Sdcch Assignment Failure

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Technical Document

File Name High SDCCH Assignment Failure Analysis &Troubleshooting Guide

File No.

Version : A

Totally 27 pages(Including the cover)

Drafted by Hongwei Gao & Chun Chen

Checked by

Consigned by

Standardized byApproved by

ZTE CORPORATION

This document contains proprietary information of ZTE Corporation and is not to be disclosed or used except in accordance with applicable agreements.


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Update RecordFile No. Version

No.Drafter /Modifier

UpdateDate

Reason Major Points Modified

A HongweiGao &

ChunChen

2007.12.19

null null

remark The first time the file is edited, Reason and Major Points Modified shall be vacant.


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Catalog

1 SDCCH Assignment Failure Troubleshooting Guide ...................................................................... 4

1.1 Procedure ............................................................................................................................... 4

1.2 Causes of Several SDCCH Assignment Failure: .................................................................. 51.2.1 Inappropriate Tx-Integer Setting Parameter: ............................................................. 5

1.2.2 Lapd delay leads to high SDCCH assignment failure rate ........................................ 7

1.2.3 Co-BCCH & Co-BSIC interference ................................................................. ......... 8

1.2.4 Over coverage ............................................................................................. ............... 9

1.2.5 Uplink Noise Interference ........................................................................................ 10

1.2.6 MS frequent location update due to poor downlink quality .................................... 10

Because receiving sensitivity of BTS is high than that of MS, BTS can receive Channel Request

sent by MS, and MS cant receive Imm Assign sent by BTS.Especially when MS is put in pockets

or under the pillows. If MS needs to do location update, Channel Request may be frequent

launched which will result in SDCCH assignment failure because Imm Assign message fails to

reach. .................................................................................................................................................. 10

2 Cases of high SDCCH assignment failure rate .............................................................................. 10

2.1 LAPD delay caused by too many paging .............................................................. ............. 10

2.1.1 Too much paging causes LAPD delay ..................................................................... 10

2.1.2 Satellite transmission delay ...................................................................................... 11

2.1.3 Transmission equipment failure causes LAPD delay .............................................. 14

2.2 High SDCCH assignment failure rate caused by other signals with the same BCCH and

the same BSIC .......................................................................................................................... 15

2.2.1 Case 1 ....................................................................................................................... 152.2.2 Case 2 ....................................................................................................................... 16

2.3 Noise signal access ............................................................................................................. 17

2.3.1 Noise signal is represented as TA oversteps practical coverage ............................. 17

2.3.2 Noise signal is represented as receiving level is lower than BTS receiving

sensitivity ................................................................................................................. ........ 19

2.4 High SDCCH assignment failure rate because frequency of target channel to hand over is

the same with that of BCCH of the problem cell, and target cell has same BSIC with that of

problem cell ............................................................................................................................... 20

2.4.1 Case 1 ....................................................................................................................... 20

2.4.2 Case 2 ....................................................................................................................... 21

2.5 SDCCH assignment failure caused by bad network coverage .......................................... . 22

2.5.1 Case 1 ....................................................................................................................... 22

2.5.2 Case 2 ....................................................................................................................... 23

2.5.3 Case 3 ....................................................................................................................... 24

2.6 SDCCH assignment failure caused by continuous access request of location update .... .. 25

2.7 SDCCH assignment failure caused by improper-set Tx-Integer ...................................... . 26


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1 SDCCH Assignment Failure Troubleshooting

Guide

1.1 Procedure

Upon receiving the complaint of high SDCCH assignment failure, extract performance report

from OMC-R Client, filter out trouble cells based on SDCCH assignment failure rate.

1 Adjust TX_INTEGER to 14 for the bad cells.

2 Check whether BCCH Trx of the bad cell share the same Lapd with that of other

cells BCCH Trx. If its the case, make a BCCH shift to other Lapd board to avoid

the multiplexing.

3 Check whether therere cells that use the same BSIC and same BCCH; If its the

case, modify it.

