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  C-V2X (Cellular Vehicle-to-Everything) is an advanced wireless communication technology currently used in ITS (Intelligent Transportation Systems) for autonomous driving; this technology extends the coverage of autonomous driving and improves blind spot detection capabilities.   I. C-V2X Technology Characteristics: Compared to commonly used traditional sensors, C-V2X is more cost-effective and more suitable for large-scale deployment. Based on the PC5 interface, C-V2X uses Sidelink technology (direct vehicle-to-vehicle communication) to achieve low-latency UrLLC (critical mission) sensor connectivity, with a communication range exceeding that of conventional wireless networks.   II.C-V2X and Autonomous Driving: In 2020, 5G (NR) technology was fully commercialized globally; mobile communication operators and relevant departments are eagerly anticipating its greater role in people's daily lives due to its low latency, high reliability, and high throughput. Level 3 (conditional automation) or Level 4 (highly automated) autonomous driving is a typical example of 5G (NR) applications, where the URLLC (ultra-reliable low-latency communication) used perfectly showcases the capabilities of mobile technology. The evolution of C-V2X and the deployment of 5G (NR) complement each other, jointly building a new ecosystem that will change the way people drive and manage traffic in the future.   III.C-V2X Applications: Given that approximately 1 million people die in road traffic accidents worldwide every year, making traffic accidents the eighth leading cause of death globally, C-V2X (Cellular Vehicle-to-Everything) is becoming a popular solution to this problem. As a complete communication system, it specifically includes four categories of applications:   V2V (Vehicle-to-Vehicle): Communication between vehicles, such as maintaining safe distance, speed, and lane changes. V2I (Vehicle-to-Infrastructure): Communication between vehicles and road infrastructure, such as road signs, traffic lights, and toll booths. V2P (Vehicle-to-Pedestrian): Communication between vehicles and pedestrians, such as sensing nearby pedestrians or cyclists. V2N (Vehicle-to-Network): Communication between vehicles and the network, such as obtaining infotainment information via the internet and sending vehicle performance data to the car manufacturer.

  I. Core Network Assistance Information in 5G: This is designed to assist RAN in optimizing User Equipment (UE) state transition control and RAN paging strategies in the RRC Inactive state. Core network assistance information includes the information set "Core Network Assisted RAN Parameter Tuning," which helps the RAN optimize UE RRC state transitions and CM state transition decisions. It also includes the information set "Core Network Assisted RAN Paging Information," which helps the RAN develop optimized paging strategies when RAN paging is triggered.   II. Core Network Assisted RAN Parameter Tuning helps the RAN minimize UE state transitions and achieve optimal network behavior. The current specifications do not define how the RAN uses core network assistance information.   Core network assisted RAN parameter tuning can be tuned by the AMF for each UE based on collected UE behavior statistics, expected UE behavior, and/or other available information about the UE (e.g., subscribed DNN, SUPI range, or other information). If the AMF maintains expected UE behavior parameters, network configuration parameters (as described in TS 23.502 [3] sections 4.15.6.3 or 4.15.6.3a), or SMF-derived core network assisted RAN parameter tuning, the AMF can use this information to select core network assisted RAN parameter values. If the AMF can derive the UE's mobility pattern (as described in section 5.3.4.2), the AMF can consider mobility pattern information when selecting core network assisted RAN parameter values. The SMF uses SMF-associated parameters (e.g., UE's expected behavior parameters or network configuration parameters) to derive SMF-derived CN assisted RAN parameter tuning. The SMF sends the SMF-derived CN assisted RAN parameter tuning to the AMF during the PDU session establishment process. If the SMF-associated parameters change, the PDU session modification procedure is applied. The AMF stores the SMF-derived CN assisted RAN parameter tuning in the PDU session level context. The AMF uses the SMF-derived CN assisted RAN parameter tuning to determine the PDU session level "expected UE activity behavior" parameter set, which may be associated with the DU session ID, as described below. Expected UE behavior parameters or network configuration parameters can be provided to the AMF or SMF by an external party via the NEF, as described in Section 5.20.   