1- Interference Problems

Interference is a key factor affecting network performance, including conversation

quality, call drop, handover, congestion, and so on.

I. Interference Sources

In the mobile telecommunication system, when the BTS is receiving the signals from
a remote MS, it will not only be interfered by other telecommunication equipments,
but also it will be interfered by the other BTSs and MSs within the system, as shown
in Figure 8-1.

Figure 8-1 Schematic drawing of mobile telecommunication interference

Hereunder introduces the interference sources affecting the GSM system.

 Intra-network interference
If the frequencies are improperly planned, or the frequency reuse is too
aggressive, intra-frequency interference or neighbor cell interference will be
caused.
 Repeater interference
At the early stage of network construction, repeaters are widely used for
extending the coverage distance of the network. However, if the repeaters are
improperly planned, the network will be interfered.
If the repeaters are not installed according to requirement, that is, there is not
enough isolation left between the donor antenna and the subscriber antenna, the
BTS to which the repeaters attach will interfered.
For the repeaters enabling broadband non-linear amplifier, the intermodulation
indexes are far greater than that required in the protocols. In this case, the
greater the power is, the greater the intermodulation will be. Therefore, the BTS
near the repeaters will be interfered.
 Interference from other big-power telecommunication equipments
 These equipments include radar, analog BTS, and other telecommunication
equipments using the same band.
 Hardware problems
TRX problem: If the performance of the TRX decreases, the system may be
interfered.
CDU problem or splitter problem: Active amplifier is used in the CDU splitter and
splitter module. When any problem occurs, the system may also be interfered.
Stray and intermodulation: If the out-band stray of the power amplifier or the TRX
of the BTS go beyond requirement, or the isolation of the transmission and the
reception of the CDU duplexer is too small, the connection channel will be
interfered. Meanwhile, the passive equipments, such as the feeder and the
antenna, will generate intermodulation.

II. Interference Positioning and Elimination

[Positioning and elimination procedure]

1) Find out the interference cell according to KPI
If the call drop rate, handover success rate, traffic volume, congestion rate, and
interference band of a cell deteriorate to a bad level abruptly, it means that
interference may exist in the cell.
In this case, you can also check the historical record of operations made in this
cell. For example, check if the hardware and software of the BTS has been
added or increased and if the data of the BTS has been modified. Generally, the
appearance of interference is related to these operations.
If these parameters are not adjusted, the interference may be from the hardware
itself out outside factors. In this case, you are suggested to check if it is hardware
problem. If it is not, you should check outside factors.
2) Check OMC alarm
Sometimes high call drop rate, low handover success rate, and high congestion
rate may be related to equipment problems. In this case, you can check OMC
alarm records. These records are related to the deterioration of these indexes.
3) Check frequency planning
If the interference is doubt in a cell, you can check the frequency planning for the
cell and the neighbor cells of the cell. For this check, you are required to make
clear the distribution of the antennas, find out the azimuth angle of each cell,
draw the topology, and mark the BCCH/TCH channel numbers. Meanwhile, you
are also required to compare the planned channel numbers with the configured
channel numbers in the BSC.
 According to the accurate frequency planning topology, you can make sure if the
intra-frequency interference or neighbor frequency interference is present in the
network.
4) Check cell parameter configuration
The cell parameters, such as CRO, threshold, handover duration, neighbor cell
relationship, and so on, may have interference against the system.
If the CRO is set to a great value, the MS may be guided to an idle cell whose
level is lower than its surrounding cells. Once the conversation is started but the
C/I cannot meet the threshold requirement (12dB), interference will be caused.
If neighbor cells are missing, the MS cannot hand over to a cell with better signal
level and quality. In this case, the interference will also be generated. If the
handover threshold and the P/N are too great, the handovers between cells are
unavailable. If the P/N is too small, however, it will result in frequent handover. In
this case, both the call drop rate and the system load will be increased.
5) Drive test
Drive test is an effective method to position the interference. There are two drive
test methods: idle mode test and dedicated mode test.
For idle mode test, the test equipment can test the signal level of both the signal
level and the neighbor cells. In addition, the test equipment can also perform the
frequency sweep test for the designate channel numbers or bands. In this case,
the interference caused by cross-cell coverage signals can be discovered.
For dedicated mode test, the test equipments can test the signal level of the
service cell and neighbor cells, the Rxqual, the TA, and so on. If the Rxlev is
equal to or greater then -80dBm and the Rxqual is equal to or greater than 6 in
an area, it can be confirmed that the interference exists in the area. Some test
equipment can display the FER (frame error rate). Generally, if the FER is
greater or equal to 25%, the conversation will not be continuous. That is, the
interference exists.
6) Interference elimination
You can eliminate the interference according to the above checked results, and
then evaluate the elimination through KPI and drive test.
[Hardware problem positioning and elimination]
When the interference is doubted in a cell, you should first check if the BTS where the
cell locates works normally. In the remote end, you should check if there is antenna
alarm, TRX alarm, or BTS clock alarm generated. In the near end, you should check if
there is antenna problem, water penetration, feeder (jumper) damage, CPU problem,
TRX problem, wrong jumper connection or clock problem occurred.
 Antenna performance decline
Antenna a passive component and its damage probability is small. However, if
the antenna is damaged or its performance declines, the voice quality will
become poor.
 Antenna connector problem
GSM RF signals are micro wave signals. If the connections between TRX, CDU,
feeder, and antenna have any problem, both the standing wave ratio and the
intermodulation will increase. In this case, the interference will be resulted.
 Inverse antenna connection
The inverse antenna connection is a commonly seen problem. If the antenna is
inversely connected, the channel numbers used by the cell and the planned
channel numbers are completely inconsistent. In this case, intra-frequency
interference, inter-frequency interference, and handover difficulty will be resulted.
Especially for the networks that have inadequate frequency resource, the inverse
antenna connection has great effect against network quality.
 Jumper problem
Many jumpers locate between antennas, so they are often wrongly connected. In
this case, high call drop rate will be resulted.
 TRX problem
If TRX problems occur, the interference will increase, the coverage distance area
will decrease, and the access is difficult.
 Clock failure
If the clock deviation is too great, it is hard for the MS to lock the frequencies of
the BTS, so the handover failure always occurs, or the MS cannot camp on any
cell of the BTS. In addition, if the clock deviation is too great, the BTS cannot
understand the signals of the BTS, which will result in bit errors. However, the
clock failure will not really introduce interference, but it is the transmission errors
that make the voice quality decrease.
 Conclusion
Any problem concerning the TRX, CDU, feeder, antenna, jumper, and connector
may cause interference or call drop. Therefore, if interference appears, you
should check the hardware of the BTS. In addition, BTS clock failure will also
cause interference and call drop.
It is easy to solve the hardware problems through changing the boards or
adjusting traffic data. If there is spectrum analyzer available, you can position the
problem more efficiently. Especially when the interference appears without any
modification of network data, you should focus on checking the hardware.
[Intra-Network Interference]
The intra-network interference is mainly from intra-frequency interference and
neighbor cell interference. When C/I is smaller than 12dB or the C/A is smaller than -
6dB, the interference is unavoidable. However, the aggressive frequency reuse
technology will increase of the occurrence probability of interference.
 Same-frequency and neighbor frequency interference
In GSM system, the frequency reuse is unavoidable. When the frequency reuse
distance of two cells using the same frequency is smaller than cell radius, same-
frequency interference will be easily caused. Past experiences show that the
frequency reuse must be avoided in many cases.
Figure 8-2 shows the cellular structure.

