WCDMA Access Problems Analysi

8/10/2019 WCDMA Access Problems Analysi

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HUAWEI Mexico Training Center 1

HUAWEI TECHNOLOGIES CO., LTD. All rights reserved

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OWJ200203 WCDMAAccess Problems Analysis

ISSUE 1.0

HUAWEI TECHNOLOGIES CO., LTD. Page 1All rights reserved

Access is associated with the call setup

success rate of the network. This KPI

can be optimized by means of Call

setup test and the consequent trouble-

shooting based on access failure

analysis.


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Upon completion of this course, you

will be able to:

Be familiar with the access

procedure in UMTS

Do analysis on access problems


Chapter 1 Analysis Flow of Access FailureChapter 1 Analysis Flow of Access Failure

Chapter 2 The Adjust Methods for Access FailureChapter 2 The Adjust Methods for Access Failure

Chapter 3 Typical access failure casesChapter 3 Typical access failure cases


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Section 1 The definitions about Access FailureSection 1 The definitions about Access Failure

Section 2 The drive test data analysis flowSection 2 The drive test data analysis flow

Section 3 The call statistic data analysis flowSection 3 The call statistic data analysis flow


The definitions of access failure

It will be considered as access failure if any following conditions

satisfied after RRC connection request initialed by UE

Receive RRC Connection Reject message

UE transmits or receives RRC Connection Release

message after receiving RRC Connection setup

UE receives any BCCH message in the call setup process

Timer expired, which means UE does not receive RRC

Connection setup in 3 seconds after RRC connection

request is transmitted.

Access failure definitionsAccess failure definitions


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Random access procedure failure UE does not

transmit RRC Connection Request message after dialing

RRC Connection Setup message is not received by UE

UE does not receive RRC connection setup message

after RRC connection request message is transmitted.

RRC Connection Setup Complete message is not transmitted

by UE

UE receives RRC Connection Reject message

UE does not receive measurement control message after

transmitting RRC Connection Setup Complete message

UE does not transmit CM service request message after

measurement control message is received.




UE receives Service Request Reject message

UE does not receive Call Proceeding message after

transmitting CM service request message

UE does not receive RB Setup message after receiving Call

proceeding message

UE does not transmit RB setup complete message after

receiving RB setup message

UE does not receive Alert or Connect after transmitting RBsetup message

UE does not transmit Connect Acknowledge message after

receiving Alert or connect message



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The drive test data analysis flow

SubSub--flows about DT data analysisflows about DT data analysis

Data analysis flow

Paging problems analysis flow

RRC connection request problems analysis flow


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N

Y

Received

pagingN

Y

Y

N

N

RRC

connection

setup failure?

Call Fail?

Paging

problems

RRC setup problems

Analysis drive test data

Calling failure? N

Y

Data analysis flowData analysis flow


N

N

N

Y

Y

Y

N

Is ciphering or

authentication

failure?

RAB setup

failure?

Due to

handover

Ciphering or

authentication

problems

RAB or RB

setup problems

Refer to call drop

due to handover

Abnormal problems

Data analysis flowData analysis flow



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Data analysis flow





Paging problems

After finishing RAB assignment and call setup process (NAS), UE

received Disconnect message from CN instead of Alerting

message

The causes of paging problem have following types:

RNC does not transmit paging message

Too lower power allocated for PRACH or PICH

Cell reselection problems

Paging problems analysis flowPaging problems analysis flow


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Y

N

N

N

Y

Paging problem

RNC

transmit

page?

Power too

low ?

Cell

reselection

problem ?

Y

Y

N

Equipment abnormal

problem

Adjust power

according to

coverage

Optimize cell

reselect parameters

Abnormal

problem

UE

receive

page



RNC does not transmit paging message

RNC has received the paging message but does not transmit paging

message through Uu interface. The reasons could be:

Capacity of paging channel is not enough

Abnormal problem could happen in equipment.

Power allocation PRACH or PICH are too low

UE does not receive paging message if RNC has transmitted.

The interference is too high comparatively, which result in the poorreceiving signal quality of PRACH or PICH if Ec/Io of PCPICH. In

this case, higher power can be allocated to these two channels

It could also be the bad coverage if Ec/Io of PCPICH is too poor



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Cell reselection

UE can not reselect to better cell in time

The cell reselection from 3G to 2G can also be the reason

by the reselection between two LAs.





