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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

Cellular Communication Networks 2 Andreas Mitschele-Thiel, Jens Mueckenheim Nov. 2015

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


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

Side effects:

coexistence of several signals without dynamic coordination

signal can only be detected if the spreading process is known

Alternatives:

Direct Sequence (UMTS)

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

detection at

receiver

interference spread

signal

signal (despreaded)

spread

interference

f f

power power


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

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

2 2

2 2

2

frequency

channel quality

1

spread signals

frequency

channel quality

1 2

3

4

5 6

small bandwidth guard band


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. 232) 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)


CDMA Multiple Access (contd.)

Principle of CDMA Communication


DSSS (Direct Sequence Spread Spectrum) I

Modulation 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 symbol rate RS

RC = RS · SF

TC = TS / SF

Processing Gain

GS = 10 · log10(SF)

user data (data rate)

code sequence (chip rate)

resulting signal (chip rate)

1

0

=

Tc

Ts


DSSS (Direct Sequence Spread Spectrum) II

X

user data

code

sequence

modulator

radio

carrier

spread

spectrum

signal transmit

signal

transmitter

demodulator

received

signal

radio

carrier

X

code

sequence

baseband

signal

receiver

integrator

products

decision

data

sums

correlator


CDMA Principle (Downlink)

Code 0

Code 1

Code 2

S

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


CDMA Principle (Uplink)

Code 0

Code 1

Code 2

S

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


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

data1 data2 data3

scrambling code1

chan. code3

chan. code2

chan. code1

data4 data5

chan. code4

chan. code1

sender1 sender2

scrambling code2


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

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

UL: several millions

DL: 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,-1,1

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


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 kbps

8 480 960 912 456 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 = +1 –1 +1 –1 –1 +1 +1

sending signal As = Ad Ac = (+1, –1, +1, –1, –1, +1, +1)

Sender B

sends Bd = –1, code sequence Bc = –1 +1 +1 –1 +1 –1 +1

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

Br = (+2, –2, 0, 0, –2, +2, 0) Bc = –2 –2 + 0 + 0 – 2 – 2 + 0 = –8, i.e. „–1“

wrong sequence CC = +1 +1 –1 –1 +1 +1 –1

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


CDMA on signal level I

data A

code A

signal A

Real systems use much longer keys resulting in a larger distance

between single code words in code space

0 1 0 Ad

Ac

As


CDMA on signal level II

signal A

data B

code B

signal B

As + Bs

0 1 1 Bd

Bc

Bs

As

+1

0

–1


CDMA on signal level III

Ac

(As + Bs)

• Ac

integrator

output

comparator

output

As + Bs

data A

0 1 0

0 1 0 Ad

+1

0

–1

+1

–1

+1

0

–1


CDMA on signal level IV

integrator

output

comparator

output

Bc

(As + Bs)

• Bc

As + Bs

data B

0 1 1

0 1 1 Bd

+1

0

–1

+1

–1

+1

0

–1


CDMA on signal level V

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

comparator

output

wrong

code C

integrator

output

(As + Bs)

• C

As + Bs

(1) (1) ?

+1

0

–1

+1

–1

+1

0

–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

Spreading

Sequence c(t)

Despreader

(Correlator)

Spreading

Sequence c(t–Td)

Receiver

synchronizes to

each multi-path

component for

de-spreading

Td


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.

260 ns


RAKE Receiver – Practical Realization


Macro-Diversity & Soft Handover

Optimal coherent combining

in the RAKE receiver (at MS)

NodeB 1 NodeB 2

UE


Multi-user CDMA

Conventional CDMA Receiver (Base Station):

• 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

Despreading

(Correlator)

Spreading

Sequence c1(t-Td1)

RAKE 1

Spreading

Sequence c2(t-Td2)

RAKE 2

Spreading

Sequence cn(t-Tdn)

RAKE n


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):

Detection mechanism takes into account interference from other users as all signals are known in the receiver (known interference can be canceled)

Multi-user Detector

(Joint Detection/

Interference Cancellation)

Despreading

(Correlator)

c1(t–Td1)

RAKE 1

c2(t–Td2)

RAKE 2

cn(t–Tdn)

RAKE n


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-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)


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


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 needed

TDD mode: Synchronization needed

Multi-rate/

Variable-rate scheme

Variable-spreading factor + Multi-code

Spreading factor: 4 – 256 (FDD) and 1 – 16 (TDD)

Channel coding

scheme

Convolutional coding (rate 1/2 – 1/3)

Turbo coding


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

Japan2110 22002170

IMT-2000MSSMSSIMT-2000

18951885 1918.1MHz

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 1920MHz

1865 1880 1945 1960

CDMA FDD-WLL

FDD-WLLCDMA

TDD-WLL

MSS: Mobile Satellite Services


UMTS Spectrum

2200 M

Hz

2000 M

Hz

2100 M

Hz

1900 M

Hz

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

Details:


References

H. Holma, A. Toskala (Ed.), “WCDMA for UMTS”, 5th edition, Wiley, 2010.

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”, Prentice-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.

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