Improvements in and relating to MAC handling for C-LTM cell switch

Abstract

Performing conditional L1 / L2-Triggered Mobility (C-LTM) cell switch by a UE. The UE is configured with a C-LTM candidate cell configuration comprising a target C-LTM candidate cell. The UE determines

Description

The present invention relates to extending L1 / L2-Triggered Mobility (LTM) to Conditional L1 / L2 Triggered Mobility (C-LTM), which is a further enhancement of mobility to reduce the latency and signalling overhead in a Radio Access Network (RAN). In the prior art, the LTM cell switch procedure is triggered according to a network decision, i.e. the network can send the User Equipment (UE) a MAC Control Element (MAC CE) to trigger the LTM cell switch execution. In contrast, a Conditional LTM (C-LTM) Cell switch is defined as an LTM cell switch that is executed by the UE when one or more LTM cell switch execution conditions are met. Therefore, the timing of LTM cell switch execution and C-LTM cell switch execution may be different and thus this will have a substantial impact on how to manage Timing Advance (TA) for LTM candidate cells in case of C-LTM cell switch. It is an aim of embodiment of the present invention to address issues in LTM cell switch procedures, whether referenced herein or not. According to a first aspect of the present invention, there is provided a method for performing a Conditional L1 / L2-Triggered Mobility, C-LTM, cell switch by a User Equipment, UE, communicatively coupled to a wireless communication system, the method comprising the steps of: • being configured with a C-LTM candidate cell configuration comprising a target C-LTM candidate cell; and • determining that a C-LTM execution condition for a switch to the target C-LTM candidate cell is met; • determining a status of a timer associated with the target C-LTM candidate cell, wherein the timer is configured to indicate a validity period for an uplink Timing Advance, TA, of the target C-LTM candidate cell; • performing a Random Access Channel, RACH, -less cell switch procedure to the target C-LTM candidate cell using a stored TA value if the timer is determined to be running; and • performing a RACH-based cell switch procedure to the target C-LTM candidate cell if the timer is determined to be not running. In an embodiment, the timer associated with the target C-LTM candidate cell is an Itm-Candidate-TimeAlignmentTimer configured by a Radio Resource Control, RRC, protocol. In an embodiment, there is further provided the steps of: • receiving a LTM Candidate Timing Advance Command MAC Control Element, CE, for the target C-LTM candidate cell; and • in response to receiving the LTM Candidate Timing Advance Command MAC CE: • storing a TA value contained in the MAC CE; and starting or restarting the timer associated with the target C-LTM candidate cell. In an embodiment, the LTM Candidate Timing Advance Command MAC CE comprises a field for a TA command index and a field for a Timing Advance Group, TAG, identifier. In an embodiment, the stored TA value used for the RACH-less cell switch procedure is received in a LTM Cell Switch Command MAC CE that triggers the C-LTM cell switch execution. In an embodiment, the RACH-based cell switch procedure is also performed if a beam failure is detected for the target C-LTM candidate cell. In an embodiment, in a dual connectivity configuration, if the RACH-less cell switch procedure is not successfully completed, the method further comprises the step of initiating a Random Access procedure if a MAC entity of the UE is associated with a Master Cell Group, MCG. In an embodiment, if the RACH-less cell switch procedure is not successfully completed, the method further comprises the step of refraining from initiating a Random Access procedure and waiting for a network instruction if the MAC entity of the UE is associated with a Secondary Cell Group, SCG. In an embodiment, there is further provided the step of maintaining security keys without performing a key update if the C-LTM cell switch occurs between a source cell and the target C-LTM candidate cell, and both cells belong to the same base station, gNB. In an embodiment, performing the RACH-based cell switch procedure comprises activating an uplink bandwidth part, UL BWP, on the target C-LTM candidate cell. In an embodiment, performing the RACH-less cell switch procedure further comprises the step of if an initial uplink transmission fails, selecting a new Synchronization Signal Block, SSB, from a set of SSBs associated with a configured uplink grant, wherein the selected SSB has a Reference Signal Received Power, RSRP, above a pre-configured threshold. According to a second aspect of the present invention, there is provided a User Equipment, UE, arranged to perform the method of the first aspect. According to the present invention there is provided an apparatus and method as set forth in the appended claims. Other features of the invention will be apparent from the dependent claims, and the description which follows. Although a few preferred embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes and modifications might be made without departing from the scope of the invention, as defined in the appended claims. For a better understanding of the invention, and to show how embodiments of the same may be carried into effect, reference will now be made, by way of example only, to the accompanying diagrammatic drawings in which: Figures 1,2 and 3 issulstrate message flows according to embodiments of the invention. In one way to address problems in the prior art, an embodiment provides that a LTM Cell Switch Command MAC CE to trigger LTM cell switch execution procedure can be used to trigger a C-LTM Cell switch execution procedure. In this way it is possible to differentiate between the timing of when to apply the measured Timing Advance (TA) value for a LTM Cell switch procedure and a C-LTM Cell switch procedure respectively so that UE can keep measuring TA values for LTM candidate cells for as long as possible, which enables appropriate TA application to prolong the validity of TA. To further enhance the maintenance of Uplink (UL) synchronization for LTM candidate cell (i.e. TA management) in the case of C-LTM, a new MAC Control Element (CE) is provided, because C-LTM needs to consider multiple LTM candidate cells and the network is not able to know when C-LTM cell switch is executed or triggered because it is based on when certain configured conditions are met (i.e. UE decides the timing for C-LTM cell switch execution). For this reason, if the network has a solution to easily update TA values for LTM candidate cells, e.g. by introducing a new MAC CE, it is helpful in managing uplink synchronization for LTM candidate cells. Therefore, a new MAC CE (e.g. LTM Candidate Timing Advance Command MAC CE) is provided, which includes TA values for cells (or LTM candidate cells or SCells or PCell) to let the UE apply it, so as to manage TA values and TA timer for each cell or TA Group (TAG). In the LTM cell switch procedure, RACH-based LTM cell switch is supported as well as RACH-less LTM cell switch. RACH-less LTM cell switch can reduce the latency involved to change the serving cell. An embodiment applies this RACH-less LTM cell switch to C-LTM cell switch, i.e. a similar principle for the RACH-less LTM cell switch can be applied to C-LTM cell switch procedure. However, given that the LTM and C-LTM can be extended to the dual connectivity (DC) case (i.e. UE configured with both MCG and SCG), consideration should be given to the cell group because the behaviour should be different for MCG and SCG, respectively. For example, for the case that the MAC entity considers the configured grant as not valid based on SSB evaluation process during RACH-less LTM cell switch process, the MAC entity should initiate Random Access procedure to have a fast re-connection with the network if the MAC entity is associated with MCG (i.e. to avoid unnecessary data interruption). However, for this case, if the MAC entity is associated with SCG, the MAC entity should not initiate Random Access procedure (i.e. should not do anything) and it can just wait for SCG’s instruction (e.g. PDCCH order or dynamic grant). In addition to this, how to select the beam for the initial UL transmission for C-LTM Cell switch should be considered as well. Furthermore, in embodiments, consideration is given as to how to manage a security update in LTM and C-LTM cell switch procedure. There are several terms or phrases used extensively herein which may be known to the skilled person, but for the sake of completeness, certain of these are reproduced below: PDCCH occasion: A time duration (i.e. one or a consecutive number of symbols) during which the MAC entity is configured to monitor the PDCCH. Serving Cell: A PCell, a PSCell, or an SCell. Special Cell (SpCell): For Dual Connectivity operation the term Special Cell refers to the PCell of the MCG or the PSCell of the SCG depending on if the MAC entity is associated to the MCG or the SCG, respectively. Otherwise the term Special Cell refers to the PCell. A Special Cell supports PUCCH transmission and contention-based Random Access, and is always activated. Timing Advance Group (TAG): A group of Serving Cells that is configured by RRC and that, for the cells with a UL configured, using the same timing reference cell and the same Timing Advance value. A Timing Advance Group containing the SpCell of a MAC entity is referred to as Primary Timing Advance Group (PTAG), whereas the term Secondary Timing Advance Group (STAG) refers to other TAGs. Msg3: Message transmitted on UL-SCH containing a C-RNTI MAC CE or CCCH SDU, submitted from upper layer and associated with the UE Contention Resolution Identity, as part of a Random Access procedure. LTM candidate cell: A candidate cell configured to the UE as specified by LTM candidate cell configuration (i.e. LTM-CandidateConfig) for LTM in RRC layer. Conditional L1 / L2 Triggered Mobility (C-LTM): a LTM cell switch procedure that is executed only when execution condition(s) are met. C-LTM candidate cell: A candidate cell configured for conditional LTM Multi-Radio Dual Connectivity (MR-DC) is a generalization of the Intra-E-UTRA Dual Connectivity (DC), where a multiple Rx / Tx capable UE may be configured to utilise resources provided by two different nodes connected via non-ideal backhaul, one providing NR access and the other one providing either E-UTRA or NR access. One node acts as the MN(Master Node or MCG(Master Cell Group)) and the other as the SN(Secondary Node or SCG(Secondary Cell Group)). The MN and SN are connected via a network interface and at least the MN is connected to the core network. MR-DC with the EPC: E-UTRAN supports MR-DC via E-UTRA-NR Dual Connectivity (EN-DC), in which a UE is connected to one eNB that acts as a MN and one en-gNB that acts as a SN. The eNB is connected to the EPC via the S1 interface and to the en-gNB via the X2 interface. The en-gNB might also be connected to the EPC via the S1-U interface and other en-gNBs via the X2-U interface. MR-DC with the 5GC: NG-RAN supports NG-RAN E-UTRA-NR Dual Connectivity (NGEN-DC), in which a UE is connected to one ng-eNB that acts as a MN and one gNB that acts as a SN. NG-RAN supports NR-E-UTRA Dual Connectivity (NE-DC), in which a UE is connected to one gNB that acts as a MN and one ng-eNB that acts as a SN. NG-RAN supports NR-NR Dual Connectivity (NR-DC), in which a UE is connected to one gNB that acts as a MN and another gNB that acts as a SN. In addition, NR-DC can also be used when a UE is connected to a single gNB, acting both as a MN and as a SN, and configuring both MCG and SCG. Throughout, the term “cell switch” is used for the procedure of triggering change of cells via the LTM feature and use the term “Subsequent LTM” for the case when cell switch between L1 / L2 mobility candidates is done without RRC reconfiguration in between. Throughout, the reception of LTM triggering MAC CE (i.e. LTE Command MAC CE) indicates the triggering of LTM cell switch execution (i.e. LTM cell switch procedure). The following sets out certain principles for LTM (L1 / L2-Triggered Mobility). LTM is a procedure in which a gNB receives L1 or L3 measurement report(s) from a UE, and on the basis of such reports, the gNB may change UE serving cell by a cell switch command signalled via a MAC CE. The cell switch command indicates an LTM candidate configuration that the gNB previously prepared and provided to the UE through RRC signalling. Then the UE applies the target configuration according to the cell switch command. The