omf000404 case analysis-call drop issue1.5

7/29/2019 OMF000404 Case Analysis-Call Drop ISSUE1.5

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Wireless Curriculum Development SectionWireless Curriculum Development SectionWireless Curriculum Development Section

ISSUEISSUE

OMF000404Case Study Call Drop

OMF000404OMF000404Case StudyCase Study Call DropCall Drop

1.51.5


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Principle of call drop

Analysis of call drop

Call drop cases

Course ContentsCourse Contents


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Principle of Call DropPrinciple of Call Drop

! Content:

" Calculation formula of TCH call drop rate and measurement

points.

" Calculation formula of SDCCH call drop rate and measurement

points.


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Principle of Call DropPrinciple of Call Drop

! There are two types of call drop: TCH call drop and SDCCH

call drop:

" TCH call drop means TCH channel is released abnormally after

it is occupied successfully.

" SDCCH call drop means SDCCH channel is released

abnormally after it is occupied successfully.


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Formula of TCH Call Drop RateFormula of TCH Call Drop Rate

! Formula of TCH call drop rate

" TCH call drop rate=TCH call drops / Successful TCH seizures

(all)

! Measurement points of TCH call drop

" When the channel seizures is TCH, BSC originates

CLEAR_REQ to MSC.

CLEAR_REQUEST the message is sent from BSC to MSC, and it shows that BSC require

MSC to release special channel resource.


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Causes of TCH Call DropCauses of TCH Call Drop

! Usually, the typical causes for sending the Clear_Request

message are as follows:

" Radio interface failure

" O&M intervention

" Equipment failure

" Protocol error

" Preemption


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Measurement Point of TCH Call DropMeasurement Point of TCH Call Drop

As shown in the above diagram, major causes for TCH call drop are connection

failure, error indication, Abis failure, O&M manual intervention.

CLEAR_REQ is the clearance request message that BSC sends to MSC.

ERR_IND is the link error message that BTS reports to BSC.


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! Measurement point of Successful TCH seizures

" When CH_ACT_ACK message is received in the immediate

assignment process. Since there is no SDCCH available, TCH

channel is directly assigned.

" When CH_ACT_ACK message is received, the calling status is

CS_WAIT_RR_EST(Waiting RR setup status) and the present

channel is TCH.

" When assignment complete message is sent in process of

assignment.


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! Measurement point of Successful TCH seizures

" When the MSG_ABIS_HO_DETECT message is received

during incoming inter BSC inter cell handover, and the handover

type is non-SDCCH handover.

" When the MSG_ABIS_HO_DETECT message is received

during internal inter cell handover, and the handover type is non-

SDCCH handover.

" When the CLEAR-CMD message with the cause of HO_SUCC

or CALL_CTRL is received from the MSC during inter-BSC

handover , and the handover cause is directed retry.



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! Immediate assignment process

MS BTS BSC MSC

Channel Request (RACH) Channel Required

Channel Activation (TCH or SDCCH)

Channel Activation Acknowledge

Immediate Assignment Command

Immediate Assignment (AGCH)


1. The immediate assignment process is triggered by the random access process . It

is intended to assign a signaling channel for MS to implement signaling transmission

in the call setup phase. Normally SDCCH is assigned. When there is no SDCCHchannel or an emergency call is originated, TCH can also be assigned (as early as

possible). Therefore, when the CH_ACT_ACK is received during immediate

assignment and the type of channel directly allocated is TCH, it will be counted into

successful TCH seizures as a measurement point.


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MS BTS BSC MSC

CH_REQCH_REQ

CH_ACT

CH_ACT_ACK

IMM_ASS

IMM_ASS

SABM

UA EST_IND CONN_REQ

IDLE

WAIT_RR_EST


2. In CS_WAIT_RR_EST status, when the CH_ACT_ACK message is received and

the current channel is TCH, it will be counted into successful TCH seizures times as a

measurement point.

CS_WAIT_RR_EST is one of main call-states. Call states include main call-state and

sub-call-state. Here the main call-state and sub-call-state are combined with each

other to form the call state in the call process processing procedure. Each main call-

state corresponds to one state processing function. Processing of different messages

is related to different sub-states. For CH_REQ processing, it is mainly to apply for a

radio channel and send the CH_ACT (channel activation) command, CCB is still in

IDLE mode.


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! Assignment process

MS BTS BSC MSC

ASS_REQ

CH_ACT

CH_ACT_ACK

ASS_CMDASS_CMD

EST_IND

ASS_CMP ASS_CMPASS_CMP


1. TCH assignment phase mainly involves assignment command and assignment completion.

The calling process starts with BTS channel request and ends with caller TCH

assignment completion.

Access phase mainly involves channel request, channel activation, channel activation

response, immediate assignment and service request. In this phase, MS sets up

a temporary fixed relation with BTS (BSC).

TCH assignment phase mainly involves assignment command and assignment

completion. In this phase, the TCH channel of the original call will be specified.

The terminate call process starts when MS receives BTS paging command and ends

when the conversation starts.

2.MSC assigns a traffic channel to MS through the assignment process. Normalmessage process is similar to immediate assignment process. The difference is

only TCH channel is assigned in the assignment process. Compared with the

original channel get from immediate assignment, if the assigned channel changes

the information type only, it is not necessary to apply for another channel, but

directly start the mode modification process.

If the channel rate type changes, it is necessary to apply for a channel again and

send the assignment command to MS.

If the channel rate type and information type both remain unchanged, return to

ASS_CMP (assignment completion) directly.


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! Intra-BSC handover process

MS MSBTS1 BTS2BSC

Measurement Report from MS

Channel_Active

Channel_Active ACK

HANDOVER COMMAND

HO_ Access

HO_DetectPHY INFO

First SABM

Establish_IND

PHY INFO

Handover CompleteHO_Performed

UA

MSC


Like assignment process, after the handover command is sent, MS will be ready to

access the new channel. At the same time, message and signaling on the original

channel will be forbidden and there is no DTAP message can be sent to MS anylonger.

In the new cell, MS sends the HO_Access message burst to BTS. After detecting it,

BTS will send the MSG_ABIS_HO_DETECT message to BSC and return the physical

information to MS, then BTS will require MS to stop sending the HO_Access

message burst. After that MS will send the SAMB frame and BTS will return the UA

frame and send the EST_IND message to BSC.

In intra BSC handover, when BSC receives the MSG_ABIS_HO_DETECT message,

the handover will be counted into successful TCH seizures times as the

measurement point if its not an SDCCH handover.


