RLF Triggered Handover

1. 25/05/2023
1 © Nokia 2014
Customer Confidential
RLF Triggered Handover
LTE1617
NPO Feature Training
FDD-LTE 15

2. 25/05/2023
2 © Nokia 2014
For internal use
Introduction (I)
• RL30 introduced the feature:
• LTE735: RRC Connection Re-establishment
• RRC connection re-establishment allows the recovery of a connection after a drop
• LTE735 supports RRC connection re-establishment within prepared cells
• original cell where drop occurred
• target cell during a handover procedure, if handover prepare has completed
UE drops and recovers within
the same cell, e.g. due to
coverage hole within cell
Cell A
Cell B
UE drops during a handover
procedure and recovers on the
target cell which has already
completed handover preparation
LTE735 – scenario 1 LTE735 – scenario 2
Scenario 2
could be intra or
inter eNode B

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Introduction (II)
• LTE735 does not allow re-establishment in target cells where handover preparation has
not been completed
• solution provided by LTE1617
• LTE1617 uses the uplink RRC Connection Re-establishment Request message to trigger
a handover into the target cell
• The target cell flags the requirement for a handover
• subsequent handover allows the re-establishment procedure to complete
Cell A
Cell B
UE drops and recovers on a
target cell belonging to the
same eNode B
LTE1617 – scenario 1
Cell A
Cell B
UE drops and recovers on a
target cell belonging to a
different eNode B
LTE1617 – scenario 2

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Requirements
• System Release FDD-LTE 15
• Flexi Release 2 LN7.0
• Flexi Release 3 LN7.0
• This feature does not require any additional hardware
• There are no pre-requisite features
• This feature does not have any additional UE requirements
• This feature is Application Software (ASW)

5. 25/05/2023
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RRC Connection Re-establishment

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3GPP Procedure (I)
• Objective is to re-establish the RRC connection, which involves:
• resumption of SRB1 (while other radio bearers remain suspended)
• re-activation of security
• configuration of only the primary cell
• Re-establishment succeeds only if the target cell is prepared, i.e. has a valid UE context
• The UE does not initiate the procedure if security has not been activated
• UE goes direct to RRC Idle

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3GPP Procedure (II)
• RRC Connection Re-establishment can be triggered by the UE when one of the
following occurs:
• upon detecting radio link failure
• upon handover failure
• upon mobility from E-UTRA failure
• upon integrity check failure indication from lower layers
• upon an RRC connection reconfiguration failure

8. 25/05/2023
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Radio Link Failure
• If N310 consecutive ‘out-of-sync’ indications are received from the lower layers:
• start T310
• If N311 consecutive ‘in-sync’ indications are received from the lower layers while T310
is running:
• stop T310
• continue with the existing connection
• If T310 expires, or if the maximum number of RLC re-transmissions occurs:
• radio link failure is detected
N310 default = 10 T310 default = 2 seconds
N311 default = 1

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3GPP Procedure (III)
• Upon triggering RRC Connection Re-establishment:
• start T311
• attempt cell selection
• If cell selection is achieved:
• stop T311
• start T301
• send RRC Connection Re-establishment Request message
• If RRC Connection Re-establishment message is received:
• stop T301
• re-establish SRB 1
• If T311 or T301 expires
then UE drops to RRC Idle
mode
T311 default = 3 seconds
T301 default = 400 ms

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RRC Connection Re-establishment Request
• The request message includes:
• UE identity, in terms of:
• C-RNTI
• PCI
• shortMAC-I
• re-establishment cause
used to verify the
integrity of the UE

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RRC Connection Re-establishment
• The RRC Connection Re-establishment message includes dedicated radio
resource configuration information
• allows SRB 1 to be re-established

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RRC Connection Re-establishment Complete
• The complete
message can
include one or more
flags
Radio Link Failure
information available
to be reported
Logged measurements
available to be
reported

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RRC Connection Re-establishment Reject
• The reject message does not include any information content

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Towards Different Cell of Same eNode B

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Activation
• RRC Connection Re-establishment towards a different cell of the same eNode B is
activated by default after installing RL70 software
• no parameter used for activation
• There must be a neighbour relation between the source and target cells
• not blacklisted

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Procedure
• UE experiences radio link failure while connected to cell ‘A’
• UE completes cell selection and identifies cell ‘B’
• UE sends RRC Connection Re-establishment Request on cell ‘B’
• cell ‘B’ is unprepared for the UE
• Neighbour relation must be defined from cell ‘A’ to cell ‘B’
• should not be blacklisted
• There should be no other procedure ongoing for the UE
• Intra-eNode B handover triggered
• Cell ‘B’ retrieves the UE context from cell ‘A’
• cell ‘B’ is then prepared for the UE
• Re-establishment procedure completes
Cell A
Cell B

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Signalling
• eNode B internal signalling is
used to retrieve the UE context
and complete the handover
preparation
• An RRC Connection Re-
establishment Reject is sent by
Cell ‘B’ if the handover
preparation fails, e.g. due to
admission control
Cell A Cell B
UE
Radio link failure while UE is connected to cell ‘A’
UE completes cell selection to cell ‘B’
Contention based PRACH preamble (MSG1)
RRC Connection Re-establishment Request (MSG3)
Random Access Response (MSG2)
Check for Neighbour
Retrieval of UE context
Handover Preparation
RRC Connection Re-establishment (MSG4)
RRC Connection Re-establishment Complete (MSG5)

