Radio Link Failure-PA2

Section 8 Radio Link Failure Analysis

Section Objectives

 Describe Radio Link Failure (RLF) Procedures.  Explain how to Analyze RLF Logs  Describe RLF Case Studies  Describe KPI Impact due to RLF

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Qualcomm Proprietary & Confidential

MAY CONTAIN U.S. AND INTERNATIONAL EXPORT CONTROLLED INFORMATION

RLF Procedures Overview

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Radio Link Failure (RLF) Overview  During LTE active data session, RRC is in RRC_CONNECTED State  In RRC_CONNECTED State, UE can declare RLF under various conditions  To Restore the RRC Connection, UE Initiates RLF Recovery procedure

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What is RLF and Recovery?  A UE in RRC Connected Mode has already established - Signaling Radio Bearers (i.e SRB1, SRB2) , - Default Radio Bearers (i.e DRBs) - Security Context is activated.  When a UE in RRC Connected Mode declares RLF, RRC Shall: - Stop all running timers except T320 - Reset Radio Resources for all RBs - Release MAC,RLC,PDCP,PHY Configurations in order to Suspend all SRBs and DRBs - Security Context is still active  UE initiates RLF Recovery to resume SRB1 and network configures SRB2 & DRBs.

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RLF Scenarios 1.DL Physical Layer Failure 2.RRC Connection Re-configuration failure

(PDCCH BLER > 10%)

3.Handover Failure (HO Target Cell Acquisition Failure)

Radio Link Failure

4.Hndover Failure (T304 expiry- RACH Failure)

5.RLC Unrecoverable Error

6.PDCP Integrity check failure

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7.MAC RACH Problem detection MAY CONTAIN U.S. AND INTERNATIONAL EXPORT CONTROLLED INFORMATION

1.RLF: Downlink Physical Layer Failure 18

Out-of-Sync indicatio 16

PDCCH /PHICH BLER > 10%

T310 started

BLER (%)

14

Qout Qin PDCCH BLER (%)

RLF declared

12 10

N310: n5 X

8

N311: n1

6

T310: 2000 ms

4

DRX Cycle: 8 radio frames

2 0 0

2000

4000

6000

8000

10000

12000

(ms) linkTime quality

14000

16000

18000

20000

• UE estimates Downlink radio in terms of PDCCH BLER % to detect Physical Layer Failure • RLF is declared after: – N310 Out-of-Sync indications (PDCCH BLER > Qout) and the * 7

– The subsequent expiry of T310 (in the absence of N311 In-Sync Additional fields not shown indicators)



Downlink Physical Layer Failure - Recovery 18

Out-of-Sync indication 16

In-Sync indication

PDCCH BLER > 10%

T310 started

14

T310 stopped (recovery)

Qout Qin PDCCH BLER (%)

BLER (%)

12 10

N310: n5

8

N311: n1

6

T310: 2000 ms

4

DRX Cycle: 8 radio frames

2 0 0

2000

4000

6000

8000

10000

12000

Time (ms)

14000

16000

18000

20000

• UE estimates Downlink radio link quality in terms of PDCCH BLER % to detect Physical Layer Failure – N310 Out-of-Sync indications (PDCCH BLER > Qout) starts T310 – T310 stopped due to reception of N311 In-Sync indicators (PDCCH BLER < Qin ) 8

– RLF not declaredQualcomm Proprietary & Confidential


2.RLF : RRC Connection Re-configuration Failure UE

eNodeB

RRC Connection Reconfig Validation of RRC Connection Reconfiguration Failure

RLF

UE Evaluates all parameters of the RRC Connection Reconfiguration message 9



3.RLF : Handover Failure UE

Measurement Report

Source eNB

Target eNB

Handover Request RRC Connection Reconfig Perform Target eNodeB Frequency Acquisition and Physical Cell ID Specified But Encounters Failure

