Drive Test Analysis Version1.0
Upon completion of this course, you will be able to: Understand the common procedure Of drive test data analysis Identify the basic problems and cause of the problems Propose the solutions to solve the network problems
Chapter 1 Common procedure of drive test data analysis Chapter 2 Case study for drive test analysis Chapter 3 Practice on drive test analysis
Common procedure of DT data analyze 1.
Check if the Drive test KPIs meet the requirement. If not, identify the problematic log files. Capture the figure and events (e.g. locations, time, identify problematic cells)
2.
Check the RxLevel and RxQual distribution, check if they are related to the points that miss the KPI requirement?
3.
(If new site or cluster) Check the distribution of each BCCH (CGI), pay attention to the crossed feeder.
4.
Analyze the points that miss the KPIs point by point. (To make the analysis more comprehensive , get more information from engineering parameters, BSC data configuration, traffic statistic and BTS alarms)
5.
Identify the possible causes of each problem points ,discuss with customers
4.
Propose solutions and get approval from customers before execute the change request
4.
Re- Drive Test to verify the problem after implemented the solutions !
Chapter 1 Common procedure of drive test data analysis Chapter 2 Case study for drive test analysis Chapter 3 Practice on drive test analysis
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Coverage • Downlink RxLevel • Coverage of single cell • Coverage of whole network
Coverage Network Coverage is evaluated by: RxLev = EIRP(dBm) – L(dB) Where: EIRP = Effective Isotropic Radiated Power L = Propagation Losses EIRP(dBm) L(dB) RxLev(dBm) Unexpected coverage (overshooting)
Good coverage
In urban area, Lower Antenna Height (<=25m), Greater downtilt (>=6 degree), and Lower EIRP (<=41 dBm) are used to control overshooting & interference. #
Coverage(one cell) The following example shows the downlink RxLevel(Dedicated Mode) decreases when MS is driving away from BTS. Downtilt: 0 drgree
RxLevel
The RxLevel decrease faster when Antenna Downtilt increase.
Variation s due to Rayleigh fading Global means
Downtilt: 8 drgree
Variation due to shadowing $
Coverage(Network) The picture shows the coverage of a city, which contains good and bad coverage areas. The distribution of downlink RxLev concentrates from -80 to -95 dBm.
Good Coverage
Bad Coverage
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Coverage Issues • • • • • •
Coverage hole Sudden Decrease on Signal Level Line of Sight Lost Lack of Dominant Server Coverage Overlapping Overshooting
Coverage Hole Legend RxLevel
Poor Coverage Areas
Bad RxQuality
RxQuality
RxLevel(Serving Cell) is about (or less than) -95 dBm RxLevel(Neighbours) are about (or less than) -95 dBm
Call Drop
Sudden Decrease on Signal Level RxLev(Serving Cell and Neighbors) decrease in a short time. Check if there is big change of testing environment, e.g. check if the test was performed on a highway and that particular area was a tunnel or not. Signal level on the chart will make a curve rather than unstable changes. This usually cause Ping Pong Handovers
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Line of Sight Lost
Signal Strength of Serving cell make fast up and down due to far away server being blocked by obstacles from the terrain. The other way, signal from the server lose line of sight(LOS) to the mobile because of a hill of something.
RxQuality goes worse when the level drops down fast &
Lack of Dominant Server Signal Level of more than one cell (Serving Cell and Neighbors) are not high enough.
This might happen because the MS is located on the cell borders and there is no any best server to keep the call.
Lack of Dominant Server Causes too many handovers
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Coverage Overlapping Signal Level of 3–4 cells are too closed to each other.
This might point overlapping cells.
This will cause quality problems because of frequency reuse and frequent HO Immediate action to optimize cell coverage should be taken by power reductions, downtilt or other configuration changes. &"
Overshooting MS is covered by the serving cell, but is outside the cell BSC thinks that it is neighbor cell A and makes a handover to cell A The MS is not near cell A so the assigned TCH for the handover is not used Cell A will have unused TCH (handover) The call might be dropped because it cannot make the handover or bad quality
Serving cell
Other cell B BSIC 1 Freq. A
BTS
Coverage spot (island effect)
Neighbour cell A BSIC 1 Freq. A
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Propose solutions for Coverage Issue
Solution of low coverage Solution of no dominant cell ,overshooting and overlapping
Solutions of Low Coverage Possible solution can be listed as below: New Site Proposal Sector Addition Site Configuration Change (Antenna Type, height, azimuth, tilt changes) Loss or Attenuation Check ( Feeders, Connectors, Jumpers, etc..)
