Handover Failure Investigation and Solution Guide_V1

Handover Failure Investigation & Solution Guide

Vodafone Proprietary Material. Vodafone’s Internal Use Only. All rights reserved ©2016. If you have any questions, please contact [email protected]

Contents 1.

Introduction .......................................................................................................................................... 3

2.

Handover Terms ................................................................................................................................... 3

3.

Handover Types .................................................................................................................................... 3

4.

3.1

Frequency Based .......................................................................................................................... 3

2.2

Interface Based ............................................................................................................................. 4

2.2.1

X2 Based Handover .............................................................................................................. 4

2.2.2

S1 Based Handover ............................................................................................................... 4

Handover Settings and Parameters ..................................................................................................... 4 4.1

Handover Power Threshold ......................................................................................................... 4

4.2

Time to Trigger (TTT) .................................................................................................................... 4

4.3

Power Measurements .................................................................................................................. 5

4.3.1

RSRP ...................................................................................................................................... 5

4.3.2

RSRQ ..................................................................................................................................... 5

5.

Handover Events .................................................................................................................................. 6

6.

Key Performance Indicators ................................................................................................................. 7

7.

8.

6.1

Handover Failure Rate.................................................................................................................. 7

6.2

Handover Delay ............................................................................................................................ 7

Handover Parameters .......................................................................................................................... 7 7.1

InterFreqHoA1A2TrigQuan .......................................................................................................... 7

7.2

InterFreqHoA2ThdRsrp ................................................................................................................ 7

7.3

InterFreqHoA1A2Hyst .................................................................................................................. 7

7.4

InterFreqHoA1A2TimeToTrig ....................................................................................................... 8

7.5

InterFreqHoA4ThdRsrp ................................................................................................................ 8

7.6

InterFreqHoA4Hyst....................................................................................................................... 8

7.7

InterFreqHoA4TimeToTrig ........................................................................................................... 8

7.8

FreqPriorityHoSwitch ................................................................................................................... 8

Case Analysis ........................................................................................................................................ 8 8.1

High Handover Failure Rates........................................................................................................ 9

8.1.1 8.2

Solution ................................................................................................................................. 9

Ping – Pong ................................................................................................................................... 9

8.2.1

Solution ............................................................................................................................... 10


1. Introduction Handover is a mechanism used to provide call continuity in LTE networks. During handover call is transferred from one cell to another without service interruption. In LTE technology, handover decision is made by eNB based on certain UE measurements.

2. Handover Terms There are a few important keywords and definitions used. Here is a list for the most important ones,     

Source eNB: This is the eNB that is currently hosting the call. UE is connected to this eNB right now. Target eNB: This is the eNB that UE will be transferred to during handover. That is, this is the new eNB. Handover Events: These are certain events that are used to make handover decisions. X2 Interface: This is the interface between two eNBs. This interface carries all control and user data during handover. S1-C Interface: This is the interface between an eNB and MME, and carries all control related information.

3. Handover Types 3.1

Frequency Based

Depending on the frequency of the source and the target eNB, handovers are classified as Intra and Inter-frequency handover. 3.1.1

Intra-Frequency Handover

This handover takes place sectors of the same eNB which operate in the same frequency. That means, this is handover between the sectors. For example, if a particular eNB operates in 2.5 GHz, and is this eNB has 3 sectors, all sectors operate in 2.5 GHz also. Thus, handover between the sectors is called intra-frequency handovers. 3.1.2

Inter-Frequency Handover


This handover type happens when target and source eNB operates in different frequencies. For instance, if target eNB operates in 1.8 GHz, and source eNB operates in 2.5 GHz, the handover between these two eNBs is called inter-frequency handover.

2.2

Interface Based

There are X2 and S1 interfaces between eNBs and between eNbs & MMEs. Handover signalling information can be carried on any of these interfaces. Depending on the interface, the name of the handover will be different. Latency on X2 based handover will be much less than the latency over S1 based handover.

2.2.1

X2 Based Handover

If the handover signalling and user data is carried over X2 interface during handover, handover is named as X2 based handover. For this to happen, there should be live X2 interface between eNBs.

2.2.2

S1 Based Handover

If handover signalling is carried over S1 interface, this type of handover is named as S1 based handover. User data can also be carried over S1, however the latency over S1 will be much higher than the latency over X2. This may cause late data arrival to target eNB, and data session cannot start where it was left at source eNB. Thus, X2 based handover is preferred over S1 based handovers.

4. Handover Settings and Parameters 4.1

Handover Power Threshold

This is the power value needed to find the target cell whose power value is larger than the defined target. There are multiple power measurements depending on the handover event type. Figure shows the power threshold value and hysteresis.

4.2

Time to Trigger (TTT)


TTT is needed for decision. This is the value that UE uses to decide when to handover. This value is given in msec. TTT is used to guarantee sudden or inconsistent HO decisions. Figure 1 shows the TTT value.

Figure 1. Handover Power Threshold and TTT value

4.3

Power Measurements

4.3.1

RSRP

This refers to received signal resource power which means the absolute signal power received on the UE side. RSRP is calculated and given as; RSRP = EPRE – PL - Shadowing Effect (Log Normal)-Fading (Rayleigh or Ricean) EPRE means Energy per Resource Element, which is the transmitted energy per RE in RB. EPRE is one of the eNodeB parameters that is set by service providers.

