4 General description of Idle mode
25.3043GPPRelease 17TSUser Equipment (UE) procedures in idle mode and procedures for cell reselection in connected mode
4.1 Overview
When a UE is switched on, a public land mobile network (PLMN) is selected and the UE searches for a suitable cell of this PLMN to camp on. Criteria for cell selection and cell re-selection between radio access technologies (RATs) described in this document only consider radio criteria. In addition to RAT, the PLMN type may differ as well. In this specification, the term PLMN is used as a generic term covering both GSM MAP and ANSI-41 type of PLMNs. According to the type of PLMN, the way to identify it can be different. If the PLMN type is GSM, the PLMN is identified by ‘PLMN identity’ and if the PLMN type is ANSI-41, the PLMN is identified by ‘SID’
The NAS shall provide a list of equivalent PLMNs, if available, that the AS shall use for cell selection and cell reselection.
The UE searches for a suitable cell of the selected PLMN and chooses that cell to provide available services, and tunes to its control channel. This choosing is known as "camping on the cell". The UE will, if necessary, then register its presence, by means of a NAS registration procedure, in the registration area of the chosen cell and as outcome of a successful Location Registration the selected PLMN becomes the registered PLMN [5].
If the UE finds a more suitable cell, it reselects onto that cell and camps on it. If the new cell is in a different registration area, location registration is performed.
If necessary, the UE shall search for higher priority PLMNs at regular time intervals as described in [9] and search for a suitable cell if another PLMN has been selected by NAS.
NOTE: For RRC connected mode the requirements for the search for higher priority PLMNs are defined in [4].
Search of available CSGs may be triggered by NAS to support manual CSG selection.
If the UE loses coverage of the registered PLMN, either a new PLMN is selected automatically (automatic mode), or an indication of which PLMNs are available is given to the user, so that a manual selection can be made (manual mode).
Registration is not performed by UEs only capable of services that need no registration.
The purpose of camping on a cell in idle mode is fourfold:
a) It enables the UE to receive system information from the PLMN.
b) When registered and if the UE wishes to establish an RRC connection, it can do this by initially accessing the network on the control channel of the cell on which it is camped.
c) If the PLMN receives a call for the registered UE, it knows (in most cases) the registration area of the cell in which the UE is camped. It can then send a "paging" message for the UE on control channels of all the cells in the registration area. The UE will then receive the paging message because it is tuned to the control channel of a cell in that registration area and the UE can respond on that control channel.
d) It enables the UE to receive cell broadcast services.
If the UE is unable to find a suitable cell to camp on, or the USIM is not inserted, or if the location registration failed (except for LR rejected with cause #12, cause #14, cause #15 or cause #25, see [5] and [16]), it attempts to camp on a cell irrespective of the PLMN identity, and enters a "limited service" state in which it can only attempt to make emergency calls, receive ETWS, and receive CMAS.
The idle mode tasks can be subdivided into four processes:
– PLMN selection;
– Cell selection and reselection;
– Location registration;
– Support for manual CSG selection.
The relationship between these processes is illustrated in Figure 1.
Figure 1: Overall Idle Mode process
When NAS indicates that PSM starts, the AS configuration (e.g. priorities provided by dedicated signalling and logged measurements) is kept, all running timers continue to run but the UE need not perform any idle mode tasks. If a timer expires while the UE is in PSM it is up to UE implementation whether it performs the corresponding action immediately or the latest when PSM ends. When NAS indicates that PSM ends, the UE shall perform all idle mode tasks.
When NAS indicates that extended DRX in Idle mode parameters have been received, the AS configuration (e.g. priorities provided by dedicated signalling and logged measurements) is kept, all running timers continue to run but the UE need not perform any idle mode tasks. When NAS indicates that extended DRX in Idle mode parameters have been received, the AS starts a timer to indicate the start of the paging transmission window.If a timer expires while the UE is in extended DRX it is up to UE implementation whether it performs the corresponding action immediately or the latest when the paging transmission window starts. When the timer of the extended DRX expires, the AS re-starts the timer of the extended DRX, the AS starts a timer to indicate the end of the paging transmission window, and the UE shall perform all idle mode tasks until the timer of the paging transmission window expires.
4.2 Functional division between AS and NAS in Idle mode
Table 1 presents the functional division between UE non-access stratum (NAS) and UE access stratum (AS) in idle mode. The NAS part is specified in [5] and the AS part in the present document. Examples of different idle mode procedures are presented in Clause 10.
