A user equipment and a method

Abstract

A user equipment, UE, 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: receiving configuration information associated with L1 / L2 triggered mobility, LTM; estimating timing advance, TA, information for a candidate access node; determining that an LTM condition or conditions for the candidate access node has been met; determining whether valid TA information, obtained from a network, is available or unavailable for the candidate access node; in dependence upon a determination that valid TA information, obtained from the network, is available for the candidate access node, triggering random-access channel, RACH,-less LTM towards the candidate node using the TA information obtained from the network; and in dependence upon a determination that valid TA information, obtained from the network, is unavailable for the candidate access node, triggering RACH-less LTM towards the candidate node using the estimated TA information.

Description

[0001] TITLE

[0002] A USER EQUIPMENT AND A METHOD

[0003] TECHNOLOGICAL FIELD

[0004] Examples of the disclosure relate to L1 / L2 triggered mobility (LTM). Some relate to conditional LTM (CLTM).

[0005] BACKGROUND

[0006] A wireless network comprises a plurality of network nodes including terminal nodes and access nodes. Communication between the terminal nodes and the access nodes is wireless.

[0007] A terminal node, such as a user equipment (UE) can be configured to perform L1 / L2 triggered mobility (LTM) from a first cell to a second, different cell. In some circumstances, it may be desirable to improve or enhance how LTM is performed.

[0008] BRIEF SUMMARY

[0009] According to various, but not necessarily all, examples there is provided a user equipment (UE) 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: receiving configuration information associated with L1 / L2 triggered mobility, LTM; estimating timing advance, TA, information for a candidate access node; determining that an LTM condition or conditions for the candidate access node has been met; determining whether valid TA information, obtained from a network, is available or unavailable for the candidate access node; in dependence upon a determination that valid TA information, obtained from the network, is available for the candidate access node, triggering random-access channel, RACH,-less LTM towards the candidate node using the TA information obtained from the network; and in dependence upon a determination that valid TA information, obtained from the network, is unavailable for the candidate access node, triggering RACH-less LTM towards the candidate access node using the estimated TA information.In some but not necessarily all examples, determining that an LTM condition or conditions for the candidate access node has been met comprises receiving, from a network node, a cell switch command, CSC.

[0010] In some but not necessarily all examples, determining that valid TA information, obtained from the network, is available for the candidate access node comprises: receiving, from the network, an indication of TA information for the candidate access node; and determining that the indicated TA information is valid.

[0011] In some but not necessarily all examples, the indication of TA information is received with an associated timer value for a timer associated with TA information validity, and determining that valid TA information, obtained from the network, is available for the candidate access node comprises: starting a timer associated with TA information validity having the timer value; and determining that the timer associated with TA information validity has not expired when the LTM condition or conditions for the candidate access node has been met.

[0012] In some but not necessarily all examples, determining that valid TA information, obtained from the network, is unavailable for the candidate access node comprises: receiving, from the network, an indication of TA information for the candidate access node; and determining that the indicated TA information is invalid.

[0013] In some but not necessarily all examples, determining that the indicated TA information is invalid comprises determining that the indicated TA information has a null value.

[0014] In some but not necessarily all examples, the indication of TA information is received with an associated timer value for a timer associated with TA information validity, and determining that valid TA information, obtained from the network, is unavailable for the candidate access node comprises: starting a timer associated with TA information validity having the timer value; and determining that the timer associated with TA information validity has expired when the LTM condition or conditions for the candidate access node has been met.In some but not necessarily all examples, determining that valid TA information, obtained from the network, is unavailable for the candidate access node comprises: determining that an indication of TA information for the candidate access node has not been received from the network when the LTM condition or conditions for the candidate access node has been met.

[0015] In some but not necessarily all examples, the instructions, when executed by the at least one processor, cause the UE to perform at least: determining whether valid TA information, obtained from the network within a cell switch command, CSC, is available or unavailable for the candidate access node; in dependence upon a determination that valid TA information, obtained from the network node within a CSC, is available for the candidate access node, triggering RACH-less LTM towards the candidate node using the TA information obtained from the network within a CSC; and in dependence upon a determination that valid TA information, obtained from the network within a CSC, is unavailable for the candidate access node, determining whether valid TA information, obtained from the network, is available or unavailable for the candidate access node.

[0016] In some but not necessarily all examples, determining that valid TA information, obtained from the network node within a CSC, is available for the candidate access node comprises: receiving, from the network, and within a CSC, an indication of TA information for the candidate access node; and determining that the indicated TA information is valid.

[0017] In some but not necessarily all examples, determining that valid TA information, obtained from the network within a CSC, is unavailable for the first candidate access node comprises: receiving, from the network, and within a CSC, an indication of TA information for the candidate access node, and determining that the indicated TA information is invalid.

[0018] In some but not necessarily all examples, determining that the indicated TA information is invalid comprises determining that the indicated TA information has a null value.In some but not necessarily all examples, determining that valid TA information, obtained from the network within a CSC, is unavailable for the candidate access node comprises: determining that an indication of TA information has not been received from the network within a CSC when the LTM condition or conditions for the candidate access node has been met.

[0019] According to various, but not necessarily all, embodiments there is provided a method comprising: receiving configuration information associated with L1 / L2 triggered mobility, LTM; estimating timing advance, TA information for a candidate access node; determining that an LTM condition or conditions for the candidate access node has been met; determining whether valid TA information, obtained from a network, is available or unavailable for the candidate access node; in dependence upon a determination that valid TA information, obtained from the network, is available for the candidate access node, triggering random-access channel, RACH,-less LTM towards the candidate access node using the TA information obtained from the network; and in dependence upon a determination that valid TA information, obtained from the network, is unavailable for the candidate access node, triggering RACH-less LTM towards the candidate access node using the estimated TA information.

[0020] According to various, but not necessarily all, embodiments there is provided an apparatus comprising

[0021] at least one processor; and

[0022] at least one memory;

[0023] the at least one memory storing instructions that, when executed by the at least one processor, cause the apparatus to perform at least a part of one or more methods described herein.

[0024] According to various, but not necessarily all, embodiments there is provided an apparatus comprising means for performing at least part of one or more methods described herein. The description of a function and / or action should additionally be considered to also disclose any means suitable for performing that function and / or action. Functions and / or actions described herein can be performed in any suitable way using any suitable method.According to various, but not necessarily all, embodiments there is provided examples as claimed in the appended claims.

[0025] While the above examples of the disclosure and optional features are described separately, it is to be understood that their provision in all possible combinations and permutations is contained within the disclosure. It is to be understood that various examples of the disclosure can comprise any or all the features described in respect of other examples of the disclosure, and vice versa. Also, it is to be appreciated that any one or more or all the features, in any combination, may be implemented by / comprised in / performable by an apparatus, a method, and / or instructions as desired, and as appropriate. The description of a function should additionally be considered to also disclose any means suitable for performing that function

[0026] BRIEF DESCRIPTION

[0027] Some examples will now be described with reference to the accompanying drawings in which:

[0028] FIG. 1 shows an example of the subject matter described herein;

[0029] FIG. 2 shows another example of the subject matter described herein;

[0030] FIG. 3 shows another example of the subject matter described herein;

[0031] FIGs 4A - 4I show further examples of the subject matter described herein;

[0032] FIGs 5A - 5D show further examples of the subject matter described herein;

[0033] FIGs 6A - 6B show further examples of the subject matter described herein;

[0034] FIGs 7A - 7D show further examples of the subject matter described herein;

[0035] FIGs 8A - 8D show further examples of the subject matter described herein;

[0036] FIG. 9 shows another example of the subject matter described herein; and

[0037] FIG. 10 shows another example of the subject matter described herein.

[0038] The figures are not necessarily to scale. Certain features and views of the figures can be shown schematically or exaggerated in scale in the interest of clarity and conciseness. For example, the dimensions of some elements in the figures can be exaggerated relative to other elements to aid explication. Similar reference numerals are used in the figures to designate similar features. For clarity, all reference numerals are not necessarily displayed in all figures.DETAILED DESCRIPTION

[0039] The Figures illustrate a user equipment (UE) 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: receiving configuration information associated with L1 / L2 triggered mobility, LTM; estimating timing advance, TA, information for a candidate access node; determining that an LTM condition or conditions for the first candidate access node has been met; determining whether valid TA information, obtained from a network, is available or unavailable for the candidate access node; in dependence upon a determination that valid TA information, obtained from the network, is available for the candidate access node, triggering random-access channel, RACH,-less LTM towards the candidate node using the TA information obtained from a network; and in dependence upon a determination that valid TA information, obtained from the network, is unavailable for the candidate access node, triggering RACH-less LTM towards the candidate node using the estimated TA information.

[0040] Fig. 1 illustrates an example of a network 100 comprising a plurality of network entities including terminal apparatus 110, node apparatus 120 and one or more network apparatus 130. The terminal apparatus 110 and node apparatus 120 communicate 124 with each other. The one or more network apparatus 130 communicate 128 with the node apparatus 120.

[0041] In some examples the one or more network apparatus 130 communicate with the terminal apparatus 110. The one or more network apparatus 130 can, in some examples, communicate with each other. The one or more node apparatus 120 can, in some examples, communicate 126 with each other.

[0042] The network 100 can be a cellular network comprising a plurality of cells 122 each served by a node apparatus 120. In this example, the interface between the terminal apparatus 110 and a node apparatus 120 defining a cell 122 is a wireless interface 124.The node apparatus 120 comprises one or more cellular radio transceivers. The terminal apparatus 110 comprises one or more cellular radio transceivers.

[0043] In the example illustrated the cellular network 100 is a third generation Partnership Project (3GPP) network in which the terminal apparatus 110 are user equipment (UE) and the node apparatus 120 can be access nodes such as base stations.

[0044] A user equipment comprises a mobile equipment. Where reference is made to user equipment that reference includes and encompasses, wherever possible, a reference to mobile equipment.

[0045] In some examples, during operation, a user equipment 110 comprises a mobile equipment comprising a smart card for authentication / encryption etc. such as a Subscriber Identity Module (SIM). In some examples, during operation, a user equipment 110 comprises mobile equipment comprising circuitry embedded as part of the user equipment 110 for authentication / encryption such as software SIM.

[0046] The node apparatus 120 can be any suitable access node such as a base station or transmission reception point. The node apparatus 120 can be a network element responsible for radio transmission and reception in one or more cells 122, to or from the UE 110. The node apparatus 120 can be a network element in a Radio Access Network (RAN), an Open-Radio Access Network (O-RAN) or any other suitable type of network.

[0047] The network apparatus 130 can be part of a core network. The network apparatus 130 can be configured to manage functions relating to connectivity for the UEs 110. For example, the network apparatus 130 can be configured to manage functions such as connectivity, mobility, authentication, authorization and / or other suitable functions. In some examples the network apparatus 130 can comprise an Access and Mobility management Function (AMF) and / or a User Plane Function (UPF) or any other suitable entities.

[0048] In the example of Fig. 1 the network apparatus 130 is shown as a single entity. In some examples the network apparatus 130 could be distributed across a plurality ofentities. For example, the network apparatus 130 could be cloud based or distributed in any other suitable manner. The network apparatus 130 can be a core network node.

[0049] The network 100 can be a 4G or 5G network, for example. It can for example be a New Radio (NR) network that uses gNB or eNB as access nodes 120. New Radio is the 3GPP name for 5G technology. In such cases the node apparatus 120 can comprise gNodeBs (gNBs) 120 configured to provide user plane and control plane protocol terminations towards the UE 110 and / or to perform any other suitable functions. The gNBs 120 are interconnected with each other by means of an X2 / Xn interface 126. The gNBs are also connected by means of the N2 interface 128 to the network apparatus 130. The gNBs can be connected to an AMF or any other suitable network apparatus 130. Other types of networks and interfaces could be used in other examples. Other types of network could comprise next generation mobile and communication network, for example, a 6G network.

[0050] Release 18 of 3GPP specifications has introduced layer 1 (L1) / L2 based mobility that is also called lower layer triggered mobility (LTM) in the literature. LTM enables a serving cell change via L1 / L2 signaling, while keeping configuration of upper layers. Advantages of the LTM compared with L3 handover include reduced latency, overhead and interruption time during handover or cell switch.

