ho optimization process
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1 © NOKIA HO Process&Optimization.PPT/ DATE / MPu
Handover Process
HO parameters and optimization related to HO process in GSM network
Nokia Networks
2 © NOKIA HO Process&Optimization.PPT/ DATE / MPu
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.
3 © NOKIA HO Process&Optimization.PPT/ DATE / MPu
Handover Process 2/2
BSC
Eq. 1 or
Eq. 1'
Adj1….. Adj2…. Adj3…. --------- -------- ------- -------- -------- -------- -------- Adj32…
Adj1… Adj3… ------ ------ ------ Adjn
Eq.2 or
Eq.2'
Target Cell Evaluation
Cell ranked according to priority/load
Adj1… Adj3… ------- Adji….
Load information
from the RR management
4 © NOKIA HO Process&Optimization.PPT/ DATE / MPu
HO&PC Diagram
0
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
No action needed
Interference Ho Quality Ho
Lev Ho
PcLevUpperThresholdLevUL/DL
HoThresholdLevDL
PcLowerThresholdLevUL/DL
HoThresholdInterferenceUL/DL
PcLowerThresholdQualUL/DL
HoThresholdLevUL (if diversity gain)
RxLevMinCell(n)
RxLevAccMin
Power increase
5 © NOKIA HO Process&Optimization.PPT/ DATE / MPu
Handover Priority
• Handover priority
1. Uplink and downlink Interference
2. Uplink quality
3. Downlink quality
4. Uplink level
5. Downlink level
6. Distance
7. Rapid Field Drop
8. Slow moving MS
9. 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)
6 © NOKIA HO Process&Optimization.PPT/ DATE / MPu
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
7 © NOKIA HO Process&Optimization.PPT/ DATE / MPu
Handover Types 2/2
target
MSC 1
MSC 2
MSC controlled
BSC 1
out-
going
intra
cell inter
BSC source
BSC 2 in-
coming
inter
cell
MSC controlled
8 © NOKIA HO Process&Optimization.PPT/ DATE / MPu
Handover Syncronization
Site A
Site B
• Synchronised HO between co-BCF cells
• Non-synchronised HO between different sites
9 © NOKIA HO Process&Optimization.PPT/ DATE / MPu
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) where
PBGT = ((msTxPwrMax - msTxPwrMax(n))-(AV_RXLEV_DL_HO - AV_RXLEV_NCELL(n))
- (btsTxPwrMax - BTS_TXPWR))
Equation 2': (If enableHoMarginLevQual = Y)
PBGT > hoMarginLev/Qual(n) where
PBGT = (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 cell
btsTxPwrMax: Transmitting power level of serving cell
10 © NOKIA HO Process&Optimization.PPT/ DATE / MPu
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
11 © NOKIA HO Process&Optimization.PPT/ DATE / MPu
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 dBm
msTxPwrMax = 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 dBm
rxLevMinCell(n) = -99 dBm
msTxPwrMax(n) = 30 dBm (= 1W)
btsTxPwrMax = 42 dBm (= 16 W)
12 © NOKIA HO Process&Optimization.PPT/ DATE / MPu
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
13 © NOKIA HO Process&Optimization.PPT/ DATE / MPu
Level Handover 2/2
Equations 1 and 2’ are used if parameter enableHoMarginLevQual is set “Yes”
2 dB
hoMarginLev = 4 dB
Trigger 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
14 © NOKIA HO Process&Optimization.PPT/ DATE / MPu
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
15 © NOKIA HO Process&Optimization.PPT/ DATE / MPu
Quality Handover 2/2
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
16 © NOKIA HO Process&Optimization.PPT/ DATE / MPu
Signal Quality in GSM
• RX Quality (RXQUAL parameter)
• RXQUAL classes 0 ... 7 bit error rate before all decoding/ corrections
RXQUAL Mean BER BER range
class (%) from... to
0 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
unusable
signal
good
acceptable
17 © NOKIA HO Process&Optimization.PPT/ DATE / MPu
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
18 © NOKIA HO Process&Optimization.PPT/ DATE / MPu
