Handling of UL timing upon LTM execution

Abstract

Described herein is a User Equipment, UE, A User Equipment, UE, served by a first cell of a network node, the UE comprising: at least one processor; and at least one memory storing instructions that, when executed by the at least one processor, cause the UE at least to: determine a first condition for triggering a switch from the first cell towards a second cell; determine a second condition indicating that a Timing Alignment, TA, value associated with the second cell is available at the UE; and determine whether the cell switch involves a random access procedure towards the second cell, wherein based Advance the determined first condition and the determined second condition and based on determining a third condition indicating that the cell switch involves the random access procedure, the UE is further caused to perform at least one action relating to at least one of the following: a Time Alignment Timer, TAT, the available TA value, and a Timing Advance Command, TAC associated with the second cell and received from the network node.

Description

Handling of UL timing upon LTM executionTECHNOLOGY

[0001] The present disclosure relates to New Radio, NR, mobility, more specifically to L1 / L2 triggered mobility, LTM, in particular to UL timing upon LTM execution.BACKGROUND

[0002] Any discussion of the background art throughout the specification should in no way be considered as an admission that such art is widely known or forms part of common general knowledge in the field.

[0003] Currently, network (NW) cannot issue UE to perform Random Access (RA) based L1 / L2 Triggered Mobility (LTM) while the UE is configured with UE based timing measurement. This hinders the usefulness of the UE based TA measurement as NWs would very likely not use it as there is no way to override it (other than additional Radio Resource Control, RRC, reconfiguration which hinders the whole use case of LTM). Further, UE based TA measurement is not very reliable since the exact time the UE performs TA measurement is left up to UE implementation and therefore there is a need to enable such NW behaviour (enforcing RA based LTM) in 3 GPP as well.

[0004] Going further, when NW triggers RA based LTM while UE based TA measurement is configured, it is not clear what should be the UE behaviour with respect to the measured TA value. Based on the current specified behaviour, the UE would ignore the TAC provided over Random Access Response (RAR) and apply the UE measured TA value regardless of the fresh timing information NW provides in RAR. This would make the Random Access procedure also useless in this case. It should be noted that this problem arises in particular with Contention Based Random Access (CBRA) while with Contention Free Random Access (CFRA) UE applies the TAC provided in RAR always.

[0005] Hence, there is a need to provide specific UE Uplink (UL) behavior with respect to the available TA value when a RA procedure is involved in a cell switch procedure; there is also a need to provide specific UE behavior with respect to the RA procedure initiated by the network node, in particular with respect to timing information provided by NW; there is also a need to improve usefulness of the RA procedure and to improve efficiency of RA based cell switch.SUMMARY

[0006] In accordance with a first aspect of the present disclosure, there is provided aUser Equipment, UE, served by a first cell of a network node, the UE comprising: at least one processor; and at least one memory storing instructions that, when executed by the at least one processor, cause the UE at least to: determine a first condition for triggering a switch from the first cell towards a second cell; determine a second condition indicating that a Timing Advance, TA, value associated with the second cell is available at the UE; and determine whether the cell switch involves a random access procedure towards the second cell, wherein based on the determined first condition and the determined second condition and based on determining a third condition indicating that the cell switch involves the random access procedure, the UE is further caused to perform at least one action relating to at least one of the following: a Time Alignment Timer, TAT, the available TA value, and a Timing Advance Command, TAC associated with the second cell and received from the network node.

[0007] In some examples, the at least one action comprises determining not to start or to stop the TAT.

[0008] In some examples, the UE is further caused to determine not to start or to stop the TAT upon performing the cell switch.

[0009] In some examples, the UE is further caused to determine not to start or to stop the TAT upon acknowledging a cell switch command received from the network node.

[0010] In some examples, the at least one action comprises determining not to apply or to ignore the available TA value.

[0011] In some examples, the UE is further caused to receive the available TA value from the network node in a cell switch command.

[0012] In some examples, the UE is configured with UE based TA measurement and is further caused to obtain the available TA value based on UE measurement.

[0013] In some examples, the at least one action comprises determining to stop the UE based TA measurement.

[0014] In some examples, the UE is further caused to: determine to stop the UE based TA measurement for one or more cells included in a Timing Alignment Group, TAG, wherein the TAG comprises the second cell.

[0015] In some examples, the at least one action comprises: when performing the cell switch, determining to apply the TAC regardless of if the TAT is running.

[0016] In some examples, the UE is further caused to: if determining to apply the TAC when the TAT is running, then start or restart the TAT after applying the TAC.

[0017] In some examples, the UE is further caused to: if determining that the cell switch does not involve the random access procedure towards the second cell, apply the available TA value and start or restart the TAT.

[0018] In some examples, the random access procedure involves at least one of a Contention Based Random Access, CBRA, procedure, or a Contention Free Random Access, CFRA, procedure.

[0019] In some examples, the TAT and / or the available TA value and / or the TAC is associated with a Timing Advance Group, TAG, comprising the second cell.

[0020] In some examples, the UE is configured to support Ll / L2-Triggered Mobility, LTM.

[0021] In some examples, the UE is configured to receive the TAC in a Random Access Response, RAR, message or a MSGB.

[0022] In some examples, the first condition comprises that the UE receives a cell switch command, preferably an LTM cell switch command Medium Access Control, MAC, Control Element, CE.

[0023] In some examples, the second condition comprises that the UE is configured with UE based TA measurement and has successfully measured the TA value.

