GSM Handover Algorithms ----Special Subject Training V1.4Suitable for staff with junior P&O skill certificates (lower than certificate III)Issued by GSM Network Planning & Optimization Dept.

Contents Overview of handoverFlow of handoversBasic handover algorithmsAdvanced handover algorithmsInter-system handovers

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Aims of handoversTo ensure normal phone callsTo improve call qualityGSM handover belongs to system hard handover.

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Aims of handoversRetrieval handoverto retrieve the on-going calls;Marginal handoverto improve the on-going call;

Intra-cell handoverto reduce interference in the cell;

Forced handoverto balance traffic dispersion in different cells;

Directed retryto improve the call establishment rate.

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Handover featuresVarious algorithmsVarious algorithms can function simultaneously;Controlled by switches;System makes handover decision automatically.Ordering of target cellsStatic priority;Resource available(%);Budget power value.Automatic traffic balanceDirected retry;Traffic load handover started by MSC;Priority adjustment by traffic load;Force directed-shift handover; Fast-fading handover algorithm Unique penalty mechanism

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Application scenes for handovers IscenesThe table is for reference only!

Algorithms

Scenes UL/DL interference handover UL/DL quality handoverUL/DL level handoverLong MS-BS distance handoverUL fast-fading handoverMacro-micro handover PBGT handoverTraffic load handover started by MSC Co-BCCH handoverForced traffic-shift handoverPath loss& TA-based concentric handover CI-based concentric handover urban Dense urban suburb

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Application scenes for handovers IInetwork typesThe table is for reference only!

Algorithms

ScenesUL/DL interference handoverUL/DL quality handoverUL/DL level handoverLong MS-BS distance handoverUL fast-fading handoverMacro-micro handover PBGT handoverTraffic load handover started by MSC Co-BCCH handoverForced traffic-shift handoverPath loss& TA-based concentric handover CI-based concentric handover Dual-band network Double-layer network CO-BCCH Long-distance coverage

Contents Overview of handoverFlow of handoverBasic handover algorithmsAdvanced handover algorithmsInter-system handover

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Overall handover flowProcess of MRDecision of handover startSelection of target cellsOrdering of target cellsStart of handoverPenalty strategy for handover failureOverall handover flow

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What is MR?DL DTX performanceUL receive level/qualityBS PowerMS power levelTADL dynamic power controlUL MRDL MRUL DTX performanceDL receive level/qualityAdjacent cell levelUL dynamic power control

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MR cycleMR is sent to BTS in SACCH UL direction;When MS is in SDCCH, MR cycle is 470ms/time;When MS is in TCH, MR cycle is 480ms/time.

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Process of MRNumber of MR shall reach the average size of window;The max number of missing MR is ZeroAllowed;When power control is on, power compensation shall be made to handover decision threshold.Consider DTX weight;Measurement report is the original data base for handover decision. ZXG10 process adopts roll average method with various weights, which can ensure smooth handover.

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The max number of missing MR - ZeroAllowedSuppose ZeroAllowed=1MR1/PR1Average_MR1MR2/PR2Average_MR2MR3/PR3MR4/PR4MR5/PR5MR6/PR6MR7/PR7MR1MR2MR3MR4MR5MR6MR7missing= =(MR1+0+MR3+MR4)/(4-1) ==(0+MR3+MR4+0)/4

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Handover preprocess at BTSSuppose HOPreprocess is openand the value is 2.MR1MR2MR3MR4BSCIf HOPreprocess=0, how to upload MR?

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Process of MR at BSCSuppose window value=4, N=4MR1/PR1Average_MR1MR2/PR2Average_MR2MR3/PR3Average_MR3MR4/PR4Average_MR4N/P decisionMR5/PR5MR6/PR6MR7/PR7

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Selection of target cellscondition 1 must be satisfied

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Settings of cell layer & priorityFunctions of cell layer structure:More reasonable control of traffic dispersion;Control of traffic congestion;Backup of network capacity redundancy.

Layer1

Layer2

Layer2

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Layer3

Layer3

Layer3

Layer3

Layer3

Layer3

Layer3

Layer3

Layer3

Layer2

Layer2

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Serial Handover causesHandover typesPriorityAfter Tmicro overtimesBefore Tmicro overtimes1CO-BCCHIntra-cell Invalid Invalid 2UL interferenceIntra-cell Invalid Invalid 3DL interferenceIntra-cell Invalid Invalid 4UL qualityInter-cell Same layer- upper layer -lower layer Upper layer same layer -lower layer 5DL qualityInter-cellSame layer- upper layer -lower layerUpper layer same layer -lower layer6UL levelInter-cellSame layer- upper layer -lower layerUpper layer same layer -lower layer7DL levelInter-cellSame layer- upper layer -lower layerUpper layer same layer -lower layer8Long distanceInter-cellSame layer- upper layer -lower layerUpper layer same layer -lower layer9UL fast fadingInter-cellFirst search for related cellsUpper layer same layer lower layer Invalid

10PBGTInter-cellAccording toPbgtHoLayer:Same layer- upper layerlower layer No cells on same layer and lower layer selected 11Macro-micro time hyteresisInter-cellHandover to lower layer No handover 12Traffic Inter-cellAccording to TrafficHoLayrCtlTrafficHoFreqCtl Invalid 13Forced directed shiftInter-cellInvalidInvalid

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Parameter-NcellLayer(NcellLayer) is a mathematical array. Each element determines the layer relation between the serving cell and its adjacent cells.The number of cells in an array rests on NcellNum.

