Method and device for supporting LTM cell switching

Abstract

The invention relates to a method and equipment for supporting L1 / L2 triggered mobility (LTM) cell handover. Embodiments of the present disclosure provide a user equipment (UE) comprising: a transceiver; and a processor coupled with the transceiver and configured to: obtain information indicating at least one candidate cell configuration, where each candidate cell is configured with at least one timer associated with a random access channel (RACH) or RACH skip; receiving indication information associated with LTM cell handover to the first candidate cell; and determining whether to perform an LTM cell handover with a RACH procedure or an LTM cell handover without a RACH procedure to the first candidate cell.

Description

Technical Field

[0001] The present disclosure relates to wireless communications, and in particular, to methods and apparatus for supporting L1 / L2 triggered mobility (LTM) cell switching. Background Art

[0002] User equipment (UE) may move from one cell to another and therefore at some point, a serving cell change needs to be performed.

[0003] Currently, serving cell change is performed through explicit radio resource control (RRC) reconfiguration signaling to trigger synchronization of the target cell based on layer 3 (L3) measurement reports, which can result in longer latency, greater overhead, and longer outage time compared to beam-level mobility.

[0004] In the 3rd Generation Partnership Project (3GPP) Release 18, a new work item on additional New Radio (NR) mobility enhancements was approved to implement serving cell changes via lower layer signaling (e.g., Layer 1 (L1) or Layer 2 (L2) signaling) to reduce latency, overhead, and interruption time. During LTM cell handover, failures may occur, so there is a need to detect failures and handle LTM cell handover failures. Summary of the invention

[0005] One objective of the present disclosure is to propose some solutions to support LTM cell handover, such as solutions for failure detection and handling of LTM cell handover.

[0006] An embodiment of the present disclosure provides a UE, comprising: a transceiver; and a processor, which is coupled to the transceiver and configured to: obtain information indicating at least one candidate cell configuration, wherein each candidate cell is configured with at least one timer associated with a random access channel (RACH) or RACH skipping; receive indication information associated with an LTM cell switch to a first candidate cell; and determine whether to perform an LTM cell switch with a RACH procedure or an LTM cell switch without a RACH procedure to the first candidate cell.

[0007] In some embodiments, the processor is further configured to: start a first timer associated with RACH to perform the LTM cell switch with RACH procedure to the first candidate cell.

[0008] In some embodiments, the processor is further configured to: start a second timer associated with RACH skipping to perform the LTM cell switch without RACH procedure to the first candidate cell.

[0009] In some embodiments, the information includes at least one of: a first indicator indicating whether to apply the stored candidate cell configuration when the candidate cell selected by the UE is an LTM candidate cell after at least one of the LTM cell switching with a RACH procedure or the LTM cell switching without a RACH procedure fails; a first timing advance (TA) value for each candidate cell corresponding to each of the at least one candidate cell configuration; or a RACH configuration for obtaining a second TA value for each candidate cell corresponding to each of the at least one candidate cell configuration.

[0010] In some embodiments, the indication information includes at least one of: a second TA value of the first candidate cell; a second indicator indicating whether to apply the stored candidate cell configuration when the candidate cell selected by the UE is an LTM candidate cell after at least one of the LTM cell switching with a RACH procedure or the LTM cell switching without a RACH procedure fails; or a third indicator indicating a timer in the at least one timer associated with the RACH or RACH skipping of the first candidate cell.

[0011] According to the UE of technical solution 1, the processor is further configured to: transmit the LTM cell switching failure without RACH procedure to a source distributed unit (DU).

[0012] In some embodiments, the processor is further configured to: receive a message from the source DU indicating to perform the LTM cell handover with RACH procedure to the first candidate cell.

[0013] In some embodiments, the processor is further configured to: apply a stored candidate cell configuration of a second candidate cell selected by the UE and perform an LTM cell switch to the second candidate cell upon receiving a first indicator or a second indicator, wherein the first indicator or the second indicator indicates that the stored candidate cell configuration is applied when the candidate cell selected by the UE is an LTM candidate cell after at least one of the LTM cell switch with a RACH procedure or the LTM cell switch without a RACH procedure fails.

[0014] In some embodiments, the processor is further configured to: report at least one of: an identifier of the first candidate cell; an identifier of the second candidate cell selected by the UE; a fourth indicator indicating that the LTM cell switching without a RACH procedure or the LTM cell switching with a RACH procedure is performed; or a fifth indicator indicating whether the LTM cell switching without a RACH procedure falls back to the LTM cell switching with a RACH procedure.

[0015] In some embodiments, the processor is further configured to: receive the second TA value of the first candidate cell from a corresponding candidate DU.

[0016] Another embodiment of the present disclosure provides a DU, comprising: a transceiver; and a processor, which is coupled to the transceiver and configured to: transmit information indicating at least one candidate cell configuration to a UE, wherein each candidate cell is configured with at least one timer associated with RACH or RACH skipping for LTM cell switching; and transmit indication information indicating an LTM cell switching to a first candidate cell to the UE.

[0017] In some embodiments, a first timer associated with RACH is used to perform said LTM cell handover with RACH procedure to a first candidate cell.

[0018] In some embodiments, a second timer associated with RACH skipping is used to perform said LTM cell handover to the first candidate cell without RACH procedure.

[0019] In some embodiments, the information further indicates a first indicator indicating whether to apply a stored candidate cell configuration when the candidate cell selected by the UE is a LTM candidate cell after at least one of the LTM cell handover with RACH procedure or the LTM cell handover without RACH procedure fails.

[0020] In some embodiments, the indication information indicates at least one of: a TA value of the first candidate cell; a second indicator indicating whether to apply the stored candidate cell configuration when the candidate cell selected by the UE is an LTM candidate cell after at least one of an LTM cell handover with a RACH procedure to the first candidate cell or an LTM cell handover without a RACH procedure to the first candidate cell fails; or a third indicator indicating a timer in the at least one timer associated with RACH or RACH skipping of the first candidate cell.

[0021] In some embodiments, the processor is further configured to: receive the LTM cell handover failure without RACH procedure from the UE.

[0022] In some embodiments, the processor is further configured to: transmit a message instructing the UE to perform the LTM cell handover with RACH procedure to the first candidate cell.

[0023] In some embodiments, the processor is further configured to: transmit a response message to a central unit (CU) indicating one or more candidate cell configurations associated with the DU; and receive the at least one candidate cell configuration from the CU.

[0024] In some embodiments, the processor is further configured to: receive a report from the CU indicating at least one of: the type of failure; an identifier of the first candidate cell; an identifier of the second candidate cell selected by the UE; a fourth indicator indicating that the LTM cell switch with a RACH procedure or the LTM cell switch without a RACH procedure is performed; or a fifth indicator indicating whether the LTM cell switch without a RACH procedure falls back to the LTM cell switch with a RACH procedure.

[0025] Yet another embodiment of the present disclosure provides a CU comprising: a transceiver; and a processor coupled to the transceiver and configured to: receive one or more candidate cell configurations from one or more DUs, wherein each DU is associated with a set of candidate cell configurations; and transmit information indicating at least one candidate cell configuration for LTM cell switching to a UE via a source DU, wherein each candidate cell is configured with at least one timer associated with RACH or RACH skipping for LTM cell switching.

[0026] In some embodiments, a first timer associated with RACH is used to perform said LTM cell handover with RACH procedure to a first candidate cell.

[0027] In some embodiments, a second timer associated with RACH skipping is used to perform said LTM cell handover to the first candidate cell without RACH procedure.

[0028] In some embodiments, the at least one timer is received from the one or more DUs or determined by the CU.

[0029] In some embodiments, the processor is further configured to: transmit a report of a failure of an LTM cell handover to a first candidate cell, the report comprising at least one of: the type of the failure; an identifier of the first candidate cell; an identifier of a second candidate cell selected by the UE; a fourth indicator indicating that the LTM cell handover with a RACH procedure to the first candidate cell or the LTM cell handover without a RACH procedure to the first candidate cell is performed; or a fifth indicator indicating whether the LTM cell handover without a RACH procedure falls back to the LTM cell handover with a RACH procedure.

[0030] In some embodiments, the processor is further configured to: receive a report of a failure of an LTM cell handover of a first candidate cell, the report comprising at least one of: an identifier of the first candidate cell; an identifier of a second candidate cell selected by the UE; a fourth indicator indicating that an LTM cell handover with a RACH procedure to the first candidate cell or the LTM cell handover without a RACH procedure to the first candidate cell is performed; or a fifth indicator indicating whether the LTM cell handover without a RACH procedure falls back to the LTM cell handover with a RACH procedure.