Proposal
Remark
Most effective solution, but the cost is highest Effective solution Sector Addition but only for Omni-Site Difficult, high cost Height Difficult, high cost Type Antenna Easy to operate Azimuth Downtilt Easy to operate Only for troubleshooting Feeders Hardware Connectors Only for troubleshooting Only for troubleshooting Jumpers New Site Proposal
How to do Using prediction tool(U-net) to identify the best locations to put new sites Using prediction tool(U-net) to identify the recommended azimuth of new sectors Using prediction tool(U-net) to identify the recommended height, type, azimuth or down tilt of sector.
Referring to hardware checking guide
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Solutions of overshooting or overlapping Possible solution can be listed as below: Site Configuration Change (Antenna Type, height, azimuth, tilt changes) Cell Configuration Change (Carrier Power Type, Static TRX Power Class, Fine Tuning of Static TRX Power…)
Proposal Antenna
Parameter
Height Type Azimuth Downtilt
Remark Difficult, high cost Difficult, high cost Easy to operate Easy to operate
Easy to operate, but with Output Power related risk of coverage reduction
How to do Using prediction tool(U-net) to identify the recommmended height, type, azimuth or downtilt of sector. Referring to Data Configuration Reference Help
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Cross Feeder What is crossed feeder issue (1) The term crossed feeder is used to describe the problem that arises when the feeders for two or more sectors in a site are inadvertently connected incorrectly. For Example, consider a new cell site that has three sectors, A, B, and C:
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What is crossed feeder issue (2) The crossed feeder problem would arise if the feeder for sector A is connected to sector C and vice versa. When this happens, the sectors typically continue to provide good coverage. However, the network parameters for the two sectors are also reversed. For example, the parameters storing the sectors' BCCH and lists of adjacent sectors would be swapped. For example, suppose we run a drive test through the area served by the cell whose feeders are crossed display the ServBCCH attribute on the Map while the cell sectors are colored by BCCH. This is what we might see:
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What is crossed feeder issue (3) There are 3 types of crossed feeder: Crossed transmit feeders
Crossed receive feeders
Crossed transmit and receive feeders
The problems raised by crossed feeder issue
Normal Situation
Crossed Feeder Situation
The problems raised by crossed feeder issue Crossed feeder will raise many problems, such as: A greater degree of interference A poor uplink signal strength A poor performance of handover
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How to detect crossed feeder issue by DT Crossed transmit feeders Crossed transmit feeders will result in the swap of 2 or more sectors BCCH frequency and TCH’s. As the sectors are pointing in the incorrect direction, performance will suffer as the frequency plan has been changed and a greater degree of interference will be present. In DT, we will find that the handset receives the signal which shouldn't have been received in the current cell.
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How to detect crossed feeder issue by DT Crossed transmit feeders
In the example shown above, point A is in the area of coverage of Cell D3, but the handset at point A receives the stronger signal of Cell D2. It indicates that TX feeders were crossed between Cell D3 and Cell D2. '
How to detect crossed feeder issue by DT Crossed receive feeders It is not easy to detect this fault by DT, because the BCCH frequencies will appear exactly as they were designed. However, the statistics for the cell would help us to detect the fault: Uplink signal strength would be very poor Link balance would be larger than expected Handover success rate would be very low
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How to detect crossed feeder issue by DT Crossed receive feeders The MR measurement is very useful in detecting this type of feeder cross. Uplink-and-Downlink Balance Measurement per TRX and TCH Receive Level Measurement per TRX are the most useful statistics . For example, if the uplink-and-downlink balance is always in level 10 or level 11, we can consider that there is something wrong in the receive path of the site, crossed receive feeder is a possible problem.
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How to detect crossed feeder issue by DT Crossed transmit and receive feeder The symptom is similar with the fault “crossed transmit feeder”, and we can detect the fault by DT easily .
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Poor Quality Issue Downlink RxQual Issue What is RxQual Issue The problems raised by RxQual issue
How to detect quality issue by DT Bad Quality due to Signal Strength – FER is Bad Bad Quality due to Signal Strength – FER is OK Bad Quality due to C/A Interference Bad Quality due to Time Dispersion
The solution to RxQual issue
What is RxQuality Issue Bad RxQual is one of the biggest problems in a Network. The RxQual that a network operator can offer to customers mostly depends on the RxLevel and the interference of the network. There are two types of RxQual problems, uplink RxQual and downlink RxQuality. We can only detect downlink RxQual problem by DT, however, if the downlink RxQual is bad in one cell, there is a risk that there would be problems on the uplink as well.