Figure 8-2 Cellular structure

As shown in this figure, there are 4 BTSs, A, B, C, and D. If the channel number N is
allocated to A3 cell, this channel number cannot be allocated to A-1, A2, B1, B2, C1,
C2, C3, D1, D2, and D3. And the channel numbers N±1 cannot be allocated to A1,
A2, A3, B1, C2, D1, and D2 (no frequency hopping).
The interference against the uplink channel numbers can be judged by the
interference band data in the traffic statistics.
For the interference against the downlink channel numbers, the existing drive test
equipments can be indirectly used to measure if the same-frequency interference is
present. First you should lock the test MS in the service cell and enable make the MS
 work in conversation mode during drive test. If you find that the Rxlev in an area is
high but the Rxqual is low, it is likely that the same-frequency is present in this area.
 Interference caused by cross coverage
In a properly designed network, each cell covers the areas around the BTS only
and the MS camps on or holds conversation in the nearest cell. Cross coverage
means that the coverage of a cell is too large and the cell can cover the areas
under the control of other BTSs. If cross coverage occurs, irrational traffic
absorption, interference, call drop, congestion, and handover failure may arise.
 Interference caused by aggressive frequency reuse
Capacity and quality always contradicts to each other. In urban areas, the
aggressive frequency reuse technology must be used for the number of
subscribers in urban areas are great. In this case, the network quality will surely
decrease. In the areas where BTSs are irrationally distributed, the aggressive
frequency reuse technology may cause the collision of same frequency and
neighbor frequencies.
 Interference caused by repeater
It is convenient to use repeater for special coverage. However, if a repeater is not
qualified or it is not properly installed, it will cause interference.
 Interference caused by outside environment
Outside environment, such as TV station, big-power radio station, micro wave,
radar, high voltage wire, analog BTS, and so on, will cause interference.

III. Interference Cases

Case 1: Interference cause by antenna performance decline

[Problem description]
There are 5 BTSs in a county. The configuration type is S4/4/4 or S6/6/6. The
interference band 5 reaches 15 according to the TCH performance measurement of
the most cells. There is no alarm found at the OMC.
[Problem positioning and solution]
1) Through monitoring and registering the interference band traffic statistics for the
problem cells all day, engineers found that the interference band 5 mostly
appeared at day time, and it seldom appeared at early morning.
2) Through sending the idle BURSTS of all the BTSs, engineers found that the
interference bands of these cells appeared in the early morning. If the sending of
these idle BURSTS stopped, these interference bands disappeared. Therefore, it
can be proved that the interference came from the network. It is not related to
other telecommunication equipments.
3) The frequencies and other data were not adjusted before the interference
appeared, so the interference is not related to the frequency planning.
4) Through surveying the RXM test interface of the CDU using the spectrum
analyzer during the traffic peak at day time, engineers found that the broadband
interference was strong and the back noise was rising.
5) There was no interference in one cell, but the interference in another two cells
was strong. Through replacing the antenna feeder of the cell with no interference
with the antenna feeders of the cells with interference and sending idle BURSTS,
engineers found that the interference went with the antenna feeder. Therefore, it
can be decided that the problem occurred at the antenna and feeder system.
6) Through changing the antenna, engineers found that the interference went with
the antenna. Therefore, the problem is likely present at the antenna.
7) Through replacing the antenna with dual polarization antenna, engineers found
that the strong interference disappeared immediately. Through replacing the old
antenna of another BTS with a new one, engineers found that the interference
also disappeared.
Case 2: Call drop caused by intra-network interference
[Problem description]
Customers in a place complaint that call drop happen frequently. Figure 8-3 shows
the topology for BTS distribution and frequency planning.

