10-troubleshooting for call drop

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Troubleshooting ManualM900/M1800 Base Station Subsystem Table of Contents

Table of Contents

Chapter 10 Troubleshooting for Call Drop................................................................................ 10-110.1 Overview........................................................................................................................ 10-1

10.1.1 Description .......................................................................................................... 10-110.1.2 Formula for Call drop ..........................................................................................10-4

10.2 Causes........................................................................................................................... 10-510.2.1 Coverage............................................................................................................. 10-510.2.2 Handover............................................................................................................. 10-810.2.3 Interference....................................................................................................... 10-1010.2.4 Uplink/downlink Unbalance Caused by Antenna & Feeder System................. 10-1210.2.5 Transmission Failure......................................................................................... 10-1310.2.6 Unreasonable Parameter Settings.................................................................... 10-1410.2.7 Others................................................................................................................ 10-15

10.3 Examples ..................................................................................................................... 10-1510.3.1 Reducing Call Drop by Optimizing Handover Related Parameter.................... 10-1510.3.2 Call Drop Caused by Interference..................................................................... 10-1610.3.3 Call Drop Caused by Interference..................................................................... 10-1810.3.4 Uplink/downlink Unbalance............................................................................... 10-1910.3.5 Call Drop Caused by Interference from Repeater............................................. 10-20

10.3.6 Call Drop Caused by Isolated Island Effect ...................................................... 10-2010.3.7 Settings of Version Related Parameters........................................................... 10-21

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Troubleshooting ManualM900/M1800 Base Station Subsystem Chapter 10 Troubleshooting for Call Drop

Chapter 10 Troubleshooting for Call Drop

10.1 Overview

For GSM network, call-dropping failure rate is an important index measuring the

quality of radio network.

This chapter analyzes the causes resulting in call drop and describes the methods

of troubleshooting for the purpose of reducing call-dropping failure rate, thus

improving the quality of network. In addition, it also introduces the measures of

dealing with worst cells caused by high call-dropping failure rate, reducing worst cell

ratio, thus decreasing call-dropping failure rate.

Definitions of worst cell ratio indices that are calculated according to different

network sizes are given below:

Worst cell ratio in super network: Number of worst cells/number of cells where the

busy time average traffic per channel exceeds 0.15Erl.

Worst cell ratio in large network: Number of worst cells/number of cells where the


Worst cell ratio in medium network: Number of worst cells/number of cells where the


Definition of worst cell: The cell where the busy time TCH congestion rate (not

including handover) is greater than 5%, or the TCH call-dropping failure rate is

greater than 3%.

Definition of call-dropping failure rate: Call-dropping failure rate = [Busy time total

TCH traffic * 60]/total number of busy time TCH call drops, in which the number of

call drops indicates the number of Clear Request messages.

10.1.1 Description

There are two types of call drops given below.

Call drop over SDCCH: Indicating the call drop occurs in the course during

which BSC assigns a SDCCH to an MS but a TCH has not been successfully

assigned yet

Call drop over TCH: Indicating the call drop occurs after BSC assigns a TCH to

MS successfully.

There are three causes resulting in call drop, which are given below.


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Radio link failure, which occurs in the course of communication and causes the

situation that messages cannot be received.

T3103 timeout. It indicates the MS cannot occupy a channel of the destination

cell or return to the original channel. System failure, such as equipment failure etc.

1) Among the three causes, radio link failure is the main factor

During a conversation, when the voice quality of a MS is too bad to be accepted

and cannot be improved via radio frequency power control or handover, the MS

will consider the radio link gets faulty and forcedly release the link, which thus

causes call drop. As stated in GSM specification, there is a counter S in the MS.

As soon as a conversation starts, the counter is assigned an initial value, which

is the parameter Radio Link Timeout. If the MS fails to decode a SACCH

message with period of 120 ms, 1 will be subtracted from S. Contrarily, every

time when the MS receives a SACCH message successfully, 2 is added to S,

but the value of S cannot be greater than the initial value. When S is 0, the MS

reports radio link failure. The Signaling procedure is shown in Figure 10-1.

Steps (1) and (2) shows SDCCH/TCH have been established, while step (3)

cannot decode the SACCH message block (uplink/downlink), thus radio link

timeout is caused.

