The whole topic is divided into three parts: Part 1: Call drop
rate calculation formula, call drop rate traffic measurement point
and call flow chart Part 2: Cause analysis of high call drop rate,
problem located principle and troubleshooting solutionsPart 3: Call
drop cases, presenting the call drop cases due to various
causes
TCH call drop rate is an important index of traffic statistics
measurement, the measurement type involves BSC general performance
and cell performance.
CLEAR_REQUEST: This message, sent from BSS to MSC, is used to
request MSC to release the allocated special radio resource. This
message is sent through BSSAP SCCP (signaling connection control
part) and used to release the allocated special channel
resource.
Shown as in the above diagram, major causes for TCH dropouts are
connection failure, error indication, Abis failure, A interface and
O&M manual intervention.
CLEAR_REQ is the clearance request message that BSC sends to
MSC.ERR_IND is the link error message that BTS reports to BSC.
1. The immediate assignment flow is triggered by the random
access flow . It is intended to assign a signaling channel for MS
to implement signaling transmission in the call setup phase.
Normally SDCCH is assigned. When there is no 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 TCH seizure success times as a
statistic point. 2. In CS_WAIT_RR_EST status, when the CH_ACT_ACK
message is received and the current channel is TCH, it will be
counted into TCH occupation success times as a statistic
point.CS_WAIT_RR_EST is one of main call-states. Call states
include main call-state and sub-call-state. Here the main
call-state and sub-call-state are combined with each other to form
the call state in the call flow processing procedure. Each main
call-state corresponds to one state processing function. Processing
of different messages is related to different sub-states. For
CH_REQ processing, it is mainly to apply for a radio channel and
send the CH_ACT (channel activation) command and CCB is still in
IDLE state.For CH_ACT_ACK (channel activation response) processing,
it is mainly to assign and install the IMM_ASS (immediate
assignment) command and CCB enters WAIT_RR_EST state. When the
CH_ACT_ACK message is received at this time and the current channel
is TCH, it will be counted into TCH occupation success times as a
statistic point.3. TCH assignment phase mainly involves assignment
command and assignment completion.The calling process starts with
BTS channel request and ends with caller TCH assignment
completion.Access phase mainly involves channel request, channel
activation, channel activation response, immediate assignment and
service request. In this phase, MS sets up a temporarily fixed
relation with BTS (BSC).TCH assignment phase mainly involves
assignment command and assignment completion. In this phase, the
voice channel of the caller will be specified.The called process
starts when MS receives BTS paging command and ends when the caller
and the called start conversation.Access phase mainly involves
channel request, channel activation, channel activation response,
immediate assignment and paging response. In this phase, MS sets up
a temporarily fixed relation with BTS (BSC).
TCH assignment phase mainly involves assignment command and
assignment completion. In this phase, the voice channel of the
called subscriber will be specified.MSC assigns a service channel
to MS through the assignment flow. Normal message flow is similar
to immediate assignment. The difference is that what is applied for
is the TCH channel. However, the message flow inside BSC is
different from immediate assignment.Compared with the original
channel, the assigned channel changes the information type only. It
is not necessary to apply for a radio channel, but directly start
the mode modification flow.If the channel rate type changes, it
will be necessary to re-apply for a channel and send the assignment
command to MS.If the channel rate type and information type both
remain unchanged, return to ASS_CMP (assignment completion)
directly.
Like assignment, after the handover command is sent, MS will be
ready to access on the new channel. In this case, information and
signaling communication on the original channel will have been
stopped and no DTAP message can be sent to MS any longer.In the new
cell, MS sends the HO_Acces message frame to BTS. After detecting
this frame, BTS will send the MSG_ABIS_HO_DETECT message to BSC and
return the physical information to MS to ask it to stop sending the
HO_Acces message frame. Then MS will send the SAMB frame and BTS
will return the UA frame response and send the EST_IND message to
BSC.In case of handover inside BSC, when BSC receives the
MSG_ABIS_HO_DETECT message, the handover will be counted into TCH
occupation success times as the statistic point if its not an SDCCH
handover.In incoming handover flow, the channel will be allocated
and activated according to the required channel type, if this
succeeds, HO_REQ_ACK (handover request response) will be returned
to MSC. Then MSC will send the handover command to the BSC of the
outgoing party. New channel access of MS after receiving the
message is similar to handover between cells and the only
difference is that HO_Detect, Est_Ind, and HO_Cmp messages must all
be sent to MSC. After the handover is completed, MSC will send
MSG_CLEAR_CMD to the source BSC and clear the old channel.When the
MSG_ABIS_HO_DETECT message is received during incoming BSC handover
and if the handover is not an SDCCH handover at this time, the
handover will be counted into TCH occupation success times as the
statistic point.The assignment often fails due to cell congestion,
or no radio resources can be assigned. However, while the service
cell of MS is short of resource, there may be enough resource in an
adjacent cell. To increase the call completion ratio and lower the
call loss ratio, the directional re-try scheme is raised for GSM
system: In case that there is a shortage of resources in the
current service cell, a service channel of an adjacent cell can be
allocated directly to MS. By means of handover flow, MS will be
handed over to an adjacent cell with enough resources, thus
implementing the assignment flow process beautifully. Therefore,
directional re-try can be regarded as a type of special handover.As
shown in the following diagram, main causes for SDCCH dropout are
connection failure, error indication, Abis failure, and so on.
