2. WCDMA Principles

W-CDMA Principles

W-CDMA Principles

W-CDMA Principles

• The Spread Spectrum Principle

• The Channelization codes & Scrambling codes. Their main properties

• The importance of Eb/No

• The concept of Power Control

• The coverage limits

• The Rake Receiver

• The macro-diversity

• Handovers

Objectives

At the end of this session, you will be able to:

W-CDMA PrinciplesW-CDMA Principles

W-CDMA Principles

Power

Frequency

Time

FrequencyTime

Power

Frequency

Time

Power

FDMA TDMA

W-CDMA

Access Technologies

W-CDMA Principles

Duplex Spacing: 190 MHz

FDD

Time

Frequency

Power

5 MHz 5 MHz

Code Multiplex

UL DL

UMTS USER 1

UMTS USER 2

Time

Frequency

Power

TDD

5 MHz

Code Multiplex&

Time Division

666.67 s

DL

UL

DL

DL

UL

UMTS USER 2

UMTS USER 1

Access Technologies

W-CDMA: FDD or TDD

W-CDMA Principles

Spread Spectrum Principle

Frequency

Po

we

r

Frequency

Po

we

r

•Narrow Band Signal

•High Power Spectral Density (Power / Hz)

•Wide Band Signal

•Low Power Spectral Density (Power / Hz)

W-CDMA Principles

Spreading / Despreading

dBkbps

kcps

RateBitUser

RateChipPG 2575.314

2.12

3840

In the receiving path, de-spreading is achieved by auto-correlation with the same code

Due to low cross-correlation properties with other codes, the received signal energy is increased compared to noise and other signal interference

The gain due to despreading is called processing gain

Example for 12.2 AMR speech:

W-CDMA Principles

Tbit

Tchip

Data sequence

spreading sequence

transmitted sequence

1/Tbit

1/Tchip

Frequency

1/Tchip

+a

-a

-1

+1

-a

+a

x

=

Data sequence

Transmitted signal

Spreading sequence generator

Modulation

x(t)Power spectrum


2 - Transmission

W-CDMA Principles

Tbit

Tchip

Data sequence

spreading sequence

received sequence

Power spectrum

1/Tbit

1/Tchip

Frequency+a

-a

-1

+1

-a

+a

x

=

1/Tchip

Received signal

Data sequence

Spreading sequence generator

Demodulation

x(t)


3 - Reception

W-CDMA Principles


4 - Code Multiplexing

Power spectrum

User 1

User 2

User 3

User 4

User 5

Spreading

Code 1

Code 2

Code 3

Code 4

Code 5

Composite signal

5 MHz

Codes discriminate usersCodes discriminate users

W-CDMA Principles

Unwanted Powerfrom other sources

Using the “right” mathematical sequences any Code Channel can be extractedfrom the received composite signal


5 - Extraction

W-CDMA Principles

+

-1 -1 -1

-1 -1 -1 -1

1 1 1 1

1 1 1 1

-1

*

1 1 1 1-1 -1 -1 -1

Cj

Ck

T0 synchronization

= 0+

-1 -1 -1

-1 -1 -1 -1

1 1 1 1

1 1 1 1

-1

*

1 1 1 11 -1 1 -1

Cj

Ck

no T0 synchronization

= 4

=> Orthogonal => Non orthogonalNo correlation Small correlation

Channelization Codes - OVSF

2 - Orthogonality

W-CDMA Principles

Spreading and scrambling codes

Downlink– Same Frequency in all Cells, OVSF codes from same Base Station are synchronised– Same OVSF Codes in all Cells -> Interference– Need for additional identifier for different Cells

Uplink– User sends information in different OVSF codes– OVSF codes from different users are not Synchronized > Not Orthogonal– Need for additional identifier for Mobiles

Same Frequency

Same OVSF Codes

Same Frequency

Same OVSF Codes

OVSF 1

OVSF 2

W-CDMA Principles

Spreading and scrambling codes

Spreading codes (channelization codes)

