10.1.1 Mobility Management in ECM-IDLE
36.3003GPPEvolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN)Overall descriptionRelease 17Stage 2TS
10.1.1.1 Cell selection
The principles of PLMN selection in E-UTRA are based on the 3GPP PLMN selection principles. Cell selection is required on transition from EMM_DETACHED to EMM-REGISTERED and from ECM-IDLE or ECM-CONNECTED.
Cell selection:
– The UE NAS layer identifies a selected PLMN and equivalent PLMNs;
– The UE searches the E-UTRA frequency bands and for each carrier frequency identifies the strongest cell. It reads cell system information broadcast to identify its PLMN(s):
– The UE may search each carrier in turn ("initial cell selection") or make use of stored information to shorten the search ("stored information cell selection").
– The UE seeks to identify a suitable cell; if it is not able to identify a suitable cell it seeks to identify an acceptable cell. When a suitable cell is found or if only an acceptable cell is found it camps on that cell and commence the cell reselection procedure:
– A suitable cell is one for which the measured cell attributes satisfy the cell selection criteria; the cell PLMN is the selected PLMN, registered or an equivalent PLMN; the cell is not barred or reserved and the cell is not part of a tracking area which is in the list of "forbidden tracking areas for roaming";
– An acceptable cell is one for which the measured cell attributes satisfy the cell selection criteria and the cell is not barred.
Transition to RRC_IDLE:
On transition from RRC_CONNECTED to RRC_IDLE, a UE should camp on the last cell for which it was in RRC_CONNECTED or a cell/any cell of set of cells or frequency be assigned by RRC in the state transition message.
Recovery from out of coverage:
The UE should attempt to find a suitable cell in the manner described for stored information or initial cell selection above. If no suitable cell is found on any frequency or RAT the UE should attempt to find an acceptable cell.
10.1.1.2 Cell reselection
A UE in RRC_IDLE performs cell reselection. The principles of the procedure are the following:
– The UE makes measurements of attributes of the serving and neighbour cells to enable the reselection process:
– There is no need to indicate neighbouring cells in the serving cell system information to enable the UE to search and measure a cell i.e. E-UTRAN relies on the UE to detect the neighbouring cells;
– For the search and measurement of inter-frequency neighbouring cells, only the carrier frequencies need to be indicated;
– Measurements may be omitted if the serving cell attribute fulfils particular search or measurement criteria.
– Cell reselection identifies the cell that the UE should camp on. It is based on cell reselection criteria which involves measurements of the serving and neighbour cells, except for NB-IoT:
– Intra-frequency reselection is based on ranking of cells;
– Inter-frequency reselection is based on absolute priorities where a UE tries to camp on the highest priority frequency available. Absolute priorities for reselection are provided only by the RPLMN and are valid only within the RPLMN; priorities are given by the system information and are valid for all UEs in a cell, specific priorities per UE can be signalled in the RRC Connection Release message. A validity time can be associated with UE specific priorities.
– Inter-frequency E-UTRAN reselection can be also based on redistribution priority. A UE may be redistributed to a redistribution target (frequency or cell) and will consider the redistribution target (frequency or cell) as having the highest priority (i.e. higher than any network configured priority) for a period of time (i.e. validity timer). The redistribution parameters are defined in system information and can be triggered by Paging.
– For inter-frequency neighbouring cells, it is possible to indicate layer-specific cell reselection parameters (e.g., layer specific offset). These parameters are common to all neighbouring cells on a frequency;
– An NCL can be provided by the serving cell to handle specific cases for intra- and inter-frequency neighbouring cells. This NCL contains cell specific cell reselection parameters (e.g., cell specific offset) for specific neighbouring cells;
– Exclude-lists can be provided to prevent the UE from reselecting to specific intra- and inter-frequency neighbouring cells;
– Cell reselection can be speed dependent (speed detection based on UTRAN solution);
– Cell reselection parameters are applicable for all UEs in a cell, but it is possible to configure specific reselection parameters per UE group or per UE.
For NB-IoT, cell reselection identifies the cell that the UE should camp on. It is based on cell reselection criteria which involve measurements of the serving and neighbour cells as follows:
– Intra-frequency reselection is based on ranking of cells (potentially with cell specific offsets);
– Inter-frequency reselection is based on ranking of frequencies (potentially with frequency specific offsets);
– Blind redirection supported for load balancing (potentially with a dedicated offset for the frequency where the UE is redirected to).
Cell access restrictions apply as for UTRAN, which consist of access class (AC) barring and cell reservation (e.g. for cells "reserved for operator use") applicable for mobiles in RRC_IDLE mode. For NB-IoT UEs, BL UEs or UEs in enhanced coverage, E-UTRAN can also restrict access to the cell based on the level of coverage enhancements that would be needed by the UE.