WO2025093244A1 - Cell change in cellular communication networks - Google Patents

Abstract

According to an example aspect of the present disclosure, there is provided a 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, 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 timing advance of the candidate target cell based on said timing advance information of said one of the at least one serving cell and said downlink measurements.

Description

CELL CHANGE IN CELLULAR COMMUNICATION NETWORKS

FIELD

[0001] Various example embodiments relate in general to cellular communication networks and more specifically, to a cell change in such networks.

BACKGROUND

[0002] Mobility of wireless terminals, such as User Equipment, UEs, needs to be enabled in various wireless communication networks and cell changes may be exploited for ensuring that a wireless terminal may move in a cellular communication network without experiencing significant connectivity issues. Mobility is thus very important in cellular communication networks, such as in networks operating according to Long Term Evolution, LTE, and/or 5G radio access technology. 5G radio access technology may also be referred to as New Radio, NR, access technology. Since its inception, LTE has been widely deployed and 3rd Generation Partnership Project, 3GPP, still develops LTE. Similarly, 3GPP also develops standards for 5G/NR. At least one of the topics in the 3GPP discussions is related to cell changes and according to the discussions there is a need to provide improved methods, apparatuses and computer programs for cell changes.

SUMMARY

[0003] According to some aspects, there is provided the subject-matter of the independent claims. Some example embodiments are defined in the dependent claims.

[0004] The scope of protection sought for various example embodiments of the disclosure is set out by the independent claims. The example embodiments and features, if any, described in this specification that do not fall under the scope of the independent claims are to be interpreted as examples useful for understanding various example embodiments of the disclosure.

[0005] According to a first aspect of the present disclosure, there is provided 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 timing advance of the candidate target cell based on said timing advance information of said one of the at least one serving cell and said downlink measurements.

[0006] Example embodiments of the first aspect may comprise at least one feature from the following bulleted list or any combination of the following features:

• wherein the stored instructions further cause, when executed by the at least one processor, the apparatus at least to receive an indication of said one of the at least one serving cell and perform said downlink measurements based on the indication of said one of the at least one serving cell;

• wherein the stored instructions further cause, when executed by the at least one processor, the apparatus at least to receive a list comprising information about multiple serving cells of the apparatus, wherein said multiple serving cells are synchronized in time with the candidate target cell;

• wherein said 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 estimation of timing advance of the candidate target cell;

• wherein the stored instructions further cause, when executed by the at least one processor, the apparatus at least to receive the indication of the candidate target cell and said timing advance information about the at least one serving cell from a central unit via one of the at least one serving cell;

• wherein the stored instructions further cause, when executed by the at least one processor, the apparatus at least to receive an indication indicating that the apparatus is to perform a User Equipment, UE, -based timing advance estimate of the candidate target cell based on said one of the at least one serving cell;

• wherein the stored instructions further cause, when executed by the at least one processor, the apparatus at least to receive an indication indicating that the at least one serving cell is synchronized with the candidate target cell; • wherein the stored instructions further cause, when executed by the at least one processor, the apparatus at least to receive an indication indicating a timing advance group to be used for determining a timing advance value for the candidate target cell, determine, based on the indication indicating the timing advance group, the timing advance value for the candidate target cell and communicate with the candidate target cell using the determined timing advance value;

• wherein said one serving cell is a secondary cell and another serving cell of the apparatus is a special cell, wherein the special cell is unsynchronized with the candidate target cell;

• wherein the stored instructions further cause, when executed by the at least one processor, the apparatus at least to receive a final configuration of a timing advance group or a target cell mapping for UE-based timing advance estimation.

[0007] According to a second aspect of the present disclosure, there is provided 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 determine a candidate target cell for a cell change of a user equipment and at least one serving cell of the user equipment, wherein the at least one serving cell is synchronized with the candidate target cell and transmit, to the at least one serving cell or to the user equipment via one of the at least one serving cell, an indication of the candidate target cell and timing advance information of the at least one serving cell.

[0008] Example embodiments of the second aspect may comprise at least one feature from the following bulleted list or any combination of the following features:

• wherein the stored instructions further cause, when executed by the at least one processor, the apparatus at least to transmit a list comprising information about multiple serving cells, wherein said multiple serving cells are synchronized in time with the candidate target cell;

• wherein the stored instructions further cause, when executed by the at least one processor, the apparatus at least to transmit the indication of the candidate target cell and said timing advance information about the at least one serving cell to the at least one serving cell or to the user equipment via one of the at least one serving cell; • wherein the stored instructions further cause, when executed by the at least one processor, the apparatus at least to receive, from a distributed unit controlling the candidate target cell, a list comprising information about multiple serving cells of the user equipment, wherein said multiple cells are synchronized in time with the candidate target cell;

• wherein the stored instructions further cause, when executed by the at least one processor, the apparatus at least to transmit identities of timing advance groups that are applicable for a User Equipment, UE, -based timing advance estimate of the candidate target cell;

• wherein the stored instructions further cause, when executed by the at least one processor, the apparatus at least to transmit a final configuration of a timing advance group or a target cell mapping for UE-based timing advance estimation to the at least one serving cell or to the user equipment via one of the at least one serving cell.

[0009] According to a third aspect, there is provided a first method comprising, receiving an indication of a candidate target cell for a cell change of the 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, 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 timing advance of the candidate target cell based on said timing advance information of said one of the at least one serving cell and said downlink measurements.

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

[0011] According to a fifth aspect of the present disclosure, there is provided an apparatus comprising means for receiving an indication of a candidate target cell for a cell change of the apparatus, means 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, means for performing downlink measurements on at least one signal received from one of the at least one serving cell, means for performing downlink measurements on at least one signal received from the candidate target cell and means for estimating timing advance of the candidate target cell based on said timing advance information of said one of the at least one serving cell and said downlink measurements.

