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The NTN (Non-Terrestrial Network) introduced by 3GPP in its standardization roadmap aims to achieve full 5G coverage and connectivity through satellites and airborne platforms. Key terminology includes:   1. NTN Definition: This is a wireless network technology approved by 3GPP, where access nodes are deployed on space-based or air-based platforms such as satellites or High Altitude Platform Stations (HAPS), rather than being fixed to ground infrastructure. NTN networks are typically used to extend coverage to areas where ground network deployment is impractical or economically unfeasible. From a 3GPP perspective, NTN is not an independent technology, but rather an extension of 5G (NR). NTN reuses and adapts NR protocols, parameters, and procedures as much as possible to support long propagation delays, high Doppler shifts, large cell sizes, and platform mobility.   2. NTN Platforms: This is the most basic classification of satellite orbits, which directly affects latency, coverage, and mobility; specifically including:   GEO (Geostationary Orbit): GEO satellites are located at an altitude of approximately 35,786 kilometers and are stationary relative to the Earth. GEO (Geosynchronous Orbit) satellites have wide coverage but high round-trip delay, making them unsuitable for latency-sensitive services. MEO (Medium Earth Orbit): MEO satellites operate at altitudes between 2,000 and 20,000 kilometers, achieving a balance between coverage and latency; this is particularly emphasized in the current 3GPP NTN specifications. LEO (Low Earth Orbit): LEO satellites operate at altitudes between 300 and 2,000 kilometers. They offer low latency and high throughput, but move very quickly relative to the Earth, leading to frequent inter-satellite handovers and significant Doppler effects. VLEO (Very Low Earth Orbit): VLEO refers to experimental satellites designed to operate at altitudes below 300 kilometers. They are expected to achieve ultra-low latency but face significant atmospheric challenges. HAPS (High Altitude Platform Station): HAPS typically operate at altitudes between 20 and 50 kilometers. HAPS platforms include: solar-powered drones, balloons, and airships. High Altitude Platform Systems (HAPS) can act as NR base stations, relays, or coverage enhancers, and compared to satellites, they have quasi-static characteristics and significantly lower latency.   3. Wireless Access (Terminology) NTN gNB: This is a 5G (NR) base station specifically modified for non-terrestrial deployment. Depending on the architecture, the NTN gNB can be fully hosted on a satellite or HAPS, partially deployed in space and partially on the ground, or entirely ground-based with the satellite acting as a relay. The functional division between space and ground is a key design choice. Transparent Payload or Bent-Pipe Architecture: In a transparent payload or bent-pipe architecture, the satellite does not perform baseband processing. This architecture aims to simplify satellite design, but its operation is highly dependent on the availability of ground infrastructure and feeder links; the transmission payload performs the following functions: Receiving radio frequency signals from user equipment (UE) Performing frequency shifting and amplification Forwarding them to the ground base station (gNB) via the feeder link Regenerative Payload: Performs part or all of Layer 1 and Layer 2 processing on the satellite. In this model, the satellite itself carries the gNB functionality. This architecture reduces feeder link latency, improves scalability, and enables localized decision-making. However, regenerative payloads increase the complexity and cost of the satellite.   4. NTN Links Service Link: Specifically refers to the wireless connection between the user equipment (UE) and the NTN platform (satellite or high-altitude platform). It uses the NR air interface waveform suitable for large cell radii and extended timing advance. Diagram of 5G NTN service link, inter-satellite link, feeder link, and ground network integration. Feeder Link: This connects the satellite to the gateway ground station, which interfaces with the 5G core network. Feeder links typically operate at higher frequencies and require high-capacity backhaul links. Inter-Satellite Link (ISL): Supports direct communication between satellites, allowing data to be routed in space without direct involvement of ground stations. ISL enhances network resilience and reduces end-to-end latency.   5. Network Architecture Gateway Earth Station: The gateway earth station acts as the interface between the satellite system and the 5G core network. It connects the feeder link and plays a crucial role in mobility and session continuity. 5GC supporting NTN: From a protocol perspective, the 5G core network (5GC) remains largely unchanged. Enhancements primarily focus on: supporting long latency, handling large cells, and optimizing processing procedures for idle and connected modes. D2D NTN (Direct-to-Device): User equipment (UE) communicates directly with satellites/high-altitude platforms (HAPS) without intermediate ground access. Hybrid NTN-TN architecture: NTN complements the terrestrial network, used for fallback, offloading, or extending coverage. Relay-based NTN: Satellites or high-altitude platforms (HAPS) act as relay nodes between user equipment (UE) and the terrestrial network.

