LTM handover

Abstract

There is provided a method performed by a first network node, the method comprising: transmitting, to a second network node associated with a candidate target cell of a layer 1 / layer 2 triggered mobility (LTM) handover, a request to assign scheduling request indicator (SRI) resources for a user equipment that may be handed over from a source cell associated with the first network node to the candidate target cell; receiving, from the second network node, an indication of the assigned SRI resources; and transmitting the indication of the assigned SRI resources to the user equipment, wherein the SRI resources are useable by the user equipment for performing a transmission in connection of the LTM handover.

Description

[0001] LTM HANDOVER

[0002] TECHNICAL FIELD

[0003] Various example embodiments relate generally to handling of resource usage in connection of mobility of a user equipment (UE).

[0004] BACKGROUND

[0005] Mobility of a user equipment (UE) may require handover from a source cell to a target cell. Performing a handover requires communication between the source and target cells, as well as to and from the UE. Further, handovers are time consuming processes. Optimization of a handover is needed.

[0006] BRIEF DESCRIPTION

[0007] According to some aspects, there is provided the subject matter of the independent claims. Some further aspects are defined in the dependent claims. The embodiments that do not fall under the scope of the claims are to be interpreted as examples useful for understanding the disclosure.

[0008] LIST OF THE DRAWINGS

[0009] In the following, the invention will be described in greater detail with reference to the embodiments and the accompanying drawings, in which

[0010] Figure 1 presents a network to which one or more embodiments are applicable;

[0011] Figure 2 shows an example of a layer 1 / layer 2 triggered mobility (LTM) handover;

[0012] Figure 3 shows a signaling flow diagram for an enhanced LTM handover, according to an embodiment;

[0013] Figures 4, 5, 6 and 7 illustrates methods, according to some embodiments; and Figure 8 illustrates an apparatus, according to an embodiment.

[0014] DESCRIPTION OF EMBODIMENTS

[0015] The following embodiments are exemplary. Although the specification may refer to “an”, “one”, or “some” embodiment(s) in several locations of the text, this does not necessarily mean that each reference is made to the same embodiment(s), or that a particular feature only applies to a single embodiment. Single features of different embodiments may also be combined to provide other embodiments. Further, when a particular feature, structure, or characteristic is described in connection of an embodiment, it is within the knowledge of one skilled in the art to apply such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described. It shall be understood that although the terms “first,” “second” and the like may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another.

[0016] For the purposes of the present disclosure, the phrases “at least one of A or B”, “at least one of A and B”, and “A and / or B” means (A), (B), or (A and B). For the purposes of the present disclosure, the phrase “A, B, and / or C” means (A), (B), (C), (A and B), (A and C), (B and C), or (A, B, and C).

[0017] Embodiments described may be implemented in a communication network, such as following any of the following radio access technologies (RATs): Worldwide Interoperability for Micro-wave Access (WiMAX), Global System for Mobile communications (GSM, 2G), GSM EDGE radio access Network (GERAN), General Packet Radio Service (GRPS), Universal Mobile Telecommunication System (UMTS, 3G) based on basic wide- band-code division multiple access (W-CDMA), high-speed packet access (HSPA), Long Term Evolution (LTE), LTE-Advanced, and enhanced LTE (eLTE), 5G (also called NR), or 6G. Moreover, communication within the communication network may utilize any proper wireless communication technology, comprising but not limited to: Code Division Multiple Access (CDMA), Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), Frequency Division Duplex (FDD), Time Division Duplex (TDD), Multiple- Input Multiple-Output (M1M0), Orthogonal Frequency Division Multiple (OFDM), and / or Discrete Fourier Transform spread OFDM (DFT-s-OFDM).

[0018] As used herein, the term “network device” or “network node” refers to a node in a communication network via which user equipment may access the network and / or which is capable of controlling radio communication and managing radio resources within a cell. The network device may be called a base station (BS), an access point (AP) or access node. The network device may be, depending on the applied technology, for example, a node B (NodeB or NB), an evolved NodeB (eNodeB or eNB), an NR NB (also referred to as a gNB), a Remote Radio Unit (RRU), a radio head (RH), a remote radio head (RRH), a relay, an Integrated Access and Backhaul (1AB) node, a low power node, a nonterrestrial network (NTN) or non-ground network device such as a satellite network device, a low earth orbit (LEO) satellite and a geosynchronous earth orbit (GEO) satellite, or an aircraft network device.

[0019] Moreover, in connection of split radio access network (RAN), the network device may refer to a centralised unit (CU) of a base station or a distributed unit (DU) of a base station. In the split RAN architecture, node operations may be carried out, at least partly, in the central / centralized unit, CU, (e.g. server, host or node) operationally coupled to the DU, (e.g. a radio head / node). One CU may control one or more DUs, acting at least as transmit / receive (Tx / Rx) nodes. In some embodiments, the DUs may comprise e.g. a radio link control (RLC), medium access control (MAC) layer and a physical (PHY) layer, whereas the CU may comprise the layers above RLC layer, such as a packet data convergence protocol (PDCP) layer, a radio resource control (RRC) and an internet protocol (IP) layers. Other functional splits are possible too. In practice, any processing task may be performed in either the CU or the DU and the boundary where the responsibility is shifted between the CU and the DU may depend on the applied implementation.

[0020] The term “terminal device” refers to any end device that may be capable of wireless communication. By way of example rather than limitation, a terminal device may also be referred to as a communication device, user equipment (UE), a Subscriber Station CSS}, or a Mobile Station (MS). The terminal device may include, but not limited to, a mobile phone, a cellular phone, a smart phone, voice over IP (VoIP) phones, wireless local loop phones a tablet, a wearable terminal device, a personal digital assistant (PDA), portable computers, desktop computer, image capture terminal devices such as digital cameras, gaming terminal devices, music storage and playback appliances, vehiclemounted wireless terminal devices, USB dongles, an Internet of Things (loT) device, a watch or other wearable, a head-mounted display (HMD), a vehicle, a drone, a medical device and applications (e.g., remote surgery), an industrial device and applications (e.g., a robot and / or other wireless devices operating in an industrial and / or an automated processing chain contexts), a consumer electronics device, a device operating on commercial and / or industrial wireless networks, and the like.

[0021] A term “resource”, as used herein, may refer to radio resources in time domain, in frequency domain, in space domain, and / or in code domain. Some examples of resources include e.g. a physical resource block (PRB), a radio frame, a subframe, a time slot, a subband, a frequency region, a sub-carrier, a beam, etc. The term “transmission” and / or “reception” may refer to wirelessly transmitting and / or receiving via a wireless propagation channel on radio resources.

[0022] Figure 1 illustrates an example of a communication network to which embodiments of the invention may be applied. The system may comprise a network node 110 providing one or more cells, such as cell 100, and a network node 112 providing one or more other cells, such as cell 102. Each cell may be, e.g., a macro cell, a micro cell, femto, or a pico cell, for example. In another point of view, the cell may define a coverage area or a service area of the corresponding access node. The network node 110 may provide a user equipment (UE) 120 (one or more UEs) with wireless access to the communication network. The wireless access may comprise downlink (DL) communication from the control node to the UE 120 and uplink (UL) communication from the UE 120 to the control node.

[0023] There may be a plurality of UEs 120, 122 in the system. Each of them may be served by the same or by different control nodes 110, 112. The UEs 120, 122 may communicate with each other, in case device-to-device (D2D) communication interface is established between them via a so-called sidelink. Such D2D communications may be also called machine-to-machine, peer-to-peer (P2P) communications, or vehicle-to-vehicle (V2V).

[0024] In the case of multiple access nodes in the communication network, the access nodes may be connected to each other with an interface. LTE specifications call such an interface as X2 interface. For IEEE 802.11 network (i.e. wireless local area network, WLAN, WiFi), a similar interface may be provided between access points. An interface between an LTE access point and a 5G access point, or between two 5G access points may be called Xn. Other communication methods between the access nodes may also be possible.

