UMTS CS Call Drop Analysis Guide V2.0

8/12/2019 UMTS CS Call Drop Analysis Guide V2.0

1/31

Internal

UMTS CS Call Drop Analysis Guide

Version: V2.0

ZTE UMTS Radio Network Planning & OptimizationDepartment


2/31

UMTS Network Planning & Optimization Guidebook

Release Notes:

Version Date Author Reviewed by Revision History

V1.0 2009/4/23 Qin Jianhan None Draft

V2.0 2009/11/25 Zhou ChangjingYin Jianhua, Gan Yi, Xu

Zhexian2nd edition


3/31

Key words:

CS, Call drop

Abstract:

This document introduces ways to evaluate, test, analyze and solve the call drop

problem.

Abbreviation :

None

Reference:

None


4/31

This document contains proprietary information of ZTE Corporation and is not to be disclosed or usedexcept in accordance with applicable agreements. I

Table of Contents

1 Introduction ....................................................................................................................................... 1

2 Definition ............................................................................................................................................ 2

2.1 Definition of Call Drop from Drive Test Aspect ............................................................................. 2

2.2 Definition of Call Drop in OMC Side............................................................................................. 2

3 Call Drop Analysis ............................................................................................................................. 4

3.1 Call Drop Reasons .......................................................................................................................... 4

3.1.1 Call Drops Caused by Insufficient Coverage ........................................................................ 4

3.1.2 Call Drop Caused by Neighbor Cells .................................................................................... 5

3.1.3 Call Drop Caused by Interference ......................................................................................... 6

3.1.4 Call Failure Caused by Two Cells Using the Same PSC ....................................................... 7

3.1.5 Call Drops Caused by Engineering Causes ........................................................................... 9

3.1.6 Call Drops Caused by 2G/3G Interoperability .................................................................... 12

3.1.7 Call Drops Caused by the System ....................................................................................... 13

3.2 Analyzing Call Drops by DT ........................................................................................................ 13

3.3 Analyzing Call Drops by Traffic Statistics ................................................................................... 15

3.3.1 Procedure of KPI Analysis .................................................................................................. 16

3.3.2 Basic Methods to Analyze KPIs .......................................................................................... 17

3.3.3 KPI Analysis Tolls ............................................................................................................... 19

3.4 Radio Parameters Involved During Optimization ......................................................................... 21

3.4.1 Radio Parameters Related with CS Call Drops ................................................................... 21

3.4.2 Timer and Counter Related with Call Drop ........................................................................ 24


5/31

Internal

This document contains proprietary information of ZTE Corporation and is not to be disclosed or usedexcept in accordance with applicable agreements. II

Figures

Figure 1 Scenario one that may cause the same PSC problem ...................................................... 7

Figure 2 Scenario two that may cause the same PSC problem ...................................................... 8

Figure 3 Scenario three that may cause the same PSC problem .................................................... 9

Figure 4 Measurement of antenna power through PMS .............................................................. 10

Figure 5 Flow chart to test call drops by DT ............................................................................... 14

Figure 6 Flow chart for top cell selection .................................................................................... 17

Figure 7 PMS cell performance measurement figure .................................................................. 19


6/31


7/31

Internal

This document contains proprietary information of ZTE Corporation and is not to be disclosed or usedexcept in accordance with applicable agreements. 1

1 IntroductionThis document is compiled to guide the network optimization engineers to solve the

call drop problem, to reduce the call drop rate, and to improve the quality of the

network. It also introduces ways to evaluate, test, analyze and solve the call drop

problem. In addition, it also includes some typical cases. In the actual network

optimization activities, handover and call drop are strongly related. In most cases,

handover failure would lead to call drops. For this kind of call drops, you may refer to

the guidebook for call drops caused by handover. This document mainly focuses on call

drops which are not caused by handover failures.


8/31

Internal


2 Definition

2.1 Defini tion of Call Drop from Drive Test Aspect

Air interface signaling at the UE side: Call drops refer to call releases caused by Not

Normal Clearing, Not Normal, or Unspecified when the message on the air interface

satisfying any of the following three conditions:

The UE receives any BCH information (system information).

The call is released for Not Normal and the UE receives the RRC

Release information.

