Fallback to GEO for recovery in multi-orbit ntn

Abstract

A method includes receiving, by a user equipment (UE) being served by a source low Earth orbit (LEO) satellite, a first message from the source LEO satellite, the first message including a first configuration for conditional handover (CHO) to a geostationary Earth orbit (GEO) satellite; determining, by the UE, a time window for monitoring notification from the GEO satellite based on the first configuration; receiving, by the UE, a notification from the GEO satellite that the GEO satellite is ready for the UE to perform CHO within the time window; and performing CHO to the GEO satellite for fallback in the time window. The first configuration indicates that CHO to the GEO satellite is for fallback after a failure to perform CHO to a target LEO satellite or a failure to perform radio link failure (RLF) recovery to the source LEO satellite.

Description

FALLBACK TO GEO FOR RECOVERY IN MULTI-ORBIT NTNFIELD

[0001] Various example embodiments relate generally to wireless networking and, more particularly, to enhanced recovery from conditional handover (CHO) failure or radio link failure (RLF) for UE in 6G multi-orbit non-terrestrial networks (NTN).BACKGROUND

[0002] 6G multi-orbit NTN includes multiple access layers provided by satellites (SAT) in different orbits such as GEO and LEO and hence is also referred to as multi-layered NTN. A user equipment (UE) may be served by a source LEO satellite in a multi-orbit NTN. CHO is a handover that is executed by the UE when one or more handover execution conditions are met. The UE starts evaluating the execution conditions upon receiving the CHO configuration, and stops evaluating the execution conditions once a handover is executed.

[0003] A CHO configuration contains a configuration of CHO candidate cells generated by a candidate gNB and execution conditions generated by a source gNB. An execution condition may consist of one or more trigger conditions. Before any CHO execution condition is satisfied, upon reception of a handover (HO) command (without CHO configuration) or lower-layer triggered mobility (LTM) cell switch command MAC control element (CE), the UE executes the HO procedure or LTM cell switch procedure, regardless of any previously received CHO configuration. While executing CHO, e.g., from the time when the UE starts synchronization with the target cell, the UE does not monitor a source cell.SUMMARY

[0004] According to some aspects, there is provided the subject matter of the independent claims. Some further aspects are defined in the dependent claims.

[0005] In accordance with aspects of the disclosure, a method includes: receiving, by a user equipment (UE) being served by a source low Earth orbit (LEO) satellite, a first message from the source LEO satellite, the first message including a first configuration for conditional handover (CHO) to a geostationary Earth orbit (GEO) satellite, wherein the first configuration indicates that CHO to the GEO satellite is for fallback after a failure to perform CHO to a target LEO satelliteor a failure to perform radio link failure (RLF) recovery to the source LEO satellite; determining, by the UE, a time window for monitoring notification from the GEO satellite based on the first configuration; receiving, by the UE, a notification from the GEO satellite that the GEO satellite is ready for the UE to perform CHO within the time window; and performing CHO to the GEO satellite for fallback in the time window.

[0006] In an aspect of the method, the first message may further include a third configuration originated from the GEO satellite and the third configuration includes a configuration for the UE to monitor notification from the GEO satellite to trigger execution of CHO to the GEO satellite.

[0007] In an aspect of the method, the first configuration may include a first conditional event for CHO to the GEO satellite.

[0008] In an aspect of the method, the first configuration may include a duration of the time window and an offset value configured to enable the UE to determine a start time of the time window.

[0009] In an aspect of the method, the first conditional event may be based on receiving the notification from the GEO satellite that the GEO satellite is ready for the UE to perform CHO within the time window.

[0010] In an aspect of the method, the first conditional event may be based on a timer associated with RLF recovery at the UE.

[0011] In an aspect of the method, the notification from the GEO satellite may be configured to trigger the UE to restart, extend or stop the time window, wherein stopping the time window includes a case where CHO to the GEO satellite for fallback is not possible and the UE is released to IDLE state.

[0012] In an aspect of the method, the configuration for the UE to monitor notification from the GEO satellite may include at least one of allocated slots, occasions, resources, or cell-specific radio network temporary identifier (C-RNTI) to be used by the UE for monitoring notification from the GEO satellite.

[0013] In an aspect of the method, the method may further include receiving, by the UE, a second message from the source LEO satellite, the second message including a second configuration and a fourth configuration for CHO to the target LEO satellite wherein the second configuration is originated from the source LEO satellite; and the fourth configuration is originated from the target LEO satellite.

[0014] In an aspect of the method, the second configuration may indicate a priority of the target LEO satellite.

[0015] In an aspect of the method, the second configuration may include a second conditional event for CHO to the target LEO satellite.

[0016] In an aspect of the method, the first conditional event may be based on the second conditional event.

[0017] In accordance with aspects of the disclosure, a user equipment (UE) apparatus includes at least one processor and at least one memory storing instructions which, when executed by the at least one processor, causes the UE apparatus at least to perform any of the foregoing methods.

[0018] In accordance with aspects of the disclosure, a processor-readable medium stores instructions which, when executed by at least one processor of a UE apparatus, cause the UE apparatus at least to perform any of the foregoing methods.

[0019] In accordance with aspects of the disclosure, a method includes receiving, by a geostationary Earth orbit (GEO) satellite from a source low Earth orbit (LEO) satellite serving a user equipment (UE), a request for conditional handover (CHO) of the UE, the request indicating that CHO to the GEO satellite is for fallback after a failure to perform CHO to a target LEO satellite or a failure to perform radio link failure (RLF) recovery to the source LEO satellite; and transmitting, by the GEO satellite to the source LEO satellite upon accepting the request, a request acknowledgement, the request acknowledgement comprising a configuration to be sent to the UE via the Source LEO satellite wherein the configuration includes a configuration for the UE to monitor notification from the GEO satellite to trigger execution of CHO to the GEO satellite.

[0020] In an aspect of the method, the method may further include receiving, by the GEO satellite from the source LEO satellite, a notification indicating at least one of a detected RLF of the UE, a time window for CHO of the UE to the target LEO satellite, or a time window for CHO of the UE to the GEO satellite to facilitate notification from the GEO satellite to the UE for CHO to the GEO satellite.

[0021] In an aspect of the method, the method may further include determining, by the GEO satellite, to start an expected time window for expecting CHO of the UE to the GEO satellite based on no notification of successful CHO or RLF recovery of the UE from one of the target LEO satellite or the source LEO satellite, before a start of the expected time window.

[0022] In an aspect of the method, the method may further include stopping, by the GEO satellite, the expected time window upon receiving the notification of successful CHO or RLF recovery of the UE during the expected time window.

[0023] In an aspect of the method, the method may further include transmitting, by the GEO satellite to the UE, a notification to trigger CHO of the UE to the GEO satellite when the GEO satellite is ready for the UE within the expected time window.

[0024] In an aspect of the method, the method may further include transmitting, by the GEO satellite to the UE, a notification that a time window for CHO of the UE to the GEO satellite is to be started.

[0025] In an aspect of the method, the method may further include transmitting, by the GEO satellite to the UE, a notification wherein the notification is configured to trigger the UE to restart, extend or stop a time window for CHO of the UE to the GEO satellite, wherein stopping the time window includes a case where CHO to the GEO satellite for fallback is not possible and the UE is released to IDLE state.

[0026] In an aspect of the method, the notification may be transmitted using the configuration.

[0027] In an aspect of the method, the configuration may include at least one of allocated slots, occasions, resources, or cell-specific radio network temporary identifier (C-RNTI) to be used by the GEO satellite for transmitting notification to the UE.

[0028] In accordance with aspects of the disclosure, a geostationary Earth orbit (GEO) satellite includes at least one processor and at least one memory storing instructions which, when executed by the at least one processor, causes the GEO satellite at least to perform any of the foregoing methods.

[0029] In accordance with aspects of the disclosure, a processor-readable medium stores instructions which, when executed by at least one processor of a geostationary Earth orbit (GEO), cause the GEO satellite at least to perform any of the foregoing methods.

[0030] According to some aspects, there is provided the subject matter of the independent claims. Some further aspects are defined in the dependent claims.BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Some example embodiments will now be described with reference to the accompanying drawings.

