microsoft powerpoint - oeo000020 lte eran6.0 access fault diagnsis issue1

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Copyright © 2013 Huawei Technologies Co., Ltd. All rights reserved.

LTE eRAN6.0 Access Fault Diagnosis


Objectives

� Upon completion of this course, you will be able to:

� Get deep understanding of UE initial access flow

� Describe the typical access fault scenarios

� Describe how to locate the access fault

� Typical methods for fault analysis

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Contents

1. EUTRAN Typical Signaling Procedure

2. Typical Access Fault Analysis and Process

3. Case Study

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UE Initialization Flow

Invisible flow

Visible flow

Partial visible flow

UE idle

behavior phase

UE access phase

UE power on

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UE Initial Attach Procedure UE

eNodeBMME

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Service Request Procedure UE eNodeB MME

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TAU Update ProcedureUE eNodeB MME

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Random Access Procedure Overview

� Purpose of random access

� Get uplink synchronization

� Send schedule request if no PUCCH resource

� Scenarios:

� Case1: UE initial attach

� Case2: RRC reestablishment after RLF

� Case3: Handover in a new cell

� Case4: UE sends uplink data while RLF occurs

� Case5: UE notices that downlink data arrival while RLF occurs

� Case6: When UE trigger LCS(Location service)

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Classification of Random Access

� UE randomly selects a

preamble and sends it,

conflict might occurs

� Can be used for all cases

UEeNodeB UE

eNodeB

Contention based Non-contention based

� Before random access,

eNodeB assigns a dedicated

preamble to UE, so there is

no conflict.

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Preamble Index Configuration

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Preamble Format 0 ms 1 ms 2 ms 3 ms

CP0.1

Sequence 0.8

0.1 Format 0

CP0.68

Sequence 0.8

Guard Period0.52

Format 1

CP0.2

Sequence 1.6

0.2 Format 2

CP0.68

Sequence 1.6

Guard Period0.72 Format 3

5102 xTs

Format 4

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Preamble Sending

� In contention based case, UE randomly select a preamble based

on RACH configuration and send it on the corresponding time slot

� Once the frequency and time is determined, the corresponding

RA_RNTI is already known by the UE, where RA_RNTI= 1 +

t_id+10*f_id (t_id indicates the start subframe number, f_id

indicate the frequency offset of RACH)

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Random Access Response

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UL Data Sending (Msg 3)

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Contention Resolution

Case1: Detect C-RNTI on PDCCH

Case2: Detect temporary C-RNTI on PDCCH and decode the corresponding MAC PDU on PDSCH.In the MAC PDU, it contains the UE ID that send in Msg 3

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RRC Connection Setup Procedure

A

B

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RRC Cause

� According to different NAS procedure, the RRC cause can

be MO-singling, MO-data, MT-access, and emergency

NAS Procedure RRC Cause Type of call

Attach MO – signaling Original signaling

TAU MO – signaling Original signaling

Service RequestMO - data Original call

MT – access Terminated call

Extended Service Request

MO – data (original CSFB) Original call

MT – access (terminated CSFB) Terminated call

Emergency (emergency CSFB) Emergency call

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UEeNodeB

MME

Initial ERAB Setup

B

A

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S1 Initial UE Message

NAS PDU

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S1 Initial Context Setup Request

NAS PDU

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Security Mode Setup

� Purpose:

� Activated AS security algorithms, including integrity protect and

ciphering

� Integrity protect: only for signaling, support AES, snow3G and Null

� Ciphering: both for signaling and data, support AES, snow3G and

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UE Capability Query

� If no UE capability in MME, eNodeB will trigger UE

capability query and store it and also inform MME

� If MME already stored UE capability, it will forward to eNode

via S1 initial contexts setup request message

UE eNodeB MME

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SRB2 Setup

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Measurement Control

Measurement Configuration

- measObjectToRemoveList- measObjectToAddModList- reportConfigToRemoveList

- reportConfigToAddModList- measIdToRemoveList

- measIdToAddModList- quantityConfig- measGapConfig

- s-Measure

RRC Connection Reconfiguration Request

RRC Connection Reconfiguration Complete

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S1 Initial Context Setup Response