III. CN-assisted RAN parameter tuning provides the RAN with methods to understand UE behavior, specifically including the following aspects: "Expected UE activity behavior," which refers to the expected pattern of UE switching between CM-CONNECTED and CM-IDLE states, or the duration of the CM-CONNECTED state. This can be obtained from sources such as statistical information, expected UE behavior, or user information. The AMF derives one or more sets of "expected UE activity behavior" parameters for the UE as follows: The AMF can derive and provide the RAN with a UE-level set of "expected UE activity behavior" parameters, which considers the expected UE behavior parameters or network configuration parameters received from the UDM (see Sections 4.15.6.3 or 4.15.6.3a of TS 23.502 [3]) and the SMF for CN-assisted RAN parameter tuning. Control plane CIoT 5GS optimization is used to tune parameters related to PDU sessions. This set of "expected UE activity behavior" parameters is valid for the UE; and The AMF can provide the RAN with a PDU session-level set of "expected UE activity behavior" parameters, for example, considering CN-assisted RAN parameter tuning derived from the SMF, for each established PDU session.   IV. The PDU session-level "expected UE activity behavior" parameter set is associated with and valid for the PDU session ID. The RAN can consider the PDU session-level "expected UE activity behavior" parameters when the user plane resources of the PDU session are activated; "Expected handover behavior," which refers to the expected interval between inter-RAN handovers. This can be derived by the AMF, for example, from mobility pattern information; "Expected UE mobility," which indicates whether the UE is expected to be stationary or mobile. For example, this information can be obtained from the following sources: statistical information, expected UE behavior parameters, or subscription information; Expected UE mobility trajectory, for example, can be obtained from statistical information, expected UE behavior parameters, or subscription information; or UE differentiation information includes expected UE behavior parameters, but does not include the expected UE mobility trajectory (see clause 4.15.6.3 of TS 23.502 [3]), to support Uu operation optimization for NB-IoT UE differentiation (if the RAT type is NB-IoT).   ----The AMF decides when to send this information as "expected UE activity behavior" to the RAN via an N2 request, through the N2 interface (see TS 38.413 [34]). ----The CN assisted information calculation, i.e., the algorithm used and relevant criteria, and the decision on when it is considered appropriate and stable to send it to the RAN, are vendor-specific.

    I. DRX (Discontinuous Reception) is a technology used in mobile communication designed to save battery power for user equipment (UE). Specifically, the mobile terminal (UE) and the network (RAN) negotiate so that the terminal's (UE) receiver only operates during data transmission and is switched off and enters a low-power state at other times.   II. DRX Framework: The 5G system supports the DRX architecture, allowing negotiation of idle mode DRX cycles between the UE and the AMF; the idle mode DRX cycle applies to: UEs in CM-IDLE state; UEs in CM-CONNECTED state that enter RRC Inactive state.   III. DRX Application: In 5G, if the UE wishes to use specific DRX parameters, it should include its preferred values during each initial registration and mobility registration process, respectively for NR/WB-EUTRA and NB-IoT; the registration and mobility registration processes performed on NB-IoT cells follow standard 5G procedures. For NB-IoT cells, the cell broadcasts an indication of support for UE-specific DRX for NB-IoT, and the UE can request UE-specific DRX for NB-IoT during the registration process regardless of whether the cell broadcasts this support indication. The AMF should determine the accepted DRX parameters based on the received UE-specific DRX parameters, and the AMF should accept the values requested by the UE, but the AMF may change the values ​​requested by the UE based on operator policy. The AMF should reply to the UE with the accepted DRX parameters for NR/WB-EUTRA and NB-IoT, respectively. ---- For detailed information on DRX parameters, please refer to TS 38.331 [28] and TS 36.331 [51].   Unless the UE has received the accepted DRX parameters for that RAT from the AMF, and for NB-IoT, the cell supports UE-specific DRX for NB-IoT; otherwise, the UE should apply the DRX cycle broadcast by the RAN in the cell. If the above parameters have been received, the UE shall apply the DRX cycle of the cell broadcast or the DRX parameters of the accepted RAT (as defined in TS 38.304 [50] and TS 36.304 [52]).   IV. The TAU and DRX periodic registration procedures do not change the UE's DRX settings. A terminal (UE) in CM-CONNECTED state and entering RRC Inactive mode will apply the DRX cycle negotiated with the AMF, the DRX cycle broadcast by the RAN, or the UE-specific DRX cycle configured by the RAN (as defined in TS 38.300 [27] and TS 38.304 [50]).

