Cell change in a cellular communications network

Abstract

According to one example aspect of this disclosure, a method is provided, the method comprising: receiving an indication of a candidate target cell for a cell change of a user equipment; receiving timing advance information of at least one serving cell of the user equipment, wherein the at least one serving cell is synchronized with the candidate target cell; performing downlink measurements on at least one signal received from one of the at least one serving cells; performing downlink measurements on at least one signal received from the candidate target cell; and estimating the timing advance of the candidate target cell based on the downlink measurements and the timing advance information of the at least one serving cell.

Description

Technical Field

[0001] The various example embodiments generally relate to cellular communication systems, and more specifically to cell changes in such networks. Background Technology

[0002] Mobility of wireless terminals (such as User Equipment) needs to be enabled in various wireless communication networks, and cell changes can be used to ensure that wireless terminals can move within cellular communication networks without encountering serious connectivity problems. Therefore, mobility is crucial in cellular communication networks, such as those operating under LTE and / or 5G radio access technologies. 5G radio access technology can also be referred to as New Radio (NR) access technology. LTE has been widely deployed since its inception, and the 3GPP (3rd Generation Partnership Project) is still developing LTE. Similarly, 3GPP has also developed standards for 5G / NR. At least one topic in the 3GPP discussions relates to cell changes, and based on these discussions, there is a need for improved methods, apparatus, and computer programs for cell changes. Summary of the Invention

[0003] The subject matter of the independent claims is provided for several aspects. Some exemplary embodiments are defined in the dependent claims.

[0004] The scope of protection sought for the various exemplary embodiments of this disclosure is defined by the independent claims. Exemplary embodiments and features (if any) described in this specification that do not fall within the scope of the independent claims are to be construed as examples that aid in understanding the various exemplary embodiments of this disclosure.

[0005] According to a first aspect of this disclosure, an apparatus is provided, the apparatus including at least one processor and at least one memory storing instructions, which, when executed by the at least one processor, cause the apparatus to at least: receive an indication of a candidate target cell for cell change of the apparatus; receive timing advance information of at least one serving cell of the apparatus, wherein the at least one serving cell is synchronized with the candidate target cell; perform a downlink measurement on at least one signal received from one of the at least one serving cell; perform a downlink measurement on at least one signal received from the candidate target cell; and estimate the timing advance of the candidate target cell based on the downlink measurement and the timing advance information of the at least one serving cell.

[0006] Example embodiments of the first aspect may include at least one feature from the following bulleted list or any combination of the following features: • The stored instructions, when executed by the at least one processor, also cause the device to at least: receive an instruction from one of the at least one serving cells; and perform the downlink measurement based on the instruction from the one of the at least one serving cells; • The stored instructions, when executed by the at least one processor, also cause the device to at least: receive a list including information about a plurality of serving cells of the device, wherein the plurality of serving cells are time-synchronized with candidate target cells; • The timing advance information of at least one serving cell includes an indication of the following: a timing advance value of at least one serving cell or a timing advance group applicable to estimating the timing advance of candidate target cells; • The stored instructions, when executed by the at least one processor, also cause the device to at least: receive from the central unit an indication of a candidate target cell and the timing advance information about the at least one serving cell via one of the at least one serving cells; • When the stored instructions are executed by the at least one processor, the device also causes the device to at least: receive an instruction instructing the device to perform a timing advance estimation of a candidate target cell based on the user equipment (UE) based on one of the at least one serving cells; • When the stored instructions are executed by the at least one processor, the device also causes the device to at least: receive an instruction instructing at least one serving cell to synchronize with a candidate target cell; • When the stored instructions are executed by the at least one processor, the apparatus further causes the apparatus to at least: receive an instruction indicating a timing advance group to be used to determine a timing advance value for a candidate target cell; determine a timing advance value for a candidate target cell based on the instruction indicating the timing advance group; and communicate with the candidate target cell using the determined timing advance value. • The serving cell is a secondary cell, and the other serving cell of the device is a special cell, wherein the special cell is out of sync with the candidate target cell; • When the stored instructions are executed by the at least one processor, the apparatus also causes the device to at least: receive the final configuration of the timing advance group or target cell mapping for timing advance estimation based on the UE.

[0007] According to a second aspect of this disclosure, an apparatus is provided, the apparatus comprising at least one processor and at least one memory storing instructions, the instructions, when executed by the at least one processor, causing the apparatus to at least: determine at least one serving cell of a user equipment and a candidate target cell for cell change of the user equipment, wherein the at least one serving cell is synchronized with the candidate target cell; and transmit an indication of the candidate target cell and timing advance information of the at least one serving cell to the user equipment or via one of the at least one serving cells.

[0008] Example embodiments of the second aspect may include at least one feature from the following bulleted list or any combination of the following features: • When executed by the at least one processor, the stored instructions also cause the device to at least: transmit a list including information about a plurality of serving cells, wherein the plurality of serving cells are time-synchronized with candidate target cells; • When the stored instructions are executed by the at least one processor, the device also causes the device to at least: send an indication of a candidate target cell and the timing advance information about the at least one serving cell to the user equipment, or via one of the at least one serving cells; • The stored instructions, when executed by the at least one processor, also cause the device to at least: receive from a distributed unit controlling candidate target cells a list including information about a plurality of serving cells of a user equipment, wherein the plurality of cells are time-synchronized with the candidate target cells; • When the stored instructions are executed by the at least one processor, the device also causes the device to at least: transmit an identifier of a timing advance group based on the timing advance estimation of the user equipment (UE) for the candidate target cell; • When the stored instructions are executed by the at least one processor, the apparatus also causes the device to at least: send the final configuration of the timing advance group or target cell mapping to the user equipment via at least one serving cell or through one of the at least one serving cells for timing advance estimation based on the UE.

[0009] According to a third aspect, a first method is provided, the first method comprising: receiving an indication of a candidate target cell for cell change of a user equipment; receiving timing advance information of at least one serving cell of the user equipment, wherein the at least one serving cell is synchronized with the candidate target cell; performing downlink measurements on at least one signal received from one of the at least one serving cells; performing downlink measurements on at least one signal received from the candidate target cell; and estimating the timing advance of the candidate target cell based on the downlink measurements and the timing advance information of the at least one serving cell.

[0010] According to the fourth aspect, a second method is provided, the second method comprising: determining at least one serving cell of a user equipment and a candidate target cell for cell change of the user equipment, wherein the at least one serving cell is synchronized with the candidate target cell; and sending an indication of the candidate target cell and timing advance information of the at least one serving cell to the user equipment or via one of the at least one serving cells.

