10 gsmp&o b-en-gsm network sdcch assignment analysis-word--201009

GSM Network SDCCH Assignment Analysis

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Contents

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

1.1 Procedure .......................................................................................................................................... 1

1.2 Causes of Several SDCCH Assignment Failure:............................................................................... 2

1.2.1 Inappropriate Tx-Integer Setting Parameter:.......................................................................... 2

1.2.2 Lapd delay leads to high SDCCH assignment failure rate ..................................................... 4

1.2.3 Co-BCCH & Co-BSIC interference ....................................................................................... 5

1.2.4 Over coverage ........................................................................................................................ 6

1.2.5 Uplink Noise Interference ...................................................................................................... 7

1.2.6 MS frequent location update due to poor downlink quality ................................................... 7

2 Cases of high SDCCH assignment failure rate........................................................................................ 9

2.1 LAPD delay caused by too many paging .......................................................................................... 9

2.1.1 Too much paging causes LAPD delay.................................................................................... 9

2.1.2 Satellite transmission delay.................................................................................................. 10

2.1.3 Transmission equipment failure causes LAPD delay........................................................... 12

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

BSIC...................................................................................................................................................... 14

2.2.1 Case 1................................................................................................................................... 14

2.2.2 Case 2................................................................................................................................... 14

2.3 Noise signal access.......................................................................................................................... 16

2.3.1 Noise signal is represented as TA oversteps practical coverage........................................... 16

2.3.2 Noise signal is represented as receiving level is lower than BTS receiving sensitivity ....... 18

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 ....... 19

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2.4.1 Case 1 ...................................................................................................................................19

2.4.2 Case 2 ...................................................................................................................................19

2.5 SDCCH assignment failure caused by bad network coverage.........................................................20

2.5.1 Case 1 ...................................................................................................................................20

2.5.2 Case 2 ...................................................................................................................................21

2.5.3 Case 3 ...................................................................................................................................22

2.6 SDCCH assignment failure caused by continuous access request of location update .....................23

2.7 SDCCH assignment failure caused by improper-set Tx-Integer......................................................24

1

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.

Adjust TX_INTEGER to 14 for the bad cells.

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.

Check whether there’re cells that use the same BSIC and same BCCH; If it’s the case,

modify it.

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

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

SDCCH assignment failure.

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.

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.

Analyze basic measurement report to see whether high SDCCH assignment failure

still exists. If it’s the case then check whether TCH assignment failure rate is high too.

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


2

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.

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.

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.

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)

3, 8, 14,50 55 41

4, 9, 16 76 52

5,10,20 109 58

6,11,25 163 86

Chapter 1 0BSDCCH Assignment Failure Troubleshooting Guide

3

7,12,32 217 115

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 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


4

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 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,

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.

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

to great paging number which will result in lapd overflow.

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

transmission.

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：


B T SMS B S C

Channel Request

Channel Required

Channel Active

Channel Active Ack

mm Assign(OK) Imm Assign Cmd

Channel Request(Re-Send）

TxInteger

Channel Required

Channel Active Channel Active Ack

Imm Assign Cmd

Imm Assign(Fail)

S change to SDCCH

LapdDelay

1.2.3 Co-BCCH & Co-BSIC interference

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

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.

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

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request resend interval is a false random number, therefore FN of MS channel 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,

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.

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, reselection will not fail due to TA_Allowed.


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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.

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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

Problem description

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%.

Analysis

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;

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

rate is only 0.02%;

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.

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 Object

Signaling

channel

congestion

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


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March 10th Bsc3 0.04 171155 69 96228 74855 56.25%

March 11th Bsc3 0.04 173784 72 97894 75796 56.36%

March 13th Bsc3 0.05 158272 87 86770 71373 54.87%

March 14th Bsc3 0 105145 5 100085 5045 95.20%

March 15th Bsc3 0 101817 0 98763 3045 97.01%

2.1.2 Satellite transmission delay

Problem description

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

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:

We can see that the average time required to successfully activate one channel is 0.58s.

