W-CDMA for UMTSCDMA for UMTS – · PDF fileW-CDMA for UMTSCDMA for UMTS –...

W-CDMA for UMTS – PrinciplesW CDMA for UMTS Principles

IntroductionIntroductionCDMA BackgroundUMTS StandardizationK P t d D l i M dKey Parameter and Duplexing Modes

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

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

UMTS General RequirementsUMTS General RequirementsFDD vs. TDDSpectrum Allocation

Summary

References

H Holma A Toskala (Ed ) “WCDMA for UMTS” 4th edition Wiley 2007H. Holma, A. Toskala (Ed.), WCDMA for UMTS , 4th edition, Wiley, 2007.

T. Benkner, C. Stepping, UMTS – Universal Mobile Telecommunications System, J. Schelmbach 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 SpectrumR.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 C i i ” A h H 1998Communication”, Artech House, 1998.

R. Prasad, W. Mohr, W. Konhäuser, “Third Generation Mobile Communications Systems”, Artech House, March 2000.y , ,

UMTS Networks 2Andreas Mitschele-Thiel, Jens Mueckenheim Oct. 2008

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 CDMA1949: 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 t d d 2000 1 / 3 (USA)1995-today: cdma2000 1x/ 3x (USA)

1998-today: UMTS (Rel.-99)

High-Speed CDMA Era

2000-today: HSDPA (Rel.-5/ 2000), E-DCH (Rel.-6/ 2002), HSPA+ (Rel.-7/ 2005)

cdma2000 1x EV-DO/DV


UMTS Standardization History

ETSI SMG2 air interface standardization for UMTS:ETSI SMG2 air interface standardization for UMTS:five groups evaluated competing concepts until end of 1997 (research was driven by FRAMES project)

No decision at SMG#24 on Dec 15 1997; result of the vote was 58 45% for W-CDMANo decision at SMG#24 on Dec 15, 1997; result of the vote was 58.45% for W-CDMA, 41.55% for TD-CDMA

Two proposals selected at SMG#24bis on Jan 28, 1998,W-CDMA (61 1%) and TD-CDMA (38 7%)W-CDMA (61.1%) and TD-CDMA (38.7%) consensus decision: W-CDMA for the paired frequency bands, TD-CDMA for the unpaired bands

3rd Generation Partnership Project (3GPP) formed Jan 1999 (ETSI ARIB T1P1 TTA3rd Generation Partnership Project (3GPP) formed Jan 1999 (ETSI, ARIB, T1P1, TTA, …); CDMA proposals: W-CDMA in Europe & Japan (3GPP), and CDMA2000 in the US (3GPP2)

Pressure from ITU and operators to harmonize W-CDMA:Pressure from ITU and operators to harmonize W-CDMA:OHG (Operator Harmonisation Group) proposal accepted by 3GPP on July 6-7, 1999, i.e. CDMA standard with 3 modes (W-CDMA, TD-CDMA and multi-carrier CDMA); chip rate = 3.84Mcpsrate 3.84Mcps

In June 1999 work on narrowband TDD started (TD-SCDMA); main proponent: China; chip rate = 1.28Mcps


W-CDMA for UMTS – Summary of Key Parameters

Multiple-Access DS-CDMA (TD-CDMA) Duplex scheme FDD (TDD) Chip rate 3.84 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)Frame length 10 ms / (15 time slots)Inter-BS synchronization

FDD mode: No accurate synchronization needed TDD mode: Synchronization needed

Multi rate/ Variable spreading factor + Multi codeMulti-rate/ Variable-rate scheme

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

Channel coding Convolutional coding (rate 1/2–1/3)Channel coding scheme

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


FDD-Mode (WCDMA)


TDD-Mode (TD-CDMA)


CDMA Key Characteristics

Based upon spread spectrum technique developed for military anti-jam applications

Wide bandwidth needed to support high bit rates and to combat fading in pp g gmulti-path radio channels

Many users share the same RF carrier

E h i i d i d d diff t t d i t lEach user is assigned a unique random code different to and approximately orthogonal to other codes

Interference limited systems; quality degrades as number of users on a channel (carrier) increase

Spreading codes keep channels apart such that the same carrier can be used in the next cell (frequency re-use is 1)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 ca use t e o e ba d dt o t e t a s ss o c a eeach sender has a unique random number, the sender XORs the signal with this random numberthe receiver can “tune” into this signal if it knows the pseudo randomthe receiver can tune into this signal if it knows the pseudo random number, tuning is done via a correlation function

Advantages: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 codedinterference (e.g. white noise) is not codedforward error correction and encryption can be easily integrated

Disadvantages:Disadvantages:higher complexity of a receiver (receiver cannot just listen into the medium and start receiving if there is a signal)ll i l h ld h th t th t i ( t l)


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 narrowProblem 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 codecode

