Lte irat-troubleshooting-guide

HUAWEI TECHNOLOGIES CO., LTD.
www.huawei.com
Huawei Confidential
Security Level: INTERNAL
LTE IRAT
Troubleshooting Guide

HUAWEI TECHNOLOGIES CO., LTD. Huawei Confidential Page 2
Content
 Why IRAT
 Applications of IRAT
 Delivery Policies of IRAT
 Signaling Procedures of IRAT
 Definitions and Observation of IRAT KPIs
 IRAT Fault Diagnosis

HUAWEI TECHNOLOGIES CO., LTD. Huawei Confidential
Why IRAT – 1
Page 3
Revenues(US$bn)
Global mobile voice vs data service
Source: Informa Telecoms & Media
 GSM, UMTS and, LTE will coexist for a long time.
 Voice service keeps creating huge revenue for operators.
 Main revenue growth will come from data services.

Why IRAT – 2
Page 4

Content
 Why IRAT

Applications of IRAT
Page 6
1. An example of LTE & GSM interoperation is TMO of Germany.
2. Examples of LTE & UMTS interoperation are Bell of Canada,
eMobile of Japan, and Genius of Hong Kong.
3. An example of LTE, GSM, and UMTS interoperation is N4M of
Sweden.
Function requirements
a) UEs preferentially receive services from the LTE network for better user
experience.
b) The continuity of the LTE services is guaranteed.
c) The services and load are appropriately shared among the LTE, GSM, and
UMTS.
d) The network upgrade causes minimum impact on the network and users.
Performance requirements
a) The KPIs must be guaranteed, for example, IRAT handover success rate
and IRAT handover delay.
b) Users of high-speed data services receive services from preferably the LTE
for better user experience. Where the LTE is unavailable, the UMTS or
GSM is used.
c) Parameters are correctly set to improve the IRAT handover success rate,
lower the impact of a handover on the service, and prevent ping-pong
handovers.
d) Minimum LTE to UMTS/GSM handover delay is guaranteed. Users are
unaware of the service degradation caused by handovers.

Content
 Why IRAT

Delivery Policies of IRAT – Overall
Page 8
LTE/EP
C
GSM/UMTS
Mobility in idle mode
Mobility of PS services
Mobility of voice services

Delivery Policies of IRAT – Overall
Page 9
LTE
UTRAN/GERA
N
Cell
reselection
to LTE
Cell reselection to
UTRAN/GERAN
UE moving into
LTE coverage
UE moving out of
LTE coverage
Camping priority:
LTE > UTRAN > GERAN
Broadcasted in
GERAN/UTRAN/LTE
Idle/PCH Mode
PS
handover/Redirection
to UTRAN/GERAN
UE moving into
LTE coverage
UE moving out of
LTE coverage
Connected Mode
CS Fallback to
UTRAN/GERAN by PS
handover/redirection

Delivery Policies of IRAT – Idle Mode – 1
Page 10
Target ：
 UEs preferentially reside in LTE except when the LTE cell has poor signal
quality.
 UE resides in the LTE cell even when the G/U cell has good signal quality.
Cell reselection from LTE to GSM/UMTS :
 The eNodeB is so configured that the priority of LTE cell
reselection is greater than that of GSM/UMTS cell reselection. If
the signal strength of the serving LTE cell is lower than the inter-
RAT measurement threshold, the GSM/UMTS measurement is
triggered. If the signal strength of the LTE cell is lower than the
serving-frequency low-priority reselection threshold and the signal
strength of the inter-RAT frequency is greater than the inter-RAT
frequency low-priority reselection threshold, the GSM/UMTS cell
reselection is triggered.

Delivery Policies of IRAT – Idle Mode – 2
Page 11
Target ：
 UEs preferentially reside in LTE except when the LTE cell has poor
signal quality.
 UEs reside in LTE cell even when the G/U cell has good signal quality.
L to G/U 小区重选 :
 在 eNodeB 配置 LTE 点的小区重先高于频选优级 GSM/UMTS ，
当 LTE 服小区的信号低于系量限触务异统测门发 GSM/UMTS 测
量，当 (LTE 小区信号 )<( 服点低先重限务频优级选门 ) 并且 ( 异
系信号统 )>( 系点低先重限异统频优级选门 ) 才会触到发
GSM/UMTS 的小区重。选
Cell reselection from GSM/UMTS to LTE:
 All LTE frequencies are configured on the BSC/RNC and the
priority of LTE cell reselection is greater than that of GSM/UMTS
cell reselection. After the UE returns to the idle or PCH mode
upon completion of a service in the GSM/UMTS network, the UE
periodically measures the LTE signal strength. If the signal
strength of the LTE cell is greater than the high-priority reselection
threshold configured on the BSC/RNC, the LTE cell reselection is
triggered.

