Method for ntn conditional and rach-less handover

Abstract

The present disclosure describes method for Non Terrestrial Networks (NTN) conditional and RACH-less handover comprising the steps: performing UE mobility prediction and grouping UEs, whereby a determination of different suitable conditional handover configurations and / or of a sequence of multiple conditional handover candidates is done, followed by pre-allocating uplink grants.

Description

[0001] TITLE

[0002] Method for NTN conditional and RACH-less handover

[0003] TECHNNICAL FIELD

[0004] The 3rd Generation Partnership Project (3GPP) is a standards organization which develops protocols for mobile telephony and is known for the development and maintenance of various standards including second generation (2G), third generation (3G), fourth generation (4G), Long Term Evolution (LTE), and fifth generation (5G) standards.

[0005] The 5G network has been designed as a Service Based Architecture (SBA) or, in other words, a system architecture in which the system functionality is achieved by a set of network functions providing services to other authorized network functions to access their services. The 5G network may comprise a plurality of base stations (e.g., Next Generation NodeB (gNB), etc.) that serve multiple cells across a particular area.

[0006] As a User Equipment (UE) moves through the particular area cell changes, known as handovers in the connected mode, occur to maintain connectivity between the UE and the serving Radio Access Network (RAN).

[0007] Moreover, cells transmit and receive data via multiple beams. Handover procedures may be triggered as a result of rotation of the UE or as a result of obstructions between the UE and base stations (e.g., a wall, etc.).

[0008] Conditional Handover (CHO) allows a source cell to prepare a UE with multiple target cells for future handover procedures. The preparation for the CHO occurs during good radio conditions between the source cell and the UE before the handover is required to maintain communication. The UE determines that a CHO is necessary based on detection of a configured condition. Upon detection of conditions necessitating the CHO, the UE executes the prepared CHO to one of the target cells. The CHO is more likely to happen because of preparation during good radio conditions and the handover between cells is triggered during poorer radio conditions.

[0009] BACKGROUND

[0010] There are defined standard specific measurement / mobility and service continuity enhancements between terrestrial and non-terrestrial networks (NTN-TN and NTN- NTN). For NTN-NTN mobility, specific cell reselection enhancements for earth moving cell, the timing based and location-based cell reselection for quasi-earth fixed cell in Release 17 are the starting point. Specific NTN-NTN handover enhancement for radio connected (RRC_CONNECTED) UEs in quasi-earth-fixed cell and earthmoving cell are to be defined to reduce the signalling overhead. Specific cell reselection enhancements for not connected (RRCJDLE / INACTIVE) UEs to reduce UE power consumption are about to be defined and NTN-TN mobility is prioritized.

[0011] At least for pre-allocated grant, for the confirmation of RACH-less handover completion, a reuse of LTE approach is known, i.e. , the UE Contention Resolution Identity MAC CE is used, but UE ignores the content of this field. RACH-less handover is limited to NTNs at same orbit and speed. The synchronization among source and target cells is not an issue in NTN RACH-less handover. The LTE approach of confirming the handover completion is reused for both pre-allocated grant and dynamic grant.

[0012] At least the scenario of intra-satellite handover within the same gNB is possible, and the scenarios of intra-satellite HO between gNBs, inter-satellite HO within the same gNB and inter-satellite HO between gNBs may also be possible, as long as UE uplink transmission synchronization can be maintained by applying correct Timing Advance (TA) pre-com pensation in the target cell. Based on the above reply the are some approaches in supporting RACH-less handover in NTN.

[0013] US 2021282061 A1 describes methods, systems, and devices for wireless communications for management of conditional handover (CHO) configurations. A source base station may configure a user equipment (UE) with one or more CHO configurations for multiple target base stations. The CHO configurations may provide, for each target base station, one or more associated conditions that may trigger the UE to initiate a handover to the particular target base station, or to deconfigure a CHO configuration, such as based on a measurement threshold of one or more target base station measurements, one or more source base station measurements, or combinations thereof. The CHO configurations may also include failure handling information for initiating one or more subsequent handovers responsive to a failure of an initial handover attempt.

[0014] US 2022030483 A1 discloses a user equipment (UE) that performs a conditional handover procedure to transfer from a source cell to a target cell of a plurality of candidate target cells. The UE may transmit, to the source cell, a conditional handover execution message to indicate the condition handover procedure to transfer from the source cell to the target cell based at least at least in part on determining to perform the conditional handover procedure. The UE may communicate with the target cell to transfer to the target cell based at least in part on determining to perform the conditional handover procedure. Numerous other aspects are provided.

[0015] US 2022240138 A1 relates to a pre-5th-Generation (5G) or 5G communication system to be provided for supporting higher data rates Beyond 4th-Generation (4G) communication system such as Long-Term Evolution (LTE). According to embodiments, a method is performed by a user equipment (UE) in a wireless communication system, and the method comprises: receiving, from a base station, a radio resource control (RRC) reconfiguration message including conditional configuration information, wherein the conditional configuration information includes: identification information for a candidate cell; and a configuration of the candidate cell; and performing one or more operations for a conditional handover based on RRC reconfiguration message.

[0016] US 2022167220 A1 discloses a handover configuration method including: receiving target handover configuration information and handover configuration valid information, the handover configuration valid information including valid area information and / or valid timer information, the valid area information being used for indicating a valid area of at least part of the target handover configuration information, the valid timer information being used for indicating a valid duration of at least part of the target handover configuration information; keeping at least part of the target handover configuration information valid in a case that a current cell of the terminal device is located in the valid area, in a case that a current moment is within the valid duration, or in a case that a current cell of the terminal device is located in the valid area and a current moment is within the valid duration.