[0025] The network nodes 110 and 112 may be further connected via another interface to a core network 116 of the cellular communication system. The LTE specifications specify the core network as an evolved packet core (EPC), and the core network may comprise e.g. a mobility management entity (MME) and a gateway node. The MME may handle mobility of terminal devices in a tracking area encompassing a plurality of cells and handle signalling connections between the terminal devices and the core network. The gateway node may handle data routing in the core network and to / from the terminal devices. The 5G specifications specify the core network as a 5G core (5GC), and there the core network may comprise e.g. an access and mobility management function (AMF) and a user plane function / gateway (UPF), to mention only a few. The AMF may handle termination of non-access stratum (NAS) signalling, NAS ciphering & integrity protection, registration management, connection management, mobility management, access authentication and authorization, security context management. The UPF node may support packet routing & forwarding, packet inspection and QoS handling, for example.

[0026] In mobility of a UE, the UE may move from one cells’ (source cell) coverage area to another cell’s (target cell) coverage area. Such mobility may trigger a handover in order to avoid radio link failure when the UE gets outside the range of the source cell. Legacy handovers include e.g. layer 3 (L3) handovers (HO). L3 HOs are triggered by L3 measurement reports, which typically are event-based measurement reports, e.g., which are triggered when a cell (e.g., a neighbour cell) has a RSRP or RSS1 (or other signal parameter) that meets some HO event criteria. The L3 measurement report may include or may be based on averaging and / or filtering of more signal samples, or averaging over a longer period of time, as compared to layer 1 (LI) measurement report. As said, L3 measurement reports are transmitted to a CU who may then, based on the L3 report, trigger a handover (e.g., L3 handover, which may be basic handover or conditional handover) of the UE to another cell.

[0027] For example, in response to receiving a L3 measurement report from a UE, a source network node (e.g., gNB, or source CU) may send a handover request to a target network node (e.g., target CU). The source network node may transmit to the UE a RRC reconfiguration message including a HO command to cause the UE to perform handover to the target cell, or a CHO configuration (to configure the conditional handover conditions that will trigger or cause the UE to initiate CHO to the target cell). Because the L3 HO (either HO or CHO) is based on a L3 measurement report, e.g., which may require more time to obtain or measure (e.g., based on more signal measurement samples and / or filtering and / or averaging of the measurement samples) as compared to a LI measurement report, the L3 measurement report may be transmitted by a UE well after radio conditions with a serving cell / serving network node have already degraded. If a handover configuration is not already prepared for a HO / CHO to the target cell, then the source network node may need to send a message to the target node to request and prepare the HO configuration for the UE, which may cause significant delay, e.g., of 100ms or more, before the UE can perform a L3 HO or CHO to the target cell. This significant delay may increase the likelihood the UE will suffer a radio link failure (RLF) or loss in connectivity.

[0028] 3GPP ReL18, a new type of inter-cell mobility is introduced, referred to as a L1 / L2 based inter-cell mobility (L1 / L2 triggered mobility, LTM). In above mentioned legacy procedures (e.g., layer 3 HO or conditional handover (CHO), the cell switch or HO decision is made based on L3 measurements, which may introduce significant latency or delay before the UE can perform the L3 HO or CHO. The LTM procedure can be used to reduce the mobility (or HO) latency, since the LTM HO is triggered by a LI measurement report (which has lower latency than the L3 measurement report). L1 / L2 measurement reports may include no averaging or filtering of measurement values or may include less averaging or filtering than what is performed for L3 measurement reports.

[0029] In LTM, the source CU may receive a L3 measurement report (possibly via a source DU), and then on that basis, may prepare one or more HO configurations for UE handover. The source CU may transmit to the UE (via a source DU that is serving the UE), a RRC reconfiguration message to provide a measurement configuration to configure the UE to perform LI measurements of cells, and a configuration (or handover configuration) of one or more prepared LTM candidate target cells. A HO configuration for each prepared LTM candidate target cell may include, e.g., beam information of the target cell and / or other configuration information for the UE to communicate with the target cell.

[0030] The UE may then measure a set of cells (e.g., UE may measure RSRP or RSRQ of reference signals received from the set of cells) and may send LI measurement reports to the source DU that is serving the UE. On the basis of these LI measurement reports from the UE, the source DU associated with the source CU for the UE may change the UE’s serving cell(s) by a cell switch command by sending to the UE a MAC control element (MAC CE), which indicates a LTM candidate (target) cell configuration that the gNB or CU previously prepared and provided to the UE through RRC signaling. Thus, a cell switch for LTM HO is triggered by the network node selecting a LTM candidate cell configuration (e.g., having a strongest RSRP or RSRQ or other signal parameter based on the received LI measurement report) as the target configuration by the network node and indicating this target cell or HO configuration for this target cell to the UE via MAC CE. The UE may then perform a random access procedure to establish a connection with the indicated target cell.

[0031] Figure 2 shows one possible L1 / L2 Triggered Mobility procedure. In step 200, a UE (e.g. UE 120) is in RRC connected mode with the source cell (e.g. with the network node 112). In step 202, the UE sends a L3 measurement report to the source cell (e.g. source CU in this step). Based on this, the source gNB in step 204 triggers LTM candidate target cell preparation and in step 206 transmits RRC message to the UE to configure the UE with the cell configurations of the LTM candidate target cells. The source gNB may also prepare some legacy HO cells to the UE. In step 208, the UE replies with RRC reconfiguration complete message. Steps 202-208 may be seen as part of so-called LTM preparation phase.

[0032] In step 210, LTM evaluation phase takes place. In this step the UE may measure the neighboring cells (e.g. the cells which were indicated in step 206) at layer 1.

[0033] In step 212, the UE sends a measurement report to the source gNB (which may be a source DU in this step, although depicted in the Figure as one gNB). In step 214, the source gNB makes the LTM decision to switch the UE to one of the prepared cells, and thus sends a cell switch command (e.g. MAC CE, instead of RRC message) in step 216. In step 218, the UE detaches from the source cell and starts applying target cell’s configuration. In step 220, the UE makes a random access process (RA- procedure / RACH-proce- dure) to the target cell. Steps 212-220 may be seen as part of so-called LTM execution phase.

[0034] Finally, in step 222, LTM is completed by the UE being connected to the target cell (and detached from the source cell).

[0035] In 3GPP Rel-18, as a further enhancement to LTM, a technique for LTM RACH- less access to target cell is considered. It means, that it is possible to skip RACH of step 220 in the LTM execution phase presented in Figure 2. As a result, UE can connect to target cell (step 222 in Figure 2) without RACH delay. To be able to start using the target cell and proceed with the LTM completion, the UE shall be provided with possibility of sending UL transmission over physical uplink shared channel (PUSCH). It should beneficially be done without additional admission control and in such a way that it is possible: a. to inform target cell about UE’s presence without involving any additional signaling. This causes issue #1: target cell must be informed that UE wants to start using it. b. to minimize interruption time between leaving source cell and starting the traffic at target cell. This causes issue #2: any kind of additional admission control increases interruption time. c. to confirm that UE can start using target cell. This causes issue #3: both UE and target cell need to be sure that UE can proceed in RRC-connected mode. d. to avoid wasting the resources at target cell as much as it is possible. This causes issue #4: target cell shall not be forced to reserve too many resources for too long. Reserved resources shall be used in the most efficient way possible.

[0036] A variety of techniques are being considered with regards to how UE can start using target cell quickly while avoiding additional communication between source cell and target cell to announce UE’s presence in target cell. In particular, three solutions have been discussed at 3GPP in the context of 3GPP Rel-18:

[0037] • Configured Grant (CG) provided to UE in LTM preparation phase and / or in LTM cell switch command to allow the UE to perform a first UL transmission to a target cell to announce its presence.

[0038] ■ This solution addresses issue #1 since UE is able to start sending RRCReconfigurationComplete message using a CG.

[0039] ■ This solution also addresses issue #2, due to having an active grant, i.e., CG, to be used right away.

[0040] ■ This solution can address issue #3 but the solution needs to be enhanced slightly to define some kind of confirmation for the UE (e.g., new transmission grant after successful reception of first PUSCH transmission at MAC level)

[0041] ■ However, this solution does not address issue #4. CG resources needs to be reserved in LTM preparation phase and kept for unknown period. It can happen that they will never be used.

[0042] ■ In addition, the target cell does not know when the UE wants to start using the CG. Therefore, the network node of the target cell must be prepared for PUSCH discontinuous transmission (DTX). In legacy cases, PUSCH DTX is always an abnormal situation (e.g., PDCCH decoding problem, or discontinuous reception (DRX) misalignment). In this case, it may have to be treated as expected situation. It means that target cell is not able to differentiate between PUSCH DTX caused by the fact that UE has not started using it, and the situation when there are real problems with PUSCH transmission from the UE.

