Lower-layer triggered mobility in mobile communication systems

Abstract

Handover in Mobile Communication Systems Example embodiments relate to UEs, network nodes, systems, methods and computer programs for handover in mobile communication systems comprising: receiving (at a network node) a measurement report from a UE of the mobile communication system, wherein the measurement report is either an L1 or an L3 measurement report; determining whether to initiate LTM switching to a target cell of one or more candidate target cells based, at least in part, on said measurement report; providing an LTM cell switch command to the UE in the event that LTM switching is initiated, the LTM cell switch command identifying said target cell; and determining whether RACH-less switching was unsuccessful due to an invalid TA value, but that subsequent RACH-based switching to the same target cell was successful

Description

[0001] LOWER-LAYER TRIGGERED MOBILITY IN MOBILE COMMUNICATION SYSTEMS

[0002] Field

[0003] Example embodiments relate to systems, methods and / or computer programs for 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. There remains a need for further developments in this field.

[0006] Summary

[0007] 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.

[0008] In a first aspect, this specification describes a user equipment, UE, of a mobile communication system, the UE comprising: an output (or some other means) for providing a measurement report to a network node of the mobile communication system, wherein the measurement report is either an L1 or an L3 measurement report; a / the input (or some other means) for receiving an LTM cell switch command (e.g. a MAC CE cell switch command) from the network node, wherein the cell switch instruction is generated at the network node in response to said measurement report; a control module (or some other means) for initiating RACH-less cell switching to a target cell in accordance with the LTM cell switch command using a timing advance, TA, value for the target cell available to the UE; a / the control module (or some other means) for initiating RACH-based switching to the target cell in the event of a failure of the RACH-less cell switching to the target cell; a / the control module (or some other means) forgenerating a radio link failure, RLF, report, in the event of a failure of the RACH-less cell switching to the target cell, wherein the RLF report includes an indication of whether L1 or L3 triggered LTM was attempted; and a / the output (or some other means) for providing said RLF report to the network node.

[0009] In some example embodiments, the indication of whether LI or L3 triggered LTM was attempted comprises an information element, IE, of the RLF report. The IE may, for example, be an " LTM type" IE.

[0010] The RLF report may comprise a lastHOtype information element, wherein the indication of whether L1 or L3 triggered LTM was attempted comprises a value of said lastHOtype IE.

[0011] In some example embodiments, the cell switch command includes the TA value for the target cell. In an alternative embodiment, the UE may determine the TA value.

[0012] The UE may further comprise a / the output for indicating whether said RACH-based switching was successful. This indication may be used, for example, in updating a KPI indicating that RACH-less switching was unsuccessful due to an invalid TA value, but that subsequent RACH-based switching to the same target cell was successful.

[0013] In a second aspect, this specification describes a network node (e.g. a source node) of a mobile communication system, the node comprising: an input (or some other means) for receiving a measurement report from a user equipment, UE, of the mobile communication system, wherein the measurement report is either an L1 or an L3 measurement report; a control module (or some other means) for determining whether to initiate LTM switching to a target cell of one or more candidate target cells based, at least in part, on said measurement report; an output (or some other means) for providing an LTM cell switch command (e.g. a MAC CE cell switch command) to the UE in the event that LTM switching is determined to be initiated, the LTM cell switch command identifyingsaid target cell; and a / the input (or some other means) for receiving a radio link failure, RLF, report, from the UE in the event of a failure of the RACH-less cell switching to the target cell, wherein the RLF report includes an indication of whether LI or L3 triggered LTM was attempted.

[0014] The indication of whether LI or L3 triggered LTM was attempted may comprise an information element, IE, of the RLF (such as an " LTM type" IE).

[0015] The RLF report may comprise a lastHOtype information element, wherein the indication of whether L1 or L3 triggered LTM was attempted comprises a value of said lastHOtype IE.

[0016] The cell switch command may include the TA value for the target cell. In an alternative embodiment, the UE determines the TA value.

[0017] The network node may further comprise a / the control module (or some other means) for determining whether RACH-less switching was unsuccessful due to an invalid TA value, but that subsequent RACH-based switching to the same target cell was successful.

[0018] The network node may further comprise a / the output (or some other means for) providing Access and Mobility information to another network node of the communication system or to another entity of the same network, wherein the Access and Mobility information comprises an indication of whether LI or L3 triggered LTM was attempted.

[0019] The network node may further comprise an input (or some other means) for receiving an indication regarding whether RACH-based switching was successful. This indication may be used, for example, in updating a KPI indicating that RACH-less switching was unsuccessful due to an invalid TA value, but that subsequent RACH-based switching to the same target cell was successful.

[0020] In a third aspect, this specification describes a user equipment, UE, of a mobilecommunication system, the UE comprising: an input (or some other means) for providing a measurement report to a network node of the mobile communication system, wherein the measurement report is either an L1 or an L3 measurement report; a / the input (or some other means) for receiving an LTM cell switch command (e.g. a MAC CE cell switch command) from the network node, wherein the cell switch instruction is generated at the network node in response to said measurement report; a control module (or some other means) for initiating RACH-less cell switching to a target cell in accordance with the LTM cell switch command using a timing advance, TA, value for the target cell available to the UE; and a / the control module (or some other means) for initiating RACH-based switching to the target cell in the event of a failure of the RACH-less cell switching to the target cell. The cell switch command may include the TA value for the target cell.

[0021] The UE may further comprise a / the control module (or some other means) for determining the TA value for the target cell. In some alternative embodiments, the TA value for the target cell is determined by the UE.

[0022] The UE may further comprise a / the control module (or some other means) for generating a radio link failure, RLF, report, in the event of a failure of the RACH-less cell switching to the target cell, wherein the RLF report includes an indication of whether L1 or L3 triggered LTM was attempted; and a / the output (or some other means)) for providing said RLF report to the network node. The indication of whether LI or L3 triggered LTM was attempted may comprise an information element, IE, of the RLF (such as an " LTM type" IE). The RLF report may comprises a lastHOtype information element, wherein the indication of whether L1 or L3 triggered LTM was attempted comprises a value of said lastHOtype IE.

