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HANDOVER PROCESSBSS Parameters S9

Contents 1. Overview 2. Handover Causes & Priorities 3. Threshold Comparison Process 4. Target Cell Evaluation Process 5. Handover Algorithms

• Power Budget (PBGT)• Level & Quality (RXLEV & RXQUAL)• Umbrella (& Combined Umbrella/PBGT)• MS Speed (FMMS & MS_SPEED_DETECTION)

6. Imperative Handovers• Distance• Rapid Field Drop (RFD) & Enhanced Rapid

Field Drop (ERFD) 7. Handover Timers

Overview

Why are handovers needed?

Call continuity - to ensure a call can be maintained as a MS moves geographical location from the coverage area of one cell to another

Call quality - to ensure that if an MS moves into a poor quality/coverage area the call can be moved from the serving cell to a neighbouring cell (with better quality) without dropping the call

Traffic Reasons - to ensure that the traffic within the network is optimally distributed between the different layers/bands of a network

Handover Causes

Timing Advance

Adjacent Cells

Downlink Quality

Uplink Quality AV_RXQUAL_UL_HO

AV_RXQUAL_DL_HO

Downlink Level

Uplink Level AV_RXLEV_UL_HO

AV_RXLEV_DL_HO

AV_RANGE_HO

AV_RXLEV_NCELL(n)

QUALITY&INTERFERENCE

LEVEL

DISTANCE

PERIODICCHECKS

UMBRELLA

POWER BUDGET

IMPERATIVE HOCHANNEL ADMINISTRATION

DIRECTED RETRY

THRESHOLD COMPARISON

RAPID FIELD DROP

MS SPEEDMS Speed AV_MS_SPEED

Others causes;- Intelligent Underlay/Overlay (IUO)- Traffic Reason Handover (TrHO)- Direct Access to Desired Layer/Band (DADL/B)

Handover Priorities

If 2 or more handover (PC) criteria are satisfied simultaneously the following priority list is used in determining which process is performed;

1. Uplink and downlink Interference2. Uplink quality3. Downlink quality4. Uplink level 5. Downlink level6. Distance7. Enhanced (RFD)8. Rapid Field Drop (RFD)9. Slow moving MS10. Better cell i.e. Periodic check (Power Budget HO or Umbrella HO)

11. PC: Lower quality/level thresholds (UL/DL)12. PC: Upper quality/level thresholds (UL/DL)

e.g if downlink quality & slow moving mobile criteria were satisfied simultaneously HO with cause downlink quality performed

Threshold Comparison Process

• Threshold comparison;• Quality• Level• Distance• Load

• Periodic checks;

• Power budget• Umbrella

IFAV_RXQUAL_DL_HO < hoThresholdsQualDLTHEN Downlink Quality HO is performed

The Handover process may be triggered by:

IFEnablePowerBudgetHO = YesTHENPBGT comparison performed everyhoPeriodPBGT sec

Averaged value obtained from measurement averaging process using hoAveragingQualDL

Target Cell Evaluation Process

Threshold levelbased on nx & px

Target Cell Evaluation (1/3)Radio conditions

AV_RXLEV_NCELL(n) > rxLevMinCell(n) + Max (0, A)A = msTxPwrMax(n) - PP = depending on MS Classmark

1.

In all Handover cases

AV_RXLEV_NCELL(n) > hoLevelUmbrella(n)1’.

Except for Umbrella Handover

PBGT > hoMarginLev/Qual(n) where PBGT = (AV_RXLEV_NCELL(n) - AV_RXLEV_DL_HO)-(btsTxPwrMax - BTS_TXPWR)(Note: enableHoMarginLevQual must = Yes) - for RxLev & RxQual handovers

2’.

PBGT > hoMarginPBGT(n) wherePBGT = ((msTxPwrMax - msTxPwrMax(n))-(AV_RXLEV_DL_HO - AV_RXLEV_NCELL(n)) - (btsTxPwrMax - BTS_TXPWR))

2.

The additional condition

For imperative handoversonly Eq. 1 has to be satisfied

Target Cell Evaluation (2/3)Cell Preference based on Load conditions

Best candidates to RR Management:• intra BSC HO max 16 cells under the same BSC as the source

cell• inter BSC HO numberOfPreferredCells

Load check of Candidates by btsLoadThreshold (0..100%)

If overloaded priority decreased by hoLoadFactor (0...7)1.

Comparison of priorities of Candidates (hoLevelPriority (0..7))2.