[0024] In some examples, the third condition comprises that the cell switch involves a random access procedure.

[0025] In some examples, that the cell switch involves a random access procedure refers to that a random access procedure is initiated towards the second cell.

[0026] In accordance with a second aspect of the present disclosure, there is provided a method of a user equipment, UE, served by a first cell of a network node, the method comprising:determining a first condition for triggering a switch from the first cell towards a second cell; determining a second condition indicating that a Timing Advance, TA, value associated with the second cell is available at the UE; and determining whether the cell switch involves a random access procedure towards the second cell, wherein based on the determined first condition and the determined second condition and based on determining a third condition indicating that the cell switch involves the random access procedure, the method further comprises performing at least one action relating to at least one of the following: a Time Alignment Timer, TAT, the available TA value, and a Timing Advance Command, TAC associated with the second cell and received from the network node.

[0027] In accordance with a third aspect of the present disclosure, there is provided a computer program comprising instructions for causing an apparatus to perform the method according to the second aspect.

[0028] In accordance with a fourth aspect of the present disclosure, there is provided a memory storing computer readable instructions for causing an apparatus to perform the method according to the second aspect.

[0029] In addition, according to some other example embodiments, there is provided, for example, a computer program product for a wireless communication device comprising at least one processor, including software code portions for performing the respective steps disclosed in the present disclosure, when said product is run on the device. The computer program product may include a computer-readable medium on which said software code portions are stored. Furthermore, the computer program product may be directly loadable into the internal memory of the computer and / or transmittable via a network by means of at least one of upload, download and push procedures.

[0030] While some example embodiments will be described herein with particular reference to the above application, it will be appreciated that the present disclosure is not limited to such a field of use, and is applicable in broader contexts.

[0031] Notably, it is understood that methods according to the present disclosure relate to methods of operating the apparatuses according to the above example embodiments andvariations thereof, and that respective statements made with regard to the apparatuses likewise apply to the corresponding methods, and vice versa, such that similar description may be omitted for the sake of conciseness. In addition, the above aspects may be combined in many ways, even if not explicitly disclosed. The skilled person will understand that these combinations of aspects and features / steps are possible unless it creates a contradiction which is explicitly excluded.

[0032] Implementations of the disclosed apparatuses may include using, but not limited to, one or more processor, one or more application specific integrated circuit (ASIC) and / or one or more field programmable gate array (FPGA). Implementations of the apparatus may also include using other conventional and / or customized hardware such as software programmable processors, such as graphics processing unit (GPU) processors.

[0033] Other and further example embodiments of the present disclosure will become apparent during the course of the following discussion and by reference to the accompanying drawings.BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Example embodiments of the disclosure will now be described, by way of example only, with reference to the accompanying drawings in which:

[0035] Figure 1 schematically illustrates an example of a signaling diagram for UE behavior in a cell switch procedure;

[0036] Figure 2 schematically illustrates an example of a signaling diagram for UE behavior in a cell switch procedure according to an example embodiment of the present disclosure; and

[0037] Figure 3 schematically illustrates an example of a signaling diagram for UE behavior in a cell switch procedure according to an example embodiment of the present disclosure.DESCRIPTION OF EXAMPLE EMBODIMENTS

[0038] In the following, different exemplifying embodiments will be described using, as an example of a communication network to which examples of embodiments may be applied, a communication network architecture based on 3 GPP standards for a communication network, such as a 5G / NR, without restricting the embodiments to such an architecture, however. It is apparent for a person skilled in the art that the embodiments mayalso be applied to other kinds of communication networks where mobile communication principles are integrated with a D2D (device-to-device) or V2X (vehicle to everything) configuration, such as SL (side link), e.g. Wi-Fi, worldwide interoperability for microwave access (WiMAX), Bluetooth®, personal communications services (PCS), ZigBee®, wideband code division multiple access (WCDMA), systems using ultra-wideband (UWB) technology, mobile ad-hoc networks (MANETs), wired access, etc. Furthermore, without loss of generality, the description of some examples of embodiments is related to a mobile communication network, but principles of the disclosure can be extended and applied to any other type of communication network, such as a wired communication network.

[0039] The following examples and embodiments are to be understood only as illustrative examples. Although the specification may refer to “an”, “one”, or “some” example(s) or embodiment s) in several locations, this does not necessarily mean that each such reference is related to the same example(s) or embodiment(s), or that the feature only applies to a single example or embodiment. Single features of different embodiments may also be combined to provide other embodiments. Furthermore, terms like “comprising” and “including” should be understood as not limiting the described embodiments to consist of only those features that have been mentioned; such examples and embodiments may also contain features, structures, units, modules, etc., that have not been specifically mentioned.

[0040] A basic system architecture of a (tele)communication network including a mobile communication system where some examples of embodiments are applicable may include an architecture of one or more communication networks including wireless access network subsystem(s) and core network(s). Such an architecture may include one or more communication network control elements or functions, access network elements, radio access network elements, access service network gateways or base transceiver stations, such as a base station (BS), an access point (AP), a NodeB (NB), an eNB or a gNB, a distributed unit (DU) or a centralized / central unit (CU), which controls a respective coverage area or cell(s) and with which one or more communication stations such as communication elements or functions, like user devices or terminal devices, like a user equipment (UE), or another device having a similar function, such as a modem chipset, a chip, a module etc., which can also be part of a station, an element, a function or an application capable of conducting a communication, such as a UE, an element or function usable in a machine-to-machine communication architecture, or attached as a separate element to such an element, function or application capable of conducting a communication, or the like, are capable tocommunicate via one or more channels via one or more communication beams for transmitting several types of data in a plurality of access domains. Furthermore, core network elements or network functions, such as gateway network elements / functions, mobility management entities, a mobile switching center, servers, databases and the like may be included.

