01. WCDMA Fundamentals

1 © NOKIA WCDMA Fundamentals.PPT/10.06.2003 / NN

WCDMA Fundamentals

MODULE 1


Module Objectives

At the end of this module the participant will be able to: • Describe the usage of scrambling and channalisation codes

• Describe how different bit rates can be achieved

• Describe how the capacity and coverage are connected to each others

• Describe different Handover types

• Describe Power Control procedures

• Explain the tasks of Admission Control, Load Control, Packet Scheduler and interconnections between them


IMT-2000 frequency allocations

2200 MHz20001900 1950 2050 2100 21501850

JapanIMT-2000

PH

S

IMT-2000

ITU

Mob

ile

Sate

llit

eIMT-2000 IMT-2000

EuropeUMTS(FDD)DEC

T

UM

TS

(T

DD

)

GSM1800 U

MTS

(T

DD

)

UMTS(FDD)

USA

PC

S

un

licen

sed

PCSPCS

UM

TS

(T

DD

)IM

T-2

00

0

(TD

D)

Mob

ile

Sate

llit

e

Mob

ile

Sate

llit

e

Mob

ile

Sate

llit

e

Mob

ile

Sate

llit

e

Mob

ile

Sate

llit

e

Mob

ile

Sate

llit

e

Mob

ile

Sate

llit

e


UMTS & GSM Network Planning

GSM900/1800: 3G (W CDM A):


Hierarchical Cell Structure

F1

F2

F2

F3

F3

F3

Micro BTSMacro BTS

Pico BTSs

1 - 10 km

50 - 100 m200 - 500 m


Differences between WCDMA & GSM

WCDMA GSM

Carrier spacing 5 MHz 200 kHz

Frequency reuse factor 1 1–18

Power controlfrequency

1500 Hz 2 Hz or lower

Quality control Radio resourcemanagement algorithms

Network planning(frequency planning)

Frequency diversity 5 MHz bandwidth givesmultipath diversity with

Rake receiver

Frequency hopping

Packet data Load-based packetscheduling

Timeslot basedscheduling with GPRS

Downlink transmitdiversity

Supported forimproving downlink

capacity

Not supported by thestandard, but can be

applied

High bit rates

Spectral efficiency

Different quality

requirements

Efficient packet data


WCDMA Technology

5 M Hz

3.84 M Hz

f

5+5 MHz in FDD mode5 MHz in TDD mode

Freq

uenc

y

TimeDirect Sequence (DS) CDMA

WCDMA Carrier

WCDMAWCDMA5 MHz, 1 carrier5 MHz, 1 carrier

TDMA (GSM)TDMA (GSM)5 MHz, 25 carriers5 MHz, 25 carriers


Spreading Code

Spread Signal

Data

Air Interface

Chips & Bits & Symbols Bits (In this drawing, 1 bit = 8 Chips)

Baseband Data

-1

+1

+1

+1

+1

+1

-1

-1

-1

-1

ChipChip

Despreadi

Despreadi

ngng


Energy Box

Freq

uenc

y Ba

nd

Duration (SF)

Pow

er

Originating Bit Received Bit

Energy per bit = const


Spreading & Processing Gain

Frequency

Pow

er

den

sity

(W

att

s/H

z)

Unspread narrowband signal Spread wideband signal

Bandwidth W (3.84 Mchip/sec)

Bit rate R

sec84.3 MchipWconstSFR

SFR

W

B

BdBG

Baerer

Up

u Gp: processing gainBUu: system chip rateBBearer: bearer symbol rateSF: spreading factor


Processing Gain Examples

Frequency (Hz)

Voice user (R=12,2 kbit/s)

Packet data user (R=384 kbit/s)

Pow

er

den

sity

(W

/Hz)

R

Frequency (Hz)

Gp=W/R=24.98 dB

Pow

er

den

sity

(W

/Hz)

R

Gp=W/R=10 dB

• Spreading sequences have a different length• Processing gain depends on the user data rate


Transmission Power

Frequency

5MHz

Power

Time

High bit rate user

Low bit rate user


DL & UL Channalisation Codes

• Walsh-Hadamard codes: orthogonal variable spreading factor codes (OVSF codes)

• SF for the DL transmission in FDD mode = {4, 8, 16, 32, 64, 128, 256, 512}

• SF for the UL transmission in FDD mode = {4, 8, 16, 32, 64, 128, 256}

• Good orthogonality properties: cross correlation value for each code pair in the code set equals 0

• Orthogonal codes are suited for channel separation, where synchronisation between different channels can be guaranteed, e.g. downlink channels under one cell, uplink channels from a single user; uplink signals from different users are not time synchronised.