  MM (Mobility Management) is a key system in wireless networks for handling terminal (UE) mobility; in the 5G service-based architecture (SBA), it is handled by the AMF (Access and Mobility Management Function) unit to support ultra-high-speed, low-latency services; the 3GPP definition of 5GC handling of terminal (UE) mobility is as follows:   I. Core Network Capabilities In the 5G system, the core network capabilities of the terminal (UE) are divided into: S1 UE network capabilities (mainly used for E-UTRAN access-related core network parameters) and UE 5GMM core network capabilities (mainly including other UE capabilities related to 5GCN or EPS interworking); TS 24.501 [47] defines and includes non-radio-related capabilities (such as NAS security algorithms), where:   S1 UE network capabilities are transmitted between all CN nodes, including AMF to AMF, AMF to MME, MME to MME, and MME to AMF handovers. UE 5GMM core network capabilities are only transmitted during AMF to AMF handovers.   II. AMF and MM To ensure that the UE MM core network capability information stored in the AMF remains up-to-date, such as when the USIM is moved to another device when out of coverage, and the original device does not send a detach message, and in the case of cross-RAT registration area updates, the UE should send the UE MM core network capability information to the AMF via NAS messages during initial registration and mobility registration update processes. The AMF should always store the latest UE MM core network capability information received from the UE; when the UE provides UE MM core network capabilities through registration signaling, any UE MM core network capability information received by the AMF from the old AMF/MME will be replaced. If the UE's MM core network capability information changes (whether in CM-CONNECTED state or CM-IDLE state), the UE should perform a mobility registration update process the next time it returns to NG-RAN coverage (see clause 4.2.2 of TS 23.502 [3]).   III. MM Capabilities of 5G Terminals include: Attaching to EPC with request type "handover" in the PDN connection request message (see clause 5.3.2.1 of TS 23.401 [26]); EPC NAS; Sending SMS via NAS; LCS; 5G SRVCC from NG-RAN to UTRAN (as described in TS 23.216 [88]); Radio Capability Signalling Optimization (RACS); Network slice-specific authentication and authorization; Receiving WUS assistance information (E-UTRA) - see clause 5.4.9; Paging subgroup support indication (NR) - see clause 5.4.12; CAG - see clause 5.30.3.3; Subscription-based network slice simultaneous registration restriction - see clause 5.15.12; NSAG support - see clause 5.15.14; Minimizing Service Interruption (MINT) - clause 5.40.   IV. Multi-SIM Card Scenario: If a UE operates two or more USIMs and supports and intends to use one or more multi-USIM functions in a PLMN (see clause 5.38), the UE shall indicate its support for these one or more multi-USIM functions in the UE 5GMM core network capabilities for that USIM in that PLMN and include the following indications: Support for connection release; Support for voice service paging cause indication; Support for rejecting paging requests; Support for paging restriction;   Otherwise, a UE with multi-USIM capabilities but not intending to use them shall not indicate support for these one or more multi-USIM functions.

