Conditional handover in mobile communication systems

Abstract

A UE, network node, method and computer program are described comprising means for: receiving an RRC Reconfiguration message from a source node of a mobile communication system at a UE of the mobile communication system, wherein the RRC Reconfiguration message includes configuration information for one or more candidate cells for C-LTM transfer; receiving, at the UE, one or more MAC CE messages including timing advance information and a timing advance maintenance timer for each of one or more of said candidate cells for C-LTM transfer; identifying, at the UE, one of the one or more candidate cells as a preferred candidate cell for C-LTM transfer based on radio conditions determined at the UE; and initiating, by the UE, C-LTM to an alternative candidate cell of the one or more candidate cells in the event that: the identified preferred candidate cell is not available due to timing advance information for the identified preferred candidate cell being unavailable or due to expiry of the timing advance maintenance timer for the preferred candidate cell; and the alternative candidate cell has a signal strength within a signal strength difference parameter of the preferred candidate cell.

Description

[0001] Conditional Handover in Mobile Communication Systems

[0002] Field

[0003] Example embodiments may relate to systems, methods and / or computer programs for conditional handover in mobile communication systems.

[0004] Background

[0005] In a mobile communication system in which a user equipment, UE, is within a particular cell, a handover arrangement may be initiated in the event that an alternative cell offers a better communication option according to some metric, such as signal strength.

[0006] Conditional LTM, C-LTM, enables a UE to determine whether or not trigger conditions for handover have occurred and, if so, handover occurs under the control of the UE. There remains a need for further developments in this field.

[0007] Summary

[0008] The scope of protection sought for various embodiments of the invention is set out by the independent claims. The 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.

[0009] In a first aspect, this specification describes a user equipment, UE, of a mobile communication system, the UE comprising: an input (or some other means) for receiving an RRC Reconfiguration message from a source node of the mobile communication system, wherein the RRC Reconfiguration message includes configuration information for one or more candidate cells for conditional LTM, C-LTM, transfer. The configuration information may include C-LTM execution conditions and / or candidate cell(s) configuration behaviour; an / the input (or some other means) for receiving one or more Medium Access Control Control Element, MAC CE, messages including timing advance information and a timing advance maintenance timer for each of one or more of said candidate cells for C-LTM transfer; a control module (or some other means) for identifying one of the one or more candidate cells as a preferred candidate cell for C-LTM transfer based on radio conditions determined at the UE; and a / the control module (or some other means) for initiating C-LTM to an alternative candidate cell of the one or more candidate cells in the event that: the identified preferred candidate cell is not available due to timing advance information for the identified preferred candidate cell being unavailable or due to expiry of the timing advance maintenance timer for the preferred candidate cell; and the alternative candidate cell meets requirements of a cell selection parameter that has been relaxed in accordance with a candidate offset relaxation parameter. The candidate offset parameter may beprovided by the network, e.g. as part of the configuration information. The candidate offset parameter could be provided as part of a TA management policy. Alternatively, the parameter could be implemented in some other way (e.g. stored at the UE).

[0010] The UE may further comprise means (e.g. the control module) for initiating C-LTM to the preferred candidate cell of the one or more candidate cells in the event that the timing advance information for the identified preferred candidate cell is available and the timing advance maintenance timer has not expired.

[0011] The UE may further comprise means (e.g. the control module) for determining whether the alternative candidate cell meets the requirements of the cell selection parameter that has been relaxed in accordance with the candidate offset relaxation parameter.

[0012] The UE may further comprise means (e.g. the control module) for determining said alternative candidate cell in the event that the identified preferred candidate cell is not available due to the timing advance information for the identified preferred candidate cell being unavailable or due to expiry of the timing advance maintenance timer for the preferred candidate cell.

[0013] The radio conditions may comprise signal strength parameters (e.g. Reference Signal Received Power, RSRP).

[0014] In some example embodiments, the preferred cell is a best candidate cell of the one or more candidate cells according to the determined radio conditions. Further, the alternative candidate cell may be the next-best candidate cell of the one or more candidate cells according to the determined radio conditions.

[0015] The candidate offset relaxation parameter may be provided by a network node of the mobile communication system. Alternatively, or in addition, the candidate offset relaxation parameter may be stored by said UE.

[0016] In some example embodiments, the RRC Reconfiguration message includes a timing advance management policy for the one or more candidate cells for C-LTM transfer. The candidate offset relaxation parameter may, for example, be provided as part of said timing advance management policy.

[0017] The UE may further comprise means (e.g. the control module) for initiating C-LTM to said alternative candidate cell of the one or more candidate cells in the event that: the identifiedpreferred candidate cell is not available due to timing advance information for the identified preferred candidate cell being unavailable or due to expiry of the timing advance maintenance timer for the preferred candidate cell; the alternative candidate cell meets requirements of said cell selection parameter; and the alternative candidate cell has a signal strength within a signal strength difference parameter of the preferred candidate cell.

[0018] The UE may further comprise means (e.g. the control module) for initiating RACH-based C-LTM (sometimes referred to as legacy C-LTM) to the preferred candidate cell in the event that the preferred candidate cell is unavailable and the requirements of the candidate offset relaxation parameters are not met for the alternative candidate cell. Thus, the RACH-based C-LTM may use the preferred candidate cell (rather than an alternative of next-best candidate cell).

[0019] In a second aspect, this specification describes a first network node (e.g. a source node) of a mobile communication system, the node comprising : an output (or some other means) for providing an RRC Reconfiguration message to a UE of the mobile communication system, wherein the RRC Reconfiguration message includes configuration information (e.g. including C-LTM execution conditions and / or candidate cell (s) configuration behaviour) for one or more candidate cells for conditional LTM, C-LTM, transfer; a / the output (or some other means) for providing one or more Medium Access Control Control Element, MAC CE, messages to the UE, the or each MAC CE message including timing advance information and a timing advance maintenance timer for each of one or more of said candidate cells for C-LTM transfer; and a control module (or some other means) for initiating C-LTM to an alternative candidate cell of the one or more candidate cells, in response to an instruction from the UE, in the event that: a preferred candidate cell identified by the UE is not available due to timing advance information for the identified preferred candidate cell being unavailable or due to expiry of the timing advance maintenance timer for the preferred candidate cell; and the alternative candidate cell meets requirements of a cell selection parameter that has been relaxed in accordance with a candidate offset relaxation parameter. The candidate offset parameter could be provided as part of a TA management policy. Alternatively, the parameter could be implemented in some other way (e.g. stored at the UE).

[0020] The candidate offset relaxation parameter may be provided by a network node of the mobile communication system, e.g. as part of the configuration information. The candidate offset parameter could be provided as part of a TA management policy. Alternatively, the parameter could be implemented in some other way (e.g. stored at the UE).In some example embodiments, the RRC Reconfiguration message includes a timing advance management policy for the one or more candidate cells for C-LTM transfer, wherein the candidate offset relaxation parameter is provided as part of said timing advance management policy.

[0021] The radio conditions may comprise signal strength parameters (e.g. Reference Signal Received Power, RSRP).

[0022] In some example embodiments, the preferred cell is a best candidate cell of the one or more candidate cells according to the determined radio conditions. Further, the alternative candidate cell may be the next-best candidate cell of the one or more candidate cells according to the determined radio conditions.

[0023] In a third aspect, this specification describes a user equipment, UE, of a mobile communication system, the UE comprising: an input (or some other means) for receiving an RRC Reconfiguration message from a source node of the mobile communication system, wherein the RRC Reconfiguration message includes configuration information (e.g. including C-LTM execution conditions and / or candidate cell (s) configuration behaviour) for one or more candidate cells for conditional LTM, C-LTM, transfer; a / the input (or some other means) for receiving one or more Medium Access Control Control Element, MAC CE, messages including timing advance information and a timing advance maintenance timer for each of one or more of said candidate cells for C-LTM transfer; a control module (or some other means) for identifying one of the one or more candidate cells as a preferred candidate cell for C-LTM transfer based on radio conditions determined at the UE; and a / the control module (or some other means) for initiating C-LTM to an alternative candidate cell of the one or more candidate cells in the event that: the identified preferred candidate cell is not available due to timing advance information for the identified preferred candidate cell being unavailable or due to expiry of the timing advance maintenance timer for the preferred candidate cell; and the alternative candidate cell has a signal strength within a signal strength difference parameter of the preferred candidate cell. The signal strength difference parameter may be an RSRP difference parameter.

