Devices and methods for reliable handover in non-terrestrial networks

Abstract

A user equipment, UE, (110) is disclosed for performing a reliable handover in a mobile network (100) with a radio access network including a non-terrestrial network, NTN, with a plurality of NTN access nodes (120a-n). The UE (110) is configured to send a handover request to one or more NTN access nodes of the plurality of NTN access nodes (120a-n) for performing a handover, wherein the UE (110) is connected to the one or more NTN access nodes of the plurality of NTN access nodes (120a- n) and wherein the handover request is indicative of at least a first NTN access node and a second NTN access node of the plurality of NTN access nodes (120a-n).

Description

[0001] DEVICES AND METHODS FOR RELIABLE HANDOVER IN NON-TERRESTRIAL NETWORKS

[0002] TECHNICAL FIELD

[0003] The present disclosure relates to wireless communications. More specifically, the present disclosure relates to devices and methods for reliable handover in non-terrestrial networks, NTNs, in particular multi-satellite MIMO communications.

[0004] BACKGROUND

[0005] Non-terrestrial networks (NTNs) specify radio access networks, where the access nodes are carried on platforms hovering high above the terrestrial surface in the air (in the form of high-altitude platforms - HAPs or unmanned aerial vehicles - UAVs, such as drones or balloons) or in space (satellites). While the 5G New Radio (NR) system has originally been designed as a terrestrial network (TN), it has later been extended to support NTN components for extending coverage for its main services, allowing to communicate via any network node through the same end device. The vision for a communication network in the future goes far beyond that concept, envisaging a full integration of NTN and TN on the network infrastructure level that results in a network convergence, where all the radio services can be seamlessly provided through any of the access nodes, while being transparent to the user.

[0006] 5G NR focused on NTN support solely for its enhanced mobile broadband (eMBB) and massive machine-type communication (mMTC) services, which both feature a high tolerance for delays; however, the most challenging 5G service to be supported by NTN (and that constituted of satellites, in particular) is the ultra-reliable low-latency communication (URLLC) service. Its requirements can be met by the latest satellite generation operated in the low earth orbit (LEO), though, which is characterized by a high constellation density and a close distance to the Earth's surface of several hundred kilometres. The latter yields relatively short communication delays - in particular, if the base station and parts of the network functions are operated on board of the satellites themselves. Moreover, the dense constellation results in the fact that several satellites will be in the field of view of any user on Earth, and hence each user can connect to more than one satellite at a time to improve the connection reliability for the communication. Connecting a user to multiple satellite nodes is also known as multi-satellite MIMO communication, enabling to transmit the same data via independent communication channels between the user and the network. A high reliability can be attained by the redundancy of the communication links: Should one of the satellite links be shadowed, the connection to that satellite is likely to break; however, the communication can reliably be maintained by the link to the other satellites.

[0007] There are various use cases in the context of vehicle-to-everything (V2X) communication that are built on URLLC service, requiring ubiquitous connectivity and highly reliable communication links. Since vehicles may drive to any area where roads have paved the way, they may easily get out of TN coverage, and hence URLLC service support via NTN can be considered essential for seamless connectivity in V2X communications. In this context, multi-satellite MIMO in dense LEO constellations may be the key enabler for meeting the requirements of those use cases in NTN.

[0008] LEO satellites move at considerable speed, resulting in the fact that a single satellite will be in a user’s field of view for a few minutes only. Hence, frequent handover between satellites will be necessary, even if the user does not move at all. If a user is connected to more than one satellite, handover may occur even more frequently, and the conventional schemes designed for single satellite connections need to be adapted accordingly. SUMMARY

[0009] It is an objective of the present disclosure to provide improved devices and methods for reliable handover in non-terrestrial networks, NTNs, in particular multi-satellite MIMO communications.

[0010] The foregoing and other objectives are achieved by the subject matter of the independent claims. Further implementation forms are apparent from the dependent claims, the description and the figures. In the following some or more of the following abbreviations and acronyms will be used:

[0011] 5G 5th generation mobile wireless communications

[0012] HO Handover

[0013] CHO Conditional Handover

[0014] DC Dual Connectivity

[0015] NTN Non-terrestrial network

[0016] TN Terrestrial network

[0017] LEO Low Earth Orbit

[0018] AMF Access and mobility management function

[0019] TAI Tracking area identity

[0020] UE User equipment

[0021] According to a first aspect a user equipment, UE, is provided for a mobile network with a radio access network including a non-terrestrial network, NTN, with a plurality of NTN access nodes (also referred to as base stations). The UE according to the first aspect is configured to send a handover request to one or more NTN access nodes of the plurality of NTN access nodes for performing a handover, wherein the UE is currently connected to the one or more NTN access nodes of a plurality of NTN access nodes and wherein the handover request is indicative of at least a first NTN access node and a second NTN access node of the plurality of NTN access nodes. Moreover, the UE according to the first aspect is configured to perform the handover from one of the connected NTN access nodes to the first NTN access node or the second NTN access node of the plurality of NTN access nodes. As will be appreciated, prior to the handover the UE is connected to the one or more NTN access nodes the handover request is sent to as well as possibly one or more further NTN access nodes of the plurality of NTN access nodes. These NTN access nodes are herein referred to as connected NTN access nodes.

