WO2025189178A1

WO2025189178A1 - Enabling early synchronization in inter-central unit lower layer triggered mobility in dual connectivity

Info

Publication number: WO2025189178A1
Application number: PCT/US2025/019079
Authority: WO
Inventors: Chih-Hsiang Wu; Ching-Jung Hsieh
Original assignee: Google LLC
Current assignee: Google LLC
Priority date: 2024-03-08
Filing date: 2025-03-08
Publication date: 2025-09-12
Anticipated expiration: 2026-09-08
Also published as: WO2025189178A8

Abstract

A candidate secondary node (C-SN) receives, from a master node (MN) that provides, with a source secondary node (S-SN), dual connectivity (DC) to a user equipment (UE), an SN request including at least one of (i) a lower layer triggered mobility (LTM) indication for a candidate cell associated with the C-SN or (ii) a request for early synchronization information for the candidate cell. The C-SN transmits, to the MN and in response to the SN request, an SN request acknowledgement including the early synchronization information.

Description

ENABLING EARLY SYNCHRONIZATION IN INTER-CENTRAL UNIT LOWER LAYER TRIGGERED MOBILITY IN DUAL CONNECTIVITY

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to and the benefit of the filing date of provisional U.S. Patent Application No. 63/563,306 entitled “Enabling Early Synchronization in InterCentral Unit Lower Layer Triggered Mobility in Dual Connectivity,” filed on March 8, 2024, and to the provisional U.S. Patent Application No. 63/567,413 entitled “Enabling Early Synchronization in Inter-Central Unit Lower Layer Triggered Mobility in Dual Connectivity,” filed on March 19, 2024. The entire content of the provisional applications is hereby expressly incorporated herein by reference.

FIELD OF THE DISCLOSURE

[0002] This disclosure relates to wireless communications and, more particularly, to enabling early uplink synchronization in inter-central unit (inter-CU) lower layer triggered mobility (LTM) in dual connectivity (DC).

BACKGROUND

[0003] This background description is provided for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.

[0004] Generally speaking, a base station operating a cellular radio access network (RAN) communicates with a user equipment (UE) using a certain radio access technology (RAT) and multiple layers of a radio protocol stack. For example, the physical layer (PHY) of a RAT provides transport channels to the Medium Access Control (MAC) sublayer, which in turn provides logical channels to the Radio Link Control (RLC) sublayer, and the RLC sublayer in turn provides data transfer services to the Packet Data Convergence Protocol (PDCP) sublayer.

[0005] The PDCP sublayer of the radio protocol stack provides services such as transfer of user-plane data, ciphering, integrity protection, etc. For example, the PDCP layer defined for the Evolved Universal Terrestrial Radio Access (EUTRA) radio interface (see 3GPP specification TS 36.323) and New Radio (NR) (see 3GPP specification TS 38.323) provides sequencing of protocol data units (PDUs) in the uplink direction (from a user device, also known as a user equipment (UE), to a base station) as well as in the downlink direction (from the base station to the UE). Further, the PDCP sublayer provides signaling radio bearers (SRBs) and data radio bearers (DRBs) to the Radio Resource Control (RRC) sublayer. Generally speaking, the UE and a base station can use SRBs to exchange RRC messages as well as non-access stratum (NAS) messages, and the UE and base station can use DRBs to transport data on a user plane.

[0006] The UE can concurrently utilize resources of multiple RAN nodes (e.g., base stations or components of a distributed base station), interconnected by a backhaul. When these multiple RAN nodes support different radio access technologies (RATs), this type of connectivity is referred to as Multi-Radio Dual Connectivity (MR-DC). When a UE operates in MR-DC, one base station operates as a master node (MN) that covers a primary cell (PCell), and the other base station operates as a secondary node (SN) that covers a primary secondary cell (PSCell). The UE communicates with the MN (via the PCell) and the SN (via the PSCell). In other scenarios, the UE utilizes resources of one base station at a time. One base station and/or the UE can determine that the UE should establish a radio connection with a second base station. For example, one base station can determine to hand the UE over to the second base station, and the base station initiate a handover procedure.

[0007] When the UE moves from the coverage area of one cell to another cell in a RAN, the RAN performs a serving cell change by configuring the UE to transmit Layer 3 (L3) measurement results. Based on L3 measurement results received from the UE, the RAN transmits an RRC reconfiguration message configuring Reconfiguration with Synchronization (e.g., the RRC reconfiguration message includes a ReconfigurationWithSync IE) for change of the serving cell (e.g., PCell or PSCell). When the UE operates in carrier aggregation (CA) of at least one secondary cell (SCell) with the PCell or PSCell, the RAN has to release the at least one SCell due to the change of the PCell or PSCell. The serving cell change involves complete L2 (and LI) resets, leading to longer latency, larger overhead, and longer interruption time. To address these concerns, 3GPP recently proposed procedures referred to as lower layer trigger mobility (LTM).

[0008] During RAN communication with the UE via a serving cell associated with the base station, the base station receives one or more layer 3 (e.g., RRC) measurement results from the UE. Based on the layer 3 (L3) measurement result(s), the base station determines to configure an LTM candidate cell for LTM cell switch. To configure the LTM candidate cell for the UE, the base station transmits an LTM candidate configuration to the UE via RRC signaling. Subsequently, the base station receives one or more layer 1 (LI) measurement results from the UE. Based on the one or more LI measurement result(s), the base station determines that the LTM candidate cell qualifies to be a serving cell for the UE. After making this determination, the base station transmits an LTM Cell Switch Command to the UE to command the UE to perform the LTM cell switch to the LTM candidate cell. The UE performs a cell change from the serving cell to the LTM candidate cell in response to the LTM Cell Switch Command. The base station includes a central unit (CU) and one or more distributed units (DUs). One of the DU(s) operating the serving cell is referred to as a serving DU. If the LTM candidate cell is operated by the serving DU, the LTM cell switch is an intra- CU intra-DU LTM cell switch. If the LTM candidate cell is operated by a candidate DU (C- DU) in the DU(s), the LTM cell switch is an intra-CU inter-DU LTM cell switch. 3GPP has enabled the intra-CU intra-DU LTM cell switch and intra-CU inter-DU LTM cell switch in Release 18 specifications. However, it is not clear how devices should handle inter-CU LTM cell switches. Lor example, if is not clear how devices should handle cases in which the LTM candidate cell is operated within a candidate DU of a different CU. Eurthermore, it is not clear how inter-CU LTM cell switches should be handled in DC scenarios.

SUMMARY

[0009] An example embodiment of these techniques is a method implemented in a candidate secondary node (C-SN). The method comprises receiving, from a master node (MN) that provides, with a source secondary node (S-SN), dual connectivity (DC) to a user equipment (UE), an SN request including at least one of (i) a lower layer triggered mobility (LTM) indication for a candidate cell associated with the C-SN or (ii) a request for early synchronization information for the candidate cell; and transmitting, to the MN and in response to the SN request, an SN request acknowledgement including the early synchronization information.

[0010] Another embodiment of these techniques is a method implemented in a source secondary node (S-SN). The method comprises determining a candidate cell for lower layer triggered mobility (LTM) of a user equipment (UE) that communicates in dual connectivity (DC) with a master node (MN) and the S-SN, the candidate cell being associated with a candidate secondary node (C-SN); and transmitting, to the MN, an SN required message including a request for early synchronization information for the candidate cell. [0011] Yet another example embodiment of these techniques is a base station comprising processing hardware and configured to implement one of the methods above.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Fig. 1A is a block diagram of an example system in which a radio access network (RAN) and a user equipment (UE) can implement the techniques of this disclosure for managing inter-central unit (CU) lower layer triggered mobility (LTM) in dual connectivity (DC);

[0013] Fig. IB is another block diagram of an example system in which (the RAN) and a UE can implement the techniques of this disclosure for managing inter-CU LTM DC;

[0014] Fig. 1C is a block diagram of an example base station including a CU and a distributed unit (DU) that can operate in the system of Fig. 1A or Fig. IB;

[0015] Fig. 2A is a block diagram of an example protocol stack according to which the UE of Fig. 1A communicates with base stations;

[0016] Fig. 2B is a block diagram of an example protocol stack according to which the UE of Fig. 1A communicates with a CU and a DU;

[0017] Fig. 3 is a messaging sequence of an example intra-DU LTM scenario;

[0018] Fig. 4 is a messaging sequence of an example inter-DU LTM scenario;

[0019] Fig. 5 is a message sequence diagram of an example inter-CU LTM scenario;

[0020] Fig. 6A is a message sequence diagram of an example inter-CU LTM scenario with a UE in DC with an MN and an SN;

[0021] Fig. 6B is a message sequence diagram of a second example inter-CU LTM scenario with a UE in DC with an MN and an SN;

[0022] Fig. 6C is a message sequence diagram of a third example inter-CU LTM scenario with a UE in DC with an MN and an SN;

[0023] Fig. 6D is a message sequence diagram of a fourth example inter-CU LTM scenario with a UE in DC with an MN and an SN;

[0024] Fig. 7 is a flow diagrams of an example method for supporting LTM in dual connectivity (DC) scenarios, which a candidate secondary node (C-SN) can implement; [0025] Fig. 8 is a flow diagrams of an example method for supporting LTM in DC scenarios, which can be implemented in a candidate central unit (C-CU);

[0026] Fig. 9A is a flow diagram of a first example method for supporting LTM in DC scenarios, which can be implemented in a candidate secondary node (C-SN);

[0027] Fig. 9B is a flow diagram of a second example method for supporting LTM in DC scenarios, which can be implemented in a C-SN;

[0028] Fig. 10A is a flow diagram of a first example method for supporting LTM in DC scenarios, which can be implemented in a C-CU;

[0029] Fig. 10B is a flow diagram of a second example method for supporting LTM in DC scenarios, which can be implemented in a C-CU;

[0030] Fig. 11 A is a flow diagram of a third example method for supporting LTM in DC scenarios, which can be implemented in a C-CU;

[0031] Fig. 1 IB is a flow diagram of a fourth example method for supporting LTM in DC scenarios, which can be implemented in a C-CU;

[0032] Fig. 12 is a flow diagram of an example method for supporting LTM in DC scenarios, which can be implemented in a source secondary node (S-SN);

[0033] Fig. 13 is a flow diagram of an example method for supporting LTM in DC scenarios, which can be implemented in a source central unit (S-CU);

[0034] Fig. 14 is a flow diagram of an example method for supporting LTM in DC scenarios, which can be implemented in a master node (MN);

[0035] Fig. 15 is a flow diagram of an example method for supporting LTM in DC scenarios, which can be implemented in a central unit (CU);

[0036] Fig. 16 is a flow diagram of an example method for supporting LTM in DC scenarios, which can be implemented in an S-SN;

[0037] Fig. 17 is a flow diagram of an example method for supporting LTM in DC scenarios, which can be implemented in an S-SN;

[0038] Fig. 18 is a flow diagram of an example method for managing early synchronization for LTM cell switches in DC, which can be implemented in a C-SN; sand [0039] Fig. 19 is a flow diagram of an example method for managing early synchronization for LTM cell switches in DC, which can be implemented in an S-SN.

DETAILED DESCRIPTION OF THE DRAWINGS

[0040] Techniques of this disclosure are available for performing LTM cell switches between different SNs while a UE is in DC mode and methods for enabling early synchronization for inter-cell LTM cell switch in these and similar scenarios.

[0041] Referring first to Fig. 1A, an example wireless communication system 100 can implement one or more of these techniques. The wireless communication system 100 includes a UE 102, a base station (BS) 104A, a base station 106A and a core network (CN) 110. The base stations 104A and 106A can operate in a radio access network (RAN) 105The UE 102 initially connects to the base station 104A. In some scenarios, the base station 104A can perform an SN addition to configure the UE 102 to operate in dual connectivity (DC) with the base station 104A and the base station 106A. The base stations 104A and 106A operate as an MN and an SN for the UE 102, respectively.

[0042] In various configurations of the wireless communication system 100, the base station 104A can operate as a master eNB (MeNB) or a master gNB (MgNB), and the base station 106A can operate as a secondary gNB (SgNB). The UE 102 can communicate with the base station 104A and the base station 106A via the same RAT such as EUTRA or NR, or different RATs. When the base station 104A is an MeNB and the base station 106 A is an SgNB, the UE 102 can be in EUTRA-NR DC (EN-DC) with the MeNB and the SgNB.

[0043] In some cases, an MeNB or an SeNB is implemented as an ng-eNB rather than an eNB. When the base station 104A is a Master ng-eNB (Mng-eNB) and the base station 106 A is an SgNB, the UE 102 can be in next generation (NG) EUTRA-NR DC (NGEN-DC) with the Mng-eNB and the SgNB. When the base station 104A is an MgNB and the base station 106 A is an SgNB, the UE 102 may be in NR- NR DC (NR-DC) with the MgNB and the SgNB. When the base station 104A is an MgNB and the base station 106 A is a Secondary ng-eNB (Sng-eNB), the UE 102 may be in NR-EUTRA DC (NE-DC) with the MgNB and the Sng-eNB.

[0044] In the scenarios in which the UE 102 hands over from the base station 104A to the base station 106A, the base stations 104A and 106A operate as the source base station (S-BS) and as a target base station (T-BS), respectively. The UE 102 can operate in DC with the base station 104A and an additional base station (not shown in Fig. 1A) for example prior to the handover. The UE 102 can continue to operate in DC with the base station 106 A and the additional base station, or the UE can operate in single connectivity (SC) with the base station 106 A, after completing the handover. In the latter case, the base stations 104A and 106 A operate as a source MN (S-MN) and a target MN (T-MN), respectively.

[0045] A core network (CN) 110 can be an evolved packet core (EPC) 111 or a fifthgeneration core (5GC) 160, both of which are depicted in Fig. 1A, although the CN 110 can support other cores. The base station 104A can be an eNB supporting an SI interface for communicating with the EPC 111, an ng-eNB supporting an NG interface for communicating with the 5GC 160, or a gNB that supports an NR radio interface as well as an NG interface for communicating with the 5GC 160. To directly exchange messages with each other during the scenarios discussed below, the base stations 104A and 106A can support an X2 or an Xn interface.

