09 Principles of Handover in WCDMA

HUAWEI TECHNOLOGIES CO., LTD.

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Huawei Confidential

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.


TS25.331 Flow of Handover and

Related Parameters

TS25.133 Measurement Process

TS25.215 Definitions of Measurement

Indexes


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


Chapter 1 Overview of Handover

Chapter 2 Handover Measurement

Chapter 3 Soft Handover

Chapter 4 Hard 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


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





1.3 Methods and Types of

Handover




CELL_DCH CELL_FACH

CELL_PCHURA_PCH

IDLEDEAD RRC connection

UE Modes and StatusUE Modes and Status


CELL_DCH CELL_FACH

CELL_PCHURA_PCH

IDLE

DEAD

- Searching a PLMN- Find a cell to camps on

- Monitering the paging channel- Cell reselecting

- Dedicated channel - Common channel, low speed service- Triggered by theCN

- Reducing activities, saving power and resource

RRC connection

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

UE Modes and Status (Status migration)UE Modes and Status (Status migration)

SMSVP/Voice/PS

Low speed PS

Cell Update No trafficAfter several Cell Updates

URA Update





1.3 Methods and Types of Handover




Handover Methods

Changing cells in the status of CELL_DCH

Handover

Directly retry

Changing residing cells in the statuses of IDLE, CELL_FACH,

CELL_PCH, and URA_PCH

Cell reselection (forward handover)


Handover Types in CELL_DCH

Classify by System Change

Classify by Signaling Flow

Classify by Frequency

ChangeDescription

Intra- system

Soft handover

Intra-frequency

Low-rate services (e.g. AMR, VP）

Hard handover

High-rate services (e.g. PS384K)

No Iur interface between to RNCs

Inter-frequency

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)

Inter-system (Inter-RAT)





1.3 Methods and Types of Handover




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

Frequencies in the cells before and after

handoverIntra-frequency

Intra-frequency

Inter-frequency or

Inter-RAT

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

None

DisadvantageOccupies more resources.

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

Call drop ratio is relatively higher

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


Soft Handover – (Connection before Disconnection)

RNCNodeB1 NodeB2


Softer Handover – (A Special Case of Soft Handover)

RNCNode B

Cell A Cell B


Hard Handover- (Disconnection before Connection)

RNCNodeB1 NodeB2


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?


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.


Chapter 1 Overview of

Handover

Chapter 2 Handover

Measurement




2.1 Basic Concepts about

Handover

2.2 Three Steps of Handover

2.3 Measurement Control

2.4 Measurement Report


Basic Concepts about Handover

Active set

Monitor set

Detection set

Event report

Event to periodic report

Periodic report

Radio link (RL)

Radio link set (RLS)

Maximum-ratio combining

Selective combining

Soft handover gain

Pilot channel (CPICH)



2.1 Basic Concepts about Handover





Three Steps of Handover

Decision

Execution

Measurement

The following figure shows handover procedure (measurement quality set to PCPICH Ec/Io)

UE measures the CPICH Ec/Io of its serving cell(s) and neighbor cells. And sends the measurement report to RNC.

UE measures the CPICH Ec/Io of its serving cell(s) and neighbor cells. And sends the measurement report to RNC.

Are handover criteria meet?Are handover criteria meet?

Perform a handover and update relative parameters

Perform a handover and update relative parameters

Measure

Decision

Execution

Y

N



2.1 Basic Concepts about Handover





Measurement Control

Measurement control is used to notify the UE of the object to be measured, neighbor cell list, report method, and event parameters.

When measurement conditions are changed, the RNC notifies the UE of new conditions.

Measurement Control, normal case

UE UTRAN

Measurement Control


Measurement Control

Measurement Control

The RNC sends a

measurement control message

according to the best cell in the

active set.