[0043] ■ Another problem with this solution is that it needs to be specified how to de-configure or de-activate CGs after synchronization to the target cell is done, and when such de-activation shall be done (typically, on-shot CG is being proposed).

[0044] • Dynamic Grant (DG) to allow UE to perform the first transmission to the target cell. It is feasible since, per agreements at 3GPP level, UE is obliged to listen to target cell physical downlink control channel (PDCCH) immediately after the cell switch.

[0045] ■ This solution addresses issue #1. However, it needs an explicit trigger sent by the source cell to the target cell once the source cell decides to trigger a cell switch of the UE. Additional communication between the source cell and the target cell increases interruption time (even in intra-DU case). It may be specifically problematic in inter-DU and inter-CU cases when the source cell communicates with the target cell over F1AP and NGAP (inter-CU scenario).

[0046] ■ This solution does not address issue #2. Although in the intra-DU case when additional communication between source cell and target cell can be quick, the loss is small.

[0047] ■ This solution can address issue #3 but the solution needs to be enhanced slightly to define some kind of confirmation for the UE (e.g., new transmission grant after successful reception of first PUSCH transmission at MAC level).

[0048] ■ This solution addresses issue #4 since the start of dynamic granting UE is explicitly indicated to the target cell by the source cell.

[0049] • A mix of two above-mentioned methods with either 3GPP-specified or vendor-specific prioritization. Typically, DG is prioritized higher than CG.

[0050] ■ A mix of solution 1 and solution 2 described above can give some benefits. E.g., the target cell can “cancel” CG and start handling UE’s PUSCH transmission in legacy way what would make hybrid automatic repeat request (HARQ) retransmissions more straightforward and could also help with DTX interpretation.

[0051] ■ However, this solution still does not help with addressing all above mentioned issues.

[0052] ■ Further, it complicates the procedure since it must be specified whether to prioritize CG over DG, or vice versa.

[0053] In all above-mentioned solutions, PUSCH transmission is assumed to be the first transmission between the UE and the target cell after the UE enters the coverage area of the target cell, and the source cell has sent the cell switch command. With this first transmitted message, the UE announces its presence in the target cell. As can be seen, the above three solutions still fail to address all issues in an efficient manner. Therefore, there is proposed an enhanced solution for RACHless LTM handover. This solution utilizes resources of a scheduling request indicator (SRI) for announcing UEs presence in the coverage area of the target cell. The following provides an overview of the proposed approach for such enhanced RACH-less LTM handover, with some references to Figures 3A and 3B which are described later in more details.

[0054] • Initial phase (UE in RRC-Connected state in the source cell) o UE is capable of SRl-based target cell access and sends a dedicated UE capability indication to the source cell (step 301 in Figure 3A). UEs which are incapable of SRl-based target cell admission may continue with legacy LTM.

[0055] • LTM preparation phase o The source cell sends LTM HO requests (step 304A in Figure 3A), asking each candidate target cell to assign LTM SRI resources (in at least one of time, frequency, spatial, code domains) for the UE. o Each candidate target cell provides a SRI configuration ‘LTM-SRl-Con- figuration’ that comprises the LTM SRI resources (i.e. SRI resource configuration) for each of one or more beams (steps 304C-304D in Figure 3A). o For example, it may be that there is one SRI configuration per beam, and one or more beams for each candidate target cell. Then, the SRI configurations may comprise: Candidate target cell 1: (beaml_l, SR11_1), (beaml_2, SR11_2); Candidate target cell 2: (beam2_l, SR12_1), (beam2_2, SR12_2), (beam2_3, SR12_3); and Candidate target cell 3: (beam3_l, SR13_1), where ‘SRlx’ stands for the SRI resource configuration. o In an embodiment, the SRI configurations for different beams may be different. However, in another embodiment, the UE could be configured with the same SRI resource configuration for all configured beams of a given candidate target cell. o In an embodiment, each SRI configuration (also called indication of the assigned SRI resources) can have separate PUCCH resource configuration other than previously configured UE-specific dedicated PUCCH configuration. In another embodiment, the SRI configuration can point to an existing PUCCH configuration with additional changes (e.g. delta configuration) to indicate different time-domain / frequency-domain location, in order to provide beam specific SRI resource(s). o The source cell configures the UE with the LTM SRI resources to be used for admission to the target cell (on per candidate target cell basis). This is reflected in step 306 of Figure 3A. This step may comprise sending the ‘LTM-SRl-Configuration’ to the UE.

[0056] • LTM evaluation phase (step 310 of Figure 3B) o The UE is kept synchronized with candidate target cells, l.e. the UE may have obtained, in connection of this step or earlier (e.g. before or in connection of the transmission of the L3 measurement report), synchronization with one or more of the candidate target cell. o The UE keeps measuring candidate target cells in accordance with L1 / L2 measurements configuration obtained from the network, possibly in connection of step 306 of Figure 3A.

[0057] • LTM execution phase o The UE sends in step 312 of Figure 3B a report of LI radio signal measurements to the source cell. o The source cell decides to trigger the UE to be LTM handed over to a certain candidate target cell in step 314 of Figure 3B. o The source cell refrains from indicating UE’s movement to a chosen candidate target cell. This is due to the SRl-based admission the UE is able to utilize. Thus, there is no need for the source cell to inform the target cell about an approaching UE which is under LTM handover, because the UE can do that itself by using the SRI resources known to both the UE and the target cell. o The source cell triggers, in step 316, LTM cell switch. The source may in the cell switch command indicate the beam to be used for the target cell, e.g. by means of a beam index. In one embodiment, SRI resources can be indicated in the cell switch command.

[0058] • LTM completion o The UE selects SRI resource(s) based on the indicated beam and sends SRI to the target cell using the selected resource(s) (step 320A of Figure 3B). o The target cell provides a grant to the UE in step 320C. As the target cell may have reserved the SRI for LTM completion purpose, the target cell knows that this SRI is for LTM HO purposes and can directly proceed to sending an uplink grant via PDCCH (CRC scrambled using target cell’s C-RNT1), where the grant can be used by the UE to send RRC-Re- configuration-complete. If the SRI is a dedicated resource for LTM purposes, the UE can consider the reception of the uplink grant as an LTM completion. o Consequently, the UE sends RRCReconfigurationComplete message to the target cell in step 320D of Figure 3B). o Once UE is granted by the target cell in a reaction to SRI transmission, the UE can be sure that it can proceed with LTM completion (sending of RRCReconfigurationComplete). This grant can thus be treated as target cell’s confirmation that the UE is allowed to continue using target cell.

[0059] • LTM Redirection based on admission control at target cell o As the target DU is not aware of the arrival of UE prior to cell switching, it may not have resources to admit the UE on reception of the assigned SRI. o In this case, if there are other LTM cells prepared for switching, the downlink DC1 of step 320C of Figure 3B may provide an alternative LTM cell-index of the same DU, instead of the uplink grant. o If no alternative target cell is available in the same DU, the target DU shall not send the PDCCH in response to the SRI. In this case UE may time-out (e.g. based on timer T304) and attempt for LTM recovery, according to legacy LTM procedures. o As another option, the UE may retransmit SRI until a predetermined number of retransmissions is met. If still no reply from the network, the UE may attempt a RA procedure. If it fails, the UE may try RRC connection re-establishment.

[0060] The benefit of allocating SRI resources for SRI transmission as the first transmission in the target cell, instead of some other resources and some other transmission, is that using SRI resources may allow to limit the amount of reserved resources. This is partly because PUCCH format 0 allows to multiplex more than one UE within single PRB. In addition, SRls allows to avoid additional messaging between the source cell and the target cell because the target cell is informed directly by the UE. Moreover, after sending the first transmission (SRI) in the target cell, the UE needs an UL grant for sending the RRCReconfigurationComplete -message. This is exactly the purpose of SRI, i.e. to request an UL grant. This allows only small changes to legacy operation.

[0061] Now, let us take a closer look at Figures 3A and 3B which depicts an example signaling flow diagram for the proposed solution. This Figure depicts the solution for in- ter-CU LTM HO. However, the same solution is applicable to intra-CU inter-DU scenario (in which case there is no need for over-NGAP messaging between CUs) and intra-CU in- tra-DU scenario.

