06-Troubleshooting for Handover

7/29/2019 06-Troubleshooting for Handover

1/27

Troubleshooting ManualM900/M1800 Base Station Subsystem Table of Contents

Table of Contents

Chapter 6 Troubleshooting for Handover................................................................................... 6-16.1 Overview............................................................................................................................ 6-1

6.1.1 Failure Classification............................................................................................... 6-16.1.2 Tool for locating the faults....................................................................................... 6-2

6.2 Trouble Handling................................................................................................................ 6-26.2.1 Locating Procedure ................................................................................................. 6-26.2.2 Locating Procedure of No Handover Starting Up.................................................... 6-36.2.3 Locating of Hardware Failure.................................................................................. 6-46.2.4 Locating of Data Configuration Problem................................................................. 6-5

6.3 Examples ........................................................................................................................... 6-56.3.1 MSC Handover Problem......................................................................................... 6-56.3.2 BSC Problems....................................................................................................... 6-126.3.3 BTS-related Problem............................................................................................. 6-186.3.4 Others.................................................................................................................... 6-23

Huawei Technologies Proprietary

i


2/27

Troubleshooting ManualM900/M1800 Base Station Subsystem Chapter 6 Troubleshooting for Handover

Chapter 6 Troubleshooting for Handover

6.1 Overview

MS continuously moves and its relative position to the BTS changes during

conversation. In order to guarantee the channel quality during conversation, MS

continuously measures the quality of radio channels of the surrounding cells and

transmits the measurement report to BSC through the BTS of service cell. BSC

implements the radio link control based on the information such as the level strength

and quality class of service cell and adjacent cell contained in the report. When MSmoves from one cell to another, the new cell instead of the original one will serve it

and guarantee the continuity of service. All cells are formed as a seamless network

through handover.

There are many reasons causing the handover failure. This part describes the

common idea and examples for the handover failure analysis.

6.1.1 Failure Classification

I. Classified by phenomenon

They can be classified into 3 kinds of problem based on the phenomenon of

handover failure.

problem of none-handover initiation;

problem of incoming cell handover

problem of outgoing cell handover

II. Classifed by reasons

They can be classified into 3 kinds of problem based on the reasons of handoverfailure.

Hardware failure, including board failure, hardware connection failure, etc.

which may only be caused by BTS, BSC or MSC, or improper matching of BTS,

BSC and MSC.

Data configuration failure, including adjacent cells with consistent BCCH and

BSIC, inconsistent CGI, unreasonable handover parameters and defect of

frequency planning.

Congestion


6-1


3/27


6.1.2 Tool for locating the faults

The traffic statistic is a good tool to analyze the handover failure, e.g. when the

handover success rate is low in an office, check if the radio handover success rateis also low through the traffic statistic. If it is, check if that is caused by the hardware

failure or too low level in handover or analyze if the handover threshold is set too

low, resulting in low success rate due to too low level in handover.

6.2 Trouble Handling

6.2.1 Locating Procedure

1) Confirm if the problem occurs in an individual cell or all cells, and the

characteristic of the failed cell, e.g. all are the adjacent cells of a cell or they

share BSC and MSC.

If the handover failure occurs between 2 cells, check with emphasis if the data

configuration between 2 cells is correct and if the hardware fails.

If the failure occurs in all adjacent cells of a cell, check with emphasis if the

data configuration of this cell is correct and if the hardware of it fails.

If the failure occurs in all the cells under the same BSC, check with emphasis

the data configuration between BSC and MSC.

If the failure occurs in all the cells under the same MSC, there may be a

problem with the matching between the opposite office and this office, such as

incompatible Signaling and unreasonable timer setting.

2) Confirm if the data are modified before the handover failure.

If the failure occurs in an individual cell, check if the data configuration related

to this cell is modified.

If the failure occurs in all the cells under the same BSC, check if the data

configuration of this BSC and opposite MSC is modified.

Similarly, if the failure occurs in all the cells under the same MSC, check if the

data configuration of opposite MSC is modified.

3) Check if the handover problem is caused by the hardware failure. See 1.2.3 for

the locating method.

4) Register the useful traffic statistic, such as the handover and TCH performance

measurements.

Pay attention to the following, but not only these are involved.

Observe if TCH occupation of failed cell is normal, e.g. if call drop rate rises up.

Observe if the success rates of incoming and outgoing handover are normal.


6-2


4/27


Observe the distribution of reasons for handover failure.

Observe if the radio handover success rate is normal.

5) Do the driving test on the failed cell and analyze the Signaling of driving test.

Pay attention to the following.

Observe if the uplink/downlink level of failed cell is balanced. The

uplink/downlink unbalance may cause the handover problem. And frequent

uplink/downlink unbalance is caused by the hardware failure.

Observe if the measurement report of failed cell contains the correct list of the

adjacent cells.

Observe if it is possible to hand over from the failed cell to the adjacent cells

and vice versa.

Analyze if the Signaling flow of handover is normal.

6.2.2 Locating Procedure of No Handover Starting Up

The MS in a cell can not initiate a handover into another cell under very weak signal

or the signal with very bad quality. This kind of problem is generally analyzed from

the following 2 aspects.

if the condition of outgoing handover is met;

if there is a candidate cell according with the condition of outgoing cell

handover.

There may be the following specific reasons.

I. The handover threshold is set too low

For the edge handover, the triggering condition is the Rx level is lower than the

handover threshold. If the handover threshold is set too low, the handover will not

initiate even when the level of adjacent cell is much higher than that of service cell.

The conversation quality will be impacted and call drop will be resulted when the

condition is serious. The handover threshold should be set based on the coverage

scope of cell. The scope of service area in the cell can be indirectly changed

through the handover threshold.

II. The relationship of adjacent cell is not set

Although the level of adjacent cells in the service cell is very high, MS does not

report this adjacent cell and the handover to this cell is unavailable as the

relationship of adjacent cells is not set. Observe the adjacent cell list of service cell

reported by MS through reselection or conversation test. If MS has already moved

to the main lobe of a cell, but there is no such a cell in the adjacent cell list, check if

the correct relationship of adjacent cells is set. Or scan BCCH frequency with

another MS in the test and observe if the BCCH frequency with stronger signal

occurs in the service cell or adjacent cell list.