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NAS Procedure

UEeNodeB MME

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Contents



3. Case Study

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Access Performance Evaluation

� From performance management

� Access successful ratio = RRC setup successful ratio x ERAB

setup successful ratio

� Random access and NAS failures are not considered in

eNodeB performance management, but these factors also

impact user experience

� From drive test tools (probe)

� It can identify access failure caused by random access failure

and NAS failure

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General Process

� Determine the scale of problem

� From the performance statistic, we can determine the scale of the

problem, is it global problem, or cell level problem or just some individual

UE problem. Then we can select bottom N cell as optimization target

� Customer complain is an effective way to locate the individual UE

problem

� For large scale access failure

� Check eNodeB alarm and basic configuration

� Check RF channel problem

� Once RF channel is abnormal, such as high VSWR, low RSSI Too Low or RSSI

is unbalanced, then the access performance should be significantly affected.

� For individual UE problem, signaling analysis can be used to locate

the detailed cause

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Handle eNodeB Generic Alarm

� Tools: M2000 or LMT

� Procedure

� Check the generic hardware fault alarm, such as hardware

fault, board unusable , or any other alarms which are related

with hardware failure or link failure.

� Solution: Replace the faulty component.

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Check DL Power Configuration

� To ensure the coverage, the DL power parameters should be

properly set

� RS power = Maximum RRU power/channel – 10lg(total subcarriers ) +

10lg(Pb+1)

� If the single antenna, Pb=0

� If 2 or 4 antennas, Pb=1

� PDSCH power = PA + RS power

� If single antenna, PA=0

� If 2 or 4 antennas, PA=-3

� Related command:

� LST PDSCHCFG: check Pb and RS power setting

� LST CELLDLPCPDSCHPA: check PA setting

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Check CRPI Rate

� If the bandwidth is 10MHz or above, the CPRI rate should

be equal or more than 2.5Gbps, otherwise, the cell can’t be

setup

� If the rate is not correct, change the CPRI modulePage31

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Check S1 Interface Status

� If SCTP link is faulty, check the

SCTP configuration, such as peer

IP address, SCTP port number

� If S1intrface state is faulty, check

the PLMN and operator ID.

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Check Cell Barred Status

� If cell is barred, use the following command to recover it

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Check RF Channel Problem

� Due to the bad feeder quality, incorrect feeder connection or

strong interference, it might affect RF channel performance

which has impact on UE access.

� Relevant problem:

� High VSWR: ALM-26529 RF Unit VSWR Threshold Crossed

� Low RSSI: ALM-26521 RF Unit RX Channel RSSI Too Low

� RSSI unbalance: ALM-26522 RF Unit RX Channel RSSI

Unbalanced

� RSSI of the main RX channel and the RSSI of the diversity RX

channel exceeds 10 dB.

� High RSSI: Caused by interference

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Cause of High VSWR

� VSWR: Voltage Standard Wave Ratio, indicate the if feeder

impedance is match or not. The normal VSWR range is 1~1.5. If

current VSWR is more than a specified threshold, then eNodeB will

generate relevant alarm.

� Alarm generation:

� Cell is activated

� RF power is more than 34dBm/channel

� VSWR is more than a specified threshold

� Possible cause:

� Incorrect VSWR alarm threshold

� RF unit hardware fault

� RRU/RFU wrong connection

� Bad feeder quality or nonstandard feeder installation

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Solution for High VSWR

� Step 1: Check if the VSWR alarm threshold is correct (default is 2dB).

� Note: RRU TX channel will automatic closed if VSWR is extremely high (more than post-

processing threshold)

� Step 2: If alarm threshold is correct, then check if relevant feeder installation and RRU

connections are meet the standard.