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

[0013] According to a seventh aspect of the present disclosure, there is provided a non-transitory computer readable medium having stored thereon a set of computer readable instructions that, when executed by at least one processor, cause an apparatus to at least perform the first method. According to an eighth aspect of the present disclosure, there is provided non-transitory computer readable medium having stored thereon a set of computer readable instructions that, when executed by at least one processor, cause an apparatus to at least perform the second method.

[0014] According to a ninth of the present disclosure, there is provided a computer program comprising instructions which, when the program is executed by an apparatus, cause the apparatus to carry out the first method. According to a tenth aspect of the present disclosure, there is provided a computer program comprising instructions which, when the program is executed by an apparatus, cause the apparatus to carry out the second method.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] FIG. 1 illustrates a network scenario in accordance with at least some example embodiments; [0016] FIG. 2 illustrates a signalling graph in accordance with at least some example embodiments;

[0017] FIG. 3 illustrates an example apparatus capable of supporting at least some example embodiments;

[0018] FIG. 4 illustrates a flow graph of a first method in accordance with at least some example embodiments; and

[0019] FIG. 5 illustrates a flow graph of a second method in accordance with at least some example embodiments.

EMBODIMENTS

[0020] Embodiments of the present disclosure provide enhancements for a cell change in cellular communication networks. More specifically, embodiments of the present disclosure enable estimation of a Timing Advance, TA, by a User Equipment, UE, for a candidate target cell of a cell change of the UE, even in at least partly nonsynchronized scenarios. UE 110 may receive timing advance information of at least one serving cell of UE 110, wherein the at least one serving cell may be synchronized with the candidate target cell. UE 110 may then estimate TA of the candidate target cell based at least on said timing advance information. Hence, a UE-based TA estimate may be acquired even if one of the serving cells (e.g., a serving Special Cell, SpCell) of the UE would not be synchronized with the candidate target cell, as long as another one of the serving cells (e.g., a serving Secondary Cell, SCell) of the UE is synchronized with the candidate target cell. In some example embodiments, the at least one serving cell may be synchronized in time with the candidate target cell.

[0021] FIG. 1 illustrates an example of a network scenario in accordance with at least some embodiments. According to the example scenario of FIG. 1, there may be a cellular communication network, which may further comprise UE 110, Source Distributed Unit, S-DU, 120 and Target DU, T-DU, 130. S-DU 120 may also comprise, or be associated with, at least one cell. Cells 120a and 120b of S-DU 120 may be referred to as source cells for a cell change. In some example embodiments, S-DU 120 may be referred to as a serving wireless node of UE 110. Cells 120a and 120b may be referred to as serving cells of UE 110. S-DU 120 may be considered as a serving DU for UE 110 before a cell change while T-DU 130 may be considered as a serving DU for UE 110 after the cell change.

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

[0023] CU 142 may be a logical node. CU 142 may perform some tasks of a Base Station, BS, such as a gNB, but not all. For example, CU 142 may transfer data of a user, control mobility, perform sharing of resources of a radio access network, positioning and/or management of sessions. CU 142 may control operation of S-DU 120 and T-DU 130. S-DU 120 and T-DU 130 may be logical nodes as well. S-DU 120 and T-DU 130 may also perform some tasks of a BS, such as a gNB, but not all. S-DU 120 and T-DU 130 may perform different tasks of a BS than CU 142. In some example embodiments, S-DU 120 and T-DU 130 may be referred to as Transmission and Reception Points, TRPs.

[0024] Locations of UE 110 at different time instants are denoted by points 102, 104, 106 and 108 in FIGURE 1. UE 110 may be located at point 102 before the cell change and be connected to S-DU 120 via air interface 115. Then, UE 110 may start moving from point 102 towards T-DU 130 via points 104 and 106. At point 108, UE 110 may have performed the cell change. Thus, at point 108 UE 110 may be connected to target DU via air interface 125, and the cell change from cell 120a to cell 130a completed.

[0025] S-DU 120 and T-DU 130 may be connected to each other directly via wired interface 135, such as a X2 or Xn interface. S-DU 120 and T-DU 130 may be connected, directly or via at least one intermediate node, with core network 140 as well. Core network 140 may be, in turn, coupled via wired interface 145 with another network (not shown in FIG. 1), via which connectivity to further networks may be obtained, for example via a worldwide interconnection network.

[0026] UE 110 may comprise, for example, a smartphone, a cellular phone, a Machine-to-Machine, M2M, node, Machine-Type Communications node, MTC, an Internet of Things, loT, node, a car telemetry unit, a laptop computer, a tablet computer or, indeed, any kind of suitable mobile wireless terminal or station. [0027] Air interface 115 between UE 110 and S-DU 120 may be configured in accordance with a first Radio Access Technology, RAT, which UE 110 and S-DU 120 are configured to support, and UE 110 may communicate with S-DU 120 via air interface 115 using the first RAT before the cell change. Similarly, air interface 125 between UE 110 and T-DU 130 may be configured in accordance with a second RAT, which UE 110 and T-DU 130 are configured to support, and UE 110 may communicate with T-DU 130 via air interface 125 using the second RAT after the cell change.

[0028] The first RAT and the second RAT may, or may not, be the same. That is to say, the cell change may be an intra-RAT or inter-RAT cell change. The first and second RATs may be cellular RATs, operating for example in accordance with at least one standard specification specified by the 3^rd Generation Partnership Project, 3GPP. Examples of cellular RATs include Long Term Evolution, LTE, New Radio, NR, which may also be known as fifth generation, 5G, radio access technology and MulteFire. In any case, embodiments of the present disclosure are not restricted to any particular wireless technology. Instead, embodiments of the present disclosure may be exploited in any wireless communication system, wherein it is desirable to perform cell changes in at least partly non-synchronized scenarios.