[0062] In step 300, the UE (e.g. UE 120 of Figure 1) is in RRC connected state with the source gNB, comprising a source central unit (S-gNB-CU) and a source distributed unit (S-gNB-DU). The source gNB may be e.g. gNB 110 of Figure 1 and the target gNB may be e.g. gNB 112 of Figure 1. A source cell may be cell 100 in Figure 1 and the target cell (candidate target cell) may be cell 102 in Figure 1 (only one shown in Figure 1 for simplicity). As can be seen from Figure 1, the UE 120 is moving from the coverage area of cell 100 to the coverage area of cell 102, which may be the cause to trigger the LTM HO for the UE 120.

[0063] In step 301, the UE may inform the source CU that the UE is capable of SR1- based admission to LTM target cell (i.e. inform UE’s capability). That is, the source CU receives, from the UE, a capability indication indicating that the UE is capable of applying SRI resources for performing a transmission in connection of the LTM handover. The messaging may go via the source DU..

[0064] In step 302, the UE sends a L3 measurement report to the source CU. The measurement report indicates results of radio signal measurements, such as of the serving cell and / or of one or more neighboring cells. Based on this measurement report, the source CU may determine that the user equipment may need to be handed over to a candidate target cell. Consequently, the source CU determines to start a preparation of LTM handover of the UE from the source cell associated with a first network node (e.g. the S- CU and / or the S-DU) to a candidate target cell associated with a second network node (e.g. T-DU and / or T-CU). The preparation of LTM handover (also called in the Figure ‘LTM preparation’ phase) comprises steps 302-308.

[0065] In step 304A, the source CU sends LTM HO request to the target CU associated with the candidate target cell. The source CU identifies the request as inter-CU request and sends an LTM HO Request to target CU (handling the candidate target cell). This request causes a preparation of one or more candidate LTM target cells. The request also comprises a request for SRI resources (which may be e.g. PUCCH Format 0 resource(s) and associated beam(s)) for the UE. That is the LTM HO now further serves as a request to assign SRI resources for the UE.

[0066] This request is sent over Xn-AP (or NG-AP through CN if there is no active Xn link available). The request may be sent to more than one candidate target cell, depending on the content of the measurement report of step 302.

[0067] Unlike shown in the Figure, in intra-CU (inter DU) scenario, the LTM HO requests of step 304A would be sent to one or more target DUs under the control of the source CU. That is, there would be one CU associated with one source DU and one or more candidate target DUs.

[0068] In step 304B, the target CU, after receiving the request, sends (e.g. over Fl-AP interface) the LTM HO Request to the relevant target DU.

[0069] In step 304C, the target DU determines / assigns / allocates SRI resources for use by the user equipment in connection of LTM handover of the UE. In one embodiment, the assigned SRI resources are dedicated for the UE and assigned per candidate target cell basis and optionally also per beam basis. The amount of time and frequency domain resources to assign for the SRI transmission may be predetermined. Each beam of the target cell may be assigned a set of SRI resources. In an embodiment, if the target DU cannot in step 304C assign SRI resources, a fallback to legacy LTM (of Figure 2) is performed (i.e. LTM continues in legacy way).

[0070] In one embodiment, the SRI resources are selected from a pool of SRI resources, wherein the SRI resources of the pool are available both for LTM purposes and for legacy SRI purposes. In one embodiment, the SRI resources are selected from a pool of SRI resources, which are dedicated for LTM purposes only (similarly as in case of CFRA resources). This embodiment may be beneficial for a system with lesser loading with limited number of active users.

[0071] In step 304D, the target DU sends to the target CU a LTM HO Response containing the requested SRI resources (e.g. PUCCH Format 0) and optionally also an indication of at least one beam to which the assigned SRI resources are associated with (i.e. the UE may use the SRI resources when contacting the cell on a specific beam).

[0072] In step 304E, the target CU identifies the LTM HO response as an inter-CU message and sends (forwards) the LTM HO response, containing the requested SRI resources (e.g. PUCCH Format 0 + beams) to the source CU. This message may be sent over Xn-AP (or NG-AP if there is no active Xn link available). This message may serve as an indication of the assigned SRI resources to the source CU.

[0073] In step 304F, UE context modification is requested within S-gNB (source DU is thereby configured with SRI resources and beam(s)). The source DU responses to the source CU regarding the UE context modification request.

[0074] In step 304G, the source CU triggers sending of an LTM HO preparation message to the UE over RRC via the source DU. This RRC message contains the assigned SRI resources (e.g. PUCCH Format 0 resources). These resources may define time and frequency domain resources for sending the SRI. The message of step 304G may also comprise the information of which beams are applicable in each of the candidate target cell(s). In an embodiment, a beam may also be seen as a resource and may be called a spatial resource for the SRI transmission, and thus a term ‘SRI resources’ (also called SRI resource configuration or SRI configuration) may include also an indication of a beam for sending the SRI.

[0075] In step 306, the source cell (source DU) sends an RRCReconfiguration message to the UE. It contains the LTM HO preparation message and thus the assigned SRI resources to be used by the UE during admission to any of candidate target cell on per candidate target cell (e.g. the SRI resources are candidate target cell specific). As will be explained, the SRI resources are useable by the UE (e.g. dedicated for this UE) for performing a transmission (e.g. the first transmission to announce the presence of the UE in the target cell) in connection of the LTM handover of the UE from the source cell to the target cell (e.g. after the UE moving to the target cell). The message of step 306 may also comprise the information of which beams are applicable in each of the candidate target cell(s).

[0076] In step 308, the UE responds back with RRCReconfigurationComplete message to the source DU. At this point the UE is aware of the assigned SRI resources (of one or multiple candidate target cells) and beams corresponding to the SRI resources, and the candidate target cells are aware of assigned SRI resources a UE may use when moving under the coverage area of a particular candidate target cell. In an embodiment, the UE is thus aware of multiple SRI resource configurations (each configuration indicating a set of resources (including beam) for a given candidate target cell).

[0077] Although shown for one candidate target cell, similar SRI resource request may in an embodiment be performed for a plurality of candidate target cells. For example, the source CU may, based on the measurement report of step 302, determine a plurality of candidate target cells for the LTM handover of the UE. Then, the source CU may send the LTM HO request with SRI resource assignment request to many candidate target cells, and then receive a set of assigned SRI resources from each of the candidate target cells. Then, the general term “assigned SRI resources” may comprise a set of assigned SRI resource for each of the candidate target cells. The RRC message of step 306 may indicate the set of SRI resources for each candidate target cell. Different sets of SRI resources may different (i.e. do not share any SRI resource) or they may be partially overlapping.

[0078] As said earlier, in an embodiment, the message comprising the assigned SRI resources further comprises at least one beam useable by the UE in connection of the LTM handover. Each of the at least one beam has at least one assigned SRI resource among the assigned SRI resources. This way the UE may know which SRI resource to use, when the UE knows or determines the beam to be used for transmitting to a particular candidate target cell.

[0079] In step 310, it is assumed that the UE keeps DL / UL synchronization with candidate target cell(s) to allow for RACH-less access in any of the respective cells. In addition, in step 310 the UE measures candidate target cell(s) and is configured with relevant LI measurements to be able to report to the source DU an LI measurement report, suitable for LTM HO purposes.

[0080] In step 312, the LI measurement report is sent by the UE. The measurement report may indicate a need of switching the cell. This may be seen to start LTM HO execution phase.

[0081] In step 314, the source DU makes LTM HO decision (e.g. determine to perform the LTM handover), based on LI measurement report. The decision comprises selecting one of the candidate target cells to which the UE is to be moved, based on the measurement report. The source DU may also determine the beam the UE is to use for connecting the selected target cell. How the source DU determines the exact beam may be done as in legacy LTM scheme. The source DU may determine the beam e.g. based on the LI measurement report obtained in step 312.

[0082] Consequently, in step 316 the source DU sends an LTM cell switch command, which may be a MAC CE, to the UE. This may be called a second indication. The cell switch command contains an indication of the target cell for the LTM handover (so that the UE may determine or become aware of the target cell to access in connection of the LTM handover) and an indication of which beam shall be used by the UE to transmit SRI. The beam may correspond to one of the at least one beam-specific set of SRI resources, i.e. the beam is associated with at least one assigned SRI resource. As the UE is aware of the beam specific SRI resources, the UE also know which time and frequency resources to use for that beam to send the SRI. In another embodiment, the cell switch command comprises a direct indication the SRI resources to be used for performing the transmission to the target cell’s target DU. In an embodiment, the indication of the beam in the cell switch command is an indication of the SRI configuration to be used for performing the transmission to the target cell’s target DU.