LTM procedure can be used to reduce the mobility latency. When configured by the network, it is possible to activate Transmission Configuration Indication (TCI) states of one or multiple cells that are different from the current serving cell. For instance, the TCI states of the LTM candidate cells can be activated in advance before any of those cells become the serving cell. This allows the UE to be Downlink (DL) synchronized with those cells, thereby facilitating a faster cell switch to one of those cells when cell switch is triggered. All the activated TCI states except those received in the cell switch command are deactivated upon LTM cell switch execution. When configured by the network, it is possible to initiate UL TA acquisition (called early TA) procedure of one or multiple cells that are different from the current serving cells. If the cell has the same Nta as the current serving cells or Nta=0, early TA acquisition procedure is not required. The network may request the UE to perform early TA acquisition of a candidate cell before a cell switch. The early TA acquisition procedure is triggered by PDCCH order or realized through UE-based TA measurement as configured by RRC. In the former case, the gNB / gNB-DU to which the candidate cell belongs calculates the TA value and sends it to the gNB / gNB-DU to which the serving cell belongs via gNB-CU. The serving cell sends the TA value in the LTM cell switch command MAC CE when triggering LTM cell switch. In the latter case, the UE performs TA measurement for the candidate cells after being configured by RRC but the exact time the UE performs TA measurement is up to UE implementation. The UE applies the TA value measured by itself and performs RACH-less LTM upon receiving the cell switch command, if it does not include any valid TA value. The network may also send a TA value in the LTM cell switch command MAC CE without early TA acquisition. The network (e.g. the serving cell or the LTM candidate cells) may also send to the UE the TA value in the Timing Advance command MAC CE (e.g. for LTM candidate cell). When two TAG IDs are configured for an LTM candidate cell, the gNB-DU to which the LTM candidate cell belongs assigns the same TAG ID pointer values for each TRP to be used by the UEs. Depending on the availability of a valid TA value, the UE performs either a RACH-less LTM or RACH-based LTM cell switch. If the valid TA value is provided in the cell switch command, the UE applies the TA value as instructed by the network. In the case where UE-based TA measurement is configured, but no valid TA value is provided in the cell switch command, the UE applies the valid TA value by itself, if available. The UE performs RACH-less LTM cell switch upon receiving the cell switch command whenever a valid TA value is available. If no valid TA value is available, the UE performs RACH-based LTM cell switch. Regardless of whether the UE is configured for UE-based TA measurement for a certain candidate cell, it will still follow the PDCCH order, which includes performing a random access procedure towards one or more candidate cells. This also applies to the candidate cells for which the UE is capable of deriving TA values by itself. Additionally, regardless of whether the UE has already performed a random access procedure towards the candidate cells, it will still follow the UE-based measurement configuration if configured by the network. For RACH-less LTM, the UE accesses the target cell using either a configured grant or a dynamic grant. The configured grant is provided in the LTM candidate configuration, and the UE selects the configured grant occasion associated with the beam indicated in the cell switch command. Upon initiation of LTM cell switch to the target cell, the UE starts to monitor PDCCH on the target cell for dynamic scheduling. Before RACH-less LTM procedure completion, the UE shall not trigger random access procedure if it does not have a valid PUCCH resource for triggered SRs. The following principles apply to LTM: In an embodiment, LTM cell switch can be performed for intra-gNB LTM cell switch cases as well as inter-gNB LTM cell switch cases, which means that security keys may not be able to be maintained upon an LTM cell switch. Given that intra-gNB LTM Cell switch is to change the serving cell within one gNB, there would be no reason to update the security keys because the termination point of the user plane entity is the same as before the cell switch. However, the inter-gNB LTM Cell switch would require security key update as the termination of user plane entity changes. An embodiment restricts the network and UE implementation to maintain security keys for a specific condition. It can be implemented by one of the following options: Option 1: Security keys are maintained upon an LTM cell switch if the source cell and the target LTM candidate cells belong to the same gNB. Option 2: Security keys are maintained upon an LTM cell switch if the LTM cell switch is not an inter-gNB LTM cell switch or if it does not correspond to inter-gNB mobility or if it corresponds to intra-gNB-DU mobility or inter-gNB-DU mobility. Option 3: Security keys are maintained upon an LTM cell switch if the security update is not configured or if the masterKeyupdate is not received (e.g. in RRCReconfiguration message) or if the sk-Counter (e.g. for SCG) is not received (e.g. in RRCReconfiguration message). Option 4: Security keys are maintained upon an LTM cell switch if there is no change of termination point for a radio bearer or if the keyToUse in the RadioBearerConfig is the same as the keyToUse in the current UE configuration. If the security keys are not maintained (i.e. the security key is updated) or the masterKeyupdate is received (e.g. in RRCReconfiguration message) or the sk-Counter (e.g. for SCG) is received (e.g. in RRCReconfiguration message), the UE should trigger RLC reestablishment or PDCP re-establishment for DRBs or SRBs in the LTM Cell switch procedure.(e.g. upon LTM Cell switch execution) Subsequent LTM is supported. LTM supports intra-gNB-DU mobility, inter-gNB-DU mobility and inter-gNB mobility. LTM supports both intra-frequency and inter-frequency mobility, including mobility to inter-frequency cell that is not a current serving cell. LTM is supported only for licensed spectrum. The following scenarios are supported: PCell change in non-CA scenario and non-DC scenario; PCell and SCell(s) change in CA scenario; Dual connectivity scenario: including PCell and MCG SCell(s) change and PSCell and SCG SCell(s) change with or without MN involvement. LTM for simultaneous PCell and PSCell change is not supported, i.e Simultaneous LTM for PCell change and LTM for PSCell change is not supported While the UE has stored LTM candidate configurations the UE can also execute any L3 handover except for DAPS handover. In the RRC message which the UE applies for any L3 handover (except DAPS), LTM candidate configurations can be added / modified / released by the target cell The following relates to Control Plane handling. Cell switch command is conveyed in a MAC CE, which contains the necessary information to perform the LTM cell switch. The overall procedure for LTM is shown in Figure 1. Subsequent LTM is done by repeating the early synchronization 4a, 4b, LTM cell switch execution 5,6, and LTM cell switch completion 8 steps without releasing other LTM candidate configurations after each LTM cell switch completion. The general procedure over the air interface is applicable to SCG LTM. The procedure, as set out in Figure 1, for LTM is as follows. The step numbers below correspond to the numbering of the steps in Figure 1. 1. The UE sends a MeasurementReport message to the source gNB. The source gNB decides to configure LTM and initiates LTM preparation. 1a. In case of inter-gNB LTM, the source gNB requests one or more candidate gNBs to configure LTM for one or more candidate cells. The candidate gNB(s) send LTM candidate configurations to the source gNB. 2. The source gNB transmits an RRCReconfiguration message to the UE including the LTM candidate configurations. 3. The UE stores the LTM candidate configurations and transmits an RRCReconfigurationComplete message to the source gNB. 4a. The UE performs DL synchronization with the LTM candidate cell(s) before receiving the cell switch command. The UE may activate and deactivate TCI states of LTM candidate cell(s), as triggered by the source gNB. 4b. The UE may perform UL synchronization with LTM candidate cell(s) before receiving the cell switch command, by using UE-based TA measurement, if configured, and / or by transmitting a preamble towards the candidate cell, as triggered by the source gNB. When UE-based TA measurement is configured, UE acquires the TA value(s) of the candidate cell(s) by measurement. UE performs early TA acquisition with the candidate cell(s) as requested by the network before receiving the cell switch command as specified in clause 9.2.6. This is done via CFRA triggered by a PDCCH order from the source cell, following which the UE sends preamble towards the indicated candidate cell. In order to minimize the data interruption of the source cell due to CFRA towards the candidate cell(s), the UE does not receive random access response from the network for the purpose of TA value acquisition and the TA value of the candidate cell is indicated in the cell switch command. The UE does not maintain the TA timer for the candidate cell and relies on network implementation to guarantee the TA validity. 5. The UE performs L1 measurements on the configured LTM candidate cell(s) and transmits L1 measurement reports to the source gNB. L1 measurement should be performed as long as RRC reconfiguration (step 2) is applicable. To report L1 measurement results quickly, L1 measurement report MAC CE can be defined and sent to the network. The MAC CE can be generated, triggered or sent to the network when a certain condition configured by the network is met. The MAC CE can include L1 measurement results, Report ID indicating corresponding measurement report ID for the configuration identity associated with this event triggered measurement report, Satisfied Quantity indicating the number of beams which fulfil the trigger event in the L1 event triggered measurement report (this field should be absent if the beam(s) not satisfying the event could not be reported), RSRh indicating the reference signalling resource index of the beam i for the L1 event triggered measurement report (i.e. SS / PBCH Block Resource indicator (SSBRI) or CSI-RS resource indicator (CRI)), etc. 6. The source gNB decides to execute cell switch to a target cell and informs the target gNB. Source gNB transmits an LTM cell switch command MAC CE triggering cell switch by including a target configuration ID which indicates the index of the candidate configuration of the target cell, a beam indicated with a TCI state or beams indicated with DL and UL TCI states, and a timing advance command for the target cell, if available. The UE switches to the target cell and applies the candidate configuration indicated by the target configuration ID. 7. The UE performs the random access procedure towards the target cell, if UE does not have valid TA of the target cell as specified in clause 5.18.35 of 3GPP publication TS 38.321. 