07-UMTS CS Call Drop Analysis-62_PPT

UMTS CS Call Drop Analysis

ZTE University

Content

n Definition of Call Drop n Reasons of Call Drop n Methods for Call Drop Analysisn Methods for Optimization of Call Dropn Case Analysis

UE Voluntarily Initiated Signaling Release

Definition of Drive Test Indicators

n The definition of drive test :l Call drop rate = Number of call drop times/Number of call setup success

timesn Number of call drop times+1 :

l 1. After the Alerting message is received, call drop is acknowledged if the Connect ACK message is not received but the System Information message is received.

l 2. After the Connect ACK message is received, if the System Information message is received but then “radioBearerReconfigurationComplete”n “physicalChannelReconfigurationComplete”n “transportChannelReconfigurationComplete”n “rrcConnectionRelease”n “rrcConnectionRelease_CCCH” messages are not received in the subsequent

five seconds, call drop is acknowledged.l 3. After the Connect ACK message is received, if the Disconnect

message is not received but the “rrcConnectionRelease” and “rrcConnectionRelease_CCCH “messages are received, call drop is acknowledged.

Definition of Drive Test Indicators

number of call setup success times+1

Definitions of Network Management Indicators

n Call drop defined by the network management statistics is as follows:RNC/Cell Radio call drop rate=(SUM( RNC/Cell data. Number of CS RABs that are requested to be released by the RAB release request. All call-drop cause-corresponding sub-measurement items)+ SUM(RNC/Cell data. Number of CS RABs that Iu connection requests to be released. All call-drop cause-corresponding sub-measurement items)+ SUM(RNC/Cell data. Number of PS RABs that are requested to be released by the RAB release request. All call-drop cause-corresponding sub-measurement items)+ SUM(RNC/Cell data. Number of PS RABs that Iu connection requests to be released. All call-drop cause-corresponding sub-measurement items) )

/ ( SUM(RNC/Cell data. Total number of released CS RABs. All sub-measurement items) + SUM(RNC/Cell data. Total number of released PS RABs. All sub-measurement items) )

Counters of Call Drop Reasons –Iu Release

Iu connection release request by UTRAN for CS domain in cell,UTRAN Generated Reason C301230323

Iu connection release request by UTRAN for CS domain in cell,Unspecified Failure C301230322

Iu connection release request by UTRAN for CS domain in cell,Release due to Overload Control C301230321

Iu connection release request by UTRAN for CS domain in cell,O&M Intervention C301230320

Iu connection release request by UTRAN for CS domain in cell, Failure in the Radio Interface Procedure C301230319

Iu connection release request by UTRAN for CS domain in cell, timer TRELOCoverall expiry C301230318

Iu connection release request by UTRAN for CS domain in cell,Radio Connection With UE Lost C301230317

Iu connection release request by UTRAN for CS domain in cell,Release due to UE generated signalling connection release C301230316

Iu connection release request by UTRAN for CS domain in cell,Repeated Integrity Checking Failure C301230315

IuRelease

ReasonCounterRelease Type

Counters of Call Drop Reasons –RAB Release

RAB release number request by UTRAN in cell for CS domain,Unspecified Failure C301230365

RAB release number request by UTRAN in cell for CS domain,Release due to Overload Control C301230364

RAB release number request by UTRAN in cell for CS domain,Iu UP Failure C301230363

RAB release number request by UTRAN in cell for CS domain,Release due to UTRAN in cell Generated Reason C301230362

RAB release number request by UTRAN in cell for CS domain,RAB pre-empted C301230361

RABrelease

ReasonCounterRelease Type

Content


Call Drop Reasons

n Poor Network Coveragen Miss configure Neighboring Celln Handover n Interference n SC Conflictionn Exceptions

Call Drop Reasons- Poor Network Coverage

n Generally, poor coverage refers to the RSCP is insufficient. At the network boundary, the RSCP may be insufficient but Ec/Io may be good because there are few cells. In this case, the network coverage is also regarded poor.

-8-90HSDPA

-10-95PS384K

-11-100CS64K

-13-105AMR12.2KRequirement of Ec/Io (dB)Requirement of RSCP (dBm)Service Type

Call Drop Reasons-Poor Network Coverage

n The decision whether it is problem of uplink or downlink poor coverage is based on the power of Dedicated Channel before call drop.

n UL Poor Coverage:l TX Power reaches the maximum;l UL BLER is poor or NodeB report “RL failure”

n DL Poor Coverage:l TX Power reaches the maximum;l DL BLER is poor

n Scanner:l If the RSCP and Ec/Io of the cell with the best coverage are poor,

you can infer that the coverage is poor.

