Timing advance maintenance during conditional ltm

Abstract

Embodiments of the disclosure describe a method for Timing Advance (TA) maintenance during Conditional Layer 1 / Layer 2 Triggered Mobility (C-LTM). The method includes receiving, at a User Equipment (UE), a TA value associated with a Layer 1 / Layer 2 Triggered Mobility (LTM) candidate gNodeB Distributed Unit (gNB-DU) cell from a candidate gNB-DU. The method includes transmitting, by the UE, the TA value to a serving gNB-DU associated with the UE via an Uplink Medium Access Control-Control Element (UL MAC-CE) message to maintain the received TA value at both the UE and the serving gNB-DU. The method includes determining, by the UE and / or network, whether the received TA value associated with the LTM candidate gNB-DU cell has expired. The method includes initiating a re-acquisition of the TA value based on either a network-based decision-making timer or a UE-initiated request in response to determining that the received TA value has expired.

Description

TIMING ADVANCE MAINTENANCE DURING CONDITIONAL LTMCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to Indian Provisional Patent Application No.202511009521, filed on February 5, 2025, and Indian Non-Provisional Patent Application No.202511009521, filed on September 25, 2025, the entire contents of which are incorporated herein by reference.FIELD

[0002] The present disclosure relates to Timing Advance (TA) maintenance during Conditional Layer 1 / Layer 2 Triggered Mobility (C-LTM).BACKGROUND

[0003] The information disclosed in this background section is only for enhancement of understanding of the general background of the disclosure and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.

[0004] Layer 1 / Layer 2 Triggered Mobility (LTM), introduced in Release 18, presents significant advancements in reducing handover latency and minimizing interruption times when compared to traditional Layer 3-based mobility solutions. However, the initial implementation of the LTM in the Release 18 has certain limitations, such as the lack of support for an inter-Central Unit (CU)LTM. The objective of the Release 19 work item is to address and mitigate these constraints, thereby enhancing the overall mobility framework.

[0005] In the context of the LTM, a disaggregated architecture 100 defined by the Third Generation Partnership Project (3GPP) delineates a decomposition of a gNodeB (gNB) into multiple logical entities, as illustrated in FIG. 1. This architecture 100 allows a single Distributed Unit (DU) to manage multiple cells, with a maximum capacity of 512 cells as specified in current standards. Within this disaggregated architecture 100, a gNB-Central Unit-Control Plane (gNB-CU-CP) is configured to host one or more Packet Data Convergence Protocol (PDCP) and Radio Resource Control (RRC) layers, while the gNB-DU accommodates the Radio Link Control (RLC), Medium Access Control (MAC), and Physical (PHY) layers. The scheduling operations are executed at the gNB-DU, ensuring efficient resource allocation and management across a network.SUMMARY

[0006] This summary is provided to introduce a selection of concepts, in a simplified format, that are further described in the detailed description of the disclosure. This summary is neither intended to identify key or essential inventive concepts of the disclosure nor is it intended for determining the scope of the disclosure.

[0007] According to one embodiment of the present disclosure, a method is disclosed. The method includes receiving, at a User Equipment (UE), a Timing Advance (TA) value associated with a Layer 1 / Layer 2 Triggered Mobility (LTM) candidate gNodeB Distributed Unit (gNB-DU) cell from a candidate gNB-DU, either directly or via a serving gNB-DU. The method further includes transmitting, by the UE, the TA value to the serving gNB-DU associated with the UE via an UplinkMedium Access Control-Control Element (UL MAC-CE) message to maintain the received TA value at both the UE and the serving gNB-DU. The method further includes determining, by the UE, whether the received TA value associated with the LTM candidate gNB-DU cell has expired. The method further includes initiating a re-acquisition of the TA value based on either a networkbased decision-making timer or a UE-initiated request in response to determining that the received TA value has expired.

[0008] According to one embodiment of the present disclosure, a method is disclosed. The method includes receiving, at a serving gNodeB Distributed Unit (gNB-DU), a Timing Advance (TA) value associated with an LTM candidate gNB-DU cell from at least one of a gNB Central Unit (CU), the candidate gNB-DU, or a User Equipment (UE). The method further includes storing the received TA value at the serving gNB-DU. The method further includes determining, after storing the received TA value, whether one or more predefined conditions are satisfied. The one or more predefined conditions comprise an expiration of a TA timer, receiving an L1 measurement report indicating that a re-acquisition of the TA value is to be initiated, a receipt of a Uplink Medium Access Control-Control Element (UL MAC-CE) message from the UE requesting a Physical Downlink Control Channel (PDCCH) order to acquire TA. The method further includes identifying that the re-acquisition of the TA value is to be initiated in response to determining that the one or more predefined conditions are satisfied. The method further includes initiating the reacquisition of the TA value by transmitting the PDCCH order to the UE. The PDCCH order indicates an instruction to the UE to re-acquire the TA.

[0009] According to one embodiment of the present disclosure, a User Equipment (UE) is disclosed. The UE is configured to receive a Timing Advance (TA) value associated with a Layer1 / Layer 2 Triggered Mobility (LTM) candidate gNodeB Distributed Unit (gNB-DU) cell from a candidate gNB-DU, either directly or via a serving gNB-DU. The UE is further configured to transmit the TA value to the serving gNB-DU associated with the UE via an Uplink Medium Access Control-Control Element (UL MAC-CE) message to maintain the received TA value at both the UE and the serving gNB-DU. The UE is further configured to determine whether the received TA value associated with the LTM candidate gNB-DU cell has expired. The UE is further configured to initiate a re-acquisition of the TA value based on either a network-based decisionmaking timer or a UE-initiated request in response to determining that the received TA value has expired.

[0010] According to one embodiment of the present disclosure, a non-transitory computer-readable medium stores instructions. The one or more instructions are executed by a User Equipment (UE) that comprises one or more processors. The one or more processors are configured to receive a Timing Advance (TA) value associated with a Layer 1 / Layer 2 Triggered Mobility (LTM) candidate gNodeB Distributed Unit (gNB-DU) cell from a candidate gNB-DU, either directly or via a serving gNB-DU. The one or more processors are further configured to transmit the TA value to the serving gNB-DU associated with the UE via an Uplink Medium Access Control-Control Element (UL MAC-CE) message to maintain the received TA value at both the UE and the serving gNB-DU. The one or more processors are further configured to determine whether the received TA value associated with the LTM candidate gNB-DU cell has expired. The one or more processors are further configured to initiate a re-acquisition of the TA value based on either a network-based decision-making timer or a UE-initiated request in response to determining that the received TA value has expired.

[0011] To further clarify the advantages and features of the present disclosure, a more particular description of the disclosure will be rendered by reference to specific embodiments thereof, which are illustrated in the appended drawings. It is appreciated that these drawings depict only typical embodiments of the disclosure and are therefore not to be considered limiting of its scope. The disclosure will be described and explained with additional specificity and detail in the accompanying drawings.BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Features, aspects, and advantages of embodiments of the disclosure will be described below with reference to the accompanying drawings, in which like reference numerals denote like elements, and wherein:FIG. 1 illustrates a disaggregated gNodeB (gNB)architecture, according to prior art; FIG. 2 is a sequence flow diagram illustrating a signalling procedure for a Layer1 / Layer 2 Triggered Mobility (LTM), according to prior art;FIG. 3 is a flow diagram illustrating a method for initiating a re-acquisition of the TA value based on either a network-based decision-making timer or a UE-initiated request, according to an embodiment as disclosed herein;FIGS. 4-5 are sequence flow diagrams illustrating a signalling procedure for a Conditional Layer 1 / Layer 2 Triggered Mobility (C-LTM), according to an embodiment as disclosed herein;FIG. 6 is a flow diagram illustrating a method for initiating the re-acquisition of the TA value based on the network-based decision-making timer, according to an embodiment as disclosed herein;FIG. 7 is a flow diagram illustrating a method for initiating the re-acquisition of the TA value based on the UE-initiated request, according to an embodiment as disclosed herein;FIG. 8 is a flow diagram illustrating a method for initiating the re-acquisition of the TA value based on the UE-initiated request, according to another embodiment as disclosed herein;FIG. 9 is a flow diagram illustrating a method for initiating the re-acquisition of the TA value based on one or more predefined conditions, according to an embodiment as disclosed herein; andFIG. 10 illustrates a diagram of example components of an apparatus, according to an embodiment as disclosed herein.DETAILED DESCRIPTION

[0013] The following detailed description of example embodiments refers to the accompanying drawings. The present disclosure provides illustrations and descriptions, but is not intended to be exhaustive or to limit the implementations to the precise form disclosed. Modifications and variations are possible in light of the present disclosure or may be acquired from practice of the implementations. Further, one or more features or components of one embodiment may be incorporated into or combined with another embodiment (or one or more features of another embodiment). Additionally, the flowchart and description of operations provided below relate to at least one of the embodiments in the present disclosure. It should be noted that it is possible to make other embodiments that do not exactly match the flowchart and its description. It is understood that in other embodiments, one or more operations may be omitted, one or moreoperations may be added, or one or more operations may be performed simultaneously (at least in part).

[0014] It will be apparent that systems and / or methods, described herein, may be implemented in different forms of hardware, software, or a combination of hardware and software. The actual specialized control hardware or software code used to implement these systems and / or methods should not limit their implementations. Thus, the operation and behavior of the systems and / or methods are described herein without reference to specific software code. It is understood that software and hardware may be designed to implement the systems and / or methods based on the description herein.

[0015] Even though particular combinations of features are recited in the claims and / or disclosed in the specification, the particular combinations are not intended to limit the disclosure of implementations. In fact, many of these features may be combined in ways not specifically recited in the claims and / or disclosed in the specification. Even if a dependent claim directly depends on only one claim, the present disclosure may indicate that the dependent claim is dependent on other claims in the claim set.

[0016] No element, act, or instruction used herein should be construed as critical or essential unless explicitly described as such. Also, as used herein, the articles “a” and “an” (in other words, nouns not mentioned in the plural) are intended to include one or more items, and may be used interchangeably with “one or more.” Also, as used herein, the terms “has,” “have,” “having,” “include,” “including,” or the like are intended to be open-ended terms. Further, the phrase “based on” is intended to mean “based, at least in part, on” unless explicitly stated otherwise. Furthermore,expressions such as “at least one of [A] and [B],” “[A] and / or [B],” or “at least one of [A] or [B]” are to be understood as including only A, only B, or both A and B.

[0017] The foregoing disclosure provides illustration and description, but is not intended to be exhaustive or to limit the implementations to the precise form disclosed. Modifications and variations are possible in light of the above disclosure or may be acquired from practice of the implementations.

[0018] Throughout this disclosure, the term “serving gNB-DU” is the one that is actively providing a radio connection to a UE. The serving gNB-DU handles the UE’s data traffic and manages the lower layers of the radio protocol stack, including the Medium Access Control (MAC), Radio Link Control (RLC), and physical (PHY) layers.

[0019] Throughout this disclosure, the term “source gNB-DU” is the serving gNB-DU at the beginning of a handover procedure. The source gNB-DU is the initial point of connection for the UE before the connection is transferred to a new target gNB-DU (e.g., candidate target gNB-DU).