[0032] FIG. 1 is a diagram of an example embodiment of wireless networking between a network system and a user equipment (UE), according to one illustrated aspect of the disclosure;

[0033] FIG. 2 is a diagram of example components of a network system, according to one illustrated aspect of the disclosure;

[0034] FIG. 3 is a diagram depicting an example UE being served by a source LEO satellite in a multi-orbit non-terrestrial network (NTN), according to one illustrated aspect of the disclosure;

[0035] FIG. 4 is a diagram of an example embodiment of signals and operations among a UE, a source LEO satellite, a target LEO satellite, and a GEO satellite, according to one illustrated aspect of the disclosure;

[0036] FIG. 5 is a diagram of an example embodiment of signals and operations among a UE, a source LEO satellite, a target LEO satellite, and a GEO satellite, according to one illustrated aspect of the disclosure;

[0037] FIG. 6 is a diagram of an example block diagram of a wireless station or node (e.g., network node (such as gNodeB (gNB)), a satellite, a user node or UE, relay node, or other node), according to one illustrated aspect of the disclosure.DETAILED DESCRIPTION

[0038] In the following description, certain specific details are set forth in order to provide a thorough understanding of the disclosed aspects. However, one skilled in the relevant art will recognize that aspects may be practiced without one or more of these specific details or with other methods, components, materials, etc. In other instances, well-known structures associated with transmitters, receivers, or transceivers have not been shown or described in detail to avoid unnecessarily obscuring descriptions of the aspects.

[0039] Reference throughout this specification to “one aspect” or “an aspect” means that a particular feature, structure, or characteristic described in connection with the aspect is included in at least one aspect. Thus, the appearances of the phrases “in one aspect” or “in an aspect” in various places throughout this specification are not necessarily all referring to the same aspect. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more aspects.

[0040] Embodiments described in the present disclosure may be implemented in wireless networking apparatuses, such as, without limitation, apparatuses utilizing WorldwideInteroperability for Microwave Access (WiMAX), Global System for Mobile communications (GSM, 2G), GSM EDGE radio access Network (GERAN), General Packet Radio Service (GRPS), Universal Mobile Telecommunication System (UMTS, 3G) based on basic wideband-code division multiple access (W-CDMA), high-speed packet access (HSPA), Long Term Evolution (LTE), LIE- Advanced, enhanced LTE (eLTE), 5G New Radio (5G NR), 5G Advance, 6G (and beyond) and 802.1 lax (Wi-Fi 6), among other wireless networking systems. The term ‘eLTE’ here denotes the LTE evolution that connects to a 5G core. LTE is also known as evolved UMTS terrestrial radio access (EUTRA) or as evolved UMTS terrestrial radio access network (EUTRAN).

[0041] The present disclosure may use the term “serving network device” to refer to a network node or network device (or a portion thereof) that services a UE. As used herein, the terms “transmit to,” “receive from,” and “cooperate with,” (and their variations) include communications that may or may not involve communications through one or more intermediate devices or nodes. The term “acquire” (and its variations) includes acquiring in the first instance or reacquiring after the first instance. The term “connection” may mean a physical connection or a logical connection.

[0042] The present disclosure uses 5G NR as an example of a wireless network and may use smartphones and / or extended reality headsets as an example of user equipments (UEs). It is intended and shall be understood that such examples are merely illustrative, and the present disclosure is applicable to other wireless networks and user equipment.

[0043] FIG. 1 is a diagram depicting an example of wireless networking between a network system 100 and a user equipment (UE) 150. The network system 100 may include one or more network nodes 120, one or more servers 110, and / or one or more network equipment 130 (e.g., test equipment). The network nodes 120 will be described in more detail below. As used herein, the term “network apparatus” may refer to any component of the network system 100, such as the server 110, the network node 120, the network equipment 130, any component(s) of the foregoing, and / or any other component(s) of the network system 100. Examples of network apparatuses include, without limitation, apparatuses implementing aspects of 5G NR, among others. The present disclosure describes embodiments related to 5GNR and embodiments that involve aspects defined by 3rd Generation Partnership Project (3GPP). However, it is contemplated that embodiments relating to other wireless networking technologies are encompassed within the scope of the present disclosure.

[0044] The following description provides further details of examples of network nodes. In a 5G NR network, a gNodeB (also known as gNB) may include, e.g., a node that provides new radio (NR) user plane and control plane protocol terminations towards the UE and that is connected via a NG interface to the 5G core (5GC), e.g., according to 3GPP TS 38.300 V16.6.0 (2021-6) section 3.2, which is hereby incorporated by reference herein.

[0045] A gNB supports various protocol layers, e.g., Layer 1 (LI) - physical layer, Layer 2 (L2), and Layer 3 (L3).

[0046] The layer 2 (L2) of NR is split into the following sublayers: Media Access Control (MAC), Radio Link Control (RLC), Packet Data Convergence Protocol (PDCP) and Service Data Adaptation Protocol (SDAP), where, e.g.: o The physical layer offers to the MAC sublayer transport channels; o The MAC sublayer offers to the RLC sublayer logical channels; o The RLC sublayer offers to the PDCP sublayer RLC channels; o The PDCP sublayer offers to the SDAP sublayer radio bearers; o The SDAP sublayer offers to 5GC quality of service (QoS) flows; o Control channels include broadcast control channel (BCCH) and physical control channel (PCCH).

[0047] Layer 3 (L3) includes, e.g., radio resource control (RRC), e.g., according to 3GPP TS 38.300 V16.6.0 (2021-6) section 6, which is hereby incorporated by reference herein.

[0048] A gNB central unit (gNB-CU) includes, e.g., a logical node hosting, e.g., radio resource control (RRC), service data adaptation protocol (SDAP), and packet data convergence protocol (PDCP) protocols of the gNB or RRC and PDCP protocols of the en-gNB, that controls the operation of one or more gNB distributed units (gNB-DUs). The gNB-CU terminates the Fl interface connected with the gNB-DU. A gNB-CU may also be referred to herein as a CU, a central unit, a centralized unit, or a control unit.

[0049] A gNB Distributed Unit (gNB-DU) includes, e.g., a logical node hosting, e.g., radio link control (RLC), media access control (MAC), and physical (PHY) layers of the gNB or en- gNB, and its operation is partly controlled by the gNB-CU. One gNB-DU supports one or multiple cells. One cell is supported by only one gNB-DU. The gNB-DU terminates the Fl interface connected with the gNB-CU. A gNB-DU may also be referred to herein as DU or a distributed unit.

[0050] As used herein, the term “network node” may refer to any of a gNB, a gNB-CU, or a gNB-DU, or any combination of them. A RAN (radio access network) node or network node such as, e.g., a gNB, gNB-CU, or gNB-DU, or parts thereof, may be implemented using, e.g., an apparatus with at least one processor and / or at least one memory with processor-readable instructions (“program”) configured to support and / or provision and / or process CU and / or DU related functionality and / or features, and / or at least one protocol (sub-)layer of a RAN (radio access network), e.g., layer 2 and / or layer 3. Different functional splits between the central and distributed units are possible. An example of such an apparatus and components will be described in connection with FIG. 6 below.

[0051] The gNB-CU and gNB-DU parts may, e.g., be co-located or physically separated. The gNB-DU may even be split further, e.g., into two parts, e.g., one including processing equipment and one including an antenna. A central unit (CU) may also be called baseband unit / radio equipment controller / cloud-RAN / virtual-RAN (BBU / REC / C-RAN / V-RAN), open-RAN (O- RAN), or part thereof. A distributed unit (DU) may also be called remote radio head / remote radio unit / radio equipment / radio unit (RRH / RRU / RE / RU), or part thereof. Hereinafter, in various example embodiments of the present disclosure, a network node, which supports at least one of central unit functionality or a layer 3 protocol of a radio access network, may be, e.g., a gNB-CU. Similarly, a network node, which supports at least one of distributed unit functionality or a layer 2 protocol of the radio access network, may be, e.g., a gNB-DU.

