GSM Cell Selection and Reselection

Cell Selection/Reselection

Network optimization SSMC Training Center

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Cell Selection / Cell Reselection

Contents 1 Cell Selection 3 1.1 Measurements for Normal Cell Selection 4 1.2 Criteria for Cell Selection 5 2 Cell Reselection 13 2.1 Necessary Actions for a Cell Reselection 14 2.2 Phase 1 MS 15 2.3 Phase 2 MS 16 2.4 Triggers for Cell Reselection 18 3 Parameters for Cell Selection/Reselection 21 4 Exercise 35

Cell selection/Reselection


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1 Cell Selection


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Cell selection/reselection are processes, which are performed by a GSM mobile station in idle mode. The MS uses the cell selection algorithm to look for a cell (of the chosen PLMN) where it can camp on ("camping on a cell"). If the MS looses coverage of that cell (caused for example by moving out of the cell) it will look for the most suitable alternative cell (of the selected PLMN) and it will camp on that cell. This is called cell reselection. The cell selection /reselection processes are described in GSM 03.22, GSM 05.08 and ETSI TS 123 122.

1.1 Measurements for Normal Cell Selection In case of normal cell selection the MS has no prior knowledge on which GSM 900 or DCS 1800 frequencies are BCCH carriers. Therefore the MS has to search in the beginning all RF carriers of the corresponding system (124 carriers for P-GSM, 174 carriers for E-GSM and 374 carriers for DCS 1800), and has to measure the signal strength on each RF carrier. The MS takes 5 measurement samples per RF carrier which are averaged:

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

A multi band MS searches all carriers within its band of operation. The number of carriers searched corresponds to the sum of carriers on each band of operation.


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1.2 Criteria for Cell Selection Based on these measurements the MS 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”:

1.2.1 C1 Criterion for Cell Selection

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

RXLEV_ACCESS_MIN: minimum received level at the MS required for access

to the system MS_TXPWR_MAX_CCH: maximum allowed MS transmit power on RACH P: Powerclass of MS specified in GSM O5.05

Power Class (GSM 05.05) Max. Output Power (GSM 900)

Max. Output Power (DCS 1800)

1 -- 1 Watt = 30 dBm

2 8 Watt = 39 dBm 0.25W = 24 dBm

3 5 Watt = 37 dBm 4 Watt = 36 dBm

4 2 Watt = 33 dBm

5 0.8 Watt= 29 dBm

Power classes of GSM 900 and DCS 1800 MS


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During cell selection procedure the MS will select only those cells which have C1 > 0. This means that the averaged received level downlink has to be greater than the threshold RXLEV_ACCESS_MIN + max (0, MS_TXPWR_MAX_CCH - P):

AV_RXLEV > RXLEV_ACCESS_MIN + max (0, MS_TXPWR_MAX_CCH - P) The maximum term is included to ensure a sufficient uplink received level even for MS of low maximum transmit power: If P < MS_TXPWR_MAX_CCH, then the C1 criterion is equivalent to:

AV_RXLEV > RXLEV_ACCESS_MIN + (MS_TXPWR_MAX_CCH - P) i.e. the averaged received level downlink has to exceed the RXLEV_ACCESS_MIN by a certain margin to ensure that the Base Station can receive the MS even in case of a MS with low maximum output power. If P > MS_TXPWR_MAX_CCH, then the C1 criterion reduces to:

AV_RXLEV > RXLEV_ACCESS_MIN


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1.2.2 Suitable Cell Beside the C1 radio criterion there are some other criteria (administrative and traffic control) for a cell to be suitable: A Suitable Cell is defined as a cell which is part of the selected PLMN, is unbarred (parameter CELL_BAR_ACCESS = 0), has a value C1 > 0, 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: An Acceptable Cell is defined as a cell which is unbarred, has a parameter C1 > 0.


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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.


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no

Cell SelectionAlgorithm

(no BCCH Info)

yes

measure allcarriers

sort by receivedlevel

carriers in list

trial carrier:best level in list

BCCH

decode BCCH

suitable cellno

yes

no

yes

Selection of anacceptable cell

try only carriers ofBCCH allocation

remove trialcarrier from list

nocellin selected

PLMN

Camp on normalpriority cell

yes

Fig. 1 Cell selection for phase 1 MS - no BCCH info is stored


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For phase 2 MS there is an additional parameter called CELL_BAR_QUALIFY (values: 0, 1) used to assign different priorities to cells: CELL_BAR_QUALIFY = 0 <=> normal priority cell CELL_BAR_QUALIFY = 1 <=> low priority cell A MS tries first to select a suitable normal priority cell. If no such cell can be found, a suitable low priority cell is selected.


