LTE Retainability Problem cause &
solution a-z step
Prepared byRajesh 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)/(PmErabRelAbnormalE
nbAct+PmErabRelNormalEnb+PmErabRelMme)]*100
Enb Initiated E-Rab Release:1> PmErabRelNormalEnb
2> PmErabRelAbnormalEnbAct
MME Initiated E-Rab Release:1> PmErabRelMme
2>PmErabRelMmeAct
RetainAbility/SARR(Session abnormal release rate) :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 Azimuth
3> 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.
RetainAbility/SARR(Session abnormal release rate) :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 precondition 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).
.
RetainAbility/SARR(Session abnormal release rate) :-
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.
RetainAbility/SARR(Session abnormal release rate) :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.
RetainAbility/SARR(Session abnormal release rate) :› 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.
RetainAbility/SARR(Session abnormal release rate) :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)
Therefore, from drive test
› DRB: 400ms (8)  1600ms (32)
analysis, it is often observed
< T310 (2000ms)
that the drop occur before
T310 time out because ReTx
hits upper limit first
LTE Retainability Tuning
› RRC Drops - T301 Timer
Timer is 1s, the UE is unable to successfully
RACH the serving cell.
› 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.
LTE Retainability Tuning
›RRC Drops - T304 Timer
Timer is 1s, The UE is
unable to RACH the Target
Cell at Handover.
› 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.
–
LTE Retainability Tuning
›RRC Drops - T310 Timer
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
LTE Retainability Tuning
›RRC Drops - T311 Timer
›Cause: Radio Link (RL) Synchronization
–UE not able to find suitable cell due to poor coverage.
LTE Retainability Tuning
› RRC Drops – Random Access failure at Handover
MAC_RA_Problem – Random Access
attempt to re-sync, for no response
after 10 retransmission (SIB2)
› 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.
LTE Retainability Tuning
› RRC Drops – Random Access failure at Handover
MAC_RA_Problem – Random Access
attempt to re-sync, for no response
after 10 retransmission (SIB2)
› 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.
LTE Retainability Tuning
› 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.
RetainAbility/SARR(Session abnormal release rate) :MOC
Parameter Name
Parameter Type
Parameter - High Cap Site
System Constant
ulInterferenceForgettingFactor (system constant 70)
Site Level Parameter
System Constant
deltaPsdRs2Tx (system constant 872)
Site Level Parameter
5
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
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
EUtranCellFDD
featureStateUlFss
pZeroNominalPucch
Site Level Parameter
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 2
Site Level Parameter
RESOURCE_FAIR (0)
QciProfilePredefined
schedulingAlgorithm - qci 3
Site Level Parameter
RESOURCE_FAIR (0)
QciProfilePredefined
schedulingAlgorithm - qci 4
Site Level Parameter
RESOURCE_FAIR (0)
QciProfilePredefined
schedulingAlgorithm - qci 5
Site Level Parameter
RESOURCE_FAIR (0)
Site Level Parameter
RESOURCE_FAIR (0)
8
1
schedulingAlgorithm - qci 6
QciProfilePredefined
QciProfilePredefined
schedulingAlgorithm - qci 7
Site Level Parameter
RESOURCE_FAIR (0)
QciProfilePredefined
schedulingAlgorithm - qci 8
Site Level Parameter
RESOURCE_FAIR (0)
QciProfilePredefined
schedulingAlgorithm - qci 9
Site Level Parameter
RESOURCE_FAIR (0)
RetainAbility/SARR(Session abnormal release rate) :-
Note: Any Suggestion/Feedback will be Highly Appreciable to Improve This
Presentation.
THANK YOU