[0046] As illustrated in Fig. 1A, the base station 104A supports cell 124A, and the base station 106A supports a cell 126A. The cells 124A and 126A can partially overlap, so that the UE 102 can communicate in DC with the base station 104A and the base station 106 A, where one of the base stations 104A and 106A is an MN and the other is an SN. The base station 104A can support additional cell(s) such as cell 124B, and the base station 106A can support additional cell(s) (not shown in Fig. 1A). The cells 124A, 124B can partially overlap, so that the UE 102 can communicate in carrier aggregation (CA) with the base station 104A. The base station 104A can operate the cells 124A, 124B and 124C via one or more transmit and receive points (TRPs). More particularly, when the UE 102 is in DC with the base station 104A and the base station 106 A, one of the base stations 104A and 106 A operates as an MeNB, an Mng-eNB or an MgNB, and the other operates as an SgNB or an Sng-eNB.

[0047] In general, the RAN 105 can include any suitable number of base stations supporting NR cells and/or EUTRA cells. More particularly, the EPC 111 or the 5GC 160 can be connected to any suitable number of base stations supporting NR cells and/or EUTRA cells. An example configuration in which the CN 110 is connected to additional base stations is discussed below with reference to Fig. IB. Although the examples below refer specifically to specific CN types (EPC, 5GC) and RAT types (5G NR and EUTRA), in general the techniques of this disclosure also can apply to other suitable radio access and/or core network technologies such as sixth generation (6G) radio access and/or 6G core network or 5G NR- 6G DC.

[0048] With continued reference to Fig. 1A, the base station 104A includes processing hardware 130 that can include one or more general-purpose processors (e.g., CPUs) and a non-transitory computer-readable memory storing instructions that the one or more general- purpose processors execute. Additionally or alternatively, the processing hardware 130 can include special-purpose processing units. The processing hardware 130 can include a PHY controller 132 configured to transmit data and control signals on physical downlink (DL) channels and DL reference signals to one or more user devices (e.g., UE 102) via one or more cells (e.g., the cell(s) 124A, and/or 124B ) and/or one or more TRPs. The PHY controller 132 is also configured to receive data and control signals on physical uplink (UL) channels and/or UL reference signals from the one or more user devices via one or more cells (e.g., the cell(s) 124A, and/orl24B) and/or one or more TRPs. The processing hardware 130 in an example implementation includes a MAC controller 134 configured to perform MAC functions with one or more user devices. The MAC functions include a random access (RA) procedure, managing UL timing advance (TA) for the one or more user devices, and/or communicating UL/DL MAC PDUs with the one or more user devices. The MAC functions include lower layer triggered mobility (LTM, also referred to herein as Ll/L2-triggered mobility or low layer triggered mobility) related functions as described below. In examples, LTM functions receive control through an LTM controller 137 in coordination or through signaling with the MAC controller 134. The processing hardware 130 can further include an RRC controller 136 to implement procedures and messaging at the RRC sublayer of the protocol communication stack. Lor example, the RRC controller 136 may be configured to support RRC messaging associated with handover procedures, and/or to support the necessary operations when the base station 104A operates as an MN relative to an SN or as an SN relative to an MN. When the base station 104A is a distributed base station, a CU LTM Controller 137A can operate in a CU, and a DU LTM Controller 137B can operate in a DU (see Eig. 1C). The base station 106A can include processing hardware 140 that is similar to processing hardware 130. In particular, components 142, 144, 146, and 147 can be similar to the components 132, 134, 136, and 137, respectively.

[0049] The UE 102 includes processing hardware 150 that can include one or more general-purpose processors such as CPUs and non-transitory computer-readable memory storing machine-readable instructions executable on the one or more general-purpose processors, and/or special-purpose processing units. The PHY controller 152 is also configured to receive data and control signals on physical DL channels and/or DL reference signals from the base station 104A or 106A via one or more cells (e.g., the cell(s) 124A, 124B, and/or 126A) and/or one or more TRPs. The PHY controller 152 is also configured to transmit data and control signal on physical UL channels and/or UL reference signals to the base station 104A or 106A via one or more cells (e.g., the cell(s) 124A, 124B, and/or 126A) and/or one or more TRPs. The processing hardware 150 in an example implementation includes a MAC controller 154 configured to perform MAC functions with base station 104A or 106A. For example, the MAC functions include a random-access procedure, managing UL timing advance for the one or more user devices, and communicating UL/DL MAC PDUs with the base station 104A or 106A. In another example, the MAC functions include LTM related functions as described below. The processing hardware 150 can further include an RRC controller 156 to implement procedures and messaging at the RRC sublayer of the protocol communication stack.

[0050] In operation, the UE 102 in DC can use a radio bearer (e.g., a DRB or an SRB) that at different times terminates at the MN 104A or the SN 106A. The UE 102 can apply one or more security keys when communicating on the radio bearer, in the uplink (UL) (from the UE 102 to a base station) and/or downlink (from a base station to the UE 102) direction. UEs can use several types of SRBs and DRBs. When operating in DC, the cells associated with the base station operating the MN define a master cell group (MCG), and the cells associated with the base station operating as SN define the secondary cell group (SCG). For example, a first type of SRB resource, referred to as SRB1 resources, carry RRC messages, which in some cases include NAS messages over the dedicated control channel (DCCH). A second type of SRB resource, referred to as SRB2 resources, support RRC messages that include logged measurement information or NAS messages, also over the DCCH but with lower priority than SRB1 resources. More generally, SRB1 and SRB2 resources allow the UE and the MN to exchange RRC messages related to the MN and to embed RRC messages related to the SN. The SRB1 and SRB2can be referred to as MCG SRBs. SRB3 resources allow the UE and the SN to exchange RRC messages related to the SN and can be referred to as SCG SRBs. Split SRBs allow the UE to exchange RRC messages directly with the MN via lower layer resources of the MN and the SN. Further, DRBs using the lower-layer resources of only the MN can be referred as MCG DRBs, DRBs using the lower-layer resources of only the SN can be referred as SCG DRBs, and DRBs using the lower-layer resources of both the MCG and the SCG can be referred to as split DRBs.

[0051] Fig. IB is another block diagram of an example system 100B in which a radio access network (RAN) and a user device can implement the techniques of this disclosure for managing procedures (e.g., LTM procedures and inter-CU LTM procedures) related to an MN or an SN. Fig. IB is similar to Fig. 1A. Fig. IB depicts CN 110 having connected thereto additional base stations 104A, 104B, 106 A and 106B communicating in cells 124A, 124B, 126 A, and 126B). Cell 125 can overlap one or more others of cells 124A, 124B, 126 A, and 126B.

[0052] Fig. 1C depicts an example distributed or disaggregated implementation of any one or more of the base station(s) shown in Fig. 1A or Fig. IB (e.g., base station 104A,106A or 106B). In this implementation, the base station i includes a central unit (CU) 172 and one or more distributed units (DUs) 174. The CU 172 includes processing hardware such as one or more general-purpose processors (e.g., CPUs) and non-transitory computer-readable memory storing machine-readable instructions executable on the one or more general-purpose processors, and/or special-purpose processing units. In one example, the CU 172 includes the processing hardware 130. In another example, the CU 172 includes the processing hardware 140 (Fig. 1A). The processing hardware 140 includes an SN RRC controller that can be similar to RRC Controller 146 (Fig. 1A) and configured to manage or control one or more RRC configurations and/or RRC procedures when the base station 106 A operates as an SN.

[0053] Each of the DUs 174A includes processing hardware that can include one or more general-purpose processors (e.g., CPUs) and non-transitory computer-readable memory storing machine-readable instructions executable on the one or more general-purpose processors, and/or special-purpose processing units. In some examples, the processing hardware includes a medium access control (MAC) controller (e.g., MAC controller 134, 144 (Fig. 1A)) configured to manage or control one or more MAC operations or procedures (e.g., a random access procedure) and a radio link control (RLC) controller configured to manage or control one or more RLC operations or procedures when the base station 106A operates as an MN or an SN. The process hardware may also include a physical layer controller configured to manage or control one or more physical layer operations or procedures.

[0054] In some implementations, the CU 172 can include a logical node CU-CP 172A that hosts the control plane of the CU 172. The CU 172 can also include logical node(s) CU-UP 172B that hosts the user plane of the CU 172. The CU-CP 172A can transmit control information (e.g., RRC messages, Fl application protocol messages), and the CU-UP 172B can transmit the data packets (e.g., SDAP PDUs or Internet Protocol packets).

[0055] The CU-CP 172A can be connected to multiple CU-UP 172B through the El interface. The CU-CP 172A selects the appropriate CU-UP 172B for the requested services for the UE 102. In some implementations, a single CU-UP 172B can be connected to multiple CU-CP 172A through the El interface. The CU-CP 172A can be connected to one or more DU(s) 174 through an Fl-C or Wl-C interface. The CU-UP 172B can be connected to one or more DU 174 through an Fl-U or Wl-U interface under the control of the same CU-CP 172A. In some implementations, one DU 174 can be connected to multiple CU-UP 172B under the control of the same CU-CP 172A. In such implementations, the connectivity between a CU-UP 172B and a DU 174 is established by the CU-CP 172A using Bearer Context Management functions.

[0056] Fig. 2A illustrates, in a simplified manner, an example protocol stack 200 according to which the UE 102 can communicate with an eNB/ng-eNB 230 or a gNB 232 (e.g., one or more of the base stations 104A, 106A).

[0057] In the example stack 200, a physical layer (PHY) 202A of EUTRA provides transport channels to the EUTRA MAC sublayer 204A, which in turn provides logical channels to the EUTRA RLC sublayer 206A. The EUTRA RLC sublayer 206A in turn provides RLC channels to an EUTRA PDCP sublayer 208 and, in some cases, to an NR PDCP sublayer 210. Similarly, the NR PHY 202B provides transport channels to the NR MAC sublayer 204B, which in turn provides logical channels to the NR RLC sublayer 206B. The NR RLC sublayer 206B in turn provides data transfer services to the NR PDCP sublayer 210. The NR PDCP sublayer 210 in turn can provide data transfer services to Service Data Adaptation Protocol (SDAP) 212 or a radio resource control (RRC) sublayer (not shown in Fig. 2A). The UE 102, in some implementations, supports both the EUTRA and the NR stack as shown in Fig. 2A, to support handover between EUTRA and NR base stations and/or to support DC over EUTRA and NR interfaces. Further, as illustrated in Fig. 2A, the UE 102 can support layering of NR PDCP 210 over EUTRA RLC 206 A, and SDAP sublayer 212 over the NR PDCP sublayer 210.

[0058] The EUTRA PDCP sublayer 208 and the NR PDCP sublayer 210 receive packets (e.g., from an Internet Protocol (IP) layer, layered directly or indirectly over the PDCP layer 208 or 210) that can be referred to as service data units (SDUs), and output packets (e.g., to the RLC layer 206A or 206B) that can be referred to as protocol data units (PDUs). Except where the difference between SDUs and PDUs is relevant, this disclosure for simplicity refers to both SDUs and PDUs as “packets.”

[0059] On a control plane, the EUTRA PDCP sublayer 208 and the NR PDCP sublayer 210 can provide signaling radio bearers (SRBs) or an RRC sublayer (not shown in Fig. 2A) to exchange RRC messages or non-access-stratum (NAS) messages, for example. On a user plane, the EUTRA PDCP sublayer 208 and the NR PDCP sublayer 210 can provide Data Radio Bearers (DRBs) to support data exchange. Data exchanged on the NR PDCP sublayer 210 can be SDAP PDUs, Internet Protocol (IP) packets or Ethernet packets.

[0060] Fig. 2B illustrates, in a simplified manner, an example protocol stack 250, on which the UE 102 can communicate with a DU (e.g., DU 174) and a CU (e.g., CU 172). The radio protocol stack 200 is functionally split as shown by the radio protocol stack 250 in Fig. 2B. The CU at any of the base stations 104A or 106A can hold all the control and upper layer functionalities (e.g., RRC 214, SDAP 212, NR PDCP 210), while the lower layer operations (e.g., NR RLC 206B, NR MAC 204B, and NR PHY 202B) are delegated to the DU. To support connection to a 5GC, NR PDCP 210 provides SRBs to RRC 214, and NR PDCP 210 provides DRBs to SDAP 212 and SRBs to RRC 214.

[0061] Next, with reference to Figs. 3-5, several example scenarios in which a base station operating in the system of Fig. 1 A transmits a configuration to the UE 102 and later activates a configuration for communication between the UE 102 and base station. Generally speaking, similar events in Figs. 3-5 are labeled with similar reference numbers that share two least significant digits, with differences discussed below where appropriate. For example, event 302 is similar to event 402 of Fig. 4 and event 502 of Fig. 5, event 390 is similar to event 490 of Fig. 4 and event 590 of Fig. 5. With the exception of the differences shown in the figures and discussed below, any of the other implementations discussed with respect to a particular event (e.g., for messaging and processing) may apply to events labeled with similar reference numbers in other figures.

[0062] Referring first to Fig. 3, in a scenario 300, the base station 104A includes a CU 172 and a DU 174 and the DU 174 operates the cell 124A. The UE 102 initially communicates 302 with the DU 174 on a serving cell (e.g., the cell 124A), using a serving DU configuration, and communicates 302 with the CU 172 via the DU 174, using a serving CU configuration. The DU 174 is a serving or a source DU (S-DU) for the UE 102. In other words, the DU 174 is a serving DU (S-DU) that communicates with the UE 102. In some implementations, the UE 102 in carrier aggregation (CA) communicates with the DU 174 on the cell 124A and other cell(s) (not shown in Fig. 1A) using the serving DU configuration. The DU 174 operates the other cell(s). The cell 124A and/or the other cell(s) are serving cell(s) for the UE 102. In other implementations, the UE 102 communicates with the DU 174 on the cell 124A only (e.g., not on other cell(s)). In some implementations, the UE 102 communicates with the DU 174 on the cell 124A and/or other cell(s) (i.e., serving cell(s)) via one or multiple TRPs. In the following description, events 394, 324, 350, 352, 354, and 326 occur on the serving cell(s). In some implementations, the cell 124A can be a PCell. In such cases, the other cell(s) include SCell(s) and/or additional cell(s) associated with the PCell or an SCell. In other implementations, the cell 124A can be an SCell, and one of the other cell(s) is a PCell. In such cases, the additional cell(s) include(s) SCell(s) and/or additional cell(s) associated with the PCell or an SCell. In the following description, the base station 104A can include the DU 174, the CU 172 or both the DU 174 and CU 172.

