bsspar chapter 06 handover control and adjacencies mo[1]

© Nokia Networks Oy

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HANDOVER CONTROL AND ADJACENCIES

Handover Control and Adjacencies

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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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Copyright © Nokia Networks Oy 2004. All rights reserved.



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Contents

6 Handover Control and Adjacencies ......................................... 4 6.1 Module Objectives ....................................................................... 4 6.2 Introduction .................................................................................. 4 6.2.1 Handover Types .......................................................................... 5 6.2.2 Signalling ..................................................................................... 6 6.2.3 Multi Layer Networks ................................................................... 9 6.3 General Strategy ........................................................................11 6.3.1 Handover Triggering ..................................................................11 6.3.2 Target Cell Selection .................................................................14 6.3.3 Handover Execution ..................................................................19 6.4 Radio Resource Handover ........................................................21 6.4.1 Handover due to Interference ....................................................21 6.4.2 Handover due to Signal Quality .................................................24 6.4.3 Handover due to Signal Level ....................................................25 6.4.4 Pure Power Budget Handover ...................................................27 6.4.5 Pure Umbrella Handover ...........................................................28 6.4.6 Combined Power Budget and Umbrella Handover ....................32 6.5 Imperative Handover .................................................................38 6.5.1 Operation and Maintenance Handover ......................................38 6.5.2 Directed Retry ............................................................................38 6.5.3 Handover due to Distance MS - BTS .........................................41 6.5.4 Handover due to Rapid Field Drop ............................................42 6.6 Traffic Reason Handover ...........................................................47 6.6.1 MSC Initiated Traffic Reason Handover ....................................48 6.6.2 Advanced Multi Layer Handling .................................................48



6 Handover Control and Adjacencies

6.1 Module Objectives

At the end of the module the participant will be able to:

• Explain the motivation for handover

• Indicate the different types of handover and multi layer network

• Describe the principle steps to be executed for handover (averaging, triggering, adjacent cell ranking, target cell selection, timer)

• Discuss the handover algorithms applied to interference, RX quality and RX level handover

• Discuss the handover algorithms applied to power budget and umbrella handover, and the impact of the MS speed

• Describe the different kinds of imperative handovers, especially the role of rapid field drop

• Explain the use of handover for traffic control

6.2 Introduction

Handover is one of the key features of a cellular network. It means basically, that a call is moved from one cell to another for one of the following reasons:

• The MS moves from the coverage area of one cell to that of another.

• The MS moves into an area of poor quality. The call therefore is moved to an adjacent cell offering a better quality.

• The call is moved to another cell to optimise the traffic distribution within the network.

The decision for a handover is made by the BSC, on the basis of the uplink and downlink measurements taken by the MS and BTS, respectively. If handovers are needed the BSC looks for a suitable target cell and tries to move the ongoing call to it.

The handover can be classified not only according the reason, but also according the type of border the MS is crossing, when moving from one cell to another. Another important classification scheme is based on the network layers invoked in the handover process.



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6.2.1 Handover Types

One can distinguish between the following handover types, which are summarized by Fig. 6-1.

6.2.1.1 Intra Cell and Inter Cell Handover

In case of an intra cell handover the MS does not leave the cell at all, but is allocated to another resource in the same cell. With an inter cell handover the MS is moved from one cell to another.

6.2.1.2 Intra and Inter BSC Handover

An inter cell handover can be an intra or an inter BSC handover. In the first case the target cell and the source cell are controlled by the same BSC. In the latter source and target cells are located in different BSC areas. The inter BSC handover has to be handled by the MSC, but nevertheless the decision about it is made by the BSC controlling the source cell.

6.2.1.3 Intra and Inter MSC Handover

In case of an inter BSC handover, the target cell might be located in a different MSC area than the source cell. For such an inter MSC handover, the current MSC / VLR must contact the target MSC / VLR and then transfer the call to it. In case of an intra MSC handover source and target cells belong to the same MSC area.

6.2.1.4 Intra and Inter PLMN Handover

Finally the MS might be moved even from one PLMN to another one. This is called inter PLMN handover or roaming.

For handover within the same PLMN (intra PLMN handover) cells must be identified on the basis of the cell identity CI and location area code LAC only. For handover to another PLMN, however, the global cell identity GCI must be used, which includes additionally the mobile network code MNC and the mobile country code MCC. The inter PLMN handover always is also an inter MSC handover.



© NO K IA BS S P AR / 10.05.2004

Handover T ypes

Intra ce llO nly other carrie r / times lot

Inte r ce llIntra BS C

Inte r BS CIntra MS C

HandoverControl &

Adjacencies

Inte r MS C

Inte r P LMNG lobal C e ll ID requiredC I + L AC + MNC + MC C

G ermany C zech R epub lic

Intra P LMNS imple C e ll ID requiredC I + L AC

Fig. 6-1: Handover types

6.2.2 Signalling

Here only an overview about the messages to be exchanged during a handover are given. One must distinguish basically between synchronized and not synchronized handover.

6.2.2.1 Synchronized Handover

If source and target cells are controlled by the same BCF, these are synchronized to each other. Thus the MS does not need timing advance information from the network. This kind of handover is restricted to inter cell handover from one sector to another one of the same site. The synchronised handover can be switched on and of with the parameter sync(ADJC) (Y/N)(N). The messages exchanged during such a handover are given by Fig. 6-2.



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© NOKIA BSSPAR / 10.05.2004

Synchronized Handover Signalling

New Channel, New Cell

ACTIVE CALL

HANDO CMD

HANDO ACC

HANDO ACC

HANDO ACC

HANDO ACC

HANDO COM

ACTIVE CALL

MS NETWORK

Old Channel, Old Cell

HandoverControl &

Adjacencies

Source and target cells controlled by same BCFNo timing advanced information required for MS

Inter cell handover from one sector to another of same BTS

Fig. 6-2: Synchronized Handover Signalling

6.2.2.2 Non Synchronized Handover

If original and target cells are controlled by different BCFs, the MS needs timing advance information from the network. This is transferred by a so-called physical info message. To ensure a successful transmission, it can be repeated a certain number of times. This is controlled by the parameter maxNumberOfRepetition (NY1)(SEG)(5..35)(5). Furthermore, there is a restricted time between the first handover access and the last physical info message. This timer always is set to 320 ms, it is fixed in GSM specifications. Usually any handover is a non synchronized handover. The messages exchanged during such a handover are given by Fig. 6-3.




New Channel, New Cell

MS NETWORK

ACTIVE CALL

HANDO CMD

HANDO ACC…….

HANDO ACC

PHYS INFO

PHYS INFO

HANDO COM

ACTIVE CALL

Old Channel, Old Cell

maxNumberOfRepetition 5..35

HandoverControl &

AdjacenciesNon Synchronized Handover Signalling

Source and target cells controlled by different BCFTiming advanced information required for MS

Inter cell handover from one BTS to another

Timer320 ms

Fig. 6-3: Non synchronized handover signalling

6.2.2.3 Handover Failure

If the signalling timer mentioned above expires or the radio link within the target cell cannot be established, the MS returns to the old cell. The corresponding messages are given by Fig. 6-4.



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© NO K IA BS S P AR / 10.05.2004

Handover Failure S ignalling

New C hannel, New C ell

HANDOVER CMD

AC T IVE C AL L

MS NETWORK

HANDOVER FAIL

AC T IVE C AL L

O ld C hannel, O ld C ell

O ld C hannel, O ld C ell

T imer expiry orR adio link failure

HandoverControl &

Adjacencies

Fig. 6-4: Handover failure signalling

6.2.3 Multi Layer Networks

A very important concept to optimise both access and traffic distribution is the multi layer network. Such a network has at least two layers, a coverage layer and a capacity layer. The coverage layer gives the user access to the network, but does not offer much capacity. After successful access therefore a handover to the capacity layer takes place, if possible. There are several types of multi layer networks, which are summarized by Fig. 6-5.

6.2.3.1 Macro and Micro Cells

This is the most common way to realise a multi layer network. When the user request access to the network, first he gets a TCH of a macro cell. Afterwards a handover to a micro cell is carried out, as the micro cell layer allows to handle a much higher user density as the macro cell layer. Only fast moving users are kept in the macro cell layer to avoid too much load introduced by handovers between micro cells. This topic will be discussed in detail in the sections Pure Umbrella Handover / Combined Power Budget & Umbrella Handover of this chapter.



6.2.3.2 Intelligent Underlay Overlay

This Nokia specific feature is based on the use of two frequency sets. The cells working with the normal frequency set give a complete coverage within a PLMN, these define the access layer. A second set of frequencies, the so-called super reuse frequencies, is restricted to users close to a BTS. This frequencies are reused more often than the normal ones, leading to a higher capacity of the network. This technique will be discussed in the Intelligent Underlay Overlay chapter.

6.2.3.3 Intelligent Coverage Enhancement

This feature uses TRXs with high and low power within one BTS. If the RX level exceeds a certain threshold, a handover from the high power to a low power TRX is executed. If it drops below a certain value, a handover back to the high power TRX is performed. This technique will be discussed in the Intelligent Coverage Enhancement chapter.