[0024] The UE may further comprise a / the control module (or some other means) for initiating C-LTM to the preferred candidate cell of the one or more candidate cells in the event that the timing advance information for the identified preferred candidate cell is available and the timing advance maintenance timer has not expired.The UE may further comprise a / the control module (or some other means) for determining whether the signal strength of the alternative candidate cell differs from a signal strength of the preferred candidate cell by less than said signal strength difference parameter.

[0025] The UE may further comprise a / the control module (or some other means) for determining the preferred candidate cell for handover based on signal strength parameters.

[0026] The UE may further comprise a / the control module (or some other means) for determining said alternative candidate cell in the event that the identified preferred candidate cell is not available due to the timing advance information for the identified preferred candidate cell being unavailable or due to expiry of the timing advance maintenance timer for the preferred candidate cell.

[0027] The radio conditions may comprise signal strength parameters (e.g. RSRP).

[0028] In some example embodiments, the preferred cell is a best candidate cell of the one or more candidate cells according to the determined radio conditions. The alternative candidate cell may be the next-best candidate cell of the one or more candidate cells according to the determined radio conditions.

[0029] The signal strength difference parameter may be provided by a network node of the mobile communication system.

[0030] The signal strength difference parameter may be stored by said UE.

[0031] The RRC Reconfiguration message may include a timing advance management policy for the one or more candidate cells for C-LTM transfer. Moreover, the signal strength difference parameter may be provided as part of said timing advance management policy.

[0032] The UE may further comprise a / the control module (or some other means) for initiating RACH-based C-LTM (sometimes referred to as legacy C-LTM) to the preferred candidate cell in the event that the preferred candidate cell is unavailable and the alternative candidate cell has a signal strength beyond the signal strength difference parameter of the preferred candidate cell. Thus, the RACH-based C-LTM uses the preferred candidate cell (rather than the alternative or next-best candidate cell).

[0033] The UE may further comprise a / the control module (or some other means) for initiating C-LTM to said alternative candidate cell of the one or more candidate cells in the eventthat: the identified preferred candidate cell is not available due to timing advance information for the identified preferred candidate cell being unavailable or due to expiry of the timing advance maintenance timer for the preferred candidate cell; the alternative candidate cell meets requirements of a cell selection parameter that has been relaxed in accordance with a candidate offset relaxation parameter; and the alternative candidate cell has a signal strength within said signal strength difference parameter of the preferred candidate cell.

[0034] In a fourth aspect, this specification describes a first network node (e.g. a source node) of a mobile communication system, the node comprising: an output (or some other means) for providing an RRC Reconfiguration message to a UE of the mobile communication system, wherein the RRC Reconfiguration message includes configuration information (e.g. including C-LTM execution conditions and / or candidate cell (s) configuration behaviour) for one or more candidate cells for conditional LTM, C-LTM, transfer; a / the output (or some other means) for providing one or more Medium Access Control Control Element, MAC CE, messages to the UE, the or each MAC CE message including timing advance information and a timing advance maintenance timer for each of one or more of said candidate cells for C-LTM transfer; and a control module (or some other means) for initiating C-LTM to an alternative candidate cell of the one or more candidate cells, in response to an instruction from the UE, in the event that a preferred candidate cell identified by the UE is not available due to timing advance information for the identified preferred candidate cell being unavailable or due to expiry of the timing advance maintenance timer for the preferred candidate cell and the alternative candidate cell has a signal strength within a signal strength difference parameter of the preferred candidate cell. The signal strength difference parameter may be an RSRP difference parameter.

[0035] The radio conditions may comprise signal strength parameters (e.g. RSRP).

[0036] In some example embodiments, the preferred cell is a best candidate cell of the one or more candidate cells according to the determined radio conditions. The alternative candidate cell may be the next-best candidate cell of the one or more candidate cells according to the determined radio conditions.

[0037] The signal strength difference parameter may be provided by a network node of the mobile communication system.

[0038] The signal strength difference parameter may be stored by said UE.The RRC Reconfiguration message may include a timing advance management policy for the one or more candidate cells for C-LTM transfer. Moreover, the signal strength difference parameter may be provided as part of said timing advance management policy.

[0039] In a fifth aspect, this specification describes a user equipment, UE, of a mobile communication system, the UE comprising: an input (or some other means) for receiving an RRC Reconfiguration message from a source node of the mobile communication system, wherein the RRC Reconfiguration message includes a precedence indication for one or more candidate cells for conditional LTM, C-LTM, transfer; a / the input (or some other means) for receiving one or more Medium Access Control Control Element, MAC CE, messages including timing advance information and a timing advance maintenance timer for each of one or more of said candidate cells for C-LTM transfer; a control module (or some other means) for identifying one of the one or more candidate cells as a preferred candidate cell for C-LTM transfer based on radio conditions determined at the UE; and a / the control module (or some other means) for initiating C-LTM to an alternative candidate cell of the one or more candidate cells in the event that: the identified preferred candidate cell is not available due to timing advance information for the identified preferred candidate cell being unavailable or due to expiry of the timing advance maintenance timer for the preferred candidate cell; and the precedence indication indicates that alternative candidate cells are allowed.

[0040] The UE may further comprise a / the control module (or some other means) for initiating C-LTM to the preferred candidate cell of the one or more candidate cells in the event that the timing advance information for the identified preferred candidate cell is available and the timing advance maintenance timer has not expired.

[0041] The UE may further comprise a / the control module (or some other means) for determining said alternative candidate cell in the event that the identified preferred candidate cell is not available due to the timing advance information for the identified preferred candidate cell being unavailable or due to expiry of the timing advance maintenance timer for the preferred candidate cell.

[0042] The radio conditions may comprise signal strength parameters (e.g. Reference Signal Received Power, RSRP).

[0043] In some example embodiments, the preferred cell is a best candidate cell of the one or more candidate cells according to the determined radio conditions. The alternative candidate cell may be the next-best candidate cell of the one or more candidate cellsaccording to the determined radio conditions.

[0044] In some example embodiments, the RRC Reconfiguration message includes a timing advance management policy for the one or more candidate cells for C-LTM transfer. The precedence indication may be provided as part of said timing advance management policy.

[0045] The UE may further comprise a / the control module (or some other means) for initiating C-LTM to said alternative candidate cell of the one or more candidate cells in the event that: the identified preferred candidate cell is not available due to timing advance information for the identified preferred candidate cell being unavailable or due to expiry of the timing advance maintenance timer for the preferred candidate cell; the precedence indication indicates that alternative candidate cells are allowed; and the alternative candidate cell meets requirements of a cell selection parameter that has been relaxed in accordance with a candidate offset relaxation parameter.

[0046] The UE may further comprise a / the control module (or some other means) for initiating C-LTM to said alternative candidate cell of the one or more candidate cells in the event that: the identified preferred candidate cell is not available due to timing advance information for the identified preferred candidate cell being unavailable or due to expiry of the timing advance maintenance timer for the preferred candidate cell; the precedence indication indicates that alternative candidate cells are allowed; and the alternative candidate cell has a signal strength within a signal strength difference parameter of the preferred candidate cell.

[0047] In some example embodiments, the precedence indication is provided by a network node of the mobile communication system. Alternatively, or in addition, the precedence indication relaxation parameter is stored by said UE.

[0048] The UE may further comprise a / the control module (or some other means) for initiating RACH-based C-LTM to the preferred candidate cell in the event that the preferred candidate cell is unavailable and the precedence indication indicates that alternative candidate cells are not allowed. Thus, the RACH-based C-LTM may use the preferred candidate cell (rather than the alternative or next-best candidate cell).