[0031] Another embodiment of the present disclosure provides a DU, comprising: a transceiver; and a processor, which is coupled to the transceiver and configured to: receive a request message from a CU; and transmit a response message to the CU indicating at least one of the following: at least one candidate cell configuration, wherein the candidate cell configuration corresponds to an LTM candidate cell; at least one timer associated with RACH or RACH skipping of each LTM candidate cell; a first TA value for each LTM candidate cell; or a RACH configuration for obtaining a second TA value for each LTM candidate cell.

[0032] In some embodiments, a first timer associated with the RACH is used to perform an LTM cell handover with a RACH procedure to a first candidate cell.

[0033] In some embodiments, a second timer associated with RACH skipping is used to perform an LTM cell handover without RACH procedure to the first candidate cell.

[0034] In some embodiments, the processor is further configured to: transmit a report associated with a failure of an LTM cell switch to a first candidate cell to the CU, the report comprising at least one of: an identifier of the first candidate cell; an identifier of a second candidate cell; a fourth indicator indicating that the LTM cell switch with a RACH procedure to the first candidate cell or the LTM cell switch without a RACH procedure to the first candidate cell is performed; or a fifth indicator indicating whether the LTM cell switch without a RACH procedure falls back to the LTM cell switch with a RACH procedure.

[0035] In some embodiments, the processor is further configured to: receive a report associated with a failure of an LTM cell switch to a first candidate cell from the CU, the report including at least one of: the type of the failure; an identifier of a second candidate cell selected by the UE; a fourth indicator indicating that the LTM cell switch with a RACH procedure to the first candidate cell or the LTM cell switch without a RACH procedure to the first candidate cell is performed; or a fifth indicator indicating whether the LTM cell switch without a RACH procedure falls back to the LTM cell switch with a RACH procedure.

[0036] Yet another embodiment of the present disclosure provides a method performed by a UE, comprising: obtaining information indicating at least one candidate cell configuration, wherein each candidate cell is configured with at least one timer associated with RACH or RACH skipping; receiving indication information associated with an LTM cell handover to a first candidate cell; and determining whether to perform an LTM cell handover with a RACH procedure to the first candidate cell or an LTM cell handover without a RACH procedure to the first candidate cell.

[0037] Yet another embodiment of the present disclosure provides a method performed by a DU, comprising: transmitting information indicating at least one candidate cell configuration to a UE, wherein each candidate cell is configured with at least one timer associated with RACH or RACH skipping for LTM cell switching; and transmitting indication information indicating an LTM cell switching to a first candidate cell to the UE.

[0038] Yet another embodiment of the present disclosure provides a method performed by a CU, comprising: receiving one or more candidate cell configurations from one or more DUs, wherein each DU is associated with a set of candidate cell configurations; transmitting information indicating at least one candidate cell configuration for LTM cell switching to a UE via a source DU, wherein each candidate cell is configured with at least one timer associated with RACH or RACH skipping for LTM cell switching. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] In order to describe the manner in which the advantages and features of the present application can be obtained, the description of the present application is presented by reference to specific embodiments of the present application illustrated in the accompanying drawings. These drawings depict only example embodiments of the present application and therefore should not be considered to limit its scope.

[0040] Figure 1A A schematic diagram illustrating intra-CU and intra-DU mobility scenarios according to some embodiments of the present disclosure.

[0041] Figure 1B A schematic diagram illustrating intra-CU inter-DU mobility scenarios according to some embodiments of the present disclosure.

[0042] Figure 2 A method for supporting LTM cell switching performed by a UE according to some embodiments of the present disclosure is described.

[0043] Figure 3 A method for supporting LTM cell switching performed by a DU according to some embodiments of the present disclosure is described.

[0044] Figure 4 A method for supporting LTM cell switching performed by a CU according to some embodiments of the present disclosure is described.

[0045] Figure 5 A method for supporting LTM cell switching performed by a DU according to some embodiments of the present disclosure is described.

[0046] Figure 6 A flow chart illustrating a failure detection and handling procedure for LTM cell handover according to some embodiments of the present disclosure.

[0047] Figure 7 A flow chart illustrating a failure detection and handling procedure for LTM cell handover according to some embodiments of the present disclosure.

[0048] Figure 8 A flow chart illustrating a failure detection and handling procedure for LTM cell handover according to some embodiments of the present disclosure.

[0049] Fig. 9 A simplified block diagram illustrating an apparatus according to some embodiments of the present disclosure. DETAILED DESCRIPTION

[0050] The detailed description of the accompanying drawings is intended as a description of the currently preferred embodiments of the present invention and is not intended to represent the only form of practicing the present invention. It should be understood that the same or equivalent functions can be accomplished by different embodiments that are intended to be encompassed within the spirit and scope of the present invention.

[0051] Although the operations are depicted in a particular order in the drawings, those skilled in the art will readily recognize that such operations may not necessarily be performed in the particular order shown or in sequential order or may not necessarily perform all of the illustrated operations to achieve the desired result; one or more operations may sometimes be skipped. In addition, the drawings may schematically depict one or more example processes in the form of a flow chart. However, other operations not depicted may be incorporated into the example processes schematically illustrated. For example, one or more additional operations may be performed before, after, simultaneously with, or between any of the illustrated operations. In some cases, multitasking and parallel processing may be advantageous.

[0052] Reference will now be made in detail to some embodiments of the present disclosure, examples of which are illustrated in the accompanying drawings. To facilitate understanding, embodiments are provided under specific network architectures and new service scenarios (e.g., cellular telephone networks, networks based on time division multiple access (TDMA), networks based on code division multiple access (CDMA), networks based on orthogonal frequency division multiple access (OFDMA), long term evolution (LTE) networks, 3GPP-based networks, LTE, LTE-Advanced (LTE-A), 3GPP 4G, 3GPP 5G NR, 3GPP Release 16 and above, satellite communication networks, high altitude platform networks, etc.). It is expected that with the development of network architectures and new service scenarios, all embodiments in the present disclosure are also applicable to similar technical problems; and in addition, the terms cited in the present disclosure may change, which should not affect the principles of the present disclosure.

[0053] When a UE moves from one cell to another, at some point it is necessary to change the UE's serving cell. The serving cell change can be performed with explicit RRC reconfiguration signaling, which can trigger synchronization of the target cell based on L3 measurement reports. However, this approach can have longer latency, more overhead, and longer outage time compared to beam-level mobility.

[0054] In 3GPP Release 18, a new work item on further NR mobility enhancement (which may be called LTM) was approved to implement serving cell change via L1 or L2 signaling to reduce latency, overhead, and interruption time. Potential application scenarios of LTM may include intra-CU intra-DU mobility and intra-CU inter-DU mobility.

[0055] Figure 1A A schematic diagram illustrating intra-CU and intra-DU mobility scenarios according to some embodiments of the present disclosure.

[0056] Figure 1A The wireless communication system in FIG. 1 includes a DU (e.g., DU 102A), a UE (e.g., UE 101A), and some access nodes (e.g., access node 103A1 and access node 103A2). Access node 103A1 and access node 103A2 are controlled by DU 102A and provide services to UEs in cell #1A and cell #2A, respectively. Figure 1A There is only one UE and two access nodes in the system, but those skilled in the art will recognize that any number of UEs and access nodes may be included in a wireless communication system.

[0057] UE 101A may include a computing device, such as a desktop computer, a laptop computer, a personal digital assistant (PDA), a tablet computer, a smart TV (such as a TV connected to the Internet), a set-top box, a game console, a security system (including a security camera), a car computer, a network device (such as a router, a switch, a modem), or the like. According to an embodiment of the present disclosure, UE 101A may include a portable wireless communication device, a smart phone, a cellular phone, a flip phone, a device with a subscriber identity module, a personal computer, a selective call receiver, or any other device capable of sending and receiving communication signals on a wireless network. In some embodiments, UE 101A includes a wearable device, such as a smart watch, a fitness bracelet, an optical head-mounted display, or the like. In addition, UE 101A may be referred to as a subscriber unit, a mobile device, a mobile station, a user, a terminal, a mobile terminal, a wireless terminal, a fixed terminal, a subscriber station, a user terminal, a device, or other terms used in the art.

[0058] Access nodes 103A1 and 103A2 may be distributed over a geographic region. In some embodiments, access nodes may also be referred to as access points, access terminals, base stations, base stations, macro cells, Node Bs, enhanced Node Bs (eNBs), gNBs, home Node Bs, relay nodes, devices, or any other terminology used in the art. Access nodes 103A1 and 103A2 may generally be part of a radio access network that may include one or more controllers communicatively coupled to one or more corresponding base stations.

