Then the CU may request the preparation of a candidate target cell controlled by the target DU by sending UE a Context Setup Request message. The target DU may provide the configuration of the UE in UE Context Setup Response messages, respectively, containing a container from DU to CU. The configuration may contain UE-specific and non-UE-specific parts.

The CU may further request the preparation of a candidate target cell controlled by the source DU by sending a UE Context Modification Request message. Similarly, the source DU may provide the configuration of the UE in UE Context Modification Response message containing a container from DU to CU. The configuration may contain UE-specific and non-UE-specific parts.

After receiving the UE configurations for the candidate target cell(s), the CU may generate an RRC Reconfiguration that is sent to the UE. Among other information, the Radio Resource Control (RRC) Reconfiguration message may contain measurement reporting configuration for Low Layer Mobility (LLM) and a configuration of the prepared candidate cell(s) which the UE needs to execute when it receives a L2 command to change the serving cell. Then the UE may confirm the RRC Reconfiguration to the network.

In the execution phase, the UE may perform a DL synchronization and TA acquisition with candidate target cell(s) (i.e., a UL synchronization). The synchronization step may not be necessarily performed right after RRC Reconfiguration is received. However, the synchronization for candidate cell(s) may be supported between the RRC Reconfiguration and cell switch command reception, at least based on System Synchronization Block (SSB). The TA acquisition of candidate cell(s) before LTM cell switch command may be supported, at least based on PDCCH ordered RACH, where the PDCCH order is only triggered by source cell.

After confirming the RRC Reconfiguration to the network, the UE may start to report the L1 beam measurement of serving and candidate target cells. It is to be understood that the UE does not necessarily wait for RRC Reconfiguration complete message to be sent to initiate the L1 measurement reporting. That is, the UE may start reporting the L1 measurements upon applying the new RRC configuration message.

Upon receiving multiple L1 measurement reports and determining that there is a target candidate cell having a better radio link/beam measurement than the serving cell, the serving cell may send a Medium Access Control (MAC) Control Element (MAC CE) to trigger the cell change to the target candidate cell. Then the handover from serving cell to target cell may be executed by the UE. Both RACH and RACH-less based cell change may be considered.

In the completion phase, the UE context may be released from the source DU.

As mentioned, the UE may perform the early UL/DL synchronization towards the candidate cells during LTM procedure. The DL synchronization is mainly performed via SSB measurements of the candidate cells where the UE can monitor those SSBs and calculate DL synchronization without interruption between UE and the serving cell DL. However, the uplink synchronization can lead both DL and UL interruption between the UE and the serving cell, which may cause the early synchronization procedure to be obsolete since the goal of the early synchronization intends to reduce the interruption during handover. Depending on the capability of the UE, the UE may need to detach from serving cell, perform RACH procedure to acquire TA for UL synchronization and attach back to the serving cell.

This interruption is inevitable for the UEs with single panel. For MPUEs, even if the MPUE can perform simultaneous UL or DL transmission on different panels, it may also be subject to interruption if the same panel is used for both serving cell connection and target cell UL synchronization. Otherwise, the UE may perform interruption free UL synchronization (when the MPUE may perform UL synchronization towards target cell on a panel different than the one that is used between MPUE and the serving cell). However, it is not known by the serving cell whether the UL synchronization triggered by the serving cell leads UL/DL interruption on the UE side.

Now the problem about the interruption may be further explained with reference to the scenario shown in FIG. 1. The terminal device 110 is arranged with antenna panels 101 and 102 and the terminal device 110 may receive 2 Reference Signal Received Power (RSRP), i.e., RSRP 1 and RSRP 2 at the panel 102 from the cell 103 and the cell 104. The terminal device 110 may report the L1-RSRP measurements to the cell 103 (the serving cell). The network is unaware which panel the UE has used for the report of the L1-RSRP measurements. In this case, if the distribute network device 130-1 (the source DU) commands TA acquisition for the cell 104 (the target cell) under same or different DU, the distribute network device 130-1 doesn't know whether the terminal device 110 will use same panel of the serving panel or different panel. Thus, when the TA acquisition is triggered by the distribute network device 130-1, it will cause interruption time as the terminal device 110 will use same panel for both cell 103 (the serving cell) UL/DL and TA acquisition of the cell 104 (the target cell).

