Method for enhanced RACH-less handover

Abstract

The present invention is related to a method for enhanced RACH-less handover, performed by an UE in wireless communication with a first base station and a second base station. The method (100) comprises the following steps: (101) receiving, by the UE, an uplink timing advance (UL TA) of the second base station, and (102) providing, by the first base station, response to the UE, including an updated timing advance, wherein the uplink timing advance is either determined by the first base station, the second base station, a combination of both first and second base stations, or determined and reported by UE to the first base station.

Description

[0001] 202404028

[0002] 1

[0003] Description

[0004] Method for enhanced RACH-less handover

[0005] The present invention relates to a method and system for enhanced RACH-less handover, generally related to heterogenous (NTN-TN) mobile communications.

[0006] TECHNNICAL FIELD

[0007] The evolution of telecommunication technologies, the ever-increasing demand for new services, and the exponential growth of smart devices fuel the development of Non-Terrestrial Network (NTN) systems as an effective solution to complement terrestrial networks in providing services over uncovered or under-served geographical areas.

[0008] A terrestrial network (5G, for example) comprises a plurality of base stations (e.g., Next Generation NodeB, gNB) that serve multiple cells across a particular area. As a User Equipment (UE) moves through a particular area, cell changes occur to maintain connectivity between the UE and the serving Radio Access Network (RAN). Moreover, cells transmit and receive data via multiple beams.

[0009] As defined by the 3rd Generation Partnership Project (3GPP), an NTN is a network where spaceborne (i.e. , GEO, MEO, LEO) or airborne (i.e., UAS and HAPS) vehicles act either as a relay node or as a base station, thus distinguishing transparent and regenerative satellite architectures.

[0010] BACKGROUND

[0011] The integration of different RANs (i.e., satellite and terrestrial) to achieve global connectivity poses several challenges due to the heterogeneity in access technologies, network architectures, and protocols. 202404028

[0012] 2

[0013] There are defined standard specific measurement / mobility and service continuity enhancements between terrestrial and non-terrestrial networks. For NTN-NTN mobility, specific cell reselection enhancements for earth moving cell, the timingbased and location-based cell reselection for quasi-earth fixed cell in Release 17 are the starting point.

[0014] One of the key issues is service continuity enhancement and, hence, seamless handover procedures. Handover may belong to intra- or inter-system types. The former may occur either between the beams generated by the same satellite (i.e. , intra-satellite handover) or between two satellites (i.e., inter-satellite handover). The latter may occur between the satellite radio access network and the terrestrial system and vice versa (i.e., vertical handover).

[0015] In order to reduce outage, timing and synchronization are crucial. The 3rd Generation Partnership Project (3GPP) specifications require to adjust the timing of an uplink frame in order to have alignment with a downlink frame in time scale; the uplink timing control procedure is based on a timing advance (TTA) provided by a base station (i.e., gNB) to UE. The uplink frame is transmitted by UE towards gNB whereas downlink frame is transmitted by gNB towards UE, and downlink and uplink are frame aligned at the uplink time synchronization reference point. Timing advance is the offset between the start of the received downlink frame and the transmitted uplink frame, and is a special command from gNB to UE that enables the UE to adjust its uplink transmission.

[0016] In this context, random access channel-less (RACH-less) handovers can be used to reduce the additional time required to complete the RACH procedure when the timing advance must be determined as part of the handover procedure.

[0017] Extensive non-patent and patent literature address outage reduction in the context of random access channel-less handover procedure.

[0018] [1] „RACH-less Handover with Early Timing Advance Acquisition for Outage Reduction" by Subhyal Bin Iqbal et al., published in 15 March 2024, 202404028

[0019] 3

[0020] DOI:10.48550 / arXiv.2403.10286, describes a RACH-less handover signaling scheme for the 3rd Generation Partnership Project (3GPP) conditional handover (CHO) mechanism. The scheme exploits the decoupling between the CHO preparation and execution phases to establish initial synchronization between the UE and the target cell through an early acquisition of the timing advance.

[0021] US 2020267609 A1 [2] discloses systems, methods and apparatus for determining a target base station timing advance value between a user equipment (UE) and target base station in connection with a random access channel (RACH)-less handover procedure. In one aspect, the UE may determine the target base station's timing advance based at least in part on a timing difference between the target base station and the source base station and based at least in part on a timing offset associated with an uplink and a downlink of the UE. In some aspects, the target base station may determine the timing advance using an uplink reference signal of the UE. In some aspects, the UE may report a timing difference between the target base station and the source base station, and the source cell may determine a timing advance based at least in part on the timing difference.

[0022] US 2022015131 A1 [3] relates to a random access channel (RACH)-less procedure. According to [3], a base station determines to transmit a pre-allocated uplink (UL) grant to a user equipment (UE), wherein the pre-allocated UL grant includes: a secondary node (SN) transmission configuration indicator (TCI) list, and a type 1 configured grant (CG). The base station transmits, to the UE, the preallocated UL grant based at least in part on determining to transmit the preallocated UL grant.

[0023] US 2019174372 A1 [4] describes RACH-less handovers used to reduce the additional time required to complete the RACH procedure when the Timing Advance (TA) must be determined as part of the handover procedure. In order to facilitate a RACH-less handover, the source base station informs the target base station of a radio transmission configuration that will be used by the UE device to transmit uplink signals to the source base station, and the source base station configures the UE device in accordance with the radio transmission configuration. 202404028

[0024] 4

[0025] The UE device transmits uplink signals to the source base station using the specified radio transmission configuration, and the target base station receives the uplink signals. The UE device transmits these uplink signals to the source base station before the handover procedure is initiated. The target base station calculates the TA, based on the uplink signals, which can be signaled to the UE device in a handover command.

[0026] US 2021345211 A1 [5] presents a technique of performing a handover procedure that involves providing a proper timing adjustment indication for the UE performing RACH-less HO. The timing adjustment provided in performing a HO in an IAB network involves computing a timing advance (TA) to be used by an IAB node prior to executing HO to an alternative serving node. Such a use of a TA may enable a bypassing of a RACH procedure when accessing a cell.

[0027] US 2016381611 A1 [6] describes techniques for implementing proper RACH-less handover control. One aspect of the present invention relates to a base station including a communication control unit configured to control radio communication with user equipment, a capable cell information storage unit configured to store capable cell information indicating a RACH-less handover capable cell and a RACH-less handover determination unit configured to determine whether the user equipment is RACH-less handover capable.

[0028] US 2023262492 A1 [7] refers to a method and apparatus for RACH-less activation in a wireless communication system is provided. The wireless device deactivates a Secondary Cell Group (SCG). The wireless device detects a beam failure of a Primary SCell (PSCell) in the SCG. The wireless device initiates a SCG failure information procedure to report the SCG failure. The wireless device skips a Media Access Control (MAC) reset procedure, wherein the MAC reset procedure includes stopping a Time Alignment Timer (TAT) for the SCG; transmitting SCG failure information. The wireless device determines that a random access procedure is not needed for activation of the SCG, based on the TAT being not expired. 202404028

[0029] 5

[0030] US 2024048227 A1 [8] describes methods and apparatuses for a timing advance report an NTN. The method of UE comprises: receiving, from a BS, first information including satellite assistance information for a target satellite; receiving, from the BS, second information including timing information, wherein the timing information includes at least one of a start time and a time duration for a synchronization operation; determining whether a RACH-less synchronization operation is configured; and initiating, based on the satellite assistance information and the timing information, the synchronization operation by performing a RA procedure upon a determination that the RACH-less synchronization operation is not configured.

