HO Optimization Process

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Handover Process

HO parameters and optimization related to HO process in GSM network

Nokia Networks


Handover Process 1/2

• Handover decisions, made by RR Management in BSC, are based on the measurement results reported by the MS / BTS, parameter sets for each cell and algorithms. Target cell evaluation is also one part of the handover process. Handovers are triggered off by threshold comparison or by periodic comparison.

• Target cell evaluation is made in RR Management in BSC and its purpose is to find the best cell for handover. The evaluation on the preferred list of the target cells is based on:

• The radio link measurements• Threshold comparison• The priority levels of the neighbouring cells• The load of the neighbouring cells which belong to the same BSC as

the serving cell.• Rx level comparison – if it is needed

• The BTS sends the list of best candidates to the BSC. First the BSC defines and selects those cells, which meet the requirements for the radio link properties. RR Management can handle up to 32 best candidates for target cell evaluation.


Handover Process 2/2

BSC

Eq. 1 or

Eq. 1'

Adj1…..Adj2….Adj3….--------------------------------------------------------Adj32…

Adj1…Adj3…------------------Adjn

Eq.2or

Eq.2'

Target Cell Evaluation

Cell ranked according to priority/loadAdj1…Adj3…-------Adji….

Load information from the RR management


HO&PC Diagram0

1

2

3

4

5

6

7-110 -105 -100 -95 -90 -85 -80 -75 -70 -65 -60 -55 -50

PcUpperThresholdQualUL/DL

HoThresholdQualUL/DL

Power decrease

Power increase

Noactionneeded

Interference HoQuality Ho

LevHo

PcLevUpperThresholdLevUL/DL

HoThresholdLevDL

PcLowerThresholdLevUL/DL

HoThresholdInterferenceUL/DL

PcLowerThresholdQualUL/DL

HoThresholdLevUL (if diversity gain)

RxLevMinCell(n)

RxLevAccMin

Power increase


Handover Priority

• Handover priority

1. Uplink and downlink Interference2. Uplink quality3. Downlink quality4. Uplink level 5. Downlink level6. Distance7. Rapid Field Drop8. Slow moving MS9. Better cell i.e. Periodic check (Power Budget HO or Umbrella HO)10. PC: Lower quality/level thresholds (UL/DL)11. PC: Upper quality/level thresholds (UL/DL)


Handover Types 1/2

• Intra Cell HO• Handover in same TRX between different timeslots• Controlled by BSC

• Intra Site HO• Handover between different sectors in same site• Controlled by BSC

• Inter Site HO• Handover between sectors of different sites• Controlled by BSC

• Inter BSC HO• Handover between sectors of different sites and different BSCs• Controlled by MSC

• Inter System HO• Handover between two MSCs• Controlled by MSC


Handover Types 2/2

target

MSC 1

MSC 2

MSC controlled

BSC 1

out-going

intracell

inter BSC source

BSC 2 in-coming

intercell

MSC controlled


Handover Syncronization

Site A

Site B

• Synchronised HO between co-BCF cells• Non-synchronised HO between different sites


HO Algorithms

Equation 1:AV_RXLEV_NCELL(n) > rxLevMinCell(n) + Max (0, A)A = msTxPwrMax(n) - P P = depending on MS Classmark

Equation 2:PBGT > hoMarginPBGT(n) wherePBGT = ((msTxPwrMax - msTxPwrMax(n))-(AV_RXLEV_DL_HO - AV_RXLEV_NCELL(n)) - (btsTxPwrMax - BTS_TXPWR))Equation 2': (If enableHoMarginLevQual = Y)PBGT > hoMarginLev/Qual(n) wherePBGT = (AV_RXLEV_NCELL(n) - AV_RXLEV_DL_HO)-(btsTxPwrMax - BTS_TXPWR)

AV_RXLEV_NCELL(n): Averaged receiving levels of adjacent cells AV_RXLEV_DL_HO: Averaged receiving level of serving cellbtsTxPwrMax: Transmitting power level of serving cell


Power Budget Handover 1/2 • Trigger

• Periodic Check ( hoPeriodPBGT )

• Candidate Selection• Equation 1 2 used • Priority and Load Considered

• Typically used between cells of the same "Layer"

• Parameters Value• HOPeriodPBGT 0 … 63 SACCH periods• enablePwrBudgetHandover Y / N• rxLevMinCell(n) -110 … -47 dBm• msTxPwrMax(n) 0 … 30 dBm • hoMarginPBGT(n) 0 … 63 dBm


Power Budget Handovers 2/2

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

PBGT = ((30dBm-30dBm)-(-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 + (30 dBm - 30 dBm) = -99 dBm

1.

