My 3GPP TS 38.304 Notes

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1. Scope

3GPP TS 38.304 document specifies the Access Stratum (AS) part of the UE procedures in RRC_IDLE state (also called Idle mode) and RRC_INACTIVE state. The non-access stratum (NAS) part of Idle mode procedures and processes is specified in TS 23.122.

The present document specifies the model for the functional division between the NAS and AS in a UE.

The present document applies to all UEs that support at least NR Radio Access, including multi-RAT UEs as described in 3GPP specifications, in the following cases:

  • When the UE is camped on a NR cell;
  • When the UE is searching for a cell to camp on;
NOTE: When the UE is camped on or searching for a cell to camp on belonging to other RATs, the UE behaviour is described in the specifications of the other RATs.

4.1 Overview of general description of RRC_IDLE state and RRC_INACTIVE state

See spec for details.

The RRC_IDLE state and RRC_INACTIVE state tasks can be subdivided into three processes:

  • PLMN selection (for UE not operating in SNPN access mode) or SNPN selection (for UE operating in SNPN access mode);
  • Cell selection and reselection;
  • Location registration and RNA update.

PLMN selection, SNPN selection, cell reselection procedures, and location registration are common for both RRC_IDLE state and RRC_INACTIVE state. RNA update is only applicable for RRC_INACTIVE state. When UE selects a new PLMN or SNPN, UE transitions from RRC_INACTIVE to RRC_IDLE, as specified in TS 24.501.

When a UE is switched on, a public land mobile network (PLMN) or a SNPN is selected by NAS. For the selected PLMN/SNPN, associated RAT(s) may be set, as specified in TS 23.122. The NAS shall provide a list of equivalent PLMNs, if available, that the AS shall use for cell selection and cell reselection.

With cell selection, the UE searches for a suitable cell of the selected PLMN or selected SNPN, chooses that cell to provide available services, and monitors its control channel. This procedure is defined as "camping on the cell".

The UE shall, if necessary, then register its presence, by means of a NAS registration procedure, in the tracking area of the chosen cell. As an outcome of a successful Location Registration, the selected PLMN/SNPN then becomes the registered PLMN/SNPN, as specified in TS 23.122.

If the UE finds a more suitable cell, according to the cell reselection criteria, it reselects onto that cell and camps on it. If the new cell does not belong to at least one tracking area to which the UE is registered, location registration is performed. In RRC_INACTIVE state, if the new cell does not belong to the configured RNA, an RNA update procedure is performed.

If necessary, the UE shall search for higher priority PLMNs at regular time intervals as described in TS 23.122 and search for a suitable cell if another PLMN has been selected by NAS.

The purpose of camping on a cell in RRC_IDLE state and RRC_INACTIVE state is as follows:

  1. It enables the UE to receive system information from the PLMN or the SNPN.
  2. When registered and if the UE wishes to establish an RRC connection or resume a suspended RRC connection, it can do this by initially accessing the network on the control channel of the cell on which it is camped.
  3. If the network needs to send a message or deliver data to the registered UE, it knows (in most cases) the set of tracking areas (in RRC_IDLE state) or RNA (in RRC_INACTIVE state) in which the UE is camped. It can then send a "paging" message for the UE on the control channels of all the cells in the corresponding set of areas. The UE will then receive the paging message and can respond.
  4. It enables the UE to receive ETWS and CMAS notifications.
  5. It enables the UE to receive MBS broadcast services.

4.2 Functional division between AS and NAS in RRC_IDLE state and RRC_INACTIVE state

See spec for details.

4.3 Service types in RRC_IDLE state

This clause defines the level of service that may be provided by the network to a UE in RRC_IDLE state. The following three levels of services are provided while a UE is in RRC_IDLE state:

  • Limited service (emergency calls, ETWS and CMAS on an acceptable cell);
  • Normal service (for public use or non-public use on a suitable cell);
  • Operator service (for operators only on a reserved cell).

4.4 Service types in RRC_INACTIVE state

This clause defines the level of service that may be provided by the network to a UE in RRC_INACTIVE state. The following two levels of services are provided while a UE is in RRC_INACTIVE state:

  • Normal service (for public use or non-public use on a suitable cell);
  • Operator service (for operators only on a reserved cell).