[0059] The wireless communication system is compatible with any type of network capable of sending and receiving wireless communication signals. For example, the wireless communication system is compatible with wireless communication networks, cellular telephone networks, TDMA-based networks, CDMA-based networks, OFDMA-based networks, LTE networks, 3GPP-based networks, 3GPP 5G networks, satellite communication networks, high altitude platform networks, and / or other communication networks.

[0060] In one embodiment, the wireless communication system is compatible with NR of the 3GPP protocol, where transmission may be performed using an OFDM modulation scheme. More generally, the wireless communication system may implement some other open or proprietary communication protocols, such as WiMAX, among others.

[0061] In other embodiments, the transmission may be performed using other communication protocols, such as the IEEE 802.11 family of wireless communication protocols. In addition, in some embodiments, the transmission may be performed via a licensed spectrum, while in other embodiments, the transmission may be performed via an unlicensed spectrum. The present disclosure is not intended to be limited to implementing any particular wireless communication system architecture or protocol. In another embodiment, the transmission may be performed using the 3GPP 5G protocol.

[0062] exist Figure 1A In the example, UE 101A is moving from cell #1A to cell #2A and may perform a handover procedure from cell #1A to access cell #2A, which is a handover procedure performed between different cells within a DU. This scenario may be referred to as intra-CU intra-DU mobility. This scenario may be referred to as intra-DU mobility for short.

[0063] Figure 1B A schematic diagram illustrating intra-CU inter-DU mobility scenarios according to some embodiments of the present disclosure.

[0064] Figure 1B The wireless communication system in FIG. 1 includes a CU (e.g., CU 104B), a UE (e.g., UE 101B), and some DUs (e.g., DU 102B1 and DU 102B2). DU 102B1 and DU 102B2 are controlled by CU 104B and provide services to UEs in cell #1B and cell #2B, respectively. Figure 1B There is only one UE and two DUs in the system, but those skilled in the art will recognize that any number of UEs and DUs may be included in a wireless communication system.

[0065] exist Figure 1B In the example, UE#101B is moving from cell#1B to cell#2B and can perform a handover procedure from cell#1B to access cell#2B, which is a handover procedure performed between different cells belonging to different DUs but within the same CU. This scenario can be referred to as intra-CU inter-DU mobility. This scenario can be referred to as inter-DU mobility for short.

[0066] During the serving cell change procedure, it may be important to determine the successful completion of the cell change procedure. In the following of the present disclosure, LTM cell switch may also be referred to as LTM cell handover, LTM cell change or the like. Those skilled in the art know that these expressions may be used interchangeably.

[0067] For LTM cell switching, the following issues need to be resolved:

[0068] Problem 1: Currently, there is no timer for LTM cell switching, and thus no start condition or stop condition of a timer for LTM cell switching. In the following of this disclosure, a timer may also be referred to as a timer value, a value of a timer, or the like.

[0069] Question 2: Since there is no timer for LTM cell switching, it is unclear what the UE behavior is when the timer for LTM cell switching expires.

[0070] Problem 3: LTM cell handover can be initiated by the source DU, and candidate cell configuration can be prepared by the candidate DU. When LTM cell handover fails, the source DU, candidate DU and / or CU need to be enabled to detect and handle LTM radio link failure (RLF). That is, it is important to optimize LTM cell handover decision and / or candidate cell configuration preparation.

[0071] Problem 4: Currently, the UE does not know how to determine whether to perform an LTM cell handover with a RACH procedure or an LTM cell handover without a RACH procedure to a candidate cell. When an LTM RLF occurs, the UE does not know whether to apply the stored candidate cell configuration when the selected cell is an LTM candidate cell.

[0072] The present disclosure proposes some solutions to solve the above problems.

[0073] Figure 2 A method performed by a UE according to some embodiments of the present disclosure is described.

[0074] In operation 201, the UE may obtain information indicating at least one candidate cell configuration, wherein each candidate cell is configured with at least one timer associated with RACH or RACH skipping. In operation 202, the UE may receive indication information associated with an LTM cell handover to a first candidate cell. In operation 203, the UE may determine whether to perform an LTM cell handover with a RACH procedure or an LTM cell handover without a RACH procedure to the first candidate cell. For example, the information may be indicated by an RRC RECONFIGURATION message transmitted from the CU to the UE via a source DU. The indication information may be received from the source DU and may be an LTM cell handover command.

[0075] In some embodiments, the UE may start a first timer associated with RACH to perform an LTM cell handover with RACH procedure to the first candidate cell. In some other embodiments, the UE may start a second timer associated with RACH skipping to perform an LTM cell handover without RACH procedure to the first candidate cell.

[0076] In some embodiments, the information transmitted from the CU to the UE via the source DU may include at least one of the following:

[0077] - a first indicator indicating whether to apply the stored candidate cell configuration when the candidate cell selected by the UE is the LTM candidate cell after at least one of the LTM cell handover with RACH procedure or the LTM cell handover without RACH procedure fails;

[0078] - a first TA value for each candidate cell corresponding to each of at least one candidate cell configuration; or

[0079] - A RACH configuration for acquiring a second TA value for each candidate cell corresponding to each of the at least one candidate cell configuration.

[0080] In some embodiments, the indication information transmitted from the source DU to the UE may include at least one of the following:

[0081] - a second TA value of the first candidate cell;

[0082] - a second indicator indicating whether to apply the stored candidate cell configuration when the candidate cell selected by the UE is the LTM candidate cell after at least one of the LTM cell handover with RACH procedure or the LTM cell handover without RACH procedure fails; or

[0083] - a third indicator indicating a timer of at least one timer associated with RACH or RACH skipping of the first candidate cell.

[0084] In some embodiments, the UE may transmit an LTM cell handover failure without a RACH procedure to the source DU.

[0085] In some embodiments, the UE may receive a message from the source DU indicating to perform an LTM cell handover with a RACH procedure to the first candidate cell.

[0086] In some embodiments, the UE may apply a stored candidate cell configuration of a second candidate cell selected by the UE and perform an LTM cell switch to the second candidate cell upon receiving a first indicator or a second indicator, wherein the first indicator or the second indicator indicates that the stored candidate cell configuration is applied when the candidate cell selected by the UE is an LTM candidate cell after at least one of an LTM cell switch with a RACH procedure or an LTM cell switch without a RACH procedure fails.

[0087] In some embodiments, the UE may report at least one of the following:

[0088] - an identifier of the first candidate cell, such as a global cell identifier of the first candidate cell;

[0089] - an identifier of a second candidate cell selected by the UE;

[0090] - a fourth indicator indicating that an LTM cell handover without a RACH procedure or an LTM cell handover with a RACH procedure is performed; or

[0091] - A fifth indicator indicating whether the LTM cell handover without RACH procedure falls back to the LTM cell handover with RACH procedure.

[0092] In some embodiments, the above information may be transmitted to the CU via the DU, and the DU may be associated with the second candidate cell selected by the UE, which may include the source DU or the candidate DU. Alternatively, the above information may be transmitted to the DU, and the DU may transmit the information to the CU.

[0093] In some embodiments, the UE may receive the second TA value of the first candidate cell from the corresponding candidate DU (ie, from the candidate DU that manages the first candidate cell).

[0094] Figure 3 A method performed by a DU (eg, a source DU) according to some embodiments of the present disclosure is described.

[0095] In operation 301, the source DU may transmit information indicating at least one candidate cell configuration to the UE, wherein each candidate cell is configured with at least one timer associated with RACH or RACH skipping for LTM cell switching. In operation 302, the source DU may transmit indication information indicating LTM cell switching to a first candidate cell to the UE. For example, the information may be indicated by an RRC RECONFIGURATION message transmitted from the CU to the UE via the source DU. The indication information may be received from the source DU and may be an LTM cell switching command.

[0096] In some embodiments, a first timer associated with RACH is used to perform an LTM cell handover with RACH procedure to a first candidate cell. In some embodiments, a second timer associated with RACH skipping is used to perform an LTM cell handover without RACH procedure to a first candidate cell.

[0097] In some embodiments, the information further indicates at least one first indicator, wherein each first indicator is associated with a corresponding one of the at least one candidate cell configuration and indicates whether the stored candidate cell configuration is applied when the candidate cell selected by the UE is an LTM candidate cell after at least one of an LTM cell switching with a RACH procedure or an LTM cell switching without a RACH procedure fails.

[0098] In some embodiments, the indication information indicates at least one of the following:

[0099] -TA value of the first candidate cell;

[0100] - a second indicator indicating whether to apply the stored candidate cell configuration when the candidate cell selected by the UE is the LTM candidate cell after at least one of an LTM cell handover with a RACH procedure to the first candidate cell or an LTM cell handover without a RACH procedure to the first candidate cell fails; or

[0101] - a third indicator indicating a timer of at least one timer associated with RACH or RACH skipping of the first candidate cell.