6.2.3.4 Dual Band

This again is very common. The network consists of both GSM 900 and GSM 1800 cells. Due to the better propagation characteristics, the coverage layer is defined by the GSM 900 cells. After successful access a handover to a GSM 1800 cell takes place, if possible. This kind of network is discussed in the Dual Band Features chapter.

© NO K IA BS S P AR / 10.05.2004

Multi L ayer Network T ypesHandoverControl &

Adjacencies

C overage L ayer: G ives acces s to the networkC apac ity L ayer: O ptimis es traffic dis tribution

Macro ce lls

Micro ce lls Inte lligent C overage E nhancementHigh power + low power T R x

Inte lligent Underlay OverlayD iffe rent frequencie s for high / low power T R x

Normalfrequency

S upe r reus efrequency

G SM 900 ce llG S M 1800 ce ll

Fig. 6-5: Multi layer network



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6.3 General Strategy

One general technique to execute a handover follows similar principles as the power control. The measurement taken by the MS and BTS are averaged by the BSC and compared with thresholds which of course are different from the power control thresholds. If a threshold is exceeded, the BSC looks for a suitable target cell. If such a cell has been found, the BSC tries to move the MS to it. There are, however, other reasons for handover than pure threshold comparison.

6.3.1 Handover Triggering

One way to decide, whether a handover is required, is that the BSC evaluates permanently the measurements taken by the MS and BTS, as described by the chapter Measurement Processing. For the serving cell the RX interference, level and quality both for the uplink and downlink are considered, together with the speed of the MS and the timing advance (which gives the distance between MS and BTS). The RX level of the adjacent cells is needed for the handover process as well, either to select a suitable target cell or e.g. to trigger a power budget handover.

The measurements averaged by the BSC are compared with threshold values. This comparison is done by the same way as described in the Power Control chapter. A certain number Nx of averaged values is considered. If at least Px values exceed a threshold, the BSC starts the handover process by looking for a suitable target cell. The threshold, Nx and Px parameters will be discussed together with the different kinds of handover mechanisms. The principle of triggering is shown by Fig. 6-6.



© NO K IA BS S P AR / 10.05.2004

Handover S trategyT riggering

threshold

Actual average samples

Nx samplesLess than Px samples exceed thresholdNo handover triggered

Nx samplesPx samples exceed thresholdhandover triggered

S ignal interference thres holdshoT hres holds Interfe renceD L /UL -110. .-47 dBm

S ignal quality thres holdshoT hres holdsQ ualD L /UL 0..7

S ignal level thres holdshoThresholdLevelDL/UL -110..-47 dBmhoThresholdRapidLevelUL -110..-47 dBm

MS speed thresholdsupper/lowerSpeedLimit 0..255 (unit = 2km/h)

Number of average samplesNx 1..32Px 1..32

Target cell selection

HandoverControl &

Adjacencies

Fig. 6-6: Handover strategy (triggering)

In dependence on the kind of threshold exceeded, several handover reasons have been defined, which have different priorities. If several reasons are present simultaneously, the handover of highest priority is carried out.

Handovers triggered by comparison of averaged message reports with threshold values are due to RX interference, RX quality, RX level, MS speed, MS distance.

For power budget and umbrella handovers there is no comparison of measured values with thresholds, but only a periodic check of the RX levels of the adjacent cells. Such a handover is triggered, if an adjacent cell offers a better power budget than the serving cell, even if no threshold is exceeded yet.

The priority of all these handovers is as follows:

1) Interference (uplink or downlink)

2) Uplink quality

3) Downlink quality

4) Uplink level

5) Downlink level

6) Distance between MS and BTS



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7) Turn around corner MS (special case of rapid field drop)

8) Any other rapid field drop

9) Fast / slow moving MS (special case of umbrella handover)

10) Any other umbrella or power budget handover

If both handover and power control are triggered simultaneously, handover has higher priority.

There are further kinds of handovers, which are not triggered by a comparison of MS / BTS measurements with thresholds. Such ones are due to operation and maintenance, (intelligent) directed retry, intelligent underlay / overlay, traffic and direct access to desired layer / band. The different kinds of handover are summarized by Fig. 6-7, their priorities by Fig. 6-8.

© NO K IA BS S P AR / 10.05.2004

Handover S trategy(Handover R eas ons )

T im ing Advance

Adjacent C e lls

D ownlink Q uality

Uplink Q uality AV _R XQUAL_UL _HO

AV_R XQUAL _DL _HO

Downlink L eve l

Uplink L eve l AV_R XL E V _UL _HO

AV_R XL E V _D L _HO

AV_R ANG E _HO

AV_R XL E V_NC E L L (n)

INT ER FER ENC E &QUAL IT Y

INT ER FER ENC E &QUAL IT Y

LEVE LLEVE L

D IS T ANC ED IS T ANC E

P E R IO D ICR X L E V E LC HE C K

UMBR E LLAUMBR E LLA

POWER B UDGE TPOWER B UDGE T

IMP E R AT IV E HOCHANNEL ADMINIS T R AT IONCHANNEL ADMINIS T R AT ION

D IR EC T ED R E TRYD IR EC T ED R E TRY

T HR E S HO LD C OMP AR IS O N

R AP ID F IE LD DROPR AP ID F IE LD DROP

MS SP EEDMS SP EED

MS S peed AV_MS _S P E E D

O thers caus es (NO T R E S HO LD C OMP AR IS O N)

- Inte lligent Underlay/O verlay (IUO )

- T raffic R eason Handover (T R HO )

- D irect Acces s to D es ired L aye r/Band (DAD L /B )

HandoverControl &

Adjacencies

Fig. 6-7: Handover strategy (handover reasons)



© NO K IA BS S P AR / 10.05.2004

Handover S trategy(P riorities )

More than one handove r crite rion fulfilled-> proces s of highe r priority pe rformed

Handove r and power control crite ria fulfilled-> handove r pe rformed

1) Interference (uplink or downlink)2) Uplink quality3) Downlink quality4) Uplink level5) Downlink level6) Distance between MS and BTS7) Turn around corner MS (special case of rapid field drop)8) Any other rapid field drop9) Fast / slow moving MS (special case of umbrella handover)10) Any other umbrella or power budget handover

e.g downlink quality & s low moving MS criterion

-> HO due to downlink quality

HandoverControl &

Adjacencies

Fig. 6-8: Handover strategy (priorities)

6.3.2 Target Cell Selection

If handover shall be carried out, a suitable target cell must be found. The standard selection process uses as criteria the traffic loads, priorities and RX levels of the adjacent cells. This mechanism can be improved, taking into account also the potential C/I ratios of the possible destination cells.

6.3.2.1 Standard Selection

First of all, the traffic load of all adjacent cells is verified. If it exceeds the threshold defined by the parameter (the unit is %) btsLoadThreshold (BLT)(SEG)(0..100)(70), the cell is overloaded.

Next the cell priorities are considered. These are defined by the parameter hoPriorityLevel (PRI)(ADJC)(0..7)(3). If a cell is overloaded, the priority is reduced by a certain value which is defined by the parameter hoLoadFactor (OF)(ADJC)(0..7)(1).

Now the cells are ranked according their corrected priority. If two cells have the same priority, they are ranked according their RX level. An example is given by Fig. 6-9.



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Information about the cell load is available only for cells belonging to the same BSC as the serving cell. In case of an inter BSC handover therefore uncorrected priorities can be used only. In both cases, up to 16 cells can be considered. In case of an inter BSC handover, the maximum cell number must be set by the parameter genHandoverRequestMessage (NPC)(BSC)(1..16)(3).

The ranking is not carried out in case of imperative and traffic reason handover.

© NO K IA BS S P AR / 10.05.2004

Handover S trategy(S tandard T arget C ell R anking)

C ell a b c

Overloaded? N/Y N N

P riority 4 3 3

hoLoadFactor 2 1 1

C orrected prio rity 4/2 3 3

R x level -75 -80 -83

No ce ll ove rloaded -> prioritie s 4,3,3 -> ce ll lis t a,b,c

A overloaded -> prioritie s 2,3,3 -> ce ll lis t b,c,a

C ell a b c

Overloaded? N/Y N N

P riority 4 3 3

hoLoadFactor 2 1 1

C orrected priority 4/2 3 3

R x level -75 -80 -83

No ce ll ove rloaded -> prioritie s 4,3,3 -> ce ll lis t a ,b,c

A overloaded -> priorities 2,3,3 -> ce ll lis t b,c,a

Ad jacent C ell L oad T hres hold C an be checked only for ce lls be longing to the s ame BS C as the s e rving onebtsLoadThreshold 0..100 %

Adjacent Cell PriorityhoPriorityLevel 0..7

Overloaded Cell -> Reduction of PriorityhoLoadFactor 0..7

Ranking (not for imperative and traffic reason handover)1) Corrected priority2) Cells with same priority -> RX level

HandoverControl &

Adjacencies

Fig. 6-9: Handover strategy (standard target cell ranking)

After the ranking of the adjacent cells, further conditions must be checked. First the average signal level AV_RXLEV_NCELL is considered, which must follow the condition

(1a) AV_RXLEV_NCELL (n) > rxLevMinCell (n) + Max (0,msTxPwrMax (n) - P)

The minimum signal level to be allowed for the potential target cell is set by the parameter rxLevMinCell (SL)(ADJC)(-110..-47)(-100). The setting of the maximum allowed MS output power msTxPwrMax has been discussed already in chapter Power Control (section Introduction). P depends on the power class of the MS. n indicates the nth adjacent cell in the ranking list. If no index is given, the variable is related to the serving cell.