[0041] The following description may provide further details of alternatives, modifications and variances: a gNB comprises e.g., a node providing NR user plane and control plane protocol terminations towards the UE, and connected via the NG interface to the 5GC, e.g., according to 3GPP TS 38.300 V16.6.0 (2021-06) section 3.2 incorporated by reference.

[0042] A gNB Central Unit (gNB-CU) comprises e.g., a logical node hosting e.g., RRC, SDAP and PDCP protocols of the gNB or RRC and PDCP protocols of the en-gNB that controls the operation of one or more gNB-DUs. The gNB-CU terminates the Fl interface connected with the gNB-DU.

[0043] A gNB Distributed Unit (gNB-DU) comprises e.g., a logical node hosting e.g., RLC, MAC and PHY layers of the gNB or en-gNB, and its operation is partly controlled by the gNB-CU. One gNB-DU supports one or multiple cells. One cell is supported by only one gNB-DU. The gNB-DU terminates the Fl interface connected with the gNB-CU.

[0044] A gNB-CU-Control Plane (gNB-CU-CP) comprises e.g., a logical node hosting e.g., the RRC and the control plane part of the PDCP protocol of the gNB-CU for an en-gNB or a gNB. The gNB-CU-CP terminates the El interface connected with the gNB-CU-UP and the Fl-C interface connected with the gNB-DU.

[0045] A gNB-CU-User Plane (gNB-CU-UP) comprises e.g., a logical node hosting e.g., the user plane part of the PDCP protocol of the gNB-CU for an en-gNB, and the user plane part of the PDCP protocol and the SDAP protocol of the gNB-CU for a gNB. The gNB-CU- UP terminates the El interface connected with the gNB-CU-CP and the Fl-U interface connected with the gNB-DU, e.g., according to 3GPP TS 38.401 V16.6.0 (2021-07) section 3.1 incorporated by reference.

[0046] Different functional splits between the central and distributed unit are possible, e.g., called options:Option 1 (lA-like split):The function split in this option is similar to the 1 A architecture in DC. RRC is in the central unit. PDCP, RLC, MAC, physical layer and RF are in the distributed unit.Option 2 (3C-like split):• The function split in this option is similar to the 3C architecture in DC. RRC and PDCP are in the central unit. RLC, MAC, physical layer and RF are in the distributed unit.Option 3 (intra RLC split):• Low RLC (partial function of RLC), MAC, physical layer and RF are in the distributed unit. PDCP and high RLC (the other partial function of RLC) are in the central unit.Option 4 (RLC-MAC split):• MAC, physical layer and RF are in the distributed unit. PDCP and RLC are in the central unit.Or else, e.g., according to 3GPP TR 38.801 V14.0.0 (2017-03) section 11 incorporated by reference.

[0047] A gNB supports different protocol layers, e.g., Layer 1 (LI) - physical layer.

[0048] The layer 2 (L2) of NR is split into the following sublayers: Medium Access Control (MAC), Radio Link Control (RLC), Packet Data Convergence Protocol (PDCP) and Service Data Adaptation Protocol (SDAP), where e.g.:• The physical layer offers to the MAC sublayer transport channels;• The MAC sublayer offers to the RLC sublayer logical channels;• The RLC sublayer offers to the PDCP sublayer RLC channels;• The PDCP sublayer offers to the SDAP sublayer radio bearers;• The SDAP sublayer offers to 5GC QoS flows;• Comp, refers to header compression and Segm. To segmentation;• Control channels include (BCCH, PCCH).

[0049] Layer 3 (L3) includes e.g., Radio Resource Control (RRC), e.g., according to 3GPP TS 38.300 V16.6.0 (2021-06) section 6 incorporated by reference.

[0050] A RAN (Radio Access Network) node or network node like e.g. a gNB, base station, gNB CU or gNB DU or parts thereof may be implemented using e.g. an apparatus with at least one processor and / or at least one memory (with computer-readable instructions (computer program)) configured to support and / or provision and / or process CU and / or DUrelated functionality and / or features, and / or at least one protocol (sub-)layer of a RAN (Radio Access Network), e.g. layer 2 and / or layer 3.

[0051] The gNB CU and gNB DU parts may e.g., be co-located or physically separated. The gNB DU may even be split further, e.g., into two parts, e.g., one including processing equipment and one including an antenna. A Central Unit (CU) may also be called BBU / REC / RCC / C-RAN / V-RAN, O-RAN, or part thereof. A Distributed Unit (DU) may also be called RRH / RRU / RE / RU, or part thereof. Hereinafter, in various example embodiments of the present disclosure, the CU-CP (or more generically, the CU) may also be referred to as a (first) network node that supports at least one of central unit control plane functionality or a layer 3 protocol of a radio access network; and similarly, the DU may be referred to as a (second) network node that supports at least one of distributed unit functionality or the layer 2 protocol of the radio access network.

[0052] A gNB-DU supports one or multiple cells, and could thus serve as e.g., a serving cell for a user equipment (UE).

