Hand Over

Handover is key technology of Mobile

communication system and make

continued conversation possible.

Handover algorithm in Huawei product

is flexible and powerful

� Upon completion this course, you will be

able to:

�Understand the type of handover.

�Master handover judgment flow

�Configure handover data

�Master handover signaling flow

ChapterChapter 1 1 Introduction of HandoverIntroduction of Handover

ChapterChapter 2 2 HO Algorithm processHO Algorithm process

ChapterChapter 3 3 HO Data ConfigurationHO Data Configuration

ChapterChapter 4 4 HO Signaling processHO Signaling process

Purposes of HO

� To keep a continuous communication with a moving MS

�To improve network service performance

− To reduce the call drop rate

− To reduce the congestion rate

� Emergency HO

�Timing Advance (TA) Emergency HO

�Bad quality (BQ) Emergency HO

�Rx_Level_Drop Emergency HO

� Interference Emergency HO

� load HO

� Normal HO

�Edge HO

�Layer HO

�Power Budget (PBGT) HO

� Speed-sensitive HO (Fast moving MS HO)

� Concentric Cell HO

Classification by Reason

Classification by Synchronization

� Synchronous handover: source

and target cell belong the same

BTS

� Asynchronous handover: source and

target cell belong the different BTS






1.1 1.1 General HO processGeneral HO process

1.2 1.2 Measurement report preprocessingMeasurement report preprocessing

1.3 1.3 Penalty processingPenalty processing

1.4 1.4 Basic ranking and Secondary rankingBasic ranking and Secondary ranking

1.5 Condition of 1.5 Condition of handoverhandover

General process of HO Algorithm

M.R.preprocessing

Penalty processing

Basic ranking

Secondary ranking

HO judgment

TA emergency HO

BQ emergency HO

RSD emergency HO

Interf. emergency HO

Load Sharing HO

Edge HO

Layer HO

PBGT HO

Processing program

OM forced HO

Directed retry

Overlaid/underlaid HO

Fast moving MSHO

1

1







Measurement Report

� Uplink MR includes uplink receiving level and quality.

� Downlink MR includes downlink receiving level, downlink

receiving quality of the serving cell and other downlink

receiving levels from the neighbor cells.

Serving cell Neighbour cell

The downlink measurement report

of the serving cell

The uplink measurement

report of MSThe downlink measurement reportof the neighbour cell (BCCH)

Measurement Report

Period of measurement report

� The downlink MR is sent to BTS in SACCH uplink

�The interval is 480ms/per time when MS is on TCH

�The interval is 470ms/per time when MS is on SDCCH

12TCH 12TCH1SACCH 1 Idle

480ms 4 TCH multi-frames

MR interpolation

�Every time BSC receives a measurement report, there will

be an update to the basic rank of the cells.

�BTS may fail to receive the measurement report from MS.

Before the rank-update, BSC needs to recover the lost

measurement reports according to Filter Table. If the lost

MR amount is within the allowed range, then recovers the

lost MR according to the algorithm.

MR MR MR MR MR

Measurement report No. n

Measurement report No. n+4

Continuous MR flow

How to interpolate MR?

Filter----Average several continuous MR.

MR MR MR MR MR MR

Continuous MR flow

filtering







Penalty Processing

� There are altogether four types of penalty process (second step of

HO algorithm process )

� Penalty on the target cell when a HO fails.

� Penalty on the original serving cell when an emergency HO

( base on BQ and TA ) is performed.

� Penalty on other high priority layer cells after a fast moving HO

is performed.

� A new HO attempt is prohibited within the penalty time after an

overlaid/underlaid HO fails.

BTS

HO failure

BSC

Cell A

Cell B

Penalty on the Target Cell

Punish the target cell when a HO fails. This is to avoid the

MS to select this cell again in next HO judgment.

BTS

BQ& TA HO

BSC

Cell A

Cell B

Penalty on the Source Cell

Punish the original serving cell when an emergency HO

( due to BQ and TA) occurs.

Back? No way!Back? No way!

Umbrella

Micro cell

penalty on Non-umbrella Layer

� Giving penalty on the other three layers after MS handovers to

Umbrella cell by fast-moving-HO. This is to keep MS staying in

the umbrella cell and avoid frequent HO.

Underlaid

Overlaid Do not attempt

again after a failed HO!

Penalty on Overlaid/underlaid Cell

� A new Overlaid/underlaid HO is prohibited within a penalty time

after an Overlaid/Underlaid HO failure.







Procedure of Ranking

� Basic ranking and secondary ranking of cells are major parts of

the HO judgment. Ranking is made through 16bits-algorithm.