The condition has to be fulfilled by any kind of handover, except the umbrella handover. For the latter there is the simplified condition



(1b) AV_RXLEV_NCELL (n) > hoLevelUmbrella (n)

The parameter hoLevelUmbrella (AUCL)(ADJC)(-110..-47)(-47) defines the minimum signal level to be allowed for an umbrella adjacent cell.

The decision process based on the RX level is summarized by Fig. 6-10.

© NO K IA BS S P AR / 10.05.2004

Handover S trategy(Dec is ion due to R X level)

AV_R XL E V_NC E L L (n) > rxLevMinCell(n) + Max (0, msTxPwrMax(n) – P )P depends on MS power clas s

1a

Any kind of handover (except umbrella)

AV_R XL E V_NC E L L (n) > hoLevelUmbrella(n)1b

Umbrella handover

HandoverControl &

Adjacencies

Minimum allowed s ignal level for target cell

rxLevMinCell -110..-47 dBm any handover (except umbrella)hoLevelUmbrella -110..-47 dBm umbrella handover

Fig. 6-10: Handover Strategy (Decision due to RX level)

For imperative handovers the use of condition (1) is sufficient. For other kinds of handovers, however, also the power budget PBGT has to be taken into account. In general, this leads to the conditions

(2a) PBGT > hoMarginPGBT(n)

PBGT = (msTxPwrMax - msTxPwrMax (n)) -

(AV_RXLEV_DL_HO - AV_RXLEV_NCELL (n)) -

(btsTxPwrMax - BTS_RXPWR)

The margin defined by the parameter (the unit is dB) hoMarginPGBT (PMRG)(ADJC)(-24..63)(6) prevents repeated handovers between adjacent cells. The setting of the maximum allowed BTS output power btsTxPwrMax has been discussed already in chapter Power Control (section Introduction).

For handover due to RX level or RX quality also the handover margins for the RX level and the RX quality are used. This leads to the modified condition



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(2b) PBGT > hoMarginLevel/Qual(n)

PBGT = (AV_RXLEV_NCELL (n) - AV_RXLEV_DL_HO) -

(btsTxPwrMax - BTS_RXPWR)

The margins defined by the parameters (unit is dB) hoMarginQual (QMRG)(ADJC)(-24..24)(0) and hoMarginLevel (LMRG)(ADJC)(-24..24)(3) prevent again repeated handovers between adjacent cells. In contradiction to the power budget margin, the RX level and RX quality margins must be enabled with the parameter enableHOMarginLevQual (MGRS)(ADJC)(Y/N)(Y).

The decision process based on the RX level is summarized by Fig. 6-11.

© NO K IA BS S P AR / 10.05.2004

P BG T > hoMarginLev/Qual(n)P BG T = (AV _R XL E V_NC E L L (n) - AV _R XL E V_D L_HO ) -

(btsTxPwrMax - B T S _T XPWR )

2b

P BG T > hoMarginPBGT(n)

P BG T = (msTxPwrMax - msTxPwrMax(n)) –

(AV _R XL E V _DL _HO - AV_R XL E V _NC E L L (n)) -


2a

G eneral cas e

Handover S trategy(Dec is ion due to P ower B udget)

HandoverControl &

Adjacencies

Handover due to R X level, R X quality, d is tance and umbrella handover

P ower budget marg in to prevent repeated handovers between ad jacent cellshoMarginPBGT -24..63 dB general case

RX level and RX quality margins to prevent repeated handovers between adjacent cellshoMarginQual -24..24 dB handover due to signal qualityhoMarginLevel -24..24 dB handover due to signal levelEnableHOMarginLevQual Y/N required to enable margins

Fig. 6-11: Handover strategy (decision due to power budget)

The check for RX level and power budget starts with the cell on top of the ranking list. If this cell does not fulfil the requirements, the process goes to the next cell and so on.

6.3.2.2 Selection Considering C/I

The target cell selection process can be improved by correcting the cell priorities additionally according their potential C/I rations. For this purpose the parameter ciEstMethod()(HOC)(AVE,MAX,NONE)(NONE) must be activated.

To determine the C/I of an adjacent cell, up to 5 reference cell can be considered for it (the reference cell must be adjacent again to the



adjacent cell). These are indicated by the location area code and the cell identification. If ciEstMethod is set to AVE, the C/I of the adjacent cell will be estimated by averaging over all reference cells. Using the setting MAX, the worst individual C/I will be taken. The estimation of C/I can be affected by the following parameters. ciEstWeight (W1-W5)(ADJC)(0..10)(1) gives each reference cell an individual weight for the average C/I. If it is set to 0, the reference cell is not used. levelAdjustment (L1-L5)(ADJC)(-63..63)(0) allows to define the signal difference (in dB) between adjacent and interfering cell.

The estimated C/I now is compared with six user defined interference bands. These are defined by the parameter lowerCILimit (L1-L6)(HOC)(-128..127)(30/25/20/17/13/9). In dependence on the interference band, to which the cell belongs, its priority is corrected according the parameter priorityAdjStep (P1-P7)(HOC)(-8..7)(3/1/0/-1/-2/-5/-8). If it is set to -8, the cell is removed from the target candidate list. An overview and example is given by Fig. 6-12 and Fig. 6-13.

Like the overload correction, the C/I correction is not used for imperative and traffic reason handovers.

The decision process based on the RX level and power budget is the same as for the standard target selection.

© NO K IA BS S P AR / 10.05.2004

Handover S trategy(T arget C ell R anking with C /I)

E s timation of C /IF or each adjacent ce ll: re fe rence cells (indicated by L AC and ce ll identification)ciEstMethod AVE -> C/I averaged over all reference cells

MAX -> C/I taken from worst reference cell

ciEstWeight 0 -> reference cell not used1..10 -> individual weight for average C/I

levelAdjustment -63..63 dB -> signal difference adjacent –reference cell

Comparison with user defined interference bandlowerCLimit -128..127 dB

Correction of prioritypriorityAdjStep -8 -> cell removed from target candidate list

-7..7 -> priority decreased/increased in dependence on C/I

HandoverControl &

Adjacencies

Fig. 6-12: Handover strategy (target cell ranking with C/I)



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© NO K IA BS S P AR / 10.05.2004

Interference band lowerC IL imit priorityAd jS tep

1 30 3

2 25 1

3 20 0

4 17 -1

5 13 -2

6 9 -5

C ell a b c

Overloaded? Y N N

P riority 4 3 3

hoLoadFactor 2 1 1

C orrected prio rity 2 3 3

C /I 28 18 22

C /I band 1 4 3

priorityAd jS tep 3 -1 0

C orrected prio rity 5 2 3

R x level -75 -80 -83

W ithout C /I correction -> prioritie s 2,3,3 -> ce ll lis t b,c,a

W ith C /I correction -> priorities 5,2,3 -> ce ll lis t a ,c,b

Interference band lowerC IL imit priorityAd jS tep

1 30 3

2 25 1

3 20 0

4 17 -1

5 13 -2

6 9 -5

C ell a b c

Overloaded? Y N N

P riority 4 3 3

hoLoadFactor 2 1 1

C orrected priority 2 3 3

C /I 28 18 22

C /I band 1 4 3

priorityAd jS tep 3 -1 0

C orrected priority 5 2 3

R x level -75 -80 -83

W ithout C /I correction -> priorities 2,3,3 -> ce ll lis t b,c,a

W ith C /I correction -> prioritie s 5,2,3 -> ce ll lis t a ,c,b

HandoverControl &

Adjacencies

Handover S trategy(Target C ell R anking with C /I)

Fig. 6-13: Handover strategy (target cell ranking with C/I)

6.3.3 Handover Execution

After a target cell has been found, the BSC tries to move the MS to it. To avoid repetitive handovers or handover attempts in case of a failure, certain time limits can be set by the parameters minIntBetweenHOReq (MIH)(HOC)(0..31)(5) and minIntBetweenUnsuccHOAttempt (MIU)(HOC)(0..31)(3) (the unit is s). The first parameter sets the minimum time between two handovers related to the same connection. The second one indicates the time the MS has to wait after an unsuccessful handover attempt, before it can try a handover again. Further important time scales are controlled by the parameters hoPeriodPBGT (HPP)(HOC)(0..63)(6) and hoPeriodUmbrella (HPU)(HOC)(0..63)(6) (the unit is SACCH periods). These indicate the time interval between consecutive estimations of the RX levels of the adjacent cells for power budget and umbrella handover. From all this time limits and scales several guard timers are derived preventing back handovers. The averaging of the measurement reports and the threshold comparisons, however, continues, even if handovers are not possible. The timers are summarized by Fig. 6-14 and Fig. 6-15.



