How to Analyze Handover in DT

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How to analyze Handover Issues in DT

www.huawei.com

HUAWEI TECHNOLOGIES CO., LTD.

Huawei Confidential

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

Handover Procedure

Cell_1 DNeighbor o

ow (Ne nlin k ig hbo RxL rs) eve l

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

l ve e L

Rx k in nl r) d ow rve an DServing m Neighbor Cell2 e mCell el (S co v Le HO x R R k kM n li in Serving Cell Neighbor Cell2 Up wnl o eDss c c u er S v o nd Ha

HO

co m m an d

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


MS in dedicated mode will continuously perform measurements on serving and neighbor cells. Measurement Reports are sent to BSC and used in the handover algorithm.

BSC

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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!!! The RxLev of Neighbors Only after SABM & UA in Layer2 Message, handover success!

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

BSC

BTS

BTS

CELL CELL CELL CELL CELL CELL CELL CELL Synchronous Asynchronous

Case 2 Case 1 Case 3

Case 4


Case 5

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• • • • •

Case1 Case2 Case3 Case4 Case5

Intra cell HO Synchronous Inter cell HO Asynchronous Inter cell HO Inter BSC HO Inter MSC HO

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Handover Types(Algorithm) In DT, the most common handover types happens in following order of priority

TA RxQuality RxLevel PBGT OL&UL


OM Forced HO Directed Retry

TA Emergency HO

Edge HO

Bad Quality HO

Layer HO

Rapid Level Drop HO

PBGT HO

Interference HO

Fast Moving MS HO HO

Load HO

Overlay/Underly HO

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PBGT Handover Target Cell

Serving Cell

PBGT HO Exp: When: RXLEV_NCELL(n) – (RXLEVEL+ PWR_DIFF) > HO_MARGIN

HO_MARGIN > 0

MS is handed over to the neighboring cell .

RXLEV_NCELL(n)

RX LEVEL + PWR_DIFF

HO_MARGIN is usually set to 3 to 6 dB.

PBGT HO

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.


Power-budget Distance

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. HUAWEI TECHNOLOGIES CO., LTD.

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. Huawei Confidential

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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. HUAWEI TECHNOLOGIES CO., LTD.

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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. 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 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.


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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 Channel. Handover Failure Half Rate MA list is 1, 17, 18, 19, 20, 21, 22, 23, 24 has not MAIOThe is 4,handover HSN is 31, successfully completed yet! TSC is 5. 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 handover threshold, and P/N Value is set too high. Dragged Handover Check if there is no dominant coverage. Referring to the solution of Low Coverage. Check traffic volume ofproblem. target cell at measuring time. checking Checkififthe there is hardware Referring to hardware guide Check if there is no dominant coverage. Referring to the solution of Low Coverage. Ping-pong Check if the parameter of handover, such as: PBGT, Quality, Handover Interference, Edge handover threshold, and P/N Value is set too low . Check if there is overlapping coverage area, which may also cause pingpong handover. Neighbor Audit. Check if there is neighbor missing according to data Missing Neighbor configuration in OMC. Find out the cause of Handover failure form Um signaling flow from Layer2 and Layer3 Message. Handover Failure 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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Thank you www.huawei.com

How to Analyze Handover in DT

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