07 - LTE Mobility Management

LTE Mobility Management

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Contents

LTE Measurements PLMN Selection Cell Selection/Re-selection Handover

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LTE Measurements  Intra LTE measurements ( from LTE to LTE)  UE measurements CQI measurements Reference Signal Received Power (RSRP) Reference Signal Received Quality (RSRQ)  e-NodB measurements Standardized: DL RS Tx Power, Received Interference Power, Thermal Noise Power Non standardized :TA, Average RSSI, Average SINR, detected PRACH preambles, transport channel BLER

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Measurements from LTE to other systems  UE measurements are mainly intended for handover. UTRA FDD: CPICH RSCP, CPICH Ec/No and carrier RSSI GSM: GSM carrier RSSI UTRA TDD: carrier RSSI, RSCP, P-CCPCH CDMA2000: 1xRTT Pilot Strength, HRPD Pilot Strength

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LTE Measurements  List of detected preambles The eNB shall report a list of detected PRACH preambles to higher layers. Higher layer utilize this info for the RACH procedure

 Transport BLER The ACK/ NACKs for each transmission of the HARQ process are reported to the MAC. Based on these ACK/NACKs the higher layers compute the BLER for RRM issues.

 TA The eNB needs to measure the initial timing advance (TA) of the uplink channels based on the RACH preamble.

 Average SINR In UL the eNB measures SINR per UE. The average SINR can be used as a quality indicator for the UL power control

 UL CSI Channel state information per PRB for each UE. The CSI shall be the received signal power averaged per PRB.

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LTE Measurements  REFERENCE SIGNAL RECEIVED POWER (RSRP) The received power on the resource elements that carry cell-specific reference signals

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LTE Measurements  Reference Signal Received Quality (RSRQ) The relation of N times the Reference Signal Received Power divided by the total received power in the channel bandwidth. Within the RSRQ also the noise and interference contributions are considered.

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LTE Measurements  Received Signal Strength Indicator (RSSI) RSSI comprises the linear average of the total received power (in [W]) observed only in OFDM symbols containing reference symbols for antenna port 0, in the measurement bandwidth, over N number of resource blocks by the UE from all sources, including co-channel serving and non-serving cells, adjacent channel interference, thermal noise etc.

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Contents

LTE Measurements PLMN Selection Cell Selection/Re-selection Handover

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PLMN Selection  When the UE is switched on, it attempts to establish a contact with a public land mobile network (PLMN) using a certain radio access technology.

 The overall process is divided into three sub-processes: • PLMN selection and reselection to search for an available mobile network. • Cell selection and reselection to search for a suitable cell belonging to the selected PLMN. • Location registration to register the UE’s presence in a registration area

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PLMN Selection  When an UE is switched on, it attempts to make contact with a PLMN. The selection of the PLMN could be either automatically or manually.  

Automatic mode: Utilizes a list of PLMN to be selected in priority order. Manual mode: The mobile station indicates available PLMN to the user.

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PLMN Selection  Service Types  LIMITED SERVICE: emergency calls on an acceptable cell UE may camp to obtain limited service like emergency call. The minimum set of requirements for initiating an emergency call in a UTRAN network are: • The cell is not barred. • The cell selection criteria are fulfilled.

 NORMAL SERVICE: for public use on a suitable cell UE may camp on to obtain normal service. Such a cell shall fulfill all the following requirements: • The cell is part of the selected / registered / equivalent PLMN • The cell is not barred • The cell is not part of a forbidden registration area • The cell selection criteria are fulfilled • In case of CSG cell it is part of the white list

 OPERATOR SERVICE: For operators only on a reserved cell. Reserved cell: When the cell status "reserved for operator use" is indicated and the Access Class of the UE is 11 or 15 the UE may select/re-select this cell if in Home-PLMN Barred cell: When cell status "barred" is indicated the UE is not permitted to select/re-select this cell, not even for limited services. This information is set by office data. 10

PLMN Selection

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Contents

LTE Measurements PLMN Selection Cell Selection/Re-selection Handover

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Cell Selection/Re-Selection  Cell Search

 The UE scans all RF channels in the UTRAN band according to its capabilities to find

available PLMNs.

 On each carrier, the UE searches for the strongest cell according to the cell search

procedure (refer cell searching) and read its system information in order to find out which PLMN the cell belongs to.