[0022] Thus, the UE according to the first aspect may signal, for instance, a pair of NTN access nodes, e.g. satellites in the HO request to any connected NTN access node, e.g. satellite in a MIMO cluster. The UE according to the first aspect is in the best condition to decide early on HO, as it will have the best knowledge of the environment and the conditions that may affect the signal quality of any satellite link. The signalling informs the system which NTN access node, e.g. satellite of the cluster should be exchanged by another. By allowing the UE according to the first aspect to send its HO request to any of the NTN access nodes, e.g. satellites it is connected to (i.e., belonging to the MIMO cluster), a sudden drop of a satellite link (due to shadowing, blockage or something similar) can be handled efficiently, as the UE can immediately trigger a HO in that case.

[0023] In a further possible implementation form, the first NTN access node is a source NTN access node and the second NTN access node is a target NTN access node for the handover.

[0024] In a further possible implementation form, the first NTN access node is one of the one or more NTN access nodes connected to the UE, and the second NTN access node is a target NTN access node for the handover.

[0025] In a further possible implementation form, the first NTN access node is a first target NTN access node and the second NTN access node is a second target NTN access node for the handover. In a further possible implementation form, the UE is configured to receive a tracking area identity, TAI, list from a network control plane entity of the mobile network, wherein the TAI list includes a plurality of NTN access node candidates the UE may connect to.

[0026] In a further possible implementation form, for each of the plurality of NTN access node candidates of the TAI list, the TAI list is indicative of a Mobile Country Code, MCC, a Mobile Network Code, MNC, a Tracking Area Code, TAC, and / or a priority indication, wherein the priority indication is indicative of an order for selecting one or more of the pluralities of NTN access node candidates for the handover from the TAI list by the UE.

[0027] In a further possible implementation form, for each of the plurality of NTN access node candidates the priority indication is based on a load of the respective NTN access node candidate.

[0028] In a further possible implementation form, the network control plane entity is an access and mobility management function, AMF, of the mobile network.

[0029] According to a second aspect, a method is provided for operating a user equipment, UE, for a mobile network with a radio access network including a non-terrestrial network, NTN, with a plurality of NTN access nodes, e.g. base stations. The method according to the second aspect comprises the following steps: sending a handover request to one or more NTN access nodes of the plurality of NTN access nodes for performing a handover, wherein the UE is currently connected to the one or more NTN access nodes of a plurality of NTN access nodes and wherein the handover request is indicative of at least a first NTN access node and a second NTN access node of the plurality of NTN access nodes, and performing the handover from one of the connected NTN access nodes to the first NTN access node or the second NTN access node of the plurality of NTN access nodes.

[0030] The method according to the second aspect can be performed by the UE according to the first aspect. Thus, further features of the method according to the second aspect result directly from the functionality of the UE according to the first aspect as well as its different implementation forms described above and below.

[0031] According to a third aspect, a non-terrestrial network, NTN, access node is provided for providing network access for a user equipment, UE, to a mobile network with a radio access network including a plurality of NTN access nodes, e.g. base stations. The NTN access node according to the third aspect is configured to receive a handover request from the UE for performing a handover, wherein the UE is currently connected to the one or more NTN access nodes of the plurality of NTN access nodes and wherein the handover request is indicative of at least a first NTN access node and a second NTN access node of the plurality of NTN access nodes. Moreover, the NTN access node according to the third aspect is configured to perform the handover or forward the handover request to a further NTN access node of the connected NTN access nodes for performing the handover from one of the connected NTN access nodes to the first NTN access node or the second NTN access node of the plurality of NTN access nodes.

[0032] In a further possible implementation form, the first NTN access node is a source NTN access node and the second NTN access node are a target NTN access node for the handover.

[0033] In a further possible implementation form, the first NTN access node is one of the connected NTN access nodes and the second NTN access node is a target NTN access node for the handover. In a further possible implementation form, the first NTN access node is a first target NTN access node and the second NTN access node are a second target NTN access node for the handover.

[0034] In a further possible implementation form, the NTN access node according to the third aspect is configured to forward a tracking area identity, TAI, list from a network control plane entity of the mobile network to the UE and wherein the TAI list includes a plurality of NTN access node candidates the UE may connect to.

