23616119-03-Mn1789eu11mn-0001-Design-Radio-Cells

Design of radio cells Siemens

MN1789EU11MN_0001 © 2005 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 3 Handover 27 3.1 General notes on handover 28 3.2 Measurement preprocessing 35 3.3 Measurement reporting and neighbor cell book-keeping 42 3.4 Threshold comparisons and handover detection algorithms 47 3.5 Target cell list generation 68 3.6 Handover signaling and timer 71 4 Handover types 75 4.1 Handover types belonging to radio criteria 76 4.2 Compression/Decompression HO Improvements 84 4.3 Handover types belonging to network criteria 87 4.4 Service dependent handover management 112 4.5 AMR-handover 115 5 Exercises 123 6 Solutions 131

Design of radio cells

Siemens Design of radio cells

MN1789EU11MN_0001© 2005 Siemens AG

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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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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. Note : For phase 2 mobile stations there is an additional cell selection 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. This parameter is relevant only during the cell selection process. 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 SelectionAlgorithm

(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 anacceptable cell

no

yes

no

no

yes

no

no yes

yes

low prioritycell found

Camp on lowpriority cell

suitable low prioritycell found

try only carriers ofBCCH allocation

try only normal prioritycells

remove trialcarrier from list

nocellin selected

PLMN

Camp on normalpriority 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.

• Downlink Signaling Failure Downlink Signaling failure Counter DSC is set to initial value DSC0 = round (90 / BS_PA_MFRMS). The counter value changes in time, dependent on whether a message on paging sub-channel is successfully or unsuccessfully decoded:

successful: → DSCt+1 = DSCt + 1 but if DSCt = DSC0 → DSCt+1 = DSC0 unsuccessful: → DSCt+1 = DSCt – 4

DSC < 0 indicates 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. It is described in the following paragraph.



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Time (unit: BS_PA_MFRMS)

Downlink SignalingCounter DSC

DSC0

DSC0=RND(90/BS_PA_MFRMS)

DSC0<=0:Downlink Signaling

Failure

- 4unsuccessful decoding

+1successful decoding

Fig. 2 Downlink signaling failure



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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.



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C2

C1CELL_RESELECT_OFFSET

TPENALTY_TIME

TEMPORARY_OFFSET

Cell included in thelist of 6 strongest

Fig. 3 Illustration of the C2 criterion

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 (Hierarchical Cell Structures).



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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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Parameters for Cell Selection/Reselection BCCH_ARFCN_NC(n) BA - BCCH Allocation Object DB Name Range Step Size Description BTS / TGTBTS BCCHFREQ 0...1023 1 ARFCN used for BCCH

by neighbor cell

BTS EFCRECEL TRUE/FALSE (FALSE)

Parameter to include/exclude serving cell BCCH frequency in/from the BA list

In each cell the absolute radio frequency number BCCH_ARFCN_NC(n) (coding as given in chapter 1) 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. The last SBS release gives an operator the opportunity to introduce the BCCH frequency of the serving cell in the BA list. This is done via parameter EFCRECEL. The benefit is much faster camp on of the MS. When switched on, due to stored last known BA list (that includes serving BA too) in a non-volatile memory, MS doesn't need to search for the BA. It camps immediately on the best cell-no need for cell reselection and therefore the call set up is also speed up.

SYS_ID - System Identifier

Object DB Name Range Description

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

System Indicator -indicates the frequency band used by traffic channels

Value to be set if cell supports Common BCCH bual band operation: GSMDCS: GSM base band available for MS phase 1, exten.band for MS phase 2 EXT900 Only phase 2 MS can be used, phase 1 not supported F2ONLY900



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CELL_BAR_ACCESS - Cell Barred for Access

Object DB Name Range Step Size Unit 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 MS access classes has to be used. Barring access for an access class does not trigger a cell reselection for MSs of that class. Access barring is done using the access classes specified by GSM (refer to GSM 08.08). There are 10 access classes (0...9) related to normal subscribers. The access class to which a subscriber belongs is derived from his IMSI. Furthermore, there are 5 classes (11...15) assigned to special high priority subscribers (e.g. police, PLMN operator.....).



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MS_TXPWR_MAX_CCH - Maximum allowed MS Transmit Power on RACH

Object DB Name Range Step Size Unit BTS MSTXPMAXCH 0...31 (5) 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 maximum output transmit power P of the MS (Power Class)

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

Power Class (GSM 05.05)

Max. Output Power (GSM 900 Phase 2)

Max. Output Power (GSM 900 Phase 1)

Max. Output Power (DCS 1800)

1 -- 20Watt = 43dBm 1 Watt = 30 dBm

2 8 Watt = 39 dBm 8 Watt = 39 dBm 0.25W = 24 dBm

3 5 Watt = 37 dBm 5 Watt = 37 dBm 4 Watt = 36 dBm

4 2 Watt = 33 dBm 2 Watt = 33 dBm

5 0.8 Watt= 29 dBm 0.8 Watt= 29 dBm



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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.



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POWER_OFFSET - Additional Powerclass for Class 3 DCS1800 MS

Object DB Name Range Step Size Unit 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

Object DB Name Range Step Size Unit BTS RXLEVAMI 0...63 (6) 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).



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(l)(h)

Cell Reselection(a) no change of location area(b) change of location area

Radius of Cell 2for 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. 4 Illustration of cell selection/reselection



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

Object DB Name Range Step Size Unit BTS CELLRESH 0...7 (2) 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: 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).



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Additional Phase 2 Parameters for Cell Selection/Reselection

CELL_BAR_QUALIFY Object DB Name Range Step Size Unit

BTS CBQ 0...1 (0) 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

Object DB Name Range Step Size Unit BTS CRESPARI 0...1 (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

Object DB Name Range Step Size Unit BTS PENTIME 0...30 and 31 (5) 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

Object DB Name Range Step Size Unit BTS TEMPOFF 0...7

(1) 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

Object DB Name Range Step Size Unit BTS CRESOFF 0...63 (1) 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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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



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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 Handover2.Intra-BSS Handover3.Intra-MSC Handover4.Inter-MSC Handover

BSC 2 4

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



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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 it is 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. The same received level in the better cell is achieved with a smaller transmit power 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.



32

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 tables 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/disable the Fast Uplink Handover

EN_INTER_ SDCCH_HO

IERCHOSDCCH Flag to enable/disable intercell SDCCH Handover

EN_INTRA_ SDCCH_HO

IRACHOSDCCH Flag to enable/ disable intracell SDCCH Handover



33

EN_LEV_HOM ELEVHOM Flag to enable/disable Level Handover Margin for Level Handover

EN_QUAL_HOM ENAQUALEVHOM

Flag to enable/disable Level Handover Margin for Quality Handover

EN_UMTS_HO EUHO Flag to enable/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

The following three flags to enable/ disable HO are administered in the object SET BSC .

