8-BSSPAR- Handover Control and Adjacencies.pdf

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BSSPAR

Handover Control & AdjacenciesTraining Document

BSSPAR- Handover Control & Adjacencies

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© Nokia Oyj 2 (78)

The information in this document is subject to change without notice and describes only the product defined in the introduction of this documentation. This document is intended for the use of Nokia Networks' customers only for the purposes of the agreement under which the document is submitted, and no part of it may be reproduced or transmitted in any form or means without the prior written permission of Nokia Networks. The document has been prepared to be used by professional and properly trained personnel, and the customer assumes full responsibility when using it. Nokia Networks welcomes customer comments as part of the process of continuous development and improvement of the documentation.

The information or statements given in this document concerning the suitability, capacity, or performance of the mentioned hardware or software products cannot be considered binding but shall be defined in the agreement made between Nokia Networks and the customer. However, Nokia Networks has made all reasonable efforts to ensure that the instructions contained in the document are adequate and free of material errors and omissions. Nokia Networks will, if necessary, explain issues which may not be covered by the document.

Nokia Networks' liability for any errors in the document is limited to the documentary correction of errors. Nokia Networks WILL NOT BE RESPONSIBLE IN ANY EVENT FOR ERRORS IN THIS DOCUMENT OR FOR ANY DAMAGES, INCIDENTAL OR CONSEQUENTIAL (INCLUDING MONETARY LOSSES), that might arise from the use of this document or the information in it.

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Other product names mentioned in this document may be trademarks of their respective companies, and they are mentioned for identification purposes only.

Copyright © Nokia Oyj 2003. All rights reserved.

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Contents

1 Module Objectives .................................................................... 5

2 Introduction to the Handover Process .................................... 6

3 Handover Decisions ................................................................. 7

4 Handover Timers ...................................................................... 9

5 Target Cell Evaluation Process ............................................. 115.1 Decision Algorithms .................................................................. 125.2 C/I Based Handover Candidate Evaluation ............................... 14

6 Radio Resource Handovers ................................................... 166.1 Power Budget Handover........................................................... 176.2 Umbrella Handover................................................................... 206.3 Combined Umbrella and Power Budget .................................... 246.3.1 Fast Moving MS (FMMS) Handling In Macro Cell ..................... 276.3.2 MS Speed Detection and Variable Window Size....................... 276.4 Handover due to Quality or Signal Level ................................... 326.4.1 Handover due to Quality ........................................................... 326.4.2 Handover due to Signal Level ................................................... 356.5 Handover Due to Interference................................................... 37

7 Imperative Handovers ............................................................ 397.1 MS – BTS Distance .................................................................. 407.2 Rapid Field Drop and Enhanced Rapid Field Drop.................... 417.2.1 Chained Adjacent Cells In Rapid Field Drop ............................. 427.2.2 Turn-Around-Corner MS ........................................................... 43

8 Traffic Reason Handovers...................................................... 488.1 MSC-Controlled Traffic Reason Handover ................................ 488.2 BSC Initiated Traffic Reason Handover (Advanced Multilayer

Handling) .................................................................................. 498.2.1 Interactions with other Features................................................ 51

9 Load control between layers: Advanced Multilayer Handling (AMH)....................................................................... 52

9.1 IUO Load Control...................................................................... 529.2 Multilayer (Dual Band/Micro Cellular) Network Load Control..... 539.3 Parameters Involved in AMH feature activation......................... 53

10 Direct Access to Desired Layer/Band (DADL/B)................... 5510.1 DADL/B interactions with other features ................................... 56


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11 Handover Parameter List ....................................................... 58

12 Adjacent Cell Parameter List ................................................. 64

13 Practical Examples of Handovers.......................................... 68

14 Handover Control Exercise.................................................... 69

15 Handover Exercise ................................................................. 70

16 Key Learning Points ............................................................... 71

17 Review Questions................................................................... 77

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1 Module ObjectivesAt the end of this module, the participant will be able to:

• List ten types of handovers, in the order in which they are prioritised.

• State the parameters defining the minimum intervals between handovers, and between handover attempts.

• Discuss the target cell evaluation process.

• Explain the parameters associated with power budget handovers.

• Name the parameters used to control umbrella handovers.

• List the parameters associated with MS speed detection and handovers.

• Discuss the mechanism and parameters for handover due to signal level or quality.

• State the various types of imperative handovers and their associated parameters.

• Explain the Advanced Multilayer Handling (AMH) concept and parameters.


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2 Introduction to the Handover ProcessHandover decisions are made by Radio Resource Management in the BSC.They are based on the DL & UL measurement results reported by the MS & BTS, which are then processed according to parameter settings for each cell, and then processed by the Handover Control Algorithm, producing a decision. Target cell evaluation is also part of the handover process. Handovers are triggered by threshold comparison, or by periodic comparison.

Why are handovers needed?

1. Call continuity: to ensure a call can be maintained as an MS moves in geographical location, from the coverage area of one cell to another.

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

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

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3 Handover DecisionsThere are numerous causes for a handover decision to be made, based on comparisons of measurements with thresholds. The HO threshold comparison includes the evaluation of the uplink and downlink level, quality, and interference. Other criteria also considered are: MS-BS distance evaluation, the evaluation of a rapid field drop, the detection of a fast or slow-moving MS, the detection of a turn-around-corner MS, power budget evaluation, and umbrella handover evaluation.

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)

Figure 1 Handover Causes

If two or more criteria for a handover are present simultaneously, their order of priority is as follows:

1. Interference (uplink or downlink)

2. Uplink quality

3. Downlink quality

4. Uplink level

5. Downlink level

6. MS-BS distance (maximum or minimum)

7. Turn-around-corner MS

8. Rapid field drop

9. Fast/Slow-moving MS

10. Better cell (Power budget or Umbrella)

11. PC due to Lower quality thresholds (uplink and downlink)


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12. PC due to Lower level thresholds (uplink and downlink)

13. PC due to Upper quality thresholds (uplink and downlink)

14. PC due to Upper level thresholds (uplink and downlink).

When two or more of the above indicated criteria (from 1. to 9.) are present simultaneously, for example Uplink quality and Uplink level, the BSC performs target cell evaluation, only according to the criterion which has the highest priority. So, out of Uplink quality and Uplink level, that would be Uplink quality. If no adjacent cell is good enough for a handover caused by this criterion, then the BSC proceeds to the next HO threshold comparison (10. Better cell), and then to the PC threshold comparisons (11. to 14.).

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4 Handover TimersThe BSC normally controls the intervals between handovers and handover attempts by the use of the following parameters:

minIntBetweenHoReq (MIH)(HOC)(ZEHO)(0..31s)(5s), andminIntBetweenUnsuccHoAttempt (MIU)(HOC)(ZEHO)(0..31s)(3s)

The first parameter (MIH) prevents repetitive handovers for the same MS, by using a timer that sets the minimum interval between handovers related to the same connection.The latter parameter (MIU) defines the minimum time that the MS must wait after a handover attempt fails, before it can try a handover attempt again.

The MML default values are (5s) and (3s) as noted above, and the recommended values for these parameters are the same as the defaults.

The measurement averaging and the HO Threshold comparison do not stop during these intervals, although handovers are not possible.

• Minimum time between consecutive handovers related to the sameconnections• minIntBetweenHoReq (MIH)(HOC)

• Minimum time between handover attempts after a failure• minIntBetweenUnsuccHoAttempt (MIU)(HOC)• Applied differently in Intercell / Intracell handovers

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

minIntBetweenUnsuccHoAttempt (MIU)(HOC) 0 ... 30 secminIntBetweenHoReq (MIH)(HOC) 0 ... 30 sec

Parameter Value

Figure 2 Handover Timers


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• Reversion to old channel OR no radio resource availableminIntBetweenUnsuccHoAttempt applied only to intra-cell handover attempts

• Other failure cases (BSS failures)minIntBetweenUnsuccHoAttempt (MIU) applied to all types of handover attempts

• Reversion to old channel( 1 + NUMBER_OF_HO_FAIL ) * minIntBetweenUnsuccHoAttempt (MIU)

applied to handover attempts to the same target cell

• No Radio Resource Available

minIntBetweenUnsuccHoAttempt applied to handover attempts to the same targetcells ( all those in the list )

• Other Failure Cases• minIntBetweenUnsuccHoAttempt applied to all types of handover attempts

Figure 3 Handover Failure Timer

• 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 (HPP)(HOC)

• an Umbrella HO back is not allowed during

GUARD_TIME = 2 * hoPeriodUmbrella (HPU)(HOC)

• 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 (MIH)(HOC)

• After a Traffic Reason HO:• Power Budget HO & Umbrella HO back to the source, not allowed during

GUARD_TIME = 20sec + minIntBetweenHOReq (MIH)(HOC)

• Repetitive Intra Cell HO not allowed duringGUARD_TIME = 4 * minIntBetweenUnsuccHoAttempt (MIU)(HOC)

Figure 4 Guard Period for back-handover

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5 Target Cell Evaluation ProcessTarget cell evaluation is made in Radio Resource Management in the BSC.The purpose of the process is to find the best target cell to satisfy the cause for the handover.Evaluation of the preferred list of target cells is based on:

1. the radio link measurements

2. threshold comparison

3. the priority levels of the neighbouring cells

4. the load of those neighbouring cells which belong to the same BSC as the serving cell

5. RX level comparison – if it is needed

To be on the target cell candidate list, the neighbouring cells must, of course, be defined as Adjacencies in the outgoing direction from the serving cell. 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 radiolink properties.RR Management can handle up to 32 of the best candidates for target cell evaluation.

The loads of the traffic channels in these candidate cells are checked by the parameter:

btsLoadThreshold (BLT)(BTS)(ZEQO)(0..100%)(70%)

If one of these candidate cells is already "overloaded", (that is loaded above the threshold, which is a soft limit), then the priority that has already been set for that cell by the parameter:

hoLevPriority (PRI)(ADJC)(ZEAO)(0..7)(3)

is decreased by the value of the parameter:

hoLoadFactor (OF)(ADJC)(ZEAO)(0..7)(1)

After the load check of each candidate, a comparison between priorities of each candidate cell is made, and the candidate with the highest priority will be selected as a target cell. If there are many cells with equally high priorities, the cell with the best received signal strength will be selected.

Note: Parameter OF must NOT be set to be greater than PRI.

With the BLT parameter, we define which proportion of reserved or unavailable channels, in all channels, is acceptable. BLT is one of the key parameters used in the handover control process. If this threshold is exceeded, the BTS (= cell, = sector) is


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considered to be "overloaded", and handovers to that BTS will be avoided, although still possible.

With the PRI parameter, we define the priority level for an adjacent cell. The priority level is used for Target Cell evaluation by the Handover Control Process. By using priority levels for the Handover Algorithm, it is possible to take into account the location of the adjacent cell.

With the OF parameter, we define by how much, the priority of the Target BTS will be decreased if the BTS is "overloaded". The parameter is used only for the BTSs under one BSC, because this BSC cannot get information about the loading of the other BTSs under another BSC.

• Threshold comparison:• Quality• Level• Distance• Load

• Periodic checks:• Power budget• Umbrella

IFAV_RXQUAL_DL_HO <hoThresholdsQualDL

THEN� Downlink Quality HO is performed

The Handover process may be triggered by:

IFEnablePowerBudgetHO = Yes

THENPBGT comparison performed

everyhoPeriodPBGT seconds

Averaged value obtained frommeasurement averaging process usinghoAveragingQualDL

Target Cell Evaluation Process

Threshold levelbased on nx & px

Figure 5 Threshold Comparison Process

5.1 Decision Algorithms

Handovers are made on the basis of algorithms, which are actually used for comparisons. Different handovers make use of different algorithms, to be successful in different situations. The causes and priorities for different kinds of handovers have been presented in Handover Decisions. Equations for the different handovers can be seen in Figure 6.

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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

Figure 6 Handover Decision Algorithms

AV_RXLEV_NCELL (n), stands for the averaged received levels of adjacent cells, and AV_RXLEV_DL_HO, stands for the averaged received level of the serving cell. So, small n or capital N always means the Nth adjacent cell. BTS_TX_PWR stands for the transmitting power level of the serving cell.

