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Content for  TS 38.304  Word version:  16.2.0

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4  General description of RRC_IDLE state and RRC_INACTIVE stateWord‑p. 9

4.1  Overview

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.
For UE not operating in SNPN access mode, search of available CAGs may be triggered by NAS to support manual CAG selection. The AS shall report available CAG-ID(s) together with their HRNN (if broadcast) and PLMN(s) to the NAS.
If the UE loses coverage of the registered PLMN/SNPN, either a new PLMN/SNPN is selected automatically (automatic mode), or an indication of available PLMNs/SNPNs is given to the user so that a manual selection can be performed (manual mode). As part of manual SNPN selection, the AS shall report available SNPN identifiers together with their HRNN (if broadcast) to the NAS.
Registration is not performed by UEs only capable of services that need no registration.
The UE may perform NR sidelink communication and/or V2X sidelink communication while in-coverage or out-of-coverage for sidelink, as specified in clause 8.
The purpose of camping on a cell in RRC_IDLE state and RRC_INACTIVE state is fourfold:
  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.
When the UE is in RRC_IDLE state, upper layers may deactivate AS layer when MICO mode is activated as specified in TS 24.501. When MICO mode is activated, the AS configuration (e.g. priorities provided by dedicated signalling) is kept and all running timers continue to run but the UE need not perform any idle mode tasks. If a timer expires while MICO mode is activated it is up to the UE implementation whether it performs the corresponding action immediately or the latest when MICO mode is deactivated. When MICO mode is deactivated, the UE shall perform all idle mode tasks.
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4.2  Functional division between AS and NAS in RRC_IDLE state and RRC_INACTIVE stateWord‑p. 11
Table 4.2-1 presents the functional division between UE non-access stratum (NAS) and UE access stratum (AS) in RRC_IDLE state and RRC_INACTIVE states. The NAS part is specified in TS 23.122 and the AS part in the present document.
RRC_IDLE and RRC_INACTIVE state Process UE Non-Access Stratum UE Access Stratum
PLMN Selection For a UE not operating in SNPN access mode, perform the following:
Maintain a list of PLMNs in priority order according to TS 23.122. Select a PLMN using automatic or manual mode as specified in TS 23.122 and request AS to select a cell belonging to this PLMN. For each PLMN, associated RAT(s) may be set.
Evaluate reports of available PLMNs and any associated CAG-IDs from AS for PLMN selection.
Maintain a list of equivalent PLMN identities.
To support manual CAG selection, provide request to search for available CAGs and evaluate reports of available CAGs from AS for CAG selection.
For a UE operating in SNPN access mode, perform the following:
Maintain a list of SNPNs according to TS 23.122. Select a SNPN using automatic or manual mode as specified in TS 23.122 and request AS to select a cell belonging to this SNPN.
Evaluate reports of available SNPNs from AS for SNPN selection.
For a UE not operating in SNPN access mode, search for available PLMNs.
If associated RAT(s) is (are) set for the PLMN, search in this (these) RAT(s) and other RAT(s) for that PLMN as specified in TS 23.122.
For a UE operating in SNPN access mode, search for available SNPNs only consider NR cells.
Perform measurements to support PLMN/SNPN selection.
Synchronise to a broadcast channel to identify found PLMNs/SNPNs.
Report available PLMNs and any associated CAG-IDs with associated RAT(s) to NAS on request from NAS or autonomously.
For a UE operating in SNPN access mode, report available SNPNs to NAS autonomously.
To support manual CAG selection, perform the following:
Search for cells broadcasting a CAG-ID.
Read the HRNN (if broadcast) for each CAG-ID if a cell broadcasting a CAG-ID is found.
Report CAG-ID(s) of found cell(s) broadcasting a CAG-ID together with the associated manual CAG selection allowed indicator, HRNN and PLMNto NAS.
On selection of a CAG by NAS, select any acceptable or suitable cell belonging to the selected CAG and give an indication to NAS that access is possible (for the registration procedure)
To support manual SNPN selection, report available SNPNs together with associated HRNNs (if available) to NAS on request from NAS.
Cell Selection Control cell selection for example by indicating RAT(s) associated with the selected PLMN to be used initially in the search of a cell in the cell selection.
Maintain a list of "Forbidden Tracking Areas" and provide the list to AS.
For a UE not operating in SNPN access mode: Maintain Allowed CAG list and optional CAG-only indication along with associated PLMN ID(s) on which the UE is allowed access and provide these lists to AS. To support manual CAG selection, select a CAG and request AS to select a cell belonging to this CAG.
Perform measurements needed to support cell selection.
Detect and synchronise to a broadcast channel. Receive and handle broadcast information. Forward NAS system information to NAS.
Search for a suitable cell. The cells broadcast one or more 'PLMN identity' or 'SNPN identity' (for a UE operating in SNPN access mode) in the system information. Respond to NAS whether such cell is found or not.
If associated RATs is (are) set for the PLMN, perform the search in this (these) RAT(s) and other RATs for that PLMN as specified in TS 23.122.
If a cell is found which satisfies cell selection criteria, camp on that cell.
Cell Reselection For a UE not operating in SNPN access mode, maintain a list of equivalent PLMN identities and provide the list to AS.
Maintain a list of "Forbidden Tracking Areas" and provide the list to AS.
For a UE not operating in SNPN access mode, maintain Allowed CAG list and optional CAG-only indication along with associated PLMN ID(s) on which the UE is allowed access and provide these lists to AS.
Perform measurements needed to support cell reselection.
Detect and synchronise to a broadcast channel. Receive and handle broadcast information. Forward NAS system information to NAS.
Change cell if a more suitable cell is found.
Location registration Register the UE as active after power on.
Register the UE's presence in a registration area, for instance regularly or when entering a new tracking area.
Deregister UE when shutting down.
Maintain a list of "Forbidden Tracking Areas".
Control and restrict location registration for a UE in eCall Only Mode.
Report registration area information to NAS.
RAN Notification Area Update Not applicable. Register the UE's presence in a RAN-based notification area (RNA), periodically or when entering a new RNA.
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4.3  Service types in RRC_IDLE stateWord‑p. 13
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 stateWord‑p. 14
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 categorised 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 fulfil 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:
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 TA that is not part of the list of "Forbidden Tracking Areas for Roaming" (TS 22.011), which belongs to a PLMN that fulfils 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 TA that is not part of the list of "Forbidden Tracking Areas" 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 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 fulfils the conditions to support NR sidelink communication or V2X sidelink communication in limited service state as specified in TS 23.287 clause 5.7, the UE may perform NR sidelink communication or V2X sidelink communication.
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