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What is RxQual Issue
Good RxQua lity
Bad RxQua lity
RxQual is divided into eight levels which are from 0 to 7, the level 0 is the best and the level 7 is the worst.
The problems raised by RxQual Issue Bad RxQual may raise many problems, such as: Difficulty in accessing the network Dropped calls due to bad quality Poor handover success rate Low MOS value (poor speech quality)
Bad Quality due to Signal Strength – FER is Bad As the signal strength drops down, the quality of the call becomes worse being effected by interference and/ or fading. Consequently the system becomes weaker to handle the interference. Drop calls and ping pong handovers usually happen in such environments. RxLev (Serving Cell and Neighbors) is not high enough (about less than -95dbm). Maybe there are interferences in this region for bad FER. Bad RxLev Bad RxQual Bad FER
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Bad Quality due to Signal Strength – FER is OK This case is similar with the previous except for FER. Signal strength is also bad in this, but FER is still fine. FER is ok, it means that there is no obvious interference in the area. The coverage is usually the problem in this situation.
Bad RxLev Bad RxQual Fine FER
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Bad Quality due to C/A Interference There is adjacent channel interference in this case and the RxQual is not good. Bad Quality due to Adjacent Interference. Adjacent BCCH between best server and best neighbor.
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Bad Quality due to Time Dispersion
Site Reflecting object
MS Time dispersion is caused by the reflections. The MS uses all or most of the received power, instead of only the direct signal, there is a larger probability to decode the information. So the RxQual will be very bad perhaps.
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Bad Quality due to Time Dispersion The MS is near the cell, it is less than 1 mile far away. The RxQual is very bad for the bad C/R, in which, R stands for reflected signal and C stands for original signal. The TA is too high. It is because that TA stands for the reflected signal but the original signal.
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The solution to RxQual issue (1) RxQual Issue
Solution Find out the interference source (e.g. frequency planning) or /and
Bad Quality due to Signal Strength – FER is
adjust the azimuth or down tilt of
Bad
the antenna or /and adjust the power of the transmitter(s) to improve coverage There is no obvious interference in
Bad Quality due to Signal Strength – FER is
this situation. Adjust the azimuth or
OK
down tilt of the antenna or /and adjust the power of the transmitter(s) to improve coverage
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The solution to RxQual issue (2) RxQual Issue
Solution
Bad Quality due to C/A
Avoided adjacent frequencies in the same cell
Interference
and preferably in neighboring cells as well.
Move the site to be placed near the reflecting Bad Quality due to Time
object to prevent time dispersion. Another
Dispersion
efficient solution is to modify the antenna arrangement, either in azimuth (horizontally) or by tilt (vertically).
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Interference Issue Hardware Problem TRX Problem Feeder & Arrester Problem Intra-Interference Adjacent channel & Co-channel Inter-modulation Interference Inter-Interference Other Equipment
Co-Channel & Adjacent Channel Interference As beside Figure show Station A~D, Supposing the frequency N have already allocated to cell A-3.so frequency N can not be allocate to A1、A2、B1、B2、B3、C1、C2、 C3、D1、D2、D3; and frequency N±1 can not be allocated to cell A1、 A2、A3、B1、C2、D1、D2 (No ,hoping)
Co-Channel & Adjacent Channel Interference As below, From the neighbor list ,BCCH 73 interfered BCCH 72,it is adjacent interference. When the co channel or adjacent channel happen
It will bring the strong interference. The typical phenomena: High Rxlev Worst Rxqual
C/I Problem C/I Definition C/I: Carrier signal /Interference signal Optimize C/I method Improve Coverage Decrease Interference
C/I Problem due to poor coverage
RxLevel(Serving Cell) is about (or less than) -95 dBm; RxLevel(Neighbours Cell) is about (or less than) -95 dBm; Result: Worst C/I due to poor coverage.
C/I Problem due to Interference (frequency)
RxLevel (Serving Cell) is about -70dBm; (strong) RxLevel (Neighbors Cell) is about -65 to -80 dBm Result: Worst C/I due to interference ( no dominant cell) !'
The solution to Interference, C/I,BER issue (1) Interference Issue
Solution
Serious interference due to Co-channel or Adjacent channel
Adjust cell frequency or adjust coverage
Serious interference due to feeder or Arrester Problem
Check whether the Feeder and antenna tightening, or direct replacement its.
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The solution to Interference,C/I,BER issue (1) C/I Issue
Worst C/I due to poor coverage Worst C/I to interference
Solution Improve coverage by adjusting antenna azimuth and downtilt or add new sites Ensure the interference source and solution as interference solution.