[0049] In a sixth aspect, this specification describes a first network node (e.g. a source node) of a mobile communication system, the node comprising: an output (or some other means) for providing an RRC Reconfiguration message to a UE of the mobile communication system, wherein the RRC Reconfiguration message includes configuration information (e.g.including C-LTM execution conditions and / or candidate cell (s) configuration behaviour) for one or more candidate cells for conditional LTM, C-LTM, transfer; a / the output (or some other means) for providing one or more Medium Access Control Control Element, MAC CE, messages to the UE, the or each MAC CE message including timing advance information and a timing advance maintenance timer for each of one or more of said candidate cells for C-LTM transfer; and a control module (or some other means) for initiating C-LTM to an alternative candidate cell of the one or more candidate cells, in response to an instruction from the UE, in the event that: a preferred candidate cell identified by the UE is not available due to timing advance information for the identified preferred candidate cell being unavailable or due to expiry of the timing advance maintenance timer for the preferred candidate cell; and precedence indication indicates that alternative candidate cells are allowed.

[0050] The radio conditions may comprise signal strength parameters (e.g. Reference Signal Received Power, RSRP).

[0051] In some example embodiments, the preferred cell is a best candidate cell of the one or more candidate cells according to the determined radio conditions. The alternative candidate cell may be the next-best candidate cell of the one or more candidate cells according to the determined radio conditions.

[0052] In some example embodiments, the RRC Reconfiguration message includes a timing advance management policy for the one or more candidate cells for C-LTM transfer. The precedence indication may be provided as part of said timing advance management policy.

[0053] In some example embodiments, the precedence indication is provided by a network node of the mobile communication system. Alternatively, or in addition, the precedence indication relaxation parameter is stored by said UE.

[0054] In a seventh aspect, this specification describes a method comprising: receiving an RRC Reconfiguration message from a source node of a mobile communication system at a UE of the mobile communication system, wherein the RRC Reconfiguration message includes configuration information for one or more candidate cells for conditional LTM, C-LTM, transfer; receiving, at the UE, one or more Medium Access Control Control Element, MAC CE, messages including timing advance information and a timing advance maintenance timer for each of one or more of said candidate cells for C-LTM transfer; identifying, at the UE, one of the one or more candidate cells as a preferred candidate cell for C-LTM transfer based on radio conditions determined at the UE; and initiating, by the UE, C-LTM to analternative candidate cell of the one or more candidate cells in the event that: the identified preferred candidate cell is not available due to timing advance information for the identified preferred candidate cell being unavailable or due to expiry of the timing advance maintenance timer for the preferred candidate cell; and the alternative candidate cell meets requirements of a cell selection parameter that has been relaxed in accordance with a candidate offset relaxation parameter. The candidate offset parameter may be provided by the network, e.g. as part of the configuration information. The candidate offset parameter could be provided as part of a TA management policy. Alternatively, the parameter could be implemented in some other way (e.g. stored at the UE).

[0055] The method may further comprise initiating C-LTM to the preferred candidate cell of the one or more candidate cells in the event that the timing advance information for the identified preferred candidate cell is available and the timing advance maintenance timer has not expired.

[0056] The method may further comprise determining whether the alternative candidate cell meets the requirements of the cell selection parameter that has been relaxed in accordance with the candidate offset relaxation parameter.

[0057] The method may further comprise determining said alternative candidate cell in the event that the identified preferred candidate cell is not available due to the timing advance information for the identified preferred candidate cell being unavailable or due to expiry of the timing advance maintenance timer for the preferred candidate cell.

[0058] The method may further comprise initiating C-LTM to said alternative candidate cell of the one or more candidate cells in the event that: the identified preferred candidate cell is not available due to timing advance information for the identified preferred candidate cell being unavailable or due to expiry of the timing advance maintenance timer for the preferred candidate cell; the alternative candidate cell meets requirements of said cell selection parameter; and the alternative candidate cell has a signal strength within a signal strength difference parameter of the preferred candidate cell.

[0059] The method may further comprise initiating RACH-based C-LTM (sometimes referred to as legacy C-LTM) to the preferred candidate cell in the event that the preferred candidate cell is unavailable and the requirements of the candidate offset relaxation parameters are not met for the alternative candidate cell. Thus, the RACH-based C-LTM may use the preferred candidate cell (rather than an alternative of next-best candidate cell).In an eighth aspect, this specification describes a method comprising: receiving an RRC Reconfiguration message from a source node of a mobile communication system at a UE of the mobile communication system, wherein the RRC Reconfiguration message includes configuration information for one or more candidate cells for conditional LTM, C-LTM, transfer; receiving, at the UE, one or more Medium Access Control Control Element, MAC CE, messages including timing advance information and a timing advance maintenance timer for each of one or more of said candidate cells for C-LTM transfer; identifying, at the UE, one of the one or more candidate cells as a preferred candidate cell for C-LTM transfer based on radio conditions determined at the UE; and initiating, by the UE, C-LTM to an alternative candidate cell of the one or more candidate cells in the event that: the identified preferred candidate cell is not available due to timing advance information for the identified preferred candidate cell being unavailable or due to expiry of the timing advance maintenance timer for the preferred candidate cell; and the alternative candidate cell has a signal strength within a signal strength difference parameter of the preferred candidate cell. The signal strength difference parameter may be an RSRP difference parameter.

[0060] The method may further comprise initiating C-LTM to the preferred candidate cell of the one or more candidate cells in the event that the timing advance information for the identified preferred candidate cell is available and the timing advance maintenance timer has not expired.

[0061] The method may further comprise determining whether the signal strength of the alternative candidate cell differs from a signal strength of the preferred candidate cell by less than said signal strength difference parameter.

[0062] The method may further comprise determining the preferred candidate cell for handover based on signal strength parameters.

[0063] The method may further comprise determining said alternative candidate cell in the event that the identified preferred candidate cell is not available due to the timing advance information for the identified preferred candidate cell being unavailable or due to expiry of the timing advance maintenance timer for the preferred candidate cell.

[0064] The method may further comprise initiating RACH-based C-LTM (sometimes referred to as legacy C-LTM) to the preferred candidate cell in the event that the preferred candidate cell is unavailable and the alternative candidate cell has a signal strength beyond the signal strength difference parameter of the preferred candidate cell. Thus, the RACH-based C-LTM uses the preferred candidate cell (rather than the alternative or next-best candidatecell).

[0065] The method may further comprise initiating C-LTM to said alternative candidate cell of the one or more candidate cells in the event that: the identified preferred candidate cell is not available due to timing advance information for the identified preferred candidate cell being unavailable or due to expiry of the timing advance maintenance timer for the preferred candidate cell; the alternative candidate cell meets requirements of a cell selection parameter that has been relaxed in accordance with a candidate offset relaxation parameter; and the alternative candidate cell has a signal strength within said signal strength difference parameter of the preferred candidate cell.

[0066] In a ninth aspect, this specification describes a method comprising: receiving an RRC Reconfiguration message from a source node of a mobile communication system at a UE of the mobile communication system, wherein the RRC Reconfiguration message includes configuration information for one or more candidate cells for conditional LTM, C-LTM, transfer; receiving, at the UE, one or more Medium Access Control Control Element, MAC CE, messages including timing advance information and a timing advance maintenance timer for each of one or more of said candidate cells for C-LTM transfer; identifying, at the UE, one of the one or more candidate cells as a preferred candidate cell for C-LTM transfer based on radio conditions determined at the UE; and initiating, by the UE, C-LTM to an alternative candidate cell of the one or more candidate cells in the event that: the identified preferred candidate cell is not available due to timing advance information for the identified preferred candidate cell being unavailable or due to expiry of the timing advance maintenance timer for the preferred candidate cell; and precedence indication indicates that alternative candidate cells are allowed.

[0067] The method may further comprise initiating C-LTM to the preferred candidate cell of the one or more candidate cells in the event that the timing advance information for the identified preferred candidate cell is available and the timing advance maintenance timer has not expired.

[0068] The method may further comprise determining said alternative candidate cell in the event that the identified preferred candidate cell is not available due to the timing advance information for the identified preferred candidate cell being unavailable or due to expiry of the timing advance maintenance timer for the preferred candidate cell.

[0069] The method may further comprise initiating C-LTM to said alternative candidate cell of the one or more candidate cells in the event that: the identified preferred candidate cell is notavailable due to timing advance information for the identified preferred candidate cell being unavailable or due to expiry of the timing advance maintenance timer for the preferred candidate cell; the precedence indication indicates that alternative candidate cells are allowed; and the alternative candidate cell meets requirements of a cell selection parameter that has been relaxed in accordance with a candidate offset relaxation parameter.

