VoLTE RAN Case Study Contents VoLTE Services Various Tools used for optimization VoLTE RAN Case Studies Take Away VoLTE SERVICES Services • • • • VoLTE VoLTE VoLTE Audit VoLTE RAN Optimization Border Optimization Parameter and Configuration E2E Analysis Analysis ( RAN & EPC ) • Drive logs • UETR ( X2, S1,RRC) • MOS scores • S11 • Rx & Gx • SGi • SIP signaling • EPC traces Tools: Accomplishments: • • • • • • • • TEMS Discovery ITK Business Object ERTT VoLTE E2E Analyzer & NEO WIRESHARK ECO • • • • • Optimized 8000+ sites for AT&T and 1700+ sites for SingTel SRVCC Border optimization MT RRC re-establishment Feature workaround DL ICIC feature testing performed Wining DFW KPI improvement challenge E2E Problem identification Tools used TEMS Discovery Drive data ( 3rd Party tool) ECO/ERA • Analyze layer 3 messages • Analyze poor MOS • Analyze VoLTE setup failures & drop calls • Identify POOR MOS calls/clips • Fetch HOIT info • Fetch NCM/Scanner data • Fetch Network type info • Used for automatic tilt analysis / recommendatio ns using drive data RF conditions as input • PCI confusion • Parameter Audit ITK / BO VoLTE End-2-End Analyzer/ Wireshark ERTT • Fetch parameters • Perform Audit OSS • Analyze UETRs & Cell traces • Analyze EPC traces • Analyze IMS traces • Used to fetch counter based statistics for analysis and provide supporting stats for recommendations. VoLTE RAN CASE STUDIES List of case studies 1. VoLTE Drop without RRC drop 2. RRC drop without VoLTE drop 3. QCI 1 Profile not defined resulting in VoLTE setup failure 4. MT RRC Work around to reduce VoLTE/LTE Drop 5. Incorrect MCS assignment Causing VoLTE Drop 6. Improvement in Retainability with DL ICIC feature activation & MT RRC Work around 7. Improvement in Retainability and SRVCC SR with change in parameter “b2threshold2rscpUtraoffset”. 1.VoLTE DROP without RRC drop 1/3 2 3 1 2 3 4 5 1.UE 1.UE sets sets up up RRC RRC connection connection (mt-access). (mt-access). 2.VoLTE 2.VoLTE bearer bearer is is established. established. 3.Event 3.Event A4 A4 is is triggered triggered indicating indicating HO HO for for IFLB, IFLB, followed followed by by handover. handover. 4.UE 4.UE performs performs HO HO (RF (RF HO HO successful) successful) 5.20 5.20 sec sec after after HO HO RRC RRC connection connection is is released released due due to to User User Inactivity. Inactivity. 1.VoLTE DROP without RRC drop 2/3 UE 1 1 1 2 2 4 1 2 3 UE UE was was served served by by cell cell B B (PCI (PCI 458). 458). Event Event A4 A4 is is triggered triggered for for IFLB IFLB HO. HO. 1.Source 1.Source cell cell B B sends sends “HO “HO Request Request ““ to to target target cell cell Z Z 4 2.Target 2.Target cell cell Z Z accepts accepts two two default default bearers( bearers( Internet Internet & & IMS) IMS) & & does does not not admit admit VoLTE VoLTE Cell Z did not have RLC - UM feature for VoLTE bearer ( QCI 1) activated bearer. bearer. 1.VoLTE DROP without RRC drop 3/3 Typical causes causes for for VoLTE VoLTE Drop Drop without without RRC RRC drop: drop: Typical RLC mode mode mismatch mismatch in in source source & & target target cell cell •• RLC PDCP SN SN length length for for QCI QCI 1 1 mismatch mismatch in in source source & & target target cell cell •• PDCP RLC SN SN length length for for QCI QCI 1 1 mismatch mismatch in in source source & & target target cell cell •• RLC RoHC feature feature must must be be activated activated on on both both source source & & target target cell cell •• RoHC IMPORTANT: In some some cases cases it it was was observed observed that that RLC RLC mode mode /PDCP /PDCP // RLC RLC SN SN length length ›› In mismatch was was observed observed in in configured configured value value & & value value found found in in layer layer 3 3 mismatch messages messages SOLUTION: SOLUTION: Site should should be be locked locked & & unlocked unlocked after after the the parameter parameter changes changes to to take take in in effect. effect. –– Site 2.RRC drop without VoLTE DROP 1/3 There is time offset of 6Hrs b/t drive logs & UETR 1 1 2 1 2 2 3 3 1&2: Dedicated VoLTE bearer is setup with EPS bearer Id:7 & QCI:1 3: UE performs successful HO 2.RRC drop without VoLTE DROP 2/3 Ne w Cel l Old Cel l 4 6 7 5 4 7 4. Due Due to to poor poor RF RF conditions, conditions, RRC RRC ReConfig ReConfig msg msg sent sent by by 4. ENB is is missed missed out. out. ENB 5. Before Before the the old old ENB ENB can can detect detect the the RLF, RLF, UE UE establishes establishes 5. new RRC RRC connection connection with with 211226. 