8. The UE completes the LTM cell switch procedure by sending RRCReconfigurationComplete message to the target cell. If the UE has performed a RA procedure in step 7 the UE considers that LTM cell switch execution is successfully completed when the random access procedure is successfully completed. For RACH-less LTM, the UE considers that LTM cell switch execution is successfully completed when the UE determines that the network has successfully received its first UL data. The steps 4-8 can be performed multiple times for subsequent LTM cell switch executions using the LTM candidate configuration(s) provided in step 2. The procedure over the air interface described in Figure 1 is applicable to both intra-gNB-DU LTM and inter-gNB-DU LTM. The overall LTM procedures over F1-C interface are captured in 3GPP publication TS 38.401. To support the above procedure, the network may configure an RRC_CONNECTED UE to perform L1 beam level measurements for SpCell or SCell or LTM candidate cell(s), including L1 RSRP or SINR. The network may configure the UE to report them in accordance with the L1 event triggered beam level measurement configuration. The measurement report is used for indicating to serving gNB of the L1 beam level measurement results from the serving cell and / or candidate cell(s) when L1 event trigger condition is satisfied. The measurement configuration is provided associated with L1 measurement resource configuration by means of RRC dedicated signalling (e.g. RRCReconfiguration message). The network may configure the UE to perform L1 event triggered beam level measurement for the following LTM purpose: Select the candidate beam / cell to trigger early synchronization; Select the target beam / cell and trigger LTM cell switch procedure. The network may configure the UE to report the following measurement information based on SS / PBCH block(s): Measurement results per SS / PBCH block; SS / PBCH block(s) resource indicator (SSBRI). The network may configure the UE to report the following measurement information based on CSI-RS resources: Measurement results per CSI-RS resource; CSI-RS resource indicator (CRI). An RRC_CONNECTED UE obtains L1 beam level measurement results by measuring one or multiple beams as configured by the network. For each L1 beam level measurement result in RRC_CONNECTED, the UE applies the layer 1 filtering , before using the measured results for evaluation of reporting criteria and measurement reporting. The MAC entity performs the evaluation of reporting criteria based on the L1 filtered measurement results from lower layer. For L1 beam level event triggered measurements report, the network can configure SS / PBCH block(s) or CSI-RS as event evaluation RS type, and L1-RSRP or SINR as trigger quantity. Reporting quantities can be any combination of quantities (i.e. only L1-RSRP; only SINR; L1-RSRP or SINR), irrespective of the trigger quantity. The purpose of this report is to transfer L1 measurement results from the UE to the network. RRC controls the L1 event triggered beam level measurement reporting by configuring the relevant parameters. The following relates to early synchronization procedure for LTM cell switch procedure. Early Downlink Synchronisation When configured by the network, it is possible for a UE in RRC_CONNECTED to be DL synchronized with a cell which is different from the current serving cell. This is possible by activating in advance TCI state(s) or a downlink BWP (Bandwidth Part) that belongs to the cell to which the early DL sync is needed. The downlink BWP can be active or activated as the BWP indicated by the firstActiveDownlinkBWP-ld in the cell configuration to which the early DL synch is needed (e.g. ServingCellConfig), which can be configured in RRCReconfiguration. The TCI state(s) can be activated on the downlink BWP. Figure 2 describes the early TCI state (or a downlink BWP) activation (early DL sync) procedure triggered by the network. The step numbers below refer to corresponding steps in Figure 2. 11. The gNB to which Cell A belongs provides a list of TCI states of Cell B to the UE within the RRCReconfiguration message. The gNB to which Cell A belongs may provide a list of TCI state(s) for one or multiple cells to which the early TCI state activation procedure may be executed by the UE. 12. The UE replies with the RRCReconfigurationComplete message. 13. The gNB to which Cell A belongs sends an early TCI state activation MAC CE to the UE in order to initiate an early TCI state activation procedure with Cell B. UE receives the early TCI state activation MAC CE from the current serving cell (i.e. Cell A (SpCell, PCell or PSCell)). The early TCI state activation MAC CE may also indicate TCI state(s) of other cells during the TCI state activation procedure, which may include / indicate index (indices) for TCI state or the corresponding Cell identity or the corresponding downlink BWP identity for early DL synchronization. To indicate these in MAC CE, a bitmap may be used to reduce the overhead of MAC CE, which can be mapped to the configuration information (e.g. cell identifiers or BWP ID) in the ascending order of the values and ‘1 ’ (or ‘0’) means the indication of the corresponding TCI state or Cell or downlink BWP. 14-1. (e.g. upon the reception of early TCI state activation MAC CE indicating a cell (e.g. Cell B) and TCI state(s) for the cell,) the UE activates the TCI state(s) of Cell B indicated in the early TCI state activation MAC CE. In other words, the UE activates the TCI state(s) of Cell B indicated in the early TCI state activation MAC CE on the indicated downlink BWP (e.g. by the firstActiveDownlinkBWP-ld for Cell B in RRCReconfiguration or by indication in MAC CE). In this case, it would be beneficial for the early uplink synchronization procedure as TCI states between the network and UE get aligned well. 14-2. In another embodiment, the UE may activate the TCI state(s) of Cell B indicated in the early TCI state activation MAC CE when the UE triggers LTM cell switch procedure (i.e. upon the reception of the first MAC CE (i.e. LTM Cell switch command MAC CE)) to save UE power consumption or UE may activate the TCI state(s) of Cell B indicated in the early TCI state activation MAC CE on the indicated downlink BWP (e.g. by the firstActiveDownlinkBWP-ld for Cell B in RRCReconfiguration or by indication in MAC CE) when UE triggers LTM cell switch procedure (i.e. upon the reception of the first MAC CE (i.e. LTM Cell switch command MAC CE)) to save UE power consumption as PDCCH monitoring on active BWP causes unnecessary UE power consumption. There are several options concerning how to handle the downlink BWP for Cell B in the early downlink synchronization procedure: o Option 1: Upon the reception of early TCI state activation MAC CE indicating a cell (e.g. Cell B) and TCI state(s) for the cell, the downlink BWP (e.g. indicated by the firstActiveDownlinkBWP-ld for the indicated cell (e.g. Cell B) in RRCReconfiguration or indicated by the MAC CE) is activated or active. The UE activates the downlink BWP and the TCI state(s) of the cell on the downlink BWP in the early downlink synchronization procedure or after completion of the early downlink synchronization procedure. o Option 2: Upon the reception of early TCI state activation MAC CE indicating a cell (e.g. Cell B) and TCI state(s) for the cell, the UE decides the TCI state(s) of the cell in the early downlink synchronization procedure or after completion of the early downlink synchronization procedure. The downlink BWP (e.g. indicated by the firstActiveDownlinkBWP-ld for the indicated cell (e.g. Cell B) in RRCReconfiguration or indicated by the MAC CE) is activated or active when UE triggers LTM cell switch procedure (i.e. upon the reception of the first MAC CE (i.e. LTM Cell switch command MAC CE)) and UE activates the TCI state(s) of the cell on the downlink BWP. The early TCI state activation MAC CE and LTM Cell switch command MAC CE may be received together in the same MAC PDU. Upon RRC (re-)configuration of firstActiveDownlinkBWP-ld and / or firstActiveUplinkBWP-ld for SpCell except for PSCell when SCG is deactivated or activation of an SCell or early downlink (or uplink) synchronization, the DL BWP and / or UL BWP indicated by firstActiveDownlinkBWP-ld and / or firstActiveUplinkBWP-ld is active without receiving PDCCH indicating a downlink assignment or an uplink grant. Upon RRC (re-)configuration of firstActiveDownlinkBWP-ld for PSCell when SCG is deactivated, the DL BWP is switched to the firstActiveDownlinkBWP-ld. The active BWP for a Serving Cell is indicated by either RRC or PDCCH or MAC CE (early TCI state activation MAC CE or LTM Cell switch command MAC CE). For unpaired spectrum, a DL BWP is paired with a UL BWP, and BWP switching is common for both UL and DL. The UE is assumed to have early DL synchronization with the gNB to which Cell B belongs. With this, the gNB to which Cell A belongs may initiate cell switch procedure to Cell B by proving a cell switch command which indicates Cell B as target cell. The cell switch command can be e.g., the LTM cell switch command MAC CE. This procedure can be applied to both MCG or SCG separately when UE is configured with dual connectivity. The early downlink synchronization can be performed before the early uplink synchronization as the exact TA value can be measured by the network when the TCI states between UE and the network are well aligned, i.e. the network can trigger the early downlink synchronization to UE first and then trigger early uplink synchronization to UE (e.g. after completion of the early downlink synchronization). In other embodiment, the network can skip the early downlink synchronization and trigger early uplink synchronization to UE. In another embodiment, the early downlink synchronization and LTM cell switch procedure can be indicated together by two MAC CEs, i.e. early TCI state activation MAC CE and LTM Cell switch command MAC CE. The two MAC CEs can be included in the same MAC PDU and the network send the MAC PDU to UE. In this way, UE activates the indicated TCI states (e.g. on the indicated downlink BWP) and triggers LTM cell switch procedure based on the two MAC CEs. Early Uplink Synchronisation When configured by the network, it is possible for a UE in RRC_CONNECTED to be UL synchronized with a cell which is different from the current serving cell. Figure 3 describes the early TA acquisition (early UL sync) procedure triggered by the network (i.e. herein, early uplink synchronization means early TA acquisition procedure). The step numbers below refer to corresponding steps in Figure 3. 21. The gNB to which Cell A belongs provides the TA acquisition configuration to the UE within the RRCReconfiguration message. The TA acquisition configuration includes RRC configuration information required to send a Random Access Preamble to Cell B so that the gNB to which Cell B belongs can calculate a TA value to be used by the UE, e.g., in case an LTM cell switch procedure is executed to Cell B. The TA acquisition configuration may include information for one or multiple cells to which the TA acquisition procedure may be executed by the UE. UE receives the TA acquisition configuration from the current serving cell (i.e. Cell A or SpCell or PCell or PSCell) by RRCReconfiguration message. 22. The UE replies with the RRCReconfigurationComplete message. 23. The gNB to which Cell A belongs sends a PDCCH order message to the UE in order to initiate a TA acquisition procedure with Cell B. The PDCCH order may include the information required to sends a Random Access Preamble to Cell B and indication whether to perform preamble transmission or preamble retransmission. The gNB to which Cell A belongs may indicate the retransmission of preamble for TA acquisition in case no TA is obtained. UE receives the PDCCH order indicating random access procedure (or preamble (re)transmission) to other cell(s) (e.g. Cell B) from the current serving cell (i.e. Cell A or SpCell or PCell or PSCell). 24. The UE sends a Random Access Preamble to Cell B so that the gNB to which Cell B belongs can calculate a TA value to be used by the UE, e.g., if an LTM cell switch procedure is triggered to Cell B. There are several options concerning how to handle the uplink BWP for Cell B to transmit the Random Access Preamble to Cell B. o Option 1: For each LTM candidate cell, the DL BWP and / or UL BWP indicated by firstActiveDownlinkBWP-ld and / or firstActiveUplinkBWP-ld respectively is active when the Random Access procedure on an LTM candidate cell is initiated by a PDCCH order for early uplink synchronization or when early downlink synchronization is initiated (or completed), i.e. UE peforms random access preamble (re)transmission on the UL BWP to the indicated cell (a LTM candidate cell indicated by PDCCH order) in the random access procedure triggered by a PDCCH order (of the serving cell (Spcell or PCell or PSCell)) for early uplink synchronization (e.g. if PRACH occasions or resources are configured on the UL BWP). The DL BWP and / or UL BWP on an LTM candidate cell is deactivated upon reception of LTM Cell Switch Command MAC CE except for the LTM candidate cell indicated by the Target Configuration ID included in the LTM Cell Switch Command MAC CE. The DL BWP and / or UL BWP for the LTM candidate cell indicated by the Target Configuration ID included in the LTM Cell Switch Command MAC CE are active for the Serving Cell. o Option 2: For each LTM candidate cell, the DL BWP and / or UL BWP indicated by firstActiveDownlinkBWP-ld and / or firstActiveUplinkBWP-ld respectively is active when the Random Access procedure on an LTM candidate cell is initiated by a PDCCH order for early uplink synchronization or when early downlink synchronization is initiated (or completed), i.e. UE peforms random access preamble (re)transmission on the UL BWP to the indicated cell (a LTM candidate cell indicated by PDCCH