[0070] The method may further comprise initiating C-LTM to said alternative candidate cell of the one or more candidate cells in the event that: the identified preferred candidate cell is not available due to timing advance information for the identified preferred candidate cell being unavailable or due to expiry of the timing advance maintenance timer for the preferred candidate cell; the precedence indication indicates that alternative candidate cells are allowed; and the alternative candidate cell has a signal strength within a signal strength difference parameter of the preferred candidate cell.

[0071] The method may further comprise initiating RACH-based C-LTM to the preferred candidate cell in the event that the preferred candidate cell is unavailable and the precedence indication indicates that alternative candidate cells are not allowed. Thus, the RACH-based C-LTM may use the preferred candidate cell (rather than the alternative or next-best candidate cell).

[0072] In a tenth aspect, this specification describes computer-readable instructions which, when executed by a computing apparatus, cause the computing apparatus to perform (at least) any method as described herein (including the methods of the seventh to ninth aspects described above).

[0073] In an eleventh aspect, this specification describes a computer-readable medium (such as a non-transitory computer-readable medium) comprising program instructions stored thereon for performing (at least) any method as described herein (including the methods of the seventh to ninth aspects described above).

[0074] In a twelfth aspect, this specification describes a computer program product comprising program instructions which, when the program is executed by an apparatus, cause the apparatus to perform (at least) any method as described herein (including the methods of the seventh to ninth aspects described above).

[0075] In a thirteenth aspect, this specification describes an apparatus comprising: at least one processor; and at least one memory storing instructions that, when executed by the atleast one processor, causes the apparatus to perform (at least) any method as described herein (including the methods of the seventh to ninth aspects described above).

[0076] Brief Description of the Drawings

[0077] Example embodiments will now be described by way of non-limiting example, with reference to the accompanying drawings, in which:

[0078] FIG. 1 is a block diagram of an example system;

[0079] FIG. 2 shows a flow chart showing an example conditional handover arrangement;

[0080] FIG. 3 shows a flow chart in accordance with an example embodiment;

[0081] FIG. 4 shows a message flow sequence in accordance with an example embodiment; FIG. 5 to 11 show flow charts in accordance with example embodiments;

[0082] FIGS. 12 to 14 show message flow sequences in accordance with example embodiments; FIG. 15 is a schematic diagram of a system that may be used to implement one or more of the example embodiments; and

[0083] FIG. 16 shows tangible media for storing computer-readable code which when run by a computer may perform methods according to example embodiments described herein.

[0084] Detailed Description

[0085] FIG. 1 is a block diagram of an example system, indicated generally by the reference numeral 10. The system 10 is part of a mobile communication system includes a user equipment, UE, 12, a source (or serving) cell 14 that is currently serving the UE 12, a first candidate cell 16 and a second candidate cell 18.

[0086] In the event that, for example, the first candidate cell 16 offers a better communication option for the UE than the source cell 14 according to some metric (such as signal strength or some other radio parameter), then a handover procedure can be initiated such that the first candidate cell 16 becomes the source cell.

[0087] Lower-layer triggered mobility, LTM, (sometimes referred to as L1 / L2 triggered mobility) is a process by which handover from the source cell 14 serving the UE to a candidate cell, such as the first candidate cell 16, serving the UE can be controlled.

[0088] Conditional LTM, C-LTM, refers to an arrangement in which the UE determines whether or not pre-configured trigger conditions for handover have occurred. If so, handover occurs under the control of the UE.FIG. 2 shows a flow chart, indicated generally by the reference numeral 20, showing an example conditional handover arrangement. The flow chart 20 is described below with reference to the system 10.

[0089] The flow chart 20 starts at step 22, where conditional LTM configuration occurs. In the step 22, the UE 12 can be provided with information defining conditions for handover, as discussed in detail below.

[0090] At step 24, the UE determines whether handover conditions (as defined in step 22) are met. For example, the decision in step 24 may be based, at least in part, on relative signal strength conditions between the UE 12 and each of the source cell 14, the first candidate cell 16, the second candidate cell 18 and any other candidate cells (not shown).

[0091] If the handover conditions are met, handover is initiated at step 26 and the flow chart 20 ends at step 28. If the handover conditions are not met, the flow chart 20 ends at step 28 without initiating handover.

[0092] In the flow chart 20, handover can be made to a "best" candidate cell according to some metric, for example, based on signal strength measurements (e.g. Reference Signal Received Power, RSRP, such as CSI-RSRP or SSB RSRP).

[0093] FIG. 3 shows a flow chart, indicated generally by the reference numeral 30, in accordance with an example embodiment. The flow chart 30 is described below with reference to the system 10 by way of example.

[0094] The flow chart starts at step 32, where LTM candidate preparation takes place. In the step 32, one or more candidate cells (such as the first and second candidate cells 16 and 18 described above) are prepared for a potential handover. The step 32 may include parameters defining the operation of the C-LTM algorithm being transferred between nodes of the system, as discussed further below.

[0095] At step 34, radio conditions at the UE 12 are monitored. The step 34 may include the UE 12 periodically determining signal strength information for signals received from a source cell (such as the source cell 14) and from one or more candidate cells (such as the first and second candidate cells 16 and 18). The signal strength information may, for example, be provided to the source cell 14.At step 36, an early synchronisation procedure is triggered based on the monitored radio conditions. As discussed in detail below, the step 36 includes obtaining timing advance (TA) information for cells that the UE may switch to. TA information is used during switching so that the UE can be synchronised with a new cell.

[0096] At step 38, LTM execution takes place to a selected candidate cell in the event that handover conditions are met.

[0097] FIG. 4 shows a message flow sequence, indicated generally by the reference numeral 40, in accordance with an example embodiment. The message flow sequence 40 shows an example implementation of steps 32 to 36 of the flow chart 30. Example implementations of the LTM execution step 38 are discussed further below.

[0098] The message flow sequence 40 shows messages transferred between a UE 42 (such as the UE 12), a next generation node B distributed unit, gNB-DU, 43 of a source cell (such as the source cell 14) a distributed unit, DU-1, 44 of a first target cell (such as the first candidate cell 16), a distributed unit, DU-2, 45, of a second target cell (such as the second candidate cell 18), and a gNB central unit, gNB-CU, 46 of the target DUs.

[0099] The message sequence 40 starts with an L1 / L3 Measurement Report (message 1) being sent by the UE 42 to the source gNB-DU 43.

[0100] An uplink (UL) RRC message transfer is sent (message 2) from the source gNB-DU 43 to gNB-CU 46 containing the L1 / L3 Measurement Report received with message 1.

[0101] The L1 / L3 Measurement Report is analyzed at the gNB-CU 46. The gNB-CU 46 triggers the LTM candidate preparation for multiple candidate cells belonging to different DUs.

[0102] A UE context setup request is sent from the gNB-CU 46 to the target-DU-1 44 (message 3) and target-DU-245 (message 4). Thus, the gNB-CU sends UE context setup requests to the first and second candidate cells.

[0103] The target-DUl 44 (the first candidate cell) sends a UE context setup response (message 5) to the gNB-CU 46. Similarly, the target DU-245 (the second candidate cell) sends a UE context setup response (message 6) to the gNB-CU 46. The UE context setup responses include conditional LTM report configurations.

[0104] Following receipt of the UE context setup responses, the gNB-CU 46 sends a downlink(DL) message to the source gNB-DU 43 containing an RRCReconfiguration for each candidate cell (i.e. the target cells) including configuration of prepared cells received from the DUs in the messages 5 and 6 discussed above. The RRC reconfiguration messages include execution conditions for the candidate cells; such execution conditions may include event conditions similar to LTM-3 and LTM-5 event conditions.

[0105] The RRC reconfiguration is sent from the source-gNB DU 43 to the UE 42 (message 8). The UE responds by sending an RRC Reconfiguration Complete message to the source-gNB DU (message 9). An uplink (UL) RRC message transfer containing the RRC Reconfiguration Complete message is then sent from the source-gNB DU 43 to the gNB-CU 46 (message 10).

[0106] The messages 1 to 10 are an example implementation of the conditional LTM configuration step 22 of the algorithm 20 described above and the LTM candidate preparation step 32 of the flow chart 30 described above.