[0053] A user equipment (UE) may include a wireless or mobile device, an apparatus with a radio interface to interact with a RAN (Radio Access Network), a smartphone, an in- vehicle apparatus, an loT device, a M2M device, or else. Such UE or apparatus may comprise: at least one processor; and at least one memory including computer program code; wherein the at least one memory and the computer program code are configured to, with the at least one processor, cause the apparatus at least to perform certain operations, like e.g. RRC connection to the RAN. A UE is e.g., configured to generate a message (e.g., including a cell ID) to be transmitted via radio towards a RAN (e.g., to reach and communicate with a serving cell). A UE may generate and transmit and receive RRC messages containing one or more RRC PDUs (Packet Data Units).

[0054] The UE may have different states (e.g., according to 3GPP TS 38.331 V16.5.0 (2021-06) sections 42.1 and 4.4, incorporated by reference).

[0055] A UE is e.g., either in RRC CONNECTED state or in RRC INACTIVE state when an RRC connection has been established.

[0056] In RRC CONNECTED state a UE may:• store the AS context;• transfer unicast data to / from the UE;• monitor control channels associated with the shared data channel to determine if data is scheduled for the data channel;• provide channel quality and feedback information;• perform neighboring cell measurements and measurement reporting.

[0057] The RRC protocol includes e.g. the following main functions:• RRC connection control;• measurement configuration and reporting;• establishment / modification / release of measurement configuration (e.g. intrafrequency, inter-frequency and inter-RAT measurements);• setup and release of measurement gaps;• measurement reporting.

[0058] The general functions and interconnections of the described elements and functions, which also depend on the actual network type, are known to those skilled in the art and described in corresponding specifications, so that a detailed description thereof may omitted herein for the sake of conciseness. However, it is to be noted that several additional network elements and signaling links may be employed for a communication to or from an element, function or application, like a communication endpoint, a communication network control element, such as a server, a gateway, a radio network controller, and other elements of the same or other communication networks besides those described in detail herein below.

[0059] A communication network architecture as being considered in examples of embodiments may also be able to communicate with other networks, such as a public switched telephone network or the Internet. The communication network may also be able to support the usage of cloud services for virtual network elements or functions thereof, wherein it is to be noted that the virtual network part of the telecommunication network can also be provided by non-cloud resources, e.g. an internal network or the like. It should be appreciated that network elements of an access system, of a core network etc., and / or respective functionalities may be implemented by using any node, host, server, access node or entity etc. being suitable for such a usage. Generally, a network function can be implemented either as a network element on a dedicated hardware, as a software instance running on a dedicated hardware, or as a virtualized function instantiated on an appropriate platform, e.g., a cloud infrastructure.

[0060] Furthermore, a network element, such as communication elements, like a UE, a terminal device, control elements or functions, such as access network elements, like a base station / BS, a gNB, a radio network controller, a core network control element or function, such as a gateway element, or other network elements or functions, as described herein, andany other elements, functions or applications may be implemented by software, e.g., by a computer program product for a computer, and / or by hardware. For executing their respective processing, correspondingly used devices, nodes, functions or network elements may include several means, modules, units, components, etc. (not shown) which are required for control, processing and / or communication / signaling functionality. Such means, modules, units and components may include, for example, one or more processors or processor units including one or more processing portions for executing instructions and / or programs and / or for processing data, storage or memory units or means for storing instructions, programs and / or data, for serving as a work area of the processor or processing portion and the like (e.g. ROM, RAM, EEPROM, and the like), input or interface means for inputting data and instructions by software (e.g. floppy disc, CD-ROM, EEPROM, and the like), a user interface for providing monitor and manipulation possibilities to a user (e.g. a screen, a keyboard and the like), other interface or means for establishing links and / or connections under the control of the processor unit or portion (e.g. wired and wireless interface means, radio interface means including e.g. an antenna unit or the like, means for forming a radio communication part etc.) and the like, wherein respective means forming an interface, such as a radio communication part, can be also located on a remote site (e.g. a radio head or a radio station etc.). It is to be noted that in the present specification processing portions should not be only considered to represent physical portions of one or more processors, but may also be considered as a logical division of the referred processing tasks performed by one or more processors. It should be appreciated that according to some examples, a so-called “liquid” or flexible network concept may be employed where the operations and functionalities of a network element, a network function, or of another entity of the network, may be performed in different entities or functions, such as in a node, host or server, in a flexible manner. In other words, a “division of labor” between involved network elements, functions or entities may vary case by case.

[0061] As illustrated above, the present disclosure generally seeks to address the issue that, during a procedure for a cell switch, a RA procedure cannot be performed when the UE is provided with an available TA value. Further, the present disclosure also generally seeks to address the issue that exact UE behavior (in particular UL timing) is not provided with respect to the available TA value and / or the obtaining or maintenance of the available TA value in order to ensure performing of the RA procedure. Further, the present disclosure alsogenerally seeks to address the issue that exact UE behavior (in particular UL timing) is not provided with respect to timing information provided by the network node for the RA procedure.

[0062] In the following, LTM is used as an example, whereas the present disclosure is not limited to an LTM cell switch.

[0063] RAN2 agreed CR in R2 -2313832 introduces LTM as follows, according to TS 38.300 V18.0.0:

[0064] In the above case where the early TA acquisition procedure is realized through UE-based TA measurement, the UE performs TA measurement for the candidate cells after being configured by the RRC, but the exact time or timing the UE performs the TA measurement is up to UE implementation. The UE for instance applies the TA value measured by itself and performs Random Access Channel (RACH)-less LTM e.g., upon receiving the cell switch command.