[0029] For example, in case of L1/L2 Triggered Mobility, LTM, a serving cell change may be needed at some point when UE 110 moves from a coverage area of one cell to a coverage area of another cell. Change of the Serving cell may be triggered by Layer 3 measurements, e.g., Radio Resource Control, RRC, measurement report from UE 110. Serving cell change may be done by downlink RRC signaling, such as RRC reconfiguration message with synchronization for change of Primary Cell, PCell, and Primary Secondary Cell, PSCell, as well as release and add for SCells, when applicable. Cases may involve complete L2 (and LI) resets, leading to longer latency, larger overhead, and longer interruption time than beam switch mobility. However, the goal of L1/L2 mobility enhancements is to enable a serving cell change via L1/L2 signaling, to reduce the latency, overhead and interruption time.

[0030] In case of LTM, there is a need to specify mechanism and procedures of L1/L2 based inter-cell mobility for mobility latency reduction. For example, configuration and maintenance for multiple candidate target cells may be needed to allow fast application of configurations for candidate target cells. Dynamic switch mechanism among candidate cells (including SpCells and SCells) may be needed for the potential applicable scenarios based on L1/L2 signaling. Also, LI enhancements may be needed for inter-cell beam management, including LI measurement and reporting, and beam indication. Alternatively, or in addition, TA management may be needed. In addition, CU-DU interface signaling should support L1/L2 mobility, if needed.

[0031] The procedure of L1/L2 based inter-cell mobility may be applicable to at least one of the following scenarios:

• Standalone, Carrier Aggregation, CA, and New Radio - Dual Connectivity, NR- DC, case with a cell change of a serving cell within one cell group, prioritizing Master Cell Group, MCG;

• Intra-DU case and intra-CU inter-DU case (applicable for Standalone and CA: new radio access network interfaces might not be needed);

• Both intra-frequency and inter-frequency;

• Both Frequency Ranges, FR, 1 and FR2, wherein FR1 may refer to frequency bands up to 6 GHz and FR2 may refer to frequency bands between 24,25 GHz and 71 GHz; or

• Source and target cells may be synchronized or non-synchronized.

[0032] Concerning TA Acquisition, there is a need to support TA acquisition of candidate target cell(s) before cell switch command is received in L1/L2 based mobility. On a mechanism to acquire TA of the candidate target cells, for example the following solutions may be further studied:

• Random Access Channel, RACH, -based solutions, e.g., Physical Downlink Control Channel, PDCCH, ordered RACH, UE-triggered RACH, higher layer triggered RACH from a network other than L3 HO command; and

• RACH-less solutions, e.g., Sounding Reference Signal, SRS, -based TA acquisition, receiver timing difference based, RACH-less mechanism as in LTE, UE-based TA measurement (including UE-based TA measurement with one TA command from a serving cell).

[0033] On mechanism to acquire TA of the candidate target cell(s) in LTM, at least PDCCH ordered RACH may need to be supported. The PDCCH order may only be triggered by a source cell, i.e., a serving cell, but not by other cells.

[0034] For PDCCH ordered RACH in LTM, at least one the following enhancements may need to be supported: • introduce indication of a candidate target cell and/or RACH occasion of candidate target cell in Downlink Control Information, DCI;

• configuration of RACH resource for candidate cell(s) is provided prior to the PDCCH order; or

• whether/how to transmit a Random Access Response, RAR.

[0035] On whether a RAR is needed for PDCCH ordered RACH for a candidate target cell in LTM, at least one of the following alternatives may be considered for further study:

• Alt 1 : RAR is needed;

• Alt 2: RAR is not needed; or

• Alt 3 : whether RAR is needed can be configured.

[0036] UE-based TA measurement, wherein UE 110 may derive TA based on received timing difference between a current serving cell and a candidate target cell, as well as TA value for the current serving cell, may need to be supported. Corresponding UE capability may be introduced to support UE-based TA measurement. For example, UE 110 may report its support of this capability. Configuration of UE-based TA measurement may be supported as well.

[0037] Due to Timing Alignment Error, TAE, downlink timing estimation error (of both serving and candidate target cell), serving cell TA resolution error and TA adjustment error, the actual uplink receiver timing error at T-DU 130 may be larger than a cyclic prefix. Even though UE 110 may be able to derive the TA, such errors may cause performance degradation at T-DU 130. However, in some specific scenarios, e.g., in FR1 where the TAE between a serving cell and a candidate target cell is within 260ns, UE 110 may be able to derive the TA based on UE-based TA measurement. UE 110 may meet uplink transmit timing requirements under good SNR condition and might not cause any performance degradation at T-DU 130. It may be hence assumed that UE-based TA measurement is feasible at least for some scenarios and UE-based TA measurement enabled for LTM.

[0038] For PDCCH ordered-RACH for candidate cell(s), RAR reception may be configured and/or indicated. If reception of a RAR is not configured and/or indicated (without RAR), a TA value of a candidate target cell may be indicated in a cell switch command. It may also need to be considered whether UE 110 should re-transmit Physical Random Access Channel, PRACH, when reception of a RAR is not configured and/or indicated, and how UE 110 may determine the transmit power of subsequent PRACH triggered by a PDCCH order.

[0039] If reception of a RAR is configured and/or indicated (with RAR), it may need to be considered whether the RAR is received from a serving cell or a candidate target cell. If the RAR is received from a candidate cell, it may also need to be considered whether Typel-PDCCH Common Search Space, CSS, of the candidate target cell is configured to UE 110. Content of the RAR may need to be decided as well.

[0040] UE 110 may report the support for combination of with RAR only and without RAR only, where support of one default scheme may be the baseline UE approach for LTM.

[0041] In some example embodiments, the LTM may be performed as follows. CU 142 may prepare the candidate target cells for LTM and provide the LTM configuration for the prepared cells. CU 142 may also configure UE 110 with LI measurement reporting needed for LTM execution. UE 110 may then perform early uplink/downlink synchronization with the target candidate cells to minimize the interruption during LTM execution. This is one of the main benefits of the LTM, which differs from the traditional handover procedure where the interruption time caused by synchronization with a candidate target cell during the handover is minimized with early synchronization procedure.