SACCH multiframe number (namely, SACCH multiframe period with a unit of

480 ms) in the cell attributes table defines uplink connection failure time. When

detecting an activated connection on the radio link is broken, BTS will report the

message Connection Failure. The system judges whether a connection fails by

the BER of uplink or by checking whether SACCH can correctly decode. As

stated by GSM protocol, if the BER of SACCH is used to judge whether a

connection fails, when the uplink BERs in N continuous SACCH multiframe

periods are greater than the set threshold, BTS will report the message

Connection Failure to BSC. N, the number of SACCH multiframe periods, has

been set during data configuration, which is no other than the SACCH

multiframe number in the cell attributes table, with a unit of 480 ms.

In addition, in case that the layer 2 frame cannot interwork with MS normally,

BTS layer 2 radio interface will report the message Error Indication to BSC, as

shown in step (3) in Figure 10-1. The cause is T200 timeout, and at this time,

BSC will release the radio link and report the message Clear REQ.


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MSC BSC BTS MS

Measurement Result

Connection Failure

Clear REQ

(Radio Interface Failure)

(1)

(2)

(3)

Figure 10-1 Signaling flow of radio link failure

2) T3103

(a) Definition: In the course of an intra-BSS or inter-BSS handover, BSC

reserves TCHs of the cell initiating the handover and the destination cell in

terms of T3103. This timer is activated as soon as BSC sends the message

Handover Command, and is cleared after receiving Handover Complete (for

intra-BSS handover), or Clear Command (for inter-BSS handover).

(b) This timer is used to reserve a channel for a long time so that MS can returnto the channel. Nevertheless, If MS lost, it will be used to release the channel.

When BSS sends a handover command to MS, the timer starts to perform the

timing function. After BSC receives a Handover Complete message from the

destination cell or a Handover Failure message from the source cell, the timer

will be reset. Following BSC sending a Handover Command message to BTS,

if no messages are received after T3103 expires, BSC will consider radio link

failure occurs to the source cell, and then release the channel of the source cell.

The Signaling flow is shown in Figure 10-2.


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MSCBSCBTS1MS

Channel Activate

BTS2

Handover Indication

Channel ACK

Handover CommandHandover Command

Handover AccessHandover Detection

Physical Info (TA)

SABMEstablish Indication

UA

Handover CompleteHandover Complete

Set T3103

Reset T3103

Figure 10-2 Call drop resulting from T3103 timeout

3) See example 8 for detailed descriptions of call drop resulting from the causes

such as equipment failure.

10.1.2 Formula for Call drop

1) TCH call-dropping failure rate = number of TCH call drops/times when TCH is

occupied successfully % 100%

2) TCH call drop measurement point: The channel currently occupied is of TCH

type when BSC sends a Clear Request message to MSC.3) The cause values for sending Clear Request are as follows:

Radio Interface Message Failure

O&M Intervention

Equipment Failure

Protocol Error Between BSS and MSC

Preemption

The Signaling flow is shown in Figure 10-3.


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BSCBTSMS

Connection or Connection Acknowledge

Connection Failure

Call Assignment Procedure Signaling

Handover Procedure Signaling

Connection or Connection AcknowledgeConnection or Connection Acknowledge

Handover Complete

Error Indication

Or.

Or.

Or.

Figure 10-3 Signaling flow for TCH call drop

Formula of SDCCH call-dropping failure rate:

SDCCH call-dropping failure rate = number of SDCCH call drops/total times when

SDCCH is occupied successfully % 100%

SDCCH call-dropping failure rate (%) = [number of radio link failures when SDCCH

is occupied (connection failure) + number of radio link failures when SDCCH is

occupied (error indication) + number of terrestrial link failures when SDCCH is

occupied (Abis)]/total times when SDCCH is occupied successfully % 100%

SDCCH call drop measurement point: The channel currently occupied is of SDCCH

type when the messages Clear REQ and Error Indication are sent to MSC.

10.2 Causes

10.2.1 Coverage

I. Analysis

1) Discontinuous coverage (blind area)

Call drop is caused by isolated BTS. As the signal is of weak strength and poor

quality at the edge of an isolated BTS, handover to other cells cannot be

implemented, and thus call drop occurs.

If BTS lies in the place where the landform is intricate and radio propagation

environment is complicated (e.g., a mountainous area), it may cause call drop

owing to discontinuous coverage.

2) Poor indoor coverage


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In the place where many buildings are located, call drop easily occurs due to

high transmission attenuation, low indoor level and great penetrate loss.

