1. Internal
OMF000404
Case Study – Call Drop
Issue 2.0 www.huawei.com
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2. References
31160978-BSC Traffic Statistic Manual
Volume I
31033203-BSS Troubleshooting
Manual
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3. Upon completion of this course, you are
supposed to be able to:
Understand the principles of call drop.
Analyze and solve call drop problems
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4. Chapter 1 Principle of call drop
Chapter 2 Analysis of call drop
Chapter 3 Call drop cases
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5. Chapter 1 Principle of call drop
Section 1 Definition of call drop
Section 2 TCH call drop
Section 3 SDCCH call drop
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6. Definition of Call Drop
There are two types of call drop: TCH call drop and SDCCH
call drop:
TCH call drop means TCH channel is released abnormally
after it is occupied successfully.
SDCCH call drop means SDCCH channel is released
abnormally after it is occupied successfully.
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7. Chapter 1 Principle of call drop
Section 1 Definition of call drop
Section 2 TCH call drop
Section 3 SDCCH call drop
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8. Causes of TCH Call Drop
Usually, the typical causes for sending the Clear_Request
message are as follows:
Radio interface failure
O&M intervention
Equipment failure
Protocol error
Preemption
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9. Formula of TCH Call Drop Rate
Formula of TCH call drop rate
TCH call drop rate=TCH call drops / Successful TCH
seizures (all)
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10. Measurement Point of TCH Call Drop
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11. Measurement Point of TCH Call Drop
TCH serves as traffic channel, BSC receives ERROR INDICATION
message from BTS.
TCH is seized as traffic channel, BSC receives CONNECT FAILURE
INDICATION message from BTS.
In assignment procedure and handover procedure, failure of decoding
HANDOVER DETECTION and HANDOVER COMPLETE message.
TCH serves as traffic channel, incoming BSC handover initiated and the
timer for the target cell to wait for HANDOVER COMPLETE message
expires.
TCH serves as traffic channel, outgoing BSC handover initiated and the
timer for the source cell to wait for CLEAR COMMAND message from MSC
expires (T8 expires).
In intra-BSC handover procedure, the target cell sends Inter Clear Request
to the source cell when the timer for the target cell to wait for HANDOVER
COMPLETE from MS expires, in this case, this item is measured in the
source cell.
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12. Measurement Point of TCH Call Drop
In Intra-BSC handover procedure, the source cell, excluding the source cell
for directed retry, measures the item when the timer for the source cell to wait
for Inter Clear Request with cause value HANDOVER COMPLETE from the
target cell expires.
In Intra-BSC handover procedure, when the target cell AM/CM net-drive fails
(due to timeout or negative acknowledgement), the target cell sends Inter
Clear Request to source cell, in this case, this item is measured in the source
cell.
In the case that MS reverses to the original channel after intra BSC handover
fails, the source cell first releases the terrestrial connection but the AM/CM re-
net-drive fails (due to timeout or negative acknowledgement). In this case this
item is measured in the source cell.
The resource of the lower_priority call will be preempted by the higher_priority
call if MSC and BSC both supports preemption, which will cause call drop.
This item is measured when the RSL link of the TRX that the call is using
disconnects, which will cause call drop.
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13. TCH Call Drop - Assignment
MS BTS BSC MSC
ASSIGNMENT REQUEST
CHANNEL ACTICATION
CHANNEL ACTIVATION ACK
ASSIGN CMMAND
SABM
ESTABLISH INDICATION
UA
ERROR INDICATION
CONNECTION FAILURE INDICATION A1
A2
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14. TCH Call Drop – Intra-BSC Handover
MS BSC' BSC Ori-Cell BSC Tag-Cell BSC''
Intercell Handover Request
CH ACT
CH ACT ACK
Intercell Handover Response
HANDOVER COMMAND
HANDOVER ACCESS
HANDOVER DETECT
SABM
UA
ERROR INDICATION
A1
CONNECTION FAILURE INDICATION
A2
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15. TCH Call Drop - Incoming BSC Handover
MS Other BSC HUAWEI BSC HUAWEI BTS MSC
HANDOVER REQUIRED
HANDOVER REQUEST
CH ACT
CH ACT ACK
HANDOVER REQUEST ACK
HANDOVER COMMAND
HANDOVER ACCESS
HANDOVER DETECT
SABM
UA
ERROR INDICATION
A1
CONNECTION FAILURE INDICATION
A2
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16. Measurement Point of Successful TCH Seizures
Measurement point of Successful TCH seizures
Upon BSC’s reception of CHANNEL ACTIVATION ACKNOWLEDGE
message from BTS in very early assignment TCH process.