order) in the random access procedure triggered by a PDCCH order (of the serving cell (Spcell or PCell or PSCell)) for early uplink synchronization (e.g. if PRACH occasions or resources are configured on the UL BWP). The DL BWP and / or UL BWP on an LTM candidate cell is deactivated upon the preamble (re)transmission (i.e. when Random Access procedure initiated by a PDCCH order for early uplink synchronization is completed). o Option 3: For each LTM candidate cell, the DL BWP and / or UL BWP indicated by firstActiveDownlinkBWP-ld and / or firstActiveUplinkBWP-ld respectively is not active (i.e. deactivated or keep the deactivate state) when the Random Access procedure on an LTM candidate cell is initiated by a PDCCH order for early uplink synchronization or when early downlink synchronization is initiated (or completed), i.e. UE peforms random access preamble (re)transmission on the UL BWP (i.e. on the deactivated UL BWP) to the indicated cell (a LTM candidate cell indicated by PDCCH order) in the random access procedure triggered by a PDCCH order (of the serving cell (Spcell or PCell or PSCell)) for early uplink synchronization (e.g. if PRACH occasions or resources are configured on the UL BWP). In this way, UE is allowed to perform preamble (re)transmission on the deactivated BWP for LTM candidate cells. The DL BWP and / or UL BWP on an LTM candidate cell is activated as SpCell (i.e. PCell or PSCell) when UE successfully completes the LTM cell switch procedure to the LTM candidate cell indicated by the Target Configuration ID included in the LTM Cell Switch Command MAC CE. 25. The gNB to which Cell A belongs provides the TA value calculated by the gNB to which Cell B belongs during the TA acquisition procedure, e.g. in LTM cell switch command MAC CE which initiate LTM cell switch procedure to Cell B in case an LTM cell switch procedure is triggered to Cell B. UE receives the first MAC CE (i.e. LTM Cell switch command MAC CE) from the current serving cell (i.e. Cell A or SpCell or PCell or PSCell), which triggers LTM cell switch procedure to other cell (e.g. Cell B). This procedure can be applied to both MCG or SCG separately when UE is configured with dual connectivity. The following concerns User Plane handling. After receiving an LTM cell switch command MAC CE, the UE performs MAC reset. For intra-gNB LTM(e.g. intra-gNB-DU mobility), whether the UE performs RLC re-establishment and PDCP data recovery during cell switch is explicitly controlled by the network through RRC signalling. For inter-gNB LTM(e.g. inter-gNB mobility), UE always performs RLC reestablishment and PDCP re-establishment during cell switch. In other words, after receiving an LTM cell switch command MAC CE, the UE performs MAC reset. For the case that the security update is not configured or the masterKeyupdate is not received (e.g. in RRCReconfiguration message) or the sk-Counter (e.g. for SCG) is not received (e.g. in RRCReconfiguration message) or there is no change of termination point for a radio bearer or the keyToUse in the RadioBearerConfig is the same as the keyToUse in the current UE configuration, whether the UE performs RLC re-establishment and PDCP data recovery during cell switch is explicitly controlled by the network through RRC signalling. For the case that the security update is configured or the masterKeyupdate is received (e.g. in RRCReconfiguration message) or the sk-Counter (e.g. for SCG) is received (e.g. in RRCReconfiguration message) or there is change of termination point for a radio bearer or the keyToUse in the RadioBearerConfig is not the same as the keyToUse in the current UE configuration, the UE always performs RLC re-establishment and PDCP re-establishment during cell switch. The following relates to RACH-less handover (LTM Cell switch). During intra-gNB HO procedure, RACH-less handover can be configured for a UE. The RACH-less handover procedure applies the following functionality: The UE uses the same timing advance value at the target cell as in the source cell or timing advance value of 0. The handover command for the UE may contain a beam identifier for the beam to be used by the UE at the target cell. The beam may be determined based on a UE measurement report and / or left up to gNB implementation, e.g., using the target cell's knowledge about the beam(s) used by the UE at the co-located source cell. The handover command may include a configured UL grant. UE can fallback to RACH when there is no valid configured uplink grant. Alternatively, an UL grant is dynamically signalled by the target cell. The UE transmits the RRCReconfigurationComplete message using the configured or dynamically signalled UL grant. Successful UL data reception on the target cell terminates the RACH-less handover execution. The following applies to Conditional L1 / L2 Triggered Mobility. A Conditional L1 / L2 Triggered Mobility (C-LTM) is defined as an LTM cell switch that is executed by the UE when one or more LTM cell switch execution conditions are met. The following principles apply to C-LTM: Source cell sends the conditional LTM configuration via RRCReconfiguration to UE, which includes the LTM candidate configurations, and the corresponding execution conditions. Source cell and each candidate cell provide their own execution condition for conditional LTM. The resulting C-LTM configuration is included as part of the LTM configuration to the UE. An execution condition can be based on events. Events can be defined and configured by the network C-LTM is supported for intra-CU LTM and C-LTM based on inter-CU LTM(inter-gNB LTM) is not supported. C-LTM can be RACH-based or can be configured to be RACH-less. The completion of C-LTM follows the same procedure as LTM cell switch procedure. The conditional LTM cell switch procedure is executed (or triggered) upon: the MAC entity determines that the event for conditional LTM is satisfied based on L1 measurements the event for conditional LTM is satisfied based on L3 measurements indicated by upper layers (e.g. RRC layers). The network may configure an RRC_CONNECTED UE to perform L1 beam level measurements for SpCell and conditional LTM candidate cell(s) for C-LTM, including L1 RSRP or SINR. The UE may use the L1 measurement for LTM cell switch conditions evaluation in accordance with the reconfiguration for C-LTM. The network may configure the UE to report the L1 measurement results for C- LTM, e.g., to trigger PDCCH ordered early RACH. Throughout this application , LTM can also indicate C-LTM, i.e. LTM cell switch can indicate C-LTM cell switch, LTM cell candidate configuration can indicate C-LTM cell candidate configuration, LTM cell switch execution can indicate C-LTM cell switch execution, etc. 2.3. Principles for Security protection The following high-level principles should be applied in embodiments. Herein, the term security protection implies ciphering or integrity protection. “Ciphering” means not only the ciphering operation but also the deciphering operation because the deciphering should be applied to the data at the receiver if data is ciphered at the transmitter. Likewise, the integrity protection means the integrity verification operation as well as the integrity protection operation because the integrity verification should be applied to the data at the receiver if a data is integrity protected at the transmitter. AS security comprises of the integrity protection and ciphering of RRC signalling (SRBs) and user data (DRBs). RRC handles the configuration of the AS security parameters which are part of the AS configuration: the integrity protection algorithm, the ciphering algorithm, if integrity protection and / or ciphering is enabled for a DRB and two parameters, namely the keySetChangelndicator and the nextHopChainingCount, which are used by the UE to determine the AS security keys upon reconfiguration with sync (with key change), connection re-establishment and / or connection resume. The integrity protection algorithm is common for SRB1, SRB2, SRB3 (if configured), SRB4 (if configured), SRBx (if configured) and DRBs configured with integrity protection, with the same keyToUse value. The ciphering algorithm is common for SRB1, SRB2, SRB3 (if configured), SRB4 (if configured), SRBx (if configured) and DRBs configured with the same keyToUse value. Neither integrity protection nor ciphering applies for SRBO. NOTE: All DRBs related to the same PDU session have the same enable / disable setting for ciphering and the same enable / disable setting for integrity protection. RRC integrity protection and ciphering are always activated together, i.e. in one message / procedure. RRC integrity protection and ciphering for SRBs are never de-activated. However, it is possible to switch to a 'NULL.' ciphering algorithm (neaO). For SRBx (if configured), RRC integrity protection and ciphering can be activated and deactivated based on configuration or indication by RRC messages (or MAC CE(Control Elemenent) or PDCP control PDU(Protocol Data Unit)), in order to reduce the UE processing burden. For SRBx (if configured), it is also possible to switch to a 'NULL.' ciphering algorithm (neaO) and the 'NULL.' integrity protection algorithm (niaO) can be used. The 'NULL.' integrity protection algorithm (niaO) is used only for SRBs and for the UE in limited service mode and when used for SRBs, integrity protection is disabled for DRBs. In case the 'NULL.' integrity protection algorithm is used, 'NULL.' ciphering algorithm is also used. NOTE: Lower layers discard RRC messages for which the integrity protection check has failed and indicate the integrity protection verification check failure to RRC. The AS applies four different security keys: one for the integrity protection of RRC signalling (KRRCint), one for the ciphering of RRC signalling (KRRCenc), one for integrity protection of user data (Kupint) and one for the ciphering of user data (Kupenc). All four AS keys are derived from the KgNB key. The K9nb key is based on the Kamp key, which is handled by upper layers. The integrity protection and ciphering algorithms can only be changed with reconfiguration with sync. The AS keys (K9nb, KRRCint, KRRCenc, Kupint and Kupenc) change upon reconfiguration with sync (if masterKeyUpdate is included), and upon connection re-establishment and connection resume. For each radio bearer an independent counter (COUNT used in PDCP layer) is maintained for each direction. For each radio bearer, the COUNT is used as input for ciphering and integrity protection. It is not allowed to use the same COUNT value more than once for a given security key. The network is responsible for avoiding reuse of the COUNT with the same RB identity and with the same key, e.g. due to the transfer of large volumes of data, release and establishment of new RBs, and multiple termination point changes for RLC-UM bearers and multiple termination point changes for RLC-AM bearer with SN terminated PDCP re-establishment (COUNT reset) due to SN only full configuration whilst the key stream inputs (i.e. bearer ID, security key) at MN have not been updated. In order to avoid such re-use, the network may e.g. use different RB identities for RB establishments, change the AS security key, or an RRC_CONNECTED to RRC_IDLE / RRC_INACTIVE and then to RRC_CONNECTED transition. In order to limit the signalling overhead, individual messages / packets include a short sequence number (PDCP SN(Sequence Number)). In addition, an overflow counter mechanism is used: the hyper frame number (HFN used in PDCP layer). The HFN needs to be synchronized between the UE and the network. For each SRB, the value provided by RRC to lower layers to derive the 5-bit BEARER parameter used as input for ciphering and for integrity protection is the value of the corresponding srb-ldentity with the MSBs padded with zeroes. For a UE provided with an sk-counter, keyToUse indicates whether the UE uses the master key (KgNB) or the secondary key (S-Ksnb or S-K9nb) for a particular DRB. The secondary key is derived from the master key and sk-Counter. Whenever there is a need to refresh the secondary key, e.g. upon change of MN with K9nb change or to avoid COUNT reuse, the security key update is used. When the UE is in NR-DC, the network may provide a UE configured with an SCG with an sk-Counter even when no DRB is setup using the secondary key (S-KgNB) in order to allow the configuration of SRB3. The network can also provide the UE with an sk-Counter, even if no SCG is configured, when using SN terminated MCG bearers. The following relates to MAC Protocol and how this is impacted by embodiments. Maintenance of Uplink Time Alignment As