Reducing service interruption time for LLM in RAN4

Abstract

The present invention relates to a user equipment (UE) served by a source cell of a first network node supporting a radio access network (RAN) distributed unit (DU) function and / or Layer 2 protocol processing. The UE includes a processor and memory, and executes instructions to select from a plurality of candidate target cells controlled by the first network node based on L3 measurements and generate L1 measurements. For L1 / L2 triggered mobility (LTM), the UE determines whether the candidate cells meet preset conditions for handover and preferably periodically transmits L1 measurement reports to the first node. The preset conditions include whether single-sideband (SSB) is detected and whether a first timer associated with the L3 measurement report has not expired since a first timestamp. The UE determines whether the timer has expired since a second timestamp, and the L3 measurement report is the most recent at the time of generating the L1 measurement report.

Description

[Technical Field]

[0001] The present disclosure relates to L1L2 triggered mobility, LTM, and in particular to interruption times in LTM handover procedures. [Background technology]

[0002] Any discussion of background art throughout this specification should in no way be taken as an admission that such art is widely known or forms part of the common general knowledge in the field.

[0003] Broadly speaking, for L1L2-triggered mobility (LTM), the UE reports L1 beam measurements to a serving DU (i.e., source DU), which can decide when to trigger a handover to one or more candidate target cells indicated by the L1 measurement report provided by the UE based on previous L3 measurements provided by the UE indicating that one or more candidate target cells are determined to be suitable cells to which the UE can be handed over.

[0004] However, before the gNB makes a decision to trigger a cell change to said one or more candidate target cells, an indeterminate amount of time may have already passed since the UE indicated one or more candidate target cells to the gNB. During this indeterminate amount of time, a previously known or detectable candidate target cell may no longer be detectable, for example, due to UE mobility, etc. This leads to the gNB relying on a received L1 measurement report indicating a "detectable" candidate target cell, but not knowing that the indicated cell is in fact no longer detectable.

[0005] Therefore, when the gNB triggers a cell change for a cell that it believes is still detectable, given this lack of knowledge of the cell's actual detectability, the UE may still have to perform a search for candidate target cells that are no longer actually known after receiving the trigger message from the gNB. The time associated with such a search, i.e., T_search (which is part of the interruption time in the LTM handover procedure), may be up to 60 milliseconds. The time associated with the search is only for cells that are still actually known to the UE. Summary of the Invention [Problem to be solved by the invention]

[0006] Therefore, it is necessary to solve the above problem that neither the network nor the UE actively tracks the actual detectability of candidate target cells for which the gNB triggers a cell change during the entire LTM handover procedure, resulting in endlessly long interruptions in the LTM handover procedure. In other words, it is necessary to propose a method and corresponding apparatus to ensure that when the gNB triggers a cell change, for candidate target cells that are still known, the T_search time is set to 0 in order to reduce the latency induced in the handover procedure, while for cells that are no longer known, it is necessary to provide a mechanism by which at least the UE and the gNB are specifically informed of the possible induced delay. [Means for solving the problem]

[0007] According to one aspect of the present disclosure, there is provided a user equipment, UE, served by a source cell of a first network node supporting a radio access network, a distributed unit of a RAN, a DU function and / or Layer 2 protocol processing, the UE comprising: at least one processor; and at least one memory that stores instructions that, when executed by the at least one processor, cause the UE to perform at least: generating L1 measurements for a plurality of candidate target cells controlled by the first network node, the candidate target cells being selected based on the L3 measurements generated by the UE; determining whether the plurality of candidate target cells comply with one or more preset conditions for a UE to perform a handover from the serving source cell to at least one of the plurality of candidate target cells for L1L2-triggered mobility (LTM); causing the first network node to transmit L1 measurement reports, preferably periodically; the L1 measurement report is provided by the UE based on the determination associated with the one or more preset conditions; One or more of the preset conditions above can be The synchronization signals / PBCH blocks, SSBs of the plurality of candidate target cells are detectable; and / or The method includes causing a first timer to not expire since a first point in time indicated by a first timestamp associated with an L3 measurement report including the L3 measurement, and the UE is further caused to determine whether the first timer has expired at a second point in time indicated by a second timestamp associated with the L1 measurement report, the L3 measurement report being the most recent L3 measurement report generated by the UE when preparing the L1 measurement report.

[0008] In some examples, the L1 measurement report includes L1 measurements for one or more candidate target cells of the plurality of candidate target cells that are determined to meet the one or more preset conditions.

[0009] In some examples, the UE further comprises: For any one of the plurality of candidate target cells that is determined not to meet the one or more preset conditions, an invalid bit is added to the L1 measurement report, and the L1 measurement report with the added invalid bit is preferably transmitted to a first network node periodically.

[0010] In some examples, the first timer has a length of 5 seconds.

[0011] In some examples, the UE is further configured to determine whether SSBs of the plurality of candidate target cells are detectable based on at least one of a Synchronization Signal Reference Signal Received Power (SS-RSRP), a CSI Reference Signal Received Power (CSI-RSRP), a Synchronization Signal Reference Signal Received Quality (SS-RSRQ), a CSI Reference Signal Received Quality (CSI-RSRQ), a Synchronization Signal Signal to Interference and Noise Ratio (SS-SINR), and a CSI Signal to Interference and Noise Ratio (CSI-SINR).

[0012] In some examples, the UE is further caused to send a cell change trigger response message to the first network node if, after receiving the cell change trigger message from the first network node, it is determined that any one candidate target cell of the plurality of candidate target cells does not meet the one or more preset conditions.

[0013] In some examples, the UE further comprises: The cell change trigger response message may include T_search information indicating a first estimated time required to search for any one of the plurality of candidate target cells that is determined not to meet the one or more preset conditions.

[0014] In some examples, the UE further comprises: The cell change trigger response message may include T_IU information indicating a second estimated time for interruption uncertainty when acquiring a first available physical random access channel, PRACH (Physical Random Access Channel), opportunity in any one of the plurality of candidate target cells that is determined not to comply with the one or more preset conditions, preferably wherein the second estimated time is determined based on whether a random access channel, RACH (Random Access Channel) or RACH-less procedure is applied by the UE.

[0015] In some examples, the cell change trigger response message is sent by lower layer signaling.

[0016] According to another aspect of the present disclosure, there is provided a first network node configured to support a radio access network, a distributed unit of a RAN, a DU function and / or Layer 2 protocol processing, the first network node comprising: at least one processor; and at least one memory storing instructions that, when executed by the at least one processor, cause the first network node to at least: determining whether a plurality of candidate target cells controlled by a first network node meet one or more preset conditions for a UE to perform a handover from a serving source cell of the first network node to at least one of the plurality of candidate target cells for L1L2 triggered mobility (LTM), wherein the determining is performed by the first network node based on L1 measurements and / or L3 measurements provided by the UE for the plurality of candidate target cells; and causing the UE to transmit a cell change trigger message based on the determination, wherein the cell change trigger message includes an instruction to trigger a change to one or more candidate target cells of the plurality of candidate target cells that are determined to meet the one or more preset conditions.

[0017] In some examples, the first network node: The method is further configured to determine, for each candidate target cell of the one or more candidate target cells, whether the each candidate target cell satisfies the one or more preset conditions, wherein the one or more preset conditions include, for each candidate target cell, an L1 measurement report including the L1 measurement received from the UE does not include an invalid bit.

[0018] In some examples, the first network node: and the first network node is further configured to: determine, for each candidate target cell of the one or more candidate target cells, whether the each candidate target cell complies with the one or more preset conditions, wherein the one or more preset conditions include, for each candidate target cell, a second timer has not expired since a first time point indicated by a first timestamp associated with an L3 measurement report received from the UE and including the L3 measurement; and At a third time point, it is determined whether the second timer has expired, the third time point being after receiving an L1 measurement report including the L1 measurement from the UE and before sending the cell change trigger message to the UE, and the L3 measurement report is the latest L3 measurement report generated by the UE when preparing the L1 measurement report.

[0019] In some examples, the first network node is further caused to transmit the cell change trigger message before the second timer expires.

[0020] In some examples, the second timer is five seconds long.

[0021] In some examples, the first network node is further configured to include a failure indication in the cell change trigger message for any one candidate target cell of the plurality of candidate target cells that is determined not to comply with the one or more preset conditions.

[0022] In some examples, the cell change trigger message is sent by lower layer signaling, preferably a MAC Control Element, MAC CE, or Downlink Control Information, DCI.

[0023] According to another aspect of the present disclosure, there is provided a second network node configured to support a radio access network, a central unit of a RAN, a CU function and / or Layer 3 protocol processing, the second network node comprising: at least one processor; and at least one memory storing instructions that, when executed by the at least one processor, cause the second network node to at least: determining a first timestamp associated with an L3 measurement report received from a UE, the UE being served by a source cell of a first network node, the L3 measurement report including L3 measurements for a plurality of candidate target cells controlled by the first network node; The first timestamp is included in CU to DU RRC information sent to the first network node; The CU to DU RRC information having the first timestamp included therein is sent to the first network node.

[0024] In some examples, the second network node is further caused to send the CU to DU RRC information in the UE context modification request message.

[0025] In some examples, the second network node further comprises: Before a third timer expires, a decision is made to perform preparation of the plurality of candidate target cells so that the UE can perform a handover from the serving source cell to at least one of the plurality of candidate target cells for L1L2 triggered mobility (LTM), and the third timer starts at a fourth point in time when the L3 measurement report is received from the UE.

[0026] In some examples, performing the preparation of the plurality of candidate target cells includes one or more of: sending a UE context setup request message to the first network node; generating and sending the RRC reconfiguration associated with the plurality of candidate target cells to the first network node; and sending a UE context modification request message to the first network node.

[0027] In some examples, the third timer has a length of 5 seconds.

[0028] According to another aspect of the present disclosure, there is provided a method for a user equipment, UE, served by a source cell of a first network node supporting a distributed unit, DU functionality and / or Layer 2 protocol processing of a radio access network, the method comprising: generating L1 measurements for a plurality of candidate target cells controlled by a first network node, the candidate target cells being selected based on L3 measurements generated by the UE; determining whether the plurality of candidate target cells meet one or more preset conditions for a UE to perform a handover from the serving source cell to at least one of the plurality of candidate target cells for L1L2 triggered mobility (LTM); transmitting L1 measurement reports to the first network node, preferably periodically; the L1 measurement report is provided by the UE based on the determination associated with the one or more preset conditions; One or more of the preset conditions above can be The synchronization signals / PBCH blocks, SSBs of the plurality of candidate target cells are detectable; and / or The method includes determining whether the first timer has expired at a second time indicated by a second time stamp associated with the L1 measurement report, the L3 measurement report including the L1 measurement, the first timer having not expired since a first time point indicated by a first time stamp associated with the L1 measurement report, the L3 measurement report being the most recent L3 measurement report generated by the UE when preparing the L1 measurement report.