The equations 1 and 2' above, are used in cases of handovers due to level, quality, or distance. In the case of an Umbrella Handover, equations 1' and 2' are used. For a Power Budget Handover, equations 1 and 2 are used.

Best candidates to RR Management:• intra BSC HO max 16 cells under the same BSC as thesource cell• inter BSC HO numberOfPreferredCells (NPC)(BSC)

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

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

Comparison of Candidates' priorities (hoPriorityLevel (PRI)(A)(0..7))

2.

If there are 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

Figure 7 Cell Preference based on Load conditions


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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

=> 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

=> 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 overloaded)=> 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 overloaded)=> cell list b,c,a

Figure 8 Load Evaluation Example

5.2 C/I Based Handover Candidate Evaluation

ciEstMethod (HOC) AVE (Average taking method) MAX (Maximum taking method) NONE (C/I estimation not used)

LAC1-5 (ADJC) Location area code of reference cell 1-5 0 ... 65535CI1-5 (ADJC) Cell identification of reference cell 1-5 0 ... 65535L1-5 (ADJC) Level adjustment for reference cell 1-5 - 63 ... 63 dBW1-5 (ADJC) C/I estimation weight parameter for reference cell 1-5 0 ... 10

• Designed to estimate the level of the potential interference for each handovercandidate (used during the target cell evaluation process)

• The purpose is to direct the mobile station to a cell which can provide aminimum interference service at the current location of the MS

• OPTIONAL - Enabled using O&M parameter ciEstMethod (HOC) on a cell-by-cell basis

• Each Adjacent Cell can have up to 5 "reference cells"• Real source of interference or just reference

Parameter Value

Figure 9 C/I Based HO candidate evaluation

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lowerCIlimit (L1-L6)(HOC) L1 (Lower C/I limit for band 1) -128 ... 127 dBL2 (Lower C/I limit for band 2) -128 ... 127 dB

……L6 (Lower C/I limit for band 6) -128 ... 127 dB

priorityAdjStepforBand1 P1 (Priority adj. for band 1) -8 ... 7 (P1-P7)(HOC) P2 (Priority adj. for band 2) -8 ... 7 ……

P6 (Priority adj. for band 6) -8 ... 7

• C/I is calculated for the handover candidates• Averaged over the reference cells(Average)

• By taking the worst interferer (Maximum)• C/I is compared to 7 user-defined bands

• Priority of the Cell is incremented by thequantity associated with the band

Parameter Value

priorityAdjStepforBand1 >= LowerC/ILimit1priorityAdjStepforBand2 >= LowerC/ILimit2 ……. …….priorityAdjStepforBand6 >= LowerC/ILimit6priorityAdjStepforBand7 >= LowerC/ILimit7

priorityAdjStepforBand1 >= LowerC/ILimit1priorityAdjStepforBand2 >= LowerC/ILimit2 ……. …….priorityAdjStepforBand6 >= LowerC/ILimit6priorityAdjStepforBand7 >= LowerC/ILimit7

-8 Eliminates the Adjacent Cell from the candidates list

Figure 10 C/I Calculation

• C/I estimates used to:• Remove any HO candidates whose predicted C/I is below acceptable level• Increase priority rating of cells with good C/I• Reduce priority rating of cells with bad C/I

• Modified cell priority equation:

PRIORITY(n) = hoPriorityLevel(n) - hoLoadFactor(n) + CI_Priority_Factor(n)

• Applies to all handover cases that use priority levels(e.g. PBGT, RxLev, RxQual, Umbrella)but not imperative handovers(e.g. DR, RFD, distance)

Figure 11 C/I Modified Cell Priority


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6 Radio Resource HandoversA handover is considered as a Radio Resource Handover if the cause is one of the following reasons:

1. Level (uplink / downlink)

2. Quality (uplink / downlink)

3. Interference (uplink / downlink)

4. Power budget

5. Umbrella

When the BSC receives measurement results from the BTS it always compares each of the processed results (averages) with the relevant thresholds:

• hoThresholdsLevDL/UL• hoThresholdsQualDL/UL• hoThresholdsInterferenceDL/UL• msSpeedThresholdNx/Px (STN)/(STP)(HOC)(ZEHO)(1..32)(6)/(3)OPTIONAL - FMMS in Macrocell, lower & upper speed thresholds, Nx, Px

Each threshold is composed of three parts: the threshold itself, the total number of samples (Nx) to be taken into account before decision is possible, and the proportionate number of samples (Px), out of the total samples (Nx), that have to be lower or higher than, or equal to the threshold, before any action is possible.

There are also other handover types, with conditions that are checked periodically.Power Budget Handover, is checked/de-activated for the HOC of a cell by:enablePwrBudgetHandover (EPB)(HOC)(ZEHO)(Yes/No)(Yes)and Umbrella Handover, is activated for the HOC of a cell by:enableUmbrellaHandover (EUM)(HOC)(ZEHO)(Yes/No)(No)The checking periods (intervals) for those handovers are controlled by the parameters:hoPeriodicPBGT (HPP)(HOC)(ZEHO)(0..63)(6 SACCH periods) andhoPeriodicUmbrella (HPU)(HOC)(ZEHO)(0..63)(6 SACCH periods)

The BSC may perform either an inter-cell or an intra-cell handover when the cause is interference. In the case of interference the parameters HUL & HDL (for UL & DL):hoPreferenceOrderInterUL/DL (HUL/HDL)(BSC)(ZEEO)(INTER/INTRA)(INTER)define the order of preference between, inter-cell or intra-cell handover.Those parameters (HUL & HDL) are set in each BSC by the ZEEM command. enableIntracellHoInterfUL/DL (EIC/EIH)(HOC)(ZEHO)(Yes/No)(Yes)indicate whether an intra-cell handover caused by UL/DL interference is enabled.These two parameters (EIC & EIH) do not affect handovers between the normal and extended areas of an extended cell.

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6.1 Power Budget Handover

The Power Budget handover procedure ensures that an 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.

The parameter: enablePwrBudgetHandover (EPB)(HOC)(ZEHO)(Yes/No)(Yes)indicates whether power budget is used as a criterion for a handover. Power Budget Handover enabled is the default setting, and is checked/de-activated for the HOC of a cell by ZEHO/ZEHG. Handover Control parameters for a cell are created with ZEHC.

If power budget handover is enabled, the BSC evaluates the radio link properties of the adjacent cells, with PBGT HO threshold comparisons, in order to find a target cell for the handover, at every interval of:hoPeriodPBGT (HPP)(HOC)(ZEHO)(0..63)(6 SACCH periods)

The BSC uses the power budget evaluation algorithm for target cell evaluation (equations 1 and 2 in Algorithms).

• Trigger• Periodic Check - hoPeriodPBGT (HPP)(HOC)

• Candidate Selection• Equation 1 & 2 used• Priority and Load considered

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

hoPeriodPBGT (HPP)(HOC) 1 ... 63 (SACCH)enablePwrBudgetHandover (EPB)(HOC) Y / N

Parameter Range

rxLevMinCell(n) (SL)(ADJC) -110 … -47 dBmmsTxPwrMax(n) (PMAX1 & PMAX2)(ADJC) 5 … 43 & 0 … 36 dBm hoMarginPBGT(n) (PMRG)(ADJC) -24 … 63 dB

Figure 12 Power Budget Handover Process and Parameters


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Figure 13 Power budget handover algorithm

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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)

Figure 14 Power Budget Handover Example

As seen in the above example, both requirements (equations) are fulfilled and the handover will be triggered.


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6.2 Umbrella Handover

If umbrella handover is enabled, the BSC evaluates the radio link properties of the adjacent cells in order to find a target cell for the handover at intervals of: hoPeriodUmbrella (HPU)(HOC)(0..63)(6 SACCH periods)

The measurement results of the adjacent cell must satisfy equation 1' (see Algorithms) before an umbrella handover is possible.

• 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) (HPU)(HOC)• Candidate Selection

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

enableUmbrellaHandover (EUM)(HOC) Y / NhoPeriodUmbrella (HPU)(HOC) 0 … 63 (SACCH)hoLevelUmbrella (AUCL)(ADJC) -110 … -47 dBm

Parameter Value

dcsMicrocellThreshold (DMIC)(BSC) 0..36 & 0..33 dBm (GSM1800 & 1900)dcsMacrocellThreshold (DMAC)(BSC) 0..36 & 0..33 dBm (GSM1800 & 1900)

Figure 15 Umbrella Handover

In addition to the radio link properties (equation 1'), umbrella HO also takes into account the power class of the MS so that only those adjacent cells which suit the type of MS are selected to be the target cells for that MS. That is, the BSC selects macrocells for vehicle MS's and portable MS's, and microcells for handhelds. The BSC uses two sets of thresholds to achieve this properly:

1. MACROCELL_THRESHOLD parameters:gsmMacrocellThreshold (GMAC)(BSC)(ZEEO)(MML default 35dBm) anddcsMacrocellThreshold (DMAC)(BSC)(ZEEO)(MML default 26dBm)

2. MICROCELL_THRESHOLD parameters:gsmMicrocellThreshold (GMIC)(BSC)(ZEEO)(MML default 33dBm) anddcsMicrocellThreshold (DMIC)(BSC)(ZEEO)(MML default 24dBm)

The maximum RF power parameters (in ADJC):msTxPwrMaxGSM (PMAX1)(ADJC)(ZEAO)(5..43dBm)(33dBm)(n) andmsTxPwrMaxGSM1x00 (PMAX2)(ADJC)(ZEAO)(0..36dBm)(30dBm)(n), that an

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MS is permitted to use on a traffic channel in the adjacent cell (n), determines the size of the adjacent cell (n), by means of the above thresholds (MACROCELL_THRESHOLD and MICROCELL_THRESHOLD), as follows:

� If msTxPwrMaxGSM/GSM1x00 (n) is greater than or equal to: MACROCELL_THRESHOLD, then the adjacent cell is considered a macrocell.

� If msTxPwrMaxGSM/GSM1x00 (n) is smaller than or equal to: MICROCELL_THRESHOLD, then the adjacent cell is considered a microcell.

� If msTxPwrMaxGSM/GSM1x00 (n) is smaller than: MACROCELL_THRESHOLD, and greater than MICROCELL_THRESHOLD, the adjacent cell is considered a "middle sized" cell.

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 )

Figure 16 Handover Comparison

When selecting the target cells, the BSC takes into account the following conditions, in addition to the radio link properties (equation 1'):

� If the maximum power capability of the MS is greater than or equal to MACROCELL_THRESHOLD, the adjacent cell must be considered a macrocell in order for the HO to be allowed.

� If the maximum power capability of the MS is smaller than or equal to MICROCELL_THRESHOLD, the adjacent cell must be considered a microcell in order for the HO to be allowed.

� If the maximum power capability of the MS is smaller than MACROCELL_THRESHOLD, and greater than MICROCELL_THRESHOLD, then the adjacent cell must be considered a "middle sized" cell in order for the HO to be allowed.

If the MACROCELL_THRESHOLD is selected so that it equals the lowest MS power level, or the MICROCELL_THRESHOLD is selected so that it equals the highest MS power level, it is possible to bypass each of these properties, and evaluate the target cells only according to radio link properties.

After the BSC has selected the target cells for the umbrella handover, the cells are ranked according to the priority levels.


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Figure 17 Umbrella Handover Algorithms

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UmbrellaHandover

A

B-90 dBmHandover due

to Level

-85 dBm

1800 Macro

1800 MicroUmbrella Handoverdown to micro layer

hoLevelUmbrella = -85dBmRR Handoverout of micros

PBGT Handoverbetween SAME

layer cells

Example - Priority � microcells

hoLevelUmbrella macro �� macro = -47 dBm(prevents Umbrella HOs between adjacent macrocells)

hoLevelUmbrella macro �� micro = -85 dBm

hoThresholdLevDL = -90 dBm

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

Figure 18 Umbrella Handover Example


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6.3 Combined Umbrella and Power Budget

If umbrella handover and power budget handover are active simultaneously in the same cell, then the target cell evaluation for a power budget handover will be different from the standard procedure described in the section "Power Budget Handover".

