UMTS Handover description

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WCDMA Handover Principal and Analysis

ISSUE 1.0

2Internal Use

Why mobile systems need handover?

UE mobility The mobile system is composed of

cells which the coverage ability is

limited. Provide continuous service in

mobile system is the basic element

in QoS.

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ObjectiveObjective

Upon completion this course, you will be able to understand:

The basic definitions of Handover

The algorithms of handover decision

The handover flow

The parameters of handover

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Chapter 1 Chapter 1 Introduction to HandoverIntroduction to Handover

Chapter 2 Handover Measurement

Chapter 3 Basic Handovers

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The Purpose of HandoverThe Purpose of Handover

Providing the continuous service in mobile system is the basic element in QoS.

The load balance: sharing the resource

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The Basic Concepts of HandoverThe Basic Concepts of Handover

Active Set Monitored Set Detected set Event reporting

Event reporting to Periodic reporting Periodic reporting Radio Link (RL) Radio Link Set (RLS) Combination way:

maximum ratio combination selection combination

The soft handover gain CPICH Soft handover, softer handover, hard handover

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Types of HandoverTypes of Handover

According to the signaling characters: Soft handover (softer handover) Hard handover

According to the properties of source cell and target cell Intra-frequency handover Inter-frequency handover Inter-mode handover (FDD <-> TDD) Inter-system handover (UMTS <-> GSM/CDMA2000)

According to the purpose of handover Based on Coverage Based on Load (Optional) Based on mobility of UE (Optional) Based on Service (Optional)

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The Characters of Different HandoversThe Characters of Different Handovers

ItemItem Soft HandoverSoft Handover Hard HandoverHard Handover

The numbers of RL in active set after handover

Several One

Interruption during handover

No Yes

The frequencies of cells

Only possible in Intra-frequency cells

Occurs in Intra-frequency cells or Inter-frequency cells

Comparison between soft handover and hard handover:

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The Characters of Different HandoversThe Characters of Different Handovers

During softer handover, the uplink signaling are combined in NodeB by maximum ratio combination, but during soft handover they are combined in RNC by selection combination.

Compare to later one, the maximum ratio combination give more gain. So the performance of maximum ratio combination is better.

Since softer handover is completed in NodeB, it does not consume a lot of transport resource of Iub.

Comparison between soft handover and softer handover:

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RNC

NodeB 2NodeB 1

Soft HandoverSoft Handover

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RNC

NodeB

Softer HandoverSofter Handover

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Hard HandoverHard Handover

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Three Steps of HandoverThree Steps of Handover

Decision

Execute

Measurement

Measurement Measurement control Measurement execution and

the result processing The measurement report Mainly accomplished by UE

Decision Based on Measurement The application and distribution

of resource Mainly accomplished by RRM in

RNC

Execution The process of signaling Support the failure drawback Measurement control refresh

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QuestionsQuestions

The differences among Soft handover, softer handover and hard handover

Typical application scenarios

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SummarySummary

This chapter focus on the purpose of handovers and the categories of handover in WCDMA.

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Chapter 1 Introduction to Handover

Chapter 2Chapter 2 Handover MeasurementHandover Measurement

Chapter 3 Basic Handovers

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Chapter 2 Handover MeasurementChapter 2 Handover Measurement

Section 1 Measurement control and

measurement report

Section 2 Basic definitions of measurement

Section 3 Intra-frequency measurement event

Section 4 Inter-frequency and

inter-system measurement event

Section 5 UE internal measurement

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Measurement Control and Measurement Measurement Control and Measurement ReportReport

Measurement Control ：

UE UTRAN

MEASUREMENT CONTROL

Measurement Control, normal case

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Measurement Control and Measurement Measurement Control and Measurement ReportReport

Measurement report ：

UE UTRAN

MEASUREMENT REPORT

Measurement report, normal case

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Chapter 2 Measurement of HandoverChapter 2 Measurement of Handover