⇒ protection against narrow band interference

detection at

interferencespread signal

signal (despreaded)

spreadinterference

power power

Side effects:

detection atreceiver

interference

f f

coexistence of several signals without dynamic coordinationtap-proof

Alternatives: Direct Sequence (UMTS)F eq enc Hopping (slo FH GSM fast FH Bl etooth)


Frequency Hopping (slow FH: GSM, fast FH: Bluetooth)

Effects of spreading and interference

Ψff

i) narrow band signal ii) spreaded signal (broadband signal)

Ψff

user signalbroadband interferencenarrowband interference

f f

sender

narrowband interference

iii) addition of interference

iv) despreadedsignal

v) application of bandpass filter

Ψff ΨffΨff

f f f


receiver

DSSS (Direct Sequence Spread Spectrum) I

XOR of the signal with pseudo random number (chipping sequence)XOR of the signal with pseudo-random number (chipping sequence)many chips per bit (e.g., 128) result in higher bandwidth of the signal

Advantagesreduces frequency selective fading

tbg

in cellular networks base stations can use the same frequency range

user data

0 1 XORtc

(data rate)

q y gseveral base stations can detect and recover the signalsoft handover

chipping sequence

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

c

(chip rate)

Disadvantagesprecise power control needed

resultingsignal

0 1 1 0 0 1 0 11 0 1 0 01 (chip rate)precise power control needed 0 1 1 0 0 1 0 11 0 1 0 01

tb: bit periodtc: chip period

( p )


DSSS (Direct Sequence Spread Spectrum) II

user data

spreadspectrumsignal

transmitsignal

Xuser data

chippingsequence

modulator

radiocarrier

signal signal

sequence carrier

transmitter

receivedlowpassfiltered products

d t

sampledsums

correlator

demodulatorsignal

radio

X

chipping

signalintegrator decision

data

carrierpp g

sequence

receiver


Digital modulation

Modulation of digital signals known as Shift KeyingModulation of digital signals known as Shift Keying

Amplitude Shift Keying (ASK):1 0 1

very simplelow bandwidth requirementsvery susceptible to interference

tvery susceptible to interference

Frequency Shift Keying (FSK):

1 0 1

Frequency Shift Keying (FSK):needs larger bandwidth t

Phase Shift Keying (PSK):more complex

1 0 1

more complexrobust against interference t


Advanced Phase Shift Keying

BPSK (Binary Phase Shift Keying):bit value 0: sine wavebit l 1 i t d i

Q

bit value 1: inverted sine wavevery simple PSKlow spectral efficiency

I01

low spectral efficiencyrobust, used e.g. in satellite systems

QPSK (Quadrature Phase Shift Keying):Q (Q y g)

2 bits coded as one symbolsymbol determines shift of sine wave

Q 1110

needs less bandwidth compared to BPSKmore complex

d i UMTS d li k

I

0100used in UMTS downlinkPuls filtering of baseband to avoid sudden phase shifts

=> reduce bandwidth of modulated signal

0100


> reduce bandwidth of modulated signal

BPSK Spread Spectrum

For example:For example:

Data rate 240 kb/s

Spreading factor 16

Chip rate 3.84 Mchip/s


UMTS/DL uses QPSK, i.e. 2 bits per symbol (chip) UL: BPSK, i.e. 1 bit per symbol for BPSK

UMTS Spreading

chip rate: 3,84 Mcps (Mega chips per second)spreading factor (= ratio code rate/data rate) 4 - 512 => 15 - 1920 kbps

The spreading comprises two parts

channelisation codescrambling code Data (z.B. 960 kbps)scrambling code ( p )

Channelisation/spreading code

(3 8 )

- +

(data rate)

(3,84 Mcps)+ + ++- -- - + + ++- -- -

( hi t )

Scrambling code(PN sequence,

3,84 Mcps)

(chip rate)

Modulation(QPSK)

3,84 Mcps)

5 MHz spectrum

(chip rate)


(QPSK) spectrum

Functionality of channelization and scrambling codes

Channelization Code Scrambling CodeUsage UL: Separation of physical data

(DPDCH) and control channels UL: Separation of terminals

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

UL: several millionsDL: 256

factor (SF)

Code Family Orthogonal Variable Spreading Factor

Long 10 ms code: Gold code

Spreading Yes, increases transmission bandwidth

No, does not affect transmission bandwidth


Properties of Spreading Sequences

C d #1

Auto correlation function (ACF)

Code sequence #1

Code sequence #2function (ACF) Code sequence #2

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

Hi h ACF kCross correlation function (CCF)

• High ACF peak

• Low ACF sidelobe →inter-symbol interference (ISI)y ( )

• Low CCF →multi-user interference (MUI)


Channelization Code: Orthogonal Code Tree

C d ti i i lCode generation principle:

X X

1111

11

11111111

11110000X X

X1100

11001100

1 11000011

X X

10

1010

1001100110100101

10101010

With interpretation x 1 x 11001

10010110

10011001

SF = 4

SF = 2With interpretation x = 1, x = -1,

these codes define „orthogonal“ channels!

