03 Mn1789eu10mn 0002 Design Radio Cells

Design of radio cells Siemens

MN1789EU10MN_0002 © 2004 Siemens AG

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Contents

1 Design of radio cells 3

2 Cell selection/reselection 5

2.1 Cell selection 6

2.2 Cell reselection 10

2.3 Cell reselection to UMTS 22

2.4 Parameters 24

3 Handover 25

3.1 General notes on handover 26

3.2 Measurement preprocessing 32

3.3 Handover criteria 40

3.4 Handover detection algorithms 44

3.5 Target cell list compilation 78

3.6 Handover signaling and timer 86

3.7 AMR-handover 92

3.8 GSM to UMTS intersystem handover 96

3.9 Service dependent handover management 126

4 Exercises 129

5 Solutions 135

Design of radio cells

Siemens Design of radio cells

MN1789EU10MN_0002

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3

1 Design of radio cells

In textbooks on cellular mobile communications radio cells are idealized by regular hexagons, which has been proved to be a good model for explaining principle effects.

However, in reality cell borders do not have this simple geometric structure. The physical cell borders are fixed on the one hand

� by the radio propagation conditions

and on the other hand

� by the algorithms which decide on changing from one serving base station to another one on the basis of “link quality” measurements.

These algorithms are called:

� Cell Selection/Reselection (for the idle mode),

� Handover (for the connected mode)

and are described in detail in the following sections.

Though it is controlled by some parameters broadcasted from the BS to the MS, the cell selection/ reselection algorithm itself is implemented in the MS. Therefore, it is specified in details by the GSM Recommendations (especially GSM 03.22 and GSM 05.08).

In contrast, the handover decision algorithm is implemented in the BSS (assisted by downlink measurements reported by the MS). Hence, some degree of freedom is left to the manufacturer to optimize the algorithm.

For this reason and because its very important to take the correct decision when the MS is in connected mode, the focal point of this chapter is the handover algorithm.


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5

2 Cell selection/reselection


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As mentioned above, the cell selection/reselection algorithm is implemented in the MS. Because the algorithm for a GSM phase 1 MSs differs from that of a phase 2 MSs, both variants are described (as far as there is a difference).

2.1 Cell selection

Normal Cell Selection:

Measurements for normal cell selection

The MS takes 5 samples of the received level on each RF carrier which are averaged:

AV_RXLEV = 1/5 * (RXLEV1 + RXLEV2 + ... + RXLEV5)

These samples are spread evenly over a period of 3 - 5 s.



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Criteria for Cell Selection

Based on these measurements one can estimate whether a cell will be an appropriate serving cell from the radio propagation point of view, i.e. whether there will be a sufficient “link quality”.

This is done by checking the criterion C1 > 0.

C1 = AV_RXLEV - RXLEV_ACCESS_MIN - Max(0, MS_TXPWR_MAX_CCH - P)

=>

AV_RXLEV > RXLEV_ACCESS_MIN + Max(0, MS_TXPWR_MAX_CCH - P)

This means that the received downlink level has to be above a threshold (RXLEV_ACCESS_MIN).

To ensure a sufficient uplink received level even for MSs of low transmit power level P a further term is included:

If P < MS_TXPWR_MAX_CCH (the maximum allowed MS transmit power level to access the random access channel), the C1 criterion is equivalent to

AV_RXLEV > RXLEV_ACCESS_MIN + (MS_TXPWR_MAX_CCH - P)

i.e. the received downlink level has to exceed the RXLEV_ACCESS_MIN by a certain margin to have a reserve for the uplink in the case of a MS of a low power class.

Beside the C1 radio criterion there are some other criteria (administrative and traffic control) for a cell to be suitable:


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Definition: a “Suitable Cell” is defined as a cell which

1. is part of the selected PLMN,

2. is unbarred (parameter CELL_BAR_ACCESS = 0),

3. has a parameter C1 > 0,

4. is not in a location area forbidden for national roaming.

To allow e.g. emergency calls the conditions for a serving cell are less restrictive:

Definition: an “Acceptable Cell” is defined as a cell which

1. is unbarred,

2. has a parameter C1 > 0.

The general strategy for cell selection is to find the “suitable cell” with the highest C1 (best estimated link quality). If no suitable cell can be found, an “acceptable cell” is selected.

For phase 2 mobile stations there is an additional parameter called CELL_BAR_QUALIFY (values: 0, 1) used to assign priorities to cells:

CELL_BAR_QUALIFY = 0 <=> normal priority cell

CELL_BAR_QUALIFY = 1 <=> low priority cell

First it is tried to select a suitable normal priority cell, if no such cell can be found, a suitable low priority cell is selected.

The complete cell selection process is illustrated in the flow chart below.

Cell Selection with Stored BCCH Information

Optionally, the MS may store information on received level on BCCH carriers when switched off.

When switched on, the MS first performs measurements on these carriers. If cell selection for the corresponding cells is not successful, normal cell selection is carried out.



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yes

Cell Selection

Algorithm

(no BCCH Info)

measure all carriers

sort by received level

carriers in list

trial carrier:

best level in list

BCCH

decode BCCH

suitable cell

normal priority

Selection of an

acceptable cell

no

yes

no

no

yes

no

no yes

yes

low priority

cell found

Camp on low

priority cell

suitable low priority

cell found

try only carriers of

BCCH allocation

try only normal priority

cells

remove trial

carrier from list

nocell

in selected

PLMN

Camp on normal

priority cell

yes

Fig. 1 Cell selection (no BCCH info stored)


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2.2 Cell reselection

While moving within the radio network in idle mode, another cell may be more appropriate to serve the MS. Therefore, cell reselection may be performed.

Preconditions: The MS camps on a cell, which is called serving cell in the following.

The following actions are performed by the MS to detect whether a cell reselection is necessary.

� Down Link Signaling Failure:

downlink signaling failure counter DSC is initialized: DSC0 = round (90 / BS_PA_MFRMS);

decode message on paging sub-channel;

successful: → DSCt+1 = DSCt + 1 but if DSCt = DSC0 → DSCt+1 = DSC0

unsuccessful: → DSCt+1 = DSCt – 4

DSC < 0 => downlink signaling failure;

� monitor all BCCH carriers given in the BCCH allocation (neighbor cells) of the serving cell,

� take at least 5 samples of the received level from the serving cell (on paging sub-channel) as well as from the neighbor cells;

=> AV_RXLEV(serving cell) and AV_RXLEV (neighbor cell);

� decoding of full BCCH data of the serving cell at least every 30 sec;

� decoding of BCCH data of the 6 strongest neighbor cells at least every 5 min.

From the radio propagation point of view it is worth to select a new (neighbor) cell if the received level from that neighbor cell exceeds the received level of the current serving cell. For phase 1 MSs this is expressed using the C1 criterion defined in the paragraph above:

C1 (neighbor cell) > C1 (serving cell).

For the reselection process for phase 1 MSs the neighbor cells are ordered according to their C1-value.

For phase 2 MSs a modified path loss criterion, the so-called C2 criterion, is used which is described in the following paragraph.



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Cell Reselection Criterion C2 for MS of Phase 2:

The C2 criterion depends upon the value of a timer T:

A timer T is started in the MS for each cell in the list of the 6 strongest neighbor cells as soon as it is placed on the list. T is reset to 0 if the cell is removed from the list.

C2 = C1 - CELL_RESELECT_OFFSET for PENALTY_TIME = 31 and arbitrary T)

C2 = C1 + CELL_RESELECT_OFFSET - TEMPORARY_OFFSET

for T < PENALTY_TIME < 31

C2 = C1 + CELL_RESELECT_OFFSET else.

The C2 criterion is illustrated in the figure below.

C2

C1CELL_RESELECT_OFFSET

TPENALTY_TIME

TEMPORARY_OFFSET

Cell included in the

list of 6 strongest

Fig. 2 Illustration of the C2 criterion


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A negative TEMPORARY_OFFSET reduces the priority of a cell in the list of strongest neighbor cells.

A positive CELL_RESELECT_OFFSET increases the priority of a cell in the list of strongest neighbor cells.

This mechanism may be applied in hierarchical cell structures to keep fast moving mobiles in the umbrella cells and slow moving mobiles in the micro cells: When a mobile reaches the coverage area of a (neighbor) micro cell given by the C1 criterion, this cell becomes effectively excluded from reselection during the PENALTY_TIME.

A fast moving mobile is assumed to have left the coverage area of the micro cell before PENALTY_TIME is reached and hence the micro cell is not selected. In contrast, a slow moving mobile is assumed to be still within the coverage area of the micro cell when PENALTY_TIME has expired. Applying the positive CELL_RESELECT_OFFSET, this cell is selected with preference.

This mechanism will be discussed in more detail when explaining the mobile speed sensitive handover in chapter 4.2 (Hierarchical Cell Structures).

Triggers for Cell Reselection

Cell reselection is triggered by the following conditions:

1. C1 < 0 for the serving cell for a period of 5 s

2. MS detects downlink signaling failure

3. Serving cell becomes barred

4.

a) Phase 1 MS C1 (serving cell) < C1 (suitable neighbor cell) if the suitable neighbor cell is in the same location area for a period of 5 sec. C1 (serving cell) + CELL_RESELECT_HYSTERESIS < C1 (suitable neighbor cell) if the suitable neighbor cell is in another location area for a period of 5 sec.

b) Phase 2 MS C2 (serving cell) < C2 (suitable neighbor cell) if the suitable neighbor cell is in the same location area for a period of 5 sec. C2 (serving cell) + CELL_RESELECT_HYSTERESIS < C2 (suitable neighbor cell) if the suitable neighbor cell is in another location area for a period of 5 sec.

5. A random access attempt is unsuccessful even after the maximum number of repetitions.

For phase 2 there is the additional trigger:

6. A location update request has been rejected with cause “location area not allowed”.



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(l)(h)

Cell Reselection

(a) no change of location area

(b) change of location area

Radius of Cell 2

for selection

(l) low power MS

(h) high power MS

(b)(a)

C1=0

C1

BTS1

BTS1

BTS2

BTS2

CELL_RESELECT_ HYSTERESIS

direction of movementPhase 1 MS

low power class MS

high power class MS

Fig. 3 Illustration of cell selection/reselection


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Parameters for Cell Selection/Reselection

BCCH_ARFCN_NC(n) BA - BCCH Allocation

Object DB Name Range Step Size Unit

BTS / TGTBTS BCCHFREQ 0...1023 1 -

In each cell the absolute radio frequency number BCCH_ARFCN_NC(n) (coding as given in chapter 2) each of its neighbor cell n has to be known. This information is broadcasted as the so called BCCH Allocation to all MSs in the respective cell. On the corresponding frequencies the MSs take measurement samples of the received level used for cell selection/reselection.

Furthermore, the BCCH of neighbor cells has to be decoded by the MS (at least every 5 min) to know the current values of the control parameters for the reselection algorithm.

SYS_ID - System Identifier


BTS SYSID BB900, DCS1800, F2ONLY900, EXT900, GSMR, PCS1900, GSMDCS, GSM850, GSM850PCS

- -

Indicates the frequency band to be used by the BCCH channel.

Standard GSM band: BB900

DCS 1800 band: DCS1800

GSM extended band: F2ONLY900

GSM mixed band EXT900

GSM railway band GSMR

PCS 1900 band PCS1900

Dual Band cell GSMDCS



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CELL_BAR_ACCESS - Cell Barred for Access


BTS CELLBARR FALSE/TRUE - -

A mobile station cannot camp on a barred cell, i.e. a barred cell is not selected by the cell selection/reselection procedure. Mobile stations which camp on a cell while it becomes barred, initialize the reselection procedure to find a new (unbarred) cell, i.e. traffic load is distributed to neighbor cells.

This means that e.g. neither a call nor a location update can start in a barred cell. However, a cell barred for access is not barred for incoming handovers. To barr a cell completely, e.g. for maintenance reasons also incoming handovers have to be avoided.

To reduce overload in a certain cell more moderately without distributing the overload to neighbor cells, barring of access classes has to be used. Barring access for an access class does not trigger a cell reselection for MSs of that class.


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MS_TXPWR_MAX_CCH - Maximum allowed MS Transmit Power on Random Access Channel


BTS MSTXPMAXCH 0...31 1 2 dB

The MS_TXPWR_MAX_CCH field is coded as the binary representation of the power control level defined in GSM rec. 05.05.

GSM900 Phase 1 GSM900 Phase 2

GSM850

DCS1800 PCS1900

0 = 43 dBm 0 = 39 dBm 0 = 30 dBm 0 = 30 dBm

1 = 41 dBm 1 = 39 dBm 1 = 28 dBm 1 = 28 dBm

2 = 39 dBm 2 = 39 dBm 2 = 26 dBm 2 = 26 dBm

3 = 37 dBm 3 = 37 dBm 3 = 24 dBm 3 = 24 dBm

: : : :

15 = 13 dBm 19 = 5 dBm 15 = 0 dBm 15 = 0 dBm

16 - 31 = 13 dBm 20 - 31 = 5 dBm 16 - 31 = 0 dBm 16 - 29 = 0 dBm

30 = 33 dBm

31 = 32 dBm

The transmit power level the MS uses for the access on the random access channel is given by the minimum of two values:

� the output transmit power P of the MS

� the maximum allowed power for access within the respective cell (given by MS_TXP WR_MAX_CCH).



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This parameter affects the

� the random access procedure,

� the cell selection procedure.

Random access: If there is a collision of channel requests on the random access channel, the one with the higher received level has a good chance to be decoded and to get a response by the BS.

Hence, MSs with higher output power are preferred. This imbalance can be avoided by choosing a low maximum allowed transmit power.

Cell Selection: To be selected by the cell selection procedure, a cell has to fulfill the C1 criterion C1 > 0 where

C1 = AV_RXLEV - RXLEV_ACCESS_MIN - Max (0, MS_TXPWR_MAX_CCH - P).

Choosing for example MS_TXPWR_MAX_CCH = Pmin where Pmin is the output power level for the minimum power class 5 (29 dBm), the C1 criterion reduces to

AV_RXLEV > RXLEV_ACCESS_MIN

for MSs of all power classes. Hence, the same idle mode cell border is seen by each mobile.

Choosing for example MS_TXPWR_MAX_CCH = Pmax where Pmax is the output power level for the maximum power class 1 (43 dBm), the C1 criterion reduces to

AV_RXLEV > RXLEV_ACCESS_MIN + (MS_TXPWR_MAX_CCH - P)

for MSs of all power classes. Hence, a larger cell radius is seen by a mobile of higher output power than by a mobile of lower output power. On the other hand one can ensure by this mechanism that a certain uplink received level is exceeded by each MS independent of its power class.

POWER_OFFSET - Additional Powerclass for Class 3 DCS1800 MS


BTS PWROFS 0...3 1 2 dB

The parameter POWER_OFFSET is only used by class 3 DCS1800 MS to calculate the C1-criterion described as follows.

C1 = AV_RXLEV - RXLEV_ACCESS_MIN - Max(0,MS_TXPWR_MAX_CCH + POWER_OFFSET- P).


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RXLEV_ACCESS_MIN - Minimum Downlink Received Level for Cell to be selected


BTS RXLEVAMI 0...63 1 1 dB

The parameter RXLEV_ACCESS_MIN determines the cell border for an MS in idle mode by means of the C1 or C2 criterion, respectively.

Choosing a high value, reduces the risk of a handover immediately after call setup.

On the other hand the value has to be low enough to achieve a sufficient overlap between adjacent cells (especially if they belong to different location areas). This is illustrated in the figure 3 for phase 1 MSs using the C1 criterion for cell reselection. It has to be observed that the overlap may be different for mobiles of different power classes (refer to MS_TXPWR_MAX_CCH).

In any case RXLEV_ACCESS_MIN has to be above the MS receiver sensitivity level (-100 dBm for DCS1800, -102 dBm for GSM handhelds, -104 dBm for other GSM MSs). Furthermore, it has to harmonize with the handover thresholds (RXLEV_MIN, L_RXLEV_HO).

CELL_RESELECT_HYSTERESIS - Hysteresis for Reselection of a Cell from another Location Area


BTS CELLRESH 0...7 1 2 dB

In idle mode the MS selects a new (neighbor) cell if the received level of the neighbor cell exceeds the received level of the current cell in order to be served by the cell with the expected best link quality. However, due to fading effects, the propagation conditions may change rapidly and therefore a reselection may occur very frequently.

If the cells involved in the reselection process belong to the same location area, frequent cell reselection does not have an effect on the network performance.

But if the involved cells belong to different location area, the reselection of a new cell triggers a location update procedure, which causes signaling load (e.g. on the SDCCH) and involves all network elements.

To avoid unnecessary signaling load by forward and backward reselection due to fading, a hysteresis given by the parameter CELL_RESELECT_HYSTERESIS is introduced, i.e. a cell from another location area is selected only if the corresponding received level exceeds the level of the current serving cell by the value of this parameter. This is expressed in terms of the C1 (phase 1) or C2 (phase 2) criterion:



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C1 (serving cell) + CELL_RESELECT_HYSTERESES (serving) < C1 (suitable neighbor cell)

or

C2 (serving cell) + CELL_RESELECT_HYSTERESES(serving) < C2 (suitable neighbor cell)

respectively.

The adjustment of CELL_RESELECT_HYSTERESES should be a compromise between

� reduction of unnecessary location updates (high value) and

� selection of the cell with best reception quality (low value).

