EP4691043A1

EP4691043A1 - Downlink pre-synchronization for time alignment establishment on ltm candidate cells

Info

Publication number: EP4691043A1
Application number: EP24716400.7A
Authority: EP
Inventors: Venkatarao Gonuguntla; Claes Tidestav; Icaro Leonardo DA SILVA; Antonino ORSINO; Maomao CHEN LARSSON
Original assignee: Telefonaktiebolaget LM Ericsson AB
Current assignee: Telefonaktiebolaget LM Ericsson AB
Priority date: 2023-04-04
Filing date: 2024-04-04
Publication date: 2026-02-11
Also published as: WO2024208963A1; KR20250164851A; AR132306A1; CN121195570A

Abstract

A method performed by a UE in a wireless communication network for performing time alignment with a L1/L2 based inter-cell mobility (LTM) candidate cell includes receiving an LTM configuration including an LTM candidate cell configuration for TA establishment and/or TA update with the LTM candidate cell, and receiving an indication to perform downlink (DL) pre-synchronization with the LTM candidate cell. The UE performs DL pre-synchronization with the LTM candidate cell in response to the indication. After performing DL pre-synchronization with the LTM candidate cell, the UE receives, from a serving cell, a trigger for TA establishment and/or TA update to the LTM candidate cell, and transmits, in response to the trigger, an UL signal to the LTM candidate cell for TA establishment and/or update.

Description

DOWNLINK PRE-SYNCHRONIZATION FOR TIME ALIGNMENT ESTABLISHMENT ON LTM CANDIDATE CELLS

TECHNICAL FIELD

[0001] The present disclosure relates to wireless communication systems, and in particular, to establishment of timing alignment in wireless communication systems.

BACKGROUND

[0002] In a wireless communication system, different user equipment devices (UEs) in the same cell may typically be situated at different locations with different distances to the base station that serves the cell. Transmissions from different UEs may therefore experience different delays until they reach the base station. To make sure that an uplink (UL) transmission from a UE reaches the base station within a corresponding receive window for the base station, an uplink timing control procedure may be used. The uplink timing procedure reduces intracell interference from occurring, both between UEs assigned to transmit in consecutive subframes and between UEs transmitting on adjacent subcarriers.

[0003] Time alignment of the uplink transmissions is achieved by applying a timing advance at the UE transmitter relative to the received downlink timing. The main purpose of this is to counteract differing propagation delays between different UEs, as shown in Figure 1 for a long term evolution (LTE) eNodeB. In particular, Figure 1 illustrates time alignment of uplink transmissions for a case (a) without timing advance and for a case (b) with timing advance.

[0004] To achieve time alignment (i.e. , to obtain UL synchronization), the base station (e.g., gNodeB, eNodeB) derives a timing advance value that the UE should use for the UL transmissions in order for its transmissions to reach the base station within the receive window. The base station provides the timing advance value to the UE. When the UE accesses a cell, it uses a random-access procedure in which the UE transmits a physical random access channel (PRACH) preamble (a so-called msgl) to the base station. The PRACH preamble is used by the base station to determine the UE’s initial timing advance value to use for UL transmissions in the cell. During the connection, the base station continuously monitors whether the UE needs to advance/delay the UL transmissions, in order to compensate for changes in propagation delay, and indicates to the UE if there is a need to change the timing advance value. [0005] Time Alignment in L3 mobility (Handover/Reconfiguration with Sync) [0006] In a legacy layer 3 (L3) mobility procedure in 5G New Radio (NR), also called a reconfiguration with sync for the Master Cell Group (MCG), when the UE changes its primary cell (PCell), the UE always performs a random access to the target PCell. As part of that procedure, the UE transmits a PRACH preamble in the UL, which enables the target gNodeB to calculate the timing advance value for the UE. The timing advance value is provided in the Random-Access Response (RAR), or msg3, from the base station, so that from msg3 onwards the UE is able to transmit UL messages on the physical uplink control channel (PUCCH) and/or physical uplink shared channel (PUSCH) with appropriate timing.

[0007] PRACH transmissions

[0008] In NR, various implementations of the cells are possible. For example, some cells (e.g., FR1 cells) may be implemented to provide large coverage, and some cells (e.g., FR2 cells) may be implemented to provide more throughput over a short coverage area. The maximum distance between base station and UE in a cell depends on the cell coverage area. Due to diverse implementations, the same set of PRACH preambles may not work well in all scenarios. To address this, different preamble formats with different lengths of PRACH preamble have been introduced in NR.

[0009] PRACH preambles are transmitted during RACH transmission occasions, or RACH occasions. The RACH occasion depends on the type of RACH being accessed. A RACH occasion is a set of resources specified in the time and frequency domain that are available or reserved for the transmission of RACH preamble by a UE.

[0010] In NR, two types of RACH are supported, namely contention-based RACH (CBRA) and contention free RACH (CFRA). For contention-based RACH, the RACH occasion is computed at the UE based on the configuration from the network (NW) and the certain conditions observed at the UE.

[0011] In NR, each beam is associated with a different synchronization signal (e.g., synchronization signal block, or SSB), which may, for example, be transmitted in a spatial direction. Each SSB is configured with certain preamble indices and certain RACH transmission occasions. Based on a SSB seen by the UE, the UE determines the preamble index to be transmitted and the RACH occasion where the preamble is to be transmitted. The NW can determine which beam UE has selected, because the NW configured the mapping between SSBs and RACH Occasions (RO). By detecting which RO a UE uses, the NW can figure out which SSB beam the UE has selected. [0012] The mapping between SSB and RACH Occasion is defined by the following two RRC parameters.

• msgl-FDM (configured in the RACH-ConfigGeneric information element (IE)).

• ssb-perRACH-OccasionAndCB-PreamblesPerSSB (configured in the RACH- ConflgCommon IE).

[0013] Contention free RACH is scheduled by the NW. The scheduling information indicates what information to transmit and what resources should be used to transmit it. This information is conveyed to the UE by a combination of radio resource control (RRC) messaging and physical downlink control channel (PDCCH) (e.g., through Downlink Control Information, DCI, message) order.

[0014] The RRC message that carries the CFRA related information is the RACH- ConflgDedicated IE.

[0015] L1/L2 inter-cell mobility or L1/L2 triggered inter-cell mobility (LTM) in

Rel-18

[0016] In Rel-18, the third generation partnership project (3GPP) has agreed on a Work Item on Further NR mobility enhancements in a technical area entitled L1/L2 based inter-cell mobility, or LTM.

[0017] According to the Work Item description for LTM, when a UE moves from the coverage area of one cell to another cell, at some point a serving cell change needs to be performed. Currently, a serving cell change is triggered by L3 measurements, and is done by RRC signalling that triggers a Reconfiguration with Synchronization for change of PCell and primary secondary cell (PSCell), as well as release/add for secondary cells (SCells) when applicable. All cases involve complete L2 (and LI) resets, leading to longer latency, larger overhead and longer interruption time than beam switch mobility. The goal of L1/L2 mobility enhancements is to enable a serving cell change via L1/L2 signalling to reduce the latency, signalling overhead and interruption time.

[0018] As part of L1-L2 inter-cell mobility measurement framework, it was agreed to support at least LI -reference signal received power (RSRP) as the reporting quantity. That means UE is required to report LI -RSRP of the candidate cells to the NW so that NW can use them for LTM handover (HO) decisions.

[0019] In Rel-17, as part of inter-cell beam management, a solution has been standardized where LI -RSRP is measured and reported on a CSI resource that are not associated to a PCI of the serving cells. [0020] Layer 3 handover requirements specified in [2] are reproduced in Table 1 below.

Table 1 - Excerpt from 3GPP TS 38.133 V18.0.0 described in Clause 9.2.5 for intra-frequency handover and Clause 9.3.4 for inter-frequency handover.

[0021] According to the requirements shown in Table 1, the L3 HO delayDhandover) equals the RRC processing delay of the HO command and the interruption time. The interruption delay includes software and hardware processing, cell search, acquisition of fine timing, and delay uncertainty of obtaining the PRACH preamble.

SUMMARY

[0022] Some embodiments described herein provide methods for reducing delay associated with a TA establishment/update procedure with an LTM candidate cell by a UE performing DL synchronization (which may also be referred to as pre-synchronization or DL pre-synchronization) with an LTM candidate cell.

[0023] A method performed by a UE in a wireless communication network for performing time alignment (TA) with a L1/L2 based inter-cell mobility (LTM) candidate cell includes receiving (602) an LTM configuration including an LTM candidate cell configuration for TA establishment and/or TA update with the LTM candidate cell, and receiving (603) an indication to perform downlink, DL, pre-synchronization with the LTM candidate cell. The UE performs (604) DL pre-synchronization with the LTM candidate cell in response to the indication. After performing DL pre-synchronization with the LTM candidate cell, the UE receives (606), from a serving cell, a trigger for TA establishment and/or TA update to the LTM candidate cell, and transmits (608), in response to the trigger, an UL signal to the LTM candidate cell for TA establishment and/or update.

[0024] The indication to perform DL pre-synchronization may be received in a same message as the LTM configuration.

[0025] The indication may be received in a medium access control, MAC, control element, CE, after receipt of the LTM configuration.

[0026] The method may further include transmitting a measurement report to a network node, wherein the measurement report contains measurements related to the LTM candidate cell, and wherein the indication may be received in response to the measurement report.

[0027] The LTM configuration may include LTM candidate cell configurations for TA establishment and/or TA update with a plurality of LTM candidate cells, the method further comprising selecting an LTM candidate cell for performing DL pre-synchronization from the plurality of LTM candidate cells. [0028] Selecting the LTM candidate cell for performing DL pre-synchronization may be performed in response to the indication.

[0029] The indication may indicate the LTM candidate cell to be selected for performing DL pre-synchronization. The trigger may be received after the indication.

[0030] The UL signal may be transmitted in a configured UL channel time/frequency resource.

[0031] The trigger may correspond to a radio resource control, RRC, message, an RRC information element, IE, a field, a parameter, a medium access control, MAC, control element, CE, or a physical downlink control channel, PDCCH, order, command or indication.

[0032] The serving cell may include a PCell, a PSCell and/or a SpCell.

[0033] The UL channel may include a physical random-access channel, PRACH, and wherein the signal may include a PRACH preamble.

[0034] The LTM configuration may be received in a radio resource control, RRC, reconfiguration message from the serving cell, and wherein the LTM configuration may include one or more LTM candidate cell configurations, to be applied upon reception of an LTM cell switch command.

[0035] The LTM candidate cell configuration may include one or more UL related parameters, the one or more UL related parameters comprising a PRACH preamble configuration, a PRACH occasion, and/or a PRACH frequency resource.

[0036] The LTM configuration may include a plurality of LTM candidate cell configurations for a plurality of LTM candidate cells, the method further comprising selecting a subset of the plurality of LTM candidate cells for performing DL presynchronization based on one or more rules.

[0037] Performing DL pre-synchronization with the LTM candidate cell may include one or more of: i) detecting and/or measuring at least one synchronization signal of the LTM candidate cell, wherein the synchronization signal may include a synchronization signal block, SSB, of the LTM candidate cell associated to an SSB index and/or identifier and transmitted in a spatial direction, a channel state information reference signal, CSI-RS, a tracking reference signal, TRS, a primary synchronization signal, PSS, and/or a secondary synchronization signal SSS; ii) performing fine time tracking and acquiring full timing information of the LTM candidate cell; iii) obtaining time boundaries of a time unit of a given LTM candidate cell, wherein the time unit may include one of a time slot, an orthogonal frequency division multiplexing, OFDM, symbol, a subframe, and/or radio frame; and iv) synchronizing a clock with the time boundaries of a time unit of a given LTM candidate cell.

