3-WCDMA Handover Principal

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WCDMA Handover Principle


Why mobile systems need handover?

It is decided by the characters of mobile system:

−The mobility of UE

−The mobile system is composed by cells which the coverage ability is limited.

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


Upon completion of this course, you will

be able to:

The basic definitions of Handover

The algorithms of handover decision

The handover flow

The parameters of handover


Chapter 1 Introduction of HandoverChapter 1 Introduction of Handover

Chapter 2 Measurement of HandoverChapter 2 Measurement of Handover

Chapter 3 The Basic HandoversChapter 3 The Basic Handovers


The Purpose of Handover

Providing the continuous service in mobile system is

the basic element in QoS.

The load balance: sharing the resource

The hierarchy divided by speed and service: high

efficiency of using resource


The 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


The Categories 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-system handover (UMTS <-> GSM)

According to the purpose of handover

Based on Coverage

Based on Load (Optional)

Based on mobility of UE (Optional)

Based on Service (Optional)


The Characters of Different Handovers

Item Soft Handover Hard Handover

The numbers of RL in

active set after handover

Several One

Interruption during

handover

No Yes

The frequencies of cells Only happened in

Intra-frequency cells

Can be happened in Intra-

frequency cells or Inter-

frequency cells

Comparison between soft handover and hard handover:


The Characters of Different Handovers

Comparison between soft handover and softer handover:

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 ration combination can get

more gain. So the performance of maximum ration combination

is better.

Since softer handover is completed in NodeB, it do not

consume transport resource of Iub.


RNC

NodeB 2NodeB 1

Soft Handover


RNC (WFMR)

NodeB

Softer Handover


Hard Handover

RNC

NodeB 2NodeB 1


Three Steps of Handover

Decision

Execute

Measurement



2.1 Measurement control and measurement report2.1 Measurement control and measurement report

2.2 The basic definitions of measurement2.2 The basic definitions of measurement

2.3 Measurement event2.3 Measurement event

2.4 Compressed mode2.4 Compressed mode


Measurement Control, normal case

UE UTRAN

MEASUREMENT CONTROL

Measurement Control


Measurement Control


UE UTRAN

MEASUREMENT CONTROL



UE UTRAN

MEASUREMENT REPORT

Measurement Report


The Basic Concepts of Measurement The measurement values of Handover

Soft handover and Intra-frequency hard handover : CPICH RSCP, CPICH Ec/N0, Path loss

Inter-frequency ： CPICH RSCP, CPICH Ec/N0

Inter-system ： GSM Carrier RSSI, BSIC Identification, BSIC Reconfirmation

The reporting methods of measurement

Periodic reporting

Event reporting

The events of reporting

Intra-frequency events ： 1A,1B,1C,1D,1F

Inter-frequency events ： 2D,2F,2B,2C

Inter-system events ： 3A,3C


Measurement Model

Layer 1filtering

Layer 3filtering

Evaluationof reporting

criteria

A DB C

C'

parameters parameters


Reporting Criterion

Reporting Criterion

Decision formula: for example, 1A event :

Path Loss

Other measurement：

),2/(10)1(1010 11

aBest

N

iiNew HRLogMWMLogWLogM

A

),2/(10)1(1010 11

aBest

N

iiNew HRLogMWMLogWLogM

A


Key Parameters


Intra-frequency Measurement Events

All intra-frequency measurement report events are identified with 1X :

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

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


Inter-frequency Measurement Events

Inter-frequency measurement events are identified with 2X

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

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


Inter-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;

3C : The estimated quality value of the other system is

higher than a certain threshold ;


The Purpose of Compressed mode

Purpose：

Measure the inter-frequency cell or inter-system cell under

FDD mode

Categories :

Downlink compressed:

Uplink compressed


Compressed Mode Sketch Map

One frame(10 ms) Transmission gap available for

inter-frequency measurements


The realization Methods of Compressed mode

Realization Methods

SF/2

Rate matching/puncturing

Higher layer scheduling



3.1 Soft Handover3.1 Soft Handover

3. 2 Intra-frequency Hard Handover3. 2 Intra-frequency Hard Handover

3.3 Inter-frequency Hard Handover3.3 Inter-frequency Hard Handover

3.4 Inter-system Hard Handover3.4 Inter-system Hard Handover


Brief Introduction of Soft Handover

Characters of Soft Handover

During handover, UE has several RLs with different cells----

active set.

The handover among different cells which are in same RLS

can be softer handover.

− Soft handover:

▪ Selection combination in uplink

▪ Maximum combination in downlink

− Softer handover

▪ Maximum combination in uplink and downlink


Brief Introduction of Soft Handover Advantages

Soft handover gain

Load balance

Decrease the possibility of call drop caused by ping-pong handover.

Disadvantages

More resource needed in downlink, especially for the code resource

of BE service.

Usually, the gain of downlink power is negative.

When the downlink power from different cells is not balanced, it will

bring side-effect.