 If the UE can read the PLMN identity, the PLMN and the measured signal strength is reported to the NAS: if: signal >= - x dBm (high quality PLMN); without the measured signal strength if < x dBm

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Cell Selection/Re-Selection  Cell Search

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Cell Selection/Re-Selection  Cell Search

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Cell Selection/Re-Selection  Cell Selection Procedure After selecting a PLMN, the cell selection process starts. The UE selects a suitable cell and the radio access mode based on idle mode measurements and cell selection criteria. The UE searches a suitable cell of that PLMN to camp on according to the following steps:

1) The UE creates a candidate list of potential cells to camp on by using one of the two search procedures: • Initial Cell Selection: UE scans all RF channels in the UTRAN band to find a suitable cell. On each carrier, UE searches for the strongest cell and reads its system information. Once the UE has found the suitable cell for the selected PLMN, the UE creates a candidate list consisting of this cell and its neighboring cells as received in measurement control information. • Stored Information Cell Selection (optionally): This procedure requires information stored from previously received measurement control information elements (cell parameters, carrier frequencies, etc). After the UE has found a suitable cell for the selected PLMN, candidate list is created same as the initial cell selection process. 2) Each cell on the candidate list is evaluated according to the selection criteria S as described below. 3) After selecting a suitable cell (S criterion fulfilled) for camp on, UE reports this event to NAS for registration procedures. If the registration is successful, the UE enters into "camped normally“ state. 16

Cell Selection/Re-Selection  Cell Selection Procedure If the UE is unable to find any suitable cell in the selected PLMN, the UE enters to "any cell selection" state. Camped normally state : UE obtains normal service and performs the following tasks: • Select and monitor the PCH of the cell. • Performs system information monitoring. • Performs necessary measurements for the cell reselection evaluation procedure. • Execute the cell reselection procedure

If after cell reselection evaluation process a better cell is found, the cell reselection is performed. If no suitable cel is found, the UE enters to next state “any cell selection” Any cell selection: UE searches an acceptable cell of any PLMN to camp on. If an acceptable cell is found, the UE reports to NAS and camp on this cell obtaining limited service. And UE enters to “camp on any cell” state. If the UE can’t find any acceptable cell, it stay in this state. Camped on any cell state: UE obtains limited service and periodically searches for a suitable cell in the selected PLMN. If a suitable cell is found the states changed to Camped normally. 17 17

Cell Selection/Re-Selection  S-Criteria R8 UE selects a eUTRA cell if the S (selection) criteria is fulfilled:

Srxlev > 0

Srxlev = Qrxlevmeas – (Qrxlevmin* + Qrxlevminoffset**) - Pcompensation UE measurement (RSRP)

SIB1 Parameter

SIB1 Parameter

Pcompensation = max (PEMAX*** – PUMAX, 0) (dB) SIB1 Parameter

* Qrxlevmin = LNCEL: qrxlevmin

qrxlevmin

** Qrxlevminoffset = LNCEL: qRxLevMinOffset (used

qRxLevMinOffset

only when camped in VPLMN)

pMaxOwnCell

*** PEMAX = LNCEL: pMaxOwnCell PUMAX is UE class specific max. UL Tx power

*PUMAX: UE class specific max. UL Tx power; 23 dBm 18

Cell Selection/Re-Selection  S-Criteria R9 UE selects a eUTRA cell if the S (selection) criteria is fulfilled: Srxlev > 0 and Squal > 0 Srxlev = Qrxlevmeas – (Qrxlevmin + Qrxlevminoffset) - Pcompensation Squal = Qqualmeas – (Qqualmin + Qqualminoffset) Pcompensation = max (PEMAX – PUMAX, 0) (dB)

Squal = Cell selection quality value (dB) Qqualmeas = Measured cell quality value (RSRQ) Qqualmin = Minimum required quality level in the cell (dB) Qqualminoffset

=

Offset to Qqualmin for a higher priority

qQualMinR9 qQualMinOffsetR9

PLMN while camped normally in a VPLMN *PUMAX: UE class specific max. UL Tx power; 23 dBm 19

Cell Selection/Re-Selection  Reselection Process Overview

Measurements trigger

Evaluate cell reselection criteria “R”

UE is measuring the neighbor cells only when the RSRP signal level is below some threshold. The measurements trigger is based primarily on absolute priorities. In case of equal priority additional thresholds are defined

For the reselection there are different cases depending on priorities of the neighbor frequency Layers (for inter-frequency and inter-RAT)

Execute cell reselection

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Cell Selection/Re-Selection  R-Criteria

The cell reselection evaluation process depends on whether Hierarchical Cell Structure (HCS) is used or not. In order to perform cell reselection UE measures and ranks the neighbor cells. For each type of neighbor cells (Intra-Frequency; Inter-Frequency; Inter-RAT, i.e. GSM) thresholds are definable. Measurements of neighbor cells will be triggered if these thresholds are reached.