Additional Phase 2 Parameters for Cell Selection/Reselection

CELL_BAR_QUALIFY


BTS CBQ 0...1 1 -

normal priority: 0

low priority: 1

Parameter used to assign a priority to a cell selection process. A suitable cell of low priority is only selected if no suitable cell of normal priority can be found. This parameter can be used e.g. in hierarchical cell structures that the MS initially selects an umbrella cell.

CELL_RESELCT_PARAM_IND_ - Phase 2 Reselection Parameter Indication


BTS CRESPARI 0...1 - -

CELL_RESELECT_PARAM_IND=1:

The cell reselection parameters CELL_RESELECT_OFFSET, TEMPORARY_OFFSET and PENALTY_TIME used for the C2 criterion as well as the parameter CELL_BAR_QUALIFY are broadcasted on the BCCH. These parameters are taken into account by phase 2 MSs, but are ignored by phase 1 Mss.

CELL_RESELECT_PARAM_IND = 0:

The cell reselection parameters and CELL_BAR_QUALIFY are not broadcasted on the BCCH. A phase 2 MS then uses the value 0 for all these parameters, i.e. C1 = C2.


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PENALTY_TIME - Time to apply a negative Offset to C2 of a Neighbor Cell


BTS PENTIME 0...30 and 31: 1 20 sec

PENALTY_TIME = 20 sec + PENTIME ∗ 20 sec

A timer T is started in the MS for each cell in the list of the 6 strongest neighbor cells as soon as it is placed on the list. T is reset to 0 if the cell is removed from the list. During Penalty Time (T < PENALTY_TIME) a negative TEMPORARY_OFFSET is applied to the C2 of the respective neighbor cell

C2 = C1 + CELL_RESELECT_OFFSET - TEMPORARY_OFFSET

which is removed after Penalty Time (T > PENALTY_TIME):

C2 = C1 + CELL_RESELECT_OFFSET.

PENALTY_TIME = 31:

C2 = C1 - CELL_RESELECT_OFFSET.

For PENALTY_TIME = 31 the priority of a neighbor cell for reselection is permanently reduced.

TEMPORARY_OFFSET


BTS TEMPOFF 0...7 1 10 dB

7: infinity

Subtracting TEMPORARY_OFFSET from CELL_RESELECT_OFFSET reduces the priority of a cell in the list of strongest neighbor cells, i.e. during run time of the timer the corresponding neighbor cell is effectively barred for cell reselection.



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CELL_RESELECT_OFFSET


BTS CRESOFF 0...63 1 2 dB

Adding CELL_RESELECT_OFFSET increases the priority of a cell in the list of strongest neighbor cells when the timer has expired.

This mechanism may be applied in hierarchical cell structures to keep fast moving mobiles in the umbrella cells and slow moving mobiles in the micro cells: When a mobile reaches the coverage area of a (neighbor) micro cell, given by the C1 criterion, this cell becomes effectively excluded from reselection during the PENALTY_TIME.

A fast moving mobile is assumed to have left the coverage area of the micro cell before PENALTY_TIME is reached and hence the micro cell is not selected. In contrast, a slow moving mobile is assumed to be still within the coverage area of the micro cell when PENALTY_TIME has expired. Applying the positive CELL_RESELECT_OFFSET, this cell is selected with preference.


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2.3 Cell reselection to UMTS

The inter-system cell reselection from GSM to UMTS allows a MS to reselect to the UMTS system.

The cell reselection feature is important to guarantee a better quality connection when the mobile detects a stronger signal from a neighbor cell, even if this cell belongs to another Radio Access Technology (RAT).

If an FDD cell is suitable, the following two conditions must be fulfilled:

1. For a period of 5 seconds, the measured RSCP value must exceed the RLA of the serving GSM and of all non-serving GSM cells by the value FDD_Qoffset.

2. The measured Ec/No of the FDD cell must be greater or equal than the value FDD_Qmin broadcast in the serving cell.

The principles of the cell reselection from GSM to UMTS are displayed in the figure below.



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5sec

Qsearch_I

start WCDMA

measurements

RLA of serving GSM cell

RSCP of WCDMA

neighbor cell

RLA of (non serving) GSM

neighbor cell

Cell Reselection

(only if CPICH Ec/No

is greater or equal

FDD_Qmin)

FDD_Qoffset

Time

Fig. 4 Principle of the GSM to UMTS cell reselection


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2.4 Parameters

Parameter Range (default) Meaning

QSRHI

UMDB98, UMDB94, UMDB90, UMDB86, UMDB82, UMDB78, UMDB74, ALWAYS, OMDB78, OMDB74, OMDB70, OMDB66, OMDB62, OMDB58, OMDB54, NEVER

Default = NEVER

This attribute defines a threshold which enables the searching for 3G cells with regard to cell reselection. It indicates, when the measurements of 3G cells have to be initiated.

FDDQMI

MDB20, MDB19, MDB18, MDB17, MDB16, MDB15, MDB14, MDB13, NULL,

Default = NULL

The FDDQMI threshold defines a minimum CPICH Ec/No for a WCDMA cell for cell re-selection.

FDDQO

ALWAYS, MDB28, MDB24, MDB20, MDB16, MDB12, MDB08, MDB04, DB00, DB04, DB08, DB12, DB16, DB20, DB24, DB28

Default: DB00

The FDDQO defines an offset value, which has to be added to the RLA of the serving and non-serving GSM cells. The result is compared with the RSCP value of the WCDMA cell.



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3 Handover


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3.1 General notes on handover

The handover algorithm is the most important algorithm in cellular mobile communications.

Its main objectives are:

� maintenance of connection in case of cell change (movement)

� channel change in case of severe disturbance (interference)

� design of cell borders and radio network structure.

Steps of the Handover Process

The handover process can be divided into several sub-processes listed in the table below together with the network elements involved within the respective process.

No. Sub-process Involved Network Element

1. Measurements

„link quality“ serving cell

received level neighbor cells

traffic load

MS, BTS

MS

BTS

2. Measurement Preprocessing BTS

3. Neighbor cell book-keeping BTS

4. Handover Decision BTS

5. Target Cell Generation BTS

6. Target Cell Evaluation

intra BSS handover

inter BSS handover

BSC

MSC

7. Selection of new channel BSC

8. Handover execution MS, BTS, BSC, MSC



27

Types of Handover

Different types of handover can be distinguished with respect to the changed region: a cell, a BSS area or an MSC area. These are illustrated in the figure below. The different types of handover can enabled or disabled by several flags.

BSC 1b

BSC 1a

MSC 1

MSC 2

1.Intracell Handover

2.Intra-BSS Handover

3.Intra-MSC Handover

4.Inter-MSC Handover

BSC 24

3

1

2

Fig. 5 Types of handover


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Handover Causes

Two criteria groups of different handover causes are defined:

Radio Criteria 1 received quality (too low/bit error rate too high)

inter-/intracell HO

2 received level (too low) intercell HO

3 received UL-level (too low) /fast measurement

intercell HO

4 MS-BS distance (too high) intercell HO

5 better cell (power budget: relative received level)

intercell HO

Network Criteria 6 serv. cell congestion > directed retry for call setup

intercell HO

7 traffic load (too high) intercell HO

8 MS-BS distance (too high/low in extended cells)

intracell HO

9 received level or MS-BS distance (too low/high in concentric cells)

intracell HO



29

The first four causes are known as mandatory or imperative causes, i.e. if one of these causes occurs, a hand-over is necessary to maintain the call. This may happen because the MS is leaving the coverage area of the serving cell (intercell handover) or because there is a strong interferer using the same channel in another cell (intracell handover).

The fifth cause is an optional one, i.e. the link quality in the serving cell is sufficiently good, but there are neighbor cells with better received level. Though its not necessary for the link quality of this specific call, there is a benefit for overall network performance to handover the call to the better cell: A call in the better cell causes less interference. Especially, if power control is applied. Since to achieve the same received level in the better cell, a smaller transmit power can be used in this cell.

In a well planned radio network “better cell” should be the overwhelming handover cause. Hence, the locations of a “better cell” handover determine the cell “boundaries”.

The sixth cause is named forced handover because it is triggered by the BSC due to a congestion situation, and not due to radio conditions on the link. This handover (directed retry) is performed from a SDCCH in the congested cell to a TCH in a neighbor cell during call setup.

The seventh cause is named 'Handover decision due to BSS resource management'. The criteria (traffic load) are evaluated in the BTS; if a BTS detects a high traffic load, a handover cause is triggered and an intercell handover execution starts.

The last two causes are intracell handovers in special cell configurations:

� in extended cells handovers are feasible from single to double timeslots and vice versa.

� In a concentric cell handovers are performed between the inner and complete area.

These handover causes can be enabled/disabled separately by corresponding flags.


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Flags to enable/disable Handover Types and Causes due to Radio Criteria

The flags to enable/disable the different handover types and causes are listed in the table below.

They are administered in the object HAND with the range TRUE / FALSE.

Specification Name DB Name Meaning

EN_INTER_HO INTERCH Flag to enable/disable all handover types and causes except for intracell handover.

EN_INTRA_HO INTRACH Flag to enable/disable intracell handover.

EN_BSS_INTER_HO LOTERCH Flag to enable/disable a BSS internal intercell handover, i.e. if disabled, the handover is handled as an inter BSS handover even if the first cell in the target cell list belongs to the same BSS as the serving cell.

EN_BSS_INTRA_HO LOTRACH Flag to enable/disable a BSS internal intracell handover, i.e. if disabled, the handover is handled as an inter BSS handover and the MSC is involved.

EN_RXQUAL_HO RXQUALHO Flag to enable/disable intercell handover due to quality.

EN_RXLEV_HO RXLEVHO Flag to enable/disable intercell handover due to level.

EN_DIST_HO DISTHO Flag to enable/disable intercell handover due to distance.

EN_PBGT_HO PBGTHO Flag to enable/disable better cell (power budget) handover.

EN_TRAFFIC_HO TRFHOE Flag to enable/disable intercell handover due to BSS resource management criteria

EN_FUL_HO EFULHO Flag to enable or disable the Fast Uplink Handover

EN_INTER_SDCCH_HO

IERCHOSDCCH

Flag to enable or disable intercell SDCCH Handover

EN_INTRA_SDCCH_HO

IRACHOSDCCH

Flag to enable or disable intracell SDCCH Handover



31

EN_UMTS_HO EUHO Flag to enable or disable Intersystem Handover to UMTS

EN_UMTS_BETTER_CELL_HO

EUBCHO Flag to enable/disable better cell (power budget) handover to UMTS

EN_UMTS_SUFF_COV_HO

EUSCHO Flag to enable/disable sufficient coverage handover to UMTS

EN_UMTS_IMPERATIVE_HO

EUIMPHO Flag to enable/disable imperative handover to UMTS

EN_UMTS_SDCCH_HO

EUSDCHO Flag to enable/disable directed retry to UMTS

Comments

� Enabling BSS internal handover has the following advantages:

reduction of signaling load on the A-interface

reduction of processing load in the MSC

faster handover execution.

Consequences:

BSS internal handover should be enabled,

BSS regions should be adapted to traffic flows to reduce the inter-BSS handover rate.

� Normally, intracell handover should be enabled to allow a handover from a channel with high interference to another one with less interference within the same cell. However, if random frequency hopping (see chapt. 6.2) is applied, it may be reasonable to disable intracell handover since interference is approximately the same on all channels and no improvement can be achieved by intracell handover.

� If distance handover is disabled, an MS could largely exceed the planned cell boundaries in the case of favorable radio conditions at the serving cell without causing a handover. As a consequence, neighboring cells may suffer from excessive interference produced by this MS. Furthermore, there is a risk that link quality decreases very suddenly (turn around a corner), i.e. there is the risk of a call drop. Hence, distance handover should be switched on.

� If power budget handover is disabled, no handovers with cause “better cell” are generated. Nevertheless, power budget is calculated and evaluated for the ranking of neighbor cells within the target cell list, which also has to be compiled for mandatory handovers (Ch. 4.2.4).


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3.2 Measurement preprocessing

Measurement Values:

The following parameters are measured and calculated each SACCH multiframe (0.48 s):

RXQUAL:

It is defined according to GSM TS 05.08 as function of the bit error rate (BER) before channel decoding:

RXQUAL = 0 : BER < 0.2% assumed value: 0.14%

RXQUAL = 1 : 0.2% < BER < 0.4% assumed value: 0.28%






RXQUAL = 7 : 12.8% < BER assumed value: 18.01%

The RXQUAL values are measured on the dedicated channel for the uplink as well as for the downlink for each TDMA frame (100 frames) within an SACCH multiframe. The measured RXQUAL values are averaged over the respective SACCH period using the assumed values of the table above. The resulting RXQUAL value is the one used within the handover algorithm in the way described below.

RXLEV:

The received level is measured on the dedicated channel for the uplink as well as for the downlink for each TDMA frame (100 frames) within an SACCH multiframe. The measured level values in [dBm] are averaged over the respective SACCH period. The average value is mapped on an RXLEV value using the table below (refer to GSM TS 05.08):

RXLEV = 0: RXLEV ≤ - 110 dBm

RXLEV = 1: - 110 dBm < RXLEV ≤ - 109 dBm



RXLEV = 63: RXLEV > - 48 dBm



33

RXLEV_NCELL(n):

The mobile measures the level received on the BCCH frequency of each neighbor cell n. The mapping is as for RXLEV above.

MS_BS_DIST:

The distance MS_BS_DIST between the MS and BS is calculated from the timing advance (TA) value measured by the BS and is coded as follows:

MS_BS_DIST = 0, 1, ... 35. Distance[Km].

Aspects of Discontinuous Transmission

When Voice Activity Detection (VAD) and Discontinuous Transmission (DTX) is applied not all TDMA frames within a SACCH multiframe may be transmitted. Hence, RXQUAL and RXLEV measurement values (SUB values) for the corresponding SACCH frames are less reliable than those for that SACCH with no silence period (FULL values). Therefore SUB and FULL values have to be distinguished within measurement preprocessing (see below).

SACCH Multiframe Occupancy

DTX not applied: 100 slots not idle

DTX applied (silence periode): 12 slots not idle

Silence description burstSACCH burstspeech burstidle slot

Fig. 6


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Measurement Values for Handover (Summary)

Measurement Range Measurement Type Description

RXLEV_DL_FULL 0 - 63 Received signal level on TCH/SDCCH (full set of TDMA frames) downlink

RXLEV_DL_SUB 0 - 63 Received signal level on TCH (subset of TDMA frames) downlink

RXQUAL_DL_FULL 0 - 7 Received signal quality on TCH/SDCCH (full set of TDMA frames downlink

RXQUAL_DL_SUB 0 - 7 Received signal quality on TCH (subset of TDMA frames) downlink

DTX_USED 0.-.1 DTX used/not used on uplink in previous frame

RXLEV_NCELL(1..6) 0 - 63 Received signal level on BCCH of up to 6 neighbor cells (downlink)

BCCH_FREQ_NCELL_(1...6) 0 - 31 BCCH RF channel number of up to 6 neighbor cells (downlink)

BSIC_NCELL (1...6) 0 - 63 Base Station Identity Code of up to 6 neighbor cells (downlink)

RXLEV_UL_FULL 0 - 63 Received signal level on TCH/SDCCH (full set of TDMA frames) uplink

RXLEV_UL_SUB 0 - 63 Received signal level on TCH (subset of TDMA frames) uplink

RXQUAL_UL_FULL 0 - 7 Received signal quality on TCH/SDCCH (full set of TDMA frames) uplink

RXQUAL_UL_SUB 0 - 7 Received signal quality on TCH (subset of TDMA frames) uplink

MS_BS_DIST 0 - 35 Absolute MS-BS distance [km]



35

Illustration of Measurement Preprocessing

The measured (and reported) data per SACCH multiframe are preprocessed within the BTS using a gliding average window. The size of the window can be set separately for RXQUAL, RXLEV, DIST and PBGT. The measured RXLEV_FULL/SUB or RXQUAL_FULL/SUB values are put into the gliding window with a multiplicity (weight) given by the parameter W_LEV_HO or W_QUAL_HO, respectively. This is illustrated in the figure below.

DTX enabled: Averaging of RXLEV

Example: average of RXLEV with a gliding window of size A_LEV_HO = 4

and a weight factor of the full values of W_LEV_HO = 2.

21292923272732

21292327323128

Gliding Window

average value = 27

Measurement Values each

SACCH Multiframe (0.48 s)

RXLEV_SUB (weight 1) RXLEV_FULL (weight 2)

Fig. 7 Illustration of weighting and averaging of measurement values


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Fast Uplink Handover BR 6.0

In areas of critical radio conditions, situations might occur in which a call drops because of long handover decision times.

Fast Uplink Handover offers a high speed handover to prevent from rapid uplink level loss. Saves connections in special places with drop call problems, when the power level of a mobile decreases rapidly, a handover can be performed to a predefined cell to save the connection.

n-1

UL/DL reportingperiod; length: 480 ms

Arrival of SACCH report containing a flagindicating if the MS used DTX.Time of arrival in task-context is uncertain.

burst of a SACCH report(4 bursts make up onSACCH)

n n+1

'end of measurements'detected in interruptcontext

Fig. 8 Save of detection time



37

The A_PBGT_HO window size (by default approx. 7.7 s) is not suitable to reflect the current situation related to the event 'Fast Uplink Handover' because it is too long. When a Fast Uplink HO is detected by evaluating the UL measurements (e.g. during the last second), a target cell has to be found that was good during that time (last second) and not during the last 7.7 seconds (during which the measurements of the A_PBGT_HO were averaged).