[0042] After that, UE 110 may send the LI beam measurements of the candidate target cells to a serving cell and S-DU 120 may decide to which cell UE 110 should be handed over. If the TA of the candidate target cell is still valid, UE 110 may skip the random access procedure during the handover, i.e., the cell change.

[0043] Alternatively, in some example embodiments, the LTM may be performed as follows. CU 142 may prepare the candidate target cells for LTM and provide the LTM configuration for the prepared cells. CU 142 may also configure UE 110 with LI measurement reporting needed for LTM execution. CU 142 may also provide, to S-DU 120, the TA acquisition triggering criteria and necessary configurations. CU 142 may also provide, to S-DU 120, the cell switch triggering criteria and necessary configurations. The triggering criteria of TA acquisition and cell switch may be similar to measurement event report triggering conditions, e.g., A3, A4 or A5 conditions or validity of acquired TA. Triggering configurations comprise a filter configuration (for LI measurements), trigger offsets, cell individual offsets, etc.

[0044] UE 110 may then start reporting the LI measurements to the S-DU 120 regarding the LTM configuration. After that, S-DU 120 may decide to trigger the TA acquisition of the T-DU/cell(s) and send a TA Acquisition Command to UE 110. UE 110 may then send a random access preamble to the T-DU/cell(s) so that the T-DU/cell(s) may estimate the TA between UE 110 and the T-DU/cell(s). After that, S-DU 120 may receive the RAR indirectly (via CU 142).

[0045] UE 110 may send the LI beam measurements of the prepared candidate target cells to S-DU 120 and S-DU 120 may decide to which candidate target cell UE 110 should be handed over. If the RAR is not received indirectly, S-DU 120 may provide the acquired TA to UE 110 via a Medium Access Control Control Element, MAC CE, command (as another alternative for receiving the TA of the candidate target cell).

[0046] If the TA of the candidate target cell is still valid, UE 110 may skip the RACH procedure when executing the HO. After that, UE 110 and the network may proceed with the completion of LTM procedure.

[0047] In case of UE-based TA acquisition, UE 110 may calculate or determine or estimate the TA of T-DU 130, or the TA of a cell of T-DU 130, while UE 110 is served by S-DU 120, e.g., as follows

TA2 = TAI + 2 * RTD — 2 * Realized (TAE) — OtherEstError (1)

[0048] Relative Time Difference, RTD, may refer to a relative difference in time of transmission of the simultaneous signals between any pair of two TRPs, such as S-DU 120 and T-DU 130. RTD may be based on the downlink measurements that UE 110 performs for both, serving and target cells. TA2 may be the TA of a candidate target cell, e.g. a cell of T-DU 130. TAI may be the TA of a serving cell, e.g. a cell of S-DU 120.

[0049] TAE may be a time alignment error of an estimator of UE 110, e.g., due to synchronization between a source and candidate target cell. TAE may be defined by the 3GPP, for example in standard specification TS 38.104, Section 6.5.3. In the context of Multiple Input Multiple Output, MIMO, transmissions from a TRP, the maximum value might not exceed 3000 ns. A similar quantity, named cell phase synchronization accuracy, may pertain to transmissions from a pair of cells and be defined by the 3GPP, for example in standard specification TS 38.133, Section 7.4. Cell phase synchronization accuracy for time division duplexing may be defined as the maximum absolute deviation in frame start timing between any pair of cells on the same frequency that have overlapping coverage areas and it should be better than 3000 ns.

[0050] Furthermore, in the context of downlink positioning, a transmission timing error may be defined by the 3GPP for example in TS 38.305, Section 3.1. The transmission timing error may be defined as the result of a transmission time delay involved in the transmission of a signal, which in turn may be defined as the time delay from the time when the digital signal is generated at the baseband to the time when the radio frequency signal is transmitted from the transmit antenna. In addition, the 3GPP may define, for example in standard specification TS 37.355, the information element NR-RTD-Info, which may be used by a location server to provide time synchronization information between a reference TRP and a list of neighbor TRPs. These definitions, information elements and relevant mechanisms in the specification may allow UE 110 to become aware of timing misalignments in the transmissions from different TRPs and take them into account in position estimation. In some example embodiments, the term “OtherEstError” may refer to any error that may be caused by the uplink/downlink reciprocity or estimator implementation and/or method error.

[0051] Synchronization mismatch between a serving cell, such as cell 120a, and a candidate target cell, such as cell 130a, may be a challenge in case of a cell change of UE 110. UE-based TA estimate may work in a scenario, where the serving cell and the candidate target cell are synchronized, i.e., the time alignment error between the serving and the target cell does not exist or negligible.

[0052] The UE-based TA estimate may use the TA of the serving cell, such as SpCell, when estimating the TA of the candidate target cell. However, if the serving cell and the candidate target cell are not synchronized, UE 110 cannot perform UE-based TA estimate as it would result in erroneous TA estimate of the candidate target cell, which may lead to a failure during a cell change to the candidate target cell. As disclosed herein, method(s) and apparatuses configured to perform the method(s) are provided to enable the UE-based TA estimate in such a scenario, e.g., for RACH-less LTM cell switch.

[0053] In some example embodiments, UE 110 may be served by a group of cells. The group of cells may be referred to as a Cell-Group (either MCG or Secondary Cell Group, SCG). The group of cells may comprise at least one SpCell. In case of CA, there may be also one or multiple SCells that serve UE 110. If the TA of the cells in the group is different, UE 110 may maintain 4 different TA values for all the cells in the group. SpCell may refer, at least in case of dual connectivity, to PCell of the MCG or the PSCell of the SCG. In some example embodiments, SpCell may be PCell+PSCell.

[0054] Although UE 110 may have multiple serving cells at a time (e.g., one SpCell and multiple SCells) and maintain at most 4 TA values, the TA of the SpCell may be used for the UE-based TA estimate. TAs of other cells than SpCell might not be used, which would limit the applicable scenarios of the UE-based TA estimate due to synchronization issue. For example, the serving SpCell and the candidate target cell might not be synchronized while one of the serving SCells may be synchronized with the candidate target cell. Even in that case, UE 110 cannot perform the UE-based TA estimate because if UE 110 would be allowed to perform UE-based TA estimate, it would use the TA of the SpCell, but not TAs of any other cells.