[0038] Some embodiments provide a method performed by a network node in a wireless communication network for LTM of a UE to a candidate cell. The method includes transmitting (702) to the UE an LTM configuration including an LTM candidate cell configuration for TA establishment and/or TA update with the LTM candidate cell, transmitting (703) an indication to the U Eto perform downlink, DL, pre-synchronization to the LTM candidate cell, and transmitting (704) a trigger to the UE for TA establishment and/or TA update to the LTM candidate cell.

[0039] Transmitting the trigger for TA establishment may be performed after the UE has performed DL pre-synchronization with the LTM candidate cell in response to the LTM configuration.

[0040] The indication to perform DL pre-synchronization may be received in a same message as the LTM configuration.

[0041] The indication may be received in a medium access control, MAC, control element, CE, after receipt of the LTM configuration.

[0042] The method may further include transmitting a measurement report to a network node, wherein the measurement report contains measurements related to the LTM candidate cell, and wherein the indication may be received in response to the measurement report.

[0043] The LTM configuration may include LTM candidate cell configurations for TA establishment and/or TA update with a plurality of LTM candidate cells.

[0044] The indication may indicate an LTM candidate cell of the plurality of LTM candidate cells to be selected by the UE for performing DL pre-synchronization.

[0045] Transmitting the trigger may be performed after transmitting the indication.

[0046] The UL signal may be transmitted in a configured UL channel time/frequency resource.

[0047] The trigger may correspond to a radio resource control, RRC, message, an RRC information element (IE), a field, a parameter, a medium access control, MAC, control element, CE, or a physical downlink control channel, PDCCH, order, command or indication. [0048] The network node may be associated with a serving cell that serves the UE, and the serving cell may include a PCell, a PSCell and/or a SpCell.

[0049] The UL channel may include a physical random access channel, PRACH, and wherein the signal may include a PRACH preamble.

[0050] The LTM configuration may be transmitted in a radio resource control, RRC, reconfiguration message from the serving cell, and wherein the LTM configuration may include one or more LTM candidate cell configurations, to be applied upon reception of an LTM cell switch command.

[0051] The LTM candidate cell configuration may include one or more UL related parameters, the one or more UL related parameters comprising a PRACH preamble configuration, a PRACH occasion, and/or a PRACH frequency resource.

[0052] The LTM configuration may include a plurality of LTM candidate cell configurations.

BRIEF DESCRIPTION OF THE DRAWINGS

[0053] Figure 1 illustrates time alignment of uplink transmissions for cases with and without timing advance.

[0054] Figure 2 illustrates a timeline for L1/L2 inter-cell mobility.

[0055] Figure 3 illustrates an example of a procedure for establishing timing alignment (TA).

[0056] Figure 4 illustrates an example of DL pre-synchronization according to some embodiments.

[0057] Figure 5 illustrates an example signaling flow according to some embodiments.

[0058] Figure 6 illustrates a method performed by a UE in a wireless communication network for performing TA with a LTM candidate cell according to some embodiments.

[0059] Figure 7 illustrates a method performed by a network node in a wireless communication network for L1/L2 based inter-cell mobility of a U Eto a candidate cell according to some embodiments.

[0060] Figure 8 shows an example of a communication system in accordance with some embodiments.

[0061] Figure 9 shows an example of a UE in accordance with some embodiments. [0062] Figure 10 shows an example of a network node in accordance with some embodiments.

DETAILED DESCRIPTION

[0063] Two potential approaches and two potential timelines for establishing timing alignment (TA) in LTM cell switching have been discussed in the Rel-18 LTM discussions. These are shown in Figures 2 and 3. In particular, Figure 2 illustrates a RAN2- agreed baseline timeline for L1/L2 inter-cell mobility, and Figure 3 illustrates an example of a procedure for establishing TA in which the UE establishes TA by transmitting a PRACH preamble and receives the timing advance value in the LTM cell switch command.

[0064] In LTM, an LTM cell switch procedure has been agreed, in which the UE receives an LTM cell switch command (e.g., a medium access control, MAC, control element, CE, including an indication of one of the configured LTM candidate cells) and accesses the indicated LTM candidate cell. To further reduce the interruption time, it has also been agreed that the UE may be configured to establish time alignment with one or more LTM candidate cells before the triggering of the LTM cell switch, so that at the moment of the LTM cell switch the UE would not be required to trigger a Random Access procedure, and instead, the first UE action at the LTM candidate cell which becomes the target cell (i.e. the new PCell) is to monitor PDCCH and/or transmit an UL signal on PUCCH and/or PUSCH, which requires UL synchronization, or UL sync, to be established.

[0065] Different options for this time alignment procedure (for UL sync establishment) are still under discussion in 3GPP, but they all rely on the UE, while still connected to the PCell, receiving a trigger (e.g., PDCCH order) from the PCell for transmitting an UL signal (e.g., PRACH preamble) to an LTM candidate cell, so that the candidate distributed unit (DU) at the network side (responsible for the LTM candidate cell) which receives the preamble, calculates a timing advance value to be provided to the UE at some point in time, e.g., at the LTM cell switch command, or in a downlink (DL) response (e.g., via PCell or via the LTM candidate cell). The timing advance value is a value for the UE and the LTM candidate cell in which the UE transmits the PRACH preamble. The TA establishment procedure may be triggered for one or multiple LTM candidate cells.

[0066] Referring to Figure 3, in a procedure for LTM cell switching, a UE sends a measurement report to the central unit (CU) of its serving cell. The measurement report includes measurements of a candidate cell served by a candidate distributed unit (DU). Based on the measurement report, the CU sends a UE context setup request to a candidate

DU and obtains an LTM candidate configuration from the candidate DU.

[0067] The serving DU (S-DU) sends the UE an RRCReconflguration message including the LTE candidate configuration and a TA establishment configuration.

[0068] The S-DU then sends a trigger to the UE to perform DL sync with the LTM candidate cell. In response to the trigger, the UE then performs DL synchronization with the candidate cell. After synchronization, the UE transmits a random access preamble to the candidate cell. However, there is a DL synchronization delay between the time of the trigger for TA establishment until the next PRACH occasion when the UE can send the random access preamble for LTM cell switch. In response to the random access preamble, the candidate DU determines a TA value for the UE and provides the TA value to the S-DU. The UE provides measurement reports for candidate cells of the candidate DU to the S-DU, and based on the measurement reports, the S-DU decides to trigger an LTM cell switch to the candidate DU using the provided TA value. After the cell switch, the UE may transmit in the LTM candidate cell.

[0069] One challenge with the time alignment establishment procedure is that before the UE transmits the PRACH preamble to the LTM candidate cell, the UE needs to first perform a downlink (DL) synchronization to one or more SSB(s) of the LTM candidate cell so that the UE will be able to transmit the PRACH preamble upon reception of a TA establishment trigger, such as a PDCCH order.

[0070] That could possibly be avoided if the TA establishment procedure is triggered early, possibly far in time to the timing to trigger an LTM cell switch. However, a typical network implementation would only trigger the TA establishment procedure when there is some level of certainty that a particular LTM candidate is a high potential candidate, which may be known at the network (e.g., the source DU, or S-DU) based on the reception of further LI and/or L3 measurements on the LTM candidate cell. However, doing that would in principle require the procedure for TA establishment to be as fast as possible, so that the timing between TA establishment and LTM cell switch is not too close in time. Another potential issue with the longer delay to transmit the PRACH preamble for TA establishment is that it may not always be possible for the UE to try to synchronize with an SSB of an LTM candidate and receive/ transmit data at the same time from the serving cell(s). This may impact the throughput/ data rates of the UE.

[0071] As PRACH occasions and SSB(s) may be sparse (e.g., 10s of milliseconds) the procedure may not be that fast. The longer the TA establishment procedure takes, the closer the UE is to the timing to perform the LTM cell switch, which may also increase the chances of failure if, for example, the radio conditions of the serving cell(s) become much worse and/or the radio conditions on the LTM candidate become much better during TA establishment.

[0072] Another potential issue involving TA establishment with one or more LTM candidate cells is that the UE may be configured with multiple LTM candidate cells as the potential target cells. Based on the measurement reports from the UE, the NW may configure the UE to be handed over to one of the candidate cells. Although the UE could measure multiple cells, the UE may not be able to maintain DL synchronization with all the candidate cells.

[0073] Certain aspects of the disclosure and their embodiments may provide solutions to these or other challenges. Some embodiments described herein provide methods for reducing delay associated with a TA establishment/update procedure with an LTM candidate cell. By reducing the delay involved in the TA establishment/update procedure, it may be possible later for the network to trigger an LTM cell switch without a random access procedure. That is, the NW may trigger an LTM cell switch to an LTM candidate cell for which the UE has established time alignment/ UL synchronization. The UE receives an LTM cell switch command and transmits an UL signal on PUCCCH and/or PUSCH.

[0074] In some embodiments, a UE performs DL synchronization (which may also be referred to as pre-synchronization or DL pre-synchronization) with an LTM candidate cell. The UE receives a trigger from a serving cell to transmit an UL signal, such as a PRACH preamble, for TA establishment/update to the LTM candidate cell while the UE is connected to a serving cell. The trigger may include, for example, a PDCCH order from the Primary Cell, or Primary secondary cell group (SCG) cell.

[0075] The UE transmits the UL signal to the LTM candidate cell in a configured UL channel resource(s) in time and frequency, such as a PRACH occasion, based on the DL synchronization performed before the reception of the trigger from the serving cell.

[0076] Figure 4 illustrates an example of DL pre-synchronization according to some embodiments. Referring to Figure 4, at step 1, the UE performs DL sync with an LTM candidate cell. At step 2, the UE receives a trigger for TA establishment for the LTM candidate cell. For example, the trigger may be a PDCCH order from the PCell serving the UE. After a delay the UE transmits at step 3 a preamble for TA establishment on a PRACH occasion before the next SSB, as the UE is already pre-synchronized to the candidate LTM cell. [0077] In some embodiments, a UE may be configured with multiple LTM candidate cell(s). The UE selects a subset of the LTM candidate cell(s) for performing DL synchronization before a TA establishment procedure is triggered. The subset of LTM candidate cells may include at least one LTM candidate cell. The selection of the subset of the LTM candidate cell(s) is based on one or more rules (or combination of rules). Multiple rules for the selection of the subset of the LTM candidate cell(s) are provided. One or more of the rules may be combined.

[0078] Certain embodiments may provide one or more technical advantages. In particular, some embodiments described herein may reduce delay in a TA establishment/update procedure towards an LTM candidate cell, in preparation for an LTM cell switch without a random access (RA) procedure.

[0079] According to some embodiments, a UE can transmit an UL signal (e.g., a PRACH preamble) to an LTM candidate cell (indicated by a trigger for TA establishment, such as a PDCCH order, RRC message or MAC from the PCell) based on a pre-acquired DL synchronization. This may reduce the time it takes to perform the UL synchronization for an LTM candidate cell during a TA establishment procedure, which may reduce the time the UE needs to be away from the serving cell(s) and, consequently, may improve the throughput/ data rates experienced by the UE.

[0080] This may improve the reliability of the overall UE connection, because a shorter time to perform the TA establishment/update procedure means that the UE may be ready to receive an LTM cell switch command from the PCell at an earlier time.