Thus, a separate average must be calculated for the Fast Uplink HO, which is based on the window-size for the Fast Uplink HO on the received UL-level. The window-size in the Fast Uplink HO's bookkeeping list is one size smaller than that of the Fast Uplink HO averaging window.

The average of the values within the Fast Uplink HO window of the neighboring cells list has to be compiled in order to be able to generate a target cell list, when necessary.


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Parameters for Measurement Preprocessing

The parameters for measurement preprocessing for handover are administered in the object HAND and are listed in the table below.

Specification Name

DB Name Range Meaning

A_QUAL_HO

HOAVQUAL AQUALHO

1-31 Averaging window size for RXQUAL values, used for handover decisions due to RXQUAL

W_QUAL_HO

HOAVQUAL WQUALHO

1-3 Weighting factor for RXQUAL_FULL values

A_LEV_HO

HOAVLEV ALEVHO

1-31 Averaging window size for RXLEV values, used for handover decisions due to RXLEV

W_LEV_HO

HOAVLEV WLEVHO

1-3 Weighting factor for RXLEV_FULL values

A_DIST_HO HOAVDIST 1-31 Averaging window size for Timing Advance values used for handover decisions due to distance.

A_PBGT_HO HOAVPWRB 1-31 Averaging window size used for power budget calculation. Averaging is applied to: RXLEV_DL PWR_C_D RXLEV_NCELL

A_LEV_FUL_HO ALEVFULHO ALEVFULHO

1...31 Averaging window size used for Fast Uplink Handover detection and target cell list generation

W_LEV_FUL_HO ALEVFULHO WLEVFULHO

1...3 Weighting factor used for Fast Uplink Handover detection and target cell list generation



39

Comments:

Range of W_XX: 1, 2, 3; as default value 3 is recommended.

� Range of A_XX: 1...31, Step Size: 1

� The adjustment of the averaging size mainly depends upon rate of change of the radio propagation conditions.

Example:

path loss (change of 3 dB at a distance

of 2000 m)

→ MS movement of ∼400 m

long term fading change of 6 dB → MS movement of ∼5...100 m

short term fading → MS movement of ∼0.15 m

Hence, at the cell border the main variation of received level is due to long and short term fading. Within one SACCH multiframes an MS moves

0.5 m for MS speed = 1 m/s = 3.6 km/h

5.0 m for MS speed = 10 m/s = 36 km/h

Using an averaging window size of 10 SACCH frames, short term fading is averaged for pedestrians (as well as for “fast” moving MSs).

Long term fading is partly averaged for fast moving MSs (the degree of average depends on the exact speed and the correlation length of long term fading, whereas there is nearly no averaging of long term fading for pedestrians.

The setting of the averaging window size has to be a compromise between a fast decision and a reliable decision.

Therefore it is recommended to use a larger window size for the optional handover (better cell) to do not cause a lot of unnecessary handovers and a smaller window size for the mandatory handover causes (quality, level, distance) to be able to react quickly on a sudden decrease of link quality.


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3.3 Handover criteria

Evaluation of handover criteria is based on:

� Up- and downlink measurements of level and quality

� The absolute MS-BS distance

� The power budget criterion of up to 32 neighbor cells

� BSC-trigger to answer with a HO Condition Indication message

Two types of handover may occur:

� Intercell HO requests the allocation of a dedicated channel outside the serving cell

� Intracell HO requests the allocation of another dedicated channel within the serving cell

GSM distinguishes three classes of handover criteria:

� Power budget HO as ‘normal criterion’

� All other TCH-causes as ‘alarm- or imperative criteria’

� Forced HO triggered by the BSC

On TCHs it is possible that the condition for more than one handover cause is fulfilled. Therefore it is necessary to rank the evaluation of handover causes. On SDCCH where only one cause is evaluated, no ranking is necessary.

Static ranking is performed according a priority list.

Priority Handover Cause HO type HO class evaluated on

Extended Cell Handover intracell imperative TCH 1

Concentric Cell Handover intracell imperative TCH

2 Quality Intercell Handover intercell imperative TCH

3 Level Handover intercell imperative TCH

4 Distance Handover intercell imperative TCH

5 UMTS Sufficient Coverage Handover

intercell normal TCH

6 Power Budget Handover intercell normal TCH

7 Quality Intracell Handover intracell imperative TCH

8 Traffic Handover intercell normal BTS

- Forced Handover intercell forced SDCCH

- Fast Uplink Handover intracell imperative UL-TCH

Handover causes priorities



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Handover Criteria Evaluation - Static Ranking

Evaluate HO

criterion

priority 1

HO

detected

Evaluate HO

criterion

priority 2

HO

detected

HO

detected

Evaluate HO

criterion

priority 3

Until evaluation

of HO criterion 7

N

Y

N

Y

N

Y

Fig. 9


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Handover Criteria Evaluation - Dynamic Ranking

In some cases it is necessary to rank priorities dynamically:

in case a extended cell HO (single/double timeslot) or an Concentric Cell HO (inner/complete area) was requested but could not be performed due to lack of resources, evaluation of these criteria is skipped the next time to make evaluation of other HO criteria possible (e.g. Quality, Level etc.).

In case the following HO-attempt is also unsuccessful or no other HO can be detected then the skipped priority is enabled again for the next try. BTS toggles.

The following flow chart shows in principle the dynamic ranking mechanism of Extended Cell HO/Concentric Cell HO:



43

Dynamic ranking of HO evaluation is also performed in case of Quality inter-/intracell HO (see chapter 0 Handover detection algorithms).

Skip evaluation

of HO criterion

(slip_flag set)

Reset skip_flag

Y

Y

N

NHO

condition

fulfilled

Set skip_flag

HO detected

Fig. 10


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3.4 Handover detection algorithms

3.4.1 Radio criteria

The standard handover algorithm for radio criteria uses the decision criteria listed in the table below where the order of processing within the overall handover algorithm is used (see Fig. 9).

These criteria will be modified for a speed sensitive handover used within hierarchical cells (refer to reg. 4.2).

Handover Causes Decision Criteria

Intercell HO due to Quality 1. RXQUAL_XX > L_RXQUAL_XX_H

2. RXLEV_XX < L_RXLEV_XX_IH *

3. XX_TXPWR = Min ( XX_TXPWR_MAX, P )

HO due to Level 1. RXLEV_XX < L_RXLEV_XX_H

2. XX_TXPWR = Min ( XX_TXPWR_MAX, P )

HO due to Distance 1. MS_BS_DIST > MS_RANGE_MAX

HO due to Power Budget 1. RXLEV_NCELL(n) > RXLEV_MIN(n) + Max ( 0, MS_TXPWR_MAX(n) - P )

2. PBGT(n) > HO_MARGIN(n)

Intracell HO due to Quality 1. RXQUAL_XX > L_RXQUAL_XX_H

2. RXLEV_XX > L_RXLEV_XX_IH

Fast HO due to UL-Level 1. RXLEV_UL < THR_RXLEV_FAST_UL_HO

* skip flag not set

Notes:

� XX: used as variable for both UL (uplink) and DL (downlink)

� MS_TXPWR_MAX: maximum allowed transmit power of the MS in the serving cell,

� MS_TXPWR_MAX(n): maximum allowed transmit power of the MS in the adjacent cell “n“

� P [dBm]: the maximum power capability of the MS (power class)

� An intercell handover due quality or level is only performed if the transmit power of the MS or BS respectively is on its maximum.



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Dynamic Ranking of Quality Inter-/Intracell Handover

When criteria for Quality Intercell HO are evaluated it is first checked if any Quality handover shall be performed (averaged UL/DL measurements show a high signal quality value). Then the criterion of Quality Intracell handover is evaluated. It’s obvious that Quality Intercell HO is detected when the criterion of Quality Intracell HO is not given.

In case a Quality Intracell HO was requested but could not be performed (HO_FAILS) than Quality Intercell HO is tried the next time (if criterion is still given). When this Intercell HO attempt also fails then BTS toggles back and tries again an Intracell HO.

To initiate a Quality Intercell HO after an unsuccessful Quality Intracell HO attempt, a skip_flag is used. This skip_flag is set after an Intracell HO attempt and is processed in case of Quality Intercell detection.

When the criterion of Quality HO is given and the skip_flag for Intracell HO is not set, it is possible to perform a Quality Intracell HO if the condition is fulfilled. In this case the evaluation of handover criteria is resumed with the next priority level.

When the criterion of Quality HO is given and the skip_flag for Intracell HO is set, an unsuccessful Intracell HO has been performed before. The dynamic ranking mechanism forces an Intercell HO instead of the previous Intracell HO by skipping the evaluation for Quality Intracell HO condition. The skip_flag is reset again to make a future Intracell HO evaluation possible in case this Intercell HO attempt also fails. BTS toggles between intracell and intercell handover.


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Limitation of Intracell HO Repetition

The BTS is informed by the BSC that a number of consecutive and successful Intra Cell HO over the same connection have been performed. Any further subsequent Intracell HO has to be disabled for a defined period of time, but if criteria are given during penalization time a Intercell HO may be tried instead. The O&M flag ‘Enable_Limitation_Intracell_HO’ is used to enable/disable this feature.

The attribute ‘Max_Intracell_HO’ specifies the maximum number of consecutive successful quality intracell handovers, which are permitted in the same BTS for a single connection. The next intracell HO is suspended when the threshold Max_Intracell_HO+1 is reached, until the ‘Timer_No_Intracell_HO’ expires.

Parameters for Intracell handover limitation

Specification Name Object DB Name Range Step Size

Unit

Enable_Limitation_ Intracell_HO

HAND ELIMITCH TRUE/ FALSE

- -

Max_Intracell_HO HAND MAIRACHO 1 .. 15 1 -

Timer_No_Intracell_HO HAND TINOIERCHO 1 .. 254 1 1 sec

The following flow charts show in principle the evaluation of Quality HO criterion (incl. dynamic ranking mechanism of Quality inter-/ intracell HO and limitation of intracell HO repetition).



47

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Dynamic Ranking of Quality Inter-/Intracell Handover

Max. power

Y

Y

Condition for

Quality HO

fulfilled

reset skip_flag

Quality Intercell

HO detected

N

N

Y

Skip Intracell HO

(skip_flag set)

N

Y RXLEV <

L_RXLEV_IH

N

Y

Fig. 11


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Dynamic Ranking of Quality Inter-/Intracell Handover & Limitation of Intracell HO Repetition

Y

set skip_flag

Quality Intracell

HO detected

N

Intracell HO

condition fulfilled

N

Y

Skip Intracell HO

(skip_flag set)

Y

N

Limit. of

intracell HO rep:

penalization time

active

set skip_flag

Fig. 12



49

Power Budget:

PBGT(n) = RXLEV_NCELL(n) - ( RXLEV_DL + PWR_C_D )

+ Min( MS_TXPWR_MAX, P ) - Min( MS_TXPWR_MAX(n), P )

> HO_MARGIN(n)

RXLEV_DL: averaged value of the measured downlink level in the serving cell,

PWR_C_D: BS_TXPWR_MAX [dBm] - BS_TXPWR [dBm]

Averaged difference between the maximum downlink RF power BS_TXPWR_MAX and the currently used downlink power BS_TXPWR (due to power control) in the serving cell.

RXLEV_NCELL(n)

: Averaged value of the measured downlink level of the adjacent cell “n”

HO_MARGIN(n): Handover margin; if path loss with respect to the serving cell exceeds the path loss with respect to the adjacent cell “n” by this margin, the adjacent cell is considered as the (much) better cell.

Loss(serving) - Loss(adjacent) =

= BS_TXPWR - RXLEV_DL - (BS_TXPWR_MAX(n) - RXLEV_NCELL(n))

= RXLEV_NCELL(n) - (RXLEV_DL + PWR_C_D)

+ BS_TXPWR_MAX - BS_TXPWR_MAX(n).

Assumption:

BS_TXPWR_MAX - BS_TXPWR_MAX(n) = MS_TXPWR_MAX - MS_TXPWR_MAX(n)

� if the link budget of the serving cell is designed for MSs of P = MS_TXPWR_MAX &

� if the link budget of the adjacent cell(n) is designed for MSs of P = MS_TXPWR_MAX(n).


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Back Handover Prevention

Back-handover (due to power budget) prevention is triggered by BSC by including the old cell and the previous handover cause (GSM 08.08 Cause) in the Channel activation message. A timer is started in BTS and until the timer for the specified cell expires,

� no handover condition due to Power Budget will be evaluated for this cell

� this cell is excluded from the target cell list in case of any other handover request due to Power Budget.

The O&M flag ‘Enable_No_Back_HO’ is used to enable/disable this feature.

Back handovers due to imperative criteria are not affected by this mechanism.

Parameters for Back Handover Prevention


Unit

Enable_No_Back_HO HAND NOBAKHO TRUE/FALSE - -

Timer_Inhibit_Back_HO ADJC TINHBAKHO 1 .. 254 1 1 sec

General Information Flow

Cell A

Cell B

In case of PBGT-HO:

Cell A is not included

in TCL

HO Cond Ind (B, C, D)

Chan Activ in B (Cell A, HO-cause)

BTS BSC

Fig. 13



51

Parameters of Handover Decision

Specification Name DB Name/ Object

Range Meaning

L_RXQUAL_DL_H L_RXQUAL_UL_H

HOLTHQUDL HOLTHQUUL

/HAND

0...7 Thresholds for downlink/uplink quality. If RXQUAL is above these thresholds, the received level is low and the transmit power has reached its maximum, a quality intercell handover is initiated.

L_RXLEV_DL_H L_RXLEV_UL_H

HOLTHLVDL HOLTHLVUL

/HAND

0...63 Thresholds for downlink/uplink level. If RXLEV is below these thresholds and the transmit power has reached its maximum a level handover is initiated.

L_RXLEV_DL_IH L_RXLEV_UL_IH

HOTDLINT HOTULINT

/HAND

0...63 If the quality falls below a threshold, but the received level is high, higher than L_RXLEV_XX_IH, an intracell handover is initiated.

MS_RANGE_MAX HOTMSRM

/HAND

0...35 If the measured timing advance value is above this threshold, a distance handover is initiated in a standard cell. Unit: 1Km

MS_RANGE_MAX_EXT HOTMSRME

/HAND

35...100 If the measured timing advance value is above this threshold, a distance handover is initiated in an extended cell. Unit: 1Km

THR_RXLEV_FAST_UL_HO

THLEVFULHO/ HAND

0...63 Receive signal strength threshold for an intercell Fast Uplink HO decision


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Range Meaning

MS_TXPWR_MAX MSTXPMAXGSM MSTXPMAXDCS MSTXPMAXPCS

/BTS

2...15 0...15 0...15, 30, 31

Maximum TXPWR an MS is allowed to used in the serving cell

GSM: 2..15 = 39..13 dBm

DCS: 0..15 = 30..0 dBm

PCS: 0..15 = 30..0 dBm

30 = 31 dBm, 31 = 33 dBm

MS_TXPWR_MAX (n) MSTXPMAXGSM MSTXPMAXDCS MSTXPMAXPCS

/BTS /TGTBTS

2...15 0...15 0...15, 30, 31

Maximum TXPWR an MS is allowed to use in the neighbor cell n

2 = 39 dBm, 15 = 13 dBm (GSM)

0 = 30 dBm, 15 = 0 dBm (DCS)

RXLEV_MIN(n) RXLEVMIN

/ADJC

0...63 The level received from a neighbor cell n has to exceed this threshold

� to initiate a better cell handover to that neighbor cell

� to include this cell in the target cell list for a mandatory handover.

HO_MARGIN(n) HOM /ADJC

0...126 The path loss difference between serving and adjacent cell has to exceed this margin for a better cell handover. – 63 dB ... + 63 dB



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Handover Region

XX=UL : Uplink

Bit-Error-Rate (RXQual)

Level (RXLev)

Handover

L_RXLev_XX_H L_RXLev_XX_IH

L_RXQual_XX_H

Inter-cell handover

due to power budget

/

Inter-cell handover

due to level

HOLTHQUXX

HOLTHLVTXX HOTXXINT

XX=DL :

Downlink

XX UL U li k

Inter-cell handover

due to quality

(if skip flag=TRUE or

if INTRACH=FALSE)

Intra-cell handover

due to quality

(if skip flag=FALSE)

Inter-cell handover

due to quality

(skip flag not evaluated)

Fig. 14 Regions of handover defined by quality and level thresholds


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Handover Scenarios

For adjusting the handover thresholds one has to distinguish two scenarios:

A) Noise Limited Scenario:

Large cells (in rural area) with low traffic load: received level at the cell border not much above the receiver limit sensitivity level.

B) Interference Limited Scenario:

“Small” cells (in urban area) with high traffic load: received level at cell border significantly exceeds the receiver sensitivity level, but C/I not much above the reference interference sensitivity.

In any case intercell handover due to quality should be avoided as far as possible, i.e.

� set L_RXQUAL_XX_H to highest value for acceptable speech quality,

� set L_RXLEV_XX_IH to a appropriate value so that in case of low RXQUAL an intracell handover is initiated for the locations within the cell area defined by the other thresholds.

Scenario A:

Main handover criterion is the level criterion and L_RXLEV_XX_H has to be set to a value just some dBs above the receiver limit sensitivity level.

Furthermore, there should be a hysteresis between the threshold RXLEV_MIN for incoming handover and the corresponding one for outgoing handover L_RXLEV_XX_H to avoid a lot of unnecessary forward and backward handover:

RXLEV_MIN - L_RXLEV_XX_H = level hysteresis > 0.

The order of magnitude for the level hysteresis is given by the standard deviation of the long term fading, i.e. RXLEV_MIN > L_RXLEV_XX_H + 4 ... 10 dB.