Figure 8-3 Topology for BTS distribution and frequency planning

In this figure, 112, 107, 120, 124, 118, 122, 104, 106, 116, 101, 110, and 113 are
BCCH channel numbers; 109, 102, 115, 96, 98, 100, 111, 114, and 108 are TCH
channel numbers.
[Problem analysis and solution]
1) Through a careful test, engineers found that there were 12 channel numbers
gathering at the call drop spot and Rxlev reached -73dBm. When the MS seized
channel number 11, the interference from channel number 112 caused the call
drop.
2) Through testing the CGI of channel number 12 using test MS, engineers found
that this channel number was one of the BCCH number of D3.
3) Through surveying BTS D, engineers found that the antenna of D3 is installed at
the top of a building. In addition, a house made of glass was found 8m away and
4m under the antenna. Engineers tested that the signal strength near the
antenna was about -45dBm, and the signal strength at near the glass was -
30dBm, which was beyond the expectation of engineers. In fact, the cause was
that the signals reflected by the glass were reflected to the call drop spot.
4) It is suggested to change the antenna installation place and channel number.
You should interchange the channel number 111 and channel number 114 of
BTS A and increase the down tilt angle of A3 cell. In addition, to avoid the
interference caused by channel number 111 after the interchange, you should
adjust the direction of channel number 113 of C1 cell.
5) Test shows that everything is normal after the adjustment. The channel number
113 of BTS C has no effect against channel number 114. And the call drop
disappears.
Case 3: Interference caused by repeater
[Problem description]
Users in an area complaint that the MS cannot seize a channel to hold conversation,
or the noise is great after channel seizure and the channel and the MS signal is
strong. Two BTSs are installed in this area. The antenna azimuth angle of cell1 rightly
directs to the north. Before user complaint, the BTS in this area ran normally and the
network indexes met the requirement. After the problem arisen, the traffic volume of
the two BTSs dropped sharply from the perspective of traffic statistics indexes. In
addition, the traffic volume of cell1 and cell3 also dropped sharply. Though the signals
for the conversation were strong, the voice quality was quite poor. According to traffic
statistics, the interference bands of the four cells were of level 3, level 4, and level 5,
and 95% of the channels were interfered. In addition, other channels were interfered
to some extent. However, no alarm messages were generated at the OMC.
[Problem analysis and solution]
6) According to user feedbacks, the possible reasons include transmission problem,
antenna feeder problem, hardware problem, intra-network interference, and
outside interference.
7) The uplink interference signals in the northwest direction might strong.
Therefore, cell1, cell2, and cell3 of the two BTSs were interfered, in which cell1
and the cell3 were seriously interfered.
8) Through on-site dialing test, engineers found that it was hard to make calls in the
areas covered by cell1 and cell3. Even if a call was put through, the voice quality
was quite poor. In addition, the voice was discontinuous and the interference
was strong. Through using MS to call a fixed phone, engineers found it was hard
to hear the voice clearly. On the contrary, they could hear the voice from the
fixed phone clearly. This has proved the above analysis. That is, the interference
might be from the outside, or the standing wave problem was occurring at the
antenna (from this perspective, it can be judged that the interference existed on
the uplink only).
9) Through using antenna feeder analyzer to perform on-site test, engineers found
no problem was existing at any BTS. A new repeater was found in this area, and
it was located two kilometers away from the BTS in the northwest direction.
Moreover, the interference appeared just when the repeater was enabled. On-
site test found that the BTS became normal state once the repeater shut down,
and the interference bands also became normal, so did the call. If the repeater
was enabled, however, it was hard to make calls and the interference was
strong. At last, the agreement to shut down the repeater was reached. After that,
the conversation became normal.
Case 4: Microwave interference
[Problem description]
During network maintenance, through analyzing BSC traffic statistics, engineers
found that the call drop rate of the cell2 and cell3 of a S2/2/2 BTS arisen abruptly, and
the value even reached 20% at some time.
[Problem analysis and solution]
10) Through checking BSC traffic statistics, engineers found that the number of idle
TCHs was increasing at the interference bands 3-5 around 8:30. Around 10:00,
the idle TCHs were found at the interference band 4 and interference band 5.
Around 22:00, the idle TCHs were found at the interference band 1. Therefore, it
could be judged that the interference existed.
11) Because the BTS ran normally, the problems cannot be related to frequency
planning.
12) According to the TRX management messages, engineers found that the
interference existed at the four boards of the cell2 and cell3 of the BTS. Because
the probability for the four boards to be damaged simultaneously is quite small,
the TRX problem can be excluded. However, one board was changed in case of
abnormal conditions, but the interference was not eliminated.
 13) Through checking all the BSC traffic statistics data, engineers found that cells of
the BTSs near the BTS and the cells of the cell2 and cell3 of the BTS were
interfered to some extent. In addition, engineers also found that the SDCCHs (16
SDCCHs in total) of the seriously-affected cells were seized at sometimes.
However, the number of subscribers determined that the probability for all the
SDCCHs to be seized simultaneously is quite small. Therefore, it could be
judged that the uplink was interfered by outside factors. However, the
interference might be related to direction only.
14) To further position the problem, engineers interchanged the jumpers of cell1 and
cell3 at the set top. In this case, the interference was found at cell1, but the
interference was disappearing from cell3. Therefore, the interference was not
related to channel number.
15) Because the interference was not related to channel number, it might be the big-
power signals that caused the interference.
16) Through using the spectrum analyzer to perform the measurement at the output
interface of the BTS splitter, engineers found that the big-power signals existed
at the 904MHz channel number (it has an interval of 5M between the used
channel number. For the BTS where the interference was strong, the signal level
can reach as high as about -25dBm. For other BTSs, the signal level was about -
50dBm. Therefore, it could be judged that it was this signal that affected the
BTS.
17) Through using the spectrum analyzer to scan the areas near the BTS, engineers
found that there was a microwave antenna outputting big-power at the channel
number 904.