In the case the target cell of directed retry is located in other BSC
and directed retry succeeds, MSC sends CLEAR COMMAND
message to the originating BSC to release the original connection.
This item is measured when the originating BSC receives this
message.
In the case the directed retry target cell is located in the same BSC
and the directed retry succeeds, target cell sends Inter Clear
Request message to the source cell to request to release the
resource and the original connection. This item is measured when
the source cell receives this message.
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17. Measurement Point of Successful TCH
seizures
Measurement point of Successful TCH seizures
This item is measured when BSC sends ASSIGNMENT COMPLETE
message to MSC after the assignment procedure is successfully
implemented.
In incoming BSC handover procedure, MS sends HANDOVER ACCESS
message to the BSC. This item is measured when BSC receives
HANDOVER DETECT message from BTS.
In the process of incoming internal inter cell handover and intracell
handover in BSC, MS sends HANDOVER ACCESS message to BSC.
BSC measures this item in the target cell when receiving HANDOVER
DETECTION message from BTS.
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18. Successful TCH Seizure – Very Early Assignment
Very early assignment process
MS BTS BSC MSC
Channel Request (RACH) Channel Required
Channel Activation (TCH or SDCCH)
Channel Activation Acknowledge
Immediate Assignment Command
Immediate Assignment (AGCH)
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19. Successful TCH Seizure – Assignment
Assignment process
MS BTS BSC MSC
ASS_REQ
CH_ACT
CH_ACT_ACK
ASS_CMD
ASS_CMD
EST_IND
ASS_CMP ASS_CMP ASS_CMP
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20. Successful TCH Seizure – Intra-BSC Handover
Intra-BSC handover process
MS BTS1 BSC BTS2 MS MSC
Measurement Report from MS
Channel_Active
Channel_Active ACK
HANDOVER COMMAND
HO_ Access
HO_Detect
PHY INFO
PHY INFO
First SABM
Establish_IND
UA
Handover Complete HO_Performed
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21. Successful TCH Seizure – Inter-BSC
Handover
Inter BSC handover process
MS BTS1 BSC1 MSC BSC2 BTS2 MS
Measure Report from MS
HO_Required
HO_Request
CH_ACT
CH_ACT_ACK
HO_Request_ACK
HO_CMD
Handover Command HO_Access
HO_Detect
HO_Detect
PHY INFO
PHY INFO
Establish_IND First SABM
HO_CMP Handover Complete
Clear_CMD
Clear_CMP
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22. Chapter 1 Principle of call drop
Section 1 Definition of call drop
Section 2 TCH call drop
Section 3 SDCCH call drop
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23. Formula of SDCCH Call Drop Rate
Formula of SDCCH call drop rate:
SDCCH call drop rate = SDCCH call drops/ successful
SDCCH seizures*100%
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24. Measurement Point of SDCCH Call Drop
This item is measured when BSC receives ERROR INDICATION
message from BTS due to an abnormal case for a radio link layer
connection.
This item is measured when BSC receives CONNECTION FAILURE
INDICATION message from BTS because an active connection has been
broken for some reason such as SDCCH link failure or hardware failure
(see GSM 0508 for details).
In incoming BSC handover procedure on SDCCH, this item is measured
in the target cell in the case of the failure of decoding HANDOVER
DETECTION and HANDOVER COMPLETE message.
In the process of incoming BSC handover on SDCCH, this item is
measured in the target cell when the timer for the target cell to wait for
the HANDOVER COMPLETE message expires.
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25. Measurement Point of SDCCH Call Drop
In the process outgoing BSC handover on SDCCH, this item is
measured when the timer for the source cell to wait for CLEAR
COMMAND message from MSC expires (T8 expires).
In the process of intra BSC handover on SDCCH, this item is measured
in the source cell when the timer for the source cell or the target cell to
wait for HANDOVER COMPLETE message expires.
This item is measured when the RSL link of the TRX that the call is
running on disconnect, which will cause call drop, this item measures
call drop on SDCCH for RSL disconnection.