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The solution to Interference,C/I,BER issue (1) BER Issue
Worst BER due to poor coverage
Solution Improve coverage by adjusting antenna azimuth and downtilt or add new sites
Worst BER due to interference
Ensure the interference source and solution as interference solution.
Worst BER due to Transmission
Checking transmission alarm in M2000
Worst BER to TRX
At first check equipment alarm in M2000,ensure which TRX is problem
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Handover Issues What is… Handover procedure in network? Handover Um signaling flow? Purpose of Handover Analysis in DT?
Handover Procedure Uplink MR (Measured by BTS) •RXLEV Uplink •RXQUAL Uplink •TA •Current BTS TX power
Neighbor Cell_1 NeighborCell Cell2 Serving
MS in dedicated mode will continuously perform measurements on serving and neighbor cells.
NeighborCell Serving Cell2
Measurement Reports are sent to BSC and used in the handover algorithm. Downlink MR (Measured by MS) •Downlink RXLEV(Sever) •Downlink RXQUAL(Sever) •RXLEV_NCELL(n) on the 6 best neighboring cells (+BSIC)
BSC
Serving BSC decides whether a handover is necessary and send HO command
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Handover signaling on Um(TEMS Message) Tell MS how to access to Target TCH Channel
Target cell Description Target channel Description BCCH of Neighbors Channel Type, Timeslot Cell: to beServing measured TSC DL DTX is ON RF hopping RxLev(sub) is -109+ 62 =--49dBm MAIO,isHSN RxQual (sub) 0
Handover Complete
Handover Success!!!
Only after SABM & UA in Layer2 Message, handover success!
The RxLev of Neighbors Inter cell Synchronous Handover Speech version: EFR
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The purpose of Handover Analysis The purpose of handover analysis in DT is understand the wireless handover performance of network. find out whether the handovers are healthy in this network. what is typical handover failure in this network. find out whether neighbor audit work is needed in this network.
To make optimization of HSR, the most effective way is based on traffic analysis, combining DT events, neighbor audit, and data configuration audit together.
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Handover Types Different handover types by network topology Different handover types by algorithm PBGT handover Quality handover Edge Handover Intracell Handover Other types of handover
Handover Types(Network Topology) Operator MSC
BSC
BTS
MSC
BSC
BTS
BSC
BTS
BTS
CELL CELL CELL CELL CELL CELL CELL CELL
BTS
BTS
Case1 Intra cell HO Case2 Synchronous Inter cell HO
Synchronous Asynchronous
Case 2 Case 1 Case 3
BSC
Case 4
Case 5
Case3 Asynchronous Inter cell HO Case4 Inter BSC HO Case5 Inter MSC HO
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Handover Types(Algorithm) OM Forced HO
In DT, the most common handover types happens in following order of priority
Directed Retry
TA Emergency HO
Edge HO
Bad Quality HO
Layer HO
Rapid Level Drop HO
PBGT HO
Interference HO
Fast Moving MS HO
Load HO
Overlay/Underly HO
TA RxQuality RxLevel PBGT OL&UL
HO
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PBGT Handover PBGT HO Exp: When: RXLEV_NCELL(n) – (RXLEVEL+ PWR_DIFF) > HO_MARGIN
MS is handed over to the neighboring cell . HO_MARGIN is usually set to 3 to 6 dB.
Target Cell
Serving Cell
HO_MARGIN > 0 RXLEV_NCELL(n)
RX LEVEL + PWR_DIFF
PBGT HO
Power-budget Distance
HO_MARGIN could be increased to reduce number of handovers. HO_MARGIN should be decreased if faster handover decision is wanted. PBGT Handover is considered as most healthy handover in a network.
A typical PBGT HO
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Quality Handover Quality Handover Exp: When: DL RxQuality >= DL Quality HO Threshold
handover caused by DL RxQuality takes place. When: UL RxQuality >= UL Quality HO Threshold
handover caused by UL RxQuality takes place. DL&UL Quality HO Thresholds are generally set to 6. UL Quality HO can’t be monitored in DT.
Handover was performed to a better quality cell just after experiencing quality problems.
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EDGE Handover Edge Handover Exp: When: DL RxLevel < Edge HO DL RX_LEV Threshold
handover caused by DL RxLevel takes place. When: UL RxLevel < Edge HO UL RX_LEV Threshold
handover caused by UL RxQuality takes place. UL EDGE handover can’t be monitored in DT. Handover was performed to the cell with higher downlink Rxlev.
