My 3GPP 38.300 Notes

Scope

The 38.300 document provides an overview and overall description of the NG-RAN and focuses on the radio interface protocol architecture of NR connected to 5GC (E-UTRA connected to 5GC is covered in the 36 series). Details of the radio interface protocols are specified in companion specifications of the 38 series.

4.1 Overall Architecture

An NG-RAN node is either:

a gNB, providing NR user plane and control plane protocol terminations towards the UE; or
an ng-eNB, providing E-UTRA user plane and control plane protocol terminations towards the UE.

The gNBs and ng-eNBs are interconnected with each other by means of the Xn interface. The gNBs and ng-eNBs are also connected by means of the NG interfaces to the 5GC, more specifically to the AMF (Access and Mobility Management Function) by means of the NG-C interface and to the UPF (User Plane Function) by means of the NG-U interface (see TS 23.501).

NOTE: The architecture and the F1 interface for a functional split are defined in TS 38.401.

4.2 Functional Split

See spec for details. Details are summarized in following diagram.

4.3 Network Interfaces

4.3.1 NG Interface

4.3.1.1 NG User Plane

The NG user plane interface (NG-U) is defined between the NG-RAN node and the UPF. The user plane protocol stack of the NG interface is shown on Figure 4.3.1.1-1. The transport network layer is built on IP transport and GTP-U is used on top of UDP/IP to carry the user plane PDUs between the NG-RAN node and the UPF.

NG-U provides non-guaranteed delivery of user plane PDUs between the NG-RAN node and the UPF.

Further details of NG-U can be found in TS 38.410.

4.3.1.2 NG Control Plane

The NG control plane interface (NG-C) is defined between the NG-RAN node and the AMF. The control plane protocol stack of the NG interface is shown on following figure. The transport network layer is built on IP transport. For the reliable transport of signalling messages, SCTP is added on top of IP. The application layer signalling protocol is referred to as NGAP (NG Application Protocol). The SCTP layer provides guaranteed delivery of application layer messages. In the transport, IP layer point-to-point transmission is used to deliver the signalling PDUs.

NG-C provides the following functions:

NG interface management;
UE context management;
UE mobility management;
Transport of NAS messages;
Paging;
PDU Session Management;
Configuration Transfer;
Warning Message Transmission.

Further details of NG-C can be found in TS 38.410.

4.3.2 Xn Interface

The Xn User plane (Xn) interface is defined between two NG-RAN nodes.

4.3.2.1 Xn User Plane

The Xn User plane (Xn-U) interface is defined between two NG-RAN nodes. The user plane protocol stack on the Xn interface is shown in following figure. The transport network layer is built on IP transport and GTP-U is used on top of UDP/IP to carry the user plane PDUs.

Xn-U provides non-guaranteed delivery of user plane PDUs and supports the following functions:

Data forwarding;
Flow control.

Further details of Xn-U can be found in TS 38.420.

4.3.2.2 Xn Control Plane

The Xn control plane interface (Xn-C) is defined between two NG-RAN nodes. The control plane protocol stack of the Xn interface is shown on following figure. The transport network layer is built on SCTP on top of IP. The application layer signalling protocol is referred to as XnAP (Xn Application Protocol). The SCTP layer provides the guaranteed delivery of application layer messages. In the transport IP layer point-to-point transmission is used to deliver the signalling PDUs.

The Xn-C interface supports the following functions:

Xn interface management;
UE mobility management, including context transfer and RAN paging;
Dual connectivity.

Further details of Xn-C can be found in TS 38.420.

4.4 Radio Protocol Architecture

4.4.1 User Plane (UP)

The figure below shows the protocol stack for the user plane, where SDAP, PDCP, RLC and MAC sublayers (terminated in gNB on the network side) perform the functions listed in clause 6.

4.4.2 Control Plane (CP)

The figure below shows the protocol stack for the control plane, where:

PDCP, RLC and MAC sublayers (terminated in gNB on the network side) perform the functions listed in clause 6;
RRC (terminated in gNB on the network side) performs the functions listed in clause 7;
NAS control protocol (terminated in AMF on the network side) performs the functions listed in TS 23.501, for instance: authentication, mobility management, security control…

5.2 Downlink

5.2.5 Physical layer procedures

5.2.5.1 Link adaptation

Link adaptation (AMC: adaptive modulation and coding) with various modulation schemes and channel coding rates is applied to the PDSCH. The same coding and modulation is applied to all groups of resource blocks belonging to the same L2 PDU scheduled to one user within one transmission duration and within a MIMO codeword.

