Lower-layer triggered mobility

Abstract

The application relates to lower-layer triggered mobility (LTM) mobility. The application describes a method, apparatus, and computer program. The method includes: receiving, from a network node, a configuration comprising a list of candidate target cells for a LTM handover. For at least one candidate target cell of the list of candidate target cells, performing an evaluation of a receive timing difference between a signal received from a serving cell and a signal received from the at least one candidate target cell and further performing a determination based on the RTD and a predetermined time window, the determination being related to an ability of the apparatus to perform layer (1) measurements for the at least one candidate target cell. The method then causes transmission of information on the determination to the network node.

Description

[0001] LOWER-LAYER TRIGGERED MOBILITY

[0002] TECHNOLOGICAL FIELD

[0003] Various example embodiments relate to wireless communication systems, and more particularly, to facilitating mobility within such wireless communication systems.

[0004] BACKGROUND

[0005] As user equipment (UE) moves through a wireless communication system, it may move through cells that comprise regions of radio coverage that are supported by one or more network access node. Maintaining an ability for a UE to communicate effectively with the wireless communication system as it moves through regions of radio coverage is typically referred to as mobility. Layer 1 or layer 2 triggered mobility, sometimes termed lower layer triggered mobility, is being increasingly considered to reduce the latency, overhead and interruption time associated with a UE moving between the different cells.

[0006] BRIEF SUMMARY

[0007] The scope of protection sought for various example embodiments of the invention is set out by the independent claims. The example embodiments and features, if any, described in this specification that do not fall under the scope of the independent claims are to be interpreted as examples useful for understanding various embodiments of the invention.

[0008] According to various, but not necessarily all, example embodiments of the invention there is provided an apparatus, comprising: at least one processor; and at least one memory storing instructions that when executed by the at least one processor cause the apparatus at least to perform: receive, from a network node, a configuration comprising a list of candidate target cells for a lower-layer triggered mobility, LTM, handover; for at least one candidate target cell of the list of candidate target cells, perform an evaluation of a receive timing difference, RTD, between a signal received from a serving cell and a signal received from the at least one candidate target cell; perform a determination based on the RTD and a predetermined time window, wherein the determination is related to an ability of the apparatus to perform layer 1, L1, measurements for the at least one candidate target cell; and cause transmission of information on the determination to the network node. In lower layer triggered mobility LTM which may be called layer 1 / layer 2 L1 / L2 triggered mobility the target cell selected for a handover will be based on lower layer measurements made at the apparatus, which may be a UE. Some apparatus only support lower layer measurements where the RTD and predetermined time window have a certain relationship, for example in some embodiments where the RTD is less than or equal to the predetermined time window. This is apparatus dependent, and the network may be unaware of the apparatus’s ability in this regard and may therefore be unaware of which lower layer measurements are reliable and which may not be. In order to address this potential problem, the apparatus evaluates the RTD for one or more candidate cells and performs a determination of its ability to perform these lower layer measurements reliably based on the RTD and the predetermined time window and provides this information to the network.

[0009] For the sake of this application LTM and L1 / L2 triggered mobility handovers are used interchangeably to mean handovers triggered by lower layer measurements.

[0010] In some example embodiments, the predetermined time window comprises a length of a cyclic prefix (CP).

[0011] In some example embodiments, the information is caused to be transmitted in at least one of the following: a L1 measurement report transmission; a L3 measurement report transmission; or a transmission that is separate from any L3 or L1 measurement report transmissions.

[0012] In some example embodiments, causing the transmission of information on the determination comprises one of the following: causing transmission of the information only in a first L1 and / or a first L3 measurement report; causing transmission of the information in a predetermined number of L1 and / or L3 measurement reports; or causing transmission of the information in all L1 and / or L3 measurement reports until said LTM handover is complete.

[0013] In some example embodiments, the apparatus is caused to perform the determination based on the RTD and the predetermined time window by: determining those candidate target cells for which the RTD is less than or equal to the predetermined time window; and the information on the determination comprises an indication of those candidate cells for which the RTD is less than or equal to the predetermined time window.

[0014] In some example embodiments, the apparatus is caused to perform the determination based on the RTD and the predetermined time window by determining those candidate target cells for which the RTD is greater than the predetermined time window; and the information on the determination comprises an indication of those candidate cells for which the RTD is greater than the predetermined time window.

[0015] In some example embodiments, the apparatus is configured to cause transmission of an indication of whether the apparatus supports or does not support L1 measurements based on whether said RTD is greater than, equal to, or less than the predetermined time window.

[0016] In some example embodiments, the apparatus is configured to cause transmission of an indication that the apparatus does support L1 measurements when the RTD is less than or equal to the predetermined time window.

[0017] In some example embodiments, the apparatus is configured to cause transmission of an indication that the apparatus does not support L1 measurements when the RTD is greater than the predetermined time window.

[0018] In some example embodiments, the apparatus is further caused to: receive, from the network node, an indication that for at least one candidate target cell of the list of candidate target cells the RTD can be assumed to be less than or equal to the predetermined time window; and based on the received indication, skip performing the evaluation of the RTD for the at least one candidate target cell with RTD assumed to be less than or equal to the predetermined time window.

[0019] In some example embodiments, the at least one candidate target cell with RTD assumed to be less than or equal to the predetermined time window is collocated with the serving cell. In some example embodiments, the apparatus is further caused to: perform L1 measurements for a set of candidate target cells for which the RTD is less than or equal to the predetermined time window.

[0020] In some example embodiments, the apparatus is further caused to: receive a configuration to perform L1 measurements only for said set of candidate cells.

[0021] In some example embodiments, the apparatus is further caused to: transmit an L1 measurement report to the network node comprising the performed L1 measurements.

[0022] In some example embodiments, the apparatus is further caused to: update the evaluation of the RTD, update the determination, and cause transmission of information on the updated determination to the network node.

[0023] According to various, but not necessarily all, example embodiments of the invention there is provided a method performed at an apparatus, comprising: receiving, from a network node, a configuration comprising a list of candidate target cells for a lower-layer triggered mobility, LTM, handover, for at least one candidate target cell of the list of candidate target cells, performing an evaluation of a receive timing difference, RTD, between a signal received from a serving cell and a signal received from the at least one candidate target cell; performing a determination based on the RTD and a predetermined time window, wherein the determination is related to an ability of the apparatus to perform layer 1 , L1, measurements for the at least one candidate target cell; and causing transmission of information on the determination to the network node.

[0024] In some example embodiments, the predetermined time window comprises a length of a cyclic prefix (CP).

[0025] In some example embodiments, the information is transmitted in a L1 measurement report transmission.

[0026] Alternatively and / or additionally the information may be transmitted in a L3 measurement report transmission. Transmitting the information in measurement reports that are transmitted periodically to the network may be an effective and signalling efficient way of providing the information.

[0027] Alternatively and / or additionally the information may be transmitted in a transmission that is separate from any layer 3 or layer 1 measurement report transmission.

[0028] This may be a more flexible arrangement than combining the indication with an L1 or L3 measurement report. However, it increases the number of transmission made by the UE.

[0029] In some example embodiments, causing the transmission of information on the determination comprises one of the following: causing transmission of the information only in a first L1 and / or a first L3 measurement report; causing transmission of the information in a predetermined number of L1 and / or L3 measurement reports; or causing transmission of the information in all L1 and / or L3 measurement reports until said LTM handover is complete.

[0030] In some example embodiments, the performing the determination based on the RTD and the predetermined time window comprises: determining those candidate target cells for which the RTD is less than or equal to the predetermined time window; and wherein the information on the determination comprises an indication of those candidate cells for which the RTD is less than or equal to the predetermined time window.

[0031] In some example embodiments, the performing the determination based on the RTD and the predetermined time window comprises: determining those candidate target cells for which the RTD is greater than the predetermined time window; and wherein the information on the determination comprises an indication of those candidate cells for which the RTD is greater than the predetermined time window.

[0032] The relevant factor in whether or not the apparatus may be able to reliably perform L1 measurements may be whether the RTD is less than or equal to the predetermined time window, thus, whether this is the case, or whether this is not the case, may be determined and indicated to the network.

[0033] In some example embodiments, the indication is a list of detected candidate target cells, or a bit map indicating candidate cells in a list, the bit map may distinguish between cells in a list that have an RTD greater than the predetermined time window and those with an RTD less than or equal to the predetermined time window.

[0034] In some example embodiments, said method comprises causing transmission of an indication of whether the apparatus supports or does not support L1 measurements based on whether the RTD is greater than, equal to, or less than the predetermined time window

[0035] In some example embodiments, said method comprises causing transmission of an indication that the apparatus supports L1 measurements when the RTD is equal to or less than the predetermined time window.

[0036] In some example embodiments, said method comprises causing transmission of an indication that the apparatus does not support L1 measurements when the RTD is greater than the predetermined time window.

[0037] In some example embodiments, the method further comprises: receiving, from the network node, an indication that for at least one candidate target cell of the list of candidate target cells the RTD can be assumed to be less than or equal to the predetermined time window; and based on the received indication, skipping performing the evaluation of the RTD for the at least one candidate target cell with RTD assumed to be less than or equal to the predetermined time window.

[0038] In some example embodiments, the at least one candidate target cell with RTD assumed to be less than or equal to the predetermined time window is collocated with the serving cell.

[0039] In some example embodiments, the method comprises performing L1 measurements for a set of candidate target cells for which the RTD is less than or equal to the predetermined time window.