Parameters:enableUmbrellaHO (EUM)(HOC) andenablePwrBudgetHandover (EPB)(HOC)In this case the power budget handover also takes into account the type (size or layer) of the adjacent cell, in addition to the radio link properties (equations 1 and 2).

macrocells

microcells

UMB,RR

PBGT,RR

PBGT,RRUMB,RR

UMB umbrella HORR radio reason HOPBGT power budget HO

• When enablePowerBudgetHo = Yes & enableUmbrellaHo = Yes (HOC)• Power Budget Handover to cells of the same layer• Umbrella Handover to cells of different layer

• Based on• gsmMacrocellThreshold, gsmMicrocellThreshold (BSC)• msTxPwrMax, msTxPwrMax(n) (BTS, ADJC)• MS classmark

Figure 19 Combined Umbrella and Power Budget Handovers

An umbrella handover has priority over a power budget handover.

The size/layer of the adjacent cell must be equal to the size/layer of the serving cell in order for the handover to be allowed. That is, the BSC performs power budget handovers only between cells of the same size/layer (the power class of the MS does not affect the evaluation procedure), when umbrella handover is employed. For example, if the serving cell is considered a microcell, the adjacent cell must also be considered a microcell, in order for the power budget handover to be allowed.

The BSC is able to determine the size/layer of a cell by means of two alternative methods. The method is selected with the parameter:

adjCellLayer (ACL)(ADJC)(ZEAO)(N/SAME/UPPER/LOWER)(N)

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UPPER layer (e.g. 900macro)

SAME layer (servinglayer)

LOWER layer(micro)

• Three layers visible to serving cell (relative toserving 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(ACL)(ADJC)

Figure 20 Adjacent Cell Layer Parameter

The BSC determines the size/layer of a cell as follows:

1. If the use of the parameter adjCellLayer (ACL)(ADJC) is disabled, that is, the layer of the adjacent cell has not been defined, then the maximum RF power an MS is permitted to use on a traffic channel in the cell, determines the size of the serving/adjacent cell, as described before.

2. If the cell layer has been defined for the adjacent cell by means of the parameter adjCellLayer (ACL)(ADJC), then the parameter adjCellLayer (ACL)(ADJC)determines the layer of the adjacent cell from the point of view of the serving cell. The layer of an adjacent cell can be either:

� the same as the layer of the serving cell;� upper to the layer of the serving cell;� lower than the layer of the serving cell.

Those adjacent cells which meet both the required radio link properties, and the required cell size/layer, are ranked according to priority levels as usual.

The parameter adjCellLayer (ACL)(ADJC) is related to traffic control, based on the MS Speed. MS Speed is an OPTIONAL feature in the BSC. If this option is not enabled, then adjCellLayer (ACL)(ADJC) is disabled as a default value.

The feature MS Speed consists of two different features: Fast moving MS in macro cell, and MS Speed Detection. These features were introduced to increase the capacity of a cellular network. Areas of high traffic density may be covered with a multi-layer network consisting of different sized cells. If a high-speed mobile is located in such an area, then it should be located in a macrocell to decrease the amount of handovers. In other words a high speed mobile locating in a microcell would result in increased signalling load and a potentially high rate of dropped calls.


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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 detectionprocess• Used to move slow MS from macro � micro & fast MS from micro �macro

Figure 21 MS Speed Based Handovers

The “Fast moving MS handling in macro cell” (S5) feature uses the adjacent cell measurements to define the ms speed and the “MS Speed Detection” (S6) feature uses a crossing rate algorithm to define the MS speed. The S5 feature is handling MSs in macrocell (with RF hopping used) and the S6 feature is handling the MSs in microcells (no RF hopping). Handovers between cells within the same layer are based on PBGT or radio reasons, and handovers between cells in different layers are based on umbrella handover reasons.

BSCBTS

BTS

macrocell with RF hopping

microcell(s) no RF hopping

fast MSs

slow MSs

meas_res

Crossing rate algorithm

BSS

6/M

S sp

eed

BSS

5/Fa

st M

S

Adjacent cell measurements

HO&PCalgorithm

Combination of Fast Moving MS Handling (BSC) and MS Speed Detection (BTS)

Figure 22 Fast / Slow Moving MS

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6.3.1 Fast Moving MS (FMMS) Handling In Macro Cell

The feature estimates whether the mobile is moving fast or slowly, by checking the number of received measurements from adjacent microcells. This is done through a counter for each adjacent microcell at the BSC, and a threshold: fastMovingThreshold (FMT) (ADJC) 0 … 255). The counter starts from zero if a measurement is received, and if the level exceeds the minimum for that cell, set by: rxLevMinCell (SL) (ADJC)) the counter is increased by 2. Otherwise it is decreased by 1.

Macro cell’s parametersfor each adjacent micro cell:

• fastMovingThreshold(FMT) 0..255• rxLevMinCell (SL)(ADJC) • hoLevelUmbrella (AUCL)(ADJC)

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 onmeasurement reports on adjacent microcells

macrocells

microcells

FMMS HOinitiated

time ‘t’

FMT Counter

HO

time ‘t’

hoLevelUmbrella = -85 dBm(AUCL)(ADJC)

FMT Threshold = 40(FMT)(ADJC)

Figure 23 FMMS Processing

If the mobile is moving slowly, then an umbrella handover to the lower layer (microcell) will be triggered, the cause being slow mobile. The feature is automatically used when there is at least one adjacent cell belonging to a layer that is lower than the serving cell.

6.3.2 MS Speed Detection and Variable Window Size

The OPTIONAL feature called FMMS handling, estimates whether the mobile is moving slow or fast by measuring MS speed. The MS speed can be verified by counting fading dips. The handover decision algorithm, called crossing rate algorithm, is based on comparison of signal strength values obtained from each burst, and their averaged value (over one SACCH-frame). The averaged value is RXLEV in the uplink direction, which the BTS reports to the BSC. The algorithm counts the rate at which the signal level crosses the averaged signal level due to fast fading.


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BTS

BTS

Adjacent cell measurements

BSC

BTS sends MS speedmeasurements to BSCevery SACCH period

(~480ms)

2

BTS 'measures' MS speed based

on zero cross rate algorithm providing

call is on non-hopping TCH (no SDCCH)

MS_SPEED_DETECTION not suitable

for 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)

Candidate Selection• Fast-moving to upper /

Slow-moving to lowerlayer adjacent cells

• Equation 1' used• Priority considered

Figure 24 MS Speed Detection Processing

If the proportion of averaged MS speed indications:msSpeedThresholdPx (STP)(HOC)(1..32)(3),out of last number of averaged MS speed indications:msSpeedThresholdNx (STN)(HOC)(1..32)(6), exceeds theupperSpeedLimit (USL)(HOC)(0..255)(0), then the MS is considered as a fast moving MS, and the call will be handed over (umbrella handover) to a suitable upper layer cell (macrocell), if any.If averaged MS speed indications out of last averaged MS speed indications is/are lower than the lowerSpeedLimit (LSL)(HOC)(0..255)(0), then the MS is considered as a slow moving MS, and the call will be handed over to a suitable lower layer cell (microcell), if any. The step size for upper/lowerSpeedLimit parameter is 1, which correlates with a speed difference of 2 km/h. If the value of either is 0, then that speed verifying is not made. (OPTIONAL FMMS feature).

If the RF hopping and the crossing rate algorithm are both set on in a BTS, then the MS speed detection indications sent to the BSC are all non-valid (255). The MS speed indication is set to non-valid also, if the uplink DTX was used during the current and/or the previous SACCH multiframe. This means that the crossing rate algorithm cannot determine the MS speed reliably, and the HO&PC algorithm in the BSC will not take into account the speed indications, and therefore no handovers will be made due to MS speed.

If MS power control is in progress, the HO&PC algorithm will discard the MS speed indications until the power control is completed (i.e. the MS has received the

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requested power level). The MS speed estimations indicated before the power control start are valid for averaging and threshold comparison.

If the BTS sends the MS speed indication to the BSC, then the HO&PC algorithm will process the MS speed indications (averaging and threshold comparison).

The Various Window Size feature was introduced in S7 release because “Speed” itself has different interpretations in different locations. Therefore, it is reasonable to have various window sizes (i.e. Better Cell Trigger / Quality Trigger) according to the speed indication.

In principle, high-speed MS should use shorter average window size, and low-speed MS should use longer average window size. Therefore, all averaging-processes should have two sets of window parameters, one set for high speed MS and one set for low speed MS. By applying various window sizes, fast-moving MSs have a shorter window size so they may hand over to the target cell faster. For a slow-moving MS, a longer window size is applied in order to prevent it from unnecessary oscillation.

The BSC may use the information on the speed of the mobile station either to control traffic between separate layers of the multi-layered cellular network or to scale the values of the averaging parameters. The function is selected with the parameter MsSpeedDetectionState (SDS) (HOC). The alternative values are the following:

0 MS speed information is used to control traffic between separate layers of the multi-layered cellular network by means of the handover procedure.

1 – 100 MS speed information is used to scale the values of the averaging parameters. The range is from 1% to 100%. That is, if the value is, for example, 80% it means that the averaging window is 80% of the normal window size.

If Px averaged MS speed indications, out of last Nx averaged MS speed indications, exceed the upper speed limit (USL), then the MS is considered as a fast-moving MS and the scaled averaging windows are used. If Px averaged MS speed indications, out of last Nx averaged MS speed indications, are lower than the lower speed limit (LSL), then the MS is considered as slow-moving and the normal averaging windows are used.

The following parameters for averaging windows and number of zero results, are scaled when the MS is detected to be fast-moving, and are returned to normal when the MS is detected as slow-moving:

� UL signal level

� DL signal level

� DL signal levels of adjacent cells


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� Number of zero results

� DL signal level of interfering cells in IUO

� Number of zero results when averaging interfering cells in IUO

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

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

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

Parameter Value

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

MS SpeedDetection

0 means "Not Used"

Figure 25 Summary of FMMS and MS Speed Parameters

These parameters concern only handover, not power control.

There is interaction with the existing feature MS Speed Detection. If the MS Speed Detection State (SDS)(HOC)(ZEHO) is set to handover (0), it is always attempted that the MS is directed to the correct cell (layer) according to the MS speed. If the MS Speed Detection State is set to variable window size, the algorithm changes only the size of the averaging windows and the number of zero results, when fast or slow speed is detected.

The crossing rate algorithm gives reliable results only in non-hopping cells, and when uplink DTX is not in use.

The HOP, BHOP, UHOP, & BUHOP parameters define the frequency hopping mode of the BTS, depending on BTS site type and SW version.

The DTX parameter defines how the MS uses DTX:

(dtxMode)(DTX)(BTS)(ZEQO)(0/1/2)(2)(2 = MS shall not use DTX).

MS speed indications are received from the BTSs (since releases B10, DF3.0, and CX(M) 2.0).

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Figure 26 Handover due to fast/slow-moving MS


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6.4 Handover due to Quality or Signal Level

If the HO threshold comparison indicates that a handover due to uplink or downlink quality or level is required, the BSC evaluates the radio link properties of the adjacent cells in order to find a target cell for the handover. Equations 1 and 2' are used in the case of uplink or downlink quality or level. (See chapter Algorithms)

6.4.1 Handover due to Quality

The parameter hoThresholdsQualUL/DL (QUR/QDR)(HOC) is used for comparing the averaged values of uplink signal quality measurements for triggering the HO due to uplink quality.

⇒ 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 (of worse quality) than or equal to the threshold before a handover is possible. The range varies from 1 to 32.

• Trigger• Threshold Comparison (hoThresholdsLevUL/DL with px/nx)(LUR/LDR)(HOC)

• Candidate Selection• Equation 1 used• Equation 2 used if enableHoMarginLevQual = N (MRGS)(ADJC)• Equation 2' used if enableHoMarginLevQual = Y• Priority and Load Considered

hoThresholdLevUL/DL (LUR/LDR)(HOC) -110 … -47 dBm px (LUP/LDP) 1 … 32nx (LUN/LDN) 1 … 32

Parameter Value

rxLevMinCell(n) (SL)(ADJC) -110 … -47 dBmmsTxPwrMax(n) (PMAX1 & PMAX2)(ADJC) 5 … 43 & 0 … 36 dBmhoMarginLev(n) (LMRG)(ADJC) -24 … 24 dB

Figure 27 Handover Due to Quality

The BSC compares the averaged measurement result AV_RXQUAL_UL/DL_HOwith hoThresholdsQualUL (QUR)(HOC). A handover whose cause is uplink quality might be required if at least Px averages out of Nx averages are greater (of worse quality) than or equal to the threshold RxQual.