[0023] The UE may further comprise a / the output (or some other means) for indicating whether RACH-based switching was successful. This indication may be used, for example, in updating a KPI indicating that RACH-less switching was unsuccessful due to an invalid TA value, but that subsequent RACH-based switching to thesame target cell was successful.

[0024] In a fourth aspect, this specification describes a network node (e.g. a source node) of a mobile communication system, the node comprising: an output (or some other means) for receiving a measurement report from a user equipment, UE, of the mobile communication system, wherein the measurement report is either an L1 or an L3 measurement report; a control module (or some other means) for determining whether to initiate LTM switching to a target cell of one or more candidate target cells based, at least in part, on said measurement report; a / the output (or some other means) for providing an LTM cell switch command (e.g. a MAC CE cell switch command) to the UE in the event that LTM switching is initiated, the LTM cell switch command identifying said target cell; and a / the control module (or some other means) for determining whether RACH-less switching was unsuccessful due to an invalid TA value, but that subsequent RACH-based switching to the same target cell was successful.

[0025] The network node may further comprise a / the control module (or some other means) for updating a key performance indicator, KPI, based on the determination of whether RACH-less switching was unsuccessful due to an invalid TA value, but that subsequent RACH-based switching to the same target cell was successful. The KPI may be stored in a register (e.g. locally or remotely).

[0026] The cell switch command may include the TA value for the target cell.

[0027] The network node may further comprise a / the input (or some other means) for receiving a radio link failure, RLF, report, from the UE in the event of a failure of the RACH-less cell switching to the target cell, wherein the RLF report includes an indication of whether LI or L3 triggered LTM was attempted.

[0028] The network node may further comprise a / the output (or some other means) for providing Access and Mobility information to another network node of the communication system or to another entity of the same network, wherein theAccess and Mobility information comprises an indication of whether LI or L3 triggered LTM was attempted.

[0029] In a fifth aspect, this specification describes a method comprising: providing a measurement report to a network node of the mobile communication system, wherein the measurement report is either an L1 or an L3 measurement report; receiving an LTM cell switch command (e.g. a MAC CE cell switch command) from the network node, wherein the cell switch instruction is generated at the network node in response to said measurement report; initiating RACH-less cell switching to a target cell in accordance with the LTM cell switch command using a timing advance, TA, value for the target cell available to the UE; initiating RACH-based switching to the target cell in the event of a failure of the RACH-less cell switching to the target cell; generating a radio link failure, RLF, report, in the event of a failure of the RACH-less cell switching to the target cell, wherein the RLF report includes an indication of whether L1 or L3 triggered LTM was attempted; and providing said RLF report to the network node.

[0030] The indication of whether LI or L3 triggered LTM was attempted comprises an information element, IE, of the RLF report (such as an " LTM type" IE).

[0031] The RLF report may comprise a lastHOtype information element, wherein the indication of whether L1 or L3 triggered LTM was attempted comprises a value of said lastHOtype IE.

[0032] In some example embodiments, the cell switch command includes the TA value for the target cell. In an alternative embodiment, the UE may determine the TA value.

[0033] The method may further comprise indicating whether said RACH-based switching was successful. This indication may be used, for example, in updating a KPI indicating that RACH-less switching was unsuccessful due to an invalid TA value, but that subsequent RACH-based switching to the same target cell was successful.In a sixth aspect, this specification describes a method comprising: receiving a measurement report from a user equipment, UE, of the mobile communication system, wherein the measurement report is either an L1 or an L3 measurement report; determining whether to initiate LTM switching to a target cell of one or more candidate target cells based, at least in part, on said measurement report; providing an LTM cell switch command (e.g. a MAC CE cell switch command) to the UE in the event that LTM switching is determined to be initiated, the LTM cell switch command identifying said target cell; and receiving a radio link failure, RLF, report, from the UE in the event of a failure of the RACH-less cell switching to the target cell, wherein the RLF report includes an indication of whether LI or L3 triggered LTM was attempted.

[0034] The indication of whether LI or L3 triggered LTM was attempted may comprise an information element, IE, of the RLF (such as an " LTM type" IE).

[0035] The RLF report may comprise a lastHOtype information element, wherein the indication of whether L1 or L3 triggered LTM was attempted comprises a value of said lastHOtype IE.

[0036] The cell switch command may include the TA value for the target cell. In an alternative embodiment, the UE determines the TA value.

[0037] The method may further comprise determining whether RACH-less switching was unsuccessful due to an invalid TA value, but that subsequent RACH-based switching to the same target cell was successful.

[0038] The method may further comprise providing Access and Mobility information to another network node of the communication system or to another entity of the same network, wherein the Access and Mobility information comprises an indication of whether LI or L3 triggered LTM was attempted.

[0039] The method may further comprise receiving an indication regarding whetherRACH-based switching was successful. This indication may be used, for example, in updating a KPI indicating that RACH-less switching was unsuccessful due to an invalid TA value, but that subsequent RACH-based switching to the same target cell was successful.

[0040] In a seventh aspect, this specification describes a method comprising: providing a measurement report to a network node of the mobile communication system, wherein the measurement report is either an L1 or an L3 measurement report; receiving an LTM cell switch command (e.g. a MAC CE cell switch command) from the network node, wherein the cell switch instruction is generated at the network node in response to said measurement report; initiating RACH-less cell switching to a target cell in accordance with the LTM cell switch command using a timing advance, TA, value for the target cell available to the UE; and initiating RACH-based switching to the target cell in the event of a failure of the RACH-less cell switching to the target cell. The cell switch command may include the TA value for the target cell.

[0041] The method may further comprise determining the TA value for the target cell. In some alternative embodiments, the TA value for the target cell is determined by the UE.