[0083] In case the RRC message of step 306 (“indication”) comprises a set of SRI resources for each of a plurality of candidate target cells, then the source DU in step 314 selects a target cell among the plurality of candidate target cells, based on the measurement report possibly indicates measurement results for many candidate target cells. In this case, the cell switch command (“the second indication”) indicates at least one SRI resource corresponding the selected target cell.

[0084] In step 317, the UE may detach from the source cell and apply target cell’s configuration received already in the RRC message of step 306. Alternatively, the detachment from the source cell may be performed only after connection to the target cell is established.

[0085] In step 320A, the UE performs a transmission to the target DU associated with the target cell on the at least one SRI resource of the assigned SRI resources. The SRI resource may be based on the target cell and beam indicated in step 316 (cell switch command). That is, the UE sends the SRI to target cell using PUCCH Format 0 resources configured by the target DU already in step 304C on the beam indicated in step 316. The beam may be associated with C-RNT1 of the target cell. The solution thus proposes a new trigger to send SRI. The new trigger comprises the UE being assigned SRI resources for LTM HO purposes and the UE moving to a target cell for which the UE has assigned SRI resources. Then the SRI may be transmitted as a first transmission after moving to the target cell. Alternatively, the trigger to send the SRI may be a legacy one, which may be e.g. a buffer status report needs to be sent.

[0086] The uplink beam to be used for transmitting the SRI may correspond to one of a plurality of downlink beams of the target cell. E.g. the UE may e.g. select the strongest DL beam (and possibly indicate that in the LI measurement report). The source DU may then in the MAC CE cell switch command of step 316 inform the beam to be used in the target cell (or the UE may already know the beam to be used when receiving information of the target cell in the cell switch command). Then, the UE may perform the first transmission to the candidate target cell on an uplink beam that corresponds to the selected DL beam (based on reciprocity) and on resources (comprised in the assigned SRI resources) that correspond to the selected beam. In one example, there may be three DL beams in the cell and the UE detects that DL beam index #2 is the strongest. Then, the UE uses an uplink beam corresponding (according to a reciprocity principle) to the DL beam index #2 for transmission of the SRI, and uses SRI resources corresponding to beam index #2.

[0087] In step 32 OB, after the target DU receiving the SRI transmission from the user equipment on at least one of the assigned SRI resources, the target DU may identify the UE based on the transmission being received on at least one of the assigned SRI resources. For example, the assigned SRI resources are specific to the UE and comprise at least one beam-specific set of SRI resources, and the transmission from the UE is received on a beam which corresponds to one of the at least one beam-specific set of SRI. This way the UE may know that the UE transmitting this SRI is the UE for which the target DU has already assigned SRI resources in step 304C. In other words, the target DU recognizes the UE based on resources in use. Hence, the target DU knows the purpose of the SRI transmission.

[0088] In the next step 320C, the target DU allocates, to the UE and based on receiving the transmission of step 320A on at least one of the assigned SRI resources, further resources for transmission of a radio resource control (RRC) reconfiguration complete message. That is, the target DU knows that PUSCH scheduling is needed for sending RRCRe- configurationComplete after recognizing the UE in step 320B. As a result, the target DU can assign enough resources to send the RRC reconfiguration complete message (instead of resources sufficient only for sending regular BSR, as would be done in response to receiving a SRI in legacy operations). The PDCCH message of step 320C may comprise an UL grant and may be CRC scrambled with the C-RNT1 of the UE in the target cell.

[0089] A UE not receiving the allocation message of step 320C, can be an indication of admission rejection but can also be the result of decoding problems. In an embodiment this is handled as follows: the UE may retransmit the SRI of step 320A up to predetermined SRI transmission threshold (which may be configured by the target DU in step 304C and indicated to the UE in the RRC message of step 306 or may be prespecified by standard specifications). If there is not response despite of re-transmissions, then legacy LTM HO failure handling may be applied.

[0090] In an embodiment, the UE keeps on using the assigned SRI resources in the target cell (=new serving cell) until they are explicitly released / re-configured by this new serving cell. This ensures at least some dedicated resources in the new serving cell. In another embodiment, SRI resources are implicitly released by the UE once the message of step 302C is received. This is efficient way of releasing the resources without dedicated messaging.

[0091] In step 320D, the UE sends the RRCReconfigurationComplete message on PUSCH to the target cell (=new serving cell) by using PUSCH resources assigned in step 320C. This completes the enhanced LTM.

[0092] As maybe deduced from above description of Figures 3A and 3B, the proposal avoids the need of RACH process (i.e. it is RACH-less access scenario). Moreover, it addresses all the issues #l-#4 raised above efficiently.

[0093] Figures 4 to 7 show example methods for the enhanced LTM handover. The methods may be computer-implemented. In these Figures it is assumed that a first network node may be e.g. the source base station (e.g. gNB 110) or a central unit of the source base station 110 (in case a distributed RAN architecture is in use), a second network node is a target base station (e.g. gNB 112) or a distributed unit of the target base station 112 (in case a distributed RAN architecture is in use), and a third network node is a distributed unit of the source base station (e.g. of the gNB 110, when distributed RAN architecture is in use).

[0094] Figure 4 shows an example method for the enhanced LTM handover. The method may be performed by the first network node. The gNB 110 may determines to start a preparation of LTM handover of a UE from the source cell (e.g. cell 100) associated with the gNB 110 to a candidate target cell (e.g. 102) associated with the second network node (e.g. gNB 112, or a CU or a DU of the target base station 112). Term “associated with” in this context throughout the application may mean e.g. that the corresponding node provides coverage area for the corresponding cell or that the node controls communication in the corresponding cell. For example, gNB 110 manages the cell 100. In case of distributed RAN, a CU of gNB 110 may control the cell 100 via the DU, or the DU of gNB 110 may itself control at least lower layers within the cell 100. In step 400, the first network node transmits, to the second network node, a request to assign SRI resources for the UE. In step 402, the first network node receives, from the second network node, an indication of the assigned SRI resources (e.g. ‘LTM-SRI-Configuration’). In step 404, the first network node transmits the indication of the assigned SRI resources to the UE, wherein the SRI resources are useable by the user equipment for performing a transmission (e.g. the first transmission to announce UE’s presence in the target cell) in connection of the LTM handover of the user equipment from the source cell 100 to the candidate target cell 102. Figure 5 shows an example method for the enhanced LTM handover. The method may be performed by the second network node. In step 500, the second network node receives, from the first network node associated with a source cell 100 of the LTM handover, a request to assign SRI resources for a UE 120 that may be handed over to the target cell 102, wherein the candidate target cell 102 is associated with the second network node. . In step 502, the second network node assigns the SRI resources for the user equipment, and in step 504 transmits an indication of the assigned SRI resources to the first network node. In step 506, the second network node receives a transmission from the UE 120 on at least one of the assigned SRI resources in connection of the LTM handover of the UE 120 from the source cell 100 to the candidate target cell 102. The second network node may in step 508 identify the UE based on the transmission being received on at least one of the assigned SRI resources.

[0095] Figure 6 shows an example method for the enhanced LTM handover.. The method may be performed by the third network node. In step 600, the third network node receives, from the first network node associated with the source cell of the LTM handover, an indication (e.g. ‘LTM-SRl-configuration’) of assigned SRI resources for the UE 120, the SRI resources being assigned by the second network node associated with the candidate target cell 102. In step 602, the third network node transmits the indication of the assigned SRI resources to the UE 120. This may happen as mere receive-and-forward operation, or the third network node may decode the message and extract the information of the indication before transmitting the indication to the UE 120. In step 604, the third network node determines to perform the LTM handover of the UE 102 from the source cell 100 to the candidate target cell 102, and in step 606 third network node transmits, to the UE 120, a second indication (e.g. ‘cell switch command’) indicating at least one of the assigned SRI resources (e.g. via indicating the beam associated to certain SRI resources) to be used by the UE 120 for transmitting a message the candidate target cell in connection of the LTM handover.