� Step3: After the feeder tuning, if TX channel is closed, then activate TX channel again

� MOD TXBRANCH

Threshold for TX shutdown

Threshold VSWR alarm

generation

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Cause of Low RSSI

� Alarm generation: If RSSI is less than a specified value

� Incorrect RX attenuation setting

� If no TMA, the attenuation should be 0

� If 12dB TMA is used, the attenuation should be 4 to 11dB

� If 24dB TMA is used, the attenuation should be 11 to 22dB

� Feeder problem

� Due to bad feeder quality, causing additional loss

� RRU fault

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Solutions for Low RSSI

� Step 1: Check if RRU RX attenuation is correct

� Step2: If attenuation is correct, then check the feeder

installation and connection

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Cause of Imbalance of RSSI

� Alarm generation: RSSI of the main RX channel and the

RSSI of the diversity RX channel exceeds 10 dB.

� Possible causes

� High interference

� RRU cross connections

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Solution of Imbalance of RSSI

� UL interference check

� From web-LMT: Perform spectrum detection to evaluate UL interference

� From M2000 client: Perform interference detect monitoring

� Find out interference source

� Check RRU connections, avoid cross connections, show as below

A

N

T

1

RRU1

A

N

T

2

RRU2

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Classification of Interference

� Passive inter-module interference:

� Due to non-linearity of radio component ,the high phase

harmonic wave might be generated and inter-modulated by RX

signal, thus the new inter-module signal will arrive the receiver

causing the interference.

� External interference:

� Interference from the other system, such as 2G, 3G, CDMA,

radio, radar eg.

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How to Located Inter-modulated Interference

� This kind of interference increases with eNodeB transmit

power, so we can simulate the DL load, and check RSSI, if

it obviously increases with the DL load incensement, then

we can determine the interference

� Load simulation:

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How to Located External Interference

� If we exclude inter-modulated interference, then it should be

external interference

� Shutdown the TX of RRU/RFU, if RSSI is still high, then we

can conclude this is external interference

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Analysis of Tracing

� Driver test tracing analysis

� We can get all UU interface messages, internal UE message as well as

RSRP value

� eNdoeB tracing analysis

� We can get UU interface message and S1 interface message

� Form the above tracing analysis, we can locate the problem with the

following causes:

� Random access failure

� RRC setup failure

� NAS procedure failure

� ERAB setup failure

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Random Access Failure

� Generic scenario: There is no “RRC connect request ”message

� Possible cause:

� UE doesn’t support the band

� Scenario: UE doesn't receive any system message

� Solution: Check if UE band is identical with eNdoeB configuration

� Poor coverage

� Scenario :Due to poor DL or UL coverage, UE or eNodeB miss receiving the

relevant random access message

� Solution: Perform RF tuning. Use TMA on uplink

� Cell is barred

� Check the cell status, should be not barred.

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RRC Connection Failure

� Scenario 1: eNodeB directly send reject message

� Possible cause: No radio resource due to no enough resource

for PUCCH and SRS

� Solution: Check the cell load status, if it is very high, consider

capacity extension

UE eNodeB

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RRC Connection Failure (Cont.)

� Scenario 2: UE doesn’t receive “RRC connection setup” message,

and repeat sending request message until timer is out

� Possible cause:

� Poor DL coverage, check RSRP and RSRQ

� Solution:

� Perform RF tuning

UE eNodeB

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RRC Connection Failure (Cont.)

� Scenario 3: eNodeB send RRC setup message, but doesn’t get

complete message

� Possible cause

� If UE doesn’t reply RRC complete, it should be UE problem

� If UE already send RRC complete, then it should be poor UL

coverage

UE eNodeB

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NAS Procedure Failure

� Note: NAS failure is not counted in performance statistic

UEeNodeB MME

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ERAB Setup FailureUE

eNodeBMME

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ERAB Setup Failure Cause

� If any failure occurs during ERAB setup, we can directly get the

failure cause form performance statistic, the relevant counters are:

� L.E-RAB.FailEst.NoReply (No response from UE)

� L.E-RAB.FailEst.MME (EPC fault)

� L.E-RAB.FailEst.TNL (Transport fault)

� L.E-RAB.FailEst.RNL (Radio network fault)

� L.E-RAB.FailEst.NoRadioRes (No radio resource)

� L.E-RAB.FailEst.SecurModeFail (Security mode setup fault)

