Coverage-Triggered Inter-Frequency Handover.pdf

Coverage-Triggered Inter-FrequencyHandover

USER DESCRIPTION

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Copyright

Ericsson AB 2011. All rights reserved. No part of this document may bereproduced in any form without the written permission of the copyright owner.

Disclaimer

The contents of this document are subject to revision without notice due tocontinued progress in methodology, design and manufacturing. Ericsson shallhave no liability for any error or damage of any kind resulting from the useof this document.

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All trademarks mentioned herein are the property of their respective owners.These are shown in the document Trademark Information.

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Contents

Contents

1 Introduction 11.1 Basic Characteristics 11.2 Benefits 21.3 Additional Information 2

2 Feature Operation 32.1 Network Requirements 42.2 Feature Operation Sequence Diagram 42.3 Process Steps 5

3 Parameters 73.1 Introduced Parameters 73.2 Affected Parameters 7

4 Network Impact 94.1 Capacity 94.2 Handover 9

5 Associated Features and Affected Functions 115.1 Prerequisite Features 115.2 Affected Features 115.3 Related Features 115.4 Affected System Functions 11

6 Performance 136.1 KPIs 136.2 Counters 136.3 Events 13

7 O&M Information 15

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Coverage-Triggered Inter-Frequency Handover

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Introduction

1 Introduction

This document describes the Coverage-Triggered Inter-Frequency Handoverfeature.

The Coverage-Triggered Inter-Frequency Session Continuity feature allows theRBS to release the User Equipment (UE) and direct it to an alternative LTEfrequency (release with redirect) in the event of poor coverage in the servingcell. The purpose of the, Coverage-Triggered Inter-Frequency Handoverfeature is to extend this with the option of initiating a handover to a cell on adifferent LTE frequency, instead of initiating a release.

The Coverage-Triggered Inter-Frequency Handover feature provides enhancedactive mode mobility between frequencies and cells in an LTE network. Whenthe feature is active, a UE can be directed to do a handover between cells ondifferent frequencies in the LTE network while maintaining the data session.

Correct configuration of the Coverage-Triggered Inter-Frequency Handoverfeature is required to ensure the following:

That the UE will be transferred to a cell on a different frequency for whichthe UE has coverage.

That an Event A5 (serving becomes worse than threshold1 and neighborbecomes better than threshold2) measurement is configured in the UEwhenever there are several candidate target cells, none of which coversthe source cell completely.

That the UE hands over directly, without an Event A5 measurement,whenever there is (at least) one target cell which is known to cover thesource cell completely.

1.1 Basic CharacteristicsThis section describes the basic characteristics of the feature.

Feature identity:FAJ 121 0877, Coverage-Triggered Inter-FrequencyHandover

Replaces: N/A

Dependencies

This feature requires the following RAN features to be active:

FAJ 121 0493, Inter-Frequency Session Continuity, Coverage-Triggered

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This feature affects no other features.

1.2 BenefitsThis section describes the benefits of this feature.

The added benefits of the Coverage-Triggered Inter-Frequency Handoverfeature, over the Coverage-Triggered Inter-Frequency Session Continuityfeature, can be summarized as follows:

Session transition to a cell on another LTE frequency is possible withreduced interruptions to data flows during the transition process.

Enhanced control over how the UE is transferred between frequenciesand cells is available.

Using handover to transfer a UE between cells enables the RBS to usedifferent admission criteria than would be used at RRC connection setup.This makes it possible to improve the probability that a service, for exampleVoice over Internet Protocol (VOIP), is not interrupted in a network withhigh load.

1.3 Additional InformationMore information about this feature and related topics can be found in thefollowing documentation:

3GPP TS 36.331 Radio Resource Control (RRC); Protocol Specification

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Feature Operation

2 Feature Operation

This section describes the Coverage-Triggered Inter-Frequency Handoverfeature in more detail, including network configuration requirements andoperation flows.

This feature extends the Coverage-Triggered Inter-Frequency SessionContinuity feature.

The Coverage-Triggered Inter-Frequency Session Continuity feature uses theEvent A2 (serving cell becomes worse than threshold) measurement processto detect poor coverage in the serving cell. The RBS determines what otherfrequencies, LTE or other Radio Access Technology (RAT) devices are targetcandidates for transferring the UE, and if the highest prioritized frequency whichthe UE can handle (has capabilities for) is an LTE frequency.The RBS can do one of two things:

Immediately release the UE with a redirection indication for this frequency

This is referred to as blind release with redirect .