[0011] According to a fifth aspect of this disclosure, an apparatus is provided, comprising: components for receiving an indication of a candidate target cell for cell change of the apparatus; components for receiving timing advance information of at least one serving cell of the apparatus, wherein the at least one serving cell is synchronized with the candidate target cell; components for performing downlink measurements on at least one signal received from one of the at least one serving cell; components for performing downlink measurements on at least one signal received from the candidate target cell; and components for estimating the timing advance of the candidate target cell based on the downlink measurements and the timing advance information of the at least one serving cell.

[0012] According to a sixth aspect of this disclosure, an apparatus is provided, comprising: components for determining at least one serving cell of a user equipment and a candidate target cell for cell change of the user equipment, wherein the at least one serving cell is synchronized with the candidate target cell; and components for transmitting an indication of the candidate target cell and timing advance information of the at least one serving cell to the user equipment or via one of the at least one serving cells.

[0013] According to a seventh aspect of this disclosure, a non-transitory computer-readable medium is provided having a computer-readable instruction set stored thereon, which, when executed by at least one processor, causes a device to perform at least a first method. According to an eighth aspect of this disclosure, a non-transitory computer-readable medium is provided having a computer-readable instruction set stored thereon, which, when executed by at least one processor, causes a device to perform at least a second method.

[0014] According to a ninth aspect of this disclosure, a computer program is provided, the computer program including instructions that, when executed by a device, cause the device to perform a first method. According to a tenth aspect of this disclosure, a computer program is provided, the computer program including instructions that, when executed by a device, cause the device to perform a second method. Attached Figure Description

[0015] Figure 1 The illustration depicts a network scenario according to at least some example embodiments;

[0016] Figure 2 The diagram illustrates a signaling diagram according to at least some example embodiments;

[0017] Figure 3 The illustration shows an example device capable of supporting at least some of the example embodiments;

[0018] Figure 4 A flowchart illustrating a first method according to at least some example embodiments is shown; and

[0019] Figure 5 A flowchart illustrating a second method according to at least some example embodiments is shown. Detailed Implementation

[0020] Embodiments of this disclosure provide enhancements for cell changes in cellular communication networks. More specifically, even in scenarios with at least partial asynchrony, embodiments of this disclosure enable timing advance (TA) estimation by a User Equipment (UE) for candidate target cells in a cell change for the UE. The UE 110 may receive timing advance information for at least one of its serving cells, wherein the at least one serving cell may be synchronized with the candidate target cell. The UE 110 may then estimate the TA of the candidate target cell based at least on the timing advance information. Therefore, as long as one of the UE's serving cells (e.g., a serving secondary cell, SCell) is synchronized with the candidate target cell, a TA estimate based on the UE can be obtained even if another serving cell (e.g., a serving special cell, SpCell) is not synchronized with the candidate target cell. In some example embodiments, at least one serving cell may be time-synchronized with the candidate target cell.

[0021] Figure 1 Examples of network scenarios according to at least some embodiments are illustrated. Figure 1 In an example scenario, a cellular communication network may exist, which may further include UE 110, source distributed unit S-DU 120, and target DU T-DU 130. S-DU 120 may also include at least one cell or be associated with at least one cell. Cells 120a and 120b of S-DU 120 may be referred to as source cells for cell change. In some example embodiments, S-DU 120 may be referred to as the serving radio node of UE 110. Cells 120a and 120b may be referred to as serving cells of UE 110. Before the cell change, S-DU 120 may be considered as the serving DU for UE 110, while after the cell change, T-DU 130 may be considered as the serving DU for UE 110.

[0022] T-DU 130 may include or be associated with cells, for example, at least three cells, such as cells 130a, 130b, and 130c. First cell 130a may be a first candidate target cell for cell change, and second cell 130b may be a second candidate target cell for cell change. First cell 130a may be the most likely cell for cell change, and second cell 130b may be the second most likely cell for cell change. The network may also include a core network 140. Central unit CU 142 may be located in the core network 140.

[0023] CU 142 can be a logical node. CU 142 can perform some, but not all, tasks of a base station (BS) (such as a gNB). For example, CU 142 can transmit user data, control mobility, perform resource sharing of the radio access network, location, and / or session management. CU 142 can control the operation of S-DU 120 and T-DU 130. S-DU 120 and T-DU 130 can also be logical nodes. S-DU 120 and T-DU 130 can also perform some, but not all, tasks of a BS (such as a gNB). S-DU 120 and T-DU 130 can perform BS tasks different from those of CU 142. In some example embodiments, S-DU 120 and T-DU 130 may be referred to as Transmit Receive Points (TRPs).

[0024] The position of UE 110 at different times is determined by Figure 1 Points 102, 104, 106, and 108 are represented in the diagram. Before the cell change, UE 110 may be located at point 102 and connected to S-DU 120 via air interface 115. Then, UE 110 may begin moving from point 102 to T-DU 130 via points 104 and 106. At point 108, UE 110 may have already performed the cell change. Therefore, at point 108, UE 110 may be connected to the target DU via air interface 125, and the cell change from cell 120a to cell 130a is complete.

[0025] S-DU 120 and T-DU 130 can be directly connected to each other via wired interface 135 (such as an X2 or Xn interface). S-DU 120 and T-DU 130 can also be connected to the core network 140 directly or via at least one intermediate node. The core network 140 can in turn connect to another network via wired interface 145. Figure 1 (Not shown in the image) coupling, through which connections to other networks can be obtained, for example, via a global interconnection network.

[0026] For example, UE 110 may include smartphones, cellular phones, machine-to-machine (M2M) nodes, machine-type communication (MTC) nodes, Internet of Things (IoT) nodes, automotive telemetry units, laptops, tablets, or virtually any type of suitable mobile wireless terminal or station.

[0027] The air interface 115 between UE 110 and S-DU 120 can be configured according to a first radio access technology (RAT). UE 110 and S-DU 120 are configured to support the first RAT, and UE 110 can communicate with S-DU 120 via air interface 115 using the first RAT before a cell change. Similarly, the air interface 125 between UE 110 and T-DU 130 can be configured according to a second RAT. UE 110 and T-DU 130 are configured to support the second RAT, and UE 110 can communicate with T-DU 130 via air interface 125 using the second RAT after a cell change.

[0028] The first RAT and the second RAT can be the same or different. That is, the cell change can be an intra-RAT or inter-RAT cell change. The first RAT and the second RAT can be, for example, cellular RATs operating according to at least one standard specification specified by the 3GPP (3rd Generation Partnership Project). Examples of cellular RATs include Long Term Evolution (LTE), New Radio (NR), also known as 5G Radio Access Technology, and MulteFire. In any case, embodiments of this disclosure are not limited to any particular wireless technology. Rather, embodiments of this disclosure can be utilized in any wireless communication system where it is desirable to perform cell changes in scenarios where there is at least partial asynchrony.

