wcdma handover principle

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Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.

WCDMA Handover Principle

Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved. Page2

Foreword

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.


Objectives

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


Contents

1. Introduction of Handover

2. Measurement of Handover

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


Questions

The differences among Soft handover, softer

handover and hard handover

Typical application scenarios


Summary

This chapter focus on the purpose of handovers and

the categories of handover in WCDMA.


Contents





Contents


2.1 Measurement control and measurement report

2.2 The basic definitions of measurement

2.3 Measurement event

2.4 Compressed mode


Measurement Control, normal case

UE UTRAN

MEASUREMENT CONTROL

Measurement Control


Measurement Control


UE UTRAN

MEASUREMENT CONTROL



UE UTRAN

MEASUREMENT REPORT

Measurement Report


Contents





2.4 Compressed mode


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

),2/(10)1(1010 11

aBest

N

iiNew HRLogMWMLogWLogM

A

),2/(10)1(1010 11

aBest

N

iiNew HRLogMWMLogWLogM

A

Reporting Criterion

Decision formula: for example, 1A event :

Path Loss

Other measurement：


Key Parameters


Contents





2.4 Compressed mode


Intra-frequency Measurement Events 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

l 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


Contents





2.4 Compressed mode


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


Questions

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?


Summary

This chapter covers intra-frequency measurement,

inter-frequency measurement, inter-system

measurement and UE internal measurement from

their definitions and application scenarios


Contents





Contents


3.1 Soft Handover

3.2 Intra-frequency Hard Handover

3.3 Inter-frequency Hard Handover

3.4 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


Contents


3.1 Soft Handover





Brief Introduction 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 no


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

Similar to soft handover Decision

1D event Execution

UE reports CFN-SFN information Synchronization hard handover Use the original DOFF value Continuous CFN

UE does not report CFN-SFN information The hard handover based on timing Re-configure DOFF Calculate CFN according to DOFF


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


Contents


3.1 Soft Handover





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

Application scenarios 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


Contents


3.1 Soft Handover





The Brief Introduction of Inter-system Hard Handover

Application scenarios 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 (I) Inter-system handover caused by coverage parameters

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

BSIC confirmed (Optional) Inter-system handover caused by coverage parameters

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

Using inter-system frequency quality handover threshold Configure the delay trigger time, hysteresis for each event


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


Questions

What are the differences between soft handover and

softer handover?

What is compressed mode?

Draw out the hard handover signaling flow.


Summary 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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wcdma handover principle

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