3) Beyond coverage (isolated island)

Owing to some reasons, the coverage of a serving cell is beyond the defined

coverage. For example, the power in cell A is so high that a MS still occupies

the signals of cell A after it moves out of the coverage of the adjacent cell B that

has been defined by cell A and reaches cell C. However, cell A has not

defined cell C as an adjacent cell yet, so at this time the MS cannot find a

proper cell when it tries to perform a handover according to the adjacent cell B

provided by cell A, thus call drop occurs, as shown in Figure 10-4.

Cell A

Cell B

Cell C

Expected Coverage

Actual Coverage

Can't find nextcell cause call

drop

Figure 10-4 Call drop resulting from overlarge coverage

4) Shortage of coverage

It may be caused by some equipment failure in a cell. For example, the

antenna is obstructed or the carrier taking BCCH (power amplifier) gets faulty.

II. Location

Get familiar with the area that is not covered enough and perform a large-scope test.

Observe the signal level, whether the handover is normal and whether call drop

occurs. In addition, by means of OMC traffic measurements check the BSC

call-dropping failure rates to find the cells with high call-dropping failure rates and

other relevant statistics, facilitating the location.

The related traffic measurement tasks and items are listed below:

1) In power control performance measurement, see whether the average

uplink/downlink signal strength is too low.


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2) In receive signal level performance measurement, see whether the proportion

of number of low receive signal levels is too large.

3) In cell/inter-cell handover performance measurement, see whether the level

class and the average receive signal level are too low when a handover isinitiated.

4) In call drop performance measurement, see whether the level is too low when

call drop occurs, and whether TA is abnormal prior to call drop.

5) Defined adjacent cell performance measurement. Based on the statistics of

adjacent cells defined in the cell adjacent relationship table and reported by MS,

you may locate in which adjacent cell the average level is too low.

6) In undefined adjacent cell performance measurement, see whether the

undefined adjacent cells in which the average level is too high exist.

7) In power control performance measurement, see whether the maximum

distance between MS and BTS, namely TA, keeps abnormal in multiple

continuous time segments.

III. Solution

1) Find the area that is short of coverage

Perform tests to find the areas that are short of coverage. For the isolated BTS

or the BTS in a mountainous area, continuous coverage can be formed by

adding BTSs or expanding the original coverage via other ways, such as

improving the maximum transmit power of BTS, adjusting azimuth, downtilt and

height of antenna etc. Analyze whether the trouble is caused by surroundings,e.g., tunnel, mall, subway entry, underground parking lot and depression.

Generally, call drop easily occurs at these places, and micro cells can be used

to handle the trouble.

2) To guarantee indoor communication quality, the signals going outside must be

strong enough. If indoor communication quality can not be improved greatly by

enhancing the maximum transmit power of BTS, adjusting azimuth, downtilt

and height of antenna, adding BTSs can be helpful. To build up indoor

coverage of main publics such as office buildings and hotels, the indoor

distribution system could be applicable.

3) For the cell where beyond coverage may occur, define its all potential adjacent

cells to reduce call drops resulting from lack of proper cell for handover. The

problem of beyond coverage can be solved by lessening the antennas downtilt

of the BTS.

4) Removing hardware failure.

Perform tests to judge whether hardware failure occurs and causes short of

coverage. If the call-dropping failure rate of a BTS rises abruptly and all other

indices remain normal, check whether the adjacent cells work normally. Such

trouble may be caused by failure of the downlink, such as failure of TRX,


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diversity unit or antenna, because faults of the uplink will cause high handover

failure rate of the original cell.

10.2.2 Handover

I. Analysis

1) Unreasonable parameters

For example, if the level of the handover candidate cell is set to be too low and

the handover threshold is set to be too little, some MSs will be handed over to

the adjacent cell when the level of the adjacent is a little stronger than that of

the serving cell for a time. But after a while, if the signal of the adjacent cell

faint, and it happens no proper cell is available for handover, call drop could

occur. See example 6 for call drop resulting from improper settings of handover

parameters.

2) Adjacent cell undefined

If an adjacent cell has not been defined yet, MS will keep communicating in the

serving cell until it goes out of its coverage. At this time, call drop shall occur

since MS cannot be handed over to a cell with stronger signals.

3) Existence of adjacent cells with the same BSIC and BCCH frequency.

4) Traffic congestion

Unbalance of traffic may cause handover failure due to lack of handover

channel available for the destination BTS. When reestablishment of handover

channel fails too, call drop occurs.

5) BTS clock out of synchronization and frequency offset beyond limits, which can

cause handover failure and call drop.