© NO K IA BS S P AR / 10.05.2004

HandoverControl &

AdjacenciesB as ic Handover T imers

Minimum time between cons ecutive handovers related to s ame connectionminIntBetweenHOReq 0..31 s

Minimum time between cons ecutive handover attempts after failureminIntBetweenUnsuccHOAttempt 0..31s

Time interval between consecutive RX level estimations of the adjacent cells for power budget and umbrella handoverhoPeriodPBGT 0..63 SACCH periodshoPeriodUmbrella 0..63 SACCH periods

Basic timers do not stop averaging and threshold comparison

Fig. 6-14: Basic handover timers

© NO K IA BS S P AR / 10.05.2004

HandoverControl &

AdjacenciesGuard Handover T imers

F irs t handover B ack handover G uard T ime

Inte rfe rence / R X quality powe r budge t 2 * hoP eriodP BG TInte rfe rence / R X quality umbre lla 2 * hoP eriodUmbre lla

Inte rfe rence s low MS 255 s

D is tance any 20 s + minIntBetweenHO R eq

T raffic reason power budge t / umbre lla 20 s + minIntBetweenHO R eq

Intra ce ll Intra ce ll 4 * m inIntBetweenUnsuccHO Attempt

Guard timers do not stop averaging and threshold comparison

Fig. 6-15: Guard handover timers



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6.4 Radio Resource Handover

To this group belong handovers due to interference, RX level, RX quality and also power budget and umbrella handovers. These are triggered on the basis of different thresholds which will be discussed in the following sections.

6.4.1 Handover due to Interference

In this case the following thresholds are important:

Threshold Nx Px

hoThresholdsInterferenceDL (IDR)(HOC)(-110..-47)(-85)

Nx (IDN)(HOC)(1..32)(1)

Px (IDP)(HOC)(1..32)(1)

hoThresholdsInterferenceUL (IUR)(HOC)(-110..-47)(-85)

Nx (IUN)(HOC)(1..32)(1)

Px (IUP)(HOC)(1..32)(1)

hoThresholdsQualDL (QDR)(HOC)(0..7)(4)

Nx (QDN)(HOC)(1..32)(6)

Px (QDP)(HOC)(1..32)(4)

hoThresholdsQualUL (QUR)(HOC)(0..7)(4)

Nx (QUN)(HOC)(1..32)(6)

Px (QUP)(HOC)(1..32)(4)

One can decide, whether an intra or inter cell handover shall be preferred. The first is recommended for a network without frequency hopping, while the second for a network with frequency hopping. The preferred handover type is set by the parameter hoPreferenceOrderInterDL/UL (HDL/HUL) (BSC) (INTER,INTRA) (INTER). The intra cell handover must be enabled using the parameter enableIntraHOInterDL/UL (EIH/EIC)(HOC)(Y/N)(Y). The parameters specific for interference handover are summarized by Fig. 6-16.



© NO K IA BS S P AR / 10.05.2004

HandoverControl &

Adjacencies

Interference Handover(P arameters )

T hres ho lds

hoThresholdsInterferenceDL/UL -110..-47 dBmhoThresholdsQualDL/UL 0..7

Intra / inter cell handover

hoPreferenceOrderInterDL/UL INTRA for network without frequency hoppingINTER for network with frequency hopping

enableIntraHOInterDL/UL Y/N required to enable intra cell handover

Fig. 6-16: Interference handover (parameters)

The interference handover is performed as follows. If both the interference and quality threshold is exceeded (too low quality in spite of sufficient RX level), the handover is triggered. The adjacent cells are ranked as already discussed, and their RX levels are checked according condition (1a). The power budgets are checked according condition (2a) or (2b), in dependence on the setting of enableHOMarginLevQual. If no suitable target cell can be found, an intra cell handover is carried out (if enabled) or an handover failure occurs (if intra cell handover is disabled). The whole process is summarized by Fig. 6-17, an example is given by Fig. 6-18.



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HandoverControl &

Adjacencies

Interference Handover(Execution)

AvRX level above hoThresholdsInterferenceDL/UL

AND

AvRX quality value ≥≥≥≥ hoThresholdsQualDL/UL

Ranking of adjacent cells

enableHOMarginLevQual = Y enableHOMarginLevQual = N

Check of adjacent cells with conditions 1a and 2b

Check of adjacent cells with conditions 1a and 2a

Suitable adjacent cell ?

Yes No

Inter cell handover If enabled Intra cell handover

Otherwise Handover failure

Fig. 6-17: Interference handover (execution) (does not include intra cell handover)

© NO K IA BS S P AR / 10.05.2004

hoThres holdQualDL = 4hoThres hold InterferenceDL = -85 dBmhoP referenceO rderInterfDL = INT E RenableHandoverMarg inL evQual = YhoMarginQual = 0 dBB TS output power = Maximum

hoT hres holdInte rfe renceD L exceeded

5

0

RX quality

HandoverControl &

Adjacencies

Interference Handover(E xample)

T hres hold

hoT hres holdQ ualD L exceededHandover triggeredAdjacent ce lls checked with conditions 1a and 2bAV_R XL E V_NC E L L – AV_R XL E V_D L_HO > hoMarginQ ual fulfilled-> indicated ce ll s uitable

S erving ce ll

Adjacent ce ll

T hreshold RX level-85 dBm

Fig. 6-18: Interference handover (example)



6.4.2 Handover due to Signal Quality

For this kind of handover only the thresholds hoThresholdsQualDL and hoThresholdsQualUL have to be taken into account. If the signal quality becomes to low, the handover is triggered. The adjacent cells are ranked and checked according conditions 1a, 2a and 2b by the same way as for interference handover. Now only inter cell handover is possible, so that the handover fails, if no suitable adjacent cell can be found. The whole process is summarized by Fig. 6-19, an example is given by Fig. 6-20.


HandoverControl &

Adjacencies

RX Quality Handover(Execution)

AvRX quality value ≥ hoThresholdsQualDL/UL






Yes No

Inter cell handover Handover failure

ThresholdshoThresholdsQualDL/UL 0..7

Fig. 6-19: RX quality handover (execution)



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© NO K IA BS S P AR / 10.05.2004

hoThres holdQualDL = 4enableHandoverMarginL evQual = YhoMarginQual = 0 dBB TS output power = Maximum

5

0

RX quality

HandoverControl &

Adjacencies

R X Quality Handover(E xample)

T hres hold

hoT hres holdQ ualD L exceededHandover triggeredAdjacent ce lls checked with conditions 1a and 2bAV_R XL E V_NC E L L – AV_R XL E V_D L_HO > hoMarginQ ual not fulfilled-> indicated ce ll cannot be used as target

S erving ce ll

Adjacent ce ll

RX level

Fig. 6-20: RX quality handover (example)

6.4.3 Handover due to Signal Level

Now the following thresholds have to be considered.

Threshold Nx Px

hoThresholdsLevDL (LDR)(HOC)(-110..-47)(-95)

Nx (LDN)(HOC)(1..32)(1)

Px (LDP)(HOC)(1..32)(1)

hoThresholdsLevUL (LUR)(HOC)(-110..-47)(-95)

Nx (LUN)(HOC)(1..32)(1)

Px (LUP)(HOC)(1..32)(1)

If the signal level becomes too low, the handover is triggered. The remaining steps of the handover process follow exactly the same scheme as for the RX quality handover. The whole process is summarized by Fig. 6-21, an example is given by Fig. 6-22.




HandoverControl &

Adjacencies

RX Level Handover(Execution)

AvRX level ≤ hoThresholdsLevDL/UL






Yes No


ThresholdshoThresholdsLevDL/UL -110..-47 dBm

Fig. 6-21: RX level handover (execution)

© NO K IA BS S P AR / 10.05.2004

hoThres ho ldL evelDL = -95 dBmenableHandoverMarg inL evQual = YhoMarg inL evel = 3 dBB TS output power = Maximum

HandoverControl &

Adjacencies

RX L evel Handover(E xample)

T hreshold

hoT hres holdL eve lD L exceededHandover trigge redAdjacent ce lls checked with conditions 1a and 2bAV_R X L E V_NC E L L – AV_R X L E V_D L_HO > hoMarginL eve l fulfilled-> indicated ce ll s uitable

S erving ce ll

Adjacent ce ll

R X level-95 dBm

6 dB

Fig. 6-22: RX level handover (example)



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6.4.4 Pure Power Budget Handover

This kind of handover is executed, if an adjacent cell offers a better power budget than the serving cell, even if the averaged measurements for the serving cell do not exceed a threshold. This way the path loss always is kept as small as possible.

The power budget handover must be enabled with the parameter enablePwrBudgetHandover (EPB)(HOC)(Y/N)(Y). Than the RX levels of the adjacent cells are checked periodically, according the value of the parameter hoPeriodPBGT (which has been explained already in section 6.3.3 Handover Execution).

The adjacent cells are ranked as for the other radio resource handovers. To decide, whether an adjacent cell can be used as target cell, now generally the conditions (1a) and (2a) are used. If an adjacent cell offers a sufficient RX level and a power budget better than hoMarginPGBT, the handover is carried out. The whole process is summarized by Fig. 6-23, an example is given by Fig. 6-24.

