11 OMO312010 BSC6000 GSM Handover Problem Analysis ISSUE1.01

7/27/2019 11 OMO312010 BSC6000 GSM Handover Problem Analysis ISSUE1.01

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Confidential Information of Huawei. No Spreading Without Permission

Handover Problem Analysis



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BCCH frequencies of all adjacent cells in BA2 table are sent to MS on system message.

MS reports measurement report to BSC. It includes the BCCH , BSIC and signal level of the

adjacent cells and serving cells.

When the measurement report is preprocessed, BSC identifies the CGI of all adjacent cells

through BCCH frequency and BSIC .

BSC executes handover judgment flow such as basic cell ranking. Once a proper target cellis found, the handover request message which includes the target cell CGI will be sent toBSC.

If the target cell is an internal cell, BSC send the “channel active” to the BTS.

If the target cell is an external cell, both the CGI of the target cell and that of service cellare sent to MSC via the handover required.

By matching the CGI of the target cell, MSC searches the target cell. Once the cell is found,MSC will confirm which BSC is belonged to, and send the handover request message tothis BSC.

If there is no CGI of the target cell in local MSC, MSC will check “Adjacent MSC Table" andsearch the target MSC, then send the handover request message to that MSC.

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Difference between " Internal Handover Success Ratio per Cell " and " Intra-BSC Radio

Handover Success Rate " :

As viewed from formulas, both numerators are success times of handover, while thedenominators are different. While viewed from the measurement points of the counter,

intra-cell/inter-cell handovers requests >= intra-cell/inter-cell cell handover commands orresponses, so Internal Handover Success Ratio per Cell <= Intra-BSC Radio Handover

Success Rate.

In practice:

If "Internal Handover Success Ratio per Cell“ = "Intra-BSC Radio Handover Success Rate",this indicates that there is no problem with data, the unsuccessful handovers are caused by

radio interfaces, and the radio reasons like interference and coverage should be checked inthe first place.

If "Internal Handover Success Ratio per Cell“ < "Intra-BSC Radio Handover Success Rate",

this indicates that failures may exist in the process from “Channel_Activate” to "HO-COMD", and there might be data problem or congestion.

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The key measurement points are as follows, same as those of intra BSC handover:

1. After sending "HO-REQUIRED", the original BSC measures "Attempted outgoing

inter BSC inter cell handovers".

2. After receiving "HO-REQUEST", the target BSC measures "Attempted incoming

inter BSC handovers".

3. After sending "HO-REQUEST ACK", the target BSC measures "incoming inter BSChandovers".

4. After receiving "HO-Command" , the original BSC measures "outgoing inter BSChandovers".

5. After receiving "HO-Complete", the target BSC measures "Successful incominginter BSC handover"

6. After receiving "Clear-COM" and the cause value is "HO-Successful", the originalBSC measures "Successful outgoing inter BSC inter cell handover".

The difference between "handover times" and "handover request times":

Handover times - After "HO-COM" is received or "HO-REQ-ACK" is sent

Handover request times - After "HO-REQUIRED" is sent or "HO-REQUEST" isreceived

Therefore, inter BSC inter cell radio handover success rate >= inter BSC inter cellhandover success rate.

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Note: Signaling flow of A interface and Abis interface of inter MSC handover is the same

as that of intra MSC handover, only the signaling between two MSC is particular for the

inter-MSC handover. As shown in above figure the signaling with "MAP" is of the MAPlayer, and signaling of A and Abis interfaces are omitted.

After receiving "HO-REQUIRED" of BSC-A (the request message includes CGI of the originalcell and target cell), if MSC-A finds that LAC of the target cell doesn’t belong to this MSC,

MSC-A will query the "REMOT LAC Table“ (including the LAC and route of the adjacentMSC), and send "Prepare-HO" message to MSC-B according to the route. CGI of the target

cell and the indicator of whether to allocate the handover number are included in thisPrepare-HO message.

According to the received "Prepare-HO" message, if the handover number needsallocating, MSC-B will request the local VLR to allocate the handover number. If VLR has

the free handover number, the handover number will be sent to MSC-B through "Send-HO-Report". If no handover number is needed, proceed to the next step.

After SCCP link between MSC-B and BSC-B is established, MSC-B sends "HO-REQUEST" toBSC-B. After that, BSC-B activates the target cell’s channel, and returns "HO-REQUEST

ACK" to MSC-B after receiving the channel activation acknowledgement. According to thismessage, MSC-B sends "Prepare-HO ACK" to MSC-A.

MSC-A establishes the route to MSC-B according to the handover number, and sends

"Initialize-Address" (IAI) to MSC-B to help the latter to identify which voice channel isreserved for MS. While MSC-B returns "Address-Complete" (ACM) to MSC-A.

