173287126 12 OMO312000 BSC6000 GSM Call Drop Problem Analysis ISSUE1 01

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Call Drop Problem Analysis

Call Drop is classified as TCH call drop and SDCCH call drop. They are respectively

registered via TCH Call Drop Ratio and SDCCH Call Drop Ratio.

TCH Call Drop Ratio

TCH Call Drop Ratio refer to the ratio of call drop to successful TCH seizures

after the BSC successfully assigns TCHs to MSs. The TCH call drop rate can

be measured from the following aspects:

TCH call drop (including handover)

TCH call drop (excluding handover)

The TCH Call Drop ratio is one of the most significant KPIs for telecom

operators, is related to retainability. It indicates the probability of call drop

due to various reasons after MSs access TCHs. A too high TCH call drop

rate adversely affects the users experience.

SDCCH Call Drop Rate

SDCCH Call Drop Rate indicates the probability of call drops when the MS

occupies the SDCCH. The SDCCH call drop rate is one of accessibility KPIs.

This KPI reflects the seizure condition of signaling channels. If the value of

this KPI is high, user experience is adversely affected.



From the perspective of the whole network, there are three reasons for call drops:

Radio link fault (When messages cannot be received normally during a

conversation)

T3103 timeout (during a handover, when the MS cannot occupy a channel of the

target cell and cannot return to the previous channel)

System fault (all possible faults, such as equipment fault)



The main reason for a call drop is radio link fault.

Downlink: In GSM specifications there is a parameter RADIO LINK TIMEOUT. When

the voice quality of an MS during the conversation is too bad to receive and cannot

be improved by radio power control or handover, the MS thinks the radio link is

faulty and releases the link, causing call drop. According to the GSM specifications,

there is a counter S in the MS that is assigned an initial value at the beginning of a

conversation, which is the value of the parameter Radio Link Timeout. When the

MS decoding the SACCH message (period: 120 ms) fails, S decrements by 1; on the

other hand, if the MS receives a correct SACCH message, S increments by 2 but

shall not exceed the initial value. When the S is equal to 0, the MS reports radio

link failure.

Uplink: SACCH multi-frames (period: 480 ms) in the cell attribute table defines the

time of uplink connect failure. When the BTS detects that an activated connection

on the radio link is damaged, the BTS sends the CONNECT FAILURE message to the

BSC. The system judges a connection failure based on the bit error rate of the

uplink SACCH. According to GSM specifications, if a connection failure is judged

on the basis of the bit error rate on the SACCH, when the bit error rate of the

uplink is greater than the preset threshold in the period of N consecutive SACCH

multi-frames, the BTS sends the CONNECT FAILURE message to the BSC. The

number of SACCH multi-frames N, which is the number of SACCH multi-frames in

the cell attribute table, is determined in the data configuration. The unit is 480 ms.



(1) Dedicated mode has been setupSDCCH/TCH)

(2) Active Abis monitor funciton

(3) SACCH message block (uplink/downlink) can not be explained resulting in radio link

timeout



When a handover command is received, the timer is set and begins to count. When a clear

command is received, the timer is reset.

When the timer is counting, two channels are retained. One is in the source cell and the

other is in the target cell. If the value of the timer is too big, the two channels are

occupied for a long time, wasting the resource.

If the value of the timer is too small, the number of call drops may increase. The handover

failure and call re-establishment failure lead to call drops. If the length of T3103 is too

short, the channel retaining time is too short for the channel in the source cell to

reconnect the MS after a handover failed. Therefore, the call drop rate rises.



When the call drop rate is high, first check whether this is a common or individual

phenomenon. Analyze whether the overall call drop rate becomes abnormal because the

call drop rates of several cells or all the BTSs are high. In the case of a common

phenomenon, check the coverage, cell parameters, and frequency planning in terms of

planning, analyze whether the link budget meets the requirements, whether the setting of

the uplink/downlink radio failure counter is proper, and whether the network interference

is too high. In addition, detect the BSC hardware, such as the clock. Then, perform drive

tests of the radio coverage of the network.

