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? • • •
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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 reauthenticate 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
• 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. Scenario D current cell
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Scenario E UE
target cell
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current cell
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Scenario F UE
target cell
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current cell
UE
target cell
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Image Courtesy: 3GPP TS 36:300-a80 page 68
Inter-RAT HO between UMTS example UE
Target RNC
S-eNB
Target SGSN
Source MME
S/P-GW GGSN
P-GW
UL/DL User Plane PDUs RRC Connection Reconfiguration Measurement Report (Consists of target UMTS Cells)
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) DL User Plane PDUs Relocation Complete
S-GW relocation or Bearer Modification 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 preconfigured 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 • Moving forward, Source eNB load is used as a trigger to perform IRAT handovers . Cell load Load threshold for mobility
time
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