HIGH MOBILITY / MEDIUM MOBILITY / NORMAL MOBILITY: For faster moving UEs the procedure alters - speed dependent scaling rules are applied. If the number of (different cells) cell reselections during the past time period TCRmax exceeds NCR_H, high mobility has been detected. If the number exceeds NCR_M, and not NCR_H, medium mobility has been detected.

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Cell Selection/Re-Selection  R-Criteria

In high/medium-mobility states, a UE: • multiplies Qhyst by "Speed dependent ScalingFactor for Qhyst for high/medium mobility state" if sent. • multiplies TreselectionRAT by "Speed dependent ScalingFactor for TreselectionRAT for * mobility state for RAT cells. (RAT = EUTRAN, UTRAN, GERAN).

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Cell Selection/Re-Selection  R-Criteria For inter-frequency and inter-system measurements, depending on the UE capability, the network allocates measurement gaps during which no data are sent for the UE, so that the UE could perform the necessary measurements using a single receiver. During the measurement gaps, the particular UE cannot be scheduled for data transmission, but the vacant resources could still be used for other UEs, because of the shared channel mechanism.

FOR INTRA-FREQUENCY AND EQUAL PRIORITY INTER-FREQUENCY CELLS: (Re-) Selected cell is a suitable cell (e.g. fulfills the S criterion) and is the best ranked cell (has the highest R). The UE shall however reselect the new cell, only if the following conditions are met: • The new cell is better ranked than the serving cell during a time interval Treselections • more than 1 second has elapsed since the UE has camped on the current serving cell. The cell-ranking criterion R is defined as shown below: Note, s – indicates the serving cell, n – indicates the candidate cell.

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Cell Selection/Re-Selection  Reselection Process Overview

Measurements trigger

Evaluate cell reselection criteria “R”

UE is measuring the neighbor cells only when the RSRP signal level is below some threshold. The measurements trigger is based primarily on absolute priorities. In case of equal priority additional thresholds are defined

For the reselection there are different cases depending on priorities of the neighbor frequency Layers (for inter-frequency and inter-RAT)

Execute cell reselection

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Cell Selection/Re-Selection  R-Criteria Neighbor Cell Measurement

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Cell Selection/Re-Selection  R-Criteria Neighbor Cell Measurement

 FOR INTER-FREQUENCY AND INTER-RAT NEIGHBOUR CELLS: If UE camps longer than 1 sec in the serving cell and:

- a higher priority neighbor fulfills (during TreselectionRAT): SnonServingCell,x > Threshhigh reselect neighbor cell.

- no cell fulfills SnonServingCell,x > Threshhigh : SServingCell < Threshserving,low and SnonServingCell,x > Threshx,low reselect neighbor cell.

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Cell Selection/Re-Selection  R-Criteria Neighbor Cell Measurement

 Cell Reselection from WCDMA to LTE

UE must measure the LTE frequencies and detect the available LTE cell in order to perform cell reselection to LTE. • UE measures two physical properties called for WCDMA signal. One is CPICH RSCP and CPICH EcNo. RSCP determines Srxlev and EcNo determines Squal. • Srxlev = Qrxlevemeas - qRxLevMin. Qrxlevemeas is RSCP level measured by UE and qRxLevMin is the value specified in SIB.

• Squal = Qqualmeas - qQualMin. Qqualmeas is EcNo level measured by UE and qQualMin is the value specified in SIB. • The detection measurement of LTE frequencies should be done at least once every 60s for higher priority LTE frequencies. 27

Cell Selection/Re-Selection  R-Criteria Neighbor Cell Measurement

 Cell Reselection from WCDMA to LTE • In following condition, detection measurements of lower priority LTE frequency is not required. Srxlev > absPrioCellRes.sPrioritySearch1 Squal > absPrioCellRes.sPrioritySearch2

• In following condition, UE should detect once every 30s for both lower and higher priority LTE frequencies Srxlev threshXHigh (SIB6), where Srxlev = Qrxlevmeas - qRxLevMin (SIB3), where Qrxlevemeas = measured RSCP level, qRxLevMin = minimum RSCP level for camping