Radio link fault: When radio link timeout is reduced to 0, the
channel will be released, a dropout will occur, and cause for this
dropout will be recorded as a radio link fault. In network running,
such dropouts 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 to be accepted. And finally as a result, MS or
network cannot decode correctly the information sent from an
opposite terminal and this cannot be controlled in any other ways.
In this case, the system will conclude that a radio link fault
occurs. In this case, MS will either start call re-establish or
forcedly disconnect the link. Forced link disconnection will lead
to a dropout. Therefore, a radio link fault will be concluded by
the system only when the communication quality is in no way
acceptable.In GSM specification, the concept of radio link timeout
is introduced.In Huawei system, in [System information Table],
define the parameter Radio link failure counter, which is used by
MS to decide when to disconnect the call (downlink) if SACCH
decoding fails. In [Cell Attribute Table], define the parameter
SACCH multi-frame number, which is used by BTS to inform BSC of the
radio link connection failure (uplink).In monitoring the radio link
fault, the control of the uplink and downlink should be consistent.
The radio link fault algorithms at the BTS side and MS side are
thus consistent. That is, when a special channel is assigned to MS,
it will start counter S. Then each time when an SACCH message
cannot be translated, S will reduce by 1, and each time when an
SACCH message is translated correctly, S will increase by 2. When
this value is reduced to 0, radio link failure will be
reported.
Counter T3103 timeout leads to a call dropWhen 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 source cell, BSC will reset counter
T3103. After sending the Handover Command to BTS, if BSC still
receives no message after counter T3103 expires, it will conclude
that there is a radio link failure in the source cell and then
release the source cell channel.
Interference mainly includes co-channel, adjacent-channel and
cross interference and external interference.When MS receives
intensive co-channel or side-channel interference signals in the
service cell, BER will be deteriorated and dropout will be resulted
in. ]When BTS is troubled by cross interference, the direct result
is that time slots cannot be allocated and thus resulting in waste
of BTS resource.]1. Equipment problems: Due to self-excitation of
TRX or tower-top amplifier, the system noise coefficient becomes
larger and the sensitivity is deteriorated. Cross interference of
the antenna is also an equipment problem.2. Analyze according to
driving test data: Interference road section and signal quality
distribution, the repetition and coverage of which cell signals are
causing interference.Adjust the dip angle of BTS antenna,
transmission power, relation between adjacent cells, handover
parameter of the relevant cells or adjust the frequency planning to
avoid interference.
1. If there are more than one idle channel 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
system -- through observation of how the uplink channel is
interfered -- via the RF resource indication message when the
carrier channel is idle. It should also be noted that the
interference band condition of this channel will not be shown in
the traffic measurement when the channel is blocked.2. If there are
too many times of high level and low quality, it means that there
is in-network co- and adjaecnt-channel interference or out-network
interference at the frequency of this TRX.3. In cell performance
measurement/inter-cell handover performance measurement, or
out-cell handover performance measurement, outgoing handover
attempts due to various causes are measured. If there are too many
times of handover caused by low quality, this indicates possible
interference. The handover times due to low uplink and downlink
quality gives proof of how the uplink and downlink are
interfered.4. Measure the average receiving quality level of TRX
for reference.5. Record the average level and quality upon dropout
for reference.6. There may exist interference in the destination
cell, just for reference.
1. Isolated island phenomenon. For some reasons, the service
cell covers a too large area, including the adjacent cells. As a
result, after MS goes beyond the coverage scope defined for the
adjacent cell B and reaches cell C, it still occupies the signal of
the original service cell A. However, cell A does not define cell C
and at this time MS will perform handover according to the adjacent
cells table provided by the original service cell A. In this case,
dropout will be caused because the appropriate destination cell
cannot be found. (Isolated island phenomenon)2. There are clear-cut
signal coverage blind areas at the common boundary between cells.3.