[0096] Figure 7 shows an example method for the enhanced LTM handover. The method may be performed by a user equipment, such as the UE 120. In step 700, the UE 120 receives, from the first network node associated with the source cell 100 of the LTM handover, an indication of assigned SRI resources for the UE 120. This may be e.g. RRC reconfiguration message. In step 702, the UE 120 determines the target cell 102 to access in connection of the LTM handover. This may be determined based on the cell switch command from the source DU. The cell switch command may also indicate the beam to be used in the target cell for sending the SRI. Therefore, the UE may also determine which beam to use for sending the SRI to the target cell 102. In step 704, the UE performs a transmission (of the SRI) to the second network node associated with the target cell 102 on at least one SRI resource of the assigned SRI resources. The SRI transmission may be performed on the determined / indicated beam.

[0097] An embodiment, as shown in Figure 8, provides an apparatus 10 comprising a control circuitry (CTRL) 12, such as at least one processor, and at least one memory 14 storing instructions that, when executed by the at least one processor, cause the apparatus at least to carry out any one of the above-described processes. The control circuitry 12 may comprise relevant circuitry / ies for performing the functions, according to any of the embodiments. In an example, the at least one memory and the computer program code (software), are configured, with the at least one processor, to cause the apparatus to carry out any one of the above-described processes. The memory may be implemented using any suitable data storage technology, such as semiconductor-based memory devices, flash memory, magnetic memory devices and systems, optical memory devices and systems, fixed memory and removable memory. The memory may comprise a database for storing data.

[0098] The apparatus may further comprise a radio interface (TRX) 16 comprising hardware and / or software for realizing communication connectivity according to one or more communication protocols. The TRX may provide the apparatus with communication capabilities. The apparatus may also comprise a user interface 18 comprising, for example, at least one keypad, a microphone, a touch display, a display, a speaker, etc. The user interface may be used to control the apparatus by the user.

[0099] In another embodiment, the apparatus 10 may be or be comprised in a first network node, such as the gNB 110 or CU of the gNB 110. The apparatus may be caused to execute some of the functionalities of the above described processes, such as the steps of Figure 4 and / or some of the steps of Figures 3A-3B.

[0100] In another embodiment, the apparatus 10 may be or be comprised in a second network node, such as the gNB 112 or DU of the gNB 112. The apparatus may be caused to execute some of the functionalities of the above described processes, such as the steps of Figure 5 and / or some of the steps of Figures 3A-3B.

[0101] In another embodiment, the apparatus 10 may be or be comprised in a third network node, such as the gNB 110 or DU of the gNB 110. The apparatus may be caused to execute some of the functionalities of the above described processes, such as the steps of Figure 6 and / or some of the steps of Figures 3A-3B.

[0102] In an embodiment, the apparatus 10 may comprise the terminal device of a communication system. In an embodiment, the apparatus 10 is or is comprised in the UE 120. The apparatus may be caused to execute some of the functionalities of the above described processes, such as the steps of Figure 7 and / or some ofthe steps of Figures 3A- 3B.

[0103] In an embodiment, an apparatus carrying out at least some of the embodiments described comprises at least one processor and at least one memory including a computer program code, wherein the at least one memory and the computer program code are configured, with the at least one processor, to cause the apparatus to carry out the functionalities according to any one of the embodiments described. According to an aspect, when the at least one processor executes the computer program code, the computer program code causes the apparatus to carry out the functionalities according to any one of the embodiments described. According to another embodiment, the apparatus carrying out at least some of the embodiments comprises the at least one processor and at least one memory including a computer program code, wherein the at least one processor and the computer program code perform at least some of the functionalities according to any one of the embodiments described. Accordingly, the at least one processor, the memory, and the computer program code form processing means for carrying out at least some of the embodiments described. According to yet another embodiment, the apparatus carrying out at least some of the embodiments comprises a circuitry including at least one processor and at least one memory including computer program code. When activated, the circuitry causes the apparatus to perform the at least some of the functionalities according to any one of the embodiments described.

[0104] As used in this application, the term ‘circuitry’ refers to all of the following: (a) hardware-only circuit implementations, such as implementations in only analog and / or digital circuitry, and (b) combinations of circuits and soft- ware (and / or firmware), such as (as applicable): (i) a combination of processor(s) or (ii) portions of processor(s) / soft- ware including digital signal processor(s), software, and memory(ies) that work together to cause an apparatus to perform various functions, and (c) circuits, such as a micropro- cessor(s) or a portion of a microprocessor(s), that require software or firmware for operation, even if the software or firmware is not physically present. This definition of ‘circuitry’ applies to all uses of this term in this application. As a further example, as used in this application, the term ‘circuitry’ would also cover an implementation of merely a processor (or multiple processors) or a portion of a processor and its (or their) accompanying software and / or firmware. The term ‘circuitry’ would also cover, for example and if applicable to the particular element, a baseband integrated circuit or applications processor integrated circuit for a mobile phone or a similar integrated circuit in a server, a cellular network device, or another network device.

[0105] In an embodiment, at least some of the processes described may be carried out by an apparatus comprising corresponding means for carrying out at least some of the described processes. Some example means for carrying out the processes may include at least one of the following: detector, processor (including dual-core and multiple-core processors), digital signal processor, controller, receiver, transmitter, encoder, decoder, memory, RAM, ROM, software, firmware, display, user interface, display circuitry, user interface circuitry, user interface software, display software, circuit, antenna, antenna circuitry, and circuitry.

[0106] A term non-transitory, as used herein, is a limitation of the medium itself [i.e. tangible, not a signal) as opposed to a limitation on data storage persistency [e.g. RAM vs. ROM).

[0107] As used herein the term “means” is to be construed in singular form, i.e. referring to a single element, or in plural form, i.e. referring to a combination of single elements. Therefore, terminology “means for [performing A, B, C]”, is to be interpreted to cover an apparatus in which there is only one means for performing A, B and C, or where there are separate means for performing A, B and C, or partially or fully overlapping means for performing A, B, C. Further, terminology “means for performing A, means for performing B, means for performing C” is to be interpreted to cover an apparatus in which there is only one means for performing A, B and C, or where there are separate means for performing A, B and C, or partially or fully overlapping means for performing A, B, C.

[0108] The techniques and methods described herein may be implemented by various means. For example, these techniques may be implemented in hardware [one or more devices), firmware [one or more devices), software [one or more modules), or combinations thereof. For a hardware implementation, the apparatuses) of embodiments may be implemented within one or more application-specific integrated circuits [ASICs), digital signal processors [DSPs), digital signal processing devices [DSPDs), programmable logic devices [PLDs), field programmable gate arrays [FPGAs), processors, controllers, micro-controllers, microprocessors, other electronic units designed to perform the functions described herein, or a combination thereof. For firmware or software, the implementation can be carried out through modules of at least one chip set [e.g. procedures, functions, and so on) that perform the functions described herein. The software codes may be stored in a memory unit and executed by processors. The memory unit may be implemented within the processor or externally to the processor. In the latter case, it can be communicatively coupled to the processor via various means, as is known in the art. Additionally, the components of the systems described herein may be rearranged and / or complemented by additional components in order to facilitate the achievements of the various aspects, etc., described with regard thereto, and they are not limited to the precise configurations set forth in the given figures, as will be appreciated by one skilled in the art.

[0109] Embodiments as described may also be carried out in the form of a computer process defined by a computer program or portions thereof. Embodiments of the methods described may be carried out by executing at least one portion of a computer program comprising corresponding instructions. The computer program may be in source code form, object code form, or in some intermediate form, and it may be stored in some sort of carrier, which may be any entity or device capable of carrying the program. For example, the computer program may be stored on a computer program distribution medium readable by a computer or a processor. The computer program medium may be, for example but not limited to, a record medium, computer memory, read-only memory, electrical carrier signal, telecommunications signal, and software distribution package, for example. The computer program medium may be a non-transitory medium. Coding of software for carrying out the embodiments as shown and described is well within the scope of a person of ordinary skill in the art.

[0110] Following is a list of some aspects of the invention.

