handover in wcdma.pdf

HUAWEI TECHNOLOGIES CO., LTD.

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Huawei Confidential

WCDMA Handover Principle

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HUAWEI TECHNOLOGIES CO., LTD. Huawei Confidential

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

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

[ Providing continuous communication service is the main purpose of handover.

l The load balance: sharing the resource

l The hierarchy divided by speed and service: high efficiency of using resource


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


Handover Methods

l Changing cells in the status of CELL_DCH

[ Handover

[ Directly retry

l Changing residing cells in the statuses of IDLE, CELL_FACH, CELL_PCH, and URA_PCH

[ Cell reselection (forward handover)


Handover Types in CELL_DCH

Inter-system (Inter-RAT)

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-frequency

High-rate services (e.g. PS384K)No Iur interface between to RNCs

Hard handover

Low-rate services (e.g. AMR, VP）

Intra-frequency

Soft handover

Intra- system

DescriptionClassify by Frequency

Change

Classify by Signaling Flow

Classify by System Change


Features Comparison between Soft Handover and Hard Handover

Call drop ratio is relatively higher

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

Disadvantage

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

Handover gain

Intra-frequencyInter-frequency or

Inter-RATIntra-frequency

Frequencies in the cells before and after

handover

YesNoInterruption during handover

OneSeveralNumber of radio links in the activate set

Hard HandoverSoft Handover (Softer Handover)Item

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

l What are the differences between soft handover, softer handover, and hard handover?

l Why intra-frequency handover is not always soft handover?

l How does handover gain arise? How much are various handover gains?


Summary

l 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.

l In addition, this chapter describes a brief flow of each type of handover.



2.1 Basic Concepts about Handover

2.2 Three Steps of Handover

2.3 Measurement Control

2.4 Measurement Report


Basic Concepts about Handover

l Active set

l Monitor set

l Detection set

l Event report

[ Event to periodic report

l Periodic report

l Radio link (RL)

l Radio link set (RLS)

l Maximum-ratio combining

l Selective combining

l Soft handover gain

l Pilot channel (CPICH)


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


Measurement Controll Measurement control is used to notify the UE of the object to be

measured, neighbor cell list, report method, and event parameters.

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

Measurement Control, normal case

UE UTRAN

Measurement Control


Measurement Controll 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

l 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

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

l Filtering parameters of layer 3 can be configured.

l 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.




2.2 Three Steps in Handover




Measurement Report

l 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

l Event report

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

l 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

l 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

l 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

l 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 thresholdvalue. - 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 Parametersl Relative threshold (reporting range)[ Unidirectional difference between two measurement results, for example,

relative threshold value between 1A event and 1B event

l Absolute threshold (threshold)[ Measurement results reach an absolute value, for example, absolute

threshold values of 1E event and 1F event

l Hysteresis[ Bi-directional difference between two measurement results

l Time to trigger[ Time that lasts until the previous threshold value is met

l Weight factor (W)[ Control calculation of comparative benchmark values


Event Report Decision

l 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

l In which cases does the RNC trigger measurement control?

l Is the start time of intra-frequency, inter-frequency, and Inter-RAT measurement the same?

l When is an event reported?

l How to report an event?

l What events does intra-frequency measurement contain?

l What events does inter-frequency measurement contain?


Summary

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



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 Handoverl 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

l 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)

l 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

l 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

l The Handover can only happened among the neighbor cells

l MML: Add IntraFreqNCell:

[ Configure neighbor cells with the same frequency

[ The maxim number of IntraFreqNCell for each cell is 31


Questions

l Draw the signaling flow chart of soft handover.

l What is the impact of parameters in soft handover on system performance?

l What parameters can affect performance of soft handover besides the parameters described in this chapter?


Summary

l This chapter describes the signaling flow and common parameters of soft handover in WCDMA.


Other Handovers

l 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.

l 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.

l 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

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

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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

l Measurement

[ It is similar to that in soft handover

l 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 Parametersl 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

l 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.


Inter-frequency Hard Handover Overview

l Features

[ The frequency is different after handover.

[ For the UE with only one transceiver, the compress mode should be started to assist measurement.

l 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)

l Disadvantages

[ The compress mode causes extra radio resource

l Application scenarios

[ Uncontinuous coverage of carrier

[ Load caused handover

[ Hierarchical cell


Inter-frequency Hard Handover- Measurement and Event

l 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)

l 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 Model 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.

l 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 Flowl Signaling flow

9. ALCAP RELEASE

3. ALCAP ESTABLISHMENT

SOARCE NODEBTARGET NODEB


Inter-frequency Hard Handover- Common Parametersl 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 - Questionsl 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.

l 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.

l Is the compress mode started immediately after the UE sends the 2D parameter?[ The compress mode is started after the RNC reconfigure the RB.


Inter-RAT Hard Handover

l Overview

l Measurement and Event

l Handover Decision Algorithm

l Handover Flow

l Common Parameters


Inter-RAT Hard Handover - Overviewl Category[ WCDMA FDD <－>GSM[ WCDMA FDD <－>WCDMA TDD[ WCDMA FDD <－>CDMA2000

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

is different.[ Need compress mode to assist measurement.

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

(2G->3G)l 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

l 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

l 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

1 . R elo cat io n R equ ired 2 . P repare

H ando ver 3 . H ando ver R equ est

4 . H ando ver R equ est Ack

1 2 . Iu R elease C o m m and

1 3 . Iu R elease C o m ple te 1 4 . Send E nd

Signa l R espo nse

5 . P repare H ando ver R espo nse

6 . R e lo catio n C o m m and

7 . D C C H : H ando ver fro m U TR AN C o m m and

8 . H ando ver D etect

9 . H ando ver C o m ple te

1 0 . H ando ver C o m plete 1 1 . Send

E nd

UE N o de B SR N C C N M S C B SC


Inter-RAT Handover - Common Parametersl 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

l According to the signaling characters

− Soft handover (softer handover)

− Hard handover

l According to the properties of source cell and target cell:

− Intra-frequency handover

− Inter-frequency handover

− Inter-RAT handover (UMTS ↔ GSM/GPRS)

l According to the purpose of handover:

− Based on Coverage

− Based on Noncoverage

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