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Figure 28 Algorithm for Handover Due to Quality

The parameter hoThresholdsLevUL/DL (LUR/LDR)(HOC) is used for comparing the averaged values of uplink signal level measurements for triggering the HO due to uplink level.


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⇒ RxLev is the threshold level for handover . The range is from -110 dBm to -47 dBm.

⇒ 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 lower than or equal to the threshold before a handover is possible. The range varies from 1 to 32.

The BSC compares the averaged measurement result AV_RXLEV_UL_HO with hoThresholdsLevUL (LUR)(HOC). A handover whose cause is uplink level might be required if at least Px averages out of Nx averages are lower than or equal to the threshold RxLev.

Equations 1 and 2’ are used if parameter enableHoMarginLevQual is set “Yes” (MRGS)(ADJC)

2 dB

hoMarginQual = 0 dB (QMRG)(ADJC)

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

Figure 29 Example of Handover Due to Quality

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6.4.2 Handover due to Signal Level

• Trigger• Threshold Comparison (hoThresholdsQualUL/DL with px/nx)(QUR/QDR)(HOC)

• Candidate Selection• Equation 1 used• Equation 2 used if enableHoMarginLevQual = N (MRGS)(ADJC)• Equation 2' used if enableHoMarginLevQual = Y• Priority and Load Considered

hoThresholdQualUL/DL (QUR/QDR)(HOC) 0 … 7 px (QUP/QDP) 1 … 32

nx (QUN/QDN) 1 … 32

Parameter Value

rxLevMinCell(n) (SL)(ADJC) -110 … -47 dBmmsTxPwrMax(n) (PMAX1 & PMAX2)(ADJC) 5 … 43 & 0 … 36 dBm hoMarginQual(n) (QMRG)(ADJC) -24 … 24 dB

Figure 30 Handover Due to Signal Level

Equations 1 and 2’ are used if parameter enableHoMarginLevQual is set “Yes” (MRGS)(ADJC)

hoMarginLev = 4dB (LMRG)(ADJC)

Cell B

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

(RxLev) Thresholddefined by;hoThresholdLevUL/DL= -92 / -95 dBm(LUR/LDR)(HOC)

2 dB

Trigger for Handover due to Level

Cell A

Figure 31 Example of Handover Due to Level


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Figure 32 Algorithm for Handover Due to Level

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6.5 Handover Due to Interference

• Trigger:• Threshold Comparison for Quality (hoThresholdsQualUL/DL with px/nx)• Threshold Comparison for Level (hoThresholdsInterferenceUL/DL withpx/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 (IUR/IDR)(HOC) -110 … -47 dBmpx (IUP/IDP) 1 … 32nx (IUN/IDN) 1 … 32

enableIntraHoInterfUL/DL (EIC/EIH)(HOC) Y / N

Parameter Value

hoPreferenceOrderInterfUL/DL (HUL/HDL)(BSC) INTER / INTRA

Figure 33 Parameters Used

Equations 1 and 2’ are used if parameter enableHandoverMarginLevQual is set “Yes” (MRGS)(ADJC)

hoThresholdQualDL = 5 (QDR)(HOC)hoThresholdInterferenceDL = -85 dBm (HOC)hoPreferenceOrderInterfDL = intra (HDL)(BSC)

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

Trigger for Handover due to Interference

Cell A

Cell B

Threshold (Interference Lev) -85 dBm

5

0

RXLEV

RXQUAL

Figure 34 Example of Handover Due to Interference


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Figure 35 Handover Due to Interference

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7 Imperative HandoversA handover is considered imperative if the cause is one of the following events:

1. Handover due to distance (MS - BTS distance)

2. O&M order to empty a cell

3. Directed Retry and Intelligent Directed Retry (IDR)

4. Chained cells in rapid field drop and enhanced rapid field drop (OPTIONAL)

The target cells for imperative handovers are ranked only according to radio link properties - priority levels are not used.


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7.1 MS – BTS Distance

In the case where distance is the reason for the handover request, the behaviour of the Mobile Station is controlled by the BSC level parameter:

msDistanceBehaviour (DISB)(BSC)(ZEEO)(0..60, 255)(255)

With this parameter we define which executions are allowed, after the Timing Advance has exceeded the threshold set by another parameter ( ).

There are three alternative parameter settings on which the Mobile can act for this Imperative Handover: It will release the call immediately (0), try a 1..60 second Imperative Handover, or just try an Imperative Handover (255). As seen in Handover Algorithms above, the Imperative Handover needs just one equation – equation 1.

Distance Process ---> msDistanceBehaviour (0,1..60,255) (DISB) (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 (EMS)(HOC) Y / NmsDistanceHoThresholdParam (MSR)(HOC) 0 … 63

px (MSP) 1 … 32nx (MSN) 1 … 32

Parameter Value

msDistanceBehaviour (DISB)(BSC) 0, 1 … 60, 255

Figure 36 MS-BTS Distance Handover

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7.2 Rapid Field Drop and Enhanced Rapid Field Drop

The BSC recognises the necessity to make a handover when the HO threshold comparison indicates that a handover whose cause is rapid field drop, might be required from the serving cell to a specified adjacent cell. The situation can take place, for example, when an MS moves so fast from the coverage area of an indoor cell to the coverage area of an outdoor cell, that the uplink radio is lost. The parameter hoThresholdsRapidLevUL (RPD)(HOC) is used for comparing the rawresults of uplink signal level measurements for triggering the HO due to rapid field drop. The parameter hoThresholdsRapidUlN (HOC) defines how many successive rapid field drop thresholds have to be triggered before a call is handed over to a chained adjacent cell.

Chained Cell

Serving Cell

• 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

hoThresholdLevUL (forRapidFieldDrop)(RPD)(HOC) -110 ... -47 dBm[hoThresholdRapidLevU1N 0 ... 32]

chainedAdjacentCell (CHAIN)(ADJC) Y / N

Parameter Value

Figure 37 Rapid Field Drop Parameters

MS Chained Cell

Serving Cell

Rapid Field Drop Handover.

.1st

2nd

-93 dBm

Serving Cell

hoThresholdRapidLevUL(RPD) = - 93 dBmhoThresholdRapidLevUlN (px) = 2chainedAdjacentCell = Yes

Example

Figure 38 Example Rapid Field Drop Handover


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7.2.1 Chained Adjacent Cells In Rapid Field Drop

When the cause is rapid field drop, only those adjacent cells, which are defined as chained adjacent cells, may be selected as target cells. The definition is made for each adjacent cell by the parameter chainedAdjacentCell (CHAIN)(ADJC)(Yes / No). In this case it suffices that the radio link properties of the chained adjacent cells satisfy equation 1, (see chapter, Algorithms) i.e. the handover is considered imperative.

IFHoThresholdsRapidLevUL/Px > 0

IFPx number of RXLEV_UL measurement < HoThresholdsRapidLevUL

THENHandover cause Rapid Field Drop

Target Cell Evaluation:IF

ChainedAdjacentCell(n) = YESAND

AV_RXLEV_NCELL(n) > RxLevMinCell(n) + MAX(0, Pa)

wherePa = MsTxPwrMaxCell(n) - PP = maximum power of MS

Order of preference of target cells:The target cells are reanked according to radio link properties

Interval between handovers and handover attempts

Figure 39 Flowchart for Handover due to Rapid Field Drop

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7.2.2 Turn-Around-Corner MS

When a MS is turning a corner, the signal strength of micro-cell may drop by 30 dB due to the losing of line-of-sight effect. The existing feature rapid field drop in chained cells directs a MS from horizontal cell to the vertical one, but it may also cause ping-pong effect to the in-house slow-moving traffic, if the Rapid field dropthreshold is set too high. Therefore, the newer feature Enhanced Rapid Field Drop with Turn-Around-Corner handover has been designed.

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

A MS turns a corner,the signal drops rapidly

Signal Level

Time Figure 7 Signal Strength of a Fast Moving MS

Figure 40 Rapid Field Drop Example

The basic idea of Enhanced Rapid Field Drop is to detect Deep Dropping Edge for serving cell signal for handover. The parameter erfdEnabled (ERFD) (HOC) can be used and it is based on either uplink measurements or downlink measurements, or both uplink and downlink measurements.

• An Enhanced Rapid Field Drop is detected when the uplink receive signal level falls by adefined level over a set period of time. (Note: ERFD does not use a threshold like RFD)

• On detection of an ERFD, the averaging window for neighbour cells, and the number of zero results allowed in a measurement report, are modified in order to speed up the monitoring of neighbour cells

• The ModifiedAveWinNcell (ERAW)(HOC) and ModifiedNOZ (ERZ)(HOC) parameters are modified over the period ErfdOver (ERD)(HOC)

• If a Handover with cause RR is requested during ErfdOver, an ERFD Handover is initiated using the Handover Target Cell Candidate list for that handover cause Note: An ERFD Handover does not use 'Chained Cells' for target cell evaluation process

• If Handover with RR cause is not detected during ErfdOver, ERFD Handover is stopped

Figure 41 Enhanced Rapid Field Drop Basics


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

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

ParameterParameter ValueValue

erfdEnablederfdEnabled (ERFD)(HOC) (ERFD)(HOC) DIS, UL, DL, or UDL (OPTIONAL feature) DIS, UL, DL, or UDL (OPTIONAL feature)ddeThresholdsLevddeThresholdsLev (ERT)(HOC) 0 … 63 dB (ERT)(HOC) 0 … 63 dB

NxNx (ERN) (ERN) 1 … 32 1 … 32PxPx (ERP) (ERP) 1 … 32 1 … 32

ddeWindowddeWindow (ERMW)(HOC) (ERMW)(HOC) 1 … 32 SACCH 1 … 32 SACCHmodifiedAveWinNcellmodifiedAveWinNcell (ERAW) (ERAW) 1 … 32 1 … 32modifiedNOZmodifiedNOZ (ERZ)(HOC) (ERZ)(HOC) 1 … 32 1 … 32erfdOvererfdOver (ERD)(HOC) (ERD)(HOC) 0 … 64 sec 0 … 64 sec

Figure 42 Enhanced Rapid Field Drop (ERFD) Parameters

The ERFD feature is particularly designed for micro cellular networks where the signal strength of MS can vary a lot. The DDE algorithm is based on monitoring continuous deep drops in the serving cell signal strength (both uplink and downlink) and the following new handover control parameters.

MS measures serving cell signal level all the time and reports the values to BSC in every SACCH frame. There is only one measurement result in one SACCH frame. The BSC calculates the dde_level value based on the raw measurement results coming from MS and defined ddeWindow (ERMW) (HOC)–parameter value. The calculated dde_level value is then compared to the Deep Dropping Edge Threshold level (ddeThresholdLev ), and if the dde_level value is above the threshold level n/p times then MS is considered to be as a Turn-Around-Corner MS

The basic idea of the ERFD target cell evaluation is to make the handover rapidly to the better adjacent cell by using smaller averaging windows (modifiedAveWinNcell (ERAW) (HOC)). The shorter averaging windows and smaller amount of zero results (modifiedNOZ (ERZ) (HOC)) are used to speed up the decision process. The new averaging windows are used after DDE is detected in the serving cell. The new averaging parameters are used as long as the timer erfdOver (ERD) (HOC) expires. This is an improvement to existing optional feature Chained Cells in Rapid Field Drop.

The old Rapid Field Drop in Chained Cells can be used simultaneously, but the new algorithm has a higher priority.

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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

Figure 43 ERFD Detection

For example, If the value of the parameter ddeWindow (ERMW) (HOC) is 3 SACCH (1.5 sec), the BSC compares the most recent measurement sample 8 (multiframe k) with the measurement sample 1 (multiframe k-3).