[0042] The method may further comprise generating a radio link failure, RLF, report, in the event of a failure of the RACH-less cell switching to the target cell, wherein the RLF report includes an indication of whether LI or L3 triggered LTM was attempted. The method may further comprise providing said RLF report to the network node. The indication of whether LI or L3 triggered LTM was attempted may comprise an information element, IE, of the RLF (such as an " LTM type" IE). The RLF report may comprises a lastHOtype information element, wherein the indication of whether L1 or L3 triggered LTM was attempted comprises a value of said lastHOtype IE.

[0043] The method may further comprise indicating whether RACH-based switching was successful. This indication may be used, for example, in updating a KPIindicating that RACH-less switching was unsuccessful due to an invalid TA value, but that subsequent RACH-based switching to the same target cell was successful.

[0044] In an eighth aspect, this specification describes a method comprising: receiving a measurement report from a user equipment, UE, of the mobile communication system, wherein the measurement report is either an L1 or an L3 measurement report; determining whether to initiate LTM switching to a target cell of one or more candidate target cells based, at least in part, on said measurement report; providing an LTM cell switch command (e.g. a MAC CE cell switch command) to the UE in the event that LTM switching is initiated, the LTM cell switch command identifying said target cell; and determining whether RACH-less switching was unsuccessful due to an invalid TA value, but that subsequent RACH-based switching to the same target cell was successful. The cell switch command may include the TA value for the target cell.

[0045] The method may further comprise updating a key performance indicator, KPI, based on the determination of whether RACH-less switching was unsuccessful due to an invalid TA value, but that subsequent RACH-based switching to the same target cell was successful.

[0046] The method may further comprise receiving a radio link failure, RLF, report, from the UE in the event of a failure of the RACH-less cell switching to the target cell, wherein the RLF report includes an indication of whether LI or L3 triggered LTM was attempted.

[0047] The method may further comprise providing Access and Mobility information to another network node of the communication system or to another entity of the same network, wherein the Access and Mobility information comprises an indication of whether LI or L3 triggered LTM was attempted.

[0048] In a ninth aspect, this specification describes computer-readable instructions which, when executed by a computing apparatus, cause the computingapparatus to perform (at least) any method as described herein (including the methods of the fifth to eighth aspects described above).

[0049] In a tenth 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 fifth to eighth aspects described above).

[0050] In an eleventh 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 fifth to eighth described above).

[0051] In an twelfth aspect, this specification describes an apparatus comprising: at least one processor; and at least one memory storing instructions that, when executed by the at least one processor, causes the apparatus to perform (at least) any method as described herein (including the methods of the fifth to eighth aspects described above).

[0052] Brief Description of the Drawings

[0053] Example embodiments will now be described by way of non-limiting example, with reference to the accompanying drawings, in which:

[0054] FIG. 1 is a block diagram of an example system;

[0055] FIG. 2 is a flow chart showing an example LTM procedure;

[0056] FIG. 3 shows a message flow sequence in accordance with an example embodiment;

[0057] FIG. 4 to 6 show flow charts in accordance with example embodiments;

[0058] FIG. 7 shows a message flow sequence in accordance with an example embodiment;

[0059] FIG. 8 is a schematic diagram of a system that may be used to implement one or more of the example embodiments; andFIG. 9 shows tangible media for storing computer-readable code which when run by a computer may perform methods according to example embodiments described herein.

[0060] Detailed Description

[0061] 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 including 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.

[0062] 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.

[0063] 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.

[0064] FIG. 2 is a flow chart, indicated generally by the reference numeral 20, showing an example LTM procedure. The flow chart 20 may be implemented using the system 10 described above.

[0065] The LTM procedure of the flow chart 20 starts with LTM candidate preparation in step 22. In the step 22, information is shared between a network node (gNB) and a UE regarding candidate cells (such as the first and second candidate cells 16 and 18) that the UE might switch to. As discussed further below, the candidate cell information is shared using RRC signalling.

[0066] LTM is a procedure in which a network node (e.g. a gNB) receives measurement report(s) from a UE, such as the UE 12, and on the basis of the measurementreports, the network node controls a change to the serving cell of the UE. As discussed further below, this is implemented using cell switch commands signaled via Medium Access Control Control Element (MAC CE) messages. The cell switch command indicates an LTM candidate cell configuration that the gNB previously prepared in the LTM candidate preparation step 22.

[0067] When configured by the network, it is possible to activate Transmission Configuration Indicator, TCI, states of one or multiple cells that are different from the current serving cell (such as the first and second candidate cells 16 and 18). For instance, the TCI states of the LTM candidate cells can be activated in advance before any of those cells become the serving cell. This allows the UE to be downlink, DL, synchronized with those cells, thereby facilitating a faster cell switch to one of those cells when cell switch is triggered.

[0068] At step 24 of the flow chart 20, an early timing advance, TA, synchronization procedure between the UE and respective candidate cells may be carried out. For example, the step 24 may involve an uplink, UL, TA acquisition procedure to one or multiple cells that are different from the current serving cell. For instance, the network may request the UE to perform early TA acquisition of a candidate cell before a cell switch. The early TA acquisition may be triggered by PDCCH order (e.g. as specified in clause 9.2.6 of 3GPP TS 38.300) or realized through UE-based TA measurement. In the former case, the gNB to which the candidate cell belongs determines the TA value and sends it to the gNB to which the serving cell belongs. The serving cell sends the TA value in the LTM cell switch command MAC CE when triggering LTM cell switch. In the latter case, the UE applies the TA value measured by itself and performs RACH-less LTM upon receiving the cell switch command.

[0069] At step 26 of the flow chart 20, LTM execution takes place, in which the respective UE switches to a target cell in accordance with a cell switch command (such as a MAC CE message). The LTM procedure can be used to reduce the mobility latency.By way of example, the UE 12 may attempt RACH-less access to the target cell if TA information for that cell is available to the UE. If network provided and UE based TA measurement are both available for the target cell, the TA signaled by the network in the cell switch command may be prioritized over the UE based measurement.