2.

AV_RXLEV_DL_HO = -90 dBmmsTxPwrMax = 30 dBm (= 1W)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) = 30 dBm (= 1W)btsTxPwrMax = 42 dBm (= 16 W)


Level Handover 1/2• Trigger

• Threshold Comparison ( hoThresholdsLevUL/DL (px - nx) )

• Candidate Selection• Equation 1 used • Equation 2 used if enableHoMarginLevQual = N• Equation 2' with hoMarginLev used if enableHoMarginLevQual = Y• Priority and Load Considered within the same BSC

• Parameters• HOThresholdLevUL/DL -110 … -47 dBm

• px 1 … 32• nx 1 … 32

• rxLevMinCell(n) -110 … -47 dBm• msTxPwrMax(n) 0 … 30 dBm• hoMarginLev(n) -24 … 24 dB


Level Handover 2/2

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

2 dB

hoMarginLev = 4 dBTrigger for Handover due to Level

A

B

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

Threshold (Lev)-95 dBm


Quality Handover 1/2• Trigger

• Threshold Comparison ( hoThresholdsQualUL/DL (px - nx) )

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

• Parameters Value• HOThresholdQualUL/DL 0 … 7

• px 1 … 32• nx 1 … 32

• rxLevMinCell(n) -100 … -47 dBm• msTxPwrMax(n) 0 … 30 dBm• hoMarginQual(n) -24 … 24 dB


Quality Handover 2/2

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

2 dB

hoMarginQual = 0 dBTrigger 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


Signal Quality in GSM

• RX Quality (RXQUAL parameter)

• RXQUAL classes 0 ... 7bit error rate before all decoding/ corrections

RXQUAL Mean BER BER rangeclass (%) from... to0 0,14 < 0,2%1 0,28 0,2 ... 0,4 %2 0,57 0,4 ... 0,8 %3 1,13 0,8 ... 1,6 %4 2,26 1,6 ... 3,2 %5 4,53 3,2 ... 6,4 %6 9,05 6,4 ... 12,8 %7 18,1 > 12,8 %

usable signal

unusablesignal

good

acceptable


Handover Due To Interference

• Trigger : Threshold Comparison • Threshold Comparison for Quality ( hoThresholdsQualUL/DL (px - nx) )• Threshold Comparison for Level ( hoThresholdsInterferenceUL/DL (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

• Parameters Value• hoThresholdInterferenceUL/DL -110 … - 47 dBm

• px 1 … 32• nx 1 … 32

• enableIntraHoInterfUL/DL Y / N• hoPreferenceOrderInterfUL/DL INTER / INTRA


Imperative Handovers

• Imperative Handovers• Handover due to Distance• Order to empty cell, from NMS• Directed Retry or IDR• Rapid Field Drop


Handover Due To 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

• Parameters Value• enableMsDistanceProcess Y / N• msDistanceHoThresholdParam 0 …63

•px 1 … 32•nx 1 … 32

• msDistanceBehaviour 0, 1 … 60, 255


Directed Retry

• Directed Retry• Directed retry is handover from serving cell SDCCH to an

adjacent cell TCH during call setup• Used when TCH congestion is disabling TCH assignment• Enabled by parameter drInUse

• Parameters Value• drInUse Y / N• DrThreshold -110 … -47 dBm


Handover Due To Rapid Field Drop

• 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

• Parameters Value• HOThresholdLevULforRapidFieldDrop -110 … - 47• HOThresholdRapidLevU1N 0 … 32• chainedAdjacentCell Y / N


HO Parameters 1/6

IntervalsminIntBetweenHoReg

Prevents repetitive handovers for the same MS with a timer that sets the minimum interval between handovers

MinIntBetweenUnsuccHoAttempt Indicates the minimum time that MS must wait after handover attempt fails and before it can try a handover attempt again


HO Parameters 2/6

Radio Resource ParametersbtsLoadThreshold

Checks the load threshold ratio of candidate cells hoLevPriority

Defines the priority level of an adjacent cell to account for locationhoLoadFactor

Defines how much the priority level will be decreased when the load threshold is exceeded

msSpeedthresholdNx/Px Compares speed threshold values


HO Parameters 3/6

Power Budget ParametersPower budget handover procedure ensures that the MS is always handed over to the cell with the minimum path loss

even though the quality and the level thresholds may not have been exceeded

EnablePowerBudgetHO Indicates if power budget is used as a criterion for a handover

HoPeriodPBGTBSC evaluates adjacent cells target cell for the handover at every defined SACCH interval


HO Parameters 4/6

Quality Level ParametersHoThresholdsQualUL/DL compares the averaged values of AVRXQUAL or AVRXLEVRxQual is the threshold level for handover.