Sample 1 2 3 4 5 6 7 8

Signal level

-71 dBm

-68 dBm

-70 dBm

-71 dBm

-69 dBm

-70 dBm

-75 dBm

-83 dBm


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ddeWindow (ERMW)(HOC) = 3 SACCH (n = 3)ddeThresholdsLev (ERT)(HOC) = 10, px = 2 & nx =3

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

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

Sample 1 2 3 4 5 6 7 8Signallevel

-71dBm

-68dBm

-70dBm

-71dBm

-69dBm

-70dBm

-75dBm

-83dBm

Figure 44 Enhanced Rapid Field Drop

Furthermore, if ddeThresholdsLev is 10 dB, Px is 2 and Nx 3, it is needed totally two DDE_LEVEL results bigger or equal than the defined threshold value before the MS is considered as a turn-around-corner MS. The minimum time for this decision will be 3 SACCH + 1 SACCH + 1 SACCH (ddeWindow (ERMW) (HOC)) frames which is about 2.5 sec. After this the averaging window sizes can be changed and handover process can start.

If modifiedAveWinNcell (ERAW) (HOC) is 2 and modifiedNOZ (ERZ) (HOC) is 1, before the handover decision can be done the averaging window should be full. This will take minimum 1 second. So totally this process will take 3.5 seconds (2.5 sec + 1 sec.) before the handover is done.

The following figure presents the process leading to the handover

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IFE r fd E n a b le d = U L

O RE r fd E n a b le d = D L

O RE r fd E n a b le d = U D L

IFD D E _ L E V E L > = d d e T h re s h o ld s L e v

T H E NT u rn -A ro u n d -C o rn e r M S

IFU p l in k /D o w n lin k in te r f e re n c e

U p lin k /D o w n l in k q u a l i tyU p l in k /D o w n l in k le v e l

B e t te r c e l l (P o w e r b u d g e t o r U m b re l la )T H E N

H a n d o v e r c a u s e T u rn -a ro u n d -c o r n e r M S

T a rg e t c e l l e v a lu a t io n a c c o rd in g to s e c o n d a r y c r i te r ia

In te rv a l b e tw e e n h a n d o v e rs a n d h a n d o v e r a t te m p ts

Figure 45 ERFD Flowchart


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8 Traffic Reason Handovers

8.1 MSC-Controlled Traffic Reason Handover

In order to share the load between cells, the MSC may request the BSC to perform a specified number of handovers, from one specified cell. In early software versions, it was possible to make traffic reason handovers, only to cells with better or equal radiolink conditions.

Now (since S5), it is possible to hand over to a ‘worse’ cell, when the field strength in that cell is above the rxLevMinCell (SL)(ADJC)(ZEAO)(-110..-47dBm)(-100), and also above the newer parameter trhoTargetLevel (TRHO)(ADJC)(ZEAO)(-109..–47dBm, N)(N) which defines the minimum signal level when a traffic reason handover is allowed to an adjacent cell. Any differences in the mobile transmit power capability in the source and target cell candidates are also taken into account. The target cell candidates are ranked only according to radiolink properties - priority levels are not used.

Figure 46 Traffic reason handover

MSC request Traffic reason handover

Target cell evaluation:AV_RXLEV_NCELL(n) > RxLevMinCell(n) + MAX(0, Pa)

ANDAV_RXLEV_NCELL(n) > TrhoTargetLevel(n) + MAX(0, Pa)

wherePa = MsTxPwrMaxCell(n) - PP = maximum power of MS

Interval between handovers and handover attempts:Power budget and umbrella handovers back to the source

cell are not allowed during the quard time:GUARD_TIME = 20 sec + MinIntBetweenHoReq

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Traffic reason handover from the serving cell to the adjacent cell in question is disabled when the value of the ADJC(n) parameter trhoTargetLevel(TRHO)(ADJC)(ZEAO)(-109..–47dBm, N)(N) is at the default value of N (not in use).

8.2 BSC Initiated Traffic Reason Handover (Advanced Multilayer Handling)

Handover

+0 dB +4dB +6 dB

Figure 47 BSC initiated TRHO (Congestion relief)

The AMH concept provides the network operator with the tools needed to relieve the load of the congested cells and smooth out the load over the network. The overall traffic load of the network can be smoothed out using the BSC initiated TRHO feature. The basic idea of the feature is to dynamically change the power budget margins and thus, direct the MSs hanging around in the cell border to less loaded adjacent cells.

BSC initiated TRHO is based on the traffic load situation of the serving cell. If the traffic load of the serving cell exceeds the parameter AmhUpperLoadThreshold (AUT) (BSC), AmhTrhoPbgtMargin (ATPM) (HOC) is used for all adjacent cells in the power budget equation instead of the existing HoMarginPBGT (PMRG) (ADJC).

The basic evaluation algorithm calculated according to radio link properties is based on the following strategy and order:

1. AV_RXLEV_NCELL (n) > TRHO_TARGET_LEVEL (n) + Max (0, (MS_TXPWR_MAX_CELL (n) - P))

2.PBGT (n) > AmhTrhoPbgtMargin and PBGT (n) < HOMarginPBGT


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Note: In the 1st equation of power budget HO the old parameter rxLevMin Cell (n) has been replaced by TRHO_TARGET_LEVEL (n), already seen in the MSC traffic reason HO.

Power Budget HO does not have to be enabled in order to make BSC initiated trho enabled.

If the PBGT value is between AmhTrhoPbgtMargin (ATPM) (HOC) and HoMarginPBGT (PMRG) (ADJC), the handover is triggered due to the BSC initiated TRHO handover.

If the PBGT value exceeds the HoMarginPBGT (PMRG) (ADJC), the handover is triggered due to PBGT handover.

Furthermore, the TRHO candidate must have less than AmhMaxLoadOfTgtCell (AML) (BSC) reserved traffic channels in order to become a target cell.

Note: Queuing is not used with the BSC initiated TRHO.

Sometimes, traffic must be directed to weaker cells. Therefore, a special mechanism for handling this kind of traffic, keeping the call in the new cell, is added to the concept. The parameter TrhoGuardTime determines the penalty time to handover back to the original cell because of PBGT handover. Therefore, consecutive handovers back to the original cell during the guard time are prevented.

Note 1: So that external AMH can be successful, the parameter "CAUSE FIELD IN HANDOVER REQUEST SUPPORTED" must have the value YES in the BSSAP version profile of the MSC. This parameter must be YES so that ho guard timers and queuing is not in use.

Note 2: Only calls from the regular layer can be redirected due to traffic reason handover.

Note 3: The new parameters are visible only when this new feature (AMH_USAGE) is in use: since it's an OPTIONAL feature it has to be configured in the BSC SW package.

Parameters involved:

AmhTrhoPbgtMargin -24 dBm - +24 dBm, N

If the parameter is set to "N", the feature is turned off.

Power budget margin will be HoMarginPBGT (PMRG) (ADJC).

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AmhUpperLoadThreshold 0 - 100%

(serving cell parameter)

AmhMaxLoadOfTgtCell 0 - 100%

(adj cell parameter)

TrhoGuardTime 0 – 120 sec

8.2.1 Interactions with other Features

MSC controlled TRHO

The AMH BSC initiated TRHO feature is not recommended to be used at the same time and in the same cell with the existing MSC controlled TRHO feature.

Meaning that the old parameter trhoTargetLevel (TRHO) (ADJC) would be set to "N", corresponding to the value –110 dBm according to the coding of level and quality done in the BSC for PC&HO algorithm.

Direct Access to Desired Layer/Band (S8)

The AMH multi layer load control has lower priority than the Direct Access to Desired Layer/Band (DADL/B) feature. It is possible to direct traffic to lower layer cells (underlaying micro cells or DCS cells) with DADL/B although AMH prevents mobiles from accessing to lower layer cells.

HSCSD (S7)

The BSC initiated TRHO is made only for single slot connections. The parameter UpperLimitCellLoadHSCSD (HCU) (BTS) should be set to a greater value than the parameter AmhUpperLoadThreshold (AUT) (BSC). Through these actions unnecessary downgrades are avoided and radio resources are used efficiently.


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9 Load control between layers: Advanced Multilayer Handling (AMH)

The Nokia Advanced Multilayer Handling (AMH) feature concept also consists of two other features related to network load. It was first introduced in BSS S8.

We have already seen the feature that relates to the same-layer situation, where congestion can be regulated via a BSC traffic-reason handover.

In networks with more than one layer, (GSM 800, 900, 1800, 1900, IUO), the AMH concept provides two more features.

AMH is used to re-distribute traffic to the appropriate layer or frequency band, according to the prevailing load on the network. Therefore AMH can even-out traffic distribution over the network.

It provides operators with the parameter setting tools necessary to enable using only the overlay network during low traffic periods. This avoids additional handovers between two layers. AMH is able to re-distribute traffic from the congested regular layer, to other cells, selecting the best cell for each mobile call, reducing the probability of abnormal termination, and giving good quality in the new serving cell.

9.1 IUO Load Control

IUO is applied to the network to increase capacity by increasing the spectral efficiency of some channels. However, during periods when the capacity needs are not so high, the IUO functionality is still performing a lot of handovers. Therefore, by avoiding the additional handovers between different frequency layers, the quality of the network can be improved.

AMH can be used to prevent the use of IUO during very light traffic and thus, keep the mobiles only in the overlay network. The solution is based on the traffic load of the serving cell.

If the traffic load of the serving cell goes under the threshold value for amhLowerLoadThreshold (ALT)(BSC)(ZEEO)(0..100%)(20%), then an IUO handover, or a Direct Access to super-reuse TRX, are not allowed. The functionality is controlled by parameter amhTrafficControlIUO (ATCI)(HOC)(ZEHO)(Y/N)(N), which indicates whether AMH is used with IUO.

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9.2 Multilayer (Dual Band/Micro Cellular) Network Load Control

Many operators build capacity by using micro cells or a Dual Band solution. The increasing number of different network layers also increases the number of handovers between layers. During very quiet traffic, for example at night, there is no need for extra capacity, and the underlay capacity is not necessary. Therefore, adequate traffic capacity can be achieved by using only the overlay network.

On the other hand, during night time, most traffic is accomplished outside buildings, and on roads, particularly from fast moving vehicles. Furthermore, fast moving mobiles in the micro cell network generate a lot of handovers with relatively high speed. Therefore, it is more reasonable to keep this traffic in the overlay network instead of the underlay, in order to provide better quality to the end users.

The OPTIONAL feature AMH can be used to prevent the use of the microcell layer or dual band layer, during very light traffic, and thus keep the mobiles only in the macrolayer (eg GSM 900) network, once they have camped on it.

If the traffic load of the serving cell goes under amhLowerLoadThreshold (ALT)(BSC)(ZEEO)(0..100%)(20%), then the lower layer cells are not allowed to use these features while parameter ACL has been set to enabled:FMMS (S5), MS Speed Detection (S6), and Umbrella handovers.ACL is: adjCellLayer (ACL)(ADJC)(ZEAO)(N/SAME/UPPER/LOWER)(N=not in use, is default) (OPTIONAL - FMMS handling in macro cell)

AMH functionality is controlled by the parameter: amhTrafficControlMCN (ATCM)(HOC)(ZEHO)(Y/N)(N) (OPTIONAL – AMH)ATCM indicates whether AMH is used with micro cells / Dual Band, or not.

Note: Access to micro cells cannot be prevented, but C2 reselection can be used to keep the fast moving mobiles in the overlay network during idle mode.

9.3 Parameters Involved in AMH feature activation

amhTrafficControlIUO (ATCI)(HOC)(ZEHO)(Y/N)(N)amhTrafficControlMCN (ATCM)(HOC)(ZEHO)(Y/N)(N)amhLowerLoadThreshold (ALT)(BSC)(ZEEO)(0..100%)(20%)


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These parameters (ATCI, ATCM, ALT) are visible only when the OPTIONAL feature AMH is built into the BSC SW package (from S8), with AMH_USAGE on(ZWOO, ZWOF, 10-53).

With the ATCI (HOC) parameter we indicate whether AMH is used with IUO.

With the ATCM (HOC) parameter we indicate whether AMH is used with Micro Cells/Dual Band, or not.