[0051] A terminal node 110, such as a user equipment (UE), can be configured to perform LTM to perform a handover procedure. For example, a terminal node 110 can be connected to a first access node 120 and can be configured to perform LTM to instead be connected to a second, different access node 120.

[0052] In examples, a terminal node 110 can be connected to a first distributed unit (DU) of a central unit (CU) and can be configured to perform LTM to instead be connected to a different DU of the CU.

[0053] In examples, a terminal node 110 can be connected to a first cell of an access node 120, and can be configured to perform LTM to instead be connected to a second, different cell of the access node 120.A terminal node 110 can be configured to perform conditional LTM (CLTM). A terminal node 110 configured to perform CLTM can be configured with at least one condition associated with at least one candidate access node 120, and can trigger the handover autonomously to a candidate access node 120 when the associated condition(s) for a candidate access node 120 are fulfilled. This differs from the conventional (network-triggered) LTM where the handover is triggered via a cell switch command received by the terminal node 110.

[0054] A candidate access node 120 can be the access node 120 with which the terminal node 110 is connected (source access node), as the CLTM can be configured in relation to a different cell of the source access node.

[0055] In examples, a terminal node 110 that is configured to perform LTM and / or CLTM can be instructed, for example using a physical downlink control channel (PDCCH) order, to perform early synchronization and acquire timing advance (TA) information for at least one candidate access node. This is referred to below as TA information estimated by the terminal node 110. The terminal node may measure a timing difference between a serving cell and a candidate cell via downlink signal measurements and the TA information of the serving cell.

[0056] In examples, information is provided to the terminal node 110 that is configured to perform LTM and / or CLTM, to control use of TA information by the terminal node 110 during CLTM. In some examples, information is provided to the terminal node 110 that is configured to perform LTM and / or CLTM, to control use of the network provided TA information and terminal node estimated TA information at the terminal node during LTM and / or CLTM.

[0057] In some examples, information is provided to the terminal node 110 that is configured to perform LTM and / or CLTM, to control estimation of TA information by the terminal node 110.

[0058] FIG. 2 illustrates an example of a method 200.In examples, FIG. 2 can be considered to illustrate a plurality of methods. For example, FIG. 2 illustrates one or more actions at a plurality of actors / entities, and, in examples, FIG. 2 can be considered to illustrate a plurality of methods performed by the individual actors / entities.

[0059] One or more of the features discussed in relation to FIG. 2 can be found in one or more of the other FIGs.

[0060] In the example of FIG. 2, a plurality of apparatuses transmit and / or receive one or more signals and / or one or more messages across and / or via and / or using a network. In examples, any suitable form of communication in any suitable network can be used. For example, at least a portion of the network 100 of FIG. 1 can be used.

[0061] Accordingly, in examples, the plurality of apparatuses in FIG. 2 form at least a portion of network 100 as described in relation to FIG. 1.

[0062] In the illustrated example, a terminal node 110 and an access node 120 transmit and / or receive one or more signals and / or one or more messages. The access node can comprise a gNodeB (gNB) and the terminal node 110 can comprise a UE.

[0063] In examples, communications and / or transmissions between elements illustrated in FIG. 2 can proceed via any number of intervening elements, including no intervening elements.

[0064] Although one terminal node 110 is illustrated in the example of FIG. 2, in examples any suitable number of terminal nodes 110, for example UEs, can be included.

[0065] Similarly, in examples, any suitable number of access nodes 120 can be included.

[0066] As described herein, a description of a function and / or action should also be considered to disclose enabling, and / or causing, and / or controlling that function and / or action. For example, a description of transmitting information should also be considered to disclose enabling, and / or causing, and / or controlling transmitting / transmission of information.In examples, at least part of method 200 can be considered a method of performing CLTM.

[0067] Additionally, or alternatively, in examples, at least part of method 200 can be considered a method of controlling TA information use at a terminal node 110, such as a UE.

[0068] Additionally, or alternatively, in examples, at least part of method 200 can be considered a method of enabling random access channel (RACH)-less CLTM.

[0069] For example, a description of an apparatus, such as a UE, transmitting information should also be considered to disclose at least one controller of the apparatus enabling, and / or causing, and / or controlling the apparatus to transmit the information. In the illustrated example, the location of blocks indicates the entity performing the functions(s) and / or action(s).

[0070] Because FIG. 2 illustrates one or more actions / features of transmitting, FIG. 2 illustrates the corresponding receiving / enabling and / or causing receiving action(s) / feature(s).

[0071] For the further discussion of FIG. 2 it will be considered that the terminal node 110 is a UE.

[0072] The preceding statements should be understood to apply to other FIGs described herein, for example FIGs 4A - 4D.

[0073] From the point of view of the UE, at block 202, the method comprises receiving configuration information associated with LTM.

[0074] The information associated with LTM can comprise any suitable information. For example, the information associated with LTM can comprise any suitable information to enable the UE to determine configuration information 186 associated with CLTM.In some, but not necessarily all, examples, the configuration information associated with LTM is received via at least one radio resource control, RRC, reconfiguration message.

[0075] In some, but not necessarily all, examples, the configuration information comprises configuration information associated with conditional LTM, CLTM.

[0076] In examples, the information associated with CLTM comprises at least one CLTM execution condition. Examples of such execution conditions are described in the 3GPP specifications, and execution conditions may be defined, for example, in terms of measured reference signal metrics of one or more serving cells and one or more candidate cells for the CLTM. In examples, the information associated with CLTM comprises at least one layer 1 (L1) measurement reporting configuration. The at least one L1 measurement reporting configuration can comprise at least one pruned L1 measurement reporting configuration.

[0077] At block 204, the method comprises estimating TA information for a candidate access node.

[0078] In some, but not necessarily all, examples, estimating TA information for a candidate access node is based, at least in part, on the received configuration information associated with LTM.

[0079] At block 206, the method comprises determining that an LTM condition or conditions for the candidate access node has been met.

[0080] In some, but not necessarily all, examples in which the configuration information comprises configuration information associated with CLTM, wherein determining that an LTM condition or conditions for a first candidate access node has been met comprises determining that a CLTM condition or conditions for a first candidate access node has been met.In some, but not necessarily all, examples, determining that an LTM condition or conditions for a first candidate access node has been met comprises receiving, from a network node, a cell switch command, CSC.

[0081] In some, but not necessarily all, examples, the CSC is received via a medium access control, MAC, control element, CE.

[0082] At block 208, the method comprises determining whether valid TA information, obtained from a network, is available or unavailable for the candidate access node.

[0083] In some, but not necessarily all, examples, determining whether valid TA information, obtained from a network, e.g. from the serving cell or serving DU or serving CU, is available or unavailable for the candidate access node comprises receiving, from the network, an indication of TA information for the candidate access node, and determining whether the indicated TA information is valid.

[0084] In some, but not necessarily all, examples, TA information is obtained from the network at least one of two different message types. The first message type comprises a cell switch command (CSC); the second message type does not comprise a CSC. In some, but not necessarily all, examples, both message types are MAC CE messages.

[0085] TA information indicated within a message of the first type is referred to as “first network TA information” below. Therefore, first network TA information is TA information that is received at the UE, from the network, within a MAC CE which comprises a CSC.

[0086] TA information indicated within a message of the second type is referred to as “second TA information” below. Therefore, second network TA information is TA information that is received at the UE, from the network, within a MAC CE which does not comprise a CSC.Where “indicated TA information” is referred to below, this may be interpreted as referring to at least one of first network TA information or second network TA information.

[0087] In some, but not necessarily all, examples, determining whether valid TA information, obtained from a network, is available or unavailable for the candidate access node comprises determining whether valid first network TA information is available or unavailable for the candidate access node, and, if valid first network TA information is unavailable for the candidate access node, determining whether valid second network TA information is available or unavailable for the candidate access node.

[0088] In some, but not necessarily all, examples, determining whether valid TA information, obtained from a network, is available or unavailable for the candidate access node comprises determining whether valid second network TA information is available or unavailable for the candidate access node and does not comprise determining whether valid first network TA information is available or unavailable for the candidate access node.

[0089] If the indicated TA information is valid, valid TA information, obtained from a network, is available for the candidate access node. If the indicated TA information is invalid, valid TA information, obtained from a network, is unavailable for the candidate access node.

[0090] In some, but not necessarily all, examples, determining whether the indicated TA information is valid or invalid comprises determining whether the indicated TA information has a null value. If the indicated TA information has a null value, the indicated TA information is invalid. If the indicated TA information does not have a null value, the indicated TA information is valid. The null value may be indicated by a value ‘FFF’ in the CSC MAC CE.

[0091] In some, but not necessarily all, examples, the indication of TA information is received with an associated timer value for a timer associated with TA information validity. Determining whether valid TA information, obtained from the network, is available for the candidate access node comprises starting a timer associated withTA information validity having the timer value and determining whether the timer has expired when the LTM condition or conditions for the candidate access node has been met. If the timer has expired when the LTM condition or conditions for the candidate access node has been met, the TA information is invalid. If the timer has not expired when the LTM condition or conditions for the candidate access node has been met, the TA information is valid.

[0092] In some, but not necessarily all, examples, TA information is valid only if it does not have a null value and the timer has not expired when the LTM condition or conditions for the candidate access node has been met. Therefore, if the TA information does not have a null value, but the timer has expired when the LTM condition or conditions for the candidate access node has been met, the TA information is invalid. If the timer has not expired when the LTM condition or conditions for the candidate access node has been met, but the TA information has a null value, the TA information is invalid.

[0093] In some, but not necessarily all, examples, determining whether valid TA information, obtained from a network, is available or unavailable for the candidate access node comprises determining whether an indication of TA information has been received from the network when the LTM condition or condition for the candidate access node has been met.

[0094] At block 210, the method comprises triggering RACH-less LTM towards the candidate node.

[0095] In dependence upon a determination that valid TA information, obtained from the network, is available for the candidate access node, triggering RACH-less LTM towards the candidate node comprises using the TA information obtained from the network node.

[0096] In dependence upon a determination that valid TA information, obtained from the network, is unavailable for the candidate access node, triggering RACH-less LTM towards the candidate node comprises using the estimated TA information.Thus, TA information obtained for the network is used in preference to the estimated TA information.

[0097] In some, but not necessarily all, examples, in dependence upon a determination that valid first network TA information is available for the candidate access node, triggering RACH-less LTM towards the candidate node comprises using the first network TA information.

[0098] In some, but not necessarily all, examples, in dependence upon a determination that valid first network TA information is unavailable for the candidate access node, and a determination that valid second network TA information is available for the candidate access node, triggering RACH-less LTM towards the candidate node comprises using the second network TA information.

[0099] In some, but not necessarily all, examples, in dependence upon a determination that valid first network TA information is unavailable for the candidate access node, and a determination that valid second network TA information is unavailable for the candidate access node, triggering RACH-less LTM towards the candidate node comprises using the estimated TA information.

[0100] Thus, first network TA information is used in preference to second network TA information, and second network TA information is used in preference to the estimated TA information.

[0101] Consequently, FIG. 2 illustrates a method 200 comprising:

[0102] at block 202, receiving configuration information associated with LTM;

[0103] at block 204, estimating TA information for a candidate access node;

[0104] at block 206, determining that an LTM condition or conditions for the candidate access node has been met;

[0105] at block 208, determining whether valid TA information, obtained from a network, is available or unavailable for the candidate access node; and

[0106] at block 210, triggering RACH-less LTM towards the candidate node.In some, but not necessarily all, examples, one or more blocks of method 200 are performed as part of a recovery process after a failed LTM or CLTM cell switch.

[0107] In a first block of the recovery process, the UE selects a candidate cell for recovery.

[0108] In a second block of the recovery process, the UE determines whether valid TA information, obtained from the network, is available or unavailable for an access node of the candidate cell.

[0109] In a third block of the recovery process, the UE triggers RACH-less recovery to the access node of the candidate cell. Triggering the recovery may be performed in dependence upon a determination that the candidate cell has a configured grant.

[0110] If the UE determines that valid TA information, obtained from the network, is available for the access node of the candidate cell, the UE triggers RACH-less recovery to the access node of the candidate cell using the TA information obtained from the network.

