umts call drop

UMTS Call Drop Analysis

ZTE University

Content

Definition of Call Drop Reasons of Call Drop Analysis of Call Drop Parameters of Call Drop Case of Call Drop

Type of Call Drop Definition

Call termination not through the normal hang up process, that is dropped calls.

Tow types of call drop definition: Definition of drive test indicators Definition of network management indicators

Definition of Drive Test Indicators

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

success times Number of call setup success times+1:After the Alerting message

is received


Number of call drop times+1:(Air interface signaling at the UE side)

The Connect ACK message is not received but the System Information message is received.

After the Connect ACK message is received, RRC Release message received and the reasons is Not Normal.

After the Connect ACK message is received, any of the CC Disconnect, CC Release, CC Release Complete message received and the reasons is Not Normal.


number of call setup success times+1

UE Voluntarily Initiated Signaling Release

Definitions of Network Management Indicators

The definition of the network management:

CSRabrelTriggedByRNC+1:(Iu interface signaling at the RNC side)

When the RNC send the Iu Release Request or RAB Release Request message to the CN.

%*SuccessCSRABSetup

iggedByRNCCSRabrelTrCDRCS 100_

Definitions of Network Management IndicatorsCN

RABRELEASE REQUEST

RNC

CNRNC

IU RELEASE REQUEST

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

C301230361RAB release number request by UTRAN in cell for CS domain,RAB pre-empted

C301230362RAB release number request by UTRAN in cell for CS domain,Release due to UTRAN in cell Generated Reason

C301230363RAB release number request by UTRAN in cell for CS domain,Iu UP Failure

C301230364RAB release number request by UTRAN in cell for CS domain,Release due to Overload Control

C301230365RAB release number request by UTRAN in cell for CS domain,Unspecified Failure

Content


Call Drop Reasons

Poor Coverage Unconfigured Neighbor Cell Handover Interference PSC Confliction Engineering Error

Poor Coverage

RSCP and Ec/Io threshold for different services

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 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 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

Missed neighbor cell. Removal of key neighbor cells caused by combination of

macro diversity. Untimely update of the external cell information.

Handover

The handover process is incomplete Radio Environment becoming bad, UE can not receive the “Active

Set Update”

UE RNC

Measurement Report (1A event)

Active Set Update

Active Set Update Complete

Handover

Ping-pong Handover In a short time, UE send different Report for delete or add cell A

UE RNC

Measurement Report (1B event, Delete cell A)

Measurement Report (1A event, Add cell A)

Active Set Update (Delete cellA)

Active Set Update (Add cell A)

Interference

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

Interference

Judgment of DL interference: CPICH RSCP of the active set is large than -85dBm Ec/Io is lower than -13dB

Reason of DL interference: Pilot pollution Unconfigured neighboring cell

Interference

Judgment of UL interference: The average RTWP of an idle cell exceeds -100dBm The max RTWP is around -90dBm

Reason of UL interference: Intra-RAT interferences Inter-RAT interferences.

Case-Interference

UL interference = -93(dBm)

Case-Interference

PSC Confliction

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

PSC Confliction

Cell A and Cell B are configured as neighbor cell for each other. Cell C and Cell B are not configured as neighbor cells for each other. Cell A and Cell C have the same PSC.

PSC Confliction

Cell A is the neighbor cell of Cell B. Cell D is the neighbor cell of Cell C. Cell A and Cell D have the same PSC.

PSC Confliction

Cell B and Cell D are not configured as neighbor cell for each other. Cell A is the neighbor cell of Cell B. Cell E is the neighbor cell of Cell D. Cell A and Cell E have the same PSC.

Engineering Error

Call drops caused by engineering error: Reversely-connected antenna An excessive VSWR Multi-band antenna problem Leakage of signals from indoor distribution system Call drop caused by unsteady transmission

Case-Engineering Error

Content


Common Analysis Methods for Call Drop

Analyzing Call Drops by DT Analyzing Call Drops by Traffic Statistics

Analyzing Call Drops by DT

Call drop data Call drop spots Stability of the primary serving cell RSCP and Ec/Io of the primary

serving cell Reproducing of problems with DT

Analyzing Call Drops by Traffic Statistics

The commonly used KPI analysis method is the TOP cell method, which means the top cells will be screened out according to certain index, then these top cells are optimized and then the top cells are selected again. After several repetitions, the related KPI can be speedily converged.

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

access handover traffic statistics time RTWP

Multi-Dimension AnalysisCell IDCell NameCS Call Drop Rate [%]Number of Successful CS RAB establishmentTotal Abnormal Release

Abnormal Iu Released Number, by Cause

Repeated Integrity Checking FailureRadio Connection With UE LostTRELOCoverall expiryFailure in the Radio Interface ProcedureUnspecified FailureUTRAN Generated Reason

Abnormal RAB Released Number, by Cause

Release due to UTRAN in cell Generated ReasonIu UP FailureUnspecified 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 cellAverage HSDPA users in cellMax HSUPA users in cellAverage 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.

Ranking Analysis

Ranking analysis Ranking analysis is carried out through classifying data into top N and bottom N

data from a large amount of data.Index 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 0 030 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.

Content


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.

Time To TriggerAbbreviated Name TrigTime[MAX_INTRA_MEAS_EVENT]

Description This parameter indicates the time difference between having detected the event generation and reporting the event. Only when the event generation is detected and still meets all requirements of event triggering after Time to trigger, the event can be triggered and reported. The larger the value is, the stricter the judgment is for the event to be triggered. The parameter should be set according to the actual requirements. Sometimes, if it is set too large, the quality of calls may decrease.