[0107] With the LTM candidate preparation phase complete, the UE 42 sends periodic LI measurement reports to source gNB-DU 43 for tracking and sharing radio conditions of source and target cells. A single measurement report (message 11) is shown for ease of reference in the message sequence 40, however multiple instances of the message 11 may be sent. The instances of the message 11 are an example implementation of the monitor radio conditions step 34 of the flow chart 30.

[0108] Based on the LI measurement reports shared by the UE 42, the source gNB-DU 43 triggers the early synchronization procedure (see step 36 of the flow chart 30). For each prepared target cell, the UE 42 shares a PDCCH order consisting of RACH preambles and other parameters (see messages 12 and 14 sent by the source gNB-DU 43 to the UE 42).

[0109] The UE conducts RACH with multiple target cells (see the PRACH messages 13 and 15 sent to the target DU-1 44 and the target DU-2 45). Timing advance (TA) information for the target DU-1 44 (the first target cell) is shared with the source gNB-DU 43 via the gNB-CU 46 (see messages 16 and 17). Similarly, TA information for the target DU-2 45 (the second target cell) is shared with the source gNB-DU via the gNB-CU 46 (see messages 18 and 19).

[0110] FIG. 5 shows a flow chart, indicated generally by the reference numeral 50, in accordance with an example embodiment. The flow chart 50 shows steps carried at a UE,such as the UE 42, when implementing C-LTM (e.g. as part of the message sequence 40). The UE periodically provides measurement reports to the source node (see messages 1 and 11 of the message sequence 40). These periodic actions are omitted from the flow chart 50.

[0111] The flow chart 50 starts at step 52, where an RRC reconfiguration takes place (see messages 8 and 9 of the message sequence 40). The step 52 is therefore an example of step 32 of the flow chart 30. In the step 52, the respective UE may receive an RRC Reconfiguration message from a source node of the mobile communication system, wherein the RRC Reconfiguration message includes configuration information for one or more candidate cells for C-LTM transfer. Thus, the source node (e.g. the gNB-CU 46 described above) may prepare the RRC reconfiguration including the C-LTM execution conditions and the C-LTM configuration of the candidate cell(s).

[0112] At step 53, the UE takes part in early synchronisation, as discussed above with reference to messages 12 to 19 of the flow chart 40. The step 53 is therefore an example of step 36 of the flow chart 30.

[0113] At step 54, the UE receives one or more MAC CE messages from the source node (e.g. from source gNB-DU 43). The MAC CE messages enable the network to inform a UE of the TA information for respective candidate cells. The MAC CE message 54 includes TA information and a TA maintenance timer indicating the validity time of the TA information (e.g. for each of one or more candidate cells for C-LTM transfer). The relative position of elements within a mobile communication system is one factor that affects timing advance (TA) data. Over time, TA information becomes out-of-date (i.e. invalid); the TA maintenance timer provides an indication of whether particular TA information remains valid. For example, UEs that are moving rapidly relative to a cell may have a lower TA maintenance timer value that UEs that are static.

[0114] The network may therefore send one or more MAC CE messages containing TA information relating to candidate cells for which the network (e.g. the source gNB-DU 43) has sent the PDCCH order based on LI or L3 measurements. Depending on the number of PDCCH orders which correspond to prepared candidate cell(s), there may, for example, be up to 8 candidate cells (if prepared for C-LTM).

[0115] The TA value(s) obtained during the early synchronization process are shared with the UE. In conventional LTM, the TA value may be stored at the serving DU and shared with the UE along with a MAC CE cell switch command during an LTM execution phase.However, in conditional LTM, there is generally no explicit cell change command from the network; thus, in the flow chart 50, the TA information may be shared (in step 53) before the MAC CE messages are sent (in step 54).

[0116] At step 55, the UE identifies and selects one or more candidate cells for transfer. Then, at step 56, C-LTM is initiated. The step 55 may involve the UE selecting one or more candidate cells as a preferred candidate cell for C-LTM transfer based on radio conditions determined at the UE.

[0117] Steps 54 to 56 of the flow chart 50 are an example implementation of step 38 of the flow chart 30 described above.

[0118] In the flow chart 50, a conditional LTM, C-LTM, handover can be made to a selected (e.g. a "best") candidate cell selected, for example, based on signal strength measurements (e.g. RSRP) for communications with different candidate cells. In the event that defined conditions are met, the C-LTM algorithm generally indicates that the UE should initiate the handover. However, if a timing advance (TA) signal has not yet been received for the selected candidate cell, then handover cannot take place since the UE cannot be synchronized with the selected candidate cell. In such circumstances, it may not be clear whether the step 56 of the flow chart should be wait for the TA signal (introducing a delay) or use an alternative candidate cell (potentially reducing performance). Similarly, if a TA signal for a best candidate cell is invalid (e.g. because the relevant TA maintenance timer has expired), it may not be clear whether the step 56 of the flow chart should be wait for the TA signal or use an alternative candidate cell.

[0119] There are a number of reasons why TA information may be unavailable or invalid. For example, a MAC CE message containing TA information along with a TA maintenance timer may be sent in the step 54, but may not have reached the UE due to link problems between the serving-DU and the UE while the conditions for the C-LTM cell switch execution has been met. It is also possible that the MAC CE message containing TA information along with a TA maintenance timer could be successfully received by the UE, however, by the time the C-LTM cell switch conditions are met, the TA maintenance timer may have expired. Thus, the TA of the candidate cell selected in step 55 may be invalid.

[0120] FIG. 6 shows a flow chart, indicated generally by the reference numeral 60, in accordance with an example embodiment. The flow chart 60 may, for example, in an example implementation of the steps 55 and 56 of the flow chart 50.The flow chart 60 starts at step 61 (which may follow step 54 of the flow chart 50). At step 61, a determination is made regarding whether or not conditional LTM conditions are met by one or more candidate cells. If so, that algorithm moves to step 61, otherwise the algorithm terminates at step 67.

[0121] A preferred or "best" candidate cell is identified (e.g. determined) according to some metric (e.g. signal strength or some other radio conditions). At step 62, a determination is made regarding whether valid timing advantage, TA, data is available for that preferred / best candidate cell. If so, the algorithm moves to step 63; otherwise the algorithm moves to step 64.

[0122] At step 63, a preferred / best candidate cell has been selected and deemed available for use (according to the TA data for the preferred / best candidate cell). Conditional LTM switching to that preferred / best candidate cell is triggered (thereby implementing step 56 of the flow chart 50). The algorithm then terminates at step 67.

[0123] At step 64, a determination has been made that valid TA data for the preferred / best candidate cell is not available. A consideration is now made regarding whether alternative conditions are met, e.g. whether an alternative or "next best" candidate cell can be used. If so, the algorithm moves to step 65, otherwise the algorithm moves to step 66.

[0124] At step 65, a "next best" candidate cell has been selected and deemed available for use (according to the TA data for the next best candidate cell). Conditional LTM switching to that next best candidate cell is triggered (thereby implementing step 56 of the flow chart 50). The algorithm then terminates at step 67. The next best candidate cell may be selected from a plurality of alternative candidate cells determined in the event that the identified preferred candidate cell is not available or due to expiry of the timing advance maintenance timer for the preferred candidate cell.

[0125] At step 66, RACH-based C-LTM switching towards the preferred / best candidate cell is carried out. This is, of course, different to the RACH-less C-LTM switching of steps 63 and 65 described above. Step 66 implements C-LTM, thereby implementing step 56 of the flow chart 50. The algorithm then terminates at step 67. Thus, if there is no TA available for any of the candidate cell (s) that meets the requirements of step 62 or step 64, then the UE triggers RACH-based C-LTM for the preferred / best candidate cell.Thus, in the flow chart 60, if a UE has received C-LTM configurations for multiple candidate cell(s), and if the network provided TA for the best available candidate cell is unavailable or has become invalid, the UE could switch to the next best available candidate cell provided a valid TA is available for that cell. Otherwise, a RACH-based C-LTM cell switch execution towards the best candidate cell may be triggered. As discussed in detail below, there are a number of mechanisms that may be provided to seek to ensure that the cell measurements of the "next best" candidate are acceptable (e.g. within a tolerable offset limit with respect to either the best candidate cell and / or the existing source / serving cell). Note that in some example embodiments, if the best and next-best candidate cells are both unavailable due to TA issues, then the third best candidate cell (if it exists) is not used.