• Orthogonal codes have bad auto correlation properties and thus not suited in an asynchronous environment


Channelisation Code Tree

C0(0)=[1]

C2(1)=[1-1]

C2(0)=[11]

C4(0)=[1111]

C4(1)=[11-1-1]

C4(2)=[1-11-1]

C4(3)=[1-1-11]

C8(0)=[11111111]

C8(1)=[1111-1-1-1-1]

C8(2)=[11-1-111-1-1]

C8(3)=[11-1-1-1-111]

C8(0)=[1-11-11-11-1]

C8(5)=[1-11-1-11-11]

C8(6)=[1-1-111-1-11]

C8(7)=[1-1-11-111-1]

C16(0)=[............]C16(1)=[............]

C16(15)=[...........]

C16(14)=[...........]

C16(13=[...........]

C16(12)=[...........]

C16(11)=[...........]

C16(10)=[...........]

C16(9)=[............]

C16(8)=[............]

C16(7)=[............]

C16(6)=[............]

C16(5)=[............]

C16(4)=[............]

C16(3)=[............]

C16(2)=[............]

SF=1

SF=2

SF=4

SF=8

SF=16

SF=256SF=512...


Physical Layer Bit Rates (DL)

Half rate speechFull rate speech

128 kbps384 kbps

2 Mbps


DL & UL Scrambling Codes

DL Scrambling Codes• Pseudo noise codes used for cell separition

• Generated with the shift register of length 18 (218-1=262 143 codes can be generated)

• The first 8192 first codes from 262 143 code set are exclusively used in DL, they are organised into hierarhical groups:

• 512 Primary Scrambling Codes• 512·15 Secondary Scrambling Codes

UL Scrambling Codes• Two different types of UL scrambling codes are generated:

• Long scrambling codes created from the Gold pseudo-noise sequence (length of 38 400 chips)

• Short scrambling codes generated by the quaternary S(2) pseudo-noise sequence (256 chips are periodicaly repeted to get the scrambling code of the frame length)

• For the common physical channels long scrambling codes must be used

• For the dedicated channels both long and short scrambling codes can be used


Scrambling Codes & Multipath Propagation

Sprambling code

C1

Sprambling code

C2

C 1+ 3

C1+2C1+

1

C2


RAKE Receiver

Del

ay

1Code usedfor the

connection

Rx

Output

Finger

t

Cell-1

Cell-1

Cell-1

Cell-2

Rx

Rx

Rx

Finger

Finger

Finger

Del

ay

2

Del

ay

3


Channelisation and Scrambling Codes


Micro Diversity

Micro Diversity Pointsmax ratio combining is used

Summed signalNode B RAKE

Receiver

MS RAKE

Receiver


Macro Diversity in the RNC

Node B

Node B

Node B

S-RNC

D-RNC

Core Network

Active cell set

Macro Diversity Pointselection combining is used


Radio Resource Management

service quality

cell coverage cell capacity

Optimizationand Tailoring

• RRM is responsible for optimal utilization of the radio resources:• Transmission power• Logical codes• Bandwidth

• The trade-off between capacity, coverage and quality is done all the time: as many users as possible will get the resources in such a way, that the minimum required quality is meet (nothing less and nothing more)

• By means of several RRM functionalities, the radio resources are continuously monitored and optimize.


RRM must be able to:

• Predict the impact on interference (power) of the admitting a new user for UL & DL

• Perform appropriate actions (e.g. new call admissions, bit rate increase/decrease etc.) in accordance with prevailing load conditions

• Provide different quality of service for real time (RT) and non-real time (NRT) users

• Take appropriate corrective action when the different cell load thresholds are exceeded in order to maintain cell stability (i.e. load control)

Overload

Load TargetOverload Margin

Pow

er

Time

Estimated capacity for NRT trafficMeasured load causedby non-controllable load (RT)

Radio Resource Management

RT services must have higher quality assurance than NRT


RRM Functionalities

• AC - Admission Control

• LC - Load Control

• PS Packet Scheduler

• RM - Resource Manager

• PC - Power Control

• HC - HO Control

PC

HCConnection based functions

LC

AC

Network based functions

PS

RM


Admission Control • Checks that admitting a new user will not sacrifice planned

coverage or quality of existing connections

• Determines whether or not a new RT RAB can be admitted to the RAN

• With PS decides whether to admit NRT RABs (PS handles all NRT connections)

• Also sets• UL/DL BLER, Ec/No targets• SIR target for outer loop power control• Initial DL transmission power for the channel• Radio Link Control parameters, e.g. transmission mode• Transport Channel (TrCH) parameters, e.g. TFS

• Provides RLC parameters to PS for NRT users;• Bearer class• Traffic handling priority• Transport Formats• MS capabilities


Admission Control

Radio Access Bearerson the air interface Uu

Inte

rfac

e Ban

dwid

th

Allowed RangeAC Procedure

...