The serving cell and the neighbor cells will be listed in a cell list

according to their 16bits value. The ranking processes include:

�M rule

�K rule

�16bits ranking

M rule � Only the cells with received signal level satisfy the following conditions

can be put into the candidate cell list.

� For serving cell

� RX_LEV (o) >MSRXMIN(o) + MAX(0,Pa(o))

� For Neighbor cell

� RX_LEV (n) > MSRXMIN(n)+ MAX(0,Pa(n))+ OFFSET

�Pa(0) : MS_TXPWR_MAX(0) – P

�Pa(n) : MS_TXPWR_MAX(n) – P

�MS_TXPWR_MAX( ) : The appointed MS transmitting power by the

BSS.

�P : Max_Power_of_MS

�Max_Power_of_MS : MS maximum transmitting power

K rule Criterion

� After the M rule , the serving cell and candidate neighbor cells are

ranked in descending order according to the receiving level only

� Both the serving cell and the neighbor cells have their own 16bits value.

The smaller the value is, the higher the priority and position the cell is in

the cell list.

� The 1st-3rd bits: bit value is decided according to the cell signal level and the penalty process taking place beforehand.

�The values come from max. 6 candidate cells and 1 serving cell

according to the level ranges from 000~110. The value for the cell

with the strongest signal level is 000.

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The 4th bit: determined by HO hysteresis

� The 4th bit: bit value is determined by inter-cell HO ( of the same layer )

hysteresis.

� The 4th bit of the serving cell is always 0,

� The receiving signal level of the neighbor cell >= The receiving level of the

serving cell + Inter-cell HO ( of the same layer ) hysteresis, bit 4th is set to 0.

� The receiving level of the neighbor cell < The receiving level of the serving

cell + Inter-cell HO ( of the same layer ) hysteresis, bit 4th is set to 1.

�Note: In PBGT HO, whichever the greater of the inter-cell ( of the same

layer ) hysteresis and PBGT threshold, that value will be used in the

PBGT HO.

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The 5th—10th bit: determined by Layer

� The 5th-10th bits: bit value is decided according to their position in

Huawei hierarchical network structure.

� When the signal level of the neighbor cells or the serving cell is lower

than the layer HO threshold and hysteresis, this function is turned off

and all bits are set to 0.

� That is to say only when the above criterions are met, then this

function take effect.

� Huawei cell layers can be divided into 4 layers and each layer can be

further divided into 16 different priorities. So there are 64 different

priorities in Huawei hierarchical cell structure.

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GSM900

Cell

Micro Cell

Umbrella

Cell GSM 900

GSM1800 GSM1800GSM1800

GSM 900 GSM 900 GSM 900

GSM900GSM900

GSM1800GSM1800

GSM900 GSM900

GSM1800 GSM1800

GSM1800

Cell

Hierarchical cell structure

The 11th bit: determined by load

� The 11th bit: bit value is decided by cell-load-sharing criterion.

� Serving cell: if Cell Load>= Start threshold of load HO, bit 11th is set to 1,

otherwise is set to 0.

� Neighbor cell: if Cell Load>=Receive threshold of load HO, bit 11th is set to 1,


� Refer to Load HO Table for the load HO threshold and load req. on

candidate cell.

�Clue : When the cell load is higher than the threshold, then the bit 11th is

set to 1.This is done in order to put the cell in a lower part of the cell list.

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The 12th/13th bit: determined by co-BSC/MSC

� 12th bit: bit value is decided by co-BSC criterion.

�Serving cell: is always set to 0.

�Neighbor cell: if co-BSC with the serving cell, 12th bit is set to 0,


�When the signal level from the neighbor cell or the serving cell is lower

than layer HO threshold and hysteresis. This function is turned off and

the value is set to 0.

� If the parameter – “Co-BSC/MSC Adj.” in the HO control table is set to

“No”, then this function is turned off and the value is 0.

� 13th bit : Bit value is decided by Co-MSC parameter, having the same

concept as the 12th bit.

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The 14th bit: determined by Layer HO

� The 14th bit: Layer HO threshold adjustment bit

� Serving cell criterion

�Receive level >= layer HO threshold – layer HO hysteresis, bit 14th

is set to 0. At the same time, bit 13th, 12th and 10th—5th bits are

set to 0.

� If the above criterion is not met, then bit 14th is set to 1.

�Example : 20-5 = 15 ( -95 dBm )

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The 14th bit: determined by Layer HO

� Neighbor cell criterion

�Receive level >=layer HO threshold + layer HO hysteresis, bit 14th is set to 0. At the same time, bit 13th, 12th and 10th—5th bits are set to 0

� If the above criterion is not met, then bit 14th is set to 1.