[0031] US 2024022972 A1 [9] relates to a handover command in non-terrestrial networks. According to [9], the method includes receiving handover information associated with an execution of a handover and performing at least one operation of the handover based on the handover information. The handover information includes at least one of: a time offset; information for generating a time offset; an absolute time for executing the handover or for accessing the target cell; a condition to be fulfilled before execution of the handover; information associated with ephemeris data of a satellite serving the target cell; information associated with an upcoming service link switch or feeder link switch; information associated with a timing advance; a physical random access channel resource; an indication to execute a preconfigured handover command; an identifier associated with a preconfigured handover command; an indication for disabling fallback to a source cell; and a parameter for determining a quality of the source cell for fallback to the source cell.

[0032] WO 2023192692 A2

[0010] describes a method and an apparatus for inter-cell crosstrip seamless mobility. According to

[0010] , a UE receives, from a source cell, a mobility pre-configuration radio resource control (RRC) message for randomaccess channel (RACH)-less access. The mobility pre-configuration RRC message indicates target timing advance (TA) assistance information. The UE receives, from the source cell, a lower layer target access command. The lower layer target access command indicates a most updated or latest network-updated time sensitive dynamic TA assistance information. The UE determines a target cell 202404028

[0033] 6

[0034] TA of a target cell based on a latest source cell TA of the source cell, a latest measured timing difference between reference signals (RSs) from the source cell and from the target cell, the most updated or latest network-updated time sensitive dynamic TA assistance information, and the target TA assistance information. The UE performs the RACH-less mobility access to the target cell based on the target cell TA.

[0035] WO 2024076379 A1

[0011] relates to realizing random access channel layer-less (RACH-less) layer 1 / layer 2 triggered mobility (LTM). Realizing RACH-less LTM includes receiving, at at least one LTM target cell of a base station, information indicative of whether handover (HO) of service for a user equipment (UE) currently being served by a serving DU of the base station can be a random access channel-less (RACH-less) HO to the at least one target cell or can be a RACH- based HO to the at least one target cell, and preparing the at least one LTM target cell, based on the information received at the at least one LTM target cell, for one of RACH-less HO and RACH-based HO.

[0036] WO 2023111619 A1

[0012] describes an apparatus and a method of wireless communication. The method includes receiving by a user equipment, UE, from a serving cell, a handover command for a handover procedure for the UE switching from the serving cell to a target cell, transmitting, to the target cell, an uplink transmission according to the handover command, and receiving, from the target cell, a message, wherein the handover command comprises a command for a random access channel-less (RACH-less) handover for the UE switching from the serving cell to the target cell.

[0037] However, it has been identified the issue that, if the timing of the UE performing its first uplink transmission (during RACH-less handover) is unclear for the network, this may lead to waste of resources (uplink grant), as the second base station or cell may try to schedule the UE unsuccessfully before it has switched.

[0038] Therefore, it is an object of the present invention to apply correct timing advance pre-compensation in the target cell, and to avoid cancelling all resource reservations. 202404028

[0039] 7

[0040] This objective is achieved according to the invention by means of the technical characteristics mentioned in the independent claims.

[0041] The invention relies on clearly defining method steps of determining the uplink timing advance either by a first base station, second base station, a combination of first and second base stations, or determined by UE and reported to the first base station.

[0042] By applying these steps, the invention aims to increase the timing advance adjustment accuracy, thereby increasing the success rate of RACH-less handover.

[0043] The dependent claims include advantageous further developments and improvements of the present invention.

[0044] According to a first aspect of the invention, there is provided a method for enhanced RACH-less handover, performed by an UE in wireless communication with a first base station and a second base station. The method comprises the following steps: receiving, by the UE, an uplink timing advance (UL TA) of second base station, and providing, by the first base station, response to the UE, including an updated timing advance, wherein the uplink timing advance is either determined by first base station, second base station, a combination of source and second base stations, or determined and reported by UE to the first base station.

[0045] The main benefit of the invention is that the reservations of uplink grant resources are done more efficiently.

[0046] In a first embodiment, when the UE receives the uplink timing advance of second base station determined by first base station, the uplink timing advance is based on: last NTAvalue or history of UE’s uplink signaling to first base station; first base station’s service time t-Service, meaning the time when the first base station stops serving a certain area; second base station’s ephemeris data, including epoch 202404028

[0047] 8 time; second base station’s t-ServiceStart time, meaning the time when second base station starts serving a certain area. Moreover, the first base station provides a UE (determined target) the uplink timing advance in handover request message to second base station.

[0048] In a second embodiment, when the UE receives the uplink timing advance of second base station determined by second base station, the uplink timing advance is based on: last NTAvalue or history of UE’s UL signaling to first base station, which is forwarded by first base station; first base station’s ephemeris data; t-Service, meaning the time when first base station stops serving a certain area, and movingReferenceLocation message of first base station, which is forwarded by first base station; second base station’s ephemeris data, including epoch time; second base station’s t-ServiceStart time, meaning the time when second base station starts serving a certain area. Moreover, the second base station considers the mentioned data for determining target uplink timing advance of UE and configuring beam-specific UL grant resources for RACH-less handover. Further on, the second base station indicates uplink timing advance of UE (target NTA) and uplink grant to first base station.

[0049] In a third embodiment, when the UE receives the uplink timing advance of second base station determined by both source and second base stations, the uplink timing advance of second base station is based on: last NTAvalue or history of UE’s uplink signaling to first base station, which is forwarded by first base station; first base station’s ephemeris data; t-Service, meaning the time when first base station stops serving a certain area, and movingReferenceLocation message of first base station, which is forwarded by first base station; target NTAproposal by first base station, second base station’s ephemeris data including epoch time, second base station’s t-ServiceStart time, meaning the time when second base station starts serving a certain area. Furthermore, the second base station considers the mentioned data and either confirms or updates the proposed target NTAby first base station. In addition, the second base station uses the confirmed or updated target NTAfor determining target UL timing advance of UE and 202404028

[0050] 9 configuring beam-specific UL grant resources for RACH-less handover. The second base station indicates the uplink timing advance of UE (target NTA) and beam-specific UL grant to first base station.

[0051] In a fourth embodiment, when the UE receives the uplink timing advance of second base station based on information determined and reported by UE to first base station, the first base station configures the UE with triggering conditions to report its predicted target NTA value for configured RACH-less candidate cells, the predicted target NTAis reported as part of measurement report. In an alternative fourth embodiment, UE applies a trained model for target NTAestimation and considers the following parameters: last NTAvalue or history of UE’s uplink signaling to first base station, previous or history of target NTAvalues that were applied in similar situations, based on stored and recorded satellite ephemeris data, geolocation data, UE movement data.

[0052] According to a second aspect of the invention, there is provided an apparatus for enhanced 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 method claims.

[0053] According to a third aspect of the invention, there is provided an user equipment for enhanced 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 method claims.