If the overall call drop rate is abnormal because the call drop rates of several cells are high,

rule out the call drops caused by equipment faults based on the traffic statistics.

Equipment fault check covers four aspects: A interface circuits, modules in the BSC, Abis

interface circuits, and BTSs. Alarms, which are reported when equipment faults occur, can

be used as references. In actual operations, most faults are caused in the microwave

transmission on the Abis interface and BTS RF components, especially DTRU boards and

DDPU boards.

After the equipment fault causes are ruled out, the following analyzes the call drops on

the radio interface in three aspects: interference, coverage, and handover.



1. Interference can be uplink interference and downlink interference.

Uplink interference can be analyzed based on the number of interference

bands into which the idle TCH falls in the cell performance measurement. It

is considered normal when the idle TCH fall into either interference band 1

or interference band 2. For a network that adopts tight frequency reuse, it

can also be partly accepted when the idle TCH falls into interference band

3. Here, take into account frequency hopping, PBGT handovers, and

coverage control comprehensively. If the idle TCH falls into interference

band 4 and over, check for the interference carefully. Generally, the higher

the traffic, the higher the interference from the inside the network.

Through the received quality measurement task and received level

measurement task, you can see that the grade of the voice quality in the

call increases when there is interference. From the inter-office handover

performance measurement, you can find that the ratio of handovers due to

poor quality increases. In addition, the number of reestablishment failures

upon handover failure also increases.

2. The two main coverage problems that cause call drops are insufficient coverage

and unbalance between uplink and downlink.

You can check whether the received signal is strong enough based on the

average received level and power grade in the power control measurement

task. When the transmitter power reaches the maximum output but the

received level remains very low, it indicates that there is poor area coverage.

In addition, you can refer to the average received level and quality during

the call drop. You can estimate the user distribution radius through the

distribution of TA values. You can analyze the cell coverage by observing

the received channel strength of the adjacent cell. Generally, you need to

perform detailed analysis in combination with drive tests.

Unbalance between the uplink and downlink may also cause call drops,

usually accompanied by the RF component faults or cable connection

problems. You can analyze the unbalance between the uplink and

downlink through the uplink/downlink balance measurement task, power

control measurement task, and call drop measurement task. In addition,

you need to pay attention to alarm messages and user complaints.

3. The MS fails to perform a handover to the best cell due to a handover failure

and a call drop may occur.

You can confirm this case by analyzing the average received level and

outgoing handover success rate of a cell with a high call drop rate. Both

cross coverage and target cell congestion may cause such a problem. In

this case, when the user moves, the call quality of the original cell keeps

falling due to a handover failure until a call drop occurs. You need to solve

such call drops by balancing the traffic and adding adjacent cell

relationships.

The analysis of the high call drop rate of an SDCCH is similar to that of a

TCH. The SDCCH, as a point-to-point signaling channel, is more sensitive to

interference that the voice channel. Generally, you can lower the call drop

rate by adjusting the access threshold and reducing interference.



Lets analyze the call drop reason and its solution on coverage, handover, interference, antenna, transmission, parameter.



1. Discontinuous coverage (blind area)

The signals on the edge of an isolated BTS are weak and of poor quality; thus,

calls are dropped as they cannot be handed over to other cells.

In complex landforms such as mountainsides, the transmission of signals is

blocked and discontinuous, which causes call drops.

2. Bad indoor coverage

Densely distributed buildings and thick walls cause great attenuation and low

indoor level, which lead to call drops.

3. Cross coverage (isolated island)

Cell B is the neighbor cell of cell A, but cell C is not. If the MS roams from cell A to

cell C and still seizes the signals of cell A, when a handover is initiated from cell A

to cell B, the MS will not find a suitable target cell and a call drop is caused.

4. Small coverage

If the hardware of a cell is faulty, for example, the radiator of the antenna is

blocked or the BCCH TRX is faulty (the power amplifier), the coverage might be

too small.



Drive test

When a user complains, find out the area of insufficient coverage and perform

large-area drive test. Observe the signal level, the handover, and the call drop.