4 Observe one day. If the problem still exists, take the next step.

5 Collect measurement report of the bad cells and get to know the time span of high

SDCCH assignment failure.

6 Analyze basic measurement report. If counter 11687(Number of other access request

attempts ) is high while counter 11639(Number of other successful access requests)

is zero, its probable that TCH handover request of other cell is mistaken for access

request of serving cell. If Mplog file contains the error message Channel

Number=Ox88 then it indicates the existence of the same BCCH handover

interference, check whether there is the cell with the same BCCH and BSIC.

7 Analyze basic measurement report, check whether counter 11643(Number of sdcch

allocation failures including handover) has value, if so then we may suspect for

transmission problems . Check whether transmission alarm exists or whether there is

lapd error printing by analyzing mplog.

8 Analyze basic measurement report to see whether high SDCCH assignment failure

still exists. If its the case then check whether TCH assignment failure rate is highThis document contains proprietary information of ZTE Corporation and is not to be disclosed or used except in accordance with applicable agreements.

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too. Generally TCH assignment failure rate is lower than SDCCH assignment failure

rate. If TCH assignment failure rate is high, it suggests that the cell is having some

interference or poor coverage. So reduce BCCH interference and improve the

coverage. If TCH assignment failure rate is good, observe signaling tracing of the

cell for 10 minutes, convert it to be MA10 format and filter Channel Request

messages. If there are many Immediate Assign failure of Channel Request with high

TA, it suggests there is overshooting or false signaling access (big difference

between TA of channel required and actual coverage range). For overlapping, adjust

BCCH transmission power or coverage range of the cell; For false signaling access,

adjust TA Allowed to filter false access.

9 If SDCCH assignment failure increases suddenly, check whether it is periodic (High

SDCCH in fixed time of each day), if its the case then the coverage area is blind

spot.

10 When SDCCH assignment failure increases suddenly but not periodic, confirm

whether its accompanied by TCH assignment failure. If its the case then the cell

may have sudden strong interference.

11 If SDCCH assignment failure is suddenly increased instead of periodic and without

accompanied with sudden high TCH assignment failure, check counter 11686 (LOC

access attempt times), counter 11638(LOC access success times) and counter

11645(SDCCH assignment failure times). If the difference between counter 11686

and counter 11638 is close to counter 11645 then most probably failures are caused

by disconnection due to Mobile Station uplink poor quality, which is common in

GSM network..1.2 Causes of Several SDCCH Assignment Failure:

1.2.1 Inappropriate Tx-Integer Setting Parameter:

The interval (TS number) between the two channel request messages is a random number in

{S,S+1,,S+T-1}. In which, T is the TS number which represents by TxInteger, S value are

as follows

TS number TxInteger represents

S(CCCH not combines with

SDCCH)

S(CCCH combines with

SDCCH)


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3, 8, 14,50 55 414, 9, 16 76 525,10,20 109 586,11,25 163 867,12,32 217 115

TxInteger value TS number represents

0 31 42 53 64 75 86 97 108 11

9 1210 1411 1612 2013 2514 3215 50

From the above two grids, we get the relation between TxInteger with the interval between

request message of the two channels

TxInteger Interval(ms)(CCCH not combined with

SDCCH)

Interval(ms)(CCCH combined with SDCCH)

12 501ms~593ms,109~129 slot 267ms~359ms,58~78 slot

13 750ms~865ms,163~188 slot 396ms~511ms,86~111slot



Generally , Tx-Integer is 14 by default. When transmission link delay is large, TxInteger is

small, it may cause many MS access requests.

Abis one-way signaling transmission delay is 60ms~100ms. For your notice,delay of one

immediate assignment without the UM delay as follows

Channel Required uplink- 60ms

Channel Activation downlink- 60ms

Channel Activation Ack uplink- 60ms

Immediate Assign downlink- 60ms

Time delay between MS sending Channel Request and receiving Imm Assign shall be around


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240ms.

