Timing advance acquisition for l3 handover

Abstract

A User Equipment, UE (110), responsive to receiving a Timing Advance, TA, value of a target cell (130b) served by a target node (120b), refrains (230) from using the TA value to perform a Random Access Channel-less Layer 3, RACH-less L3, handover to the target cell (130b) until a handover triggering event occurs The UE (110) then performs (240) the RACH- less L3 handover to the target cell (130b) using the TA value responsive to the handover triggering event occurring. A source node (120a) and a target node (120b) support the RACH- less L3 handover.

Description

[0001] TIMING ADVANCE ACQUISITION FOR L3 HANDOVER

[0002] RELATED APPLICATIONS

[0003] This application claims priority to U.S. Provisional Patent Application Serial Number 63 / 530,652 filed August 3, 2023, the entire contents of which are incorporated by reference herein.

[0004] TECHNICAL FIELD

[0005] The present disclosure generally relates to the technical field of wireless communication and, more particularly, to efficiently performing device handover within a wireless communication network.

[0006] BACKGROUND

[0007] Radio Resource Control (RRC) is a Layer 3 (L3) protocol in the protocol stack of several generations of wireless communication technologies. A device that supports RRC may be in one of several RRC states. When a User Equipment (UE) is in the RRC_CONNECTED state, the UE is configured with a dedicated Signaling Radio Bearer (SRB) and with resources for transfer of unicast data to and from a radio network. Sometimes, when a UE is in the RRC_CONNECTED state, there is a need to update these resources. To do so, the UE may undergo an L3 handover.

[0008] One example of when an L3 handover may occur is when the UE is subject to mobility; e.g., when a UE is moving from a source cell into a target cell and therefore needs to be reconfigured with resources from the target cell. Such a scenario is typically referred to as an inter-cell handover (or a reconfiguration with sync) procedure. In an inter-cell handover, the UE applies resources of the target cell and releases resources of the source cell.

[0009] SUMMARY

[0010] Embodiments of the present disclosure enable a UE to acquire an appropriate Timing Advance (TA) value of a target cell served by a target node in advance of performing a RACH- less L3 handover to the target node. In many embodiments, the TA value enables the UE to perform handover with reduced signaling overhead and reduced interruption time relative to traditional approaches.

[0011] Particular embodiments include a method implemented by a UE. The method comprises, responsive to receiving a TA value of a target cell served by a target node, refraining from using the TA value to perform a Random Access Channel-less Layer 3 (RACH-less L3), handover to the target cell until a handover triggering event occurs. The method further comprises performing the RACH-less L3 handover to the target cell using the TA value responsive to the handover triggering event occurring.

[0012] In some embodiments, the method further comprises receiving, from a source node, a TA acquisition configuration for acquiring the TA value of the target node. The method further comprises using the TA acquisition configuration to acquire the TA value of the target node. In some such embodiments, the TA acquisition configuration comprises a Physical Random Access Channel (PRACH) configuration. Additionally or alternatively, the method may further comprise using the PRACH configuration to transmit a preamble to the target cell. In some such embodiments, the TA acquisition configuration indicates whether to expect a response to the preamble transmission from the source node. In some such embodiments, the TA acquisition configuration indicates a time window in which to expect the response to the preamble transmission from the source node. In some such embodiments, the method further comprises, responsive to the time window elapsing without having received the expected response, retransmitting the preamble to the target cell or transmitting a different preamble to the target cell. In some such embodiments, retransmitting the preamble or transmitting the different preamble comprises retransmitting or transmitting with increased power relative to the preamble transmission.

[0013] In some embodiments, the method further comprises receiving a duration in which the TA value is valid.

[0014] Other embodiments include a UE comprising interface circuitry and processing circuitry communicatively connected to the interface circuitry. The processing circuitry is configured to, responsive to receiving, via the interface circuitry, a TA value of a target cell served by a target node, refrain from using the TA value to perform a RACH-less L3 handover to the target cell until a handover triggering event occurs. The processing circuitry is further configured to perform the RACH-less L3 handover to the target cell using the TA value responsive to the handover triggering event occurring.

[0015] In some embodiments, the processing circuitry is further configured to perform any one of the UE methods described above.

[0016] Other embodiments include a computer program comprising instructions which, when executed on processing circuitry of a UE, cause the processing circuitry to carry out any one of the methods described above.

[0017] Other embodiments include a carrier containing said computer program. The carrier is one of an electronic signal, optical signal, radio signal, or computer readable storage medium.

[0018] Yet other embodiments include a method implemented by a source node. The method comprises transmitting a TA acquisition configuration to a UE. The method further comprises receiving, from a target node, a TA value of a target cell served by the target node. The method further comprises transmitting the TA value of the target cell to the UE to enable RACH-less L3 handover of the UE toward the target cell. The method further comprises sending a handover request to the target node to trigger the RACH-less L3 handover of the UE to the target cell.

[0019] In some embodiments, the method further comprises transmitting a TA acquisition procedure request to the target node and receiving a TA acquisition configuration in response. In some embodiments, transmitting the TA acquisition configuration to the UE comprises transmitting the TA acquisition configuration in an RRC message, a Medium Access Control (MAC) Control Element (CE), or Downlink Control Information (DCI).

[0020] In some embodiments, the method further comprises signaling the UE to perform TA acquisition. In some such embodiments, signaling the UE to perform TA acquisition comprises signaling the UE to transmit a PRACH preamble in the target cell according to the TA acquisition configuration.

[0021] In some embodiments, transmitting the TA value to the UE comprises transmitting the TA value in a MAC CE or RRC message.

[0022] In some embodiments, the handover request comprises an indication that the UE has acquired the TA value for the target cell.

[0023] In some embodiments, the method further comprises receiving, from the target node, a handover request acknowledgement comprising a handover command. In some such embodiments, the method further comprises transmitting, to the UE, an RRC Reconfiguration message comprising the handover command. In some such embodiments, the RRC Reconfiguration message indicates a timer value usable by the UE to determine the remaining time in which the TA value is valid for the target cell.

[0024] In some embodiments, the handover command comprises a RACH-less handover configuration for performing handover to the target cell.

[0025] In some embodiments, the method further comprises receiving, from the target node an amount of time remaining in which the TA value is valid in the target cell.

[0026] Other embodiments include a source node comprising interface circuitry and processing circuitry communicatively coupled to the interface circuitry. The processing circuitry is configured to transmit, via the interface circuitry, a TA acquisition configuration to a UE. The processing circuitry is further configured to receive, from a target node via the interface circuitry, a TA value of a target cell served by the target node. The processing circuitry is further configured to transmit the TA value of the target cell to the UE via the interface circuitry to enable RACH-less L3 handover of the UE toward the target cell. The processing circuitry is further configured to send a handover request to the target node via the interface circuitry to trigger the RACH-less L3 handover of the UE to the target cell.

[0027] In some embodiments, the processing circuitry is further configured to perform any one of the source node methods described above.

[0028] Other embodiments include a computer program comprising instructions which, when executed on processing circuitry of a source node, cause the processing circuitry to carry out any one of the source node methods described above.

[0029] Yet other embodiments include a carrier containing said computer program. The carrier is one of an electronic signal, optical signal, radio signal, or computer readable storage medium. Other embodiments include a method implemented by a target node. The method comprises transmitting, to a source node, a TA acquisition configuration for acquiring a TA value of a target cell served by the target node. The method further comprises receiving an uplink transmission from a UE in accordance with the TA acquisition configuration. The method further comprises transmitting a TA value of the target cell to the source node in response to the uplink transmission. The method further comprises receiving, from the source node, a handover request for RACH-less L3 handover of the UE to the target cell. The method further comprises triggering the RACH-less L3 handover of the UE to the target cell.

[0030] In some embodiments, transmitting the TA acquisition configuration is responsive to receiving a TA acquisition configuration request from the source node.

[0031] In some embodiments, the method further comprises transmitting an indication to the source node, separate from the TA acquisition configuration, indicating that the UE should acquire the TA value according to the TA acquisition configuration.

[0032] In some embodiments, receiving the uplink transmission from the UE comprises receiving a PRACH preamble.

[0033] In some embodiments, the handover request comprises an indication that the UE has acquired the TA value for the target cell.

[0034] In some embodiments, the handover request comprises a request that the RACH-less L3 handover be configured for the UE.

[0035] In some embodiments, triggering the RACH-less L3 handover comprises transmitting, to the source node, a handover command.

[0036] In some embodiments, the method further comprises transmitting, to the source node, a handover configuration for use by the UE in performing the RACH-less L3 handover to the target cell.

[0037] In some embodiments, the method further comprises transmitting, to the source node, an indication of how much longer the TA value will remain valid.

[0038] In some embodiments, the method further comprises determining how much longer the TA value will remain valid based on when the uplink transmission was received from the UE and / or a configured validity of a time alignment timer for the target cell.

[0039] In some embodiments, triggering the RACH-less L3 handover comprises transmitting an updated TA value for the target cell to the source node in response to the handover request.

[0040] Other embodiments include a target node comprising interface circuitry and processing circuitry communicatively coupled to the interface circuitry. The processing circuitry is configured to transmit, to a source node via the interface circuitry, a TA acquisition configuration for acquiring a TA value of a target cell served by the target node. The processing circuitry is further configured to receive, via the interface circuitry, an uplink transmission from a UE in accordance with the TA acquisition configuration. The processing circuitry is further configured to transmit, via the interface circuitry, a TA value of the target cell to the source node in response to the uplink transmission. The processing circuitry is further configured to receive, from the source node via the interface circuitry, a handover request for RACH-less L3 handover of the UE to the target cell. The processing circuitry is further configured to trigger the RACH- less L3 handover of the UE to the target cell.

[0041] In some embodiments, the processing circuitry is further configured to perform any one of the target cell methods described above.

[0042] Other embodiments include a computer program comprising instructions which, when executed on processing circuitry of a target node cause the processing circuitry to carry out any one of the target cell methods described above.

[0043] Yet other embodiments include a carrier containing said computer program. The carrier is one of an electronic signal, optical signal, radio signal, or computer readable storage medium.

[0044] BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Aspects of the present disclosure are illustrated by way of example and are not limited by the accompanying figures with like references indicating like elements. In general, the use of a reference numeral should be regarded as referring to the depicted subject matter according to one or more embodiments, whereas discussion of a specific instance of an illustrated element will append a letter designation thereto (e.g., discussion of a UE 110, generally, as opposed to discussion of particular instances of UEs 110a, 110b).

[0046] FIG. 1 is a schematic diagram illustrating an example wireless network, according to one or more embodiments of the present disclosure.

[0047] FIG. 2 is a signaling diagram illustrating an example signaling to perform handover, according to the prior art.

[0048] FIG. 3 is a signaling diagram illustrating traditional four step random access procedure, according to the prior art.

[0049] FIG. 4 is a flow diagram illustrating an example method implemented by a UE, according to one or more embodiments of the present disclosure.

[0050] FIG. 5 is a flow diagram illustrating an example method implemented by a source node, according to one or more embodiments of the present disclosure.

[0051] FIG. 6 is a flow diagram illustrating an example method implemented by a target node, according to one or more embodiments of the present disclosure.

[0052] FIGS. 7-16 are signaling diagrams illustrating example signaling between network nodes according to corresponding embodiments of the present disclosure.

[0053] FIG. 17 is a schematic block diagram illustrating an example UE, according to one or more embodiments of the present disclosure.