[0020] FIG. 1 illustrates a disaggregated gNodeB (gNB)Architecture 100, according to prior art.FIG. 1 illustrates one or more components and interfaces of the gNB in a 5G network. Central to this disaggregated gNB Architecture 100 is a gNB Central Unit (CU) and a gNB Distributed Unit (DU). The gNB-CU is divided into two parts: a gNB-CU Control Plane (CP), which manages signaling, user equipment registration, and mobility management, and a gNB-CU User Plane (UP), responsible for user data transmission to a User Plane Function (UPF). The gNB-DU handles realtime radio signal processing, including channel coding and resource management, allowing for reduced latency by being closer to a radio environment. The disaggregated gNB Architecture 100 may utilize one or more key interfaces, including an El interface that connects the gNB-DU to thegNB-CU, and Fl interfaces, with Fl -C managing control signaling and Fl-U facilitating user data transfer between the two units.

[0021] In order to support L1 / L2 triggered mobility (i.e., change of serving cell) in the disaggregated gNB architecture 100, a new mechanism is needed in which Handover (HO) preparation may take place at the gNB-CU-CP, but executed autonomously by the gNB-DU, without further interaction with the upper layers.

[0022] The primary objective of a Work Item (WI) is to define support for Conditional LTM (C-LTM) across Radio Access Network (RAN) 2, RAN3, and RANI. This involves establishing the conditions evaluated by a User Equipment (UE) that trigger the activation of the LTM. The focus is on facilitating the C-LTM, which includes subsequent LTM processes, while prioritizing intraCentral Unit (CU) LTM implementations. Additionally, a review checkpoint for this objective is scheduled at RAN#105, and activities within the RAN Working Group may not commence prior to this checkpoint.

[0023] In the context of the NR, mobility has evolved over several releases. Conditional Hand-Over (CHO) and other conditional mobility procedures (e.g., Conditional PSCell Change (CPAC), Subsequent Conditional PSCell Change (SCPAC), etc.) were developed to achieve high robustness by enabling the procedure to be executed without necessitating a signalling exchange with a source cell beforehand. The LTM, as introduced in Release- 18, offers a short interruption time but not with the same level of robustness as the conditional L3 mobility procedures. In Release- 19, enhancements should be specified so that the system may benefit from both high robustness and short interruptions.

[0024] The C-LTM is a combination of techniques used for the CHO and the LTM. In C-LTM, the source cell sends a C-LTM configuration of a candidate cell via a Radio Resource Control (RRC) reconfiguration message to the UE, which includes LTM candidate configurations and corresponding execution conditions. Each candidate cell provides its own execution condition for the C-LTM. In the C-LTM, the UE is configured with one or more LTM candidate cells and specific execution conditions. Once the conditions are satisfied, the UE detaches from the source cell and applies the stored corresponding C-LTM candidate cell configuration for that selected candidate cell. The UE then performs a RACH-less C-LTM handover to the candidate cell and finalizes the procedure by sending an RRC reconfiguration complete message to the target gNB-DU.

[0025] According to 3GPP Technical Specification (TS) 38.300, the LTM is a procedure in which the gNB receives LI measurement report(s) from the UE, and on this basis, the gNB may change a UE serving cell by a cell switch command signalled via a Medium Access Control (MAC) Control Element (MAC CE). The cell switch command indicates an LTM candidate configuration that the gNB previously prepared and provided to the UE through RRC signalling. Then, the UE switches to a target configuration according to the cell switch command. The LTM procedure can be used to reduce the mobility latency. When configured by a network, there is a possibility to activate Transmission Configuration Indicator (TCI) states of one or multiple cells that are different from the current serving cell. For instance, the TCI states of the LTM candidate cells can be activated in advance before any of those cells become the serving cell. This allows the UE to be DL synchronized with those cells, thereby facilitating a faster cell switch to one of those cellswhen the cell switch is triggered. All the activated TCI states except those received in the cell switch command are deactivated upon LTM cell switch execution.

[0026] When configured by the network, there is a possibility to initiate a Uplink Timing Advance Acquisition (UL TA) acquisition (e.g., called an early TA) procedure of one or multiple cells that are different from the current serving cells. If the cell has the same NTA as the current serving cells or NTA=0, an early TA acquisition procedure is not required. The network may request the UE to perform the early TA acquisition of a candidate cell before a cell switch. The early TA acquisition procedure is triggered by a Physical Downlink Control Channel (PDCCH) order as specified in clause 9.2.6 or realized through a UE-based TA measurement as configured by an RRC. In the former case, the gNB / gNB-DU, to which the candidate cell belongs, calculates the TA value and sends it to the gNB / gNB-DU to which the serving cell belongs via the gNB-CU. The serving cell sends the TA value in the LTM cell switch command MAC CE when triggering the LTM cell switch. In the latter case, the UE performs the TA measurement for the candidate cells after being configured by the RRC, but the exact time the UE performs the TA measurement is up to the UE implementation. The UE applies the TA value measured by itself and performs RACH-less LTM upon receiving the cell switch command, if it does not include any valid TA value. The network may also send a TA value in the LTM cell switch command MAC CE without the early TA acquisition.

[0027] Depending on the availability of a valid TA value, the UE performs either the RACH-less LTM or a RACH-based LTM cell switch. If the valid TA value is provided in the cell switch command, the UE applies the TA value as instructed by the network. In the case where UE-based TA measurement is configured, but no valid TA value is provided in the cell switch command, theUE applies the valid TA value by itself, if available. The UE performs the RACH-less LTM cell switch upon receiving the cell switch command whenever a valid TA value is available. If no valid TA value is available, the UE performs the RACH-based LTM cell switch.

[0028] In the context of handover execution, the cell switch command is conveyed in the MAC CE, which contains the necessary information to perform the LTM cell switch. The overall procedure for the LTM is shown in FIG. 2. Subsequently, the LTM is performed by repeating the early synchronization, LTM cell switch execution, and LTM cell switch completion steps without releasing other LTM candidate configurations after each LTM cell switch completion. The general procedure over the air interface applies to Secondary Cell Group (SCG) LTM. Further details of SCG LTM may be found in TS 37.340

[0021] ,

[0029] FIG. 2 is a sequence flow diagram illustrating a signalling procedure 200 for the LTM, according to prior art. The sequence flow diagram includes several operations outlined as follows.

[0030] At operation-201, the UE 200a sends a measurement report message to the gNB 200b, wherein the UE 200a is in an RRC connected mode. The gNB 200b decides to configure the LTM and initiates LTM preparation. At operation-202, the gNB 200b transmits an RRC reconfiguration message to the UE 200a, including the LTM candidate configurations. At operation-203, the UE 200a stores the LTM candidate configurations and transmits an RRC reconfiguration complete message to the gNB 200b.

[0031] At operation-204a, the UE 200a performs the DL synchronization with the LTM candidate cell(s) before receiving the cell switch command. The UE 200a may activate and deactivate the TCI states of the LTM candidate cell(s), as triggered by the gNB 200b. At operation-204b, the UE 200a may perform the UL synchronization with the LTM candidate cell(s) before receiving thecell switch command, by using the UE-based TA measurement, if configured, and / or by transmitting a preamble towards the candidate cell, as triggered by the gNB 200b. When the UE-based TA measurement is configured, the UE 200a acquires the TA value(s) of the candidate cell(s) by measurement. The UE 200a performs the early TA acquisition with the candidate cell(s) as requested by the network before receiving the cell switch command as specified in clause 9.2.6. This is performed via a Contention-Free Random Access (CFRA) triggered by a PDCCH order from the source cell, following which the UE 200a sends a preamble towards the indicated candidate cell. In order to minimize the data interruption of the source cell due to CFRA towards the candidate cell(s), the UE 200a does not receive a random access response from the network for the purpose of TA value acquisition, and the TA value of the candidate cell is indicated in the cell switch command. The UE 200a does not maintain a Timing Advance (TA) timer for the candidate cell and relies on network implementation to guarantee the TA validity.

[0032] At operation-205, the UE 200a performs L1 measurements on the configured LTM candidate cell(s) and transmits L1 measurement reports to the gNB 200b. L1 measurement should be performed as long as RRC reconfiguration (operation 202) is applicable. At operation-206, the gNB 200b decides to execute a cell switch to a target cell. The gNB 200b transmits an LTM cell switch command MAC CE triggering cell switch by including a target configuration ID which indicates an index of the candidate configuration of the target cell, a beam indicated with a TCI state or beams indicated with DL and UL TCI states, and a timing advance command for the target cell, if available. The UE 200a switches to the target cell and applies the candidate configuration indicated by the target configuration ID.

[0033] At operation-207, the UE 200a performs the random access procedure towards the target cell, if the UE 200a does not have a valid TA of the target cell as specified in clause 5.18.35 of TS 38.321 [6], At operation-208, the UE 200a completes the LTM cell switch procedure by sending the RRC reconfiguration complete message to the target cell. If the UE 200a has performed the RA procedure in operation 207, the UE 200a considers that LTM cell switch execution is successfully completed when the random access procedure is successfully completed. For the RACH-less LTM, the UE 200a considers that the LTM cell switch execution is successfully completed when the UE 200a determines that the network has successfully received its first UL data.

[0034] The operations 204-208 may be performed multiple times for subsequent LTM cell switch executions using the LTM candidate configuration(s) provided in operation 202.

[0035] In addition, the procedure over the air interface described in FIG. 2 applies to both intra-gNB-DU LTM and inter-gNB-DU LTM. The overall LTM procedures over the F1-C interface are captured in TS 38.401[4]. Due to the above-mentioned various functionalities or operations, the LTM may offer several advantages. For instance, the LTM enhances network efficiency by allowing quicker handovers between cells, reducing latency, and improving user experience during calls or data sessions. The LTM optimizes resource allocation, enabling better management of bandwidth and reducing congestion. Additionally, the LTM supports seamless connectivity for users on the move, ensuring consistent service quality. By operating at lower layers, the LTM minimizes the overhead associated with higher-layer protocols, leading to faster response times and improved overall system performance.

[0036] Despite the above mentioned advantages, several challenges / problems / limitations are encountered in the existing methods (e.g., LTM, C-LTM, etc.), which are mentioned below.

[0037] RAN2#127-bis established specific agreements, including support for RACH-less Conditional intra-CU LTM and PDCCH-ordered early Timing Advance (TA) acquisition for the C-LTM. Even during the C-LTM, a User Equipment (UE) must acquire TA for the C-LTM candidate cell by performing UL-Sync (RACH) based on a PDCCH order from the serving gNB-DU (e.g., the gNB 200b). TA is essential for the UE 200a to execute a RACH-less C-LTM cell switch to the target cell.

[0038] In traditional LTM, the serving gNB-DU manages the UE’s TA, being aware of when the TA for a particular candidate cell is active or expired. The serving gNB-DU is in charge of LTM execution by sending a Downlink MAC Control Element (DL MAC CE), allowing detection of when the UE's TA for a specific LTM candidate cell requires refreshing, necessitating a RACH procedure to reacquire TA. During an LTM cell switch execution, the source gNB-DU may transmit all candidate cell IDs along with their respective active TA to the target gNB-DU.

[0039] Two methods exist for delivering TA to the UE 200a in C-LTM:a. The serving / source gNB-DU may provide TA along with the cell switch command, a method employed in traditional LTM, orb. The target gNB-DU may send TA directly to the UE 200a in a random-access response when the UE 200a performs RACH to acquire TA from the candidate cell.