Admission Control

Planned uplink interference margin; defines the optimum operating point up to which the AC can operate.

Defines the limit (the first UL overload threshold) for the UL interference margin, after which the BS starts its load control actions to prevent overload.

0

5

10

15

20

25

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Load

Inte

rfere

nce M

arg

in (

dB

)

Offset


Load Control• Cell load is defined as a function of interference – main criterion in

WCDMA

• The load control function within RRM can be divided into:• Preventive load control (e.g. congestion)• Overload control (e.g. dropping of calls in worst case)

• The load control functionality is done by measuring both UL (received interference) and DL (transmit power) periodically on a cell basis

• Load control is performed for UL and DL separately (asymmetric traffic)

• Preventive actions are performed before the cell is overloaded (threshold y)

• Overload actions are performed after cell is overloaded (threshold x)

• RNP parameters define the thresholds for the RRM functionalities

• The thresholds define a stable functionality within a cell and with surrounding cells


Load Control

Estimated capacity for NRT traffic.Measured load caused by non-controllable load (RT)

Overloadthreshold x

Load Targetthreshold y

Pow

er

Time

Preventive Load Control

Overload Control


Load Control• LC performs the function of load control in association with AC &

PS (LC works as glue between these two functions)

• Updates load status using measurements & estimations provided by AC and PS

• Continuously feeds cell load information to PS and AC;• Interference levels• BTS power levels• Non-controllable load

LC

AC

PSNRT load

Load change info

Load status


Packet Scheduler• A non-real time call constitutes of a bursty sequence of packets.

• In the downlink, the Packet Scheduler decides which channel to use, DCH or FACH.

• The load target can be reached by scheduling the transmission of NRT packets .

time

packet service session

packet call

reading time

packet size packet arrival interval


Packet Scheduler

• PS allocation times need to be fast to accommodate changing conditions & accurate (up-to-date load info)

• Capacity requests sent via traffic volume measurement reports (governed by RNP parameters)

• PS comprises two parts: MS specific & Cell specific

power

time

non-controllable load

controllable load

Total Load

Target threshold

Overload threshold

• Responsible for scheduling radio resources for both UL and DL NRT RABs

• Scheduling period defined by RNP parameters

• PS relies on up-to-date information from AC and PS

• Capacity allocated on a needs basis using ‘best effort’ approach


Packet Scheduler (PS)• PS also responsible for:

• TFCS selection• Initial channel selection based on RNP parameters & RLC buffer load;

• small amounts of NRT data sent on control channels (Cell _FACH state)

• long and frequent data sent on dedicated channels (Cell _DCH state)

• Queuing of unscheduled NRT capacity requests (queuing algorithm)

• Capacity request handling policy

• Increasing/decreasing user bit rates (governed by RNP parameters)

• PS preventive & overload actions


Packet Scheduler

Radio network planning parameters

Periodical cell measurements

Periodical radio link measurements

RB setup/reconfiguration/release

information

Traffic volume measurements (triggers for

DCH allocation)

Updated power estimations

Control of traffic volume measurements

DCH allocations for NRT RB

PacketPacketschedulerscheduler


Resource Manager• Responsible for managing the logical radio resources of the RNC in

co-operation with AC and PS

• On request for resources, from either AC(RT) or PS(NRT), RM allocates:

• DL spreading code• UL srambling code

• Also looks after code tree management (to maintain orthogonality);• Initial code selection – codes concentrated to the same branch• Code re-fragmentation – dynamic reallocation of codes as users

enter/leave the system

Code Type Uplink Downlink

Scrambling codes

Spreading codes

User separation Cell separation

Data & control channels from same UEUsers within one cell


Outer Loop Power Control

Power Control (PC)

Open Loop Power Control (Initial Access)

Closed Loop Power Control

RNC

BS

MS


Power control in WCDMA

• Fast, accurate power control is of utmost importance – particularly in UL;• UEs transmit continuously• WCMDA often uses 1 frequency• Poor PC leads to increase interference > reduced capacity

