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7/15/2019 GO_NA20_E1_1 GSM Network SDCCH Assignment Analysis-62
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SDCCH Assignment Analysis
& Solutions
Zte university
7/15/2019 GO_NA20_E1_1 GSM Network SDCCH Assignment Analysis-62
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Contents
Overview
Analysis of signaling and counters related to
immediate assignment
Radio parameters Instructions on checking of SDCCH assignment
failure
Typical cases on SDCCH assignment
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Definition of SDCCH
SDCCH: the Standalone Dedicated ControlChannel is used to transmit information likechannel assignment, which falls into the followingtwo types:
SDCCH/8: the standalone dedicated control channel; SDCCH/4: the SDCCH that is combined with CCCH.
In brief, the following processes shall be taken intoconsideration in the process of occupying SDCCH:
Location update, periodical location update; IMSI attach/detach
Call setup
SMS
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Signaling flow of immediate assignment
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Counters related to SDCCH assignment &
corresponding signaling messages V3
C900060242
Number of
SDCCH
assignment
success
Function:
After BSC sends out the immediate assignment message (IMM_ASS),this counter counts the number of successful MS accesses to thecorresponding SDCCH.
Sampling:
when BSC receives the correct EST_IND or the message of assignmentcomplete.
C900060243
Number of
SDCCH
assignment
failure
Function:
After BSC sends out the immediate assignment message (IMM_ASS),this counter counts the number of failed MS accesses to the allocatedSDCCH.
Sampling:
when BSC receives the wrong EST_IND, or when T3101 expires.
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SDCCH assignment success rate
KPI SDCCH assignment success rate
Definition Number of successful SDCCH assignments*100/(Number of successful
SDCCH assignments + Number of failed SDCCH assignments)
Counter formula
V2 C11644*100%/( C11644+ C11645)
V3 V6.2 C900060242*100%/(C900060242+C900060243)
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Difference: Random access success rate
Definition: Number of successful randomaccesses / Number of random accessrequests*100%
Number of random access requests
Definition: MS applies for a channel in the idle mode. Trigger point: it counts the message of CHANNEL
REQUIRED received by BSC from MS. (A1)
Number of successful random accesses
Definition: BSC successfully assigns a dedicatedchannel for MS.
Trigger point: it counts the message of IMMEDLATE ASSIGNMENT sent from BSC to MS. (A2)
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Contents
Overview
Analysis of signaling and counters related to
immediate assignment
Radio parameters Instructions on checking of SDCCH assignment
failure
Typical cases on SDCCH assignment
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Analysis of Channel Request cause
Establishment Cause
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Analysis of Channel Request cause
Establishment Cause (continued)
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Analysis of Channel Request cause
Summary on Establishment Cause Emergency call
Call re-establishment
Paging response(MTC)
Mobile originating call(MOC)
Location update(LOC)
Other access causes
One-step access
LMU service
MBMS service
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Channel Required
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Channel Required
Request Reference
RA(Random access reference): it continues to use the Cause and
Random Reference in the Channel Request.
Byte 3 and 4 (T1, T2, T3): receive the frame number(42432) of the
burst pulse.
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Channel Required
Access DelayThe estimated TA
Physical Context
including Rxlev_UL
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Immediate AssignmentPage Mode = same as before
Packet Response Type and Dedicated mode or TBF
Downlink assignment to mobile in Ready state: no meaning
TBF or dedicated mode: this message assigns a dedicated mode resource
PR Type: immediate assignment procedure for RR connection establishment
Channel Description
Type = SDCCH/8[0]
Timeslot Number: 1
Training Sequence Code: 0h
ARFCN: 104
Request Reference
Random Access:
Establish Cause: E0h = Originating call and TCH/F is needed, or originating call
and the network does not set NECI bit to 1Random Reference: 12h
N32: 13h; N51: 1Fh; N26: 0Dh
Timing Advance: 1 = 0,6 km
Mobile allocation
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Establish Indication
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Establish Indication
T represents the sub-channel number.