The UE receives CC Disconnect , CC Release Complete , and CC

Release information.

Signaling recorded at the RNC side: Call drops refer to call releases when the RNC has

sent the Iu Release Request to the CN through the Iu interface, or when the RNC has

sent the RAB Release Request information to the CN through the user panel.

2.2 Defin it ion of Call Drop at OMC SideThe definition of call drop in a broad sense contains the call drop rates at both the CN

and UTRAN sides. Since the network optimization focuses on the call drop rate at the

UTRAN side, this document only focuses on the KPI analysis at the UTRAN side.

The KPIs at the UTRAN side refers to the number of released RABs of different

services triggered by the RNC. Two aspects are involved: (1) After the RAB is

established, the RNC sends the RAB RELEASE REQUEST information to the CN. (2)

After the RAB is established, the RNC sends the IU RELEASE REQUEST to the CN,

and then it receives the IU RELEASE COMMAND from the CN. The statistics can be

collected based on specific services.

Meanwhile the traffic statistics also imply reasons that the RNC triggers the release of

the RABs of different services.

The call drop rate can be calculated by the following formula:


9/31

Internal

%*SuccessCSRABSetup

iggedByRNC CSRabrelTr CDRCS 100 _

=

CSRabrelTriggedByRNC contains the number of RABs included in RAB RELEASE

REQUEST for CS services and that included in IU RELEASE REQUEST for CS

services.

%*SuccessPSRABSetup

iggedByRNC PSRabrelTr CDRPS 100 _

=

RabrelTriggedByRNC contains the number of RABs included in RAB RELEASEREQUEST for PS services and that included in IU RELEASE REQUEST for PS

services.

It should be specified that the RNC traffic statistics calculates the times of call drops

through the signaling at the Iu interface, and counts the number of RAB RELEASE

REQUEST and the number of IU RELEASE REQUEST initiated by the RNC. While

call drops in the drive test aspect emphasizes the information at the air interface and

non-access stratum and their cause value. It is different from call drops at the OMC

side.



10/31

Internal


3 Call Drop Analysis

Many reasons may lead to the call drop problem, and call drop is an expression of the

deep network problems. This chapter focuses on the call drop reasons, commonly-used

call drop analysis methods, and main call-drop optimization instruments.

3.1 Call Drop Reasons

3.1.1 Call Drops Caused by Poor Coverage

In the definition of network coverage, the requirements of effective coverage for a

certain sampling point is that its RSCP and Ec/Io should be better than the specified

threshold. In this section, bad coverage is represented by poor RSCP value. Note that

coverage at cell edges is a special case. Coverage at cell edges would have bad RSCP

value and excellent Ec/Io owing to little cell number, but still the coverage in these cell

edges is defined as bad coverage.

In UMTS network, initiation and maintenance of different services would have

different requirements on coverage. Table 1 lists the reference values.

Table 1 RSCP and Ec/Io threshold for different services

Service Type RSCP [dBm] Ec/Io [dB]

AMR12.2K -105 -13

CS64K -100 -11

PS384K -95 -10

HSDPA -90 -8

The coverage condition at the UL and DL of the network can be estimated through the

power of the dedicated channels for the UL and DL before call drops, which can be performed through the following methods.

If the UL TX power before the call drop has reached the maximum value and the UL

BLER is bad, or it is found out through the single user tracing record at the RNC that

the NodeB has reported RL failure, then the call drop is caused by bad UL coverage. If

the DL TX power before the call drop has reached the maximum value and the DL

BLER is bad, then the call drop is caused by bad DL coverage.


11/31

Internal


For the coverage optimization method, see the WCDMA Radio Network Optimization

Guide .

3.1.2 Call Drop Caused by Neighbor Cells

1. Missed neighbor cell

Neighbor cell optimization is an important link of radio network optimization. If

certain cells should be included but excluded from the neighbor cell list of one cell,

then call drop would happen and the interference in the network would also increase

and system capacity would be impacted. Therefore, neighbor cell optimization is an

important part of the engineering optimization.

It is easy to estimate whether the cell is configured with any neighbor cell, and you can

playback the call drop data, perform NCOS analysis, and analyze the scanner data.