6-3


5/27


III. The hysteresis is set unreasonable

Only when the difference of signal levels of handover candidate cell and service cell

is bigger than the hysteresis, can it be taken as the destination cell. When the

hysteresis is set too big, handover may be hard to be triggered.

IV. The parameter N-P is set unreasonable

In normal handover, MS sorts the sequence of handover candidate cells through

N-P principle. If a candidate cell is the optimal cell for P seconds in N seconds, it

can be taken as the destination cell of handover.

When 2 good candidate cells become the optimal cells alternatively, it is very hard

for the handover decision algorithm to find an optimal cell satisfying N-P principle,

resulting in hard handover. Adjust the setting of N and P, and reduce the statistictime to make the handover decision be more sensitive to the level change. This

case occurs in optimization of a network. The original statistic time of a cell is N=5

and P=4 and after they are adjusted as N=4 and P=3, the handover is normal.

When the land form of service cell is very complex, the received-signal level of

moving MS fluctuates a lot. In this case it is hard for the candidate cell to satisfy the

N-P principle, resulting in hard handover.

6.2.3 Locating of Hardware Failure

If the data configuration of failed cell and adjacent cell have not been modified

recently and the handover problem occurs suddenly, first consider if it is caused by

BTS hardware failure.

1) If similar problem occurs in the shared BTS cell of this cell, consider if it is

caused by the shared hardware failure of cells, e.g. if TMU fails.

2) If only one cell has the problem with handover under this BTS, consider if it is

caused by the cell hardware failure, e.g. some carriers are damaged and the

failure of handover to the carrier is resulted. Some carriers can be blocked to

verify this problem. If the handover success rate is recovered normal after acarrier is blocked, check if it is caused by the failure of this carrier, the related

CDU or antenna. If the signal uplink/downlink of a carrier is seriously

unbalanced, the handover problem will be resulted, e.g. frequent handover or

decreasing of handover success rate.

3) Observe if the Signaling of this cell is normal through Abis interface tracing,

including if the uplink/downlink Rx quality in the measurement report is good. If

the Rx quality in the report is bad, there will be failure with the hardware or

serious interference in this cell, as a result, the Signaling can not be normally

interacted and the handover problem is caused.


6-4


6/27


6.2.4 Locating of Data Configuration Problem

As there are many handover problems caused by the data configuration error, the

analysis and solution are introduced below.1) MSC independent networking mode: If the handover of incoming or outgoing

MSC is abnormal, check first if the Signaling matching of 2 MSCs is correct and

second if the data have been modified recently for the opposite MSC and local

MSC.

2) Shared MSC networking mode: If the handover is between BSC from different

suppliers and the handover abnormity occurs, check first if the Signaling

matching between BSC is correct and second if the data have been modified

for BSC.

3) If the handover abnormity only occurs in a cell, make the analysis based on the

specific condition of abnormity.

4) If the incoming cell handover is abnormal, observe if the handover from all

other cells to this cell is abnormal (the general problem for abnormity is the

handover success rate is low and the handover to this cell is unavailable).

If the handover from all other cells to this cell is abnormal, it is generally caused

by the data configuration of this cell. The data configuration includes not only

that of this cell but also that of other cells related to this cell. For instance, CGI

may be correct in the data configuration table of this cell but incorrect when

configured in other adjacent cells.

The incoming cell handover is abnormal, but only the handover of one cell to

this cell is abnormal and that of others to this cell is normal. If so, check if the

data configuration of adjacent cells about this cell is correct and if the hardware

of this cell is normal other than checking if the data configuration of adjacent

cells is correct in that of this cell.

For the abnormity of outgoing cell handover, the analysis idea is similar to the

incoming cell handover, so it is not described here.

6.3 Examples

6.3.1 MSC Handover Problem

I. The charging data modification of supplier A leads the success rate for

the incoming MSC handover of dual band network to 0%.

Description

After a dual band network was activated, the indices were normal all the time. But

someday the incoming MSC handover success rate suddenly became 0%.


6-5


7/27


Handling Process

1) When the handover was suddenly abnormal, check if the data configuration of

GSM1800 network had been modified.

2) When the Signaling at MSC side of GSM1800 was traced, it was discoveredafter MSC of GSM900 transmitted the handover request to that of GSM1800,

the latter responded the handover response message (with handover

command) to the former. And after 2 seconds, MSC of opposite GSM900

responded the Abort message to that of GSM1800.

3) When the Signaling of MSC for GSM900 was traced, it was discovered the Ho

Detect message was reported by BSC of GSM900 when handed over from the

BTS of GSM900 to that of GSM1800. But that message transmitted from

GSM900 was not received at the MSC side of GSM1800, so the handover

failure was resulted.

4) It was checked If the data of opposite MSC were modified and it was

discovered the MSC data of GSM900 had been modified.

5) The handover was normal after the MSC data of GSM900 were modified.

II. MSC data configuration error leads to the handover failure.

Description

The GSM1800 network of an office was of Huawei and the GSM900 network was of

supplier S. The former was the configuration of 1 MSC (MSC4) and the latter was

that of 2 MSC (MSC1 and MSC2). MS was handed over from MSC4 to MSC1 andthe handover was normal. But when MS was handed over from MSC4 to MSC2, the

handover was unsuccessful.

Handling Process

1) As the handover from MSC4 to MSC1 was normal, the reason of failure may be

the problem with routing from MSC4 to MSC2.

2) When checking the Signaling link of interface E between MSC2 and MSC4, all

was normal and there was no problem with the conversation between MSC2

and MSC4.

3) The Signaling of interface E was analyzed. MS under MSC4 transmitted thehandover request. MSC2 responded the handover request response and

transmitted the return handover roaming number but the latter flow was

interrupted.

4) The latter flow was MSC4 established the service channel at interface E based

on the handover roaming number addressing. It was possible MSC4 did not

check out the routing based on the handover roaming number.

5) When checking the data configuration of MSC4, it was discovered the called

number attribute of roaming number returned by MSC in the analysis table of

called number was configured as MSISDN.

