Cell Selection/Reselection
Cell Selection / Cell Reselection
Contents
1
1.1
1.2
2
2.1
2.2
2.3
2.4
3
4
Cell Selection
Measurements for Normal Cell Selection
Criteria for Cell Selection
Cell Reselection
Necessary Actions for a Cell Reselection
Phase 1 MS
Phase 2 MS
Triggers for Cell Reselection
Parameters for Cell Selection/Reselection
Exercise
Network optimization
3
4
5
13
14
15
16
18
21
35
SSMC Training Center
1
Cell selection/Reselection
Network optimization
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2
Cell Selection/Reselection
1
Cell Selection
Network optimization
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3
Cell selection/Reselection
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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4
Cell Selection/Reselection
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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5
Cell selection/Reselection
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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6
Cell Selection/Reselection
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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7
Cell selection/Reselection
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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8
Cell Selection/Reselection
Cell Selection
Algorithm
(no BCCH Info)
measure all
carriers
sort by received
level
carriers in list
no
Selection of an
acceptable cell
yes
trial carrier:
best level in list
no
remove trial
carrier from list
BCCH
yes
decode BCCH
suitable cell
yes
no
cell
in selected
PLMN
no
yes
try only carriers of
BCCH allocation
Camp on normal
priority cell
Fig. 1 Cell selection for phase 1 MS - no BCCH info is stored
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9
Cell selection/Reselection
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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10
Cell Selection/Reselection
Cell Selection
Algorithm
(no BCCH Info)
measure all carriers
sort by received level
carriers in list
no
yes
yes
Camp on low
priority cell
no
trial carrier:
best level in list
BCCH
low priority
cell found
Selection of an
acceptable cell
no
remove trial
carrier from list
yes
decode BCCH
suitable cell
no
yes
cell
in selected
PLMN
no
yes
try only carriers of
BCCH allocation
normal priority
no
suitable low priority
cell found
try only normal priority
cells
yes
Camp on normal
priority cell
Fig. 2 Cell selection for phase 2 MS - no BCCH info is stored
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11
Cell selection/Reselection
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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12
Cell Selection/Reselection
2
Cell Reselection
Network optimization
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13
Cell selection/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.
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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14
Cell Selection/Reselection
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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15
Cell selection/Reselection
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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16
Cell Selection/Reselection
Cell included in the
list of 6 strongest
C2
TEMPORARY_OFFSET
CELL_RESELECT_OFFSET
C1
PENALTY_TIME
T
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.
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17
Cell selection/Reselection
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 allowed”.
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18
Cell Selection/Reselection
C1
direction of movement
Phase 1 MS
BTS2
BTS1
high power class MS
low power class MS
BTS1
BTS2
CELL_RESELECT_
HYSTERESIS
(h)
C1=0
(l)
(a)
(b)
Cell Reselection
(a) no change of location area
(b) change of location area
Radius of Cell 2
for selection
(l) low power MS
(h) high power MS
Fig. 4 Illustration of cell selection / reselection
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SSMC Training Center
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19
Cell selection/Reselection
Network optimization
SSMC Training Center
20
Cell Selection/Reselection
3
Parameters for Cell Selection/Reselection
Network optimization
SSMC Training Center
21
Cell selection/Reselection
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
0...1023
1
-
(Taken from
TGTCELL)
BCCH_ARFCN_NC(n)
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Cell Selection/Reselection
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.
Mobile stations which camp on a cell while it becomes barred initialize the reselection
procedure to find a new (unbarred) cell, i.e. traffic load is distributed to neighbor cells.
This means that e.g. neither a call nor a location update can start in a barred cell.
However, a cell barred for access is not barred for incoming handovers. To barr a cell
completely, e.g. for maintenance reasons also incoming handovers have to be
avoided.
To reduce overload in a certain cell more moderately without distributing the overload
to neighbor cells, barring of access classes has to be used. Barring access for an
access class does not trigger a cell reselection for MSs of that class.
Object
DB Name
Range
Step Size
Unit
BTS
CELLBARR
FALSE/TRUE
-
-
CELL_BAR_ACCESS - Cell Barred for Access
default: FALSE
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23
Cell selection/Reselection
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 between 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.
Network optimization
SSMC Training Center
24
Cell Selection/Reselection
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 Channel 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.
Network optimization
SSMC Training Center
25
Cell selection/Reselection
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
Network optimization
SSMC Training Center
26
Cell Selection/Reselection
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.
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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27
Cell selection/Reselection
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, 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).
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
Network optimization
SSMC Training Center
28
Cell Selection/Reselection
CELL_RESELCT_PARAM_IND_ - Phase 2 Reselection Parameter Indication
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.
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.
Object
DB Name
BTS
Range
CRESPARI
Step Size
0...1
Unit
-
-
CELL_RESELCT_PARAM_IND_ - Phase 2 Reselection Parameter Indication
default: 1
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29
Cell selection/Reselection
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:
Network optimization
1
SSMC Training Center
30
Cell Selection/Reselection
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
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Network optimization
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31
Cell selection/Reselection
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
Network optimization
SSMC Training Center
32
Cell Selection/Reselection
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 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.
Object
DB Name
Range
Step Size
Unit
BTS
CRESOFF
0...63
1
2 dB
CELL_RESELECT_OFFSET
default: 1
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SSMC Training Center
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33
Cell selection/Reselection
Network optimization
SSMC Training Center
34
Cell Selection/Reselection
4
Exercise
Network optimization
SSMC Training Center
35
Cell selection/Reselection
Network optimization
SSMC Training Center
36
Cell Selection/Reselection
Exercise 1
Fill in the relevant parameters for Cell Selection/Reselection:
Specification Name
(GSM Name)
Network optimization
BSC-Database Name Value Range
Remarks
SSMC Training Center
37