[0055] There is therefore a need to make UE 110 use the TA of a serving cell, such as an SCell, that is synchronized with the candidate target cell. Otherwise, UE 110 could not exploit its UE-based TA estimate capability and perform RACH-less cell change by using an estimated TA. It is hence desirable to make it possible for UE 110 to perform the UE-based TA estimate even if one of the serving cells, such as an SpCell, would not be synchronized with the candidate target cell.

[0056] In some example embodiments, the serving SpCell and the candidate target cell might not be synchronized but one of the serving SCells may be synchronized with the candidate target cell. In such a case, UE 110 may exploit information about the serving SCell, if the serving SCell is synchronized with the target cell. UE 110 may perform the UE-based TA estimate by using the SCell’ s TA that is synchronized with the target cell.

[0057] FIG. 2 illustrates a signalling graph in accordance with at least some example embodiments. On the vertical axes are disposed, from the left to the right, UE 110, S-DU 120, first T-DU 130, second T-DU 132 and CU 142.

[0058] At step 202, UE 110 may transmit a measurement report to S-DU 120. At step 204, S-DU 120 may forward the measurement report of UE 110 to CU 142. UE 110 may hence transmit the measurement report to CU 142 via S-DU 120, to trigger preparation of at least one candidate target cell, such as cells 130a, 130b and 130c of first

T-DU 130 illustrated in FIG. 1.

[0059] At steps 206 and 208, CU 142 may transmit a UE context setup request to first T-DU 130 and second T-DU 132, to set up a context of UE 110. First T-DU 130 may control two cells (cell-1.1 and cell-1.2, such as cells 130a and 130b illustrated in FIG. 1) while second T-DU 132 may control two other cells (cell-2.1 and cell-2.2). At steps 206 and 208, CU 142 may hence initiate preparation of cells 1.1 and 1.2 at first T-DU 130 and preparation of cells 2.1 and 2.2 at second T-DU 132, by transmitting the UE context setup requests. At steps 206 and 208, CU 142 may also request certain cells to be prepared for the cell change of UE 110. First T-DU 130 and second T-DU 132 may accept all or subset of the requested cells to be prepared. However, first T-DU 130 and second T-DU 132 might not be allowed, and/or configured, to prepare any cell that was not requested.

[0060] At steps 210 and 212, first T-DU 130 and second T-DU 132 may transmit UE context setup responses to CU 142, respectively. At step 210, first T-DU 130 may transmit 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, second T-DU 132 may transmit 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. Hence, first T-DU 130 and second T-DU 132 may respond with the UE context setup responses and indicate that the cells requested are prepared.

[0061] In some example embodiments, first T-DU 130 and second T-DU 132 may indicate, for each candidate target cell, which serving cells of S-DU 120 are synchronized with each candidate target cell. That is, first T-DU 130 may transmit a list comprising information about multiple serving cells of UE 110, wherein said multiple serving cells are synchronized in time with candidate target cell-1.1 and another list comprising information about multiple serving cells of UE 110, wherein said multiple cells are synchronized in time with candidate target cell-1.2. Similarly, second T-DU 132 may transmit a list comprising information about multiple serving cells of UE 110, wherein said multiple serving cells are synchronized in time with candidate target cell-2.1 and another list comprising information about multiple serving cells, wherein said multiple serving cells are synchronized in time with candidate target cell-2.2. The lists may comprise at least one of SpCell or SCell. [0062] At step 214, CU 142 may transmit a list of candidate target cells that are prepared for a 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 of the candidate target cells. S-DU 120 may transmit, responsive to the request, a list of serving cells of UE 110 which are synchronized with each of the candidate target cells. That is, S-DU 130 may transmit a list comprising information about multiple serving cells, wherein said multiple serving cells are synchronized in time with candidate target cell- 1.1 and another list comprising information about multiple serving cells, wherein said multiple serving cells are synchronized in time with candidate target cell-1.2. The lists may comprise at least one of SpCell or SCell.

[0063] In some example embodiments, S-DU 120 may transmit to CU 142 a list of Timing Advance Groups, TAGs, that are applicable for UE-based TA estimate of each candidate cell. For example, TAG 2 (or TAG-ID2) may be applicable for UE-based TA estimate of cell 1.1., TAG 3 and TAG 4 (or TAG-ID 3 and TAG 4, respectively) may be applicable for UE-based TA estimate of cell 1.2. In some example embodiments, S-DU 120 may transmit to CU 142, for each TAG-ID, a list of applicable candidate cells.

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

[0065] In some example embodiments, CU 142 may determine, for each candidate target cell, a list of serving cells that are synchronized with each candidate target cell. Alternatively, or in addition, CU 142 may determine for each candidate target cell a list of TAGs applicable for UE-based TA estimate. For example, in case that CU 142 determines the synchronization between the serving cells and the candidate target cells, CU 142 may determine the TAG that may be used for each candidate cell. [0066] At step 218, CU 142 may transmit at least an indication of a candidate target cell for a cell change of UE 110 and TA information of at least one serving cell, wherein the at least one serving cell is synchronized in time with the candidate target cell. Said TA information of the at least one serving cell may comprise an indication of a TA value of the at least one serving cell or a TAG applicable for estimation of TA of the candidate target cell.

[0067] In some example embodiments, CU 142 may transmit a list of serving cells that are synchronized with each candidate target cell or a list of TAGs that are applicable for UE-based TA estimate of each candidate target cell. At step 220, S-DU 120 may transmit the indication and said information to UE 110.

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

[0069] At step 222, UE 110 may transmit an acknowledgement message to S-DU 120, to acknowledge reception of the indication and said TA information. UE 110 may for example respond with an RRCReconfigurationComplete message after successfully decoding the received configuration. S-DU 120 may, at step 224, forward the acknowledgement message to CU 142, e.g., transmit the RRCReconfigurationComplete message.