[0065] Additionally or alternatively, the TA value may be received at the UE in the cell switch command from the NW and the UE performs a RACH-less LTM cell switch e.g., upon receiving the cell switch command.

[0066] Therefore, in the above case, when performing a cell switch, the UE always performs a RACH-less procedure if a TA value is available. That is, the UE cannot be issued to perform a RACH-based procedure for the cell switch, as long as a TA value is available.

[0067] Currently, when the UE performs a RA procedure while it has valid TA (e.g., TAT running), the following is specified (TS 38.321 vl8.0.0):

[0068] In the above procedure, when a Contention Free Random Access (CFRA) procedure is involved, the UE applies the TAC received in the RAR message, whereas whena Contention Based Random Access (CBRA) procedure is involved, the UE ignores the TAC.

[0069] Hence, according to the above procedure, depending on whether CFRA or CBRA procedure is performed, the UE either applies the received TAC or ignores it, respectively. The ignoring of the TAC in case of CBRA is due to the reason that the contention resolution of the CBRA attempt may fail and then the UE would lose its timing towards the cell completely (due to stopping the TAT upon unsuccessful contention resolution).

[0070] Currently, the LTM command MAC Control Element (CE) is specified as follows (R2-2313860), according to TS 38.321 vl8.0.0:

[0071] According to the above procedure, if the UE is configured with TA measurement and the UE has successfully measured the TA for the indicated LTM target cell, the UE considers a RACH-less LTM cell switch to be ongoing.

[0072] Hence, the UE cannot be issued to perform a RACH LTM cell switch with UE- based TA measurement.

[0073] An LTM Cell Switch Command MAC CE is configured with the following elements.

[0074] Therein, in the LTM Cell Switch Command MAC CE, the Timing Advance Command is indicated via the Timing Advance Command field that indicates the index value TA used to control the amount of timing adjustment that the MAC entity has to apply in TS 38.213.

[0075] References are now made to the figures. In particular, it is to be noted that identical or like reference numbers used in the figures of the present disclosure may, unless indicated otherwise, indicate identical or like elements, such that repeated description thereof may be omitted for reasons of conciseness.

[0076] Figure 1 schematically illustrates an example of a signaling diagram for UE behavior in a cell switch procedure and illustrates the issue caused by the above procedures. Figure 1 is shown with the example that the cell switch is an LTM cell switch.

[0077] As shown in Figure 1, the UE is served by a source cell of a network node, wherein the network node initiates switching of the serving cell, for instance after receiving LI measurement report(s) from the UE. For instance, the network node provides the UE with configurations of candidate target cells, namely (target) cells that are available for being switched to. The candidate target cells may be controlled by another network node that is different from the network node that controls the source cell.

[0078] At step S10: the UE receives LTM candidate configuration, i.e., configuration(s) of one or more LTM candidate target cells, i.e., (target) cells that are available for being switched to.

[0079] At step SI 1 : the UE obtains, based on the received LTM candidate configuration, information relating to candidate LTM target cell(s) and UE based TA measurement configuration. The UE is therefore configured with UE based TA measurement.

[0080] At step S 12: the UE measures TA value(s) for the indicated or selected LTM target cell(s) that are e.g., selected from the one or more LTM candidate target cells indicated with the LTM candidate configuration.

[0081] At step S13 : the UE receives an LTM Cell Switch Command (CSC) MAC CE to trigger the LTM cell switch towards the indicated or selected LTM target cell(s) with a random access procedure.

[0082] At step S 14: the UE triggers the LTM cell switch towards the indicated or selected LTM target cell(s).

[0083] At step S15: the UE applies the UE-measured TA value(s) and starts the Time Alignment Timer (TAT).

[0084] At step SI 6: the random access procedure is started for the UE to switch from the source cell to the selected LTM target cell(s).

[0085] At step SI 7: the UE ignores the Timing Advance Command (TAC) that is received from the network node in a random access response (RAR) message since the UE has obtained the TA value based on UE-measurement.

[0086] As shown in Figure 1, due to the availability of the TA value at the UE at the time of initiation of the RA procedure (in the case of Figure 1 by UE based TA measurement), at step S15 the UE always applies the available TA value and at step S17 the TAC from the network node is ignored at the UE and therefore the network node cannot indicate the UE to perform RA based LTM cell switch with the timing information indicated by the network node. Further, the network node may consider the UE measurement of the TA value as incorrect and may not further apply the UE measured TA value. Therefore, the usefulness of the UE measured TA value is low.

[0087] In view of the above, it is proposed in accordance with the present disclosure that the UE applies always the TAC provided by the NW upon LTM execution. The reason why ignoring the UE based timing is that the LTM handover cannot be completed without completing the RA procedure.

[0088] The following example embodiments are provided in accordance with the present disclosure.

[0089] When UE is configured with UE based TA measurement for the LTM candidate target cell and the UE has successfully measured the TA for the indicated LTM target cell (e.g., in LTM Cell Switch Command MAC CE) and NW indicates the UE to perform Random Access for LTM cell switch, the following options are proposed.

[0090] In one example embodiment, the UE does not start or the UE stops the Time Alignment Timer (TAT) upon LTM. The TAT may be associated with a Primary TAG (PTAG) or a Secondary TAG (STAG).