But if transmission link delay is large and TxInteger is inappropriately set, for example if Tx

integer is set to 15 it corresponds that the interval between two-channel request is about

300ms. Ie. MS resends Channel Request before receiving Imm Assign. At the same time

when MS sents the second attempt for channel request it receives Channel Request Imm

Assign of the first attempt and complete access procedure, during this process the Channel

Request of the second attempt fails.

1.2.2 Lapd delay leads to high SDCCH assignment failure rate

There are several causes leading to lapd delay,

1 If Lapd1*4 multiplex is adopted, its probable that multiple BCCH share the same

lapd which will lead to great lapd throughput and finally time delay.

2 Large lapd flow results in delay. For example, inappropriate LAC allocation will lead

to great paging number which will result in lapd overflow.

3 Transmission equipment has great delay. For example, ABIS adopts satellite

transmission.

4 PS effect. PS is sensitive to network delay. If lapd has delay, PS message will be

resend which will add to the throughput and finally results in lapd delay and so on

its vicious circle.

If lapd delay reaches certain degree which leads to MS resending Channel Request, SDCCH

assignments may fail. Please refer to the following diagram


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1.2.3 Co-BCCH & Co-BSIC interference

There are two situations, which lead to co-BCCH & co-BSIC interference:

1 Two cells sharing BCCH & BSIC: Channel Request sent by MS is received by two

cells simultaneously and is assigned by SDCCH. MS can only provide access of one

SDCCH thus SDCCH assignment of another cell will fail.

Firstly six-bit color codes are added to the original eight information bits. These six-

bit color codes are generated by (BSIC and six-bit parity codes) mode 2.

Co-BCCH and co-BSIC may make BTS decode it as MS initial access of other site,

which may result in SDCCH assignment failure. Due to limited information of

random access signaling (8bit and BSIC), two cells of same bcch and bsic may

cause false access. Therefore we should avoid co-BCCH and co-BSIC.

2 When one cell uses the BCCH same as other cells TCH/BCCH and CO-BSIC,during the time of handover, handover access message happened in the TCH TS

will be decoded by both the targeted cell as well as other cell that uses the same

BCCH/BSIC and so the cell which is not the original target cell will also decode as

Channel Request message. Hence the case will result in great SDCCH assignment

failure.

When MS initiate Channel Request, RA is random and not consistent. MS access

request resend interval is a false random number, therefore FN of MS channelThis document contains proprietary information of ZTE Corporation and is not to be disclosed or used except in accordance with applicable agreements.

Channel Request

Channel Required

MS

Channel Active

Channel Active Ack

Imm Assign(OK)Imm AssignCmd

BTS

Channel Request(Re-Send

TxInt eger LapdDelay

Channel Required

Channel ActiveChannel Active Ack

Imm AssignCmdImm Assign(Fail)

MSchange toSDCCH

BSC

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requests are not continuous. According to the protocol, if handover access message

sent by MS is the same with random access request message in format(AB frame)

but MS handover access message content of one time is consistent and FN is

continuous. If BCCH of one cell shares the same frequency with TCH of another

cell, handover access on TCH can be considered as random access request, which

will lead to SDCCH assignment failure. For checking these type of problem we can

analyze the signal trace, when we find RA is the same, TA is same while FN is

continuous. Thus we can conclude that great number of Channel Request is false

access caused by handover access of co-BCCH cell.

Thus if the two cell share the use the same BCCH and is close, downlink interference may

occur which will also lead to SDCCH assignment failure.

1.2.4 Over coverage

There are two situation causing over coverage,

1 Coverage area is large leading to poor downlink reception quality of cell edge.

Because sensitivity of BTS is high than that of MS, BTS can receive Channel

Request sent by MS and MS cant receive Imm Assign sent by BTS.

2 Cell coverage is large which results in co-BCCH and co-BSIC with the cell far away.

To solve the problem, antenna-engineering parameter needs to be adjusted for coverage

control purpose. TA_allowed can also be used to solve the problem but it may cause MS to

fail to get access to the network. Therefore, threshold of TA_allowed shall be greater than

actual coverage range of the cell. Repeater influence shall be taken into account when

calculating cell coverage range.