4.3.2

RSRQ

This is known as signal quality parameter in LTE networks. It stands for Reference Signal Received Quality (RSRQ), and it is used for handoff purposes in the network. RSRQ is measured using the resource element that carries the reference symbol. Vodafone Proprietary Material. Vodafone’s Internal Use Only. All rights reserved ©2016. If you have any questions, please contact [email protected]

5. Handover Events There are 5 different handover types. Figure 2 shows the Event A1 and Event A2. Event A1 is triggered when the power value of the serving cell is better than the threshold. Event A2 is triggered when the power value of the serving cell becomes worse than the threshold.

Figure 2. A1 and A2 Handover Event Types


Figure 3. A3, A4 and A5 Handover Event Types Figure 3 shows the A3, A4 and A5 event types. Event A3 is triggered when the power of the target cell is greater than the power of the source cell by an offset. Event A4 is triggered when the power of the target cell becomes better than a defined power threshold. Event A5 is triggered when the target becomes better than a threshold and when the source becomes worse than a threshold.

6. Key Performance Indicators 6.1

Handover Failure Rate

Handover is considered as failed when the transmission of one RRC HO-Involved message exceeds a predefined delay. 280ms nearly corresponds to 4-5 RLC retransmission.

6.2

Handover Delay

Handover delay is defined as the time difference between the time UE transmits measurement report (RSRP, RSRQ), and the time ‘HO confirm’ is received at the target eNB.

7. Handover Parameters 7.1

InterFreqHoA1A2TrigQuan

Indicates the trigger quantity for event A1/A2 related to event-A4/A5-triggered inter-frequency handover. The quantity can be RSRP, RSRQ, or both. Default value is RSRP.

7.2

InterFreqHoA2ThdRsrp

Indicates the RSRP threshold for event A2 related to event-A4/A5-triggered inter-frequency handover. When the measured RSRP value is lower than this threshold, a measurement report will be sent. Default value -109 dBm

7.3

InterFreqHoA1A2Hyst


Indicates the hysteresis of event A1/A2 for triggering inter-frequency measurement. This parameter is used to prevent frequent triggering of event evaluation caused by radio signal fluctuation. Default value is 2 dB.

7.4

InterFreqHoA1A2TimeToTrig

Indicates the time-to-trigger of event A1/A2 for triggering inter-frequency measurement. Default value is 640 msec.

7.5

InterFreqHoA4ThdRsrp

Indicates the RSRP threshold for event A4 related to coverage-based inter-frequency handover. When the measured RSRP value exceeds this threshold, event A4 is reported. Default value is -106 dBm.

7.6

InterFreqHoA4Hyst

Indicates the hysteresis for event A4. This parameter is used to prevent frequent triggering of event evaluation caused by radio signal fluctuation. Default value is 2 dB.

7.7

InterFreqHoA4TimeToTrig

Indicates the time-to-trigger for event A4 for the inter-frequency handover. Default value is 640 msec.

7.8

FreqPriorityHoSwitch

Indicates whether to enable frequency-priority-based inter-frequency handover algorithms. Default values are FreqPriorIFHOSwitch:Off, FreqPriorIFBlindHOSwitch:On, MlbBasedFreqPriHoSwitch:Off, A2BasedFreqPriHoSwitch:Off

8. Case Analysis Vodafone Proprietary Material. Vodafone’s Internal Use Only. All rights reserved ©2016. If you have any questions, please contact [email protected]

8.1

High Handover Failure Rates

Inter HO failure rate is high at a particular region. Failure occurs when UE cannot attach to the target eNB, and as a result call drops.

8.1.1

Solution

1- You should increase TTT value in order to make handover harder to happen. Parameters to change are; A. InterFreqHoA1A2TimeToTrig B. InterFreqHoA4TimeToTrig 2- You should increase the hysteresis value. Parameters to change are; A. InterFreqHoA1A2Hyst B. InterFreqHoA4Hyst

Figure 4. New TTT Value after the Change

8.2

Ping – Pong

Ping-Pong happens when UE becomes indecisive when UE wants to handover to a particular eNB. At the time UE sends measurement report to source eNB, source eNB makes handover decision to target eNB. After UE handovers to target eNB, because of the RF environment, UE immediately sends Vodafone Proprietary Material. Vodafone’s Internal Use Only. All rights reserved ©2016. If you have any questions, please contact [email protected]

measurement report to the new source eNB. Source eNB makes handover decision to the target eNB (old source), and this process continues. Figure 5 shows the general structure of ping-pong effect.

Figure 5. Ping Pong Effect

8.2.1

Solution

There are certain HO parameters that can be used to mitigate the ping pong effect. These parameters are given in the table below; Parameter Name

FilterRsrp

GAP measurement pattern

Definition Indicates the alpha filtering coefficient used by the UE during RSRP measurement for path loss estimation. Indicates the measurement gap pattern

Default Value FC6

Recommended Value Lower value than FC6. Possible Values: FC0-5

Pattern 1


Pattern 2


Handover Failure Investigation and Solution Guide_V1

Recommend Documents