4 UMTS CS Call Drop Analysis

UMTS Voice Drop Analysis

ZTE University

Content

Definition of Call DropReasons of Call DropAnalysis of Call DropOptimization of Call DropCase of Call Drop

UE Voluntarily Initiated Signaling Release

Definition of Drive Test Indicators The definition of drive test :

Call drop rate= Number of call drop times/Number of call setup success timesNumber of call drop times+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.After the Connect ACK message is received, if the System Information message is received but the.

“radioBearerReconfigurationComplete”“physicalChannelReconfigurationComplete”“transportChannelReconfigurationComplete”“rrcConnectionRelease”“rrcConnectionRelease_CCCH” messages are not received in the subsequent five seconds, call drop is acknowledged.

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

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

Release Type Counter Reason

IuRelease

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

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

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

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

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

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

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

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

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

Counters of Call Drop Reasons

Release Type Counter Reason

RABrelease

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

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

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

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

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

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Call Drop Reasons

Poor Network CoverageUnconfigured Neighboring CellHandover Interference SC ConflictionExceptions

Poor Network Coverage

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

CS64K -100 -11

PS384K -95 -10

HSDPA -90 -8

Poor Network Coverage

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

TX Power reaches the maximum;UL BLER is poor or NodeB report “RL failure”

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

Scanner:If the RSCP and Ec/Io of the cell with the best coverage are poor, you can infer that the coverage is poor.

Unconfigured Neighboring Cell

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 thespecified threshold for several seconds.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.Some UE may report the Detected Set message. If the detected setinformation before call drop includes relevant SC information, you can infer that unconfigured cells exist.

Handover

The handover process is incomplete

UE RNC

Measurement Report (1A event)

Active Set Update

Active Set Update Complete

Radio Environment becoming bad,

UE can not receive the “Active Set Update”

Handover

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)

Downlink Interference

Reasons for Pilot Pollution:Cross-cell coverage of high Node-BNode-B in ring layoutSignal distortion caused by street effect or strong reflection

Uplink Interference

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

Case-Interference

UL interference = -93(dBm)

Case-Interference

SC Confliction

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

Content


Common Analysis Methods for Call Drop

multi-dimension analysistrend analysisaccident analysis comparison analysisranking analysiscause-and-effect analysis

Multi-Dimension Analysis

Multi-dimension analysis is carried out from different perspectives.For the call drop problem, not only the call drop itself, but also related factors such as

accesshandovertraffic statisticstimeRTWP

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

Cell ID

Cell Name

CS Call Drop Rate [%]

Number of Successful CS RAB establishment

Total Abnormal Release

Abnormal Iu Released Number,by Cause

Repeated Integrity Checking Failure

Radio Connection With UE Lost

TRELOCoverall expiry

Failure in the Radio Interface Procedure

Unspecified Failure

UTRAN Generated Reason

Abnormal RAB Released Number,by Cause

Release due to UTRAN in cell Generated Reason

Iu UP Failure

Unspecified Failure

CS Traffic [Erl]

PS Traffic [Kbyte]

HSDPA RLC Throughput [Mbps]

Max Cell Freq RTWP [dBm]

Average Cell Freq RTWP [dBm]

Max Cell Freq Tcp [dBm]

Average Cell Freq Tcp [dBm]

Max HSDPA users in cell

Average HSDPA users in cell

Max HSUPA users in cell

Average HSUPA users in cell

Trend Analysis

Accident Analysis

Check the equipment alarm and system log of this period to find out hardware problems;Check the transmission of this period;Check whether the upgrade or cell blocking is performed during this period;Check whether there is an occasion with abrupt high traffic requirements such as a concert, game, or exhibition.

Comparison Analysis and Ranking Analysis

Comparison analysisComparison analysis is carried out through comparing various data sets from the same perspective and finds out the differences.

Ranking analysisRanking analysis is carried out through classifying data into top N and bottom N data from a large amount of data.

Ranking AnalysisIndex Reason of failure for Handover Failure times Percent(%)1 Failure when getting the decision of handover from database 0 02 Timeout for setup of service channel in handover 0 03 Failure for decision of NodeB hard handover 0 04 Failure when build the service channel 0 05 Timeout for waiting for the handover of UE complete 0 010 Other errors 3104 9.2412 Basic channel switch, cannot find appointed frequency 0 013 Basic channel switch, failed to allocate the resource 0 014 Basic channel switch, failed to setup channel board 0 021 Not enough resource for channel 0 022 OVSF code is not engough 0 024 Timeout for handover of UE 29966 89.1625 The frequency is configured in the neighbouring cell list 0 026 Overload for power allocation of the frequency for the service cell 0 027 There is no idle channel in service cell 0 029 There is not suitable frequency in service cell 0 0

30 Different reason of failure for several service cells 0 032 Can not find UE 416 1.2434 Abnormal release in CN 9 0.09

Cause-and-Effect Analysis

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

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Optimization Methods for Call Drop

Engineer optimization:Directional angle, downtilt, position of Antenna, type of Antenna, Transmit power of BS, position of BS, new BS.

Radio parameter optimization: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 Trigger

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.

Neighboring Cell Offset CIO

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

The compression mode is used in inter-frequency and inter-system handovers. The compression mode is enabled before the handover. Currently, the compression mode is enabled by the 2D event and disabled by the 2F event. The measurement can be RSCP or Ec/Io. By default, the RSCP is currently used.

Maximum Downlink Transmit Power of Radio Link

If call drop occurs frequently in a cell due to coverage problem, increase the maximum downlink transmit power of services. However, a user in the edge area may consume great transmit power, which affects other users and reduces the downlink capacity of the system.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 handover

When the measured value of inter-frequency and inter-system neighboring cell signal exceeds the specified threshold, handover is triggered. 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 Name Description Value Range Default

Value

T312 Connected

T312 of connection mode, that is, the time when the UE waits for L1 synchronization indicator when the special physical channel is set up

(1..15)s 1s

N312 Connected

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

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

1

T313 Waiting time after the DPCCH channel set up in CELL_DCH mode loses synchronization (0..15)s 3s

N313 Maximum number of lost synchronization indicators that the UE receives continuously from the L1

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

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

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

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

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

N315Maximum number of synchronization indicators that the UE receives continuously from L1 in T313 activated state

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

1

T309 Waiting time after initiating requests to access other RATs, such as GSM (1..8)s 3s

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Neighboring Relation Adjustment

ReasonHigh call drop rate caused by improper neighbor list configuration

DescriptionThe 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.


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


9

74


Main parameters:Cells in the system

Troubleshooting process: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.

Solution:Adjust the neighbor relation: Add the cell with SC 9 to the neighbor list of the cell with SC 74.

Result:In the same test, the cell with SC 9 is in the active set of theserving 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 withSC 9.


9

74

9

74

Soft Handover Parameter Optimization

Reason:Low handover success rate because of improper soft handover parameter configuration

Description: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.

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


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.

434436BS-1

BS-2


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


ResultAfter 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. 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

ReasonCall drop because of incorrect data configuration

DescriptionWhen 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 handoverfails in the east to west direction.

Main parametersBSC ID, LAC, NCC, and BCCH of the 2G neighboring cell

3G

2G

fail


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


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

And the cause of the repositioning failure is as follows:

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.


Solution: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.

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

Suggestion for similar problems: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.

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

4 UMTS CS Call Drop Analysis

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