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  I. ATSSS is an abbreviation for Access Traffic Steering, Switching, Splitting; this is a function introduced by 3GPP for 5G (NR) that allows mobile devices (UEs) to simultaneously use 3GPP and non-3GPP access, manage user data traffic, control new data flows, select (new) access networks, switch all ongoing data to different access networks to maintain data continuity, and split individual data flows, allocating them to multiple access networks to improve performance or achieve redundancy. Specifically:   Control:The network determines which access method (e.g., 5G and Wi-Fi) a new data flow should use based on operator-defined rules and real-time conditions. Switching:The network transfers an ongoing data session from one access network to another. For example, a video call can be switched from Wi-Fi to 5G without interruption. Splitting:The network can simultaneously allocate a single data flow to two or more access networks. This can be used to increase bandwidth (link aggregation) or ensure reliability (redundancy). II. Working Principle ATSSS can operate at the IP layer (using protocols such as MPTCP) or below the IP layer (using underlying routing functions). Control is handled by the 5G core network's PCF (Policy Control Function), based on operator-defined rules and performance measurement data from the User Equipment (UE) and the network itself.   III. ATSSS Modes The main ATSSS modes are as follows: Primary/Backup Mode:Traffic is sent through the active link. If the active link fails, it switches to the backup link. Load Balancing Mode:Traffic is distributed among available access networks, typically based on a percentage to balance the load. Minimum Latency Mode:Traffic is routed to the access network with the lowest latency (round-trip time). Priority Mode:Traffic is initially sent through a high-priority link. If that link becomes congested, traffic is split or diverted to a lower-priority link. IV. Architecture Expansion and Functionality The 5G system architecture has been expanded to support ATSSS functionality (see Figures 4.2.10-1, 4.2.10-2, and 4.2.10-3); the 5G terminal (UE) supports one or more flow control functions, namely MPTCP, MPQUIC, and ATSSS-LL. Each flow control function in the UE can perform flow control, handover, and splitting between 3GPP and non-3GPP access networks according to the ATSSS rules provided by the network. For Ethernet-type MA PDU sessions, the UE must have ATSSS-LL functionality, with the following specific requirements for the UPF: - The UPF can support MPTCP proxy functionality, which communicates with the MPTCP function in the UE using the MPTCP protocol (IETF RFC 8684 [81]). - UPF can support MPQUIC proxy functionality, which communicates with the MPQUIC function in the UE using the QUIC protocol (RFC9000 [166], RFC9001 [167], RFC9002 [168]) and its multipath extension (draft-ietf-quic-multipath [174]). - UPF can support ATSSS-LL functionality, which is similar to the ATSSS-LL functionality defined for the UE. IV. ATSSS Application Characteristics 4.1 Ethernet type MA PDU sessions require the ATSSS-LL functionality (conversion) in 5GC. In addition: - UPF supports Performance Measurement Function (PMF), which the UE can use to obtain access performance measurements on the 3GPP access user plane and/or non-3GPP access user plane. - AMF, SMF, and PCF extend new functionality, which is discussed further in Section 5.32. 4.2 ATSSS control may require interaction between the UE and the PCF (as specified in TS 23.503[45]).   4.3 The UPF shown in Figure 4.2.10-1 can be connected via the N9 reference point instead of the N3 reference point.   V. Roaming Scenarios 5.1 Figure 4.2.10-2 shows ATSSS support in a roaming scenario for the 5G system architecture; this scenario includes home-roaming traffic, and the UE is registered to the same VPLMN via 3GPP and non-3GPP access. In this case, the MPTCP proxy function, MPQUIC proxy function, ATSSS-LL function, and PMF are located in the H-UPF. 5.2 Figure 4.2.10-3 shows ATSSS support in a roaming scenario for the 5G system architecture, this scenario includes home-roaming traffic, and the UE is registered to the VPLMN via 3GPP access and to the HPLMN via non-3GPP access (i.e., the UE is registered to different PLMNs). In this case, the MPTCP proxy function, MPQUIC proxy function, ATSSS-LL function, and PMF are all located in H-UPF.