[0040] In some example embodiments, the method comprises receiving a configuration to perform L1 measurements only for said set of candidate cells.

[0041] In some example embodiments, the method comprises causing transmission of an L1 measurement report to the network node comprising the performed L1 measurements. In some example embodiments, the method comprises updating the evaluation of the RTD, updating the determination, and causing transmission of information on the updated determination to the network node.

[0042] According to various, but not necessarily all, example embodiments of the invention there is provided a non-transitory computer readable medium comprising program instructions stored thereon which when executed by a processor on an apparatus cause said apparatus to perform at least the following: receive from a network node, a configuration comprising a list of candidate target cells for a lower-layer triggered mobility, LTM, handover, for at least one candidate target cell of the list of candidate target cells, perform an evaluation of a receive timing difference, RTD, between a signal received from a serving cell and a signal received from the at least one candidate target cell; perform a determination based on the RTD and a predetermined time window, wherein the determination is related to an ability of the apparatus to perform layer 1 , L1, measurements for the at least one candidate target cell; and transmit information on the determination to the network node.

[0043] According to various, but not necessarily all, example embodiments of the invention there is provided a computer program which when executed by a processor on an apparatus cause said apparatus to perform at least the following: receive from a network node, a configuration comprising a list of candidate target cells for a lower-layer triggered mobility, LTM, handover, for at least one candidate target cell of the list of candidate target cells, perform an evaluation of a receive timing difference, RTD, between a signal received from a serving cell and a signal received from the at least one candidate target cell; perform a determination based on the RTD and a predetermined time window, wherein the determination is related to an ability of the apparatus to perform layer 1 , L1, measurements for the at least one candidate target cell; and cause transmission of information on the determination to the network node.

[0044] The computer program may cause the apparatus to perform the optional features set out in relation to the method mentioned above.

[0045] According to various, but not necessarily all, example embodiments of the invention there is provided an apparatus, comprising: means for receiving, from a network node, a configuration comprising a list of candidate target cells for a lower-layer triggered mobility, LTM, handover; means for performing for at least one candidate target cell of the list of candidate target cells, an evaluation of a receive timing difference, RTD, between a signal received from a serving cell and a signal received from the at least one candidate target cell; means for performing a determination based on the RTD and a predetermined time window, wherein the determination is related to an ability of the apparatus to perform layer 1 , L1 , measurements for the at least one candidate target cell; and means for causing transmission of information on the determination to the network node.

[0046] The means may perform the optional features set out in relation to the apparatus mentioned above.

[0047] According to various, but not necessarily all, example embodiments of the invention there is provided an apparatus, comprising circuitry configured to receive, from a network node, a configuration comprising a list of candidate target cells for a lower-layer triggered mobility, LTM, handover, circuitry configured to perform, for at least one candidate target cell of the list of candidate target cells, an evaluation of a receive timing difference, RTD, between a signal received from a serving cell and a signal received from the at least one candidate target cell; circuitry configured to perform a determination based on the RTD and a predetermined time window, wherein the determination is related to an ability of the apparatus to perform layer 1 , L1 , measurements for the at least one candidate target cell; and circuitry configured to cause transmission of information on the determination to the network node.

[0048] The circuitry may be configured perform the optional features set out in relation to the apparatus mentioned above.

[0049] According to various, but not necessarily all, example embodiments of the invention there is provided an apparatus, comprising: at least one processor; and at least one memory storing instructions that when executed by the at least one processor cause the apparatus at least to perform: cause transmission, to a user equipment, UE, of a configuration comprising a list of candidate target cells for a lower-layer triggered mobility (LTM) handover; receive from the UE, information on a determination by the UE for at least one candidate target cell of the list of candidate target cells, wherein the determination is based on a receive timing difference, RTD, between a signal received by the UE from a serving cell and a signal received by the UE from the at least one candidate target cell and a predetermined time window, wherein the determination is related to an ability of the UE to perform layer 1 , L1 , measurements for the at least one candidate cell; based on the received information, select a subset of candidate target cells of the list of candidate target cells; cause transmission of an indication of the subset of candidate target cells to the UE.

[0050] In some example embodiments, the predetermined time window comprises a length of a cyclic prefix (CP).

[0051] In some example embodiments, the apparatus is further caused to perform: receive, from the UE, an L1 measurement report comprising L1 measurements for the subset; and selecting a target cell for the LTM handover based on the L1 measurement report.

[0052] In some example embodiments, the apparatus is further caused to perform transmission of an indication of the selected target cell to the UE comprised within a cell switch command.

[0053] In some example embodiments, the subset comprises those candidate target cells for which the RTD is less than or equal to the predetermined time window.

[0054] In some example embodiments, the apparatus is further caused to: cause transmission of an indication that for at least one candidate target cell of the list of candidate target cells the RTD can be assumed to be less than or equal to the predetermined time window to said UE.

[0055] In some example embodiments, the indication of the at least one candidate target cell with RTD assumed to be less than or equal to the predetermined time window is comprised within a transmission also comprising the configuration.

[0056] In other embodiments, the indication of the at least one candidate target cell with RTD assumed to be less than or equal to the predetermined time window is comprised in a transmission that does not include said candidate target cell configuration.

[0057] In some example embodiments, the at least one candidate target cell with RTD assumed to be less than or equal to the predetermined time window is collocated with the serving cell. In some example embodiments, the apparatus is further caused to perform transmission of an instruction to the UE to skip performing an evaluation of the RTD for the at least one candidate target cell with RTD assumed to be less than or equal to the predetermined time window.

[0058] In some example embodiments, the apparatus is further caused to: when selecting the subset of candidate cells, reconfigure the list of candidate target cells for the LTM handover to exclude those candidate target cells for which the RTD is greater than the predetermined time window; and when causing transmission of an indication of the subset of candidate target cells to the UE, cause transmission of an indication of the reconfigured list of candidate target cells to the UE.

[0059] In some example embodiments, the apparatus is further caused to perform receiving from the UE an indication of candidate target cells detected by the UE, preferably wherein the indication of the detected candidate target cells is comprised in a layer 3 measurement report.

[0060] In some example embodiments, the apparatus comprises a network node.

[0061] In some example embodiments, the network node comprises a gNodeB.

[0062] According to various, but not necessarily all, example embodiments of the invention there is provided a method performed at an apparatus, comprising: causing transmission, to a user equipment, UE, of a configuration comprising a list of candidate target cells for a lower-layer triggered mobility (LTM) handover; receiving, from the UE, information on a determination by the UE for at least one candidate target cell of the list of candidate target cells, wherein the determination is based on a receive timing difference, RTD, between a signal received by the UE from a serving cell and a signal received by the UE from the at least one candidate target cell and a predetermined time window, wherein the determination is related to an ability of the UE to perform layer 1 , L1, measurements for the at least one candidate cell; based on the received information, selecting a subset of candidate target cells of the list of candidate target cells; causing transmission of an indication of the subset of candidate target cells to the UE. In some example embodiments, the predetermined time window comprises a length of a cyclic prefix (CP).

[0063] In some example embodiments, the subset comprises those candidate target cells for which the RTD is less than or equal to the predetermined time window.

[0064] In some example embodiments, the subset of candidate target cells may be those for the UE to perform L1 measurements on.

[0065] In some example embodiments, the subset comprises those candidate target cells for which the RTD is greater than the predetermined time window.

[0066] In some example embodiments, the subset indicates those candidate target cells the UE should not perform L1 measurements on.

[0067] In some example embodiments, the method further comprises: receiving, from the UE, an L1 measurement report comprising L1 measurements for the subset; and selecting a target cell for the LTM handover based on the L1 measurement report.

[0068] In some example embodiments, the method further comprises causing transmission of an indication of the selected target cell to the UE comprised within a cell switch command.

[0069] In some example embodiments, the cell switch command is comprised within a MAC CE transmission.

[0070] In some example embodiments, the selecting the target cell comprises selecting a candidate target cell for which the RTD is less than or equal to the predetermined time window.

[0071] In some example embodiments, the selecting the target cell comprises opting not to select a candidate target cell for which the RTD is greater than the predetermined time window.

[0072] In some example embodiments, the method further comprises causing transmission of an indication that for at least one candidate target cell of the list of candidate target cells the RTD can be assumed to be less than or equal to the predetermined time window to said UE.

[0073] In some example embodiments, the indication of the at least one candidate target cell with RTD assumed to be less than or equal to the predetermined time window is comprised within a transmission also comprising the configuration. In other embodiments, the indication of the at least one candidate target cell with RTD assumed to be less than or equal to the predetermined time window is comprised in a transmission that does not include said candidate target cell configuration.

[0074] In some example embodiments, the at least one candidate target cell with RTD assumed to be less than or equal to the predetermined time window is collocated with the serving cell.

[0075] In some example embodiments, the method further comprises causing transmission of an instruction to the UE to skip performing an evaluation of the RTD for the at least one candidate target cell with RTD assumed to be less than or equal to the predetermined time window.

[0076] In some example embodiments, the step of selecting the subset of candidate cells comprises reconfiguring the list of candidate target cells for the LTM handover to exclude those candidate target cells for which the RTD is greater than the predetermined time window; and said step of causing transmission of an indication of the subset of candidate target cells to the UE comprises causing transmission of an indication of the reconfigured list of candidate target cells to the UE.

[0077] In some example embodiments, the list of candidate target cells comprises eight candidate target cells.