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Frequent handover: Handover in GSM is hard handover, it is meaning, before handover to

the new channel, MS must release the original channel firstly. So frequent handover will

result in word-loss in handover and break-make of conversation, thus affecting theconversation quality.

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In isolated cell coverage area, when MS moves towards the cell edge, the signal becomes

weaker and weaker, and since there is no adjacent cell available around, handover cannot

be triggered, thus call drop will occur.

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For BTS clock, BSC clock, and MSC clock, the following standards are used to judge

whether they are out of synchronization:

MSC:△f/f (frequency deviation) ≤ 1E-8

BSC/BTS:△f/f (frequency deviation) ≤5E-8

Inaccurate clock will cause that MS cannot decode BSIC of the adjacent cell correctly.

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For service cell: Outgoing inter cell handover will fail.

For illusory target cell: Incoming inter-cell handover will fail, and successful rate of

handover = successful rate of radio handover.

Notes:

CI can not be configured as "FFFF".

"Transmitting BS/MS Power Level": If the measurement report preprocess isenabled, this parameter must be set to "Yes".

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The following possibilities can lead to the problem that a call is dropped, that there is no

signal during call drop, and that the signal is recovered after several seconds:

The most common possibility is that there is no neighbor cell handover relation, soan MS cannot be handed over to the neighbor cell, hence the call drops. After the

call drops, the MS re-selects a cell and resides in a good cell, so the signal isrecovered.

Isolated island caused by cross-coverage also leads to this problem. When a signalexceeds the coverage, an MS uses the signal even when the MS is far away from

the cell that provides the signal, but at this time there is no cell that is a neighborof the cell. When the cell signal becomes weak, handover is impossible, leading to

the call drop. After the call drop, the MS re-selects a good cell and displays signalagain.

At that place, the signal is indeed too weak to maintain conversation due togeographical factors of buildings and mountains. When an MS goes to that place,

call drop occurs. After the MS passes the place, the signal is recovered again. Inthe urban area where there are many BTSs, this problem does not happen. In the

suburb area where only one BTS is used for coverage, this problem sometimesoccurs.

At that place, due to buildings, the signal of a cell suddenly fades drastically, and

the MS has not time to hand over, hence the call drop. After the call drop, the MSresides in a good cell again.

Signal is interrupted due to the BTS equipment or intermittent transmission.

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Cell 1 of BTS C provides coverage along the road and the signal reaches far away.

When a vehicle drives very fast, sometimes the MS has not time to hand over to

cell 3 of BTS B or cell 2 of BTS A. Cell 1 of BTS C is far away from cell 3 of BTS Dand the two cells are not defined as neighbor cells. Thus an isolated island isgenerated, handover cannot be performed, and call drop arises. The special thing

about this isolated island effect is that it occurs at high speed. Thereforetroubleshooting is difficult.

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 handoverdecision time is reduced so that the handover can happen timely.

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Signaling tracing is very helpful if there are problems in cooperation with other

manufacturers’ equipment.

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To solve the problem of low handover attempts, we firstly compared the handover timeswith other BSCs under the same scenario and found out the attempts are less than other

BSCs even under the same condition, in addition, we also checked the neighbor cellconfiguration to see whether there are enough neighbors to ensure the handover properand prompt, for some cells, there were some neighboring cells missing, but after adding,the problem still existed, and we checked the parameters, found out There is a parameter"NCC permitted" relative to the low handover times, the definition of this parameter is asfollows:

NCC permitted

Range: 0 allowed~7 allowed

Default: 1111111

Description: Network color code permitted. The value 1 stands for permitted and 0for forbidden. This parameter is sent in system information 2 and 6. When the

cell's NCC is consistent with the value of NCC permitted, then this cell will bemeasured by MS. And MS will report the measurement report to BTS. Thisparameter consists of one byte (8bit). Each bit is corresponding to an NCC (0~7)and the last bit is corresponding to NCC 0. If bit N is 0, then MS will not measurethe cell level with NCC being N.

Note:As MS cannot report the adjacent cell information where NCC is set to 0, theincorrect setting of this parameter will cause MS to be unable to hand over duringconversation. See Protocol 0508.

This parameter can be used to make MS 's measurements on some adjacent cellsoptionally.

We found that the configuration in the problem BSC is set to be 11110000, that is,

enabling NCC 4~7 handover, but from the feedback, the neighboring cells' NCC is 0~3.

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(3). The requirements of the PBGT handover are as follows:

The PBGT handover is implemented only between the cells with the same layer and

same level.

The priority of the target cell is higher than that of the serving cell.

The PBGT handover is triggered only over the TCH. In the 16-bit handover algorithm, other bits with higher priorities than the priority

of the level are:

Inter-layer handover threshold adjust bit

Co-BSC/MSC adjust bit

Load adjust bit

Handover layer bit

The priorities of these bits are higher than the priority of the level. If they are set

improperly, the priorities of many neighbor cells with good levels are lower thanthe priority of the local cell. Thus, the PBGT handover fails to be triggered.

