umts handover performance optimization

Product Type Technical Proposal

UMTS Handover Performance Optimization Guide

R1.0

LEGAL INFORMATION

By accepting this certain document of ZTE CORPORATION you agree to the following terms. If you do not agree to the following terms, please notice that you are not allowed to use this document.Copyright 2010 ZTE CORPORATION. Any rights not expressly granted herein are reserved. This document contains proprietary information of ZTE CORPORATION. Any reproduction, transfer, distribution, use or disclosure of this document or any portion of this document, in any form by any means, without the prior written consent of ZTE CORPORATION is prohibited. and are registered trademarks of ZTE CORPORATION. ZTEs company name, logo and product names referenced herein are either trademarks or registered trademarks of ZTE CORPORATION. Other product and company names mentioned herein may be trademarks or trade names of their respective owners. Without the prior written consent of ZTE CORPORATION or the third party owner thereof, anyones access to this document should not be construed as granting, by implication, estopped or otherwise, any license or right to use any marks appearing in the document.The design of this product complies with requirements of environmental protection and personal security. This product shall be stored, used or discarded in accordance with product manual, relevant contract or laws and regulations in relevant country (countries).This document is provided as is and as available. Information contained in this document is subject to continuous update without further notice due to improvement and update of ZTE CORPORATIONs products and technologies.

ZTE CORPORATION

Address:NO. 55Hi-tech Road SouthShenZhenP.R.China518057

Website:http://dms.zte.com.cn (Technical Support)

Email:[email protected]

Revision HistoryProduct VersionDocument VersionSerial NumberReason for Revision

R1.0First published

AuthorDateDocument VersionPrepared byReviewed byApproved by

2010-01-10R1.0Wang JianWang ZhenhaiJin Zhengtuan

Applicable to: UMTS network optimization engineersProposal: Before reading this document, you had better have the following knowledge and skills.SEQKnowledge and skillsReference material

1NullNull

2

3

Follow-up document: After reading this document, you may need the following information.SEQReference materialInformation

1NullNull

2

3

About This DocumentSummaryChapterDescription

1 Handover Performance Analysis and OptimizationHandover performance analysis

2 Cases of Handover Performance OptimizationCase analysis

TABLE OF CONTENTS11Handover Performance Analysis and Optimization

11.1DT Optimization Analysis

11.1.1Causes for Intra-Frequency Handover Failures

121.1.2Corner Effect

171.1.3Causes for Inter-Frequency Handover Failures

171.1.4Causes for Inter-RAT Handover Failures

181.2Optimization of the Traffic Statistics

191.2.1Causes for Soft Handover Failure

221.2.2Causes for Inter-Frequency Handover Failure

251.2.3Analysis of the Reason for the Outgoing CS Inter-RAT Handover Failure

301.2.4Analysis of the Reason for the Outgoing PS Inter-RAT Handover Failure

322Cases of Handover Performance Optimization

322.1Cases of Intra-RAT Handover Optimization

322.1.1Handover Failure and Call Drop Caused by Corner Effect

362.1.2Handover Failure and Call Drop Caused by Pinpoint Effect

402.1.3Handover Failure and Call Drop Caused by Insufficient PSC Reuse Distance

472.1.4Handover Failure and Call Drop Caused by Limited System Resources

482.2Inter-RAT Handover Cases

482.2.1Handover Failure Caused by Improper Intra-RAT Handover Parameter Configuration

522.2.2Handover Failure Caused by Wrong Configuration of Intra-RAT Neighboring cells

542.2.3Inter-RAT PS Domain Handover Failure Caused by No Routing Between SGSNs

FIGURES1Figure 1-1 Analysis flow for intra-frequency handover failures

3Figure 1-2 Judging missing neighboring cells by Best SC of the UE and the Scanner (1)

4Figure 1-3 Judging missing neighboring cells by Best SC of the UE and the Scanner (2)

5Figure 1-4 Pilot set of the UE before the call drop

5Figure 1-5 Pilot set of the UE after the call drop

6Figure 1-6 Using pilot pollution analysis algorithm 2 to check for missing neighboring cells

7Figure 1-7 Parameter setting for pilot pollution analysis algorithms

8Figure 1-8 Example of pilot pollution analysis algorithm one

8Figure 1-9 Example of pilot pollution analysis algorithm two

9Figure 1-10 Example of pilot pollution analysis algorithm three

10Figure 1-11 Analysis chart of pilot pollution algorithm three

13Figure 1-12 Corner effect and signal changes

15Figure 1-13 Pinpoint effect and signal changes

19Figure 1-14 Handover traffic statistic analysis flow

20Figure 1-15 Soft handover failure analysis flow

22Figure 1-16 Inter-frequency handover failure analysis flow

33Figure 21 Signaling out of the elevator before optimization

33Figure 22 Signaling in the elevator before optimization

34Figure 23 Analysis of the call drop caused by corner effect

35Figure 24 Signaling out of the elevator after the optimization

35Figure 25 Signaling at the moment of the elevator being closed after optimization

37Figure 26 Handover and call drop caused by Pinpoint effect (The main service cell is PSC53 in moment 1)

37Figure 27 Handover and call drop caused by Pinpoint effect (PSC53 cell leaves the active set in moment 2)

38Figure 28 Handover and call drop caused by Pinpoint effect (Call drop happens because the PSC53 cell cannot join in the active set in moment 3)

38Figure 29 Handover and call drop caused by tip effect

39Figure 210 Main service cell PSC53

40Figure 211 Main service cell changed to be PSC48

40Figure 212 Main service cell changed to be PSC53

41Figure 213 Call drop near Yi Yuan Restaurant

42Figure 214 Distribution diagram of the NodeB and scrambling near Yi Yuan Restaurant

43Figure 215 Signaling of PSC79 entering the active set in UE log

43Figure 216 Corresponding RNC signaling

44Figure 217 Active set list

44Figure 218 Reported Event 1B

45Figure 219 Call drop caused by out of sync of the Radio Link

46Figure 220 Screenshot of Signaling near YiYuan Restaurant after adjustment

49Figure 221 Site diagram of Tai Lam Tunnel

49Figure 222 Indicator change after the 2D/2F parameter modification

52Figure 223 Call drop caused by the inter-RAT handover failure

53Figure 224 GSM cell information delivered by the system

54Figure 225 Actual GSM cell information (BCCH=124)

54Figure 226 Normal Inter-RAT handover after the GSM neighboring cell update

55Figure 227 Background signaling 1

56Figure 228 Background signaling 2

TABLES16Table 11 Handover Failure Scenarios

21Table 12 Causes for RL Addition/Setup Failure on the Iub Interface

21Table 13 General Causes for Active Set Update Failure

24Table 14 Reasons for Uu Interface Handover Failure

26Table 15 Relevant Counters in the Outgoing CS Inter-RAT Handover Preparation Process

28Table 16 Relevant Counters During the Outgoing CS Inter-RAT Handover on the Uu Interface

30Table 17 Reasons for the Outgoing PS Inter-RAT Handover Failure

50Table 21 Indicator Change After the 2D/2F Parameter Modification

1 Handover Performance Analysis and Optimization1.1 DT Optimization Analysis

1.1.1 Causes for Intra-Frequency Handover Failures

Figure 1-1 Analysis flow for intra-frequency handover failures

1.1.1.1 Missing Neighboring cells

Judgment of missing neighboring cells

Neighboring cells refer to cells that may enter the active set, and cells that the UE may reselect as the serving cell or may be handed over to. When the UE is at idle or FACH state, it can read the neighboring cell information from the system information, and it will measure these cells when the conditions are fulfilled. Therefore, missing neighboring cells may cause that the cell reselection cannot be finished in time, which further leads to call setup failure. When the UE is at DCH state, the RNC will deliver the neighboring cell information to the UE through the measurement control command, and instruct the UE to measure all the neighboring cells that it may be handed over to. In this condition, missing neighboring cells may lead to call drops. Generally, the following three methods can be used to judge missing neighboring cells:

1.Best SC of the UE and Best SC of the Scanner

Missing neighboring cells may lead to call drops. If both the UE and the Scanner are used in the test, we can check whether there are any missing neighboring cells by comparing the Ec/Io of the optimal serving cell in the active set of the UE and the Ec/Io of the optimal serving cells measured by the Scanner. If the Ec/Io of the optimal serving cell within the active set of the UE is poor, while that measured by the Scanner is good, and the PSCs of the optimal serving cell recorded by the Scanner is not included in the Measurement Control signaling delivered by the RNC, it is certain that the some neighboring cells are missing.

As shown in the figure below, on one road, the Best SC by Ec/Io measured by the UE is poor, lower than -11dB, while that recorded by the Scanner is good, higher than -8 dB. The optimal serving cell recorded by the Scanner is 194 and 409, while that of the UE is 160. Through the Measurement Control signaling received by the UE, we find out that Cell 194 and cell 409 are not included in the neighboring cell list. From this, it is safe to ascertain that some neighboring cells are missing.

