Retention of timing advance (TA) values at subsequent conditional layer 1 / layer 2 triggered mobility (CLTM) execution

Abstract

A user equipment, UE (22) is provided. UE (22) is configured to receive a conditional Layer 1 / Layer 2, L1 / L2, triggered mobility, CLTM, configuration for one or more CLTM candidate cells (18), where the CLTM configuration includes one or more CLTM execution conditions for executing a CLTM procedure. UE (22) is configured to, before at least one of the one or more CLTM execution conditions is fulfilled, perform uplink, UL, synchronization with the one or more CLTM candidate cells (18) to acquire one or more timing advance, TA, values, and when at least one of the one or more CLTM execution conditions is fulfilled: execute the CLTM procedure for a first CLTM candidate cell of the one or more CLTM candidate cells (18); and retain the one or more TA values.

Description

[0001] RETENTION OF TIMING ADVANCE (TA) VALUES AT SUBSEQUENT CONDITIONAL LAYER 1 / LAYER 2 TRIGGERED MOBILITY (CLTM) EXECUTION

[0002] TECHNICAL FIELD

[0003] The present disclosure relates to wireless communications, and in particular, to retention of TA values at subsequent conditional lower-layer triggered mobility (e.g., LTM) execution.

[0004] BACKGROUND

[0005] The Third Generation Partnership Project (3GPP) has developed and is developing standards for Fourth Generation (4G) (also referred to as Long Term Evolution (LTE)) and Fifth Generation (5G) (also referred to as New Radio (NR)) wireless communication systems. Such systems provide, among other features, broadband communication between network nodes, such as base stations, and mobile user equipments (UE), as well as communication between network nodes (NNs) and between UEs. The 3GPP is also developing standards for Sixth Generation (6G) wireless communication networks.

[0006] Timing Advance and UL Synchronization in NR

[0007] UEs within a cell are typically located at different positions and distances from the network nodes such as a base station, e.g., gNB. Consequently, the transmissions from different UEs experience different delays until they are received at the base station. The reception of these uplink (UL) transmissions at the base station within the corresponding receive window is ensured by UL timing control procedure. This procedure helps mitigate the intracell interference which occurs between the UEs transmitting in consecutive subframes and the UEs which are allocated adjacent subcarriers for transmission. Time alignment of the UL transmissions may be performed by adjusting the UE transmitter's timing relative to the received downlink timing, primarily to offset propagation delays among different UEs.

[0008] The time alignment for UL synchronization may be obtained by the calculation of the Timing Advance (TA) value at the base station and is indicated to the UE. The TA may either be represented as an actual timing adjustment value and / or an index pointing to a timing adjustment value. During the initial cell access, the UE performs the randomaccess (RA) procedure in which the received Physical Random Access Channel (PRACH) preamble, i.e., Msgl, is utilized by the base station to determine the UE initial TA values for UL transmissions within the cell. The UE then may use that TA value in sending the UL transmissions to the network node. Throughout the connection, the network node continuously monitors whether any adjustments are required for the UE to advance or delay the UL transmissions, compensating for changes in propagation delay. The UE may be indicated if there is a need to modify the TA value.

[0009] Early TA Acquisition Procedure in LTM

[0010] L1 / L2 Triggered Mobility (LTM) is an inter-cell mobility procedure, proposed in 3GPP Release 18 (Rel-18), to reduce the overall handover latency by leveraging lower layer signaling. To achieve this, the UE is first configured with the Radio Resource Control (RRC) configuration of one or multiple LTM candidate cells. Then the UE performs LI -reference signal received power (RSRP) measurements on the synchronization signal block (SSBs), i.e., beams, of the configured LTM candidate cells and reports the measurement over user control information (UCI) via physical uplink control channel (PUCCH) or physical uplink shared channel (PUSCH). The reduction in handover interruption may be attained by means of early uplink (UL) and downlink (DL) synchronization with the target cell before the transmission of LTM cell switch MAC CE command.

[0011] The DL pre-synchronization refers to the pre-activation of certain transmission configuration indicator (TCI) states in the LTM candidate cells before any of those cells becomes the target cell. DL pre-synchronization reduces the overall duration required for cell-search, fine-tracking and acquisition of S SB-based RRM Measurement Timing Configuration window (SMTC), and the time spent in SSB post-processing, thus facilitating a faster cell switch.

[0012] The UL pre-synchronization corresponds to the early acquisition of LTM candidate cell TA value before the UE switches to that cell. The early UL synchronization allows the UE to switch to the LTM candidate cell upon receiving the LTM cell switch indication, without any random-access (RA). An example UL pre-synchronization procedure is summarized in the steps below:

[0013] • The serving cell transmits a PDCCH order to the UE, which may include information about the RA parameters for TA acquisition.

[0014] • The UE sends the PRACH preamble to the LTM candidate cell, according to the indicated PDCCH order.

[0015] • The LTM candidate cell calculates the TA values, according to the PRACH preamble received and then sends the TA value to the serving cell. • In this case, the TA value is provided to the UE via cell switch MAC CE during the eventual LTM handover execution.

[0016] The early TA acquisition procedure in LTM is illustrated in the example of FIG. 1. More specifically, at step 1, a network node (e.g., gNB (CellA)) transmits an RRCReconfiguration message including TA acquisition configuration of Cell B. At step 2, the UE transmits an RRCReconfigurationComplete message. At step 3, the network node (e.g., gNB (CellA)) transmits a PDCCH order indicating the UE to perform TA acquisition procedure to Cell B. At step 4, UE performs a random access preamble transmission, and at step 5, network node (e.g., gNB (CellA)) transmits a TA value (e.g., in cell switch command indicating Cell B as targeted cell. The UE may not be required to maintain the time alignment timer for the LTM candidate cell in this context and it is up to the network implementation to determine the TA validity.

[0017] Subsequent LTM

[0018] A UE can also perform multiple LTM cell switch executions without any requirement to receive an RRC Reconfiguration message by the network between subsequent LTM cell switch procedures, a concept referred to as Subsequent LTM in Rel- 18 LTM. In other words, the subsequent LTM is supported by repeating the early UL and DL synchronization steps, LTM cell switch execution, and LTM cell switch completion steps without releasing other LTM candidate configurations after each LTM cell switch completion.

[0019] In Subsequent LTM, the UE is first configured with multiple LTM candidate cells via RRC Reconfiguration such that the UE is provided with a separate “Reference Configuration” which is stored at the UE, and a “Delta Configuration” per LTM candidate cell. Upon LTM cell switch to a certain LTM candidate cell, the UE generates a complete LTM candidate cell configuration using the delta configuration of the LTM candidate cell on top of the stored reference configuration and then applies that configuration.

[0020] However, when the UE performs the LTM cell switch execution in subsequent LTM, there is a requirement to update the lower layer configuration of the target cell, which becomes the new source cell after LTM cell switch, at each subsequent execution. This may be accomplished by sending a UE CONTEXT MODIFCATION REQUEST message, including early UL and DL configuration information, from a gNB-CU to the candidate RAN node, i.e., C-DU. In response, the C-DU sends the UE CONTEXT MODIFICATION RESPONSE message to gNB-CU which contains information about the updated lower layer configuration, e.g., LTM-CSI-report-configuration. Moreover, the source RAN node, i.e., S-DU may also send the DU-CU CELL SWITCH NOTIFICATION message to gNB-CU which includes the target cell ID, TCI state ID(s) for the serving beam(s) in the target cell, and the TA value(s) for subsequent LTM, at LTM cell switch execution. This information is transmitted by gNB-CU to the C- DU (i.e., another / new S-DU) in the CU-DU CELL SWITCH NOTIFICATION message. This means that the valid TA value(s) for the LTM candidate cell(s), for which the UE performed TA value acquisition before LTM cell switch, are retained at the network after LTM cell switch completion in subsequent LTM. It is to be noted that the maintenance of the time alignment timer or the determination of the TA validity for the shared TA value(s) in Cell Switch Notification messages during subsequent LTM is up to network implementation. In other words, the standard specifications do not define any procedure related to the maintenance of Time alignment timer and TA validity check for the TA values shared in subsequent LTM execution. Also, the UE may not be required to retain the TA values for the other LTM candidate cells during subsequent LTM execution and the time alignment timers associated to the TA values for the other LTM candidate cells are reset.

[0021] Conditional LTM in 3GPP Rel-19

[0022] Conditional handover (CHO) and the related conditional mobility procedures were introduced in NR for improving the mobility robustness by preparing the UE (and the CHO candidate cells) in advance before there are any radio link outages. The UE may be provided the RRC configuration of the candidate CHO cells, like LTM, and some CHO execution conditions, which once fulfilled lead the UE to directly perform the handover without sending measurement report to the network, unlike LTM. However, there are other differences between the legacy CHO and LTM, for example, the CHO does not include the procedure of early synchronization in 3GPP Rel-18.

[0023] To facilitate both the advantages of short handover interruption as well as better robustness, 3GPP Release 19 (Rel-19) aims at introducing Conditional LTM as part of the mobility-related enhancements. The following Conditional LTM-related objectives have been agreed upon in NR mobility enhancements phase 4 work item (WI):

[0024] • Specify support of conditional LTM [RAN2, RAN3, RANI]

[0025] • Specify UE evaluated conditions for triggering LTM.

[0026] • Aim to support conditional LTM including subsequent LTM.

[0027] In Conditional LTM, the UE is configured with the conditional LTM execution conditions along with the LTM candidate cell configuration. The UE may perform early UL and DL synchronization procedures before the cell switch and shall execute LTM cell switch upon the fulfillment of the provided execution conditions. The Conditional LTM in Rel-19 can also be configured with the subsequent LTM executions, in the same way as the subsequent LTM in 3GPP Rel-18.

[0028] The support for subsequent conditional LTM is regarded as one of the objectives of 3GPP Rel-19 Mobility enhancements WI. The concept of subsequent conditional LTM is similar to the subsequent LTM, in that it requires the UE to be configured with the LTM candidate cells such that the UE may switch to one of the LTM candidate cells in the group, without requiring any further RRC Reconfiguration. In other words, the UE does not require any RRC reconfiguration in between the subsequent LTM cell switches. The main difference between the subsequent LTM and subsequent conditional LTM is that the UE is configured with the conditional LTM execution conditions to be evaluated at the subsequent LTM cell switches, rather than receiving LTM cell switch MAC CE from the network.

[0029] As for the other lower layer mobility cases (e.g., LTM, subsequent LTM or Conditional LTM), the subsequent conditional LTM may also benefit from the UE performing early synchronization procedure. That is, the UE can have the one or multiple beam(s) in LTM candidate cell(s) with activated TCI states before subsequent Conditional LTM execution, i.e., DL pre-synchronization with the LTM candidate cell. Also, the UE can have the timing advance (TA) values associated to one or more LTM candidate cell(s) before subsequent Conditional LTM execution.

[0030] The procedures for TA value retention and TA validity check in subsequent LTM have the following functional characteristics, as discussed earlier:

[0031] • Source DU (S-DU) keeps track of TA validity for the UE, as per the network implementation. If the TA value becomes invalid, the S-DU either sends LTM cell switch command without any valid TA value, in which case the UE executes RACH-based LTM cell switch, or the S-DU sends another PDCCH order related to the LTM candidate cell for the UE to acquire an updated TA value.

[0032] • Since the UE relies on network to keep track of TA validity for LTM candidate cells, the UE does not maintain any time alignment timer for the TA values acquired for the LTM candidate cells).

[0033] • The valid TA values associated to the other LTM candidate cells for a UE in the previous source cell (located in source gNB-DU) are shared using LTM Cell Switch Notification messages with the next source cell (located in candidate DU (C-DU)).

[0034] However, subsequent Conditional LTM execution is different from the subsequent LTM execution in that there is no transmission of LTM cell switch medium access control (MAC) control element (CE) to the UE or the transmission of CELL SWITCH NOTIFICATION between the network nodes, and the cell switch is performed upon the fulfillment of Conditional LTM execution conditions. This results in the following problems:

[0035] • There is no LTM cell switch MAC CE in Conditional LTM, and cell switch execution is subject to the fulfillment of CLTM execution conditions. Therefore, there is no way for the UE to determine whether the TA value for a certain LTM candidate cell is valid or not at the time of the fulfillment of CLTM execution conditions.

[0036] • In subsequent Conditional LTM execution, there is no CELL SWITCH NOTIFICATION transmission from S-DU to C-DU and therefore, no procedure is defined for sending the valid TA values for the other LTM candidate cells from old S-DU to the new S-DU / C-DU.

[0037] In the context of subsequent Conditional LTM, there is a possibility that the UE performs early TA acquisition for two or more Conditional LTM candidate cells and executes the LTM cell switch for only one of those conditional LTM candidate cells. Upon cell switch, the TA value(s) associated to the remaining one or more LTM candidate cell(s) are still valid. In other words, the time alignment timer corresponding to the other conditional LTM candidate cell(s), for which the UE performed early TA acquisition, is still running. In this case, the next S-DU is not aware of that the UE has a valid TA value for the other LTM candidate cell(s) and may then trigger a TA acquisition procedure towards those other candidate cell(s) even though it is not required.

[0038] SUMMARY

[0039] Some embodiments advantageously provide methods, systems, and apparatuses for retention of TA values at subsequent conditional LTM execution. A UE centric process or solution and a network centric process or solution are described.

[0040] In some embodiments associated the UE centric process, a method is implemented at a UE configured with subsequent Conditional LTM, where the UE receives LTM candidate cell configuration for multiple LTM candidate cells(s) and their respective associated execution conditions for Conditional LTM such that there is no requirement for the UE to receive an RRC Reconfiguration after LTM cell switch to a new source cell, during the subsequent Conditional LTM execution(s). The UE may perform early TA value acquisition for one or multiple LTM candidate cells before the fulfilment of CLTM execution condition(s) and switches to one of those candidate target cells once the CLTM execution conditions are met. According to the method, the UE does not discard the TA values associated to the other LTM candidate cells, for which the execution condition(s) are not fulfilled or for which the execution condition(s) are fulfilled but that the UE has not selected for execution. The method also includes the UE also indicating to the next source cell (i.e., the LTM candidate cell which is selected for execution) the TA values which are still valid (e.g., for which Time alignment timer at the UE is still running) after the cell switch execution.

[0041] In some other embodiments associated with the UE centric process, a first network node (operating as the new source cell in C-DU) receives an RRC Reconfiguration Complete Message or any other RRC message (existing or new) including one or more valid TA value(s) for the other LTM candidate cell(s) and the related information (e.g., time alignment timer information, such as information about remaining time left until expiry), transmitted by the UE upon the triggering or completion of a Conditional LTM cell switch execution.

[0042] In some embodiments associated with the network centric process, a method at the network nodes is described. The method is for sharing the valid TA values for the other LTM candidate cells such that the TA values can be applied by the UE in performing RACH-less LTM cell switch during subsequent conditional LTM execution, without any requirement for the network to trigger PDCCH order for the LTM candidate cell with valid TA value and the UE to send PRACH preamble to the LTM candidate cell with valid TA value.

[0043] According to one aspect of the present disclosure, a method in a user equipment, UE, is provided. A conditional Layer 1 / Layer 2, L1 / L2, triggered mobility, CLTM, configuration for one or more CLTM candidate cells is received, where the CLTM configuration includes one or more CLTM execution conditions for executing a CLTM procedure. Before at least one of the one or more CLTM execution conditions is fulfilled, uplink, UL, synchronization is performed with the one or more CLTM candidate cells to acquire one or more timing advance, TA, values. When at least one of the one or more CLTM execution conditions is fulfilled: executing the CLTM procedure is executed for a first CLTM candidate cell of the one or more CLTM candidate cells; and the one or more TA values are retained.

[0044] According to one or more embodiments of this aspect, the retaining of the one or more TA values comprises, after the conditional CLTM execution to the first CLTM candidate cell, retaining at least one TA value for at least a second CLTM candidate cell of the one or more CLTM candidate cells.

[0045] According to one or more embodiments of this aspect, the at least one TA value for the second CLTM candidate cell is retained based on a time left for expiry of at least one timing alignment timer associated with the at least one TA value meeting a threshold criterion.

[0046] According to one or more embodiments of this aspect, the at least one TA value for the second CLTM candidate cell is retained based on the UE being configured to one or both of: retain the at least one TA value; and retain at least one time alignment timer value associated with the at least one TA value.

[0047] According to one or more embodiments of this aspect, the one or more TA values comprises a TA value for a third CLTM candidate cell; and information associated with the TA value for the third CLTM candidate cell is indicated to the second CLTM candidate cell.

[0048] According to one or more embodiments of this aspect, the indication is provided by an RRC message or a UE assistance information message.

[0049] According to one or more embodiments of this aspect, a measurement report for an event associated with early UL synchronization of a second CLTM candidate cell of the one or more candidate cells is prevented from being triggered, if a TA value for second CLTM candidate cell was or had been acquired before the CLTM execution in a previous source cell and is: still valid; or a time alignment timer that is still running.

[0050] According to one or more embodiments of this aspect, information is maintained for at least one TA value for at least the second CLTM candidate, where the information indicates a current state of the at least one time alignment time associated with the at least one TA value.

[0051] According to another aspect of the present disclosure, a user equipment, UE, is provided. The UE is configured to receive a conditional Layer 1 / Layer 2, L1 / L2, triggered mobility, CLTM, configuration for one or more CLTM candidate cells, where the CLTM configuration includes one or more CLTM execution conditions for executing a CLTM procedure. The UE is configured to, before at least one of the one or more CLTM execution conditions is fulfilled, perform uplink, UL, synchronization with the one or more CLTM candidate cells to acquire one or more timing advance, TA, values. The UE is configured to, when at least one of the one or more CLTM execution conditions is fulfilled, execute the CLTM procedure for a first CLTM candidate cell of the one or more CLTM candidate cells and retain the one or more TA values.

[0052] According to one or more embodiments of this aspect, the UE is further configured to, after conditional LTM execution to the first CLTM candidate cell, retain at least one TA value for at least a second CLTM candidate cell of the one or more CLTM candidate cells.

[0053] According to one or more embodiments of this aspect, the at least one TA value for the second CLTM candidate cell is retained when a time left for expiry of at least one timing alignment timer associated with the at least one TA value meets a threshold criterion.

[0054] According to one or more embodiments of this aspect, the at least one TA value for the second CLTM candidate cell is retained when the UE is configured to one or both of: retain the at least one TA value; and retain at least one time alignment timer value associated with the at least one TA value.

[0055] According to one or more embodiments of this aspect, the at least one TA value comprises a TA value for a third CLTM candidate cell; and the UE is further configured to indicate, to the second CLTM candidate cell, information associated with the TA value for the third CLTM candidate cell.

[0056] According to one or more embodiments of this aspect, the indication is provided by an RRC message or a UE assistance information message.

[0057] According to one or more embodiments of this aspect, the UE is further configured to: prevent triggering a measurement report for an event associated with early UL synchronization of a second CLTM candidate cell of the one or more candidate cells, if a TA value for second CLTM candidate cell was acquired before the CLTM execution in a previous source cell and is: still valid; or a time alignment timer that is still running.

[0058] According to one or more embodiments of this aspect, the UE is further configured to maintain information for at least one TA value for at least the second CLTM candidate, where the information indicates a current state of the at least one time alignment time associated with the at least one TA value.

[0059] According to another aspect of the present disclosure, a method implemented by a network node that is configured to communicate with a user equipment, UE, is provided. The network node is configured as a centralized unit, CU. The UE is configured with a conditional layer 1 / layer 2, L1 / L2, triggered mobility, CLTM, configuration for at least a first CLTM candidate cell and a second CLTM candidate cell, where the CLTM configuration includes one or more CLTM execution conditions for executing a CLTM procedure. A timing advance, TA, value associated with the first CLTM candidate cell or a second candidate CLTM cell is received from a first CLTM candidate cell and based on the configuration, where the TA value was or had been acquired by the UE before at least one of the one or more CLTM execution conditions were fulfilled.

[0060] According to one or more embodiments of this aspect, the TA value is associated with the first candidate CLTM cell, and a TA value associated with the second candidate CLTM cell is received and the TA value associated with the second candidate CLTM cell is indicated to the first candidate CLTM cell.

[0061] According to one or more embodiments of this aspect, the indication is signaled via one of: a UE context modification request message; a TA information transfer message; or an radio resource control, RRC, message.

