handover

Abstract

An apparatus, method and computer program is described comprising: sending a handover request from a source network node to a target network node, requesting a RRC configuration (LTM) and a second RRC configuration (DAPS); receiving a handover response from the target network node, the handover response including the requested first and second RRC configurations; providing an RRC reconfiguration message to a user device, wherein the RRC reconfiguration message includes the first and second RRC configurations; receiving at least one measurement report from the UE; transmitting, to the user device an indication indicating which of two handover types, associated with the first and second RRC configuration respectively, is to be used.

Description

[0001] Handover

[0002] Field

[0003] The present specification relates to performing handovers in radio communication systems.

[0004] Background

[0005] A number of arrangements are known for performing handovers in radio communication systems. For example, a UE connection that is initially connected to a source node of a network may change to being connected to a target node of the network in accordance with a handover protocol. Different handover protocols have different advantages and disadvantages. There remains a need for further developments in this field.

[0006] Summary

[0007] In a first aspect, this specification describes an apparatus (e.g. a source cell or a source central unit) comprising: means for sending a handover request (e.g. a DAPS with LTM handover request) from a source network node (e.g. a central unit or a distributed unit of a serving network node) to a target network node, requesting a first radio resource control, RRC, configuration and a second RRC configuration, wherein the first RRC configuration comprises a low-layer triggered mobility, LTM, configuration and the second RRC configuration comprises a dual active protocol stack, DAPS, configuration; means for receiving a handover response from the target network node, the handover response including the requested first and second RRC configurations; means for providing an RRC reconfiguration message to a user device, wherein the RRC reconfiguration message includes the first and second RRC configurations; means for receiving at least one measurement report from the UE; and means for transmitting, to the user device an indication indicating which of two handover types, associated with the first and second RRC configuration respectively, is to be used. The handover types may be LTM and DAPS handovers. The first RRC configuration may be a delta configuration compatible with low-layer triggered mobility (LTM) reference configuration. The apparatus may form part of the source network node. The source network node may, for example, be a source distributed unit, DU, or a source centralized unit, CU.

[0008] The handover request may include at least one measurement resource configuration for the at least one measurement. Some example embodiments further comprise means for deciding which of the two handover types is selected.

[0009] One of the two handover types may be selected based, at least in part, on the at least one measurement report. For example, the handover type may be based on source link quality (e.g. RSRP values) or different configured threshold values.

[0010] Some example embodiments further comprise means for informing the user device indicating a time or a trigger event where the user device needs to release a source cell of the source network node.

[0011] Some example embodiments further comprise means for informing the user device indicating a time or a trigger event where the user device needs to apply a configuration associated with the indicated handover type.

[0012] Some example embodiments further comprise means for modifying the DAPS configuration by adding measurement resource configuration (e.g. LI measurement resource configuration) of at least one of: the source cell of the source network node; or the target cell of the target network node.

[0013] The first RRC configuration may contain at least one measurement resource configuration (e.g. at least one LI measurement resource configuration) of at least one of: the source cell of the source network node; or the target cell of the target network node.

[0014] The second RRC configuration may contain at least one measurement resource configuration of at least one of: the source cell of the source network node; or the target cell of the target network node.

[0015] In a second aspect, this specification describes an apparatus (e.g. a target network node or target cell) comprising: means for receiving a handover request, from a source network node at a handover target network node, requesting both a first radio resource control, RRC, configuration and a second RRC configuration, wherein the first RRC configurations comprises an LTM configuration and the second RRC configuration comprises a DAPS configuration; means for providing a handover response to the source network node, the handover response including the first and second RRC configurations; and means for exchanging RRC reconfiguration messages with a user device, the RRC reconfiguration messages include an RRC reconfiguration complete message received from the user device, the RRC reconfiguration complete message including an indication of which of two handover types, associated with the first and second RRC configuration respectively, is to be used. The apparatus may form part of the handover target network node.

[0016] Some example embodiments further comprise means for indicating that the handover source cell is to be released.

[0017] Some example embodiments further comprise means for sending a medium access control control element, MAC-CE, command to the user device indicating release of the source cell and an application of LTM configuration.

[0018] The handover request may include at least one measurement resource configuration for the at least one measurement.

[0019] The first RRC configuration may contain at least one measurement resource configuration (e.g. at least one LI measurement resource configuration) of at least one of: the source cell of the source network node; or the target cell of the target network node.

[0020] The second RRC configuration may contain at least one measurement resource configuration of at least one of: the source cell of the source network node; or the target cell of the target network node.

[0021] In a third aspect, this specification describes an apparatus (e.g. a user device or UE) comprising: means for receiving a radio resource control, RRC, reconfiguration message from a source network node, wherein the RRC reconfiguration message includes a first RRC configuration and a second RRC configuration, wherein the first RRC configuration comprises a low-layer triggered mobility, LTM, configuration and the second RRC configuration comprises a dual active protocol stack, DAPS, configuration; means for transmitting at least one measurement report to the source network node; means for receiving, from the source network node, an indication indicating which of two handover types, associated with the first and second RRC configuration respectively, is to be used; and means for performing a handover execution phase based on the indication. The apparatus may form part of the user device. Some example embodiments further comprise means for receiving information indicating a time or a trigger event where the user device needs to release a source cell of the source network node.

[0022] Some example embodiments further comprise means for receiving information indicating a time or a trigger event where the user device needs to apply the configuration associated with the indicated handover type.

[0023] Some example embodiments further comprise means for transmitting an RRC Reconfiguration Complete message to the target network node indicating a selected handover type.

[0024] Some example embodiments further comprise means for receiving a medium access control control element, MAC-CE, command from the target network node indicating release of the source cell and an application of the LTM configuration.

[0025] The first RRC configuration may contain at least one measurement resource configuration (e.g. at least one LI measurement resource configuration) of at least one of: the source cell of the source network node; or the target cell of the target network node.

[0026] The second RRC configuration may contain at least one measurement resource configuration of at least one of: the source cell of the source network node; or the target cell of the target network node.

[0027] In a fourth aspect, this specification describes a method comprising: sending a handover request from a source network node to a target network node, requesting a first radio resource control, RRC, configuration and a second RRC configuration, wherein the first RRC configuration comprises a low-layer triggered mobility, LTM, configuration and the second RRC configuration comprises a dual active protocol stack, DAPS, configuration; receiving a handover response from the target network node, the handover response including the requested first and second RRC configurations; providing an RRC reconfiguration message to a user device, wherein the RRC reconfiguration message includes the first and second RRC configurations; receiving at least one measurement report from the UE; and transmitting, to the user device an indication indicating which of two handover types, associated with the first and second RRC configuration respectively, is to be used. The handover types may be LTM and DAPS handovers. The first RRC configuration may be a delta configuration compatible with low-layer triggered mobility (LTM) reference configuration.

[0028] The handover request may include at least one measurement resource configuration for the at least one measurement.

[0029] Some example embodiments further comprise deciding which of the two handover types is selected.

[0030] One of the two handover types may be selected based, at least in part, on the at least one measurement report. For example, the handover type may be based on source link quality (e.g. RSRP values) or different configured threshold values.

[0031] Some example embodiments further comprise informing the user device indicating a time or a trigger event where the user device needs to release a source cell of the source network node.

[0032] Some example embodiments further comprise informing the user device indicating a time or a trigger event where the user device needs to apply a configuration associated with the indicated handover type.

[0033] Some example embodiments further comprise modifying the DAPS configuration by adding measurement resource configuration (e.g. LI measurement resource configuration) of at least one of: the source cell of the source network node; or the target cell of the target network node.