Chapter 2 1BCases of high SDCCH assignment failure rate

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

Required sent by the same mobile phone

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.

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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.

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.

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

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.

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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

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

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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

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%.

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.

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

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.

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)

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.

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

2.2.2 Case 2

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.


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.

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

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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

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 immediate

assignment being sent

1 6 location update 05-49-52.640

2 7 location update

3 7 location update

4 6 MTC

5 5 location update

6 5 MTC 05-56-43.343

7 0 MTC 05-57-24.531

8 4 MTC 06-00-02.109

9 1 MTC 06-00-02.890

10 63 MOC 06-05-56.828

11 61 MOC 06-06-12.656


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12 59 MOC 06-06-23.578


14 53 MOC 06-06-57.718


16 46 MOC 06-07-42.718

17 43 MTC 06-08-08.515

18 41 MOC 06-08-14.765

19 40 MTC 06-08-20.578

20 18 Call reset 06-08-22.203

21 38 MOC 06-08-27.546


23 5 MTC 06-08-52.140

24 5 MTC 06-08-52.625

25 34 MOC 06-08-52.796

26 5 MTC 06-08-53.281

27 35 MOC 06-08-55.375

28 36 MTC 06-08-55.562

29 35 MOC 06-08-55.984

30 34 MTC 06-08-56.578

31 32 MOC 06-09-11.640

32 30 MTC 06-09-24.546

33 27 MTC 06-09-38.031

34 27 MTC 06-09-38.578

35 27 MTC 06-09-39.109

36 0 MOC 06-09-57.171

37 24 MOC 06-09-57.828

38 10 MOC 06-11-15.406

39 2 MOC 06-12-12.781

40 0 MOC 06-12-52.671

41 0 MOC 06-12-53.218


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

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 ~ 24:00

UserLabel Object

identifier

Cell and

Location Area

Cell(LAC-CI)

SDCCH

assign

successful

number

SDCCH

assign

failure

number

SDCCH

assign

failure

rate

TCH

Assignme

nt Success

Number

TCH

assign

failure

number

TCH

assign

failure

rate

SUNKADA

KATTE

-PIPLINE-3

Bsc107-Sit

e35-Bts3

LAC107-CI1

7353 14479 4490 23.63 4678 122 2.54

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.

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

through setting RACHMin.

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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.

2.4.2 Case 2

A cell’s SDCCH assignment failure rate soars on busy hour, but TCH assignment

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

UserLabel Object identifier Cell and Location

Area Cell(LAC-CI) Pmdatatime

SDCCH

assign

failure

rate

TCH

assign

failure

rate

GOLLARAHTTI-XCEL-2 Bsc107-Site64-Bts2 LAC116-CI17642 19:00-20:00 15.85 0.68



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.

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

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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.

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.

UserLabel Object identifier

Handover

success

rate (%)

SDCCH

assignmen

t success

number

SDCCH

assignment

failure

number

SDCCH

assignment

failure rate

TCH

assignment

success

number

TCH

assignment

failure

number

TCH

assignment

failure rate

C11658*100/

C11611

GAYATRIPU

RAM-MYS-3

Bsc23-Site19-Bt

s3 20 1915 3001 58.67 1033 1325 56.19

21


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

SDCCH

assign

failure

rate

TCH Assignment

Success Rate

NANJANGUD-1 Bsc36-Site19-Bts1 LAC1036-CI36191 2007-12-17 20:00-21:00 10.8 81.75




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.

22


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.

23


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.

24

DATETIME BSC_NAME BSCID CELL_ID SITE_NAME MYHOUR SD_ASSN_FAIL_RATE

13-Dec-07 JAYANAGAR-BSC 102 12282 THAYAGRAJNAGAR-2-s 21 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.


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

Tx-Integer into 14.

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10 gsmp&o b-en-gsm network sdcch assignment analysis-word--201009

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