[0062] According to one or more embodiments of this aspect, the TA value is associated with the first candidate CLTM cell. The TA value associated with the first candidate CLTM cell is indicated to the second candidate CLTM cell.

[0063] According to one or more embodiments of this aspect, a TA value associated with the second candidate CLTM cell is received from the second candidate CLTM cell and based on the configuration, where the TA value was or had been acquired by the UE before at least one of the one or more CLTM execution conditions were fulfilled.

[0064] According to one or more embodiments of this aspect, the receiving of the TA value is part of a CU-Distributed Unit, DU, information transfer.

[0065] According to one or more embodiments of this aspect, the TA value for subsequent conditional CLTM execution is retained.

[0066] According to another aspect of the present disclosure, a network node configured to communicate with a user equipment, UE, is provided. The network node is configured as a centralized unit, CU. The network node is configured to configure the UE with a conditional L1 / L2 triggered mobility, CLTM, configuration for at least a first CLTM candidate cell and a second CLTM candidate cell, where the CLTM configuration includes one or more CLTM execution conditions for executing a CLTM procedure. The network node is configured to receive, from a first CLTM candidate cell and based on the configuration, a timing advance, TA, value associated with the first CLTM candidate cell or a second candidate CLTM cell, where the TA value was acquired by the UE before at least one of the one or more CLTM execution conditions were fulfilled.

[0067] According to one or more embodiments of this aspect, the TA value is associated with the first candidate CLTM cell, and the network node is further configured to: receive a TA value associated with the second candidate CLTM cell, and indicate, to the first candidate CLTM cell, the TA value associated with the second candidate CLTM cell.

[0068] According to one or more embodiments of this aspect, the indication is signaled via one of: a UE context modification request message; a TA information transfer message; or a radio resource control, RRC, message.

[0069] According to one or more embodiments of this aspect, the TA value is associated with the first candidate CLTM cell, and the network node is further configured to indicate, to the second candidate CLTM cell, the TA value associated with the first candidate CLTM cell.

[0070] According to one or more embodiments of this aspect, the network node is further configured to receive, from the second candidate CLTM cell and based on the configuration, a TA value associated with the second candidate CLTM cell, where the TA value was acquired by the UE before at least one of the one or more CLTM execution conditions were fulfilled.

[0071] According to one or more embodiments of this aspect, the receiving of the TA value is part of a CU-Distributed Unit, DU, information transfer.

[0072] According to one or more embodiments of this aspect, the network node is further configured to retain the TA value for subsequent conditional CLTM execution.

[0073] BRIEF DESCRIPTION OF THE DRAWINGS

[0074] A more complete understanding of the present embodiments, and the attendant advantages and features thereof, will be more readily understood by reference to the following detailed description when considered in conjunction with the accompanying drawings wherein:

[0075] FIG. 1 shows an example early TA acquisition procedure in LTM;

[0076] FIG. 2 is a schematic diagram of an example network architecture illustrating a communication system according to principles disclosed herein;

[0077] FIG. 3 is a block diagram of a network node in communication with a user equipment over a wireless connection according to some embodiments of the present disclosure; FIG. 4 is a flowchart of an example process in a user equipment according to some embodiments of the present disclosure;

[0078] FIG. 5 is a flowchart of another example process in a user equipment according to some embodiments of the present disclosure;

[0079] FIG. 6 is a flowchart of an example process in a network node according to some embodiments of the present disclosure;

[0080] FIG. 7 is a flowchart of an example process in another network node according to some embodiments of the present disclosure;

[0081] FIG. 8 is a flowchart of an example process in another network node according to some embodiments of the present disclosure;

[0082] FIG. 9 is a flowchart of an example process in another network node according to some embodiments of the present disclosure;

[0083] FIG. 10 is a diagram of an example process related to TA value retention in subsequent Conditional LTM execution according to some embodiments of the present disclosure;

[0084] FIG. 11 is a diagram of an example process associated with an indication of TA validity for other LTM candidate cells according to some embodiments of the present disclosure;

[0085] FIG. 12 is a diagram of another example process associated with an indication of TA validity for the other LTM candidate cells during early TA acquisition according to some embodiments of the present disclosure;

[0086] FIG. 13 is a diagram of an example process associated with an indication of TA validity for the other LTM candidate cells according to some embodiments of the present disclosure;

[0087] FIG. 14 is a diagram of an example UE Context Modification procedure according to some embodiments of the present disclosure; and

[0088] FIG. 15 is a diagram of a Retained TA Information Transfer procedure according to some embodiments of the present disclosure.

[0089] DETAILED DESCRIPTION

[0090] Before describing in detail exemplary embodiments, it is noted that the embodiments reside primarily in combinations of apparatus components and processing steps related to the retention of TA values at subsequent conditional LTM execution. Accordingly, components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.

[0091] As used herein, relational terms, such as “first” and “second,” “top” and “bottom,” and the like, may be used solely to distinguish one entity or element from another entity or element without necessarily requiring or implying any physical or logical relationship or order between such entities or elements. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the concepts described herein. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises,” “comprising,” “includes” and / or “including” when used herein, specify the presence of stated features, integers, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof.

[0092] In embodiments described herein, the joining term, “in communication with” and the like, may be used to indicate electrical or data communication, which may be accomplished by physical contact, induction, electromagnetic radiation, radio signaling, infrared signaling or optical signaling, for example. One having ordinary skill in the art will appreciate that multiple components may interoperate, and modifications and variations are possible of achieving the electrical and data communication.

[0093] In some embodiments described herein, the term “coupled,” “connected,” and the like, may be used herein to indicate a connection, although not necessarily directly, and may include wired and / or wireless connections.

[0094] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the concepts described herein. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises,” “comprising,” “includes” and / or “including” when used herein, specify the presence of stated features, integers, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof. The term “network node” used herein can be any kind of network node comprised in a radio network which may further comprise any of base station (BS), radio base station, base transceiver station (BTS), base station controller (BSC), radio network controller (RNC), g Node B (gNB), evolved Node B (eNB or eNodeB), Node B, multistandard radio (MSR) radio node such as MSR BS, multi-cell / multicast coordination entity (MCE), relay node, donor node controlling relay, radio access point (AP), transmission points, transmission nodes, Remote Radio Unit (RRU) Remote Radio Head (RRH), a core network node (e.g., mobile management entity (MME), self-organizing network (SON) node, a coordinating node, positioning node, MDT node, etc.), an external node (e.g., 3rd party node, a node external to the current network), nodes in distributed antenna system (DAS), a spectrum access system (SAS) node, an element management system (EMS), a distributed unit (DU) (e.g.,. source DU (S-DU), candidate DU (C-DU), etc.), a centralized unit (CU), etc. The network node may also comprise test equipment. The term “radio node” used herein may be used to also denote a user equipment (UE) such as a wireless device (WD) or a radio network node.

[0095] In some embodiments, the non-limiting terms wireless device (WD) or a user equipment (UE) are used interchangeably. The UE herein can be any type of user equipment capable of communicating with a network node or another UE over radio signals, such as a wireless device (WD). The UE may also be a radio communication device, target device, device to device (D2D) UE, machine type UE or UE capable of machine to machine communication (M2M), low-cost and / or low-complexity UE, a sensor equipped with UE, Tablet, mobile terminals, smart phone, laptop embedded equipped (LEE), laptop mounted equipment (LME), USB dongles, Customer Premises Equipment (CPE), an Internet of Things (loT) device, or a Narrowband loT (NB-IOT) device etc.

[0096] Also, in some embodiments the generic term “radio network node” is used. It can be any kind of a radio network node which may comprise any of base station, radio base station, base transceiver station, base station controller, network controller, RNC, evolved Node B (eNB), Node B, gNB, Multi-cell / multicast Coordination Entity (MCE), relay node, access point, radio access point, Remote Radio Unit (RRU) Remote Radio Head (RRH).

[0097] Note that although terminology from one particular wireless system, such as, for example, 3GPP LTE and / or New Radio (NR), may be used in this disclosure, this should not be seen as limiting the scope of the disclosure to only the aforementioned system. Other wireless systems, including without limitation Wide Band Code Division Multiple Access (WCDMA), Worldwide Interoperability for Microwave Access (WiMax), Ultra Mobile Broadband (UMB) and Global System for Mobile Communications (GSM), may also benefit from exploiting the ideas covered within this disclosure.

[0098] Note further, that functions described herein as being performed by a user equipment or a network node may be distributed over a plurality of user equipments and / or network nodes. In other words, it is contemplated that the functions of the network node and user equipment described herein are not limited to performance by a single physical device and, in fact, can be distributed among several physical devices.

[0099] Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. It will be further understood that terms used herein should be interpreted as having a meaning that is consistent with their meaning in the context of this specification and the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

[0100] Referring again to the drawing figures, in which like elements are referred to by like reference numerals, there is shown in FIG. 2 a schematic diagram of a communication system 10, according to an embodiment, such as a 3GPP-type cellular network that may support standards such as LTE and / or NR (5G), which comprises an access network 12, such as a radio access network, and a core network 14. The access network 12 comprises a plurality of network nodes 16a, 16b, 16c (referred to collectively as network nodes 16), such as NBs, eNBs, gNBs or other types of wireless access points, each defining a corresponding coverage area 18a, 18b, 18c (referred to collectively as coverage areas 18). In some embodiments, coverage area 18 corresponds to a cell (e.g., CLTM cell 18), which may serve or be used for communication with other devices of system 10. In some embodiments, a cell may be referred to as cell 18 (which may provide one or more coverage areas as described herein). Further, a cell may be of one or more types, such as a serving cell, target cell, neighbor cell, etc. Each network node 16a, 16b, 16c is connectable to the core network 14 over a wired or wireless connection 20. A first user equipment (UE) 22a located in coverage area 18a is configured to wirelessly connect to, or be paged by, the corresponding network node 16a. A second UE 22b in coverage area 18b is wirelessly connectable to the corresponding network node 16b. While a plurality of UEs 22a, 22b (collectively referred to as user equipments 22) are illustrated in this example, the disclosed embodiments are equally applicable to a situation where a sole UE is in the coverage area or where a sole UE is connecting to the corresponding network node 16. Note that although only two UEs 22 and three network nodes 16 are shown for convenience, the communication system may include many more UEs 22 and network nodes 16.

[0101] Also, it is contemplated that a UE 22 can be in simultaneous communication and / or configured to separately communicate with more than one network node 16 and more than one type of network node 16. For example, a UE 22 can have dual connectivity with a network node 16 that supports LTE and the same or a different network node 16 that supports NR. As an example, UE 22 can be in communication with an eNB for LTE / E-UTRAN and a gNB for NR / NG-RAN.

[0102] A network node 16 (eNB or gNB) is configured to include a node management unit 24 which is configured to perform any step and / or task and / or process and / or method and / or feature described in the present disclosure, e.g., network node functions. A user equipment 22 is configured to include a UE management unit 26 which is configured to perform any step and / or task and / or process and / or method and / or feature described in the present disclosure, e.g., UE functions.

[0103] Example implementations, in accordance with an embodiment, of the UE 22 and network node 16 discussed in the preceding paragraphs will now be described with reference to FIG. 3.

[0104] The communication system 10 includes a network node 16 provided in a communication system 10 and includes hardware 28 enabling it to communicate with the UE 22. The hardware 28 may include a radio interface 30 for setting up and maintaining at least a wireless connection 32 with a UE 22 located in a coverage area 18 served by the network node 16. The radio interface 30 may be formed as or may include, for example, one or more RF transmitters, one or more RF receivers, and / or one or more RF transceivers. The radio interface 30 includes an array of antennas 34 to radiate and receive signal(s) carrying electromagnetic waves. Hardware 28 may also include communication interface 31 configured for setting up and maintaining communication with other network nodes 16, via wired / wireless connection(s).

[0105] In the embodiment shown, the hardware 28 of the network node 16 further includes processing circuitry 36. The processing circuitry 36 may include a processor 38 and a memory 40. In particular, in addition to or instead of a processor, such as a central processing unit, and memory, the processing circuitry 36 may comprise integrated circuitry for processing and / or control, e.g., one or more processors and / or processor cores and / or FPGAs (Field Programmable Gate Array) and / or ASICs (Application Specific Integrated Circuitry) adapted to execute instructions. The processor 38 may be configured to access (e.g., write to and / or read from) the memory 40, which may comprise any kind of volatile and / or nonvolatile memory, e.g., cache and / or buffer memory and / or RAM (Random Access Memory) and / or ROM (Read-Only Memory) and / or optical memory and / or EPROM (Erasable Programmable Read-Only Memory).

[0106] Thus, the network node 16 further has software 42 stored internally in, for example, memory 40, or stored in external memory (e.g., database, storage array, network storage device, etc.) accessible by the network node 16 via an external connection. The software 42 may be executable by the processing circuitry 36. The processing circuitry 36 may be configured to control any of the methods and / or processes described herein and / or to cause such methods, and / or processes to be performed, e.g., by network node 16. Processor 38 corresponds to one or more processors 38 for performing network node 16 functions described herein. The memory 40 is configured to store data, programmatic software code and / or other information described herein. In some embodiments, the software 42 may include instructions that, when executed by the processor 38 and / or processing circuitry 36, causes the processor 38 and / or processing circuitry 36 to perform the processes described herein with respect to network node 16. For example, processing circuitry 36 of the network node 16 may include a node management unit 24 which is configured to perform any step and / or task and / or process and / or method and / or feature described in the present disclosure, e.g., network node functions. Processing circuitry 36 of the network node 16 may also include DU 100 configured to perform DU functions and CU 102 configured to perform CU functions.

[0107] The communication system 10 further includes the UE 22 already referred to. The UE 22 may have hardware 44 that may include a radio interface 46 configured to set up and maintain a wireless connection 32 with a network node 16 serving a coverage area 18 in which the UE 22 is currently located. The radio interface 46 may be formed as or may include, for example, one or more RF transmitters, one or more RF receivers, and / or one or more RF transceivers. The radio interface 46 includes an array of antennas 48 to radiate and receive signal(s) carrying electromagnetic waves.

[0108] The hardware 44 of the UE 22 further includes processing circuitry 50. The processing circuitry 50 may include a processor 52 and memory 54. In particular, in addition to or instead of a processor, such as a central processing unit, and memory, the processing circuitry 50 may comprise integrated circuitry for processing and / or control, e.g., one or more processors and / or processor cores and / or FPGAs (Field Programmable Gate Array) and / or ASICs (Application Specific Integrated Circuitry) adapted to execute instructions. The processor 52 may be configured to access (e.g., write to and / or read from) memory 54, which may comprise any kind of volatile and / or nonvolatile memory, e.g., cache and / or buffer memory and / or RAM (Random Access Memory) and / or ROM (Read-Only Memory) and / or optical memory and / or EPROM (Erasable Programmable Read-Only Memory).

[0109] Thus, the UE 22 may further comprise software 56, which is stored in, for example, memory 54 at the UE 22, or stored in external memory (e.g., database, storage array, network storage device, etc.) accessible by the UE 22. The software 56 may be executable by the processing circuitry 50. The software 56 may include a client application 58. The client application 58 may be operable to provide a service to a human or non-human user via the UE 22.

[0110] The processing circuitry 50 may be configured to control any of the methods and / or processes described herein and / or to cause such methods, and / or processes to be performed, e.g., by UE 22. The processor 52 corresponds to one or more processors 52 for performing UE 22 functions described herein. The UE 22 includes memory 54 that is configured to store data, programmatic software code and / or other information described herein. In some embodiments, the software 56 and / or the client application 58 may include instructions that, when executed by the processor 52 and / or processing circuitry 50, causes the processor 52 and / or processing circuitry 50 to perform the processes described herein with respect to UE 22. For example, the processing circuitry 50 of the user equipment 22 may include a UE management unit 26 which is configured to perform any step and / or task and / or process and / or method and / or feature described in the present disclosure, e.g., UE functions.

[0111] In some embodiments, the inner workings of the network node 16 and UE 22 may be as shown in FIG. 3 and independently, the surrounding network topology may be that of FIG. 2.

[0112] The wireless connection 32 between the UE 22 and the network node 16 is in accordance with the teachings of the embodiments described throughout this disclosure. More precisely, the teachings of some of these embodiments may improve the data rate, latency, and / or power consumption and thereby provide benefits such as reduced user waiting time, relaxed restriction on file size, better responsiveness, extended battery lifetime, etc. In some embodiments, a measurement procedure may be provided for the purpose of monitoring data rate, latency and other factors on which the one or more embodiments improve.

[0113] Although FIGS. 2 and 3 show various “units” such as node management unit 24 and UE management unit 26 as being within a respective processor, it is contemplated that these units may be implemented such that a portion of the unit is stored in a corresponding memory within the processing circuitry. In other words, the units may be implemented in hardware or in a combination of hardware and software within the processing circuitry.

[0114] FIG. 4 is a flowchart of an example process in a user equipment 22 according to some embodiments of the present disclosure. One or more blocks described herein may be performed by one or more elements of user equipment 22 such as by one or more of processing circuitry 50 (including the UE management unit 26), processor 52, and / or radio interface 46. User equipment 22 such as via processing circuitry 50 and / or processor 52 and / or radio interface 46 is configured to: (A) receive (Block S100) a conditional L1 / L2 triggered mobility (CLTM) configuration for one or more CLTM candidate cells, where the CLTM configuration includes one or more CLTM execution conditions; (B) in response to receiving the CLTM configuration, evaluate (Block SI 02) the one or more CLTM execution conditions; (C) before at least one of the one or more the CLTM execution conditions is fulfilled for the one or more CLTM candidate cells 18, perform (Block SI 04) uplink (UL) synchronization with the one or more CLTM candidate cells 18; and (D) when at least one of the one or more CLTM execution condition is fulfilled for the one or more CLTM candidate cells 18, execute (Block SI 06) a CLTM procedure according to the CLTM configuration for the one or more CLTM candidate cells 18.

[0115] In some embodiments, one or more of: (A) the method further includes retaining one or more valid timing advance (TA) values for other CLTM candidate cells 18 according to the information provided by the network or according to the UE implementation, for which the UE 22 performed early TA acquisition but did not execute LTM cell switch, after conditional LTM execution to one of the one or more CLTM candidate cells 18; (B) the method further includes maintaining other relevant information about TA values, for the other CLTM candidate cells 18; (C) the other relevant information about TA values includes information about a current ‘state’ of the corresponding Time alignment timers; and (D) the information about the current ‘state’ includes a current value or a remaining time left to expiry or a time instant when a TA value was obtained.. In some other embodiments, the method further includes retaining one or more TA values for the other CLTM candidate cells 18, after conditional LTM execution to one of the one or more CLTM candidate cells 18, when a time left for expiry of associated time alignment timer is above a threshold value and / or when the UE 22 is explicitly configured to retain TA values and / or associated time alignment timer values for other CLTM candidate cells 18.

[0116] In some embodiments, one or more of: (A) the method further includes sharing information about retained TA values for other CLTM candidate cells 18 with the network node 16; (B) the information about the retained TA values is shared via a radio resource control (RRC) message, another existing message, an existing lower layer signaling message, or new lower layer signaling message;(C) the RRC message includes RRC Reconfiguration Complete Message; and (D) the existing lower layer signaling message or the new lower layer signaling message includes a TA Status reporting medium access control (MAC) control element (CE).

[0117] In one or more embodiments, the method further includes preventing triggering a measurement report for an event associated with early UL synchronization of a CLTM candidate cell 18, if a TA value for that cell 18 was acquired before the CLTM execution in a previous source cell 18 and is still valid, or a time alignment timer corresponding to the TA value of the CLTM candidate cell 18 is still running.

[0118] In some other embodiments, the method further includes releasing TA values for other CLTM candidate cells 18, upon conditional LTM execution to one of CLTM candidate cells 18, if the UE 22 does not retain the TA values for other LTM candidate cells 18.

[0119] FIG. 5 is a flowchart of another example process in a user equipment 22 according to some embodiments of the present disclosure. One or more blocks described herein may be performed by one or more elements of user equipment 22 such as by one or more of processing circuitry 50 (including the UE management unit 26), processor 52, and / or radio interface 46. UE 22 is configured to receive (Block SI 08) a conditional Layer 1 / Layer 2, L1 / L2, triggered mobility, CLTM, configuration for one or more CLTM candidate cells 18, where the CLTM configuration includes one or more CLTM execution conditions for executing a CLTM procedure, as described herein. UE 22 is configured to, before at least one of the one or more CLTM execution conditions is fulfilled, perform (Block SI 10) uplink, UL, synchronization with the one or more CLTM candidate cells 18 to acquire one or more timing advance, TA, values, as described herein. UE 22 is configured to (Block SI 12), when at least one of the one or more CLTM execution conditions is fulfilled: execute the CLTM procedure for a first CLTM candidate cell 18 of the one or more CLTM candidate cells 18; and retain the one or more TA values, as described herein.