[0063] In the event 302, the UE 102 can transmit UL PDUs and/or UL control signals to the base station 104A on the cell 124A and/or other cell(s) via one or multiple TRPs. In some implementations, the UE 102 communicates UL PDUs and/or DL PDUs with the base station 104A via radio bearers which can include SRBs and/or DRB(s). The base station 104A can configure the radio bearers to the UE 102. In some implementations, UL control signals include UL control information, channel state information, hybrid automatic repeat request (HARQ) acknowledgements (ACKs), HARQ negative ACKs, scheduling request(s), and/or sounding reference signal(s). Similarly, the UE 102 can receive DL PDUs and/or DL control signals from the base station 104A on the cell 124A and/or other cell(s) via one or multiple TRPs. In some implementations, the DL control signals include downlink control information (DCIs) and reference signals (e.g., synchronization signal block(s), channel state information reference signal(s) (CSI-RS(s)), and/or tracking reference signal(s)). The base station 104A can transmit the DCIs on physical downlink control channel(s) (PDCCH(s)) monitored by the UE 102, on the cell 124A, and/or on other cell(s) via one or multiple TRPs.

[0064] In some implementations, the serving DU configuration includes physical layer configuration parameters, MAC configuration parameters, and/or RLC configuration parameters. In some implementations, the serving DU configuration includes at least one first non-LTM transmission configuration indicator (TCI) state configuration for the serving cell(s). In some implementations, the DU 174 can transmit these configuration parameters and/or the first non-LTM TCI state configuration(s) to the CU 172. The CU 172 generates one or more messages (e.g., RRC reconfiguration message(s)) including the configuration parameters and/or the first non-LTM TCI state configuration(s). The CU 172 transmits these one or more messages to the UE 102 via the DU 174. In other implementations, the DU 174 transmits the configuration parameters and/or the first non-LTM TCI state configuration(s) to the UE 102 directly. In some implementations, the serving DU configuration is CellGroupConfig IE defined in 3GPP specification 38.331. In other implementations, the serving DU configuration includes configuration parameters in the CellGroupConfig IE. In some implementations, the serving CU configuration includes PDCP configuration parameters, measurement configuration parameters, and/or radio bearer configuration parameters. In some implementations, the serving CU configuration includes a MeasConfig IE and/or a RadioBearerConfig IE defined in 3GPP specification 38.331, or the serving CU configuration includes configuration parameters in the MeasConfig IE and/or the RadioBearerConfig IE. The radio configuration parameters or the RadioBearerConfig IE configures one or more DRB(s). In some implementations, the serving DU configuration includes a CSl-MeasConfig IE or configuration parameters for channel state information (CSI) measurement and reporting. In other implementations, the serving CU configuration includes a CSl-MeasConfig IE or configuration parameters for CSI measurement and reporting. In some implementations, the UE 102 receives the serving CU configuration or the configuration parameters in the serving CU configuration from the CU 172 via the DU 174. In other implementations, the UE 102 receives a portion of the serving CU configuration and/or a portion of the serving DU configuration from a base station other than the base station 104A, and the UE receives the remaining portion of these configuration parameters from the base station 104A.

[0065] In some implementations, the DU 174 and the UE 102 communicate with each other using first non-LTM TCI state configuration(s), e.g., in the events 302, 318, 320, 324,350, 354, and/or 326. In some implementations, the DU 174 transmits at least one first non-LTM TCI States Activation/Deactivation command (e.g., a MAC control element (CE)) to the UE 102 to activate the first non-LTM TCI state configuration(s). The UE 102 activates the first non-LTM TCI state configuration(s) in response to the first non-LTM TCI States Activation/Deactivation command(s). In some implementations, the DU 174 includes a serving cell ID (e.g., a serving cell index) in each of the first non-LTM TCI States Activation/Deactivation command(s) to identify the first non-LTM TCI state configuration(s). Each of the serving cell ID(s) indicates a respective serving cell of the serving cell(s). In some implementations, the serving DU configuration includes the serving cell ID(s) and configures association(s) between the serving cell ID(s) and the first non-LTM TCI state configuration(s).

[0066] While communicating with the base station 104A, the UE 102 transmits 304 at least one measurement report to the DU 174. In some implementations, the measurement report(s) includes measurement results for a serving cell (e.g., the cell 124A) of the UE 102 and/or at least one non-serving cell. For each of the measurement report(s), the DU 174 transmits 306 a DU-to-CU message including the measurement report to the CU 172. In some implementations, the DU-to-CU message(s) is/are Fl application protocol (F1AP) message(s) (e.g., UL RRC Message Transfer message(s)). The at least one serving cell includes the cell 124A and/or other cell(s), and the at least one non-serving cell includes the cell 124B and/or additional cell(s) not shown in Fig. 1A. In some implementations, the serving CU configuration includes at least one measurement configuration. In accordance with the measurement configuration(s), the UE 102 performs measurements and transmits 304 the measurement report(s) to the DU 174. In some implementations, the measurement configuration(s) includes Layer 3 (L3) measurement configuration(s) (e.g., MeasConfig IE(s)) and the measurement report(s) include L3 measurement report(s).

[0067] After (e.g., in response to) receiving one or some of the measurement report(s) from the UE 102, the CU 172 determines to prepare a first cell (e.g., cell 1 such as the cell 124B) as an LTM candidate cell for the UE 102. In some implementations, the base station 104A determines to prepare the first cell for the UE 102 because the measurement report(s) indicates that the first cell could be used by the base station 104A to communicate with the UE 102. In some implementations, the base station 104A determines to prepare the first cell for the UE 102 because the measurement report(s) indicates that the first cell qualifies to be an LTM candidate cell that could be used for communication with the UE 102. In some implementations, if the L3 measurement report(s) indicates that the signal strength and/or the quality of the first cell is above a first predetermined threshold, is better than the signal strength and/or the quality of the serving cell (e.g., cell 124A), and/or is better than the signal strength and/or the quality of the serving cell by a first predetermined threshold, the CU 172 determines to prepare the first cell for the UE 102. Alternatively, the CU 172 determines to prepare the first cell for the UE 102 regardless of whether a measure report is received from the UE 102.

[0068] In response to determining to prepare the first cell for LTM, the CU 172 transmits 308 a first CU-to-DU message to the DU 174 to prepare the first cell for the UE 102. In some implementations, the CU 172 includes a cell identity (ID) 1 of the first cell in the first CU-to-DU message to request the DU 174 to prepare the first cell for LTM for the UE 102. For example, the cell ID 1 includes a cell global identity (CGI). In another example, the cell ID is a portion of the CGI. In yet another example, the cell ID is a physical cell ID (PCI). In some implementations, the CU 172 includes an LTM indicator in the first CU-to-DU message to indicate the DU 174 to prepare the first cell for LTM. In some implementations, the CU 172 includes the LTM indicator in an LTM Information Setup IE and includes the LTM Information Setup IE in the first CU-to-DU message. In yet other implementations, the CU 172 includes the LTM indicator in an LTM Information Modify IE and includes the LTM Information Modify IE in the first CU-to-DU message.

[0069] In response to the first CU-to-DU message, the DU 174 generates a first LTM DU configuration (referred to herein after as LTM DU configuration 1) for the UE 102, which configures the first cell for LTM. In some implementations, the CU 172 includes a first LTM configuration ID (referred to herein after as LTM ID 1) in the first CU-to-DU message and the DU 174 associates the LTM ID 1 and/or the cell ID 1 with the LTM DU configuration 1. The DU 174 then transmits 310 a first DU-to-CU message including the LTM DU configuration 1 to the CU 172 in response to the first CU-to-DU message.

[0070] The events 385 and 310 are collectively referred to in Fig. 3 as an LTM preparation procedure 390.

[0071] In some implementations, the DU 174 includes, in the first DU-to-CU message, the cell ID 1 of the first cell associated with the LTM DU configuration 1 to indicate that the LTM DU configuration 1 is configured for or associated with the first cell. When the CU 172 performs multiple LTM preparation procedures (e.g., the procedure 390 and the LTM preparation procedure 2, ..., N described below) with the DU 174 to prepare multiple LTM candidate cells, the CU 172 can determine that the LTM DU configuration 1 is configured for or associated with the first cell, based on the cell ID 1 in the first DU-to-CU message.

[0072] In some implementations, the CU 172 does not include an LTM reference DU configuration in the first CU-to-DU message. In such cases, the DU 174 generates an LTM reference DU configuration, and the DU 174 includes the LTM reference DU configuration in the first DU-to-CU message. In some implementations, the DU 174 generates the LTM DU configuration 1 as a delta configuration to augment the LTM reference DU configuration. In other implementations, the DU 174 generates the LTM DU configuration 1 as a complete configuration, i.e., not merely to augment the LTM reference DU configuration.

[0073] In some implementations, the CU 172 includes an LTM reference DU configuration request in the first CU-to-DU message, and the DU 174 generates the LTM reference DU configuration and includes the LTM reference DU configuration in the first DU-to-CU message in response to the request. In some implementations, the CU 172 determines whether the UE 102 supports an LTM reference configuration. In the context of embodiments and as described in 3GPP specification 38.331, a reference configuration is a configuration provided by the network to the UE that is common, within the same cell group, to a group of configured non-complete candidate configurations. A candidate configuration can include a complete candidate configuration or a delta configuration relative to the reference configuration. If the CU 172 determines that the UE 102 supports an LTM reference configuration, the CU 172 includes the LTM reference DU configuration request in the first CU-to-DU message. The DU 174 includes the LTM reference DU configuration in the first DU-to-CU message in response to the LTM reference DU configuration request. Otherwise, if the CU 172 determines that the UE 102 does not support an LTM reference configuration, the CU 172 does not include the LTM reference DU configuration request in the first CU-to- DU message. In this case, the DU 174 may generate the LTM DU configuration 1 as a complete configuration and may not include a/the LTM reference DU configuration in the first DU-to-CU message. In other implementations, the CU 172 transmits an additional CU- to-DU message including the LTM reference DU configuration request to the DU 174, rather than transmitting the LTM reference DU configuration request in the first CU-to-DU message. In response, the DU 174 transmits, to the CU 172, an additional DU-to-CU message including the LTM reference DU configuration. In yet other implementations, the DU 174 determines whether the UE 102 supports an LTM reference configuration. If the DU 174 determines that the UE 102 supports an LTM reference configuration, the DU 174 includes the LTM reference DU configuration in the first DU-to-CU message. Otherwise, if the DU 174 determines that the UE 102 does not support an LTM reference configuration, the DU 174 does not include a/the LTM reference DU configuration in the first DU-to-CU message. [0074] In some implementations, the CU 172 includes an LTM reference DU configuration in the first CU-to-DU message. In some implementations, the CU 172 receives the LTM reference DU configuration from an additional DU during an LTM preparation procedure as described above and with reference to Fig. 4. In other implementations, the CU 172 is preconfigured with the LTM reference DU configuration. In some implementations, the DU 174 generates the LTM DU configuration 1 as a delta configuration to augment the LTM reference DU configuration. In other implementations, the DU 174 disregards the LTM reference DU configuration and generates the LTM DU configuration 1 as a complete configuration, i.e., not merely to augment the LTM reference DU configuration.

[0075] If the DU 174 generates the LTM DU configuration 1 as a complete configuration, the DU 174 may include a complete configuration indication in the first DU-to-CU message to indicate that the LTM DU configuration 1 is a complete configuration. In some implementations, if the first CU-to-DU message does not include a/the LTM reference DU configuration, the CU 172 may determine that the LTM DU configuration 1 is a complete configuration. Otherwise, if the first CU-to-DU message includes a/the LTM reference DU configuration, the CU 172 may determine that the LTM DU configuration 1 is a delta configuration.

[0076] In some implementations, the LTM reference DU configuration is different from the serving DU configuration. In some implementations, a portion of the LTM reference DU configuration is the same as a portion of the serving DU configuration and the remainder of the LTM reference DU configuration is different from the other portions of the serving DU configuration. In yet other implementations, the LTM reference DU configuration is the same as the serving DU configuration. In some implementations, the LTM reference DU configuration includes physical layer configuration parameters, MAC configuration parameters, and/or RLC configuration parameters. In some implementations, the LTM reference DU configuration is the CellGroupConfig IE defined in 3GPP specification 38.331. In other implementations, the LTM reference DU configuration includes configuration parameters in the CellGroupConfig IE. In some implementations, the LTM reference DU configuration includes a CSl-MeasConfig IE or configuration parameters for channel state information (CSI) measurement and/or reporting.

[0077] In some implementations, the LTM reference DU configuration is different from the serving DU configuration. In some implementations, a portion of the LTM reference DU configuration is the same as a portion of the serving DU configuration and the remainder of the LTM reference DU configuration is different from the other portions of the serving DU configuration. In other implementations, the LTM reference DU configuration is the same as the serving DU configuration.

[0078] To prepare the first cell as an LTM candidate cell for the UE 102, the CU 172 may transmit 312 a second CU-to-DU message to the DU 174, including a CSI resource configuration (e.g., CSI resource configuration 1) and/or an LTM SSB configuration (i.e., LTM SSB configuration 1) to request that the DU 174 generate one or more CSI report configurations (e.g., (LTM) CSI report configuration(s) 1). The CSI resource configuration (e.g., (LTM) CSI resource configuration(s) 1) include configuration parameters configuring at least one reference signal (RS) transmitted on the first cell. The RS(s) include SSB(s) and/or CSLRS(s). The LTM SSB configuration include SSB configuration parameters configuring an SSB frequency, a subcarrier spacing, an SSB periodicity, SSB positions and/or SSB power for SSB(s) transmitted on the first cell.

[0079] After (e.g., in response to) receiving the CSI resource configuration, the DU 174 generates one or more CSI report configurations based on the CSI resource configuration and includes the CSI report configuration(s) in a serving DU configuration (referred to as a second serving DU configuration to distinguish from the serving DU configuration in event 302). In some implementations, the CSI report configuration(s) configures the UE 102 to transmit CSI reports based on measurements of the RS(s). The DU 174 transmits 314 a second DU-to-CU message including the second serving DU configuration to the CU 172. In some implementations, the CSI resource configuration comprises (e.g., is or includes) one or more LTM-CSl-ResourceConfig-rl8 IES. In other implementations, the CSI resource configuration comprises an Itm-CSl-ResourceConfigToAddModList field/IE. In some implementations, the second serving DU configuration is a CellGroupConfig IE.