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! Inter BSC handover processMS MSBTS1 BTS2BSC1 BSC2MSC

Measure Report from MS

HO_Required

HO_Request

CH_ACT

CH_ACT_ACKHO_Request_ACK

HO_CMDHandover Command HO_Access

HO_DetectHO_Detect

PHY INFO

PHY INFO

First SABMEstablish_IND

Handover CompleteHO_CMPClear_CMD

Clear_CMP

Measurement point of TCH Call DropMeasurement point of TCH Call Drop

In incoming handover process, the channel will be allocated and activated according

to the required channel type, if this succeeds, BSC will send HO_REQ_ACK

(handover request response) to MSC. Then MSC will send the handover command tothe original BSC. After receiving handover command MS will access the network on

new channel, which is similar to handover between cells, and the difference is that

HO_Detect, Establish_Ind and HO_Cmp messages must be sent to MSC in this case.

After the handover is completed, MSC will send MSG_CLEAR_CMD to the original

BSC and release the old channel.

After MSG_ABIS_HO_DETECT message is received during incoming BSC handover,

if the handover is not an SDCCH handover, the handover will be counted into

successful TCH seizures times as the measurement point.


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! Direct retry process

" When the CLEAR-CMD message, whose cause values is

HO_SUCC or CALL_CTRL, is received from the MSC during

inter BSC handover , and at the same time the handover cause

is direct retry, one measurement point will be counted and

added to successful TCH seizures times

Measurement point of TCH Call DropMeasurement point of TCH Call Drop

The assignment often fails due to cell congestion, or no radio resources can be

assigned. However, while the service cell of MS is short of resource, there may be

enough resource in an adjacent cell. To increase the call completion rate and lowerthe call loss rate, the direct retry is designed for GSM system. In case that there is a

shortage of resources in the current service cell, a service channel of an adjacent cell

can be allocated directly to MS. By means of handover process, MS will handover to

an adjacent cell with enough resources, thus implementing the assignment process

process successfully. Therefore, direct retry can be regarded as a type of special

handover.


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Formula of SDCCH Call Drop RateFormula of SDCCH Call Drop Rate

! Formula of SDCCH call drop rate:

" SDCCH call drop rate=SDCCH call drops/ successful SDCCH

seizures*100%

! SDCCH call drop rate = (Number of SDCCH lost connections

(connect failure) + Number of SDCCH lost connections (error

indications) + Number of unsuccessful SDCCH seizures due

to terrestrial links (ABIS)) / successful SDCCH seizures *

100%


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Measurement Point of SDCCH

Call Drop


Call Drop

! Measurement point of SDCCH call drop

" When the CLEAR_REQ and ERR_IND messages are sent to

the MSC, and the channel currently seized is the SDCCH.

! Measurement point of successful SDCCH seizures

" CH_ACT_ACK is received in the immediate assignment process

and the channel type is SDCCH.

" CH_ACT_ACK is received in CS_WAIT_RR_EST status and the

current channel is SDCCH

" HO_DETECT is received during incoming inter BSC SDCCH

handover.

" HO_DETECT is received during intra-BSC SDCCH handover.


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MS BTS BSC MSC

Or:Or:

Channel Request Channel Required

Channel Activation (SDCCH)

Channel Activation Acknowledge

Immediate Assignment CommandImmediate Assignment

Establish Indication (L3 Info)

Connection Failure

Error Indication

Abis FailureCell SDCCH Call Drop

(Subject to different cases)


Call Drop


Call Drop

As shown in the above diagram, main causes of SDCCH call drop are connection

failure, error indication, Abis failure, and so on.


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! The following conditions can lead to SDCCH call drop:

" When the HO_DETECT message is illegal during incoming

SDCCH handover

" When the HO_CMP message is illegal during incoming SDCCH

handover

" When HO_CMP message transfer fails during incoming SDCCH

handover

" When TN_WAIT_HO_DETECT and TN_WAIT_HO_CMP

(SDCCH handover) are timeout

" When TN_WAIT_INTER_HO_CMP(SDCCH handover) is

timeout

" When TN_T8 (Out-BSC handover complete) is timeout

" When internal clearing is caused by other causes


Call Drop


Call Drop


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Call drop cases



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Analysis of Call DropAnalysis of Call Drop

! content

" main causes of high call drop rate

" troubleshooting of high call drop rate


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!According to the definition of call drop measurement point, call

drop is usually caused by the following:

" Radio link fault. During the communication, messages can not

be received correctly.

" T3103 counter is timeout.

" Other system faults (for example, the cooperation between BSC

timer and MSC timer)

! Timers that may cause call drops (BSC timer):

" T3103: starting from sending HANDOVER CMD and ending at

receiving HANDOVER CMP. Time out of the timer will cause call

drop.

" T3109: starting from sending CHAN REL and ending at

receiving REL IND. Time out of the timer will cause call drop.

Analysis of Call DropAnalysis of Call Drop


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Radio Link FaultRadio Link Fault

! Signaling process chart of radio link fault

MS BTS BSC MSC

Measurement Report

Measurement Result

Connection Failure

Clear_REQ (Radio Interface Failure)

(1)

(2)

(3)

(1) Dadicated mode is created. (SDCCH/TCH)

(2) Activate Abis monitoring function.

(3)SACCH message block can not be decoded(uplink/downlink),

resulting in radio link timeout.

Radio link fault: When radio link timeout timer is reduced to 0, the channel will be

released, a call drop will occur, and cause of this call drop will be recorded as a radio

link fault. In network running, such call drops are the most common.


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! Diagram of radio link timeout

Radio link fault means that the communication link is lost during communication.

During communication, due to interference in the system or low receiving level, the

voice or data often becomes too deteriorated to be accepted. And finally as a result,MS or network cannot decode correctly the information sent from an opposite

terminal, moreover this cannot be controlled in any other ways. In this case, the

system will conclude that a radio link fault occurs. In this case, MS will either start call

re-establish or forcedly disconnect the link. Forced link disconnection will lead to a

call drop. Therefore, a radio link fault will be concluded by the system only when the

communication quality is unacceptable.

In Huawei system, in [System Information Table], define the parameter Radio Link

Timeout, which is used by MS to decide when to disconnect the call (downlink) if

SACCH decoding fails. In [Cell Attribute Table], define the parameter SACCH multi-

frame number, which is used by BTS to inform BSC of the radio link connectionfailure (uplink).

In monitoring the radio link fault, the control of the uplink and downlink should be

consistent. The radio link fault algorithms at BTS and MS are thus consistent. That is,

when a special channel is assigned to MS, it will start counter S. Then each time

when an SACCH message cannot be translated, S will reduce by 1, and each time

when an SACCH message is translated correctly, S will increase by 2. When this

value is reduced to 0, radio link failure will be reported.


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T3103 is TimeoutT3103 is Timeout

! Handover process

MS BTS1 BSC MSCBTS2

Handover Indication

CH_ACT

CH_ACT_ACK

Handover CommandHandover Command

Handover Access HO_Detect

SABM

Physical Information (TA)

UA

Handover CompleteHandover Complete

EST_IND

Set T3103

Reset T3103

Counter T3103 timeout leads to a call drop.