4.5 Cell Categories

The cells are categorized according to which services they offer:

acceptable cell
An "acceptable cell" is a cell on which the UE may camp to obtain limited service (originate emergency calls and receive ETWS and CMAS notifications). Such a cell shall fulfill the following requirements, which is the minimum set of requirements to initiate an emergency call and to receive ETWS and CMAS notification in an NR network:
  • The cell is not barred, see clause 5.3.1;
  • The cell selection criteria are fulfilled, see clause 5.2.3.2.
suitable cell
For UE not operating in SNPN Access Mode, a cell is considered as suitable if the following conditions are fulfilled:
  • The cell is part of either the selected PLMN or the registered PLMN or PLMN of the Equivalent PLMN list, and for that PLMN either:
    • The PLMN-ID of that PLMN is broadcast by the cell with no associated CAG-IDs and CAG-only indication in the UE for that PLMN (TS 23.501) is absent or false;
    • Allowed CAG list in the UE for that PLMN (TS 23.501) includes a CAG-ID broadcast by the cell for that PLMN;
  • The cell selection criteria are fulfilled, see clause 5.2.3.2.

According to the latest information provided by NAS:

  • The cell is not barred, see clause 5.3.1;
  • The cell is part of at least one Tracking Area (TA) that is not part of the list of "Forbidden Tracking Areas for Roaming" (TS 22.011), which belongs to a PLMN that fulfills the first bullet above.

For UE operating in SNPN Access Mode, a cell is considered as suitable if the following conditions are fulfilled:

  • The cell is part of either the selected SNPN or the registered SNPN of the UE;
  • The cell selection criteria are fulfilled, see clause 5.2.3.2;

According to the latest information provided by NAS:

  • The cell is not barred, see clause 5.3.1;
  • The cell is part of at least one Tracking Area (TA) that is not part of the list of "Forbidden Tracking Areas for Roaming" which belongs to either the selected SNPN or the registered SNPN of the UE.
barred cell
A cell is barred if it is so indicated in the system information, as specified in TS 38.331.
reserved cell
A cell is reserved if it is so indicated in system information, as specified in TS 38.331.

Following exception to these definitions are applicable for UEs:

  • if a UE has an ongoing emergency call, all acceptable cells of that PLMN/SNPN are treated as suitable for the duration of the emergency call.
  • camped on a cell that belongs to a tracking area that is forbidden for regional provision of service; a cell that belongs to a tracking area that is forbidden for regional provision service (TS 23.122, TS 24.501) is suitable but provides only limited service.
  • if the UE in RRC_IDLE fulfills the conditions to support NR sidelink communication or V2X sidelink communication in limited service state as specified in TS23.287 clause 5.7, the UE may perform NR sidelink communication or V2X sidelink communication.
NOTE: UE is not required to support manual search and selection of PLMN or CAG or SNPN while in RRC CONNECTED state. The UE may use local release of RRC connection to perform manual search if it is not possible to perform the search while RRC connected.

5.1 PLMN selection and SNPN selection

5.1.1 Support for PLMN selection

5.1.1.1 General

On request of the NAS, the AS shall perform a search for available PLMNs and report them to NAS.

5.1.1.2 NR case

The UE shall scan all RF channels in the NR bands according to its capabilities to find available PLMNs and available Closed Access Groups (CAGs). On each carrier, the UE shall search for the strongest cell and read its system information, in order to find out which PLMN(s) the cell belongs to and any associated CAG(s). For operation with shared spectrum channel access, the UE may also read the system information of multiple strongest cell(s). If the UE can read one or several PLMN identities in the strongest cell or the multiple strongest cell(s) in case of operation with shared spectrum channel access, each found PLMN (see the PLMN reading in TS 38.331) shall be reported to the NAS as a high quality PLMN (but without the RSRP value) and any associated CAG-ID, provided that the following high-quality criterion is fulfilled:

  • For an NR cell, the measured RSRP value shall be greater than or equal to -110 dBm.

Found PLMNs that do not satisfy the high-quality criterion but for which the UE has been able to read the PLMN identities are reported to the NAS together with their corresponding RSRP values and any associated CAG-ID. The quality measure reported by the UE to NAS shall be the same for each PLMN found in one cell.