Call Drop Due to Unconfigured Neighboring Cell n The unconfigured neighboring cells can be spotted by the

following methods:l 1. During the drive test, the UE obtains the neighbor list from the

base station, Scanner will make a record all the value of Ec/Io,If a certain main SC is not in the neighbor list and its strength exceeds the specified threshold for several seconds.

l 2. If call drop occurs, the SC of the UE residing cell is inconsistent with the SC before call drop. It may be caused by unconfigured cells. Check “Measurement Control” message.

l 3. Some UE may report the Detected Set message. If the detected set information before call drop includes relevant SC information, you can infer that unconfigured cells exist.

n The method for inter-frequency and inter-system neighboring cells is similar.

Call Drop Reasons--Handover

n The handover process is incompleteUE RNC

Measurement Report (1A event)

Active Set Update

Active Set Update Complete

Radio Environment becoming bad,

UE can not receive the “Active Set Update”

Call Drop Reasons--Handover

n Ping-pong Handover

UE RNC

Measurement Report (1B event, Delete cell A)

Measurement Report (1A event, Add cell A)

In a short time, UE send

different Report for delete or add

cell A

Active Set Update (Delete cellA)

Active Set Update (Add cell A)

Interference Call Drop - DownLink

n The downlink interference is mainly caused by the Pilot Pollution or Unconfigured Neighboring Cells.

n The typical feature of downlink interference call drop is that the RNC sends the Active Set Update message to the UE but the UE cannot receive it. As a result, RL Failure call drop occurs.

n Reasons for Pilot Pollution:l Cross-cell coverage of high BSl Base stations in ring layoutl Signal distortion caused by street effect or strong reflection

Interference Call Drop - UpLink

n You can check whether uplink interference exists according to the cell Average RTWP and Max RTWP of the OMC-R.

n The average RTWP of an idle cell and is around -105 dBm and for 50% uplink load is -102 dBm.

n If the Average RTWP in the idle state exceeds -100 dBm and the Max RTWP is around -90 dBm, you can infer that uplink interference exists.

Interference Call Drop-case

UL interference = -93(dBm)

Interference Call Drop-case

In this case, the signal of the UE cannot be received by the uplink. The value of UL interference in the SIB7 is -93 dBm. It indicates that the uplink RTWP is high. Then, you can infer that the call drop is caused by uplink interference. At the same time, you can verify the interference using the RTWP statistics of the OMC.

Call Drop Due to SC Confliction

n When analyzing such call drop, check Cell ID in the call drop signaling besides SCs because the neighbor relation is identified by CellID.

Call Drop Due to Exceptions

n call drop may be caused by equipment faults. In this case, you should check the alarm information and system logs.

n The transmission condition in some overseas countries is poor and the transmission is not stable. Thus, the call drop caused by transmission problems has profound impact.

Content

n Definition of Call Drop n Reasons of Call Drop n Methods for Call Drop Analysis n Methods for Optimization of Call Drop n Case Analysis

Common Analysis Methods for Call Drop

n The common analysis methods: l multi-dimension analysisl trend analysisl accident analysis l comparison analysisl ranking analysisl cause-and-effect analysis.

Multi-Dimension Analysis

n Multi-dimension analysis is carried out from different perspectives.

n For the call drop problem, not only the call drop itself, but also related factors such asl access, l handover, l traffic statistics,l time, l RTWP

n Next page is a table which contains some important parameter from the statistic of OMCR.

Average HSUPA users in cell

Max HSUPA users in cell

Average HSDPA users in cell

Max HSDPA users in cell

Average Cell Freq Tcp [dBm]

Max Cell Freq Tcp [dBm]

Average Cell Freq RTWP [dBm]

Max Cell Freq RTWP [dBm]

HSDPA RLC Throughput [Mbps]

PS Traffic [Kbyte]

CS Traffic [Erl]

Unspecified Failure

Iu UP Failure

Release due to UTRAN in cell Generated Reason

Abnormal RAB Released Number,by Cause

UTRAN Generated Reason

Unspecified Failure

Failure in the Radio Interface Procedure

TRELOCoverall expiry

Radio Connection With UE Lost

Repeated Integrity Checking Failure

Abnormal Iu Released Number,by Cause

Total Abnormal Release

Number of Successful CS RAB establishment

CS Call Drop Rate [%]

Cell Name

Cell ID

Trend Analysis

Accident Analysis

n Check the equipment alarm and system log of this period to find out hardware problems;

n Check the transmission of this period;

n Check whether the upgrade or cell blocking is performed during this period;

n Check whether there is an occasion with abrupt high traffic requirements such as a concert, game, or exhibition.

Comparison Analysis and Ranking Analysis

n Comparison analysisl Comparison analysis is carried out through comparing

various data sets from the same perspective and finds out the differences.

n Ranking analysisl Ranking analysis is carried out through classifying data

into top N and bottom N data from a large amount of data.