− Measurement identity

− Measurement command

− Measurement type

− Measurement object

− Criteria of measurement report

− Measurement report mode


Measurement Objects

Measurement objects

Intra-frequency measurement

− CPICH RSCP, CPICH Ec/Io, or Pathloss (normally Ec/Io)

Inter-frequency measurement

− CPICH RSCP, CPICH Ec/Io

Inter-RAT measurement

− GSM Carrier RSSI, BSIC Identification, and BSIC Reconfirmation


Measurement Model

Measurement time of layer 1 is 200 ms. The vendor can decide the filtering method.

Filtering parameters of layer 3 can be configured.

Measurement results are reported when the reporting criteria are met.

Layer 1filtering

Layer 3filtering Evaluation

of reportingcriteria

A DB C

C'

parameters parameters


Measurement Model - Layer 3 Filtering

Filtering of measurement results by the UE is calculated according to the following formula:

Fn ＝ (1-α)Fn-1+αMn

Fn: new measurement result after L3 filtering

Fn-1: the last measurement result after filtering

Mn: the latest measurement result from the physical layer of the UE

α ＝ 0.5k/2, K is the parameter in filter coefficient in the measurement control message (default=3 ). If it is set to 0 , layer 3 filtering is unavailable.



2.1 Basic Concepts about

Handover

2.2 Three Steps in Handover




Measurement Report

When the measurement reporting criteria are met, the UE

reports the results to the RNC as an event.

Measurement Report

UE UTRAN

Measurement Report


Measurement Report Methods

Event report

The measurement report is sent when the reporting criteria are met.

Periodic report (event to periodic report)

If the RNC does not respond after several events are reported, the

UE switches to periodic report.

The interval between report and total report times are controlled

through parameters.


Intra-frequency Measurement Events

Intra-frequency measurement events are identified through 1X:

1A: A CPICH signal from monitor set enters the reporting range. Means a

monitor cell is good enough that the UE can add it into active set.

1B ： A CPICH signal in active set leaves the reporting range. Means a cell

in the active set is not good enough according to the best cell.

1C: Replace event. A CPICH signal from a monitor set is better than one in

the active set.

1D: Event of changing the best serving cell.

1F: Event that measurement results of the cells in the active set are lower

than an absolute threshold.


Inter-frequency Measurement Events

Inter-frequency measurement events are identified by 2X:

2B event: The quality of the serving cell is lower than an absolute

threshold value and the quality of another inter-frequency neighbor

cell is higher than another threshold value. - Handover based on

Coverage

2C event: The quality of the inter-frequency neighbor cell is higher

than an absolute threshold value. - Handover based on Non-Coverage

(e.g. Load)

2D event: The quality of the serving cell is lower than an absolute

threshold value. It is used to start the compress mode.

2F event: The quality of the serving cell is higher than an absolute

threshold value. It is used to stop the compress mode.


Inter-RAT Measurement Events

Inter-RAT Measurement events are identified through 3X:

3A event: The quality of the currently used frequency is lower than an

absolute threshold value, but the quality in a GSM cell is higher than

another absolute threshold value. - Handover based on Coverage

3C event: The quality in a GSM cell is higher than an absolute threshold

value. - Handover based on Non-Coverage (e.g. Load)

2D event: The quality of the currently used frequency is lower than an

absolute threshold value. It is used to start the compress mode.

2F event: The quality of the currently used frequency is higher than

another absolute threshold value. It is used to stop the compress mode.


Some Event Report Control Parameters Relative threshold (reporting range)

Unidirectional difference between two measurement results, for example,

relative threshold value between 1A event and 1B event

Absolute threshold (threshold)

Measurement results reach an absolute value, for example, absolute

threshold values of 1E event and 1F event

Hysteresis

Bi-directional difference between two measurement results

Time to trigger

Time that lasts until the previous threshold value is met

Weight factor (W)

Control calculation of comparative benchmark values


Event Report Decision

Deciding according to the judgment inequation

Example: decision inequation of 1A event

If W = 0, for RSCP and Ec/Io,

10LogM(New)≥10LogM(Best) - (R1a-H1a/2)

M: measurement results

R: relative threshold values

H: hysteresis

W: weight factor. It is 0 generally.