[0040] In the case of the C-LTM, the LTM cell switch occurs autonomously at the LEE 200a based on predefined execution conditions. Since the source gNB-DU lacks knowledge of whether the UE’s TA for a specific cell is active or expired, the source gNB-DU cannot guarantee that the UE’sTA for a particular C-LTM candidate cell remains active (active at all times), especially during the execution of the C-LTM cell switch. This lack of awareness may lead to potential failure in cell switching, resulting in service interruption or degraded performance for the UE 200a.

[0041] To address the above-mentioned challenges / problems / limitations, a disclosed method provides a unique strategy for the TA maintenance during the C-LTM, as described in conjunction with FIG. 3 to FIG. 10

[0042] Referring now to the drawings, and more particularly to FIGS. 3 to 10, where similar reference characters denote corresponding features consistently throughout the figures, their preferred embodiments are shown.

[0043] FIG. 3 is a flow diagram illustrating a method 300 for initiating a re-acquisition of a TA value based on either a network-based decision-making timer or a UE-initiated request, according to an embodiment as disclosed herein. The method 300 may execute multiple operations for initiating a re-acquisition of the TA value, which are given below.

[0044] At operation 301, the method 300 includes receiving, at a User Equipment (UE), the TA value associated with a LTM candidate gNB-DU cell from a candidate gNB-DU. At operation 302, the method 300 includes transmitting, by the UE, the TA value to a serving gNB-DU associated with the UE via an Uplink Medium Access Control-Control Element (UL MAC-CE) message to maintain the received TA value at both the UE and the serving gNB-DU. At operation 303, the method 300 includes determining, by the UE and / or network, whether the received TA value associated with the LTM candidate gNB-DU cell has expired. At operation 304, the method 300 includes initiating the re-acquisition of the TA value based on either the network-based decision-making timer or the UE-initiated request, as described in conjunction with FIGS.4-5, FIG.6, FIG.7, and FIG. 8.

[0045] Further, a detailed description related to the various operations of FIG.3 is covered in the description related to FIGS. 4-5, FIG. 6, FIG. 7, and FIG. 8 and is omitted herein for the sake of brevity.

[0046] In some example embodiments, the method 300 has several advantages over the existing method. First, the method 300 maintains the TA value at both the UE and the serving gNB-DU, which enhances synchronization, which is crucial for efficient data transmission and reception. This synchronization minimizes latency and improves overall network performance. Second, the ability to determine whether the TA value has expired allows for proactive management of the connection. By initiating the re-acquisition of the TA value either through the network-based decision-making timer or the UE-initiated request, the method 300 ensures that the UE remains connected to the appropriate cell without interruptions. This proactive approach reduces the likelihood of dropped connections and enhances user experience.

[0047] Additionally, the implementation of RACH-less C-LTM streamlines the process of cell transition. By eliminating the need for RACH procedures, the method 300 facilitates quicker and more seamless handovers. This results in improved mobility management, particularly in environments with high user density or rapid movement. Overall, the method 300 promotes efficient resource utilization, enhances connection reliability, and supports seamless mobility, contributing to a more robust and responsive communication network.

[0048] In some example embodiments, the TA value is received directly from the candidate gNB- DU in the random access response message or via the serving gNB-DU associated with the UE.

[0049] In some example embodiments, the TA value is received from the candidate gNB-DU through the gNB-CU and the serving gNB-DU. The TA value is received in a downlink MAC CE message.

[0050] FIGS. 4-5 are sequence flow diagrams illustrating a signalling procedure (hereinafter referred to as “method 400”) for the C-LTM, according to an embodiment as disclosed herein. The method 400 may execute multiple operations to maintain / manage TA during the C-LTM, which are given below. The signaling procedure involves multiple entities, specifically the UE 400a, the source gNB-DU 400b, the candidate gNB-DU 400c, and the gNB-CU 400d.

[0051] In the signaling procedure, at an initial stage, user data travels from the UE 400a to the source gNB-DU 400b and is then forwarded by the source gNB-DU 400b to the gNB-CU 400d (e.g., gNB-CU-UP) for processing and onward transmission to the core network.

[0052] At operation-401, the procedure initiates with the execution of one or more operations pertaining to Layer 3 (L3) measurement control and reporting procedure. The operation-401 is critical for assessing the quality of the radio environment. At operation-402, the gNB-CU 400d evaluates and makes a decision regarding the addition of an LTM candidate cell, based on the L3 measurement report received from the UE 400a. At operation-403, the gNB-CU 400d issues a UE context setup request to the candidate gNB-DU 400c to prepare the LTM candidate cell configuration. This request is essential for establishing the context necessary for the UE’s LTM cell switch operation in the candidate gNB-DU 400c. At operation-404, the candidate gNB-DU 400c responds to the gNB-CU 400d with a UE context setup response. This UE context setup response returns the LTM candidate cell configuration and confirms the successful establishment of the UE context in the candidate gNB-DU 400c.

[0053] At operation-405, following the context setup, the gNB-CU 400d sends an UE context setup modification request to the source gNB-DU 400b. This UE context setup modification request is aimed at modifying the existing UE context to notify the LTM candidate cell configuration and accommodate the transition. At operation-406, the source gNB-DU 400b processes the modification request and sends back a UE context setup modification response to the gNB-CU 400d, indicating the success or failure of the modification.

[0054] At operation-407, the gNB-CU 400d then transmits a UE context modification request to the candidate gNB-DU 400c. This is necessary to synchronize the CSI-RS related configuration, as consolidated by the source gNB-DU 400b. At operation-408, the candidate gNB-DU 400c replies with an UE context modification response to the gNB-CU 400d, confirming the acceptance of the modifications made to the UE context. At operation-409, the gNB-CU 400d proceeds to send a downlink RRC message transfer, specifically an RRC Reconfiguration message, to the source gNB-DU 400b. This message is pivotal for sending the LTM candidate cell configuration to the UE 400a. At operation-410, the source gNB-DU 400b forwards the RRC reconfiguration message to the UE 400a, ensuring that the LTM candidate cell configuration provided by the candidate gNB-DU 400c is delivered to the UE 400a.

[0055] At operation-411, the UE 400a acknowledges the successful reconfiguration by transmitting an RRC reconfiguration complete message back to the source gNB-DU 400b. At operation-412, the source gNB-DU 400b sends an uplink RRC message transfer to the gNB-CU 400d that contains the RRC message from the UE 400a, completing the signaling procedure and ensuring that all entities are synchronized with the LTM candidate cell configuration.

[0056] At operation-413, the UE 400a sends a L1 measurement report to the source gNB-DU 400b. At operation-414 and 415, the source gNB-DU 400b transmits the PDCCH order to perform early TA acquisition to the UE 400a, and the UE 400a executes a RACH procedure with the candidate gNB-DU 400c to acquire candidate cell RA. At operation-416, the candidate gNB-DU 400c transmits DU-CU TA information to the gNB-CU 400d. At operation-417, the gNB-CU 400d transmits CU-DU TA information to the source gNB-DU 400b. At operation-418, the source gNB-DU 400b receives the UE’s TA of the candidate cell from the gNB-CU 400d (after the UE 400a has performed the RACH procedure). The source gNB-DU 400b sends the TA and the TCI state info in a MAC CE to the UE 400a, which may relate to existing art as mentioned prior, " The serving gNB-DU may provide TA along with the cell switch command, a method employed in traditional LTM”. Herein, in one embodiment, the TA (also known as Timing Advance (TA) value) is associated with the LTM candidate gNB-DU cell from the candidate gNB-DU 400c.

[0057] At operation-419, the UE 400a determines that the TA has expired. At operation-420, upon expiration of the TA, the UE 400a verifies the TA acquisition criteria (e.g., Reference Signal Received Power (RSRP) threshold of the candidate cell) and sends a MAC CE (e.g., UL MAC-CE message) along with the target cell ID to the source gNB-DU 400b for requesting a Physical Downlink Control Channel (PDCCH) order by providing the Cell ID or configuration index. Alternatively, the source gNB-DU 400b may also determine that the TA timer has expired by maintaining a candidate cell TA timer. In such a scenario, the source gNB-DU 400b may initiate the PDCCH order to acquire the TA, if the given candidate cell continues to be good enough for the UE 400a i.e RRM conditions to acquire the TA are satisfactory. Therefore, the UE 400a doesn’t have to request for the PDCCH order.

[0058] At operation-421, the UE 400a receives the PDCCH order (cell ID) from the serving gNB- DU (e.g., source gNB-DU 400b). In other words, the serving gNB-DU (e.g., source gNB-DU 400b) sends the PDCCH order to re-acquire the TA. At operation-422, the UE 400a sends the RACH request to the candidate gNB-DU 400c, to re-acquire the TA.

[0059] At operation-423, the UE 400a makes a decision to trigger the C-LTM cell switch, when an execution condition is satisfied. At operation-424, the UE 400a executes the RACH-less C-LTM procedure with the identified candidate gNB-DU 400c. At operation-425, the source gNB-DU 400b transmits a DU-CU cell switch notification to the gNB-Central Unit (gNB-CU 400d), which includes a target cell identifier (ID) and a Transmission Configuration Indicator (TCI) state ID. At operation-426, in response, the gNB-CU 400d forwards a CU-DU cell switch notification to the candidate gNB-DU 400c, also containing the target cell ID and TCI state ID. It has to be noted here that operations 425 and 426 are feasible only if the UE 400a reports the selected target cell ID and TCI state to the source gNB-DU 400b before executing the CLTM cell switch. At operation-427, concurrently, the source gNB-DU 400b provides a downlink data delivery status update to the gNB-CU 400d.

[0060] At operation-428, the candidate / target gNB-DU 400c detects the UE 400a access / acknowl edges the access attempt from the UE 400a. At operation-429, the candidate gNB-DU 400c confirms successful access to the gNB-CU 400d. At operation-430, the UE 400a completes the RRC reconfiguration process by sending an RRC reconfiguration complete message to the candidate gNB-DU 400c. At operation-431, the candidate gNB-DU 400c transmits an uplink RRC message transfer, specifically the RRC reconfiguration complete notification, to the gNB-CU 400d

[0061] At operation-432, the gNB-CU 400d issues a UE context release command to the source gNB-DU 400b to terminate the existing context. At operation-433, the source gNB-DU 400b acknowledges the release of the UE context by sending a UE context release complete message back to the gNB-CU 400d. As a result, at the final stage, user data travels from the UE 400a to the candidate gNB-DU 400c and is then forwarded by the candidate gNB-DU 400c to the gNB-CU 400d (e.g., gNB-CU-UP) for processing and onward transmission to the core network.

[0062] FIG. 6 is a flow diagram illustrating a method 600 for initiating the re-acquisition of the TA value based on the network-based decision-making timer, according to an embodiment as disclosed herein. The method 600 may execute multiple operations for initiating the re-acquisition of the TA value, which are given below.

[0063] At operation 601, the method 600 includes receiving, at the UE 400a, the PDCCH order from the serving gNB-DU 400b. The serving gNB-DU 400b transmits the PDCCH order based on the TA timer expiry. The PDCCH order indicates an instruction to the UE 400a to re-acquire the TA from the LTM candidate gNB-DU cell. At operation 602, the method 600 includes initiating the re-acquisition of the TA from the LTM candidate gNB-DU cell in response to receiving the PDCCH order. Further, a detailed description related to the various operations of FIG. 6 is covered in the description related to FIGS. 4A-4B and FIG. 5, and is omitted herein for the sake of brevity.

