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  In 5G (NR) wireless networks, mobile terminal equipment (UEs) can employ two types of link adaptation: inner-loop link adaptation and outer-loop link adaptation. Their characteristics are as follows: ILLA – Inner-loop link adaptation; OLLA – Outer-loop link adaptation. I. ILLA (Inner-loop Link Adaptive) performs fast and direct adjustments based on the Channel Quality Indicator (CQI) reported by each UE. The UE measures downlink quality (e.g., using CSI-RS). It reports the CQI to the gNB, which maps the CQI (via a static lookup table) to the MCS index for the next transmission. This mapping reflects the link condition estimate for that time slot/TTI. ILLA applies a three-step process as follows:   The UE measures the CSI-RS and reports CQI=11. The gNB maps CQI=11 to MCS=20. The MCS is used to calculate the transport block for the next time slot.   ILLA's advantage lies in its ability to adapt very quickly to channel changes; however, it has limitations in terms of false detections, CQI errors, and noise. Specifically, the BLER target value may shift if the channel is not ideal or the feedback is imperfect.   II. OLLA (Outer Loop Link Adaptive) uses a feedback mechanism to fine-tune the MCS target value to compensate for the actual link performance observed through HARQ ACK/NACK responses. For each transmission, the gNB receives either an ACK (success) or NACK (failure); where: If the BLER is higher than the set target value (e.g., 10%), OLLA adjusts downwards by a correction offset (Δoffset), i.e., reducing the aggressiveness of the MCS. If the BLER is lower than the target value, the offset is adjusted upwards, i.e., increasing the aggressiveness of the MCS. The offset is added to the SINR→CQI mapping in ILLA, thus ensuring that the BLER eventually converges to the target value—even if the input signal is not ideal.   OLLA's advantage lies in its ability to maintain a robust and stable BLER and adapt to slowly changing system errors in the SINR/CQI report. Due to its slower response speed, the optimal setting of the step size (i.e., Δup and Δdown) requires a trade-off between stability and response speed. In the OLLA mechanism, feedback is used to fine-tune the MCS target to compensate for the actual link performance observed through HARQ ACK/NACK responses.   III. Comparison of 4G and 5G Link Adaptation The table below compares 4G and 5G link adaptation.   Feature 5G NR 4G LTE CSI CQI + PMI + RI + CRI Mainly CQI Adaptation Speed Up to 0.125 ms 1 ms Traffic Types eMBB, URLLC, mMTC eMBB mainly MCS Mapping ML-optimized, Vendor-driven Fixed table Beamforming MassiveMIMO,Beam selection Minimal Scheduler Fully integrated & Intelligent Basic CQI, PF                     In 5G (NR) networks, Link Adaptive (LA) plays a crucial role in ensuring high-performance and reliable connectivity. Unlike the slower, fixed-table approach of 4G (LTE), 5G systems employ smarter and faster technologies, including AI/ML and real-time feedback. This enables the network to adapt to changing environments in real time and utilize radio resources more efficiently.