[0070] For RACH-less LTM, the UE may access the target cell via a configured grant provided in the LTM candidate cell configuration and selects the configured grant occasion associated with the beam indicated in the cell switch command. If the LTM candidate cell configuration does not include a configured grant, the UE may monitor PDCCH for dynamic scheduling from the target cell upon LTM cell switch.

[0071] FIG. 3 shows a message flow sequence, indicated generally by the reference numeral 30, in accordance with an example embodiment. The message flow sequence 30 shows messages transmitted between, and actions taken at, a UE 32 and a gNB 34. The message flow sequence 30 allows cell switch triggering based on L3 measurements (see step 5 of the message sequence 30 described below).

[0072] Initially, the UE 32 is connected to a serving cell and operating in RRC_Connected mode.

[0073] At step 1 of the message flow sequence 30, the UE 32 performs L1 measurements on the configured LTM candidate cell(s) and transmits L1 measurement reports to the gNB 34. L1 measurement should be performed as long as RRC reconfiguration (step 2) is applicable.

[0074] In response to the L1 measurement report sent in step 1, the gNB 34 prepares one or more candidate cells for potential LTM transfer. An LTM candidate preparation step (see step 22 of the flow chart 20) can be completed with the transfer of RRC reconfiguration messages between the UE 32 and the gNB 34 (see messages 2 and 3 of the message sequence 30).Following LTM preparation, early synchronization may take place (see step 24 of the flow chart 20). For example, DL synchronization may take place with LTM candidate cells (see messages 4a) and / or UL synchronization may take place with LTM candidate cells (see messages 4b).

[0075] Following LTM preparation and early synchronization, LTM execution may take place (see step 26 of the flow chart 20).

[0076] The LTM execution phase of the message sequence 30 involves the UE 32 sending a measurement report to the gNB 34 (see step 5 of the message sequence 30) and the gNB taking an LTM decision based on the measurement report. As discussed above, the UE may perform L1 measurements.

[0077] Alternatively, or in addition, the UE may perform L3 measurement reporting to the gNB, including beam level measurement results on cell(s) which are configured as LTM candidate cell(s) according to the received network configuration. Thus, the message 5 of the message sequence 30 may be an L1 measurement report or an L3 measurement report.

[0078] Regarding L3 measurement reporting, it should be noted that LTM execution may be extended to be applicable to FR2 implementations with no LI measurements. In such scenarios, rather than cell switch being triggered based on L1 measurements from a UE, cell switching can be triggered based on L3 measurements used also for other handover methodologies. An advantage of using L3 measurements in LTM execution may be that network may communicate the TA value of candidate target cell to a UE as for Rel. 18 LTM, which may decrease the mobility interruption time. The TA may be obtained via Early TA acquisition procedure, as discussed above.

[0079] Handover failures can occur in implementations of the message sequence 30. A Mobility Robustness Optimization, MRO, framework has been developed to detect and describe connection failures. One of the functions of MRO is to detect connection failures that occur due to handover that occur too early or too late, or handovers to the wrong cell. These problems are defined as follows:• Intra-system Too Late Handover: a radio link failure, RLF, occurs after the UE has stayed for a long period of time in the cell; the UE attempts to reestablish the radio link connection in a different cell.

[0080] • Intra-system Too Early Handover: an RLF occurs shortly after a successful handover from a source cell to a target cell or a handover failure occurs during the handover procedure; the UE attempts to re-establish the radio link connection in the source cell.

[0081] • Intra-system Handover to Wrong Cell: an RLF occurs shortly after a successful handover from a source cell to a target cell or a handover failure occurs during the handover procedure; the UE attempts to re-establish the radio link connection in a cell other than the source cell and the target cell.

[0082] Limitations of, and potential extensions to, the MRO framework are discussed further below.

[0083] In some example implementations of the message sequence 30, the network is not able to distinguish between LTM execution based on L1 measurement and LTM execution based on L3 measurements, especially if both types were configured for the UE. The UE typically only reports that an LTM based cell switch has occurred / was attempted with no differentiation on L1 or L3 based measurements. The distinction may be useful as different entities are responsible for optimization based on the type of measurements used. For example, for L1-triggered LTM, an invalid TA may be optimized by source Distributed Unit (DU), whilst for L3-triggered LTM, an invalid TA may be optimized by a Central Unit Control Plane (CU CP). In the event that the L3 LTM decision is made at a DU, then the DU may also be responsible for optimization.

[0084] FIG. 4 shows a flow chart, indicated generally by the reference numeral 40, in accordance with an example embodiment. The flow chart 40 may be implemented at a UE of a mobile communication system, such as the UE 12 or the UE 32 described above. The flow chart 40 is generally described below with reference to the message sequence 30 by way of example only.The flow chart 40 begins at step 42 with a measurement report being provided from a UE (such as the UE 32) to a network node (such as the gNB 34) of a mobile communication system. As discussed above, the measurement report is either an L1 measurement report or an L3 measurement report. The step 42 may, for example, be step 5 of the message sequence 30 described above.

[0085] In response to the measurement report provided in step 42, the UE receives (in step 43) an LTM cell switch command (e.g. a MAC CE cell switch command) from the network node. The LTM cell switch command (which may be step 6 of the message sequence 30) is generated at the network node in response to the measurement report received in the step 42 and may be generated following an LTM decision made at the network node (as shown in the message sequence 30).

[0086] In step 44, the UE initiates RACH-less cell switching to a target cell in accordance with the LTM cell switch command received in step 43. The RACH-less cell switching uses a timing advance, TA, value for the target cell available to the UE. The TA value may be provided as part of the cell switch command, although alternatives implementations are possible (e.g. the TA value may be determined by the UE).

[0087] At step 45, if a cell switch failure occurs, the flow chart 40 moves to step 46; otherwise the flow chart 40 terminates at step 49 (with the cell switching being complete).

