4. Overall architecture and functional distinction
4.1 Overall architecture
NG-RAN can be:
gNB: Provides NR user plane and control plane to UE Protocol termination
ng-eNB: Provide UE with E-UTRA user plane and control plane protocol termination
4.2 Functional distinction
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gNb and ng-eNB carry the following functions:
Radio resource management functions: radio bearer control, radio access control, mobility control, in the uplink and downlink Dynamic allocation of resources to the UE in the route (scheduling);
IP header compression, encryption and integrity protection;
When the UE accesses, it cannot be determined from the information provided by the UE to the AMF When routing, select AMF for UE;
Route user plane data to UPF;
Route control plane data to AMF;
Connection establishment and release ;
Paging scheduling and transmission;
System broadcast information scheduling and transmission (from AMF or OAM);
Mobility and scheduling measurement and Measurement report configuration;
Transport level packet marking in the uplink (?)
Session management;
Support network slicing;
QoS Flow management and mapping to drb;
Support RRC_INACTIVE state of UE;
NAS message distribution;
Wireless network sharing;
Dual link;
Close intercommunication between NR-EUTRA;
AMF carries the following functions:
NAS signaling Termination;
NAS signaling security
AS security control;
3GPP network movement, signaling between CN nodes;
< p>Idle mode UE reachability (including the control and execution of Paging retransmission);
Registration area management;
Support intra-system and inter-system mobility;
Access authentication;
Roaming access authorization;
Mobility management control (subscription and policy)
Support network slicing;
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SMF selection;
UPF carries the following main functions:
Inter-system and intra-system mobility anchor points (when applicable);
External PDU conversation point interconnected with data network
Packet routing and forwarding
Data packet inspection and user plane department Implementation of sub-policy rules
Traffic usage report;
Upstream classifier that routes traffic to the data network;
Branch point to support multi-homed PDU sessions< /p>
User plane QoS processing, such as packet filtering, gating, UL/DL rate enhancement;
Upstream traffic verification (SDF to QoS flow mapping)
Downlink packet buffering and downlink data notification triggering
Session Management Function (SMF) carries the following main functions:
Session Management;
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UE IP address allocation and management;
Configure traffic diversion in UPF to route traffic to the correct target;
The control part of policy implementation and QoS;
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Downlink data notification;
4.3 network interface
4.3.1NG interface
4.3.1.1 NG user plane interface
NG user plane interface (NG-U) is defined between NG-RAN node and UPF. The transmission network layer is constructed based on IP transmission, and GTP-U is used on top of UDP/IP to carry user plane PDUs between NG-RAN nodes and UPF.
NG-U provides non-guaranteed delivery of user plane PDUs between NG-RAN nodes and UPF. For further details on NG-U, please refer to TS 38.410
4.3.1.2NG control plane interface
NG control plane interface (NG -C) Defined between NG-RAN node and AMF. The transmission network layer is constructed based on IP transmission. In order to reliably transmit signaling messages, SCTP was added to IP. The application layer signaling protocol is called NGAP (NG Application Protocol). The SCTP layer provides guaranteed delivery of application layer messages. In transmission, point-to-point transmission at the IP layer is used to transmit signaling PDUs.
NG-C provides the following functions: NG interface management; UE context management; UE mobility management; NAS message transmission; Paging; PDU session management; configuration transmission; warning message transmission.
For further details on NG-C, please refer to TS 38.410 [16].
4.3.2 Xn interface
4.3.2.1 Xn user interface
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Xn user plane (Xn-U) interface is defined between two NG-RAN nodes. The transmission network layer is constructed based on IP transmission, and GTP-U is used to carry user plane PDUs on top of UDP/IP.
Xn-U provides non-guaranteed delivery of user plane PDUs, and supports the following functions: data forwarding; flow control
about Xn-U For more information, please refer to TS 38.420
4.3.2.2 Control plane interface
Xn control plane interface (Xn-C) Defined between NG-RAN nodes. The transport network layer is built on SCTP over IP. The application layer signaling protocol is called XnAP (Xn Application Protocol). The SCTP layer provides guaranteed delivery of application layer messages. In transmission, point-to-point transmission at the IP layer is used to transmit signaling PDUs.
Xn-C interface supports the following functions: Xn interface management; UE mobility management, including context transmission and RAN Paging; dual connectivity.