[0102] In some embodiments, the source DU may receive an LTM cell handover failure without RACH procedure from the UE, for example, an LTM RLF report.

[0103] In some embodiments, the source DU may transmit a message instructing the UE to perform an LTM cell handover with a RACH procedure to the first candidate cell.

[0104] In some embodiments, the source DU may transmit a response message to the CU indicating one or more candidate cell configurations associated with the DU and receive at least one candidate cell configuration from the CU.

[0105] In some embodiments, the source DU may receive a report from the CU indicating at least one of the following:

[0106] - Type of failure, such as too late LTM cell handover, too early LTM cell handover or LTM cell handover to the wrong cell;

[0107] - an identifier of the first candidate cell, such as a global cell identifier of the first candidate cell;

[0108] - an identifier of a second candidate cell selected by the UE;

[0109] - a fourth indicator indicating that LTM cell handover with RACH procedure or LTM cell handover without RACH procedure is performed; or

[0110] - A fifth indicator indicating whether the LTM cell handover without RACH procedure falls back to the LTM cell handover with RACH procedure.

[0111] Figure 4 A method performed by a CU according to some embodiments of the present disclosure is described.

[0112] In operation 401, the CU may receive one or more candidate cell configurations from one or more DUs, wherein each DU is associated with a set of candidate cell configurations. In operation 402, the CU may transmit information indicating at least one candidate cell configuration for LTM cell switching to the UE via a source DU, wherein each candidate cell is configured with at least one timer associated with RACH or RACH skipping for LTM cell switching.

[0113] In some embodiments, at least one timer is received from one or more DUs or determined by the CU.

[0114] In some embodiments, the CU may transmit a report of a LTM cell handover failure to the first candidate cell, the report comprising at least one of the following:

[0115] - Type of failure;

[0116] - an identifier of the first candidate cell, such as a global cell identifier of the first candidate cell;

[0117] - an identifier of a second candidate cell selected by the UE;

[0118] - a fourth indicator indicating that an LTM cell handover with a RACH procedure to the first candidate cell or an LTM cell handover without a RACH procedure to the first candidate cell is performed; or

[0119] - A fifth indicator indicating whether the LTM cell handover without RACH procedure falls back to the LTM cell handover with RACH procedure.

[0120] In some embodiments, the CU may receive a report of a LTM cell handover failure of the first candidate cell, the report including at least one of the following:

[0121] - an identifier of the first candidate cell, such as a global cell identifier of the first candidate cell;

[0122] - an identifier of a second candidate cell selected by the UE;

[0123] - a fourth indicator indicating that an LTM cell handover with a RACH procedure to the first candidate cell through a RACH procedure or an LTM cell handover without a RACH procedure to the first candidate cell is performed; or

[0124] - A fifth indicator indicating whether the LTM cell handover without RACH procedure falls back to the LTM cell handover with RACH procedure.

[0125] Figure 5 A method performed by a DU (eg, a candidate DU) according to some embodiments of the present disclosure is described.

[0126] In operation 501, the candidate DU may receive a request message from the CU; and in operation 502, the candidate DU may transmit a response message to the CU indicating at least one of the following:

[0127] - at least one candidate cell configuration, wherein the candidate cell configuration corresponds to an LTM candidate cell;

[0128] - at least one timer associated with RACH or RACH skipping for each LTM candidate cell;

[0129] - the first TA value of each LTM candidate cell; or

[0130] - RACH configuration, which is used to obtain the second TA value for each LTM candidate cell.

[0131] In some embodiments, the candidate DU may transmit a report associated with a failure of an LTM cell handover to a first candidate cell to the CU, the report comprising at least one of:

[0132] - an identifier of the first candidate cell;

[0133] - an identifier of a second candidate cell;

[0134] - a fourth indicator indicating that an LTM cell handover with a RACH procedure to the first candidate cell or an LTM cell handover without a RACH procedure to the first candidate cell is performed; or

[0135] - A fifth indicator indicating whether the LTM cell handover without RACH procedure falls back to the LTM cell handover with RACH procedure.

[0136] In some embodiments, the candidate DU may receive a report from the CU associated with a failure of an LTM cell handover to the first candidate cell, the report comprising at least one of:

[0137] - Type of failure, such as too late LTM cell handover, too early LTM cell handover or LTM cell handover to the wrong cell;

[0138] - an identifier of a second candidate cell selected by the UE;

[0139] - a fourth indicator indicating that an LTM cell handover with a RACH procedure to the first candidate cell or an LTM cell handover without a RACH procedure to the first candidate cell is performed; or

[0140] - A fifth indicator indicating whether the LTM cell handover without RACH procedure falls back to the LTM cell handover with RACH procedure.

[0141] Figure 6 A flow chart illustrating a failure detection and handling procedure for LTM cell handover according to some embodiments of the present disclosure.

[0142] exist Figure 6 In the process, there are UE, source DU (which may also be called serving DU), one or more candidate DUs (which may also be called Figure 6The source DU and one or more candidate DUs are within the CU, and the UE is moving from the source DU to one of the one or more candidate DUs. The source DU may manage (or control) the serving cell and one or more source cells of the UE. The serving cell and one or more source cells are considered to be in the source DU. Each candidate DU in the candidate DUs may manage one or more candidate cells, and one or more candidate cells are considered to be in each candidate DU. One of the one or more candidate DUs manages the candidate cell to which the UE switches. In some embodiments, the candidate cell to which the UE switches may be referred to as a "target cell" or a "candidate target cell".

[0143] The handover mode (e.g., LTM cell handover without RACH procedure to the candidate cell (or LTM cell handover with RACH skipping procedure) or LTM cell handover with RACH procedure to the candidate cell) may be determined by the source DU. The expression "RACH skipping" may also be referred to as no RACH, non-RACH, no RACH or the like, which may refer to a procedure for accessing a candidate cell without a RACH procedure.

[0144] In operation 601, the UE may transmit a message (e.g., a UE CONTEXT SETUP REQUEST message) indicating a number of candidate cell IDs (simply represented as K candidate cell IDs) to one candidate DU when the K candidate cells are managed by one candidate DU (or to multiple candidate DUs when the K candidate cells are managed by multiple candidate DUs). Hereinafter, one candidate DU is used to describe the solution, and those skilled in the art know that the solution may also be applicable to scenarios including multiple candidate DUs. The message may request preparation of the K candidate cells controlled by the candidate DU to create a UE context and set up one or more data bearers.

[0145] In some embodiments, a UE CONTEXT SETUP REQUEST message may be sent for each candidate cell. That is, the UE may transmit K UE CONTEXT SETUP REQUEST messages, where each UE CONTEXT SETUP REQUEST message may indicate a candidate cell ID.

[0146] In some other embodiments, the UE CONTEXT SETUP REQUEST message may include multiple candidate cell IDs (eg, a list of K candidate cell IDs) and be transmitted to the candidate DUs controlling the multiple candidate cells.

[0147] In operation 602, the preparation request may be accepted and the candidate DU may transmit a response message to the CU, such as a UECONTEXT SETUP RESPONSE message. The UECONTEXT SETUP RESPONSE message may indicate the candidate cell IDs in the candidate DU requested by the CU and the candidate cell configuration of each candidate cell.

[0148] The UE CONTEXT SETUP RESPONSE message may include at least one timer for a candidate cell, and each candidate cell may be associated with at least one timer. At least one timer may be used for failure detection and handling of the UE, and the following types of timers may exist:

[0149] 1. Timer for RACH. This timer can be used for LTM cell switching with RACH procedure. In the following of this disclosure, the timer for RACH may be referred to as the first timer. The UE CONTEXT SETUP RESPONSE message may indicate the timer value used during the LTM cell switching to the corresponding candidate cell using the RACH-based mechanism. The timer value may be enumerated, such as 50ms, 100ms, 150ms. Starting condition of the timer for RACH: After the LTM cell switching command is successfully received, the LTM cell switching command may be received from the source DU and may instruct the UE to perform an LTM cell switching with a RACH procedure. Alternatively, if the UE can send a hybrid automatic repeat request-acknowledgement (HARQ-ACK) for the LTM cell switching command, the UE may start the timer for RACH when the UE applies the LTM cell switching command.

[0150] Stop condition of the timer for RACH: after the random access procedure to the corresponding candidate cell is successfully completed, for example, when the MAC layer successfully completes the random access procedure.

[0151] 2. Timer for RACH skipping. This timer can be used for LTM cell handover without RACH procedure. In the following of the present disclosure, the timer for RACH skipping may be referred to as the second timer. The UE CONTEXT SETUP RESPONSE message may indicate the timer value used during the LTM cell handover to the corresponding candidate cell using the RACH-based mechanism. The timer value may be enumerated, such as 10ms, 20ms, 50ms, etc.

