Early uplink synchronization through network-initiated physical random access channel preamble transmission

Abstract

A technical solution is provided, which enables early uplink (UL) synchronization through a network-initiated physical random access channel (PRACH) preamble transmission in case of using mobility candidates each comprising one or more candidate cells For this purpose, a UE is provided with one or more early UL sync configurations and two or more signal synchronization block (SSB) configurations per mobility candidate, with each of the early UL sync configurations being associated with an SSB configuration, directly or indirectly (via a timing advance group (TAG)), and a TAG, directly or indirectly (via an SSB configuration). Depending on the association between the early UL sync configuration(s), the SSB configurations and the TAGs, different options for the content of a network-initiated PRACH preamble transmission order (e.g., physical downlink control channel (PDCCH) order) are provided.

Description

[0001] EARLY UPLINK SYNCHRONIZATION THROUGH NETWORK-INITIATED PHYSICAL RANDOM ACCESS CHANNEL PREAMBLE TRANSMISSION

[0002] TECHNICAL FIELD

[0003] The present disclosure relates generally to the field of wireless communications. In particular, the present disclosure relates to techniques for enabling early uplink (UL) synchronization through a network-initiated physical random access channel (PRACH) preamble transmission.

[0004] BACKGROUND

[0005] In 3rd generation partnership project (3GPP) release 18 (Rel-18), a new mobility mechanism has been introduced, namely lower layer-triggered mobility (LTM) (also referred to as layer 1 (L1) / layer (L2) based mobility), which aims to reduce interruption time during handover (HO). One of the key goals of the Rel-18 LTM mechanism is to reduce the interruption caused by a cell switch from layer 3 (L3)-driven HO. This is achieved through the following early procedures, namely:

[0006] - early downlink (DL) synchronization through early transmission configuration indicator (TCI) state activation;

[0007] -early UL synchronization through physical downlink control channel (PDCCH)-ordered RACH or UE-based timing advance (TA) estimation; and

[0008] - early Abstract Syntax Notation One (ASN.l) decoding and validity check.

[0009] More specifically, the Rel-18 LTM mechanism supports said early UL synchronization / TA acquisition through:

[0010] - PDCCH-ordered RACH transmission, where a source gNB sends a PDCCH order to a user equipment (UE) for a candidate cell, and the UE performs the RACH transmission on that cell before a cell switch. A target candidate gNB sends a TA value to the source gNB, which gives the TA value to the UE in a cell switch command. Then, the UE applies the TA value for a first UL transmission on a target cell, and hence performs RACH-less cell switch;- UE autonomous TA estimation, where the source gNB configures the UE to autonomously estimate the TA value for one or more candidate cells. When the UE receives a cell switch command for such candidate cell without any TA value, the UE shall apply the UE-estimated TA for a first UL transmission and hence perform the RACH-less cell switch; and

[0011] -a serving cell TA value utilized by the source gNB (i.e., when the TA of the target cell is same as that of the serving cell) or an indication of TA = 0 in the cell switch command, which also leads to the RACH-less cell switch.

[0012] If none of the above is applicable, the UE performs RACH-based cell switch, i.e. performs the conventional RACH procedure after receiving the cell switch command.

[0013] In the Rel. 18 LTM mechanism, a mobility candidate (comprising one or more candidate cells) can have a radio resource control (RRC) configuration with carrier aggregation (CA), but early TA acquisition for two or more candidate cells belonging to the same mobility candidate is not supported.

[0014] SUMMARY

[0015] This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the detailed description. This summary is not intended to identify key features of the present disclosure, nor is it intended to be used to limit the scope of the present disclosure.

[0016] It is an objective of the present disclosure to provide a technical solution that enables early UL synchronization through a network-initiated PRACH preamble transmission in case of using mobility candidates each comprising one or more candidate cells.

[0017] The objective above is achieved by the features of the independent claims in the appended claims. Further embodiments and examples are apparent from the dependent claims, the detailed description, and the accompanying drawings.

[0018] According to a first aspect, a UE in a wireless communication network is provided. The UE comprises at least one processor and at least one memory storing instructions that, when executed by the at least one processor, cause the UE to operate at least as follows. At first,the UE receives configuration information for at least one mobility candidate from a network node serving a source cell. Each of the at least one mobility candidate comprises at least two candidate cells. The configuration information comprises: (i) at least two synchronization signal block (SSB) configurations each associated with one of the at least two candidate cells, and (ii) at least one early UL sync configuration each associated with one of the at least two SSB configurations. Then, the UE receives an order for a PRACH preamble transmission, which indicates either an early UL sync configuration of interest among the at least one early UL sync configuration, or an SSB configuration of interest among the at least two SSB configurations. If the order for the PRACH preamble transmission indicates the early UL sync configuration of interest, the UE determines the SSB configuration of interest among the at least two SSB configurations based on the configuration. If the order for the PRACH preamble transmission indicates the SSB configuration of interest, the UE determines the early ULsync configuration of interest based on the configuration information. Next, the UE uses the SSB configuration of interest and the early UL sync configuration of interest to perform the PRACH preamble transmission. By using such configuration information and order for the PRACH preamble transmission, the UE may properly determine the SSB and early UL sync configurations to be used for the PRACH preamble transmission towards corresponding one of the at least two candidate cells included in the mobility candidate (which may be further indicated as a target cell in a subsequent cell switch command from the serving network node).

[0019] In one example embodiment of the first aspect, the UE is further caused to receive a TA value in response to the PRACH preamble transmission. The provision of the TA value may be either by a candidate network node or by the serving network node (e.g., the candidate network node may estimate the TA value and provide it to the serving network node which then sends it to the UE). The UE may use the TA value for subsequent RACH-less cell switch. More specifically, the serving or candidate network node may estimate the TA value by receiving the PRACH preamble transmitted by the UE and then provide the TA value to the UE. The TA value may be explicitly or implicitly associated with a TA group (TAG), meaning that for accessing the cells belonging to the TAG, the UE must use the associated TA value. In one example embodiment of the first aspect, each of the at least two SSB configurations has an identifier (ID), and each of the at least one early UL sync configuration comprises theID of one of the at least two SSB configurations. Such ID-based association may simplify the determination of the SSB configuration of interest or the early UL sync configuration of interest in response to the order for the PRACH preamble transmission.

[0020] In one example embodiment of the first aspect, each of the at least two SSB configurations has an ID, and each of the at least one early UL sync configuration has an ID. In this embodiment, the order for the PRACH preamble transmission indicates either the ID of the early UL sync configuration of interest, or the ID of the SSB configuration of interest. Again, the use of such an ID in the order for the PRACH preamble transmission may enable the UE to identify the SSB or early UL sync configuration of interest.

[0021] In one example embodiment of the first aspect, each of the at least two SSB configurations is associated with a TAG. By providing the additional association between the SSB configurations and the TAGs, it is possible to allow uplink synchronization of multiple component carriers in a carrier aggregation (CA) scenario.

[0022] In one example embodiment of the first aspect, each of the at least two SSB configurations has an ID, and each of the TAGs has an ID. In this embodiment, the configuration information further comprises an association list comprising at least two elements. Each of the at least two elements has an index that indicates the ID of one of the at least two SSB configurations and a value that indicates the ID of the TAG associated with said one of the at least two SSB configurations. Such an ID-based association list may allow the UE to easily and quickly determine the TAG to be used for subsequent RACH-less cell switch.

[0023] In one example embodiment of the first aspect, the TAG of each of the at least two SSB configurations has an ID, and each of the at least two SSB configurations comprises the ID of the TAG associated therewith. Such ID-based association may simplify the determination of the TAG to be used for subsequent RACH-less cell switch.

[0024] In one example embodiment of the first aspect, the order for the PRACH preamble transmission indicates the SSB configuration of interest by indicating an ID of the TAG associated with the SSB configuration of interest. Thus, each early UL sync configuration may be associated with the corresponding TAG indirectly via the corresponding SSB configuration. This may provide additional flexibility in the control signaling used in the wireless communication network.In one example embodiment of the first aspect, the UE is caused to receive, after the PRACH preamble transmission, a cell switch command comprising two or more TA values each being associated with a TAG. By using the TA values, the UE may be properly configured for subsequent RACH-less cell switch.

[0025] In one example embodiment of the first aspect, the UE is caused to receive the configuration information via a RRC signaling. Thus, the UE may be provided with one or more early UL sync configurations and two or more SSB configurations per mobility candidate via an existing (modified) RRC message (e.g., an RRC reconfiguration message). That is, the configuration information may be used in the wireless communication network without having to increase control signaling therein.

[0026] According to a second aspect, a network node in a wireless communication network is provided. The network node comprises at least one processor and at least one memory storing instructions that, when executed by the at least one processor, cause the network node to operate at least as follows. At first, the network node obtains configuration information for at least one mobility candidate. Each of the at least one mobility candidate comprises at least two candidate cells. The configuration information comprises: (i) at least two SSB configurations each associated with one of the at least two candidate cells, and (ii) at least one early UL sync configuration each associated with one of the at least two SSB configurations. Then, the network node transmits the configuration information to a UE. After that, the network node transmits, to the UE, an order for a PRACH preamble transmission, which indicates an early UL sync configuration of interest among the at least one early UL sync configuration or an SSB configuration of interest among the at least two SSB configurations. By using such configuration information and order for the PRACH preamble transmission, the UE may properly determine the SSB and early UL sync configurations to be used for the PRACH preamble transmission towards corresponding one of the at least two candidate cells included in the mobility candidate (which may be further indicated as a target cell in a subsequent cell switch command from the serving network node).

[0027] In one example embodiment of the second aspect, the at least one mobility candidate comprises a first mobility candidate supported by a first other network node and a second mobility candidate supported by a second other network node. In this embodiment, thenetwork node is further caused to operate as follows. The network node receives, from the first other network node, the at least two SSB configurations and the at least one early UL sync configuration for the first mobility candidate, and receives, from the second other network node, the at least two SSB configurations and the at least one early UL sync configuration for the second mobility candidate. Then, the network node obtains the configuration information based on the at least two SSB configurations and the at least one early UL sync configuration for each of the first mobility candidate and the second mobility candidate. Furthermore, the network node transmits, to the first other network node, the at least two SSB configurations and the at least one early UL sync configuration for the second mobility candidate and transmits, to the second other network node, the at least two SSB configurations and the at least one early UL sync configuration for the first mobility candidate. Knowing the early UL sync configuration(s) and the SSB configurations for the mobility candidate supported by the second (first) other network node, the first (second) other network node (when it becomes a serving network node) will be able to issue the order for the PRACH preamble transmission, as discussed above.