  In 5G (NR) systems, due to the large amount of data in terminal (UE) radio capability information, only the basic content is usually transmitted to the relevant core network units during the access registration phase; when the core network queries other related functions of the terminal (such as VoNR support), it will match its radio (support) capabilities with the radio network (paging is required when the terminal is in idle state); the specific process is as follows:   I. Radio Capability Matching Request: If the AMF needs more information about UE radio capability support to set the IMS VoPS session support indication (see Section 5.16.3), the AMF can send a UE Radio Capability Matching Request message to the NG-RAN. This process is typically used during the registration process or when the AMF has not yet received the voice support matching indication (as part of the 5GMM context); where:   During the registration process, if the AMF has not yet obtained the UE's radio capabilities and if the RAT where the UE is located requires the establishment of an AN security context before retrieving radio capabilities, the AMF needs to provide a security context to the 5G-AN according to the "Initial Context Establishment" procedure defined in TS 38.413 [34] before sending the UE Radio Capability Matching Request message. ​ II.Paging Assistance Information is information related to the user equipment (UE) radio in the 5G system, used to assist the radio access network (RAN) in efficient paging. Paging assistance information includes:   2.1 UE Radio Capability Information: The UE radio capability information used for paging includes information derived from the UE radio capability information of the next-generation radio access network (NG-RAN) node (such as frequency band support information);   The AMF (Automatic Management Function) stores this information and needs to understand its content. Since the AMF only prompts the NG-RAN to retrieve and upload UE radio capability information (i.e., UE radio capability information) to the AMF in very few cases (e.g., during initial registration), and the AMF may be connected to multiple NG-RAN radio access technologies (RATs), the NG-RAN is responsible for ensuring that the UE radio capability information used for paging (derived by the NG-RAN node) includes all NG-RAN RAT information supported by the UE in that PLMN. To assist the NG-RAN in completing this task, the AMF, as described in TS 38.413 [34], provides its stored UE paging radio capability information in each NG-AP initial context setup request message sent to the NG-RAN. During AMF reselection, the terminal (UE) paging radio capability information is maintained in the core network and stored in the UCMF along with the radio capability information associated with the UE radio capability ID.   2.2 Paging Recommended Cell and RAN Node Information · Based on the information sent by the NG-RAN, the AMF uses this information when paging the UE to help determine which NG-RAN nodes to page and provides recommended cell information to each RAN node to optimize the paging success rate while minimizing the signaling load on the radio path. The RAN provides this information during N2 release.

  I. RACS Background: With the expansion of terminal (UE) radio capabilities (due to new features, frequency bands, and combinations in E-UTRA and NR, etc.), the number of bytes carrying capability information is increasing. RACS (Radio Capability Signaling Optimization) defines an efficient method for transmitting UE capability information through the radio interface and other network interfaces—RACS is not applicable to NB-IoT.   II.Working Principle: RACS assigns an identifier to a set of UE radio capabilities; this identifier is called the UE Radio Capability ID. This ID can be assigned by the manufacturer or by the PLMN (see 5.9.10 for specific regulations). The UE Radio Capability ID is an alternative signaling method for UE radio capability information, transmitted through the radio interface within NG-RAN, from NG-RAN to E-UTRAN, from AMF to NG-RAN, and between CN nodes; ​ III.RACS Support: In 5G (NR) systems, PLMN-assigned UE Radio Capability ID configurations are reassigned to the UE through update commands or registration reception (as defined in TS 23.502[3]). The specific configuration of PLMN-assigned UE Radio Capability ID versions by UCMF is defined in section 5.9.10.   