Inter-IRAT Handover in LTE

Jaymin Kavaiya

EETS 8316

SMU ID: 31096862

What is Inter-RAT mobility?

2G/3G

LTE

Intra-RAT mobility

Inter-RAT mobility LTE to 3G inter-RAT -Done when there is limited LTE coverage -To balance load of new system 3G to LTE inter-RAT -Triggered by traffic management policies

Why Inter-RAT HO is done?

• LTE will be available only in Urban areas during initial phase. Therefore mobility across Radio Access Technology will be critical.

• Historically, handover is main reason for call drop so it is significant to minimize failures and drops during HO.

• Non-real time data should be considered as important and should not be dropped while executing HO procedure.

• Some instances Applications may have to re-authenticate to already established services e.g. streaming have to be restarted.

Handovers

• Intra LTE inter cells HO (LTE HO within eNB)

• Intra LTE inter eNB HO (LTE HO between eNB)

• Inter LTE HO (HO between LTE and other access technologies)

– Between E-UTRAN and UTRAN (3G)

– Between E-UTRAN and GSM

– Between E-UTRAN and non 3GPP Access

How Inter-RAT HO is done?

Handover Steps • Setting up measurements • Evaluating measurement reports • Handover Preparation • Handover Execution • Handover Completion • Data forwarding • If inter-RAT is to GSM, NACC(Network assisted Cell

Change) is used and no resources are prepared in the target system. UE enter GSM system in RRC_Idle mode and start the RRC Connection Setup Procedure.

Measurements types in E-UTRAN

I. Intra-frequency E-UTRAN measurements

II. Inter-frequency E-UTRAN measurements

III. Inter-RAT measurements for UTRAN and GERAN

IV. Inter-RAT measurements of CDMA2000 HRPD or 1xRTT frequencies

Any LTE capable UE is classified in any three measurements types.

How to collect Measurements?

• Each Measurement Type will define on one or several Measurement Object(s) (Carrier Frequency).

• Reporting Configurations are defined on each Measurement Object (i.e. criteria like Event triggered, periodic or event triggered periodic reporting).

• Measurement quantities and Reporting Configurations are considered separately for each Measurement Type.

• Measurement Object and Measurement Configurations are related with the help of Measurement id.

• eNB configures each Measurement Type using a Measurement Identity and instructs the UE when to report the measurements results.

• Measurement commands are used by eNB to order the UE to start, modify or stop measurements specified by RRC as in UTRAN Measurement_Control.

What UE measures? • After instructions from eNB; UE measures two parameters on

reference signal: cell RSSI & CPICH RSCP or CPICH En/lo.

• The handover measurement report triggering are based on cell RSSI & CPICH RSCP in GSM or CPICH Ec/Io for UMTS for inter-RAT.

• Serving Cell’s RSRP (purple) is degrading threshold 1, while neighbor cell RSSI or CPICH RSCP or CPICH En/lo (orange) increasing threshold 2 will trigger the event as shown in graph.

• Similarly, hysteresis (to prevent ping pong between E-UTRAN and UTRAN) and time to trigger events could be set.

Measurements

current cell UE target cell

fcfc

current cell UE target cell

fcfc

Scenario D Scenario E

current cell UE target cell

fc

fc

Scenario F

Image Courtesy: 3GPP TS 36:300-a80 page 68

• Inter-frequency neighbor (cell) measurements are done as shown below in image; performed by the UE when the neighbor cell operates on a different carrier frequency, compared to the current cell.

• This type of measurements are carried out by UE based on its capability and generally measured with measurement gaps.

• HO will be performed to the best cell according to prioritization (As per 3GPP 36.304, absolute priorities of different E-UTRAN frequencies or inter-RAT frequencies may be provided to the UE in various SIBs.)

• eNB triggers the process “RRC connection release with redirect” based on downlink Reference Signal Received Power (RSRP) measurements in the source cell.

Inter-RAT HO between UMTS example

UETarget

RNC

Source

MME

Target

SGSN

S/P-GW

GGSNS-eNB P-GW

UL/DL User Plane PDUs

RRC Connection Reconfiguration

Measurement Report (Consists of target UMTS Cells)

S1AP: Handover Required (target UMTS Cell id)

Handover Decision

Handover Preparation

S1AP: Handover Command

RRC Mobility from E-UTRAN Command

UTRAN Access Procedures

Handover to UTRAN Complete

DL User Plane PDUs

Direct FWDing

In-direct Forwarding (if same S-GW/GGSN serving subscriber)

Relocation Complete

S-GW relocation or Bearer Modification

DL User Plane PDUs

UL User Plane PDUs

Inter-RAT HO between UMTS example (1/2)

• eNB configure the UMTS measurements in the UE using the “RRC Connection Reconfiguration” message. – It sets thresholds (serving Cell and inter-RAT neighbor

cell) for activations and set events

– It gives WCDMA center frequency and scrambling code to be used.

• UE measures UMTS cells and sends RRC: Measurement Report

• eNB takes decision to perform HO to UMTS

Inter-RAT HO between UMTS example (2/2) • Preparation Phase:

– In order to prepare target UMTS cell, S-eNB sends S1AP: “Handover Required” Message to MME.

– This message contains target UMTS cell id – MME then contacts SGSN based on UMTS Cell id and

allocates the resources (using FWD Relocation Req. : Out of the scope or this discussion)

– Once MME gets confirmation (receipt of Relocation Complete message from SGSN) MME sends S1AP: “Handover Command” to eNB

• eNB then sends “RRC: Mobility from E-UTRA Command” to UE indicating move to UTRAN.

Coverage based HO

• Handovers in E-UTRAN are network controlled and usually triggered by measurement reports sent by UE.

• Measurement reports are sent according pre-configured reporting criteria (like we have seen in the example, neighboring cells becomes better or serving cell becomes worst than absolute threshold).

• RRC Reconfiguration message provide list of intra-LTE and inter-RAT measured cells.

• It also provides handover related parameters such as threshold and cell prioritization for measurement reports.

To balance the load of new system

time

Cell load

Load threshold for mobility

• Moving forward, Source eNB load is used as a trigger to perform IRAT handovers .

LTE states and inter-RAT Handover

Image Courtesy: Joona Vehanen of Aalto University http://lib.tkk.fi/Dipl/2011/urn100472.pdf

Summary

• Intro to Inter-RAT Handover

• Inter-RAT HO Applications

• Setting up measurements

• Evaluating measurement reports

• Handover Preparation

• Handover Execution

• Handover Completion

• Data forwarding

Acknowledgements:

• 3GPP TS 23.401 (E-UTRAN) access • 3GPP TS 36.300 Overall description • 3GPP TS 36.214 Physical Measurements • http://wired-n-

wireless.blogspot.com/2009/12/lte-inter-rat-handover-e-utran-to-utran.html

• http://lib.tkk.fi/Dipl/2011/urn100472.pdf • http://lteworld.org/blog/measurements-lte-e-

utran • Radisys Whitepaper - UE Handovers in LTE