[0054] FIG. 18 is a schematic block diagram illustrating an example network node, according to one or more embodiments of the present disclosure. DETAILED DESCRIPTION

[0055] FIG. 1 is a schematic diagram illustrating an example network 100 comprising a plurality of radio nodes. More specifically, the network 100 comprises a UE 110, a first network node 120a, and a second network node 120b. Each of the network nodes 120a-b serves a respective cell 130a-b. The UE 110 is initially served cell 130a by network node 120a. The UE 110 subsequently moves from within the coverage area of cell 130a into the coverage area of cell 130b. In order to reconfigure the UE 110 to be served cell 130b by network 120b, the UE 110 participates in an L3 handover. Because the UE 110 changes from using cell 130a to using cell 130b, cell 130a may be referred to as a source cell. Correspondingly, cell 130b may be referred to as a target cell. Similarly, network node 120a may be referred to as a source node, whereas network node 120b may be referred to as a target node.

[0056] The source cell 130a may be a standalone serving cell or may be part of a Dual or MultiConnectivity configuration of the UE 110. Thus, the source cell 130a may be, e.g., a Primary Cell (PCell), a Secondary Cell (SCell), a Primary Secondary Cell (PSCell) or a Special Cell (SpCell). Correspondingly, the source node 120a may, for example, be a network node in a Master Cell Group (MCG), in which case the source node 120a may be referred to as a Source Master Node (S-MN) for the UE 110.

[0057] For example, in some embodiments, the source node 120a may be an S-MN in an MCG and may serve the source cell 130a to the UE 110, and the source cell 130a may be a PCell. Correspondingly, the target cell 130b served by the target node 120b may replace the source cell 130a within the Dual or Multi-Connectivity configuration. Accordingly, the target node 120b may be referred to as a Target Master Node (T-MN), in some embodiments.

[0058] For purposes of explanation, examples described herein may use terminology that is particular to specific releases of the Third Generation Partnership Project (3GPP) specifications. For example, embodiments herein may use Fifth Generation (5G) nomenclature to refer to a network node 120 as a gNB. However, it should be understood that the embodiments described herein may be used in a variety of different technologies and are not limited solely to a particular wireless network generation, Radio Access Technology (RAT), or 3GPP release. Other embodiments, for example, may be applied to other generations and RATs without substantially deviating from the inventive concepts described herein.

[0059] FIG. 2 is a signaling diagram depicting basic intra-New Radio (NR) handover from a source cell 130a served by a source node 120a to a target cell 130b served by a target node 120b. In this example, the core network nodes Access and Mobility Management Function (AMF) 140 and User Plane Function (UPF) 150 do not change for the UE 110. This handover is often referred to as a L3 handover because the measurement configuration and reporting as well as the configuration and execution of the procedure are done on the RRC level as specified in the RRC specification, i.e. , on L3 (the network layer). L3 handover is traditionally divided into two phases; namely, a preparation phase 510 and an execution phase 520. During the preparation phase 510 the source node 120a and the target node 120b prepare the reconfiguration of resources from the source node 120a and source cell 130a to the target node 120b and target cell 130b. In the execution phase 520, the reconfiguration message (in this example, the handover command) is sent by the source node 120a to the UE 110. When the UE 110 receives the reconfiguration message, the UE 110 applies the received configuration and starts the procedure to access the target cell 130b served by the target node 120b (and leave the source cell 130a served by the source node 120a). Accordingly, during handover completion 530, the source node 120a will forward the UE’s user traffic to the target node 120b, and the UE 110 will exchange user data with the target node 120b (rather than the source node 120a) thereafter.

[0060] To access the target node 120b and target cell 130b, as part of the handover the UE 110 synchronizes to the target cell 120b and initiates an uplink transmission in the target cell 130b. The UE 110 then typically performs a random access procedure 10 in the target cell 130b. An example random access procedure 10 is illustrated in the signaling diagram of FIG. 3.

[0061] In the random access procedure 10, the first uplink transmission from the UE 110 to the target node 120b is traditionally referred to as Msg1 (step 20). Msg1 comprises a Physical Random Access Channel (PRACH) preamble. The target node 120b responds to Msg1 by transmitting a Random Access Response (RAR) (often referred to as Msg2) to the UE 110 (step 30). The RAR may provide, among other things, Timing Advance (TA) information to the UE 110. TA is information about the timing that the UE 110 should use for uplink transmissions in the target cell 130b in order for subsequent messages to reach the target node 120b at an appropriate point in time. The RAR often also comprises an uplink (UL) grant to use UL resource when transmitting information back to the network in Msg3 (step 40). The TA is retrieved by the target node 120b from the preceding PRACH preamble that is received from the UE 110. The target node 120b then acknowledges Msg3 by transmitting Msg4 back to the UE 110. Msg4 is often referred to as the contention resolution message (step 50).

[0062] As part of 3GPP Rel-14, Random Access Channel (RACH)-less handover and RACH- less Secondary Cell Group (SCG) change were specified for Long Term Evolution (LTE) in order to decrease the interruption time at handover and SCG change, respectively. In these RACH-less procedures, Msg1 and Msg2 are not transmitted when the UE 110 accesses the target cell 130b.

[0063] Because Msg1 (PRACH preamble) and Msg2 (RAR) are not transmitted in a RACH-less handover, the UE 110 is traditionally unable to receive a TA value when accessing the target cell 130b during the handover. Instead, the UE 110 is traditionally provided with a TA value prior to the procedure in the RRC Reconfiguration message (Handover Command) that configures the handover. The RACH-less handover in LTE Rel-14 is therefore limited to certain scenarios where the TA value can be known in advance (without any PRACH preamble to determine the correct TA value). These scenarios are available when the target cell 130b is known to have the same TA value as another cell where the UE 110 already has a connection when the handover is configured (the source PCell, or a serving SCell, or a PSCell), or that the target cell 130b is known to have a TA value=0 (e.g., when the target cell 130b is a small cell where only TA=0 is applicable).

[0064] The UL grant for Msg3 transmission in the target cell 130b, at the LTE Rel-14 RACH- less handover / SCG change, is traditionally provided to the UE 110 using one of two techniques. In a first technique, the UL grant is pre-allocated to the UE 110 in the Handover Command message (i.e., within the RRCConnectionReconfiguration message instructing the UE 110 to perform the handover or SCG change). In a second technique, the target node 120b schedules the UE 110 with UL grants (Physical Uplink Shared Channel (PUSCH) resources) using the Physical Downlink Control Channel (PDCCH). The UE 110 is then scheduled through the PDCCH using the Cell-Radio Network Temporary Identifier (C-RNTI) that the UE has been configured with for the target cell 130b (in the Handover Command message).

[0065] A UE 110 in a Non-Terrestrial Network (NTN) can achieve synchronization (and autonomously pre-compensate the TA) with an NTN cell based on the Global Navigation Satellite System (GNSS) position, received ephemeris information, and common TA parameters for the cell. Because the position of a satellite is knowable in advance, the distance between the satellite and the UE 110 can be determined. Accordingly, to enable RACH-less handover to an NTN cell, the UE 110 may determine the TA value to use for initial PUSCH transmission in the target NTN cell using this same synchronization mechanism.

[0066] Although inter-cell mobility procedures (such as handover) are typically triggered on L3 (using RRC procedures), 3GPP Release 18 has introduced for NR an inter-cell mobility procedure that is instead triggered on Layer 1 and Layer 2. Such a procedure is called L1 / L2 Triggered Mobility (LTM). In LTM, the UE 110 is configured before the inter-cell mobility procedure (i.e., the LTM cell switch procedure) with candidate RRC configurations for one or more LTM candidate target cells. The UE 110 applies an appropriate configuration when an LTM cell switch procedure is triggered to the corresponding candidate target cell.

[0067] LTM enables reduced interruption time during the cell switch as compared to a normal handover. This is achieved by the UE 110 synchronizing to the target cell 120b before the actual cell switch procedure to that target cell being triggered. This pre-synchronization is done both for the downlink and for the uplink and results in an actual cell switch procedure that is quicker and has shorter interruption time.

[0068] To pre-synchronize with the uplink, the UE 110 acquires a TA value for the target cell 130b in advance through transmission of a RACH preamble to the target cell 130b before the LTM cell switch procedure is triggered. Because the UE 110 then has a TA value for the target cell 130b, it is possible to perform a RACH-less access to that target cell 130b when actual cell switch procedure is performed. Thus, the Msg1 PRACH preamble transmission from the UE 110 to the network and the Random Access Response (RAR) message in response can be skipped. This decreases the interruption time during the LTM cell switch procedure.

[0069] An LTM procedure for acquiring a valid TA value for an LTM candidate target cell in advance to enable RACH-less access may comprise the UE 110 transmitting a PRACH preamble in the LTM candidate target cell after reception of a PDCCH order in the serving (i.e. , source) cell. Thus, the serving node in the serving cell triggers the UE 110 to perform TA acquisition for the candidate target cell by transmission of a PDCCH order to the UE 110. The serving node may, e.g., be the serving Centralized Unit (CU) or the serving Distributed Unit (DU) handling the serving cell.

[0070] That said, there are a variety of possible ways in which the UE 110 could receive the TA value for the LTM candidate target cell. For example, the UE 110 could receive the TA value in a RAR in the corresponding LTM candidate target cell, in a RAR in the serving cell, or in the LTM cell switch command that triggers the actual LTM cell switch. In one particular example, for PDCCH ordered early TA acquisition without RAR, there is no need for the UE 110 to maintain the TA timer for the target cell 130b. Accordingly, the TA may be given in the cell switch Medium Access Control (MAC) Control Element (CE) when available in the network, which then triggers the actual LTM cell switch to the target cell 130b.

[0071] However, as of 3GPP Release 18, only intra-CU LTM cell switches are addressed (i.e., where the source cell 130a and the target cell 130b belong to the same CU). NR currently does not support RACH-less handover between nodes comprising different CUs. Embodiments of the present disclosure provide techniques to support RACH-less handover in such contexts, e.g., in order to advantageously decrease interruption time during handover.

[0072] Although RACH-less handover exists in LTE as of 3GPP Release 14, these early versions of RACH-less handover are restricted to scenarios in which the TA value for the target cell 130b is the same as a source cell 130a, or where the target cell 130b is so small that TA is always equal to 0. Therefore, LTE RACH-less handover cannot be used in many scenarios. Adoption of similar solutions in NR would lead to similar limitations. Although RACH-less handover solutions have been contemplated in NTN (based on the known distance between the UE 110 and the satellite using e.g. satellite ephemeris to determine an initial TA value) this approach is not available for normal, terrestrial networks / cells. The information for calculating such a solution is not available for terrestrial cells. Therefore, the UE cannot determine an initial TA value for a terrestrial target cell 130b in the same way as can be determined for an NTN cell.

[0073] At least some embodiments herein advantageously support RACH-less handover for L3 handover in a terrestrial network toward a target cell 130b that has a non-zero TA value that is different from the source cell 130a.

[0074] Existing TA acquisition for LTM also faces limitations that are overcome by embodiments described herein. Traditional TA acquisition for LTM is based on the UE 110 being configured with candidate cells for which the TA acquisition may be ordered in advance (through a PDCCH order). The request to acquire the TA value from the candidate target cell is triggered by the source cell 130a (i.e., the serving cell) sending this PDCCH order to the UE 110. It is thus the source node 120a serving the source cell 130a (e.g., the serving CU or serving DU) that initiates the TA acquisition procedure towards the UE 110 in such procedures. It is then also the source node 120a serving the source cell 130a that decides that the TA acquisition procedure should be performed and when it should be initiated.

[0075] In contrast, it is the target node 120b serving the target cell 130b that decides the configuration (e.g., RACH resources) that the UE should use in the target cell 130b during traditional L3 handover procedure. Accordingly, embodiments described herein support L3 handover in which the decision of whether a RACH-less handover should be used or not is made by the target node 120b serving the target cell 130b. Further, according to one or more embodiments, the target node 120b decides whether or not to perform a TA acquisition towards the target cell 130b.