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26. Measurement Point of SDCCH Call Drop
MS BTS BSC MSC
Channel Request Channel Required
Channel Activation (SDCCH)
Channel Activation Acknowledge
Immediate Assignment Command
Immediate Assignment
Establish Indication (L3 Info)
Connection Failure
Or:
Or: Error Indication
Abis Failure
Cell SDCCH Call Drop
(Subject to different cases)
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27. Measurement Point of Successful SDCCH
Seizures
In immediate SDCCH assignment procedure, this item is
measured when BSC receives CHANNEL ACTIVATION
ACKNOWLEDGE message from BTS.
In the process of incoming BSC handover on SDCCH. This
item is measured when BSC receives HANDOVER
DETECTION from BTS.
In the process of incoming internal inter cell handover and
intracell handover in BSC.BSC measures this item in the target
cell when receiving HANDOVER DETECTION from BTS.
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28. Chapter 1 Principle of call drop
Chapter 2 Analysis of call drop
Chapter 3 Call drop cases
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29. Analysis of Call Drop
content
main causes of high call drop rate
troubleshooting of high call drop rate
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30. Analysis of Call Drop
According to the definition of call drop measurement point, call drop is usually
caused by the following:
Radio link fault. During the communication, messages can not be received
correctly.
Abis link broken during conversation.
Call drop during handover.
Other system faults.
Timers that may cause call drops (BSC timer):
T3103: starting from sending HANDOVER CMD and ending at receiving
HANDOVER CMP. Time out of the timer will cause call drop.
T3109: starting from sending CHAN REL and ending at receiving REL IND.
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31. Radio Link Fault
Signaling process chart of radio link fault
MS BTS BSC MSC
(1)
Measurement Report
(2)
Measurement Result
Connection Failure
Clear_REQ (Radio Interface Failure)
(3)
(1) Dadicated mode is created. (SDCCH/TCH)
(2) Activate Abis monitoring function.
(3)SACCH message block can not be decoded(uplink/downlink),
resulting in radio link timeout.
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32. Radio Link Fault
Diagram of radio link timeout
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33. T3103 Times Out
Handover process
MS BTS1 BTS2 BSC MSC
Handover Indication
CH_ACT
CH_ACT_ACK
Handover Command
Handover Command Set T3103
Handover Access HO_Detect
Physical Information (TA)
SABM
EST_IND
UA
Handover Complete
Handover Complete
Reset T3103
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34. Causes of Radio Link Fault
The causes of radio link fault can be
Interference
− Internal interference
− External interference
− Equipment interference
Poor coverage
− Coverage hole
− Isolated island
− Uplink/downlink imbalance
Improper parameter setting
− Radio link timeout, SACCH multi-frames
− Handover parameters
− Power control parameters
Equipment problem (Antenna, feeder, combiner, TRX)
Clock problem
Transmission problem
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35. Radio Link Fault - Interference
Category
Co-channel interference
Adjacent-channel interference
Inter-modulation interference and other external
interference
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36. Radio Link Fault - Interference
Solution
First check equipment problems.
Make an drive test, check the interference area and distribution of
signal quality. Find the interference frequency.
Further search for the interference source with the spectrum analyzer.
Activate hopping, DTX and power control functions to lower the
internal interference of the system
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37. Radio Link Fault - Interference
Judgment Process
Analyze the occurrence regularity of interference band in the
traffic measurement.
Observe the receiving level performance
Find the poor quality handover rate
Observe receiving quality performance
Observe call drop performance
Observe whether there are many handover failures and reversion
failures.
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38. Radio Link Fault - Coverage
Coverage:
Overshooting
Coverage hole
Signal attenuation
Incomplete definition of adjacent cells
Imbalance of uplink/downlink
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39. Radio Link Fault - Coverage
Judgment Process
Power control measurement function
Receiving level measurement function
Cell measurement function/inter-cell handover measurement function
Call drop measurement function
Defined adjacent cell measurement function
Undefined adjacent cell measurement function
Outgoing inter-cell handover measurement function
Up-down link balance measurement function
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40. Radio Link Fault - Coverage
Solution
Adjust network parameters
Add BTS
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41. Radio Link Fault - Parameters
Main parameters that may affect the call drop rate:
Radio link timeout and SACCH multi-frames
RACH busy threshold and RACH minimum access level.
MS minimum receiving signal level
Call re-establishment permitted.