[0017] US 2022361055 A1 is describing a method comprising: checking if an indication indicates that a target cell is configured to store concurrently first and second configurations of a terminal, wherein the first configuration is based on a first conditional handover request, and the second configuration data set is based on a second conditional handover request; monitoring if the source cell intends performing a radio reconfiguration of the terminal after the source cell configured the terminal for the first conditional handover; canceling the first conditional handover if the source cell intends performing the radio reconfiguration and the indication does not indicate the concurrently storing; instructing the source cell to request the second conditional handover from the target cell if the source cell intends performing the radio reconfiguration and the indication indicates the concurrently storing.

[0018] US 2022264397 A1 describes a method by a wireless device includes receiving, from a network node, an indication identifying at least a first configuration identifier for modification. The first configuration identifier is associated with a conditional reconfiguration. The wireless device replaces at least a portion of the conditional reconfiguration based on the indication. The wireless devices determines that the first configuration identifier is associated with at least a first trigger condition for performing a conditional handover and stops monitoring for the first trigger condition.

[0019] US 2022078684 A1 describes a user equipment supports conditional handovers to one or more cells of a cellular network. The user equipment is configured to receive configuration data from the cellular network. The configuration data indicates for a plurality of target cells of the cellular network, conditions to trigger a conditional handover to any of the target cells. The user equipment is further configured to check the conditions with respect to appliance, and, in case of appliance, trigger the conditional handover.

[0020] US 2020154326 A1 describes a wireless transmit / receive unit (WTRU) may receive from a serving cell based on the measurement report, a conditional reconfiguration message that includes a trigger condition and configuration information for handover to a target cell based on the trigger condition. The conditional reconfiguration message may be stored and the WTRU monitors trigger quantities of the serving cell and the target cell for the trigger condition being met. When the trigger condition is met within a validity period, then the WTRU applies the received configuration information and starts the handover to the target cell.

[0021] US 2021051537 A1 discloses apparatus and methods are provided for handover robustness. The UE receives a conditional handover (CHO) command from a source gNB containing a set of candidate cells with corresponding triggering conditions; detects a handover conditions for a target gNB belonging to the set of candidate cells; and performs HO procedure towards the target gNB. In one embodiment, the conditional HO command further configures a validity timer that controls a validity of handover conditions for the set of candidate cells. The validity timer is started upon receiving the conditional HO command, the validity timer is stopped upon detecting at least one events comprising a handover condition is met and a normal handover command is received, and the conditional HO command is set to be invalid upon the expiration of the validity timer.

[0022] US 2021360495 A1 is a disclosure relates to techniques for performing conditional handovers and cell re-selections along known routes. A cellular base station may select a conditional handover set for a wireless device. The conditional handover set may include information for multiple conditional handovers. The wireless device may receive the conditional handover set. The wireless device may perform conditional handover to at least a first cell indicated in the conditional handover set based at least in part on the conditional handover set. US 2022377633 A1 describes a user equipment receives multiple conditional reconfigurations associated with respective conditions. The user equipment is to execute a conditional configuration upon fulfillment of an associated condition. The user equipment accordingly executes one of the multiple conditional configurations upon fulfillment of an associated condition. The user equipment transmits, to a target radio network node, an indication that indicates which one of the multiple conditional reconfigurations the user equipment executed.

[0023] US 2022338076 A1 provides handover method and apparatus. The handover method includes: A terminal device sends path information used to determine a first cell to a network device; and the terminal device receives conditional handover CHO configuration information that is corresponding to the first cell and that is sent by the network device, and determines a target cell based on the CHO configuration information corresponding to the first cell. According to the technical solutions provided in this application, a handover success rate can be improved when the terminal device performs cell handover.

[0024] US 2016345222 A1 describes a handover procedure is described where a user equipment, UE, is pre-configured by a network node of a source cell with one or more target cells and associated conditions to be fulfilled for the UE to autonomously decide when to execute the handover. The UE may be pre-configured while in good radio conditions, being connected to the source cell, and hence the UE does not risk going out of source cell coverage before being handed over to a target cell.

[0025] US 2019387440 A1 provides methods, systems, and storage media for exiting conditional handovers and for estimating a user equipment mobility state. Other embodiments may be described and / or claimed.

[0026] US 2021051534 A1 discloses techniques for performing consecutive conditional handovers. A cellular base station may provide conditional handover information for multiple conditional handovers to a wireless device. The conditional handover information may include information for performing a conditional handover from a cell provided by the cellular base station to at least one cell, and information for performing a conditional handover from one or more of those cells to at least one cell. The wireless device may perform multiple consecutive conditional handovers using the conditional handover information.

[0027] US 2022408323 A1 describes a wireless terminal that comprises receiver circuitry and processor circuitry. The receiver circuitry is configured to receive a message. The message comprises one or more conditional handover configurations; at least one indication; and, at least one triggering condition; each of the one or more conditional handover configurations comprising at least one identity of a candidate target cell, the at least one indication indicating whether or not the message is provided as full configuration. The processor circuitry is configured to: store the one or more conditional handover configurations and the at least one indication; perform a handover to a target cell; and determine validity of the one or more conditional handover configurations based on the indication upon or after performing the handover.

[0028] Earth-moving NTN cells have very high velocity (i.e. , UE mobility is negligible in comparison), and one satellite can operate multiple cells / beams. Depending on NTN constellation and beam footprints, a high number of UEs recurringly perform a conditional and / or RACH-less handover, which results in recurring high network signaling load and overhead and service continuity challenges.