[0029] For example, in the case of L1 / L2 triggered mobility (LTM), when UE 110 moves from the coverage area of ​​one cell to the coverage area of ​​another, a serving cell change may be required at some point. The serving cell change can be triggered by Layer 3 measurements, such as Radio Resource Control (RRC) measurement reports from UE 110. The serving cell change can be performed via downlink RRC signaling, such as an RRC reconfiguration message with synchronization for changes to the primary cell PCell and primary / secondary cell PSCell, and, where applicable, synchronization for the release and addition of SCells. This can involve a full L2 (and L1) reset, resulting in longer latency, greater overhead, and longer downtime than beam-switching mobility. However, the goal of L1 / L2 mobility enhancement is to enable serving cell changes via L1 / L2 signaling to reduce latency, overhead, and downtime.

[0030] In the LTM scenario, mechanisms and procedures for L1 / L2-based inter-cell mobility need to be specified to reduce mobility latency. For example, configuration and maintenance for multiple candidate target cells may be required to allow for rapid application of configurations for these cells. For potential applications based on L1 / L2 signaling, dynamic handover mechanisms between candidate cells (including SpCell and SCell) may be necessary. Furthermore, L1 enhancements, including L1 measurement and reporting, and beam indication, may be required for inter-cell beam management. Alternatively or additionally, TA management may be needed. Additionally, if required, CU-DU interface signaling should support L1 / L2 mobility.

[0031] The process based on L1 / L2 inter-cell mobility can be applied to at least one of the following scenarios: • In standalone, carrier aggregation (CA), and new radio-dual connectivity (NR-DC) scenarios, where a cell change occurs within a cell group, the primary cell group (MCG) takes precedence. • Intra-DU and intra-CU DU scenarios (applicable to standalone and CA: new radio access network interfaces may not be required). • Both within and between frequencies; • Frequency ranges include both FR1 and FR2, where FR1 can refer to a frequency band up to 6 GHz, and FR2 can refer to a frequency band between 24, 25 GHz and 71 GHz; or • The source cell and the target cell can be synchronized or not.

[0032] Regarding TA acquisition, before receiving a cell handover command in L1 / L2-based mobility, it is necessary to support TA acquisition for (multiple) candidate target cells. For example, the following solutions regarding the mechanism for acquiring the TA of candidate target cells can be further investigated: • Solutions based on the Random Access Channel (RACH), such as RACH from the network's Physical Downlink Control Channel (PDCCH) command, UE-triggered RACH, and higher-layer triggered RACH (excluding L3 HO commands); and • RACH-free solutions, such as TA acquisition based on the sounding reference signal (SRS), RACH-free mechanisms based on receiver timing difference in LTE, and UE-based TA measurement (including UE-based TA measurement via a TA command from the serving cell).

[0033] The mechanism for obtaining the TA (Target Acquisition Target) of (multiple) candidate target cells in LTM may require at least RACH (Rapid Access Control) that supports PDCCH commands. PDCCH commands can be triggered solely by the source cell (i.e., the serving cell) and not by other cells.

[0034] For RACH of the PDCCH command in LTM, support for at least one of the following enhancements may be required: • Introduce an indication of the candidate target cell and / or the RACH timing of the candidate target cell in the downlink control information (DCI); • The configuration of RACH resources for (multiple) candidate cells is provided before the PDCCH command; or • Whether / how to send a random access response (RAR).

[0035] Regarding the RACH for PDCCH commands used in LTM candidate target cells, whether RAR is needed, at least one of the following alternatives can be considered for further investigation: • Alternative Option 1: Requires RAR; • Alternative Option 2: No RAR required; or • Alternative Option 3: Whether RAR needs to be configurable.

[0036] Support for UE-based TA measurement may be required, where UE 110 can derive the TA based on the received timing difference between the current serving cell and the candidate target cell, and the TA value used for the current serving cell. Corresponding UE capabilities can be introduced to support UE-based TA measurement. For example, UE 110 can report its support for this capability. Configuration for UE-based TA measurement can also be supported.

[0037] Due to timing alignment error (TAE), downlink timing estimation error (between the serving and candidate target cells), serving cell TA resolution error, and TA adjustment error, the actual uplink receiver timing error at T-DU 130 can be greater than the cyclic prefix. Even if UE 110 can derive the TA, such errors can lead to a performance degradation of T-DU 130. However, in certain scenarios, such as FR1, where the TAE between the serving and candidate target cells is within 260 ns, UE 110 can derive the TA based on UE-based TA measurements. UE 110 can meet uplink transmission timing requirements under good SNR conditions without causing any performance degradation of T-DU 130. Therefore, it can be assumed that UE-based TA measurements are feasible at least in some scenarios, and that UE-based TA measurements are enabled for LTM.

[0038] For the RACH used in PDCCH commands for (multiple) candidate cells, RAR reception can be configured and / or indicated. If RAR reception is not configured and / or indicated (no RAR), the TA value of the candidate target cell can be indicated in the cell handover command. It may also be necessary to consider whether UE 110 should retransmit the Physical Random Access Channel (PRACH) when RAR reception is not configured and / or indicated, and how UE 110 determines the transmission power of subsequent PRACH triggered by the PDCCH command.

[0039] If RAR reception is configured and / or indicated (with RAR), it may be necessary to consider whether the RAR was received from the serving cell or the candidate target cell. If the RAR was received from the candidate cell, it may also be necessary to consider whether the Type 1-PDCCH common search space (CSS) of the candidate target cell is configured for UE 110. The context of the RAR may also need to be determined.

[0040] UE 110 can report support for combinations with and without RAR only, where support for a default scheme can be the baseline UE method for LTM.

[0041] In some example embodiments, LTM can be performed as follows. CU 142 can prepare candidate target cells for LTM and provide LTM configuration for the prepared cells. CU 142 can also configure L1 measurement reports required for LTM execution for UE 110. UE 110 can then perform early uplink / downlink synchronization with the target candidate cells to minimize interruptions during LTM execution. This is one of the main advantages of LTM, which differs from traditional handover procedures where interruptions caused by synchronization with candidate target cells during the handover process are minimized through early synchronization.

[0042] Afterwards, UE 110 can send the L1 beam measurement of the candidate target cell to the serving cell, and S-DU 120 can determine which cell UE 110 should hand over to. If the TA of the candidate target cell is still valid, UE 110 can skip the random access procedure during the handover (i.e., cell change).

[0043] Alternatively, in some example embodiments, LTM can be performed as follows: CU 142 can prepare candidate target cells for LTM and provide LTM configuration for the prepared cells. CU 142 can also configure the UE 110 with the L1 measurement reports required for LTM execution. CU 142 can also provide S-DU 120 with TA acquisition trigger criteria and necessary configurations. CU 142 can also provide S-DU 120 with cell handover trigger criteria and necessary configurations. The trigger criteria for TA acquisition and cell handover can be similar to measurement event reporting trigger conditions, such as A3, A4, or A5 conditions or the validity of the acquired TA. Trigger configurations include filter configurations (for L1 measurements), trigger offsets, cell-specific offsets, etc.