[0054] According to a fourth aspect of the invention, there is provided a base station comprising an apparatus for enhanced RACH-less handover.

[0055] According to a fourth aspect of the invention, there is provided a wireless communication system comprising a base station with a processor coupled with a memory in which computer program instructions are stored, said instructions being configured to implement steps of method claims, in communication with at least one UE configured to implement steps of the method claims. 202404028

[0056] 10

[0057] For a better understanding of the principle of the present invention, embodiments of the invention will be explained in more detail below with reference to the figures. Like reference numerals are used in the figures for the same or equivalent elements and are not necessarily described again for each figure. It is to be understood that the invention is not limited to the illustrated embodiments and that the features described may also be combined or modified without departing from the scope of the invention as defined in the appended claims.

[0058] Figures

[0059] Fig. 1 shows the timing advance adjustment mechanism, as illustrated by 3GPP TS38.211 standard specification,

[0060] Fig. 2 shows a schematic diagram of a standard handover procedure, including the step of uplink synchronization,

[0061] Fig. 3 presents an illustration of handover in a non-geostationary network scenario,

[0062] Fig. 4 presents an illustration of handover in a multi-orbit scenario, with a comparison of a geo-stationary vs. a non-geostationary network, Fig. 5 presents a sketch of a method for enhanced RACH-less handover, according to invention,

[0063] Figures 6a-b illustrate a first embodiment of the method, wherein an uplink timing advance is determined by first base station,

[0064] Fig. 7 illustrates the UE behavior when performing the first embodiment of the method,

[0065] Fig. 8 illustrates the first base station behavior when performing the first embodiment of the method,

[0066] Fig. 9 illustrates the second base station behavior when performing the first embodiment of the method,

[0067] Figures 10a-b show a second embodiment of the method, wherein an uplink timing advance is determined by second base station,

[0068] Fig. 11 illustrates the UE behavior when performing the second embodiment of the method, 202404028

[0069] 11

[0070] Fig. 12 illustrates the first base station behavior when performing the second embodiment of the method,

[0071] Fig. 13 illustrates the second base station behavior when performing the second embodiment of the method,

[0072] Figures 14a-b show a third embodiment of the method, wherein an uplink timing advance is determined by both first and second base stations,

[0073] Fig. 15 illustrates the UE behavior when performing the third embodiment of the method,

[0074] Fig. 16 illustrates the first base station behavior when performing the third embodiment of the method,

[0075] Fig. 17 illustrates the second base station behavior when performing the third embodiment of the method,

[0076] Figures 18a-c show a fourth embodiment of the method, wherein the uplink timing advance of second base station is based on information determined by UE and reported to first base station,

[0077] Fig. 19 illustrates the UE behavior when performing the fourth embodiment of the method,

[0078] Fig. 20 illustrates schematically a series of parameters used by the inventive method in the fourth embodiment.

[0079] Fig. 21 illustrates the first base station behavior when performing the fourth embodiment of the method,

[0080] Fig. 22 illustrates the second base station behavior when performing the fourth embodiment of the method,

[0081] Fig. 23 illustrate a variant of the fourth embodiment of the method,

[0082] Fig. 24 presents the UE behavior when performing the variant of the fourth embodiment of the method,

[0083] Fig. 25 illustrates the first base station behavior when performing the variant of the fourth embodiment of the method,

[0084] Fig. 26 illustrates the second base station behavior when performing the variant of the fourth embodiment of the method.

[0085] Detailed description 202404028

[0086] 12

[0087] 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.

[0088] 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.

[0089] 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.

[0090] 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 202404028

[0091] 13 communicates with a UE (directly or via another node) and / or with another network node. Examples of network nodes are NodeB, MeNB, ENB, Integrated Access and Backhaul (IAB) node, 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.

[0092] 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.

[0093] 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. The same applies to “first base station” and “second base station”; throughout this description, at some point, “first base station” is equivaled with a source or serving gNB / cell, and “second base station” is equivaled with a target gNB / cell. 202404028

[0094] 14

[0095] 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.

[0096] 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.

[0097] 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.

[0098] 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.

[0099] 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 202404028

[0100] 15 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.

[0101] 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”)).

[0102] 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 202404028

[0103] 16 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.

[0104] 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.

[0105] 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.

[0106] 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 202404028

[0107] 17 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.

[0108] 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).

[0109] 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.

[0110] Figure 1 schematically shows standard timing and synchronization as described by 3GPP standard specifications. A base station (i.e., NR gNB) provides a UE with a configurable timer, TTA, used to control how long the UE is considered uplink time aligned. The timer is normally restarted whenever a new timing avdance is given by the base station.

[0111] The time alignment timer is configured like this:

[0112] TTA: TimeAlignmentTimer ::= ENUMERATED {ms500, ms750, ms1280, ms1920, ms2560, ms5120, ms10240, infinity} 202404028

[0113] 18

[0114] For non-terrestrial networks, the long propagation delay requires timing relationship adaptations and UE pre-compensation - e.g., GEO with x100s ms and LEO with x10s ms Round Trip Delays.

[0115] To accommodate the propagation delay in non-terrestrial networks, several timing relationships are enhanced by a common timing advance mechanism (Common TA) and two offsets, KOffSet and kmac. Common TA is a configured timing offset that is equal to the round trip-time between the reference point and the non-terrestrial network payload. KOffSet is a configured scheduling offset that needs to be larger or equal to the sum of the service link round trip-time and the Common TA, and kmacis a configured offset that is approximately equal to the round trip-time between the reference point and the gNB.

[0116] According to TS 38.213, downlink DL and uplink UL are frame aligned at the uplink time synchronization reference point with an offset given by NTA, offset- NTA, offset is derived from SIB19 data, as in the following:

[0117] SIB19: ServingCellConfigCommonSIB ::= SEQUENCE {

[0118] [•••] NTA, offset n-TimingAdvanceOffset ENUMERATED { nO, n25560, n39936 } OPTIONAL, - Need S

[0119] The UE can be provided a value NTA OffSet of a timing advance offset for a serving cell by n-TimingAdvanceOffset for the serving cell. Using higher-layer ephemeris parameters for a serving satellite (if provided), a UE pre-compensates the two-way transmission delay on the service link based on N^A adj that the UE determines using the serving satellite position and its own position. To pre-compensate the two-way transmission delay between the uplink time synchronization reference point and the serving satellite, the UE determines Ny^dj00(as described by clause 4, TS 38.211) based on one-way propagation delay determined by the UE. The propagation delay provides a distance at time t between the serving satellite and the uplink time synchronization reference point divided by the speed of light. 202404028

[0120] 19

[0121] NTArepresents reception of the random access response or absolute timing advance command (MAC CE).

[0122] Fig. 2 presents a generic standard handover procedure of a UE from a first base station (i.e. , a source gNB) to a second base station (i.e. , a target gNB); the uplink synchronization is supported via either RACH-based or RACH-less access. RCReconfiguration message triggering the RACH-less handover includes a timing adjustment indication and either a configured grant or a beam indication for accessing the target cell. UE synchronizes to the target cell by applying the timing adjustment indication and transmits the RRCReconfigurationComplete message using the configured uplink grant if included. UE can fallback to RACH when there is no valid configured uplink grant.

