What are Handover Failure Counters in LTE? Possible Solutions

This article is about Counter Measurements Related to Handover (LTE), Intra, Inter RAT handover measurement for both incoming and outgoing and we collected list of counters which shows handover failures. After, we discussed one by one RF optimization possible solutions and actions can be taken by RF 4G optimization engineer.

Handover Failure Counters and Possible Solutions

Incoming Intra-RAT Handover Measurement:

Counter Name	Failure Cause	Investigation	Possible Reason
L.HHO.Prep.FailIn.FlowCtrl	flow control	CPU usage is high.	Check whether the measured maximum number of UEs of the target cell exceeds the specification. Check PRB/PUCCH/SRS utilisation Check Alarm/ Board Overload Check Admission and congestion control parameters
L.HHO.PrepAttIn.disc.FlowCtrl	flow control (without returning a preparation failure message)	The call attempts per second (CAPS) is limited and the	Same as above.
L.HHO.Prep.FailIn	Preperation failure	The CAPS is limited.	Same as above.
L.HHO.Prep.FailIn.AdmitFail.SRS	admission failure caused by insufficient SRS resources in a cell	The CAPS is limited.	Same as above.
L.HHO.Prep.FailIn.AdmitFail.PUCCH	admission failure caused by insufficient PUCCH resources in a cell	The CAPS is limited.	Same as above.
L.HHO.Prep.FailIn.AdmitFail.ULThrpLic	admission failure caused by insufficient licensed uplink traffic volume in a cell	The CAPS is limited.	Same as above.
L.HHO.Prep.FailIn.AdmitFail.DLThrpLic	admission failure caused by insufficient licensed downlink traffic volume in a cell	The CAPS is limited.	Same as above.
L.HHO.Prep.FailIn.AdmitFail.UserLic	admission failures caused by insufficient licensed number RRC-connected UEs in a cell	The CAPS is limited.	Same as above.
L.HHO.Prep.FailIn.AdmitFail.UserSpec	admission failure caused by limited specification on the number RRC-connected UEs in a cell	The CAPS is limited.	Same as above.
L.HHO.Prep.FailIn.Other	Other Cause	The CAPS is limited.	Same as above.
L.HHO.Prep.FailIn.AdmitFail.Other	admission failure due to other causes in a cell	The CAPS is limited.	Same as above.
L.HHO.Prep.FailIn.RNL	faults at the radio network layer	The CAPS is limited.	Raise with Core Team
L.HHO.Prep.FailIn.AdmitFail.VoIPPref	voice service preferential access	The CAPS is limited.	Same as above.
L.HHO.Prep.FailIn.AdmitFail	admission failure	The target cell is congested, or the transport is abnormal.	Check whether the measured maximum number of UEs of the target cell exceeds the specification. Check PRB/PUCCH/SRS utilisation Check Alarm/ Board Overload Check Admission and congestion control parameters
L.HHO.Prep.FailIn.AdmitFail.Load	load-based admission failure in a cell	The target cell is overloaded.	Same as above.
L.HHO.Prep.FailIn.AdmitFail.TNL.DLRes	admission failure caused by insufficient downlink transport resources in a cell	The CAPS is limited.	Same as above.
L.HHO.Prep.FailIn.AdmitFail.TNL.ULRes	admission failure caused by insufficient uplink transport resources in a cell	The CAPS is limited.	Same as above.
L.HHO.Prep.FailIn.TNL	faults at the transport network layer	The CAPS is limited.	Same as above.
L.HHO.Prep.FailIn.HOCancel	the target cell receives handover cancellation messages	UEs may initiate RRC connection reestablishments in the	Coverage and Interference Improvement.

Incoming Intra-RAT Handover Measurement over X2:

Counter Name	Failure Cause	Investigation Possible Reason	Investigation Possible Reason
L.HHO.X2.Prep.FailIn.UnknownMmeCode	Unknown MME Code or Unknown MME Group ID	Core	GlobalProcSwitch.HoProcCtrlSwitch
L.HHO.X2.InterFreq.Load.Failin.PrepFail	high load in a cell	Target cell congested	Check whether the measured maximum number of UEs of the target cell exceeds the specification. Check PRB/PUCCH/SRS utilisation Check Alarm/ Board Overload Check Admission and congestion control parameters

LTE Cell-to-LTE Cell Outgoing Handover Measurement:

Counter Name	Failure Cause	Investigation Possible Reason	Investigation Possible Reason
L.HHO.Ncell.Prep.FailOut.NoReply	No response from the target cell	The target cell is congested, or the transport is abnormal.	Check whether the measured maximum number of UEs of the target cell exceeds the specification. Check PRB/PUCCH/SRS utilisation Check Alarm/ Board Overload Check Admission and congestion control parameters
L.HHO.Ncell.Prep.FailOut.PrepFailure	the handover preparation failure messages sent from the target cell	The target cell is congested, or the transport is abnormal.	Same as above.