© NO K IA BS S P AR / 10.05.2004

HandoverControl &

Adjacencies

P ower B udget Handover(E xecution)

Estimation of RX levels of adjacent cells



Adjacent cell with sufficient RX level / power budget

Yes No

Inter cell handover No handover

Mus t be enabled withenablePwrBudgetHandover Y/N

S erv ing cell meas urements need not exceed a thres hold

Periodic check of RX levels of adjacent cellshoPeriodPBGT 0..63 SACCH periods

Fig. 6-23: Power budget handover (execution)



© NO K IA BS S P AR / 10.05.2004

P BG T = (msTxPwrMax - msTxPwrMax(n)) –

(AV _R XL E V_DL _HO - AV_R XL E V _NC E L L (n)) -


P BG T = (33dBm - 33dBm) - (-90 - (-80)) - (42dBm - 42dBm) = 10 dB > 6 dB

P BG T > hoMarginP G G T (n) fulfilled

AV _R XL E V_NC E L L (n) > rxLevMinCell(n) + Max (0, msTxPwrMax(n) -msTxPwrMax)

-80 dBm > -99 dBm + (33 dBm - 33 dBm) = -99 dBm fulfilled

1a

2a

AV_R X L E V_D L_HO = -90 dBmms TxPwrMax = 33 dBmBT S _T X _P WR = 42 dBmbts T xPwrMax = 42 dBm

Serv ing C ell: B es t Ad jacent C ell:

AV_R X LE V_NC E L L (n) = -80 dBmrxL evMinC ell(n) = -99 dBmms TxPwrMax(n) = 33 dBmbts T xPwrMax = 42 dBmhoMarg inP BG T (n) = 6 dB

HandoverControl &

Adjacencies

P ower B udget Handover(E xample)

Fig. 6-24: Power budget handover (example)

6.4.5 Pure Umbrella Handover

This kind of handover is used for multi layer networks, especially for such ones with macro and micro cells. Its purpose is to move the MS to a suitable layer in dependence on its power class. Umbrella handover must be enabled by setting the parameter enableUmbrellaHandover (EUM)(HOC)(Y/N)(N) to Y. Than the RX levels of the adjacent cells are checked periodically, according the value of the parameter hoPeriodUmbrella (which has been explained already in section 6.3.3 Handover Execution).

Like the power budget handover, the umbrella handover can take place, even if the serving cell measurements do not exceed a threshold. The adjacent cells are ranked as usual. To decide, whether such one can be used as target cell, now condition (1b) is used. To move the MS to the layer corresponding to its power class, additionally the following thresholds are considered.

Threshold Remark

gsmMacrocellThreshold (GMAC)(BSC)(5..43)(35)

Defines macro cell size by setting the maximum output power (in dBm) of the MS within a GSM 900 cell. With the



29 (50)

minimum setting multi layering is not used.

dscMacrocellThreshold (DMAC)(BSC)(0..36/32)(26)

Is used like gsmMacrocellThreshold, but for GSM 1800 / 1900.

gsmMicrocellThreshold (GMIC)(BSC)(5..43)(33)

Defines micro cell size by setting the maximum output power (in dBm) of the MS within a GSM 900 cell. With the maximum setting multi layering is not used.

dscMicrocellThreshold (DMIC)(BSC)(0..36/32)(24)

Is used like gsmMicrocellThreshold, but for GSM 1800 / 1900.

The umbrella handover specific parameters are summarized by Fig. 6-25.

© NO K IA B S S P AR / 10.05.2004

HandoverControl &

Adjacencies

Umbrella Handover(P arameters )

Mus t be enabled withenableUmbrellaHandover Y/N

S erving cell meas urements need not exceed a thres hold

Periodic check of RX levels of adjacent cellshoPeriodUmbrella 0..63 SACCH periods

T hres ho lds to determ ine layers of ad jacent ce llsgsmMacrocellThreshold 5..43 dBm for GSM 900 / 800dcsMacrocellThreshold 0..36/32 dBm for GSM 1800 / 1900gsmMicrocellThreshold 5..43 dBm for GSM 900 / 800dcsMicrocellThreshold 0..36/32 dBm for GSM 1800 / 1900

Fig. 6-25: Umbrella handover (parameters)

The maximum output of the MS allowed on a TCH within a cell PTCH is defined by the parameters msTxPwrMaxGSM for GSM 900 and msTxPwrMaxGSM1x00 for GSM 1800 / 1900 (see chapter Power Control, section Introduction). In dependence on the value defined for an adjacent cell, this is adopted to be a macro, middle size or micro cell according the following rules:

• PTCH >= macro cell threshold -> macro cell



• Macro cell threshold > PTCH > micro cell threshold -> middle size cell

• PTCH <= micro cell threshold -> micro cell

The cell type allowed for a handover depends on the maximum output power capability PMAX of the MS. The rules are fully analogous to those just mentioned above:

• PMAX >= macro cell threshold -> macro cell

• Macro cell threshold > PMAX > micro cell threshold -> middle size cell

• PMAX <= micro cell threshold -> micro cell

Cell classification and handover restrictions are summarized by Fig. 6-26. The whole process is summarized by Fig. 6-27, an example is given by Fig. 6-28.

© NO K IA BS S P AR / 10.05.2004

P T CH >= Macrocell Threshold -> adjacent cell = macro cell

Microcell Threshold < P T CH <Macrocell Threshold -> adjacent cell = middle size cell

P T CH <= MicrocellThreshold -> adjacent cell = micro cell

HandoverControl &

Adjacencies

Umbrella Handover(Ad jacent C ell C las s ification)

Maximum MS output power allowed on T C H: P TC H -> defined for every ad jacent ce llMaximum MS output capab ility : P MAX -> defined by power c las s

P MAX >= Macrocell Threshold -> only handover to macro cellMicrocell Threshold < PMAX < Macrocell Threshold -> only handover to middle size cell

P MAX <= MicrocellThreshold -> only handover to micro cell

C e ll clas s ification crite ria : cons ider P T CH

Handove r crite ria : cons ider PMAX

Fig. 6-26: Umbrella handover (adjacent cell classification)



31 (50)

© NO K IA BS S P AR / 10.05.2004

HandoverControl &

Adjacencies

Umbrella Handover(E xecution)

Estimation of RX levels and P T C H check of adjacent cells

Ranking and layer classification of adjacent cells

Check of adjacent cells with condition 1b

Adjacent cell with sufficient RX level and of correct layer

Yes No

Inter cell handover No handover

Fig. 6-27: Umbrella handover (execution)

© NO K IA BS S P AR / 10.05.2004

Umbre lla handoverto micro ce ll

A

B

R X leve l handoverback to macro ce ll

1800 Macro

1800 Micro

hoLevelUmbrella = -85 dBmgsmMacrocellThreshold = 30 dBmgsmMicrocellThreshold = 20 dBmmsTxPwrMax(n) = 15 dBm -> micro ce llPMAX = 15 dBm -> handove r to m icro ce ll onlyhoThresholdLevelDL = -95 dBm

HandoverControl &

Adjacencies

Umbrella Handover(E xample)

T hres hold R X leve l handover –95 dBm

T hres hold umbre lla handover –85 dBm

Fig. 6-28: Umbrella handover (example)



6.4.6 Combined Power Budget and Umbrella Handover

Power budget and umbrella handover can be enabled simultaneously. If for both kinds the requirements are fulfilled, the umbrella handover has higher priority.

The power budget handover now takes place only between cells belonging to the same layer, which complicates the target cell selection. An overview about the different handover types within a multi layer network is given by Fig. 6-29.

© NO K IA BS S P AR / 10.05.2004

macrocells

microcells

UMB,RR

PBGT,RR

PBGT,RRUMB,RR

UMB umbrella HORR radio reason HOPBGT power budget HO

C ombined Umbrella & P ower B udget HO(Overview)

enablePowerBudgetHo = Y es & enableUmbrellaHo = Y es

P ower budget handove r to ce lls of the s ame layer

Umbre lla handover to ce lls of diffe rent laye r

HandoverControl &

Adjacencies

Fig. 6-29: Combined umbrella & power budget HO (overview)

6.4.6.1 Adjacent Cell Classification

The layer, to which an adjacent cell belongs, can be determined as described already above or predefined. The latter can be realized by the parameter adjCellLayer (ACL)(ADJC) (N,SAME,UPPER,LOWER)(N). The adjacent cell can belong either to the same layer as the serving cell, to the next lower or next upper layer. With the default setting no predefinition is used. The modified classification technique is summarized by Fig. 6-30. The cell ranking and the evaluation of the RX level and power budget are done by the same way as already described above. This feature can be combined with Fast Moving MS Handling in Macrocells



33 (50)

© NO K IA BS S P AR / 10.05.2004

UP P E R layer (macro)

S AME laye r (s e rving laye r)

L O WE R laye r (micro)

P redefinition of laye r pos s ible by ad jC ellL ayer

T hree laye rs vis ible re lative to se rving ce ll

U s ed for target ce ll evaluation• C ombined umbre lla and power budget • Handover bas ed on MS s peed• F as t moving MS handling in macro ce ll

N (no predefinition)

C ombined Umbrella & P ower B udget HO(Ad jacent C ell C las s ification)

HandoverControl &

Adjacencies

Fig. 6-30: Combined umbrella & power budget HO (adjacent cell classification)

6.4.6.2 MS Speed Handling

In networks using cells of different sizes, the speed of the MS is an important factor. A fast moving MS should be kept in the macro cell layer to avoid huge signalling traffic caused by frequent crossings of micro cell boarders. Slow moving MSs should enter the micro cell layer to achieve an optimum use of the capacity resources. So umbrella handover shall be carried out due to the MS speed. Cell ranking and target cell selection are done by the same way as for the ordinary umbrella handover.