  I. Link Adaptation In mobile communication networks, the wireless environments of any two end users (UEs) are never exactly the same. Some users may be right next to a 5G base station with excellent wireless signal, while others may be deep inside buildings, moving at high speeds, or at the edge of a cell. However, they all expect a fast and stable network experience. To achieve the highest possible throughput and optimal reliable connection, "Link Adaptation" technology was developed. Link adaptation can be viewed as an "automatic mode" of the 5G physical layer, continuously monitoring the wireless environment and adjusting transmission parameters in real time to provide the best data rate while controlling errors.   II. Link Adaptation (AMC) in 5G In 5G networks, link adaptation refers to the process of dynamically adjusting transmission parameters (such as modulation, coding, and transmit power) to optimize the communication link between the base station (gNodeB) and the user equipment (UE). The goal of link adaptation is to maximize spectral efficiency, throughput, and reliability while adapting to constantly changing channel conditions and user needs. Figure 1. 5G Link Adaptive Process   III. Characteristics of 5G Link Adaptive Process   Modulation and Coding Scheme (MCS) Selection:Link adaptive process involves selecting a suitable modulation and coding scheme based on channel conditions, signal-to-noise ratio (SNR), and interference levels. Higher modulation schemes offer higher data rates but are more demanding on channel conditions; lower modulation schemes are more robust under adverse conditions. Transmit Power Control: Link adaptive process also includes adjusting transmit power to optimize signal quality and coverage while minimizing interference and power consumption. Transmit power control helps maintain a balance between signal strength and interference levels, especially in dense network deployments. Channel Quality Feedback: Link adaptive process relies on feedback mechanisms to provide information about channel conditions, such as Channel State Information (CSI), Received Signal Strength Index (RSSI), and Signal-to-Interference-Ratio (SINR). This feedback enables the gNodeB to make informed decisions regarding modulation, coding, and power adjustments. Adaptive Modulation and Coding (AMC): AMC is a key feature of link adaptive process; it dynamically adjusts modulation and coding parameters based on real-time channel conditions. By adapting to changes in channel quality, AMC maximizes data rates and spectral efficiency while ensuring reliable communication. Fast Link Adaptation: In rapidly changing channel environments, such as high-mobility scenarios or fading channels, fast link adaptation technology is used to quickly adjust transmission parameters to cope with channel fluctuations. This helps maintain a stable and reliable communication link under changing channel conditions.   In wireless systems, link adaptation plays a crucial role in optimizing wireless communication system performance by continuously adjusting transmission parameters to match current channel conditions and user needs. By maximizing spectral efficiency and reliability, link adaptation helps achieve high data rates, low latency, and seamless connectivity in 5G networks.

  As 5G (NR) supports increasingly more connections and functions, the number of network functions and entities in the system is also constantly increasing. 3GPP defines network functions and entities in Release 18.5 as follows:   I. Network Function (NF) Units The 5G system includes the following functional units:  AUSF (Authentication Server Function); AMF (Access and Mobility Management Function); DN (Data Network), specifically including: operator services, internet access, or third-party services; UDSF (Unstructured Data Storage Function); NEF (Network Exposure Function); NRF (Network Repository Function); NSACF (Network Slice Admission Control Function); NSSAAF (Network Slice-Specific and SNPN Authentication and Authorization Function); NSSF (Network Slice Selection Function); PCF (Policy Control Function); SMF (Session Management Function); UDM (Unified Data Management); UDR (Unified Data Repository). - UPF (User Plane Functions). UCMF (UE Radio Capability Management Functions). AF (Application Functions). UE (User Equipment). RAN (Radio Access Network). 5G-EIR (5G Device Identity Registration). NWDAF (Network Data Analysis Functions). CHF (Charging Functions). TSN AF (Time-Sensitive Network Adapter). TSCTSF (Time-Sensitive Communications and Time Synchronization Functions). DCCF (Data Collection Coordination Functions). ADRF (Analysis Data Repository Functions). MFAF (Message Frame Adapter Functions). NSWOF (Non-Seamless WLAN Offload Functions). EASDF (Edge Application Server Discovery Functions). *Functions provided by DCCF or ADRF can also be carried by NWDAF.   II. Network Entities The 5G system, supporting connectivity with non-3GPP Wi-Fi, WLAN, and wired access networks, also includes the following entity units in its architecture: SCP (Service Communication Agent). SEPP (Secure Edge Protection Agent). N3IWF (Non-3GPP Interoperability Function). TNGF (Trusted Non-3GPP Gateway Function). W-AGF (Wired Access Gateway Function). TWIF (Trusted WLAN Interoperability Function).

