Definition of Call Drop
Reasons of Call Drop
Analysis of Call Drop
Parameters of Call Drop
Case of Call Drop
Definition of Call Drop
Type of Call Drop
Definition
- Call termination not through the normal hang up process, that is dropped calls.
- Tow types of call drop definition:
- Definition of drive test indicators
- Definition of network management indicators
Definition of Drive Test
Indicators
- The definition of drive test:
- Call drop rate= Number of call drop times/Number of call setup success times
- Number of call setup success times+1:After the Alerting message is received
- Number of call drop times+1:(Air interface signaling at the UE side)
- The Connect ACK message is not received but the System Information message is received.
- After the Connect ACK message is received, RRC Release message received and the reasons is Not Normal.
- After the Connect ACK message is received, any of the CC Disconnect, CC Release, CC Release Complete message received and the reasons is Not Normal.
UE Voluntarily Initiated
Signaling Release
Definitions of Network
Management Indicators
Definitions of Network
Management Indicators
Call Drop Reasons
- Poor Coverage
- Un-configured Neighbor Cell
- Handover
- Interference
- PSC Confliction
- Engineering Error
Poor Coverage
- The decision whether it is problem of uplink or downlink poor coverage is based on the power of dedicated channel before call drop.
- UL Poor Coverage:
- TX power reaches the maximum
- UL BLER is poor
- NodeB report “RL failure”
- DL Poor Coverage:
- TX power reaches the maximum
- DL BLER is poor
- Scanner:
- If the RSCP and Ec/Io of the cell with the best coverage are poor, you can infer that the coverage is poor.
Unconfigured Neighboring Cell
- Missed neighbor cell.
- Removal of key neighbor cells caused by combination of macro diversity.
- Untimely update of the external cell information.
Handover
- The handover process is incomplete
- Radio Environment becoming bad, UE can not receive the “Active Set Update”
- Ping-pong Handover
- In a short time, UE send different Report for delete or add cell A
Interference
- Reasons for Pilot Pollution:
- Cross-cell coverage of high Node-B
- Node-B in ring layout
- Signal distortion caused by street effect or strong reflection
- Judgment of DL interference:
- CPICH RSCP of the active set is large than -85dBm
- Ec/Io is lower than -13dB
- Reason of DL interference:
- Pilot pollution
- Unconfigured neighboring cell
- Judgment of UL interference:
- The average RTWP of an idle cell exceeds -100dBm
- The max RTWP is around -90dBm
- Reason of UL interference:
- Intra-RAT interferences
- Inter-RAT interferences.
Case-Interference
PSC Confliction
- Cell A and Cell B are configured as neighbor cell for each other.
- Cell C and Cell B are not configured as neighbor cells for each other.
- Cell A and Cell C have the same PSC.
Engineering Error
- Call drops caused by engineering error:
- Reversely-connected antenna
- An excessive VSWR
- Multi-band antenna problem
- Leakage of signals from indoor distribution system
- Call drop caused by unsteady transmission
Common Analysis Methods
for Call Drop
- Analyzing Call Drops by DT
- Analyzing Call Drops by Traffic Statistics
Analyzing Call Drops by
DT
- Call drop data
- Call drop spots
- Stability of the primary serving cell
- RSCP and Ec/Io of the primary serving cell
- Reproducing of problems with DT
Analyzing Call Drops by
Traffic Statistics
- The commonly used KPI analysis method is the TOP cell method, which means the top cells will be screened out according to certain index, then these top cells are optimized and then the top cells are selected again. After several repetitions, the related KPI can be speedily converged.
Multi-Dimension Analysis
- Multi-dimension analysis is carried out from different perspectives.
- For the call drop problem, not only the call drop itself, but also related factors such as
- access
- handover
- traffic statistics
- time
- RTWP
Accident Analysis
- Check the equipment alarm and system log of this period to find out hardware problems;
- Check the transmission of this period;
- Check whether the upgrade or cell blocking is performed during this period;
- Check whether there is an occasion with abrupt high traffic requirements such as a concert, game, or exhibition.
Ranking Analysis
- Ranking analysis
- Ranking analysis is carried out through classifying data into top N and bottom N data from a large amount of data.