[0080] In some implementations, the CU 172 includes the cell ID 1 and/or the LTM ID 1 in the second CU-to-DU message (e.g., at event 312). In one implementation, the CU 172 includes the cell ID 1 and/or the LTM ID 1 in the second CU-to-DU message to indicate that the CSI resource configuration(s) is/are associated with the first cell. In another implementation, the CU 172 includes the cell ID 1 and/or the LTM ID 1 in the second CU-to- DU message to allow for the DU 174 to associate the LTM ID 1 with the first cell, the cell ID 1, the LTM DU configuration 1, and/or the CSI report configuration(s). Based on the above implementation(s), the DU 174 can associate the LTM ID 1 and/or the cell ID 1 with configurations (e.g., LTM DU configuration 1, the CSI resource configuration, and/or the CSI report configuration(s)) related to the first cell. In such implementations, in some embodiments, the CU 172 includes the LTM ID 1 in the first CU-to-DU message. In other embodiments, the CU 172 does not include the LTM ID in the first CU-to-DU message.

[0081] In some alternative implementations, the CU 172 includes the CSI resource configuration in the first CU-to-DU message and the DU 174 includes the CSI report configuration(s) in the first DU-to-CU message.

[0082] In some implementations, the DU 174 transmits the LTM SSB configuration or the SSB configuration parameters to the CU 172, e.g., in the first DU-to-CU message, the second DU-to-CU message or in an additional DU-to-CU message. In some implementations, the DU 174 transmits the additional DU-to-CU message in response to receiving an additional CU-to-DU message from the CU 172. In other implementations, the DU 174 transmits the fourth DU-to-CU message (e.g., a UE Context Modification Required message) in response to receiving the first CU-to-DU message or the second CU-to-DU message.

[0083] The events 312 and 314 are collectively referred to in Fig. 3 as an LTM CSI report configuration and/or an LTM ID configuration procedure 392.

[0084] After receiving the first DU-to-CU message, the CU 172 generates a first LTM candidate configuration (i.e., LTM candidate configuration 1) including the LTM DU configuration 1 and generates a first RRC reconfiguration message including the LTM candidate configuration 1 and the LTM ID 1. In some implementations, the CU 172 includes LTM CU configuration 1 in the LTM candidate configuration 1. In other implementations, the CU 172 does not include an LTM CU configuration in the LTM candidate configuration 1. The CU 172 transmits 316 a third CU-to-DU message including the first RRC reconfiguration message to the DU 174. In turn, the DU 174 transmits 318 the first RRC reconfiguration message to the UE 102. In response, the UE 102 transmits 320 a first RRC reconfiguration complete message to the DU 174. The DU 174 then transmits 322 a third DU-to-CU message including the first RRC reconfiguration complete message to the CU 172.

[0085] If the first DU-to-CU message includes the LTM reference DU configuration, the

CU 172 generates an LTM reference configuration including the LTM reference DU configuration. In such cases, the CU 172 may include the LTM reference configuration in the first RRC reconfiguration message. In some implementations, the CU 172 includes an LTM reference CU configuration in the LTM reference configuration. In such cases, the CU 172 may generate the LTM CU configuration 1 as a delta configuration based on the LTM reference CU configuration. In other implementations, the CU 172 does not include an LTM reference CU configuration in the LTM reference configuration. In such cases, the CU 172 may generate the LTM CU configuration 1 as a complete configuration. Alternatively, the CU 172 transmits a second RRC reconfiguration message including the LTM reference configuration to the UE 102 via the DU 174, similar to the events 316 and 318. In response, the UE 102 transmits a second RRC reconfiguration complete message to the CU 172 via the DU 174, similar to the events 320 and 322. In some implementations, if the CU 172 does not receive an LTM reference DU configuration, the CU 172 may generate an LTM reference configuration including only the LTM reference CU configuration. In other implementations, if the CU 172 does not receive an LTM reference DU configuration, the CU 172 may not generate an LTM reference configuration.

[0086] In some implementations, if the first DU-to-CU message includes the complete configuration indication, the CU 172 may determine that the LTM DU configuration 1 is a complete configuration. Otherwise, if the first DU-to-CU message does not include the complete configuration indication, the CU 172 may determine that the LTM DU configuration 1 is a delta configuration. In some implementations, if the LTM DU configuration 1 is a complete configuration, the CU 172 generates the LTM candidate configuration 1 as a complete configuration. Otherwise, if the LTM DU configuration 1 is a delta configuration, the CU 172 generates the LTM candidate configuration 1 as a delta configuration. If the LTM candidate configuration 1 is a complete configuration, the CU 172 includes, in the first RRC reconfiguration message, a complete configuration indication to indicate that the LTM candidate configuration 1 is a complete configuration. If the LTM candidate configuration 1 is a delta configuration, the CU 172 excludes the complete configuration indication from the first RRC reconfiguration message to indicate that the LTM candidate configuration 1 is a delta configuration.

[0087] When the CU 172 performs the procedure 392, the CU 172 may include the second serving DU configuration in the first RRC reconfiguration message. Alternatively, the CU 172 transmits a third RRC reconfiguration message including the second serving DU configuration to the UE 102 via the DU 174, in an event similar to the events 316 and 318. In response, the UE 102 transmits a third RRC reconfiguration complete message to the CU 172 via the DU 174, in an event similar to the events 320 and 322. [0088] In some implementations, the CU 172 includes the CSI resource configuration in the first RRC reconfiguration message, the second RRC reconfiguration message, or the third RRC reconfiguration message. In other implementations, the CU 172 transmits a fourth RRC reconfiguration message including the CSI resource configuration to the UE 102 via the DU 174, in an event similar to the events 316 and 318. In response, the UE 102 transmits a fourth RRC reconfiguration complete message to the CU 172 via the DU 174, in an event similar to the events 320 and 322.

[0089] In some implementations, the DU 174 transmits a DU-to-CU message to the CU 172, including early synchronization information for the UE 102. The DU-to-CU message may be the first DU-to-CU message, the second DU-to-CU message or a fourth DU-to-CU message. In some implementations, the DU 174 transmits the fourth DU-to-CU message in response to receiving a fourth CU-to-DU message from the CU 172. In other implementations, the DU 174 transmits the fourth DU-to-CU message (e.g., a UE Context Modification Required message) in response to receiving the first CU-to-DU message or the second CU-to-DU message. In one implementation, the DU 174 transmits the fourth DU-to- CU message if the DU 174 determines that the UE 102 supports (i.e., is capable of) early UL synchronization with an LTM candidate cell (e.g., early TA acquisition with an LTM candidate cell, early RA on an LTM candidate cell, or UE measured TA). Otherwise, if the DU 174 determines that the UE 102 does not support the early UL synchronization with an LTM candidate cell, the DU 174 does not transmit the early synchronization information to the CU 172. In other implementations, the CU 172 transmits a CU-to-DU message including an early synchronization information request (e.g., an IE) to the DU 174, and the DU 174 includes the early synchronization information in the DU-to-CU message in response to the early synchronization information request. The CU-to-DU message may be the first CU-to- DU message, the second CU-to-DU message or the fourth CU-to-DU message. In one implementation, the CU 172 may transmit an early synchronization information request if the CU 172 determines that the UE 102 supports the early UL synchronization with an LTM candidate cell. Otherwise, if the CU 172 determines that the UE 102 does not support the early UL synchronization with an LTM candidate cell, the CU 172 does not request the DU 174 to provide the early synchronization information for the UE 102. If the CU 172 receives the early synchronization information, the CU 172 includes the early synchronization information in the first, second, third or fourth RRC reconfiguration message. Alternatively, the CU 172 transmits a fifth RRC reconfiguration message including the early synchronization information to the UE 102 via the DU 174, in an event similar to the events 316 and 318. In response, the UE 102 transmits a fifth RRC reconfiguration complete message to the CU 172 via the DU 174, in an event similar to the events 320 and 322.

[0090] In some implementations, the early synchronization information includes a Random-Access Channel (RACH) configuration (i.e., RACH configuration 1) and/or one or more TCI state configurations (i.e., TCI state configuration(s) 1). In some implementations, the early synchronization request may include a request for a RACH configuration. If the early synchronization request includes the request for a RACH configuration, the DU 174 includes the RACH configuration in the early synchronization information or in the DU-to- CU message (e.g., be the first, second or fourth DU-to-CU message). Otherwise, if the early synchronization request does not include the request for a RACH configuration, the DU 174 does not include the RACH configuration in either the early synchronization information or in the DU-to-CU message.

[0091] In some implementations, the CU 172 includes, in the first, second, third, fourth, and/or fifth RRC reconfiguration messages, one or more other LTM related configurations for the first cell. For example, the other LTM related configuration(s) include a PCI of the first cell and/or of the LTM SSB configuration.

[0092] The events 316, 318, 320, 322 are collectively referred to in Fig. 3 as an LTM configuration delivery procedure 394. The LTM configuration delivery procedure 394 can further include the second, third, fourth, and/or fifth RRC reconfiguration message(s)) and the second, third, fourth, and/or fifth RRC reconfiguration complete message(s), the related CU-to-DU message(s), and/or the related DU-to-CU message(s) In some implementations, the RRC reconfiguration message and the RRC reconfiguration complete message described above are an RRCReconfiguration message and an RRCReconfigurationComplete message, respectively.

[0093] In some implementations, the first CU-to-DU message is a UE Context Modification Request message, and the first DU-to-CU message is a UE Context Modification Response message. In some implementations, the second CU-to-DU message is a UE Context Modification Request message, and the second DU-to-CU message is a UE Context Modification Response message, or a UE Context Modification Required message. In the case of the UE Context Modification Required message, the CU 172 can transmit a UE Context Modification Confirm message to the DU 174 in response to UE Context Modification Required message. In some implementations, the third CU-to-DU message is a DE RRC Message Transfer message. In other implementations, the third CU-to-DU message is a UE Context Modification Request message. In some implementations, the third DU-to- CU message is a UE RRC Message Transfer message. In other implementations, the third DU-to-CU message is a UE Context Modification Response message.

[0094] In some implementations, the LTM reference CU configuration is different from the serving CU configuration. In some implementations, a portion of the LTM reference CU configuration is the same as a portion of the serving CU configuration and the remaining portions of the LTM reference CU configuration are different from the remaining portions of the serving CU configuration. In yet other implementations, the LTM reference CU configuration is the same as the serving CU configuration.

[0095] In some implementations, the LTM reference CU configuration includes PDCP configuration parameters, measurement configuration parameters, and/or radio bearer configuration parameters. In some implementations, the LTM CU configuration 1 includes a MeasConfig IE and/or a RadioBearerConfig IE defined in 3GPP specification 38.331 or the LTM CU configuration 1 includes configuration parameters in the MeasConfig IE and/or RadioBearerConfig IE.

[0096] In some implementations, the LTM CU configuration 1 and/or the LTM reference CU configuration include PDCP configuration parameters, measurement configuration parameters, and/or radio bearer configuration parameters. In some implementations, the LTM CU configuration 1 or the LTM reference CU configuration includes a MeasConfig IE and/or a RadioBearerConfig IE defined in 3GPP specification 38.331, or the LTM CU configuration 1 or the LTM reference CU configuration includes configuration parameters in the MeasConfig IE and/or RadioBearerConfig IE.

[0097] In some implementations, the LTM DU configuration 1 includes a plurality of configuration parameters for the UE 102 to communicate with the DU 174 on the first cell. In some implementations, the plurality of configuration parameters includes physical layer configuration parameters (e.g., PhysicalCellGroupConfig IE), MAC layer configuration parameters (e.g., MAC-CellGroupConfig IE), and/or RLC configuration parameters (e.g., REC-BearerConfig IE(s)). In some further implementations, the plurality of configuration parameters includes a special cell configuration (e.g., SpCellConfig IE) and/or one or more SCell configurations (e.g., SCellConfig IE(s)). In some implementations, the LTM DU configuration 1 is CellGroupConfig IE defined in 3GPP specification 38.331. In other implementations, the LTM DU configuration 1 includes configuration parameters in the CellGroupConfig IE.

[0098] In some implementations, the LTM DU configuration 1 includes a first LI measurement configuration (e.g., a CSl-MeasConfig IE) and/or at least one first transmission configuration indicator (TCI) state configuration. In other implementations, the LTM CU configuration 1 includes the first TCI state configuration(s). In some implementations, the first LI measurement configuration includes at least one first RS resource configuration and/or at least one first report configuration. In some implementations, the first RS resource configuration(s) configures one or more RS(s) or one or more RS resources associated with the cell 1. The RS(s) includes SSB(s) and/or CSLRS(s). The RS resource(s) includes SSB resource(s) and/or CSLRS resource(s). In some implementations, each of the first RS resource configuration(s) includes a RS resource configuration ID. In some implementations, the first RS resource configuration(s) is/are (similar to) CSl-ResourceConfig IE(s). In some implementations, the first report configuration(s) configures one or more UL resources (e.g., PUCCH resources or PUSCH resources) on the first cell for the UE 102 to transmit measurement results (e.g., CSI reports or LTM CSI reports). In some implementations, each of the first report configuration(s) includes one or more RS resource configuration IDs identifying one or more RS resource configurations included in the first RS resource configuration(s). In some implementations, each of the first TCI state configuration(s) configures a TCI state that associates one or two DL RSs with a corresponding quasicolocation (QCL) type. The DL RS(s) is/are associated with the cell 1.