Imperative Handovers
• Imperative Handovers • Handover due to Distance • Order to empty cell, from NMS • Directed Retry or IDR • Rapid Field Drop
19 © NOKIA HO Process&Optimization.PPT/ DATE / MPu
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
20 © NOKIA HO Process&Optimization.PPT/ DATE / MPu
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
21 © NOKIA HO Process&Optimization.PPT/ DATE / MPu
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
22 © NOKIA HO Process&Optimization.PPT/ DATE / MPu
HO Parameters 1/6
Intervals
minIntBetweenHoReg
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
23 © NOKIA HO Process&Optimization.PPT/ DATE / MPu
HO Parameters 2/6
Radio Resource Parameters
btsLoadThreshold
Checks the load threshold ratio of candidate cells
hoLevPriority
Defines the priority level of an adjacent cell to account for location
hoLoadFactor
Defines how much the priority level will be decreased when the load threshold is
exceeded
msSpeedthresholdNx/Px
Compares speed threshold values
24 © NOKIA HO Process&Optimization.PPT/ DATE / MPu
HO Parameters 3/6
Power Budget Parameters
Power 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
HoPeriodPBGT
BSC evaluates adjacent cells target cell for the handover at every defined SACCH
interval
25 © NOKIA HO Process&Optimization.PPT/ DATE / MPu
HO Parameters 4/6
Quality Level Parameters
HoThresholdsQualUL/DL compares the averaged values of AVRXQUAL or AVRXLEV
RxQual is the threshold level for handover.
The range is <0.2% - >12.8% bit error rate
Nx is the total number of averages to be taken into account before decision is possible.
The range varies from 1 to 32
Px 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
26 © NOKIA HO Process&Optimization.PPT/ DATE / MPu
HO Parametrs 5/6
MS to BTS Distance
When 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 immediately
Try a 1 to 60 seconds Imperative Handover
Try an Imperative Handover
27 © NOKIA HO Process&Optimization.PPT/ DATE / MPu
HO Parametrs 6/6
Sharing the Load
The 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’ cells
The field strength in the adjacent cell has to be above two parameter
rxLevMinCell
trhoTargetLevel
The target cells are only ranked according to radio link properties
priority levels are not used
The feature is disabled by setting the value to 'not used'
28 © NOKIA HO Process&Optimization.PPT/ DATE / MPu
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
29 © NOKIA HO Process&Optimization.PPT/ DATE / MPu
Handover Flowcharts
Handover Failure
New Channel, New Cell
HANDOVER CMD
ACTIVE CALL
MS NETWORK
HANDOVER FAIL
ACTIVE CALL
Old Channel, Old Cell
Old Channel, Old Cell
Timer T3124 expiry or
Radio Link Failure
30 © NOKIA HO Process&Optimization.PPT/ DATE / MPu
Syncronized Handover
New Channel, New Cell
ACTIVE CALL
HANDO CMD
HANDO ACC
HANDO ACC
HANDO ACC
HANDO ACC
HANDO COM
ACTIVE CALL
MS NETWORK
Old Channel, Old Cell
Serving BTS Neighbor BTS
(FACCH)
(FACCH)
(FACCH)
(FACCH)
(FACCH)
(FACCH)
31 © NOKIA HO Process&Optimization.PPT/ DATE / MPu
Non-Syncronized Handover
New Channel, New Cell
MS NETWORK
ACTIVE CALL
HANDO CMD
HANDO ACC
…….
HANDO ACC
PHYS INFO
PHYS INFO
HANDO COM
ACTIVE CALL
Old Channel, Old Cell
Ny1
Serving BTS Serving BTS
T3124
(in MS)
32 © NOKIA HO Process&Optimization.PPT/ DATE / MPu
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
33 © NOKIA HO Process&Optimization.PPT/ DATE / MPu
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.
34 © NOKIA HO Process&Optimization.PPT/ DATE / MPu
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.
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