211226. new 6. UE UE requests requests for for all all 33 bearers. bearers. When When MME MME receives receives 6. initial context context request request ,, it it realizes realizes that that UE UE is is no no longer longer initial connected to to 210918. 210918. It It transfers transfers all all the the three three bearers bearers connected to new new ENB. ENB. to 7. MME MME commands commands old old ENB ENB to to release release all all the the bearers bearers (as (as 7. UE is is now now connected connected to to new new cell) cell) UE VolTE call call continues continues as as the the UE UE gets gets back back all all the the three three VolTE bearers bearers 2.RRC drop without VoLTE DROP 3/3 EFFECT of of R Radio adio L Link ink F Failure ailure on on VOLTE VOLTE call: call: EFFECT RLF can can be be UE UE detected detected or or ENB ENB detected detected ••RLF If the the RLF RLF is is UE UE detected detected & & if if UE UE performs performs a a successful successful RRC RRC ••If connection before before MME MME releases releases the the UE UE context context then then VOLTE VOLTE connection call will will continue continue call If the the RLF RLF is is ENB ENB detected detected then then ENB ENB will will request request MME MME to to ••If release the the bearers bearers resulting resulting in in VoLTE VoLTE drop. drop. release It is important to note that RRC drop alone do not conclude that the VoLTE call has dropped. Further analysis on bearer level needs to be performed to ascertain if VoLTE call dropped. 3.QCI-1 profile not defined 1 1 1 2 1 2 3 3 1. Default Default bearers bearers (Internet (Internet and and IMS) IMS) are are setup setup 1. 2. MME MME Requests Requests ENB ENB to to setup setup QCI1 QCI1 bearer bearer (( VoLTE VoLTE bearer) bearer) 2. 3. ENB ENB fails fails to to setup setup QCI QCI 11 & & responds responds with with “not-supported-QCI-Value” “not-supported-QCI-Value” 3. CAUSE: QCI-1 Profile was not activated in the site. 4.Multi target RRC connection Re-establishment HOW MULTI MULTI TARGET TARGET RRC RRC CONNECTION CONNECTION RE-ESTABLISHMENT RE-ESTABLISHMENT WORKS: WORKS: HOW 1. UE UE detects detects radio radio link link failure failure 1. 2. UE UE selects selects aa new new cell cell (( on on same same or or different different frequency) frequency) and and attempts attempts RRC RRC CONNECTION CONNECTION RERE2. ESTABLISHMENT ESTABLISHMENT 3. UE UE sends sends PCI, PCI, C-RNTI, C-RNTI, short short MAC-I MAC-I of of the the last last serving serving cell cell in in re-establishment re-establishment message. message. 3. Short MAC-I MAC-I is is used used to to verify verify the the credibility credibility of of the the Re-establishment Re-establishment message. message. Short 4. New New cell cell attempts attempts to to fetch fetch UE UE context context from from cells cells having having the the same same PCI PCI through through X2 X2 interface interface 4. 5. If If the the context context fetch fetch is is successful, successful, RRC RRC connection connection re-establishment re-establishment procedure procedure is is completed completed 5. For multi multi target target RRC RRC connection connection re-establishment re-establishment to to be be successful successful neighbor neighbor relations relations must must be be For defined from from source source to to target target cell cell and and vice vice versa. versa. defined 4.Context fetch timeout ( IFHO NBR not defined) 4 1 2 3 1. UE UE is is served served by by cell cell first first carrier carrier (band (band 17) 17) cell cell (PCI(PCI1. 