Delivery Policies of IRAT – Connected Status
 Events and meaning related to the LTE system
Page 12
Event Type Meaning
A1 Serving becomes better than threshold
A2 Serving becomes worse than threshold
A3 Neighbor becomes offset better than serving
A4 Neighbor becomes better than threshold
A5
Serving becomes worse than threshold and neighbor becomes better
than threshold2
B1 Inter RAT neighbor becomes better than threshold
B2
Serving becomes worse than threshold and inter RAT neighbor
becomes better than threshold2

Delivery Policies of IRAT – Connected Status
 Coverage-based: When a UE moves out of the LTE coverage, the
LTE signal worsens. After the UE reports event A2, the eNodeB
determines whether the UE supports inter-RAT measurement.

If the UE supports inter-RAT measurement, the eNodeB delivers the
inter-RAT measurement control. After the UE reports inter-RAT
measurement event B1, the inter-RAT PS handover or redirection is
triggered. If both the UE and network support PS handover, PS
handover is preferred to obtain shorter service interruption. If the
operator does not want to upgrade the GERAN/UTRAN to support PS
handover or the UE does not support PS handover, redirection is
performed. In this case, the PS handover switch must be turned off.

If the UE does not support inter-RAT measurement, the eNodeB
performs the UMTS blind redirection triggered by event A2.
Page 13

Delivery Policies of IRAT – Voice Service
 Before the IMS becomes mature, CSFB is a solution for providing
voice service over the LTE system. When the UE performs a
mobile-originated or mobile-terminated voice service over the
LTE, the CSFB works as follows:

If the UE supports inter-RAT measurement, the eNodeB delivers the
inter-RAT measurement control. After the UE reports inter-RAT
measurement event B1, CSFB based on inter-RAT PS handover or
redirection is triggered. If both the UE and network support PS
handover, PS handover is preferred to obtain shorter service
interruption. If the operator does not want to upgrade the
GERAN/UTRAN to support PS handover or the UE does not support
PS handover, redirection is performed. In this case, the PS handover
switch must be turned off.

If the UE does not support inter-RAT measurement, the CSFB is
based on inter-RAT blind redirection.
Page 14

Content
 Why IRAT

IRAT Signaling Procedure – Reselection
Page 16
Reselection procedure

IRAT Signaling Procedure – Reselection
Page 17
Signaling procedure of the UE

IRAT Signaling Procedure - Redirection
Page 18
 The eNodeB sends RRC CONNECTION RELEASE to UE with IE:
RedirectedCarrierInfo including UMTS or GERAN carrier frequency.

IRAT Signaling Procedure – Redirection
Page 19
Measurement control
Measurement report
Redirect command
Redirected
frequency

IRAT Signaling Procedure – PS Handover
Page 20

IRAT Signaling Procedure – PS Handover
Page 21
A1/A2 measurement control
A2 measurement
report
B1 measurement
control
B1 measurement report
Handover command, instructing the
UE to hand over to the inter-RAT

IRAT Signaling Procedure – Redirection-
based CSFB
Page 22

IRAT Signaling Procedure – CSFB
Based on PS Handover
Page 23

Content
 Why IRAT

Definitions and Observation of IRAT KPIs
Page 25
KPI Meaning
L2U redirection control-
plane delay
From reception of RRC CONN REL on the LTE to transmission of
RRC CONN REQ on the UMTS
L2G redirection control-
plane delay
From reception of RRC CONN REL on the LTE to transmission of
Channel Request on the GERAN.
PSHO control-plane delay
From reception of Handover Command to reply of Handover
Complete
CSFB voice service setup
delay
From transmission of Extend Service Request to reception or
transmission of the Alerting message on the UMTS/GERAN
User-plane interruption
delay (uplink)
Average E2E IP-layer interruption delay. Observed on the FTP
server. From reception of the last data packet on the LTE to
reception of the first data packet on the UMTS/GERAN
User-plane interruption
delay (downlink)
Average E2E IP-layer interruption delay. Observed on the UE. From
reception of the last data packet on the LTE to reception of the first
data packet on the UMTS/GERAN

Definitions and Observation of IRAT
KPIs – QXDM
Page 26

Definitions and Observation of IRAT KPIs - Probe
 The CSFB KPIs, including CS setup delay and success rate, can
be observed on the Probe/Assistant.
Page 27