Signal fading: Serious fading occurs during signal propagation so
that handover cannot be implemented in time and this causes a
dropout. 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, dropout occurs.5. If the uplink
signal coverage is larger than the downlink signal coverage, the
downlink signal of the cell edge will become weak and can easily be
submerged by the intensive signals of other cells. If the downlink
signal coverage is larger than the uplink signal coverage, MS has
to remain under this intensive signal. However, if the uplink
signal is too weak or the voice quality is too bad, dropout will
occur.
Driving test is the most direct means to track down a coverage
problem. Traffic measurement can be another useful method.1. In
power control performance measurement, the average uplink and
downlink signal intensity is too low.2. In receiving level
performance measurement, the proportion of low receiving level
times is too large.3. In cell performance measurement/inter-cell
handover performance measurement, the level when originating a
handover is too low and the average receiving level is too low.4.
In dropout performance measurement, the level during a dropout is
too low and the TA value before a dropout is abnormal.5. In
adjacent cell definition performance measurement, the too low
average level of which adjacent cell can be located and the
adjacent cell definition average receiving level is too high
(cross-cell coverage).6. The average level of undefined adjacent
cells is too high (isolated island).7. In power control performance
measurement, the Max. distance between MS and BTS in multiple
consecutive time segments exceeds the normal value.8. In out-cell
handover performance measurement, the handover completion ratio to
an adjacent cell is low.9. Register uplink and downlink balance
performance measurement in traffic measurement and analyze whether
uplink and downlink are virtually unbalanced. Whether uplink and
downlink are balanced should be checked depending on the system
performance indices.
1. According to traffic measurement analysis, make sure that the
cell has a high dropout ratio and a high out-cell handover failure
ratio. And most handovers are caused by emergency handover, but all
other indices are normal. If this is the case, it is necessary to
check whether there exists the coverage problem. The driving test
can be feasible to find out the areas with insufficient coverage.
Analyze whether it is caused by landform reasons, such as tunnels,
large shopping centers, entrances of subway. Generally speaking,
most of this kind of dropouts 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 stations or in some other ways, increasing the Max. BTS
transmission power, adopting the zero fill antenna, and changing
the directional and dip angles of the antenna.3. Restrict the BTS
coverage scope. In terms of hardware, adjust BTS dip angle or
antenna height. As for software, lower the Max. transmission power
or increase the Max. receiving level (RXLEV ACCESS MIN) and RACH
busy threshold parameters so as to restrict the coverage range.4.
Check whether the adjacent cells are defined completely and whether
they are in mutual symmetrical relation. Pay more attention to
external adjacent cell data.5. Judge whether the uplink and
downlink are balanced.1> Check the tower-top amplifier, power
amplifier, antenna feeder and combiner, and check the alarm
condition to make comprehensive analysis.2> Check whether the
BTS antenna direction is consistent with what is designed. If the
diversity receiving antenna is connected inversely, the diversity
receiving gain of the antenna will be greatly reduced and the
uplink signal will become worse than the downlink signal.3>
Observe the MA10 measurement report. Check BSC data configuration
(MS, BTS transmission power, the Min. access level of MS).4>
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 can be dropped out very easily after accessing
the network.
a) Radio link failure counter and number of SACCH
multi-framesSet the radio link failure counter and number of SACCH
multi-frames to ensure that a radio link failure will be concluded
by the system only when the communication quality is in no way
acceptable. When these two parameters are too small, the radio link
fault is easily incurred and thus dropout will happen.If difficult
access or serious dropout is resulted in from high BER caused by
too bad downlink radio environment or low-quality link, properly
increase the value of the radio link failure counter parameter. If
difficult access or serious dropout is resulted in from high BER
caused by too bad uplink radio environment or low-quality link,
properly increase the value of multi-frames of parameter SACCH.b)
RACH busy threshold and RACH Min. access level RACH busy
thresholdIn BTS20, it indicates the level threshold used by the
system to judge MS random access In BTS30, it has no bearing on MS
access. RACH Min. access levelThe RACH Min. access level function
of BTS3X later than 03.0529 version affects the MS access. It
indicates the level threshold value that the system uses to judge
MS random access.Setting of these two values may affect the dropout
ratio and access scope.It should be noted that: For BTS20, the RACH
busy threshold parameter should be set with reference to the actual
BTS sensitivity and the Min. MS access level so as to ensure the
balance between uplink and downlink. To control uplink access
(call, paging response, handover), compromise between the coverage
and dropout ratio and add the parameter of RACH Min. access level.