For more information about Xn-C, please refer to TS 38.420
4.4 Wireless protocol architecture
4.5 MRDC
NG-RAN supports MR-DC operation, Among them, the UE in RRC_CONNECTED is configured to use the radio resources provided by two different schedulers. These schedulers are located in two different NG-RAN nodes and are connected through non-ideal backhaul. One provides NR access and the other provides E-UTRA. Or NR visit. For more detailed information about MR-DC operation, please refer to TS 37.340
4.6 Network Sharing
NG-RAN supports TS 23.501 [3 ] Is defined in the wireless access network sharing. If NR access is shared, the system information broadcast in the shared unit indicates the TAC and unit IDs (up to 12) of each PLPN subset. NR access only provides one TAC and one unit identifier for each unit of each PLMN. Each unit identifier associated with the PLPN subset identifies its serving NG-RAN node.
5.Physical Layer
6.Layer 2
7 RRC
7.1 Services and functions
The main services and functions of the RRC sublayer include:
AS and NAS related System information broadcast;
Paging triggered by 5GC and NG-RAN;
establish, maintain, and Release RRC connection,
Including: adding, modifying and releasing CA;
Adding and modifying between NR or E-UTRA and NR And release dual connection
including the security function of key management;
SRB and DRB establishment configuration maintenance and release;
< p align="left">Mobile function: handover and context transmission; UE cell selection and reselection and its control; inter-system mobility;
QOS management function;
UE measurement report and report control;
RLF check and recovery;
NAS message transmission
7.2 Protocol status
RRC supports the following statuses:
RRC_IDLE:
PLMN selection;
system information broadcast;
Select mobility;
Paging triggered by 5gc;
NAS is DRX configured for CN Paging;
RRC_INACTIVE:
PLMN selection;
system information broadcast;
Select mobility;
Paing triggered by NG-RAN;
NG-RAN RNA (RAN-based notification area) management;< /p>
NG-RAN is the DRX configured for RNA Paging;
establishes a 5GC-NG-RAN connection (control plane and user plane) for UE;
Wrong UE AS context is stored in NG-RAN and UE;
NG-RAN knows which RNA the UE belongs to;
RRC_CONNECTED:
Establish 5GC-NG-RAN connection (control plane and user plane) for UE;
The wrong UE AS context is stored in NG-RAN and UE;
NG-RAN knows the RNA to which UE belongs;
Transmit to UE Unicast data;
Includes measuring mobility controlled by the network;
7.3 System information processing
System information (SI) consists of MIB and multiple SIBs, which are divided into Minimum SI and Other SI.
Minimum SI includes basic information required for initial access and information required to obtain any other SI
Minimum SI includes:
MIB contains cell prohibition status information and basic physical layer information that the cell needs to further accept for system information, such as CORESET#0 configuration. The MIB is periodically broadcast on the BCH.
SIB1 defines the scheduling of other system information blocks and contains the information required for initial access. SIB1, also known as Remaining Minimum SI (RMSI), is periodically broadcast on DL-SCH, or is sent to UEs in RRC_CONNECTED in a dedicated manner on DL-SCH.
Other SI includes all SIBs that are not broadcast in the smallest SI. These IBs can be broadcast on the DL-SCH periodically, on-demand on the DL-SCH (that is, broadcast at the request of the UE in RRC_IDLE or RRC_INACTIVE), or sent on the DL-SCH to the UE in RRC_CONNECTED in a dedicated manner.
Other SI includes:
SIB2 contains cell reselection information, mainly related to the serving cell;
SIB3 contains information about the service frequency and neighboring cells within the frequency related to cell reselection (including frequency-common cell reselection parameters and cell-specific reselection parameters)
SIB4 contains information about other NR frequencies and inter-frequency neighboring cells related to cell reselection (including frequency-common cell reselection parameters and cell-specific reselection parameters)
SIB5 contains information about E-UTRA frequencies and E-UTRA neighboring cells related to cell reselection (including frequency-common cell reselection parameters and cell-specific reselection parameters)
SIB6 contains the main notification of ETWS.
SIB7 includes ETWS auxiliary notice.
SIB8 contains CMAS warning notifications.
SIB9 contains information related to GPS time and Coordinated Universal Time (UTC).
For UE Considering the cell/frequency for camping, the UE does not need to obtain the minimum SI content of the cell/frequency from another cell/frequency layer. This does not exclude the case where the UE applies the stored SI from the previously visited cell.
If the UE cannot determine the entire content of the minimum SI of the cell (by receiving from the cell or from the previously stored SI in the previous cell), the UE should treat the cell as barred of.
When multiple numbers are mixed on a single carrier, only the default value is used for system information broadcast and paging.
7.3.2 scheduling
MIB maps and performs BCH on BCCH, and All other SI messages are mapped on the BCCH and dynamically performed on the DL-SCH. SIB1 indicates the scheduling of SI messages in the Other SI part.