Scenario B:

In this scenario the better cell criterion should be the main handover criterion, since

� it is the most suitable criterion for designing well defined cell borders,

� it guarantees that the mobile is served by the cell with (nearly) the lowest path loss and therefore offers the greatest potential for power control to reduce interference.

To avoid a lot of unnecessary forward and backward power budget handover caused by long term fading fluctuations of the received levels from the respective BTSs, a hysteresis has to be introduced:

HO_MARGIN(cell1 -> cell2) + HO_MARGIN(cell2 -> cell1)

= power budget hysteresis > 0.

Usually, the handover margin is chosen symmetrically; its value should be a compromise between ideal power budget handover (low value) and a low rate of forward and backward handovers (high value).


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By choosing unsymmetrical values for the handover margin, one can adapt the cell area to the traffic load, e.g. increasing HO_MARGIN(cell1 -> cell2) while keeping the power budget hysteresis constant (i.e. reducing HO_MARGIN(cell2 -> cell1) by the same amount), increases the effective area of cell 1 while reducing that of cell 2).

RXLEV_MIN(n) should be set to a value so that RXLEV_NCELL(n) > RXLEV_MIN(n) for almost all locations where PBGT(n) > HO_MARGIN(n), i.e. a better cell handover is really initiated if the power budget condition is fulfilled.

This means that there should be an overlap of the outgoing power budget area of one cell and the incoming RXLEV_MIN area of the neighbor cell n.

Furthermore, as for scenario A, there should be a level hysteresis between RXLEV_MIN and L_RXLEV_XX_H.

This is illustrated for an ideal situation without long term fading in the figure below:



57

L_RXLEV_XX_H

ideal power budget

cell border

RXLEV_MIN

BTS3

BTS1 BTS2

Fig. 15 Cell borders defined by handover thresholds

BTS

receiver limit sensitivity

RXLEV_MIN (incoming HO)

L_RXLEV_XX_H (outgoing level HO)

Fig. 16 Relation between handover level thresholds


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3.4.2 Network criteria

3.4.2.1 Extended cell handover

Maximum propagation delay within one timeslot allows a maximum BS-MS distance of 35 km. In “extended cells“ the operator can configure TCHs optionally as double timeslot channels where two subsequent timeslots are used for transmission to provide coverage farther than 35km.

Extended Cell Handover is the intracell handover between a single timeslot channel and a double timeslot channel and vice versa. Handover detection is based on comparison of actual BS-MS distance with a threshold (O&M parameter).

Extended Cell handover can be enabled/disabled via O&M flag ‘ENABLE_EXTENDED_CELL_HO’ only if there are double and single timeslots configured in the cell.

Parameters for Extended Cell Handover


Unit

ENABLE_EXTENDED_CELL_HO HAND EXTCHO TRUE/FALSE - -

HO_MS_TA_MAX HAND HOMSTAM 0 .. 34 1 km

HO_MARGIN_TA HAND HOMRGTA 0 .. 35 1 km

A single-to-double handover is detected when the actual BS-MS distance exceeds the threshold ‘HO_MS_TA_MAX’ (handover alarm distance_near_far); a double-to-single handover is detected when the actual BS-MS distance falls below the threshold minus a hysteresis factor ‘HO_MARGIN_TA’ (handover alarm distance_far_near).

Following conditions must be fulfilled for a extended cell handover:

� Enable_Extended_Cell_HO = TRUE

� skip_flag = not set (see dynamic ranking).

The following additional condition must be fulfilled for a single to double timeslot handover:

� actual BS-MS distance > HO_MS_TA_MAX

or the following additional condition must be fulfilled for a double to single timeslot handover:

� actual BS-MS distance < HOMSTAM - HOMRGTA

(no double to single HO will be performed in case of HOMSTAM - HOMRGTA < 0 ).



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3.4.2.2 Concentric cell handover

Concentric Cells means two logical cells within one GSM/DCS cell: Inner area and complete area. The TX power of the TRX’s belonging to the inner area is strongly reduced, which results in a lower radius.

Inner Area

Complete Area

Fig. 17 Cell configured as concentric


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This feature can be used to introduce an additional frequency re-use pattern for the frequencies in the inner area, i.e. a new frequency re-use pattern can be used with a shorter re-use distance. This will increase the network capacity without appreciable increase of interference.

A flag is used to define whether the cell is a concentric one or not. Furthermore it has to be specified per TRX whether this TRX should belong to the inner area or to the complete area. One set of signaling channels -BCCH & SDCCH- in the complete area is used for both areas.

Objects and parameters in SBS for installation of concentric cell configuration:

Object DB Name Range Meaning

BTS CONCELL TRUE/ FALSE

Flag indicates whether concentric cell configuration is used or not

TRX PWRRED 0...6 Static reduction of TRX output power: BS_TXPWR_MAX = PBTS - 2 * PWRREDunit: 2 dB

TRX TRXAREA NONE INNER COMPLETE

This parameter specifies the area the TRX belongs to



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Intracell handovers between the two areas are depending on field strength level and the location of the mobile. These handovers are executed on level / distance conditions defined by appropriate thresholds in the handover package. Moreover, during the call setup procedure in a concentric cell the same values are also evaluated to determine whether the call is set up on a TCH belonging to an inner or complete area TRX

Parameters for Concentric Cell handover behavior

EN_CON_CELL_DIST


HAND CCDIST TRUE/ FALSE

Flag to enable/disable whether the distance should also have an impact on the intracell handover decision in addition to the HORXLVDLI / HORXLVDLO within the concentric cell intracell handover algorithm.

HO_RXLEV_DL_INNER


HAND HORXLVDLI 0...63 1 1 dB

This attribute defines the receive signal strength threshold on downlink which is evaluated for the intracell handover from a TRX belonging to the inner area to a TRX belonging to the complete area of a concentric cell.

The condition, which must be fulfilled to perform an intracell handover from inner- to complete area, is:

RXLEV_DL(serv. cell) < HORXLVDLI

The RXLEV_DL(serv. cell) is the averaged value derived from the handover measurement preprocessing; refer to chapter 3.2.2.


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HO_RXLEV_DL_OUTER


HAND HORXLVDLO 0...63 1 1 dB

This attribute defines the receive signal threshold level on downlink which is evaluated for the intracell handover from a TRX belonging to the complete area to a TRX belonging to the inner area of a concentric cell, furthermore the TCH assignment is influenced by this threshold.

The condition, which must be fulfilled, to perform an intracell handover from complete- to inner area, or to assign a TCH directly in the inner area is:

RXLEV_DL(serv. cell) > HORXLVDLO

HO_CON_CELL_DIST


HAND HOCCDIST 0...35 1 1 Km

This attribute specifies the distance limit between inner and complete area of a concentric cell which is taken into account for the intracell handover in addition to the attribute hoRxlevDinner respectively hoRxLevDlouter.

The condition, which must be fulfilled to perform an intracell handover from inner- to complete area, is:

RXLEV_DL(serv. cell) < HORXLVDLI or

BS_MS_DIST > HOCCDIST (if CCDIST=TRUE)

The condition, which must be fulfilled, to perform an intracell handover from complete- to inner area, or to assign a TCH directly in the inner area is:

RXLEV_DL(serv. cell) > HORXLVDLO and

BS_MS_DIST < HOCCDIST (if CCDIST=TRUE)



63

EN_INNER_INNER_HO


HAND ININHO TRUE/ FALSE

Flag to enable/disable the intercell handover from inner to inner area in sectorized concentric cells.

COLOCATED_CELL


HAND CCELL1, CCELL2

pathname, e.g. BTSM:0/BTS:2 BTS ID

The attribute defines the cells belonging to the sectorized concentric cells for which it is possible to perform an intercell handover into the inner area.

The condition, which must be fulfilled to perform an intercell handover into the inner area of a collocated neighbor cell, is:

RXLEV_NCELL(adj. cell) > HORXLVDLO(n) and

BS_MS_DIST < HOCCDIST(n) (if CCDIST(n)=TRUE)

In this case the BSC sends a message: 'preferred_area_request' to the serving BTS together with the relevant parameters of the target cell. The BTS calculates whether a intercell handover directly into the inner area is possible or not. The BSC is informed with message: 'preferred_area' and assigns an appropriate TCH.

To avoid handover oscillation between inner and complete area, the following recommendations have to be considered:

HORXLVDLO > HORXLVDLI

HORXLVDLO - HORXLVDLI > BS_TXPWR_MAX(Complete) - BS_TXPWR_MAX(Inner)


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Common BCCH for GSM 900/1800 Dual Band Operation

This feature enables operators to be able to configure a single cell with the frequencies required for both GSM900 and GSM1800 bands, while having a common BCCH in only one of the bands.

A dual band cell using a common BCCH has to be configured as a concentric cell as it is described in the previous pages.

The BCCH can either belong to the GSM900 or the GSM1800 band. The operator can configure a single cell with frequencies in both GSM900 and GSM1800 bands while the cell still has one common cell identity. In order to optimize signal reception within the concentric cell feature the BTS selects an intracell handover from the complete area (GSM900) to the inner area (GSM1800), or vice versa.

In a single-band concentric cell the intracell HO decision is made by analyzing the downlink received level and -if applied- the measured MS-BS distance, the uplink conditions are not important because the mobile is not affected when the call is handed over between the two areas inside the cell

In contrast to this, in a dual-band concentric cell when the mobile is moved from the GSM area to the DCS one or vice versa, the uplink conditions change due to the fact that the mobile has different power classes in the two bands.

Therefore the algorithm for intracell HO in a dual band cell from complete to inner area is extended as follows:

RXLEV_DL(serv. cell) > HORXLVDLO + Max [ 0, MS_TXPWR_MAX(inner) - P(inner) ]

A cell using frequencies from different bands must be configured with different parameters to define the maximum allowed MS transmit power per band. Additionally the parameter, which defines the used frequency band in a cell, has to be extended.


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3.4.2.3 Directed retry

Directed retry is the transition (handover) from a SDCCH in one cell to a TCH in another cell during call setup because of unavailability of an empty TCH within the first cell.

Directed retry is a means to control the traffic distribution between cells and to avoid a call rejection because of congestion in one cell. If Queuing of ASS REQs is not supported within a BSC Directed retry is merely triggered by the BSC by sending a Forced HO Request message to the BTS, which has to respond with an "initiated" Intercell HO Cond. Indic. message.

It can happen that the Intercell HO Cond. Indic. message does contain only an empty target cell list (If triggered by a Forced HO Request message the BTS has to send a Intercell HO Cond. Indic. message even if no suitable neighbor cell exists - in this case the target cell list is empty!). In this case a TCH cannot be assigned and the BSC shall not send a HO RQD message to the MSC of course but shall send an ASS FAILURE (cause "no radio resource available").

If the target cell list contains cells from inside and outside the BSC area and if e.g. the first and second cell is inside, the third outside and the fourth inside the BSC area than the Directed retry attempts shall be carried out as BSC controlled Directed retries to the first and second one. If these Directed retries are not possible for any reason (e.g. no empty TCH) than the third attempt and all following attempts (independently whether the fourth and the following cells lie in- or outside the BSC area) shall be executed as MSC controlled directed retry.

If in case of a MSC controlled handover the MS cannot access the new cell and the MSC receives a HO FAILURE (cause "radio interface failure, reversion to old channel") from the old BSS, it can happen that the MSC generally releases this SDCCH connection by sending a CLEAR CMD message (cause "radio interface failure, reversion to old channel") to the BSC independently.

If the MSC does not support Directed retry HO´s the BSC may perform BSC controlled Directed retries (approximately 75% of all Directed retries) only. In this case the EN_INTER_SDCCH_HO flag in the BSC shall be set to "disabled" and the BSC has to check the target cell list of Intercell HO_Cond_Ind messages belonging to a SDCCH connection. All cell identifiers not belonging to the BSC area shall be skipped and if there remain cell identifiers belonging to the BSC area the corresponding HO shall be performed to strongest (if impossible to the second strongest, third strongest etc.) remaining cell. If the target cell list does not contain a remaining or any cell identifier of the same BSC area, this Intercell HO Cond. Indic. message shall be discarded and the BSC shall release this SDCCH connection (Sending of an ASS FAILURE with cause "no radio resource available").



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Flowchart of Directed Retry

Directed Retry

Procedure

Is the incoming

message an

Assignment_REQ ?

BSC sends to BTS

Forced_HO_Request

BTS sends to BSC

Intercell_HO_Cond_Ind

with target cell list

Cell within

BSC area ?

N

Y

N

Y

EN_INTER_SDCCH_HO

set to enable ?

N

Assign TCH

Start

Queueing

Procedure

BSC assigns TCH

internally and sends

Assignmend_Complete

to MSC

Suiteble cell

in list ?

Y

N

Directed Retry towards

MSC successful ?

Y

NY

Fig. 18


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HO Algorithm/Generation of the Target Cell List

The BTS has to send the Intercell HO Cond. Indic. messages toward the BSC. Please note that for Directed retry the sending of an Intercell HO Cond. Indic. message for a SDCCH may only be triggered by a BTS external event: The BSC sends a Forced HO Request because of "no TCH available".

If an Intercell HO Cond. Indic. message is to be sent, the target cell list shall contain all neighbor cells with:

RXLEV > RXLEV_MIN + Max(0, MS_TXPWR_MAX-P) + FHO_RXLEV_MIN_OFFSET

in the order of decreasing values of (PBGT - HO_MARGIN <=> 0). Additional parameters specific to speed sensitive HO shall be taken into account for the ranking of the target cells. Even if no suitable neighbor cell exists, the BTS shall send an Intercell HO Cond. Indic. message. In this case the target cell list ("Cell Identif. List Pref. IE") shall be empty! The cause of the Intercell HO Cond. Indic. message shall be FORCED. FHO_RXLEV_MIN_OFFSET is a cell specific O&M-parameter to select only target cells for forced HO which the MS can access without any problems. It is a result of radio planning for each individual cell. It allows to influence the amount of Forced HO´s failed because of empty target cell list, the amount of HO attempts back to the "old" cell and the success rate of HO ACCESSes to the target cell.

Prevention of "Back-HO´s"

A major general problem of forced HO (Directed retry is one sort of forced HO!) is the probability of HO due to PBGT back to the "old" (congested) cell. Its drawbacks are:

1. increased load at the Abis-interface because of periodic sending of Intercell HO_Cond_Ind messages in intervals of T7

2. increased load at the A-interface in case of inter-BSC-HO because of the same reason

3. additional processor capacity in BSC (and MSC) is required for HO trials for which it is known in advance that they are useless

4. the load in the congested cell will not be reduced for a certain time, but it will be kept at a permanent high level.

For the Channel Activation message a new optional information element "Cell Identifier List (no target)" is defined. This information element contains the cell identifier (CI) of a cell from which a handover request (intra- or inter-BSC) because of forced HO was received. If this information element exists in the Channel Activation message, the BTS

� shall not trigger a (TCH-)HO due to PBGT for the time Tbho if the PBGT condition is fulfilled for the indicated cell only and

� shall not include the indicated CI’s in the target cell list in this case for the time Tbho (i.e. for the condition HO due QUAL/LEV/DIST the indicated cell identifier may be part of the target cell list).



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Tbho is a timer that limits the mentioned prohibitions. It has to be set by O&M command.

If a HO is necessary the target BSC has to generate the Channel Activation message. The target BSC shall insert the Cell Identifier List (no target) IE into this message.

The BSC shall derive the CI for the Cell Identifier List (no target) IE from the stored context in case of intra BSC HO or from the mandatory Cell Identifier (serving) IE of the HO REQ message in case of inter BSC HO with cause "Directed retry".

EN_FORCED_HO


BSC ENFORCHO ENABLE DISABLE

- -

This BSC specific O&M flag allows to enable/disable the sending of Forced HO Request messages for running SDCCH connections (e.g. queued or not queued ASS REQ´s which do not find an empty TCH). It is used to enable/disable Directed retry.

EN_INTER_SDCCH_HO


BSC EISDCCHHO ENABLE DISABLE

- -

This BSC specific O&M flag allows to enable/disable inter BSC SDCCH-HOs (i.e. SDCCH-SDCCH-HO and Directed retry). It simply prevents the sending of HO RQD messages for SDCCH connections to the MSC. This flag should be set to "disable" by an operator if in a network the MSC, which the BSS is connected to or other adjacent BSSs do not support the prevention of "back-HO". If it is set to "disable" the BSC shall skip all cell identifiers of the target cell list of the Intercell HO Cond. Ind. message, which belong to another BSC area.


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FHO_RXLEV_MIN_OFFSET


ADJC FHORLMO 0...24 1 1 dB

FHO_RXLEV_MIN_OFFSET ("RXLEV_MIN offset for forced-handover") is a cell specific O&M-parameter used within the BTS to select only target cells for forced HO which the MS can access without any problems. It is a result of radio planning for each individual cell. It allows to influence the amount of Forced HO´s failed because of empty target cell list, the amount of HO attempts back to the "old" cell and the success rate of HO ACCESSes to the target cell.

Default: 6.

Tbho


ADJC TIMERFHO 1...320 1 10 sec

Tbho (bho=back handover) is a neighbor cell specific O&M parameter. It is the value of a timer running in the BTS that controls the duration how long a former serving cell from which forced HO was performed to the new serving cell may not be considered in the PBGT HO decision algorithm of the new serving cell and may not be contained in the target cell list. It is started at the reception of a Channel Activation message containing a Cell Identifier (no target) IE.

Default: 12.