Section 1 Measurement control and

measurement report

Section 2 Basic concepts of measurement

Section 3 Intra-frequency measurement event

Section 4 Inter-frequency and

inter-system measurement event

Section 5 UE Internal Measurement

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Basic Concepts of MeasurementBasic Concepts of Measurement

Measurement values of Handover Intra-frequency and inter-frequency:

CPICH RSCP, CPICH Ec/No, Path loss Inter-frequency ：

CPICH RSCP, CPICH Ec/No Inter-system ：

GSM Carrier RSSI, BSIC Identification, BSIC Reconfirmation

Reporting methods of measurement Periodic reporting Event reporting

The events of reporting Intra-frequency events ： 1A,1B,1C,1D,1E,1F Inter-frequency events ： 2D,2F,2B,2C Inter-system events ： 3A,3C Others ： 6G,6F

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Measurement ModelMeasurement Model

Layer 1filtering

Layer 3filtering Evaluation

of reportingcriteria

A DB C

C'

parameters parameters

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Reporting CriteriaReporting Criteria

Reporting Criteria Decision formula: For example, 1A event :

1.Path Loss

2.Other measurement ：

Relative thresholdRelative threshold, Absolute threshold, Hysteresis, TimHysteresis, Time to Triggere to Trigger

),2/(10)1(1010 11

aBest

N

iiNew HRLogMWMLogWLogM

A

),2/(10)1(1010 11

aBest

N

iiNew HRLogMWMLogWLogM

A

Mnew : measurement result of cell entering reporting rangeMi : measurement result of cells in active setNA : number of cells in active setMbest : measurement result of best cell in active setW : weight factorR : Reporting rangeH1a : hysteresis value of 1A event

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Key ParametersKey Parameters

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Chapter 2 Measurement of HandoverChapter 2 Measurement of Handover

Section 1 Measurement control and

measurement report

Section 2 Basic definitions of measurement

Section 3 Intra-frequency measurement events

Section 4 Inter-frequency and

inter-system measurement events

Section 5 UE Internal Measurement

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Intra-frequency Measurement EventsIntra-frequency Measurement Events

Intra-frequency measurement events are identified with 1x :

1A : A primary pilot channel enters the reporting range.

If active set of UE is full, UE stops reporting 1A event;

1B : A primary pilot channel leaves the reporting range;

1C : The primary pilot channel in a non active set is better than the primary pilot channel in an active set;

1D : The best cell changes;

1E : The measurement value of a primary pilot channel exceeds the absolute threshold

1F : The measurement value of a primary pilot channel is lower

than the absolute threshold

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Chapter 2 Measurement of HandoverChapter 2 Measurement of Handover

Section 1 Measurement control and

measurement report

Section 2 Basic definitions of measurement

Section 3 Intra-frequency measurement event

s

Section 4 Inter-frequency and

inter-system measurement events

Section 5 UE Internal Measurement

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Inter-frequency Measurement EventsInter-frequency Measurement Events

Inter-frequency measurement events are identified with 2x:

2A : The best frequency changes

2B : The quality of the current cell frequency is lower than a certain threshold, but that of the non-used

frequency is higher than a certain threshold

2C : The estimated quality of the non-used frequency is higher than a certain threshold

2D : The estimated quality of the used frequency is lower than a certain threshold

2E : The estimated quality of the non-used frequency is lower than a certain threshold

2F : The estimated quality of the used frequency is higher than a certain threshold

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Inter-system Measurement EventsInter-system Measurement Events

Inter-system measurement events are identified with 3x:

3A: The estimated quality value of the used UTRAN frequency is lower than a certain threshold, and that of the other system is higher than a certain threshold;

3B: The estimated quality value of the other system is lower than a certain threshold ;

3C: The estimated quality value of the other system is higher than a certain threshold ;

3D: The best cell in the other system changes

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Chapter 2 Chapter 2 Measurement of HandoverMeasurement of Handover

Section 1 Measurement control and

measurement report

Section 2 The basic definitions of measurement

Section 3 Intra-frequency measurement events

Section 4 Inter-frequency and

inter-system measurement events

Section 5 UE Internal Measurement

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UE Internal MeasurementUE Internal Measurement

UE Internal measurement events are identified with 6x :

6G : The time difference between downlink receiving and uplink transmission of the UE is smalle

r than an absolute threshold;

6F : The time difference between downlink receiving and uplink transmission of the UE is bigger

than an absolute threshold;

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QuestionsQuestions

How many events are there in intra-frequency measurement? What are they?