[0035] In a further possible implementation form, for each of the plurality of NTN access node candidates of the TAI list, the TAI list is indicative of a Mobile Country Code; MCC, a Mobile Network Code, MNC, a Tracking Area Code, TAC, and / or a priority indication, wherein the priority indication is indicative of an order for selecting one or more of the pluralities of NTN access node candidates from the TAI list for the handover by the UE.

[0036] In a further possible implementation form, for each of the plurality of NTN access node candidates the priority indication is based on a load of the respective NTN access node candidate.

[0037] In a further possible implementation form, the network control plane entity is an access and mobility management function, AMF, of the mobile network.

[0038] In a further possible implementation form, the NTN access node according to the third aspect is a master NTN access node of the connected NTN access nodes or the NTN access node according to the third aspect is configured to forward the handover request to a master NTN access node of the connected NTN access nodes.

[0039] In a further possible implementation form, the NTN access node according to the third aspect is the current master node and configured to re-assign a current master node status, if the current master node is the NTN access node that will be released after handover.

[0040] According to a fourth aspect a method is provided for operating a non-terrestrial network, NTN, access node for providing network access for a user equipment, UE, to a mobile network with a radio access network including a plurality of NTN access nodes, e.g. base stations. The method according to the fourth aspect comprises: receiving a handover request from the UE for performing a handover, wherein the UE is currently connected to the one or more NTN access nodes of the plurality of NTN access nodes and wherein the handover request is indicative of at least a first NTN access node and a second NTN access node of the plurality of NTN access nodes, and performing the handover or forwarding the handover request to a further NTN access node of the connected NTN access nodes for performing the handover from one of the connected NTN access nodes to the first NTN access node or the second NTN access node of the plurality of NTN access nodes.

[0041] The method according to the fourth aspect can be performed by the NTN access node according to the third aspect. Thus, further features of the method according to the fourth aspect result directly from the functionality of the NTN access node according to the third aspect as well as its different implementation forms described above and below.

[0042] According to a fifth aspect, a computer program product is provided, comprising a computer-readable storage medium for storing program code which causes a computer or a processor to perform the method according to the second aspect or the method according to the fourth aspect, when the program code is executed by the computer or the processor.

[0043] Details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description, drawings, and claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] In the following, embodiments of the present disclosure are described in more detail with reference to the attached figures and drawings, in which:

[0045] Fig. 1 shows a schematic diagram illustrating a mobile network 100 with a radio access network including a non-terrestrial network, NTN, with a plurality of NTN access nodes according to an embodiment for performing a handover of a UE according to an embodiment;

[0046] Fig. 2a shows a signalling diagram illustrating a conventional handover scheme for terrestrial networks;

[0047] Fig. 2b shows a signalling diagram illustrating a conventional conditional handover scheme;

[0048] Fig. 2c shows a schematic diagram illustrating a conventional handover scheme between clusters of satellites;

[0049] Fig. 2d shows a signalling diagram illustrating a conventional handover scheme in dual connectivity mode, where the handover between secondary nodes is initiated by the master node;

[0050] Fig . 3 shows a schematic diagram illustrating processing steps implemented by a AMF for maintaining a TAI list and supporting a handover of a UE according to an embodiment;

[0051] Fig. 4a shows a signalling diagram illustrating a handover scheme implemented by a UE and a plurality of NTN access nodes according to an embodiment;

[0052] Fig. 4b shows a signalling diagram illustrating a variant of the handover scheme of figure 4a;

[0053] Fig. 5 shows a flow diagram illustrating a method of operating a UE according to an embodiment; and

[0054] Fig. 6 shows a flow diagram illustrating a method of operating a NTN access node according to an embodiment.

[0055] In the following, identical reference signs refer to identical or at least functionally equivalent features.

[0056] DETAILED DESCRIPTION OF THE EMBODIMENTS

[0057] In the following description, reference is made to the accompanying figures, which form part of the disclosure, and which show, by way of illustration, specific aspects of embodiments of the present disclosure or specific aspects in which embodiments of the present disclosure may be used. It is understood that embodiments of the present disclosure may be used in other aspects and comprise structural or logical changes not depicted in the figures. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present disclosure is defined by the appended claims.

[0058] For instance, it is to be understood that a disclosure in connection with a described method may also hold true for a corresponding device or system configured to perform the method and vice versa. For example, if one or a plurality of specific method steps are described, a corresponding device may include one or a plurality of units, e.g. functional units, to perform the described one or plurality of method steps (e.g. one unit performing the one or plurality of steps, or a plurality of units each performing one or more of the plurality of steps), even if such one or more units are not explicitly described or illustrated in the figures. On the other hand, for example, if a specific apparatus is described based on one or a plurality of units, e.g. functional units, a corresponding method may include one step to perform the functionality of the one or plurality of units (e.g. one step performing the functionality of the one or plurality of units, or a plurality of steps each performing the functionality of one or more of the plurality of units), even if such one or plurality of steps are not explicitly described or illustrated in the figures. Further, it is understood that the features of the various exemplary embodiments and / or aspects described herein may be combined with each other, unless specifically noted otherwise.