),2/(10)1(1010 11

aBest

N

iiNew HRLogMWMLogWLogM

A

2. Other measurement variables (e.g. RSCP, Ec/Io)

),2/(10)1(1010 11

aBest

N

iiNew HRLogMWMLogWLogM

A

1. Path loss


Event Report Decision - Example of 1A Event Measurement Report

Measurement results in the primary cell

1A event occurs


Event Report Decision (Continued)

Trigger criteria of 1B event:

)2/(10)1(1010 111

bbBest

N

iinew HRLogMWMLogWLogM

A

,2/11 ffNew HTM Trigger criteria of 1F event:

,2/1dBestNotBest HMM Trigger criteria of 1D event:

,2/1cInASNew HMM Trigger criteria of 1C event:


Event Report Decision - Example of 1B Event Measurement Report

1B event is reported

Delete the cell which primary scrambling code is 23


Event Report


Event Report

Replace cell 1 by the cell 2


Event Report


Questions

In which cases does the RNC trigger

measurement control?

Is the start time of intra-frequency,

inter-frequency, and Inter-RAT

measurement the same?

When is an event reported?

How to report an event?

What events does intra-frequency

measurement contain?

What events does inter-frequency

measurement contain?


Summary

This chapter describes concepts related to handover, explains measurement control, which covers measurement objects, event report.



Chapter 2 Handover

Measurement




3.1 Flow of Soft Handover

3.2 Common Parameters and Intra

Frequency Neighbor Cell


Flow of Soft Handover - Adding Radio Link

(Reporting 1A event)

(Add new radio link)


Flow of Soft Handover - Adding Radio Link


Flow of Soft Handover - Deleting Radio Link

(Reporting 1B event)

(Del old radio link)


Chapter 3 Flow of Soft Handover

3.1 Flow of Soft Handover

3.2 Common Parameters and Intra

Frequency Neighbor Cell


Setting Common Parameters of Soft Handover Relative threshold

1A and 1B event have their individual values.

The value of 1B should be larger than that of 1A, to avoid ping-pong

handover. (1A < 1B)

− Default setting: 1A: 3 dB; 1B: 6 dB

Hysteresis

Configure it for each event separately.

Usually the threshold values of 1A event and 1B event are 0 dB and those

of 1C, 1D, 1F are 4 dB.

This value affects the soft handover area of the UE.

MML command for adding cell-class intra-frequency handover parameters: ADD CELLINTRAFREQHOMML Command for setting RNC-class intra-frequency handover parameters: SET INTRAFREQHO


Common Parameters of Soft Handover (Continued)

Time to trigger

Configured it for each event separately.

The value of 1B should be larger than that of 1A, to avoid ping-pong

handover. (1A < 1B)

− Default setting: 1A: 320 ms; 1B: 640 ms; 1C, 1D: 640 ms

Lay 3 filtering coefficient

It is shared by all types of intra-frequency measurement.

This parameter is sensitive to the delay of event triggering and ping-

pong handover.

It is usually set to 3.


Intra Frequency Neighbor Cell

The Handover can only happened among the neighbor cells

MML: Add IntraFreqNCell:

Configure neighbor cells with the same frequency

The maxim number of IntraFreqNCell for each cell is 31


Questions

Draw the signaling flow chart of soft

handover.

What is the impact of parameters in

soft handover on system performance?

What parameters can affect

performance of soft handover besides

the parameters described in this

chapter?


Summary

This chapter describes the signaling

flow and common parameters of soft

handover in WCDMA.


Other Handovers

The WCDMA system supports many handover technologies.

Besides soft (softer) handover described previously, there are

intra-frequency hard handover, inter-frequency hard handover,

and Inter-RAT handover.

If the system supports HSDPA, the WCDMA system also

supports handover between carrier frequency of HSDPA and

ordinary carrier frequency to balance load and to utilize network

resources efficiently.