[0064] FIG. 7 is a flow diagram illustrating a method 700 for initiating the re-acquisition of the TA value based on the UE-initiated request, according to an embodiment as disclosed herein. The method 700 may execute multiple operations for initiating the re-acquisition of the TA value, which are given below.

[0065] At operation 701, the method 700 includes determining, by the UE 400a, whether a radio condition associated with the LTM candidate gNB-DU cell indicates that the LTM candidate gNB-DU cell satisfies a criterion to acquire the TA value. At operation 702, the method 700 includes transmitting, by the UE 400a, the UL MAC-CE message to the serving gNB-DU 400b for requesting the PDCCH order in response to determining that the LTM candidate gNB-DU cell satisfies the criterion to re-acquire the TA value. The PDCCH order indicates an instruction to the UE 400a to re-acquire the TA. The UL MAC-CE message comprises either the C-LTM candidate cell ID or the configuration index of the candidate gNB-DU 400c.

[0066] At operation 703, the method 700 includes receiving, at the UE 400a, the PDCCH order from the serving gNB-DU 400b. At operation 704, the method 700 includes initiating, by the UE 400a, the re-acquisition of the TA value in response to receiving the PDCCH order. Further, a detailed description related to the various operations of FIG.7 is covered in the description related to FIGS. 4A-4B and FIG. 5, and is omitted herein for the sake of brevity.

[0067] In some example embodiments, the method may perform various operations to initiate the re-acquisition of the TA value based on the UE-initiated request. For instance, the method includes receiving, at the UE 400a, configuring from the serving gNB-DU 400b to transmit an L1 measurement report to the serving gNB-DU 400b. The method further includes transmitting, by the UE 400a, the L1 measurement report to the serving gNB-DU 400b. The serving gNB-DU 400b determines whether the TA value needs to be re-acquired based on the L1 measurement report. The method further includes receiving, by the UE 400a, an indication from the serving gNB-DU 400b to initiate the re-acquisition of the TA value in response to determining that the TA value needs to be re-acquired.

[0068] FIG. 8 is a flow diagram illustrating a method 800 for initiating the re-acquisition of the TA value based on the UE-initiated request, according to another embodiment as disclosed herein. The method 800 may execute multiple operations for initiating the re-acquisition of the TA value, which are given below.

[0069] At operation 801, the method 800 includes receiving, at the UE 400a, the TA value associated with the LTM candidate gNB-DU cell. At operation 802, the method 800 includes maintaining, at the UE 400a, the received TA value associated with the LTM candidate gNB-DU cell. At operation 803, the method 800 includes determining, by the UE 400a, whether the received TA value has expired. At operation 804, the method 800 includes initiating the re-acquisition of the TA value based on the UE-initiated request in response to determining that the received TA value has expired. Further, a detailed description related to the various operations of FIG. 8 is covered in the description related to FIGS. 4A-4B and FIG. 5, and is omitted herein for the sake of brevity.

[0070] In some example embodiments, the method may perform various operations to initiate the re-acquisition of the TA value based on the UE-initiated request. For instance, the method includes determining, by the UE 400a, whether a radio condition indicates that the LTM candidate gNB-DU cell satisfies a criterion to acquire the TA value. The method further includes transmitting, by the UE 400a, the UL MAC-CE message to the serving gNB-DU 400b for requesting the PDCCH order in response to determining that the candidate gNB-DU 400c still satisfies the criterion to acquire the TA value. The method further includes receiving, at the UE 400a, the PDCCH order from the serving gNB-DU 400b. The method further includes initiating, by the UE 400a, the reacquisition of the TA value in response to receiving the PDCCH order.

[0071] In some example embodiments, the method may determine whether a TA timer is active. The method may further configure the UE 400a to reduce a number of periodic L1 measurement reports, using the RRC reconfiguration message.

[0072] In some example embodiments, the method may not prevent the UE 400a from transmitting an L1 measurement report when a Timing Advance Timer (TAT) (i.e., related to TA value) of the LTM candidate gNB-DU cell is to expire.

[0073] In some example embodiments, the method may configure the UE 400a for one or more event-based L1 measurements at the same time and indicate via an RRC reconfiguration message that a particular event-based measurement configuration is for TA acquisition.

[0074] FIG. 9 is a flow diagram illustrating a method 900 for initiating the re-acquisition of the TA value based on one or more predefined conditions, according to an embodiment as disclosed herein. The method 900 may execute multiple operations for initiating the re-acquisition of the TA value, which are given below.

[0075] At operation 901, the method 900 includes receiving, at the serving gNB-DU 400b, the TA value associated with the LTM candidate gNB-DU cell from at least one of the gNB-CU 400d, the candidate gNB-DU 400c, or the UE 400a. At operation 902, the method 900 includes storing the received TA value at the serving gNB-DU 400b.

[0076] At operation 903, the method 900 includes determining, after storing the received TA value, whether one or more predefined conditions are satisfied. For instance, one or more predefined conditions may include an expiration of the TA timer, receiving the L1 measurement report indicating that the re-acquisition of the TA value is to be initiated, a receipt of the UL MAC-CE message from the UE 400a requesting the PDCCH order.

[0077] At operation 904, the method 900 includes identifying that the re-acquisition of the TA value is to be initiated in response to determining that the one or more predefined conditions are satisfied. At operation 905, the method 900 includes initiating the re-acquisition of the TA value by transmitting the PDCCH order to the UE 400a. Herein, the PDCCH order indicates an instruction to the UE 400a to re-acquire the TA. Further, a detailed description related to the various operations of FIG. 9 is covered in the description related to FIGS. 4A-4B, and is omitted herein for the sake of brevity.

[0078] In some example embodiments, the method 900 has several advantages over the existing method. First, the method 900 provides effective maintenance of the TA ensures that the TA remains active and valid at all times, which is crucial for maintaining optimal communication between the UE 400a and the serving gNB-DU 400b / candidate gNB-DU 400c. This proactive approach minimizes potential disruptions in service, leading to improved reliability.

[0079] Additionally, the ability to determine whether the one or more predefined conditions are met before initiating the re-acquisition process streamlines operations. Moreover, the method 900 supports the RACH-less C-LTM procedure, which reduces the need for RACH procedures. This efficiency not only decreases latency but also optimizes resource usage, allowing for better overall network performance. By transmitting the PDCCH order to the UE 400a, the method 900 facilitates a seamless re-acquisition of the TA value, enhancing the user experience through consistent and high-quality connectivity.

[0080] FIG. 10 illustrates a diagram of example components of an apparatus 1000, according to an embodiment as disclosed herein. As shown in FIG. 10, the apparatus 1000 comprises aprocessor 1010, a memory 1020, a storage component 1030, an input component 1040, an output component 1050, a communication interface 1060, and a bus 1070.

[0081] In some example embodiments, the apparatus 1000 may relate to at least one of the UE 400a, the source gNB-DU, and / or service gNB-DU 400b, the candidate gNB-DU 400c, and the gNB-CU 400d.

[0082] In some example embodiments, the processor 1010, as used herein, means any type of computational circuit that may comprise hardware elements and software elements. The processor 1010 may be embodied as a multi-core processor, a single core processor, or a combination of one or more multi-core processors and / or one or more single core processors, a distributed processing system, or the like. The processor 1010 may be a Central Processing Unit (CPU), a Graphics Processing Unit (GPU), an Accelerated Processing Unit (APU), an Application-Specific Integrated Circuit (ASIC), or another type of processing component.

[0083] In some example embodiments, the memory 1020 includes a non-transitory computer readable medium. Memory 1020 includes a Random-Access Memory (RAM), a Read Only Memory (ROM), and / or another type of dynamic or static storage device (e.g., a flash memory, a magnetic memory, and / or an optical memory) that stores information and / or instructions for use by processor 1010. The memory 1020 comprises machine-readable instructions which are executable by the processor 1010. These machine-readable instructions, when executed by the processor 1010 cause the processor 1010 to perform one or more method steps of an embodiment described above.

[0084] In some example embodiments, the storage component 1030 stores information and / or software related to the operation and use of the apparatus 1000. For example, the storagecomponent 1030 may include a hard disk (e.g., a magnetic disk, an optical disk, a magneto-optic disk, and / or a solid-state disk), a Compact Disc (CD), a Digital Versatile Disc (DVD), a floppy disk, a cartridge, a magnetictape, and / or another type of non-transitory computer-readable medium, along with a corresponding drive.

[0085] In some example embodiments, the input component 1040 is configured to receive information, such as user input. For example, the input component 1040 may include, but not be limited to, a touchscreen display, a keyboard, a keypad, a mouse, a button, a switch, and / or a microphone. Additionally, or alternatively, the input component 1040 may include a sensor for sensing information (e.g., a Global Positioning System (GPS), an accelerometer, a gyroscope, and / or an actuator).

[0086] In some example embodiments, the output component 1050 is configured to provide output information from the apparatus 1000. For example, the output component 1050 may be, but is not limited to, a display, a speaker, instructions to an external device, and / or one or more Light-Emitting Diodes (LEDs).

[0087] In some example embodiments, the communication interface 1060 is an interface that provides a communication connection to other devices, such as external devices and internal devices. The connection by the communication interface 1060 can be a wired connection, a wireless connection, or a combination of wired and wireless connections, and can be a direct connection or an indirect connection via a communication network that exists between the apparatus 1000 and other devices. In other words, the standard of the communication interface 1060 is not limited.

[0088] In some example embodiments, the bus 1070 acts as an interconnect between the processor 1010, the memory 1020, the storage component 1030, the input component 1040, the output component 1050, and the communication interface 1060 of the apparatus 1000. The bus 1070 may include a wired interconnection or a wireless interconnection.

[0089] In some example embodiments, the number and arrangement of components shown in FIG.10 are provided as an example. In practice, the apparatus 1000 may include additional components, fewer components, different components, or differently arranged components than those shown in FIG. 10. Additionally, or alternatively, a set of components (e.g., one or more components) of the apparatus 1000 may perform one or more functions described as being performed by another set of components of the apparatus 1000. Further, one or more method steps / operations described in any of the embodiments may be performed utilizing the apparatus 1000 in communication with one another.

[0090] In some example embodiments, the disclosure may represent techniques related to a document titled “Conditional LTM” for discussion and decision at the 3GPP TSG-RAN Working Group 2 (WG2) Meeting R2-129, held in Athens, Greece, from February 17 to 21, 2025. The document, identified as R2-2500286, falls under Agenda Item 8.6.4 and is associated with the NR Mob Ph4-Core Release 19.