The interference disappeared after the microwave equipment was shut down

Call Drop Cases

Case 1: Call drop caused by frequency hopping collision

[Problem analysis]
A BTS uses 1 x 3 RF frequency hopping. After capacity expansion, the TCH allocation
failure rate is still high due to radio link problems. In addition, the TCH call drop rate
and incoming handover failure rate are high. The SDCCH call drop rate is normal.
[Problem positioning and solution]
Because high call drop rate and high incoming handover failure rate come together
with the TCH allocation rate, it can be judged that the problem may arise during TCH
assignment or the channel numbers or timeslots seized by the call are interfered or
unstable. Because the SDCCH call drop rate is normal, it can be judged that the
probability for the BCCH carriers and BCCH numbers to the interfered are small, but
the non-BCCH carriers and non-BCCH numbers may be greatly interfered.
Through checking the hardware, antenna feeder, and transmission, engineers found
no problem. According to drive test, engineers found that the signal level was high but
the quality was poor. Through on-site dialing test, engineers found that the
conversation quality was poor. Through checking engineering parameters, engineers
found that the MAIO of the new carrier was the same as that of the old carrier.
Therefore, it can be judged that the call drop was caused by the frequency hopping
collision. After modifying the MAIO, engineers found that call drop rate became
normal.
Case 2: Call drop caused by isolated island effect
[Problem description]
Users complained that call drop always occurred above the fifth floor of a building.
[Problem analysis]
18) Through on-site test, engineers found that call drop and noise existed here. As
far as the test MS was concerned, it was always in the service area of the other
BTS (hereunder called BTS B) other than the local BTS (hereunder called BTS
C) before the call drop.
19) It is estimated that the service cell belongs to BTS B, which is 3-4 kilometers
away from the building. Therefore, it can be judged that the signals from the BTS
B are reflected signals. As a result, an area similar to an isolated island is
formed.
20) Through checking data configuration, engineers found that only the cell 2 of BTS
A has the neighbor cell relationship with BTS B. Therefore, when the MS is using
the signals in cell 2 of the BTS B, if the signals in cell 3 of BTS A were strong,
and if the cell 2 of BTS B has no neighbor cell relationship with the cell 3 of the
BTS A, the handover cannot be performed.
The signals from the cell 2 of BTS B are reflected many times. Therefore, when
signals (from BTS B) received by the MS became poor dramatically, emergent
handover may be initiated. In this case, however, either the cell 2 or cell 3 of the
BTS A is not an ideal candidate cell for the cell 2 of BTS B. As a result, the MS
may hand over to other BTS (hereunder called BTS C), but the MS cannot
receive the signals from BTS C. Therefore, call drop occurs.
[Solution]
You are recommended to change the data in the BA1 (BCCH) list, BA2 (SACCH) list,
and neighbor cell relationship list. For example, you can configure the cell 3 of BTS A
as the neighbor cell of cell 2 of BTS B. To eliminate the isolated island effect, you
should also further optimize the engineering parameters. After that, the call drop
problem can be solved.
Case 3: Reduce call drop rate through optimizing handover parameters
[Problem description]
The drive test in an area found that the call drop rate at a cave near the BTS high
because the handover cannot be performed in due time.
[Problem analysis and solution]
The cave is near the BTS. The signal level of the target cell is about -80dBm in the
cave, but the signal level of the old cell drops below -100dBm. The downlink power of
the two cells outside the cave is good, so the handover cannot be initiated. However,
the signal level deteriorates dramatically in the cave, so the call drop occurs before
the measurement time is arriving.
To reduce the call drop rate, you can optimize and adjust the handover parameters:
1) If no ping-pong handover is present and the conversation is continuous, you can
make the PBGT handover happen as easily as possible.
2) Set the threshold to trigger the emergent handover rationally so that the
emergent handover can be triggered before call drop.
For the parameter modification, see

Table 8-1 Handover parameter optimization

Parameter Name Before Modification After Modification

PBGT handover measurement time 5 3

PBGT handover duration 4 2

PBGT handover threshold 72 68

Uplink quality threshold for emergent
70 60
handover

Minimum downlink power for handover

10 15
candidate cell

Case 4: Call drop caused by clock problem

[Problem description]
The cell A of an 1800MHz network has been cutover. After the establishment of a cell
at site B, the calls made in the cell handing over to the GSM900 MHz cell that shares
the same BTS site drops in the GSM900 MHz cell. And the call drop rate is great.
[Problem analysis and solution]
 Engineers find that the clock of the GSM900 MHz BTS and that of the GSM1800 MHz
BTS are asynchronous. When the calls established on the GSM1800 MHz cell intend
to hand over to the GSM900 MHz cell, the drive test data shows that the FER arises
dramatically first, and then gradually disappears to none. If the handover is from a
GSM900 MHz cell to a GSM1800 MHz cell, this phenomenon is also present.
Through monitoring signaling, engineers find that the conversation held several
seconds before the call drop is just process for call re-establishment. However, the
test MS shows that the call has been handed over to the GSM900 MHz cell.
Therefore, it can be judged that the clocks are seriously asynchronous. To solve this
problem, the carrier concerned and the GSM900 equipment provider cooperate with
each other on clock calibration. After that, the abnormal call drop disappears.
Therefore, for dual-band handover, the clock of the GSM900 MHz BTS and that of the
GSM1800 MHz BTS must be synchronous.

8.1.2 Handover Problems

The MS is always moving during conversation. To ensure channel quality, the MS

must measure the quality of the channels of the surrounding channels without stop,
and then send the measurement report to the BSC through the service cell. The BSC
will perform radio link control according to the signal level and quality contained in the
measurement report. If the MS moves to another cell, the new cell will replace the old
cell to ensure the continuity of the service. The handover enables each cell to form a
seamless network.