When BSC sends the Handover Command to BTS, counter T3103 will start counting.

When receiving Handover Complete from the handover destination cell or Handover

Failure from the original cell, BSC will reset counter T3103. After sending the

Handover Command to BTS, if BSC still can not receive the message after counter

T3103 is timeout, it will conclude that there is a radio link failure in the original cell

and then release its channel.


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Analysis of Causes of Radio Link FaultAnalysis of Causes of Radio Link Fault

!Analyze the causes of the fault of radio links. The causes can

be:

" Interference

# Internal interference, external interference and the equipment

interference

" Poor coverage

# Coverage hole, isolated island, uplink/downlink unbalance

" parameter setting not proper

# radio link timeout counter, SACCH multi-frame number, handover,

power control etc.

" Equipment problem (Antenna---Feeder---CDU---TRX)

" Clock problem

" Transmission problem


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Radio Link Fault - interferenceRadio Link Fault - interference

! Interference

" Co-channel interference

" Adjacent-channel interference

" Inter-modulation interference and other external interference

Interference mainly includes co-channel, adjacent-channel and inter-modulation

interference and external interference.

When MS receives intensive co-channel or adjacent-channel interference signals in

the service cell, BER will be deteriorated and will resulted in call drop.

When there is serious inter-modulation interference in BTS, it will result in call drop.


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! Solution

" First check equipment problems.

" Make an actual drive test, check the interference area and

distribution of signal quality. Find the interference frequency.

" Further search for the interference source with the spectrum

analyzer.

" Activate hopping, DTX and power control functions to rapidly

lower the internal interference of the system


Equipment problems: Due to self-excitation of TRX or tower-mount amplifier, the

system noise coefficient becomes larger and the sensitivity is deteriorated. Inter-

modulation of the antenna is also an equipment problem.

Analyze according to driving test data: Interference area, signal quality distribution

and the overlapping that cause interference.

Adjust the downtilt of BTS antenna, transmission power, relation between adjacent

cells, handover parameter of the relevant cells or adjust the frequency planning to

avoid interference.


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! Judgment Process

" Analyze the occurring laws of interference band in the

traffic measurement.

" Observe the receiving level performance

" Find the poor quality handover rate

" Observe receiving quality performance

" Observe call drop performance

" The handover fails, calling re-establishment also fail too many

times.

1. 1. If there are many data in the interference band 3, 4, and 5, usually

the interference problem should be taken into consideration.

The interference band is reported to BSC by the BTS -- through observation of how

the uplink channel is interfered -- via the RF resource indication message when the

carrier channel is idle. It should also be noted that the interference band condition of

this channel will not be shown in the traffic measurement when the channel is blocked

or busy.

2. If there are too many times of high level and low quality, it means that there is co-

frequency and adjacent frequency interference or external interference at the

frequency of this TRX.

3. In cell measurement function/inter-cell handover measurement function, or

outgoing inter-cell handover measurement function, outgoing handover attempts dueto various causes are measured. If there are too many times of handover caused by

low quality, this indicates possible interference. The handover times due to low uplink

and downlink quality gives proof of how the uplink and downlink are interfered.

4. Measure the average receiving quality level of TRX for reference.

5. Record the average level and quality upon call drop for reference.

6. There may exist interference in the target cell, just for reference.


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Radio Link Fault - CoverageRadio Link Fault - Coverage

! Coverage:

" Coverage over shooting

" Coverage hole

" Signal attenuation

" Incomplete definition of adjacent cells

" unbalance of uplink/downlink

1. Isolated island phenomenon.

For some reasons, the service cell covers a too large area, including the adjacent

cells. As a result, after MS goes beyond the coverage scope defined for the adjacent

cell B and reaches cell C, it still occupies the signal of the original service cell A.

However, cell A does not define cell C and at this time MS will perform handover

according to the adjacent cells table provided by the original service cell A. In this

case, call drop will be occurred because the appropriate target cell cannot be found.

(Isolated island phenomenon)

2. There are clear-cut signal coverage hold at the common boundary between cells.

3. Signal fading: Serious fading occurs during signal propagation so that handover

cannot be implemented in time and this causes a call drop.

4. The adjacent cell definition is incomplete so that MS keeps conversation in the

current cell until it goes beyond this cell coverage edge and as a result, call drop

occurs.

5. If the uplink signal coverage is larger than the downlink signal coverage, the

downlink signal of the cell edge will become weak and can easily be submerged by

the intensive signals of other cells.

If the downlink signal coverage is larger than the uplink signal coverage, MS has to

remain under this intensive signal. However, if the uplink signal is too weak or the

voice quality is too bad, call drop will occur.


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! Judgment Process

" Power control measurement function

" Receiving level measurement function

" Cell measurement function/inter-cell handover measurement

function

" Call drop measurement function

" Defined adjacent cell measurement function

" Undefined adjacent cell measurement function

" Outgoing inter cell handover measurement function

" Up-down link balance measurement function

Driving test is the most direct means to track down a coverage problem. Traffic

measurement can be another useful method.

1. In power control measurement function, the average uplink and downlink signalintensity is too low.

2. In receiving level measurement function, the proportion of low receiving level times

is too large.

3. In cell measurement function/inter-cell handover measurement function, the level

when originating a handover is too low and the average receiving level is too low.

4. In call drop measurement function, the level during a call drop is too low and the

TA value before a call drop is abnormal.

5. In undefined adjacent cell measurement function, the undefined adjacent cell

average receiving level is too high (over-shooting coverage).

6. The average level of undefined adjacent cells is too high (isolated islandphenomenon).

7. In power control measurement function, the Max. distance between MS and BTS in

multiple consecutive time segments exceeds the normal value.

8. In out-cell handover measurement function, the handover success rate to an

adjacent cell is low.

9. Register uplink and downlink balance measurement function in traffic

measurement and analyze whether uplink and downlink are virtually unbalanced.

Whether uplink and downlink are balanced should be checked depending on the

system performance indices.


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! Solution

" Adjust network parameter

" Add BTS


1. According to traffic measurement analysis, make sure that the cell has a high call

drop rate and a high outgoing inter-cell handover failure rate. And most handovers

are caused by low signal level or bad quality, but all other indices are normal. If this isthe case, it is necessary to check whether there exists the coverage problem. The

driving test can be feasible to find out the areas with insufficient coverage. Analyze

whether it is caused by landform reasons, such as tunnels, large shopping centers,

entrances of subway. Generally speaking, most of this kind of call drops occur in one

direction and this problem can be solved by adding a micro-cell.

2. Find out the areas with insufficient coverage. The BTS coverage can be increased

by adding new BTS or in some other ways, increasing the Max. BTS transmission

power, adopting the zero fill antenna, and changing azimuth and downtilt of the

antenna.