We do adjust TA_Allowed parameter in order to avoid reselection failure due to TA

TA_Allowed. We do experiment concerning this problem and the result is when MS selects a

cell with strongest power . If TA_allow area update is unsuccessful, MS will select another

cell with less strong power(C1>0)access of this cell is allowed. Interval of cell reselection is

decided by TX_Integer and maximum resend time and is generally several seconds.

Computing method is as follows,

Cell reselection interval = TX_Integer * MaxRetrans +T3126

Judging from the result, we can see that cell that fails to reselect will be punished, as a result,


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reselection will not fail due to TA_Allowed.

Remark: Other manufacturers have similar parameters like TAallowed. For example, Nortel

has RNDACCTIMADVTHRESHOLD and it is described as follows,avoid SDCCH

assignment by making the parameter link to actual coverage area range and setting proper

threshold to filter false RACH request. It shows that for cells with small radius coverage,

RNDACCTIMADVTHRESHOLD is set to be 35Km and RACH misjudgement ,system

demodulate the noise to be RACH burst by mistake is almost 30% of all RACH requests.

When rndAccTimAdvThreshold is changed to be 2, RACH misjudgement seldom occurs.

1.2.5 Uplink Noise Interference

BTS receiving sensitivity is generally between -112dbm and -125dbm and random access

signaling with less sensitivity has noise interference, which will definitely lead to SDCCH

assignment failure.

RACHMin(-dbm) is a parameter set to filter noise signaling. Random access signaling with

receiving level lower than RACHMin will be discarded as noise interference. By adjusting

RACHMin, SDCCH assignment success rate can be greatly increased.

False noise access signaling reflects in two aspects: 1- receiving level is week, 2- TA is

greater than actual coverage range. Hence RACHMin integrated with TA_allowed can

reduce the influence of noise interference to a larger extent.

Notice: RachMin shall be carefully set. If its too high, it may affect paging success rate.

1.2.6 MS frequent location update due to poor downlink quality

Because receiving sensitivity of BTS is high than that of MS, BTS can receive Channel

Request sent by MS, and MS cant receive Imm Assign sent by BTS.Especially when MS is

put in pockets or under the pillows. If MS needs to do location update, Channel Request may

be frequent launched which will result in SDCCH assignment failure because Imm Assign

message fails to reach.

2 Cases of high SDCCH assignment failure rate

2.1 LAPD delay caused by too many paging

2.1.1 Too much paging causes LAPD delay

1. Problem description


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We discover that ZTE BSC3 SD assignment success rate is relatively low in a certain place

while analyzing its performance parameters and the rate is especially low on late busy hour,

only about 60%.

2. Analysis

1 After checking statistic we find out that almost each cells SD assignment failure

rate is high, and the bad assignment isnt caused by cells radio parameters;

2 We can see that SD channel has no congestion generally from statistic, the

congestion rate is only 0.02%;

3 ZTE BSC1/2/4 SD assignment success rates are all above 95%, thats a normal

figure. Only BSC3 is abnormal. Because BSC3 is separately controlled by MSC7,

after contacting China Unicom we find out that SD assignment success rates of all

BSC controlled by MSC7 (including Siemens BSC) are about 60%, and MSC7

paging success rate is also low. We get to know from China Unicom that there is

only one LAC controlled by MSC7. We have to page all cells in LAC while paging,

then more traffic, more paging.

3. Adjustment and effect

Contact Siemens and ask them to add one more LAC to MSC7 and update LAC SN of

cells controlled by part of BSC (Siemens). After the adjustment, BSC3 SD assignment

success rate comes back to normal and reaches above 95%.

