019 wcdma call drop problem analysis

HUAWEI Mexico Training Center 1

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Call Drop Problem Analysis

ISSUE 1.0


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

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

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

message”RAB 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_release

message 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 Failure

Signaling 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 unbalance

�parameter 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 cells

�unbalance 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 there’s no

neighbor relation between SC209 and SC128.


Case 1

� Trouble-shooting

add 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

� Troubleshooting

the 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 coverage

�2)Analyze the uplink coverage in conjunction with the

downlink coverage

�3)Analyze the scanner data in conjunction with the UE

data

�4)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

� Analysis

3. 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

� Conclusion

handover 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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