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Establish Indication
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Establish Indication
Information on layer3: CM SERVICE REQUEST
LOCATION UPDATING REQUEST
IMSI DETACH
PAGING RESPONSE
CM RE-ESTABLISHMENT REQUEST
NOTIFICATION RESPONSE
IMMEDIATE SETUP
RR INITIALISATION REQUEST
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Establish Indication
CM SERVICE REQUEST Originate call
Emergency call (Access statistics show that emergency
call is not included in MOC )
SMS Supplementary service
Group call establishment
Voice broadcast call
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Access counters
Basic measurement
Counter Number Counter name
C900060001 Number of MTC access requests
C900060002 Number of MTC access successes
C900060131 Number of CM SERVICE REQ of MOC
C900060136 Number of MOC access requests
C900060137 Number of accesses due to paging response
C900060236 Number of MOC access successes
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Access counters
Radio access measurement (I)Counter Number Counter name
C901110001 Number of invalid access requests
C901110003 Number of successful process for MOC access
C901110006 Number of successful process for MTC access
C901110008 Number of call re-establishment access requests
C901110009Number of successful process for call re-
establishment access
C901110010 Number of call re-establishment access success
C901110011 Number of emergency call access requests
C901110012Number of successful process for emergency call
access
C901110013 Number of emergency call access success
C901110014 Number of LOC access requests
C901110015 Number of successful process for LOC access
C901110016 Number of LOC access success
C901110017 Number of access requests due to other causes
C901110018 Number of successful process for other causes’ access
C901110019 Number of access success of other causes
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Access counters
Radio access measurement (II)C901110020 Number of LMU Establishment access requests
C901110021 Number of successful process for LMU Establishmentaccess
C901110022 Number of LMU Establishment access success
C901110023 Number of accesses due to location update
C901110024 Number of accesses due to CM SERVICE REQ
C901110026 Number of Emergency Call (CM SERVICE REQ)accesses
C901110027 Number of SMS (CM SERVICE REQ ) accesses
C901110028 Number of supplementary service (CM SERVICE REQ)accesses
C901110029 Number of accesses for LCS (CM SERVICE REQ )accesses
C901110031 Number of accesses due to call re-establishment
C901110032 Number of accesses due to IMSI de-activation
C901110033 Number of accesses due to other causes
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Contents
Overview Analysis of signaling and counters related to
immediate assignment
Radio parameters Instructions on checking of SDCCH assignment
failure
Typical cases on SDCCH assignment
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TxInteger
Before response to the previous “channel request”is received, MS waits for a period of time at
random and sends the request again after
expiration. TxInteger is to decide the random
waiting time. The interval (number of timeslots) from MS originating
the immediate assignment to the transmission of the
first “channel request” message is a random number
among { 0,1,…,Max(T,8)-1 }. The interval (number of timeslots) between two
consecutive “channel request” is a random number
among {S,S+1,…,S+T-1}.
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TxInteger
T(Number of
timeslots
Of TxInteger)
S
(CCCH is NOT
combined with
SDCCH)
S
(CCCH is
combined with
SDCCH)
3, 8, 14,50 55 41
4, 9, 16 76 52
5,10,20 109 58
6,11,25 163 86
7,12,32 217 115
TxInteger Number of
timeslots (T)
0 3
1 4
2 5
3 6
4 7
5 8
6 9
7 10
8 11
9 12
10 14
11 1612 20
13 25
14 32
15 50
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MaxRetrans
Because RACH is a ALOHA channel, in order toimprove MS access success rate, the network
allows MS to send several Channel Request
messages before it receives the Immediate Assign
message. The max number of Channel Requestssent by MS is decided by MaxRetrans.
MaxRetrans Max number of retransmission
0 1
1 2
2 4
3 7
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TaAllowed
It represents the max TA allowed for access to thecell.
It is used to filter out fake accesses.
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RachAccessMin
New parameter for iBSC 6.20.100e Used to filter out fake access, but not
recommended because it will affect the paging
performance.
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Contents
Overview Analysis of signaling and counters related to
immediate assignment
Radio parameters Instructions on checking of SDCCH
assignment failure
Typical cases on SDCCH assignment
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Explanation on common causes of SDCCH
assignment failure
MS frequently originates location update due topoor downlink quality;
Improper setting of Tx-Integer;
High SD assignment failure rate due to LAPD
delay
Co-channel/co-BSIC interference
Uplink interference
Overshooting
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Improper setting of Tx-Integer
The default of Tx-Integer is 14, which is also the
max value.
Usually, the one-way signaling transmission delay
at Abis interface is 60ms~100ms; there should be
a delay of about 240ms from MS originates
Channel Request till it receives Immediate Assign.
When the transmission link delay is long, while
TxInteger is set with a small value, it will result in
MS sending too many access requests. However,
MS only responds to the first Immediate Assign it
receives.