�Example : 20+5 = 25 ( -85 dBm )

� note

�The layer HO threshold and hierarchical hysteresis correspond to the value of that individual cell’s value.

�Usual situation : When the neighbor cells are of the same layer, each of the neighbor cell’s layer HO threshold value will be the same. Same concept goes for the layer HO hysteresis. This can maintain the entire hierarchical layers of the cell.

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The 15th bit: determined by Cell Type

� The 15th bit: Bit value is decided by cell type

− Serving cell or Neighbor cells:

− When cell type is extension cell �1.

− When cell type is normal cell �0.

� The 16th bit: Reserved bit

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Types of HO

� Emergency HO

�TA HO

�BQ HO

� interference HO

�Rx_Level_Drop HO

� Load HO

� Normal HO

�Edge HO, layer cell HO and PBGT HO

� Fast moving HO

� Overlaid/underlaid HO

TA&Bad Quality HO

� TA HO criterion :

�TA of the serving cell > TA Thrsh.

� BQ HO criterion :

�The average value of the uplink quality of the serving cell >

� UL Qual. Thrsh.

�The average value of the downlink quality of the serving cell >

DL Qual. Thrsh.

� Requirements on the target cell (same as the above two types of

HO)

�Select the first cell in the neighbor cell list i.e. lowest 16bits

value.

Rx_Level_Drop HO

� Rx_Level_Drop HO

�Due to downlink signal level drop

�Triggered upon detecting rapid level drop during MS busy

mode.

� Requirements for the target cell:

�The target cell is the first cell in the neighbor cell list.

Interference HO

� Interference HO (DL&UL) :

�When the receiving level > receiving threshold level. But

Receiving quality < threshold of quality interference.


�The target cell is in the cell list.

Cell Load HO

� Cell Load HO Criterions :

�System load of BSC < Permissible load of HO threshold

�Load of serving cell > Load HO threshold

� Requirements for the target cell

�Load of target cell < Load HO threshold

BTS

BSC

Edge HO

� Edge HO Criterion:

� In N seconds, when there are P seconds that neighbor

cell’s DL or MS’s UL signal level is lower than the Edge HO

threshold. Then the criterion is met and Edge HO occurs.

This method utilizes the P/N rule.


�The target cell should be ranked in front of the serving cell.

Layer HO

� Layer HO criterions:

�Serving cell :

− No requirement.

�Target cell :

− Layer of the target cell is lower than the serving cell.

− Receive level of the target cell > layer cell threshold +

layer cell hysteresis.

− Target cell should be ranked in front of the serving cell.

The priority of target cell should be higher than the

serving cell’s.

PBGT HO

� PBGT HO Criterions :

�Target cell’s path loss is smaller than the serving cell’s path

loss by the PBGT threshold value.

�Satisfying the P/N rule.

�Target cell should be ranked in front of the serving cell.

� Note :

�PBGT HO can only occur between same-priority cell. If the

system permits PBGT HO for the cell, PBGT HO can occur

in either inter-BSC or inter-MSC.

Fast-Moving HO

� When the serving cell is micro cell :

�When the MS has traveled through P numbers of cell, and

there are Q (=<P) numbers of cell that the MS has traveled

in high speed, the criteria is satisfied. MS will be handed

over to umbrella cell.

� When the serving cell is umbrella cell :

�When the MS is traveling high speed in umbrella cell, a

greater penalty can be given to the micro cell for a duration

of time (penalty time). In this way, the MS will not use the

micro cell. Note :In this case, the micro cell is only used

under urgency conditions( Poor TA and BQ ).

Overlaid/Underlaid Cell

Assume, 6TRX in one cell, one CDU and one SCU are adopted

Where can we configure BCCH CH? CDU or SCU?

SCU

CDU

Antenna

Antenna

TRX

TRX

TRX

TRX

TRX

TRX

Overlaid/Underlaid Cell

� Purpose

�Maximize coverage area

�Reduce interference and improve frequency reuse density

� Construct method

�Different combiner loss

�Decrease the power of TRX

�By HO parameter, it is called IUO

� Classify

�Normal Underlaid/Overlaid

� IUO: Enhance Underlaid/Overlaid

How to Allocate SDCCH/TCH?

� SDCCH

�SDCCH are always allocated in Underlaid

�Layer is allocated according the transmission delay of

access burst

� TCH

�The receiving level

�TA

�They are included in MR of SDCCH CH on uplink reported

by BTS

Factor of Handover between Under/Overlaid

� Normal Underlaid/Overlaid

�Receiving level

�Time Advance

�Quality

� Enhanced Underlaid/Overlaid

�Receiving level

�Time Advance

�Quality

�Traffic Load of Underlaid

underlaid

overlaid

:

Division of underlaid and overlaid is decided by MS

downlink receive level ,TA value and quality.