[0152] Since there is no RACH procedure in LTM cell switching, the timer value of RACH skipping can be shorter than the timer value of RACH.

[0153] Starting condition of the timer for RACH skipping: After the LTM cell switching command is successfully received, the LTM cell switching command can be received from the source DU and instructs the UE to perform an LTM cell switching without a RACH procedure. Alternatively, if the UE can send a HARQ-ACK for the LTM cell switching command, the UE can start the timer for RACH skipping when the UE applies the LTM cell switching command.

[0154] Stop condition of the timer for RACH skipping: after the procedure for accessing the corresponding candidate cell is successfully completed, for example, when MAC indicates successful reception of the Physical Downlink Control Channel (PDCCH) transmission address of the cell Radio Network Temporary Identifier (C-RNTI).

[0155] In some embodiments, a UE CONTEXT SETUP RESPONSE message may be sent for each candidate cell. That is, the candidate DU may transmit K UE CONTEXT SETUP RESPONSE messages, wherein each UE CONTEXT SETUP RESPONSE message may indicate the candidate cell configuration and a corresponding timer, wherein the corresponding timer may include the first timer, the second timer, or both.

[0156] In some other embodiments, the UE CONTEXT SETUP RESPONSE message may include multiple candidate cell configurations and corresponding timers. For example, the candidate DU may transmit all candidate cell configurations and corresponding timers with one UE CONTEXT SETUP RESPONSE message.

[0157] In some cases, the UE CONTEXT SETUP RESPONSE message from the candidate DU may only include the candidate cell configuration and may not include the corresponding timer. The CU may determine the corresponding timer for the corresponding candidate cell.

[0158] In operation 603, the CU may send a message to the source DU, such as a DL RRC MESSAGE TRANSFER message (or a UECONTEXT MODIFICATION REQUEST message or other message), which may include the candidate cell configuration, the timer, and the generated RRC RECONFIGURATION message.

[0159] If the candidate DU has transmitted timers to the CU in operation 602, the CU may incorporate these received timers into a message (e.g., a DL RRC MESSAGE TRANSFER message) and the generated RRC RECONFIGURATION message. These received timers are per MAC layer or may be referred to as per cell, and each candidate cell may be associated with its own corresponding timer. The timers for different candidate cells may be different.

[0160] If the candidate DU has not transmitted the timer to the CU in operation 602, the CU may determine the timer and may incorporate the determined timer into a message (eg, DL RRC MESSAGE TRANSFER message) and an RRC RECONFIGURATION message. The determined timer is per RRC layer and all candidate cells have the same timer.

[0161] The RRC RECONFIGURATION message may further include one or more indicators (hereinafter referred to as first indicators for clarity), which may indicate whether to apply the stored candidate cell configuration when the selected cell is an LTM candidate cell after the LTM cell handover failure, wherein the UE has stored the candidate cell configuration of the selected cell. For example, the one or more first indicators may be: an enumeration value "true", which indicates that the stored candidate cell configuration is applied; or an enumeration value "false", which indicates that the stored candidate cell configuration is not applied. Other values ​​such as "1", "0" or the like may also be used to indicate whether to apply the stored candidate cell configuration when the selected cell is an LTM candidate cell after the LTM cell handover failure.

[0162] In some embodiments, there may be one first indicator in the RRC RECONFIGURATION message, and the first indicator may be applicable to all candidate cells.

[0163] In some other embodiments, there may be multiple first indicators, and each first indicator is associated with a candidate cell. When more than one candidate cell is associated with a first indicator with a value of "true", the UE may select one from the candidate cells based on UE implementation after LTM cell handover failure.

[0164] In operation 604, the source DU may forward a message (eg, an RRC RECONFIGURATION message) to the UE, and the RRC RECONFIGURATION message may include: 1) candidate cell configurations; and 2) a timer associated with each candidate cell configuration. In addition, the RRC RECONFIGURATION message may include one or more first indicators.

[0165] In operation 605, the UE may transmit a message (eg, an RRC RECONFIGURATION COMPLETE message) to the source DU as a response to the RRC RECONFIGURATION message.

[0166] In operation 606, the source DU may forward the RRC RECONFIGURATION COMPLETE message to the CU via a UL RRC MESSAGE TRANSFER message (or a UE CONTEXT MODIFICATION RESPONSE message or other message).

[0167] In operation 607 , the UE may start reporting L1 measurements of the serving cell, the source cell, and / or the candidate cell.

[0168] In operation 608, the source DU may determine that an LTM cell handover to a candidate cell is required.

[0169] In operation 609, the source DU may send an LTM cell switching command to the UE to trigger the UE to change from the current serving cell to a candidate cell, which for clarity is referred to as candidate cell 1 hereinafter.

[0170] The LTM cell handover command may include an indicator (hereinafter referred to as the second indicator for clarity) indicating whether to perform an LTM cell handover with a RACH procedure or an LTM cell handover without a RACH procedure to the candidate cell 1.

[0171] In some embodiments, the source DU may determine whether to perform an LTM cell handover with a RACH procedure or an LTM cell handover without a RACH procedure to the candidate cell 1 based on whether the candidate cell 1 has an available TA. If the source DU obtains an early TA of the candidate cell 1, the source DU may determine that the UE may perform an LTM cell handover without a RACH procedure to the candidate cell 1. The source DU may further include a TA value of the candidate cell 1 in the LTM cell handover command. If the source DU does not obtain a TA of the candidate cell 1, the source DU may determine that the UE may perform an LTM cell handover, RACH procedure for the candidate cell 1.

[0172] The second indicator may be separated from the TA value of candidate cell 1. In other words, the second indicator and the TA value of candidate cell 1 may be two different fields, two different indicators, or two different components.

[0173] Alternatively, the second indicator may be a TA value of candidate cell 1. That is, the TA value may be used to determine whether to perform an LTM cell handover with a RACH procedure or an LTM cell handover without a RACH procedure to candidate cell 1. For example, if the TA value is set (e.g., has a non-zero value), the UE may perform an LTM cell handover without a RACH procedure to candidate cell 1. If the TA value is empty (e.g., has a zero value), the UE may perform a RACH procedure for candidate cell 1.

[0174] The LTM cell handover command may include another indicator (hereinafter referred to as the third indicator for clarity), which may indicate whether to apply the stored candidate cell configuration when the selected cell is an LTM candidate cell after the LTM cell handover fails. For example, the third indicator may be: an enumeration value of "true", which indicates that the stored candidate cell configuration is applied; or an enumeration value of "false", which indicates that the stored candidate cell configuration is not applied. Other values ​​such as "1", "0" or the like may also be used to indicate whether to apply the stored candidate cell configuration when the selected cell is an LTM candidate cell after the LTM cell handover fails.

[0175] In some embodiments, the LTM cell switching command may be a medium access control element (MAC CE) command, and the MAC CE may include at least one of the above indicators: 1) the second indicator; 2) the TA value of candidate cell 1; or 3) the third indicator.

[0176] In operation 610 , the UE may perform LTM cell handover to the candidate cell 1 .

[0177] The second indicator in the LTM cell switching command may indicate that the UE performs an LTM cell switching with a RACH procedure to the candidate cell 1. Alternatively, the second indicator may indicate that the TA value of the candidate cell 1 is 0, the UE may start a timer for RACH (which is associated with the candidate cell 1) after successfully receiving the LTM cell switching command and may perform a random access procedure on the candidate cell 1 by starting a preamble (i.e., the first step of the RACH procedure). Alternatively, if the UE sends a HARQ-ACK for the LTM cell switching command in the physical layer, the UE will start a timer for RACH when the UE applies the LTM cell switching command.

[0178] Alternatively, the second indicator in the LTM cell switching command may indicate that the UE performs an LTM cell switching without a RACH procedure to the candidate cell 1. Alternatively, the second indicator may indicate that the TA value of the candidate cell 1 may be a non-zero value, the UE may start a timer for RACH skipping (which is associated with the candidate cell 1) after successfully receiving the LTM cell switching command and may perform an access procedure (i.e., without a RACH procedure) on the candidate cell 1 by sending an initial physical uplink shared channel (PUSCH) transmission to the candidate cell 1. Alternatively, if the UE may send a HARQ-ACK for the LTM cell switching command in the physical layer, the UE may start a timer for RACH skipping when the UE applies the LTM cell switching command.

[0179] During the LTM cell handover to the candidate cell 1, the UE may determine whether the LTM cell handover is successful.