The MS speed can be estimated by two techniques. To check, whether a MS moves slowly or fast, the number of measurement reports from adjacent micro cells is determined. This is called fast moving MS support. The speed of a MS can be measured also explicitly. This method is called MS speed detection. A summary about MS speed handling is given by Fig. 6-31.



© NO K IA BS S P AR / 10.05.2004

Mobile dis tribution in multi laye r ne tworks bas ed on speed

•S low moving MS -> lower laye r (micro) ce lls -> optimum capacity us e•F as t moving MS -> upper laye r (macro) ce lls -> reduction of s ignalling traffic

T wo proprie tary Nokia features

•Fas t Moving Mobile S upport (FMMS )•E s timation of MS s peed bas ed on number of measurement reports fromadjacent micro ce lls

•used to move MS from UP P E R (macro) to L OWE R (micro) ce ll

•MS´Speed Detection•Measurement of MS s peed by counting fading dips•Us ed to move s low/fas t MS from macro/micro to micro/micro ce ll

C ombined Umbrella & P ower B udget HO(Impact of MS S peed)

HandoverControl &

Adjacencies

Fig. 6-31: Combined umbrella & power budget handover (impact of MS speed)

As already discussed in the chapter Measurement Processing, handover due to MS speed must be activated by setting the parameter msSpeedDetectionState to 0. Otherwise only the average window is scaled to the indicated percentage, which nevertheless is useful, as the average process is speed up.

The fast moving MS support works as follows. When the MS just has entered a macro cell, a counter is started. Each time a measurement report from an adjacent micro cell with a sufficient RX level following condition (1a) is received by the BSC, the counter is incremented by 2. Otherwise it is decreased by 1. If the MS moves slowly, a lot of measurement reports will be generated, before it leaves the current macro cell and enters the next one. Thus it is moved to a micro cell, if the number of SACCH frames exceeds the limit defined by the parameter fastMovingThreshold (FMT)(ADJC)(0.255)(0). In case of the default setting the feature is deactivated. The fast moving MS support method is summarized by Fig. 6-32.



35 (50)

© NO K IA BS S P AR / 10.05.2004

Macro ce ll

Micro ce ll

time ‘t’

C ounte r

S low moving MShandover triggered

time ‘t’

C ombined Umbrella & P ower B udget HO(Fas t Moving MS Support)

HandoverControl &

Adjacencies

E s timation of MS s peed bas ed on measurement reports from adjacent micro ce lls

MS ente rs macro ce ll -> counte r is s tarted

rxL evMinC e ll -110..-47 dBm if m icro ce ll has higher leve l -> counte r +2

othe rwis e counte r –1

fas tMovingT hre shold 0..255 S AC C H frames if exceeded -> handover to m icro ce ll trigge red

targe t ce lls s e lected as for umbre lla handove r

T hreshold umbre llahandover –85 dBm

F as t movingthres hold 40

Micro ce ll has sufficient R X leve lHandover pos s ible

Fig. 6-32: Combined umbrella & power budget HO (fast moving MS support)

The idea of the MS speed detection method is the following. The signal strength of each burst is compared with the signal strength averaged over one SACCH period (the uplink is used). The speed is derived from the so-called crossing rate, i.e. how often the burst signal level crosses the average level because of fading. The method cannot be used, if frequency hopping is activated. In this case all MS speed indications sent to the BSC are set to 255 = invalid. If DTX has been used during the current or the previous SACCH frames, reliable MS speed measurements are not available, too. If power control is carried out, the MS speed indications are discarded, until it is finished. The measurements taken before the start of the power control process, however, are reliable and can be used for handover decision.

This is based on the thresholds defined by the parameters upperSpeedLimit (USL)(HOC)(0..255)(0) and lowerSpeedLimit (LSL)(HOC)(0..255)(0). The unit is 2 km/h. In case of the default setting, the threshold will not be used. If the upper limit is exceeded, the MS will be moved to an upper layer cell (macro cell), if possible. If the lower limit is exceeded, the target cell will be a lower layer cell (micro cell). The threshold will be considered as exceeded, if this is true for at least Px (STP)(HOC)(1..32)(3) of the last Nx (STN)(HOC)(1..32)(6) averaged MS speed indications. The averaging and threshold comparison process for the MS speed follows the same scheme as for the RX level and quality.



© NO K IA BS S P AR / 10.05.2004

HandoverControl &

Adjacencies

Meas urement of MS s peed bas ed on number of fad ing d ips c ros s ing the average s ignal level

S peed thres ho lds

upperSpeedLimit 0..255 units of 2 km/h if exceeded -> handover to upper layer (macro) celllowerSpeedLimit 0..255 units of 2 km/h if exceeded -> handover to lower layer (micro) cellNx 1..32 Number of ave raged va lues taken for comparis onPx 1..32 Number of ave raged va lues which mus t exceed

thre shold

C ros s ing rate meas urements not reliab le for

F requency hopping ne tworkD T X during current or previous S AC C H frameP ower control

C ombined Umbrella & P ower B udget HO(MS S peed Detection)

Fig. 6-33: Combined umbrella & power budget HO (MS speed detection)

How both kinds of MS speed handling work together, is shown by Fig. 6-34. For the macro cell layer usually frequency hopping is active, so only the Fast Moving MS Support method is available. If a MS is classified as slow moving MS, it is moved to a micro cell, if possible. For the micro cell layer normally frequency hopping is not applied, so that the more accurate MS speed detection method is available. If the upper speed threshold is exceeded, it is moved back to a macro cell. The whole process is summarized by Fig. 6-35.



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© NO K IA BS S P AR / 10.05.2004

B SCB TS

B T S

Macro cell with R F hopping

Micro cells without R F hopping

fas t MS s

s low MSs C ros s ing rate algorithmas no R F hopp ing

Adjacent cell

meas urements

Handoveralgorithm

C ombined Umbrella & P ower B udget HO(C ombined MS S peed Handling)

HandoverControl &

Adjacencies

Upper s peed thres holdexceeded

S low MS

No cros s ing rate algorithmbecaus e of R F hoppingBut fas t mov ing MS s upport

Fig. 6-34: Combined umbrella & power budget HO (combined MS speed handling)

© NO K IA BS S P AR / 10.05.2004

HandoverControl &

Adjacencies

C ombined Umbrella & P ower B udget HO(MS Speed Hand ling E xecution)

MS jus t ente red macro ce ll C ounte r = 0

Check of adjacent cells with condition 1a

Fulfilled -> counter + 2 Not fulfilled -> counter -1

Yes

Slow moving MS HO to micro cell

Counter > fast moving threshold

MS jus t ente red micro ce ll

Measurement of MS speed

MS speed > upper speed threshold

No

Fast moving MS No HO

Yes

Fast moving MS HO to macro cell

No

Slow moving MS No HO



Fig. 6-35: Combined umbrella & power budget HO (MS speed handling execution)

6.5 Imperative Handover

To this group belong handovers due to operation and maintenance, large distance between MS and BTS and rapid field drop, but also directed retry. They are triggered by different events which will be explained in the following.

6.5.1 Operation and Maintenance Handover

If a hardware device, e.g. a BTS has to be switched off, all the running calls supported by it must be allocated to other TCHs. Each MS is moved to the best cell according the adjacent cell ranking list.

6.5.2 Directed Retry

6.5.2.1 Normal Directed Retry

If no TCH is available during call set up in the serving cell, a TCH of another cell can be allocated to the MS. This is done by a handover from the SDCCH to this TCH.

Directed retry must be enabled by setting the parameter drInUse (DR)(SEG)(Y/N)(N) to Y. The adjacent cell ranking is done as usual, and the target cell selection is based on condition (1a). There are now, however, two possibilities for the threshold to be exceeded by the RX level. This is controlled by the parameter drMethod (DRM)(SEG)(0,1)(0).

In case of the default setting one works with the threshold rxLevMinCell as usual. Selecting method 1, instead the threshold defined by the parameter drThreshold (DRT)(ADJ)(-110..-47)(-100) can be used. drThreshold must be higher than rxLevMinCell, as otherwise the latter is used as threshold. With method 1 a better signal level and quality for the target cell of a directed retry handover is achieved.

Directed retry uses two special timers. minTimeLimitDirectedRetry (MIDR)(SEG)(0..14)(0) defined the time in s from the TCH assignment request, during which a target cell evaluation and thus directed retry is not allowed. The MS can use this time to decode the BSIC of adjacent cells. maxTimeLimitDirectedRetry (MADR)(SEG)(1..15)(5) is the time



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in s from the TCH assignment request, during which target cell evaluation is allowed. The directed retry specific parameters are summarized by Fig. 6-36, the whole process by Fig. 6-37.