Use this parameter in version 0592 or later versions of BTS3X. This
parameter can increase upward by degrees from -110dBm. It will
process the uplink access of MS and not restrict the threshold of
handover access. This parameter may affect the dropout ratio and
access scope (coverage) and it must have the support of versions
later than BSC04.0520.MS Min. receiving signal
levelRXLEV-ACCESS-MIN indicates the Min. receiving signal level for
MS to access the system.If this parameter is too low, the
requirement of the access signal level will also be low. As a
result, many MSs will try to stay in this cell and the cell load
will be increased and dropout be more easily incurred. Therefore,
it should be set according to the balance condition between uplink
and downlink.Whether to allocate SDCCH dynamicallyThe host version
G3BSC32.10100.07.0520B and later versions support the forced
recovery function of SDCCH channel. When this parameter is set as
No on the data management console, all SDCCH channels adjusted from
TCH will recover to TCH channels immediately. In this case, all
conversations set up on this TCH channel will drop out.Call
re-creation permittedRE is set to determine whether to permit call
re-creation. In case of the dropout due to a radio link fault
because of sudden interference or building blindspot, MS will start
the call re-creation process to recover conversation.Call
re-creation enabled can lower the average dropout ratio, but it
takes longer. It applies to suburbs or city areas with bad
coverage.Nation color code NCC permittedNCC permitted NCC permitted
is sent in system messages 2 and 6. It lists the combinations of
cell NCC codes that MS must measure. MS will report the measurement
report with the value of 1 in this parameter to BTS. Since MS
cannot report the adjacent information of the network whose NCC is
set as 0, if this parameter is not set properly, cross-cell dropout
will be caused if MS cannot normally originate handover in
conversation status.Set handover-related parameters properly to
lower the dropout ratio.For example: While ensuring that there is
no toggle handover and there is not much voice interruption, PBGT
handover easily take place, so as to avoid interference and lower
the dropout ratio. Set the emergency handover trigger threshold
properly so that emergency handover can be triggered before a
dropout so as to reduce dropouts.Optimize the power control
parameter to make it more sensitive. In addition, the level after
power control must be intensive enough to ensure normal
conversation.System message data table: Radio link failure counter
(effective on MS), 32 or 56 recommended.Cell attribute table:
Number of SACCH multi-frames, 31 recommended for both BTS2X and
versions later than 05.0529.Radio link connection timer, Max. times
of physical message repetition is related to handover dropout.Find
out dropout causes through dropout ratio performance traffic
measurementJudge from the cause of dropout: The cause error
indication (when the cause value is 0x01)is usually caused by T200
timeout. For BTS3X BS, check related data.The cause connection
failureis usually caused by that 14 (number of SACCH multi-frames)
consecutive measurement reports cannot be decoded. This is often
the problem of radio link quality.Add adjacent cells
reasonably.Adjacent cell missing is the ultimate cause for
unavailable handover and dropout in the end.Adjust improper
handover parameters.Common adjustment parameters: Min. DL level
candidate cell, Min. access level offset , inter-cell handover
hysteresis, PBGT threshold, various PN judgement duration time, and
edge HO UL_EX LEV threshold.
Power control parameter adjustment:Modify the power control
parameter to make the power control more sensible, and make the
level after power control intensive enough to ensure normal
conversation.For example : If the uplink expectation level is too
low and the filter length is too long, power control will become
insensible.
Popular methods of judging dropout resulted from equipment
problemsA. Hardware troubleshooting1. For dropout due to hardware
problems, view relevant hardware alarms via OMC.If there is no
hardware alarm information in OMC, there may be something wrong
with the TRX or diversity receiver part. The allocation failure
ratio and the uplink and downlink quality handover ratio at this
time are certainly very high. In this case, monitor Abis interface
or close all other TRXs in the cell to perform dialing test on the
suspected TRX to locate the fault. Generally speaking, when there
is something wrong with the frame processing unit, the assignment
failure rate and uplink and downlink quality handover will be quite
serious. When there is something wrong with the receiving part, the
assignment and uplink quality handover will be relatively serious.
When there is something wrong with the transmission part, the
assignment failure ratio and downlink quality handover will be
quite serious. It is also advisable to judge whether there is some
problem with TRX according to the TCH assignment failure ratio.
When there is some problem with TRX, usually the TCH congestion
ratio and access failure ratio are very high. Monitor Abis
interface to locate the faulty TRX (whose equipment No. is TEI).
Analyze according to the occupation ratio of handover trigger
causes. If there exists some hardware problem in the uplink, the
receiving level of the MS occupying this TRX will be very weak and
the receiving quality will also be poor. When the downlink quality
or the ratio of handover triggered by the level is high, the fault
may exist on the downlink equipment hardware, such as TRX, power
amplifier, transmission antenna or feeder. When the ratio of
handover triggered by the uplink quality is rather high, the fault
may exist on the equipment hardware of the radio antenna uplink.2.