For UEs in RRC_IDLE and RRC_INACTIVE, Other SI requests trigger the random access procedure (see 9.2.6). If the requested SI is associated with a subset of PRACH resources, MSG3 contains SI Request message, MSG1 is used for indication of Other SI request. When using MSG1, the minimum request granularity is one SI message (that is, a set of SIB). One RACH preamble and/or PRACH resource can be used to request multiple SI messages and gNB to request confirmation in MSG2. When using MSG 3, gNB confirms the request in MSG4.
Other SI broadcasts with a configurable period and a certain duration. When the UE requests in RRC_IDLE/RRC_INACTIVE, it can also broadcast Other SI.
To allow the UE to camp on a cell, it must obtain the minimum SI content from the cell. There may be cells that do not broadcast Minimum SI in the system, so the UE cannot camp on such cells.
7.3.3 SI modification
Change of system information (ETWS/CMAS, Please refer to Article 16.4) It only occurs on a specific radio frame, that is, the concept of a modification period is used. System information can be transmitted multiple times with the same content during the modification period (defined by its schedule). The modification period is configured by system information.
7.4 Access control
NG-RAN supports overload and access control functions, Such as RACH fallback, RRC connection rejection, RRC connection release and UE-based access prohibition mechanism.
A unified access control framework specified in TS 22.261 applies to all UE states of NR (RRC_IDLE, RRC_INACTIVE and RRC_CONN). NG-RAN broadcast prohibits the control information associated with the access category and access identifier (in the case of network sharing, the prohibition control information can be set separately for each PLMN). UE according to the selected PLMN’s prohibited information broadcast and the selected access category and access identifier of the access attempt (determine whether the access attempt is authorized:
For the request triggered by the NAS, the NAS determines the access category And access identification;
For AS-triggered requests, RRC determines the access category, and NAS determines the access identification.
gNB is processing a ” When the high-priority access attempts of “emergency “, “mps-PriorityAccess” and “mcs-PriorityAccess” (ie emergency call, MPS, MCS subscribers), use RRC_REJECt to reject these only in extreme network situations that may threaten the stability of gNB Access attempts.
7.5 UE capabilities
The UE reports its UE capabilities, at least when requested by the network, these functions are static. gNB can Which functions are requested by the UE according to the band information.
7.6 NAS message transmission
NR provides reliable ordering on SRB in RRC Delivery of NAS messages, but PDCP reconstruction during handover may cause loss or duplication. In RRC, NAS messages are transparently transmitted. Piggybacking of NAS messages may occur in the following scenarios:
Bearer establishment/modification/release in DL.
Initial NAS message is transmitted during connection establishment and connection restoration in UL.
NOTE: In addition to integrity protection and encryption performed by NAS, NAS messages can also be integrity protected and encrypted by PDCP
During the establishment or modification of PDU session resources, you can Multiple NAS messages are sent in a single downlink RRC message. In this case, the sequence of the NAS messages contained in the RRC message should be the same as the sequence of the corresponding NG-AP messages to ensure the orderly delivery of NAS messages.
7.7 CA
7.8 Bandwidth Adaptation
7.9 UE Assistance Information
8 NG Identities
8.1 UE Identities
here In the sentence, the identity used by the NR connected to the 5GC is listed. For cell-level scheduling, the following identifiers are used:
C_RNTI: Unique UE identifier, used for RRC connection and scheduling.
CS_RNTI, INT_RNTI, MCS_C_RNTI, P_RNTI, SI_RNTI, SP_CSI_RNTI
SFI_RNTI, TPC_PUCCH_RNTI, TPC_PUSCH_RNTI, TPC_SRS_RNTI
RA_RNTI, Temporary C_RNTI, Random value for contention resolution
I_RNTI
8.2 Network Identities
In NG-RAN, the following identifiers are used to identify specific network entities:
AMF Name,
NR Cell Global Identifier (NCGI),
gNB Identifier(gNB ID),
Global gNB ID,
TAI(Tracking Area identity),
S-NSSAI
9 mobility And state transition
Load balancing is implemented in NR through handover, redirection mechanism when RRC is released, and through the use of inter-frequency and inter-system absolute priority and inter-frequency Qoffset parameters.
The measurement performed by the linked UE for mobility is divided into at least three measurement types:
Same frequency NR measurement, inter-frequency NR measurement , Different system E-UTRA measurement
For each measurement type, one or more measurement objects can be defined (the measurement objects define the carrier frequency to be monitored).
For each measurement object, one or more report configurations can be defined (the report configuration defines the report conditions). Three reporting standards are used: event-triggered report, periodic report, and event-triggered periodic report.