[0120] According to one or more embodiments, the UE 22 is further configured to, after conditional LTM execution to the first CLTM candidate cell 18, retain at least one TA value for at least a second CLTM candidate cell 18 of the one or more CLTM candidate cells 18.

[0121] According to one or more embodiments, the at least one TA value for the second CLTM candidate cell 18 is retained when a time left for expiry of at least one timing alignment timer associated with the at least one TA value meets a threshold criterion.

[0122] According to one or more embodiments, the at least one TA value for the second CLTM candidate cell 18 is retained when the UE 22 is configured to one or both of: retain the at least one TA value; and retain at least one time alignment timer value associated with the at least one TA value.

[0123] According to one or more embodiments, the at least one TA value comprises a TA value for a third CLTM candidate cell 18; and the UE 22 is further configured to indicate, to the second CLTM candidate cell 18, information associated with the TA value for the third CLTM candidate cell 18.

[0124] According to one or more embodiments, the indication is provided by an RRC message or a UE assistance information message.

[0125] According to one or more embodiments, the UE 22 is further configured to: prevent triggering a measurement report for an event associated with early UL synchronization of a second CLTM candidate cell 18 of the one or more candidate cells 18, if a TA value for second CLTM candidate cell 18 was acquired before the CLTM execution in a previous source cell 18 and is: still valid; or a time alignment timer that is still running.

[0126] According to one or more embodiments, the UE 22 is further configured to maintain information for at least one TA value for at least the second CLTM candidate, where the information indicates a current state of the at least one time alignment time associated with the at least one TA value.

[0127] FIG. 6 is a flowchart of an example process in a network node 16. The network node 16 may be configured as a source radio access network node 16 or an S-DU 100 for the UE 22. One or more blocks described herein may be performed by one or more elements of network node 16 such as by one or more of processing circuitry 36 (including the node management unit 24), processor 38, and / or radio interface 30. Network node 16 such as via processing circuitry 36 and / or processor 38 and / or radio interface 30 is configured to: (A) determine (Block SI 14) whether to configure the UE 22 with a conditional L1 / L2 triggered mobility (CLTM) configuration for one or more CLTM candidate cells 18, where the CLTM configuration includes one or more CLTM execution conditions; (B) transmit (Block SI 16) a request for configuration of CLTM for a CLTM candidate cell 18 to a candidate network node 16 configured for handling the CLTM candidate cell 18; (C) receive (Block SI 18) a configuration for CLTM for the CLTM candidate cell 18 from the candidate network node 16; (D) transmit (Block S120) the CLTM configuration for the CLTM candidate cell 18, with an associated CLTM execution condition, to the UE 22; (E) transmit (Block SI 22) a physical downlink control channel (PDCCH) order for triggering uplink (UL) synchronization for the one or more CLTM candidate cells 18; and (F) maintain (Block S124) one or more timing advance (TA) values associated with the one or more CLTM candidate cells 18, for a predetermined UE 22, for which the predetermined UE 22 performed early TA acquisition.

[0128] In some embodiments, another S-DU 100 is configured to, after conditional LTM execution, receive one or more of a Class 1 Message from a C-DU 100, a Class 2 Message from the C-DU, Class 1 Message from a gNodeB centralized unit (gNB-CU) 102, a Class 2 Message from the gNB-CU 102, another indication from the C-DU 100, and another indication from the gNB-CU 102 which triggers the S-DU 100 to provide information about the one or more TA values received during the UL synchronization to one or more CLTM candidate cells 18 and validity of the one or more TA values.

[0129] In some other embodiments, another S-DU 100 is configured to, after conditional LTM execution, provide information about the one or more TA values received during the UL synchronization to one or more CLTM candidate cells 18 and validity of the one or more TA values by sending one or more of a Class 1 Message to a candidate distributed unit (C-DU) 100, a Class 2 Message to the C-DU 100, a Class 1 Message to a gNodeB centralized unit (gNB-CU) 102, a Class 2 Message to the gNB-CU 102.

[0130] FIG. 7 is a flowchart of an example process in a network node 16. The network node 16 may be configured as a candidate radio access network node 16 or a C-DU 100 for the UE 22. One or more blocks described herein may be performed by one or more elements of network node 16 such as by one or more of processing circuitry 36 (including the node management unit 24), processor 38, and / or radio interface 30. Network node 16 such as via processing circuitry 36 and / or processor 38 and / or radio interface 30 is configured to: receive (Block S126) a request from a serving network node 16 for the UE 22, where the request requests a conditional L1 / L2 triggered mobility (CLTM) configuration for one or more CLTM candidate cells 18 handled by the network node 16; (B) transmit (Block S128) a configuration for a CLTM for a CLTM candidate cell 18 to a serving network node 16; (C) receive (Block SI 30) a random access message transmitted by the UE 22 for timing advance (TA) acquisition; transmit (Block SI 32) a calculated TA value to other network nodes 16 or directly to the UE 22; and maintain (Block S134) one or more TA values associated with one or more CLTM candidate cells 18 located in the candidate distributed unit (C-DU), for a predetermined UE 22, for which the predetermined UE 22 performed TA acquisition.

[0131] In some embodiments, the C-DU becomes a source distributed unit (S-DU) 100 after Conditional LTM execution.

[0132] In some other embodiments, the C-DU 100 receives a message or a lower layer indication transmitted by the UE 22 including information about one or more retained TA values for other CLTM candidate cells 18 and about a current state of corresponding time alignment timers.

[0133] In some embodiments, one or both of the message is a radio resource control (RRC) Message and the RRC Message is an RRC Reconfiguration Complete Message.

[0134] In some other embodiments, one or both of: (A) the C-DU 100 receives another existing RRC Message or a new RRC Message, transmitted by the UE 22 including information about one or more retained TA values for other CLTM candidate cells 18 and about a current state of corresponding time alignment timers; and (B) the other existing RRC Message is a UEAssistancelnformation Message.

[0135] In some embodiments, one or more of: (A) the C-DU 100 receives the one or more TA values obtained during TA acquisitions and the relevant information about TA validity for the one or more CLTM candidate cells 18 from a corresponding gNB-CU 102 over an Fl interface using a FIAP protocol Message; (B) the relevant information about TA validity includes current ‘state’ of the corresponding Time alignment timers; and (C) and the current state includes a current value or remaining time left to expiry.

[0136] In some other embodiments, wherein the C-DU 100 receives the one or more TA values obtained during TA acquisitions and the relevant information about TA validity of the corresponding Time alignment timers, for the one or more CLTM candidate cells 18 from a corresponding gNB-CU 102 over an Fl interface using the FIAP protocol Message, upon sending an ACCESS SUCCESS or RRC Reconfiguration Complete Message to the gNB-CU 102.

[0137] In some embodiments, the C-DU 100 receives the one or more TA values obtained during TA acquisitions and the relevant information about TA validity of the corresponding Time alignment timers, for the one or more CLTM candidate cells 18 from a corresponding gNB-CU 100 over an Fl interface using FIAP protocol Message, upon sending one or more of a Class 1 Message to a source distributed unit (S-DU) 100, a Class 2 Message the S-DU 100, a Class 1 Message to the gNB-CU 102, a Class 2 Message to the gNB-CU 102, another indication to S-DU 100, and another indication to the gNB-CU 102.

[0138] In some other embodiments, the C-DU 100 receives the one or more TA values obtained during TA acquisitions and the relevant information about TA validity of the corresponding Time alignment timers, for the one or more CLTM candidate cells 18 from a corresponding gNB-CU 102 over an Xn interface using XnAP protocol Message, upon sending an ACCESS SUCCESS or RRC Reconfiguration Complete Message to the gNB- CU 102.

[0139] In some embodiments, the C-DU 102 receives the one or more TA values obtained during TA acquisitions and the relevant information about TA validity of the corresponding Time alignment timers, for the one or more CLTM candidate cells 18 from a corresponding gNB-CU 102 over an Xn interface using XnAP protocol Message, upon sending one or more of a Class 1 Message to a source distributed unit (S-DU) 100, a Class 2 Message to the S-DU 100, a Class 1 Message to the gNB-CU, a Class 2 Message to the gNB-CU 102, another indication to the S-DU 100, and another indication to the gNB-CU 102.

[0140] In some other embodiments, one or both of: (A) the C-DU 102 receives the one or more TA values obtained during TA acquisitions and the relevant information about TA validity of the corresponding Time alignment timers, for the one or more CLTM candidate cells 18, from a corresponding gNB-CU 102 over an Fl interface, using FIAP protocol Message before the UE 22 performs Conditional LTM execution to one of the configured LTM candidate cells 18, while early TA acquisition is being performed for the one or more CLTM candidate cells 18; and (B) the FIAP protocol Message is a CU-DU TA INFORMATION TRANSFER Message.

[0141] FIG. 8 is a flowchart of an example process in a network node 16. The network node 16 may be configured as one of (A) a gNB-CU 102 that serves a source distributed unit S-DU 100 and configures the UE 22 with a LTM candidate cell configuration; and (B) a gNB-CU 102 that serves a candidate distributed unit C-DU 100. One or more blocks described herein may be performed by one or more elements of network node 16 such as by one or more of processing circuitry 36 (including the node management unit 24), processor 38, and / or radio interface 30. Network node 16 such as via processing circuitry 36 and / or processor 38 and / or radio interface 30 is configured to maintain (Block S136) one or more timing advance (TA) values associated with one or more LTM candidate cells 18, for a predetermined UE 22, for which the predetermined UE 22 performed early TA acquisition.

[0142] In some embodiments, one or both of: (A) the gNB-CU 102 serving the C-DU 100 or a new S-DU 100 decodes information about retained TA values for other LTM candidate cells 18, and the C-DU 100 receives information about a current state of corresponding time alignment timers validity, wherein the information is received via a new radio resource control (RRC) Message or an existing RRC Message; and the new RRC Message or the existing RRC Message includes a Reconfiguration Complete Message or a UEAssistancelnformation Message.

[0143] In some other embodiments, the gNB-CU 102 serving the C-DU 100 or a new S- DU 100 transmits the decoded information about the retained TA value(s) and the information about the current state of corresponding time alignment timers validity via a message.

[0144] In one or more embodiments, the gNB-CU 102 serving the C-DU 100 or a new S- DU 100, after CLTM execution, provides information about retained TA values and information about a current state of corresponding time alignment timers validity via one or more of a Class 1 Message or a Class 2 Message to the C-DU 100.

[0145] In some embodiments, the gNB-CU 102 serving a previous S-DU 100, after conditional LTM execution, retains the one or more TA value(s) received during uplink synchronization to the one or more LTM candidate cells 18 and keeps track of the validity of the one or more TA values, based on one or more conditions.

[0146] In some other embodiments, the gNB-CU 102 serving a previous S-DU 100, after the conditional LTM execution, provides the one or more TA values received during UL synchronization to one or more LTM candidate cells 18 and provides relevant TA validity information to another S-DU 100.

[0147] In some embodiments, one or both of: (A) the gNB-CU 102 serving a previous S- DU sends obtained one or more TA values and relevant information about TA validity for the one or more LTM candidate cells 18 to the S-DU 100 as well as to one or more C-DUs 100 of the same gNB over an Fl interface, using a FIAP protocol Message before the UE 22 performs CLTM execution to one of the configured LTM candidate cells 18; and (B) the FIAP protocol Message is a CU-DU TA INFORMATION TRANSFER Message.

[0148] In some other embodiments, one or more of: (A) the gNB-CU 102 serving a previous S-DU 100, sends obtained one or more TA values and relevant information about TA validity for the one or more LTM candidate cells 18 to the S-DU 100 as well as to one or more C-DUs 100 of a different gNB over an Xn interface, using an XnAP protocol Message, before the UE 22 performs CLTM execution to one of the configured LTM candidate cells 18; (B) the gNB which received the information about the obtained one or more TA values and the relevant information about the TA validity for the one or more LTM candidate cells 18 sends this information to the C-DU it serves over an Fl interface, using a FIAP protocol Message; (C) the XnAP protocol Message is a TA Information Transfer Message; and (D) the FIAP protocol Message is a CU-DU TA INFORMATION TRANSFER Message.

[0149] In some embodiments, the relevant TA values and the TA validity information, received during UL synchronization to one or more LTM candidate cells 18, is provided via one or more of a Class 1 Message, a Class 2 Message, and an XnAP protocol Message.

[0150] In some other embodiments, the gNB-CU 102 serving a previous S-DU 100, after the Conditional LTM execution, requests and receives information about one or more TA values and the relevant TA validity information received during the early UL synchronization to one or more LTM candidate cells 18 and TA values validity from the previous S-DU 100.

[0151] In some embodiments, the TA value and the relevant TA validity information is requested and / or provided via one or more of a Class 1 Message, a Class 2 Message, and an XnAP protocol Message.

[0152] In some other embodiments, the gNB-CU 102 serving anew S-DU 100 or a candidate distributed unit (C-DU) 100, after the CLTM execution, requests and receives information about one or more TA values and the relevant TA validity information received during the early UL synchronization to one or more LTM candidate cells 18 and TA values validity from a previous S-DU 100.

[0153] In some embodiments, the TA value and relevant TA validity information is one or both requested and provided via one or more of a Class 1 Message, a Class 2 Message, and an XnAP protocol Message. FIG. 9 is a flowchart of another example process in a network node 16. The network node 16 is configured as a central unit (CU). One or more blocks described herein may be performed by one or more elements of network node 16 such as by one or more of processing circuitry 36 (including the node management unit 24), processor 38, and / or radio interface 30. Network node 16 is configured to configure (Block S138) the UE 22 with a conditional L1 / L2 triggered mobility, CLTM, configuration for at least a first CLTM candidate cell 18 and a second CLTM candidate cell 18, where the CLTM configuration includes one or more CLTM execution conditions for executing a CLTM procedure, as described herein. Network node 16 is configured to receive (Block SI 40), from a first CLTM candidate cell 18 and based on the configuration, a timing advance, TA, value associated with the first CLTM candidate cell 18 or a second candidate CLTM cell 18, where the TA value had been acquired by the UE 22 before at least one of the one or more CLTM execution conditions were fulfilled.

[0154] According to one or more embodiments, the TA value is associated with the first candidate CLTM cell 18; and the network node 16 is further configured to: receive a TA value associated with the second candidate CLTM cell 18; and indicate, to the first candidate CLTM cell 18, the TA value associated with the second candidate CLTM cell 18.

[0155] According to one or more embodiments, the indication is signaled via one of: a UE context modification request message; a TA information transfer message; or an radio resource control, RRC, message.

[0156] According to one or more embodiments, the TA value is associated with the first candidate CLTM cell 18; and the network node 16 is further configured to indicate, to the second candidate CLTM cell 18, the TA value associated with the first candidate CLTM cell 18.

[0157] According to one or more embodiments, the network node 16 is further configured to receive, from the second candidate CLTM cell 18 and based on the configuration, a TA value associated with the second candidate CLTM cell 18, where the TA value had been acquired by the UE 22 before at least one of the one or more CLTM execution conditions were fulfilled.

[0158] According to one or more embodiments, the receiving of the TA value is part of a CU-Distributed Unit, DU, information transfer.

[0159] According to one or more embodiments, the network node 16 is further configured to retain the TA value for subsequent conditional CLTM execution. Having described the general process flow of arrangements of the disclosure and having provided examples of hardware and software arrangements for implementing the processes and functions of the disclosure, the sections below provide details and examples of arrangements for retention of TA values at subsequent conditional LTM execution.

[0160] In some embodiments, various units such as DUs 100 (e.g., S-DUs, C-DUs, etc.) and CUs 102 are described. Each DU 100 may be associated with or may be configured to provide source cell functions, candidate cell functions, etc. In some embodiments, one or more DUs 100 and / or one or more CUs 102 may be comprised in one or more network nodes 16. In some embodiments, the term “first cell” is used and may refer to “Celli”, and the term “second cell” is also used and may refer to “Cell2”, which are types of cells 18. The first cell and the second cell may be candidate cells but may be other types of cells as well. Further, the term “first DU” is used and may refer to DU1, and the term “second DU” is used and may refer to “DU2”.

[0161] FIG. 10 is a signal flow diagram showing an example process with a problem related to TA value retention in subsequent Conditional LTM execution. At step S200, CU 102 provides an LTM candidate cell configuration (including CLTM execution conditions) to the S-DU 100a (associated with source cell 18 and / or configured to perform source cell functions). At step S202, S-DU 100a may transmit the LTM candidate cell configuration (including CLTM execution conditions) to UE 22. At step S204, UE 22 transmits an RRCReconfiguration complete message. At step S206, S-DU 100a indicates an early TA acquisition for a first cell 18. At step S208, UE 22 transmits a random access preamble to C-DU 100b (e.g., associated with the first cell 18 and / or configured to perform source first cell functions or candidate cell functions). At step S210, the TA value is updated for the first cell 18. At step S212, S-DU 100a indicates an early TA acquisition for a second cell 18. At step S214, UE 22 transmits a random access preamble to C-DU 100c (e.g., associated with the second cell 18 and / or configured to perform source second cell functions or candidate cell functions). At step S216, the TA value is updated for the second cell 18. At step S218, UE 22 determines that execution conditions are fulfilled for the first cell 18. At step S220, an RRCReconfigurationComplete message is transmitted by UE 22 to C-DU 100b, and at step S222, the first cell 18 associated with C-DU 100b triggers early TA acquisition for the second cell 18, unaware that the TA value for the second cell 18 is still valid.

[0162] In other words, the UE 22 is in the source cell 18 (also termed as previous source cell) in which it performed the early synchronization for the first cell 18 and the second cell 18, after receiving the PDCCH order for early TA acquisition for both the cells 18. The UE 22 switches to the first cell 18 which is now the new source cell (or the next source cell), but the TA value for the second cell 18 is still valid. Since there is no LTM cell switch MAC CE in conditional LTM, and cell switch execution is subject to the fulfillment of CLTM execution conditions, there would not be a transmission of LTM Cell Switch Notification messages between the previous S-DU 100a and the next S-DU 100 to carry any valid TA values associated to other LTM candidate cells 18. Therefore, the first cell 18 triggers the early TA acquisition for the second cell 18 for the possible subsequent Conditional LTM execution to the second cell 18.

[0163] One or more embodiments provide UE centric solutions and network centric solutions for the problem described.

[0164] UE Centric Solution

[0165] The solution described below is termed as “UE Centric Solution” as it provides UE methods for solving the aforementioned problems. However, the UE centric solution may also include network node related embodiments.

[0166] One or more embodiments provide a method at the UE 22 configured with subsequent Conditional LTM. The UE 22 receives LTM candidate cell configuration for multiple LTM candidate cell(s) 18 and their respective associated execution conditions for Conditional LTM such that there is no requirement for the UE 22 to receive an RRC Reconfiguration after LTM cell switch to another source cell 18, during the subsequent Conditional LTM execution(s). The UE 22 performs early TA value acquisition for one or multiple LTM candidate cells 18 before the fulfilment of CLTM execution condition(s) and switches to one of those candidate target cells 18 once the CLTM execution conditions are met. According to the nonlimiting method, the UE 22 does not discard the TA values associated to the other LTM candidate cells 18, for which the execution condition(s) are not fulfilled or for which the execution condition(s) are fulfilled but the UE 22 has not selected for execution. The method also includes the UE 22 also indicating to the next source cell 18 (i.e., the LTM candidate cell which is selected for execution) the TA values which are still valid (e.g., for which Time alignment timer at the UE 22 is still running) after the cell switch execution.

[0167] In one example of the method, the valid TA values for the other LTM candidate cells 18 are transmitted by the UE 22 in an RRC Reconfiguration Complete message to the another / new source cell 18. In this method, once the Conditional LTM execution conditions are fulfilled, and the UE 22 prepares to switch to the candidate target cell 18, the UE 22 sends the information about any valid TA values associated to the other LTM candidate cells 18, including any other relevant information, e.g., the information about the current ‘state’ (e.g., current value or remaining time left to expiry) of the corresponding time alignment timers, which the UE 22 received while being in the old source cell 18, via RRC Reconfiguration Complete message to the other / new source cell 18.