  In 5G (NR) systems, the PSA (PDU Session Anchor) is the UPF (User Plane Function). It acts as a gateway connecting to the external DN (Data Network) via the N6 interface of the PDU session. As the anchor point for user data sessions, the PSA manages data flow and establishes connections to services such as the Internet.   I.There are three PSA modes: SSC Mode 1, SSC Mode 2, and SSC Mode 3. SSC Mode 1: In this mode, the 5G network maintains the UE connection service. For IPv4, IPv6, or IPv4v6 class PDU sessions, the IP address is reserved. In this case, the User Plane Function (UPF) acting as the PDU session anchor remains unchanged until the UE releases the PDU session. SSC Mode 2: In this mode, the 5G network can release the connection to the UE, i.e., release the PDU session. If the PDU session was used to transmit IP packets, the allocated IP address will also be released. One application scenario for this mode is when the anchor UPF requires load balancing, allowing the network to release connections. In this case, the PDU session can be transferred to a different anchor UPF by releasing the existing PDU session and subsequently establishing a new one. It uses a "disconnect + establish" framework, meaning the PDU session is released from the first serving UPF and then a new PDU session is established on the new UPF. SSC Mode 3: In this mode, the 5G network maintains the connection provided to the UE, but some impacts may occur during certain processes. For example, if the anchor UPF changes, the IP address assigned to the UE will be updated, but the change process ensures that the connection is maintained; that is, a connection to the new anchor UPF is established before releasing the connection with the old anchor UPF. 3GPP Release 15 only supports Mode 3 for IP-based PDU sessions. II. The main uses of the PDU session anchor point include: Data Termination Point: The PSA is the UPF where the PDU session terminates its connection with the external data network. Data Routing: It routes user data packets between the user equipment (UE) and the external DN. IP Address Allocation: The PSA is associated with an IP address pool. The UE's IP address is allocated from this pool, either by the UPF itself or through an external server (e.g., a DHCP server). The Session Management Function (SMF) manages this address pool. Data Path Control: The SMF controls the data path of the PDU session, selects the PSA, and manages the termination of the N6 interface.

  I. Characteristics of Repeaters In mobile communication systems, a repeater (Mobile Repeater), also known as a signal amplifier (repeater) or mobile signal booster, is a device that amplifies existing mobile phone signals to improve signal strength in weak areas. Its working principle involves using an external antenna to receive weak signals, transmitting them to a signal amplifier for amplification, and then rebroadcasting the enhanced signal through an internal antenna. This improves mobile phone connectivity within its effective range, making it particularly suitable for rural areas, large concrete and metal structures, or vehicles.   II. Repeater Standards Signal boosters used in 5G (NR) systems are classified into: Repeaters, NCRs (Network Control Repeaters), and auxiliary equipment; among them, NCRs are further divided into NCR-Fwd and NCR-MT. The applicable requirements, procedures, test conditions, performance evaluation, and performance standards for different types of base stations in wireless networks are as follows:   NR repeaters equipped with antenna connectors that can be terminated during EMC testing meet the RF requirements for type 1-C repeaters in TS 38.106[2] and demonstrate compliance with TS 38.115-1[3]. NR repeaters without antenna connectors, i.e., antenna elements do not radiate during EMC testing, meet the RF requirements for type 2-O repeaters in TS 38.106[2] and demonstrate compliance with TS 38.115-2[4]. NCRs equipped with antennas or TAB connectors that can be terminated during EMC testing meet the RF requirements for NCR-Fwd/MT type 1-C and type 1-H in TS 38.106[2] and demonstrate compliance with TS 38.115-1[3]. The NCR is not equipped with an antenna connector, meaning that the antenna element was not radiated during EMC testing, which complies with the NCR-Fwd/MT 2-O type RF requirements in TS 38.106 [2] and demonstrates its compliance by conforming to TS38.115-2 [4]. The repeater usage environment classification refers to the residential, commercial, and light industrial environment classifications used in IEC 61000-6-1 [6], IEC 61000-6-3 [7], and IEC 61000-6-8 [24]. These EMC requirements were chosen to ensure that the equipment is sufficiently compatible in residential, commercial, and light industrial environments. However, these levels do not cover extreme situations that may occur in any location but with a low probability.