Ranking Analysis

0.099Abnormal release in CN341.24416Can not find UE3200Different reason of failure for several service cells 30

00There is not suitable frequency in service cell2900There is no idle channel in service cell2700Overload for power allocation of the frequency for the service cell2600The frequency is configured in the neighbouring cell list2589.1629966Timeout for handover of UE2400OVSF code is not engough2200Not enough resource for channel2100Basic channel switch, failed to setup channel board1400Basic channel switch, failed to allocate the resource1300Basic channel switch, cannot find appointed frequency129.243104Other errors1000Timeout for waiting for the handover of UE complete500Failure when build the service channel400Failure for decision of NodeB hard handover300Timeout for setup of service channel in handover200Failure when getting the decision of handover from database1Percent(%)Failure timesReason of failure for Handover Index

Cause-and-Effect Analysis

n For a certain effect, the cause-and-effect analysis is performed to locate the causes that may result in the effect and to determine the influence of the causes.

n For example, the call congestion of a cell may be caused by insufficient capacity of the hardware, of the downlink, or of the uplink.

Content


Methods for Optimization of Call Drop

n There are two main optimization methodsl Engineer optimization:

n Directional angle, downtilt, position of Antenna, type of Antenna, Transmit power of BS, position of BS, new BS.

l Radio parameter optimization:n Time to Trigger, CIO, threshold of enabling/disabling

Compression Mode, Maximum transmit power of DL RL, Threshold of Inter-frequency and Inter-system.

Soft Handover Event Detection and Reporting Time To Triggern Time To Trigger (TTT) refers to the interval between the

detection and reporting of events (1A, 1B, 1C, and 1D). The setting of TTT has an impact on the promptness of handovers.

n When adjusting handover parameters, pay attention to two factors: l 1. The signals from different cells must be definitely overlapped.

Then, you can adjust related radio parameters so that the time for the UE crossing the handover region is longer than the whole handover delay. Thus, the service continuity is guaranteed.

l 2. The size of the handover region determined by both the signaland the radio parameters cannot too large. If the size of the handover region is too large, the overhead used by the handover is high and the resource usage decreases.

Neighboring Cell Offset CIO

n A higher value of this parameter results in easier soft handovers, more UEs in the soft handover state, and more used resources.

n A lower value results in more difficult handovers.n The CIO has an impact on the non-best cell. In

detail, the CIO is effective for 1a events in neighboring cells and effective for 1b events in cells to be deleted.

Threshold of Enabling/Disabling Compression Mode n The compression mode is used in inter-frequency

and inter-system handovers. n The compression mode is enabled before the

handover. n Currently, the compression mode is enabled by

the 2D event and disabled by the 2F event. n The measurement can be RSCP or Ec/Io. By

default, the RSCP is currently used.

Maximum Downlink Transmit Power of Radio Link

n If call drop occurs frequently in a cell due to coverage problem, increase the maximum downlink transmit power of services.

n However, a user in the edge area may consume great transmit power, which affects other users and reduces the downlink capacity of the system.

n If users fail to access a cell due to heavy traffic, consider changing the value of this parameter to a smaller value.

Threshold of Inter-frequency and Inter-system handovern When the measured value of inter-frequency and

inter-system neighboring cell signal exceeds the specified threshold, handover is triggered.

n If you set this parameter to a small value, handover is triggered ahead of time. If you set this parameter to a great value, handover is delayed.

Timer and Counter Related to Call Drop

3s(1..8)sWaiting time after initiating requests to access other RATs, such as GSM

T309

1(1, 2, 4, 10, 20, 50, 100, 200, 400, 600, 800, 1000)

Maximum number of synchronization indicators that the UE receives continuously from L1 in T313 activated state

N315

30s(0,10, 30, 60, 180, 600, 1200, 1800)s

Time of cell update, existing in T315-related radio bearer when wireless connection fails

T315

4s(0, 2, 4, 6, 8, 12, 16, 20)s

Time of cell update, existing in T314-related radio bearer when wireless connection fails

T314

20(1, 2, 4, 10, 20, 50, 100, 200)

Maximum number of lost synchronization indicators that the UE receives continuously from the L1

N313

3s(0..15)sWaiting time after the DPCCH channel set up in CELL_DCH mode loses synchronization

T313

1(1, 2, 4, 10, 20, 50, 100, 200, 400, 600, 800, 1000)

N312 of connection mode, that is, the number of synchronization indicators that the UE should receive continuously from L1 before the special channel is set up successfully

N312 Connected

1s(1..15)sT312 of connection mode, that is, the time when the UE waits for L1 synchronization indicator when the special physical channel is set up

T312 Connected

Default Value

Value RangeDescription Name

Content


Neighboring Relation Adjustment

n Reasonl High call drop rate caused by improper neighbor list

configurationn Description

l The call drop rate of the TRI135W-1 cell corresponding to RNC1 in Libya is always around 3%, and no hardware alarm is generated. The cell coverage is mainly on the sea.


n After the neighbor relation is adjusted, the CS call drop rate of TRI135W-1 decreases from 3％ to 1.3%.