[0029] According to another aspect of the present disclosure, there is provided a method of a first network node configured to support a distributed unit, DU functionality and / or Layer 2 protocol processing of a radio access network, the method comprising: determining whether a plurality of candidate target cells controlled by a first network node meet one or more preset conditions for a UE to perform a handover from a serving source cell of the first network node to at least one of the plurality of candidate target cells for L1L2 triggered mobility (LTM), wherein the determining is performed by the first network node based on L1 measurements and / or L3 measurements provided by the UE for the plurality of candidate target cells; transmitting a cell change trigger message to the UE based on the determination, wherein the cell change trigger message includes an instruction to trigger a change to one or more candidate target cells of the plurality of candidate target cells that are determined to meet the one or more preset conditions.

[0030] According to another aspect of the present disclosure, there is provided a method of a second network node configured to support a radio access network, a central unit of a RAN, a CU function and / or Layer 3 protocol processing, the method comprising: determining a first timestamp associated with an L3 measurement report received from a UE, the UE being served by a source cell of a first network node, the L3 measurement report including L3 measurements for a plurality of candidate target cells controlled by the first network node; including the first timestamp in CU to DU RRC information sent to the first network node; and transmitting the CU to DU RRC information with the first timestamp included therein to the first network node.

[0031] According to another aspect of the present disclosure, there is provided a user equipment, UE, configured to connect to a source cell of a radio access network, a distributed unit of a RAN, a first network node supporting DU functionality and / or Layer 2 protocol processing, the UE comprising: at least one processor; and at least one memory that stores instructions that, when executed by the at least one processor, cause the UE to perform at least: generating L1 measurements for at least one candidate target cell for possible L1L2 triggered mobility, LTM; determining whether at least one of the at least one candidate target cell meets one or more preset conditions for the UE; generating and transmitting to a first network node an L1 measurement report, the L1 measurement report being provided by the UE based on the determination related to the one or more preset conditions; One or more of the preset conditions above can be a first timer not expiring relative to a first time point associated with a corresponding L3 measurement; The UE is further caused to determine a timing offset by the expiration of the first timer and include an indication related to the determined timing offset in the L1 measurement report.

[0032] It may be advantageous to calculate the timing offset at the UE and provide this information to the DU. In this way, the reported candidate cell is considered known at the UE, and the DU is therefore informed how much time is still available before T_search becomes zero, so that handover can be performed quickly without further delay. The DU typically bases this decision to handover on one or more criteria or parameters, one of which may be the time remaining until the end of the first timer, another may be, for example, the load conditions at the DU or the DU to which the UE may switch.

[0033] In some examples, an indication related to the determined timing offset is included in the L1 measurement report to enable the first network node to decide on a handover to one of the candidate cells in the L1 report before the first timer expires.

[0034] In some examples, the indication may include an absolute value of the time remaining until the first timer expires.

[0035] According to another aspect of the present disclosure, there is provided a first network node configured to support a radio access network, a distributed unit of a RAN, a DU function and / or Layer 2 protocol processing, the first network node comprising: at least one processor; and at least one memory storing instructions that, when executed by the at least one processor, cause the first network node to at least: receiving, for at least one candidate cell for a possible L1L2 triggered mobility, LTM, from a user equipment connected to a source cell supported by the first network node; causing the first network node to determine whether a handover from a source cell to a candidate cell shall be triggered; causing the UE to send a cell change trigger message based on the determination, the determination being based on one or more preset conditions, one condition including whether a first timer has not expired in relation to a first time point associated with the corresponding L3 measurement.

[0036] It may be advantageous to calculate the timing offset at the UE and provide this information to the DU. In this way, the reported candidate cell is considered known at the UE, and the DU is therefore informed how much time is still available before T_search becomes zero, so that handover can be performed quickly without further delay. The DU typically bases this decision on handover on one or more criteria or parameters, one of which may be the time remaining until the expiration of the first timer, and another may be, for example, the load conditions at the DU to which the DU or UE may switch. Instead of the timing offset, a first point in time associated with the corresponding L3 measurement may be transmitted to the DU, thus enabling the DU to independently calculate the time remaining until the expiration and decide to take this into account. This timing information may be transmitted to the DU by the UE or CU. To reduce the amount of signaling, the timing information may be transmitted, for example, together with the L1 measurement report or the DU's configuration for LTM. The DU may be instructed by the CU to make a decision within the indicated time limit, e.g., before the expiration of the timer, to enable handover with low latency, e.g., T_search=0. Alternatively, the DU may be instructed by the CU to make a decision within the indicated time limit if possible, e.g., if such a decision would not compromise other decisions. The transfer of timing information and / or its use may be optional. The DU may be configured to use or not use the provided timing information. If it is advantageous for the DU, the DU may decide to trigger handover before the indicated timer expires, but may also decide to trigger handover after the timer has already expired. In the latter case, the DU knows that the UE will have a T_search not equal to zero and will likely take appropriate action on the network side.The calculation of the timer expiration may include calculating the time needed for the trigger to be generated and sent to the UE, and possibly also the time it takes for the UE to process the trigger message and perform the handover. Thus, the DU predicts the time the UE will need to perform the handover, e.g., sending a random access message to the new DU, and takes this time into account when selecting the time to trigger the handover, such that when performing the handover, the UE still knows the indicated target candidate cell.

[0037] In some examples, the indication related to the first point in time is received from a radio access network, a central unit of a RAN, a user equipment or a second network node configured to support a CU function and / or Layer 3 protocol processing prior to the determination.

[0038] In some examples, an indication related to the determined timing offset is included in the L1 measurement report to enable the first network node to decide on a handover to one of the candidate cells in the L1 report before the first timer expires.

[0039] In some examples, the indication may include an absolute value of the time remaining until the first timer expires.

[0040] In some examples, the L1 measurement report includes an instruction to determine a handover to one of the at least one candidate cells in the L1 report, the handover being associated with a timing offset by the end of a first timer associated with a first time point associated with a corresponding L3 measurement of the at least one candidate cell, before the first timer expires.

[0041] According to another aspect of the present disclosure, there is provided a second network node configured to support a radio access network, a central unit of a RAN, a CU function and / or Layer 3 protocol processing, the second network node comprising: at least one processor; and at least one memory storing instructions that, when executed by the at least one processor, cause the second network node to at least: determining a first timestamp associated with an L3 measurement report received from a UE, the UE being served by a source cell of a first network node, the L3 measurement report including L3 measurements for at least one candidate target cell for handover; including an indication related to said first timestamp in a message sent to said first network node; causing the first network node to transmit a message having a first timestamp-related indication included therein;

[0042] In some examples, the message is a message that triggers possible L1L2 triggered mobility, LTM, and configures the first network node to receive L1 measurement reports from the user equipment.

[0043] In some examples, the indication may include an absolute value of the time remaining until the first timer expires.

[0044] According to another aspect of the present disclosure, there is provided a user equipment, UE, configured to connect to a source cell of a first network node supporting a radio access network, a distributed unit of a RAN, a DU function and / or Layer 2 protocol processing, the UE comprising: at least one processor; and at least one memory that stores instructions that, when executed by the at least one processor, cause the UE to perform at least: generating L1 measurements for at least one candidate target cell for possible L1L2 triggered mobility, LTM; determining whether at least one of the at least one candidate target cell meets one or more preset conditions for the UE; generating and transmitting to a first network node an L1 measurement report, the L1 measurement report being provided by the UE based on the determination related to the one or more preset conditions; One or more preset conditions can be a first timer not expiring relative to a first time point associated with a corresponding L3 measurement; The UE is further caused to send a timing indication to the first network node; The timing indication relates to a first time point associated with a corresponding L3 measurement.

[0045] In some examples, the indication may include an absolute value of the time remaining until the first timer expires or a timestamp of the first point in time.

[0046] According to some other exemplary embodiments, a computer program comprising instructions for causing an apparatus to perform the methods disclosed in the present disclosure is also provided.

[0047] According to some other exemplary embodiments, a memory storing computer-readable instructions for causing an apparatus to perform the methods disclosed in the present disclosure is also provided.

[0048] Furthermore, according to some other exemplary embodiments, for example, a computer program product is provided for a wireless communication device having at least one processor, the computer program product including software code portions for performing each of the steps disclosed in the present disclosure when the product is executed on the device. The computer program product may include a computer-readable medium on which the software code portions are stored. Furthermore, the computer program product may be directly loadable into the internal memory of a computer and / or transmittable over a network by at least one of an upload, download, and push procedure.

[0049] Although some exemplary embodiments will be described herein with particular reference to the above applications, it will be recognized that the present disclosure is not limited to such fields of use but is applicable in broader contexts.

[0050] In particular, it is understood that methods according to the present disclosure relate to methods of operating apparatuses according to the above exemplary embodiments and variations thereof, and that each statement made with respect to an apparatus applies equally to the corresponding method, and vice versa, whereby similar descriptions may be omitted for the sake of brevity. Furthermore, the above aspects may be combined in many ways, even if not explicitly disclosed. Those skilled in the art will understand that these combinations of aspects and features / steps are possible unless they create a contradiction that is expressly excluded.

[0051] Implementations of the disclosed apparatus may include, but are not limited to, using one or more processors, one or more application specific integrated circuits (ASICs), and / or one or more field programmable gate arrays (FPGAs). Implementations of the apparatus may also include using other conventional and / or customized hardware, such as software programmable processors, such as graphics processing unit (GPU) processors.

[0052] Other and further exemplary embodiments of the present disclosure will become apparent during the course of the following discussion and by reference to the accompanying drawings.