If two or more Adjacent cells with equal priorities

-> Ranking based on radio properties (RxLev)3.

Only for Adjacent Cellsof the same BSC(intra-BSC) analysis

Target Cell Evaluation (3/3)Load Evaluation Example

Case 1: All cells have equal priority

Cell a b c

Rx_Level -75 -80 -831. Load overl. overl. n.overl. hoLoadFactor 1 1 12. Priority 3 3 3 New Priority 2 2 3

3. Rx_Level -75 -80 -83

=> cell list c , a ,b

Case 2 : One cell with higher priority

cell a b c

Rx_Level -75 -80 -831. Load n./overl. n.overl. n.overl. hoLoadFactor 2 1 12. Priority 4 3 3 New Priority 4/2 3 3

3. Rx_Level -75 -80/-80 -83

=> cell list a,b,c (if cell a is not overload)=> cell list b,c,a

Power Budget Handover (1/2)Process & Parameters

Trigger ◦ Periodic Check ( hoPeriodPBGT )

Candidate Selection◦ Equation 1 & 2 used ◦ Priority and Load Considered

When used in association with Umbrella HO (& AdjCellLayer) PBGT handovers are only between cells of the SAME layer

hoPeriodPBGT 1 ... 63 (SACCH)enablePwrBudgetHandover Y / N

Parameter Range

rxLevMinCell(n) -110 … -47 dBmmsTxPwrMax(n) 0 … 36 dBmhoMarginPBGT(n) -24 … 63 dB

PBGT = ((msTxPwrMax- msTxPwrMax(n)) - (AV_RXLEV_DL_HO-AV_RXLEV_NCELL(n)) - (btsTxPwrMax - BTS_TXPWR)

PBGT = ((33dBm-33dBm)-(-90 - -80)-(42dBm-42dBm) = 10 dB

10 dB > 6 dB OK !!!!

AV_RXLEV_NCELL(n) > rxLevMinCell(n) + Max (0, msTxPwrMax(n) - msTxPwrMax)-80 dBm > -99 dBm + (33 dBm - 33 dBm) = -99 dBm

1.

2.

Equations 1 and 2 are used

AV_RXLEV_DL_HO = -90 dBmmsTxPwrMax = 33 dBm (= 2W)btsTxPwrMax = 42 dBm (= 16 W)BTS_TX_PWR = 42 dBm = (16 W)hoMarginPBGT(n) = 6 dB

Serving Cell: Best Adjacent Cell:

AV_RXLEV_NCELL(n) = -80 dBmrxLevMinCell(n) = -99 dBmmsTxPwrMax(n) = 33 dBm (= 2W)btsTxPwrMax = 42 dBm (= 16 W)

Power Budget Handover (2/2)Example

Handover due to Level (1/2)Process

Trigger ◦ Threshold Comparison (hoThresholdsLevUL/DL with px / nx )

Candidate Selection◦ Equation 1 used ◦ Equation 2 used if enableHoMarginLevQual = N◦ Equation 2' used if enableHoMarginLevQual = Y◦ Priority and Load Considered

hoThresholdLevUL/DL -110 … -47 dBm

px 1 … 32 nx 1 … 32

Parameter Value

rxLevMinCell(n) -110 … -47 dBmmsTxPwrMax(n) 0 … 36 dBmhoMarginLev(n) -24 … 24 dB

Handover due to Level (2/2)Example

Equations 1 and 2’ are used if parameter enableHoMarginLevQual is set “Yes”

hoMarginLev = 4dB

Cell B

Cell B is not selected as candidate for HO due to level since 2dB < 4 dB

(RxLev) Threshold defined by;hoThresholdLevUL/DL= -92 / -95 dBm

2 dB

Trigger for Handover due to Level

Cell A

Handover due to Quality (1/2)Process

Trigger ◦ Threshold Comparison (hoThresholdsQualUL/DL with px / nx)

Candidate Selection◦ Equation 1 used ◦ Equation 2 used if enableHoMarginLevQual = N◦ Equation 2' used if enableHoMarginLevQual = Y◦ Priority and Load Considered

hoThresholdQualUL/DL 0 … 7

px 1 … 32 nx 1 … 32

Parameter Value

rxLevMinCell(n) -110 … -47 dBmmsTxPwrMax(n) 0 … 36 dBmhoMarginQual(n) -24 … 24 dB

Handover due to Quality (2/2)Example

Equations 1 and 2’ are used if parameter enableHoMarginLevQual is set “Yes”