Figure 1-2 Judging missing neighboring cells by Best SC of the UE and the Scanner (1)

Figure 1-3 Judging missing neighboring cells by Best SC of the UE and the Scanner (2)

2.Pilot set of the UE

If only the UE is used in the test, we can judge whether there are missing neighboring cells by checking the pilot set changes before and after the call drops. It is safe to ascertain that some neighboring cells are missing when the following three conditions are all met: 1. One cell with strong signals exists in the detective set around the call drop point; 2. The PSC of the cell that the UE accesses after the call drop is different from the one that the UE accessed before the call drop; 3. There is no record about this cell in the last Measurement Control message that the UE received before the call drop.

As shown in the figure below, Cell227 with strong pilot strength is included in the detective set. After the call drop, the UE accesses Cell227. However, Cell227 is excluded in the neighboring cell list delivered by the last Measurement Control message. Then, we can ascertain that Cell227 is the missing neighboring cell.

Figure 1-4 Pilot set of the UE before the call drop

Figure 1-5 Pilot set of the UE after the call drop

3.CNA pilot pollution analysis algorithm 2

CNA pilot pollution analysis algorithm 2 is used to check the test data of the UE. Pilot pollution refers to the condition that the pilots that exceed the pilot pollution threshold are not included in the active set after certain lag time. When the UE is used in the test, the condition that the missing neighboring cells that cannot join the active set in a short period of time deteriorates the Ec/Io of the serving cell and generates extra interference. The pilot pollution analysis algorithm 2 can speedily locate the potential area with missing neighboring cells. Then, the pilot set of the UE can be used to check whether the problem exists.

As shown in the figure below, you can locate the pilot pollution area by pilot pollution analysis algorithm 2, and then use the UE to judge whether the pilot pollution is caused by missing neighboring cells. From the pilot set of the UE, we can judge that Cell194 is the missing neighboring cell.

Figure 1-6 Using pilot pollution analysis algorithm 2 to check for missing neighboring cells

Solution for missing neighboring cells

Once the missing neighboring cells are detected, you should add the cells to the neighboring cell list in the OMC-R. Note that more configured neighboring cells does not necessarily represent the network performance is better. It is the quality of the neighboring cells that impacts the network performance. If too many neighboring cells are configured, the neighboring cell searching period will be prolonged, and then the equipment performance will be impacted; on the contrary, if some neighboring cells are missing, unnecessary interference will exist, and call drop may occur. At the initial stage of network construction, the network engineers set the neighboring cell relation based on field inspection and distribution of the base stations. After the network is put into commercial operation, the network load is added with the increase of subscribers. In this condition, the engineer can optimize the neighboring cell configuration by tracing the detected set and MR.

1.1.1.2 Pilot Pollution

Judgment of pilot pollution

Pilot pollution is the most common problem in UMTS. In plain terms, it refers to the condition that the pilot signals received from different cells are similar (either strong or weak) at one testing point, and there is no primary pilot signal. At present, most UMTS terminals support a maximum of three active sets. In other words, if more than three cells have similar Ec/Ios, the three RLs in the active set will be interfered.

Pilot pollution is generally caused by improper design of the network coverage. The following causes may lead to poor coverage: overshooting of high sites, ring-shaped NodeB distribution, wave-guide effect, and large reflector. All these factors lead to the deformation of signals. Areas with severe pilot pollution will have low call setup success rate, low call setup success rate for high-speed data services, high handover failure rate, and compromised capacity.

CNA has defined three algorithms to calculate the pilot pollution. Click Tools > Analysis Parameter Setup to set the parameters related to the algorithms. These algorithms can speedily and accurately analyze the pilot pollution problem, and timely locate the pilot pollution area.

Figure 1-7 Parameter setting for pilot pollution analysis algorithms

The above figure shows the setting of parameters related to pilot pollution and their default value.

Algorithm one considers the number of pilots in the active set, and is applicable to the test data of the Scanner and the UE. Algorithm one is defined as the number of pilots that has exceeded the pilot pollution threshold and the active set threshold. In algorithm one, pilot pollution is defined as follows: RSCP > -95 dBm (RSCP Threshold), and Ec/Io > 13 dB (Ec/Io Threshold). We can get the number of pilot pollutions by subtracting 3 (active set) from the total number of pilots that have exceeded the pilot pollution threshold. As shown in the figure below, when the parameters related to pilot pollution is set as shown in the figure above, three pilots have exceeded the pilot pollution threshold, and the number of the active set is set to 3, so the number of pilot pollution is 0. If the RSCP Threshold is set to be larger than -105 dBm, and the Ec/Io Threshold is set to be larger than -18 dB, then four pilots have exceeded the pilot pollution threshold. Because the active set is set to 3, the number of pilot pollution is 1.

Figure 1-8 Example of pilot pollution analysis algorithm one

Algorithm two does not consider the number of pilots in the active set, and it is applicable to the test data of the UE. Pilot pollution refers to the condition that the pilots that exceed the pilot pollution threshold are not included in the active set after certain lag time. In algorithm two, pilot pollution is defined as follows: RSCP > -95 dBm (RSCP Threshold), and Ec/Io > 13 dB (Ec/Io Threshold). Number of the pilot pollutions refers to the pilots that have exceeded the pilot pollution threshold, and are not included in the active set after certain lag time.

When the parameters related to pilot pollution is set as shown in the figure below, three pilots have exceeded the pilot pollution threshold. After 1 s, pilot 222 and pilot 123 still are not included in the active set, so the number of pilot pollution is 2.

Figure 1-9 Example of pilot pollution analysis algorithm two

Algorithm three, defined by China Unicom, is applicable to the test data of the Scanner and the UE. Here, pilot pollution refers to the number of pilots that has exceeded the pilot pollution threshold and the active set threshold. In algorithm three, pilot pollution is defined as follows: RSCP > -100 dBm (RSCP Threshold), and Ec/Io > 1BestServingCell - 5dB (Ec/Io Threshold). We can get the number of pilot pollutions by subtracting 3 (active set) from the total number of pilots that has exceeded the pilot pollution threshold.

As shown in the figure below, when the parameters related to pilot pollution is set as shown in Figure 1-7, six pilots have exceeded the pilot pollution threshold. Because the active set is set to 3, the number of pilot pollution is 3.

Figure 1-10 Example of pilot pollution analysis algorithm three

Note:The parameter settings of the three algorithms are different, so their conclusions also may differ. Pilot pollution analysis can help to locate the area with possible pilot pollution problems. Therefore these three algorithms with different parameter settings can be used to analyze the condition of the network from different aspects. Then, we can put forward a more customized network optimization solution. For China Unicom projects, it is recommended to use algorithm three to evaluate the pilot pollution level.

The following figure shows the analysis chart of pilot pollution algorithm three. 0, 1, 2, and 3 represent the number of pilot pollution.

Figure 1-11 Analysis chart of pilot pollution algorithm three

Solution for pilot pollution

To solve the pilot pollution problem, we have to find one main coverage pilot within the pilot pollution area, which can be performed by the following steps:

1.Add base stations.

If pilot pollution happens in areas with strong coverage signals, reconstruct the ambient base stations to omni cells or 2-sector base stations to reduce the number of pilots; if pilot pollution happens in areas with poor coverage signals, add a new base station in the polluted area. It is common to add base stations in the pilot polluted areas. The working principle is as follows: the path loss of a newly constructed site will be far less than that of the cells in the pilot polluted areas owning to the distance causes. So the pilot power of this site will obviously larger than that of the other sites, and the new site will work as the main serving cell of the UE. For the sectors of the original sites, the introduction of the new site helps to increase the Io value of this area, so the Ec/Io reduces, and the pilot pollution problem is solved. This method has its own shortcomings, for example, it may waste some resources, PSC resources, or even capacity. Whats more, it makes the PSC planning become more complicated, and increase the investment.

2.Adjust TX power of the cell.

Increase or decrease the TX power of one or multiple cells within the pilot polluted area, then a main pilot will appear. If the TX power of the cell is reduced, the Io value within the area also will reduce. Then, the EC/Io will be increased under the precondition that the power of the rest pilots remains unchanged. In this way, you can enlarge the difference between the main pilot and the rest PSCs in Ec/Io, thereby removing the pilot pollution. ZTE has proved that there will not be significant changes in the cell capability when the TX power of the pilot is reduced.

Similarly, increase of the TX power of one or two sectors can help to increase the Ec/Io of the sector(s), and then Ec/Io of the other sectors is reduced for the Io value is increased. Thereby, the pilot pollution problem is solved. Different from reducing the TX power, when you increase the TX power of the sectors, you must ensure that this operation will not generate extra interference to other cells or even new pilot pollutions in other areas. All these problems may appear.

Surely, this adjustment method also has its own shortcomings:

If the power of the pilot increases, the sync channel power and paging channel power also increase, and then the traffic channel power will reduce.

If the pilot power is reduced, the penetration of the signals will be greatly reduced, and then the communication quality will be affected. Therefore increasing rather than decreasing of the power is recommended.

Because the TX power of the sector is adjusted, the coverage of the cell and ambient cells may be affected. When optimizing the pilot pollution issue, you must fully consider the impact of this operation on system coverage.

3.Adjust the antenna parameters.

This mainly refers to the azimuth and downtilt. The solution is to generate a main pilot signal strong enough for the pilot pollution area, and to reduce the pilot quality of the other signals. The adjustable range of the antenna downtilt is small. Therefore you can adjust the downtilts of multiple cells to expand the adjustment range. This may also influence the coverage of the adjusted cell and its ambient cells. Compared with adjusting the TX power of the sectors, adjusting the antenna downtilt will have limited influences on coverage. Only the coverage of the ambient cells will be slightly impacted for the cell breathing effect.