Furthermore, if the distribute network device 130-1 is aware, the terminal device will use panel 101 to perform random access procedure towards the cell 104 and the panel 101 is not the serving panel that is used for the serving cell, the distribute network device 130-1 can trigger TA acquisition without worrying about any interruption time. However, there is no such information available in L1 measurement report. Thus, even if the distribute network device 130-1 is aware the terminal device 110 has multiple panels, the distribute network device 130-1 is unaware that whether TA acquisition will cause any interruption time.

Also considering that a report of panel diversity information to the centralized network device 120 through Layer 3 RRC messages may cause minimum 120 ms delay, the issue of latency minimization through efficient TA information acquisition under LTM scenario by MPUEs may need to be further discussed.

The solution of the present disclosure proposes a mechanism for interruption time reduction in LTM for MPUEs. In this solution, the terminal device 110 obtains a configuration of the L1 measurement for cell change at least indicating the L1 measurement report may be provided by the terminal device 110 with an indication of interruption free TA acquisition for a cell associated with the cell change or per a panel indication of the terminal device. Then the terminal device 110 transmit to a distributed network device 130-1, a report of the layer 1 measurement based on the configuration.

Example embodiments of the present disclosure will be described in detail below with reference to the accompanying drawings.

LLM is one of the objectives for mobility enhancement. In contrast to L3 mobility procedures where the handover between two cells is decided by the RRC layer, the LLM decision may be made by the MAC layer terminated in the distributed network device, i.e., DU.

FIGS. 2 and 3 show signalling diagrams of LLM from a serving cell in source DU to a target cell in target DU (i.e., inter-DU intra-CU scenario). It is to be understood that the processes shown in FIGS. 2 and 3 may also be applied as well in case of intra-DU intra-CU cell change where source DU would be the same as target DU. The procedure may be split in three phases, namely the preparation phase, the execution phase, and the completion phase. The FIG. 2 shows the process of the preparation phase while FIG. 3 shows the process of the execution and the completion phases.

Reference is now made to FIG. 2, which shows a signaling chart 200 for communication according to some example embodiments of the present disclosure. As shown in FIG. 2, the signaling chart 200 involves a terminal device 110, a centralized network device 120 and distributed network devices 130-1 and 130-2. For the purpose of discussion, reference is made to FIG. 1 to describe the signaling chart 200.

In an exemplary scenario, the distributed network device 130-1 may act as a source DU and manage a serving cell 103 of the terminal device 110, while the further distributed network device 130-2 may act as a target DU and manage a target cell 104 of the terminal device 110. It is to be understood that more target cells may also be available for the process 200.

As shown in FIG. 2, in the preparation phase, the terminal device 130 may transmit 202, to the distributed network device 130-1, a L3 measurement report containing the measurements of the serving cell 103 and a target cell 104 (more target cells are possible). The L3 measurement report may be forwarded 204 from the distributed network device 130-1 to the centralized network device 120.

Using the reported measurements, the centralized network device 120 may identify 206 a potential set of candidate target cells to which the terminal device 110 may be handed over to. In the exemplary scenario shown in FIG. 2, the centralized network device 120 may identify candidate target cells that are served by either distributed network device 130-1 (source DU) or the further distributed network device 130-2 (i.e., target DU) which are controlled by the same CU, i.e., the centralized network device 120.

Then the centralized network device 120 may request 208 the preparation of a candidate target cell controlled by the further distributed network device 130-2 by sending UE Context Setup Request message. The further distributed network device 130-2 may provide 210 the configuration of the UE in UE Context Setup Response messages, respectively, containing a container from DU to CU. The configuration may contain UE-specific and non-UE-specific parts.

Then the centralized network device 120 may request 212 the preparation of a candidate target cell (i.e., cell 103) controlled by the distributed network device 130-1 by sending UE Context Modification Request message. The distributed network device 130-1 may provide 214 the configuration of the terminal device 110 in UE Context Modification Response message containing a container from DU to CU. The configuration may contain UE-specific and non-UE-specific parts.

After receiving the UE Context Modification Response message, the centralized network device 120 may generate 216 a RRC configuration message that includes the measurement configuration of L1 cell change of the terminal device 110. The RRC configuration message may also indicate that the terminal device 110 may report the measurement with an indication of the interruption free TA acquisition for one or more cells that are reported in the L1 measurements. For example, 1 bit flag may be used to indicate whether the interruption free TA acquisition can be supported by the cell.