[0028] According to a third aspect, a method for operating a UE in a wireless communication network is provided. The method starts with the step of receiving configuration information for at least one mobility candidate from a network node serving a source cell. Each of the at least one mobility candidate comprises at least two candidate cells. The configuration information comprises: (i) at least two SSB configurations each associated with one of the at least two candidate cells, and (ii) at least one early UL sync configuration each associated with one of the at least two SSB configurations. The method further proceeds to the step of receiving an order for a PRACH preamble transmission, which indicates either an early UL sync configuration of interest among the at least one early UL sync configuration, or an SSB configuration of interest among the at least two SSB configurations. If the order for the PRACH preamble transmission indicates the early UL sync configuration of interest, the method goes on to the step of determining the SSB configuration of interest among the at least two SSB configurations based on the configuration information. If the order for the PRACH preamble transmission indicates the SSB configuration of interest, the method goes on to the step of determining the early UL sync configuration of interest based on the configuration information. After that, the method proceeds to the step of using the SSBconfiguration of interest and the early UL sync configuration of interest to perform the PRACH preamble transmission. By using such configuration information and order for the PRACH preamble transmission, the UE may properly determine the SSB and early UL sync configurations to be used for the PRACH preamble transmission towards corresponding one of the at least two candidate cells included in the mobility candidate (which may be further indicated as a target cell in a subsequent cell switch command from the serving network node).

[0029] According to a fourth aspect, a method for operating a network node in a wireless communication network is provided. The method starts with the step of generating configuration information for at least one mobility candidate. Each of the at least one mobility candidate comprises at least two candidate cells. The configuration information comprises: (i) at least two SSB configurations each associated with one of the at least two candidate cells, and (ii) at least one early UL sync configuration each associated with one of the at least two SSB configurations. Then, the method proceeds to the step of transmitting the configuration information to a UE. After that, the method goes on to the step of transmitting, to the UE, an order for a PRACH preamble transmission, which indicates an early UL sync configuration of interest among the at least one early UL sync configuration or an SSB configuration of interest among the at least two SSB configurations. By using such configuration information and order for the PRACH preamble transmission, the UE may properly determine the SSB and early UL sync configurations to be used for the PRACH preamble transmission towards corresponding one of the at least two candidate cells included in the mobility candidate (which may be further indicated as a target cell in a subsequent cell switch command from the serving network node).

[0030] According to a fifth aspect, a UE in a wireless communication network is provided. The UE comprises at least one processor and at least one memory storing instructions that, when executed by the at least one processor, cause the UE to operate at least as follows. At first, the UE receives configuration information for at least one mobility candidate from a network node serving a source cell. Each of the at least one mobility candidate comprises at least two candidate cells. The configuration information comprises: (i) at least two SSB configurations each associated with one of the at least two candidate cells, and (ii) at least one early UL sync configuration each associated with one of the at least two SSB configurations andfurther associated with a TAG. Then, the UE receives an order for a PRACH preamble transmission, which indicates one of: an early UL sync configuration of interest among the at least one early UL sync configuration, an SSB configuration of interest among the at least two SSB configurations, and the TAG associated with the early UL sync configuration of interest. If the order for the PRACH preamble transmission indicates the early UL sync configuration of interest, the UE determines the SSB configuration of interest among the at least two SSB configurations based on the configuration. If the order for the PRACH preamble transmission indicates the SSB configuration of interest, the UE determines the early ULsync configuration of interest based on the configuration information. If the order for the PRACH preamble transmission indicates the TAG associated with the early UL sync configuration of interest, the UE determine the early UL sync configuration of interest and the SSB configuration of interest based on the configuration information. Next, the UE uses the SSB configuration of interest and the early UL sync configuration of interest to perform the PRACH preamble transmission. By using such configuration information and order for the PRACH preamble transmission, the UE may properly determine the SSB and early UL sync configurations to be used for the PRACH preamble transmission towards corresponding one of the at least two candidate cells included in the mobility candidate (which may be further indicated as a target cell in a subsequent cell switch command from the serving network node).

[0031] In one example embodiment of the fifth aspect, the UE is further caused to receive, in response to the PRACH preamble transmission, a TA value. The provision of the TA value may be either by a candidate network node or by the serving network node (e.g., the candidate network node may estimate the TA value and provide it to the serving network node which then sends it to the UE). The UE may use the TA value for subsequent RACH-less cell switch. In one example embodiment of the fifth aspect, each of the at least two SSB configurations has an ID and the TAG of each of the at least one early UL sync configuration has an ID. In this embodiment, each of the at least one early UL sync configuration comprises the ID of one of the at least two SSB configurations and the ID of the TAG associated with the early UL sync configuration. Such ID-based association may simplify the determination of the SSB configuration of interest or the early UL sync configuration of interest in response to the order for the PRACH preamble transmission.In one example embodiment of the fifth aspect, each of the at least two SSB configurations has an ID, each of the at least one early UL sync configuration has an ID, and the TAG of each of the at least one early UL sync configuration has an ID. In this embodiment, the order for the PRACH preamble transmission comprises one of: the ID of the early UL sync configuration of interest, the ID of the SSB configuration of interest, and the ID of the TAG associated with the early UL sync configuration of interest. Again, the use of such IDs in the order for the PRACH preamble transmission may enable the UE to identify the SSB or early UL sync configuration of interest.

[0032] In one example embodiment of the fifth aspect, each of the at least two SSB configurations has an ID, and each of the at least one early UL sync configuration has an ID. In this embodiment, the configuration information further comprises an association list comprising at least two elements. Each of the at least two elements has an index that indicates the ID of one of the at least one early UL sync configuration and a value that indicates the ID of one of the at least two SSB configurations which is associated with said one of the at least one early UL sync configuration. Such an ID-based association list may allow the UE to easily and quickly determine the TAG to be used for subsequent RACH-less cell switch.

[0033] In one example embodiment of the fifth aspect, the UE is caused to receive, after that PRACH preamble transmission, a cell switch command comprising two or more TA values each being associated with a TAG. By using the TA values, the UE may be properly configured for subsequent RACH-less cell switch.

[0034] In one example embodiment of the fifth aspect, the UE is caused to receive the configuration information via a RRC signaling. Thus, the configuration information may be provided to the UE without having to increase control signaling in the wireless communication network. According to a sixth aspect, a network node in a wireless communication network is provided. The network node comprises at least one processor and at least one memory storing instructions that, when executed by the at least one processor, cause the network node to operate at least as follows. At first, the network node obtains configuration information for at least one mobility candidate. Each of the at least one mobility candidate comprises at least two candidate cells. The configuration information comprises: (i) at least two SSB configurations each associated with one of the at least two candidate cells, and (ii)at least one early UL sync configuration each associated with one of the at least two SSB configurations and a TAG. Then, the network node transmits the configuration information to a UE. After that, the network node transmits, to the UE, an order for a PRACH preamble transmission, which indicates one of: an early UL sync configuration of interest among the at least one early UL sync configuration, an SSB configuration of interest among the at least two SSB configurations, and the TAG associated with the early UL sync configuration of interest. By using such configuration information and order for the PRACH preamble transmission, the UE may properly determine the SSB and early UL sync configurations to be used for the PRACH preamble transmission towards corresponding one of the at least two candidate cells included in the mobility candidate (which may be further indicated as a target cell in a subsequent cell switch command from the serving network node).

[0035] In one example embodiment of the sixth aspect, the at least one mobility candidate comprises a first mobility candidate supported by a first other network node and a second mobility candidate supported by a second other network node. In this embodiment, the network node is further caused to operate as follows. The network node receives, from the first other network node, the at least two SSB configurations and the at least one early UL sync configuration for the first mobility candidate, and receives, from the second other network node, the at least two SSB configurations and the at least one early UL sync configuration for the second mobility candidate. Then, the network node obtains the configuration information based on the at least two SSB configurations and the at least one early UL sync configuration for each of the first mobility candidate and the second mobility candidate. Furthermore, the network node transmits, to the first other network node, the at least two SSB configurations and the at least one early UL sync configuration for the second mobility candidate and transmits, to the second other network node, the at least two SSB configurations and the at least one early UL sync configuration for the first mobility candidate. Knowing the early UL sync configuration(s) and the SSB configurations for the mobility candidate supported by the second (first) other network node, the first (second) other network node (when it becomes a serving network node) will be able to issue the order for the PRACH preamble transmission, as discussed above.

[0036] According to a seventh aspect, a method for operating a UE in a wireless communication network is provided. The method starts with the step of receiving configuration informationfor at least one mobility candidate from a network node serving a source cell. Each of the at least one mobility candidate comprises at least two candidate cells. The configuration information comprises: (i) at least two SSB configurations each associated with one of the at least two candidate cells, and (ii) at least one early UL sync configuration each associated with one of the at least two SSB configurations and further associated with a TAG. Then, the method proceeds to the step of receiving an order for a PRACH preamble transmission, which indicates one of: an early UL sync configuration of interest among the at least one early UL sync configuration, an SSB configuration of interest among the at least two SSB configurations, and the TAG associated with the early UL sync configuration of interest. If the order for the PRACH preamble transmission indicates the early UL sync configuration of interest, the method goes on to the step of determining the SSB configuration of interest among the at least two SSB configurations based on the configuration. If the order for the PRACH preamble transmission indicates the SSB configuration of interest, the method goes on to the step of determining the early UL sync configuration of interest based on the configuration information. If the order for the PRACH preamble transmission indicates the TAG associated with the early UL sync configuration of interest, the method goes on to the step of determining the early UL sync configuration of interest and the SSB configuration of interest based on the configuration information. Next, the method proceeds to the step of using the SSB configuration of interest and the early UL sync configuration of interest to perform the PRACH preamble transmission. By using such configuration information and order for the PRACH preamble transmission, the UE may properly determine the SSB and early UL sync configurations to be used for the PRACH preamble transmission towards corresponding one of the at least two candidate cells included in the mobility candidate (which may be further indicated as a target cell in a subsequent cell switch command from the serving network node).

[0037] According to an eighth aspect, a method for operating a network node in a wireless communication network is provided. The method starts with the step of obtaining configuration information for at least one mobility candidate. Each of the at least one mobility candidate comprises at least two candidate cells. The configuration information comprises: (i) at least two SSB configurations each associated with one of the at least two candidate cells, and (ii) at least one early UL sync configuration each associated with one ofthe at least two SSB configurations and a TAG. Then, the method proceeds to the step of transmitting the configuration information to a UE. After that, the method goes one to the step of transmitting, to the UE, an order for a PRACH preamble transmission, which indicates one of: an early UL sync configuration of interest among the at least one early UL sync configuration, an SSB configuration of interest among the at least two SSB configurations, and the TAG associated with the early UL sync configuration of interest. By using such configuration information and order for the PRACH preamble transmission, the UE may properly determine the SSB and early UL sync configurations to be used for the PRACH preamble transmission towards corresponding one of the at least two candidate cells included in the mobility candidate (which may be further indicated as a target cell in a subsequent cell switch command from the serving network node).