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Cell Selection/Re-Selection  R-Criteria

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Contents

LTE Measurements PLMN Selection Cell Selection/Re-selection Handover

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Handover  HO Process Overview

Measurements activation/de-activation

Event based neighbor reports

Measurement Reports from UE

HO target & mode selection in eNodeB

HO execution

UE continuously monitors the serving cell. Events A2/A1 are used to activate/de-activate neighbors measurements when radio conditions are getting worse The reports of neighbors are triggered by A3 & A5 events for intraRAT A4 for Inter frequency and B1, B2 events for inter-RAT The “Measurement Report” message contains a prioritized list of neighbors (best neighbor first) eNodeB decides the urgency of HO and identifies a prioritized list of HO target cells. The eNodeB selects the target cell for HO as well as the handover mode. The HO/NACC mode could be: -Intra eNB HO -Intra LTE inter eNB via X2 -Intra LTE inter eNB via S1 -HO to WCDMA -NACC to GSM 34

Handover  HO Options • • •

Intra cell handovers used for UE operation change within cell Intra-LTE (or Intra-RAT) handovers: Intra frequency handovers → within the same frequency band. There are 2 scenarios: • Intra-eNodeB handover (i.e. inter sector/ intra site); • Inter-eNodeB handover (i.e. inter site); • Inter-eNodeB handover can happen via the X2 interface or via the S1 interface. The DL data forwarding is applied for lossless data path switching.

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Inter-frequency handovers that allow service continuity for LTE deployments in different frequency bands and also for LTE deployments within one frequency band but with different center frequencies.

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Inter-RAT mobility: • Inter-RAT PS handover These are the transfers to other radio access technologies (RAT) to provide continuous PS coverage. • Network assisted cell change (NACC) to GSM; Unlike in the handover procedure no resources are prepared in the target system. The UE will enter GSM in RRC Idle mode and start the RRC Connection Setup procedure in GSM. • SRVCC – an Inter RAT HO of PS voice bearers to CS voice bearers.

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Handover  HO in LTE

• Hard handovers: resources are prepared in the target cell before the UE is commanded to move to the target cell • Lossless: Packets are forwarded from the source to the target cell.

• Network controlled: The target cell is selected by the network not by the UE. The handover control is in the e-UTRAN not in the Core Network. • UE-assisted: Measurements are made and reported by the UE to the network although it is the network (eNodeB) which triggers those measurements.

• Late path switch: Only when the inter eNodeB handover is successful, the packet core is involved (X2 required).

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Handover  HO Signaling HANDOVER INTRA E-UTRAN

 Intra E-UTRAN Handover is used to hand over a UE from a source eNodeB to a target eNodeB using X2 when the MME is unchanged.  The intra E-UTRAN HO in RRC_CONNECTED state is UE assisted NW controlled HO, with HO preparation signalling in E-UTRAN.  To prepare the HO, the source eNB passes all necessary information to the target eNB (e.g. ERAB attributes and RRC context) and UE accesses the target cell via RACH following a contention-free procedure using a dedicated RACH preamble.  The HO procedure is performed without EPC involvement, i.e. preparation messages are directly exchanged between the eNBs.

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Handover  HO Signaling

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Handover  HO Signaling

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Handover  HO Signaling

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Handover  HO Events 1/2

If we can see the algorithm, it looks like 3G HO event concept: “hysteresis” – “time to trigger” – “cell individual offset”

Event Type

Description

Event A1 Serving becomes better than threshold Event A2 Serving becomes worse than threshld Event A3 Neighbour becomes offset better than serving Event A4 Neighbour becomes better than threshold Event A5 Serving becomes worse than threshold1 and neighbour becomes better than threshold2 Event B1 Inter RAT neighbour becomes better than threshold Serving becomes worse than threshold1 and inter RAT neighbour becomes better than Event B2 threshold2

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Handover  HO Events 2/2

• •

Intra-LTE (Intra-frequency & inter-frequency) measurements RSRP & RSRQ as event-triggering & reporting quantity

• Resulting configurable events for reporting to E-UTRAN – – – – –

Event A1: Serving cells RSRP upcrossing certain RSRP threshold Event A2: Serving cells RSRP/Q downg certain RSRP/Q threshold Event A3: Neighbor cell’s RSRP is a predefined offset better than serving cell’s RSRP Event A4: Neighbor cell’s RSRP is a better than threshold Event A5: Serving cell’s RSRP downcrossing certain threshold, while neighbor cell’s RSRP upcrossing an other threshold