[0111] If the UE determines that valid TA information, obtained from the network, is not available for the access node of the candidate cell, the UE may determine whether the UE estimated TA for the access node of the candidate cell is valid. If the UE determines that the UE estimated TA for the access node of the candidate cell is valid, the UE triggers RACH-less recovery to the access node of the candidate cell using the UE estimated TA.

[0112] In some, but not necessarily all, examples, if the UE determines that valid TA information, obtained from the network, is not available for the access node of the candidate cell, and that the UE estimated TA for the candidate cell is invalid, the UE performs RRC recovery. From the point of view of the network node, at block 202, the method comprises transmitting, to the UE, configuration information associated with LTM.

[0113] Consequently, FIG. 2 illustrates a method 200 comprising:

[0114] at block 202, transmitting, to the UE, configuration information associated with LTM.FIG. 3 illustrates an example of a method 300.

[0115] The method 300 can be performed by any suitable apparatus comprising any suitable means for performing the method, for example an apparatus as described in relation to FIG. 9.

[0116] In examples, the method 300 can be performed by a terminal node 110, such as a UE.

[0117] At block 302, the method 300 comprises receiving configuration information associated with LTM.

[0118] At block 304, the method 300 comprises estimating TA information for a candidate access node.

[0119] At block 306, the method 300 comprises determining that an LTM condition or conditions for the candidate access node has been met.

[0120] At block 308, the method 300 comprises determining whether valid TA information, obtained from a network, is available or unavailable for the candidate access node.

[0121] At block 310, the method 300 comprises triggering RACH-less LTM towards the candidate node.

[0122] Consequently, FIG. 3 illustrates a method 300 comprising:

[0123] at block 302, receiving configuration information associated with LTM;

[0124] at block 304, estimating TA information for a candidate access node;

[0125] at block 306, determining that an LTM condition or conditions for the candidate access node has been met;

[0126] at block 308, determining whether valid TA information, obtained from a network, is available or unavailable for the candidate access node; and

[0127] at block 310, triggering RACH-less LTM towards the candidate node.

[0128] FIGs. 4A - 4I illustrate example methods.In some, but not necessarily all, examples, the method illustrated in FIG. 4A may be considered a configuration method. The method illustrated in FIG. 4A may be succeeded, with no intervening method blocks, or with one or more intervening method blocks, by any one or more of the methods illustrated in FIGs 4B, 40, 4D, 4E, 4F, 4G, 4H, or4l.

[0129] From the point of view of the UE, the method comprises, at block 401 , performing a preparation phase for CLTM for the candidate cell(s) configured with UE based TA information and, optionally, for LTM.

[0130] At block 402, the method comprises estimating TA information.

[0131] At block 403, the method comprises receiving, from the source DU, a PDCCH order for a candidate cell under a target DU.

[0132] At block 404, the method comprises transmitting, to the target DU, a RACH preamble, for acquisition of an indication of TA information for the candidate cell.

[0133] Consequently, FIG. 4A illustrates a method comprising:

[0134] at block 401, performing a preparation phase for CLTM for the candidate cell(s) configured with UE based TA information and, optionally, for LTM;

[0135] at block 402, estimating TA information;

[0136] at block 403, receiving, from the source DU, a PDCCH order for a candidate cell under a target DU; and

[0137] at block 404, transmitting, to the target DU, a RACH preamble.

[0138] From the point of view of the source DU, the method comprises, at block 401 , performing a preparation phase for CLTM for the candidate cell(s) configured with UE based TA information and, optionally, for LTM.

[0139] At block 403, the method comprises triggering a PDCCH order for a candidate cell under a target DU.At block 406, the method comprises receiving an indication of TA information for the candidate cell, with an associated timer value for a timer associated with TA information validity.

[0140] Consequently, FIG. 4A illustrates a method comprising:

[0141] at block 401, performing a preparation phase for CLTM for the candidate cell(s) configured with UE based TA information and, optionally, for LTM;

[0142] at block 403, triggering a PDCCH order for a candidate cell under a target DU; and at block 406, receiving an indication of TA information for the candidate cell, with an associated timer value for a timer associated with TA information validity.

[0143] From the point of view of the target DU, the method comprises at block 401 , performing a preparation phase for CLTM for the candidate cell(s) configured with UE based TA information and, optionally, for LTM.

[0144] At block 404, the method comprises receiving, from the UE, a RACH preamble, for acquisition of an indication of TA information for the candidate cell.

[0145] At block 405, the method comprises transmitting, to the CU, an indication of TA information for the candidate cell, with an associated timer value for a timer associated with TA information validity.

[0146] Consequently, FIG. 4A illustrates a method comprising:

[0147] at block 401, performing a preparation phase for CLTM for the candidate cell(s) configured with UE based TA information and, optionally, for LTM;

[0148] at block 404, receiving, from the UE, a RACH preamble, for acquisition of an indication of TA information for the candidate cell; and

[0149] at block 405, transmitting, to the CU, an indication of TA information for the candidate cell, with an associated timer value for a timer associated with TA information validity.

[0150] From the point of view of the CU, the method comprises, at block 401 , performing a preparation phase for CLTM for the candidate cell(s) configured with UE based TA information and, optionally, for LTM.At block 405, the method comprises receiving, from the target DU, an indication of TA information for the candidate cell, with an associated timer value for a timer associated with TA information validity.

[0151] At block 406, the method comprises transmitting, to the source DU, the indication of TA information for the candidate cell, with an associated timer value for a timer associated with TA information validity.

[0152] Consequently, FIG. 4A illustrates a method comprising:

[0153] at block 401, performing a preparation phase for CLTM for the candidate cell(s) configured with UE based TA information and, optionally, for LTM;

[0154] at block 405, receiving, from the target DU, an indication of TA information for the candidate cell, with an associated timer value for a timer associated with TA information validity; and

[0155] at block 406, transmitting, to the source DU, the indication of TA information for the candidate cell, with an associated timer value for a timer associated with TA information validity.

[0156] FIG. 4B illustrates an example method for performing CLTM.

[0157] From the point of view of the UE, the method 410 comprises, at block 411, receiving, from the source DU, a MAC CE containing an indication of TA information for the candidate cell, with an associated timer value for a timer associated with TA information validity. In the example of FIG. 4B, the indicated TA information is valid.

[0158] At block 412, the method comprises determining that a CLTM condition or conditions for the candidate access node has been met.

[0159] At block 413, the method comprises determining whether valid TA information, obtained from the network, is available or unavailable for the candidate access node. In the example of FIG. 4B, the timer has not expired when it is determined that a CLTM condition or conditions for the candidate access node has been met. Thus, the UE determines that valid TA information, obtained from the network, is available forthe candidate access node, and that the UE estimated TA information should not be used.

[0160] At block 414, the method comprises triggering RACH-less CLTM using the TA information, obtained from the network, for the candidate access node.

[0161] Consequently, FIG. 4B illustrates a method comprising:

[0162] at block 411, receiving, from the source DU, a MAC CE containing an indication of TA information for the candidate cell, with an associated timer value for a timer associated with TA information validity;

[0163] at block 412, determining that an LTM condition or conditions for the candidate access node has been met;

[0164] at block 413, determining whether valid TA information, obtained from the network, is available or unavailable for the candidate access node; and

[0165] at block 414, triggering RACH-less CLTM using the TA information, obtained from the network, for the candidate access node.

[0166] From the point of view of the source DU, the method comprises, at block 411 , transmitting, to the UE, a MAC CE containing an indication of TA information for the candidate cell, with an associated timer value for a timer associated with TA information validity.

[0167] Consequently, FIG. 4B illustrates a method comprising:

[0168] at block 411 , transmitting, to the UE, a MAC CE containing an indication of TA information for the candidate cell, with an associated timer value for a timer associated with TA information validity.

[0169] FIG. 4C illustrates another example method for performing CLTM.

[0170] From the point of view of the UE, the method comprises, at block 421, receiving, from the source DU, a MAC CE containing an indication of TA information for the candidate cell, with an associated timer value for a timer associated with TA information validity. In the example of FIG. 4C, the indicated TA information is valid.At block 422, the method comprises determining that a CLTM condition or conditions for the candidate access node has been met.

[0171] At block 423, the method comprises determining whether valid TA information, obtained from the network, is available or unavailable for the candidate access node. In the example of FIG. 40, the timer has expired when it is determined that a CLTM condition or conditions for the candidate access node has been met. Thus, the UE determines that valid TA information, obtained from the network, is unavailable for the candidate access node.

[0172] At block 424, the method comprises triggering RACH-less CLTM using the UE estimated TA information.

[0173] Consequently, FIG. 4C illustrates a method comprising:

[0174] at block 421, receiving, from the source DU, a MAC CE containing an indication of TA information for the candidate cell, with an associated timer value for a timer associated with TA information validity;

[0175] at block 422, determining that a CLTM condition or conditions for the candidate access node has been met;

[0176] at block 423, determining whether valid TA information, obtained from the network, is available or unavailable for the candidate access node; and

[0177] at block 424, triggering RACH-less CLTM using the UE estimated TA information.

[0178] From the point of view of the source DU, the method comprises, at block 421 , transmitting, to the UE, a MAC CE containing an indication of TA information for the candidate cell, with an associated timer value for a timer associated with TA information validity.

[0179] Consequently, FIG. 4C illustrates a method comprising:

[0180] at block 421, transmitting, to the UE, a MAC CE containing an indication of TA information for the candidate cell, with an associated timer value for a timer associated with TA information validity.

[0181] FIG. 4D illustrates a further example method for performing CLTM.From the point of view of the UE, the method comprises, at block 431 , receiving, from the source DU, a MAC CE containing an indication of TA information for the candidate cell, with an associated timer value for a timer associated with TA information validity. In the example of FIG. 4D, the indicated TA information is valid.

[0182] At block 432, the method comprises determining that a CLTM condition or conditions for the candidate access node has been met.

[0183] At block 433, the method comprises determining whether valid TA information, obtained from the network, is available or unavailable for the candidate access node. Because the indicated TA information is invalid, the UE determines that valid TA information, obtained from the network, is not available for the candidate access node.

[0184] At block 434, the method comprises triggering RACH-less CLTM using the UE estimated TA information.

[0185] Consequently, FIG. 4D illustrates a method comprising:

[0186] at block 431, receiving, from the source DU, a MAC CE containing an indication of TA information for the candidate cell, with an associated timer value for a timer associated with TA information validity;

[0187] at block 432, determining that a CLTM condition or conditions for the candidate access node has been met;

[0188] at block 433, determining whether valid TA information, obtained from the network, is available or unavailable for the candidate access node; and

[0189] at block 434, triggering RACH-less CLTM using the UE estimated TA information.

[0190] From the point of view of the source DU, the method comprises, at block 431 , transmitting, to the UE, a MAC CE containing an indication of TA information for the candidate cell, with an associated timer value for a timer associated with TA information validity.

[0191] Consequently, FIG. 4D illustrates a method comprising:at block 431, transmitting, to the UE, a MAC CE containing an indication of TA information for the candidate cell, with an associated timer value for a timer associated with TA information validity.

[0192] FIG. 4E illustrates another example method for performing CLTM.

[0193] From the point of view of the UE, the method comprises, at block 441 , determining that a CLTM condition or conditions for the candidate access node has been met.

[0194] At block 442, the method comprises determining whether valid TA information, obtained from the network, is available or unavailable for the candidate access node. In the example of FIG. 4E, no indication of TA information from the candidate cell has been received from the network. Thus, the UE determines that valid TA information, obtained from the network, is not available for the candidate access node.

[0195] At block 443, the method comprises triggering RACH-less CLTM using the UE estimated TA information.

[0196] Consequently, FIG. 4E illustrates a method comprising:

[0197] at block 441 , determining that a CLTM condition or conditions for the candidate access node has been met;

[0198] at block 442, determining whether valid TA information, obtained from the network, is available or unavailable for the candidate access node; and

[0199] at block 443, triggering RACH-less CLTM using the UE estimated TA information.

[0200] FIG. 4F illustrates an example method for performing LTM, in which the UE is additionally configured to perform CLTM.