In 5G (NR) systems, the policy management and execution of network and terminal service capabilities are entirely guaranteed by the PCF (Policy Control Function) and AMF (Mobility Function), which are also known as AM policy management. Application examples are as follows:   Example 1: AM/UE Policy Control Based on Consumption Limits This is a new function introduced by 3GPP in Rel-18, allowing the PCF responsible for the UE to perform AM/UE policy decisions in non-roaming scenarios based on available consumption limit information (such as whether the user's daily/weekly/monthly mobile data consumption limit has been reached or is close to being reached). This example demonstrates how to implement the operator's AM/UE policy management policy in the PCF.   The PCF interacts with the CHF (Charging Function) to request and/or subscribe to receive consumption limit-related reports for one or more "policy counters" (i.e., consumption limit indicators). Once configured, the CHF will notify the PCF of any changes to the current or pending status of subscribed policy counters, and optionally, the activation time of pending statuses (e.g., due to an upcoming billing cycle expiration). The PCF will then use all these dynamically collected policy counter states and related information as input to its internal policy decisions to apply relevant pre-configured operator-defined actions. With this functionality, operators can dynamically configure, establish, and execute AM/UE policy decisions (such as downgrading or upgrading the UE-AMBR, changing URSP rules, and updating service area restrictions) based on expenditure limit information.   In 3GPP Rel-19, this functionality is further extended to roaming scenarios to support dynamic changes to UE policies based on expenditure limit information.   Example 2: Network-Assisted Performance Level Enhancement Using Frequency Management Recommendations AM policy management plays a crucial role in improving network performance by enhancing RFSP index management.   The PCF can implement more dynamic and differentiated mobility control policies. The PCF can provide RFSP index values ​​to the AMF to assist in frequency selection and enable finer-grained radio resource management at the UE end. PCF determines the RFSP index values ​​to provide based on multiple factors, such as cumulative usage information (e.g., usage volume, usage duration, or both), network analysis data from NWDAF (including current load levels of relevant network slice instances or UE communication-related information), UE communication behavior information, user data congestion information, and perceived service experience. This flexible frequency selection and mobility management policy framework enhances user experience, optimizes network efficiency, and supports differentiated service delivery across different user groups and network conditions.   With the introduction of 5G-A (3GPP Rel-18 and later) and artificial intelligence technologies, these capabilities will be further enhanced, enabling more autonomous, dynamic, and intelligent network management. This paves the way for increased control over how the network treats user equipment (UEs), such as: real-time policy management based on AI-native network architecture and intent-driven automation; more granular UE differentiation for personalized experiences; and efficient connection of a large number and diverse range of UEs (e.g., IoT devices, sensors). We look forward to the rollout of these exciting new features and application scenarios in the future.

  The User Plane Function (UPF) is one of the most important network functions (NFs) in the 5G core network. It is the second network functional unit that the Radio Network (RAN) interacts with during PDU flows in 5G (NR). As a key element in the evolution of Control Plane and User Plane Separation (CUPS), the UPF is responsible for inspecting, routing, and forwarding packets within QoS flows in subscription policies. It uses the SMF to send SDF templates via the N4 interface to enforce uplink (UL) and downlink (DL) traffic rules. When the corresponding service ends, the UPF allocates or terminates QoS flows in the PDU session.   I. User Plane Establishment When initially accessing the 5G system, the terminal (UE) needs to establish a user plane channel with the data center according to the control plane guidance for service data transmission. During this process:   When the terminal (UE) wants to access the 5G network, it first undergoes a registration process. After completing all control plane procedures, the SMF processes all session-related information during the user plane establishment phase. The AMF requests the downlink DL TEID (Terminal Equipment Identifier) ​​of all PDU sessions passed to the SMF. The SMF then selects the best UPF for the UE within the specified range and sends a session establishment request containing all parameters for the default PDU session establishment. Afterwards, a session default QoS flow (non-GBR) is created to exchange with the data network (DN) for traffic. The service traffic includes a longer route for calculating latency and maintaining traffic. Figure 1. 