[0076] Traditional approaches to L3 handover are unsuitable for RACH-less procedures because in traditional L3 handover the UE 110 is not configured with any candidate cells. Instead, the configuration to be used is generated by the target node 120b for use in the target cell 130b during the preparation phase 510 of the handover. As a result, a Handover Command message for the target cell 130b is sent from the target node 120b to the UE 110 via the source node 120a serving the source cell 120a. This renders the LTM TA acquisition unsuitable for use with a normal L3 handover.

[0077] In view of the above, embodiments of the present disclosure include a UE that acquires a TA value for a target cell 130b of an L3 handover before execution of the L3 handover procedure (i.e., before the UE 110 accesses the target cell 130b as part of the handover). The acquired TA value allows the UE 110 to perform a RACH-less handover to the target cell 130b. Other embodiments are directed to corresponding network nodes that act as a Source Master Node (S-MN) and a Target Master Node (T-MN), respectively, during the handover.

[0078] In different embodiments, the TA acquisition procedure is performed either before the handover preparation procedure, during the handover preparation procedure, or after the UE has received a Handover Command message.

[0079] As used herein, the term “handover” shall be interpreted to include, but are not limited to, a Reconfiguration with Sync procedure. According to one particular example, in response to a radio link failure between the UE 110 and a source node 120a, the UE 110 may access a target node 120b. That said, the term “handover” shall also be interpreted to include a mobility procedure in which the serving cell of a UE 110 changes (e.g., in response to a Handover Command).

[0080] The term “L3 handover” as used herein refers to a mobility procedure for a UE 110 that is triggered by reception of an RRC message (e.g., an RRC Reconfiguration message, which may include a ReconfigurationWithSync IE, an RRC Handover Command message, or other RRC message). In particular, execution of the L3 handover procedure may be triggered when the UE 110 receives a corresponding Handover Command message (which may include a corresponding handover configuration). As used herein, L3 handover does not refer to a handover procedure in which a handover configuration is received in advance as a candidate configuration that might be applied at a later point in time when an associated condition is fulfilled.

[0081] The term “RACH-less,” as used herein, refers to a procedure in which Msg1 and Msg2 are not exchanged between the UE 110 and the target cell 120b. According to embodiments of the present disclosure, a UE 110 acquires a TA value for a target cell 130b in advance of L3 handover, which enables the UE 110 to avoid transmission of a PRACH preamble (and reception of a corresponding RAR) during access to the target cell when the L3 handover is performed. That is, early TA acquisition enables the UE 110 to perform RACH-less access to the target cell 130b during a RACH-less handover procedure in which at least the PRACH preamble and the RAR are skipped as compared to a traditional RACH procedure performed during a normal handover or Reconfiguration With Sync procedure.

[0082] That said, performance of a RACH-less handover, for example and within the meaning of this disclosure, should not be understood to mean that no preambles or RAR-like messages are ever exchanged between the UE 110 and the target node 120b. In particular, embodiments of the present disclosure exchange certain information in advance of handover so that RACH- less handover can be subsequently performed at an appropriate time.

[0083] In one set of embodiments, the invention includes methods for a UE 110 to acquire a TA value for a target cell 130b (e.g., an inter-frequency neighboring cell, an intra-frequency neighboring cell) and then perform a subsequent L3 Handover procedure (which may include a Reconfiguration With Sync) to that target cell 130b that includes using the previously acquired TA value to access the target cell 130b.

[0084] Embodiments of the present disclosure include a method 200, implemented by a UE 110, e.g., as illustrated in the example of FIG. 4. The method 200 comprises, responsive to receiving a TA value of a target cell 130b served by a target node 120b, refraining from using the TA value to perform RACH-less L3 handover to the target cell 130b until a handover triggering event occurs (block 230). The method 200 further comprises performing the RACH- less L3 handover to the target cell 130b using the TA value responsive to the handover triggering event occurring (block 240).

[0085] In some embodiments of the method 200, the UE 110 obtains a TA acquisition configuration and uses it to obtain the TA value from the target network node 130b. Accordingly, the method 200 may further comprise receiving, from a source network node 120a, a TA acquisition configuration for acquiring the TA value from the target node 120b (block 210). The method 200 may further comprise using the TA acquisition configuration to acquire the TA value from the target node 120b (block 220). Given that the target cell 130b is the target of an L3 handover procedure, the target cell 130b may, in some embodiments, correspond to a cell that is not an LTM candidate cell (i.e. , a cell for which the UE is not configured with LTM) and is not a serving cell. The target cell 130b may be an intra-frequency neighboring cell (i.e., a cell using the same Synchronization Signal Block (SSB) frequency and same subcarrier spacing as the source cell 130a) or an interfrequency neighboring cell (i.e., a cell using a different SSB frequency and / or different subcarrier spacing than the source cell 130a). The target cell 130b may, in some embodiments, be a cell of a different Radio Access Technology (RAT) than the source cell 130a of the UE 110 prior to the handover.

[0086] In some embodiments, the TA acquisition configuration comprises one or more of the following: an UL channel configuration (e.g., a PRACH configuration, which may include PRACH time and / or frequency resource(s) and / or preamble(s), possibly associated with one or more SSBs of the target cell 130b); and / or an indication of whether or not to expect a response message from a serving cell (e.g. a MAC CE acknowledging a preamble transmission) in response to the preamble transmission to the target cell 130b for TA acquisition; and / or an indication of a response window for the response message, based on which the UE 110 may trigger a new preamble transmission (or re-transmission), possibly with power ramping applied for the new transmission or re-transmission; and / or an index mapped to a predefined set of one or more values usable by the UE 110 to trigger TA acquisition.

[0087] For example, in some embodiments, the UE 110 ramps the power of a new preamble transmission or retransmission of a previous preamble (e.g., in response to the response window elapsing). Additionally or alternatively, the power ramping could be applied for every new transmission or re-transmission, or it could be applied only for some of the new transmissions or re-transmissions. Alternatively, the UE 110 may not perform power ramping for the new transmission or re-transmission. Alternatively, the UE 110 may perform power ramping of new transmissions and / or re-transmissions according to an indication from the network. Such an indication may, e.g., be included in the TA acquisition configuration.

[0088] In some embodiments, the UE 110 starts the TA acquisition procedure (e.g., by initiating a random access procedure) towards the target cell 130b according to the received TA acquisition configuration after receiving a corresponding indication from the network. The indication may comprise, for example, an RRC message. In one particular example, the RRC message that indicates to the UE 110 that the UE 110 should start the TA acquisition procedure includes TA acquisition configuration. Alternatively, the indication may comprise a different RRC message, e.g., an RRC message that the UE 110 receives after the TA acquisition configuration (which may be comprised in its own RRC message or other signaling). The indication to trigger TA acquisition may be a parameter and / or field and / or Information Element (IE) in an RRC message. The indication may indicate that the TA acquisition procedure is to be performed towards one or more target cells 130b by including, per target cell 130b, a cell identity (such as the Physical Cell Identity (PCI) and / or the SSB frequency, or other identifier from which the target cell 130b is identifiable) of the target cell 130b. One example of an identifier from which the target cell 130b may be identifiable may include an abbreviated cell identifier for the target cell 130b that is encoded with fewer bits as compared to a PCI or PCI + SSB frequency.

[0089] Alternatively, the indication may indicate that the TA acquisition procedure is to be performed by sending, to the UE 110, a reference to a corresponding TA acquisition configuration. In one embodiment, the indication or the RRC message triggers the UE 110 to perform the TA acquisition procedure towards each target cell 130b for which the UE 110 has received a TA acquisition configuration.

[0090] In another example, the UE 110 receives lower layer signaling (i.e. , lower than L3, such as Downlink Control Information (DCI) or a MAC CE) from the source cell 120a (e.g., a serving cell, a PCell) and is triggered by the lower layer signaling to perform the TA acquisition procedure. As discussed above, the source cell 120a may be a PCell or other serving cell, depending on the embodiment. In one example, the lower layer signaling is DCI comprising a PDCCH order that indicates, to the UE 110, to perform the TA acquisition procedure. As noted above, the TA acquisition procedure can be toward one or more neighboring cells, and the indication may identify which cells should be included in the TA acquisition procedure (e.g., by specifying a PCI or other identifier as previously described).

[0091] In one particular example, the UE 110 may receive a list of configurations. Each configuration may correspond to a respective target cell 130b. The signaling that triggers the UE 110 to perform the TA acquisition procedure may comprise an index that indicates which configuration in the list is to be used for TA acquisition.

[0092] In some embodiments, the indication for the UE 110 to perform the TA acquisition procedure includes the identity of the target cell 130b and the UE 110 assumes that the SSB frequency and subcarrier spacing of the target cell 130b is the same as the SSB frequency and subcarrier spacing of another cell. In this regard, the UE 110 may assume that the SSB frequency and subcarrier spacing of the target cell 130b is the same as, e.g., the source cell 130a. In some embodiments, the UE 110 may be connected to the network via Dual Connectivity (DC) or Multi-Connectivity (MC) and may assume that the SSB frequency and subcarrier spacing of the target cell 130b is the same as a PCell or an SCell, depending on the embodiment.

[0093] In some embodiments, the TA acquisition configuration is provided to the UE 110 together with the indication to perform the TA acquisition procedure. For example, the UE 110 may receive an RRC message that includes the TA acquisition configuration and performs the TA acquisition procedure in response. In this regard, the UE 110 may be triggered implicitly to perform the TA acquisition procedure (e.g., upon receipt of the configuration or some time thereafter) or based on the UE 110 also having received (e.g., together with or within the same RRC message) the indication to perform the TA acquisition procedure as described above. It should be further noted that the UE 110 may receive one or more indications, each of which may indicate that the UE 110 should perform TA acquisition toward a respective target cell 130b.

[0094] In some embodiments, the UE 110 is configured to perform the TA acquisition procedure responsive to one or more events. In this regard, the UE 110 may be configured to detect whether the event has occurred and trigger the TA acquisition procedure in response. The UE 110 may use a PRACH preamble and / or RACH occasion previously received in a TA acquisition configuration in order to obtain the TA value for the target cell 130b. Any one or more of the events may, for example, be based on one or more radio measurements, such as a condition detected based on the radio measurement(s). The measurements may be taken with respect to the source cell 130a and / or the target cell 130b, depending on the embodiment. In one particular example, the UE 110 may be configured to detect that a first event relating to the target cell 130b has occurred, and in response, detect that a second event relating to the source cell 130a has occurred. In response to detecting that the second event has occurred, the UE 110 may be configured to acquire the TA value for the target cell 130b using the TA acquisition configuration. In other words, one or more of the events may be a trigger to begin monitoring for another of the events, and the events may relate to the same cell or different cells, depending on the embodiment.

[0095] In some embodiments, the event relates to whether a detected value (e.g., signal quality, signal strength) has crossed a threshold. For example, the event may relate to signal strength being above or below a certain threshold, depending on the embodiment. Alternatively, the event may relate the difference in signal strength or signal quality between the source cell 130a and the target cell 130b being below or above a threshold amount.

[0096] In one example, the event relates to the source cell 130a, and the UE 110 is configured to perform the TA acquisition procedure with respect to a plurality of target cells 130b in response to detecting the event. As noted above, the UE 110 may have received a TA acquisition configuration for each target cell 130b.