[0029] The problem is that the source gNB sends the “handover cancel” message toward the other signaling connections or other candidate target gNBs, if any, to cancel conditional handover for the UE. Hence, all resource reservations are canceled, although UEs will be arriving after the first conditional handover to other configured target cell.

[0030] The proposed solution aims at mitigating peak load during random access procedure and further enhancing conditional handover configurations.

[0031] This objective is achieved according to the invention by means of the technical characteristics mentioned in the independent claims. Main benefit(s) of the invention are less handover messages, efficient network signalling, and overall, an improved service continuity.

[0032] BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Fig. 1 shows timing advance from gNB to UE that enables the UE to adjust its uplink transmission;

[0034] Fig. 2 shows timing advance and frequency pre-compensation;

[0035] Fig. 3 shows NTN cells broadcast;

[0036] Fig. 4 shows conditional handover;

[0037] Fig. 5 shows the scenario of random access handover for a very large number of UEs;

[0038] Fig. 6 shows conditional handover with handover cancel signaling;

[0039] Fig. 7 shows the scenario of NTN with satellites;

[0040] Fig. 8a / b shows the Earth-moving cell scenario with indication of distance threshold;

[0041] Fig. 9a / b shows Earth-moving cell scenario with conditional handover candidates;

[0042] Fig. 10 shows conditional handover configuration;

[0043] Fig. 11 shows conditional and RACH-less handover in RRC CONNECTED mode;

[0044] Fig. 12 shows conditional and RACH-less handover in admission control of the first target gNB in RRC CONNECTED mode;

[0045] Fig. 13 shows conditional and RACH-less handover with UE evaluating conditional handover conditions;

[0046] Fig. 14 shows the flowchart of the method, according to invention, on UE-side;

[0047] Fig. 15 shows the flowchart of the method according to invention, on gNB-side;

[0048] Fig. 16 shows an embodiment of the invention with indicated value range und cell identifier.

[0049] DETAILED DESCRIPTION

[0050] The detailed description set forth below, with reference to annexed drawings, is intended as a description of various configurations and is not intended to represent the only configurations in which the concepts described herein may be practiced. The detailed description includes specific details for the purpose of providing a thorough understanding of the various concepts. However, it will be apparent to those skilled in the art that these concepts may be practiced without these specific details. In particular, although terminology from 3GPP 5G NR may be used in this disclosure to exemplify embodiments herein, this should not be seen as limiting the scope of the invention.

[0051] Some of the embodiments contemplated herein will now be described more fully with reference to the accompanying drawings. Other embodiments, however, are contained within the scope of the subject matter disclosed herein, the disclosed subject matter should not be construed as limited to only the embodiments set forth herein; rather, these embodiments are provided by way of example to convey the scope of the subject matter to those skilled in the art.

[0052] Generally, all terms used herein are to be interpreted according to their ordinary meaning in the relevant technical field, unless a different meaning is clearly given and / or is implied from the context in which it is used. All references to a / an / the element, apparatus, component, means, step, etc. are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, step, etc., unless explicitly stated otherwise. The steps of any methods disclosed herein do not have to be performed in the exact order disclosed, unless a step is explicitly described as following or preceding another step and / or where it is implicit that a step must follow or precede another step. Any feature of any of the embodiments disclosed herein may be applied to any other embodiment, wherever appropriate. Likewise, any advantage of any of the embodiments may apply to any other embodiments, and vice versa. Other objectives, features and advantages of the enclosed embodiments will be apparent from the following description.

[0053] In some embodiments, a more general term “network node” may be used and may correspond to any type of radio network node or any network node, which communicates with a UE (directly or via another node) and / or with another network node. Examples of network nodes are NodeB, MeNB, ENB, a network node belonging to MCG or SCG, base station (BS), multi-standard radio (MSR) radio node such as MSR BS, eNodeB, gNodeB, network controller, radio network controller (RNC), base station controller (BSC), relay, donor node controlling relay, base transceiver station (BTS), access point (AP), transmission points, transmission nodes, RRU, RRH, nodes in distributed antenna system (DAS), core network node (e.g. Mobile Switching Center (MSC), Mobility Management Entity (MME), etc.), Operations & Maintenance (O&M), Operations Support System (OSS), Self Optimized Network (SON), positioning node (e.g. Evolved- Serving Mobile Location Centre (E-SMLC)), Minimization of Drive Tests (MDT), test equipment (physical node or software), etc.

[0054] In some embodiments, the non-limiting term user equipment (UE) or wireless device may be used and may refer to any type of wireless device communicating with a network node and / or with another UE in a cellular or mobile communication system. Examples of UE are target device, device to device (D2D) UE, machine type UE or UE capable of machine to machine (M2M) communication, PDA, PAD, Tablet, mobile terminals, smart phone, laptop embedded equipped (LEE), laptop mounted equipment (LME), USB dongles, UE category Ml, UE category M2, ProSe UE, V2V UE, V2X UE, etc.

[0055] Additionally, terminologies such as base station / gNodeB and UE should be considered non-limiting and do in particular not imply a certain hierarchical relation between the two; in general, “gNodeB” could be considered as device 1 and “UE” could be considered as device 2 and these two devices communicate with each other over some radio channel. And in the following the transmitter or receiver could be either gNodeB (gNB), or UE.