In addition, the hardware fault may also generate internal
interference that leads to a dropout Due to possible mistake in TRX
production or performance degradation of some of its parts, TRX
amplification circuit self-activation may occur, thus internal
interference will be generated. CDU or SPL fault. The active
amplifier is used in CDU divider or SPL. If the performance is not
good, self-activation interference can also be generated easily. If
the out-band stray of the power amplification module exceeds
limits, or the separation of receiving and sending of the duplexer
in CDU is too small, the interference on the receiving channel will
occur. In addition, if CDU 3-order intermodulation indices do not
meet requirements, the intermodulation component of transmission
signals will also enter the receiving channel to generate
interference.For interference caused by hardware problems, we can
start with various sections of the radio signals to find out the
cause for interference with the exclusive method.BTS RF signal
path: outside - antenna - feeder - CDU - TRX.Interference may be
generated in any link of this path. The spectrum analyser can be
used to test step by step from bottom to top, so as to identify the
interference source. Here well take BTS3X as an example to see the
basic procedure for positioning the commonest uplink
interference.1. Register traffic measurement, mainly TCH
performance measurement, cell performance measurement, uplink
frequency band scanning, uplink and downlink balance measurement.
The period of traffic measurement can be set as 30 minutes or even
shorter.2. Enable a TRX and disable all others in this BTS to
observe the traffic measurement result. This step is intended to
view whether it is the intermodulation interference. If the
interference band disappears, it means that it is the
intermodulation interference, perform step 6. If the interference
band does not disappear, perform step 3.3. Disconnect the two
main/diversity receiving input cables of TRX and connect the dummy
load (existing at the receiving ports, not used by CDU), observe
the interference band reported by Abis interface. If the
interference band is high, it means that the interference comes
from the TRX and this TRX should be replaced. If the interference
band is completely within the interference band 1, it means that
the interference comes from before the TRX step , perform step 4.4.
Recover the connectors and cable of the TRX and connect the power
meter dummy load to the CDU connection input place (TX/RX). Set the
tributary input signal of main receiving as 0 while absorbing its
output power. And at the same time, disconnect the CDU diversity
receiving cable, connect the matching load, and also set its input
signal as 0. Observe the interference band reported by Abis
interface. If the interference band is very high, it means that the
interference comes from CDU, which should be replaced. If CDU and
TRX replacement takes no effect, there may be some problem with the
BTS clock. Check TMU13M clock, the clock bus between TMU and TRX,
the clock-matching DIP switch, and the rack-top clock matcher. If
interference bands fall completely within interference band 1, it
means that the interference comes from steps before CDU, then
perform step 5.5. Recover CDU connectors and cable, disconnect the
RF soft jumper of the rack-top TX/RX and RXD of this cell, and
connect the matching load to the rack-top TX/RX and RXD ports. Then
observe the interference band reported by Abis interface. If the
interference band is very high, it means that the interference
comes from the RF cable between CDU and the rack-top ports, which
should be replaced. If all interference bands fall within
interference band 1, it means that the interference comes from
steps before rack-top, and perform step 6.6. Start all TRXs,
replace the antenna feeders of this cell and the adjacent cell
(where there is no interference) at the rack top, then observe the
interference band reported by Abis interface. If the interference
band is very high, it means that the interference comes from BTS
internal intermodulation interference. In this case, replace CDU
and TRX. If the interference band is completely within interference
band 1, it means that the interference comes from steps before
antenna or feeder, perform step 7.7. Change the frequency band and
observe the interference band reported by Abis interface. If the
interference band is still very high, it means that the
interference comes from the BTS antenna feeder or broadband
out-network interference. In this case, check the antenna feeder,
and replace the antenna feeder or use the spectrometer and
directional antenna to find the external interference source. If
the interference band is completely within interference band 1, it
means that the interference comes from the in-network
co-channel/side-channel interference. In this case, perform
frequency band optimization. B. Transmission troubleshootingIf the
transmission link is unstable, it will cause signaling loss and
low-layer link instability, and even dropout. Many Abis dropouts
can be avoided if stable transmission quality is ensured.Fault
messages of Abis interface are: BSC does not receive the
measurement report from BTS, some signaling failures during
handover and internal causes, or BER interference on Abis
interface. There are less A interface failures, which are mainly
handover (between BSCs or MSCs) failures. The reason is that the
data in the handover exchange are incomplete or the destination BTS
lacks handover capability.C. Antenna and feeder
troubleshootingJudge dropout due to antenna and feeder problems1.