Traffic measurement

Find out the BSC call drop rate via iM2000 to , find out the cells with high call drop

rate. Use other traffic measurements to help your analysis and judgment.

The following lists (in the next page) some traffic measurement tasks and measurement

items.



1. The power control performance measurement shows whether the average uplink and

downlink signal strength is low.

2. The power control performance measurement shows whether the maximum distance (TA

value) between the MS and BTS is exceptional for consecutive time segments.

3. The receiving level performance measurement shows whether the ratio of receiving low

level is high.

4. The cell performance measurement / inter-cell handover performance measurement show

whether the initiated handover level is low and whether the average receiving level is low.

5. The call drop performance measurement shows whether the level of call drop is low and

whether the TA value before call drop is exceptional.

6. The neighbor cell performance measurement shows whether the average level of a

neighbor cell defined in the cell neighbor relation table reported by the MS is low.

Whether there is an undefined neighbor cell with high average level.

7. The uplink/downlink balance performance measurement shows whether the uplink signal

and downlink signal within coverage is balanced.



1. Find out the area with insufficient coverage

Perform drive test to find out the area with insufficient coverage. For isolated BTSs and mountainside BTSs where there is no continuous coverage, more BTSs can be added to provide continuous coverage. Or you can use other means to improve the BTS coverage, such as improving the maximum transmit power of the BTS and changing the azimuth, tilt angle, and height of the antenna. Analyze whether the call drop is caused by geographical feature. In places such as tunnels, supermarkets, subway entrances, parking lots, or depressed areas, call drops happen easily. Micro cells can be used to provide the coverage for these areas.

2. To ensure the indoor communication quality, the outdoor signal must be strong enough. To increase the outdoor signal, you can increase the maximum transmit power of the BTS, change the azimuth, tilt angle, and height of the antenna. When these means cannot improve the indoor communication quality, more BTSs can be added. The indoor distributed system can be used to provide enhanced indoor coverage for office buildings and hotels.

3. For cells whose neighbor relation is not defined due to overshooting coverage, define the neighbor relation to reduce the call drops because no appropriate cell is available for handover. Reduce the tilt angle of the BTS to eliminate overshooting coverage.

4. Remove hardware problem

Perform drive test to see whether the hardware is faulty and whether the coverage is small. If the call drop rate suddenly rises and the other counters of the BTS are normal, check whether the neighbor cells work normally. (It is likely that the downlink is faulty. For example, the TRX, diversity unit, or antenna is faulty. An uplink fault may lead to the problem that the outgoing cell handover failure rate is high.)



1. Unreasonable parameters

If the signal level of the area where two cells intersect is low, or the level of the

handover candidate cell is set low, or the handover threshold is set low, some MSs

will enter a neighbor cell whose level is slightly stronger than the serving cell for a

time segment. Shortly after the handover, if it happens that the cell signal

weakens and there is no appropriate cell for a handover to occur again, call drop

occurs.

2. Incomplete neighbor cells

Incomplete definition of neighbor cells leads to the problem that the MS

maintains the conversation in the current cell, until the MS moves beyond the

edge of the cell coverage and cannot be handed over to a cell with stronger signal,

hence the call drops.

3. There is a neighbor cell with the same BCCH and the same BSIC

4. Traffic congestion

Due to unbalanced traffic, there is no available channel in the target BTS for

handover, causing handover failure. When reestablishment also fails, call drop

occurs.

5. BTS clock is out of synchronization, frequency offset exceeds a predefined threshold,

resulting in handover. When the handover fails, call drop arises.

6. T3103 timer overflows, causing call drop. For details, see the preceding description.



Analyze the traffic measurement

Check for cells that have low handover success rate, many times of handover

failures and reestablishment failures, and high call drop rate.

Use traffic measurement to analyze what causes the handover. Examples are

handover caused by uplink and downlink receiving levels, handover caused by

uplink and downlink receiving quality, handover caused by PBGT, call directed retry,

and handover caused by traffic.