[0056] As will be appreciated by one skilled in the art, aspects of the embodiments may be embodied as a system, apparatus, method, or program product. Accordingly, embodiments may take the form of an entirely hardware embodiment, an entirely software embodiment (including firmware, resident software, micro-code, etc.) or an embodiment combining software and hardware aspects. For example, the disclosed embodiments may be implemented as a hardware circuit comprising custom very-large-scale integration (“VLSI”) circuits or gate arrays, off- the-shelf semiconductors such as logic chips, transistors, or other discrete components. The disclosed embodiments may also be implemented in programmable hardware devices such as field programmable gate arrays, programmable array logic, programmable logic devices, or the like. As another example, the disclosed embodiments may include one or more physical or logical blocks of executable code which may, for instance, be organized as an object, procedure, or function.

[0057] Furthermore, embodiments may take the form of a program product embodied in one or more computer readable storage devices storing machine readable code, computer readable code, and / or program code, referred hereafter as code. The storage devices may be tangible, non- transitory, and / or non-transmission. The storage devices may not embody signals. In a certain embodiment, the storage devices only employ signals for accessing code.

[0058] Any combination of one or more computer readable medium may be utilized. The computer readable medium may be a computer readable storage medium. The computer readable storage medium may be a storage device storing the code. The storage device may be, for example, but not limited to, an electronic, magnetic, optical, electromagnetic, infrared, holographic, micromechanical, or semiconductor system, apparatus, or device, or any suitable combination of the foregoing.

[0059] More specific examples (a non-exhaustive list) of the storage device would include the following: an electrical connection having one or more wires, a portable computer diskette, a hard disk, a random-access memory (“RAM”), a read-only memory (“ROM”), an erasable programmable read-only memory (“EPROM” or Flash memory), a portable compact disc read-only memory (“CD-ROM”), an optical storage device, a magnetic storage device, or any suitable combination of the foregoing. In the context of this document, a computer readable storage medium may be any tangible medium that can contain or store a program for use by or in connection with an instruction execution system, apparatus, or device. Code for carrying out operations for embodiments may be any number of lines and may be written in any combination of one or more programming languages including an object- oriented programming language such as Python, Ruby, Java, Smalltalk, C++, or the like, and conventional procedural programming languages, such as the “C” programming language, or the like, and / or machine languages such as assembly languages. The code may execute entirely on the user’s computer, partly on the user’s computer, as a stand-alone software package, partly on the user’s computer and partly on a remote computer or entirely on the remote computer or server. In the latter scenario, the remote computer may be connected to the user’s computer through any type of network, including a local area network (“LAN”), wireless LAN (“WLAN”), or a wide area network (“WAN”), or the connection may be made to an external computer (for example, through the Internet using an Internet Service Provider (“ISP”)).

[0060] Furthermore, the described features, structures, or characteristics of the embodiments may be combined in any suitable manner. In the following description, numerous specific details are provided, such as examples of programming, software modules, user selections, network transactions, database queries, database structures, hardware modules, hardware circuits, hardware chips, etc., to provide a thorough understanding of embodiments. One skilled in the relevant art will recognize, however, that embodiments may be practiced without one or more of the specific details, or with other methods, components, materials, and so forth. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of an embodiment. Reference throughout this specification to “one embodiment,” “an embodiment,” or similar language means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, appearances of the phrases “in one embodiment,” “in an embodiment,” and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment, but mean “one or more but not all embodiments” unless expressly specified otherwise. The terms “including,” “comprising,” “having,” and variations thereof mean “including but not limited to,” unless expressly specified otherwise. An enumerated listing of items does not imply that any or all of the items are mutually exclusive, unless expressly specified otherwise. The terms “a,” “an,” and “the” also refer to “one or more” unless expressly specified otherwise.

[0061] Aspects of the embodiments are described below with reference to schematic flowchart diagrams and / or schematic block diagrams of methods, apparatuses, systems, and program products according to embodiments. It will be understood that each block of the schematic flowchart diagrams and / or schematic block diagrams, and combinations of blocks in the schematic flowchart diagrams and / or schematic block diagrams, can be implemented by code. This code may be provided to a processor of a general-purpose computer, special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the fimctions / acts specified in the flowchart diagrams and / or block diagrams.

[0062] The code may also be stored in a storage device that can direct a computer, other programmable data processing apparatus, or other devices to function in a particular manner, such that the instructions stored in the storage device produce an article of manufacture including instructions which implement the function / act specified in the flowchart diagrams and / or block diagrams.

[0063] The code may also be loaded onto a computer, other programmable data processing apparatus, or other devices to cause a series of operational steps to be performed on the computer, other programmable apparatus, or other devices to produce a computer implemented process such that the code which execute on the computer or other programmable apparatus provide processes for implementing the functions / acts specified in the flowchart diagrams and / or block diagrams.

[0064] The flowchart diagrams and / or block diagrams in the Figures illustrate the architecture, functionality, and operation of possible implementations of apparatuses, systems, methods, and program products according to various embodiments. In this regard, each block in the flowchart diagrams and / or block diagrams may represent a module, segment, or portion of code, which includes one or more executable instructions of the code for implementing the specified logical function(s).

[0065] It should also be noted that, in some alternative implementations, the functions noted in the block may occur out of the order noted in the Figures. For example, two blocks shown in succession may, in fact, be executed substantially concurrently, or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved. Other steps and methods may be conceived that are equivalent in function, logic, or effect to one or more blocks, or portions thereof, of the illustrated Figures.

[0066] Although various arrow types and line types may be employed in the flowchart and / or block diagrams, they are understood not to limit the scope of the corresponding embodiments. Indeed, some arrows or other connectors may be used to indicate only the logical flow of the depicted embodiment. For instance, an arrow may indicate a waiting or monitoring period of unspecified duration between enumerated steps of the depicted embodiment. It will also be noted that each block of the block diagrams and / or flowchart diagrams, and combinations of blocks in the block diagrams and / or flowchart diagrams, can be implemented by special purpose hardware-based systems that perform the specified functions or acts, or combinations of special purpose hardware and code.