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42. Radio Link Fault - Parameters
Main parameters that may affect the call drop rate:
NCC permitted
Handover related parameters.
Power control related parameters.
Frequency planning parameters
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43. Radio Link Fault
Judgment process
System information data
Cell data
Radio link connection timer (T3105)
Maximum times of physical information
Call drop performance measurement function
Judge from the cause of call drop
− error indication
− connection failure.
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44. Handover Problem
Judgment process :
In inter-cell handover measurement function, it occurs
frequently that the handover fails and the reversion also
fails.
In inter-cell handover measurement function, handover
failures with successful reversions occur many times.
In undefined adjacent cell measurement function, observe
the receiving level of the undefined adjacent cells and
number of the measurement reports.
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45. Handover Problem
Judgment process
In outgoing inter-cell handover measurement function, the
outgoing inter-cell handover success rate is low (for a
certain cell). Find the adjacent cell to which the handover
success rate is low and find the cause.
Incoming inter-cell handover success rate is low. The
handover judgment parameter setting of the target cell may
be improper.
In TCH measurement function, handover times are not in
normal proportion to successful TCH seizures for call.
(handovers/calls>3)
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46. Equipment Problem
Call drop arising from equipment problem
Hardware problem
Transmission problem
Antenna and feeder fault
Other causes
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47. Chapter 1 Principle of call drop
Chapter 2 Analysis of call drop
Chapter 3 Call drop cases
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48. Call Drop Case 1
Fault Description
The BTS distribution of an area is as illustrated in the
diagram (red numbers stand for BCCH frequencies. No
hopping, no DTX). Some subscribers complain that call
drop in second sector of base station C is serious.
Hardware fault has been ruled out.
please confirm whether the
frequency distribution in the
cells are resonable?
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49. Call Drop Case 1
Analysis
From the analysis of BTS topology, it can be concluded that
the frequencies are well planned.
Next, check the interference band of traffic statistic.
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50. Call Drop Case 1
Analysis
09:00-10:00 IB1 IB2 IB3 IB4 IB5
cell 1 2.85 14.25 1.14 0.27 0.54
cell 2 4.09 12.57 3.14 0.03 0.01
cell 3 0 2.92 13.27 0.25 0.37
03:00~04:00 IB1 IB2 IB3 IB4 IB5
cell 1 2.85 4.28 0.00 0.00 0.00
cell 2 4.09 2.89 0.00 0.00 0.00
cell 3 0 2.12 0.00 0.00 0.00
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51. Call Drop Case 1
Troubleshooting
Conducting a drive tests, it is found that the quality is bad
when the receiving strength is high.
Check traffic statistic and it is found that when the call drop
rate is high, handovers are mostly caused by quality
reasons, and channel assignment failure rate is also high.
The conclusion is that there is interference from the
analysis of comprehensive traffic statistic and drive test.
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52. Call Drop Case 1
Troubleshooting
A site investigation shows that the operator has a repeater.
It is a broadband repeater. It transmits the signals from a
remote TACS site. TACS signals are amplified and then
there is interference in second sector of base station C.
Problem has been located: interference causes the call
drop.
Finally, lower the power of the repeater. The interference
band reduce to IB1. Now the high call drop rate problem at
site C is solved.
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53. Call Drop Case 1
Common methods of checking and clearing call drop due to interference
Rule out the internal interference caused by equipment problems and check
the separation of BTS transceivers, antenna feeder installation, and so on.
Check the interference band
Drive test
Check traffic statistic of handover causes to get judgment
Clear uplink interference
Clear downlink interference
Check whether DTX, frequency hopping technology and power control
application are reasonable
Use PBGT handover algorithm flexibly to avoid co-channel and adjacent-
channel interference effectively.
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54. Call Drop Case 2
Fault description
The call drop rate in cell 3 of a BTS is 10% accompanied
with high congestion rate, but call drop rate and congestion
rate in cell 1 and cell 2 are normal.
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55. Call Drop Case 2
Analysis
Check the related traffic statistic
− Check whether there is high interference band in TCH
measurement function.
− Check the situation of call drop in call drop measurement
function.
− Check whether handover of the cell is normal.
Check whether there is interference through checking frequency
planning, moreover confirm whether there is external interference
with spectrum analyzer.