[0123] RACH-less handover as specified in TS 38.321 and TS 38.331 is supported in non-terrestrial networks. Nevertheless, during mobility between non-terrestrial and terrestrial networks, the UE is not required to connect to both NTN and TN at the same time. UE may support mobility between base stations operating with NTN payloads in different orbits (e.g., GSO, NGSO at different altitudes).

[0124] Fig. 3 illustrates a non-terrestrial (for example, non-geostationary satellite GSO) network scenario, in communication with a terrestrial gateway, with satellites serving cells indentified by physical cell identifiers PCI1 (i.e., serving cell) and PCI2 (i.e., target cell). UE is about to detach from PCI1 , covered by a first satellite acting as base station (referred as source gNB) and to attach to PCI2, covered by a second sattelite acting as base station (referred as target gNB), following a handover procedure. The source and target satellite, respectively, are placed on different altitudes. In this case, most probably, although UE autonomously detects target satellite and adjust timing, due to altitude differences, time shift occur.

[0125] Fig. 4 shows a multi-orbit scenario, employing both non-geostrationary (NGSO) and geostationary (GSO) satellites, in communication with a terrestrial gateway. In this case, again, due to altitude difference, timing adjustment has to take into consideration time shifts as well. 202404028

[0126] 20

[0127] Exemplary satellite altitudes are:

[0128] (FR2) Starlink satellite altitudes (in m):

[0129] Sat. #1 : -541624

[0130] Sat. #2: -544312

[0131] Sat. #3: -542295

[0132] Altitude differences:

[0133] Sat. #2 - #1 : -2688m

[0134] Time offset: ~9ps

[0135] Sat. #2 - #3: -2017m

[0136] Time offset: ~7ps

[0137] (FR1) Iridium NEXT altitudes (in m):

[0138] Sat. #1 : -789640

[0139] Sat. #2: -788106

[0140] Sat. #3: -781418

[0141] Altitude differences:

[0142] Sat. #2 - #1 : -1534m

[0143] Time offset: ~5ps

[0144] Sat. #2 - #3: -6688m

[0145] Time offset: ~22ps

[0146] (FR1) Globalstar altitudes (in m):

[0147] Sat. #1 : -1573175

[0148] Sat. #2: -1415023

[0149] Sat. #3: -1596689

[0150] Altitude differences:

[0151] Sat. #2 - #1 : -158152m

[0152] Time offset: ~528ps

[0153] Sat. #2 - #3: -181666m

[0154] Time offset: ~606ps

[0155] Default values: 202404028

[0156] NTA,offset(FR1): 0 or ~13ps

[0157] NTA,offset(FR2): ~7ps

[0158] Values are calculated for a speed of light in vacuum of 299,792,458 m / s.

[0159] In NTN RACH-less handover field descriptions from current standard specification TS 38.331 , target NTA is a timing adjustment parameter indicating the NTAvalue which the UE shall use for handover, under CellGroupConfig information element:

[0160] RACH-LessHO-r18 ::= SEQUENCE { targetNTA-r18 ENUMERATED {zero, source} OPTIONAL, - Need N beamlndication-r18 CHOICE { tci-StatelD-r18 TCI-Stateld, ssb-lndex-r18 SSB-lndex

[0161] OPTIONAL, - Need N

[0162] }

[0163] }

[0164] The value zero corresponds to NTA= 0, while the value source corresponds to the NTA value of the source serving cell. Only value source is configured by the network in case source cell is a mobile IAB cell.

[0165] However, if the timing of the UE performing its first uplink transmission (during RACH-less handover) is unclear for the network (in terms of who determine what, and how), this may lead to waste of resources (uplink grant), as the target gNB / cell may try to schedule the UE unsuccessfully before it has switched. Therefore, a clear allocation of tasks concerning an accurate timing advance is needed.

[0166] According to invention, there is provided a method 100 for enhanced RACH-less handover, performed by an UE in wireless communication with a first base station (i.e., source gNB / cell) and a second base station (i.e., target gNB / cell). First and second base stations are mainly component of an integrated NTN-TN wireless communication system. 202404028

[0167] 22

[0168] The method comprises the following steps: receiving 101 , by the UE, an uplink timing advance UL TA of the second base station and providing 102, by the first base station, response to the UE, including an updated timing advance. The uplink timing advance UL TA is determined by either the first base station, the second base station, a combination of both first and second base stations, or determined by UE and reported to the first base station.

[0169] In the following description, first base station is, for example, a source gNB / cell, and second base station is a target gNB / cell.

[0170] When the source gNB / cell decides to handover UE, the source gNB / cell shares the determined uplink timing advance UL TA of UE with the target gNB / cell.

[0171] The target gNB / cell considers the provided uplink timing advance UL TA of UE when configuring beam-specific uplink grant resources for RACH-less handover. The target gNB / cell indicates pre-configured beam-specific uplink resources, together with periodicity and validity, to the source gNB / cell; further on, the target gNB / cell includes timer value for resource validity.

[0172] Further on, the source gNB / cell includes beam-specific UL grant configuration, including validity timer value of the target gNB / cell to the UE in a handover command message. When the UE receives the handover command message from the source gNB / cell, validity timer starts. When the validity timer expires, uplink grant resources are released.

[0173] Figures 5 illustrate a first embodiment of the method, wherein an uplink timing advance is determined by first base station, i.e., source gNB / cell.

[0174] First embodiment of the method refers to the case when the UE receives the uplink timing advance of the target gNB / cell, the uplink timing advance UL TA 202404028

[0175] 23 being determined by the source gNB / cell. In this case, uplink timing advance UL TA of UE is based on:

[0176] - last NTA value or history of UE’s uplink signaling (e.g., PUSCH, PUCCH) to the source gNB / cell (i.e., history of NTA),

[0177] - source gNB’s service time t-Service, meaning the time when the source gNB / cell stops serving a certain area;

[0178] - target gNB’s ephemeris data, including epoch time;

[0179] - target gNB’s t-ServiceStart time, meaning the time when target gNB / cell starts serving a certain area.

[0180] Further on, the source gNB / cell provides a determined target UE the uplink timing advance in handover request message to target gNB / cell.

[0181] Fig. 6a, 6b show a flowchart of first embodiment, initiating the handover by detaching UE from serving gNB / cell and initiating re-synchronization, depending on t-Service. In this case, RRCReconfiguration message triggering the RACH-less handover includes targetNTA as timing adjustment indication and configured uplink UL grant for accessing the target cell. UE detaches from serving gNB and initiates re-synchronization with target cell, depending on t-Service indicator.

[0182] Uplink syncronization acquisition via RACH-less is performed by using data derived or included in SIB19, targetNTA and uplink UL grant.

[0183] The mentioned indicators are broadcasted by NTN cells; they are either included or derived from SIB19 data, for example: ntn-Config-r17 (ephemeris data, common TA parameters, KOffSet> validity duration for uplink UL synchronization information and epoch), referenceLocation-r17 (reference location of the serving cell, e.g., used for measurement initiation in IDLE / INACTIVE mode), distanceThresh-r17 (distance from the serving cell reference location), t-Service-r17 (indicates the time information on when a cell provided via NTN quasi-Earth fixed system is going to stop serving the area it is currently covering), moving ReferenceLocation-r18 indicates reference location of the serving cell of an NTN Earth moving system at a time reference, 202404028

[0184] 24 t-ServiceStart-r18 indicates the time information on when the target satellite is going to start serving the area currently covered by the serving satellite.