[0070] At step 226, UE 110 may transmit 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 start LI measurement reporting that comprises the LI measurements of said one of the at least one serving cell and the candidate target cells so that S-DU 120 may monitor the quality of said one of the at least one serving cell and the candidate target cells before triggering the TA estimate or a cell change. Said reporting may be periodic and the periodicity may be configured using an RRCReconfiguration message.

[0071] UE 110 may perform downlink measurements on at least one signal received from at least one candidate target cell. UE 110 may then calculate RTD based on the downlink measurements for signals received from the serving cell and the candidate target cell. RTD may be needed, e.g., for estimating the TA of the candidate target cell using the equation 1 above.

[0072] Either an UE-autonomous or a network triggered procedure may trigger the UE-based TA acquisition. In case of the network triggered procedure, S-DU 120 may trigger UE 110 to initiate the UE-based TA acquisition by transmitting, at step 226, a trigger for that. S-DU 120 may also transmit an indication of said one of the at least one serving cell and UE 110 may perform said downlink measurements based on the indication of said one of the at least one serving cell. Hence, S-DU 120 may indicate said one of the at least one serving cell and UE 110 should evaluate the TA of the candidate target cell using the TA of said one of the at least one serving cell. Alternatively, UE 110 may make a decision on which cell’s TA is to be estimated for the candidate target cell. In case of the network triggered procedure, S-DU 120 may indicate the TAG-ID (or list of TAG-IDs) that are applicable for UE-based TA estimate along with the UE-based TA estimate trigger of the candidate target cell 1.2.

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

[0074] At step 228, UE 110 may use the TA of said one of the at least one serving cell that is synchronized with the candidate target cell 1.2. UE 110 may estimate the TA of the candidate target cell based on the TA information of said one of the at least one serving cell and said downlink measurements. In case of UE-autonomous triggered UE- based TA estimate, 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 transmit an UE-based TA acquisition report, i.e., successful estimation or initiation of the TA estimate procedure. 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 of UE 110 towards a candidate target cell, such as cell 1.2. S-DU 120 may decide that UE-based TA estimate is to be used for RACH-less cell change. In some example embodiments, S- DU 120 may determine the TAG-ID of the TAG that is applicable for UE-based TA estimate of the candidate target cell, such as cell 1.2.

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

[0078] In some example embodiments, S-DU 120 may transmit the serving SpCell or SCell ID (or list of IDs including SpCell and SCells) that are synchronized with the candidate cell 1.2 so that UE 110 may determine which TA is to be used for UE-based TA estimate of the candidate target cell.

[0079] At step 236, UE 110 may use said TA information of said one of the at least one serving cell synchronized with the candidate target cell. For example, UE 110 may use the TA of said one of the serving cell that is synchronized with the candidate target cell. In some example embodiments, UE 110 may use the information provided at step 226 or at step 234, in the MAC CE. UE 110 may use TA of the TAG to which the TAG- ID refers to, for candidate target cell TA estimate. [0080] At step 238, UE 110 may transmit a reconfiguration complete message, such as RRCReconfiguration Complete, to first T-DU 130. UE 110 may hence proceed with the RACH-less procedure and complete the cell change towards cell 1.2. At step 240, first T-DU 130 may transmit an uplink RRC message transfer request to CU 142. At step 242, CU 142 may transmit a UE context release command to S-DU and receive in response a UE context release complete message.

[0081] UE 110 may therefore perform UE-based TA estimate by using the TA of said one serving cell, such as an SCell, of UE 110, that is synchronized with the candidate target cell. UE-based TA estimate may be performed even if another serving cell, such as an SpCell, would not be synchronized with the candidate target cell.

[0082] FIG. 3 illustrates an example apparatus capable of supporting at least some example embodiments. Illustrated is device 300, which may comprise, 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 the functioning thereof, possibly when installed therein. Comprised in device 300 is processor 310, which may comprise, for example, a single- or multi-core processor wherein a single-core processor comprises one processing core and a multicore processor comprises more than one processing core. Processor 310 may comprise, in general, a control device. Processor 310 may comprise more than one processor. Processor 310 may be a control device. A processing core may comprise, for example, a Cortex-A8 processing core manufactured by ARM Holdings or a Steamroller processing core produced by Advanced Micro Devices Corporation. Processor 310 may comprise at least one Qualcomm Snapdragon and/or Intel Atom processor. Processor 310 may comprise at least one application-specific integrated circuit, ASIC. Processor 310 may comprise at least one field-programmable gate array, FPGA. Processor 310 may be means 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 comprise circuitry, or be constituted as circuitry or circuitries, the circuitry or circuitries being configured to perform phases of methods in accordance with example embodiments described herein. As used in this application, the term “circuitry” may refer to one or more or all of the following: (a) hardware-only circuit implementations, such as implementations in only analog and/or digital circuitry, and (b) combinations of hardware circuits and software, such as, as applicable: (i) a combination of analog and/or digital hardware circuit(s) with software/firmware and (ii) any portions of hardware processor(s) with software (including digital signal processor(s)), software, and memory(ies) that work together to cause an apparatus, such as a mobile phone or server, to perform various functions) and (c) hardware circuit(s) and or processor(s), such as a microprocessor s) or a portion of a microprocessor s), that requires software (e.g., firmware) for operation, but the software may not be present when it is not needed for operation.

[0084] This definition of circuitry applies to all uses of this term in this application, including in any claims. As a further example, as used in this application, the term circuitry also covers an implementation of merely a hardware circuit or processor (or multiple processors) or portion of a hardware circuit or processor and its (or their) accompanying software and/or firmware. The term circuitry also covers, for example and if applicable to the particular claim element, a baseband integrated circuit or processor integrated circuit for a mobile device or a similar integrated circuit in server, a cellular network device, or other computing or network device.