[0078] In some example embodiments, the method further comprises receiving from the UE an indication of candidate target cells detected by the UE, preferably wherein the indication of the detected candidate target cells is comprised in a layer 3 measurement report.

[0079] According to various, but not necessarily all, example embodiments of the invention there is provided a non-transitory computer readable medium comprising program instructions stored thereon which when executed by a processor on an apparatus cause said apparatus to perform at least the following: receive from a network node, a configuration comprising a list of candidate target cells for a lower-layer triggered mobility, LTM, handover, for at least one candidate target cell of the list of candidate target cells, perform an evaluation of a receive timing difference, RTD, between a signal received from a serving cell and a signal received from the at least one candidate target cell; perform a determination based on the RTD and a predetermined time window, wherein the determination is related to an ability of the apparatus to perform layer 1 , L1, measurements for the at least one candidate target cell; and cause transmission of information on the determination to the network node.

[0080] According to various, but not necessarily all, example embodiments of the invention there is provided a computer program which when executed by a processor on an apparatus cause said apparatus to perform at least the following: cause transmission, to a user equipment, UE, of a configuration comprising a list of candidate target cells for a lower- layer triggered mobility (LTM) handover; receive from the UE, information on a determination by the UE for at least one candidate target cell of the list of candidate target cells, wherein the determination is based on a receive timing difference, RTD, between a signal received by the UE from a serving cell and a signal received by the UE from the at least one candidate target cell and a predetermined time window, wherein the determination is related to an ability of the UE to perform layer 1 , L1 , measurements for the at least one candidate cell; based on the received information, select a subset of candidate target cells of the list of candidate target cells; cause transmission of an indication of the subset of candidate target cells to the UE.

[0081] The computer program may cause the apparatus to perform the optional features set out in relation to the method mentioned above.

[0082] According to various, but not necessarily all, example embodiments of the invention there is provided an apparatus, comprising: means for transmitting to a user equipment, UE, a configuration comprising a list of candidate target cells for a lower-layer triggered mobility (LTM) handover; means for receiving from the UE, information on a determination by the UE for at least one candidate target cell of the list of candidate target cells, wherein the determination is based on a receive timing difference, RTD, between a signal received by the UE from a serving cell and a signal received by the UE from the at least one candidate target cell and a predetermined time window, wherein the determination is related to an ability of the UE to perform layer 1 , L1 , measurements for the at least one candidate cell; means for selecting a subset of candidate target cells of the list of candidate target cells based on the received information; wherein said means for transmitting is configured to transmit an indication of the subset of candidate target cells to the UE.

[0083] The means may perform the optional features set out in relation to the apparatus mentioned above.

[0084] According to various, but not necessarily all, example embodiments of the invention there is provided an apparatus, comprising circuitry configured to cause transmission to a user equipment, UE, a configuration comprising a list of candidate target cells for a lower-layer triggered mobility (LTM) handover; means for receiving from the UE, information on a determination by the UE for at least one candidate target cell of the list of candidate target cells, wherein the determination is based on a receive timing difference, RTD, between a signal received by the UE from a serving cell and a signal received by the UE from the at least one candidate target cell and a predetermined time window, wherein the determination is related to an ability of the UE to perform layer 1 , L1, measurements for the at least one candidate cell; circuitry configured to select a subset of candidate target cells of the list of candidate target cells based on the received information; wherein said circuitry configured to cause transmission is configured to cause transmission of an indication of the subset of candidate target cells to the UE.

[0085] The circuitry may be configured to perform the optional features set out in relation to the apparatus mentioned above.

[0086] Further particular and preferred aspects are set out in the accompanying independent and dependent claims. Features of the dependent claims may be combined with features of the independent claims as appropriate, and in combinations other than those explicitly set out in the claims.

[0087] Where an apparatus feature is described as being operable to provide a function, it will be appreciated that this includes an apparatus feature which provides that function or which is adapted or configured to provide that function.

[0088] BRIEF DESCRIPTION Some example embodiments will now be described with reference to the accompanying drawings in which:

[0089] FIG. 1 schematically illustrates an example embodiment of a network;

[0090] FIG. 2 illustrates an example embodiment of a measurement selection process;

[0091] FIG. 3 illustrates an example embodiment of a signalling diagram for a LTM handover;

[0092] FIG. 4 illustrates an example embodiment of a timeline for a LTM handover;

[0093] FIG. 5 illustrates an example embodiment of a timeline for a LTM handover;

[0094] FIG. 6 illustrates an example embodiment of a signalling diagram for LTM candidate configuration;

[0095] FIG. 7 illustrates an example embodiment of a signalling diagram for NW RTD indication; and

[0096] FIG. 8 schematically illustrates an example embodiment of a UE and network node.

[0097] DETAILED DESCRIPTION

[0098] Before discussing the example embodiments in any more detail, first an overview will be provided.

[0099] In lower-layer (Layer 1 (L1) and / or Layer 2 (L2)) triggered mobility (LTM), the network configures a set of candidate target cells for the UE for a LTM handover. The network transmits an indication of these candidate target cells to the UE. In response, the UE may indicate to the network via L3 measurement reporting which of the candidate target cells have been detected by the UE.

[0100] Prior to a handover to one or more of the candidate target cells, the UE may perform L1 measurements on the detected candidate target cells such that an optimal target cell can be selected for the handover. An indication of the L1 measurements may be transmitted in an L1 measurement report to the network which may determine which target candidate cell should be used for the handover based on the L1 measurement report. L1 measurements may include, for example, L1 RSRP measurements.

[0101] Some UEs support L1 measurements where the receive timing difference (RTD) is greater than a predetermined time window such as the cyclic prefix (CP) but other UEs do not. The RTD is the receive timing difference between DL signals received from the serving cell and the potential LTM target cells DL signal. The RTD is highly dynamic and it may change rapidly (from milliseconds to seconds) depending on UE radio conditions and UE mobility. A CP is repeated information at the beginning and end of a symbol which acts as a guard period to avoid interference between signals arriving via different paths.

[0102] Given that the ability to perform L1 measurements on cells for which the RTD is greater than the CP is optional, some UEs will be able to perform reliable L1 measurements when the RTD is greater than the CP and some will not. For UEs which do not support L1 measurements on cells for which the RTD is greater than the CP, L1 measurements may not be possible, may not be reliable or may not be trustworthy. As a result, a problem arises in that the network is not aware that a particular UE has determined that the RTD for one or more candidate cells is greater than the CP and that it may not be able to perform L1 measurements for some of the candidate cells. This can lead the network to instruct the UE to perform L1 measurements for candidate cells it cannot perform L1 measurements on resulting in an unnecessary UE power consumption and possibly unreliable L1 measurements. Moreover, the network may then select a target cell which is a sub-optimal choice for the LTM handover.

[0103] In view of the above, in some embodiments, the UE evaluates the RTD for each candidate target cell and determines if the RTD is greater than, equal to, or less than the CP and in some cases whether the UE is capable of performing L1 measurements for each cell. The UE transmits an indication of a result of this determination to the network such that the network is aware of the cells for which the UE cannot perform L1 measurements. The network can then avoid instructing the UE to perform L1 measurements on these cells. In this way, embodiments can reduce or avoid unnecessary L1 measurements, thereby improving UE battery life. Furthermore, depending on the capabilities of the UE and the received information, the network may select a cell for LTM handover. For example, the network may select a target cell for which reliable L1 measurement information has been received. In this way, embodiments can mitigate the risk of handovers to sub-optimal target cells.

[0104] In summary, the determination as to whether the RTD is greater than, equal to, or less than a predetermined time window may allow an apparatus such as a UE to determine for which target cells useable L1 measurements may be performed. In the context of a LTM handover, providing an indication of this determination to the network node allows the network node to ultimately select a target cell for the LTM handover based on reliable L1 measurements. Power consumption at a UE may also be reduced because unnecessary L1 measurements (i.e. , L1 measurements for target cells where RTD > time window) can be avoided using knowledge of the determination by the UE.

[0105] L1 / L2 triggered or Lower-Layer Triggered Mobility

[0106] As part of the Further NR Mobility Enhancements work item, 3GPP is specifying mechanism and procedures of L1 / L2 triggered mobility (LTM) for mobility latency reduction. The main difference between existing mobility and LTM based mobility is that the existing (layer 3 (L3)) mobility uses L3 measurements and L3 measurement reporting (which is based on RRC layer messaging). LTM aims to use means other than RRC signaling for measurement reporting to reduce latency. Additionally, in order to reduce handover (HO) latency, the aim is to change the actual cell switch (handover) from being an L3 command using RRC signaling to use means other than RRC signaling (i.e., an LTM switch command using, for example, MAC messaging).

[0107] LTM is a procedure in which a user eguipment (UE) is changing serving cells based on an L1 / L2 triggered command. The UE is configured with one or more candidate cells with RRC configuration. In LTM, the network receives L1 measurement reports from UEs and the network decides and requests changes to the UE serving cell(s) by a cell switch command (e.g., MAC control element (MAC CE)). The MAC CE is used to indicate which configured LTM candidate cell (e.g., cell id) is to be used as target for a cell switch.

[0108] Hence, indicating a LTM HO.

[0109] L3 measurements may be assumed to be performed in parallel with any LTM related measurements. It is assumed that the UE performs intra-frequency and possibly interfrequency measurements as required.

[0110] The cell switch is triggered by selecting, for example, one of the LTM candidate cell configurations as the target configuration by the gNB. An LTM candidate cell configuration can only be added, modified, and released by the network via RRC signaling.