Check the alarms

Check for clock alarms related to the BTS. Check whether the BTS clock runs

normally. If necessary, check the BTS clock to remove the clock problem.

Perform drive test

Check for handover problem during the drive test. Perform multiple drive tests around the

faulty cell. Find out the call drop problem related to handover from multiple perspectives.

Optimize the handover to reduce call drop.



1. The inter-cell handover performance measurement shows whether the number of

handover failures and reestablishment failures is large.

2. The inter-cell handover performance measurement shows whether the number of

handover and the number of reestablishment successes are large.

3. The neighbor cells performance measurement shows whether the levels of the undefined

neighbor cells and the number of measurement reports exceed the threshold.

4. The outgoing cell handover performance measurement shows whether the outgoing cell

handover success rate is low (specific for a cell). Find out the success rate of the handover

to which neighbor cell is low and further investigate reasons from the target cell.

5. The incoming cell handover success rate is low. The handover parameter setting of the

peer cell is unreasonable.

6. The TCH performance measurement shows whether the ratio of the number of handovers

to the number of TCH call seizure successes is out of proportion and whether the number

of handovers is large. (handover/call > 3)



1. Check the parameters impacting handover, such as settings of hierarchical levels,

handover thresholds, handover hysteresis, handover valid time, handover watch time, and

minimum access level of the handover candidate cell. For example, to reduce call drop

caused by handover, you can increase the minimum access level of a handover candidate

cell from -100 dBm to -95 dBm, that is, from level 10 to level 15. For example, you can

increase the value of the parameter Max Resend Times of Phy. Info. When handover

becomes slow or the success rate is lowered due to clock problems or poor transmission.

In short, you need to optimize the handover by taking into account the actual situation.

2. For call drops caused by unbalanced traffic that leads no available handover channel in the

target BTS, the solution is to adjust the traffic. For example, you can adjust the tilt angle

and azimuth of the antenna, control the cell coverage, adjust the network parameters

such as CRO to let an MS reside in an idle cell, set the layer priority to let an MS in

conversation hand over to an idle cell, use load handover to balance the traffic, or directly

expand the TRXs to solve the problem.

3. If the BTS clock is faulty, calibrate the clock to provide good clock synchronization.



The main interferences are co-channel interference, adjacent-channel interference, and

cross-modulation interference.

When an MS receives strong co-channel or adjacent-channel interference from the

serving cell, the bit error rate deteriorates and the MS cannot correctly demodulate

the BISC of the neighbor cells or the BTS cannot receive the measurement reports

from the MS, causing call drop.

When a BTS is affected by cross-modulation interference, the consequence is that

the timeslots cannot be allocated, wasting the BTS resource.

The interference thresholds are co-channel carrier/interference ratio C/I = 9 dB and

adjacent-channel carrier/interference ratio C/A = -9 dB. When the interference

exceeds the thresholds, the communication in the network is interfered,

deteriorating the communication quality and causing call drops.



Interference can be from inside the network or outside the network, in the uplink signal or

downlink signal. We can use several methods to locate the interference. The following

methods are commonly used:

1. Analyze the traffic measurements to find out the interference.

2. When there is a user complaint, perform drive test at the place complained by the

user and check for downlink interference. Use drive testers to find out places with

strong receiving level but poor communication quality. Use test MS to lock a

frequency and perform dialing test to check whether a frequency is interfered.

3. Check the frequency planning to see whether improper planning may lead to co-

channel and adjacent-channel interference.

4. Adjust the interfered frequency to see whether the interference can be avoided

and minimized.

5. Check for interference caused by the equipment.

6. If the above methods cannot remove the interference, use spectrum analyzer to

scan the frequencies and find out the interfered frequency and the interfering

source.



1. Analyze the interference band in the traffic measurement and check for uplink

interference.