[0053] Exemplary embodiments of the present disclosure will now be described, by way of example only, with reference to the accompanying drawings, in which: [Brief explanation of the drawings]

[0054] [Figure 1] FIG. 10 is a diagram illustrating an example of a RAN2 timing diagram. [Figure 2] FIG. 10 is a schematic diagram illustrating an example of an LTM case in rel-18 with inter-CU, intra-CU, and inter-DU cell switches. [Figure 3] FIG. 10 is a schematic diagram of a signaling diagram of LTM behavior showing T_search. [Figure 4] FIG. 10 is a diagram illustrating an example of an LTM interruption time. [Figure 5] 10A-10C are diagrams illustrating schematic examples of LTM interruption times in RACH-LESS and RACH cases, showing T_IU; [Figure 6] FIG. 10 illustrates schematically an example of validity checks at the CU and UE in the LTM procedure. [Figure 7] FIG. 10 is a diagram illustrating an example of validity checks at a CU, a source DU, and a UE in an LTM procedure. DETAILED DESCRIPTION OF THE INVENTION

[0055] In the following, different exemplary embodiments are described using a communication network architecture based on 3GPP® standards for communication networks, such as 5G / NR, as an example of a communication network to which the example embodiments may be applied, without, however, limiting the embodiments to such an architecture. It will be clear to those skilled in the art that the embodiments may also be applied to other types of communication networks, in which mobile communication principles are integrated with D2D (device-to-device) or V2X (vehicle-to-everything) configurations, such as SL (side link), system technologies using, for example, WiFi, WiMAX (worldwide interoperability for microwave access), Bluetooth®, personal communications services (PCS), ZigBee®, wideband code division multiple access (WCDMA), ultra-wideband (UWB), mobile ad-hoc networks (MANET), wired access, etc. Furthermore, without loss of generality, although the description of some example embodiments relates to a mobile communication network, the principles of the present disclosure may be extended and applied to any other type of communication network, such as a wired communication network.

[0056] The following examples and embodiments are to be understood as merely illustrative examples. Although the specification may refer to "an," "one," or "some" examples or embodiments in several places, this does not necessarily mean that each such reference relates to the same example or embodiment, or that a feature only applies to a single example or embodiment. Single features of different embodiments may be combined to provide other embodiments. Furthermore, terms such as "comprising" and "including" should not be understood to limit the described embodiments to consisting only of the stated features; such examples and embodiments may include features, structures, units, modules, etc. that are not specifically stated.

[0057] The basic system architecture of a (tele)communication network, including a mobile communication system, to which some example embodiments are applicable, may include the architecture of one or more communication networks, including a wireless access network subsystem and a core network. Such an architecture may include one or more communication network control elements or functions, access network elements, radio access network elements, access service network gateways, or base transceiver stations such as base stations (BS), access points (AP), Node Bs (NBs), eNBs, or gNBs, distributed units (DUs), or centralized / central units (CUs), that control respective coverage areas or cells, and whereby one or more communication stations, such as communication elements or functions, such as user devices or terminal devices, such as user equipments (UEs), or another device having similar functions, such as modem chipsets, chips, modules, etc., that may also be part of a station, element, function, or application capable of communicating, such as a UE, an element or function usable in a machine-to-machine communication architecture, or that may be attached as a separate element to such an element, function, or application or the like, capable of communicating, are able to communicate over one or more channels by one or more communication beams for transmitting several types of data in multiple access domains. Additionally, core network elements or network functions may be included, such as gateway network elements / functions, mobility management entities, mobile switching centers, servers, databases, and the like.

[0058] The following description may provide further details of alternatives, modifications, and variations: The gNB comprises a node that provides NR user plane and control plane protocol termination towards the UE and is connected to the 5GC by an NG interface, for example, in accordance with 3GPP TS 38.300 V16.6.0(2021-06) Section 3.2, which is incorporated by reference.

[0059] The gNB Central Unit (gNB-CU) comprises, for example, a logical node that hosts, for example, the RRC, SDAP, and PDCP protocols of the gNB and the RRC and PDCP protocols of the en-gNB that controls the operation of one or more gNB-DUs. The gNB-CU terminates the F1 interface that connects to the gNB-DU.

[0060] The gNB Distributed Unit (gNB-DU) comprises, for example, a logical node that hosts, for example, the RLC, MAC, and PHY layers of a gNB or en-gNB, and its operation is partially controlled by the gNB-CU. One gNB-DU supports one or more cells. One cell is supported by only one gNB-DU. The gNB-DU terminates the F1 interface that connects to the gNB-CU.

[0061] The gNB-CU-Control Plane (gNB-CU-CP) comprises, for example, a logical node that hosts the control plane portion of the RRC and PDCP protocols of the gNB-CU for, for example, an en-gNB or a gNB. The gNB-CU-CP terminates the E1 interface connected to the gNB-CU-UP and the F1-C interface connected to the gNB-DU.

[0062] The gNB-CU-User Plane (gNB-CU-UP) comprises, for example, a logical node that hosts, for example, the user plane portion of the PDCP protocol of the gNB-CU for the en-gNB and the user plane portion of the PDCP protocol and SDAP protocol of the gNB-CU for the gNB. The gNB-CU-UP terminates the E1 interface connected to the gNB-CU-CP and the F1-U interface connected to the gNB-DU, for example, in accordance with 3GPP TS 38.401 V16.6.0(2021-07) Section 3.1, which is incorporated by reference.

[0063] For example, different functional splits between the central and distributed units, called options, are possible: Option 1 (1A-like division): The functional division in this option is similar to the 1A architecture in DC: RRC is in the central unit; PDCP, RLC, MAC, physical layer and RF are in the distributed units. Option 2 (3C-like division): The functional division in this option is similar to the 3C architecture in DC: RRC and PDCP are in the central unit; RLC, MAC, physical layer and RF are in the distributed units. Option 3 (intraRLC splitting): Low RLC (partial functions of RLC), MAC, physical layer, and RF are in the distributed units. PDCP and high RLC (other partial functions of RLC) are in the central unit. Option 4 (RLC-MAC Split): MAC, physical layer, and RF are in the distributed units. PDCP and RLC are in the central unit. Otherwise, for example, according to 3GPP TS 38.801 V14.0.0(2017-03) Section 3.1, which is incorporated by reference.

[0064] The gNB supports different protocol layers, for example, Layer 1 (L1) - Physical Layer.

[0065] Layer 2 (L2) of NR is divided into the following sublayers: Medium Access Control (MAC), Radio Link Control (RLC), Packet Data Convergence Protocol (PDCP), and Service Data Adaptation Protocol (SDAP), where, for example: o The physical layer provides transport channels to the MAC sublayer; o The MAC sublayer provides logical channels to the RLC sublayer; The RLC sublayer provides the RLC channel to the PDCP sublayer; The PDCP sublayer provides radio bearers to the SDAP sublayer; The SDAP sublayer provides QoS flows for 5GC; o Comp. refers to header compression and Segm. refers to segmentation; Control channels include (BCCH, PCCH).

[0066] Layer 3 (L3) includes, for example, Radio Resource Control (RRC), for example, according to 3GPP TS 38.300 V16.6.0 (2021-06) Section 6, which is incorporated by reference.

[0067] For example, a RAN (Radio Access Network) node or network node, such as a gNB, base station, gNB CU or gNB DU, or a specified portion thereof, may be implemented using an apparatus having at least one processor and / or at least one memory (having computer readable instructions (computer program)) configured to support and / or provide and / or process, for example, CU and / or DU related functions and / or features, and / or at least one protocol (sub)layer of the RAN (Radio Access Network), e.g., Layer 2 and / or Layer 3.

[0068] The gNB CU and gNB DU portions may, for example, be co-located or physically separated. The gNB DU may be further divided, for example, into two portions (e.g., one including processing equipment and one including antennas). The central unit (CU) may be referred to as a BBU / REC / RCC / C-RAN / V-RAN, O-RAN, or a predetermined portion thereof. The distributed unit (DU) may be referred to as an RRH / RRU / RE / RU, or a predetermined portion thereof. Hereinafter, in various exemplary embodiments of the present disclosure, the CU-CP (or, more generally, the CU) may be referred to as a (first) network node supporting at least one of central unit control plane functions or Layer 3 protocol processing of the radio access network, and similarly, the DU may be referred to as a (second) network node supporting at least one of distributed unit functions or Layer 2 protocol processing of the radio access network.

[0069] A gNB-DU supports one or more cells and may therefore serve, for example, as a serving cell for a user equipment (UE).

[0070] A user equipment (UE) may include a wireless or mobile device, a device having a radio interface that interacts with a RAN (Radio Access Network), a smartphone, an in-vehicle device, an IoT device, an M2M device, etc. Such a UE or device may include at least one processor and at least one memory including computer program code configured by the at least one processor to cause the device to perform at least a specific operation, such as, for example, an RRC connection to the RAN. The UE may be configured, for example, to generate a message (e.g., including a cell ID) to be transmitted wirelessly toward the RAN (e.g., to reach and communicate with a serving cell). The UE may generate, send, and receive RRC messages including one or more RRC PDUs (Packet Data Units).

[0071] The UE may have different states (in accordance with 3GPP TS 38.331 V16.6.0(2021-06) sections 42.1 and 4.4, which are incorporated by reference).

[0072] A UE may be in an RRC_CONNECTED or RRC_INACTIVE state, for example, once an RRC connection is established.

[0073] In the RRC_CONNECTED state, the UE: o May remember AS context; o May forward unicast data to / from the UE; may monitor a control channel associated with the shared data channel to determine whether data is scheduled for the data channel; o May provide channel quality and feedback information; Neighbor cell measurements and measurement reports may be performed.

[0074] The RRC protocol includes, for example, the following main functions: 〇RRC connection control; Measurement configuration and reporting; Establishment / modification / release of measurement configurations (e.g., intra-frequency, inter-frequency, and inter-RAT measurements); o Setting up and releasing measurement gaps; 〇Measurement reporting.

[0075] The general functions and interconnections of the described elements and functions, which also depend on the actual network type, are known to those skilled in the art and are described in the corresponding specifications, and therefore a detailed description thereof is omitted herein for the sake of brevity. However, it is noted that several further network elements and signaling links may be utilized for communication to or from elements, functions, or applications, such as communication endpoints, servers, gateways, communication network control elements such as radio network controllers, and other elements of the same or other communication networks other than those described in detail herein below.

[0076] The communications network architecture considered in the example embodiments may be capable of communicating with other networks, such as the Public Switched Telephone Network or the Internet. The communications network may be capable of supporting the use of cloud services for virtual network elements or their functions, and it is noted that virtual network portions of a telecommunications network may similarly be provided by non-cloud resources, e.g., internal networks or the like. It should be appreciated that network elements and / or their respective functions of an access system, such as a core network, may be implemented by using any node, host, server, access node, entity, etc., suitable for such use. In general, network functions may be implemented as network elements on dedicated hardware, as software instances running on dedicated hardware, or as virtualized functions instantiated on a suitable platform, e.g., a cloud infrastructure.