[0168] In one example of the method, the valid TA values for the other LTM candidate cells 18 are transmitted by the UE 22 using the UEAssistancelnformation message (or another existing or new RRC message) to the new source cell 18. In this method, once the Conditional LTM execution conditions are fulfilled, and the UE 22 executed a conditional LTM cell switch procedure to the selected candidate target cell 18, the UE 22 sends the information about any valid TA values associated to the other LTM candidate cells 18, including any other relevant information, e.g., the information about the current ‘state’ (e.g., current value or remaining time left to expiry) of the corresponding Time alignment timers

[0169] In another example of the method at the UE 22, the UE 22 retains (keeps stored, does not delete) the valid TA value(s) for the other LTM candidate cells 18 after Conditional LTM execution and cell switch to the new source cell. While being in the next source cell 18, the UE 22 does not trigger the measurement report for the event, defined for early UL synchronization, if the UE 22 has valid TA value for the given LTM candidate cell 18. In other words, the UE 22 does not send a measurement report for the event associated with the early UL synchronization of the LTM candidate cell 18 if the time alignment timer corresponding to the TA value of that LTM candidate cell 18 is still running and / or is not about to expire soon.

[0170] In one example of the method implemented at the UE 22, the UE 22 retains (e.g., keeps stored, does not delete, etc.) the valid TA value(s) for the other LTM candidate cells 18 after Conditional LTM execution and sends to the new source cell 18 lower layer signaling (e.g., a new MAC CE, such as a “TA Status reporting” MAC CE) comprising information related to TA for at least one of the LTM candidate cells 18, such as the TA value, and / or whether a TA value acquired during Early TA acquisition for a certain candidate cell 18 is valid or not, and / or whether a TA value associated to a certain candidate cell 18 has been acquired by the UE 22 or not. The lower layer signalling can optionally comprise information related to the TA value(s), associated to the TimeAlignmentTimer, such as the remaining time until expiry of the TimeAlignmentTimer for the cell 18.

[0171] In another nonlimiting example of the method implemented at the UE 22, the UE 22 only retains (keeps stored, does not delete) a valid TA value for one of the other LTM candidate cells 18 after Conditional LTM execution and cell switch to the new source cell 18 depending on one or more conditions:

[0172] • when the time left for expiry of the associated time alignment timer is above a threshold value. Such a threshold value may be configured at the UE 22 e.g., as part of the LTM configuration, possibly per candidate cell 18 or source / candidate cell pair.

[0173] • when the UE 22 is explicitly configured to retain TA values and / or associated time alignment timer values for other LTM candidate cells 18, after LTM cell switch.

[0174] When the UE 22 does not retain the TA values for LTM candidate cells 18, the UE 22 releases the TA values.

[0175] In some embodiments, on the UE 22 side, the UE 22 has received one or more TA values, each associated to an LTM candidate cell 18. When the UE 22 executes LTM Cell Switch to one of the candidates (e.g., upon fulfillment of the CLTM execution condition), the UE 22 retains the TA values of the other candidate cells 18. In one option, the UE 22 keeps running the time alignment timer associated to each TA value.

[0176] At the network side, for the UE centric solution, a first network node 16 (operating as the new source cell 18 in C-DU 100) receives an RRC Reconfiguration Complete Message or any other RRC message (existing or new), including one or more valid TA value(s) for the other LTM candidate cell(s) 18 and the related information (e.g., time alignment timer information, such as information about remaining time left until expiry), transmitted by the UE 22 upon the triggering or completion of a Conditional LTM cell switch execution. According to some options, the old source cell 18 and / or one or more of the candidate cells 18, during the CLTM preparation can send instructions to the UE 22, instructing the UE 22 to send, upon CLTM execution, to the new source cell 18 (e.g., the C-DU), lower layer signaling comprising TA related information (e.g., TA values, whether TA values acquired for candidate LTM cells 18 are valid at the UE 22). The network node 16 (e.g., the C-DU 100) receiving the lower layer signaling can send this information to another network node 16 (e.g., the C-CU 102 controlling the C-DU 100). As part of this solution, the gNB-CU 102 controlling the new source cell 18 receives from the UE 22 (e.g. in an RRC Reconfiguration Complete Message) the one or more TA values and associated information (such as info about remaining time left until expiry) for at least one of the other LTM candidate cells 18. The gNB-CU 102 transmits to the C-DU 100 or the new source cell 18 the one or more TA values and associated information (such as info about remaining time left until expiry) for at least one of the other LTM candidate cells 18. Since the gNB-CU 102 receives such information from the UE 22 in an RRCReconfigurationComplete (via the C-DU 100 of the new source cell 18) or UE Assistance information, the message is encrypted and cannot be decoded in the C- DU 100 when received directly from the UE 22. Thus, assuming that the C-DU 100 of the new source cell 18 is the network node 16 monitoring the validity of the newly received TA values, and assuming that the C-DU 100 is also capable of performing legacy LTM, the gNB-CU 102 transmits the received TA values and associated information, e.g., remaining time left until TA value expiry, to the C-DU of the new source cell 18.

[0177] Network Centric Solution

[0178] The solution described below is called “Network Centric Solution” because it provides a signaling methods between the network nodes for solving the aforementioned problems. However, the embodiments are not limited as such, and the network centric solution may include steps performed by the UE 22.

[0179] One or more embodiments provide a method at the network nodes 16 for sharing the valid TA values for the other LTM candidate cells 18 such that the TA values can be applied by the UE 22 in performing RACH-less LTM cell switch during subsequent conditional LTM execution, without any requirement for the network node 16 to trigger PDCCH order for the LTM candidate cell 18 with valid TA value and the UE 22 to send PRACH preamble to the LTM candidate cell 18 with valid TA value. In other words, the S-DU 100 and / or the gNB-CU 102 in which a TA value associated to at least one of the other LTM candidate cells 18 (not selected for CLTM execution) is stored (and associated information, e.g., time left for Time alignment timer expiry), transmits the TA value and associated information to the C-DU 100 of the new source cell 118, so that the C-DU 102 (which is now the new S-DU 102) does not need to trigger TA acquisition, while the TA value is valid.

[0180] In one example of the method, after an early TA acquisition is triggered for an LTM candidate cell 18 (e.g., S-DU 100 sending PDCCH order to the UE 22, in response to which the UE 22 transmits a RACH preamble to a C-DU 100), the gNB-CU 102 receives the TA value from the C-DU 100 (C-DU 100 associated to that LTM candidate cell 18) which calculated it after reception of the PRACH preamble from the UE 22. The gNB-CU 102 then sends the obtained TA value to the S-DU 102 as well as to one or more C-DU(s) 102(of the same network node 16 (gNB) or another network node 16 (gNB)) which are part of subsequent Conditional LTM configuration. In other words, the TA information is transferred during (or shortly after) the early UL synchronization phase, e.g., before any Conditional LTM execution is detected at the candidate gNB-CU 102 and / or at the S-DU 100. In another sub-option of the method, the message is used to forward the obtained TA value, and any other relevant information (e.g., the information about the current ‘state’ (e.g., current value or remaining time left to expiry) of the corresponding time alignment timers) to one or more C-DU(s) 100 which are a part of subsequent Conditional LTM configuration. Upon early TA acquisition by the UE 22, an existing or a new message is sent over a protocol which connect a DU 100 to a CU 102 (and vice versa) or a first CU 102 with a second CU 102 (and vice versa). In this case, such protocols can be the F1AP and / or the XnAP and / or the NGAP protocols.

[0181] In another example of the method, the source gNB-CU 102 keeps track of the validity of TA value(s) received during the early UL synchronization to one or more LTM candidate cell(s) 18, e.g., by starting a time alignment timer when the TA value is received by C-DU(s) 100. Upon Conditional LTM execution, the gNB-CU 102 receives from a C- DU 100 in which the UE 22 accesses, an indication via DU-CU signaling (e.g., UL RRC Message Transfer, Access Success message) that the cell switch to the LTM candidate cell 18 (associated to that C-DU 100) has been completed. When the time alignment timer expires for a TA value before the gNB-CU 102 receives the indication, the gNB-CU 102 discards the TA value. When at least one TA value which is valid when the gNB-CU 102 receives the indication, the gNB-CU 102 then provides the valid TA values of the other LTM candidate cells 18 (previously obtained during the early TA acquisition) to the new S-DU 100 (associated to the new source cell 18) using CU-DU signaling (e.g., UE Context Modification Request Message). In other words, the TA information is transferred after the Conditional LTM execution. In another alternative of this example, the gNB-CU 102 does not store the TA values received during the early UL synchronization but rather receives TA values from the previous S-DU 100 (e.g., in response to a request), associated to the S-DU 100, to provide the valid TA values for the other LTM candidate cells 18, after receiving the LTM cell switch execution indication from C-DU 100. The old S-DU 100 provides information about the valid TA values for the other LTM candidate cells 18 to the gNB-CU 102 which is then forwarded to the new S-DU 100 using CU-DU signaling.

[0182] In another sub-option of the method, the previous S-DU 100 and the next S-DU 100 are in different gNB-CUs 102. In this case, the gNB-CU 102, upon receiving the ACCESS SUCCESS Message or RRC Reconfiguration Complete Message, requests information about the valid TA values for the other LTM candidate cells 18, and any other relevant information, e.g., the information about the current ‘state’ (e.g., current value or remaining time left to expiry) of the corresponding Time alignment timers, over a protocol which connects it with the other gNB-CU 102 in which the S-DU 100 is located, via an existing or a new message. Such protocols can be the Fl and / or the Xn interfaces.

[0183] In addition, different options are described, such as for the indication to the gNB- CU 102 (e.g., ACCESS SUCCESS message, UL transfer including the RRC Reconfiguration Complete message from the UE 22) and for the message to the C-DU 100 (new S-DU 100) in which the TA values and related information are included.

[0184] In another example of the method, the gNB-CU 102 keeps track of the validity of TA value(s) received during the early UL synchronization to one or more LTM candidate cell(s) 18. Upon Conditional LTM execution, the gNB-CU 102 receives an indication from C-DU 100 via some DU-CU signaling (e.g., UL RRC Message Transfer) that the cell switch to the LTM candidate cell 18 (associated to that C-DU 100) has been completed. In this case, the gNB-CU 102 provides the valid TA values for the other LTM candidate cells 18 (previously obtained during the early TA acquisition) to the new source cell 18 (associated to that C-DU 100) only when one or more of the following conditions are met:

[0185] • The remaining validity of the TA value is above a threshold.

[0186] • The TA value has been received below “X” time ago (where time in this case can be second, millisecond, hour, or any other time unit).

[0187] • The TA value belongs to a DU 100 which is connected to the same gNB-CU 102 controlling the DU 100 serving the UE 22.

[0188] In another example of the method, the S-DU 100 keeps track of the validity of TA value(s) received during the early UL synchronization to one or more LTM candidate cell(s) 18 , e.g., by starting a Time alignment timer when the TA value is received by C- DU(s) 100. Upon Conditional LTM execution, the gNB-CU 102 receives an indication from the gNB-CU 102 that the cell switch to the LTM candidate cell 18 (associated to that C-DU 100) has been completed. When the Time Alignment timer expires for a TA value before the S-DU 100 receives such indication of cell switch / CLTM execution, the C-DU 100 discards the TA value. When at least one TA value which is valid when the S-DU 100 receives the indication, the S-DU 100 provides the valid TA values for the other LTM candidate cells 18 (previously obtained during the early TA acquisition) to the new source cell 18 (associated to that C-DU 100) using CU-DU signaling (e.g., UE Context Modification Request Message). In other words, the TA information is transferred between the network nodes 16 after the Conditional LTM execution.

[0189] When the S-DU 100 provides the TA values to the CU 102 to be provided to the C-DU 100, for sub-sequent LTM, the S-DU 100 also provides for a TA value a remaining value of a time alignment timer. When the CU 102 provides the TA value to the C-DU 100 for sub-sequent LTM, the CU 102 also includes that remaining time alignment timer value associated to the TA value.

[0190] The following are nonlimiting example embodiments:

[0191] AL A method at a UE 22 comprising one or more of:

[0192] • Receiving a conditional LTM (CLTM) configuration for at least one CLTM candidate cell 18, with an associated CLTM execution condition.

[0193] • In response to receiving the CLTM configuration, evaluating the CLTM execution condition(s).

[0194] • Before the CLTM execution condition is fulfilled for the CLTM candidate cell 18, the UE 22 performs early UL synchronization with one or multiple LTM candidate cell(s) 18; and

[0195] • When the CLTM execution condition(s) is / are fulfilled for the LTM candidate cell 18, executing the conditional LTM procedure according to the LTM configuration for the LTM candidate cell 18.

[0196] A2. A method of Al in which the UE 22 retains the valid TA value(s) for the other LTM candidate cell(s) 18, for which it performed early TA acquisition but did not execute LTM cell switch, after Conditional LTM execution to one of the LTM candidate cell(s) 18.

[0197] A3. A method of Al and A2 in which the UE 22 also maintains the other relevant information about the TA value(s) for the other LTM candidate cell(s) 18, e.g., the information about the current ‘state’ (e.g., current value or remaining time left to expiry) of the corresponding time alignment timers, after Conditional LTM execution to one of the LTM candidate cell(s) 18.

[0198] A4. A method of Al, A2 and A3 in which the UE 22 retains the TA value(s) for the other LTM candidate cell(s) 18, after Conditional LTM execution to one of the LTM candidate cell(s) 18, when the time left for expiry of the associated Time alignment timer is above a threshold value.

[0199] A5. A method of Al, A2 and A3 in which the UE 22 retains the TA value(s) for the other LTM candidate cell(s) 18, after Conditional LTM execution to one of the LTM candidate cell(s) 18, when the UE 22 is explicitly configured to retain TA values and / or associated Time alignment timer values for other LTM candidate cells 18.

[0200] A6. A method of Al, A2 and A3 in which the UE 22 releases the TA value(s) for the other LTM candidate cell(s) 18, upon Conditional LTM execution to one of the LTM candidate cell(s) 18, if the UE 22 does not retain the TA value(s) for the other LTM candidate cell(s) 18.

[0201] A7. A method of Al to A5 in which the UE 22 shares the information about the retained TA value(s) for the other LTM candidate cell(s) 18 with the network node 16, i.e., C-DU 100, to which it performed Conditional LTM execution, via an RRC Message, e.g., RRC Reconfiguration Complete Message.

[0202] A8. A method of Al to A5 in which the UE 22 shares the information about the retained TA value(s) for the other LTM candidate cell(s) 18 with the network node 16, i.e., C-DU 100, to which it performed Conditional LTM execution, via a new or another existing message, e.g., UEAssistancelnformation Message.

[0203] A9. A method of Al to A5 in which the UE 22 shares the information about the retained TA value(s) for the other LTM candidate cell(s) 18 with the network node 16, i.e., C-DU 100, to which it performed Conditional LTM execution, via lower layer signaling, e.g., TA Status reporting MAC CE.

[0204] A10. A method of Al to A5 in which the UE 22 does not send a measurement report for the event associated with the early UL synchronization of the LTM candidate cell 18, if the TA value for that cell 18 was acquired before the Conditional LTM execution in a previous source cell 18 and the time alignment timer corresponding to the TA value of that LTM candidate cell 18 is still running.

[0205] BL A method at a network node 16, acting as a source RAN node or S-DU 100 for the UE 22, comprising one or more of:

[0206] • Determining to configure the UE 22 with a conditional LTM (CLTM) configuration for at least one CLTM candidate cell 18, with an associated CLTM execution condition.

[0207] • Sending a request for configuration of CLTM for the CLTM candidate cell 18, to a candidate network node, handling the CLTM candidate cell 18. • Receiving a configuration for CLTM for the CLTM candidate cell 18 from the candidate network node.

[0208] • Sending a conditional LTM (CLTM) configuration for the CLTM candidate cell 18, with an associated CLTM execution condition, to the UE 22.

[0209] • Sending a PDCCH order for triggering the early UL synchronization for one or more LTM candidate cell(s) 18.

[0210] • Maintaining the TA value(s) associated to one or more LTM candidate cell(s) 18, for a given UE 22, for which the UE 22 performed early TA acquisition.

[0211] B2. A method of Bl in which the old S-DU 100, after Conditional LTM execution, receives information / configuration / message to provide the information about TA value(s) received during the early UL synchronization to one or more LTM candidate cell(s) 18 and the TA value(s) validity via anew or existing Class 1 Message sent by gNB-CU 102, e g., UE CONTEXT MODIFICATION REQUEST Message.

[0212] B3. A method of Bl and B2 in which the old S-DU 100, after Conditional LTM execution, provides the information about TA value(s) received during the early UL synchronization to one or more LTM candidate cell(s) 18 and the TA value(s) validity to the gNB-CU 102 via a Class 1 Message, e g., UE CONTEXT MODIFICATION REQUEST Message, or another existing or new message, e.g., LTM CS NOTIFICATION DU->CU).

[0213] CL A method at a network node 16, acting as candidate RAN node, i.e., C-DU 100, for the UE 22, comprising:

[0214] • Receiving a request from a serving node for a UE 22, for configuration of a conditional LTM (CLTM) configuration for at least one CLTM candidate cell 18 that is handled by the candidate node.

[0215] • Sending a configuration for CLTM for the CLTM candidate cell 18 to the serving network node 16.

[0216] • Receiving the random-access transmitted by the UE 22 for early TA acquisition and sending the calculated TA value to the other network nodes 16 or directly to the UE 22.

[0217] • Maintaining the TA value(s) associated to one or more LTM candidate cell(s) 18 located in the C-DU 100, for a given UE 22, for which the UE 22 performed early TA acquisition.

[0218] C2. A method of Cl, in which the C-DU 100 becomes the new S-DU 100 after Conditional LTM execution and receives an RRC Reconfiguration Complete Message sent by the UE 22, which includes information about the retained TA value(s) for the other LTM candidate cell(s) 18, and the about the current ‘state’ (e.g., current value or remaining time left to expiry) of the corresponding time alignment timers.

[0219] C3. A method of Cl, in which the C-DU 100 becomes the new S-DU 100 after Conditional LTM execution and receives a new or another existing message, e.g., UEAssistancelnformation Message, sent by the UE 22, which includes information about the retained TA value(s) for the other LTM candidate cell(s) 18, and the about the current ‘state’ (e.g., current value or remaining time left to expiry) of the corresponding time alignment timers.

[0220] C4. A method of Cl, in which the C-DU 100 becomes the new S-DU 100 after Conditional LTM execution and receives a lower layer indication, e.g., TA Status reporting MAC CE, sent by the UE 22, which includes information about the retained TA value(s) for the other LTM candidate cell(s) 18, and the about the current ‘state’ (e.g., current value or remaining time left to expiry) of the corresponding time alignment timers.

[0221] C5. A method of Cl in which the C-DU 100 receives the TA value(s) obtained during early TA acquisitions and the relevant information about the TA validity for the LTM candidate cell(s) 18 from its gNB-CU 102 over Fl interface, using FIAP protocol Message, e g., CU-DU TA INFORMATION TRANSFER Message.

[0222] C6. A method of Cl in which the C-DU 100 becomes the new S-DU 100 after Conditional LTM execution and receives the information about the retained TA value(s) for the other LTM candidate cell(s) 18, and the about the current ‘state’ (e.g., current value or remaining time left to expiry) of the corresponding Time alignment timers, over a Class 1 Message, e.g., UE CONTEXT MODIFICATION REQUEST Message, upon sending the ACCESS SUCCESS Message or RRC Reconfiguration Complete Message.

[0223] C7. A method of Cl in which the C-DU 100 becomes the new S-DU 100 after Conditional LTM execution and receives the information about the retained TA value(s) for the other LTM candidate cell(s) 18, and the about the current ‘state’ (e.g., current value or remaining time left to expiry) of the corresponding Time alignment timers, over a new or existing Class 2 Message, upon sending the ACCESS SUCCESS Message or RRC Reconfiguration Complete Message.

[0224] C8. A method of Cl in which the C-DU 100 becomes the new S-DU 100 after Conditional LTM execution and receives the information about the retained TA value(s) for the other LTM candidate cell(s) 18, and the about the current ‘state’ (e.g., current value or remaining time left to expiry) of the corresponding time alignment timers, over a new or existing F1AP or XnAP protocol Message, upon sending the ACCESS SUCCESS Message or RRC Reconfiguration Complete Message.