3. Restrict the BTS coverage scope. In terms of hardware, adjust BTS downtilt or

antenna height. As for software, lower the Max. transmission power or increase the

Max. receiving level (RXLEV ACCESS MIN) and RACH min.access level parameters

so as to restrict the coverage range.

4. Check whether the adjacent cells are defined completely and whether they are in

mutual symmetrical relation. Pay more attention to external adjacent cell data.


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! Solution

" Adjust network parameter

" Add BTS

5. Judge whether the uplink and downlink are balanced.

Check the tower-mount amplifier, power amplifier, antenna feeder and

combiner, and check the alarm condition to make comprehensive analysis.

Check the BTS antenna. If the diversity receiving antenna is connected

inversely, the diversity receiving gain of the antenna will be greatly reduced

and the uplink signal will become worse than the downlink signal.

Observe the MA10 measurement report. Check BSC data configuration (MS,

BTS transmitting power, the RXLEV_ ACCESS_MIN).

According to the TA value, check whether BTS coverage scope is too large or

whether the signal is reflected. In this case, some weak uplink MS signals will

be dropped out very easily after accessing the network.


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! Main parameters that may affect the call drop rate:

" Radio link timeout counter and SACCH Multi-Frames

" RACH busy threshold and RACH minimum access level.

" MS minimum receiving signal level

" Call re-establishment permitted.

" Network color code (NCC) permitted

" Frequency planning parameters

" Handover related parameters.

" Power control related parameters.

1. Radio Link Timeout and SACCH Multi-Frames

When these two parameters are too small, the radio link fault is easily occurred and

thus call drop will happen.

If difficult access or serious call drop is caused by too bad downlink radio

environment, properly increase the value of the Radio Link Timeout parameter. If

difficult access or serious call drop is caused by too bad uplink radio environment,

properly increase SACCH multi-frames.

2. RACH min. access level

It indicates the level threshold value that the system uses to judge MS random access.

It is used to control uplink access (call, paging response, handover). Adjustment of

this parameter can get a compromise between coverage and call drop rate.

3. RXLEV_ACCESS_MIN

RXLEV-ACCESS-MIN indicates the Min. receiving signal level for MS to access the

system.

If this parameter is too low, the requirement of the access signal level will also be low.

As a result, many MS will try to reside in this cell and the cell load will be increased

and call drop be more frequent occurred. Therefore, it should be set according to the

balance condition between uplink and downlink.


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! Main parameters that may affect the call drop rate:

" Radio link timeout counter and SACCH Multi-Frames

" RACH busy threshold and RACH minimum access level.

" MS minimum receiving signal level

" Call re-establishment permitted.

" Network color code (NCC) permitted

" Frequency planning parameters

" Handover related parameters.

" Power control related parameters.


4. Call re-establishment permitted

In case of the call drop due to a radio link fault because of sudden interference

or building coverage hole, MS will start the call re-establishment process torecover conversation. Call re-establishment enabled can lower the average

call drop rate, but it takes longer time .It applies to suburbs or city areas with

bad coverage.

5. NCC permitted

NCC permitted, NCC permitted is sent in system messages 2 and 6. It lists the

combinations of cell NCC codes that MS must measure. MS will not report the

adjacent cells measurement report whose NCC is set as 0, if this parameter is

not set properly, call drop will be caused for MS cannot normally originate

handover in dedicated mode.

6. Set handover-related parameters properly to lower the call drop rate.

For example: PBGT handover easily take place, so as to avoid interference

and lower the call drop rate. Set the emergency handover threshold properly

so that emergency handover can be triggered before a call drop so as to

reduce call drops.

7.Optimize the power control parameter to make it more sensitive. In addition,

the level after power control must be intensive enough to ensure normal

conversation.


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! Judgment process

" System information table

" Cell attribute table

" Radio link connection counter(T3105)

" Max. retrans times of physical information

" Call drop measurement function

" Judge from the cause of call drop: error indication and

connection failure.


System information table: Radio Link Timeout (effective on MS), 32 or 56

recommended.

Cell attribute table: SACCH multi-frames, 31 recommended..

Radio link connection timer, Max. retrans times of physical information is related to

handover call drop.

Find out call drop causes through call drop rate performance traffic measurement

Judge from the cause of call drop: The cause error indication (when the cause value

is 0x01)is usually caused by T200 timeout.


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Handover ProblemHandover Problem

! Judgment process :

" 1. Inter-cell handover measurement function: it occurs frequently

that the handover fails and the calling re-establishment also fails.

" 2. Inter-cell handover measurement function: handover occurs

many times and re-establishment succeeds many times.

" 3.Undefined adjacent cell measurement function: level of the

undefined adjacent cells and number of reports.


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! Judgment process

" 4. Outgoing inter-cell handover measurement function: the

successful rate of outgoing inter-cell handover is low (for a

certain cell). Find the adjacent cell where the handover

successful rate is low and find the cause.

" 5. Incoming inter-cell handover successful rate is low. The

handover judgment parameter setting of the target cell is

improper.

" 6. TCH measurement function: handover times are not in

proportion to the successful times of TCH call attempt.

(handover/call>3)



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! Solution:

" Add adjacent cells as necessary

" Adjust improper handover parameters

1510Min. DL level candidate cell

6070UL Qual. Thrsh.

6872PBGT HO Thrsh.

34PBGT valid time

45PBGT watch time

Handover

Value after

change

Value before

changeParameter name


Add adjacent cells reasonably.

Adjacent cell missing is the ultimate cause for unavailable handover and call drop in

the end.

Adjust improper handover parameters.

Common adjustment parameters: Min. DL level on candidate cell, Min. access level

offset , Inter-cell HO hysteresis, PBGT Thrsh., various PN judgement duration

time, and Edge HO UL RX_LEV Thrsh..


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Power Control ProblemPower Control Problem

610

Filter Length for Stable

RX_LEV

58PC interval

8050UL RX_LEV

compensationPower

control

Value after

change

Value before

changeParameter name

Power control parameter adjustment:

Modify the power control parameter to make the power control more sensible, and

make the level after power control intensive enough to ensure normal conversation.

For example : If the uplink expectation level is too low and the filter length is too long,

power control will become insensible.


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Equipment ProblemEquipment Problem

! Call drop arising from equipment problem

" Hardware problem


" Antenna and feeder fault

" Other causes

Popular methods of judging call drop resulted from equipment problems

A. Hardware troubleshooting

1. For call drop due to hardware problems, check relevant hardware alarms in OMC.If

there is no hardware alarm information in OMC, there may be something wrong with

the TRX or diversity receiver part. The allocation failure rate and the uplink and

downlink quality handover rate at this time are certainly very high. In this case,

monitor Abis interface or block all other TRX in the cell to perform calling test on the

suspected TRX to locate the fault. Generally speaking, when there is something

wrong with the frame processing unit, the assignment failure rate and uplink and

downlink quality handover will be quite serious. When there is something wrong with

the receiving part, the assignment and uplink quality handover will be relatively

serious. When there is something wrong with the transmission part, the assignment

failure rate and downlink quality handover will be quite serious. It is also advisable tojudge whether there are some problems with TRX according to the TCH assignment

failure rate. When there are some problems with TRX, usually the TCH congestion

rate and access failure rate are very high, monitor Abis interface to locate the faulty

TRX (whose equipment No. is TEI).