[0091] In one example embodiment, the UE does not start or the UE stops the Time Alignment Timer (TAT) upon acknowledging the LTM cell switch command.

[0092] In one example embodiment, the UE does not apply the measured TA for a TAG.

[0093] In one further example embodiment, the UE stops the UE based TA measurement and / or the UE based TA maintenance / adjustment for the target LTM cell. The UE may stop the UE based TA measurement for any candidate cells.

[0094] In one additional / altemative example embodiment, the UE applies the TAC (Timing Advance Command) provided in RAR or MSGB for PTAG upon LTM regardless of if the TAT for the associated TAG is running.

[0095] In one example embodiment, the UE does not ignore the TAC (Timing Advance Command) provided in RAR or MSGB for PTAG upon LTM regardless of if the TAT for the associated TAG is running.

[0096] Figure 2 schematically illustrates an example of a signaling diagram for UE behavior in a cell switch procedure according to an example embodiment of the present disclosure.

[0097] Similar to Figure 1, the UE is currently served by a first cell of a network node, wherein the network node initiates switching of the serving cell, i.e., changing the serving cell from the first cell to a second cell, for instance after receiving LI measurement report(s) from the UE. For instance, the network node provides the UE with configurations of (candidate) target cells, namely (target) cells that are available for being switched to. The candidate target cells may be controlled by another network node that is different from the network node that controls the first cell.

[0098] At step S20: the UE determines a first condition for triggering a switch from the first cell towards a second cell. The cell switch may be an LTM cell switch.

[0099] Therein, the first condition may include that a cell switch from the first cell towards a second cell is or has been triggered. Therefore, preferably, at step S20, the UE determines whether a cell switch from the first cell towards a second cell is or has been triggered.

[0100] Additionally or alternatively, the first condition may include that the UE generates or has generated LI measurements report and / or transmits or has transmitted the generated LI measurements report to the network node.

[0101] Additionally or alternatively, the first condition may include that the UE receives or has received from the network node a cell switch command (for instance an LTM cell switch command) that is e.g., signalled via a MAC CE.

[0102] S21 : the UE determines a second condition indicating that a Timing Advance, TA, value associated with the second cell is available at the UE. The available TA value may be provided by the network node, and / or may be provided by the UE based on UE measurement. The TA value may further be associated with a TAG that includes the second cell, wherein the TAG may be a PTAG or an STAG.

[0103] Therein, the second condition may include that a TA value associated with the second cell is available at the UE. Therefore, preferably, at step S21, the UE determines whether a TA value associated with the second cell is available at the UE.

[0104] Additionally or alternatively, the second condition may include that the UE is configured with UE based TA measurement and has successfully measured the TA.

[0105] Additionally or alternatively, the second condition may include that an early TA acquisition procedure is triggered by PDCCH.

[0106] Additionally or alternatively, the second condition may include that the second cell has a different NTA value as the current serving cell(s), i.e., the first cell, wherein NTA is a timing offset between uplink and downlink radio frames at the UE.

[0107] S22: the UE determines whether the cell switch involves a random access, RA, procedure towards the second cell.

[0108] Therein, that the cell switch involves an RA procedure refers to that an RA procedure is triggered or to be triggered towards the second cell. Preferably, that the cell switch involves an RA procedure refers to that the UE is indicated to trigger or is to trigger or triggers an RA procedure towards the second cell.

[0109] Additionally or alternatively, that the cell switch involves an RA procedure refers to e.g., that the LTM CSC MAC CE indicates the UE to initiate an RA procedure or it indicates a CFRA resource which also indicates to initiate a RA procedure.

[0110] S23 : based on the determined first condition and the determined second condition and based on determining a third condition indicating that the cell switch involves the random access procedure, the UE performs at least one action relating to at least one of the following: a Time Alignment Timer, TAT; the available TA value; and a Timing Advance Command, TAC associated with the second cell and received from the network node.

[0111] Therein, the third condition may include that the cell switch involves the random access procedure.

[0112] Additionally or alternatively, the third condition may include that the UE has requested a random access procedure towards the second cell.

[0113] Additionally or alternatively, the third condition may include that a Random Access Preamble is selected by the UE among contention-based Random Access Preamble(s).

[0114] Additionally or alternatively, the third condition may include that the UE has transmitted to the network node a Random Access Preamble, preferably on a RA channel, RACH, selected by the UE.

[0115] At step S23, that the UE performs at least one action relating to at least one of the TAT, the available TA value and the TAC refers to that the UE actively applies, or actively determines not to apply (i.e., does not apply, or actively ignores, or stops, or stops applying) any one of the TAT, the available TA value and the TAC.

[0116] It is therefore proposed in accordance with the present disclosure clear-defined UE behavior (more specifically UL timing) for / upon execution of RA based cell switch, with respect to the configurations / information at the UE relating to the available TA value and / or the TAC provided by the network node (in particular the timing information provided by the network node at the time of the RA based cell switch). As a result, the UE always applies the TAC from the network node and RA based cell switched is performed, with improved usefulness of the RA procedure.

[0117] Figure 3 schematically illustrates examples of a signaling diagram for UE behavior in a cell switch procedure according to an example embodiment of the present disclosure.