Outgoing Inter-RAT Handover Measurement:

Counter Name

Failure Cause

Investigation

Possible Reason

L.IRATHO.E2W.Prep.FailOut.MME	Faults on the MME side	The core network works abnormally.
L.IRATHO.E2W.CSFB.Prep.FailOut.MME	the MME side causes	The core network works abnormally.	Raise with Core Team.
L.IRATHO.E2W.SRVCC.Prep.FailOut.MME	the MME side causes	The core network works abnormally.	Check Alarm/ Board Overload.
L.IRATHO.E2W.CSFB.SRVCC.Prep.FailOut.MME	MME-caused preparation failures	The core network works abnormally.
L.IRATHO.E2N.Prep.FailOut.MME	faults on the core network side	The core network works abnormally.	Check whether the measured maximum number of UEs of the target cell exceeds the specification. Check PRB/PUCCH/SRS utilisation Check Alarm/ Board Overload Check Admission and congestion control parameters.
L.IRATHO.E2N.Prep.FailOut.MME.Voice	core network reasons for voice services	The core network works abnormally.	Same as above.
L.IRATHO.E2W.Prep.FailOut.NoReply	no responses from WCDMA network	For S1-based handovers, the core network may also work	Same as above.
L.IRATHO.E2W.Prep.FailOut.PrepFailure	WCDMA network cells send handover preparation failure messages	The target cell is congested, or the transport is abnormal.	Raise with Core Team
L.IRATHO.E2W.CSFB.Prep.FailOut.PrepFailure	the response handover preparation failure from WCDMA network	The target cell is congested, or the transport is abnormal.	Same as above.
L.IRATHO.E2W.CSFB.Prep.FailOut.NoReply	no response from WCDMA network	The target cell is congested, or the transport is abnormal.	Same as above.
L.IRATHO.E2W.SRVCC.Prep.FailOut.PrepFailure	the response handover preparation failure from WCDMA network	The target cell is congested, or the transport is abnormal.	Same as above.
L.IRATHO.E2W.SRVCC.Prep.FailOut.NoReply	no response from WCDMA network	The target cell is congested, or the transport is abnormal.	Same as above.
L.IRATHO.E2W.CSFB.SRVCC.Prep.FailOut.PrepFailure	the response handover preparation failure sent by the WCDMA networks	The target cell is congested, or the transport is abnormal.	Check whether the measured maximum number of UEs of the target cell exceeds the specification. Check PRB/PUCCH/SRS utilisation Check Alarm/ Board Overload Check Admission and congestion control parameters
L.IRATHO.E2W.CSFB.SRVCC.Prep.FailOut.NoReply	no response from the WCDMA networks	The target cell is congested, or the transport is abnormal.	Same as above.
L.IRATHO.E2N.Prep.FailOut.PrepFailure	NG-RAN cells respond with handover preparation failure messages	The target cell is congested, or the transport is abnormal.	Same as above.
L.IRATHO.E2N.Prep.FailOut.NoReply	no responses from the NG-RAN	The target cell is congested, or the transport is abnormal.	Same as above.
L.IRATHO.E2N.Prep.FailOut.PrepFailure.Voice	responses preparation failures from the NG-RAN for voice services	The target cell is congested, or the transport is abnormal.	Same as above.
L.IRATHO.E2N.Prep.FailOut.NoReply.Voice	no responses from the NG-RAN for voice services	The target cell is congested, or the transport is abnormal.	Same as above.
L.IRATHO.E2W.NoNRT	The adjacent cell is not in the NRT the source cell	Neighboring cells are not configured.	Same as above.
L.IRATHO.E2HRPD.NoNRT	The adjacent cell is not on the NRT the source cell	Neighboring cells are not configured.	Same as above.
L.IRATHO.E2W.NoNRT.PLMN	the adjacent cell is not on the NRT the source cell for a specific operator	Neighboring cells are not configured.	Same as above.
L.CSFB.PrepFail.Conflict	Process conflict	Handover and CSFB procedures conflict.