[0038] According to a ninth aspect, a UE in a wireless communication network is provided. The UE comprises at least one processor and at least one memory storing instructions that, when executed by the at least one processor, cause the UE to operate at least as follows. At first, the UE receives, from a network node serving a source cell, configuration information for at least one mobility candidate. Each of the at least one mobility candidate comprises at least two candidate cells. The configuration information comprises: (i) at least one early UL sync configuration each associated with a TAG, and (ii) at least two SSB configurations each associated with one of the at least two candidate cells. Each of the at least two SSB configurations is further associated with the TAG of one of the at least one early UL sync configuration. Then, the UE receives an order for a PRACH preamble transmission, which indicates either an early UL sync configuration of interest among the at least one early UL sync configuration, or the TAG associated with each of the early UL sync configuration of interest and an SSB configuration of interest. The SSB configuration of interest is among the at least two SSB configurations. If the order for the PRACH preamble transmission indicates the early UL sync configuration of interest, the UE determines the TAG associated with the early UL sync configuration of interest based on the configuration information and determine the SSB configuration of interest based on the determined TAG and the configuration information. If the order for the PRACH preamble transmission indicates the TAG associated with each of the early UL sync configuration of interest and the SSB configuration of interest, the UE determines the early UL sync configuration of interest andthe SSB configuration of interest based on the configuration information and the indicated TAG. After that, the UE uses the SSB configuration of interest and the early UL sync configuration of interest to perform the PRACH preamble transmission. By using such configuration information and order for the PRACH preamble transmission, the UE may properly determine the SSB and early UL sync configurations to be used for the PRACH preamble transmission towards corresponding one of the at least two candidate cells included in the mobility candidate (which may be further indicated as a target cell in a subsequent cell switch command from the serving network node).

[0039] In one example embodiment of the ninth aspect, the UE is further caused to receive a TA value in response to the PRACH preamble transmission. The provision of the TA value may be either by a candidate network node or by the serving network node (e.g., the candidate network node may estimate the TA value and provide it to the serving network node which then sends it to the UE). The UE may use the TA value for subsequent RACH-less cell switch. In one example embodiment of the ninth aspect, the TAG associated with each of the at least one early UL sync configuration has an ID. In this embodiment, each of the at least one early UL sync configuration comprises the ID of the TAG. Such ID-based association may simplify the determination of the SSB configuration of interest or the early UL sync configuration of interest in response to the order for the PRACH preamble transmission.

[0040] In one example embodiment of the ninth aspect, each of the at least one early UL sync configuration has an ID, and the TAG associated with each of the at least one early UL sync configuration and each of the at least two SSB configurations has an ID. In this embodiment, the order for the PRACH preamble transmission comprises the ID of the early UL sync configuration of interest, or the ID of the TAG associated with each of the early UL sync configuration of interest and the SSB configuration of interest. The use of such IDs in the order for the PRACH preamble transmission may enable the UE to identify the SSB or early UL sync configuration of interest.

[0041] In one example embodiment of the ninth aspect, each of the at least two SSB configurations has an ID, and the TAG associated with each of the at least one early UL sync configuration and each of the at least two SSB configurations has an ID. In this embodiment, the configuration information further comprises an association list comprising at least twoelements. Each of the at least two elements has an index that indicates the ID of one of the at least two SSB configurations and a value that indicates the ID of the TAG associated with said one of the at least two SSB configurations and one of the at least one early UL sync configuration. Such an ID-based association list may allow the UE to easily and quickly determine the TAG to be used for subsequent RACH-less cell switch.

[0042] In one example embodiment of the ninth aspect, the UE is caused to receive, after the PRACH preamble transmission, a cell switch command comprising two or more TA values each being associated with a TAG. By using the TA values, the UE may be properly configured for subsequent RACH-less cell switch.

[0043] In one example embodiment of the ninth aspect, the UE is caused to receive the configuration information via a RRC signaling. Thus, the configuration information may be provided to the UE without having to increase control signaling in the wireless communication network.

[0044] According to a tenth aspect, a network node in a wireless communication network is provided. The network node comprises at least one processor and at least one memory storing instructions that, when executed by the at least one processor, cause the network node to operate at least as follows. At first, the network node obtains configuration information for at least one mobility candidate. Each of the at least one mobility candidate comprises at least two candidate cells. The configuration information comprises: (i) at least one early UL sync configuration each associated with a TAG, and (ii) at least two SSB configurations each associated with one of the at least two candidate cells. Each of the at least two SSB configurations is further associated with the TAG of one of the at least one early UL sync configuration. Then, the network node transmits the configuration information to a UE. After that, the network node transmits, to the UE, an order for a PRACH preamble transmission, which indicates an early UL sync configuration of interest among the at least one early UL sync configuration, or the TAG associated with each of the early UL sync configuration of interest and an SSB configuration of interest. The SSB configuration of interest is among the at least two SSB configurations. By using such configuration information and order for the PRACH preamble transmission, the UE may properly determine the SSB and early UL sync configurations to be used for the PRACH preamble transmission towards corresponding one of the at least two candidate cells included in themobility candidate (which may be further indicated as a target cell in a subsequent cell switch command from the serving network node).

[0045] In one example embodiment of the tenth aspect, the at least one mobility candidate comprises a first mobility candidate supported by a first other network node and a second mobility candidate supported by a second other network node. In this embodiment, the network node is further caused to operate as follows. The network node receives, from the first other network node, the at least two SSB configurations and the at least one early UL sync configuration for the first mobility candidate, and receives, from the second other network node, the at least two SSB configurations and the at least one early UL sync configuration for the second mobility candidate. Then, the network node obtains the configuration information based on the at least two SSB configurations and the at least one early UL sync configuration for each of the first mobility candidate and the second mobility candidate. Furthermore, the network node transmits, to the first other network node, the at least two SSB configurations and the at least one early UL sync configuration for the second mobility candidate and transmits, to the second other network node, the at least two SSB configurations and the at least one early UL sync configuration for the first mobility candidate. Knowing the early UL sync configuration(s) and the SSB configurations for the mobility candidate supported by the second (first) other network node, the first (second) other network node (when it becomes a serving network node) will be able to issue the order for the PRACH preamble transmission, as discussed above.

[0046] According to an eleventh aspect, a method for operating a UE in a wireless communication network is provided. The method starts with the step of receiving, from a network node serving a source cell, configuration information for at least one mobility candidate. Each of the at least one mobility candidate comprises at least two candidate cells. The configuration information comprises: (i) at least one early UL sync configuration each associated with a TAG, and (ii) at least two SSB configurations each associated with one of the at least two candidate cells. Each of the at least two SSB configurations is further associated with the TAG of one of the at least one early UL sync configuration. Then, the method proceeds to the step of receiving an order for a PRACH preamble transmission, which indicates either an early UL sync configuration of interest among the at least one early UL sync configuration, or the TAG associated with each of the early UL sync configuration of interest and an SSB configurationof interest. The SSB configuration of interest is among the at least two SSB configurations. If the order for the PRACH preamble transmission indicates the early UL sync configuration of interest, the method goes on to the step of determining the TAG associated with the early UL sync configuration of interest based on the configuration information and determining the SSB configuration of interest based on the determined TAG and the configuration information. If the order for the PRACH preamble transmission indicates the TAG associated with each of the early UL sync configuration of interest and the SSB configuration of interest, the method goes on to the step of determining the early UL sync configuration of interest and the SSB configuration of interest based on the configuration information and the indicated TAG. After that, the method proceeds to the step of using the SSB configuration of interest and the early UL sync configuration of interest to perform the PRACH preamble transmission. By using such configuration information and order for the PRACH preamble transmission, the UE may properly determine the SSB and early UL sync configurations to be used for the PRACH preamble transmission towards corresponding one of the at least two candidate cells included in the mobility candidate (which may be further indicated as a target cell in a subsequent cell switch command from the serving network node).

[0047] According to a twelfth aspect, a method for operating a network node in a wireless communication network is provided. The method starts with the step of obtaining configuration information for at least one mobility candidate. Each of the at least one mobility candidate comprises at least two candidate cells. The configuration information comprises: (i) at least one early UL sync configuration each associated with a TAG, and (ii) at least two SSB configurations each associated with one of the at least two candidate cells. Each of the at least two SSB configurations is further associated with the TAG of one of the at least one early UL sync configuration. Then, the method proceeds to the step of transmitting the configuration information to a UE. After that, the method goes on to the step of transmitting, to the UE, an order for a PRACH preamble transmission, which indicates an early UL sync configuration of interest among the at least one early UL sync configuration, or the TAG associated with each of the early UL sync configuration of interest and an SSB configuration of interest. The SSB configuration of interest is among the at least two SSB configurations. By using such configuration information and order for the PRACH preamble transmission, the UE may properly determine the SSB and early UL sync configurations to beused for the PRACH preamble transmission towards corresponding one of the at least two candidate cells included in the mobility candidate (which may be further indicated as a target cell in a subsequent cell switch command from the serving network node).

[0048] According to a thirteenth aspect, a UE in a wireless communication network is provided. The UE comprises at least one processor and at least one memory storing instructions that, when executed by the at least one processor, cause the UE to operate at least as follows. At first, the UE receives, from a network node serving a source cell, configuration information for at least one mobility candidate. Each of the at least one mobility candidate comprises at least one candidate cell belonging to a TAG. The configuration information comprises: (i) at least two SSB configuration, and (ii) an early UL sync configuration associated with one of the at least two SSB configurations. At least one of the at least two SSB configurations is associated with the at least one candidate cell and at least one of the at least two SSB configurations is associated with at least one other cell belonging to the same TAG as the at least one candidate cell. Then, the UE receives an order for a PRACH preamble transmission. In response to the order for the PRACH preamble transmission, the UE determines an SSB configuration of interest among the at least two SSB configurations based on the configuration information. The early UL sync configuration is associated with the SSB configuration of interest. Next, the UE uses the SSB configuration of interest and the early UL sync configuration to perform the PRACH preamble transmission. By using such configuration information and order for the PRACH preamble transmission, the UE may properly determine the SSB and early UL sync configurations to be used for the PRACH preamble transmission towards corresponding one of the at least two candidate cells included in the mobility candidate (which may be further indicated as a target cell in a subsequent cell switch command from the serving network node).

[0049] In one example embodiment of the thirteenth aspect, each of the at least two SSB configurations has an ID. In this embodiment, the early UL sync configuration comprises the ID of one of the at least two SSB configurations. Such ID-based association may simplify the determination of the SSB configuration of interest in response to the order for the PRACH preamble transmission.

[0050] In one example embodiment of the thirteenth aspect, the TAG is a primary TAG (PTAG).In one example embodiment of the thirteenth aspect, the configuration information further comprises at least one other early UL sync configuration each associated with one other of the at least two SSB configurations, and the order for the PRACH preamble transmission comprises an indication of the early UL sync configuration to be used for the PRACH preamble transmission. In this embodiment, the UE is caused to determine the SSB configuration of interest based on the indication of the early UL sync configuration and the configuration information. Thus, the UE may be properly informed about the early UL sync configuration to be used for determining the SSB configuration of interest.

[0051] In one example embodiment of the thirteenth aspect, the order for the PRACH preamble transmission comprises an ID of the early UL sync configuration. The use of such an ID in the order for the PRACH preamble transmission may enable the UE to identify the associated early UL sync configuration first and then the SSB configuration of interest.

[0052] In one example embodiment of the thirteenth aspect, the UE is caused to receive the configuration information via a RRC signaling. Thus, the configuration information may be provided to the UE without having to increase control signaling in the wireless communication network.