The UCMF (UE Radio Capability Management Function) stores the mapping relationship of all UE radio capability IDs in the PLMN and is responsible for assigning a UE radio capability ID to each UE in that PLMN (see Section 6.2.21); the UCMF stores UE radio capability ID information and corresponding radio paging capabilities. Each UE radio capability ID stored in the UCMF can be associated with one or two UE radio capability formats specified in TS 36.331 [51] and TS 38.331 [28]. ---These two UE radio capability formats should be recognizable by the AMF and UCMF, and the AMF should only store the TS 38.331 [28] format.   IV.NG-RAN supporting RACS: When providing UE radio capabilities to the AMF, the NG-RAN can be configured in one of two operating modes. When the NG-RAN performs the UE radio capability query procedure (see TS 38.331 [28]) to obtain radio capabilities from the UE, the NG-RAN performs the following operations:   Operating Mode A: The NG-RAN provides the AMF with both formats (i.e., TS 38.331 [28] format and TS 36.331 [51] format); the NG-RAN uses local transcoding to extract E-UTRAN UE paging radio capabilities and NR UE paging radio capabilities from the other format received from the UE. Operating Mode B: The NG-RAN only provides the AMF with the TS 38.331 [28] format. ----In a PLMN that only supports 5GS, Mode B should be configured.   V.4G+5G: If the PLMN supports RACS for both EPS and 5GS, then:   If the RAN nodes in EPS and 5GS are configured in Mode B, the UCMF should be able to transcode between TS 36.331 [51] and TS 38.331 [28] formats, and the UCMF should be able to generate RAT-specific UE radio capabilities for paging information from the UE radio capabilities.​​ If the NG-RAN is configured to operate in Mode A, E-UTRA should also be configured to operate in Mode A, and the UCMF does not need to transcode between TS 36.331 [51] and TS 38.331 [28] formats. The system performs transcoding between the 36.331[51] and TS 38.331[28] formats, and the AMF should provide UE radio capability information for paging.

  1.5G paging is based on operator configuration, and 5GS supports AMF and NG-RAN in applying different paging strategies for different types of traffic; specifically:   When the UE is in CM-IDLE state, the AMF performs paging and determines the paging strategy based on information such as local configuration, the NF that triggered the paging, and the information available in the request that triggered the paging. If NWDAF is deployed, the AMF can also use analytical data (i.e., statistical or predictive data - see TS 23.288 [86]) provided by NWDAF regarding UE mobility. When the UE is in CM-CONNECTED state and the RRC connection is in RRC_INACTIVE state, the NG-RAN performs paging and determines the paging strategy based on information such as local configuration and information received from the AMF (as described in TS 23.501 Section 5.4.6.3) and SMF (as described in TS 23.501 Section 5.4.3.2).   2.SMF Service Triggered Paging: For network-triggered service requests from the SMF, the SMF determines the 5QI and ARP based on the following information:   Downlink data packets (if the SMF performs buffering) or downlink data reports received from the UPF (if the UPF performs buffering). The SMF includes the 5QI and ARP corresponding to the QoS flow of the received downlink PDU in the request sent to the AMF. If the UE is in CM-IDLE state, the AMF can use, for example, the 5QI and ARP to derive different paging strategies, as described in Section 4.2.3.3 of TS 23.502 [3]. ----The AMF uses the 5QI to determine the appropriate paging strategy.   