Time Objec

t

Signaling

channel

congestio

n rate (%)

Total call

attempts in

signaling

channel

Total

overflow

times of

signaling

channel

SDCCH

assignment

success

Number

SDCCH

assignment

failure

Number

SD

assignment

success

rate

March

10 th

Bsc3 0.04 171155 69 96228 74855 56.25%

March

11 th

Bsc3 0.04 173784 72 97894 75796 56.36%

March

13 th

Bsc3 0.05 158272 87 86770 71373 54.87%

March

14 th

Bsc3 0 105145 5 100085 5045 95.20%

March

15 th

Bsc3 0 101817 0 98763 3045 97.01%

2.1.2 Satellite transmission delay

1. Problem descriptionThis document contains proprietary information of ZTE Corporation and is not to be disclosed or used except in accordance with applicable agreements.

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There are total 4 sites, TBT-G, TBT-D, GWD-G and GJR-G all of which are controlled by

BSC01 but by different periphery modules. From the following performance parameter

form we can see that these sites SD assignment failure rates are as high as 50% or even

higher.

Object CI LAC FreqBand CellName SDCCHASS FailureRate

2_1_92_1 10921 2008 E-GSM900 GWD1 50.31

2_1_92_2 10922 2008 E-GSM900 GWD2 50.64

2_1_92_3 10923 2008 E-GSM900 GWD3 50.3

2_1_252_1 12521 2008 E-GSM900 TBT1 55.74

2_1_252_2 12522 2008 E-GSM900 TBT2 50.2

2_1_252_3 12523 2008 E-GSM900 TBT3 63.57

2_1_326_1 13261 2008 E-GSM900 GAR1 49.62_1_326_2 13262 2008 E-GSM900 GAR2 50.26

2_1_326_3 13263 2008 E-GSM900 GAR3 50.54

2_1_999_1 12524 2008 DCS1800 TBT4 50.24

2_1_999_2 12525 2008 DCS1800 TBT5 49.38

2_1_999_3 12526 2008 DCS1800 TBT6 49.01

2. Analysis

We record signaling on Abis interface of TBT1, 4, 5, 6, GAR and GWD. Set TBT5 signaling

as an example, the analysis is as follows:

1 We can see that the average time required to successfully activate one channel

is 0.58s.

2 We figure out from the following signaling whether the two signaling is

Channel Required sent by the same mobile phone


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We can calculate frame numbers of the two channels by T1, T2 and T3. The

formula is FN=T1*26*51+((T3-T2)mod 26)*51+T3

The frame number difference of the two channels is 32454-32227=227(frames)

Tracking the whole process of the first channel request we can see it is a

complete signaling process to turn off the mobile phone. Tracking the whole

process of the second channel request we can see immediate assignment failure,

BSC doesnt receive Establish Indication message, and T3101 is overtime and

then channel is released.

The two signaling has the same Access delay: 3. Meanwhile the max retransmission

times that system configured is 4, TX Integer=14(T=32,S=217). Then the interval for

any mobile phone to send two Channel Require messages in one call is a random one

from 217 to 248 timeslot, which means the shortest interval between two requests sent

by mobile phone is 1001ms, and the longest interval is 1144ms.


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The interval between these two Channel Require messages received by BSC is 1.906-

0.875=1.031s. We suppose that uplink and downlink signaling transmission delays

between BTS and BSC are the same, then signaling length of the whole immediate

assignment process is 0.58*2=1.16s, which is close to 1.031s.

According to upper calculation of frame number, the real interval between these two

messages is 227 frames (1048), and then we deduce that the two Channel Require

messages are sent by one mobile phone in one call attempt.

3. Conclusion

We have to use satellite to transmit because these sites are far away from the urban area. The

satellite transmission delay on one direction is around 260ms, and then transmission delay

of 4 signaling is 1040ms, which accords with the upper signaling analysis.

2.1.3 Transmission equipment failure causes LAPD delay

1. Problem description: massive SDCCH assignment failures occur in 3 cells of a site in

India accompanied with lots of TCH assignment failures. Basic measure data are shown

in the following figure.

