173296829 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 cell

is found, the handover request message which includes the target cell CGI will be sent to

BSC.

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 cell

are 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 to

this BSC.

If there is no CGI of the target cell in local MSC, MSC will check Adjacent MSC Table" and search 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 the

denominators are different. While viewed from the measurement points of the counter,

intra-cell/inter-cell handovers requests >= intra-cell/inter-cell cell handover commands or

responses, so Internal Handover Success Ratio per Cell

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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 BSC

handovers".

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

handovers".

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

inter BSC handover"

6. After receiving "Clear-COM" and the cause value is "HO-Successful", the original

BSC 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" is

received

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

handover 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 MAP

layer, and signaling of A and Abis interfaces are omitted.

After receiving "HO-REQUIRED" of BSC-A (the request message includes CGI of the original

cell and target cell), if MSC-A finds that LAC of the target cell doesnt belong to this MSC, MSC-A will query the "REMOT LAC Table (including the LAC and route of the adjacent MSC), 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 this

Prepare-HO message.

According to the received "Prepare-HO" message, if the handover number needs

allocating, 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" to

BSC-B. After that, BSC-B activates the target cells channel, and returns "HO-REQUEST ACK" to MSC-B after receiving the channel activation acknowledgement. According to this

message, 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 is

reserved 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 the

conversation 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 is

enabled, 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, so

an 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 is

recovered.

Isolated island caused by cross-coverage also leads to this problem. When a signal

exceeds 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 neighbor

of 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 signal

again.

At that place, the signal is indeed too weak to maintain conversation due to

geographical 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. In

the 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 sometimes

occurs.

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 MS

resides in a good cell again.

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

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The customer service technicians go to the site to perform drive test and discovers

that the road from south to north is covered by the signals from four cells, which

are cell1 (northward) of BTS C, cell 3 (southwestward) of BTS B, cell 2

(southeastward) of BTS A, and cell 3 (southwestward) of BTS D.

In the complained place the signal of several cells is strong and is -70 to -80 dBm.

It is unlikely that the problem is caused by bad signal. The drive test shows that the

handover relation is normal. The drive route is such that cell 1 of BTS C is first

occupied; then the vehicle drives northward and the MS can hand over to cell 3 of

BTS B and cell 2 of BTS A. As the vehicle drives on northward, the MS hands over

to cell 3 of BTS D. Multiple drive tests along the road show normal result and the

problem complained by the subscriber does not occur.

To investigate the problem, we obtain further information from the subscriber and

know that the driver at that time drove very fast. Therefore we think this problem

may occur under high speed movement. We perform drive test again under high

speed for several times and the call drop recurs.

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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 D

and the two cells are not defined as neighbor cells. Thus an isolated island is

generated, handover cannot be performed, and call drop arises. The special thing

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

troubleshooting 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 handover

decision 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 times with 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 cell configuration to see whether there are enough neighbors to ensure the handover proper and 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 as follows:

NCC permitted

Range: 0 allowed~7 allowed

Default: 1111111

Description: Network color code permitted. The value 1 stands for permitted and 0 for 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 be measured by MS. And MS will report the measurement report to BTS. This parameter 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 measure the cell level with NCC being N.

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

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

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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(1). The PBGT handover algorithm is used to find a cell that meets the following conditions

on a real-time basis and determine whether to perform a handover:

The cell has less path loss.

The level of the neighbor cell is higher than the threshold for the local cell.

The cell meets the system requirements.

PBGT Handover Proportion = PBGT Handover Requests/Total Handover Requests

(2). The possible causes of the decrease in the PBGT handover proportion are as follows:

The number of other handovers with a higher priority than that of the PBGT

handover increases. The possible causes are the interference or coverage problems.

The system parameters that the PBGT handover requires change, thus causing an

increase in the number of requirements for the PBGT handover and a decrease in

the number of PBGT handover attempts.

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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 than

the 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 traffic measurement. 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 is operational. The OMC data shows that not all parameters are mapped according to the original network during the swap from the BSC32 to the BSC6000. All BTSs are configured according to the initial template of the BSC6000.

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

(3). Check the settings of the 16-bit priority. The inter-layer handover threshold and hysteresis are greatly different from those configured on the original BSC. For the existing network, 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, the PBGT 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 great probability that the receiving level of these sites is 25. Therefore, the settings of 16-bit priority 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, the PBGT handover proportion increases to 55%-60%, which is close to the original BSC index value.

-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 indexes

may 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 traffic

measurement have problems.

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

.If the serving cell level >= inter-layer handover thresholdhysteresis, which is >=88 dBm, bit14 is 0, bits 5~10 of the serving cell are the same as those of the neighbor cell, so

bits 12, 13 and 14 are all 0.

.If the serving cell level < inter-layer handover thresholdhysteresis, 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.

.If neighbor cell level >= inter-layer handover threshold + hysteresis, which is >= 82 dBm, bit 14 is 0, bits 5~10 of the internal cells are the same as those of the serving cell, so

bits 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.

.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.

.The actual situation is, when the level of the serving cell is higher than 88 dBm, all the most 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 of different 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 other

words, 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 to initiate.

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.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 is

lower 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, no matter if the level of the neighbor cells is greater than 82 dBm, the cells can be correctly sorted and can initiate normal handover.

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



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Documents

173296829 11 OMO312010 BSC6000 GSM Handover Problem Analysis ISSUE1 01