[0052] A gNB-CU may support one or multiple gNB-DUs. A gNB-DU may support one or multiple cells and, thus, could support a serving cell for a user equipment (UE) or support a candidate cell for handover, dual connectivity, and / or carrier aggregation, among other procedures.

[0053] The user equipment (UE) 150 may be or include a wireless or mobile device, an apparatus with a radio interface to interact with a RAN (radio access network), a smartphone, an in-vehicle apparatus, an loT device, or a machine-to-machine (M2M) device, among other types of user equipment. Such UE 150 may include: at least one processor; and at least one memory including program code; where the at least one memory and the computer program code are configured to, with the at least one processor, cause the apparatus at least to perform certain operations, such as, e.g., RRC connection to the RAN. An example of components of a UE will be described in connection with FIG. 6. In embodiments, the UE 150 may be configured to generate a message (e.g., including a cell ID) to be transmitted via radio towards a RAN (e.g., to reach andcommunicate with a serving cell). In embodiments, the UE 150 may generate and transmit and receive RRC messages containing one or more RRC PDUs (packet data units). Persons skilled in the art will understand RRC protocol as well as other procedures a UE may perform.

[0054] With continuing reference to FIG. 1, in the example of a 5G NR network, the network system 100 provides one or more cells, which define a coverage area of the network system 100. As described above, the network system 100 may include a gNB of a 5G NR network or may include any other apparatus configured to control radio communication and manage radio resources within a cell. As used herein, the term “resource” may refer to radio resources, such as a resource block (RB), a physical resource block (PRB), a radio frame, a subframe, a time slot, a sub-band, a frequency region, a sub-carrier, a beam, etc. In embodiments, the network node 120 may be called a base station.

[0055] FIG. 1 provides an example and is merely illustrative of a network system 100 and a UE 150. Persons skilled in the art will understand that the network system 100 includes components not illustrated in FIG. 1 and will understand that other user equipment may be in communication with the network system 100.

[0056] FIG. 2 is a block diagram of example components of the network system 100 of FIG. 1. A 5G NR network may be described as an example of the network system 100, and it is intended that aspects of the following description shall be applicable to other types of network systems, as well. The network system may operate in accordance with the signals and connections shown in FIG. 1 such that the UE 150 is in communication with the network system 100 through the radio access network 225. Additionally, the network system may be divided into user plane components and functions and control plane components and functions, as shown and described herein. Unless indicated otherwise, the terms “component,” “function,” and “service” may be used interchangeably herein, and they may refer to and be implemented by instructions executed by one or more processors.

[0057] Example functions of the components are described below. The example functions are merely illustrative, and it shall be understood that additional operations and functions may be performed by the components described herein. Additionally, the connections between components may be virtual connections over service-based interfaces such that any component may communicate with any other component. In this manner, any component may act as a service“producer,” for any other component that is a service “consumer,” to provide services for network functions.

[0058] For example, a core network 210 is described in the control plane of the network system. The core network 210 may include an authentication server function (AUSF) 211, an access and mobility function (AMF) 212, and a session management function (SMF) 213. The core network 210 may also include a network slice selection function (NSSF) 214, a network exposure function (NEF) 215, a network repository function (NRF) 216, and a unified data management function (UDM) 217, which may include a uniform data repository (UDR) 224.

[0059] Additional components and functions of the core network 210 may include an application function 218, policy control function (PCF) 219, network data analytics function (NWDAF) 220, analytics data repository function (ADRF) 221, management data analytics function (MDAF) 222, and operations and management function (0AM) 223.

[0060] The user plane includes the UE 150, a radio access network (RAN) 225, a user plane function (UPF) 226, and a data network (DN) 227. The RAN 225 may include one or more components described in connection with FIG. 1, such as one or more network nodes. However, the RAN 225 may not be limited to such components. The UPF 226 provides connection for data being transmitted over the RAN 225. The DN 226 identifies services from service providers, Internet access, and third-party services, for example.

[0061] The AMF 212 processes connection and mobility tasks. The AUSF 211 receives authentication requests from the AMF 212 and interacts with UDM 217 to authenticate and validate network responses to determine successful authentication. The SMF 213 conducts packet data unit (PDU) session management and manages session context with the UPF 226.

[0062] The NSSF 214 may select a network slicing instance (NSI) and determine the allowed network slice selection assistance information (NSSAI). This selection and determination are utilized to set the AMF 212 to provide service to the UE 150. The NEF 215 secures access to network services for third parties to create specialized network services. The NRF 216 acts as a repository to store network functions to allow the functions to register with and discover each other.

[0063] The UDM 217 generates authentication vectors for use by the AUSF 211 and ADM 212 and provides user identification handling. The UDM 217 may be connected to the UDR 224 which stores data associated with authentication, applications, or the like. The AF 218 providesapplication services to a user (e.g., streaming services, etc.). The PCF 219 provides policy control functionality. For example, the PCF 219 may assist in network slicing and mobility management, as well as provide quality of service (QoS) and charging functionality.

[0064] The NWDAF 220 collects data (e.g., from the UE 150 and the network system) to perform network analytics and provide insight to functions that utilize the analytics in the providing of services. The ADRF 221 allows the storage, retrieval, and removal of data and analytics by consumers. The MDAF 222 provides additional data analytics services for network functions. The 0AM 223 provides provisioning and management processing functions to manage elements in or connected to the network (e.g., UE 150, network nodes, etc.).

[0065] FIG. 2 is merely an example of components of a network system, and variations are contemplated to be within the scope of the present disclosure. In embodiments, the network system may include other components not illustrated in FIG. 2. In embodiments, the network system may not include every component illustrated in FIG. 2. In embodiments, the components and connections may be implemented with different connections than those illustrated in FIG. 2. Such and other embodiments are contemplated to be within the scope of the present disclosure.

[0066] FIG. 3 is a diagram depicting an example UE 150 being served by a source LEO satellite 302 in a multi-orbit non-terrestrial network (NTN), such as network system 100, according to one illustrated aspect of the disclosure. The UE 150 is capable of communicating with a serving network for ongoing service via either a LEO satellite 302, 304 or a GEO satellite 306. The use of a LEO layer may be more preferable and / or have higher priority for the UE 150, e.g., for enhanced link budget and / or energy efficiency, as compared to the use of a GEO layer. It is to be noted that LEO satellite 302, 304 or GEO satellite 306 represents a gNB or a radio access node of a multiorbit NTN in the present disclosure.

[0067] In various embodiments, the UE 150 may expect frequent handover (HO) and / or radio link failure (RLF) while being served by LEO satellites 302, 304, as illustrated in FIG 3. Handovers between LEO satellites 302, 304 may be more challenging than handovers between terrestrial network nodes, as radio conditions may be impacted by factors such as long distances, varying Doppler frequencies, and / or shifting delays. Availability of a GEO satellite 306 for the UE 150 is generally more stable for a longer period of time than a LEO satellite 302, 304, thus, the GEO satellite 306 may be used to enhance CHO and / or RLF recovery on a LEO layer for the UE 150.

[0068] The UE 150 may be selected among a group of active UEs being served by the network system 100. In various embodiments, the network system 100 is configured to provide service continuity and / or minimized service interruption for a targeted active UE such as a high-altitude UAV or airplane UE, which may suffer from frequent coverage issues, as illustrated in FIG. 3. However, the targeted active UE may be of any supported UE types or services.

[0069] In various embodiments, the GEO satellite 306 is configured to provide fallback and / or assistance to the UE in case of RLF. The network system 100 may provide fallback to the UE 150 for RLF recovery with CHO of the UE 150 to the GEO satellite 306 when the UE 150 is capable of communicating with the GEO satellite 306 in both UL and DL. Moreover, the GEO satellite 306 may provide assistance to the UE for CHO to the target LEO satellite 304 for RLF recovery when the UE is not capable of transmitting to the GEO satellite 306 in UL but can receive from the GEO satellite 306 in DL. Thus, using the GEO satellite 306 for providing fallback and / or assistance to the UE 150 provides the benefits of enhanced radio connection mobility management and / or RLF recovery for the UE 150 (e.g., when the UE fails to execute a CHO to the target LEO satellite 304 and / or when the UE fails to identify RLF and / or fails to perform RLF recovery to either the source LEO satellite 302 or the target LEO satellite 304 within a configured time window) while prioritizing the use of LEO layer for serving the UE 150.