described above, C-LTM is an LTM cell switch that is executed by the UE when one or more LTM cell switch execution conditions are met. To enable C-LTM correctly, we also need to consider how to manage TA for the LTM candidate cells configured for C-LTM (or LTM) cell switch. The legacy timeAlignmentTimer can be used and applied to manage TA for the LTM candidate cells configured for C-LTM (or LTM) cell switch. For example, the timeAlignmentTimer associated with TAG that a LTM candidate cell belongs to can be used to manage TA for the LTM candidate cell for C-LTM and also be applied to the maintenance procedure of uplink time alignment thereof. Another solution is to introduce a new timeAlignmentTimer (e.g. Itm-Candidate-TimeAlignmentTimer) to be used for LTM candidate cell for C-LTM (or LTM) cell switch as proposed below. The legacy timeAlignmentTimer may play a role as the newly introduced timeAlignmentTimer (e.g. Itm-Candidate-TimeAlignmentTimer). LTM Cell Switch Command MAC CE can be used to trigger either a LTM Cell switch procedure or a C-LTM Cell switch procedure. Therefore, we need to differentiate the timing when to apply the measured TA value for a LTM Cell switch procedure and a C-LTM Cell switch procedure respectively so that UE can keep measuring TA values for LTM candidate cells as much as possible, which enables appropriate TA application to prolong the validity of TA. RRC configures the following parameters for the maintenance of UL time alignment: timeAlignmentTimer (per TAG) which controls how long the MAC entity considers the Serving Cells to the associated TAG to be uplink time aligned for the TAG, which can be called as TAT timer in this invention; inactivePosSRS-TimeAlignmentTimer which controls how long the MAC entity considers the Positioning SRS transmission in RRCJNACTIVE to be uplink time aligned; cg-SDT-TimeAlignmentTimer which controls how long the MAC entity considers the uplink transmission for CG-SDT to be uplink time aligned; inactivePosSRS-ValidityAreaTAT which controls how long the MAC entity considers Positioning SRS transmission in RRCJNACTIVE to be uplink time aligned when SRS positioning validity area is configured. Itm-Candidate-TimeAlignmentTimer which controls how long the MAC entity considers the CLTM candidate cells to be uplink time aligned or how long C-LTM cell switch is performed. The MAC entity shall: 1> when an LTM Cell Switch Command MAC CE is received and the Timing Advance Command is not set as FFF: 2> apply the Timing Advance Command for the PTAG; 2> start or restart the timeAlignmentTimer associated with the PTAG. 1> when an LTM Cell Switch Command MAC CE is received and the LTM Cell Switch Command MAC CE is not for C-LTM Cell Switch, and the Timing Advance Command is set as FFF, and the UE has successfully measured the Timing Advance: 2> apply the measured Timing Advance for the PTAG; 2> start or restart the timeAlignmentTimer associated with the PTAG. Together with the above procedure, for the C-LTM procedure, the following procedure can be used to manage TA: 1> when a conditional LTM cell switch procedure is executed or triggered for a LTM candidate cell as in Section 2.1.4: 2> apply the stored (or measured) TA value associated with the LTM target cell (e.g. one of LTM candidate cells) for the PTAG 2> start or restart the timeAlignmentTimer associated with the PTAG 1> when a conditional LTM cell switch is executed or triggered for a LTM candidate cell as in Section 2.1.4, and the UE has successfully measured the Timing Advance (or received the Timing Advance from the received MAC CE): 2> apply the measured (or received) Timing Advance for the PTAG; 2> start or restart the timeAlignmentTimer associated with the PTAG.] For the C-LTM procedure, another procedure to manage TA is used: 1> when a conditional LTM cell switch procedure is executed or triggered for a LTM candidate cell as in Section 2.1.4 and the Itm-Candidate-TimeAlignmentTimer associated with the LTM candidate cell is running: 2> apply the stored (or measured) TA value associated with the LTM target cell (e.g. one of LTM candidate cells) for the PTAG 2> start or restart the timeAlignmentTimer (or Itm-Candidate-TimeAlignmentTimer) associated with the PTAG (or associated with the corresponding LTM candidate cell) 1> when a conditional LTM cell switch is executed or triggered for a LTM candidate cell as in Section 2.1.4, and the Itm-Candidate-TimeAlignmentTimer associated with the LTM candidate cell is not running, and the UE has successfully measured the Timing Advance (or received the Timing Advance from the received MAC CE): 2> apply the measured (or received) Timing Advance for the PTAG; 2> start or restart the timeAlignmentTimer (or Itm-Candidate-TimeAlignmentTimer) associated with the PTAG (or associated with the corresponding LTM candidate cell). For the C-LTM procedure, another procedure is used to manage TA: 1> when a conditional LTM cell switch procedure is executed or triggered for a LTM candidate cell as above and the Itm-Candidate-TimeAlignmentTimer associated with the LTM candidate cell is running: 2> apply the stored (or measured) TA value associated with the LTM target cell (e.g. one of LTM candidate cells) for the PTAG 2> start or restart the timeAlignmentTimer (or Itm-Candidate-TimeAlignmentTimer) associated with the PTAG (or associated with the corresponding LTM candidate cell), if not running 1> when a conditional LTM cell switch is executed or triggered for a LTM candidate cell as above, and the Itm-Candidate-TimeAlignmentTimer associated with the LTM candidate cell is not running, and the UE has successfully measured the Timing Advance (or received the Timing Advance from the received MAC CE): 2> apply the measured (or received) Timing Advance for the PTAG; 2> start or restart the timeAlignmentTimer (or Itm-Candidate-TimeAlignmentTimer) associated with the PTAG (or associated with the corresponding LTM candidate cell), if not running C-LTM needs to consider multiple LTM candidate cells in the C-LTM cell switch procedure and the network is not able to know when C-LTM cell switch is executed or triggered because it is based on when the configured conditions are met (i.e. UE decides the timing for C-LTM cell switch execution) as described above. For this reason, if the network has a solution to easily update TA values for LTM candidate cells, it would be helpful to manage uplink synchronization for LTM candidate cell. Therefore, a new MAC CE (e.g. LTM Candidate Timing Advance Command MAC CE) is defined, which includes TA values for cells (or LTM candidate cells or SCells or PCell) to let UE apply it to manage TA values and TAT timer for each cell or TAG. The maintenance of UL Synchronization for LTM candidate cell can be controlled or managed by a new MAC CE (e.g. LTM Candidate Timing Advance Command MAC CE) as below : The MAC entity shall for each LTM candidate cell: 1> when a LTM Candidate Timing Advance Command MAC CE is received 2> store the TA value in the Timing Advance Command MAC CE for the indicated LTM candidate cell; 2> start or restart the timeAlignmentTimer (or Itm-Candidate-TimeAlignmentTimer) associated with the PTAG (or associated with the indicated LTM candidate cell) For the new MAC CE (e.g. LTM Candidate Timing Advance Command MAC CE), another procedure is defined to manage TA (In this procedure, the Itm-Candidate-TimeAlignmentTimer can be extended whenever the new MAC CE is received, in order to prolong C-LTM cell switch procedure): The MAC entity shall for each LTM candidate cell: 1> when a LTM Candidate Timing Advance Command MAC CE is received 2> store the TA value in the Timing Advance Command MAC CE for the indicated LTM candidate cell; 2> if C-LTM cell switch procedure is ongoing (or triggered or executed): 3> start or restart the Itm-Candidate-TimeAlignmentTimer (e.g. associated with the PTAG (or associated with the indicated LTM candidate cell)) 2> else: 3> start or restart the timeAlignmentTimer associated with PTAG. For the new MAC CE (e.g. LTM Candidate Timing Advance Command MAC CE), another procedure is defined to manage TA(ln this procedure, the new MAC CE includes the field indicating a special value (e.g. FFF): The MAC entity shall for each LTM candidate cell: 1> when a LTM Candidate Timing Advance Command MAC CE is received and the Timing Advance Command is not set as FFF: 2> store the TA value in the Timing Advance Command MAC CE for the indicated LTM candidate cell; 2> start or restart the timeAlignmentTimer (or Itm-Candidate-TimeAlignmentTimer) associated with the PTAG (or associated with the indicated LTM candidate cell) 1> when a LTM Candidate Timing Advance Command MAC CE described is received and the Timing Advance Command is set as FFF: 2> stop the timeAlignmentTimer (or Itm-Candidate-TimeAlignmentTimer) associated with the PTAG (or associated with the indicated LTM candidate cell) The LTM Candidate Timing Advance Command MAC CE is identified by MAC subheader with eLCID. It has a fixed size and consists of two octets: Timing Advance Command: This field indicates the index value Ta (0, 1,2... 63) used to control the amount of timing adjustment that MAC entity has to apply when UE switches to the candidate cell (e.g. during LTM or C-LTM), where the LTM or C-LTM candidate cell is indicated by the latest PDCCH order before UE receives this MAC CE. The length of the field is 12 bits; Tl: If two TAGs are configured for the LTM or C-LTM candidate cell indicated by the latest PDCCH order before UE receives this MAC CE, this field indicates one of the two TAGs to which the Timing Advance Command is applied. The field set to 0 indicates the tag2-ld and the field set to 1 indicates the tag-Id of the LTM or C-LTM candidate cell; The following relates to Configured grant handling for RACH-less C-LTM cell switch. As described above, RACH-less LTM cell switch can reduce the latency to change the serving cell. Embodiments may apply RACH-less LTM cell switch to C-LTM cell switch, i.e. a similar principle for the RACH-less LTM cell switch can be applied to C-LTM cell switch procedure. However, given that the LTM and C-LTM can be extended to the dual connectivity case (i.e. UE configured with MCG and SCG), we need to consider the cell group carefully because the behaviour should be different for MCG and SCG, respectively. For example, for the case that the MAC entity considers the configured grant as not valid based on SSB evaluation process during RACH-less LTM cell switch process, the MAC entity should initiate Random Access procedure to have fast re-connection with the network if the MAC entity is associated with MCG (i.e. to avoid unnecessary data interruption). However, for this case, if the MAC entity is associated with SCG, the MAC entity should not initiate Random Access procedure (i.e. should not do anything) and it can just wait for SCG’s instruction (e.g. PDCCH order or dynamic grant). In addition to this, how to select the beam for the initial UL transmission for C-LTM Cell switch should be considered as well. To achieve the above, the following procedure is defined: For the uplink grant configured for configured grant Type 1 for RACH-less handover or RACH-less C-LTM, if the configured uplink grant is valid, the MAC entity shall: 1> if, after the initial transmission of RACH-less handover or RACH-less C-LTM has been performed according to Section 2.2, RACH-less handover is not successfully completed: 2> if the SSB corresponding to the configured UL grant has the same SSB index as the SSB selected for the initial transmission of RACH-less handover (or RACH-less C-LTM) (i.e., retransmission of initial transmission of RACH-less handover or RACH-less C-LTM): 3> select this SSB; 3> indicate the SSB index corresponding to the configured uplink grant to the lower layer; 3> consider this configured uplink grant as valid. 1> else if at least one SSB corresponding to the configured uplink grant with SS-RSRP above cg-RRC-RSRP-ThresholdSSB is available: 2> select an SSB with SS-RSRP above cg-RRC-RSRP-ThresholdSSB amongst the SSB(s) associated with the configured uplink grant; 2> indicate the selected SSB index to the lower layer; 2> consider this configured uplink grant as valid. 