2 dB

hoMarginQual = 0 dB

Trigger for Handover HO due to Quality

A

B

Cell B is selected as potential candidate for HO due to Quality since 2 dB > 0 dB

Handover due to Interference (1/3)Process

Trigger: ◦ Threshold Comparison for Quality (hoThresholdsQualUL/DL with px / nx)◦ Threshold Comparison for Level (hoThresholdsInterferenceUL/DL with

px / nx) Candidate Selection

◦ Priority for InterCell / Intracell HO selected at BSC independently for UL / DL ◦ Priority InterCell HO◦ Quality HO if any candidate◦ If not IntraCell HO◦ Priority IntraCell HO

hoThresholdInterferenceUL/DL -110 … -47 dBm px 1 … 32 nx 1 … 32

enableIntraHoInterfUL/DL Y / N

Parameter Value

hoPreferenceOrderInterfUL/DL INTER / INTRA

Equations 1 and 2’ are used if parameter enableHandoverMarginQual is set “Yes” hoThresholdQual = 5hoThresholdInterferenceDL = -85 dBmhoPreferenceOrderInterfDL = intra

• Field strength higher than threshold (AV_RXLEV_DL_HO > hoThresholdsInterferenceDL• Bad quality (AV_RXQUAL_DL hoThresholdsQualDL Handover due to DL interference intra cell handover !!

Handover due to Interference (3/3)Example

Trigger for Handover due to Interference

Cell A

Cell BThreshold (Interference Lev) -85 dBm

5

0

RXLEV

RXQUAL

Umbrella Handover (1/3)Process

Used in multi-layer/band networks (better for bands - no speed criterion) Typically used in association with PBGT (Combined PBGT/Umbrella feature) Trigger

◦ Periodic Check (hoPeriodUmbrella) Candidate Selection

◦ Equation 1' used ◦ Consistency between MS classmark and target cell power constraints◦ Priority and Load Considered

enableUmbrellaHandover Y / NhoPeriodUmbrella 0 … 63 (SACCH)hoLevelUmbrella -110 … -47 dBm

Parameter Value

gsmMicrocellThreshold 0 … 36 dBmgsmMacrocellThreshold 0 … 36 dBm

Umbrella Handover (2/3)MS Classmark & Target Cell Power Comparison

Max power capability of MS >= gsmMacrocellThreshold

HO allowed only to a macrocell ( MS_TXPWR_MAX(n) >= gsmMacrocellThreshold )

gsmMicrocellThreshold < Max power capability of MS < gsmMacrocellThreshold

HO only to middle size cell ( gsmMicrocellThreshold < MS_TXPWR_MAX(n) < gsmMacrocellThreshold )

Max power capability of MS <= gsm MicrocellThreshold

HO allowed only to microcell ( MS_TXPWR_MAX(n) <= gsmMicrocellThreshold )

Umbrella Handover (3/3)Example

UmbrellaHandover

A

B-90 dBmHandover dueto Level

-85 dBm

1800 Macro

1800 MicroUmbrella Handoverdown to micro layerhoLevelUmbrella = -85dBm

RR Handoverout of micros

PBGT Handoverbetween SAMElayer cells

Example - Priority microcellshoLevelUmbrella macro macro = -47 dBm(prevents Umbrella HOs between adjacent macrocells)hoLevelUmbrella macro micro = -85 dBmhoThresholdLevDL = -90 dBm

GSM MS class 4 (33 dBm)gsmMacrocellThreshold = 35 dBmgsmMicrocellThreshold = 33 dBmmsTxPwrMax(n) = 33 dBm

macrocells

microcells

UMB,RR

PBGT,RR

PBGT,RRUMB,RR

UMB umbrella HORR radio reason HOPBGT power budget HO

Combined Umbrella & Power Budget

When enablePowerBudgetHo = Yes & enableUmbrellaHo = Yes◦ Power Budget Handover to cells of the same layer◦ Umbrella Handover to cells of different layer

Based on ◦ gsmMacrocellThreshold, gsmMicrocellThreshold◦ msTxPwrMax, msTxPwrMax(n)◦ MS classmark

UPPER layer (e.g. 900 macro)

SAME layer (serving layer)

LOWER layer (micro)

AdjCellLayer

Three layers visible to serving cell (relative to serving cell)

Used in target cell evaluation for; ◦ Fast moving MS handling in macro cell◦ HOs based on MS speed (BSS6) ◦ Combined umbrella and power budget

N (not in use)