For channel state estimation purposes, the UE may be configured to measure CSI-RS and estimate the downlink channel state based on the CSI-RS measurements. The UE feeds the estimated channel state back to the gNB to be used in link adaptation.

5.2.5.2 Power Control

Downlink power control can be used.

5.2.5.3 Cell search

Cell search is the procedure by which a UE acquires time and frequency synchronization with a cell and detects the Cell ID of that cell. NR cell search is based on the primary and secondary synchronization signals, and Physical Broadcast Channel (PBCH) Demodulation Reference Signal (DMRS), located on the synchronization raster.

5.2.5.4 HARQ

Asynchronous Incremental Redundancy Hybrid Automatic Repeat ReQuest (ARQ), or HARQ, is supported. The gNB provides the UE with the HARQ-ACK feedback timing either dynamically in the Downlink Control Information (DCI) or semi-statically in an Radio Resource Controller (RRC) configuration. Retransmission of HARQ-ACK feedback is supported by using enhanced dynamic codebook and/or one-shot triggering of HARQ-ACK transmission for (i) all configured CCs and HARQ processes in the Physical Uplink Control Channel (PUCCH) group, (ii) a configured subset of CCs and/or HARQ processes in the PUCCH group, or (iii) a dynamically indicated HARQ-ACK feedback instance. For HARQ-ACK of Semi-Persistent Scheduling (SPS) Physical Downlink Shared Channel (PDSCH) without associated PDCCH, in case of HARQ-ACK dropping due to Time Division Duplexing (TDD) specific collisions, the HARQ-ACK feedback can be deferred to a next available PUCCH transmission occasion.

The UE may be configured to receive code block group based transmissions where retransmissions may be scheduled to carry a sub-set of all the code blocks of a Transport Block (TB).

5.2.5.5 Reception of SIB1

The Master Information Block (MIB) on Physical Broadcast Channel (PBCH) provides the UE with parameters (e.g. CORESET#0 configuration) for monitoring of PDCCH for scheduling PDSCH that carries the System Information Block 1 (SIB1). PBCH may also indicate that there is no associated SIB1, in which case the UE may be pointed to another frequency from where to search for an Synchronization Signal (SS)/Physical Broadcast Channel (PBCH) block (SSB) that is associated with a SIB1 as well as a frequency range where the UE may assume no SSB associated with SIB1 is present. The indicated frequency range is confined within a contiguous spectrum allocation of the same operator in which SSB is detected.

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 characterized 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.

7.6 Transport of NAS Messages

New Radio (NR) provides reliable in-sequence delivery of Non-Access Stratum (NAS) messages over Signalling Radio Bearers (SRBs) in Radio Resource Controller (RRC), except at handover where losses or duplication can occur when Packet Data Convergence Protocol (PDCP) is re-established. In RRC, NAS messages are sent in transparent containers. Piggybacking of NAS messages can occur in the following scenarios:

At bearer establishment/modification/release in the Downlink (DL);
For transferring the initial NAS message during connection setup and connection resume in the Uplink (UL).

NOTE: In addition to the integrity protection and ciphering performed by NAS, NAS messages can also be integrity protected and ciphered by PDCP.

Multiple NAS messages can be sent in a single downlink RRC message during Protocol Data Unit (PDU) Session Resource establishment or modification. In this case, the order of the NAS messages contained in the RRC message shall be in the same order as that in the corresponding Next Generation Application Protocol (NGAP) message in order to ensure the in-sequence delivery of NAS messages.

NG-RAN node may trigger the NAS Non Delivery Indication procedure to report the non-delivery of the non PDU Session related NAS PDU received from the AMF as specified in TS 38.413.

Section 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.

Section 9 Mobility and State Transitions

9.1 Overview

Load balancing is achieved in New Radio (NR) with handover, redirection mechanisms upon Radio Resource Controller (RRC) release and through the usage of inter-frequency and inter-RAT (Radio Access Type) 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.

Single Radio Voice Continuity (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.

9.2 Intra-NR

My Intra-NR Notes

To My Radio Technology beyond LTE - 38 series notes