[0085] Device 300 may comprise memory 320. Memory 320 may comprise random-access memory and/or permanent memory. Memory 320 may comprise at least one RAM chip. Memory 320 may comprise solid-state, magnetic, optical and/or holographic memory, for example. Memory 320 may be at least in part accessible to processor 310. Memory 320 may be at least in part comprised in processor 310. Memory 320 may be means for storing information. Memory 320 may comprise computer instructions that processor 310 is configured to execute. When computer instructions configured to cause processor 310 to perform certain actions are stored in memory 320, and device 300 overall is configured to run under the direction of processor 310 using computer instructions from memory 320, processor 310 and/or its at least one processing core may be considered to be configured to perform said certain actions. Memory 320 may be at least in part comprised in processor 310. Memory 320 may be at least in part external to device 300 but accessible to device 300.

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

[0087] Device 300 may comprise a Near-Field Communication, NFC, transceiver 350. NFC transceiver 350 may support at least one NFC technology, such as Bluetooth, Wibree or similar technologies.

[0088] Device 300 may comprise User Interface, UI, 360. UI 360 may comprise at least one of a display, a keyboard, a touchscreen, a vibrator arranged to signal to a user by causing device 300 to vibrate, a speaker and a microphone. A user may be able to operate device 300 via UI 360, for example to accept incoming telephone calls, to originate telephone calls or video calls, to browse the Internet, to manage digital files stored in memory 320 or on a cloud accessible via transmitter 330 and receiver 340, or via NFC transceiver 350, and/or to play games.

[0089] Device 300 may comprise or be arranged to accept a user identity module 370. User identity module 370 may comprise, for example, a Subscriber Identity Module, SIM, card installable in device 300. A user identity module 370 may comprise information identifying a subscription of a user of device 300. A user identity module 370 may comprise cryptographic information usable to verify the identity of a user of device 300 and/or to facilitate encryption of communicated information and billing of the user of device 300 for communication effected via device 300.

[0090] Processor 310 may be furnished with a transmitter arranged to output information from processor 310, via electrical leads internal to device 300, to other devices comprised in device 300. Such a transmitter may comprise a serial bus transmitter arranged to, for example, output information via at least one electrical lead to memory 320 for storage therein. Alternatively to a serial bus, the transmitter may comprise a parallel bus transmitter. Likewise processor 310 may comprise a receiver arranged to receive information in processor 310, via electrical leads internal to device 300, from other devices comprised in device 300. Such a receiver may comprise a serial bus receiver arranged to, for example, receive information via at least one electrical lead from receiver 340 for processing in processor 310. Alternatively to a serial bus, the receiver may comprise a parallel bus receiver.

[0091] Device 300 may comprise further devices not illustrated in FIG. 3. For example, where device 300 comprises a smartphone, it may comprise at least one digital camera. Some devices 300 may comprise a back-facing camera and a front-facing camera, wherein the back-facing camera may be intended for digital photography and the frontfacing camera for video telephony. Device 300 may comprise a fingerprint sensor arranged to authenticate, at least in part, a user of device 300. In some example embodiments, device 300 lacks at least one device described above. For example, some devices 300 may lack a NFC transceiver 350 and/or user identity module 370.

[0092] Processor 310, memory 320, transmitter 330, receiver 340, NFC transceiver 350, UI 360 and/or user identity module 370 may be interconnected by electrical leads internal to device 300 in a multitude of different ways. For example, each of the aforementioned devices may be separately connected to a master bus internal to device 300, to allow for the devices to exchange information. However, as the skilled person will appreciate, this is only one example and depending on the example embodiment, various ways of interconnecting at least two of the aforementioned devices may be selected without departing from the scope of the example embodiments.

[0093] FIG. 4 is a flow graph of a first method in accordance with at least some embodiments. The method may be for, and/or performed by, UE 110 of FIG. 1, or a device controlling functioning thereof.

[0094] The first method may comprise, at step 410, receiving an indication of a candidate target cell for a cell change of a user equipment. The first method may also comprise, 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 also comprise, at step 430, performing downlink measurements on at least one signal received from one of the at least one serving cell. In addition, the first method may comprise, at step 440, performing downlink measurements on at least one signal received from the candidate target cell. Finally, the first method may comprise, at step 450, estimating timing advance of the candidate target cell based on said timing advance information of said one of the at least one serving cell and said downlink measurements.

[0095] FIG. 5 is a flow graph of a second method in accordance with at least some embodiments. The method may be for, and/or performed by, CU 142 of FIG. 1, or a device controlling functioning thereof. [0096] The second method may comprise, at step 510, determining a candidate target cell for a cell change of a user equipment and at least one serving cell of the user equipment, wherein the at least one serving cell is synchronized with the candidate target cell. The second method may also comprise, at step 520, transmitting, to the at least one serving cell or to the user equipment via one of the at least one serving cell, an indication of the candidate target cell and timing advance information of the at least one serving cell.

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

[0098] Reference throughout this specification to one embodiment or an embodiment means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Where reference is made to a numerical value using a term such as, for example, about or substantially, the exact numerical value is also disclosed.

[0099] As used herein, a plurality of items, structural elements, compositional elements, and/or materials may be presented in a common list for convenience. However, these lists should be construed as though each member of the list is individually identified as a separate and unique member. Thus, no individual member of such list should be construed as a de facto equivalent of any other member of the same list solely based on their presentation in a common group without indications to the contrary. In addition, various embodiments and examples may be referred to herein along with alternatives for the various components thereof. It is understood that such embodiments, examples, and alternatives are not to be construed as de facto equivalents of one another, but are to be considered as separate and autonomous representations.

[00100] In an example embodiment, an apparatus, like UE 110, S-DU 120, first T- DU 130, second T-DU 132 or CU 142, or a device controlling functioning thereof, may comprise means for carrying out the embodiments described above and any combination thereof. [00101] In an example embodiment, a computer program comprising instructions which, when the program is executed by an apparatus, may cause the apparatus to carry out the first method or the second method. in accordance with the embodiments described above and any combination thereof. In an example embodiment, a computer program product, embodied on a non-transitory computer readable medium, may be configured to control a processor to perform a process comprising the embodiments described above and any combination thereof.