Start an Event A5 (serving becomes worse than threshold1 and neighborbecomes better than threshold2) measurement in the UE to determine ifthere is a cell on this frequency which is covering the UE

The RBS then waits until the UE sends a report for this measurement andrelease the UE with a redirection indication for this frequency.

TheCoverage-Triggered Inter-Frequency Handover feature extends this withtwo new options:

Immediately initiate a handover for the UE to a cell on the target frequencywhich is known by the RBS to cover the source cell completely

This is referred to as blind inter-frequency handover.

Start an Event A5 measurement in the UE to determine whether there is acell on this frequency which is covering the UE

The RBS waits until the UE sends a report for this measurement and initiatea handover for the UE to one of the cells received in the measurementreport. This is referred to as measurement based inter-frequencyhandover.

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2.1 Network RequirementsThe following must be fulfilled at feature activation:

License keys must be activated to operate both the Coverage-TriggeredInter-Frequency Session Continuity and Coverage-TriggeredInter-Frequency Handover features. The features are called InterFrequencySessionContinuity and InterFrequencyLTEHandover.License Management provides details on the process required to load andactivate a license.

The Coverage-Triggered Inter-Frequency Session Continuity feature mustbe configured correctly. Coverage-Triggered Inter-Frequency SessionContinuityprovides details on how to do this is provided.

There must be UE in the network that has the capability do inter-frequencyhandover. This is indicated in bit 13 of the featureGroupIndicators whichis part of the UE-EUTRA-Capability information element as described in3GPP TS 36.331

2.2 Feature Operation Sequence DiagramThe Coverage-Triggered Inter-Frequency Handover feature operates as follows:

1. The Source RBS configures an Event A2 (serving cell becomes worse thanthreshold) measurement in the UE as described in Coverage-TriggeredInter-Frequency Session Continuity.

2. When the Event A2 measurement report is received from the UE by theserving RBS, the RBS determines the set of LTE candidate frequencieswhere the UE can be transferred, as described in Coverage-TriggeredInter-Frequency Session Continuity.

3. If any of the LTE candidate frequencies is an LTE frequency, the RBSchecks whether it has information about a cell on this frequency whichis fully covering the source cell, and if so selects this cell as the targetfor (blind) handover.

4. If none of the LTE frequency candidates has a cell which covers the UE,and the RBS is configured to start measurements in the case of a Event A2measurement report, an Event A5 measurement (serving becomes worsethan threshold1 and neighbor becomes better than threshold2) for each ofthe candidate frequencies will be configured in the UE. When an Event A5measurement report is received from the UE, the RBS will investigate thereported cells and select a target cell which is possible to do a handoverto. If none of the reported cells are possible to use for handover, theUE will be released with redirect, as described in Coverage-TriggeredInter-Frequency Session Continuity, to the frequency reported bye theEvent A5 measurement.

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Feature Operation

5. If the RBS is configured to not start measurements in the case of a EventA2 measurement report, no target cell will be selected and instead the UEwill be released with redirect to one of the candidate frequencies.

6. If a target cell was selected for handover, the RBS proceeds to prepareand execute the handover attempt. This is done in the same way as isdescribed in Intra-LTE Handover, including the decision on whether to usean X2 or S1 connection when communicating with the RBS handling thetarget cell, in the case when the target cell is handled by a different RBSthan the RBS handling the source cell. The events sent from the RBSduring handover preparation and execution will contain information sayingthat it is an inter-frequency handover and whether the handover wasinitiated based on a pre-configured target cell (blind handover), or if anEvent A5 measurement was used to find a target cell (measurement basedhandover), see Section 6.3 on page 13.

2.3 Process StepsThis section describes the main process steps for the feature flow.

2.3.1 Configure for Blind Inter-Frequency Handover

When the RBS has selected an LTE frequency as a candidate target fortransferring the UE, the RBS will check if the UE should be transferredthere with a blind release with redirect, a blind handover, or if an Event A5measurement should be started in the UE. To indicate that blind handovershould be used for this target frequency, the operator needs to configure theRBS in the following way:

The covTriggerdBlindHoAllowed attribute for the serving cell must beset to the value true.