Refer to supplementary contents for description of intra-

frequency hard handover, inter-frequency hard handover, and

Inter-RAT handover. This part is provided for self-study.


Course Summary

Course Summary

This course describers the principles, algorithm, related parameters and general flow of soft handover in the WCDMA system.

Thank you

www.huawei.com



4.1 Intra-frequency Hard Handover

4.2 Inter-frequency Hard Handover

4.3 Inter-RAT Hard Handover


Intra-frequency Hard Handover - Measurement and Event

Measurement

It is similar to that in soft handover

Related events

1D event


Intra-frequency Hard Handover - Handover Flow

TARGET NODEB SOARCE NODEB

3. ALCAP ESTABLISHMENT

9. ALCAP RELEASE


Intra-frequency Hard Handover - Common Parameters BeBitrateThd: BE service handover rate decision threshold.

If the maximum rate of transmission channel of the BE service is not more than this threshold, the system performs soft handover for the users using this service.

If the maximum rate of transmission channel of the BE service is greater than this threshold, the system performs intra-frequency hard handover for the users using this service.

MML: SET HOCOMM

Parameters related to the 1D event

Time to trigger (640ms), hysteresis (4dB)

The parameters should be appropriate so the system can not only track changes of the signal to perform handover, but also avoid ping-pong handover.



4.1 Intra-frequency Hard

Handover

4.2 Inter-frequency Hard

Handover

4.3 Inter-RAT Hard

Handover


Inter-frequency Hard Handover Overview

Features

The frequency is different after handover.

For the UE with only one transceiver, the compress mode should be

started to assist measurement.

Advantages

The handover success rate is higher than that of intra-frequency hard

handover.

The load on carrier frequencies are balanced.


Inter-frequency Hard Handover Overview (Continued)

Disadvantages

The compress mode causes extra radio resource

Application scenarios

Uncontinuous coverage of carrier

Load caused handover

Hierarchical cell


Inter-frequency Hard Handover - Measurement and Event

Measurement in inter-frequency hard handover

Measurement variables:

− CPICH RSCP, CPICH Ec/N0

Different measurement types are used for different handover purposes:

− Edge of carrier coverage area: CPICH RSCP

− Center of carrier coverage area: CPICH Ec/Io


Inter-frequency Hard Handover - Measurement and Event (Continued)

Measurement Report Methods

Event report

− 2D event: The quality of current frequency is lower than a certain absolute

threshold value. It is used to start the compress mode.

− 2F event: The quality of current frequency is higher than a certain absolute

threshold value. It is used to stop the compress mode.

− 2B event: The quality of current frequency is lower than the absolute

threshold value, and the quality of the neighbor frequency is higher than

another absolute threshold value. It is used in coverage handover trigger.

− 2C event: The quality of the neighbor frequency is higher than a certain

absolute threshold value. It is used in load handover trigger.

Periodic report


Inter-frequency Hard Handover - Starting and Stopping of the Compression Mode Starting conditions of measurement

2D event: If the quality of the currently frequency is lower than a certain absolute threshold value, the compress mode is started.

Use the velocity estimation algorithm to decide starting of inter-frequency handover measurement.

Stopping conditions of measurement 2F event

After the best cell is changed, the new best cell does not have any inter-frequency neighbor cell.

The timer of inter-frequency measurement expires.

The speed estimate status of the UE changes, and related inter-frequency handover measurement is disabled according to the hierarchical cell algorithm

In hard handover, if the coverage reason is triggered, handover is stopped simultaneously.


Inter-frequency Hard Handover Flow

Signaling flow

9. ALCAP RELEASE

3. ALCAP ESTABLISHMENT

SOARCE NODEBTARGET NODEB


Inter-frequency Hard Handover - Common Parameters Inter-frequency coverage handover parameters:

Methods for report inter-frequency measurement: periodic or event

Inter-frequency measurement variables: CPICH Ec/Io or CPICH RSCP.