[0118] Similar to Figure 1, the UE is currently served by a source cell of a network node, wherein the network node initiates switching of the serving cell, i.e., changing the serving cell from the source cell to a target cell, for instance after receiving LI measurement report(s) from the UE. For instance, the network node provides the UE with configurations of (candidate) target cells, namely (target) cells that are available for being selected to be switched to. The candidate target cells may be controlled by another network node that is different from the network node that controls the first cell.

[0119] In the present disclosure, the “source cell” refers to the first cell that is currently serving the UE, i.e., the serving cell before the cell switch; and the “target cell” refers to the second cell for serving the UE after the cell switch.

[0120] At step S30: the UE determines that cell switch towards a target cell is triggered and a TA value for the target cell is available. The network node has prepared for the UE one or more candidate target cells for being selected, i.e., one or more target cells that arecandidates for being selected, so as to switch from the current serving cell, i.e., the source cell, to e.g., one or more selected candidate target cells. The cell switch may be an LTM cells switch.

[0121] At step S31 : the UE determines that the cell switch involves a random access procedure. In particular, the UE determines that for instance the random access procedure is to be triggered or is triggered towards the selected candidate target cell.

[0122] At step S32: based on the above determinations, the UE performs at least one of the following: ignoring or not applying the available TA value: not starting the TAT associated with the TA value or stopping the TAT associated with the TA value; stopping the UE based TA measurement and / or maintenance; and applying the TAC received in the RAR message.

[0123] Therein, the Time Alignment Timer is a UE side operation / configuration that eventually may then be associated to a Timing Advance Group, TAG (which may include the source cell or eventually the target cell), for instance after the cell switch. The TAG may be a PTAG or an STAG.

[0124] It is therefore proposed in accordance with the present disclosure specific UE behavior for execution of RA based cell switch, with respect to the configurations / information at the UE relating to the available TA value and / or the TAC (in particular the timing information) provided by the network node for the RA based cell switch. As a result, the UE is configured to always apply the TAC from the network node and RA based cell switched is performed, with improved usefulness of the RA procedure.

[0125] Some example implementation options for the various embodiments of the present disclosure are provided below:

[0126] According to the above example embodiments, for the indicated cell switch, if the TAT associated with the target cell (or the TAG including this target cell) is running and a RA procedure is initiated by for instance a LTM Cell Switch Command MAC CE, the UE applies the TAC for instance associated with the TAG regardless of the fact that the TAT is running, and starts or restarts the TAT associated with this TAG after apply this TAC.

[0127] According to the above example embodiments, when a RA procedure is not initiated according to an LTM CSC MAC CE, the UE applies the available TA, as well as starts or restarts the TAT associated with the target cell.

[0128] According to the above example embodiments, when an LTM CSC MAC CE is received and the Random Access procedure is initiated by receiving this LTM CSC MAC CE, the UE stops the TAT associated with the target cell (or the TAG including this target cell, the TAG may more specifically being a PTAG or an STAG).

[0129] In summary, in accordance with the present disclosure, the UE is configured with specific and clear-defined UL behavior (in particular UL timing) when performing RA based cell switch, wherein the most accurate timing is always applied for the UL transmissions by the UE.

[0130] Further, in accordance with the present disclosure, regardless of the availability of the TA value (obtained via UE based TA measurement or from NW), NW has always the possibility to override that with timing acquired through RA procedure, since the UE in accordance with the present disclosure is specifically configured either to ignore the TA value available at the UE (or the TAT associated with the target / second cell), or to apply the TAC from the NW.

[0131] As a result, in accordance with the present disclosure, the UE always applies the TAC provided (e.g., in RAR / MSGB) in the Random Access procedure towards the target cell, without being impacted by the TA available at the UE before initiated cell switch, leading to improved usefulness of the RA and improved efficiency of RA based cell switch.

[0132] It is noted that, although in the above-illustrated example embodiments (with reference to the figures), the messages communi cated / exchanged between the network components / elements may appear to have specific / explicit names, depending on various implementations (e.g., the underlining technologies), these messages may have different names and / or be communi cated / exchanged in different forms / formats, as can be understood and appreciated by the skilled person.

[0133] According to some example embodiments, there are also provided corresponding methods suitable to be carried out by the apparatuses (network elements / components) as described above, such as the UE, the CU, the DU, etc.

[0134] It should nevertheless be noted that the apparatus (device) features described above correspond to respective method features that may however not be explicitly described, for reasons of conciseness. The disclosure of the present document is considered to extend also to such method features. In particular, the present disclosure is understood to relate to methods of operating the devices described above, and / or to providing and / or arranging respective elements of these devices.

[0135] Further, according to some further example embodiments, there is also provided a respective apparatus (e.g., implementing the UE, the CU, the DU, etc., as described above) that comprises at least one processing circuitry, and at least one memory for storing instructions to be executed by the processing circuitry, wherein the at least one memory and the instructions are configured to, with the at least one processing circuitry, cause the respective apparatus to at least perform the respective steps as described above.

[0136] Yet in some other example embodiments, there is provided a respective apparatus (e.g., implementing the UE, the CU, the DU, etc., as described above) that comprises respective means configured to at least perform the respective steps as described above.

[0137] It is to be noted that examples of embodiments of the disclosure are applicable to various different network configurations. In other words, the examples shown in the above described figures, which are used as a basis for the above discussed examples, are only illustrative and do not limit the present disclosure in any way. That is, additional furtherexisting and proposed new functionalities available in a corresponding operating environment may be used in connection with examples of embodiments of the disclosure based on the principles defined.