[0067] The description of elements in each figure may refer to elements of proceeding figures. Like numbers refer to like elements in all figures, including alternate embodiments of like elements.

[0068] The detailed description set forth below, with reference to annexed drawings, is intended as a description of various configurations and is not intended to represent the only configurations in which the concepts described herein may be practiced. The detailed description includes specific details for the purpose of providing a thorough understanding of the various concepts. However, it will be apparent to those skilled in the art that these concepts may be practiced without these specific details. Although terminology from 3GPP 5G NR may be used in this disclosure to exemplify embodiments herein, this should not be seen as limiting the scope of the invention.

[0069] Fig. 1 shows a timing advance TA mechanism from gNB to UE that enables the UE to adjust its uplink transmission; the timing advance is a special command (notification) from gNB to UE that enables the UE to adjust its uplink transmission as shown here.

[0070] This uplink adjustment applies to PUSCH, PUCCH and SRS. For non-terrestrial networks NTN, long propagation delay requires timing relationship adaptations and UE pre-com pensation - e.g., GEO with x100s ms and LEO with x10s ms round trip delays.

[0071] To accommodate the propagation delay in NTNs, several timing relationships are enhanced by a common timing advance (Common TA) and two scheduling offsets K_offset and k_mac:

[0072] Common TA is a configured offset that corresponds to the round trip time RTT between the reference point (RP) and the NTN payload;

[0073] K_offset is a configured scheduling offset that need to be larger or equal to the sum of the service link round trip time RTT and the Common TA; k_mac is a configured offset that need to be larger or equal to the round trip time RTT between the reference point RP and the gNB.

[0074] NTA: reception of the random access response or absolute timing advance command (MAC CE). Downlink DL and uplink UL are frame aligned at the uplink time synchronization reference point RP with an offset given by NTA, offset.

[0075] The offset k_mac is used to delay the application of a downlink configuration indicated by a MAC CE command on PDSCH, and estimation of UE-gNB RTT. It may be provided by the network when downlink and uplink frame timing are not aligned at gNB. The k_mac is also used in the random access procedure, to determine the start time of RAR window / MsgB window after a Msg1 / MsgA transmission. Fig. 2 shows timing advance and frequency pre-com pensation mechanism. For the serving cell, the network broadcast valid ephemeris information and Common TA parameters. The UE shall have valid GNSS position as well as ephemeris and Common TA before connecting to a NTN cell. To achieve synchronization, before and during connection to the NTN cell, the UE shall compute the round trip time between UE and the reference point based on the GNSS position, the ephemeris, and the Common TA parameters, and autonomously pre-compensate the target time to arrival TTA for the round trip time between UE and the RP. In connected mode, the UE shall be able to continuously update the timing advance and frequency precompensation. The UE may be configured to report timing advance during random access procedures or in connected mode. In connected mode, event-triggered reporting of the timing advance is supported.

[0076] Fig. 3 shows NTN cells broadcast (SIB19), comprising: ephemeris data, common TA parameters, k_offset, validity duration for uplink synchronisation information and epoch (NTN configuration data); reference location of the serving cell, e.g., used for measurement initiation in IDLE / INACTIVE mode; distance from the serving cell reference location; time information on when a cell provided via NTN quasi-Earth fixed system is going to stop serving the area it is currently covering; a list of NTN neighbour cells including their NTN configuration data, carrier frequency and identifiers of physical cells. This list includes all elements of NTN neighbored cells configurations. If cell beam points straight to ground, the cell reference location coincides with sub-satellite point.

[0077] Fig. 4 shows a standard conditional handover (CHO) mechanism. A conditional handover (CHO) is defined as a handover that is executed by the UE when one or more handover execution conditions are met. The UE starts evaluating the execution condition(s) upon receiving the CHO configuration and stops evaluating the execution condition(s) once a handover is executed. Up to 8 CHO candidate cells can be configured.

[0078] Fig. 5 shows the scenario of random access / handover for a very large number of UEs. Considering the large cell size of non-terrestrial networks, many devices may be served within a single cell. Depending on constellation assumptions (e.g., propagation delay and satellite speed) and UE density, a potentially very large number of UEs may need to perform handover at a given time, leading to possibly large signalling overhead and service continuity challenges.

[0079] Handover signalling concentration can be briefly mitigated by distributing the signaling of handover command and handover execution timing of UEs. The network can provide configuration of target cell by conditional handover configuration in advance. Furthermore, time-based conditional handover can be utilized to trigger conditional handover at intended time. For example, the network configures various entering condition of a time-based conditional event, i.e. , CondEvent T1 among UEs. In addition, the network also configures a RRM-based conditional event, e.g., CondEvent A4 to be satisfied at configured entering condition of CondEvent T1 for each UE. By combining two conditions, UE automatically performs conditional handover at configured timing leading to distributed HO execution among UEs. Consequently, conditional handover-based pre-configuration can reduce the number of handover signalling by skipping triggering indication, and alleviates RACH congestion thanks to distributed handover execution.

[0080] During a conditional handover, the ephemeris data of the satellite serving the candidate target cell, as well as the associated common timing advance parameters and K_mac, are part of the essential configuration a UE needs to initiate random access to an NTN candidate target cell. These are commonly included in the handover command (“RRCReconfiguration”). However, the conditional handover configuration may be stored a non-negligible time in the UE before the conditional handover is executed, and the validity time of the ephemeris data and common timing advance parameters may expire.