3.Paging Strategy Area: This is an optional function that allows the AMF to apply different paging strategies to different traffic or service types provided within the same PDU session, based on operator configuration. In the R18 version specifications, this function is only applicable to IP type PDU sessions, where, when 5GS supports Paging Policy Differentiation (PPD) functionality, the DSCP value (TOS in IPv4/TC in IPv6) is set by the application to indicate which paging policy the 5GS should apply to a specific IP packet, as described in TS23.228 [15]. The P-CSCF can support paging policy differentiation by marking packets related to specific IMS services (such as session voice defined in IMS multimedia telephony services) to be sent to the UE. ----This PPD function can be used to determine the paging cause indication for voice services, as described in Section 5.38.3 of TS23501. Operators should be able to configure the SMF to apply the paging policy differentiation function only to certain HPLMNs, DNNs, and 5QIs. For HR roaming, this configuration is done in the SMF in the VPLMN.   4.Roaming Paging: Paging Policy Differentiation (PPD) support in HR roaming requires inter-operator agreements, including the DSCP values associated with this function, where:   For network-triggered service requests and cases where the UPF caches downlink data packets, the UPF should include the TOS (IPv4)/TC (IPv6) value in the downlink data packet IP header and the corresponding QoS flow indication in the downlink data report sent to the SMF. When PPD is applicable, the SMF determines the Paging Policy Indicator (PPI) based on the DSCP value received from the UPF. For network-triggered service requests and cases where the SMF buffers downlink data packets, when PPD is applicable, the SMF determines the PPI based on the TOS (IPv4)/TC (IPv6) value in the received downlink data packet IP header and identifies the corresponding QoS flow from the QFI of the received downlink data packet. The SMF includes the PPI, ARP, and 5QI of the corresponding QoS flow in the N11 message sent to the AMF. If the UE is in CM-IDLE state, the AMF uses this information to generate a paging policy and sends a paging message to the NGRAN via N2.   Network configuration needs to ensure that the information used as a trigger for paging policy indication does not change during the 5GS period; the network configuration needs to ensure that the specific DSCP in the TOS (IPv4)/TC (IPv6) values ​​used as triggers for paging policy indication is correctly managed to avoid unintended use of certain paging policies; where: for a UE in RRC_INACTIVE state, the NG-RAN can enforce a specific paging policy in the case of NG-RAN paging, based on the 5QI, ARP, and PPI associated with the incoming downlink PDU. To achieve this, the SMF instructs the UPF to detect the DSCP in the TOS (IPv4)/TC (IPv6) value in the downlink PDU IP header (by using a downlink PDR containing the DSCP for that traffic) and transmit the corresponding PPI in the CN tunnel header (by using a QER containing the PPI value). The NG-RAN can then utilize the PPI in the CN tunnel header of the received downlink PDU to apply the corresponding paging policy for paging when the UE is in RRC_INACTIVE state.   ----In the case of home-routed roaming, the V-SMF is responsible for controlling the UPF settings for the PPI. In the case of a PDU session established with the I-SMF, the I-SMF is responsible for controlling the UPF settings for the PPI.   5.Paging Priority: This is a function that allows the AMF to include an indication in the paging message sent to the NG-RAN, indicating that the UE needs to be paged with priority. Whether the AMF includes paging priority in the paging message depends on the ARP value of the IP packet received from the SMF and waiting to be delivered in the UPF. If:   The ARP value is associated with a specific priority service (e.g., MPS, MCS), the AMF will include paging priority in the paging message. When the NG-RAN receives a paging message containing paging priority, it will prioritize the processing of that paging. When the AMF is waiting for a response from the UE to a paging message that does not include priority, if it receives another message from the SMF whose ARP value is associated with a specific priority service (e.g., MPS, MCS), the AMF will send another paging message containing paging priority to the (RAN). For subsequent messages, the AMF may decide, based on local policy, whether to send a paging message with a higher paging priority.   For UEs in the RRC Inactive state, the NG-RAN determines the paging priority based on the ARP associated with the QoS flow, configured according to the operator's policy, and core network-assisted RAN paging information from the AMF (as described in Section 5.4.6.3).