[0201] From the point of view of the UE, the method comprises, at block 451, receiving, from the source DU, a MAC CE containing a first indication of TA information for the candidate cell, with an associated timer value for a timer associated with TA information validity. In the example of FIG. 4F, the first indicated TA information is valid.At block 453, the method comprises receiving, from the gNB-Dll, a CSC, the CSC comprising a second indication of TA information for the candidate cell. In the example of FIG. 4F, the second indicated TA information is valid.

[0202] At block 454, the method comprises determining that an LTM condition or conditions for the candidate access node has been met.

[0203] At block 455, the method comprises determining whether valid TA information, obtained from the network within a CSC, is available or unavailable for the candidate access node. Because the second indicated TA information is valid, the UE determines that valid TA information, obtained from the network within a CSC, is available for the candidate access node, and that neither the first indicated TA information, nor the UE estimated TA information, should be used.

[0204] At block 456, the method comprises triggering RACH-less LTM using the second indicated TA information.

[0205] Consequently, FIG. 4F illustrates a method comprising:

[0206] at block 451, receiving, from the source DU, a MAC CE containing a first indication of TA information for the candidate cell, with an associated timer value for a timer associated with TA information validity;

[0207] at block 453, receiving, from the source DU, a CSC, the CSC comprising a second indication of TA information for the candidate cell;

[0208] at block 454, determining that an LTM condition or conditions for the candidate access node has been met;

[0209] at block 455, determining whether valid TA information, obtained from the network within a CSC, is available or unavailable for the candidate access node; and at block 456, triggering RACH-less LTM using the second indicated TA information.

[0210] From the point of view of the source DU, the method comprises, at block 451 , transmitting, to the UE, a MAC CE containing an indication of TA information for the candidate cell, with an associated timer value for a timer associated with TA information validity.At block 452, the method comprises determining that a cell switch, from a current cell to a candidate cell, should occur.

[0211] At block 453, the method comprises transmitting, to the UE, a CSC, the CSC comprising a second indication of TA information for the candidate cell.

[0212] Consequently, FIG. 4F illustrates a method comprising:

[0213] at block 451, transmitting, to the UE, a MAC CE containing an indication of TA information for the candidate cell, with an associated timer value for a timer associated with TA information validity;

[0214] at block 452, determining that a cell switch, from a current cell to a candidate cell, should occur; and

[0215] at block 453, transmitting, to the UE, a CSC, the CSC comprising a second indication of TA information for the candidate cell.

[0216] FIG. 4G illustrates a further example method for performing LTM, in which the UE is additionally configured to perform CLTM.

[0217] From the point of view of the UE, the method comprises, at block 461 , receiving, from the source DU, a MAC CE containing a first indication of TA information for the candidate cell, with an associated timer value for a timer associated with TA information validity. In the example of FIG. 4G, the first indicated TA information is valid.

[0218] At block 463, the method comprises receiving, from the gNB-DU, a CSC, the CSC comprising a second indication of TA information for the candidate cell. In the example of FIG. 4G, the second indicated TA information has a null value and thus is invalid.

[0219] At block 464, the method comprises determining that an LTM condition or conditions for the candidate access node has been met.

[0220] At block 465, the method comprises determining whether valid TA information, obtained from the network within a CSC, is available or unavailable for the candidateaccess node. Because the second indicated TA information is invalid, the UE determines that valid TA information, obtained from the network within a CSC, is unavailable for the candidate access node.

[0221] At block 466, the method comprises determining whether valid TA information, obtained from the network, is available or unavailable for the candidate access node. In the example of FIG. 4G, the timer has not expired when the CSC is received. Thus, the UE determines that valid TA information, obtained from the network, is available for thee candidate access node, and that the UE estimated TA information should not be used.

[0222] At block 467, the method comprises triggering RACH-less LTM using the first indicated TA information.

[0223] Consequently, FIG. 4G illustrates a method comprising:

[0224] at block 461 , receiving, from the source DU, a MAC CE containing a first indication of TA information for the candidate cell, with an associated timer value for a timer associated with TA information validity;

[0225] at block 463, receiving, from the source DU, a CSC, the CSC comprising a second indication of TA information for the candidate cell;

[0226] at block 464, determining that an LTM condition or conditions for the candidate access node has been met;

[0227] at block 465, determining whether valid TA information, obtained from the network within a CSC, is available or unavailable for the candidate access node;

[0228] at block 466, determining whether valid TA information, obtained from the network, is available or unavailable for the candidate access node; and

[0229] at block 467, triggering RACH-less LTM using the second indicated TA information.

[0230] From the point of view of the source DU, the method comprises, at block 461 , transmitting, to the UE, a MAC CE containing an indication of TA information for the candidate cell, with an associated timer value for a timer associated with TA information validity.At block 462, the method comprises determining that a cell switch, from a current cell to a candidate cell, should occur.

[0231] At block 463, the method comprises transmitting, to the UE, a CSC, the CSC comprising a second indication of TA information for the candidate cell, the second indication of TA information having a null value.

[0232] Consequently, FIG. 4G illustrates a method comprising:

[0233] at block 461 , transmitting, to the UE, a MAC CE containing an indication of TA information for the candidate cell, with an associated timer value for a timer associated with TA information validity;

[0234] at block 462, determining that a cell switch, from a current cell to a candidate cell, should occur; and

[0235] at block 463, transmitting, to the UE, a CSC, the CSC comprising a second indication of TA information for the candidate cell.

[0236] FIG. 4H illustrates a further example method for performing LTM, in which the UE is additionally configured to perform CLTM.

[0237] From the point of view of the UE, the method comprises, at block 471 , receiving, from the source DU, a MAC CE containing a first indication of TA information for the candidate cell, with an associated timer value for a timer associated with TA information validity. In the example of FIG. 4H, the first indicated TA information is valid.

[0238] At block 473, the method comprises receiving, from the gNB-DU, a CSC, the CSC comprising a second indication of TA information for the candidate cell. In the example of FIG. 4H, the second indicated TA information has a null value and thus is invalid.

[0239] At block 474, the method comprises determining that an LTM condition or conditions for the candidate access node has been met.At block 475, the method comprises determining whether valid TA information, obtained from the network within a CSC, is available or unavailable for the candidate access node. Because the second indicated TA information is invalid, the UE determines that valid TA information, obtained from the network within a CSC, is unavailable for the candidate access node.

[0240] At block 476, the method comprises determining whether valid TA information, obtained from the network, is available or unavailable for the candidate access node. In the example of FIG. 4H, the timer has expired when the CSC is received. Thus, the UE determines that valid TA information, obtained from the network, is unavailable for the candidate access node.

[0241] At block 477, the method comprises triggering RACH-less LTM using the UE estimated TA information.

[0242] Consequently, FIG. 4H illustrates a method comprising:

[0243] at block 471 , receiving, from the source DU, a MAC CE containing a first indication of TA information for the candidate cell, with an associated timer value for a timer associated with TA information validity;

[0244] at block 473, receiving, from the source DU, a CSC, the CSC comprising a second indication of TA information for the candidate cell;

[0245] at block 474, determining that an LTM condition or conditions for the candidate access node has been met;

[0246] at block 475, determining whether valid TA information, obtained from the network within a CSC, is available or unavailable for the candidate access node;

[0247] at block 476, determining whether valid TA information, obtained from the network, is available or unavailable for the candidate access node; and

[0248] at block 477, triggering RACH-less LTM using the UE estimated TA information.

[0249] From the point of view of the source DU, the method comprises, at block 471 , transmitting, to the UE, a MAC CE containing an indication of TA information for the candidate cell, with an associated timer value for a timer associated with TA information validity.At block 472, the method comprises determining that a cell switch, from a current cell to a candidate cell, should occur.

[0250] At block 473, the method comprises transmitting, to the UE, a CSC, the CSC comprising a second indication of TA information for the candidate cell, the second indication of TA information having a null value.

[0251] Consequently, FIG. 4H illustrates a method comprising:

[0252] at block 471 , transmitting, to the UE, a MAC CE containing an indication of TA information for the candidate cell, with an associated timer value for a timer associated with TA information validity;

[0253] at block 472, determining that a cell switch, from a current cell to a candidate cell, should occur; and

[0254] at block 473, transmitting, to the UE, a CSC, the CSC comprising a second indication of TA information for the candidate cell.

[0255] FIG. 4I illustrates a further example method for performing LTM, in which the UE is additionally configured to perform CLTM.

[0256] From the point of view of the UE, the method comprises, at block 481 , receiving, from the source DU, a MAC CE containing a first indication of TA information for the candidate cell, with an associated timer value for a timer associated with TA information validity. In the example of FIG. 4I, the first indicated TA information is invalid.

[0257] At block 483, the method comprises receiving, from the gNB-DU, a CSC, the CSC comprising a second indication of TA information for the candidate cell. In the example of FIG. 4I, the second indicated TA information has a null value and thus is invalid.

[0258] At block 484, the method comprises determining that an LTM condition or conditions for the candidate access node has been met.At block 485, the method comprises determining whether valid TA information, obtained from the network within a CSC, is available or unavailable for the candidate access node. Because the second indicated TA information is invalid, the UE determines that valid TA information, obtained from the network within a CSC, is unavailable for the candidate access node.

[0259] At block 486, the method comprises determining whether valid TA information, obtained from the network, is available or unavailable for the candidate access node. Because the first indicated TA information is invalid, the UE determines that valid TA information, obtained from the network, is unavailable for the candidate access node.

[0260] At block 487, the method comprises triggering RACH-less LTM using the UE estimated TA information.

[0261] In some, but not necessarily all, examples, the UE interprets the reception of first indicated TA information having a null value as an indication not to use the UE estimated TA information to avoid loss of synchronization between the candidate DU and the source DU. In some such examples, receiving second indicating TA information having a null value causes the UE to perform a RACH-based LTM cell switch.

[0262] Consequently, FIG. 4I illustrates a method comprising:

[0263] at block 481 , receiving, from the source DU, a MAC CE containing a first indication of TA information for the candidate cell, with an associated timer value for a timer associated with TA information validity;

[0264] at block 483, receiving, from the source DU, a CSC, the CSC comprising a second indication of TA information for the candidate cell;

[0265] at block 484, determining that an LTM condition or conditions for the candidate access node has been met;

[0266] at block 485, determining whether valid TA information, obtained from the network within a CSC, is available or unavailable for the candidate access node;

[0267] at block 486, determining whether valid TA information, obtained from the network, is available or unavailable for the candidate access node; and

[0268] at block 487, triggering RACH-less LTM using the UE estimated TA information.From the point of view of the source DU, the method comprises, at block 481 , transmitting, to the UE, a MAC CE containing an indication of TA information for the candidate cell, with an associated timer value for a timer associated with TA information validity.

[0269] At block 482, the method comprises determining that a cell switch, from a current cell to a candidate cell, should occur.

[0270] At block 483, the method comprises transmitting, to the UE, a CSC, the CSC comprising a second indication of TA information for the candidate cell, the second indication of TA information having a null value.

[0271] Consequently, FIG. 4I illustrates a method comprising:

[0272] at block 481 , transmitting, to the UE, a MAC CE containing an indication of TA information for the candidate cell, with an associated timer value for a timer associated with TA information validity;

[0273] at block 482, determining that a cell switch, from a current cell to a candidate cell, should occur; and

[0274] at block 483, transmitting, to the UE, a CSC, the CSC comprising a second indication of TA information for the candidate cell.

[0275] The early synchronization behaviour for conditional LTM is like that of conventional LTM with exception in terms of delivery of the TA information to the UE by the serving DU. In case of conventional LTM, the TA value acquired during the early synchronization phase is stored in the serving DU and shared with the UE along with the MAC CE cell switch command during the execution phase based on UE measurements. However, in case of conditional LTM there is no explicit cell change command from the network and the TA information needs to be shared with the UE in advance by the network. Unlike in Release-18 where it is assumed that it is up to the network to ensure that a valid TA is provided to the UE, in Release-19 a new TA maintenance timer for the candidate cell was proposed which could possibly be maintained and updated at the UE.In case of conventional LTM, TA which is acquired by the network (S-Dll) during early synchronization, is stored and sent to the UE along with the cell switch command (CSC) over MAC CE. As UE receives the CSC, the TA information included in the CSC over-rides the UE estimated TA.