Analyze according to the attempt ratio of handover trigger causes. If there are some

hardware problem in the uplink, the receiving level of the MS seizing this TRX will be

very weak and the receiving quality will also bad. When the downlink quality or the

ratio of handover triggered by the level is high, the fault may exist on the downlink

equipment hardware, such as TRX, power amplifier, transmission antenna or feeder.

When the ratio of handover triggered by the uplink quality is rather high, the fault mayexist on the equipment hardware of the radio antenna uplink.


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" Hardware problem



" Other causes


2. In addition, the hardware fault may also generate internal interference that leads

to a call drop

Due to possible mistake in TRX or performance decrease of some parts, TRXamplification circuit self-activation may occur, thus internal interference will be

generated.

CDU or SPL fault. The active amplifier is used in CDU divider or SPL. If the

performance is not good, self-activation interference can also be generated easily.

If the out-band stray of the power amplification module exceeds limits, or the

separation of receiving and sending of the duplexer in CDU is too small, the

interference on the receiving channel will occur. In addition, if CDU intermodulation

indices do not meet requirements, the intermodulation component of transmission

signals will also enter the receiving channel to generate interference.

For interference caused by hardware problems, we can start with various sections

of the radio signals to find out the cause for interference with the exclusive method.

BTS RF signal path: outside - antenna - feeder - CDU - TRX.


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" Hardware problem



" Other causes

Interference may be generated in any link of this path. The spectrum analyzer can

be used to test step by step from bottom to top, so as to identify the interference

source. Here well take BTS3X as an example to see the basic procedure for

locating the uplink interference.

1. Register traffic statistic, mainly TCH measurement function, cell measurement

function, cell frequency scanning, up-downlink balance measurement function. The

period of traffic statistic can be set as 30 minutes or even shorter.

2. Leave one TRX and block all others in this BTS to observe the traffic statistic

result. This step is intended to view whether it is the intermodulation interference. If

the interference band disappears, it means that it is the intermodulation interference,

perform step 6. If the interference band does not disappear, perform step 3.

3. Disconnect the main/diversity input cables of TRX and connect the dummy load

(existing at the unused receiving ports of CDU), observe the interference band

reported by Abis interface. If the interference band is high, it means that the

interference comes from the TRX and this TRX should be replaced. If the

interference band is completely within the interference band 1, it means that the

interference comes from before TRX, perform step 4.


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" Hardware problem



" Other causes

4. Recover the connectors and cable of the TRX and connect the power meter and

dummy load to the CDU connection input port (TX/RX). Set the tributary input

signal of main receiving as 0 while absorbing its output power. And at the same

time, disconnect the CDU diversity receiving cable, connect the matching load, and

also set its input signal as 0. Observe the interference band reported by Abis

interface. If the interference band is very high, it means that the interference comes

from CDU, which should be replaced. If CDU and TRX replacement takes no effect,

there may be some problem with the BTS clock. Check TMU13M clock, the clock

bus between TMU and TRX, the clock-matching DIP switch and the rack-top clock

matcher. If interference bands fall completely within interference band 1, it means

that the interference comes before CDU, then perform step 5.

5. Recover CDU connectors and cable, disconnect the RF jumper of the rack-top

TX/RX and RXD of this cell, and connect the matching load to the rack-top TX/RX

and RXD ports. Then observe the interference band reported by Abis interface. If

the interference band is very high, it means that the interference comes from the

RF cable between CDU and the rack-top ports, which should be replaced. If all

interference bands fall within interference band 1, it means that the interference

comes before rack-top, and perform step 6.


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" Hardware problem



" Other causes

6. Unblock all TRX, interchange the antenna & feeders of this cell with the adjacent

cell (where there is no interference) at the rack top, then observe the interference

band reported by Abis interface. If the interference band is very high, it means that

the interference comes from BTS internal intermodulation interference. In this case,replace CDU and TRX. If the interference band is completely within interference

band 1, it means that the interference comes from steps before antenna or feeder,

perform step 7.

7. Change the frequency and observe the interference band reported by Abis

interface. If the interference band is still very high, it means that the interference

comes from the BTS antenna & feeder or broadband external interference. In this

case, check the antenna & feeder, and replace the antenna & feeder or use the

spectrum analyzer and directional antenna to find the external interference source. If

the interference band is completely within interference band 1, it means that the

interference comes from the internal co-frequency and adjacent frequency

interference. In this case, perform frequency optimization.

B. Transmission troubleshooting

If the transmission link is unstable, it will cause signaling loss and low-layer link

instability, and even call drop. Many Abis call drops can be avoided if stable

transmission quality is ensured.

Fault messages of Abis interface are: BSC does not receive the measurement report

from BTS, some signaling failures during handover and internal causes, or BER

influence on Abis interface.

There are less A interface failures, which are mainly handover (between BSC or

MSC) failures. The reason is that the data for handover are improper, or the target

cell can not be accessed via handover.


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" Hardware problem



" Other causes


C. Antenna and feeder troubleshooting

Judge call drop due to antenna and feeder problems

1. Azimuth and downtilt of the antenna

If the azimuth of these two antennas of one sector are different, possibly the MS can

receive SDCCH, MS maybe call drop because of the TCH signal strength very poor.


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! Judgment process

" TCH measurement function

# TCH availability abnormal.

# Excessive call drop and disconnection times of terrestrial links.

" If a cell always suffers from high call drop rate and congestion

rate, some equipment in this cell may be faulty.


Solutions:

Observe transmission and board alarms (TC board fault, A interface PCM loss alarm,

LAPD link disconnection, power amplifier, HPA, TRX board alarm, and CUI/FPU

alarm). According to the alarm data, analyze whether transmission is interrupted or

there is a faulty board (such as transmission grounding mode, TRX damage or poor

contact).

The fault can be located by blocking TRX and registering temporary traffic statistics

tasks (call drop rate and congestion rate).


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Call drop cases



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Call Drop Case 1Call Drop Case 1

! Fault Description

" The BTS distribution of an area is as illustrated in the diagram

(red numbers stand for BCCH frequency. No hopping, DTX).

Some subscribers complained that call drop in second sector of

base station C is serious. (Hardware fault is ruled out).

please confirm

whether the

frequency

distribution inthe BTS cells

are correct?

