02-OEO109060 LTE eRAN8.1 Intra-RAT Handover Feature ISSUE 1.00(bulgaria).pdf

LTE eRAN8.1 Intra-RAT Handover Feature

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As shown above,the handover process consists of three phases: handover measurement,handover decision, and handover execution. A blind handover does not require handover measurement. 

In the handover measurement phase, the eNodeB uses the RRC Connection Reconfiguration message to deliver the measurement configuration to the UE and waits for a measurement report from the UE.

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In the handover decision phase, the eNodeB checks the measurement results reported by the UE and determines whether to initiate a handover. In the handover execution phase, the eNodeB controls the procedure of UE handover to the target cell based on the decision, to perform the handover.




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When a UE establishes a radio bearer, the eNodeB delivers the intra-frequency measurement configuration to the UE through an RRC Connection Reconfiguration message by default. Then, the UE performs intra-frequency measurements by default.

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When measurement gaps need to be set up, the eNodeB delivers the inter-frequency and/or inter-RAT measurement configuration to the UE. After that, the UE performs gapassisted inter-frequency and/or inter-RAT measurements. Inter-frequency and inter-RAT measurements can use the same gap pattern, but the eNodeB is able to differentiate between the gap configurations for inter-frequency and inter-RAT measurements.



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For intra- frequency neighbor, we don’t need defined the frequency information, it will use the same configuration as current cell.

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For the inter-frequency or inter-RAT frequencies, we must add these frequencies separately.

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eNodeB complies with all 3Gpp events to trigger UE report.

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After an event is reported for the first time, the measurement results associated with the event are reported periodically. This reporting mode is called event-triggered periodical reporting. The UE sends the measurement results to the eNodeB in event-triggered periodical reporting mode. The periodical reporting is stopped when the leaving condition is met or after the UE receives feedback from eNdoeB.

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Event-triggered periodical reporting has the following benefits: 

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Reduces the impact of loss of measurement reports or failures in internal processing on handovers. Enables retries of access in case of admission rejection Provides complete measurement results by updating cell information in periodical reports, since a single measurement report may not contain information about all neighboring cells that meet an event triggering condition and neighboring cells meeting the condition may change with UE movement.



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A measurement gap is a time period during which the UE performs measurements on a neighboring frequency of the serving frequency. Measurement gaps are applicable to interfrequency and inter-RAT measurements. The UE performs inter-frequency or inter-RAT measurements only within the measurement gaps. One UE normally has only one receiver, and consequently one UE can receive the signals on only one frequency at a time.

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When inter-frequency or inter-RAT measurements are triggered, the eNodeB delivers the measurement gap configuration, and then the UE starts gap-assisted measurements accordingly. As shown above, Tperiod denotes the repetition period of measurement gaps, and TGAP denotes the gap width, within which the UE performs measurements.

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Gap-assisted measurements for the following types of handover may co-exist: coveragebased, load-based, and service-based handovers. When two or all of the preceding types of gap-assisted measurement co-exist, the eNodeB records the measurements based on these types. These different types of measurement are called member gap-assisted measurements. The member gap-assisted measurements can share the measurement gap configuration. A UE releases measurement gaps only when all member measurements are stopped.

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There are two measurement gap patterns available: pattern 1 and pattern 2. In pattern 1, TGAP is 6 ms and Tperiod is 40 ms. In pattern 2, TGAP is 6 ms and Tperiod is 80 ms. The pattern to be used is specified through the GapPatternType parameter.



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Before evaluating the reporting criteria and sending measurement reports, the UE performs layer 1 (L1) filtering and L3 filtering on the measurement results. The L1 filtering is performed by the UE at the physical layer to eliminate the impact of fast fading on the measurement results. No user configuration is required for the L1 filtering. The L3 filtering aims at eliminating the impact of shadow fading and certain fast fading. In this way, better measurement data can be provided for the evaluation of the reporting criteria. Based on the triggering quantity, two L3 filtering coefficients are applicable: one for RSRP measurements and the other for RSRQ measurements.



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A neighboring relation is a relation between the serving cell and each candidate cell involved in a handover. Neighboring relation management covers the following aspects:

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Whether to allow automatic removal of a neighboring relation by ANR or not

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Whether to allow handovers of UEs between two cells or not

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Whether to allow handovers over an X2 interface or not

Neighboring relations are planned in the network design stage. They can be automatically adjusted by ANR. The ANR function reduces the risk of missing neighboring cells and solves the problems of inappropriate neighboring relations caused by collisions of physical cell IDs or by physical positions. In this way, the call drop rate is reduced and the handover success rate is increased.