[0044] UE 110 can then begin reporting L1 measurements regarding the LTM configuration to S-DU 120. S-DU 120 can then decide to trigger TA acquisition for T-DU / (multiple) cells and send a TA acquisition command to UE 110. UE 110 can then send a random access preamble to the T-DU / (multiple) cells so that the T-DU / (multiple) cells can estimate the TA between UE 110 and the T-DU / (multiple) cells. S-DU 120 can then indirectly (via CU 142) receive RAR.

[0045] UE 110 can send the prepared L1 beam measurements of candidate target cells to S-DU 120, and S-DU 120 can determine which candidate target cell UE 110 should hand over to. If RAR is not received indirectly, S-DU 120 can provide the acquired TA (as an alternative for receiving the TA of the candidate target cell) to UE 110 via Media Access Control Unit (MAC CE) command.

[0046] If the target cell's TA is still valid, UE 110 can skip the RACH procedure when performing the HO. Afterwards, UE 110 and the network can continue to complete the LTM procedure.

[0047] In the case of TA acquisition based on UE, when UE 110 is served by S-DU 120, UE 110 can calculate, determine, or estimate the TA of T-DU 130, or the TA of the cell of T-DU 130, for example, as shown below. (1)

[0048] The relative time difference (RTD) can refer to the relative difference in transmission time of simultaneous signals between any pair of two TRPs (such as S-DU 120 and T-DU 130). The RTD can be based on downlink measurements performed by UE 110 for both the serving cell and the target cell. TA2 can be the TA of the candidate target cell (e.g., the cell of T-DU 130). TA1 can be the TA of the serving cell (e.g., the cell of S-DU 120).

[0049] The Time Alignment Error (TAE) can be the time alignment error of the UE 110 estimator, for example, due to synchronization between the source cell and the candidate target cell. The TAE can be defined by 3GPP, for example in section 6.5.3 of standard specification TS 38.104. In the context of multiple-input multiple-output MIMO transmissions from the TRP, the maximum value may not exceed 3000 ns. A similar quantity (referred to as cell phase synchronization accuracy) can relate to transmissions from cell pairs and is defined by 3GPP, for example in section 7.4 of standard specification TS 38.133. Cell phase synchronization accuracy for time division duplex can be defined as the maximum absolute deviation of frame start timing between any cell pairs on the same frequency with overlapping coverage areas, and should be better than 3000 ns.

[0050] Furthermore, in the context of downlink positioning, transmission timing errors can be defined by 3GPP, for example, in Section 3.1 of TS38.305. Transmission timing errors can be defined as the result of transmission time delays involved in signal transmission, which can be defined as the time delay from the time the digital signal is generated at baseband to the time the radio frequency signal is transmitted from the transmit antenna. Additionally, 3GPP can define the information element NR-RTD-Info, for example in the standard specification TS 37.355, which can be used by the location server to provide time synchronization information between the reference TRP and the list of adjacent TRPs. These definitions, information elements, and related mechanisms in the specification allow the UE 110 to be aware of timing misalignments in transmissions from different TRPs and to account for them in location estimation. In some example embodiments, the term "OtherEstError" can refer to any error caused by uplink / downlink reciprocity or estimator implementation and / or method errors.

[0051] When the cell of UE 110 changes, synchronization mismatch between the serving cell (e.g., cell 120a) and the candidate target cell (e.g., cell 130a) can be a challenge. UE-based time alignment estimation can work in scenarios where the serving cell and the candidate target cell are synchronized (i.e., the time alignment error between the serving cell and the target cell is non-existent or negligible).

[0052] When estimating the TA of a candidate target cell, UE-based TA estimation can use the TA of the serving cell (such as SpCell). However, if the serving cell and the candidate target cell are out of sync, UE 110 cannot perform UE-based TA estimation because this would result in an incorrect TA estimation of the candidate target cell, which could lead to failure during cell change to the candidate target cell. As disclosed herein, several methods and apparatus configured to perform these methods are provided to enable UE-based TA estimation in such scenarios, for example, for LTM cell handover without RACH.

[0053] In some example embodiments, UE 110 may be served by a group of cells. This group of cells may be referred to as a cell group (MCG or secondary cell group SCG). The cell group may include at least one SpCell. In the case of CA, there may also be one or more SCells serving UE 110. If the TAs of the cells in the group are different, UE 110 may maintain four different TA values ​​for all cells in the group. At least in the dual connectivity case, SpCell may refer to the PCell of the MCG or the PSCell of the SCG. In some example embodiments, SpCell may be PCell+PSCell.

[0054] Although UE 110 can have multiple serving cells at a time (e.g., one SpCell and multiple SCells) and maintain a maximum of four TA values, the TA of the SpCell can be used for UE-based TA estimation. The TAs of other cells besides the SpCell may not be used, which limits the applicability of UE-based TA estimation due to synchronization issues. For example, the serving SpCell and the candidate target cell may be out of sync, while one of the serving SCells may be synchronized with the candidate target cell. Even in this case, UE 110 cannot perform UE-based TA estimation because if UE 110 were allowed to perform UE-based TA estimation, it would use the TA of the SpCell but not the TAs of any other cells.

[0055] Therefore, UE 110 needs to use the TA of the serving cell (such as SCell) that is synchronized with the candidate target cell. Otherwise, UE 110 cannot utilize its UE-based TA estimation capability and perform RACH-free cell change by using the estimated TA. Therefore, even if one of the serving cells (such as SpCell) is not synchronized with the candidate target cell, UE 110 is expected to be able to perform UE-based TA estimation.

[0056] In some example embodiments, the serving SpCell and the candidate target cell may be out of sync, but one of the serving SCells may be synchronized with the candidate target cell. In this case, if the serving SCell is synchronized with the target cell, UE 110 can utilize information about the serving SCell. UE 110 can perform UE-based TA estimation by using the TA of the SCell synchronized with the target cell.

[0057] Figure 2 The diagram illustrates a signaling diagram according to at least some example embodiments. On the vertical axis, from left to right, are arranged UE110, S-DU 120, first T-DU 130, second T-DU 132, and CU 142.

[0058] At step 202, UE 110 can send a measurement report to S-DU 120. At step 204, S-DU 120 can forward UE 110's measurement report to CU 142. UE 110 can therefore send a measurement report to CU 142 via S-DU 120 to trigger at least one candidate target cell (such as...) Figure 1 Preparation of cells 130a, 130b and 130c of the first T-DU 130 shown.

