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Content for  TS 43.022  Word version:  16.0.0

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3  Requirements and technical solutions

The following subclauses list the main requirements of idle mode operation and give an outline of the technical solution.

3.1  PLMN selection and roaming

See TS 23.122.

3.2  Camping on a cell

3.2.1  Normal camping

For normal service, the MS has to camp on a suitable cell, tune to that cell's control channel(s), and possibly register within the PLMN so that the MS can:
  1. Receive system information from the PLMN , e.g., the cell options ;
  2. Receive paging messages from the PLMN, e.g., when there is an incoming call for the MS;
  3. Initiate call setup for outgoing calls or other actions from the MS (where possible, see subclause 3.5.3 and subclause 3.5.4).
The choice of such a suitable cell for the purpose of receiving normal service is referred to as "normal camping". There are various requirements that a cell must satisfy before an MS can perform normal camping on it:
  1. It should be a cell of the selected PLMN or, if the selected PLMN is equal to the last registered PLMN, an equivalent PLMN;
  2. It should not be "barred" (see subclause 3.5.1);
  3. It should not be in an LA which is in the list of "forbidden LAs for roaming";
  4. The radio path loss between MS and BTS must be below a threshold set by the PLMN operator. This is estimated as shown in subclause 3.6;
  5. It should not be a SoLSA exclusive cell to which MS does not subscribe. This requirement is only valid for MSs supporting SoLSA.
Initially, the MS looks for a cell which satisfies these 5 constraints ("suitable cell") by checking cells in descending order of received signal strength. If a suitable cell is found, the MS camps on it and performs any registration necessary. Cells can have two levels of priority, suitable cells which are of low priority are only camped on if there are no other suitable cells of normal priority. (This is called "cell selection").
When camped on a cell the MS regularly looks to see if there is a better cell in terms of a cell re-selection criterion, and if there is, the better cell is selected. Also if one of the other criteria changes, (e.g., the current serving cell becomes barred), or there is a downlink signalling failure (see subclause 3.6), a new cell is selected. (This is called "cell reselection"). A MS that has enabled PEO or EC operation has relaxed requirements for how often it verifies the suitability of its serving cell and the suitability of neighbour cells for re-selection (see TS 45.008 and TS 44.018).
In order to speed up these processes, a list of the RF channels containing BCCH or CPBCCH carriers of the same PLMN is broadcast in the system information messages, see subclause 4.8. Also, the MS does not need to search all possible RF channels to find a suitable cell. If, after searching the number of RF channels, given for each frequency band below, with the strongest received signal level, a BCCH or CPBCCH carrier has been found but no suitable cell of the selected PLMN has been found, the MS can stop the attempt to find a suitable cell of the selected PLMN.
The number of channels to be searched are 15 for T-GSM 380, 15 for T-GSM 410, 15 for GSM 450, 15 for GSM 480, 25 for GSM 710, 25 for GSM 750, 25 for T-GSM 810, 30 for GSM 850 Band, 30 for GSM 900, 15 for T-GSM 900 and 40 for DCS 1800 and PCS 1900.
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3.2.2  "Camp on any cell"Word‑p. 10

If the MS cannot find a suitable cell, it attempts to camp on any acceptable cell and enter a "limited service state", as defined in subclause 3.7. An MS attempting to camp on any acceptable cell shall not scan for CPBCCH.
There are various requirements that a cell must satisfy before being considered an acceptable cell to be camped on in "limited service state":
  1. It should not be "barred" (see subclause 3.5.1);
  2. The radio path loss between MS and BTS must be below a threshold set by the PLMN operator. This is estimated as shown in subclause 3.6.
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3.3  Regional provision of service

See TS 23.122.

3.4  Borders between registration areas

If the MS is moving in a border area between registration areas, it might repeatedly change between cells of different registration areas. Each change of registration area would require an LR, which would cause a heavy signalling load and increase the risk of a paging message being lost. To prevent this, a "CELL_RESELECT_HYSTERESIS" (CRH) parameter is used. A cell in a different registration area is only selected if it is "better", in terms of the path loss criterion (see subclause 3.6), than all the cells in the current registration area by at least the value of CRH. The CRH parameter is broadcast as system information. As the value of CRH broadcast may be different on different cells, the CRH parameter to be used is that broadcast on the current serving cell. There is also a lower limit on the time interval between reselection of cells on different registration areas. Instead of CRH a GPRS MS uses a "GPRS_CELL_RESELECT_HYSTERESIS" if provided.
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3.5  Barred cells and access control

3.5.1  Barred cells

The PLMN operator may decide not to allow MSs to camp on certain cells. (These cells may, for example, only be used for hand over traffic, i.e. calls which need to be handed over to other cells). Barred cell information is broadcast as system information to instruct MSs not to camp on these cells. The barred cell status may in fact change dynamically; hence the MS needs to regularly check the system information for this parameter.
The barred status of a cell depends on the parameters CELL_BAR_ACCESS and CELL_BAR_QUALIFY, further described in TS 45.008. For a MS that has enabled EC operation, the barred status of a cell is given by the Implicit Reject Status broadcast in EC-SCH, see TS 44.018.
If a cell is barred this applies both for cell selection and reselection.
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3.5.2  Prioritizing cells

In general, cell prioritization is a means of encouraging MSs to select some suitable cells in preference to others. Since the priority comparison is only between suitable cells, prioritization does not affect coverage. Operators may prefer a certain type of cell not to be selected unless it is the only suitable type. For example, umbrella cells due to their large frequency reuse distance, or microcells because the MS could be travelling too fast for them.