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(1). The problem may be caused by interference and coverage. Check the related trafficmeasurement. The interference handover proportion before the swap is low. The traffic

indexes of the interference band before and after the swap are low. The BTS hardware isoperational.The OMC data shows that not all parameters are mapped according to the originalnetwork during the swap from the BSC32 to the BSC6000. All BTSs are configuredaccording to the initial template of the BSC6000.

(2). Check the parameters related to the PBGT handover, PBGT Handover Threshold andDecision Time. The parameters of the handovers with higher priories than PBGT handoverare proper.

(3). Check the settings of the 16-bit priority. The inter-layer handover threshold andhysteresis are greatly different from those configured on the original BSC. For the existingnetwork, the initial threshold is set to 25 and the hysteresis is set to 3. For the original BSC,

the initial threshold is set to 40 and the hysteresis is set to 1. If the following conditions are met, the priority of the neighbor cell is lower than that of

the local cell. Even the level of the neighbor cell is higher than that of the local cell, thePBGT handover fails to be triggered.

The threshold for the receiving level is set to 25.

The level of the neighbor cell is lower than the value of threshold + hysteresis.

The level of the local cell is not lower than the value of threshold - hysteresis.

In addition, the sites covered by the BSC are located in the suburb areas. There is a greatprobability that the receiving level of these sites is 25. Therefore, the settings of 16-bitpriority have great impacts on the PBGT handover.

After the threshold is set to 40 and the hysteresis is set to 1 in batches on the BSC, thePBGT handover proportion increases to 55%-60%, which is close to the original BSC indexvalue.

-82 --88 -71

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(1). When analyzing the handover traffic measurement, you need to take the impacts of

the factors in the handover algorithm on the handover priority and handover triggering

conditions into consideration. (2). The mapping of all parameters at the cell level is essential to the BSC swap. If

parameters are not configured according to the original network, the network indexesmay be affected. There are multiple parameters at the cell level. Therefore, the workload

of analyzing and checking the parameters is large if the indexes related to the trafficmeasurement have problems.

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Since Co-BSC/MSC Adj is set to YES, of the 16bit sequence number,

①.If the serving cell level >= inter-layer handover threshold–hysteresis, which is >=–88

dBm, bit14 is 0, bits 5~10 of the serving cell are the same as those of the neighbor cell, sobits 12, 13 and 14 are all 0.

②.If the serving cell level < inter-layer handover threshold–hysteresis, which is < –88 dBm,bit14 is 1, bits 5~10 of the serving cell are the same as those of the neighbor cell, so bits

12 and 13 are 0, and bit 14 is 1.

③.If neighbor cell level >= inter-layer handover threshold + hysteresis, which is >= –82dBm, bit 14 is 0, bits 5~10 of the internal cells are the same as those of the serving cell, sobits 12, 13, and 14 of the internal cells are all 0, while bits 5~10 of the external cells are

the same as those of the serving cell, so bit 12 is 1, and bits 13 and 14 are 0.

④.If neighbor cell level < inter-layer handover threshold + hysteresis, which is < –82 dBm,bit 14 is 1, bits 5~10, 12, and 13 are all 0, for both internal cells and external cells.

⑤.The actual situation is, when the level of the serving cell is higher than –88 dBm, all themost significant bits of the serving cell are 0. For cells of different BSC, even when the level

of the serving cell is higher than –82 dBm, bit 14 is 0. But because bit 12 is 1, the cells ofdifferent BSC are still sorted after the serving cell and no normal handover can be initiated

(including edge handover, PBGT handover, and inter-layer inter-priority handover). In otherwords, when Co-BSC/MSC Adj is YES and the vicinity of Huawei BTS is BTSs of different

BSCs, if the level of the serving cell is not lower than –88 dBm, handover is very difficult toinitiate.

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⑥.When the level of the serving cell is lower than –88 dBm, bit 14 of the serving cell is 1,bits 5~10, 12, and 13 are 0. If the receiving level of a neighbor cell of different BSC islower than –82 dBm, bit 14 is 1, bits 5~10, 12, and 13 are 0. In this case all the cells are

sorted by the level. If the receiving level of a neighbor cell of different BSC is higher than –82 dBm, bit 14 is 0 and the cell is sorted at the front. That is, if the vicinity of Huawei BTS

is BTSs of different BSCs, when the level of the serving cell is lower than –88 dBm, nomatter if the level of the neighbor cells is greater than –82 dBm, the cells can be correctly

sorted and can initiate normal handover.

⑦.Therefore, if a Huawei BTS is randomly distributed among other BSCs and the vicinity isBTSs of other BSCs, it is recommended to set Co-BSC/MSC Adj to NO.



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11 OMO312010 BSC6000 GSM Handover Problem Analysis ISSUE1.01

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