I UMTS di f t (SF) f 4 512 (DL) / 4 256 (UL) d

SF = 8

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

Channel bit rate (kbps)

DPDCH channel bit

Maximum user data rate withfactor symbol rate

(kbps)rate (kbps) channel bit

rate range (kbps)

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 2880 5760 5616 2 3 Mbps4, with 3 parallel codes

2880 5760 5616 2.3 Mbps


CDMA Principle (Downlink)

sender (base station) receiver (terminal)

Code 0 Code 0

Code 1

data 0

Code 1

data 0

Code 2

Σdata 1

Code 2

data 1Transmission overair interface

data 2 data 2


CDMA Principle (Uplink)

sender (terminal) receiver (base station)

Code 0 Code 0transmission overair interface

Code 1

data 0

Code 1

data 0

Code 2

Σdata 1

Code 2

data 1

Code 2

data 2 data 2


CDMA by example

S 1

data stream A & BCode 1

spreadingSource 1 spread

spreaded signalSource 1 Code 1 Source 1 spread

Source 2 Code 2 Source 2 spread


CDMA by example

Despread Source 1

Sum of Sources Spread Sum of Sources Spread + Noise

decoding and

+Despread Source 2

and despreading

overlay of signals transmission and y gdistortion (noise and interference)


CDMA in theory

Sender A

sends Ad = 1, key Ak = 010011 (assign: „0“= -1, „1“= +1)

sending signal A = Ad * Ak = (-1 +1 -1 -1 +1 +1)sending signal As Ad Ak ( 1, +1, 1, 1, +1, +1)

Sender B

sends Bd = 0, key Bk = 110101 (assign: „0“= -1, „1“= +1)

sending signal Bs = Bd * Bk = (-1, -1, +1, -1, +1, -1)

Both signals superimpose in space

interference neglected (noise etc )interference neglected (noise etc.)

As + Bs = (-2, 0, 0, -2, +2, 0)

Receiver wants to receive signal from sender Ag

apply key Ak bitwise (inner product)

Ae = (-2, 0, 0, -2, +2, 0) • Ak = 2 + 0 + 0 + 2 + 2 + 0 = 6

l h h f l b “result greater than 0, therefore, original bit was „1“

receiving B

Be = (-2, 0, 0, -2, +2, 0) • Bk = -2 + 0 + 0 - 2 - 2 + 0 = -6, i.e. „0“


Be ( 2, 0, 0, 2, +2, 0) • Bk 2 + 0 + 0 2 2 + 0 6, i.e. „0

CDMA on signal level I

data A1 0 1 Ad

key A

keysequence A 10 0 1 0 0 1 0 0 0 1 0 1 1 0 0 1 1 Ak

i l A

data ⊕ keysequence A

01 1 0 1 1 1 0 0 0 1 0 0 0 1 1 0 0

Assignal A As

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


CDMA on signal level II

signal A As

data B 1 0 0 Bd

key Bkey

sequence B00 0 1 1 0 1 0 1 0 0 0 0 1 0 1 1 1 Bksequence B

data ⊕ key11 1 0 0 1 1 0 1 0 0 0 0 1 0 1 1 1

Bssignal B s

10

As + Bs

0-1


CDMA on signal level III

data A1 0 1 Ad

1As + Bs

10-1

Ak

1

1

(As + Bs) 10

-1

* Ak

integrator

-1

output

comparatoroutput

1 0 1


CDMA on signal level IV

data B1 0 0 Bd

1As + Bs

10-1

Bk

1

1

(As + Bs) * B

-110

integrator

* Bk -1

output

comparatoroutput

1 0 0


CDMA on signal level V

1As + Bs

10-1

wrongkey K

1

(As + Bs)

-1

10* K 0-1

comparator

integratoroutput

comparatoroutput (0) (0) ?

Assumptionsth lit f k


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

Mobile Radio Channel (wrap-up)


Channel Impulse Response

l h d f h lMulti-path components cause time dispersion of the signalTime dispersion is measured by delay spread σ

Channel impulse response (CIR)

|CIR| Typical (mean) delay spread σ:example|CIR| yp ( ) y p

• macro-cells 5 μs• micro-cells 1 μs• pico cells 500 ns

example of impulse response

• pico-cells 500 ns• indoor 100 ns

approximation(neg. exponential)

τ

Exponentially decaying channel impulse response:f

σ


a level of -30dB is reached at τ = 7σ

Multi-path Transmission

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

DespreaderSpreader (Correlator)

SpreadingSequence c(t)

SpreadingSequence c(t-Td)

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


Rake Receiver

Correlate and track each multi-path component separately

Rake receiver with K fingers

• trackers: independent tracking

of dominant pathsof dominant paths

• searchers: scan a time window to

search (the pilot channel) for ( p )

dominant multi-path components

• time resolution in UMTS approx.

