885 2110

PHS

20101980 2025Japan

2110 22002170

IMT-2000 MSSMSSIMT-2000

18951885 1918.1 MHz

1980 2110 22002170

IMT-2000 MSS

19001880

DECT

2010

MSSIMT-2000

2025 MHz

Europe

2110 220021652150

Reserve MSSBroadcast Auxilary

1910 1930 1990 2025

MSS

1850

PCS*PCSA B CD E F

PCSA B CD E F

MHz

USA

20101980 2025

China2110 22002170

MSSMSS

1900 1920 MHz1865 1880 1945 1960

CDMA FDD-WLL

FDD-WLLCDMA

TDD-WLL

MSS: Mobile Satellite Services


UMTS Spectrum

2200 MH

z

2000 MH

z

2100 MH

z

1900 MH

z

Unpaired Band: 20 + 15MHz (1900-1920 and 2010-2025MHz) for TDD

Paired Band: 2 x 60MHz (1920-1980 and 2110-2170MHz) for FDD

Up-link Down-link

Satellite Band: 2 x 30MHz (1980-2010 and 2170-2200MHz)

1 2 3 11 12. . .

1920 MHz 1980 MHz

1 2 3 11 12. . .

2110 MHz 2170 MHz

5 MHz

Uplink Downlink

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