[0185] Epoch time is used to indicate the epoch time for the NTN assistance information, and it is defined as the starting time of a DL sub-frame. The reference point for EpochTime of the serving or neighbour NTN payload ephemeris and Common TA parameters is the uplink time synchronization reference point when this field is provided in an NTN cell and the gNB when this field is provided in a TN cell.

[0186] EpochTime information element is listed as he following:

[0187] - ASN1 START

[0188] - TAG-EPOCHTIME-START

[0189] EpochTime-r17 ::= SEQUENCE { sfn-r17 INTEGER(O..1O23), subFrameNR-r17 INTEGER(0..9)

[0190] }

[0191] - TAG-EPOCHTIME-STOP

[0192] - ASN1STOP

[0193] For example, ephemeris data, common TA parameters, KoffSet, validity duration for uplink UL synchronization information and epoch are expressed via ntn-Config parameter; t-Service and t-ServiceStart time. ntn-Config’. provides parameters needed for the UE to access NR via NTN access such as Ephemeris data, common TA parameters, k_offset, validity duration for UL sync information and epoch. In a TN cell, this field is only present in ntn- NeighCellConfigList and ntn-NeighCellConfigListExt. referenceLocation is the reference location of the serving cell provided via NTN quasi-Earth fixed system and is used in location-based measurement initiation in RRCJDLE and RRCJNACTIVE. 202404028

[0194] 25 distanceThresh is the distance from the serving cell reference location and is used in location-based measurement initiation. Each step represents 50m. moving ReferenceLocation is the reference location of the serving cell of an NTN Earth moving system at a time reference. The time reference of this field is indicated by epochTime in ntn-Config of the serving cell. ntn-NeighCellConfigList, ntn-NeighCellConfigListExt provides a list of NTN neighbour cells including their ntn-Config, carrier frequency and PhysCellld. t-Service: indicates the time information on when a cell provided via NTN system is going to stop serving the area it is currently covering. t-ServiceStart: / ndicates the time information on when the target satellite is going to start serving the area currently covered by the serving satellite. The reference point for t-ServiceStart is the uplink time synchronization reference point of the serving satellite.

[0195] However, it worthy to mention that, in an NTN cell, the indicated time is referenced at the uplink time synchronization reference point (RP), i.e., UE should take into account the propagation delay between UE and RP when determining the UTC time at the UE.

[0196] Fig. 7 illustrates the UE behavior when performing first embodiment of the method. UE receives SIB19 from serving gNB, receives, from the serving gNB, RRC reconfiguration including targetNTA and configuration for uplink transmission, determines to detach from serving gNB and initiates re-synchronization with target gNB. Finally, it performs RACH-less handover and applies the configuration received for uplink UL transmission.

[0197] Fig. 8 illustrates the first base station behavior when performing first embodiment of the method. First base station (i.e., source or serving gNB / cell) decides to 202404028

[0198] 26 switch UE from serving to target gNB, determines targetNTA value for UE, sends handover request to target gNB, including the previously determined targetNTA value for UE, receives handover request acknowledge message, including a configured uplink UL grant. Provides updated RRC re-configuration to UE (including targetNTA value and the configured uplink UL grant). Forwards user data to target gNB. Finally, releases UE context, when indication from target gNB is received.

[0199] Fig. 9 illustrates the second base station behavior when performing the first embodiment of the method. Second base station (i.e., target gNB) receives a handover request from serving gNB, including the determined targetNTA value for UE. Schedules UL resources for UL transmission based on the determined targetNTA value for UE. Sends handover request acknowledge message, including the configured UL grant for UE to serving gNB. Buffers user data forwarded by serving gNB. Indicates switching completed to serving gNB.

[0200] Indicates path switch to Access and Mobility Management Function or User Plan Function AMF / UPF.

[0201] Second embodiment of the method refers to the case when the UE receives the uplink timing advance of the target gNB / cell determined by target gNB / cell, the uplink timing advance being based on:

[0202] - last NTAvalue or history of UE’s UL signaling (e.g., PUSCH, PUCCH) to source gNB (i.e., history of NTA), which is forwarded by source gNB;

[0203] - source gNB’s ephemeris data;

[0204] - t-Service, meaning the time when source gNB stops serving a certain area, and

[0205] - movingReferenceLocation message of source gNB, which is forwarded by source gNB;

[0206] - target gNB’s ephemeris data, including epoch time;

[0207] - target gNB’s t-ServiceStart time, meaning the time when target gNB starts serving a certain area.

[0208] As in the case of first embodiment, these indicators are either included or derived in SIB 19. 202404028

[0209] 27

[0210] Fig. 10a, 10b show a flowchart of few method steps of the second embodiment of the method. Target gNB considers before-mentioned data for determining target UL timing advancement of UE and configuring beam-specific UL grant resources for RACH-less handover. The target gNB indicates uplink timing advance of UE (target NTA) and uplink grant to source gNB.

[0211] Fig. 11 illustrates the UE behavior when performing the second embodiment of the method. The UE behavior is identical with that for the first embodiment of the method.

[0212] Fig. 12 illustrates the serving gNB / cell behavior when performing the second embodiment of the method. Serving gNB decides to switch UE from serving to target gNB, sends handover request to target gNB, including UE’s uplink timing advance UL TA value(s), t-Service, moving reference location. Receives handover request acknowledge message, including configured uplink UL grant. Provides an updated RRC re-configuration to UE (including targetNTA value and configured UL grant). Forwards user data to target gNB. Finally, releases UE context, when indication from target gNB is received.

[0213] Fig. 13 illustrates the second base station behavior when performing the second embodiment of the method. Target gNB receives handover request from serving gNB, including UE’s uplink timing advance UL TA value(s), t-Service, moving reference location, determines targetNTA value for UE, schedules UL resources for UL transmission based on determined target NTA value for UE. Sends handover request acknowledge message, including the determined targetNTA value and configured UL grant for UE to serving gNB. Buffers user data forwarded by serving gNB. Indicates switching completed to serving gNB. Indicates path switch to AMF / UPF.

[0214] Figures 14 show a third embodiment of the method, when the UE receives the uplink timing advance of target gNB determined by both source and target gNBs. In this case, the uplink timing advance of target gNB is based on: 202404028

[0215] 28

[0216] - last NTAvalue or history of UE’s uplink signaling to source gNB, which is forwarded by source gNB;

[0217] - source gNB’s ephemeris data;

[0218] - t-Service, meaning the time when source gNB stops serving a certain area, and - movingReferenceLocation message of source gNB, which is forwarded by source gNB;

[0219] - target NTAproposal by source gNB,

[0220] - target gNB’s ephemeris data including epoch time,

[0221] - target gNB’s t-ServiceStart time, meaning the time when target gNB starts serving a certain area.

[0222] As in the case of first and embodiment, these indicators are either included or derived in SIB 19, except for target NTAproposal, which is either predetermined, estimated or predicted.