200 ns

Optimal coherent combining


Optimal coherent combining

Macro-Diversity & Soft Handover

NodeB 1NodeB 1NodeB 2NodeB 2

UEUE

Optimal coherent combiningOptimal coherent combiningin the RAKE receiver (at MS)in the RAKE receiver (at MS)


( )( )

Multi-user CDMA

Conventional CDMA Receiver (Base Station):

Despreader(Correlator) • coherent (amplitude and phase) RF

demodulation at base stationdemodulation at base station• separate despreading and demodulation of

each signal at base stationR k i ith K fi i l

SpreadingSequence c1(t-Td1)

SpreadingSequence c2(t-Td2)

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

mobiles

SpreadingS (t T )Sequence cn(t-Tdn)


Near-Far Problem - Power Control

Near-Far Problem:• Spreading sequences are not orthogonal

(multi-user interference)• Near mobile dominate• Signal to interference ratio is lower for far

UE 1UE 1

gmobiles and performance degrades

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

AND/OR: by means of joint multi-user detection

NodeBNodeB

AND/OR: by means of joint multi-user detection

UE 2UE 2


Interference Cancellation

Multi-user Interference Cancellation (Joint Detection):

Matched Filter toSequence c1(t)

MF

Detection mechanism takes into account interference f th ll i l

Matched Filter toSequence c2(t)

MF1

Multi-userDetector

(J i t from other users as all signalsare known in the receiver(intra-cell interference can becanceled)

q ( )

Matched Filter to

MF2

(Joint Detection

Interference Cancellation)

canceled)Matched Filter toSequence cn(t)

MFn


FDD vs. TDD Mode

UMTS supports FDD and TDD

FDD mode: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)


TDD Mode (TD-CDMA)

• TDD: each time frame contains at least one time slot for the uplink and one time slot for the downlink

• Only spreading factors between 1 and 16 are used resulting in the same transmission rates as in FDDsame transmission rates as in FDD

• As in FDD, FDMA can be used if a Network operator provides of further , p pfrequency channels

S it hi i t ld b ft h ti l t thi ld l i i i• Switching points could be after each time slot, this would also minimize transmission delay, puts however high demands on hardware in NodeB and UE.


TDD-Mode (TD-CDMA) Summary


TDD Mode Switching

multiple switching points, symmetric DL/UL allocation

multiple switching points, asymmetric DL/UL allocationp g p , y

single switching point, symmetric DL/UL allocation

single switching point, asymmetric DL / UL allocation

1 Frame (10ms) of 15 Slots


1 Frame (10ms) of 15 Slots

Global Spectrum Allocations for IMT-2000

IMT-2000ITU

2010 20251980MSS MSS*

1930

IMT-2000 MSSMSS*IMT 2000

2160 2170 2200 MHz

*Region2

1885 2110

IMT-2000 MSSDECT MSSIMT-2000Europe

PHS

20101980 2025Japan

2110 22002170

IMT-2000 MSSMSSIMT-2000

18951885 1918 1

1980 2110 2200217019001880 2010 2025 MHz

Europe

20101980 2025 2110 2200217018951885 1918.1 MHz

20101980 2025

China2110 22002170

MSSMSS

1900 1920 MHz1865 1880 1945 1960

CDMA FDD-WLL

FDD-WLLCDMA

TDD-WLL

2110 220021652150

Reserve MSSBroadcast Auxilary

1910 1930 1990 2025

MSS

1850

PCS*PCSA B CD E F

PCSA B CD E F

MHz

USAMHz


MSS: Mobile Satellite Services

UMTS Spectrum

2200

2000

2100

1900

Up-link Down-linkM

Hz

MH

z

MH

z

MH

z

U i d B d 20 15MH (1900 1920 d 2010 2025MH ) f TDDUnpaired Band: 20 + 15MHz (1900-1920 and 2010-2025MHz) for TDD

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

Uplink Downlink

Details:

1 2 3 11 12. . .

1920 MHz 1980 MHz

1 2 3 11 12. . .

2110 MHz 2170 MHzp

5 MHz


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

Spectrum per Operator

GSM Operator UMTS Operator 1 UMTS Operator 2

Micro Layer Macro Layer

5 MHz5 MHz4 4 MHz3 MHz 5 MHz5 MHz4.4 MHz3 MHz

2.7+4.4+5.0+2.5 = 14.6 MHz

• Frequency reuse 1, i.e. typically 1 macro layer frequency band• No frequency planning• Border problems are to be solved


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