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Improper setting of Tx-Integer
Flow chart of repeated assignment failure
Channel Request
Channel Required
M S
Channel Active
Channel Active Ack
Imm Assign(OK) Imm Assign Cmd
B T S
Channel Request(Re-Send)
TxInteger
Lapd
Delay
Channel Required
Channel Active
Channel Active Ack
Imm Assign Cmd Imm Assign(Fail)
MS change
to SDCCH
B S C
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LAPD delay
Possible causes of LAPD delay Application of LAPD 1:4 multiplexing will lead to the situation that
several BCCH TRXs are multiplexed on one LAPD, which willcause heavy flow on the LAPD and hence delay.
Heavy flow on LAPD leads to delay. For example, improper LACdivision will lead to large amount of paging and hence LAPD flow
control. Transmission equipment fault leads to loss of messages on LAPD
or long LAPD delay. These phenomena are often accompaniedwith SDCCH assignment failure.
The transmission equipment’s own delay, such as the delay
caused by satellite transmission at Abis interface. Impact of PS service: PS service is more sensitive to network delay.
Any LAPD delay will leads to re-transmission of PS servicemessage, which increases the flow on LAPD and causes longer LAPD delay, then a malicious circle will be resulted.
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Co-channel & co-BSIC
Two cells have same BCCH and same BSIC The Channel Request sent by MS is received by two
cells and they assign SDCCH at the same time, but MS
can only accept one SDCCH, therefore, one of the two
cells will inevitably experience SDCCH assignment
failure.
For RACH coding, first add in 6bit color code, which is
obtained through taking mod2 of 6bit BSIC and 6bit
parity checking code. Therefore, co-BCCH and co-BSIC
may cause the BTS to incorrectly decode MS access
bursts to other sites, which will lead to SDCCH
assignment failure
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Co-channel & co-BSIC
Two cells have same BSIC and the TCH Arfcn of one cellis same as the BCCH Arfcn in the other cell.
The handover access request occurring on the TCH timeslot is
received as Channel Request by the other cell, which thereafter
performs assignment. This certainly leads to SDCCH assignment
failure. It’s stipulated in protocols that the MS-started handover access
information and the random access request share the same format,
which is AB frame; the difference is that the handover access
information content (RA) in one handover started by MS is the
same, and the FN is in consecution. Signaling related to this problem displays that the RA is the same,
TA is in consistence and FN in consecution. It’s confirmed that all
the large amount and consecutive Channel Requests are fake
accesses caused by handovers between co-channel cells.
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Overshooting
If the coverage of cell is too large, the DL Rxqual at the cellmargin will be poor. In this case, BTS can receive ChannelRequest sent by MS, but MS can not receive Immediate
Assign sent by BTS, for BTS is more sensitive than MS,
If the coverage of cell is too large, the cell may share
channel and BSIC with the cell which is far away. Solution to overshooting:
Adjust the engineering parameters of antenna to limit the cellcoverage.
TA_allowed can effectively decrease SDCCH assignment failures
caused by overshooting. The side effect it brings is that the distantMS is not able to access network. Therefore, the threshold of TA_allowed shall be set a bit higher than the cell’s actual coverage.Besides, we should take into account the transmission distance of repeater when calculating the cell coverage range.
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Uplink Interference --- Fake Access
BTS receiving sensitivity is -112dbm~-125dbm. If the random access signal
strength received by BTS is lower than BTS sensitivity, it usually is confirmed to
be interference. The interference can be decoded as random access, which iscalled as fake access, and will definitely lead to SDCCH assignment failure.
Another feature of fake access is that TA is larger than that needed for the actual
coverage range.
Solution: TA_allowed
Note:① RachAccessMin is not recommended to use
② As for TA-allowed, the corresponding name used by Nortel is RNDACCTIMADVTHRESHOLD,
whose description is as follows: adjust the parameter according to the cell’s actual coverage range.
Fake RACH request can be filtered out through setting proper threshold, therefore unnecessary
SDCCH assignment can be avoided. Test results prove that if TA-allowed is set 35Km for cells with
small coverage radius, fake RACH (the system demodulate the noise into RACH pulse by mistake)accounts for almost 30% of all RACH requests. After rndAccTimAdvThreshold is changed to 2, fake
RACH is totally filtered out.
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Frequent location update started by MS
If MS needs to make location update, while theradio environment is poor, it will retransmit
Channel Request with the cause of location
update again and again, but it can never receive
Immediate Assign message. The frequent location update will cause
fluctuations in SDCCH assignment indicators.
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Frequent location update started by MS
Number of SDCCHassignment
successes
Number of SDCCHassignment failures
SDCCH assignmentsuccess rate
Number of
MOC access
requests
Number of
MOC access
successes
Number of
MTC access
requests
Number of
MTC access
successes
Number of
SDCCH
occupation
attempts (for
assignment)
(MOC+MT
C)
assignment
success rate
(MOC+MT
C)
proportion
Reference
indicators
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Troubleshooting instructions
Check TxInteger of the problem cell, along with LAPDdelay observed from signaling.