• used to differentiate mobiles and services

• different lengths (spreading factor) according to service in UMTS

• Orthogonal Variable Spreading Factor (OVSF) in UMTS

• Low Cross Correlation, High Auto Correlation

Scrambling codes

• used to differentiate un-synchronized codes (from other UEs or Node-Bs)

• 1 scrambling code per sector on downlink

• PN code family in UMTS

DL

ULUE

Descrambling Despreading

SpreadingOVSF

(Service identifier)

ScramblingPN

(User identifier)

Node B

SpreadingOVSF

(Service/ user identifier)

ScramblingPN

( Cell identifier)

DescramblingDespreading

W-CDMA Principles

Scrambling codeScrambling code

Channelization code 1Channelization code 1



User 1 signal

User 2 signal

User 3 signal

BTS

Codes Multiplexing

1 - Downlink Transmission on a Cell Level

W-CDMA Principles

BTS

Channelization code

Codes Multiplexing

2 - Uplink Transmission on a Cell Level

Scrambling code 2

User 2 signal

Scrambling code 3

User 3 signal

Channelization code

Channelization code

Scrambling code 1

User 1 signal

W-CDMA Principles

Cch,1,0 = 1

Cch,2,0 = 1 1

Cch,4,0 = 1 1 1 1

Cch,4,1 = 1 1 -1 -1

Cch,2,1 = 1 -1

Cch,4,2 = 1 -1 1 -1

Cch,4,3 = 1 -1 -1 1

SF = 1 SF = 2 SF = 4 SF = 8 SF = 16, 32, 64, 128, 256, 512.

Cch,2,0 = 1 1

Cch,2,1 = 1 -1

Cch,4,0 = 1 1 1 1

Cch,4,1 = 1 1 -1 -1

Cch,4,2 = 1 -1 1 -1

Cch,4,3 = 1 -1 -1 1

Channelization Codes - OVSF

1 - Orthogonal Variable Spreading Factor: code tree generator

W-CDMA Principles

Physical Layer Structure

Frame #0 Frame #1 Frame #i Frame #4095

System frame = 4096 frames = 40.96 seconds

Slot #0 Slot #1 Slot #j Slot #14

Frame = 15 slots = 10 ms = 38400 chips

Slot = 0.667 ms = 2560 chips

UMTS Frame Format

(38400*1000/10 = 3.84 Mcps)

W-CDMA Principles

Basic W-CDMA Terminologies

1 - Eb/No

C

IN

C

CEb/No

W-CDMATDMA-GSM

Power spectrum

1

1

11

1

1

1

2

2

2

2

3

3

3

3

3

2

4

4

4

4

4

W-CDMA Principles

Basic W-CDMA Terminologies

Eb/No -> Eb = Energy per bit, No = Noise Spectral Density

[ Sensitivity of Base Station]

Uplink Eb/No = Minimum Signal/Noise to achieve any Service

BER (Bit Error Rate) = Function of Eb/No

SNR = C/I = Eb/No - Processing Gain

CS 12.2 CS 64 PS 64 PS 128 PS 384

Bit rate (kbps) 12.2 64 64 128 384UL Eb/ No (dB) 4.9 3 3.2 2.6 2.1Spreading Factor 256

Processing gain (dB) 25 18 18 15 10UL C/ I (dB) -20 -15 -15 -12 -8

VendorDependent

W-CDMA Principles

Interference level

Example: 2 UEs at the same distance from the BTS using 2 data rates

Eb/No required

SF

= 1

28

Service provided: SpeechService provided: Speech

Interference level

Eb/No required

SF

= 8

Service provided: Data 144Service provided: Data 144

User 2 needs more power for the UL & DL for the same quality as

user 1

UE2UE1

Speech 8 kbps Data 144 kbpsThe higher the SF, the less power requiredThe higher the SF, the less power required

BTS

Received powerReceived power

Coverage Limits

W-CDMA Principles

SF = 128

Speech 8 kbps Data 64 kbps Data 384 kbps

BTS

SF = 32

SF = 4

Coverage Limits

The coverage limits are determined by

the Uplink link Budget

The coverage limits are determined by

the Uplink link Budget

W-CDMA Principles

WCDMA : Coverage and Capacity

COVERAGE:

– Uplink : Depends on C/I Requirement

– Downlink : Depends on Power Received at MS

CAPACITY:

– Uplink : Interference

– Downlink : Power [20 W per carrier is OK for non HSDPA]

– Uplink/Downlink : Codes (512 codes)

W-CDMA Principles

UE 1

UE 2

Before despreading After despreading

“Near-Far-Problem”

– Up to around 80 dB attenuation between UE1 and UE2

– If UE1 and UE2 transmitted with the same power, UE1 would jam UE2 : so-called “near-far” effect

– Solution : power control

– Need for an efficient power control able to fight against slow AND fast fading!

W-CDMA Principles

Power Control

TX Power is adjusted regularly so that each connection is received with the required Eb/No of its service

– Uplink: Avoid „Near-Far-Problem“

– Downlink: Power share allocation

Policy: “No one gets a higher quality (Eb/No) than he needs. Everyone gets exactly the required quality or is not served at all“

– no unnecessary increase of interference for other mobiles

– no waste of common power resource in the downlink

W-CDMA Principles

Interference Limit

When the number of users in the cell increases, the interference level increases (noise rise), the required received power at the base station to

reach a given Eb/No (quality) increases

For high interference level, the required received power becomes infinite: power control is unstable pole capacity

Coverage and capacity are linked in CDMA systems

0

2

4

6

8

10

12

14

16

18

20

0 10 20 30 40 50 60 70

Number of simultaneous users per sector

Inte

rfere

nce

leve

l rela

tive to N

ois

e le

vel

(dB

)

W-CDMA Principles

Interference level as a function of capacity

0

5

10

15

20

25

30

35

0 10 20 30 40 50 60 70 80 90 100

Cell loading (%)

50% of cell load (3dB of interference)

max loading : 75%

Inte

rfere

nce

level (d

B)

)1log(10 ULXNoiseRise

Note:For cell load above 75 %, the system gets unstable

Uplink Cell load (monoservice)

The UL cell load is directly linked to the so called ‘Noise Rise’ or interference level

100 % UL cell load means infinite mobile power required

monoservice

W-CDMA Principles

WCDMA Capacity : Uplink Cell Load

servN

j jIC

jIC

jUL Nfx1 1

..1UL Eb/No (dB) = 4.9

Bit Rate = 12.2 Kbps

Processing Gain (dB) = 10*log10(3840/12.2) = 25 dB

UL = Eb/No – Processing Gain = 4.9 – 25 = -20.1 dB

= OCIF Factor = 0.8

C/I (in absolute value) = 10^(()/10) = 0.00982

For one simultaneous Voice user, N = 1

Applying Formula,

Uplink Cell Load = = (1+0.8)* 1 * 0.00982/(1+0.00982) =

0.0175 = 1.75%

For Maximum Uplink Cell Load = 50%

No. of simultaneous Voice users = 50%/1.75% = 28

Voice users have activity factor = 50%

Hence Simultaneous Voice Users = 28/(50%) = 56

W-CDMA Principles

Downlink Power Limit : Cell breathing

Considering the limitation of maximal transmit power, the increase of required received power due to high traffic will lead to decrease the cell

range

The cell coverage decreases when the traffic increases : so-called “cell breathing” phenomenon

Coverage and capacity are linked in CDMA systems

W-CDMA Principles

Tra

ffic d

en

sity

incre

ases

Deployed inter site distanceDeployed inter site distance

Load control

In order to avoid power control instability and coverage holes due to high traffic level, the level of interference received by a base station should be controlled

by means of admission and load control algorithms

W-CDMA Principles

TXD(t)

Delay 0

Delay 1

C(t-0)

+C(t-1)

Delay (1)

RX

C(t-n)

Delay (0)

Delay (n)RX

RX

C(t)

0

1

n

Take advantage of multipath diversityTake advantage of multipath diversity

BTS

Taking advantage of Multipath: Rake Receiver

UE

Spreading &

Scrambling

Spreading &

Scrambling

W-CDMA Principles

Macro-Diversity

Softer Hand Over

Node B(BTS)