[0053] According to a fourteenth aspect, a network node in a wireless communication network is provided. The network node comprises at least one processor and at least one memory storing instructions that, when executed by the at least one processor, cause the network node to operate at least as follows. At first, the network node obtains configuration information for at least one mobility candidate. Each of the at least one mobility candidate comprises at least one candidate cell belonging to a TAG. The configuration information comprises (i) at least two SSB configurations, and (ii) an early UL sync configuration associated with one of the at least two SSB configurations. At least one of the at least two SSB configurations is associated with the at least one candidate cell and at least one of the at least two SSB configurations is associated with at least one other cell belonging to the same TAG as the at least one candidate cell. Then, the network node transmits the configuration information to a UE. After that, the network node transmits, to the UE, an order for a PRACH preamble transmission. By using such configuration information, the UE may properly determine the SSB and early UL sync configurations to be used for the PRACH preamble transmission towards corresponding one of the at least two candidate cellsincluded in the mobility candidate (which may be further indicated as a target cell in a subsequent cell switch command from the serving network node).

[0054] In one example embodiment of the fourteenth aspect, the at least one mobility candidate comprises a first mobility candidate supported by a first other network node and a second mobility candidate supported by a second other network node. In this embodiment, the network node is further caused to operate as follows. The network node receives, from the first other network node, the at least two SSB configurations and the early UL sync configuration for the first mobility candidate, and receives, from the second other network node, the at least two SSB configurations and the early UL sync configuration for the second mobility candidate. Then, the network node obtains the configuration information based on the at least two SSB configurations and the early UL sync configuration for each of the first mobility candidate and the second mobility candidate. Furthermore, the network node transmits, to the first other network node, the at least two SSB configurations and the early UL sync configuration for the second mobility candidate and transmits, to the second other network node, the at least two SSB configurations and the early UL sync configuration for the first mobility candidate. Knowing the early UL sync configuration and the SSB configurations for the mobility candidate supported by the second (first) other network node, the first (second) other network node (when it becomes a serving network node) will be able to issue the order for the PRACH preamble transmission, as discussed above.

[0055] According to a fifteenth aspect, a method for operating a UE in a wireless communication network is provided. The method starts with the step of receiving, from a network node serving a source cell, configuration information for at least one mobility candidate. Each of the at least one mobility candidate comprises at least one candidate cell belonging to a TAG. The configuration information comprises: (i) at least two SSB configuration, and (ii) an early UL sync configuration associated with one of the at least two SSB configurations. At least one of the at least two SSB configurations is associated with the at least one candidate cell and at least one of the at least two SSB configurations is associated with at least one other cell belonging to the same TAG as the at least one candidate cell. Then, the method proceeds to the step of receiving an order for a PRACH preamble transmission. In response to the order for the PRACH preamble transmission, the method goes on to the step of determining an SSB configuration of interest among the at least two SSB configurations based on theconfiguration information. The early UL sync configuration is associated with the SSB configuration of interest. Next, the method proceeds to the step of using the SSB configuration of interest and the early UL sync configuration to perform the PRACH preamble transmission. By using such configuration information, the UE may properly determine the SSB and early UL sync configurations to be used for the PRACH preamble transmission towards corresponding one of the at least two candidate cells included in the mobility candidate (which may be further indicated as a target cell in a subsequent cell switch command from the serving network node).

[0056] According to a sixteenth aspect, a method for operating a network node in a wireless communication network is provided. The method starts with the step of obtaining configuration information for at least one mobility candidate. Each of the at least one mobility candidate comprises at least one candidate cell belonging to a TAG. The configuration information comprises (i) at least two SSB configurations, and (ii) an early UL sync configuration associated with one of the at least two SSB configurations. At least one of the at least two SSB configurations is associated with the at least one candidate cell and at least one of the at least two SSB configurations is associated with at least one other cell belonging to the same TAG as the at least one candidate cell. Then, the method proceeds to the step of transmitting the configuration information to a UE. After that, the method goes on to the step of transmitting, to the UE, an order for a PRACH preamble transmission. By using such configuration information, the UE may properly determine the SSB and early UL sync configurations to be used for the PRACH preamble transmission towards corresponding one of the at least two candidate cells included in the mobility candidate (which may be further indicated as a target cell in a subsequent cell switch command from the serving network node).

[0057] According to a seventeenth aspect, a computer program product is provided. The computer program product comprises a non-transitory computer-readable storage medium that stores a computer code. Being executed by at least one processor, the computer code causes the at least one processor to perform the method according to any of the third, seventh, eleventh and fifteenth aspects. By using such a computer program product, it is possible to simplify the implementation of the methods according to the third, seventh, eleventh andfifteenth aspects in the UEs according to the first, fifth, ninth and thirteenth aspects, respectively.

[0058] According to an eighteenth aspect, a computer program product is provided. The computer program product comprises a non-transitory computer-readable storage medium that stores a computer code. Being executed by at least one processor, the computer code causes the at least one processor to perform the method according to any of the fourth, eighth, twelfth and sixteenth aspects. By using such a computer program product, it is possible to simplify the implementation of the methods according to the fourth, eighth, twelfth and sixteenth aspects in the network nodes according to the second, sixth, tenth and fourteenth aspects, respectively.

[0059] According to a nineteenth aspect, a UE in a wireless communication network is provided. The UE comprises one or more means configured to perform the operations in accordance with any of the third, seventh, eleventh and fifteenth aspects.

[0060] According to a twentieth aspect, a network node in a wireless communication network is provided. The network node comprises one or more means configured to perform the operations in accordance with any of the second, sixth, tenth and fourteenth aspects.

[0061] Other features and advantages of the present disclosure will be apparent upon reading the following detailed description and reviewing the accompanying drawings.

[0062] BRIEF DESCRIPTION OF THE DRAWINGS

[0063] The present disclosure is explained below with reference to the accompanying drawings in which:

[0064] FIG. 1 schematically illustrates a signaling procedure used in the conventional LTM mechanism;

[0065] FIG. 2 shows a block diagram of a UE in a wireless communication network in accordance with one example embodiment;

[0066] FIG. 3 shows a flowchart of a method for operating the UE of FIG. 2 in accordance with a first example embodiment;FIG. 4 schematically illustrates the content of LTM configuration information that may be used in the method of FIG. 3;

[0067] FIG. 5 shows a flowchart of a method for operating the UE of FIG. 2 in accordance with a second example embodiment;

[0068] FIG. 6 schematically illustrates the content of LTM configuration information that may be used in the method of FIG. 5;

[0069] FIG. 7 shows a flowchart of a method for operating the UE of FIG. 2 in accordance with a third example embodiment;

[0070] FIG. 8 schematically illustrates the content of LTM configuration information that may be used in the method of FIG. 7;

[0071] FIG. 9 shows a flowchart of a method for operating the UE of FIG. 2 in accordance with a fourth example embodiment; and

[0072] FIGs. 10A-10C schematically illustrate a signaling procedure used in a modified LTM mechanism according to one example embodiment.

[0073] DETAILED DESCRIPTION

[0074] Various embodiments of the present disclosure are further described in more detail with reference to the accompanying drawings. However, the present disclosure can be embodied in many other forms and should not be construed as limited to any certain structure or function discussed in the following description. In contrast, these embodiments are provided to make the description of the present disclosure detailed and complete.

[0075] According to the detailed description, it will be apparent to the ones skilled in the art that the scope of the present disclosure encompasses any embodiment thereof, which is disclosed herein, irrespective of whether this embodiment is implemented independently or in concert with any other embodiment of the present disclosure. For example, the apparatuses and methods disclosed herein can be implemented in practice by using any numbers of the embodiments provided herein. Furthermore, it should be understood thatany embodiment of the present disclosure can be implemented using one or more of the elements presented in the appended claims.

[0076] Unless otherwise stated, any embodiment recited herein as "example embodiment" should not be construed as preferable or having an advantage over other embodiments.

[0077] Furthermore, although the numerative terminology, such as "first", "second", etc., may be used herein to describe various embodiments, elements or features, it should be understood that these embodiments, elements or features should not be limited by this numerative terminology. This numerative terminology is used herein only to distinguish one embodiment, element or feature from another embodiment, element or feature.

[0078] According to the example embodiments disclosed herein, a user equipment (UE) may refer to an electronic computing device that is configured to perform wireless communications. The UE may be implemented as a mobile station, a mobile terminal, a mobile subscriber unit, a mobile phone, a cellular phone, a smart phone, a cordless phone, a personal digital assistant (PDA), a wireless communication device, a desktop computer, a laptop computer, a tablet computer, a gaming device, a netbook, a smartbook, an ultrabook, a medical mobile device or equipment, a biometric sensor, a wearable device (e.g., a smart watch, smart glasses, a smart wrist band, etc.), an entertainment device (e.g., an audio player, a video player, etc.), a vehicular component or sensor (e.g., a driver-assistance system), a smart meter / sensor, an unmanned vehicle (e.g., an industrial robot, a quadcopter, etc.) and its component (e.g., a self-driving car computer), industrial manufacturing equipment, a global positioning system (GPS) device, an internet-of-things (loT) device, an industrial loT (I loT) device, a machine-type communication (MTC) device, a group of massive loT (MIoT) or massive MTC (mMTC) devices / sensors, or any other suitable mobile device configured to support wireless communications. In some embodiments, the UE may refer to at least two collocated and inter-connected UEs thus defined.

[0079] As used in the example embodiments disclosed herein, a network node may refer to a fixed point of communication or communication node for a UE in a particular wireless communication network. More specifically, the network node may be used to connect the UE to a data network (DN) through a core network (CN) and may be referred to as a base transceiver station (BTS) in terms of the 2G communication technology, a NodeB in terms ofthe 3G communication technology, an evolved NodeB (eNodeB or eNB) in terms of the 4G communication technology, and a gNB in terms of the 5G new radio (NR) communication technology. The network node may serve different cells, such as a macrocell, a microcell, a picocell, a femtocell, and / or other types of cells. The macrocell may cover a relatively large geographic area (e.g., at least several kilometers in radius). The microcell may cover a geographic area less than two kilometers in radius, for example. The picocell may cover a relatively small geographic area, such, for example, as offices, shopping malls, train stations, stock exchanges, etc. The femtocell may cover an even smaller geographic area (e.g., a home). Correspondingly, the network node serving the macrocell may be referred to as a macro node, the network node serving the microcell may be referred to as a micro node, and so on.

[0080] According to the example embodiments disclosed herein, a wireless communication network, in which a UE and a network node communicate with each other, may refer to a cellular or mobile network, a wireless local area network (WLAN), a wireless personal area networks (WPAN), a wireless wide area network (WWAN), a satellite communication (SATCOM) system, or any other type of wireless communication networks. Each of these types of wireless communication networks supports wireless communications according to one or more communication protocol standards. For example, the cellular network may operate according to the global system for mobile communications (GSM) standard, the code-division multiple access (CDMA) standard, the wide-band code-division multiple access (WCDM) standard, the time-division multiple access (TDMA) standard, or any other communication protocol standard, the WLAN may operate according to one or more versions of the IEEE 802.11 standards, the WPAN may operate according to the infrared data association (IrDA), wireless USB, Bluetooth, or ZigBee standard, and the WWAN may operate according to the worldwide interoperability for microwave access (WiMAX) standard.