6) After the called number attribute was modified, the failure was eliminated.


6-6


8/27


Recommendation

As the incoming/outgoing handover is related to the selection of routing, the analysis

of failure reason is complicated. It is recommended to make good use of the

analyzer function and find out the failure reasons from the flow.

III. Improper BSC parameter setting of Company S causes low handover

success rate to Huawei MSC.

Description

In an office, MSC, BSC and BTS from both Huawei and company S existed at the

same time. The handover success rate from MSC of Huawei to that of company S

was generally maintained about 80%, but the handover success rate from MSC of

company S to that of Huawei was lower and the lowest was about 30%.

Analysis

There are many factors impacting the across-MSC handover. They include the

network optimization, data configuration and protocol.

Handover includes 3 processes:

Handover Required Indication;

Handover Resource Allocation;

Handover Execution;

Note: The Handover Required Indication process allows BSS to execute handover

for an MS request

Handling Process

1) Adjust the radio parameters such as frequency, power and handover

relationship of both sites near to the coverage area of equipment from company

S, but the effect is not obvious.

2) The Signaling of both interfaces was traced through the Signaling analyzer and

it was discovered the main reason for unsuccessful handover was BSC of

company S did not deliver the Handover Command message to MS, resulting

in the time out of timer waiting for MS access of Huawei MSC.

Figure 6-1 shows the Signaling flow of typical handover failure.


6-7


9/27


S MSC Huawei MSC

Handover Failure (MAP)

Prepare Handover (MAP)

Prepare Handover ACK (MAP)

IAI (TUP)

ACM (TUP)

7s Timer Overtime

Abort (MAP)

CLF

RLG

Figure 6-1 Signaling flow of typical handover failure

The Signaling result showed for most of handover, the time from Prepare

Handover to receive MAP Prepare Handover ACK (MAP) was about 2s. With

the delay on the interface A, the whole continuous process exceeded 2s in

most cases. For a small amount of handovers trying within 1s, it succeeded in

most cases. The speed of continuous process for handover was related to not

only the switch but also the radio environment. The varied algorithm of different

equipment also led to the possibility of handover delay.

3) RACHBT parameter of BSC from company S was modified (the min. interval of

service channel handover, corresponding to Huawei BSC parameter as the min.

interval of channel handover) from 3s to 5s, then the problem was resolved.

(BSC parameter of Huawei is 4s.)

Figure 6-2 shows the Signaling flow of successful handover.


6-8


10/27


S MSC Huawei MSC

Prepare Handover (MAP)

Prepare Handover ACK (MAP)

IAI (TUP)

ACM (TUP)

Handover Complete

CLF

RLG

Handover Detect

ANN

Figure 6-2 Signaling flow of successful handover

4) After the problem was resolved, the handover success rate of both parties

reached about 80%.

Recommendation

The handover across MSC is related to BTS, BSC and MSC of both parties. So the

analysis is difficult and there are many problems to be resolved. As long as they are

analyzed and excluded through checking, all problems will be finally resolved.

IV. The Signaling matching problem leads to handover failure.

Description

The BSC office (1AM+2BM) was newly established in a place and connected to

MSC60 of Huawei. The BSC controlled 50 BTSs.

All was normal when handover was among the cells within BSC. But when the

outgoing BSC was handed over to the BSC connected to DX2000 (MSC) of supplier

N, the handover was unsuccessful. The handover from BSC of supplier N to that of

Huawei could be successful and the consequent handover back to BSC of supplier

N could also be successful.

Analysis

1) Unsuccessful handover might be due to the problem with data configuration at

BSC side, e.g. external description error, cell adjacent relationship error, BA1

table and BA2 table description error.


6-9


11/27


2) It might be due to the problem with data configuration of cell description in

MSC60 of Huawei BSC.

3) As the inter-office handover between MSC of Huawei and DX2000 (MSC) of

supplier N was involved, it might be due to the external description error of BSCconnected to DX2000 on BSC of Huawei.

Handling Process

1) When checking out the reasons at BSC side of Huawei, the data table of

external cell description, cell adjacent relationship table, BA1 (BCCH) table and

BA2 (SACCH) table, the maintenance personnel did not find any error. The

data of foreground/background were consistent based on checking.

2) Let BSC of other party check if the external cell description data of Huawei

BSC cell were correctly set. They were correct based on checking.

3) Let MSC of both parties confirm if the data were correctly set. They were bothcorrect based on the confirmation.

4) The Signaling at interface A was traced and it was discovered when Huawei

BSC cell delivered compulsory handover to the cell of other party, there was

only handover request command but no handover command delivered by

upper level MSC of Huawei, and CGI of destination cell was correctly reported

in the handover request. But the Signaling was completely correct when the

compulsory handover was transmitted from the cell of other party to that of

Huawei and the handover was successful. It could be fundamentally ensured

there was no problem with data configuration and there might be the problem

with the Signaling flow between MSC.

5) According to inter-office handover flow, when Huawei BSC delivered the

compulsory outgoing BSC handover to BSC of supplier N, Huawei BSC first

transmitted a handover request to upper level MSC60 which transmitted

Perform Handover to DX200. The CGI of destination cell was included in this

message. VLR of other party allocated an Handover Number and returned it to

Huawei MSC60 through Radio Channel ACK. If the Signaling flow was normal,

MSC60 transmitted an IAM to DX2000 which transmitted an ACM to MSC60.

After the session was established, MSC60 delivered Handover Command to

Huawei BSC and then MS could be handed over to BSC of other party.

6) MAP message was traced with analyzer between MSC. It was discovered

MSC60 did not transmit IAM after it received the message Handover Number

transmitted by the other party and thus the Signaling flow was terminated.

When this message was checked, it was discovered DX2000 had found the

CGI of destination cell.

7) When Handover Number from other party was analyzed, it was discovered the

handover number of other party was transmitted in the mode of 130%%%%%

%%%% without 86 added in front. As Huawei equipment did not accept this

format, the handover failure was resulted.


6-10


12/27


8) Coordinated with supplier N for adding 86 before the handover number, the

problem was resolved.

V. The equipment matching problem between different supplier leads to low

outgoing BSC handover success rate.