[0126] FIG. 7 shows a flow chart, indicated generally by the reference numeral 70, in accordance with an example embodiment. The flow chart 70 starts at step 72, which is an example implementation of step 64 of the flow chart 60 described above.

[0127] As discussed above with reference to step 64, at step 72, a determination has been made that valid TA data for the preferred / best candidate cell is not available (e.g. the TA data may be missing or expired). A consideration is now made regarding whether alternative conditions are met, e.g. whether an alternative or "next best" candidate cell meets the requirements of the cell selection parameter that has been relaxed in accordance with a candidate offset relaxation parameter. If so, conditional LTM switching to that next best candidate is initiated at step 65 (as discussed above), otherwise RACH based C-LTM switching is to the preferred / best candidate cell is initiated at step 66 (as also discussed above).

[0128] The step 72 is implemented using a "candidate offset relaxation indication" provided to (or otherwise available to) the UE; for example, the candidate offset relaxation indication may be provided to the UE as part of the MAC CE message discussed above with reference to step 54 of the flow chart 50. If the best available candidate cell cannot be used (e.g. because the TA information for that cell is unavailable or invalid), then the UE determines whether the next best candidate cell is better than the existing serving cell to within a more relaxed handover condition (i.e. more relaxed than the normal handover threshold that would have been applied when determining that a switch to the "best" candidate should take place) as defined by the candidate offset relaxation indication. If so, the cell switch to the next best candidate cell takes place (and is implemented in the step 65). If not, the algorithm falls back on the existing RACH-based switching method to switch to the best candidate cell (as is implemented in the step 66 described above),i.e. in the event that the TA information for the preferred candidate cell is unavailable and the required requirements of the candidate offset relaxation parameters are not met for the alternative candidate cell.

[0129] In one example embodiment, the candidate offset relaxation indication is 2dB, meaning that handover to the "next best" candidate cell can be initiated if a determined signal strength of the next best candidate is within 2dB of the signal strength used to identify whether the preferred / best candidate cell should be used.

[0130] The candidate offset relaxation indication can be used to enable faster cell switch towards the next best candidate cell by mitigating the cell switching conditions by relaxing the candidate cell offset condition by a pre-configured value (the value indicated by the candidate offset relaxation indication; 2dB in the example given above) which could be provided by the network or could be left up to the UE implementation. Whilst this relaxation of the candidate offset value may improve interruption time, it could lead to a degradation of the target cell signal quality. Thus, there is a trade-off in terms of signal quality and interruption time.

[0131] When applying the candidate cell offset condition, the UE may, for example, relax the LI or L3 requirements by a value provided in the candidate offset relaxation indication to meet the C-LTM conditions early such that the new TA maintenance timer of the selected candidate is still running (has valid TA) and RACH-less C-LTM cell switch execution can be performed in a timely manner. The candidate offset relaxation indication may be implemented just before or upon the expiry of the new TA maintenance timer of the best available candidate cell.

[0132] FIG. 8 shows a flow chart, indicated generally by the reference numeral 80, in accordance with an example embodiment. The flow chart 80 starts at step 82, which is an example implementation of step 64 of the flow chart 60 described above.

[0133] As discussed above with reference to step 64, at step 82, a determination has been made that valid TA data for the preferred / best candidate cell is not available. A consideration is now made regarding whether alternative conditions are met, e.g. whether an alternative or "next best" cell can be used. If so, conditional LTM switching to that next best candidate is initiated at step 65 (as discussed above), otherwise RACH based C-LTM switching to the preferred / best candidate cell is initiated at step 66 (as also discussed above).The step 82 is implemented using a "signal strength condition", e.g. in the form of an RSRP difference threshold, that is provided to (or otherwise available to) the UE; for example, the signal strength condition may be provided to the UE as part of the MAC CE message discussed above with reference to step 54 of the flow chart 50. If the best available candidate cell cannot be used (e.g. because the TA information for that cell is unavailable or invalid), then the UE determines whether the signal strength (e.g. RSRP, such as CSI-RSRP or SSB-RSRSP, or some similar metric) difference between the best candidate cell and the next available candidate cell is below this threshold. If so, the cell switch to the next best candidate cell takes place (and is implemented in the step 65). If not, the algorithm falls back on the existing RACH-based switching method to switch to the best candidate cell (as is implemented in the step 66 described above).

[0134] The step 82 may include the UE determining the signal strength of the respective candidate cell(s).

[0135] Thus, in the flow chart 80, if the difference of the signal strength (e.g. RSRP) of the strongest triggered candidate cell and RSRP of the strongest triggered candidate cell for which a valid TA is available is smaller than the configured threshold, then the UE selects the latter for cell switch.

[0136] In other words, let:

[0137] • cell A be the candidate cell with the strongest RSRP (cell- or beam-level evaluation, such as RSRP) for which a cell switch condition has been triggered;

[0138] • cell B be the candidate cell with the strongest RSRP (cell- or beam-level evaluation, such as RSRP) for which a cell switch condition has been triggered and a valid TA is available; and

[0139] • D be the configured RSRP difference threshold. In some example embodiments, the value D may be 2dB.

[0140] Then, if \RSRPcellA-RSRPcellB\ < D, cell B is selected as the target cell for cell switch; otherwise UACH-based C-LTM is carried out. Herein, the term D is generally referred to as the RSRP difference threshold for synchronized cell selection.

[0141] FIG. 9 shows a flow chart, indicated generally by the reference numeral 90, in accordance with an example embodiment. The flow chart 90 makes use of both the candidate offset relaxation indication of step 72 and the signal strength condition of step 82.The flow chart 90 starts at step 92, which is an example implementation of step 64 of the flow chart 60 described above. As discussed above, at step 92, a determination has been made that valid TA data for the preferred / best candidate cell is not available. A consideration is now made regarding whether alternative conditions are met, e.g. whether an alternative or "next best" cell can be used. If so, conditional LTM switching to that next best candidate is initiated at step 65 (as discussed above), otherwise RACH based C-LTM switching is to the preferred / best candidate cell is initiated at step 66 (as also discussed above).

[0142] At step 92, the candidate offset relaxation indication discussed above with reference to step 72 is used to determine whether the next best candidate cell is better than the existing serving cell to within a more relaxed handover condition (i.e. more relaxed than the normal handover threshold that would have been applied when determining that a switch to the "best" candidate should take place) as defined by the candidate offset relaxation indication. If so, the flow chart 90 moves to step 94; otherwise the flow chart 90 moves to step 66 (where the existing RACH-based switching method is used to switch to the best candidate cell).

[0143] In the step 94, the signal strength condition discussed above with reference to step 82 is used to determine whether the signal strength (e.g. RSRP) difference between the best candidate cell and the next available candidate cell is below this threshold. If so, the cell switch to the next best candidate cell takes place (and is implemented in the step 65). If not, the algorithm falls back on the existing RACH-based switching method to switch to the best candidate cell (as is implemented in the step 66).

[0144] Of course, the steps 92 and 94 of the flow chart 90 could be reversed, or could be implemented at the same time. Moreover, additional steps could be implemented in addition to the steps 92 and 94 for determining whether a next best candidate cell could be used.

[0145] FIG. 10 shows a flow chart, indicated generally by the reference numeral 100, in accordance with an example embodiment. The flow chart 100 starts at step 102, which is an example implementation of step 64 of the flow chart 60 described above.

[0146] As discussed above with reference to step 64, at step 102, a determination has been made that valid TA data for the preferred / best candidate cell is not available. A consideration is now made regarding whether alternative conditions are met, e.g. whether an alternative or "next best" cell can be used. If so, conditional LTM switching tothat next best candidate is initiated at step 65 (as discussed above), otherwise RACH based C-LTM switching is to the preferred / best candidate cell is initiated at step 66 (as also discussed above).

[0147] The step 102 is implemented by determining whether a TA precedence indication (e.g. "Early TA Precedence indication") has been provided to the UE (e.g. as part of the MAC CE message discussed above with reference to step 54 of the flow chart 50). If the best available candidate cell cannot be used (e.g. because the TA information for that cell is unavailable or invalid), then the presence of the TA precedence indication indicates that the UE can use the next best candidate cell. The TA precedence indication may be in the form of a flag that is set to "true" or "false" depending on whether the value is set.