Data analysis flow




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RRC connection setup process includes following steps:

UE transmits RRC Connection Request by RACH

RNC transmits RRC Connection Setup by FACH

UE transmits RRC Connection Setup CMP by DCH

The reasons for RRC setup failure could be :

Uplink RACH problem

Power allocation for Downlink FACH

Cell reselection parameters

Low initial power of downlink DPCH

Uplink power control

Congestion problem

Equipment abnormal problem

RRC connection problems analysis flowRRC connection problems analysis flow




RRC setup problem

Does UE transmit RRC

connection request?

Does RNC receive

RRC connection

request?

Does RNC transmit

RRC connection

setup?

UE abnormal

problem

adjust PRACH parameters

Other problem

N

Y

N

Y

N

Y


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Does UE transmit

RRC connection

setup complete


Does UE receive

RRC connection

setup

Does RNC receive

RRC connection

setup complete

Adjust FACH power

Adjust downlink initial

transmit power parameter

Adjust uplink DPCCH open

loop power control parameter

Does cell

reselection

happen?

Optimize cell

reselection parameters

Y

N

Y

Y

N

N

Y

N

Y



The possible reasons for RRC connection establishment failure:

RACH

Usually, it could be the problem on RACH if the RNC does not receive

RRC Connection Request message. Meanwhile, the Ec/Io of CPICH is

normal

The cell reselection parameters

The improper parameters setting could result in that UE can not stay in

the best cell to initiate the access procedure

Lower power allocation for initial transmission power of downlink DPCH

The insufficient power of DPCH can cause the out of synchronization. Uplink initial transmission power

The initial transmission power of PRACH

The initial transmission power of uplink DPCH


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The drive test data analysis flowThe drive test data analysis flow

RRC connectionRRC connection

1. CCCH: RRC Connection Request

5. Downlink Synchronisation

UENode B

Serving RNS

Serving

RNC

DCH - FP

Allocate RNTISelect L1 and L2parameters

RRCRRC

NBAPNBAP

3. Radio Link Setup Response

NBAPNBAP2. Radio Link Setup Request

RRCRRC7. CCCH: RRC Connection Set up

Start RXdescription

Start TX

description

4. ALCAP Iub Data Transport Bearer Setup

RRCRRC9. DCCH: RRC Connection Setup Complete

6. Uplink Synchronisation

NBAPNBAP8. Radio Link Restore Indication

DCH - FP

DCH - FP

DCH - FP







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The call statistic data analysis flow

Check RAB setup success rate and RRC setup success rate intraffic statistics data

Get familiar with the network operating situation, and then position thespecific area for the problems

The RRC/RAB setup success rate are calculated by the followingformulas:

%100=ervicesfour QoS sts in allRRC requesNumber of

ervicesfour QoS sup in allRRCs setNumber ofccRateRRCSetupSu

%100=ervicesfour QoS sts in allRAB requesNumber of

ervicesfour QoS sup in allRABs setNumber ofccRateRABSetupSu

%100=

ccRateRABSetupSuccRateRRCSetupSuuccRateCallSetupS



Analysis accessindex about cell

Check systemalarm

Solve problem ofequipment abnormal

RRC setupsuccess rate istoo low

Solve problem of

RRC setup failure

Y

Y

N

Analysis access indices inRNC call statistic data

N

Call Statistic Data Analysis Flow chartCall Statistic Data Analysis Flow chart


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RAB setup

success rate istoo low

RB setupsuccess rate istoo low

pagingsuccess rateis too low

Solve problem of

RAB setupfailure

Solve problem ofRB setup failure

Solve problem ofpaging failure

over

Y

Y

Y

N

N

N

Y

Call Statistic Data Analysis Flow chartCall Statistic Data Analysis Flow chart






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Section 1 project parametersSection 1 project parameters

Section 2 cell parametersSection 2 cell parameters


Project parameters

Project parameters main include antenna azimuth, antenna

tilt, antenna gain, antenna direction,etc.

In general, project parameters can be adjusted only in poor

coverage condition.

The influence on the original coverage quality has to be

considered when adjusting the project parameters.