[0223] Fig. 14a, 14b show a third embodiment of the method. The target gNB considers the mentioned data and either confirms or updates the proposed target NTAby source gNB; the target gNB uses the confirmed or updated target NTAfor determining target UL timing advance of UE and configuring beam-specific UL grant resources for RACH-less handover. In addition, the target gNB indicates the uplink timing advance of UE (targetNTA) and beam-specific UL grant to source gNB. Uplink UL synchronization acquisition is performed via RACH-less handover (by means of SIB19, target NTA (confirmed or updated), and uplink grant.

[0224] Fig. 15 illustrates the UE behavior when performing the third embodiment of the method; it is easily observed the UE behavior is identical with that for the first embodiment and the second embodiment of the method.

[0225] Fig. 16 illustrates the serving gNB behavior when performing the third embodiment of the method. Serving gNB decides to switch UE from serving to target gNB, determines targetNTAproposal for UE, sends handover request to target gNB, including UE uplink timing advance value(s), t-Service, moving reference location, and a target NTAproposal, receives handover request acknowledge message 202404028

[0226] 29 including configured or updated target NTAvalue and configured uplink grant, provides an updated RRC re-configuration to UE, including target NTAvalue and the configured uplink grant, forwards user data to target gNB, and releases UE context, when indication from target gNB is received.

[0227] Fig. 17 illustrates the target gNB behavior when performing the third embodiment of the method. Target gNB receives handover request from a serving gNB, including UE uplink timing advance value(s), t-Service, moving reference location, and a target NTAproposal, confirms or updates the proposed target NTAfor UE, schedules uplink resources for uplink transmission based on the confirmed or updated target NTAvalue for UE, sends the handover request acknowledge message including confirmed or updated target NTAvalue and configured uplink grant for UE to the serving gNB, buffers user data forwarded by the serving gNB, indicates switching completed to the serving gNB, and indicates path switch to AMF / UPF.

[0228] Figures 18 show a fourth embodiment of the method, wherein the uplink timing advance of second base station is based on information determined by UE and reported to first base station, i.e., serving gNB / cell.

[0229] In this case, the uplink time advance UL TA is determined and reported by UE to source gNB (e.g., via L1 or L2 signaling). Source gNB configures the UE with triggering conditions to report its predicted or estimated target NTA value (e.g., as part of measurement report) for configured RACH-less candidate cells. UE applies an AI / ML model for target NTA estimation and considers the following parameters:

[0230] - last NTA value (or history) of UE’s UL signaling (e.g., PUSCH, PUCCH) to source gNB, (i.e., history of NTA),

[0231] - previous (or history of) target NTA values that were applied when in similar situations (e.g., based on stored and recorded satellite ephemeris data, geolocation data, UE movement data);

[0232] - source gNB’s ephemeris data (from SIB19),

[0233] - t-Service (time when source gNB stops serving a certain area) and movingReferenceLocation of source gNB (from SIB19), 202404028

[0234] 30

[0235] - targetNTA of RACH-LessHO configuration (e.g., RRC re-configuration message),

[0236] - target gNB’s ephemeris data (including epoch time) (from SIB19, most precisely from ntn-NeighCellConfigList),

[0237] - target gNB’s t-ServiceStart time (time when target gNB starts serving a certain area) (from SIB19).

[0238] Figures 18a, 18b, 18c show a fourth embodiment of the method. When triggering conditions are met, UE reports predicted target NTA value for configured conditional handover (C)HO candidate cells to source gNB. Serving gNB checks the reported, predicted target NTA value and either uses the same or updates the target NTA value for configured RACH-less handover cells (e.g., the source gNB uses an own AI / ML model for predicting target NTA values and for validating UE reported estimation values). As a mention, training the model by AI / ML techniques is not part of this invention.

[0239] Source gNB provides the same or updated target NTA value in handover request message to target gNB. Target gNB uses the same or updated target NTA value for configuring beam-specific UL grant resources for RACH-less handover. Finally, target gNB indicates beam-specific uplink UL grant resources to source gNB.

[0240] The difference between this embodiment and the other embodiments described so far consists in the set of triggering conditions for target NTA estimation and reporting.

[0241] Most precisely, base station configures UE to measure timing advance TA based on the following triggering criteria: if the distance from UE to reference location is greater than a threshold (distanceThreshTA), then UE determines or updates timing advance TA; if the determined or updated timing advance is greater than a reporting threshold (taTresh), UE sends report to source gNB; if the distance from UE to reference location is greater than a scaling threshold (scalDistanceThresh*distanceThresh), then UE determines or updates timing advance; if the determined / updated timing 202404028

[0242] 31 advance is greater than the reporting threshold (taTresh), UE sends report to source gNB.

[0243] Fig. 19 illustrates schematically details of parameters (fields) used for triggering criteria.

[0244] New distanceThreshTA parameter is used when UE distance to referenceLocation is greater than distanceThreshTA, UE determines / updates TA.

[0245] When determined / updated TA is greater than reporting threshold taTresh, UE sends report to gNB. New scaling parameter scalDistanceThresh is used when UE distance to referenceLocation is greater than (scalDistanceThresh*distanceThresh); UE determines / updates TA.

[0246] When determined / updated TA is greater than reporting threshold taTresh, UE sends report to gNB.

[0247] Fig. 20 illustrates the UE behavior when performing the fourth embodiment of the method. UE receives SIB19 from serving gNB, receives measurement configuration from serving gNB (including triggering conditions for target NTA estimation and reporting). If the conditions for NTA estimation are met, then UE performs target NTA estimation according to configuration. If not met, then another evaluation of the triggering conditions is done. Further on, If reporting conditions for NTA estimation are met, then UE sends measurement report (including target NTA estimation(s)) to serving gNB. If not met, then another evaluation of the reporting conditions is done. The UE receives RRC re-configuration (including target NTA and configuration for uplink UL transmission), determines to detach from serving gNB and initiates re-synchronization with target gNB, performs RACH-less handover and applies configuration for uplink UL transmission.

[0248] Fig. 21 illustrates the serving gNB / cell behavior when performing the fourth embodiment of the method. Serving gNB / cell configures UE to perform target NTA estimation(s) and reporting, receives measurement report(s) including predicted target NTA value(s), decides to switch UE from serving to target gNB, validates or updates received target NTA value for UE. Sends handover request to target gNB, including same or updated target NTA value for UE. Receives handover request 202404028

[0249] 32 acknowledge message including configured UL grant. Provides updated RRC reconfiguration to UE (including same or updated target NTA value and configured UL grant), forwards user data to target gNB, and releases UE context, when indication from target gNB is received.

[0250] Fig. 22 illustrates the target gNB / cell behavior when performing the fourth embodiment of the method. Target gNB receives handover request from a serving gNB, including validated or updated target NTA value for UE, schedules uplink resources for uplink transmission based on the validated or updated target NTAvalue for UE, sends handover request acknowledge message including confirmed or updated target NTA value and configured uplink grant for UE to the serving gNB, buffers user data forwarded by the serving gNB, indicates switching completed to the serving gNB, and indicates path switch to AMF / UPF.

[0251] Figures 23a, b, c illustrate a variant of the fourth embodiment of the method, whereby instead of estimation, there is used a predicted target NTA value.