Outgoing Intra-RAT Handover Measurement:

Counter Name	Failure Cause	Investigation	Possible Reason
L.HHO.Prep.FailOut.MME	faults on the MME side	The core network works abnormally.	Neigbour plan , Neighbour parameter configuration
L.HHO.IntraFreq.Prep.FailOut.MME	the MME side causes	The core network works abnormally.	Same as above.
L.HHO.Prep.FailOut.NoReply	no responses from the target cell	The target cell is congested, or the transport is abnormal.	Raise with Core Team
L.HHO.Prep.FailOut.PrepFailure	the handover preparation failure messages sent from the target cell	The target cell is congested, or the transport is abnormal.	Raise with Core Team
L.HHO.IntraFreq.Prep.FailOut.NoReply	the target cell does not respond	The target cell is congested, or the transport is abnormal.	Raise with Core Team
L.HHO.IntraFreq.Prep.FailOut.PrepFailure	the target cell sends handover preparation failure messages	The target cell is congested, or the transport is abnormal.	Raise with Core Team
L.HHO.Prep.FailOut.TNL	faults at the transport network layer	Transmission Fault	Check whether the measured maximum number of UEs of the target cell exceeds the specification. Check PRB/PUCCH/SRS utilisation Check Alarm/ Board Overload Check Admission and congestion control parameters
L.IntraFreqHO.NoNRT	the adjacent cell is not on the NRT the source cell	Neighboring cells are not configured.	Same as above.
L.InterFreqHO.NoNRT	the target cell is not configured as a neighboring cell for the source cell	Neighboring cells are not configured.	Same as above.
L.HHO.Prep.FailOut.RNL	faults at the radio network layer	Radio Fault	Same as above.
L.HHO.Prep.FailOut.HOCancel	the handover cancellation messages sent from the source cell	UEs may initiate RRC connection reestablishments in the	Raise with Tx Team
L.HHO.IntraFreq.Prep.FailOut.HOCancel	the source cell sends handover cancellation messages	UEs may initiate RRC connection reestablishments in the	Coverage and Interference Improvement
L.HHO.Prep.FailOut.Other	Other Cause		Same as above.
L.HHO.Prep.FailOut.TargetIllegal	the handover response messages sent by the target eNodeB fail the validity check		Same as above.
L.HHO.IntraFreq.Prep.FailOut.TargetIllegal	the handover response messages sent by the target eNodeB fail the validity check		Same as above.

Outgoing Intra-RAT Handover Measurement over X2:

Counter Name	Failure Cause	Investigation	Possible Reason
L.HHO.X2.Prep.FailOut.MME	the MME side causes	The core network works abnormally.	Neigbour plan , Neighbour parameter configuration
L.HHO.X2.IntraFreq.Prep.FailOut.MME	the MME side causes	The core network works abnormally.	Same as above.
L.HHO.X2.IntraFreq.Exec.FailOut.MMERel	the MME side causes	The core network works abnormally.	Same as above.
L.HHO.X2.Prep.FailOut.PrepFailure.UnknownMmeCode	Unknown MME Code or Unknown MME Group ID	The target cell is congested, or the transport is abnormal.	Check whether the measured maximum number of UEs of the target cell exceeds the specification. Check PRB/PUCCH/SRS utilisation Check Alarm/ Board Overload Check Admission and congestion control parameters.
L.HHO.X2.InterFreq.Load.FailOut.PrepFail	high load in a cell	The target cell is congested, or the transport is abnormal.	Same as above.
L.HHO.X2.Prep.FailOut.NoReply	the target cell does not respond	The target cell is congested, or the transport is abnormal.	Same as above.
L.HHO.X2.Prep.FailOut.PrepFailure	the target cell sends handover preparation failure messages	The target cell is congested, or the transport is abnormal.	Same as above.
L.HHO.X2.IntraFreq.Prep.FailOut.NoReply	the target cell does not respond	The target cell is congested, or the transport is abnormal.	Same as above.
L.HHO.X2.IntraFreq.Prep.FailOut.PrepFailure	the target cell sends handover preparation failure messages	The target cell is congested, or the transport is abnormal.	Same as above.
L.HHO.X2.Prep.FailOut.HOCancel	the source cell sends handover cancellation messages	UEs may initiate RRC connection reestablishments in the	Coverage and Interference Improvement.
L.HHO.X2.IntraFreq.Prep.FailOut.HOCancel	the source cell sends handover cancellation messages	UEs may initiate RRC connection reestablishments in the	Same as above.
L.HHO.X2.Prep.FailOut.TargetIllegal	the handover response messages sent by the target eNodeB fail the validity check.		Same as above.
L.HHO.X2.IntraFreq.Prep.FailOut.TargetIllegal	the handover response messages sent by the target eNodeB fail the validity check.		Same as above.

See Also:

1. eRAB Drop Counters and Possible Solutions.
2. eRAB Failure Counters with RF Optimization Solutions.