Table 1: Functional division between AS and NAS in idle mode
|
Idle Mode Process |
UE Non-Access Stratum |
UE Access Stratum |
|---|---|---|
|
PLMN Selection |
Maintain the list of allowed PLMN types. It can be GSM-MAP only, ANSI-41 only or both. Maintain a list of PLMNs in priority order according to [5]. Select a PLMN using automatic or manual mode as specified in [5] 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 from AS for PLMN selection. Maintain a list of equivalent PLMN identities. |
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 [5]. Perform measurements to support PLMN selection. Synchronise to a broadcast channel to identify found PLMNs. Report available PLMNs with associated PLMN type and RAT to NAS on request from NAS or autonomously. It shall respect allowed PLMN types indications from NAS. |
|
Cell |
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. NAS is also maintaining lists of forbidden registration areas and a Permitted CSG list. NAS provides these lists to AS. |
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 their ‘PLMN identity’ (GSM-MAP) or ‘SID’ and may broadcast a CSG ID and CSG indicator 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 [5]. If such a cell is found, the cell is selected to camp on. |
|
Cell |
Control cell reselection by for example, maintaining lists of forbidden registration areas. Maintain a list of equivalent PLMN identities and provide the list to AS. Maintain a list of forbidden LAs and provide the list to AS. Maintain a Permitted CSG list and provide the list 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 registration area. Maintain lists of forbidden LAs. Deregister UE when shutting down. |
Report registration area information to NAS. |
|
MBMS |
Maintain priority information for various services and participate in service selection. Indicate to AS if an offered session has previously been received. |
Maintain a list of TMGIs of activated services. Notify NAS of service availability and request NAS to prioritise in case of service conflicts. Report to NAS when an activated service requires action. Receive and handle the MCCH and MTCH, and, if applicable, the MSCH and MICH. |
|
Support for manual CSG selection |
Provide request to search for available CSG(s). Evaluate reports of available CSG(s) from AS for CSG selection. Select a CSG and request AS to select a suitable cell belonging to this CSG. |
Search for cells with a CSG ID. Read the HNB name from system information if a cell with a CSG ID is found. Report CSG ID(s) of the found cells broadcasting a CSG ID together with the HNB name and PLMN(s) to NAS. On selection of a CSG by NAS, select any cell belonging to the selected CSG fulfilling the cell selection criteria and not barred or reserved for operator use for UEs not belonging to AC 11 or 15 and give an indication to NAS that access is possible (for the registration procedure). |
4.3 Service type in Idle and Connected Mode
This clause defines the level of service that may be provided by the network to a UE in Idle mode and Connected Mode.
The action of camping on a cell is necessary to get access to some services. Three levels of services are defined for UE:
– Limited service (emergency calls, ETWS if supported, and CMAS if supported on an acceptable cell)
– Normal service (for public use on a suitable cell)
– Operator service (for operators only on a reserved cell)
Furthermore, 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, receive ETWS if supported, and receive CMAS if supported). Such a cell shall fulfil the following requirements, which is the minimum set of requirements to initiate an emergency call and to receive ETWS primary notification and ETWS secondary notification and to receive CMAS notification in a UTRAN network:
– The cell is not barred, see subclause 5.3.1.1;
– The cell selection criteria are fulfilled, see subclause 5.2.3.1.2;
suitable cell:
A "suitable cell" is a cell on which the UE may camp on to obtain normal service. Such a cell shall fulfil all the following requirements.
– The cell shall be part of either:
– the selected PLMN, or:
– the registered PLMN, or:
– a PLMN of the Equivalent PLMN list
– For a CSG cell, the cell is a CSG member cell for the UE;
According to the latest information provided by the NAS.
– The cell is not barred, see subclause 5.3.1.1;
– The cell is part of at least one LA that is not part of the list of "forbidden LAs for roaming" [9], which belongs to a PLMN that fulfills the first bullet above;
– The cell selection criteria are fulfilled, see subclause 5.2.3.1.2.
If the IE "Multiple PLMN List" [4] is broadcast in the cell, the cell is considered to be part of all LAs with LAIs constructed from the PLMN identities in the "Multiple PLMN List" and the LAC broadcast in the cell.
barred cell:
A cell is barred if it is so indicated in the system information [4].
reserved cell:
A cell is reserved if it is so indicated in system information [4].
Following exceptions to these definitions are applicable for UEs:
– camped on a cell that belongs to a LA that is forbidden for regional provision of service, and during emergency calls.
– as an outcome of the manual CSG selection procedure the UE is allowed to access an acceptable cell which fulfils the cell selection criteria and is not barred or reserved for operator use for UEs not belonging to AC 11 or 15 and inform NAS that access is possible (for location registration procedure).
NOTE: UE is not required to support manual search and selection of CSG(s) while in RRC CONNECTED state.
A cell that belongs to a LA that is forbidden for regional provision service ([5], [16]) is suitable but provides only limited service.
If a UE has an ongoing emergency call, all acceptable cells of that PLMN are treated as suitable for cell reselection for the duration of the emergency call.
suitable MBSFN cluster:
A cell is part of an MBSFN cluster if it is indicated in system information [4]. An MBSFN cluster provides exclusively MBMS services. A suitable MBSFN cluster shall fulfil all the following requirements:
– For FDD, 3.84 Mcps TDD IMB and 3.84/7.68 Mcps TDD the MBSFN cluster shall be part of either:
– the registered PLMN, or:
– a PLMN of the Equivalent PLMN list
The MBSFN cluster shall be indicated as:
– providing MBSFN only service
For FDD, 3.84 Mcps TDD IMB and 3.84/7.68 Mcps TDD MBSFN cluster selection/reselection process for MBSFN clusters operating on the same frequency shall be based on reception quality. Details of the process are not specified.