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Intracell Handover Based on Interference The Intra–cell Handover feature aims to maintain good quality by performing a handover to a new channel within the same cell when uplink or downlink interference is detected. Interference is defined by bad RxQual and high enough RxLEV. Intracell HO also can be triggered by: • Rate change between Full rate and Half rate • Handover between overlaid and underlaid subcell.
An example of intracell handover after experiencing quality problems. '(
Other types of handover Other types of handover are not easy to judge or not easy to encounter in DT test, for example : TA handover Directed Retry Load handover Rapid Level Drop HO Fast Moving MS HO For analysis of different types of handover or handover caused by uplink, it is recommended by traffic analysis or single-user signaling tracing.
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Handover Issues What is … Dragged Handover Power Control Effect Ping–Pong Handover Missing Neighbor Relation Fake Neighbor Handover Failure
Dragged Handover There will be such cases that you will notice handover process taking place a little late, which is called dragged handover. First thing to check will be handover margins between the neighbors. If margins for level, quality or power budget handovers are not set correctly, If margins are too much, handover will happen late, vice versa.
A Quality handover should happen earlier
Dragged handover events often occur after a GSM serving cell loses dominance for a significant time period. Dragged Handover often cause Call Drop or Handover Failure.
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Power Control Effect Power Control Effect sometimes may mislead us, you might think that handover is happening too late between two neighbors.
Power control
When the call is continuing on a timeslot that belongs to TCHTRX, power control feature will try to reduce output power as much as possible until a quality problem occurs. That’s why you will see serving cell signal level is less than neighbor’s level. It looks less but in reality, the signal level on BCCH TRX is still higher than neighbors broadcasting level.
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Ping-pong Handover Ping-pong handovers occur when the MS is handed over from one cell to another but is quickly handed back to the original cell. The shot shows two pingpong handovers displayed on the map along with the Servering CI attribute. The first ping-pong handover is from cell 12424 to cell 12368 and back and the second is from cell 12424 to cell 12366 and back. Changes in the value of the CI is shown below the route. It can be clearly visualized by lines to cells display.
Ping-pong HO
For most cases, Ping-pong Handover is caused by no dominant cell. There was a handover back to cell 12366 after the second ping-pong handover, but this was outside of the defined window. The change in dominance after the handover was 9dB, which represents a much healthier handover. '"
Missing Neighbor Relation If a handoff is not performed to a neighbor cell that seems to be best server, there is a possibility of a missing neighbor relation. This will happen with sudden appearance of strong cell in the neighbor list just after a handover.
The shot shows after second handover in the red box, a much stronger Serving Cell appears. ( suspect there might be missing neighbor in the 1st HO attempt) At this time Neighbor audit should be performed.
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Fake Neighbor Relation Sometimes you will see a good handover candidate in the neighbor list but handover will not take place and call will drop.
Although that overshooting cell with a very good signal level appears in neighbor list, in reality it is not. Just because the serving cell has another neighbor cell use same with the same BCCH&BSIC ,so the measurement of the overshooting cell appears in the list.
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Handover Failure Handover Failure: Handover attempt was failed and the call returned back to its all channel.
Target Cell is BCCH is 13 BSIC is 65 Target channel is 900M RF hopping Handover Failure Channel. Half Rate MA list is 1, 17, 18, 19, 20, 21, 22, 23, 24 MAIO is 4, HSN is 31, The TSC is 5. handover has not successfully completed yet! Channel mode is AMR(SV3) Handover Failure is Another example: Excessive Number Handover caused by T200ofExpired
Failure due to Hardware Problem
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Proposal of Handover Issue Dragged Handover Ping–Pong Handover Missing Neighbor Relation Handover Failure
Solutions of Handover Issues Issue
Proposal Check if the parameter of handover, such as: PBGT, Quality, Interference, Edge
Dragged Handover
handover threshold, and P/N Value is set too high. Check if there is no dominant coverage. Referring to the solution of Low Coverage. Check if the traffic volume of target cell is high or has congestion at measuring time. Check if there is hardware problem. Referring to hardware checking guide Check if there is no dominant coverage. Referring to the solution of Low Coverage.
Ping-pong Handover
Check if the parameter of handover, such as: PBGT, Quality, Interference, Edge handover threshold, and P/N Value is set too low . Check if there is overlapping coverage area, which may also cause ping-pong handover.
Missing Neighbor
Neighbor Audit. Check if there is neighbor missing according to data configuration in OMC. Find out the cause of Handover failure form Um signaling flow from Layer2 and
Handover Failure
Layer3 Message. Check if the HSR between source and target cell is low. If it is, find the root cause of handover failure reasons by traffic statistic analysis.
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