Frequency planning principles:

1. Same frequency cannot exist in the same BTS.

2. The frequency interval of BCCH and TCH in the same cell should better be above

400K.

3. When there is no frequency hopping, the frequency interval between TCHs in the

same cell should better be above 400K.

4. In non- 1*3 frequency reuse mode, the direct adjacent BTS cannot have the same

frequency (even if the directions of the antenna main lobes are the same, the

interference of side lobes and back lobes cannot be predicted easily due to antenna

and environment causes).

5. In consideration of the complexity of antenna height and propagation environment,

the two opposite cells cannot be arranged same frequency.

6. Usually, 1*3 frequency reuse should ensure that the number of hopping frequency

should be more than two times of the carriers participating hopping.

7. Make sure to avoid the situation that the same BCCH or BSIC exists in adjacent

areas.

8. Enable PBGT handover. After the adjacent frequency interference is under-

controlled through parameter adjustment, the adjacent frequency can be used in

direct adjacent cells.


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!Analysis

" From the analysis of BTS topology, it can be conclude that the

frequencies are well planned.

" Next step: Check the interference band of traffic statistic.


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!Analysis

09:00~10:00 IB1 IB2 IB3 IB4 IB5

cell 1: 2.85 14.25 1.14 0.27 0.54

cell 2: 4.09 12.57 3.14 0.03 0.01

cell 3: 0 2.92 13.27 0.25 0.37

03:00~04:00 IB 1 IB2 IB3 IB4 IB5

cell 1: 2.85 4.28 0.00 0.00 0.00

cell 2: 4.09 2.89 0.00 0.00 0.00

cell 3: 0 2.12 0.00 0.00 0.00

Why the uplink interference can be confirmed when there is an interference band as

high as of level 3 - 5?

1. Concept of interference band: BSS measures the uplink characteristics of the radio

channel , and calculates and reports the channel interference condition of Idle

channel, thus to provide judgement bases for BSC to assign channels. Interference is

divided into six levels artificially according to the intensity of interference signals. The

level amount used to divide these levels is called interference band threshold.

2. When there are many channels working in interference mode, it can be concluded

that there is interference in the system. Therefore, the interference band

measurements result can be used as the basis for various threshold settings and

interference analysis.

3. For internal interference, generally the interference band increases with the traffic

while external interference has little bearing on the traffic (of course, the interference

caused by the TACS network will also vary with the traffic).


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! Troubleshooting

" 1.Actual drive tests: It is found that the quality is bad when the

receiving strength is high.

" 2.Check traffic statistic: It is found that when the call drop rate is

high, the handover is mostly caused by quality problems and

channel assignment failure rate is also high.

" 3.The conclusion is interference from the analysis of

comprehensive traffic statistic and drive test.


1. Interference bands as high as of level 3-5 appear.

2. High congestion rate. During signaling transmission, since the signaling channel

suffers from external interference, SDCCH or TCH assignment fails.

3. The call drop rate is far higher than the normal. Due to external interference, too

high BER will lead to handover failure.

4. High BER. Sometimes even if the uplink receiving level is up to -70dBm, the

receiving BER may also be bigger than 12.8%.

5. Check the traffic statistic of handover causes to make judgment

When there are many handovers triggered by uplink signal quality deterioration, it can

be judged as to be caused by uplink interference or hardware fault.

When there are many handovers triggered by downlink signal quality deterioration, it

can be judged that downlink interference or hardware fault cause it.

If there are many handovers triggered by both uplink and downlink signal quality

deterioration, hardware fault should be ruled out firstly, and then check interference.


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! Troubleshooting

" 4.A site investigation shows that the operator has a repeater . It

is a broadband repeater . It transmits the signals from a remote

TACS site through optical fiber for amplification and sends it. In

this way, digital signals are amplified and then there is

interference in second sector of base station C.

" 5.Fault has been located: Interference causes the call drop.

The repeater will amplify the interference signals while amplifying the desired uplink

and downlink signals of BTS. As a result, the signal quality will decrease and finally

call drop will occur. The accompanying phenomenon is the obvious increase ofchannel assignment failure rate.


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! Troubleshooting

" Finally, lower the power of the repeater. The interference band

reduce to IB1. Now the high call drop rate problem at site C is

solved.


The repeater is widely used in early phase of network construction to extend the BTS

coverage. Due to its own characteristics, it will interfere the BTS when being used

improperly. The repeater has the two following interference modes:

1) As installation of the repeater doesnt conform to relevant standard, there is no

enough separation between the host antenna and transmit antenna. As a result, self-

activation is formed and the normal work of the BTS which this repeater relies to is

affected.

2) Since the repeater is a broad frequency band and non-linear amplifier, its

intermodulation indices are far larger than protocol requirements. If the power is too

large, its intermodulation signal will also be strong and it can interfere adjacent BTS

very easily.

Why the repeater can interfere the BTS: When the uplink background noise level of

the repeater host is too large, the BTS channel will be blocked.


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! Common methods of checking and clearing call drop due to

interference

" 1. Rule out the internal interference caused by equipment

problems and check the separation of BTS transceivers,

antenna feeder installation, and so on.

" 2. Check the interference band

" 3. Driving test

" 4. Check traffic statistic of handover causes to get judgment

" 5. Clear uplink interference

" 6. Clear downlink interference


Clear uplink interference

Currently this is the major interference, which mainly occurs in traffic peak time and

originates from co-channel interference, or external interference. Co-channel

interference is related to the traffic of the co-channel cell. If the traffic is high, the

interference will also be high. External interference is mainly intermodulation

interference. The uplink interference can be analyzed and cleared by modifying the

co-channel frequency of the co-channel cell base on analyzing related reports in

driving test, increasing the distance between two co-channel cells. Interference can

also be reduced by diversity receiving and effective power control.

Clear downlink interference

Downlink interference are mainly co-channel interference and adjacent-channel

interference of some cells due to inappropriate frequency planning. The interference

source can be found out with the spectrum analyzer.


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! Common methods of checking and clearing call drop due to

interference

" 7. Check whether DTX, frequency hopping technology and

power control application are reasonable

" 8. Use PBGT handover algorithm flexibly to avoid co-channel

and adjacent-channel interference effectively.



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! Fault Description

" 1 3 RF hopping is used in a specific site. After expansion, TCH

channel assignment failure rate is continuously high (due to

radio link fault), accompanied by high TCH call drop rate and

incoming inter cell handover failure rate. SDCCH call drop rate

is normal.

The channel assignment failure rate and incoming handover

failure rate are high, what are the causes?

Judge whether there is some problem with TRX according to the TCH assignment

failure rate. When there is something wrong with TRX, generally the TCH congestion

rate will be very high and the assignment failure rate will also be rather high. MeasureAbis interface to find the faulty TRX (equipment No. is TEI).

When there is something wrong with the receiving part, the assignment failure rate

and the uplink quality handover will be relatively serious.