ParameterAdjCellLayer

MS Speed based HandoversOverview

Mobile distribution in multi-layer networks based on speed of mobile

• Slow moving MS lower layer (micro) cells• Fast moving MS upper layer (macro) cells

Two proprietary Nokia features;• Fast Moving Mobile Support (FMMS)

• Estimation of MS speed based on duration of stay in target cell• used to move MSs from UPPER (macro) to LOWER (micro) cells

• MS_SPEED_DETECTION• Measurement of MS speed based on zero crossing detection process• Used to move slow MS from macro micro & fast MS from micro macro

Macro cell’s parametersfor each adjacent micro cell:

• fastMovingThreshold 0 .. 255• RxLevMinCell• hoLevelUmbrella

Counter for each adjacent micro cell +2 measurement and over rxLevMinCell -1 no meas. or bad level

Target cell selection based on adjacentcell RX_LEVEL and on hoLevelUmbrella

FMMS used in macrocell layer to 'estimate' the speed of a mobile based on measurement reports on adjacent microcells

macrocells

microcells

FMMS HOinitiated

time ‘t’

FMT Counter

HO

time ‘t’

hoLevelUmbrella = -85 dBm

FMT Threshold = 40

Fast Moving Mobile Support (FMMS)Process

RxLevMinCell = -85 dBm

BTS

BTS

Adjacent cell measurements

BSC

BTS sends MS speedmeasurements to BSCevery SACCH period(~480ms)

2

BTS 'measures' MS speed basedon zero cross rate algorithm providingcall is on non-hoping TCHMS_SPEED_DETECTION not suitablefor use with frequency hopping networks

1

BSC averages speedindications usingmsSpeedAveraging AV_MS_SPEED BSC ignores indications if;

• UL DTx used during SACCH• MS changing power during SACCH

3

4AV_MS_SPEED is compared withthresholds;• LowerSpeedLimit (slow MS)• UpperSpeedLimit (fast MS)to direct MS to appropriate layer(cell priorities used)

MS_SPEED_DETECTIONProcess

Candidate Selection Fast-moving to upper /

Slow-moving to lower layer adjacent cells

Equation 1' used Priority considered

ParametersFMMS & MS_SPEED_DETECTION

msSpeedAveraging (MSA) 1 ... 32 (SACCH frames)

adjCellLayer (ACL) N / Same / Upper / LowerhoLevelUmbrella (AUCL) -110 ... -47 dBm

lowerSpeedLimit (LSL) 0 … 255 (1 step 2km/h) upperSpeedLimit (USL) 0 … 255 (1 step 2km/h) msSpeedThresholdNx 1 … 32msSpeedThresholdPx 1 … 32

Parameter Value

adjCellLayer (ACL) N / Same / Upper / LowerhoLevelUmbrella (AUCL) -110 ... -47 dBmFastMovingThreshold (FMT) 0 … 255 (SACCH frames) FMMS

MS SpeedDetection

0 means "Not Used"

Imperative Handover Handover considered to be imperative:

◦ Handover due to Distance◦ Order to empty a cell ( from O&M )◦ Directed Retry and IDR◦ Rapid Field Drop (RFD)◦ Enhanced Rapid Field Drop (ERFD)

Imperative Handover: Distance

Distance Process ---> msDistanceBehaviour (0,1..60,255) in BSC• 0 : Release immediately• 1 - 60 : Release after certain time 1 - 60 s, try handover

during that time• 255 : No release, only imperative Handover attempt

Distance Process

enableMsDistanceProcess Y / NmsDistanceHoThresholdParam 0 … 63

px 1 … 32 nx 1 … 32

Parameter Value

msDistanceBehaviour 0, 1 … 60, 255

Chained CellServing

Cell

Rapid Field Drop (1/2)Process

Trigger ◦ Threshold Comparison

( HoThresholdRapidLevUl (px) Rx_Lev_UL (Not averaged / Only UL)

Candidate Selection◦ Only Chained adjacent cell ◦ Equation 1 only / no priority

Multi-Layered Network

hoThresholdLevULforRapidFieldDrop -110 ... -47 dBmhoThresholdRapidLevUIN 0 ... 32

chainedAdjacentCell Y / N

Parameter Value

MS Chained CellServing

Cell

Rapid Field Drop Handover..1st

2nd

-93 dBm

Serving Cell

hoThresholdRapidLevUl = - 93 dBmhoThresholdRapidLevUIN (px) = 2chainedAdjacentCell = Yes

Example

Rapid Field Drop (2/2)Example

Enhanced Rapid Field Drop (1/5)Process

• In case of DDE (Deep Dropping Edge), the averaging window sizes and power budget period are reduced