[0225] DI. A method at a network node 16, acting as gNB-CU 102 which serves the S- DU 100 and configures the UE 22 with LTM candidate cell configuration, or gNB-CU 102 which serves the C-DU 100, comprising.

[0226] • Maintaining the TA value(s) associated to one or more LTM candidate cell(s) 18, for a given UE 22, for which the UE 22 performed early TA acquisition.

[0227] D2. A method of DI in which the gNB-CU 102 serving the C-DU 100 or the new S-DU 100 decodes information about the retained TA value(s) for the other LTM candidate cell(s) 18, and the information about the current ‘state’ (e.g., current value or remaining time left to expiry) of the corresponding Time alignment timers, received by the C-DU 100 via RRC Reconfiguration Complete Message or UEAssistancelnformation Message.

[0228] D2a. A method of DI and D2 in which the gNB-CU 102 serving the C-DU 100, or the new S-DU 100 provides the decoded information about the retained TA value(s) for the other LTM candidate cell(s) 18, and the information about the current ‘state’ (e.g., current value or remaining time left to expiry) of the corresponding Time alignment timers, via a class 1 Message, e g., UE CONTEXT MODIFICATION REQUEST Message.

[0229] D2b. A method of DI and D2 in which the gNB-CU 102 serving the C-DU 100, or the new S-DU 100 provides the decoded information about the retained TA value(s) for the other LTM candidate cell(s) 18, and the information about the current ‘state’ (e.g., current value or remaining time left to expiry) of the corresponding Time alignment timers, via an existing or new class 2 Message.

[0230] D3. A method of DI in which the gNB-CU 102 serving the S-DU 100, before the Conditional LTM execution, sends the obtained TA value(s) and the relevant information about the TA validity for the LTM candidate cell(s) 18 to the S-DU 100 as well as to one or more C-DU(s) 100 of the same gNB over Fl interface, using FIAP protocol Message, e g., CU-DU TA INFORMATION TRANSFER Message.

[0231] D3a. A method of DI in which the gNB-CU 102 serving the S-DU 100, before the Conditional LTM execution, sends the obtained TA value(s) and the relevant information about the TA validity for the LTM candidate cell(s) 18 to the S-DU 100 as well as to one or more C-DU(s) 100 of a different gNB over Xn interface, using XnAP protocol Message, e.g., e.g., TA Information Transfer Message. The network node 16 (gNB) which received the information about the obtained TA value(s) and the relevant information about the TA validity for the LTM candidate cell(s) 18, sends this information to the C- DU 100 it serves over Fl interface, using FIAP protocol Message, e.g., CU-DU TA INFORMATION TRANSFER Message.

[0232] D4. A method of DI in which the gNB-CU 102 serving the previous S-DU 100, after the Conditional LTM execution, retains the TA value(s) received during the early UL synchronization to one or more LTM candidate cell(s) 18 and keeps track of the validity of those TA value(s), based on one or more conditions:

[0233] • The remaining validity of the TA value is above a threshold.

[0234] • The TA value has been received below “X” time ago (where time in this case can be second, millisecond, hour, or any other time unit).

[0235] • The TA value belongs to a DU 100 which is connected to the same gNB-CU 102 controlling the DU 100 serving the UE 22.

[0236] D5. A method of DI and D4, in which the gNB-CU 102 serving the previous S- DU 100, after the Conditional LTM execution, provides the TA value(s) received during the early UL synchronization to one or more LTM candidate cell(s) 18 and the relevant TA validity information, to the new S-DU 100 via a Class 1 Message, e.g., UE CONTEXT MODIFICATION REQUEST Message, upon receiving the ACCESS SUCCESS Message or RRC Reconfiguration Complete Message.

[0237] D5a. A method of DI and D4, in which the gNB-CU 102 serving the previous S- DU 100, after the Conditional LTM execution, provides the TA value(s) received during the early UL synchronization to one or more LTM candidate cell(s) 18 and the relevant TA validity information, to the new S-DU 100 via a new or existing Class 2 Message, e.g., LTM Cell Switch Notification (CU->DU), upon receiving the ACCESS SUCCESS Message or RRC Reconfiguration Complete Message from the new S-DU 100.

[0238] D5b. A method of DI and D4, in which the gNB-CU 102 serving the previous the S-DU 100, after the Conditional LTM execution, provides the TA value(s) received during the early UL synchronization to one or more LTM candidate cell(s) 18 and the relevant TA validity information, to the new S-DU 100, which is located in another network node 16 (gNB), over Xn interface, using a new or existing XnAP protocol Message, upon receiving the ACCESS SUCCESS Message or RRC Reconfiguration Complete Message from the new S-DU 100. The network node 16 which received the information about the obtained TA value(s) and the relevant information about the TA validity for the LTM candidate cell(s) 18, sends this information to the new S-DU 100 it serves over Fl interface, using a new or existing FIAP protocol Message.

[0239] D6. A method of DI in which the gNB-CU 102 serving the previous S-DU 100, after the Conditional LTM execution, requests the information about TA value(s) received during the early UL synchronization to one or more LTM candidate cell(s) 18 and the TA value(s) validity from the old S-DU 100 via anew or existing Class 1 Message, e.g., UE CONTEXT MODIFICATION REQUEST Message.

[0240] D7. A method of DI and D6 in which the gNB-CU 102 serving the previous S-DU 100, after the Conditional LTM execution, receives the information about TA value(s) received during the early UL synchronization to one or more LTM candidate cell(s) 18 and the TA value(s) validity sent by the old S-DU 100 via a new or existing Class 1 Message, e g., UE CONTEXT MODIFICATION RESPONSE Message.

[0241] D8. A method of DI, D6 and D7 in which the gNB-CU 102 serving the previous S-DU 100, after the Conditional LTM execution, provides the information about TA value(s) received during the early UL synchronization to one or more LTM candidate cell(s) 18 and the TA value(s) validity to the new S-DU 100 via anew or existing Class 1 Message, e g., UE CONTEXT MODIFICATION REQUEST Message.

[0242] D8a. A method of DI, D6 and D7 in which the gNB-CU 102 serving the previous S-DU 100, after the Conditional LTM execution, provides the information about TA value(s) received during the early UL synchronization to one or more LTM candidate cell(s) 18 and the TA value(s) validity to the new S-DU 100 via anew or existing Class 2 Message, e.g., LTM Cell Switch Notification (CU- DU).

[0243] D8b. A method of DI, D6 and D7 in which the gNB-CU 102 serving the previous S-DU 100, after the Conditional LTM execution, provides the information about TA value(s) received during the early UL synchronization to one or more LTM candidate cell(s) 18 and the TA value(s) validity to the new S-DU 100, which is located in another gNB, via an existing or a new message over Fl and / or the Xn interfaces

[0244] One or more embodiments are relevant to the context of subsequent Conditional LTM network, in which multiple LTM candidate cells 18 are simultaneously configured with subsequent conditional LTM and the associated CLTM execution conditions. The invention offers advantages in the case in which an early UL synchronization has been accomplished for multiple LTM candidate cell(s) 18 in the network with subsequent Conditional LTM, and the UE 22 performs LTM cell switch to one of those LTM candidate cells 18 according to the fulfillment of CLTM execution, while retaining the valid TA values for the other LTM candidate cells 18. The primary advantages provided by the proposed methods are mentioned below:

[0245] • There is no requirement for the repeated PDCCH-order transmission by network and random-access to the network for the LTM candidate cells 18 for which the UE 22 already has valid TA values after LTM cell switch in the subsequent conditional LTM execution. This helps in saving the energy which is consumed at the UE 22 and network nodes during repeated transmission, reception, and processing of the PDCCH order, PRACH preamble and TA value provision.

[0246] • The method also enables better utilization of time-frequency and PRACH resources (e.g., allocated preambles for early TA acquisition) in the UL and DL due to reduced repeated signaling overhead.

[0247] In the embodiments where the UE 22 retains the valid TA values for the other LTM candidate cells 18 and indicates this information to the new source cell 18 after LTM cell switch in subsequent CLTM execution, there is a possibility that the CLTM execution condition(s) are fulfilled for one of the LTM candidate cells 18 with valid TA values in the subsequent CLTM network before there is any time to repeat the TA acquisition procedure for the other LTM candidate cells 18. In that case, the UE 22 can perform RACH-less LTM cell switch (because the UE 22 retains the valid TA value after each subsequent CLTM execution) which decreases the handover interruption

[0248] One or more embodiments provide Conditional LTM (CLTM), which may refer to a form of conditional reconfiguration. In CLTM, the UE 22 is configured with at least one LTM candidate cell 18 (denoted as a CLTM candidate cell 18), by receiving an LTM candidate cell configuration, as in legacy LTM, and referred to herein as a Conditional LTM candidate cell configuration, and an associated execution condition, denoted as CLTM execution condition. The evaluation of CLTM execution condition associated to a CLTM candidate cell 18 is performed by the assessment of lower layer measurements, such as Layer 1 reference signal received power (Ll-RSRP) and / or SS-RSRP, derived from SSBs and / or CSI-RSs of either the source cell 18 and / or an CLTM candidate cell 18. Lower layer measurements, in this context, are measurements reported to support lower layer procedures like beam management, TCI state activations / deactivations, early timing advance (TA) acquisition, and link adaptation, and they are not filtered based on Layer 3 (L3) parameters, though there may be some filtering of these measurements based on lower layer parameters. The reception of CLTM execution condition may also involve receiving an indication of the condition and / or configuring it with parameters such as event identifier(s), offset(s), threshold(s), reference signal (RS) type, trigger quantity such as RSRP, reference signal received quality (RSRQ) or signal-to-interference-plus-noise ratio (SINR), time-to-trigger (TTT), etc.

[0249] In the context of CLTM, the UE 22 relies on evaluating one or two condition(s), referred to as CLTM execution condition(s), LTM execution condition(s), or triggering condition(s), or a combination thereof. When the condition(s) for a CLTM candidate cell 18 is (are) fulfilled, the UE 22 performs a cell switch, which may be seen as a kind of LTM execution which is not triggered by the reception of an LTM cell switch command; this may also be considered as a kind of LTM cell switch, or Conditional LTM cell switch, or Conditional LTM execution, or CLTM execution. During the execution, the UE 22 may apply a message, parts of a message, or at least one information element (IE), or perform a serving cell switch or change. According to the methods outlined in the invention, upon satisfaction of the execution condition(s), the UE 22 initiates an LTM Cell Switch.

[0250] One or more embodiments provide an LTM candidate cell 18 within the framework of Conditional LTM. The candidate cell 18 may be referred to as a CLTM candidate cell 18, CLTM cell 18, simply candidate cell 18, candidate target cell 18, simply target cell 18, LTM candidate cell 18, or L1 / L2 inter-cell mobility candidate cell 18, depending on the context or terminology used in the invention. That is, it denotes a cell 18 to which the UE 22 switches to in the event of executing a conditional L1 / L2 inter-cell mobility procedure after meeting the associated execution condition(s). These cells 18 may also be termed as candidate cells 18, mobility candidates 18, non-serving cells 18, additional cells 18, candidate target cell 18, simply target cell 18 or deactivated cells 18. An LTM candidate cell 18 may also pertain to a candidate cell 18 in a 5G Radio Access Technology like NR or a 6G Radio Access Technology.

[0251] In the method, the UE 22 receives an LTM candidate cell configuration for Conditional LTM, typically through an RRC Reconfiguration message, which is stored in the UE 22 and applied upon fulfillment of the associated CLTM execution conditions. This configuration includes parameters necessary for the UE 22 to operate within the designated LTM candidate cell 18 after the Conditional LTM Cell execution.

[0252] The method also discusses the concept of subsequent Conditional LTM execution in which the UE 22 is provided a reference LTM candidate cell configuration and one or more delta configuration(s) corresponding to the LTM candidate cell(s) 18. In that case, the UE 22 may apply the LTM candidate cell configuration on top of the UE current configuration and / or reference configuration which is also received by the UE 22 as part of a Conditional LTM configuration. Conversely, the UE 22 may also receive a complete RRC configuration associated to a conditional LTM candidate cell 18.

[0253] The method disclosed is applied to subsequent Conditional LTM execution in which the UE 22 can perform subsequent LTM cell switches, as determined by the fulfillment of CLTM execution conditions, without any requirement to be RRC Reconfigured in between. However, the UE 22 may perform early UL synchronization and early DL synchronization with the target cell 18 before the Conditional LTM execution. In the context of subsequent Conditional LTM, the text refers to the serving cell 18 before the LTM cell switch as source cell 18, old source cell 18, or previous source cell 18 whereas the serving cell 18 after the LTM cell switch is referred to as target cell 18, candidate target cell 18, new source cell 18 or the next source cell 18.

[0254] In some embodiments the term “beam” is used and may correspond to a spatial direction in which a Reference signal (RS), such as Synchronization Signal Block-RS (SSB-RS), Mobility Reference Signal (MRS), a Channel State Information - RS (CSI- RS), or a RS defined for a 6G radio interface, is transmitted (e.g., by a network node) or received (e.g. by the UE 22), or a spatial filter applied to a signal which is transmitted or received. Thus, transmitting signals in different beams corresponds to transmitting signals in different spatial directions. The beam measurement corresponds to a measurement on an RS transmitted in that beam e.g., an S SB measurement and involves determining a measurement quantity value such as a Synchronization Signal based Reference Signal Received Power (SS-RSRP) and / or Synchronization Signal based Reference Signal Received Quality (SS-RSRQ) and / or Synchronization Signal based Signal to Noise and Interference Ratio (SS-SINR). In the text the beam is identified by beam index and / or a Reference Signal (RS) index or identifier, such as an SSB index, or a CSI-RS resource identifier.

[0255] In some embodiments, a beam or RS may be linked to or associated with a Transmission Configuration Information (TCI) state, for example, by configuring the RS (e.g., SSB) as the Quasi Co-Located (QCL) source of a TCI state configuration. An activated TCI state of an LTM candidate cell 18 could also be termed as a pre-activated TCI state. This is because the UE 22 receives the TCI state activation MAC CE command for one or more beam(s) (e.g., SSBs) within the CLTM candidate cell 18 before the CLTM execution. This is done to activate the LTM candidate cell 18 before the UE 22 receives the command to perform an LTM Cell Switch or before fulfilling the LTM execution condition(s) in the case of Conditional LTM.

[0256] In one or more embodiments, the method refers to a RACH-less Conditional LTM execution procedure in which the UE 22 does not transmit a Physical Random- Access Channel (PRACH) preamble to the LTM candidate cell 18 as its first UL message, after the fulfillment of CLTM execution conditions and the cell switch. Instead, the UE 22 either: i) transmits a Scheduling Request (SR) over the Physical Uplink Control Channel (PUCCH), or any other UL control channel which requires the UE 22 to be UL synchronized (or UL time aligned); or ii) transmits UL payload (bits associated to a complete message, e.g., RRC Reconfiguration Complete), e.g., using at least a preconfigured grant, over a Physical Uplink Shared Channel (PUSCH).

[0257] In some other embodiments, a TA value is obtained for the UE 22 for early UL synchronization. This allows the UE 22 to prevent the transmission of a PRACH (Physical Random-Access Channel) preamble, and consequently, the reception of a corresponding Random- Access Response (RAR) message during the access to that target cell 18 and reduce handover interruption. This acquisition facilitates the UE 22 to execute a RACH- less access to the target cell 18 during the handover procedure. In a RACH-less access, the UE 22 has a valid timing advance (TA) for the selected LTM or CLTM candidate cell 18. In other words. The time alignment timer for the earlier acquired TA value for an LTM candidate cell 18 is still running or not expired yet.

[0258] In some embodiments, the overall architecture (e.g., of system 10) for Conditional LTM includes a Centralized Unit (CU) 102 and a Distributed Unit (DU) 100 in a Radio Access Network (RAN). The RAN (e.g., access network 12) is a Next-Generation RAN (NG-RAN), which may be referred to as the 5G RAN, however, the method is applicable to any RAN such as a 6G RAN architecture. The RAN (e.g., NG-RAN) consists of a set of RAN nodes (e.g., gNBs) connected to a Core Network (e.g., a 5GC) through a RAN / CN interface (e.g., NG interface). A network node 16 (gNB) may include gNB-CU 102 and one or more gNB-DU(s) 100. A gNB-CU 102 and a gNB-DU 100 may be connected via Fl interface. Further, the source cell 18 where the UE 22 is located in belongs to (or served / controlled by) the source gNB-DU 100 or S-DU 100 whereas the candidate target cell 18 is associated to the candidate gNB-DU 100 or C-DU 100. The source and target cells 18 in CLTM execution may be controlled by the same network node 16 (e.g., gNB), which sometimes is referred to as the intra-gNB case, or when the network node 16 uses a distributed CU-DU RAN architecture, the intra-CU 102 inter-DU case or the intra-CU 102 intra-DU case (depending on whether the cells 18 are controlled by the same DU 100 or different DUs 100). The source and target cells 18 in CLTM execution may be controlled by different network nodes 16, which sometimes is referred to as the inter-gNB scenario, or the inter-CU 102 inter-DU scenario, in which case the two gNBs may be directly connected via an Xn interface or indirectly connected (via one or more CN nodes) via an NG interface. In some embodiments, in the context of subsequent Conditional LTM, the old source cell 18 or the previous source cell 18 belongs to the source RAN node, i.e., S- DU 100 (also referred to as the previous S-DU 100 or old S-DU 100) whereas the new source cell 18 or the next source cell 18 is associated to the candidate RAN node, i.e., C- DU 100 (also referred to as the next S-DU 100 or new S-DU 100). The gNB-CU 102 which serves the S-DU 100 may be referred to as source gNB-CU 102 while the gNB-CU 102 which serves the C-DU 100 may be termed as candidate gNB-CU 102, without any loss of meaning. The method is presented as applicable to the NG-RAN as an example, however, the method is also applicable to any RAN architecture, such as a 6G RAN.

[0259] Example Embodiments

[0260] The disclosure describes various embodiments related to a UE 22 performing early UL synchronization on multiple LTM candidate cells 18, which form a subsequent Conditional LTM execution setup, and acquiring Timing Advance (TA) values for those LTM candidate cells 18, such that the UE 22 performs LTM cell switch to one of those LTM cells 18 upon the fulfillment of execution condition(s) while still retaining the valid TA values for the other LTM candidate cells 18 after CLTM execution. According to the method, the network is not required to send the PDCCH order for the LTM candidate cells 18 which still have valid TA values after the cell switch and the UE 22 can perform RACH-less CLTM execution to one of those LTM candidate cells 18 without sending any random-access and re-acquiring the TA value.

[0261] As an example, there is a network with multiple LTM candidate cells 18 which are configured with subsequent Conditional LTM. There is a “source cell” 18 located in source the source RAN node, i.e., S-DU 100, an LTM candidate cell “Cell 1” (i.e., a first cell 18 or first candidate cell 18) located in candidate RAN node or C-DU 100 (e.g., the first candidate DU 100) and another LTM candidate cell “Cell 2” (i.e., a second cell 18 or second candidate cell 18) located in another candidate RAN node or C-DU 100 (e.g., the second candidate DU 100). All the gNB-DUs 100 are served by the same gNB-CU 102 and connected to the gNB-CU 102 via Fl interface. The UE 22 is first located in the source cell 18 and can perform subsequent Conditional LTM execution to any of the cells 18 in the network, i.e., the first and second candidate cells 18. Put simply, the UE 22 can perform subsequent Conditional LTM execution to any of the cells 18 in the network, i.e., source cell 18, first cell 18, and second cell 18, given that the UE 22 was served by one of these cells 18 before the cell switch.

[0262] In order to execute RACH-less subsequent CLTM execution between these cells 18, i.e., source cell 18, the first cell 18, and the second cell 18, without any repeated TA acquisition towards these cells 18, the UE 22 may be required to retain the valid TA values after each subsequent CLTM execution, and the network may be required to be informed about the valid TA values available at the UE 22.