RNC

Data UL

Data UL1Data UL2 Data UL

Data UL

Data DLData DL

Data DL1

Data DL1Data DL2

Data DL

UE

Data DL2

Data UL

CoreNetwork

W-CDMA Principles

Macro-Diversity

Soft Hand Over Intra RNC

RNC

Data UL1


Data UL

Data DL

Data DL1

Data DL1

Data DL1Data DL2

Data DL

UE

CoreNetwork

Data DL2

Data UL

Data DL2

Data UL2

Data UL2

Data UL1

Node B(BTS)

Node B(BTS)

W-CDMA Principles

Macro-Diversity

Soft Hand Over Inter RNC: Serving RNC (SRNC) and Drift RNC (DRNC)

Node B(BTS)

SRNC

DRNCNode B(BTS)

Data UL

Data UL

Data ULData UL1

Data UL2

Data UL2


Data UL

Data DLData DL2

Data DL2

Data DL1

Data DL2

Data DL1

Data DL1Data DL2 Data DL

UE

CoreNetwork

W-CDMA Principles

Different Types of Handover

Soft HandoverSoft Handover Softer HandoverSofter Handover Hard HandoverHard Handover

SRNC DRNC

Node B

UE

Core Network

SRNC

Node B

UE

Core NetworkSRNC

UE

Core Network

GSM / GPRSBSS

SRNC

UE

Core Network

GSM / GPRSBSS

Inter RNC Intra Node B

W-CDMA Principles

In UL selection of the best signal on a frame basis at RNC level -

‘selection diversity’

In DL Maximum Ratio combining due to RAKE receiver at UE

For UL & DL good decorrelation due to different locations of Node

Bs many multipaths

In UL Maximum. Ratio Combining at Node B

In DL Maximum Ratio combining due to RAKE receiver at UE

For UL & DL less decorrelation due to “same” location of sectors less multipaths

Soft HO

Softer HO

Soft Handover (SHO) Macrodiversity gain

RNC

RNC

W-CDMA Principles

Physical channels: Real transmission resource

Transport Channel: How and with what characteristics the information is transferred

Transport Channel: How and with what characteristics the information is transferred

Different messagetypes

=Different Logical

Channels with differentQoS Requirements

(Transport channels)

Channels

3 Types of Channel

High rate data

Physical ChannelSpeech

Signaling

Logical Channel: What type of information is transferredLogical Channel: What type of information is transferred

W-CDMA Principles

Channels

Logical Channels: UE Protocol Layers

Logical ChannelsLogical Channels

Transport Channels

Transport sublayer

Physical Channels

Physical sublayer

PHY(PHYsical Layer)

RLC (Radio Link Control)

MAC (Medium Access Control)

W-CDMA Principles

CEM

CCM

CEM

Digital modules

Iub (ATM or IP/Ethernet)

3G NodeB Architecture

TRDUCEMCEMCEM

RRHRemote RadioSolutions

Radio Modules

TRM MCPA Filter

Node B Portfolio architectureFlexibility around a common software platform

ANC

SUM

TRX

2G BTS

Architecture

W-CDMA Principles

CHANNEL ELMENTS : UTRAN RADIO RESOURCE

A Channel Elements is a Radio Resource used at any instance of time.

Different RABs require different no. of CEMs.

It is different for Uplink and Downlink

CEM requirements are vendor dependent

W-CDMA Principles

Comparison 2G and 3G Cell Range

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Cell Range (urban) for Voice

GSM 900

UMTS 900

GSM 1800

UMTS 2100

Te

ch

no

log

y

Comparison 2G and 3G Cell Range

UMTS 2100

GSM 1800

UMTS 900

GSM 900

W-CDMA Principles

Comparison 2G and 3G Voice Capacity (AMR 12.2)

With 5 MHz Bandwidth, FDD

2G:

With good KPI commitments -> 3,3,2 config

Total = 36 Erlangs per site

3G:

56 Erlangs per Sector

168 Erlangs per Site

Approx -> 5 times higher capacity in 3G

2. WCDMA Principles

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