How many events are there in UE internal measurement? How many events are there in inter-frequency

measurement? What are they?

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SummarySummary

This chapter covers intra-frequency measurement, inter-frequency measurement, inter-system measurement and UE internal measurement from their definitions and application scenarios.

SummarySummary

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Chapter 1 Introduction to Handover

Chapter 2 Handover Measurement

Chapter 3Chapter 3 Basic HandoversBasic Handovers

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Chapter 3 Basic HandoversChapter 3 Basic Handovers

Section 1 Soft Handover

Section 2 Intra-frequency Hard Handover

Section 3 Inter-frequency Hard Handover

Section 4 Inter-system Hard Handover

Section 5 Compressed Mode

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Introduction of Soft HandoverIntroduction of Soft Handover

Soft Handover Features

UE has several RLs with different cells----active set.

The handover among different cells which are in same RLS is softer handover.

Soft handover Combination:– Selection combination in uplink– Maximum combination in downlink

Softer handover Combination:– Maximum combination in uplink and downlink

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Introduction of Soft HandoverIntroduction of Soft Handover

AdvantagesAdvantages

Soft handover gain: Multi-Cell gain: Multiple unrelated radio links can reduces the required

fading margin. Macro Diversity Combining gain: Gain for the link demodulation of the soft

handover:

Load balance: Different cells receive the signal from a UE in uplink, which can decrease

the transmission power of UE. Similarly, UE receive signal from different cells, which also can decrease

the required transmission power of base station. Decrease the possibility of call drop caused by ping-pong handover.

DisadvantagesDisadvantages

More resource needed in downlink, especially for the code resource of BE service.

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Measurement of Soft Handover Measurement of Soft Handover

The measurement of soft/softer handover

Measurement value ： CPICH RSCP, CPICH Ec/No, Pathloss

Process of Measurement ： Layer 1 filter, Layer 2 filter

Reporting way

Periodic reporting Event reporting

– Event type ： 1A, 1B, 1C, 1D, 1F– Reporting rules: Trigger condition, Relative threshold (or

Absolute threshold), Hysteresis, Time_to_Trigger– Event reporting to periodic reporting

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The Events of Soft Handover MeasurementThe Events of Soft Handover Measurement

Soft/softer handover measurement events

Intra-frequency events reporting: 1A : A primary pilot channel enters the reporting range.

If active set of UE is full, UE stops reporting 1A event; 1B : A primary pilot channel leaves the reporting range; 1C : The primary pilot channel in a non active set is better

than the primary pilot channel in an active set; 1D : The best cell changes;

1E : The measurement value of a primary pilot channel

exceeds the absolute threshold 1F : The measurement value of a primary pilot channel is

lower than the absolute threshold

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Decision Algorithm of Soft HandoverDecision Algorithm of Soft Handover

Soft/softer handover decision

1A : Add RL into active set ； 1B : Delete RL from active set ； 1C : Replace cell ； 1D : Change best cell

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Execution of Soft HandoverExecution of Soft Handover

Soft handover execution

The principal of measurement control Configure the neighbor cells and the parameters acco

rding to best cell

RLC mode UM mode is used for measurement control AM mode is used for measurement reporting

The compensation and restriction of soft handover From event reporting to periodic reporting

– Parameters ： Periodic reporting time interval Number for reporting ti

mes

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Soft Handover Flow (Add RL)Soft Handover Flow (Add RL)

The execution of soft handover (Add RL)

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Soft Handover Flow (Del RL)Soft Handover Flow (Del RL)

The execution of soft handover (Del RL)

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Key Parameters To OptimizeKey Parameters To Optimize