Similarly, if you adjust the azimuth adequately, the signal power of this sector in the polluted area will decrease or increase, then the difference among the signal power of different sectors will be enlarged, and the pilot pollution problem can be solved. However, it is not easy to control the TX power of the sector within the range by adjusting the sector azimuth. Whats more, the coverage of this sector may be affected. Actually, azimuth is mainly used to adjust the coverage.

From the reasons above, we can see that adjusting antenna azimuth and downtilt are good optimization methods to solve the pilot pollution problem. It requires few project loads, and has little impact on system coverage. It is recommended to use the planning software with advanced algorithms and high accuracy to do the simulation. During the DT, several engineers can work together to modify the parameters.

4.Replace the original antenna with Remote Electrical Tilt (RET).

When the downtilt of mechanical antennas is increased, the antenna beam will be flattened, and more signals cover the side lobe. Then coverage of the side lobe to other cells will increase, and the new pilot pollution may occur. For the RET, this kind of problem never appears. Therefore, you can replace the mechanical antenna with RET when the other methods do not work.

5.Adjust handover parameters.

From the handover aspect, you can increase the handover threshold properly so signals with poor Ec/Io cannot enter the active set. However, this may cause the problem that the handover cannot be completed timely, and the handover failure rate may arise.

1.1.2 Corner Effect

Description

Corner effect: After the UE reports that the target cell satisfies the requirement for Event 1A or Event 1C, the Ec/Io of the source cell decreases abruptly, while that of the target cell increases abruptly. From the background signal tracing we can see that the RNC delivers the active set upgrade command after receiving the measurement report. However, the UE fails to receive this command, and call drop happens.

The figure below is the scenario that the handover happens at the crossroads. The UE moves from cell2 to cell1 at a high speed. The UE reports that cell1 satisfies Event 1A through the link of Cell2. When it crosses the road, signals of Cell2 decreases while that of Cell1 increases, and Ec/Io of Cell2 deteriorates. Through the RNC delivers the command to upgrade the active set, the UE cannot receive the command for the poor RLs of Cell2. Then, the UE cannot perform the handover in time, and call drop happens.

Figure 1-12 Corner effect and signal changes

Solution:

From the above analysis, we can see that corner effect is mainly caused by abrupt drop of the Ec/Io in the source cell and increase of Ec/Io in the target cell. To counter the corner effect, you should include the target cell into the active set before the signal of the source cell drops abruptly. The following methods can be used to counter the corner effect.

1.Adjust cell CIO.

Set the CIO of the target cell to a positive value, so the target cell can report Event 1A as soon as possible, and then it can enter the active set before the signal of the source cell drops abruptly. This setting also makes it difficult for the target cell to report Event 1B, and then the cell handover proportion is increased.

2.Adjust cell R1a (RptRange), H1a (Hysteresis), and Time to Trigger.

By increasing R1a, decreasing H1a and Time to Trigger of the source cell, its neighboring cells can report Event 1A early, and the target cell also can enter the active set early.

Compared with adjusting the cell CIO, adjusting R1a, H1a, and Time to Trigger may influence the handover between the source cell and the neighboring cells.

3.Adjust the RF parameters.

By adjusting the RF parameters, the antenna of the target cell can cover across the corner, so the handover happens before the UE reaches the corner. Or, you can make the antenna of the source cell cover across the corner to avoid the abrupt drop of the signals. Then, the call drop rate reduces. In actual application, the adjustment of the RF parameters and whether the antenna can cover across the corner depend on the experience of the engineers, which makes this method hard to apply.

1.1.2.1 Pinpoint EffectDescription:

Pinpoint effect mainly happens in the shadow area of the buildings or areas where indoor signals are leaked outdoors. As to the signal changes, the Ec/Io of the source cell increases after a certain periods of decreasing, and the Ec/Io of the target cell surges in a short period of time. As to the signaling changes, the UE reports that the target cell satisfies Event 1A or 1C, and then reports that the source cell satisfies Event 1B or 1C. Then, the UE removes the source cell from the active set. However, the source cell reports Event 1A just after the removal, and at this moment, the signal quality of the target cell plunges. The UE cannot receive the active set upgrade command delivered by the RNC, and then handover fails. The figure below shows the typical pinpoint effect and the typical signal changes. There is one building in Cell1, and the area behind the building is the shadow area of Cell1, and the signals from Cell2 are very strong in this area. Initially, the UE is connected to Cell1, and then it moves to the shadow area of Cell1. At moment one, the UE gets through the shadow area of Cell1, Cell2 reports Event 1A, and the UE adds Cell2 to the active set. At moment 2, the signal of Cell1 plunges for the blocking of the building and reports Event 1B, and the UE removes Cell1 from the active set, and then only Cell2 remains in the active set. At moment 3, the UE moves out of the shadow area of Cell1, the signal of Cell1 upsurges, ad reports Event 1A. However the UE cannot receive the active set upgrade command delivered by the RNC for the signal of Cell2 plunges, and the handover fails.

Figure 1-13 Pinpoint effect and signal changes

Solution:

From the above analysis, we can see the pinpoint effect is mainly caused by abrupt drop and the subsequent arise of Ec/Io in the source cell in a short period of time.

Because the signal quality in the source cell only drops in a short period of time, to counter the corner effect, you should ensure that the source cell remains in the active set when its signals drop abruptly. The following methods can be used to counter the pinpoint effect.

1.Adjust cell CIO.

Set the CIO of the target cell to a positive value, which makes it hard for the source cell to report Event 1B, and then it can remain in the active set when its signals drop abruptly.

2.Adjust cell R1a (RptRange), H1a (Hysteresis), and Time to Trigger.

By increasing R1a and Time to Trigger and decreasing H1a of the target cell, you can make it hard for the neighboring cells of the target cell to report Event 1B, and then the source cell can remain in the active set when its signals drop abruptly.

Compared with adjusting the cell CIO, adjusting R1a, H1a, and Time to Trigger may influence the handover between the target cell and its neighboring cells.

3.Adjust the RF parameters.

Adjust the RF parameters, and then the signals both of the source and target cell can become smoother.

1.1.2.2 Conjoint Analysis of DT Data and Background Signaling

Based on the signaling flow, the handover mainly happens in the following scenarios:

Table 11 Handover Failure Scenarios

ScenariosCauses

When the UE enters a new cell, the RNC does not receive the Measurement Report message. Check the RRC messages. If the UE sends the Measurement Report message, while the RNC fails to receive it. The UL is faulty.

Some neighboring cells are missing.

The pilot pollution problem exists.

There are too many cells in the monitoring set of the UE, so the cell searching period is too long.

The moving speed of the UE is fast, which makes the handover detection fails.

The handover area is too small.

The handover parameters are improperly set.

The RNC receives the Measurement Report message, but does not send the Active Set Update message. If the RNC does not send the radio Link Set Up/Addition Request message, the network may be congested or the hardware is faulty.

The DL is faulty.

The RNC sends the Active Set Update message, but the UE fails to receive it.The DL is faulty.

The UE receives the Active Set Update message, but does not respond. The UE is faulty.

The UE sends the Active Set Update Complete/Failure message, but the RNC fails to receive it.The UL is faulty, or the timer for active set update expires, and the connection is released.

The handover-target NodeB does not send the Radio Link Restore Indication message to the RNC. The UL of the target NodeB is not synchronized with the UE.

1.1.3 Causes for Inter-Frequency Handover Failures

Inter-frequency handover mainly happens among the inter-frequency neighboring cells. The place and condition of inter-frequency handovers must be properly set. Generally, if the threshold for enabling the compressed mode of the inter-frequency handover is too high, the UEs frequently enables the compressed modes when they are near the cell edge, and then much signaling overhead are wasted, and system interference is generated. If the threshold is too low, the compressed mode measurement is triggered too late and the handover cannot be performed timely. Then, call drops will occur.

The main cause for the inter-frequency handover failure is that the handover area is too narrow, so the UE reaches the cell edge just after it started the inter-frequency measurement. The handover measurement cannot be finished.

During the indoor/outdoor handover, if the indoor and outdoor cells are at different frequencies, the handover success rate will be lower than the soft handover success rate. To solve this problem, you can use the following methods:

1.Adjust the handover parameters to control the indoor and outdoor handover area.

2.Set the handover areas at places with little traffic.

3.Construct a transitional cell in indoor area, so the inter-frequency handover can happen indoors.

4.Install the antenna in a proper location and control the power at the antenna port to adjust the area of hard handover.

1.1.4 Causes for Inter-RAT Handover FailuresThe general causes for inter-RAT handover failures are as follows:

1.The quality of UL is poor.

2.The threshold for Event 2D is set improperly.

3.The parameter for Event 3A is set improperly.

4.Some GSM neighboring cells of the current UMTS cell are missing.

5.Too many GSM neighboring cells are configured for the UMTS cell.

6.No radio resources are available for the target GSM cell.