Specification Name

DB Name Meaning

EN_FORCED_ INTRA_HO

EFOIAHO Flag to enable/disable forced Intracell Handover due to multislot call

EN_INTER_ SDCCH_HO

EISDCCHHO Flag to enable/disable Inter BSC SDCCH Handover.

EN_FORCED HO ENFORCHO Flag to enable /disable Forced Handover



34

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.



35

3.2 Measurement preprocessing

3.2.1 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 = 2 : 0.4% < BER < 0.8% assumed value: 0.57% RXQUAL = 3 : 0.8% < BER < 1.6% assumed value: 1.13% RXQUAL = 4 : 1.6% < BER < 3.2% assumed value: 2.26% RXQUAL = 5 : 3.2% < BER < 6.4% assumed value: 4.53% RXQUAL = 6 : 6.4% < BER < 12.8% assumed value: 9.05% 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 in [dBm] 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 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



36

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



37

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) NCC-BCC

0 - 7 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]



38

3.2.2 Averaging algorithm for Measurement Preprocessing The measured (and reported) data per SACCH multiframe are preprocessed within the BTS using a gliding average window, as the window works as a queue: when a new measurement is received the oldest one is removed from the window. The ''averaging period'' defines the size of the gliding averaging window for the measured values. the size of the averaging size determines the number of measurement samples) the new measurement sample is received every 480ms from the BTS or the MS) over which the BTS calculates the arithmetic average. 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. The DTX weighting factor determines how much more the FULL values shall be weighted for radio measurements results measured over a period with voice activity. The current weighting factor given by the parameter W_LEV_HO or W_QUAL_HO is stored in parallel under the same offset as illustrated in the figure below. The averaging window total is calculated by adding up all sample values currently stored within the averaging window while a single sample is added number of "weight" times. Then the total is divided by the "weight" total (all ''weight'' values within the averaging window are added up).

Example: Averaging of RXLEV when DTX enabled: : 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.



39

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

wi

ai21282227323128

21282227323128

Gliding WindowAveraging size=4

average value = 28

Measurement sample valueobtained each SACCH Multiframe (0.48s or 0.471s

RXLEV_SUB (weight 1) RXLEV_FULL (weight 2)

1212211

ai sample valuewi waiting factor of the sample value

1212211

Fig. 7 Illustration of weighting and averaging of measurement values



40

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 (6)

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

W_QUAL_HO

HOAVQUAL WQUALHO

1-3 (2)

Weighting factor for RXQUAL_FULL values

A_LEV_HO

HOAVLEV ALEVHO

1-31 (8)

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

W_LEV_HO

HOAVLEV WLEVHO

1-3 (2)

Weighting factor for RXLEV_FULL values

A_DIST_HO HOAVDIST 1-31 (8)

Averaging window size for Timing Advance values used for handover decisions due to distance.

A_PBGT_HO HOAVPWRB 1-31 (8)

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 (2)

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

W_LEV_FUL_HO ALEVFULHO WLEVFULHO

1...3 (2)

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



41

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.



42

3.3 Measurement reporting and neighbor cell book-keeping

These tasks are preceded in several steps:

• Definition of Neighbor Cells (Maximum Number = 64) 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. The choice is performed among the geographical neighbor cells if there is a traffic flow from the serving cell into these cells and eventually some alternative cells if there is a congestion in the preferred direct neighbor cell;

• Definition of 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 example) and 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.

NOTE Number of adjacent neighbor cells (object Create ADJC) is extended from 32 to 64. Up to 32 of them can be active adjacent cells used for cell reselection or/and HO if the parameter USG (object Create ADJC), if SI_2 or SI_5/SI_2_5 respectively. The parameter may have an additional new value introduced in BR8.0 USG=INACTIVE which indicates that adjacent cell is inactive, i.e. not to be used for cell reselection or/and HO but for measurements purposes only. A cell's BCCH is then included into 'measure extra BCCH frequencies' but only if the new parameter in the object Create BTS MEAEXTBCCH=TRUE. Up to 64 inactive and in active adjacent cells can be configured in total. If less then 32 adjacent cells are active more then 32 can be included in extra BCCH measurements.



43

Parameters for measurement reporting and neighbor cell book-keeping The parameters for measurement reporting and target cell generation of the object BTS are summarized in the table below.

DB Name Object Range Meaning 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

BCCHFREQ BTS /TGTBTS

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

BSIC = NCC + BCC 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

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

NCELL HAND 0…15 (6) Number of preferred cells for HO

MEAEXTBCCH BTS TRUE/ (FALSE=

Enable /disable feature 'Measure Extra BCCH'

USG HAND SI_2, SI_5. SI_2_5 INACTIVE

Parameter indicates whether the adjacent cell must be sent over System Info 2, 5 or both or is used only for measurement purposes (inactive)



44

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



45

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. 8

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



46



47

3.4 Threshold comparisons and handover detection algorithms

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



48

3.4.1 Decision 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 (chapter 4).

Handover Causes Decision Criteria Intercell HO due to Quality 9. RXQUAL_XX > L_RXQUAL_XX_H

10. RXLEV_XX < L_RXLEV_XX_IH * 11. XX_TXPWR = Min ( XX_TXPWR_MAX, P )

HO due to Level 12. RXLEV_XX < L_RXLEV_XX_H 13. XX_TXPWR = Min ( XX_TXPWR_MAX, P )

HO due to Distance 14. MS_BS_DIST > MS_RANGE_MAX

HO due to Power Budget 15. RXLEV_NCELL(n) > RXLEV_MIN(n) + Max ( 0, MS_TXPWR_MAX(n) - P )

16. PBGT(n) > HO_MARGIN(n)

Intracell HO due to Quality 17. RXQUAL_XX > L_RXQUAL_XX_H 18. RXLEV_XX > L_RXLEV_XX_IH

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

* parameter INTRACH =TRUE

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 to quality or level is only performed if the transmit power of the MS or BS respectively is on its maximum.



49

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

Handover Regions

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. 9 Regions of handover defined by quality and level thresholds



50

Power Budget: Power Budget is calculated in the following way: 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).



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 (5)

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 (10/8)

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 (35)

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 (34)

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 (99)

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 (8)

Receive signal strength threshold for an intercell Fast Uplink HO decision



52


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 (12)

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 3 (69)

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



53

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.