© NO K IA BS S P AR / 10.05.2004

HandoverControl &

Adjacencies

D irected R etry(P arameters )

No T CH available during call s et up in s erv ing cell -> handover to T CH of other cell

Mus t be enabled withdrInUse Y/N

T hres holds to be exceeded by target cell according condition (1a)drMethod 0/1 D efined type of thresholdrxLevMinCell -110..-47 dBm Used if drMethod = 0drThreshold -110..-47 dBm Used if drMethod = 1 and

drThreshold > rxLevMin CellTimers (to be counted from TCH assignment)minTimeLimitDirectedRetry 0..14 s No target cell evaluation allowedmaxTimeLimitDirectedRetry 1..15 s Target cell evaluation allowed

SDCC HT CH

conges ted TimeAs s ignmentR eques t

minT imeL imitDR

maxT imeL im itDR

DR not allowed DR allowed

Fig. 6-36: Directed retry (parameters)

© NO K IA BS S P AR / 10.05.2004

HandoverControl &

Adjacencies

T C H ava ilable in s e rving ce ll

D irected R etry(E xecution)

Yes No

Estimation of RX levels of adjacent cells


Check of adjacent cells with condition 1a

Adjacent cell with sufficient RX level

Yes No

Directed retry handover Call set up fails

Normal call set up



Fig. 6-37: Directed retry (execution)

6.5.2.2 Intelligent Directed Retry

Here the target cell is selected on only according the RX level, but also according its cell type, defined by the parameter cellType (CTY)(ADJ)(GSM/MCN)(GSM). Thus it is applied especially to multi layer networks. Intelligent directed retry must be activated by setting the parameter idrUsed (IDR)(SEG)(Y/N)(N) to Y.

Intelligent directed retry requires to classify the MS as GSM or MCN one. A GSM MS can have as target cell both a macro or micro cell, while a MCN MS only a micro cell. The classification can be done either on the basis of the power class (see chapter Power Control) or the subscriber priority. The latter has to be defined by the network administrator, the corresponding data are stored in the MSC.

Directed retry to a target cell belonging to another BSC than the serving cell is possible only, if this external BSC and the MSC support this feature. Furthermore, the parameter disableExtDr (DEXDR)(BSC)(Y/N)(N) must be kept to N. If directed retry is not supported by all BSCs of the network, it must be set to Y. Intelligent directed retry is summarized by Fig. 6-38.

© NO K IA BS S P AR / 10.05.2004

Intelligent D irected R etry

conges tion

macro ce ll (G S M ce ll)

micro ce lls (MC N ce lls )

G SM subs criberBoth G SM and MCN ce llsa llowed as target

conges tion

macro ce ll (G SM ce ll)

micro ce lls (MCN ce lls )

NOKIA TELECOMMUNICATIONS

HandoverControl &

Adjacencies

T arget cell s election inc luding cell and s ubs criber type

Mus t be enabled withidrUsed Y/N

C ell type c las s ificationce llT ype G SM/MC N G SM = macro ce ll, MCN = micro ce ll

Subscriber classification byPower classPriority (defined by network administrator)

Directed Retry to external BSSdisableExtDr Y/N N possible only, if directed retry supported

by all BSCs and the MSC

MCN s ubs criberO nly MC N ce llsa llowed as target

Fig. 6-38: Intelligent directed retry



41 (50)

6.5.3 Handover due to Distance MS - BTS

An imperative handover can be triggered also, if the distance between the MS and BTS becomes too large. This feature must be enabled by setting the parameter enableMSDistanceProcess (EMS)(HOC)(Y/N)(N) to Y.

The distance to be BTS is determined on the basis of the timing advance. This property principally undergoes the same averaging and threshold comparison process as RX level, RX quality and MS speed. The handover is triggered, if the threshold defined by the parameter msDistanceHoThresholdParam (MSR)(HOC)(0..63)(63) is exceeded. The setting 63 corresponds to the maximum allowed distance. The threshold is considered as exceeded, if at least Px (MSP)(HOC)(1..32)(1) of the last Nx (MSN)(HOC)(1..32)(1) average samples are above it.

If the handover is triggered, there are three possibilities to continue, which is controlled by the parameter msDistanceBehaviour (DISB)(BSC)(0..60/255)(255). In case of the setting 0 no handover is tried at all, i.e. the call is released immediately. The settings 1 to 60 define the time in s, during which handover is tried. In case of unsuccessful attempts, afterwards the call is released. In case of the default setting just the imperative handover is tried; an unsuccessful attempt does not lead to a release of the call.

The imperative handover due to distance is performed like the normal directed retry, i.e. the target cell is selected on basis of the usual ranking and condition (1a). The MS-BTS distance handover is summarized by Fig. 6-39.

© NO K IA BS S P AR / 10.05.2004

MS – B TS D is tance HandoverHandoverControl &

Adjacencies

T arget cell s election inc luding cell and s ubs criber type

Mus t be enabled withenableMSDistanceProcess Y/N

Dis tance thres holdsmsDistanceHoThresholdParam 0..63 63 = maximum allowed distanceNx 1. .32 Number of ave raged values taken for comparis onP x 1..32 Number of ave raged values which mus t exceed thres hold

P rocedures after handover triggeringmsDistanceBehaviour 0 Call released immediately

1..60 s Handover attempt within given timeUnsuccessful attempt -> call released

255 Just handover attemptUnsuccessful attempt -> call still supported by old cell



Fig. 6-39: MS-BTS distance handover

6.5.4 Handover due to Rapid Field Drop

6.5.4.1 Normal Rapid Field Drop

The normal RX level handover mechanism is too slowly, when a rapid field drop occurs, e.g. the MS moves rapidly from an indoor cell to an outdoor cell. The reason for this is, that the RX level handover is based on measurements averaged over a certain time. Therefore an additional handover algorithm has been introduced which is triggered by raw measurements of the uplink RX level.

First of all it is checked, whether the threshold defined by the parameter hoThresholdsRapidLevUL (RPD)(HOC)(-110..-47)(-110) is exceeded. If this occurs for hoThresholdsRapidLevUIN (CNT)(HOC)(0..32)(0) consecutive measurements, the handover is triggered. In case of the default setting the rapid field drop handover is disabled.

The adjacent cells now are ranked only according the RX level, the priorities described in section 6.3 General Strategy are not taken into account. Furthermore, only cells defined as chained adjacent cells can be used as target. This is done by setting the parameter chainedAdjacentCell (CHAIN)(ADJC)(Y/N)(N) to Y. A target cell must fulfil condition (1a) as for any other kind of imperative handover.

The specific parameters are summarized by Fig. 6-40, the whole process by Fig. 6-41. An example is given by Fig. 6-42.



43 (50)

© NO K IA BS S P AR / 10.05.2004

HandoverControl &

Adjacencies

R ap id F ie ld D rop Handover(P arameters )

Normal R X leve l handover -> trigge red by averaged measurements-> too s lowly in case of rapid fie ld drop

R apid fie ld drop handover -> trigge red by raw measurements-> can react immediate ly

T hres ho ldshoThresholdsRapidLevUL -110..-47 dBm Raw uplink level to be exceededhoThresholdsRapidLevUIN 0..32 1..32 = number of consecutive measurements

which must exceed RX level threshold0 = rapid field drop handover disabled

Target cell selectionchainedAdjacentCell Y/N Cell must be defined as chained cell

Fig. 6-40: Rapid field drop handover (parameters)

© NO K IA BS S P AR / 10.05.2004

HandoverControl &

Adjacencies

R apid F ield D rop Handover(E xecution)

RX level hoThresholdsRapidLevUIN consecutive times below hoThresholdsRapidLevUL

Ranking of chained adjacent cells only according RX level

Check of adjacent cells with conditions 1a


Yes No


T hres holdshoThresholdsRapidLevUL -110..-47 dBmhoThresholdsRapidLevUIN 1..32

Fig. 6-41: Rapid field drop handover (execution)



© NO K IA BS S P AR / 10.05.2004

C hainedC ell

S erv ing C ell

..

1s t

2nd

Serv ing cell

hoThresholdRapidLevUL = -93 dBm

hoThresholdRapidLevUlN = 2

chainedAdjacentCell = Y

HandoverControl &

Adjacencies

R apid F ield D rop Handover(E xample)

T hres hold rapid fie ld drophandover –93 dBm

Handovertriggered

Fig. 6-42: Rapid field drop handover (example)

6.5.4.2 Enhanced Rapid Field Drop

When the MS turns around a corner and looses therefore direct line of sight to the BTS, a very rapid field drop of up to 30 dB occurs. This should be distinguishable from the more gradual field drop, when the MS moves away from the cell site. This is possible with the enhanced rapid field drop method, which does not measure simply the RX level, but the drop the signal suffers. Typical scenarios are shown by Fig. 6-43.