The optimization of the handover between UMTS and GSM is to optimize the boundaries of the UMTS and GSM networks. For example, if the GSM cell can provide favorable signals at the UMTS boundary, the handover to GSM is favored. If the GSM signals are weak, the failure possibility of inter-RAT measurement or signaling interaction will increase. Then, call drops will occur. Therefore the signal coverage within the UTMS network should be continuous, and the number of weak coverage areas or blind areas should be minimum. And the inter-RAT handovers should happen at the boundary of the UMTS network, and the times should be minimum. In addition, the inter-RAT handovers should happen in areas with small population density, and the handover times should be minimum. This can avoid the signaling interaction delay or failure caused by insufficient processing capability, which may further lead to call drops.

1.2 Optimization of the Traffic Statistics

After the network is put into commercial operation and enters the O&M stage, the engineers can analyze the network performance by monitoring the traffic statistics, with DT data as the assistance.

The following figure displays the handover traffic statistic analysis flow.

Figure 1-14 Handover traffic statistic analysis flow

1.2.1 Causes for Soft Handover Failure

The soft handover failure can be analyzed from the following aspects: association data, foreground and background signaling tracing, and association KPI analysis. The following figure displays the flow to analyze the soft handover failure.

Figure 1-15 Soft handover failure analysis flow

The handover procedure contains three stages: handover decision stage, handover preparation stage, and handover on the Uu interface.

Handover decision stage

The UE reports Event 1A, 1B, and 1C, and then the RNC admission control module determines whether to let the UE to be handed over to the target cell based on the current system resources of the target cell. If the target cell can provide sufficient resources, the RNC triggers the handover preparation. The RNC sends the Radio Link Setup Request message to request the NodeB to build the corresponding RL; if the target cell cannot provide sufficient resources, the RNC triggers the congestion control flow, and postpones the handling of the events. You can check whether the handover failure is caused by the congested target cell by the handover congestion rate.

Handover preparation stage

The RNC receives the measurement report sent by the UE, and then instructs the target NodeB to build the corresponding RLs through the Radio Link Setup Request message. After the RLs are successfully built, the handover on the Uu interface begins. If the NodeB returns the Radio Link Setup Failed message, the handover preparation fails. The causes that may lead to handover preparation failure are included in the following table.

Table 12 Causes for RL Addition/Setup Failure on the Iub Interface

Causes for RL Addition/Setup Failure on the Iub Interface

TNBAP_ul_sf_not_supported

TNBAP_dl_sF_not_supported

TNBAP_cm_not_supported

TNBAP_transport_resource_unavailable

TNBAP_requested_configuration_not_supported

TNBAP_unspecified_2

No response

Handover on the Uu interface stage

After the target NodeB finishes building the RLs, the RNC instructs the UE to perform the handover through the Active Set Update signaling. After the handover is completed, the UE reports the Active Set Update Complete message. If the UE returns Active Set Update Failed or the RNC receives no response from the UE, then the handover on the Uu interface also fails. The causes that may lead to handover failure on the Uu interface are included in the following table.

Table 13 General Causes for Active Set Update Failure

General Causes for Active Set Update Failure

Configuration unsupported

Physical channel failure

Incompatible simultaneous reconfiguration

Compressed mode runtime error

Protocol error

Cell update occurred

No response

Current the soft handover success rate only refers to the handover success rate on the Uu interface.

1.2.2 Causes for Inter-Frequency Handover Failure

Figure 1-16 Inter-frequency handover failure analysis flow

The inter-frequency handover procedure contains three stages: handover decision stage, handover preparation stage, and handover on the Uu interface.

The handover decision stage is mainly to judge whether the inter-frequency cells can provide sufficient resources to build the new services.

The inter-frequency handover preparation stage is similar to that of the soft handover.

For the handover on the Uu interface, the RNC adopts the Radio Bearer Reconfiguration\Transport Channel Reconfiguration\Physical Channel Reconfiguration messages to instruct the UE to perform the hard handover, and then the UE reports the Radio Bearer Reconfiguration complete\Transport Channel Reconfiguration complete\ Physical Channel Reconfiguration Complete message after the hard handover is completed. If the UE returns Radio Bearer Reconfiguration Failed\Transport Channel Reconfiguration Failed\Physical Channel Reconfiguration Failed or the RNC receives no response from the UE, then the handover on the Uu interface also fails. The causes that may lead to handover failure on the Uu interface are included in the following table.

Table 14 Reasons for Uu Interface Handover Failure

Failure CourseReason for Active Set Update Failure

Configuration unsupported

If the hard handover of the UE to the target cell is decided to be made, the RNC will send the reconfiguration message to the UE. The reconfiguration message includes multiple kinds of parameter configuration, such as RB, transmission channel, and physical channel. If the parameter configuration is beyond the UEs capability, the UE will return the reconfiguration failure message to the RNC, and the reason for the failure is that the configuration is not supported. If the parameter value is not in the allowed range, or the mutex parameters appear in the message at the same time, or the parameters in the message are not consistent with each other, the UE will send the reconfiguration failure message to the RNC, and the reason is Configuration unsupported.

Physical channel failureIf the hard handover of the UE to the target cell is decided to be made, the RNC will send the reconfiguration message to the UE. If the UE is in the dedicated status before the hard handover, it tries to establish the dedicated physical channel according to the parameter configuration in the message, and start to synchronize the physical layer with that at the network side. If the synchronization fails, the UE will send the RNC the reconfiguration failure message, and the reason is physical channel failure. At the same time, the UE returns to the status before receiving the message.

Incompatible simultaneous reconfiguration

If the hard handover of the UE to the target cell is decided to be made, the RNC will send the reconfiguration message to the UE. If the UE is busy with dealing the other messages when receiving this message, the UE will send the reconfiguration failure message to the RNC, and the reason is incompatible simultaneous reconfiguration.

Compressed mode runtime error

If the hard handover of the UE to the target cell is decided to be made, the RNC will send the reconfiguration message to the UE. If the reconfiguration message carries the compressed mode parameter, and the multi activated transmission gap pattern sequences create the transmission gaps in the same radio frame, the UE will send the reconfiguration failure message to the RNC, and the reason is the compression mode runtime error. One radio frame can only include the transmission gap of one function. The transmission gap is the time slot used for the inter-frequency/inter-RAT measurement in one radio frame.

Protocol error

If the hard handover of the UE to the target cell is decided to be made, the RNC will send the reconfiguration message to the UE. If some parameters in the reconfiguration message do not conform to the protocol (such as grammar mistakes), the UE will send the reconfiguration failure message to the RNC, and the reason is protocol error.

Cell update occurred

If the hard handover of the UE to the target cell is decided to be made, the RNC will send the reconfiguration message to the UE. If the radio link failure makes the UE activate the cell update in the reconfiguration process, the UE will send the message about reconfiguration failure to the RNC after the cell update, and the reason is cell update occurred.

No reply

If the hard handover of the UE to the target cell is decided to be made, the RNC will send the reconfiguration message to the UE, start the timer, and wait for the response from the UE. If the reconfiguration message or the reconfiguration response message is lost in the transmission process, the timer at the RNC side will expire, that is to say, the UE does not response in the waiting time, which is mainly caused by the poor radio environment.

1.2.3 Analysis of the Reason for the Outgoing CS Inter-RAT Handover Failure

Outgoing CS inter-RAT handover includes the following two phases: the outgoing CS inter-RAT handover preparation at the lu interface and the outgoing inter-RAT handover request on the Uu interface. The outgoing CS inter-RAT handover preparation at the lu interface corresponds to the message of relocation preparation, and the outgoing inter-RAT handover requirement on the Uu interface corresponds to the HANDOVER FROM UTRAN COMMAND message.

The outgoing CS inter-RAT handover preparation is the relocation process between the RNC and the BSC. After the RNC receives Event 3A/3C reported by the UE, the RNC judges whether the handover from 3G to 2G is possible. If it is, the RNC will send the Relocation Required message to the CN. Then, the CN will send the handover request to the BSC. If the handover is allowed, the BSC will allocate the necessary resources and returns the confirming message to the CN. The CN sends the RANAP message Relocation Command to the SRNC, and the handover preparation is finished. The relevant counters in the outgoing CS inter-RAT handover preparation process are described in the following table.

Table 15 Relevant Counters in the Outgoing CS Inter-RAT Handover Preparation Process

CounterDescription

Number of attempted relocation

preparation for outgoing CS inter-RAT

handovers, time critical relocationThe measurement object of the counter is the cell. The counter is used to count the number of the inter-RAT CS domain handover outgoing preparation requests and the request cause is Time Critical Relocation. Time critical indicates all emergent cases (such as, transmission resources are blocked at Iub interface). If the relocation is not performed immediately, the call may drop.



handovers, resource optimization relocationThe measurement object of the counter is the cell. The counter

is used to count the number of the inter-RAT CS domain handover outgoing preparation requests and the request cause is Resource Optimization Relocation. Resource optimization indicates that the SRNC re-configures all types of the radio transmission resources according to the maximum utilization rate, such as, code resources. Thus, the UE is handed over into different system.



handovers, relocation desirable for radio reasonsThe measurement object of the counter is the cell. The counter is used to count the number of the inter-RAT CS domain handover outgoing preparation requests and the request cause is Relocation Desirable for Radio Reasons. The UE is handed over into different system because the quality of the radio link is not good at the RNC side but good in another system.

At this time the reason of the accompanied relocation is the radio quality.



handovers, directed retryThe measurement object of the counter is the cell. The counter is used to count the number of the inter-RAT CS domain handover outgoing preparation requests and the request cause is Directed Retry.