6) T3103 timeout

II. Location

According to traffic measurement indices analyze whether there are cells with lowhandover success rate, high call-dropping failure rate, multiple handover and

reestablishment failures. By ways of traffic measurement analyze the causes

resulting in handover, such as uplink/downlink receive signal level, uplink/downlink

receive quality, power budget (PBGT), call directed retry and traffic. Observe

whether there are BTS related clock alarms and whether BTS clock runs normally.

Check BTS clock and remove clock fault if necessary. Perform a road test to find the

cell in which handover is abnormal. Perform multiple road tests near the problem

cell to find handover related call drop, and optimize handover parameters to reduce

call-dropping failure rate.


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The following problems might be detected during traffic measurement:

1) Too many handover failures and reestablishment failures in inter-cell handover

performance measurement.

2) Too many handovers and successful re-establishments in inter-cell handoverperformance measurement.

3) The number of measurement reports including undefined adjacent levels in

undefined adjacent cell performance measurement is beyond limits.

4) Low outgoing handover success rate (for a cell) in outgoing handover

performance measurement. Find the adjacent cell to which the handover

success rate is low, and further search the cause in the destination cell.

5) Low incoming handover success rate and unreasonable settings of handover

parameters of the opposite cell.

6) Number of handovers in disproportion with number of successful TCH

occupancies and too much handover in TCH performance measurement.

(number of handovers/number of calls >3)

III. Solution

1) Check the parameters impacting handover, e.g., settings of stratum levels,

handover thresholds, handover hystereses, handover measurement time,

handover duration and minimum access level of handover candidate cell. For

instance, to decrease call drop resulting from handover, the minimum access

level of the handover candidate cell can be improved from -100dBm to -95dBm.

That is, it is changed from grade 10 to grade 15. Also, when handover is slowor handover success rate is low owing to clock problem or poor transmission,

the value of the parameter NY1, which is the maximum retransmission times of

physical information could be set greater, In a word, handover optimization

should be based on the actual conditions. Example 6 introduces how to reduce

call drop by adjusting handover parameters. See it for details.

2) Traffic adjustment is to handle the call drop resulting from no handover channel

available for the destination BTS due to unbalance of traffic. For example,

control the coverage of cell by adjusting the engineering parameters such as

downtilt and azimuth of antenna, or lead MS to stay in an idle cell via network

parameters such as CRO, or lead MS in conversation to hand over to an idle

cell by setting stratum level priority, or balance traffic by adopting load

handover or directly expand the carrier.

3) Calibrate the BTS clock that is faulty until clock synchronization.


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10.2.3 Interference

I. Analysis

There are co-channel interference, adjacent-channel interference and

inter-modulation interference. When MS receives signals in the serving cell with

strong co-channel or adjacent-channel interference, it may aggravate BER and

make MS cannot accurately demodulate BSIC of the adjacent cell or BTS cannot

correctly receive measurement reports of MS.

The interference threshold is set as co-channel carrier-to-interference ratio C/I9dB

and adjacent-channel carrier-to-interference ratio C/A9dB. When the interference

index is so bad that it exceeds the threshold, conversations in network shall be

interfered, thus conversation of poor quality and call drop might occur.

II. Location

Interference may be from inside or outside of the network and exists in uplink or

downlink signals. The following methods can be used to locate interference.

1) Find the position that may be interfered by analyzing traffic measurement.

2) Perform road tests at the position that may be interfered according to

complaints of the users and search downlink interference. With road test tools

check whether the position where the receive signal level is strong but the

conversation quality is poor exists. Or use a test MS to perform dialing tests ata locked frequency to observe whether interference occurs at the frequency.

3) Check whether there is co-channel interference caused by improper frequency

planning.

4) Adjust the frequencies that might be interfered to try to reduce even avoid

interference.

5) Remove the interference caused by equipment failure.

6) If interference still remains, perform frequency scan with a spectrum analyzer to

search the frequency that is interfered and to further find the interference

sources.

See examples for detailed analysis of interference. List below the traffic

measurement indices used for interference analysis.

1) Interference band for observing uplink interference

If an idle channel appears in interference bands 3, 4 and 5, generally it

indicates there is interference. For intra-network interference, the interference

may increase with the augmentation of traffic, while out-network interference

has nothing to do with it. Note that interference band is reported to BSC by the

BTS carrier channel in an idle state via radio frequency resource indication, it

indicates the uplink characteristics of the radio channel occupied by MS,


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namely interference severity of uplink signals. If the channel is busy, it is

difficult for it to report resource indication, therefore, the measurement of

interference band should be comprehensive.