Relative thresholdRelative threshold Set 1A, 1B value separately 1A < 1B ， which makes deleting RL is more difficult, and it can avoid ping-pong

handover Usually 1A:Usually 1A: 3dB;3dB; 1B: 6dB

Time to triggerTime to trigger Each event can be set separately Usually, 1B>1A ， which makes deleting RL is more difficult, and it can avoid pin

g-pong handover Usually, 1A: 320ms, 1B: 640ms

Layer 3 filter coefficientLayer 3 filter coefficient Only one value for all intra-frequency measurement Sensitive to the delay of event trigger and ping-pong handover Usually ： 3

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Chapter 3 The Basic HandoversChapter 3 The Basic Handovers

Section 1 Soft Handover

Section 2 Intra-frequency Hard Handover

Section 3 Inter-frequency Hard Handover

Section 4 Inter-system Hard Handover

Section 5 Compressed Mode

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Characteristic of Intra-frequency Hard HandoverCharacteristic of Intra-frequency Hard Handover

Characteristic UE has one RL with one cell in the process of handover Disconnect UE from the original cell, then hand it over to target cell The working frequency of source cell and target cell are same

Advantages Enhance the efficiency of the OVSF code and hardware resource

Disadvantages High call drop possibility because of the intra-frequency interference

Application scenarios No Iur interface between two RNCs or the Iur interface congested (only

happened in inter-RNC handover).

Handover of BE services that exceeds threshold rate.

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Measurement, Decision and Execution of Intra-Measurement, Decision and Execution of Intra-frequency Hard Handoverfrequency Hard Handover

Measurement Similar to soft handover

Decision 1D event

Execution

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Intra-frequency Hard Handover FlowIntra-frequency Hard Handover Flow

Intra-frequency hard handover flow

UE

1.RADIO LINK SETUP REQUEST

Target NODEB

Source NODEB SRNC

2. RADIO LINK SETUP RESPONSE

3.ALCAP establish

4. PHYSICAL CHANNEL RECONFIGURATION

5. RADIO LINK FAILURE INDICATION

6. PHYSICAL CHANNEL RECONFIGURATION COMPLETE

7. RADIO LINK DELETION REQUEST

8. RADIO LINK DELETION RESPONSE

9.ALCAP release

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Key ParametersKey Parameters

Handover decision threshold based on BE rate

UE should do soft handover when the rate of BE service is less than the threshold.

UE should do intra-frequency hard handover when the rate of BE service is greater than the threshold.

The parameters about 1D event:

Time to trigger ， Hysteresis The parameters should be set accord with the Qos

MML Command:MML Command: SET HOCOMMSET HOCOMM

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Chapter 3 The Basic HandoversChapter 3 The Basic Handovers

Section 1 Soft Handover

Section 2 Intra-frequency Hard Handover

Section 3 Inter-frequency Hard Handover

Section 4 Inter-system Hard Handover

Section 5 Compressed Mode

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Introduction of Inter-frequency Hard HandoverIntroduction of Inter-frequency Hard Handover

CharacteristicCharacteristic The working frequency is different after handover The compressed mode needed if the UE only has one receiver

AdvantagesAdvantages Compare to intra-frequency hard handover, the success possibility is

higher To balance the load among cells of different frequencies

DisadvantagesDisadvantages Compressed mode results in extra radio resource occupied The timing re-initiation hard handover increase the time of handover and

the risk of call drop

Application scenarios Application scenarios Disconnected coverage Handover based on load

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Inter-frequency Hard Handover Measurement Inter-frequency Hard Handover Measurement Values and EventsValues and Events

Inter-frequency hard handover measurement values

Measurement values: CPICH RSCP, CPICH Ec/No

Different handover purpose for different measurement type: At the edge of carrier coverage: CPICH RSCP At the center of carrier coverage: CPICH Ec/No

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Measurement Values and Events of Inter-frequency Measurement Values and Events of Inter-frequency Hard Handover Hard Handover

Measurement reporting Periodic reporting Event reporting

2A: The best frequency changes;