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Faulty Signaling of No UE Response

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Fault Signaling of Other Cause

� Scenario: eNodeB receives “initial context setup request” or

“E-RAB setup request” message, after that, if failures

happen, then eNB replies response message with failure

cause

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Analysis of EPC Fault

� Cause：� S1 message validity check failure, including

� Error exist in S1 message

� Duplicated ERAB ID

� Conflict with other procedures

� Solution:

� Contact EPC engineer to locate the problem

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Analysis of Transport Fault

� Possible causes:

� Transport admission failure

� S1 link failure during S1 message exchange

� Incorrect IP path configuration

� Wrong QoS configuration in EPC

� Solutions:

� Check transport admission algorithm, consider extend S1 bandwidth

� Check S1 interface

� Check IP path configuration

� Check relevant QoS configuration, AMBR should be more than 0, and for initial

ERAB setup, the QCI must be 6~9

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Analysis of Radio Network Fault

� Possible cause

� UU message process failure due to UE capability (UE doesn’t

support relevant service)

� eNodeB failure during ERAB setup

� Solutions:

� Check UE capability

� Check eNodeB status

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Analysis of No Radio Resource

� Possible cause

� Radio admission failure for ERAB

� Solution:

� Check if admission parameters is correct

� Consider to enable load balance algorithm

� Consider capacity extension

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Analysis of Security Mode Failure

� Possible cause:

� UE security capability is not identical with eNodeB security

capability

� Solution:

� Modify the eNB security algorithm to match it.

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Contents



3. Case Study

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Case 1 – IP Path Configuration Leads to Low Access Ratio

� Description: In one live network, ERAB setup ratio is very

low, but it’s not zero.

� Alarm info: None

� Analysis:

� Check the performance statistic of ERAB release, we found

that most of failures are caused by transport resource not

available

� From the message tracing, we can also confirm this result

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Case 1 –IP Path Configuration Leads to Low Access Ratio (Cont.)

� Now we can locate that the problem is due to IP path

configuration. We check eNodeB configuration and find a

valid IP path, the connection is valid. And it is also weird

that this problem occurs with a probability

� Then we analyze the message again, and find that in the

“UE context setup request” message delivers variable

GTPU address, and these address is not identical with

eNodeB configuration.

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Case 1 –IP Path Configuration Leads to Low Access Ratio (Cont.)

� Conclusion :As the SGW deliveries multiple GTUP address

to eNodeB, but we only configure one IP path to SGW, so it

causes the partly ERAB setup failure

� Solutions: We confirm this with EPC engineer, SGW pool

function is used, so there are multiple IP addresses for

SGW. Then we make up the IP path for all address, the

problem is cleared.

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Case 2 – Multi-mode UE Attach Failure

� Description: During the commissioning of one live network, we found that UE

attach fails.

� Alarm info: None

� Message Tracing: From the message tracing, we can see that most of access

procedures are normal, after ERAB setup, MME release the connection with

the cause “normal release”

UU trace

S1 trace

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Case 2 – Multi-mode UE Attach Failure (Cont.)

� Analysis

� Since the most of procedures are normal, and eNodeB consider

that it’s a normal release, so we locate that the problem is due to

NAS failure.

� So we continue to analyze the previous NAS message, and in the

DL NAS message, we can see that MME response the attach with

the cause “MSC-temporatily-not-reachable”, as show above. now

we can confirm that this is root cause of attach failure

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� Analysis (Cont.)

� Why should UE need attach to MSC ? We check the UE model,

it ‘s Huawei E398, with multi-modes(GSM/UMTS/LTE). So we

assume that this UE performs combined attach. From the

attach request message we verify our assumption.

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� Conclusion:

� We check that there is no CS domain configuration in current EPS network, so

MME only replies PS attach accept, and also inform that MSC can’t reachable

� Temporary solution:

� Change UE attach mode with PS only

� Add CS domain configuration in EPC

� Final solution:

� Update MME to compatible with combined attach even there is no CS domain

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Copyright © 2013 Huawei Technologies Co., Ltd. All rights reserved. Page67

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