Inter-frequency measurement layer 3 filtering coefficient, time to trigger, and hysteresis

Starting and stopping threshold values of inter-frequency measurement: Configure it respectively for CPICH Ec/Io, CPICH RSCP, and configure 2D, 2F event for CS and PS.

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

Quality threshold value of used frequency in inter-frequency hard handover

Lowest access threshold value in inter-frequency handover


Inter-frequency Hard Handover - Questions Why the compress mode threshold for the CS and PS are

configured different?

The Eb/No required by these two services are different, so the RSCPs are also different.

If the UE uses the CS service and PS service at the same time, the 2D and 2F parameters of which service are used to decide starting the compress mode?

If the parameters of any service meet the threshold, the compress mode will started.

Is the compress mode started immediately after the UE sends the 2D parameter?

The compress mode is started after the RNC reconfigure the RB.



4.1 Intra-frequency Hard Handover

4.2 Inter-frequency Hard Handover

4.3 Inter-RAT Hard Handover


Inter-RAT Hard Handover

Overview

Measurement and Event

Handover Decision Algorithm

Handover Flow

Common Parameters


Inter-RAT Hard Handover - Overview Category

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

Features The radio access technology (RAT) adopted in the system after handover

is different. Need compress mode to assist measurement.

Advantages Coverage: Solve the problem of transition between different systems. Capacity: Improve the utilization of old equipments to the greatest extent

(2G->3G)

Disadvantages The flow is complicated, and the requirement for compatibility of

equipment is high. The UE is complicated.


Inter-RAT Hard Handover - Measurement and Event

Measurement in Inter-RAT hard handover (GSM measurement)

Measurement types:

− GSM Carrier RSSI

− BSIC Identification

− BSIC Reconfirmation

Measurement processing: layer-1 filtering, layer-3 filtering


Inter-RAT Hard Handover - Measurement and Event (Continued)

Measurement Report

− Event report

▪ 2D event: starting GSM measurement

▪ 2F event: stopping GSM measurement

▪ 3A event: The quality of WCDMA is lower than an absolute

threshold but quality of a GSM neighbor cell is higher than

another absolute threshold. - Handover based on Coverage

▪ 3C event: The quality of a GSM neighbor cell is higher than an

absolute threshold. - Handover based on Non-Coverage (e.g. Load)


Inter-RAT Hard Handover - Decision Algorithm

Decision of Inter-RAT handover

Inter-RAT handover based on coverage

− Event

▪ 3A event: The quality of the currently used frequency is lower

than an absolute threshold value, but quality of a GSM cell is

higher than another absolute threshold value.

− Periodic

▪ Evaluation: Confirm the status according to the GSM RSSI

measurement value and the BSIC of the target cell in GSM and

evaluate the cell of which the GSM RSSI exceeds the absolute

threshold value. If both conditions are met, consider the cell of

which the BSIC is confirmed.


Inter-RAT Hard Handover - Decision Algorithm (Continued)

Inter-RAT handover based on non-coverage

− Event

▪ The 3C event means that the quality of the GSM cell is

higher than an absolute threshold value.


Inter-RAT Handover Flow


Inter-RAT Handover - Common Parameters

Inter-RAT coverage handover parameters

Starting and stopping thresholds of Inter-RAT measurement: They

determine the thresholds of the 2D event and 2F event. Configure CPICH

Ec/Io, CPICH RSCP for the CS service and PS service respectively.

Measurement variables for the 2D event and 2F event in Inter-RAT

handover: Select CPICH Ec/Io or PICH RSCP.

BSIC selection confirmation switch

Inter-RAT coverage handover threshold: GSM RSSI threshold in Inter-RAT

coverage handover. Configure it for the CS service and PS service

respectively.

Quality threshold of used frequency in Inter-RAT handover

Configure time to trigger and hysteresis for each event respectively

Penalty duration


Summary - 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-RAT handover (UMTS ↔ GSM/GPRS)

According to the purpose of handover:

− Based on Coverage

− Based on Noncoverage

Documents

09 Principles of Handover in WCDMA