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3.4.2.4 HO decision due to BSS resource management criteria

The purpose of this feature is to increase the network efficiency by redistributing of traffic between the cells of a BSC area.

The handover for traffic reason has been introduced in order to move calls from heavily loaded cells into adjacent cells with lower utilization. The expected benefit from this feature is to be able to free some channels in a high traffic cell in order to use the released channels for new calls.

The ‘Traffic’ cause is not foreseen on the A interface and the serving BSC cannot control the channel occupancy of a cell belonging to another BSC, consequently the handover for traffic reason may be performed only between cells belonging to the same BSC.

This feature implementation can avoid the planning of new resources into a cell and the channel reservation for high traffic situation.

The handover for traffic reason is the lowest priority one, because the technical necessary handovers shall be proceeded first in order not to disturb the normal network behavior too much.

The HO due to resource criteria is not triggered for extended cells. In concentric cells only the outer area is checked for the resource criteria.

This feature as well as Directed Retry has the scope of increasing the capacity of the network. Whereas Directed Retry acts on new connections by moving them towards adjacent cells, H/O Decision due to BSS Resource Management Criteria moves connected users towards adjacent cells allowing in the previously used cells new connections to be set up. A connection moved for Directed Retry can be far from the ideal cell border, while the connection moved by H/O Decision due to BSS Resource Management Criteria is chosen near to the ideal cell border.


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If the function is enabled in a cell (Traffic_Handover_Enable = TRUE), the following procedure is applied:

1. The BSC periodically (Traffic_Control_Timer) evaluates the traffic level for that cell. If the percentage becomes higher or equal a given threshold (Traffic_High_Threshold), the cell is regarded as a high traffic cell and the handover cause due to traffic is enabled. Otherwise the cell will be regarded as a low traffic cell and the handover cause due to traffic is disabled. In order to evaluate correctly the percentage of busy channels in case of FR- and HR-channels in use, the FR-channels are considered with weight 2 and the HR-channels with weight 1. If the handover cause due to traffic changes from disabled to enabled or vice versa, the BTS will be informed.

2. In the BTS two tasks (parent /dedicated) are implemented to proceed the evaluation of the handover due to BSS resource management criteria. In case of cause detection and evaluation of appropriate neighbor cell candidate(s), the BSC will be informed via Intercell_Handover_Cond_Ind message that includes the handover cause "traffic" together with the list of preferred target cells.

3. When receiving the message "Intercell_Handover_Cond_Ind" with cause "traffic", the BSC analyses the target cell list in order to discard all cells with a high traffic load where the percentage of busy channels is higher than a given threshold (Traffic_Low_Threshold).



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Parameters to control traffic HO


Range Meaning

Traffic_Control_Timer TRFCT

/ BSC

10...200 unit = 0.5 sec

This parameter establishes for each BTS the period of time to wait before evaluating the traffic level

Traffic_HO_Enable TRFHOE

/ HAND

TRUE FALSE

This parameter allows to enable/disable the handover for traffic reason feature in the BTS

Traffic_High_Threshold TRFHITH / HAND

50...100 unit = %

This parameter defines the high traffic level threshold in order to establish if the handover for traffic reason has to be enabled or disabled.

Traffic_Low_Threshold TRFLTH / HAND

0...85 unit = %

This parameter defines the low traffic level threshold in order to establish if a cell can be a candidate to receive handover for traffic reason.


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Description of tasks in the BTS

If the cause TrafficHo is changed the message O_DMXX_LDMXX_PARA_CHANGE is sent from the BSC to the initialization task of the BTS. The initialization task sends the message O_RLRL_TRAF_HO_EN_DIS to the parent task. On receipt of this message the parent task updates the counter m and starts timer Traffic_Ho_Timer, if necessary. The counter m is set to 1 if it was 0, m remains untouched if its value was greater than 0.

Every time the timer expires the following procedure is started:

1. If the HO due to traffic reason is enabled the value m is increased by one up to a value which is not higher than Traffic_Margin_Max / Traffic_Margin_Step. If m has already reached this value m stays constant. In any case the timer is restarted.

2. If the HO due to traffic reasons is disabled the value m is decreased by one and the timer is restarted. If the value reaches 0 the timer is not started again.

Parent Task

HO cause

enabled

N

Y

m = 0N

Y

set m = 1

start timer

Traffic_HO_Timer

O_RLRL_TRAF_HO_EN_DIS

received

Traffic_HO_Timer

expired

HO cause

enabled

N

Y

Increment m

by one

decrement m

by one

restart

Traffic_HO_Timer

m = 0N

Y

End

Fig. 19 Parent task in BTS to perform traffic HO



75

The counter m is now used in the dedicated task to determine if the traffic HO has to be evaluated.

Traffic HO

Evaluation

Y

N

Y

m > 0

Traffic HO

condition fulfilled

for adj. cell

BTS sends to BSC

Intercell_HO_Cond_Ind

with target cell list

N

End

Fig. 20 Dedicated task in BTS to perform traffic HO


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The traffic HO condition is fulfilled if one or more internal adjacent cells are evaluated with a positive result according to the following algorithm:

Case 1 - HCS disabled (HIERC = FALSE)

RXLEV_NCELL(n) > RXLEV_MIN(n) + max[0, MS_TXPWR_MAX(n) - P]

& PBGT(n) > TRAFFIC_HO_MARGIN(n) - K

Case 2 - HCS enabled (HIERC = TRUE)

RXLEV_NCELL(n) > RXLEV_MIN(n) + max[0, MS_TXPWR_MAX(n) - P]

& PBGT(n) > TRAFFIC_HO_MARGIN(n) - K

& (PLNC(n) = PL if parameter Traffic_Keep_Priority is set to TRUE or

PLNC(n) ≤ PL if parameter Traffic_Keep_Priority is set to False)

Where K = m * Traffic_Margin_Step

with m = 1, 2, 3,.....,Traffic_Margin_Max div Traffic_Margin_Step

Back Handover Prevention in case of traffic HO

Back-handover prevention is triggered by BSC by including the old cell and the handover cause (traffic) into the Channel activation message.

A timer (Back_HO_Forbidden_Timer) is started in the BTS and until the timer for the originated cell expires, the back HO due to BSS resource criteria or due to power budget is inhibited.



77

Parameters to control traffic HO in the BTS BR6.0


Range Meaning

Traffic_HO_Timer TRFHOT

/ HAND

2...20 unit = sec

This parameter represents the timer which is used to establish the period of time to wait before updating the m value. The m value is increased if the traffic HO cause is enabled and decreased if the traffic HO cause is disabled, m is updated until it reaches value 0

Traffic_Margin_Step TRFMS

/ HAND

1...6 unit = 1 dB

This parameter establishes the minimum reduction for TRFHOM

Traffic_Margin_Max TRFMMA

/ HAND

1...48 unit = 1 dB

This parameter establishes the maximum reduction for TRFHOM.

Traffic_Keep_Priority TRFKPRI

/ HAND

TRUE FALSE

This parameter determines whether candidate cells have to be of the same priority as the serving cell or may be of the same or higher priority

Traffic_HO_Margin TRFHOM

/ ADJC

0...126 unit = 1 dB (-63 dB .... +63 dB)

This parameter defines the nominal cell border between cells for traffic handover reason

Back_HO_Forbidden Timer

BHOFOT

/ ADJC

1...120 unit = sec

This parameter represents the timer used to establish the time for which a back handover due to traffic reason or PBGT (Power Budget) has to be avoid.


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3.5 Target cell list compilation

The target cell compilation proceeds in several steps:

Definition of Neighbor Cells (Maximum Number = 32) by CI_NCELL(n):

CI_NCELL(n) = Location Area Code (LAC) + Cell Identifier (CI) of Neighbor Cell n. The selection of neighbor cells affects handover traffic flow. The number of target cells should be kept to a minimum:

� geographical neighbor cells if there is a traffic flow from the serving cell into these cells

� eventually some alternative cells if there is a congestion in the preferred direct neighbor cell

� MS needs the BCCH frequency for each neighbor cell n: Absolute Radio Frequency Channel Number of BCCH: ARFCN_NCELL(n)

� MS reports to BTS level measured on a certain ARFCN(n) together with

Relative BCCH frequency number BCCH_FREQ_NCELL(n) (see Fig. 16)

Decoded Base Station Identity Code BSIC(n) Neighbor Cells n1 and n2 using the same BCCH frequency ARFCN(n1) = ARFCN(n2) need different Base Station Identity Codes !!!

BSIC: = NCC +BCC

NCC: National Color Code (3 bits)

BCC: Base Station Color Code (3 bits), has to be chosen by the network operator in accordance with rule given above.

Example

Neighbor Cell ARFCN_NCELL BSIC BCCH_FREQ_NCELL

1 4 01 0

2 4 02 0

3 11 01 1

4 18 01 2

5 25 01 3

6 32 03 4

7 39 02 5

8 39 04 5

BCCH_FREQ_NCELL(n) and BSIC(n) → CI_NCELL(n)uniquely!



79

Illustration of Measurement Reporting and Neighbor Cell Book-Keeping Process

BSIC BCCH_FREQ_NCELL

RXLEV_NCELL(n)

01 1 48

02 0 37

03 4 36

04 5 29

01 2 27

For not reported neighbor cellsRXLEV_NCELL is set to 0

Measurement Report by MSreporting of the strongest cells with known and

allowed BSIC; maximum: 6 cells

each SACCH-Multiframe

Book-Keeping at BS

Neighbour Cell ARFCN BSIC BCCH_FREQ_NCELL

RXLEV_NCELL(n)

1 4 01 0 0

2 4 02 0 37

3 11 01 1 48

4 18 01 2 27

5 25 01 3 0

6 32 03 4 36

7 39 02 5 0

8 39 04 5 29

Fig. 21


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Uniqueness of Neighbor Cell Measurements:

BSIC (n1) ≠ BSIC (n2)

for adjacent cells n1 and n2 with

ARFCN(N1) = ARFCN(n2)

Generation of the Target Cell List

The target cell list is generated when a handover cause is detected.

The maximum number of cells to be included in the target cell list is given by the parameter N_CELL (Parameter NCELL in object HAND, Range: 0 ... 15).

Conditions for Neighbor Cells to be included in the Target Cell List:

� for Quality, Distance and Traffic Intercell Handover:

RXLEV_NCELL(n) > RXLEV_MIN(n) + MAX(0, MS_TXPWR_MAX(n) - P)

� for Level Intercell Handover:


& RXLEV_NCELL(n) > RXLEV_DL + LEVEL_HO_MARGIN(n)*)

� for Fast Uplink Level Intercell Handover:

RXLEV_NCELL(n) > RXLEV_MIN(n) + MAX(0, MS_TXPWR_MAX(n) - P) +

FAST_UL_RXLEV_MIN_OFFSET(n)

� for Power Budget Handover:


& PBGT(n) - HO_MARGIN(n) > 0

*) Level Handover Margin

To guarantee that a Level HO will be performed to a cell with higher level than that from serving cell the additional level handover margin condition shall be satisfied, with LEVHOM as a new parameter. In general a positive value should be chosen for this parameter. But for special applications it is possible to choose a negative value to force a HO even in case of lower level in the target cell.



81

Order Criterion for Handover Candidate Cells within the Target Cell List:

PRIO_NCELL(n) = BCV(n (Better Cell Value)

BCV(n) = PBGT(n) - HO_MARGIN(n)

BCV(n) = PBGT(n) - TRAFFIC_HO_MARGIN(n) [ if HO-cause = traffic ]

PBGT(n): averaged value of the power budget

Order Criterion for Fast Uplink HO Candidate Cells within the Target Cell List:

1. Order Criterion: FAST_UL_HO_CELL(n) = TRUE FAST_UL_HO_CELL(n) = FALSE

2. Order Criterion: PBGT(n)


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Parameters for Target Cell Generation Process

The parameters for measurement reporting and target cell generation are summarized in the table below.

Specification Name

DB Name/ Object

Range Meaning

CI_NCELL MCC-MNC-LAC-CI

CELLGLID /BTS /TGTBTS

0...999 - 0...999 - 1...65535 -0...65535

Global cell identifier of the adjacent cell consisting of mobile country code, mobile network code, location area identifier and cell identity

ARFCN_NCELL BCCHFREQ /BTS /TGTBTS

0...1023 Absolute radio frequency channel number of the BCCH frequency of the neighbor cell.

BSIC = NCC + BCC BSIC /BTS /TGTBTS

0...7 0...7

Base station identity code consisting of national- and base station-color code. Neighbor cell measurement are identified using BSIC and BCCH_FREQ_NCELL

PLMN_ PERMITTED

PLMNP/ BTS

0...255 The MS includes only received level values of those cells within the measurement report which are defined as cells of a permitted PLMN



83

Parameters for Level Handover Margin & Fast Uplink Handover BR6.0

Specification Name

DB Name/ Object

Range Meaning

ENABLE_LEVEL_ HO_MARGIN

ELEVHOM/ HAND

TRUE FALSE

Indicates if the level handover margin condition is enabled or disabled.

LEVEL_HO_ MARGIN

LEVHOM ADJC

0 ... 126

-63 dB ... +63 dB [1 dB step] Threshold to guarantee a handover to a target cell with a higher level than the serving cell

FAST_UL_HO_ CELL

FULHOC/ ADJC

TRUE FALSE

Indicates a cell as a predefined FULHO target cell. Cells that have the attribute set to TRUE will be preferred to cells that have this attribute set to FALSE

FAST_UL_RXLEV _MIN_OFFSET

FULRXLVMOFF/ADJC

0...126 [-63 dB ... +63 dB] Value is used to select a target cell for FULHO


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Evaluation of Target Cell List

� Intracell Handover:

HO Condition Indication message with cause Intracell HO without target cell list is sent from the BTS to the BSC. BSC selects new channel for the call within the same BTS.

� Intercell Handover:

HO Condition Indication message with cause and target cell list is sent from the BTS to the BSC. If the first cell within the target cell list is within its BSS area, the BSC selects a channel at the corresponding BTS. If no channel is available at that BTS, the next cell within the target cell is tried. If the first target cell (or the ones tried in further steps) does not belong to the own BSS area, a Handover Required message is sent to the MSC. This message contains a reduced target cell list (without the cells tried internally). This is illustrated in the following figure:



85

Illustration of Target Cell Evaluation

HO Required

- cause

- (reduced) target cell list

Handover

Detection

BTS

yes

yes

no

yes

try

next cell

select

channel

Handover

Failure

HO Cond Ind

- cause

- target cell list

BSC

next cell

available

next cell

external

no

channel

available

no

MSC

yesChannel

Activation

HO cause = traffic

& cell load > TRFLTH

no

yes

Fig. 22


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3.6 Handover signaling and timer

Measurements

MS

Measurements

BTS

serving

HO Cond Ind

BSC

serving

HO Required

MSC

HO Request

BSS

target

HO Request AckHO Command

HO CommandHO Command

HO Cond Ind HO RequiredHO Access

HO Access

Physical Info

HO Detect

T3105HO Complete

Physical Info

HO Failure

HO CompleteClear Command

Channel Release

messages at timer expiry, e.g.

messages for successful handover, e.g.

Channel Release

HO Required

HO Failure

NY1

T_HAND_REQ

T3124

T8

T7

Fig. 23 Non synchronized handover



87

Parameter and Timer for Handover Signaling

T_HAND_REQ


HAND THORQST 0...31 1 2 * SACCH multiframe

Purpose: minimum time for HANDOVER CONDITION INDICATION messages for the same connection

Start: sending of HANDOVER CONDITION INDICATION by BTS

Stop: � HANDOVER COMMAND received

� reason for handover has disappeared

� communication with MS is lost

� transaction has ended, call cleared

Action expiry: repetition of HANDOVER CONDITION INDICATION

Default: 4

T7


BSC BSCT7 unit * (0...255) 1 MS100 = 100 msec HLFSEC = 0.5 sec

SEC5 = 5 sec

Purpose: minimum time for HANDOVER REQUIRED messages for the same connection

Start: sending of HANDOVER REQUIRED by BSC

Stop: � HANDOVER COMMAND received

� reason for handover has disappeared

� communication with MS is lost

� transaction has ended, call cleared

Action expiry: repetition of HANDOVER REQUIRED

Default: HLFSEC-4


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T8


BSC BSCT8 unit * (0...255) 1 MS100 = 100 msec HLFSEC = 0.5 sec

SEC5 = 5 sec

Purpose: keep the old channel sufficient long to be able to return to it, and to release the channels if the MS is lost

Start: reception of HANDOVER COMMAND at BSC

Stop: reception of CLEAR COMMAND from MSC or HANDOVER FAILURE from MS at BTS

Action expiry: release of old channels

Default: HLFSEC-20

T3124 - MS Timer, not adjustable by parameter,

Purpose: detect the lack of answer from the network at handover access.

Start: sending of first HANDOVER ACCESS by MS

Stop: reception of PHYSICAL INFORMATION by MS

Action expiry: deactivation of new channel, reactivation of old channel, send HANDOVER FAILURE

Default: 675 msec for SDCCH - 320 msec else



89

T3105


BTS T3105 unit * (0...255) 1 MS10 = 10 msec

Purpose: period for repetition of PHYSICAL INFORMATION

Start: sending of PHYSICAL INFORMATION by BTS

Stop: reception of correctly decoded signaling or TCH frame on new channel from MS at BTS

Action expiry: repetition of PHYSICAL INFORMATION; if the maximum number of repetitions has been reached: release of new channel

Default: MS10-10

NY1


BTS NY1 0...254 1 -

NY1 is the maximum number of repetitions of the physical information by the BTS.