[0081] Multiple sets of ephemeris and common timing advance parameters, each with its own epoch (increasingly further into the future) and validity time can be transmitted in the conditional handover configuration message (“RRCReconfiguration”).

[0082] Fig. 6 shows a standard conditional handover with handover cancel signalling. The source gNB decides to use conditional handover and requests conditional handover for one or more candidate cells belonging to one or more candidate gNBs. A conditional handover request message is sent for each candidate cell. The candidate gNB(s) sends conditional handover response (“HO REQUEST ACKNOWLEDGE”) including configuration of conditional handover candidate cell(s) to the source gNB. The conditional handover response message is sent for each candidate cell. The source gNB sends an RRCReconfiguration message to the UE, containing the configuration of conditional handover candidate cell(s) and conditional handover execution condition(s). The UE sends an “RRCReconfigurationComplete” message to the source gNB. If early data forwarding is applied, the source gNB sends an “EARLY STATUS TRANSFER” message. The UE maintains connection with the source gNB after receiving conditional handover configuration, and starts evaluating the conditional handover execution conditions for the candidate cell(s). If at least one conditional handover candidate cell satisfies the corresponding conditional handover execution condition, the UE detaches from the source gNB, applies the stored corresponding configuration for that selected candidate cell, synchronises to that candidate cell and completes the RRC handover procedure by sending “RRCReconfigurationComplete” message to the target gNB. The UE releases stored conditional handover configurations after successful completion of RRC handover procedure. The target gNB sends the “HANDOVER SUCCESS” message to the source gNB to inform that the UE has successfully accessed the target cell. In return, the source gNB sends the “SN STATUS TRANSFER” message. The source gNB sends the “HANDOVER CANCEL” message toward the other signalling connections or other candidate target gNBs, if any, to cancel conditional handover for the UE. Fig. 7 shows a scenario of NTN with satellites serving several cells indentified by means of physical cell identifiers PCI 1 to PCI 14. Source gNB sends the “HANDOVER CANCEL” message toward other signalling connections or other candidate target gNBs (if any), to cancel conditional handover for the UE. Hence, all resource reservations will be canceled, although UEs will be arriving after the first conditional handover to other configured target cell.

[0083] When uplink grant is configured, a timer is allocated by a target cell (during admission control). The timer starts when UE receives the RRC reconfiguration message or sends the message “HANDOVER COMPLETE”. The timer stops when UE uses the uplink grant. If timer expire, UE release the uplink grant.

[0084] The other option is that, after conditional handover procedure, this is defined by further variation: if congestion is determined in any target cell, target cell indicate to source cell to release configured uplink grant resources (update conditional handover configuration data) resulting in that any grant-related validity timer is over-written and stopped. Source cell to provide indication in different ways: RRC, MAC CE, PDCCH.

[0085] Benefit is that the target cell does not reserve grant resources infinitely, but can flexibly revoke allocated grants.

[0086] This solution leverages the fact that satellite movements are deterministic and recurring. Therefore, either network or UE perform UE mobility prediction: Network performs UE mobility prediction based on consecutive UE measurement reports (e.g., RSRP values of serving and neighboring cells).

[0087] UE determines and reports distance to serving cell’s reference point (SIB19) and angle with respect to reference vector and own mobility vector (UE mobility information).

[0088] Network groups UEs and determine different suitable CHO configurations (meaning a set or a sequence of multiple CHO candidates; a pre-defined catalogue (almanac) of conditional handover configuration identifier ID, where each identifier represent suitable, location-specific sequence of conditional handover candidates), e.g., depending on NTN ephemeris, UE measurements and / or UE mobility info.

[0089] UE only discards conditional handover candidate cell, if no measurements could be obtained for the last / V times or new / updated CHO configuration is provided.

[0090] The source gNB sends the “HANDOVER CANCEL” message toward the other signalling connections or other candidate target gNBs (if any), which - based on conditional handover configuration identifier and UE mobility predictions - will not become target gNBs, to cancel conditional handover for the UE (i.e. , to free preallocated resources).

[0091] Network uses UE’s QoS requirements and mobility characteristics for determining and pre-allocating uplink grants, e.g., in a recurring / semi-persistent manner.

[0092] If network predicts significant cell load increase, target cell will indicate to source cell to release configured UL grant resources (update conditional handover configuration), and thus, over-write or stop any running grant-related validity timer.

[0093] If network predicts significant cell load increase, network may configure additional randomization in conditional handover trigger timer of corresponding candidate cell(s) (e.g., based on UE’s QoS requirements and mobility characteristics), which is signaled via an updated (delta) configuration.

[0094] Fig. 8a and 8b show the Earth-moving cell scenario with indicating the distance threshold. UE determines and reports distance to reference point and angle with respect to reference vector (pre-defined, e.g., latitudinal direction) and own mobility vector, to enable UE mobility prediction conditional handover candidate sequence: {cell #1 , cell #2, cell #3}

[0095] Fig. 9 shows an Earth-moving cell scenario with conditional handover candidates with cell #4 with conditional handover candidate sequence: {cell #1 , cell #2, cell #4} in Fig. 9a and beam or SSB-specific grant in Fig. 9b. Fig. 10 shows a conditional handover configuration. A terrestrial network is aware of non-terrestrial network constellation characteristics, where cell neighborhood relationships are pre-defined and deterministic. Thus, an almanac of possible conditional handover configuration that is based on cell neighborhood relationships and possible UE movement directions can be created. gNBs will use conditional handover configuration identifier ID for preparing or reserving multiple conditional handover candidate target cells. Each conditional handover configuration identifier ID represents a set or a sequence of / V neighboring conditional handover candidate cells, which are known by each gNB neighbor and enable more efficient signalling between gNBs.