NcellNum valueMeaning 0N, undefined1SAME, the serving cell and its adjacent cells are onthe same layer 2UPPER, adjacent cells are on the serving cells uplayer 3LOWER, adjacent cells are on the serving cells lowlayer Other valuesReserved

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Layer parameter-TmicroTmicro is started when the MS accesses into new channel;The layer selection priority of target cells will be affected, if handover is started within the time set on Tmicro. Default: 80(8s).

TmicroTime duration505s515.1s20020s

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Ordering of target cellsLayer priority of candidate cellsStatic priorityResource available(%)Power budget marginDynamic priority

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Penalty strategy of inter-cell handover failure

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TMaxIHo overtimesPenalty strategy of intra-cell handover failure

Contents Overview of handoverFlow of handoverBasic handover algorithmsAdvanced handover algorithmsInter-system handover

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UL/DL interference handoverIdle TCH available in cell?UL/DL interference handoverintra-cell handoverUL/DL qualityHandoverinter-cell handoverYNDecision of handover start:Its suggested that UL/DL interference be closed in network with HF open.

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UL/DL quality handoverDecision of handover start:AvRxQualUL>=HoUlQualThs AvRxQualDL>=HoDlQualThs Selection of target cells:1AvRxLevNCell(n) > RXLEV_MIN(n) + MAX(0,(MS_TXPWR_MAX(n)- P(n)))2AvRxLevNCell(n) > AvRxLevDL + HO_MARGIN_QUAL(n)That is: Adjacent cells level must be larger than the minimum receive level set for it; Adjacent cells level shall be larger than the sum of quality handover threshold and the serving cells DL level.

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UL/DL level handover Decision of handover start:AvRxLevelUL= RXLEV_MIN(n) + MAX(0,(MS_TXPWR_MAX(n)- P(n))) 2AvRxLevNCell(n) > AvRxLevDL + HO_MARGIN_LEVEL(n) That is:Adjacent cells level must be larger than the minimum receive level set for it; Adjacent cells level shall be larger than the sum of level handover threshold and the serving cells DL level.

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Long BS-MS distance handover Decision of handover start:AvDistance>DistanceThreshold(TA) Selection of target cells:1AvRxLevNCell(n) > RXLEV_MIN(n) + MAX(0,(MS_TXPWR_MAX(n)- P(n))) The only condition is that the adjacent cells level is larger than the minimum receive level set for it.Radio environment quality is not regarded as decision standard.Application range:This kind of handover is usually used in networks with long distance coverage.

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PBGT handoverDecision of handover start:Selection of target cells:1AvRxLevNCell(n) > RXLEV_MIN(n) + MAX(0,(MS_TXPWR_MAX(n)- P(n))) 2 PBGT(n ) > HO_MARGIN(n) That is: 1Adjacent cells level shall be larger than the minimum receive level set for it; 2Its power budget value shall be larger than the PBGT handover threshold set for adjacent cell. Make handover decision to each adjacent cell in rotation; when an adjacent cells PBGT value is higher than 0, handover condition is satisfied.This handover happens mostly in urban area;It isnt for saving handover, but for better call quality.

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Directed retry-decision of handover startMS applies for TCHUsable TCH available?Assignment successForce disconnetion, force handover and ordering attemptSuccessful force disconnetion, force handover and orderingDirected retry switch open or notDirected retryYessuccessfulYesNounsuccessfulTCH occupancy failureNo

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Directed retry handoverIt happens in the process of calling access;It dynamically adjusts call traffic between cells; It can lower call loss rate;Selection of target cells must fulfill condition 1.Get to know the flow of internal & external directed retryHow to control 2/3G handover priority?Its suggested that ordering process be opened first, since its effect is better than that of directed retry.

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Penalty on directed retry failureNo candidate cells found Failure due to radio link problemCandidate cells have no radio resources

Contents Overview of handoverFlow of handoverBasic handover algorithmsAdvanced handover algorithmsInter-system handover

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UL fast fading handoverThis is a kind of algorithm for saving handover due to UL level decrease; It is flexibly used in dense urban areas, like micro-cells, highways, corners and slopes, where in-time outgoing handovers are needed;Selection of target cell shall fulfill condition 1;Given condition 1 is fulfilled, handover to relevant cells is preferred.MR1/PR1MR2/PR2MR3/PR3MR4/PR4Number of consecutive handover decisions (N)Directly report MR to BTSP unavailable

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Macro-micro handoverHandover trigger conditions: measured values of adjacent cells signal strength is higher than threshold, and this reaches certain consecutive times.

Selection of target cell:Condition 1 is fulfilled;Layer relation with neighbor cell NcellLayer is Lower.AvRxLevUL(n)>= MacroMicroHoThs

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Traffic handover When the serving cell suffers from congestion, for MS which has established a call, BSC will hand it over to an adjacent cell which can fulfill traffic handover conditions. Adjust call traffic in different cells to reach traffic balance.Decision of handover start: TCHBusyPercent>TrafficThsExcluding congested TCHs;Regardless dynamic HR and dynamic PDTCH.Selection of target cell:Condition 1PBGT(n) > TrafficLevThs

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MSC-started traffic load handover Its started by MSC;Select at most NumOfMS calls;Handover from high loaded cells to low loaded ones. Inter-BSC load adjustment;No data configuration needed at radio side.Note: Precondition for this handover algorithm: MSC starts the function, and BSC supports load report.Currently, no manufacturer makes this function open. ?????

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Forced directed-shift handoverFeatures The last means to adjust traffic; Mainly used in CO-BCCH cells;Applicable scenes:900M1800M cell of the same direction;Target cell must be forced firected shift handover cell.

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Forced directed-shift handover strategyDecision of handover start:PathLoss(n)

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