I. Handover Problem Positioning Steps

1) Find out if the problem occurs at an individual cell or all cells and find out the
characteristics of the problem cells. For example, if the cells are the neighbors
cell of a cell, or if they are co-BSC cells, or if they are co-MSC cells.
If the handover between two cells fails, you should focus on checking if the data
of the two cells is correctly configured. In addition, you should also check the
hardware of the two cells.
If the problem is found in all the neighbor cells of a cell, you should focus on
checking of the data of this cell is correctly configured. In addition, you should
also check the hardware of the cell.
If the problem is found in all the cells under the same BSC, you should focus on
checking the data configuration between the BSC and MSC.
If the problem is found in all the cells under the same MSC, the cooperation
between the local exchange and the opposite exchange may fail. For example,
the signaling is incompatible and the timer setting is irrational.
2) Check if the data has been modified before handover problems occur.
 If the problem is found in an individual cell, you should focus on checking if the
data configuration for this cell has been modified.
If the problem is found in all the cells under the same BSC, you should focus on
checking the data configuration for the local BSC and the opposite MSC has
been modified.
If the problem is found in the cells under the same MSC, you should check if the
data configuration for the opposite MSC has been modified.
3) Check if it is the hardware failure that causes the handover problem.
4) Register the related traffic statistics items, such as the handover performance
measurement and TCH performance measurement.
 Check if the TCH seizure of the problem cell is normal. For example, if the
call drop rate is high.
 Check if the outgoing handover success rate is normal.
 Find out the causes for the handover failure.
 Check if the radio handover success rate is normal.
5) Perform drive test for the problem cell and analyze the drive test signaling.
 Check if the uplink and downlink of the problem cells are balanced,
because unbalanced path may cause handover problem (BTS problem
may cause the unbalance).
 Check if the measurement report for the problem cell contains correct
neighbor cell list.
 Check if a call can hand over from a problem cell to a neighbor cell
correctly and check if it can hand over from a neighbor cell to the problem
cell.
 Analyze if the signaling procedure for the handover is normal.

II. Handover Problem Analysis Methods

i) Handover cannot be initiated

If the MS is in a cell where the signal is poor, it cannot hand over to another cell. In
this case, you should consider of the handover conditions are met and if there is an
outgoing cell available.
Hereunder details the possible reasons:
 The handover threshold is set to a low value
For edge handover, the handover triggering condition is that the Rxlev must be
smaller than the handover threshold. If the edge handover threshold is set to a
too small value, the signal level of the neighbor cells will be far higher than that of
the service cell. In this case, the handover cannot be initiated. As a result, the
conversation quality will be affected, or even the call drop will be resulted. The
setting of the handover threshold depends on the coverage scope of the cell. You
 can change the size of the service area of the cell through changing the
handover threshold.
 Neighbor cell relationship is not set
Though the signal level in the neighbor cells of the service cell is high, the MS
will not report the neighbor cells if the neighbor cell relationship is not set. In this
case, the MS cannot hand over to a neighbor cell. Through performing cell
reselection or dialing test, you can check the neighbor cell list reported by the
MS. If the MS has moved to the major lobe of a cell but this cell is not found in
the neighbor cell list, you should check if the correct neighbor cell relationship
has been correctly set. During the drive test, you can use another MS to scan the
BCCH numbers to check if the strong BCCH numbers are in the service cell or in
the neighbor cell list.
 Handover hysteresis is irrationally set
If the difference between the signal level of the handover candidate cell and that
of the service cell is greater than handover hysteresis, the cell can work as a
target cell. If the hysteresis is set to a too great value, the handover is hard to be
initiated.
 The best measurement time “N” and “P” are irrationally set
During normal handover, the MS uses N-P rules to list the handover candidate
cells in a certain order. If a candidate cell is the best cell within P seconds out of
N seconds, it will be treated as the best cell.
When there are two cells become the best cell alternately, the MS may find it
hard to select a best cell through N-P rule, which makes the handover difficult. In
this case, you can adjust the values of N and P and reduce the measurement
time to make the handover decision more sensitive to level change.
If the landform and the ground objects of the service cell are quite complicated,
the signals received by the moving MS will fluctuate greatly. In this case, the
handover candidate cell cannot meet N-P rule, which will make the handover
difficult.
ii) Handover problem caused by hardware failure
If the data configuration for the problem cell and the neighbor cells has not been
modified recently but the handover problems occur abruptly, you should first
consider if the problems are caused by BTS hardware equipment.
If the cells sharing the same base station with the cell have similar problem, you
should consider if the problem is caused by the common hardware of the cells.
If the problem is found in only one cell under the base station, you should
consider if it is the hardware of this cell that causes the problem. For example, if
some of the carriers are damaged. To test the problems of this kind, you can
 disable some of the carriers. If the handover success rate returns to normal state
after a carrier is disabled, you can check if the problem is present at this carrier
or if the CDU and antenna feeder part related to this carrier fails. If signals of a
cell on the uplink and downlink are seriously unbalanced, frequent handover will
be caused and the handover success rate will decrease.
To check if the signaling flow of the cell is normal and if the uplink Rxqual and
downlink Rxqual are good, you can monitor the messages sent across the Abis
interface. If the Rxqual is poor, it means that the hardware equipment of the fails
or serious interference is present in the cell. In this case, the signaling exchange
is unavailable and the handover problem will occur.