The inter-RAT handover occurs because the voice service establishment fails due to the resource congestion at the SRNC side.



handovers, reduce load in serving cellThe measurement object of the counter is the cell. The counter

is used to count the number of the inter-RAT CS domain handover outgoing preparation requests and the request cause is Reduce Load in Serving Cell. The inter-RAT handover occurs because the load at the SRNC side is too high.



handovers, access restricted due to shared

networksThe measurement object of the counter is the cell. The counter is used to count the number of the inter-RAT CS domain handover outgoing preparation requests and the request cause is Access Restricted Due to Shared Networks.

The inter-RAT handover occurs because the cell in the sharing network does not authorize the UE.



handovers, other causesThe measurement object of the counter is the cell. The counter is used to count the number of the inter-RAT CS domain handover outgoing preparation requests and the request cause is not listed above.

This counter will count the number of preparation requests when the inter-RAT handover occurs due to the cause not listed above.

Number of failed relocation


handovers, TRELOCalloc expiryThe measurement object of the counter is the cell. The counter is used to count the number of the inter-RAT CS domain handover outgoing preparation failures and the failure cause is Trelocprep Expiry.

When the SRNC sends the RELOCATION REQUIRED message to the CN, the RNC begins to set the relocation preparation timer after receiving the RELOCATION COMMAND message from the CN, the RNC stops the relocation preparation timer. If the RNC fails to receive the response from the CN after the relocation preparation timer timeout, the SRNC cancels the relocation flow.



handovers, relocation failure in target CN/RNC

or target systemThe measurement object of the counter is the cell. The counter is used to count the number of the inter-RAT CS domain handover outgoing preparation failures and the failure cause is Relocation Failure In Target CN/RNC Or Target System.

After the SRNC sends the RELOCATION REQUIRED message to the CN and if the relocation fails due to the operation error at the CN or 2G side, the CN sends the RELOCATION PREPARATION FAILURE message with this cause value.



handovers, relocation not supported in target

RNC or target systemThe measurement object of the counter is the cell. The counter

is used to count the number of the inter-RAT CS domain handover outgoing preparation failures and the failure cause is Relocation Not Supported In Target RNC Or Target System.

After the SRNC sends the RELOCATION REQUIRED message to the CN and if the incoming relocation is not supported at the 2G side, the CN sends the RELOCATION PREPARATION FAILURE message with this cause value.



handovers, relocation target not allowedThe measurement object of the counter is the cell. The counter is used to count the number of the inter-RAT CS domain handover outgoing preparation failures and the failure cause is Relocation Target not allowed.After the SRNC sends the RELOCATION REQUIRED message to the CN and if the DRNC does not authorize the UE to access the target cell at the 2G side , the CN sends the RELOCATION PREPARATION FAILURE message to the SRNC with this cause value.



handovers, no radio resources available in

target cellThe measurement object of the counter is the cell. The counter

is used to count the number of the inter-RAT CS domain handover outgoing preparation failures and the failure cause is No Radio Resources Available in Target Cell.

After the SRNC sends the RELOCATION REQUIRED message to the CN and if the DRNC cannot allocate the radio resources due to high load, the CN sends the RELOCATION PREPARATION FAILURE message to the SRNC with this cause value.



handovers, traffic load in the target cell higher

than in the source cellThe measurement object of the counter is the cell. The counter is used to count the number of the inter-RAT CS domain handover outgoing preparation failures and the failure cause is Traffic Load In The Target Cell Higher Than In The Source Cell.

After the SRNC sends the RELOCATION REQUIRED message to the CN and if the load in the target cell is higher than that in the source cell, the 2G side rejects the relocation request and the CN sends the RELOCATION PREPARATIONFAILURE message to the SRNC with this cause value.



handovers, unknown target RNCThe measurement object of the counter is the cell. The counter is used to count the number of the inter-RAT CS domain handover outgoing preparation failures and the failure cause is Unknown target RNC.

After the SRNC sends the RELOCATION REQUIRED message to the CN and if the CN has no idea about the 2G BSC and does not know how to access, the CN sends the RELOCATION PREPARATION FAILURE message with this cause value.



handovers, other causesThe measurement object of the counter is the cell. The counter is used to count the number of the inter-RAT CS domain handover outgoing preparation failures and the failure cause is not listed above.

After the SRNC sends the RELOCATION REQUIRED message to the CN and if the cause is not listed above, the CN sends the RELOCATION PREPARATION FAILURE message to the RNC with this cause value.



handovers, no replyThe measurement object of the counter is the cell. The counter is used to count the number of the inter-RAT CS domain handover outgoing preparation failures and the failure cause is no reply.

After the SRNC sends the RELOCATION REQUIRED message to the CN, the CN does not response and the relocation preparation timeout happens.

After the handover preparation is completed, the RNC sends the Handover form Utran command to the UE to start the handover process on the Uu interface. If the UE responses with the Handover form Utran Failed message or the RNC does not receive any response from the UE, the handover on the Uu interface fails. The main reasons for the handover on the Uu interface are described as follows:

Table 16 Relevant Counters During the Outgoing CS Inter-RAT Handover on the Uu Interface

Failure CourseCourse for Active Set Update Failure

Configuration unacceptableIf the hard handover of the UE to the 2G cell is decided to be made, the RNC will send the HANDOVER FROM UTRAN COMMAND message to the UE, and the message includes multiple kinds of parameters. If the UE does not support the configured parameters, for example, the UE does not support the inter-RAT handover in the PS domain, the UE will return the failure message to the RNC, and the reason is Configuration unacceptable.

Physical channel failureIf the hard handover of the UE to the 2G cell is decided to be made, the RNC will send the HANDOVER FROM UTRAN COMMAND message to the UE. After receiving the message, the UE will try to establish connection with the target system. If the connection fails, the UE will reply to the RNC with the failure message, and the reason is physical channel failure, and the UE returns to the status before receiving the message.

Protocol errorIf the hard handover of the UE to the 2G cell is decided to be made, the RNC will send the HANDOVER FROM UTRAN COMMAND message to the UE. If the message includes the parameter not conforming to the protocol, the UE will reply to the RNC with the failure message, and the reason is protocol error.

Inter-RAT protocol errorIf the hard handover of the UE to the 2G cell is decided to be made, the RNC will send the HANDOVER FROM UTRAN COMMAND message to the UE. If cell "Inter-RAT message" in this message includes the parameter not conforming to the protocol,.the UE will reply to the RNC with the failure message, and the reason is Inter-RAT protocol error.

UnspecifiedIf the hard handover of the UE to the 2G cell is decided to be made, the RNC will send the HANDOVER FROM UTRAN COMMAND message to the UE. If the handover fails, and the reason is not listed in the above content, the UE sends the message about the failure with the unspecified reason.

Timer timeoutThe measurement object of the counter is the cell. The counter is used to count the number of the inter-RAT CS domain handover outgoing failures and the failure cause is Inter-RAT handover timeout. If there are several cells in the active set, the measurement object is the best cell with the best Ec/No or RSCP at the SRNC side. If all the cells are at the DRNC side, the counter is not counted. If the hard handover of the UE to the 2G cell is decided to be made, the RNC will send the HANDOVER FROM UTRAN COMMAND message to the UE, start the timer, and wait for the IU RELEASE COMMAND message sent by the CN. If the HANDOVER FROM UTRAN COMMAND message or the response message is lost during the transmission, the timer at the RNC side will expire. And the RNC cannot receive the IU RELEASE COMMAND message sent by the CN, or the response about the failure from the UE.

1.2.4 Analysis of the Reason for the Outgoing PS Inter-RAT Handover Failure

The outgoing PS inter-RAT handover does not have the preparation phase. The outgoing PS handover at the network side is for the UE in the CELL_DCH and CELL_FACH status, and it involves outgoing PS handover process on the Uu interface and the context information acquisition and transmission process at the lu interface.

Outgoing PS handover on the Uu interface corresponds to the CELL CHANGE ORDER FROM UTRAN message, and the UE starts the routing area update flow to the 2G network after it receives the CELL CHANGE ORDER FROM UTRAN message. Then the SGSN launches the context information acquisition and data transmission through the lu interface. Context Information acquisition at the lu interface corresponds to the SRNS CONTEXT REQUEST/ SRNC CONTEXT RESPONSE message, and the data transmission process corresponds to the SRNS DATA FORWARD COMMAND message.

The following conditions indicate the outgoing PS inter-RAT handover failure.

1.The RNC receives the UE CELL CHANGE ORDER FROM UTRAN FAIL message.

2.If the outgoing inter-RAT handover is successful, the cause value of the Iu Release Command message may be Successful Relocation or Normal Release. When the RNC receives the lu connection release command message with cause value Successful Relocation or Normal Release, the outgoing inter-RAT handover is successful.

3.The inter-RAT handover times out. If the UE will be handed over to the 2G cell, the RNC will send the CELL CHANGE ORDER FROM UTRAN message to the UE, start the timer, and wait for the SRNS CONTEXT REQUEST message sent by the CN. If the CELL CHANGE ORDER FROM UTRAN message or the response message is lost in the transmission process, the RNCs waiting will time out, and then the RNC cannot receive the SRNS CONTEXT REQUEST message delivered by the CN, or the failure response from the UE.

