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Content for  TS 38.300  Word version:  18.4.0

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9 Mobility and State Transitions  9.1 Overview  9.2 Intra-NR  9.2.1 Mobility in RRC_IDLE  9.2.1.1 Cell Selection  9.2.1.2 Cell Reselection  9.2.1.3 State Transitions 
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9  Mobility and State Transitions  p. 75

9.1  Overview  p. 75

Load balancing is achieved in NR with handover, redirection mechanisms upon RRC release and through the usage of inter-frequency and inter-RAT absolute priorities and inter-frequency Qoffset parameters.
Measurements to be performed by a UE for connected mode mobility are classified in at least four measurement types:
  • Intra-frequency NR measurements;
  • Inter-frequency NR measurements;
  • Inter-RAT measurements for E-UTRA;
  • Inter-RAT measurements for UTRA.
For each measurement type one or several measurement objects can be defined (a measurement object defines e.g. the carrier frequency to be monitored).
For each measurement object one or several reporting configurations can be defined (a reporting configuration defines the reporting criteria). Three reporting criteria are used: event triggered reporting, periodic reporting and event triggered periodic reporting.
The association between a measurement object and a reporting configuration is created by a measurement identity (a measurement identity links together one measurement object and one reporting configuration of the same RAT). By using several measurement identities (one for each measurement object, reporting configuration pair) it is then possible to:
  • Associate several reporting configurations to one measurement object and;
  • Associate one reporting configuration to several measurement objects.
The measurements identity is used as well when reporting results of the measurements.
Measurement quantities are considered separately for each RAT.
Measurement commands are used by NG-RAN to order the UE to start, modify or stop measurements.
Handover can be performed within the same RAT and/or CN, or it can involve a change of the RAT and/or CN.
Inter system fallback towards E-UTRAN is performed when 5GC does not support emergency services, voice services, for load balancing etc. Depending on factors such as CN interface availability, network configuration and radio conditions, the fallback procedure results in either RRC_CONNECTED state mobility (handover procedure) or RRC_IDLE state mobility (redirection), see TS 23.501 and TS 38.331.
SRVCC from 5G to UTRAN, if supported by both the UE and the network, may be performed to handover a UE with an ongoing voice call from NR to UTRAN. The overall procedure is described in TS 23.216. See also TS 38.331 and TS 38.413.
In the NG-C signalling procedure, the AMF based on support for emergency services, voice service, any other services or for load balancing etc, may indicate the target CN type as EPC or 5GC to the gNB node. When the target CN type is received by gNB, the target CN type is also conveyed to the UE in RRCRelease Message.
Inter-gNB CSI-RS based mobility, i.e. handover, is supported between two neighbour gNBs by enabling that neighbour gNBs can exchange and forward their own CSI-RS configurations and on/off status.
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9.2  Intra-NR  p. 76

9.2.1  Mobility in RRC_IDLE  p. 76

9.2.1.1  Cell Selection  p. 76

The principles of PLMN selection in NR are based on the 3GPP PLMN selection principles. Cell selection is required on transition from RM-DEREGISTERED to RM-REGISTERED, from CM-IDLE to CM-CONNECTED and from CM-CONNECTED to CM-IDLE and is based on the following principles:
  • The UE NAS layer identifies a selected PLMN and equivalent PLMNs;
  • Cell selection is always based on CD-SSBs located on the synchronization raster (see clause 5.2.4):
    • The UE searches the NR frequency bands and for each carrier frequency identifies the strongest cell as per the CD-SSB. It then 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.
  • The IAB-MT and NCR-MT apply the cell selection procedure as described for the UE with the following differences:
    • The IAB-MT and NCR-MT ignore cell-barring or cell-reservation indications contained in cell system information broadcast;
    • The IAB-MT only considers a cell as a candidate for cell selection if the cell system information broadcast indicates IAB support for the selected PLMN or the selected SNPN, and the NCR-MT only considers a cell as a candidate for cell selection if the cell system information broadcast indicates Network-Controlled Repeater support.
  • The mobile IAB-MT applies the cell selection procedure as described for the IAB-MT with the following differences:
    • The mobile IAB-MT only considers a cell as a candidate cell for cell selection if the cell system information broadcast indicates mobile IAB support.
Transition to RRC_IDLE:
  • On transition from RRC_CONNECTED or RRC_INACTIVE to RRC_IDLE, a UE should camp on a cell as result of cell selection according to the frequency be assigned by RRC in the state transition message if any.
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.
In multi-beam operations, the cell quality is derived amongst the beams corresponding to the same cell (see clause 9.2.4).
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9.2.1.2  Cell Reselection  p. 77

A UE in RRC_IDLE performs cell reselection. The principles of the procedure are the following:
  • Cell reselection is always based on CD-SSBs located on the synchronization raster (see clause 5.2.4).
  • The UE makes measurements of attributes of the serving and neighbour cells to enable the reselection process:
    • For the search and measurement of inter-frequency neighbouring cells, only the carrier frequencies need to be indicated.
  • 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:
    • 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;
    • A Neighbour Cell List (NCL) can be provided by the serving cell to handle specific cases for intra- and inter-frequency neighbouring cells;
    • Exclude-lists can be provided to prevent the UE from reselecting to specific intra- and inter-frequency neighbouring cells;
    • Allow-lists can be provided to request the UE to reselect to only specific intra- and inter-frequency neighbouring cells;
    • Cell reselection can be speed dependent;
    • Service specific prioritisation;
    • Slice-based cell reselection information can be provided to facilitate the UE to reselect a cell that supports specific slices.
In multi-beam operations, the cell quality is derived amongst the beams corresponding to the same cell (see clause 9.2.4).
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9.2.1.3  State Transitions  p. 77

The following Figure describes the UE triggered transition from RRC_IDLE to RRC_CONNECTED (for the NAS part, see TS 23.502):
Reproduction of 3GPP TS 38.300, Fig. 9.2.1.3-1: UE triggered transition from RRC_IDLE to RRC_CONNECTED
Figure 9.2.1.3-1: UE triggered transition from RRC_IDLE to RRC_CONNECTED
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Step 1.
The UE requests to setup a new connection from RRC_IDLE.
Step 2/2a.
The gNB completes the RRC setup procedure.
Step 3.
The first NAS message from the UE, piggybacked in RRCSetupComplete, is sent to AMF.
Step 4/4a/5/5a.
Additional NAS messages may be exchanged between UE and AMF, see TS 23.502.
Step 6.
The AMF prepares the UE context data (including PDU session context, the Security Key, UE Radio Capability and UE Security Capabilities, etc.) and sends it to the gNB.
Step 7/7a.
The gNB activates the AS security with the UE.
Step 8/8a.
The gNB performs the reconfiguration to setup SRB2 and DRBs for UE, or SRB2 and optionally DRBs for IAB-MT.
Step 9.
The gNB informs the AMF that the setup procedure is completed.
The following Figure describes the rejection from the network when the UE attempts to setup a connection from RRC_IDLE:
Reproduction of 3GPP TS 38.300, Fig. 9.2.1.3-2: Rejection of UE triggered transition from RRC_IDLE
Figure 9.2.1.3-2: Rejection of UE triggered transition from RRC_IDLE
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Step 1.
UE attempts to setup a new connection from RRC_IDLE.
Step 2.
The gNB is not able to handle the procedure, for instance due to congestion.
Step 3.
The gNB sends RRCReject (with a wait time) to keep the UE in RRC_IDLE.

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