[0263] In one embodiment, the UE 22 retains the valid TA values for the other LTM candidate cells 18 and sends the information about the valid TA values to the network after the conditional LTM execution using “RRC Reconfiguration Complete” message. FIG. 11 shows an example process associated with an indication of TA validity for other LTM candidate cells 18 using RRC Reconfiguration Complete message. At step S300, a DL RRC message transfer is performed (including LTM candidate cell configuration and CLTM execution conditions). At step S202, a DL RRC message is transferred to UE 22. At step S304, UE 22 transmits an RRCReconfiguration complete message. At step S306, S-DU 100a transmits a PDCCH order for the first cell 18. At step S308, UE 22 transmits a random access preamble to C-DU 100b (e.g., associated with the first cell 18 and / or configured to perform source first cell functions or candidate cell functions). At step S310, early TA acquisition for the first cell 18 is performed. At step S312, S-DU 100a transmits a PDCCH order for the second cell 18. At step S314, UE 22 transmits a random access preamble to C-DU 100c (e.g., associated with the second cell 18 and / or configured to perform source second cell functions or candidate cell functions). At step S316, early TA acquisition for the second cell 18 is performed. At step S318, UE 22 determines that execution conditions are fulfilled for the first cell 18. At step S320, an RRCReconfigurationComplete message is transmitted by UE 22 to C-DU 100b. The message may indicate that the first cell 18 becomes the new S-DU. Further, UE 22 informs about the valid TA values for the other LTM candidate cells 18 (e.g., the second cell 18 in this case). At step S322, C-DU 100b indicates that access is successful (e.g., providing target cell ID). At step S324, UL RRC message transfer is performed, where RRC reconfiguration complete may be indicated and the valid TA values for the other LTM candidate cells 18 (e.g., second cell 18) may be provided. At step S326, CU 102 may transmit a UE CONTEXT MODIFICATION REQUEST, with valid TA values for the other LTM candidate cells 18. At step S328, a response is transmitted by C-DU 100b.

[0264] Put differently, one or more of the following may be performed:

[0265] • UE 22 receives the subsequent Conditional LTM configuration while being in the source cell 18 (located in S-DU 100), along with the CLTM execution condition(s).

[0266] • The UE 22 performs early TA acquisition on the first cell 18 (located in the first candidate DU 100) by sending PRACH preamble to the first cell 18.

[0267] • The network node, i.e., a first C-DU 100, calculates the TA value according to the received preamble and sends the calculated TA value to the controlling gNB-CU 102, e.g., via a DU-CU TA information transfer F1AP message, which in turn is sent by the gNB-CU 102 to the S-DU 100, e.g., using a CU- DU TA information transfer Fl AP message. o In another sub-option of the method, the message used to forward the obtained TA value, and any other relevant information, is an existing or a new message which is sent over a protocol which connect a first CU 102 with a second CU 102 (and vice versa), e.g., TA Information Transfer Message. In this case, such protocol can be the XnAP and / or the NGAP protocols. And then the TA value and the related information is shared with the S-DU 100 using a CU-DU TA information transfer F1AP message.

[0268] • The UE 22 receives the acquired TA value for the first cell 18 from S-DU 100 including any other relevant information e.g., time alignment timer value. o In one sub-option, the time alignment timer value associated to a certain LTM candidate cell 18 is provided to the UE 22 during LTM candidate configuration in the LTM preparation phase. o In another sub-option, the time alignment timer value associated to a certain LTM candidate cell 18 is maintained and provided by C-DU 100 to which random-access was sent for early TA acquisition, e.g., the first candidate DU 100, or S-DU, or gNB-CU 102 which serves the S-DU 100, or gNB-CU 102 which serves the C-DU 100.

[0269] • The process for early TA acquisition is repeated for the second cell 18, served by the second candidate DU 100.

[0270] • While monitoring for CLTM execution condition(s), the execution condition(s) for the first cell 18 are fulfilled at the UE 22. • The UE 22 performs RACH-less LTM cell switch to the first cell 18 because of the availability of a valid TA value associated to the first cell 18 and sends a message, e.g., an RRC “RRC Reconfiguration Complete” message to the first candidate DU 100.

[0271] • After receiving the RRC Reconfiguration Complete Message, the first candidate DU 100 sends the ACCESS SUCCESS Message and the UL RRC Message to the gNB-CU 102. The gNB-CU 102 obtains the information about the valid TA values for the other LTM candidate cells 18, including any other relevant information e.g., the information about the current ‘state’ (e.g., current value or remaining time left to expiry) of the corresponding Time alignment timers. o Alternatively, the first candidate DU 100 sends a message, e.g., an ACCESS SUCCESS F1AP message to the CU 102 in response to the reception from the UE 22 of an UL message on lower layers, such as a random-access preamble and / or a msg3 in a random-access procedure, or a scheduling request on PUCCH and / or a message over PUSCH with provided UL grant(s).

[0272] • After receiving the indication of cell switch completion, e.g., via UL RRC Message transfer, the gNB-CU 102 forwards the information about the valid TA value(s) for the other LTM candidate cell(s) 18, and any other relevant information e.g., the information about the current ‘state’ (e.g., current value or remaining time left to expiry) of the corresponding Time alignment timers, via a class 1 message, e.g., UE CONTEXT MODIFICATION REQUEST MESSAGE. o In another sub-option, the gNB-CU 102 forwards the information about the valid TA value(s) for the other LTM candidate cell(s) 18, and any other relevant information e.g., the information about the current ‘state’ (e.g., current value or remaining time left to expiry) of the corresponding Time alignment timers, via a new class 2 message, after receiving the indication of cell switch completion.

[0273] • UE 22 determines that the TA value for the second cell 18 is still valid, so the UE 22 includes this information about the TA validity, including any other relevant information e.g., the information about the current ‘state’ (e.g., current value or remaining time left to expiry) of the corresponding time alignment timers, for the second cell 18 in a message towards the first candidate DU 100 (e.g., the same RRC “RRC Reconfiguration Complete” message described above). o In one sub-option, the UE 22 retains a valid TA value for one of the other LTM candidate cells 18 after Conditional LTM execution when the time left for expiry of the associated Time alignment timer is above a threshold value. Such a threshold value may be configured at the UE 22 e.g., as part of the LTM configuration, possibly per candidate cell 18 or source / candidate cell pair. o In another sub-option, the UE 22 retains a valid TA value for one of the other LTM candidate cells 18 after Conditional LTM execution when the UE 22 is explicitly configured to retain TA values and / or associated Time alignment timer values for other LTM candidate cells 18, after LTM cell switch.

[0274] • The first candidate DU 100, which is informed about the TA validity for the second cell 18 at the UE 22, does not trigger PDCCH order towards the UE 22 to perform early TA acquisition for the second cell 18, and the UE 22 can perform RACH-less CLTM execution to the second cell 18, upon CLTM execution condition(s) fulfilment (as long as the TA value for the second cell 18 is valid), without any random-access transmission.

[0275] In another embodiment, upon CLTM execution, the UE 22 sends to the new source cell 18 (e.g., the C-DU 100) a lower layer signalling (e.g., anew MAC CE such as a “TA Status reporting” MAC CE) comprising for at least one of the LTM candidate cells 18 information related to TA, such as the TA value, and / or whether a TA value acquired during Early TA acquisition for a certain candidate cell 18 is valid or not, and / or whether a TA value associated to a certain candidate cell 18 has been acquired by the UE 22 or not, and / or whether the TA value associated to a certain candidate cell 18 has expired or about to expire). The lower layer signalling can optionally comprise information related to the TA value(s), associated to the TimeAlignmentTimer , such as the remaining time until expiry of the TimeAlignmentTimer for the cell 18 and / or the configured TimeAlignmentTimer for that cell 18 and / or the time since the latest TA value was acquired / received for the cell 18.

[0276] The information received by the network node via the lower layer signalling can be sent to another network node. For instance, the C-DU 100 can forward the information related to TA values and / or its relation to the TimeAlignmentTimer to the C-CU 102. In a related embodiment, before the UE 22 sends the lower layer signaling related to the status of the TA values and information associated to its relation to the TimeAlignmentTimer, the UE 22 may have been configured by the network (e.g., the source CU 102) to do so, e.g., as part of the CLTM preparation. For instance, the source CU 102 and / or one of the C-CU(s) 102 includes in its prepared RRC Reconfiguration message which the source CU 102 sends to the UE 22 during the CLTM configuration a flag, instructing the UE 22 to send the lower layer signaling containing the information described above on the TA value(s) and associated information upon CLTM execution.

[0277] In another embodiment, the UE 22 is configured with event-triggered lower layer reporting or event-triggered L3-reporting to send measurement reports to the network for triggering PDCCH order to acquire TA values for LTM candidate cells 18. That is, a lower layer or L3 -measurement report shall be transmitted by the UE 22 for early TA acquisition (also termed as UL pre-synchronization) of one or more LTM candidate cell(s) 18. According to the method, the UE 22 can acquire TA values for multiple LTM candidate cells 18 (e.g., the first cell 18 in the first candidate DU 100 and the second cell 18 in the second candidate DU 100) while being in the source cell 18 and perform CLTM execution to one of these LTM candidate cells 18 as the CLTM execution condition(s) for the cell 18 are fulfilled. However, the UE 22 retains the valid TA values for the other LTM candidate cells 18 (e.g., the second cell 18 in the second candidate DU 100) but does not notify the network or the new source cell 18 (e.g., the first cell 18 in the first candidate DU 100) about the validity of the TA values associated to the other LTM candidate cells 18 (e.g., the second cell 18 in the second candidate DU 100). In one sub-option, the UE 22 retains a valid TA value for one of the other LTM candidate cells 18 after Conditional LTM execution when the time left for expiry of the associated Time alignment timer is above a threshold value. Such a threshold value may be configured at the UE 22 e.g., as part of the LTM configuration, possibly per candidate cell 18 or source / candidate cell pair. In another sub-option, the UE 22 retains a valid TA value for one of the other LTM candidate cells 18 after Conditional LTM execution when the UE 22 is explicitly configured to retain TA values and / or associated Time alignment timer values for other LTM candidate cells 18, after LTM cell switch. Instead, the UE 22 does not trigger any measurement report for the events which are associated with the early TA acquisition for the LTM candidate cell(s) 18 whose valid TA value(s) is(are) available at the UE 22. For example, the UE 22 would not transmit any measurement report to trigger PDCCH order for early TA acquisition of the second cell 18 while being in the first cell 18 because the UE 22 still has a valid TA value associated to the second cell 18. If the condition(s) for CLTM execution are fulfilled for the second cell 18 while the TA value for the second cell 18 is still valid, the UE 22 can perform RACH-less cell switch from the first cell 18 to the second cell 18 without performing any random-access.

[0278] In one embodiment, the UE 22 informs its new source cell 18 about the valid TA values of the other LTM candidate cells 18 only after that the conditional LTM cell switch procedure has been successfully executed. This means that the UE 22 first executes the conditional LTM cell switch procedure, after that it starts to operate in the new serving cell 18, and only then it informs about the valid TA values of the other LTM candidate cell(s) 18. In this case, the UE 22 may use the UEAssistancelnformation message or any other new or existing RRC message to deliver the valid TA values, including any other relevant information e.g., the information about the current ‘state’ (e.g., current value or remaining time left to expiry) of the corresponding Time alignment timers.

[0279] In another embodiment, the UE 22 informs its new source cell 18 about the valid TA values of the other LTM candidate cells 18 only if the new source cell requests the UE 22 to do so. In this case, the UE 22 first executes the conditional LTM cell switch procedure, that it starts to operate in the new serving cell 18, and only then it may receive a request from the new serving cell 18 to inform about the valid TA values of the other LTM candidate cell(s) 18. The request can be a simple indication to indicate to the UE 22 to send the valid TA value(s) of the other LTM candidate cell(s) 18 or it may include additional information such as for which LTM candidate cell 18 the valid TA value should be sent.

[0280] In one embodiment, the UE 22, when it receives a request from the new serving cell 18 to perform a TA acquisition towards another LTM candidate cell 18 for which the UE 22 already has a valid TA value, does not execute the corresponding TA acquisition procedure towards the indicate cell 18 but instead sends back an indication to the new source cell 18 that it has a valid TA value for that cell 18. In one example, the UE 22 uses the UEAssistancelnformation message or any other new or existing RRC message to indicate to the new source cell 18 that it has a valid TA value for the candidate cell 18 for which the request to perform a TA acquisition was received. In another example, the UE 22 uses a MAC CE or a PUCCH message to indicate that it has a valid TA value for the corresponding candidate cell 18. The message indicating that the UE 22 has a valid TA value for the candidate cell 18, for which the request for TA acquisition was received, may also include information about e.g., the remaining time until expiry of the TimeAlignmentTimer for the cell 18 and / or the configured TimeAlignmentTimer for that cell 18 and / or the time since the latest TA value was acquired / received for the cell 18. The message may also include corresponding information about other candidate cells 18 for which the UE 22 has a valid TA value.

[0281] In some examples, the UE 22 indicates to the new source cell 18 that it has a valid TA value for the previous source cell 18, where the latest update of the TA value was received based on uplink transmission in that cell 18 as part of the operation in that cell 18, e.g., uplink transmissions on the PUSCH. The UE 22 may also indicate (to the new source cell 18) the time since the last received update of the TA value in the previous source cell 18 or the corresponding remaining time until expiry of the TA value for the previous source cell 18.

[0282] In some embodiments, the UE 22 indicates in the message to the new source cell 18 that multiple candidate cells 18, for which the UE 22 has a valid TA value, share the same TA value. In other words, the UE 22 indicates that the same information, such as e.g., that it has a valid TA value and / or the remaining time until expiry for the corresponding TA value and / or information about the time since the corresponding TA value was acquired, is valid for more than one candidate cell 18. In one example, the UE 22 indicates this as a list of cells 18 for which the information is valid. In another example, the UE 22 indicates an identity related to a group of cells 18 that share the same TA value, e.g., as a configured TAG or another group identity associated to one or more LTM candidate cells 18.

[0283] FIG. 12 shows another example process associated with an indication of TA validity for the other LTM candidate cells 18 during early TA acquisition via CU-DU TA information transfer. More specifically, at step S400 a DL RRC message transfer is performed, and an RRC reconfiguration (e.g., including LTM candidate cell configuration and CLTM execution conditions) is performed, at step S402. At step S404, an RRC configuration message is transmitted, and at step S406, PDCCH order for the first cell 18 is transmitted. At step S408, a random access preamble is transmitted to C-DU 100b. At step S410, a TA estimation for the first cell 18 is performed, and at step S412, a DU-CU TA information transfer is performed, which may include S-DU ID, the first cell ID, TA value, and preamble index. At step S414, the TA information is also transferred to S-DU 100a. At step S416, TA information for the first cell 18 in TA, MAC CE / RAR is provided to UE 22. At step S420, PDCCH order for the second cell 18 is transmitted to UE 22, and the UE 22 transmits random access preamble to the second cell 18. At step S424, TA estimation is performed for the second cell 18, and at step S426, DU-CU TA information transfer is performed, which may include S-DU ID, the second cell ID, TA value, and preamble index. At step S428, CU-DU TA information transfer is performed, which may include S-DU ID, the second cell ID, TA value, and preamble index. At step S430, TA information for the second cell 18 in TA MAC CE / RAR is provided. At step S432, CU- TU TA information is transferred from CU 102 to C-DU 100b, which may include TA value for the second cell 18, S-DU ID, the second cell ID, UE ID. Further, the first cell 18 is updated with the TA information for the UE 22 for possible subsequent conditional LTM execution to the second cell 18.

[0284] Put differently, the information about the valid TA values for LTM candidate cells 18 in a subsequent Conditional LTM network is shared with the other LTM candidate cells 18 via gNB-CU 102 as the TA value associated to an LTM candidate cell 18 is calculated. This method may include one or more of:

[0285] • UE 22 receives the subsequent Conditional LTM configuration while being in the source cell 18 (located in S-DU 100), along with the CLTM execution condition(s).

[0286] • The UE 22 performs early TA acquisition on the first cell 18 (located in the first candidate DU 100) by sending PRACH preamble to the first cell 18.

[0287] • The network node, i.e., the first candidate DU 100, calculates the TA value according to the received preamble and sends the calculated TA value to the gNB-CU 102 via DU-CU TA information transfer. o In another dependent step, the message used to forward the obtained TA value, and any other relevant information, is an existing or a new message which is sent over a protocol which connect a first CU 102 with a second CU 102 (and vice versa), e.g., TA Information Transfer Message. In this case, such protocols can be the Fl AP and / or the XnAP and / or the NGAP protocols.

[0288] • The gNB-CU 102 shares the TA value for LTM candidate cell 18, i.e., the first cell 18, with the source cell 18, located in S-DU 100, via CU-DU TA information transfer.

[0289] • In addition to sharing the TA value for Celli with the source cell 18, the gNB- CU 102 also shares the information about the TA validity for the first cell 18 with the other LTM candidate cells 18 (e.g., the second cell 18 located in the second candidate DU 100). While sharing the TA validity information with the other LTM candidate cells 18, the gNB-CU 102 also informs the other LTM candidate cells 18 (e.g., the second cell 18 in the second candidate DU 100) about the UE ID for which the TA value was calculated, S-DU ID of the source cell 18 in which the UE 22 is currently located and the LTM candidate cell ID for which the TA value is calculated (i.e., the first cell 18 located in the first candidate DU 100). The additional information helps the LTM candidate cell 18 build up a context about the shared TA information for which the LTM candidate cell 18 did not trigger PDCCH order. o In another sub-option of the method, the message used to forward the obtained TA value, and any other relevant information e.g., the information about the current ‘state’ (e.g., current value or remaining time left to expiry) of the corresponding Time alignment timers, to one or more C- DU(s) 100 which are a part of subsequent Conditional LTM configuration, upon early TA acquisition by the UE 22, is an existing or a new message which is sent over a protocol which connect a DU 100 to a CU 102 (and vice versa), e.g., CU-DU TA Information Transfer Message, or a first CU 102 with a second CU 102 (and vice versa), e.g., TA Information Transfer Message. In this case, such protocols can be the F1AP and / or the XnAP and / or the NGAP protocols.

[0290] • The same procedure may be repeated while performing early TA acquisition for the second cell 18.

[0291] • As a result of the proposed method, the LTM candidate cells 18 in the subsequent Conditional LTM network are already informed about the valid TA values associated to the other LTM candidate cells 18. For example, the first cell 18 knows about the validity of the TA value for the second cell 18 and vice versa.

[0292] • While monitoring for CLTM execution condition(s), the execution condition(s) for the first cell 18 are fulfilled at the UE 22.

[0293] • The UE 22 performs LTM cell switch to the first cell 18 because of the availability of a valid TA value associated to the first cell 18 which was acquired during UL pre-synchronization with the first cell 18. During Conditional LTM execution, the UE 22 retains the valid TA values for the other LTM candidate cells 18 (i.e., the second cell 18 located in the second candidate DU 100).

[0294] • After the Conditional LTM execution, the UE 22 only retains the valid TA value(s) for one or more LTM candidate cell(s) 18: o In one sub-option, the UE 22 retains a valid TA value for one of the other LTM candidate cells 18 after Conditional LTM execution when the time left for expiry of the associated Time alignment timer is above a threshold value. Such a threshold value may be configured at the UE 22 e.g., as part of the LTM configuration, possibly per candidate cell 18 or source / candidate cell pair. o In another sub-option, the UE 22 retains a valid TA value for one of the other LTM candidate cells 18 after Conditional LTM execution when the UE 22 is explicitly configured to retain TA values and / or associated Time alignment timer values for other LTM candidate cells 18, after LTM cell switch.

[0295] The first candidate DU 100 (which is informed about the TA validity for the second cell 18 at the UE 22 during early TA acquisition via modified CU-DU TA information transfer (shown in FIG. 12) through the first candidate DU 100) does not trigger PDCCH order towards the UE 22 to perform early TA acquisition for the second cell 18, and the UE 22 can perform RACH-less CLTM execution to the second cell 18, upon CLTM execution condition(s) fulfilment, without any random-access transmission.

[0296] In another embodiment of the solution, the information about the valid TA values for the other LTM candidate cells 18 in a subsequent Conditional LTM network is shared with the new source cell via gNB-CU 102 after receiving the notification of cell switch completion using a modified UE CONTEXT MODIFICATION REQUEST message or another new message. FIG. 13 shows an example process associated with an indication of TA validity for the other LTM candidate cells 18 by gNB-CU 102 after the reception of LTM cell switch completion notification from the new source cell 18. At step S500 a DL RRC message transfer is performed, and an RRC reconfiguration (e.g., including LTM candidate cell configuration and CLTM execution conditions) is performed, at step S502. At step S504, an RRC configuration complete message is transmitted. At step S506, a TA estimation for the first cell 18 is performed. At step S508, TA estimation is performed for the second cell 18. At step S510, execution conditions fulfilled for the first cell 18 are determined. At step S512, an RRC reconfiguration complete message is transmitted to C- DU 100b, which may indicate that the first cell 18 becomes the new S-DU. At step SI 4, an access success message is transmitted (which may include the target cell ID. At step S516, UL RRC message transfer is performed which may include an indication that the RRC reconfiguration is complete. CU 102 may retain valid TA values for the other LTM candidate cells 18 for possible subsequent conditional LTM execution. At step S518, a UE CONTEXT MODIFICATION REQUEST message (including valid TA value for the other LTM candidate cell, i.e., the second cell 18) may be transmitted, and at step S520, C-DU 100b may provide a UE CONTEXT MODIFICATION RESPONSE message.