Directional angle and pitch angle of the antennaDuring BTS
installation, each directional cell has two antennas(main and
diversity). BCCH and SDCCH of the cell may be sent out from these
two antennas respectively. If the coverage ranges of these two
antennas are different, the user can receive BCCH signals while he
cannot occupy SDCCH sent out by another antenna when originating a
call. As a result, dropout occurs.If the directional angles of
these two antennas are different, possibly the user can receive
signaling channel SDCCH, while if he is assigned to TCH sent out by
another antenna, dropout will occur.
Solutions:Observe transmission and board alarms (TC board fault,
A interface PCM loss alarm, LAPD link disconnection, power
amplifier, HPA, TRX board alarm, and CUI/FPU alarm). According to
the alarm data, analyze whether transmission is interrupted or
there exists a faulty board (such transmission grounding mode, TRX
damage or poor contact).The fault can be located by blocking TRX
and registering temporary traffic measurement tasks (dropout ratio
and congestion ratio). Frequency planning principles:1) Frequency
bands of the 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 multiplexing mode, the immediately adjacent
BTSs cannot have the same frequency (even if the directions of the
antenna main lobes are the same, the interference of side lobes and
back lobes cannot be predicted easily due to antenna and
environment causes).5) In consideration of the complexity of
antenna height and propagation environment, same frequency opposite
(including skew opposite) should be avoided for BTSs near to each
other. 6) Usually, 1*3 multiplexing should ensure that the hopping
frequency bands should be two times the number of the carriers
participating hopping.7) Focus on co-channel multiplexing to avoid
the situation that the same BCCH or BSIC exists in adjacent
areas.8) Enable PBGT handover. After the side-channel suppression
ratio is ensured through parameter adjustment, the side channel can
be used in immediately adjacent opposite cells.
Why the uplink interference can be concluded when there is an
interference band as high as of level 3 - 5?1. Concept of
interference band:BSS measures the uplink characteristics of the
radio channel , and calculates and reports the channel interference
condition of Idle channel, thus to provide judgement bases for BSC
to assign channels. Interference is divided into six levels
artificially according to the intensity of interference signals.
The level amount used to divide these levels is called interference
band threshold.2. When there are many channels working in
interference mode, it can be concluded that there exists
interference phenomenon in the system. Therefore, the interference
band statistics result can be used as the basis for various
threshold settings and interference analysis.3. For in-network
interference, generally the interference band increases with the
traffic while out-band interference has little bearing on the
traffic (of course, the interference caused by the TACS network
will also vary with the traffic).
1. Interference bands as high as of level 3-5 appear.2. High
congestion ratio. During signaling transmission, since the
signaling channel suffers from outside interference, SDCCH or TCH
assignment fails.)3. The dropout ratio is far higher than the
normal . Due to outside interference, too bad handover command
signaling BER or voice channel will lead to handover failure.)4.
High BER. (Sometimes even if the uplink receiving level is up to
-70dBm, the receiving BER may also be bigger than 12.8%.)5. Observe
the absolute value of the average level of RACH request to judge
whether there exists uplink interference.6. Observe the traffic
measurement of handover causes to make judgmentWhen there are many
handovers triggered by uplink signal quality deterioration, it can
be judged as to be caused by uplink interference or hardware
fault.When there are many handovers triggered by downlink signal
quality deterioration, it can be judged as to be caused by downlink
interference or hardware fault.When there are many handovers
triggered by both uplink and downlink signal quality deterioration,
first track down the cause on the hardware fault problem and then
on the uplink or downlink interference.
Interference mainly includes co-channel, side-channel,
intermodulation interference and other external interference.When
MS receives very intensive co-channel or side-channel interference
signals in the service cell, this will lead to BER deterioration so
that MS cannot correctly demodulate the BSIC codes of adjacent
cells or correctly receive the measurement report of the MS.When
the BS suffers from intermodulation interference, the immediate
result is that timeslots cannot be allocated, which will cause
waste of BS resources.The repeater station will amplify the
interference signals while amplifying the desired uplink and
downlink signals of BTS. As a result, the signal quality will
degrade and finally dropout will occur. The accompanying phenomenon
is the obvious increase of the allocation failure rate.
The repeater is an effective mode recommended to be widely
adopted in early phase of network construction to extend the BTS
coverage distance. Due to its own characteristics, it interferes
the BTS easily when being used improperly. The repeater has the two
following interference modes:1) As installation of the repeater
station doesnt conform to relevant standard, there is no enough
separation between the donor antenna and user antenna. As a result,
self-activation is formed and the normal work of the BTS which this
repeater station relies to is affected.2) Since the repeater
station 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 task will also be
heavy and it can interfere adjacent BTSs very easily.Reason why the
repeater station can interfere the BTS: When the uplink background
noise level of the repeater station host is too large, the BTS
channel will be blocked when the noise level transmitted to the BTS
is of high intensity.