III. Handover Cases

Case 1: No handover candidate cell is available due to CGI error

[Problem description]
The handover in an area is abnormal. When the MS moves from cell A to cell B, the
signals in cell B are stronger than that of cell A, but the handover does not happen.
After the MS moves from cell B to cell C, the MS hands over from cell A to cell C.
[Cause analysis]
If a cell can work as a service cell and can hand over to other cells, but the incoming
handover is unavailable, you can check if the CGI, BSIC, BCCH number of the cell
are correct.
[Problem solution]
1) Use the test MS to lock the BCCH numbers of cell B. The call is normal. The MS
can hand over to any other cell by force.
2) Make a call after locking the BCCH umber of any neighbor cell of cell B, and then
force the MS to hand over to cell B, but the handover does not happen, because
no handover command is seen in the drive test software.
3) The handover procedure requires the MS detecting the neighbor cell signals and
reporting the detected signals to BSC with a measurement report. Upon
receiving the measurement report, the BSC must make the handover decision. If
the handover conditions are met, the BSC should activate the TCH of the service
cell, and then send a handover command to the MS.
4) If the signals of cell B are far stronger than that of cell A and the handover
conditions are met (the PBGT handover threshold is 70), but no handover
command is sent, it means that errors occur during the activation of the target
cell TCH.
5) If the cell B works as the target cell but the TCH cannot be activated, the data
may be incorrectly configured for the cell. In this case, the BSC that contains the
 cell cannot find the target cell, so the TCH cannot be activated and no handover
command can be found in the service cell.
6) The CGI error is found in cell B through data checking. The handover is normal
after the CGI is changed to correct value.
Case 2: Unbalanced path causes low handover success rate
[Problem description]
The incoming BSC handover success rate is quite low for the two cells under a BTS,
ranging from 10% to 30%.
[Cause analysis]
Generally, if the data problems, such as CGI error or intra-frequency interference,
exists and if there is dead zones in heavy-traffic areas, or if it is hard for the MS to
access the cell due to poor uplink signals, the incoming BSC handover success rate
is low.
[Solution]
1) The cell data is found normal.
2) Through checking traffic statistics items, engineers found that all incoming cell
handover success rates were low.
3) Through drive test, engineers found that frequent handover attempts were made
in the area 2km away from the BTS, but the handover always failed. Even if a
successful handover was made, call drop occurred immediately. During the
handover, engineers found that the downlink level was about -85dBm. Engineers
made 10 dialing tests with frequency locked, all the originating calls failed. For
the answering calls, they can be connected but cannot be called out.
4) It is estimated that the CDU uplink channel loss is great, or the jumpers are
incorrectly connected at the BTS top. In this case, the uplink signals will be poor,
which causes the problem.
5) After changing the CDU, engineers found that the incoming handover success
rate increased to 95%.
Case 3: Improper antenna planning causes low handover success rate
[Problem analysis]
The handover success rate among the three cells under a BTS is quite low according
to traffic statistics. For the handover from cell1 to cell3 and the handover from cell2 to
cell3, the success rate is lower than 30%.
[Cause analysis]
Generally, low handover success rate is caused by board failure, handover data error,
or improper antenna planning.
[Solution]
1) The BTS hardware is normal and no alarm concerning handover parameters is
generated, so the hardware problem and parameter setting problem can be
excluded.
2) The BTS locates at the eastern side of a south-north road and is 700m away
from the road. The azimuth angles of the three cells are 0°, 80°and 160°. They
three cells direct to the two directions and the open resident areas lying under a
hill in the east respectively. Among the three cells, the down tilts of two cells are
7°. To make the coverage as specified as possible, engineers concentrated the
antenna azimuth angles of the three cells in design. In this case, however, the
cells of the BTS are seriously overlapped in the east. For the areas just in the
west, the coverage is provided by the side lobes and back lobes of the three
cells. Therefore, when the MS is moving on this road, first it is covered by cell1.
When it moves to the west, the signals of the three cells are poor and fluctuating
greatly. In addition, since the handover measurement time and the handover
duration is set to a small value, the handover is rather sensitive, and that’s why
the frequent handover failure occurs.
3) After setting the azimuth angles of the three cells to 60°, 180°, and 350°,
engineers found that the handover success rate of among the three cells
increased to 95%.
Case 4: Problems concerning the cooperation of different carriers’ equipment
cause low outgoing BSC handover success rate
[Problem description]
There is a dual-band network in which the GSM900 MHz network and the GSM1800
MHz network are stand-alone. After the two networks completed cell reselection and
handover parameter setting, engineers found that the dual-band handover success
rate was low; especially the success rate of the handover from the GSM1800 MHz
network to the GSM900 MHz network was low, ranging from 60% to 80%. However,
the success rate of the handover from the GSM900 MHz network to the GSM1800
MHz network was higher than 92%.
[Cause analysis]
For a dual-band network, if the problems concerning the cooperation of different
carriers’ equipment are found, you must know the data configuration of the
equipment. For example, if the equipment supports Phase 2+ and EFR.
[Solution]
1) Through using signaling analyzer to analyze the message flowing across the A-
interface and E-interface, engineers found that the MSC of the GSM1800 MHz
network would send back a Handover Reject message to the BSC of the
GSM1800 MHz network when the BSC sent a Handover Required message to
the MSC.
 2) The MSC of the GSM1800 MHz network sent a Prepare Handover message to
the MSC of the GSM900 MHz network. Upon receiving the message, the MSC of
the GSM900 MHz network sent back an Abort message.
3) Because the success rate of the handover from GSM900 MHz network to the
GSM1800 MHz network was high, engineers found that the voice version carried
in the Prepare Handover message (from the GSM900 MSC to GSM1800 MSC)
is half rate version 1, but the voice versions carried in the Prepare Handover
message (from GSM1800 MSC to GSM900 MSC) are full rate version 1, full rate
version 2, and half rate version 1, which belong to PHASE 2+. However, MSC of
provider A does not support the PHASE 2+, so the handover failure is caused.
4) Through modifying the MSC data of the circuit MSC data at the A-interface and
selecting the full rate version 1 only, engineers found that the voice versions
carried in the Prepare Handover message (from GSM1800 MSC to GSM900
MSC) are full rate 1 and 2. After that, the dual-band handover success rate was
greatly increased.

8.1.3 Congestion Problems

This section introduces the methods to handle SDCCH congestion and TCH
congestion, in which TCH congestion indicates SDCCH seizure all busy. The TCH
congestion has two cases. One is TCH seizure all busy. For this case, the real
channels cannot be allocated to the MS, so the MS will fail to request the channels.
The other one is that the TCH assignment fails after an assignment is sent due to
various reasons.