[0081] In the following description, the term “L1/L2 based inter-cell mobility” is used as in the Work Item Description in 3GPP. The term may be used interchangeably with the terms L1/L2 mobility, Ll-mobility, LI based mobility, Ll/L2-centric inter-cell mobility, L1/L2 inter-cell mobility, or L1/L2 triggered Mobility (LTM).

[0082] The basic principle of LTM is that a UE is first configured with one or more LTM candidate cells (via RRC). After the UE reports LI measurements on one or more LTM candidate cells, the UE receives a lower layer signaling from the network indicating to the UE a change (or switch or activation) of its serving cell (e.g., change of PCell, from a source to a target PCell), wherein a lower layer signaling is a message/ signaling of a lower layer protocol, which may be referred as a L1/L2 inter-cell mobility execution command (or LTM cell switch command). The change of serving cell (e.g., change of PCell) may also lead to a change in SCell(s) for the same cell group, e.g., in case the command triggers the UE to change to another cell group configuration of the same type (e.g., another MCG configuration). Before the UE receives the LTM cell switch command, the UE is configured by the network with one or more LTM candidate cells (e.g., reception of an RRCReconfiguration message, with at least one candidate cell configuration) A candidate cell configuration may include parameters in the IE CellGroupConflg per candidate cell and/or an embedded RRCReconfiguration per candidate cell.

[0083] A "lower layer protocol" refers to a lower layer protocol in the air interface protocol stack compared to RRC protocol. For example, the Medium Access Control (MAC) is considered a lower layer protocol as it is “below” RRC in the air interface protocol stack, and in this case a lower layer signaling/ message may correspond to a MAC Control Element (MAC CE). Another example of lower layer protocol is the Layer 1 (or Physical Layer, LI), and in this case a lower layer signaling/ message may correspond to a Downlink Control Information (DCI). Signaling information in a protocol layer lower than RRC reduces the processing time and, consequently, reduces the interruption time during mobility; in addition, it may also increase the mobility robustness as the network may respond to faster changes in the channel conditions. Another relevant aspect in L1/L2 inter-cell mobility is that in multibeam scenario, a cell can be associated to multiple SSBs, and during a half-frame, different SSBs may be transmitted in different spatial directions (i.e. using different beams, spanning the coverage area of a cell). Similar reasoning may be applicable to CSI-RS resources, which may also be transmitted in different spatial directions. Hence, in L1/L2 inter-cell mobility (LTM), the reception of a lower layer signaling indicates the UE to change from one beam in the serving cell, to another beam in a neighbour cell (which is a configured candidate cell), and by that changing serving cell.

[0084] The term "LTM cell switch procedure" (or simply cell switch) refers to the process of a UE changing its cell from a source cell to a target cell (which may be called here a candidate cell), using L1/L2 -triggered mobility (also called LTM). In the context of L1/L2 based inter-cell mobility or L1/L2 -triggered mobility, the LTM cell switch procedure may sometimes also be known as dynamic switch, LTM switch, (LTM) cell switch, (LTM) serving cell change or (LTM) cell change. Even if the term change of cell is used, that may comprise a change of a whole cell group configuration, which includes a change in the SpCell (e.g., change of PCell, or change of PSCell) and a change in SCells of the cell group (e.g., addition, modification and/or release of one or more SCells).

[0085] The term “LTM candidate cell” refers to a cell the UE is configured with when configured with L1/L2 inter-cell mobility. That is a cell the UE can move to in a L1/L2 inter-cell mobility procedure, upon reception of a lower layer signaling. These cells may also be called candidate cells, candidates, mobility candidates, non-serving cells, additional cells, etc. This is a cell the UE perform measurements on (e.g., Ll-RSRP measurements or CSI measurements) as disclosed herein, so that the UE reports these measurements and network may take educated decision on which beam (e.g., TCI state) and/or cell the UE is to be switched to. A L1/L2 inter-cell mobility candidate cell may be a candidate to be a target PCell or PSCell, or an SCell of a cell group (e.g., MCG SCell). In that sense, when the text refers to a resource configuration to indicate SSs and/or RSs for the UE to measure for CSI for reporting, it may be referring to SSs and/or RSs of a candidate SCell of the MCG, a candidate SCell of the SCG, a candidate PSCell and/or a candidate PCell.

[0086] According to some embodiments, related to L1/L2 inter-cell mobility, a UE may be capable of acquiring DL and/or UL synchronization before receiving the LTM cell switch command (e.g., a MAC CE indicating an LTM candidate cell and/or LTM candidate cell configuration). For a UE that is capable of acquiring DL synchronization before receiving the LTM cell switch command, there may be a limitation on the number of cells for which the UE can acquire such a synchronization before receiving the LTM cell switch command. Also, for acquiring the UL synchronization, a UE needs to transmit UL signals, such as PRACH preamble or Sounding Reference Signal (SRS), to the LTM candidate cell (e.g., of a target gNB and/or candidate DU). Unless the UE has acquired DL synchronization, the UE does not transmit PRACH or SRS to acquire the UL synchronization. As discussed above, each PRACH preamble may be associated with an SSB and a RACH occasion (RO) where the preamble can be transmitted. The RO can be a periodically repeating occasion. For example, the first RO associated with a PRACH preamble is at 10ms, then the 2nd RO associated with the same preamble may be at 10ms + (160ms) and 3rd RO associated with the same preamble may be at 10ms +( 2* 160ms), and the 4th RO is 10ms + 3* 160ms, etc.

[0087] In general, when the UE is configured to measure on neighboring cells (e.g., L3-RSRP or Ll-RSRP) for LTM, the UE is not required to meet any synchronization requirements for measuring the neighbor cells. As part of the measurement requirements, the UE needs to meet only the accuracy requirement of the measurement and measurement delay of the measurement and the number of cells or beams that the UE is required to measure.

[0088] According to some embodiments, the UE transmits an UL signal (e.g., a PRACH preamble) to an LTM candidate cell for establishing time alignment and UL synchronization (TA establishment procedure), so that the network can calculate a timing advance value to be provided to the UE before the UE accesses the LTM candidate cell in an LTM cell switch. The description also refers to a “TA establishment/ update procedure” in which the UE which had established TA and UL sync with an LTM candidate cell but may have lost UL sync (e.g., because a TA timer expired) and needs to transmit an UL signal again to the LTM candidate cell. The procedures for establishment and update of TA are similar from the point at which the UE transmits the UL signal.

[0089] An example of a signaling flow according to some embodiments is shown in Figure 5, with some of the steps also performed by network nodes involved, such as the Source DU (S-DU), the Central Unit (CU) and the Candidate DU (C-DU).

[0090] Referring to Figure 5, a UE transmits a measurement report 502 to a network node, and in particular to a CU of a network node, containing requested measurements. The measurements may include measurements of a cell of associated with a candidate DU. The CU determines to configure the UE to perform an LTM procedure by the UE towards the candidate DU. The CU sends a UE context setup request 504 to the candidate DU requesting LTM of the UE towards the candidate DU. The candidate DU responds with a UE context response 506 including a candidate LTM configuration to be provided to the UE. The serving CU/DU then transmits an RRCReconflguration message 508 to the UE including an LTM candidate configuration for the candidate DU.

[0091] After replying to the CU/DU with an RRCReconflgurationComplete, and before TA establishment, the UE performs DL synchronization with the candidate cell as preparation for TA establishment at block 510.

[0092] The serving CU/DU then sends a trigger 512 to the UE for TA establishment. In response to the trigger, the UE transmits a random access preamble 514 for TA establishment to the candidate DU. The candidate DU calculates a timing advance value at block 516 and then transmits the timing advance value back to the UE via the serving CU/DU.

[0093] The UE may provide LI measurement reports 518 for the candidate cell to the CU/DU. The serving CU/DU then determines to trigger an LTM cell switch at block 520, and sends an LTM cell switch command 522 to the UE. The UE may then transmit a UL transmission 524 in the candidate cell.

[0094] The UE performing DL synchronization (also called DL presynchronization, pre-synchronization, pre-sync) with an LTM candidate cell comprises the UE detecting and/or measuring at least one synchronization signal of the LTM candidate cell, such as an Synchronization Signal Block (SSB), e.g., an SSB of the LTM candidate cell associated to an SSB index and/or identifier and transmitted in a spatial direction (beam), and/ or a Channel State Information - Reference Signal (CSI-RS) and/or a Tracking Reference Signal (TRS) and/or a Primary Sync Signal (PSS) and/or a Secondary Sync Signal (SSS); in this context, measuring comprises determining a measurement quantity value such as a Synchronization Signal based Reference Signal Received Power (SS-RSRP) and/or Synchronization Signal based Reference Signal Received Quality (SS-RSRQ) and/or Synchronization Signal based Signal to Noise and Interference Ratio (SS-SINR).

[0095] The UE performing DL synchronization with an LTM candidate cell comprises the UE performing fine time tracking and acquiring full timing information of the LTM candidate cell. Timing acquisition comprises obtaining the time boundaries of time units of a given LTM candidate cell such as time slot, OFDM symbol, subframe, radio frame. Timing acquisition comprises synchronizing a clock with the boundaries of time units of a given LTM candidate cell such as time slot, OFDM symbol, subframe, radio frame. The acquired fine timing is used as reference point for PRACH transmission and UE uplink transmissions.

[0096] In some embodiments, after the UE obtains DL sync with one or more LTM candidate cells, the UE starts a timer (e.g., configured with a timer value in the RRC configuration). While the timer is running the UE is required to keep the DL synchronization, e.g., the UE may need to perform further measurements and/or obtain timing information. When the timer expires, the UE is not required to maintain DL sync with that LTM candidate cell(s). When the UE receives the trigger for the TA establishment, the UE stops the timer. Similar functionality could be achieved by the UE counting a number of SSB occasions (e.g., configured with a timer value in the RRC configuration) or counting a number of PRACH occasions. The timer value for such a timer may be configured by the network to the UE, e.g., received in an RRC Reconfiguration.

[0097] In some embodiments, the UE receives an RRC message (e.g., from the NW node) configuring lower layer measurements (also called beam measurement) of one or more LTM candidate cells, and performs one or more of the following:

Performing measurements on the group of N1 cells (group 1) that are configured as LTM candidate cells; and acquiring and maintaining the DL synchronization to at least a group of N2 cells (group 2) among the configured cells (group 1 or N1 cells). In other words, the UE performs DL sync for a first subset of LTM candidate cells the UE is being configured with.

Receiving a request (sigl) from NW node (e.g., from a cell 1 ) for transmission of UL synchronization signal (sig2) to the LTM candidate cell (cell2), for TA establishment/ update. Determining the transmission occasion of the sig2 (e.g., PRACH occasion /

RACH occasion) to the LTM candidate cell 2 with respect to the sigl.

[0098] In some embodiments, sigl may be a PDCCH order. In some other embodiments sigl may be a MAC CE message or RRC message.

[0099] In some embodiments sig2 may be PRACH preamble. In some other embodiments sig2 may be SRS. In some other embodiments UL synchronization signal may be a signal specially designed for acquiring UL synchronization.

[0100] In some examples if the sig2 is PRACH preamble, PRACH is configured with multiple RACH occasions.

[0101] In some embodiments, if the cell2 belong to group 2, the transmission occasion of the sig2 is the first transmission occasion configured for transmission of sig2.

[0102] In some embodiments, the number of cells (N2) (i.e., the number of cells UE could maintain DL synchronization) in group 2 is indicated as a UE capability, which is reported to the network by the UE, e.g., during a transition from IDLE to CONNECTED states.

