019 WCDMA Call Drop Problem Analysis

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Call Drop Problem Analysis

ISSUE 1.0

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Call drop analysis is a useful way for optimization and trouble-shooting. Following discussion will give general ideas on this special topic.



Upon completion of this course, you will be able to:

Describe the categories for call drop causes

Describe the flow of general call drop analysis

Analyze common call drop cases


Chapter 1 Principle of call dropChapter 1 Principle of call drop

Chapter 2 Analysis of call dropChapter 2 Analysis of call drop

Chapter 3 Call drop casesChapter 3 Call drop cases



Principle of Call Drop

Content:

Calculation formula of call drop rate on signaling plane and measurement points.

Calculation formula of call drop rate on user plane and measurement points.


Principle of Call Drop

There are two types of call drop: signaling plane call drop and user plane call drop:

Signaling plane call drop means RNC will send the message IU RELEASE REQUEST to CN after CN receives the IU connection setup complete

User plane call drop means RNC will send the messageRAB release request to CN after CN receives the RAB setup complete



Formula of Signaling Plane Call Drop Rate

The formula of Call Drop Rate of CS Signaling Plane:

Call Drop Rate of CS Signaling Plane = RNC-originated CS Domain Iu Connection Release / RNC-originated CS Domain Iu Connection Setup Success 100%

The formula of Call Drop Rate of PS Signaling Plane:

Call Drop Rate of PS Signaling Plane = RNC-originated PS Domain Iu Connection Release / RNC-originated PS Domain Iu Connection Setup Success 100%


Measurement points of Signaling plane call drop

Measurement points of Signaling plane call drop

Call Drop Rate of Signaling Plane is calculated by counting RNC-originated Iu connection release

A

IU RELEASE COMMAND

IU RELEASE COMPLETE

RNC CN

IU RELEASE REQUEST



Cause of Signaling Plane call drop

Usually, the typical causes for sending the IU_releasemessage are as follows:

OM intervention

Repeated integrity checking failure

RNC generated signaling connection release

RL out-of-synchronization


Formula of User Plane Call Drop Rate

The formula of Call Drop Rate of CS User Plane:

Call Drop Rate of CS User Plane = RNC-originated CS Domain RAB Release / RNC-originated CS Domain RAB Setup Success 100%

The formula of Call Drop Rate of PS User Plane:

Call Drop Rate of PS User Plane = RNC-originated PS Domain RAB Release / RNC-originated PS Domain RAB Setup Success 100%



Measurement points of User plane call drop

Measurement points of User plane call drop

Call Drop Rate of User Plane is calculated by counting RNC-originated RAB connection release

A

RNC CN

RAB RELEASE REQUEST







Analysis of Call Drop

content

main causes of high call drop rate

troubleshooting of high call drop rate


Analysis of Call Drop

According to the definition of call drop measurement point, call drop is usually caused by the following:

Equipment problem. That is, call drop due to the equipment defects, such as UE , NodeB, RNC,CN;

RF problem. that is, call drop due to coverage problem or unsuitable radio layer parameters.



Equipment Problem

Call drop arising from equipment problem

Hardware problem

Transmission problem

Antenna and feeder fault

Other causes


Radio Link FailureSignaling process chart of radio link failureSignaling process chart of radio link failure

UE NodeB RNC CN

Measurement Report

Measurement Result

RL Failure INDICATION

IU RELEASE REQUEST (RL Failure)



Analysis of Causes of Radio Link Failure

Analyze the causes of the failure of radio links. The causes can be: Interference

Internal interference, external interference and the equipment interference

Poor coverage Coverage hole, dead zone, isolated island,

uplink/downlink unbalanceparameter setting not proper

radio link timeout counter, handover, power control etc.


Radio Link Fault - Coverage

Coverage:

Coverage over shooting

Coverage hole

Signal fading

Incomplete definition of adjacent cellsunbalance of uplink/downlink



Radio Link Fault - Interference

Interference:

Strong downlink interference (CPICH RSCP is not low, but Ec/Io is low)

Strong uplink interference. On the basis of the RSSI scanning result or analysis of the SIB7, there is uplink interference


Handover Problem

1 Handover is too late 1)The soft handover threshold is too high or the time-to-trigger

setting is too big. 2) The soft handover area is too small.

2 Ping Pong Handover In the handover area, because of the signal fluctuation, the UE

will continually handover between source cell and target cell. For intra-frequency hard handover, since there are intra-frequency interferences, call drops are likely to occur; for inter-frequency handover, since the compressed mode measurement is required, ping pong handover will bring serious influence on the communication quality.







