My Radio Technology beyond LTE - 38 series notes
38.300
Section 7 Radio Resource Controller (RRC)
7.1 Services and Functions
The main services and functions of the RRC sublayer over the Uu interface include:
- Broadcast of System Information related to AS and NAS;
- Paging initiated by 5GC or NG-RAN;
- Establishment, maintenance and release of an RRC connection between the UE and NG-RAN including:
- Addition, modification and release of carrier aggregation;
- Addition, modification and release of Dual Connectivity in NR or between E-UTRA and NR.
- Security functions including key management;
- Establishment, configuration, maintenance and release of Signalling Radio Bearers (SRBs) and Data Radio Bearers (DRBs);
- Mobility functions including:
- Handover and context transfer;
- UE cell selection and reselection and control of cell selection and reselection;
- Inter-RAT mobility.
- QoS management functions;
- UE measurement reporting and control of the reporting;
- Detection of and recovery from radio link failure;
- NAS message transfer to/from NAS from/to UE.
The sidelink specific services and functions of the RRC sublayer over the Uu interface include:
- Configuration of sidelink resource allocation via system information or dedicated signalling;
- Reporting of UE sidelink information;
- Measurement configuration and reporting related to sidelink;
- Reporting of UE assistance information for SL traffic pattern(s).
7.2 Protocol States
RRC supports the following states which can be characterised as follows:
- RRC_IDLE:
- PLMN selection;
- Broadcast of system information;
- Cell re-selection mobility;
- Paging for mobile terminated data is initiated by 5GC;
- DRX for CN paging configured by NAS.
- RRC_INACTIVE:
- PLMN selection;
- Broadcast of system information;
- Cell re-selection mobility;
- Paging is initiated by NG-RAN (RAN paging);
- RAN-based notification area (RNA) is managed by NG- RAN;
- DRX for RAN paging configured by NG-RAN;
- 5GC - NG-RAN connection (both C/U-planes) is established for UE;
- The UE Inactive AS context is stored in NG-RAN and the UE;
- NG-RAN knows the RNA which the UE belongs to;
- Transfer of unicast data and/or signalling to/from the UE over radio bearers configured for SDT.
- RRC_CONNECTED:
- 5GC - NG-RAN connection (both C/U-planes) is established for UE;
- The UE AS context is stored in NG-RAN and the UE;
- NG-RAN knows the cell which the UE belongs to;
- Transfer of unicast data to/from the UE;
- Network controlled mobility including measurements.
7.3 System Information Handling
7.3.1 Overview
System Information (SI) consists of a MIB and a number of SIBs, which are divided into Minimum SI and Other SI:
- Minimum SI comprises basic information required for initial access and information for acquiring any other SI. Minimum SI consists of:
- MIB contains cell barred status information and essential physical layer information of the cell required to receive further system information, e.g. CORESET#0 configuration. MIB is periodically broadcast on BCH.
- SIB1 defines the scheduling of other system information blocks and contains information required for initial access. SIB1 is also referred to as Remaining Minimum SI (RMSI) and is periodically broadcast on DL-SCH or sent in a dedicated manner on DL-SCH to UEs in RRC_CONNECTED.
- Other SI encompasses all SIBs not broadcast in the Minimum SI. Those SIBs can either be periodically broadcast on DL-SCH, broadcast on-demand on DL-SCH (i.e. upon request from UEs in RRC_IDLE, RRC_INACTIVE, or RRC_CONNECTED), or sent in a dedicated manner on DL-SCH to UEs in RRC_CONNECTED (i.e., upon request, if configured by the network, from UEs in RRC_CONNECTED or when the UE has an active BWP with no common search space configured or when the UE configured with inter cell beam management is receiving DL-SCH from a TRP with PCI different from serving cell's PCI). See spec for details what SIBx are within Other SI.