[0034] The first RRC configuration may contain at least one measurement resource configuration (e.g. at least one LI measurement resource configuration) of at least one of: the source cell of the source network node; or the target cell of the target network node.

[0035] The second RRC configuration may contain at least one measurement resource configuration of at least one of: the source cell of the source network node; or the target cell of the target network node.

[0036] In a fifth aspect this specification describes a method comprising: receiving a handover request, from a source network node at a handover target network node, requesting both a first radio resource control, RRC, configuration and a second RRC configuration, wherein the first RRC configurations comprises an LTM configuration and the second RRC configuration comprises a DAPS configuration; providing a handover response to the source network node, the handover response including the first and second RRC configurations; and exchanging RRC reconfiguration messages with a user device, the RRC reconfiguration messages include an RRC reconfiguration complete message received from the user device, the RRC reconfiguration complete message including an indication of which of two handover types, associated with the first and second RRC configuration respectively, is to be used. The method may be implemented (at least in part) at handover target network node.

[0037] The handover request may include at least one measurement resource configuration for the at least one measurement.

[0038] The first RRC configuration may contain at least one measurement resource configuration (e.g. at least one LI measurement resource configuration) of at least one of: the source cell of the source network node; or the target cell of the target network node.

[0039] The second RRC configuration may contain at least one measurement resource configuration of at least one of: the source cell of the source network node; or the target cell of the target network node.

[0040] Some example embodiments further comprise indicating that the handover source cell is to be released.

[0041] In a sixth aspect, this specification describes a method comprising: receiving a radio resource control, RRC, reconfiguration message from a source network node, wherein the RRC reconfiguration message includes a first RRC configuration and a second RRC configuration, wherein the first RRC configuration comprises a low-layer triggered mobility, LTM, configuration and the second RRC configuration comprises a dual active protocol stack, DAPS, configuration; transmitting at least one measurement report to the source network node; receiving, from the source network node, an indication indicating which of two handover types, associated with the first and second RRC configuration respectively, is to be used; and performing a handover execution phase based on the indication. The method may be implemented, at least in part, at a user device. Some example embodiments further comprise receiving information indicating a time or a trigger event where the user device needs to release a source cell of the source network node.

[0042] Some example embodiments further comprise receiving information indicating a time or a trigger event where the user device needs to apply the configuration associated with the indicated handover type.

[0043] Some example embodiments further comprise transmitting an RRC Reconfiguration Complete message to the target network node indicating a selected handover type.

[0044] Some example embodiments further comprise receiving a medium access control control element, MAC-CE, command from the target network node indicating release of the source cell and an application of the LTM configuration.

[0045] The first RRC configuration may contain at least one measurement resource configuration (e.g. at least one LI measurement resource configuration) of at least one of: the source cell of the source network node; or the target cell of the target network node.

[0046] The second RRC configuration may contain at least one measurement resource configuration of at least one of: the source cell of the source network node; or the target cell of the target network node.

[0047] In a seventh 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 method of the fourth to sixth aspects described above).

[0048] In an eighth 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 fourth to sixth aspects described above).

[0049] In a ninth aspect, this specification describes an apparatus comprising: at least one processor; and at least one memory including computer program code which, 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 fourth to sixth aspects described above).

[0050] In a tenth aspect, this specification describes a computer program comprising instructions for causing an apparatus to perform at least the following: sending a handover request from a source network node to a target network node, requesting a first radio resource control, RRC, configuration and a second RRC configuration, wherein the first RRC configuration comprises a low-layer triggered mobility, LTM, configuration and the second RRC configuration comprises a dual active protocol stack, DAPS, configuration; receiving a handover response from the target network node, the handover response including the requested first and second RRC configurations; providing an RRC reconfiguration message to a user device, wherein the RRC reconfiguration message includes the first and second RRC configurations; receiving at least one measurement report from the UE; and transmitting, to the user device an indication indicating which of two handover types, associated with the first and second RRC configuration respectively, is to be used.

[0051] In an eleventh aspect this specification describes a computer program comprising instructions for causing an apparatus to perform at least the following: receiving a handover request, from a source network node at a handover target network node, requesting both a first radio resource control, RRC, configuration and a second RRC configuration, wherein the first RRC configurations comprises an LTM configuration and the second RRC configuration comprises a DAPS configuration; providing a handover response to the source network node, the handover response including the first and second RRC configurations; and exchanging RRC reconfiguration messages with a user device, the RRC reconfiguration messages include an RRC reconfiguration complete message received from the user device, the RRC reconfiguration complete message including an indication of which of two handover types, associated with the first and second RRC configuration respectively, is to be used.

[0052] In a twelfth aspect this specification describes a computer program comprising instructions for causing an apparatus to perform at least the following: receiving a radio resource control, RRC, reconfiguration message from a source network node, wherein the RRC reconfiguration message includes a first RRC configuration and a second RRC configuration, wherein the first RRC configuration comprises a low-layer triggered mobility, LTM, configuration and the second RRC configuration comprises a dual active protocol stack, DAPS, configuration; transmitting at least one measurement report to the source network node; receiving, from the source network node, an indication indicating which of two handover types, associated with the first and second RRC configuration respectively, is to be used; and means for performing a handover execution phase based on the indication.

[0053] Brief description of the drawings

[0054] Example embodiments will now be described, by way of example only, with reference to the following schematic drawings, in which:

[0055] FIG. 1 is a block diagram of a system in accordance with an example embodiment; FIG. 2 is a flow chart in accordance with an example embodiment;

[0056] FIG. 3 to 8 are message flow sequences in accordance with example embodiments;

[0057] FIG. 9 is a flow chart in accordance with an example embodiment;

[0058] FIG. 10 is a block diagram of a system in accordance with an example embodiment;

[0059] FIG. 11 is a flow chart in accordance with an example embodiment;

[0060] FIG. 12 to 14 are message flow sequences in accordance with example embodiments;

[0061] FIG. 15 is a flow chart in accordance with an example embodiment;

[0062] FIG. 16 is a flow chart in accordance with an example embodiment;

[0063] FIG. 17 is a block diagram of components of a system in accordance with an example embodiment; and

[0064] FIG. 18 shows an example of tangible media for storing computer-readable code which when run by a computer may perform methods according to example embodiments described above.

[0065] Detailed description

[0066] The scope of protection sought for various embodiments of the disclosure is set out by the independent claims. The embodiments and features, if any, described in the 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 disclosure.

[0067] In the description and drawings, like reference numerals refer to like elements throughout.

[0068] FIG. 1 is a block diagram of a system, indicated generally by the reference numeral 10, in accordance with an example embodiment.

[0069] The system 10 shows a user equipment (UE) moving from a first position 12a to a second position 12b. Assume that the UE moves from the coverage area of one cell (in the position 12a) to the coverage area of another cell (in the position 12b). In response, a serving cell change needs to be performed. The handover from one cell to another may, for example, be implemented using the low-layer (e.g., LI (e.g., physical layer) and / or L2 (e.g., MAC layer)) triggered mobility (LTM) framework, as discussed further below. Hereinafter a handover (HO) that may be performed under the LTM framework is called a LTM HO.

[0070] The serving cell change may, for example, be triggered by Layer 3 (L3) measurements (e.g., RRC Measurement Report from UE) and implemented by downlink RRC signalling, e.g. using an RRC Reconfiguration message for change of Primary Cell (PCell) or Primary Secondary Cell (PSCell), as well as release and add for Secondary Cells (SCells) when applicable.