[0059] Figure 1 shows a schematic diagram illustrating a mobile network 100 with a radio access network including a non-terrestrial network, NTN, with a plurality of NTN access nodes 120a-n according to an embodiment for performing a handover of a UE 110 according to an embodiment. In an embodiment, the mobile network 100 may be a current or future 3rd Generation Partnership Project (3GPP) mobile network 100, for instance, a 5G or a communication network in the future network. The NTN access nodes 120a-n according to an embodiment may be implemented, for instance, in the form of high-altitude platforms, HAPs, unmanned aerial vehicles, UAVs, such as drones or balloons, or satellites. Latest satellite technology allows LEO satellites to accommodate strong computing capabilities, enabling them to act as full base stations. The corresponding satellite architecture is denoted as a regenerative payload. With regenerative payload, a satellite can perform independent routing of data and control the communication processes in the links to the UEs and ground stations, and it essentially enables the cooperative communication of a UE with more than one satellite at a time.

[0060] Existing solutions for handover in satellite communications have been adopted from the 5G NR terrestrial network, where all of these rely on a connection to a single source cell (= satellite). For NTN, the following two handover schemes have been adopted: Conventional Handover (HO) and Conditional Handover (CHO).

[0061] The conventional HO scheme illustrated in figure 2a is a network-driven process comprising the steps described in the following. If a measurement event is triggered (in TN, typically the SNR to the source cell falling below a threshold, while for NTN further triggers have been defined), the UE measures the target cell(s) and sends a measurement report (MR) to the source cell. The source cell takes the decision to hand over based on the quality of the MR as well as the quality of the actual connection to the UE. The source cell prepares the target cell for HO by sending a HO request to the target cell. The target cell runs through the admission control process and if successful, sends an acknowledgment to the source cell. After receiving the acknowledgment, the source cell sends the HO command to the UE. The UE configures for communication with the target cell. More specifically, the UE synchronizes with the target cell. The UE receives uplink (UL) allocation from the target cell. The UE completes the reconfiguration process and acknowledges the target cell.

[0062] As will be appreciated, the conventional HO scheme illustrated in figure 2a has the disadvantage of a potential connection interruption resulting in a delay during the HO process, since the HO process is triggered when the quality of the link to the source cell drops, and hence the connection could break before the new connection to the target cell has been established (also called “break before make” principle). To avoid this potential delay and / or interruption, enhanced HO schemes have been developed following the “make before break” principle, where a number of potential target cells are prepared for the HO before the HO event is triggered, such as the conditional handover (CHO) scheme illustrated in figure 2b. CHO represents a UE-driven process comprising the following modifications. The UE measures several target cells and sends measurement reports (MRs) for a list of candidate target cells for HO. The source cell sends CHO requests to candidate target cells to prepare them for handover. For all target cells that acknowledged the CHO request, the source cell sends CHO reconfiguration information to the UE. The UE monitors CHO conditions for the target cells that have been prepared for HO. If the CHO condition is fulfilled for any of the target cells, the UE configures for communication with the selected target cell by running through the same reconfiguration as described in the context of the conventional HO, namely: UE synchronizes with the target cell; UE receives uplink (UL) allocation from target cell; and UE completes the reconfiguration process and acknowledges the selected target cell.

[0063] For performing a HO in multi-satellite MIMO communications, a clustering concept has been proposed, where a single satellite acting as master node controls the satellites inside a cluster (called “cluster master” - CMA). Handover is considered between different satellite clusters, where each one is controlled by its own master node, as illustrated in figure 2c. In essence, the handover process is performed between the two master nodes of the clusters, and hence the HO schemes known for single-cell attachment (source cell and target cell) can be reused directly.

[0064] Dual connectivity (DC) is a concept in 5G NR that allows to maintain links to two different network nodes at the same time, facilitating the simultaneous transmission of data via the different network nodes, and it can be adopted in NTN for establishing connections to more than one satellite. In DC, one access node acts as the master, and the other as secondary node (which is actually similar to the clustering concept described above for two satellites). The master node establishes the control plane (CP), while the secondary node provides resources for increasing the capacity for data transmission only. Since the CP runs through the master, the entire control, including HO, is managed by the master, and if the connection to the master breaks, the entire connection to the master and secondary node breaks down. Hence, there is a strong hierarchical structure in DC, implying that the master node should provide the most reliable connection.

[0065] The HO process in DC is specified dependent on the node type, i.e., whether it is HO between master or secondary nodes (SN). The standardized procedure for HO between secondary nodes (source-SN and target-SN) initiated by the master node (MN) is specified in 3GPP TS 37.340 and shown in the signalling diagram of figure 2d. The single steps in the signalling diagram resemble those from conventional HO and CHO described above, while now the full control is managed by the MN (the measurement event trigger and MR have been left out in this case).