Check whether the LAPD link of BCCH TRX in the problemcell is multiplexed with that of other cells.
Check whether any of the adjacent cells have same Arfcn
and BSIC with the problem cell. Check whether the value of counter “number of access
attempts due to other causes” is big. If so, and the counter “number of access successes due to other causes” is zero,it is possible that “handover access” on other TCH TRXsare decoded as “channel request” by the problem cell.
Error Report with Channel Number 0x88 is available in themplog file.
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Troubleshooting instructions
Check SDCCH allocation KPIs and transmissionalarms.
If SDCCH &TCH assignment indicators are all bad,
the problem shall be related to radio environment.
Analyze signaling and check if Channel Requestwith large TA, if so, fake access exist and
TA_allowed restriction can be used.
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Contents
Overview Analysis of signaling and counters related to
immediate assignment
Radio parameters Instructions on checking of SDCCH assignment
failure
Typical cases on SDCCH assignment
LAPD d l C 1 L t f
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LAPD delay—Case 1: Large amount of
paging
Problem description: It’s found through performance
analysis that ZTE BSC3 has low SD assignment success
rate, which is only about 60% on late busy hours.
Problem analysis:
It’s observed that all the cells are experiencing high SD assignment
failure rate, so impact from radio parameters is excluded.
Indicators of other BSCs are normal; the SD assignment success
rate is low in only BSC3 and the Siemens BSC, both of which are
under MSC7.
The paging success rate in MSC7 is also very low; as the traffic
volume increases, the amount of paging increases as well.
LAPD d l C 1 L t f
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LAPD delay—Case 1: Large amount of
paging
Adjustment measure:
Add one LAC under MSC7. After the adjustment, the SD assignment
success rate of BSC3 returns to normal, reaching above 95%.
50%
60%
70%
80%
90%
100%
0
20000
40000
60000
80000
100000
3月10日 3月11日 3月12日 3月13日 3月14日 3月15日
BSC3 SDCCH指配成功率对比
SDCCH指配成功次数 SDCCH指配失败次数 SD指配成功率
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LAPD delay—Case 2: Satellite transmission
Problem description: 4BTSs are under BSC01, butbelong to different peripheral modules. The SD
assignment failure rate of the 4BTSs reaches as
high as 50%.
The time stamp shows that it takes an average of 0.58s to successfully activate a channel.
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LAPD delay—Case 2: Satellite transmission
How to confirm that two Channel Requests arestarted by the same call attempt?
They should have the same Establish Cause;
The same Access Delay;
The frame number interval corresponds to the setting of TxInteger:
Calculation formula: FN=T1*26*51+((T3-T2)mod 26)*51+T3
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LAPD delay—Case 3: Transmission equipment fault
Problem description: Massive SDCCH assignmentfailures occur in 3 cells of a site, accompanied
with lots of SDCCH allocation failures.
Problem analysis: SDCCH allocation failure
usually means transmission equipment fault. After checking mpLog printing, there are lots of LAPD
Errors.
Also There are a lot of transmission alarms.
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Improper setting of Tx-Integer
A cell’s ordinary SDCCH assignment failure rate remains at around
20% and hits 30% in busy hour. However, other KPIs(such as TCHassignment failure rate, handover success rate) are all good.
Problem analysis: After analyzing the cell’s signaling, we find thereusually are Channel Request messages appearing in couples in thecell (with the same TA and cause). The Imm Assignmentcorresponding to the first Channel Request was successful, but theone corresponding to the second Channel Request failed.
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Improper setting of Tx-Integer
Problem analysis: Tx-Integer=12, which means “channel request”
retransmission interval is 109~128
FN of the first Channel Request is 964; that of the second
Channel Request is 1086; there is a difference of 124 frames.
It’s confirmed that the two Channel Requests are sent by the
same MS.
Solution: change Tx-Integer to be 14. After the
adjustment, the SDCCH assignment failure rate drops
to below 10%.
Access of interference signal Case 1: TA
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Access of interference signal—Case 1: TA
exceeding the actual coverage range
Problem description: the SDCCH assignmentsuccess rate in a cell is very poor.