[0138] It should also to be noted that the disclosed example embodiments can be implemented in many ways using hardware and / or software configurations. For example, the disclosed embodiments may be implemented using dedicated hardware and / or hardware in association with software executable thereon. The components and / or elements in the figures are examples only and do not limit the scope of use or functionality of any hardware, software in combination with hardware, firmware, embedded logic component, or a combination of two or more such components implementing particular embodiments of the present disclosure.

[0139] It should further be noted that the description and drawings merely illustrate the principles of the present disclosure. Those skilled in the art will be able to implement various arrangements that, although not explicitly described or shown herein, embody the principles of the present disclosure and are included within its spirit and scope. Furthermore, all examples and embodiment outlined in the present disclosure are principally intended expressly to be only for explanatory purposes to help the reader in understanding the principles of the proposed method. Furthermore, all statements herein providing principles, aspects, and embodiments of the present disclosure, as well as specific examples thereof, are intended to encompass equivalents thereof.List of abbreviations:LTM L1 / L2 triggered mobilityTA Timing AdvanceTAG Timing Advance GroupTAT Time Alignment TimerTAC Timing Advance CommandPT AG Primary TAGSTAG Secondary TAGRA Random AccessRAR Random Access ResponseCBRA Contention Based Random AccessCFRA Contention Free Random Access

Claims

CLAIMS1. A User Equipment, UE, served by a first cell of a network node, the UE comprising: at least one processor; and at least one memory storing instructions that, when executed by the at least one processor, cause the UE at least to: determine a first condition for triggering a switch from the first cell towards a second cell; determine a second condition indicating that a Timing Advance, TA, value associated with the second cell is available at the UE; and determine whether the cell switch involves a random access procedure towards the second cell, wherein based on the determined first condition and the determined second condition and based on determining a third condition indicating that the cell switch involves the random access procedure, the UE is further caused to perform at least one action relating to at least one of the following: a Time Alignment Timer, TAT, the available TA value, and a Timing Advance Command, TAC associated with the second cell and received from the network node.

2. The UE according to claim 1, wherein the at least one action comprises: determining not to start or to stop the TAT.

3. The UE according to claim 2, wherein the UE is further caused to: determine not to start or to stop the TAT upon performing the cell switch.

4. The UE according to claim 2, wherein the UE is further caused to: determine not to start or to stop the TAT upon acknowledging a cell switch command received from the network node.

5. The UE according to any one of claims 1 to 4, wherein the at least one action comprises: determining not to apply or to ignore the available TA value.

6. The UE according to any one of claims 1 to 5, wherein the UE is further caused to: receive the available TA value from the network node in a cell switch command.

7. The UE according to any one of claims 1 to 6, wherein the UE is configured with UE based TA measurement and is further caused to obtain the available TA value based on UE measurement.

8. The UE according to claim 7, wherein the at least one action comprises: determining to stop the UE based TA measurement.

9. The UE according to claim 8, wherein the UE is further caused to: determine to stop the UE based TA measurement for one or more cells included in a Timing Alignment Group, TAG, wherein the TAG comprises the second cell.

10. The UE according to any one of claims 1 to 9, wherein the at least one action comprises: when performing the cell switch, determining to apply the TAC regardless of if the TAT is running.

11. The UE according to claim 10, wherein the UE is further caused to: if determining to apply the TAC when the TAT is running, then start or restart the TAT after applying the TAC.

12. The UE according to any one of claims 1 to 11, wherein the UE is further caused to: if determining that the cell switch does not involve the random access procedure towards the second cell, apply the available TA value and start or restart the TAT.

13. The UE according to any one of claims 1 to 12, wherein the random access procedure involves at least one of a Contention Based Random Access, CBRA, procedure, or a Contention Free Random Access, CFRA, procedure.

14. The UE according to any one of claims 1 to 13, wherein the TAT and / or the available TA value and / or the TAC is associated with a Timing Alignment Group comprising the second cell.

15. The UE according to any one of claims 1 to 14, wherein the UE is configured to support Ll / L2-Triggered Mobility, LTM.

16. The UE according to any one of claims 1 to 15, wherein the UE is configured to receive the TAC in a Random Access Response, RAR, message or a MSGB.

17. The UE according to any one of claims 1 to 16, wherein the first condition comprises that the UE receives a cell switch command comprising an LTM cell switch command Medium Access Control, MAC, Control Element, CE.

18. The UE according to any one of claims 1 to 17, wherein the second condition comprises the UE being configured with UE based TA measurement and has successfully measured the TA value.

19. The UE according to any one of claims 1 to 18, wherein the third condition comprises that the cell switch involves an initiation of a random access procedure towards the second cell.

20. A method of a user equipment, UE, served by a first cell of a network node, the method comprising: determining a first condition for triggering a switch from the first cell towards a second cell; determining a second condition indicating that a Timing Advance, TA, value associated with the second cell is available at the UE; and determining whether the cell switch involves a random access procedure towards the second cell, wherein based on the determined first condition and the determined second condition and based on determining a third condition indicating that the cell switch involvesthe random access procedure, the method further comprises performing at least one action relating to at least one of the following: a Time Alignment Timer, TAT, the available TA value, and a Timing Advance Command, TAC associated with the second cell and received from the network node.

21. A computer program comprising instructions for causing an apparatus to perform the method according to claim 20.

22. A memory storing computer readable instructions for causing an apparatus to perform the method according to claim 20.

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