Content
 Why IRAT

Common Problems of LTE IRAT
Type Common Problem
Capability Support
IRAT not supported by UE
IRAT not supported by GSM or UMTS BTS version
IRAT not supported by core network version
Parameter
Configuration
eNodeB IRAT switch not turned on
Inter-RAT neighboring cell not configured or incorrectly
configured
Core network interface not configured or incorrectly configured
Interference
Coverage
Large interference
Poor quality of the Uu interface
Known Problem
Known bugs of earlier eNodeB versions
Known bugs of the core network
Known bugs of the UE

IRAT Troubleshooting Flowchart
Page 30

Discover a Problem
Page 31
 Understand the scenario of the problem.
 Check each signaling procedure to determine where the problem
occurs.
eRAN 2.1 provides performance statistics for only L2U, L2G, and L2C outgoing
handovers for calculating the handover success rate. eRAN3.0 provides additional
performance statistics for redirection and CSFB. For details, see the performance
indexes of eRAN3.0.

Page 32

Determining the UE Capability
Page 33

Page 34

IRAT Parameter Configuration – Reselection
 The following command configures the LTE cell reselection priority and parameters:
MOD CELLRESEL: LocalCellId=0, SNonIntraSearchCfgInd=CFG,
SNonIntraSearch=8, ThrshServLow=7, CellReselPriority=6;
where CellReselPriority indicates the LTE cell reselection priority and
SnonInterSearch indicates the time that the UE starts the inter-RAT measurement.
CellReselPriority ranges from 0 to 7. To let the UE preferentially reside in the LTE
system, this parameter is set to a high value.
 The following command configures the UMTS frequency and reselection priority:
ADD UTRANNFREQ: LocalCellId=0, UtranDlArfcn=2825,
UtranFddTddType=UTRAN_FDD, UtranUlArfcnCfgInd=NOT_CFG,
CellReselPriorityCfgInd=CFG, CellReselPriority=3, ThreshXLow=6;
where UTRANDLARFCN indicates the UMTS downlink frequency and
CELLRESELPRIORITY indicates the UMTS cell reselection priority ranging from 0 to 7.
If the LTE signal quality is less than ThrshServLow and the UMTS signal quality is
greater than ThreshXLow, the UE reselects the UMTS.
 The following command adds the UMTS cell reselection information:
ADD CELLRESELUTRAN: LocalCellId=0, SpeedStateSfCfgInd=NOT_CFG;
After this command is configured, the SIB6 can be delivered.
Page 35

IRAT Parameter Configuration –
Redirection and Handover
 The following command adds the UMTS frequency:
ADD
UTRANNFREQ:LOCALCELLID=0,UTRANDLARFCN=2825,UTRANFDDTDDTYPE=UTRAN_FDD,UTRANULARFC
NCFGIND=NOT_CFG,CELLRESELPRIORITYCFGIND=CFG,CELLRESELPRIORITY=3;
where UTRANDLARFCN indicates the UMTS downlink frequency.
 The following command adds the UMTS external cell:
ADD
UTRANEXTERNALCELL:MCC="460",MNC="00",UTRANCELLID=18,UTRANDLARFCN=2825,UTRANULARF
CNCFGIND=NOT_CFG,RNCID=20,RACCFGIND=CFG,RAC=5,PSCRAMBCODE=18,LAC=3;
where RACCFGIND and RAC are mandatory for PS handovers.
 The following command adds the UMTS neighboring relationship:
ADD UTRANNCELL:LOCALCELLID=0,MCC="460",MNC="00",UTRANCELLID=18,BLINDHOPRIORITY=5;
where BLINDHOPRIORITY indicates the priority of the blind handover and blind redirection and has 32
values. The value has a positive correlation with the priority. Value 0 indicates that blind handover is
prohibited. A non-zero value must not be duplicate with the value of existing neighboring cell of the same type
in the local cell.
 The following command configures the handover and redirection switches:
ADD MOD ENODEBALGOSWITCH: HoModeSwitch=UtranPsHoSwitch-1&UtranRedirectSwitch-1;
If both the UtranPsHoSwitch and UtranRedirectSwitch are turned on, the former has a higher priority.
 The following commands configure the handover and redirection thresholds:
MOD
INTERRATHOCOMMGROUP:LOCALCELLID=0,INTERRATHOCOMMGROUPID=0,INTERRATHOA1THDRSRP=-
111,INTERRATHOA2THDRSRP=-115;
MOD INTERRATHOUTRANGROUP: LocalCellId=0, InterRatHoUtranGroupId=0,
InterRatHoUtranB1ThdRscp=-103;
where the INTERRATHOCOMMGROUPID and InterRatHoUtranGroupId of a QCI are queried by running
the LST CELLSTANDARDQCI. These two parameters are used to set the coverage-based inter-RAT A1, A2,
and UTRAN B1 thresholds.
Page 36

