013 wcdma access problems analysis

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OWJ200203 WCDMA Access Problems Analysis

ISSUE 1.0


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.



� Upon completion of this course, you

will be able to:

�Be familiar with the access

procedure in UMTS

�Do analysis on access problems

HUAWEI TECHNOLOGIES CO., LTD. Page 3All rights reserved

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

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

Chapter 3 Typical access failure casesChapter 3 Typical access failure cases



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

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




� 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 “RB

setup ” message

� UE does not transmit “Connect Acknowledge” message after

receiving “Alert or connect” message









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




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











� 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





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 poor

receiving 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





� 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.











� 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




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



The drive test data analysis flowThe drive test data analysis flow

RRC connectionRRC connection

1. CCCH : RRC Connection Request

5. Downlink Synchronisation

UENode BServing 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 RX description

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








The call statistic data analysis flow

� Check “RAB setup success rate” and “RRC setup success rate” in traffic statistics data

� Get familiar with the network operating situation, and then position the

specific area for the problems

� The RRC/RAB setup success rate are calculated by the following formulas:

%100

×=

ervicesfour QoS sts in all RRC requesNumber of

ervicesfour QoS sup in all RRCs setNumber of ccRateRRCSetupSu

%100

×=

ervicesfour QoS sts in all RAB requesNumber of

ervicesfour QoS sup in all RABs setNumber of ccRateRABSetupSu

%100××= ccRateRABSetupSuccRateRRCSetupSuuccRateCallSetupS



Analysis access

index about cell

Check system

alarmSolve problem of

equipment abnormal

”RRC setup

success rate” is too low

Solve problem of

RRC setup failure

Y

Y

N

Analysis access indices in

RNC call statistic data

N

Call Statistic Data Analysis Flow chartCall Statistic Data Analysis Flow chart




RAB setup

success rate” is

too low

RB setup

success rate” is

too low

paging

success rate”is too low

Solve problem of

RAB setup

failure

Solve problem of RB setup failure

Solve problem of

paging failure

over

Y

Y

Y

N

N

N

Y

Call Statistic Data Analysis Flow chartCall Statistic Data Analysis Flow chart



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





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.




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 the

interference, 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

� PRACH

�The maximum times of retransmission for preamble

�The power ramping step

�The power offset between preamble and message

Physical channels parametersPhysical channels parameters



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+2×Sintrasearch, 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







Typical Access Failure Cases

� Inappropriate Setting for the power of PCH and PICH

� UE can’t 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




� 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

happened

�Do 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




� 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 abnormal

UE 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 the open 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




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




� 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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013 wcdma access problems analysis

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