2. Problem analysis: commonly SDCCH assignment failure represents transmission

failure. After checking mpLog print we find out that the site has lots of LAPD Errors

get printed: Site 36, BTS 3, TRX 2 catch errorF: receive SABME in Mulframe or


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TimerRec, Site 36, BTS 1, TRX 0 catch errorF: receive SABME in Mulframe or

TimerRec. Then we check alarm, there are lots of transmission alarm:

Therefore we can confirm that there is transmission problem that causes lots of LAPD

breaks.

2.2 High SDCCH assignment failure rate caused by other signals with the sameBCCH and the same BSIC

2.2.1 Case 1

1. Problem description: In a place the high SDCCH assignment failure rate problem hasnt

been solved for a long time, and SDCCH assignment failure rates of lots of cells in the

whole network are higher than 25%.

2. Solution: The problem cant be solved by replacing all the hardware. When TA=20, a

signal from another cell with the same frequency and the same BSIC is received that

causes SDCCH assignment failure. Therefore we re-plan over 10 cells BSIC in the

whole network. After re-planning, parameters of all cells whose BSIC has been

modified come back to normal.

3. Conclusion of fault analysis: Within 5km, if a mobile phone locates in an area covered

by two cells with the same BCCH and the same BSIC, then SDCCH assignment failure

may occur. The failure is triggered by these two cells with the same BCCH and the


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same BSIC, two cells SDCCH timeslots are synchronous. After mobile phone

synchronizes with BTS and chooses to be accessed into one of these two cells, it will be

interference to another cell.

4. So we have two solutions for high SDCCH assignment failure problem (within a certain

multiplexing distance, high SDCCH assignment failure rate caused by other signals

with the same BCCH and the same BSIC)

1) Reset cells CMM whose assignment failure rate is high, reset clock to avoid

SDCCH timeslot synchronization, so as to decrease the impact. This is just a

temporary solution. In field situation we shall modify parameters and then change

them back, it is the reason to reset CMM.

2) The basic solution is to avoid signal with same frequency and same BSIC.

2.2.2 Case 2

1. Problem description: SDCCH assignment failure rate keeps high in a cell in Indian

spice network, but its TCH assignment rate is normal. Basic measure data are shown in

the following figure.

2. Problem Analysis: after tracking signaling of troubled cell, we find out that SDCCH

assignment failure is caused by lots of random signal access whose TA>37.


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We figure out that there is a cell 20Km away using same frequency and BSIC after

checking the cells surrounding frequencies and BSIC.

Analysis conclusion: Indian Spice doesnt have much frequency resource and its sites

distribute densely, therefore it is inevitable that cells use same frequency and BSIC

within 34km. We have to change antenna down tilt angle or modify TA_Allowed to

shrink cells coverage, so as to solve SDCCH assignment failure caused by cells use

same frequency and BSIC.

2.3 Noise signal access

2.3.1 Noise signal is represented as TA oversteps practical coverage

1. Problem description: a cells SDCCH assignment success rate is low. The following

table shows the cells basic data measured on April 26 th busy hour.

Time Alias 11644(Number of SDCCHAssignmentSuccessful)

11645(Number of SDCCHAssignment

Failure)

2007-4-26 19:15 ASHOKA PILLAR-1 191 15

2007-4-26 19:30 ASHOKA PILLAR-1 190 24

2007-4-26 19:45 ASHOKA PILLAR-1 177 33

2007-4-26 20:00 ASHOKA PILLAR-1 192 26


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2. Problem analysis: after analyzing signaling on Abis interface, the summary of

immediate assignment failure signal accesses into TA (in channel required) is as

follows:

SN TA Reason Time when immediateassignment beingsent

1 6 location update 05-49-52.6402 7 location update 3 7 location update 4 6 MTC 5 5 location update 6 5 MTC 05-56-43.3437 0 MTC 05-57-24.5318 4 MTC 06-00-02.109

9 1 MTC 06-00-02.89010 63 MOC 06-05-56.82811 61 MOC 06-06-12.65612 59 MOC 06-06-23.57813 5 location update 06-06-43.07814 53 MOC 06-06-57.71815 49 location update 06-07-22.57816 46 MOC 06-07-42.71817 43 MTC 06-08-08.51518 41 MOC 06-08-14.765