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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. 2 Cell selection for phase 2 MS - no BCCH info is stored


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1.2.3 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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2 Cell Reselection


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While moving within the radio network in idle mode another cell may be more appro-priate to serve the MS. Therefore cell reselection may be performed. For the following discussion the MS shall camp on a cell which is called serving cell.

2.1 Necessary Actions for a Cell Reselection The following actions are performed by the MS to detect whether or not a cell reselection is necessary: Detection of a Downlink Signaling Failure:

The downlink signaling failure counter DSC is a counter running in the MS. The starting value of this counter is called DSC0 and is set by the MS to the following value: DSC0 = round(90 / BS_PA_MFRMS) The value of BS_PA_MFRMS is broadcasted in the system informations on the BCCH of the serving cell. Each BS_PA_MFRMS, the MS attempts to decode the messages on its paging sub-channel. Depending on whether or not this decoding is successful, the value of the downlink signaling failure counter DSC is modified in the following way :

successful decoding: DSC new = DSC old + 1 if DSC new + 1 > DSC0 → DSC new = DSC old unsuccessful decoding: DSC new = DSC old - 4

If DSC < 0 the MS has detected a downlink signaling failure.


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Monitoring of all BCCH carriers given in the BCCH allocation (neighbor cells) of the serving cell Taking 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 minutes

2.2 Phase 1 MS 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 MS this is expressed using the C1 criterion

C1 (neighbor cell) > C1 (serving cell). For the reselection process for phase 1 MS the neighbor cells are ordered according to their C1-value.


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2.3 Phase 2 MS

For phase 2 MS a modified path loss criterion, the C2 criterion, is used The C2 criterion depends on the value of a timer T, which is called PENALTY_TIMER. The 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. Timer T is reset to 0 if the cell is removed from the list. During penalty time: C2 = C1 + CELL_RESELECT_OFFSET - TEMPORARY_OFFSET After penalty time: C2 = C1 + CELL_RESELECT_OFFSET For penalty time = 31 the whole time: C2 = C1 - CELL_RESELECT_OFFSET CELL_RESELECT_OFFSET, TEMPORARY_OFFSET, and PENALTY_TIME are cell reselection parameters which are broadcasted on the BCCH of the cell in the system informations if CELL_RESLECT_PARAM_IND (Cell Reselection Parameter Indication) is set to 1.


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Cell included in thelist of 6 strongest

C2

C1

T

CELL_RESELECT_OFFSETTEMPORARY_OFFSET

PENALTY_TIME

Fig. 3 Illustration of the C2 criterion

A negative TEMPORARY_OFFSET reduces the priority of a cell in the list of strong-est 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 be-fore 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 prefer-ence.


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2.4 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 a downlink signaling failure 3. Serving cell becomes barred 4. Phase 1 MS

C1 (serving cell) < C1 (suitable neighbor cell) if the suitable neighbor cell is in the same location area 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.

5. Phase 2 MS C2 (serving cell) < C2 (suitable neighbor cell) if the suitable neighbor cell is in the same location area 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.

6. A random access attempt is unsuccessful even after the maximum number of repetitions. For phase 2 there is the additional trigger:

7. A location update request has been rejected with cause ”location area not al-lowed”.


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(l)(h)

Radius of Cell 2for selection(l) low power MS(h) high power MS

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

C1=0

C1

BTS1 BTS2

CELL_RESELECT_HYSTERESIS

direction of movementPhase 1 MS

high power class MS

low power class MS

BTS1 BTS2

Fig. 4 Illustration of cell selection / reselection

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3 Parameters for Cell Selection/Reselection


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BA (BCCH) - BCCH Allocation broadcasted on BCCH In each cell the absolute radio frequency carrier number BCCH_ARFCN_NC(n) of each of its neighbor cell n has to be known. This information is broadcasted as the so-called BCCH Allocation to all MS in the respective cell. On the corresponding frequencies the MS 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.

Object DB Name Range Step Size Unit ADJC BCCHFREQ

(Taken from TGTCELL)

0...1023 1 -

BCCH_ARFCN_NC(n)


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CELL_BAR_ACCESS - Cell Barred for Access CELL_BAR_ACCESS - Cell Barred for Access A mobile station cannot camp on a barred cell, i.e. a barred cell is not selected by the cell selection/reselection procedure. 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. 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. 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.

However, a cell barred for access is not barred for incoming handovers. To barr a cell completely, e.g. for maintenance reasons also incoming handovers have to be avoided. To reduce overload in a certain cell more moderately without distributing the overload to neighbor cells, barring of access classes has to be used. Barring access for an access class does not trigger a cell reselection for MSs of that class.