Drive test
Check the hardware
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56. Call Drop Case 2
Troubleshooting
Block TRX in turn and the congestion rate is always quite high no
matter which TRX is blocked.
Check and analyze the traffic statistic, interference band and traffic
volume and call drop rate, and it is found that the interference
becomes more serious as the traffic gets high.
Change frequency. The frequency of cell 3 is changed to 1MHz away
from the original value. But the problem persists.
Judge whether the equipment is faulty.
Locate external interference.
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57. Call Drop Case 2
Troubleshooting
Make a scanning test with a spectrum analyzer.
− A suspect signal with 904.14MHz center frequency, 300K
bandwidth is found. It is similar to an analog signal and it exists
continuously.
− At the distributor output port of cell 3, the signal strength is –
27dBm. cell 2 is –40dBm, cell 1 is –60dBm. It accords with the
degree of interference.
− Traffic volume is higher in the day time than that at night.
Now the problem is found: 904MHz external interference source.
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58. Call Drop Case 2
Conclusion: solution of interference
Solve internal interference through checking frequency planning.
After internal interference is excluded, we can locate external
interference with spectrum analyzer.
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59. Call Drop Case 3
Fault description
Subscribers complain that call drops often happen from on
the 5th floor and above in a building.
Subscriber complaint is also an important source of information
about the network quality.
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60. Call Drop Case 3
Analysis
Perform on-site test
− There are call drops and noise on the site
− The test mobile phone shows that before the call drop the
serving cell is BTS-B. But this building should be covered by
BTS-A.
Check traffic statistic
− BTS-B is about 9 kilometers away from this building. It is
determined that the BTS-B signal received in this area is
coming from some obstacles’ reflection. Thus an isolated
island coverage is formed in this area.
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61. Call Drop Case 3
Analysis
Check data configuration
− In BSC data configuration, BTS-A is not configured as the adjacency of
BTS-B
Cause analysis of call drop
− When the MS uses the signal of cell 2 of BTS-B in this area, the signal
of cell 3 of BTS-A is strong. But cell 2 of BTS-B and cell 3 of BTS-A are
not adjacent, therefore, handover cannot happen.
− The signal in cell 2 of BTS-B is the result of multiple reflections. When
the signal of BTS-B received by the mobile phone gets weak suddenly,
an emergency handover is needed. But there is no adjacent cell of
BTS-B, so call drops will occur.
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62. Call Drop Case 3
Troubleshooting
Modify the data in BA1 table, BA2 table and add adjacent cell
relationship, set cell 3 of BTS-A as an adjacent cell of cell 2 of BTS-B.
Optimize the network parameters to eliminate the isolated island.
The test results show that the call drop problem is solved.
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63. Call Drop Case 3
Conclusion ： two methods to solve isolated island problem
Adjust the antenna of the isolated cell, to eliminate the isolated island
problem.
Define new adjacent cells for the isolated cell.
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64. Call Drop Case 4
Fault description
In a drive test from A to B, it is found that there are many call drops
at entrance of a tunnel near a BTS due to slow handover.
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65. Call Drop Case 4
Analysis
The tunnel is near the BTS. When the MS enters the tunnel, the power
of the target cell is -80dBm. But the signal of source cell goes down
quickly to less than -100dBm. Before the MS enters the tunnel, the
downlink power of the two cells is good and no handover is triggered.
When the MS enters the tunnel, the level of the source cell goes down
rapidly. The call drop occurs before any handover is triggered.
Think it over: How to solve problems of this type?
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66. Call Drop Case 4
Troubleshooting
The adjusted parameters are listed below.
Parameter name Value before Value after
change change
PBGT watch time 5 3
PBGT valid time 4 2
PBGT HO threshold 72 68
UL Qual. Thrsh.
(Emergency handover) 70 60
Min. DL level on 10 15
candidate cell
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67. Call Drop Case 4
Conclusion: optimize and adjust handover parameters to reduce call drop
On condition that there is no ping-pang handover and excessive voice
interruption, PBGT handover can help to reduce interference and lower
call drop rate.
Set emergency handover thresholds properly, and make sure the
emergency handover can be triggered in time before the call drop so as
to reduce call drops.
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68. Call Drop Case 5
Fault description
In the dial test, many call drops are found in cell 2.