Use ZTE CNA to playback the call drop data. If the blue pillar

(representing the detected set) in the histogram of the pilot signals is the

longest, then the missed neighbor cell problem exists.

Use the automatic analysis tool of ZTE NCOS, it would study the

handover data of the network, and automatically add the missed neighbor cell.

For details, see the operation guide of NCOS.

During the drive test, the UE would acquire the neighbor cell list from

the NodeB, and the scanner would scan the 512 PSCs and record the Ec/Io. If

one of the PSCS is not included in the neighbor cell list, and its pilot strength

is stronger than the threshold, and the phenomenon lasts for a few seconds,

then the missed neighbor cell problem exists.

2. Removal of key neighbor cells caused by combination of macro diversity

Assign the priority of the neighbor cell when performing the initial neighbor cell planning, then optimize the priority and number of neighbor cells periodically with

NCOS as the traffic volume increases.

3. Untimely update of the external cell information

Check the external cells of the RNC periodically, and ensure the cells in the list are

correct.


12/31

Internal


3.1.3 Call Drop Caused by Interference

Distinguish the UL and DL interferences.

The interferences from the UL and DL would both lead to call drop. Generally, when

the CPICH RSCP of the active set is large than -85dBm, and the comprehensive Ec/Io

is lower than -13dB, call drop occurs, then the call drop is caused by the DL

interference. Note that when the handover is not timely, the RSCP of the serving cell

may be good, but the Ec/Io is bad. However, the RSCP and Ec/Io of the monitored set

are both excellent under this condition. When the UL RTWP is 10dB higher than the

normal value, which is -107~-105, and the interference duration is 2s or 3s longer, call

drop may happen and the problem must be solved.

Two reasons may cause DL interferences, which are pilot pollution and missed

neighbor cell. The missed neighbor cell has already been discussed in the above part

and would not be repeated here. In the pilot pollution area, signals of multiple cells

exist, the RSCP of these cells is good, while Ec/Io is bad, the UE would frequently

reselect the neighbor cell or perform the handover, and the incoming and outcoming of

calls can hardly reach the UE. Generally, three factors would lead to pilot pollution in

the network.

Overshooting of sites at a high location

NodeBs in ring-shaped distribution

Wave-guide effect, large reflectors, and some other effects that may

cause the distortion of signals.

The typical feature of DL call drops is that the RNC sends the Active Set Update

message, while the UE cannot receive it, then the call is dropped for RL Failure.

You can judge whether the UL interferences exist by the Average RTWP and Max

RTWP on the OMC-R. For an idle cell, the Average RTWP is about -105dBm; for acell carrying 50% of UL load, the Average RTWP is around -102dBm. If the Average

RTWP of an idle cell exceeds -100dBm, we can believe that UL interferences exist.

The UL interferences make the UL TX power of the cell in connected mode increase,

and then an excessively high BLER is generated. Then call drop happens. During

handover, the newly-added link is out of sync for UL interferences, which further leads


13/31

Internal

to failed handovers and call drops. The UL interference may be intra-RAT or inter-RAT

interferences. In most cases, the UL interferences are inter-RAT interferences.

When DL interference exists, the UL TX power is very small or the UL BLER may

converge, however, when the DL TX power of the UE reaches the maximum value, the

DL BLER does not converge. If UL interferences exist, the same problem would insist.

Thus, in actual analysis, this method can be used to distinguish whether interferences

exist.

For methods to investigate the interferences, see the UMTS Interference Investigation

Guidebook .

3.1.4 Call Failure Caused by Two Cells Using the Same PSC

3.1.4.1 Scenario One

Figure 1 Scenario one that may cause the same PSC problem

Cell A and Cell B (source cell) are configured as neighbor cell for each other, however,

the geographical distance between Cell A and Cell B is huge. Cell A and Cell C has thesame PSC, and Cell C and Cell B (source cell) is very close, however, Cell C and Cell

B are not configured as neighbor cells for each other.