The association between the measurement object and the report configuration is created by the measurement identifier (the measurement identifier links a measurement object and a report configuration of the same RAT). By using multiple measurement identifiers (one for each measurement object, report configuration pair), you can:
Associate multiple report configurations with one measurement object
Associate a report configuration with multiple measurement objects
When reporting the measurement results, the measurement identifier is also used.
The measurement of each RAT is considered separately.
NG-RAN uses measurement commands to instruct UE to start, modify or stop measurement.
The handover can be performed in the same RAT and/or CN, or it can involve changes to the RAT and/or CN.
When 5GC does not support emergency services, voice services, load balancing, etc., it will perform a fallback to the E-UTRAN system. According to factors such as CN interface availability, network configuration and wireless conditions, the fallback process will cause the RRC_CONNECTED state to move (handover process) or RRC_IDLE state to move (redirection), please refer to TS 23.501 [3] and TS38.331 [12]
9.2 NR system
9.2.1 RRC_IDLE move
9.2.1.1 Cell Selection
The PLMN selection principle in NR is based on the 3GPP PLMN selection principle. Cell selection needs to be 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:
UE NAS layer Identify the selected PLMN and the equivalent PLMN
Cell selection is always based on the CD-SSB located on the synchronization grid (see clause 5.2.4)
The UE searches for the NR frequency band, and each carrier frequency identifies the strongest cell according to the CD-SSB. Then read the unit system information broadcast to identify its PLMN
UE can search each carrier in turn (“initial unit selection”) or use stored information to shorten the search (“stored information unit Select “)
The UE seeks to determine a suitable cell; if it cannot identify a suitable cell, it seeks to identify an acceptable cell. When a suitable cell is found or only an acceptable cell is found, camp on that cell and start the cell reselection process.
The appropriate cell is the cell attribute that meets the cell selection conditions; the cell PLMN is the selected PLMN, registered or equivalent PLMN; the cell is not forbidden or reserved, and it is not “forbidden” Part of the tracking area in the “Roaming Tracking Area” list
Acceptable cells are the cells whose measured cell attributes meet the cell selection conditions and are not prohibited.
Go to RRC_IDLE state:
When transitioning from RRC_CONNECTED or RRC_INACTIVE to RRC_IDLE, the UE shall be based on the RRC allocation in the state transition message The frequency (if any) selects the cell to camp on.
Recovering from coverage:
The UE should try to find a suitable cell in the manner described above for storing information or initial cell selection. If no suitable cell is found on any frequency or RAT, the UE should try to find an acceptable cell.
In multi-beam operation, the cell quality is derived in the beam corresponding to the same cell (see subclause 9.2.4)
9.2 .1.2 Cell reselection
The UE in RRC_IDLE performs cell reselection. The principle of the procedure is as follows:
Cell reselection is always based on the CD-SSB located on the synchronization grid (see clause 5.2.4)
UE measures the attributes of the serving cell and neighboring cells to enable the reselection process:
For search and measurement of inter-frequency neighboring cells, you only need to indicate the carrier frequency
< p align="left">Cell reselection identifies the cell where the UE should reside. It is based on the cell reselection standard, which involves the measurement of the serving cell and neighboring cells.
The same frequency reselection is based on the cell ranking.
Inter-frequency reselection of absolute priority, where the UE attempts to camp on the highest-priority frequency available.
Provide blacklist to prevent UE from reselecting to specific same-frequency and different-frequency cells.
Cell reselection can depend on speed.
Service specific priority.
In multi-beam operation, the cell quality is derived in the beam corresponding to the same cell (see subclause 9.2.4)
9.2 .1.3 State transition
The following figure describes the transition from RRC_IDLE to RRC_CONNECTED triggered by the UE (for NAS components, please refer to TS 23.502)
1 UE requests to set up a new connection from RRC_IDLE
2/2a gNB completes RRC establishment
3 The first NAS message of the UE is sent to AMF through RRCSetupCmp and Initial Ue Message
4/4a/5/5a It is possible to exchange other NAS messages between UE and AMF.
6. AMF prepares UE context data (including PDU session context, security keys, UE wireless capabilities and UE security capabilities, etc.) and sends it to the gNB.
7/7a gNb activation safety
8/8a gNB establishes SRB2 and DRB through retake
9 gNB informs AMF that the establishment process has been completed.
9.2.2 RRC_INACTIVE mobile
9.2.2.1 overview
< p align="left">RRC_INACTIVE means that the UE maintains the CM connection state and can move within the area (RNA) configured by the NG-RAN without notifying the NG-RAN. In RRC_INACTIVE, the last serving gNB node retains the UE context and the NG connection associated with the UE serving AMF and UPF.
