Sdcch Assignment Failure Troubleshooting Guide

Secret ▲

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 by Approved 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.

Secret ▲ File No.

Version No. A

Drafter / Modifier Hongwei Gao & Chun Chen

Update Record Update Date 2007.12.1 9

null

Reason

Major Points Modified null

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

2

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

Secret ▲

Catalog 1 SDCCH Assignment Failure Troubleshooting Guide......................................................................4 1.1 Procedure...............................................................................................................................4 1.2 Causes of Several SDCCH Assignment Failure:..................................................................5 1.2.1 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 can’t 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.......................................................................................................................15 2.2.2 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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This document contains proprietary information of ZTE Corporation and is not to be disclosed or used except in accordance with applicable agreements.

Secret ▲

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 cell’s BCCH Trx. If it’s the case, make a BCCH shift to other Lapd board to avoid the multiplexing. （3） Check whether there’re cells that use the same BSIC and same BCCH; If it’s 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, it’s 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 4 still exists. If it’s the case then check whether TCH assignment failure rate is high This document contains proprietary information of ZTE Corporation and is not to be disclosed or used except in accordance with applicable agreements.

Secret ▲ 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 it’s the case then the coverage area is blind spot. （10） When SDCCH assignment failure increases suddenly but not periodic, confirm whether it’s accompanied by TCH assignment failure. If it’s 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 1.2.1

Causes of Several SDCCH Assignment Failure: 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)

5

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

Secret ▲ 3, 8, 14,50 4, 9, 16 5,10,20 6,11,25 7,12,32

55 76 109 163 217

41 52 58 86 115

TxInteger value

TS number represents 0 3 1 4 2 5 3 6 4 7 5 8 6 9 7 10 8 11 9 12 10 14 11 16 12 20 13 25 14 32 15 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 combined with SDCCH)

12

Interval(ms) (CCCH not combined with SDCCH) 501ms~593ms,109~129 slot

13

750ms~865ms,163~188 slot

396ms~511ms,86~111slot

14

998ms~1146ms,217~249 slot

529ms~676ms,115~147 slot

15

253ms~483ms,55~105 slot

189ms~419ms,41~91 slot

267ms~359ms,58~78 slot

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 Channel Activation

uplink- 60ms 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 6 This document contains proprietary information of ZTE Corporation and is not to be disclosed or used except in accordance with applicable agreements.

Secret ▲ 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, it’s 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 it’s 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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Secret ▲

MS

BTS

BSC

Channel Request Channel Required Channel Active TxInteger Channel Request(Re-Send） MS change to SDCCH

Imm Assign(OK)

Imm Assign(Fail)

1.2.3

Channel Active Ack

Lapd Delay

Channel Required Imm Assign Cmd Channel Active Channel Active Ack Imm Assign Cmd

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 sixbit 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

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

Secret ▲ 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 can’t 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, 9

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

Secret ▲ 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 it’s 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 can’t 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 10

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

Secret ▲ 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 cell’s SD assignment failure rate is high, and the bad assignment isn’t caused by cell’s 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%, that’s 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

March

Objec

Signaling

Total call

Total

SDCCH

SDCCH

SD

t

channel

attempts in

overflow

assignment

assignment

assignment

congestio

signaling

times of

success

failure

success

n rate (%)

channel

signaling

Number

Number

rate

Bsc3

0.04

171155

channel 69

96228

74855

56.25%

Bsc3

0.04

173784

72

97894

75796

56.36%

Bsc3

0.05

158272

87

86770

71373

54.87%

Bsc3

0

105145

5

100085

5045

95.20%

Bsc3

0

101817

0

98763

3045

97.01%

th

10 March th

11 March th

13 March th

14 March 15

th

2.1.2

Satellite transmission delay 11 1. Problem description This document contains proprietary information of ZTE Corporation and is not to be disclosed or used except in accordance with applicable agreements.

Secret ▲ 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

SDCCH ASS Failure Rate

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

49.6

2_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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Secret ▲

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 doesn’t 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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Secret ▲ The interval between these two Channel Require messages received by BSC is 1.9060.875=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 14

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Secret ▲ 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 same BCCH and the same BSIC

2.2.1

Case 1 1. Problem description: In a place the high SDCCH assignment failure rate problem hasn’t 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 can’t 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 15

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

Secret ▲ 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 cell’s 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)

2.2.2

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

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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Secret ▲

We figure out that there is a cell 20Km away using same frequency and BSIC after checking the cell’s surrounding frequencies and BSIC. Analysis conclusion: Indian Spice doesn’t 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 cell’s 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 cell’s SDCCH assignment success rate is low. The following table shows the cell’s basic data measured on April 26th busy hour. Time

Alias

11644(Number of SDCCH Assignment Successful)

11645(Number of SDCCH Assignment 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

17

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Secret ▲ 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 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