When there is something wrong with the transmitting part, the assignment failure rate

and the downlink quality handover will be relatively serious.


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!Analysis

" Since assignment failure is accompanied with high call drop rate

and incoming inter cell handover failure rate, the causes may

be as follows:

#A problem occurs when TCH channel is assigned

# The timeslot seizures in communication is not stable or affected by

interference

" Since SDCCH call drop rate is normal, it is unlikely that the

interference comes from the carriers of BCCH. Accordingly, the

TCH carrier of and hopping frequency may attribute to the

interference.


Therefore, first check whether there is something wrong with the equipment and then

judge the interference by the traffic statistic analysis, driving test data analysis and

parameters checking.


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! Troubleshooting

" Check the equipment hardware, antenna and feeder, the

transmission stability. No problem is found.

" In the driving test, it is found that high level and bad quality

problem is very serious.

" Make dialing test nearby this site , it is found that

communication quality is bad.

"

Check the parameter, it is found that the MAIO of the newcarrier is the same as that of another carrier.

" The fault: There exists conflict of hopping.


Frequency hopping can effectively improve the quality of radio signal, especially the

quality of slowly moving MS. The reason is that frequency hopping can make the

transmission carrier jump on the basis of burst pulse sequence, so that the co-channel interference and frequency selective fading effect can be decreased greatly.

However, note whether there is conflicting frequency hopping offset (MAIO) between

TRX in one cell. The unreasonable setting of relevant parameters such as MA HSN

can also lead to call drop.


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! Troubleshooting

" After setting a new value for the MAIO of the new carrier, the

related indices such as call drop rate is normal.

What other hopping parameters may cause the

co-channel or adjacent-channel conflict?


MAIO (Mobile Allocation Index Offset)

Position: Radio channel configuration table.

Value range: 0 ~ (N - 1). Here N is the number of frequency in MA. Pay attention to

the hopping modes supported by the BTS when planning MAIO and avoid co-channel

and adjacent-channel conflicts under the same cell of the same BTS.

HSN (Hopping Sequence Number)

Position: Hopping data table

Value range: 0 ~ 63. Here 0 represents the cyclic hopping (0 is prohibited for some

devices) and other values represent pseudo random hopping. In principle, all

channels of all TRX in the same BTS must use the same HSN, so as to avoid co-

channel and adjacent-channel conflicts by properly setting MAIO. Since theconsistency of frame numbers cannot be ensured in different BTS, keep HSN of

different BTS different to minimize conflict possibility, especially when the same

hopping frequency group is used.


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! Conclusion

" It is important to check frequency planning and parameter

configuration for solving frequency interference.



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! Fault description

" In a driving test, it is found that an MS occupies a cell, but it can

not make an original call. Communication is unidirectional. Call

drop also occurs frequently at a specific distance from the cell

after frequently handover.

AnalysisIt may be caused by ?


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!Analysis

" The problem as mentioned above is usually caused by

uplink/downlink unbalance

! Troubleshooting

" Make a driving test : MS move to the cell boundary. At the same

time, trace and capture data at the BTS side with a MA10

signaling analyzer. (see the figure below).

A radio link has two directions: uplink and downlink. The actual coverage range

should depend on the weaker one. If the uplink signal coverage is larger than the

downlink signal coverage, the cell edge downlink signal will be relatively weak andeasy to be covered by intensive signals of other cells. If the downlink signal coverage

is larger than the uplink signal coverage, the MS will be forced to stay under this

intensive signal. However, if the uplink signal is too weak, MS will not be able to

originate a call or it may lead to bad quality or even call drop.


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! Capture data with MA10


Compare the receiving levels of BTS and MS respectively and observe whether the

downlink signal is still good enough for MS to stay in this cell when the uplink signal

reaches the lowest receiving threshold level.


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! Troubleshooting

" Check whether the service area of the cell is too large.

" When the uplink power control is enabled, improper setting of

power control parameters will also cause obvious unbalanced

link.

# First confirm that the static power level of MS is set properly (900 is

level 5 and 1800 is level 0).

#An investigation shows that, in the System Information Table, no

matter it is a 900 cell or a 1800 cell, the corresponding MS Max.

transmitting power levels are all set as 5. In this case, the

corresponding DCS1800 MS output power is much more lower than

its MAX. transmitting power 1W (30dBm).

In areas with very complicated landforms, make power control as sensitive as

possible. When the power control makes the power of MS decrease and the uplink

signal turns poor suddenly, call drop will occur due to too weak uplink signal if thepower control is very dull at this time.


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! Troubleshooting

" Adjust Max. transmitting power levels of GSM 1800 cell,

the parameter is changed to 0, the problem is solved.


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! Conclusion: solution for call drop due to unbalanced

uplink/downlink

" If the cell coverage is too large, reduce the BTS transmitting

power or increase MS access threshold and handover threshold

of the cell.

" Higher the uplink compensation factor and shorten the stable

signal filter.



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! Fault description

" The call drop rate in cell 3 of a BTS is 10%, but call drop rate

and congestion rate in cell 1 and cell 2 are normal.


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!Analysis

" Check the related traffic statistic

# Check whether there is high interference band in TCH

measurement function.

# Check the situation of call drop in call drop measurement function.

# Check whether handover of the cell is normal.

" Check whether there is interference through checking frequency

planning, moreover confirm whether there is external

interference with spectrum analyzer.

" Driver test

" Check the hardware



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! Troubleshooting

" 1.The congestion rate always is quite high no matter which

channel is blocked.

" 2.Check and analyze the traffic statistic, interference band and

traffic volume and call drop rate. They are all regular.

" 3. Change frequency. The frequency interval of cell 3 is changed

to 1M . But the problem persists.

"

4. Judge whether the equipment is faulty." 5. Locate external interference.


1. Because no matter which channel of this cell is blocked, the congestion rate is

always relatively high. There can be interference or the terrain in the coverage range

of the cell is possibly complicated.

2. It is concluded that, by viewing and analyzing the traffic statistic data, the

interference band of cell 3 basically stays at 4 or 5 in daytime, and it stays at band 1

or band 2 between 23:00 FM and 7:00 AM. In addition, the call drop rate and the

interference band are regular.

3. First take co-channel and adjacent-channel interference into consideration. Change

the frequency. The frequency interval of cell 3 is changed to 1M . But the problem

persists.

4. Then consider the equipment problems. Interchange the antenna& feeder of cell 3

with that of cell 1, but cell 3 interference remains the same. Therefore, it can basicallybe concluded that there is no problem with the BTS devices below the antenna and

feeder. After the above possibilities are excluded, the fault can be located as external

interference.


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! Troubleshooting

" 6. Make a scanning test with a spectrum analyzer.

#A suspect signal with 904.14 center frequency, 300K bandwidth is

found. It is similar to an analog signal and it exists continuously.