[0088] At step 46, it has been determined that a failure of the RACH-less cell switching initiated in step 44 has occurred. In response, an alternative method of cell switching takes place. In the example flow chart 40, at step 46 RACH-based switching to the target cell is initiated. The network node may receive an indication (or otherwise determine) that the RACH-based switching has been successful (e.g. that RACH-less switching was unsuccessful due to an invalid TA value, but that subsequent RACH-based switching to the same target cell was successful).At step 47 of the flow chart 40, a radio link failure, RLF, report, is generated. The RLF report includes an indication of whether L1 or L3 triggered LTM was attempted. The generated RLF report is provided to the network node in step 48 following which the flow chart terminates at the step 49.

[0089] As discussed in detail below, the indication in the RLF report of whether L1 or L3 triggered LTM was attempted may comprise an information element, IE, of the RLF report. The IE (which may be an "LTM type" IE) may have a value of L1 or L3 and may be optional. Alternatively, or in addition, the RLF report may comprise a "lastHOtype" information element, wherein the indication of whether L1 or L3 triggered LTM was attempted comprises a value of said lastHOtype IE.

[0090] An example RLF format is provided below by way of example:

[0091] RLF-Report-rl6:: = CHOICE {

[0092] nr-RLF-Report-r16 SEQUENCE { measResultLastServCell-r16 MeasResultRLFNR-r16, measResultNeighCells-r16 SEQUENCE { measResultListNR-r16 MeasResultList2NR-r16 OPTIONAL,

[0093] measResultListEUTRA-r16 MeasResultList2EUTRA-r16 OPTIONAL

[0094] } OPTIONAL, c-RNTI-r16 RNTI-Value, previousPCellId-r16 CHOICE { nrPreviousCell-r16 CGI-Info-Logging-r16, eutraPreviousCell-r16 CGI-InfoEUTRALogging }

[0095] OPTIONAL,

[0096] failedPCellId-r16 CHOICE { nrFailedPCellId-r16 CHOICE { cellGlobalId-r16 CGI-Info-Logging-r16,

[0097] pci-arfcn-r16 PCI-ARFCN-NR-r16 },

[0098] eutraFailedPCellId-r16 CHOICE { cellGlobalId-rl6 CGI-InfoEUTRALogging, pci-arfcn-r16 PCI-ARFCN-EUTRA-r16 }

[0099] },

[0100] reconnectCellId-r16 CHOICE {

[0101] nr Re connected 11 d-rl 6 CGI-Info-Logging-r16,eutraReconnectCell!d-rl6 CGI-InfoEUTRALogging }

[0102] OPTIONAL,

[0103] timeUntilReconnection-r16 TimeUntilReconnection-r16 OPTIONAL,

[0104] reestablishmentCellId-r16 CGI-Info-Logging-r16 OPTIONAL,

[0105] timeConnFailure-r16 INTEGER (0..1023) OPTIONAL,

[0106] timeSinceFailure-r16 TimeSinceFailure-r16, connectionFailureType-r16 ENUMERATED {rlf, hof}, rlf-Cause-r16 ENUMERATED {t310-Expiry, randomAccessProblem, rlc-MaxNumRetx,

[0107] beamFailureRecoveryFailure, lbtFailure-r16

[0108] bh-rlf RecoveryFailure, t312-expiry-rl7, sparel }

[0109] locationInfo-r16 LocationInfo-r16 OPTIONAL,

[0110] noSuitableCellFound-r16 ENUMERATED {true} OPTIONAL,

[0111] ra-InformationCommon-r16 RA-InformationCommon-r16 OPTIONAL,

[0112] csi-rsRLMConfigBitmap-v1650 BIT STRING (SIZE (96)) OPTIONAL

[0113] lastHO-Type-r17 ENUMERATED {cho, daps, ltm-l3, spare2, spare1}

[0114] OPTIONAL,

[0115] ltm-Type-rel19 ENUMERATED {l1, l3} OPTIONAL,

[0116] timeConnSourceDAPS-Failure-r17 TimeConnSourceDAPS-Failure-r17 OPTIONAL,

[0117] timeSinceCHO-Reconfig-r17 TimeSinceCHO-Reconfig-r17 OPTIONAL,

[0118] choCellId-rl7 CHOICE { cellGlobalId-rl7 CGI-lnfo-Loqqinq-r16 pci-arfcn-r!7 PCI-ARFCN-NR-r16 OPTIONAL,

[0119] choCandidateCellList-r17 ChoCandidateCellList-r17 OPTIONAL

[0120] pSCellId-r18 CHOICE { cellGlobalId-r18 CGI-Info-Logging-r16, pci-arfcn-r18 PCI-ARFCN-NR-r16

[0121]

[0122] As discussed above, the example RLF report includes a "lastHO-type" IE that optionally includes an indication of the LTM type and / or an LTM-type IE that indicates the LTM type.

[0123] FIG. 5 shows a flow chart, indicated generally by the reference numeral 50, in accordance with an example embodiment. The flow chart 50 may be implemented at a network node of a mobile communication system, such as the source cell 14 or the gNB 34 described above. The flow chart 50 is generally described below with reference to the message sequence 30 by way of example only. The flow charts 40 and 50 may be considered together.

[0124] The flow chart 50 starts at step 52, where a measurement report is received (e.g. at the gNB 34) from a UE (e.g. the UE 32) of the mobile communication system, wherein the measurement report is either an L1 or an L3 measurement report. The measurement report received in the step 52 may be the measurement report provided in step 42 of the flow chart 40 described above.

[0125] At step 53, the network node determines whether to initiate LTM switching to a target cell of one or more candidate target cells (e.g. one or more of the first and second candidate cells 16 and 18 described above) based, at least in part, on the measurement report received in step 52. If a cell switch is to be initiated, the flow chart moves to step 54; otherwise the flow chart 50 terminates at step 57.