TA 6 7 7 6 5 5 0 4 1 63 61 59 5 53 49 46 43 41 40 18 38 36 5 5 34 5 35 36 35 34 32 30 27 27 27 0 24 10 2 0

Reason location update location update location update MTC location update MTC MTC MTC MTC MOC MOC MOC location update MOC location update MOC MTC MOC MTC Call reset MOC location update MTC MTC MOC MTC MOC MTC MOC MTC MOC MTC MTC MTC MTC MOC MOC MOC MOC MOC

Time when immediate assignment being sent 05-49-52.640 　 　 　 　 05-56-43.343 05-57-24.531 06-00-02.109 06-00-02.890 06-05-56.828 06-06-12.656 06-06-23.578 06-06-43.078 06-06-57.718 06-07-22.578 06-07-42.718 06-08-08.515 06-08-14.765 06-08-20.578 06-08-22.203 06-08-27.546 06-08-45.062 06-08-52.140 06-08-52.625 06-08-52.796 06-08-53.281 06-08-55.375 06-08-55.562 06-08-55.984 06-08-56.578 06-09-11.640 06-09-24.546 06-09-38.031 06-09-38.578 06-09-39.109 06-09-57.171 06-09-57.828 06-11-15.406 06-12-12.781 06-12-52.671

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Secret ▲ 41 42

0 1

MOC location update

06-12-53.218 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 cell’s SDCCH assignment failure rate keeps high but TCH assignment rate is ok.

查询时间：2007-12-23 统计时段：0:00 ~ UserLabel Object Cell and identifier Location Area Cell(LACCI) SUNKADAKATT E -PIPLINE-3

Bsc107Site35Bts3

LAC107CI17353

24:00 SDCCH assign successf ul number

SDCCH assign failure numbe r

SDCCH assign failure rate

14479

4490

23.63

TCH Assign ment Succes s Numbe r 4678

TCH assign failure numbe r

TCH assign failure rate

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 cell’s 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.

19

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Secret ▲

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 cell’s 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.

20

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Secret ▲

2.4.2

Case 2 A cell’s SDCCH assignment failure rate soars on busy hour, but TCH assignment failure rate is low, network performance parameters are in the following table: UserLabel

GOLLARAHTTI-XCEL-2 GOLLARAHTTI-XCEL-2 GOLLARAHTTI-XCEL-2

Object identifier

Bsc107-Site64Bts2 Bsc107-Site64Bts2 Bsc107-Site64Bts2

Cell and Location Area Cell(LAC-CI)

LAC116CI17642 LAC116CI17642 LAC116CI17642

Pmdatati me

19:0020:00 21:0022:00 20:0021:00

SDCCH assign failure rate 15.85

TCH assig n failur e rate 0.68

12.78

0.71

11.27

1.36

After tracking the site’s 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.

21

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

Secret ▲

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 isn’t solved after resetting TRX and the site. Problem analysis: we can see from the cell’s 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 cell’s uplink signal has interference or the coverage is unfavorable.

22

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

Secret ▲

2.5.2

Case 2 Problem description: a cell’s 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.

UserLabe l

GAYATRIP URAMMYS-3

Object identifier

Bsc23Site19Bts3

Handove r success rate (%)

SDCCH assignme nt success number

SDCCH assignme nt failure number

SDCCH assignme nt failure rate

TCH assignme nt success number

TCH assignme nt failure number

20

1915

3001

58.67

1033

1325

23

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

TCH assignme nt failure rate C11658*1 00/C1161 1 56.19

Secret ▲

Problem analysis: after checking the cell’s 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 cell’s own coverage is bad, but the cell has overlapping problem and co-channel interference.

2.5.3

Case 3 A cell’s SDCCH assignment failure rate reaches as high as 20% on busy hour.

UserLabel

Object identifier

Cell and Location Area Cell(LAC-CI)

Pmdatatime

NANJANGUD-1

Bsc36-Site19-Bts1

LAC1036-CI36191

NANJANGUD-1

Bsc36-Site19-Bts1

LAC1036-CI36191

NANJANGUD-1

Bsc36-Site19-Bts1

LAC1036-CI36191

NANJANGUD-1

Bsc36-Site19-Bts1

LAC1036-CI36191

2007-12-17 20:00-21:00 2007-12-17 21:00-22:00 2007-12-18 20:00-21:00 2007-12-18 21:00-22:00

SDCCH assign failure rate

TCH Assignment Success Rate

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 cell’s has downlink interference. We can solve it through re-planning frequencies, or using downlink power control of surrounding cells. 24

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

Secret ▲

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 cell’s 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.

25

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

Secret ▲

2.7

SDCCH assignment failure caused by improper-set Tx-Integer Problem description: a cell’s 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 13-Dec-07

BSC_NAME JAYANAGAR-BSC

BSCID 102

CELL_ID 12282

SITE_NAME THAYAGRAJNAGAR-2-s

MYHOUR 21

SD_ASSN_FAIL_RATE 30.21

Problem analysis: after tracking the cell’s 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.

26

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Secret ▲

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 cell’s SDCCH assignment failure rate decreases lower than 10% after changing TxInteger into 14.

27

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