Cause-and-Effect
Analysis
- For a certain effect, the cause-and-effect analysis is performed to locate the causes that may result in the effect and to determine the influence of the causes.
- For example, the call congestion of a cell may be caused by insufficient capacity of the hardware, of the downlink, or of the uplink.
Optimization Methods for
Call Drop
- Engineer optimization:
- Directional angle, downtilt, position of Antenna, type of Antenna, Transmit power of BS, position of BS, new BS.
- Radio parameter optimization:
- Time to Trigger, CIO, threshold of enabling/disabling Compression Mode, Maximum transmit power of DL RL, Threshold of Inter-frequency and Inter-system.
Time To Trigger
Cell Individual Offset
Threshold of
Enabling/Disabling Compression Mode
Maximum Downlink
Transmit Power of Radio Link
Maximum Downlink
Transmit Power of Radio Link
Threshold of
Inter-system handover
Timer and Counter
Related to Call Drop
Neighboring Relation
Adjustment
- Reason
- High call drop rate caused by improper neighbor list configuration
- Description
- The call drop rate of the TRI135W-1 cell corresponding to RNC1 in Libya is always around 3%, and no hardware alarm is generated. The cell coverage is mainly on the sea.
Neighboring Relation
Adjustment
Unconfigured Neighboring Cell
Unconfigured Neighboring Cell
- Main parameters:
- Cells in the system
- Troubleshooting process:
- The cell with SC 9 is in the detection set and cannot be added to the active set when the quality of the serving cell with SC 74 is extremely poor. This is a typical unconfigured neighboring cell.
- Solution:
- Adjust the neighbor relation: Add the cell with SC 9 to the neighbor list of the cell with SC 74.
- Result:
- In the same test, the cell with SC 9 is in the active set of the serving cell with SC 74. When the signal cell with SC 9 is strong enough, the UE hands over from the serving cell to the cell with SC 9.
Soft Handover Parameter
Optimization
- Reason:
- Low handover success rate because of improper soft handover parameter configuration
- Description:
- The success rate of the handover from sector 1 (SC 436) of the Shuqian Lu site to sector 2 (SC 434) of the Meihuacun hotel is low. This area is within the Shuqian Lu section.
- Main parameters:
- Soft handover 1a/1b event handover threshold, trigger time
Soft Handover Parameter
Optimization
- Adjust the 1a/1b event handover threshold and trigger time of cell 436.
- Lower the 1a event handover threshold and shorten the trigger time to ensure that the cells with good signal quality can firstly enter the active set.
- Raise the 1b event handover threshold and extend the trigger time so that the cells are not deleted too early due to drastic signal deterioration.
- Result
- After the parameter optimization, cell 434 of the BS-1 (Meihuacun hotel site) can be added to the active set quickly and is not deleted too early.
- According to the drive test result from more than 100 times of handover tests, the success rate of the handover between the BS-2 (Shuqian Lu site) and the BS-1 (Meihuacun hotel site) increases greatly.
Data Configuration
Optimization in 2G/3G Handover
- Reason
- Call drop because of incorrect data configuration
- Description
- When the 2G/3G handover tests are performed at the boundary of the 3G network, the handover from the 3G network to the 2G network succeeds in the west to east direction, but the handover fails in the east to west direction.
- Main parameters
- BSC ID, LAC, NCC, and BCCH of the 2G neighboring cell
- Troubleshooting process
- Because the handover from 3G network to 2G network fails, you should firstly check whether 2G neighboring cells are configured. If all 2G neighboring cells are configured, go to the next step.
- Record whether the 2G Sagem UE starts the compression mode when the signal quality of a 3G UE is lower than the threshold for starting the compression mode, and record the CI of the 2G cell where the compression mode is started.
- You can find that the UE starts the compression mode after the preceding step is performed. The signaling is as follows:
- Solution:
- Check the BSC and LAC of the target 2G cell on the CN. You can find that the LAC is not configured. Then, reconfigure the LACs of all 2G neighboring cells on the CN.
- Result:
- After the data is configured again, all 3G-to-2G handovers succeed.
- Suggestion for similar problems:
- In the 3G-to-2G handover, the BSC ID, LAC, NCC, and BCCH of 2G neighboring cells must be configured in the 2G neighboring cell database of the OMCR and on the CN.
Call Drop Case-Handover
End of Course