2B: The estimated quality value of the used frequency is lower than a certain threshold, and that of the non-used frequency is higher than a certain threshold;

2C: The estimated quality value of the non-used frequency is higher than a certain threshold;

2D: The estimated quality value of the used frequency is lower than a certain threshold

2E:The estimated quality value of the non-used frequency is lower than a certain threshold

2F:The estimated quality value of the used frequency is higher than a certain threshold

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Compressed Mode ICompressed Mode Initiatnitiation in Inter-frequency ion in Inter-frequency Hard HandoverHard Handover

Conditions to initiate Compressed Mode (CM) measurement

2D event2D event Used to enable the compressed mode to perform inter-

frequency measurement.

Conditions to stop measurement

2F event2F event Used to stop compressed mode. When used frequency quality

exceeds the threshold.

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Inter-frequency Hard Handover Decision AlgorithmInter-frequency Hard Handover Decision Algorithm

The inter-frequency hard handover decision

Coverage trigger handoverCoverage trigger handover 2B event2B event ：

– the quality of current serving cell is lower than absolute threshold, but the quality in other cell is higher than another absolute threshold.

– Both cells are of different frequency

Load triggers handoverLoad triggers handover 2C event2C event ：

– the quality of another frequency is higher than an absolute threshold

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Execution of Inter-frequency Hard HandoverExecution of Inter-frequency Hard Handover

Execution of Inter-frequency hard handover

UE report the timing information UE with two receivers does not need compressed mode. Synchronization hard handover Using the original DOFF

UE does not report timing information UE with a single receiver has to initiate compressed mode If the NodeB of target cell is different from the cells in active set :

– Timing re-initiation hard handover– Re-configure DOFF

If the NodeB of target cell is the same which the cells in active set:– Calculate the target cell timing relationship– Synchronization hard handover– Using the original DOFF

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Inter-frequency Hard Handover FlowInter-frequency Hard Handover Flow

Signaling flow

UEUE

1.RADIO LINK SETUP REQUEST

Target Target NodeBNodeB

SourceSourceNodeBNodeB SRNCSRNC

2. RADIO LINK SETUP RESPONSE

3.ALCAP established

4. PHYSICAL CHANNEL RECONFIGURATION

5. RADIO LINK FAILURE INDICATION

6. PHYSICAL CHANNEL RECONFIGURATION COMPLETE

7. RADIO LINK DELETION REQUEST

8. RADIO LINK DELETION RESPONSE

9.ALCAP released

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Key Parameters (I)Key Parameters (I)

Inter-frequency coverage handover parameters:

Inter-frequency measurement reporting: periodic reporting or event reporting

Inter-frequency measurement values: CPICH Ec/No or CPICH RSCP

Inter-frequency measurement layer 3 filter coefficient, delay trigger time and hysteresis

Inter-frequency initiated and stopped threshold: Considering the different demands of CPICH Ec/No and CPICH RS

CP for PS domain and CS domain, the different 2D and 2F parameters are configured.

Inter-frequency handover based on coverage: the quality threshold of target cell

Using frequency quality threshold in inter-frequency hard handover The minimum access threshold for inter-frequency handover

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Chapter 3 The Basic HandoversChapter 3 The Basic Handovers

Section 1 Soft Handover

Section 2 Intra-frequency Hard Handover

Section 3 Inter-frequency Hard Handover

Section 4 Inter-system Hard Handover

Section 5 Compressed Mode

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Introduction of Inter-system Hard HandoverIntroduction of Inter-system Hard Handover

Application scenarios WCDMA FDD < － >GSM WCDMA FDD < － >WCDMA TDD WCDMA FDD < － >CDMA2000

Characteristic Different radio access technology is used after handover Usually, compressed mode should be used to help the measurem

ent

Advantages For coverage, it can solve the problems from one system to anoth

er system For capacity, it can enhance the utilization efficiency of old equip

ments(2G->3G)

Disadvantages The flow is complicated, and it demands higher compatibility for e

quipments Demands more complicated UE

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Measurement for Inter-systemMeasurement for Inter-system ：： Compressed MoCompressed Mode Initiatedde Initiated