Normal Overlaid/Underlaid HONormal Overlaid/Underlaid HO

The quality boundary is elided in this figure

Normal Overlaid/Underlaid HO

� Criterion for HO from overlaid to underlaid:

�TA value => TA threshold + TA hysteresis OR

� RX_LEV <= RX_LEV threshold －RX_LEV hysteresis OR

� Qua. =>Qua. threshold

�Satisfying P/N rule

� Criterion for HO from underlaid to overlaid:

�TA value =< TA threshold － TA hysteresis AND

� RX_LEV >=RX_LEV threshold + RX_LEV hysteresis AND

� Qua. =<Qua. threshold


Enhance Overlaid/Underlaid HO

� Criterion for HO from overlaid to underlaid:

�TA value => TA threshold + TA hysteresis OR

� RX_LEV <= O to U level OR

� Qua. =>Qua. threshold


� Criterion for HO from underlaid to overlaid:

�TA value =< TA threshold － TA hysteresis AND

� RX_LEV >= U to O level AND

� Qua. =<Qua. Threshold

� If the box “U to O HO traffic threshold” is checked, just in the condition

that underlaid load is more than this threshold, U to O handover based

on previous conditions can be triggered






ChapterChapter 4 4 HO Signaling process HO Signaling process

1.1 1.1 Intra BSC HandoverIntra BSC Handover

1.2 1.2 Intra MSC HandoverIntra MSC Handover

1.3 1.3 Inter MSC HandoverInter MSC Handover

Intra-BSC Handover Signaling process

MS MSBTS1 BTS2BSC MSC

Measurement Report from MS

Channel_Active

Channel_Active ACK

HANDOVER COMMAND

Handover Access

Handover_DetectPHY INFO

First SABM

Establish_IND

PHY INFO

Handover Complete

Handover_Performed

Intra-BSC Handover Signaling process

� Attention

� In asynchronous HO, if MS could not reach the new TCH

channel after the target cell has sent PHY INFO up to max

times, the target cell reports CONN FAIL IND to BSC with

the reason: HO access failure.

�After the above message is received, BSC release the

assigned TCH channel in the target cell .

�Max resend times of physical information*Radio link

connection timer > Time interval between EST IND and HO

DETECT (120~180ms). This is to make sure that the

physical information reach MS.

MS BTS BTSBSC MSCMeasurement Report

Measurement Report

Channel_Activate

Channel_Activate ACK

Handover Command (Old FACCH)

Handover Access (New FACCH)

Handover Complete (New FACCH)

RF Channel ReleaseHandover Performed

T09++

T12++

T10++

T13++

Attempted outgoing internal inter cell handovers

Attempted incoming internal inter cell handovers

Successful incoming internal inter cell handovers

Successful outgoing internal inter cell handovers

(Original) (Target)

Intra-BSC Handover Signaling processIntra-BSC Handover Signaling process

Measurement Points of Intra BSC Handover

� Handover formula definition

� Internal inter cell radio handover success rate

�=(Successful incoming internal inter cell handovers +

Successful outgoing internal inter cell handovers) /

(Incoming internal inter cell handovers + Outgoing internal

inter cell handovers )

� Internal inter cell handover success rate

� =(Successful incoming internal inter cell handovers +

Successful outgoing internal inter cell handovers) /

(Attempted incoming internal inter cell handovers +

Attempted outgoing internal inter cell handovers)

� Internal inter cell radio handover success rate >= Internal inter

cell handover success rate





MS BTS BTSBSC1 BSC2MSC(original) (Target)

Measurement Report

Measurement ReportHandover Required

Handover Request

Channel_Active

Channel_Active_ACKHandover_Request_ACK

Handover Command

Handover Access

Handover Detect

Handover Complete

Handover Complete

Clear Command (HO Successful)