Under this situation, the UE detects signals from Cell C and sends Event 1A request to

be soft handed over to Cell C. The PSC in the Event 1A request is 123. After receiving

the Event 1A request, the RNC checks from the neighbor cell list of Cell B (source cell)

for cells with PSC of 123, then it finds Cell A. Then the RNC tries to build the radio



14/31

Internal

link on Cell A. The RNC instructs the UE to add Cell A to its active set. Then, the

update of the active set times out for the cell measured by the UE is different from the

cell where the radio link is built.

3.1.4.2 Scenario Two

Figure 2 Scenario two that may cause the same PSC problem

In this scenario, the UE has established the radio link with two cells, Cell B and Cell C.

Cell A is the neighbor cell of Cell B, and Cell D is the neighbor cell of Cell C, and

these two cells have the same PSC. When the UE is in soft handover state, the RNC

would combine the neighbor cell lists of Cell B and Cell C, then the same PSC problem

would happen.



15/31


16/31

Internal

Figure 4 Measurement of antenna power on PMS

The balance level checking of two antennas in whole network can be implemented by

OMCB measurement. However, you need to manually process the acquired data.

2. An excessive VSWR

You can check the VSWR of the current site at the RNC SDR. If the VSWR is large

than or equals to 1.4, then it must be adjusted.

3. Multi-band antenna problem

In the network of some cities, multi-band antennas exist. The operator usually refuses

to adjust the parameters of the multi-band antenna for fearing of affecting the

subscribers of the existing 2G network. Then pilot pollution or overshooting may occur.

To solve this problem, you should try to persuade the operator to change the antenna,

so that 2G and 3G networks can have separate antennas. If these antennas cannot be

changed, then the specific environment must be carefully studied before taking any

actions. You can optimize the neighbor cells to avoid call drops.

4. Leakage of signals from indoor distribution system



17/31

Internal


In most cities, call drops caused by signal leakage from indoor distribution system exist.

You should persuade the operator to reconstruct the indoor distribution system. Or, the

indoor distribution system can be merged to the whole network, which can be done by

optimizing of the coverage of the ambient outdoor cells and addition of neighbor cells.

5. Call drop caused by unsteady transmission

As the bottom level of transmission medium, E1 would report the loss of E1 electrical

signals and reception failures at the remote end. Meanwhile, several E1s would be

bound together as a group, and then E1 would report the fault of IMA group in

non-operating mode.

The following table lists several E1 faults that must be handled and the related

handling suggestions.

Table 2 Common E1 faults and handling suggestions

Fault Causes Solutions

Lost of E1

electrical signals

The RX end detects no line circuit pulse or

cannot detect logic 1 within continuous

periods, then the LOS alarm is reported. This

alarm is generally caused by the RX fault of

the E1/T1 or broken lines, then the E1/T1cannot detect the signals from the remote end.

1. Check whether the SA board is

secure, and whether the E1 adapter

is slack.

2. Check whether the pins of the

adapter are damaged.3. Check whether the joint

connector of the E1 cable is

damaged, and whether the joint

connector is securely connected

with the E1 cable.

4. Check whether the cabling of

the E1 cable satisfies the

engineering specification, whether

the E1 cable bears any external

force.5. Use the E1 self-loop cable to

recycle the line, if the alarm is

cleared, then check the E1 cable at

the peer end.

Remote reception

failure of the E1

It indicates the E1/T1 remote alarm. This

alarm indicates the abnormal receptions at the

remote end. The remote end inserts the RAI

indicator bit to the signals and then sends it to

1. The TX line is faulty or

broken. Check whether the TX line

is correctly connected. For details,

see the Handling suggestions for the


18/31

Internal


the local end, and the local end reports the

alarm after detecting the alarm. The remote

reception error is reported.

LOS Alarm.

2. Check whether the frame

structures of the E1 frame at thelocal end and remote match. The E1

frame at both ends must both work

at dual-frame or multi-frame mode.

3. Check for error codes at the

TX line.

E1 frame out of

sync

The first bit of slot 0 of both E1 and T1

carries the synchronous clock signals, which

inform the RX end of the start of one frame. If

the RX ends of the E1 and T1 are out of sync,

then data frames would be lost and the LOFalarm is reported.

1. Whether E1 and T1 work at the

same state.

2. Check whether E1 frames are

of the same modes

(dual-frame/multi-frame).3. Check whether the impedance

modes of E1/T1 matches.