[0111] LTM supports intra-gNB-DU, intra-gNB-CU, and inter-gNB-DU mobility. LTM also supports inter-frequency mobility, including mobility to inter-frequency cell that is not a current serving cell. LTM currently applies for UEs in NR connected mode. However, the procedure may also apply to other states and systems like 6G in the future.

[0112] LTM Procedure

[0113] The current design of the LTM procedure based on latest RAN1 and RAN2 agreements can be summarized in the following steps:

[0114] 1. A LTM capable UE is connected to a cell (serving cell) and is performing L3 measurements on one or more neighboring cells.

[0115] 2. Network provides to the UE through RRC message a LTM candidate cell configuration comprising one or multiple LTM candidate cells.

[0116] - LTM Candidate cell configuration can be a reference configuration or delta configuration on top of the reference configuration. Each LTM candidate cell configuration can be provided as delta configuration on top of a reference configuration, which is used to form a complete candidate cell configuration. The reference configuration can be managed separately, and a UE stores the reference configuration as a separate configuration.

[0117] - When a complete candidate cell configuration is applied, it replaces the current UE configuration at the time of cell switch. Although the reconfiguration procedure makes replacement, it doesn’t necessarily reset MAC, RLC or PDCP layer.

[0118] - In LTM, User plane is continued without reset (e.g. intra-DU LTM), if it is configured in RRC signalling, with the target to avoid data loss and the additional delay of data recovery.

[0119] 3. UE performs L1 measurements on one or more of the configured candidate cells and reports the results to the network.

[0120] 4. Based on the UE reported L1 measurement results, the network may configure the UE to activate one or more transmission configuration indication (TCI) states with MAC CE for one or multiple candidate cells. Unified TCI state framework is used in LTM. Related to the target cell TCI control, there are at least two options:

[0121] - The network may activate TCI state(s) for candidate cell(s) before the cell switch command in a separate MAC CE activation command, while TCI state indication follows in the cell switch command.

[0122] - Alternatively, TCI state activation and indication may both be done at the cell switch command (step 5)

[0123] 5. From the one or more activated LTM candidate cells, the network may select one of the LTM candidate cells as the target cell and request the UE to switch cell using a MAC-CE LTM cell switch command to the UE to have the UE to execute the LTM handover to the target cell.

[0124] - The cell switch command comprises (at least) of:

[0125] - The index of LTM candidate target configuration to apply for LTM cell switch

[0126] - Indication (and possibly activation if not done before the cell switch as in step 4) of DL and UL (or joint) TCI state ID for the LTM target cell

[0127] - DL and UL BWP ID

[0128] 6. UE detaches from the source cell and LTM handover is completed to the target cell.

[0129] Optional / additional steps:

[0130] As an additional or optional step, the network may request the UE to prepare UL synchronization to the potential LTM target cell before the cell switch command. This step may be executed between steps 4 and 5 above.

[0131] To prepare one or multiple LTM candidate cells, the network may initiate PDCCH ordered random access on one or multiple candidate cells before the network request the actual LTM cell switch to the target cell(s).

[0132] On the potential LTM target cell, using random access channel (RACH), the UE may send preamble to enable the network to acquire the UL timing, timing advance (TA) value, of the target cell before the cell switch command.

[0133] However, UL synchronization may also be executed after the LTM cell switch, for example if early TA acquisition has not been performed by PDCCH order before the cell switch command.

[0134] Subsequent LTM between configured LTM candidate cells is supported. In subsequent LTM UE does not release other LTM candidate cell configurations after LTM is triggered. Therefore, subsequent LTM can be performed without RRC reconfiguration.

[0135] Unified TCI State Framework

[0136] The unified TCI state framework was specified by 3GPP in Rel-17. In Rel-15 / 16, TCI states were only configured for downlink, while uplink spatial relation covered the beam indication for uplink. With the unified TCI state concept, TCI states are configured for both downlink and uplink. The configuration is either joint, where same TCI state covers both UL and DL or separate, where there are separate TCI states configured for DL and UL.

[0137] Unified TCI state framework uses a common TCI state concept, where one common TCI / indicated TCI state at a time provides spatial assumption for the set of signals and channels (PDCCH, PDSCH, PUCCH, PUSCH, CSI-RS, SRS).

[0138] Configuration and control of the unified TCI states is done with the following steps:

[0139] 1. RRC-based configuration of up to 128 TCI states

[0140] 2. MAC-CE-based activation of up to 16 UL or DL TCI states or 8 joint TCI states into 8 codepoints

[0141] - In LTM, the activation may be done in a separate MAC CE before the cell switch command or in the cell switch command

[0142] 3. DCI-based indication of a single joint TCI state or TCI state pair of DL and UL TCI states

[0143] - In LTM, the indication is done in the cell switch command, which is a MAC CE

[0144] L1 and L3 Measurements

[0145] In NR, RSRP measurements are performed and reported at Layer 1 (physical layer) and Layer 3 (RRC layer). Measurements can be SSB based or, if configured in CS / - MeasConfig, also CSI-RS based.

[0146] L3 measurements are cell or beam level measurements. Beam level measurements are generated from the L1 measurements by applying L3 filtering. Cell level measurements are derived from the L1 measurements using certain rules. L3 RSRP measurements are reported in a RRC measurement report in MeasResults information element.

[0147] L1 measurements are beam level measurements. L1 RSRP measurements are reported by the UE at channel state information (CSI) based on CSI-ReportConfig. CSI reporting is defined in TS 38.214, section 5.2.1. The time and frequency resources that can be used by the UE to report CSI are controlled by the gNB. CSI may consist of Channel Quality Indicator (CQI), precoding matrix indicator (PMI), CSI-RS resource indicator (CRI), SS / PBCH Block Resource indicator (SSBRI), layer indicator (LI), rank indicator (Rl), L1- RSRP, L1-SINR or Capabilityindex. Each Reporting Setting CSI-ReportConfig [TS 38.331] is associated with a single downlink BWP given in the associated CSI-ResourceConfig for channel measurement and contains the parameter(s) for one CSI reporting band. Each CSI Resource Setting CSI-ResourceConfig contains a configuration of a list of S>1 CSI Resource Sets, where the list is comprised of references to either or both of NZP CSI-RS resource set(s) and SS / PBCH block set(s) or the list is comprised of references to CSI-IM resource set(s). Each CSI Resource Setting is located in the DL BWP identified by the higher layer parameter BWP-id, and all CSI Resource Settings linked to a CSI Report Setting have the same DL BWP. The reporting configuration for CSI can be aperiodic (using PLISCH), periodic (using PLICCH) or semi-persistent (using PLICCH, and DCI activated PLISCH).

[0148] For L1-RSRP reporting, if the higher layer parameter nrofReportedRS in CSI- ReportConfig is configured to be one, the reported L1-RSRP value is defined by a 7-bit value in the range [-140, -44] dBm with 1dB step size. If the higher layer parameter nrofReportedRS is configured to be larger than one, or if the higher layer parameter groupBasedBeamReporting is configured as 'enabled', or if the higher layer parameter groupBasedBeamReporting-r17 is configured, the UE shall use differential L1-RSRP based reporting, where the largest measured value of L1-RSRP is quantized to a 7-bit value in the range [-140, -44] dBm with 1dB step size, and the differential L1-RSRP is quantized to a 4-bit value. The differential L1-RSRP value is computed with 2 dB step size with a reference to the largest measured L1-RSRP value which is part of the same L1- RSRP reporting instance. The mapping between the reported L1-RSRP value and the measured quantity is described in TS 38.133.

[0149] LTM Measurements

[0150] LTM handover is planned to be based on L1 measurements. However, the details of these measurements are still open in RAN4 (how to measure, what to measure, when to measure etc). In some examples, L3 measurements are included in an L1 measurement report to be used in making the decision about the LTM target cell.

[0151] Importantly, UEs may not be capable of performing L1 measurements on all candidate cells that are included in the LTM candidate cell configuration. There will be UEs that support L1 measurements on candidate target cells for which the RTD is greater than, equal to, or less than a predetermined time window, such as a cyclic prefix (CP), and there will be UEs which only support measurements on candidate target cells for which the RTD is less than or equal to the predetermined time window as further discussed below. It follows that the number of cells on which the UE can perform LTM L1 measurements may be smaller than the maximum number of candidate cells in the LTM candidate cell configuration (which may contain up to 8 cells).

[0152] Given that the support of LTM L1 -measurements, e.g., L1-RSRP measurements, on candidate target cells for which the RTD is greater than the predetermined time window, which in many cases may be the cyclic prefix CP, is an optional UE capability, some UEs cannot reliably measure L1-RSRP for the candidate cells when the RTD observed on the UE side is larger than CP.

[0153] This RTD condition relaxation may be problematic because the network does not know the UE observed the RTD between the serving and a neighbour cell at the UE. Therefore, the network does not know for which of the candidate cells the UE is able to perform L1 measurements, is able to perform reliable L1 measurements, or where the measurement results can be trusted (i.e., the candidate cells for which the RTD <= CP). As the network does not know on which candidate cells the UE can perform L1 measurements, the network may indicate to the UE a target cell for LTM handover for which the UE cannot perform L1 reliable measurements.

[0154] A LTM capable UE not being able to perform LTM L1 -measurements on all the configured candidate cells may therefore lead to unreliable L1 measurements being taken, unnecessary L1 measurements being taken draining the resources of the UE, and handovers to suboptimal candidate cells.