If there is an idle channel in the interference band 3, 4, and 5, it indicates that

there is interference. In case of interference from inside the network, the

interference increases as the traffic increases. In case of interference from outside

the network, usually the interference is not related to the traffic increase. It should

be noted that the interference band is reported by the BTS TRX channel under the

idle state to the BSC through the RF resource indication message. The interference

band indicates the uplink feature of the radio channel occupied by the MS, that is,

the interference of the uplink signal. If the current channel is busy, the RF

resource indication message cannot be reported. Therefore analysis of the

interference band should consider such factors.

2. Receiving level performance measurement (describes the matrix relation between the level

and the quality)

This is a measurement task for the TRXs. If there are many times of high level and

low quality, it indicates that the frequencies of the TRX board have co-channel

interference, adjacent-channel interference, or interference from outside the

network.

3. BQ handover ratio

The cell performance measurement, inter-cell handover performance

measurement, and cell handover performance measurement count the number of

outgoing handover attempts caused by various reasons. If the number of

handovers caused by bad quality is large, it indicates interference. If there are

many uplink BQ handovers, it indicates uplink interference; if there are many

downlink BQ handovers, it indicates downlink interference.



4. Receiving quality performance measurement

This counts the average receiving quality of a TRX for reference.

5. Call drop performance measurement

This records the average level and quality during a call drop for reference.

6. Both handover failures and reestablishment failures are many

It is likely that the target cell has interference. This is for reference.



Interference from outside the network and interference from inside the network are

solved by different methods. Invalid interference from outside the network is solved by

the Radio Committee; interference from inside the network is solved by adjusting the

network.

1. Perform drive tests. Check the places where interference occurs and distribution of

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

interference. According to the actual condition, adjust the related BTS transmit

power, tilt angle of antenna, or frequency planning, to prevent interference.

2. Use discontinuous transmission (DTX), frequency hopping, power control, and

diversity technologies.

These methods can lower the system noise and improve the anti-

interference capability of the system. DTX is divided into uplink DTX and

downlink DTX. DTX reduces the effective transmission time and lowers

system interference level. But DTX must work with the actual radio

environment and be adjusted according to the relation with the neighbor

cells. When the MS receiving signal is bad, using DTX may cause call drop.

This is because when DTX downlink function is enabled and an MS

establishes a call, the BTS transmit power increases when the user is

talking and lowers during the gap of the call. This lowers the interference

on other BTSs on the one hand, but on the other hand, if there is

interference around the BTS, the discontinuous transmission of the

downlink signal worsens the communication quality. When the BTS lowers

the transmit power, in the place with low receiving level and strong

interference signal, the communication quality deteriorates and call drop

even arises.

3. Exclude the interference caused by the equipment (such as self-excitation of the

TRX board and cross-modulation interference of the antenna).



The following section analyzes call drops due to unbalanced uplink and downlink caused

by antenna feeder (tower amplifier, power amplifier, and antenna feeder)

1. If the antenna feeders of a cell are wrongly connected, for example, if the transmit

antennas between two cells are inversely connected, the uplink signal of the cell is

worse than the downlink signal. In places far from the BTS, problems like call drop,

monolog, and difficulty in making a call may arise.

2. A cell that uses uni-polarization antenna has two antennas. The difference of the

antenna downtilt Angle causes call drop.

When a directional cell has a main antenna and a diversity antenna, the

BCCH and SDCCH of the cell may be transmitted from two different

antennas of the cell. When the pitch angles of the two antennas are

different, the coverage of the two antennas is also different, in which case

a user can receive the BCCH signal but cannot occupy the SDCCH

transmitted by another antenna when initiating a call, causing call drop.

3. Call drops caused by the azimuths of the two antennas

When the azimuths of the two antennas are different, a user can receive

the SDCCH, but once the user is assigned to the TCH transmitted by

another antenna, call drop arises.

4. Call drops caused by antenna feeder.

If the antenna feeder is damaged, watered, bended, or if the connector is

in bad contact, the transmit power and receive sensitivity are lowered,

causing severe call drops. You can confirm the problem by testing the

standing wave ratio.



Methods to Analyze and Judge a Call Drop Caused by Antenna Feeder

1. Check for alarms related to combiner, DDPU, tower amplifier, and standing wave

ratio.