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Intra-frequency neighboring cell 

An intra-frequency neighboring cell is a neighboring cell whose DL E-UTRA Absolute Radio Frequency Channel Number (EARFCN) is the same as the DL EARFCN of the serving cell.

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Inter-frequency neighboring cell 

An inter-frequency neighboring cell is a neighboring cell whose DL EARFCN is different from the DL EARFCN of the serving cell, and can be located on a maximum of 8 neighboring EUTRAN frequencies. An E-UTRAN cell can be configured totally with a maximum of 256 intra-frequency & inter-frequency neighboring cells, TDD cells can also be configured as inter-frequency neighboring cells of FDD cells. Huawei eNodeB supports interoperability between LTE FDD and LTE TDD.



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Inter-RAT neighboring cell 

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Inter-RAT neighboring cells are neighboring UTRAN cells, neighboring GERAN cells, and neighboring CDMA2000 cells. The maximum configurations of neighboring cells and frequencies for an E-UTRAN cell are as follows: 128 neighboring UTRAN cells and 16 neighboring UTRAN frequencies; 64 neighboring GSM cells and 16 neighboring GSM frequency groups; 32 neighboring CDMA2000 EVDO cells and 32 neighboring CDMA2000 1x cells.

Black cell: 

Blacklisted cells are not considered in event evaluation or measurement reporting.

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This list should be configured manually and delivered to UE via SIB




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If intra-frequency handover switch is set “ON”, then eNodeB will deliver intra-frequency measurement message by default.

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Relevant switch:



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If we configure the specific neighbors as high measurement priority, then eNodeB will deliver theses cells information in measurement control message, and after that UE will prior to measure theses cells

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Relevant configuration



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In intra-frequency measurement control message, A3 event configuration will be delivered by default, and A1, A2 events are optional if inter-frequency or inter-RAT handover is enabled.




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Mn: The measurement result of the neighboring cell

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Ofn: The frequency-specific offset for the frequency of the neighboring cell, it is not valid during intra-frequency handover.

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Ocn: The cell-specific offset (CIO) for the neighboring cell, related command: MOD EUTRANINTERFREQNCELL

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Ms: The measurement result of the serving cell

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Ofs: The frequency-specific offset for the serving frequency

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Ocs: The cell-specific offset for the serving cell

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Hys: The hysteresis for event A3. It is specified through the IntraFreqHoA3Hyst parameter and contained in the delivered measurement configuration.

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Off: The offset for event A3. It is specified through the IntraFreqHoA3Offset parameter.

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If the entering condition of the event is continuously met during the time specified through IntraFreqHoA3TimeToTrig, the UE reports event A3 and starts the event-triggered periodical reporting. Then, if the leaving condition of the event is continuously met during the time specified through IntraFreqHoA3TimeToTrig, the UE stops reporting event A3.




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After receiving a measurement report from the UE, the eNodeB generates a list of candidate cells, which meet the triggering condition of the specific event. As a second step, the eNodeB filters the candidate cells. If the measurement result of an intra-eNodeB cell is the same as that of an inter-eNodeB cell among the candidate cells, the eNodeB prioritizes the intra-eNodeB cell to prevent signaling and data forwarding required in an inter-eNodeB handover.




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In the handover execution phase, the UE and the eNodeB exchange signaling over the radio interface according to 3GPP TS 36.331. During an inter-eNodeB handover, the source eNodeB and the target eNodeB exchange signaling and data through X2/S1 adaptation. The LTE system uses hard handover, that is, only one radio link is connected to a UE at a time. Therefore, to prevent user data loss at the eNodeB during the handover, data forwarding is performed to ensure eNodeB data integrity. The loss of data may cause a decrease in the data transfer ratio and an increase in the data transfer delay.

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In the case of an intra-MME inter-eNodeB handover, the source eNodeB checks whether the X2 interface is available between the source and target eNodeBs or not and then automatically selects a path for the handover as follows: 

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If the X2 interface is available, the handover request is sent over the X2 interface. Data forwarding is also performed over the X2 interface. If the X2 interface is unavailable, the handover request is sent over the S1 interface. Data forwarding is also performed over the S1 interface.