Measurement of Soft Handover

The measurement of soft handover/softer handover

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

Process of Measurement ： Layer 1 filter, Layer 3 filter

Reporting way

− Periodic reporting

− Event reporting

▪ Event type ： 1A 、 1B 、 1C 、 1D 、 1F

▪ UE measures the time difference between CFN and SFN

▪ Reporting rules: trigger condition, Relative threshold, Absolute threshold, Hysteresis, Time to trigger

− Event reporting to periodic reporting


The Decision Algorithm of Soft Handover

Soft/softer handover decision

1A : Radio Link Addition

1B : Radio Link Removal

1C : Combined Radio Link Addition

and Removal

1D : change best cell


The Execution of Soft Handover

Soft handover execution

The update principal of measurement control

− Configure the neighbor cells and the parameters according to best cell

RLC mode

− AM mode is used for measurement control

− UM mode is used for measurement report

The compensation and restriction of soft handover

− From event reporting to periodic reporting


Soft Handover Flow (add RL)


Soft Handover Flow (del RL)


Key Parameters

Relative threshold

Time to trigger

Layer 3 filter coefficient

Absolute threshold of soft handover


The Characters of Intra-frequency Hard Handover Characters

UE only has one RL with one cell in the process of handover

Disconnect UE with the original cell, then hand it over to target cell

The working frequency of source cell and target cell are same

Advantages

Enhance the using efficiency of the OVSF code and hardware resource

Disadvantages

High call drop possibility because of the intra-frequency interference

Application scenarios

Without Iur interface between two RNCs or the Iur interface jam (only happened in inter-RNC handover)

The different strategies can be used for different conditions, such as code resource condition, the QoS condition and so on.


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


Key Parameters

Handover decision threshold based on BE speed

UE should do soft handover when the speed of BE

service is less than the threshold.

UE should do intra-frequency hard handover when the

speed 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


The Brief Introduction of Inter-frequency Hard Handover

Characters

The working frequency is different after handover

The compressed mode needed if the UE only has one

receiver

Usually, the timing re-initiation hard handover is used

for hard handover


Disconnected coverage

Handover based on load

Hierarchy cells


Measurement Values and Events

The Inter-frequency hard handover measurement

values

Measurement values:

− CPICH RSCP 、 CPICH Ec/N0

Different handover purpose for different

measurement type:

− In the edge of carrier coverage: CPICH RSCP

− In the center of carrier coverage: CPICH Ec/No


Compressed Mode Initiation

Conditions for measurement initiated

2D event : The estimated quality value of the used

frequency is lower than a certain threshold

Conditions for Measurement stopping

2F event : The estimated quality value of the used

frequency is higher than a certain threshold


Cells Hierarchy Algorithm Based on UE Moving Speed

When UE is in one of the hierarchy cells, the moving speed

estimation algorithm is initiated

Handover events in a while decides whether the speed

of UE satisfies the current cell hierarchy condition

UE’s moving speed decided the result.


Decision Algorithm

The inter-frequency hard handover decision

The coverage triggers the handover

− 2B event：▪ the quality in the used frequency is lower than

absolute threshold, but the quality in another non-used frequency is higher than another absolute threshold.

The load triggers handover

− 2C event：▪ the quality in another frequency is higher than an

absolute threshold


Inter-frequency Hard Handover Signaling Flow

UE

1.RADIO LINK SETUP REQUEST

Target NODEB

Source NODEB SRNC

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


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 RSCP 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


Key Parameters (II)

Inter-frequency handover parameters caused by

non-coverage ability

Inter-frequency measurement layer 3 filter

coefficient, delay trigger time and hysteresis

Inter-frequency handover based on non-

coverage ability


The Brief Introduction of Inter-system Hard Handover


WCDMA FDD < － >GSM

Characters

Different radio access technology is used after handover

Usually, compressed mode should be used to help the measurement

Advantages

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

For capacity, it can enhance the utilizing efficiency of old equipments(2G->3G)

Disadvantages

The flow is complicated, and it demands higher compatibility for equipments

Demands more complicated UE


Compressed Mode Initiated

The inter-system measurement (GSM measurement)

Measurement type:

− GSM Carrier RSSI

− BSIC Identification

− BSIC Reconfirmation

The process of measurement: layer 1 filter, layer 3 filter

Measurement reporting

− Event reporting

▪ 2D Event: initiate GSM measurement

▪ 2F Event: stop GSM measurement


Decision Algorithm

Inter-system handover caused by coverage

Event reporting:

− 3A event ： 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 .

Periodic reporting:

− Evaluation ： According to the periodic reported GSM RSSI

measurement value and the BSIC confirming state of target cell of

GSM system, and meanwhile the UE evaluates the GSM RSSI of

target cell is greater than the absolute threshold, then consider

the cell confirmed by BSIC.


Decision Algorithm

Inter-system handover caused by non-coverage

Event reporting

3C event: The estimated quality value of the other

system is higher than a absolute threshold.


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

Inter-system Handover Flow


Key Parameters (II)

Inter-system handover caused by non-coverage parameters

The period of inter-system measurement report

The decision threshold for inter-system handover: configure the

CS, PS, signaling separately

Time delay trigger : configure BSIC with acknowledged and

unacknowledged mode

Inter-frequency measurement layer 3 filter coefficient and

hysteresis

Penalty time

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3-WCDMA Handover Principal

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