The range is <0.2% - >12.8% bit error rateNx is the total number of averages to be taken into account before decision is possible.

The range varies from 1 to 32Px is the number of averages out of total averages that have to be greater than or equal to the threshold before a handover is possible.

The range varies from 1 to 32


HO Parametrs 5/6

MS to BTS DistanceWhen distance is the reason for the handover request msDistanceBehaviour controls the behaviour of the MS There are three alternatives in which the Mobile will act in the case of this Imperative Handover:

It will release the call immediatelyTry a 1 to 60 seconds Imperative HandoverTry an Imperative Handover


HO Parametrs 6/6

Sharing the LoadThe MSC may request the BSC to perform a specified number of handovers from one specified cell for load sharing

It is possible to handover to ‘worse’ cellsThe field strength in the adjacent cell has to be above two parameter

rxLevMinCell trhoTargetLevel

The target cells are only ranked according to radio link propertiespriority levels are not usedThe feature is disabled by setting the value to 'not used'


Network Doctor Reports

• Useful Handover Network Doctor reports for HO optimization and analysis

• 150, Cells having high HO failure ratio• 153, Adjacencies having high HO failure ratio• 154, Handover cause distribution• 157, Cells having high HO att./call ratio• 158, Intra BSS handover observation statistics

• Other ND reports for HO analysis&studies• 060, Adjacency discrepancies• 061, Non-symmetrical adjacencies• 062, Adjacent cells with same or adjacent frequency• 067, HO syncronization• 074, Adjacencies of cells• 111, Frequency plan• 196, UL, DL quality, UL interference per TRX• 216, Cell analyzer


Handover Flowcharts

Handover Failure

New Channel, New Cell

HANDOVER CMD

ACTIVE CALL

MS NETWORK

HANDOVER FAIL

ACTIVE CALL

Old Channel, Old Cell


Timer T3124 expiry or Radio Link Failure


Syncronized Handover


ACTIVE CALL

HANDO CMD

HANDO ACCHANDO ACCHANDO ACCHANDO ACC

HANDO COM

ACTIVE CALL

MSNETWORK


Serving BTS Neighbor BTS

(FACCH)

(FACCH)

(FACCH)

(FACCH)

(FACCH)

(FACCH)


Non-Syncronized Handover


MSNETWORK

ACTIVE CALL

HANDO CMD

HANDO ACC …….

HANDO ACC

PHYS INFO

PHYS INFO

HANDO COM

ACTIVE CALL


Ny1

Serving BTS Serving BTS

T3124(in MS)


Handover Analysis

• Click here to go HO analysis flowchart

• Compare Network Configuration against Network Design• Adjacencies to Non-existing Cells• Non-symmetrical Adjacencies • Adjacent Cell with same or adjacent Frequency• Synchronised HO• Adjacency Discrepancies• BTS Default Parameter Audit


Drive Test Analysis 1/2

• If drive test data available• Find all the HOF failures using analysis tools• Study reason for HO failure using

• Graphical data provided by analysis tool• Layer 3 messages provided by analysis tool• NMS data (ND reports)

• Common reasons for HO failures• Handover executed too late (source cell quality or interference allready

too bad)• HOPBGT margin, HOLevel, HO window averaging size

• Target cell quality bad, can't make HO stays at old channel• Frequency planning, coverage/dominance area planning

• No good cell to make HO• Coverage planning, lack of sites or azimuth/tilt/antenna optimization

• Hardware failure HO timer expires• BTS/T1 syncronization problem

• Discrepancies in BTS/BSC parameters• Check all BTS parameters (CI, BSIC, LAC, BSC, Freq., Adjacencies,

HO parameters) with ND reports/NMS comparing to real drive data.


Drive Test Analysis 2/2

• After drive test data and NMS report analysis plan needed parameter changes.

• Execute change requests implementing new parameter values using NMS.

• Make new drive test at same area after parameter changes or if not possible study NMS data with ND reports (needs real traffic).

• Start analysis and study if more changes are required.

• Handover failure does not necessarily mean drop call it can still stay at old channel and make new HO attempt later either to same adjacent cell or some other adjacent cell.

HO Optimization Process

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