2) Receive signal level performance measurement (denoting the matrixrelationship of level and quality)

This is a measurement item used for carrier. If there are too many times when

level is high but quality is poor for a carrier board, it indicates co-channel or

adjacent-channel or out-network interference occurs to the frequency of the

board.

3) Handover ratio on poor quality communication

In cell performance measurement/inter-cell handover or outgoing handover

performance measurement, handover attempts due to various reasons are

measured. If there is too much handover caused by poor quality

communication, it means there could be interference. Furthermore, if there is

lots of handover resulting from uplink communication of poor quality, and vice

versa.

4) Receive quality performance measurement

Measure average received quality level for carrier, which serves as a reference.

5) Call drop performance measurement

Record average level and quality in case of call drop, which serves as a

reference.6) Too many handover failures and reestablishment failures

It might be caused by interference in destination cell, serving as a reference.

III. Solution

Out-network interference could be solved with the help of operators, while

intra-network interference can be handled by adjusting network planning.

1) Perform actual road tests, check the places where interference occurs and

distribution of signal quality and analyze the coverage overlap of which cells

causes the interference. Then according to the actual condition, adjust the

related BTS transmit power, downtilt/azimuth of antenna or frequency planning

to prevent interference.

2) Application of discontinuous transmission (DTX), frequency hopping, power

control and diversity

These technologies can help to reduce system noise and improve

anti-interference capability. DTX is classified into uplink DTX and downlink DTX,

which can help to reduce the effective time for transmission, thus to decrease

the interference level of the system. Nevertheless DTX should be adjusted

properly considering the actual radio surroundings and relationships with the


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adjacent cells. When MS receives signals of poor quality, the application of

DTX might cause call drop. Because after MS sets up a conversation, the BTS

transmit power will be stronger during the conversation due to the activation of

DTX downlink function, while in the interval of conversations, the power willdecrease. In this manner, it may reduce the interference to other BTSs, but on

the other hand, if interference exists around the BTS, DTX of downlink signals

may aggravate the quality of conversation. As a result, when the BTS transmit

power decreases, debasement of conversation quality even call drop may

easily occur at the position where the receive signal level is low but

interference signal is strong.

3) Remove the interference caused by equipment itself (e.g., carrier board

self-excitation, antenna inter-modulation interference).

10.2.4 Uplink/downlink Unbalance Caused by Antenna & Feeder System

I. Analysis

1) Improper installation of antenna and/or feeder. For example, the Tx antenna

between two cells is installed just reversedly, which shall make the uplink signal

level is much poorer than the downlink one, thus cause call drop, single pass or

difficult connection occurring far from the BTS.

2) If single polarization antenna is adopted, a cell has two sets of such antennas.

If their azimuths are different, call drop might occur.

A directional cell has a main antenna and a diversity antenna, so it is possible

that BCCH and SDCCH of this cell come from the two different antennas.

Different azimuths will cause different coverage, consequently, although the

user can receive BCCH signal, it cannot occupy SDCCH sent by another

antenna when originating a call, thus call drop occurs.

3) Different azimuths of two antennas may cause call drop.

Different azimuths of two antennas will cause the situation that the user can

receive SDCCH, but call drop shall occur once it is assigned to TCH

transmitted by another antenna.4) Antenna problem also can cause call drop.

Mar, watering, bend and connector of poor contact all can reduce Tx power and

Rx sensitivity, thus cause serious call drop, which can be confirmed via

standing wave ratio.

II. Location

1) Check whether there are combiner, CDU, tower top amplifier and standing

wave ratio alarms.


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2) View whether all boards of BTS work normally via remote maintenance.

Analyze whether uplink/downlink unbalance appears from traffic measurement.

3) Trace relevant Abis interfaces by performing Abis interface tracing function or

with a Signaling analyzer. Further observe whether uplink/downlink signals arebalanced from the measurement report about Signaling messages.

4) Perform road tests and dialing tests. Make sure the BCCH frequency of the

serving cell is consistent with the expected one and the Tx antenna is installed

correctly prior to road tests.

5) After full remote analysis, perform on-site inspections and tests. Check whether

the azimuth and the downtilt of the antenna are designed normatively and

whether the feeder and jumper are connected accurately. Make sure the

antenna & feeder connector is in good contact and the feeder is in good

condition. Test whether the standing wave ratio is normal.

6) Judge whether BTS hardware failure causes the uplink/downlink unbalance.