Description

The independent networking of Huawei GSM1800 in an office was matched through

dual frequency with GSM900 of supplier A and supplier B. After the reselection and

handover data were completed for both parties, it was observed through the traffic

statistic the dual frequency handover success rate was low. It was represented that

the handover success rate from GSM1800 to GSM900 was as low as about 60% to

80% and that from GSM900 to GSM1800 was high.

Analysis

When there was problem with the interconnection with the equipment from other

suppliers, the parameter and details of the other party should be known on time, e.g.

if Phase 2+, EFR was supported.

Handling Process

1) The message of interface A and interface E was analyzed through a Signaling

analyzer. It was discovered after BSC of GSM1800 transmitted the message

Handover Required, MSC of GSM1800 responded the message handover

REJECT and declined handover.

2) The corresponding interface E (GSM1800 MSC-GSM900 MSC) GSM 1800

MSC transmitted Prepare Handover to GSM900 MSC which responded the

message Abort.

3) As the handover success rate from GSM900 to GSM1800 was high, it was

observed in the message Prepare Handover transmitted from GSM900 MSC to

GSM1800 MSC, the voice version provided was the full rate version 1. But in

the message Prepare Handover transmitted from GSM1800 MSC to GSM900

MSC, the voice version provided was the full rate version 1 and 2 and half rate

version 1 belonging to PHASE 2+ version. MSC of supplier A did not acceptthis version, so handover failure was resulted.

4) Only the full rate version 1 was selected through the modification on circuit pool

table of interface A of MSC data. After loaded, it was discovered the voice

versions provided in the message Prepare Handover from GSM1800 to

GSM900 were all the full rate version 1 and 2. The dual frequency handover

success rate was greatly increased.


6-11


13/27


6.3.2 BSC Problems

I. Incorrect CGI leads to low handover success rate.

Description

MSC of an office is the equipment of supplier M while BSC and BTS are the

equipment of Huawei. The traffic statistic indices of a day were observed and it is

discovered in "Inter-cell Handover Performance Measurement", "success rate of

inter-cell handover" was very low for a cell (cell 24 of No.3 module of BSC) at the

period of 10:00 to 11:00, and it was 73.12%. It was discovered that was mainly

because the outgoing cell handover success rate in the cell 46000****OCFB was

very low and the handover failure times reached 10.

Analysis

The main reasons causing the failure of handover between cells follow.

1) unreasonable handover data configuration

2) problem with equipment (individual TRX damaged)

3) congestion

4) interference

5) clock problem

6) coverage

7) uplink/downlink unbalance

Handover flow:

1) All BCCH frequencies of adjacent cells in "BA2 Table" are delivered to MS

through the system message type 5.

2) MS reports the BCCH frequencies, BSIC and level value of 6 adjacent cells and

service cell with the strongest level to BSS (through the measurement report).

3) After the measurement report is preprocessed, the module, cell number and

CGI of all cells are confirmed by BSC through BCCH frequency and BSIC to

"Cell Adjacent Relationship Table" and "Cell Description Data Table" (external

cell description data table). If "Cell Adjacent Relationship Table" is not

configured with an adjacent cell, the information of this adjacent cell will not be

indexed, thus handover can not be transmitted. If the frequency and BSIC of 2

adjacent cells are same, they will be indexed to the first adjacent cell, thus

handover failure will be resulted. If the frequency of active BCCH in cell A is the

same as that of a TCH in cell B which has the same BSIC with cell C, the

asynchronous handover access on a time slot (this time slot is aligned with the

active BCCH time slot in cell A) of this TCH may by encoded by mistake

through cell A into its own random access. For instance, MS retransmits the

handover access to cell B in this time slot for several times due to some reason


6-12


14/27


(e.g. handover failure), cell A may generate SDCCH congestion and

assignment failure.

4) BSC executes the handover decision flow (completed in GLAP) such as basic

sequence of cell. Once it finds the proper destination, it will transmit thehandover request information with destination cell CGI to BSCGMPU. GMPU

confirms the number of module where the cell locates in "cell module

information table".

5) GMPU transmits the handover request to this module and makes the statistic

for "Outgoing Cell Handover Request".

6) If there is no CGI in the "Cell Module Information Table", BSC will think the

destination cell is an external cell and transmit the CGI of destination cell and

service cell to MSC through the handover request.

7) MSC first checks the cell matching with the destination cell CGI in "Location

Area Cell Table", confirms the ''Destination Signaling" of this cell, i.e. BSC, and

transmits the handover request message to this BSC.

8) If there is no CGI of destination cell in "Location Area Cell Table", look it up in

the adjacent cell. If found, transmit the handover request message to this MSC,

then to BSC.

Handling Process

1) There are no full busy times in the traffic statistic, so the problem with

congestion can be excluded.

2) There is only interference for the interference band 2 in "TCH Performance

Measurement", so the handover failure due to interference can be excluded.

3) Check the board status in "BTS Maintenance System", it is normal (so the

problem with equipment can be excluded). Right click TMU-check the

board-close and double click BAM-WH long message. The final digits are

"12-01-", showing the clock is normal.

4) There are many failure times for outgoing BSC handover. The maintenance

personnel checked BA2 table, cell adjacent relationship table, external cell

description data table and location area cell table, but did not find the data

inconsistency and consistent frequency and BSIC. After checking, the

maintenance personnel discovered the CGI about this cell in the external cell

description table was incorrect.

5) CGI was modified. All modules were transmitted after setting. The command

character was configured for all the cells taking this external cell as the

adjacent cell, i.e. the handover data were configured. The indices were

observed an hour later and all was normal.

II. BSIC modification leads to low handover success rate.

Description


6-13


15/27


When the result of traffic statistic was checked, it was discovered the handover

success rate in some cells was low. Based on detailed analysis, it was discovered

the outgoing cell handover of these cells was normal but the time of incoming cell

handover is 0.

Handling Process

1) It was checked whether there was the cell with consistent frequency and BSIC

in the adjacent cell of failed cell but it was not discovered, so this reason was

excluded.

2) It was checked whether BCCH of failed cell had been modified and it was

discovered not all adjacent cells of failed cell were set. But based on the record,

the frequency of failed cell had never been modified, so this reason was

excluded.