54

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).

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:



55

RXLEV_MIN L_RXLEV_XX_H

BT S2

BT S3

BT S1

ideal powerbudget cell

border

Fig. 10 Cell borders defined by HO thresholds

L_RXLEV_XX_IH (inter / intracell quality HO)

RXLEV_MIN (incoming HO)

L_RXLEV_XX_H (outgoing level HO)

receiver limit sensitivity

BTS

Fig. 11 Relation between HO level thresholds



56

3.4.2 Priorities of handover causes 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 given in the following table.

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 intercell imperative UL-TCH



57

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

Handover Criteria Evaluation - Static Ranking

Evaluate HOcriterionpriority 1

HOdetected


HOdetected

HOdetected


Until evaluationof HO criterion 7

N

Y

N

Y

N

Y

Fig. 12 Static ranking of the HO criteria



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Handover Criteria Evaluation - Dynamic Ranking In some cases it is necessary to rank priorities dynamically to avoid hanging on one HO criteria: 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. Dynamic ranking of HO evaluation is also performed in case of Quality inter/intracell HO. The following flow chart shows in principle the dynamic ranking mechanism of Extended Cell HO/Concentric Cell HO.



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

Skip evaluationof HO criterion(skip_flag set)

Reset skip_flag

Y

Y

N

NHOconditionfulfilled

Set skip_flag

HO detected

Fig. 13 Dynamic ranking of handover causes



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3.4.3 Quality intracell handover

3.4.3.1 Intracell 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 algorithm is used. This skip_flag is set after an Intracell HO attempt and is processed in case of Quality Intercell detection. 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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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Max. power

Y

Y

Condition forQuality HO

fulfilled

reset skip_flag

Quality IntercellHO detected

N

N

Y

Skip Intracell HO(skip_flag set)

N

Y RXLEV <L_RXLEV_IH

N

Y

Fig. 14 Dynamic ranking of Quality Inter/Intracell Handover



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3.4.3.2 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 (2) 1 -

Timer_No_Intracell_HO HAND TINOIERCHO 1.. 254 (60) 1 1 sec

The following configuration rule should be regarded: TINOIERCHO (BTS timer) > THORQST (BTS timer) > T7 (BSC timer).

This condition allows the possibility to perform an intercell handover.



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BTS

TRX:0

TRX:1

TRX:2TRX:3

TRX:4

Fig. 15 Limitation of intracell handovers



64

3.4.4 Back Handover (due to Power Budget) Prevention In some cases back handovers, i.e. handovers to a cell just left before, should be avoided. 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 Specification Name Object DB Name Range Step

Size Unit

Enable_No_Back_HO HAND NOBAKHO TRUE/FALSE - -

Timer_Inhibit_Back_HO ADJC TINHBAKHO 1 .. 254 (30) 1 1 sec

TIMER_FORCED_HO ADJC TIMERFHO 0…300 (12) 1 10sec

TINHBACKHO defines the time how long a power budget back handover is prohibited to a cell just left with an Imperative Handover. TIMERFHO defines the time how long a power budget back handover is prohibited to a cell just left with an Forced Handover (refer to section Directed retry).



65

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. 16 General information flow

Cell A Cell B

Imperative HO

Power Budget HO

TINBACKHO

Fig. 17 Prevention of power budget back handover for time TINHBAKHO



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3.4.4.1 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

Specification Name Object DB Name Range Step Size

Unit

Enable_No_Failure_Rep_HO HAND NOFREPHO TRUE/FALSE - -

Max_Failure_HO HAND MAXFAILHO 1 .. 15 (2) 1 -

Timer_Inhibit_Failure_HO ADJC TINHFAIHO 1 .. 254 (7) 1 1 sec

The following configuration rule should be regarded: TINHFAIHO (BTS timer) > THORQST (BTS timer) > T7 (BSC timer).

This condition allows the possibility to perform the handover towards the second cell indicated in the target cell list.



67

Cell A

Cell B is not included in TCL

for a definedperiod time

BTS

HO Failure (cell B)

HO Failure (cell B)

after MAXFAILHOconsecutive HOfailures on the

same adjacent cells

BSC



HO Failure Ind (B)

HO Cond Ind (C, D)

Fig. 18 General information flow

CELL A CELL B

1

4

MAXFAILHO

...

TINFAILHO

23

Fig. 19 Concept of handover failure repetition prevention



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3.5 Target cell list generation

Generation of the Target Cell List The target cell list is generated by the BTS when a handover cause is detected. This list contains possible HO candidates i.e. neighbor cause to where the call can be handed over. 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 Only those neighbor cells are included in the target cell list which fulfill the following conditions:

• for Quality, Level, Distance and Traffic Intercell Handover: RXLEV_NCELL(n) > RXLEV_MIN(n) + MAX(0, MS_TXPWR_MAX(n) - P)

("minimum condition")

• for Power Budget Handover: RXLEV_NCELL(n) > RXLEV_MIN(n) + MAX(0, MS_TXPWR_MAX(n) - P) & PBGT(n) - HO_MARGIN(n) > 0

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.

The cell with the highest PRIO_NCELL(n) value will be listed first.



69

HO Required- cause- (reduced) target cell list

HandoverDetection

BTS

yes

yes

no

yes

try next cell

selectchannel

Handover Failure

HO Cond Ind- cause- target cell list

BSC

next cellavailable

next cell external

no

channel available

no

MSC

yesChannel Activation

HO cause = traffic& cell load > TRFLTH

no

yes

Fig. 20 Evaluation of Target Call List



70

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 figure above:



71

3.6 Handover signaling and timer

Measurements

MS

Measurements

BTSserving

HO Cond Ind

BSCserving

HO Required

MSC

HO Request

BSStarget

HO Request AckHO CommandHO CommandHO Command

HO Cond Ind HO RequiredHO Access

HO Access

Physical Info

HO Detect

T3105HO Complete

Physical Info

HO FailureHO Complete

Clear CommandChannel 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. 21 Non synchronized handover



72

Parameter and Timer for Handover Signaling T_HAND_REQ Object DB Name Range Step Size Unit HAND THORQST 0...31

(5) 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 T7 Object DB Name Range Step Size Unit BSC BSCT7 unit * 0...255

(HLFSEC-4) 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



73

T8 Object DB Name Range Step Size Unit BSC BSCT8 unit * 0...255

(HLFSEC-10) 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 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



74

T3105 Object DB Name Range Step Size Unit

BTS T3105 unit * 0...255 (MS10-10)

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

NY1 Object DB Name Range Step Size Unit

BTS NY1 0...254 (50)