With parameter: amhTrhoPbgtMargin (ATPM)(HOC)(ZEHO)(-24..24dBm/N)(N), we define the power budget margin used in AMH (suggested value 6dBm), when the load of the cell exceeds the value that has been defined with the parameter AUT (default value 80%).

With the ALT (BSC) parameter we define the lower threshold for the load of a Base Station, to trigger AMH functionality with the features IUO and/or Dual Band/Microcell. (default value 20%) (to change: ZEEM:ALT=v;)

These Dual Band Adjacencies need to be checked: (ZEAC, ZEQF / DBC=Y).Parameter multiBandCell (DBC)(BTS)(ZEQO)(Y/N)(Y=900&1800; N=800&1900)defines whether adjacent cells with a BCCH allocated from a different band to the serving cell BCCH, are taken into account in handovers, and in Idle mode cell selection and reselection. (OPTIONAL feature Dual Band GSM900-1800 / 800-1900)

AMH must be allowed with each Dual Band / Micro cell (ZEHG:BTS=n:ATCM=Y;).Handovers between the cells must be enabled (ZEAM:BTS=n:ABTS=n:PMRG=v;).Parameter hoMarginPBGT (PMRG)(ADJC)(ZEAO)(-24..63dB)(6dB default),defines a threshold in the power budget process, preventing repeated handovers between adjacent cells.

With the AML (BSC) parameter we define the maximum traffic load (Adjacent Cell) allowed for the Target Cell of a Traffic Reason Handover (TRHO) (default value 70%).

With the AUT (BSC) parameter we define the upper threshold for the load of a Base Station, to trigger BSC-controlled traffic reason handovers (default value 80%).

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10 Direct Access to Desired Layer/Band (DADL/B)

The purpose of the Direct Access to Desired Layer/Band (DADL/B) feature is to direct traffic in the call setup phase from the SDCCH of macro cell / GSM900 cell to the TCH of micro cell / GSM1800 cell whenever possible.

Also the extended GSM900 band capability of the MS is taken into account. The functionality of this feature is similar to Directed Retry.

The difference is that the handover in DADL/B is triggered due to cell load and the target cell selection criteria are tighter.

Note: the functionality of this direct access feature is different from direct access in IUO.

The feature is OPTIONAL and was first released in BSS S8.

This feature makes it possible to reduce the number of unnecessary TCH reservations of dual band mobiles in GSM 900 cell and thus provides more trunking efficiency in the network. The DADL/B handover can also be made in another direction, from micro cell to macro cell or from GSM1800 cell to GSM 900 cell. The use of DADL/B is flexible with adjacent cell specific indications and DADL/B can be tailored to different environments and concepts. Note also that this DADL/B feature is also applicable to phase 1 mobiles which do not support C2 cell re-selection.

The DADL/B will be applied if the DADL/B is enabled in BSC and if the load of the accessed cell is higher than the BTSLoadThreshold (BLT) (BTS) defined for the accessed cell.

There must also be are adjacent cells defined as DADL/B handover target cells with the dadlbTargetCel (DADL) (ADJC)l (n) parameter (the only new parameter added for this feature).

The adjacent cells are verified according to the MS capabilities (single band, dual band, and tri band) read from the classmark.

Directed Retry or Direct Access to Desired Layer/Band

If there is real TCH congestion in the accessed cell, then a DR due to congestion with or without queuing will be made.

If there are TCHs available in accessed cell, then BSC will act according to the DADL/B usage determination and cell load. If DADL/B is applied and the TCH load in the accessed cell is higher than the BTSLoadThreshold (BLT) (BTS), then DADL/B handover procedure will be started. If DADL/B is not applied or the TCH load is lower than the BTSLoadThreshold (BLT) (BTS, a TCH will be allocated from


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the accessed cell. Note that pre-emption always has higher priority. The TCH will be allocated from the accessed cell and thus no DADL/B handover will be attempted.

The target cells for DADL/B handover are selected according to the following criteria:

Adjacent cells handled by the same BSC handling the accessed cell, considered as DADL/B target cells with signal level exceeding HOLevelUmbrella (AUCL) (ADJC), will be sorted according to adjacent cell priority and load factor (HOPriorityLevel (PRI) (ADJC) or HOPriorityLevel (PRI) (ADJC) (n) –LoadFactor ( ) ( ) (n) if adjacent cell is loaded) to be used as target cells for DADL/B handover.

The DADL/B handover attempt can fail due to he following reasons:

- TCH could not be allocated from any cell included in the DADL/B list (TCH

congestion in DADL/B target cells)

- DADL/B handover fails and MS returns to old channel (SDCCH).

- The timer monitoring DADL/B target cell list availability triggers. This timer is the same as this one used in Directed Retry (MinTimeLimitDR).

In these cases, the TCH is tried to allocate from the accessed cell. The DR and queuing are possible in the second TCH request if no TCHs are available in the accessed cell. The BSC will evaluate the target cells for DR. When at least one target cell for DR is found.

Note that the DADL/B handovers are possible only as intra BSC handovers.

10.1 DADL/B interactions with other features

Management of SDCCH-SDCCH handovers in ABIPRB

If an SDCCH-SDCCH handover is started and ABIPRB receives an abis_assignment_ s message, the SDCCH-SDCCH handover will be stopped and the assignment request will be handled. If ABIPRB has received an "abis_assignment_s" message and is handling the assignment request, then the starts will not be made to the SDCCH-SDCCH handovers and IUO DAC handovers.

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DADL/B handover vs. IUO DAC and SDCCH-SDCCH handovers

The DADL/B handover case cannot happen simultaneously with the IUO DAC handover and SDCCH-SDCCH handover, because DADL/B handover is triggered after ABIPRB has received the "abis_assignment_s" message (as stated above).

IUODR vs. DADL/B

In radio resource allocation (i.e. when allocating a TCH for a call) the IUODR has higher priority than the DADL/B.

Other interactions

There are other interactions in Directed Retry (DR), as described in the functional description of Directed Retry Procedure in the BSC.


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11 Handover Parameter List1) BSC level HO parameters

Parameters Value Range

hoPreferenceOrderInterfDL INTER / INTRA

hoPreferenceOrderInterfUL INTER / INTRA

msDistanceBehaviour 0 ... 60, 255

genHandoverRequestMessage 1 ... 16

disableIntHo (DINHO) Yes / No (default)

2) HO Control Process

Parameters Value/ Range

BtsLoadThreshold (BLT)(BTS) 0 ... 100%

maxNumberOfRepetitions 5 ... 35

3) Averaging Adjacent Cells


averagingWindowSizeAdjCell (AWS)(HOC)

1 ... 32 SACCH

numberOfZeroResults (NOZ)(HOC) 0 ... 7

allAdjacentCellsAveraged (AAC)(HOC) (Yes=all, No=6 best)

Yes / No

4) Ho Measurement Averaging Method


EnaFastAveCallSetup Yes / No

EnaFastAveHo Yes / No

EnaFastAvePC Yes / No

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5) Minimum Intervals between handovers


MinIntBetweenUnsuccHoAttempt 0 ... 30

MinIntBetweenHoReq 0 ... 30

6) HO types allowed


EnableIntraHoInterfUL Yes / No

EnableIntraHoInterfDL Yes / No

EnablePwrBudgetHandover Yes / No

EnableUmbrellaHandover Yes / No

EnableMSDistanceProcess Yes / No

EnableSDCCHHandover Yes / No

7) Periodic handovers


HoPeriodPBGT 0 ... 63

HoPeriodUmbrella 0 ... 63

8) Averaging windows and weighting values


HoAveragingLevDL 1 ... 32

Weighting 1 ... 3

HoAveragingLevUL 1 ... 32

Weighting 1 ... 3

HoAveragingQualDL 1 ... 32

Weighting 1 ... 3

HoAveragingQualUL 1 ... 32

Weighting 1 ... 3


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MsDistanceAveragingParam 1 ... 32

MsSpeedAveraging 1 ... 32

9) Thresholds


HoThresholdsLevDL -110 ... -47

Px 1 ... 32

Nx 1 ... 32

HoThresholdsLevUL -110 ... -47

Px 1 ... 32

Nx 1 ... 32

HoThresholdsQualDL 0 ... 7

Px 1 ... 32

Nx 1 ... 32

HoThresholdsQualUL 0 ... 7

Px 1 ... 32

Nx 1 ... 32

HoThresholdsInterferenceDL -110 ... -47

Px 1 ... 32

Nx 1 ... 32

HoThresholdsInterferenceUL -110 ... -47

Px 1 ... 32

Nx 1 ... 32

MsDistanceHoThresholdParam 0 ... 63

Px 1 ... 32

Nx 1 ... 32

10) Fast / Slow Moving Ms


LowerSpeedLimit 0 ... 255

UpperSpeedLimit 0 ... 255

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MsSpeedThresholdNx 1 ... 32

MsSpeedThresholdPx 1 ... 32

MsSpeedDetectionState 0 … 100

11) Rapid Field Drop and Enhanced Rapid Field Drop


ErfdEnabled DIS / UL / DL / UDL

ErfdOver 1 … 64

DdeWindow 1 … 32

ModifiedAveWinNcell 1 … 32

ModifiedNOZ 1 … 32

DdeThresholdsLev 0 … 63

Px 1 … 32

Nx 1 … 32

HOThresholdsRapidLevUl -110 ... -47

HoThresholdsRapidLevUlN (px) 0 ... 32

12) C/I Based Ho Candidate Evaluation


CiEstMethod AVE / MAX / NONE

LowerCILimit -128 ... 127

L1 -128 ... 127

L2 -128 ... 127

L3 -128 ... 127

L4 -128 ... 127

L5 -128 ... 127

L6 -128 ... 127

PriorityAdjStep -8 ... 7

P1 -8 ... 7

P2 -8 ... 7

P3 -8 ... 7


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P4 -8 ... 7

P5 -8 ... 7

P6 -8 ... 7

P7 -8 ... 7

13) Directed Retry handover


drInUse Yes / No

IdrUse Yes / No

maxTimeLimitDirectedRetry 1 ... 15

minTimeLimitDirectedRetry 0 ... 14

disableExtDr Yes / No

drMethod 0 … 1

drThreshold -47 … -110

14) Extended cell radius (Improved solution)(OPTIONAL)


maxMSDistanceHOThreshold (MAX)(HOC)

0 ... 63

minMSDistanceHOThreshold (MIN)(HOC)

0 ... 63

15) AMH


AmhUpperLoadThreshold 0 … 100 %

AmhLowerLoadThreshold 0 ... 100 %

AmhMaxLoadOfTgtCell 0 ... 100 %

AnhTrhoGuardTime 0 … 120 s

AmhTrhoPbgtMArgin -24 … 24 dBm / N

AmhTrafficControlMCN Y / N

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AmhTrafficControlIUO Y / N

16) Dynamic Hotspot


SoftBlockingStartReg 0 … 255

SoftBlockingStartSup 0 … 255

BadQualLimit 0 … 100 %

GoodQualLimit 0 … 100 %

SigQualLimit1 0 … 100 %

SigQualLimit2 0 … 100 %

TchProbability1 0 … 100 %



In the above HO parameters list, in 1) BSC level, the parameter: disableIntHo (DINHO)(BSC)(Y/N)(N), for: disable internal handover, was been added in S7: It defines whether all handovers are controlled by the MSC or not.


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12 Adjacent Cell Parameter ListThe handover process always requires two cells: a serving cell and a target cell. The target cell has to be defined as an adjacent cell to the serving cell. This is because the Mobile Station measures only the serving cell and the defined adjacent cells. Some information is needed about the adjacent cells in order to know the conditions of these cells: their IDs, frequencies, cell priorities, minimum access levels, margins for handovers, etc. All the parameters required for adjacent cells (to S10.5) are presented below. The parameter names are given in two formats, as Q3 (long), and as short, with the object levels, and range of values. It is intended that the default values, and recommended values are discussed during the BSSPAR training course, then notes made by the participant, on these pages.

