HandOver For LTE_Intra-frequency

Mobility Management Overview



Redirection Introduction

Redirection is the substitute  solution for handover ,used in the following scenario:

UE or network doesn’t support handover

There is no neighbor relation

When redirection is triggered, eNodeB sends the UE an “RRC Connection Release” message which contains information about a neighboring frequency in the LTE system or in another RAT system.

Handover Procedure & Entities

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.
• 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.

Handover Trigger Scenarios


Measurement Control Information

eNodeB should deliver all the measurement control parameters to UE via RRC reconfiguration, including measurement object, report configuration, measurement ID and filter configuration .

Besides, for inter-frequency/inter-RAT measurement, it also includes gap configuration for gap-assisted measurement.

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.

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 gap-assisted 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.

Measurement Object – Frequency

• For intra- frequency neighbor, we don’t need defined the frequency information, it will use the same configuration as current cell.
• For the inter-frequency or inter-RAT frequencies, we must add these frequencies separately.

Event Triggered Report

eNodeB complies with all 3GGP event to trigger UE report.

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.

Event-triggered periodical reporting has the following benefits:

• 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.

Measurement Gap Configuration

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 inter-frequency 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.

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.

Gap-assisted measurements for the following types of handover may co-exist: coverage-based, 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.

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.

 Measurement Filleting Operation
 
 

A: Measurement value at the physical layer

B: Measurement value obtained after L1 filtering.

C :Measurement value obtained after L3 filtering.

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.

eNodeB Neighbor Management
 

 

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:

• Whether to allow automatic removal of a neighboring relation by ANR or not
• Whether to allow handovers of UEs between two cells or not
• 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.

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. An E-UTRAN cell can be configured with a maximum of 32 intra-frequency neighboring cells.

lInter-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. An E-UTRAN cell can be configured with a maximum of 64 inter-frequency neighboring cells, which can be located on a maximum of 8 neighboring E-UTRAN frequencies. TDD cells can also be configured as inter-frequency neighboring cells of FDD cells. Huawei eNodeB supports interoperability between LTE FDD and LTE TDD.

 

Intra-frequency Handover

 Intra-frequency Handover Procedure
 
Event A3 Measurement Report

Intra-frequency handover is triggered by event A3, which is reported in event-triggered periodical reporting mode.

• Enter condition: Mn + Ofn + Ocn – Hys > Ms + Ofs + Ocs + A3Off
• Leave condition: Mn + Ofn + Ocn + Hys < Ms + Ofs + Ocs + A3Off

Mn: The measurement result of the neighboring cell

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

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

Ms: The measurement result of the serving cell

Ofs: The frequency-specific offset for the serving frequency

Ocs: The cell-specific offset for the serving cell

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

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

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.

Handover Decision – Filtering List
 
 

After receive measurement report, eNodeB makes candidate list after filtering the following

• Neighboring cells that have a different PLMN from the serving cell if the inter-PLMN
• Neighboring cells with a handover prohibition flag

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.

Handover Decision – Target Decision 
 

After get candidate list, eNodeB will rank the list with the following priority:

• The best RSRP/RSRQ neighbor cell
• Intra-eNodeB neighbor cell if reported result is the same
• Inter-eNdoeB neighbor cell with X2 interface if reported result is the same.

The eNodeB then sends a handover request to the target cell at the top of the candidate cell list.If the handover request fails, the eNodeB sends the handover request to the next target cell.    

 Handover Execution – S1/X2 Adaption
 
 

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.

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:

• 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.

Retry and Penalty
 If admission failure with target eNodeB, source eNodeB will try the next target until it gets succeed feedback from it. If all the target are failed for handover preparation, source eNodeB will wait for the next UE report.

The eNodeB imposes a penalty on a cell where a non-resource-induced admission failure has occurred. In a handover attempt, the eNodeB does not send a handover request to such a cell. This increases the handover success rate and reduces the number ofdropped calls caused by handover failures.

Typical Handover Flow
 
 

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.
• 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.