3) It was checked whether the failed cell had been severely interfered. Seen fromthe traffic statistic, the interference band was all normal and there were not

many times for call drop of failed cell and there were few times of handover

caused by the bad quality of conversation. If there was interference, it would be

impossible there was no successful incoming cell handover. So this reason was

also excluded.

4) It was checked whether TRX worked normally, and the channel was observed,

but no problem was found out.

5) The data of failed cell were checked and it was discovered BSCI had been

modified. Not all adjacent cells might be dynamically set. It was through BCCH

and BSIC that BSC located the destination cell when it transmitted the

handover request. When one of them was modified, all adjacent cells of the

modified cell should be informed.

6) After all adjacent cells of failed cell were dynamically set, the traffic statistic was

observed the next day and it was discovered all was normal for the incoming

cell handover.

III. Consistent BCCH and BSIC lead to failures of TCH occupation and

incoming cell handover.

Description

The traffic statistic indices of an office were observed and it was discovered in "TCH

Performance Measurement", there were many "assignment occupation failure times

(all)" for 2 cells (cell 7 and 8 of No.2 module of BSC) in a period of time, which were

respectively 89 times and 61 times. But "TCH call occupation failure times" were 0.

Handling Process

1) "TCH assignment failure times" were consisted of "TCH call occupation failure

times" and "TCH assignment failure time in handover". As "TCH call occupation

failure times" were 0, all TCH assignment failures occurred in handover.


6-14


16/27


2) Fifteen minutes of "incoming cell handover performance measurement" of the 2

cells was registered to check between which adjacent cells on earth and this

cell the failure occurred. With the observation of several period of time, all

handover failures were from a specified cell (CGI=********1768) to the 2 cellsand the handover was not caused by congestion. So it was estimated the

failure was caused by the handover to wrong cell due to data error of cell with

consistent frequency and BSIC.

3) Under the state of conversation, MS measured the level value of frequency

corresponding to the adjacent cell specified in BA2 table delivered by the

system through SACCH, and reported the measurement result to BSC. Based

on the reported BCCH and BSIC, BSC found the module number and cell

number of related adjacent cell in [Cell Description Data Table]. Then it

confirmed the found cell was geographically and logically adjacent to the

service cell through checking the module number and cell number of adjacent

cell of service cell in [Cell Adjacent Relationship Table]. And it found the

module transmitted by the handover request in [Cell Module Information Table]

through finally-confirmed CGI (this CGI was of the adjacent cell recorded by the

system when it was looking for [Cell Description Data Table]). Thus the course

of looking up the information of adjacent cell and routing was completed.

4) Based on the course above, it could be judged there was the cell with

consistent BCCH and BSIC in [Cell Description Data Table] and this group of

cells with consistent BCCH and BSIC were exactly the adjacent cells of the

same cell.

5) Based on the flow of the table checking above, the module number and cell

number of all adjacent cells of the cell with CGI=********1768 were recorded in

[Cell Adjacent Relationship Table], then the corresponding BCCH and BSIC

were checked and recorded in [Cell Description Data Table]. It was discovered

the cell 7 of No.2 module and cell 27 of No.3 module were the cells with

consistent BCCH and BSIC, and the cell 8 of No.2 module and cell 36 of

module 5 were the cells with consistent BCCH and BSIC, too. After the reason

was found, BSIC of cell was modified to reduce the work amount for modifying

the data. After the data were set, the related indices were normally recovered.

Recommendation

1) When the network capacity is very large, the cells with consistent BCCH and

BSIC are unavoidable. To reduce the error probability, it is required to make

good planning on frequency and BSIC in the network planning and this is

important in guaranteeing the network quality.

2) If there are adjacent cells with consistent BCCH and BSIC in a cell, the success

rate will surely be 0 when the handover is from the cell to those adjacent cells.

It is shown in the traffic statistic that the success times of incoming cell

handover of the specified cell in "Incoming cell Handover Performance

Measurement" of a cell are always 0 (the failure reason is not congestion). And


6-15


17/27


there are "TCH occupation failure times (all)", but the times of "TCH call

occupation failure" are 0.

3) Being familiar with the flow of handover and that of system table checking is the

precondition for quick decision and location of handover failure.

IV. Cell BCCH modification leads to lots of incoming handover failure and

SDCCH congestion.

Description

1) There was the adjacent frequency interference for a cell due to the frequency

planning, resulting in high call drop and bad conversation quality. So the data

management console modified and dynamically set the active BCCH frequency.

The next day the traffic statistic showed there were lots of BSC inner-incoming

handover and BSC inter-incoming handover in cell 1, but most wereunsuccessful. Meanwhile the cell 1 SDCCH was seriously congested (max.

congestion rate was 75% for being busy).

2) When the channel state was observed in the remote maintenance of BTS, it

was discovered there was no SDCCH occupation before lots of TCH

occupation in cell 1 (such TCH occupation should be BSC inner or

inter-incoming handover to TCH). And the TCH occupation time was basically

3s to 5s, but few was 10s. Sometimes 3 to 4 TCH became "A" at the same time

and quickly became "I". Although SDCCH was seldom occupied, it was very

common that 8 SDCCH became from "I" into "A".

Handling Process

1) After the abnormity of cell 1 was found out, the operation of a day was

reviewed based on the traffic statistic and it was discovered the frequency

planning was modified and dynamically set just before lots of incoming

handover failure and SDCCH congestion rate increment. So the problem was

probably caused by the data setting.

2) In order to confirm the congestion was not caused by the hardware problem,

the equipment in this BTS was checked and verified. And the channel state of

cell 1 was observed at the remote end. Finally it was located that the problem

was caused by the data modification and setting. After the frequency of cell

was modified, this cell and another cell had consistent BCCH and BSIC. When

BSC implemented the handover and decision based on the BCCH and BSIC of

adjacent cells reported by MS, it possibly handed over the request which

should be to another cell to this cell. Even if no power was transmitted from this

cell, the incoming handover request would be generated. After the incoming

handover, it was discovered the channel quality was very bad, and became a

bit better after a while. So it was discovered the occupation TCH time of

incoming handover was very short (3s to 4s). If the traffic of another cell was

large, the times of request for incoming handover to this cell would increase,


6-16


18/27


resulting in the failure of lots of incoming handover. This cell received lots of

instant assignment request and assigned SDCCH, resulting in serious SDCCH

congestion.