[0097] In some embodiments, the TA acquisition procedure comprises transmitting a preamble to an UL time and frequency resource of the target cell 130b. In one such embodiment, the preamble and the UL time and frequency resource of the target cell 130b corresponds to a random access configuration of the target cell 130b. In one particular example, the UE 110 expects a message from the source cell 130a in response to the preamble transmitted to the target cell 130b. In some embodiments, the UE 110 performs the TA acquisition procedure using dedicated random access resources. The dedicated random access resources may be dedicated to the UE 110 and / or dedicated to the purpose of acquiring TA in advance of RACH- less handover. This can be done by using PRACH preambles and / or RACH resources that are reserved in the target cell 130b for a particular UE 110. Alternatively, the dedicated resources may be dedicated for any UE 110 that may wish to acquire TA for the target cell 130b. In some embodiments, the same preamble may be reserved for multiple UEs and / or the RACH occasion may be derived at the UE 110 based on a rule. The rule may be based on a UE identifier in the cell or on some variable indicated in the RACH configuration, for example. In this regard, the target cell 130b may be configured to refrain from sending Msg 3 in response to receiving a preamble that is dedicated to enabling subsequent RACH-less handover. Notwithstanding, the target node 130b may nonetheless send the TA to the UE 110, e.g., through a message transmitted in the target cell 130b or in the source cell 130a (e.g., in a MAC CE, an RRC message, a Handover Command message).

[0098] In some embodiments, the UE 110 may follow legacy principles relating to Contention- Based Random Access (CBRA) to select an appropriate RACH preamble and RACH occasion in which to perform the TA acquisition in advance of actual handover. In this regard, the target cell 130b may send an UL grant to the UE 110 and the UE 110 may send Msg3 in accordance with the UL grant. The UE 110 may, in Msg3, indicate that the random access procedure was initiated for the purpose of early TA acquisition. Accordingly, although certain CBRA techniques may be performed in the TA acquisition procedure, the TA acquisition procedure does not, itself, result in an actual handover.

[0099] In some embodiments, the PDCCH scheduling the response to the PRACH transmission is scrambled using a C-RNTI. In other cases, the PDCCH scheduling the response to the PRACH transmission is be scrambled using an identity other than the C-RNTI. In one example, the C-RNTI (or other identity) used for scrambling the PDCCH that schedules the response to the PRACH transmission is provided to the UE 110 as part of the TA acquisition configuration for the corresponding target cell 130b.

[0100] Through the TA acquisition procedure towards the target cell 130b, the network can determine a TA value for the UE 110 in that target cell 130b. Embodiments of the present disclosure include different alternatives for providing this TA value from the network to the UE 110, which may subsequently be used in a handover (or Reconfiguration With Sync) procedure toward that target cell 130b (and possibly for later UL transmission in that cell.

[0101] The UE 110 may receive the TA value for the target cell 130b in the RRC message that triggers the UE 110 to perform a mobility procedure (e.g., a Handover procedure). The RRC message may, e.g., be a Handover Command message. One example of a handover message includes an RRCReconfiguration message. In some embodiments, the RRCReconfiguration message comprises a Reconfiguration With Sync IE. Alternatively, the UE 110 may receive the TA value for the target cell 130b in a MAC CE. The MAC CE may, in some embodiments, be received together with an RRC message from the source cell 130a that triggers the Handover procedure. For example, the UE 110 may receive a MAC CE in the same MAC Packet Data Unit (PDU) that includes the RRC message (e.g., during the same Transmission Time Interval (TTI), radio frame, subframe, time-slot, or other unit of time).

[0102] Alternatively, the UE 110 may receive the TA value for the target cell 130b before the Handover procedure is triggered. In one such example, the UE 110 receives the TA value in a MAC CE that is included in a different MAC PDU (i.e., an earlier PDU) than the RRC message triggering the mobility procedure. The UE 110 then stores the TA value for the target cell 130b, receives a command (e.g. RRCReconfiguration message) to perform a Handover procedure with that target cell 130b, and then uses the TA value during that procedure.

[0103] The TA value for the target cell 130b may be received by the UE 110 via a message in the source cell 130a. Alternatively, the UE 110 may receive the TA value for the target cell 130b via a message in the target cell 130b. The message may comprise an RRC message, MAC CE, or MAC PDU (e.g., in a RAR), depending on the embodiment.

[0104] In one example, the UE 110 may receive the TA value for the target cell 130b in a MAC PDU in the target cell 130b, i.e., in the same cell in which the UE 110 has previously sent a PRACH preamble as part of the TA acquisition procedure.

[0105] In another example, the TA value is received by the UE 110 in a MAC PDU in a different target cell 130b, which, e.g., could be another target 130b cell where the UE also performs a TA acquisition procedure. In one particular example, the UE 110 may receive the TA value from a first target cell and use the TA value to handover to a different, second target cell.

[0106] In another example, the TA value may be received by the UE 110 in a MAC PDU in the target cell 130b during the execution of the handover procedure. For instance, if the UE 110 performs a RACH-less handover and has no configured grant available while listening to the PDCCH to receive a dynamic grant from the network, the UE 110 may receive in the same MAC PDU the dynamic grant and the MAC CE carrying the TA value to be applied in the target cell 130b. Alternatively, the network may decide to first send the TA value in a MAC CE in one PDCCH signaling and in a separate PDCCH signaling to send the dynamic grant.

[0107] In one alternative, the UE 110 starts a TA validity timer when it receives the TA value for the target cell 130b. The TA validity timer tracks a duration in which the TA value is valid. For example, after a threshold amount of time has elapsed on the TA validity timer, the UE 110 may consider the TA value invalid. Accordingly, the UE 110 may discard the TA value upon expiration of the TA validity timer. In some embodiments, in response to expiration of the TA validity timer, the UE 110 may perform the TA acquisition procedure again to obtain a new TA value. The new TA value may be an updated TA value for the same target cell or may be a TA value for a different target cell. In one embodiment, the UE 110 receives a value for the TA validity timer in the same message in which the UE 110 receives the TA value (e.g., in an RRC message that also triggers the Handover procedure as described above or in an RRC message that is sent before such a trigger). The source node 120a and / or the target node 120b may know how much time has elapsed from when the UE transmitted the preamble until the time in which the source node 120a transmits the handover command. Accordingly, the amount of time elapsed between these transmissions may be compensated for in the TA validity timer value.

[0108] In another embodiment, the UE 110 starts the timer with a value that it has previously received, e.g. within or together with the TA acquisition configuration.

[0109] In one example, the UE 110 considers the TA value of a target cell 130b to be valid as long as the network refrains from sending a new TA value for that target cell 130b.

[0110] Additionally or alternatively, the UE 110 may consider the TA value of the target cell 130b to be valid as long as the network refrains from explicitly indicating to the UE 110 that the TA value is no longer valid. In some such embodiments, the validity of the TA value may be enforced by the network. Accordingly, in some embodiments, the TA validity timer may be started within the network (e.g., at the source node 120a, at the target node 120b) when the TA value is sent to the UE 110.

[0111] Other embodiments are directed to a method implemented by a source node 120a serving a source cell 130a to a UE 110. An example of such a method 300 is illustrated in FIG. 5. As noted above, the TA value may be provided to the UE 110 before execution of a handover procedure to the target cell 130b. Accordingly, handover of the UE 110 from the source cell 130a to the target cell 130b may be triggered after the TA value is acquired by the UE 110.

[0112] The method 300 comprises transmitting a TA acquisition configuration to a UE 110 (block 310). The method 300 further comprises receiving, from a target node 120b, a TA value of a target cell 130b served by the target node 120b (block 320). The method 300 further comprises transmitting the TA value of the target cell 130b to the UE 110 to enable RACH-less handover of the UE 110 toward the target cell 130b (block 330). The method 300 further comprises sending a handover request to the target node 120b to trigger the RACH-less handover of the UE 110 to the target cell 130b (block 340).

[0113] In some embodiments, the method further comprises transmitting a TA acquisition procedure request to the target node 120b and receiving a TA acquisition configuration in response (block 305). In some such embodiments, transmitting the TA acquisition procedure request comprises transmitting the TA acquisition procedure request in an XnAP message. In some embodiments, receiving the TA acquisition configuration comprises receiving the TA acquisition configuration in an acknowledgement or confirmation of the TA acquisition procedure request. In some embodiments, transmitting the TA acquisition configuration to the UE comprises transmitting the TA acquisition configuration in an RRC message, a MAC CE, or a DCI. In some particular embodiments, transmitting the TA acquisition configuration to the UE comprises transmitting the TA acquisition configuration in a PDCCH order. In other particular embodiments, transmitting the TA acquisition configuration to the UE comprises transmitting the TA acquisition configuration in an RRC message.

[0114] In some embodiments, the method 300 further comprises signaling the UE 110 to perform TA acquisition (e.g., via lower layer signaling) (block 315). The lower layer signaling may comprise a MAC CE or DCI. In one particular example, the lower layer signaling comprises a PDCCH order. In some embodiments, signaling the UE to perform TA acquisition comprises signaling the UE to transmit a PRACH preamble in the target cell 130b according to the TA acquisition configuration.

[0115] In some embodiments, transmitting the TA value to the UE 110 comprises transmitting the TA value in a MAC CE or RRC message.

[0116] In some embodiments, the handover request comprises an indication that the UE 110 has an acquired TA value for the target cell 130b. Additionally or alternatively, the handover request may specifically indicate that the requested handover be RACH-less.

[0117] In some embodiments, the method 300 further comprises receiving a handover request acknowledgement from the target node 120b (block 350). In some such embodiments, the handover request acknowledgement comprises a Handover Command message. The Handover Command message may comprise a RACH-less handover configuration for performing handover to the target cell 130b. The UE 110 uses the acquired TA value for handover to the target cell 130b.

[0118] In some such embodiments, the handover request acknowledgement indicates an amount of time remaining in which the TA value is valid in the target cell 130b. In other embodiments, the handover request acknowledgement indicates a timer value usable by the UE 110 for determining the remaining time in which the TA value is valid in the target cell 130b.

[0119] In some embodiments, the method 300 further comprises transmitting, to the UE 110, an RRC Reconfiguration message (block 360). The RRC Reconfiguration message may, e.g., comprise the Handover Command. In one example, the RRC Reconfiguration message indicates a timer value usable by the UE to determine the remaining time in which the TA value is valid for the target cell 130b.

[0120] Other embodiments are directed to a method 400 implemented by a target node 120b, e.g., as illustrated in FIG. 6. As noted above, the target node 120b may, in some embodiments, be T-MN for a UE 110. The target node 120b may enable the UE to acquire a TA value of a target cell 130b served by the target node 120b in advance of handover from, e.g., an S-MN.

[0121] The method 400 comprises transmitting, to a source node 120a, a TA acquisition configuration for acquiring a TA value of a target cell served by the target node 120b (block 410). In some embodiments, transmitting the TA acquisition configuration is responsive to receiving a TA acquisition configuration request from the source node 120a (block 405).

[0122] The method 400 further comprises receiving an uplink transmission from a UE 110 in accordance with the TA acquisition configuration (block 420). The method 400 further comprises transmitting a TA value of the target cell 130b to the source node 120a in response to the uplink transmission (block 430). The method 400 further comprises receiving, from the source node 120a, a handover request for RACH-less L3 handover of the UE 110 to the target cell 130b (block 440). The method 400 further comprises triggering the RACH-less L3 handover of the UE 110 to the target cell 130b (block 450). In some embodiments, triggering the RACH- less L3 handover comprises transmitting, to the source node 120a, a handover command.

[0123] In some embodiments, the method further comprises transmitting an indication to the source node 120a, separate from the TA acquisition configuration, indicating that the UE 110 should acquire the TA value according to the TA acquisition configuration (block 415).