[0071] FIG. 2 is a flow chart, indicated generally by the reference numeral 20, in accordance with an example embodiment. The flow chart 20 may be used to implement a handover, such as that required by the system 10 described above.

[0072] The flow chart 20 starts at operation 22, where a LTM HO preparation is carried out. In the operation 22, one or more target cells (such as coverage area of in the position 12b described above) are prepared for potential handover by a central unit (CU). The CU may provide support for the higher layers of the protocol stack such as a service data adaptation protocol (SDAP) layer, packet data convergence protocol (PDCP) layer and radio resource control (RRC) layer, while a distributed unit (DU) may provide support for the lower layers of the protocol stack such as radio link control (RLC), medium access control (MAC) and Physical layer.

[0073] At operation 24, the early synchronisation is performed in which the UE 12 synchronises with candidate cells in order to minimise the interruption time during a possible later handover.

[0074] As operation 26, L1 / L2 triggered mobility (LTM) HO is executed (i.e. handover occurs). The flow chart 20 then ends with an LTM HO completion at stage 28.

[0075] FIG. 3 is a message flow sequence, indicated generally by the reference numeral 30, in accordance with an example embodiment. The sequence 30 shows messages between a user equipment (UE) 32 and a network node (gNB) 34.

[0076] The UE 32 may be the UE 12 described above. The sequence 30 may be used in an example implementation of the flow chart 20. More specifically, steps 1 to 3 of the message sequence 30 may be used to implement the LTM HO preparation operation 22, step 4 may be used to implement the early synchronisation operation 24, steps 5 to 7 may be used to implement the LTM execution operation 26, and step 8 may be used to implement the LTM complete operation 28.

[0077] The message sequence 30 starts with the UE 32 in RRC_Connected mode. The UE 32 sends an L3 measurement report to the gNB 34 (step 1). The gNB 34 (acting as a central unit (CU)) prepares a set of one or more candidate cells for LTM HO (operation 36). The gNB / CU 34 provides LTM configuration information for the prepared cells (in an RRC reconfiguration message - step 2), in response to which the UE 32 provides an RRC reconfiguration complete message (step 3). The CU also configures the UE 32 for LI measurement reporting needed for LTM execution (implemented later in the message sequence).

[0078] In step 4 of the message sequence 30, the UE 32 performs early UL / DL synchronisation with the candidate cells identified in operation 36 (and communicated in step 2). As noted above, early synchronisation seeks to avoid interruption time caused by synchronisation with target cell during a potential future handover.

[0079] In step 5 of the message sequence 30, the UE sends LI beam measurements of the target cells to the source cell (as an LI Measurement Report). The gNB 34 decides which cell the UE should handover to (operation 37) and provides a cell switch command to the UE 32 (step 6). After receiving the cell switch command, the UE detaches from the source cell and applies a configuration for the target cell (e.g., LTM configuration) at stage 38 of Fig. 3.

[0080] The UE 32 may perform a random access procedure to access the target cell (step 7), although if the timing advance (TA) of the target cell is still valid (from step 4). The UE may skip the random access procedure (step 7) during handover when the noncontention based RACH has been configured.

[0081] Finally, the flow chart is completed at LTM Completion step 8.

[0082] FIGS. 4 and 5 show a message flow sequence, indicated generally by the reference numerals 40 and 50, in accordance with an example embodiment. The sequences 40 and 50 shows messages sent between, and operations performed at, a UE 42, a source distributed unit (S-DU) or a source cell 44, a target distributed unit (T-DU) or a target cell 46, and a central unit 48.

[0083] The sequence 40 shown in FIG. 4 is an example sequence for the L1 / L2 triggered mobility (LTM) and may be used to implement the operation 22 of the flow chart 20 described above (see also steps 1 to 3 of the message sequence 30).

[0084] In step 1 of the sequence 40, an L3 Measurement Report is sent from the UE 42 to the source DU / cell 44 (as in the message sequence 30 described above). The L3 Measurement Report is sent to the CU 48 as part of an RRC message transfer (step 2). In step 3 of the sequence 40, a target cell preparation decision is made at the CU (e.g. based, at least in part, on the L3 Measurement Report).

[0085] The CU 48 sends a UE context setup request to the target DU / cell 46 (step 4) and receives a UE context setup response (step 5). The CU 48 also sends a UE context modification request to the source DU / cell 44 (step 6) and receives a UE context modification response (step 7).

[0086] At this stage, the CU 48 generates an RRC reconfiguration (step 8), for use in handover. The RRC reconfiguration includes a measurement configuration of the LI cell change and a configuration of prepared cells. Step 8 of the sequence 40 completes operation 36 of the sequence 30 described above.

[0087] The RRC reconfiguration is provided from the CU 48 to the UE 42 via the source DU / cell 44 (steps 9 and 10). The RRC reconfiguration message may include TA acquisition criteria and cell switch criteria. An RRC Reconfiguration Complete message is sent from the UE (step 11) and transferred via the source DU to the CU (step 12) - these correspond to steps 2 and 3 of the sequence 30.

[0088] In this way, in steps 1 to 12, the central unit 48:

[0089] • prepares target cells for LTM;

[0090] • provides LTM configurations for prepared cells;

[0091] • configures the UE 42 with LI measurement reporting needed for LTM execution;

[0092] • provides TA Acquisition triggering criteria and necessary configurations to the S- DU 44; and

[0093] • provides cell switch triggering criteria and necessary configurations to the S-DU

[0094] 44. The triggering criteria of TA acquisition and cell switch can be similar to measurement event report triggering conditions, e.g., A3, A4 or A5 conditions or validity of acquired TA. Triggering configurations can be a filter configuration (for LI measurements), trigger offsets, cell individual offsets, etc.

[0095] The message sequence 50 (FIG. 5) shows the HO execution (steps 13 to 23) and completion (steps 24 and 25) phases of the message sequence (and may be used to in an example implementation of steps 26 and 28 of the message sequence 20).

[0096] In step 13, the UE 42 starts reporting the LI measurements to the Source DU (S-DU) 44 regarding the LTM configuration. In step 14 and 15, the S-DU 44 decides to trigger the TA acquisition of the T-DU / cell(s) and sends the TA Acquisition Command to the S- DU (step 15).

[0097] In step 16, the UE 42 sends the Random-Access Preamble to the T-DU / cell(s) 46 so that the T-DU / cell can estimate the TA between the UE and the T-DU / cell(s). In step 17, the UE receives the Random-Access Response (RAR) indirectly (via CU and S-DU).

[0098] In step 18-20, the UE 42 sends the at least one LI beam Measurement Reports (step 18) of the prepared target cells and the S-DU 44 decides to which target cell the UE should handover (step 19). In step 20, if the RAR is not received by the UE 42 in step 17, the S-DU 44 provides the acquired TA to the UE via MAC CE command (as another alternative for receiving the TA of the target cell).

[0099] If the TA of the target cell is still valid (received in step 17), the UE may skip the RACH procedure of step 21 when executing the HO.

[0100] In step 22 to 25, the UE and the network proceed with the completion of LTM procedure. (Note that although the steps 22 and 23 are shown as part of the "Execution" phase in FIG. 5, they could be considered to form part of the "Completion" phase.)

[0101] The message sequences 40 and 50 relates to an intra-central unit (CU) mobility framework. The principles described herein are also applicable to an inter-central unit mobility framework, as discussed further below.

[0102] FIGS. 6 and 7 show a message flow sequence, indicated generally by the reference numerals 60 and 70 respectively, in accordance with an example embodiment. The message flow sequences 60 and 70 relate to an inter-CU mobility framework, specifically an L1 / L2 triggered mobility framework for inter-CU mobility.