[0070] FIG. 4 is a diagram of an example embodiment of signals and operations among a UE, a source LEO satellite, a target LEO satellite, and a GEO satellite, according to one illustrated aspect of the disclosure. In various embodiments, FIG. 4 shows an example method according to one illustrated aspect of the disclosure. In various embodiments, the components depicted in FIG. 4 may correspond to similar components described above in FIGS. 1-3. It will be understood that a described signal may have associated operations, and a described operation may have associated signals. Further details relating to the signals and operations shown in FIG. 4 are described herein below.

[0071] The source LEO satellite, based on UE capabilities and QoS policies associated with the UE and the UE’s services, may determine whether to configure the UE with CHO to the GEO satellite for fallback in case of RLF. It is noted that in FIG. 4, the UE is implied to be configured with CHO to the GEO satellite for fallback, such that the UE is capable of communicating with the GEO SAT in both UL and DL.

[0072] A first configuration for CHO to the GEO satellite that is signaled from the source LEO satellite to the UE indicates to the UE that CHO to the GEO satellite is for fallback in case of RLF, also referred to as the fallback (otherwise, CHO to the GEO satellite is considered as a legacy CHO). A first conditional event for CHO to the GEO satellite, such as the starting time tl GEO of a time window [tl GEO, t2_GEO] during which the UE may perform CHO to the GEO satellite (also known as condEventTl in TS 38.331), is associated with and / or dependent on a second conditional event, a timer associated with RLF recovery at the UE, and / or a notification from the GEO satellite to the UE. It is noted that the second conditional event is for CHO to a selected target LEO SAT from the target LEO satellite(s) (e.g., one or more target LEO satellites), of which a second configuration may be signaled from the source LEO satellite to the UE before, after, and / or together with the first configuration.

[0073] The first conditional event for CHO to the GEO satellite, e.g., how the UE may trigger CHO to the GEO satellite for the fallback in various embodiments, is described below. Different options are provided, considering that direct Xn interface between the GEO satellite and LEO satellite may or may not be available.

[0074] In various embodiment, the UE initiates CHO to the GEO satellite for fallback, which may address a case when Xn interface between the GEO satellite and the LEO satellite is not available (e.g., operations 407a to 408a).

[0075] In aspects, the first configuration may include a duration of the time window and an offset value configured to enable the UE to determine a start time of the time window for the first conditional event. For example, the offset may be zero, and in this case the offset may not need to be included in the first configuration.

[0076] In case CHO to the selected target LEO satellite is configured to the UE with condEventTl (the second conditional event) given by a time window of [tl LEO, t2_LEO], the time window [tl GEO, t2_GEO] (the first conditional event) for CHO to the GEO satellite is associated with and / or dependent on the time window [tl LEO, t2_LEO] for CHO to the selected target LEO satellite as follows: [tl GEO, t2_GEO] is an extension of [tl LEO, t2_LEO] such that tl_GEO=t2_LEO+offset; or [tl GEO, t2_GEO] is overlapping with the last portion of [tl LEO, t2_LEO] such that tl_GEO=t2_LEO-duration_GEO+offset, and the UE may perform CHO to the selected target LEO satellite until tl GEO-offset, for example, and if that fails then the UE may perform CHO to the GEO satellite within [tl GEO, t2_GEO], The first configuration may includethe duration of [tl GEO, t2_GEO] and the offset value to allow the UE to determine the first conditional event, e.g., condEventTl of [tl GEO, t2_GEO] in the above examples. This is preferable, considering that the first configuration for CHO to the GEO satellite may be provided to the UE before the second configuration for CHO to at least one target LEO satellite or there may be more than one target LEO satellite(s) and the UE needs to select the target LEO satellite to perform CHO.

[0077] In case CHO to the selected target LEO satellite is configured to the UE with condEventDl or condEventD2 to trigger the UE to perform CHO to the selected target LEO satellite when the distance between the UE and the source LEO satellite dl>Dl or the distance between the UE and the selected target LEO satellite d2<D2, tl LEO, in this case, starts when condEventDl or condEventD2 is met and based on that the first conditional event or, that is, the time window [tl GEO, t2_GEO] for CHO to the GEO satellite may be determined as in the above case. In case the UE is experiencing a RLF towards the serving source LEO satellite before CHO to a target LEO satellite may be triggered or, that is, the configured second conditional event is not met in this case, the first conditional event or, that is, the time window [tl GEO, t2_GEO] for CHO to the GEO satellite may be determined based on using the same scheme as above where [tl LEO, t2_LEO] is replaced with the running RLF recovery timer. It is noted that in this case the UE may or may not have received the second configuration for CHO to the LEO SAT(s).

[0078] In various embodiments, the GEO satellite initiates CHO for more network-controlled fallback, which may address a case when the Xn interface between GEO SAT and LEO SAT is available (e.g., operations 407b to 412).

[0079] The UE is configured to monitor a notification from the GEO satellite for CHO to the GEO satellite during a specified time window [tl*_GEO, t2*_GEO], which can be determined by the UE in a similar way as for [tl GEO, t2_GEO] above. The notification may trigger the UE to perform CHO to the GEO satellite. Thus, if the notification from the GEO satellite is not received, no CHO to the GEO satellite is performed. The notification may trigger the UE to extend and / or stop the corresponding time window [tl*_GEO, t2*_GEO], If the UE is triggered to stop the corresponding time window, fallback to the GEO SAT is generally not possible, and the UE is released to IDLE. The notification may be monitored in configured or allocated slots and / or occasions and / or resources during [tl*_GEO, t2*_GEO] using a dedicated cell radio network temporary identifier (C-RNTI), as indicated in a third configuration originated from the GEOsatellite to the UE, which is sent to the UE via the source LEO satellite. The third configuration is sent to the UE by the source LEO satellite together with the first configuration. It is noted that while executing CHO, the UE may not need to monitor the source cell. Thus, during [tl*_GEO, t2*_GEO], the UE may not need to monitor the source LEO satellite, and therefore, no additional gaps need to be specified or configured for the UE to monitor notification from the GEO satellite.

[0080] In various embodiments, coordination between LEO satellite and GEO satellite may be used for facilitating or enhancing the GEO satellite initiated CHO of the UE or the UE initiated CHO to the GEO satellite for the fallback. The source LEO satellite may inform and / or notify the GEO satellite about timing information related to condEventTl configured for CHO to the GEO satellite, such as expected [tl GEO, t2_GEO], This allows the GEO satellite to prepare and / or determine to trigger the UE to perform CHO to the GEO satellite efficiently using the notification. This step may be triggered based on, e.g., condEventTl configured for CHO to the target LEO satellite(s) or upon detecting RLF of the UE. The selected target LEO satellite may inform and / or notify the GEO satellite about a successful CHO of the UE, as requested by the source LEO satellite, which enables the GEO satellite to determine early release of the third configuration of the UE and thus avoid wasting resources to notify the UE for possible CHO to the GEO satellite for the fallback.

[0081] It is noted that after a successful fallback to the GEO satellite, the UE may be handed back to the LEO layer by the GEO satellite when the UE is in good coverage of a LEO satellite in the LEO layer. This HO of the UE from the GEO satellite to the LEO satellite can be performed using legacy procedures.

[0082] Referring to FIG. 4, at operation 400, a UE is in RRC CONNECTED state to a source LEO satellite or, that is, the UE is currently active and being served by the source LEO satellite. The UE is assumed to transmit a UE capability indication to the source LEO satellite as part of operation 400. Specifically, the UE transmits a message to a source LEO satellite, indicating that the UE is capable of communicating with the GEO satellite in both UL and DL and is authorized to communicate with the GEO satellite. The UE may transmit (e.g., indicate and / or report) an ID of the GEO satellite along with the indication of GEO access capability.