Table 17 Reasons for the Outgoing PS Inter-RAT Handover Failure

Failure courseCause for Active Set Update Failure

Configuration unacceptableIf the hard handover of the UE to the 2G cell is decided to be made, the RNC will send the CELL CHANGE ORDER FROM UTRAN message to the UE. The message includes multiple kinds of parameters. If the UE does not support the configuration parameter, the UE will reply to the RNC with the failure message, and the reason is configuration unacceptable. This failure is very common in other countries, and it brings the inter-RAT handover failure for most UEs, except some UEs with excellent compatibility. The main reason is that the 2G equipment does not support SAI cell, and the encryption setting information is not complete. The solution is updating the BSC equipment or using ZTE's CN adaptor.

Physical channel failureThis is the most ordinary failure by now. This failure indicates the T3124 timer timeout and the 2G RACH channel cannot be accessed. The reasons are listed as follows:

1. Incorrect neighbouring cell configuration.

2. Because of the serious uplink interference of the GSM network, the UE cannot be synchronized to the GSM channel. The solution is checking the 2G cells handover success rate and deleting the 2G neighboring cells with poor indicators.

3. The GSM signal of the handover area is very unstable. After the UE reports the Event 3A, the GSM signal become weaker immediately.

4. The handover target refuses the service marketing strategy of the operator, and parts of the users are not allowed to register in the 2G network.

5. The UE is abnormal.

Protocol errorIf the handover of the UE to the 2G cell is decided to be made, the RNC will send the CELL CHANGE ORDER FROM UTRAN message to the UE. If the message includes the parameters not conforming to the protocol (grammar mistake), the UE will send the failure message to the RNC, and the failure reason is protocol error.

UnspecifiedWhen the handover of the UE to the 2G cell is decided to be made, the RNC will send the CELL CHANGE ORDER FROM UTRAN message to the UE. If the handover fails, and the failure reason is not listed above, The content of the failure message returned by the UE is unspecified.

No responseWhen the handover of the UE to the 2G cell is decided to be made, the RNC will send the CELL CHANGE ORDER FROM UTRAN message to the UE, and start the timer to wait the SRNS CONTEXT REQUEST message sent by the CN. If the CELL CHANGE ORDER FROM UTRAN message or the response message is lost during the transmission, the waiting at the RNC side will be timeout, and the RNC will receive neither the SRNS CONTEXT REQUEST message from the CN nor the failure response from the UE.

2 Cases of Handover Performance Optimization

2.1 Cases of Intra-RAT Handover Optimization

2.1.1 Handover Failure and Call Drop Caused by Corner Effect

Problem Description

According to the CQT test, the engineer finds the call drop when the elevator is opened or closed at some indoor distributed sites.

Analysis

The main reasons include that the neighboring cell relation between the cells inside and outside of the elevator is improperly set, and the handover is not processed in time. After the repeated continuous calling test in the elevator where the call drop happens, the engineer finds that the call drop happens at the moment of the elevator being closed. And the test shows that the neighboring cell relation between the cell inside and outside the elevator is properly set, so the reason of the call drop may be the fact that the handover is not processed in time.

The following figure shows the signaling outside of the elevator, which is mainly covered by the cell with PSC 378.

Figure 21 Signaling out of the elevator before optimization

The signaling of the scrambling 368 cell becomes stronger at the moment the UE enters the elevator and its door is being closed, as shown in the following figure.

Figure 22 Signaling in the elevator before optimization

According to the test signaling, this kind of call drop is the typical corner effect. As shown in the following figure, at the moment of the elevator being closed, the signaling quality of the scrambling 378 cell becomes poor, the scrambling 378 cell leaves the active set, but the scrambling 368 cell is not included the active set in time, which causes the call drop.

Figure 23 Analysis of the call drop caused by corner effect

Optimization Plan

Because the area around the elevator is covered by the indoor distributed sites, it is difficult to reconstruct the indoor antenna and feeder system. So the optimization can be done by modifying the parameters. The general optimization plan is that the PSC368 cell should report Event 1A as soon as possible to enter the active set. In order to make the cell handover finished before the elevator is closed, and the offset of the cell in the elevator should be increased from 0 dB to 4 dB.

Optimization Result

Figure 24 Signaling out of the elevator after the optimization

Figure 25 Signaling at the moment of the elevator being closed after optimization

After the adjustment, judging from the signaling, the engineer can view that the scrambling 368 cell has being included in the active set and become the main service cell before the elevator is closed. After the elevator is closed, the scrambling 378 cell leaves the active set because the signal quality becomes poor, however, the scrambling 368 is still the main service cell.

In the area where the signals changes dramatically such as the elevator and the corner, call drop always happens because the handover is not processed in time. If the problem cannot be solved by adjusting the antenna and feeder system, the engineer can adjust the cell offset to achieve the optimization. Based on the recent optimization experience of sever indoor distributed sites, the engineer realizes that it is proper to change the cell offset to 4 dB or 5 dB to solve the problem that the handover in the elevator is not processed in time.

2.1.2 Handover Failure and Call Drop Caused by Pinpoint Effect

Problem DescriptionIn moment 1, the cell phone moves along the route shown by the red arrow, and the handover failure and call drop of the voice service happen. The two related cells are BKC0044U (PSC48) and BKCOO74U (PSC53). At the beginning, the cell is in the macro diversity status, the main serving cell is the third cell (PSC53) of site BKC0074U, and the pilot quality Ec/Io is -9.83 dB.

Figure 26 Handover and call drop caused by Pinpoint effect (The main service cell is PSC53 in moment 1)

In moment 2, the main serving cell of the UE is changed to be the third cell (PSC48) of BKC0044U, and the pilot quality Ec/Io is -10.31 dB. The third cell (PSC53) of BKC0074U leaves the active set and appears in the monitoring set.

Figure 27 Handover and call drop caused by Pinpoint effect (PSC53 cell leaves the active set in moment 2)

In moment 3, after one second, the signal quality of the third cell (PSC53) of BKC0074U is stronger than that of the third cell (PSC48) of BKC0044U, and the Ec/Io is -2.39 dB. And the third cell (PSC53) of BKC0074U reports Event 1A and tries to enter the active set again. However, as the main serving cell, the third cell (PSC48) of BKC0044U has poor pilot quality, and the Ec/Io is -21.05 dB. As shown in the following figure, after the UE reports Event 1A, the downlink cannot receive the handover command, and finally the call drop happens.

Figure 28 Handover and call drop caused by Pinpoint effect (Call drop happens because the PSC53 cell cannot join in the active set in moment 3)

AnalysisThis is the typical call drop caused by the Pinpoint effect. The signal of PSC48 cell becomes stronger suddenly, which makes the Ec/Io become poorer of the original PSC53 cell, and the PSC53 cell reports Event 1B and leaves the active set. Then, the signal of the PSC48 becomes weaker suddenly, and the call drop happens because the PSC53 do not have enough time to enter the active set. Because the signal of PSC53 cell becomes weaker in a short time, the general plan to solve this problem is to make it hard for PSC53 cell to trigger Event 1B and leave the active set.

Figure 29 Handover and call drop caused by tip effect

Optimization Plan

In order to prevent the situation that the call drop happens because it is difficult for the third cell (PSC53) of BKC0074U to enter the active set again after it leaves the active set, the CellIndivOffset (utranCell) of the third cell (PSC53) of BKC0074U in the active set should be modified from 0 to 3 dB, so the cell will remain in the active set.

Optimization Result

After modifying the parameter, the engineer finds that the call drop point disappears in the test. In the call hold status, the UE still moves along the arrows direction, and the main service cell is the third cell (PSC53) of BKC0074U.

Figure 210 Main service cell PSC53

Later, the signal quality of the third cell (PSC53) of BKC0074U becomes poor, and the Ec/Io is -13.23, weaker than that of the third cell (PSC48) of BKC0044U. Although the main service cell changes, the PSC53 cell still stays in the active set, as shown in the following cell.

Figure 211 Main service cell changed to be PSC48

Finally, PSC53 cell is the main service cell again, as shown in the following cell, and the call drop disappears.

Figure 212 Main service cell changed to be PSC53

2.1.3 Handover Failure and Call Drop Caused by Insufficient PSC Reuse Distance

Problem DescriptionIn the whole-network optimization process near the BaiShiYi in JiuLongPo district of ChongQing, call drop happens in every test on the road near Yi Yuan Restaurant. The specific locations are shown in the following figure.

Figure 213 Call drop near Yi Yuan Restaurant

AnalysisFirst, check the signal coverage in the area where the call drop happens, and the engineer finds that the signal coverage strength and the signal quality are good in this area according to the drive test. The signal strength is around -75 dBm, and the signal quality is about -6 dB. With this kind of signal coverage, it is impossible that the call drop is caused by the weak coverage or the poor signal quality or pilot pollution in the radio environment.

The NodeB distribution and the scrambling allocation near Yi Yuan Restaurant are shown in the following figure.

Figure 214 Distribution diagram of the NodeB and scrambling near Yi Yuan Restaurant

From the above figure, the engineer finds that the scrambling at Yi Yuan Restaurant is the same with those of the NodeBs at BeiDiYiYuan and ZouMaTaoHuaLin (PSC79), and the distance between two sites is less than 5 kilometers. Then, the signal planned to be sent to Yi Yuan Restaurant may be sent to BeiDiYiYuan or ZouMaTaoHualin, and the signal interaction is unsuccessful, which causes the call drop.