[0091] In RAN #105, the Rel-19 WID on NR mobility enhancements Phase 4 was updated [1], where the objectives relevant to conditional LTM (CLTM) are described below:Specify support of conditional Intra-CU LTM [RAN2, RAN3, RANI]o Specify UE evaluated conditions for triggering LTMo Aim to support conditional LTM, including subsequent LTMo Limit specifying the conditional LTM to the scenario where the UE is in non-DC Checkpoint at RAN# 107 to review the objective on whether Intra-CU conditional LTM can be specified to DC scenarios, and if so, to which cases. RAN WG work to not start before this checkpoint

[0092] In RAN2#127bis, the following progress was made:Agreements on C-LTM:1. Source cell sends the conditional LTM configuration via RRCReconfiguration to UE, which includes the LTM candidate configurations, and the corresponding execution conditions.2. Event LTM3-like and LTM5-like are used as the conditional LTM execution condition.FFS on reuse of CHO conditions.3. The source cell and each candidate cell provide its own execution condition for conditional LTM.4. It is DU to generate the LI execution condition. FFS on a case where L3 measurement is used.5. RACH-less Conditional intra-CU LTM is supported.6. RACH-based conditional intra-CU LTM is supported.7. UE-based TA measurement mechanism is supported for conditional intra-CU LTM. 8. PDCCH-ordered early TA acquisition is supported for conditional LTM.9. Rel-18 Early candidate TCI State activation / deactivation is supported for conditional intra-CU LTM.10. For RACH-less conditional LTM, CG-based first UL transmission on the target cell is supported. FFS on DG-based approach.11. The LTM completion defined for Rel-18 intra-CU LTM is reused for conditional LTM.

[0093] In RAN2 128, the following progress was made:Agreements on C-LTM:1. The triggering condition of conditional LTM can be based on L3 measurement.2. The CondEventA3 and CondEventA5 conditions can be a baseline for the conditional LTM execution.3. The L1 execution condition of a candidate cell is associated to only one triggering event.4. For L3 execution condition, it may consist of one or two triggering condition(s). If there are two triggering conditions associated with the same candidate cell, the UE shall consider the execution condition fulfilled only when both triggering conditions are met. Only a single RS type is supported, and at most two different trigger quantities can be configured simultaneously for the evaluation of the execution condition of a single candidate cell.5. To support initial and subsequent conditional LTM, the following items can be considered for the configuration of execution conditions:a) The CLTM configuration of each candidate cell shall include the execution condition for initial conditional LTM, which is generated by the initial source cell to trigger the CLTM for the candidate cell.b) The CLTM configuration of each candidate cell may include execution conditions for subsequent conditional LTM, which is generated by the candidate cell to trigger the CLTM for other candidate cells when the candidate cell becomes a serving cell.6. The network can configure measurement reports, e.g., LI periodic, semi-persistent, aperiodic, and event-triggered reports, or L3 measurement reports for conditional LTM, e.g., to trigger PDCCH ordered early RACH.7. For CLTM, the Candidate Cell TCI States Activation / Deactivation MAC CE is reused for the early activation / deactivation of TCI state(s) of a CLTM candidate configuration.8. The Early TA is signalled to the UE from the source cell (i.e., not from the candidate cell directly to the UE). This agreement will be included in the LS to RAN1 / 3 / 4.9. The network can inform the candidate cell's TA information to the UE via a new MAC CE, which is the TA value when the UE switches to that candidate cell during CLTM.10. Candidate cell TA is maintained by a new timer.11. For L1-based conditional LTM, the condition evaluation is at the MAC level, and for L3-based conditional LTM, the condition evaluation is at the RRC level.

[0094] The primary motivation for CLTM is to introduce robustness to the LTM procedure where the UE may not be able to receive the LTM cell-switch command from the serving gNB-DU,which may lead to radio link failure. The conditional LTM is expected to bring the benefits of robustness while retaining the interruption time reduction benefits of LTM.

[0095] In this contribution, the disclosure discusses CLTM and the related key issues.

[0096] Open issues from RAN2 #128:

[0097] For Beam(s) used for C-LTM event evaluation: in RAN2#128, there were two contrasting proposals discussed regarding how and when the UE evaluates the execution condition for the candidate beam and triggers the CLTM cell switch procedure. One proposal was that the CLTM cell switch is triggered when at least one beam fulfills the associated execution condition. The counter proposal was based on the UE evaluating multiple beams of candidate cells and triggering the CLTM cell switch only when all of them satisfy the execution condition.

[0098] TTT for L1-based measurement events is already agreed to be based on individual beams. Therefore, there is no reason to evaluate multiple beams to trigger the CLTM cell switch. The UE evaluates the execution condition for each candidate cell beam, and when at least one of the candidate cell beams fulfills the execution condition, the UE triggers the LTM cell switch procedure.

[0099] Proposal 1: UE evaluates the execution condition for each candidate beam and triggers the LTM cell switch procedure when at least one beam fulfils the associated execution condition.

[0100] For timing advance maintenance during CLTM: The agreement below was made in RAN2#127bis,8. The Early TA is signalled to the UE from the source cell (i.e., not from the candidate cell directly to the UE). This agreement will be included in the LS to RAN1 / 3 / 4.

[0101] In Rel 18 LTM, the serving DU delivers the TA within the LTM cell switch MAC CE, and hence it was agreed that the serving DU checks the validity of the TA value before sending it to the UE, i.e, maintains the TA of the UE in the candidate cell. In CLTM, since the TA is received by the UE in advance of the CLTM cell switch, it makes sense for the UE to maintain this TA. Hence, propose that the UE maintains the CLTM candidate cell TA, corresponding to each CLTM candidate cell.

[0102] Once the RACH-less CLTM is completed and signalled to the target gNB-DU, the UE should be able to inform all the valid and active TAs of different candidate cells. This is required to prevent redundant RACH-based TA acquisition at the target gNB-DU.

[0103] Proposal 2: The validity of the CLTM candidate cell TA value is maintained by the UE.

[0104] Proposal 3: Whenever a TA value is received, the UE starts a Time Alignment Timer (TAT), which is specific to each CLTM candidate cell configuration.

[0105] Proposal 4: If the TAT of a CLTM candidate cell expires, the UE may request a PDCCH order from the gNB-DU to re-acquire the TA. This request could be made by sending an L1 measurement report with an indication or the UL MAC CE, subject to the candidate cell satisfying the TA acquisition requirement.

[0106] Proposal 5: If the TAT expires before the execution of a CLTM cell switch, the UE executes a RACH-based CLTM cell switch.

[0107] Proposal 6: After indicating a successful RACH-less CLTM cell switch, the UE informs the target gNB-DU of all the valid and active CLTM candidate cell TAs.

[0108] For UL and DL sync for L3 measurements based on the CLTM: In RAN2 128, it was agreed that CLTM is supported based on L3 measurements also. In RAN2#127bis, it was agreed that early UL and DL sync is supported for intra gNB-CU CLTM.

[0109] In L3 measurements based on the CLTM, the gNB-CU is responsible for triggering early UL and DL sync. This could be performed by configuring beam-level L3 measurements. In addition, it requires the gNB-CU to trigger the gNB-DU to initiate the UL / DL sync procedures at the appropriate time by providing the CLTM candidate cell ID and beam ID. The candidate cell ID is required for the UE to perform RACH-based TA acquisition, and the beam ID is required to activate the TCI states in advance.

[0110] Proposal 7: RAN2 discusses and agrees on all the parameters to be sent from gNB-CU to gNB-DU to initiate early UL and DL sync for L3 measurements based on the CLTM.

[0111] For RACH-based TA acquisition during the CLTM: One of the important advantages of LTM in R18 is the reduction of service interruption during HO, due to RACH-less cell switch. It was agreed in one of the previous meetings to support RACH-less CLTM, and at least PDCCH ordered early TA acquisition.

[0112] A UE configured with CLTM performs a cell switch without receiving a cell switch command, when the CLTM execution condition of a candidate cell is satisfied. Therefore, the existing mechanism of TA information transfer to the UE in the MAC CE cell switch command is not feasible in CLTM, and an alternate mechanism is required for delivering TA for RACH-less CLTM cell switches. TA delivery using random-access response (RAR) to the UE was ruled out for non-conditional intra- and inter-CU LTM and left as FFS for conditional LTM.

[0113] In the case of PDCCH-ordered RACH-based TA acquisition, one solution is that the TA could be provided by the source gNB-DU in a MAC CE, similar to R18.

[0114] While this is some improvement compared to explicitly triggered LTM cell switch, there is still some dependency on the source gNB-DU to assist the cell switch, i.e, an event-triggered UL MAC CE to report LI measurements, and a DL MAC CE to provide TA, before the CLTM.

[0115] To overcome this signaling and latency with the source gNB-DU, a conditional early UL sync could be employed, where the TA value can be obtained based on a new execution condition (ahead of cell switching), therefore reducing the amount of time needed during the cell switch itself. It also ensures that the RACH for TA acquisition is performed without the event-triggered LI measurement report to the source gNB-DU in UL and PDCCH order in DL. The early UL sync and the cell switch could therefore be performed completely based on execution conditions being satisfied at the UE.

[0116] For this conditional early UL sync, a dedicated RACH preamble may have to be allocated at the time of CLTM target cell preparation, which enables the target gNB-DU to identify the UE’s objective of performing RACH.

[0117] Together with this, the conditional trigger for performing RACH would need to be set such that the condition is met earlier than the condition for performing CLTM cell switch.

[0118] A conditional uplink sync could be perfectly complemented by delivering the TA directly to the UE using RAR.

[0119] Proposal 8: RAR-based TA acquisition directly from the target gNB-DU is supported for conditional LTM.

[0120] Direct RAR-based delivery of TA was also discussed in Rel 18 LTM, and the main drawback identified was that, depending on UE capability and cell frequency, it may lead to increased interruption time at the source cell, since the UE must monitor the PDCCH of the candidate cell and may not be able to receive data from the source while monitoring for RAR.

[0121] In addition, delivery of target gNB-DU RAR via the source gNB-DU is fully compatible with the Rel-18 early UL synchronization procedure, and very minimal changes are required to support the inclusion of RAR over Fl.

[0122] Therefore, to address the requirements of UEs that don’t have the capability to monitor PDCCH of the target cell without its serving cell data transmission getting impacted, propose the delivery of RAR via the serving gNB-DU. This could be delivered by having RAR as a payload in a MAC CE.

[0123] Proposal 9: Delivery of target gNB-DU RAR to the UE via the serving gNB-DU is supported.

[0124] For Conditional TA acquisition during CLTM: In one of the previous meetings, it was agreed to support explicit triggering of early UL sync (using PDCCH order for TA acquisition) and early DL sync (Rel-18 Early candidate TCI State activation / deactivation).

[0125] As explained above, to reduce the signaling and latency overhead with the source gNB-DU, a conditional early UL sync could be configured at the UE, where the RACH-based TA acquisition could be performed based on a new execution condition.

[0126] The CLTM execution condition could be a trigger to execute the LTM cell switch, or it could be a trigger for performing early DL / UL sync (e.g., activate TCI state on candidate cell, perform a RACH procedure for TA acquisition).

[0127] In summary, it should be possible to configure an execution condition (e.g., beam-based, cell-based threshold) to perform an action (cell switch, UL sync, DL sync).

[0128] In L3 measurements based on the CLTM, the decision to trigger early DL and UL sync is triggered based on L3 measurements, and this involves additional signaling from gNB-CU to gNB-DU, as the early UL / DL sync is initiated by the gNB-DU. Conditional execution based early UL / DL sync avoids this overhead.

[0129] Proposal 10: Conditional execution, i.e, triggered based on an execution condition, of the following is supported for both L1 measurements based on the CLTM and L3 measurements based on the CLTM.a. CLTM cell switchb. Early DL sync (TCI state activation)c. Early UL sync (RACH-based for TA acquisition).