[0111] According to a first aspect, there is provided a method performed by a first network node, comprising: transmitting, to a second network node associated with a candidate target cell of a layer 1 / layer 2 triggered mobility (LTM) handover, a request to assign scheduling request indicator (SRI) resources for a user equipment that may be handed over from a source cell associated with the first network node to the candidate target cell; receiving, from the second network node, an indication of the assigned SRI resources; and transmitting the indication of the assigned SRI resources to the user equipment, wherein the SRI resources are useable by the user equipment for performing a transmission in connection of the LTM handover.

[0112] Various embodiments of the first aspect may comprise at least one feature from the following bulleted list:

[0113] • receiving, from the user equipment, a capability indication indicating that the user equipment is capable of applying SRI resources for performing the transmission in connection of the LTM handover.

[0114] • wherein the assigned SRI resources comprise physical uplink control channel (PUCCH) format 0 resources.

[0115] • transmitting the indication to the user equipment in an RRC Reconfiguration message.

[0116] • determining a plurality of candidate target cells for the LTM handover of the user equipment; request and receive a set of assigned SRI resources from each of the candidate target cells; and transmitting the received sets of assigned SRI resources within the indication to the user equipment.

[0117] • wherein the indication further comprises an indication of at least one beam useable by the user equipment in connection of the LTM handover, wherein each of the at least one beam corresponds to at least one assigned SRI resource among the assigned SRI resources.

[0118] According to a second aspect, there is provided a method performed by a second network node, comprising: receiving, from a first network node associated with a source cell of a layer 1 / layer 2 triggered mobility (LTM) handover, a request to assign scheduling request indicator (SRI) resources for a user equipment that may be handed over to candidate target cell, wherein the candidate target cell is associated with the second network node; assigning the SRI resources for the user equipment; transmitting an indication of the assigned SRI resources to the first network node; receiving a transmission from the user equipment on at least one of the assigned SRI resources in connection of the LTM handover of the user equipment from the source cell to the target cell; and identifying the user equipment based on the transmission received on at least one of the assigned SRI resources.

[0119] Various embodiments of the second aspect may comprise at least one feature from the following bulleted list:

[0120] • wherein the SRI resources are assigned from a pool of SRI resources which are reserved for being used in connection of the LTM handover.

[0121] • wherein the assigned SRI resources comprise at least one beam-specific set of SRI resources, and wherein the transmission from the user equipment is received on a beam which corresponds to one of the at least one beam-specific set of SRI resources.

[0122] • allocating, to the user equipment and based on receiving the transmission on at least one of the assigned SRI resources, further resources for transmission of a radio resource control (RRC) reconfiguration complete message.

[0123] According to a third aspect, there is provided a method performed by a third network node, comprising: receiving, from a first network node associated with a source cell of a layer 1 / layer 2 triggered mobility (LTM) handover, an indication of assigned scheduling request indicator (SRI) resources for a user equipment, the SRI resources being assigned by a second network node associated with a candidate target cell of the LTM handover; transmitting the indication of the assigned SRI resources to the user equipment; determining to perform the LTM handover of the user equipment from the source cell to the candidate target cell; transmitting, to the user equipment, a second indication indicating at least one of the assigned SRI resources to be used by the user equipment for transmitting a message in connection of the LTM handover in the candidate target cell.

[0124] Various embodiments of the third aspect may comprise at least one feature from the following bulleted list:

[0125] • wherein the indication of the assigned SRI resources comprises a set of SRI resources for each of a plurality of candidate target cells, and the method further comprises selecting a target cell among the plurality of candidate target cells, wherein the second indication indicates at least one SRI resource corresponding the selected target cell. • wherein the assigned SRI resources comprise at least one beam-specific set of SRI resources, and wherein the second indication indicates a beam which corresponds to one of the at least one beam-specific set of SRI resources, wherein the indicated beam is to be used by the user equipment for transmitting the message.

[0126] According to a fourth aspect, there is provided a method performed by a user equipment, comprising: receiving, from a first network node associated with a source cell of a layer 1 / layer 2 triggered mobility (LTM) handover, an indication of assigned scheduling request indicator (SRI) resources for the user equipment; determining a target cell to access in connection of the LTM handover; and performing a transmission to a second network node associated with a target cell on at least one SRI resource of the assigned SRI resources.

[0127] Various embodiments of the fourth aspect may comprise at least one feature from the following bulleted list:

[0128] • wherein determining the target cell comprises receiving a cell switch command, the cell switch command including an indication of the target cell.

[0129] • wherein the cell switch command includes an indication of at least one SRI resource to be used for performing the transmission to the second network node.

[0130] • wherein the cell switch command comprises an indication of a beam to be used by the user equipment when performing the transmission to the second network node, wherein the beam is associated with at least one assigned SRI resource.

[0131] • wherein the transmission is a first transmission in the target cell after moving to the target cell.

[0132] According to a fifth aspect, there is provided a first network node, comprising: at least one processor; and at least one memory storing instructions that, when executed by the at least one processor, cause the first network node at least to: transmit, to a second network node associated with a candidate target cell of a layer 1 / layer 2 triggered mobility (LTM) handover, a request to assign scheduling request indicator (SRI) resources for a user equipment that may be handed over from a source cell associated with the first network node to the candidate target cell; receive, from the second network node, an indication of the assigned SRI resources; and transmit the indication of the assigned SRI resources to the user equipment, wherein the SRI resources are useable by the user equipment for performing a transmission in connection of the LTM handover. Various embodiments of the fifth aspect may comprise at least one feature from the bulleted list under the first aspect. According to a sixth aspect, there is provided a second network node, comprising: at least one processor; and at least one memory storing instructions that, when executed by the at least one processor, cause the second network node at least to: receive, from a first network node associated with a source cell of a layer 1 / layer 2 triggered mobility (LTM) handover, a request to assign scheduling request indicator (SRI) resources for a user equipment that may be handed over to candidate target cell, wherein the candidate target cell is associated with the second network node; assign the SRI resources for the user equipment; transmit an indication of the assigned SRI resources to the first network node; receive a transmission from the user equipment on at least one of the assigned SRI resources in connection of the LTM handover of the user equipment from the source cell to the target cell; and identify the user equipment based on the transmission received on at least one of the assigned SRI resources. Various embodiments of the sixth aspect may comprise at least one feature from the bulleted list under the second aspect.

[0133] According to a seventh aspect, there is provided a third network node, comprising: at least one processor; and at least one memory storing instructions that, when executed by the at least one processor, cause the third network node at least to: receive, from a first network node associated with a source cell of a layer 1 / layer 2 triggered mobility (LTM) handover, an indication of assigned scheduling request indicator (SRI) resources for a user equipment, the SRI resources being assigned by a second network node associated with a candidate target cell of the LTM handover; transmit the indication of the assigned SRI resources to the user equipment; determine to perform the LTM handover of the user equipment from the source cell to the candidate target cell; transmit, to the user equipment, a second indication indicating at least one of the assigned SRI resources to be used by the user equipment for transmitting a message in connection of the LTM handover in the candidate target cell. Various embodiments of the seventh aspect may comprise at least one feature from the bulleted list under the third aspect.

[0134] According to an eight aspect, there is provided a user equipment, comprising: at least one processor; and at least one memory storing instructions that, when executed by the at least one processor, cause the user equipment at least to: receive, from a first network node associated with a source cell of a layer 1 / layer 2 triggered mobility (LTM) handover, an indication of assigned scheduling request indicator (SRI) resources for the user equipment; determine a target cell to access in connection of the LTM handover; and perform a transmission to a second network node associated with a target cell on at least one SRI resource of the assigned SRI resources. Various embodiments of the eight aspect may comprise at least one feature from the bulleted list under the fourth aspect.

[0135] According to a ninth aspect, there is provided a computer program product embodied on a distribution medium and comprising program instructions which, when executed by an apparatus, cause the apparatus to carry out the method according to any of the first to fourth aspects.

[0136] According to a tenth aspect, there is provided a computer program product comprising program instructions which, when executed by an apparatus, cause the apparatus to carry out the method according to any of the first to fourth aspects.

[0137] According to an eleventh aspect, there is provided an apparatus, comprising means for performing the method according to any of the first to fourth aspects, and / or means configured to cause the apparatus to perform the method according to any of the first to fourth aspects.

[0138] According to a twelfth aspect, there is provided computer implemented system, comprising: a server and at least one radio node; and at least one processor and at least one memory storing instructions that, when executed by the at least one processor, cause the system at least to carry out the method according to any of the first to fourth aspects.