19 40 MTC 06-08-20.57820 18 Call reset 06-08-22.20321 38 MOC 06-08-27.54622 36 location update 06-08-45.06223 5 MTC 06-08-52.14024 5 MTC 06-08-52.62525 34 MOC 06-08-52.79626 5 MTC 06-08-53.28127 35 MOC 06-08-55.37528 36 MTC 06-08-55.56229 35 MOC 06-08-55.98430 34 MTC 06-08-56.57831 32 MOC 06-09-11.64032 30 MTC 06-09-24.54633 27 MTC 06-09-38.03134 27 MTC 06-09-38.57835 27 MTC 06-09-39.10936 0 MOC 06-09-57.17137 24 MOC 06-09-57.82838 10 MOC 06-11-15.40639 2 MOC 06-12-12.781

40 0 MOC 06-12-52.671


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41 0 MOC 06-12-53.21842 1 location update 06-15-13.140

The surrounding sites of the site distribute densely with distances lesser than 1km. TA

values are unreal on the table, we suppose there are fake signals. Further more,

assignment failure in half an hour almost focuses on 5 minutes. We shall use

TA_Allowed parameter to filter fake signals.

2.3.2 Noise signal is represented as receiving level is lower than BTS receiving sensitivity

1. Problem description: a cells SDCCH assignment failure rate keeps high but TCH

assignment rate is ok.

2007-12-23 0:00 ~ 24:00

UserLabel Objectidentifier

Cell andLocation

AreaCell(LAC-

CI)

SDCCHassign

successf ul

number

SDCCHassignfailurenumbe

r

SDCCHassignfailure

rate

TCHAssignment

Success

Number

TCHassignfailurenumbe

r

TCHassignfailure

rate

SUNKADAKATTE-PIPLINE-3

Bsc107-Site35-Bts3

LAC107-CI17353

14479 4490 23.63 4678 122 2.54

2. Problem analysis: the cell uses EDGE TRX; in this version, random accessed receiving

level can be reported in physical context by Channel Request. Through observing the

cells signaling tracking data, we find out there are lots of Channel Request messages

whose receiving level is -135dbm 0x87 which cause large numbers of SDCCH

assignment failures.


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We can confirm that most of these Channel Requests are noise signal; we can solve it

through setting RACHMin.

2.4 High SDCCH assignment failure rate because frequency of target channel

to hand over is the same with that of BCCH of the problem cell, and target

cell has same BSIC with that of problem cell

2.4.1 Case 1

Problem description: the following are a cells signaling. We can see that there are

continuous Channel Requests with same RA, TA, and their frame numbers are consecutive.

All of the SDCCH assignments corresponding to these Channel Requests are failed with no

exception. Besides, other access request attempt times in basic measurement are

extremely high. Then we can confirm that all these abundant and consecutive Channel

Requests are fake access caused by incoming handover of co-channel cells.


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2.4.2 Case 2

A cells SDCCH assignment failure rate soars on busy hour, but TCH assignment failure rate

is low, network performance parameters are in the following table:

UserLabel Object identifier Cell andLocation AreaCell(LAC-CI)

Pmdatatime

SDCCHassignfailure

rate

TCHassig

nfailur

erate

GOLLARAHTTI-XCEL-2 Bsc107-Site64-Bts2

LAC116-CI17642

19:00-20:00

15.85 0.68


LAC116-CI17642

21:00-22:00

12.78 0.71


LAC116-CI17642

20:00-21:00

11.27 1.36

After tracking the sites signaling we find out there are lots of consecutive abnormal random

access whose Channel Request RA and TA are the same and frame number are consecutive.


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We find out there is a cell uses same frequency and same BSIC 14km away from the site

after checking frequency planning. The problem is solved by re-planning the frequency.