3.5.2.1  For cell selectionWord‑p. 11

During cell selection (see subclause 3.2.1), a cell with low priority indication will only be selected if a suitable cell of normal priority cannot be found.

3.5.2.2  For cell reselection

Cell prioritization can also be achieved during cell reselection by the use of the reselection parameters optionally broadcast. Cells are reselected on the basis of a parameter called C2 and the C2 value for each cell is given a positive or negative offset to encourage or discourage MSs to reselect that cell. A full range of positive and negative offsets is provided to allow the incorporation of this feature into already operational networks. Instead of C2 a GPRS MS uses the GPRS cell reselection parameter if provided.
An MS supporting SoLSA with SoLSA subscription shall use the SoLSA cell re-selection parameters.
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3.5.3  Access control

Due to problems in certain areas, Network Operators may decide to restrict access from some MSs (e.g., in case of congestion on the AGCH), and for this reason the access control mechanism is provided.
At subscription one or more access control classes are allocated to the subscriber and stored in the SIM. The information providing all authorized classes is broadcast as system information (together with a bit indicating whether emergency calls may be made). This information is modified dynamically and therefore the MS has to check the system information before each attempt to access.
The MS ignores the Access Control information when selecting a cell to camp on, i.e. it shall not reject a cell for camping on because access on that cell is not allowed.
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3.5.4  Forbidden LA for regional provision of service

If the MS has received the cause "LA not allowed", it shall ignore this fact when selecting a cell to camp on, i.e. it shall not reject a cell for camping on because that cell is part of a LA where this cause has been received.

3.5.5  Barred cell due to failed network authentication check

If upper layers have determined that the network has failed an authentication check (TS 24.008), the MS shall treat this cell as if the barred status broadcasted in the system information is set and reselect another cell. The MS shall treat the cell as barred for a period equal to the MM timer T3212 minus 60 seconds if the MM authentication procedure was performed or equal to the GMM timer T3302 minus 60 seconds if the GMM authentication was performed (see TS 24.008). If the timer T3212 or T3302 are disabled by the network a default value of 12 hours shall be used.
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3.6  Radio constraints

The MS uses a "path loss criterion" parameter C1 to determine whether a cell is suitable to camp on. C1 depends on 4 parameters:
  1. The received signal level (suitably averaged);
  2. The parameter RXLEV_ACCESS_MIN, which is broadcast as system information, and is related to the minimum signal that the operator wants the network to receive when being initially accessed by an MS;
  3. The parameter MS_TXPWR_MAX_CCH, which is also broadcast as system information, and is the maximum power that an MS may use when initially accessing the network;
  4. The maximum power of the MS.
The formula for determining C1 is given in TS 45.008.
Use of the parameter C1 enables the MS to determine whether communication is possible with the network in the absence of interference. However because of the possibility of interference degrading the communications, an additional safeguard is used. This is to monitor the success rate of the MS in decoding signalling blocks of the paging subchannel except when EC operation is enabled in which case the MS monitors the time required to decode the EC-SCH. If there is a downlink signalling failure, i.e. the success rate drops too low (or, in case EC operation is enabled, the maximum time to decode EC-SCH is exceeded), this indicates probable interference on the downlink, and the MS attempts to find another suitable cell. Downlink signalling failure monitoring is specified in TS 45.008.
In order to optimize cell reselection, additional cell reselection parameters can be broadcast as system information of each cell. The cell reselection process employs a parameter C2 which depends on these parameters.
The parameters used to calculate C2 are as follows:
  1. CELL_RESELECT_OFFSET (see subclause 3.5.2.2);
  2. PENALTY_TIME;
    When the MS places the cell on the list of the strongest carriers as specified in TS 45.008, it starts a timer which expires after the PENALTY_TIME. This timer will be reset when the cell is taken off the list. For the duration of this timer, C2 is given a negative offset. This will tend to prevent fast moving MSs from selecting the cell.
  3. TEMPORARY_OFFSET;
    This is the amount of the negative offset described in (ii) above. An infinite value can be applied, but a number of finite values are also possible.
The permitted values of these parameters and the way in which they are combined to calculate C2 are defined in TS 45.008.
Instead of the parameter C2, a GPRS MS applies the corresponding GPRS parameter if provided.
A MS that supports EC-GSM-IoT applies EC-specific C1 and C2 parameters.
An MS supporting SoLSA with SoLSA subscription shall use the SoLSA cell re-selection parameters.
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3.7  No suitable cell (limited service state)Word‑p. 12

If the MS is unable to obtain normal service from a PLMN (see TS 23.122), the MS attempts to camp on an acceptable cell, irrespective of its PLMN identity, so that emergency calls can be made or warning notifications can be received if necessary. To minimize the time taken to find new available PLMNs while maintaining battery life, discontinuous search schemes may be used, see TS 22.011. Cell reselection takes place as normal, except that a zero dB value of CRH will be used.
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3.8  CTS fixed part selection

See TS 23.122.

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