Analysis
Check the traffic statistic and find out that TCH congestion rate of this cell
is over 10% and internal inter-cell handover failure rate is high. It is found
that one TRX board of this cell is abnormal in OMC. A preliminary
conclusion is that TRX board problem causes the call drop.
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69. Call Drop Case 5
Troubleshooting
Lock the frequency with a test mobile phone and perform dial test for
many times. It is found that call drops only happen in timeslots 1, 3, 5, 7
while communications in timeslots 2, 4, 6, 8 are normal.
Move this board to another slot, and the problem still exists.
Move another good board to this slot, and the communication is normal.
Move this defective board to other cabinet, the problem arises.
When it is replaced, the communication is recovered.
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E E NOL OGIE CO., L D.
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70. Call Drop Case 5
Conclusion
The BTS test should guarantee that communication should be successful
not only in each TRX but also in each timeslot of the TRX.
It must be ensured that each TCH channel provides bidirectional high
quality communication.
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E E NOL OGIE CO., L D.
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71. Call Drop Case 6
Fault description
In dual-band network, when a call is setup in a GSM1800 cell and being
handed over to a GSM900 cell in the same site, the call drops in the
GSM900 cell in 2 to 5 seconds. The call drop rate in the GSM900 cell is
quite high.
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E E NOL OGIE CO., L D.
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72. Call Drop Case 6
Analysis
In the test it is found that the clock of GSM900 cell and GSM1800 cell
are not synchronized.
When a call set up in a GSM1800 cell and is handed over to a GSM900
cell, the drive test tool shows that FER increases to the maximum value
suddenly and then it goes down to zero gradually.
It is the same with the handover from GSM900 to GSM1800.
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E E NOL OGIE CO., L D.
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73. Call Drop Case 6
Troubleshooting
After adjusting GSM900 clock system, the abnormal call drop
problem is solved.
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E E NOL OGIE CO., L D.
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74. Call Drop Case 6
Conclusion
Clocks of GSM900 and GSM1800 should be exactly synchronized
with each other in a dual band network, otherwise there will be call
drops and handover failures.
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E E NOL OGIE CO., L D.
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75. Summary
Types of call drop
Measurement points of TCH call drop
Measurement points of SDCCH call
drop
Causes of call drop
Cases
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CLEAR_REQUEST ： the message is sent from BSC to MSC, and it shows that BSC require MSC to release special channel resource.
As shown in the above diagram, major causes for TCH call drop are connection failure, error indication, Abis failure, O&M intervention. CLEAR_REQ is the clearance request message that BSC sends to MSC. ERR_IND is the link error message that BTS reports to BSC.
The immediate assignment process is triggered by the random access process . It is intended to assign a signaling channel for MS to implement signaling transmission in the call setup process. Normally SDCCH is assigned. When there is no SDCCH channel or an emergency call is originated, TCH can also be assigned (as early as possible). Therefore, when the CH_ACT_ACK is received during immediate assignment and the type of channel directly allocated is TCH, it will be counted into successful TCH seizures as a measurement point.
As shown in the above diagram, main causes of SDCCH call drop are connection failure, error indication, Abis failure, and so on.
Radio link fault: when radio link timeout timer is reduced to 0, the channel will be released, and a call drop will occur whose cause will be recorded as a radio link fault. In network running, such call drops are the most common.
Radio link fault means that the communication link is lost during communication. During communication, due to interference in the system or low receiving level, the voice or data often becomes too deteriorated. And finally as a result, MS or network cannot correctly decode the information sent from the opposite side; moreover this cannot be controlled in any other ways. In this case, the system will conclude that a radio link fault occurs and the MS will either start call re-establish or forcedly disconnect the link. Forced link disconnection will lead to a call drop. Therefore, a radio link fault will be concluded by the system only when the communication quality is unacceptable. In Huawei system, the parameter Radio Link Timeout is defined in system information data, which is used by MS to decide when to disconnect the call (downlink) if SACCH decoding fails. In cell property data, the parameter SACCH multi-frames is defined, which is used by BTS as a criterion to decide the disconnection of uplink and inform the BSC a radio link connection failure (uplink). The radio link fault algorithms at BTS and MS are consistent. That is, when a dedicated channel is assigned to MS, it will start counter S. Then each time when an SACCH message cannot be decoded, S will reduce by 1, and each time when an SACCH message is decoded correctly, S will increase by 2. When this value is reduced to 0, radio link failure will be reported.