Range and Step (0, 10, 20, 40, 60, 80, 100, 120, 160, 200, 240,320, 640, 1280, 2560, 5000)ms

Default Value UE Event Report Parameters for CPICH Ec/No:[200,640,320,320,200,200, 320]msUE Event Report Parameters for CPICH RSCP:[200,200,200,200,200,200,200]msDetected Set Measurement Parameters for CPICH Ec/No: [200]msDetected Set Measurement Parameters for CPICH RSCP: [200]ms

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.

Cell Individual Offset

Abbreviated Name CellIndivOffset

Description An offset is allocated for each cell being monitored. The offset can be positive or negative. Before the UE judges whether an event occurs, adds the offset to the measurement result. If the PCPICH uses a positive offset, the UE sends the measurement report just like PCPICH is x dB better than the actual case. Or, if the PCPICH uses a negative offset, the PCPICH report is limited. When the cell individual offset is used, the corresponding cell is possibly (at least temporarily) the target cell of the handover or removed from the active set.

Range and Step OMCR: [-10, 10] dB, step 0.5RNC: D=(P+10)*2, [0, 40]

Default Value 0dB

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 ModeAbbreviated Name ThreshUsedFreq[MAX_INTER_MEAS_EVENT]

Description This parameter indicates the absolute threshold that is required to be configured for event 2b/2d/2f (used when judging the quality of the currently used frequency.

Range and Step CPICH RSCP: [-115, -25] dBm, step 1dBmCPICH Ec/No: [-24, 0] dB, step 1dB

Default Value UE Inter-frequency Event Report Parameters for CPICH Ec/No: [-24,-13, -24,-13,-24,-8]dBUE Inter-frequency Event Report Parameters for CPICH RSCP: [-115,-95,-115,-95,-115,-80]dBm

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 LinkAbbreviated Name MaxDlDpchPwr

Description This parameter indicates the maximum downlink DPCH transmission power. When performing the downlink inner loop power control, the new transmission power must be smaller than or equal to the configured DPCH Maximum DL Power. If the newly computed transmission power is larger than the configured DPCH Maximum DL Power, make it equal to the configured DPCH Maximum DL Power.

Range and Step [-35, 15] dB, step 0.1 dB

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.

Maximum Downlink Transmit Power of Radio Link Conversational DL 3.4kbps Signaling(AM): 0 dB Conversational DL 13.6kbps Signaling:0dB Conversational DL WAMR 6.60~23.85kbps: 0dB Conversational DL 64kbps (PS Conversational Video): 3dB Conversational DL NAMR 4.75~12.2kbps: 0dB Conversational DL CS 28.8kbps: -6dB Conversational DL CS 32kbps: -4dB Conversational DL CS 64kbps: 3dB Streaming DL CS 14.4kbps: -7dB Streaming DL CS 28.8kbps: -6dB Streaming DL CS 57.6kbps: -4dB Streaming DL CS 64kbps: 3dB Streaming DL PS 16kbps:-4dB Streaming DL PS64kbps: 1dB Streaming DL PS384kbps: 4dB Streaming DL PS128kbps: 2dB Interactive DL PS 8kbps:-8dB Interactive DL PS 16kbps:-4dB Interactive DL PS64kbps: 1dB Interactive DL PS384kbps: 4dB Interactive DL PS128kbps: 2dB Background DL PS 8kbps:-8dB Background DL PS 16kbps:-4dB Background DL PS64kbps: 1dB Background DL PS384kbps: 4dB Background DL PS128kbps: 2dB

Maximum Downlink Transmit Power of Radio Link Streaming DL CS64kbps: 3dB Interactive DL PS 1.2Mbps: 4dB Interactive DL PS 3.65Mbps: 4dB Interactive DL PS 7.2Mbps: 4dB Interactive DL PS 10.1Mbps: 4dB Interactive DL PS 14.0Mbps: 4dB Background DL PS 1.2Mbps: 4dB Background DL PS 3.65Mbps: 4dB Background DL PS 7.2Mbps: 4dB Background DL PS 10.1Mbps: 4dB Background DL PS 14.0Mbps: 4dB Streaming DL PS 1.2Mbps: 4dB Streaming DL PS 3.65Mbps: 4dB Streaming DL PS 7.2Mbps: 4dB Streaming DL PS 10.1Mbps: 4dB Streaming DL PS 14.0Mbps: 4dB Interactive DL PS 17.6Mbps: 4dB Interactive DL PS 21.096Mbps: 4dB Background DL PS 17.6Mbps: 4dB Background DL PS 21.096Mbps: 4dB Streaming DL PS 17.6Mbps: 4dB Streaming DL PS 21.096Mbps: 4dB

Threshold of Inter-system handover

Abbreviated Name Thresh[MAX_RAT_MEAS_EVENT]

Description This parameter indicates the absolute threshold of the UTRAN cell quality that UE uses to judge event 3a. The range and unit of the parameter relate to the measurement quantity of UTRAN system.

Range and Step CPICH RSCP: [-115, -25] dBm, step 1dBmCPICH Ec/No: [-24, 0] dB, step 1dB

Default Value UE Event Report Parameters for Own System CPICH Ec/No: [-6,-24,-24,-24]UE Event Report Parameters for Own System CPICH RSCP: [-95,-115,-115,-115]

When the measured value of 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

N313Maximum 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

Content


Neighboring Relation Adjustment

Reason High call drop rate caused by improper neighbor list configuration

Description 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.


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 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

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.


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.


Result 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.

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 Reason

Call drop because of incorrect data configuration

Description 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.

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

3G

2G

fail

Data Configuration Optimization in 2G/3G Handover Troubleshooting process

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.

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:

Data Configuration Optimization in 2G/3G Handover 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.

Data Configuration Optimization in 2G/3G Handover 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

umts call drop

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