[0099] After receiving the LTM-related configurations and the second serving DU configuration or after receiving the RRC reconfiguration message(s) described above, the UE 102 performs measurements on at least one first RS, generates at least one first LI measurement result based on the measurements, and transmits 324 at least one first LI measurement report including the first LI measurement result(s) to the DU 174. The first RS(s) may comprise SSB(s) and/or CSLRS(s). In some implementations, the first RS(s) and/or transmission pattern(s) of the first RS(s) are configured in the CSI report configuration(s), the LTM SSB configuration, and/or the CSI resource configuration. The UE 102 performs the measurements on the first RS(s) in accordance with the CSI report configuration(s), the LTM SSB configuration, and/or the CSI resource configuration. [0100] After receiving the LTM-related configurations and the second serving DU configuration or after receiving the RRC reconfiguration message(s) described above, the UE 102 performs measurements on at least one second RS, generates at least one second LI measurement result based on the measurements, and transmits at least one second LI measurement report including the first LI measurement result(s) to the DU 174. The second RS(s) may comprise SSB(s) and/or CSLRS(s). In some implementations, the second RS(s) and/or transmission pattern(s) of the second RS(s) are configured in one or more second CSI report configuration(s) and/or in one or more second CSI resource configuration(s) that is/are included in the serving DU configuration in event 302 and/or the second serving DU configuration. The UE 102 performs the measurements on the second RS(s) in accordance with the second CSI report configuration(s) and/or the second CSI resource configuration(s). The second CSI report configuration(s) may include non-LTM CSI report configuration(s) and/or LTM CSI report configuration(s). The second CSI resource configuration(s) may include non-LTM CSI resource configuration(s) and/or LTM CSI resource configuration(s).

[0101] After transmitting the RACH configuration to the UE 102 via the CU 172, the DU 174 may transmit 327 a PDCCH order to the UE 102 to command the UE 102 to transmit an RA preamble on the first cell. In response to the PDCCH order, the UE 102 transmits an RA preamble on the first cell. The DU 174 includes PDCCH order information in the PDCCH order. The PDCCH order information includes an RA preamble index, a UL or a supplemental UL indicator, an SSB index and/or a physical RACH mask index. In some implementations, the DU 174 includes the LTM ID 1 in the PDCCH order to indicate the first cell. The UE 102 identifies the first cell based on the LTM ID 1 in the PDCCH order and transmits 352 the RA preamble on the first cell to the DU 174, using the PDCCH order information. Correspondingly, the DU 174 receives 352 the RA preamble in accordance with the PDCCH order information. In some implementations, the DU 174 may determine the SSB index, based on LI measurement report(s) 324, and/or the CSI resource configuration, the CSI report configuration, and/or the LTM SSB configuration. In some implementations, the LI measurement report(s) may include the SSB index. In other implementations, the LI measurement report(s) include an SS/PBCH Block Resource Indicator (SSBRI) corresponding to the SSB index. Thus, the DU 174 determines the SSB index based on the SSBRI.

[0102] In some implementations, the DU 174 determines whether to transmit the PDCCH order based on the LI measurement result(s) 324. In some implementations, if the LI measurement result(s) indicate that the first cell qualifies for the UE 102 to perform RA for early UL synchronization, the DU 174 transmits the PDCCH order. Otherwise, if the LI measurement result(s) indicate that the first cell does not qualify for the UE 102 to perform RA for early UL synchronization, the DU 174 refrains from transmitting the PDCCH order. In other implementations, if the LI measurement result(s) indicate that the first cell qualifies for the UE 102 to access, the DU 174 transmits the PDCCH order. Otherwise, if the LI measurement result(s) indicate that the first cell does not qualify for the UE 102 to access, the DU 174 refrains from transmitting the PDCCH order. In yet other implementations, the DU 174 transmits the PDCCH order after receiving 320 the RRC reconfiguration complete message, regardless of the LI measurement result(s) 324.

[0103] After transmitting 324 the LI measurement report(s) or 352 the RA preamble, the UE 102 may transmit 354 additional LI measurement report(s) to the DU 174, similar to the event 324. The DU 174 determines to command that the UE 102 t perform an LTM cell switch to the first cell based on the additional LI measurement report(s) and/or the LI measurement report(s) 324. In response to the determination, the DU 174 generates an LTM Cell Switch Command (e.g., a MAC CE) including the LTM ID 1 (e.g., configuration identifier) and transmits 326 the LTM Cell Switch Command to the UE 102. In response to the determination, the DU 174 may transmit 328 a DU-CU Cell Switch Notification message to the CU 172. In response to the LTM Cell Switch Command, the UE 102 performs an LTM cell switch to the first cell. In the LTM cell switch, the UE 102 accesses 332 the first cell and transmits 336 an RRC reconfiguration complete message to the DU 174 via the first cell. The DU 174 transmits 338 a DU-to-CU message (e.g., UL RRC Message Transfer message) including the RRC reconfiguration complete message to the CU 172. When the UE 102 receives the LTM Cell Switch Command, the UE 102 identifies the LTM candidate configuration 1 from the LTM ID 1 and accesses 332 the first cell using the LTM candidate configuration 1. Depending on implementations, the UE 102 may stop communicating on the serving cell(s) in response to the LTM Cell Switch Command. In turn, the DU 174 transmits a fifth DU-to-CU message including the RRC reconfiguration complete message to the CU 172. When the DU 174 detects that the UE 102 accesses the first cell in the event 332, the DU 174 may transmit 334 an Access Success message to the CU 172 to indicate that the UE 102 has accessed the first cell.

[0104] In some implementations, the DU 174 includes, in the LTM Cell Switch Command, a TA value for UL synchronization with the first cell. In one implementation, the DU 174 1 derives the TA value based on the RA preamble (e.g., reception timing of the RA preamble). In another implementation, the DU 174 derives the TA value from a UL transmission on the serving cell (e.g., the cell 124A) from the UE 102. The UE 102 applies the TA value to synchronize with the first cell in UL transmission. After applying the TA value, the UE 102 transmits the first UL transmission on the first cell based on the LTM candidate configuration 1 without performing an RA procedure on the first cell. In some implementations, the UE 102 transmits the first UL transmission on the first cell using a UL grant. In such cases, the first UL transmission is a PUSCH transmission. In some implementations, the PUSCH transmission includes the RRC reconfiguration complete message 336. In some implementations, the UL grant is a configured grant and the LTM candidate configuration 1 or the LTM DU configuration 1 includes the configured grant configuration configuring the configured grant. In other implementations, the UL grant is a dynamic grant that the UE 102 receives on a PDCCH on the first cell. After transmitting the first UL transmission, the UE 102 receives a PDCCH transmission addressed to a C-RNTI of the UE 102 and determines that the LTM cell switch is completed successfully in response to receiving the PDCCH transmission.

[0105] The PDCCH transmission may include a UL grant or a DL assignment. If the PDCCH transmission includes a UL grant, the UE 102 transmits a PUSCH transmission to the DU 174 on the first cell using the UL grant. If the PDCCH transmission includes a DL assignment, the DU 174 transmits a PDSCH transmission to the UE 102 on the first cell in accordance with the DL assignment. The DU 174 may transmit 334 the Access Success message to the CU 172 after receiving (e.g., in response to) the first UL transmission, transmitting the PDCCH transmission, receiving the PUSH transmission, or transmitting the PDSCH transmission.

[0106] In other implementations, the DU 174 does not include a TA value in the LTM Cell Switch Command. If the LTM Cell Switch Command does not include a/the TA value, the UE 102 performs 332 an RA procedure on the first cell in accordance with the RA configuration parameters. In some implementations, the RA configuration parameters are included in the LTM candidate configuration 1 or in the LTM DU configuration 1. In some implementations, the RA configuration parameters configure PRACH resources, an association between SSB and PRACH resources, and/or one or more PRACH occasions. If the UE 102 successfully completes the RA procedure, the UE 102 determines the LTM cell switch to the first cell is completed successfully. Depending on implementation and/or on the RA configuration parameters, the RA procedure can be a four- step RA procedure or a two- step RA procedure. During the four-step RA procedure, the UE 102 transmits a Message 3 on the first cell and the DU 174 transmits a Message 4 on the first cell to the UE 102 in response. During the two-step RA procedure, the UE 102 transmits a Message A on the first cell and the DU 174 transmits a Message B to the UE 102 on the first cell in response. The UE 102 may include the RRC reconfiguration complete message 336 in the Message 3 or Message A. Alternatively, the UE 102 transmits the RRC reconfiguration complete message 336 after completing the RA procedure. The DU 174 may transmit 334 the Access Success message to the CU 172, after receiving the Message 3, Message A or the RRC reconfiguration complete message 336 or after transmitting the Message 4 or Message B.

[0107] After successfully completing the LTM cell switch to the first cell as described above, the UE 102 communicates 340 with the DU 174 and the CU 172 via the first cell, using the LTM candidate configuration 1. In the case of the LTM reference configuration, the UE 102 applies the LTM reference configuration first and then applies the LTM candidate configuration 1 to augment the LTM reference configuration.

[0108] In some implementations, each of the TCI state configuration(s) include a TCI state ID. In some implementations, the DU 174 includes, in the DU-CU Cell Switch Notification message and/or the LTM Cell Switch Command, a first TCI state ID indicating a first one of the TCI state configuration(s). The UE 102 identifies the first one of the TCI state configuration(s) based on the first TCI state ID and applies the first TCI state configuration to communicate UL transmissions and/or DL transmissions with the DU 174 in the events 332, 336, and/or 340. The DU 174 applies the first TCI state configuration to communicate UL transmissions and/or DL transmissions with the UE 102 in the events 332, 336, and/or 340. In other implementations, in case that separate TCI states are used, the DL TCI State and the UL TCI state use different TCI State IDs. In such cases, the DU 174 includes, in the DU-CU Cell Switch Notification message and/or the LTM Cell Switch Command, a first TCI State ID for DL and/or a second TCI State ID for UL that identify a first one and a second one of the TCI state configuration(s). The UE 102 identifies the first TCI state configuration and the second of the TCI state configuration based on the first TCI state ID and the second TCI state ID respectively. The UE 102 applies the first TCI state configuration and the second TCI state configuration to communicate DL transmissions and UL transmissions, respectively, with the DU 174 in the events 332, 336 and/or 340. The DU 174 identifies the first TCI state configuration and the second of the TCI state configuration based on the first TCI state ID and the second TCI state ID respectively. The DU 174 applies the first TCI state configuration and the second TCI state configuration to communicate DL transmissions and UL transmissions, respectively, with the UE 102 in the events 332, 336, and/or 340.

[0109] In some implementations, the CU 172 may prepare additional cell(s) (i.e., cell(s) 2, ..., N) as LTM candidate cell(s) for the UE 102 with the DU 174, before or after transmitting the LTM Cell Switch Command or during, before or after the procedure 390 or 392, as described above. N is an integer and larger than 1. For example, the CU 172 performs additional LTM preparation procedure(s) 2, ..., N with the DU 174 to prepare the cell(s) 2, ..., N respectively. Each of the LTM preparation procedure(s) 2, ..., N is similar to the procedure 390. In the LTM preparation procedure(s) 2, ..., N, the CU 172 receives LTM DU configuration(s) 2, ..., N configuring the cell(s) 2, ..., N for LTM, respectively. The CU 172 generates LTM candidate configuration(s) 2, ..., N including the LTM DU configuration(s) 2, ..., N, respectively. The CU 172 assigns LTM ID(s) 2, ..., N to identify the LTM DU configuration(s) 2, ..., N and the LTM candidate configuration(s) 2, ..., N, respectively. The CU 172 may obtain CSI resource configuration 2, ..., N and perform CSI report configuration and/or LTM ID configuration procedure(s) 2, ..., N with the DU 174 to obtain the CSI report configuration(s) 2, ..., N, respectively, as described for the CSI resource configuration 1 and the CSI report configuration(s) 1. The CU 172 may obtain RACH configuration 2, ..., N for the cell(s) 2, ..., N respectively, as described for the RACH configuration 1. The CU 172 may obtain TCI state configuration(s) 2, ..., N for the cell(s) 2, ..., N respectively, as described for the TCI state configuration(s) 1. Each of the CSI report configuration and/or LTM ID configuration procedure(s) 2, ..., N is similar to the procedure 392. The CU 172 may obtain LTM SSB configuration 2, ..., N for the cell(s) 2, ..., N, respectively, as described for the LTM SSB configuration 1. In some implementations, the CU 172 may perform LTM configuration delivery procedure 2, ..., N with the UE 102 to transmit a list including: {LTM ID 2, the LTM candidate configuration 2, the CSI resource configuration 2 (if obtained), the TCI state configuration 2 (if obtained), the RACH configuration 2 (if obtained), the LTM SSB configuration 2 (if obtained)}, ..., {the LTM ID N, the LTM candidate configuration N, the CSI resource configuration N, the TCI state configuration(s) N (if obtained), the RACH configuration N (if obtained), the LTM SSB configuration N (if obtained)} to the UE 102, respectively. Each of the LTM configuration delivery procedure(s) 2, ..., N is similar to the procedure 394. In other implementations, the CU 172 includes the list in the first RRC reconfiguration message. [0110] In some implementations, after receiving 334 the Access Success message or 338 the DU-to-CU message, the CU 172 may transmit 342 a CU-to-DU message to the DU 174. In one implementation, the CU 172 transmits 342 the CU-to-DU message to release radio resources and/or configurations of the serving cell(s) configured for the UE 102. In another implementation, the CU 172 transmits 342 the CU-to-DU message to release some of the LTM candidate cell(s) 2, ..., N. In this case, the CU 172 releases any information or settings associated with respective LTM candidate cell(s). In response to the CU-to-DU message 342, the DU 174 transmits 344 a DU-to-CU message to the CU 172. In some implementations, the CU-to-DU message 342 and the DU-to-CU message 344 are a UE Context Modification Request message and a UE Context Modification Response message, respectively.

[0111] In some implementations, an LTM ID is provided in a different format depending on which command or message provides the LTM ID. Lor example, the LTM ID may be provided differently in a PDCCH order, in an LTM Cell Switch Command and in an RRC reconfiguration message. Lor example, the PDCCH order or the LTM Cell Switch Command includes a first field to include the LTM ID 1 and the first RRC reconfiguration message includes a second field to include the LTM ID 1. In some implementations, the first field and the second field have different formats or coding schemes. Lor example, the first field uses a binary format (i.e., 3 bits) with a value range of 0, ..., 7 and the second field uses an integer format with a value range of 1, ..., 8. In this example, the first field with binary value 000b is equivalent to the second field with integer value 1, the first field with binary value 001b is equivalent to the second field with integer value 2, ... , and the first field with binary value 11 lb is equivalent to the second field with integer value 8.