Default: 20

HOSYNC


BSC HOSYNC NONSYNC/SYNC - -

Purpose: finely synchronized Handover (GSM 04.08)

The finely synchronized handover is restricted to internal BTSE handover. In this case the MS only sends four HANDOVER ACCESS bursts (continuously in case of non-synchronized handover). Furthermore no PHYSICAL INFORMATIO is sent.

The synchronized handover improves the speech quality during handover by a shorter handover procedure at the air interface.


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Prevention of Handover Failure Repetition

To prevent handover repetition after consecutive HO failures to the same cell a Handover Failure Indication message is received from the BSC. A timer is started and until expiry of the timer the defined cell is excluded from the target cell list for any kind of handover.

The penalization time is defined by the O&M parameter ‘Timer_Inhibit_Failure_HO’, the number of permitted HO failures is defined by the O&M parameter ‘Max_Failure_HO’. The O&M flag ‘Enable_No_Failure_Rep_HO’ is used to enable/disable this feature.

Parameters for Prevention of Handover Failure Repetition


Unit

Enable_No_Failure_Rep_HO HAND NOFREPHO TRUE/FALSE - -

Max_Failure_HO HAND MAXFAILHO 1 .. 15 1 -

Timer_Inhibit_Failure_HO ADJC TINHFAIHO 1 .. 254 1 1 sec



91

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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

General Information Flow

Cell A

Cell B is not

included in TCL

for a defined

period time

BTS

HO Failure (cell B)

HO Failure (cell B)

after MAXFAILHO

consecutive HO

failures on the

same adjacent cells

BSC



HO Failure Ind (B)

HO Cond Ind (C, D)

Fig. 24


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3.7 AMR-handover

The handover (and power control) decision for AMR calls is in principle the same as for 'normal' speech calls. The only exception is that for quality based decisions not the RXQUAL but the C/I is taken.

3.7.1 AMR compression handover

The strategy for (intracell) codec handover (controlled from BSC) is as follows:

1. The AMR HO from HR to FR is always enabled (disregarding the cell load, i.e. if at least one TCH/F is free, HO is performed even if the percentage of busy TCH/F exceeds the thresholds HRACTT1/HRACTT2).

2. The AMR HO from FR to HR is only enabled when the percentage of busy TCH is higher than a predefined threshold (c.f. HRACTT1, HRACTT2).



93

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BTS BSC

TRFCT

TRFCT

...

Check, whether or not traffic load > HRACTT1If TRUE, AMR compression HO is started in that cell:

SET ATTRIBUTE: "AMR COMPRESSION HO ENABLED"

Check, whether or not traffic load > HRACTT1If TRUE, AMR compression HO is started in that cell:

SET ATTRIBUTE: "AMR COMPRESSION HO ENABLED"

Fig. 25 AMR traffic control


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Parameter Object Range Description

HRACTT1 BTS 0..10000

Default value: 6000

This parameter is the threshold that indicates the percentage of busy TCHs in case of standard cell or complete area of a concentric cell or far area of an extended cell. If the traffic load in the cell exceeds the threshold HRACTT1, the BSC enables the AMR compression handover.

HRACTT2 BTS 0..10000

Default value: 6000

This parameter is the threshold that indicates the percentage of busy TCHs in case of inner area of a concentric cell or near area of an extended cell. If the traffic load in the cell exceeds the threshold HRACTT2, the BSC enables the AMR compression handover.

HOTHAMRCDL

HOTHAMRCUL

HAND 0 ... 30 (step size 1 dB, default: 23 dB)

hoThresAMRComprDL

hoThresAMRComprUL

Handover FR�HR is triggered only if both thresholds HOTHAMRCDL and HOTHAMRCUL are exceeded.

HOTHAMRDDL

HOTHAMRDUL

HAND 0 ... 30 (step size 1 dB, default: 10 dB)

hoThresAMRDecomprDL

hoThresAMRDecomprUL

Handover HR�FR is triggered whenever any of the thresholds HOTHAMRDDL or HOTHAMRDUL is exceeded.



95

3.7.2 AMR quality handover

For an AMR call, the quality handover are not based on averaged RXQUAL but on averaged C/I. The thresholds used for AMR calls (non-AMR calls in brackets) are

1. hoLowerThresQualAMRDL (hoLowerThresholdQualDL) and

2. hoLowerThresQualAMRUL (hoLowerThresholdQualUL).

Parameter Object Range Description

AMRACMRDL HAND 1 ... 63 (unit CMR, default: 5 CMR)

aMRAveragedCMRDL

Size of averaging window for Codec Mode Requests received from MS during AMR call (for Abis satellited AMRACMRDL should be set to 63)

HOLTHQAMRDL HAND 0 ... 30 (step size 1 dB, default: 8 dB)

hoLowerThresQualAMRDL

Quality HO for DL is triggered if the DL quality (C/I) exceeds HOLTHQAMRDL. For good granularity a minimum averaging window size of 4 for averaging C/I is used for AMR calls.

HOLTHQAMRUL HAND 0 ... 30 (step size 1 dB, default: 8 dB)

hoLowerThresQualAMRUL

Quality HO for UL is triggered if the UL quality (C/I) exceeds the value HOLTHQAMRUL.


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3.8 GSM to UMTS intersystem handover

3.8.1 Basics

This chapter describes the intersystem handover from a serving cell of a GSM network to a UMTS target neighboring cell of a UMTS network.

HO decisions are always based on measurements of MS/UE and BTS. Therefore the UMTS neighbor cells have to be added to the database and have be taken into consideration during the whole order of events of the measurement processing and the handover decision.

All handover specific evaluations are performed by the GSM base station system (BSS) and not by the UMTS radio network system (RNS). The BSS informs the mobiles about UMTS neighboring cells via appropriate system info messages. Multi-RAT capability (GSM, UMTS) of mobiles is indicated to the network, as the mobiles send the class mark information to the BSS.

Calls once established within the GSM system can be handed over to a UMTS system due to one of the following reasons:

3. Emergency Handover (quality, signal strength, distance, forced)

4. Better cell

5. Sufficient UMTS coverage.

As for circuit-switched GSM services, priorities for their Hierarchical Cell Structure (HCS) can be defined, with GSM or UMTS as the preferred layer.



97

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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Uu

RNC

(Target) UMTS Cell (Source) GSM Cell

NodeB

BSC

BTS

Um

2G-3G-MSC (Visited)

(Circuit Switched) Voice & Data

Mobile (MS/UE)

Iub Abis

Fig. 26 HO from GERAN to UTRAN


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3.8.2 General

For the handover the mobiles use the UMTS cell information. The hierarchical cell structure concept of the BSC and BTS is enhanced to include UMTS cells.

The BSS initiates a handover to UMTS according to one of the following reasons:

1. Emergency Handover

2. Better cell

3. Sufficient UMTS coverage.

Handover causes for the emergency handover are triggered by

1. uplink quality

2. downlink quality

3. uplink strength

4. downlink strength

5. distance or

6. forced, which is related to directed retry, O&M intervention, preemption.

Although the link quality within the serving cell may be sufficient, the better cell handover is triggered if there is available a neighbor cell with a better receive level. Consequently there is a benefit in terms of overall network performance as a call in a better cell causes less interference.

The operator can configure, if either the “handover due to better cell functionality” or “handover due to sufficient UMTS coverage” should be applied by the system.

An handover due to sufficient UMTS coverage is triggered by the system as soon as distinct thresholds (RSCP – received signal code power from UMTS, Ec/No – quality related for UMTS cells) have been exceeded regarding to UMTS availability.

The procedure for the intersystem handover of circuit-switched voice and data services is specified and recommended in 3GPP TS 23.009.



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3.8.3 Better cell HO from GSM to UMTS

The main objective of the "Better cell HO" (Power-Budget HO) is to minimize the RF power needed for communication between the mobile station MS/UE and the network (base station). Minimizing the RF power reduces the radio interference in the whole network and increases the battery life of the MS/UE.

The better cell HO decision (power budget HO decision) concerning UMTS neighbor cells is the same than GSM neighbor cells except an additional adjustment value which is needed to adapt the UMTS measurement values to the GSM values because of a difference in measurement method.

Whether or not the conditions for an intercell handover due to power-budget are met are changed due to the new UMTS neighbor cells.

UMTS neighbor cells are only taken into consideration if the following conditions are met:

1. MS/UE is UMTS capable

2. Service HO Information Element (in the ASSIGNMENT REQUEST or HANDOVER REQUEST) is not set to “Handover to UTRAN or cdma2000 should not be performed ” or "Handover to UTRAN or cdma2000 shall not be performed”

3. HO_from_GSM_to_UMTS is enabled (enableUmtsHO)

4. Better_Cell_HO_to_UMTS is enabled (enable UmtsBetterCellHO)

5. FDD_REP_QUANT == 0 ( 0=RSCP, 1=Ec/No)

6. Flag "ASCI talker" is not set to TRUE

Notes:

1. The GSM system internal Power-Budget HO is not modified!

2. ASCI is a GSM feature and not supported by the UMTS system. Therefore, a handover from GSM to UMTS shall not be performed for an ASCI talker. I.e. the ASCI talker shall be kept within the GSM system. The Information about ASCI is sent from the BSC to the BTS in the IE “Channel Options” of the “Channel Activation” message and of the "Forced HO Request" message. The value of the flag ‘ASCI talker’ indicates whether or not the channel is activated for an ASCI talker. The BTS stores the information and includes it in the HO decision. If the ASCI-Talker flag is set to TRUE, no HO to UMTS takes place.

3. Handover to cdma2000 is not supported.

4. For: HO "should be" performed:

BTS initiates a "Sufficient UMTS coverage HO" for this MS/UE if the condition of the "Sufficient UMTS coverage HO" is met.


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5. For HO "shall not be" performed:

BTS never performs a handover to UMTS for this MS/UE; i.e. the MS/UE is kept in the GSM system. The BTS generates a TCL without UMTS target cells.

6. For HO "should not be" performed:

The MS/UE is kept in the GSM system as long as possible.

In order to achieve this:

the BTS only includes UMTS cells in the target cell list (TCL) in case of an "imperative" handover. I.e., in case of a "Better cell HO", the BTS generates a TCL without UMTS target cells for this MS/UE. The "Sufficient UMTS Coverage" HO is disabled by the BTS for this MS/UE.

the sorting of the TCL in case of an "imperative" handover is modified for this MS/UE because it must be avoided that an UMTS cell is sorted before a GSM cell. Therefore, GSM and UMTS cells in a "mixed" TCL, i.e. in a TCL which contains GSM and UMTS cells, are not sorted together according to HCS priority and PRIO_PBGT. Instead, the UMTS cells and GSM cells are sorted separately. The GSM cells are sorted relative to other GSM cells according to HCS priority and PRIO_PBGT (GSM-sublist). The UMTS cells are sorted relative to other UMTS cells according to HCS priority and PRIO_PBGT (UMTS-sublist). The GSM-sublist is sorted before the UMTS-sublist to form the complete TCL. This modification of the TCL sorting ensures, that an UMTS neighbor cell will only be on top of the TCL if there is no GSM neighbor cell available. Thus, the MS/UE is kept in GSM as long as possible.



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The "Power-Budget" PBGT condition concerning UMTS neighbor cells, which compares the serving cell downlink receive level RXLEV_DL and the neighbor cell received level RXLEV_NCELL, is met under the following circumstance :

PBGT := [ Min(MS_TXPWR_MAX(s), P(s)) - RXLEV_DL - PWR_C_D ]

- [ Min(MS_TXPWR_MAX(n), P(s)) - RXLEV_NCELL(n) ]

The main condition that the BTS initiates a "better cell HO" (Power-Budget HO) is:

PRIO_PBGT > 0

PRIO_PBGT := PBGT - HOMARGIN - UMTS_ADJUST

In addition the following minimum condition must be fulfilled:

RXLEV_NCELL(n) > RXLEVMIN(n) + Max (0, Pa)

Pa := (MS_TXPWR_MAX(n) - P(n))

In addition, for HCS the following condition must be fulfilled:

PRIO_NCELL ≤ PL


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Notes:

1. (s) denotes serving cell, (n) denotes neighbor cell

2. UMTS_ADJUST(n) is a parameter which is introduced for GSM to UMTS handover and not used for GSM to GSM handover. It is an inter-system adjustment parameter in order to have the flexibility to adjust the RXLEV_NCELL(n) of the UMTS neighbor cells (n) to the RXLEV_DL of the GSM serving cell.

3. For UMTS neighbor cells (n), the UMTS measurement quantity RSCP, which the MS/UE reports in its Measurement Report, is used for RXLEV_NCELL(n).

4. Not new in the above equations are the following parameters (already in use for GSM-internal HO):

RXLEV_NCELL(n) is the neighbor cell received level (GSM or UMTS neighbor cell) measured by the MS/UE.

RXLEV_DL is the GSM serving cell received level (downlink) measured by the MS/UE.

PWR_C_D is the difference between the maximum downlink RF power permitted in the GSM serving cell and the actual downlink power due to the BSS power control.

RXLEVMIN(n) defines the minimum received level which is required to perform a handover to the neighbor cell (GSM or UMTS neighbor cell).

MS_TXPWR_MAX(s) is the maximum RF TX power an MS/UE is permitted to use on a traffic channel in the GSM serving cell.

MS_TXPWR_MAX(n) is the maximum RF TX power an MS/UE is permitted to use in the neighbor cell (GSM or UMTS neighbor cell).

P(s) is the maximum TX power capability of the MS/UE in the GSM serving cell.

'P(n) is the maximum TX power capability of the MS/UE in the neighbor cell (GSM or UMTS neighbor cell).

HOMARGIN(n) is a threshold which can be used to prevent handover oscillations between the serving GSM cell and neighbor cells (GSM or UMTS neighbor cell).

PL is the HCS priority value of the GSM serving cell.

PRIO_NCELL is the HCS priority value of the neighbor cell (GSM or UMTS neighbor cell).

In order to perform the handover, the HCS priority of the neighbor cell must be equal to or higher than the priority of the GSM serving cell; i.e. the priority value PRIO_NCELL of the neighbor cell must be equal to or smaller than the priority value PL of the GSM serving cell.



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3.8.4 Sufficient UMTS coverage HO from GSM to UMTS

The additional HO cause "Sufficient UMTS coverage HO" is introduced. The main objective of the GSM to UMTS "Sufficient UMTS coverage HO" is to perform a handover to the UMTS system as soon as possible, i.e. as soon as the UMTS measurement quantity (RSCP or Ec/No for FDD cells, RSCP for TDD cells) exceeds a certain "sufficient" threshold in order to direct as much traffic as possible to areas which offer "sufficient" UMTS coverage. "Sufficient UMTS coverage HO" has to be enabled by the operator. Sufficient UMTS Coverage HO has a relatively high priority (less than emergency HO but higher than power budget HO).

UMTS neighbor cells are only taken in consideration if the following conditions are met:

1. MS/UE is UMTS capable

2. Service HO Information is not set to “Handover to UTRAN or cdma2000 should not be performed ” or "Handover to UTRAN or cdma2000 shall not be performed”

3. HO_from_GSM_to_UMTS is enabled (enableUmtsHO)

4. UMTS_Sufficient_Coverage_HO is enabled (enableUmtsSuffCovHO)

5. Flag "ASCI talker" is not set to TRUE

For a "Sufficient UMTS coverage" HO from GSM to UMTS, the BTS compares the RXLEV_NCELL(n) of the UMTS neighbor cell (n) with a "sufficient" threshold UMTS_SUFFICIENT(n); the condition that the BTS initiates a "Sufficient UMTS coverage" HO from GSM to UMTS is:

RXLEV_NCELL(n) > UMTS_SUFFICIENT(n) + Max (0, Pa)

Pa := (MS_TXPWR_MAX(n) - P(n))

The "sufficient" threshold UMTS_SUFFICIENT(n) depends on the UMTS measurement quantity.

UMTS_SUFFICIENT_RSCP(n) and UMTS_SUFFICIENT_Ec/No(n) are "sufficient" threshold parameters which are used for GSM to UMTS "Sufficient UMTS coverage" HO and not used for GSM to GSM handover. They define the "sufficient" threshold which is required to perform a handover to the neighbor UMTS cell. The kind of threshold which has to be used depends on the parameter fdd_rep_quant, which indicates the measurement quantity type.


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3.8.5 Imperative HO from GSM to UMTS

The BTS initiates an imperative handover in the same way as for a GSM internal handover. Imperative HO is done if the connection run the risk of connection loss because of bad receive level or distance. The decision basis is the same than in pure GSM systems because the imperative HO decision depends on the measurement quantities of the current (active) connection and do not require a comparison with other cells. This is in contrast to the power budget HO.

The imperative (emergency) HO causes which are currently used for GSM-system internal handovers are used in the same way for UMTS handover (HO causes: "uplink quality", "downlink quality", "uplink strength", "downlink strength", "distance").

Imperative HO to UMTS cells is enabled / disabled by the parameter enableUmtsImperativeHO.

If an imperative HO takes place, the UMTS neighbor cells have to be taken into account for the target cell list generation.



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3.8.6 Mobile speed sensitive HO from GSM to UMTS

The idea of the Mobile Speed Sensitive HO is to perform a handover to microcells only for "slow moving" and not for "fast moving" mobile stations.

Within the current concept of "Mobile Speed Sensitive HO", a timer of duration HOMDTIME is used to distinguish "fast moving" and "slow moving" mobile stations. The timer is started when the MS/UE crosses the border of the microcell. If the MS/UE leaves the cell within HOMDTIME, the MS/UE is considered to be "fast moving". If the MS/UE is still located within the cell after duration HOMDTIME, the MS/UE is considered "slow moving".

