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W-CDMA for UMTS Principles

Introduction

CDMA Background/ History

Key Parameters

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 Spectrum Allocation

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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. Ojanper, R. Prasad, Wideband CDMA for Third Generation Mobile

Communication, Artech House, 1998.

R. Prasad, W. Mohr, W. Konhuser, Third Generation Mobile Communications

Systems, Artech House, March 2000.

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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)

19921995: RACE project CODIT (UMTS Code Division Testbed, PKI, Ericsson, Telia, etc.)

Wide-band CDMA Era

19951999: 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

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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

multi-path radio channels

Many users share the same RF carrier

Each 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 achannel (carrier) increases

Spreading codes keep channels apart such that the same carrier can be used

in the next cell (frequency re-use is 1)

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CDMA Multiple Access

CDMA (Code Division Multiple Access)

all terminals send on the same frequency probably at the same time andcan use the whole bandwidth of the transmission channel

each sender has a unique random number (spreading sequence), thesender XORs the signal with this random number

the receiver can tune into this signal if it knows the pseudo randomnumber, tuning is done via a correlation function

Advantages: all terminals can use the same frequency, less planning needed

huge code space (e.g. 232) compared to frequency 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 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)

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Spread Spectrum Technology

Problem of radio transmission: frequency dependent fading can wipe out narrowband signals for duration of the interference

Solution: spread the narrow band signal into a broad band signal using a specialcode

protection against narrow band interference

Side effects:

coexistence of several signals without dynamic coordination

tap-proof

Alternatives:

Direct Sequence(UMTS)

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

detection atreceiver

interferencespreadsignal

signal (despreaded)

spread

interference

f f

power power

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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 channelselective fading

CDMA: relatively large bandwidth ofthe spread signal

Frequency selective fading causesonly some reduction in the level ofthe received signal

Users are separated by thespreading sequence

22

22

2

frequency

channelquality

1

spreadsignals

frequency

channelquality

1 2

3

4

5 6

small bandwidth guard band

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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 thesignal

Spreading factor SF: ratio betweenchip rate RC and data rate Rb

RC = Rb SF

tb = tC SF

Processing Gain

GS = 10 log10(SF)

user data

code

sequence

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 duration

tc: chip duration

(data rate)

(chip rate)

(chip rate)

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DSSS (Direct Sequence Spread Spectrum) II

X

user data

codesequence

modulator

radiocarrier

spread

spectrumsignal

transmitsignal

transmitter

demodulator

receivedsignal

radio

carrier

X

code

sequence

basebandsignal

receiver

integrator

products

decision

data

sums

correlator

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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 over

air interface

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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 over

air interface

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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 orthogonalchannelization codes

No need for precise synchronization as the scrambling codes remainquasi-orthogonal

data1 data2 data3

scramblingcode1

chan.code3

chan.code2

chan.code1

data4 data5

chan.code4

chan.code1

sender1 sender2

scramblingcode2

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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 connectionsto 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 chipsNumber of codes Number of codes under 1

scrambling code = spreadingfactor (SF)

UL: several millions

DL: 256

Code Family Orthogonal Variable SpreadingFactor

Long 10 ms code: Gold code

Spreading Yes, increases transmissionbandwidth

No, does not affect transmissionbandwidth

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OVSF-Coding Tree

1

1,1

1,-1

1,1,1,1

1,1,-1,-1

X

X,X

X,-X1,-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

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

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Downlink Dedicated Channel Symbol and Bit Rates

Spreadingfactor Channelsymbol rate(kbps)

Channel bitrate (kbps) DPDCHchannel bitrate range

(kbps)

Maximum userdata rate with1/2-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 kbps

8 480 960 912 456 kbps

4 960 1920 1872 936 kbps

4, with 3parallelcodes

2880 5760 5616 2.3 Mbps

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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 B

sends Bd = 0, code sequence Bc = 0110101

sending 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 A

apply sequence AC chipwise (inner product)