[0103] The sequences 60 and 70 shows messages sent between, and operations performed at, a UE 61, a source distributed unit (S-DU) or cell 62, a source central unit (CU) 63, a target central unit (CU) 64 and a target distributed unit (T-DU) or cell 65.

[0104] The sequence 60 shown in FIG. 6 may be used to implement the operations 22 and 24 of the flow chart 20 described above. The sequence 70 shown in FIG. 7 may be used to implement the operations 26 and 28 of the flow chart 20.

[0105] Some differences between the inter-CU algorithm of the message sequences 60 and 70 and the intra-CU algorithm of the sequences 40 and 50 include:

[0106] • In steps 4-8 of the message sequence 60, the target CU 64 receives an LTM HO request from the source CU. After performing LTM admission control, the target CU requests the target DU / cell 65 to setup the UE context. The relevant UE configuration is received in a UE context setup response (step 7). In step 8, the target CU 64 provides an LTM HO response message to the source CU 63, including the R.RC configuration of the UE for the target DU / cell.

[0107] • In steps 13 and 14 of the message sequence 60, the R.RC reconfiguration message includes parameters for new security key determination. In step 26 of the sequence 70, the application of target cell configuration includes new security key determination and PDCP re-establishment with security key change.

[0108] • In step 32 of the message sequence 70, the target CU 64 sends an LTM HO complete message to the source CU 63, so that the source CU is informed about the successful completion of the HO and can in turn command the source DU to release the UE context.

[0109] The Intra-CU and inter-CU LTM algorithms described above seek to reduce interruption time during a handover. Despite the reduction achieved through LTM, a residual interruption time (e.g. of about 5-10ms) may occur. This residual interruption time may be larger for inter-CU LTM due to the required security reestablishment. More specifically, the UE must use a new security key for de-ciphering the Packet Data Convergence Protocol (PDCP), whose required re-establishment introduces an additional delay compared to intra-CU LTM (see step 26 of the message sequence 60). A further reduction or even elimination of the interruption time would be advantageous. FIG. 8 is a message flow sequence, indicated generally by the reference numeral 80, in accordance with an example embodiment. The message sequence 80 shows an alternative handover protocol, referred to as Dual Active Protocol Stack (DAPS) handover. The message sequence 80 shows messages sent between, and performed at, a UE 82, a source node 84, a target node 86 and a serving gateway (S-GW) or user plane function (UPF) 88.

[0110] DAPS Handover seeks to reduce the interruption time in downlink (DL) and uplink (UL). In case of DAPS handover, the UE continues the downlink user data reception from the source gNB until the source cell 84 is released and continues the uplink user data transmission to the source gNB until a successful random access procedure to the target gNB is completed.

[0111] In DAPS, both the source node 84 and the target node 86 have a full L2 protocol stack with their own security key for ciphering from the source node (and deciphering of the Packet Data Convergence Protocol (PDCP) Service Data Units (SDUs). The UE establishes a new radio link with the target node (step 9 - 11 of the message sequence 80) before detaching from the source node at step 21. As shown in FIG. 8, the UE 82 receives data from both source node 84 (step 12) and the target node 86 (step 13) before releasing the source. If the procedure fails (when the UE does not manage to set up a connection with the target cell, i.e., a handover failure case), the UE 82 may fall back to the source if it still has a sufficient radio link (e.g. if timer T310 for radio link monitoring has not expired).

[0112] The target cell may delay the release of the source node 84, to ensure that the newly established link (with the target cell) is stable. The sequence 80 presents one example implementation of such a procedure. In this example, the UE 82 may provide measurements to the target cell and the target cell evaluates the link; when the link is deemed to be stable, then the target can decide to release the source link. The transmission of measurements to the target cell can be done via proper configuration of the UE either by suitable setting of A3 event or via indicating to the UE to perform periodic measurement reporting during the DAPS handover.

[0113] Especially in FR2, where the UE has multiple panels (i.e., the UE is a multi-panel UE), where a panel consists of a set of antennas, with receive beamforming and spatial interference suppression capability, high link quality may be maintained with both source and target cell at the same time. This is because, depending on the UE orientation, the multiple panels of the UE provide significant spatial gain. As discussed in detail below, example embodiments seek to exploit advantages of both the LTM and DAPS handover procedures discussed above. Exploiting the advantages of both LTM and DAPS handover procedures is not trivial.

[0114] In DAPS handover algorithms, the target CU generates and provides to the source CU (with a cell switch / HO command) an RRC configuration of the UE to be applied for the dual protocol stack operation of the UE during the execution (after cell switch / HO command and before source release). Then, upon reception of the source release command, the UE switches to single protocol stack with the RRC configuration of the target. Since DAPS can only operate without secondary-cells, the network will not configure target with secondary-cells. In inter-CU LTM, the target CU generates and provides to the UE during the handover preparation phase an RRC configuration of the UE, which is to be applied after the UE receives a cell switch command. The RRC configuration may comprise a reference configuration, which is common for all candidate cells, and a candidate delta configuration, which is candidate cell-specific and is applied on top of the reference configuration. When multiple target cells are prepared, such signaling of the RRC configuration is resource efficient, as the reference configuration, whose signaling may require more resources, must be sent only once, while the candidate-specific delta configurations are more compact, as they describe only the differences of the candidate configuration with respect to the reference. It is possible for the LTM configuration to have reference or candidate configuration with multiple secondary cells already. From the above, a number of questions arise, such as: what configuration(s) should be generated and provided to the UE during the HO preparation phase; how is the target CU triggered to prepare such configuration(s) when are these configurations applied; and how are the involved entities informed.

[0115] In conventional DAPS, after successful establishment of the link between the UE and the target cell (i.e., after successful random access of the UE to the target cell - see step 10 of the message sequence 80), and reception from the UE of the RRC reconfiguration complete message (see step 11 of the message sequence 80), the target cell informs the source cell that the handover has been completed, so that the source cell can release the UE context. After that, the UE is also commanded through an RRC Reconfiguration message to release the source link (see step 21 of the message sequence 80). The transmission of the HO complete message may be conditioned on L3 measurements of the target and / or the source cell. In DAPS combined with LTM as described further below, the conditional transmission of the HO complete message to the source cell may be based on LI measurements, which are not available at the target CU, but may be available at the target DU. A number of problems may arise. For example:

[0116] • In LTM, the target cell LI measurements typically cannot be configured as an independent configuration. Some part of this configuration is given to the UE along with the LTM candidate configuration (not inside the candidate configuration but along with it). Hence, it may be difficult to configure the DAPS along with the LI measurements that cannot be configured without LTM candidate configuration.

[0117] • Deciding when the target DU should trigger the release of the source cell is not trivial.

[0118] • Similarly, deciding how to inform the target CU and the UE that the source cell-UE link should be released is not trivial.

[0119] FIG. 9 is a flow chart, indicated generally by the reference numeral 90, in accordance with an example embodiment. The flow chart 90 may be used to implement a handover and has many similarities with the flow chart 20 described above. As discussed in detail below, the flow chart 90 seeks to exploit advantages of using both the LTM and DAPS handover procedures discussed above.

[0120] The flow chart 90 starts at operation 91, where a handover request is issued, for example by a source network node (that is currently in communication with a user device) to a target network node. A handover response is then received, at operation 92, from the target node.