This "Mobile Speed Sensitive HO" is not modified for GSM-system-internal handovers. The decision whether a cell border is crossed or not depends on the PRIO_PBGT as described below. For UMTS cells the additional parameter UMTS_ADJUST is introduced into PRIO_PBGT.

For Better Cell (Power Budget) GSM to UMTS handovers, concerning UMTS cells, the term "PRIO_PBGT" is modified by UMTS_ADJUST and is defined:

PRIO_PBGT = PBGT - HOMARGIN - UMTS_ADJUST

For Sufficient UMTS coverage HO, the hoMarginDelayTimer is started when the above "Sufficient UMTS coverage" condition is met.

The "Sufficient UMTS coverage" HO shall be performed for a "slow moving" MS/UE, i.e. for a MS/UE for which the "Sufficient UMTS coverage" condition is still met for t >= HOMDTIME.

The "Sufficient UMTS coverage" HO shall not be performed for a "fast moving" MS/UE, i.e. for a MS/UE for which the "Sufficient UMTS coverage" condition is violated before the timer HOMDTIME expires.


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This means:

Within the current concept of "Mobile Speed Sensitive HO", the Power-Budget HO is penalized for t < HOMDTIME by adding a penalization value HOMSOFF to the Power-Budget equation and by substituting the HCS cell priority PRIO_NCELL to a low value PPLNC:

For GSM cells:

PRIO_PBGT = PBGT - (HOMARGIN + HOMSOFF)

PRIO_NCELL is substituted by PPLNC.

For UMTS cells:

PRIO_PBGT = PBGT - (HOMARGIN + HOMSOFF + UMTS_ADJUST)

PRIO_NCELL is substituted by PPLNC.

After timer expiry, i.e. for t >= HOMDTIME, the above Power-Budget penalization is compensated with HOMDOFF and the HCS cell priority is set back to the original value PRIO_NCELL (i.e. a handover is possible for the "slow moving" MS/UE):

For GSM cells:

PRIO_PBGT = PBGT - (HOMARGIN + HOMSOFF - HOMDOFF)

HCS cell priority PPLNC is set back to PRIO_NCELL.

For UMTS cells:

PRIO_PBGT = PBGT - (HOMARGIN + HOMSOFF - HOMDOFF + UMTS_ADJUST)

HCS cell priority PPLNC is set back to PRIO_NCELL.



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3.8.7 Directed retry from GSM to UMTS

During call setup the BSC indicates, that there is a HO necessary and sends a “Forced HO Request” message to the BTS.

The “Forced HO Request” contains the IE “Service Handover” received from the “ ASSIGNMENT REQUEST” message and the IE Channel Options. The IE "Service Handover" specifies whether a handover to UMTS should, should not or shall not be performed. The IE Channel Options contains information concerning ASCI-Talker.

The BTS answers the “Forced HO request” if possible (TCL available) with an "Inter-cell HO Condition Indication" message which contains the Target Cell List (TCL). The BTS sorts the TCL as for an "Imperative HO" taking into account the setting of the IE "Service Handover".

It is possible to enable/disable the Directed Retry to UMTS via the parameter "enableUmtsSdcch". If set to FALSE, the BTS does not include UMTS cells into the TCL and does not initiate a handover to UMTS for a MS/UE which is on a SDCCH.


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3.8.8 Generation of target cell list

If the BTS has detected that a handover is necessary; i.e. if the conditions for handover initiation are met, a target cell list TCL is built by the BTS. The TCL contains the neighbor cells which are handover candidates. The target cell list is part of the message Handover Condition Indication which will be exchanged from BTS to BSC.

Apart from the additions/modifications due to GSM to UMTS handover described below, the BTS shall include GSM neighbor cells into the TCL in the same way as for a GSM-internal handover.

UMTS target cells will only be taken into consideration if:

7. HO from GSM to UMTS is enabled,

8. the flag “ASCI talker” is FALSE,

9. the IE “Service Handover” is not set to "Handover to UTRAN or cdma2000 shall not be performed”.

Remarks:

1. If the HO cause is CRL_UMTS_SUFFICIENT the TCL only contains UMTS target cells.

2. If enableUmtsImperativeHO is true and the HO cause is CRL_DL_LEV, CRL_UL_LEV, CRL_UL_QUAL, CRL_DL_QUAL, CRL_DIST or FORCED the UMTS target cells are sorted in the TCL in the same way as GSM target cell lists. If EC/No is used, UMTS cells are sorted after the GSM cells.

3. If enableUmtsBetterCellHO is true and FDD_REP_QUANT is 0 , the HO cause is CRL_PBGT, and the UMTS target cells are sorted in the TCL in the same way as GSM target cell lists.

The sorting of the UMTS-cells in relation to the GSM-cells depends on the Service Handover Information. If Service Handover is set to:

1. "Handover to UTRAN or cdma2000 shall not be performed" no UMTS cell in TCL

2. "Handover to UTRAN or cdma2000 should not be performed" in case of imperative HO the UMTS cells are sorted after the GSM cells. In case of other HO reasons (e.g. Power Budget) no UMTS cell in the TCL

3. "Handover to UTRAN or cdma2000 should be performed" the GSM cells are sorted after the UMTS-sublist.

4. If Service Handover is not set, a mixed TCL list is generated. This means UMTS and GSM target cell list are sorted according to their priority and level.



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UMTS cells and GSM cells internally are sorted according to their HCS priority. The cell with the highest HCS priority is sorted to the top position of the TCL-sublist. If the HCS priority is the same, GSM cells are sorted in the same way as for a GSM-internal HO.

Detailed description of TCL sorting in dependence of Service HO information:

1. If the IE "Service Handover" is set to "Handover to UTRAN or cdma2000 shall not be performed", the BSC/BTS will not select an UMTS cell; i.e. the MS/UE is never handed over from GSM to UMTS. The MS/UE is kept within the GSM system. The BTS generates a TCL without UMTS target cells. The "Sufficient UMTS Coverage" HO is not possible for this MS/UE.

2. If the IE "Service Handover" is set to "Handover to UTRAN or cdma2000 should not be performed", the MS/UE should be kept in GSM as long as possible. In order to achieve this, the BTS only includes UMTS cells in the target cell list (TCL) in case of an "imperative HO". I.e., in case of a "Better cell HO", the BTS generates a TCL without UMTS target cells for this MS/UE. The "Sufficient UMTS Coverage" HO is not possible for this MS/UE.

In order to achieve that the MS/UE is kept in GSM as long as possible, the sorting of the TCL in case of an "imperative HO" must be modified for this MS/UE, because it must be avoided that an UMTS cell is sorted before a GSM cell. Therefore, GSM and UMTS cells in a "mixed" TCL, i.e. in a TCL which contains GSM and UMTS cells, are not sorted together according to HCS priority and PRIO_PBGT.

Instead, the UMTS cells and GSM cells will be sorted separately; the GSM cells will be sorted relative to other GSM cells according to HCS priority and PRIO_PBGT (GSM-sublist). The UMTS cells will be sorted relative to other UMTS cells according to HCS priority and PRIO_PBGT (UMTS-sublist in case of FDD_REP_QUANT = 0); and the GSM-sublist will be sorted before the UMTS-sublist to form the complete TCL (for FDD_REP_QUANT = 1).

This modification of the TCL sorting ensures, that an UMTS neighbor cell will only be on top of the TCL if there is no GSM neighbor cell available.


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3. If the IE "Service Handover" is set to "Handover to UTRAN or cdma2000 should be performed", the BSC/BTS should handover the MS/UE from GSM to UMTS.

The BTS uses the "Sufficient UMTS Coverage" HO in order to support this IE service handover, i.e. the BTS will initiate a "Sufficient UMTS Coverage" HO for this MS/UE when the "sufficient threshold" condition of the "Sufficient UMTS Coverage" HO is met. The BTS initiates a "Sufficient UMTS Coverage" HO for this MS/UE even if the "Sufficient UMTS Coverage" HO is generally disabled (i.e. Enable_Sufficient_UMTS_Coverage_HO_to_UMTS is set to FALSE) in order to allow the handover to UMTS for this MS/UE when the "sufficient threshold" condition is met.

In order to achieve that the MS/UE is handed over to UMTS as soon as possible also in case of an "Imperative HO", the sorting of the TCL in case of an "Imperative HO" must be modified for this MS/UE because it must be avoided that a GSM cell is sorted before an UMTS cell. Therefore, GSM and UMTS cells in a "mixed" TCL, i.e. in a TCL which contains GSM and UMTS cells, will not be sorted together according to HCS priority and PRIO_PBGT.

Instead, the UMTS cells and GSM cells are sorted separately; the GSM cells are sorted relative to other GSM cells according to HCS priority and PRIO_PBGT (GSM-sublist); the UMTS cells are sorted relative to other UMTS cells according to HCS priority and PRIO_PBGT (UMTS-sublist in case of FDD_REP_QUANT = 0); and the GSM-sublist is sorted after the UMTS-sublist to form the complete TCL (for FDD_REP_QUANT = 1).

This modification of the TCL sorting ensures, that a GSM neighbor cell will only be on top of the TCL if there is no UMTS neighbor cell available.

4. If the IE "Service Handover" is not set, it is not important whether a MS/UE is handed over to UMTS or kept GSM. The “best” cells will be taken. The BTS will select UMTS and GSM cells. A mixed TCL is sorted according to HCS priority and PRIO_PBGT.



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Sorting of UMTS and GSM cells which have different HCS priority:

The BTS sorts the UMTS cells and the GSM cells according to their HCS priority. I.e. cells with a higher HCS priority (= lowest value of PRIO_NCELL) are sorted before cells with lower HCS priority. The cell with the highest HCS priority is sorted to the top position of the TCL.

In the following cases, GSM and UMTS cells in a "mixed" TCL, i.e. in a TCL which contains GSM and UMTS cells, are not sorted together according to HCS priority:

1. In case of GSM to GSM "Better cell" HO

1. If FDD_REP_QUANT = 1: Only the GSM cells are sorted according to HCS priority. UMTS cells are not included in the TCL.

2. If MS/UE is on SDCCH and Enable_SDCCH_HO_to_UMTS = FALSE: Only GSM cells are sorted according to HCS priority. UMTS cells are not included in the TCL for this MS/UE.

3. If ASCI_talker = TRUE: Only GSM cells are sorted according to HCS priority. UMTS cells are not included in the TCL for this MS/UE.

2. In case of GSM to UMTS "Better cell" HO

1. If FDD_REP_QUANT = 1: The "Better cell HO" from GSM to UMTS is disabled.

2. If MS/UE is on SDCCH and Enable_SDCCH_HO_to_UMTS = FALSE: The "Better cell HO" from GSM to UMTS is disabled for this MS/UE.

3. If ASCI_talker = TRUE: The "Better cell HO" from GSM to UMTS is disabled for this MS/UE.


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3. In case of a "Sufficient UMTS coverage" HO:

A TCL, which only contains UMTS cells, is generated and sorted according to HCS priority.

1. If MS/UE is on SDCCH and Enable_SDCCH_HO_to_UMTS = FALSE: The GSM to UMTS "Sufficient UMTS coverage HO" is disabled for this MS/UE.

2. If ASCI_talker = TRUE: The GSM to UMTS "Sufficient UMTS coverage HO" is disabled for this MS/UE.

4. In case of an "imperative" HO

1. If MS/UE is on SDCCH and Enable_SDCCH_HO_to_UMTS = FALSE: Only GSM cells are sorted according to HCS priority. UMTS cells are not included in the TCL for this MS/UE.

2. If ASCI_talker = TRUE: Only GSM cells are sorted according to HCS priority. UMTS cells are not included in the TCL for this MS/UE.



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Sorting of UMTS and GSM cells which have same HCS priority:

GSM cells are sorted in the same way as for a GSM-internal HO, i.e. they are sorted according to the Power-Budget term "PRIO_PBGT". I.e. GSM cells with a higher "PRIO_PBGT" are sorted before GSM cells with lower "PRIO_PBGT".

For GSM cells, the term "PRIO_PBGT" is defined as:

PRIO_PBGT = PBGT - HOMARGIN

For UMTS cells, the term "PRIO_PBGT" is defined as:

PRIO_PBGT = PBGT - HOMARGIN - UMTS_ADJUST

In the following cases, GSM and UMTS cells in a "mixed" TCLare not sorted together according to PRIO_PBGT:

1. In case of GSM to GSM "Better cell" HO

1. If FDD_REP_QUANT = 1: In case of a "Better cell" HO from GSM to GSM, a "mixed" TCL is not sorted according to PRIO_PBGT if FDD_REP_QUANT is set to 1, because the UMTS FDD measurement quantity Ec/No is not a suitable parameter for a comparison with the GSM received level (Ec/No is a quality parameter and not a receive level parameter). Instead, only the GSM cells are sorted according to PRIO_PBGT. UMTS cells are not included in the TCL.

2. If MS/UE is on SDCCH and Enable_SDCCH_HO_to_UMTS = FALSE: Only GSM cells are sorted according to PRIO_PBGT. UMTS cells are not included in the TCL for this MS/UE.

3. If ASCI_talker = TRUE: Only GSM cells are sorted according to PRIO_PBGT. UMTS cells are not included in the TCL for this MS/UE.

2. In case of GSM to UMTS "Better cell" HO

1. If FDD_REP_QUANT = 1: The "Better cell HO" from GSM to UMTS is disabled if FDD_REP_QUANT = 1 because the UMTS FDD measurement quantity Ec/No is not a suitable parameter for a comparison with the GSM received level.

2. If MS/UE is on SDCCH and Enable_SDCCH_HO_to_UMTS = FALSE: The "Better cell HO" from GSM to UMTS is disabled for this MS/UE.

3. If ASCI_talker = TRUE: The "Better cell HO" from GSM to UMTS is disabled for this MS/UE.


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3. In case of a "Sufficient UMTS coverage" HO:

In case of a "Sufficient UMTS coverage" HO, a "mixed" TCL is not sorted according to PRIO_PBGT in order to avoid that a GSM cell is sorted before a UMTS cell.

Instead, in case of a "Sufficient UMTS coverage" HO, a TCL which only contains UMTS cells is generated. If FDD_REP_QUANT = 0, the UMTS cells (FDD and TDD) are sorted according to RSCP. If FDD_REP_QUANT = 1, the FDD cells are sorted according to Ec/No and are sorted before the TDD cells (which are sorted according to RSCP). The main objective of the GSM to UMTS "Sufficient UMTS coverage HO" is to perform a handover to the UMTS system as soon as possible, i.e. as soon as the UMTS measurement quantity (RSCP or Ec/No for FDD cells, RSCP for TDD cells) exceeds a certain "sufficient" threshold in order to direct as much traffic as possible to areas which offer "sufficient" UMTS coverage.

1. If MS/UE is on SDCCH and Enable_SDCCH_HO_to_UMTS = FALSE: The GSM to UMTS "Sufficient UMTS coverage HO" is disabled for this MS/UE.

2. If ASCI_talker = TRUE: The GSM to UMTS "Sufficient UMTS coverage HO" is disabled for this MS/UE.



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4. In case of an "imperative" HO:

1. If FDD_REP_QUANT = 1: An "imperative" HO (imperative HO causes: "uplink quality", "downlink quality", "uplink strength", "downlink strength", "distance", "forced") is initiated e.g. because for the GSM serving cell the received level is too low or the quality is too low or the MS-BTS distance is too large.

In case of an "imperative" HO, a "mixed" TCL is not sorted according to PRIO_PBGT if FDD_REP_QUANT is set to 1 because the UMTS FDD measurement quantity Ec/No is not a suitable parameter for a comparison with the GSM received level. Instead, the UMTS cells and the GSM cells are sorted separately; only the GSM cells are sorted according to PRIO_PBGT (GSM-sublist). The UMTS FDD cells are sorted according to Ec/No and are sorted before the TDD cells (which are sorted according to RSCP) in order to form the UMTS-sublist.

To form the complete TCL, the GSM-sublist is sorted before the UMTS-sublist if the IE "Service Handover" is set to "Handover to UTRAN or cdma2000 should not be performed" or if this IE is missing at the A-Interface. The GSM-sublist is sorted after the UMTS-sublist if the IE "Service Handover" is set to "Handover to UTRAN or cdma2000 should be performed". If the IE "Service Handover" is set to "Handover to UTRAN or cdma2000 shall not be performed", UMTS cells shall not be included in the TCL because this MS/UE shall never be handed over to UMTS.

2. If FDD_REP_QUANT = 0 the sorting depends on IE "Service Handover"

3. If MS/UE is on SDCCH and Enable_SDCCH_HO_to_UMTS = FALSE: Only GSM cells are sorted according to PRIO_PBGT. UMTS cells are not included in the TCL for this MS/UE.

4. If ASCI_talker = TRUE: Only GSM cells are sorted according to PRIO_PBGT. UMTS cells are not included in the TCL for this MS/UE.


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Short Summary:

If the reason for HO is Power Budget and measurement quantity RSCP is used, the UMTS cells are sorted in the same way as GSM neighbor cells except the usage of the UMTS_ADJUST parameter. If measurement quantity Ec/No is used, UMTS target cells are not allowed.

If the reason for HO is imperative HO and RSCP is used, the UMTS cells are sorted like in Power Budget HO. The UMTS FDD cells are sorted according to Ec/No and are sorted before the TDD cells (which are sorted according to RSCP).