[0083] At operation 401, the source LEO satellite determines a need for CHO to the GEO satellite as a fallback option for the UE in case of RLF. The operation 401 determination may be based on UE capabilities and / or a need to provide enhanced service continuity and / or minimizedservice interruption for the UE. The source LEO satellite (e.g., network implementation) may decide when and / or how to perform operation 401. However, as RLF may happen to the UE at any time, operation 401 may be performed in advance for the UE.

[0084] At operation 402a, the source LEO satellite transmits a HO Request to the GEO satellite indicating that the HO Request is for fallback. For example, the source LEO satellite may transmit a HO request including a cause indication and the cause may be set to “recovery” implying the fallback.

[0085] At operation 403 a, the GEO satellite, upon accepting the HO Request, responds to the Source LEO satellite with a HO Request Ack including a third configuration to be sent to the UE via the Source LEO satellite. The third configuration may include configuration for the UE to monitor a notification from the GEO satellite to trigger CHO execution, for example, for use with operation 407b.

[0086] In various embodiments, the Source LEO satellite may initiate a regular HO Request to the GEO satellite, for example, a request without indicating the fallback or recovery cause. The GEO satellite may accept the regular HO Request but only for the fallback and therefore indicating as such in the HO Request Ack when responding to the Source LEO satellite.

[0087] At operation 404a, the Source LEO satellite determines a first configuration including a first conditional event (e.g., condEventTl of [tl GEO, t2_GEO]) for CHO of the UE to the GEO satellite. The first configuration is associated with the third configuration and the first conditional event is associated with and / or dependent on a second conditional event configured for CHO of the UE to a selected target LEO satellite (provided in operation 404b and operation 405b).

[0088] At operation 405a, the source LEO satellite reconfigures the UE with the first and third configurations for CHO to the GEO satellite for the fallback.

[0089] At operation 406a, the UE, upon accepting the reconfiguration, responds to the source LEO satellite with a reconfiguration complete message.

[0090] Operations 402b to 406b are for preparation of CHO to at least one target LEO satellite as prioritized option for the UE.

[0091] At operation 402b, the source LEO satellite transmits a HO Request to the target LEO satellite including an identification (ID) of the GEO satellite, which indicates that the GEO satellite is used for the fallback. The source LEO satellite may further include a UE ID in the HO request, which can be used with or without the ID of the source GEO satellite to identify the UE betweenthe target satellite LEO(s) and the GEO satellite. For example, the existing C-RNTI of the UE assigned by the source LEO satellite coupled with an associated cell ID and / or satellite ID of the source LEO satellite may be used for identifying the UE between the GEO satellite and the target LEO satellite(s). Thus, a selected target LEO satellite among the target LEO satellite(s) may be able to notify the GEO satellite of a successful CHO of the UE to the selected target LEO satellite.

[0092] The outcome of operations 402b to 406b is that the UE is reconfigured by the source LEO satellite with the second and fourth configurations for CHO to each of the target LEO satellite(s). The fourth configuration is from a corresponding target LEO satellite among the target LEO satellite(s) and the second configuration, which is associated with the fourth configuration, is from the source LEO satellite. It should be noted that operations 402b-406b may happen before, after, and / or in parallel with operations 402a-406a, though after operation 406a may be more likely to happen.

[0093] In various embodiments, the second configuration may indicate a priority of each target LEO satellite(s). This can be considered by the UE to select a target LEO satellite among the LEO satellite(s) to perform CHO. It is noted that other factors can be considered for the selection, for example, a timing of each target LEO satellite’s availability and / or a radio condition such as reference signal received power (RSRP) towards each target LEO satellite.

[0094] Operations 407a to 408a relate to a UE initiated CHO to the GEO satellite.

[0095] At operation 407a, the UE determines the time window (e.g., from tl GEO to t2_GEO) for the first conditional event based on the first and second configurations, as described above.

[0096] At operation 408a, the UE fails to perform CHO to a selected target LEO satellite among the at least one target LEO satellite prior to the start or the end of the time window, i.e., tl GEO or t2_GEO, and therefore performs CHO to the GEO satellite for the fallback, as described above.

[0097] Operations 407b to 412 relate to GEO satellite-initiated CHO to the GEO satellite.

[0098] At operation 407b, the UE determines the time window [tl*_GEO, t2*_GEO] for monitoring notification from the GEO satellite based on the first configuration or in case of RLF before CHO to a target LEO satellite can be performed or after a failure to perform CHO to the selected target LEO satellite.

[0099] At operation 408b, after determining an expected time window [tl GEO, t2_GEO] based on at least one second configuration for CHO to at least one target LEO satellite or timingof detected RLF, the source LEO satellite transmits a notification to the GEO satellite of the expected time window [tl GEO, t2_GEO] to facilitate notification from the GEO satellite to the UE for CHO to the GEO satellite in Step 10. It is noted that the Source LEO satellite may not know which target LEO satellite among the at least one target LEO satellite the UE selects to perform CHO to, and hence the expected time window [tl GEO, t2_GEO] may be based on, e.g., the latest [tl LEO, t2_LEO] of the at least one target LEO satellite. CondEventTl or condEventDl may be assumed for CHO to the at least one target LEO satellite to enable the Source LEO satellite to determine the expected time window [tl GEO, t2_GEO], It is noted that the expected time window [tl GEO, t2_GEO] is not necessarily the same as time window [tl*_GEO, t2*_GEO] determined at the UE but at least overlapping as much as possible. For example, the GEO satellite receives from the source LEO satellite, a notification indicating at least one of a detected RLF of the UE, a time window for CHO of the UE to the target LEO satellite, or a time window for CHO of the UE to the GEO satellite to facilitate notification from the GEO satellite to the UE for CHO to the GEO satellite.

[0100] At operation 409, the GEO satellite determines to start the expected time window [tl GEO, t2_GEO] for expecting CHO of the UE to the GEO satellite based on that no notification of successful CHO or RLF recovery of the UE from one of the at least one target LEO satellites or the Source LEO satellite before the expected tl GEO. The GEO satellite may stop the expected time window upon receiving the notification of successful CHO or RLF recovery of the UE during the expected time window.

[0101] At operation 410, the GEO satellite may transmit a notification to the UE for CHO to the GEO satellite when the GEO satellite is ready for the UE within [tl GEO, t2_GEO], The notification may be transmitted and received on preconfigured or allocated slots, occasions, resources, or C-RNTI indicated in the third configuration, as proposed above.

[0102] At operations 411 and 412, upon receiving the notification from the GEO satellite, the UE starts a time window [tl GEO, t2_GEO] and performs CHO to the GEO satellite for the fallback within its time window [tl GEO, t2_GEO],

[0103] At operation 411, the GEO satellite transmits to the UE, a notification that a time window for CHO of the UE to the GEO satellite is to be started, causing the UE to start a corresponding time window and performs CHO to the GEO satellite for fallback within the corresponding time window.

[0104] In various embodiments, there may be coordination between the source LEO satellite, the selected target LEO satellite, and the GEO satellite. For example, the GEO satellite may be able to reserve actual resources for the UE later or release the reserved resources and UE contexts earlier based upon receiving a corresponding notification from the source LEO satellite or selected target LEO satellite.

[0105] The operations of FIG. 4 are merely illustrative, and variations are contemplated to be within the scope of the present disclosure. In embodiments, the operations may include other operations not illustrated in FIG. 4. In embodiments, the operations may not include every operation illustrated in FIG. 4. In embodiments, the operations may be implemented in a different order than that illustrated in FIG. 4. Such and other embodiments are contemplated to be within the scope of the present disclosure. Persons of skill in the art will appreciate that, although various example components are described as performing various functions, other components may perform those functions described in FIG. 4.

[0106] FIG. 5 is a diagram of an example embodiment of signals and operations among a UE, a source LEO satellite, a candidate LEO satellite, and a GEO satellite. In various embodiments, FIG. 5 shows an example method for enhancing service continuity or minimizing service interruption for targeted service or service class of targeted UE or UE class in multi-orbit nonterrestrial networks (NTN) environment wherein LEO or, in general, NGSO layer is used primarily or, that is, with higher priority for providing better link budget and energy efficiency for the UE, compared to GEO or, in general, GSO layer, according to one illustrated aspect of the disclosure. In various embodiments, the components depicted in FIG. 5 may correspond to similar components described above in FIGS. 1 and 2. It will be understood that a described signal may have associated operations, and a described operation may have associated signals.