[0059] At steps 206 and 208, CU 142 can send a UE context establishment request to both the first T-DU 130 and the second T-DU 132 to establish the context of UE 110. The first T-DU 130 can control two cells (cell-1.1 and cell-1.2, such as...) Figure 1 Cells 130a and 130b are shown, while the second T-DU 132 can control two other cells (cell-2.1 and cell-2.2). At steps 206 and 208, CU 142 can therefore initiate preparation for cells 1.1 and 1.2 at the first T-DU 130 and cells 2.1 and 2.2 at the second T-DU 132 by sending a UE context establishment request. At steps 206 and 208, CU 142 can also request the preparation of certain cells for cell change for UE 110. The first T-DU 130 and the second T-DU 132 can accept all or a subset of the requested cells to be prepared. However, the first T-DU 130 and the second T-DU 132 may not be permitted and / or configured to prepare any cells not requested.

[0060] At steps 210 and 212, the first T-DU 130 and the second T-DU 132 can respectively send a UE context establishment response to the CU 142. At step 210, the first T-DU 130 can send a list of S-DU serving cells synchronized with cell-1.1 and a list of S-DU serving cells synchronized with cell-1.2. At step 212, the second T-DU 132 can send a list of S-DU serving cells synchronized with cell-2.1 and a list of S-DU serving cells synchronized with cell-2.2. Therefore, the first T-DU 130 and the second T-DU 132 can respond with a UE context establishment response, indicating that the requested cell is ready.

[0061] In some example embodiments, for each candidate target cell, the first T-DU 130 and the second T-DU 132 may indicate which serving cells of the S-DU 120 are synchronized with each candidate target cell. That is, the first T-DU 130 may send a list including information about multiple serving cells of UE 110, wherein these multiple serving cells are synchronized in time with candidate target cell-1.1; and send another list including information about multiple serving cells of UE 110, wherein these multiple cells are synchronized in time with candidate target cell-1.2. Similarly, the second T-DU 132 may send a list including information about multiple serving cells of UE 110, wherein these multiple serving cells are synchronized in time with candidate target cell-2.1; and send another list including information about multiple serving cells, wherein these multiple serving cells are synchronized in time with candidate target cell-2.2. These lists may include at least one of SpCell or SCell.

[0062] At step 214, CU 142 may send a list of candidate target cells prepared for cell change of UE 110 to S-DU 120, and request from S-DU 120 information about which serving cells of UE 110 are synchronized with each candidate target cell. S-DU 120 may, in response to this request, send a list of serving cells of UE 110 synchronized with each candidate target cell. That is, S-DU 130 may send a list including information about multiple serving cells that are time-synchronized with candidate target cell-1.1; and send another list including information about multiple serving cells that are time-synchronized with candidate target cell-1.2. These lists may include at least one of SpCell or SCell.

[0063] In some example embodiments, S-DU 120 may send to CU 142 a list of timing advance groups (TAGs) for each candidate cell's UE-based TA estimation. For example, TAG 2 (or TAG-ID 2) may be applicable to the UE-based TA estimation for cell 1.1, and TAG 3 and TAG 4 (or corresponding TAG-ID 3 and TAG 4) may be applicable to the UE-based TA estimation for cell 1.2. In some example embodiments, S-DU 120 may send to CU 142 a list of applicable candidate cells for each TAG-ID.

[0064] At step 216, CU 142 may determine a candidate target cell (such as cell 130a) and at least one serving cell (such as cell 120a) for cell change of UE 110. At least one serving cell may be time-synchronized with a candidate cell. For example, at least one serving cell may be synchronized with a candidate target cell when the clocks of at least one serving cell and the candidate target cell are synchronized. In some example embodiments, at least one serving cell may be considered synchronized with a candidate target cell when the cell phase synchronization accuracy is higher than a threshold. For example, cell phase synchronization accuracy may be higher than a threshold for time division duplex when the maximum absolute deviation in frame start timing between any pair of cells on the same frequency with overlapping coverage areas is better than 3000 ns.

[0065] In some example embodiments, CU 142 may determine a list of serving cells synchronized with each candidate target cell. Alternatively or additionally, CU 142 may determine a list of TAGs applicable to UE-based TA estimation for each candidate target cell. For example, when CU 142 determines synchronization between serving cells and candidate target cells, CU 142 may determine TAGs that can be used for each candidate cell.

[0066] At step 218, CU 142 may send at least TA information of at least one serving cell and an indication of a candidate target cell for cell change of UE 110, wherein at least one serving cell is time-synchronized with the candidate target cell. The TA information of at least one serving cell may include an indication of the following: the TA value of at least one serving cell or a TAG applicable to estimating the TA of the candidate target cell.

[0067] In some example embodiments, CU 142 may send a list of serving cells synchronized with each candidate target cell or a list of TAGs based on UE-based TA estimation applicable to each candidate target cell. At step 220, S-DU 120 may send this indication and the aforementioned information to UE 110.

[0068] For example, CU 142 may provide these lists as LTM candidate configurations to UE 110 via S-DU 120 in an RRCReconfiguration message. CU 142 may provide the final configuration of TAG and candidate target cell mappings for UE-based TA estimation to S-DU 120 and / or UE 110. S-DU or UE may need to know at least one of the following: which serving cells are synchronized with which candidate target cells (a mapping between serving cells and candidate target cells in terms of synchronization); or which TAG IDs are applicable to which candidate target cells for UE-based TA estimation (a mapping between candidate target cells and applicable TAG IDs for UE-based TA estimation). For example, UE 110 may receive a list including information about multiple serving cells of UE 110, such as at least one TAG-ID, wherein these multiple serving cells are time-synchronized with candidate target cells. Alternatively or additionally, UE 110 may receive an indication of candidate target cells and the timing advance information, such as TAG-IDs, of at least one serving cell from CU 110 via one of the at least one serving cells.

[0069] At step 222, UE 110 may send an acknowledgment message to S-DU 120 to acknowledge receipt of the instruction and the TA information. UE 110 may respond, for example, by sending an RRCReconfigurationComplete message after successfully decoding the received configuration. At step 224, S-DU 120 may forward the acknowledgment message to CU 142, for example, by sending an RRCReconfigurationComplete message.

[0070] At step 226, UE 110 may send a measurement report to S-DU 120. For the measurement report, UE 110 may perform downlink measurements on at least one signal received from one of the at least one serving cell. For example, UE 110 may initiate an L1 measurement report that includes L1 measurements of the candidate target cell and the one of the at least one serving cells, so that S-DU 120 can monitor the quality of the candidate target cell and the one of the at least one serving cells before triggering TA estimation or cell change. The report may be periodic, and the period can be configured using an RRCReconfiguration message.

[0071] UE 110 can perform downlink measurements on at least one signal received from at least one candidate target cell. UE 110 can then calculate RTD based on the downlink measurements for signals received from the serving cell and the candidate target cell. For example, RTD may be needed to estimate the TA of the candidate target cell using Equation 1 above.