[0112] The events 304, 306, 390, 392, 394, and 324 are collectively referred to in Eig. 3 as an intra-CU intra-DU LTM configuration procedure 396. The events 304, 306, 390, 392, 394, 324, 350, 352, 354, 326, 328, 332, 334, 336, 338, and 340 are collectively referred to in Eig.

3 as an intra-CU intra-DU LTM procedure 380.

[0113] Referring next to Eig. 4, in a scenario 400, the base station 104A includes a CU 172, an S-DU 174A and a candidate DU (C-DU) 174B. The S-DU 174A operates the cell 124A and optionally operates additional cell(s), while the C-DU 174B operates a different cell (e.g., cell 124B or cell 126 (Eig. 1A)). The scenario 400 is an intra-CU inter-DU scenario, similar to the scenario 300. Thus, the descriptions for the scenario 300 can generally apply to the scenario 400. Some descriptions for the DU 174 in Eig. 3 may apply to the S-DU 174A in Fig. 4, and some descriptions for the DU 174 in Fig. 3 may apply to the C-DU 174B. The differences between the scenarios 300 and 400 are described below.

[0114] Initially, the UE 102 communicates 402 with the S-DU 174A on one or more serving cells (e.g., the cell 124A and/or other cell(s)) using a serving DU configuration and the UE 102 communicates with the CU 172 via the S-DU 174A using a serving CU configuration. In some implementations, the CU 172 and S-DU 174A may perform the LTM configuration procedure 496 or the LTM procedure 480 with the UE 102, similar to the procedures 396 and 380, respectively, as described with reference to Fig. 3. In the case of the procedure 480, the UE 102 may perform an LTM cell switch to the first cell (e.g. cell 124B) as described for Fig. 3. Upon successfully completing the LTM cell switch, the first cell becomes a serving cell and cell 124A and/or the other cell(s) is/are no longer serving cell(s) for the UE 102. In the case of the procedure 496, the UE 102 does not perform an LTM cell switch. During the communication 402, the UE 102 transmits 404, 406 at least one measurement report (e.g., L3 measurement report(s)) to the CU 172 via the S-DU 174A. Based on the measurement report(s), the CU 172 determines to prepare cell 1 (e.g., a cell operated by the C-DU 174B) for LTM for the UE 102. The cell 1 is identified by a cell ID (i.e., cell ID 1). In response to the determination, the CU 172 performs 490 an LTM preparation procedure with the C-DU 174B to prepare or to request the C-DU 174B to prepare the cell 1 as an LTM candidate cell for the UE 102. In the LTM preparation procedure 490, the CU 172 transmits a first CU-to-DU message including a cell ID 1 of the cell 1 to the C-DU 174B to request the C-DU 174B to prepare the cell 1 as an LTM candidate cell for the UE 102, similar to the event 385. In response, the C-DU 174B transmits a first DU-to-CU message including an LTM DU configuration (e.g., LTM DU configuration 1) to the CU 172, similar to the event 310. In some implementations, the CU 172 requests an LTM reference DU configuration in the first CU-to-DU message, as described with reference to Fig. 3. In other implementations, the CU 172 does not request an LTM reference DU configuration in the first CU-to-DU message. In some implementations, the C-DU 174B includes an LTM reference DU configuration in the first DU-to-CU message, as described with reference to Fig. 3. In other implementations, the C-DU 174B does not include an LTM reference DU configuration in the first DU-to-CU message.

[0115] In some implementations, if the CU 172 receives an LTM reference DU configuration from the S-DU 174A as described with reference to Fig. 3, the CU 172 may include the LTM reference DU configuration in the first CU-to-DU message and the C-DU 174B may generate the LTM DU configuration as a delta configuration based on the LTM reference DU configuration. In such cases, the C-DU 174B does not transmit an LTM reference DU configuration for the UE 102 to the CU 172. In other implementations, the CU 172 receives an LTM reference DU configuration from the C-DU 174B, e.g., in the first DU- to-CU message or an additional DU-to-CU message as described with reference to Fig. 3. In such cases, the CU 172 may generate an LTM reference configuration including the LTM reference DU configuration. In some implementations, the CU 172 includes an LTM reference CU configuration in the LTM reference configuration. In other implementations, the CU 172 does not include an LTM reference CU configuration in the LTM reference configuration.

[0116] To prepare the cell 1 for LTM, the CU 172 may perform 492 an LTM CSI report configuration and/or LTM ID configuration procedure with the S-DU 174A. In the procedure 492, the CU 172 transmits a second CU-to-DU message including a CSI resource configuration (e.g., CSI resource configuration 1) and/or an LTM SSB configuration (i.e., LTM SSB configuration 1) to the S-DU 174A, similar to the event 312. In response, the S- DU 174A transmits a second DU-to-CU message including one or more CSI report configurations (e.g., CSI report configuration(s) 1) to the CU 172. In some implementations, the CU 172 generates an LTM candidate configuration (e.g., LTM candidate configuration 1) including the LTM DU configuration and assigns an LTM ID (e.g., LTM ID 1) for identifying the LTM DU configuration and/or the LTM candidate configuration as described with reference to Fig. 3. In some implementations, the CU 172 includes {the LTM ID 1, the cell ID 1 } as a tuple in the second CU-to-DU message.

[0117] To prepare the cell 1 as an LTM candidate cell for the UE 102, the CU 172 may receive early synchronization information for the cell 1 in a DU-to-CU message (e.g., the first DU-to-CU message or an additional DU-to-CU message) from the C-DU 174B. In some implementations, the C-DU 174B transmits the additional DU-to-CU message in response to receiving an additional CU-to-DU message from the CU 172. In other implementations, the C-DU 174B transmits the additional DU-to-CU message (e.g., a UE Context Modification Required message) in response to receiving the first CU-to-DU message. The early synchronization information includes a RACH configuration (e.g., RACH configuration 1) and/or at least one TCI state configuration (e.g., TCI state configuration(s) 1), as described with reference to Fig. 3. In some implementations, the C-DU 174B includes, in the early synchronization information or in the DU-to-CU message, the PDCCH order information (PDCCH order information 1) for early UL synchronization with the first cell.

[0118] In some implementations, the CU 172 may receive the LTM SSB configuration or SSB configuration parameters in the LTM SSB configuration from the C-DU 174B, e.g., in the first DU-to-CU message or in an additional DU-to-CU message. In some implementations, the CU 172 may receive a PCI of the cell 1 from the C-DU 174B in the first DU-to-CU message or in the additional DU-to-CU message. In some implementations, the C- DU 174B transmits the additional DU-to-CU message in response to receiving an additional CU-to-DU message from the CU 172. In other implementations, the C-DU 174B transmits the additional DU-to-CU message (e.g., a UE Context Modification Required message) in response to receiving the first CU-to-DU message.

[0119] As described with reference to Fig. 3, the CU 172 performs 494 an LTM configuration delivery procedure with the UE 102 to transmit the LTM ID 1 and the LTM candidate configuration to the UE 102. In some implementations, the CU 172 transmits {LTM ID, the LTM candidate configuration] as a tuple in a first RRC reconfiguration message in the procedure 494. Depending on the implementations, the CU 172 may include the LTM reference configuration, the CSI report configuration(s), the CSI resource configuration, the RACH configuration, the TCI state configuration(s), the LTM SSB configuration and/or the PCI of the cell 1 in the first RRC reconfiguration message to the UE 102 and/or other RRC reconfiguration message(s) transmitted to the UE 102, as described with reference to Fig. 3. The CU 172 includes the LTM ID in the first RRC reconfiguration or the other RRC reconfiguration message(s) to indicate that the CSI resource configuration, the RACH configuration, the TCI state configuration(s), the LTM SSB configuration, and/or the PCI of the cell 1 are associated with the cell 1. For example, the CU 172 includes the CSI resource configuration, the RACH configuration, the TCI state configuration(s), the LTM SSB configuration, and/or the PCI of the cell 1 in the tuple. In another example, the CU 172 includes {LTM ID, CSI resource configuration, RACH configuration, TCI state configuration(s), LTM SSB configuration, and/or the PCI of the cell 1 } as a tuple in the other RRC reconfiguration message(s). In response to each of the other RRC reconfiguration message(s), the UE 102 transmits an RRC reconfiguration complete message to the CU 172 via the S-DU 174A.

[0120] In some implementations, the PDCCH order information includes a frequency domain resource assignment, an RA preamble index, a UL or a supplemental UL indicator, an SSB index and/or a physical RACH mask index. The CU 172 may transmit a CU-to-DU message (e.g., the second CU-to-DU message or an additional CU-to-DU message) including the PDCCH order information to the S-DU 174A. In some implementations, the CU 172 includes the LTM ID 1 or the cell ID 1 in the CU-to-DU message to indicate that the PDCCH order information is associated with the LTM ID 1 or the cell ID 1. For example, when the CU 172 transmits the PDCCH order information in the second CU-to-DU message, the CU 172 includes {the cell ID 1, the PDCCH order information} as a tuple in the CU-to-DU message. When the CU 172 transmits an additional CU-to-DU message, the S-DU 174A may transmit an additional DU-to-CU message to the CU 172 in response. The S-DU 174A transmits 450 a PDCCH order to the UE 102, based on the PDCCH order information. For example, The S-DU 174A transmits 450 a PDCCH order to the UE 102, including the PDCCH order information. In some implementations, the S-DU 174A may determine an SSB index included in the PDCCH order, based on LI measurement report(s) 424, and/or the CSI resource configuration, the CSI report configuration and/or the LTM SSB configuration. In some implementations, the S-DU 174A includes the LTM ID 1 in the PDCCH order to indicate the cell 1. When the CU 172 transmits the PDCCH order information in the additional CU-to-DU message, the S-DU 174A may transmit an additional DU-to-CU message to the CU 172 in response. The UE 102 transmits 452 an RA preamble to the C-DU 174B on the cell 1, using the RACH configuration and/or the PDCCH order information. The C-DU 174B derives a TA value based on the RA preamble. The C-DU 174B transmits 456 a DU-CU TA Information Transfer message including the TA value to the CU 172. The CU 172 in turn transmits 458 a CU-DU TA Information Transfer message including the TA value to the S-DU 174A. In some implementations, the C-DU 174B includes the cell ID 1, the RA preamble index, an RA radio network temporary identifier (RA-RNTI) and/or a DU ID of the S-DU 174A in the message 456. In such cases, the CU 172 includes the cell ID 1, the RA preamble index, the RA-RNTI, and/or the DU ID of the S-DU 174A in the message 458. In response to determining to command the UE 102 to perform an LTM cell switch to the cell 1, the S-DU 174A transmits 456 the LTM Cell Switch Command including the LTM ID 1 to the UE 102. If the S-DU 174A receives a TA value as described above, the S-DU 174A may include the TA value in the LTM Cell Switch Command. The S-DU 174A may include a first TCI state ID in the LTM Cell Switch Command. The first TCI state ID indicates a first one of the TCI state configuration(s). [0121] In some implementations, the C-DU 174B determines the RA-RNTI based on a PRACH occasion in which the C-DU 174B receives the RA preamble 452. In some implementations, the C-DU 174B calculates the RA-RNTI as:

RA-RNTI = 1 + s_id + 14 x t_id + 14 x 80 x f_id + 14 x 80 x 8 x ul_carrier_id where s_id is the index of the first OFDM symbol of the PRACH occasion (0 < s_id < 14), t_id is the index of the first slot of the PRACH occasion in a system frame (0 < t_id < 80), where the subcarrier spacing to determine t_id is based on the value of p specified in clause 5.3.2 in 3GPP TS 38.211 for p = {0, 1, 2, 3}, and for p = {5, 6}, t_id is the index of the 120 kHz slot in a system frame that contains the PRACH occasion (0 < t_id < 80), f_id is the index of the PRACH occasion in the frequency domain (0 < f_id < 8), and ul_carrier_id is the UL carrier used for the RA Preamble transmission (0 for NUL carrier, and 1 for SUL carrier).

[0122] In response to determining to command the UE 102 to perform an LTM cell switch or transmitting 426 the LTM Cell Switch Command, the S-DU 174A transmits 428 a DU-CU Cell Switch Notification message to the CU 172 to indicate that the UE 102 performs an LTM cell switch to the cell 1. In response, the CU 172 transmits 430 a CU-DU Cell Switch Notification message to the C-DU 174B to indicate that the UE 102 performs an LTM cell switch to the cell 1. In some implementations, the S-DU 174A includes the first TCI state ID in the DU-CU Cell Switch Notification message and the CU 172 in turn includes the first TCI state ID in the CU-DU Cell Switch Notification message. The UE 102 and the C-DU 174B identify the first one of the TCI state configuration(s) based on the first TCI state ID and apply the first TCI state configuration to communicate UL transmissions and/or DL transmissions in the events 432, 436, and/or 440. The C-DU 174B transmits 434 an Access Success message to the CU 172.

[0123] In other implementations, when separate TCI states are used, the DL TCI State and the UL TCI state use different TCI State IDs. In such cases, the S-DU 174A includes, in the DU-CU Cell Switch Notification message and/or the LTM Cell Switch Command, a first TCI State ID for DL, and a second TCI State ID for UL that identify a first and a second TCI state configuration(s), respectively. The CU 172, in response to the DU-CU Cell Switch Notification message, in turn includes the first TCI State ID and the second TCI state ID in the CU-DU Cell Switch Notification message to the C-DU 174B. The UE 102 identifies the first TCI state configuration and the second TCI state configuration based on the first TCI state ID and the second TCI state ID. The UE 102 applies the first TCI state configuration and the second TCI state configuration to receive DL transmissions and transmit UL transmissions, respectively, with the C-DU 174B in the events 432, 436, and/or 440. The C- DU 174B identifies the first TCI state configuration and the second the TCI state configuration based on the first TCI state ID and the second TCI state ID respectively. The C- DU 174B applies the first TCI state configuration and the second TCI state configuration to communicate DL transmissions and UL transmissions, respectively, with the UE 102 in the events 432, 436, and/or 440.