[0107] FIG. 5 describes the case where the UE is not configured with conditional handover (CHO) to a geostationary Earth orbit (GEO) satellite for fallback but configured to listen to assistance from the GEO satellite for radio link failure (RLF) recovery using CHO to a target LEO satellite. This case is applicable when the UE is not capable of communicating with the GEO satellite in both UL and DL but only in DL or when the UE is not able to get access to the GEO for its network connection. Figure 3 provides an illustration of some examples for this case.

[0108] At operation 500, the UE is in an RRC CONNECTED state to the source low Earth orbit (LEO) satellite. The UE transmits a UE capability indication to the source LEO satelliteindicating that the UE is not capable of communicating with GEO satellite in both UL and DL but only in DL and authorized to communicate with the GEO satellite in DL, for example. The UE may indicate / report the identification (ID) of the GEO satellite along with its indication of GEO access capability.

[0109] At operations 501, 502a, and 503a are now for assistance from the GEO satellite for the UE in case of RLF, instead of CHO to the GEO satellite in FIG. 4. The third configuration provided in Step 3a to the UE is now for only monitoring assistance from the GEO satellite. The assistance request procedure may be implemented as a new procedure or based on an existing procedure such as a handover (HO) request procedure. At operation 502a, the source LEO satellite transmits to the GEO satellite an assistance request message. At operation 503a, the GEO satellite transmits to the source LEO satellite an assistance request acknowledgment message. The assistance request acknowledgment message includes the third configuration.

[0110] The source LEO satellite may determine to request the GEO satellite to provide assistance, instead of CHO, for the UE in case of RLF even when the UE is capable of and authorized for accessing the GEO satellite in both UL and DL. In an embodiment, the source LEO satellite may request HO to the GEO satellite for the UE in case of RLF, but the GEO satellite may not accept the HO request but accept to provide assistance instead.

[0111] At operation 504a, the first configuration is now for configuring the UE to listen to the GEO satellite for assistance in case of RLF using the third configuration, as compared to FIG. 4.

[0112] At operations 505a and 506a, the source LEO satellite reconfigures the UE for monitoring assistance from the GEO satellite in case of RLF.

[0113] At operation 505a, the source LEO satellite transmits to the UE an RRC reconfiguration message, including the first and the third configurations.

[0114] At operation 506a, the UE transmits to the source LEO satellite an RRC reconfiguration complete message.

[0115] Operations 502b-506b are similar to operations 502b-506b of FIG. 4.

[0116] At operations 507a and 507b, RLF is detected at the source LEO satellite and at the UE before CHO to a target LEO satellite may be performed. Detection of RLF in the source LEO satellite and UE may not happen simultaneously. It may happen that the source LEO satellite detects the RLF while UE has not detected the RLF yet. In this case, the notification from the GEOsatellite for assistance may reduce the amount of time it takes for the UE to perform the RLF recovery.

[0117] At operation 508, RLF recovery may be performed between the UE and the Source LEO satellite. The assistance may be applied when this RLF recovery fails or is not performed by the UE due to, for example, the UE did not yet detect RLF.

[0118] At operation 509a, the UE starts a time window (e.g., from tl*_GEO to t2*_GEO) to monitor assistance from the GEO satellite.

[0119] At operation 509b, the source LEO satellite transmits a notification to the GEO satellite of the UE’s RLF to trigger assistance from the GEO satellite for the UE.

[0120] At operation 510, the GEO satellite starts a time window to provide assistance for the UE. The assistance may be a notification to trigger the UE to start a time window to perform CHO to a selected target LEO satellite among the at least one target LEO satellite configured for the UE. In embodiments, the notification may indicate a time duration during which CHO to a selected target LEO satellite may be performed by the UE. The time duration may start at the UE upon reception of the notification. In embodiments, the notification may indicate the selected target LEO satellite for the UE. In embodiments, the notification may trigger a restart or an extension of the time window to perform CHO to a selected target LEO satellite for the UE. In embodiments, the notification may stop the time window and release the UE to IDLE. In embodiments, the notification may configure the UE to perform CHO to a target LEO satellite. For example, the source LEO satellite, after detecting RLF of the UE, may request CHO for the UE to the target LEO satellite if the source LEO satellite has not prepared and configured CHO for the UE before. The third configuration from the target LEO satellite to the UE via the source LEO satellite and the first configuration from the source LEO satellite in this case is delivered to the UE from the source LEO satellite via the GEO satellite in form of the notification.

[0121] At operation 511, the GEO satellite transmits the notification to the UE, according to the third configuration.

[0122] At operations 512 and 513, the UE starts the time window and performs CHO to the selected LEO satellite within the time window based on the notification received from the GEO satellite.

[0123] It is noted that the notification received from the GEO satellite may overwrite conditional event or priority of CHO associated with each of the at least one target LEO satellite that has been configured to the UE before RLF by the source LEO satellite.

[0124] The operations of FIG. 5 are merely illustrative, and variations are contemplated to be within the scope of the present disclosure. In embodiments, the operations may include other operations not illustrated in FIG. 5. In embodiments, the operations may not include every operation illustrated in FIG. 5. In embodiments, the operations may be implemented in a different order than that illustrated in FIG. 5. Such and other embodiments are contemplated to be within the scope of the present disclosure. Persons of skill in the art will appreciate that, although various example components are described as performing various functions, other components may perform those functions described in FIG. 5.

[0125] The following describes operations from the perspective of a UE. From such a perspective, a method may include: receiving, by a user equipment (UE) being served by a source low Earth orbit (LEO) satellite, a first message from the source LEO satellite, the first message including a first configuration and a third configuration for conditional handover (CHO) to a geostationary Earth orbit (GEO) satellite; determining, by the UE, a time window for monitoring notification from the GEO satellite based on the first configuration; receiving, by the UE, a notification from the GEO satellite that the GEO satellite is ready for the UE to perform CHO within the time window; and performing CHO to the GEO satellite for fallback in the time window. The first configuration may originate from the source LEO satellite. The third configuration may originate from the GEO satellite. The first configuration may indicate that CHO to the GEO satellite is for fallback after a failure to perform CHO to a target LEO satellite or a failure to perform radio link failure (RLF) recovery to the source LEO satellite. The third configuration may include a configuration for the UE to monitor notification from the GEO satellite to trigger execution of CHO to the GEO satellite.

[0126] The following describes operations from the perspective of a geostationary Earth orbit (GEO) satellite. From such as perspective, a method may include: receiving, by a GEO satellite from a source low Earth orbit (LEO) satellite serving a user equipment (UE), a request for conditional handover (CHO) of the UE, the request indicating that CHO to the GEO satellite is for fallback after a failure to perform CHO to a target LEO satellite or a failure to perform radio link failure (RLF) recovery to the source LEO satellite; and transmitting, by the GEO satellite to thesource LEO satellite upon accepting the request, a request acknowledgement, the request acknowledgement comprising a configuration to be sent to the UE via the Source LEO satellite. The configuration may include a configuration for the UE to monitor notification from the GEO satellite to trigger execution of CHO to the GEO satellite.

[0127] FIG. 6 is a block diagram of a wireless station or node (e.g., UE, user device, AP, BS, eNB, gNB, RAN node, network node, TRP, or other node) 600, according to one illustrated aspect of the present disclosure. The wireless station 400 may include, for example, one or more (e.g., two as shown in FIG. 6) RF (radio frequency) or wireless transceivers 602A, 602B, where each wireless transceiver includes a transmitter to transmit signals and a receiver to receive signals. The wireless station also includes a processor or control unit / entity (controller) 604 to execute instructions or software and control transmission and receptions of signals, and a memory 606 to store data and / or instructions.

