02-Cell Selection and Reselction-Libre

 

 

Cell Selection/Reselection

Cell Selection / Cell Reselection

Contents 1  1.1  1.2  2  2.1 

Cell Selection Measurements for Normal Cell Selection Criteria for Cell Selection Cell Reselection Necessary Actions for a Cell Reselection

3  4  5  13  14 

2.2  2.3  2.4  3  4 

Phase 1 MS Phase 2 MS Triggers for Cell Reselection Parameters for Cell Selection/Reselection Exercise

15  16  18  21  35 

Network optimization

SSMC Training Center

1

 

Cell selection/Reselection

Network optimization

SSMC Training Center

2

 

 

Cell Selection/Reselection

1

Cell Selection

Network optimization

SSMC Training Center

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.

Network optimization

SSMC Training Center

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

Network optimization

SSMC Training Center

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

Network optimization

SSMC Training Center

6

 

 

Cell Selection/Reselection

1.2.2

Suitable Cell

Beside the C1 radio criterion there are some other criteria (administrative and traffic tr affic control) for a cell to be suitable: A Suitable Cell is 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 Cell is defined as a cell which 

is unbarred,



has a parameter C1 > 0.

Network optimization

SSMC Training Center

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.

Network optimization

SSMC Training Center

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

Network optimization

SSMC Training Center

9

 

Cell selection/Reselection

For phase 2 MS there is an additional parameter called CELL_BAR_QUALIFY 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.

Network optimization

SSMC Training Center

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

Network optimization

SSMC Training Center

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.

Network optimization

SSMC Training Center

12

 

 

Cell Selection/Reselection

2

Cell Reselection

Network optimization

SSMC Training Center

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 t he system informations on the BCCH of the serving cell. Each BS_PA_MFRMS, the MS attempts to t o 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.

Network optimization

SSMC Training Center

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 levelphase from that neighbor cell exceeds thethe received level of  the current serving cell. For 1 MS this MS  this is expressed using C1 criterion  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.

Network optimization

SSMC Training Center

15

 

Cell selection/Reselection

2.3

Phase 2 MS

For phase 2 MS a MS a modified path loss criterion, the C2 criterion, 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 i f the cell is removed from the list. During penalty time: C2 = C1 + CELL_RESELECT_OF CELL_RESELECT_OFFSET FSET - TEMPORARY_OFFSET After penalty time: C2 = C1 + CELL_RESELECT_OF CELL_RESELECT_OFFSET FSET For penalty time = 31 the whole time: C2 = C1 - CELL_RESELECT_OFF CELL_RESELECT_OFFSET SET CELL_RESELECT_OFFSET, TEMPORARY_OFFSET, and PENALTY_TIME are cell CELL_RESELECT_OFFSET, 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.

Network optimization

SSMC Training Center

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 reduces the priority of a cell in the list of strongest neighbor cells. A positive CELL_RESELECT_OFFSET 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, CELL_RESELECT_OFFSET, this cell is selected with preference.

Network optimization

SSMC Training Center

17

 

Cell selection/Reselection

2.4

Triggers for Cell Reselection

Cell reselection is triggered by the following conditions: 1. C1 < 0 for the serv serving ing cell for a period o off 5 s 2. MS detects a downlink signaling failure 3. Serving cell becomes barred 4. Phase 1 MS  MS  C1 (serving cell) < C1 C1 (serving C1 (suitable  (suitable neighbor cell) if the suitable neighbor cell is in the same location area C1 (serving cell) + CELL_RESELECT C1 (serving CELL_RESELECT_HYSTERESIS _HYSTERESIS < C1 C1 (suitable  (suitable neighbor cell) if the suitable neighbor cell is in another location area for a period of 5 sec. 5. Phase 2 MS  MS  C2 (serving cell) < C2 C2 (serving C2 (suitable  (suitable neighbor cell) if the suitable neighbor cell is in the same location area C2 C2 (serving  (serving cell) + neighbor CELL_RESELECT_HYSTERESIS < CELL_RESELECT_HYSTERESIS  < C2 C2 (suitable  (suitable neighbor cell) if the suitable cell is in another location area for a period of 5 sec. 6. A random access attempt is unsuccessful even a after fter the maximum n number umber of repetitions. For phase 2 there is the additional trigger: 7. A location update requ request est has been rejected with cause ”location area not allowed”.

Network optimization

SSMC Training Center

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

. . . . . . . .

. . . . . . . .

. . . . . . . .

. . . . . . . .

. . . . . . . .

. . . . . . . .

. . . . . . . .

. . . . . . . .

. . . . . . . .

. . . . . . . .