[0130] For the Dynamic grant for first UL transmission in RACH-less CLTM: During RAN2#127bis, it was agreed that CG-based first UL transmission towards CLTM target cell is supported to indicate a successful RACH-less CLTM. FFS on DG-based approach.

[0131] Conditional LTM execution is triggered by the UE, and the target cell beam selection is also performed by the UE. Hence, the candidate cell is not aware of the beam the UE will select during conditional LTM execution.

[0132] Unlike Rel 18 LTM, there is no notification from source gNB-DU to the target gNB-DU (via gNB-CU) about the LTM cell switch execution, where the selected beam is indicated. Therefore, some assistance information is required to allocate a dynamic grant to the UE at the target cell.

[0133] During conditional RACH-less LTM, if the NW is not aware of the cell ID and the beam ID of the target cell selected by the UE, there is no possibility for the target gNB-DU to know when and on which beam to perform dynamic scheduling for the first PUSCH transmission.

[0134] An obvious solution could be that each candidate cell keeps sending the dynamic scheduling information on each candidate beam or keeps monitoring the UL transmission among all CG occasions after the preparation phase. This is very inefficient and sub-optimal.

[0135] Hence, propose that whenever C-LTM is configured, the UE periodically sends an L1 measurement report to the source cell, which indicates the potential target cells / beams. The source cell can inform all the corresponding potential target gNB-DUs to send dynamic scheduling information on each candidate beam and / or monitor the UL transmission among all CG occasions in those beams.

[0136] Additionally, whenever the L1 execution condition of a C-LTM candidate cell is satisfied and the target cell beam selection is performed, if the UE’s radio link with the source cell is intact, the UE shall indicate the selected target cell and beam information to the source cell. In such a case, the Rel 18 mechanism can be reused.

[0137] Proposal 11: UE is configured to periodically report the potential target cell and the corresponding beam information to the source cell during C-LTM.

[0138] DG DG-based solution is necessary for CLTM and is also appropriate, as it alleviates the need to reserve radio resources for a long time, as the network doesn’t have any control over the CLTM cell switch lead time.

[0139] In the DG-based solution, the target gNB-DU needs to be aware when the UE has performed CLTM cell switch and can receive DCI on the PDCCH of the new cell, to allocate the dynamic grants.

[0140] The feasible solutions for this are:a. Indicate to the source gNB-DU (e.g., an event-triggered measurement report in MAC CE, or using a dedicated SR), the selected target cell and beam. The source gNB-DU notifies the target gNB-DU via the gNB-CU, as in Rel 18 LTM. b. Indicate directly to the target gNB-DU (E.g., send a dedicated SR to the target gNB- DU).

[0141] Without an indication / request to either source or target, it is not feasible to schedule a UE on the target gNB-DU post CLTM cell switch.

[0142] Proposal 12: For DG-based RACH-less CLTM, the UE notifies the completion of a successful CLTM by sending a UL MAC CE to the source gNB-DU, which includes the selected target cell and beam information, or sending a dedicated SR to the target gNB-DU or both.

[0143] For CLTM cell switch notification to the gNB-CU: In Rel 18 LTM, the gNB-DU notifies the gNB-CU after the LTM cell switch command is delivered to the UE. This ensures that the gNB-CU is always aware of the UE’s current serving cell.

[0144] In CLTM, the gNB-DU doesn’t deliver a cell switch command; instead, the CLTM cell switch is based on the CLTM execution condition. Hence, there is a need to address this issue using an alternative mechanism.

[0145] The UE may inform the source gNB-DU of the selected CLTM target cell right when the CLTM execution condition is satisfied, but this may not be reliable, as there could be a too-late HO, and the message may not successfully reach the source gNB-DU.

[0146] Hence, propose that the UE informs the target gNB-DU of the target cell configuration index sent over RRC, after a successful CLTM cell switch. The target gNB-DU can send the same to the gNB-CU, using which the gNB-CU can identify the UE’s current serving cell.

[0147] Proposal 13: UE informs the target gNB-DU of the target cell configuration index sent over RRC, after a successful CLTM cell switch.

[0148] For Co-existence of LTM and CLTM: CLTM could be seen as an extension of Rel 18 LTM. LTM and CLTM candidate cell configurations are not mutually exclusive and hence configurable for the same UE at the same time. The gNB-CU needs to ensure that the maximum limit of 8 candidate cells is not exceeded, and the same cell is not configured for both LTM and CLTM.

[0149] Proposal 14: A UE can be configured with both LTM and CLTM candidate cells at the same time, with a maximum limit of 8 candidate cells.

[0150] For CLTM for a multiTRP UE: When a UE is configured with mTRP and with C-LTM, we have the following.a. There are at least two TRPs configured for the UE.b. The TRPs could belong to different cells, which could belong to the same or different gNB-DU, but the same gNB.c. The UE can be configured with one or more C-LTM candidate cells coupled with a CLTM execution condition.d. The C-LTM candidate cells and the execution condition are TRP-specific, i.e, candidate cells and execution conditions of one TRP don’t apply to the other.

[0151] So, there is a possibility that only one of the TRPs satisfies the CLTM execution condition, while the other does not. There are the following feasible alternatives when only one of the TRPs satisfies the CLTM execution condition.a. The UE performs RACH-less C-LTM cell switch for only that TRP. The other TRPs are retained with their serving cell, until either of the following occurs. i. The TRP execution condition(s) are satisfied, and the C-LTM cell switch can be executed.ii. UE detects RLF for the TRP, and the TRP is autonomously discarded by the UE. No RRC re-establishment is performed after RLF detection, as there is another TRP active. TRP recovery can be attempted, if configured and feasible.b. One of the TRPs is designated as the master TRP by the gNB, and the UE performs C-LTM cell switch only if the execution condition(s) of the master TRP are satisfied.c. C-LTM cell switch is performed only when both TRPs have the execution condition(s) satisfied.d. C-LTM candidates are never prepared with mTRP (only prepared with a single TRP). If the C-LTM execution condition is satisfied for one TRP at the source, the UE performs a C-LTM cell switch. The network may configure mTRP again after a successful C-LTM cell switch.

[0152] Proposal 15: RAN2 discusses and agrees on a way forward for CLTM handling of a mTRP UE.

[0153] The following documents were referred to during the course of this work to understand the technical discussions and decisions relevant to the ongoing mobility improvements.a. RP-242356, “Revised Work Item: NR mobility enhancements Phase 4”. b. RAN2 Chairman Notes R2_127bis_ChairNotes.c. RAN2 Chairman Notes R2_128_ChairNotes.

[0154] Examples of the techniques and apparatus described herein include, but are not limited to, the following enumerated embodiments:[1] A method comprising:receiving, at a User Equipment (UE), a Timing Advance (TA) value associated with a Layer 1 / Layer 2 Triggered Mobility (LTM) candidate gNodeB Distributed Unit (gNB- DU) cell from a candidate gNB-DU, either directly or via a serving gNB-DU, transmitting, by the UE, the TA value to the serving gNB-DU associated with the UE via an Uplink Medium Access Control-Control Element (UL MAC-CE) message to maintain the received TA value at both the UE and the serving gNB-DU;determining, by the UE, whether the received TA value associated with the LTM candidate gNB-DU cell has expired; andin response to determining that the received TA value has expired, initiating a reacquisition of the TA value based on either a network-based decision-making timer or a UE-initiated request.[2] The method as described in [1], wherein initiating the re-acquisition of the TA value based on the network-based decision-making timer comprises:receiving, at the UE, a Physical Downlink Control Channel (PDCCH) order from the serving gNB-DU,wherein the serving gNB-DU transmits the PDCCH order based on a TA timer expiry,wherein the PDCCH order indicates an instruction to the UE to re-acquire the TA from the LTM candidate gNB-DU cell; andin response to receiving the PDCCH order, initiating the re-acquisition of the TA from the LTM candidate gNB-DU cell.[3] The method as described in any of [1] to [2], wherein initiating the re-acquisition of the TA value based on the UE-initiated request comprises:determining, by the UE, whether a radio condition associated with the LTM candidate gNB-DU cell indicates that the LTM candidate gNB-DU cell satisfies a criterion to acquire the TA value;in response to determining that the LTM candidate gNB-DU cell satisfies the criterion to re-acquire the TA value,transmitting, by the UE, the UL MAC-CE message to the serving gNB-DU for requesting a Physical Downlink Control Channel (PDCCH) order, wherein the PDCCH order indicates an instruction to the UE to reacquire the TA, andwherein the UL MAC-CE message comprises either a conditional Lower Layer Triggered Mobility (C-LTM) candidate cell ID or a configuration index of the candidate gNB-DU;receiving, at the UE, the PDCCH order from the serving gNB-DU; and in response to receiving the PDCCH order, initiating, by the UE, the reacquisition of the TA value.[4] The method as described in any of [1] to [3], wherein initiating the re-acquisition of the TA value based on the UE-initiated request comprises:receiving, at the UE, configuring from the serving gNB-DU to transmit an LI measurement report to the serving gNB-DU;transmitting, by the UE, the LI measurement report to the serving gNB-DU, wherein the serving gNB-DU determines whether the TA value needs to be re-acquired based on the LI measurement report; andin response to determining that the TA value needs to be re-acquired, receiving, by the UE, an indication from the serving gNB-DU to initiate the re-acquisition of the TA value.[5] The method as described in any of [1] to [4], comprising:receiving, at the UE, the TA value associated with the LTM candidate gNB-DU cell from the candidate gNodeB Distributed Unit (gNB-DU) through a gNB Central Unit (CU) and the serving gNB-DU,wherein the TA value is received in a downlink MAC CE message.[6] The method as described in any of [1] to [5], comprising:in response to receiving, at the UE, the TA value associated with the LTM candidate gNB-DU cell,maintaining, at the UE, the received TA value associated with the LTM candidate gNB-DU cell;determining, by the UE, whether the received TA value has expired; and in response to determining that the received TA value has expired, initiating a re-acquisition of the TA value based on the UE-initiated request.[7] The method as described in any of [1] to [6], wherein initiating the re-acquisition of the TA value based on the UE-initiated request comprises:determining, by the UE, whether a radio condition indicates that the LTM candidate gNB-DU cell satisfies a criterion to acquire the TA value;in response to determining that the candidate gNB-DU still satisfies the criterion to acquire the TA value,transmitting, by the UE, the UL MAC-CE message to the serving gNB-DU for requesting a Physical Downlink Control Channel (PDCCH) order, wherein the PDCCH order indicates an instruction to the UE to reacquire the TA, andwherein the UL MAC-CE message comprises either a conditional Lower Layer Triggered Mobility (C-LTM) candidate cell ID or a configuration index of the candidate gNB-DU;receiving, at the UE, the PDCCH order from the serving gNB-DU; and in response to receiving the PDCCH order, initiating, by the UE, the reacquisition of the TA value.[8] The method as described in any of [1] to [7], comprising:determining whether a TA timer is active; andin response to determining that the TA timer is active, configuring the UE to reduce a number of periodic L1 Measurement Report (MR), using a Radio Resource Control (RRC) reconfiguration message.[9] The method as described in any of [1] to [8], comprising:not preventing the UE from transmitting an LI Measurement Report (MR) when a Timing Advance Timer (TAT) of the LTM candidate gNB-DU cell is to expire.