5G Terminal User Plane Establishment Process (Messages) [5] New UE establishment request, requires creating session context [1] Set UPF address [5] [10] Request to create session with UPF [3] Session context response [4] [5] Get default session update [3] Default QoS, AMBR [3] Add default downlink and uplink PDR rules for IMSI II. First Uplink/Downlink Data Transmission When actual data transmission (i.e., uplink or downlink data) occurs, the AMF sends another SM context request to the SMF, in which:   The SMF sends a session modification request containing information related to the requested session type. The UPF establishes a PDU session within the rules and regulations according to user requirements. The UPF then adds QoS flow mapping, sets the TEID, inserts various rules (such as PDR, FAR, URR, etc.), and some session-related policies to the PDU session. It also bills each packet exchange and adds a unique session ID to distinguish it from other PDU sessions. The UPF also adds an IMSI number to identify the UE to which the current session belongs. The session context is prepared by the UPF and sent to the AMF via the SMF, which then forwards it to the gNB. It contains information such as the UPF's local TEID, QoS context, and session release message. Figure 2.5G Terminal User Plane First Data Transmission Flow (Message) [2] QoS Policy Management (Policy Type) [2] Dynamic Rule Setting [2] Static and Dynamic Rule Update [3] Mapping FDR, PDR, QDR, BAR, URR [3] Attaching Rules to Session [3] Creating a New TEID and Inserting it into the PDR [2] Setting the TEID to be Passed to UPF [2] QoS/Bearer Management [5] Creating a Session Request [9] Updating and Creating a Session [6] Handling Rule Scheduling [7] Receiving Charging Authorization [2] Initializing Charging Credits [2] Obtaining All Active Policies [10] Setting Up UPF Session [4] Reading, Creating, Updating, and Searching for Sessions [8] Reading and Writing Sessions, and Serializing and Deserializing All Session Vectors [5] Inactive State When PDU Session Moves to Idle State [6] Handling Session Update Response [5] Process setup messages from AMF (initial request or existing PDU session) [3] Update state change notifications sent to AMF [3] Prepare responses (session context) to send to AMF for forwarding to gNB [3] Send the UPF local TEID to AMF for use by gNB [3] Send the appropriate QoS context to AMF [5] Obtain the PDU session ID from the RAT context [5] Request AMF to send a message to release the session

  Similar to previous generations of mobile communication, the services supported by the terminal (UE) are stored in the core network. The UE can only be executed by the radio network after completing authentication and encryption actions upon power-on. In 5G (NR) systems supporting NSSF (Network Slice Selection Function), after "RRC connection establishment, UE context, UE ID allocation, and security authentication," the terminal (UE) will obtain specific subscription data based on the activation status and perform user plane settings. The specific process is as follows:   I. Subscription Data Acquisition: The AMF searches for the NSSF (Network Slice Selection Function) through the N22 interface to select the best available network slice for the user's requested service. Then, it searches the UDM to retrieve all subscription data related to AM (Access Management), SM (Session Management), and UE (Terminal). The AMF connects to the UDM via the N10 interface to obtain subscription data. The process (message) is as follows: [21] Fill in the slice information in the PDU session establishment accept message [8] Obtain the AMF context based on the UE identifier [8] Obtain the SMF context from the mapping [20] Set the SMF context in the AMF context [8] The AMF creates a new UE context   ---The AMF configures the PCF (Policy Control Function) to retrieve the AM policy through the N15 interface accessible to the UE, and the SMF allocates services accordingly.   ---The AMF has collected all UE contexts, and now it creates another identifier for the UE, the AMF UE NGAP ID, to add it to the network.   II. User Plane Setup The AMF selects the SMF (which performs all session management operations in the 4G system MME (as well as SGW-C and PGW-C)) to manage all session management operations itself. Message exchange between the AMF and SMF is conducted through the N11 interface. The SMF then finds the best UPF (User Plane Function) for the UE and creates a session during the UL and DL data streams. Interaction between SMF and UPF is performed via PFCP (Packet Forwarding Control Protocol) on the N4 interface; the specific process (message) is as follows:   [3] Check the session ID of the existing PDU session [3] Send a PDU session establishment acceptance message to the UE and gNB [3] Send a PDU session resource establishment request message to the gNB [4] Process the PDU session resource establishment response [4] Process the PDU session resource release response [20] AMF processes PDU session establishment rejection [20] Send a PDU session rejection message to the UE [3] Set the session AMBR [20] Update the IP address information in the SMF context and send a downlink transmission message with a 5GMM reason to the gNB [3] [5] Retrieve the user QoS profile and UPF GTP TEID IP address from the SMF context [1] Send an activation PDU session context request message [5] Add a security header to the AMF PDU session transmission request [3] [6] Generate a new AMF NGAP UE ID [8] Notify NGAP of the new AMF NGAP ID