Handover Request Ack SN Status Transfer

RLF RRC Connection Reconfiguration specifies Target eNodeB C-RNTI, Radio Resource info, Dedicated RACH Info, SecurityConfigHO etc. 10



4.RLF : Handover Failure (T304 ExpiryRACH Failure) UE

Measurement Report

Source eNB

Target eNB

Handover Request RRC Connection Reconfig T304 Begins Perform Target eNodeB Frequency Acquisition and Physical Cell ID Specified

Handover Request Ack SN Status Transfer

MIB and SIB Overhead Messages RACH Failure & T304 Expiry RLF

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Handover Failure – T304 Expiry Handover Requested Acquisition Successful, RACH started RLF declared

RACH preamble transmissions

Preamble Trans Max: 4

T304

X

UE declares RLF following failure to successfully complete the handover within T304 12



5.RLF : RLC Retransmission Error eNB

UE

X X

1st PDU transmission PDU retransmissions RLF is declared

Max Re-Tx Threshold: 6

X X X X X

RLF

Failure to send a specific RLC PDU after attempting Max Retransmissions causes RLF. 13



6.RLF : PDCP Integrity Check Failure RRC

PDCP

Signaling Integrity protection Failure Integrity Protection Failure RLF

All Signaling Messages are both Ciphering and Integrity protected * 14

Additional fields not shown Qualcomm Proprietary & Confidential


7.RLF : MAC RACH Problem Detection

(Connected Mode: UL Time Alignment Timer expired or RACH Failure) UE in Connected state eNB stops grants Random Access is started

Random Access

Data (bytes)

New UL Data

RLF is declared

Preamble Trans Max: 6 RA-Response Size: sf3 (3 ms)

0

30

60

90

X RLF

(preamble sent again after (3+3)ms if no response or backoff)

Time (ms)

Failure to RACH [when UL/DL data arrives] in Connected state causes RLF. 15



RLF Recovery Procedure (1 of 2)  UE starts T311 Timer and initiates Cell Selection Procedure  After Successful Cell Selection Procedure (successful MIB decoding), UE stops T311 Timer and reads SIBs from PDSCH  UE Initiates T301 Timer & RRC Re-establishment procedures and RRC Configures lower layers to activate SRB1  SRB2 and DRBs are configured upon receiving RRC Connection Re-configuration message from E-UTRAN after RLF Recovery

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RLF Recovery Procedure (2 of 2) RLF Declared, Cell Reselection started Cell Reselection successful Re-establishment successful

Reselection and Connection Reestablishment procedures are used to recover from RLF

Transition to RRC IDLE state

T301 Connection Reestablishment

T311

SRB1 resumes

SRB2 & DRB resumes

Cell Reselection

• If AS Security has been activated prior to RLF, UE remains in the RRC Connected state – Suspends SRB1; releases SRB2 and DRBs – Trigger recovery procedures • If AS Security has not been activated, UE transitions to RRC Idle state 17



RLF Recovery Call flow eNB

UE

RLF Suspend SRB1 & Release SRB2,DRBs & initiate Cell Selection Process RRC Connection Re-establishment Request msg

Resume SRB1 Resume SRB2 & DRB

RRC Connection Re-establishment RRC Connection Re-establishment Complete RRC Connection Re-Configuration RRC Connection Re-Configuration Complete

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RLF Log Analysis

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Various Events (1 of 2)  EVENT_LTE_RRC_RADIO_LINK_FAILURE This Counter indicates how many times UE declared RLF since power-up.

 EVENT_LTE_RRC_TIMER_STATUS

Timer Value indicates the remaining amount of time.

 EVENT_LTE_RRC_NEW_CELL_IND

Cause indicates Cell Selection, Re Selection or Handover

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Various Events (2 of 2)  EVENT_LTE_RACH_ACCESS_START RACH Cause indicates what caused the UE to start RACH Procedure

 EVENT_LTE_RACH_ACCESS_RESULT RACH Result indicates whether RACH is aborted, successful or has problem

 EVENT_LTE_MAC_TIMER Timer Type indicates whether it is Timing Advance (TA), Random Access (RA), Contention Timer or RACH Back off Timer

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LTE ML1 Initial Acquisition Results (0xB176)  This log packet provides information about each initial acquisition attempt. It is sent whenever RRC requests initial acquisition.