[0124] In some implementations, each of the TCI state configuration(s) includes or is associated with a TCI state ID. In some implementations, the CU 172 may transmit a CU-to- DU message (e.g., the second CU-to-DU message or an additional CU-to-DU message) including the TCI state configuration(s) and/or the associated TCI state ID(s) to the S-DU 174A. In some implementations, the CU 172 includes the LTM ID 1 or the cell ID 1 in the CU-to-DU message to indicate that the TCI state configuration(s) is associated with the LTM ID 1 or the cell ID 1. For example, when the CU 172 transmits the TCI state configuration(s) and/or the associated TCI state ID(s) in the second CU-to-DU message, the CU 172 includes {the cell ID 1, the TCI state configuration(s)} as a tuple in the CU-to-DU message. When the CU 172 transmits an additional CU-to-DU message, the S-DU 174A may transmit an additional DU-to-CU message to the CU 172 in response. In some implementations, the S- DU 174A includes, in the LTM Cell Switch Command 426, a first TCI state ID indicating a first one of the TCI state configuration(s). In some implementations, the S-DU 174A determines the first TCI state configuration or the first TCI state ID. The UE 102 identifies the one of the TCI state configuration(s) based on the first TCI state ID and applies the first TCI state configuration in UL transmissions and/or DL receptions in the events 432, 436, and/or 440.

[0125] In some implementations, the CU 172 may prepare additional cell(s) (i.e., cell(s) 2, ..., N) as LTM candidate cell(s) for the UE 102 with the C-DU 174B, before or after transmitting the LTM Cell Switch Command or during, before, or after the procedure 49,0 or 492, as described with reference to Fig. 3.

[0126] In some implementations, the first CU-to-DU message and the first DU-to-CU message are a UE Context Setup Request message and a UE Context Setup Response message. In some implementations, the first CU-to-DU message and the first DU-to-CU message are a UE Context Modification Request message and a UE Context Modification Response message. In some implementations, the LTM preparation procedure 490 is a UE Context Setup procedure, and the additional LTM preparation procedure is a UE Context Modification procedure. In other implementations, the LTM procedure 490 and the additional LTM preparation procedures are UE Context Setup procedures. In yet other implementations, the LTM procedure 490 and the additional LTM preparation procedures are UE Context Modification procedures.

[0127] The events 404, 406, 490, 492, 494, and 424 are collectively referred to in Eig. 4 as an intra-CU inter-DU LTM configuration procedure 496. The events 404, 406, 490, 492, 494, 424, 450, 452, 454456, 458, 426, 428, 430, 432, 434, 436, 438, 440, 442, and 444 are collectively referred to in Eig. 4 as an intra-CU inter-DU LTM procedure 480.

[0128] Referring next to Eig. 5, in a scenario 500, the base station 104A operates as a serving or source base station (S-BS), and the base station 106A operates as a candidate base station (C-BS). The C-BS 106A includes a CU 172 and a DU 174. The scenario 500 is similar to the scenarios 300 and 400, except that the scenario 500 is an inter-CU scenario (i.e., inter-base station scenario) while the scenarios 300 and 400 are intra-CU (i.e., intra-base station) scenarios. The S-BS 104A can include a CU and a DU (not shown in Eig. 5), similar to the base station 104A as shown in Figs. 3 and 4. Initially, the UE 102 communicates 502 with the S-BS 104A via serving cell(s) using a serving configuration. In some implementations, the S-BS 104A includes an S-DU and a CU and the serving configuration may include a serving CU configuration and a serving DU configuration, as described with reference to Figs. 3 and 4. While the communicating 502 with the UE 102, the S-BS 104A may perform 580 intra-CU LTM procedure(s) with the UE 102, similar to the procedures 380 and/or 480. Alternatively, while the communicating 502 with the UE 102, the S-BS 104A may perform 596 intra-CU LTM configuration procedure(s) with the UE 102, similar to the procedures 396 and/or 496.

[0129] While communicating with the S-BS 104A, the UE 102 transmits 504 at least one measurement report to the S-BS 104A. The measurement report(s) include measurement results for a serving cell of the UE 102 and/or at least one non-serving cell (e.g., cell 126A). The S-BS 104A determines to prepare a first cell (e.g., the cell 126A) as an LTM candidate cell for the UE 102, based on the measurement report(s). For example, the measurement report(s) include a PCI of the first cell and measurement result(s) of the cell 126A. The S-BS 104A identifies that the first cell is operated by the base station 106 A based on the PCI and determines that the first cell qualifies for LTM preparation based on the measurement result(s). [0130] After (e.g., in response to) determining to prepare the first cell as an LTM candidate cell for the UE 102, the S-BS 104A (e.g., the CU of the S-BS 104A) generates a Handover Request message including a first cell ID (i.e., cell ID 1) of the first cell (i.e., cell 1). The S- BS 104A transmits 505 the Handover Request message to the CU 172. In some implementations, the Handover Request message includes an LTM indicator indicating the Handover Request message concerns LTM for the first cell ID. After (e.g., in response to) receiving the Handover Request message, the CU 172 performs an LTM preparation procedure 590 with the DU 174 to prepare the first cell as an LTM candidate cell for the UE 102, similar to the procedure 390 or 490. In the procedure 590, the CU 172 transmits a first CU-to-DU message including the first cell ID to the DU 174 to request preparing the first cell, similarly to the event 385. In response, the CU 172 may receive a first DU-to-CU message including an LTM DU configuration 1 from the DU 174, similar to the event 310. The CU 172 generates a first LTM candidate configuration (LTM candidate configuration 1). In response to the Handover Request message, the CU 172 transmits 507 a Handover Request Acknowledge message including the first LTM candidate configuration to the S-BS 104A. In some implementations, the CU 172 includes the first cell ID in the Handover Request Acknowledge message to indicate that the first LTM candidate configuration is provided for or associated with the first cell (ID).

[0131] The events 505, 590, 507 are collectively referred to in Eig. 5 as an inter-CU LTM preparation procedure (or, alternatively, an inter-MN LTM preparation procedure) 598.

[0132] In some implementations, the Handover Request message includes a DU ID of the S-DU of the S-BS 104A. In such cases, the CU 172 includes the DU ID in the first CU-to-DU message. In some implementations, the Handover Request message includes a BS ID of the S-BS 104A. When receiving the BS ID, the CU 172 may include the BS ID in the first CU- to-DU message. Lor example, the BS ID may be a gNB ID.

[0133] In some implementations, the CU 172 requests an LTM reference DU configuration in the procedure 590, as described with reference to Eigs. 3 and 4. In other implementations, the CU 172 does not request an LTM reference DU configuration in the procedure 590. In some implementations, the DU 174 transmits an LTM reference DU configuration to the CU 172 in the procedure 590, as described with reference to Eigs. 3 and 4. In other implementations, the DU 174 does not transmit an LTM reference DU configuration to the CU 172 in the procedure 590. In some implementations, the S-BS 104A (e.g., the CU of the S-BS 104A) may obtain an LTM reference configuration, as described with reference to Eigs. 3 and 4. In other implementations, the S-BS 104A may receive an LTM reference configuration from another BS (not shown in Fig. 5) in another inter-CU LTM preparation procedure as described above and later herein. If the S-BS 104A obtains an LTM reference configuration, the S-BS 104A may include the LTM reference configuration (S-BS generated LTM reference configuration) in the Handover Request message. In some implementations, the S-BS 104A includes the LTM reference configuration in the inter-node RRC message HandoverP reparationinformation or as an Xn Application Protocol (XnAP) IE or field and includes the inter-node RRC message or the XnAP IE in the Handover Request message. Alternatively, the S-BS 104A determines to request or cause the C-BS 106A to provide a complete LTM candidate configuration in which case the S-BS 104A does not include the LTM reference configuration in the Handover Request message. If the S-BS 104A does not obtain an LTM reference configuration, the S-BS 104A does not include an LTM reference configuration in the Handover Request message. If the Handover Request message includes an LTM reference configuration, the CU 172 may include the LTM reference configuration in the first CU-to-DU message. The DU 174 may extract an LTM reference DU configuration from the LTM reference configuration. Alternatively, the CU 172 extracts an LTM reference DU configuration from the LTM reference configuration and includes the LTM reference DU configuration in the first CU-to-DU message. The DU 174 may generate an LTM DU configuration as a delta configuration based on the LTM reference DU configuration, as described with reference to Fig. 3. Alternatively, the DU 174 may ignore the LTM reference (DU) configuration and generate an LTM DU configuration as a complete configuration, as described with reference to Fig. 3.

[0134] Otherwise, if the Handover Request message does not include an LTM reference configuration, in some embodiments, the CU 172 receives an LTM reference DU configuration from the DU 174 as described with reference to Fig. 3. In other embodiments, the CU 172 does not receive an LTM reference DU configuration from the DU 174. If the CU 172 receives an LTM reference DU configuration (e.g., in the first DU-to-CU message), the CU 172 generates an LTM reference configuration (C-BS generated LTM reference configuration) including the LTM reference DU configuration. The CU 172 may include an LTM reference CU configuration (candidate CU (C-CU) generated LTM reference CU configuration). Otherwise, if the CU 172 does not receive an LTM reference DU configuration from the DU 174 as described with reference to Fig. 3, the CU 172 does not generate an LTM reference configuration. Alternatively, the CU 172 generates an LTM reference configuration (C-BS generated LTM reference configuration) only including a C- CU generated LTM reference CU configuration. When the CU 172 generates an LTM reference configuration (C-BS generated LTM reference configuration), the CU 172 includes the C-BS generated LTM reference configuration in the Handover Request Acknowledge message.

[0135] In some implementations, if the LTM DU configuration 1 is a complete configuration, the CU 172 generates the LTM candidate configuration 1 as a complete configuration. The CU 172 may include a complete configuration indication (e.g., a BS-to-BS interface protocol field/IE) in the Handover Request Acknowledge message to indicate that the LTM candidate configuration 1 is a complete configuration. In some implementations, the BS-to-BS interface protocol is an Xn application protocol defined in 3GPP specification 38.423. In some implementations, the complete configuration indication is a dedicated field/IE (e.g., LTM specific) to reduce or eliminate an impact to non-LTM configurations. In other implementations, the complete configuration indication is an existing field/IE defined in 3GPP specification 38.423. Otherwise, if the LTM DU configuration 1 is a delta configuration, the CU 172 generates the LTM candidate configuration 1 as a delta configuration. The CU 172 may exclude the complete configuration indication from the Handover Request Acknowledge message to indicate that the LTM candidate configuration 1 is a delta configuration. Alternatively, the CU 172 may include a delta configuration indication (e.g., a BS-to-BS interface protocol field/IE) in the Handover Request Acknowledge message to indicate that the LTM candidate configuration 1 is a delta configuration. In some implementations, the BS-to-BS interface protocol is an Xn application protocol defined in 3GPP specification 38.423. In some implementations, the delta configuration indication is a dedicated field/IE (e.g., LTM specific) to reduce or eliminate an impact to non-LTM configurations. In other implementations, the delta configuration indication is an existing field/IE defined in 3GPP specification 38.423. In some implementations, the BS-to-BS interface protocol field/IE can have one of at least two possible values (i.e., a first value and a second value). The BS-to-BS interface protocol field/IE can have the first value to indicate a complete configuration indication and the BS-to- BS interface protocol field/IE can have the second value to indicate a delta configuration.

[0136] In some implementations, the S-BS 104A is preconfigured with a CSI resource configuration (e.g., LTM CSI resource configuration 1 or LTM CSI resource configuration 1 and/or an LTM SSB configuration (LTM SSB configuration 1) for the first cell. In other implementations, the S-BS 104A receives the CSI resource configuration and/or the LTM SSB configuration from an OAM (Operations, Administration and Maintenance) node. In yet other implementations, the S-BS 104A receives the CSI resource configuration and/or the LTM SSB configuration from the CU 172. For example, the CU 172 includes the CSI resource configuration and/or the LTM SSB configuration in the Handover Request Acknowledge message. In some implementations, the CU 172 includes a PCI (e.g., PCI 1) of the first cell in the Handover Request Acknowledge message. To prepare the first cell as a candidate LTM cell for the UE 102, the CU of the S-BS 104A performs an LTM CSI report configuration and/or LTM ID configuration procedure (not shown in Fig. 5) with an S-DU of the S-BS 104A, similar to the procedure 392. In the LTM CSI report configuration and/or LTM ID configuration procedure, the CU of the S-BS 104A transmits the CSI resource configuration and/or the LTM SSB configuration to the S-DU of the S-BS 104A. In response, the CU of the S-BS 104A receives one or more CSI report configurations for the UE 102 from the S-DU of the S-BS 104A. In some implementations, the CU of the S-BS 104A receives the CSI report configuration(s) in a second serving DU configuration from the S- DU.

[0137] To prepare the first cell as an LTM candidate cell for the UE 102, the CU 172 may receive early synchronization information (e.g., early synchronization information 1) for the first cell from the DU 174 in a DU-to-CU message (e.g., the first DU-to-CU message or an additional DU-to-CU message). In some implementations, the DU 174 transmits the additional DU-to-CU message in response to receiving an additional CU-to-DU message from the CU 172. In other implementations, the DU 174 transmits the additional DU-to-CU message (e.g., a UE Context Modification Required message) in response to receiving the first CU-to-DU message. The CU 172 includes the early synchronization information in the Handover Request Acknowledge message. The early synchronization information includes a RACH configuration (e.g., RACH configuration 1) and/or at least one TCI state configuration (e.g., TCI state configuration(s) 1). In some implementations, the DU 174 includes, in the early synchronization information or in the DU-to-CU message, PDCCH order information for early UL synchronization with the first cell. The CU 172 includes the PDCCH order information in the Handover Request Acknowledge message.

[0138] In some implementations, the CU 172 assigns an LTM ID (e.g., LTM ID 1) for identifying the first LTM candidate configuration and includes the LTM ID in the Handover Request Acknowledge message. In other implementations, the S-BS 104A assigns an LTM ID (e.g., LTM ID 1) for identifying the first LTM candidate configuration.

