Handover Optimization

Content

Basic Concepts
Handover Signaling Procedure
Handover-related Radio Parameters
Case Study

Basic Concepts

• All handovers in LTE are hard handovers 
• There are not such concepts as “idle handover” and “access handover” in LTE. The counterpart concept is “cell reselection” 
• Intra-eNodeB handover 
• Inter-eNodeB handover 
• S1-based 
• X2-based
• S1: The interface between the E-UTRAN and the EPC. It includes two parts

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• X2: The interface between eNodeBs for transmitting 
• Load and interference information 
• Handover information

• Sent to the UE via the RRCConnectionReconfiguration message 
• Measurement configuration parameters includes: 
• Measurement objects 
• Reporting configurations: including reporting criterion and reporting format 
• Measurement identities (IDs): One ID links one object 
• Quantity configuration: One quantity configuration is configured per RAT type. The quantity configuration defines the measurement quantities and associated filtering. 
• Measurement gaps
• Intra-RAT handover events: A1, A2, A3, A4, A5 Event A1: Serving cell becomes better than absolute threshold Event A2: Serving cell becomes worse than absolute threshold Event A3: Neighbour cell becomes better than an offset relative to the serving cell Event A4: Neighbour cell becomes better than absolute threshold Event A5: Serving cell becomes worse than one absolute threshold and neighbour cell becomes better than another absolute threshold 
• Inter-RAT handover events: B1, B2 Event B1: Neighbour cell becomes better than absolute threshold Event B2: Serving cell becomes worse than one absolute threshold and neighbour cell becomes better than another absolute threshold

Handover Signaling Procedure

Handover-related Radio Parameters

Radio Parameters (1) -- Measurement 

• Configure handover-related measurement parameters in the OMC 
• 1. Find the measurement configuration index.
• 2.Configure the index for handover

Radio Parameters (2) – Handover Mode

• S1-based handovers are certain to occur 
• X2-based handovers may take place 
• If X2 interface is configured, the X2-based handover is preferred 
• In the case of X2 interface association, configure the remote port as 36422 
• Enable X2 in neighbor cell configuration (The X2 is enabled by default) 
• Reporting mode 
• Event-triggered reporting (Event A3) 
• Periodical reporting

Radio Parameters (3) – A3-triggered Reporting

Radio Parameters (3) – A3-triggered Reporting

Radio Parameters (3) – A3 Threshold

• A3-Offset 
• Off in the entering/leaving condition is A3-Offset 
• Its default value is 3 dB 
• The recommend value is 2, 3 or 4 dB. A greater or smaller value is not recommended because it may lead to delayed handover and too many handovers at a time 
• This offset is applicable to all neighbor cells

Radio Parameters (4) – TimeToTrigger

• TimeToTrigger (TTT) 
• This parameter specifies the value range used for time to trigger parameter, which concerns the time during which specific criteria for the event needs to be met in order to trigger a measurement report. 
• Value range: enumerated (0, 40, 64, 80, 100, 128, 160, 256, 320, 480, 512, 640, 1024, 1280, 2560, 5120) 
• Default value: 320 ms。 
• A high TTT can put off measurement reporting and reduce handovers. 
• Recommended values: 256, 320, 480, 512 or 640 ms.

Radio Parameters (5) – Hysteresis

• Hysteresis 
• Hysteresis, used to adjust the entering/leaving threshold of event-triggered reporting, determines the period the UE stays at Event A3.

• Default value is 0。 
• If ReportOnLeave is not enabled，the recommended value is 0。

Radio Parameters (6) – ReportOnLeave

Radio Parameters (7) – CIO

Radio Parameters (8) – filterCoefficient

Radio Parameters (8) – filterCoefficient

Radio Parameters (9) – ReportInterval and ReportAmount

• ReportInterval and reportAmount 
• Event triggering: reportInterval shall be used with reportAmount. When reportAmount is greater than 1, reportInterval is valid. 
• Periodical reporting: ReportInterval is the period to report the measurement 
• Value range of ReportInterval: enumerated (120, 240, 480, 640, 1024, 2048, 5120, 10240, 60000, 360000, 720000, 1800000, 3600000), unit: ms 
• Value range of ReportAmount: enumerated (1, 2, 4, 8, 16, 32, 64, Infinity), unit: times Recommend value for ReportInterval is 480ms, and that for ReportAmount is 4

Radio Parameters (10) – T304

• T304 
• Timer waiting for handover success 
• Value range: enumerated (50, 100, 150, 200, 500, 1000, 2000), unit: ms 
• Default value: 1000 ms

Case Study

• Handover failure due to improper radio parameter settings 
• Problem description 
• The cluster KPI analysis report shows there is about 5% call drop because of MR (measurement report) not processed. 
• In a cluster, the call drops because of MR (measurement report) not processed even account for over 50% of the call drops excluding those arisen from poor coverage. 
• Handover configuration: S1-based; the handover request is triggered when the neighbor cell’s RSRP is 3.5 dB higher than the serving cell’s.

• Problem analysis 
• Current parameter configuration 
• HO Prepare Timer = 1 s (Upon receiving one UE’s MR request, the source eNB sends the “handoverRequired” messages to MME at an interval of 1 s.) 
• Intra-frequency Reporting Interval for Event = 1024 ms 
• Intra-frequency Amount of Reporting for Event = 1 (If the target cell meets the MR reporting criterion, the UE reports MR every 1024ms and the total times is 1.) If the UE performs handover to two neighbor cells one by one in 1 second, and the amount of reporting is only 1, the second handover request message will be bypassed by the 1s timer on the core network side. As a result, the handover fails and call drop occurs.

Summary

• Common handover optimization methods 
• Neighbor cell optimization 
• RF optimization 
• Radio parameter optimization 
• ANR

End of Course