[0148] Thus, if it is determined at step 102 that a TA precedence indication has been received at the UE, then the cell switch to the next best candidate cell takes place (and is implemented in the step 65). If not, the algorithm falls back on the existing RACH-based switching method to switch to the best candidate cell (as is implemented in the step 66).

[0149] Accordingly, in the flow chart 100, if the C-LTM conditions are met but TA information for the best candidate cell is either unavailable to the UE or is invalid, the UE can select another candidate cell configured for C-LTM which is not the best candidate cell, but for which TA information is available. In this embodiment, there is not necessarily a check of how the "next best" candidate cell compares with the best candidate cell or the existing cell. If no check is provided, then the cell switching is simple (which could be an advantage), but risks switching to an inferior cell.

[0150] The consideration of whether a TA precedence indication has been received at the UE could be combined with either the relaxing condition of step 72 and / or the signal strength condition of step 82, as discussed further below.

[0151] FIG. 11 shows a flow chart, indicated generally by the reference numeral 110, in accordance with an example embodiment. The flow chart 110 starts at step 112, which is an example implementation of step 64 of the flow chart 60 described above.

[0152] As discussed above with reference to step 64, at step 112, a determination has been made that valid TA data for the preferred / best candidate cell is not available. A consideration is now made regarding whether alternative conditions are met, e.g. whether when an alternative or "next best" cell can be used.The step 112 may be implemented by determining whether a TA precedence indication has provided to the UE (e.g. as part of the MAC CE message discussed above with reference to step 54 of the flow chart 50). Thus, the step 112 is similar to the step 102 described above. If it is determined at step 112 that a TA precedence indication (e.g. a TA precedence indication flag set to "true") has been received at the UE, then the flow chart moves to step 114. If not, the algorithm falls back on the existing RACH-based switching method to switch to the best candidate cell (as is implemented in the step 66).

[0153] At step 114, the candidate offset relaxation indication discussed above with reference to steps 72 and 92 is used to determine whether the next best candidate cell is better than the existing serving cell to within a more relaxed handover condition (i.e. more relaxed than the normal handover threshold that would have been applied when determining that a switch to the "best" candidate should take place) as defined by the candidate offset relaxation indication. If so, the flow chart 90 moves to step 116; otherwise the flow chart 110 moves to step 66 (where the existing RACH-based switching method to switch to the best candidate cell).

[0154] In the step 116, the signal strength condition discussed above with reference to step 82 and 94 is used to determine whether the signal strength difference between the best candidate cell and the next available candidate cell is below this threshold. If so, the cell switch to the next best candidate cell takes place (and is implemented in the step 65). If not, the algorithm falls back on the existing RACH-based switching method to switch to the best candidate cell (as is implemented in the step 66).

[0155] In the flow chart 110, the TA precedence indication may be a flag that indicates that the candidate offset relaxation and / or the RSRP difference threshold should be applied to determine whether early switching should be implemented in preference to waiting for valid TA information for the best candidate.

[0156] Of course, the 114 and 116 of the flow chart 110 could be reversed, or could be implemented at the same time. Moreover, one or both of those steps could be omitted and / or other steps included. Note that both steps were omitted in the flow chart 100 discussed above.

[0157] It should be noted that the various algorithms defined above have a fall-back to using RACH-based C-LTM cell switching to the preferred candidate cell. This is not essential to all example embodiments. RACH-based switching may be omitted or some other fallback arrangement may be provided.A TA Management Policy may be provided to the UE 42 providing information for implementing the algorithms discussed above with reference to the flow charts 70, 80, 90, 100 and 110. The TA Management Policy may include one or more of the candidate relaxation offset information, the RSRP difference threshold and the TA precedence indication (and / or some similar data).

[0158] FIG. 12 shows a message flow sequence, indicated generally by the reference numeral 120, in accordance with an example embodiment. The message flow sequence shows messages transferred between the UE 42 and the source node 43 described above. The message flow sequence 120 is similar to the message flow sequence 40 discussed above (and may additionally include steps of the sequence 40 not shown in the message sequence 120).

[0159] The message sequence 120 starts with the source node 43 sending an RRC reconfiguration 121 to the UE 42. This may be the message 8 of the flow chart 40 and may be sent following messages 1 to 7 of the flow chart 40 (or some similar messages). The RRC reconfiguration may include a TA Management Policy. As discussed above, the TA Management Policy may include one or more of the candidate relaxation offset information, the RSRP difference threshold and the TA precedence indication described above (and / or some similar data). The TA Management Policy and / or parameters provided by the TA Management Policy may be stored by the UE.

[0160] Following the receipt of the RRC Reconfiguration, early synchronization may be triggered in step 122. The step 122 may implement steps 14 to 21 of the flow chart 40 (and may be triggered by the source node 43 based on LI measurement reports shared by the UE 42, not shown in the flow chart 120).

[0161] The UE 42 receives a first MAC CE message 123 and a second MAC CE message 124. The MAC CE messages relate to candidate cells for transfer and may include TA information and a TA maintenance timer for the respective cell. In the message flow sequence 120, the first MAC CE message 123 may relate to a first candidate cell and the second MAC CE message 124 may relate to a second candidate cell.

[0162] At step 125 of the message flow sequence, the UE makes a decision regarding cell switching. For example, if the the first candidate cell of MAC CE message 123 is considered to be the best candidate cell but does not include TA information (or has an invalid TA maintenance timer), the UE needs to decide how to respond.As discussed in detail above, the UE 42 may make use of one or more of: a TA precedence indication; relaxed conditions; and signal strength comparisons when deciding how to respond to the invalid or missing TA information.

[0163] If the UE determines that conditional LTM cell switching should take place, this is triggered in step 126 of the message sequence 120

[0164] FIG. 13 shows a message flow sequence, indicated generally by the reference numeral 130, in accordance with an example embodiment. The message flow sequence shows messages transferred between the UE 42 and the source node 43 described above.

[0165] The message sequence 130 shows the source node 43 sending a plurality of MAC CE messages to the UE 42. In the message sequence 130, first, second and third MAC CE messages 132, 133 and 134, but any number may be sent (and each may relate to a different candidate cell). By way of example, the first and second MAC CE messages 132 and 133 may be the first and second MAC CE messages 123 and 124 described above.

[0166] FIG. 14 shows a message flow sequence, indicated generally by the reference numeral 140, in accordance with an example embodiment. The message flow sequence shows messages transferred between the UE 42 and the source node 43 described above.

[0167] The message flow sequence 140 differs from the message flow sequence 130 in that a single MAC CE message received at the UE 42 that includes TA data (e.g. TA measurements and TA timer data) for multiple candidate cells in the same message. For example, the TA information and TA maintenance timer information of the MAC CE messages 132 to 134 may be merged into the single MAC CE message 142.

[0168] The information included in the MAC CE messages 132, 133, 134 and 142 may include one or more of: a TA precedence indication, relaxed conditions or signal strength comparisons

[0169] Example Apparatus

[0170] FIG. 15 shows an apparatus according to some example embodiments, which may comprise the UE 42. The apparatus may be configured to perform the operations described herein, for example operations described with reference to any disclosed process. The apparatus comprises at least one processor 302 and at least one memory 304 directly or closely connected to the processor. The memory 304 may include atleast one random access memory (RAM) and at least one read-only memory (ROM). Computer program code (software) 305 is stored in the ROM. The apparatus may be connected to a transmitter (TX) and / or a receiver (RX), for example via radio interface 306. The apparatus may, optionally, be connected with a user interface, UI, 308 for instructing the apparatus and / or for outputting data. The at least one processor 302, with the at least one memory 304 and the computer program code 305 are arranged to cause the apparatus to at least perform at least the method according to any preceding process, for example as disclosed in relation to the flow charts and message flow sequences of FIGs. 2 to 14 and related features thereof.

[0171] FIG. 16 shows a non-transitory media 350 according to some embodiments. The non-transitory media 350 is a computer readable storage medium. It may be e.g. a CD, a DVD, a USB stick, a blue ray disk, etc. The non-transitory media 350 stores computer program code, causing an apparatus to perform the method of any preceding process for example as disclosed in relation to the flow diagrams and message flow sequences of FIGS. 2 to 14 and related features thereof.