General call drop analysis flow:



Call drop location analysis

Call drop location analysis: based on the location of the call drop event in the map window in the post processing tool, following information can be got:a. The place of the drop call b. At the edge of the 3G coverage or not c. Related cells d. Distance between related 3G cells and the drop

call point


Neighbor list analysis

Neighbor list analysis: to find out any cell not defined in the neighbor list, following information need to be got:

a.Best serving cell before the call dropped

b.Best serving cell after the call dropped

c. Existing neighbor list of the best serving 3G cell



Coverage analysis

Coverage analysis Analyze the pilot coverage to get information about: i) CPICH transmission power ii) Whether following coverage events take place or not:

a. Coverage limited b. System interference c. Poor uplink coverage d. Poor downlink coverage e. Pilot pollution


Coverage analysis

Coverage analysis Analyze the service coverage to get information about: a. Service related Maximum downlink transmission

power b. SIR before the call dropped c. Max uplink transmission power allowed for UE d. Actual uplink transmission power before the call

dropped



Handover analysis

Handover analysis to get information about: i) Signal strength change of the target call and

the serving cell a. insufficient handover area b. too late to start CM c. too late to add target cell d. interaction problem among 3 type of

handover ii) Following cause exist or not:


Signaling analysis

Signaling analysis:

a. Signaling of different interface including: Iu, Iub, Iur, Uu

b. Signaling of different layer including : NAS, RRC

c. In addition, analyze signaling in UE side in conjunction with signaling in network side.



Case 1 Fault description: The position of the drop call was not in the

edge of the 3G coverage. The call dropped on cell SC209.


Case1

Analysis:

1.Poor coverage :from the page shown ,we can find that

1) prior to the drop, the best server for the UE is not the same as that of the scanner.

2) the CPICH Ec/Io (and CPICH RSCP) degrades for UE ONLY while scanner shows no degradation.



Case 1

Analysis:

2:Compared UE data with scanner data, it can be concluded that the best serving cell is not the signal strongest cell. The best serving cell should be picked out by the latest measurement control message and corresponding event measurement report.

3: Check the latest measurement control message . Corresponding measurement control shows theres no neighbor relation between SC209 and SC128.


Case 1

Trouble-shootingadd the neighbor relation between SC209 and SC128



Case 2 Fault description : The position of the drop call was in the edge of the

3G coverage, the call dropped on cell SC314, the distance between cell SC314 was very far away from the drop call point.


Case 2 Analysis

1. analyze the active set and monitor set information before call drop from UE measurement results

only sc314 in the active set, no cell in the monitor set.



Case2 Analysis

2 Analyze the scanner data in conjunction with the UE data


case2

Troubleshootingthe coverage should be enhanced here to avoid the call drop in this place



case2

Conclusion To analyze the coverage, need to:1)Analyze the pilot coverage in conjunction with the

service coverage2)Analyze the uplink coverage in conjunction with the

downlink coverage3)Analyze the scanner data in conjunction with the UE

data4)Analyze the CPICH RSCP in conjunction with CPICH

Ec/Io, SC


case3 Fault description: The call drop happened in the area of frequent

change of best server shown by the scanner



case3 Analysis

1.Compare Ec/Io from both scanner and UE at the time of the drop: the UE Ec/Io drop to < -18 dB while the scanner remained above -6 dB :


case3 Analysis

2.Comparing the best servers from the UE and the scanner at the time of drop: for the scanner and UE SC506 is the best server prior to the drop. However, before the drop, the scanner selected SC505 as the best server while the UE continued to have only SC506 in its active set resulting in the drop call. Immediately after the drop, the UE camps on SC505:



Case 3

Analysis3. according the latest measurement control message, we can find SC506 is in the list. The drop call is not the case of missing neighbor.


Case 3 Analysis

4. It seems that the best cell changes from SC506 to SC505 too fast for the UE to perform soft handover on time.



Case 3

Analysis

As the handover Reporting range of 1A event is 4.5dB, the hysteresis is 0dB, the delay trigger time is 320ms, the filter coefficient is 3 (1 second is required), so the requirement for the handover area is: the lasting time when (Ec/Io of 506 cell Ec/Io of 505 cell) < 4.5dB is about 2 seconds . The value in the circle as shown in the diagram cannot meet the requirement.


Case3 Analysis

Uu Interface messages that 1A event of SC505 was triggered at 54:04:



Case3 Analysis

But SC506 went bad too severe for the procedure of Active Set Update to be finished:


Case3

Troubleshooting

Increase the value of range-to-report

decrease the value of trigger-to-time



Case3

Conclusionhandover analysis to get information about:

1)the signal strength change of the target call and the serving cell

2)whether following cause exist or not: insufficient handover area, too late to start CM, too late to add target cell, too early to delete serving cell, interaction problem among 3 type of handover

3)whether ping-pang handover exist or not

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019 WCDMA Call Drop Problem Analysis

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