LTE Retainability Problem cause & solution a-z step Prepared by- Rajesh Kumar

LTE Retainability Problem cause & solution a-z step

Prepared by- Rajesh Kumar

RetainAbility/SARR(Session abnormal release rate) :-

Definition: Ability of a user to retain the E-RAB once Connected for the desired duration.

Point:-E-RAB release can be initiated by ENB & MME

Definition based on Counter:-SARR=[(PmErabRelAbnormalEnbAct+PmErabRelMmeAct)/(PmErabRelAbnormalEnbAct+PmErabRelNormalEnb+PmErabRelMme)]*100

Enb Initiated E-Rab Release:-1> PmErabRelNormalEnb2> PmErabRelAbnormalEnbAct

MME Initiated E-Rab Release:-1> PmErabRelMme2>PmErabRelMmeAct


1> pmErabRelAbnormalEnbActCdt(The total number of abnormal ERAB releases by the eNB per cell due cell down time (manual intervention) with the pre-condition that the Initial Context Establishment procedure must first have been successfully completed and that there was data in either the UL or DL buffer (i.e. active).Soln: Check Outage report or Fluctuation Report.

2>pmErabRelAbnormalEnbActHo(The total number of abnormal ERAB releases per cell by the eNB due to handover execution failure and that there was data in either the UL or DL buffer (i.e. active).Soln:1>PCI Collision issue check(PCI Planning should be correct).There should no PCI re-use within near by cell(1tier,2tier neib).2> Coverage issue(make Any PCI Dominant which should be based on Clutter as per Common sense).Make EDT,Change Azimuth3> High UL Interference issue on Target cell(PmRadiorecInterferencePwr). Check Connection b/w Antenna Connector & RRU Connector.4> Also Repeater is responsile sometimes for Hi-UL Interference.Might be due to Power Leakage.


4> H/W issue,Alarm issue.Correct the issue based on Problem u Identify.Take help of Alex if you are not Understanding the Alarm meaning.

3> pmErabRelAbnormalEnbActUeLost(The total number of abnormal ERAB releases by the eNB per cell due that the contact with the UE is lost with the pre-condition that the Initial Context Establishment procedure must first have been successfully completed and that there was data in either the UL or DL buffer (i.e. active).Soln: 1> Solve the Poor Coverage issue by making Tilt,Azimuth change or Parameter change(Part of Sector Pwr,ENB Power), for uplink Coverage(P0NominalPucch,P0NominalPusch).2> High Ul Interference issue(As per earlier mentioned.Also make sure Nominal pwr Should not set very high otherwise it will create interference in neib cell).

.


4> pmErabRelAbnormalEnbLic(The total number of abnormal E-RAB Releases initiated by the RBS due to license reject (per cell).Soln:1. Recently this issue I have faced in “Sprint NV Project”.After analysis it was found that there was issue with LKF file.Once New LKF file uploaded issue get resolved.2. If LKF is correctly loaded then This counter will Peg only When user have crossed the ENB License limit.Pls upgrade the License.Once u think that every basic Optimization u have done.5>pmErabRelAbnormalEnbActTnFail(The total number of abnormal ERAB releases per cell due to S1 interface down, "X2 interface down" or "Transport Resource Unavailable", with the pre-condition that the Initial Context Establishment procedure must first have been successfully completed and that there was data in either the UL or DL buffer (i.e. active).Soln: 1> Raise Ticket to NOC or IM Team to resolved the issue.


6>pmErabRelAbnormalEnbActHpr(The total number of abnormal ERAB releases by the eNB per cell due to handover preparation and that there was data in either the UL or DL buffer (i.e. active).

Soln:1>MME might be Locked/ disabled.2> MME mismatch b/w Scource & Target cell.3> Tranport issue suspected.4> License issue suspected.5> Site Configuration issue.6> Hi-Load on Target Cell.


› From drive test analysis, it was observed that a large portion of the drop calls were due to poor radio environment.

› Just before HO, it was observed that the UE SINR was poor and this caused the UE to drag the call in poor radio environment rather than HO faster to a neighbour cell with better RSRP.

› In this optimization project, 2 parameters were tuned; A3offset and a2TresholdRsrpPrim.

– A3offset : change from 3 to 2 db– a2TresholdRsrpPrim : change from -120 to -110 dbm

› A trial was also conducted to evaluate the impact of changing the values below for a few sites with high DCR:

– DataRadioBearer : dlMaxRetxThreshold / ulMaxRetxThreshold› 8 to 32

– SignalingRadioBearer : dlMaxRetxThreshold / ulMaxRetxThreshold › 8 to 32

Retainibility/SARR(Session abnormal release rate) :-

With current A3 setting, A3 event handover will be performed when neighbour cell is greater than a3Offset + hysteresis (RSRP) of the current serving cell.

a3Offset = 3 dB, hysteresis = 1dB Handover will only be performed once Neighbour Cell RSRP > 4dB of Serving Cell RSRP

Thus, it is important to expedite the HO with reduction of a3Offset to 2 dB.After the parameter change, it was observed that the drop calls due to call dragging was reduced in a cluster of 20 sites.