  Besides defining SA (Standalone) as the standard 5G configuration, Release 16 5G enhances many features to support numerous improvements to the air interface, including unlicensed spectrum in the millimeter wave (mmW) band, and support for Industrial Internet of Things (IIoT) and Ultra-Reliable Low-Latency Communication (URLLC), making it more powerful. Specific additions are as follows:   I. Feature Enhancements As 5G network deployment progresses, the capacity requirements of the Radio Access Network (RAN) continue to grow, and the flexibility of network deployment is also increasing, including support for dedicated networks; RAN capacity and performance have become key to solving problems;   1.1 Capacity Enhancements include:   MIMO (Multiple-Input Multiple-Output) Improvements: Enhanced CSI II codebook to support MU-MIMO, multiple transmissions and receptions (multiple TRPs/panel transmissions), multi-beam operation in the millimeter wave band FR2, and low peak-to-average power ratio (PAPR) reference signals. Unlicensed Spectrum Applications: Similar to Licensed Assisted Access (LAA) and Enhanced LAA, 3GPP Release 16 supports unlicensed spectrum for NR access to improve the throughput and capacity of Wi-Fi in the 5-6 GHz band. 1.2 Performance Improvements:   RACS (Radio Access Capability Signaling) Optimization: Establishing RACS IDs and mapping them to device radio capabilities optimizes signaling for UE radio capabilities. Multiple UEs can share the same RACS ID, which is stored in the Next Generation Radio Access Network (NG-RAN) and Access and Mobility Management Function (AMF). Additionally, a new network function called UCMF (UE Capability Management Function) is introduced. TDD Applications: NR is primarily used in high-frequency time-division duplex bands: Due to electromagnetic wave reflection and refraction, the downlink of one cell can interfere with the uplink of another cell; this cross-link interference is inherent. NR Release 16 supports remote interference management to mitigate this cross-link interference. II. Flexible Network Deployment R16's IAB (Integrated Access and Backhaul) functionality can increase network capacity by rapidly deploying denser access points. Additionally: Non-Public Networks (NPNs): R16 supports two types of NPNs: Standalone NPN (SNPN) and Public Network Integrated NPN (PNI-NPN).  Flexible SMF and UPF Deployment: R16 introduces management flexibility for Session Management Functions (SMFs) and User Plane Functions (UPFs), allowing multiple SMFs to control a single UPF, and the UPF can assign IP addresses in place of the SMF. Enhanced Network Slicing Capabilities: R16 adds Network Slice-Specific Authentication and Authorization (NSSAA) to support individual authentication and authorization for services within a given network slice. Enhanced eSBA (Service-Based Architecture): R16 enhances service discovery and routing capabilities, including the introduction of a new Service Communication Broker (SCP) network function. R16 also enhances Network Automation Architecture (eNA). Release 15 supports data collection and network analytics public functionality. In Release 16, network analytics IDs can be used to assign specific analytics data, such as network usage per network slice, UE mobility information, and network performance, enabling the Network Data Analytics Function (NWDAF) to collect specific data associated with that analytics ID.

  3GPP introduced LTE in Release 8 and LTE-Advanced in Release 10. As the first version of the 5G specification, Release 15 defined the 5G (NR) air interface and the 5G radio access network and core network. Release 16 (R16) introduced standalone (SA) and non-standalone (NSA) deployments, allowing operators to take advantage of the additional benefits of 5G.   I. Evolution from 4G to 5G In Release 16 (R16), 3GPP enhanced 5G capabilities to support several improvements to the NR air interface, including unlicensed spectrum in the millimeter-wave (mmW) band and improved support for Industrial Internet of Things (IIoT) and Ultra-Reliable Low-Latency Communication (URLLC). The network also underwent several enhancements to improve deployment flexibility and performance.   II. R16 Support for 5G Applications 5G was developed to meet the diverse application scenarios of wirelessly connected devices, covering enhanced mobile broadband (eMBB), massive Internet of Things (mIoT), and ultra-reliable low-latency communication (URLLC). Release R15 primarily focused on eMBB, with limited support for other application scenarios. Release R16 enhances URLLC and IoT capabilities and adds support for 5G vehicle-to-everything (V2X) communication.   III. Key 5G Application Scenarios include:   1. Ultra-reliable low-latency communication New enhancements provide low-latency communication to support industrial automation, connected cars, and telemedicine applications; specifically: The Time-Sensitive Networking (TSN) architecture supports redundant transmissions, thus supporting URLLC applications. Furthermore, the TSN service provides time synchronization for packet transmissions through integration with external networks. R16 enhances the uplink synchronization (RACH) process by supporting low latency and reducing signaling overhead, enabling two-step RACH compared to the previous four-step approach. New mobility enhancements reduce downtime and improve reliability during 5G connected device handover. 2. Internet of Things (IoT): 5G-supported Industrial Internet of Things (IIoT) capabilities can meet the service needs of industries such as manufacturing, logistics, oil and gas, transportation, energy, mining, and aviation.   Cellular Internet of Things (CIoT), now available in 5G, offers similar functionality to that provided in LTE (LTE-M and NB-IoT), allowing IoT traffic to be carried in network signaling. Energy-saving features such as enhanced discontinuous reception (DRX), relaxed radio resource management for idle devices, and enhanced scheduling can extend the battery life of IoT devices. 3. Vehicle-to-Everything (V2X): Release 16 goes beyond the V2X service capabilities supported by LTE in Release 14, leveraging 5G (NR) access to enhance V2X in several ways, such as enhanced autonomous driving, accelerated network effects, and energy-saving features.

  Release 15, finalized in June 2018, paved the way for the commercialization of 5G (NR) technology. R15 laid the foundation for 5G networks through Standalone (SA) and Non-Standalone (NSA) architectures, introducing a service-based virtualized core network and new physical layer technologies to enhance capacity, reduce latency, and improve flexibility. During this period, 3GPP Radio Working Groups RAN1-RAN5 made significant contributions to the standardization of 5G (NR) technology. The work and key technical points of each group are as follows:   I. RAN1 (Physical Layer Innovation) Key work areas include waveforms, parameter sets, multiple access, MIMO, and reference signals: 1. Flexible subcarrier spacing and frame structure; Introduction of scalable subcarrier spacing: Support for different latency and frequency ranges (FR1 and FR2); Support for low latency (

  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.