[0010] The method as described in any of [1] to [9], comprising:configuring the UE for one or more event-based L1 measurements at the same time and indicating via a RRC reconfiguration message that a particular event-based measurement configuration is for TA acquisition.

[0011] The method as described in any of [1] to

[0010] , wherein the TA value is received directly from the candidate gNB-DU in the random access response message or via the serving gNB-DU associated with the UE.

[0012] A method comprising:receiving, at a serving gNodeB Distributed Unit (gNB-DU), a Timing Advance (TA) value associated with a LTM candidate gNB-DU. cell from at least one of a gNB Central Unit (CU), the candidate gNB-DU, or a User Equipment (UE);storing the received TA value at the serving gNB-DU;determining, after storing the received TA value, whether one or more predefined conditions are satisfied,wherein the one or more predefined conditions comprise an expiration of a TA timer, receiving an L1 measurement report indicating that a re-acquisition of the TA value is to be initiated, a receipt of a Uplink Medium Access Control- Control Element (UL MAC-CE) message from the UE requesting a Physical Downlink Control Channel (PDCCH) order to acquire TA;identifying that the re-acquisition of the TA value is to be initiated in response to determining that the one or more predefined conditions are satisfied; andinitiating the re-acquisition of the TA value by transmitting the PDCCH order to the UE, wherein the PDCCH order indicates an instruction to the UE to re-acquire the TA.

[0013] A User Equipment (UE) configured to:receive, at the UE, a Timing Advance (TA) value associated with a Layer 1 / Layer 2 Triggered Mobility (LTM) candidate gNodeB Distributed Unit (gNB-DU) cell from a candidate gNB-DU, either directly or via a serving gNB-DU,transmit the TA value to the serving gNB-DU associated with the UE via an Uplink Medium Access Control-Control Element (UL MAC-CE) message to maintain the received TA value at both the UE and the serving gNB-DU;determine whether the received TA value associated with the LTM candidate gNB- DU cell has expired; andin response to determine that the received TA value has expired, initiate a reacquisition of the TA value based on either a network-based decision-making timer or a UE-initiated request.

[0014] The UE as described in

[0013] , wherein to initiate the re-acquisition of the TA value based on the network-based decision-making timer, the UE is configured to:receive, at the UE, a Physical Downlink Control Channel (PDCCH) order from the serving gNB-DU,wherein the serving gNB-DU transmits the PDCCH order based on a TA timer expiry,wherein the PDCCH order indicates an instruction to the UE to re-acquire the TA from the LTM candidate gNB-DU cell; andin response to receive the PDCCH order, initiate the re-acquisition of the TA from the LTM candidate gNB-DU cell.

[0015] The UE as described in any of

[0013] to

[0014] , wherein to initiate the re-acquisition of the TA value based on the UE-initiated request, the UE is configured to:determine whether a radio condition associated with the LTM candidate gNB-DU cell indicates that the LTM candidate gNB-DU cell satisfies a criterion to acquire the TA value;in response to determine that the LTM candidate gNB-DU cell satisfies the criterion to re-acquire the TA value,transmit the UL MAC-CE message to the serving gNB-DU for requesting a Physical Downlink Control Channel (PDCCH) order,wherein the PDCCH order indicates an instruction to the UE to reacquire the TA, andwherein the UL MAC-CE message comprises either a conditional Lower Layer Triggered Mobility (C-LTM) candidate cell ID or a configuration index of the candidate gNB-DU;receive the PDCCH order from the serving gNB-DU; and in response to receive the PDCCH order, initiate the re-acquisition of the TA value.

[0016] The UE as described in any of

[0013] to

[0015] , wherein to initiate the re-acquisition of the TA value based on the UE-initiated request, the UE is configured to:receive configuring from the serving gNB-DU to transmit an L1 measurement report to the serving gNB-DU;transmit the L1 measurement report to the serving gNB-DU, wherein the serving gNB-DU determines whether the TA value needs to be re-acquired based on the L1 measurement report; andin response to determine that the TA value needs to be re-acquired, receive an indication from the serving gNB-DU to initiate the re-acquisition of the TA value.

[0017] The UE as described in any of

[0013] to

[0016] , the UE is configured to:receive the TA value associated with the LTM candidate gNB-DU cell from the candidate gNodeB Distributed Unit (gNB-DU) through a gNB Central Unit (CU) and the serving gNB-DU,wherein the TA value is received in a downlink MAC CE message.

[0018] The UE as described in any of

[0013] to

[0017] , the UE is configured to:in response to receive, at the UE, the TA value associated with the LTM candidate gNB-DU cell,maintain the received TA value associated with the LTM candidate gNB- DU cell;determine whether the received TA value has expired; and in response to determine that the received TA value has expired, initiate a re-acquisition of the TA value based on the UE-initiated request.

[0019] The UE as described in any of

[0013] to

[0018] , wherein to initiate the re-acquisition of the TA value based on the UE-initiated request, the UE is configured to:determine whether a radio condition indicates that the LTM candidate gNB-DU cell satisfies a criterion to acquire the TA value;in response to determine that the candidate gNB-DU still satisfies the criterion to acquire the TA value,transmit the UL MAC-CE message to the serving gNB-DU for requesting a Physical Downlink Control Channel (PDCCH) order,wherein the PDCCH order indicates an instruction to the UE to reacquire the TA, andwherein the UL MAC-CE message comprises either a conditional Lower Layer Triggered Mobility (C-LTM) candidate cell ID or a configuration index of the candidate gNB-DU;receiving, at the UE, the PDCCH order from the serving gNB-DU; and in response to receive the PDCCH order, initiate the re-acquisition of the TA value.

[0020] The UE as described in any of

[0013] to

[0019] , the UE is configured to:determine whether a TA timer is active; andin response to determine that the TA timer is active, configure the UE to reduce a number of periodic L1 Measurement Report (MR), using a Radio Resource Control (RRC) reconfiguration message.

[0021] The UE as described in any of

[0013] to

[0020] , the UE is configured to:not prevent the UE from transmitting an L1 Measurement Report (MR) when a Timing Advance Timer (TAT) of the LTM candidate gNB-DU cell is to expire.

[0022] The UE as described in any of

[0013] to

[0021] , the UE is configured to:configure the UE for one or more event-based L1 measurements at the same time and indicating via a RRC reconfiguration message that a particular event-based measurement configuration is for TA acquisition.P3] The UE as described in any of

[0013] to

[0022] , wherein the TA value is received directly from the candidate gNB-DU in the random access response message or via the serving gNB-DU associated with the UE.

[0024] A serving gNodeB Distributed Unit (gNB-DU) configured to:receive, at the serving gNB-DU, a Timing Advance (TA) value associated with a LTM candidate gNB-DU. cell from at least one of a gNB Central Unit (CU), the candidate gNB-DU, or a User Equipment (UE);store the received TA value at the serving gNB-DU;determine, after storing the received TA value, whether one or more predefined conditions are satisfied,wherein the one or more predefined conditions comprise an expiration of a TA timer, receiving an L1 measurement report indicating that a re-acquisition of the TA value is to be initiated, a receipt of a Uplink Medium Access Control- Control Element (UL MAC-CE) message from the UE requesting a Physical Downlink Control Channel (PDCCH) order to acquire TA;identify that the re-acquisition of the TA value is to be initiated in response to determining that the one or more predefined conditions are satisfied; andinitiate the re-acquisition of the TA value by transmitting the PDCCH order to the UE, wherein the PDCCH order indicates an instruction to the UE to re-acquire the TA.

[0025] A non-transitory computer-readable medium storing instructions, the instructions comprising:one or more instructions that, when executed by a User Equipment (UE), the apparatus comprising one or more processors, cause the one or more processors to:receive, at the UE, a Timing Advance (TA) value associated with a Layer 1 / Layer 2 Triggered Mobility (LTM) candidate gNodeB Distributed Unit (gNB- DU) cell from a candidate gNB-DU, either directly or via a serving gNB-DU, transmit the TA value to the serving gNB-DU associated with the UE via an Uplink Medium Access Control-Control Element (UL MAC-CE) message to maintain the received TA value at both the UE and the serving gNB-DU;determine whether the received TA value associated with the LTM candidate gNB-DU cell has expired; andin response to determine that the received TA value has expired, initiate a re-acquisition of the TA value based on either a network-based decision-making timer or a UE-initiated request.

[0155] The embodiments disclosed herein can be implemented through at least one software program running on at least one hardware device and performing network management functions to control the elements. The elements can be at least one of a hardware device or a combination of hardware devices and software modules.

[0156] While specific language has been used to describe the disclosure, any limitations arising on account of the same are not intended. As would be apparent to a person in the art, various working modifications may be made to the method in order to implement the inventive concept as taught herein.

[0157] The drawings and the forgoing description give examples of embodiments. Those skilled in the art will appreciate that one or more of the described elements may well be combined into a single functional element. Alternatively, certain elements may be split into multiple functional elements. Elements from one embodiment may be added to another embodiment. For example, orders of processes described herein may be changed and are not limited to the manner described herein.

[0158] Moreover, the actions of any flow diagram need not be implemented in the order shown; nor do all of the acts necessarily need to be performed. Also, those acts that are not dependent on other acts may be performed in parallel with the other acts. The scope ofembodiments is by no means limited by these specific examples. Numerous variations, whether explicitly given in the specification or not, such as differences in structure, dimension, and use of material, are possible. The scope of embodiments is at least as broad as given by the following claims.

[0159] Benefits, other advantages, and solutions to problems have been described above with regard to specific embodiments. However, the benefits, advantages, solutions to problems, and any component(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential feature or component of any or all the claims.

[0160] The foregoing description of the specific embodiments will so fully reveal the general nature of the embodiments herein that others can, by applying current knowledge, readily modify and / or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments herein have been described in terms of at least one embodiment, those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the embodiments as described herein.

Claims

We claim:

1. A method comprising:receiving, at a User Equipment (UE), a Timing Advance (TA) value associated with a Layer 1 / Layer 2 Triggered Mobility (LTM) candidate gNodeB Distributed Unit (gNB- DU) cell from a candidate gNB-DU, either directly or via a serving gNB-DU;transmitting, by the UE, the TA value to the serving gNB-DU associated with the UE via an Uplink Medium Access Control-Control Element (UL MAC-CE) message to maintain the received TA value at both the UE and the serving gNB-DU;determining, by the UE, whether the received TA value associated with the LTM candidate gNB-DU cell has expired; andin response to determining that the received TA value has expired, initiating a reacquisition of the TA value based on either a network-based decision-making timer or a UE-initiated request.

2. The method as claimed in claim 1, wherein initiating the re-acquisition of the TA value based on the network-based decision-making timer comprises:receiving, at the UE, a Physical Downlink Control Channel (PDCCH) order from the serving gNB-DU,wherein the serving gNB-DU transmits the PDCCH order based on a TA timer expiry, andwherein the PDCCH order indicates an instruction to the UE to re-acquire the TA from the LTM candidate gNB-DU cell; andin response to receiving the PDCCH order, initiating the re-acquisition of the TA from the LTM candidate gNB-DU cell.