The following figure shows the signaling at the moment that the PSC79 cell enters the active set before the call drop in UE log.

Figure 215 Signaling of PSC79 entering the active set in UE log

The message authentification code 00101010 11011111 11010000 11011011B is converted to hexadecimal 0x2ADFD0DB, and the corresponding RNC signaling is shown in the following figure:

Figure 216 Corresponding RNC signaling

From the internal signaling after the completion of active set upgrade, the new active set list can be checked, and the 2nd cell of Yi Yuan Restaurant and the 2nd cell of ZouMaTaoHuaLin use the same scrambling PSC79.

Figure 217 Active set list

Then the UE reports Event 1B to activate PSC77 and PSC78.

Figure 218 Reported Event 1B

The call drop happens before the UE deletes PSC78 from the active set. Then, the UE sends the cell update message, and the reason is radiolink failure.

Figure 219 Call drop caused by out of sync of the Radio Link

Check the recent two measure reports before the call drop, the signal quality of PSC79 is good.

modeSpecificInfo fdd :

{

primaryCPICH-Info

{

primaryScramblingCode 79

},

cpich-Ec-N0 40,

cpich-RSCP 28

}

modeSpecificInfo fdd :

{

primaryCPICH-Info

{

primaryScramblingCode 79

},

cpich-Ec-N0 40,

cpich-RSCP 33

}

The reason of radio link failure is analyzed as follows: The UE reports Event 1A after measuring cell26913 (PSC79), but the RNC establishes one RL in cell24722 (PSC79) according to the neighboring cell relationship. The UE measures the FrameOffset of cell26913, but uses the FrameOffset in cell24722, which causes the out of sync of the RL node.

Optimization PlanFrom the Figure 214, the engineer can view that there is scrambling reuse in the area where the call drop happens. Through the subsequent signaling analysis, the engineer knows that the scrambling reuse causes the call drop. The solution of this problem is reasonable planning of the scrambling allocation in this area. In order to verify the problem, the cells at ZouMaTaoHuaLin and BeiDiYiYuan using the same scrambling should be disabled. And then, the relative signaling should be adjusted as shown in the following figure.

Figure 220 Screenshot of Signaling near YiYuan Restaurant after adjustment

After disabling the antennas in the cells at ZouMaTaoHuaLin and BeiDiYiYuan, the engineer finds that there is no call drop near the Yi Yuan Restaurant through the repeated tests. Therefore the problem is solved after the scramblings in the cells of the three NodeBs are adjusted.

2.1.4 Handover Failure and Call Drop Caused by Limited System Resources

Problem DescriptionThe background KPI statistical indicators show that the successful rate of the soft handover is relatively low when the cell is busy. And the drive test at the busy hours indicates that the UE does not receive the active set update command delivered by the RNC after it reports Event 1A. The background signaling RNC has received the measuring report of the UE, but the RNC does not deal with it.

AnalysisAfter the RNC receives Event 1A reported by the UE, it makes the resource decision for the target cell through the admission control algorithm. If the resource of the target cell is limited, the RNC will not deliver the Radio Link Setup request to the NodeB. Through the analysis of the signaling captured at the background at the busy hours, the engineer finds that the downlink CE resources of the target NodeB is limited, so the RNC decision fails to be admitted.

TCrmRlmmCacResponse.tCauseStack.atRnlcFailCause[0].btInterfaceCause = 0:CAC_REFUSE,

TCrmRlmmCacResponse.tCauseStack.atRnlcFailCause[0].btInnerCause = 393235:CAC_FAILURE_FOR_HIGHPRIO_BLOCK: current cell has high priority ue to be accessed ,

TCrmRlmmCacResponse.tCauseStack.atRnlcFailCause[0].btFlowExceptionInfo = 0:

TCrmRlmmCacResponse.tCauseStack.atRnlcFailCause[0].btFlowStatus = 0:

TCrmRlmmCacResponse.tCauseStack.atRnlcFailCause[0].btFlowStatusExt = 0:

TCrmRlmmCacResponse.tCauseStack.atRnlcFailCause[0].btExtendInfoType = 0:eExtendInfoType_Self,

TCrmRlmmCacResponse.tCauseStack.atRnlcFailCause[0].btExtendInfo = 147456:CAC_FAILURE_FOR_UL_CELLCE_RESOURCE_LIMIT | CAC_FAILURE_FOR_DL_CELLCE_RESOURCE_LIMITTCrmRlmmCacResponse.tCauseStack.atRnlcFailCause[0].dwFileCode = 0:

TCrmRlmmCacResponse.tCauseStack.atRnlcFailCause[0].dwLineNo = 0:

TCrmRlmmCacResponse.wQueueTime = 0

Optimization Plan and Result

The successful rate of soft handover is increased to 99.5% or higher after the BPC board is added.

2.2 Inter-RAT Handover Cases

2.2.1 Handover Failure Caused by Improper Intra-RAT Handover Parameter Configuration

One going to Tsuen Wan from JinShang Road passes Tai Lam Mountain. There are two tunnels in Tai Lam Mountain, one for Tsing Long Highway and the other for West Rail. There is no 3G coverage in these two tunnels. Then, it is necessary to configure GSM neighboring cells for the 3G sites to cover the exits of the tunnels. The site covering the northern exit of the Tai Lam Tunnel is SUT, and the CS call drop rate of the relative cells is 15%~20%.

Figure 221 Site diagram of Tai Lam Tunnel

Problem Analysis and Optimization Plan

In this scenario, the user enters the 2G covered area from the 3G covered area outside of the tunnel at a high speed. The general optimization plan is to let the users handover from 3G network to 2G network become quick enough. During the period of middle and late May, the engineer tried three kinds of 2D/2F parameter configuration for the SUT, to make the UE start the measure early enough by adjusting the 2D threshold.

The following table shows the KPI indicators of SUT call drop recorded by day after the 2D/2F parameter is modified. From this table, two parameter configuration mistakes which cause the abnormal call drop KPI (in yellow) can be viewed. The first mistake is that F1 and F3 do not use the same 2D/2F parameter (16th May~18th May), and the second mistake is that the failure of the handover from 3G to 2G caused by the wrong importing sequence of the parameter templates (26th May~27th May).

Figure 222 Indicator change after the 2D/2F parameter modification

Table 21 Indicator Change After the 2D/2F Parameter ModificationIndexBegin TimeEnd TimeTime GranularityCell Call Drop Rate, CSNumber Of Call Drop, CSNumber Of Successful RAB Establishment, CSRemark

12009-05-112009-05-1224 hours17.66%4662342For F1/F3 cells 2D/2F = -115/-106;Hys_2d/Hys_2f = 4/4; TTT_2d/TTT_2f = 640ms/640ms; Inter-RAT Handover Tactic = 1 (Event 3A trigger) 3A_Utrancell = -95; 3A_OtherRat = -90; Hys = 4; TTT_3A = 100 ms

22009-05-122009-05-1324 hours19.70%6002642

32009-05-132009-05-1424 hours19.44%5652460

42009-05-142009-05-1524 hours19.08%5272339

52009-05-152009-05-1624 hours10.40%3112578

62009-05-162009-05-1724 hours5.80%1622540For F1 cells 2D/2F = -115/-106; Hys_2d/Hys_2f = 4/4; TTT_2d/TTT_2f = 640ms/640ms; Inter-RAT Handover Tactic = 1 (Event 3A trigger) 3A_Utrancell = -95; 3A_OtherRat = -90; Hys=4; TTT_3A = 100 ms For F3 cells2D/2F = -95/-85; Hys_2d/Hys_2f = 4/4; TTT_2d/TTT_2f = 640ms/640ms; Inter-RAT Handover Tactic = 1 (Event 3A trigger) 3A_Utrancell = -95; 3A_OtherRat = -90; Hys = 4; TTT_3A = 100 ms

72009-05-172009-05-1824 hours7.02%1421821

82009-05-182009-05-1924 Hours5.28%1302184

92009-05-192009-05-2024 Hours2.08%532273For F1/F3 cells2D/2F = -95/-85; Hys_2d/Hys_2f = 4/4; TTT_2d/TTT_2f = 640 ms/640 ms; Inter-RAT Handover Tactic = 1 (Event 3A trigger) 3A_Utrancell = -95; 3A_OtherRat = -90; Hys =4; TTT_3A = 100 ms

102009-05-202009-05-2124 Hours2.89%662081

112009-05-212009-05-2224 Hours2.07%512174

122009-05-222009-05-2324 Hours2.62%672230

132009-05-232009-05-2424 Hours2.11%672768

142009-05-242009-05-2524 Hours2.46%632295

152009-05-252009-05-2624 Hours5.69%1953165The wrong sequence of the imported parameter template causes the failure of the handover from 3G to 2G

162009-05-26009-05-2724 Hours8.13%3163499

172009-05-272009-05-2824 Hours1.25%503453For F1/F3 cells2D/2F = -90/-80; Hys_2d/Hys_2f = 4/4; TTT_2d/TTT_2f = 640 ms/640 ms; Inter-RAT Handover Tactic = 2(Event 3C trigger) 3C_OtherRat = -95; Hys = 4; TTT_3C = 100 ms

18009-05-282009-05-2924 Hours0.91%272642

192009-05-292009-05-3024 Hours1.39%553538

202009-05-302009-05-3124 Hours1.03%423668

212009-05-312009-06-0124 Hours1.12%342743

222009-06-012009-06-024 Hours1.38%563591

232009-06-022009-06-0324 Hours1.42%573636

From the change of the KPI of the SUT call drop, the engineer can know: 2D/2F=-115/-106, and the call drop rate is 17%~19%; 2D/2F=-95/-85, and the call drop is 2%~3%; 2D/2F=-90/-80, and the call drop is 1%~1.4%. Therefore, the setting of parameter 2D/2F=-90/-80 is suitable for the SUT in the scenario of fast handover from 3G to 2G. If this suit of parameter uses Event 3A trigger, the corresponding threshold should be adjusted. The 3C event trigger can also be used.

