011- OMO312010 BSC6000 GSM V9R8C12 Handover Problem Analysis ISSUE1_01-Edited [兼容模式]

Case Analysis —— Handover www.huawei.com

Copyright © 2010 Huawei Technologies Co., Ltd. All rights reserved.

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References l

BSC Counter Reference(V900R008C12)

l

BSS KPI Reference(V900R008C12)

Copyright © 2010 Huawei Technologies Co., Ltd. All rights reserved. PDF created with FinePrint pdfFactory Pro trial version http://www.fineprint.com

Page2

Objectives l

Upon completion of this course, you will be able to: p

Understand the Measurement Points and Performance Index of handover

p

Analyze, locate and solve handover problems


Page3

Contents 1. Handover Process Flow 2. Analysis of Handover Problem 3. Handover Cases


Page4

Handover Process Flow MSC2 Search target cell（7）

Handover judgement（4）

BSC1

MSC1 Inter-MSC H.O （8）

CGI（6）

H.O request（7）

BTS1

BA2 table（1）

MS

Channel active （5） Preprocessed MR （3）

BSC2

BTS2

MR（2）


Page5



Page6

Analysis of Handover Problem l

Types of handover problems

l

Locating handover problem

l

Causes of handover problem


Page7

Types of Handover Problems l

Types － Possible Results p

No handover － Result in call drop

p

Handover failure － Affect the conversation quality, and call drop

p

Frequent handover － Affect the conversation quality and increase load of the system


Page8

Method of Locating Handover Problem l

Traffic statistics analysis p

Handover Failure/Attempt/Success Measurement per BSC

p

Incoming/Outgoing Internal/External Inter-Cell Handover Measurement per Cell

l

p

GSM Cell to GSM Cell Incoming/Outgoing Handover Measurement

p

Neighbor Cell Level Measurement per Cell

View alarm p

Board fault, transmission, clock, etc.

l

Drive test

l

Signaling analysis p

Abis interface, A interface, E interface


Page9

Causes of Handover Problem l

Coverage

l

Interference

l

Antenna and feeder system

l

BTS hardware

l

Transmission

l

BSC hardware

l

Data Configuration

l

A interface

l

Target cell congestion

l

Cooperation with equipment of other manufacturers


Page10

Problem in Coverage and Interference p

Coverage n

Poor coverage: forest, complicated topography, building direction and indoor coverage

p

n

Isolated site: no adjacent cell

n

Over shooting: island effect result in no adjacent cell

Interference n

MS can not access network or receive any signal.


Page11

Island Effect Results in Handover Failure There is no adjacent cell, so handover becomes impossible.

Non-adjacent cell

Adjacent cell N1

Service cell Non-adjacent Adjacent

cell

Cell N2 Non-adjacent

Adjacent

cell

Cell N3

Isolated island resulting from over shooting


Page12

Problem in Antenna and Feeder System p

High Voltage Standing Wave Ratio (VSWR)

p

Antenna is not properly installed

p

Antenna is not parallel

p

The azimuth and downtilt are not correct

p

Poor antenna isolation value

p

RF cables, connectors are loose or incorrect


Page13

Problem in BTS Hardware p

RFU, splitter/combiner failure

p

TRX failure

p

DTMU failure

p

Clock failure

p

Internal communication cable failure


Page14

Problem in Transmission and BSC Hardware l

l

Transmission p

Transmission is not stable

p

Serious BER in transmission

Fault of BSC Hardware p

Clock board: the faulty clock board causes clock inconsistency between base stations.


Page15

Problem in Data Configuration p

Unsuitable setting of handover hysteresis and handover priority.

p

Unsuitable frequency and adjacent relationship configured in BA1/BA2 table

p

CGI, BCCH and BSIC in "External Cell Description Table" are different from those in corresponding BSC.

p

The DPC of BSC in MSC "LAI and GCI Table" is incorrect.


Page16

Problem in A Interface and Target Cell l

A interface problem p

Basically, the insufficient link resource results in the abnormal handover, as well as abnormal communication.

p

Circuit pool numbers are different ,causing the handover failure.

l

Target cell congestion p

The target cell is congested, which causes the handover failure. Then the target cell should be expanded or reduce its traffic .


Page17

Problem in Cooperation between manufactures l

Handover between equipment of different manufacturers p

The signalings at A interface, E interface of the opposite equipment don’t matched with our equipment and can not be recognized or supported, which causes the handover failure, such as voice version, handover number, addressing mode (CGI or LAI).