4. Check for interferences from

digital devices around E1/T1.

5. Check whether the clock

signals are normal.

SSCOP link error This alarm is caused by that the SSCOP

signaling link is unsuccessfully established or

the SSCOP signaling from the remote end is

not received within a certain period. Then the

SSCOP link would be broken off, and this

alarm is reported.

See the handling suggestions for

E1 faults.

IMA group in

non-operating

mode

After the IMA group is successfully

configured, if IMA remains in non-operating

mode for over 1s, then this alarm is reported.

See the handling suggestions for

E1 faults.

Currently, some sites are configured with IP transmission. Therefore, the alarm of

"Lack Ethernet electrical signals" also should be handled on site.

3.1.6 Call Drops Caused by 2G/3G Interoperabi li ty1. Optimization of 2G neighbor cells configured for 3G cells

If the 2G cells are congested, or interfered, then the success rate of 3G -> 2G

handovers is low. During the neighbor cell optimization, this kind of neighbor cells

must be removed from the list.

2. Parameters must be refined based on different scenarios.


19/31


20/31

Internal

Figure 5 Flow chart to test call drops by DT

1. Call drop data

The call drop data refers to the CNT test data and RNC signaling tracing data.

2. Call drop spots

Use CNA to analyze the call drops to acquire the location where call drops happen.

Then acquire the following data: pilot data collected before and after call drops, active

set and monitoring set information collected by the cell phone, and signaling flow.

3. Stability of the primary serving cell



21/31

Internal


The stability of the primary serving cell refers to its changes. If the primary serving cell

is stable, then analyze RSCP and Ec/Io. If the primary serving cell changes frequently,

then the handover parameters should be changed to avoid the ping-ping effect.

4. RSCP and Ec/Io of the primary serving cell

Check the RSCP and Ec/Io of the optimal cell, and then

When the RSCP is bad, the coverage is poor.

When the RSCP is normal, while the Ec/Io is bad, pilot pollution or DL

interference exists.

When RSCP and Ec/Io are both normal, if cells in the active set of theUE are not the optimal cells (which can be checked through playback of data),

then the call drops may be caused by missed neighbor cell or untimely

handovers; if cells in the active set of the UE are the optimal cells, then call

drops may be caused by UL interferences or abnormal call drops.

5. Reproducing of problems with DT

Since you cannot collect all necessary information by one DT, then multiple DTs shall

be performed to collect sufficient data. In addition, multiple DTs can also help to

ascertain whether the call drop is random or always happens at the same spot.

Generally, if call drops always happen at the same spot, this problem must be solved,

or if call drops happen randomly, multiple DTs must be performed to find inner

reasons.

3.3 Analyzing Call Drops by Traffic Statis tics

When analyzing the traffic statistics, check the call drops index on the RNC firstly to

learn the operating status of the whole network. Meanwhile, a cell-by-cell analysis can

be performed to acquire the detailed call drop indexes of each cell. During the analysis,

the traffic statistic analyzing tool can be used to analyze the call drop situations of

different services and the possible causes.

Acquire data about cells with abnormal KPIs through the traffic statistics. If KPIs of

these cells used to be normal, then the abnormal KPIs may be brought by software

version, hardware, transmission, antenna, or data, then you can check these aspects


22/31

Internal


based on the alarms. If no obvious abnormal cells exist, the statistics can be classified

based on the carrier in each sector, then cells with poor KPIs can be screened out.

Further analyze the traffic statistics of these cells, such as analyzing more related KPIs,

such as analyzing data at a shorter interval, or analyzing KPIs that are more likely to

cause call drops, such as handover. Meanwhile, you can analyze the reasons for call

drops based on system logs. During the analysis, you should consider the effect of

other KPIs when focusing on a certain KPI. It should be specified that the result of

traffic statistics is meaningful only when the traffic volume reaches a certain amount.

For example, a 50% of call drop rate does not mean that the network is bad. This value

is meaningful only when the calling number, succeed calling number, call drop times

all make statistical significances.