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Section 1 project parametersSection 1 project parameters

Section 2 cell parametersSection 2 cell parameters


Cell parameters

Transmission power of FACH, PCH and PICH

In the edge of cell, UE can not receive the signaling or

service correctly if the transmission power is too low. But on

the contrary, higher transmission power can increase theinterference, which can result in the decline of adjacent

cells capacity

Transmission power of AICH

In the edge of cell, UE can not receive the AI if the power is

too low

PRACHThe maximum times of retransmission for preamble

The power ramping step

The power offset between preamble and message

Physical channels parametersPhysical channels parameters


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Cell parameters

Qhyst:

This parameter is use to avoid ping-pang cell reselection in

the cell edge. Ping-pang cell reselection will cause signaling

load increase and waste power of UE because of frequent

cell reselection, LA update or URA update.

The parameter value is related to local slowing fade

characteristics.

Treselections

The duration time that target cell signal quality is better than

current camp cell is longer than Treselections. The cell will

be selected to the camping cell.

The parameter is use to avoid ping-pang cell reselection

Cell reselection parametersCell reselection parameters


Cell parameters

Sintrasearch

Sintrasearch is the threshold for intra-frequency cells

measurement. When the RSCP of current cell is less than

Qrxlevmin+2Sintrasearch, the measurement will be initiated.

It will influence on the rapidity of cell reselection, and then the

access success rate.

This value could be set as great as possible if the UE battery is not

a problem

Cell reselection parametersCell reselection parameters


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Typical Access Failure Cases

Inappropriate Setting for the power of PCH and PICH

UE cant receive paging message correctly when PICH and PCH

power are too low. The power could be increased with the poor

network coverage

The paging message is being transmitted while UE is doing LA update

It mainly happens the procedure of inter-system cell reselection.

UE leaves the network without detach procedure

CN does not receive the LA update from UE periodically

UE is out of service area for a long time

Inappropriate operation, such as unplug battery of UE directly, or

SIM card directly

Paging problemsPaging problems


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UE transmitted RRC Connection Request message twice in different cells

And the interval between them is 1.2s

Cell reselection problemCell reselection problem



Reasons

UE did not initiate the reselection in time

Solution methods

The Tselections could be minimized, but the default value is

already the minimum.

Minimize the Qhyst2 value, but the reselection is still not in

time. On the contrary, the ping-pong cell reselection

happenedDo not modify Qhyst2, but increase the Sintrasearch value.

Since the measurement is initiated earlier, the reselection

can happed earlier too.

Cell reselection problemCell reselection problem


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The calling success rate and called success rate are greater than 97% from KPI,

but RRC establishment success rate is about 94% from traffic statistic

After analysis, the RRC Connection Request retransmission is the result of

following reasons:

Cell phone abnormalUE dose not transmit RRC Connection Request after

receiving paging

Other reasons

After cell reselection, UE retransmits the RRC

Connection Request in new cell

After UE transmits RRC Connection Request

message, the cell quality decreased sharply, and UE

has to do cell reselection immediately

UE can not receive RRC Connection Setup from RNC in

downlinkThe area with poor downlink coverage

RNC can not receive the RRC Connection Request

message from UE in uplinkThe area with good downlink coverage

RRC connection problemsRRC connection problems


RRC connection problemsRRC connection problems


Solution

Adjust the maximum preamble retransmission times from 8 to 20

Increase the transmission power of PRACH and FACH

Experimental conclusion

The error of initial preamble transmission power, which is calculated by theopen loop power control, could be +/-9dB, or even +/-12dB

The preamble power climbing step is 2dB, and the error could be +/-1.5dB

If the outside interference is higher 10dB, enough margin should be given

for the preamble transmission power

Increase the FACH power from the default value -1dB to +1dB

Increase the access success rate in the condition that Ec/Io is -14dB


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RAB or RB establishment problemsRAB or RB establishment problems

Establishment request is refused

The register information in HLR is not identical with the service

request

The request is beyond the ability of UE

If the service request is beyond the ability of UE, SGSN will

renegotiate QoS to initiate a new RAB assignment again till the

ability of UE can satisfy the QoS. Usually, the maximum data rate is

same as the maximum ability of UE

If the maximum ability of UE still can not support the QoS, the RAB

establishment is failed.



Handover can also happen

in the process from RRC

Connection complete to

RAB assignment complete

or RB establishment

complete

The following figure is the

RNC signaling trace for

single UE

Handover problemsHandover problems


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Cell signaling strength record

From the record, the strength of cell 121 decreased sharply.

Meanwhile, RNC transmitted ActiveSet Update. But UE can not

receive it To solve this problem, the handover parameters should be adjusted to

make it happen earlier

Handover problemsHandover problems

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WCDMA Access Problems Analysi

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