#At the distributor output port of cell 3, the signal strength is 27dBm.

cell 2 is 40dBm, cell 1 is 60dBm. It accords with the degree of

interference.

# Traffic volume is higher in the day time than that at night.

" Now the problem is found: 904M external interference source.


1. Although there is a 10M distance between this frequency band and that used in this

cell, it is a continuous signal and it can be more possibly to conflict and

intermodulation with other signals. Some parts of intermodulation components mayfall within the receiving band, thus forming interference.

2. In daytime the traffic is larger than that at night so the intermodulation

components(interference) are also more than those at night..


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! Conclusion: solution of interference

" Solve internal interference through checking frequency

planning.

" After internal interference is excluded, we can locate external

interference with spectrum analyzer.



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! Fault description

" Subscriber complained it is often call drop from the 5th floor and

above in a building.

Subscriber complaint is also an important source of information

about the network quality.


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! analysis

" Step 1: Perform on-site test

# There are call drops and noise on the site

# The test mobile phone shows that before the call drop the serving

cell is BTS-B. we have known this building should be covered by

BTS-A.

" Step 2: Check traffic statistic

# Make sure that the BTS-B cell is about 9 kilometers away from this

building. It is determined that the BTS-B signal received in this area

is coming from some obstacles reflection. Thus an isolated island

coverage is formed in this area.


It is found that, after multiple on-site dialing tests, there really exist call drops and

noise. However, it can be seen from the test MS that MS always stays in a service

cell of a non-local BTS A before call drop, and its TA value is about 17, and thereceiving signal strength is about -80dBm. Therefore, it can be concluded that MS

stays at the second cell of a BTS about 9km away from the dialing position.

Thus, a coverage area like an isolated island is formed in this area.


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!Analysis

" Step 3: Check data configuration

# In BSC data configuration, BTS-A is not configured as the

adjacency of BTS-B

" Cause analysis of call drop

# When the MS uses the signal of cell 2 of BTS-B in this area, the

signal of cell 3 of BTS-A is strong. But cell 2 of BTS-B and cell 3 of

BTS-A are not adjacent, therefore, handover fails.

# The signal in cell 2 of BTS-B is the result of multiple reflections.

When the signal of BTS-B received by the mobile phone is reduced

suddenly, an emergency handover is needed. But there is no

adjacent cell of BTS-B, so call drops will occur.



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! Troubleshooting

" Modify the data in BA1 table, BA2 table and Adjacent Cell

Relation Table.

" Set cell 3 of BTS-A as an adjacent cell of cell 2 of BTS-B.

" Optimize the network parameter to eliminate the isolated island.

" The test results show that the call drop problem is solved.



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! Conclusion two methods to solve isolated island problem

" Adjust the antenna of the isolated cell, to eliminate the isolated

island problem.

" Define new adjacent cells for the isolated cell.

Isolated island problem: When there is an isolated coverage island from a cell in an

area, if MS stay in this cell at the island area and make a call. No matter how the

signal changes, handover cannot be implemented normally until a call drop occurs.To avoid such situation, two means can be used. The better one is to adjust the

antenna of the cell to eliminate the isolated island phenomenon. However, due to the

complexity of radio propagation, usually multiple experiments are required to

eliminate the isolated island effect while the coverage area is not obviously affected.

In addition, it is difficult to completely eliminate the isolated island phenomenon of

high buildings. The another means is to define new adjacent cells for the cell with

isolated island.


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! Fault description

" In a drive test from A to B, it is found that there are many call

drops at the tunnel near the BTS due to slow handover.


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!Analysis

" The tunnel is near the BTS. When the MS enters the tunnel, the

power of the target cell is -80dBm. But the signal of source cell

goes down quickly to less than -100dBm. Before the MS enters

the tunnel, the downlink power of the two cells is good and no

handover is triggered. When the MS enters the tunnel, the level

of the source cell goes down rapidly. The call drop occurs before

any handover is triggered.

Think it over: How to solve problems of this type?


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! Troubleshooting

" The adjusted parameter tables are as follows

6872PBGT HO Thrsh.

24PBGT valid time

35PBGT watch timeHandover

Value after

change

Value before

change

Parameter name



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! Troubleshooting

" The adjusted parameter tables are as follows

1510Min. DL Level on

Candidate Cell

6070UL Qual. Thrsh.

(Emergency handover)

Handover

Value after

change

Value before

change

Parameter name



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! Conclusion: optimize and adjust handover parameter to

reduce call drop

" On condition that there is no ping-pang handover and excessive

voice interruption, PBGT handover will help to reduce

interference and lower call drop rate.

" Set emergency handover trigger threshold properly, make sure

the emergency handover is triggered in time before the call drop

so as to reduce call drops.


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! Fault description

" In the dialing test, many call drops are found in cell 2.

!Analysis

" Check the traffic statistic and find out that TCH congestion rate

of this cell is over 10% and internal handover failure rate is high.

It is found that one TRX board of this cell is abnormal in OMC.

A preliminary conclusion is that TRX board problem causes the

call drop.


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! Troubleshooting

" Lock the frequency with a test mobile phone and perform dial

test for many times. It is found that call drops only happen in

timeslots 1, 3, 5, 7 while communications in timeslots 2, 4, 6, 8

are normal. Move this board to another slot, and the problem still

exists. Move other good boards to this slot, and the

communication is normal. Move this defective board to other

cabinet, the problem arises. This TRX proves defective. When it

is replaced with a standby board, the communication is

recovered.



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! Conclusion

" The BTS test should guarantee that communication should be

successful not only in each RC but also in each timeslot of each

RC. It must be ensured that each TCH channel provides

bidirectional high quality communication.



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! Fault description

" In dual-band network, When a call is setup a GSM1800 cell, the

call is handed over to a GSM900 cell from the same site. After

2~5 seconds, the call dropped in the GSM900 cell. The call drop

rate in the GSM900 cell is quite high.


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!Analysis

" In the test it is found that the clock of GSM900 and GSM1800 is

not synchronized. When a call set up in a GSM1800 cell is

handed over to a GSM900 cell, the drive test tool shows that

FER increase to the maximum value suddenly and then it goes

down to zero gradually. And it is the same with the handover

from GSM900 to GSM1800.

" Trace the signaling and find that the conversation before the call

drop for several seconds is actually call re-establish, but test

mobile phone indicates the call is already handover to cell

GSM900 successfully. The clock synchronization problem is

serious.



88/90

88

! Troubleshooting

" After adjusting GSM900 clock system, the abnormal call drop

problem is solved.



89/90

89


! Conclusion

" Clocks of GSM900 and GSM1800 should be exactly

synchronized with each other in a dual band network, otherwise,

there will be call drops and handover failures.


90/90

omf000404 case analysis-call drop issue1.5

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