[0124] In some embodiments, receiving the uplink transmission from the UE 110 comprises receiving a PRACH preamble.

[0125] In some embodiments, the handover request comprises an indication that the UE has acquired the TA value for the target cell 130b.

[0126] In some embodiments, the handover request comprises a request that the RACH-less L3 handover be configured for the UE 110.

[0127] In some embodiments, triggering the RACH-less L3 handover comprises transmitting, to the source node 120a, a handover command.

[0128] In some embodiments, the method further comprises transmitting, to the source node 120a, a handover configuration for use by the UE 110 in performing the RACH-less L3 handover to the target cell 130b.

[0129] In some embodiments, the method further comprises transmitting, to the source node 120a, an indication of how much longer the TA value will remain valid.

[0130] In some embodiments, the method further comprises determining how much longer the TA value will remain valid based on when the uplink transmission was received from the UE 110 and / or a configured validity of a time alignment timer for the target cell 130b.

[0131] In some embodiments, triggering the RACH-less L3 handover comprises transmitting an updated TA value for the target cell 130b to the source node 120a in response to the handover request.

[0132] Particular embodiments vary the above in certain respects. For example, TA acquisition for a possible target PCell during HO preparation procedure may be performed in a variety of ways. In one set of embodiments, a network node acting as an S-MN for a UE 110 enables acquisition of a TA value of a target cell 130b belonging to a T-MN. The TA value may be acquired by (and provided to) the UE 110 during a handover preparation procedure to the target cell 130b before execution of a RACH-less L3 handover procedure to the target cell 130b. In some such embodiments, the S-MN decides to trigger a handover (or a reconfiguration with sync procedure) for the UE 110 to a target cell 130b belonging to the T-MN. The S-MN transmits a message to the T-MN to request a handover procedure for the UE 110 to the target cell 130b (e.g., in a Handover Request message). In one alternative, the S-MN includes in the message an indication that RACH-less handover is desirable or requested by the S-MN.

[0133] The S-MN may receive a message from the T-MN requesting a TA acquisition procedure for the UE to the target cell 130b. In one example, the message is a new XnAP message. Additionally or alternatively, the message may include a configuration for the UE 110 to perform a TA acquisition procedure for the target cell. Alternatively, the message may include an indication that the UE 110 should perform a TA acquisition procedure and the S-MN initiates the TA acquisition procedure for the UE 110 (towards the target cell 130b) using configurations that the S-MN has received earlier.

[0134] In some embodiments, the S-MN transmits the configuration and / or indication to the UE 110 to perform the TA acquisition procedure towards the target cell 130b. The configuration and / or indication can be sent to the UE 110 in an RRC message, a MAC CE, a DCI, a PDCCH order, or other data structure. In some embodiments, the configuration for TA acquisition is sent in an RRC message and the indication to perform the TA acquisition (e.g., to transmit a PRACH preamble in the target cell) is provided via lower layer signaling, e.g. a MAC CE or DCI, e.g. a PDCCH order.

[0135] In some embodiments, the S-MN receives a message confirming the handover from the T-MN (e.g. a Handover Request Acknowledge message) including a configuration for handover to the target cell (e.g., a Handover Command, which may include a configuration for a RACH- less handover). In one example, the Handover Command message includes a TA value to be used by the UE during the RACH-less handover procedure to the target cell 130b. In one example, the message to the S-MN includes the TA value that the UE 110 should use in the target cell 130b, so that the S-MN provides the TA value to the UE 110. In one example, the message includes an indication about a timer value for the (remaining) validity for the TA value that the UE 110 has acquired for the target cell 130b. In one example, the Handover Command message includes an indication about a timer value that the UE 110 should use to determine the (remaining) validity for the acquired TA value for the target cell 130b.

[0136] In some embodiments, the S-MN transmits an RRC Reconfiguration message with the Handover Command to the UE 110. In one example, the S-MN includes the TA value received from the T-MN that the UE 110 should use in the target cell 130b for the handover execution procedure together with the RRC Reconfiguration message. In one example, the S-MN includes an indication about a timer value that the UE 110 should use to determine the remaining validity for the acquired TA value for the target cell 130b. Other corresponding embodiments relate to the T-MN. The T-MN enables acquisition of a TA value for the target cell 130b during a handover preparation procedure while the UE 110 is still in a serving cell served by an S-MN. The handover preparation procedure is followed by a corresponding handover execution procedure.

[0137] According to particular embodiments, the T-MN receives a message from the S-MN requesting a handover procedure for the UE 110 to a target cell 130b that belongs to the T-MN (e.g. a Handover Request message). In one alternative, the S-MN includes in the message an indication that RACH-less handover is desirable or requested by the S-MN.

[0138] In some embodiments, the S-MN decides that a TA acquisition procedure is needed or wanted for the UE 110 towards the target cell 130b for the requested handover procedure. The T-MN transmits a message to the S-MN to request a TA acquisition procedure for the UE 110 to the target cell 130b. In one example, the message is a new XnAP message. In one example, the message also includes a configuration for the UE 110 to perform a TA acquisition procedure for the target cell 130b. In one alternative, the message includes an indication that the UE 110 should perform a TA acquisition procedure and that the S-MN shall initiate the TA acquisition procedure for the UE 110 (towards the target cell 130b) using a configuration that the T-MN has provided earlier to the S-MN.

[0139] In some embodiments, the T-MN receives an UL transmission from the UE 110 in the target cell 130b, e.g. a PRACH preamble according to the TA acquisition configuration that the T-MN has provided, and determines the TA value for the UE 110 in the target cell 130b.

[0140] In some embodiments, the T-MN transmits a message confirming the handover to the S- MN, e.g. a Handover Request Acknowledge message, including a configuration for handover to the target cell 130b (a Handover Command, e.g. with a configuration for a RACH-less handover). In one example, the T-MN includes in the Handover Command message a TA value to be used by the UE 110 during the RACH-less handover procedure to the target cell 130b. In one example, the T-MN includes in the message to the S-MN the TA value that the UE 110 should use in the target cell 130b so that the S-MN provides the value to the UE 110. In other words, the TA value may be included in a part of the message that is read by the S-MN. In another example, the T-MN determines a remaining validity for the TA value that the UE 110 has acquired for the target cell 130b and provides this information to the S-MN in the message. In one example, the T-MN determines a remaining validity for the TA value that the UE 110 has acquired for the target cell 130b and includes this information in the Handover Command message that it includes for transmission to the UE 110. In another example, the T-MN determines a remaining validity for the TA value that the UE has acquired for the target cell based on the time of the reception of the UL transmission from the UE in the target cell (when the provided TA value was determined by the T-MN) and / or a configured validity for the Time Alignment Timer for the target cell. Alternatively, the T-MN may send the TA value for the UE 110 in the target cell 130b to the S-MN in a separate message (instead of including it within the Handover Command message or the Handover Request Acknowledge message), and the S-MN may provide the TA value to the UE 110 within, or together with, the RRC Reconfiguration message that comprises a Handover Command to that target cell 130b.

[0141] In one alternative, the S-MN includes a request (or an indication) that a TA acquisition / establishment procedure should be performed for the UE 110 for the target cell 130b, in the message to the T-MN to request a handover procedure for the UE 110 to the target cell 130b (e.g. a Handover Request message).

[0142] Particular embodiments include TA acquisition for a possible target PCell during HO execution procedure. Accordingly, particular embodiments, include a UE that acquires a TA value for the target cell 130b during a Handover execution procedure while still being connected to the source cell 130a, and finalizes the Handover execution towards the target cell 130b.

[0143] According to particular embodiments, the UE receives, from the network, a Handover Command message to perform a handover (e.g., Reconfiguration with Sync) to a target cell 130b and an indication and / or configuration to perform a TA acquisition for the target cell during the handover procedure. The UE 110 performs a TA acquisition procedure towards the target cell 130b, e.g. by transmitting one or more PRACH preambles to the target cell while still being connected to the source cell 130a. The UE 110 receives an indication / information about the TA value, to use in the target cell 130b, from the source cell 120a, e.g., in a Target_Cell_TA_Value message. The UE 110 finalizes the handover towards the target cell 130b by transmitting a PUSCH message to the target cell 130b. The UE 110 uses the TA value for the target cell 130b that it received from the source cell 130a.

[0144] While the UE 110 transmits the PRACH preamble(s) to the target cell it stays connected to the source cell 130a, thus avoiding interruption due to e.g. waiting for a RAR message in response to the PRACH preamble in the target cell 130b, which would cause an interruption using, e.g., legacy L3 handover procedures. As the UE 110 in this example stays connected to the source cell 130a, the UE 110 may, in some embodiments, continue reception of downlink transmissions and perform uplink transmissions in the source cell 130a. In a procedure where the UE 110 instead performs a Random Access procedure in the target cell 130b as part of traditional handover, there would be an interruption due to the UE 110 transmitting the PRACH preamble and having to wait for the downlink response message, i.e., the RAR (Msg2) or MsgB after having already left the source cell connection. In legacy approaches this introduces an interruption in the downlink reception during handover.

[0145] In an alternative embodiment, the indication / information about the TA value (e.g., in a Target_Cell_TA_Value message) also corresponds to a trigger for the UE 110 to perform a PUSCH transmission in the target cell 130b. The indication / information can e.g. be provided through an RRC message, in a MAC CE or a MAC PDU (e.g. as a Random Access Response) in the source cell 130a.

[0146] In one example, the UE 110 is also provided with a configuration of UL grants (e.g. PLISCH resources), to use for the UL transmission in the target cell, in the same message (or transmitted together with) that includes the indication / information about the TA value to use in the target cell.

[0147] In some embodiments, the reception of the indication / information about the TA value and / or of UL grants for the target cell, e.g. in a Target_Cell_TA_Value message, is considered as a trigger to perform a so called RACH-less handover to the target cell, corresponding to at least the PRACH preamble transmission to the target cell and the corresponding RAR reception from the target cell are skipped when performing the actual cell change. In one example, the UE performs RACH-less handover to the target cell only if it receives an indication / information about the TA value and / or of UL grants for the target cell in the Target_Cell_TA_Value message. In one example the UE performs a normal RACH procedure as part of the cell change procedure if it has not received any indication / information about the TA value to use in the target cell in the Target_Cell_TA_Value message. In one example the UE performs a normal RACH procedure as part of the cell change procedure if it has not received any UL grants to use in the target cell in the Target_Cell_TA_Value message. The TA value may, in some embodiments, be part of the configuration of UL grant received by the UE to be used for UL transmission in the target cell.

[0148] Other example embodiments include methods implemented by an S-MN to enable TA value acquisition for a target cell 130b belonging to a T-MN while the UE 110 is still connected to the source cell 120a. Example methods includes the S-MN transmitting a Handover Request message to the T-MN to request a handover procedure for the UE to a target cell 130b. One or more methods may further comprise receiving a Handover Request Acknowledge message from the T-MN confirming the handover (including a configuration for handover to the target cell, a Handover Command, e.g. with a configuration for a RACH-less handover) and with a configuration for the UE to perform a TA acquisition procedure for the target cell.

[0149] In some embodiments, the message includes an indication that the UE should perform a TA acquisition procedure and the S-MN initiates the TA acquisition procedure for the UE (towards the target cell) using configurations that the S-MN has received earlier.

[0150] In some embodiments, the message includes an indication that the UE should perform a TA acquisition procedure and the S-MN initiates the TA acquisition procedure by first requesting the needed TA acquisition configuration, e.g. towards the T-MN or another node.