[0180] For LTM cell handover with RACH procedure to candidate cell 1, the UE may stop the timer for RACH when random access on candidate cell 1 is successfully completed. For example, the UE may stop the timer for RACH when the MAC layer successfully completes the random access procedure.

[0181] For LTM cell handover without RACH procedure to candidate cell 1, the UE may stop the timer for RACH skipping if RACH skipping-based access on candidate cell 1 is successfully completed. For example, when the MAC layer indicates that the PDCCH transmission address of the cell radio network temporary identifier (C-RNTI) of candidate cell 1 is successfully received, the UE may stop the timer for RACH skipping.

[0182] Otherwise, when the timer (the timer for RACH or the timer for RACH skipping) expires, the UE may determine that the LTM cell switching has failed.

[0183] Due to different types of failures, the UE may take different actions.

[0184] Case 1: The UE performs LTM cell handover without RACH procedure to candidate cell 1, and the LTM cell handover without RACH procedure has failed, the UE may perform the following operations 611 to 613.

[0185] In operation 611, the UE may notify the source DU of LTM cell handover failure information via MAC CE.

[0186] In operation 612, the source DU may send another LTM cell switching command (i.e., an LTM cell switching command different from the LTM cell switching command in operation 609) to the UE, which may trigger the UE to perform an LTM cell switching with a RACH procedure from the current serving cell to the candidate cell 1. That is, the LTM cell switching without a RACH procedure falls back to the LTM switching with a RACH procedure.

[0187] The LTM cell handover command may include a second indicator. The value of the second indicator may be set to a value indicating that the UE performs an LTM cell handover and a RACH procedure on the candidate cell 1.

[0188] The LTM cell handover command may include a third indicator, which may indicate whether to apply the stored candidate cell configuration when the selected cell is an LTM candidate cell after the LTM cell handover fails. For example, the third indicator may be an enumeration value of "true".

[0189] In some embodiments, the LTM cell switching command may be a MAC CE command, and the MAC CE may include at least one of the above indicators: 1) the second indicator; or 2) the third indicator.

[0190] In operation 613, the UE may perform an access procedure to the candidate cell 1 using a RACH-based mechanism, such as an LTM cell handover with a RACH procedure.

[0191] Case 2: The UE performs an LTM cell handover with a RACH procedure to candidate cell 1, and the LTM cell handover with a RACH procedure has failed. In addition, the UE has received a third indicator indicating that the stored candidate cell configuration should be applied when the selected cell is an LTM candidate cell after the LTM cell handover fails. For example, the third indicator may be an enumeration value of "true". The UE may perform the following operations 614 and 615.

[0192] In operation 614, the LTM cell handover with RACH procedure has failed, for example, the UE has performed a RACH procedure on candidate cell 1 and the timer for RACH has expired. The UE may perform a cell selection procedure to select a suitable cell. If the selected cell (hereinafter referred to as candidate cell 2 for clarity) is an LTM candidate cell, then based on the third indicator, the UE may apply a stored candidate cell configuration corresponding to the selected cell.

[0193] Otherwise, the UE may initiate the RRC re-establishment procedure.

[0194] In operation 615 , the UE may perform an access procedure on the candidate cell 2 .

[0195] In operation 616 , the UE may send an LTM RLF report to the candidate DU managing the candidate cell 2 .

[0196] The LTM RLF report may include at least one of the following:

[0197] - An identifier of a candidate cell selected by the UE, such as candidate cell 2, which may indicate that the UE selects a candidate cell for LTM based recovery, such as the ID of candidate cell 2. The UE may set the LTM cell ID of candidate cell 2 to the global cell identifier of candidate cell 2 (if available), otherwise to the physical cell identifier and carrier frequency of candidate cell 2.

[0198] - An identifier of the failed candidate cell 1, such as a global cell identifier or a physical cell identifier and a carrier frequency.

[0199] - Access mechanism of failed candidate cell 1, such as with RACH procedure or without RACH procedure.

[0200] - A fifth indicator indicating whether the LTM cell handover without RACH procedure to the candidate cell 1 falls back to the LTM cell handover with RACH procedure during the procedure of accessing to the candidate cell 1.

[0201] In operation 617, the candidate DU may forward the LTM RLF report to the CU, for example, via a UL RRC MESSAGETRANSFER message.

[0202] Figure 7 A flow chart illustrating a failure detection and handling procedure for LTM cell handover according to some other embodiments of the present disclosure.

[0203] Figure 7 Contains Figure 6 The same components as described in and the details are omitted here.

[0204] The handover mode (eg, LTM cell handover without RACH procedure to a candidate cell (or LTM cell handover with RACH skipping procedure) or LTM cell handover with RACH procedure to a candidate cell) may be determined by the UE itself.

[0205] Operation 701 is similar to Figure 6 Operation 601 described in , and the details are omitted here.

[0206] In operation 702, the preparation request may be accepted, and the candidate DU may transmit a response message to the CU, such as a UECONTEXT SETUP RESPONSE message. In addition to the contents included in the UE CONTEXT SETUP RESPONSE message (e.g., 1) the candidate cell ID requested from the CU; 2) the candidate cell configuration of each candidate cell; and 3) the timer of each candidate cell, including a timer for RACH and / or a timer for RACH skipping, it is similar to Figure 6 In addition to the UE CONTEXT SETUP RESPONSE message described in operation 602 of , the UE CONTEXT SETUP RESPONSE message may further include: 4) a first TA for each candidate target cell; and / or 5) a RACH configuration for acquiring a second TA for each candidate target cell. The value of the first TA may be 100 ms or the like, and the value of the second TA may be 20 ms or the like.

[0207] The RACH configuration for acquiring the second TA of each candidate target cell may include at least one of the following:

[0208] -RACH opportunity: indicates the time-frequency domain resources used for preamble transmission.

[0209] - Synchronization Signal and Physical Broadcast Channel (SS / PBCH) Index: Indicates the SS / PBCH that can be used to determine the RACH opportunity for PRACH transmission.

[0210] - Physical Random Access Channel (PRACH) Mask Index: Indicates the RACH opportunity associated with the SS / PBCH indicated by the "SS / PBCH Index" for PRACH transmission.

[0211] In some embodiments, a UE CONTEXT SETUP RESPONSE message may be sent for each candidate cell. That is, the CU may transmit K UE CONTEXT SETUP RESPONSE messages, each of which may indicate a candidate cell configuration and a corresponding timer, wherein the corresponding timer may include the first timer, the second timer, or both.

[0212] In some other embodiments, the UE CONTEXT SETUP RESPONSE message may include multiple candidate cell configurations and corresponding timers.

[0213] In some cases, the UE CONTEXT SETUP RESPONSE message from the candidate DU may only include the candidate cell configuration and may not include the corresponding timer. The CU may determine the corresponding timer for the corresponding candidate cell.

[0214] In operation 703, the CU may send a message (e.g., a DL RRC MESSAGE TRANSFER message) (or a UECONTEXT MODIFICATION REQUEST message or other message) to the source DU, which may include the candidate cell configuration and the generated RRC RECONFIGURATION message. In some cases, the DL RRC MESSAGE TRANSFER message may also include a timer associated with each candidate cell configuration.

[0215] In operation 704, the source DU may forward a message (eg, an RRC RECONFIGURATION message) to the UE, and the RRC RECONFIGURATION message may include: 1) candidate cell configurations; and 2) a timer associated with each candidate cell configuration.

[0216] If the candidate DU has transmitted timers to the CU in operation 702, the CU may incorporate these received timers into the generated RRC RECONFIGURATION message. These received timers are per MAC layer or may be referred to as per cell, and each candidate cell may be associated with its own corresponding timer. The timers for different candidate cells may be different.

[0217] If the candidate DU has not transmitted the timer to the CU in operation 702, the CU may determine the timer and may incorporate the determined timer into the generated RRC RECONFIGURATION message. The determined timer is per RRC layer, and all candidate cells have the same timer.

[0218] The RRC RECONFIGURATION message may further include one or more first indicators, which may indicate whether to apply the stored candidate cell configuration when the selected cell is an LTM candidate cell after the LTM cell handover failure, wherein the UE has stored the candidate cell configuration of the selected cell. For example, the one or more first indicators may be an enumeration value "true", which indicates that the stored candidate cell configuration is applied, and an enumeration value "false" indicates that the stored candidate cell configuration is not applied. Other values ​​such as "1", "0" or the like may also be used to indicate whether to apply the stored candidate cell configuration when the selected cell is an LTM candidate cell after the LTM cell handover failure.

[0219] In some embodiments, there may be one first indicator in the RRC RECONFIGURATION message, and the first indicator may be applicable to all candidate cells.