If the reason for HO is Sufficient UMTS Coverage the UMTS cells are sorted according to their measurement quantity. The TCL only contains UMTS target cells. If FDD_REP_QUANT = 0, the UMTS cells (FDD and TDD) are sorted according to RSCP. If FDD_REP_QUANT = 1, the FDD cells are sorted according to Ec/No and are sorted before the TDD cells (which are sorted according to RSCP).



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3.8.9 Bookkeeping of adjacent cells

The bookkeeping of the adjacent cell list is upgraded to include the UMTS neighbor cells. The current BTS bookkeeping list contains the averaged MS neighbor cell measurements of up to 32 neighbor cells.

Since the neighbor cell list of the MS/UE may contain more than 32 neighbor cells, the current BTS bookkeeping mechanism needs to be adapted in order to handle MS measurements of more than 32 neighbor cells. In order to avoid an increased BTS memory consumption, the size of the bookkeeping list is not increased beyond 32 neighbor cells.

Instead, if the bookkeeping list is already filled with 32 neighbor cells and the MS reports an additional new cell, this new cell replaces another cell already contained in the bookkeeping list. The cell with the lowest neighbor cell received level average is replaced by the new cell.

If a neighbor cell is overwritten, no relevant results have been measured in the last measurement periods.

The different measurement quantities RSCP and Ec/No are registered in the bookkeeping list equally. (The parameter fdd_rep_quant indicates, whether the reporting quantity is RSCP or Ec/No.)


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3.8.10 Message flow

The BSS initiates a handover to UMTS by including the UMTS target cells in the “Handover –Required” message.

The 3G-MSC performs the UMTS service mapping (for speech and data) and transcoder mapping as well as the GSM to UMTS cipher key conversion. The 3G-MSC generates and sends the “Iu relocation request” message to the UMTS radio network system (RNS).

After having receipt the “Iu-Relocation-Request-Ack” message, the 3G-MSC sends the “handover command” to the BSS, which sends the “RI-HO-Command” message (Radio Interface to UTRAN handover command) to the UE/MS. Now the UE/MS is instructed to tune to a new radio resource.

After the UE/MS has performed the radio access to the UMTS cell, the RNS sends the “Iu relocation detect” message to the 3G-MSC. As soon as the UE/MS has completed the handover and is successfully communicating with the target RNS, it will send the “radio resource control handover complete” message to the target RNS. The target RNS sends the “Iu relocation complete” message to the 3G-MSC. Then the 3G-MSC sends the “clear command” message to the source BSS, which starts releasing resources because they are no longer in use. After this, the source BSS returns the “clear complete” message to the 3G-MSC, and the execution of the handover is completed.



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UE

BSS-A 3G MSC RNS

Handover-Required

Iu-Relocation-Request

Iu-Relocation-Request-Ack

Handover-Command

RI-HO-Command

Iu-Relocation-Detect RRC-HO-Complete

Iu-Relocation-Complete

Clear-Command

Clear-Complete

UE/MS

Fig. 27 The message flow of a GSM to UMTS handover


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3.8.11 Database parameters

The parameters for the handover from GSM to UMTS are located in the following objects:

1. BTS

2. BSC

3. HAND

4. ADJC3G (Adjacent Cell 3G)

5. TGTFDD (Target Cell FDD)

6. TGTTDD (Target Cell TDD)



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Name Range (Default) Object Description

EUHO TRUE, FALSE (def.) HAND “Main switch” for GSM to UMTS HOs

EUBCHO TRUE, FALSE (def.), NULL

HAND Enable Better Cell HO from GSM to UMTS (cannot be TRUE if FDDREPQTY=1)

EUSCHO TRUE, FALSE (def.) HAND Enable Sufficient UMTS Coverage HO

EUIMPHO TRUE, FALSE (def.) HAND Enable Imperative HO from GSM to UMTS (risk for call drop: no comparison with other cells – no change with UMTS neighbor cells)

EUSDCHO TRUE, FALSE (def.), NULL

BSC Enable Directed Retry from GSM to UMTS

UADJ 0 (-63 dB) ..63 (0 dB, default) .. 126 (63 dB)

ADJC3G For Better Cell HO: adjustment of level for UMTS cell to level of GSM serving cell (63 def.)

USRCP 0 (RSCP < -115 dBm) .. 63 (>=53 dBm);

ADJC3G Threshold required for “sufficient UMTS cell coverage” (FDD: FDDREPQTY=0 required)

USECNO 0 (Ec/N0 < 24 dB) .. 49 (Ec/N0 >= 0 dB)

ADJC3G Threshold required for “sufficient UMTS cell coverage” (FDD: FDDREPQTY=1; 23 def.)

UMECNO 0 (Ec/N0 < 24 dB) .. 49 (Ec/N0 >= 0 dB)

ADJC3G Min. Level required to include a UMTS cell in target cell list for imp. HO (FDDREPQTY=1 req; 19 def.)

FDDREPQTY 0 (RSCP, def.), 1 (Ec/N0)

BTS Reporting quantity for UTRAN FDD cells

Parameters in HAND, BSC, ADJC3G and BTS object


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CELLGLID MCC–MNC–LAC-CI TGTFDD Cell Global ID

RNCID 0 .. 4095 TGTFDD RNC identifier

FDDARFCN 412 .. 10838 TGTFDD Absolute radio frequency carrier no. (FDD)

FDDSCRMC 0 .. 511 TGTFDD Primary scrambling code

FDDDIV Diversity (1), No_Diversity (0)

TGTFDD Indicates if diversity is used (or not) in FDD cell

Parameters in TGTFDD object


CELLGLID MCC–MNC–LAC-CI TGTTDD Cell Global ID

RNCID 0 .. 4095 TGTTDD RNC identifier

BNDWIDTDD 0 (HCR), 1 (LCR); TGTTDD Bandwidth TDD cell

TDDARFCN 9262 .. 10113 TGTTDD Absolute radio frequency carrier no. (TDD)

TDDCPAR 0 .. 127 TGTTDD TDD cell parameter

TDDSYNCCAS SYNC1, SYNC2 TGTTDD Synchronization case for TDD cell

TDDDIV Diversity (1), No_Diversity (0)

TGTTDD Indicates if diversity is used (or not) in TDD cell

Parameters in TGTTDD object



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QSRHC 0 (< -98 dB), 1 (<-94 dB) .. 7 (always) .. 14 (>-54 dB), 15 (never, def.)

BTS Threshold which enables the search for 3G cells for HO measurements on other RATs

QSRHCINI QSEARCHI (0, def.), Always (1)

BTS Qsearch values which is used before QsearchC is received

FDDMURREP, TDDMURREP

0 .. 3 BTS Number of FDD / TDD cells to be included in the measurement report

MSTXPMAXUMTS

0 (- infinity), 1 (- 28 dB), 2 (- 24 dB) .. 15 (28 dB)

BTS Offset applied to level of FDD / TDD serving cell (def. 8 – 0 dB)

Parameters in BTS object


TGTCELL pathname

(i.e. OBJ1:2/OBJ2:1/ OBJ3:5)

ADJC3G This attribute indicates the adjacent cell, namely the BTS object if it belongs to the same BSC (TGTCELL = BTSM:#/BTS:#) or the TGTBTS if it belongs to another BSC (TGTCELL = TGTBTS:#), or the TGTFDD if it is a UMTS one.

PLNC 0 .. 15 ADJC3G Priority layer of the adjacent cell

Parameters in ADJC3G object


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RXLEVMINC 0: RSCP < - 115 dBm

1: - 115 dBm <= RSCP < - 114 dBm

... ...

62: - 54 dBm <= RSCP < - 53 dBm

63: - 53 dBm <= RSCP

ADJC3G This parameter determines the minimum received signal level the adjacent UMTS cell must provide to be regarded as suitable target cell for handover.

TINHFAIHO 1 .. 254 (seconds) ADJC3G The BTS timer is started when MAXFAILHO has been reached. While the timer is running, the corresponding cell is excluded from the target cell list.

MICROCELL TRUE, FALSE (def.) ADJ3G To enable Mobile Speed Sensitive Handover

HOM 0 (- 63 dB), 1 (-62 dB) .. 69 (6 dB, def.) .. 126 (+63 dB)

ADJ3G Threshold to prevent repetitive HO if caused by RX level or power budget

HOMDTIME 0 (def.) .. 255 (SACCH multiframes)

ADJ3G This parameter is relevant for speed sensitive handover; it specifies the time an immediate handover request is delayed when a power budget handover is requested.

HOMSOFF 0 (def.) .. 127 (in dB) ADJ3G Static offset for HO margin (during run-time of HOMDTIME)

PPLNC 0 (def.) .. 15 ADJ3G Temporary penalty priority layer of the adjacent UMTS cell

HOMDOFF 0 (def.) .. 127 (dB) ADJ3G Dynamic offset for reduction of HO margin after HOMDTIME has expired

Parameters in ADJC3G object



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3.9 Service dependent handover management

The feature Service dependent Handover Management distinguishes fourteen service groups. This concerns Circuit-Switched services (CS) on Half Rate (HR), Full Rate (FR), Enhanced Full Rate (EFR), Adaptive Multi-Rate (AMR), Advanced Speech Call Items (ASCI), Voice Broadcast Services (VBS), Voice Group Call Services (VGCS), and High Speed Circuit-Switched Data services (HSCSD).

For each service group relevant threshold parameters for power control can be defined individually.

The different service groups are summarized in the table below:

Service Group Description

SG-1 Signaling on hopping channel

SG-2 Signaling on non-hopping channel

SG-3 CS speech (FR, EFR, ASCI VBS, ASCI VGCS) on hopping channel

SG-4 CS speech (FR, EFR, ASCI VBS, ASCI VGCS) on non-hopping channel

SG-5 CS speech (HR) on hopping channel

SG-6 CS speech (HR) on non-hopping channel

SG-7 CS data up to 9,6kbit/s or HSCSD 9,6kbit/s on hopping channel

SG-8 CS data up to 9,6kbit/s or HSCSD 9,6kbit/s on non-hopping channel

SG-9 CS data up to 14,4kbit/s or HSCSD 14,4kbit/s on hopping channel

SG-10 CS data up to 14,4kbit/s or HSCSD 14,4kbit/s on non-hopping channel

SG-11 CS speech (AMR-FR) on hopping channel

SG-12 CS speech (AMR-FR) on non-hopping channel

SG-13 CS speech (AMR-HR) on hopping channel

SG-14 CS speech (AMR-HR) on non-hopping channel

If parameters are set for a specific service group, the system will use those values for the handover algorithm, otherwise the global parameter settings are used.



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Settings for

Service groups activated

in the BSC area

HAND object

General settings for

BSC area for all SGs that

are not activated HAND parameters for SG1:

• HOLTHLVDL

• HOLTHQUDL

• …

HAND parameters for SG14:

• HOLTHLVDL

• HOLTHQUDL

• …

HAND parameters:

• INTERCH

• INTRACH

• HOLTHLVDL

• HOLTHQUDL

• …

• SG1PCPAR

• SG2PCPAR

• …

• SG14PCPAR

Fig. 28 Principle of service dependent handover management



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4 Exercises



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Exercise 1

Title: Cell Reselection

Task

Consider a static (not moving) MS GSM phase 1 of power class 3 camping on cell 1 in idle mode.

The MS monitors the BCCH of cell 1 and cell 2 and measures the following levels:

AV_RXLEV = 26 in cell 1

AV_RXLEV = 20 in cell 2

The following parameters are set:

Cell 1: MS_TXPWR_MAX_CCH = 39 dBm RXLEV_ACCESS_MIN = 20 CELL_RESELECT_HYSTERESIS = 4dB

Cell 2: MS_TXPWR_MAX_CCH = 33 dBm RXLEV_ACCESS_MIN = 14

Does the MS perform a cell reselection

1. if cell 1 and cell 2 belong to one location area

2. if cell 1 and cell 2 belong to different location areas



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Exercise 2

Title: Handover Decision

Task

Consider of output power P = 39 dBm in connected mode served by cell 1. In cell 1 the following handover parameter are valid (consider only the downlink in the following):

A_QUAL_HO = 7 L_RXQUAL_DL_H = 5

A_LEV_HO = 7 L_RXLEV_DL_H = 10

A_PBGT_HO = 8 L_RXLEV_DL_IH = 13

W_QUAL_HO = 3

W_LEV_HO = 3 MS_TXPWR_MAX = 39

Furthermore, cell 1 has the following adjacent cells:

Parameters for Adjacent Cells.

adjacent cell parameter cell 2 cell 3

ARFCN_NCELL 35 13

BSIC 00 00

MS_TXPWR_MAX 37 39

RXLEV_MIN 16 16

HO_MARGIN 6 dB ?

The measurement reports for the last 8 SACCH frames have contained the following values:


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Measurement Values: F: FULL, S: SUB.

SACCH frame 1 2 3 4 5 6 7 8

RXQUAL_SERV 3 S 4 S 4 F 4 F 6 F 4 S 5 S 7 F

RXLEV_SERV 21 S 16 S 15 F 16 F 12 F 16 S 14 S 10 F

BCCH_FREQ_NCELL/ BSIC

0 / 00

0 / 00

0 / 00

0 / 00

0 / 00

0 / 00

0 / 00

0 / 00

RXLEV_NCELL 14 15 16 17 17 18 19 20

BCCH_FREQ_NCELL/ BSIC

1/ 00

1/ 00

1/ 00

1/ 00

1/ 00

1/ 00

1/ 00

1/ 00

RXLEV_NCELL 15 16 17 18 18 19 20 21

Due to DL power control the BTS transmit power level is reduced by the following values:

PWR_C_D 8 8 8 6 6 6 4 2

1. Which cell corresponds to BCCH_FREQ_NCELL = 0?

2. What are average values for RXLEV_DL RXQUAL_DL

PBGT (→ cell 2), PBGT (→ cell 3)?

3. Is cell 2 included within the target cell list for a handover?

4. Which value for HO_MARGIN (cell 1 → cell 3) is required to allow a better cell handover to cell 3?

5. Assume that a better cell handover to cell 3 occurs. Some seconds after the handover the received level form cell1 has increased by 6 dB, while the received level from the new serving cell 3 remains constant. To which HO_MARGIN (cell 3

→ cell 1) has to be set to prevent a „back-handover“ to cell?



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5 Solutions



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Solution 1

Title: Cell Reselection

Task

For a phase 1 MS the following conditions for cell reselection have to be fulfilled:

for

1. own location area C1 (cell 2) > C1 (cell 1),

for

2. different location areas C1 (cell 2) > C1 (cell 1) + CELL_RESELECT_HYSTERESIS.

The C1 value is given by

C1 = AV_RXLEV - ACCESS_MIN - Max (0, MS_TXPWR_MAX_CCH - P)

i.e. for this scenario one has (power class 3 = 37 dBm):

C1 (cell 1) = 26 - 20 - Max (0, 39-37) = 4

C1 (cell 2) = 20 - 14 - Max (0, 33-37) = 6

If cell 1 and cell 2 belong to the same location area, cell reselection takes place. If they belong to different location areas and if CELL_RESELECT_HYSTERESIS > 2 dB, no cell reselection takes place.



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Solution 2

Title: Handover Decision

Task

1. The cell with BCCH_FREQ_NCELL = 0 is the lowest ARFCN of the bcch frequencies (among the neighbor cells of cell 1), i.e. cell 3.

2. The averaging windows for RXLEV and RXQUAL of the serving cell contain 7 values. Since the weight for full values is three, the value of frame 8 is taken with multiplicity 3; frame 7 and 6 is taken with multiplicity 1. The remaining 2 values for the averaging window are take from the full value of frame 5. Therefore one has: AV_RXQUAL = 1/7 * (3 * 7 + 5 + 4 + 2 * 6) = 6 AV_RXLEV = 1/7 * (3 * 10 + 14 + 16 + 2 * 12) = 12 For calculation of the power budget the values of SACCH frame 1...8 are averaged without taking into account multiplicities. The power budget is given by the following formula: PBGT(n) = RXLEV_NCELL(n) - (RXLEV_DL + PWR_C_D) + Min(MS_TXPWR_MAX, P) - Min(MS_TXPWR_MAX(n), P) For the average values one has: Min (MS_TXPWR_MAX, P) - Min (MS_TXPWR_MAX (cell 2), P) = 2 Min (MS_TXPWR_MAX, P) - Min (MS_TXPWR_MAX (cell 3), P) = 0 RXLEV_DL + PWR_C_D = 15 + 6 = 21 RXLEV_NCELL (cell 2) = 18 RXLEV_NCELL (cell 3) = 17 => PBGT (cell 2) = 18 - 21 + 2 = - 1 PBGT (cell 3) = 17 - 21 + 0 = - 4

3. Although one has AV_RXQUAL > L_RXQUAL_DL_H no quality handover is initiated: no intracell handover since AV_RXLEV < L_RXLEV_DL_IH no intercell handover since the BTS transmit power is not at its maximum. Furthermore, no level handover is initiated. Since for a power budget handover the target cell list only contains cells with PBGT (n) > HO_MARGIN(n) - which is not fulfilled by cell 2 - cell 2 is not included in the target cell list.


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4. To allow a better cell handover to cell 3, the HO_MARGIN (cell1 → cell 3) has to be set to - 5 or a lower value.

5. PBGT (cell 3 → cell 1) = - PBGT (cell 1 → cell 3) + 6 dB improvement of cell1 = 10 dB hence to prevent a power budget handover from cell 3 to cell1,

HO_MARGIN (cell 1→ cell 3) has to be set to 10 dB or higher.

03 Mn1789eu10mn 0002 Design Radio Cells

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