[0151] One or more methods may further comprise transmitting the configuration and / or indication to the UE 110 to perform the TA acquisition procedure towards the target cell 130b. The configuration and / or indication can be sent to the UE in an RRC message, a MAC CE or a DCI, e.g. a PDCCH order. In one example, the configuration for TA acquisition is sent in an RRC message and the indication to perform the TA acquisition (e.g. to transmit a PRACH preamble in the target cell) is provided via lower layer signaling, e.g. a MAC CE or DCI, e.g. a PDCCH order

[0152] One or more methods may further comprise receiving a message from the T-MN with the TA value for the UE that has been calculated / acquired for the UE in the target cell (based on the UE transmission in the target cell).

[0153] One or more methods may further comprise transmitting the TA value to the UE and transmits the Handover Command to the UE. In one example, the S-MN transmits the TA value to the UE in a separate message, e.g. a message similar to a Random Access Response message (but here in response to a message transmitted by the UE in the target cell), or another MAC CE, and the HO Command in another message (a RRC Reconfiguration message). In another example, the S-MN includes the TA value (received from the T-MN) within the RRC Reconfiguration message (the stored Handover Command) that it sends to the UE, or it sends the TA value together with the RRC Reconfiguration message.

[0154] Other embodiments are directed to methods implemented by a T-MN to enable acquisition of a TA value of a target cell of the T-MN by a UE while the UE is still connected to a source cell belonging to an S-MN. One or more methods may comprise the T-MN receiving a Handover Request message from the S-MN requesting a handover procedure for the UE to a target cell belonging to the T-MN. One or more methods may further comprise the T-MN deciding to configure a RACH-less handover for the UE to the target cell and to initiate a TA acquisition procedure to the target cell in order to acquire a TA value to use during the RACH- less access.

[0155] One or more methods may further comprise the T-MN transmitting a Handover Request Acknowledge message to the S-MN to confirm the handover and provide the configuration for the handover to the target cell (a Handover Command), e.g. with a configuration for a RACH- less handover, and with a configuration for the UE to perform a TA acquisition procedure for the target cell. Alternatively, the T-MN may have been earlier provided a corresponding configuration for the TA acquisition to the S-MN and, in this message includes an indication that the UE should perform a TA acquisition procedure. The S-MN then initiates the TA acquisition procedure for the UE (towards the target cell) using the configuration that the T-MN provided earlier. In another alternative, the message includes an indication to the S-MN that the UE should perform a TA acquisition procedure. In one option, the T-MN then receives a request for the TA acquisition configuration from the S-MN and sends back the TA acquisition configuration to use.

[0156] One or more methods may further comprise the T-MN receiving an UL transmission from the UE in the target cell, e.g. a PRACH preamble according to the TA acquisition configuration that the T-MN has provided, and determines the TA value for the UE in the target cell. One or more methods may further comprise the T-MN transmitting the TA value for the UE in the target cell to the S-MN, e.g. in a new message.

[0157] Alternatively, in contrast to the above, the T-MN may not transmit a Handover Request Acknowledge message (with the handover configuration) to the S-MN in response to the Handover Request, but instead may respond with a message to the S-MN to initiate (and / or configure) a TA acquisition procedure for the UE in the target cell. The S-MN then initiates the TA acquisition procedure towards the UE. When the T-MN has acquired the TA value for the UE in the target cell (e.g. based on reception of a UL transmission, such as a PRACH transmission according to the TA acquisition configuration) it sends the Handover Request Acknowledge message to the S-MN, including the TA value. In one example, the T-MN includes TA value in the Handover Command message that it generates and includes in the Handover Request Acknowledge message to the S-MN, for transmission to the UE.

[0158] In another alternative, the S-MN may include a request for TA acquisition in the same message that includes the request for handover (e.g. Handover Request message) to the T-MN. In one example, the S-MN includes an indication to the T-MN whether TA acquisition procedure is supported and / or desirable for the requested handover. This could e.g. correspond to whether the S-MN supports the TA acquisition procedure, whether the current type of service and / or buffer levels for the UE require a RACH-less handover (and thus a TA acquisition for the target cell) and whether the S-MN considers it needed to acquire a TA value (using the TA acquisition procedure) for the target cell, or if it could be determined in any other way.

[0159] When the T-MN receives a request for the TA acquisition procedure from the S-MN, the T-MN may, e.g., accept the request and respond with the needed TA acquisition configuration, if any, to the S-MN. That said, the T-MN may instead, in one option, reject the request for the TA acquisition procedure. In one example, the T-MN then accepts the request for handover to the target cell but rejects the request for TA acquisition for the target cell. In one alternative, the T- MN indicates the reject of the TA acquisition procedure in the message that is used to accept the request for handover (e.g. in the Handover Request Acknowledge message).

[0160] The TA acquisition procedure for a target cell concerns a particular cell in the T- MN / target gNB. That cell is then typically hosted by a specific DU in the target gNB. In one alternative for the different embodiments in the invention, when the T-MN receives a request from the S-MN for a TA acquisition procedure or when the Centralized Unit (CU) in the T-MN (i.e. the CU in the target gNB) determines that a TA acquisition procedure should be performed for the UE to the target cell, the CU of the target gNB contacts the DU that is hosting that cell. It will then be this DU that provides to the T-MN (the CU of the target gNB) the configuration for the TA acquisition. The T-MN (i.e. the CU of the target gNB) will then include the TA configuration within an RRC reconfiguration message and send it to the S-MN.

[0161] In one alternative for the above embodiments for the S-MN and the T-MN, corresponding to the embodiments for the UE in this section, the T-MN sends the Handover Request Acknowledge message to the S-MN with both the handover configuration for the target cell (e.g. the Handover Command message for a RACH-less handover) and the TA acquisition configuration for the target cell. In one example, the TA acquisition configuration is included within the Handover Command message. The S-MN then sends the RRC Reconfiguration message to trigger the handover to the UE (with the Handover Command message from the T- MN), including the TA acquisition configuration. In one example, the TA acquisition configuration is transmitted together with the Handover Command message to the UE. Based on this / these message(s) the UE starts the TA acquisition procedure towards the target cell, while still being connected to the source cell while waiting for a TA value for the target cell. When the T-MN can determine a TA value for the UE in the target cell, e.g. based on an UL transmission such as a PRACH preamble, it sends the TA value to the S-MN. The S-MN, in turn sends the TA value to the UE, whereby the UE initiates the actual handover execution procedure, such as transmission of UL messages / data (e.g. for transmission of an RRC Reconfiguration Complete message, or a MAC CE or a MAC PDU in the target cell).

[0162] FIGS. 7-16 are signaling diagrams illustrating examples for signaling between network nodes, (e.g., an S-MN, T-MN, and a UE). In some of the options the TA acquisition procedure is initiated by the source node 120a and in some of the options it is initiated by the target node 120b. In particular FIGS. 7-12 illustrate examples in which the TA acquisition procedure is initiated by the source node 120a. FIGS. 13-15 illustrate examples in which the TA acquisition procedure is initiated by the target node 120b. FIG. 16 illustrates an example in which a common configuration for TA acquisition is shared between the source node 120a and the target node 120b.

[0163] As shown in FIGS. 7-12, some embodiments include the S-MN deciding that TA acquisition should be performed by the UE 110 prior to the handover execution, in order to support that a RACH-less handover. The decision to trigger a handover is done by the S-MN, e.g., based on measurement reports that it has received from the UE 110 that indicate that a handover should be performed to a certain target cell 130b. The decision to trigger a TA acquisition for the handover may then be based on, e.g., UE capabilities (such as that the UE supports TA acquisition and a corresponding RACH-less handover), information about or from the T-MN that it supports TA acquisition and RACH-less handover and / or would like that a RACH-less handover should be performed to the target cell 130b, and / or the current type of service that the UE 110 is running in the network. As an example, in case the UE 110 has a service that requires low interruptions at the handover, then a RACH-less handover (and thus TA acquisition by the S-MN) may be initiated. In one example, the TA acquisition is also only triggered towards target cells where the TA value for the UE cannot be determined in other ways.

[0164] FIG. 7 illustrates a first example in which TA acquisition / establishment is decided by the source node 120a. In this example, the source node 120a (in this example, a source gNB, which may be an S-MN) decides to trigger a TA acquisition for the UE for a target cell (to which a handover is to be performed). The TA acquisition procedure is performed before the actual handover preparation is initiated, i.e. before the request for handover is transmitted to the target gNB.

[0165] In FIG. 7 steps 1-2, the source gNodeB of the UE decides to trigger a TA acquisition procedure for a target cell that belongs to target gNB. It transmits a request message for the TA acquisition to the target gNodeB (e.g. a TA establishment request message).

[0166] In FIG. 7 step 3, the target gNodeB accepts the request for TA acquisition for the UE and sends back the corresponding TA acquisition / establishment configuration to the source gNB, e.g. in a TA establishment acknowledge message.

[0167] In FIG. 7, steps 4-5, the source gNodeB transmits the configuration for TA acquisition in the target cell to the UE, including the corresponding configuration to the be used for TA acquisition as received from the target gNB.

[0168] In FIG. 7, step 6 (optional), the actual TA acquisition is initiated by the UE when it receives a lower layer signaling from the source gNB that triggers the TA acquisition.

[0169] In FIG. 7, step 7, the UE transmits a PRACH preamble to the target cell based on the received TA acquisition configuration and, possibly, indications in the lower layer signaling.

[0170] In FIG. 7, step 8, the target gNB determines a TA value for the UE (in the target cell) based on the received PRACH preamble and transmits the information about the TA value to the source gNB.

[0171] In FIG. 7, step 9, the source gNodeB transmits message to the UE with the TA value for the target cell. The message can e.g. be a RAR like message or a MAC CE that is sent in the source cell but concerns the target cell.

[0172] In FIG. 7, step 10, the source gNodeB transmits a request message for the handover (e.g. a Handover Request message) to the target gNodeB. In one option, the message includes an indication that the UE has an acquired TA value for the target cell.

[0173] In FIG. 7, step 11, the target gNodeB acknowledges the request for a handover and includes a target configuration (for the target cell) for the handover in the response message to the source gNB, e.g. in a Handover Request Acknowledge message. The handover configuration corresponds to a RACH-less handover and may take into account that the UE has an acquired TA value for the target cell.

[0174] In FIG. 7, step 12, the source gNodeB transmits a RRC Reconfiguration message to the UE to trigger the handover execution to the target cell (i.e. the Handover Command message). The handover configuration corresponds to a RACH-less handover.

[0175] In FIG. 7, step 13-14, the UE executes the RACH-less handover and uses the TA value that was earlier acquired and received in step 9 for the UL transmission in the target cell during the handover execution (the RACH-less access). In FIG. 8 and FIG. 9, options similar to those in FIG. 7 are illustrated, but with the difference that the TA value is provided to the UE 110 in other ways. In FIG. 10, FIG. 11, and FIG. 12, examples can be seen where the TA acquisition is initiated as part of the handover preparation procedure with different options for how the TA value is provided to the UE 110.

[0176] FIGS. 13-15 illustrate examples in which the T-MN decides whether a RACH-less handover or a (normal) RACH-based handover should be performed towards the target cell. It should therefore be possible for the T-MN to decide whether an associated TA acquisition procedure should be performed towards the target cell prior to the actual handover execution procedure to the target cell (i.e. before the actual access to the target cell). In some of the options in the invention it is the target node (the T-MN) that decides that a TA acquisition procedure should be performed towards the target cell and also initiates the procedure.

[0177] In the example of FIG. 13, the target gNB (target MN, T-MN) receives a request for a handover to one of its cells (a target cell) from the source gNB (source MN, S-MN) and decides to configure a RACH-less handover for the UE and that a corresponding TA acquisition procedure for the target cell shall be performed.