[0220] In some other embodiments, there may be multiple first indicators, and each first indicator is associated with a candidate cell. When more than one candidate cell is associated with a first indicator with a value of "true", the UE may select one from the candidate cells based on UE implementation after LTM cell handover failure.

[0221] The RRC RECONFIGURATION message may further include a first TA of each candidate target cell (received from the candidate DU in operation 702) and / or a RACH configuration for acquiring a second TA of each candidate target cell (received from the candidate DU in operation 702).

[0222] In operation 705, the UE may transmit a message (eg, an RRC RECONFIGURATION COMPLETE message) to the source DU as a response to the RRC RECONFIGURATION message.

[0223] In operation 706, the source DU may forward the RRC RECONFIGURATION COMPLETE message to the CU via a UL RRC MESSAGE TRANSFER message (or a UE CONTEXT MODIFICATION RESPONSE message or other message).

[0224] In some embodiments, the UE may use the first TA of the candidate cell to access the candidate cell. In some other embodiments, the first TA may not be so accurate, for example, the UE's position has changed a lot since receiving the first TA, and therefore the UE may acquire the second TA of the candidate cell based on the RACH configuration for acquiring the second TA of each candidate target cell. More specifically, the UE may perform operations 707 and 708. In operation 707, the UE may send a random access preamble to some or all target cells (e.g., candidate cell 1) in the candidate DU. In operation 708, the candidate DU may send a request to the UE for the second TA of the candidate target cell.

[0225] In operation 709 , the UE may start reporting L1 measurements of the serving cell, the source cell, and / or the candidate cell.

[0226] In operation 710, the source DU may determine that an LTM cell handover to a candidate cell is required.

[0227] In operation 711 , the source DU may send an LTM cell switching command (eg, a MAC CE command) to the UE to trigger the UE to change from the current serving cell to the candidate cell 1 .

[0228] After receiving the LTM cell handover command, the UE may determine whether to perform an LTM cell handover with a RACH procedure or an LTM cell handover without a RACH procedure to the candidate cell 1.

[0229] If the RRC RECONFIGURATION message transmitted to the UE in operation 704 includes the first TA of the candidate cell 1 or the UE obtains the second TA of the candidate cell 1 in operation 708 or the UE obtains both the first TA and the second TA of the candidate cell 1, the UE may determine to perform an LTM cell handover without a RACH procedure to the candidate cell. Otherwise, the UE may determine to perform an LTM cell handover with a RACH procedure to the candidate cell 1.

[0230] If the UE determines to perform the LTM cell handover without the RACH procedure, then in operation 712 , the UE may perform the LTM cell handover without the RACH procedure to the candidate cell 1 .

[0231] The UE may start a timer for RACH skipping (which is associated with candidate cell 1) after successfully receiving the LTM cell switching command and may perform an access procedure (i.e., without a RACH procedure) on candidate cell 1 by sending an initial physical uplink shared channel (PUSCH) transmission to candidate cell 1. Alternatively, if the UE may send a HARQ-ACK for the LTM cell switching command in the physical layer, the UE may start a timer for RACH skipping when the UE applies the LTM cell switching command.

[0232] During the LTM cell handover without RACH procedure to candidate cell 1, the UE may determine whether the LTM cell handover is successfully completed. For example, when the MAC layer indicates that the PDCCH transmission address of the C-RNTI of candidate cell 1 is successfully received, the UE may stop the timer for RACH skipping and determine that the LTM cell handover is successfully completed.

[0233] Otherwise, when the timer (the timer for RACH or the timer for RACH skip) expires, the UE may determine that the LTM cell handover has failed. The UE may perform operation 713, such as LTM cell handover with RACH procedure. That is, the LTM cell handover without RACH procedure falls back to LTM handover with RACH procedure. In some embodiments, the UE may directly perform operation 714 without performing operation 713.

[0234] In operation 713, the UE may determine to perform an LTM cell handover with a RACH procedure to the candidate cell 1. Due to failure of the LTM cell handover without a RACH procedure or due to lack of a TA of the candidate cell 1, the LTM cell handover with a RACH procedure may be performed.

[0235] The UE may start a timer for RACH (which is associated with candidate cell 1) after successfully receiving the LTM cell switching command and may perform an access procedure for candidate cell 1 by starting the preamble. Alternatively, if the UE may send a HARQ-ACK for the LTM cell switching command in the physical layer, the UE may start a timer for RACH when the UE applies the LTM cell switching command.

[0236] During the LTM cell handover with RACH procedure to candidate cell 1, the UE may determine whether the LTM cell handover is successful.

[0237] For LTM cell handover with RACH procedure to candidate cell 1, the UE may stop the timer for RACH when random access on candidate cell 1 is successfully completed. For example, the UE may stop the timer for RACH when the MAC layer successfully completes the random access procedure.

[0238] Otherwise, when the timer for RACH expires, the UE may determine that the LTM cell handover with RACH procedure has failed.

[0239] In some embodiments, the first indicator received in operation 704 indicates that the stored candidate cell configuration is applied when the selected cell is an LTM candidate cell after the LTM cell handover failure. For example, the first indicator may be an enumeration value of “true”. The UE may perform operation 714.

[0240] Operations 714 to 717 are similar to Figure 6 Operations 614 to 617 described in , and the details are omitted here.

[0241] Figure 8 A flow chart illustrating a failure detection and handling procedure for LTM cell handover according to some other embodiments of the present disclosure.

[0242] Figure 8 Contains Figure 6 The same components as described in and the details are omitted here.

[0243] In operation 801, the source DU may send an LTM cell switching command to the UE to trigger the UE to change from the current serving cell to the candidate cell 1. Figure 6 Operation 609 or Figure 7 A detailed description of operation 711 in FIG. 7 is given, and the details are omitted here.

[0244] In operation 802, the source DU may send an LTM handover command report to the CU, the LTM handover command report indicating that the LTM handover command has been sent to the UE. In some other embodiments, the source DU may not transmit this report to the CU. The LTM handover command report may include a candidate target cell identity toward which the LTM cell handover is initiated, such as an identity of candidate cell 1.

[0245] In operation 803, the UE may perform an access procedure on the candidate target cell 1. Figure 6 A detailed description of operations 610 to 615 in FIG. 6 is given, and the details are omitted here.

[0246] In operation 804, the candidate DU managing the cell currently providing service to the UE may transmit a message (e.g., an access success message) to the CU, which may indicate to the CU that the UE has successfully accessed the candidate cell. The message may indicate an identifier of the candidate cell, such as candidate target cell 2. Alternatively, the candidate DU may not transmit the message to the CU.

[0247] In operation 805, the UE may transmit an LTM RLF report to the candidate DU. Figure 6 The UE may transmit the LTM RLF report to the candidate DU in operation 616, and the details are omitted here. Alternatively, the UE may not transmit the LTM RLF report to the candidate DU.

[0248] In operation 806, the candidate DU may forward the LTM RLF report to the CU, for example, via a UL RRC MESSAGETRANSFER message. Alternatively, the candidate DU may not transmit the LTM RLF report to the CU.

[0249] In operation 807a and operation 807b, the CU may determine a problem during the LTM cell switch and may send a message, such as an LTM switch report, to the source DU in operation 807a and / or to the candidate DU in operation 807b.

[0250] The LTM handover report may include at least one of the following:

[0251] 1. Type of failure;

[0252] Types of failures can include the following:

[0253] i. Late LTM cell switching: Radio link failure occurs after the UE stays in the source cell at the source DU for a long time; and the UE attempts to re-establish the radio link connection for LTM in a different cell.

[0254] ii. Premature LTM cell handover: Radio link failure occurs shortly after a successful LTM cell handover from a source cell to a candidate target cell or LTM cell handover failure occurs during the LTM procedure;

[0255] And the UE attempts to re-establish the radio link connection in the source cell at the source DU.

[0256] iii. LTM cell handover to wrong cell: Radio link failure occurs shortly after a successful LTM cell handover from a source cell to a candidate target cell or LTM cell handover failure occurs during the LTM procedure; and the UE attempts to re-establish the radio link connection in a cell other than the source cell and the candidate target cell.

[0257] 2. The content in the LTM RLF report, which may be received from the UE via the candidate DU, may include at least one of the following:

[0258] i. an identifier of the first candidate cell, such as a global cell identifier of the first candidate cell;

[0259] ii. an identifier of a second candidate cell selected by the UE;

[0260] iii. an indicator indicating that an LTM cell handover with a RACH procedure to the first candidate cell or an LTM cell handover without a RACH procedure to the first candidate cell is performed; or

[0261] iv. An indicator indicating whether LTM cell handover without RACH procedure falls back to LTM cell handover with RACH procedure.

[0262] Fig. 9 A simplified block diagram illustrating an apparatus according to some embodiments of the present disclosure.