1 -

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

Object DB Name Range Step Size Unit 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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4 Handover types



76

4.1 Handover types belonging to radio criteria

4.1.1 Fast Uplink Handover In areas of critical radio conditions, situations might occur in which a call drops because of long handover decision time. In case of an uplink level loss shorter than 3s the normal handover triggering with averaging windows (>4s) is too slow. A moving mobile will be lost at the cell boundary or in the shadowed areas. Fast Uplink Handover offers a high speed handover to prevent from rapid uplink level loss, thus saving connections in special places with call drop problems, where the power level of a mobile decreases rapidly. The handover can be performed to a predefined cell to save the connection. The BTS maintains a Fast Uplink Handover specific bookkeeping list for each possible adjacent cell (up to 32) in which the neighbor cell measurements of the mobile, the downlink RXLEV (RXLEV of serving cell measured by the mobile) and the BTS transmit power are compiled and averaged. This bookkeeping list is the basis for generation and sorting of the target cell list. As said above 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 (RXLEVUL<THLEVFULHO) 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.



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n-1

UL/DL reportingperiod length: 480 ms

Arrival of SACCH report containing a flagindicating if the MS used DTX

.

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

n n+1

'end of measurements'detected in interruptcontext

Fig. 22 Save of detection time



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Generation of the target cell list As a candidate for the target cell list, the minimum condition enhanced by an additional fast up link handover offset applies for the neighboring cell n:

RXLEV_NCELL(n) > RXLEVMIN(n) + max(0; Pa) + FULRXLVMOFF; where Pa = MSTXPMAX(n) – P MSTXPMAX(n) - maximum RF transmission power that a mobile station is permitted to use on a traffic channel in an adjacent cell. P- maximum transmission power capabilities of the mobile station.

Order Criterion for Fast Uplink HO Candidate Cells within the Target Cell List The target cell list is sorted into two sections, the first (high priority) section contains cells marked as predefined cells that have the attribute FastULHOCell (FULHOC) set to TRUE. The second section contains cells that are not predefined cells:

• Order Criterion: FAST_UL_HO_CELL(n) = TRUE FAST_UL_HO_CELL(n) = FALSE

• Order Criterion: PBGT(n)

Both sections are sorted in decreasing order by the power budget PBGT of the neighboring cells, i.e. the cell with the lowest value of PBGT is ranked last. Prevention of a FUHO The FUHO is not detected

• until the averaging window is full (end of measurements)

• as long as the timer for handovers is running (another handover has been triggered and is pending)

• when an assignment is pending (when the call is between assignment command and establish indication and due to these measurements are explicitly not compiled and averaged).

Other HO prevention algorithms (e.g. back-handover to the old cell) are not explicitly implemented so as to reserve the FUHO as a last opportunity to maintain a call.



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Database parameters DB Name Object Range Description EFULHO HAND TRUE,

FALSE (default) Flag used to enable/disable the FUHO. (Note: also internal intercell HO must be enabled, parameter INTERCH)

THLEVFULHO HAND 0 (< -110 dBm)... 63 (> -48 dBm), (8)

FUHO is possible when (FUHO average) RXLEVUL decreases below THLEVFULHO.

ALEVFULHO HAND aLevFuHo=1-31 (1 SACCH multiframe step size, default: 2) wLevFuHo=1-3 (step size 1, default: 1)

Averaging parameters used for FUHO signal strength measurements. aLevFuHo gives averaging window size (smaller than normal window size), wLevFuHo indicates weighting factor for "full" measurements (optional)

FULHOC ADJC TRUE, FALSE (default)

When searching for FUHO target cells, cells with attribute FULHOC "TRUE" are preferred to cells with FULHOC "FALSE"

FULRXLVMOFF ADJC 0, ... , 126 step size 1 dB, where 0=-63 dB, 126=63 dB, (default 69=6 dB)

RX level necessary for a neighbor cell to be included in the FUHO target cell list.



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4.1.2 Introduction of "Level Handover Margin" Parameters "Level Handover Margin" Parameters provide a new target cell criteria that takes the different requirements for level handover and quality handover into consideration. The creation of the target cell list is based on these new criteria.

4.1.2.1 Level Handover Margin for Level Handover 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 (parameter LEVHOM) condition should be satisfied. 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. HO procedure When the handover detection algorithm evaluates a handover condition, a target cell list is generated, i.e. a list of all adjacent cells which are considered as handover candidates. To guarantee that a level handover will be performed to a cell with a higher level than that of the serving cell, the candidate cell will now be included in the target cell list if additionally to the previously used condition

RXLEV_NCELL(n) > RXLEV_MIN(n) + max (0,MS_TXPWR_MAX-P) also condition

PBGT(n) > LEVHOM(n) is fulfilled. The operator on a cell basis can define this LEVHOM. The implementation of both conditions offers the operator the choice to prevent level handover below a certain level that the operator defines as not suitable for handover by setting RXLEVMIN. This new algorithm is very flexible since the operator still has the possibility to decide that RXLEVMIN does not impact the level handover by setting it to a low level.



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Database Parameters The operator has the possibility to enable or disable this feature. If this feature is disabled, the traditional condition

RXLEV_NCELL(n) > RXLEV_MIN(n) + max(0,MS_TXPWR_MAX-P) is chosen.

Object DB Name Range (Default)

Description

HAND ELEVHOM TRUE/ FALSE (FALSE)

Flag to enable or disable "Level Handover Margin".

ADJC LEVHOM 0...126, steps of 1 dB. 0 = - 63 dB, 126 = + 63 dB (69= 6 dB)

Only if the power budget of neighbor cells is higher than Level HO margin these neighbor cells will be included in the target cell list (if ELEVHOM=TRUE).



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4.1.2.2 Level Handover Margin for Quality Handover The single use of the minimum level criteria for the target cell may not guarantee in all cases a sufficient signal level to maintain the connection after HO. To guarantee a target cell with a higher level than the serving cell an additional condition "Quality handover condition" is introduced. Neighbor cell is regarded as a suitable target cell and is thus inserted into the target cell list if it fulfills:

RXLEV_NCELL(n) > RXLEV_MIN(n) + max (0, MS_TXPWR_MAX-P) (handover minimum condition) and RXLEV_NCELL(n) > RXLEV_DL + QUALLEVHOM(n) (additional condition).

It results the target cell receive level is, in case of both conditions fulfilled, better than the serving cell. So the number of unsuccessful handovers, back handovers or ping pong HOs can be reduced, less calls are dropped. Target Cell List Generation In order to evaluate the additional condition and to include in the Target Cell List cells that fulfill both criteria, the feature has to be enabled (SET HAND: ENAQUALEVHOM = 1, means TRUE).