[0072] UE-based TA acquisition can be triggered either UE-initiated or network-triggered. In the case of a network-triggered process, S-DU 120 can trigger UE 110 to initiate UE-based TA acquisition at step 226 by sending a trigger for this operation. S-DU 120 can also send an indication of at least one of the serving cells, and UE 110 can perform the downlink measurement based on the indication of the at least one serving cell. Therefore, S-DU 120 can indicate the at least one serving cell, and UE 110 should use the TA of the at least one serving cell to evaluate the TA of the candidate target cell. Alternatively, UE 110 can decide which cell's TA will be evaluated for the candidate target cell. In the case of a network-triggered process, S-DU 120 can indicate the TAG-ID (or a list of TAG-IDs) applicable to the UE-based TA estimation and the UE-based TA estimation trigger for the candidate target cell 1.2.

[0073] In some example embodiments, UE 110 may receive an indication of a candidate target cell for cell change of UE 110 and TA information about at least one serving cell, wherein the at least one serving cell is time-synchronized with the candidate target cell. For example, UE 110 may receive the serving SpCell or SCell ID (or a list including the IDs of SpCell and SCell) synchronized with the candidate target (such as cell 1.2) so that UE 110 can determine which TA will be used for UE-based TA estimation of the candidate target cell.

[0074] At step 228, UE 110 may use the TA of one of the at least one serving cell synchronized with candidate target cell 1.2. UE 110 may estimate the TA of the candidate target cell based on the downlink measurement and the TA information of the one of the at least one serving cell. In the case of UE-initiated UE-based TA estimation, UE 110 may use the information provided at step 220. Otherwise, UE 110 may use the information provided at step 226.

[0075] At step 230, UE 110 may send a UE-based TA acquisition report, i.e., a successful estimation or initiation of the TA estimation process. At step 230, UE 110 may also perform one or more transmissions of measurement reports.

[0076] At step 232, S-DU 120 may decide to trigger a serving cell change for UE 110 to a candidate target cell (such as cell 1.2). S-DU 120 may decide that the TA estimate based on the UE will be used for a cell change without RACH. In some example embodiments, S-DU 120 may determine the TAG-ID of the TAG based on the TA estimate of the UE applicable to the candidate target cell (such as cell 1.2).

[0077] At step 234, S-DU 120 may send a control message, such as MAC CE, to UE 110 to trigger a cell change. Upon receiving the control message, UE 110 may determine that a cell change to the candidate cell will be performed. In some example embodiments, S-DU 120 may send an indication of a TAG-ID, which UE 110 should use for UE-based TA estimation of the candidate target cell. S-DU 120 may indicate to UE 110 the TAG-ID (or a list of TAG-IDs) applicable to the UE-based TA estimation and the UE-based TA estimation trigger for the candidate target cell.

[0078] In some example embodiments, S-DU 120 may send the service SpCell or SCell ID (or a list including the IDs of SpCell and SCell) synchronized with candidate cell 120 so that UE 110 can determine which TA will be used for UE-based TA estimation of the candidate target cell.

[0079] At step 236, UE 110 may use the TA information of one of the at least one serving cell synchronized with the candidate target cell. For example, UE 110 may use the TA of the one serving cell synchronized with the candidate target cell. In some example embodiments, UE 110 may use information from the MAC CE provided at step 226 or step 234. UE 110 may use the TA of the TAG indicated by the TAG-ID for candidate target cell TA estimation.

[0080] At step 238, UE 110 can send a reconfiguration complete message, such as RRCReconfigurationComplete, to the first T-DU 130. UE 110 can then continue the RACH-free procedure and complete the cell change to cell 1.2. At step 240, the first T-DU 130 can send an uplink RRC message transmission request to CU 142. At step 242, CU 142 can send a UE context release command to the S-DU and receive a UE context release complete message in response.

[0081] Therefore, UE 110 can perform UE-based TA estimation by using the TA of one serving cell (such as SCell) of UE 110 that is synchronized with the candidate target cell. UE-based TA estimation can be performed even if another serving cell (such as SpCell) is not synchronized with the candidate target cell.

[0082] Figure 3 An example device capable of supporting at least some of the example embodiments is illustrated. The illustrated device is 300, which may include, for example, UE 110, S-DU 120, first T-DU 130, second T-DU 132, or CU 142, or a control device configured to control its functions (possibly when installed therein). Device 300 includes a processor 310, which may include, for example, a single-core or multi-core processor, wherein a single-core processor includes one processing core, and a multi-core processor includes more than one processing core. Processor 310 typically includes a control device. Processor 310 may include more than one processor. Processor 310 may be a control device. For example, the processing core may include a Cortex-A8 processing core manufactured by ARM Holdings or a Steamroller processing core manufactured by Advanced Micro Devices Corporation. Processor 310 may include at least one Qualcomm Snapdragon and / or Intel Atom processor. Processor 310 may include at least one application-specific integrated circuit (ASIC). Processor 310 may include at least one field-programmable gate array (FPGA). Processor 310 may be a component for performing method steps in device 300. Processor 310 may be configured, at least in part, by computer instructions to perform actions.

[0083] A processor may include, or be configured as, one or more circuit systems configured to perform various stages of the methods according to the example embodiments described herein. As used herein, the term “circuit system” may refer to one or more or all of the following: (a) a hardware circuit implementation only, such as an implementation in an analog and / or digital circuit system only; and (b) a combination of hardware circuitry and software, such as, where applicable: (i) a combination of (multiple) analog and / or digital hardware circuitry with software / firmware; and (ii) any portion of (multiple) hardware processors having software (including (multiple) digital signal processors), software, and (multiple) memories, which work together to enable a device (such as a mobile phone or a server) to perform various functions; and (c) (multiple) hardware circuitry and / or (multiple) processors, such as (multiple) microprocessors or a portion thereof, which require software (e.g., firmware) to operate, but may be absent when the software is not required to operate.

[0084] This definition of circuit system applies to all uses of the term in this application, including in any claim. As another example, as used in this application, the term circuit system also covers only hardware circuitry or a processor (or multiple processors), or a portion of hardware circuitry or a processor and its accompanying software and / or firmware implementation. The term circuit system also covers (e.g., and if applicable to a particular claim element) baseband integrated circuits or processor integrated circuits for mobile devices, or similar integrated circuits in servers, cellular network devices, or other computing or network devices.

[0085] Device 300 may include memory 320. Memory 320 may include random access memory and / or permanent memory. Memory 320 may include at least one RAM chip. Memory 320 may include, for example, solid-state, magnetic, optical, and / or holographic memory. Memory 320 may be at least partially accessible by processor 310. Memory 320 may be at least partially included in processor 310. Memory 320 may be a component for storing information. Memory 320 may include computer instructions configured to be executed by processor 310. When computer instructions configured to cause processor 310 to perform certain actions are stored in memory 320, and device 300 is configured as a whole to operate using computer instructions from memory 320 under the guidance of processor 310, processor 310 and / or at least one of its processing cores may be considered to be configured to perform said certain actions. Memory 320 may be at least partially included in processor 310. Memory 320 may be at least partially located outside device 300, but device 300 may access the memory.