[0081] In the lower layer-triggered mobility (LTM) mechanism, a cell switch procedure has been proposed, in which a UE receives a cell switch command (e.g., a media access control (MAC) control element (CE)) including an indication of one of configured LTM candidate cells and accesses the indicated LTM candidate cell (which is a target cell which the UE should be handed over to). In response to the cell switch command, the UE can access the LTM candidate cell based on a random access (RA) procedure. More specifically, when the UEreceives the cell switch command from a serving cell (e.g., primary cell (PCell)), the UE transmits a physical random access channel (PRACH) preamble to the LTM candidate cell and receives a random access response. However, before the UE transmits the PRACH preamble to the LTM candidate cell, the UE needs first to perform downlink (DL) synchronization, e.g., by detecting and receiving one or more synchronization signal blocks (SSBs) of that LTM candidate cell, so that the UE can determine the PRACH occasion(s) and transmit the PRACH preamble upon reception of a corresponding order for the PRACH preamble transmission (e.g., a physical downlink control channel (PDCCH) order). This step (i.e., the DL synchronization) will result in an increase in the delay to access the LTM candidate cell, i.e., the time required to perform the HO process increases (which can adversely affect delaysensitive communication services used by the UE).

[0082] FIG. 1 schematically illustrates a signaling procedure 100 used in the conventional LTM mechanism. More specifically, the signaling procedure 100 is defined in TS38.300v18 and is used to reduce interruption time during HO (as compared to layer 3 (L3)-driven HO).

[0083] The signaling procedure 100 starts with a step S102, in which a UE sends a measurement report message to a gNB. The gNB decides to configure LTM and initiates LTM preparation. In a subsequent step S104, the gNB transmits an RRC reconfiguration message to the UE. The RRC reconfiguration message includes LTM candidate configuration. Each mobility or LTM candidate may contain one or more candidate cells configured by the gNB for the UE. The LTM candidate configuration may contain:

[0084] RRC configurations of one or more mobility candidates;

[0085] SSB configuration of one candidate cell (e.g., a candidate primary cell (PCell)) whose RRC configuration is included in the LTM candidate configuration; and

[0086] Early UL sync configuration of one candidate cell (e.g., the candidate PCell) whose RRC configuration is included in the LTM candidate configuration.

[0087] Next, in a step S106, the UE stores the LTM candidate configuration and transmits an RRC reconfiguration complete message to the gNB.

[0088] Then, the signaling procedure 100 goes on to a step S108, in which the UE performs DL synchronization with the candidate cell(s) before receiving a cell switch command.To perform UL synchronization with the candidate cell(s), the UE needs to obtain one or more TA values in a step S110. When UE-based TA measurement is configured, the UE acquires the TA value(s) of the candidate cell(s) by measurement. The UE performs early TA acquisition (for early UL synchronization) with the candidate cell(s) as requested by the gNB before receiving the cell switch command. This is done via contention-free random access (CFRA) triggered by a PDCCH order from the source cell (served by the gNB), following which the UE sends a PRACH preamble towards the indicated candidate cell. To minimize the data interruption of the source cell due to the CFRA towards the candidate cell(s), the UE does not receive a random access (RA) response from the gNB for the purpose of TA value acquisition, and the TA value of the candidate cell is instead indicated in the cell switch command. The UE does not maintain a TA timer for the candidate cell and relies on network implementation to guarantee the TA validity.

[0089] The signaling procedure 100 further proceeds to a step S112, in which the UE performs LI measurements on the configured candidate cell(s) and transmits LI measurement reports to the gNB. The LI measurements should be performed as long as the RRC reconfiguration (from the step S104) is applicable.

[0090] In a subsequent step S114, the gNB decides to execute cell switch to a target cell and transmits a MAC CE triggering cell switch by including the candidate configuration index of the target cell. The UE then switches to the target cell and applies the configuration indicated by the candidate configuration index.

[0091] The signaling procedure 100 then proceeds to a step S116, in which the UE performs the RA procedure towards the target cell if the UE does not have a valid TA of the target cell, as specified in TS 38.321.

[0092] The signaling procedure 100 then proceeds to a step S118, in which the UE completes the LTM cell switch procedure by sending an RRC reconfiguration complete message to the target cell.

[0093] It should be noted that if the UE has performed the RA procedure in the step S116, then the UE considers that LTM cell switch execution is successfully completed when the RA procedure is successfully completed. For RACH-less LTM, the UE considers that LTM cellswitch execution is successfully completed when the UE determines that the network (i.e., the network node or gNB serving the target cell) has successfully received its first UL data. One of the key goals of the Rel-18 LTM mechanism is to reduce the interruption caused by the cell switch from the L3 HO. This can be done, among others, using early UL synchronization (or sync for short) through PDCCH ordered RACH- or UE-based TA estimation. The Rel-18 information elements (lEs) that are relevant for the RACH-based early TA acquisition are as follows (see TS 38.331):

[0094] LTM-Candidate

[0095] LTM-Candidate-r18 ::= SEQUENCE {

[0096] ltm-CandidateId-r18 LTM-CandidateId-r18,

[0097] ltm-CandidatePCI-r18 PhysCellId OPTIONAL, -- Need M

[0098] ltm-SSB-Config-r18 LTM-SSB-Config-r18 OPTIONAL, -- Need M

[0099] ltm-CandidateConfig-r18 OCTET STRING (CONTAINING RRCReconfiguration) OPTIONAL, -- Need M

[0100] ltm-ConfigComplete-r18 ENUMERATED {true} OPTIONAL, -- Need R

[0101] ltm-EarlyUL-SyncConfig-r18 OCTET STRING (CONTAINING EarlyUL-SyncConfig-r18) OPTIONAL, -- Need Rltm-EarlyUL-SyncConfigSUL-r18 OCTET STRING (CONTAINING EarlyUL-SyncConfig-r18) OPTIONAL, -- Need Rltm-TCI-Info-r18 LTM-TCI-Info-r18 OPTIONAL, -- Need Mltm-NoResetID-r18 INTEGER (1..maxNrofLTM-Configs-plus1-r18) OPTIONAL, -- Need Mltm-UE-MeasuredTA-ID-r18 INTEGER (1..maxNrofLTM-Configs-plus1-r18) OPTIONAL, -- Need M

[0102]

[0103] LTM-SSB-Config

[0104] LTM-SSB-Config-r18 ::= SEQUENCE {

[0105] ssb-Frequency-r18 ARFCN-ValueNR,

[0106] subcarrierSpacing-r18 SubcarrierSpacing,

[0107] ssb-Periodicity-r18 ENUMERATED {ms5, ms10, ms20, ms40, ms80, ms160, spare2, spare1} OPTIONAL, -- Need R

[0108] ssb-PositionsInBurst-r18 CHOICE {

[0109] shortBitmap BIT STRING (SIZE (4)),

[0110] mediumBitmap BIT STRING (SIZE (8)),

[0111] longBitmap BIT STRING (SIZE (64))

[0112] } OPTIONAL, — Need R

[0113] ss-PBCH-BlockPower-r18 INTEGER (-60..50) OPTIONAL, -- Need R

[0114] }

[0115] EarlyUL-SyncConfig

[0116] The IE EarlyUL-SyncConfig is used to configure random access resources for the early UL synchronization procedure.

[0117] EarlyUL-SyncConfig-r18 ::= SEQUENCE {

[0118] frequencyInfoUL-r18 FrequencyInfoUL,

[0119] rach-ConfigGeneric-r18 RACH-ConfigGeneric,

[0120] bwp-GenericParameters-r18 BWP,

[0121] ssb-PerRACH-Occasion-r18 ENUMERATED {oneEighth, oneFourth, oneHalf, one, two, four, eight, sixteen} OPTIONAL, -- Need R

[0122] prach-RootSequenceIndex-r18 CHOICE {

[0123] l839 INTEGER (0..837),

[0124] l139 INTEGER (0..137)

[0125] } OPTIONAL, — Need R

[0126] ltm-PRACH-SubcarrierSpacing-r18 SubcarrierSpacing OPTIONAL, -- Cond L139n-TimingAdvanceOffset-r18 ENUMERATED {n0, n25600, n39936, spare1} OPTIONAL, -- Need R

[0127] [ [

[0128] ltm-tdd-UL-DL-ConfigurationCommon-r18 TDD-UL-DL-ConfigCommon OPTIONAL, -- Cond TDDltm-restrictedSetConfig-r18 ENUMERATED {unrestrictedSet, restrictedSetTypeA, restrictedSetTypeB} OPTIONAL -- Need R

[0129] ]]

[0130] }

[0131] The parameter "ssb-PerRACH-Occasion-rl8" indicates the number of SSBs per RACH occasion. Together with "ssb-PositionsInBurst" (provided in "ltm-SSB-Config"), it allows the UE to determine the mapping of SSB indexes to RACH occasions.

[0132] It should be noted that the early UL sync configuration contains all the configurations required by the UE to perform a RA preamble transmission to the corresponding candidate cell, but it does not contain RA preambles to be used for early TA acquisition. The reason for this choice was to allow a serving network node to dynamically allocate the CFRA preambles to UEs in the PDCCH order, so that the limited CFRA resources can be shared among the UEs and, therefore, can be more efficiently utilized.

[0133] In the Rel-18 LTM mechanism, a mobility candidate can have an RRC configuration with carrier aggregation (CA), but early TA acquisition using candidate cells (e.g., secondary cells (SCells)) belonging to the mobility candidate is not supported. It is not straightforward to extend the Rel-18 framework, as it is not necessary that early UL sync configuration needs to be provided for every SCell and RRC configuration and lower level signaling (i.e., PDCCH order) to enable TA acquisition for multiple TAGs. For example, it is not obvious which SSBs the UE should use for obtaining DL timing and determining the appropriate RACH occasion when receiving a PDCCH order and how SSBs and / or early UL sync configurations are associated with TAGs.

[0134] Furthermore, even when only the TA of a primary TAG (PTAG) is considered (e.g., for a UE that supports CA but only with a single TAG, i.e., the same TA value for all candidate cells), as in the Rel-18 LTM mechanism, the PRACH resources (e.g., preamble sequences, RACH occasions) of a candidate PCell might be congested and there may not be available CFRA resources for early TA acquisition. Hence, it will be impossible for the network to trigger early TA acquisition for that candidate PCell until CFRA resources are freed up.

[0135] The example embodiments disclosed herein provide a technical solution that enables early UL synchronization through a network-initiated PRACH preamble transmission in case ofusing mobility candidates each comprising one or more candidate cells. For this purpose, a UE is provided with one or more early UL sync configurations and two or more SSB configurations per mobility candidate, with each of the early UL sync configurations being associated with an SSB configuration, directly or indirectly (via a TAG), and a TAG, directly or indirectly (via an SSB configuration). Depending on the association between the early ULsync configuration(s), the SSB configurations and the TAGs, different options for the content of a network-initiated PRACH preamble transmission order (e.g., PDCCH order) are provided. In the embodiments disclosed herein, the order for the PRACH preamble transmission refers to an instruction or indication for the UE to perform the PRACH preamble transmission. It should be noted that, unlike the currently used LTM configuration information, the example embodiments disclosed herein are based on its modified version that contains:

[0136] RRC configuration of one or more candidates (the same as in the Rel-18 LTM mechanism);

[0137] SSB configurations of two or more candidate cells (e.g., a PCell and at least one SCell) whose RRC configuration is included in the LTM configuration for each mobility candidate (not used in the Rel-18 LTM mechanism); and

[0138] Early UL sync configuration of one or more of said two or more candidate cells (any of the PCell or the Scell(s)) whose RRC configuration is included in the candidate configuration (the early UL sync configuration of SCell(s) is not used in the Rel-18 LTM mechanism).