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

Object DB Name Range Step Size Unit BTS CELLBARR FALSE/TRUE - -

CELL_BAR_ACCESS - Cell Barred for Access default: FALSE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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RXLEV_ACCESS_MIN - Minimum Downlink Received Level for Cell to be selected The parameter RXLEV_ACCESS_MIN determines the cell border for a 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 be-tween adjacent cells (especially if they belong to different location areas). 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 MS). Furthermore, it has to harmonize with the handover thresholds (RXLEV_MIN, L_RXLEV_HO).

Object DB Name Range Step Size Unit BTS RXLEVAMI 0...63 1 RXLEV

RXLEV_ACCESS_MIN - Minimum downlink received level for cell to be selected default: 6 RXLEV 0 = less than -110 dBm. RXLEV 1 = -110 dBm to -109 dBm. ... RXLEV 62 = -49 dBm to-48 dBm. RXLEV 63 = greater than -48 dBm.


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MS_TXPWR_MAX_CCH - Maximum allowed MS Transmit Power on RACH The transmit power level the MS uses for the access on the Random Access Chan-nel is given by the minimum of two values: the maximum possible transmit power of the MS (given by the power class P of the

MS) the maximum allowed power for access within the respective cell

(given by MS_TXP WR_MAX_CCH). 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 BTS. Hence MS 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.


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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.

Object DB Name Range Step Size Unit BTS MSTXPMAXCH 0...31 1 2 dB

MS_TXPWR_MAX_CCH - Maximum allowed MS transmit power on RACH default: 2 For the values of MS_TXPWR_MAX_CCH see the power control level defined in GSM rec. 05.05:

MS GSM900 Phase 1 MS GSM900 Phase 2 MS DCS1800 0 := 43 dBm 0 := 39 dBm 0 := 30 dBm

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

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

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

... ... ...

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

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


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POWER_OFFSET - Additional Parameter for Class 3 DCS1800 MS POWER_OFFSET - Additional Parameter for Class 3 DCS1800 MS The parameter POWER_OFFSET is only used by class 3 DCS1800 MS to calculate the C1-criterion described as follows. 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). C1 = AV_RXLEV - RXLEV_ACCESS_MIN - Max(0,MS_TXPWR_MAX_CCH + POWER_OFFSET- P).

Object DB Name Range Step Size Unit BTS PWROFS 0...3 1 2 dB

Power_Offset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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CELL_RESELECT_HYSTERESIS - Hysteresis for Reselection of a Cell from another Location Area 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, fre-quent 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 be-tween reduction of unnecessary location updates (high value) and selection of the cell with best reception quality (low value).

Object DB Name Range Step Size Unit BTS CELLRESH 0...7 1 2 dB

CELL_RESELECT_HYSTERESIS - Hysteresis for reselection of a cell from another location area default: 2


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CELL_RESELCT_PARAM_IND_ - Phase 2 Reselection Parameter Indication CELL_RESELCT_PARAM_IND_ - Phase 2 Reselection Parameter Indication CELL_RESELECT_PARAM_IND=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 MS, but are ignored by phase 1 MS.

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 MS, but are ignored by phase 1 MS. CELL_RESELECT_PARAM_IND = 0: CELL_RESELECT_PARAM_IND = 0: The cell reselection parameters and CELL_BAR_QUALIFY are not broadcasted on the BCCH. 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. A phase 2 MS then uses the value 0 for all these parameters, i.e. C1 = C2.

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

CELL_RESELCT_PARAM_IND_ - Phase 2 Reselection Parameter Indication default: 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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CELL_BAR_QUALIFY Parameter is 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.

Object DB Name Range Step Size Unit BTS CBQ 0...1 1 -

CELL_BAR_QUALIFY default: 0 normal priority: 0 low priority: 1


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PENALTY_TIME - Time to Apply a Negative Offset to C2 of a Neighbor Cell PENALTY_TIME < 31: 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.

Object DB Name Range Step Size Unit BTS PENTIME 0...30 and 31:

special meaning 1 20 sec

PENALTY_TIME - Time to apply a negative offset to C2 of a neighbor cell default: 5


TEMPORARY_OFFSET 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.

Object DB Name Range Step Size Unit BTS TEMPOFF 0...7 1 10 dB

7: infinity

TEMPORARY_OFFSET default: 1


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CELL_RESELECT_OFFSET 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 be-fore 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.

Object DB Name Range Step Size Unit BTS CRESOFF 0...63 1 2 dB

CELL_RESELECT_OFFSET default: 1

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34


4 Exercise


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36


Exercise 1

Fill in the relevant parameters for Cell Selection/Reselection:

Specification Name (GSM Name)

BSC-Database Name Value Range Remarks


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Documents

GSM Cell Selection and Reselection