Optionally or alternatively, the RRC configuration message may also indicate that the terminal device 110 may report the measurement with a panel indication per L1 measurement based on MPUE capability of the terminal device 110.

For example, the trigger for L1 cell change and panel indication through L1 measurement may be based on an average latency experienced by the terminal device at the serving cell 103, a ratio of MPUE to single panel UE with the serving and the target cells, dynamic blockages and expected outage duration for MPUEs, L1 behaviour like the number of multi-paths components of the radio signal received at the multiple panels of the terminal device 110 and/or geographical area information with higher number of average radio link failures/handover failure (HOF).

Furthermore, the centralized network device 120 may also generate a DU configuration such that the distributed network device 130-1 will consider the interruption of the TA acquisition either by using 1 bit flag or panel diversity that is reported in L1 measurements from the terminal device 110 to the distributed network device 130-1, which may be outside of the RRC message.

Then the centralized network device 120 transmits 218 the RRC reconfiguration message for the terminal device 110 to report the L1 measurement, to the distributed network device 130-1 and the the distributed network device 130-1 may forward 220 the RR(′ reconfiguration message to the terminal device 110.

It is to be understood that the distributed network device 130-1 cannot decode the RRC message and therefore the distributed network device 130-1 may simply relay the RR(reconfiguration message to the terminal device 110. If the centralized network device 120 also transmits the DU configuration as mentioned above, the distributed network device 130-1 may receive this DU configuration with the RRC message and know the interruption of the TA acquisition either by using 1 bit flag or panel diversity may be reported by the terminal device 110 along with the report of the L1 measurements.

Among other information as described above, the RRC Reconfiguration message may also include a measurement reporting configuration for LLM, i.e., a configuration on how to report the L1 beam measurements of serving and target cells and/or a configuration of the prepared candidate cell(s) which the terminal device 110 needs to execute when it receives a L2 command to change the serving cell, i.e., perform the handover.

Then the terminal device 110 may confirm 222 the RR(′ Reconfiguration to the distributed network device 130-1 which may also be transferred 224 from the distributed network device 130-1 to the centralized network device 120.

Reference is now made to FIG. 3, which shows a signaling chart 300 for communication according to some example embodiments of the present disclosure. As shown in FIG. 3, the signaling chart 300 involves a terminal device 110, a centralized network device 120 and distributed network devices 130-1 and 130-2. For the purpose of discussion, reference is made to FIG. 1 to describe the signaling chart 300.

In an exemplary scenario, the distributed network device 130-1 may act as a source DU and manage a serving cell 103 of the terminal device 110, while the further distributed network device 130-2 may act as a target DU and manage a target cell 104 of the terminal device 110. It is to be understood that more target cells may also be available for the process 300.

As shown in FIG. 3, in the execution phase, after confirming the RRC′ Reconfiguration, the terminal device 110 may start 302 to report the L1 beam measurement of serving and candidate target cells with 1 bit flag for interruption free TA acquisition or per panel indication periodically to the distributed network device 130-1. The distributed network device 130-1 may monitor the L1 measurement to monitor the radio condition of the serving cell (i.e., cell 103) and the target cell (i.e., cell 104).

For example, the terminal device 110 may add a flag to indicate whether interruption free TA acquisition is supported for the target cell. As another option, the terminal device 110 may add a flag with the L1 measurement(s) associated with the target cell to indicate if the measurement was done using the serving panel or a different panel. It is also possible that the panel index may be added with the target cell measurements.

It is to be understood that there could be multiple L1 measurement reports with panel indication received by the distributed network device 130-1 from the distributed network device 130-1. In case if the target cell radio condition is weaker than the serving cell, the distributed network device 130-1 will not trigger early synchronization and keep on measuring the L1 measurement reports. The other reason for not triggering early synchronization as soon as the terminal device 110 receives the RR(Reconfiguration because if the distributed network device 130-1 triggers early synchronization for TA acquisition and due to the weak radio condition, the decision to change the serving cell may be delayed, the TA information becomes old and inaccurate.

In some example embodiments, the distributed network device 130-1 may determine, based on the report the L1 beam measurement received from the terminal device 110, the L1 measurement for the serving and the candidate target cell is received at the different panels. In this case, the distributed network device 130-1 may decide 304 to trigger early synchronization response.