[0139] Even though the invention has been described above with reference to an example according to the accompanying drawings, it is clear that the invention is not restricted thereto but can be modified in several ways within the scope of the appended claims. Therefore, all words and expressions should be interpreted broadly and they are intended to illustrate, not to restrict, the embodiment. It will be obvious to a person skilled in the art that, as technology advances, the inventive concept can be implemented in various ways. Further, it is clear to a person skilled in the art that the described embodiments may, but are not required to, be combined with other embodiments in various ways.

Claims

CLAIMS1. A first network node, comprising: at least one processor; and at least one memory storing instructions that, when executed by the at least one processor, cause the first network node at least to: transmit, to a second network node associated with a candidate target cell of a layer 1 / layer 2 triggered mobility (LTM) handover, a request to assign scheduling request indicator (SRI) resources for a user equipment that may be handed over from a source cell associated with the first network node to the candidate target cell; receive, from the second network node, an indication of the assigned SRI resources; and transmit the indication of the assigned SRI resources to the user equipment, wherein the SRI resources are useable by the user equipment for performing a transmission in connection of the LTM handover.

2. The first network node of claim 1, wherein the instructions, when executed by the at least one processor, cause the first network node further to: receive, from the user equipment, a capability indication indicating that the user equipment is capable of applying SRI resources for performing the transmission in connection of the LTM handover.

3. The first network node of any of claims 1 to 2, wherein the assigned SRI resources comprise physical uplink control channel (PUCCH) format 0 resources.

4. The first network node of any of claims 1 to 3, wherein the instructions, when executed by the at least one processor, cause the first network node further to: transmit the indication to the user equipment in an RRC Reconfiguration message.

5. The first network node of any of claims 1 to 4, wherein the instructions, when executed by the at least one processor, cause the first network node further to: determine a plurality of candidate target cells for the LTM handover of the user equipment; request and receive a set of assigned SRI resources from each of the candidate target cells; and transmit the received sets of assigned SRI resources within the indication to the user equipment.

6. The first network node of any of claims 1 to 5, wherein the indication further comprises an indication of at least one beam useable by the user equipment in connection of the LTM handover, wherein each of the at least one beam corresponds to at least one assigned SRI resource among the assigned SRI resources.

7. A second network node, comprising: at least one processor; and at least one memory storing instructions that, when executed by the at least one processor, cause the second network node at least to: receive, from a first network node associated with a source cell of a layer 1 / layer 2 triggered mobility (LTM) handover, a request to assign scheduling request indicator (SRI) resources for a user equipment that may be handed over to candidate target cell, wherein the candidate target cell is associated with the second network node; assign the SRI resources for the user equipment; transmit an indication of the assigned SRI resources to the first network node; receive a transmission from the user equipment on at least one of the assigned SRI resources in connection of the LTM handover of the user equipment from the source cell to the target cell; and identify the user equipment based on the transmission received on at least one of the assigned SRI resources.

8. The second network node of claim 7, wherein the SRI resources are assigned from a pool of SRI resources which are reserved for being used in connection of the LTM handover.

9. The second network node of any of claims 7 to 8, wherein the assigned SRI resources comprise at least one beam-specific set of SRI resources, and wherein the transmission from the user equipment is received on a beam which corresponds to one of the at least one beam-specific set of SRI resources.

10. The second network node of any of claims 7 to 9, wherein the instructions, when executed by the at least one processor, cause the second network node further to: allocate, to the user equipment and based on receiving the transmission on at least one of the assigned SRI resources, further resources for transmission of a radio resource control (RRC) reconfiguration complete message.

11. A third network node, comprising: at least one processor; andat least one memory storing instructions that, when executed by the at least one processor, cause the third network node at least to: receive, from a first network node associated with a source cell of a layer 1 / layer 2 triggered mobility (LTM) handover, an indication of assigned scheduling request indicator (SRI) resources for a user equipment, the SRI resources being assigned by a second network node associated with a candidate target cell of the LTM handover; transmit the indication of the assigned SRI resources to the user equipment; determine to perform the LTM handover of the user equipment from the source cell to the candidate target cell; transmit, to the user equipment, a second indication indicating at least one of the assigned SRI resources to be used by the user equipment for transmitting a message in connection of the LTM handover in the candidate target cell.

12. The third network node of claim 11, wherein the indication of the assigned SRI resources comprises a set of SRI resources for each of a plurality of candidate target cells, and wherein the instructions, when executed by the at least one processor, cause the third network node further to: select a target cell among the plurality of candidate target cells, wherein the second indication indicates at least one SRI resource corresponding the selected target cell.

13. The third network node of any of claims 11 to 12, wherein the assigned SRI resources comprise at least one beam-specific set of SRI resources, and wherein the second indication indicates a beam which corresponds to one of the at least one beamspecific set of SRI resources, wherein the indicated beam is to be used by the user equipment for transmitting the message.

14. A user equipment, comprising: at least one processor; and at least one memory storing instructions that, when executed by the at least one processor, cause the user equipment at least to: receive, from a first network node associated with a source cell of a layer 1 / layer 2 triggered mobility (LTM) handover, an indication of assigned scheduling request indicator (SRI) resources for the user equipment; determine a target cell to access in connection of the LTM handover; and perform a transmission to a second network node associated with a target cell on at least one SRI resource of the assigned SRI resources.

15. The user equipment of claim 14, wherein determining the target cell comprises receiving a cell switch command, the cell switch command including an indication of the target cell.

16. The user equipment of any of claims 14 to 15, wherein the cell switch command includes an indication of at least one SRI resource to be used for performing the transmission to the second network node.

17. The user equipment of any of claims 14 to 16, wherein the cell switch command comprises an indication of a beam to be used by the user equipment when performing the transmission to the second network node, wherein the beam is associated with at least one assigned SRI resource.

18. The user equipment of any of claims 14 to 17, wherein the transmission is a first transmission in the target cell after moving to the target cell.

19. A method performed by a first network node, comprising: transmitting, to a second network node associated with a candidate target cell of a layer 1 / layer 2 triggered mobility (LTM) handover, a request to assign scheduling request indicator (SRI) resources for a user equipment that may be handed over from a source cell associated with the first network node to the candidate target cell; receiving, from the second network node, an indication of the assigned SRI resources; and transmitting the indication of the assigned SRI resources to the user equipment, wherein the SRI resources are useable by the user equipment for performing a transmission in connection of the LTM handover.

20. A method performed by a second network node, comprising: receiving, from a first network node associated with a source cell of a layer 1 / layer 2 triggered mobility (LTM) handover, a request to assign scheduling request indicator (SRI) resources for a user equipment that may be handed over to candidate target cell, wherein the candidate target cell is associated with the second network node; assigning the SRI resources for the user equipment; transmitting an indication of the assigned SRI resources to the first network node; receiving a transmission from the user equipment on at least one of the assigned SRI resources in connection of the LTM handover of the user equipment from the source cell to the target cell; andidentifying the user equipment based on the transmission received on at least one of the assigned SRI resources.

21. A method performed by a third network node, comprising: receiving, from a first network node associated with a source cell of a layer 1 / layer 2 triggered mobility (LTM) handover, an indication of assigned scheduling request indicator (SRI) resources for a user equipment, the SRI resources being assigned by a second network node associated with a candidate target cell of the LTM handover; transmitting the indication of the assigned SRI resources to the user equipment; determining to perform the LTM handover of the user equipment from the source cell to the candidate target cell; transmitting, to the user equipment, a second indication indicating at least one of the assigned SRI resources to be used by the user equipment for transmitting a message in connection of the LTM handover in the candidate target cell.

22. A method performed by a user equipment, comprising: receiving, from a first network node associated with a source cell of a layer 1 / layer 2 triggered mobility (LTM) handover, an indication of assigned scheduling request indicator (SRI) resources for the user equipment; determining a target cell to access in connection of the LTM handover; and performing a transmission to a second network node associated with a target cell on at least one SRI resource of the assigned SRI resources.

23. A computer program product embodied on a distribution medium readable by a computer and comprising program instructions which, when the program is executed by an apparatus, cause the apparatus to carry out the method according to any of claims 19 to 22.

24. A computer program product comprising program instructions which, when the program is executed by an apparatus, cause the apparatus to carry out the method according to any of claims 19 to 22.

25. An apparatus, comprising means for performing the method according to any of claims 19 to 22

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