[0103] In some examples, N2 may be a fixed value specified in 3GPP specification.

[0104] In some other embodiments N2 may be signalled to the UE by NW and in this case N1 can be a configurable parameter through RRC message or MAC message or DCI message.

[0105] In some other embodiments N2 may be signaled from the UE to the network. In such a case, the network may ensure that N1 is equal to N2 in order to not exceed the UE capability.

[0106] In one alternative, the network may configure N1 to be greater than N2 and it may indicate separately with N2 cells the UE should keep the pre-sync.

[0107] In one alternative, the network may configure N1 to be greater than N2 and leave to the UE how to select the N2 cells to which to keep the pre-sync.

[0108] In one alternative, the network may configure N2 to be lower with respect to the maximum N2 that is supported by the UE (according to its capabilities).

[0109] The method further comprises the UE determining which N2 cells UE maintains synchronization among the configured N1 cells. In some embodiments, the N2 cells in group 2 are the N2 strongest cells from the N1 cells in group 1. In some examples, the strongest N2 cells are determined based on the signal strength measured at UE, where the signal strength may be dependent on the measurement quantity configured for the UE. In some examples, measurement quantity may be RSRP (e.g., Ll-RSRP or L3-RSRP or SS- RSRP or CSI-RS-RSRP.) or RSRQ or SINR.

[0110] In some embodiments, UE acquires and maintains DL synchronization using any synchronization signals (e.g., SSB) or any tracking signals (e.g., tracking reference signal, TRS) or combination of both SSB and TRS. UE may further acquire DL synchronization from SSB or TRS using correlation or matched filter approach or any proprietary algorithm.

[0111] In some embodiments, the UE maintains the DL synchronization to all the cells in group 2 also after the transmission of the UL synchronization signal. In other embodiments, the UE maintains the DL synchronization to a subset of the cells in group 2 after the transmission of the UL synchronization signal. In one embodiment, the UE maintains the DL synchronization only to cell2 after the transmission of the UL synchronization signal.

[0112] In some embodiments, the network provides the UE with N1 LTM candidate cells and requests the UE to perform DL pre-sync with all of them (therefore N1 equal to N2). However, the UE performs the pre-sync only with N3 LTM candidate cell and, in this case, it reports the number N3 and an identification for each of the N3 LTM candidate cell. In this case, the identification may be one or a combination of a LTM candidate cell configuration identifier (ID); a serving cell ID that is included within the LTM candidate cell configuration ID; a Transmission configuration indication (TCI) state ID; a beam ID; and a measurement ID.

[0113] The reason on why the UE may decide to do a pre-sync only with an N3 LTM candidate cell may be due to a capability limitation, due to signal strength measured with starting to perform the pre-sync, geographical location, or other UE-implementation specific criteria.

[0114] In some embodiments, a UE performs DL synchronization with an LTM candidate cell. The UE receives a trigger from a serving cell to transmit an UL signal for TA establishment/update to the LTM candidate cell while the UE is connected to a serving cell, such as a PCell. The UE transmits the UL signal to the LTM candidate cell in a configured UL channel resource(s) in time and frequency, such as a PRACH occasion, based on the DL synchronization performed before the reception of the trigger from a serving cell to transmit an UL signal for TA establishment/update to the LTM candidate cell.

[0115] Example embodiments are illustrated, for example in Figure 6 which shows a method performed by a UE in a wireless communication network for performing TA with a LTM candidate cell. The method includes receiving an LTM configuration including an LTM candidate cell configuration for TA establishment and/or TA update with one or more LTM candidate cells (block 602), and receiving an indication to perform DL synchronization with one of the LTM candidate cells (block 603).

[0116] In response to the indication, the UE performs DL pre-synchronization with the LTM candidate cell (block 604). After performing DL pre-synchronization with the LTM candidate cell, the UE receives, from a serving cell, a trigger for TA establishment and/or TA update to the LTM candidate cell (block 606). The UE then transmits, in response to the trigger, a UL signal to the LTM candidate cell for TA establishment and/or update (block 608).

[0117] Figure 7 illustrates a method performed by a network node in a wireless communication network for L1/L2 based inter-cell mobility of a UE to a candidate cell according to some embodiments. The method includes transmitting to the UE an LTM configuration including an LTM candidate cell configuration for TA establishment and/or TA update with one or more LTM candidate cells (block 702). The network node transmits an indication to the UE to perform DL pre-synchronization with an LTM candidate cell (block 703).

[0118] In response to the indication, the UE performs DL pre-synchronization to the candidate cell (block 704). After the UE has performed DL pre-synchronization with the LTM candidate cell in response to the LTM configuration, the network node transmits a trigger to the UE for TA establishment and/or TA update to the LTM candidate cell (block 706). The UE then transmits, in response to the trigger, a UL signal to the LTM candidate cell for TA establishment and/or update (block 708).

[0119] According to some embodiments, the trigger may correspond to an RRC message (e.g., RRC Reconfiguration), an RRC Information Element (IE), a field, a parameter, a MAC Control Element (MAC CE) or a Physical Downlink Control Channel (PDCCH) order/ command or indication. The trigger may be received from the Primary Cell (e.g., PCell, PSCell, SpCell) triggering the UE to transmit an UL signal (e.g., PRACH preamble) to the LTM candidate cell for TA establishment/update.

[0120] According to some embodiments, the UL signal may correspond to a PRACH preamble that is transmitted to an UL channel of the LTM candidate cell. The UL channel of the LTM candidate cell may be a PRACH. Alternatively, the UL signal may correspond to a Sounding Reference Signal (SRS) transmitted to an UL channel of the LTM candidate cell configured for SRS transmission (e.g., PUCCH/ PUSCH). [0121] In some embodiments, prior to receiving the trigger from a serving cell for TA establishment/update to the LTM candidate cell while the UE is connected to a serving cell (e.g., a PCell), the UE receives a configuration (e.g., an LTM configuration within an RRC Reconfiguration message) from the serving cell with one or more LTM candidate cell configuration(s), to be applied upon reception of an LTM cell switch command. This is equivalent to the UE being configured with LTM by the network.

[0122] In some embodiments, prior to receiving the trigger from a serving cell for TA establishment/update to the LTM candidate cell while the UE is connected to a serving cell, the UE receives a configuration for TA establishment/update with the LTM candidate cell. The configuration includes one or more UL related parameters, such as a PRACH preamble configuration, PRACH occasion(s), PRACH frequency resource(s), etc. In some embodiments, the configuration for TA establishment/update is included in the same RRC message configuring the UE with LTM, such as an RRC Reconfiguration message. In further embodiments, the configuration for TA establishment/update may be included in a second RRC message while the UE receives a first RRC message configuring the UE with LTM.

[0123] According to some embodiments, the UE is configured with multiple LTM candidate cell(s) (i.e. , more than one candidate cell). The UE selects a subset of the LTM candidate cell(s) for performing DL synchronization before a TA establishment procedure is triggered. The selection of the subset of the LTM candidate cell(s) may be based on one or more rules (or combination of rules).

[0124] According to some embodiments, the UE is configured with multiple LTM candidate cell(s) (i.e., more than one candidate cell) and selects a subset of the LTM candidate cell(s), including at least one LTM candidate cell, for performing DL synchronization before a TA establishment procedure is triggered. The selection of the subset of the LTM candidate cell(s) is based on one or more rules (or combination of rules), which may include one or more of the rules described below.

[0125] Rule 1) The UE selects all LTM candidate cell(s) which the UE is configured with, when the number of LTM candidate cell(s) do not exceed a UE capability corresponding to a maximum number of cells in which the UE is capable of performing DL sync before TA establishment.

[0126] For example, when the UE reports a capability which indicates that the UE is capable of performing DL synchronization before a TA establishment with a number “K” of LTM candidate cells and the UE receives a message (e.g., RRC Reconfiguration) including an LTM configuration for a number K* LTM candidate cells with K*<= K, the UE performs DL sync to all configured LTM candidate cell(s) after receiving the LTM candidate cell configuration and before the TA establishment procedure is triggered, e.g., before the reception of the PDCCH order triggering the transmission of the UL preamble for TA establishment/ update.

[0127] The capability described above may be reported when the UE transitions from IDLE to CONNECTED state.

[0128] Rule 2) The UE selects the LTM candidate cell(s) with a measurement quantity (e.g., RSRP) above a threshold, e.g., the UE selects the LTM candidate cell(s) with RSRP above a threshold.

[0129] In one option the threshold is configurable, for example, per LTM candidate cell, or per LTM configuration (applicable for multiple/all LTM candidate cell(s)). In other words, the UE may receive an RRC Reconfiguration message including the configuration for performing TA establishment/ update which includes that threshold.

[0130] In one option, the measurement quantity (e.g., RSRP, RSRQ, SINR) is configurable.

[0131] In one option, the measurement quantity is a cell-based measurement quantity, e.g., a cell RSRP for the LTM candidate cell.

[0132] In one option, the measurement quantity is a beam/SSB/CSI-RS based measurement quantity. For example, the UE selects the cells whose strongest SS-RSRP is above the threshold.

[0133] In one option, this rule may be combined with Rule 1) as follows: the UE selects the LTM candidate cell whose measurement quantity (e.g., RSRP) is above a threshold, when the number of LTM candidates whose measurement quantity above the threshold do not exceed the maximum number of LTM candidate cells the UE is capable of performing DL sync before TA establishment.

[0134] Rule 3) The UE selects “K” strongest LTM candidate cells out of “N” configured LTM candidate cells according to a measurement quantity, wherein a measurement quantity may correspond to RSRP, RSRQ, SINR, etc.

[0135] In one option, the UE reports a capability which indicates that the UE is capable of performing DL synchronization before a TA establishment with a number “K” of LTM candidate cells and the UE receives a message (e.g., RRC Reconfiguration) including an LTM configuration for a number “N” LTM candidate cells with N > K, so that the UE performs DL sync to the K strongest LTM candidate cell(s) for a measurement quantity, after receiving the LTM candidate cell configuration and before the TA establishment procedure is triggered, e.g., before the reception of the PDCCH order triggering the transmission of the UL preamble for TA establishment/update.

[0136] In one option, the measurement quantity (e.g., RSRP, RSRQ, SINR) is configurable.

[0137] In one option, the maximum number of LTM candidate cells for which the UE establishes DL sync before the reception of the trigger for TA establishment/ update is configurable, e.g., a parameter received in an RRC Reconfiguration.

[0138] In one option, when the UE reports a UE capability indicating a maximum number of LTM candidate cells for DL sync before reception of the trigger for TA establishment trigger, the UE receives the parameter indicating a maximum number of LTM candidates for establishing TA before the TA trigger which does not exceed the reported UE capability.

[0139] In one option, the K strongest LTM candidate cells are selected based on the cell quality of the LTM candidate cell, e.g., UE selects the LTM candidates whose cell RSRP values are the strongest.

[0140] In one option, the K strongest LTM candidate cells are selected based on a beam/SSB/CSI-RS quality of the LTM candidate cell, e.g., UE selects the LTM candidates whose strongest beam level RSRP values (SS-RSRP values) are the strongest.

[0141] Rule 4) The UE selects all LTM candidate cell(s) configured for TA establishment/update, e.g., upon reception of the RRC message including the TA establishment configuration, when the number of LTM candidate cell(s) configured for TA establishment/update do not exceed a UE capability corresponding to a maximum number of cells in which the UE is capable of performing DL sync before TA establishment.