1) Frequency

Parameters Value / Range

BCCHFrequency(FREQ)(ADJC) also (BFREQ)(ADJC) incl ND

ARFCN

DUAL (900 & 1800) (OPTIONAL)

0 …

…

1023

GSM800 128 … 251

P-GSM900 1 … 124

E-GSM900 975 … 1023, 0

GSM1800 512 … 885

GSM1900 512 … 810

2) Cell Accessing Parameters


MsTxPwrMaxGSM (800/900) (PMAX1)(ADJC)

5 ... 43 dBm

MsTxPwrMaxGSM1x00 (1800/1900) (PMAX2)(ADJC)

0 ... 36 dBm

rxLevMinCell (SL)(ADJC) -110 ... -47 dBm

3) Other handover parameters

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synchronised (SYNC)(ADJC) Yes / No

hoPriorityLevel (PRI)(ADJC) 0 ... 7

hoLoadFactor (OF)(ADJC)(<PRI) 0 ... 7

msPwrOptLevel (POPT)(ADJC) -110 ... -47, N dBm

4) Handover Margins


enableHoMarginLevQual (MRGS)(A) Yes / No (Y)

hoMarginLev (LMRG)(ADJC) -24 ... 24 dB

hoMarginQual (QMRG)(ADJC) -24 ... 24 dB

hoMarginPBGT (PMRG)(ADJC) -24 ... 63 dB

5) Identity parameters for adjacent cells


AdjacentCellId (LAC & CI)(ADJC)

LAC 0 … 65535

CI 0 … 65535

BSIC = NCC + BCC

bs IdentityCode (NCC & BNCC) (ADJC)

0 … 7 (ND)

bs IdentityCode (BCC & BBCC) (ADJC)

0 … 7 (ND)

6) Handover Access Level and Threshold


hoLevelUmbrella (AUCL)(ADJC) -110 ... -47 dBm

hcsThreshold (HCS)(ADJC) -110 ... -48 dBm, N

7) Directed Retry (DR) & Intelligent Directed Retry (IDR) (both OPTIONAL)


drThreshold (DRT)(ADJC) -110 … -47 dBm


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CellType (CTY)(ADJC) GSM / MCN

8) Fast moving MS handling in macro cell (OPTIONAL)


adjCellLayer (ACL)(ADJC) N / SAME / LOWER / UPPER

fastMovingThreshold (FMT)(ADJC) 0 ... 255

9) Chained cells in rapid field drop (OPTIONAL)

Parameter Value / Range

chainedAdjacentCell (CHAIN)(ADJC)

Yes / No

10) Traffic reason handover


trhoTargetLevel (TRHO)(ADJC) -109 ... -47, N dBm

11) Improved solution for extended range cell (OPTIONAL)


hoTargetArea (HOTA)(ADJC) 0 ... 3

12) C/I Based Handover Candidate Evaluation (OPTIONAL)


LA Code of ref cell (LAC1-LAC5) 0 … 65535

referCell (CI1-CI5) 0 … 65535

ciEstWeight (W1-W5) 0 ... 10

levelAdjustment (L1-L5) -63 ... 63

13) Half Rate or EFR Codec (OPTIONAL)


tchRateInternalHo (HRI)(BSC) 1 .. 5

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14) AMR Codec (OPTIONAL)


amrConfInHandovers (ACH)(BSC) 1 .. 2

15) Direct Access to Desired Layer/Band (DADL/B)(OPTIONAL)


amrDadlbTargetCell (DADLA)(ADJC) Yes / No

dadlbTargetCell (DADL)(ADJC) Yes / No

16) Dynamic Hotspot (OPTIONAL)


backgroundDBAttributes: interferedCell (BIC)(ADJC)

0 … 3, ND

interferedCell (IC)(ADJC) 0 … 3


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13 Practical Examples of HandoversSome practical examples will be given in the figure below. The first one is quite a normal case, when the adjacent channel frequency is found in the adjacent cell. This is an example which shows that the specifications are not tight enough in this matter. The GSM specifications say that the first adjacent channel can be 9 dB stronger than the serving channel, and based on this fact, the hoMarginLev (LMRG)(ADJC) could be set at 6 dB. In practice there already is interference when the adjacent channel is 4-5 dB higher than the serving channel. This is the reason that the value of hoMarginLev (LMRG)(ADJC) has to be set lower than 6 dB. This, however, more easily causes the Ping-Pong effect. It is common practice to set LMRG to 3 dB, which is the default value. For the LMRG setting to have effect, the MRGS (ADJC)parameter must be set to Yes, which is its default value. Also the DL RX level handover threshold LDR, LDP, LDN (HOC) will affect LMRG action.

The second example is of some cells with very large coverage areas, such as with a high antenna mast, or with the BTS in a hilly area within Line Of Site of the MS. This can sometimes be avoided with "double adjacent cell list".

Practical Examples

1. Adjacent Channel in Adjacent Cell

2. Cell with Very Large Coverage Area

• In practice after -6 dB -> interferencies + quality goes down to 4-5

• hoMarginLev < -6 dB -> Ping-Pong !!

• MS switched off in cell A and transferred to area of cell X

C I - 9 dBa =•

A X

20 km

• •xx

• MS switched on in new place -> MS tries first old channel + neighbourgs• MS camped on cell A which is not in neighbourg list of cell X -> do not listen BCCH of cell X -> no HOs to cell X !!!

Figure 48 Practical Examples of Handovers

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14 Handover Control Exercise1) Look-up the default and recommended values for the thresholds named on this

diagram, and compare those values to the values that are shown on the diagram.

2) State the optimum range of values for RX level & RX quality, for both UL & DL.


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15 Handover and Power Control Exercise 1) Discuss the reasons why the HOC & POC thresholds are at different levels.

2) State the optimum range of values for RX level & RX quality, for both UL & DL, in both HOC & POC.

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16 Key Learning PointsHandover decisions are made by Radio Resource (RR) Management in the BSC, based on (a) measurement results from the MS and BTS, (b) cell parameters, and (c) algorithms.

Handovers are triggered either by threshold comparison or by periodic comparison.

Handover thresholds are compared and prioritised in the following order: 1.Interference (uplink or downlink), 2.Uplink quality, 3.Downlink quality, 4.Uplink level, 5.Downlink level, 6.MS-BS distance (maximum or minimum), 7.Turn-around-corner MS, 8.Rapid field drop, 9.Fast or Slow-moving MS, 10.Better cell (Power budget or Umbrella), 11.PC due to Lower quality thresholds (uplink and downlink), 12.PC due to Lower level thresholds (uplink and downlink), 13.PC due to Upper quality thresholds (uplink and downlink), and 14.PC due to Upper level thresholds (uplink and downlink).

When two or more threshold comparisons are present simultaneously, the one with the highest priority is selected, and the BSC performs target cell evaluation using this threshold criterion on adjacent cells. If no cell is good enough for the handover, the BSC performs the same operation on the next highest threshold comparison.

The BSC controls the intervals between handovers and handover attempts using:

• MinIntBetweenHoReq (MIH)(HOC)(0..30s: prevents repetitive handovers for the same MS, with a timer that sets the minimum interval between handovers

• MinIntBetweenUnsuccHoAttempt (MIU)(HOC)(0..30s): sets the minimum time between an unsuccessful handover and the next attempted handover

The purpose of target cell evaluation is to find the best adjacent cell for handover. The steps are

1.BTS sends the list of best candidates to the BSC based on radio link measurements.

2. The BSC defines and selects those cells (up to 32), which meet the requirements for radio link properties and threshold comparison.

3. Load Check: loads of candidate cells are checked using the following parameters

If current btsLoad > btsLoadThreshold (BLT) (BTS) (0 ... 100 %).

Then hoLevPriority = hoLevPriority - hoLaodFactor

btsLoadThreshold (BLT) (BTS) (0 ... 100 %) load threshold for the BTS.

hoLevPriority (PRI) (ADJC) (0 ... 7) : handover level priority of a cell.

hoLoadFactor (OF) (ADJC) (0 ... 7) used to decrease handover priority.

4. The candidate cell with highest priority is selected as target cell. If there are many cells of equal priorities, the cell with the best receiving signal strength will be selected.

Radio Resource Handovers are due to level (uplink / downlink), quality (uplink / downlink), interference (uplink / downlink), power budget or umbrella. The relevant thresholds are:


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• hoThresholdsLevDL/UL

• hoThresholdsQualDL/UL

• hoThresholdsInterferenceDL/UL

• msSpeedThresholdNx/Px (STN)/(STP) (HOC)

Each above threshold is composed of three parts:

(a) the threshold itself,

(b) Px: total samples that form sample space,

(c) Nx: number of samples out of Px that have to be above/below threshold.

Power Budget Handover ensures an MS is handed over to a cell with minimum path loss, even though quality and level thresholds may not have been exceeded.

The parameters used for enabling and checking period for power budget handovers are

• EnablePwrBudgetHandover (EPB) (HOC) (Yes/No) used to activate it.

• HoPeriodPBGT (HPP) (HOC) (0 ... 63 SACCH): checking periods of radio link properties of adjacent cells for power budget handover

The power budget evaluation algorithm for target cell evaluation is 1. AV_RXLEV_NCELL(n) > RxLevMinCell(n) + Max (0, MsTxPwrMax(n)-MsTxPwrMax)

2. PBGT > HoMarginPBGT(n)

where PBGT = MsTxPwrMax -AV_RXLEV_DL_HO-(BTSTxPwrMax-BTS_TXPWR)-(msTxPwrMax(n) - AV_RXLEV_NCELL(n))

MsTxPwrMaxCell (n) is the maximum power an MS is permitted to use on a traffic channel in the adjacent cell (n)

AV_RXLEV_NCELL (n) stands for averaged receiving levels of adjacent cells

AV_RXLEV_DL_HO stands for the averaged receiving level of the serving cell.

Umbrella Handover are controlled by

• EnableUmbrellaHandover (EUM) (HOC) (Yes/No): used to activate it.

• HoPeriodUmbrella (HPU) (HOC) (0 ... 63 SACCH): checking period for umbrella handovers

BSC selects target cells for umbrella handover which satisfy the following criteria, and then the cells are ranked according to the priority level:

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

During umbrella handover only adjacent cells that fit MS power class are chosen. BSC selects macrocells for vehicle and portable MSs and microcells for handhelds using parameters

gsmMacrocellThreshold (GMAC) (BSC)

dcsMacrocellThreshold (DMAC) (BSC))

gsmMicrocellThreshold (GMIC) (BSC)

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dcsMicrocellThreshold (DMIC) (BSC)).

Handovers due to combined umbrella and power budget if activated simultaneously then(a) power budget handover can only take place between cells of the same layer and size in addition to the radio link properties (equations 1 and 2). and

(b) umbrella handover has higher priority over power budget handover when both are possible

The feature MS Speed consists in two different features: Fast moving MS in macro cell and MS Speed Detection. These features are used to increase the capacity of a cellular network in high traffic density areas which are covered with a multi-layer network consisting of different sized cells. If a high speed mobile is located in such an area it should be located in a macrocell to decrease the amount of handovers.

Fast Moving MS Handling In Macro Cell checks if the MS is moving fast by counting the number of received measurements from each adjacent microcells and checking if a threshold (fastMovingThreshold (FMT) (ADJC) 0 … 255) has been passed. The counter starts from zero and if a measurement is received and the received signal level exceeds the minimum for that cell (rxLevMinCell (SL) (ADJC)), then the counter is increased by 2. Otherwise it is decreased by 1.

If the mobile is moving slow an umbrella handover to the lower layer (microcell) will be triggered. The feature is automatically used when there is at least one adjacent cell belonging to a lower layer than the serving cell.

Handovers due to MS Speed Detection feature estimates MS speed by counting fading dips. The crossing rate algorithm counts the rate with which the signal strength levels obtained from each burst crosses the averaged signal level (RXLEV in the uplink direction over one SACCH frame) due to fast fading.

If averaged MS speed (msSpeedThresholdPx (STP) (HOC) (1 ... 32)) out of last averaged MS speed indications (msSpeedThresholdNx (STN) (HOC) (1 ... 32)) exceeds upperSpeedLimit (USL) (HOC) (0 ... 254), then MS is considered fast moving and the call will be handed over (umbrella handover) to a suitable upper layer cell (macrocell) if any.