[0128] Processor 604 may also make decisions or determinations, generate frames, packets or messages for transmission, decode received frames or messages for further processing, and other tasks or functions described herein. Processor 604, which may be a baseband processor, for example, may generate messages, packets, frames, or other signals for transmission via wireless transceiver 602 (602A or 602B). Processor 604 may control transmission of signals or messages over a wireless network and may control the reception of signals or messages, etc., via a wireless network (e.g., after being down-converted by wireless transceiver 602, for example). Processor 604 may be programmable and capable of executing software or other instructions stored in memory or on other computer media to perform the various tasks and functions described above, such as one or more of the tasks or methods described above. Processor 604 may be (or may include), for example, hardware, programmable logic, a programmable processor that executes software or firmware, and / or any combination of these. Using other terminology, processor 604 and transceiver 602 together may be considered as a wireless transmitter / receiver system, for example.

[0129] In addition, referring to FIG. 6, a controller (or processor) 608 may execute software and instructions, and may provide overall control for the station 600, and may provide control for other systems not shown in FIG. 6, such as controlling input / output devices (e.g., display, keypad), and / or may execute software for one or more applications that may be provided on wireless station 600, such as, for example, an email program, audio / video applications, a word processor, a Voiceover IP application, or other application or software.

[0130] In addition, a storage medium may be provided that includes stored instructions, which when executed by a controller or processor may result in the processor 604, or other controller or processor, performing one or more of the functions or tasks described above.

[0131] According to another example embodiment, RF or wireless transceiver(s) 602A / 602B may receive signals or data and / or transmit or send signals or data. Processor 604 (and possibly transceivers 602A / 602B) may control the RF or wireless transceiver 602A or 602B to receive, send, broadcast or transmit signals or data.

[0132] Example embodiments are provided or described for each of the example methods, including: An apparatus (e.g., 600, FIG. 6) including means (e.g., processor 604, RF transceivers 602A and / or 602B, and / or memory 606, in FIG. 6) for carrying out any of the methods; a non- transitory computer-readable storage medium (e.g., memory 606, FIG. 6) comprising instructions stored thereon that, when executed by at least one processor (processor 604, FIG. 6), are configured to cause a computing system (e.g., 600, FIG. 6) to perform any of the example methods; and an apparatus (e.g., 600, FIG. 6) including at least one processor (e.g., processor 604, FIG. 6), and at least one memory (e.g., memory 606, FIG. 6) including computer program code, the at least one memory (606) and the computer program code configured to, with the at least one processor (604), cause the apparatus (e.g., 600) at least to perform any of the example methods.

[0133] Further embodiments of the present disclosure include the following examples.

[0134] Example 1.1. A user equipment (UE), comprising: means for receiving, by a user equipment (UE) being served by a source low Earth orbit (LEO) satellite, a first message from the source LEO satellite, the first message including a first configuration for conditional handover (CHO) to a geostationary Earth orbit (GEO) satellite, wherein the first configuration indicates that CHO to the GEO satellite is for fallback after a failure to perform CHO to a target LEO satellite or a failure to perform radio link failure (RLF) recovery to the source LEO satellite; means for determining, by the UE, a time window for monitoring notification from the GEO satellite based on the first configuration; means for receiving, by the UE, a notification from the GEO satellite that the GEO satellite is ready for the UE to perform CHO within the time window; and means for performing CHO to the GEO satellite for fallback in the time window.

[0135] Example 1.2. The UE of Example 1.1, wherein the first message further includes a third configuration originated from the GEO satellite and the third configuration includes a configuration for the UE to monitor notification from the GEO satellite to trigger execution of CHO to the GEO satellite.

[0136] Example 1.3. The UE of Example 1.1, wherein the first configuration includes a first conditional event for CHO to the GEO satellite.

[0137] Example 1.4. The UE of Example 1.3, wherein the first configuration includes a duration of the time window and an offset value configured to enable the UE to determine a start time of the time window.

[0138] Example 1.5. The UE of Example 1.4, wherein the first conditional event is based on receiving the notification from the GEO satellite that the GEO satellite is ready for the UE to perform CHO within the time window.

[0139] Example 1.6. The UE of Example 1.3, wherein the first conditional event is based on a timer associated with RLF recovery at the UE.

[0140] Example 1.7. The UE of Example 1.1, wherein the notification from the GEO satellite is configured to trigger the UE to restart, extend or stop the time window, wherein stopping the time window includes a case where CHO to the GEO satellite for fallback is not possible and the UE is released to IDLE state.

[0141] Example 1.8. The UE of Example 1.1, wherein the notification from the GEO satellite is configured to trigger the UE to restart, extend or stop the time window, wherein stopping the time window includes a case where CHO to the GEO satellite for fallback is not possible and the UE is released to IDLE state.

[0142] Example 1.9. The UE of Example 1.1, further comprising: means for receiving, by the UE, a second message from the source LEO satellite, the second message including a second configuration and a fourth configuration for CHO to the target LEO satellite wherein the second configuration is originated from the source LEO satellite; and the fourth configuration is originated from the target LEO satellite.

[0143] Example 1.10. The UE of Example 1.9, wherein the second configuration indicates a priority of the target LEO satellite.

[0144] Example l.il. The UE of Example 1.9, wherein the second configuration includes a second conditional event for CHO to the target LEO satellite.

[0145] Example 1.12. The UE of Example 1.3 or Example 1.11, wherein the first conditional event is based on the second conditional event.

[0146] Example 1.15. A geostationary Earth orbit (GEO) satellite, comprising: means for receiving, by a geostationary Earth orbit (GEO) satellite from a source low Earth orbit (LEO) satellite serving a user equipment (UE), a request for conditional handover (CHO) of the UE, the request indicating that CHO to the GEO satellite is for fallback after a failure to perform CHO to a target LEO satellite or a failure to perform radio link failure (RLF) recovery to the source LEO satellite; and means for transmitting, by the GEO satellite to the source LEO satellite upon accepting the request, a request acknowledgement, the request acknowledgement comprising a configuration to be sent to the UE via the Source LEO satellite wherein the configuration includes a configuration for the UE to monitor notification from the GEO satellite to trigger execution of CHO to the GEO satellite.

[0147] Example 1.16. The GEO satellite of Example 1.15, further comprising: means for receiving, by the GEO satellite from the source LEO satellite, a notification indicating at least one of a detected RLF of the UE, a time window for CHO of the UE to the target LEO satellite, or a time window for CHO of the UE to the GEO satellite to facilitate notification from the GEO satellite to the UE for CHO to the GEO satellite.

[0148] Example 1.17. The GEO satellite of Example 1.15, further comprising: means for determining, by the GEO satellite, to start an expected time window for expecting CHO of the UE to the GEO satellite based on no notification of successful CHO or RLF recovery of the UE from one of the target LEO satellite or the source LEO satellite, before a start of the expected time window.

[0149] Example 1.18. The GEO satellite of Example 1.17, further comprising: means for stopping, by the GEO satellite, the expected time window upon receiving the notification of successful CHO or RLF recovery of the UE during the expected time window.

[0150] Example 1.19. The GEO satellite of Example 1.17, further comprising: means for transmitting, by the GEO satellite to the UE, a notification to trigger CHO of the UE to the GEO satellite when the GEO satellite is ready for the UE within the expected time window.

[0151] Example 1.20. The GEO satellite of Example 1.19, further comprising: means for transmitting, by the GEO satellite to the UE, a notification that a time window for CHO of the UE to the GEO satellite is to be started.

[0152] Example 1.21. The UE of Example 1.17, further comprising: means for transmitting, by the GEO satellite to the UE, a notification wherein the notification is configured to trigger the UE to restart, extend or stop a time window for CHO of the UE to the GEO satellite, wherein stopping the time window includes a case where CHO to the GEO satellite for fallback is not possible and the UE is released to IDLE state.

[0153] Example 1.22. The GEO satellite of any of Example 1.15 or Examples 1.19-1.21 wherein the notification is transmitted using the configuration.

[0154] Example 1.23. The GEO satellite of Example 1.21, wherein the configuration includes at least one of allocated slots, occasions, resources, or cell-specific radio network temporary identifier (C-RNTI) to be used by the GEO satellite for transmitting notification to the UE.