3) After the frequency of active BCCH of cell was modified and the data werereset, the problem was resolved. The incoming handover times of cell 1 was

recovered to the normal level as usual.

V. CGI error of cell description data table leads to no incoming handover to

this cell.

Description

The handover of a GSM network was abnormal. When the handover was from cell A

to cell B, the signal of cell B was much stronger than that of cell A, but the handover

was not generated. Only after the handover was across cell B coverage area to cellC coverage area was the handover from cell A to cell C available.

Analysis

If a cell can be taken as the service cell providing service and be formally handed

over to other cells but not be handed over in, it can be checked whether CGI, BSIC,

BCCH frequency number, etc. of this cell in [Cell Description Data Table] are correct.

Normally it is caused by the incorrect setting of those data.

Handling Process

1) The BCCH frequency of cell B was locked by the test MS. It was normal when

dialing. Compulsory handover could be available to any adjacent cell.

2) The BCCH frequency of any adjacent cell in cell B was locked for dialing, then

it was compulsorily handed over to cell B, but the handover was unavailable. It

was discovered from the software of driving test that there was no handover

command in the network.

3) Based on the handover flow, it should be MS that detected the signal of

adjacent cell and reported to BSC in the measurement report. BSC made the

handover decision based on the measurement report. If the handover condition

was met, the service channel of destination cell would be activated and thehandover command was transmitted to MS.

4) The signal of cell B was obviously stronger than that of cell A. The handover

requirement (PBGT handover threshold 70) was surely met, but the handover

command was not transmitted. It showed there was an error in the course of

activating the service channel of destination cell.

5) Only 2 items, frequency and BSIC, were contained in the adjacent cell

information reported by MS. BSC looked up the cell which had the adjacent

relationship with the service cell in [Cell Description Data Table] and [External

Cell Description Data Table] based on the frequency and BSIC, and found the

CGI of destination cell. If the destination cell was the external cell, the handover


6-17


19/27


request would be transmitted to MSC. The portable parameter was the CGI of

destination cell. The course of channel activation was completed by BSC in

which the destination cell located. Then the handover command was

transmitted to MS by the service cell. If the destination cell was the inner cell ofBSC, the module number and cell number of destination cell would be

determined by CGI, then the channel activation was completed and the

handover command was transmitted.

6) If cell B could not activate the channel when taken as the destination cell, it

might be due to the error of [Cell Description Data Table]. As a result, BSC in

which the destination cell located could not find the destination cell or activate

the channel, and the service cell did not transmit the handover command.

7) When [Cell Description Data Table] was checked, it was discovered there was

an error for the CGI of cell B. After the CGI was modified and dynamically set,

the handover was normal.

6.3.3 BTS-related Problem

I. Too high busy threshold of RACH of BTS2.0 leads to low handover

success rate.

Description

The handover success rate of BSC in the area A was very low all the time and it was

about 83%. Normally the success rate of integral handover of BSC is above 90%.

Handling Process

1) After the cell with very low handover success rate was analyzed, it was

discovered the request assignment times of TCH in the TCH performance

statistic was much more than the successful assignment. It could be thought

BSC had transmitted the handover request to BTS, just the success rate of

TCH occupation was very low.

2) The BTS in which the cell located was BTS2.0, so it was suspected the busy

threshold of RACH of BTS2.0 was higher, resulting in difficult TCH occupation.

And the access of new channel in the course of handover was impacted, too.

3) After the busy threshold of RACH was decreased from 8 to 5, the integral BSC

handover success rate was increased to about 90%.

II. Uplink/downlink unbalance due to CDU failure leads to low handover

success rate.

Description

The incoming cell radio handover success rate in BSC of cell 2 of a BTS was very

low as 10% to 30%.


6-18


20/27


Analysis

The low BSC inner incoming cell radio handover success rate is normally because

of the problem with data (e.g. CGI error in [Cell Description Data Table], lack of

measurement frequency for BA1 and BA2, adjacent frequency interference, etc.). It

is also because there is blind coverage area for high traffic or difficulty for uplink

weak MS access.

Handling Process

1) When the hardware was checked, it was discovered the state of BTS

maintenance board was normal. From the channel state it was seen TCH had

been occupied for long but the times were few. So it could be judged there was

no problem with conversation data.

2) The handover data were all normal based on checking.

3) "Incoming Cell Handover Performance Measurement" of this cell was

registered in the traffic statistic, and it was discovered the incoming handover

for all adjacent cells was bad.

4) In the driving test, at a place 2km away from BTS, it was discovered the

handover was tried frequently but the handover did not succeed and returned

to the original cell. If it occasionally succeeded, call drop would immediately

occurred. The downlink level was about 85 dBm in time of handover. The

dialing test with frequency lock was done for decades of times. When it was

taken as the caller, the tests all failed; and when taken as the called, the test all

passed but calling out was unavailable.5) So it was inferred the loss of CDU uplink channel was excessively big or BTS

cabinet top wiring was incorrect, resulting in weak uplink signal.

6) After CDU was replaced, the incoming handover success rate was increased to

over 95%.

III. BTS clock free oscillation leads to low handover success rate.

Description

A BSC with multi-module was configured with 3 BM. No.1 BM controlled 3 BTS30s.

The whole network handover success rate was maintained about 95%, and

decreased to 90% after running for some time.

Analysis

If the data were not modified and the network handover success rate was changed

from normal to abnormal, it was normally because one or several BTSs were normal,

and as a result the handover success rate was impacted. The abnormal BTS could

be found through the traffic statistic and the problem could be handled according to

the BTS.

Handling Process


6-19


21/27


1) When the traffic statistic was analyzed, it was discovered the handover success

rate for the cell of 3 BTS30s and adjacent cells were generally low. When the

working state of 3 BTS30s was checked, it was discovered the clock was in the

state of free oscillation, resulting in low handover success rate.2) The 3 BTSs located in the same group of BIE which was so suspected failed,

resulting in the losing lock of BTS clock. This group of BIE was exchanged with

another BIE, but the problem still existed.