[0276] However, there can be scenarios when the candidate cell(s) are configured with conditional LTM and while UE evaluates the conditions for cell switch execution, the network may send a MAC CE which is a cell switch command. As per the legacy behaviour, if a cell switch command is received by the UE from the network, UE shall always give precedence to the network triggered cell switch. In this case the UE behaviour of handling the network provided or UE estimated early Timing Advance (TA) is not very clear.

[0277] FIG. 5A: Problem-1 (Step-7 to Step-9): UE is configured with UE estimated TA and it starts the estimation of the early TA right after receiving the RRC Reconfiguration during the preparation phase (Step-2). Now, the network decides to trigger early uplink synchronization and the serving -DU sends PDCCH order to the UE. The UE sends PRACH preambles to the candidate cell in the target DU and the target DU shares the TA with the CU. The gNB-CU forwards the TA along with the new maintenance to the serving-DU. The serving DU now sends the TA along with the new maintenance timer over the new MAC CE to the UE containing the TA value which is not set as “FFF”. Or null along with the new timer. Now, UE checks the timer and finds it is still running which means the TA is valid. In this case whether UE should use the TA provided by the network or it can use the TA it estimated needs to be explicitly defined which is not clear from current MAC specification as this TA is not received over a MAC CE which is a cell switch command (CSC MAC CE) for which the UE behavior is clearly defined where UE is expected to use the TA provided over the CSC MAC CE.

[0278] The timer to check the TA validity can be configured and included in the agreed new MAC CE which is Optional. Else the TA validity timer (TAT) can be configured in RRC Reconfiguration messageFIG. 5B: Problem-2(Step-10 to Step-12): If the TA value in the CSC MAC CE is set as “FFF” and if UE is configured with UE based TA, in that case UE may use the UE estimated TA to perform RACH-less cell switch, this is the legacy behavior.

[0279] Similarly, in case if the network set the TA value as “FFF” in the new MAC CE for any reason (for e,g, serving cell couldn’t obtain the TA from the candidate, problem to transmit PDCCH order etc.), UE should be able to fall back to UE estimated TA. However, unlike the legacy behavior, UE behavior is not clearly defined in the specification that when TA included in the new MAC CE is set as “FFF”, then whether UE can use the UE estimated TA to perform RACH-less C-LTM execution.

[0280] FIG. 5C: Problem-3 (Step-13 to Step-15): If this newly proposed TA maintenance timer or the legacy TA validity timer expires before the conditions for C-LTM execution is met, then UE may determine that the TA provided by the network is invalid. In that case, UE in general is expected to fall back on a RACH-based C-LTM cell switch execution. However, from the previous agreements it is also observed that UE based TA measurement mechanism is supported for C-LTM.

[0281] Therefore, in such scenario must be allowed to use the UE estimated TA to perform the RACH-less C-LTM cell switch execution. However, such UE behaviour is not captured in the current MAC specification.

[0282] FIG. 5D: Problem-4 (Step-16 to Step-17): There can be scenario that UE never received the new MAC CE due to link problem between the UE and the S-DU. This is a special case of the more general case of the UE not having a RACH-based TA estimate when the cell switch is triggered (e.g., no RACH-based TA acquisition has been triggered). In this scenario if no valid RACH-based TA is available, then UE is expected to use the UE-based TA if UE is configured with UE based TA which is not currently specified in the MAC specification.

[0283] The network many initially configure the candidate cell(s) for conditional LTM and at the same time it configures UE for UE estimated TA, During the execution phase while the conditions for C-LTM is being evaluated based on L1 or L3 measurements, the network may decide to trigger cell switch by its own by sending a MAC CE which is a cell switch command (CSC MAC CE) for the same candidate cell. In that case UE will need to immediately trigger cell switch to the indicated candidate cell.FIG. 6A: Problem 5 (Step-7 to Step-11): For a UE configured with C-LTM, a RACH-based early TA acquisition procedure is triggered by the network (e.g., PDCCH order), the UE transmits the random-access preamble to the candidate cell and then receives new MAC CE the TA and the new TA maintenance timer.

[0284] Even though the UE is configured with CLTM, the network may still decide to command the UE to perform a cell switch using a CSC MAC CE for the same candidate cell. Since the TA has been previously provided to the UE, it may be unnecessary to send again the TA in the CSC.

[0285] For network triggered LTM, as per Release-18 it is mandatory for the network to include the TA in the CSC MAC CE while in Release-19 it could be optional. Thus TA may not be included in the CSC MAC CE or may be its value is set as “FFF” or null. Therefore, if network decides to trigger RACH-less LTM cell switch, it is not clear if it can use the TA that was provided by the network using the new MAC CE which was primarily meant to trigger conditional LTM cell switch.

[0286] FIG. 6B: Problem 6 (Step-12 to Step-16): If there is no TA value provided in the CSC MAC CE and the TA provided in the new MAC CE which was meant for conditional LTM has become invalid due to the expiry of the new TA maintenance timer, in that case UE can’t execute a RACH-less cel switch.

[0287] If UE has a valid UE estimated TA, the same can be used to perform a RACH-less cell switch execution for the same candidate cell.

[0288] However, there are no clear indications in current MAC specification which enables UE to use the UE estimated TA in case there is no valid TA available from the network either from CSC MAC CE or from the new MAC CE

[0289] Examples for Conditional-LTM with UE based TA estimation configured (No Conditional-LTM and Conventional-LTM inter-working involved)

[0290] Network initially configures the with conditional-LTM (C-LTM) and UE based TA estimation. The network may eventually trigger PDCCH order to the UE to perform RACH process towards the candidate cell(s) for early Uplink synchronization to obtain the early timing advance (TA) which is conveyed to the UE by the serving-DU via new MAC CE. This new MAC CE also contains the new TA maintenance timer.#1: For UE configured with Conditional LTM and UE based TA estimation, if early synchronization is triggered by the network and UE receives the early TA via new MAC CE from serving-DU, UE will always prioritize the use of the TA provided by the network and ignore the UE estimated TA. This UE behavior can be achieved and implemented through the enhancements in the current MAC specification TS 38.321. The proposed enhancements are captured the Text Proposal included in the Annexure-1.

[0291] #2: For UE configured with Conditional LTM and UE based TA estimation, if early synchronization is triggered by the network and UE receives the early TA via new MAC CE from serving-DU, if the TA provided in the new MAC CE becomes invalid due to the expiry of the related TA maintenance timer, UE can then use the available valid UE estimated TA. This UE behaviour can be achieved and implemented through the enhancements in the current MAC specification TS 38.321. The proposed enhancements are captured the Text Proposal included in the Annexure-1. #3: For UE configured with Conditional LTM and UE based TA estimation, if early synchronization is triggered by the network and UE receives the early TA via new MAC CE from serving-DU, if the TA provided in the new MAC CE is set as “FFF” (null value) or no TA is included or no new MAC CE is received at the UE, UE can then use the available UE estimated TA. This UE behavior can be achieved and implemented through the enhancements in the current MAC specification TS 38,321. The proposed enhancements are captured the Text Proposal included in the Annexure-1.

[0292] Examples for Conditional-LTM with UE based TA estimation configured ( Conditional-LTM and Conventional-LTM inter-working involved)

[0293] Network initially configures the UE with conditional-LTM (C-LTM) and UE based TA estimation. The network may eventually trigger PDCCH order to the UE to perform RACH process towards the candidate cell(s) for early Uplink synchronization to obtain the early timing advance (TA) which is conveyed to the UE by the serving-DU via a new MAC CE. This new MAC CE also contains the new TA maintenance timer (Optional). During the cell switch execution phase while the C-LTM conditions are being evaluated, the network may trigger cell switch by sending MAC CE which is a cell switch command (CSC MAC CE).#4: For UE configured with Conditional LTM and UE based TA estimation, during the C-LTM execution phase if UE receives a CSC MAC CE from the network which contains a valid TA (TA value not set as “FFF”) for the same candidate cell, the UE will use the latest TA which is provided via CSC MAC CE and trigger RACH-less cell switch ignoring the TA which was provided earlier over new MAC CE or UE estimated TA. This UE behaviour can be achieved and implemented through the enhancements in the current MAC specification TS 38.321. The proposed enhancements are captured the Text Proposal included in the Annexure-1.

[0294] #5: For UE configured with Conditional LTM and UE based TA estimation, during the C-LTM execution phase if UE receives a CSC MAC CE from the network which does not contain a valid TA (TA value set as “FFF” or no TA value included for the same candidate cell, the UE can use the latest TA (if valid) which is provided via new MAC CE for C-LTM TA and execute RACH-less cell switch ignoring the UE estimated TA. This UE behaviour can be achieved and implemented through the enhancements in the current MAC specification TS 38.321. The proposed enhancements are captured the Text Proposal included in the Annexure-1.

[0295] #6: For UE configured with Conditional LTM and UE based TA estimation, during the C-LTM execution phase if UE receives a CSC MAC CE from the network which does not contain a valid TA (TA value set as “FFF” or no TA value included for the same candidate cell, and at that time the TA acquired by the UE over the new MAC CE has become invalid due to the new TA maintenance timer expiry, UE can then use the UE estimated TA and execute RACH-less cell switch. This UE behaviour can be achieved and implemented through the enhancements in the current MAC specification TS 38.321. The proposed enhancements are captured the Text Proposal included in the Annexure-1.

[0296] #7: If both new MAC CE or CSC MAC CE doesn’t contain any TA value (set as “FFF”) or if the new MAC CE is not received at the UE while no TA is included CSC MAC CE, UE can then use the UE estimated TA in case UE is configured for UE based TA procedure and UE has successfully measured the TA and execute RACH-less cell switch. This UE behaviour can be achieved and implemented through the enhancements in the current MAC specification TS 38.321. The proposed enhancements are captured the Text Proposal included in the Annexure-1.

[0297] #8: If a UE performing recovery after a failed LTM or CLTM cell switch selects a cell which is an LTM candidate and if it has previously received a MAC CE containing TAinformation for this candidate and the TA value is still valid, then the UE can perform RACH-less recovery to the candidate, if it has a configured grant.

[0298] Annexture-1

[0299] This new UE behavior can also be implemented with the following procedural enhancements in the 3GPP MAC specification (TS 38.321).

[0300] TS 38.321 (Section 5.2- Maintenance of Uplink Time Alignment)

[0301] LTM CSC MAC-CE-based cell switch and conditional LTM cell switch treated separately

[0302] TP assuming TAC in CSC is mandatory (as in Rel-18)

[0303] when an LTM Cell Switch Command MAC CE is received:

[0304] ■ if the Timing Advance Command contained in the CSC MAC CE is not set as FFF:

[0305] apply the Timing Advance Command contained in the CSC MAC CE for the PTAG as specified in clause 6.1.3.75

[0306] start or restart the timeAlignmentTimer associated with the PTAG as specified in clause 6.1.3.75

[0307] ■ else (Timing Advance Command contained in the CSC MAC CE is set as FFF):

[0308] if a MAC CE has been previously received containing a TAC for the LTM or conditional LTM candidate indicated in the LTM CSC MAC CE and the Timing Advance contained in the MAC CE is valid (e.g., the timer associated with the received TA value is not expired)

[0309] ■ apply the Timing Advance Command contained in the MAC CE for the PTAG as specified in clause 6.1.3.75

[0310] ■ start or restart the timeAlignmentTimer associated with the PTAG as specified in clause 6.1.3.75.

[0311] else if the UE has successfully measured the Timing Advance as in clause 5.18.35

[0312] apply the measured Timing Advance for the PTAG;

[0313] start or restart the timeAlignmentTimer associated with the PTAG.TP assuming TAC in CSC is optional (valid possibility for Rel-19, as it is unnecessary to send the TA again to a UE, if already done so with the new MAC-CE)

[0314] when an LTM Cell Switch Command MAC CE is received:

[0315] ■ if the LTM CSC MAC CE contains a TAC and the TAC is not set as FFF: apply the Timing Advance Command contained in the CSC MAC CE for the PTAG as specified in clause 6.1.3.75

[0316] start or restart the timeAlignmentTimer associated with the PTAG as specified in clause 6.1.3.75

[0317] ■ else (LTM CSC MAC does not contain a TAC or the TAC contained in the CSC MAC CE is set as FFF):

[0318] if a MAC CE has been previously received containing a TAC for the LTM or conditional LTM candidate indicated in the LTM CSC MAC CE and the Timing Advance contained in the MAC CE is valid (e.g., the timer associated with the received TA value is not expired)

[0319] ■ apply the Timing Advance Command contained in the MAC CE for the PTAG as specified in clause 6.1.3.75

[0320] ■ start or restart the timeAlignmentTimer associated with the PTAG as specified in clause 6.1.3.75.