[0139] FIG. 2 shows a block diagram of a UE 200 in a wireless communication network in accordance with one example embodiment. As shown in FIG. 2, the UE 200 comprises a processor 202 and a memory 204. The memory 204 stores processor-executable instructions 206 which, when executed by the processor 202, cause the processor 202 to perform the aspects of the present disclosure, as will be described below in more detail. It should be noted that the number, arrangement, and interconnection of the constructive elements constituting the UE 200, which are shown in FIG. 2, are not intended to be any limitation of the present disclosure, but merely used to provide a general idea of how the constructive elements may be implemented within the UE 200. For example, the processor 202 may be replaced with several processors, as well as the memory 204 may be replaced with several removable and / or fixed storage devices, depending on particular applications. Furthermore, in someembodiments, the processor 202 may perform different operations required to perform data reception and transmission, such, for example, as signal modulation / demodulation, encoding / decoding, etc. Alternatively, the UE 200 may further comprise an individual transceiver which can be configured to perform the required operations for data reception and transmission based on commands from the processor 202.

[0140] The processor 202 may be implemented as a CPU, general-purpose processor, singlepurpose processor, microcontroller, microprocessor, application specific integrated circuit (ASIC), field programmable gate array (FPGA), digital signal processor (DSP), complex programmable logic device, etc. It should be also noted that the processor 202 may be implemented as any combination of one or more of the aforesaid. As an example, the processor 202 may be a combination of two or more microprocessors.

[0141] The memory 204 may be implemented as a classical nonvolatile or volatile memory used in the modern electronic computing machines. As an example, the nonvolatile memory may include Read-Only Memory (ROM), ferroelectric Random-Access Memory (RAM), Programmable ROM (PROM), Electrically Erasable PROM (EEPROM), solid state drive (SSD), flash memory, magnetic disk storage (such as hard drives and magnetic tapes), optical disc storage (such as CD, DVD and Blu-ray discs), etc. As for the volatile memory, examples thereof include Dynamic RAM, Synchronous DRAM (SDRAM), Double Data Rate SDRAM (DDR SDRAM), Static RAM, etc.

[0142] The processor-executable instructions 206 stored in the memory 204 may be configured as a computer-executable program code which causes the processor 202 to perform the aspects of the present disclosure. The computer-executable program code for carrying out operations or steps for the aspects of the present disclosure may be written in any combination of one or more programming languages, such as Java, C++, Python, or the like. In some examples, the computer-executable program code may be in the form of a high-level language or in a pre-compiled form and be generated by an interpreter (also pre-stored in the memory 204) on the fly.

[0143] FIG. 3 shows a flowchart of a method 300 for operating the UE 200 in accordance with a first example embodiment. The method 300 starts with a step S302, in which the processor 202 receives (e.g., via the transceiver) configuration information for one or more mobilitycandidates from a network node (e.g., gNB) serving a source cell. Each mobility candidate comprises two or more candidate cells. The configuration information itself comprises: (i) two or more SSB configurations each associated with one of the (two or more) candidate cells, and (ii) one or more early UL sync configurations each associated with one of the (two or more) SSB configurations. Thus, the number of early UL sync configurations should be less than or equal to the number of SSB configurations. The configuration information may be provided to the UE 200 via dedicated signaling, such as RRC signaling (e.g., in an RRC reconfiguration message).

[0144] After the configuration information is received, the method 300 proceeds to a step S304, in which the processor 202 receives (e.g., via the transceiver) an order for a PRACH preamble transmission. The order may be a PDCCH order, and it indicates either an early UL sync configuration of interest among said one or more early UL sync configurations, or an SSB configuration of interest among said two or more SSB configurations.

[0145] If the order for the PRACH preamble transmission indicates the early UL sync configuration of interest, the processor 202 determines the SSB configuration of interest based on the configuration information in a subsequent step S306. That is, the processor 202 determines the SSB configuration of interest through the corresponding association between the early UL sync configuration of interest (indicated in the order) and the SSB configuration of interest. Alternatively, if the order for the PRACH preamble transmission indicates the SSB configuration of interest, the step S306 is performed in the opposite direction, i.e., the early UL sync configuration of interest is determined based on its association with the SSB configuration of interest.

[0146] The method 300 further goes to a step S308, in which the processor 202 uses the SSB configuration of interest and the early UL sync configuration of interest to perform the PRACH preamble transmission.

[0147] FIG. 4 schematically illustrates the content of LTM configuration information 400 that may be used in the method 300. As shown in FIG. 4, each of two early UL sync configurations (i.e., " LTM-EarlyUL-SyncConfig" with ID 1 and " LTM-EarlyUL-SyncConfig" with ID 2) is associated with one of two SSB configurations (i.e., " LTM-SSB-Config" with ID 1 and " LTM-SSB-Config" with ID 2), e.g., through the ID of the corresponding SSB configuration. It is not possible toassociate multiple early UL configurations with the same SSB configuration; in other words, an SSB configuration ID can be included in at most one early UL sync configuration.

[0148] Optionally, the LTM configuration information 400 may contain the association of the SSB configurations with corresponding TAGs. This association may be presented either as a separate association list provided as part of " LTM-Candidate" or by including the TAG ID to " LTM-SSB-Config". It should be noted that the indication of a TAG in the LTM configuration information may be useful when conditional LTM is configured for the UE 200, since the UE 200, in determining that cell switch should be executed to a mobility target based on the fulfillment of a configured condition, may determine what TA value to use to access a target cell in the determined mobility target based on the TAG to which the target cell belongs. Given the LTM configuration information 400, the order for the PRACH preamble transmission (e.g., PDCCH order) may be implemented as one of the following options:

[0149] Option A: the order indicates the early UL sync configuration ID, with the following two sub-options:

[0150] • Sub-option A-1: if the TAG ID is indicated in the PDCCH order and the number of early UL sync configurations is equal to the number of TAGs, a one-to-one mapping between the TAGs and the early UL sync configurations is required to unambiguously determine the early UL sync configuration of interest for the PRACH preamble transmission. However, enforcing such a one-to-one mapping is restrictive, because it may be advantageous to provide multiple early UL sync configurations per TAG;

[0151] • Sub-option A-2: The UE 200 identifies the early UL sync configuration to be used for the PRACH preamble transmission from the indicated ID (i.e., the order indicates the early UL sync configuration of interest). The SSB configuration of interest is further identified by the SSB configuration ID included in the identified early UL sync configuration and is used for determining the PRACH occasion, DL timing either directly from the indicated SSB belonging to the identified SSB configuration or from potentially activated TCI states associated with the indicated SSB configuration, as well as the path loss to be used for computation of the transmit power of the PRACH preamble. The TAG ID is identified by the association of the identified SSBconfiguration with the TAG, as provided in the LTM configuration information 400;

[0152] Option B: the PDCCH order indicates the SSB configuration ID. In this option, the UE 200 identifies the early UL sync configuration to be used for the PRACH preamble transmission from the association of the indicated SSB configuration ID with one of said one or more early UL sync configurations. The SSB configuration of interest is identified by the indicated SSB configuration ID and the TAG ID is identified by the association of the indicated SSB configuration with the TAG, as provided in the LTM configuration information 400; and

[0153] Option C: the PDCCH order indicates the TAG ID. As described above, if the TAG ID is indicated in the PDCCH order, the 1-1 mapping between the TAGs and the early UL sync configurations is required to unambiguously determine the early UL sync configuration to be used for the PRACH preamble transmission. The SSB configuration of interest is determined from the association of the SSB configurations with the TAGs (which is provided within the LTM configuration information), and the early UL sync configuration ID is determined from the association of the early UL sync configurations with the SSB configurations. FIG. 5 shows a flowchart of a method 500 for operating the UE 200 in accordance with a second example embodiment. The method 500 starts with a step S502, in which the processor 202 receives (e.g., via the transceiver) configuration information for at least one mobility candidate from a network node (e.g., gNB) serving a source cell. Again, each mobility candidate comprises at least two candidate cells. However, unlike the method 300, the configuration information in the method 500 comprises: (i) two or more SSB configurations each associated with one of said two or more candidate cells, and (ii) one or more early UL sync configurations each associated with one of said two or more SSB configurations and further associated with a TAG. The configuration information may be again provided to the UE 200 via dedicated signaling, such as RRC signaling (e.g., in an RRC reconfiguration message).

[0154] After the configuration information is received, the method 500 goes on to a step S504, in which the processor 202 receives (e.g., via the transceiver) an order for a PRACH preamble transmission. Unlike the order used in the method 300, the order used in the method 500may indicate not only an early UL sync configuration of interest among said one or more early UL sync configurations or an SSB configuration of interest among said two or more SSB configurations, but also the TAG associated with the early UL sync configuration of interest. A subsequent step S506 may resemble the step S306 if the order for the PRACH preamble transmission indicates the early UL sync configuration of interest or the SSB configuration of interest. However, if the order for the PRACH preamble transmission indicates the TAG associated with the early UL sync configuration of interest, the step S506 consists in determining, by the processor 202, the early UL sync configuration of interest from its association with the indicated TAG, and then determining the SSB configuration of interest from its association with the determined early UL sync configuration of interest.

[0155] The method 500 further goes to a step S508 (like the step S308), in which the processor 202 uses the SSB configuration of interest and the early UL sync configuration of interest to perform the PRACH preamble transmission.

[0156] FIG. 6 schematically illustrates the content of LTM configuration information 600 that may be used in the method 500. As shown in FIG. 6, each of two early UL sync configurations (i.e., " LTM-EarlyUL-SyncConfig" with ID 1 and " LTM-EarlyUL-SyncConfig" with ID 2) is associated with one of two SSB configurations (i.e., " LTM-SSB-Config" with ID 1 and " LTM-SSB-Config" with ID 2), e.g., by including the SSB configuration ID to the early UL sync configuration, or by providing an association list in the LTM configuration information 600. Furthermore, each of the two early UL sync configurations is associated with a TAG, e.g., through including the TAG ID in the early UL sync configuration.

[0157] Given the LTM configuration information 600, the order for the PRACH preamble transmission (e.g., PDCCH order) may indicate the early UL sync configuration ID, the SSB configuration ID or the TAG ID.

[0158] The method 500 requires less signaling compared to the method 300. However, if no early UL sync configuration is associated with the SSB configuration, then it is unclear how UE-based TA estimates related to such an SSB configuration will be associated with a TAG. FIG. 7 shows a flowchart of a method 700 for operating the UE 200 in accordance with a third example embodiment. The method 700 starts with a step S702, in which the processor 202 receives (e.g., via the transceiver) configuration information for at least one mobilitycandidate from a network node (e.g., gNB) serving a source cell. Again, each mobility candidate comprises at least two candidate cells. However, unlike the methods 300 and 500, the configuration information in the method 700 comprises: (i) one or more early UL sync configurations each associated with a TAG, and (ii) two or more SSB configurations each associated with one of said two or more candidate cells and further associated with the TAG of one of said one or more early UL sync configurations. The configuration information may be again provided to the UE 200 via dedicated signaling, such as RRC signaling (e.g., in an RRC reconfiguration message).