Unconfigured Neighboring Cell

9

74


n Main parameters:l Cells in the system

n Troubleshooting process:l The cell with SC 9 is in the detection set and cannot be

added to the active set when the quality of the serving cell with SC 74 is extremely poor. This is a typical unconfigured neighboring cell.

n Solution:l Adjust the neighbor relation: Add the cell with SC 9 to

the neighbor list of the cell with SC 74.n Result:

l In the same test, the cell with SC 9 is in the active set of the serving cell with SC 74. When the signal cell with SC 9 is strong enough, the UE hands over from the serving cell to the cell with SC 9.


9

74

9

74

Soft Handover Parameter Optimization

n Reason:l Low handover success rate because of improper soft

handover parameter configurationn Description:

l The success rate of the handover from sector 1 (SC 436) of the Shuqian Lu site to sector 2 (SC 434) of the Meihuacun hotel is low. This area is within the Shuqian Lu section.

n Main parameters:l Soft handover 1a/1b event handover threshold, trigger

time


n The signal quality of the Shuqian Lu section is poor and unstable because there are overpasses in this section, Because the comparative threshold decision algorithm is used, a cell with poor signal quality may be added to the active set if the 1a threshold is excessively high. If the RNC sends the ActiveSet Update Command message to instruct the UE to enter this cell, the soft handover may fail because the radio link cannot be set up due to poor and unstable signal quality of this cell.

434436

BS-1

BS-2


n Adjust the 1a/1b event handover threshold and trigger time of cell 436.

n Lower the 1a event handover threshold and shorten the trigger time to ensure that the cells with good signal quality can firstly enter the active set.

n Raise the 1b event handover threshold and extend the trigger time so that the cells are not deleted too early due to drastic signal deterioration.


n Resultl After the parameter optimization, cell 434 of the BS-1

(Meihuacun hotel site) can be added to the active set quickly and is not deleted too early.

l According to the drive test result from more than 100 times of handover tests, the success rate of the handover between the BS-2 (Shuqian Lu site) and the BS-1 (Meihuacun hotel site) increases greatly.

Data Configuration Optimization in 2G/3G Handover

n Reasonl Call drop because of incorrect data configuration

n Descriptionl When the 2G/3G handover tests are performed at the

boundary of the 3G network, the handover from the 3G network to the 2G network succeeds in the west to east direction, but the handover fails in the east to west direction.

n Main parametersl BSC ID, LAC, NCC, and BCCH of the 2G neighboring

cell


n Troubleshooting process:n Because the handover from 3G network to 2G network fails, you

should firstly check whether 2G neighboring cells are configured. If all 2G neighboring cells are configured, go to the next step.

n Record whether the 2G Sagem UE starts the compression mode when the signal quality of a 3G UE is lower than the threshold for starting the compression mode, and record the CI of the 2G cell where the compression mode is started.

n You can find that the UE starts the compression mode after the preceding step is performed. The signaling is as follows:


n However, after the UE starts the compression mode, the repositioning fails. The signaling is as follows:

n And the cause of the repositioning failure is as follows:

n According to the preceding signaling analysis, you can infer that the UE does not recognize the BSC of the 2G cell during the 3G-to-2G handover. In this case, the failure may be caused by unconfigured BSC ID or LAC.


n Solution:n Check the BSC and LAC of the target 2G cell on the CN.

You can find that the LAC is not configured. Then, reconfigure the LACs of all 2G neighboring cells on the CN.

n Result:n After the data is configured again, all 3G-to-2G handovers

succeed.n Suggestion for similar problems:n In the 3G-to-2G handover, the BSC ID, LAC, NCC, and

BCCH of 2G neighboring cells must be configured in the 2G neighboring cell database of the OMCR and on the CN.

Open your mind to Analysis this Case from next page

Call Drop Case-Handover


Delete Cell 51 and Cell 53


Report e1A to add cell 51 and Cell 53

into Active Set after 0.4 second


Report e1a to add cell 51,64,53,52 to

Active Set


Continue report e1a to add stronger cells to Active Set

But UE cannot receive the AcitiveUpdate message because

the poor signal of the cells in ActiveSet.


Call Drop Happened

07-UMTS CS Call Drop Analysis-62_PPT

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