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Database Parameters The operator has the possibility to enable or disable this feature. If this feature is disabled, the traditional solution

RXLEV_NCELL(n) > RXLEV_MIN(n) + max(0, MS_TXPWR_MAX-P) is chosen.

Object DB Name Range (Default)

Description

HAND ENAQUALEVHOM 1 - TRUE/ 0 - FALSE (FALSE)

The attribute indicates whether the "Level HO Margin for Quality HO" is enabled or disabled

ADJC QUALLEVHOM 0...126, in dB. 0 = - 63dB,126 = + 63 dB (69= 6 dB)

The parameter defines the margin by which the DL power level of a target cell shell exceed the DL power level of the serving cell to be entered into the target cell list in case of a quality HO



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4.2 Compression/Decompression HO Improvements The feature Compression/Decompression Handover (HO) Improvements is realizing intracell handover between the following Codecs:

• EFR GSM HR

• GSM FR GSM HR The Compression/Decompression of AMR calls (AMRFR AMRHR, i.e. handover between AMR Codecs) has been realized since release BR6.0 and was enhanced in BR7.0 by taking into account not only quality but also level conditions. This new feature impacts the existing AMR Compression/Decompression HO in sense that system has to decide which call to hand over in the case of cell load dependent conditions. It provides the selection of the best suited connection for compression HO out of all allocated calls (AMR calls as well as connections using standard codecs).Therefore some parameters introduced specify in addition whether AMR mobiles should be included in the algorithm while the parameters concerning AMR thresholds will be given in the AMR HO section 4.5.3. For Compression HO the BTS keeps the following:

• A list of connections for which condition for compression is fulfilled (exceeded C/I thresholds for compression, i.e. HOTHxxCMPyy, where xx specifies EFR or FR, and yy DL or UL direction)

• definition of the preferred codec type for compression HO (EADVCMPHOAMR or/and EADVCMPHOSC and ADVCMPHOOAMR to specify the codec specific preference)

• determination of the best suited connection from the candidate list (connection with the highest C/I).

Decompression HO is triggered in the following cases:

• decompression handover due to bad radio conditions ( always enabled - exceeded C/I threshold for decompression HOTHHRDCMPyy))

• traffic load dependent triggering of decompression HO (FRACTTH and FRACTAMRTH)

• due to relaxing traffic load ongoing connections can be shifted from half-rate mode to full rate mode

• connection that is closest its decompression threshold is selected (HRDCMLIMTH).



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Database Parameters DB Name Object Range Description

HOTHEFRCMPDL

HAND 0, 1…, 30 dB

(18 dB)

This parameter specifies a C/I threshold for compression handover from EFR to GSM HR in downlink

HOTHEFRCMPUL

HAND 0, 1…, 30 dB

(18 dB)

This parameter specifies a C/I threshold for compression handover from EFR to GSM HR in uplink

HOTHFRCMPDL

HAND 0, 1…, 30 dB

(18 dB)

This parameter specifies a C/I threshold for compression handover from FR to GSM HR in downlink

HOTHFRCMPUL

HAND 0, 1…, 30 dB

(18 dB)

This parameter specifies a C/I threshold for compression handover from FR to GSM HR in uplink

HOTHHRDCMDL

HAND 0, 1…, 30 dB

(13 dB)

This parameter specifies a C/I threshold for decompression handover from GSM HR to EFR respectively to GSM FR in downlink.

HOTHHRDUL

HAND 0, 1…, 30 dB

(13 dB)

This parameter specifies a C/I threshold for decompression handover from GSM HR to EFR respectively to GSM FR in uplink.

.

EADVCMPHOAMR

HAND TRUE /FALSE

(TRUE)

This parameter specifies if AMR HR capable mobiles are selected as candidates for compression handover

EADVCMPHOSC

HAND TRUE /FALSE

(TRUE)

This parameter specifies if GSM HR capable mobiles are selected as candidates for compression handover



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DB Name Object Range Description

ADVCMPHOOAMR

HAND -30, 29,…, 0,…, 29, 30 dB

(6 dB)

This parameter specifies the codec specific preference (offset of AMR compared to standard codecs)

HRDCMLIMTH

HAND 0…100 dB

(6 dB)

Threshold for limitation of load dependent decompression HO. Connection that are not far than this threshold to decompression threshold are selected

FRACTTH1

BTS 0, 1, …, 10000

(3000)

Load dependent threshold for decompression handover for standard cells or complete area of concentric cell, far area of extended call for standard codecs

FRACTTH2

BTS 0, 1, …, 10000

(3000)

Load dependent threshold for decompression handover for inner area of concentric cell, near area of extended call for standard codecs

FRACTAMRTH1

BTS 0, 1, …, 10000

(3000)

Load dependent threshold for decompression handover for standard cells or complete area of concentric cell, far area of extended call for AMR calls

FRACTAMRT2

BTS 0, 1, …, 10000

(3000)

Load dependent threshold for decompression handover for inner area of concentric cell, near area of extended call for AMR calls

• advComprHoOffsetAMR > 0 preferred AMR mobiles for compression HO

• advComprHoOffsetAMR < 0 preferred mobiles using standard codecs for compression HO



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4.3 Handover types belonging to network criteria



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4.3.1 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 further 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 ( CHANTYPE=EXTMODE). Parameters for Extended Cell Handover

Specification Name Object DB Name Range Step Size

Unit

ENABLE_EXTENDED_CELL_HO HAND EXTCHO TRUE/FALSE

- -

HO_MS_TA_MAX HAND HOMSTAM 0 .. 34 (32)

1 km

HO_MARGIN_TA HAND HOMRGTA 0 .. 35 (4)

1 km

HO_TH_MS_RANGE_MAX_EXT HAND HOTMSRME 35…100 (99)

A single-to-double timeslot 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 timeslot 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). 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 timeslot HO will be performed in case of HOMSTAM - HOMRGTA < 0).