  Since 4G (LTE), mobile communications have implemented encryption and integrity protection during terminal (UE) access to ensure personal privacy and security during communication. The specific processes for these, along with service resources and data transmission, in the 5G (NR) system are as follows:   I. AS Security and RRC Reconfiguration: First, the AMF sends a UE Initial Context Establishment Request and Registration Acceptance Message to the gNB to update the UE context existing in the gNB. The gNB then performs the RRC reconfiguration and SMC procedures so that the UE can access the encrypted channel using derived keys (e.g., k-gNB, k-RRC, k-UP-int).   [17] AMF sends SAP [1] Update the GUTI assigned to AMF SAP [9] Process AMF AS SAP connection establishment request [9] [16] Process AMF AS SAP connection establishment rejection [9] Process AMF AS SAP connection establishment confirmation [18] Notify AMF AS SAP that it needs to send a security mode command message to the UE [9] Process AMF AS SAP security request primitive [17] Set security request when data is transmitted to the lower layer [1] Notify AS SAP that registration is rejected [10] Obtain a new security context from the upper layer [23] Encrypt/decrypt/decode Layer 3 NAS message [8] Register UE context [1] Execute registration signaling process [1] Process registration completion message [1] AMF sends registration acceptance message   II. Uplink (downlink) data transmission When the user plane is set to uplink or downlink purpose, the PDU session update message is transmitted from AMF to SMF. The specific process is as follows;   [3] Transfer gNB IP and TEID are stored in the corresponding SMF context [3] Session creation response message received from SMF [3] Prepare and send gN establishment response message to SMF via gRPC [9] QoS flow establishment list [20] Function to check if the maximum number of PDU sessions has been reached

In the 5G (NR) protocol stack, RRC (Radio Resource Control) is Layer 3, specifically responsible for the control and management of radio resource connections between the UE (UE) and gNB (gNB), including: establishing and managing connections, broadcasting system information, and processing mobility radio bearer configuration. 5G terminal RRC connections have three states: RRC_IDLE, RRC_CONNECTED, and RRC_INACTIVE; "RRC_INACTIVE" was introduced to improve battery efficiency and speed up reconnection.   I. RRC Connection Establishment Process: As shown in Figure (1), after power-on, the terminal (UE) initiates the establishment of an RRC connection with the gNB; subsequently, the gNB sends an initial NAS message to the AMF via the N2 interface, containing the RAN UE NGAP ID, UE context registration request, location information, 5G S-TMSI, and the reason for RRC establishment. Figure 1. RRC establishment process of 5G terminal (UE)   II. Initial NAS message + UE context reacquisition These parameters are the identity provided for the terminal (UE) to help the AMF obtain the UE context from the old serving AMF or by re-executing the entire process (only when the serving AMF cannot find traces of the old AMF); the whole process is completed through the N14 interface, and the specific process (message) is as follows: Figure 2. Initial NAS message and UE context of 5G terminal (UE)   [8] Release the previous registration request context [3] gNB sends the initial NAS message through the new RRC connection [23] Decode the security-protected NAS message [3][9] Process the NGAP initial UE NAS message [4] Process the initial UE message from NGAP [9] Mobility management message [16] Store the registration type in the parameters [1] Create the registration request process [9] Encode the initial NAS information message [7] Process the NAS encoded message and send it to the NGAP task [23] Decode the plain text NAS message [8] Check if there are old parameters (e.g. UE context (GUTI, IMSI, gNB ID, etc.) [3] Update AMF UE context with new gNB UE NGAP ID. Assuming the new AMF cannot find any old AMF traces in the network, it will be unable to close the NR call process. At this time, the AMF will begin identity, authentication, and security procedures for the UE in order to add a more explicit identity to the UE.