[0172] Names of network elements, protocols, and methods are based on current standards. In other versions or other technologies, the names of these network elements and / or protocols and / or methods may be different, as long as they provide a corresponding functionality. For example, embodiments may be deployed in 2G / 3G / 4G / 5G networks and further generations of 3GPP but also in non-3GPP radio networks such as WiFi.

[0173] A memory may be volatile or non-volatile. It may be e.g. a RAM, a SRAM, a flash memory, a FPGA block ram, a DCD, a CD, a USB stick, and a blue ray disk.

[0174] If not otherwise stated or otherwise made clear from the context, the statement that two entities are different means that they perform different functions. It does not necessarily mean that they are based on different hardware. That is, each of the entities described in the present description may be based on a different hardware, or some or all of the entities may be based on the same hardware. It does not necessarily mean that they are based on different software. That is, each of the entities described in the present description may be based on different software, or some or all of the entities may be based on the same software. Each of the entities described in the present description may be embodied in the cloud.

[0175] Implementations of any of the above-described blocks, apparatuses, systems, techniques or methods include, as non-limiting examples, implementations as hardware,software, firmware, special purpose circuits or logic, general purpose hardware or controller or other computing devices, or some combination thereof. Some embodiments may be implemented in the cloud.

[0176] It is to be understood that what is described above is what is presently considered the preferred embodiments. However, it should be noted that the description of the preferred embodiments is given by way of example only and that various modifications may be made without departing from the scope as defined by the appended claims.

Claims

Claims1. A user equipment, UE, of a mobile communication system, the UE comprising: at least one processor; andat least one memory storing instructions that, when executed by the at least one processor cause the apparatus to:receive an RRC Reconfiguration message from a source node of the mobile communication system, wherein the RRC Reconfiguration message includes configuration information for one or more candidate cells for conditional LTM, C-LTM, transfer;receive one or more Medium Access Control Control Element, MAC CE, messages including timing advance information and a timing advance maintenance timer for each of one or more of said candidate cells for C-LTM transfer;identify one of the one or more candidate cells as a preferred candidate cell for C-LTM transfer based on radio conditions determined at the UE; andinitiate C-LTM to an alternative candidate cell of the one or more candidate cells in the event that: the identified preferred candidate cell is not available due to timing advance information for the identified preferred candidate cell being unavailable or due to expiry of the timing advance maintenance timer for the preferred candidate cell; and the alternative candidate cell has a signal strength within a signal strength difference parameter of the preferred candidate cell.

2. The UE as claimed in claim 1, wherein the instructions further cause the node at least to:initiate C-LTM to the preferred candidate cell of the one or more candidate cells in the event that the timing advance information for the identified preferred candidate cell is available and the timing advance maintenance timer has not expired.

3. The UE as claimed in claim 1 or claim 2, wherein the instructions further cause the node at least to:determine whether the signal strength of the alternative candidate cell differs from a signal strength of the preferred candidate cell by less than said signal strength difference parameter.

4. An apparatus as claimed in any one of claims 1 to 3, wherein the instructions are further configured to:determine the preferred candidate cell for handover based on signal strength parameters.

5. The UE of any one of the preceding claims, wherein the instructions further cause the node at least to:determine said alternative candidate cell in the event that the identified preferred candidate cell is not available due to the timing advance information for the identified preferred candidate cell being unavailable or due to expiry of the timing advance maintenance timer for the preferred candidate cell.

6. The UE of any one of the preceding claims, wherein the radio conditions comprise signal strength parameters.

7. The UE of any one of the preceding claims, wherein the preferred cell is a best candidate cell of the one or more candidate cells according to the determined radio conditions.

8. The UE of claim 7, wherein the alternative candidate cell is the next-best candidate cell of the one or more candidate cells according to the determined radio conditions.

9. The UE of any one of the preceding claims, wherein the signal strength difference parameter is provided by a network node of the mobile communication system.

10. The UE of any one of the preceding claims, wherein the signal strength difference parameter is stored by said UE.

11. The UE of any one of the preceding claims, wherein the RRC Reconfiguration message includes a timing advance management policy for the one or more candidate cells for C-LTM transfer.

12. The UE of claim 11, wherein the signal strength difference parameter is provided as part of said timing advance management policy.

13. The UE of any one of the preceding claims, wherein the instructions further cause the node at least to:initiate RACH-based C-LTM to the preferred candidate cell in the event that the preferred candidate cell is unavailable and the alternative candidate cell has a signal strength beyond the signal strength difference parameter of the preferred candidate cell.

14. The UE of any one of the preceding claims, wherein the instructions cause thenode at least to initiate C-LTM to said alternative candidate cell of the one or more candidate cells in the event that:the identified preferred candidate cell is not available due to timing advance information for the identified preferred candidate cell being unavailable or due to expiry of the timing advance maintenance timer for the preferred candidate cell;the alternative candidate cell meets requirements of a cell selection parameter that has been relaxed in accordance with a candidate offset relaxation parameter; and the alternative candidate cell has a signal strength within said signal strength difference parameter of the preferred candidate cell.

15. A first network node of a mobile communication system, the node comprising: at least one processor; andat least one memory storing instructions that, when executed by the at least one processor, cause the node at least to:provide an RRC Reconfiguration message to a UE of the mobile communication system, wherein the RRC Reconfiguration message includes configuration information for one or more candidate cells for conditional LTM, C-LTM, transfer;provide one or more Medium Access Control Control Element, MAC CE, messages to the UE, the or each MAC CE message including timing advance information and a timing advance maintenance timer for each of one or more of said candidate cells for C-LTM transfer; andinitiate C-LTM to an alternative candidate cell of the one or more candidate cells, in response to an instruction from the UE, in the event that a preferred candidate cell identified by the UE is not available due to timing advance information for the identified preferred candidate cell being unavailable or due to expiry of the timing advance maintenance timer for the preferred candidate cell and the alternative candidate cell has a signal strength within a signal strength difference parameter of the preferred candidate cell.

16. The network node of claim 15, wherein the preferred cell is a best candidate cell of the one or more candidate cells according to the determined radio conditions and the alternative candidate cell is the next-best candidate cell of the one or more candidate cells according to the determined radio conditions.

17. The network node of claim 15 or claim 16, wherein the RRC Reconfiguration message includes a timing advance management policy for the one or more candidate cells for C-LTM transfer, wherein the signal strength difference parameter is provided as part of said timing advance management policy.

18. A method comprising:receiving an RRC Reconfiguration message from a source node of a mobile communication system at a UE of the mobile communication system, wherein the RRC Reconfiguration message includes configuration information for one or more candidate cells for conditional LTM, C-LTM, transfer;receiving, at the UE, one or more Medium Access Control Control Element, MAC CE, messages including timing advance information and a timing advance maintenance timer for each of one or more of said candidate cells for C-LTM transfer;identifying, at the UE, one of the one or more candidate cells as a preferred candidate cell for C-LTM transfer based on radio conditions determined at the UE; and initiating, by the UE, C-LTM to an alternative candidate cell of the one or more candidate cells in the event that: the identified preferred candidate cell is not available due to timing advance information for the identified preferred candidate cell being unavailable or due to expiry of the timing advance maintenance timer for the preferred candidate cell; and the alternative candidate cell has a signal strength within a signal strength difference parameter of the preferred candidate cell.

19. A computer program product comprising program instructions which, when executed by a computing apparatus, cause the computing apparatus to:receive an RRC Reconfiguration message from a source node of the mobile communication system, wherein the RRC Reconfiguration message includes configuration information for one or more candidate cells for conditional LTM, C-LTM, transfer;receive one or more Medium Access Control Control Element, MAC CE, messages including timing advance information and a timing advance maintenance timer for each of one or more of said candidate cells for C-LTM transfer;identify one of the one or more candidate cells as a preferred candidate cell for C-LTM transfer based on radio conditions determined at the UE; andinitiate C-LTM to an alternative candidate cell of the one or more candidate cells in the event that: the identified preferred candidate cell is not available due to timing advance information for the identified preferred candidate cell being unavailable or due to expiry of the timing advance maintenance timer for the preferred candidate cell; and the alternative candidate cell has a signal strength within a signal strength difference parameter of the preferred candidate cell.

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