[0121] At operation 93, RRC configuration messages are provided to the user device. Then, at operation 94, early synchronisation can be performed (based on the RRC configurations provided in operation 93). As discussed above with reference to the flow chart 20, early synchronisation enables the user device to synchronise with candidate cells in order to minimise the interruption time during a possible later handover procedure (e.g., HO excution phase and / or completion phase).

[0122] As operation 95, the handover is executed (e.g. so that the user device is in communication with the target node). The flow chart 90 then ends with a completion stage 96.

[0123] FIG. 10 is a block diagram of a system, indicated generally by the reference numeral

[0124] 100, in accordance with an example embodiment. The system 100 comprises a user equipment (UE 102), a source network node 104 and at least one target network node 106. The system 100 may be used to implement the flow chart 90, in which the UE 102 is initially in communication with the source node 104, before a handover to the target node 106 occurs. The system 100 may also be used in other example embodiments (such as the embodiments described herein with reference to other flow charts). As discussed in detail below, the source network node 104 may comprise at least one source distributed unit (S-DU) and a source central unit (S-CU). Similarly, the at least one target network node may comprise at least one target distributed unit (T-DU) and a target central unit T-CU).

[0125] FIG. 11 is a flow chart, indicated generally by the reference numeral 110, in accordance with an example embodiment. The flow chart 110 may be implanted at the source network node 104 of the system 100 described above. The flow chart 110 may be used in an implementation of the algorithm 90 described above.

[0126] The flow chart 110 starts at operation 111, where a handover request (e.g. a DAPS with LTM handover, as discussed in detail below) from a source network node (such as the source node 104) to a target network node (e.g. the target node 106). The handover request 111 may request a first radio resource control (RRC) configuration and a second RRC configuration, wherein the first RRC configuration is an LTM configuration, including a delta configuration compatible with an LTM reference configuration, and the second RRC configuration is a DAPS configuration. For example, at 111, the HO request may be transmitted by a S-CU to a T-CU.

[0127] At operation 112, the source network node receives a handover response from the target network node (e.g. the node 106). The handover response includes the requested first and second RRC configurations. For example, the handover response may be transmitted by the T-CU to the S-CU.

[0128] At operation 113, an RRC reconfiguration message (including the first and second RRC configurations) is provided by the source network node to a user device (e.g. the UE 102). For example, the first and the second RRC configurations may be transferred by the S-CU to one of the S-DUs and the S-DU may transmit the RRC reconfiguration message including the first and the second RRC configuration message to the user device 102.

[0129] At operation 114, at least one measurement report (e.g. an LI Measurement Report) is received at the source network node from the user device. For example, the user device 102 may perform at least one LI and / or L3 measurements and transmit the at least one measurement report to the source network node (e.g., the S-DU or S-CU).

[0130] At operation 115, an indication is transmitted from the source node to the user device means for indicating which of two handover types (e.g. LTM HO or DAPS HO), associated with the first and second RRC configuration respectively, is to be used. The indication may take the form of a cell change trigger instructing a change to the target network node. The indication may be transmitted via a handover type message, cell change trigger, or cell switch type information.

[0131] FIGS. 12 to 14 show a message flow sequence, indicated generally by the reference numerals 120, 130 and 140 respectively, in accordance with an example embodiment. The sequences 120, 130 and 140 show messages sent between, and operations performed at, a UE 122, at least one source distributed unit (S-DU) 123, a source central unit (S-CU), at least one target distributed unit (T-DU) 125, and a target central unit (T-CU) 126. The UE 122 may be the UE 32 of Fig. 3, UE 42 of Fig. 4 and 5, UE 61 of Fig. 6 and 7, UE 82 of Fig. 8, the UE 102 of the system 100, or the UE of Fig. 11 described above. The source distributed unit 123 and source central unit may form part of the source network node 104 described in the above example embodiments. Similarly, the target distributed unit 125 and target central unit 126 may form part of the target network node 106 described in the above example embodiments.

[0132] The sequence 120 represents a handover preparation and early synchronization phases and is an example implementation of aspects of the flow charts 90 and 110 described above and is, for example, an example implementation of the operations 91 to 94 of the flow chart 90.

[0133] In step 1, at least one L3 Measurement report is sent from the UE 122 to the S-DU 123. In step 2, The S-DU transfers the L3 measurement via UL RRC message to the S-CU 124. In step 3, the T-CU 124 decides a target cell preparation based at least partially on L3 measurement report.

[0134] In steps 4 and 5, a UE context modification request is sent from the S-CU to the S-DU (step 4) and acknowledged (step 5).

[0135] At step 6, the S-CU 124 sends a handover request to the T-CU 125. In this example embodiment, the handover request is a DAPS with LTM handover request. In some example embodiments, if the T-CU 125 is able to supports DAPS with LTM handover request, then the S-CU may be configured to use this format when sending a handover request (on the basis that this methodology combines some advantages of DAPS handover and some advantages of LTM handover).

[0136] The handover request indicates to the target CU that (unlike inter-CU LTM or DAPS handover requests, for example), the target network node is required to provide two RRC configurations in the handover response. The RRC configurations includes an LTM configuration. The LTM configuration may comprise a delta configuration (which is compatible with LTM reference configuration, potentially including secondary cells configurations, which is generally referred to herein as an LTM configuration) and a DAPS configuration, which does not include secondary cells. Note that secondary cells are used in carrier aggregation, allowing a UE to communicate simultaneously with multiple cells, thereby potentially achieving higher data rates. For DAPS HO, secondary cells, if configured, should be released and, after the DAPS HO is completed, then link re-established from scratch.

[0137] The handover request sent in step 6 may also include the source cell's measurement resource configuration for LI measurement that may be embedded in the target cell's DAPS configuration. More specifically, the source cell's measurement resource configuration may be required for the compilation of the LI measurement reporting configuration, which is part of the target cell's RRC configuration.

[0138] At step 7, the T-CU performs admission control, wherein the T-CU determines whether the candidate target cell(s) can serve the UE.

[0139] At step 8, the T-CU provides a request message to the T-DU 126. For example, The request message may be a user equipment context setup request message. The target CU may provide configuration information to the target DU about the conditional release of the source cell, which shall be determined by the target DU based on a configured condition (e.g., configured event). The request message may comprise a configuration for conditional release of a source cell, the configuration for conditional release being determined based on the configured condition.

[0140] The configured condition (or event) defined by step 8 may take a number of forms, for example:

[0141] • The absolute LI measurements of the target cell or the relative LI measurements of the target cell with respect to the LI measurements of the source cell. • Relative LI measurements of the target cell with respect to a first threshold conditioned on the relative LI measurements of the source cell with respect to a second threshold.

[0142] • Relative time or number of measured / reported samples that the absolute LI measurements of the target cell are above a first threshold with respect to the time the absolute LI measurements of the source cell are below a second threshold.

[0143] • LI measurements involved in the above examples may start being performed by the UE and reported to the target cell / DU at the beginning of the completion stage discussed further below.

[0144] Also, in step 8, a request to the T-DU to provide the UL resources for LI measurement reporting of the UE may be provided after DAPS execution (i.e., after Step 24) is included.

[0145] In step 9 The T-DU 126 transmits / provides, to T-CU 125, scheduling information indicating UL resources for one or more LI measurement reports. The scheduling information may be transmitted via a response message in response to the request message is provided by the T-DU 126 to the T-CU 125. For example, the response message may be a UE context setup response message. The UE context setup response message may include the scheduling information and the target cell's LI measurement resource configuration. The one or more LI measurement reports may be transmitted from the UE on the UL resources to the target DU after DAPS execution.