[0155] Some embodiments introduce signalling that indicates to the network the cells for which the RTD is above, below or equal to the predetermined time window, such as the CP. In some embodiments, issues are addressed by performing a down selection, based on the UE capability, among the configured cells to select candidate cells on which LTM L1 measurements are to be performed.

[0156] Fig. 1 shows an embodiment of a network comprising a user equipment (UE) 10, a current serving cell 20 and a number of neighbouring cells 22a, 22b which may act as candidate target cells for a LTM handover. The number of network nodes may vary from what is shown. In the example of Fig. 1 , the UE is only capable of performing L1 measurements on candidate target cells for which the RTD is less than or equal to a predetermined time window such as the CP. The UE does not support performing L1 measurements on candidate target cells for which the RTD is greater than the predetermined time window.

[0157] Fig. 1 shows that the UE evaluates the RTD (RTD_1 , RTD_2) with respect to each candidate cell 22a, 22b. RTD1 is measured between a signal received at the UE 10 from serving cell 20 and a signal received at the UE 10 from the neighbour cell 22a. RTD2 is measured between a signal received at the UE 10 from the serving cell 20 and a signal received at the UE 10 from the neighbour cell 22b. In some example embodiments, where the UE is configured with a primary serving cell and one or more secondary cells in a dual connectivity arrangement, the UE may evaluate an RTD associated with the one or more secondary cells. In this case, the RTD is measured between a signal received at the UE from the primary serving cell and a signal received at the UE from the one or more secondary cells.

[0158] The UE is configured to cause transmission of an indication of the RTD evaluations to the network. This message is called UE LTM measurement indication. The UE LTM measurement indication indicates to the network which cells the UE can perform reliable L1 measurements on (i.e. , which cells are currently fulfilling the criteria RTD <= CP). When the UE determines that the RTD > the CP, as the UE does not support L1 measurements in this situation, the UE may not be able to perform measurements which meet the requirements or any L1 measurement results may be invalid. If the UE has performed L1 measurements, the indication may be included in the L1 measurement report to tell the network in which conditions the measurements have been performed. The network may take this information into account in its decision making, such as in generating a LTM RRC candidate cell configuration or in LTM cell switch decisions.

[0159] Fig. 1 further shows the optional transmission from the network to the UE of an indication that the RTD can be assumed to be less than or equal to the CP for one or more candidate target cells. This message is called network RTD indication and is discussed in more detail below. Fig. 2 shows a timeline of events according to an embodiment. At stage 1001 , the network provides to the UE a configuration comprising a list of eight candidate target cells 22 for a LTM handover. In some networks, eight is the maximum number of candidate target cells in an LTM candidate cell configuration. In some examples, the list of candidate target cells may comprise some other number of candidates, e.g., smaller than eight.

[0160] At stage 1002, the UE detects one or more of the candidate target cells 22. In this example, cells 1 to 4 have been detected by the UE. This information may be transmitted to the network in an L3 measurement report. Note that L3 measurements are performed periodically by the UE.

[0161] At stage 1003, the UE has evaluated the RTD for each of the detected cells and performed a determination whether the RTD is greater than, equal to, or less than a predetermined time window, such as the CP. In this example, the UE does not support L1 measurements on cells for which the RTD is greater than the CP. Out of the detected cells, only cells 1 and 2 are determined to have an RTD less than or equal to the CP. The UE transmits an indication of this determination using a UE LTM measurement indication transmission as shown in Fig. 1. In response to receipt of this information, the network may indicate a subset of the detected candidate target cells upon which the UE should perform L1 measurements. The subset may comprise one or more cells. In this case, the subset includes cell 1 and cell 2. The L1 measurements may be reported to the network node in an L1 measurement report for consideration by the network node in its decision making. In some embodiments, the network updates the list of candidate target cells. In some embodiments, the network selects one or more target cells for LTM handover.

[0162] Fig. 3 shows a signalling diagram demonstrating how the UE LTM measurement indication may be implemented according to an embodiment.

[0163] Initially, the UE is in an RRC connected state.

[0164] At step 1 , the UE transmits one or more L3 measurement reports containing an indication of L3 measurements performed by the UE. The L3 measurement and measurement report events may be periodic. The L3 measurements are not a pre-requisite for RRC configuration but they are required for the UE to send L1 measurement reports. At step D1, the network node serving cell, gNB Source, prepares a configuration comprising a list of candidate target cells for LTM handover.

[0165] At step 2, the network node transmits to the UE the prepared LTM candidate cell configuration with up to N (e.g., 8) candidate cells, where N indicates the number of configurable LTM cells. At step 3, the UE transmits an RRC reconfiguration complete acknowledgement.

[0166] At step D2, the UE evaluates the RTD for at least one of the candidate target cells of the list of candidate target cells. The list of candidate target cells has been received from the network node. In some embodiments, the UE evaluates the RTD for all candidate target cells that have been detected by the UE (stage 1002 of Fig. 2).

[0167] In step D2, the UE may further perform a determination based on the capability of the UE, the RTD and a predetermined time window, such as the length of the cyclic prefix, to identify which of the configured candidate cells it can perform L1 measurements on. The cell is assumed to be detected and remains detected during the evaluation and / or reporting procedure. Alternatively, this determination may be made by the network node. In this case, the UE transmits an indication of the evaluated RTD and the capability of the UE to the network node as per steps 4, 6, and / or 8.

[0168] In some embodiments, the UE is not capable of performing L1 measurements on cells for which the RTD is greater than the CP. In this case, the UE shall evaluate the RTD compared to the serving cell for each detected cell in the candidate cell configuration and select the cells for which the RTD is less than or equal to the CP to report to the network.

[0169] The evaluation procedure may be triggered independently or with signalling (e.g., DCI or MAC CE), or it may be part of a procedure, such as down selection, where the evaluation is performed across multiple cells based on L1 / L3 measurement report.

[0170] In steps 4, 6 and 8, the UE indicates to the network the cells it is able to perform L1 measurements on based on the RTD evaluation and the capability of the UE. Specifically, based on the comparison of the evaluated RTD to the CP in step D2, the UE transmits an LTM candidate cell indication as an indication of the one or more LTM candidate cells which the UE is capable of performing L1 measurements on. In some embodiments, the capability of the UE is limited to performing L1 measurements on cells for which the RTD is less than or equal to the CP. Thus, the LTM candidate cell indication may indicate cells which meet this criterium. In some embodiments, the UE indicates cells which do not meet this criterium. In this case, the network should avoid instructing the UE to perform L1 measurements on these cells or avoid selecting these cells for a LTM handover.

[0171] Steps 4, 6 and 8 will now be discussed in more detail.

[0172] In step 4, the LTM candidate cell indication is transmitted as a part of an LTM L1 measurement report. This could be an existing L1 measurement report or an LTM-specific measurement report. In step 6, a similar procedure occurs except that the LTM candidate cell indication is transmitted as part of an L3 measurement report.

[0173] The LTM candidate cell L1 or L3 measurement report comprises:

[0174] • First part:

[0175] The UE LTM measurement indication. This may include the index / measurement ID / Cell ID of each candidate cell the UE is able to perform L1 measurements on (e.g., cells that have RTD <= CP). Alternatively, the list can be an exclude list where the UE reports cells where RTD >= CP or RTD > CP (i.e. the cells on which the UE is not able to perform L1 measurements). Another alternative is that the indication may be a flag or a bit (e.g., RTD=true / false or 1 / 0 which indicates whether CP <= (or >=, >, <) than RTD.

[0176] • Second Part:

[0177] If the UE has already performed L1 measurements for a subset or for all of the indicated candidate cells, the L1-RSRP values for the corresponding cells.

[0178] In some embodiments, the UE includes the UE LTM measurement indication in the first part of the measurement report: a. only in the first L1 / L3 measurement report after the LTM candidate cell configuration, i.e. after the UE has received a configuration comprising at least one candidate target cell for LTM, and / or b. in all L1 / L3 measurement reports until the cell switch command, and / or c. if RTD <= CP conditions have changed, and / or d. after number of evaluations / reports have passed (e.g. evaluations are counted with a counter), and / or e. based on a network request.

[0179] In the first part of the LTM L1 measurement report, the UE includes the UE LTM measurement indication including all the candidate cells for which the UE can perform L1 measurements. The number of indicated cells may be up to the number of cells in the LTM candidate cell configuration. While reporting, the UE may update this part of the report according to the UE conditions as discussed in more detail below.

[0180] In the second part of the report, the UE includes L1-RSRP values (absolute or differential value, SSB or CSI-RS based) for each cell the UE has measured. This may be all the cells indicated in the first part of the report, if the UE is capable of measuring all of them, or it may be a subset of the indicated cells based on some kind of further UE or network based down selection.

[0181] In an example embodiment, when the network configures candidate cells #1-8 to the UE, the UE is able to measure and actually also measuring and reporting cells #1-4, the report would contain these parts:

[0182] First part (UE LTM measurement indication):

[0183] Candidate cell #1

[0184] Candidate cell #2

[0185] Candidate cell #3

[0186] Candidate cell #4

[0187] Second part (L1-RSRP value for (a subset of) LTM candidate cells the UE has measured):

[0188] L1-RSRP for Candidate cell #1

[0189] L1-RSRP for Candidate cell #2

[0190] L1-RSRP for Candidate cell #3

[0191] L1-RSRP for Candidate cell #4

[0192] Fig. 4 illustrates an example LTM L1 measurement report that may be used in step 4. Fig. 4 is disclosed in more detail further down. In step 8, the LTM candidate cell indication is transmitted as part of a separate existing or a new LTM-based message (e.g., MAC CE, DCI, RRC).

