ch04handovergvl-130619023100-phpapp01
TRANSCRIPT
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
QUALITY&INTERFERENCE
LEVELLEVEL
DISTANCEDISTANCE
PERIODICCHECKS
UMBRELLAUMBRELLA
POWER BUDGETPOWER BUDGET
IMPERATIVE HOCHANNEL ADMINISTRATIONCHANNEL ADMINISTRATION
DIRECTED RETRYDIRECTED RETRY
THRESHOLD COMPARISON
RAPID FIELD DROPRAPID FIELD DROP
MS SPEEDMS SPEED
MS 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
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
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 B
Threshold (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 dBm
Handover dueto Level
-85 dBm
1800 Macro
1800 Micro
Umbrella Handoverdown to micro layerhoLevelUmbrella = -85dBm
RR Handoverout of micros
PBGT Handoverbetween SAMElayer cells
Example - Priority microcells
hoLevelUmbrella macro macro = -47 dBm(prevents Umbrella HOs between adjacent macrocells)
hoLevelUmbrella macro micro = -85 dBm
hoThresholdLevDL = -90 dBmGSM 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)
Parameter
AdjCellLayer
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 Cell
Serving 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 Cell
Serving 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 Available
MinIntBetweenUnsuccHoAttempt applied to handover attempts to the same target cells ( all those in the list )
Other Failure Cases
MinIntBetweenUnsuccHoAttempt 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