2.2.2 Handover Failure Caused by Wrong Configuration of Intra-RAT Neighboring cells

Problem Description

On a main road from north to south, the inter-RAT handover from SC27 (KHTK3) cell failed, which caused the call drop, as shown in the following figure.

Figure 223 Call drop caused by the inter-RAT handover failure

SHAPE \* MERGEFORMAT

Analysis

The background inter-RAT parameter 2D and 3A are set to -95 dBm, and the signal strength RSCP of the main service cell SC27 (KHTK3) has been less than -100 dBm. The UE reports Event 2D for many times, but never reports Event 3A, as shown in the following figure. At the same time, the background signaling tracing record shows that the compressed mode is enabled, but the handover cannot be started.

Modify the compressed mode and the handover threshold to -90 dBm. In the following test, the compressed mode is enabled normally, but the UE still does not report Event 3A, and the handover fails.

Slow down the car speed, and modify the time to trigger of Event 3A from 100 ms to 80 ms. In the following test, the compressed mode is enabled normally, but the UE still does not report Event 3A, and the handover fails.

Check the neighboring cell list in the measure control command. When the UE is in the UMTS service cell, the BCCH of the GSM neighboring cell in the measuring report is 112, as shown in the Figure 224. When the UE is locked in the GSM network, use CNT to check the GSM signals, and the BCCH parameter delivered by the GSM cell is 124, as shown in Figure 225. Judging from the above facts, the engineer can be sure that the reason of the problem is that the delivered GSM neighboring cell information is different from the actual GSM neighboring cell information.

Figure 224 GSM cell information delivered by the system

Figure 225 Actual GSM cell information (BCCH=124)

Optimization Plan

Let the background update the cell information of all the 2G sites in the neighboring cell list.

Result

As shown in the measurement result, the UE reports the Event 2D and Event 3A in time, and the inter-RAT handover is normal.

Figure 226 Normal Inter-RAT handover after the GSM neighboring cell update

SHAPE \* MERGEFORMAT

2.2.3 Inter-RAT PS Domain Handover Failure Caused by No Routing Between SGSNs

Problem Description

In the drive test, the inter-RAT handover of the PS service is unsuccessful. The UE reports the Routing Area Update Request message, but does not receive the Routing Area Update Accept message. The routing area update fails.

Figure 227 Background signaling 1

Analysis

From the background signaling analysis, the engineer finds that the RNC delivers the cellChangeFromUtran command after the UE reports Event 3A, but the RNC receives the Iu_Release Command from the CN before starting the context information acquisition and data transmission on the lu interface. Viewing from the Uu interface, the UE sent the Routing Area Update Request message, but it does not receive the Routing Area Update Accept message.

Figure 228 Background signaling 2

General Plan

1.First, the Router Update Request message is not found at the 2-SGSN terminal, but, viewing from the Uu interface, the engineer finds that the UE sent this message. Actually, 2-SGSN terminal can only record some simple signaling, but it cannot trace the signaling. Therefore, this phenomenon does not necessarily represent that 2-SGSN did not receive the message completely. 2.Suppose that 2-SGSN received the Router Update Request message in the last step, and then check whether the new 2-SGSN sent the SGSN Context Request message to the old 3-SGSN, and whether the old 3-SGSN received this message. The result shows that the new 2-SGSN terminal does not has the record of sending this message, and the reason is the same with that demonstrated in the last step, with nothing new in this. At the same time, the old 3-SGSN terminal does not have the record of receiving this message, but this terminal can show the signaling in detail. Therefore, this problem locates at the transmission of the SGSN Context Request message between 2-SGSN and 3-SGSN.

What is more, judging from the reason MS identity cannot be derived by the network of the routing area update failure, the engineer realizes that the most possible reason is that the new 2-SGSN does not get SGSN Context from the old 3-SGSN.

3.Three possibilities are shown as follows:

i.The new 2-SGSN did not send the SGSN Context Request message to the old 3-SGSN.

ii.The new 2-SGSN sent the SGSN Context Request message to the old 3-SGSN, but the old 3-SGSN could not receive the message because the connection between 2-SGSN and 3-SGSN was blocked.iii.The new 2-SGSN sent the SGSN Context Request message to the old 3-SGSN, and the old 3-SGSN received this message. But the old 3-SGSN could not read this message because of the difference in format between the two systems.After communicating with the SGSN vendor, we find that the reason of the routing area update failure belongs to the second kind. The reason leading to the situation is that 3G SGSN and 2G SGSN have not been cut over.

4.Provide the parameters of routing area and location area for the SGSN vendor, and the test is successful after the cutover.

VI 2010ZTE CORPORATION. All rights reserved.ZTE Confidential Proprietary

ZTE Confidential Proprietary 2010 ZTE Corporation. All rights reserved.4(15)

_1347193655.vsd

Prepare the data: association log, foreground and background signaling

The RNC receives the measurement message (1A\1B\1C)

The RNC sends the RL Setup message to the NodeB

The coverage is poor, adjust the RF or handover parameters

The handover block rate of the target cell is high

The load of the target cell is heavy

Yes

The NodeB returns the RB Setup Complete message to the RNC

The RNC sends the ActiveSet Update message

Yes

Yes

No

No

Yes

The UE receives the Active Set Update message


The RNC receives the RL Setup Failed message or no response

No

Check RNC hardware

No

The UE returns the Active Set Update Complete or Active Set Update Failed message

The UE is faulty

The RNC receives the Active Set Update Complete message

The RNC receives the Active Set Update Failed message or no response

Yes

Yes

Yes

No

No

No

Check the RNC hardware

No

The UE reports the measurement message (1A\1B\1C)

Yes

No

1. The UE is faulty2. The coverage is poor, adjust the RF or handover parameters

The preparation of the handover fails, analyze the cause value

The Uu interface handover fails, analyze the cause value

Yes

1. Adjust the access control threshold2. Adjust the RF parameters of the target cell, narrow the coverage scope3. Expand the capacity, such as adding carriers and baseband boards4. Add sites

_1347193656.vsd

Collect traffic statistic data at busy hours

RNC indexes satisfy the requirement

Indexes of TopN cells satisfy the requirement

Divide TopN cells into different sections

RNC alarms, RNC board failure

Locate the problem

No

No

No

Yes

NodeB alarms, NodeB board failure

No

Association log and association KPI analysis

Perform DT to locate the problem

No

Yes

Yes

Implement the optimization solution

No

_1347193612.vsd

DT data

Background siganling

Engineering parameters

Call drop

Access failure

Missing neighbor cells

Pilot pollution

Improperly configured handover parameters

Equipment abnormality

Insufficient reuse distance for PSCs

No

High proportion of soft handover

Yes

Improper RF parameters

Improper handover parameters

No

Adjust the equipment, verify the adjustment by DT

Problem solved

End

Yes

No

No

No

No

No

Yes

Yes

_1347193654.vsd

Prepare the data: association log, foreground and background signaling

The RNC receives the measurement message (2D)

The RNC sends the Inter-Measurement Control message to the UE


The handover block rate of the source cell is high


Yes

The UE receives the Inter-Measurement Control message to the

The UE sends the measurement report (2A)

Yes

Yes

No

No

The RNC receives the measurement report (2A)



No

Adjust the handover parameters

No

The RNC sends the RL setup request to the target NodeB

The handover block rate of the target cell is high

The RNC receives the RL setup success message or no response

The preparation of the handover fails, analyze the cause value

Yes

Yes

Yes

No

No

No


The UE reports measurement message (2D)

Yes

1. The UE is faulty2. The coverage is poor, adjust the RF or handover parameters

No


The RNC sends the PC, TC, and RB reassignment message

Yes

Check RNC hardware

No

The UE receives the PC, TC, and RB reassignment message


The UE returns the PC, TC, and RB reassignment complete or failed message

The UE is faulty

No

Yes

No

The RNC receives the PC, TC, and RB reassignment complete message

The RNC receives the RL setup failed message or no response

Yes

The RNC receives the PC, TC, and RB reassignment failed message or no response

The Uu interface handover fails, analyze the cause value

Yes

No

Yes


1. Adjust the access control threshold2. Adjust the RF parameters of the target cell, narrow the coverage scope3. Expand the capacity, such as adding carriers and baseband board4. Add sites

umts handover performance optimization

Documents

Transcript of umts handover performance optimization