If averaged MS speed out of last averaged MS speed indications is/are lower than the lowerSpeedLimit (LSL) (HOC) (0 ... 254), then MS is considered slow moving and call will be handed over to a suitable lower layer cell (microcell) if any. The step size for Upper/LowerSpeedLimit parameter is 1, which correlates with a speed of 2 km/h.

MS speed indication is set to non-valid (255) and sent to BSC if

(a) If RF hopping and the crossing rate algorithm are both set on in a BTS.

(b) If uplink DTX was used during current and/or previous SACCH multiframe.

(c) If MS power control is in progress, the HO&PC algorithm will discard the MS speed indications until the power control is completed.

Various Window Size feature is introduced in release S7 because “Speed” has different interpretations in different locations. It is reasonable to have various window size (i.e. Better Cell Trigger / Quality Trigger) according to speed indication. High-


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speed MS should use shorter average window size and low-speed MS should use longer average window size.

BSC may use the information on the speed of the mobile station either to control traffic between separate layers of the multi-layered cellular network or to scale the values of the averaging parameters. The function is selected with the parameter MsSpeedDetectionState (SDS) (HOC). The alternative values are the following:

0 MS speed information is used to control traffic between separate layers of the multi-layered cellular network by means of the handover procedure.

1 – 100 MS speed information is used to scale values of averaging parameters. If the value=80% the averaging window is 80% of the normal window size.

If Px averaged MS speed out of last Nx averaged MS speed indications exceed upper speed limit (USL), then MS is fast moving and scaled averaging windows are used.

If Px averaged MS speed out of last Nx averaged MS speed indications are lower than lower speed limit (LSL), then MS is slow moving and normal averaging windows are used.

Handover Due To Quality Or Signal Level

If HO threshold comparison indicates that a handover due to uplink or downlink quality or level is required, the BSC evaluates the radio link properties of the adjacent cells in order to find a target cell for the handover. Equations 1 and 2' are used in case of uplink or downlink quality or level.

The BSC compares averaged measurement result AV_RXQUAL_UL/DL_HO with HoThresholdsQualUL/DL for triggering the HO due to uplink quality.

The BSC compares the averaged measurement result AV_RXLEV_UL/DL_HO with HoThresholdsLevUL/DL.

Imperative handover are caused by one of the following events: 1. MS - BTS distance, O&M order to empty the cell, directed retry, chained cells in a rapid field drop (optional), The target cells for imperative handovers are ranked only according to radio link properties (equation 1); priority levels are not used.

1. When MS-BTS distance is the reason for handover request, the MS behaviour is controlled using parameter msDistanceBehaviour (DISB) (BSC) (0 ... 60, 255). There are three alternatives: release the call immediately, try a 1..60 seconds imperative handover, or try an imperative handover (255).

2. Handovers due to rapid field drop are controlled by parameters:

HoThresholdsRapidLevUL (RPD)(HOC) is used for comparing the raw results of uplink signal level measurements for triggering the HO due to rapid field drop.

hoThresholdsRapidUlN (HOC defines number of rapid field drop thresholds that have to be triggered before a call is handed over to a chained adjacent cell.

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ChainedAdjacentCell (CHAIN)(ADJC)(Yes/No) only those adjacent cells which are defined as chained adjacent cells may be selected as target cells. The radio link properties of the chained adjacent cells satisfy equation 1.

The field strength of Turn-Around-Corner MS may drop by 30 dB due to the loss of line- of-sight. The rapid field drop in chained cells directs a MS from horizontal cell to the vertical one, but it may also cause ping-pong effect to the in-house slow-moving traffic, if the Rapid field drop threshold is set too high.

The DDE (Deep Drop Edge) algorithm is used to detect deep drops in the serving cell signal strength (both uplink and downlink) and uses the following parameters

ddeWindow (ERMW)(HOC)(1..32 SACCH) a window of raw measurement results from MS that the BSC uses to calculate the dde_level value.

ddeThresholdsLev (ERT)(HOC)(0..63dB) The calculated dde_level value above, is then compared to the deep dropping edge threshold (ERT), which is the size of the signal drop that will trigger a handover.

If dde_level value > threshold level n/p times then MS is considered to be as a Turn-Around-Corner MS.

Enhanced Rapid Field Drop (ERFD) is used to detect deep dropping edge in transmit signal strength (on uplink, downlink or both) due to turn around corner MS. It uses shorter measurement windows and smaller number of zeros compared to DDE. The relevant parameters are:

erfdEnabled (ERFD)(HOC): enables ERFD.

modifiedAveWinNcell (ERAW)(HOC): averaging windows used in ERFD.

modifiedNOZ (ERZ)(HOC) number of zero results

erfdOver (ERD)(HOC) timer associated with the new averaging parameters.

The new averaging windows are used after DDE is detected in the serving cell.

MSC Initiated Traffic Reason Handovers are performed by the MSC to share the loadbetween cells. MSC requests BSC to perform a handover. It is possible to hand over to ‘worse’ cells with better or equal radio link conditions. Traffic reason handover from serving cell to the adjacent cell is disabled when trhoTargetLevel(TRHO)(ADJC) is 'not used'.

Target cell selection involves satisfying the following conditions and then ranking target cells according to radio link properties (priority levels are not used)1. AV_RXLEV_NCELL(n) > RxLevMinCell(n) + Max (0, MsTxPwrMax(n)-MsTxPwrMax)

AV_RXLEV_NCELL(n) > TrHoTargetLevel(n) + Max (0, MsTxPwrMax(n)-MsTxPwrMax)

where:

rxLevMinCell (SL)(ADJC) is the minimum field strength level in the adjacent cell.

trhoTargetLevel (TRHO)(ADJC)(-109..–47, N) is the target level for traffic reason handover.

BSC initiated traffic reason handover concept is used by operators to relieve the load in congested cells and distribute load over the network. It involves dynamically


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changing the power budget margins and thus directing MSs in the cell border to hand over to less loaded adjacent cells. It is based on the traffic load of the serving cell as follows:1. AV_RXLEV_NCELL (n) > TRHO_TARGET_LEVEL (n) + Max (0, (MS_TXPWR_MAX_CELL (n) - P))

2. PBGT (n) > AmhTrhoPbgtMargin and PBGT (n) < HOMarginPBGT

If the PBGT value is between amhTrhoPbgtMargin (ATPM)(HOC) and hoMarginPBGT (PMRG)(ADJC), the handover is triggered due to the BSC initiated TRHO handover. If the PBGT value exceeds the hoMarginPBGT (PMRG)(ADJC), the handover is triggered due to PBGT handover. The TRHO candidate must have less than amhMaxLoadOfTgtCell (AML)(BSC) reserved traffic channels in order to become a target cell.

During TRHO, traffic directed to weaker cells must be kept in the new cell for a period of time to prevent power budget handovers to the original cell. This guard time is set using the parameter amh trhoGuardTime (TGT)(BSC).

Advanced Multilayer Handling (AMH) concept has three features related to regulating the network load.

(a) BSC traffic reason handover used to regulate congestion in same-layer situation.

(b) Used to redistribute traffic to the appropriate layer or frequency band according to load, especially for networks with more than one layer (GSM 800/900/1800/1900 or/and IUO).

(c) Providing operators with the tools necessary to use only the overlay network during low traffic periods, thus avoiding additional handovers between two layers.

AMH can be used to prevent the use of IUO during very light traffic and thus keep the mobiles only in the overlay network. If the traffic load of the serving cell goes under amhLowerLoadThreshold (ALT)(BSC), the IUO handover and the Direct Access to super-reuse TRX are not allowed. The functionality is controlled by parameter amhTrafficControlIUO (ATCI)(HOC).

If the traffic load of the serving cell goes under amhLowerLoadThreshold (ALT) (BSC), the FMMS, MS Speed Detection and Umbrella handovers (adjCellLayer (ACL)(ADJC) enabled) are not allowed for lower layer cells.

The Direct Access to Desired Layer/Band (DADL/B) OPTIONAL feature is to:direct traffic in the call setup phase, from the SDCCH of a macro cell / GSM900 cell, to the TCH of a micro cell / GSM1800 cell, and vice versa.

DADL/B is applied if:(a) DADL/B is enabled in the BSC, and(b) load of accessed cell is higher than btsLoadThreshold (BLT)(BTS) defined for the accessed cell, and(c) there must also be adjacent cells defined as DADL/B handover target cells with the dadlbTargetCell (DADL)(ADJC)(n)(Y/N). The adjacent cells are verified according to the MS capabilities (single band, dual band, and tri band) read from the classmark.

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17 Review QuestionsQ1. If the following handover threshold comparisons are present simultaneously, which one is used by the BSC?

a. Turn-around-corner MS

b. Rapid field drop

c. Downlink quality

d. Uplink level

e. Downlink level

f. MS-BS distance (maximum or minimum)

Q2. Which parameter defines the minimum interval between handovers for the same MS?

a. btsLoadThreshold (BLT)(BTS)(0..100 %)

b. hoLevPriority (PRI)(ADJC)(0..7)

c. minIntBetweenHoReq (MIH)(HOC)(0..30s)

d. minIntBetweenUnsuccHoAttempt (MIU)(HOC)(0..30s)

e. hoLevPriority (PRI)(ADJC)(0..7)

f. All of the above

Q3. Which of the following parameters are used to define the loads of candidate cells?

a. btsLoadThreshold (BLT)(BTS)(0..100 %)

b. hoLevPriority (PRI)(ADJC)(0..7)

c. hoLoadFactor (OF)(ADJC)(0..7)

d. All of the above

Q4. Which of the following is true about power budget handovers?

a. They ensure an MS is handed over to a cell with minimum path loss.

b. Enabled using enablePwrBudgetHandover (EPB)(HOC)(Yes/No)

c. Handover period is set using hoPeriodPBGT (HPP)(HOC)

d. The algorithm for target cell evaluation is based on equation 1 and 2.

e. None of the above

Q5. Which of the following parameter is not used by Umbrella Handover?

a. enableUmbrellaHandover (EUM)(HOC)(Yes/No)

b. hoPeriodUmbrella (HPU)(HOC)(0..63 SACCH)

c. AV_RXLEV_NCELL(n)

d. hoLevelUmbrella(n)


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e. hoPeriodPBGT (HPP)(HOC)

Q6. Which MS speed indication parameter has to be exceeded in averaged measurements for the call to be handed over to a suitable upper layer macrocell?

a. fastMovingThreshold (FMT)(ADJC)(0..255)

b. rxLevMinCell (SL)(ADJC)

c. msSpeedThresholdPx (STP)(HOC)(1..32)

d. upperSpeedLimit (USL)(HOC)(0..254)

e. lowerSpeedLimit (LSL)(HOC)(0..254)

Q7. Which threshold parameters are used for handover due to uplink signal level?

a. hoThresholdsQualUL (QUR, QUP, QUN)(ZEHO)(0..7, 1..32, 1..32)(4, 4, 6)

b. hoThresholdsLevDL (LDR, LDP, LDN)(ZEHO)(-110..-47, 1..32)(-95, 1, 1)

c. hoThresholdsQualDL (QDR, QDP, QDN)(ZEHO)(0..7, 1..32, 1..32)(4, 4, 6)

d. hoThresholdsLevUL (LUR, LUP, LUN)(ZEHO)(-110..-47, 1..32)(-95, 1, 1)

e. All of the above

Q8. Imperative handovers are caused by which of the following reasons?

a. MS - BTS distance (DISB)(BSC)

b. O&M order to empty the cell (Forced Handover)

c. Directed Retry (DR)(BTS) (OPTIONAL feature)

d. Intelligent Directed Retry (DRM)(BTS) (OPTIONAL feature)

e. Chained cells in rapid field drop (CNT)(HOC) (OPTIONAL feature)

f. All of the above

Q9. Which of the following is part of Advanced Multilayer Handling (AMH)?

a) BSC traffic reason handover to regulate congestion in a same-layer situation

b) Re-distribution of traffic to the appropriate layer or frequency band according to load, especially for networks with more than one layer (GSM 800/900/1800/1900/IUO).

c) Providing operators with the tools necessary to use only the overlay network during low traffic periods, thus avoiding additional handovers between two layers.

d) All of the above

8-BSSPAR- Handover Control and Adjacencies.pdf

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