• From BTS perspective every UE accessing network increase interference

• WCMDA capacity is proportional to interference level > minimise interference

• PC maintains link quality by adjusting UE (UL) and BTS (DL) powers every slot

• Mitigates 'near far effect', by providing minimum required power for each connection

• UEs and BTSs should always be at the lowest possible transmission power

• PC utilises Signal-to-Interference Ratio (SIR) – independently for each connection

• Provides protection against shadowing and fast fading


Open Loop Power Control• Controlled by UE• Determines how much power UE should use during random

access procedure (UL)• Network informs UE of current network status;

• CPICH power (RNP parameter)• UL required C/I ratio (RNP parameter)• UL interference

• UE uses these parameters to calculate initial power of RACH preamble

• If access request is not detected power of preamble is increased in steps

• After detection of MS signal, the initial SIR is calculated in RNC

Preamble PreamblePreamble

Preamble

MS

Ou

tpu

t Pow

er

AICH

Mesage Part

RACH


Fast Loop Power Control

• Located in BTS and UE• Controls the power of the dedicated physical channels• Power control changes can occur every slot (i.e. 1500 times per

second)• BTS and UE continuously compare recevied SIR with SIR target

and inform each other to either increase or decrease its power (using TPC commands)

With OptimumPower Control

WithoutPower Control

MS1

MS2

MS3

MS4

MS1 MS2 MS3 MS4

Rec

eive

d p

ower

at

BS

Rec

eive

d p

ower

at

BS

MS1MS2

MS3

MS4


Closed Loop Power Control

• Adjusts the SIR for every user based on BER/FER observation. Initial, max. and min. SIR values are set by AC

• Needed to track changes in radio environment• Aims to provide required quality• UL quality evaluation is made after MDC• RNP parameters control the threshold comparison process for SIR

target and the reporting of these results• If SIR target reaches its maximum (I.e. radio conditions

deteriorate even though SIR target is inceased, system has to take action;

• inter-frequency / inter-system handover• RRC connnection release


Handover Control• HC is responsible for:

• Managing the mobility aspects of an RRC connection as UE moves around the network

• Maintaining high capacity by ensuring UE is always served by strongest cell

Hard handover: MS handover between different frequencies or between WCDMA and GSM.

Soft handover: MS handover between different base stations.Softer handover: MS handover within one base station but between different sectors.

• Soft handover keeps simultaneous connection to different base stations thus providing a way to improve call quality during handover. However, this feature has a direct impact on network capacity and therefore is a trade-off between quality and capacity. It has also an effect to coverage due cell breathing.

• Optimisation has an important role in controlling the handover performance during the pre-launch optimisation (initial setting). This role is especially essential in continuous optimisation when traffic increases and levelling of traffic between base stations becomes more important.


Hanover Control

BS1

BS2

BS3

Received signal strength

BS3

Distance from BS1

Threshold

Base station diversity

BS2

BS1


Handover Control

Intra-layerInter-layer

Intra-System

Soft(er) Handover

Hard Handover

Inter-frequency

Intra-frequency

Intra-frequency

Inter-System

(Inter-RAT)

Hard Handover

Inter-frequency

WCDMA to GSMWCDMA to GPRSGSM to WCDMAGPRS to WCDMA

WCDMA to WCDMA




Requires Compressed

Mode


Increased load 800 kbps Decreased coverage

Increased load 800 kbps Decreased coverage

Low load 200 kbps Large coverage

Low load 200 kbps Large coverage

128 kbps

64 kbps

8 kbps144 kbps

64 kbps

64 kbps

144 kbps

144 kbps

64 kbps64 kbps

• Traffic load has direct effect on the cell size

• Radio Resource Management provides means to control cell breathing in network optimisation

• Traffic load has direct effect on the cell size

• Radio Resource Management provides means to control cell breathing in network optimisation

Cell Breathing


WCDMA Key Benefits

• Soft Handover• Call is connected before handover is completed, reducing the

probability of a dropped call

• Processing Gain• Basic CDMA benefit => the wider the transmitted bandwidth

compared to the user datarate the less power is needed for the transmission

• Advanced Radio Resource Management (RRM)• RRM will control call admission and packet scheduling and all

RRM building blocks are closely related to each other

• Multipath Signal Processing • Combines power for increased signal integrity => RAKE

receiver

01. WCDMA Fundamentals

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