[0193] In some embodiments, the UE sends the UE LTM measurement indication to the network in a message that is separate from any L1 or L3 measurement reports. The indication may comprise the index / measurement ID / Cell ID of each candidate cell the UE is able to perform L1 measurements on (e.g. cells that have RTD <= CP). Alternatively, the list can be an exclude list where the UE reports cells where RTD >= CP or RTD > CP (i.e. the cells on which the UE is not able to perform L1 measurements).

[0194] In some embodiments, the UE may send the indication: a. once after receiving the candidate cell command, and / or b. if RTD =< CP conditions have changed and / or c. after number of evaluations have passed (e.g. evaluations are counted with a counter), and / or d. based on a network request.

[0195] In some embodiments, the UE shall report the L1 / L3 measurement results separately based on the existing reporting.

[0196] When the UE LTM measurement indication is sent in a message separate from the measurement report, the UE shall include all the candidate cells for which it can perform L1 measurements in the indication part of the message. The number of indicated cells may be up to the number of cells in the LTM candidate cell configuration. The indication may be included in some existing message (e.g. UE assistance information, MAC CE) or a new LTM specific message.

[0197] In an example embodiment, when the network configures candidate cells #1-8 to the UE and the UE is able to measure cells #1-4, the indication included in an existing or new message would be:

[0198] Candidate cell #1

[0199] Candidate cell #2

[0200] Candidate cell #3

[0201] Candidate cell #4 An example of such a report that may be used in step 8 is shown in Fig. 5 which will be discussed in more detail below.

[0202] It will be appreciated that steps 4, 6 and 8 are not mutually exclusive. The LTM candidate cell indication may be transmitted using any one of these signals or any combination thereof.

[0203] In some embodiments, based on the evaluation and indication, the network and / or UE (dynamically) select the LTM candidate cells the UE shall perform L1 measurements on, e.g., based on UE capability. Although not shown in Fig. 3, the network may transmit to the UE an indication of the cells on which the UE should perform L1 measurements. In some embodiments, the network indicates to perform L1 measurements on all cells for which the RTD is less than or equal to the CP. The L1 measurements may be transmitted to the network in an L1 measurement report.

[0204] Steps 10, 11 , 12 and D3 represent the network behaviour upon receiving the indication. In response to receipt of the indication, the network node takes this information into account in the cell switch I handover I measurement procedure. The network selects the target cell based on the UE L1 measurements (and the RTD evaluation results and the determination of the UE’s ability to perform L1 measurements for the cells) and, in step 13, indicates a cell switch (LTM handover) to the target cell in an LTM cell switch command transmission.

[0205] The following examples consider the network behaviour when RTD > CP as it poses the problems discussed above. However, other cases such as the other way around where RTD <= CP may also be used as the relevant factor and the decisions made accordingly.

[0206] The network may use the UE LTM measurement indication in various ways.

[0207] • LTM candidate configuration

[0208] If indication (e.g., RTD > CP) was sent in L3 measurement report, the network may decide not to configure LTM candidates for the cells with conditions of RTD > CP.

[0209] • LTM Cell switch decision

[0210] The network may decide not to perform LTM cell switch on a cell where RTD > CP (this may be together or independently decided if UE is supporting RTD > CP or not). For example, where UE indicates that RTD > CP: if UE supports RTD > CP, network may perform cell switch.

[0211] If UE does not support RTD > CP, the network does not perform cell switch on that neighbour cell.

[0212] If UE supports RTD > CP and indicates RTD > CP, network may still decide not to use that cell for a cell switch.

[0213] • LTM down selection process when RTD > CP UE based down selection:

[0214] Network may reconfigure UE and exclude cells where RTD > CP

[0215] - Alternatively, the network may indicate in a configuration for UE not to perform down selection when RTD > CP (down selection impact)

[0216] Network indicated down selection:

[0217] The network may exclude the cells from the list of candidate cells where the following conditions have been met o UE indicates RTD > CP and UE does not support RTD > CP capability

[0218] Updating the determination

[0219] In some embodiments, due to the fact that the RTD is dynamic and depends on network conditions and UE mobility, the UE is configured to perform periodical / semi-persistent or event triggered RTD evaluation and reporting. Updating the evaluation of the RTD in this way allows an updated determination to be made on whether the RTD is greater than, equal to, or less than the predetermined time window (e.g., the CP). Once reported to the network, the network can update the list of candidate target cells, decide which candidate target cells the UE should perform L1 measurements on, and / or select a target cell for LTM handover based on more up-to-date information. In a periodical / semi-persistent configuration, the RTD evaluation is performed within certain (configurable or static) time intervals.

[0220] Fig. 4 shows a timeline 100 of events when step 4 of Fig. 3 is implemented according to an embodiment. Initially, the network supplies the UE with a candidate cell configuration at step 102. Then, at steps 104 and 106 respectively, the UE performs RTD evaluations and reports the information in an LTM L1 measurement report to the network. The LTM L1 measurement report comprises a first part including a list of candidate cells the UE can perform L1 measurements on and an empty second part as no measurements have been made yet. In fig. 4, it is assumed that the UE capability allows the UE to measure all the indicated cells, i.e., no further down selection of the measured cells is assumed in addition to the RTD-based down selection.

[0221] In step 108, the UE re-evaluates the RTD of the candidate cells after a predetermined time or after being instructed to do so. After the re-evaluation / updated evaluation, an updated LTM L1 measurement report is sent in step 110, with an updated list of cells which the UE can measure in the first part and any L1 measurements in the second part.

[0222] Taking L1 measurements and / or re-evaluating the RTD may occur any number of times (e.g., once more in step 112) before the UE receives a cell switch command in step 114 from the network indicating which target cell is to be used for the handover, i.e., candidate cell #6. It will be appreciated that the timeline shown may include additional steps.

[0223] Fig. 5 shows a timeline 200 of events when step 8 of Fig. 3 is implemented according to an embodiment. Initially, the network supplies the UE with a candidate cell configuration in step 202. Then, in steps 204 and 206 respectively, the UE performs RTD evaluations and reports the information in a dedicated UE LTM measurement indication transmission. After L1 measurements are performed, these are transmitted to the network in a normal LTM L1 measurement report in step 208. In Fig. 5, it is assumed that the UE is capable of measuring all the indicated cells, i.e., no further down selection of the measured cells is assumed in addition to the RTD-based down selection.

[0224] In step 210, the UE re-evaluates the RTD of the candidate cells after a predetermined time or after being instructed to do so. After the re-evaluation / updated evaluation, updated results are transmitted to the network using another dedicated UE LTM measurement indication transmission in step 212. An updated L1 measurements report is also sent in step 214.

[0225] Taking L1 measurements and / or re-evaluating the RTD and reporting it may occur any number of times before the UE receives a cell switch command in step 216 from the network indicating which target cell is to be used for the handover, i.e., candidate cell #6. It will be appreciated that the timeline shown may include additional steps.

[0226] Fig. 6 shows an example embodiment where the network influences the LTM configuration based on the UE RTD evaluation. Depending on UE mobility, the RTD conditions may change, and therefore UE may be required to transmit RTD evaluation information either in L3 report as shown, or in an L1 report. The network may use this information differently. For example, the network may not configure LTM for cells where RTD > CP if UE does not support RTD > CP capability.

[0227] In this case, the UE may, or may not, be configured with LTM but it is required to perform an RTD > CP evaluation for L3 measurements. This may be, for instance, if the UE supports LTM (LTM capability) and the UE supports RTD > CP (RTD > CP capability).

[0228] The UE may be required to report in the L3 measurement report per cell, or per beam, if during the measurements, RTD > CP as shown in step 5. Alternatively, the UE may always be required to add the information, or only if it has changed.

[0229] Upon receiving the report, the network may decide whether to configure LTM candidates where RTD > CP. In step 6, the network transmits a new LTM candidate configuration which does not include cells for which the RTD is greater than the CP. In some embodiments, the network may still configure these candidate cells but it does not perform LTM cell switch before the UE indicates that the RTD is less than the CP either via L3 measurement report or L1 measurement report.

[0230] In some embodiments, within the LTM candidate cell configuration (step 2 in Fig. 3) or in a separate message, the network may additionally send a NW RTD indication to inform the UE of candidate cells for which the UE can assume that the RTD is within the predetermined time window such as the CP.

[0231] In some embodiments, this indication is related to collocation information of the candidate target cell relative to the serving cell. The UE is not aware whether the cells are collocated, so it is necessary for the network to indicate this scenario to the UE. When such an indication is received from the network, the UE can assume that the RTD <= the CP for that candidate target cell. Therefore, when receiving the NW RTD indication from the network, the UE can skip the evaluation of the RTD for the indicated cells to avoid performing unnecessary RTD evaluations. In some embodiments, the indication is a part of the LTM candidate cell configuration, e.g., as a one-bit value related to each candidate cell, or it can be included in a separate message.

[0232] In some example embodiments, in some collocated scenarios (e.g., when the cells are physically collocated), the network may assume that the RTD between the serving cell and the corresponding candidate cell observed on UE side will be within the CP. In this case, the network may assist the UE by indicating that the RTD is always below the CP. Signalling may, for example, already be included in the LTM candidate cell configuration. Alternatively, this could be indicated in a separate message. With this indication, the UE can skip the RTD evaluation for these candidate cells.