[0263] like Fig. 9 , an example of apparatus 900 may include at least one processor 904 and at least one transceiver 902 coupled to the processor 904. Apparatus 900 may be a UE, a BS, a CU, a candidate DU, a source DU, a target DU, or any other device having similar functionality.

[0264] Although elements such as at least one transceiver 902 and a processor 904 are described in singular form in this figure, plural form may be considered unless explicitly stated to be limited to the singular form. In some embodiments of the present disclosure, the transceiver 902 may be divided into two devices, such as a receiving circuit system and a transmitting circuit system. In some embodiments of the present disclosure, the device 900 may further include an input device, a memory and / or other components.

[0265] In some embodiments of the present disclosure, the device 900 may be a UE. The transceiver 902 and the processor 904 may interact with each other to perform the operations of the UE described in any of Figures 1 to 8. In some embodiments of the present disclosure, the device 900 may be a CU, a candidate DU, a source DU, a target DU, or the like. The transceiver 902 and the processor 904 may interact with each other to perform the operations of the CU or DU described in any of Figures 1 to 8.

[0266] In some embodiments of the present disclosure, the apparatus 900 may further include at least one non-transitory computer-readable medium.

[0267] For example, in some embodiments of the present disclosure, a non-transitory computer-readable medium may store computer-executable instructions to cause the processor 904 to implement the method described above with respect to the UE. For example, when the computer-executable instructions are executed, the processor 904 interacts with the transceiver 902 to perform the operation of the UE described in any of Figures 1 to 8.

[0268] In some embodiments of the present disclosure, a non-transitory computer-readable medium may store computer-executable instructions to cause the processor 904 to implement the method described above with respect to a CU, a candidate DU, a source DU, a target DU, or a candidate DU. For example, when the computer-executable instructions are executed, the processor 904 interacts with the transceiver 902 to perform the operation of the CU or DU described in any of Figures 1 to 8.

[0269] The methods of the present disclosure may be implemented on a programmed processor. However, the controller, flow charts, and modules may also be implemented on a general or special purpose computer, a programmed microprocessor or microcontroller and peripheral integrated circuit components, an integrated circuit, a hardware electronic or logic circuit (e.g., a discrete component circuit), a programmable logic device, or the like. In general, any device having a finite state machine capable of implementing the flow charts shown in the figures may be used to implement the processing functions of the present disclosure.

[0270] Although the present disclosure has been described using specific embodiments of the present disclosure, it is apparent that many substitutions, modifications, and variations will be apparent to those skilled in the art. For example, the various components of the embodiments may be interchangeable, added, or substituted in other embodiments. Moreover, the operation of the disclosed embodiments may not necessarily require all of the elements shown in each figure. For example, a person skilled in the art of the disclosed embodiments will be able to make and use the teachings of the present disclosure by simply adopting the elements of the independent claims. Therefore, the embodiments of the present disclosure set forth herein are intended to be illustrative rather than restrictive. Various changes may be made without departing from the spirit and scope of the present disclosure.

[0271] In the present disclosure, for example, the relational terms of "first", "second" and the like can only be used to distinguish entities or actions from each other and do not necessarily need or imply any actual this relationship or order between such entities or actions. The term "comprising" or any other deformation thereof is intended to cover non-exclusive inclusion, so that the process, method, article or equipment comprising a series of elements not only comprises these elements, but also can comprise other elements that are not clearly listed or inherent to this process, method, article or equipment. The element starting with "one" or the like does not exclude the existence of additional identical elements in the process, method, article or equipment comprising the element when there are no more constraints. Moreover, the term "another" is defined as at least the second or more. As used herein, the terms "comprising", "having" and the like are defined as "comprising".

Claims

1. A user equipment UE, comprising: Transceiver; and a processor coupled to the transceiver and configured to: Obtaining information indicating at least one candidate cell configuration, wherein each candidate cell is configured with at least one timer associated with a random access channel RACH or RACH skipping; receiving indication information associated with an L1 / L2 triggered mobility LTM cell handover to a first candidate cell; and It is determined whether to perform an LTM cell handover with a RACH procedure or an LTM cell handover without a RACH procedure to the first candidate cell.

2. The UE of claim 1, wherein the processor is further configured to: A first timer associated with RACH is started to perform the LTM cell handover with RACH procedure to the first candidate cell.

3. The UE of claim 1, wherein the processor is further configured to: A second timer associated with RACH skipping is started to perform the LTM cell handover without RACH procedure to the first candidate cell.

4. The UE according to claim 1, wherein the information includes at least one of the following: a first indicator indicating whether to apply a stored candidate cell configuration when a candidate cell selected by the UE after at least one of the LTM cell handover with a RACH procedure or the LTM cell handover without a RACH procedure fails is a LTM candidate cell; a first timing advance, TA, value for each candidate cell corresponding to each of the at least one candidate cell configuration; or A RACH configuration is used to obtain a second TA value for each candidate cell corresponding to each of the at least one candidate cell configuration.

5. The UE according to claim 1, wherein the indication information comprises at least one of the following: a second TA value of the first candidate cell; a second indicator indicating whether to apply a stored candidate cell configuration when a candidate cell selected by the UE is an LTM candidate cell after at least one of the LTM cell handover with a RACH procedure or the LTM cell handover without a RACH procedure fails; or and a third indicator indicating a timer among the at least one timer associated with RACH or RACH skipping of the first candidate cell.

6. The UE of claim 1, wherein the processor is further configured to: The LTM cell handover failure is transmitted without RACH procedure to the source distributed unit DU.

7. The UE of claim 1, wherein the processor is further configured to: Applying a stored candidate cell configuration of a second candidate cell selected by the UE and performing an LTM cell handover to the second candidate cell upon receiving a first indicator or a second indicator, wherein the first indicator or the second indicator indicates that the stored candidate cell configuration is applied when the candidate cell selected by the UE is an LTM candidate cell after at least one of the LTM cell handover with a RACH procedure or the LTM cell handover without a RACH procedure fails.

8. The UE of claim 7, wherein the processor is further configured to: Report at least one of the following: an identifier of the first candidate cell; an identifier of the second candidate cell selected by the UE; a fourth indicator indicating that the LTM cell handover without a RACH procedure or the LTM cell handover with a RACH procedure is performed; or A fifth indicator indicates whether the LTM cell handover without RACH procedure falls back to the LTM cell handover with RACH procedure.

9. The UE of claim 3, wherein the processor is further configured to: The second TA value of the first candidate cell is received from a corresponding candidate DU.

10. A distributed unit DU, comprising: Transceiver; and a processor coupled to the transceiver and configured to: transmitting information indicating at least one candidate cell configuration to a user equipment UE, wherein each candidate cell is configured with at least one timer associated with a random access channel RACH or RACH skipping for L1 / L2 triggered mobility LTM cell switching; and Indication information indicating LTM cell switching to a first candidate cell is transmitted to the UE.

11. The DU of claim 10, wherein the information further indicates a first indicator to indicate whether to apply a stored candidate cell configuration when the candidate cell selected by the UE is an LTM candidate cell after at least one of the LTM cell handover with a RACH procedure or the LTM cell handover without a RACH procedure fails.

12. The DU according to claim 10, wherein the indication information indicates at least one of the following: a timing advance TA value of the first candidate cell; a second indicator indicating whether to apply the stored candidate cell configuration when the candidate cell selected by the UE is the LTM candidate cell after at least one of an LTM cell handover with a RACH procedure to the first candidate cell or an LTM cell handover without a RACH procedure to the first candidate cell fails; or and a third indicator indicating a timer among the at least one timer associated with RACH or RACH skipping of the first candidate cell.

13. The DU of claim 12, wherein the processor is further configured to: The LTM cell handover failure is received without a RACH procedure from the UE.

14. The DU of claim 12, wherein the processor is further configured to: A report is received from the central unit CU indicating at least one of the following: the type of failure; an identifier of the first candidate cell; an identifier of a second candidate cell selected by the UE; a fourth indicator indicating that the LTM cell handover with a RACH procedure or the LTM cell handover without a RACH procedure is performed; or A fifth indicator indicates whether the LTM cell handover without the RACH procedure falls back to the LTM cell handover with the RACH procedure.

15. A central unit CU, comprising: Transceiver; and a processor coupled to the transceiver and configured to: receiving one or more candidate cell configurations from one or more distributed units DU, wherein each DU is associated with a set of candidate cell configurations; Information indicating at least one candidate cell configuration for L1 / L2 triggered mobility LTM cell switching is transmitted to the user equipment UE via the source DU, wherein each candidate cell is configured with at least one timer associated with a random access channel RACH or RACH skipping for LTM cell switching.

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