[0077] Furthermore, network elements such as communication elements like UEs, terminal devices, control elements or functions of access network elements like base stations / BSs, gNBs, radio network controllers, core network control elements or functions like gateway elements or other network elements or functions described herein, and any other elements, functions, or applications may be implemented by software, e.g., by a computer program product for a computer, and / or by hardware. To perform their respective processing, the correspondingly used devices, nodes, functions, or network elements may include several means, modules, units, components, etc. (not shown) required for control, processing, and / or communication / signaling functions. Such means, modules, units and components may include, for example, one or more processors or processor units including one or more processing portions for executing instructions and / or programs and / or processing data, storage or memory units (e.g., ROM, RAM, EEPROM and the like) for storing instructions, programs and / or data to serve as working areas for the processors or processing portions and the like, inputs or input means (e.g., floppy disks, CD-ROM, EEPROM and the like) for inputting data and instructions by means of software, user interfaces (e.g., screens, keyboards and the like) for providing monitoring and operability to a user, other interfaces or means for establishing links and / or connections under the control of the processor units or portions (e.g., wired and wireless interface means, e.g., radio interface means including antenna units or the like, means for forming wireless communication portions, etc.), and the like, wherein each means for forming an interface such as a wireless communication portion may be located at a remote site (e.g., radio head or radio station, etc.).It is noted that, in this specification, a processing portion should not only be considered to refer to a physical portion of one or more processors, but may also be considered as a logical division of the referenced processing tasks performed by one or more processors. According to some examples, a so-called "liquid" or flexible network concept may be used, and it should be recognized that the operations and functions of a network entity, network function, or another entity of the network may be implemented in different entities or functions, such as in a node, host, or server, in a flexible manner. In other words, the "division of labor" between the involved network elements, functions, or entities may vary from case to case.

[0078] Reference is now made to the figures. In particular, it is noted that identical or similar reference numerals used in the figures of the present disclosure, unless otherwise indicated, indicate identical or similar elements, whereby repeated descriptions thereof may be omitted for the sake of brevity. As can be understood and appreciated by those skilled in the art, it is further noted that even if the figures appear to refer to some specific / explicit message names / types, these messages may certainly have different names and / or be communicated / exchanged in different forms / formats depending on various implementations (e.g., underlying technologies).

[0079] The 3GPP RAN group is discussing L1L2 triggered mobility (LTM). The RAN plenary, in accordance with 3GPP RP-221799, has set the following objectives for this work: Specifically, it specifies mechanisms and procedures for L1 / L2-based inter-cell mobility to reduce mobility latency: configuration and maintenance for multiple candidate cells to enable fast application of configurations for candidate cells [RAN2, RAN3]; dynamic switch mechanisms between candidate serving cells (including SpCells and SCells) for possible applicable scenarios based on L1 / L2 signaling [RAN2, RAN1]; L1 enhancements for inter-cell beam management and beam direction, including L1 measurement and reporting [RAN1, RAN2] (Note 1: Early RAN2 involvement is required, including the possibility of further clarifying the interaction between this bullet point and the previous bullet point); Timing Advance management [RAN1, RAN2]; and, if needed, CU-DU interface signaling to support L1 / L2 mobility [RAN3]. Note 2: FR2-specific enhancements, if present, are not excluded. Note 3: The L1 / L2 based inter-cell mobility procedure is applicable to the following scenarios: standalone, CA, and NR-DC cases with serving cell change within one CG; Intra-DU and intra-CU inter-DU cases (applicable for standalone and CA: no new RAN interface is foreseen); both intra-frequency and inter-frequency; both FR1 and FR2. The source and target cells may or may not be synchronized.

[0080] The main goal of this work is to reduce handover latency in the scenarios described above.

[0081] In general, for L1L2 triggered mobility (LTM), it is expected that the UE will report L1 beam measurements to the serving DU, so that the serving DU can decide when to trigger a handover. The purpose of this is to simplify many of the network and UE mobility procedures, as well as reduce the disruption time / delay introduced by mobility. Furthermore, it is expected that network data transfer, scheduling will benefit from LTM as well. In LTM, the UE may maintain configurations of multiple cells in order to apply fast adaptation of each configuration.

[0082] LTM may or may not include a serving cell change and may use RACH or be RACH-less.

[0083] In LTM, the serving DU triggers the execution of the prepared target cell configuration based on lower layer signaling, which could be a MAC Control Element (MAC CE) or Downlink Control Information (DCI). Upon triggering the cell change, the serving DU informs the CU to stop sending any RRC reconfiguration over the serving cell radio link and initiate data transfer to the target cell, if needed.

[0084] LTM is expected to use L1 measurements. These measurements have the benefit of faster reaction time to radio link degradation in the serving link because the network can avoid the delays introduced by L3 filtering and Time to Trigger (TTT) for handover decisions. This should result in a reduced number of radio link failures compared to baseline handovers.

[0085] For further discussion related to LTM topics, reference is made to 3GPP R4-2219443, R4-2219444, R4-2219445, R4-2219446.

[0086] In recent RAN2 discussions, it was agreed that the handover (HO) interruption time for LTM is defined as the time from when the UE receives the cell switch command to when the UE performs the first DL / UL reception / transmission on the indicated beam of the target cell. The timing diagram in Table 1 shows the components that contribute to HO interruption for LTM.

[0087] The meaning of each component is further shown in Table 1 below: [Table 1]

[0088] RAN4 has discussed T_search (the time required to search for a target cell), and the following summarizes the RAN2 agreement and current status:

[0089] RAN2 Agreement: R2 assumes that the following items may be discussed by RAN1 and RAN4 (and may be scenario specific): - Whether to perform DL synchronization to the candidate / target cell before receiving the cell switch command. R2 assumes that this is feasible at least if the target cell is already the active serving cell; - Whether to support performing TRS tracking and CSI measurements of the candidate / target cell before / with the cell switch command.

[0090] T_search shown above refers to the time required to search for the target cell. After the MAC-CE cell trigger from the DU is sent to the UE, the following conditions are specified in current 3GPP for the value of T_search: 0 ms if the cell is known; up to 60 ms if the cell is not known.

[0091] To illustrate the different LTM scenarios, Figure 2 shows the difference between inter-CU (L3 handover) cell switch and intra-CU, inter-DU cell switch. Release-18 work initially focuses on intra-DU or inter-DU cases. The scope may be further expanded in future releases.

[0092] For example, according to NR38.133, V17.6.0, "Technical Specification Group Radio Access Network; NR; Requirements for support of radio resource management", section 9.13.2, the known cell conditions in 3GPP are specified as follows: A cell with a different Physical Cell ID (PCI) from the serving cell is considered known if the following conditions are met in this requirement: - The SSB of a cell with a different PCI from the serving cell has the same SCS and center frequency as the SSB of the PCell; - the timing difference of arrival at the UE between the SSB of the serving cell and the SSB of a cell with a different PCI is shorter than the CP length of the corresponding SCS; - The UE has sent a valid L3 measurement report within the last 5 seconds; - SSBs from cells with different PCIs remain detectable in accordance with the cell identification requirements specified in clause 9.2; - Otherwise, the cell is unknown.

[0093] The above conditions are defined from a UE perspective and with reference to a Synchronization Signal / PBCH Block (SSB).

[0094] The first condition states that "The SSB of the cell with different PCI from the serving cell has the same SCS and center frequency as the SSB of the PCell." This is the provision that RAN4 agreed on in the last meeting about same-frequency measurements. From the tdoc review from the current meeting, there is a tendency that different-frequency measurements are considered in LTM. As a result, the above known condition will have to be revised for the LTM case.

[0095] The second condition states that "The timing difference of arrival at the UE between the SSBs of the serving cell and the SSB of the cell with different PCI is less than the CP length of the corresponding SCS." This is a provision for synchronous or asynchronous transmission. It is discussed similarly if this concept is to be adopted in the case of LTM.

[0096] The last two criteria are concise as they are not related to regulations and are self-explanatory in the measurement report or measurements, therefore these two criteria will be discussed in this disclosure.

[0097] The detectability of SSB as defined in NR38.133, Articles 9.2 / 9.3, is "The UE shall be able to identify new intra / inter-frequency cells and perform SS-RSRP, SS-RSRQ, and SS-SINR measurements of identified intra / inter-frequency cells if carrier frequency information is provided by the PCell or the PSCell, even if no explicit neighbor list with physical layer cell identities is provided."

[0098] For intra / inter-frequency cells, for each relevant SSB: - the SS-RSRP related conditions given in NR 38.133, clauses 10.1.4 and 10.1.5, for FR1 and FR2, respectively, are met for the corresponding bands; - the SS-RSRQ related conditions given in NR 38.133, clauses 10.1.9 and 10.1.10, for FR1 and FR2, respectively, are fulfilled for the corresponding bands; - the SS-SINR related conditions given in NR 38.133, clauses 10.1.14 and 10.1.15, for FR1 and FR2, respectively, are met for the corresponding bands; - SSB_RP and SSB according to NR38.133, Annex B.2.3

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[0099] As described above, candidate cell RRC configuration is prepared by the CU based on L3 measurements. The UE performed L1 measurements for the candidate cell, and the DU triggered a serving cell change based on those L1 measurements. Here, one problem arises due to the difference between L1 and L3 measurements. The term "L3 measurement" suggests that L3 performs filtering on the L1 values ​​and final reporting. The filtering is performed to remove the effects of fast fading and ignore short-term fluctuations. While L1 may collect measurements more frequently, L3 may report them with a longer periodicity than configured. Thus, L3 obtains a long-term view of the channel conditions. The maximum L3 measurement report can last up to 30 minutes.

[0100] Second, there is no timer specified in RAN2 for when the DU must make a decision about a serving cell change. Considering that L3 measurement reports can also be configured with a much longer periodicity, mobility can cause cells that were once known to the UE for L3 measurements to no longer be known to the UE.

[0101] Therefore, in the present disclosure, it is investigated what conditions shall be configured to ensure that T_search is zero in the context of LTM, or alternatively, what methods may be implemented so that the network knows whether the UE has learned of a cell before, for example, a 5-second timer expires. Of course, the 5-second time period is only used herein as an example value for illustrative purposes, and it shall not be construed as limiting to the scope of the present disclosure.