[0066] As will be appreciated, conventional HO schemes adopted from TN do not support connections to multiple satellites. DC can enable multi-satellite MIMO connections with corresponding HO schemes, but the hierarchical structure does not match well to the nature of satellite connections, where it is barely possible to predict the most reliable link in advance.

[0067] The conventional clustering scheme for multi-satellite MIMO has the following drawbacks. Clusters may be extended by satellite nodes, but all are controlled by the master node. A loss of the link to the master node cuts off the control of the cluster and requires a HO to the new cluster and its master, causing delays. Moreover, highly-reliable communications with delay-free HO are not supported. The link to the master node must be maintained, otherwise connection is interrupted or even lost. The HO follows the „break before make“ principle. No dynamic formation of clusters is supported.

[0068] Embodiments disclosed herein provide a scheme for reliable handover, for instance, in multi-satellite MIMO communications, such as the scenario illustrated in figure 1, where the UE 110 is connected to a plurality of NTN access nodes in the form of a cluster of visible satellites 120a-n at the same time. Embodiments disclosed herein build on the concept of dual-connectivity, as introduced by 3GPP.

[0069] Embodiments disclosed herein are based on one or more of the following considerations. If connections from one UE 110 to multiple satellites 120a-n exist, the network should know which of the satellites 120a-n requires a HO. Considering that the UE 110 most likely will be well aware of its environment in future radio systems, thanks to radio maps downloaded from the network and enhancing and maintaining those at the UE 110 by its sensing capabilities, the UE 110 may well be configured to predict which satellite 120a-n is prone to be shadowed and which other satellite 120a-n may be the best to handover to.

[0070] For the above reasons, the HO scheme disclosed herein is UE-driven. Since each satellite 120a-n has equal likelihood to be shadowed, according to embodiments disclosed herein, there is no dedicated control channel to a single satellite node 120a-n (as it is the case in conventional DC). Instead, according to embodiments disclosed herein, each connected satellite 120a-n is capable of receiving at least the HO control signal. Typically, there may be several satellites 120a-n as potential candidates for HO, and according to embodiments disclosed herein, any of these may be selected. As in CHO, according to embodiments disclosed herein, the network may be configured to keep control of the list of candidates, allowing the network to perform load balancing and thus avoid assigning additional UEs to satellites 120a-d with a high traffic load. More specifically, as indicated in figure 1, the UE 110 is configured to send a HO request to one or more NTN access nodes of the plurality of NTN access nodes 120a-n, e.g. satellites for performing a handover. As further indicated, the UE 110 is connected to the one or more NTN access nodes of the plurality of NTN access nodes 120a-n (in the example shown in figure 1 , the UE is connected to the NTN access node SI 120a and the NTN access node S4 120n). As will be described in more detail further below, the HO request is indicative of at least a first NTN access node and a second NTN access node of the plurality of NTN access nodes 120a-n. Moreover, the UE 110 is configured to perform the handover from one of the one or more NTN access nodes connected to the UE 110 to the first NTN access node or the second NTN access node of the plurality of NTN access nodes 120a-n.

[0071] As already mentioned above, in the embodiment illustrated in figure 1, the UE 110 sends the HO request containing a first and second, i.e. a pair of satellite nodes, which indicates the source and target satellite for the HO, such as (SI, S2) in the example shown in figure 1. In a further embodiment, the HO request may indicate the set, e.g. pair of satellites 120a-n the UE 110 wants to be connected to in the future, such as (S2, S4) in the example shown in figure 1. As already described above, the HO request may be sent by the UE 110 to any of the connected satellites, as shown in figure 1 for the connected satellites SI 120a and S4 120n.

[0072] As already mentioned above, embodiments disclosed herein support network control of the satellite candidates for HO. To this end, the tracking area identity (TAI) list, which is a list maintained in the access and mobility management function (AMF) 130 (illustrated in figure 3) in the Core Network for defining the mobility restriction area for any UE 110, may be used with the following enhancements. The TAI list may be maintained individually for each UE 110 and contains the available access nodes 120a-n the UE 110 is allowed to connect to. The AMF 130 may provide the TAI list to the UE 110 as part of the UE registration process. Each available entry in the conventional TAI list is comprised of the following three fields: Mobile Country Code (MCC), Mobile Network Code (MNC), Tracking Area Code (TAC). To allow the network to control, the selection of satellite candidates for HO for the UE 110, according to embodiments disclosed herein, the conventional TAI list is enhanced with the addition of prioritization of its entries. To facilitate this, each entry in the TAI list (denoted as TAI itself) may be extended with a field named “Priority” (Prio), which indicates the order of candidate selection for the HO of the corresponding UE 110. In other words, according to embodiments disclosed herein, the TAI may be represented as follows:

[0073] TAI = (MCC + MNC + TAC + Prio).