IRAT Parameter Configuration – CSFB
 For the command to add UMTS neighboring cells for CSFB, see the ADD
UTRANNFREQ, ADD UTRANEXTERNALCELL, and ADD UTRANNCELL commands
described in the previous page.
 The following command configures the CSFB switch:
MOD ENODEBALGOSWITCH: HoAlgoSwitch=UtranCsfbSwitch-1;
This command configures UtranCsfbSwitch for CSFB to UMTS.
CSFB can be PSHO-based or redirection-based, if UtranPsHoSwitch or
UtranRedirectSwitch is turned on, respectively. These two parameter are configured by the
ADD MOD ENODEBALGOSWITCH command, as described in the previous page.
 The following command configures the blind handover switch for CSFB:
MOD ENODEBALGOSWITCH: HoModeSwitch=BlindHoSwitch-1;
 The following command configures the RAT priority for blind CSFB:
MOD
CSFALLBACKBLINDHOCFG:CNOPERATORID=0,INTERRATHIGHESTPRI=UTRAN,INTERRAT
SECONDPRI=GERAN;
During a blind CSFB, the RAT of highest priority is selected, and then the highest-priority cell
of the highest-priority RAT is selected for blind handover or redirection.
 The following command configures the CSFB threshold:
MOD CSFALLBACKHO:LOCALCELLID=0,CSFBHOUTRANB1THDRSCP=-103;
This command configures the B1 threshold for CSFB.
Page 37

Page 38

Known Problem
For details, see eRAN2.1 FDD LTE Fault Diagnosis Guide –
Inter-RAT section 6.3. This section will be periodically
refreshed to contain the latest common problems.
Page 39

Page 40

Collecting Log
• Uu interface trace, S1 interface trace
• One-click log of the main control board
• GSM and UMTS user trace
• UE log (QXDM and Probe)
• Core network log
Page 41

Case Study – 1
• In a test, the eNodeB does not deliver the L2G redirect command. However,
the L2U redirect test is normal.
• Diagnosis
The UE capability is queried. The result shows that the UE does not support
GERAN. According to 3GPP TS 36.331, the eNodeB does not deliver the L2G
redirect command. Our recommendation is that the testing engineer should
obtain the UE that supports the GSM band to test the L2G redirect command
again.
Page 42

Case Study – 2
• During a test of the LTE PS service, after the CSFB is
initiated, the PS service does not fall back to the UMTS but is
interrupted.
• Diagnosis
The trace result shows that the LTE eNodeB processes the
redirection-based CSFB normally. After delivering the redirect
command, the eNodeB releases the UE. The UE initiates the RAU
procedure in the UMTS network, but the RAU procedure is rejected
with the cause value "MS identity can not be derived by the network".
The SGSN engineer explains that this problem is most likely caused
by the DNS configuration error. After the testing engineer modifies
the DNS configuration on the SGSN, the problem is solved.
Page 43

Case Study – 3
• In a test, the HTC UE is used to perform the L2U CSFB. The core network
delivers the CSFB indication but the eNodeB does not respond. The CSFB
fails.
• Diagnosis
The standard interface trace result shows that the eNodeB does not process
the CSFB indication received from the core network. The UE capability and
inter-RAT configuration on the eNodeB are queried and the results show no
problem. The one-click logs returned by the testing engineers show that on
December 20, an operation to delete the UTRAN neighboring cell was
performed and on December 23, an operation to add the UTRAN neighboring
cell was performed. The problem occurred on December 21, when the
UTRAN neighboring cell was missing. Therefore, the eNodeB does not
process the L2U CSFB indication.
Page 44

Case Study – 4
• In a test of coverage-based L2G blind redirection, the Samsung UE cannot perform the
redirection.
• Diagnosis
The UE capability and eNodeB configuration are queried and the result shows no
problem. The logs show no problem. The DTS database is queried and the result shows
that the problem is similar to the following problem described in the DTS database:
DTS2011110702473 [LTE_eRAN2.1_Inter-RAT redirection] The UE does not support
GSM and UMTS measurement. If the GERAN redirection switch is turned on and the
local cell ID is non-zero, the blind redirection to GERAN fails.
The eNodeB configuration is queried and the result shows that the local cell ID is 1. After
the local cell ID is changed to 0, the L2G redirection succeeds. This problem is solved in
SPC430.
Page 45

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