2. Check the BTS from the remote maintenance console to see whether the boards

are normal. Analyze the traffic measurement to see whether the uplink and

downlink are unbalanced.

3. Use the Abis interface tracing on the LMT or use MA10 to trace the Abis interface

and observe the measurement report of the signaling message to see whether the

uplink and downlink are balanced.

4. Perform drive test and dialing test. During the drive test, check whether the BCCH

of the serving cell is consistent with the planning, that is, whether the transmit

antenna of the cell is installed correctly.

5. With remote thorough analysis, go to the BTS site to check and test. Check

whether the azimuths and pitch angles of the antennas comply with the design

specifications and whether the feeders and jumpers are connected correctly.

Check whether the connectors of the antenna feeder are in good contact and

whether the antenna feeders are damaged. Check whether the standing wave

ratio is correct. The preceding methods exclude the faults of the antenna feeders.

6. Check whether the BTS hardware fault causes unbalanced uplink and downlink,

resulting in call drop. If you suspect that a component is faulty, you can replace

the component. Or close other TRXs of the cell and make dialing test over the

suspected TRX to find out the fault. Once a component is found to be faulty, the

component must be replaced timely. If there is no spare part, block the faulty

component to avoid call drops that affect the network operation quality.



Use the following traffic measurement to analyze the uplink and downlink balance and to

help check whether the antenna feeders are faulty.

1. Register "performance measurement of uplink and downlink balance" in the traffic

measurement to analyze whether the uplink and downlink are balanced.

2. Register "call drop performance measurement" in the traffic measurement to

analyze the average uplink and downlink levels and uplink and downlink qualities

during the call drop.

3. Register "power control performance measurement" in the traffic measurement to

analyze the average receiving levels of the uplink and downlink.



The following section analyzes call drops caused by transmission.

Because there are the Abis interface and the A interface links, if the transmission

quality is bad and the transmission link is unstable, call drop arises.

Methods to analyze and solve call drops caused by transmission:

1. 1. Observe the transmission and board alarms (GTPU/GTPUX fault, A interface

PCM out-of-synchronization alarm, LAPD disconnection, power amplifier board,

HPA, and DTRU alarms). Check for intermittent transmission and faulty boards

according to the alarms (such as bad TRX board and poor contact).

2. 2. Check whether the transmission paths, bit error rate, 2M connectors, and

equipment grounding are normal. Ensure stable transmission quality to reduce call

drops.

3. 3. Observe the traffic measurement to see whether there are many call drops

caused by transmission.

TCH performance measurement shows the A interface failures during TCH seizure

TCH performance measurement shows whether the TCH availability is normal.

TCH performance measurement shows call drops due to interruption of terrestrial links



Check the parameter configuration. Reasonably configure the parameters in accordance with the data configuration specifications. The parameters are described below.

1. Radio Link Timeout

If the value of this parameter is small, when the receiving level of the MS suddenly fades drastically due to geographical reasons, call drop arises. If the value is large, though the voice quality can no longer be accepted, the network cannot release the related resource until radio link timeout, lowering the resource usage ratio. The value of this parameter can be set large for remote areas with small traffic and small for areas with large traffic.

2. SACCH Multi-frames

The BTS uses this parameter to notify the BSC of radio link connection failure. The BSS determines a radio link connection failure according to the BER on the uplink SACCH.

When the BTS receives the measurement report sent on the SACCH, the counter for determining the radio link fault is set equal to the value of this parameter. Each time the SACCH measurement report sent by the MS cannot be decoded, the counter decreases by 1. If the SACCH measurement report is correctly decoded, the counter increases by 2.When the value of the counter is 0, it indicates that the radio link fails. The BTS sends a connection failure indication message to the BSC. The number of SACCH multi-frames and the radio link failure counter in the system information define the radio link failure times of the uplink and the downlink respectively. A failure is determined if the SACCH message cannot be correctly decoded.