Page18



Page19

Handover Case 1 l

Fault Description p

A subscriber complains that the signal in a place on his way to office is not good. Call drops occurs several times when he drives his car. The phenomenon is that the conversation is suddenly interrupted. The handset signal bar indicates no signal, and then normal again after several seconds.


Page20

Handover Case 1 l

Analysis p

Neighbor cell relationship

p

Isolated island effect

p

Geographical factors of buildings and mountains result in the weak signal

p

The signal of a cell suddenly fades drastically due to buildings

p

BTS equipment or intermittent transmission


Page21

Handover Case 1 l

Solution p

Via DT, it shows that the signal of several cells is

D

strong and is -70 to -80 dBm, so it is unlikely that the problem is caused by bad signal. p

3

A 2

We obtain further information from the

B 3

subscriber and know that the driver at that time drive very fast. Therefore it supposed that this problem may occur under high speed movement. Then perform drive test again under high speed for several times and the call drop recurs.


1

C

Page22

Handover Cases 1 l

Solution p

Cell 1 of BTS C is far away from cell 3 of BTS D and the two cells are not defined as neighbor

D

cells. Thus an isolated island is generated,

3

handover cannot be performed, and call drop arises. The special thing about this isolated island

A 2

B

effect is that it occurs at high speed. Therefore

3

troubleshooting is difficult. p

After a neighbor relation is added, this problem is solved. On the other hand, because this problem occurs due to insufficient time to hand over, the handover decision time is reduced so that the handover can happen timely.


Page23

1

C

Handover Cases 2 l

Fault Description p

This case occurs during the drive test at a new site.

p

Even when the receive level of serving cell B is higher than the recei ve level of neighboring cell M, handover is triggered from cell B to c ell M.


Page24

Handover Cases 2 l

Analysis p

The possible causes of the handover from a cell with a higher level t o a cell with a lower level cell are as follows: 1 The handover is emergency handover due to the heavy load or lo w quality of the original cell with the higher level. 2. The priority of the cell with the higher level is lower than the prio rity of the cell with the lower level.


Page25

Handover Cases 2 l

Solution p

Analyze the drive test data and find that the downlink receive qualit y and level of the serving cell are normal. Thus, the handover is not associated with the downlink receive quality and level.

p

Check the data configurations and find that the handover due to he avy load is disabled. Thus, the handover is not due to heavy load.


Page26

Handover Cases 2 l

Solution p

Check the priority settings of the cells. The result shows that cell B is at lev el 2 on layer 3 and that cell M is at level 1 on layer 3. Check the inter-layer handover threshold and hysteresis. The result shows that the interlayer handover threshold is 20 and that the interlayer handover hysteresis is 5. Thus, when the receive level of cell M is hig her than -85 dBm, handover is initiated from cell B to cell M.

p

Modify the priority level of cell B to level 1. In this way, the cells are at the same priority level. The handover problem is solved.


Page27

Handover Cases 3 l

Fault Description p

At a suburban area in city A, Huawei BTS3012 M-2 cell, incoming handover success rate lower than 70%, the number of incoming handover failures at busy hours is greater than 35.


Page28

Handover Cases 3 l

Analysis p p

p

p

The hardware such as clock board is checked and no problem is found. Register the traffic measurement of the cell incoming handover, it is found that most of the handover failures occur from the external N-1 cell of the BTS of company S. The field drive test shows that in times of handover failure, there occurs a neighbor frequency in the neighbor cell frequency list. It is suspected that the handover failures are caused by downlink inter-frequency interference. We trace and analyze the handover failure signaling and finds that sometimes the handover failures occur when the MS receives the handover command and sends access pulses to the target cell and sometimes the handover failures occur when the MS reports the SABM frame.


Page29

Handover Cases 3 l

Solution p

Therefore the conclusion is that the MS fails to correctly decode the physical message due to the downlink inter-frequency interference, or the MS fails to receive the UA frame from the target cell, resulting in handover failure.

p

The cell frequency is planned again and is changed from 113 to 121. The problem is solved.


Page30

Handover Cases 3 l

Handling Procedure p

The BTS hardware alarms are checked.

p

The cell incoming handover traffic measurement is registered.

p

Drive test to trace the signaling.

p

The cell frequency is modified.

p

l

The traffic measurement is checked and it is confirmed that the number of incoming handover failures decreases and the incoming handover success rate reaches 92%.

Suggestion and Summary p

The downlink interference is not represented in the interference band and the range affected by the partial downlink interference is small. We may miss the faulty place during the drive test due to selection of the drive route. It is desirable to perform careful drive test on the abnormal cells.