[0146] At step 10, the T-CU 125 sends a handover response message in response to the handover request message at step 6. For example, the handover response message may be a DAPS with LTM HO response message, which includes the two RRC configurations described above embodiments. The RRC configuration includes DAPS configuration for dual protocol stack operation. The DAPS configuration may include at least one of:

[0147] • the source cell's LI measurement resource configuration (provided to the target

[0148] CU in step 6);

[0149] • the target cell's LI measurement resource configuration (provided to the target CU in step 9); or

[0150] • the LI measurement reporting configuration, such as scheduling information for providing one or more LI measurement reports to the target DU and uplink resources relating to where the report message is to be transmitted. The other RRC configuration includes the LTM configuration. The LTM configuration may include at least one of:

[0151] • the LTM candidate delta configuration;

[0152] • a configuration of secondary cells; or

[0153] • the target cell's LI measurement resource configuration (provided to the target CU in step 9).

[0154] The DAPS configuration and the LTM configuration may be transmitted via a transparent container to the S-DU at step 10.

[0155] An exemplary schematic representation of the relevant information elements included in the handover response is provided in Table 1 below.

[0156] Table 1

[0157] In one example embodiment, the source cell's LI measurement resource configuration may not be included in the DAPS configuration. In this case:

[0158] • The source CU may modify the DAPS configuration received from the target CU, adding the source cell's LI measurement resource configuration.

[0159] • Alternatively, the source CU may provide the source cell's LI measurement configuration to the UE separately from the DAPS configuration, e.g., it may be provided in the LTM configuration (separately from the LTM candidate delta configuration).

[0160] In an example embodiment, the target cell's LI measurement configuration may not be included in the DAPS configuration, but only provided separately, e.g., in the LTM configuration. In a further embodiment, the source CU may modify the DAPS configuration received from the target CU, by adding the target cell's LI measurement resource configuration.

[0161] In one embodiment the target CU informs the source CU about the conditions under which the source DU / CU can trigger DAPS executions (e.g., RSRP values / threshold). It can also provide the source CU / DU about the conditions that are going to be applied at the target DU / CU for DAPS termination. In another embodiment, the above conditions can be provided from the source DU / CU to the target DU / CU, e.g., in the HO request (step 6).

[0162] In step 11 of the message sequence 120, the S-CU 124 generates at least one RRC configuration, based at least on the information received at the S-CU in step 10 (including the two RRC configurations). As described above with respect to step 10, if the DAPS configuration does not include the source / target cell's measurement resource configuration, the source CU may modify the DAPS configuration to include them. As in conventional LTM, it also provides the measurement configuration of LI measurements on which the cell change is based, which includes the source and target LI measurement resource configuration and the LI measurement report configuration for reporting measurements to the source DU to be used for cell switch decision. In steps 12-13, the S-CU 124 provides the generated RRC configuration to the UE 122, be sending an RRC Reconfiguration message which is transferred to the UE through the S-DU 123.

[0163] The message sequence 130 is an example implementation of the handover execution operation 95 of the flow chart 90 described above (see the steps 11 to 17 of FIGS, 4 and 5, and the step 14 to 21 of FIG. 6, for example).

[0164] In step 22 of Fig. 13, the UE 122 provides at least one LI Measurement Reports to the S-DU 123. Then, at step 23, S-DU 123 makes a serving cell change decision. The serving cell change decision may includes a decision on the handover type (DAPS HO or LTM HO). The source DU may decide on serving cell change and HO type based on LI measurements, for example:

[0165] • If the highest LI RSRP measurement is from a target cell in a different CU, an inter-CU HO is decided;

[0166] • If the source link quality is poor, e.g., LI RSRP measurement of the source cell is below a configured threshold, LTM switch is selected. Otherwise, DAPS switch is selected; and / or The threshold may be service-dependent, e.g. lower for interruption-sensitive service (compared to throughput-sensitive service) so that DAPS is more likely to be used.

[0167] At step 24, the S-DU 123 sends an indication indicating which of two handover types, associated with the first and the second RRC configuration respectively, to be used. The indication may be transmitted via a cell switch command MAC-CE to the UE 122, indicating the handover type, as selected in step 23.

[0168] At step 25, if the indicated handover type is LTM HO, the UE applies the LTM configuration, which may include the application of the target delta configuration on top of the LTM reference configuration. Then, the procedure continues as in conventional inter-CU LTM.

[0169] Alternatively, if the indicated handover type is DAPS HO, the UE maintains the connection with the source cell and instantiates a separate protocol stack, where the target configuration is applied. The procedure may continue with the UE maintaining the link to the source cell, continuing bidirectional data transmission with it. In step 26, the S-DU informs the S-CU that a cell switch has been triggered, optionally informing also about the decided HO type. In addition, after step 26, data can be forwarded to the target CU.

[0170] In Step 27, the S-CU 124 may optionally inform the T-CU 125 about the handover type. This may be useful, for example, when the HO type is LTM. Upon receiving the indication that the HO type is LTM, the T-CU 125 can inform the T-DU 126 that it can release the resources used for reporting of LI measurements to decide on the release of the source cell link, as they are not needed in an LTM HO.

[0171] In step 28, the UE may optionally perform random access to the target cell for UL synchronization, if a valid timing advance (TA) value for accessing the target is not available at the UE.

[0172] In step 29, the T-DU 126 may optionally send an access notification message to the target CU 125 to inform the T-CU 125 that the UE has accessed the target cell.

[0173] For the notification of the T-CU and / or the T-DU about the HO type, the following two options are provided:

[0174] • Option 1 : o Step 30: The UE 122 may send an RRC reconfiguration complete message to the target CU 125, via the target DU 126, indicating the successful completion of the HO and the HO type. After step 29, the target CU can start forwarding data to the target DU / cell. o Step 31 : The T-CU 125 sends a cell switch indication to the T-DU 126 including an indication of the HO type, if this has not been already provided to the T-DU.

[0175] • Option 2: o Step 32: The UE may send a MAC-CE to the T-DU 126 indicating the HO type. o Step 33: The T-DU 126 sends a cell switch indication to the T-CU 125 including an indication of the HO type, if this has not been already provided to the T-CU.

[0176] After step 30 (or step 32), the UE switches its uplink to the target cell (as in conventional DAPS)

[0177] In Step 34, the target cell / DU can transmit PDCCH to the UE scrambled with the UE's C-RNTI (of the target).

[0178] The message sequence 140 of Fig. 14 is an example implementation of the handover completion operation 96 of the flow chart 90 described above.

[0179] At step 35, the UE 122 transmits at least one LI Measurement Report to the target DU / cell 126. Then, at step 36, based on the at least one LI measurement report provided by the UE and the configuration for conditional release it received in step 8, the target DU / cell determines that the condition to release the source cell is met.

[0180] In the following steps, the target DU / cell 126 indicates to the target CU that the condition for the source cell release has been met and the source cell should be released. Two alternative options for this are:

[0181] Option 1 :

[0182] • Step 37 (Optional): The target DU / cell 126 informs the target CU that the condition to release the source cell has been met. This option is included, so that the target CU can send "DAPS with LTM HO complete message" (step 46) without having to wait to first receive the RRC reconfiguration complete message. • Step 38: The target DU / cell 126 sends a new MAC-CE to indicate to the UE 122 to release the source cell.

[0183] • Step 39: The UE 122 releases the source cell, switches to single protocol stack and applies the target LTM configuration.