367) and attempts HO to PCI 71 which fails. RRC 367) and attempts HO to PCI 71 which fails. RRC connection Re-establishment Re-establishment complete message message connection complete does not not reach reach ENB. ENB. (not (not seen seen in in UETR) UETR) does 2. UE UE latches latches on on to to aa new new cell cell on on second second carrier carrier (Band (Band 2. 4). 4). UE dropped dropped on on first first carrier carrier & & latched latched on on to to second second UE carrier cell carrier cell 3. UE UE attempts attempts RRC RRC CONNECTION CONNECTION RE-ESTABLISHMENT RE-ESTABLISHMENT 3. (multi target) target) (multi 4. Context Context fetch fetch result result shows shows cause cause as as “2” “2” .. 22 4. represents context context fetch fetch timeout. timeout. represents Operator has has 1C 1C & & 2C 2C on on Band Band 17 17 & & 44 respectively. respectively. Operator Neighbor is not defined from 1C to 2C due to which which ReReNeighbor is not defined from 1C to 2C due to establishment fails. 1 PENDING 5 4 4 When the the new new cells cells tries tries to to fetch fetch context context from from When old cell, cell, the the old old cell cell is is now now aware aware that that UE UE is is no no old longer connected connected to to it, it, hence hence it it releases releases the the longer UE context context (#5) (#5) UE This will will always always result result in in VOLTE VOLTE DROP DROP This 4. MT RRC RE-ESTABLISHMENT Workaround Purpose: Purpose: To improve improve the the performance performance of of Multi Multi Target Target RRC RRC Connection Connection re-establishment re-establishment feature feature & & To improve LTE LTE & & VoLTE VoLTE retainability. retainability. improve MT RRC RRC feature feature does does not not work work ifif cell cell relation relation is is missing missing and and result result in in VoLTE VoLTE as as well well as as LTE LTE ›› MT Drop. Drop. In this this case case as as per per Operator Operator strategy strategy Handover Handover is is not not allowed allowed from from 11stst carrier carrier to to second second ›› In carrier hence hence there there was was no no frequency frequency relation relation defined defined either. either. So So ,, following following solutions solutions were were carrier implemented to to reduce reduce the the limitation: limitation: implemented Add 2C 2C Frequency Frequency in in 1C 1C cells cells ›› Add Relax the the IF IF ANR ANR parameters parameters ›› Relax Increase the the neighbor neighbor removal removal time time ›› Increase Block the the neighbor neighbor so so ANR ANR can can not not delete delete it it ›› Block Do not not allow allow IFHO IFHO from from 1C 1C to to 2C 2C ›› Do Parameter Value Parameters Adjusted: Adjusted: Parameters connectedModeMobilityPrio -1 voicePrio -1 a5Threshold1Rsrp (cell level) -112 a5Threshold2Rsrp (cell level) -112 a5Threshold1RsrpAnrDelta (cell level) 10 a5Threshold2RsrpAnrDelta (cell level) 1 IsRemove Allowed FALSE 5.Incorrect MCS leading to VOLTE DROP1/2 1 1 2 3 1 3 4 1. 1. 2. 2. 3. 3. 2 UE reports reports MR MR for for HO HO UE Source ENB ENB responds responds RRC RRC connection connection reconfiguration. reconfiguration. RSRP RSRP = = -108.2 -108.2 // SINR SINR = = -0.3 -0.3 dB dB Source UE misses misses the the RRC RRC connection connection reconfiguration reconfiguration message. message. UE UE assumes assumes that that HO HO is is not not UE initiated & & sends sends multiple multiple MR MR for for HO. HO. initiated 4. Tx2Relocoverall Tx2Relocoverall timer timer expires expires (( 55 sec) sec) & & UE UE context context is is released. released. 