[0297] In other words, one or more of:

[0298] • UE 22 receives the subsequent Conditional LTM configuration while being in the source cell 18 (located in S-DU 100), along with the CLTM execution condition(s).

[0299] • The UE 22 performs early TA acquisition on the first cell 18 (located in the first candidate DU 100) by sending PRACH preamble to the first cell 18.

[0300] • The network node, i.e., the first candidate DU 100, calculates the TA value according to the received preamble and sends the calculated TA value to the CU 102 via DU-CU TA information transfer, which in turn is shared by CU 102 with the S-DU 100 using CU-DU TA information transfer. o In another sub-option of the method, the message used to forward the obtained TA value, and any other relevant information, is an existing or a new message which is sent over a protocol which connect a first CU 102 with a second CU 102 (and vice versa), e.g., TA Information Transfer Message. In this case, such protocol can be the XnAP and / or the NGAP protocols. And then the TA value and the related information is shared with the S-DU 100 using a CU-DU TA information transfer F1AP message.

[0301] • The UE 22 receives the acquired TA value for the first cell 18 from S-DU 100, including any other relevant information e.g., Time alignment timer value. o In one sub-option, the Time alignment timer value associated to a certain LTM candidate cell 18 is provided to the UE 22 during LTM candidate configuration in the LTM preparation phase. o In another sub-option, the Time alignment timer value associated to a certain LTM candidate cell 18 is maintained and provided by C-DU 100 to which random-access was sent for early TA acquisition, e.g., the first candidate DU 100, or S-DU 100, or gNB-CU 102 which serves the S-DU 100, or gNB-CU 102 which serves the C-DU 100.

[0302] • The process for early TA acquisition is repeated for the second cell 18, located in the second candidate DU 100.

[0303] • While monitoring for CLTM execution condition(s), the execution condition(s) for the first cell 18 are fulfilled at the UE 22.

[0304] • The UE 22 performs LTM cell switch to the first cell 18 because of the availability of a valid TA value associated to the first cell 18 which was acquired during UL pre-synchronization with the first cell 18. During Conditional LTM execution, the UE 22 retains the valid TA values for the other LTM candidate cells 18 (i.e., the second cell 18 located in the second candidate DU 100).

[0305] • Upon receiving the RRC Reconfiguration Complete message transmitted by the UE 22 which indicates the completion of LTM cell switch, the new source cell 18 (also termed as the next source cell 18), i.e., the first cell 18 in the first candidate DU 100 notifies the gNB-CU 102 about the cell switch completion using “UL RRC message transfer”.

[0306] • Assuming that the gNB-CU 102 and C-DU 100 do not rely on the ACCESS SUCCESS i.e. the gNB-CU 102 and C-DU 100 acknowledge the successful LTM cell switch execution based on the RRC Reconfiguration Complete, the gNB-CU 102, upon receiving the cell switch completion indication, e.g., via UL RRC Message Transfer, transmits to the S-DU 100 a UE CONTEXT MOD REQUEST. The S-DU 100, in response, transmits a UE CONTEXT MOD RESPONSE including the TA values that it had provided to the UE, and any other relevant information e.g., the information about the current ‘state’ (e.g., current value or remaining time left to expiry) of the corresponding Time alignment timers, (or in another message e.g. LTM CS NOTIFICATION DU- CU). Then, the CU 102 needs to provide these TA values, and any other relevant information e.g., the information about the current ‘state’ (e.g., current value or remaining time left to expiry) of the corresponding Time alignment timers, to the new S-DU 100 which previously was C-DU 100 e.g. in an LTM Cell Switch Notification (CU- DU).

[0307] • Assuming that the gNB-CU 102 and C-DU 100 rely on the ACCESS SUCCESS. The gNB-CU 102 acknowledges the successful LTM cell switch execution upon reception from an ACCESS SUCCESS message from the C- DU 100. In response, the gNB-CU 102 transmits to the S-DU 100 a UE CONTEXT MODIFICATION REQUEST. The S-DU 100, in response, transmits a UE CONTEXT MOD RESPONSE including the TA values (or in another message e.g. LTM CS NOTIFICATION DU^CU 102) that it had provided to the UE 22, and any other relevant information e.g., the information about the current ‘state’ (e.g., current value or remaining time left to expiry) of the corresponding Time alignment timers. Then, the CU 102 needs to provide these TA values to the new S-DU 100 which previously was C-DU 100 e.g. in an LTM Cell Switch Notification (CU- DU).

[0308] • The gNB-CU 102 sends a modified UE CONTEXT MODIFICATION REQUEST message to the first cell 18 (located in the first candidate DU 100), informing about the valid TA values, and any other relevant information e.g., the information about the current ‘state’ (e.g., current value or remaining time left to expiry) of the corresponding Time alignment timers, for the other LTM candidate cell 18 (e.g., the second cell 18 in the second candidate DU 100). The first candidate DU 100 receives the UE CONTEXT MODIFICATION REQUEST message and responds with the UE CONTEXT MODIFICATION RESPONSE message to the gNB-CU 102. o In one sub-option, the gNB-CU 102 forwards the information about the valid TA value(s) for the other LTM candidate cell(s) 18, and any other relevant information e.g., the information about the current ‘state’ (e.g., current value or remaining time left to expiry) of the corresponding Time alignment timers, via a new class 2 message, after receiving the valid TA values, and any other relevant information from S-DU 100. o In another sub-option of the method, the message used to forward the obtained TA values for the other LTM candidate cells 18 which are a part of subsequent Conditional LTM configuration, and any other relevant information e.g., the information about the current ‘state’ (e.g., current value or remaining time left to expiry) of the corresponding Time alignment timers, after Conditional LTM execution, is an existing or a new message which is sent over a protocol which connect a DU 100 to a CU 102 (and vice versa), e.g., UE CONTEXT MODIFICATION REQUEST Message, or a first CU 102 with a second CU 102 (and vice versa). In this case, such protocols can be the Fl AP and / or the XnAP and / or the NGAP protocols.

[0309] • After the Conditional LTM execution, the UE 22 only retains the valid TA value(s) for one or more LTM candidate cell(s) 18: o In one sub-option, the UE 22 retains a valid TA value for one of the other LTM candidate cells 18 after Conditional LTM execution when the time left for expiry of the associated Time alignment timer is above a threshold value. Such a threshold value may be configured at the UE 22 e.g., as part of the LTM configuration, possibly per candidate cell 18 or source / candidate cell pair. o In another sub-option, the UE 22 retains a valid TA value for one of the other LTM candidate cells 18 after Conditional LTM execution when the UE 22 is explicitly configured to retain TA values and / or associated Time alignment timer values for other LTM candidate cells 18, after LTM cell switch.

[0310] • The first candidate DU 100, which is now informed about the TA validity for the second cell 18 at the UE 22, does not trigger PDCCH order towards the UE 22 to perform early TA acquisition for the second cell 18, and the UE 22 can perform RACH-less CLTM execution to the second cell 18, upon CLTM execution condition(s) fulfilment (as long as the TA value for the second cell 18 is valid), without any random-access transmission. o In another alternative of the method above, the gNB-CU 102 retains the information about the valid TA values for the other LTM candidate cells 18 (i.e., the second cell 18 in the second candidate DU 100) after early UL synchronization with one or more LTM candidate cell(s) 18 located in different C-DU(s) 100. After the LTM cell switch and upon receiving the ACCESS SUCCESS message, the gNB-CU 102 shares this information with the new source cell 18 (e.g., the first cell 18 in the first candidate DU 100) using CU-DU signalling (e.g., UE CONTEXT MODIFICATION REQUEST message).

[0311] In another sub-option, the gNB-CU 102 forwards the information about the valid TA value(s) for the other LTM candidate cell(s) 18, and any other relevant information e.g., the information about the current ‘state’ (e.g., current value or remaining time left to expiry) of the corresponding Time alignment timers, via a new class 2 message, after receiving the indication of cell switch completion.

[0312] Below is an example implementation in the 3GPP TS 38.473, V18.2.0 (2024-06), Fl application protocol (F1AP).

[0313] 8.3.4 UE Context Modification (gNB-CU initiated)

[0314] 8.3.4.1 General

[0315] The purpose of the UE Context Modification procedure is to modify the established UE Context, e.g., establishing, modifying and releasing radio resources or sidelink resources. This procedure is also used to command the gNB-DU to stop data transmission for the UE for mobility (see TS 38.401 [4]). The procedure uses UE- associated signaling. FIG. 14 (corresponding to Figure 8.3.4.2-1 of 3GPP TS 38.473, V18.2.0) shows an example UE Context Modification procedure, e.g., a successful operation.

[0316] The UE CONTEXT MODIFICATION REQUEST message is initiated by the gNB-CU (step S600).

[0317] Upon reception of the UE CONTEXT MODIFICATION REQUEST message, the gNB-DU shall perform the modifications, and if successful reports the update in the UE CONTEXT MODIFICATION RESPONSE message (step S602).

[0318] **Text omitted**

[0319] If the TA Information IE is included in the UE CONTEXT MODIFICATION REQUEST message, the gNB-DU shall, if supported, consider that the TA values included are valid and shall use it as specified in TS 38.401 [4],

[0320] 9.2.2.7 UE CONTEXT MODIFICATION REQUEST

[0321] This message is sent by the gNB-CU to provide UE Context information changes to the gNB-DU.

[0322] Direction: gNB-CU gNB-DU

[0323] Below is another example implementation in the 3GPP TS 38.423, vl 8.2.0 (2024-

[0324] 06), Xn application protocol (XnAP).

[0325] 8.4.x Retained TA Information Transfer 8.4.x.1 General

[0326] The purpose of the Retained TA Information Transfer procedure is to enable the NG-RAN nodei to send valid TA related information to NG-RAN node2 for a specific UE.

[0327] The procedure uses UE-associated signalling.

[0328] 8.4.X.2 Successful Operation FIG. 15 (corresponding to Figure 8.4.X.2-1 of 3GPP TS 38.423, V18.2.0) shows (step S700) a Retained TA Information Transfer procedure, successful operation.

[0329] The NG-RAN nodei initiates the procedure by sending a Retained TA Information Transfer message. Upon reception of the Retained TA Information Transfer message, the NG-RAN node2 may, if supported, consider that the received information is the valid TA information from the candidate cell(s) that is indicated by the included Candidate Cell ID IE.

[0330] If the TA Validity IE is included, the NG-RAN node2 shall, if supported, understand that it indicates the remaining valid time for the TA value.

[0331] 8.4.x.3 Unsuccessful Operation

[0332] Not applicable.

[0333] 8.4.X.4 Abnormal Conditions

[0334] Not applicable. 9.1.3.x RETAINED TA INFORMATION TRANSFER

[0335] This message is sent by a NG-RAN nodei to a neighbouring NG-RAN node2 to transfer valid Timing Advance value and related information for a UE.

[0336] Direction: NG-RAN nodei a NG-RAN node2.

[0337] An example of lower layer signaling which the UE can send to the new source cell upon CLTM execution to inform the network of the validity of TA values acquired for the LTM candidate cells during Early TA acquisition is reported below.

[0338] 6.1.3. xx TA Status Reporting MAC CE

[0339] The TA Status Reporting MAC CE is identified by MAC subheader with eLCID as specified in Table 6.2.1-lb. It has a variable size with following fields (Figure 6.1.3.xx-l):

[0340] R: Reserved bit, set to 0;

[0341] Target Configuration ID: This field indicates the index of candidate target configuration to apply for LTM cell switch, corresponding to Itm-Candidateld minus 1 as specified in TS 38.331 [5], The length of the field is 3 bits;

[0342] Timing Advance Status: This field indicates whether the TA is valid for the LTM target cell (i.e. the SpCell corresponding to the target configuration indicated by Target Configuration ID field). If the value of this field is set to FFF, this field indicates that no valid timing adjustment is available for the PTAG of the LTM target cell; otherwise, this field indicates the remaining time until expiry of the TimeAlignmentTimer for the LTM target cell.

[0343] Some Examples

[0344] Example AL A method in a user equipment (UE 22) for communicating with a network node 16, the method comprising: receiving a conditional L1 / L2 triggered mobility (CLTM) configuration for one or more CLTM candidate cells 18, the CLTM configuration including one or more CLTM execution conditions; in response to receiving the CLTM configuration, evaluating the one or more CLTM execution conditions; before at least one of the one or more the CLTM execution conditions is fulfilled for the one or more CLTM candidate cells, performing uplink (UL) synchronization with the one or more CLTM candidate cells; and when at least one of the one or more CLTM execution condition is fulfilled for the one or more CLTM candidate cells, executing a CLTM procedure according to the CLTM configuration for the one or more CLTM candidate cells 18.

[0345] Example A2. The UE 22 of Example Al, wherein one or more of: the method further includes retaining one or more valid timing advance (TA) values for other CLTM candidate cells 18 according to the information provided by the network or according to the UE implementation, for which the UE 22 performed early TA acquisition but did not execute LTM cell switch, after conditional LTM execution to one of the one or more CLTM candidate cells 18; the method further includes maintaining other relevant information about TA values, for the other CLTM candidate cells 18; the other relevant information about TA values includes information about a current ‘state’ of the corresponding Time alignment timers; and the information about the current ‘state’ includes a current value or a remaining time left to expiry or a time instant when a TA value was obtained.

[0346] Example A3. The UE 22 of any one of Examples Al and A2, wherein the method further includes: retaining one or more TA values for the other CLTM candidate cells 18, after conditional LTM execution to one of the one or more CLTM candidate cells 18, when a time left for expiry of associated time alignment timer is above a threshold value and / or when the UE 22 is explicitly configured to retain TA values and / or associated time alignment timer values for other CLTM candidate cells 18.

[0347] Example A4. The UE 22 of any of any one of Examples Al -A3, wherein one or more of: the method further includes sharing information about retained TA values for other CLTM candidate cells 18 with the network node 16; the information about the retained TA values is shared via a radio resource control (RRC) message, another existing message, an existing lower layer signaling message, or new lower layer signaling message; the RRC message includes RRC Reconfiguration Complete Message; and the existing lower layer signaling message or the new lower layer signaling message includes a TA Status reporting medium access control (MAC) control element (CE).

[0348] Example A5. The UE 22 of any of any one of Examples A1-A4, wherein the method further includes: preventing triggering a measurement report for an event associated with early UL synchronization of a CLTM candidate cell 18, if a TA value for that cell was acquired before the CLTM execution in a previous source cell and is still valid, or a time alignment timer corresponding to the TA value of the CLTM candidate cell 18 is still running.

[0349] Example A6. The UE 22 of any of Example Al, wherein the method further includes: releasing TA values for other CLTM candidate cells, upon conditional LTM execution to one of CLTM candidate cells 18, if the UE 22 does not retain the TA values for other LTM candidate cells 18.

[0350] Example BL A user equipment (UE 22) for communicating with a network node 16, the UE 22 being configured to, and / or comprising a radio interface 46 and / or processing circuitry 50 configured to perform any one of the steps of Examples A1-A6.

[0351] Example CL A method in a network node 16 for communicating with a user equipment (UE 22), the network node 16 being configured as a source radio access network node 16 or a source distributed unit (S-DU 100) for the UE 22, the method comprising: determining whether to configure the UE 22 with a conditional L1 / L2 triggered mobility (CLTM) configuration for one or more CLTM candidate cells 18, the CLTM configuration including one or more CLTM execution conditions; transmitting a request for configuration of CLTM for a CLTM candidate cell 18 to a candidate network node 16 configured for handling the CLTM candidate cell 18; receiving a configuration for CLTM for the CLTM candidate cell 18 from the candidate network node 16; transmitting the CLTM configuration for the CLTM candidate cell 18, with an associated CLTM execution condition, to the UE 22; transmiting a physical downlink control channel (PDCCH) order for triggering uplink (UL) synchronization for the one or more CLTM candidate cells 18; and maintaining one or more timing advance (TA) values associated with the one or more CLTM candidate cells 18, for a predetermined UE 22, for which the predetermined UE 22 performed early TA acquisition.

[0352] Example C2. The method of Example Cl, wherein another S-DU 100 is configured to, after conditional LTM execution, receive one or more of a Class 1 Message from a candidate distributed unit (C-DU), a Class 2 Message from the C-DU, Class 1 Message from a gNodeB centralized unit (gNB-CU 102), a Class 2 Message from the gNB-CU 102, another indication from the C-DU 100, and another indication from the gNB-CU 102which triggers the S-DU 100 to provide information about the one or more TA values received during the UL synchronization to one or more CLTM candidate cells 18 and validity of the one or more TA values.

[0353] Example C3. The method of any one of Examples Cl and C2, wherein another S- DU 100 is configured to, after conditional LTM execution, provide information about the one or more TA values received during the UL synchronization to one or more CLTM candidate cells 18 and validity of the one or more TA values by sending one or more of a Class 1 Message to a candidate distributed unit (C-DU 100), a Class 2 Message to the C- DU 100, a Class 1 Message to a gNodeB centralized unit (gNB-CU 102), a Class 2 Message to the gNB-CU 102.

[0354] Example DI. A network node 16 for communicating with a user equipment (UE 22), the network (e.g., network node 16) being configured as a source radio access network node 16 or a source distributed unit (S-DU 100) for the UE 22, the network node 16 being configured to, and / or comprising a radio interface 30 and / or processing circuitry 36 configured to perform any one of the steps of Examples Cl and C3.

[0355] Example EL A method in a network node 16 for communicating with a user equipment (UE 22), the network (e.g., network node 16) being configured as a candidate radio access network node 16 or a candidate distributed unit (C-DU) for the UE 22, the method comprising: receiving a request from a serving network node 16 for the UE 22, the request requesting a conditional L1 / L2 triggered mobility (CLTM) configuration for one or more CLTM candidate cells 18 handled by the network node 16; transmitting a configuration for a CLTM for a CLTM candidate cell 18 to a serving network node 16; receiving a random access message transmitted by the UE 22 for timing advance (TA) acquisition; transmitting a calculated TA value to other network nodes 16 or directly to the UE 22; and maintaining one or more TA values associated with one or more CLTM candidate cells 18 located in the candidate distributed unit (C-DU 100), for a predetermined UE 22, for which the predetermined UE 22 performed TA acquisition.

[0356] Example E2. The method of Example El, wherein the C-DU 100 becomes a source distributed unit (S-DU 100) after Conditional LTM execution.

[0357] Example E3. The method of Example E2, wherein the C-DU 100 receives a message or a lower layer indication transmitted by the UE 22 including information about one or more retained TA values for other CLTM candidate cells 18 and about a current state of corresponding time alignment timers.

[0358] Example E4. The method of Example E3, wherein one or both of the message is a radio resource control (RRC) Message and the RRC Message is an RRC Reconfiguration Complete Message.

[0359] Example E5. The method of any one of Example E1-E4, wherein one or both of: the C-DU 100 receives another existing RRC Message or a new RRC Message, transmitted by the UE 22 including information about one or more retained TA values for other CLTM candidate cells 18 and about a current state of corresponding time alignment timers; and the other existing RRC Message is a UEAssistancelnformation Message.

[0360] Example E6. The method of any one of Examples E1-E2, wherein one or more of: the C-DU 100 receives the one or more TA values obtained during TA acquisitions and the relevant information about TA validity for the one or more CLTM candidate cells 18 from a corresponding gNB-CU 102 over an Fl interface using a FIAP protocol Message; the relevant information about TA validity includes current ‘state’ of the corresponding Time alignment timers; and and the current state includes a current value or remaining time left to expiry.

[0361] Example E7. The method of Example E6, wherein the C-DU 100 receives the one or more TA values obtained during TA acquisitions and the relevant information about TA validity of the corresponding Time alignment timers, for the one or more CLTM candidate cells 18 from a corresponding gNB-CU 102 over an Fl interface using the FIAP protocol Message, upon sending an ACCESS SUCCESS or RRC Reconfiguration Complete Message to the gNB-CU 102.