1> else: 2> consider this configured uplink grant as not valid; 2> initiate Random Access procedure in clause 5.1 if the MAC is associated with MCG. After an uplink grant is configured for a configured grant Type 2, the MAC entity shall consider sequentially that the configured uplink grant, or the first configured uplink grant in a multi-PUSCH configured grant, in the Nth (N >0) periodicity occurs. Similar LTM principles apply for MCG LTM and SCG LTM in NR-DC. Simultaneous LTM for PCell change and LTM for PSCell change is not supported. Only SN-initiated SCG LTM is supported. The following cases for simultaneous configuration of MCG LTM and SCG LTM for a UE are supported: Inter-MN MCG LTM and intra-MN MCG LTM; Inter-MN MCG LTM and intra-SN SCG LTM; Inter-SN SCG LTM and intra-SN SCG LTM; Inter-SN SCG LTM and intra-MN MCG LTM. In order to not support simultaneous LTM cell changes for MCG and SCG, the UE releases the LTM candidate configuration for MCG(i.e. Itm-Config) and LTM candidate configuration for SCG (i.e. Itm-ConfigSCG, if configured) if the LTM cell switch (e.g. LTM cell switch execution) is triggered on the MCG, which avoids possible simultaneous LTM cell changes for the case not supported by the above scenarios. However, if the LTM cell switch (e.g. LTM cell switch execution) is triggered on the SCG, UE release the LTM candidate configuration for SCG (i.e. Itm-Config), i.e. UE can keep the LTM candidate configuration for MCG, if configured. The following relates to LTH execution procedure (or LTM command). In an embodiment, TA acquisition of candidate cell(s) before LTM cell switch command is supported as described above. By this, as the source cell / DU gets to know the value and the validity of candidate cell TA, it can determine whether it can initiate a RACH-less solution for LTM cell switch and then determine whether it needs to include a beam indication (e.g. TCI state) and TA information in the first MAC CE (i.e. LTM Command MAC CE). Therefore, the network can indicate a valid TA to the UE or indicate whether a TA is still valid in the first MAC CE. Upon the reception of the TA information indicated in LTM MAC CE, the UE can apply the TA value and start the TA timer for the target LTM candidate cell upon LTM execution (i.e. LTM cell switch) and UE can perform LTM cell switch without Random access procedure (i.e. with RACH-less solution) if TAT for the target LTM candidate cell is running (i.e. TA value is valid) or if Beam failure is not detected for the target LTM candidate cell, which means that UE can monitor PDCCH from the target LTM candidate cell or UE can use configured grants the first UL data transmission to the target cell for RACH-less LTM execution (LTM cell switch). Otherwise, the UE can perform LTM execution procedure with Random Access procedure. In an embodiment, the first MAC CE to be sent to UE can be generated by the source cell (or gNB), i.e. the MAC entity of the source cell (or gNB) generates the first MAC CE including the contents (e.g. TA value or BWP ID, Configuration Identity, etc as described abive) and sends it to the UE in order to trigger LTM cell switch procedure. In another embodiment, the first MAC CE to be sent to UE can be generated by the target cell (or gNB or CU(Central Unit)), i.e. the MAC entity of the target cell (or gNB or CU(Central Unit)) generates the first MAC CE including the contents (e.g. TA value or BWP ID, Configuration Identity, etc as described above) and forwards it to the source cell (or DU(Distributed Unit)) (e.g. in Xn message via Xn interface or in RRC message or in F1-AP message), and the source cell (or DU) sends it to the UE in order to trigger LTM cell switch procedure. To maintain Uplink time alignment efficiently, one of the following options for the behaviours of the MAC entity can be implemented: RRC configures the following parameters for the maintenance of UL time alignment: timeAlignmentTimer (per TAG) which controls how long the MAC entity considers the Serving Cells belonging to the associated TAG to be uplink time aligned; Option 1: In this option, the Timing Advance Command value (or field) is optional (i.e. can be either present or absent) in the first MAC CE (LTM Command MAC CE). Upon the reception of the first MAC CE (LTM Command MAC CE), the corresponding MAC behaviour is as follows: The MAC entity shall: 1> if the MAC entity receives an LTM Command MAC CE on a Serving Cell: 2> indicate to upper layers that the LTM Command MAC CE is received (triggering the LTM cell switch procedure); 2> perform MAC reset; 2> indicate to upper layers the Target Configuration ID (i.e. identifier for target LTM candidate cell) included in the MAC CE; 2> if Timing Advance Command value is present (or included) or if Timing Advance Command indicates that the Timing Advance value needs to be updated or is not valid anymore or if keeping the Timing Advance or using the TA of the source cell(or Serving cell) is indicated (i.e. the LTM candidate cell indicated by Target Configuration ID (or Serving cell ID) in the first MAC CE, belongs to PTAG) or if Timing Advance Command value is not set as a special value (e.g. 000..0 or 111...1) (e.g. the value indicates RACH-less LTM candidate cell switch) of if RRC parameter indicating RACH-less LTM execution (or indicating the same value as the serving cell) is configured : 3> process the received Timing Advance Command. In another embodiment, UE can process the received Timing Advance Command if Timing Advance Command indicates that the Timing Advance value needs to be updated or is not valid anymore or if keeping the Timing Advance or using the TA of the source cell(or Serving cell) is not indicated, in order to avoid unnecessary processing; 3> when an LTM Command MAC CE including a Timing Advance Command is received (or if Timing Advance Command indicates that the Timing Advance value needs to be updated or is not valid anymore or if keeping the Timing Advance or using the TA of the source cell(or Serving cell) is not indicated): 4> apply the Timing Advance Command for the PTAG or the target LTM candidate cell(or the indicated LTM candidate cell) (i.e. UE can apply and store the Timing Advance value for the PTAG or the taget LTM candidate cell); 4> start or restart the timeAlignmentTimer associated with the PTAG or the target LTM candidate cell(or the indicated LTM candidate cell); In another embodiment, UE can start or restart the timeAlignmentTimer associated with the PTAG or the target LTM candidate cell(or the indicated LTM candidate cell) only if the timeAlignmentTimer associated with PTAG or the target LTM candidate cell(or the indicated LTM candidate cell)is not running, in order to avoid unnecessary update procedure. 3> indicate to upper layers to skip the Random Access procedure for this LTM cell switch or the target LTM candidate cell(or the indicated LTM candidate cell). In another embodiment, UE can indicate to upper layers to skip the Random Access procedure for this LTM cell switch or the target LTM candidate cell(or the indicated LTM candidate cell) if TAT(timeAlignmentTimer) for the target / indicated LTM candidate cell (or PTAG) is running (i.e. TA value is vaild) or if Beam failure is not detected for the target LTM candidate cell (i.e. if BFI_COUNTER <beamFailurelnstanceMaxCount for the target / indicated LTM candidate cell (the number of Beam failure Indication is smaller than the maximum number for beam failure detection). 2> else (if Timing Advance Command value is absent (or not included) or if keeping the Timing Advance or using the TA of the source cell(or Serving cell) is not indicated (i.e. the LTM candidate cell indicated by Target Configuration ID (or Serving cell ID) in the first MAC CE, does not belong to PTAG) or if Timing Advance Command value is set as a special value (e.g. 000..0 or 111...1) (e.g. the value indicates RACH-based LTM candidate cell switch) or of if RRC parameter indicating RACH-less LTM execution (or indicating the same value as the serving cell) is not configured: 3> indicate to upper layers that a Random Access Procedure is needed for this LTM cell switch or the target LTM candidate cell(or the indicated LTM candidate cell) or indicate to upper layers to trigger the Random Access procedure for this LTM cell switch or the target LTM candidate cell(or the indicated LTM candidate cell); In another embodiment, UE can indicate to upper layers that a Random Access Procedure is needed for this LTM cell switch or the target LTM candidate cell(or the indicated LTM candidate cell) if T / \T(timeAlignmentTimer) for the target / indicated LTM candidate cell (or PTAG) is not running (i.e. TA value is not vaild) or if Beam failure is detected for the target / indicated LTM candidate cell (i.e. if BFI_COUNTER >= beamFailurelnstanceMaxCount for the target / indicated LTM candidate cell(or PTAG) (the number of Beam failure Indication is larger than or equal to the maximum number for beam failure detection). 3> UE can ignore the received Timinng Advance Command in order to avoid unnecessary update procedure. 2> if TCI state information is included: 3> consider the SSB corresponding to the indicated TCI state as the selected SSB for the initial uplink transmisision towards the candidate cell; 3> indicate to lower layers the information regarding the TCI state information included in the LTM Command MAC CE. Option 2: In this option, the Timing Advance Command value (or field) is always present in the first MAC CE (LTM Command MAC CE). Upon the reception of the first MAC CE (LTM Command MAC CE), the corresponding MAC behaviour is as follows: The MAC entity shall: 1> if the MAC entity receives an LTM Command MAC CE on a Serving Cell: 2> indicate to upper layers that the LTM Command MAC CE is received (triggering the LTM cell switch procedure); 2> perform MAC reset; 2> indicate to upper layers the Target Configuration ID (i.e. identifier for target LTM candidate cell) included in the MAC CE; 2> if Timing Advance Command value is not set as a special value (e.g. 000..0 or 111...1) or if Timing Advance Command indicates that the Timing Advance value needs to be updated or is not valid anymore of if RRC parameter indicating RACH-less LTM execution (or indicating the same value as the serving cell) is configured: 3> process the received Timing Advance Command; 3> when an LTM Command MAC CE including a Timing Advance Command is received: 4> apply the Timing Advance Command for the PTAG or the target LTM candidate cell(or the indicated LTM candidate cell) (i.e. UE can apply and store the Timing Advance value for the PTAG or the taget LTM candidate cell); 4> start or restart the timeAlignmentTimer associated with the PTAG or the target LTM candidate cell(or the indicated LTM candidate cell); In another embodiment, UE can start or restart the timeAlignmentTimer associated with the PTAG or the target LTM candidate cell(or the indicated LTM candidate cell) only if the timeAlignmentTimer associated with PTAG or the target LTM candidate cell(or the indicated LTM candidate cell)is not running, in order to avoid unnecessary update procedure. 3> indicate to upper layers to skip the Random Access procedure for this LTM cell switch or the target LTM candidate cell(or the indicated LTM candidate cell). In another embodiment, UE can indicate to upper layers to skip the Random Access procedure for this LTM cell switch or the target LTM candidate cell(or the indicated LTM candidate cell) if TAT(timeAlignmentTimer) for the target / indicated LTM candidate cell (or PTAG) is running (i.e. TA value is vaild) or if Beam failure is not detected for the target LTM candidate cell (i.e. if BFI_COUNTER <beamFailurelnstanceMaxCount for the target / indicated LTM candidate cell (the number of Beam failure Indication is smaller than the maximum number for beam failure detection). 2> else (if Timing Advance Command value is set as a special value (e.g. 000..0 or 111...1)) of if RRC parameter indicating RACH-less LTM execution (or indicating the same value as the serving cell) is not configured: 3> indicate to upper layers that a Random Access Procedure is needed for this LTM cell switch or the target LTM candidate cell(or the indicated LTM candidate cell) or indicate to upper layers to trigger the Random Access procedure for this LTM cell switch or the target LTM candidate cell(or the indicated LTM candidate cell); In another embodiment, UE can indicate to upper layers that a Random Access Procedure is needed for this LTM cell switch or the target LTM candidate cell(or the indicated LTM candidate cell) if TAT(timeAlignmentTimer) for the target / indicated LTM candidate cell (or PTAG) is not running (i.e. TA value is not vaild) or if Beam failure is detected for the target / indicated LTM candidate cell (i.e. if BFI_COUNTER >= beamFailurelnstanceMaxCount for the target / indicated LTM candidate cell(or PTAG) (the number of Beam failure Indication is larger than or equal to the maximum number for beam failure detection). 3> UE can ignore the received Timinng Advance Command in order to avoid unnecessary update procedure. 