UMTS Voice Drop Analysis

ZTE University

Content

Definition of Call Drop

Reasons of Call Drop
Analysis of Call Drop
Optimization of Call Drop
Case of Call Drop

UE Voluntarily Initiated Signaling Release

Definition of Drive Test Indicators

The definition of drive test :

z

Call drop rate= Number of call drop times/Number of call setup success times

Number of call drop times+1 :

z

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

Counters of Call Drop Reasons

Release
Type

Counter
C301230315
C301230316
C301230317
C301230318

Iu
Release

C301230319
C301230320
C301230321
C301230322
C301230323

Reason
Iu connection release request by UTRAN for CS domain in cell,
Repeated Integrity Checking Failure
Iu connection release request by UTRAN for CS domain in cell,
Release due to UE generated signalling connection release
Iu connection release request by UTRAN for CS domain in cell,
Radio Connection With UE Lost
Iu connection release request by UTRAN for CS domain in cell,
timer TRELOCoverall expiry
Iu connection release request by UTRAN for CS domain in cell,
Failure in the Radio Interface Procedure
Iu connection release request by UTRAN for CS domain in cell,
O&M Intervention
Iu connection release request by UTRAN for CS domain in cell,
Release due to Overload Control
Iu connection release request by UTRAN for CS domain in cell,
Unspecified Failure
Iu connection release request by UTRAN for CS domain in cell,
UTRAN Generated Reason

Counters of Call Drop Reasons

Release
Type

RAB
release

Counter

Reason

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

Content

Definition of Call Drop

Reasons of Call Drop
Analysis of Call Drop
Optimization of Call Drop
Case of Call Drop

Call Drop Reasons

Poor Network Coverage

Unconfigured Neighboring Cell
Handover
Interference
SC Confliction
Exceptions

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:
z
z

DL Poor Coverage:
z
z

TX Power reaches the maximum;

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

Scanner:
z

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 the
specified 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 set
information before call drop includes relevant SC information, you can
infer that unconfigured cells exist.

Handover

The handover process is incomplete

UE
Radio Environment
becoming bad,
UE can not receive the
Active Set Update

RNC
Measurement Report (1A event)

Active Set Update

Active Set Update Complete

Handover

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

RNC
Measurement Report (1B event,
Delete cell A)
Active Set Update (Delete cellA)
Measurement Report (1A event,
Add cell A)
Active Set Update (Add cell A)

Downlink Interference

Reasons for Pilot Pollution:

z
z
z

Cross-cell coverage of high Node-B

Node-B in ring layout
Signal 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

Definition of Call Drop

Reasons of Call Drop
Analysis of Call Drop
Optimization of Call Drop
Case of Call Drop

Common Analysis Methods for Call Drop

multi-dimension analysis
trend analysis
accident analysis
comparison analysis
ranking analysis
cause-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
z
z
z
z
z

access
handover
traffic statistics
time
RTWP

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
Repeated Integrity Checking Failure
Radio Connection With UE Lost
TRELOCoverall expiry
Abnormal Iu Released Number,by Cause
Failure in the Radio Interface Procedure
Unspecified Failure
UTRAN Generated Reason
Release due to UTRAN in cell Generated Reason
Abnormal RAB Released Number,by Cause

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 analysis
z

Comparison analysis is carried out through comparing various data

sets from the same perspective and finds out the differences.

Ranking analysis
z

Ranking analysis is carried out through classifying data into top N

and bottom N data from a large amount of data.

Ranking Analysis
Index
1
2
3
4
5

Reason of failure for Handover

Failure when getting the decision of handover from database
Timeout for setup of service channel in handover
Failure for decision of NodeB hard handover
Failure when build the service channel
Timeout for waiting for the handover of UE complete

Failure times
0
0
0
0
0

Percent(%)
0
0
0
0
0

10
12
13
14
21
22
24
25
26
27
29

Other errors
Basic channel switch, cannot find appointed frequency
Basic channel switch, failed to allocate the resource
Basic channel switch, failed to setup channel board
Not enough resource for channel
OVSF code is not engough
Timeout for handover of UE
The frequency is configured in the neighbouring cell list
Overload for power allocation of the frequency for the service cell
There is no idle channel in service cell
There is not suitable frequency in service cell

3104
0
0
0
0
0
29966
0
0
0
0

9.24
0
0
0
0
0
89.16
0
0
0
0

30
32
34

Different reason of failure for several service cells

Can not find UE
Abnormal release in CN

0
416
9

0
1.24
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

Definition of Call Drop

Reasons of Call Drop
Analysis of Call Drop
Optimization of Call Drop
Case of Call Drop

Optimization Methods for Call Drop

Engineer optimization:
z

Directional angle, downtilt, position of Antenna, type of Antenna,

Transmit power of BS, position of BS, new BS.

Radio parameter optimization:

z

Time to Trigger, CIO, threshold of enabling/disabling Compression

Mode, Maximum transmit power of DL RL, Threshold of Interfrequency 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 intersystem 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

Value Range

Default
Value

Name

Description

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)

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

N315

Maximum 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)

T309

Waiting time after initiating requests to access

other RATs, such as GSM

(1..8)s

3s

Content

Definition of Call Drop

Reasons of Call Drop
Analysis of Call Drop
Optimization of Call Drop
Case of Call Drop

Neighboring Relation Adjustment

Reason
z

High call drop rate caused by improper neighbor list configuration

Description
z

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.

Neighboring Relation Adjustment

Neighboring Relation Adjustment

After the neighbor relation is adjusted, the CS call drop

rate of TRI135W-1 decreases from 3 to 1.3%.

Unconfigured Neighboring Cell

74

Unconfigured Neighboring Cell

Main parameters:
z

Troubleshooting process:
z

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:
z

Cells in the system

Adjust the neighbor relation: Add the cell with SC 9 to the

neighbor list of the cell with SC 74.

Result:
z

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.

Unconfigured Neighboring Cell

9

74

74

Soft Handover Parameter Optimization

Reason:
z

Description:
z

Low handover success rate because of improper soft handover

parameter configuration
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:
z

Soft handover 1a/1b event handover threshold, trigger time

Soft Handover Parameter Optimization

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.

BS-1
436
BS-2

434

Soft Handover Parameter Optimization

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.

Soft Handover Parameter Optimization

Result
z

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
z

Description
z

Call drop because of incorrect data configuration

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
z

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

fail
2G

3G

Data Configuration Optimization in 2G/3G

Handover

Troubleshooting process
z

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:
z

Result:
z

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

Suggestion for similar problems:

z

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

Call Drop Case-Handover

Delete Cell 51 and

Cell 53

Call Drop Case-Handover

Report e1A to add

cell 51 and Cell 53
into Active Set after
0.4 second

Call Drop Case-Handover

Report e1a to add

cell 51,64,53,52 to
Active Set

Call Drop Case-Handover

Call Drop Case-Handover

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 Case-Handover

Call Drop Case-Handover

Call Drop
Happened