[0126] At step 54, a decision has been taken to initiate LTM cell switching. Accordingly, an LTM cell switch command (e.g. a MAC CE cell switch command) is provided to the UE to initiate RACH-less cell switching. The LTM cell switch command identifies the target cell and may include the TA value of the target cell. The cell switch command provided in the step 54 may be the cell switch command received in step 43 of the message sequence 40.

[0127] In the event that a cell switch failure occurs, a radio link failure, R. LF, report maybe received from the UE in step 55 of the flow chart 50. The RLF report includes an indication of whether L1 or L3 triggered LTM was attempted. The RLF report received in step 55 may be the RLF report provided in step 48 of the flow chart 40.

[0128] As discussed above, the indication (included in the RLF report) of whether L1 or L3 triggered LTM was attempted may comprise an IE (e.g. an LTM type IE) of the RLF or may be provided as a value of another IE (e.g. a lastHOtype IE).

[0129] In some example embodiments, the flow chart 50 may include step 56 in which Access and Mobility information is provided to another network node of the communication system, or to another entity of the same network. The Access and Mobility information comprises an indication of whether LI or L3 triggered LTM was attempted.

[0130] The step 56 may be useful, for example, in the event of L1 or L3 triggered LTM decisions are taken at a Distributed Unit (DU) so that the relevant information can be provided from a central unit (CU) to the DU, thereby enabling the DU to optimize the TA acquisition procedure and to seek to avoid future failures due to invalid TA. The step 56 may be implemented by adding the new " LTM type" to an ACCESS AND MOBILITY INDICATION message (sent from CU to DU) via introducing a new IE as shown below:

[0131] lE / Group Name Presenc Range IE type Semantics Criticalit Assigne e and description y d reference Criticalit y Message Type M 9.2.3.1 YES ignore RA Report 0..1 YES ignore > RA Report 1.. < —

[0132] List Item maxno

[0133] of R A Re

[0134] ports >

[0135] »RA Report M OCTET Includes the

[0136] Container STRING RA- ReportList I E

[0137]

[0138] as defined inlE / Group Name Presenc Range IE type Semantics Criticalit Assigne e and description y d reference Criticalit y subclause

[0139] 6.2.2 in TS

[0140] 38.331

[0010] ,

[0141] »UE 0 NG-RAN YES ignore Assistant node UE

[0142] Identifier XnAP ID

[0143] 9.2.3.16

[0144] »NRCell List 0 OCTET Includes the YES ignore Container STRING CellldListNR

[0145] IE as defined

[0146] in subclause

[0147] 6.2.2 in TS

[0148] 36.331

[0014] ,

[0149] RLF Report 0..1 YES ignore Information

[0150] List

[0151] > RLF Report 1..

[0152] Information Item <maxn

[0153] oofRLF

[0154] Reports

[0155] >

[0156] »NR UE RLF M OCTET Includes the

[0157] Report STRING nr-RLF- Container Report-r16 IE

[0158] contained in

[0159] the

[0160] UElnformatio

[0161] nResponse

[0162] message

[0163] defined in TS

[0164] 38.331 [8],

[0165] »UE 0 gNB-DU - Assistant UE F1AP

[0166] Identifier ID

[0167] 9.3.1.5

[0168] »C-RNTI 0 9.3.1.32 C-RNTI YES ignore allocated at

[0169] the source

[0170] gNB-DU.

[0171] »RLF Report 0 ENUMER YES ignore Failure Type ATED (too

[0172] late LTM,

[0173] too early

[0174] LTM, LTM

[0175] to wrong

[0176] cell,...)

[0177]

[0178] »LTM type 0 ENUMER YES ignorelE / Group Name Presenc Range IE type Semantics Criticalit Assigne e and description y d reference Criticalit y ATED (L1,

[0179] L3,...)

[0180] Successful HO 0..1 YES ignore Report

[0181] Information List

[0182] Successful 1..

[0183] HO Report <maxn

[0184] Information oofSuc

[0185] Item cessful

[0186] HORep

[0187] orts>

[0188] »Successful M OCTET Includes the

[0189] PSCell STRING SuccessPSC

[0190] Change ell-Report I E

[0191] Report as defined in

[0192] Container TS 38.331

[0193]

[0010]

[0194] »SN Mobility 0 BIT Mobility

[0195] Information STRING Information in

[0196] (SIZE the PSCell of

[0197] (32)) the source

[0198] SN in case

[0199] this message

[0200] is sent from

[0201] the MN to the

[0202] source SN;

[0203] Mobility

[0204] Information in

[0205] the PSCell of

[0206] the target SN

[0207] in case this

[0208] message is

[0209] sent from the

[0210] MN to the

[0211] target SN.

[0212] DL LBT Failure 0..1 YES ignore Information List

[0213] > DL LBT 1..

[0214] Failure <maxn

[0215] Information oof LBT

[0216] Item Failure /

[0217] nformat

[0218] ion>

[0219] »DL LBT M 9.2.3.174 —

[0220] Failure

[0221]

[0222] InformationTable 1: Access and Mobile Indication that may be sent from a gNB-CU to a gNB-DU to provide access and mobility information to the gNB-DU.

[0223] FIG. 6 shows a flow chart, indicated generally by the reference numeral 70, in accordance with an example embodiment. The flow chart 60 may be implemented at a network node of a mobile communication system, such as the source cell 14 or the gNB 34 described above. The flow chart 60 is generally described below with reference to the message sequences 30 and the flow chart 40 by way of example only. The flow chart 60 is similar to the flow chart 50 described above.

[0224] The flow chart 70 starts at step 52 (as discussed above with reference to the flow chart 50), where a measurement report is received (e.g. at the gNB 34) from a UE (e.g. the UE 32) of the mobile communication system, wherein the measurement report is either an L1 measurement report or an L3 measurement report. The measurement report received in the step 52 may be the measurement report provided in step 42 of the flow chart 40 described above.