3.3.1 Procedure of KPI Analysi s

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. At the initial stage of network construction, there are few

subscribers in the network. At this stage, the KPIs of many cells might be unstable,

such as call drop rate. You can collect the data in seven days or longer periods, thenselect the top cell and then perform the optimization. For example, optimization of call

drop rate of CS services. When selecting top cells, you can select the cell with call drop

numbers exceeding the specified threshold, and then arrange the priority based on the

call drop rate.

The procedures of top cell selection are the same as the procedures of handling input

information from other team of engineers (complains or single site acceptance), and are

shown in the following figure.


23/31

Internal

Figure 6 Flow chart for top cell selection

3.3.2 Basic Methods to Analyze KPIs

3.3.2.1 Speedily Collect ing the Field Data

To locate the problem, you have collect data from many different spots between the UE

and the pdn server. While, speedy and accurate collection of the field data is essential

to locate and solve the problem and to improve the KPIs. Data collection can bedivided into multiple layers.

1. Collecting UE log, RNC signaling, KPI data, alarms, abnormal probes, and packet

captured at the Iub interface

2. NodeB and RNC debug log

Some common skills are required to collect data of the first layer, and the network

optimization & maintenance personnel can easily master these skills. At present, most



24/31

Internal


field questions can be located through the data analysis at this layer. Collection of the

debug log of the second layer should be performed or remotely supported by the

relative R&D engineers. Data at this layer can help to solve some deep layer problems.

The following chapter focuses on the data collection tool and method for the first layer

data, and only gives a brief introduction to that of the second layer.

3.3.2.2 Health Check of Sites

For sites where alarms are reported, you should first perform the health check for the

site, which mainly covers the following aspects:

Alarms

Frequently added or removed common transport channels

UL & DL power

Radio link restore

Balance level between two antennas

Statistics of service failures

The RL restore rate is shown by the NodeB cell measurement recorded by PMC as

shown in the following figure, and is accumulated since the establishment of cells. If

the RL restore rate of a cell is lower than 80%, the cell is treated as abnormal, and the

possible causes are as follows:

UL interferences

Insufficient cell radius or overshooting

Reuse of the same PSC

Abnormal UL RF channel

For these possible causes, you may check them combining other measurement results

and data analysis.


25/31

Internal

Figure 7 PMS cell performance measurement figure

3.3.3 KPI Analysis Tool s

3.3.3.1 Signal Trace

This tool traces signaling of RNC, you can trace the signaling at the Iu, Iur, Iub, and Uu

interfaces, TNL signaling, and RNL signaling through this tool. The most commonly

used method to check the KPIs is to trace the RNL signaling. This tool is very useful,

and can trace the signaling on the basis of cell (trace signaling of multiple UEs) and

IMSI (trace signaling of one UE).

It should be emphasized that signaling tracing by cells can only trace the UE that

initiates the call from this cell. The UE can be traced as long as it remains in the same

RNC, even if it is handed over to other cells. However, if a UE initiates the call fromother cells and then is handed over this call, and its call drop happens in this cell, it

cannot be traced. Therefore, when you trace the signaling of a cell with high call-drop

rate, the signaling of cells in close handover relation with this cell should also be traced,

then the result would be more comprehensive.

The RNC R&D engineers also develop a RNC signaling tracing tool, WinSigAn, which

can mark the call drop spots more clearly.



26/31

Internal


3.3.3.2 RNC Associ ation Log

This tool helps to record the context of the abnormal system flow, and then the context

would be counted and analyzed to locate the network problem.

It is usually used when the system is abnormal and no RNC signaling is traced. It can

help to locate the problem by the time when the system exception happens. The

exception can be queried on the basis of IMSI and CELL ID.

3.3.3.3 NodeB LMT

Besides all functions of OMCB, NodeB Local Maintenance Terminal (LMT) can also

provide detailed cell and UE information.

The NodeB LMT consists of EOMS, EFMS, DMS, and PMS.

3.3.3.4 NodeB Except ion Probe

In the field of the WCDMA commercial network, this tool can effectively help to

monitor the operating status of the NodeB. Different modules of the NodeB would

record the information when exceptions happen, thus facilitating the location of

problems. However, specialized knowledge is required. You have to understand the

functions and interfaces of different boards. If the field engineers cannot analyze the

report, they can simply send these data to the R&D engineers.