Common methods of checking and clearing dropout due to
interference1. Rule out the internal interference caused by
equipment problems and check the separation of GSM BTS
transceivers, antenna feeder installation, and so on.2. Observe the
interference band3. Driving test4. Observe traffic measurement of
handover causes to make judgment5. Clear uplink
interferenceCurrently this is the major interference, which mainly
occurs in traffic peak time and originates from co-channel
interference, or external interference. Co-channel interference is
related to the traffic of the co-channel cell. If the traffic is
high, the interference will also be high. External interference is
mainly intermodulation interference. The uplink interference can be
analyzed and cleared by analyzing related reports in driving test,
modifying the co-channel frequency of the co-channel cell,
increasing the distance between two co-channel cells or using the
spectrum analyzer. It can also be reduced by diversity receiving
and effective power control.6. Clear downlink interferenceDownlink
interference are mainly co-channel interference and side-channel
interference of some BTSs due to inappropriate frequency planning.
The interference source can be found out with the spectrum
analyzer.7. Check whether DTX, frequency hopping technology and
power control application are reasonable8. Use PBGT handover
algorithm flexibly to avoid co-channel and side-channel
interference effectively.
Judge whether there is some problem with TRX according to the
TCH assignment failure rate. When there is something wrong with
TRX, generally the TCH congestion rate will be very high and the
access failure rate will also be rather high. Measure Abis
interface to find the faulty TRX (whose equipment No. is TEI).When
there is something wrong with the receiving part, the assignment
failure rate and the uplink quality handover will be relatively
serious.When there is something wrong with the transmitting part,
the assignment failure rate and the downlink quality handover will
be relatively serious.
Therefore, first check whether there is something wrong with the
equipment and then judge the interference using the traffic
measurement analysis tools in partnership with the driving test and
parameters check.Frequency hopping can effectively improve the
transmission quality of radio signals, especially the transmission
quality of slowly moving entities. The reason is that frequency
hopping can make the transmission carrier jump on the basis of
burst pulse sequence, so that the co-channel interference and
frequency selective fading effect can be decreased greatly.However,
note whether there is conflicting frequency hopping offset (MAIO)
between in-cell TRXs. The unreasonable setting of relevant
parameters such as whether interference exists in frequency hopping
bands can also lead to dropout.
MAIO (Mobile Allocation Index Offset)Position: Radio channel
configuration table.Value range: 0 ~ (N - 1). Here N is the number
of frequency bands in MA. Pay attention to the hopping modes
supported by the BTS when planning MAIO and try best to avoid
co-channel and side-channel conflicts under the same cell of the
same BTS.HSN (Hopping Sequence Number)Position: Hopping data
tableValue range: 0 ~ 63. Here 0 represents the cyclic hopping (0
is prohibited for some devices) and other values represent pseudo
random hopping. In principle, all channels of all TRXs to which
co-channel and side channel may occur under the same BTS must use
the same HSN, so as to avoid co-channel and side-channel conflicts
by properly setting MAIO. Since the consistency of frame numbers
cannot be ensured in different BTSs, try best to make HSNs of
different BTSs inconsistent, especially when the same hopping
frequency group is used. Only when HSNs are inconsistent, their
conflict relevance can possibly be minimized.
A radio link has two directions: uplink and downlink. The actual
coverage range should depend on the direction with weaker signals.
If the uplink signal coverage is larger than the downlink signal
coverage, the cell edge downlink signal will be relatively weak and
easy to be covered by intensive signals of other cells. If the
downlink signal coverage is larger than the uplink signal coverage,
the MS will be forced to stay under this intensive signal. However,
if the uplink signal is too weak, MS will not be able to originate
a call or it may lead to bad voice quality, unidirectional
conversation or even dropout after conversation. Of course, balance
does not mean absolute equality.
Compare the receiving levels of BTS and MS respectively and
observe whether the downlink signal is still good enough for MS to
stay in this cell when the uplink signal reaches the lowest
receiving threshold level.
First confirm that the static power level of MS is set properly
(900 is level 5 and 1800 is level 0). It is once found that
unidirectional conversation of the 1800 mobile phone is caused by
the imbalance of the uplink and downlink. An investigation shows
that, in the system message table, no matter it is a 900 cell or a
1800 cell, the corresponding MS Max. transmission power levels are
all set as 5. In this case, the corresponding DCS1800 MS output
power is 0dBm, lower than its Min. transmission power 0.25W
(24dBm).In areas with very complicated landforms, make power
control as sensitive as possible. When the power control makes the
power of MS decrease and the uplink signal turns bad suddenly, bad
signal or dropout will occur due to too weak uplink signal if the
power control is very dull at this time.