When the UE is in the RRC_INACTIVE state, if the last serving gNB receives downlink data from UPF or downlink UE-associated signaling from AMF (except for the UE context release command message), Paging in the RNA corresponding to this cell, if the RNA includes sending XnAP paging across the gNB neighboring cell to the neighboring gNB
When the UE receives the UE context release command message in RRC_INACTIVE, Paging in the RNA corresponding to this cell, if the RNA contains a cross-gNB neighboring cell, send XnAP paging to the neighboring gNB to show the release of the UE.
When the UE receives the NG RESET message in RRC_INACTIVE, it will page in the RNA corresponding to this cell. If the RNA contains the cross-gNB neighboring cell to send XnAP paging to the neighboring gNB, The UE is released.
When RAN paging fails, gNB handles it according to TS 23.501.
AMF provides core network auxiliary information to NG-RAN node to help NG-RAN node decide whether it can switch UE to RRC_INACTIVE. The core network assistance information includes the registration area configured for the UE, the regular registration update timer, and the UE identification index value, and may include UE-specific DRX, indicating whether the UE is configured by the AMF with only mobile initiated connection (MICO) mode and the expected UE behavior. When configuring RNA, the NG-RAN node will consider the UE registration area. The NG-RAN node uses the UE-specific DRX and UE identification index value for RAN paging. NG-RAN nodes will consider registering update timers periodically to configure regular RNA update timers. The NG-RAN node will consider the expected UE behavior to help the UE RRC state transition decision.
When transitioning to RRC_INACTIVE, the NG-RAN node may use the regular RNA update timer value to configure the UE. When the regular RNA update timer expires, no UE notification is required. The gNB behavior is specified in TS 23.501.
If the UE accesses a gNB other than the last serving gNB, the receiving gNB will trigger XnAP The process of retrieving the UE context in order to obtain the UE context from the last serving gNB, and may also trigger the Xn-U address indication process, including the possibility of recovering data from the last serving gNB. After successfully retrieving the UE context, the receiving gNB should perform slice-aware admission control when receiving slice information, and become the serving gNB, and further trigger the NGAP path switch request and the applicable RRC process. After the path switching process, the serving gNB triggers the release of the UE context at the last serving gNB through the XnAP UE context release process.
If the UE cannot be accessed in the last serving gNB, the gNB should:
fail any AMF-initiated UE-associated Class 1 process, It is allowed to signal unsuccessful operation in the corresponding response message
Send NAS non-delivery indication program, report that any NAS PDU received from UE AMF is not delivered
If the UE accesses a gNB other than the last serving gNB, and the receiving gNB cannot find a valid UE context, the receiving gNB can establish a new RRC connection instead of restoring the previous RRC connection. UE context retrieval will also fail. Therefore, if the service AMF changes, a new RRC connection needs to be established
When the UE moves out of the configured RNA, the UE in the RRC_INACTIVE state needs to start the RNA Update the program. When receiving an RNA update request from the UE, the receiving gNB will trigger the XnAP to retrieve the UE context process in order to obtain the UE context from the last serving gNB, and may decide to send the UE back to the RRC_INACTIVE state, move the UE to the RRC_CONNECTED state, or send the UE To RRC_IDLE. In the case of regular RNA update, if the last serving gNB decides not to relocate the UE context, it fails to retrieve the UE context process and sends the UE back to RRC_INACTIVE, or directly to RRC_IDLE through the encapsulated RRCRelease message
9.2.2.2 Cell reselection
The UE in RC_INACTIVE performs cell reselection. The principle of this process is the same as that of the RRC_IDLE state (see clause 9.2.1.2).
9.2.2.3 RAN-Based Notification Area
The UE in the RRC_INACTIVE state can be configured with RNA by the last served NG-RAN node, Among them:
RNA can cover single or multiple cells, and should be included in the CN registration area; in this version, Xn connection should be available in RNA;
The RAN-based notification area update (RNAU) is sent periodically by the UE, and is also sent when the cell reselection process of the UE selects a cell that does not belong to the configured RNA
There are several different alternatives in how to configure RNA:
Cell list: UE provides a clear list of cells (one or more) that make up RNA
RAN area list: Provide UE (at least one) RAN area ID, where RAN area is a subset of CN tracking area or equal to CN tracking area. The RAN area is specified by a RAN area ID, which is composed of TAC and optional RAN area code; the cell broadcasts one or more RAN area IDs in the system information