The inter-system measurement (GSM measurement) Measurement type:

GSM Carrier RSSI BSIC Identification BSIC Reconfirmation

Measurement reporting Event reporting

– 2D Event: initiate GSM measurement– 2F Event: stop GSM measurement

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Inter-system Hard Handover Decision AlgorithmInter-system Hard Handover Decision Algorithm

The inter-system hard handover decision

Inter-system handover due to coverage issue Event reporting:

– 3A event3A event ：– The estimated quality value of UTRAN frequency is lower than

a certain threshold, and that of the other system is higher than a certain threshold

Periodic reporting:– Evaluation ： According to periodic report GSM RSSI measure

ment value and the BSIC confirming state of target cell of GSM system, and UE evaluates GSM RSSI of target cell is greater than the absolute threshold, then consider the cell.

Inter-system handover not because of coverage issue Event reporting

– 3C event3C event: – The estimated quality value of the other system is higher than

an absolute threshold.

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Inter-system Handover FlowInter-system Handover Flow

The execution of inter-system handover

1. Relocation Required 2. Prepare

Handover 3. Handover

Request

4. Handover Request Ack

12. Iu Release Command

13. Iu Release Complete 14. Send End

Signal Response

5. Prepare Handover Response

6. Relocation Command

7. DCCH : Handover from UTRAN Command

8. Handover

Detect

9. Handover Complete

10. Handover Complete

11. Send End

Signal Request

UE Node B SRNC CN MSC BSC

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Key Parameters (I)Key Parameters (I)

Parameters for Inter-system handover

Inter-system measurement initiated and stopped thresholdInter-system measurement initiated and stopped threshold: Considering different demands of CPICH Ec/No and CPICH RSCP for PS domain and CS domain, the different 2D and 2F parameters are configured

Inter-system measurement values (2D, 2F)Inter-system measurement values (2D, 2F) CPICH Ec/No CPICH RSCP

Configure the GSM RSSI threshold of CS domain and PS domain separately

Using inter-system frequency quality handover threshold

Trigger time delay, Hysteresis for each event

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Chapter 3 The Basic HandoversChapter 3 The Basic Handovers

Section 1 Soft Handover

Section 2 Intra-frequency Hard Handover

Section 3 Inter-frequency Hard Handover

Section 4 Inter-system Hard Handover

Section 5 Compressed Mode

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Purpose of Compressed modePurpose of Compressed mode

Purpose ： Measure the inter-frequency cell or inter-system cell under FDD

mode

Cause: Since one receiver only can work in one frequency, the UE has t

o stop working in current frequency if it is going to measure the signal from another frequency cell. To ensure the downlink service unaffected, the remained data should be sent in the limited time.

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Compressed Mode Sketch MapCompressed Mode Sketch Map

One frame(10 ms) Transmission gap available for

inter-frequency measurements

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Realization Methods of Compressed modeRealization Methods of Compressed mode

CM MethodsCM Methods

Reduce SF by halfReduce SF by half This double the data rate. But since amount of data not changed, it halves the

time in which it is sent, open up a gap. Advantage: easy to handle for RNC Disadvantage: occupied the resource of NodeB, decrease the utilizing

efficiency of OVSF

PuncturingPuncturing Decrease the coding redundancy Advantage: easy to handle for RNC; Disadvantage: limited to the property of channel coding; decreased the coding

gain

Higher layer schedulingHigher layer scheduling Higher layer permit only some transport format to be used in CM, to generate

gap. Appropriate for variable-rate service. Complicated method

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QuestionsQuestions

What are the differences between soft handover and softer handover?

What is compressed mode? Draw out the hard handover signaling flow.

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SummarySummary

This chapter focus on the basic handovers in WCDMA: soft handover, softer handover, intra-frequency hard handover, inter-frequency hard handover and inter-system hard handover. It also introduced the application scenarios for these different handovers

Meanwhile, compressed mode is introduced in this chapter

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