Idle mode
Cell selection and reselection
How an idle or inactive UE selects a cell by the S-criterion and keeps the best one by priority and ranking.
Overview
Cell selection requires the S-criterion Srxlev > 0 and Squal > 0, where Srxlev = Qrxlevmeas − (Qrxlevmin + Qrxlevminoffset) − Pcompensation − Qoffsettemp and Squal = Qqualmeas − (Qqualmin + Qqualminoffset) − Qoffsettemp (TS 38.304).
For intra-frequency and equal-priority inter-frequency reselection, cells are ranked with Rs = Qmeas,s + Qhyst − Qoffsettemp for the serving cell and Rn = Qmeas,n − Qoffset − Qoffsettemp for neighbours. Measurement of intra-frequency neighbours is gated by whether the serving cell satisfies Srxlev > SIntraSearchP and Squal > SIntraSearchQ. Speed-dependent scaling can be applied to TreselectionNR and TreselectionEUTRA when broadcast.
In NTN, SIB19 can give a location-based measurement initiation rule (distanceThresh with referenceLocation, or movingReferenceLocation for Earth-moving cells) and a time-based rule using t-Service.
Call flow
- 1UE
Cell selection
Check the S-criterion on a detected cell.
- 2UEtoServing cell
Read reselection system information
NR: SIB2 (common), SIB3 (intra-frequency), SIB4 (inter-frequency), SIB5 (inter-RAT, E-UTRA). LTE: SIB3 (common), SIB4 (intra-frequency neighbours), SIB5 (inter-frequency).
- 3UE
Apply measurement rules
TS 38.304 clause 5.2.4.2.
- 4UE
Rank and reselect
Using the criteria in clauses 5.2.4.5 and 5.2.4.6.
Timers and parameters
| Name | Where | Role |
|---|---|---|
| Treselection | Broadcast | Time a candidate must satisfy the reselection criteria. NR has TreselectionNR and TreselectionEUTRA, with optional speed-dependent scaling. |
Specifications
| Spec | What it covers |
|---|---|
| TS 38.304 | Cell selection process (criterion in 5.2.3.2). Reselection evaluation is 5.2.4: priorities 5.2.4.1, measurement rules 5.2.4.2, criteria 5.2.4.5 and 5.2.4.6.(see clause 5.2.3) |
| TS 36.304 | E-UTRA idle-mode procedures, including NTN and eDRX paging. |
| TS 38.331 | NR system information carrying reselection parameters (SIB2 to SIB5, SIB19). |
| TS 36.331 | LTE system information (SIB3 to SIB5). |
Changes by release
| Release | Change |
|---|---|
| Rel-17 | NTN system information (SIB19) with location- and time-based measurement initiation. |
Related procedures
Registration
5G registration and 4G tracking area update
NAS mobility management: registration in 5GS, tracking area update in EPS, and the reject causes.
NTN
NR-NTN mobility and satellite switch
Satellite assistance in SIB19 and how NR uses it for synchronisation, measurement initiation, conditional handover and satellite switch.
IoT power saving
NB-IoT and LTE-M power saving: PSM and eDRX
Power Saving Mode and extended idle-mode DRX, and what they mean for reachability.
Inactive state
RRC_INACTIVE: resume and RNA update
How an NR UE in RRC_INACTIVE resumes a connection or updates its RAN-based notification area, with UE context retrieval over Xn.