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single timeslotchannel

double timeslotchannel

HOMSTAM

HOTMSRME

Excessive Distance

BCCH

double

TCH

double

TCH

double

TCH

single

TCH

single

Channels mustnot be used

Fig. 23 Explanation of HOMSTAM parameter

HOTMSRME

HOMRGTA

Distance MS - BTSE

HOMSTAM

Fig. 24 Criterion for extended cell intracell handover



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4.3.2 Cell configurations

4.3.2.1 Introduction on concentric cell The term Concentric Cell 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. This feature can be used to introduce an additional frequency re-use pattern for the frequencies in the inner area, i.e. a 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 by MSs camping in both areas. Intracell handovers between the two areas depend 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. Possible intracell handovers:

• From complete area to inner area of a concentric cell

• From inner area to complete area of a concentric cell. Possible intercell handovers:

• From complete area of concentric cell A to complete area of concentric cell B

• From complete area of concentric cell A to inner area of concentric cell B

• From inner area of concentric cell A to complete area of concentric cell B

• From inner area of concentric cell A to inner area of concentric cell B only for sectorized concentric cells, i.e. both concentric cells are served by the same BTSE.



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Inner Area

Complete Area

Fig. 25 Cell configured as concentric

complete areaof cell B

complete areaof cell A

inner area

of cell A

inner area

of cell B

Fig. 26 Inner and complete area of two adjacent concentric cells A and B



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4.3.2.2 Dual band operation and Common BCCH Dual Band Cells Dual band operation network offers the possibility to establish the GSM service in two different GSM specified frequency bands (i.e. GSM900 and GSM1800 or GSM850 and GSM1900). Therefore the operation of "dual band" MS is supported in such network as well as the handover between cells or cell areas belonging to different bands. The transceiver equipment for frequency bands can be implemented in one BTSE (site). The operator has several configuration possibilities when configure cells in a site:

• to configure each cell working in a single and different frequency bands (BCCH and BSIC have to be planned for each cell),

• to configure cells each working in both frequency bands with common BCCH using concentric cell approach (so called Common BCCH Dual Band Cell),

• to configure cells each working in both frequency bands with common BCCH using standard cell approach (so called Dual Band Standard Cell).

Up to BR7.0, the common BCCH implementation is based on the concentric cell approach. One frequency band covers the inner area, while the other covers the complete area. The common BCCH (also SDCCH) always belongs to the frequency band of the complete area. This type of cell configuration makes planning easier as Common BCCH Dual Band cell has cell identity as single band cell. The disadvantages of this cell structure are:

• increased load due to introducing the specific intra-cell handover from inner to complete area and vice versa (propagation difference between two bands and different power classes of the MS have to be considered and call set up level conditions are evaluated to determine whether the call is set up on TCH in inner or complete area),

• incoming handover only possible to the complete area (exception collocated cells),

• GPRS/EGPRS support restricted only to the BCCH band,

• DMA not supported. With BR8.0 a Dual Band Standard Cell (or Common BCCH Standard Cell) is introduced as a new type of cell architecture.



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F5 GSM 1800

F2 GSM 900

F6 GSM 900

F3 GSM 900

F4 (GSM 900)

F1 (GSM 1800)

BTSEcomplete area

inner area

Concentric Cell

Dual Band Concentric Cell

Dual Band Standard Cell

Fig. 27 Cell Configuration



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The Dual Band Standard cell must be planned so that the two frequency bands have well matched cell boundaries, i.e. overlapping coverage. In that case a service can be allocated on either one of the frequency bands independent of its location in the dual band standard cell. This feature can support the entire range of applicable frequency band combinations, i.e. 900/1800, 850/1800 and 850/1900 MHz. The choice of the frequency band for the allocation of the BCCH is not restricted. Another characteristics are:

• GPRS/EGPRS can be allocated in both frequency bands,

• no load balancing is present,

• radio resources allocation is performed according to the feature improvement of Multi-layer service support,

• BCCH can belong to any frequency band,

• two DMA layers per cell can be configured.

Database parameters for cell configuration: DB Name Object Range Meaning CELLTYP BTS STDCELL,

EXTCELL, DBSTDCELL

Specifies a cell type: standard cell with max MS distance 35km, extended cell with max Ma distance 100km, dual band standard cell

SYSID BTS/ TGTBTS

BB900, GSMR, DSC1800, EXT900, PCS1900, GSM850, GSMDCS, GSM850PSC, GSM850DCS

Specifies frequency band/bands supported by the cell. The attribute CELLTYP can not assume value DBSTDCELL with SYSID different than GSMDCS, GSM850PCS, GSM850DCS

CONCELL BTS 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

TRXAREA TRX NONE INNER COMPLETE

Specifies the area the TRX belongs to a concentric cell, and if so which area it serves. For DBSTDCELL TRXAREA must be set to NONE.

GRXT BTS NULL, ALL900, ALLFREQ

Specifies availability of GPRS/EGPRS: only on BCCH (NULL), on 900MHz frequencies (ALL900), or all frequencies



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4.3.3 Concentric cell handover

4.3.3.1 Single Band Concentric Cell handover

Database Parameters for Single Band Concentric Cell handover

EN_CON_CELL_DIST Object DB Name Range Step Size Unit HAND CCDIST TRUE/

FALSE

This is the flag to enable/disable the impact of a distance on the intracell handover decision in addition to the HORXLVDLI / HORXLVDLO condition within the concentric cell intracell handover algorithm.

HO_RXLEV_DL_INNER Object DB Name Range Step Size Unit HAND HORXLVDLI 0...63 (26) 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.



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HO_RXLEV_DL_OUTER Object DB Name Range Step Size Unit HAND HORXLVDLO 0...63 (32) 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 Object DB Name Range Step Size Unit HAND HOCCDIST 0...35 (5) 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)

To avoid handover oscillation between inner and complete area, the following recommendations have to be considered: HORXLVDLO > HORXLVDLI and HORXLVDLO - HORXLVDLI > BS_TXPWR_MAX(Complete) - BS_TXPWR_MAX(Inner)



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HORXLVDLI

HORXVLO

Distance MS - BTSERXLVDL

Inner Area

Complete Area

HOCCDIST

Fig. 28 Concentric cell handover

CCELL1

Serving cellInner Area

Complete AreaCCELL2Inner Area

Fig. 29 Collocated cells



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4.3.3.2 INNER-INNER area Handover In sectorized concentric cells inner to inner area handover is possible.

COLOCATED_CELL Object DB Name Range Step Size Unit 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. Collocated cell1 defines the first of two possible adjacent sectorized concentric cells to which an intercell HO from inner to inner area shall be possible.

EN_INNER_INNER_HO Object DB Name Range Step Size Unit HAND ININHO TRUE/

FALSE

Flag to enable/disable the intercell handover from inner to inner area in sectorized concentric cells. Under normal conditions (with ININHO=FALSE) the incoming handover would be executed to the complete area of the collocated cell first, after that a complete-to-inner handover would follow. 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.