[0074] Setting the priority to, e.g., zero or a negative integer allows to exclude a satellite 120a-n from the list of HO candidates. The priority may be dynamically adjusted, resulting in an updated TAI list, which is then shared again between the Core Network (e.g., through the AMF 130) and the UE 110.

[0075] Figure 3 illustrates steps implemented by the AMF 130 for maintaining and updating the enhanced TAI list described above. In step 301 of figure 3, the AMF 130 retrieves the conventional TAI list for the UE 110. In step 302 of figure 3, the AMF 130 obtains load information about the access nodes 120a-n. In step 303 of figure 3, the AMF 130 determines and / or updates the list of priorities for the UE 110. In step 304 of figure 3, the AMF 130 includes the list of priorities (from step 303) in the conventional TAI list (from step 301). In step 305 of figure 3, the AMF 130 sends the enhanced TAI list, i.e. including the priorities, to the UE 110.

[0076] The embodiments shown in figures 4a and 4b illustrate for the example shown in figure 1 the signalling between the UE 110, the satellites SI 120a, S2 120b, S4 120n and the AMF 130 for performing the satellite handover triggered by the UE 110. As already described above, in the example of figure 1, the UE 110 is connected to satellites SI and S4, and a HO from SI to S2 is supposed to be executed. According to embodiments disclosed herein, the DC scheme is adopted for supporting simultaneous connection of the UE 110 to multiple satellites 120a-n, which allows reuse of the HO process for changing the secondary node without any modifications. Hence, the signalling for supporting HO in multi-satellite MEMO connections may be realized with minimal changes to the standardized scheme. Since DC requires the definition of a master node, two options are possible in the given scenario: Either the satellite SI is the master node (which is affected by the intended HO), or S4 is the master node. Both these options are considered in the following two embodiments, where the signalling is illustrated in corresponding signalling diagrams.

[0077] In the embodiment shown in figure 4a, SI 120a is considered to be the master node according to the DC concept.

[0078] In step 401 of figure 4a, the UE 110 sends its registration request to the AMF 130. This request may be in accordance with 3GPP TS 23.502.

[0079] Based on the request, the AMF 130 generates in step 402 of figure 4a the TAI list for the UE 110 and assigns priorities to each of the TAIs in the list (for instance, in the way described in the context of figure 3 above), and sends this TAI list (including the priorities) to the UE 110 (see step 403 of figure 4a).

[0080] Once the UE 110 detects that a HO for one of its connected satellites is necessary, the UE 110 sends in step 404 of figure 4a a HO request containing the pair of satellites for performing the handover (by way of example, the pair SI, S2) to any of the two connected satellites SI and S4.

[0081] If S4 receives the HO request, it forwards the request to the master satellite SI (see step 405 of figure 4a), which may be paired with a request to become master.

[0082] SI reads from the HO request that it is the one to be handed over from, and hence it assigns the master status to S4 (or any other connected satellite, if available) and forwards the HO request to the new master satellite S4 (see step 406 of figure 4a).

[0083] S4 accepts to become master node and sends a notification message to the UE 110 to inform it on the change of the master node (see step 407 of figure 4a).

[0084] Once S4 has taken the role of the master node, the HO can be performed according to the standardized procedure for secondary node change in DC, as detailed in 3GPP TS 37.340. These steps are briefly described in the following. The new master node S4 sends a node addition request to the satellite S2 (see step 408 of figure 4a), which triggers S2 to allocate resources for the UE 110 to be served by S2 after handover. After successful resource allocation, S2 sends an acknowledgement back to S4 (see step 409 of figure 4a). Master node S4 then sends a node release request to source node SI (see step 410 of figure 4a), which triggers SI to stop providing user data to the UE 110 and release corresponding resources. The request is acknowledged by SI (see step 411 of figure 4a). The master node S4 then sends the RRC connection configuration containing the information for connecting the UE 110 to the target node S2 (see step 412 of figure 4a). The UE 110 applies the new configuration and informs the master node S4 about the completion of the connection configuration (see step 413 of figure 4a). Finally, the master node S4 informs the target node S2 about the completion of the reconfiguration (see step 414 of figure 4a), and S2 can commence serving the UE 110.