[0184] • Option 1A: o Step 40: The UE 122 sends an RRC reconfiguration complete message to the target CU, via the target DU, to inform the T-CU that it released the source cell and applied the LTM configuration

[0185] • Option IB: o Step 41: The UE sends back a HARQ acknowledgement to inform the T- DU that it successfully decoded the MAC-CE, released the source cell and applied the LTM configuration. o Step 42: After reception of the HARQ acknowledgement from the UE, the target DU / cell indicates to the target CU to release the source cell.

[0186] Option 2:

[0187] • Step 43: The target DU / cell 126 indicates to the target CU 125 to release the source cell.

[0188] • Step 44: The target CU 125 sends an RRC reconfiguration message to the UE (via the target DU / cell 126) indicating to the UE to release the source cell. The RRC reconfiguration message is transparently transmitted to the UE 122 via the T-DU 126.

[0189] • Steps 45-46: The UE 122 responds to the target CU 125 with an RRC reconfiguration complete message, confirming the release of the source cell. The RRC reconfiguration complete message is transparently transmitted to the T-CU 125 via the T-DU 126.

[0190] The procedure is completed with the following steps:

[0191] • In steps 47-49, the target CU 125 informs the source CU 124 that the DAPS with LTM HO has been completed successfully and the source CU commands the source DU to release the UE context.

[0192] • After step 49, the source DU stops forwarding data.

[0193] • In Step 50, path switch takes place to switch the access path for data traffic from the source network node to the target network node.

[0194] FIG. 15 is a flow chart, indicated generally by the reference numeral 150, in accordance with an example embodiment. The flow chart 150 shows, at a high level how target and source cells are handled during a handover. The steps of the flow chart 150 may form part of any of the algorithms described with reference to the flow charts and message flow sequences described above (such as the algorithm of message flow sequences 120, 130 and 150).

[0195] The flow chart 150 starts at operation 152, means a handover request is received (e.g. at a handover target network node from a source network node at a handover target). As discussed in detail above, the handover request may request both a first RRC configuration including a delta configuration compatible with an LTM reference configuration and a second RRC configuration including a DAPS configuration.

[0196] At operation 154, a handover response is provided in response to the handover request. The handover response may include the first and second RRC configurations.

[0197] At operation 156, a handover to the target cell referred to above may be carried.

[0198] Finally, at operation 158, the source cell may be released (e.g. once a stable transfer to the target cell has been completed). The operation 150 may include providing a first source cell release instruction to a user device for releasing the source network node and applying the RRC configuration and providing a second source cell release instruction to the source network node for releasing a user device context at the source network node.

[0199] FIG. 16 is a flow chart, indicated generally by the reference numeral 160. in accordance with an example embodiment. The flow chart 160 provides further high- level details regarding how target and source cells are handled during a handover. The steps of the flow chart 160 may form part of any of the algorithms described with reference to the flow charts and message flow sequences described above (such as the algorithm of message flow sequences 120, 130 and 150).

[0200] At operation 161 of the flow chart (which may be omitted or implemented in some other way), handover (HO) request is sent from a central unit of a source network node (S-CU) to a central unit of a target network node (T-CU).

[0201] At operation 162 (which may be omitted or implemented in some other way), a user device context setup request is sent from the T-CU to a distributed unit of the target network node (T-DU).

[0202] At operation 163, a configuration for conditional release of a source cell based on a configured condition is received at the T-DU from the T-CU. At operation 164, scheduling information of uplink resources for one or more LI measurement reports is provided to the central unit of the target network node from a user device (for example in a response provided by the T-DU to the T-CU, e.g. in response to a request).

[0203] At operation 165, the one or more LI measurement reports are received at the T-DU from the user device in accordance with the scheduling information.

[0204] At operation 166, the T-DU evaluates whether the configured condition has been met, based on the one or more received LI measurement reports and the received condition configuration for conditional release.

[0205] At operation 167, an indication indicative of the user device to release the source cell in the event that the configured condition has been met is transmitted by the T-DU to the user device, in response to which a confirmation message is received (at the T- CU) from the user device confirming release of the source cell.

[0206] Finally, at operation 168, the T-CU provides a handover response in response to the handover request received in the operation 161. The handover response includes details of said uplink resources of LI measurement reports.

[0207] For completeness, FIG. 17 is a schematic diagram of components of one or more of the example embodiments described previously, which hereafter are referred to generically as a processing system 300. The processing system 300 may, for example, be the apparatus referred to in the claims below. For example, one or more of the nodes 122 to 126 may include an instance of the processing system 300 (or parts of that system).

[0208] The processing system 300 may have a processor 302, a memory 304 closely coupled to the processor and comprised of a RAM 314 and a ROM 312, and, optionally, a user input 310 and a display 318. The processing system 300 may comprise one or more network / apparatus interfaces 308 for connection to a network / apparatus, e.g. a modem which may be wired or wireless. The network / apparatus interface 308 may also operate as a connection to other apparatus such as device / apparatus which is not network side apparatus. Thus, direct connection between devices / apparatus without network participation is possible. The processor 302 is connected to each of the other components in order to control operation thereof.

[0209] The memory 304 may comprise a non-volatile memory, such as a hard disk drive (HDD) or a solid state drive (SSD). The ROM 312 of the memory 304 stores, amongst other things, an operating system 315 and may store software applications 316. The RAM 314 of the memory 304 is used by the processor 302 for the temporary storage of data. The operating system 315 may contain code which, when executed by the processor implements aspects of the flow charts and message sequences 20, 30, 40, 50, 60, 70, 80, 90, 110, 120, 130, 140, 150 and 150 described above. Note that in the case of small device / apparatus the memory can be most suitable for small size usage i.e. not always a hard disk drive (HDD) or a solid state drive (SSD) is used.

[0210] The processor 302 may take any suitable form. For instance, it may be a microcontroller, a plurality of microcontrollers, a processor, or a plurality of processors.

[0211] The processing system 300 may be a standalone computer, a server, a console, or a network thereof. The processing system 300 and needed structural parts may be all inside device / apparatus such as loT device / apparatus i.e. embedded to very small size.

[0212] In some example embodiments, the processing system 300 may also be associated with external software applications. These may be applications stored on a remote server device / apparatus and may run partly or exclusively on the remote server device / apparatus. These applications may be termed cloud-hosted applications. The processing system 300 may be in communication with the remote server device / apparatus in order to utilize the software application stored there.

[0213] FIG. 18 shows a tangible media, in the form of a removable memory unit 365, storing computer-readable code which when run by a computer may perform methods according to example embodiments described above. The removable memory unit 365 may be a memory stick, e.g. a USB memory stick, having internal memory 366 storing the computer-readable code. The internal memory 366 may be accessed by a computer system via a connector 367. Of course, other forms of tangible storage media may be used, as will be readily apparent to those of ordinary skilled in the art. Tangible media can be any device / apparatus capable of storing data / information which data / information can be exchanged between devices / apparatus / network. Embodiments of the present invention may be implemented in software, hardware, application logic or a combination of software, hardware and application logic. The software, application logic and / or hardware may reside on memory, or any computer media. In an example embodiment, the application logic, software or an instruction set is maintained on any one of various conventional computer-readable media. In the context of this document, a "memory" or "computer-readable medium" may be any non-transitory media or means that can contain, store, communicate, propagate or transport the instructions for use by or in connection with an instruction execution system, apparatus, or device, such as a computer.