[0102] To facilitate understanding of the problem, Figure 3 shows an example signaling diagram of LTM behavior, illustrating T_search.

[0103] It is noted that in the present disclosure, the terms "candidate cell", "candidate target cell", and "target cell" refer to a cell that is detected by the UE based on L3 measurements of the serving source cell and neighboring cells, and is therefore determined and selected by the UE from said serving source cell and neighboring cells as suitable for performing a handover from the serving source cell.

[0104] Furthermore, it is noted that in this disclosure, the term "known condition(s)" or "detectability condition(s)" of a (candidate target) cell refers to one or more conditions defined in 3GPP for said cell to be considered detectable to a UE. For example, meeting said condition(s) when a UE performs a cell change leads to the UE not needing to search for said (candidate target) cell, i.e., the interruption time T_search is 0.

[0105] In the following, with reference to Figure 3, five cases are highlighted where the candidate cells prepared by the CU may not be known to the UE or gNB, which means that T_search is not guaranteed to be zero, leading to increased interruption time in the LTM HO service. In this example, it is assumed that cell 1, cell 2, and cell 3 (based on L3 measurements) are detected by the UE as candidate target cells for performing HO.

[0106] As shown in Figure 3, the UE is connected to the source DU. Furthermore, the UE performs L3 measurements to detect cell 1, cell 2, and cell 3 (not shown in Figure 3).

[0107] In steps S301 to S302: The UE reports cells 1 to 3 to the CU. It is noted that up to this point, cells 1 to 3 have always been known to the network.

[0108] In step S303, the CU decides to configure L1 measurements. It is noted that up until this point, cells 1 to 3 are not always known to the network. The reason is that it is uncertain how quickly the gNB will act according to the L3 measurement report received from the UE. This uncertain reaction scenario of the CU is referred to as Case 1 in this disclosure.

[0109] In steps S304 to S310: The CU instructs the UE to perform L1 measurements for cells 1 to 3. It is noted that up to this point, cells 1 to 3 are always unknown. The reason is the long periodicity of the L3 measurement reports explained above. Cells 1 to 3 may already become unknown during the long periodicity of the L3 measurement reports, but the gNB does not know that cells 1 to 3 have become unknown during the course of the long periodicity of the L3 measurement reports, because all the gNB has are the previous L3 reports received from the UE. As mentioned above, the maximum L3 measurement reports can be configured to have a periodicity as long as 30 minutes.

[0110] In step S311, it is identified in accordance with the present disclosure that there may be two cases S311a and S311b (not shown in FIG. 3).

[0111] At S311a: The UE performs L1 measurements and still detects cell 1, cell 2, but does not further detect cell 3, for example, or there is at least one detectable cell. This scenario, in which the UE also detects at least one candidate target cell that has previously been detected based on L3 measurements when performing L1 measurements, is referred to as Case 2 in this disclosure.

[0112] At S311b: The UE performs L1 measurements and does not further detect cell 1, cell 2, or cell 3. This scenario, in which the UE performs L1 measurements and does not detect any of the candidate target cells that have been previously detected based on L3 measurements, is referred to as Case 3 in this disclosure.

[0113] However, the prior art does not provide a discussion of the above cases 2 and 3, which may occur due to the long periodicity of L3 measurements. As a result, the gNB is not informed of such possible changes in the detectability of candidate target cells, i.e., whether the candidate target cells (identified by L3 measurements) also meet known conditions when the UE performs L1 measurements.

[0114] At S312: The UE reports L1 measurements to the gNB, preferably periodically.

[0115] At S313: The DU decides to trigger handover, for example, for cells 1 and 2. It is noted that at this point, cells 1 and 2 (known at the time of L1 measurement according to the example of Case 2 above) may also become unknown, even if they were known at S312 or earlier or by a new L3 measurement report, for example, if a 5-second timer expires. When the gNB decides to trigger handover to one of the candidate target cells, the one candidate target cell, however, has already become unknown, for example, because a certain period of time has expired since the gNB received the L1 measurement report from the UE. This scenario is referred to as Case 4 in the present disclosure.

[0116] At S314: The DU sends a trigger for cell change to the UE. For a gNB, as discussed with respect to Case 4 above, it is noted that a candidate target cell may become unknown, for example, since the end of a certain period during which the UE has moved, leading to the candidate target cell no longer being detectable. In this case, the T_search time will be up to 60 milliseconds. That is, the T_search time is not 0 for this currently unknown candidate target cell. In this example, it is assumed that cell 2 becomes unknown. This scenario in which the UE must search for a candidate target cell is referred to as Case 5 in this disclosure.

[0117] In steps S315 to S320: The UE performs a handover, for example, to known cell 1, where T_search for cell 1 is zero. Since cell 3 becomes unknown as described in step S311 and cell 2 becomes unknown as described in steps S313 and S314, only cell 1 is known when the gNB decides to trigger a handover. As a result, the T_search time for cell 1 is zero and the UE does not need to search for cell 1 while performing the cell change, whereas the UE must search for cells 2 and 3 since they are in fact no longer known to the UE.

[0118] Due to the problems identified for the above five cases, the following method of the present disclosure is proposed.

[0119] In the most recently published NR 38.133, the following L3 handover service interruption time is specified: The service interruption time for L3 handover is defined as the time between the end of the last TTI containing an RRC command on the old PDSCH and when the UE starts transmitting the new PRACH, excluding the RRC procedure delay. The service interruption time is given by the following formula: T_interrupt = T_seaerch + T_IU + T_processing + T_Δ + T_margin milliseconds

[0120] The description of each component in the above formula is given in the table below: [Table 2]

[0121] Figure 4 shows the definition of service disruption time from the perspective of RAN2 and RAN4. In RAN2, UL sync can be RACH-less. In this case, the end of the disruption time can be as early as the first UL / DL data transmission before RACH.

[0122] In order to reduce the service interruption time, the present disclosure provides a method by which T_search can be set to zero. In particular, with the aim of solving the above problems identified with respect to Cases 1 to 5 above, an apparatus (UE, DU, CU, or the like) and a corresponding method are proposed to address the above-mentioned problems / observations in a particularly efficient and flexible manner. In particular, it may be seen that in a broad sense, the present disclosure generally seeks to propose a solution to reducing interruption time in LTM handover services, in particular reducing the T_search time required by the UE to search for candidate target cells that are no longer detectable when the UE performs a gNB-triggered handover or when the gNB triggers a cell change.

[0123] The method proposed by the present disclosure consists, inter alia, in the following aspects:

[0124] Aspect 1: Inserting a timer check (plausibility check) in the CU before making the HO decision. This aims to solve the problem related to Case 1 above, which is the endlessly long time required by the CU to react to L3 measurements from the UE. A condition is preferably inserted in the CU that, for example, steps S304 to S310 shown in Figure 3 must be performed within a predetermined period of, for example, 5 to 10 seconds since the CU received the L3 measurements or since the timestamp associated with the corresponding L3 measurement report. This ensures reduced latency and interruption in the handover procedure.

[0125] In the present disclosure, the timestamp associated with an L1 or L3 report may be, for example, the time when the UE generates the report or the time when the UE transmits the report, which is for illustrative purposes and not limited by the mere examples discussed in the present disclosure.

[0126] Aspect 2: Validity check at the UE side, i.e. the timestamp difference between the current L1 measurement report and the latest L3 report of the same (candidate target) cell is not longer than, for example, 5 seconds, where the current measurement satisfies the following "known" or "detectability" conditions: The UE shall only return an L1 report invalid bit (invalid bit) together with the latest L1 measurement report (for said candidate target cell) if the measurement (for said candidate target cell) does not satisfy the following conditions: - the SS-RSRP related conditions given in NR 38.133, clauses 10.1.4 and 10.1.5, for FR1 and FR2, respectively, for the corresponding bands; - the SS-RSRQ related conditions given in NR38.133, clauses 10.1.9 and 10.1.10, for the corresponding bands FR1 and FR2, respectively; - the SS-SINR related conditions given in NR38.133, clauses 10.1.14 and 10.1.15, for FR1 and FR2, respectively, for the corresponding bands, and - SSB_RP and SSB according to NR38.133, Annex B.2.3 for the corresponding bands

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[0127] This aims to solve the problems related to Cases 2 and 3 above, where one or more candidate target cells previously detected by the UE based on L3 measurements may become unknown due to the long periodicity of the L3 measurements. By configuring a timer in the UE that controls the time elapsed between the current L1 measurement report and the latest L3 measurement report of the same (candidate target) cell, it can be controlled by the UE that only a predetermined period should elapse since the last L3 measurement report, thereby reducing the impact of the periodicity of the L3 measurements. Any cells that are no longer known when the UE performs L1 measurements are preferably indicated with an invalidity bit appended to the L1 measurement report generated by the UE, so that the gNB is informed of the invalidity of those cells and may, for example, trigger a cell change only for cells that are still known.

[0128] For example, to address case 2, following the above step S311 shown in Figure 3, step S312a may be provided: the UE reports L1 measurements including the L1 report invalid bit for all cells 1-2 and cell 3 detectable to the DU.

[0129] For example, to address case 3, following the above step S311 shown in Figure 3, step S312b may be provided: the UE reports L1 report invalid for all cells, or reports a respective invalid bit for each of the unknown cells 1 to 3.

[0130] Additionally or alternatively, if the DU knows about the expiration of, for example, a 5-second timer, it can decide to handover and send a MAC CE within 5 seconds, which would ideally reduce T_search to zero; the time until the timer expires may be sent to the DU by the CU or by the UE.

[0131] Aspect 3: Validity check on both the UE side and the gNB side. This is similar to aspect 2, with the difference that the timer is moved to the gNB side. That is, the UE checks the current L1 measurements of the candidate target cell to determine whether the candidate target cell meets the following condition: The UE shall only return the L1 report invalid bit along with the latest L1 measurement report if the measurements do not meet the following condition: Then, for example, to address case 2: the UE reports L1 measurements including all detectable cells to the DU + L1 report invalid for any undetectable cell. Furthermore, for example, to address case 3: the UE reports L1 report invalid for all undetectable cells. - the SS-RSRP related conditions given in NR 38.133, clauses 10.1.4 and 10.1.5, for FR1 and FR2, respectively, for the corresponding bands; - the SS-RSRQ related conditions given in NR38.133, clauses 10.1.9 and 10.1.10, for the corresponding bands FR1 and FR2, respectively; - the SS-SINR related conditions given in NR38.133, clauses 10.1.14 and 10.1.15, for FR1 and FR2 for the corresponding bands, and - SSB_RP and SSB according to NR38.133, Annex B.2.3 for the corresponding bands

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[0132] Further, according to aspect 3, when the gNB, for example, checks validity and before the gNB triggers a cell change, the gNB checks the L1 RSRP report (invalid / valid indicated by the presence or absence of an invalid bit in the L1 report) received from the UE and / or the timestamp from the L3 measurement against the current timestamp at the gNB side. For example, the gNB shall trigger a MAC-CE cell switch within a predetermined period since the timestamp associated with the L3 measurement report, for example, within 5 seconds from the L3 measurement.