[0085] In the variant shown in figure 4b, S4 is the master node according to the DC concept. The signalling is similar to the signalling described above in the context of figure 4a, except for the handling of the HO request received by nodes SI and S4, respectively, which is described in more detail in the following. If SI receives the HO request, it forwards it in step 404a of figure 4b to the master satellite S4 (without any further changes or additions). S4 reads from the HO request that SI is the satellite to be handed over from to S2 and performs the HO according to the standardized procedure for secondary node change in DC, as detailed in 3GPP TS 37.340. Figure 5 shows a flow diagram illustrating a method 500 for operating the UE 110 in the mobile network 100 with a radio access network including a non-terrestrial network, NTN, with the plurality of NTN access nodes 120a-n. The method 500 comprises a step 501 of sending a handover request to one or more NTN access nodes of the plurality of NTN access nodes 120a-n for performing a handover. As already described above, the UE 110 is connected to the one or more NTN access nodes of the plurality of NTN access nodes 120a-n and the handover request is indicative of at least a first NTN access node and a second NTN access node of the plurality of NTN access nodes 120a-n. Moreover, the method 500 comprises a step 503 of performing the handover from one of the one or more connected NTN access nodes to the first NTN access node or the second NTN access node of the plurality of NTN access nodes 120a-n.

[0086] The method 500 can be performed by the UE 110. Thus, further features of the method 500 result directly from the functionality of the UE 110 as well as the different embodiments thereof described above and below.

[0087] Figure 6 shows a flow diagram illustrating a method 600 for operating each of the non-NTN access nodes 120a-n for providing network access for the UE 110 to the mobile network 100 with a radio access network including the plurality of NTN access nodes 120a-n. The method 600 comprises a step 601 of receiving a handover request from the UE 110 for performing a handover. As already described above, the UE 110 is connected to the one or more NTN access nodes of the plurality of NTN access nodes 120a-n and the handover request is indicative of at least a first NTN access node and a second NTN access node of the plurality of NTN access nodes 120a-n. Moreover, the method 600 comprises a step 603 of performing the handover or forwarding the handover request to a further NTN access node of the connected NTN access nodes for performing the handover from one of the connected NTN access nodes to the first NTN access node or the second NTN access node of the plurality of NTN access nodes 120a-n.

[0088] The method 600 can be performed by any of the plurality of NTN access nodes 120a-n. Thus, further features of the method 600 result directly from the functionality of the NTN access nodes 120a-n as well as the different embodiments thereof described above and below.

[0089] The person skilled in the art will understand that the "blocks" ("units") of the various figures (method and apparatus) represent or describe functionalities of embodiments of the present disclosure (rather than necessarily individual "units" in hardware or software) and thus describe equally functions or features of apparatus embodiments as well as method embodiments (unit = step).

[0090] In the several embodiments provided in the present application, it should be understood that the disclosed system, apparatus, and method may be implemented in other manners. For example, the described embodiment of an apparatus is merely exemplary. For example, the unit division is merely a logical function division and may be another division in an actual implementation. For example, a plurality of units or components may be combined or integrated into another system, or some features may be ignored or not performed. In addition, the displayed or discussed mutual couplings or direct couplings or communication connections may be implemented by using some interfaces. The indirect couplings or communication connections between the apparatuses or units may be implemented in electronic, mechanical, or other forms.

[0091] The units described as separate parts may or may not be physically separate, and parts displayed as units may or may not be physical units, may be located in one position, or may be distributed on a plurality of network units. Some or all of the units may be selected according to actual needs to achieve the objectives of the solutions of the embodiments.

[0092] In addition, functional units in the embodiments of the disclosure may be integrated into one processing unit, or each of the units may exist alone physically, or two or more units may be integrated into one unit.

Claims

CLAIMS1. A user equipment, UE, (110) for a mobile network (100) with a radio access network including a non-terrestrial network, NTN, with a plurality of NTN access nodes (120a-n), wherein the UE (110) is configured to: send a handover request to one or more NTN access nodes of the plurality of NTN access nodes (120a-n) for performing a handover, wherein the UE (110) is connected to the one or more NTN access nodes of the plurality of NTN access nodes (120a-n) and wherein the handover request is indicative of at least a first NTN access node and a second NTN access node of the plurality of NTN access nodes ( 120a-n); and perform the handover from one of the one or more NTN access nodes connected to the UE (110) to the first NTN access node or the second NTN access node of the plurality of NTN access nodes (120a-n).

2. The UE (110) of claim 1, wherein the first NTN access node is a source NTN access node and the second NTN access node is a target NTN access node for the handover.

3. The UE (110) of claim 1, wherein the first NTN access node is one of the one or more NTN access nodes connected to the UE (110) and the second NTN access node is a target NTN access node for the handover.

4. The UE (110) of claim 1, wherein the first NTN access node is a first target NTN access node and the second NTN access node is a second target NTN access node for the handover.

5. The UE (110) of any one of the preceding claims, whereinthe UE (110) is configured to receive a tracking area identity, TAI, list from a network control plane entity (130) of the mobile network (100) and wherein the TAI list includes a plurality of NTN access node candidates the UE (110) may connect to.

6. The UE (110) of claim 5, wherein, for each of the plurality of NTN access node candidates of the TAI list, the TAI list is indicative of a Mobile Country Code, MCC, a Mobile Network Code, MNC, a Tracking Area Code, TAC, and / or a priority indication, wherein the priority indication is indicative of an order for selecting one or more of the plurality of NTN access node candidates for the handover from the TAI list by the UE (110).