[0096] According to invention, a conditional handover configuration identifier ID in combination with UE mobility information is used to potentially update UE-specific conditional handover configurations as well as to request or release handover or grant resources from potential target cells. Highly mobile UEs receive several conditional handover configurations with different conditional handover identifiers IDs. For example: first ring has 6 neighbors, second ring has 12 neighbors, resulting an almanac of 6x12 possible configurations for configuring two subsequent conditional handovers.

[0097] Fig. 11 shows an embodiment of the invention, namely the method of conditional and RACH-less handover in RRC CONNECTED MODE, where UE and serving gNB are in RRC_CONNECTED. UE uses SIB19 to determine own mobility vector with respect to NTN movements. UE sends a measurement report (i.e. , UE mobility vector) to the serving gNB. Serving gNB groups UEs by using QoS requirements, UE mobility information and available further meta information and determines the set or sequence of suitable conditional handover target candidates (using ephemeris data). Serving gNB defines conditional handover decision. Serving gNB sends a handover request (including QoS, mobility information, conditional handover identifier) to a first target gNB, and sends handover request(s) (including QoS, mobility information, conditional handover identifier) to other target gNBs. Fig. 12 shows another embodiment of the invention, namely conditional and RACH- less handover in admission control of the first target gNB. First target gNB and other target gNBs have admission control. First target gNB sends a handover request acknowledge message (including pre-allocated UL grants and validity timer, NTA and validity timer, cell load status) to the serving gNB. Other target gNBs send handover request(s) acknowledge (including pre-allocated UL grantsand validity timer, NTA and validity timer, cell load status) to the serving gNB. Serving gNB determines conditional handover configuration (including set of conditional handover cell or beam candidates, UL grants and validity, NTA and validity, additional back-off). Serving gNB sends conditional handover configuration (“RRCReconfiguration”) to the UE. UE send “RRCReconfigurationComplete” message back to the serving gNB.

[0098] Fig. 13 shows yet another embodiment of the invention, namely conditional and RACH-less handover with evaluation of conditional handover conditions. UE evaluates conditional handover conditions, detach from old cell, synchronize with new cell and send “RRCReconfigurationComplete” message (including conditional handover identifier) to the first target gNB. Serving gNB sends the early status transfer message to other target gNBs before UE has been sending the message “RRCReconfigurationComplete” (including conditional handover identifier) to the first target gNB. First target gNB uses the conditional handover identifier and future UE mobility information for conditional handover updated (delta) configuration and sends “HANDOVER SUCCESS” message (including conditional handover identifier) to serving gNB. Serving gNB sends the status transfer to first target gNB. Serving gNB determines (using conditional handover identifier) which handover or resource reservations to cancel. Serving gNB sends “HANDOVER CANCEL” (unfeasible target gNBs) to other target gNBs.

[0099] Fig. 14 shows the flowchart of the inventive method on the UE-side. UE receives SIB19 from serving cell, determines and reports UE mobility info (i.e., measurement report), receives conditional handover updated (delta) configuration, evaluates conditional handover condition, checks if the conditional handover conditions are met. If they are met, UE performs conditional handover and reports the used conditional handover identifier and terminates the flow. If they are not met, UE evaluates conditional handover condition.

[0100] Fig. 15 shows the flowchart of the inventive method on gNB side: gNB configures measurement reporting (including UE mobility information), receives UE measurement report (including UE mobility information), uses UE QoS requirements and / or UE mobility information for grouping UEs, uses NTN ephemeris data (of neighbor NTN cells or nodes) as well as beam characteristics to determine a set or sequence of CHO candidate cells or beams. It requests handover and allocation of uplink grants from identified neighbor NTN cells / nodes, receives handover acknowledge message including uplink grantsand validity times, NTA and validity times, and load status from neighbor NTN cells / nodes it creates and sends (delta) conditional handover configuration (including conditional handover candidates, NTA and validity times, uplink grants and validity times, and additional back-off timers (determined based on neighbor cell load), receives conditional handover complete message (including conditional handover identifier), determine which handover / resource reservations to cancel and the flow ends.

[0101] Fig. 16 shows an embodiment with indicated value range und cell identifiers. For NTN cells, high and recurring occurrence of handovers is expected. High number of UEs that try to perform conditional handover may create significant congestion in the target cell. Serving gNB contacts potential conditional handover target cell candidates for requesting or reserving handover and grant resources. Based on predefined load threshold, target cell may request to introduce additional back-off timer to delay conditional handover execution or provide a mapping to select conditional handover target cell. Mapping is pre-determined by network / target cell.

[0102] In Fig. 16 the situation is issued by Cell #7:

[0103] The invention is relevant to wireless communications of mobile or loT devices communicating via NTN.

[0104] Abbreviations:

[0105] CHO Conditional Handover

[0106] DL Downlink

[0107] PCI Physical Cell Identifier

[0108] PDCCH Physical downlink control channel

[0109] PDSCH Physical downlink shared channel

[0110] PUCCH Physical uplink control channel

[0111] PRS Positioning reference signal

[0112] RS Reference signal

[0113] RTT Round trip time

[0114] SCI Sidelink control information

[0115] SR Scheduling Request SRS Sounding reference signal

[0116] SS Synchronisation signal SSS Secondary Synchronisation signal SS-RSRP SS reference signal received power SS-RSRQ SS reference signal received quality TTA Target time to arrival UE User equipment UL Uplink

Claims

CLAIMS1. Method for Non Terrestrial Networks (NTN) conditional and RACH-less handover comprising the steps: performing UE mobility prediction and grouping UEs, whereby a determination of different suitable conditional handover configurations and / or of a sequence of multiple conditional handover candidates is done, pre-allocating uplink grants.