2.5 SDCCH assignment failure caused by bad network coverage

2.5.1 Case 1

Problem description: a cell has high SDCCH assignment failure rate problem accompanied

with high TCH assignment failure rate. Outgoing handover attempts are frequent and call

drop rate is high. Customer has complaints about it. The problem isnt solved after resetting

TRX and the site.

Problem analysis: we can see from the cells basic measurement report that access reasons

are various for SDCCH assignment failure, both MOC access and MTC access account for a

certain proportion. The number of uplink sampling whose RQ 3 is quite large, uplink

quality is very bad. Therefore we can confirm that the cells uplink signal has interference or

the coverage is unfavorable.


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2.5.2 Case 2

Problem description: a cells SDCCH assignment failure rate reaches as high as 58% on

busy hour, accompanied with 56% TCH assignment failure rate, handover success rate is

only 20%. After tracking signaling we find out there are lots of consecutive Channel

Requests whose TA are 63 get accessed abnormally.

UserLabel

Objectidentifier

Handover successrate (%)

SDCCHassignme

ntsuccessnumber

SDCCHassignment failurenumber

SDCCHassignment failure

rate

TCHassignme

ntsuccessnumber

TCHassignment failurenumber

TCHassignment failure

rateC11658*100/C1161

1GAYATRIPURAM-MYS-3

Bsc23-Site19-Bts3

20 1915 3001 58.67 1033 1325 56.19


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Problem analysis: after checking the cells performance data of these days, we find out that

its TCH assignment failure rate, call drop rate and handover failure rate keep high but

without any alarm. DT engineer discovers that not only the cells own coverage is bad, but

the cell has overlapping problem and co-channel interference.

2.5.3 Case 3

A cells SDCCH assignment failure rate reaches as high as 20% on busy hour.

UserLabel Object identifier Cell and Location Area Cell(LAC-CI)

Pmdatatime SDCCHassignfailure

rate

TCH Assignment

SuccessRate

NANJANGUD-1 Bsc36-Site19-Bts1 LAC1036-CI36191 2007-12-1720:00-21:00

10.8 81.75


21.52 85.32


18.28 82.17


22.24 84.7

TCH assignment failure rate is as high as 20% while SDCCH assignment failure rate rises.

These two parameters are ok when traffic is low, which means the cells has downlink

interference. We can solve it through re-planning frequencies, or using downlink power

control of surrounding cells.


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2.6 SDCCH assignment failure caused by continuous access request of location

update

Problem description: in some boundary sites and suburban sites of city A, their SDCCH

assignment failure rates soar without any rule, but meanwhile cells other parameters are

normal.

The following are signaling and basic measure data recorded when SDCCH assignment

failure rate is high. We can see from signaling that the reason for one MS keeps sending

access attempts is Channel Request due to location update, but all attempts are failed.


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2.7 SDCCH assignment failure caused by improper-set Tx-Integer

Problem description: a cells ordinary SDCCH assignment failure rate keeps around 20%,

hits 30% on busy hour. But other parameters (such as TCH assignment failure rate,

handover success rate) are all normal.

DATETIME BSC_NAME BSCID CELL_ID SITE_NAME MYHOUR SD_ASSN_FAIL_RATE13-Dec-07 JAYANAGAR-BSC 102 12282 THAYAGRAJNAGAR-2-s 21 30.21

Problem analysis: after tracking the cells signaling, we find out there are couples of

Channel Request messages of the cell commonly appearing together (with same TA and for

same reason). The Imm Assign corresponding to the first Channel Request is successful, but

the Imm Assign corresponding to the second Channel Request is failed.


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Set colored messages in the upper figure as an example; the FN of the first Channel Request

message is 964, the FN of the second Channel Request message is 1086, the FN difference

is 124. While Tx-Integer=12. Then we can confirm these two Channel Request messages are

sent by one MS. Because there is a certain delay on transmission link, then MS resends

Channel Request.

The cells SDCCH assignment failure rate decreases lower than 10% after changing Tx-

Integer into 14.

This document contains proprietary information of ZTE Corporation and is not to be disclosed or

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Sdcch Assignment Failure

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