. . . . . . . .

. . . . . . . .

. . . . . . . .

. . . . . . . .

. . . . . . . .

. . . . . . . .

. . . . . . . .

. . . . . . . .

. . . . . . . .

. . . . . . . .

. . . . . . . .

. . . . . . . .

. . . . . . . .

. . . . . . . .

. . . . . . . .

. . . . . . . .

. . . . . . . .

. . . . . . . .

. . . . . . . .

. . . . . . . .

. . . . . . . .

. . . . . . . .

. . . . . . . .

. . . . . . . .

. . . . . . . .

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Network optimization

SSMC Training Center

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)

Network optimization

SSMC Training Center

22

 

 

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 t raffic 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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Network optimization

SSMC Training Center

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. r espectively. 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 t he 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 BTS

DB Name RXLEVAMI

Range 0...63

Step Size 1

Unit 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 t wo 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 i mbalance 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 t he 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_M RXLEV_ACCESS_MIN IN + ( 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_C Max(0,MS_TXPWR_MAX_CCH CH + POWER_OFFSET- P). Object

DB Name

Range

Step Size

Unit

BTS

PWROFS

0...3

1

2 dB

Power_Offset

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Network optimization

SSMC Training Center

27

 

Cell selection/Reselection

CELL_RESELECT_HYSTERESIS - Hysteresis for Reselection of a Cell from CELL_RESELECT_HYSTERESIS 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_HYST CELL_RESELECT_HYSTERESIS ERESIS 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_HY CELL_RESELECT_HYSTERESES(serving) STERESES(serving) < C1 (suitable neighbor cell) or C2 (serving cell) + CELL_RESELECT_HY CELL_RESELECT_HYSTERESES(serving) STERESES(serving) < C2 (suitable neighbor cell) respectively. The adjustment of CELL_RESELECT_ CELL_RESELECT_HYSTERESES 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 CELL_RESELECT_HYSTERESIS location area default: 2

Network optimization

SSMC Training Center

28

 

 

Cell Selection/Reselection

CELL_RESELCT_PARAM_IND_ CELL_RESELCT_PA RAM_IND_ - Phase 2 Reselection Parameter Indication I ndication CELL_RESELECT_PARAM_IND=1: The cell reselection parameters CELL_RESELECT_OFFS CELL_RESELECT_OFFSET, ET, TEMPORARY_OFFSET TEMPORARY_OFFSE T 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 CELL_RESELECT_P ARAM_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_ CELL_RESELCT_PA RAM_IND_ - Phase 2 Reselection Parameter Indication default: 1

. . . . . . .

. . . . . . .

. . . . . . .

. . . . . . .

. . . . . . .

. . . . . . .

. . . . . . .

. . . . . . .

. . . . . . .

. . . . . . .

. . . . . . .

. . . . . . .

. . . . . . .

. . . . . . .

. . . . . . .

. . . . . . .

. . . . . . .

. . . . . . .

. . . . . . .

. . . . . . .

. . . . . . .

. . . . . . .

. . . . . . .

. . . . . . .

. . . . . . .

. . . . . . .

. . . . . . .

. . . . . . .

. . . . . . .

. . . . . . .

. . . . . . .

. . . . . . .

. . . . . . .

. . . . . . .

. . . . . . .

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Network optimization

SSMC Training Center

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) PENALTY_TIME) a negative TEMPORARY_OFFSET is applied to the C2 of the respective neighbor cell C2 = C1 + CELL_RESELECT_OF CELL_RESELECT_OFFSET FSET - TEMPORARY_OFFSET which is removed after Penalty Time (T (T > PENALTY_TIME): PENALTY_TIME): C2 = C1 + CELL_RESELECT_OFFSET. PENALTY_TIME = 31: 31: C2 = C1 - CELL_RESELECT_OF CELL_RESELECT_OFFSET FSET.. 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 . . . . . . . . . . . .

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Network optimization

SSMC Training Center

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 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, CELL_RESELECT_O FFSET, 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 . . . . . .

. . . . . .

. . . . . .

. . . . . .

. . . . . .

. . . . . .

. . . . . .

. . . . . .

. . . . . .

. . . . . .

. . . . . .

. . . . . .

. . . . . .

. . . . . .

. . . . . .

. . . . . .

. . . . . .

. . . . . .

. . . . . .

. . . . . .

. . . . . .

. . . . . .

. . . . . .

. . . . . .

. . . . . .

. . . . . .

. . . . . .

. . . . . .

. . . . . .

. . . . . .

. . . . . .

. . . . . .

. . . . . .

. . . . . .

. . . . . .

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Network optimization

SSMC Training Center

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