Recommended value: BTS3X 14



3. RXLEV_ACCESS_MIN, RACH Min Access Level

Because there are uplink and downlink signals, the coverage is determined by the weaker signal. If the coverage of the uplink signal is greater than the coverage of the downlink signal, the downlink signal at the cell edge is weak and can be overwhelmed by strong signals from other cells. If the coverage of the downlink signal is greater than the coverage of the uplink signal, the MS has to reside at the strong signal, but the uplink signal is weak and the MS cannot make a call, resulting in poor voice quality, monolog, or even call drop. Therefore the uplink signal and the downlink signal should be balanced.

RXLEV_ACCESS_MIN

This parameter indicates the minimum receive level required for an MS to access the system. The parameter refers to the downlink signal. If the value of the parameter is small, the requirement for the access signal level is low, the MS easily accesses the network, and the coverage is large. But an MS at the cell edge wants to reside in the local cell, increasing the cell load and the danger of call drop. A large value of the parameter restricts an MS with low receiving level to access the network. This is conducive for reducing call drops at the cost of small coverage. You should trade off the coverage and call drop rate to set the parameter and shall not set a large value to lower the call drop rate while reduce the coverage. Set this parameter reasonably to balance the uplink and the downlink.

RACH Min Access Level

This parameter indicates the minimum receiving signal level required for the MS to access the system in the BTS3X. This parameter affects an MS access. The system judges the level threshold of a random MS access.

This parameter is similar to RXLEV_ACCESS_MIN. You should trade off the coverage and call drop rate to set this parameter.

For details about setting the parameter, refer to network planning data configuration specifications.

4. Adjust unreasonable handover parameters

Reasonably add neighbor cells. Missing neighbor cells is one of the main reasons that cause failure handover, resulting in call drop.

The parameters that are frequently adjusted are: Min DL Level on Candidate Cell, Inter-cell HO Hysteresis, PBGT HO Threshold, valid times, watch times, BQ HO Threshold, and Edge HO Threshold.

5. Adjustment of the power control parameters

Modify the power control parameters to make power control more sensitive and the power-controlled level strong enough to ensure normal communication.

For example, if the expected uplink level is low or the filter length is long, the power control will not be sensitive.



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 immediate adjacent BTSs cannot use the

same frequency (even if the directions of the antenna main lobes are not the same, the

interference of side lobes and back lobes may cause strong interference).

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 frequencies

should be more than twice of the hopping carriers.

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



From the above we can see, the interference goes along with the traffic. This is the

characteristic of internal interference. But we cannot exclude the possibility of that

the interference come from another wireless communication system.



The traffic statistics which can help to identify the conclusion are as follow:

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

2. Interference is quite a possible cause of congestion.

3. The call drop rate is far higher than the normal.

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

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

can be caused by uplink interference or hardware fault.

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

can be caused by 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.



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

coverage. Due to its own characteristics, it will bring interference 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 not enough separation between the donor 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 with adjacent BTS very

easily.



Clear uplink interference

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

high. External interference is mainly intermodulation interference. The uplink interference

can be cleared by modifying the co-channel frequency of the co-channel cell base on

analyzing related results in drive test, increasing the distance between two co-channel

cells. Interference can also be reduced by frequency hopping, DTX 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.



1. Because no matter which TRX 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 complex.

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 PM 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 and feeder of cell 3

with that of cell 1, but cell 3 interference remains the same. Therefore, it can basically be

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.



1. Although there is a 10MHz 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 inter-modulate

with other signals. Some parts of intermodulation components may fall in the receiving

band and form interference.

2. In daytime the traffic is larger than that at nightso the intermodulation components (interference) are also more than those at night.



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 it always stays in a service cell of a remote

BTS A before call drop, and its TA value is about 17, and the receiving signal strength is

about -80dBm.



When there is an isolated coverage island from a cell in an area, if MS stays in this cell at

the island area and make a call, no matter how the signal changes, handover cannot be

implemented normally and 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.



Documents

173287126 12 OMO312000 BSC6000 GSM Call Drop Problem Analysis ISSUE1 01