In this situation, the terminal device 110 may transmit 306 a RACH preamble to the further distributed network device 130-2 (managing a target cell associated with the cell change of the terminal device 110) with an indication that L1 measurement for the serving and the candidate target cell is performed by using different panels, i.e., the serving panel and the non-serving panel(s) or an interruption free TA acquisition is supported for the target cell. Then the further distributed network device 130-2 may estimate 308 the TA and transmit 310 the TA information directly to the non-serving panel of the terminal device 110. Thus, there is no DL interruption in this case.

In some other example embodiments, the distributed network device 130-1 may determine, based on the report the L1 beam measurement received from the terminal device 110, the L1 measurement for the serving and the candidate target cell is received at the same panel, i.e., the serving panel of the terminal device 110. In this case, the distributed network device 130-1 may decide 304 to wait to trigger early synchronization response.

If the early synchronization is triggered by the distributed network device 130-1 using the serving panel, the terminal device 110 may transmit 312 a RACH preamble to the further distributed network device 130-2 an indication that L1 measurement for the serving and the candidate target cell is performed by using a same panel or an interruption free TA acquisition is not supported. Then the further distributed network device 130-2 may estimate 314 the TA and relay 316 the TA information to the centralized network device 120. The centralized network device 120 may further relay 318 the TA information to the distributed network device 130-1. Then the TA information may be transferred 320 from the distributed network device 130-1 to the terminal device 110. For example, the MAC CE of the distributed network device 130-1 may trigger to transmit the TA to the terminal device 110. In this way the DL interruption may be avoid.

Then the distributed network device 130-1 may decode 324 for serving cell change based on the L1 measurement received 322 from the terminal device 110 and the MAC CE of the distributed network device 130-1 may trigger 326 the cell change. A RACHless handover mechanism between the terminal device 110 and the further distributed network device 130-2 is accomplished 328 and the RRC Reconfiguration Complete message may be sent 330 from the terminal device 110 to the further distributed network device 130-2 and UL RRC message transfer may be sent 332 from the further distributed network device 130-2 to the centralized network device 120.

In the completion phase, the UE context may be released 324 from the distributed network device 130-1 and a path switch may be performed to the further distributed network device 130-2 (the new serving DU) and UJE context release message may be sent from the distributed network device 130-1 to the centralized network device 120.

Furthermore, in the solution of the present disclosure, the MPUEs capabilities of the terminal device 110 may also be reported or indicated. The MPUEs capabilities of the terminal device 110 that could be reported or indicated may comprise a total number of panels in the terminal device 110, for example, via the L3 measurement report; the panel used for the serving cell and the target cell measurement, for example, by a flag indication; if best panel measurement is reported for each cell or only the serving panel, for example, by a flag indication; information about Tx/Rx chains to support multiple simultaneous UL transmission; and/or Ofn (Individual Panel Offset) or Array Directivity Order (ADO) of the serving and non-serving panel.

In the solution of the present disclosure, a mechanism to inform the serving DU/cell whether the TA acquisition is subject to UL/DL interruption between UE and the serving DU/cell upon the TA acquisition is triggered. The simultaneous data transmission both in UL/DL may be through one panel with the serving cell while the other panels may be used to send the PRACH message (e.g., Message 1) to the target cell and acquire TA information. In this way, the interruption may be avoided and the panels of the MPUEs may be better utilized.

FIG. 4 shows a flowchart of an example method 400 of interruption time reduction according to some example embodiments of the present disclosure. The method 400 may be implemented at the terminal device 110 as shown in FIG. 1. For the purpose of discussion, the method 400 will be described with reference to FIG. 1.

At 410, the terminal device 110 receives, from a centralized network device, a configuration at least indicating that a L1 measurement for cell change is to be reported by the terminal device with an indication of interruption free TA acquisition for a cell associated with the cell change, or per a panel indication of the terminal device.

At 420, the terminal device 110 transmits, to a distributed network device, a report of the L1 measurement based on the configuration.

In some example embodiments, the configuration is received via a RRC message.

In some example embodiments, the configuration is forwarded from the centralized network device to the terminal device by the distributed network device.

In some example embodiments, the configuration further comprises at least one of a measurement reporting configuration for a low layer mobility, or a configuration of one or more candidate cells associated with the cell change.