[0233] Fig. 7 shows a signalling diagram according to an embodiment which uses a NW RTD indication. When the candidate cell is collocated with the serving cell and potentially under some other conditions, the network can assume the RTD between serving and candidate cell to be below or equal to the CP. In this case, the network sends a NW RTD indication in step 1 to indicate to the UE that the RTD is below the CP using the LTM candidate cell indication in or, in other embodiments, in a separate message.

[0234] The indication may be, e.g., a one-bit value for an additional parameter / field in the LTM candidate cell configuration: RTDbelowCP = 0 / 1 (0 = RTD above CP or unknown to the network, 1 = RTD below CP).

[0235] For example:

[0236] Candidate cell #1 , RTDbelowCP = 1

[0237] Candidate cell #2, RTDbelowCP = 0

[0238] Candidate cell #3, RTDbelowCP = 0

[0239] Candidate cell #4, RTDbelowCP = 1

[0240] Candidate cell #5, RTDbelowCP = 0 Candidate cell #6, RTDbelowCP = 0 Candidate cell #7, RTDbelowCP = 0 Candidate cell #8, RTDbelowCP = 0

[0241] The UE can assume that the RTD <= the CP for the cells indicated and include these in the UE LTM measurement indication without evaluating the RTD. Fig. 8 shows a UE 10, a source or serving network node 20 and a candidate network node 22 according to an embodiment. Network node 20 supports providing radio coverage in a cell 30 and may comprise a distributed unit of a gNB, which communicates with a central unit. Alternatively, network node 20 may comprise a base station, or a femto network node, for example.

[0242] UE 10 comprises monitoring circuitry 18 for monitoring the power and / or quality of links between the UE 10 and neighbouring candidate cells 32 supported by other network nodes 22. UE 10 also comprises a data store 12 for storing candidate cell configuration information received from the network node 20, and RTD evaluating circuitry 14 for estimating a receive timing difference RTD between a signal received from the serving cell 30 and that from the candidate cell 32. The UE also comprises determining circuitry 17 determining based on the RTD and the CP the ability of the UE to perform L1 measurements and generation circuitry 18 configured to generate an indication of this.

[0243] There is also transmitting and receiving circuitry 16 for transmitting and receiving messages comprising information and data to a network node 20. The transmitted messages may include L1 / L3 measurement reports generated by monitoring circuitry 18 indicating the quality and / or power of links to one or more neighbouring cells and they may include one or more indications of the ability of the UE to perform L1 measurements for a candidate cell. This indication may be included within one or more of the L1 / L3 measurement reports or it may be transmitted in a separate message.

[0244] The received messages may include information regarding candidate target cells, this information may be received periodically and may be updated by the network node in response to signals such as the RTD indication transmitted by the UE.

[0245] Network node 20 comprises a data store 22, transmitting and receiving circuitry 26 for transmitting and receiving messages with the user equipment 10, and in some cases a target network node.

[0246] The receiving circuitry 26 receives L1 and L3 measurement reports and an indication of the UEs ability to perform L1 measurements. The network node 20 comprises selecting circuitry 24 for selecting based on the indication, a set of candidate target cells from a list of candidate target cells, the set being those for which the UE can perform L1 measurements. The network node also comprises message generating circuitry for generating messages indicating the candidate target cells for transmission to the UE. In response to the selecting circuitry 24 indicating an updated set of candidate cells, the message generating circuitry 25 may generate an updated message indicating an updated set of candidate target cells and transmit it to the UE using transmitting and receiving circuitry 26.

[0247] A person of skill in the art would readily recognize that steps of various above-described methods can be performed by programmed computers. Herein, some embodiments are also intended to cover program storage devices, e.g., digital data storage media, which are machine or computer readable and encode machine-executable or computerexecutable programs of instructions, wherein said instructions perform some or all of the steps of said above-described methods. The program storage devices may be, e.g., digital memories, magnetic storage media such as a magnetic disks and magnetic tapes, hard drives, or optically readable digital data storage media. The embodiments are also intended to cover computers programmed to perform said steps of the above-described methods. The tern 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).

[0248] As used in this application, the term “circuitry” may refer to one or more or all of the following:

[0249] (a) hardware-only circuit implementations (such as implementations in only analog and / or digital circuitry) and

[0250] (b) combinations of hardware circuits and software, such as (as applicable):

[0251] (i) a combination of analog and / or digital hardware circuit(s) with software / firmware and

[0252] (ii) any portions of hardware processor(s) with software (including digital signal processor(s)), software, and memory(ies) that work together to cause an apparatus, such as a mobile phone or server, to perform various functions) and

[0253] (c) hardware circuit(s) and or processor(s), such as a microprocessor(s) or a portion of a microprocessor(s), that requires software (e.g., firmware) for operation, but the software may not be present when it is not needed for operation. 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 particular claim element, a baseband integrated circuit or processor integrated circuit for a mobile device, e.g. UE, or a similar integrated circuit in server, a cellular network device, or other computing or network device.

[0254] Although example embodiments of the present invention 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 invention as claimed.

[0255] Features described in the preceding description may be used in combinations other than the combinations explicitly described.

[0256] Although functions have been described with reference to certain features, those functions may be performable by other features whether described or not.

[0257] Although features have been described with reference to certain embodiments, those features may also be present in other embodiments whether described or not.

[0258] Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and / or shown in the drawings whether or not particular emphasis has been placed thereon.

[0259] Further aspects and examples may be described by the following numbered paragraphs:

[0260] 1. A method performed at an apparatus, comprising: receiving, from a network node, a configuration comprising a list of candidate target cells for a lower-layer triggered mobility, LTM, handover, for at least one candidate target cell of the list of candidate target cells, performing an evaluation of a receive timing difference, RTD, between a signal received from a serving cell and a signal received from the at least one candidate target cell; performing a determination based on the RTD and a predetermined time window, wherein the determination is related to an ability of the apparatus to perform layer 1 , L1, measurements for the at least one candidate target cell; and causing transmission of information on the determination to the network node.

[0261] 2. The method according to paragraph 1 , wherein the predetermined time window comprises a length of a cyclic prefix (CP).

[0262] 3. The method according to paragraph 1 or paragraph 2, wherein the information is transmitted in a L1 measurement report transmission.

[0263] 4. The method according to any preceding paragraph, wherein the information is transmitted in a layer 3, L3, measurement report transmission.

[0264] 5. The method according to paragraph 1 or paragraph 2, wherein causing the transmission of information on the determination comprises one of the following: causing transmission of the information only in a first L1 and / or a first L3 measurement report; causing transmission of the information in a predetermined number of L1 and / or L3 measurement reports; or causing transmission of the information in all L1 and / or L3 measurement reports until said LTM handover is complete.

[0265] 6. The method according to any preceding paragraph, wherein the information is transmitted in a transmission that is separate from any layer 3 or layer 1 measurement report transmission.

[0266] 7. The method according to any preceding paragraph, wherein the performing the determination based on the RTD and the predetermined time window comprises: determining those candidate target cells for which the RTD is less than or equal to the predetermined time window; and wherein the information on the determination comprises an indication of those candidate cells for which the RTD is less than or equal to the predetermined time window.

[0267] 8. The method according to any preceding paragraph, wherein the performing the determination based on the RTD and the predetermined time window comprises: determining those candidate target cells for which the RTD is greater than the predetermined time window; and wherein the information on the determination comprises an indication of those candidate cells for which the RTD is greater than the predetermined time window.

[0268] 9. The method according to any preceding paragraph, comprising causing transmission of an indication of whether the apparatus supports or does not support L1 measurements based on whether said RTD is greater than, equal to, or less than the predetermined time window.

[0269] 10. The method according to any preceding paragraph, comprising causing transmission of an indication that the apparatus supports L1 measurements when said RTD is equal to, or less than the predetermined time window.

[0270] 11. The method according to any preceding paragraph, further comprising: receiving, from the network node, an indication that for at least one candidate target cell of the list of candidate target cells the RTD can be assumed to be less than or equal to the predetermined time window; and based on the received indication, skipping performing the evaluation of the RTD for the at least one candidate target cell with RTD assumed to be less than or equal to the predetermined time window.

[0271] 12. The method according to paragraph 11, wherein the at least one candidate target cell with RTD assumed to be less than or equal to the predetermined time window is collocated with the serving cell.

[0272] 13. The method according to any preceding paragraph, further comprising performing L1 measurements for a set of candidate target cells for which the RTD is less than or equal to the predetermined time window. 14. The method according to paragraph 12, receiving a configuration to perform L1 measurements only for said set of candidate cells.

[0273] 15. The method according to any preceding paragraph, further comprising updating the evaluation of the RTD, updating the determination, and causing transmission of information on the updated determination to the network node.

[0274] 16. An apparatus, comprising: at least one processor; and at least one memory storing instructions that when executed by the at least one processor cause the apparatus at least to perform: receiving, from a network node, a configuration comprising a list of candidate target cells for a lower-layer triggered mobility, LTM, handover; for at least one candidate target cell of the list of candidate target cells, performing an evaluation of a receive timing difference, RTD, between a signal received from a serving cell and a signal received from the at least one candidate target cell; performing a determination based on the RTD and a predetermined time window, wherein the determination is related to an ability of the apparatus to perform layer 1 , L1 , measurements for the at least one candidate target cell; and causing transmission of information on the determination to the network node.