3) After the corresponding HW wires of this group of BIE were replaced and the

network board was switched, the lock loss of BTS clock still existed. That the

losing lock of clocks of 3 BTSs was caused by BSC should be excluded.

4) The Tx 2MHz tributary and TMU were replaced, but BTS still could not lock the

clock.

5) As there were no frequency meter and other test instruments in the field, the

frequency offset test could not be done on 2MHz and 13MHz clocks of BTS,

and DA value could not be precisely adjusted. TMU of all BTS working normally

was checked and DA value was recorded. Then the DA value which could be

normally locked on other TMU was set in TMU of the 3 BTS30s. After being

tried for several times, the clock of 3 BTSs was in the state of lock.

6) Based on a day of observation, it was discovered 3 BTS30s ran normally. And

the handover success rate was increased to about 90%.

Recommendation

Manually set DA value of BTS if there are no test instruments in the field, and

observe whether BTS is locked. It normally takes over 30 minutes. If DA value

needs to be adjusted several times, the efficiency will be very low. Set the new DA

value and check TMU 2 to 3 minutes later. If at the moment the difference of

discriminator displayed is the same as the set DA value, the DA value can not finally

make TMU be locked, and if not the same, TMU can be finally locked. By this

method, the adjustment speed can be increased.

IV. TRX performance deterioration leads to low success rate of incoming

cell handover.

Description

The handover success rate of cell A with high traffic was low (below 70%), but other

indices were normal.

Handling Process

1) When observing the traffic statistic of this cell A, the maintenance personnel

found out high traffic was maintained all the time soon after last expansion, and

the handover success rate was low. The proportion of incoming/outgoing cell

failed handovers was 4:1. As the incoming handover success rate was not high,

the handover success rate of whole cell was decreased.


6-20


22/27


2) As there were few "incoming cell failed handovers (no usable channels)" and

most of failed handovers were "incoming cell failed handovers (other reasons)",

so the reason of congestion could be excluded.

3) The traffic statistic task of "outgoing/incoming cell handover performancemeasurement" of cell A was registered and the outgoing/incoming cell

handover performance were respectively observed.

4) In the "outgoing cell handover performance measurement" task the

maintenance personnel found out most of outgoing cell failed handover were

concentrated on several specified destination cells. The reason might be the

congestion of destination cells. "Incoming cell handover failure (no usable

channels)" of these destination cells was registered. It was discovered the

incoming cell failed handovers from the cell A were basically "incoming cell

failed handovers (no usable channel)". The outgoing cell failed handovers of

the cell A were located as being caused by congestion.

5) In the "incoming cell handover performance measurement" task it was

discovered almost none of incoming cell handover success rates was high (all

about 60%). Normally "none ofl handover success rates is high" is due to the

bad frequency planning or external interference, resulting in bad network

quality. But based on "there is no quality problem with outgoing cell handover",

the frequency planning defect and external interference can be excluded. When

observing the indices such as the call drop rate and interference band of the

cell A, maintenance personnel found out they were all very low, so such a

conclusion could be confirmed.

6) Based on the data checking, there were no cells with consistent BCCH and

BSIC. Meanwhile there was no adjacent cell with very low handover success

rate with cell A based on the traffic statistic observing, so the problem with data

was excluded.

7) The HF data were checked. TSC was consistent with BCC and there were no

other problems with data. Meanwhile the channel occupation of TCH carrier

was observed, the occupation was found normal. So the problem with HP data

could be excluded.

8) The handover-related parameters were checked, they were basically consistent

with the parameters of other cells with high traffic in the downtown, so the

reason of unreasonable parameters could be excluded.

9) The channel occupation was observed and several SDCCH were occupied for

BCCH carrier. It was normal for the cell with high traffic.

10) The channel occupation was observed in detail and a special case was found

out: 4 to 6 SDCCHs were occupied at the same time and then released at the

same time. Such cases took 60% of observation time. Under normal condition,

there is almost no possibility that 4 to 6 SDCCH are occupied and released at

the same time. It is estimated that is due to the bad TRX performance, resulting

in abnormal SDCCH occupation and release, and bad handover performance.


6-21


23/27


11) The data were checked. After it was confirmed that the "carrier mutual

assistance" function was used, the active BCCH carrier was blocked and

replaced.

12) The carrier was unblocked and the channel occupation and traffic statisticindices were observed. The channel occupation was recovered normal. There

was no case that several SDCCH were occupied and released at the same

time. The handover success rate in the traffic statistic indices was increased to

95% and the problem was resolved.

V. Improper antenna planning leads to low handover success rate.

Description

The handover success rate and the traffic statistic indices for 3 cells in a BTS were

very low, especially for the handover from cell 1 to cell 2 and cell 3, the success ratewas lower than 30%.

Analysis

The low handover success rate is generally due to the assignment failure of

hardware board defect, handover data error or improper antenna planning.

Handling Process

1) When the BTS hardware was checked, it was running normally, and there was

no related alarm. The handover-related parameter setting in the traffic statistic

was normal. So the problem with hardware and parameter setting could be

excluded.

2) The BTS located at the east side of north-south strike road, 700m away from

the road. The azimuth angle of 3 cells were 0, 80 and 160, respectively

pointing to 2 directions of the road and the open residence area in the east, and

the down tilt angels of 2 cells were 7. The directions of 3 antennas were

excessively concentrated in the design. Only the pertinence of coverage target

was considered, while no consideration was taken on the seriousness of cell

overlapping in the east of BTS. The west was only covered by the side lobe

and back lobe of 3 cells. So when the user passed this section of road, it wasfirst under the coverage of cell 1. The signal of 3 cells was very weak when

getting to the west section of road of BTS and it fluctuated a lot. The handover

statistic time and duration were set very short and the handover was very

sensitive, resulting in frequent handover failure.

3) After the azimuth angles of 3 cells were adjusted to be 60, 180 and 350, the

handover success rate of 3 cells was immediately increased to over 90%.