RF Channel Release

Clear Complete

Attempted outgoing interBSC inter cell handovers

Attempted incoming interBSC inter cell handovers

Successful incoming inter BSC handovers

Successful outgoing interBSC inter cell handovers

Intra-MSC HO Signaling processIntra-MSC HO Signaling process

Channel Release process

MS BTS BSC MSCDisconnect

Release

Release Complete

Clear_CMD

Clear_CMPChannel Release

Deactive_SACCH

First DISC

UA

Release_IND

RF_Release_REQ

RF_Release_REQ_ACK





MSC-BMSC-A VLR-BBSC-A BSC-B

HO-REQUIREDMAP_Prepare_HO

MAP_Prepare_HO_ACK

MAP_Allocate_HO_NUM

MAP_Send_HO_Report

MAP_Send_HO_Report_ACK

MS

HO-REQUEST

HO-REQUEST-ACK

HO-Command

MS

HO-AccessMAP_Process_Access_Signalling

HO-CompleteMAP_Send_End_Signal

Clear-Command

Clear-Complete

MAP_Send_End_Signal_ACK

Some intermediate steps are omitted

IAI

ACM

Signaling process between MSC

MSC-BMSC-A VLR-BBSC-A BSC-B

HO-REQUIREDMAP_Prepare_HO

MAP_Prepare_HO_ACK

MAP_Allocate_HO_NUM

MAP_Send_HO_Report

MAP_Send_HO_Report_ACK

MS

HO-REQUEST

HO-REQUEST-ACK

HO-Command

MS

HO-AccessMAP_Process_Access_Signalling

HO-CompleteMAP_Send_End_Signal

Clear-Command

Clear-Complete

MAP_Send_End_Signal_ACK

Some intermediate steps are omitted

IAI

ACM

Signaling process between MSC

Inter-MSC HO Signaling process

� Signaling process – Abnormal conditions

�The following conditions will cause HO failure

− MSC-B fails to identify the target cell.

− MSC-B does not allow HO to the indicated target cell.

− The target cell has no channel available.

− VLR-B has no HO number available.

− HO error or unsuitable data.

Roaming

E

MS

MSCa MSCbMSCb'

VLRbVLRb'

BSS2

BSS2'

Radio transmission signal measurementHO REQUIRED (target cell table)

Perform subsequent HO(MAP) (target cell ID, serving cell ID, MSC number)

Perform HO

(target cell ID, serving cell ID, channel type)HO REQUEST (PCM&Channel type)

HO REQUEST ACKNOWLmargin (including New TCH number and HO number)

Allocate HO number

Send HO report(HON)Radio channel ack. (MAP) (includes New TCH number and HON)

IAI

ACM

Subsequent HO ack.

HO COMMAND HO DETECT

HO COMPLETESend end signal (MAP)

ANS

End signal (MAP)Release HO report Release HON

CLEAR COMMAND

CLEAR COMPLETERelease (TUP)

Cut physical connection between MSCa and MSCb

End signal (MAP)

Release (TUP/ISUP)Release HO report Release HON

Cut physical connection between MSCAa and MSCb'

~~ ~~

MS

Inter MSC HO—Subsequent HO process

Highway

MSC-AMSC-C

MSC-B MSC-C

Inter MSC HO—Subsequent HO process

� Subsequent HO

Inter MSC HO Signaling process

� Statistics counter—same as Intra MSC HO, Statistics is

handled by BSC

� HO formula-- same as Intra MSC HO

Major differences

� There is no “HO request” information for intra-BSC HO, and all of the HO are analyzed

and processed in BSC. Once the target cell as required is found in the BSC, “Channel

activation” information is sent to it directly.

� When the target cell is not in the same BSC, BSC reports CGI numbers of the serving

cell and target cell, and HO cause to MSC through “Ho-Required”. When MSC finds

the LAC of the target cell is in the MSC, it sends “Ho-Request” to the BSC of the target

cell, and the target BSC activates the target cell channel to complete the following

procedure.

� When MSC finds that the target cell LAC does not belong to the MSC, it will query its

“LAI and GCI Table” (including LAC and router address of the adjacent MSC), and

send “Prepare-HO” message to the target MSC-B according to the router address.

The message includes CGI of the target cell and indication whether or not to allocate

HO number, etc. According to the message, the target MSC-B sends “HO-Request”

message to the target BSC-B after demanding HO number (unless it is not required in

the indication) from VLR-B, and sends “Prepare-HO acknowledgement” to serving

MSC after received “HO-Request acknowledgement”, to execute the next procedure.

Major differences

� Inter BSC HO transfers “HO-REQ” message through MSC,

with CGI of the serving cell and target cell carried in the

message.

� Intra BSC HO does not have any CGI in any messages, it is

handled inside BSC.

� Intra BSC HO only sends “HO-Performed” to MSC upon

completion of HO, and MSC is not involved before that time.

� In inter BSC HO, MSC is involved since the HO request .

Summary

In this course, we have learned:

� Classify of handover

� Judgment and Ranking step

� Handover Data Configuration

� Handover signaling Flow

SummarySummary

Date post:	17-Nov-2014
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