[0142] For example, when the UE reports a capability which indicates that the UE is capable of performing DL synchronization before a TA establishment with a number “K” of LTM candidate cells and the UE receives a message (e.g., RRC Reconfiguration) including an LTM configuration including the TA establishment configuration for a number K* of LTM candidate cells with K*<= K, the UE performs DL sync to all configured LTM candidate cell(s) (configured for TA establishment) after receiving the LTM candidate cell configuration and before the TA establishment procedure is triggered, e.g., before the reception of the PDCCH order triggering the transmission of the UL preamble for TA establishment/ update.

[0143] The capability described above may be reported when the UE transitions from IDLE to CONNECTED state. [0144] Rule 5) The UE selects the LTM candidate cell(s) configured for TA establishment/update with a measurement quantity (e.g., RSRP) above a threshold, e.g., the UE selects the LTM candidate cell(s) configured for TA establishment with RSRP above a threshold.

[0145] In one option the threshold is configurable, e.g., per LTM candidate cell, or per LTM configuration (applicable for multiple / all LTM candidate cell(s)), possibly in parts of the TA establishment/ update configuration. In other words, the UE may receive an RRC Reconfiguration message including the configuration for performing TA establishment/ update which includes that threshold.

[0146] In one option, the measurement quantity (e.g., RSRP, RSRQ, SINR) is also configurable.

[0147] In one option, the measurement quantity is a cell based measurement quantity, e.g., a cell RSRP for the LTM candidate cell.

[0148] In one option, the measurement quantity is a beam/ SSB/ CSI-RS based measurement quantity. For example, the UE selects the cells whose strongest SS-RSRP is above the threshold.

[0149] In one option, this rule may be combined with the rule 4) as follows: the UE selects the LTM candidate cell with TA establishment configuration whose measurement quantity (e.g., RSRP) is above a threshold, when the number of LTM candidates with a TA establishment configuration whose measurement quantity above the threshold do not exceed the maximum number of LTM candidate cells the UE is capable of performing DL sync before TA establishment.

[0150] Rule 6) The UE selects “K” strongest LTM candidate cells configured for TA establishment out of “N” configured LTM candidate cells configured for TA establishment according to a measurement quantity, wherein a measurement quantity may correspond to RSRP, RSRQ, SINR, etc.

[0151] In one option, the UE reports a capability which indicates that the UE is capable of performing DL synchronization before a TA establishment with a number “K” of LTM candidate cells configured for TA establishment and the UE receives a message (e.g., RRC Reconfiguration) including an LTM configuration for a number “N” LTM candidate cells with N > K, so that the UE performs DL sync to the K strongest LTM candidate cell(s) for a measurement quantity, after receiving the LTM candidate cell configuration and before the TA establishment procedure is triggered, e.g., before the reception of the PDCCH order triggering the transmission of the UL preamble for TA establishment/ update. [0152] In one option, the measurement quantity (e.g., RSRP, RSRQ, SINR) is configurable.

[0153] In one option, the maximum number of LTM candidate cells for which the UE establishes DL sync before the reception of the trigger for TA establishment/ update is configurable, e.g., a parameter received in an RRC Reconfiguration.

[0154] In one option, when the UE reports a UE capability indicating a maximum number of LTM candidate cells for DL sync before reception of the trigger for TA establishment trigger, the UE receives the parameter indicating a maximum number of LTM candidates for establishing TA before the TA trigger which does not exceed the reported UE capability.

[0155] In one option, the K strongest LTM candidate cells is selected based on the cell quality of the LTM candidate cell, e.g., UE selects the LTM candidates whose cell RSRP values are the strongest.

[0156] In one option, the K strongest LTM candidate cells is selected based on a beam / SSB / CSI-RS quality of the LTM candidate cell, e.g., UE selects the LTM candidates whose strongest beam level RSRP values (SS-RSRP values) are the strongest.

[0157] Rule 7) The UE selects one or more LTM candidate cell(s) based on an indication or configuration received from the network, wherein the indication indicates which LTM candidate the UE needs to perform DL synchronization before it receives the trigger for TA establishment.

[0158] In one option, the indication is received in an RRC Reconfiguration message, e.g., as part of the TA establishment/ update configuration which for a given candidate includes an indication that the LTM candidate is a candidate for which the UE performs DL sync before the TA establishment trigger (e.g., PDCCH order) is received.

[0159] In one option, the indication is received in a MAC CE (or other form or lower layer protocol signaling), e.g., received after the reception of the TA establishment/ update configuration, but before the reception of the trigger for transmitting the UL signal to the LTM candidate for TA establishment.

[0160] In one option, the indication is received in a Layer 1 (LI) signaling, e.g., in a DL physical channel.

[0161] In one option, the indication is received in a Layer 1 / Layer 2 (L1/L2) signaling.

[0162] In one option, the indication is received in a PDCCH order. [0163] In one option, the UE receives an RRC configuration including an indication of which of the LTM candidate cells the UE is to perform DL sync before the reception of a trigger for TA establishment. The indication may indicate a subset of the LTM candidate cells which have been configured for TA establishment (but no trigger has been received)

[0164] For example, the UE is configured with LTM candidates A, B, C, D, but only A and B are configured for TA establishment (e.g., B, C may already be UL synchronized); Then, the UE receives an indication that it needs to perform DL synch for LTM candidate cell A

[0165] In one option, the network indicates an LTM candidate cell to the UE for DL sync based on one or more measurements reported by the UE.

[0166] For example, the UE transmits one or more LI measurements for an LTM candidate cell, so that the network (e.g., the CU, the S-DU) determines that the reported LTM candidate cell is a cell the network wants the UE to perform TA establishment without delays, so that it indicates to the UE that this is a cell for performing DL sync, before the UE receives the trigger for TA establishment.

[0167] Rule 8) The UE selects one or more LTM candidate cell(s) based on latest LI measurement reports, e.g., SS-RSRP of an LTM candidate cell which has been reported.

[0168] In one option, the UE is configured to perform LI measurements on one or more LTM candidate cells, e.g., CSI measurements, SS-RSRP measurements, etc. Then, the UE selects the LTM candidate cells to perform DL sync as the cells for which the UE has transmitted the LI reports. The reasoning could be that these are also the LTM candidate cells which are more like to be requested from the network (e.g., S-DU) for TA establishment.

[0169] In one option, this feature is configured.

[0170] In one option, the UE does that with a number of LTM candidate cells before its capability is exceeded, e.g., the UE reports a number of cells up to the number of LTM candidate cells in which the UE can perform DL sync before the TA establishment.

[0171] In one option, the UE updates the LTM candidate cells in which the UE performs DL sync, depending on the cells which are being reported.

[0172] Rule 9) The UE selects one or more LTM candidate cell(s) based on L3 measurements, e.g., cell based RSRP of an LTM candidate cell.

[0173] In one option, the UE is configured to perform L3 measurements on one or more LTM candidate cells, e.g., Radio Resource Management (RRM) measurements like L3 filtered cell based RSRP, RSRQ, SINR. Then, the UE selects the LTM candidate cells to perform DL sync as the cells for which the UE has transmitted a L3 measurement report, e.g., triggered cells, fulfilling the condition(s) of an event configured in the reporting configuration, e.g., A3 or A5 event. The reasoning could be that these triggered cells fulfilling the event(s) may be configured as LTM candidate cells by the network and be requested from the network (e.g., S-DU) for TA establishment.

[0174] In one option, this feature is configured.

[0175] In one option, the UE does that with a number of neighbour cells (e.g., which are triggered cells) before its capability is exceeded, e.g., the UE reports a number of cells up to the number of LTM candidate cells in which the UE can perform DL sync before the TA establishment.

[0176] In one option, the UE updates the neighbour cells (e.g., triggered cells) in which the UE performs DL sync, depending on the cells which are being reported.

[0177] Rule 10) The UE selects at least one LTM candidate cell(s) for which TA had been established but have been lost.

[0178] In one option, the UE performs DL sync for an LTM candidate cell and receives the trigger for transmitting the UL signal (e.g., PRACH preamble). The UE may start a Time Alignment timer (or equivalent timer), so that while the timer is running the UE considers itself to be UL synchronized with that LTM candidate cell. When that timer expires, the UE re-starts the action of obtaining DL sync with that LTM candidate cell, as the network (e.g., S-DU) may likely request the UE to transmit another UL signal to that LTM candidate cell.

[0179] Rule 11) UE selects cells in higher frequencies and/or in a specific frequency range, e.g., FR2 cells, as these could take longer to synchronized.

[0180] In one option, the UE performs DL sync for an LTM candidate cell (or a subset of the LTM candidate cells) whose SSB(s) are in a high frequencies and/or in a specific frequency range (FR2). The reasoning is that for these cells, it may take longer to obtain DL sync so that if the UE waits for the trigger to transmit the UL signal and first needs to perform DL sync, it would take too much time.

[0181] Rule 12) UE selects cells with “long” SSB periodicity, because UE knows these may take longer to sync.

[0182] In one option, the UE performs DL sync for an LTM candidate cell (or a subset of the LTM candidate cells) whose SSB(s) are with long periodicity, e.g., above 20ms. [0183] In one option, the long periodicity is configured (e.g., periodicity threshold), so that the UE perform DL sync to cells with periodicity longer than the configured value.

[0184] According to some embodiments, the one or more rules may be based on one or more parameters the UE is configured with. In some of the rules above, examples of parameter(s) have been provided. The UE may receive the configuration of the one or more parameters in an RRC Reconfiguration message, wherein the one or more parameters may be set for one or more LTM candidate cell(s).

[0185] Figure 8 shows an example of a communication system 800 in accordance with some embodiments.

[0186] In the example, the communication system 800 includes a telecommunication network 802 that includes an access network 804, such as a radio access network (RAN), and a core network 806, which includes one or more core network nodes 808. The access network 804 includes one or more access network nodes, such as network nodes 810a and 810b (one or more of which may be generally referred to as network nodes 810), or any other similar 3^rd Generation Partnership Project (3GPP) access nodes or non- 3GPP access points. Moreover, as will be appreciated by those of skill in the art, a network node is not necessarily limited to an implementation in which a radio portion and a baseband portion are supplied and integrated by a single vendor. Thus, it will be understood that network nodes include disaggregated implementations or portions thereof. For example, in some embodiments, the telecommunication network 802 includes one or more Open-RAN (ORAN) network nodes. An ORAN network node is a node in the telecommunication network 802 that supports an ORAN specification (e.g., a specification published by the O- RAN Alliance, or any similar organization) and may operate alone or together with other nodes to implement one or more functionalities of any node in the telecommunication network 802, including one or more network nodes 810 and/or core network nodes 808.

[0187] Examples of an ORAN network node include an open radio unit (O-RU), an open distributed unit (O-DU), an open central unit (O-CU), including an O-CU control plane (O-CU-CP) or an O-CU user plane (O-CU-UP), a RAN intelligent controller (near-real time or non-real time) hosting software or software plug-ins, such as a near-real time control application (e.g., xApp) or anon-real time control application (e.g., rApp), or any combination thereof (the adjective “open” designating support of an ORAN specification). The network node may support a specification by, for example, supporting an interface defined by the ORAN specification, such as an Al, Fl, Wl, El, E2, X2, Xn interface, an open fronthaul user plane interface, or an open fronthaul management plane interface.