Page31

Handover Cases 4 l

Fault Description p

One BSC6900 has very low HO attempt & during many drive test no HO at all.

p

KPI indicate that there is very low HO attempt with good success rate.

p

From Drive test using TEMS, we found out “Missing Neighbor Detection” message appears in neighbor list when MS still try to make HO to serving cell.

p

The figure below and the corresponding KPI statistics table is presented as below. Corresponding KPI Statistics Table


Page32

Handover Cases 4


Page33

Handover Cases 4 l

Analysis p

Neighbors cell configuration is incorrect

p

Data configuration has some problem, such as “NCC Permitted”


Page34

Handover Cases 4 l

Solution p

Compared with other BSC, found out the handover attempts are less than other BSCs even under the same condition.

p

Checked the neighbor cell configuration, for some cells, there were some neighboring cells missing, but after adding, the problem still existed.

p

Checked "NCC permitted" , we found that the configuration in the problem BSC is set to be 11110000, that is, enabling NCC 4~7 handover while disabling NCC0~3, but the neighboring cells' NCC is 0~3.

p

Changed “NCC Permitted” to be 11111111, the problem is solved.


Page35

Handover Cases 4 l

Suggestions and Summary: p

“NCC Permitted” parameter is an important parameter to impact on handover but always been neglected, and if the similar phenomenon happened, this is a clue to solve this problem.


Page36

Handover Cases 5 l

Fault Description p

Huawei BTSs belong to Huawei BSC independently are inserted into a GSM900 network. After the cutover, the user complains that Huawei BTS handover is not good and that the BSC outgoing handover success rate is below 85%. The drive test shows that the serving cell is Huawei cell A and that then drive toward cell B. The receiving level of cell A is -80 dBm (no downlink power control), while the receiving level of the neighbor cell B of another BSC is -60 dBm. Handover does not happen until MS passes across the cell B. Non-Huawei BSC Huawei BSC

No handover for a long time？ -80dBm

-60dBm

Huawei Cell A Copyright © 2010 Huawei Technologies Co., Ltd. All rights reserved. PDF created with FinePrint pdfFactory Pro trial version http://www.fineprint.com

Cell B

Page37

Handover Cases 5 l

Analysis p

The reason that the receiving level of the neighbor cell is strong enough to prevent handover is as follows: n

The BA2 table has the BCCH frequency of cell B but the handover data, including parameters such as external cell CGI, BCCH, BSIC, and "whether co-MSC“ are wrong.

n

Congestion of cell B results in no handover.

n

Other parameters are set incorrectly, resulting in no handover.


Page38

Handover Cases 5 l

Solution p

Drive test shows that handover does not happen. To verify reasonableness of the data, test MS is used to perform the forced handover. Handover succeeds.

p

The traffic statistic of the non-Huawei BSCs is checked and confirm that there is no congestion in the target cell B.

p

Via drive test, it is found that all the new cells of Huawei BSC have this problem. It is suspected that the problem is caused by a common parameter.

p

The traffic measurement is analyzed and it is found that no PBGT handover happens in the new BSC, while only edge handover happens when the edge handover condition is satisfied, resulting in serious handover delay.


Page39

Handover Cases 5 l

Solution p

One of PBGT handover condition is target cell must be the first position in the 16bit criterion. If “Co-BSC/MSC Adj” is set to YES, the cells that do not in the same BSC/MSC are sorted at the last positions. Therefore the PBGT handover of the BSC outgoing handover does not happen. After “CoBSC/MSC Adj” is set to NO, the problem persists.

p

The data is further checked and it is found that the default parameter configuration shows that internal cells of Huawei BSC are "layer 3, priority 1" and the external cells are "layer 3, priority 2". Because PBGT handover only happens at the same layer and the same priority, the default configuration does not trigger PBGT handover.

p

After changing all internal cells priority of Huawei BSC from 1 to 2, it shows that PBGT handover happens by analyzing the traffic measurement. Drive test shows that BSC outgoing handover is not delayed.


Page40

Handover Cases 5 l

Suggestion and Summary p

Note that the following two default parameters need to be modified in the BTS and BSC insertion plan 1. Set “Co-BSC/MSC Adj” to NO 2. If there is no requirement for traffic balance, set the layer and priority of the internal cells and external cells to the same layer and the same priority.


Page41

Thank you www.huawei.com

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011- OMO312010 BSC6000 GSM V9R8C12 Handover Problem Analysis ISSUE1_01-Edited [兼容模式]

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