2> if TCI state information is included: 3> consider the SSB corresponding to the indicated TCI state as the selected SSB for the initial uplink transmisision towards the candidate cell; 3> indicate to lower layers the information regarding the TCI state information included in the LTM Command MAC CE. In an embodiment, alternatively, the TA value (e.g. Timing Advance Command) can be configured in each LTM candidate cell configuration in RRCReconfiguration message, and can be applied to UE or the maintenance of TAT timers. The network may activate and deactivate the TCI states of LTM candidate cell(s) configured in RRC configuration by sending the fourth MAC CE (i.e. LTM Candidate Cell TCI States Activation / Deactivation MAC CE described above). To enable this, several options are available to activate and deactivate the TCI states upon LTM execution and one of the options can be implemented: Option 1: In this option, we can restrict the transmission for the fourth MAC CE to the transmission together with the first MAC CE (LTM Command MAC CE described in Section 4.1). For example, the network can send the fourth MAC CE together with the first MAC CE (i.e. both MAC CEs can be included in the same MAC PDU) to activate and deactivate the TCI states for LTM cell switch. If the MAC entity receives a Candidate Cell TCI States Activation / Deactivation MAC CE on a Serving Cell, it indicates to lower layers (i.e. PHY layer (Physical layer)) the information regarding the Candidate Cell TCI States Activation / Deactivation MAC CE for the indicated LTM candidate cell from the first MAC CE. If the fourth MAC CE is not received (e.g. with the first MAC CE), the MAC entity indicates to the lower layers the usage of the indicated (or configured) TCI by RRCReconfiguration for the indicated LTM candidate cell from the first MAC CE, or the lower layers uses the the indicated (or configured) TCI by RRCReconfiguration for the indicated LTM candidate cell from the first MAC CE. Option 2: In this option, we do not restrict the transmission for the fourth MAC CE, i.e. the network can send the fourth MAC CE before the transmission of the first MAC CE or regardless of the transmission of the first MAC CE (LTM Command MAC CE described in Section 4.1), in order to activate and deactivate the TCI states for LTM cell switch.. However, even if the MAC entity receives a Candidate Cell TCI States Activation / Deactivation MAC CE on a Serving Cell, it indicates to lower layers (i.e. PHY layer (Physical layer)) the information regarding the Candidate Cell TCI States Activation / Deactivation MAC CE for the indicated LTM candidate cell from the first MAC CE upon the recepton of the first MAC CE or upon LTM execution. If the fourth MAC CE was not received, the MAC entity indicates to the lower layers the usage of the indicated (or configured) TCI by RRCReconfiguration for the indicated LTM candidate cell from the first MAC CE or the lower layers uses the the indicated (or configured) TCI by RRCReconfiguration for the indicated LTM candidate cell from the first MAC CE upon the recepton of the first MAC CE or upon LTM execution. The LTM Cell Switch Command MAC CE is identified by MAC subheader with eLCID. It has a variable size with following fields: R: Reserved bit, set to 0; Target Configuration ID: This field indicates the index of candidate target configuration to apply for LTM cell switch, corresponding to Itm-Candidateld minus 1. The length of the field is 3 bits; Timing Advance Command: This field indicates whether the TA is valid for the LTM target cell (i.e. the SpCell corresponding to the target configuration indicated by Target Configuration ID field). If the value of this field is set to FFF, this field indicates that no valid timing adjustment is available for the PTAG of the LTM target cell; otherwise, this field indicates the index value Ta used to control the amount of timing adjustment that the MAC entity has to apply], and that the UE can skip the Random Access procedure for this LTM cell switch. If tag-ld-ptr is configured for the TCI state indicated by the UL TCI state ID field, if present, or by the TCI state ID field otherwise, in the LTM target cell and tag-ld-ptr is set to value n1, this field indicates the TA for the TAG indicated by the tag2-ld of the LTM target cell; otherwise, this field indicates the TA for the TAG indicated by the tag-id of the LTM target cell. The length of the field is 12 bits; TCI state ID: This field indicates and activates the TCI state for the LTM target cell (i.e. the SpCell of the target configuration indicated by the Target Configuration ID field). The TCI state is identified by TCI-Stateld in Itm-DL-OrJointTCI-StateToAddModList. If the value of unifiedTCI-StateType in the Itm-TCI-Info of the configuration indicated by Target Configuration ID field is joint, this field is for joint TCI state, otherwise, this field is for downlink TCI state. The length of the field is 7 bits; UL TCI state ID: This field indicates and activates the uplink TCI state for the LTM target cell (i.e. the SpCell of the target configuration indicated by the Target Configuration ID field). The UL TCI state is identified by TCI-UL-Stateld in Itm-UL-TCI-StateToAddModList. The octet containing this field (i.e. this field and the two reserved bits in the same octet) is included if the value of unifiedTCI-StateType in the Itm-TCI-Info of the configuration indicated by Target Configuration ID field is separate. The length of the field is 6 bits; C: This field indicates the presence of the contention-free Random Access Resources fields. If the value of this field is set to 1, the following fields are present: Random Access Preamble index field, S / U field, SS / PBCH index field, PRACH Mask index field, Repetition number field and the reserved bits in the same octet. If the value of this field is set to 0, these fields are absent. S / U: This field indicates which UL carrier to transmit the PRACH of the contention-free Random Access Resources. If the value of this field is set to 1, SUL is used; otherwise, NUL is used. The length of the field is 1 bit; Random Access Preamble index: This field indicates the Random Access Preamble index of the contention-free Random Access Resources. This field should not be set to ObOOOOOO. The length of the field is 6 bits; SS / PBCH index: This field indicates the SS / PBCH that shall be used to determine the RACH occasion for the PRACH transmission of the contention-free Random Access Resources. The length of the field is 6 bits; PRACH Mask index: This field indicates the RACH occasion(s) associated with the SS / PBCH indicated by 'SS / PBCH index' for the PRACH transmission of the contention-free Random Access Resources. It indicates a subset of RACH occasion(s) from the rach-ConfigDedicated for the UL carrier (indicated by S / U field), (if provided, otherwise it indicates a subset of RACH occasion(s) from the rach-ConfigCommon for the UL carrier (indicated by S / U field) in the UL BWP configuration of firstActiveUplinkBWP-ld. When the repetition number field is not set to 0, the UE ignores this field. The length of the field is 4 bits; Repetition number: This field indicates the Msg1 repetition number to be applied to the contention-free Random Access. If this field is set to 0, Msg1 repetition number does not apply. If this field is set to 1, the Msg1 repetition number is 2. If this field is set to 2, the Msg1 repetition number is 4. If this field is set to 3, the Msg1 repetition number is 8. The length of the field is 2 bits. At least some of the example embodiments described herein may be constructed, partially or wholly, using dedicated special-purpose hardware. Terms such as ‘component’, ‘module’ or ‘unit’ used herein may include, but are not limited to, a hardware device, such as circuitry in the form of discrete or integrated components, a Field Programmable Gate Array (FPGA) or Application Specific Integrated Circuit (ASIC), which performs certain tasks or provides the associated functionality. In some embodiments, the described elements may be configured to reside on a tangible, persistent, addressable storage medium and may be configured to execute on one or more processors. These functional elements may in some embodiments include, by way of example, components, such as software components, object-oriented software components, class components and task components, processes, functions, attributes, procedures, subroutines, segments of program code, drivers, firmware, microcode, circuitry, data, databases, data structures, tables, arrays, and variables. Although the example embodiments have been described with reference to the components, modules and units discussed herein, such functional elements may be combined into fewer elements or separated into additional elements. Various combinations of optional features have been described herein, and it will be appreciated that described features may be combined in any suitable combination. In particular, the features of any one example embodiment may be combined with features of any other embodiment, as appropriate, except where such combinations are mutually exclusive. Throughout this specification, the term “comprising” or “comprises” means including the component(s) specified but not to the exclusion of the presence of others. Attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and / or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and / or steps are mutually exclusive. Each feature disclosed in this specification (including any accompanying claims, abstract and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features. The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed. Glossary of terms. Throughout this specification, various acronyms are used. The following list includes certain relevant acronyms / abbreviations. BCCH Broadcast Control Channel BCH Broadcast Channel BFD Beam Failure Detection C-RNTI Cell RNTI CBRA Contention Based Random Access CFRA Contention Free Random Access CG Configured Grant CHO Conditional Handover CORESET Control Resource Set CPA Conditional PSCell Addition CPC Conditional PSCell Change DAPS Dual Active Protocol Stack DCI Downlink Control Information DCP DCI with CRC scrambled by PS-RNTI G-CS-RNTI Group Configured Scheduling RNTI G-RNTI Group RNTI IAB Integrated Access and Backhaul L2 Layer-2 L3 Layer-3 LTM L1 / L2-Triggered Mobility : a PCell (or PSCell) cell switch procedure that the network triggers via MAC CE based on L1 measurements. RACH-less LTM indicates that a UE skips the Random Access procedure at LTM cell switch. Subsequent LTM indicates LTM between candidate cells without RRC reconfiguration in between MBS Multicast / Broadcast Services MCCH MBS Control Channel MIB Master Information Block MNO Mobile Network Operator MRB MBS Radio Bearer MT Mobile Termination MTCH MBS Traffic Channel MUSIM Multi-Universal Subscriber Identity Module NB-loT Narrow Band Internet of Things NR NR Radio Access NTN Non-Terrestrial Network P-RNTI Paging RNTI PCH Paging Channel PCI Physical Cell Identifier PDCCH Physical Downlink Control Channel PDSCH Physical Downlink Shared Channel PLMN Public Land Mobile Network PO Paging Occasion PRACH Physical Random Access Channel PRB Physical Resource Block PS-RNTI Power Saving RNTI PSS Primary Synchronisation Signal PTM Point to Multipoint PTP Point to Point PUCCH Physical Uplink Control Channel PUSCH Physical Uplink Shared Channel PWS Public Warning System QAM Quadrature Amplitude Modulation QFI QoS Flow ID QoE Quality of Experience RA Random Access RA-RNTI Random Access RNTI RACH Random Access Channel RMSI Remaining Minimum SI RNA RAN-based Notification Area RNAU RAN-based Notification Area Update RNTI Radio Network Temporary Identifier RSRP Reference Signal Received Power RSRQ Reference Signal Received Quality SDAP Service Data Adaptation Protocol SDT Small Data Transmission SIB System Information Block SI-RNTI System Information RNTI SMC Security Mode Command SMF Session Management Function SMTC SS / PBCH block Measurement Timing Configuration SpCell SPS Special Cell (i.e. PCell or PSCell or PCell in MCG or PSCell in SCG) Semi-Persistent Scheduling SR Scheduling Request SRS Sounding Reference Signal SS Synchronization Signal SSB SS / PBCH block SSS Secondary Synchronisation Signal TA Timing Advance TB Transport Block TPC Transmit Power Control TRP Transmit / Receive Point TRS Tracking Reference Signal UCI Uplink Control Information UDC Uplink Data Compression 10 UL-SCH Uplink Shared Channel UPF User Plane Function URLLC Ultra-Reliable and Low Latency Communications