Moreover, an ORAN access node may be a logical node in a physical node. Furthermore, an ORAN network node may be implemented in a virtualization environment (described further below) in which one or more network functions are virtualized. For example, the virtualization environment may include an O-Cloud computing platform orchestrated by a Service Management and Orchestration Framework via an 0-2 interface defined by the O- RAN Alliance or comparable technologies. The network nodes 810 facilitate direct or indirect connection of user equipment (UE), such as by connecting UEs 812a, 812b, 812c, and 812d (one or more of which may be generally referred to as UEs 812) to the core network 806 over one or more wireless connections.

[0188] Example wireless communications over a wireless connection include transmitting and/or receiving wireless signals using electromagnetic waves, radio waves, infrared waves, and/or other types of signals suitable for conveying information without the use of wires, cables, or other material conductors. Moreover, in different embodiments, the communication system 800 may include any number of wired or wireless networks, network nodes, UEs, and/or any other components or systems that may facilitate or participate in the communication of data and/or signals whether via wired or wireless connections. The communication system 800 may include and/or interface with any type of communication, telecommunication, data, cellular, radio network, and/or other similar type of system.

[0189] The UEs 812 may be any of a wide variety of communication devices, including wireless devices arranged, configured, and/or operable to communicate wirelessly with the network nodes 810 and other communication devices. Similarly, the network nodes 810 are arranged, capable, configured, and/or operable to communicate directly or indirectly with the UEs 812 and/or with other network nodes or equipment in the telecommunication network 802 to enable and/or provide network access, such as wireless network access, and/or to perform other functions, such as administration in the telecommunication network 802.

[0190] In the depicted example, the core network 806 connects the network nodes 810 to one or more hosts, such as host 816. These connections may be direct or indirect via one or more intermediary networks or devices. In other examples, network nodes may be directly coupled to hosts. The core network 806 includes one more core network nodes (e.g., core network node 808) that are structured with hardware and software components. Features of these components may be substantially similar to those described with respect to the UEs, network nodes, and/or hosts, such that the descriptions thereof are generally applicable to the corresponding components of the core network node 808. Example core network nodes include functions of one or more of a Mobile Switching Center (MSC), Mobility Management Entity (MME), Home Subscriber Server (HSS), Access and Mobility Management Function (AMF), Session Management Function (SMF), Authentication Server Function (AUSF), Subscription Identifier De-concealing function (SIDF), Unified Data Management (UDM), Security Edge Protection Proxy (SEPP), Network Exposure Function (NEF), and/or a User Plane Function (UPF).

[0191] The host 816 may be under the ownership or control of a service provider other than an operator or provider of the access network 804 and/or the telecommunication network 802, and may be operated by the service provider or on behalf of the service provider. The host 816 may host a variety of applications to provide one or more service. Examples of such applications include live and pre-recorded audio/video content, data collection services such as retrieving and compiling data on various ambient conditions detected by a plurality of UEs, analytics functionality, social media, functions for controlling or otherwise interacting with remote devices, functions for an alarm and surveillance center, or any other such function performed by a server.

[0192] As a whole, the communication system 800 of Figure 8 enables connectivity between the UEs, network nodes, and hosts. In that sense, the communication system may be configured to operate according to predefined rules or procedures, such as specific standards that include, but are not limited to: Global System for Mobile Communications (GSM); Universal Mobile Telecommunications System (UMTS); Long Term Evolution (LTE), and/or other suitable 2G, 3G, 4G, 5G standards, or any applicable future generation standard (e.g., 6G); wireless local area network (WLAN) standards, such as the Institute of Electrical and Electronics Engineers (IEEE) 802.11 standards (WiFi); and/or any other appropriate wireless communication standard, such as the Worldwide Interoperability for Microwave Access (WiMax), Bluetooth, Z-Wave, Near Field Communication (NFC) ZigBee, LiFi, and/or any low-power wide-area network (LPWAN) standards such as LoRa and Sigfox.

[0193] In some examples, the telecommunication network 802 is a cellular network that implements 3GPP standardized features. Accordingly, the telecommunications network 802 may support network slicing to provide different logical networks to different devices that are connected to the telecommunication network 802. For example, the telecommunications network 802 may provide Ultra Reliable Low Latency Communication (URLLC) services to some UEs, while providing Enhanced Mobile Broadband (eMBB) services to other UEs, and/or Massive Machine Type Communication (mMTC)ZMassive loT services to yet further UEs.

[0194] In some examples, the UEs 812 are configured to transmit and/or receive information without direct human interaction. For instance, a UE may be designed to transmit information to the access network 804 on a predetermined schedule, when triggered by an internal or external event, or in response to requests from the access network 804. Additionally, a UE may be configured for operating in single- or multi-RAT or multistandard mode. For example, a UE may operate with any one or combination of Wi-Fi, NR (New Radio) and LTE, i.e. being configured for multi -radio dual connectivity (MR-DC), such as E-UTRAN (Evolved-UMTS Terrestrial Radio Access Network) New Radio - Dual Connectivity (EN-DC).

[0195] In the example, the hub 814 communicates with the access network 804 to facilitate indirect communication between one or more UEs (e.g., UE 812c and/or 812d) and network nodes (e.g., network node 810b). In some examples, the hub 814 may be a controller, router, content source and analytics, or any of the other communication devices described herein regarding UEs. For example, the hub 814 may be a broadband router enabling access to the core network 806 for the UEs. As another example, the hub 814 may be a controller that sends commands or instructions to one or more actuators in the UEs. Commands or instructions may be received from the UEs, network nodes 810, or by executable code, script, process, or other instructions in the hub 814. As another example, the hub 814 may be a data collector that acts as temporary storage for UE data and, in some embodiments, may perform analysis or other processing of the data. As another example, the hub 814 may be a content source. For example, for a UE that is a VR headset, display, loudspeaker or other media delivery device, the hub 814 may retrieve VR assets, video, audio, or other media or data related to sensory information via a network node, which the hub 814 then provides to the UE either directly, after performing local processing, and/or after adding additional local content. In still another example, the hub 814 acts as a proxy server or orchestrator for the UEs, in particular if one or more of the UEs are low energy loT devices.

[0196] The hub 814 may have a constant/persi stent or intermittent connection to the network node 810b. The hub 814 may also allow for a different communication scheme and/or schedule between the hub 814 and UEs (e.g., UE 812c and/or 812d), and between the hub 814 and the core network 806. In other examples, the hub 814 is connected to the core network 806 and/or one or more UEs via a wired connection. Moreover, the hub 814 may be configured to connect to an M2M service provider over the access network 804 and/or to another UE over a direct connection. In some scenarios, UEs may establish a wireless connection with the network nodes 810 while still connected via the hub 814 via a wired or wireless connection. In some embodiments, the hub 814 may be a dedicated hub - that is, a hub whose primary function is to route communications to/from the UEs from/to the network node 810b. In other embodiments, the hub 814 may be a non-dedicated hub - that is, a device which is capable of operating to route communications between the UEs and network node 810b, but which is additionally capable of operating as a communication start and/or end point for certain data channels.

[0197] Figure 9 shows a UE 900 in accordance with some embodiments. As used herein, a UE refers to a device capable, configured, arranged and/or operable to communicate wirelessly with network nodes and/or other UEs. Examples of a UE include, but are not limited to, a smart phone, mobile phone, cell phone, voice over IP (VoIP) phone, wireless local loop phone, desktop computer, personal digital assistant (PDA), wireless cameras, gaming console or device, music storage device, playback appliance, wearable terminal device, wireless endpoint, mobile station, tablet, laptop, laptop-embedded equipment (LEE), laptop-mounted equipment (LME), smart device, wireless customer-premise equipment (CPE), vehicle, vehicle-mounted or vehicle embedded/integrated wireless device, etc. Other examples include any UE identified by the 3rd Generation Partnership Project (3GPP), including a narrow band internet of things (NB-IoT) UE, a machine type communication (MTC) UE, and/or an enhanced MTC (eMTC) UE.

[0198] A UE may support device-to-device (D2D) communication, for example by implementing a 3GPP standard for sidelink communication, Dedicated Short-Range Communication (DSRC), vehicle-to-vehicle (V2V), vehicle-to-infrastructure (V2I), or vehicle-to-everything (V2X). In other examples, a UE may not necessarily have a user in the sense of a human user who owns and/or operates the relevant device. Instead, a UE may represent a device that is intended for sale to, or operation by, a human user but which may not, or which may not initially, be associated with a specific human user (e.g., a smart sprinkler controller). Alternatively, a UE may represent a device that is not intended for sale to, or operation by, an end user but which may be associated with or operated for the benefit of a user (e.g., a smart power meter).

[0199] The UE 900 includes processing circuitry 902 that is operatively coupled via a bus 904 to an input/output interface 906, a power source 908, a memory 910, a communication interface 912, and/or any other component, or any combination thereof. Certain UEs may utilize all or a subset of the components shown in Figure 9. The level of integration between the components may vary from one UE to another UE. Further, certain UEs may contain multiple instances of a component, such as multiple processors, memories, transceivers, transmitters, receivers, etc.

[0200] The processing circuitry 902 is configured to process instructions and data and may be configured to implement any sequential state machine operative to execute instructions stored as machine-readable computer programs in the memory 910. The processing circuitry 902 may be implemented as one or more hardware-implemented state machines (e.g., in discrete logic, field-programmable gate arrays (FPGAs), application specific integrated circuits (ASICs), etc.); programmable logic together with appropriate firmware; one or more stored computer programs, general-purpose processors, such as a microprocessor or digital signal processor (DSP), together with appropriate software; or any combination of the above. For example, the processing circuitry 902 may include multiple central processing units (CPUs).

[0201] In the example, the input/output interface 906 may be configured to provide an interface or interfaces to an input device, output device, or one or more input and/or output devices. Examples of an output device include a speaker, a sound card, a video card, a display, a monitor, a printer, an actuator, an emitter, a smartcard, another output device, or any combination thereof. An input device may allow a user to capture information into the UE 900. Examples of an input device include a touch-sensitive or presence-sensitive display, a camera (e.g., a digital camera, a digital video camera, a web camera, etc.), a microphone, a sensor, a mouse, a trackball, a directional pad, a trackpad, a scroll wheel, a smartcard, and the like. The presence-sensitive display may include a capacitive or resistive touch sensor to sense input from a user. A sensor may be, for instance, an accelerometer, a gyroscope, a tilt sensor, a force sensor, a magnetometer, an optical sensor, a proximity sensor, a biometric sensor, etc., or any combination thereof. An output device may use the same type of interface port as an input device. For example, a Universal Serial Bus (USB) port may be used to provide an input device and an output device.

[0202] In some embodiments, the power source 908 is structured as a battery or battery pack. Other types of power sources, such as an external power source (e.g., an electricity outlet), photovoltaic device, or power cell, may be used. The power source 908 may further include power circuitry for delivering power from the power source 908 itself, and/or an external power source, to the various parts of the UE 900 via input circuitry or an interface such as an electrical power cable. Delivering power may be, for example, for charging of the power source 908. Power circuitry may perform any formatting, converting, or other modification to the power from the power source 908 to make the power suitable for the respective components of the UE 900 to which power is supplied.

[0203] The memory 910 may be or be configured to include memory such as random access memory (RAM), read-only memory (ROM), programmable read-only memory (PROM), erasable programmable read-only memory (EPROM), electrically erasable programmable read-only memory (EEPROM), magnetic disks, optical disks, hard disks, removable cartridges, flash drives, and so forth. In one example, the memory 910 includes one or more application programs 914, such as an operating system, web browser application, a widget, gadget engine, or other application, and corresponding data 916. The memory 910 may store, for use by the UE 900, any of a variety of various operating systems or combinations of operating systems.