For hardware failure, replace the part that might be faulty or disable other

carriers in the cell before performing dialing test on the doubtful carrier to locate

the fault point. Once a part is found in a faulty state, it should be replaced in

time. If no alternative part is available, block the faulty board first lest call drop

should occur to impact the running quality of the network.

List below some traffic measurement items used for analysis of uplink/downlink

balance:

1) From Up-Down Link Balance Measurement, analyze whether uplink/downlink

unbalance exists.

2) From Call Drop Measurement, analyze the average uplink/downlink levels

and qualities in case of call drop.

3) From Power-Control Measurement, analyze uplink/downlink average receive

signal levels.

10.2.5 Transmission Failure

As there are Abis interface and A interface link, poor quality transmission and

unstable transmission link also may cause call drop.

I. Analysis and solution:

1) Observe transmission and board alarms (e.g., FTC failure alarm, A interface

PCM out of sync alarm, LAPD link break alarm, power amplifier alarm, HPA

alarm, TRX alarm, CUI/FPU alarm). Based on alarm data, analyze whether

transmission is intermittent or whether there are faulty boards (e.g., the carrier

board is faulty or in poor contact).

2) Check transmission paths, test BER and check whether E1 connector or

grounding of equipment is reasonable, thus decrease call drops by ensuring

stable transmission quality.


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3) Observe whether there are too many call drops caused by transmission

problem via traffic measurement.

a) in TCH performance measurement of traffic measurement observe whether

there are too many A interface failures when TCH is occupied.

b)In TCH performance measurement observe whether the TCH availability rate

is abnormal.

c) In TCH performance measurement observe whether there are too many call

drops caused by interruption of terrestrial link.

10.2.6 Unreasonable Parameter Settings

Check relevant parameter configurations and make sure they are configured

reasonably, which are as follows:

1) System message data table: Radio link failure counter

If the value is too little, call drop may occur easily when the receive signal level

of MS declines greatly and abruptly due to some reasons such as fluky

landform. If it is too great, only when the radio link expires can the network

release the related resource although the quality of voices is too bad to tolerate,

which thus reduces the resource utilization. Generally, this value should be set

greater for the area with low than that for the area with high traffic.

2) Cell attribute table: SACCH multiframe numberRecommended value: BTS3X 14 (31 for version 05.0529 or newer)

BTS2X 31

3) System message data table: MS minimum received signal grade, RACH

minimum receive signal level, RACH busy threshold.

In virtue of existence of uplink/downlink signals, the actual coverage is subject

to the weaker signal. If in a cell the coverage of the uplink signal is larger than

that of the downlink signal, the downlink signal is weaker at the edge of the cell

and can be submerged easily by stronger signals from other cells. Contrarily,

if the coverage of the downlink signal is larger than that of the uplink coverage,

MS shall have to stay in the strong signal. However, MS cannot originate a call

owing to weak uplink signal, or although it can set up a call, the voice quality is

very poor, or signal pass even call drop may occur. Therefore, it is necessary to

ensure the uplink/downlink balance as possibly as you can.

MS minimum received signal grade:

It indicates the minimum receive signal level required for MS accessing the

system, which is for the downlink signal. If the value of this parameter in a cell

is too little, MS in the cell can access network easily, and the coverage is large.


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But MS at the edge of the cell tries to stay in the cell, which shall cause greater

load on the cell and increase the possibility of call drop. If it is too great, the MS

with low receive signal level cannot access network, which helps to reduce

call-dropping failure rate but lessens the coverage. Therefore, both coverageand call-dropping failure rate should be taken into account for setting of this

parameter. Call-dropping failure rate cannot be reduced at the cost of lessening

of coverage.

RACH minimum receive signal level

It indicates the minimum receive signal level required for MSs uplink access to

the system (RACH busy threshold used in BTS20 is similar to MS minimum

receive signal level. Both coverage and call-dropping failure rate should be

fully considered for setting of this parameter.)

See M900/M1800 Base Station Controller Data Configuration Reference

Network Planning Parameters for details.

10.2.7 Others

There are many other reasons causing call drop. For example, when the version of

TRX in a BTS is inconsistent with that of FPU, it may increase the number of call

drops occurring to the whole network. Or, improper use of BTS version related

parameters also causes call drop, as shown in example 7.

10.3 Examples

10.3.1 Reducing Call Drop by Optimizing Handover Related Parameter

I. Description

Too many call drops that occurred at the mouth of the cave near the BTS and were

caused by the situation that handover cannot be executed immediately were found

during road tests from place A to place B.