[0362] Example E8. The method of Example E6, wherein the C-DU 100 receives the one or more TA values obtained during TA acquisitions and the relevant information about TA validity of the corresponding Time alignment timers, for the one or more CLTM candidate cells 18 from a corresponding gNB-CU 102 over an Fl interface using FIAP protocol Message, upon sending one or more of a Class 1 Message to a source distributed unit (S-DU 100), a Class 2 Message the S-DU, a Class 1 Message to the gNB-CU 102, a Class 2 Message to the gNB-CU 102, another indication to S-DU 100, and another indication to the gNB-CU 102.

[0363] Example E9. The method of any Example E6, wherein the C-DU 100 receives the one or more TA values obtained during TA acquisitions and the relevant information about TA validity of the corresponding Time alignment timers, for the one or more CLTM candidate cells 18 from a corresponding gNB-CU 102 over an Xn interface using XnAP protocol Message, upon sending an ACCESS SUCCESS or RRC Reconfiguration Complete Message to the gNB-CU 102.

[0364] Example E10. The method of Example E6, wherein the C-DU 100 receives the one or more TA values obtained during TA acquisitions and the relevant information about TA validity of the corresponding Time alignment timers, for the one or more CLTM candidate cells 18 from a corresponding gNB-CU 102 over an Xn interface using XnAP protocol Message, upon sending one or more of a Class 1 Message to a source distributed unit (S- DU), a Class 2 Message to the S-DU 100, a Class 1 Message to the gNB-CU102 , a Class 2 Message to the gNB-CU 102, another indication to the S-DU 100, and another indication to the gNB-CU 102.

[0365] Example Ell. The method of Example E6, wherein one or both of: the C-DU 100 receives the one or more TA values obtained during TA acquisitions and the relevant information about TA validity of the corresponding Time alignment timers, for the one or more CLTM candidate cells 18, from a corresponding gNB-CU 102 over an Fl interface, using FIAP protocol Message before the UE 22 performs Conditional LTM execution to one of the configured LTM candidate cells 18, while early TA acquisition is being performed for the one or more CLTM candidate cells 18; and the FIAP protocol Message is a CU-DU TA INFORMATION TRANSFER Message. Example Fl. A network node 16 for communicating with a user equipment (UE 22), the network (e.g., network node 16) being configured as a candidate radio access network node 16 or a candidate distributed unit (C-DU 100) for the UE 22, the network node 16 being configured to, and / or comprising a radio interface 30 and / or processing circuitry 36 configured to perform any one of the steps of Examples El-El l.

[0366] Example G1. A method in a network node 16 for communicating with a user equipment (UE 22), the network (e.g., network node 16) being configured as one of (A) a gNodeB centralized unit (gNB-CU 102) that serves a source distributed unit (S-DU 100) and configures the UE 22 with an L1 / L2 triggered mobility (LTM) candidate cell configuration; and (B) a gNB-CU that serves a candidate distributed unit (C-DU 100), the method comprising: maintaining one or more timing advance (TA) values associated with one or more LTM candidate cells 18, for a predetermined UE 22, for which the predetermined UE 22 performed early TA acquisition.

[0367] Example G2. The method of Example Gl, wherein one or both of: the gNB-CU 102 serving the C-DU 100 or anew S-DU 100 decodes information about retained TA values for other LTM candidate cells, the C-DU 100 receiving information about a current state of corresponding time alignment timers validity, wherein the information is received via a new radio resource control (RRC) Message or an existing RRC Message; and the new RRC Message or the existing RRC Message includes a Reconfiguration Complete Message or a UEAssistancelnformation Message.

[0368] Example G3. The method of Example G2, wherein the gNB-CU 102 serving the C-DU 100 or a new S-DU 100 transmits the decoded information about the retained TA value(s) and the information about the current state of corresponding time alignment timers validity via a message.

[0369] Example G4 The method of any one of Examples G1-G3, wherein the gNB-CU 102 serving the C-DU 100 or anew S-DU 100, after CLTM execution, provides information about retained TA values and information about a current state of corresponding time alignment timers validity via one or more of a Class 1 Message or a Class 2 Message to the C-DU 100.

[0370] Example G5. The method of any one of Examples Gl, wherein the gNB-CU 102 serving a previous S-DU 100, after conditional LTM execution, retains the one or more TA values received during uplink synchronization to the one or more LTM candidate cells 18 and keeps track of the validity of the one or more TA values, based on one or more conditions.

[0371] Example G6. The method of any one of Examples G1 and G4, wherein the gNB- CU 102 serving a previous S-DU 100, after the conditional LTM execution, provides the one or more TA values received during UL synchronization to one or more LTM candidate cells 18 and provides relevant TA validity information to another S-DU 100.

[0372] Example G7. The method of any one of Example Gl, wherein one or both of: the gNB-CU 102 serving a previous S-DU 100 sends obtained one or more TA values and relevant information about TA validity for the one or more LTM candidate cells 18 to the S-DU 100 as well as to one or more C-DUs 100 of the same gNB over an Fl interface, using a FIAP protocol Message before the UE performs CLTM execution to one of the configured LTM candidate cells 18; and the FIAP protocol Message is a CU-DU TA INFORMATION TRANSFER Message.

[0373] Example G8. The method of Example Gl, wherein one or more of: the gNB-CU 102 serving a previous S-DU 100, sends obtained one or more TA values and relevant information about TA validity for the one or more LTM candidate cells 18 to the S-DU 100 as well as to one or more C-DUs 100 of a different gNB over an Xn interface, using an XnAP protocol Message, before the UE performs CLTM execution to one of the configured CLTM candidate cells 18; the gNB 16 which received the information about the obtained one or more TA values and the relevant information about the TA validity for the one or more LTM candidate cells sends this information to the C-DU 100 it serves over an Fl interface, using a FIAP protocol Message; the XnAP protocol Message is a TA Information Transfer Message; and the FIAP protocol Message is a CU-DU TA INFORMATION TRANSFER Message.

[0374] Example G9. The method of Example G6, wherein the TA values and the relevant TA validity information, received during UL synchronization to one or more LTM candidate cells, is provided via one or more of a Class 1 Message, a Class 2 Message, and an XnAP protocol Message.

[0375] Example G10. The method of any one of Examples Gl, wherein the gNB-CU 102 serving a previous S-DU 100, after the Conditional LTM execution, requests and receives information about one or more TA values and the relevant TA validity information received during the early UL synchronization to one or more LTM candidate cells 18 and TA values validity from the previous S-DU 100.

[0376] Example Gil. The method of Example GIO, wherein the TA value and relevant TA validity information is requested and / or provided via one or more of a Class 1 Message, a Class 2 Message, and an XnAP protocol Message.

[0377] Example G12. The method of Examples Gl, wherein the gNB-CU 102 serving a new S-DU 100 or a candidate distributed unit (C-DU 100), after the CLTM execution, requests and receives information about one or more TA values and the relevant TA validity information received during the early UL synchronization to one or more LTM candidate cells 18 and TA values validity from a previous S-DU 100.

[0378] Example G13. The method of Example G12, wherein the TA value and relevant TA validity information is one or both requested and provided via one or more of a Class 1 Message, a Class 2 Message, and an XnAP protocol Message.

[0379] Example HL A network node 16 for communicating with a user equipment (UE 22), the network (e.g., network node 16) being configured as a candidate radio access network node 16 or a candidate distributed unit (C-DU 100) for the UE 22, the network node 16 being configured to, and / or comprising a radio interface 30 and / or processing circuitry 36 configured to perform any one of the steps of Examples G1-G13.

[0380] As will be appreciated by one of skill in the art, the concepts described herein may be embodied as a method, data processing system, computer program product and / or computer storage media storing an executable computer program. Accordingly, the concepts described herein may take the form of an entirely hardware embodiment, an entirely software embodiment or an embodiment combining software and hardware aspects all generally referred to herein as a “circuit” or “module.” Any process, step, action and / or functionality described herein may be performed by, and / or associated to, a corresponding module, which may be implemented in software and / or firmware and / or hardware. Furthermore, the disclosure may take the form of a computer program product on a tangible computer usable storage medium having computer program code embodied in the medium that can be executed by a computer. Any suitable tangible computer readable medium may be utilized including hard disks, CD-ROMs, electronic storage devices, optical storage devices, or magnetic storage devices.

[0381] Some embodiments are described herein with reference to flowchart illustrations and / or block diagrams of methods, systems and computer program products. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general purpose computer (to thereby create a special purpose computer), special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions / acts specified in the flowchart and / or block diagram block or blocks.

[0382] These computer program instructions may also be stored in a computer readable memory or storage medium that can direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer readable memory produce an article of manufacture including instruction means which implement the function / act specified in the flowchart and / or block diagram block or blocks.

[0383] The computer program instructions may also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the functions / acts specified in the flowchart and / or block diagram block or blocks.

[0384] It is to be understood that the functions / acts noted in the blocks may occur out of the order noted in the operational illustrations. For example, two blocks shown in succession may in fact be executed substantially concurrently or the blocks may sometimes be executed in the reverse order, depending upon the functionality / acts involved. Although some of the diagrams include arrows on communication paths to show a primary direction of communication, it is to be understood that communication may occur in the opposite direction to the depicted arrows.

[0385] Computer program code for carrying out operations of the concepts described herein may be written in an object oriented programming language such as Python, Java® or C++. However, the computer program code for carrying out operations of the disclosure may also be written in conventional procedural programming languages, such as the "C" programming language. The program code may execute entirely on the user's computer, partly on the user's computer, as a stand-alone software package, partly on the user's computer and partly on a remote computer or entirely on the remote computer. In the latter scenario, the remote computer may be connected to the user's computer through a local area network (LAN) or a wide area network (WAN), or the connection may be made to an external computer (for example, through the Internet using an Internet Service Provider).

[0386] Many different embodiments have been disclosed herein, in connection with the above description and the drawings. It will be understood that it would be unduly repetitious and obfuscating to literally describe and illustrate every combination and subcombination of these embodiments. Accordingly, all embodiments can be combined in any way and / or combination, and the present specification, including the drawings, shall be construed to constitute a complete written description of all combinations and subcombinations of the embodiments described herein, and of the manner and process of making and using them, and shall support claims to any such combination or subcombination.

[0387] Abbreviations that may be used in the preceding description include:

[0388] C-RNTI Cell Radio Network Temporary Identifier

[0389] CE Control Element

[0390] CHO Conditional Handover

[0391] CLTM Conditional LTM

[0392] CSI-RS Channel State Information Reference Signal

[0393] CU Centralized Unit

[0394] DL Downlink

[0395] DU Distributed Unit

[0396] IE Information Element

[0397] LTM L1 / L2 -Triggered Mobility

[0398] MAC Medium Access Control

[0399] MAC CE MAC Control Element

[0400] NR New Radio

[0401] PCell Primary Cell

[0402] PDCCH Physical Downlink Control Channel

[0403] PRACH Physical Random-Access Channel

[0404] PSCell Primary Secondary Cell (in LTE) or Primary SCG Cell (in NR)

[0405] PUCCH Physical Uplink Control Channel

[0406] PUSCH Physical Uplink Shared Channel

[0407] QCL Quasi Co-Located RACH Random Access Channel

[0408] RAR Random Access Response

[0409] RLF Radio Link Failure

[0410] RRC Radio Resource Control

[0411] RSRP Reference Signal Received Power

[0412] RSRQ Reference Signal Received Quality

[0413] SCell Secondary Cell

[0414] SCG Secondary cell group

[0415] SINR Signal to Interference plus Noise Ratio

[0416] SR Scheduling Request

[0417] SSB Synchronization Signal Block

[0418] SpCell Special Cell, the primary cell of a master or secondary cell group

[0419] TA Timing Advance

[0420] TAG Timing Advance Group

[0421] TCI Transmission Configuration Indication

[0422] UCI Uplink Control Information

[0423] UE User Equipment

[0424] UL Uplink

[0425] It will be appreciated by persons skilled in the art that the embodiments described herein are not limited to what has been particularly shown and described herein above. In addition, unless mention was made above to the contrary, it should be noted that all of the accompanying drawings are not to scale. A variety of modifications and variations are possible in light of the above teachings without departing from the scope of the following claims.

Claims

CLAIMS1. A method in a user equipment, UE (22), the method comprising: receiving (SI 08) a conditional Layer 1 / Layer 2, L1 / L2, triggered mobility, CLTM, configuration for one or more CLTM candidate cells (18), the CLTM configuration including one or more CLTM execution conditions for executing a CLTM procedure; before at least one of the one or more CLTM execution conditions is fulfilled, performing (SI 10) uplink, UL, synchronization with the one or more CLTM candidate cells (18) to acquire one or more timing advance, TA, values; and when at least one of the one or more CLTM execution conditions is fulfilled(SI 12): executing the CLTM procedure for a first CLTM candidate cell (18) of the one or more CLTM candidate cells (18); and retaining the one or more TA values.

2. The method of Claim 1, wherein the retaining of the one or more TA values comprises, after the conditional CLTM execution to the first CLTM candidate cell (18), retaining at least one TA value for at least a second CLTM candidate cell (18) of the one or more CLTM candidate cells (18).

3. The method of Claim 2, wherein the at least one TA value for the second CLTM candidate cell (18) is retained based on a time left for expiry of at least one timing alignment timer associated with the at least one TA value meeting a threshold criterion.

4. The method of Claim 2, wherein the at least one TA value for the second CLTM candidate cell (18) is retained based on the UE (22) being configured to one or both of: retain the at least one TA value; and retain at least one time alignment timer value associated with the at least one TA value.

5. The method of any one of Claims 2-4, wherein the one or more TA values comprises a TA value for a third CLTM candidate cell (18); andthe method further comprising indicating, to the second CLTM candidate cell (18), information associated with the TA value for the third CLTM candidate cell (18).

6. The method of Claim 5, wherein the indication is provided by an RRC message or a UE assistance information message.

7. The method of any one of Claims 1-6, further comprising: preventing triggering a measurement report for an event associated with early UL synchronization of a second CLTM candidate cell (18) of the one or more candidate cells (18), if a TA value for second CLTM candidate cell (18) was acquired before the CLTM execution in a previous source cell (18) and is: still valid; or a time alignment timer that is still running.

8. The method of any one of Claims 1-7, further comprising maintaining information for at least one TA value for at least the second CLTM candidate, the information indicating a current state of the at least one time alignment time associated with the at least one TA value.

9. A user equipment, UE (22), configured to: receive a conditional Layer 1 / Layer 2, L1 / L2, triggered mobility, CLTM, configuration for one or more CLTM candidate cells (18), the CLTM configuration including one or more CLTM execution conditions for executing a CLTM procedure; before at least one of the one or more CLTM execution conditions is fulfilled, perform uplink, UL, synchronization with the one or more CLTM candidate cells (18) to acquire one or more timing advance, TA, values; and when at least one of the one or more CLTM execution conditions is fulfilled: execute the CLTM procedure for a first CLTM candidate cell of the one or more CLTM candidate cells (18); and retain the one or more TA values.

10. The UE (22) of Claim 9, wherein the UE (22) is further configured to, after conditional LTM execution to the first CLTM candidate cell (18), retain at least one TAvalue for at least a second CLTM candidate cell (18) of the one or more CLTM candidate cells (18).

11. The UE (22) of Claim 10, wherein the at least one TA value for the second CLTM candidate cell (18) is retained when a time left for expiry of at least one timing alignment timer associated with the at least one TA value meets a threshold criterion.

12. The UE (22) of Claim 10, wherein the at least one TA value for the second CLTM candidate cell is retained when the UE (22) is configured to one or both of: retain the at least one TA value; and retain at least one time alignment timer value associated with the at least one TA value.

13. The UE (22) of any one of Claims 10-12, wherein the at least one TA value comprises a TA value for a third CLTM candidate cell (18); and the UE (22) is further configured to indicate, to the second CLTM candidate cell (18), information associated with the TA value for the third CLTM candidate cell (18).

14. The UE (22) of Claim 13, wherein the indication is provided by an RRC message or a UE assistance information message.

15. The UE (22) of any one of Claims 9-14, wherein the UE (22) is further configured to: prevent triggering a measurement report for an event associated with early UL synchronization of a second CLTM candidate cell (18) of the one or more candidate cells (18), if a TA value for second CLTM candidate cell (18) was acquired before the CLTM execution in a previous source cell (18) and is: still valid; or a time alignment timer that is still running.

16. The UE (22) of any one of Claims 9-15, wherein the UE (22) is further configured to maintain information for at least one TA value for at least the second CLTM candidate, the information indicating a current state of the at least one time alignment time associated with the at least one TA value.

17. A method implemented by a network node (16) that is configured to communicate with a user equipment, UE (22), the network node (16) being configured as a centralized unit, CU, the method comprising: configuring (S138) the UE (22) with a conditional Layer 1 / Layer 2, L1 / L2, triggered mobility, CLTM, configuration for at least a first CLTM candidate cell (18) and a second CLTM candidate cell (18), the CLTM configuration including one or more CLTM execution conditions for executing a CLTM procedure; and receiving (SI 40), from a first CLTM candidate cell (18) and based on the configuration, a timing advance, TA, value associated with the first CLTM candidate cell (18) or a second candidate CLTM cell (18), the TA value having been acquired by the UE (22) before at least one of the one or more CLTM execution conditions were fulfilled.

18. The method of Claim 17, wherein the TA value is associated with the first candidate CLTM cell (18); the method further comprising: receiving a TA value associated with the second candidate CLTM cell (18); and indicating, to the first candidate CLTM cell (18), the TA value associated with the second candidate CLTM cell (18).

19. The method of Claim 18, wherein the indication is signaled via one of: a UE context modification request message; a TA information transfer message; or an radio resource control, RRC, message.

20. The method of Claim 17, wherein the TA value is associated with the first candidate CLTM cell (18); and the method further comprising indicating, to the second candidate CLTM cell (18), the TA value associated with the first candidate CLTM cell (18).

21. The method of Claim 20, further comprising receiving, from the second candidate CLTM cell (18) and based on the configuration, a TA value associated with thesecond candidate CLTM cell (18), the TA value having been acquired by the UE (22) before at least one of the one or more CLTM execution conditions were fulfilled.

22. The method of any one of Claims 20-21, wherein the receiving of the TA value is part of a CU-Distributed Unit, DU, information transfer.

23. The method of any one of Claims 17-22, further comprising retaining the TA value for subsequent conditional CLTM execution.

24. A network node (16) configured to communicate with a user equipment, UE (22), the network node (16) being configured as a centralized unit, CU (102), the network node (16) configured to: configure the UE (22) with a conditional Layer 1 / Layer 2, L1 / L2, triggered mobility, CLTM, configuration for at least a first CLTM candidate cell (18) and a second CLTM candidate cell (18), the CLTM configuration including one or more CLTM execution conditions for executing a CLTM procedure; and receive, from a first CLTM candidate cell (18) and based on the configuration, a timing advance, TA, value associated with the first CLTM candidate cell (18) or a second candidate CLTM cell (18), the TA value having been acquired by the UE (22) before at least one of the one or more CLTM execution conditions were fulfilled.

25. The network node of Claim 24, wherein the TA value is associated with the first candidate CLTM cell (18); and the network node (16) is further configured to: receive a TA value associated with the second candidate CLTM cell (18); and indicate, to the first candidate CLTM cell (18), the TA value associated with the second candidate CLTM cell (18).

26. The network node (16) of Claim 25, wherein the indication is signaled via one of: a UE context modification request message; a TA information transfer message; or an radio resource control, RRC, message.

27. The network node (16) of Claim 24, wherein the TA value is associated with the first candidate CLTM cell (18); and the network node (16) is further configured to indicate, to the second candidate CLTM cell (18), the TA value associated with the first candidate CLTM cell (18).

28. The network node (16) of Claim 27, wherein the network node (16) is further configured to receive, from the second candidate CLTM cell (18) and based on the configuration, a TA value associated with the second candidate CLTM cell (18), the TA value having been acquired by the UE (16) before at least one of the one or more CLTM execution conditions were fulfilled.

29. The network node (16) of any one of Claims 27-28, wherein the receiving of the TA value is part of a CU-Distributed Unit, DU (100), information transfer.

30. The network node (16) of any one of Claims 24-29, wherein the network node (16) is further configured to retain the TA value for subsequent conditional CLTM execution.

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