I. Congestion Problem Solutions

 Congestion caused by heavy traffic

You can check if the SDCCH traffic and TCH traffic are normal through viewing
traffic statistics. If the congestion is caused by heavy traffic, the most efficient
method to solve the problem is to expand the capacity of the network. In addition,
you can adopt traffic sharing technologies to ease the congestion. For example,
you can modify the CRO, enable direct retry or load handover function.
 SDCCH congestion caused by burst traffic
If the SDCCH congestion rate is high and the traffic is heavy but the TCH traffic is
normal, the SDCCH congestion may be caused by burst traffic. The SDCCH
congestion always occurs at BTSs along railways and tunnel exits, because the
BTSs are installed in remote places and the capacity of a BTS is small. As a
result, when the train moves fast along the railways or stops at a railway station,
most of MSs failing to capture a network will perform location update, which will
result in SDCCH congestion. In addition, when short messages are sent at a
 concentrated time, the SDCCH congestion will also occur easily. SDCCH
congestion cannot be completely avoided, but some measures can be taken to
ease the congestion. For example, you can add the number of SDCCHs, or
enable the dynamic conversion between SDCCH and TCH.
 Congestion caused by TRX problems
When a carrier configured in a multi-TRX cell cannot provide services, the
channel congestion will also occur. To solve the problem, you should replace the
problem TRX with a sound one. If the TRX problem cannot be positioned, you
should check if the antenna feeder connection is correct and if the antenna
standing wave is normal. If yes, recheck the TRX where the problem may be
generated.
 Congestion caused by interference
The interference present across the radio interfaces will also cause congestion.
In this case, you should solve the interference problem first.
 Channel assignment failure caused by inconsistent coverage
If the concentric technology is not used, the transmit power of the TRXs within
the same cell will be inconsistent, which will result in inconsistent coverage. In
this case, the channel assignment failure will easily occur. To position this
problem, you can check if the connection between the splitter and connector and
if the connection between CDU and SCU are correct.
When a cell uses multiple transmitter antennas, inconsistent coverage will be
inconsistent, which will result in channel assignment failure. To solve this
problem, you should make the coverage of each transmitter antenna as
consistent as possible through engineering adjustment.
In addition, if the transmitter antenna and the receiver antenna of a cell is not in
the same plane or the antenna tilt angles are inconsistent, the channel
assignment failure will also be caused. In this case, you can calibrate the
antennas to solve the problem.
.

II. Congestion Cases

Case 1: SDCCH congestion caused by wrong LAC configuration

[Description]
A BTS is configured as S1/1/1. It is found that the SDCCH congestion rate for 2 cells
reaches as high as 8%.
[Problem analysis and solution]
1) Through checking the measurement indexes for TCH and SDCCH, engineers
found that the TCH traffic was not heavy. The traffic volume for each cell during
busy hours is lower than 2.2Erl. However, the requests for SDCCH seizure are
great, reaching 3032 times during busy hours. The SDCCH traffic reaches
1.86Erl, and the congestion rate reaches 8%.
2) The main reasons for SDCCH seizure include the messages sent before call
setup, the messages sent during handover, the location update messages sent
under the idle mode, and other short messages.
3) The TCH traffic is normal, the requests for TCH seizure (including handover) are
normal (318 times), and the handover requests are also normal (146 times).
Therefore, the SDCCH congestion may be caused by a large number of location
update messages or short messages.
4) The LAC of the BTS is 0500, and the LACs of other cells of the surrounding cells
are 0520. After changing the LAC of the BTS to 0520, engineers found that the
requests for SDCCH seizure during busy hours were 298, the SDCCH traffic was
0.27Erl, and the congestion rate reduced to 0.
Case 2: SDCCH congestion caused by burst location updates
[Problem description]
The radio connected ratio of a local network is lower than average level. According to
traffic statistics analysis, it is found that the SDCCH congestion happened at several
BTSs.
[Problem analysis and solution]
1) Through analyzing traffic statistics, engineers found that the SDCCHs of the
congested cells were seized for 300 to 400 times during busy hours. Here the
BTS was configured as S1/1/1 and each cell was configured with 8 SDCCHs.
Therefore, the SDCCHs can be seized by 400 times, but the SDCCHs were
congested for tens of times during busy hours.
2) As far as the registered traffic statistics items were concerned, most of the
SDCCH seizures were caused by location update. Taking the BTS location into
consideration, engineers found that most of the BTSs were installed at the
intersections of two railways. Therefore, it might be the burst location update that
caused SDCCH congestion.
3) To verify if it was the burst location update that caused the congestion, engineers
registered the traffic statistics items in 5 minutes and found that most of the
location update happened within the five seconds. Through querying the train
time table, engineers found that there were 4 to 5 trains passing by within the
five seconds. When the trains passed the intersections, a large number of
location updates were generated in a short time. In this case, the congestion was
caused.
Therefore, if the BTSs are installed at the railway intersections, you are suggested to
enable the SDCCH dynamic allocation function and configure a suitable margin for
the SDCCH.
Case 3: Great TCH congestion rate caused by the inconsistent tilt angles of two
antennas under the same cell
[Problem description]
It is found that the TCH congestion rate of a cell is great (greater than 5%) according
to traffic statistics.
[Problem analysis and solution]
1) Through checking BSC traffic statistics, engineers found that it was the TCH
seizure failure that directly caused the great TCH congestion rate.
2) Generally, TCH seizure failure is caused by TCH assignment failure. Through
monitoring Abis interface, engineers found that most of the TCH assignment
failure occurred at the No.4 TRX and No.5 TRX, and the probability for the
assignment failure rate for the No.4 TRX was near that for the No.5 TRX.
3) Through checking the antenna feeder part, engineers found that the tile angle
corresponding to the transmitter antennas of the two TRXs were too great (it is
10 degrees than that of the antenna for BCCH, because the antenna nuts were
found loosen. Therefore, if the MS is far away from the BTS, it can receive the
BCCH signals but cannot receive the TCH signals. If the TCH is assigned to the
MS when the MS starts a call, the TCH seizure failure will occur.
4) To solve the problem, you can enable the tilt angel of the TCH antenna and that
of the BCCH antenna to be consistent with each other. In this case, the TCH
congestion rate can be reduced to 2% or lower.
Case 4: High TCH congestion rate caused by downlink interference
[Problem description]
A cell of BTS is responsible for covering a large area of sea surface along the coast.
According to the registered traffic statistics items, engineers found that when the
traffic volume was lower than 1Erl, the TCH congestion of the cell reached 10% at
some time. However, no alarm was generated. All the interference bands fell within
the interference band1 and the hardware and RF connections were normal.
[Problem analysis and solution]
Because all the interference bands fell within the interference band1, the uplink
interference is impossible. Considering that the coverage distance reaches 60 to 70
kilometers and multiple normal cells are present along the coast, the probability for
the channel numbers of the downlink areas and the cell to be interfered is high.
Through modifying the channel numbers of the cell, engineers found that the TCH
congestion rate was improved. Through further optimizing the channel numbers,
 engineers found that the TCH congestion rate was lower than 1%. Therefore, it can
be judged that the 10% of congestion rate is caused by the downlink interference of
some areas.