In some example embodiments, the terminal device may transmit the report of the layer 1 measurement associated with the cell with at least one of an indication whether the layer 1 measurement is performed by a serving panel of the terminal device or a non-serving panel of the terminal device, an index of a panel of the terminal device that performs the layer 1 measurement of the cell, or an indication whether the interruption free TA is to be performed for the cell.

In some example embodiments, in accordance with a determination that an early synchronization is triggered by the distributed network device, the terminal device may transmit, to a further distributed network device, a preamble for a random access with an indication whether a serving panel of the terminal device or a non-serving panel of the terminal device is used the layer 1 measurement, or whether an interruption free TA acquisition is performed for a cell associated with the cell change; and receive TA information of the further distributed network device.

In some example embodiments, the terminal device may receive the TA information from the further distributed network device or from the distributed network device forwarded by the centralized network device.

FIG. 5 shows a flowchart of an example method 500 of interruption time reduction according to some example embodiments of the present disclosure. The method 500 may be implemented at the centralized network device 120 as shown in FIG. 1. For the purpose of discussion, the method 500 will be described with reference to FIG. 1.

At 510, the centralized network device 120 transmits, to a terminal device, a configuration at least indicating that a L1 measurement for cell change is to be reported by the terminal device with an indication of interruption free TA acquisition for a cell associated with the cell change, or per a panel indication of the terminal device.

In some example embodiments, the configuration is received via a RRC message.

In some example embodiments, the configuration is forwarded from the centralized network device to the terminal device by the distributed network device.

In some example embodiments, the configuration further comprises at least one of a measurement reporting configuration for a low layer mobility, or a configuration of one or more candidate cells associated with the cell change.

In some example embodiments, the centralized network device 120 may transmit, to the distributed network device, an indication that the layer 1 measurement for the cell change is to be reported by the terminal device with the indication of interruption free TA acquisition for the cell associated with the cell change, or per the panel indication of the terminal device.

In some example embodiments, the centralized network device 120 may receive, from a further distributed network device, TA information of the further distributed network device; and transmit the TA information to the terminal device via the distributed network device.

FIG. 6 shows a flowchart of an example method 600 of interruption time reduction according to some example embodiments of the present disclosure. The method 600 may be implemented at the distributed network device 130-1 shown in FIG. 1. For the purpose of discussion, the method 600 will be described with reference to FIG. 1.

At 610, the distributed network device 130-1 receives, from a terminal device, a report of a L1 measurement for cell change of the terminal device with an indication of interruption free TA acquisition for a cell associated with the cell change, or per a panel indication of the terminal device.

At 620, the distributed network device 130-1 causes an early synchronization to be triggered based on the report.

In some example embodiments, the distributed network device 130-1 may receive the report of the layer 1 measurement associated with the cell with at least one of an indication whether the layer 1 measurement is performed by a serving panel of the terminal device or a non-serving panel of the terminal device, an index of a panel of the terminal device that performs the layer 1 measurement of the cell, or an indication whether the interruption free TA is to be performed for the cell.

In some example embodiments, the distributed network device may forward, from a centralized network device and to the terminal device, a configuration at least indicating that a layer 1 measurement for cell change is to be reported by the terminal device with an indication of interruption free timing advance, TA, acquisition for a cell associated with the cell change, or per a panel indication of the terminal device.

In some example embodiments, the distributed network device may receive, from a centralized network device, an indication that the layer 1 measurement for the cell change is to be reported by the terminal device with the indication of interruption free TA acquisition for the cell associated with the cell change, or per the panel indication of the terminal device.

In some example embodiments, the distributed network device may receiving, from a centralized network device, TA information of a further distributed network device; and transmitting the TA information to the terminal device.

FIG. 7 shows a flowchart of an example method 700 of interruption time reduction according to some example embodiments of the present disclosure. The method 700 may be implemented at the further distributed network device 130-2 as shown in FIG. 1. For the purpose of discussion, the method 700 will be described with reference to FIG. 1.

At 710, the further distributed network device 130-2 receives, from a terminal device, a preamble for a random access with an indication whether a serving panel of the terminal device or a non-serving panel of the terminal device is used the layer 1 measurement, or whether an interruption free TA acquisition is performed for a cell associated with the cell change.