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4.3.3.3 Common BCCH Dual Band Cell handover In order to optimize signal reception within the concentric cell the Common BCCH Dual Band cell selects an intracell handover from the complete area (i.e. GSM900) to the inner area (i.e. 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. 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.



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4.3.4 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 RetryProcedure

Is the incomingmessage an

Assignment_REQ ?

BSC sends to BTSForced_HO_Request

BTS sends to BSCIntercell_HO_Cond_Ind

with target cell list

Cell withinBSC area ?

N

Y

N

Y

EN_INTER_SDCCH_HOset to enable ?

N

Assign TCH

StartQueueingProcedure

BSC assigns TCHinternally and sends

Assignmend_Completeto MSC

Suiteble cellin list ?

Y

N

Directed Retry towardsMSC successful ?

Y

NY

Fig. 30 Directed retry flow -chart



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HO Algorithm and 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 The ranking of the neighbor cells in the target cell list is performed 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 HOs 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 Power Budget Back-Handovers 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.



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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).

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".



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Database Parameters for Directed Retry

EN_FORCED_HO Object DB Name Range Step Size Unit 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 Object DB Name Range Step Size Unit 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. 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".



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EN_FORCED_HO Object DB Name Range Step Size Unit 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 Object DB Name Range Step Size Unit BSC EISDCCHHO ENABLE

DISABLE (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.

FHO_RXLEV_MIN_OFFSET Object DB Name Range Step Size Unit ADJC FHORLMO 0...24 (6) 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.



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Tbho Object DB Name Range Step Size Unit ADJC TIMERFHO 1...320 (12) 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.

4.3.5 HO decision due to BSS resource management criteria (HO due to traffic load)

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 preceded 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, HO Decision due to BSS Resource Management Criteria moves connected users towards adjacent cells allowing in the previously used cell 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 HO 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. When this message is received by the BTS it starts timer TRFHOT and reduces the handover margin towards neighbor cells by one Margin Step Size. 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.

2. 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.

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

Every time the timer TRFHOT expires the following procedure is started: 1. If the HO due to traffic reason is enabled the counter 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.



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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 / 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.



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Traffic HOEvaluation

Y

N

Y

m > 0

Traffic HOcondition fulfilled

for adj. cell

BTS sends to BSCIntercell_HO_Cond_Ind

with target cell list

N

End

Fig. 31 Dedicated task in BTS to perform traffic HO



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Parameters to control traffic HO in the BTS


Range Meaning

Traffic_Control_Timer TRFCT / BSC

10...200 (20)

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 (90)

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 (70)

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.

Traffic_HO_Timer TRFHOT / HAND

2...20 (10)

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 1…48 (9)

unit = 1 dB This parameter establishes the minimum reduction for TRFHOM



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Range Meaning

Traffic_Margin_Max TRFMMA / HAND

1...48 (9)

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 (67=4dB)

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 (30)

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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4.4 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 handover 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 these values for the handover algorithm when applied on the ongoing service belonging to the service group, otherwise the global parameter settings are used.



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Settings forService groups activated

in the BSC areaHAND object

General settings forBSC 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• …• SG1HOPAR• SG2HOPAR• …• SG14HOPAR

Fig. 32 Principle of service dependent handover management

Example: SET HAND: SG1HOPAR=10-10-35-35-26-32-5-5;

Each field represents a specific parameter which is already known from the standard parameter's list. The meaning of these eight fields in the attribute SG1HOPAR are: SG1HOPAR = 10 - 10 - 35 - 35 - 26 - 32 -

-HOLTHLVDL HOLTHLVUL HOTDLINT HOTULINT HORXLVDLI HORXLVDLO- 5 - 5 HOLTHQUDL HOLTHQUUL



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4.5 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. Measured and reported RXQUAL values are mapped into the C/I values in the BTS.

4.5.1 AMR quality intercell handover Intercell HO downlink / uplink due to quality for an AMR call is triggered if the quality (C/I value) drops below the thresholds determined by the parameters 1. hoLowerThresQualAMRDL for downlink and 2. hoLowerThresQualAMRUL for uplink. The above parameters are relevant only if the intercell HO due to quality is enabled (RXQUALHO=TRUE).

Parameter Object Range Description

HOLTHQAMRDL HAND 0 ... 30 Unit=1 dB, (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 Unit=1 dB, (8 dB)

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



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4.5.2 AMR compression/decompression handover The purpose of the AMR compression HO is to transfer AMR FR calls with suitably good radio link quality to an AMR HR TCH in order to use the TCH resources more efficiently. The intracell AMR compression HO is not continuously enabled in the BTS but temporarily enabled/disabled by the BSC dependent on availability of radio channels and Abis resources. On every expiry of the timer TRFCT the BSC checks the traffic load in its cells and compares it to the threshold HRACTAMRT1 (or HRACTAMRT2) as illustrated in the figure below. The AMR HO from FR to HR is only enabled when the percentage of busy TCH is higher than these predefined thresholds. The purpose of the AMR decompression HO is to transfer AMR HR calls with poor radio link quality to an AMR FR TCH in order to improve speech quality. In contrast to AMR compression HO the AMR decompression HO is continuously enabled in the BTS disregarding the cell load, i.e. if at least one TCH/F is free, HO is performed. AMR compression/decompression HO is always performed in the BSC if the BSC parameter HRSPEECH is set to TRUE. The parameter EADVCMPDCMHO (Enable Advanced Compression/Decompression HO) just allows switching between standard and advanced mode of AMR Compression/Decompression HO.

4.5.2.1 Standard AMR compression/decompression handover Standard AMR compression/decompression HO (EADVCMPDCMHO=FALSE) decision is solely based on quality criteria defined by the parameters HOTHAMRCDL, HOTHAMRCUL, HOTHAMRDDL and HOTHAMRDUL. Standard AMR compression HO criterion is: C/I DL > HOTHAMRCDL and C/I UL > HOTHAMRCUL. Standard AMR decompression HO criteria are: C/I DL < HOTHAMRDDL or C/I UL < HOTHAMRDUL.



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BTS BSC

TRFCT

TRFCT

...