[00102] In an example embodiment, an apparatus, like UE 110, S-DU 120, first T- DU 130, second T-DU 132 or CU 142, or a device controlling functioning thereof, may comprise at least one processor, and at least one memory including computer program code, wherein the at least one memory and the computer program code are configured to, with the at least one processor, cause the apparatus at least to perform the embodiments described above and any combination thereof.

[00103] Furthermore, the described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. In the preceding description, numerous specific details are provided, such as examples of lengths, widths, shapes, etc., to provide a thorough understanding of embodiments of the disclosure. One skilled in the relevant art will recognize, however, that the disclosure can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the disclosure.

[00104] While the forgoing examples are illustrative of the principles of the embodiments in one or more particular applications, it will be apparent to those of ordinary skill in the art that numerous modifications in form, usage and details of implementation can be made without the exercise of inventive faculty, and without departing from the principles and concepts of the disclosure. Accordingly, it is not intended that the disclosure be limited, except as by the claims set forth below.

[00105] The verbs “to comprise” and “to include” are used in this document as open limitations that neither exclude nor require the existence of also un-recited features. The features recited in depending claims are mutually freely combinable unless otherwise explicitly stated. Furthermore, it is to be understood that the use of "a" or "an", that is, a singular form, throughout this document does not exclude a plurality. INDUSTRIAL APPLICABILITY

[00106] At least some example embodiments find industrial application in communication networks, for example in cellular communication networks, such as 3 GPP networks.

ACRONYMS LIST

3 GPP 3^rd Generation Partnership Project

BS Base Station

CA Carrier Aggregation

CSS Common Search Space

FR Frequency Range loT Internet of Things

LTE Long Term Evolution

LTM L1/L2 Triggered Mobility

M2M Machine-to-Machine

MAC CE Media Access Control Control Element

MCG Master Cell Group

MIMO Multiple Input Multiple Output

MTC Machine-Type Communications

NR New Radio

NR-DC New Radio - Dual Connectivity

PCell Primary Cell

PSCell Primary Secondary Cell

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 Cell SCG Secondary Cell Group

SpCell Special Cell

SRS Sounding Reference Signal

TA Timing Advance TAE Timing Alignment Error

TAG Timing Advance Group

TRP Transmission and Reception Point

UE User Equipment

WiMAX Worldwide Interoperability for Microwave Access WLAN Wireless Local Area Network

REFERENCE SIGNS LIST

Claims

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 timing advance of the candidate target cell based on said timing advance information of said one of the at least one serving cell and said downlink measurements.

2. The apparatus according to claim 1, wherein the stored instructions further cause, when executed by the at least one processor, the apparatus at least to:

- receive an indication of said one of the at least one serving cell; and

- perform said downlink measurements based on the indication of said one of the at least one serving cell.

3. The apparatus according to claim 1 or claim 2, wherein the stored instructions further cause, when executed by the at least one processor, the apparatus at least to:

- receive a list comprising information about multiple serving cells of the apparatus, wherein said multiple serving cells are synchronized in time with the candidate target cell.

4. The apparatus according to any of the preceding claims, wherein said 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 estimation of timing advance of the candidate target cell.

5. The apparatus according to any of the preceding claims, wherein the stored instructions further cause, when executed by the at least one processor, the apparatus at least to:

- receive the indication of the candidate target cell and said 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 according to any of the preceding claims, wherein the stored instructions further cause, when executed by the at least one processor, the apparatus at least to:

- receive an indication indicating that the apparatus is to perform a User Equipment, UE, -based timing advance estimate of the candidate target cell based on said one of the at least one serving cell.

7. The apparatus according to any of the preceding claims, wherein the stored instructions further cause, when executed by the at least one processor, 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 according to any of the preceding claims, wherein the stored instructions further cause, when executed by the at least one processor, the apparatus at least to:

- receive an indication indicating a timing advance group to be used for determining a timing advance value for the candidate target cell;

- determine, based on the indication indicating the timing advance group, the timing advance value for the candidate target cell; and

- communicate with the candidate target cell using the determined timing advance value.

9. The apparatus according to any of the preceding claims, wherein said one serving cell is a secondary cell and another serving cell of the apparatus is a special cell, wherein the special cell is unsynchronized with the candidate target cell.

10. 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: - determine a candidate target cell for a cell change of a user equipment and at least one serving cell of the user equipment, wherein the at least one serving cell is synchronized with the candidate target cell; and

- transmit, to the at least one serving cell or to the user equipment via one of the at least one serving cell, an indication of the candidate target cell and timing advance information of the at least one serving cell.

11. The apparatus according to 10, wherein the stored instructions further cause, when executed by the at least one processor, the apparatus at least to:

- transmit a list comprising information about multiple serving cells, wherein said multiple serving cells are synchronized in time with the candidate target cell.

12. The apparatus according to claim 10 or claim 11, wherein the stored instructions further cause, when executed by the at least one processor, the apparatus at least to:

- transmit the indication of the candidate target cell and said timing advance information of the at least one serving cell to the at least one serving cell or to the user equipment via said one of the at least one serving cell.

13. The apparatus according to any of the claims 10 to 12, wherein the stored instructions further cause, when executed by the at least one processor, the apparatus at least to:

- receive, from a distributed unit controlling the candidate target cell, a list comprising information about multiple serving cells of the user equipment, wherein said multiple cells are synchronized in time with the candidate target cell.

14. The apparatus according to any of the claims 10 to 13, wherein the stored instructions further cause, when executed by the at least one processor, the apparatus at least to:

- transmit identities of timing advance groups that are applicable for a User Equipment, UE, -based timing advance estimate of the candidate target cell.

15. A 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 cell;

- performing downlink measurements on at least one signal received from the candidate target cell; and

- estimating timing advance of the candidate target cell based on said timing advance information of said one of the at least one serving cell and said downlink measurements.