If handover to the target cell fails, the MS should return to the original cell. So, in the source cell, the original channel should be reserved for some time to accept the MS if it fails in accessing the target cell. T3103 is the timer for remaining the original channel. T3103 timeout leads to a call drop. When BSC sends the Handover Command to BTS, counter T3103 will start counting. When receiving Handover Complete from the handover destination cell or Handover Failure from the original cell, BSC will reset counter T3103. After sending the Handover Command to BTS, if BSC still cannot receive the message after counter T3103 times out, it means the MS failed in accessing the target cell and cannot return to the original channel. Then the BSC will release the reserved channel in the original cell.
Interference mainly includes co-channel, adjacent-channel, inter-modulation and external interference. When MS receives intensive co-channel or adjacent-channel interference signals in the serving cell, BER will be deteriorated and will result in call drop. When there is serious inter-modulation interference in BTS, it will also result in call drop.
Equipment problems: due to self-excitation of TRX or tower-mount amplifier, the system noise coefficient becomes larger and the sensitivity is deteriorated. Inter-modulation of the antenna is also an equipment problem. Analyze according to drive test data: interference area, signal quality distribution and the overlapping that causes interference. Adjust the BTS antenna downtilt, transmission power, adjacent cell relationship, handover parameters of the relevant cells or adjust the frequency planning to avoid interference.
1. If there are results in the interference band 3, 4, and 5, usually the interference problem should be taken into consideration. The interference band is reported to BSC by the BTS via the RF resource indication message when the carrier channel is idle. It should also be noted that the interference band condition of this channel will not be shown in the traffic measurement when the 3. In cell measurement function/inter-cell handover measurement function, or outgoing inter-cell channel is blocked or busy. 2. If there are too many times of high receiving level with low quality, it means that there is co-frequency or adjacent frequency interference or external interference. handover measurement function, outgoing handover attempts due to various causes are measured. If there are too many times of handover caused by low quality, it indicates possibly there is interference. The handover times due to low uplink and downlink quality shows how serious the uplink and downlink are interfered. 4. Take the average receiving quality level of TRX for reference. 5. Record the average level and quality upon call drop for reference.
1. Isolated island effect. For some uncertain reasons, the service cell may cover a too large area, superposing the adjacent cells. As a result, after MS goes beyond the coverage scope defined for the adjacent cell B and reaches cell C, it still occupies the signal of the original service cell A. However, cell A does not define cell C in the adjacent cell list and at this time MS will perform handover according to the adjacent cells table provided by the original service cell A. In this case, call drop will occur because the appropriate target cell cannot be found. (Isolated island phenomenon) 2. Coverage hole: there is uncovered area between cells. 3. Signal attenuation: serious fading occurs during signal propagation so that handover cannot be implemented in time and this causes a call drop. 4. The adjacent cell definition is incomplete so that MS keeps conversation in the current cell until it goes beyond this cell coverage edge and as a result, call drop occurs. 5. If the uplink signal coverage is larger than the downlink signal coverage, the downlink signal of the cell edge will become weak and can easily be “submerged” by the intensive signals of other cells. While if the downlink signal coverage is larger than the uplink signal coverage, MS has to remain under this intensive signal. However, if the uplink signal is too weak or the voice quality is too bad, call drop will occur.
Drive test is the most direct way to locate down a coverage problem. Traffic measurement can be another useful method. 1. In power control measurement function, the average uplink and downlink signal intensity is too low. 2. In receiving level measurement function, the proportion of low receiving level times is too large. 3. In cell measurement function/inter-cell handover measurement function, the level when originating a handover is too low and the average receiving level is too low. 4. In call drop measurement function, the level during a call drop is too low and the TA value before a call drop is abnormal. 5. In undefined adjacent cell measurement function, the undefined adjacent cell average receiving level is too high (over-shooting coverage). 6. The average level of undefined adjacent cells is too high (isolated island phenomenon). 7. In power control measurement function, the maximum distance between MS and BTS exceeds the normal value frequently. 8. In outgoing-cell handover measurement function, the handover success rate to a certain adjacent cell is low. 9. Register “uplink and downlink balance measurement function” in traffic measurement and analyze whether uplink and downlink are imbalanced.