MO Class Parameter * Range Default

dlMaxRetxThreshold 1, 2, 3, 4, 6, 8, 16, 32 8

ulMaxRetxThreshold 1, 2, 3, 4, 6, 8, 16, 32 8

dlMaxRetxThreshold 1, 2, 3, 4, 6, 8, 16, 32 8

ulMaxRetxThreshold 1, 2, 3, 4, 6, 8, 16, 32 8

DataRadioBearer

SignalingRadioBearer

› RL Failure Timing– t-PollRetransmit = 45ms (SRB), 50ms (DRB)– If retransmission limit increases from 8 32, RL failure due to

retransmission limit reached:› SRB: 360ms (8) 1440ms (32) › DRB: 400ms (8) 1600ms (32) Therefore, from drive test

analysis, it is often observed that the drop occur before T310 time out because ReTx hits upper limit first

< T310 (2000ms)

LTE Retainability Tuning

Timer is 1s, the UE is unable to successfully RACH the serving cell.

› RRC Drops - T301 Timer

› Cause: Radio Link problem–Poor RSRP, e.g. below -105dB can cause the up link synch be lost.–10 seconds later a new RRC Connection Request is sent.


›RRC Drops - T304 Timer

› Cause: Handover Failure– Check RF Conditions, UE TX Power and missing relation (PCI).– If the issue is seen in good RF conditions, troubleshoot with UETR, tuning of

HysterisisA3, TimetoTriggerA3 and filterCoefficientEUtraRsrp parameters.

–

Timer is 1s, The UE is unable to RACH the Target Cell at Handover.



The UE in RRC-CONNECTED mode, evaluates in the downlink the quality of the radio link of the serving cell. The UE bases its evaluation on the cell-specific reference signal relative to thresholds Qout and Qin.

Qout is defined as the level at which the downlink radio link cannot be reliably received and Qin is defined as the level at which the downlink radio link can be received with significant reliability.

At the expiry of the timer T310, the UE switches off uplink transmission.

Cause: Radio Link (RL) Synchronization

- Poor RSRP and SINR



›Cause: Radio Link (RL) Synchronization–UE not able to find suitable cell due to poor coverage.


› RRC Drops – Random Access failure at Handover

› Cause: RA failure, UE is not able to synch to target eNB.– Poor RF, Timing Advance alignment with target cell.– UE can reach max TX power increasing UL noise.

MAC_RA_Problem – Random Access attempt to re-sync, for no response after 10 retransmission (SIB2)


MAC_RA_Problem – Random Access attempt to re-sync, for no response after 10 retransmission (SIB2)

› RRC Drops – Random Access failure at Handover

› Cause: RA failure, UE is not able to synch to target eNB.–Poor RF, Timing Advance alignment with target cell.–UE can reach max TX power increasing UL noise.


› RRC Drops – Maximum RLC Retransmissions

› Cause: Repeated RLC Retransmissions due to unsuccessful RLC PDU reception (either UL or DL).

– This occurs when maximum retransmission has occurred at RLC level (ARQ). – UL/DL interference and Poor RF conditions.– UE will consider RL failed and initiate RRC Connection Re-establishment.

MOC Parameter Name Parameter Type Parameter - High Cap Site

System Constant ulInterferenceForgettingFactor (system constant 70) Site Level Parameter 5

System Constant deltaPsdRs2Tx (system constant 872) Site Level Parameter 0

System Constant SpdcchLaGinrMargin (system constant 220) Site Level Parameter 30

System Constant numInitialAccessAndIncomingHo (system constant 556) Site Level Parameter 10

Hidden Parameter tPollRetransmitUl - DataRadioBearer Site Level Parameter 160

Hidden Parameter tPollRetransmitUl - SignallingRadioBearer Site Level Parameter 160

Hidden Parameter tPollRetransmitDl - DataRadioBearer Site Level Parameter 160

Hidden Parameter tPollRetransmitDl - SignallingRadioBearer Site Level Parameter 160

Hidden Parameter dlMaxRetxThreshold - DataRadioBearer Site Level Parameter 8

Hidden Parameter ulMaxRetxThreshold - DataRadioBearer Site Level Parameter 8

EUtranCellFDD noOfPucchCqiUsers Sector Level Parameter 640

EUtranCellFDD noOfPucchSrUsers Sector Level Parameter 590

EUtranCellFDD pdcchCfiMode Sector Level Parameter CF_Static_3 (3)

EUtranCellFDD ulSrsEnable Sector Level Parameter TRUE

UlFss featureStateUlFss Site Level Parameter 1EUtranCellFDD pZeroNominalPucch Sector Level Parameter -120

EUtranCellFDD pZeroNominalPusch Sector Level Parameter -102

QciProfilePredefined schedulingAlgorithm - default Site Level Parameter RESOURCE_FAIR (0)

QciProfilePredefined schedulingAlgorithm - qci 1 Site Level Parameter RESOURCE_FAIR (0)





QciProfilePredefined

schedulingAlgorithm - qci 6

Site Level Parameter RESOURCE_FAIR (0)






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LTE Retainability Problem cause & solution a-z step Prepared by- Rajesh Kumar