[0178] In FIG. 13, step 1, the source gNodeB determines to trigger a handover to a target cell that belongs to the target gNB.

[0179] In FIG. 13, step 2, the source gNodeB transmits a request message for the handover (e.g. a Handover Request message) to the target gNodeB.

[0180] In FIG. 13, step 3, the target gNodeB decides to configure a RACH-less handover for the UE and that a corresponding TA acquisition procedure for the target cell shall be performed.

[0181] In FIG. 13, step 4, the target gNodeB acknowledges the request for the handover to the source gNB, e.g. by transmitting a Handover Request Acknowledge message. The message includes the handover configuration (Handover Command) and a configuration for the TA acquisition.

[0182] In FIG. 13, steps 5-6, the source gNodeB transmits the configuration for TA acquisition in the target cell to the UE, including the corresponding configuration to the be used for TA acquisition as received from the target gNB.

[0183] In FIG. 13, step 7 (optional), the actual TA acquisition is initiated by the UE when it receives a lower layer signaling from the source gNB that triggers the TA acquisition.

[0184] In FIG. 13, step 8a, the UE transmits a PRACH preamble to the target cell based on the received TA acquisition configuration and, possibly, indications in the lower layer signaling. In step 8b, the target gNB determines a TA value for the UE (in the target cell) based on the received PRACH preamble and transmits the information about the TA value to the source gNB.

[0185] In FIG. 13, step 9, the source gNodeB transmits message to the UE with the TA value for the target cell. The message can e.g. be a RAR like message or a MAC CE that is sent in the source cell but concerns the target cell. In FIG. 13, step 10, the source gNodeB transmits a RRC Reconfiguration message to the UE to trigger the handover execution to the target cell (i.e. the Handover Command message). The handover configuration corresponds to a RACH-less handover.

[0186] In FIG. 13, step 11-12, the UE executes the RACH-less handover and uses the TA value that was earlier acquired and received in step 9 for the UL transmission in the target cell during the handover execution (the RACH-less access).

[0187] FIG. 14 and FIG. 15 illustrate other examples in which the TA value is provided to the UE 110 in other ways.

[0188] In another possibility the S-MN requests the TA acquisition for a Handover, and, in response the T-MN sends the configuration for TA acquisition for the UE. Here is worth clarifying that when the T-MN receives the request from the S-MN for a TA acquisition procedure, since this procedure refers to a particular neighboring / target cell, the T-MN will (at least in one example) contact the DU that is hosting that neighboring / target cell and it will be this DU that provides to the T-MN the configuration for the TA acquisition. Then, when the T-MN receives the preamble and calculates the TA value for that UE, it generates the HO command (which includes the calculated TA value). This would require a two-step procedure: a first step for TA acquisition, a second step for the actual HO command. However, that would still be optimized, as the second step is not initiated by the S-MN, but maybe by the T-MN upon reception of the preamble. In a sub-option, as there may be some timing difference between the reception of the TA configuration and the preamble transmission (e.g. S-MN sending a PDCCH order for the target cell to the UE), the S-MN may indicate to the T-MN that it has sent the PDCCH to the UE, so the T-MN gets prepared.

[0189] FIG. 16 illustrates an example in which a common configuration for TA acquisition is shared between the source node 120a and the target node 120b. In one set of embodiments, a first node provides to another (e.g., a neighbouring) node a set of configurations to be used for TA acquisition in one or more cells that belongs to the first node. These configurations for TA acquisition can be provided to the neighbouring node prior to initialization of any TA acquisition procedure in the first node (for a UE that is located in a cell belonging to the corresponding neighbouring node), and could e.g. be provided during the procedure to setup the connection between the two nodes (e.g. during Xn establishment). The configurations for TA acquisition are here not UE specific but can instead be used for multiple UEs at different points in time. They can e.g. include RACH resources to be used for the TA acquisition in the cell, such as e.g. PRACH preambles and / or time and / or frequency resources to be used for the PRACH preamble transmission. By providing these configurations to the neighbouring node, that neighbouring node can, when it is a source node for a UE, trigger the TA acquisition / establishment procedure to a target cell in the first node without first requesting those resources from the first node (which then is the target node for the UE). In one option, the first node partitions different resources / configurations for TA acquisition for one or more of its cells to different neighbouring nodes. By providing different resources / configurations (e.g. different PRACH preambles and / or RACH resources) to different neighbouring nodes, it knows where to transit the corresponding calculated / determined TA value after reception of the PRACH preamble for TA acquisition. In one example, the first node provides different TA acquisition resources (for a cell) per neighbouring MN. In another example, the first node provides different TA acquisition resources (for a cell) per neighbouring cell. In this case, the first node (when it is a target node for a UE) knows in what (source) cell the UE has received the configuration to perform the TA acquisition.

[0190] Yet other embodiments vary in other respects. For example, in one embodiment, the request message from the S-MN to the T-MN (e.g. Handover Request message), which includes a TA acquisition request, includes an indication that the request is for Conditional Handover (CHO). The S-MN does then not have to configure the UE with CHO, and the UE does not even need to be capable of CHO, but from a Target gNB perspective, this is configured as in CHO because the timing between the request from the S-MN and the timing in which the UE accesses the target cell may be slightly longer than in the case of a legacy (nonconditional) HO. This is because the UE may first need to acquire a TA value from the target cell. Alternatively, when the HO request includes the TA acquisition, the T-MN interprets that the UE may not come shortly after, but that first the T-MN needs to receive a preamble for TA acquisition and provide to the S-MN a TA value, to be provided to the UE at the timing of the Handover.

[0191] In some embodiments described herein, the TA acquisition for the target cell and the handover execution to the target cell are performed in two separate steps by the UE, where the two steps are triggered separately by the network (e.g. by the S-MN). In one alternative, the S- MN triggers the second step (transmission of the actual Handover Command message to the UE) based on further measurement reports from the UE. The first step (TA acquisition) can then be triggered based on a first set of measurement reports (e.g. a single measurement report) and the second step is triggered based on another set of measurement reports (e.g. another single measurement report). In one example, the two sets of measurement reports have different configured reporting events / conditions, e.g. different thresholds that need to be fulfilled for triggering the measurement report. In another example, the two sets of measurement reports have the same configured reporting events / conditions, but transmission of the second set of measurement reports is triggered by another condition, e.g. that a certain time has elapsed since the transmission of the first set of measurement reports, or that the UE has transmitted the preamble for TA acquisition to the corresponding target / neighbouring cell.

[0192] In one alternative, the S-MN determines to reinitiate the TA acquisition (e.g. by sending a new message to the UE, such as a new RRC message and / or a new lower layer signaling, or by requesting a new TA acquisition configuration from the T-MN) for a target cell instead of transmitting the Handover Command to the UE. This could e.g. be in case the S-MN has not received any TA value for the UE in the target cell.

[0193] In an alternative to some embodiments, the S-MN sends an indication to the T-MN when it has configured a UE to perform a TA acquisition to a target cell of the T-MN (or when the S- MN has sent an indication to the UE to perform the TA acquisition). In one example, the S-MN sends the indication to the T-MN before the S-MN sends the corresponding configuration or indication to the UE. The T-MN can then, based on the indication from the S-MN, be prepared that the UE will perform the TA acquisition procedure towards the target cell (e.g. by transmission of a PRACH preamble). In one option, the indication from the S-MN to the T-MN provides information about what specific target cell, UE and / or RACH resource (e.g. PRACH preamble) that the TA acquisition procedure will be performed for / with.

[0194] In view of the above, the UE 110 may, for example, be implemented as schematically illustrated in the example of FIG. 17. The UE 110 of FIG. 17 comprises processing circuitry 610, memory circuitry 620, and interface circuitry 630. The processing circuitry 610 is communicatively coupled to the memory circuitry 620 and the interface circuitry 630, e.g., via a bus 604. The processing circuitry 610 may comprise one or more microprocessors, microcontrollers, hardware circuits, discrete logic circuits, hardware registers, digital signal processors (DSPs), field-programmable gate arrays (FPGAs), application-specific integrated circuits (ASICs), or a combination thereof. For example, the processing circuitry 610 may be programmable hardware capable of executing software instructions stored, e.g., as a machine- readable computer program 640 in the memory circuitry 620. The memory circuitry 620 of the various embodiments may comprise any non-transitory machine-readable media known in the art or that may be developed, whether volatile or non-volatile, including but not limited to solid state media (e.g., SRAM, DRAM, DDRAM, ROM, PROM, EPROM, flash memory, solid state drive, etc.), removable storage devices (e.g., Secure Digital (SD) card, miniSD card, microSD card, memory stick, thumb-drive, USB flash drive, ROM cartridge, Universal Media Disc), fixed drive (e.g., magnetic hard disk drive), or the like, wholly or in any combination.

[0195] The interface circuitry 630 may be a controller hub configured to control the input and output (I / O) data paths of the UE 110. Such I / O data paths may include data paths for exchanging signals over a network. The interface circuitry 630 may be implemented as a unitary physical component, or as a plurality of physical components that are contiguously or separately arranged, any of which may be communicatively coupled to any other, or may communicate with any other via the processing circuitry 610. For example, the interface circuitry 630 may comprise a transmitter 632 configured to send wireless communication signals and a receiver 634 configured to receive wireless communication signals.

[0196] The UE 110 may be configured to perform the method 200 described above. In one example, the processing circuitry 610 may be configured to, responsive to receiving, via the interface circuitry 630, a TA value of a target cell 130b served by a target node 120b, refrain from using the TA value to perform RACH-less L3 handover to the target cell 130b until a handover triggering event occurs. The processing circuitry 610 may be further configured to perform the RACH-less L3 handover to the target cell 130b using the TA value responsive to the handover triggering event occurring.

[0197] Still other embodiments include a control program 640 comprising instructions that, when executed on processing circuitry 610 of a UE 110, cause the UE 110 to carry out the method 200 described above.

[0198] Yet other embodiments include a carrier containing the control program 640. The carrier is one of an electronic signal, optical signal, radio signal, or computer readable storage medium.

[0199] Correspondingly, a network node 120 may be implemented as schematically illustrated in the example of FIG. 18. The network node 120 of FIG. 18 comprises processing circuitry 710, memory circuitry 720, and interface circuitry 730. The processing circuitry 710 is communicatively coupled to the memory circuitry 720 and the interface circuitry 730, e.g., via a bus 704. The processing circuitry 710 may comprise one or more microprocessors, microcontrollers, hardware circuits, discrete logic circuits, hardware registers, digital signal processors (DSPs), field-programmable gate arrays (FPGAs), application-specific integrated circuits (ASICs), or a combination thereof. For example, the processing circuitry 710 may be programmable hardware capable of executing software instructions stored, e.g., as a machine- readable computer program 740 in the memory circuitry 720. The memory circuitry 720 of the various embodiments may comprise any non-transitory machine-readable media known in the art or that may be developed, whether volatile or non-volatile, including but not limited to solid state media (e.g., SRAM, DRAM, DDRAM, ROM, PROM, EPROM, flash memory, solid state drive, etc.), removable storage devices (e.g., Secure Digital (SD) card, miniSD card, microSD card, memory stick, thumb-drive, USB flash drive, ROM cartridge, Universal Media Disc), fixed drive (e.g., magnetic hard disk drive), or the like, wholly or in any combination.

[0200] The interface circuitry 730 may be a controller hub configured to control the input and output (I / O) data paths of the network node 120. Such I / O data paths may include data paths for exchanging signals over a network. The interface circuitry 730 may be implemented as a unitary physical component, or as a plurality of physical components that are contiguously or separately arranged, any of which may be communicatively coupled to any other, or may communicate with any other via the processing circuitry 710. For example, the interface circuitry 730 may comprise a transmitter 732 configured to send wireless communication signals and a receiver 734 configured to receive wireless communication signals.