[0252] Fig. 24 presents the UE behavior when performing the variant of the fourth embodiment. UE receives SIB19 from serving gNB, receives measurement configuration from the serving gNB, including triggering conditions for target NTA prediction and reporting, performs target NTA prediction according to the received configuration, and checks if the reporting conditions are met. If not met, then another evaluation of the reporting conditions is done. If reporting conditions for NTA estimation are met, then UE sends measurement report (including target NTA estimation(s)) to serving gNB. The UE receives RRC re-configuration (including target NTA and configuration for uplink UL transmission), determines to detach from serving gNB and initiates re-synchronization with target gNB, performs RACH-less handover and applies configuration for uplink UL transmission.

[0253] Fig. 25 illustrates the first base station behavior when performing the variant of the fourth embodiment. Serving gNB / cell configures UE to perform target NTAestimations or predictions and reporting, receives measurement reports including predicted target NTA ■ decides to switch UE from serving to target gNB, validates or 202404028

[0254] 33 updates the received target NTfor UE, sends handover request to a target gNB, including same or updated target NTAvalue for UE, receives handover request acknowledge message including configured or updated target NTAvalue and configured uplink grant, provides an updated RRC re-configuration to UE, including target NTAvalue and the configured uplink grant, forwards user data to target gNB, and releases UE context, when indication from target gNB is received.

[0255] Fig. 26 illustrates the second base station behavior when performing the variant of the fourth embodiment. Target gNB / cell receives handover request from a serving gNB, including validated or updated target NTAvalue for UE, schedules uplink resources for uplink transmission based on the validated or updated target NTAvalue for UE, sends the handover request acknowledge message including confirmed or updated target NTAvalue and configured uplink grant for UE to the serving gNB, buffers user data forwarded by the serving gNB, indicates switching completed to the serving gNB, and indicates path switch to AMF / UPF.

[0256] 202404028

[0257] 34

[0258] Abbreviations:

[0259] AMF Access and Mobility Management Function

[0260] UPF User Plan Function

[0261] CHO Conditional Handover

[0262] UL Uplink

[0263] DL Downlink

[0264] PCI Physical Cell Identifier

[0265] PDCCH Physical downlink control channel

[0266] PDSCH Physical downlink shared channel

[0267] PUCCH Physical uplink control channel

[0268] PUSCH Physical uplink shared channel

[0269] RACH Random Access Channel

[0270] RP Reference Point

[0271] RTT Round trip time

[0272] TA Timing Advance

[0273] UE User equipment

Claims

20240402835Patent claims1. A method (100) for enhanced RACH-less handover, performed by an UE in wireless communication with a first base station and a second base station, the method comprising the following steps:(101) Receiving, by the UE, an uplink timing advance (UL TA) of the second base station, and(102) Providing, by the first base station, response to the UE, including an updated timing advance, wherein the uplink timing advance is either determined by the first base station, the second base station, a combination of both first and second base stations, or determined and reported by UE to the first base station.

2. The method of claim 1, c h a r a c t e r i z e d in that, when the first base station decides to handover UE, the first base station shares determined uplink timing advance of UE with the second base station.

3. The method of previous claims, c h a r a ct e r i z e d in that further on, the second base station considers the provided uplink timing advance of UE when configuring beam-specific UL grant resources for RACH-less handover.

4. The method of previous claims, characterized in that, in addition, the second base station indicates pre-configured beam-specific uplink UL resources, together with periodicity and validity, to the first base station.

5. The method of previous claims, characterized in that, in addition, the second base station includes timer value for resource validity.

6. The method of previous claims, characterized in that, in addition, the first base station includes beam-specific UL grant configuration, including validity timer value of the second base station to the UE in handover command message.202404028367. The method of previous claims, c h a r a c t e r i z e d i n that, further on, when the UE receives the handover command message from the first base station, validity timer starts.

8. The method of previous claims, c h a r a c t e r i z e d i n that, further on, when the validity timer expires, uplink grant resources are released.

9. The method of claim 1 , c h a r a c t e r i z e d i n that, when the UE receives the uplink timing advance of second base station determined by first base station, the uplink timing advance is based on: last NTAvalue or history of UE’s uplink signaling to first base station; first base station’s service time t-Service, meaning the time when the first base station stops serving a certain area; second base station’s ephemeris data, including epoch time; second base station’s t- ServiceStart time, meaning the time when second base station starts serving a certain area.

10. The method of claim 9, c h a r a c t e r i z e d i n that, wherein the first base station provides (a determined target) UE the uplink timing advance in handover request message to second base station.

11. The method of claim 1 , c h a r a c t e r i z e d i n that, when the UE receives the uplink timing advance of second base station determined by second base station, the uplink timing advance is based on: last NTA value or history of UE’s UL signaling to first base station, which is forwarded by first base station; first base station’s ephemeris data; t-Service, meaning the time when first base station stops serving a certain area, and movingReferenceLocation message of first base station, which is forwarded by first base station; second base station’s ephemeris data, including epoch time; second base station’s t-ServiceStart time, meaning the time when second base station starts serving a certain area.2024040283712. The method of claim 11 , c h a r a c t e r i z e d i n that, the second base station considers the mentioned data for determining target uplink timing advance of UE and configuring beam-specific UL grant resources for RACH-less handover.

13. The method of claim 12, c h a r a c t e r i z e d i n that, further on, the second base station indicates uplink timing advance of UE (target NTA) and uplink grant to first base station.

14. The method of claim 1 , c h a r a c t e r i z e d i n that, when the UE receives the uplink timing advance of second base station determined by both source and second base stations, the uplink timing advance of second base station is based on: last NTA value or history of UE’s uplink signaling to first base station, which is forwarded by first base station; first base station’s ephemeris data; t-Service, meaning the time when first base station stops serving a certain area, and movingReferenceLocation message of first base station, which is forwarded by first base station; target NTAproposal by first base station, second base station’s ephemeris data including epoch time, second base station’s t-ServiceStart time, meaning the time when second base station starts serving a certain area.

15. The method of claim 14, c h a r a c t e r i z e d i n that, further on, the second base station considers the mentioned data and either confirms or updates the proposed target NTAby first base station.

16. The method of claim 15, c h a r a c t e r i z e d i n that, in addition, the second base station uses the confirmed or updated target NTAfor determining target UL timing advance of UE and configuring beam-specific UL grant resources for RACH-less handover.2024040283817. The method of claim 16, c h a r a c t e r i z e d i n that, in addition, the second base station indicates the uplink timing advance of UE (target NTA) and beamspecific UL grant to first base station.

18. The method of claim 1 , c h a r a c t e r i z e d i n that, when the UE receives the uplink timing advance of second base station based on information determined and reported by UE to first base station, the first base station configures the UE with triggering conditions to report its predicted or estimated target NTA value for configured RACH-less candidate cells.

19. The method of claim 18, c h a r a c t e r i z e d i n that, the predicted target NTA is reported as part of measurement report.