The search for PLMNs may be stopped on request from the NAS. The UE may optimise PLMN search by using stored information e.g. frequencies and optionally also information on cell parameters from previously received measurement control information elements.

Once the UE has selected a PLMN, the cell selection procedure shall be performed in order to select a suitable cell of that PLMN to camp on.

To support manual CAG selection, the UE shall upon request by NAS report available CAG-ID(s) together with their manual CAG selection allowed indicator (if broadcast), HRNN (if broadcast) and PLMN(s) to the NAS. If NAS has selected a CAG and provided this selection to AS, the UE shall search for an acceptable or suitable cell belonging to the selected CAG to camp on.

5.1.1.3 E-UTRA case

Support for PLMN selection in E-UTRA is described in TS 36.304.

5.2 Cell selection and reselection

5.2.1 Introduction

UE shall perform measurements for cell selection and reselection purposes as specified in TS 38.133.

When evaluating Srxlev and Squal of non-serving cells for reselection evaluation purposes, the UE shall use parameters provided by the serving cell and for the final check on cell selection criterion, the UE shall use parameters provided by the target cell for cell reselection.

The NAS can control the RAT(s) in which the cell selection should be performed, for instance by indicating RAT(s) associated with the selected PLMN, and by maintaining a list of forbidden registration area(s) and a list of equivalent PLMNs. The UE shall select a suitable cell based on RRC_IDLE or RRC_INACTIVE state measurements and cell selection criteria.

In order to expedite the cell selection process, stored information for several RATs, if available, may be used by the UE.

When camped on a cell, the UE shall regularly search for a better cell according to the cell reselection criteria. If a better cell is found, that cell is selected. The change of cell may imply a change of RAT. Details on performance requirements for cell reselection can be found in TS 38.133.

The NAS is informed if the cell selection and reselection result in changes in the received system information relevant for NAS.

For normal service, the UE shall camp on a suitable cell, monitor control channel(s) of that cell so that the UE can:

  • receive system information from the PLMN or SNPN; and
    • receive registration area information from the PLMN or SNPN, e.g., tracking area information; and
    • receive other AS and NAS Information; and
  • if registered:
    • receive paging and notification messages from the PLMN or SNPN; and
    • initiate transfer to Connected mode.

For cell selection in multi-beam operations, measurement quantity of a cell is up to UE implementation.

For cell reselection in multi-beam operations, including inter-RAT reselection from E-UTRA to NR, the measurement quantity of this cell is derived amongst the beams corresponding to the same cell based on SS/PBCH block as follows:

  • if nrofSS-BlocksToAverage (maxRS-IndexCellQual in E-UTRA) is not configured in SIB2/SIB4 (SIB24 in E-UTRA); or
  • if absThreshSS-BlocksConsolidation (threshRS-Index in E-UTRA) is not configured in SIB2/SIB4 (SIB24 in E-UTRA); or
  • if the highest beam measurement quantity value is below or equal to absThreshSS-BlocksConsolidation (threshRS-Index in E-UTRA):
    • derive a cell measurement quantity as the highest beam measurement quantity value, where each beam measurement quantity is described in TS 38.215.
  • else:
    • derive a cell measurement quantity as the linear average of the power values of up to nrofSS-BlocksToAverage (maxRS-IndexCellQual in E-UTRA) of highest beam measurement quantity values above absThreshSS-BlocksConsolidation (threshRS-Index in E-UTRA).
Editor's note: Location assisted cell reselection, with the distance between UE and the reference location of the cell (serving cell and/or neighbour cell) taken into account, is supported for quasi-earth fixed cell. FFS on how UE performs location acquisition. When UE uses location based cell reselection enhancements, it's up to UE implementation to guarantee that a valid location information is available.
Editor's note: Distance based cell reselection criteria for quasi-earth fixed cell is supported.

5.2.2 States and state transitions in RRC_IDLE state and RRC_INACTIVE state

See spec for figure 5.2.2-1 that shows the states and state transitions and procedures in RRC_IDLE and RRC_INACTIVE. Whenever a new PLMN selection or new SNPN selection is performed, it causes an exit to number 1.



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