    In 5G networks, the network has two CM (Connection Management) connection states for terminals: CM-Idle and CM-CONNECTED. The CM-CONNECTED state is crucial for achieving seamless data flow and supports low-latency, large-scale IoT, and smart city applications. The reachability of the terminal (UE) in the CM-CONNECTED state is defined by 3GPP in TS 23.501 as follows:   I. Reachability in CM-CONNECTED state specifically includes: The AMF knows the UE's location at the serving (RAN) node granularity; When the UE becomes unreachable from the RAN's perspective, the NG-RAN notifies the AMF.   II. RRC Inactive State Terminal (UE): For terminals (UEs) in the RRC Inactive state, the radio access network (RAN) uses UE RAN reachability management (see TS 38.300 [27]). The location of the terminal (UE) in the RRC Inactive state is determined by the RAN at its (RAN) notification area granularity. Terminals (UEs) in the RRC Inactive state are paged in the cells of the RAN notification area assigned to that UE. The RAN notification area can be a subset of the cells configured in the UE's registration area, or all cells configured in the UE's registration area. A UE in the RRC Inactive state performs a RAN notification area update when it enters a cell that does not belong to the RAN notification area assigned to that UE.   Radio Access Network (RAN) Communication Area: In the 5G system, the RNA (Radio Access Network Notification Area) is a geographical area located within the 5GC registration area; this area consists of one or more cells belonging to one or more gNBs; where: When the UE transitions to the RRC Inactive state, the RAN configures a periodic RAN notification area update timer value for the UE, and the timer in the UE restarts with this initial timer value. After the periodic RAN notification area update timer in the UE expires, the UE in the RRC Inactive state performs a periodic RAN notification area update as specified in TS 38.300 [27].   To assist in UE reachability management in the AMF, the RAN uses a guard timer, whose value is longer than the RAN notification area update timer value provided to the UE. In the RAN, after the periodic RAN notification area update protection timer expires, the RAN should initiate the AN release procedure as specified in TS 23.502 [3]; the RAN may provide the AMF with the time elapsed since the RAN last contacted the UE.

  During the initial registration or mobility registration update process, the 5G terminal (UE) will initiate a connection with the network, which is the MICO (Mobile Initiated Connection Only) connection mode; where:   I. The MICO mode allows the AMF to determine whether to allow the UE to use MICO mode and indicate this to the UE during the registration process, based on local configuration, expected UE behavior and/or network configuration parameters (if available from the UDM), UE indicated preferences, UE subscription information, and network policies, or any combination thereof.   If NWDAF is deployed, the AMF may also use UE mobility and/or UE communication analysis data generated by NWDAF (see TS 23.288 [86]) to determine MICO mode parameters. If the UE does not indicate its preference for MICO mode during the registration process, the AMF should not activate MICO mode for that UE. II. The UE and AMF renegotiate the MICO mode during each subsequent registration process; when the UE is in the CM-CONNECTED state, the AMF can deactivate the MICO mode by triggering a mobility registration update process; this process is performed through the UE configuration update process as described in Section 4.2.4 of TS 23.502 [3]; where:   During the registration process, the AMF assigns a registration area to the UE. When the AMF indicates that the UE is in MICO mode, the registration area is not limited by the paging area size. If the AMF's service area covers the entire PLMN, the AMF may decide to provide the UE with a "full PLMN" registration area based on local policies and user information. In this case, re-registration due to mobility within the same PLMN is not applicable. If mobility restrictions are applied to a UE in MICO mode, the AMF needs to assign an allowed area/disallowed area to the UE as specified in Section 5.3.4.1. When the AMF indicates MICO mode to the UE, if the UE's CM state in the AMF is CM-IDLE, the AMF always considers the UE unreachable. For a UE in MICO mode and whose CM state in the AMF is CM-IDLE, the AMF will reject any downlink data transmission requests and provide the corresponding rejection reason. For NAS-based MT-SMS, the AMF will notify the SMSF that the UE is unreachable and then execute the mobile terminal SMS sending failure handling procedure described in TS 23.502 [3, Section 4.13.3.9]. III. Delayed Location Services: The AMF will enable delayed location services, allowing mobile terminal data or signaling communication only for UEs in MICO mode and only when they are in the CM-CONNECTED state.   IV. CM-IDLE State: UEs in CM-IDLE state do not need to listen for paging. UEs in MICO mode can stop any access layer procedures in the CM-IDLE state until the UE initiates a transition from CM-IDLE to CM-CONNECTED due to one of the following trigger conditions: The UE undergoes a change (e.g., configuration change) requiring an update of its registration information in the network. The periodic registration timer expires. MO signaling is pending (e.g., an SM procedure has been initiated). If the registration area assigned to a UE in MICO mode is not the "all PLMNs" registration area, the UE will determine whether it is within that registration area when it has MO data or MO signaling. If the UE is not within the registration area, before initiating MO data or MO signaling,   V. UE and Emergency Services: The UE will perform a mobility registration update; a UE initiating emergency services shall not indicate MICO preference during the registration process. When MICO mode is activated in the UE, the UE and AMF locally disable MICO mode after the successful completion of the emergency service PDU session establishment process. The UE and AMF shall not enable MICO mode until the AMF accepts the use of MICO mode during the next registration process. To enable emergency callback, the UE should wait for a UE implementation-specific duration after the emergency PDU session release before requesting the use of MICO mode.   VI. MT Mode: To achieve energy saving for mobile terminal (UE) MT reachability (e.g., for cellular IoT), enhancements to the MICO mode are specified in the following clauses: MICO mode with extended connection time; MICO mode with active time; MICO mode with periodic registration timer control.