[0086] Device 300 may include a transmitter 330. Device 300 may include a receiver 340. Transmitter 330 and receiver 340 may be configured to transmit and receive information according to at least one cellular or non-cellular standard, respectively. Transmitter 330 may include more than one transmitter. Receiver 340 may include more than one receiver. Transmitter 330 and / or receiver 340 may be configured to operate according to, for example, Global System for Mobile Communications (GSM), Wideband Code Division Multiple Access (WCDMA), Long Term Evolution (LTE), and / or 5G / NR standards.

[0087] Device 300 may include a near-field communication (NFC) transceiver 350. The NFC transceiver 350 may support at least one NFC technology, such as Bluetooth, Wibree, or similar technologies.

[0088] Device 300 may include a user interface (UI) 360. UI 360 may include at least one of the following: a display, a keyboard, a touchscreen, a vibrator arranged to signal to the user by causing device 300 to vibrate, a speaker, and a microphone. The user can operate device 300 via UI 360, for example, by receiving incoming calls, initiating phone or video calls, browsing the internet, managing digital files stored in memory 320 or accessible in the cloud via transmitter 330 and receiver 340 or via NFC transceiver 350, and / or playing games.

[0089] Device 300 may include or be arranged to accept a subscriber identity module 370. Subscriber identity module 370 may include, for example, a subscriber identity module SIM card that can be installed in device 300. Subscriber identity module 370 may include subscription information identifying the user of device 300. Subscriber identity module 370 may include password information that can be used to verify the identity of the user of device 300 and / or facilitate the encryption of communication information and billing of communications conducted by the user of device 300 via device 300.

[0090] Processor 310 may be equipped with a transmitter arranged to output information from processor 310 to other devices included in device 300 via electrical wires within device 300. Such a transmitter may include a serial bus transmitter arranged to output information to memory 320, for example, via at least one electrical wire, for storing the information therein. Alternatively, the transmitter may include a parallel bus transmitter. Similarly, processor 310 may include a receiver arranged to receive information from other devices included in device 300 via electrical wires within device 300. Such a receiver may include a serial bus receiver arranged to receive information from receiver 340, for example, via at least one electrical wire, for processing within processor 310. Alternatively, the receiver may include a parallel bus receiver.

[0091] Device 300 may include Figure 3 Other devices not shown. For example, in the case where device 300 includes a smartphone, it may include at least one digital camera. Some devices 300 may include a rear camera and a front camera, wherein the rear camera can be used for digital photography and the front camera is used for video calls. Device 300 may include a fingerprint sensor arranged to at least partially authenticate the user of device 300. In some example embodiments, device 300 lacks at least one of the above-mentioned devices. For example, some devices 300 may lack an NFC transceiver 350 and / or a user identification module 370.

[0092] Processor 310, memory 320, transmitter 330, receiver 340, NFC transceiver 350, UI 360, and / or user identity module 370 can be interconnected in various ways via electrical wires within device 300. For example, each of the above-described devices can be individually connected to the main bus within device 300 to allow the devices to exchange information. However, those skilled in the art will understand that this is only one example, and various ways of interconnecting at least two of the above-described devices can be selected depending on the exemplary embodiment, without departing from the scope of the exemplary embodiment.

[0093] Figure 4 This is a flowchart of a first method according to at least some embodiments. The method can be used for... Figure 1 The UE 110 or the device that controls its functions, and / or the device that performs them.

[0094] The first method may include, at step 410, receiving an indication of a candidate target cell for cell change of the user equipment. The first method may further include, at step 420, receiving timing advance information of at least one serving cell of the user equipment, wherein the at least one serving cell is synchronized with the candidate target cell. The first method may further include, at step 430, performing downlink measurements on at least one signal received from one of the at least one serving cell. Additionally, the first method may include, at step 440, performing downlink measurements on at least one signal received from the candidate target cell. Finally, the first method may include, at step 450, estimating the timing advance of the candidate target cell based on the downlink measurements and the timing advance information of the at least one serving cell.

[0095] Figure 5 This is a flowchart of a second method according to at least some embodiments. This method can be used for... Figure 1 The CU 142 or the device that controls its functions, and / or the device that performs them.

[0096] The second method may include, at step 510, determining at least one serving cell of the user equipment and a candidate target cell for cell change of the user equipment, wherein the at least one serving cell is synchronized with the candidate target cell. The second method may further include, at step 520, sending an indication of the candidate target cell and timing advance information of the at least one serving cell to the user equipment, either to or via one of the at least one serving cell.

[0097] It should be understood that the disclosed embodiments are not limited to the specific structures, process steps, or materials disclosed herein, but can be extended to equivalents that will be recognized by those skilled in the art. It should also be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting.

[0098] References to an embodiment or an embodiment throughout this specification mean that a particular feature, structure, or characteristic described in connection with that embodiment is included in at least one embodiment. Therefore, the appearance of the phrase "in one embodiment" or "in an embodiment" in various places throughout the specification does not necessarily refer to the same embodiment. When numerical values ​​are referred to using terms such as, for example, approximately, or substantially, exact numerical values ​​are also disclosed.

[0099] As used herein, for convenience, multiple items, structural elements, constituent elements, and / or materials may be presented in a common list. However, these lists should be interpreted as each member being individually identified as a separate and unique member. Therefore, no individual member in such a list should be construed as a de facto equivalent of any other member in the same list solely based on its presentation in the common group (without any indication to the contrary). Additionally, various embodiments and examples, as well as alternatives for their various components, may be referenced herein. It should be understood that such embodiments, examples, and alternatives should not be construed as de facto equivalents of each other, but should be considered as separate and autonomous representations.

[0100] In example embodiments, a device (such as UE 110, S-DU 120, first T-DU 130, second T-DU 132 or CU 142) or a device for controlling its functions may include components for performing the above embodiments and any combination thereof.

[0101] In an example embodiment, a computer program includes instructions that, when executed by a device, cause the device to perform a first or second method according to the above embodiments and any combination thereof. In an example embodiment, a computer program product embodied on a non-transitory computer-readable medium can be configured to control a processor to perform processes including the above embodiments and any combination thereof.

[0102] In an example embodiment, an apparatus (such as UE 110, S-DU 120, first T-DU 130, second T-DU 132, or CU 142) or a device for controlling its functions may include at least one processor and at least one memory including computer program code, wherein the at least one memory and the computer program code are configured, together with the at least one processor, to cause the apparatus to perform at least the embodiments described above and any combination thereof.

