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WCDMA Handover
Principle
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 Why mobile systems need handover?

 Mobile systems are composed by cells
which the coverage is limited.
 The mobility of the UE.
 Providing the continuous service - the
basic element in QoS.

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 TS25.331 Flow of Handover and

Related Parameters
 TS25.133 Measurement Process
 TS25.215 Definitions of Measurement

Indexes

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 After studying this course, you will:



Understand basic concepts about
handover



Understand the flow of soft handover



Master basic parameters about soft
handover



Master soft handover decision algorithms


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Chapter 1 Overview of Handover
Chapter 2 Handover Measurement
Chapter 3 Soft Handover
Chapter 4 Hard Handover

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Chapter 1

Overview of Handover

1.1

Purposes of Handover

1.2

Modes and Status of the UE


1.3

Methods and Types of
Handover

1.4

Comparison between Soft
Handover and Hard Handover

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Purposes of Handover
 Providing the continuous service in mobile system is the basic

element in QoS


Providing continuous communication service is the main purpose
of handover.

 The load balance: sharing the resource
 The hierarchy divided by speed and service: high efficiency of


using resource

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Chapter 1

Overview of Handover

1.1

Purposes of Handover

1.2

Modes and Status of the UE

1.3

Methods and Types of
Handover

1.4

Comparison between Soft
Handover and Hard Handover


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UE Modes and Status

URA_PCH

DEAD

IDLE

RRC connection
CELL_DCH

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CELL_PCH

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CELL_FACH

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UE Modes and Status (Status migration)
- Reducing activities, DRX, saving power and resource

RRC connection
URA_PCH

CELL_PCH
URA Update

After several Cell
Updates

Cell Update No traffic

CELL_DCH

CELL_FACH
Low speed PS

- Dedicated channel
VP/Voice/PS

These statuses are only applied
in the UTRAN and UE. They
are transparent to the CN.

- Common channel,
low speed service
- Triggered by theCN


SMS

IDLE

- Monitering the paging channel
- Cell reselecting

- Searching a PLMN
- Find a cell to camps on

DEAD

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Chapter 1

Overview of Handover

1.1

Purposes of Handover

1.2

Modes and Status of the UE


1.3

Methods and Types of Handover

1.4

Comparison between Soft
Handover and Hard Handover

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Handover Methods
 Changing cells in the status of CELL_DCH



Handover



Direct retry

 Changing residing cells in the statuses of IDLE, CELL_FACH,


CELL_PCH, and URA_PCH


Cell reselection (forward handover)

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Handover Types in CELL_DCH
Classify by
Classify by
System Change Signaling Flow

Classify by
Frequency
Change

Soft handover

Low-rate services (such as AMR ）

Intra-frequency
Intra- system
Hard handover
Inter-system
(Inter-RAT)


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Description

Inter-frequency

High-rate services (such as
PS384K)
Without the Iur interface
For the purpose of coverage or
load balancing (e.g. handover
between carriers with different
load, handover to the carrier that
supports the HSDPA service.
WCDMA-to-GSM handover)

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Chapter 1

Overview of Handover

1.1

Purposes of Handover


1.2

Modes and Status of the UE

1.3

Methods and Types of Handover

1.4

Comparison between Soft
Handover and Hard Handover

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Features Comparison between Soft Handover
and Hard Handover
Item

Soft Handover (Softer Handover)

Hard Handover

Number of radio links in
the activate set


Several

One

Interruption during
handover

No

Yes

Intra-frequency

Intra-frequency
Inter-frequency or
Inter-RAT

Frequencies in the cells
before and after
handover
Handover gain

Maximum-ratio combining or selective
combining which can reduce the impact of
fading and transmission power of the UE.

Disadvantage

Occupies more resources.

Problems occur when the power of soft
handover cells is unbalanced.

None

Call drop ratio is
relatively higher

In a word, some resources are wasted to obtain optimal system performance in soft handover.

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Soft Handover – (Connection before Disconnection)

NodeB1

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NodeB2

RNC

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Softer Handover – (A Special Case of Soft Handover)
RNC

Node B
Cell A

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Cell B

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Hard Handover- (Disconnection before Connection)

NodeB1

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NodeB2

RNC

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Questions

 What are the differences between

soft handover, softer handover, and
hard handover?
 Why intra-frequency handover is not

always soft handover?
 How does handover gain arise? How

much are various handover gains?

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Summary

 This chapter describes the purposes of

handover, working modes of the UE and
common handover types in the WCDMA
system, compares soft handover and hard
handover and describes respective

advantages and disadvantages.
 In addition, this chapter describes a brief

flow of each type of handover.

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Chapter 1

Overview of Handover

Chapter 2
Handover
Measurement
Chapter 3

Soft Handover

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Chapter 2

Handover Measurement

2.1

Basic Concepts about
Handover

2.2

Three Steps of Handover

2.3

Measurement Control

2.4

Measurement Report

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Basic Concepts about Handover
 Active set


Radio link (RL)

 monitor set

Radio link set (RLS)

 Detection set

Maximum-ratio combining

 Event report

Selective combining



Event to periodic report

 Periodic report

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Soft handover gain
Pilot channel (CPICH)

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Chapter 2

Handover Measurement

2.1

Basic Concepts about Handover

2.2

Three Steps of Handover

2.3

Measurement Control

2.4

Measurement Report

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Three Steps of Handover
The following figure shows handover

procedure (measurement quality set
to PCPICH Ec/No)
UE
UEmeasures
measuresthe
theCPICH
CPICH
Ec/No
of
its
serving
Ec/No of its servingcell(s)
cell(s)and
and
neighbor
cells.
And
sends
the
neighbor cells. And sends the
measurement
measurementreport
report totoRNC.
RNC.

Measurement

Decision

Measure


N
Are
Arehandover
handovercriteria
criteriameet?
meet?

Decision

Y

Execution
Perform
Performaahandover
handover
and
update
and updaterelative
relative
parameters
parameters

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Execution