• level downlink window size• level uplink window size• adjacent cell averaging window size• handover period power budget

Parameter Value

erfdEnabled DIS, UL, DL or UDL

ddeThresholdLev 0 … 63 dB

Nx 1 … 32

Px 1 … 32

ddeWindow 1 … 32 SACCH

modifiedAveWinNcell 1 … 32

modifiedNOZ 1 … 32

erfdOver 1 … 64 sec

Enhanced Rapid Field Drop (2/5)Scenarios for Fast & Slow MSs

A MS moves away from cell site,the signal is dropping gradually

A MS turns a corner,the signal drops rapidly

Sign

al L

evel

Time Figure 7 Signal Strength of a Fast Moving MS

MS moves away from cell site,the signal is dropping gradually

MS turns a corner, thesignal drops faster thanmoving in straight line

Sign

al L

evel

TimeFigure 8 Signal Strength of a Slow Moving MS

Handover

A handover due to Enhanced Rapid Field Drop is defined by the following parameters;

• ErfdEnabled - Enables ERFD Handovers• DdeThresholdLev - Defines the drop in signal level that must be observed before ERFD detection. Uses voting nx : px• DdeWindow - Sets the window size over which ERFD is to be monitored• ModifiedAveWinNcell - Defines the modified

averaging window size to be used for Ncells during period ErfdOver• ModifiedNOZ - Modified # of zero results used during period ErfdOver• ErfdOver - Time period over which parameters, ModifiedAveWinNcell and ModifiedNOZ are used to monitor Ncells

Serving cell

Ncell #1

25dB > 20dBDdeThreshold

hoThresholdLevXL

ErfdOver

ERFD HO initiatedto Ncell #1

XL = DL or UL

DdeWindow = 2 n(1):p(1)

ERFD Detection

-83 -87-63-61-60-60 -89 -91 -94 -89 -89

averagingWindowSizeAdjCell = 4

modifiedAveWinNcell = 2

ERFD HO

Enhanced Rapid Field Drop (4/5)Process

ddeWindow = 3 SACCH (n = 3)ddeThresholdLev = 10, px = 2 and nx =3

the BSC compares the most recent measurement sample 8 (multiframe k) with the measurement sample 5 (multiframe k-n).

DDE_LEVEL = RXLEV(k- ddeWindow) – RXLEV(k) = -69 dBm – (-83 dBm) = 14 dB

Enhanced Rapid Field Drop (5/5)Example

Sample 1 2 3 4 5 6 7 8Signallevel

-71dBm

-68dBm

-70dBm

-71dBm

-69dBm

-70dBm

-75dBm

-83dBm

Handover Timers (1/3) Minimum time between consecutive handovers related to the same

connections◦ MinIntBetweenHoReq

Minimum time between handover attempts after a failure ◦ MinIntBetweenUnsuccHoAttempt◦ Applied differently in Intercell / Intracell handovers

Additional Guard Periods are used for Back-handovers◦ Quality◦ Distance◦ Traffic reason◦ Interference

minIntBetweenUnsuccHoAttempt 0 ... 30 sec

minIntBetweenHoReq 0 ... 30 sec

Parameter Value

Reversion to old channel ( 1 + NUMBER_OF_HO_FAIL ) *

MinIntBetweenUnsuccHoAttempt applied to handover attempts to the same target cell

No Radio Resource AvailableMinIntBetweenUnsuccHoAttempt applied to handover attempts to the same target cells ( all those in the list )

Other Failure CasesMinIntBetweenUnsuccHoAttempt applied to all types of

handover attempts

Handover Timers (2/3)After a HO Failure

Handover Timers (3/3)Guard period for back HO

After a HO due to quality (UL/DL) and interference (UL/DL):◦ a Power Budget HO back is not allowed during

GUARD_TIME = 2 * HoPeriodPBGT

◦ an Umbrella HO back is not allowed during

GUARD_TIME = 2 * HoPeriodUmbrella

After an inter-cell HO due to MS-BS distance:

◦ a HO back to the source cell is not allowed during

GUARD_TIME = 20sec + MinIntBetweenHOReq

After a Traffic Reason HO:

◦ Power Budget HO and Umbrella HO back to the source are not allowed during

GUARD_TIME = 20sec + MinIntBetweenHOReq

Repetitive Intra Cell HO not allower during

GUARD_TIME = 4 * MinIntBetweenUnsuccHoAttempt