II. Analysis

The mouth of the cave lay just near the BTS. In the cave, the power of the

destination cell can be about 80dBm, but the signal power of the serving cell rapidly

declined to be less than 100dBm. Handover cannot be triggered since the downlink

power of the two cells was good enough, but the signal level of the serving cell

decreased rapidly in the cave, which caused the situation that call drop occurred

before the measurement ended.


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III. Handling process

The related parameters shown in Table 10-1 should be modified.

Table 10-1 Modification table of parameters

Parameter nameValue beforemodification

Value after modification

PBGT handover measurement time 5 3

PBGT handover duration 4 2

PBGT handover threshold 72 68

Emergency handover uplink qualitythreshold

70 60

Candidate cell minimum downlinkpower

10 15

Optimizing handover related parameters could help to reduce the call-dropping

failure rate.

1) Make PBGT handover occur easily so as to achieve anti-interference and

reduce call-dropping failure rate on the premise of no toggle handover that may

cause too much voice discontinuity.

2) Reasonably set the emergency handover trigger threshold so that emergency

handover can be triggered in time before call drop occurs, thus to reduce the

call-dropping failure rate.

10.3.2 Call Drop Caused by Interference

I. Description

The BTSs distribution of an area is shown in Figure 10-5. (The red digits indicate

BCCH frequencies. DTX is adopted without frequency hopping). As shown in the

figure, it could be seen that there were too many call drops occurring in cell 2 of BTSC. (The reason that hardware failure could cause such trouble has been excluded.)


10-16


18/23


19/23


6) A repeater was found through the on-site inspection. The repeater was a set of

broadband equipment, amplifying the signals of a remote analog BTS sent to

the near end via optical fiber and transmitting them. Also, the digital signals

were amplified by the repeater and then the cell 2 of BTS C was interfered.


The maintenance personnel reduced the transmit power of the repeater so that the

interference level could degrade from bands 2 and 3 to band 1. Consequently, the

high call-dropping failure rate at BTS C was solved.

10.3.3 Call Drop Caused by Interference

I. Description

A BTS adopted 1%3 RF hopping. After it was expanded, TCH assignment failure

rate kept high owing to radio link failure, accompanied with high TCH call-dropping

failure rate and high handover failure rate. Nevertheless the SDCCH call-dropping

failure rate remained normal.

II. Analysis

Considering high call-dropping rate and high handover failure rate accompanied

high assignment failure rate, it could be caused by two reasons as follows.

1) TCH was assigned incorrectly.

2) The frequency or time slot occupied for this conversation was interfered or

unstable.

As the SDCCH call-dropping rate remained normal, it is almost impossible that the

carrier carrying BCCH frequency and BCCH frequency itself were interfered. But the

carriers carrying non-BCCH frequencies and hopping frequencies might be

interfered.


No faults were found during the check of equipment, antenna & feeder and

transmission stability. It was found that the situation of high level with poor quality

was serious during road tests. Through an on-site dialing test the voice quality was

found very poor, and MAIO of the newly added carrier was found the same as that

of another carrier during the check of parameters.

Fault point: The hopping frequencies collided.


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10.3.4 Uplink/downlink Unbalance

I. Description

The MS occupied a cell but cannot originate a call. Single pass occurred. Call drop

always occurred at the place away from the cell. Call drop could occur after

frequency handover.

II. Analysis

The unbalance between the uplink signal level and the downlink signal level might

cause such trouble.


Perform on-site tests. Make the MS move to the edge of the cell during the test and

trace data with a Signaling analyzer at BTS so as to observe the receive signal

levels of the BTS and the MS.

Figure 10-6 Explanation of measurement report MA10

As shown in Figure 10-6, the uplink signal level is 98dBm (highlighted with a red

circle) and is much lower than the downlink signal level that is 66dBm. If the level

is lower than 98dBm, it means the signal is too weak, which can cause call drop

easily.


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10.3.5 Call Drop Caused by Interference from Repeater

I. Description

The call-dropping failure rate in cell 3 of a BTS reached 10%, while the call-dropping

failure rates and congestion rates in cells 1 and 2 kept normal.

II. Handling process

1) High congestion rate always existed no matter how to block the carrier channel

of the cell.

2) The maintenance personnel found the interference band was regular by

viewing and analyzing traffic measurement data. Generally it was high at days

but low at nights. That is to say, when traffic was high at days, interference

become high, and vice versa.