4. Since the the UE UE context context is is released, released, this this will will always always result result in in VoLTE VoLTE drop. drop. Since SINR was was poor poor but but not not the the kind kind of of SINR SINR that that UE UE would would not not be be able able decode decode the the L3 L3 message. message. SINR Further investigation investigation was was performed performed on on such such behavior behavior Further 5.Incorrect MCS leading to VOLTE DROP2/2 Further investigation investigation revealed revealed that that ›› Further target ENB ENB sent sent the the RRC RRC connection connection target reconfiguration with with aa different different MCS MCS reconfiguration (modulation coding coding scheme) scheme) (modulation The UE’s UE’s used used for for drive drive test test was was RelRel›› The 8, MCS MCS coding coding assigned assigned to to UE UE was was in in 8, accordance with with Rel-10 Rel-10 accordance Rel-10 UE’s UE’s can can decode decode messages messages ›› Rel-10 with relative relative higher higher MCS MCS when when with compared to to Rel-8 Rel-8 device device for for aa compared specific SINR SINR specific Hence, higher higher MCS MCS was was assigned assigned ›› Hence, RRC reconfiguration reconfiguration message message which which RRC the Rel-8 Rel-8 UE UE could could not not decode. decode. the Solution: Solution: ENB ENB software software was was upgraded upgraded to to L13B L13B EP14 EP14 which which fixed fixed this this issue issue L13B L13B EP EP 14 14 Software Software rollout rollout Green Green CL CL drive stopped. drive stopped. 6. DL ICIC feature activation 1/3 Purpose: Purpose: DL ICIC ICIC is is used used to to randomize randomize PRB PRB allocation allocation to to reduce reduce the the inter-cell inter-cell interference. interference. DL With the the current current baseline, baseline, the the scheduler scheduler on on the the DL DL assigns assigns the the PRBs PRBs from from the the lower lower to to higher higher ›› With frequency that that is is from from left left to to right. right. Due Due to to this, this, sites sites with with coverage coverage overlap overlap will will cause cause frequency interference. In In typical typical scenarios scenarios without without DL DL ICIC ICIC feature, feature, the the resource resource allocation allocation for for all all the the cells cells interference. with start start from from PRB PRB 0. 0. with When the the "Inter-Cell "Inter-Cell Interference Interference Coordination Coordination (ICIC) (ICIC) -- autonomous autonomous resource resource allocation" allocation" feature feature ›› When is activated activated in in downlink, downlink, aa random random selection selection is is made made as as to to where where the the allocation allocation starts. starts. With With aa is randomized allocation, allocation, the the expectation expectation is is that that the the overall overall interference interference will will be be reduced. reduced. randomized Sites with with high high PRB PRB utilization utilization will will not not benefit benefit from from DL DL ICIC ICIC since since most most of of the the PRBs PRBs are are •• Sites allocated. allocated. Sites with with lower lower PRB PRB utilization, utilization, can can benefit benefit since since the the allocation allocation is is spread spread across across the the available available •• Sites PRBs. PRBs. Note that that the the scheduler scheduler will will try try to to allocate allocate all all the the PRBs PRBs for for aa data data user user within within the the same same TTI TTI for for •• Note large data data users. users. This This can can reduce reduce the the benefit benefit of of DL DL ICIC ICIC since since the the PRBs PRBs are are being being fully fully utilized. utilized. large Since VoLTE VoLTE packets packets are are small, small, the the PRBs PRBs would would not not be be fully fully utilized utilized so so DL DL ICIC ICIC can can be be Since . beneficial beneficial Parameters Adjusted: Adjusted: Parameters EUtranCellFDD dlInterferenceManagementActive=true dlInterferenceManagementActive=true ›› EUtranCellFDD dlInterferenceManagementDuration = = 100ms 100ms ›› dlInterferenceManagementDuration 6. KPI TREND after DL ICIC & MT RRC Work around LTE Retainability pm data VoLTE Retainability pm data MARKET TREND LTE Reestablishment pm data LTE Throughput pm data Observation: Observation: Improvement Improvement in in LTE LTE retainability~0.02 retainability~0.02 ,, VoLTE~1. VoLTE~1. No. No. of of radio radio link link failures failures reduced reduced significantly, significantly, No No significant significant degradation degradation in in throughput. throughput. 