[0155] The embodiments and aspects disclosed herein are examples of the present disclosure and may be embodied in various forms. For instance, although certain embodiments herein are described as separate embodiments, each of the embodiments herein may be combined with one or more of the other embodiments herein. Specific structural and functional details disclosed herein are not to be interpreted as limiting, but as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present disclosure in virtually any appropriately detailed structure. Like reference numerals may refer to similar or identical elements throughout the description of the figures.

[0156] The phrases “in an aspect,” “in aspects,” “in various aspects,” “in some aspects,” or “in other aspects” may each refer to one or more of the same or different aspects in accordance with this present disclosure. The phrase “a plurality of’ may refer to two or more.

[0157] In various embodiments, the terms “first message” and “second message,” as well as any subsequent messages may refer to any messages that are transmitted or received in an order and are not necessarily limited to any particular message.

[0158] The phrases “in an embodiment,” “in embodiments,” “in various embodiments,” “in some embodiments,” or “in other embodiments” may each refer to one or more of the same or different embodiments in accordance with the present disclosure. A phrase in the form “A or B”means “(A), (B), or (A and B).” A phrase in the form “at least one of A, B, or C” means “(A); (B); (C); (A and B); (A and C); (B and C); or (A, B, and C) ”

[0159] Any of the herein described methods, programs, algorithms or codes may be converted to, or expressed in, a programming language or computer program. The terms “programming language” and “computer program,” as used herein, each include any language used to specify instructions to a computer, and include (but is not limited to) the following languages and their derivatives: Assembler, Basic, Batch files, BCPL, C, C+, C++, Delphi, Fortran, Java, JavaScript, machine code, operating system command languages, Pascal, Perl, PL1, Python, scripting languages, Visual Basic, metalanguages which themselves specify programs, and all first, second, third, fourth, fifth, or further generation computer languages. Also included are database and other data schemas, and any other meta- languages. No distinction is made between languages which are interpreted, compiled, or use both compiled and interpreted approaches. No distinction is made between compiled and source versions of a program. Thus, reference to a program, where the programming language could exist in more than one state (such as source, compiled, object, or linked) is a reference to any and all such states. Reference to a program may encompass the actual instructions and / or the intent of those instructions.

[0160] While aspects of the present disclosure have been shown in the drawings, it is not intended that the present disclosure be limited thereto, as it is intended that the present disclosure be as broad in scope as the art will allow and that the specification be read likewise. Therefore, the above description should not be construed as limiting, but merely as exemplifications of particular aspects. Those skilled in the art will envision other modifications within the scope and spirit of the claims appended hereto.

Claims

29WHAT IS CLAIMED IS:

1. A method, comprising: receiving, by a user equipment (UE) being served by a source low Earth orbit (LEO) satellite, a first message from the source LEO satellite, the first message including a first configuration for conditional handover (CHO) to a geostationary Earth orbit (GEO) satellite, wherein the first configuration indicates that CHO to the GEO satellite is for fallback after a failure to perform CHO to a target LEO satellite or a failure to perform radio link failure (RLF) recovery to the source LEO satellite; determining, by the UE, a time window for monitoring notification from the GEO satellite based on the first configuration; receiving, by the UE, a notification from the GEO satellite that the GEO satellite is ready for the UE to perform CHO within the time window; and performing CHO to the GEO satellite for fallback in the time window.

2. The method of claim 1 , wherein the first message further includes a third configuration originated from the GEO satellite and the third configuration includes a configuration for the UE to monitor notification from the GEO satellite to trigger execution of CHO to the GEO satellite.

3. The method of claim 1, wherein the first configuration includes a first conditional event for CHO to the GEO satellite.

4. The method of claim 3, wherein the first configuration includes a duration of the time window and an offset value configured to enable the UE to determine a start time of the time window.

5. The method of claim 4, wherein the first conditional event is based on receiving the notification from the GEO satellite that the GEO satellite is ready for the UE to perform CHO within the time window.

306. The method of claim 3, wherein the first conditional event is based on a timer associated with RLF recovery at the UE.

7. The method of claim 1, wherein the notification from the GEO satellite is configured to trigger the UE to restart, extend or stop the time window, wherein stopping the time window includes a case where CHO to the GEO satellite for fallback is not possible and the UE is released to IDLE state.

8. The method of claim 2, wherein the configuration for the UE to monitor notification from the GEO satellite includes at least one of allocated slots, occasions, resources, or cell-specific radio network temporary identifier (C-RNTI).

9. The method of claim 1, wherein the method further comprises: receiving, by the UE, a second message from the source LEO satellite, the second message including a second configuration and a fourth configuration for CHO to the target LEO satellite wherein: the second configuration is originated from the source LEO satellite; and the fourth configuration is originated from the target LEO satellite.

10. The method of claim 9, wherein the second configuration indicates a priority of the target LEO satellite.

11. The method of claim 9, wherein the second configuration includes a second conditional event for CHO to the target LEO satellite.

12. The method of one of claim 3 or claim 11, wherein the first conditional event is based on the second conditional event.

13. A user equipment (UE), comprising: at least one processor; andat least one memory storing instructions which, when executed by the at least one processor, cause the UE at least to perform a method as in any one of claims 1 to 12.

14. A processor-readable medium storing instructions which, when executed by at least one processor of a user equipment (UE), cause the UE at least to perform a method as in any one of claims 1 to 12.

15. A method, comprising: receiving, by a geostationary Earth orbit (GEO) satellite from a source low Earth orbit (LEO) satellite serving a user equipment (UE), a request for conditional handover (CHO) of the UE, the request indicating that CHO to the GEO satellite is for fallback after a failure to perform CHO to a target LEO satellite or a failure to perform radio link failure (RLF) recovery to the source LEO satellite; and transmitting, by the GEO satellite to the source LEO satellite upon accepting the request, a request acknowledgement, the request acknowledgement comprising a configuration to be sent to the UE via the Source LEO satellite wherein the configuration includes a configuration for the UE to monitor notification from the GEO satellite to trigger execution of CHO to the GEO satellite.

16. The method of claim 15, further comprising: receiving, by the GEO satellite from the source LEO satellite, a notification indicating at least one of a detected RLF of the UE, a time window for CHO of the UE to the target LEO satellite, or a time window for CHO of the UE to the GEO satellite to facilitate notification from the GEO satellite to the UE.

17. The method of claim 16, further comprising: determining, by the GEO satellite, to start an expected time window for expecting CHO of the UE to the GEO satellite based on no notification of successful CHO or RLF recovery of the UE from one of the target LEO satellite or the source LEO satellite, before a start of the expected time window.

18. The method of claim 17, further comprising: stopping, by the GEO satellite, the expected time window upon receiving the notification of successful CHO or RLF recovery of the UE during the expected time window.

19. The method of claim 17, further comprising: transmitting, by the GEO satellite to the UE, a notification to trigger CHO of the UE to the GEO satellite when the GEO satellite is ready for the UE within the expected time window.

20. The method of claim 19, further comprising: transmitting, by the GEO satellite to the UE, a notification that a time window for CHO of the UE to the GEO satellite is to be started.

21. The method of claim 17, further comprising: transmitting, by the GEO satellite to the UE, a notification wherein the notification is configured to trigger the UE to restart, extend or stop a time window for CHO of the UE to the GEO satellite, wherein stopping the time window includes a case where CHO to the GEO satellite for fallback is not possible and the UE is released to IDLE state.

22. The method of one of claim 15 or claims 19-21, wherein the notification is transmitted using the configuration.

23. The method of claim 22 wherein the configuration includes at least one of allocated slots, occasions, resources, or cell-specific radio network temporary identifier (C-RNH) to be used by the GEO satellite for transmitting notification to the UE.

24. A geostationary Earth orbit (GEO) satellite, comprising: at least one processor; and at least one memory storing instructions which, when executed by the at least one processor, cause the GEO satellite at least to perform a method as in any one of claims 15 to 23.3325. A processor-readable medium storing instructions which, when executed by at least one processor of a geostationary Earth orbit (GEO) satellite, cause the GEO satellite at least to perform a method as in any one of claims 15 to 23.

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