[0321] else if the UE has successfully measured the Timing Advance as in clause 5.18.35

[0322] ■ apply the measured Timing Advance for the PTAG;

[0323] ■ start or restart the timeAlignmentTimer associated with the PTAG.

[0324] When a conditional LTM cell switch to a target is triggered

[0325] ■ if a MAC CE has been previously received containing a TAC for the target and the Timing Advance contained in the MAC CE is valid (e.g., the timer associated with the received TA value is not expired) apply the Timing Advance Command contained in the MAC CE for the PTAG as specified in clause 6.1.3.75

[0326] start or restart the timeAlignmentTimer associated with the PTAG as specified in clause 6.1.3.75

[0327] ■ else if the UE has successfully measured the Timing Advance as in clause 5.18.35

[0328] apply the measured Timing Advance for the PTAG;start or restart the timeAlignmentTimer associated with the PTAG.

[0329] ■ else

[0330] Initiate RACH-based cell switch execution.

[0331] LTM CSC MAC-CE-based cell switch and conditional LTM cell switch treated jointly when an LTM cell switch to a target is triggered by the reception of an LTM Cell Switch Command MAC or by a conditional LTM cell switch condition being met:

[0332] ■ if a CSC MAC CE is received and the Timing Advance Command contained in the CSC MAC CE is not set as FFF:

[0333] apply the Timing Advance Command contained in the CSC MAC CE for the PTAG as specified in clause 6.1.3.75

[0334] start or restart the timeAlignmentTimer associated with the PTAG as specified in clause 6.1.3.75

[0335] ■ else (LTM CSC MAC CE not received, i.e. , conditional LTM cell switch triggered, or Timing Advance Command contained in the CSC MAC CE is set as FFF):

[0336] if a MAC CE has been previously received containing a TAC for the cell switch target and the Timing Advance contained in the MAC CE is valid (e.g., the timer associated with the received TA value is not expired)

[0337] ■ apply the Timing Advance Command contained in the MAC CE for the PTAG as specified in clause 6.1.3.75

[0338] ■ start or restart the timeAlignmentTimer associated with the PTAG as specified in clause 6.1.3.75.

[0339] else if the UE has successfully measured the Timing Advance as in clause 5.18.35

[0340] ■ apply the measured Timing Advance for the PTAG;

[0341] ■ start or restart the timeAlignmentTimer associated with the PTAG.

[0342] TP assuming TAC in CSC is optional (valid possibility for Rel-19, as it is unnecessary to send the TA again to a UE, if already done so with the new MAC-CE)

[0343] when an LTM cell switch to a target is triggered by the reception of an LTM Cell Switch Command MAC or by a conditional LTM cell switch condition being met:if an LTM CSC MAC CE containing a TAC is received and the TAC is not set as FFF:

[0344] apply the Timing Advance Command contained in the CSC MAC CE for the PTAG as specified in clause 6.1.3.75

[0345] start or restart the timeAlignmentTimer associated with the PTAG as specified in clause 6.1.3.75

[0346] ■ else (LTM CSC MAC CE not received, LTM CSC MAC CE does not contain a TAC or the TAC contained in the CSC MAC CE is set as FFF):

[0347] if a MAC CE has been previously received containing a TAC for the LTM or conditional LTM candidate indicated in the LTM CSC MAC CE and the Timing Advance contained in the MAC CE is valid (e.g., the timer associated with the received TA value is not expired)

[0348] ■ apply the Timing Advance Command contained in the MAC CE for the PTAG as specified in clause 6.1.3.75

[0349] ■ start or restart the timeAlignmentTimer associated with the PTAG as specified in clause 6.1.3.75.

[0350] else if the UE has successfully measured the Timing Advance as in clause 5.18.35

[0351] ■ apply the measured Timing Advance for the PTAG;

[0352] ■ start or restart the timeAlignmentTimer associated with the PTAG.

[0353] FIGS 7A - 7D: Step-1: Preparation phase for the conditional LTM for the candidate cell(s) configured with UE based TA estimation.

[0354] Step-2: UE estimates early TA

[0355] Step-3: Network triggers PDCCH order for a given candidate cell (Candidate cell-1) under target -DU.

[0356] Step-4: UE sends RACH preamble to the candidate cell (Candidate cell-1) under the target DU to acquire the early TA.

[0357] Step-5: Candidate cell (Candidate cell-1) under the target DU sends the TA value along with the new maintenance timer value to the S-CU.

[0358] Step-6: S-CU forwards the TA of the candidate cell (Candidate cell-1) value along with the value of the new maintenance timer to the S-DU and S-DU stores it

[0359] FIG. 7A: Scenario-1: Valid NW based TA obtained over new MAC CEStep-7: S-DU sends the new MAC CE to the UE containing the TA value (not set as “FFF”) for the candidate cell ((Candidate cell-1) along with the value of the new TA maintenance timer.

[0360] Step-8: UE find the TA value obtained in Step-6 as valid and C-LTM execution conditions are met.

[0361] Step-9: UE decides to ignore the UE estimated TA.

[0362] Step-10: UE applies the valid TA obtained via new MAC CE for the same candidate cell.

[0363] Step-11: UE initiates UE triggered RACH-less C-LTM using network provided TA over new MAC CE

[0364] FIG. 7B: Scenario-2: NW based TA obtained over new MAC CE expires

[0365] Step-12: S-DU sends the new MAC CE to the UE containing the TA value (not set as “FFF”) for the candidate cell ((Candidate cell-1) along with the value of the new TA maintenance timer.

[0366] Step-13: UE finds C-LTM executions conditions are met.

[0367] Step-14: UE determines the TA value obtained in Step-12 has become invalid as the new TA maintenance timer expired.

[0368] Step-15: UE initiates UE triggered RACH-less C-LTM using UE estimated TA.

[0369] FIG. 7C: Scenario-3: NW based TA obtained over new MAC CE set to "FFF" Step-16: S-DU sends the new MAC CE to the UE containing the TA value (set as “FFF”) for the candidate cell ((Candidate cell-1) along with the value of the new TA maintenance timer.

[0370] Step-17: UE finds C-LTM executions conditions are met.

[0371] Step-18: UE determines the TA value obtained in Step-16 is set to "FFF" and UE decides to use the UE estimated TA.

[0372] Step-19: UE initiates UE triggered RACH-less C-LTM using UE estimated TA

[0373] FIG. 7D: Scenario-4: No new MAC CE containing the TA is received at the UE Step-20: UE finds C-LTM executions conditions are met.

[0374] Step-21: UE determines that it has not received the new MAC CE for a given duration after receiving PDCCH order and decides to use UE estimated TA.

[0375] Step-22: UE initiates UE triggered RACH-less C-LTM using UE estimated TA.FIGs 8A - 8D: Step-1 : Preparation phase for the conditional LTM and network triggered LTM for the candidate cell(s).

[0376] Step-2: UE estimates the early TA.

[0377] Step-3: Network triggers PDCCH order for a given candidate cell (Candidate cell-1) under target -DU.

[0378] Step-4: UE sends RACH preamble to the candidate cell (Candidate cell-1) under the target DU to acquire the early TA.

[0379] Step-5: Candidate cell (Candidate cell-1) under the target DU sends the TA value along with the new maintenance timer value to the S-CU.

[0380] Step-6: S-CU forwards the TA of the candidate cell (Candidate cell-1) value along with the value of the new maintenance timer to the S-DU and S-DU stores it

[0381] FIG. 8A: Scenario-1: Valid NW based TA obtained over CSC MAC CE

[0382] Step-7: S-DU sends the new MAC CE to the UE containing the TA value (not set as “FFF”) for the candidate cell ((Candidate cell-1) along with the value of the new TA maintenance timer

[0383] Step-8: S-DU decides to trigger a network triggered LTM.

[0384] Step-9: S-DU sends the CSC MAC CE to the UE containing the TA value (not set as “FFF”) for the candidate cell ((Candidate cell-1).

[0385] Step-10: UE ignores the TA obtained via new MAC CE in Step-7.

[0386] Step-11 : UE applies the valid TA obtained via CSC MAC CE for the same candidate cell.

[0387] Step-12: UE initiates network triggered RACH-less LTM using network provided TA over CSC MAC CE.

[0388] FIG. 8B: Scenario-2: NW based TA obtained over CSC MAC CE set to "FFF" while TA obtained over new MAC CE not set as "FFF"

[0389] Step-13: S-DU sends the new MAC CE to the UE containing the TA value (not set as “FFF”) for the candidate cell ((Candidate cell-1) along with the value of the new TA maintenance timer.

[0390] Step-14: S-DU decides to trigger a network triggered LTM.

[0391] Step-15: S-DU sends the CSC MAC CE to the UE containing the TA value (set as “FFF”) for the candidate cell ((Candidate cell-1).Step-16: UE checks the validity of the TA received in Step-12.

[0392] Step-17: If the TA obtained in Step-12 over the new MAC CE for Candidate cell-1 is valid, it applies the TA

[0393] Step-18: UE initiates network triggered RACH-less LTM using TA obtained via new MAC CE

[0394] FIG. 8C: Scenario-3: TA to be provided in the CSC MAC CE could be optional in Rel-19 and TA obtained in new MAC CE has expired

[0395] Step-19: S-DU sends the new MAC CE to the UE containing the TA value (not set as “FFF”) for the candidate cell ((Candidate cell-1) along with the value of the new TA maintenance timer.

[0396] Step-20: S-DU decides to trigger a network triggered LTM.

[0397] Step-21: S-DU sends the CSC MAC CE with no TA value included for the candidate cell (Candidate cell-1) as per Rel-19 behaviour

[0398] Step-22: UE finds the TA value obtains using new MAC CE in Step-19 has become invalid and it decides to use the UE estimated TA.

[0399] Step-23: UE triggers RACH-less network triggered cell switch with UE based TA,

[0400] FIG. 8D: Scenario-4: NW based TA obtained over CSC MAC CE and TA obtained over new MAC CE both set as "FFF"

[0401] Step-24: S-DU sends the new MAC CE to the UE containing the TA value (set as “FFF”) for the candidate cell ((Candidate cell-1) along with the value of the new TA maintenance timer.

[0402] Step-25: S-DU decides to trigger a network triggered LTM.

[0403] Step-26: S-DU sends the CSC MAC CE to the UE containing the TA value (set as “FFF”) for the candidate cell ((Candidate cell-1).

[0404] Step-27: If TA in Step-24 and Step-26 for the same cell is set as "FFF" or the new MAC CE cis not receive dat the UE, UE applies the valid estimated TA for the same cell candidate cell (candidate cell-1)

[0405] Step-28: UE initiates network triggered RACH-less LTM using UE estimated TA.

[0406] Alternative to steps 27 and 28: The UE interprets the reception of a new MAC CE with TA value set to “FFF” as indication to not use the UE-based TA estimate in case it switches to that candidate cell (e.g., because sync between candidate and sourcecell is lost). Then, if the UE receives a CSC with the TA value set to “FFF”, the UE performs a RACH-based LTM cell switch.

[0407] The FIGs illustrate methods performed by a system comprising interaction between different system entities. The FIGs also illustrate a collection of separate methods performed separately by the different system entities.

[0408] There is present a set of numbered paragraphs A1.1, A1.2; A2.1, A2.2. The first paragraph in the set of paragraphs could form the basis for an independent claim.

[0409] A1.1. A user equipment, UE, 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: receiving configuration information associated with L1 / L2 triggered mobility, LTM; and triggering random-access channel, RACH,-less LTM towards a candidate access node, using: timing advance, TA, information, obtained from a network, if valid TA information obtained from the network is available for the candidate access node; or TA information estimated by the UE, if valid TA information obtained from the network is unavailable for the candidate access node.