2/3 6. Drive KPI TREND after DL ICIC & MT RRC Work around 3/3 MARKET TREND – DRIVE DATA 7. Retainability & SRVCC SR improvement B2Threshold2RscpUtraOffset change B2Threshold2RscpUtra Parameter ›› B2Threshold2RscpUtra Parameter defines the the threshold threshold which which UMTS UMTS defines cell must must meet meet by by hysteresis hysteresis for for aa cell period of of timetotrigger timetotrigger in in order order to to period being reported reported by by the the UE UE provided provided being LTE Cell is worse than LTE Cell is worse than B2Threshold1Rsrp+Hysteresis for for aa B2Threshold1Rsrp+Hysteresis period of of timetotrigger. timetotrigger. period Default value for ›› Default value for B2Threshold2RscpUtra is is set set to to -115 -115 B2Threshold2RscpUtra dBm resulting resulting in in cells cells having having poor poor dBm UMTS coverage coverage being being selected selected for for UMTS SRVCC and and eventually eventually dropping dropping the the SRVCC call. Changing Changing this this parameter parameter will will call. affect LTE LTE as as well well .. affect B2Threshold2RscpUtraoffset is is QCI QCI ›› B2Threshold2RscpUtraoffset specific offset offset site site level level parameter parameter ,, specific so changes changes were were made made to to QCI=1. QCI=1. so B2Threshold2RscpUtraoffset was ›› B2Threshold2RscpUtraoffset was changed from from 00 to to 10 10 effectively effectively changed reducing B2Threshold2RscpUtra B2Threshold2RscpUtra to to reducing -105dBm. -105dBm. 1/2 7. Retainability & SRVCC SR improvement B2Threshold2RscpUtraOffset change 2/2 Observation: Observation: Counter Counter Based Based B2Threshold2RscpUtraoffset B2Threshold2RscpUtraoffset was was changed changed from from 00 to to 10 10 effectively effectively reducing reducing B2Threshold2RscpUtra B2Threshold2RscpUtra to to -105dBm. -105dBm. Retainability Retainability shows shows slight slight improvement improvement after after the the changes changes .. SRVCC SRVCC attempt attempt reduced reduced as as expected expected and and SRVCC SRVCC success success rate rate improved improved when when compared compared to to same same amount amount of of attempts attempts before. before. Take Away 1. Normal RRC connection release need not necessarily mean not end to VoLTE call. Mismatch in the below parameters will lead to bearer drop. rlcMode rlcSnLength pdcpSnLength rohcEnabled 2. It is important to note that RRC drop alone do not conclude that the VoLTE call has dropped. Further analysis on bearer level needs to be performed to ascertain if VoLTE call dropped. QCI 1 profile must be defined to enable VoLTE services. Multi Target RRC Connection re-establishment will not be successful (L13B) if neighbors are not defined in both the directions. The problem could be more severe when layering strategy does not allow handover from one carrier to other(IFHO). A work around is possible to reduce the impact of the limitation. Incorrect MCS assigned by ENB to Rel 8 UE was causing the VoLTE drops in poor SINRs, this was a eNB software bug and was rectified by EP14 rollout. A significant improvement in retainability was observed. DL ICIC feature shows good improvement in VoLTE Retainability. QCI 1 specific “b2Threshold2RscpUtraOffset” helps to improve VoLTE retainability as well as SRVCC performance. Offset value of 10dB showed good results. 3. 4. 5. 6. 7.