[0225] At step 53, the network node determines whether to initiate LTM switching to a target cell of one or more candidate target cells (e.g. one or more of the first and second candidate cells 16 and 18 described above) based, at least in part, on said measurement report received in step 52. If a cell switch is to be initiated, the flow chart moves to step 54; otherwise the flow chart 50 terminates at step 77.

[0226] At step 54, a decision has been taken to initiate LTM cell switching. Accordingly, an LTM cell switch command (e.g. a MAC CE cell switch command) is provided to the UE to initiate RACH-less cell switching. The LTM cell switch command identifies the target cell and may include the TA value of the target cell.

[0227] The steps 52 to 54 are the same as steps 52 to 54 of the flow chart 50 described above.At step 55 of the flow chart 70, the network receives an RLF report. The RLF report may be the RLF report provided in step 48 of the flow chart 40 described above. Note that the step 55 may be omitted if the RLF report is not required. As discussed in detail above, the RLF report (if received in the step 55) may include an indication of whether LI or L3 triggered LTM was attempted. As discussed above, the indication (included in the RLF report) of whether LI or L3 triggered LTM was attempted may comprise an IE (e.g. an LTM type IE) of the RLF or may be provided as a value of another IE (e.g. a lastHOtype IE).

[0228] The network node may receive an indication (or otherwise determine) that the RACH-based switching has been successful (e.g. that RACH-less switching was unsuccessful due to an invalid TA value, but that subsequent RACH-based switching to the same target cell was successful).

[0229] In some example embodiments, the flow chart 70 may include a step 74 in which a key performance indicator, KPI, may be updated based on the determination of whether RACH-less switching was unsuccessful due to an invalid TA value, but that subsequent RACH-based switching to the same target cell was successful and / or Access and Mobility information may be provided to another network node of the communication system, or to another entity of the same network, wherein the Access and Mobility information comprises an indication of whether LI or L3 triggered LTM was attempted.

[0230] The KPI may be updated based on a determination of success of a RACH-based cell switching. The updated KPI may include an LTM-Invalid TA Value Handover Failure KPI indicating that an RLF occurred during the RACH-less based LTM cell switch procedure from source to target cell which is triggered by network via sending the cell switch command with included TA value to UE, but that the UE successfully connected to the same target cell with RACH based access via LTM recovery / re-establishment procedure when new TA value provided by the network in a Random Access Response, RAR, message. This provides an additional KPI that may be useful for MRO.As noted above, the step 74 may include providing Access and Mobility information to another network node of the communication system, or to another entity of the same network, wherein the Access and Mobility information comprises an indication of whether LI or L3 triggered LTM was attempted. As discussed above, with reference to the step 56, the step 74 may be useful, for example, in the event of LI or L3 triggered LTM with decisions in a Distributed Unit (DU) so that the relevant information can be provided by the central unit (CU) to the DU, thereby enabling the DU to optimize the TA acquisition procedure and to seek to avoid future failures due to invalid TA. The step 56 may be implemented by adding the new " LTM type" to an ACCESS AND MOBILITY INDICATION message (sent from CU to DU) via introducing a new IE, as discussed in detail above.

[0231] FIG. 7 shows a message flow sequence, indicated generally by the reference numeral 80, in accordance with an example embodiment. The message flow sequence 70 shows messages transmitted between, and actions taken at, the UE 32 and the gNB 34 described above with reference to the flow chart 30. The message flow sequence 80 allows cell switch triggering based on L3 measurements.

[0232] The message sequence 80 starts with LTM candidate preparation in step 22. As discussed above with reference to the flow chart 20, in the step 22, information is shared between the UE 32 and the gNB 34 regarding candidate cells (such as the first and second candidate cells 16 and 18) that the UE might switch to. The candidate cell information is shared using RRC signalling (see, by way of example, the R. R. C messages in the LTM preparation steps of the message sequence 30 described above).

[0233] At step 24 of the message sequence 80, an early timing advance (TA) synchronization procedure between the UE and respective candidate cells may be carried out (as discussed with reference to the flow chart 20).

[0234] A measurement report 81 is sent from the UE 32 to the gNB 34. As discussed inthe steps 42 and 52 described above, the measurement report is either an L1 measurement report or an L3 measurement report.

[0235] At step 83, the gNB 34 determines whether to initiate LTM switching to a target cell of one or more candidate target cells (e.g. one or more of the first and second candidate cells 16 and 18 described above) based, at least in part, on the measurement report 81. If a cell switch is to be initiated, an LTM cell switch command 84 (e.g. a MAC CE cell switch command) is sent by the network node to the UE to initiate RACH-less cell switching. The LTM cell switch command identifies the target cell and may include the TA value of the target cell.

[0236] In step 85, the UE 32 initiates RACH-less cell switching to a target cell in accordance with the LTM cell switch command received in the LTM switch command 84. The RACH-less cell switches uses a timing advance, TA, value for the target cell available to the UE (which may be provided as part of the LTM switch command). As discussed above, for RACH-less LTM, the UE accesses the target cell via a configured grant provided in the LTM candidate cell configuration and selects the configured grant occasion associated with the beam indicated in the cell switch command.

[0237] At step 86, it has been determined that a failure of the RACH-less cell switching initiated in response to the LTM switch command message 84 has occurred. In response, RACH-based switching to the target cell is initiated.

[0238] In the event that RACH-less cell switching in unsuccessful, an RLF report 87 is sent from the UE 32 to the gNB 34. As discussed in detail above, the RLF report includes an indication of whether LI or L3 triggered LTM was attempted. The network may make use of the LTM type provided in the RLF report as part of handover optimisation (since different entities may be responsible for optimisation depending on whether the LTM was based on LI or M3 measurements). Further, the LTM type can be used as part of analysis to identify the root cause of the failure and to seek to avoid invalid TA usage in the future.At step 88, a KPI is updated based on the determination of whether RACH-less switching (in step 85) was unsuccessful due to an invalid TA value, but subsequent RACH-based switching to the same target cell (in step 86) was successful. The KPI may be updated based on the receipt of a RACH success indication that may form part of the step 86. As discussed above, the KPI may be useful for MRO. For example, the KPI may assist in identifying the root cause of a failure and to seek to avoid invalid TA usage in the future.