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Towards Cell of different eNode B

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Activation (I)
• RRC Connection Re-establishment towards a different eNode B is activated using
the actRLFbasedRCR parameter
Name Range Default Description
act​RLFbasedRCR
(LNBTS)
0 (false),
1 (true)
0 (false) Activates the feature 'Radio Link Failure
(RLF) based RRC connection re-
establishment (RCR)'
• The parameter must be set to ‘true’ at both the source and target eNode B
• In addition, there must be a neighbour relation between the source and target cells

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Activation (II)
• The rlfBasedRCRsupported parameter must be set to ‘true’ for the relevant
LNADJ at both eNode B
Name Range Default Description
rlfBasedRCRsupported
(LNADJ)
0 (false),
1 (true)
0 (false) Indicates whether RLF based connection
re-establishment is supported by the
neighbour eNB. The setting must be
aligned with the feature activation
parameter actRLFbasedRCR in the
neighbour eNB.
• This parameter is automatically set to ‘true’ for all LNADJ when enabling the feature
using the actRLFbasedRCR parameter

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Activation (III)
• The rlfBasedRCRdefault parameter defines the value which is automatically
allocated to the LNADJ rlfBasedRCRsupported parameter when an LNADJ is
created by ANR
Name Range Default Description
rlfBasedRCRdefault
(LNBTS)
0 (false),
1 (true)
0 (false) Defines the default value for the LNADJ
parameter rlfBasedRCRsupported which is
used if an LNADJ instance is autonomously
created by eNB due to an activated ANR
feature.

22. 25/05/2023
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Procedure
• UE experiences radio link failure while connected to cell ‘A’
• UE completes cell selection and identifies cell ‘B’
• UE sends RRC Connection Re-establishment Request on cell
‘B’
• cell ‘B’ is unprepared for the UE
• Neighbour relation must be defined from cell ‘A’ to cell ‘B’
• BTS 2 sends X2AP: RLF Indication to BTS 1 and BTS 3
• BTS 3 recognises that it does not have a UE context:
• discards the message
• BTS 1 recognises that it has a UE context:
• triggers the handover procedure
• X2 signalling between BTS 1 and 2 used to prepare BTS 2
• Re-establishment procedure completes on BTS 2
Cell B
Cell A
Cell C
Cells ‘A’ and ‘C’ have
the same PCI
BTS 2
BTS 1
BTS 3

23. 25/05/2023
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Signalling (I)
• RRC Connection Re-
establishment Request
message triggers the
handover procedure
• Target BTS applies
admission control after
receiving X2AP: Handover
Request message
• Handover command is not
sent to the UE because the
UE is already ‘connected’
to the target cell
BTS 1
UE
Radio link failure while UE is
connected to cell belonging to BTS 1
UE completes cell selection
to cell belonging to BTS 2
Contention based PRACH preamble (MSG1)
RRC Connection Re-establishment Request (MSG3)
Random Access Response (MSG2)
BTS 2 BTS 3
X2AP: RLF Indication X2AP: RLF Indication
Message
discarded
Admission
Control
X2AP: Handover Request
X2AP: Handover Request Acknowledge

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X2AP: RLF Indication
Cell A
Cell B
PCI of cell where original
drop was experienced
Cell Global Identity (CGI) of cell where UE
sent the RRC Connection Re-establishment
Request
UE identity
Integrity check
Can provide the Radio Link Failure report
provided by the UE
(not applicable to described scenario since
UE has only sent RRC Connection Re-
establishment Request

25. 25/05/2023
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Signalling (II)
• The target cell sends the
RRC Connection Re-
establishment to the UE
BTS 1
UE BTS 2 BTS 3
RRC Connection Re-establishment (MSG4)
RRC Connection Re-establishment
Complete (MSG5)
• If Admission Control at the target BTS generates a negative result:
• X2AP: Handover Preparation Failure is returned to the source BTS
• RRC Connection Re-establishment Reject is sent to the UE

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Counters

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Counters (I)
• LTE1617 introduces the M8027, LTE Handover (RLF trigger) counter table
Identity Name
M8027C0 HO_RLF_PREP_ATT
M8027C1 HO_RLF_PREP_SUCC
M8027C2 HO_RLF_SUCC
M8027C3 RCR_UE_CONTEXT_RETR_ATT
M8027C4 RCR_UE_CONTEXT_RETR_SUCC
M8027C5 RCR_UE_CONTEXT_RETR_FAIL_
TIM
• The reporting period for this
counter table is configured using
the mtHoRlf PMRNL parameter
• Default value of 15 minutes

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Counters (II)
• The mtHoRlf parameter defines the reporting period for the LTE Handover (RLF
trigger) counter table
Name Range Default Description
mtHoRlf
(PMRNL)
disabled (0),
15min (15),
30min (30),
60min (60),
360min (360),
720min (720),
1440min (1440)
15min (15) configuration of measurement type
LTEHoRlf

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Customer Confidential