Time Alias
11644(Number of
SDCCH Assignment
Success)
11645(Number of
SDCCH Assignment
Failure)
2007-4-26 19:15 Cell A 191 15
2007-4-26 19:30 Cell A 190 24
2007-4-26 19:45 Cell A 177 33
2007-4-26 20:00 Cell A 192 26
Access of noisy signal—Case 1: TA exceeding the
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Access of noisy signal—Case 1: TA exceeding the
actual range
Problem analysis: analyze ABIS signaling; the TA of
failed random access Immediate Assign failure is asfollows; the neighboring sites are near each other ,
with a distance less than 1 Km.
Serial No. TA CauseCorresponding time stamp for sending Immediate
Assign
1 35 MOC 06-08-55.3752 36 MTC 06-08-55.562
3 35 MOC 06-08-55.984
4 34 MTC 06-08-56.578
5 32 MOC 06-09-11.640
6 30 MTC 06-09-24.546
7 27 MTC 06-09-38.031
8 27 MTC 06-09-38.578
9 27 MTC 06-09-39.10910 0 MOC 06-09-57.171
11 24 MOC 06-09-57.828
12 10 MOC 06-11-15.406
13 2 MOC 06-12-12.781
14 0 MOC 06-12-52.671
15 0 MOC 06-12-53.218
16 1 LAC update 06-15-13.140
Access of noisy signal Case 2: the Rxlev
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Access of noisy signal—Case 2: the Rxlev
lower than BTS sensitivity
Problem description: A cell’s SDCCH assignmentfailure rate keeps high, but the TCH assignment
rate is acceptable.
SDCCH
assign
successfulnumber
SDCCH
assign
failurenumber
SDCCH
assign
failure rate
TCH
Assignment
SuccessNumber
TCH assign
failure
number
TCH assign
failure rate
14479 4490 23.63 4678 122 2.54
Access of nois signal Case 2 the R le
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Access of noisy signal—Case 2: the Rxlev
lower than BTS sensitivity
Problem analysis: The Physical Context carried by
Channel Required message reports the Rxlev of random
accesses, in which we find lots of Channel Request
messages whose Rxlev is -135dbm(0x87).
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Co-BCCH & co-BSIC— Overshooting
Problem description: the SDCCH assignment failure rate
in many cells exceeds 25%. Process procedure:
After all the hardware is changed, the problem still exists.
Through signaling trace we find that the co- BCCH/co-BSIC
signals received when TA=20 lead to SDCCH assignment failure. Based on the above finding, re-plan the BSIC of more than 10
cells in the network. After the re-planning, coverage of the cells
returns to normal.
Solution:
Temporary solution: the CMM of cells with high reset failure rateenables the clock to reset, which lead to synchronous malposition
of SDCCH timeslot.
Ultimate solution: to avoid co-channel/co-BSIC.
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Co-Channel & co-BSIC—Handover
Problem description: a cell experiences a suddenincrease of SDCCH assignment failure rate in
busy hour; the TCH assignment indicators are
good.
Cell ID PmdatatimeSDCCH assign
failure rate
TCH assign
faliure rate
Cell A 19:00-20:00 15.85 0.68
Cell A 21:00-22:00 12.78 0.71
Cell A 20:00-21:00 11.27 1.36
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Co-Channel & co-BSIC—Handover
Problem analysis: Through signaling trace, we find that there is a
large number of continuous random accesses; these ChannelRequests have the same RA, TA, and consecutive frame numbers.
Solution: After checking frequency planning, we find there are co-
channel & co-BSIC cells which are located 14km away from the BTS.
After re-planning of frequency, the problem disappears.
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Weak coverage
Problem description: The SDCCH assignment failure rate in a cell reaches
58% in busy hour, and TCH assignment failure rate
56%; handover success rate in only 20%.
Network performance statistics of fore-and-aft daysdisplay that the TCH assignment failure rate, call drop
rate and handover failure rate have remained high.
UserLabel Handover success rate(%)
SDCCH assignfailure rate
TCH assignfailure rate
Cell A 20 58.67 56.19
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Weak coverage
Problem analysis:
DT result shows that the problem cell not only experiences weakcoverage, but also overshooting and co-channel interference.
Signaling trace shows a large number of abnormal accesses of
consecutive Channel Requests with TA =63.
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Consecutive LOC update request
Problem description: some sites at LAC boundaries and suburb
experience sudden increase of SDCCH assignment failure rate, whichmoves in no certain pattern; while other indicators of the cell are quite
normal.
Problem analysis:
The basic measurement data shows that LOC access attempts and
failures count for a large proportion of the SDCCH assignment failures.
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Consecutive LAC update access request
Problem analysis:
Signaling analysis shows that MS continuously starts Channel
Requests with cause of LAC update, which all end in failure.
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