In the case of an inter-MME inter-eNodeB handover, the handover request is sent over the S1 interface. In addition, the source eNodeB checks whether the X2 interface is available between the source and target eNodeBs or not and then automatically selects a path for data forwarding as follows:  

If the X2 interface is available, data forwarding is performed over the X2 interface. If the X2 interface is unavailable, data forwarding is performed over the S1 interface.



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Each parameters are described as below: 

Max report cell number: Indicates the maximum number of cells to be included in the measurement report after an event for the intra- or inter-frequency handover within the E-UTRAN is triggered.

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Measurement report amount: Indicates the number of periodic measurement reports to be sent after an event for the intra- or inter-frequency handover within the E-UTRAN is triggered. It is used to prevent the impact of measurement report loss and internal processing failure on the handover.

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A3 measurement trigger quantity: Indicates the quantity used to evaluate the triggering condition for the intra-frequency handover event. The quantity can be RSRP or RSRQ. Recommended value: RSRP A3 measurement report quantity: Indicates the quantity to be included in the measurement report for the intra-frequency handover event Intrafreq measurement report interval: Indicates the interval between each periodic report

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Measurement A4 report quantity: The same meaning as A3 measurement

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A1A2 Measurement trigger quantity: The same meaning as A3 measurement

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Interfreq measurement report interval: The same as itrafreq measurement report interval



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Specific Cell Individual Offset (CIO) values can be set for the serving cell and its neighboring cells. (Ocs and Ocn described in the subsequent chapters denote the CIO for the serving cell and the CIO for the neighboring cell respectively.) When the quality of signals fluctuates, the probability of triggering handovers to or from a specific cell can be adjusted by changing the value of CIO. This reduces the risk of call drops. The CIO values can be adjusted automatically by the MRO function.



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In LTE, 9 QCI are defined as QoS level. Each QCI is mapped with a series handover group. The related command is : MOD CELLSTANDARDQCI

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The main parameters for intra-handover decision algorithm are shown as following 

Intrafreq handover hysteresis: Indicates the hysteresis to be used in the triggering condition for the intra-frequency handover event. This parameter helps reduce the number of times the event is triggered because of radio signal fluctuation. Thus, the probability of ping-pong handovers or wrong handover decisions is reduced.

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Intrafreq handover offset: Indicates the quality offset of the neighboring cell over the serving cell to be used in the triggering condition of the intra-frequency handover event. Intrafreq handover time to trigger: Indicates the time-to-trigger for intra-frequency handover event A3.When detecting that the signal quality in the serving cell and that in at least one neighboring cell meet the entering condition, the UE does not send a measurement report to the eNodeB immediately. Instead, the UE sends a report only when the signal quality continuously meets the entering condition during the time-to-trigger



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The signaling procedure shown in above figure is described as follows: 

When the UE establishes a radio bearer, the source eNodeB sends the UE an RRC Connection Reconfiguration message that contains the measurement configuration. The measurement configuration is set by the source eNodeB to control the measurements of the UE in connected mode.

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The UE sends measurement reports to the source eNodeB based on the measurement results. The source eNodeB makes a handover decision based on the measurement reports. After deciding that a handover is preferred, the source eNodeB sends a Handover Request message to the target eNodeB, to request the target eNodeB to prepare for the handover. The target eNodeB makes admission decisions. If resources can be granted by the target eNodeB, the target eNodeB performs admission control depending on the QoS information about the Evolved Packet System (EPS) bearer. The target eNodeB prepares L1/L2 resources for the handover and then sends a Handover Request Acknowledge message to the source eNodeB. The source eNodeB sends the UE an RRC Connection Reconfiguration message that contains the mobilityControlInfo IE, indicating that the handover shall start.



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The UE performs a procedure of random access towards the target eNodeB, to achieve the UL synchronization of the UE with the target eNodeB.

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After successfully accessing the target cell, the UE sends the target eNodeB an RRC Connection Reconfiguration Complete message, indicating that the handover procedure is complete. At this time, the target eNodeB can start sending data to the UE.

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The DL data path switching is performed. The target eNodeB sends the source eNodeB a UE Context Release message, to inform the source eNodeB of the success of the handover and to trigger the release of the resources at the source eNodeB.

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After receiving the UE Context Release message, the source eNodeB releases the radio and control-plane resources associated with the UE context.