[0204] The memory 910 may be configured to include a number of physical drive units, such as redundant array of independent disks (RAID), flash memory, USB flash drive, external hard disk drive, thumb drive, pen drive, key drive, high-density digital versatile disc (HD-DVD) optical disc drive, internal hard disk drive, Blu-Ray optical disc drive, holographic digital data storage (HDDS) optical disc drive, external mini-dual in-line memory module (DIMM), synchronous dynamic random access memory (SDRAM), external micro-DIMM SDRAM, smartcard memory such as tamper resistant module in the form of a universal integrated circuit card (UICC) including one or more subscriber identity modules (SIMs), such as a USIM and/or ISIM, other memory, or any combination thereof. The UICC may for example be an embedded UICC (eUICC), integrated UICC (iUICC) or a removable UICC commonly known as ‘SIM card.’ The memory 910 may allow the UE 900 to access instructions, application programs and the like, stored on transitory or non-transitory memory media, to off-load data, or to upload data. An article of manufacture, such as one utilizing a communication system may be tangibly embodied as or in the memory 910, which may be or comprise a device-readable storage medium.

[0205] The processing circuitry 902 may be configured to communicate with an access network or other network using the communication interface 912. The communication interface 912 may comprise one or more communication subsystems and may include or be communicatively coupled to an antenna 922. The communication interface 912 may include one or more transceivers used to communicate, such as by communicating with one or more remote transceivers of another device capable of wireless communication (e.g., another UE or a network node in an access network). Each transceiver may include a transmitter 918 and/or a receiver 920 appropriate to provide network communications (e.g., optical, electrical, frequency allocations, and so forth). Moreover, the transmitter 918 and receiver 920 may be coupled to one or more antennas (e.g., antenna 922) and may share circuit components, software or firmware, or alternatively be implemented separately.

[0206] In the illustrated embodiment, communication functions of the communication interface 912 may include cellular communication, Wi-Fi communication, LPWAN communication, data communication, voice communication, multimedia communication, short-range communications such as Bluetooth, near-field communication, location-based communication such as the use of the global positioning system (GPS) to determine a location, another like communication function, or any combination thereof. Communications may be implemented in according to one or more communication protocols and/or standards, such as IEEE 802.11, Code Division Multiplexing Access (CDMA), Wideband Code Division Multiple Access (WCDMA), GSM, LTE, New Radio (NR), UMTS, WiMax, Ethernet, transmission control protocol/intemet protocol (TCP/IP), synchronous optical networking (SONET), Asynchronous Transfer Mode (ATM), QUIC, Hypertext Transfer Protocol (HTTP), and so forth.

[0207] Regardless of the type of sensor, a UE may provide an output of data captured by its sensors, through its communication interface 912, via a wireless connection to a network node. Data captured by sensors of a UE can be communicated through a wireless connection to a network node via another UE. The output may be periodic (e.g., once every 15 minutes if it reports the sensed temperature), random (e.g., to even out the load from reporting from several sensors), in response to a triggering event (e.g., when moisture is detected an alert is sent), in response to a request (e.g., a user initiated request), or a continuous stream (e.g., a live video feed of a patient).

[0208] As another example, a UE comprises an actuator, a motor, or a switch, related to a communication interface configured to receive wireless input from a network node via a wireless connection. In response to the received wireless input the states of the actuator, the motor, or the switch may change. For example, the UE may comprise a motor that adjusts the control surfaces or rotors of a drone in flight according to the received input or to a robotic arm performing a medical procedure according to the received input.

[0209] A UE, when in the form of an Internet of Things (loT) device, may be a device for use in one or more application domains, these domains comprising, but not limited to, city wearable technology, extended industrial application and healthcare. Non-limiting examples of such an loT device are a device which is or which is embedded in: a connected refrigerator or freezer, a TV, a connected lighting device, an electricity meter, a robot vacuum cleaner, a voice controlled smart speaker, a home security camera, a motion detector, a thermostat, a smoke detector, a door/window sensor, a flood/moisture sensor, an electrical door lock, a connected doorbell, an air conditioning system like a heat pump, an autonomous vehicle, a surveillance system, a weather monitoring device, a vehicle parking monitoring device, an electric vehicle charging station, a smart watch, a fitness tracker, a head-mounted display for Augmented Reality (AR) or Virtual Reality (VR), a wearable for tactile augmentation or sensory enhancement, a water sprinkler, an animal- or item-tracking device, a sensor for monitoring a plant or animal, an industrial robot, an Unmanned Aerial Vehicle (UAV), and any kind of medical device, like a heart rate monitor or a remote controlled surgical robot. A UE in the form of an loT device comprises circuitry and/or software in dependence of the intended application of the loT device in addition to other components as described in relation to the UE 900 shown in Figure 9.

[0210] As yet another specific example, in an loT scenario, a UE may represent a machine or other device that performs monitoring and/or measurements, and transmits the results of such monitoring and/or measurements to another UE and/or a network node. The UE may in this case be an M2M device, which may in a 3GPP context be referred to as an MTC device. As one particular example, the UE may implement the 3GPP NB-IoT standard. In other scenarios, a UE may represent a vehicle, such as a car, a bus, a truck, a ship and an airplane, or other equipment that is capable of monitoring and/or reporting on its operational status or other functions associated with its operation.

[0211] In practice, any number of UEs may be used together with respect to a single use case. For example, a first UE might be or be integrated in a drone and provide the drone’s speed information (obtained through a speed sensor) to a second UE that is a remote controller operating the drone. When the user makes changes from the remote controller, the first UE may adjust the throttle on the drone (e.g., by controlling an actuator) to increase or decrease the drone’s speed. The first and/or the second UE can also include more than one of the functionalities described above. For example, a UE might comprise the sensor and the actuator, and handle communication of data for both the speed sensor and the actuators.

[0212] Figure 10 shows a network node 1000 in accordance with some embodiments. As used herein, network node refers to equipment capable, configured, arranged and/or operable to communicate directly or indirectly with a UE and/or with other network nodes or equipment, in a telecommunication network. Examples of network nodes include, but are not limited to, access points (APs) (e.g., radio access points), base stations (BSs) (e.g., radio base stations, Node Bs, evolved Node Bs (eNBs) and NR NodeBs (gNBs)), O-RAN nodes or components of an O-RAN node (e.g., O-RU, O-DU, O-CU).

[0213] Base stations may be categorized based on the amount of coverage they provide (or, stated differently, their transmit power level) and so, depending on the provided amount of coverage, may be referred to as femto base stations, pico base stations, micro base stations, or macro base stations. A base station may be a relay node or a relay donor node controlling a relay. A network node may also include one or more (or all) parts of a distributed radio base station such as centralized digital units, distributed units (e.g., in an O- RAN access node) and/or remote radio units (RRUs), sometimes referred to as Remote Radio Heads (RRHs). Such remote radio units may or may not be integrated with an antenna as an antenna integrated radio. Parts of a distributed radio base station may also be referred to as nodes in a distributed antenna system (DAS).

[0214] Other examples of network nodes include multiple transmission point (multi-TRP) 5G access nodes, multi-standard radio (MSR) equipment such as MSR BSs, network controllers such as radio network controllers (RNCs) or base station controllers (BSCs), base transceiver stations (BTSs), transmission points, transmission nodes, multi- cell/multicast coordination entities (MCEs), Operation and Maintenance (O&M) nodes, Operations Support System (OSS) nodes, Self-Organizing Network (SON) nodes, positioning nodes (e.g., Evolved Serving Mobile Location Centers (E-SMLCs)), and/or Minimization of Drive Tests (MDTs).

[0215] The network node 1000 includes a processing circuitry 1002, a memory 1004, a communication interface 1006, and a power source 1008. The network node 1000 may be composed of multiple physically separate components (e.g., aNodeB component and a RNC component, or a BTS component and a BSC component, etc.), which may each have their own respective components. In certain scenarios in which the network node 1000 comprises multiple separate components (e.g., BTS and BSC components), one or more of the separate components may be shared among several network nodes. For example, a single RNC may control multiple NodeBs. In such a scenario, each unique NodeB and RNC pair, may in some instances be considered a single separate network node. In some embodiments, the network node 1000 may be configured to support multiple radio access technologies (RATs). In such embodiments, some components may be duplicated (e.g., separate memory 1004 for different RATs) and some components may be reused (e.g., a same antenna 1010 may be shared by different RATs). The network node 1000 may also include multiple sets of the various illustrated components for different wireless technologies integrated into network node 1000, for example GSM, WCDMA, LTE, NR, WiFi, Zigbee, Z-wave, LoRaWAN, Radio Frequency Identification (RFID) or Bluetooth wireless technologies. These wireless technologies may be integrated into the same or different chip or set of chips and other components within network node 1000.

[0216] The processing circuitry 1002 may comprise a combination of one or more of a microprocessor, controller, microcontroller, central processing unit, digital signal processor, application-specific integrated circuit, field programmable gate array, or any other suitable computing device, resource, or combination of hardware, software and/or encoded logic operable to provide, either alone or in conjunction with other network node 1000 components, such as the memory 1004, to provide network node 1000 functionality.

[0217] In some embodiments, the processing circuitry 1002 includes a system on a chip (SOC). In some embodiments, the processing circuitry 1002 includes one or more of radio frequency (RF) transceiver circuitry 1012 and baseband processing circuitry 1014. In some embodiments, the radio frequency (RF) transceiver circuitry 1012 and the baseband processing circuitry 1014 may be on separate chips (or sets of chips), boards, or units, such as radio units and digital units. In alternative embodiments, part or all of RF transceiver circuitry 1012 and baseband processing circuitry 1014 may be on the same chip or set of chips, boards, or units.

[0218] The memory 1004 may comprise any form of volatile or non-volatile computer-readable memory including, without limitation, persistent storage, solid-state memory, remotely mounted memory, magnetic media, optical media, random access memory (RAM), read-only memory (ROM), mass storage media (for example, a hard disk), removable storage media (for example, a flash drive, a Compact Disk (CD) or a Digital Video Disk (DVD)), and/or any other volatile or non-volatile, non-transitory device-readable and/or computer-executable memory devices that store information, data, and/or instructions that may be used by the processing circuitry 1002. The memory 1004 may store any suitable instructions, data, or information, including a computer program, software, an application including one or more of logic, rules, code, tables, and/or other instructions capable of being executed by the processing circuitry 1002 and utilized by the network node 1000. The memory 1004 may be used to store any calculations made by the processing circuitry 1002 and/or any data received via the communication interface 1006. In some embodiments, the processing circuitry 1002 and memory 1004 is integrated.

[0219] The communication interface 1006 is used in wired or wireless communication of signaling and/or data between a network node, access network, and/or UE. As illustrated, the communication interface 1006 comprises port(s)/terminal(s) 1016 to send and receive data, for example to and from a network over a wired connection. The communication interface 1006 also includes radio front-end circuitry 1018 that may be coupled to, or in certain embodiments a part of, the antenna 1010. Radio front-end circuitry 1018 comprises filters 1020 and amplifiers 1022. The radio front-end circuitry 1018 may be connected to an antenna 1010 and processing circuitry 1002. The radio front-end circuitry may be configured to condition signals communicated between antenna 1010 and processing circuitry 1002. The radio front-end circuitry 1018 may receive digital data that is to be sent out to other network nodes or UEs via a wireless connection. The radio front-end circuitry 1018 may convert the digital data into a radio signal having the appropriate channel and bandwidth parameters using a combination of filters 1020 and/or amplifiers 1022. The radio signal may then be transmitted via the antenna 1010. Similarly, when receiving data, the antenna 1010 may collect radio signals which are then converted into digital data by the radio front-end circuitry 1018. The digital data may be passed to the processing circuitry 1002. In other embodiments, the communication interface may comprise different components and/or different combinations of components.