[0133] This aims to solve the problem identified with case 4 above, where the gNB takes too long to trigger a cell change, during which time previously known candidate target cells become unknown. In this case, it is proposed in accordance with the present disclosure that the gNB must trigger a MAC-CE cell switch within a predetermined period since the timestamp associated with the last L3 report. Therefore, it is preferred in accordance with the present disclosure that the gNB triggers a cell change for candidate target cells that still meet the detectability or "known" condition, provided that the predetermined period since the timestamp associated with the L3 measurement has not expired. This ensures that handover is performed by the UE only for valid cells, and that the T_search time is 0 for those valid candidate target cells.

[0134] According to aspect 3, it is preferable that the UE only checks the validity of the measurements, and the gNB checks both the validity of the report (e.g., as indicated by the invalid bit) and the timing requirements. Thus, while the cell is known to the UE by satisfying only partial requirement conditions related to the measurements, the gNB further checks the report and finds that the report may fail the timing requirements at the time the gNB triggers a serving cell change decision. More preferably, the gNB should also add this failure information to the UE. Generally speaking, if the cell is known to the UE at the time of the L1 measurement report, when the gNB checks the timing requirements at the time of deciding to trigger a cell change, if the timing requirements fail, the gNB should also inform the UE of this information.

[0135] Aspect 4: ACK from UE side: If the report in the above option is disabled, a MAC-CE ACK shall be returned to the gNB after the MAC-CE cell switch to indicate the estimated T_search and possible T_IU depending on whether RACH-LESS or RACH is forwarded.

[0136] Estimated T_search: For simplicity, the UE may return the beam sweeping time for all panels in initial access. Alternatively, if the report is invalid, it indicates that the original candidate cell with the indicated Transmission Configuration Indication (TCI) state is no longer valid. The UE may perform a beam sweep using fewer available UE beams. The time taken in this case is proportional to the beam sweeping time in initial access.

[0137] As discussed above, for the estimation of T_IU, which refers to the interruption uncertainty when acquiring the first available PRACH opportunity in a new cell, reference is made to Figure 5, which shows an example of the time associated with T_IU in the RACH-LESS and RACH cases.

[0138] Indicate T_IU: RACH-LESS. For the RACH-LESS procedure, the endpoint of the service interruption time is the first DL / UL data transmission for the indicated beam of the target cell. T_IU is then the interruption time for the first uplink data transmission for the indicated beam of the target cell, as indicated by the T_first data shown in FIG. 5.

[0139] Indicates T_IU: RACH. For RACH procedures, the endpoint of the service interruption time is the transmission of the PRACH message. T_IU is then the interruption uncertainty when acquiring the first available PRACH opportunity in the new cell.

[0140] Uplink resource scheduling information should be included in the MAC-CE trigger for both the RACH-LESS and RACH cases above. The T_IU indication here, assuming T_search is non-zero here, indicates when the UE considers it possible to make uplink resources scheduled by the gNB from the UE side and the UE must perform fine time tracking (T_delta) and SSB post-processing (T_margin). The indication here will be only one bit, with 0 meaning negative and 1 meaning positive.

[0141] Furthermore, a further aspect is provided in accordance with the present disclosure that aims to address the above problem identified with respect to Case 5. Namely, the gNB inserts a failure indication in the MAC CE triggered cell change message to indicate to the UE that one or more of the candidate target cells failed a validity check at the gNB, meaning that the T_search time for these currently unknown cells will be non-zero and that the UE will have to search for these cells. Thus, the UE is notified in advance that these candidate target cells will have to be searched.

[0142] FIG. 6 shows a schematic example of validity checks at the CU and UE in an LTM procedure.

[0143] In steps S601-S602, the UE transmits a measurement report including cell quality measurements of the serving cell and neighboring cells. The UE may be configured by the serving cell to transmit the message report early when the UE still has a good connection to the serving cell. Using the reported cell quality measurements, the CU can identify a possible set of candidate target cells to which the UE can be handed over, which may belong to the same DU or different DUs, as shown in the current example.

[0144] In step S603, the CU makes a decision about the preparation of a new cell and checks that the timestamp of this decision is within, for example, 5 seconds from the timestamp of the received L3 measurement report.

[0145] In step S604, the CU requests the preparation of candidate target cells controlled by the target DU by sending a UE context setup request message.

[0146] In step S605, the target DU provides the UE configuration to the CU in a UE Context Setup Request message containing a container from the DU. The same steps S604 / S605 are performed for other target DUs to prepare the target cell.

[0147] Upon receiving the UE configuration for the candidate target cell, the CU generates an RRC reconfiguration message to be sent to the UE in step S606. Among other information, the RRC reconfiguration message includes measurement reporting configuration for L1 / L2 handover and prepared candidate cell configurations that the UE needs to execute when it receives a MAC CE command to change serving cell (perform handover).

[0148] In steps S607-S610, the configuration is provided to the UE, which acknowledges receipt and reconfiguration in the same manner.

[0149] In step S611, the UE compares the timestamp of the latest L3 measurement of the same candidate target cell with the current timestamp when the L1 measurement report is generated. The UE also compares measurements of the same candidate target cell to check whether the measurements meet one or more of the conditions outlined in, for example, NR38.133 clause 10.1. If the measurements are valid, and within, for example, 5 seconds, the UE will not instruct the gNB. This step of verification is for LTM L1 measurements and addresses cases 2 and 3, as discussed above.

[0150] As shown in step S612, the UE begins to periodically report L1 beam measurements of the serving cell and candidate target cells.

[0151] If it is determined that there is a candidate target cell with better radio link / beam measurements than the serving cell (step S613), e.g., L1-RSRP of target beam measurement > L1-RSRP of serving beam measurement + an amount of time, e.g., an offset for time to trigger (TTT), the serving cell sends a MAC Control Element (MAC CE) or L1 message in step S614 to trigger a cell change to the target candidate cell.

[0152] Step S615: If the L1 report is invalid, the UE estimates a possible T_search and indicates T_IU.

[0153] Step S616: The UE will update the gNB with a MAC-CE ACK after the MAC-CE cell switch to indicate the estimated T_search time and T_IU.

[0154] The handover from the serving cell to the target cell is performed by the UE in steps S617 to S619. Both RACH-based cell change and RACH-less-based cell change are considered as discussed above.

[0155] In step S620, the target DU provides an RRC reconfiguration end to the CU, which indicates to the CU that the UE will now be served by the target DU.

[0156] In steps S621 / S612, the UE context is released from the source DU.

[0157] Furthermore, a path switch is performed to the new serving DU (not shown in FIG. 6).

[0158] The termination phase is not expected to be affected in Rel. 18, similar to previous NR mobility enhancements in Rel. 16 and 17.

[0159] FIG. 7 illustrates schematically an example of validity checks at the CU, DU, and UE in an LTM procedure.

[0160] In steps S701-S702, the UE transmits a measurement report including cell quality measurements of the serving cell and neighboring cells. The UE may be configured by the serving cell to transmit the message report early when the UE still has a good connection to the serving cell. Using the reported cell quality measurements, the CU can identify a possible set of candidate target cells to which the UE can be handed over, which may belong to the same DU or different DUs, as shown in the current example.

[0161] In step S703, the CU decides on the preparation of a new cell and checks that the timestamp of this decision is within, for example, 5 seconds from the received L3 measurement report, and in step S704, the CU requests the preparation of a candidate target cell controlled by the target DU by sending a UE context setup request message.

[0162] In step S705, the target DU provides the UE configuration to the CU in a UE Context Setup Request message containing a container from the DU. The same steps S704 / S705 are performed for other target DUs to prepare the target cell.

[0163] Upon receiving the UE configuration for the candidate target cell, the CU generates an RRC reconfiguration message that is sent to the UE in step S706. Among other information, the RRC reconfiguration message includes measurement reporting configuration for L1 / L2 handover and prepared candidate cell configurations that the UE needs to execute when it receives a MAC CE command to change serving cell (perform handover).

[0164] In step S707: The CU includes the L3 measurement report timestamp for the CU to DU RRC information IE in the UE context modification request message.

[0165] In step S708: The DU responds with a UE context modification response message.

[0166] In steps S709-S712, the configuration is provided to the UE, which acknowledges receipt and reconfiguration in the same manner.

[0167] In step S713: The UE compares the measurements of the candidate target cells and checks whether the measurements meet one or more of the conditions outlined, for example, in NR38.133 clause 10.1. This step of verification is for LTM L1 measurements and to address cases 2 and 3, as discussed in detail above.

[0168] As shown in step S714, the UE begins to periodically report L1 beam measurements of the serving cell and candidate target cells.

[0169] Step S715: After receiving L1 measurement reports of various cells according to the LTM configuration from the UE, the gNB checks whether L1 RSRP is enabled, and at the same time, the gNB checks whether the timing requirement is older than, for example, 5 seconds from its corresponding L3 measurement report to confirm the validity of the timing requirement.

[0170] If it determines that there is a candidate target cell with better radio link / beam measurements than the serving cell (step S716), e.g., L1-RSRP of target beam measurements > L1-RSRP of serving beam measurements + an amount of time, e.g., an offset for time to trigger (TTT), the serving cell sends a MAC Control Element (MAC CE) or L1 message in step S717 to trigger a cell change for the candidate target cell that passed the validity check. If the timer validity check fails in S715, the gNB will add such an indication in the MAC-CE to provide the UE with earlier information.

[0171] Step S718: If the L1 report is invalid or the timer fails, the UE estimates T_search and indicates T_IU.

[0172] Step S719: If the L1 report is invalid or the timer fails, the UE shall update the gNB with a MAC-CE ACK after the MAC-CE cell switch to indicate the estimated T_search time and / or T_IU time.