RSRP

RSRP A2 trigger

A2 threshold

RSRP/Q serving move direction

RSRP A3 neighbor trigger

RSRP A4 threshold

RSRP neighbor

RSRP

A5 trigger

A4 trigger A5 thold 2

A3 offset

RSRP serving

move direction

RSRP neighbor move direction

A5 thold 1 RSRP serving move direction

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Handover  HO Measurement De-activation

HANDOVER EVENTS : A1

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Handover  HO Measurement Activation 1/2 HANDOVER EVENTS : A2

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Handover  HO Measurement Activation 2/2

radio coverage by RSRP decreases

RSRP (reported) Threshold2GERAN

Intra-frequency & Interfrequency & GERAN & UMTS measurements

Threshold2Wcdma

Intra-frequency & Interfrequency & UMTS measurements

Threshold2InterFreq

Intra-frequency & Interfrequency measurements

Threshold1

Intrafrequency measurements only

No neighbors measurements except the serving cell

Assumptions* for this example: Threshold2GERAN < Threshold2Wcdma < Threshold2InterFreq< Threshold1

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Handover  Intra-Frequency Handover 1/4 HANDOVER EVENTS A3: Better Cell Handover

• Better Cell Handover aims to keep the UE always on best cell (measured by RSRP)

• Based on 3GPP reporting event A3 : – Event A3 defines relative offset LNCEL: a3-offset – Timer to Trigger LNCEL: a3-TimeToTrigger – Reporting Interval LNCEL: a3-ReportInterval

RSRP at serving cell + a3Offset < RSRP at neighbor cell

• Better Cell Handover can be enabled / disabled:

enableBetterCellHo 46

Handover  Intra-Frequency Handover 2/4 HANDOVER EVENTS A3: Better Cell Handover

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Handover  Intra-Frequency Handover 3/4 HANDOVER EVENTS A5: Coverage Based

• Coverage Handover is used to HO to other eUTRA cell in case: • •

serving cell RSRP gets below an absolute threshold and (&&) neighbor cell RSRP gets better than an absolute threshold

• Based on 3GPP reporting event A5 : –

3GPP ThresholdEUTRA used to define absolute R“RP level for a) serving cell: LNCEL: threshold3 and b) neighbor cell: LNCEL: threshold3a – Timer to Trigger LNCEL: a5-TimeToTrigger – Reporting Interval LNCEL: a5-ReportInterval

• Coverage Handover can be enabled / disabled: enableCovHo 3GPP event A5 is UE optional; 48

Handover  Intra-Frequency Handover 4/4 HANDOVER EVENTS A5: Coverage Based

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Handover  Inter-Frequency Handover 1/2

UE needs to support both bands and inter-frequency HO – Service continuity for LTE deployment in different frequency bands as well as for LTE deployments within one frequency band but with different center frequencies – The UE performs the measurements as configured by eNodeB: eutraCarrierInfo parameter specifies the inter-frequency band – The trigger for this procedure is: • better neighbor cell (frequency) coverage (A3, RSRP) • better neighbor cell (frequency) quality (A3, RSRQ) • limited serving cell (frequency) & sufficient neighbor cell (frequency) coverage (A5)

– Measurement gaps • if needed then gap pattern 0 is used (6ms gap each 40ms) 50

Handover  Inter-Frequency Handover 2/2 Example of A3 based inter frequency handover

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Handover  Inter-RAT Handover 1/5

LTE Support Handover to other technologies:  UMTS  GSM  Other non 3GPP Systems like CDMA, HRPD and 1xRTT... Basic features related to inter-RAT Handovers are  CSFB to 2G or 3G  SRVCC to 2G or 3G

Inter RAT Handovers are based on events A2 for measurement Activation and event B1, event B2 for Handovers.

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Handover  Inter-RAT Handover 2/5 HANDOVER EVENTS B1

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Handover  Inter-RAT Handover 3/5 HANDOVER EVENTS B2

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Handover  Inter-RAT Handover 4/5 HANDOVER IRAT TO 2G/3G

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Handover  Inter-RAT Handover 5/5 HANDOVER IRAT TO CDMA 2000

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Handover  Speed Dependent Handover Speed dependent scaling of measurement parameters

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Thanks for your attention

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