20. The method of claim 18, c h a r a c t e r i z e d i n that, further on, UE applies a trained model for target NTAestimation and considers the following parameters: last NTA value or history of UE’s uplink signaling to first base station, previous or history of target NTA values that were applied in similar situations, based on stored and recorded satellite ephemeris data, geolocation data, UE movement data; first base station’s ephemeris data, derived from SIB19, t-Service, meaning the time when first base station stops serving a certain area, and movingReferenceLocation of first base station, derived from SIB19, target NTAof RACH-less handover configuration, derived from RRC reconfiguration message, second base station’s ephemeris data, including epoch time, derived from SIB19, namely from ntn-NeighCellConfigList, second base station’s t-ServiceStart time, meaning the time when second base station starts serving a certain area, derived from SIB19.

21. The method of claims 18-20, c h a r a c t e r i z e d i n that, when triggering conditions are met, the UE reports predicted target NTA value for configured (conditional) handover candidate cells to first base station.2024040283922. The method of claim 21, c h a r a c t e r i z e d in that, further on, the first base station checks the reported, predicted target NTAvalue and either uses the same or updates the target NTA value for configured RACH-less handover cells, in particular the first base station uses the trained model for predicting target NTAvalues and for validating UE reported estimation values.

23. The method of claim 21-22, characterized in that, further on, the first base station provides the same or updated target NTAvalue in handover request message to second base station.

24. The method of claim 21-23, c h a ra ct e ri z e d in that, further on, the second base station uses the same or updated target NTA value for configuring beam-specific UL grant resources for RACH-less handover.

25. The method of claim 21-24, c h a r a ct e r i z e d in that, further on, the second base station indicates beam-specific uplink grant resources to first base station.

26. The method of claim 18, characterized in that the first base station configures UE to measure the timing advance based on the following triggering criterion: if the distance from UE to reference location is greater than a threshold (distanceThreshTA), then UE determines or updates timing advance TA; if the determined or updated timing advance is greater than a reporting threshold (taTresh), UE sends report to first base station;27. The method of claim 18, characterized in that the first base station configures UE to measure the timing advance based on the further following triggering criterion: if the distance from UE to reference location is greater than a scaling threshold (scalDistanceThresh*distanceThresh), then UE determines or updates timing advance; if the determined / updated timing advance is greater than the reporting threshold (taTresh), UE sends report to first base station.2024040284028. Apparatus for enhanced 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 27.

29. User Equipment (UE) comprising an apparatus according to claim 28.

30. User Equipment (UE) according to claim 29, configured to implement steps of the claims 1-17, whereby the UE receives SIB19 from serving gNB, receives RRC re-configuration from the serving gNB, including target NTAand configuration for uplink transmission, determines to detach from the serving gNB and initiates resynchronization with second base station, performs RACH-less handover and applies configuration for uplink transmission.

31. User Equipment (UE) according to claim 29, configured to implement steps of the claims 18-27, whereby the UE receives SIB19 from serving gNB, receives measurement configuration from the serving gNB, including triggering conditions for target NTAestimation or prediction and reporting, performs target NTAestimation or prediction according to the received configuration, and if the reporting conditions are met, sends measurement report, including target NTA predictions to the serving gNB.

32. Base station comprising an apparatus according to claim 28.

33. Base station according to claim 32, configured as serving gNB to implement steps of the method claims, whereby it decides to switch UE from serving to second base station, determines target NTAvalue for UE, sends handover request to a second base station, including the determined target NTAvalue for UE, receives handover request acknowledge message including configured uplink grant, provides an updated RRC re-configuration to UE, including the determined target NTAvalue and the configured uplink grant, forwards user data to second20240402841 base station, and releases UE context, when indication from second base station is received.

34. Base station according to claim 32, configured as serving gNB to implement steps of the claims 11-13, whereby it decides to switch UE from serving to second base station, sends handover request to a second base station, including UE uplink timing advance value(s), the t-Service, and moving reference location, receives handover request acknowledge message including configured uplink grant, provides an updated RRC re-configuration to UE, including target NTAvalue and the configured uplink grant, forwards user data to second base station, and releases UE context, when indication from second base station is received.

35. Base station according to claim 32, configured as serving gNB to implement steps of the claims 14-17, whereby it decides to switch UE from serving to second base station, determines target NTAproposal for UE, sends handover request to a second base station, including UE uplink timing advance value(s), the t-Service, moving reference location, and the target NTAproposal, receives handover request acknowledge message including configured or updated target NTAvalue and configured uplink grant, provides an updated RRC re-configuration to UE, including target NTAvalue and the configured uplink grant, forwards user data to second base station, and releases UE context, when indication from second base station is received.

36. Base station according to claim 32, configured as serving gNB to implement steps of the claims 18-27, whereby it configures UE to perform target NTAestimations or predictions and reporting, receives measurement reports including estimated or predicted target NTA, decides to switch UE from serving to second base station, validates or updates the received target NTAfor UE, sends handover request to a second base station, including same or updated target NTAvalue for UE, receives handover request acknowledge message including configured or updated target NTAvalue and configured uplink grant, provides an updated RRC re-configuration to UE, including target NTAvalue and the configured uplink grant,20240402842 forwards user data to second base station, and releases UE context, when indication from second base station is received.

37. Base station according to claim 32, configured as second base station to implement steps of the method claims 1-10, whereby it receives handover request from a serving gNB, including a determined target NTAvalue for UE, schedules uplink resources for uplink transmission based on the determined target NTA value for UE, sends the handover request acknowledge message including uplink grant to the serving gNB, buffers user data forwarded by the serving gNB, indicates switching completed to the serving gNB, and indicates path switch to AMF / UPF.

38. Base station according to claim 32, configured as second base station to implement steps of the method claims 11-13, whereby it receives handover request from a serving gNB, including UE uplink timing advance value(s), the t- Service, and moving reference location, determines target NTAvalue for UE, schedules uplink resources for uplink transmission based on the determined target NTAvalue for UE, sends the handover request acknowledge message including uplink grant to the serving gNB, buffers user data forwarded by the serving gNB, indicates switching completed to the serving gNB, and indicates path switch to AMF / UPF.

39. Base station according to claim 32, configured as second base station to implement steps of the method claims 14-17, whereby it receives handover request from a serving gNB, including UE uplink timing advance value(s), the t- Service, moving reference location, and target NTAproposal, confirms or updates the proposed target NTAfor UE, schedules uplink resources for uplink transmission based on the confirmed or updated target NTAvalue for UE, sends the handover request acknowledge message including confirmed or updated target NTAvalue and configured uplink grant for UE to the serving gNB, buffers user data forwarded by the serving gNB, indicates switching completed to the serving gNB, and indicates path switch to AMF / UPF.2024040284340. Base station according to claim 32, configured as second base station to implement steps of the method claims 18-27, whereby it receives handover request from a serving gNB, including validated or updated target NTA value for UE, schedules uplink resources for uplink transmission based on the validated or updated target NTAvalue for UE, sends the handover request acknowledge message including confirmed or updated target NTAvalue and configured uplink grant for UE to the serving gNB, buffers user data forwarded by the serving gNB, indicates switching completed to the serving gNB, and indicates path switch to AMF / UPF.

41. Wireless communication system comprising a base station according to claim 32 with a processor coupled with a memory in which computer program instructions are stored, said instructions being configured to implement steps of claims 1 to 27, in communication with at least one UE according to claim 29, wherein the UE 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 27.

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