The mobilityAction attribute must have the value HANDOVER for theEUtranFreqRelation Managed Object (MO) representing the targetfrequency under the source cell in the Managed Object Model (MOM).

There has to exist a EUtranCellRelation below the targetEUtranFreqRelation that indicates a cell that is covering the servingcell completely, and therefore is safe to send the UE to from the servingcell without measuring. This is achieved in the EUtranCellRelation bysetting the coverageIndicator attribute to the COVERS value. Also,the isHoAllowed attribute must have the value true. This cell relationrepresents the target cell in the resulting blind handover.

Note: The ueMeasurementActive attribute need not have the valuefalse. Only if there are no cell relations that can be used for blindhandover will the value of ueMeasurementActive be used todetermine if a blind release with redirect should be triggered, or if anEvent A5 measurement should be started in the UE.

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2.3.2 Configure for Measurement Based Inter-Frequency Handover

When the RBS has selected an LTE frequency as a candidate target fortransferring the UE, the RBS will check if the UE should be transferredthere with a blind release with redirect, a blind handover, or if an Event A5measurement should be started in the UE. If the decision is to start an Event A5measurement, the RBS will then wait for the resulting report and then decideif the UE should be transferred to the target frequency with a release withredirect, or a handover to one of the cells included in the measurement report.

To indicate that measurement based handover should be used for this targetfrequency, the operator needs to configure the RBS as follows:

The ueMeasurementActive attribute must be set to the value true.

The mobilityAction attribute must have the value HANDOVER for theEUtranFreqRelation MO representing the target frequency under thesource cell in the MOM.

For every cell that might be reported by the Event A5 measurement and thatshould be possible to use as target cells, a EUtranCellRelation mustexist below the target EUtranFreqRelation where the isHoAllowedattribute has the value true.

Note: If the received Event A5 measurement report only contains cells whicheither do not exist in the MOM, or for which there is no cell relation fromthe source cell, or for which the isHoAllowed attribute has the valuefalse, the RBS will transfer the UE to the target frequency using arelease with redirect.

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Parameters

3 Parameters

This section describes the parameters introduced by the Coverage-TriggeredInter-Frequency Handover feature and parameters affected by activating thefeature.

3.1 Introduced ParametersTable 1 describes the parameters introduced by the feature.

Table 1 Introduced ParametersParameter Description

coverageIndicator Indicates whether the coverage area of the cell indicated by thecell relation has approximately the same coverage area (overlap)as this cell, or whether it fully covers this cell, or if it is fullycontained in this cell

mobilityAction Preferred method to transfer a UE in connected mode to a specificLTE frequency

covTriggerdBlindHoAllowed Indicates whether a blind handover from this cell can be initiatedwhen a UE reports poor coverage(1)

(1) Even if the value is true, the RBS will only initiate a blind handover if it finds a suitable cell.

3.2 Affected ParametersThe implementation of this feature affects no parameters.

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Network Impact

4 Network Impact

This section describes how the Coverage-Triggered Inter-Frequency Handoverfeature impacts the network functions and capabilities.

4.1 CapacityThis section provides information on the impact of the feature on networkcapacity.

The Coverage-Triggered Inter-Frequency Handover feature ensures thatnetwork capacity is enhanced by improving the possibility that the UE is servedby the best available frequency within the network.

4.2 HandoverThe Coverage-Triggered Inter-Frequency Handover feature improves the timetaken for to make connected mode transitions from a cell on one frequency toa cell on another frequency.

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Associated Features and Affected Functions

5 Associated Features and AffectedFunctions

This section describes how the Coverage-Triggered Inter-Frequency Handoverfeature affects other features and functions.

5.1 Prerequisite FeaturesTable 2 lists prerequisite features to the feature, that is, theymust be active prior to introducing the feature.

Table 2 Prerequisite FeaturesFeature Description

Coverage-Triggered Inter-FrequencySession Continuity

5.2 Affected FeaturesThis feature affects no other feature.

5.3 Related FeaturesTable 3 lists features that are related to the feature.

Table 3 Related FeaturesFeature Description

Intra-LTE Handover Intra-LTE intra-frequency Handover

5.4 Affected System FunctionsThis feature affects no system functions.

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Performance

6 Performance

This section describes performance indicators, counters, and events associatedwith the Coverage-Triggered Inter-Frequency Handover feature.

