how to analyze handover in dt

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How to analyze Handover Issues in DT

Handover

What is…• Handover procedure in network?• Handover Um signaling flow?• Purpose of Handover Analysis in DT?


Neighbor Cell_1Neighbor Cell_1

Serving CellServing Cell

Neighbor Cell2Neighbor Cell2

Handover Procedure

Downlink R

xLevel

(Serv

er)

Downlink RxLevel

(Neighbors)

Downlink RxLevel

(Neighbors)

Uplink R

xLevel

Downlink M

R

Downlink MR (Measured by MS)•Downlink RXLEV(Sever)•Downlink RXQUAL(Sever)•RXLEV_NCELL(n) on the 6 best neighboring cells (+BSIC)

Uplink MR (Measured by BTS)•RXLEV Uplink•RXQUAL Uplink•TA•Current BTS TX power

HO command

HO command

BSC

HO command

HO command

� MS in dedicated mode will continuously perform measurements on servingand neighbor cells.

�Measurement Reports are sent to BSC and used in the handover algorithm.

� Serving BSC decides whether a handover is necessary and send HO command

Serving CellServing Cell

Neighbor Cell2Neighbor Cell2

Handover Success


Handover signaling on Um(TEMS Message)

BCCH of Neighbors to be measuredServing Cell:

DL DTX is ONRxLev(sub) is -109+ 62 =--49dBmRxQual (sub) is 0

The RxLev ofNeighbors

Tell MS how to access to Target TCH Channel

Target cell Description

Target channel DescriptionChannel Type, TimeslotTSC RF hoppingMAIO, HSN

Inter cell Synchronous Handover

Speech version: EFR

Handover Complete ≠Handover Success!!!

Only after SABM & UA in Layer2 Message, handover success!


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 aud it, and data configuration audit together.

The purpose of Handover Analysis

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


• Case1 Intra cell HO• Case2 Synchronous Inter cell HO• Case3 Asynchronous Inter cell HO• Case4 Inter BSC HO• Case5 Inter MSC HO

Handover Types(Network Topology)

Operator

CELL CELL CELL CELL

Case 1

Synchronous

Case 2Asynchronous

Case 3Case 4 Case 5

CELL CELL CELL CELL

MSC MSC

BSC BSC BSC BSC

BTS BTS BTS BTS BTS BTS


Handover Types(Algorithm)

� In DT, the most common handover types happens in following order of priority

� TA� RxQuality� RxLevel� PBGT� OL&UL

HO

OM Forced HOOM Forced HO

Directed RetryDirected Retry

TA Emergency HOTA Emergency HO

Bad Quality HOBad Quality HO

Rapid Level Drop HORapid Level Drop HO

Load HOLoad HO

Interference HOInterference HO

Edge HOEdge HO

Layer HOLayer HO

PBGT HOPBGT HO

Fast Moving MS HOFast Moving MS HO

Overlay/Underly HOOverlay/Underly HO


� PBGT HO Exp:

When: RXLEV_NCELL(n) – (RXLEVEL+ PWR_DIFF) > HO_MARGINMS is handed over to the neighboring cell .

� HO_MARGIN is usually set to 3 to 6 dB.

� 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.

PBGT Handover

Distance

ServingCell

TargetCell

RXLEV_NCELL(n)

RX LEVEL + PWR_DIFF

Power-budgetPBGT HO

HO_MARGIN > 0

A typical PBGT HO


Quality Handover Exp:

When:DL RxQuality >= DL Quality HO Thresholdhandover caused by DL RxQuality takes place.

When:UL RxQuality >= UL Quality HO Thresholdhandover 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.

Quality Handover

Handover was performed to a better quality cell just after experiencing quality problems.Handover was performed to a better quality cell just after experiencing quality problems.


EDGE Handover

Edge Handover Exp:

When:DL RxLevel < Edge HO DL RX_LEV Thresholdhandover caused by DL RxLevel takes place.

When:UL RxLevel < Edge HO UL RX_LEV Thresholdhandover 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.Handover was performed to the cell with higher downlink Rxlev.


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.An example of intracell handover afterexperiencing quality problems.


Other types of handover are not easy to judge or no t easy to encounter in DT test, for example :�TA handover� Directed Retry� Load handover� Rapid Level Drop HO� Fast Moving MS HO

Other types of handover

For analysis of different types of handover or hand over caused by uplink, it is recommended by traffic analysis or si ngle-user signaling tracing.

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 hando ver process taking place a little late, which is called dragged handov er.

� 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.Dragged Handover often cause Call Drop or Handover Failure.


Power Control Effect

� Power Control Effect sometimes may mislead us, you might think that handover is happening too late between two neighbors.

� 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.

Power control


Ping-pong Handover

Ping-pong handovers occur when the MS is handed ove r from one cell toanother but is quickly handed back to the original cell.

Ping-pong HO

� The shot shows two ping-pong 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.

� 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 domina nce after the handover was 9dB, which represents a much healthier handover.


Missing Neighbor Relation

If a handoff is not performed to a neighbor cell th at seems to be best server, there is a possibility of a missing neighbo r 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.

� At this time Neighbor audit should be performed.


� Sometimes you will see a good handover candidate in the neighbor list but handover will not take place and call will drop.

� Although that neighboring cell with a very good signal level appears to be a neighbor, in reality it is not. Just because the serving cell has another neighbor with the same BCCH, this cell appears in the list.

Fake Neighbor Relation


Handover Failure

Handover Failure: Handover attempt was failed and t he call returned back to its all channel.

Handover Failure

Target Cell is � BCCH is 13� BSIC is 65Target channel is � 900M RF hopping Channel. � Half Rate� MA list is 1, 17, 18, 19, 20, 21, 22, 23, 24� MAIO is 4, HSN is 31, � TSC is 5.� Channel mode is AMR(SV3)

The handover has not successfully completed yet!

Handover Failure is caused by T200 ExpiredAnother example: Excessive Number of Handover

Failure due to Hardware Problem

Proposal of Handover Issue

•Dragged Handover•Ping–Pong Handover•Missing Neighbor Relation•Handover Failure


Solutions of Handover Issues

Issue Proposal

Dragged Handover

�Check if the parameter of handover, such as: PBGT, Quality, Interference, Edge 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 at measuring time.

Ping-pong Handover

� 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. � 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.

Handover Failure

� Find out the cause of Handover failure form Um signaling flow from Layer2 and 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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how to analyze handover in dt

Documents

experiencing

edge handover

neighbor audit

hardware problem

handover failure

traffic analysis

dominant coverage

pong handover