  Reachability management in the 5G (NR) system is responsible for detecting whether a UE is reachable and providing the UE's location (i.e., access node) so that the network can easily access the terminal (UE); this can be achieved through paging the UE and (UE) location tracking; UE location tracking includes: registration area tracking (i.e., UE registration area update) and reachability tracking (i.e., UE periodic registration area update); the reachability management function can be located in the 5GC (CM-IDLE state) or NG-RAN (CM-CONNECTED state).   I. CM-IDLE reachability is the result of negotiation between the UE and the AMF during the registration process. UE reachability in the CM-IDLE state is divided into two types:   1. UE Data Transmission Reachability   The network determines the UE's location based on the tracking area list granularity. Applicable to paging procedures. Applicable to CM-CONNECTED and CM-IDLE states supporting mobile-initiated data and mobile terminal data.   2. MICO (Mobile Initiated Connection Only) mode:   Applicable to CM-CONNECTED and CM-IDLE states supporting mobile-initiated data. Mobile terminal data is only supported when the UE is in the CM-CONNECTED state.   II.When a UE in the RM-REGISTERED state enters the CM-IDLE state, it starts a periodic registration timer based on the periodic registration timer value received from the AMF during the registration process; during this period,   the AMF assigns a periodic registration timer value to the UE based on local policies, subscription information, and information provided by the UE. After the periodic registration timer expires, the UE should perform periodic registration. If the UE moves out of network coverage when its periodic registration timer expires, the UE should perform the registration procedure when it returns to coverage. The AMF runs a mobile reachability timer for the UE. When the CM state of a UE in the RM-REGISTERED state changes to CM-IDLE, this timer starts with a value greater than the UE's periodic registration timer. If the AMF receives elapsed time from the RAN when the RAN initiates UE context release and indicates that the UE is unreachable, the AMF should infer the mobile reachability timer value based on the elapsed time received from the RAN and the normal mobile reachability timer value. If the UE CM state in the AMF changes to CM-CONNECTED state, the AMF stops the mobility reachability timer. If the mobility reachability timer expires, the AMF determines that the UE is reachable. However, the AMF does not know the duration of UE unreachability, so the AMF should not immediately deregister the UE. Instead, after the mobility reachability timer expires, the AMF should clear the PPF (Paging Proceed Flag) and start an implicit deregistration timer, which should have a relatively large value.   III.CM-CONNECTED: If the UE CM state in the AMF changes to CM-CONNECTED state, the AMF should stop the implicit deregistration timer and set the PPF (If the UE CM state in the AMF is CM-IDLE, and the UE is in MICO mode - see Section 5.4.1.3, the AMF considers the UE to be always unreachable).   If the PPF is not set, the AMF will not page the UE and should reject any requests to send downlink signaling or data to that UE. If the implicit deregistration timer expires before the UE contacts the network, the AMF implicitly deregisters the UE.   As part of a specific access (3GPP or non-3GPP) deregistration, the AMF should request the relevant SMF of the UE to release the PDU sessions established on that access.