8.1.4 Other Problems

I. Subscriber Is Not in Service Area

When a subscriber is not in service area, the MS works as the called party when the
signals received by the MS are good and the calling party hears a voice saying that
the subscriber is not in the service area. If the coverage, operation, and data
configuration of a network are good, the occurrence probability for the problem must
be lower than 1%, otherwise it is other causes that result in the problem.
Generally, if a subscriber is not in the service area, the following causes may be
present:
 Coverage problem
If the subscriber complaint happens at cell edges, the problem may be related to
coverage. In this case, the probability for the calling party failure is equal to the
probability for the “subscriber is not in service area”.
 Parameter setting
If the subscriber is not in service area, the parameters concerning the paging, access,
and immediate assignment may be not properly set. In this case, you can check if the
messages concerning RACH overload, PCH overload, and SDCCH overload are
generated through querying traffic statistics and alarms. If yes, the subscriber may not
in the service area.
 System capacity limit or overload
If system capacity limit or overload occur (for example, HDB overload, CPU overload,
or capacity overload during busy hours), the system may fail to read the subscriber
information, or subscribers cannot access the radio network. In this case, the
subscriber may not in the service area. To solve this problem, you can take measures
to expand the system capacity.
 Transmission problem
If the links between systems (such as the LAPD link of the Abis interface and the links
of the each network entity) and the links within a system (such as the link among the
modules of BCS/MSC) are not stable, the messages sent through these links may be
missing. In this case, subscribers may not in the service area. To position the
problem, you can check the alarms.
 Equipment causes
 If the designs concerning MSC and BSC are incomplete, the probability of “subscriber
is not in service area” will increase.
 MS causes
If the RF parts or the software parts of the MS have problems, for example, the
reception capability of the MS is poor; the frequency deviation goes beyond the
requirements defined in the protocols; and the dual-band performance of the MS is
poor, this problem may also occur.

II. Signal Fluctuation

Signal fluctuation indicates that change of the MS signal strength. The following
factors may cause signal fluctuation.
 Radio wave propagation
The strength of the signals received by the MS is the amplitude of the sum of the
vectors of various propagation paths. Because the propagation environment is ever-
changing, the attenuation of radio channels is ever-changing. Therefore, even if the
MS does not move, the strength of the received signals will change.
 Cell reselection and handover
When the MS moves from the old cell to the new cell through handover or cell
reselection, the signals of the MS will fluctuate because the signal strength of the old
cell and that of the new cell are inconsistent.
 Antenna shake
When the antenna shakes, the antenna gain will change, so the signals will also
fluctuate.
 Location update or channel assignment occurs at non-BCCH TRX
If the SDCCH is assigned to a non-BCCH TRX during location update and power
control is available on SDCCH, the signal strength may fluctuate.

III. Voice Discontinuity

Voice discontinuity stands for pauses or words loss occurs in conversation. If the
voice continuity is remarkable, the conversation quality will be affected.
The following factors may cause voice discontinuity
 Frequent handovers
Only hard handover is available in GSM system. Therefore, when the MS hands over
from the source channel to the target channel, the downlink frames may loss at the
Abis interface. As a result, the voice continuity is unavoidable during handover.
Generally, frequent handovers may occur at cell edges or during cross coverage. In
this case, the voice discontinuity will become a headache of subscribers. To avoid the
frequent handovers, you can adjust the antenna tilt and height and configure the data,
such as uplink and downlink quality threshold and restriction properly.
 Radio link interference
Radio link interference will increase the bit error rate, which will cause voice
discontinuity. In addition, the signals always fluctuate greatly at cell edges, so the
conversation quality at the cell edges is bad.
 Poor network coverage
If the network coverage is poor, the Rxlev and Rxqual will become poor, thus the
conversation quality will be affected.
 BTS transmission problem
BTS transmission problems will affect conversation quality. For connectors, you
should check of the connection among the connector is good. For optical
transmission, you should check if the optical headers are clean and if the
transmission errors are great. For micro wave transmission, it may be affected by
weather. If there is large amount of dust in the equipment, the conversation quality
may also be affected. If both micro wave transmission and optical transmission are
used, you should pay attention to the cooperation of the transmission impedance at
the equipment interfaces.
 TRX board failure
Hardware problem will result in poor conversation quality. In this case, you should
replace the problem hardware with the sound one.