[0201] The network node 120 may be configured to perform any of the network node methods 300, 400 described above. According to particular embodiments, the network node 120 is configured to operate as a source node 120a. The processing circuitry 710 is configured to transmit, via the interface circuitry 730, a TA acquisition configuration to a UE 110. The processing circuitry 710 is further configured to receive, from a target node (120b) via the interface circuitry 730, a TA value of a target cell 130b served by the target node 120b. The processing circuitry 710 is further configured to transmit the TA value of the target cell 130b to the UE 110 via the interface circuitry 730 to enable RACH-less L3 handover of the UE 110 toward the target cell 130b. The processing circuitry 710 is further configured to send a handover request to the target node 120b via the interface circuitry 730 to trigger the RACH-less L3 handover of the UE 110 to the target cell 130b.

[0202] In another example, the network node 120 may be configured to operate as a target node 120b. In at least one such example, the processing circuitry 710 is configured to transmit, to a source node 120a via the interface circuitry 730, a TA acquisition configuration for acquiring a TA value of a target cell 130b served by the target node 120b. The processing circuitry 710 is further configured to receive, via the interface circuitry 730, an uplink transmission from a UE 110 in accordance with the TA acquisition configuration. The processing circuitry 710 is further configured to transmit, via the interface circuitry 730, a TA value of the target cell 130b to the source node 120a in response to the uplink transmission. The processing circuitry 710 is further configured to receive, from the source node 120a via the interface circuitry 730, a handover request for RACH-less L3 handover of the U E 110 to the target cell 130b. The processing circuitry 710 is further configured to trigger the RACH-less L3 handover of the UE 110 to the target cell 130b.

[0203] Still other embodiments include a control program 740 comprising instructions that, when executed on processing circuitry 710 of a network node 120, cause the network node 120 to carry out any one of the methods 300, 400 described above.

[0204] Yet other embodiments include a carrier containing the control program 740. The carrier is one of an electronic signal, optical signal, radio signal, or computer readable storage medium.

[0205] Although the computing devices described herein may include the illustrated combination of hardware components, other embodiments may comprise computing and / or communication hardware with different combinations of components. It is to be understood that these computing devices may comprise any suitable combination of hardware and / or software needed to perform the tasks, features, functions, and methods disclosed herein. Further, while components are depicted as single boxes located within a larger box, or nested within multiple boxes, in practice, the devices described herein may comprise multiple different physical components that make up a single illustrated component, and functionality may be partitioned between separate components.

Claims

CLAIMSWhat is claimed is:1 . A method (200), implemented by a User Equipment, UE (110), the method comprising: responsive to receiving a Timing Advance, TA, value of a target cell (130b) served by a target node (120b), refraining (230) from using the TA value to perform a Random Access Channel-less Layer 3, RACH-less L3, handover to the target cell (130b) until a handover triggering event occurs; and performing (240) the RACH-less L3 handover to the target cell (130b) using the TA value responsive to the handover triggering event occurring.

2. The method of claim 1 , further comprising: receiving (210), from a source node (120a), a TA acquisition configuration for acquiring the TA value of the target node (120b); and using (220) the TA acquisition configuration to acquire the TA value of the target node (120b).

3. The method of claim 2, wherein the TA acquisition configuration comprises a Physical Random Access Channel, PRACH, configuration.

4. The method of any one of claims 2-3, further comprising using the PRACH configuration to transmit a preamble to the target cell (130b).

5. The method of claim 4, wherein the TA acquisition configuration indicates whether to expect a response to the preamble transmission from the source node (120a).

6. The method of claim 5, wherein the TA acquisition configuration indicates a time window in which to expect the response to the preamble transmission from the source node (120a).

7. The method of claim 6, further comprising, responsive to the time window elapsing without having received the expected response: retransmitting the preamble to the target cell (130b) with increased power relative to the preamble transmission; or transmitting a different preamble to the target cell (130b) with increased power relative to the preamble transmission.

8. The method of any one of claims 1-7, further comprising receiving a duration in which the TA value is valid.

9. A User Equipment, UE (110), comprising: interface circuitry (630) and processing circuitry (610) communicatively connected to the interface circuitry (630), wherein the processing circuitry (610) is configured to: responsive to receiving, via the interface circuitry (630), a Timing Advance (TA) value of a target cell (130b) served by a target node (120b), refrain from using the TA value to perform a Random Access Channel-less Layer 3, RACH-less L3, handover to the target cell (130b) until a handover triggering event occurs; and perform the RACH-less L3 handover to the target cell (130b) using the TA value responsive to the handover triggering event occurring.

10. The UE of the preceding claim, wherein the processing circuitry (610) is further configured to perform the method of any one of claims 2-8.

11. A computer program, comprising instructions which, when executed on processing circuitry (610) of a User Equipment, UE (110), cause the processing circuitry (610) to carry out the method according to any one of claims 1-8.

12. A method (300), implemented by a source node (120a), the method comprising: transmitting (310) a Timing Advance, TA, acquisition configuration to a UE (110); receiving (320), from a target node (120b), a TA value of a target cell (130b) served by the target node (120b); transmitting (330) the TA value of the target cell (130b) to the UE (110) to enable Random Access Channel-less Layer 3, RACH-less L3, handover of the UE (110) toward the target cell (130b); sending (340) a handover request to the target node (120b) to trigger the RACH-less L3 handover of the UE (110) to the target cell (130b).

13. The method of claim 12, further comprising transmitting (305) a TA acquisition procedure request to the target node (120b) and receiving a TA acquisition configuration in response.

14. The method of any one of claims 12-13, wherein transmitting the TA acquisition configuration to the UE comprises transmitting the TA acquisition configuration in a Radio Resource Control, RRC, message, a Medium Access Control, MAC, Control Element, CE, or Downlink Control Information, DCI.

15. The method of any one of claims 12-13, further comprising signaling (315) the UE (110) to perform TA acquisition.

16. The method of claim 15, wherein signaling the UE (110) to perform TA acquisition comprises signaling the UE (110) to transmit a Physical Random Access Channel, PRACH, preamble in the target cell (130b) according to the TA acquisition configuration.

17. The method of any one of claims 12-16, wherein transmitting the TA value to the UE (110) comprises transmitting the TA value in a MAC CE or RRC message.

18. The method of any one of claims 12-17, wherein the handover request comprises an indication that the UE (110) has acquired the TA value for the target cell (130b).

19. The method of any one of claims 12-18, further comprising receiving (350), from the target node (120b), a handover request acknowledgement comprising a handover command.

20. The method of claim 19, further comprising transmitting (360), to the UE (110), an RRC Reconfiguration message comprising the handover command, wherein the RRC Reconfiguration message indicates a timer value usable by the UE (110) to determine the remaining time in which the TA value is valid for the target cell (130b).

21. The method of any one of claims 19-20, wherein the handover command comprises a RACH-less handover configuration for performing handover to the target cell (130b).

22. The method of any one of claims 12-21 , further comprising receiving, from the target node (120b) an amount of time remaining in which the TA value is valid in the target cell (130b).

23. A source node (120a) comprising: interface circuitry (730) and processing circuitry (710) communicatively coupled to the interface circuitry (730), wherein the processing circuitry (710) is configured to: transmit, via the interface circuitry (730), a Timing Advance, TA, acquisition configuration to a UE (110); receive, from a target node (120b) via the interface circuitry (730), a TA value of a target cell (130b) served by the target node (120b); transmit the TA value of the target cell (130b) to the UE (110) via the interface circuitry (730) to enable Random Access Channel-less Layer 3, RACH-less L3, handover of the UE (110) toward the target cell (130b); send a handover request to the target node (120b) via the interface circuitry (730) to trigger the RACH-less L3 handover of the UE (110) to the target cell (130b).

24. The source node of the preceding claim, wherein the processing circuitry (710) is further configured to perform the method of any one of claims 13-22.

25. A computer program, comprising instructions which, when executed on processing circuitry (710) of a source node (120a) cause the processing circuitry (710) to carry out the method according to any one of claims 12-22.

26. A method (400), implemented by a target node (120b), the method comprising: transmitting (410), to a source node (120a), a Timing Advance, TA, acquisition configuration for acquiring a TA value of a target cell (130b) served by the target node (120b); receiving (420) an uplink transmission from a User Equipment, UE (110), in accordance with the TA acquisition configuration; transmitting (430) a TA value of the target cell (130b) to the source node (120a) in response to the uplink transmission; receiving (440), from the source node (120a), a handover request for Random Access Channel-less Layer 3, RACH-less L3, handover of the UE (110) to the target cell (130b); triggering (450) the RACH-less L3 handover of the UE (110) to the target cell (130b).

27. The method of claim 26, wherein transmitting the TA acquisition configuration is responsive to receiving (405) a TA acquisition configuration request from the source node (120a).

28. The method of any one of claims 26-27, further comprising transmitting (415) an indication to the source node (120a), separate from the TA acquisition configuration, indicating that the UE should acquire the TA value according to the TA acquisition configuration.

29. The method of any one of claims 26-28, wherein receiving the uplink transmission from the UE (110) comprises receiving a Physical Random Access Channel, PRACH, preamble.

30. The method of any one of claims 26-29, wherein the handover request comprises an indication that the UE has acquired the TA value for the target cell (130b).

31. The method of any one of claims 26-30, wherein the handover request comprises a request that the RACH-less L3 handover be configured for the UE (110).

32. The method of any one of claims 26-31 , wherein triggering the RACH-less L3 handover comprises transmitting, to the source node (120a), a handover command.

33. The method of any one of claims 26-32, further comprising transmitting, to the source node (120a) a handover configuration for use by the UE (110) in performing the RACH-less L3 handover to the target cell (130b).

34. The method of any one of claims 26-33, further comprising transmitting, to the source node (120a), an indication of how much longer the TA value will remain valid.

35. The method of any one of claims 26-34, further comprising determining how much longer the TA value will remain valid based on when the uplink transmission was received from the UE (110) and / or a configured validity of a time alignment timer for the target cell (130b).

36. The method of any one of claims 26-35, wherein triggering the RACH-less L3 handover comprises transmitting an updated TA value for the target cell (130b) to the source node (120a) in response to the handover request.

37. A target node (120b) comprising: interface circuitry (730) and processing circuitry (710) communicatively coupled to the interface circuitry (730), wherein the processing circuitry (710) is configured to: transmit, to a source node (120a) via the interface circuitry (730), a TimingAdvance, TA, acquisition configuration for acquiring a TA value of a target cell (130b) served by the target node (120b); receive, via the interface circuitry (730), an uplink transmission from a User Equipment, UE (110), in accordance with the TA acquisition configuration; transmit, via the interface circuitry (730), a TA value of the target cell (130b) to the source node (120a) in response to the uplink transmission; receive, from the source node (120a) via the interface circuitry (730), a handover request for Random Access Channel-less Layer 3, RACH-less L3, handover of the UE (110) to the target cell (130b); trigger the RACH-less L3 handover of the UE (110) to the target cell (130b).

38. The target node of the previous claim, wherein the processing circuitry (710) is further configured to perform the method (400) of any one of claims 27-36.

39. A computer program, comprising instructions which, when executed on processing circuitry (710) of a target node (120b) cause the processing circuitry (710) to carry out the method according to any one of claims 26-36.

40. A carrier containing the computer program of any one of claims 11 , 25, or 39, wherein the carrier is one of an electronic signal, optical signal, radio signal, or computer readable storage medium.

Images