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When a handover fails, the UE performs a cell selection procedure and then initiates a procedure of RRC connection re-establishment towards the selected cell. The eNodeB makes a decision based on whether the context of the UE is present. If the eNodeB accepts the re-establishment request, the UE accesses the selected cell, avoiding a dropped call caused by the handover failure. The eNodeB will the status of UE context, if it exists, the procedure could be proceed, otherwise, eNodeB rejects reestablishment.




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Blind handover switch

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Once blind HO is activated, eNodeB directly decide the HO target based on the priority configuration of each neighbor




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The switch for each scenario 

eNodeB level switch: coverage based, service based, UL quality based

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Cell level switch: frequency priority based and distance based


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This execution is mandatory to for inter-frequency measurement, must be done by manual configuration.

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If it is configured, then A1/A2 event parameters will be delivered by eNB by default, thus to trigger inter-frequency measurement.



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In a coverage-based inter-frequency handover, event A2 triggers inter-frequency measurements. The triggering of this event indicates that the signal quality in the serving cell is lower than a specified threshold.

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Ms: The measurement result of the serving cell

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Hys: The hysteresis for event A2

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Thresh: The threshold for event A2, it can be defined separately with RSRP or RSRQ



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A1 event is opposite as A2 event.

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Note: A1 threshold must be larger than A2 threshold so that there is no overlapping area for UE activate/deactivate measurement.



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UE has different service experience when stay in FDD or TDD network, and each operators may have respective FDD/TDD resource. So eRAN6.0 offer flexible InterFreq A2 configure policy.



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In coverage-based inter-frequency handovers, the eNodeB delivers a measurement configuration related to event A2 for blind handover if the signal quality of the serving cell deteriorates to a specified level and the UE has not been handed over. In addition to delivering the measurement configuration related to event A2 for blind handover, the eNodeB also delivers the measurement configuration related to event A1 used to stop a blind handover in case that the signal quality in the serving cell further deteriorates. If the eNodeB receives a report of event A1 used to stop a blind handover from the UE before the blind handover starts, the eNodeB will not perform the blind handover.



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If neighbor cell blind handover priority is configured, then eNodeB select the target based on it. Otherwise, eNodeB only chooses the highest frequency priority and perform redirection procedure.



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Note: This A1 threshold is dedicated, different from other A1 threshold.

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Event A1 triggers measurements for frequency-priority-based handover.

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The FreqPriInterFreqHoA1TrigQuan parameter specifies the event A1 triggering quantity for frequency-priority-based measurement, which can be either RSRP or RSRQ. If the triggering quantity is RSRP, the threshold for this event is specified by the FreqPriInterFreqHoA1ThdRsrp parameter. If the triggering quantity is RSRQ, the threshold for this event is specified by the FreqPriInterFreqHoA1ThdRsrq parameter.

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Other parameters related to event A1 for frequency-priority-based measurement are the same as those for coverage-based inter-frequency measurement.

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After receiving an event A1 report from a UE, the eNodeB proceeds as follows: 

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If frequency-priority-based blind handover is enabled,the eNodeB selects the neighboring cell with the highest blind-handover priority to perform a handover. However, if load balancing from that neighboring cell to the serving cell is in progress, the eNodeB selects the neighboring cell with the second highest priority. If the eNodeB traverses all candidate cells but fails to find a cell that fulfills the criteria, the eNodeB will decide not to perform a handover. If frequency-priority-based blind handover is disabled, the eNodeB delivers the inter-frequency measurement configuration to the UE. If the triggering condition of event A4 is fulfilled, the UE sends a frequency-priority-based event A4 report to the eNodeB. (The triggering of event A4 for frequency-priority-based handover is similar to that for coverage-based inter-frequency handover




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Service-based inter-frequency handovers are applicable to two neighboring E-UTRAN frequencies that cover the same area. Based on the QCIs of the services that are running on a UE, the eNodeB can divert the UE to an appropriate co-coverage E-UTRAN frequency by means of a service-based inter-frequency handover.

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To implement service-based inter-frequency handovers, operators need to configure service-based inter-frequency handover policies. Each service-based inter-frequency handover policy has an ID (ServiceIfHoCfgGroupId) and specifies the frequency (DlEarfcn) that is preferentially used to carry the services of a specific operator (CnOperatorId). Since the services of each operator are further classified by QCIs, each QCI can be associated with a service-based inter-frequency handover policy so that the frequency specified by the policy preferentially carries the services with this QCI.