[0173] The handover from the serving cell to the target cell is performed by the UE in steps S720-S722. Both RACH-based cell change and RACH-less-based cell change are considered as discussed above.

[0174] In step S723, the target DU provides an RRC reconfiguration end to the CU, which indicates to the CU that the UE will now be served by the target DU.

[0175] In steps S724 / S725, the UE context is released from the source DU.

[0176] A path switch is performed to the new serving DU (not shown in FIG. 7).

[0177] The termination phase is not expected to be affected in Rel. 18, similar to previous NR mobility enhancements in Rel. 16 and 17.

[0178] To summarise the above, the exemplary embodiments described above with reference to Figures 6 and 7 may generally be regarded as proposing to include conventional LTM procedure validity checks at the CU and / or at the source DU and / or at the UE in order to improve the efficiency of handover preparation and execution for LTM.

[0179] More specifically, it is proposed to perform a validity check by determining whether measurements provided by the UE, preferably by L1 measurements, of a selected candidate target cell (selected based on L3 measurements performed by the UE) satisfy preset conditions. The preset conditions are used to determine whether the selected candidate target cell is known or detectable to the UE, and may include the conditions indicated in NR38.133, clauses 10.1.4 and 10.1.5 for FR1 and FR2, respectively; NR38.133, clauses 10.1.9 and 10.1 for FR1 and FR2, respectively; NR38.133, clauses 10.1.14 and 10.1.15 for FR1 and FR2, respectively; and Annex B.2.3. This is, of course, merely illustrative and shall not be construed as limiting to the scope of the present disclosure.

[0180] Following such a detectability check at the UE, the UE preferably provides to the gNB an L1 measurement report having measurements for candidate target cells that are determined to be detectable at the UE, and more preferably, the UE adds an invalid bit to the L1 measurement report for any candidate target cells that are no longer known or detectable, e.g., due to the long periodicity of the L3 measurements.

[0181] Furthermore, in addition to the validity check for the detectability conditions of the L1 measurements of the selected candidate target cell, the UE preferably checks whether a predetermined time (e.g., 5 seconds) has elapsed since the timestamp associated with the latest L3 measurement report (e.g., the generation of such L3 measurement report). Provided that the predetermined time has not yet expired, the UE preferably provides the gNB with an L1 measurement report having measurements for candidate target cells determined to be detectable at the UE. Thus, it is ensured that the validity or invalidity of candidate target cells is tracked by the UE within the predetermined time since the timestamp of the latest L3 measurement report. In the case of any invalid cell (the L1 measurements do not meet the preset conditions and / or the predetermined time has expired), the UE preferably adds an invalid bit to the L1 measurement report, and the gNB thus receives such an indication informing the gNB which cells are still known and which are no longer known.

[0182] Additionally or alternatively, the gNB preferably also checks whether a predetermined time (e.g., 5 seconds) has elapsed, for example, since the timestamp of the latest L3 measurement or since the gNB received the L3 measurement. That is, in addition to checking whether an invalid bit for any one of the candidate target cells is present in the L1 measurement report received from the UE, the gNB also checks whether the predetermined time has expired when the gNB decides to trigger a cell change for said any one of the candidate target cells. In other words, the gNB is preferably configured to trigger a cell change only for cells that are still detectable, provided that the predetermined time has not expired since the timestamp of the latest L3 measurement. More preferably, the gNB notifies the UE of any failure in the validity check. This ensures that the gNB tracks the validity (and possibly any changes thereto) of candidate target cells indicated by L1 measurement reports received from the UE within the predetermined time. As a result, the T_search time is ensured to be set to 0 for cells that are still detectable within a predetermined time set at the gNB since the timestamp of the L3 measurement report, for example.

[0183] For cells that do not meet the detectability condition for which the timer expires, the T_search time and T_IU time are preferably indicated by the UE so that the gNB is informed of possible interruptions in the handover procedure, leading to improved reliability.

[0184] Furthermore, it is preferably proposed according to the present disclosure to include a plausibility check in the CU before making the HO decision, i.e., the CU is configured to prepare candidate target cells for HO within a predetermined time (e.g., 5 seconds), which leads to a reduced delay in the HO procedure.

[0185] It can be seen that the above method proposed by the present disclosure reduces the impact of the long periodicity of L3 measurements on the certainty of the validity state of candidate target cells (selected based on L3 measurements). For any candidate target cells that are detectable when the gNB triggers the corresponding cell change and / or when the UE performs such a cell change, the T_search time is set to 0, while further ensuring that the UE and gNB keep track of any possible delays induced by candidate target cells that are no longer known or detectable. As a result, the interruption time in LTM handover is reduced with improved certainty and reliability.

[0186] As noted above, in the exemplary embodiments illustrated above (with reference to the figures), although messages communicated / exchanged between network components / elements may appear to have specific / explicit names depending on various implementation aspects (e.g., underlying technologies), these messages may have different names and / or may be communicated / exchanged in different forms / formats, as can be understood and appreciated by those skilled in the art.

[0187] According to some example embodiments, corresponding methods suitable to be implemented by the above-described devices (network elements / components), such as a UE, a CU, a DU, etc., are also provided.

[0188] It should nevertheless be noted that the above-described device features correspond to respective method features that may not, for brevity's sake, however, be explicitly described. The disclosure herein is considered to extend to such method features as well. In particular, it is understood that the disclosure relates to methods of operating the above-described devices and / or providing and / or arranging the respective elements of these devices.

[0189] Furthermore, according to some further exemplary embodiments, there is also provided a respective device (e.g., implementing a UE, CU, DU, etc., as described above), each device including at least one processing circuitry and at least one memory for storing instructions executed by the processing circuitry, the at least one memory and instructions configured to cause the respective device to perform at least the respective steps described above, by the at least one processing circuitry.

[0190] In yet some other exemplary embodiments, respective apparatuses (e.g., implementing a UE, CU, DU, etc., as described above) are provided that comprise respective means configured to perform at least the respective steps described above.

[0191] It is noted that example embodiments of the present disclosure are applicable to a variety of different network configurations. In other words, the examples shown in the above-described figures used as a basis for the above-discussed examples are merely illustrative and do not limit the present disclosure in any way. That is, additional and further existing functions and proposed new functions available in the corresponding operating environment may be used in connection with example embodiments of the present disclosure based on the principles defined.

[0192] It should also be noted that the disclosed exemplary embodiments can be implemented in many ways using hardware and / or software configurations. For example, the disclosed embodiments may be implemented using dedicated hardware and / or hardware in conjunction with software executable on the hardware. The components and / or elements in the figures are merely examples and do not limit the scope of use or functionality of any hardware, software in combination with hardware, firmware, embedded logic component, or combination of two or more such components to implement a particular embodiment of the present disclosure.

[0193] It should further be noted that the description and drawings merely illustrate the principles of the present invention. Those skilled in the art will be able to implement various arrangements, which, although not explicitly described or shown herein, embody the principles of the present disclosure and are included within its spirit and scope. Moreover, all examples and embodiments outlined in this disclosure are expressly intended to be merely illustrative, primarily to help the reader understand the principles of the proposed method. Moreover, all statements herein providing principles, aspects, and embodiments of the present disclosure, as well as specific examples thereof, are intended to encompass equivalents thereof.

Claims

1. A user equipment, UE, configured to connect to a source cell of a first network node supporting a radio access network, a distributed unit of a RAN, a DU function and / or Layer 2 protocol processing, comprising: at least one processor; and at least one memory storing instructions that, when executed by the at least one processor, cause the UE to perform at least: generating L1 measurements for at least one candidate target cell of a plurality of candidate target cells controlled by the first network node or another network node, the candidate target cell being selected based on L3 measurements generated by the UE; determining whether the at least one candidate target cell of the plurality of candidate target cells complies with one or more preset conditions for enabling a UE to perform a handover from the serving source cell to the at least one candidate target cell of the plurality of candidate target cells for L1L2 triggered mobility (LTM); causing the first network node to transmit L1 measurement reports, preferably periodically; the L1 measurement report is provided by the UE based on the determination related to the one or more preset conditions; The one or more preset conditions are: The synchronization signal / PBCH block, SSB of at least one of the plurality of candidate target cells is detectable; and / or a first timer has not expired since a first point in time indicated by a first timestamp associated with an L3 measurement report including the L3 measurement, and the UE is further caused to determine whether the first timer has expired at a second point in time indicated by a second timestamp associated with the L1 measurement report, the L3 measurement report being the most recent L3 measurement report generated by the UE when preparing the L1 measurement report.

2. The UE of claim 1 , wherein the L1 measurement report includes L1 measurements for one or more candidate target cells of the plurality of candidate target cells that are determined to meet the one or more preset conditions.

3. The UE further 3. The UE of claim 1 or 2, further comprising: a UE configured to add an invalid bit to the L1 measurement report for any one of the plurality of candidate target cells that is determined not to meet the one or more preset conditions; and to transmit the L1 measurement report with the added invalid bit to the first network node, preferably periodically.

4. The UE according to any one of claims 1 to 3, wherein the length of the first timer is 5 seconds.

5. 5. The UE of claim 1, further configured to determine whether SSBs of the plurality of candidate target cells are detectable based on at least one of a synchronization signal reference signal received power, SS-RSRP, a CSI reference signal received power, CSI-RSRP, a synchronization signal reference signal received quality, SS-RSRQ, a CSI reference signal received quality, CSI-RSRQ, a synchronization signal signal to interference and noise ratio, SS-SINR, and a CSI signal to interference and noise ratio, CSI-SINR.

6. The UE further 6. The UE according to claim 1, wherein, after receiving a cell change trigger message from the first network node, if any one of the plurality of candidate target cells is determined to not comply with the one or more preset conditions, the UE is caused to send a cell change trigger response message to the first network node.

7. The UE further 7. The UE of claim 6, wherein the cell change trigger response message includes T_search information indicating a first estimated time required to search for any one of the plurality of candidate target cells that is determined not to meet the one or more preset conditions.

8. The UE further 8. The UE of claim 6 or 7, wherein the cell change trigger response message includes T_IU information indicating a second estimated time for interruption uncertainty when acquiring a first available physical random access channel, PRACH, opportunity in any one of the plurality of candidate target cells that is determined not to meet the one or more preset conditions, and preferably the second estimated time is determined based on whether a random access channel, RACH procedure is applied by the UE or a random access channel, RACH-less procedure is applied by the UE.

9. The UE according to any one of claims 6 to 8, wherein the cell change trigger response message is transmitted by lower layer signaling.