[0159] After the configuration information is received, the method 700 goes on to a step S704, in which the processor 202 receives (e.g., via the transceiver) an order for a PRACH preamble transmission. Unlike the orders used in the methods 300 and 500, the order used in the method 700 may indicate either an early UL sync configuration of interest among said one or more early UL sync configurations, or the TAG associated with each of the early UL sync configuration of interest and the SSB configuration of interest.

[0160] A subsequent step S706 may resemble the steps S306 and S506 if the order for the PRACH preamble transmission indicates the early UL sync configuration of interest. However, if the order for the PRACH preamble transmission indicates the TAG associated with each of the early UL sync configuration of interest and the SSB configuration of interest, the step S706 consists in determining, by the processor 202, the SSB and early UL sync configurations of interest from their association with the indicated TAG.

[0161] The method 700 further goes to a step S708 (like the steps S308 and S508), in which the processor 202 uses the SSB configuration of interest and the early UL sync configuration of interest to perform the PRACH preamble transmission.

[0162] FIG. 8 schematically illustrates the content of LTM configuration information 800 that may be used in the method 700. As shown in FIG. 8, each of two early UL sync configurations (i.e., " LTM-EarlyUL-SyncConfig" with ID 1 and " LTM-EarlyUL-SyncConfig" with ID 2) is associated with a TAG, e.g., through including the TAG ID in the early UL sync configuration. The LTM configuration information 800 also contains the association of each of two SSB configurations (i.e., " LTM-SSB-Config" with ID 1 and " LTM-SSB-Config" with ID 2) with the same TAGs.Given the LTM configuration information 800, the order for the PRACH preamble transmission (e.g., PDCCH order) may be implemented as one of the following options:

[0163] Option A: the PDCCH order may indicate the early UL sync configuration ID. In this option, the UE 200 may unambiguously determine the appropriate SSB configuration only if the TAG is associated with only one SSB configuration;

[0164] Option B: the PDCCH order may indicate the SSB configuration ID. In this option, the UE 200 may unambiguously determine the appropriate SSB configuration only if the TAG is associated with only one early UL sync configuration;

[0165] Option C: the PDCCH order may indicate the TAG ID. In this option, the UE 200 may unambiguously determine the appropriate SSB configuration only if the TAG is associated with only one SSB configuration.

[0166] Unlike the methods 300 and 500, no signaling reduction and more restricted configuration are provided in the method 700.

[0167] FIG. 9 shows a flowchart of a method 900 for operating the UE 200 in accordance with a fourth example embodiment. The method 900 starts with a step S902, in which the processor 202 receives (e.g., via the transceiver) configuration information for at least one mobility candidate from a network node (e.g., gNB) serving a source cell. In this embodiment, each mobility candidate comprises at least one candidate cell that is assumed to belong to a TAG. The method 900 is characterized in that the configuration information comprises: (i) two or more SSB configurations, and (ii) an early UL sync configuration associated with one of said two or more SSB configurations. At least one of the at least two SSB configurations is associated with the at least one candidate cell and at least one of the at least two SSB configurations is associated with at least one other cell belonging to the same TAG as the at least one candidate cell. The configuration information may be again provided to the UE 200 via dedicated signaling, such as RRC signaling (e.g., in an RRC reconfiguration message). After the configuration information is received, the method 900 goes on to a step S904, in which the processor 202 receives (e.g., via the transceiver) an order for a PRACH preamble transmission. Unlike the orders used in the methods 300 and 500, the order used in the method 900 does not indicate any SSB configuration of interest; it is used only as an indication for the UE 200 to perform the PRACH preamble transmission. Since there is onlyone early UL sync configuration initially configured for each mobility candidate, it should be used as the early UL sync configuration of interest. The SSB configuration of interest is determined by the processor 202, in a subsequent step S906, from its association with the single early UL sync configuration.

[0168] The method 900 further goes to a step S908 (like the steps S308, S508 and S708), in which the processor 202 uses the SSB configuration of interest and the early UL sync configuration to perform the PRACH preamble transmission.

[0169] It should be noted that the method 900 refers to a special case when the proposed signaling can be deployed to solve the problem of congested candidate PCell RACH. In one variant, the serving network node may provide to the UE 200 the early UL sync configuration of a candidate SCell (or another cell controlled by the serving network node) that belongs to the PTAG instead of that of the candidate PCell, along with the corresponding SSB configuration of the candidate SCell. Then, upon receiving the PDCCH order, which is the same as in the Rel-18 LTM mechanism, the UE 200 uses the early UL sync configuration of the candidate SCell and the corresponding SSBs to transmit the PRACH preamble to the candidate SCell and the received PRACH preamble on the network side is used to estimate the TA of the PTAG. In another variant, two or more early UL sync configurations of candidate cells under the same LTM candidate belonging to the PTAG are provided to the UE 200 and an indication in the PDCCH order is used by the UE 200 to determine which is to be used for early TA acquisition. In another variant, the SSB configuration for an SCell can be provided in the early UL sync configuration. This way, the association of the early UL sync configuration with the SSB configuration is directly determined from the presence of the SSB configuration in the early UL sync configuration. This option is attractive in cases where the SSB configuration is only provided to support the early TA acquisition using an SCell, e.g., no SSB measurement reporting for the SCell is required.

[0170] Given below are examples of modifications (put in bold) which may be used in the Rel-18 lEs to reflect the aspects of the embodiments disclosed herein.

[0171] Modified LTM-Candidate

[0172] LTM-Candidate-r20:: = SEQUENCE (

[0173] ltm-CandidateId-r18 LTM-CandidateId-r18,

[0174] ltm-CandidatePCI-r18 PhysCellId OPTIONAL, -- Need Mltm-SSB-ConfigList-r20 SEQUENCE (SIZE (1..maxSSBConfigsPerLTMCandidate)) OF LTM-SSB-Config-r20 OPTIONAL, -- Need M

[0175] ltm-CandidateConfig-r18 OCTET STRING (CONTAINING RRCReconfiguration) OPTIONAL, -- Need M

[0176] ltm-ConfigComplete-r18 ENUMERATED {true} OPTIONAL, -- Need R

[0177] ltm-EarlyUL-SyncConfigList-r20 SEQUENCE (SIZE (1..maxEarlyULSyncConfigsPerLTMCandidate)) OF EarlyUL-SyncConfig-r18 OPTIONAL, -- Need Rltm-EarlyUL-SyncConfigSULList-r20 SEQUENCE (SIZE (1..maxEarlyULSyncConfigsPerLTMCandidate)) OF EarlyUL-SyncConfig-r18 OPTIONAL, -- Need Rltm-SSB-ConfigIdToTAGID SEQUENCE (SIZE (1..maxSSBConfigsPerLTMCandidate)) OF SSB-ConfigId OPTIONAL, -- Need R?ltm-TCI-Info-r18 LTM-TCI-Info-r18 OPTIONAL, -- Need Mltm-NoResetID-r18 INTEGER (1..maxNrofLTM-Configs-plus1-r18) OPTIONAL, -- Need Mltm-UE-MeasuredTA-ID-r18 INTEGER (1..maxNrofLTM-Configs-plus1-r18) OPTIONAL, -- Need M

[0178] Modified LTM-SSB-Config

[0179] LTM-SSB-Conf ig-r20:: = SEQUENCE { Itm-SSB-Conf igId-r20 INTEGER (0.. maxSSBConf igsPerLTMCandidate-1 ), s sb- Frequen - - NR, subcarri er Si s i i i i 1 er Spacing, s sb- Periodi - D { ms 5, ms l O, ms 20, ms 4 0, ms 80, ms l 60, spare2, sparel } OPTIONAL, - ssb-PositionsInBurst-r18 CHOICE { shortBitmap BIT STRING (SIZE (4)), mediumBitmap BIT STRING (SIZE (8)), longBitmap BIT STRING (SIZE (64)) } OPTIONAL, — Need R s s - PBCH-BlockPower-rl I - 60.. 50 ) OPTIONAL,

[0180]

[0181] } Modified EarlyUL-SyncConfig

[0182] EarlyUL-SyncConfig-r20 ::= SEQUENCE {

[0183] earlyUL-SyncConfigId-r20 INTEGER (0..maxEarlyULSyncConfigsPerLTMCandidate-1) OPTIONAL, -- Need R?

[0184] ltm-ssb-ConfigId-r20 LTM-SSB-ConfigId-r20 OPTIONAL, -- Need R?

[0185] frequencyInfoUL-r18 FrequencyInfoUL,

[0186] rach-ConfigGeneric-r18 RACH-ConfigGeneric,

[0187] bwp-GenericParameters-r18 BWP,

[0188] ssb-PerRACH-Occasion-r18 ENUMERATED {oneEighth, oneFourth, oneHalf, one, two, four, eight, sixteen} OPTIONAL, -- Need R

[0189] prach-RootSequenceIndex-r18 CHOICE {

[0190] l839 INTEGER (0..837),

[0191] l139 INTEGER (0..137)

[0192] } OPTIONAL, — Need R

[0193] ltm-PRACH-SubcarrierSpacing-r18 SubcarrierSpacing OPTIONAL, -- Cond L139

[0194] n-TimingAdvanceOffset-r18 ENUMERATED {n0, n25600, n39936, spare1} OPTIONAL, -- Need R

[0195] ltm-tdd-UL-DL-ConfigurationCommon-r18 TDD-UL-DL-ConfigCommon OPTIONAL, -- Cond TDDltm-restrictedSetConfig-r18 ENUMERATED {unrestrictedSet, restrictedSetTypeA, restrictedSetTypeB} OPTIONAL -- Need R

[0196] ] ]

[0197] FIGs. 10A-10C schematically illustrate a signaling procedure 1000 used in the modified LTM mechanism according to one example embodiment. For convenience, the signaling procedure 1000 is divided into three phases: preparation, early UL synchronization, andexecution - and each of FIGs. 10A-10C refers to one of the three phases. Furthermore, the signaling procedure 1000 is shown to be implemented in a split (or disaggregated) RAN architecture, where a gNB is intended to be separated into a gNB Central Unit (CU) and two gNB Distributed Units (DUs), i.e., source DU and target DU 1. It is also assumed that the target DU 1 serves or controls a candidate PCell and at least one candidate SCell. In this case, the signaling diagram 1000 refers to inter-DU mobility without changing the CU. It is worth noting that the split RAN architecture is selected for illustrative purposes only and should not be construed as any limitation of the present disclosure - in some other embodiments, the signaling diagram 1000 may be implemented between a UE, a source gNB and a target gNB.

[0198] FIG. 10A refers to the preparation phase of the signaling diagram 1000, which starts with steps S1002-S1008 relating to the standard HO preparation procedure, whereupon their detailed description is omitted herein. It should be only noted that the CU decides, in the step S1006, that the UE needs to be handed over to the target DU 1, and sends a corresponding UE context setup request to the target DU 1 in the step S1008.