7. The UE (110) of claim 6, wherein for each of the plurality of NTN access node candidates the priority indication is based on a load of the respective NTN access node candidate.

8. The UE (110) of any one of claims 5 to 7, wherein the network control plane entity (130) is an access and mobility management function, AMF, (130) of the mobile network (100).

9. A method (500) of operating a user equipment, UE, (110) for a mobile network (100) with a radio access network including a non-terrestrial network, NTN, with a plurality of NTN access nodes (120a-n), wherein the method (500) comprises: sending (501) a handover request to one or more NTN access nodes of the plurality of NTN access nodes (120a-n) for performing a handover, wherein the UE (110) is connected to the one or more NTN access nodes of the plurality of NTN access nodes (120a-n) and wherein the handover request is indicative of at least a first NTN access node and a second NTN access node of the plurality of NTN access nodes (120a-n); andperforming (503) the handover from one of the one or more connected NTN access nodes to the first NTN access node or the second NTN access node of the plurality of NTN access nodes (120a-n).

10. A non-terrestrial network, NTN, access node (120a-n) for providing network access for a user equipment, UE, (110) to a mobile network (100) with a radio access network including a plurality of NTN access nodes (120a-n), wherein the NTN access node (120a-n) is configured to: receive a handover request from the UE (110) for performing a handover, wherein the UE is (110) connected to the one or more NTN access nodes of the plurality of NTN access nodes and wherein the handover request is indicative of at least a first NTN access node and a second NTN access node of the plurality of NTN access nodes; and perform the handover or forward the handover request to a further NTN access node of the connected NTN access nodes for performing the handover from one of the connected NTN access nodes to the first NTN access node or the second NTN access node of the plurality of NTN access nodes (120a-n).

11. The NTN access node (120a-n) of claim 10, wherein the first NTN access node is a source NTN access node and the second NTN access node are a target NTN access node for the handover.

12. The NTN access node (120a-n) of claim 10, wherein the first NTN access node is one of the connected NTN access nodes and the second NTN access node are a target NTN access node for the handover.

13. The NTN access node (120a-n) of claim 10, wherein the first NTN access node is a first target NTN access node and the second NTN access node are a second target NTN access node for the handover.

14. The NTN access node (120a-n) of any one of claims 10 to 13, wherein the NTN access node (120a-n) is configured to forward a tracking area identity, TAI, list from a network control plane entity (130) of the mobile network (100) to the UE (110) and wherein the TAI list includes a plurality of NTN access node candidates the UE (110) may connect to.

15. The NTN access node (120a-n) of claim 14, wherein, for each of the plurality of NTN access node candidates of the TAI list, the TAI list is indicative of a Mobile Country Code, MCC, a Mobile Network Code, MNC, a Tracking Area Code, TAC, and / or a priority indication, wherein the priority indication is indicative of an order for selecting one or more of the plurality of NTN access node candidates from the TAI list for the handover by the UE (110).

16. The NTN access node (120a-n) of claim 15, wherein for each of the plurality of NTN access node candidates the priority indication is based on a load of the respective NTN access node candidate.

17. The NTN access node (120a-n) of any one of claims 14 to 16, wherein the network control plane entity (130) is an access and mobility management function, AMF, (130) of the mobile network (100).

18. The NTN access node ( 120a-n) of any one of claims 10 to 17, wherein the NTN access node ( 120a-n) is a master NTN access node of the connected NTN access nodes or wherein the NTN access node (120a-n) is configured to forward the handover request to a master NTN access node of the connected NTN access nodes.

19. The NTN access node (120a-n) of any one of claims 10 to 17, wherein the NTN access node (120a-n) is configured to re-assign a master node status, if the master node is the NTN access node (120a-n) to be released after handover.

20. A method (600) for operating a non-terrestrial network, NTN, access node (120a-n) for providing network access for a user equipment, UE, (110) to a mobile network (100) with a radio access network including a plurality of NTN access nodes (120a-n), wherein the method (600) comprises: receiving (601) a handover request from the UE (110) for performing a handover, wherein the UE (110) is connected to the one or more NTN access nodes of the plurality of NTN access nodes (120a-n) and wherein the handover request is indicative of at least a first NTN access node and a second NTN access node of the plurality of NTN access nodes (120a-n); and performing (603) the handover or forwarding the handover request to a further NTN access node of the connected NTN access nodes for performing the handover from one of the connected NTN access nodes to the first NTN access node or the second NTN access node of the plurality of NTN access nodes (120a-n).

21. A computer program product comprising a computer-readable storage medium for storing program code which causes a computer or a processor to perform the method (500) of claim 9 or the method (600) of claim 20, when the program code is executed by the computer or the processor.

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