10. A first network node configured to support a radio access network, a distributed unit of a RAN, a DU function and / or Layer 2 protocol processing, at least one processor; and at least one memory storing instructions that, when executed by the at least one processor, cause a first network node to: determining whether at least one candidate target cell of a plurality of candidate target cells controlled by a first network node meets one or more preset conditions for a UE to perform a handover from a serving source cell of the first network node to at least one candidate target cell of the plurality of candidate target cells for L1L2 triggered mobility (LTM), wherein the determining is performed by the first network node based on L1 measurements and / or L3 measurements provided by the UE for the plurality of candidate target cells; and a first network node causing the UE to transmit a cell change trigger message based on the determination, the cell change trigger message including instructions for triggering a change to one or more candidate target cells of the candidate target cell determined to meet the one or more preset conditions.

11. The first network node 11. The first network node of claim 10, further configured to: determine, for each candidate target cell of the one or more candidate target cells, whether the each candidate target cell complies with the one or more preset conditions, the one or more preset conditions including: for each candidate target cell, an L1 measurement report including the L1 measurement received from the UE does not include an invalid bit.

12. The first network node and the first network node is further configured to: determine, for each candidate target cell of the one or more candidate target cells, whether the each candidate target cell complies with the one or more preset conditions, wherein the one or more preset conditions include, for each candidate target cell, a second timer has not expired since a first point in time indicated by a first timestamp associated with an L3 measurement report including the L3 measurement received from the UE; and the first network node is further configured to:

12. The first network node according to claim 10 or 11, wherein the first network node is caused to determine whether the second timer has expired at a third time point, the third time point being after receiving an L1 measurement report including the L1 measurements from the UE and before sending the cell change trigger message to the UE, and the L3 measurement report is a latest L3 measurement report generated by the UE when preparing the L1 measurement report.

13. The first network node of claim 12 , further comprising: a second timer configured to transmit the cell change trigger message before the second timer expires.

14. 14. The first network node according to claim 12 or 13, wherein the length of the second timer is 5 seconds.

15. The first network node according to any one of claims 10 to 14, further comprising: a failure indication included in the cell change trigger message for any one candidate target cell of the plurality of candidate target cells that is determined not to comply with the one or more preset conditions.

16. The first network node according to any one of claims 10 to 15, wherein the cell change trigger message is transmitted by lower layer signaling, preferably a MAC Control Element, MAC CE or Downlink Control Information, DCI.

17. a second network node configured to support a radio access network, a central unit of a RAN, a CU function and / or Layer 3 protocol processing, at least one processor; and at least one memory storing instructions that, when executed by the at least one processor, cause a second network node to: determining a first timestamp associated with an L3 measurement report received from a UE, the UE being served by a source cell of a first network node, the L3 measurement report including L3 measurements for a plurality of candidate target cells controlled by the first network node; The first timestamp is included in CU to DU information transmitted to the first network node; A second network node is caused to transmit the CU to DU information having the first timestamp included therein to the first network node.

18. The second network node of claim 17, further configured to transmit the CU to DU information in a UE Context Modify Request message.

19. moreover, 19. The second network node according to claim 17 or claim 18, wherein the second network node is caused to decide to perform preparation of the plurality of candidate target cells in order for the UE to perform handover from the serving source cell to at least one of the plurality of candidate target cells for L1L2 triggered mobility (LTM) before a third timer expires, and the third timer starts at a fourth time point when the second network node receives the L3 measurement report from the UE.

20. 20. The second network node of claim 19, wherein performing the preparation of the plurality of candidate target cells includes one or more of: sending a UE context setup request message to the first network node; generating and sending RRC reconfiguration associated with the plurality of candidate target cells to the first network node; and sending a UE context modification request message to the first network node.

21. 21. The second network node of claim 19 or claim 20, wherein the third timer has a length of 5 seconds.

22. 1. A method for a user equipment, UE, served by a source cell of a first network node supporting a distributed unit, DU functionality and / or Layer 2 protocol processing of a radio access network, comprising: generating L1 measurements for a plurality of candidate target cells controlled by the first network node, the candidate target cells being selected based on L3 measurements generated by the UE; determining whether the plurality of candidate target cells meet one or more preset conditions for the UE to perform handover from the serving source cell to at least one of the plurality of candidate target cells for L1L2 triggered mobility (LTM); transmitting L1 measurement reports to the first network node, preferably periodically; the L1 measurement report is provided by the UE based on the determination related to the one or more preset conditions; The one or more preset conditions are: the synchronization signals / PBCH blocks, SSBs of the plurality of candidate target cells are detectable; and / or a first timer has not expired since a first point in time indicated by a first timestamp associated with an L3 measurement report including the L3 measurement, the method further comprising determining whether the first timer has expired at a second point in time indicated by a second timestamp associated with the L1 measurement report, the L3 measurement report being the most recent L3 measurement report generated by the UE when preparing the L1 measurement report.

23. 1. A method of a first network node configured to support distributed unit, DU functionality and / or Layer 2 protocol processing of a radio access network, comprising: determining whether a plurality of candidate target cells controlled by the first network node or another network node meet one or more preset conditions for a UE to perform a handover from a serving source cell of the first network node to at least one of the plurality of candidate target cells for L1L2 triggered mobility (LTM), wherein the determining is performed by the first network node based on L1 measurements and / or L3 measurements provided by the UE for the plurality of candidate target cells; transmitting a cell change trigger message to the UE based on the determination, the cell change trigger message including an instruction to trigger a change to one or more candidate target cells of the plurality of candidate target cells that are determined to meet the one or more preset conditions.

24. A method of a second network node configured to support a radio access network, a central unit of a RAN, a CU function and / or Layer 3 protocol processing, comprising: determining a first timestamp associated with an L3 measurement report received from a UE, the UE being served by a source cell of a first network node, the L3 measurement report including L3 measurements for a plurality of candidate target cells controlled by the first network node; including the first timestamp in CU to DU information sent to the first network node; and transmitting the CU to DU information having a first timestamp included therein to the first network node.

25. A computer program comprising instructions for causing an apparatus to carry out the method of any one of claims 22 to 24.

26. A memory storing computer readable instructions for causing an apparatus to perform the method of any one of claims 22 to 24.

27. A user equipment, UE, configured to connect to a source cell of a first network node supporting a radio access network, a distributed unit of a RAN, a DU function and / or Layer 2 protocol processing, comprising: at least one processor; and at least one memory storing instructions that, when executed by the at least one processor, cause the UE to perform at least: generating L1 measurements for at least one candidate target cell for possible L1L2 triggered mobility, LTM; determining whether at least one of the at least one candidate target cell meets one or more preset conditions for the UE; generating and transmitting to the first network node an L1 measurement report, the L1 measurement report being provided by the UE based on the determination related to the one or more preset conditions; The one or more preset conditions are: a first timer not expiring relative to a first time point associated with the corresponding L3 measurement; The UE is further configured to determine a timing offset by the end of the first timer and include an indication related to the determined timing offset in the L1 measurement report.

28. 28. The user equipment of claim 27, wherein the indication related to the determined timing offset is included in the L1 measurement report to enable the first network node to decide on a handover to one of the candidate cells in the L1 report before the first timer expires.

29. 28. The user equipment of claim 27, wherein the indication may include an absolute value of the time remaining until the first timer expires.

30. A first network node configured to support a radio access network, a distributed unit of a RAN, a DU function and / or Layer 2 protocol processing, at least one processor; and at least one memory storing instructions that, when executed by the at least one processor, cause a first network node to: receiving, for at least one candidate cell for a possible L1L2 triggered mobility, LTM, from a user equipment connected to a source cell supported by the first network node; causing a first network node to determine whether a handover from the source cell to a candidate cell shall be triggered; a first network node configured to cause the UE to transmit a cell change trigger message based on the determination, the determination being based on one or more preset conditions, one condition including whether a first timer has not expired in relation to a first time point associated with a corresponding L3 measurement.

31. 31. The first network node of claim 30, wherein the indication related to the first point in time is received from the user equipment or a second network node configured to support a radio access network, a central unit of a RAN, a CU function and / or Layer 3 protocol processing before the determination.

32. 32. The first network node of claim 31 , wherein the indication related to the determined timing offset is included in the L1 measurement report to enable the first network node to decide on a handover to one of the candidate cells in the L1 report before the first timer expires.

33. 32. The first network node of claim 31, wherein the indication may include an absolute value of the time remaining until the first timer expires.

34. 32. The first network node of claim 31 , wherein the L1 measurement report includes an instruction for determining a handover of the at least one candidate cell to one candidate cell in the L1 report before expiration of a first timer associated with a timing offset, the first timer being associated with a first time point associated with a corresponding L3 measurement of the at least one candidate cell.

35. a second network node configured to support a radio access network, a central unit of a RAN, a CU function and / or Layer 3 protocol processing, at least one processor; and at least one memory storing instructions that, when executed by the at least one processor, cause a second network node to: determining a first timestamp associated with an L3 measurement report received from a UE, the UE being served by a source cell of a first network node, the L3 measurement report including L3 measurements for at least one candidate target cell for handover; including an indication related to the first timestamp in a message sent to the first network node; A second network node causes the first network node to transmit the message having the first timestamp-related indication included therein.

36. 36. The second network node of claim 35, wherein the message is a message to trigger possible L1L2 triggered mobility, LTM, and to configure the first network node to receive L1 measurement reports from the user equipment.

37. 36. The second network node of claim 35, wherein the indication may include an absolute value of the time remaining until the first timer expires.

38. A user equipment, UE, configured to connect to a source cell of a first network node supporting a radio access network, a distributed unit of a RAN, a DU function and / or Layer 2 protocol processing, comprising: at least one processor; and at least one memory storing instructions that, when executed by the at least one processor, cause the UE to perform at least: generating L1 measurements for at least one candidate target cell for possible L1L2 triggered mobility, LTM; determining whether at least one of the at least one candidate target cell meets one or more preset conditions for the UE; generating and transmitting to the first network node an L1 measurement report, the L1 measurement report being provided by the UE based on the determination related to the one or more preset conditions; The one or more preset conditions are: a first timer not expiring relative to a first time point associated with the corresponding L3 measurement; The UE is further caused to send a timing indication to the first network node; The timing indication relates to the first point in time associated with the corresponding L3 measurement.

39. 39. The user equipment of claim 38, wherein the indication may include an absolute value of the time remaining until the first timer expires or a timestamp of the first point in time.

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