Claims

1. A method for performing a Conditional L1 / L2-Triggered Mobility, C-LTM, cell switch by a User Equipment, UE, communicatively coupled to a wireless communication system, the method comprising the steps of:• being configured with a C-LTM candidate cell configuration comprising a target C-LTM candidate cell; and• determining that a C-LTM execution condition for a switch to the target C-LTM candidate cell is met;• determining a status of a timer associated with the target C-LTM candidate cell, wherein the timer is configured to indicate a validity period for an uplink Timing Advance, TA, of the target C-LTM candidate cell;• performing a Random Access Channel, RACH, -less cell switch procedure to the target C-LTM candidate cell using a stored TA value if the timer is determined to be running; and• performing a RACH-based cell switch procedure to the target C-LTM candidate cell if the timer is determined to be not running.

2. A method according to claim 1, wherein the timer associated with the target C-LTM candidate cell is an Itm-Candidate-TimeAlignmentTimer configured by a Radio Resource Control, RRC, protocol.

3. A method according to claim 1 or 2, further comprising the steps of:• receiving a LTM Candidate Timing Advance Command MAC Control Element, CE, for the target C-LTM candidate cell; and• in response to receiving the LTM Candidate Timing Advance Command MAC CE:• storing a TA value contained in the MAC CE; andstarting or restarting the timer associated with the target C-LTM candidate cell.

4. A method according to claim 3, wherein the LTM Candidate Timing Advance Command MAC CE comprises a field for a TA command index and a field for a Timing Advance Group, TAG, identifier.

5. A method according to claim 1, wherein the stored TA value used for the RACH-less cell switch procedure is received in a LTM Cell Switch Command MAC CE that triggers the C-LTM cell switch execution.

6. A method according to any of the preceding claims, wherein the RACH-based cell switch procedure is also performed if a beam failure is detected for the target C-LTM candidate cell.

7. A method according to any of the preceding claims, wherein, in a dual connectivity configuration, if the RACH-less cell switch procedure is not successfully completed, the method further comprises the step of initiating a Random Access procedure if a MAC entity of the UE is associated with a Master Cell Group, MCG.

8. A method according to claim 7, wherein, if the RACH-less cell switch procedure is not successfully completed, the method further comprises the step of refraining from initiating a Random Access procedure and waiting for a network instruction if the MAC entity of the UE is associated with a Secondary Cell Group, SCG.

9. A method according to any of the preceding claims, further comprising the step of maintaining security keys without performing a key update if the C-LTM cell switch occurs between a source cell and the target C-LTM candidate cell, and both cells belong to the same base station, gNB.

10. A method according to any of the preceding claims, wherein performing the RACH-based cell switch procedure comprises activating an uplink bandwidth part, UL BWP, on the target C-LTM candidate cell.

11. A method according to any of the preceding claims, wherein performing the RACH-less cell switch procedure further comprises the step of if an initial uplink transmission fails, selecting a new Synchronization Signal Block, SSB, from a set of SSBs associated with a configured uplink grant, wherein the selected SSB has a Reference Signal Received Power, RSRP, above a pre-configured threshold.

12. A UE arranged to perform the method of any preceding claim.A

Images