Ar = (+2, 2, 0, 0, 2, +2, 0) Ac = 2 + 2 + 0 + 0 + 2 + 2 + 0 = 8

result greater than 0, therefore, original bit was 1

receiving B

Be = (+2, 2, 0, 0, 2, +2, 0) Bc = 2 2 + 0 + 0 2 2 + 0 = 8, i.e. 0

wrong sequence CC = 1100110

Cr = (+2, 2, 0, 0, 2, +2, 0) Cc = 0, decision impossible

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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 distancebetween 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 1

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

Ad

Ak

As

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CDMA on signal level II

signal A

data B

key B

key

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 1

11 1 0 0 1 1 0 1 0 0 0 0 1 0 1 1 1

Bd

Bk

Bs

As

1

0

-1

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CDMA on signal level III

Ak

(As + Bs)* Ak

integrator

outputcomparator

output

As + Bs

data A

1 0 1

1 0 1 Ad

1

0

-1

1

-1

1

0

-1

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CDMA on signal level IV

integrator

outputcomparator

output

Bk

(As + Bs)* Bk

As + Bs

data B

1 0 0

1 0 0 Bd

1

0

-1

1

-11

0

-1

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comparator

output

CDMA on signal level V

wrongkey K

integratoroutput

(As + Bs)

* K

As + Bs

(0) (0) ?

Assumptions orthogonality of keys neglectance of noise no differences in signal level => precise power control

1

0

-1

1

-1

1

0-1

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Properties of Spreading Sequences

Cross correlation function (CCF)

Auto correlationfunction (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)

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Multi-path Transmission

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

Spreader

Spreading

Sequence c(t)

Despreader

(Correlator)

Spreading

Sequence c(t-Td)

Receiversynchronizes toeach multi-pathcomponent forde-spreading

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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

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RAKE Receiver Practical Realization

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Macro-Diversity & Soft Handover

Optimal coherent combiningin the RAKE receiver (at MS)

NodeB 1NodeB 2

UE

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Multi-user CDMA

Conventional CDMA Receiver (Base Station):

Despreader

(Correlator)

Spreading

Sequence c2(t-Td2)

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

Spreading

Sequence c1(t-Td1)

Spreading

Sequence cn(t-Tdn)

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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 throughdynamic power control to equalize allreceived power levels

AND/OR

By means of joint multi-user detection

Near-Far Problem Power Control

NodeB

UE 1

UE 2

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Interference Cancellation

Multi-user Interference Cancellation (Joint Detection):

Matched Filter toSequence c1(t)

Detection mechanism takesinto 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

(JointDetection

InterferenceCancellation)

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Interference Cancellation Realization

Subtractive interference cancellation

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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-cellmulti-user interference cancellation (spreading factor 1 - 16)

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FDD vs. TDD Mode (contd.)

TDD-Mode

FDD-Mode(one direction)

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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

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Global Spectrum Allocations for IMT-2000

ITU2010 20251980

MSS MSS*

1930

IMT-2000MSSMSS*

IMT-2000

2160 2170 2200 MHz

*Region2

1885 2110

PHS

20101980 2025

Japan

2110 22002170

IMT-2000MSSMSSIMT-2000

18951885 1918.1 MHz

1980 2110 22002170

IMT-2000MSS

19001880

DECT

2010

MSSIMT-2000

2025 MHz

Europe

2110 220021652150

Reserve MSSBroadcast Auxilary

1910 1930 1990 2025

MSS

1850

PCS*PCS

A B CD E F

PCS

A B CD E F

MHz

USA

20101980 2025

China

2110 22002170

MSSMSS

1900 1920 MHz1865 1880 1945 1960

CDMA FDD-WLL

FDD-WLLCDMA

TDD-WLL

MSS: Mobile Satellite Services

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UMTS Spectrum

22

00MHz

20

00MHz

21

00MHz

19

00MHz

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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