1. No matter how the traffic channel of this cell is blocked,
the congestion rate is always relatively high. There can be
interference or the terrain in the coverage range of the cell is
possibly complicated.2. It is concluded that, by viewing and
analyzing the traffic measurement task data, the interference band
of CELL3 basically stays at 4 or 5 in daytime, and it stays at 1 or
2 between 23:00 FM and 7:00 AM. In addition, the dropout rate and
the interference band are regular.3. First take co-channel and
side-channel interference into consideration. Change the frequency
band. Add more than 1M to the original frequency band of CELL3 and
the situation remains unchanged. Therefore, co-channel and
side-channel interference is ruled out.4. Then consider the
equipment problems. Change the antenna feeder of CELL3 with that of
CELL with little interference, but CELL3 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 ruled out, the fault can be located as
external interference.
1. Although there is a 10M distance between this frequency band
and that used by us, it is a continuous signal and it can be more
possibly to conflict and intermodulate with other signals. Some
parts of such intermodulation components may fall within the
receiving band, thus forming interference.2. In daytime the traffic
is larger than that at night and the intermodulation components are
also more than those at night, so the interference is also
large.
Perform driving test and positioning with the spectrometer and
the 904M signal source is not found.Perform all tests on the
building roof, the fault is finally located on a small antenna.It
is confirmed through test interruption that the interference signal
is sent by this antenna. After it is powered off, the interference
will disappear and performance indices of BTS A return to
normal.
For such interference source positioning, the effect on the land
is not very good. In addition to interference of buildings, some
key factors can hardly be found. Therefore, a roof test should
better be performed.It is found that, after multiple on-site
dialing tests with the test MS, there really exist dropout and
noise. However, it can be seen from the test MS that, MS always
stays in a service cell of a non-local BTS A before dropout, and
its TA value is about 17, and the receiving signal intensity is
about -80dBm. Therefore, it can be concluded that MS stays at the
second cell of a BTS 8-9km away from the dialing position.It is
found, by data viewing, that this BTS is BTS B about 3-4km away
from this building. Therefore, it can be concluded that the signal
of the second cell of BTS B received here is what reflected back by
an obstacle. Thus, a coverage area like an isolated island is
formed in this area. The place closest to this building is BTS A,
about 800 meters away. Carry out a test at the place where dropout
frequently occurs inside the building and find that the signal of
the second and third cells of BTS A is about -75-85dBm and this
building is just at the border between the second cell and the
third cell of BTS A closest to it. Originally, handover of MS at
this place is relatively frequent.In BSC data configuration, only
the second cell of BTS A is configured for the adjacent relation to
BTS B, and the third cell is not configured. Therefore, when MS
uses the signal of the second cell of BTS B at this area, the
signal of the third cell of BTS A is relatively intensive. In
addition, there is no adjacent cell relation between the second
cell of BTS B and the third cell of BTS A. That explains why
handover cannot be implemented.In addition, the signal of the
second cell of BTS B is that reflected for multiple times. When the
signal of BTS B that MS receives becomes weak suddenly for some
reason, it may be necessary to originate emergency handover.
However, at this time, the second and third cells of BTS A is not
the best candidate for the second cell of BTS B. Therefore, it may
be handed over to another BTS C, but at this time, MS cannot
receive the signal of BTS C. Thus, dropout is generated.
Take the third cell of BTS A as the adjacent cell of the second
cell of BTS B by modifying the data in BA1 (BCCH) table, BA2
(SACCH) table and cell adjacent relation table in BSC data
configuration. After the test, the dropout problem is solved. Then
further optimize network engineering parameters to eliminate the
isolated island effect.Isolated island problem: When there appears
a cell coverage isolated island in an area, MS will stay in the
call origination service cell since the pre-defined adjacent cell
cannot be monitored at such spot. No matter how the signal changes,
handover cannot be implemented normally until a dropout occurs. To
avoid such situation, two means can be used. The better one is to
adjust the antenna of the isolated island cell to eliminate the
isolated island phenomenon. However, due to the complexity of
electric wave 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 other means is to define new adjacent cells for the
isolated island cell. The definition principle of its parameter is
as follows: The handover/location update from the isolated island
cell to the normal cell is prior to the inverse handover/location
update.
Adjustment of handover parameters:1. On condition that there is
no toggle handover or too much voice interruption, make PBGT
handover easier, so as to avoid interference and lower the dropout
ratio.Adjustment of handover parameters:2. Set the emergency
handover trigger threshold reasonably, so as to trigger emergency
handover in time before dropout to reduce dropouts.