[0199] Further, each of the target DU 1 and the source DU provides one or more early UL sync configuration with SSB and TAG ID association to the CU in steps S1010 and S1014, respectively. The CU provides a list of other candidates' early UL sync configurations with SSB and TAG ID association to the source DU and the target DU 1 in steps S1012 and S1016, respectively. In steps S1018-S1024, the CU provides, through the source DU, RRC reconfiguration to the UE, which includes the early ULsyncconfiguration(s) with the SSB and TAG ID association. The UE completes the RRC reconfiguration and reports this to the source DU in a step S1026, and the source DU forwards a corresponding UL RRC message to the CU in a step S1028.

[0200] It should be noted in an inter-CU setup, a target DU (under a target CU different from a source CU) provides one or more early UL sync configurations with SSB and TAG ID association to the target CU. The target CU in turn provides it to the source CU which provides an RRC reconfiguration to the UE. The source CU provides, to the target CU, a list of early UL sync configurations with SSB and TAG ID association of other candidates that belong to a CU other than the target CU. The target CU subsequently provides a list of othercandidates' early UL sync configurations with SSB and TAG ID association to the target DU(s) under the target CU.

[0201] FIG. 10B refers to the early UL synchronization phase of the signaling procedure 1000, which is performed after the preparation phase shown in FIG. 10A. In a step S1030, the UE sends an L1 / L3 measurement report to the source DU which may (or may not) then forward it to the CU. In a step S1032, either the source DU or the CU uses the L1 / L3 measurement report to decide which early UL configuration to use for the UE. The UE is informed of the required early UL configuration (also referred to herein as the early UL configuration of interest) via a PDCCH order in a subsequent step S1034. In a step S1036, the UE determines the SSB configuration associated with the early UL configuration of interest and uses the determined SSB configuration to determine DL timing and RACH occasion(s). After that, the UE transmits a PRACH preamble to the SCell served by the target DU 1 in a next step S1038. The target DU 1 determines a TA to be used for the SCell and informs the CU about it, and the CU in turn provides this information to the source DU in a step S1040. The source DU then forwards the TA information to the UE in a step S1042, whereupon the early UL synchronization phase is over.

[0202] FIG. 10C refers to the (last) execution phase of the signaling procedure 1000, which is performed after the early UL synchronization phase shown in FIG. 10B is over. The execution phase covers two cell switch scenarios / cases: network-initiated and UE-initiated. The former is initiated by the source DU which sends a corresponding cell switch command to the UE in a step S1044 (with one or more TA(G) IDs), while the latter refers to a conditional HO which is performed by the UE when one or more previously configured HO conditions are fulfilled (see a step S1046). In a subsequent step S1048, the UE sends an RRC reconfiguration complete message to the PCell of the target DU 1. The target DU 1 then informs the CU about the completed RRC reconfiguration in a step S1050, whereupon the UE context release procedure is performed by the CU and the source DU in steps S1052 and S1054.

[0203] It should be noted that each step or operation of the methods 300, 500, 700 and 900, and / or the signaling procedure 1000, or any combinations of the steps or operations, can be implemented by various means, such as hardware, firmware, and / or software. As an example, one or more of the steps or operations described above can be embodied by processor executable instructions, data structures, program modules, and other suitabledata representations. Furthermore, the processor-executable instructions which embody the steps or operations described above can be stored on a corresponding data carrier and executed by the processor 202. This data carrier can be implemented as any computer-readable storage medium configured to be readable by said at least one processor to execute the processor executable instructions. Such computer-readable storage media can include both volatile and nonvolatile media, removable and non-removable media. By way of example, and not limitation, the computer-readable media comprise media implemented in any method or technology suitable for storing information. In more detail, the practical examples of the computer-readable media include, but are not limited to information-delivery media, RAM, ROM, EEPROM, flash memory or other memory technology, CD-ROM, digital versatile discs (DVD), holographic media or other optical disc storage, magnetic tape, magnetic cassettes, magnetic disk storage, and other magnetic storage devices.

[0204] Although the example embodiments of the present disclosure are described herein, it should be noted that any various changes and modifications could be made in the embodiments of the present disclosure, without departing from the scope of legal protection which is defined by the appended claims. In the appended claims, the word "comprising" does not exclude other elements or operations, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

CLAIMS1. A user equipment (UE) in a wireless communication network, comprising:at least one processor; andat least one memory storing instructions that, when executed by the at least one processor, cause the UE at least to:receive, from a network node serving a source cell, configuration information for at least one mobility candidate, each of the at least one mobility candidate comprising at least two candidate cells, the configuration information comprising:at least two synchronization signal block (SSB) configurations each associated with one of the at least two candidate cells; andat least one early uplink (UL) sync configuration each associated with one of the at least two SSB configurations and a timing advance group (TAG); receive an order for a physical random access channel (PRACH) preamble transmission, the orderforthe PRACH preamble transmission indicating one of: (i) an early UL sync configuration of interest among the at least one early UL sync configuration, (ii) an SSB configuration of interest among the at least two SSB configurations, and (iii) the TAG associated with the early UL sync configuration of interest;if the order for the PRACH preamble transmission indicates the early UL sync configuration of interest, determine the SSB configuration of interest among the at least two SSB configurations based on the configuration information, or if the order for the PRACH preamble transmission indicates the SSB configuration of interest, determine the early UL sync configuration of interest based on the configuration information, or if the order for the PRACH preamble transmission indicates the TAG associated with the early UL sync configuration of interest, determine the early UL sync configuration of interest and the SSB configuration of interest based on the configuration information; andbased on the SSB configuration of interest and the early UL sync configuration of interest, perform the PRACH preamble transmission.

2. The UE of claim 1, wherein the UE is further caused to receive, in response to the PRACH preamble transmission, a timing advance (TA) value.

3. The UE of claim 1 or 2, wherein each of the at least two SSB configurations has an identifier (ID) and the TAG of each of the at least one early UL sync configuration has an ID, and wherein each of the at least one early UL sync configuration comprises the ID of one of the at least two SSB configurations and the ID of the TAG associated with the early UL sync configuration.

4. The UE of any one of claims 1 to 3, wherein each of the at least two SSB configurations has an ID, each of the at least one early UL sync configuration has an ID, and the TAG of each of the at least one early UL sync configuration has an ID, and wherein the order for the PRACH preamble transmission comprises one of:the ID of the early UL sync configuration of interest;the ID of the SSB configuration of interest; andthe ID of the TAG associated with the early UL sync configuration of interest.

5. The UE of any one of claims 1 to 4, wherein each of the at least two SSB configurations has an ID, and each of the at least one early UL sync configuration has an ID, and wherein the configuration information further comprises an association list comprising at least two elements, each of the at least two elements having an index that indicates the ID of one of the at least one early ULsync configuration and a value that indicates the ID of one of the at least two SSB configurations which is associated with said one of the at least one early UL sync configuration.

6. The UE of any one of claims 1 to 5, wherein the UE is caused to receive, after that PRACH preamble transmission, a cell switch command comprising two or more TA values each being associated with a TAG.

7. The UE of any one of claims 1 to 6, wherein the UE is caused to receive the configuration information via a radio resource control (RRC) signaling.

8. A network node in a wireless communication network, comprising:at least one processor; andat least one memory storing instructions that, when executed by the at least one processor, cause the network node at least to:obtain configuration information for at least one mobility candidate, each of the at least one mobility candidate comprising at least two candidate cells, the configuration information comprising:at least two synchronization signal block (SSB) configurations each associated with one of the at least two candidate cells; andat least one early uplink (UL) sync configuration each associated with one of the at least two SSB configurations and a timing advance group (TAG); transmit the configuration information to a user equipment (UE); and transmit, to the UE, an order for a physical random access channel (PRACH) preamble transmission, the order for the PRACH preamble transmission indicating one of: (i) an early UL sync configuration of interest among the at least one early UL sync configuration, (ii) an SSB configuration of interest among the at least two SSB configurations, and (iii) the TAG associated with the early UL sync configuration of interest.

9. The network node of claim 8, wherein the at least one mobility candidate comprises a first mobility candidate supported by a first other network node and a second mobility candidate supported by a second other network node, and wherein the network node is further caused to:receive, from the first other network node, the at least two SSB configurations and the at least one early UL sync configuration for the first mobility candidate; receive, from the second other network node, the at least two SSB configurations and the at least one early UL sync configuration for the second mobility candidate;obtain the configuration information based on the at least two SSB configurations and the at least one early UL sync configuration for each of the first mobility candidate and the second mobility candidate;transmit, to the first other network node, the at least two SSB configurations and the at least one early UL sync configuration for the second mobility candidate; andtransmit, to the second other network node, the at least two SSB configurations and the at least one early UL sync configuration for the first mobility candidate.

10. A method for operating a user equipment (UE) in a wireless communication network, comprising:receiving, from a network node serving a source cell, configuration information for at least one mobility candidate, each of the at least one mobility candidate comprising at least two candidate cells, the configuration information comprising:at least two synchronization signal block (SSB) configurations each associated with one of the at least two candidate cells; andat least one early uplink (UL) sync configuration each associated with one of the at least two SSB configurations and a timing advance group (TAG); receiving an order for a physical random access channel (PRACH) preamble transmission, the orderforthe PRACH preamble transmission indicating one of: (i) an early UL sync configuration of interest among the at least one early UL sync configuration, (ii) an SSB configuration of interest among the at least two SSB configurations, and (iii) the TAG associated with the early UL sync configuration of interest;if the order for the PRACH preamble transmission indicates the early UL sync configuration of interest, determining the SSB configuration of interest among the at least two SSB configurations based on the configuration information, or if the order for the PRACH preamble transmission indicates the SSB configuration of interest, determining the early UL sync configuration of interest based on the configuration information, or if the order for the PRACH preamble transmission indicates the TAG associated with the early UL sync configuration of interest, determining the early UL sync configuration of interest and the SSB configuration of interest based on the configuration information; andbased on the SSB configuration of interest and the early UL sync configuration of interest, performing the PRACH preamble transmission.

11. A method for operating a network node in a wireless communication network, comprising:obtaining configuration information for at least one mobility candidate, each of the at least one mobility candidate comprising at least two candidate cells, the configuration information comprising:at least two synchronization signal block (SSB) configurations each associated with one of the at least two candidate cells; andat least one early uplink (UL) sync configuration each associated with one of the at least two SSB configurations and a timing advance group (TAG); transmitting the configuration information to a user equipment (UE); and transmitting, to the UE, an order for a physical random access channel (PRACH) preamble transmission, the order for the PRACH preamble transmission indicating one of: (i) an early UL sync configuration of interest among the at least one early UL sync configuration, (ii) an SSB configuration of interest among the at least two SSB configurations, and (iii) the TAG associated with the early UL sync configuration of interest.

12. A computer program product comprising a computer-readable storage medium, wherein the computer-readable storage medium stores a computer code which, when executed by at least one processor, causes the at least one processor to perform the method according to claim 10.

13. A computer program product comprising a computer-readable storage medium, wherein the computer-readable storage medium stores a computer code which, when executed by at least one processor, causes the at least one processor to perform the method according to claim 11.

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