[0410] A1.2. A UE as defined in paragraph A1.1, wherein the instructions, when executed by the at least one processor, cause the UE to: receive an LTM cell switch command comprising a TA command from the network; and perform triggering RACH-less LTM towards the candidate access node by using TA information comprised in the TA command, if the TA command is not set as FFF.

[0411] A1.3. A UE as defined in paragraph A1.2, wherein the instructions, when executed by the at least one processor, cause the UE to trigger the RACH-less LTM towards the candidate access node, if the TA command in the LTM cell switch command is set as FFF, and if a medium access control, MAC, control element comprising a TA element has been previously received and the TA information contained in the MAC control element is valid, using the TA information contained in the MAC control element.A1.4. A UE as defined in paragraph A1. 3, wherein the instructions, when executed by the at least one processor, cause the UE to trigger the RACH-less LTM towards the candidate access node, if the TA command in the LTM cell switch command is set as FFF, and if no MAC control element containing a TA command has been previously received or if the TA information contained in the MAC control element is invalid, and if the UE has successfully estimated TA information, using the TA information estimated by the UE.

[0412] A1.5. A UE as defined in paragraph A1. 1, wherein the instructions, when executed by the at least one processor, cause the UE to: receive an LTM cell switch command; and if the cell switch command comprises a TA command, perform triggering RACH-less LTM towards the candidate access node by using TA information comprised in the TA command, if the TA command is not set as FFF.

[0413] A1.6. A UE as defined in paragraph A1. 5, wherein the instructions, when executed by the at least one processor, cause the UE to trigger the RACH-less LTM towards the candidate access node, if the cell switch command does not comprise a TA command or if the TA command in the LTM cell switch command is set as FFF, and if a MAC control element containing a TA command has been previously received and the TA information contained in the MAC control element is valid, using the TA information contained in the MAC control element.

[0414] A1.7. A UE as defined in paragraph A1. 6, wherein the instructions, when executed by the at least one processor, cause the UE to trigger the RACH-less LTM towards the candidate access node, if no TA command is received in the LTM cell switch command or if the TA command in the LTM cell switch command is set as FFF, and if no MAC control element containing a TA command has been previously received or if the TA information contained in the MAC control element is invalid, and if the UE has successfully estimated TA information, using the TA information estimated by the UE.

[0415] A1.8. A UE as defined in paragraph A1. 1, wherein the instructions, when executed by the at least one processor, cause the UE to: determine that a condition for conditional LTM, CLTM, cell switch has been met; and trigger the RACH-less LTMtowards the candidate access node, if a MAC control element containing a TA command has been previously received and the TA information contained in the MAC control element is valid, using the TA information contained in the MAC control element.

[0416] A1.9. A UE as defined in paragraph A1. 8, wherein the instructions, when executed by the at least one processor, cause the UE to trigger the RACH-less LTM towards the candidate access node, if no MAC control element containing a TA command has been previously received or if the TA information contained in the MAC control element is invalid, and if the UE has successfully estimated TA information, using the TA information estimated by the UE.

[0417] A1.10. A UE as defined in paragraph A1. 9, wherein the instructions, when executed by the at least one processor, cause the UE to trigger the RACH-less LTM towards the candidate access node, if no MAC control element containing a TA command has been previously received or if the TA information contained in the MAC control element is invalid, and if the UE has not successfully estimated TA information, triggering RACH-based cell switch execution to the candidate access node.

[0418] A1.11. A UE as defined in paragraph A1. 1, wherein the instructions, when executed by the at least one processor, cause the UE to: determining whether an LTM cell switch has been received; if an LTM cell switch command has been received, and if the LTM cell switch command comprises a TA command, triggering RACH-less LTM towards the candidate access node by using TA information comprised in the TA command, if the TA command is not set as FFF; if an LTM cell switch command has not been received, determining whether a condition for CLTM cell switch has been met; and if a condition for CLTM cell switch has been met, triggering RACH-less CLTM towards the candidate access node, if a MAC control element containing a TA command has been previously received and the TA information contained in the MAC control element is valid, using the TA information contained in the MAC control element.

[0419] A1.12. A UE as defined in paragraph A1. 11, wherein the instructions, when executed by the at least one processor, cause the UE to, if an LTM cell switchcommand has been received, trigger the RACH-less LTM towards the candidate access node, if the LTM cell switch command does not comprise a TA command or if the TA command in the LTM cell switch command is set as FFF, and if a MAC control element containing a TA command has been previously received and the TA information contained in the MAC control element is valid, using the TA information contained in the MAC control element.

[0420] A1.13. A UE as defined in paragraph A1. 12, wherein the instructions, when executed by the sat least one processor, cause the UE to, if an LTM cell switch command has not been received, trigger the RACH-less LTM towards the candidate access node, if no MAC control element containing a TA command has been previously received or the TA information contained in the MAC control element is invalid, and if the UE has successfully estimated TA information, using the TA information estimated by the UE.

[0421] A1.14. A UE as defined in paragraph A1. 1, wherein the instructions, when executed by the at least one processor, cause the UE to: determine that an LTM cell switch to the candidate access node has been triggered; if an LTM cell switch command has been received, and if the LTM cell switch command comprises a TA command, triggering RACH-less LTM towards the candidate access node, if the TA command is not set as FFF.

[0422] A1.15. A UE as defined in paragraph A1. 14, wherein the instructions, when executed by the at least one processor, cause the UE to, if an LTM cell switch command has been received, trigger the RACH-less LTM towards the candidate access node, if the LTM cell switch command does not comprise a TA command or if the TA command in the LTM cell switch command is set as FFF, and if a MAC control element containing a TA command has been previously received and the TA information contained in the MAC control element is valid, using the TA information contained in the MAC control element.

[0423] A1.16. A UE as defined in paragraph A1. 15, wherein the instructions, when executed by the at least one processor, cause the UE to, if an LTM cell switch command has not been received, trigger the RACH-less LTM towards the candidatenode, if a MAC control element containing a TA command has been previously received and the TA information contained in the MAC control element is valid, using the TA information contained in the MAC control element.

[0424] A1 , 17. A UE as defined in paragraph A1. 16, wherein the instructions, when executed by the at least one processor, cause the UE to, if an LTM cell switch command has not been received, trigger the RACH-less LTM towards the candidate node, if no MAC control element containing a TA command has been previously received or if the TA information contained in the MAC control element is invalid, using the TA information estimated by the UE.

[0425] A1.18. A UE as defined in any of paragraphs A1.1 - A1.17, wherein the instructions, when executed by the at least one processor, cause the UE to, in dependence upon triggering the RACH-less LTM towards the candidate node, starting or restarting a timer associated with TA information validity.

[0426] A2.1. A method comprising: receiving configuration information associated with L1 / L2 triggered mobility, LTM; and triggering random-access channel, RACH,-less LTM towards a candidate access node, using: timing advance, TA, information, obtained from a network, if valid TA information obtained from the network is available for the candidate access node; or TA information estimated by the UE, if valid TA information obtained from the network is unavailable for the candidate access node.

[0427] FIG. 9 illustrates an example of a controller 900 suitable for use in an apparatus 910. Implementation of a controller 900 may be as controller circuitry. The controller 900 may be implemented in hardware alone, have certain aspects in software including firmware alone or can be a combination of hardware and software (including firmware).

[0428] As illustrated in FIG. 9 the controller 900 may be implemented using instructions that enable hardware functionality, for example, by using executable instructions 906 in a general-purpose or special-purpose processor 902 that may be stored on a machinereadable storage medium (disk, memory etc.) to be executed by such a processor 902.

[0429] The processor 902 is configured to read from and write to the memory 904. The processor 902 may also comprise an output interface via which data and / or commands are output by the processor 902 and an input interface via which data and / or commands are input to the processor 902.

[0430] The memory 904 stores instructions, program, or code 906 that controls the operation of the apparatus 910 when loaded into the processor 902. The instructions, program, or code 906, provide the logic and routines that enables the apparatus 910 to perform the methods illustrated in the accompanying FIGs. The processor 902 by reading the memory 904 is configured to load and execute the instructions, program, or code 906.

[0431] The apparatus 910 comprises:

[0432] at least one processor 902; and

[0433] at least one memory 904 storing instructions that, when executed by the at least one processor 902, cause the apparatus at least to:

[0434] receive configuration information associated with L1 / L2 triggered mobility, LTM; estimate timing advance, TA, information for a candidate access node; determine that an LTM condition or conditions for the first candidate access node has been met;

[0435] determine whether valid TA information, obtained from a network, is available or unavailable for the candidate access node;

[0436] in dependence upon a determination that valid TA information, obtained from the network, is available for the candidate access node, trigger random-access channel, RACH,-less LTM towards the candidate node using the TA information obtained from a network; and

[0437] in dependence upon a determination that valid TA information, obtained from the network, is unavailable for the candidate access node, trigger RACH-less LTM towards the candidate node using the estimated TA information.

[0438] In some examples, there is a (computer implemented) system comprising:controlling receiving configuration information associated with L1 / L2 triggered mobility, LTM;

[0439] controlling estimating timing advance, TA, information for a candidate access node; controlling determining that an LTM condition or conditions for the first candidate access node has been met;

[0440] controlling determining whether valid TA information, obtained from a network, is available or unavailable for the candidate access node;

[0441] in dependence upon a determination that valid TA information, obtained from the network, is available for the candidate access node, controlling triggering randomaccess channel, RACH,-less LTM towards the candidate node using the TA information obtained from a network; and

[0442] in dependence upon a determination that valid TA information, obtained from the network, is unavailable for the candidate access node, controlling triggering RACH-less LTM towards the candidate node using the estimated TA information.

[0443] As illustrated in FIG. 10, the instructions, program, or code 906 may arrive at the apparatus 910 via any suitable delivery mechanism 908. The delivery mechanism 908 may be, for example, a machine readable medium, a computer-readable medium, a non-transitory computer-readable storage medium, a computer program product, a memory device, a record medium such as a solid-state memory, an article of manufacture that comprises or tangibly embodies the instructions 906. The delivery mechanism may be a signal configured to reliably transfer the instructions 906. The apparatus 910 may propagate or transmit the instructions906 as a data signal.

[0444] The term “non-transitory” as used herein, is a limitation of the medium itself (i.e., tangible, not a signal ) as opposed to a limitation on data storage persistency (e.g., RAM vs. ROM).

[0445] The instructions 906 cause an apparatus to perform at least the following: causing estimating timing advance, TA, information for a candidate access node; causing determining that an LTM condition or conditions for the first candidate access node has been met;causing determining whether valid TA information, obtained from a network, is available or unavailable for the candidate access node;

[0446] in dependence upon a determination that valid TA information, obtained from the network, is available for the candidate access node, causing triggering random-access channel, RACH,-less LTM towards the candidate node using the TA information obtained from a network; and

[0447] in dependence upon a determination that valid TA information, obtained from the network, is unavailable for the candidate access node, causing triggering RACH-less LTM towards the candidate node using the estimated TA information.

[0448] The instructions 906 may be comprised in a computer program, a non-transitory computer readable medium, a computer program product, a machine readable medium. In some but not necessarily all examples, the instructions 906 may be distributed over more than one computer program.

[0449] Although the memory 904 is illustrated as a single component / circuitry it may be implemented as one or more separate components / circuitry some or all of which may be integrated / removable and / or may provide permanent / semi-permanent / dynamic / cached storage.

[0450] Although the processor 902 is illustrated as a single component / circuitry it may be implemented as one or more separate components / circuitry some or all of which may be integrated / removable. The processor 902 may be a single core or multi-core processor.

[0451] References to ‘computer-readable storage medium’, ‘computer program product’, ‘tangibly embodied computer program’ etc. or a ‘controller’, ‘computer’, ‘processor’ etc. should be understood to encompass not only computers having different architectures such as single / multi- processor architectures and sequential (Von Neumann) / parallel architectures but also specialized circuits such as field-programmable gate arrays (FPGA), application specific circuits (ASIC), signal processing devices and other processing circuitry including quantum processing circuitry. References to computer program, instructions, code etc. should be understood to encompass software for a programmable processor or firmware suchas, for example, the programmable content of a hardware device whether instructions for a processor, or configuration settings for a fixed-function device, gate array or programmable logic device etc.