45 (50)

© NO K IA BS S P AR / 10.05.2004

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

MS turns a corner, the signal drops rapidly

Sig

nal L

evel

Time

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

MS turns a corner, the signal drops faster than moving in straight line

Sig

nal L

evel

Time

HandoverControl &

Adjacencies

Enhanced R apid F ield D rop Handover(S cenarios )

F as t moving MS S low moving MS

Fig. 6-43: Enhanced rapid field drop handover (scenarios)

The process is activated with the parameter erfdEnabled (ERFD)(HOC)(DIS,UL,DL,UDL)(DIS). It can be used not only for the uplink (setting UL), but also the downlink (setting DL) and even both (setting UDL). With the default setting it is disabled.

A rapid field drop is detected as follows. With every SACCH frame the BSC is informed about the RX level in the serving cell. The BSC takes the most recent raw measurement n and compares it with the measurement n - k. k is the deep drop edge window size (given in SACCH frames) to be set by the parameter ddeWindow (ERMW)(HOC)(1..32)(2). The measured signal drop is compared with the threshold defined by the parameter ddeThresholdsLev (ERT)(HOC)(0..63)(10). If at least Px (ERP)(HOC)(1..32)(2) from Nx (ERN)(HOC)(1..32)(3) individual measurements exceed the threshold, the MS is considered as a turn around corner MS. The parameter required for the field drop measurements are summarized by Fig. 6-44, an example is given by Fig. 6-45.



© NO K IA BS S P AR / 10.05.2004

HandoverControl &

Adjacencies

Enhanced R apid F ield D rop Handover(P arameters )

Mas t be enabled witherfdEnabled D IS ,UL ,D L ,UDL D IS = dis abled

UL /D L = only downlink/uplink cons ide redUD L = both cons ide red

T hres ho ldsddeThresholdsLev 0..63 dB Field drop suffered by signal to be exceededNx 1. .32 Number of s ingle va lue s taken for comparis onP x 1..32 Number of s ingle va lue s which mus t exceed threshold

Deep drop edge windowddeWindow 1..32 Field drop is determined by comparing current

measurement n with measurement n - ddeWindow

Fig. 6-44: Enhanced rapid field drop handover (parameters)

© NO K IA BS S P AR / 10.05.2004

Handover

S erving ce ll

Adjacent ce ll

F ie ld drop threshold 20 dB

ddeW indow

Handover triggered

-83 -87-63-61-60-60 -89 -91 -94 -89 -89

HandoverControl &

Adjacencies

Enhanced R apid F ield D rop Handover(F ield D rop Detection)

ddeWindow = 2 S AC C H frames

ddeThresholdsLev = 20 dB

px = 2

nx = 3

F ie ld drop 1 3 23 24

px 1 2

nx 1 2 3 3

Fig. 6-45: Enhanced rapid field drop handover (field drop detection)



47 (50)

The target cells are selected and checked according condition (1a) as for the normal rapid field drop handover. To speed up the target cell evaluation, a faster averaging of the adjacent cell measurements is possible, using the parameter modifiedAveWinNcell (ERAW)(HOC)(1..32)(2). Then a smaller averaging window is applied immediately, after a deep drop edge has been detected. The number of allowed zero results after such a drop can be controlled as well, using the parameter modifiedNOZ (ERZ)(HOC)(1..32)(1). After rapid field drop detection a timer set by the parameter erfdOver (ERD)(HOC)(0..64)(10) is started. The modified averaging parameters are valid until the timer expires.

© NO K IA BS S P AR / 10.05.2004

Handover

S erving ce ll

Adjacent ce ll

F ie ld drop thres hold 20 dB

ddeW indow

Handover trigge red

-83 -87-63-61-60-60 -89 -91 -94 -89 -89

HandoverControl &

Adjacencies

Enhanced R apid F ie ld D rop Handover(Handover S peed Up)

averagingWindowSizeAdjCell = 4

modifiedAveWinNcell = 2

E rfdO ver

Handover executed

Handover triggered -> modified averag ing parameters for ad jacent cell R X levels

modifiedAveWinNcell 1. .32 S AC C H frames S horte r averaging windowmodified NOZ 1..32 Modified number of allowed zero resultserfdOver 0..64 s New parameters valid until timer expires

F ie ld drop detection -> 5 S AC C H frames

Handover execution -> 2 S AC C H frames

T otal -> 7 S AC C H frames

Fig. 6-46: Enhanced rapid field drop handover (handover speed up)

6.6 Traffic Reason Handover

Sometimes a handover is required not because of the signal quality or level, or because of the distance or speed of the MS, but just to avoid overload. This is called a traffic reason handover and is initiated by the MSC or the BSC. The target cell can be worse then the serving cell. A guard timer discussed already in section 6.3 General Strategy prevents an immediate back handover to the original cell in this case.



6.6.1 MSC Initiated Traffic Reason Handover

In this case the adjacent cells are ranked only according their RX levels. The priorities described in section 6.3 General Strategy are not taken into account. Condition (1a) must be fulfilled simultaneously for two thresholds, for rxLevMinCell as usual and additionally for trhoTargetLevel (TRHO)(ADJC)(-109..-47,N)(N). In case of the default setting this parameter is not used. The process is summarized by Fig. 6-47.

© NO K IA BS S P AR / 10.05.2004

HandoverControl &

Adjacencies

MS C handove r reques t

T raffic R eas on Handover(MSC Initiated)

Ranking of adjacent cells only according RX level

Check of adjacent cells with condition 1a for

rxLevMinCell and trhoTargetLevel

Adjacent cell with sufficient RX level

Yes No


Fig. 6-47: Traffic reason handover (MSC initiated)

6.6.2 Advanced Multi Layer Handling

The Advanced Multi Layer Handling concept consists of 3 different features providing an optimised traffic control within multi layer networks. These can be based on macro and micro cells, dual band or intelligent underlay overlay.

6.6.2.1 BSC Initiated Traffic Reason Handover

To avoid an overload situation, the BSC can initiate a handover, if the load of the serving cell exceeds the percentage defined by the parameter amhUpperLoadThreshold (AUT)(BSC)(0..100)(80).



49 (50)

Like for the MSC initiated traffic reason handover, the adjacent cells are ranked only according their RX levels. A target cell must fulfil condition (1a) for the threshold trhoTargetLevel, and condition (2a) for the power budget margin set by the parameter amhTrhoPbgtMargin (ATPM)(HOC)(-24..24,N)(N). If the power budget exceeds the margin set by hoMarginPGBT, then a power budget instead of a traffic reason handover is carried out. The load of the target cell must not exceed the percentage defined by the parameter amhMaxLoadOfTgtCell (AML)(SEG)(0..100,N)(N). The process is summarized by Fig. 6-48.

BSC initiated traffic reason must not be used together with MSC initiated traffic reason handover. To deactivate it, amhTrhoPbgtMargin must be set to its default value N.

© NO K IA BS S P AR / 10.05.2004

HandoverControl &

Adjacencies

S e rving ce ll with load > amhUpperLoadThreshold

T raffic R eas on Handover(B SC Initiated)

Ranking of adjacent cells only according RX level

Check of adjacent cells withcondition 1a for trhoTargetLevel condition 2a for amhTrhoPbgtMargin

Adjacent cell with sufficient RX level / power budget load < amhMaxLoadOfTgtCell

Yes No

Power budget handover

Handover failurePower budget >=hoMarginPGBT

Power budget < hoMarginPGBT

Traffic reason handover

Mus t not be us ed together w ithMSC initiated handover

DeactivationamhTrhoPbgtMargin = N

Fig. 6-48: Traffic reason handover (BSC initiated)

6.6.2.2 Intelligent Underlay Overlay Load Control

This features enables to keep all MSs in the overlay network, if there is very little traffic. It is activated by setting the parameter amhTrafficControlIUO (ATCI)(HOC)(Y/N)(N) to Y. If now the load of the serving cell falls below the percentage defined by the parameter amhLowerLoadThreshold (ALT)(BSC)(0..100)(20), an IUO handover or direct access to a super reuse TRX is not allowed. A summary is given by Fig. 6-49. IUO is discussed in detail in the following chapter Intelligent Underlay Overlay.



6.6.2.3 Multi Layer Network Load Control

This feature enables to keep all MSs in the coverage layer, if there is very little traffic. It can be applied both to macro / micro cell and dual band networks, by setting the parameter amhTrafficControlMCN (ATCM)(HOC)(Y/N)(N) to Y. If now the load of the serving cell falls below the percentage defined by the parameter amhLowerLoadThreshold (ALT)(BSC)(0..100)(20), fast moving MS support, MS speed detection and umbrella handover to the capacity layer are not allowed. A summary is given by Fig. 6-49.

© NO K IA BS S P AR / 10.05.2004

HandoverControl &

Adjacencies

Multi L ayer / IUONetwork L oad C ontro l

Mas t be enabled withamhTrafficControlIUO Y/N Acts for IUO networkamhTrafficControlMCN Y /N Acts both for macro / m icro ce ll and dual band

network

T hres holdsamhLowerLoadThreshold 0..100 % If there is little traffic in serving cell, the following

handovers are not allowedIUO handover / direct access to super reuse TRX (for IUO network)Fast moving MS support / MS speed detection / umbrella handover (for MCN or dual band network)-> MS is kept in coverage layer

Fig. 6-49: Multi layer / IUO network load control

bsspar chapter 06 handover control and adjacencies mo[1]

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