In competitive random access, after a terminal (UE) receives a RAR message and sends a request for RRC connection establishment, whether it receives permission to establish the connection is crucial for determining the success of the competition. In the NTN scenario, the duration of the contention resolution timer presents another challenge for the terminal (UE).   I. Timer Challenges: During the RACH process, after the terminal (UE) sends the RRC connection request MSG3, it waits for the contention resolution message MSG4 to determine whether its random access attempt was successful. The duration for which the UE listens for MSG4 is controlled by the ra-ContentionResolutionTimer – this timer starts immediately after MSG3 is sent. In NTN systems, the distance between the UE and the satellite base station is much greater, resulting in significantly higher round-trip delays compared to terrestrial systems. While the maximum configurable value of the ra-ContentionResolutionTimer can theoretically cover these longer delays, this approach is inefficient and may unnecessarily consume power at the UE. NTN typically requires energy-efficient operation, especially in remote or battery-constrained applications. Therefore, the default settings of the ra-ContentionResolutionTimer must be adjusted to better accommodate NTN propagation delays while conserving UE power.   II. Potential Solution: One solution is to introduce an offset for the start of the ra-ContentionResolutionTimer in the NTN scenario. The timer would not start immediately after MSG3 transmission, but only after an offset period that accounts for the expected round-trip delay in NTN. This adjustment ensures that the timer is only active during the time period when MSG4 is expected to be received; by aligning the timer with the NTN-specific delay, the UE can avoid unnecessary monitoring during periods when MSG4 is unlikely to arrive. This saves power consumption and ensures compatibility with the longer latency of NTN. The advantages of offset-based timer adjustment include:   Power Efficiency: The UE only monitors when a message is actually likely to arrive, thus reducing unnecessary power consumption. Adaptability to Different Orbits: The offset can be configured according to the type of NTN (GEO or LEO), as the propagation delay differs significantly between these systems. Scalability: This method can adapt to NTNs of different scales and propagation delay characteristics without requiring significant modifications to the standard conflict resolution process. Robustness: Aligning the timer with the actual delay prevents the conflict resolution timer from timing out prematurely, which could otherwise lead to unnecessary retransmissions or failures in NTN communication.

  In the 5G system, the AMF is responsible not only for terminal (UE) access and mobility management, but also for processing and notifying other units about terminal (UE) service requests and data transmission. The key points of the interaction with related networks during this process are as follows:   I. The AMF is responsible for SMF selection according to the procedures described in clause 6.3.2; for this purpose, it obtains subscription data from the UDM as defined in that clause. In addition, it obtains the subscribed UE-AMBR from the UDM and, based on the operator's local policy, obtains the dynamic service network UE-AMBR (optional) from the PCF; then it sends it to the (R)AN as defined in clause 5.7.2; AMF-SMF interaction supporting LADN is defined in clause 5.6.5.   To support billing and meet regulatory requirements (NPLI (Network Provided Location Information) as defined in TS 23.228 [15]) related to IMS voice call establishment, modification and release or SMS transfer, the following provisions apply:   If the AMF possesses the UE's PEI during PDU session establishment, the AMF will provide the PEI to the SMF. When the AMF forwards UL NAS or N2 signaling to a peer NF (such as SMF or SMSF) or during PDU session UP connection activation, it will provide any user location information received from the 5G-AN, as well as the AN access type (3GPP-non 3GPP) of the received UL NAS or N2 signaling. The AMF will also provide the corresponding UE time zone. In addition, to meet regulatory requirements (i.e., providing Network Provided Location Information (NPLI) as defined in TS 23.228 [15]); when the access method is non-3GPP, if the UE is still connected to the same AMF for 3GPP access (i.e., user location information is valid), the AMF can also provide the last known 3GPP access user location information and its validity period.   II.The SMF may further provide user location information, access type, and UE time zone to the PCF. The PCF can obtain this information from the SMF to provide NPLI to applications that have requested NPLI (such as IMS). User location information may include:   For 3GPP access: Cell ID, even if the AMF receives the primary cell ID from the auxiliary RAN node in NG-RAN, the AMF only includes the primary cell ID. For untrusted non-3GPP access: The local IP address used by the UE to connect to the N3IWF, and (if NAT is detected) the UDP source port number (optional).   III.Trusted non-3GPP   For trusted non-3GPP access: TNAP/TWAP identifier, the local IP address used by the UE/N5CW device to connect to the TNGF/TWIF, and (if NAT is detected) the UDP source port number (optional). When the UE connects to the TNGF using WLAN based on IEEE 802.11 technology, the TNAP identifier should include the SSID of the access point to which the UE is connected. The TNAP identifier should include at least one of the following elements, unless otherwise specified by the TWAN operator's policy: BSSID (see IEEE Std 802.11-2012 [106]); Address information of the TNAP to which the UE is connected.   IV. The TWAP identifier should include the SSID of the access point to which the NC5W is connected; unless otherwise specified by the TWAN operator's policy, the TWAP identifier should also include at least one of the following: BSSID (see IEEE Std 802.11-2012 [106]); Address information of the TWAP to which the UE is connected.   In addition: Multiple TNAPs/TWAPs may use the same SSID, and the SSID alone may not provide location information, but may be sufficient for billing purposes. It is assumed that the BSSID associated with the TNAP/TWAP is static.   V. User location information for W-5GAN access is defined in TS 23.316 [84]. When the SMF receives a request to provide access network information report, and there are no operations required to be performed on the 5G-AN or UE (e.g., no QoS flows need to be created/updated/modified), the SMF may request user location information from the AMF. The interaction between the AMF and SMF for the insertion, relocation, or removal of the I-SMF in a PDU session is described in Section 5.34.