The exception probe reported by different NodeBs can be saved on different OMCB

servers based on the RNC they belong to. Then, this tool would download the file from

different OMCB FTP, and then analyze them.

3.3.3.5 CTS

CTS is the tool for the CN, and it can be used to perform deep signaling by IMSI.

Unlike SignalTrace, which is applicable to the signaling tracing within one RNC, CTS

can perform the signaling tracing across the RNC border, Therefore, it is applicable to

the signaling tracing of VIPs.

CTS can trace the interactive signaling among different NEs within the CN, and can

trace the signaling at the Iu and Uu interfaces, and this is called deep tracing. The

working principles of CTS is as follows: First establish an IMSI task on CTS server,

and then sent this IMSI task to the CN, which is further sent to different modules

through the arranged interfaces, then each module collects the signaling related to IMSI,


27/31

Internal


and then the collected signaling is transmitted back to the CTS server through the CN.

The above interfaces are all private interfaces, thus this tool only work on ZTE CN and

RNC.

3.3.3.6 UE Log

DT is an important means to analyze KPIs. Many problems, signaling tracing at the

network side and tracing of problems which are hard to be located, can be finally

located after combining the UE logs. The commonly used DT software is

QXDM/APEX(QCAT), CNT/CNA, and TEMS.

3.4 Radio Parameters Involved During Optimization

3.4.1 Radio Parameters Related with CS Call Drops

Time To Trigger

Time To Trigger is the interval between the moment that the events (1A, 1B, 1C, and

1D) are monitored and the moment that the events are reported. The setting of TTT

would influence timely handover.

The adjustment of handover parameters should first ensure that this cell is overlapped

by other cells, then you can adjust the related radio parameters to ensure that the time

that the UE passes the handover area is longer than the handover delay of the whole

system, thus ensuring the continuity of the services. The other is to ensure that the

handover area ascertained by the signals and radio parameters cannot be too large to

avoid the increase of handover overhead and reduction of resource utilization ratio.

For areas where the signals may change greatly, the trigger time of Event 1A must be

reduced, and that of Event 1B must be increased. Meanwhile, the CIO of the

corresponding neighbor cells should be adjusted so that Event 1A can happen earlier

and Event 1B would happen later, thus ensuring successful handovers.

For highways, the cells are sparsely distributed. If the vehicles drive too quickly and

cannot access the new cell in time, call drops would happen. The optimization is the

same as that for the optimization for street corners in dense urban, which is to make

cells with good signals join the active set speedily to ensure continuity of services.


28/31

Internal

For the adjustment of the related parameters, a whole new set of parameters must be

assigned to the target cell.


3.4.1.1 Cell Individ ual Offset

The sum of the value of Cell Individual Offset (CIO) and the actually measured value

is used in the evaluation of the events of the UE. The UE would use the original

measurement value of this cell plus the CIO as the measurement result for the

intra-frequency handover judgment. CIO can help to ascertain the cell edge.

The larger this parameter is set, the easier the soft handover will be, and more UEs will

be in soft handover state. However, more resources are consumed. This smaller is

parameter is set, the more difficult the soft handover is.

CIO is valid only for the neighbor cell. For Event 1A, the CIO can be set in the

neighbor cell; for Event 1B, the CIO can be set in the cell to be removed. The formula

is as follows:

Formula of Event 1A triggering:

),2/(10)1(1010 111

aa Best

N

ii New New H R LogM W M LogW CIO LogM

A

+

+

= M New is the measurement of the to-be-evaluated cells that has entered the report range.

M i is the mean measurement result of cells (exclude the best cell) in an active set.

NA is the current cell number (exclude the best cell) in the active set.

MBest is the measurement result of the optimal cell in the active set.

W is the weight proportion of the best cell to the rest cells in the active set.

R 1a is the reporting range of Event 1A.

H1a is the reporting hysteresis of Event 1A.

Formula of Event 1B triggering:

Mnew is the measurement of the to-be-evaluated cells that has entered the report range.

Mi is the mean measurement result of cells (exclude the best cell) in an active set.

NA is the current cell number (exclude the best cell) in the active set.