[0220] In certain alternative embodiments, the network node 1000 does not include separate radio front-end circuitry 1018, instead, the processing circuitry 1002 includes radio front-end circuitry and is connected to the antenna 1010. Similarly, in some embodiments, all or some of the RF transceiver circuitry 1012 is part of the communication interface 1006. In still other embodiments, the communication interface 1006 includes one or more ports or terminals 1016, the radio front-end circuitry 1018, and the RF transceiver circuitry 1012, as part of a radio unit (not shown), and the communication interface 1006 communicates with the baseband processing circuitry 1014, which is part of a digital unit (not shown).

[0221] The antenna 1010 may include one or more antennas, or antenna arrays, configured to send and/or receive wireless signals. The antenna 1010 may be coupled to the radio front-end circuitry 1018 and may be any type of antenna capable of transmitting and receiving data and/or signals wirelessly. In certain embodiments, the antenna 1010 is separate from the network node 1000 and connectable to the network node 1000 through an interface or port.

[0222] The antenna 1010, communication interface 1006, and/or the processing circuitry 1002 may be configured to perform any receiving operations and/or certain obtaining operations described herein as being performed by the network node. Any information, data and/or signals may be received from a UE, another network node and/or any other network equipment. Similarly, the antenna 1010, the communication interface 1006, and/or the processing circuitry 1002 may be configured to perform any transmitting operations described herein as being performed by the network node. Any information, data and/or signals may be transmitted to a UE, another network node and/or any other network equipment.

[0223] The power source 1008 provides power to the various components of network node 1000 in a form suitable for the respective components (e.g., at a voltage and current level needed for each respective component). The power source 1008 may further comprise, or be coupled to, power management circuitry to supply the components of the network node 1000 with power for performing the functionality described herein. For example, the network node 1000 may be connectable to an external power source (e.g., the power grid, an electricity outlet) via an input circuitry or interface such as an electrical cable, whereby the external power source supplies power to power circuitry of the power source 1008. As a further example, the power source 1008 may comprise a source of power in the form of a battery or battery pack which is connected to, or integrated in, power circuitry. The battery may provide backup power should the external power source fail.

[0224] Embodiments of the network node 1000 may include additional components beyond those shown in Figure 10 for providing certain aspects of the network node’s functionality, including any of the functionality described herein and/or any functionality necessary to support the subject matter described herein. For example, the network node 1000 may include user interface equipment to allow input of information into the network node 1000 and to allow output of information from the network node 1000. This may allow a user to perform diagnostic, maintenance, repair, and other administrative functions for the network node 1000.

[0225] Although the computing devices described herein (e.g., UEs, network nodes, hosts) may include the illustrated combination of hardware components, other embodiments may comprise computing devices with different combinations of components. It is to be understood that these computing devices may comprise any suitable combination of hardware and/or software needed to perform the tasks, features, functions and methods disclosed herein. Determining, calculating, obtaining or similar operations described herein may be performed by processing circuitry, which may process information by, for example, converting the obtained information into other information, comparing the obtained information or converted information to information stored in the network node, and/or performing one or more operations based on the obtained information or converted information, and as a result of said processing making a determination. Moreover, while components are depicted as single boxes located within a larger box, or nested within multiple boxes, in practice, computing devices may comprise multiple different physical components that make up a single illustrated component, and functionality may be partitioned between separate components. For example, a communication interface may be configured to include any of the components described herein, and/or the functionality of the components may be partitioned between the processing circuitry and the communication interface. In another example, non-computationally intensive functions of any of such components may be implemented in software or firmware and computationally intensive functions may be implemented in hardware.

[0226] In certain embodiments, some or all of the functionality described herein may be provided by processing circuitry executing instructions stored on in memory, which in certain embodiments may be a computer program product in the form of a non-transitory computer-readable storage medium. In alternative embodiments, some or all of the functionality may be provided by the processing circuitry without executing instructions stored on a separate or discrete device-readable storage medium, such as in a hard-wired manner. In any of those particular embodiments, whether executing instructions stored on a non-transitory computer-readable storage medium or not, the processing circuitry can be configured to perform the described functionality. The benefits provided by such functionality are not limited to the processing circuitry alone or to other components of the computing device, but are enjoyed by the computing device as a whole, and/or by end users and a wireless network generally.

[0227] REFERENCES

[1] 3GPP TS 38.331 V17.1.0

[2] 3GPP TS 38.133 V18.0.0

[3] 3GPP RP -213565

Claims

1. A method performed by a user equipment, UE, in a wireless communication network for performing time alignment, TA, with a L1/L2 based inter-cell mobility, LTM, candidate cell, the method comprising: receiving (602) an LTM configuration including an LTM candidate cell configuration for TA establishment and/or TA update with the LTM candidate cell; receiving (603) an indication to perform downlink, DL, pre-synchronization with the LTM candidate cell; performing (604) DL pre-synchronization with the LTM candidate cell in response to the indication; after performing DL pre-synchronization with the LTM candidate cell, receiving (606), from a serving cell, a trigger for TA establishment and/or TA update to the LTM candidate cell; and transmitting (608), in response to the trigger, an uplink, UL, signal to the LTM candidate cell for TA establishment and/or update.

2. The method of Claim 1, wherein the indication to perform DL presynchronization is received in a same message as the LTM configuration.

3. The method of Claim 1, wherein the indication is received in a medium access control, MAC, control element, CE, after receipt of the LTM configuration.

4. The method of any previous Claim, further comprising: transmitting a measurement report to a network node, wherein the measurement report contains measurements related to the LTM candidate cell, and wherein the indication is received in response to the measurement report.

5. The method of any previous Claim, wherein the LTM configuration comprises LTM candidate cell configurations for TA establishment and/or TA update with a plurality of LTM candidate cells, the method further comprising selecting an LTM candidate cell for performing DL pre-synchronization from the plurality of LTM candidate cells.

6. The method of Claim 5, wherein selecting the LTM candidate cell for performing DL pre-synchronization is performed in response to the indication.

7. The method of Claim 6, wherein the indication indicates the LTM candidate cell to be selected for performing DL pre-synchronization.

8. The method of any previous Claim, wherein the trigger is received after the indication.

9. The method of Claim 1, wherein the UL signal is transmitted in a configured UL channel time/frequency resource.

10. The method of any previous Claim, wherein the trigger corresponds to a radio resource control, RRC, message, an RRC information element, IE, a field, a parameter, a medium access control, MAC, control element, CE, or a physical downlink control channel, PDCCH, order, command or indication.

11. The method of any of any previous Claim, wherein the serving cell comprises a PCell, a PSCell and/or a SpCell.

12. The method of any previous Claim, wherein the UL channel comprises a physical random-access channel, PRACH, and wherein the signal comprises a PRACH preamble.

13. The method of any previous Claim, wherein the LTM configuration is received in a radio resource control, RRC, reconfiguration message from the serving cell, and wherein the LTM configuration comprises one or more LTM candidate cell configurations, to be applied upon reception of an LTM cell switch command.

14. The method of any previous Claim, wherein the LTM candidate cell configuration comprises one or more UL related parameters, the one or more UL related parameters comprising a PRACH preamble configuration, a PRACH occasion, and/or a PRACH frequency resource.

15. The method of any previous Claim, wherein the LTM configuration comprises a plurality of LTM candidate cell configurations for a plurality of LTM candidate cells, the method further comprising selecting a subset of the plurality of LTM candidate cells for performing DL pre-synchronization based on one or more rules.

16. The method of any previous Claim, wherein performing DL pre-synchronization with the LTM candidate cell comprises one or more of: i) detecting and/or measuring at least one synchronization signal of the LTM candidate cell, wherein the synchronization signal comprises a synchronization signal block, SSB, of the LTM candidate cell associated to an SSB index and/or identifier and transmitted in a spatial direction, a channel state information reference signal, CSI-RS, a tracking reference signal, TRS, a primary synchronization signal, PSS, and/or a secondary synchronization signal SSS; ii) performing fine time tracking and acquiring full timing information of the LTM candidate cell; iii) obtaining time boundaries of a time unit of a given LTM candidate cell, wherein the time unit comprises one of a time slot, an orthogonal frequency division multiplexing, OFDM, symbol, a subframe, and/or radio frame; and iv) synchronizing a clock with the time boundaries of a time unit of a given LTM candidate cell.

17. A method performed by a network node in a wireless communication network for L1/L2 based inter-cell mobility, LTM, of a user equipment, UE, to a candidate cell, the method comprising: transmitting (702) to the UE an LTM configuration including an LTM candidate cell configuration for TA establishment and/or TA update with the LTM candidate cell; transmitting (703) an indication to the U Eto perform downlink, DL, presynchronization to the LTM candidate cell; and transmitting (704) a trigger to the UE for TA establishment and/or TA update to the LTM candidate cell.

18. The method of Claim 17, wherein transmitting the trigger for TA establishment is performed after the UE has performed DL pre-synchronization with the LTM candidate cell in response to the LTM configuration.

19. The method of Claim 17 or 18, wherein the indication to perform DL presynchronization is received in a same message as the LTM configuration.

20. The method of Claim 17 or 18, wherein the indication is received in a medium access control, MAC, control element, CE, after receipt of the LTM configuration.

21. The method of any of Claims 17 to 20, further comprising: transmitting a measurement report to a network node, wherein the measurement report contains measurements related to the LTM candidate cell, and wherein the indication is received in response to the measurement report.

22. The method of any of Claims 17 to 21, wherein the LTM configuration comprises LTM candidate cell configurations for TA establishment and/or TA update with a plurality of LTM candidate cells.

23. The method of Claim 22, wherein the indication indicates an LTM candidate cell of the plurality of LTM candidate cells to be selected by the UE for performing DL presynchronization.

24. The method of any of Claims 17 to 23, wherein transmitting the trigger is performed after transmitting the indication.

25. The method of any of Claims 17 to 24, wherein the UL signal is transmitted in a configured UL channel time/frequency resource.

26. The method of any of Claims 17 to 25, wherein the trigger corresponds to a radio resource control, RRC, message, an RRC information element (IE), a field, a parameter, a medium access control, MAC, control element, CE, or a physical downlink control channel, PDCCH, order, command or indication.

27. The method of any of Claims 17 to 26, wherein the network node is associated with a serving cell that serves the UE, and the serving cell comprises a PCell, a PSCell and/or a SpCell.

28. The method of any of Claims 17 to 27, wherein the UL channel comprises a physical random access channel, PRACH, and wherein the signal comprises a PRACH preamble.

29. The method of any of Claims 7 to 28, wherein the LTM configuration is transmitted in a radio resource control, RRC, reconfiguration message from the serving cell, and wherein the LTM configuration comprises one or more LTM candidate cell configurations, to be applied upon reception of an LTM cell switch command.

30. The method of any of Claims 17 to 29, wherein the LTM candidate cell configuration comprises one or more UL related parameters, the one or more UL related parameters comprising a PRACH preamble configuration, a PRACH occasion, and/or a PRACH frequency resource.

31. The method of any of Claims 17 to 30, wherein the LTM configuration comprises a plurality of LTM candidate cell configurations.