1. According to traffic measurement analysis, make sure that the cell has a high call drop rate and a high outgoing inter-cell handover failure rate. And most handovers are caused by low signal level or bad quality, but all other indices are normal. If this is the case, it is necessary to check whether there is the coverage problem. Drive test is feasible to find out the areas with insufficient coverage. Analyze whether it is caused by terrain reasons, such as tunnels, large shopping centers, entrances of subway. Generally speaking, most of this kind of call drops occur in one direction and this problem can be solved by adding a micro-cell. 2. Find out the areas with insufficient coverage. The BTS coverage can be increased by adding new BTS or in some other ways such as increasing the maximum BTS transmission power, adopting the zero fill antenna, and changing azimuth and downtilt of the antenna. 3. Restrict the BTS coverage scope. In terms of hardware, adjust BTS downtilt or antenna height. As for software, lower the maximum transmission power or increase the minimum receiving level (RXLEV ACCESS MIN) and RACH minimum access level so as to restrict the coverage range. 4. Check whether the adjacent cells are defined resonably and whether they are in mutual symmetrical relation. Pay more attention to external adjacent cell data. 5. Judge whether the uplink and downlink are balanced. Check the tower-mount amplifier, power amplifier, antenna feeder and combiner, and check the alarm to make comprehensive analysis. Check the BTS antenna. If the diversity receiving antenna is wrongly connected, the diversity receiving gain of the antenna will be greatly reduced and the uplink signal will become worse than the downlink signal. According to the TA value, check whether BTS coverage scope is too large or whether the signal is reflected. In this case, some weak uplink MS signals will drop out very easily after accessing the network.
1. Radio Link Timeout and SACCH Multi-Frames When these two parameters are too small, the radio link fault can easily occur and thus call drop will happen. If difficult access or serious call drop is caused by too bad downlink radio environment, properly increase the value of the Radio Link Timeout parameter. If difficult access or serious call drop is caused by too bad uplink radio environment, properly increase SACCH multi-frames. 2. RACH minimum access level It indicates the signal level threshold that the system uses to judge effective MS random access. It is used to control uplink access (call, paging response, handover). Adjustment of this parameter can get a compromise between coverage and call drop rate. 3. RXLEV_ACCESS_MIN RXLEV_ACCESS_MIN indicates the minimum receiving signal level for MS to access the cell. If this parameter is too low, as a result, many MSs with low receiving level will reside in the cell, so the cell load will be increased and call drop occurs more frequently. Therefore, it should be set according to the link balance between uplink and downlink. 4. Call re-establishment permitted In case of the call drop due to a radio link fault because of sudden interference or coverage hole, MS will start the call re-establishment process to recover conversation if permitted. Call re-establishment can make the service better, but it takes long time .It can be applied to suburbs or city areas with bad coverage.
5. NCC permitted NCC permitted, NCC permitted is sent in system messages 2 and 6. It lists the combinations of NCCs that MS must measure. MS will not report the adjacent cell’s measurement report whose NCC is not permitted. If this parameter is not set properly, call drop will be caused for necessary handover cannot be triggered. 6. Set handover-related parameters properly to lower the call drop rate. For example, let PBGT handover easily take place, so as to avoid interference and lower the call drop rate. Set the emergency handover threshold properly so that emergency handover can be triggered before a call drop so as to reduce call drops. 7. Optimize the power control parameters to make it more sensitive. In addition, the level after power control must be intensive enough to ensure normal conversation. 8. Unsuitable frequency planning will bring interference in the system. A good frequency planning can minimize the call drops obviously.
System information data: Radio Link Timeout (effective on MS), 32 or 56 recommended. Cell data: SACCH multi-frames, 31 recommended.. Radio link connection timer, maximum resend times of physical information is related to call drop during handover. Find out call drop causes through call drop performance traffic measurement. Judge from the cause of call drop: the cause “error indication” is usually caused by T200 timeout.
Hardware problem may cause instability when it processes traffic leading to call drop; hardware fault may also generate internal interference that leads to a call drop. Due to possible fault in TRX or performance decrease of some parts, TRX amplification circuit self-activation may occur, thus internal interference will be generated. If the transmission link is unstable, it will cause signaling loss and low-layer link instability, and even call drop. Many Abis call drops can be avoided if stable transmission quality is ensured. If the azimuths of two antennas of one sector are different, possibly the MS can receive SDCCH, MS may drop because of the TCH signal strength very poor.
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.
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 doesn’t 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 night ， so 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.