2. The method according to claim 1 , wherein the uplink grants is configured, a timer is allocated by a target cell.

3. The method according to claims 1 and 2, wherein the timer starts it receives handover configuration.

4. The method according to claims 1-3, wherein the timer stops when UE uses the uplink grant, and when the timer expire, resources allocated to uplink grant are released.

5. The method according to claim 1 , whereby the following steps are executed: reporting which conditional handover configuration identifier ID was used in handover complete message; providing only configuration update by a new cell, discarding only the conditional handover candidate cell, if no measurements were obtained for the last N times or a new or updated conditional handover configuration is provided; sending the handover cancellation by source gNB only towards those candidate target gNBs which, based on conditional handover configuration identifier and UE mobility predictions, are not prone to become target gNBs.

6. The method according to any previous claims, whereby the following steps are executed:RECTIFIED SHEET (RULE 91 ) ISA / EPif network predicts significant cell load increase, target cell will indicate to source cell to release configured uplink grant resources, and thus, overwrite or stop any running grant-related validity timer.

7. The method according to any previous claims, whereby the following steps are executed:If network predicts significant cell load increase, network configures additional randomizationin conditional handover trigger timer of corresponding candidate cell(s).

8. The method according to any previous claims, wherein UE and a serving gNB being in connected mode, the UE uses SIB19 to determine own mobility vector with respect to NTN movements, UE sends measurement report and UE mobility vector to the serving gNB, the serving gNB groups UEs by using QoS requirements, UE mobility and available further meta-information and determines a set or a sequence of suitable conditional handover target candidates using ephemeris data, the serving gNB defines the conditional handover decision, and the serving gNB sends the handover request including QoS, mobility information, conditional handover configuration identifier ID identifier to a first target gNB and to other target gNBs.

9. The method according to any previous claims, whereby the first and other target gNBs have admission control, the first target gNB and other target gNBs send handover request(s) acknowledge including pre-allocated uplink grants and validity timer, NTA and validity timer, cell load status to serving gNB, the serving gNB determines conditional handover configuration, including a set of conditional handover cell or beam candidates, uplink grants and validity, NTA and validity, additional back-off, the serving gNB sends conditional handover configuration (RRCReconfiguration) to the UE and UE sends conditional handover complete (RRCReconfigurationComplete) back to the serving gNB.

10. The method according to any previous claims, whereby UE evaluates conditional handover conditions, detaches from old cell, synchronize with new cell and complete handover by sending a message including the conditional handover configuration identifier CHO ID to first target gNB, the serving gNB sends an early status transferRECTIFIED SHEET (RULE 91) ISA / EPmessage to other target gNBs before UE has been completed the conditional handover, including the conditional handover identifier to the first target gNB, the first target gNB uses the conditional handover configuration identifier CHO ID and future UE mobility information for updating conditional handover configuration and announce the handover being successful, including conditional handover configuration identifier CHO ID to the serving gNB, the serving gNB sends status transfer to first target gNB, serving gNB determines using conditional handover identifier CHO ID which handover or resource reservations to cancel, the serving gNB sends handover cancellation unfeasible to other target gNBs.11 . The method according to any previous claims, wherein gNBs use conditional handover configuration identifier for preparing or reserving multiple conditional handover candidate target cells, each conditional handover configuration identifier representing a set or a sequence of / V neighboring conditional handover candidate cells, which are known by each gNB neighbor and enable more efficient signalling between gNBs.

12. The method according to any previous claims, wherein highly mobile UEs receive several conditional handover configurations with different conditional handover identifiers.

13. Apparatus for Non Terrestrial Networks (NTN) conditional and RACH-less handover, comprising a wireless transceiver, a processor coupled with a memory in which computer program instructions are stored, said instructions being configured to implement steps of the claims 1 to 10.

14. User Equipment (UE) comprising an apparatus according to claim 11 , whereby the UE receives SIB19 from serving cell, determines and reports UE mobility information by means of measurement report, receives conditional handover updated configuration, evaluates conditional handover conditions, and if they are met, performs conditional handover and reports used conditional handover configuration identifier and terminate the flow, or if they are met, UE evaluates conditional handover condition.RECTIFIED SHEET (RULE 91) ISA / EP13. Terrestrial network gNB (base station gNB, first target gNB, other target gNBs) comprising an apparatus according to claim 11 , whereby the gNB configures measurement reporting, receives UE measurement report, uses UE QoS requirements and / or UE mobility information for grouping UEs, uses NTN ephemeris data of neighbor NTN cells or nodes, as well as beam characteristics to determine a set or a sequence of conditional handover candidate cells or beams, request handover and allocation of uplink grants from identified neighbor NTN cells or nodes, receives handover acknowledge message, including uplink grants and validity times, NTA + validity times, and load status from neighbor NTN cells or nodes, creates and sends updated conditional handover including conditional handover candidates, NTAS and validity times, uplink grantsand validity times, and additional back-off timers determined based on neighbor cell load, receives conditional handover complete message including conditional handover configuration identifier, and determines which handover or resource reservations to cancel.

14. Non Terrestrial Wireless communication system, wherein the terrestrial network gNB according to claim 13 comprises a processor coupled with a memory in which computer program instructions are stored, said instructions being configured to implement steps of claims 1 to 10, wherein the user equipment (UE) according to claim 12 comprises a processor coupled with a memory in which computer program instructions are stored, said instructions being configured to implement steps of the claims 1 to 10.

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