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LTE ML1 Random Access Request (0xB167)  Random Access Request (MSG1) transmission is used by UE to transmit PRACH Preamble and UE uses RA RNTI Identifier

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LTE ML1 Random Access Response (0xB168)  Random Access Response (MSG 2) Report transmitted on DL-SCH carries UE Uplink Timing Advance, RNTI Type and its value, PUSCH Resources for MSG3 Transmission.

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LTE ML1 UE Identification Message (0xB169)  UE Identification Message (MSG 3) carries the UE Identifier and is transmitted using PUSCH Resources in MSG2 .

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LTE ML1 Contention Resolution Msg (0xB16A)  Contention Resolution Message (MSG 4) resolves the contention due to multiple UEs trying to access the system using the same Random Access Resource.

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RRC Connection Re-establishment Request (0xB0C0)  RRC Connection Re-establishment Request is sent by the UE to initiate RRC Connection Re-establishment.

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RRC Connection Re-establishment Response (0xB0C0)  RRC Connection Re-establishment Response is sent by the eNodeB to establish SRB1.

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RRC Connection Re-establishment Complete (0xB0C0)  UE sends RRC Connection Re-establishment Complete message after successful SRB1 Configuration .

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RRC Connection Re-configuration (0xB0C0)  Network sends RRC Connection Re-configuration including SRB2 Resources .

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RRC Connection Re-configuration Complete (0xB0C0)  After Successful SRB2 Configuration, UE sends RRC Connection Re-configuration Message .

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LTE RLC UL AM ALL PDU (0xB092)  This Packet is used for logging all control and data

AM PDUs for SRB as well as DRB sent on UL

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RLF Case Studies

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RLF Recovery (1 of 3) 1. UE Received RRC Connection Reconfiguration (0xB0C0) @ 00:00:30:789 • UE is configured with UL MAC and PHY Layers 2. UE Declared RLF (01608) @ 00:00:30:791 due to RRC Connection Reconfiguration Validation Failure (Reason not captured in RLF Event) 3. UE Initiated RLF Recovery and successfully acquired Cell Id 397 (0xB176) @ 00:00:30:886 4. UE Sent RRC Connection Reestablishment Request (0xB0C0) @ 00:00:00:955 5. eNodeB Responded with RRC Connection Reestablishment message including SRB1 Configuration (0xB0C0) @ 00:00:31:045

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RLF Recovery (2 of 3) 6. After Successful SRB1 Configuration, UE sends RRC ConnectionReestablishmentComplete (0xB0C0) @ 00:00:31:051 7. eNodeB Configures SRB2 and DRB through RRC Connection Reconfiguration (0xB0C0) @ 00:00:31:133 8. After Successful Reconfiguration of SRB2 and DRB, UE sends RRC Connection Reconfiguration Complete Message (0xB0C0) @ 00:00:31:137

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RLF Recovery (3 of 3)

HTML_Format_RLF_Recovery_Procedure

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RLF : RRC Connection Reconfiguration Failure (1 of 4) 1. UE Received RRC Connection Reconfiguration (0xB0C0) @ 02:32:13:360 • UE is configured with UL MAC and PHY Layers 2. UE Declared RLF (01608) @ 02:32:13:363

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RLF : RRC Connection Reconfiguration Failure (2 of 4) Parameters

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RLF: RRC Connection Reconfiguration Failure (3 of 4)

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RLF: RRC Connection Reconfiguration Failure (4 of 4)