1 11

10 10 (1 ) 10 ( / 2), A N

Old Old i Best b bi

LogM CIO W Log M W LogM R H =

+ + +


29/31

Internal


MBest is the measurement result of the optimal cell in the active set.

W is the weight proportion of the best cell to the rest cells in the active set.R 1bis the reporting range of Event 1B.

H1b is the reporting hysteresis of Event 1B.

3.4.1.2 Start/Stop Thresho ld fo r Compressed Mode

Compressed mode is frequently used during inter-frequency and inter-RAT handovers.

The compressed mode is started before the handover, and the system can use the time

slot brought by compressed mode to perform the signal quality test on the

inter-frequency or inter-RAT neighbor cells. In the current system, the compressedmode is started through Event 2D, and stopped through Event 2F. The measurement

value of RSCP or Ec/Io can be selected. Currently, the default value is RSCP.

Generally, the quality and other related information of the target cell (inter-frequency

or inter-RAT) must be acquired for the compressed mode. Meanwhile, the moving of

the UE would lead to the deteriorate of the quality of the cell, therefore, the start

threshold of the compressed mode should satisfy the condition that the UE can finish

the measurement of the target cell and report for handover before call drops happens.

For the stop threshold, it should help to avoid the frequent start or stop of compressedmode.

In dense urban, the continuous coverage of the 3G should be ensured, thus avoiding

unnecessary inter-RAT handovers and increase of system load. For edges of the 3G

network and highways, the UEs should be handed over to the 2G network before the

heavy fading. Under this condition, the trigger threshold of Event 2D should be raised

so that the UE can initiate the compressed mode as early as possible.

3.4.1.3 Maximum DL TX pow er of the Radio Lin k

If large amounts of call drops happen due to coverage causes, then the maximum DL

TX power of the services can be increased appropriately. However, this is at the risk

that the UEs at cell edges may consume too much power, and then affect the other UEs,

and reduce the DL capacity of the system. For cells with a great deal of access failures

caused by excessive load, this parameter can be set to a small value.


30/31

Internal


3.4.1.4 Inter-Frequency/Inter-RAT Handover Thresho ld

The UE can be handed over to the inter-RAT/frequency neighbor cells when the

measured value of the signals from these cells is higher than the threshold. This

parameter can be set combining the start threshold of the compressed mode. If this

parameter is configured with a little value, then the handover can be triggered early. If

this parameter is configured with a large value, then the handover will be prolonged.

3.4.2 Timer and Counter Related with Call Drop

The following table lists the timer and counter related to the UE.

Table 3 Timer and counter related to the UE

Name Description Value Range Default

Value

T312

Connected

T312 in connected mode, and indicates the

time that UE waits from the synchronization

indicator from L1 when it starts to establish

the DPCCH.

(1..15)s 1s

N312

Connected

T312 in connected mode, and indicates the

number of synchronization indicator that the

UE received from L1 before the DPCCH is

established.

(1, 2, 4, 10, 20, 50,

100, 200, 400, 600,

800, 1000)

1

T313 Indicates the waiting time of the UE in

CELL_DCH state after the DPCCH channel is

established.

(0..15)s 3s

N313 Indicates the number of maximum number of

out of sync indicators that the UE receives

from L1.

(1, 2, 4, 10, 20, 50,

100, 200)

20

T314 Start: When the criteria for radio link failure

are fulfilled. The timer is started if radio

bearer(s) that are associated with T314 exist or

if only RRC connection exists only to the CS

domain.

(0, 2, 4, 6, 8, 12, 16,

20)s

4s

T315 Start: When the criteria for radio link failure

are fulfilled. The timer is started if radio

bearer(s) that are associated with T314 exist or

if only RRC connection exists only to the CS

domain.

(0,10, 30, 60, 180,

600, 1200, 1800)s

30s

N315 Indicates the maximum number of (1, 2, 4, 10, 20, 50, 1


31/31

Internal


synchronization indicators that the UE

received from L1 after T313 is activated.

100, 200, 400, 600,

800, 1000)

T309 Indicates the waiting time of the UE aftersends the inter-RAT handover requests.

(1..8)s 3s

Documents

UMTS CS Call Drop Analysis Guide V2.0