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Service-based inter-frequency handovers are controlled by the corresponding switch under the HoAlgoSwitch parameter. If this switch is turned on, the eNodeB triggers a servicebased inter-frequency handover for a UE based on the highest-priority QCI of the services running on the UE. If the highest-priority QCI is associated with a service-based interfrequency handover policy with the InterFreqHoState parameter set to PERMIT_HO, and there is at least one neighboring cell working on the frequency, the eNodeB delivers the measurement configuration related to event A4 to the UE. The triggering of event A4 in service-based inter-frequency handovers is the same as that in coverage-based interfrequency handovers.


LTE eRAN8.1 Intra-RAT Handover Feature 

Related commands:



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As shown above, F1 and F2 indicate two frequencies: Cell 1 uses F1, and Cells 2, 3, and 4 all use F2. The gray area in the figure (Cell 1) is covered by F1, while Cells 2, 3, and 4 is covered by F2. It is obvious that cell 1 exerts cross-cell coverage to the other three cells. Assume that the UE moves along the direction indicated by the arrow. As the UE moves, F1 provides sustained signal quality and therefore inter-frequency measurements are not triggered for the UE even when the UE has been in the coverage of F2 for a long time. Gradually, the UE enters the overlap coverage between Cell 1 and Cell 4. Considering the long distance between the two cells, it is unlikely that Cell 4 is configured as a neighboring cell of Cell 1. As a result, as soon as the UE leaves the coverage of Cell 1, it experiences a call drop. To prevent such a call drop and ensure service continuity, the UE should have been handed over to F2 earlier.

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Related command:



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UL-quality-based inter-frequency handovers are triggered based on the UL quality. When the UL quality is unsatisfactory, call drops may occur if handovers are not performed in time. Therefore, UL-quality-based inter-frequency handovers are introduced to reduce call drops caused by poor UL quality. The eNodeB uses the modulation and coding scheme (MCS) and initial block error rate (IBLER) to determine the UL quality.




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Related command:


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Mn: The measurement result of the neighboring cell.

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Ofn: The frequency-specific offset for the frequency of the neighboring cell.

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Ocn: The cell-specific offset for the neighboring cell.

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Hys: The hysteresis for event A4

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Thresh: The threshold for event A4


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Each parameters are described as below: 

Max report cell number: Indicates the maximum number of cells to be included in the measurement report after an event for the intra- or inter-frequency handover within the E-UTRAN is triggered.

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Measurement report amount: Indicates the number of periodic measurement reports to be sent after an event for the intra- or inter-frequency handover within the E-UTRAN is triggered. It is used to prevent the impact of measurement report loss and internal processing failure on the handover.

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A3 measurement trigger quantity: Indicates the quantity used to evaluate the triggering condition for the intra-frequency handover event. The quantity can be RSRP or RSRQ. Recommended value: RSRP A3 measurement report quantity: Indicates the quantity to be included in the measurement report for the intra-frequency handover event Intrafreq measurement report interval: Indicates the interval between each periodic report

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Measurement A4 report quantity: The same meaning as A3 measurement

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A1A2 Measurement trigger quantity: The same meaning as A3 measurement

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Interfreq measurement report interval: The same as itrafreq measurement report interval




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Parameter

Description

FddIfHoA2ThdRsr pOffset

Indicates the offset of the RSRP threshold for LTE FDD inter-frequency measurement event A2 relative to the RSRP threshold for inter-frequency measurement event A2 (which is specified by InterFreqHoA2ThdRsrp).

TddIfHoA2ThdRsr pOffset

Indicates the offset of the RSRP threshold for LTE TDD inter-frequency measurement event A2 relative to the RSRP threshold for inter-frequency measurement event A2 (which is specified by InterFreqHoA2ThdRsrp).

BlindHoA1A2ThdR srp

Indicates the RSRP threshold for events A1 and A2 that are used for inter-frequency and inter-RAT blind handovers based on coverage.

BlindHoA1A2ThdR srq

Indicates the RSRQ threshold for events A1 and A2 that are used for inter-frequency and inter-RAT blind handovers based on coverage.


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02-OEO109060 LTE eRAN8.1 Intra-RAT Handover Feature ISSUE 1.00(bulgaria).pdf

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