6.1 KPIsTable 4 lists the main Key Performance Indicators (KPIs) associated with the feature.

Table 4 KPIsKPI Description

Mobility success rate More information can be found in Key Performance Indicatorsin the sectionon Mobility.

6.2 CountersTable 5 lists the counters associated with the feature.

Table 5 CountersCounter Description

pmBadCovEvalReport Total number of traffic Bad Coverage measurement reportsreceived

pmHoExeAttLteInterF Number of outgoing intra-LTE inter-frequency handover executionattempts

pmHoExeSuccLteInterF Number of successful outgoing intra-LTE inter-frequencyhandovers

pmHoPrepAttLteInterF Number of attempts to start outgoing intra-LTE inter-frequencyhandover preparation

pmHoPrepSuccLteInterF Number of successful outgoing intra-LTE inter-frequencyhandover preparations

More information about counters can be found in Managed Object Model RBS.

6.3 EventsTable 6 lists the events associated with the feature.

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Table 6 EventsEvent Event Parameter DescriptionINTERNAL_PROC_HO_PREP_S1_OUT

EVENT_PARAM_HO_SOURCE_OR_TARGET_TYPE

Indicates the RAT of the target cellWill have the value EVENT_VALUE_LTE_INTER_FREQ whenan LTE inter-frequency Handoverpreparation was performed.

INTERNAL_PROC_HO_PREP_S1_OUT

EVENT_PARAM_HO_TARGET_SELECTION_TYPE

Type of selection for the handovertarget cell: blind (non measurementbased, that is, based onpreconfigured information), ormeasurement based

INTERNAL_PROC_HO_PREP_S1_IN


Indicates the RAT of the source cell

Will have the value EVENT_VALUE_LTE_INTER_FREQ whenan LTE inter-frequency Handoverpreparation was performed.

INTERNAL_PROC_HO_EXEC_S1_OUT


Indicates the RAT of the target cellWill have the value EVENT_VALUE_LTE_INTER_FREQ when an LTEinter-frequency Handover executionwas performed.

INTERNAL_PROC_HO_EXEC_S1_OUT



INTERNAL_PROC_HO_PREP_X2_OUT


Indicates the RAT of the target cellWill have the value EVENT_VALUE_LTE_INTER_FREQ whenan LTE inter-frequency Handoverpreparation was performed.

INTERNAL_PROC_HO_PREP_X2_OUT


Type of selection for thehandover target cell: blind (nonmeasurement based,that is,basedon preconfigured information), ormeasurement based

INTERNAL_PROC_HO_PREP_X2_IN


Indicates the RAT of the source cell

Will have the value EVENT_VALUE_LTE_INTER_FREQ whenan LTE inter-frequency Handoverpreparation was performed.

INTERNAL_PROC_HO_EXEC_X2_OUT


Indicates the RAT of the target cellWill have the value EVENT_VALUE_LTE_INTER_FREQ when an LTEinter-frequency Handover executionwas performed.

INTERNAL_PROC_HO_EXEC_X2_OUT



More information about events can be found in PM Event List.

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O&M Information

7 O&M Information

This section provides Operation and Maintenance (O&M) information for thefeature.

A license and parameter setting activates the Coverage-TriggeredInter-Frequency Handover feature. License Managementprovides details ofthe license activation process.

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toc1 Introduction1.1 Basic CharacteristicsDependencies1.2 Benefits1.3 Additional Information

2 Feature Operation2.1 Network Requirements2.2 Feature Operation Sequence Diagram2.3 Process Steps2.3.1 Configure for Blind Inter-Frequency Handover2.3.2 Configure for Measurement Based Inter-Frequency Handover

3 Parameters3.1 Introduced Parameters3.2 Affected Parameters

4 Network Impact4.1 Capacity4.2 Handover

5 Associated Features and Affected Functions5.1 Prerequisite Features5.2 Affected Features5.3 Related Features5.4 Affected System Functions

6 Performance 6.1 KPIs6.2 Counters6.3 Events

7 O&M Information

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Coverage-Triggered Inter-Frequency Handover.pdf