RF Optimization Crash Course [Handover]

Handover is a key function in a GSM network. If the handover performance is poor the subscriber will perceive the quality of the network as bad.

Handover performance statistics should preferably be measured on 24 hour data or longer.

The following checks should be performed:
1. Check for handover relations with low traffic (e.g. less than 10% of average number of handovers per relation).

2. Check for unbalanced handover relations (e.g. relations which have not approximate the same number of handovers in either direction. Remark: Unbalanced handovers relation indicate problem if measured on whole days, assuming that the same number of people go in and out of the cell coverage area.

3. Check if the reason for unbalanced handover is congestion.



Probable Reasons of Bad Handover Performance

Bad Locating Parameter Setting
A bad setting of locating control parameters might result that the locating will seldom rank the cell as a candidate.
Action: Check parameter setting.
Solution: Correct bad parameter setting.
Unnecessary Neighboring Cell Relation
None or very few handovers might indicate an unnecessary neighbouring cell relation.
Action: Check neighbor cell relations
Solution: Check if the relations really should be defined. Remove unnecessary cell relations.

The Base Station is Defined But Not in Service.
Action: Check reason for BTS not in service.
Solution: Take action to set the base station in service.

HW faults.
Action: Check BTS error log.

Missed measurement frequencies in BA-list
This will cause no handover decisions to the target cell.
Action: Check measurement frequencies list.
Solution: Add missed frequencies.

Permitted Network Color Code problem
If NCCPERM doesn’t include neighboring cell’s NCC, there will be no handovers.
Action: Check NCCPERM.
Solution: Add NCC of neighbors to NCCPERM.

Wrong Use of HCS
Action: Check HCS related parameters
Solution: Change to normal value.

Poor inter-MSC/BSC handover performance
If the cell is at the border of inter-BSC or inter-MSC, poor inter-MSC/BSC handover
performance will cause few or no handover attempts.
Action: Check inter-MSC/BSC handover performance

Unsuccessful Handovers
There can be two reasons why an attempt is counted as unsuccessful: either the mobile station was lost or the call was reverted to the old cell and channel.Unsuccessful handover may lead to a dropped call. Investigate if there is any difference between incoming and outgoing handovers. This can give further reasons of the problem area.

The lost handovers are registered at outgoing handover. Note that also dropped call counters are
stepped.


Congestion
A high congestion might lead to dragged calls (handover performed at a not intended location) and a lot of unsuccessful handovers.
Action: Check TCH congestion.
Solution: Add more TCH capacity. Activate DHA or DYMA.

Timer Expire After MS is Lost
The MS never answers the base station.
Action: Check coverage. Check interference.

Link Connection or HW Failure
Action: Check BTS error log. Perform site visit. Perform link performance measurements.
Solution: Repair faulty equipment.

Bad Antenna Installation
Action: Perform site survey and check antenna installation. Check antenna cabling.
Solution: Adjust antenna installation, antenna type or cabling.

Antennas Connected to Wrong Feeder
Action: Perform site survey and check antenna installation. Check antenna cabling.
Solution: Correct the antenna to the right sector.

Incorrect Down Tilt
Action: Perform site survey and check antenna installation.
Solution: Correct antenna tilting.

The MS Measures Signal Strength of Another Co- or Adjacent than Presumed.
Action: Check frequency plan..Perform drive tests.
Solution: Decrease interference.

Wrong Neighboring Cell is Defined

This can happen when a new site is added and old definitions are not undefined and new relations are not inserted.

Action: Check parameter setting. Run Undefined Neighboring Cell Recording.
Solution: Define or remove neighboring cells.

Missing Handover Relations

Missing handover relations will act as coverage holes. Suddenly the MS can not perform handover to the best server (i.e. not in the BA-list), instead a handover is performed to the second best candidate.

Action: Check existing handover relations. Check with actual site positions and coverage plots if more relations need to be added. Run Undefined Neighboring Cell Recording.
Solution: Add missing handover relations.

Wrong Output Power due to Faulty Transceiver
Action: Check output power setting. Check BTS error log

Poor Inter-MSC/BSC Handover Performance
For outer or external cell, wrong definitions in either MSC or BSC may be reason for the problem.
Action: Check inter-MSC/BSC handover performance.

Many Neighbors Defined

Many defined measurement frequencies defined (>16) will decrease the accuracy of the mobile measurements to locate the best six servers. Many measurement frequencies mean few samples per frequency and problem for mobiles to decode the BSIC.

Action: Check number of definitions.
Solution: Remove unnecessary definitions.

Too Many Measurement Channels Defined
Action: Check number of measurement channels
Solution: Remove unnecessary measurement channels.

Bad Use of Radio Network Features
Incorrect use of radio features such as Dynamic Power Control, Locating, Intra-Cell Handover, Frequency Hopping, etc.
Action: Check Feature parameter setting.
Solution: Correct strange and erroneous parameter setting.

Delayed Handover Decision
A delayed handover decision can be due to congestion in the target cell.
Action: Check handover parameters.

Wrong Locating Parameter Setting
Action: Check locating parameters
Solution: Correct parameter setting.

Bad Radio Coverage
Action: Check coverage plots.

High Interference, Co-Channel or Adjacent

The potential handover candidate is disturbed by interference. Outgoing handover due to bad uplink quality may indicate interference from co-channel another MS. On the border, the quality may be rather bad and the signal strength low. Bad downlink quality may indicate interference from another co-channel base station.

Action: Check interference. Check if many handovers are performed due to downlink or uplink bad quality.
Solution: Change frequency plan.

Lower Output Power on TCH than on the BCCH in the Target Cell
Action: Check output power. Check BSTXPWR and BSPWR definition

Not good neighboring cell relation
Action: Delete the relation if not necessary.

Receiver Antenna Problem or RBS HW problems (in candidate cell)
Action: Check antenna installation. Check RBS HW and Error log of the target cell