Check, whether or not traffic load > HRACTAMRT1If TRUE, AMR compression HO is started in that cell:

SET ATTRIBUTE: "AMR COMPRESSION HO ENABLED"

Check, whether or not traffic load > HRACTAMRT1If TRUE, AMR compression HO is started in that cell:

SET ATTRIBUTE: "AMR COMPRESSION HO ENABLED"

Fig. 33 AMR traffic control



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4.5.2.2 Advanced AMR compression/decompression handover Advanced AMR compression/decompression HO (EADVCMPDCMHO=TRUE) algorithm additionally considers current level and PWRC situation when deciding on whether to compress or decompress the call. In this context the new parameters HOTHCMPLVDL, HOTHCMPLVUL, HOTHDCMLVDL and HOTHDCMLVUL are introduced and the new decision criteria defined. An advanced AMR compression HO from AMR FR to AMR HR is triggered if for a particular AMR FR call the following conditions are fulfilled: RXLEVUL>HOTHCMPLVUL and C/I UL>=C/Imax or C/I UL + MS_PWRRED>HOTHAMRCUL and RXLEVDL>HOTHCMPLVDL and C/I DL>=C/Imax or C/I DL + BS_PWRRED>HOTHAMRCDL. An advanced AMR decompression HO from AMR HR to AMR FR is triggered if for a particular AMR HRcall the following conditions are fulfilled: RXLEVUL<HOTHCMPLVUL or C/I UL<C/Imax and C/I UL + MS_PWRRED<HOTHAMRDUL or RXLEVDL<HOTHDCMLVDL or C/I DL<C/Imax and C/I DL + BS_PWRRED<HOTHAMRDDL, where MS_PWRRED=MS power reduction due to power control (in dB) BS_PWRRED=BS power reduction due to power control (in dB) C/Imax=20dBm.



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4.5.3 Activation of Half Rate for AMR calls EHRACTAMR parameter enables the Cell Load Dependent Activation of Half Rate for AMR calls on BTS basis. It has the same functionality as the parameter EHRACT for ordinary speech calls( see chapter1, section 4.2). This feature controls the allocation of AMR HR TCHs in such a way that AMR half rate TCHs are only assigned if the percentage of busy radio TCHs in the BTS or/and the percentage of busy Abis TCHs in the BTSM Abis channel pool have exceeded a configurable threshold. This threshold is defined by the parameters Half Rate Activation AMR threshold, HRACTAMRT1 and HRACTAMRT2 The parameters are used for two different features related to AMR calls - Cell Load Dependent Activation of Half Rate for AMR Calls - AMR Compression Handover. In case of the Cell Load Dependent Activation of Half Rate for AMR calls thresholds are relevant only if the parameter EHRACTAMR is set to TRUE. HRACTAMRT1 is the equivalent to the parameter HRACTT1 (introduced in BR6.0 for non-AMR as well as for AMR calls) for AMR and defines a traffic load threshold which is evaluated for the forced speech version selection for incoming AMR TCH seizures. If the cell traffic load exceeds the percentage defined by HRACTAMRT1 (or HRACTAMRT2), all incoming AMR calls or incoming AMR handovers, for which AMR HR is indicated as supported speech version, are forced to an AMR HR TCH. If the cell traffic load is below the percentage defined by HRACTAMRT1, all incoming calls are forced to AMR FR.



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Database Parameters for intracell AMR handover


EHRACTAMR BTS TRUE/FALSE (FALSE)

Parameter to enable/disable Call Load Dependent Activation of HR for AMR calls

EADVCMPDCMHO BTS TRUE/FALSE (FALSE)

Parameter to enable/disable AMR Advanced compression/decompression HO

HRACTAMRT1 BTS 0..10000 Unit=0.01% (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 HRACTAMRT1, the BSC enables the AMR compression handover.

HRACTAMRT2 BTS 0..10000 Unit=0.01% ( 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.



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HOTHAMRCDL HOTHAMRCUL

HAND 0 ... 30 Unit=1 dB, (23 dB)

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

HOTHAMRDDL HOTHAMRDUL

HAND 0 ... 30 Unit=1 dB, (23 dB)

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

HOTHCMPLVDL HOTHCMPLVUL

HAND 0 ... 63, NULL Unit=1 dB, (NULL) initial value 40 (-70dBm)

Handover threshold for compression Downlink and Uplink if Advanced compression/decompression HO is selected.

HOTHDCMLVDL HOTHDCMLVUL

HAND 0 ... 63, NULL Unit=1 dB, (NULL) initial value 24 (-84dBm)

Handover threshold for decompression Downlink and Uplink if Advanced compression/decompression HO is selected.



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5 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: Cell Reselection

Task Calculate the time in which mobile station will perform cell reselection due to Downlink Signaling Failure. The Number of Multiframes between Paging blocks is set to value NFRAMEPG=4.



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Exercise 3 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 = 4 A_LEV_HO = 7 L_RXLEV_DL_H = 12 A_PBGT_HO = 8 L_RXLEV_DL_IH = 25 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 3 S 4 F 4 F 6 F 4 S 5 S 7 F

RXLEV_SERV 25S 16 S 13 F 15 F 12 F 16 S 13 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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6 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: Cell Reselection

Task MS sets its Downlink Signaling Counter initial value according to the value of the NFRAMEPG broadcasted in the BCCH signal: DSC0 = round (90/NFRAMEPG) =round 90/4=22 Thus the initial DSC counter value will be set to 22. In each BS_PA_MFRMS time period MS tries to decode paging message and if the decoding is unsuccessful the counter value is decremented by 4, i.e. DSCt+1 = DSCt – 4. After the expiry of the first BS_PA_MFRMS time period (4x235ms= 0,94s) as MS can not decode paging message the counter value is decremented DSC1 = DSC0 – 4 = 18; after the second BS_PA_MFRMS time period (2x4x235ms≈ 1,9s) counter value becomes DSC2 = DSC1 – 4 = 14; …. After expiry of the sixth BS_PA_MFRMS period counter value becomes DSC6 = DSC5 – 4 = 2-4=-2<0 and the condition for cell reselection due to Downlink Signaling Failure is fulfilled. Needed time for cell reselection is 6xBS_PA_MFRMS=6x0,94s≈5,6s.



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Solution 3 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 FULL values of SACCH frames 2…8 are taken with multiplicity 3; while SUB values are taken with multiplicity 1, sum up and the sum is divided with the sum of their weighting factors according to the new measurement preprocessing algorithm. Therefore one has: AV_RXQUAL = (3*7+1*5+1*4+3*6+3*4+3*4+1*3) / (3+1+1+3+3+3+1) = 5 AV_RXLEV = (1*16+3*13+3*15+3*12+1*16+1*13+3*10) / (3+1+1+3+3+3+1) = 13 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 AV_RXQUAL > L_RXQUAL_DL_H no quality handover is initiated: no intracell quality handover since AV_RXLEV < L_RXLEV_DL_IH no intercell quality handover since the BTS transmit power is not at its maximum according to given values for the power. 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 – 5dB 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.

Documents

23616119-03-Mn1789eu11mn-0001-Design-Radio-Cells