8 Next Generation (NG) Identities
8.1 UE Identities
In this clause, the identities used by NR connected to 5GC are listed. For scheduling at cell level, the following identities are used:
- C-RNTI: unique UE identification used as an identifier of the RRC Connection and for scheduling;
- CI-RNTI: identification of cancellation in the uplink;
- CS-RNTI: unique UE identification used for Semi-Persistent Scheduling in the downlink or configured grant in the uplink;
- INT-RNTI: identification of pre-emption in the downlink;
- MCS-C-RNTI: unique UE identification used for indicating an alternative MCS table for PDSCH and PUSCH;
- P-RNTI: identification of Paging and System Information change notification in the downlink;
- SI-RNTI: identification of Broadcast and System Information in the downlink;
- SP-CSI-RNTI: unique UE identification used for semi-persistent CSI reporting on PUSCH.
For power and slot format control, the following identities are used:
- SFI-RNTI: identification of slot format;
- TPC-PUCCH-RNTI: unique UE identification to control the power of PUCCH;
- TPC-PUSCH-RNTI: unique UE identification to control the power of PUSCH;
- TPC-SRS-RNTI: unique UE identification to control the power of SRS.
During the random access procedure, the following identities are also used:
- RA-RNTI: identification of the Random Access Response in the downlink;
- Temporary C-RNTI: UE identification temporarily used for scheduling during the random access procedure;
- Random value for contention resolution: UE identification temporarily used for contention resolution purposes during the random access procedure.
For NR connected to 5GC, the following UE identities are used at NG-RAN level:
- I-RNTI: used to identify the UE context in RRC_INACTIVE.
For UE power saving purpose during DRX, the following identity is used:
- PS-RNTI: used to determine if the UE needs to monitor PDCCH on the next occurrence of the connected mode DRX on-duration.
For IAB the following identity is used:
- AI-RNTI: identification of the DCI carrying availability indication for soft symbols of an IAB-DU.
For MBS, the following identities are used:
- G-RNTI: Identifies dynamically scheduled PTM transmissions of MTCH(s);
- G-CS-RNTI: Identifies configured scheduled PTM transmissions of MTCH(s);
- MCCH-RNTI: Identifies transmissions of MCCH and MCCH change notification.
8.2 Network Identities
The following identities are used in NG-RAN for identifying a specific network entity:
- AMF Name: used to identify an AMF.
- NR Cell Global Identifier (NCGI): used to identify NR cells globally. The NCGI is constructed from the PLMN identity the cell belongs to and the NR Cell Identity (NCI) of the cell. The PLMN ID included in the NCGI should be the first PLMN ID within the set of PLMN IDs associated to the NR Cell Identity in SIB1, following the order of broadcast.
NOTE 1: How to manage the scenario where a different PLMN ID has been allocated by the operator for an NCGI is left to OAM and/or implementation.
- gNB Identifier (gNB ID): used to identify gNBs within a PLMN. The gNB ID is contained within the NCI of its cells.
- Global gNB ID: used to identify gNBs globally. The Global gNB ID is constructed from the PLMN identity the gNB belongs to and the gNB ID. The MCC and MNC are the same as included in the NCGI.
NOTE 2: It is not precluded that a cell served by a gNB does not broadcast the PLMN ID included in the Global gNB ID.
- Tracking Area identity (TAI): used to identify tracking areas. The TAI is constructed from the PLMN identity the tracking area belongs to and the TAC (Tracking Area Code) of the Tracking Area.
- Single Network Slice Selection Assistance information (S-NSSAI): identifies a network slice.
- Network Slice AS Group (NSAG): identifies a slice or a set of slices. An NSAG is defined within a TA, used for slice specific cell reselection and/or slice specific RACH configuration.
- Network Identifier (NID): identifies an SNPN in combination with a PLMN ID.
- Closed Access Group Identifier: identifies a CAG within a PLMN.
- Local NG-RAN Node Identifier: used as reference to the NG-RAN node in the I-RNTI.
9 Mobility and State Transitions
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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.
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