3) The maintenance personnel set the frequency of cell 3 to be over 1 MHz

higher/lower than the original one, but the trouble still existed. Therefore,

co-and adjacent-channel interference could be excluded.

4) The maintenance personnel checked the equipment and excluded the

possibility of equipment fault.

5) The maintenance personnel located the trouble was caused by external

interference.

6) The maintenance personnel performed the frequency scan test with a spectrum

analyzer and found a suspicious signal that was similar to a spectrum with thecentral frequency of 904.14MHz and broadband of 300KHz. And the signal

existed continuously and stably.

7) The strength of the interference signal at the mouth of the divider in cell 1 was

27dBm, and those in cells 2 and 3 were 40dBm and 60dBm respectively.

Since traffic at days is higher than that at nights, inter-modulation occurs at

days more easily than at nights. Therefore, it can be located that the trouble

was caused by the external interference source of 904 MHz.

8) The maintenance personnel couldnt locate the interference source by

performing road tests with a spectrum analyzer. Then he performed all tests at

the roof and found the interference came from the little antenna of a repeater.

He interrupted the signal test of the repeater, and the interference disappeared.

10.3.6 Call Drop Caused by Isolated Island Effect

I. Description

The user complained call drop always occurred at the fifth floor or above of a

building.


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II. Analysis

There are two ways for eliminating isolated island effect.

1) Adjust the antenna of the isolated cell.2) Define new adjacent cells for the isolated cell.


1) After the on-site test, the maintenance personnel found call drop and noise

existed. And from the test MS, he found the MS had always stayed in a serving

cell not belonging to the local BTS A before call drop occurred.

2) The cell belongs to BTS B that is 3~4km away from the building. Therefore, he

concluded that the signal received here was the signal reflected by an

interrupter, consequently a coverage equivalent to an isolated island was

formed.

3) By viewing the data configuration, the maintenance personnel found that only

cell 2 of BTS A had been configured in adjacent relationship between A and B

of BSC data configuration. When a MS adopts the signal of cell 2 of BTS B in

the area, the signal of cell 3 of BTS A is stronger but no adjacent relationship

has been defined for cell 2 of BTS B and cell 3 of BTS A. As a result, handover

cannot be implemented.

4) As the signal of cell 2 of BTS B has been reflected for many times, when the

signal from BTS B received by the MS weakens abruptly owing to a certain

reason, an emergency handover might occur. However, for cell 2 of BTS Bcells 2 and 3 of BTS A are not the most ideal candidate cells, thus handover to

another BTS (e.g., BTS C) might occur. Nevertheless, the MS cannot receive

the signal from BTS C at this time, hence call drop occurs.

5) By modifying the data in [BA1(BCCH) Table], [BA2(SACCH) Table] and

[Adjacent cell relationship table] in BSC data configuration, the maintenance

personnel set cell 3 of BTS A as an adjacent cell of cell 2 of BTS B and further

optimized the network engineering parameters to eliminate the isolated island

effect.

6) Solving the trouble was confirmed after tests.

10.3.7 Settings of Version Related Parameters

I. Description

After an expansion, the call-dropping failure rates of five BTSs in an area reached

5%, and the number of call drops in each cell reached 100. In addition, the

call-dropping failure rate of a cell that was not expanded rose too. All these troubles

were the RF call drop. But the maintenance personnel had no idea whats the cause

since no interference and no hardware faults were found.


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II. Handling process

1) The maintenance personnel checked data, frequency planning and BSIC

planning.

2) He observed the traffic measurement and found that the interference band kept

normal and no interference occurred.

3) The handover success rate remained above 93%.

4) He checked the versions of all TRXs and FPUs of the BTS and found the

version of TRX was inconsistent with that of FPU after the expansion. Then he

upgraded them to make them consistent. But the trouble still remained.

5) He checked the data again and found that the BTS after expansion was in 15:1

multiplexing and enabled the measurement report pre-processing function for

all BTSs of 2.0 versions, but parts of older versions didnt support the function,

which hence caused the increasing of the call-dropping failure rate.

III. Summary

After the system is adjusted greatly, e.g., cut-over access of new BTS, expansion of

BTS, re-planning of frequency, upgrading and patching etc., the related parameters

should be checked and adjusted correspondingly, especially the adjacent cell

relationship, frequency interference, frequency hopping and cell parameters etc.

And the version of BTS should be fully taken into account as well.


10-troubleshooting for call drop

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