[0103] Furthermore, the described features, structures, or characteristics can be combined in any suitable manner in one or more embodiments. In the foregoing description, numerous specific details, such as examples of length, width, shape, etc., have been provided to provide a complete understanding of embodiments of this disclosure. However, those skilled in the art will recognize that this disclosure can be practiced without one or more specific details, or by other methods, components, materials, etc. In other instances, well-known structures, materials, or operations have not been shown or described in detail to avoid obscuring aspects of this disclosure.

[0104] While the foregoing examples illustrate the principles of embodiments in one or more specific applications, those skilled in the art will understand that many modifications can be made to the form, use, and details of the implementation without inventive effort and without departing from the principles and concepts of this disclosure. Therefore, this disclosure is not intended to be restrictive, except as limited by the claims set forth below.

[0105] In this document, the verbs “comprising” and “including” are used as open-ended restrictions, neither excluding nor requiring the presence of unreferenced features. Unless otherwise expressly stated, features referenced in dependent claims may be freely combined with each other. Furthermore, it should be understood that the use of “an” or “a” (i.e., the singular form) throughout the document does not exclude the plural form. Industrial applicability

[0106] At least some example implementations have found industrial applications in communication networks, such as cellular communication networks (like 3GPP networks). List of abbreviations 3GPP: Third Generation Partnership Project BS: Base Station CA: Carrier Aggregation CSS: Public Search Space FR: Frequency range IoT: Internet of Things LTE: Long Term Evolution LTM: L1 / L2 triggered mobility M2M: Machine to Machine MAC CE: Media Access Control Unit MCG: Main Cell Group MIMO: Multiple Input Multiple Output MTC: Machine Type Communication NR: New Radio NR-DC: New Radio - Dual Connectivity PCell: Main Cell PSCell: Primary and Secondary Communities PDCCH: Physical Downlink Control Channel PRACH: Physical Random Access Channel RACH: Random Access Channel RAR: Random Access Response RAT: Radio Access Technology RRC: Radio Resource Control RTD: Relative Time Difference SCell: Secondary Community SCG: Auxiliary Community Group SpCell: Special Cell SRS: Detection Reference Signal TA: Scheduled in advance TAE: Timing Alignment Error TAG: Timed advance group TRP: Transmitter / Receiver Point UE: User Equipment WiMAX: Global Microwave Access Interoperability WLAN: Wireless Local Area Network List of reference numerals

Claims

1. An apparatus comprising: at least one processor; and at least one memory storing instructions that, when executed by the at least one processor, cause the apparatus at least to: - receive an indication of a candidate target cell for a cell change of the apparatus; - receive timing advance information of at least one serving cell of the apparatus, wherein the at least one serving cell is synchronized with the candidate target cell; - perform downlink measurements on at least one signal received from one of the at least one serving cell; - perform downlink measurements on at least one signal received from the candidate target cell; and - estimate a timing advance of the candidate target cell based on the downlink measurements and the timing advance information of the one of the at least one serving cell.

2. The apparatus of claim 1, wherein the stored instructions, when executed by the at least one processor, further cause the apparatus at least to: - receive an indication of the one of the at least one serving cell; and - perform the downlink measurements based on the indication of the one of the at least one serving cell.

3. The apparatus of claim 1 or claim 2, wherein the stored instructions, when executed by the at least one processor, further cause the apparatus at least to: - receive a list comprising information on a plurality of serving cells of the apparatus, wherein the plurality of serving cells are synchronized in time with the candidate target cell.

4. The apparatus of any one of the preceding claims, wherein the timing advance information of the at least one serving cell comprises an indication of a timing advance value of the at least one serving cell or a timing advance group applicable for estimating a timing advance of the candidate target cell.

5. The apparatus of any one of the preceding claims, wherein the stored instructions, when executed by the at least one processor, further cause the apparatus at least to: - receive the indication of the candidate target cell and the timing advance information of the at least one serving cell from a central unit via one of the at least one serving cell.

6. The apparatus of any one of the preceding claims, wherein the stored instructions, when executed by the at least one processor, further cause the apparatus at least to: - receive an indication indicating that the apparatus is to perform a user equipment, UE, based timing advance estimation of the candidate target cell based on the one of the at least one serving cell.

7. The apparatus of any one of the preceding claims, wherein the stored instructions, when executed by the at least one processor, further cause the apparatus at least to: - receive an indication indicating that the at least one serving cell is synchronized with the candidate target cell.

8. The apparatus of any one of the preceding claims, wherein the stored instructions, when executed by the at least one processor, further cause the apparatus at least to: ​ - The receiving indication will be used to determine the timing advance group for the timing advance value of the candidate target cell; - Determine the timing advance value for the candidate target cell based on the indication that indicates the timing advance group; as well as - Use the determined timing advance value to communicate with the candidate target cell.

9. The apparatus according to any one of the preceding claims, wherein one serving cell is a secondary cell, and another serving cell of the apparatus is a special cell, wherein the special cell is not synchronized with the candidate target cell.

10. An apparatus comprising: At least one processor; as well as At least one memory, the at least one memory storing instructions, the instructions, when executed by the at least one processor, causing the device to at least: - Identify at least one serving cell for a user equipment and a candidate target cell for cell change of the user equipment, wherein the at least one serving cell is synchronized with the candidate target cell; as well as - Send the indication of the candidate target cell and the timing advance information of the at least one serving cell to the user equipment, or via one of the at least one serving cells.

11. The apparatus of claim 10, wherein the stored instructions, when executed by the at least one processor, further cause the apparatus to at least: - Send a list including information about multiple serving cells, wherein the multiple serving cells are synchronized in time with the candidate target cell.

12. The apparatus of claim 10 or claim 11, wherein the stored instructions, when executed by the at least one processor, further cause the apparatus to at least: - Send the indication of the candidate target cell and the timing advance information of the at least one serving cell to the user equipment, or via the at least one serving cell.

13. The apparatus according to any one of claims 10 to 12, wherein the stored instructions, when executed by the at least one processor, further cause the apparatus to at least: - Receive a list from the distributed unit controlling the candidate target cell, including information about multiple serving cells of the user equipment, wherein the multiple cells are time-synchronized with the candidate target cell.

14. The apparatus according to any one of claims 10 to 13, wherein the stored instructions, when executed by the at least one processor, further cause the apparatus to at least: - Send the identifier of the timing advance group based on the timing advance estimation of the user equipment (UE) for the candidate target cell.

15. A method comprising: - Receive an instruction for a candidate target cell for cell change of user equipment; - Receive timing advance information of at least one serving cell of the user equipment, wherein the at least one serving cell is synchronized with the candidate target cell; - Perform downlink measurements on at least one signal received from one of the at least one serving cells; - Perform downlink measurements on at least one signal received from the candidate target cell; as well as - Estimate the timing advance of the candidate target cell based on the downlink measurements and the timing advance information of one of the at least one serving cells.

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