HTML_Format_RLF_RRC_Connection_Reconfiguration_Failure

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RLF: RACH Problem (1 of 3) 1. UE Received SIB2 Message with PRACH Configuration (0xB0C0) @ 06:00:23:610 • PreambleTransMax = n6 2. UE UL Data arrival triggered contention based RACH Procedure . Look for EVENT_LTE_RACH_ACCESS_START @ 06:00:38:730 3. UE sent 6 PRACH Max Preambles @ 06:00:38:707/716/722/730/738/762 4. UE MAC Layer Random Access Timer expired (EVENT_LTE_MAC_TIMER) @ 06:00:38:730/745 5. EVENT_LTE_RACH_ACCESS_RESULT Indicated RACH has problem @ 06:00:38:745 6. EVENT_LTE_RRC_RADIO_LINK_FAILURE declared @ 06:00:38:745

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RLF: RACH Problem (2 of 3)

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HTML_Format_RLF_RACH_Problem

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RLF: Handoff Failure and SIB Read Failure (1 of 5) 1. UE Received RRC Connection Reconfiguration Message from Source Cell ID = 397 including MobilityControlInfo (0xB0C0) @ 06:59:38:212 • TargetPhysCellID = 116 2. From LTE LL1 SSS Results, it is observed that UE failed to Acquire Cell ID=116 @ 06:59:38:228 3. UE Started T304 =500ms Timer EVENT_LTE_RRC_TIMER_STATUS @06:59:38:232 4. UE Stopped T304 at 462ms , EVENT_LTE_RRC_TIMER_STATUS@ 06:59:38:232 5. EVENT_LTE_HO_FAILURE is declared by UE and RLF @ 06:59:38:232

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RLF: Handoff Failure & SIB Read Failure (2 of 5) 6. UE Declared 1ST RLF, EVENT_LTE_RRC_RADIO_LINK_FAILURE @ 06:59:38:232 7.UE Attempted RLF Recovery and RRC Connection Reestablishment Request @ 06:59:38:626 • RLF Recovery Tried on Target PhysCellID = 116 8. From LTE LL1 SSS Results, it is observed that UE failed to Acquire Cell ID=116 @ 06:59:38:228 9. UE Failed to read the SIB Information on Cell ID = 116 , EVENET_LTE_RRC_SIB_READ_FAILURE @06:59:39:771 10. UE Declared 2nd RLF, EVENT_LTE_RRC_RADIO_LINK_FAILURE @ 06:59:39:771

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RLF: Handoff Failure (3 of 5)

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RLF: SIB Read Failure during RLF Recovery

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RLF: Handoff Failure and SIB Read Failure

HTML_Format_RLF_Handoff_Failure_&_SIB_Read_Failure

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RLF: Max RLC Retransmission Failure (1 of 5) 1. UE Received SRB2 and DRB1 Configuration through RRC Connection Reconfiguration Message @ 08:00:11:280 • For DRB Uplink RLC, maxRetxThreshold = t8 2. During Uplink Data Transfer, RLC PDU Sequence Number (SN) 973 is transmitted @ 08:01:11:826 3. The same SN is Retransmitted 8 times based on maxRetxThreshold configuration . RLC Retransmission is done to recover the UL Data Transmission Failures. (LTE RLC UL AM ALL PDU) 4. Retransmission Time Stamps: 08:01:11:847/873/895/921/945/972, 08:01:12:017/045 5. UE declared RLF LTE_RRC_RADIO_LINK_FAILURE @ 08:01:12:061

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RLF: Max RLC Retransmission Error

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HTML_Format_RLF_Max_RLC_Retransmission_Error

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KPI Impact

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PDSCH Throughput Impact due to RLF

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Low RSRP when RLF Happened

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Low SINR when RLF Happened

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What did we learn?

 What is Radio Link Failure (RLF) and its Recovery Procedure.  Analysis of RLF Logs  RLF Case Studies  KPI Impact due to RLF

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Test Plan aspects and performance metrics

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Radio Link Failure-PA2

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