[0275] 17. The method according to any one of paragraphs 1 to 15, or the apparatus according to paragraph 16, wherein the apparatus comprises a user equipment.

[0276] 18. A method performed at an apparatus, comprising: causing transmission, to a user equipment, UE, of a configuration comprising a list of candidate target cells for a lower-layer triggered mobility (LTM) handover; receiving, from the UE, information on a determination by the UE for at least one candidate target cell of the list of candidate target cells, wherein the determination is based on a receive timing difference, RTD, between a signal received by the UE from a serving cell and a signal received by the UE from the at least one candidate target cell and a predetermined time window, wherein the determination is related to an ability of the UE to perform layer 1 , L1 , measurements for the at least one candidate cell; based on the received information, selecting a subset of candidate target cells of the list of candidate target cells; causing transmission of an indication of the subset of candidate target cells to the UE.

[0277] 19. The method according to paragraph 18, wherein the predetermined time window comprises a length of a cyclic prefix (CP).

[0278] 20. The method according to paragraph 18 or paragraph 19, wherein the subset comprises those candidate target cells for which the RTD is less than or equal to the predetermined time window.

[0279] 21. The method according to any one of paragraphs 18 to 20, further comprising causing transmission of an indication that for at least one candidate target cell of the list of candidate target cells the RTD can be assumed to be less than or equal to the predetermined time window to said UE.

[0280] 22. The method according to paragraph 21 , wherein the at least one candidate target cell with RTD assumed to be less than or equal to the predetermined time window is collocated with the serving cell.

[0281] 23. The method according to paragraph 21 or paragraph 22, further comprising causing transmission of an instruction to the UE to skip performing an evaluation of the RTD for the at least one candidate target cell with RTD assumed to be less than or equal to the predetermined time window.

[0282] 24. The method according to any one of paragraphs 18 to 23, wherein the step of selecting the subset of candidate cells comprises reconfiguring the list of candidate target cells for the LTM handover to exclude those candidate target cells for which the RTD is greater than the predetermined time window; and said step of causing transmission of an indication of the subset of candidate target cells to the UE comprises causing transmission of an indication of the reconfigured list of candidate target cells to the UE.

[0283] 25. An apparatus, comprising: at least one processor; and at least one memory storing instructions that when executed by the at least one processor cause the apparatus at least to perform: cause transmission, to a user equipment, UE, of a configuration comprising a list of candidate target cells for a lower-layer triggered mobility (LTM) handover; receive, from the UE, information on a determination by the UE for at least one candidate target cell of the list of candidate target cells, wherein the determination is based on a receive timing difference, RTD, between a signal received by the UE from a serving cell and a signal received by the UE from the at least one candidate target cell and a predetermined time window, wherein the determination is related to an ability of the UE to perform layer 1, L1 , measurements for the at least one candidate cell; based on the received information, selecting a subset of candidate target cells of the list of candidate target cells; cause transmission of an indication of the subset of candidate target cells to the UE.

[0284] 26. The method according to any one of paragraphs 18 to 24, or the apparatus according to paragraph 24, wherein the apparatus comprises a network node.

[0285] 27. A computer program which when executed by a processor on an apparatus cause said apparatus to perform at least the following: receive from a network node, a configuration comprising a list of candidate target cells for a lower-layer triggered mobility, LTM, handover; for at least one candidate target cell of the list of candidate target cells, perform an evaluation of a receive timing difference, RTD, between a signal received from a serving cell and a signal received from the at least one candidate target cell; perform a determination based on the RTD and a predetermined time window, wherein the determination is related to an ability of the apparatus to perform layer 1 , L1, measurements for the at least one candidate target cell; and cause transmission of information on the determination to the network node.

[0286] 28. A computer program which when executed by a processor on an apparatus cause said apparatus to perform at least the following: cause transmission, to a user equipment, UE, of a configuration comprising a list of candidate target cells for a lower-layer triggered mobility (LTM) handover; receive from the UE, information on a determination by the UE for at least one candidate target cell of the list of candidate target cells, wherein the determination is based on a receive timing difference, RTD, between a signal received by the UE from a serving cell and a signal received by the UE from the at least one candidate target cell and a predetermined time window, wherein the determination is related to an ability of the UE to perform layer 1 , L1 , measurements for the at least one candidate cell; based on the received information, select a subset of candidate target cells of the list of candidate target cells; cause transmission of an indication of the subset of candidate target cells to the

[0287] UE.

[0288] List of Abbreviations

[0289] BWP Bandwidth part

[0290] CP Cyclic prefix

[0291] CSI-RS Channel State Information Reference Signal

[0292] DCI Downlink control information

[0293] DL Downlink

[0294] ICBM Inter-cell beam management

[0295] L1 / L2 Layer 1 , Layer 2

[0296] LTM L1 / L2 triggered mobility

[0297] MAC CE Medium access control control element

[0298] NR New radio

[0299] PDCCH Physical downlink control channel

[0300] PDSCH Physical downlink shared channel

[0301] PUCCH Physical uplink control channel

[0302] PUSCH Physical uplink shared channel

[0303] RSRP Reference Signals Received Power

[0304] RTD Receive timing difference

[0305] SRS Sounding reference singal

[0306] SSB Synchronization signal block

[0307] TA Timing advance

[0308] TCI Transmission configuration indication

[0309] UE User equipment

[0310] UL Uplink

Claims

CLAIMS1. An apparatus, comprising: at least one processor; and at least one memory storing instructions that when executed by the at least one processor cause the apparatus at least to perform: receive, from a network node, a configuration comprising a list of candidate target cells for a lower-layer triggered mobility, LTM, handover; for at least one candidate target cell of the list of candidate target cells, perform an evaluation of a receive timing difference, RTD, between a signal received from a serving cell and a signal received from the at least one candidate target cell; perform a determination based on the RTD and a predetermined time window, wherein the determination is related to an ability of the apparatus to perform layer 1 , L1 , measurements for the at least one candidate target cell; and cause transmission of information on the determination to the network node.

2. The apparatus according to claim 1 , wherein the predetermined time window comprises a length of a cyclic prefix (CP).

3. The apparatus according to claim 1 or claim 2, wherein the information is caused to be transmitted in a L1 measurement report transmission.

4. The apparatus according to any preceding claim, wherein the information is caused to be transmitted in a layer 3, L3, measurement report transmission.

5. The apparatus according to any preceding claim, wherein the apparatus is caused to perform the determination based on the RTD and the predetermined time window by: determining those candidate target cells for which the RTD is less than or equal to the predetermined time window; and the information on the determination comprises an indication of those candidate cells for which the RTD is less than or equal to the predetermined time window.

6. The apparatus according to any preceding claim, wherein the apparatus is caused to perform the determination based on the RTD and the predetermined time window bydetermining those candidate target cells for which the RTD is greater than the predetermined time window; and the information on the determination comprises an indication of those candidate cells for which the RTD is greater than the predetermined time window.

7. The apparatus according to any preceding claim, wherein the apparatus is configured to cause transmission of an indication of whether the apparatus supports or does not support L1 measurements based on whether said RTD is greater than, equal to, or less than the predetermined time window.

8. The apparatus according to any preceding claim, wherein the apparatus is configured to cause transmission of an indication that the apparatus supports L1 measurements when the RTD is less than or equal to the predetermined time window.

9. The apparatus according to any preceding claim, wherein the apparatus is further caused to: receive, from the network node, an indication that for at least one candidate target cell of the list of candidate target cells the RTD can be assumed to be less than or equal to the predetermined time window; and based on the received indication, skip performing the evaluation of the RTD for the at least one candidate target cell with RTD assumed to be less than or equal to the predetermined time window.

10. The apparatus according to any preceding claim, wherein the apparatus is further caused to: perform L1 measurements for a set of candidate target cells for which the RTD is less than or equal to the predetermined time window.

11. An apparatus, comprising: at least one processor; and at least one memory storing instructions that when executed by the at least one processor cause the apparatus at least to perform: cause transmission, to a user equipment, UE, of a configuration comprising a list of candidate target cells for a lower-layer triggered mobility (LTM) handover; receive, from the UE, information on a determination by the UE for at least one candidate target cell of the list of candidate target cells, wherein thedetermination is based on a receive timing difference, RTD, between a signal received by the UE from a serving cell and a signal received by the UE from the at least one candidate target cell and a predetermined time window, wherein the determination is related to an ability of the UE to perform layer 1 , L1, measurements for the at least one candidate cell; based on the received information, select a subset of candidate target cells of the list of candidate target cells; cause transmission of an indication of the subset of candidate target cells to the UE.

12. The apparatus according to claim 11 , wherein the predetermined time window comprises a length of a cyclic prefix (CP).

13. The apparatus according to claim 11 or claim 12, wherein the subset comprises those candidate target cells for which the RTD is less than or equal to the predetermined time window.

14. The apparatus according to any one of claims 11 to 13, wherein the apparatus is further caused to: cause transmission of an indication that for at least one candidate target cell of the list of candidate target cells the RTD can be assumed to be less than or equal to the predetermined time window to said UE.

15. The apparatus according to any one of claims 11 to 14, wherein the apparatus is further caused to: when selecting the subset of candidate cells, reconfigure the list of candidate target cells for the LTM handover to exclude those candidate target cells for which the RTD is greater than the predetermined time window; and when causing transmission of an indication of the subset of candidate target cells to the UE, cause transmission of an indication of the reconfigured list of candidate target cells to the UE.

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