Recommendation

The coverage target and handover should be both considered in the planning of

azimuth angle for antenna. The directions should be equally distributed to avoid


6-22


24/27


serious overlapping between cells, or blind coverage area impacting normal

handover. The problem with traffic could not be absorbed but be guaranteed through

carriers.

6.3.4 Others

I. Locate the problem with low incoming BSC handover success rate

through the traffic statistic.

Description

For a dual frequency network, GSM900 network is of supplier S and GSM1800

network is of Huawei. In a big adjustment on data (including modification on

frequency and CGI of part cells), GSM900 network and GSM1800 network wereboth reloaded with data, and it was discovered the incoming BSC handover success

rate of GSM1800 decreased.

Analysis

As all handover threshold parameters did not change after data loading, the radio

environment was basically consistent before and after data loading, and the

incoming BSC handover success rate greatly decreased, so the problem was

related to the error of configuration data.

Handling Process

1) When the traffic statistic before after data modification was compared, it was

discovered there was little change for the following items. See Table 7-1 for the

data comparison.

Table 6-1 Traffic statistic indices before/after data modification

before datamodification

after datamodification

incoming BSC handover request times (to GSM1800) 1885 times 1613 times

incoming BSC handover success times (to GSM1800) 1684 times 1460 timesincoming BSC handover failure times 185 times 216 times

From the data above, it was discovered that before data modification, the

incoming BSC handover request times minus incoming BSC handover success

times (to GSM1800) was bigger than the incoming BSC failure times. But after

the data modification, the incoming BSC handover request times (to GSM1800)

minus the incoming BSC handover success times (to GSM1800) was smaller

than the incoming BSC failure times.


6-23


25/27


Meanwhile based on the meaning of several items in the traffic statistic above,

the difference between the incoming BSC handover request times (to

GSM1800) and the incoming BSC handover success times (to GSM1800)

represented the handover failure times from the destination cell GSM1800received the handover request to the handover success message was

reported.

The occurrence after data modification showed many incoming BSC handover

requests were abnegated before the destination cell GSM1800 received the

handover request.

2) After the data modification, there was no alarm for BSC equipment. There were

2 possible reasons for loosing the incoming BSC handover request.

There was an error for GSM900 network. The CGI of destination cell

transmitted could not be found in GSM1800 system, resulting in messageabnegation.

There was an error for GSM1800 network data. Due to CGI error in the module

message table, the module message forwarding in BSC could not find the

destination cell, resulting in the message abnegation.

3) For the second reason, if there was CGI error for the cell module message

table, there would be problem for all incoming cell handovers of this cell. The

BSC inner incoming cell handover trials and BSC inter incoming handover

requests in the cell performance measurement were checked. It was

discovered the 2 indices in 3 cells of BTS A were all 0, so it was judged there

was CGI configuration error for 3 cells of this BTS in the module message

table.

4) The CGI of cell with configuration error was modified and the whole table was

set.

5) The traffic statistic data were checked. All indices were normal and the problem

was resolved.

II. Locate the handover problem through the radio handover success rate

and handover success rate difference.

Description

The radio handover success rate and the handover success rate were close and

low.

Analysis

The difference between the handovers of radio handover success rate and that of

handover success rate was not big, and it showed the course in Figure 1-2 was

successful.


6-24


26/27


MSC Target BSC

Handover Request

Handover Request ACK

Handover Required

Handover Command

Source BSCMS

Handover Command

Radio Tx Signal Measurement

Figure 6-3 Flow of part handovers

The handover success times and the handovers of 2 statistic points were much

different. It showed MS access in the destination cell was unsuccessful after it

received Handover Command. That is to say the destination cell did not receiveHandover Complete, and the source cell could not receive Clear Command

delivered by MSC. So the possible reasons for the failure at Um port include the

following.

The destination cell uplink signal was weak, so it could not be accessed.

The destination cell was not a real one, but the dummy destination with

consistent BCCH and BSIC.

The CGI of destination cell in the external cell table of BSC did not correspond

to BCCH and BSIC, resulting in CGI error of destination cell. After channels

were activated in other cells, it could not be accessed as it was very far fromMS.

Handling Process

The data were checked and the case with consistent BSIC was modified to

guarantee there was no error for external cell data. The problem was resolved.

III. Sometimes the incoming handover may be to the cell which is not

allowed by NCC.

Description

The equipment of Huawei and supplier D were used in a place. BSS of Huawei was

attached under MSC of supplier E. The Flower networking was used. There were

many handover relationships between the equipment of 2 suppliers. Based on the

traffic statistic result of Huawei BSS, the general handover request times from cell E

to cell Huawei were over 40% when busy, and the handover success rate was about

80%. While the general handover request times from cell Huawei to cell E were 800

when busy.

Handling Process


6-25


27/27


1) Based on the test result, it was presumed there might be an error for "NCC

Permitted" of cell E.

2) When the data of cell E were checked, it was discovered this parameter was

set as 7 by all cells E.3) So the case with "NCC Permitted" as 6 was added for test, and it was

discovered BSIC of cell Huawei could be resolved when the service cell was

cell E.

4) After this value was changed, the handovers form cell E to cell Huawei were

increased from over 30 to over 600.

IV. Excessively high min. downlink power of candidate cell leads to

handover unavailability.

Description

The BTS of Huawei and that of supplier A in a place were continuously covered but

the successful handover to the cell of supplier A was unavailable.

Handling Process

1) The data configuration of adjacent cells of both were checked but there was no

error.

2) It was discovered in the test that the Rx level was low in the handover area and

that was about 92 dBm. But the compulsory handover was successful.

3) As the BTS of Huawei was configured as S (1/1/1) model, the uplink/downlink

unbalance was serious. To improve the handover success rate, the downlink

power of candidate cell was adjusted as 95 dBm. But within the handover

band of this cell and supplier A, the actual downlink power level was low. That

the min. downlink power of candidate cell was set excessively high led to this

cell could not pass M regulation, resulting in unavailable handover to this cell.

4) The min. downlink power of candidate cell was adjusted as 100 dBm, then the

handover was normal.

H i T h l i P i t

Documents

06-Troubleshooting for Handover