[0239] At step 89, Access and Mobility information is provided to another network node of the communication system, or to another entity of the same network. The Access and Mobility information comprises an indication of whether LI or L3 triggered LTM was attempted. As discussed above, the step 89 may be useful, for example, in the event of LI or L3 triggered LTM with decisions in a Distributed Unit (DU) so that the relevant information can be provided by a central unit (CU) to the DU, thereby enabling the DU to optimize the TA acquisition procedure and to seek to avoid future failures due to invalid TA. As noted above, the steps 88 and 89 may be carried out in a different order, or in parallel. Moreover, one or both of the steps 88 and 89 may be omitted in some example embodiments.

[0240] Example Apparatus

[0241] FIG. 8 shows an apparatus according to some example embodiments. The apparatus may be configured to perform the operations described herein, for example operations described with reference to any disclosed methods and message sequences. 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 at least 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 tocause 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 7 and related features thereof.

[0242] FIG. 9 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 7 and related features thereof.

[0243] 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.

[0244] 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.

[0245] 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.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.

[0246] 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:provide a measurement report to a network node of the mobile communication system, wherein the measurement report is either an L1 or an L3 measurement report;receive an LTM cell switch command from the network node, wherein the cell switch instruction is generated at the network node in response to said measurement report;initiate RACH-less cell switching to a target cell in accordance with the LTM cell switch command using a timing advance, TA, value for the target cell available to the UE; andinitiate RACH-based switching to the target cell in the event of a failure of the RACH-less cell switching to the target cell.

2. The UE of claim 1, wherein the cell switch command is a MAC CE cell switch command.

3. The UE of claim 1 or claim 2, wherein the cell switch command includes the TA value for the target cell.

4. The UE of claim 1 or claim 2, whether the instructions further cause the UE at least to:determine the TA value for the target cell.

5. The UE of any one of the preceding claims, whether the instructions further cause the UE at least to:generate a radio link failure, RLF, report, in the event of a failure of the RACH-less cell switching to the target cell, wherein the RLF report includes anindication of whether L1 or L3 triggered LTM was attempted; andprovide said RLF report to the network node.

6. The UE of claim 5, wherein the indication of whether L1 or L3 triggered LTM was attempted comprises an information element, IE, of the RLF.

7. The UE of claim 5 or claim 6, wherein the RLF report comprises a lastHOtype information element, wherein the indication of whether L1 or L3 triggered LTM was attempted comprises a value of said lastHOtype IE.

8. A network node of a mobile communication system, the network node 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 a measurement report from a user equipment, UE, of the mobile communication system, wherein the measurement report is either an L1 or an L3 measurement report;determine whether to initiate LTM switching to a target cell of one or more candidate target cells based, at least in part, on said measurement report;provide an LTM cell switch command to the UE in the event that LTM switching is initiated, the LTM cell switch command identifying said target cell; anddetermine whether RACH-less switching was unsuccessful due to an invalid TA value, but that subsequent RACH-based switching to the same target cell was successful.

9. The network node of claim 8, whether the instructions further cause the network node at least to:update a key performance indicator, KPI, based on the determination of whether RACH-less switching was unsuccessful due to an invalid TA value, but that subsequent RACH-based switching to the same target cell was successful.

10. The network node of claim 8 or claim 9, wherein the cell switch command is a MAC CE cell switch command.

11. The network node of claim 10, wherein the cell switch command includes the TA value for the target cell.

12. The network node of any one of claims 8 to 11, whether the instructions further cause the network node at least to:receive a radio link failure, RLF, report, from the UE in the event of a failure of the RACH-less cell switching to the target cell, wherein the RLF report includes an indication of whether LI or L3 triggered LTM was attempted.

13. The network node of claim 12, wherein the instructions further cause the network node at least to:provide Access and Mobility information to another network node of the communication system or to another entity of the same network, wherein the Access and Mobility information comprises an indication of whether LI or L3 triggered LTM was attempted.

14. A method comprising:providing a measurement report to a network node of the mobile communication system, wherein the measurement report is either an L1 or an L3 measurement report;receiving an LTM cell switch command from the network node, wherein the cell switch instruction is generated at the network node in response to said measurement report;initiating RACH-less cell switching to a target cell in accordance with the LTM cell switch command using a timing advance, TA, value for the target cell available to the UE; andinitiating RACH-based switching to the target cell in the event of a failure of the RACH-less cell switching to the target cell.

15. A method comprising:receiving a measurement report from a user equipment, UE, of the mobile communication system, wherein the measurement report is either an L1 or an L3 measurement report;determining whether to initiate LTM switching to a target cell of one or more candidate target cells based, at least in part, on said measurement report;providing an LTM cell switch command to the UE in the event that LTM switching is initiated, the LTM cell switch command identifying said target cell; anddetermining whether RACH-less switching was unsuccessful due to an invalid TA value, but that subsequent RACH-based switching to the same target cell was successful.

16. The method of claim 15, further comprising:updating a key performance indicator, KPI, based on the determination of whether RACH-less switching was unsuccessful due to an invalid TA value, but that subsequent RACH-based switching to the same target cell was successful.

17. A computer program product comprising program instructions which, when executed by a computing apparatus, cause the computing apparatus to:provide a measurement report to a network node of the mobile communication system, wherein the measurement report is either an L1 or an L3 measurement report;receive an LTM cell switch command from the network node, wherein the cell switch instruction is generated at the network node in response to said measurement report;initiate RACH-less cell switching to a target cell in accordance with the LTM cell switch command using a timing advance, TA, value for the target cell available to the UE; andinitiate RACH-based switching to the target cell in the event of a failure of the RACH-less cell switching to the target cell.

Images