3. The method as claimed in claim 1, wherein initiating the re-acquisition of the TA value based on the UE-initiated request comprises:determining, by the UE, whether a radio condition associated with the LTM candidate gNB-DU cell indicates that the LTM candidate gNB-DU cell satisfies a criterion to acquire the TA value;in response to determining that the LTM candidate gNB-DU cell satisfies the criterion to re-acquire the TA value,transmitting, by the UE, the UL MAC-CE message to the serving gNB-DU for requesting a Physical Downlink Control Channel (PDCCH) order, wherein the PDCCH order indicates an instruction to the UE to reacquire the TA, andwherein the UL MAC-CE message comprises either a conditional Lower Layer Triggered Mobility (C-LTM) candidate cell ID or a configuration index of the candidate gNB-DU;receiving, at the UE, the PDCCH order from the serving gNB-DU; and in response to receiving the PDCCH order, initiating, by the UE, the reacquisition of the TA value.

4. The method as claimed in claim 1, wherein initiating the re-acquisition of the TA value based on the UE-initiated request comprises:receiving, at the UE, configuring from the serving gNB-DU, to transmit an L1 measurement report to the serving gNB-DU;transmitting, by the UE, the L1 measurement report to the serving gNB-DU, wherein the serving gNB-DU determines whether the TA value needs to be re-acquired based on the L1 measurement report; andin response to determining that the TA value needs to be re-acquired, receiving, by the UE, an indication from the serving gNB-DU to initiate the re-acquisition of the TA value.

5. The method as claimed in claim 1, further comprising:receiving, at the UE, the TA value associated with the LTM candidate gNB-DU cell from the candidate gNodeB Distributed Unit (gNB-DU) through a gNB Central Unit (CU) and the serving gNB-DU,wherein the TA value is received in a downlink MAC CE message.

6. The method as claimed in claim 1, further comprising:in response to receiving, at the UE, the TA value associated with the LTM candidate gNB-DU cell,maintaining, at the UE, the received TA value associated with the LTM candidate gNB-DU cell;determining, by the UE, whether the received TA value has expired; and in response to determining that the received TA value has expired, initiating a re-acquisition of the TA value based on the UE-initiated request.

7. The method as claimed in claim 6, wherein initiating the re-acquisition of the TA value based on the UE-initiated request comprises:determining, by the UE, whether a radio condition indicates that the LTM candidate gNB-DU cell satisfies a criterion to acquire the TA value;in response to determining that the candidate gNB-DU still satisfies the criterion to acquire the TA value,transmitting, by the UE, the UL MAC-CE message to the serving gNB-DU for requesting a Physical Downlink Control Channel (PDCCH) order, wherein the PDCCH order indicates an instruction to the UE to reacquire the TA, andwherein the UL MAC-CE message comprises either a conditional Lower Layer Triggered Mobility (C-LTM) candidate cell ID or a configuration index of the candidate gNB-DU;receiving, at the UE, the PDCCH order from the serving gNB-DU; and in response to receiving the PDCCH order, initiating, by the UE, the reacquisition of the TA value.

8. The method as claimed in claim 1, further comprising:determining whether a TA timer is active; andin response to determining that the TA timer is active, configuring the UE to reduce a number of periodic L1 Measurement Report (MR), using a Radio Resource Control (RRC) reconfiguration message.

9. The method as claimed in claim 1, further comprising:allowing the UE to transmit an L1 Measurement Report (MR) when a Timing Advance Timer (TAT) of the LTM candidate gNB-DU cell is to expire.

10. The method as claimed in claim 1, further comprising:configuring the UE for one or more event-based L1 measurements at the same time and indicating via a RRC reconfiguration message that a particular event-based measurement configuration is for TA acquisition.

11. The method as claimed in claim 1, wherein the TA value is received directly from the candidate gNB-DU in the random access response message or via the serving gNB-DU associated with the UE.

12. A method comprising:receiving, at a serving gNodeB Distributed Unit (gNB-DU), a Timing Advance (TA) value associated with a LTM candidate gNB-DU. cell from at least one of a gNB Central Unit (CU), the candidate gNB-DU, or a User Equipment (UE);storing the received TA value at the serving gNB-DU;determining, after storing the received TA value, whether one or more predefined conditions are satisfied,wherein the one or more predefined conditions comprise an expiration of a TA timer, receiving an L1 measurement report indicating that a re-acquisition of the TA value is to be initiated, a receipt of a Uplink Medium Access Control- Control Element (UL MAC-CE) message from the UE requesting a Physical Downlink Control Channel (PDCCH) order to acquire TA;identifying that the re-acquisition of the TA value is to be initiated in response to determining that the one or more predefined conditions are satisfied; andinitiating the re-acquisition of the TA value by transmitting the PDCCH order to the UE, wherein the PDCCH order indicates an instruction to the UE to re-acquire the TA.

13. A User Equipment (UE) configured to:receive, at the UE, a Timing Advance (TA) value associated with a Layer 1 / Layer 2 Triggered Mobility (LTM) candidate gNodeB Distributed Unit (gNB-DU) cell from a candidate gNB-DU, either directly or via a serving gNB-DU;transmit the TA value to the serving gNB-DU associated with the UE via an Uplink Medium Access Control-Control Element (UL MAC-CE) message to maintain the received TA value at both the UE and the serving gNB-DU;determine whether the received TA value associated with the LTM candidate gNB- DU cell has expired; andin response to determining that the received TA value has expired, initiate a reacquisition of the TA value based on either a network-based decision-making timer or a UE-initiated request.

14. The UE as claimed in claim 13, wherein to initiate the re-acquisition of the TA value based on the network-based decision-making timer, the UE is configured to:receive, at the UE, a Physical Downlink Control Channel (PDCCH) order from the serving gNB-DU,wherein the serving gNB-DU transmits the PDCCH order based on a TA timer expiry, andwherein the PDCCH order indicates an instruction to the UE to re-acquire the TA from the LTM candidate gNB-DU cell; andin response to receiving the PDCCH order, initiate the re-acquisition of the TA from the LTM candidate gNB-DU cell.

15. The UE as claimed in claim 13, wherein to initiate the re-acquisition of the TA value based on the UE-initiated request, the UE is configured to:determine whether a radio condition associated with the LTM candidate gNB-DU cell indicates that the LTM candidate gNB-DU cell satisfies a criterion to acquire the TA value;in response to determining that the LTM candidate gNB-DU cell satisfies the criterion to re-acquire the TA value,transmit the UL MAC-CE message to the serving gNB-DU for requesting a Physical Downlink Control Channel (PDCCH) order,wherein the PDCCH order indicates an instruction to the UE to reacquire the TA, andwherein the UL MAC-CE message comprises either a conditional Lower Layer Triggered Mobility (C-LTM) candidate cell ID or a configuration index of the candidate gNB-DU;receive the PDCCH order from the serving gNB-DU; and in response to receiving the PDCCH order, initiate the re-acquisition of the TA value.

16. The UE as claimed in claim 13, wherein to initiate the re-acquisition of the TA value based on the UE-initiated request, the UE is configured to:receive configuring from the serving gNB-DU to transmit an L1 measurement report to the serving gNB-DU;transmit the L1 measurement report to the serving gNB-DU, wherein the serving gNB-DU determines whether the TA value needs to be re-acquired based on the L1 measurement report; andin response to determining that the TA value needs to be re-acquired, receive an indication from the serving gNB-DU to initiate the re-acquisition of the TA value.

17. The UE as claimed in claim 13, further configured to:receive the TA value associated with the LTM candidate gNB-DU cell from the candidate gNodeB Distributed Unit (gNB-DU) through a gNB Central Unit (CU) and the serving gNB-DU,wherein the TA value is received in a downlink MAC CE message.

18. The UE as claimed in claim 13, further configured to:in response to receiving, at the UE, the TA value associated with the LTM candidate gNB-DU cell,maintain the received TA value associated with the LTM candidate gNB- DU cell;determine whether the received TA value has expired; and in response to determining that the received TA value has expired, initiate a re-acquisition of the TA value based on the UE-initiated request.

19. The UE as claimed in claim 18, wherein to initiate the re-acquisition of the TA value based on the UE-initiated request, the UE is configured to:determine whether a radio condition indicates that the LTM candidate gNB-DU cell satisfies a criterion to acquire the TA value;in response to determining that the candidate gNB-DU still satisfies the criterion to acquire the TA value,transmit the UL MAC-CE message to the serving gNB-DU for requesting a Physical Downlink Control Channel (PDCCH) order,wherein the PDCCH order indicates an instruction to the UE to reacquire the TA, andwherein the UL MAC-CE message comprises either a conditional Lower Layer Triggered Mobility (C-LTM) candidate cell ID or a configuration index of the candidate gNB-DU;receive, at the UE, the PDCCH order from the serving gNB-DU; and in response to receiving the PDCCH order, initiate the re-acquisition of the TA value.

20. The UE as claimed in claim 13, further configured to:determine whether a TA timer is active; andin response to determining that the TA timer is active, configure the UE to reduce a number of periodic L1 Measurement Report (MR), using a Radio Resource Control (RRC) reconfiguration message.

21. The UE as claimed in claim 13, further configured to:allow the UE to transmit an L1 Measurement Report (MR) when a Timing Advance Timer (TAT) of the LTM candidate gNB-DU cell is to expire.

22. The UE as claimed in claim 13, further configured to:configure the UE for one or more event-based L1 measurements at the same time and indicate via a RRC reconfiguration message that a particular event-based measurement configuration is for TA acquisition.

23. The UE as claimed in claim 13, wherein the TA value is received directly from the candidate gNB-DU in the random access response message or via the serving gNB-DU associated with the UE.

24. A serving gNodeB Distributed Unit (gNB-DU) configured to:receive, at the serving gNB-DU, a Timing Advance (TA) value associated with a LTM candidate gNB-DU. cell from at least one of a gNB Central Unit (CU), the candidate gNB-DU, or a User Equipment (UE);store the received TA value at the serving gNB-DU;determine, after storing the received TA value, whether one or more predefined conditions are satisfied,wherein the one or more predefined conditions comprise an expiration of a TA timer, receiving an L1 measurement report indicating that a re-acquisition of the TA value is to be initiated, a receipt of a Uplink Medium Access Control- Control Element (UL MAC-CE) message from the UE requesting a Physical Downlink Control Channel (PDCCH) order to acquire TA;identify that the re-acquisition of the TA value is to be initiated in response to determining that the one or more predefined conditions are satisfied; andinitiate the re-acquisition of the TA value by transmitting the PDCCH order to the UE, wherein the PDCCH order indicates an instruction to the UE to re-acquire the TA.

25. A non-transitory computer-readable medium storing instructions, the instructions comprising:one or more instructions that, when executed by a User Equipment (UE) comprising one or more processors, cause the one or more processors to:receive, at the UE, a Timing Advance (TA) value associated with a Layer 1 / Layer 2 Triggered Mobility (LTM) candidate gNodeB Distributed Unit (gNB- DU) cell from a candidate gNB-DU, either directly or via a serving gNB-DU, transmit the TA value to the serving gNB-DU associated with the UE via an Uplink Medium Access Control-Control Element (UL MAC-CE) message to maintain the received TA value at both the UE and the serving gNB-DU; determine whether the received TA value associated with the LTM candidate gNB-DU cell has expired; andin response to determining that the received TA value has expired, initiate a re-acquisition of the TA value based on either a network-based decision-making timer or a UE-initiated request.

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