[0214] Reference to, where relevant, "computer-readable medium", "computer program product", "tangibly embodied computer program" etc., or a "processor" or "processing circuitry" etc. should be understood to encompass not only computers having differing architectures such as single / multi-processor architectures and sequencers / parallel architectures, but also specialised circuits such as field programmable gate arrays FPGA, application specify circuits ASIC, signal processing devices / apparatus and other devices / apparatus. References to computer program, instructions, code etc. should be understood to express software for a programmable processor firmware such as the programmable content of a hardware device / apparatus as instructions for a processor or configured or configuration settings for a fixed function device / apparatus, gate array, programmable logic device / apparatus, etc.

[0215] If desired, the different functions discussed herein may be performed in a different order and / or concurrently with each other. Furthermore, if desired, one or more of the above-described functions may be optional or may be combined. Similarly, it will also be appreciated that the flow diagrams and sequences of Figures 2 to 9 and 11 to 16 are examples only and that various operations depicted therein may be omitted, reordered and / or combined.

[0216] It will be appreciated that the above-described example embodiments are purely illustrative and are not limiting on the scope of the invention. Other variations and modifications will be apparent to persons skilled in the art upon reading the present specification.

[0217] Moreover, the disclosure of the present application should be understood to include any novel features or any novel combination of features either explicitly or implicitly disclosed herein or any generalization thereof and during the prosecution of the present application or of any application derived therefrom, new claims may be formulated to cover any such features and / or combination of such features.

[0218] Although various aspects of the invention are set out in the independent claims, other aspects of the invention comprise other combinations of features from the described example embodiments and / or the dependent claims with the features of the independent claims, and not solely the combinations explicitly set out in the claims.

[0219] It is also noted herein that while the above describes various examples, these descriptions should not be viewed in a limiting sense. Rather, there are several variations and modifications which may be made without departing from the scope of the present invention as defined in the appended claims.

Claims

Claims1. An apparatus comprising: means for sending a handover request from a source network node to a target network node, requesting a first radio resource control, RRC, configuration and a second RRC configuration, wherein the first RRC configuration comprises a low-layer triggered mobility, LTM, reference configuration and the second RRC configuration comprises a dual active protocol stack, DAPS, configuration; means for receiving a handover response from the target network node, the handover response including the requested first and second RRC configurations; means for providing an RRC reconfiguration message to a user device, wherein the RRC reconfiguration message includes the first and second RRC configurations; means for receiving at least one measurement report from the UE; and means for transmitting, to the user device an indication indicating which of two handover types, associated with the first and second RRC configuration respectively, is to be used.

2. An apparatus as claimed in claim 1, wherein the handover request includes at least one measurement resource configuration for the at least one measurement.

3. An apparatus as claimed in claim 1 or 2, further comprising means for deciding which of the two handover types is selected.

4. An apparatus as claimed in claim 3, wherein one of the two handover type is selected based, at least in part, on the at least one measurement report.

5. An apparatus as claimed in any one of the preceding claims, further comprising: means for informing the user device indicating a time or a trigger event where the user device needs to release a source cell of the source network node.

6. An apparatus as claimed in any one of the preceding claims, further comprising: means for informing the user device indicating a time or a trigger event where the user device needs to apply a configuration associated with the indicated handover type.

7. An apparatus as claimed in any one of the preceding claims, further comprising: means for modifying the DAPS configuration by adding measurement resource configuration of at least one of: the source cell of the source network node; the target cell of the target network node.

8. An apparatus as claimed in any one of the preceding claims, wherein the apparatus forms part of the source network node, and wherein the source network node is a source distributed unit, DU, or a source centralized unit, CU.

9. An apparatus comprising: means for receiving a handover request, from a source network node at a handover target network node, requesting both a first radio resource control, RRC, configuration and a second RRC configuration, wherein the first RRC configurations comprises an LTM reference configuration and the second RRC configuration comprises a DAPS configuration; means for providing a handover response to the source network node, the handover response including the first and second RRC configurations; and means for exchanging RRC reconfiguration messages with a user device, the RRC reconfiguration messages include an RRC reconfiguration complete message received from the user device and the RRC reconfiguration complete message including an indication of which of two handover types, associated with the first and second RRC configuration respectively, is to be used.

10. An apparatus as claimed in claim 9, further comprising means for receiving, from the user device, a radio resource control, RRC, Reconfiguration Complete message indicating a selected handover type.

11. An apparatus as claimed in claim 9 or claim 10, further comprising means for indicating that the handover source cell is to be released.

12. An apparatus as claimed in any one of claims 9 to 11, further comprising means for sending a medium access control control element, MAC-CE, command to the user device indicating release of the source cell and an application of LTM configuration13. An apparatus as claimed in any one of claims 9 to 12, wherein the apparatus forms part of the handover target network node.

14. An apparatus comprising: means for receiving a radio resource control, RRC, reconfiguration message from a source network node, wherein the RRC reconfiguration message includes a first RRC configuration and a second RRC configuration, wherein the first RRC configuration comprises a low-layer triggered mobility, LTM, reference configuration and the second RRC configuration comprises a dual active protocol stack, DAPS, configuration; means for transmitting at least one measurement report to the source network node; means for receiving, from the source network node, an indication indicating which of two handover types, associated with the first and second RRC configuration respectively, is to be used; and means for performing a handover execution phase based on the indication.

15. An apparatus as claimed in claim 14, further comprising: means for receiving information indicating a time or a trigger event where the user device needs to release a source cell of the source network node.

16. An apparatus as claimed in claim 14 or claim 15, further comprising: means for receiving information indicating a time or a trigger event where the user device needs to apply the configuration associated with the indicated handover type.

17. An apparatus as claimed in any one of claims 14 to 16, further comprising means for transmitting an RRC Reconfiguration Complete message to the target network node indicating a selected handover type.

18. An apparatus as claimed in any one of claims 14 to 17, further comprising means for receiving a medium access control control element, MAC-CE, command from the target network node indicating release of the source cell and an application of the LTM configuration.

19. An apparatus as claimed in any one of claims 14 to 18, wherein the apparatus forms part of the user device.

20. An apparatus as claimed in any one of the preceding claims, wherein the first RRC configuration contains at least one measurement resource configuration of at least one of: the source cell of the source network node; the target cell of the target network node.

21. An apparatus as claimed in any one of the preceding claims, wherein the second RRC configuration contains at least one measurement resource configuration of at least one of: the source cell of the source network node; the target cell of the target network node.

22. A method comprising: sending a handover request from a source network node to a target network node, requesting a first radio resource control, RRC, configuration and a second RRC configuration, wherein the first RRC configuration comprises a low-layer triggered mobility, LTM, reference configuration and the second RRC configuration comprises a dual active protocol stack, DAPS, configuration; receiving a handover response from the target network node, the handover response including the requested first and second RRC configurations; providing an RRC reconfiguration message to a user device, wherein the RRC reconfiguration message includes the first and second RRC configurations; receiving at least one measurement report from the UE; and transmitting, to the user device an indication indicating which of two handover types, associated with the first and second RRC configuration respectively, is to be used.

23. A computer program comprising instructions for causing an apparatus to perform at least the following: sending a handover request from a source network node to a target network node, requesting a first radio resource control, RRC, configuration and a second RRC configuration, wherein the first RRC configuration comprises a low-layer triggered mobility, LTM, reference configuration and the second RRC configuration comprises a dual active protocol stack, DAPS, configuration; receiving a handover response from the target network node, the handover response including the requested first and second RRC configurations; providing an RRC reconfiguration message to a user device, wherein the RRC reconfiguration message includes the first and second RRC configurations;receiving at least one measurement report from the UE; and transmitting, to the user device an indication indicating which of two handover types, associated with the first and second RRC configuration respectively, is to be used.

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