[0452] As used in this application, the term ‘circuitry’ may refer to one or more or all the following:

[0453] (a) hardware-only circuitry implementations (such as implementations in analog, digital and / or quantum circuitry) and

[0454] (b) combinations of hardware circuit(s) and software, such as (as applicable):

[0455] i. a combination of analog, digital and / or quantum hardware circuit(s) with software / firmware and

[0456] ii. any or all portions of hardware processor(s) (including digital and / or quantum processor(s)) with software, and memory(ies) that work together to cause an apparatus, such as a mobile device, computing device or server, to perform various functions and

[0457] (c) any or all portions of hardware circuit(s), such as a microprocessor(s) and / or quantum processors , that requires software (for example, firmware) for operation, but the software may not be present when it is not needed for operation.

[0458] This definition of circuitry applies to all uses of this term in this application, including in any claims. As a further example, as used in this application, the term circuitry also covers an implementation of merely a hardware circuit or processor (or multiple processors) or portion of a hardware circuit or processor and its (or their) accompanying software and / or firmware. The term circuitry also covers, for example and if applicable to the claim element, a baseband integrated circuit or processor integrated circuit for a mobile device or a similar integrated circuit in a server, a cellular network device, or other computing or network device.

[0459] The blocks illustrated in the accompanying Figs may represent steps in a method and / or sections of code in the instructions 906. The illustration of a particular order to the blocks does not necessarily imply that there is a required or preferred order for the blocks and the order and arrangement of the block may be varied. Furthermore, it may be possible for some blocks to be omitted.As used here ‘module’ refers to a unit or apparatus that excludes certain parts / components that would be added by an end manufacturer or a user. The apparatus 910 can, for example be a module. A controller 900 of the apparatus 910 can, for example be a module.

[0460] Where a structural feature has been described, it may be replaced by means for performing one or more of the functions of the structural feature whether that function or those functions are explicitly or implicitly described.

[0461] The above-described examples find application as enabling components of: automotive systems; telecommunication systems; electronic systems including consumer electronic products; distributed computing systems; media systems for generating or rendering media content including audio, visual and audio visual content and mixed, mediated, virtual and / or augmented reality; personal systems including personal health systems or personal fitness systems; navigation systems; user interfaces also known as human machine interfaces; networks including cellular, non-cellular, and optical networks; ad-hoc networks; the internet; the internet of things; virtualized networks; and related software and services.

[0462] The apparatus can be provided in an electronic device, for example, a mobile terminal, according to an example of the present disclosure. It should be understood, however, that a mobile terminal is merely illustrative of an electronic device that would benefit from examples of implementations of the present disclosure and, therefore, should not be taken to limit the scope of the present disclosure to the same. While in certain implementation examples, the apparatus can be provided in a mobile terminal, other types of electronic devices, such as, but not limited to: mobile communication devices, hand portable electronic devices, wearable computing devices, portable digital assistants (PDAs), pagers, mobile computers, desktop computers, televisions, gaming devices, laptop computers, cameras, video recorders, GPS devices and other types of electronic systems, can readily employ examples of the present disclosure. Furthermore, devices can readily employ examples of the present disclosure regardless of their intent to provide mobility.The term ‘comprise’ is used in this document with an inclusive not an exclusive meaning. That is any reference to X comprising Y indicates that X may comprise only one Y or may comprise more than one Y. If it is intended to use ‘comprise’ with an exclusive meaning then it will be made clear in the context by referring to ‘comprising only one...’ or by using ‘consisting.’

[0463] In this description, the wording ‘connect’, ‘couple’ and ‘communication’ and their derivatives mean operationally connected / coupled / in communication. It should be appreciated that any number or combination of intervening components can exist (including no intervening components), i.e. , to provide direct or indirect connection / coupling / communication. Any such intervening components can include hardware and / or software components.

[0464] As used herein, the term "determine / determining" (and grammatical variants thereof) can include, not least: calculating, computing, processing, deriving, measuring, investigating, identifying, looking up (for example, looking up in a table, a database, or another data structure), ascertaining and the like. Also, "determining" can include receiving (for example, receiving information), accessing (for example, accessing data in a memory), obtaining and the like. Also, " determine / determining" can include resolving, selecting, choosing, establishing, and the like.

[0465] In this description, reference has been made to various examples. The description of features or functions in relation to an example indicates that those features or functions are present in that example. The use of the term ‘example’ or ‘for example’ or ‘can’ or ‘may’ in the text denotes, whether explicitly stated or not, that such features or functions are present in at least the described example, whether described as an example or not, and that they can be, but are not necessarily, present in some of or all other examples. Thus ‘example’, ‘for example’, ‘can’, or ‘may’ refers to a particular instance in a class of examples. A property of the instance can be a property of only that instance or a property of the class or a property of a sub-class of the class that includes some but not all the instances in the class. It is therefore implicitly disclosed that a feature described with reference to one example but not with reference to another example, can where possible be used in that other example as part of a working combination but does not necessarily have to be used in that other example.As used herein, “at least one of the following: ” and “at least one of ” and similar wording, where the list of two or more elements are joined by “and” or “or” mean at least any one of the elements, or at least any two or more of the elements, or at least all the elements.

[0466] Although examples have been described in the preceding paragraphs with reference to various examples, it should be appreciated that modifications to the examples given can be made without departing from the scope of the claims.

[0467] Features described in the preceding description may be used in combinations other than the combinations explicitly described above.

[0468] Although functions have been described with reference to certain features, those functions may be performable by other features whether described or not.

[0469] The description of a feature, such as an apparatus or a component of an apparatus, configured to perform a function, or for performing a function, should additionally be considered to also disclose a method of performing that function. For example, description of an apparatus configured to perform one or more actions, or for performing one or more actions, should additionally be considered to disclose a method of performing those one or more actions with or without the apparatus.

[0470] Although features have been described with reference to certain examples, those features may also be present in other examples whether described or not.

[0471] The term ‘a’, ‘an’ or ‘the’ is used in this document with an inclusive not an exclusive meaning. That is any reference to X comprising a / an / the Y indicates that X may comprise only one Y or may comprise more than one Y unless the context clearly indicates the contrary. If it is intended to use ‘a’, ‘an’ or ‘the’ with an exclusive meaning then it will be made clear in the context. In some circumstances the use of ‘at least one’ or ‘one or more’ may be used to emphasis an inclusive meaning but the absence of these terms should not be taken to infer any exclusive meaning.The presence of a feature (or combination of features) in a claim is a reference to that feature or (combination of features) itself and to features that achieve substantially the same technical effect (equivalent features). The equivalent features include, for example, features that are variants and achieve substantially the same result in substantially the same way. The equivalent features include, for example, features that perform substantially the same function, in substantially the same way to achieve substantially the same result.

[0472] In this description, reference has been made to various examples using adjectives or adjectival phrases to describe characteristics of the examples. Such a description of a characteristic in relation to an example indicates that the characteristic is present in some examples exactly as described and is present in other examples substantially as described.

[0473] As used herein, the terms “the at least one” and “the one or more” mean “any one of the at least one” and “any one of the one or mor” respectively.

[0474] The above description describes some examples of the present disclosure however those of ordinary skill in the art will be aware of possible alternative structures and method features which offer equivalent functionality to the specific examples of such structures and features described herein above and which for the sake of brevity and clarity have been omitted from the above description. Nonetheless, the above description should be read as implicitly including reference to such alternative structures and method features which provide equivalent functionality unless such alternative structures or method features are explicitly excluded in the above description of the examples of the present disclosure.

[0475] Whilst endeavoring in the foregoing specification to draw attention to those features believed to be of importance the Applicant may seek protection via the claims in respect of any patentable feature or combination of features hereinbefore referred to and / or shown in the drawings whether or not emphasis has been placed thereon.

[0476] l / we claim:

Claims

59CLAIMS1. A user equipment, UE, 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:receiving configuration information associated with L1 / L2 triggered mobility, LTM;estimating timing advance, TA, information for a candidate access node; determining that an LTM condition or conditions for the candidate access node has been met;determining whether valid TA information, obtained from a network, is available or unavailable for the candidate access node;in dependence upon a determination that valid TA information, obtained from the network, is available for the candidate access node, triggering random-access channel, RACH,-less LTM towards the candidate node using the TA information obtained from the network; andin dependence upon a determination that valid TA information, obtained from the network, is unavailable for the candidate access node, triggering RACH-less LTM towards the candidate access node using the estimated TA information.

2. A UE as claimed in claim 1, wherein determining that an LTM condition or conditions for the candidate access node has been met comprises receiving, from a network node, a cell switch command, CSC.

3. A UE as claimed in any preceding claim, wherein determining that valid TA information, obtained from the network, is available for the candidate access node comprises:receiving, from the network, an indication of TA information for the candidate access node; anddetermining that the indicated TA information is valid.

4. A UE as claimed in claim 3, wherein the indication of TA information is received with an associated timer value for a timer associated with TA information validity, and wherein determining that valid TA information, obtained from the network, is available for the candidate access node comprises:60starting a timer associated with TA information validity having the timer value; anddetermining that the timer associated with TA information validity has not expired when the LTM condition or conditions for the candidate access node has been met.

5. A UE as claimed in any of claims 3 to 4, wherein determining that valid TA information, obtained from the network, is unavailable for the candidate access node comprises:receiving, from the network, an indication of TA information for the candidate access node; anddetermining that the indicated TA information is invalid.

6. A UE as claimed in claim 5, wherein determining that the indicated TA information is invalid comprises determining that the indicated TA information has a null value.

7. A UE as claimed in claim 5 or claim 6, wherein the indication of TA information is received with an associated timer value for a timer associated with TA information validity, and wherein determining that valid TA information, obtained from the network, is unavailable for the candidate access node comprises:starting a timer associated with TA information validity having the timer value; anddetermining that the timer associated with TA information validity has expired when the LTM condition or conditions for the candidate access node has been met.

8. A UE as claimed in any of claims 3 to 7, wherein determining that valid TA information, obtained from the network, is unavailable for the candidate access node comprises:determining that an indication of TA information for the candidate access node has not been received from the network when the LTM condition or conditions for the candidate access node has been met.

619. A UE as claimed in any of claims 2 to 8, wherein the instructions, when executed by the at least one processor, cause the UE to perform at least:determining whether valid TA information, obtained from the network within a cell switch command, CSC, is available or unavailable for the candidate access node;in dependence upon a determination that valid TA information, obtained from the network within a CSC, is available for the candidate access node, triggering RACH-less LTM towards the candidate node using the TA information obtained from the network within a CSC; andin dependence upon a determination that valid TA information, obtained from the network within a CSC, is unavailable for the candidate access node, determining whether valid TA information, obtained from the network, is available or unavailable for the candidate access node.

10. A UE as claimed in claim 9, wherein determining that valid TA information, obtained from the network within a CSC, is available for the candidate access node comprises:receiving, from the network, and within a CSC, an indication of TA information for the candidate access node; anddetermining that the indicated TA information is valid.

11. A UE as claimed in any of claims 9 to 10, wherein determining that valid TA information, obtained from the network within a CSC, is unavailable for the candidate access node comprises:receiving, from the network, and within a CSC, an indication of TA information for the candidate access node; anddetermining that the indicated TA information is invalid.

12. A UE as claimed in claim 11, wherein determining that the indicated TA information is invalid comprises determining that the indicated TA information has a null value.6213. A UE as claimed in any of claims 9 to 12, wherein determining that valid TA information, obtained from the network within a CSC, is unavailable for the candidate access node comprises:determining that an indication of TA information has not been received from the network within a CSC when the LTM condition or conditions for the candidate access node has been met.

14. A method comprising:receiving configuration information associated with L1 / L2 triggered mobility, LTM; estimating timing advance, TA, information for a candidate access node; determining that an LTM condition or conditions for the candidate access node has been met;determining whether valid TA information, obtained from a network, is available or unavailable for the candidate access node;in dependence upon a determination that valid TA information, obtained from the network, is available for the candidate access node, triggering random-access channel, RACH,-less LTM towards the candidate access node using the TA information obtained from the network; andin dependence upon a determination that valid TA information, obtained from the network, is unavailable for the candidate access node, triggering RACH-less LTM towards the candidate access node using the estimated TA information.

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