[0139] After (e.g., in response to) receiving the Handover Request Acknowledge message, the S-BS 104A (e.g., the CU of the S-BS 104A) transmits 518 a first RRC reconfiguration message to the UE 102, including {the LTM ID 1, the LTM candidate configuration 1 } as a tuple, similar to the event 318. If the Handover Request Acknowledge message includes the LTM reference configuration, the S-BS 104A may include the LTM reference configuration in the first RRC reconfiguration message. The S-BS 104A may include the CSI resource configuration 1, the TCI state configuration(s) 1, the RACH configuration 1, and/or the LTM SSB configuration 1 in the tuple, if the S-BS 104A received this information in the Handover Request Acknowledge message. Alternatively, the S-BS 104A transmits one or more additional RRC reconfiguration messages to the UE 102, including the CSI resource configuration 1, the TCI state configuration(s) 1, the RACH configuration 1, the LTM SSB configuration 1, and/or the PCI of the first cell. In each of the additional RRC reconfiguration message(s), the S-BS 104A includes the LTM ID 1 to indicate that the CSI resource configuration 1, the TCI state configuration(s) 1, the RACH configuration 1, the LTM SSB configuration 1, and/or the PCI of the first cell are associated with the first cell or configured for the first cell. The UE 102 transmits 520 a first RRC reconfiguration complete message to the S-BS 104A in response to the first RRC reconfiguration message. The UE 102 transmits an additional RRC reconfiguration complete message to the S-BS 104A in response to each of the additional RRC reconfiguration complete message. The RRC reconfiguration message(s) (i.e., the first RRC reconfiguration message and/or the additional RRC reconfiguration message(s)) and the RRC reconfiguration complete message(s) (i.e., the first RRC reconfiguration complete message and/or the additional RRC reconfiguration complete message(s)) form an LTM configuration delivery procedure. The S-BS 104A may include the second serving DU configuration in the first RRC reconfiguration message or in one of the additional RRC reconfiguration message(s).

[0140] In some implementations, if the Handover Request Acknowledge message includes the complete configuration indication to indicate that the LTM candidate configuration 1 is a complete configuration, the S-BS 104A includes, in the first RRC reconfiguration message, a complete configuration indication (e.g., an RRC field/IE) to indicate that the LTM candidate configuration 1 is a complete configuration. Otherwise, if the Handover Request Acknowledge message does not include the complete configuration indication (or if the Handover Request Acknowledge message includes the delta configuration indication to indicate that the LTM candidate configuration 1 is a delta configuration), the S-BS 104A indicates that the LTM candidate configuration 1 is a delta configuration by excluding the complete configuration indication (e.g., an RRC field/IE) from the first RRC reconfiguration message.

[0141] If the Handover Request Acknowledge message includes the PDCCH order information (e.g., PDCCH order information 1), the S-BS 104A transmits 550 a PDCCH order based on the PDCCH order information. If the S-BS 104A is a distributed base station, the CU of the S-BS 104A may transmit the PDCCH order information to the S-DU of the S- BS 104A. For example, the S-BS 104A or the S-DU of the S-BS 104A transmits 550 a PDCCH order to the UE 102, including the PDCCH order information. In some implementations, the S-BS 104A or the S-DU of the S-BS 104A may determine an SSB index included in the PDCCH order, based on LI measurement report(s) 524, and/or the CSI resource configuration, the CSI report configuration, and/or the LTM SSB configuration. In some implementations, the S-DU or the S-BS 104A includes the LTM ID 1 in the PDCCH order to indicate the first cell. The UE 102 transmits 552 an RA preamble to the DU 174 on the first cell, using the RACH configuration and/or the PDCCH order information. The S-DU or the S-BS 104A derives a TA value based on the RA preamble. The DU 174 transmits 556 a DU-CU TA Information Transfer message including the TA value to the CU 172. The CU 172 transmits 558 a CU-CU TA Information Transfer message including the TA value to the S-BS 104A (e.g., the CU of the S-BS 104A). In some implementations, the DU 174 includes the cell ID 1, the RA preamble index, an RA-RNTI, the DU ID of the S-DU of the S-BS 104A, and/or the BS ID of the S-BS 104A in the message 556. In some implementations, the DU 174 does not include the BS ID in the message 556. In some implementations, the CU 172 includes the cell ID 1, the RA preamble index, the RA-RNTI, the DU ID of the S-DU, and/or the BS ID of the S-BS 104A in the message 558. In some implementations, the CU 172 does not include the BS ID in the message 558. The CU of the S-BS 104A transmits a CU-DU TA Information Transfer message including the TA value, the cell ID 1, the RA preamble index, the RA-RNTI, and/or the DU ID of the S-DU, and/or the BS ID of the S-BS 104A to the S-DU of the S-BS 104A. In some implementations, the CU of the S-BS 104A does not include the BS ID in the CU-DU TA Information Transfer message.

[0142] In some implementations, the CU 172 determines an address (e.g., an IP address) of the S-BS 104A or the CU of the S-BS 104A, based on the BS ID of the S-BS 104A. In other implementations, the CU 172 determines an address (e.g., an IP address) of the S-BS 104A or the CU of the S-BS 104A based on the DU ID of the S-DU of the S-BS 104A. With these implementations, the CU 172 sends the CU-CU TA Information Transfer message to the S- BS 104A or the CU of the S-BS 104A in accordance with the address.

[0143] In response to determining to command the UE 102 to perform an LTM cell switch to the first cell (e.g., based on the measurement report(s) 524 and/or 554), the S-DU or the S- BS 104A transmits 526 the LTM Cell Switch Command including the LTM ID 1 (e.g., configuration identifier) to the UE 102. In response to the LTM Cell Switch Command, the UE 102 may stop communication on the serving cell(s). In response to the LTM Cell Switch Command, the UE 102 accesses 532 the first cell and transmits 536 an RRC reconfiguration complete message to DU 174. The DU 174 in turn transmits 538 a DU-to-CU message including the RRC reconfiguration complete message to the CU 172. After receiving 538 the DU-to-CU message or the RRC reconfiguration complete message, the C-BS 106A communicates 540 with the UE 102 in accordance with the first LTM candidate configuration and/or the LTM reference configuration. In some implementations, the UE 102 transmits 536 the RRC reconfiguration complete message including the LTM ID 1 to indicate that the UE 102 applies the first LTM candidate configuration. In accordance with the LTM ID 1, the CU 172 identifies the first LTM candidate configuration and/or the LTM reference configuration. In other implementations, the CU 172 identifies the first LTM candidate configuration and/or the LTM reference configuration based on the first cell ID included in the Access Success message 534. In such implementations, the CU 172 maintains or stores association information between the first cell ID, and the first LTM candidate configuration and/or the LTM reference configuration. When the CU 172 receives 534 the first cell ID in the Access Success message, the CU 172 identifies the first LTM candidate configuration and/or the LTM reference configuration in accordance with the first cell ID and the association information.

[0144] In some implementations, if the S-DU or the S-BS 104A receives a TA value as described above, the S-DU or the S-BS 104A may include the TA value in the LTM Cell Switch Command. In some implementations, the S-DU or the S-BS 104A includes a first TCI state ID in the LTM Cell Switch Command. The first TCI state ID indicates a first one of the TCI state configuration(s).

[0145] In response to determining to command the UE 102 to perform the LTM cell switch or transmitting 526 the LTM Cell Switch Command, the S-DU of the S-BS 104A transmits a DU-CU Cell Switch Notification message to the CU of the S-BS 104A to indicate that the UE 102 performs or is performing an LTM cell switch to the first cell. In response to receiving the DU-CU Cell Switch Notification message, the CU of the S-BS 104A transmits 527 a CU- CU Cell Switch Notification message to the CU 172 to indicate that the UE 102 performs an LTM cell switch to the first cell. In response, the CU 172 transmits 530 a CU-DU Cell Switch Notification message to the DU 174 to indicate that the UE 102 performs an LTM cell switch to the first cell. In some implementations, the S-BS 104A includes the first TCI state ID in the CU-CU Cell Switch Notification message and the CU 172 then includes the first TCI state ID in the CU-DU Cell Switch Notification message. The UE 102 and the DU 174 identify the first one of the TCI state configuration(s) based on the first TCI state ID and the UE 102 and the DU 174 apply the first TCI state configuration to communicate UL transmissions and/or DL transmissions in the events 532, 536, and/or 540. In other implementations, when separate TCI states are used, the DL TCI State and the UL TCI state use different TCI State IDs. In such cases, the S-DU of the S-BS 104A includes, in the DU-CU Cell Switch Notification message and/or the LTM Cell Switch Command, a first TCI State ID for DL and a second TCI State ID for UL that identify a first and a second TCI state configuration(s), respectively. The CU of the S-BS 104A, in response to the DU-CU Cell Switch Notification message, in turn includes the first TCI State ID and the second TCI state ID in the CU-CU Cell Switch Notification message to the CU 172. The CU 172 then includes the first TCI state ID and the second TCI state ID in the CU-DU Cell Switch Notification message to the DU 174. The UE 102 identifies the first TCI state configuration and the second TCI state configuration based on the first TCI state ID and the second TCI state ID, respectively. The UE 102 applies the first TCI state configuration and the second TCI state configuration to receive DL transmissions and transmit UL transmissions, respectively, with the DU 174 in the events 532, 536, and/or 540. The DU 174 identifies the first TCI state configuration and the second TCI state configuration based on the first TCI state ID and second TCI state ID, respectively. The DU 174 applies the first and/or the second TCI state configuration to transmit DL transmissions and/or receive UL transmissions, respectively, to/from the UE 102 in the events 532, 536, and/or 540.

[0146] In some implementations, after (e.g., in response to) determining to command the UE 102 to perform the LTM cell switch, transmitting 526 the LTM Cell Switch Command, or receiving 527 the DU-CU Cell Switch Notification message, the S-BS 104A (e.g., the CU of the S-BS 104A) transmits 531 one or more Early Status Transfer messages to the CU 172, each including a DL COUNT value or a DISCARD DL COUNT value for a DRB over which the UE 102 and the S-BS 104A communicate 502 data with each other. In some implementations, after receiving 534 the Access Success message or after receiving 538 the DU-to-CU message or the RRC reconfiguration complete message, the CU 172 transmits 539 an LTM Success message to the S-BS 104A (e.g., the CU of the S-BS 104A) to indicate that the LTM cell switch is completed successfully. In some implementations, the LTM Success message is a Handover Success message. In some implementations, the CU 172 includes the first cell ID in the LTM Success message. In other implementations, the CU 172 does not transmit a BS-to-BS message to the S-BS 104A (e.g., the CU of the S-BS 104A) to indicate that the LTM cell switch is completed successfully.

[0147] In some implementations, after (e.g., in response to) determining to command the UE 102 to perform the LTM cell switch, transmitting 526 the LTM Cell Switch Command, receiving 527 the DU-CU Cell Switch Notification message, or receiving 539 the LTM Success message, the S-BS 104A (e.g., the CU of the S-BS 104A) transmits 541 an SN Status Transfer message to the CU 172, including a DL COUNT value and/or a UL COUNT value for a/the DRB over which the UE 102 and the S-BS 104A communicate 502 data with each other.

[0148] In some implementations, after (e.g., in response to) receiving 534 the Access Success message, receiving 538 the DU-to-CU message or the RRC reconfiguration complete message or receiving 541 the SN Status Transfer message, the CU 172 transmits 543 a UE Context Release message to the S-BS 104A. In response to the UE Context Release message, the S-BS 104A releases a UE context of the UE 102.

[0149] In some implementations, the S-BS 104A (e.g., the CU of the S-BS 104A) may prepare additional cell(s) (i.e., cell(s) 2, ..., N) as LTM candidate cell(s) for the UE 102 with the CU 172, before or after transmitting the LTM Cell Switch Command or during, before or after the procedure 598, as described above. The cell(s) 2, ..., N are identified by cell ID(s) 2, ..., N, respectively and operated by the DU 174 and/or other DU(s) of the C-BS 106A. N is an integer and larger than 1. Lor example, the S-BS 104A performs additional inter-CU LTM preparation procedure(s) 2, ... , N with the CU 172 to prepare the cell(s) 2, ... , N, respectively. Each of the inter-CU LTM preparation procedure(s) 2, ..., N is similar to the procedure 598. In the inter-CU LTM preparation procedure(s) 2, ..., N, the S-BS 104A receives LTM candidate configuration(s) 2, .. N configuring the cell(s) 2, .. N for LTM, respectively. As described above, the S-BS 104A or the C-BS 106A assigns LTM ID(s) 2, N to identify the LTM candidate configuration(s) 2, N, respectively. The S-BS 104A may obtain CSI resource configuration 2, N for the cell(s) 2, .. N respectively, as described for the CSI resource configuration 1. The S-BS 104A may obtain CSI report configuration(s) 2, .. N or the cell(s) 2, .. N, respectively, as described for the CSI report configuration(s)

1. The S-BS 104A may obtain RACH configuration 2, ..., N for the cell(s) 2, ..., N respectively, as described for the RACH configuration 1. The S-BS 104A may obtain TCI state configuration(s) 2, ..., N for the cell(s) 2, ..., N respectively, as described for the TCI state configuration(s) 1. The S-BS 104A may obtain LTM SSB configuration 2, ..., N for the cell(s) 2, ..., N, respectively, as described for LTM SSB configuration 1. The S-BS 104A may obtain PCI(s) 2, ..., N for the cell(s) 2, ..., N respectively, as described for the PCI 1. In some implementations, the S-BS 104A may perform LTM configuration delivery procedure

2, ..., N with the UE 102 to transmit a list of the {LTM ID 2, the LTM candidate configuration 2, the CSI resource configuration 2 (if obtained), the TCI state configuration 2 (if obtained), the RACH configuration 2 (if obtained), the LTM SSB configuration 2 (if obtained), the PCI 2 (if obtained)}, ..., {the LTM ID N, the LTM candidate configuration N, the CSI resource configuration N, the TCI state configuration(s) N (if obtained), the RACH configuration N (if obtained), the LTM SSB configuration N (if obtained), the PCI N (if obtained)} to the UE 102, respectively. Each of the LTM configuration delivery procedure 2, ..., N is similar to the procedures 394, and/or 494. In other implementations, the S-BS 104A includes the list in the first RRC reconfiguration message.

[0150] In other implementations, the S-BS 104A performs the procedure 598 with the CU 172 to prepare one or more of the cell(s) 1, ..., N as LTM candidate cell(s) for the UE 102. In such implementations, the S-BS 104A includes the cell ID(s) 1, ..., N in the Handover Request message 505 for LTM, as described for the cell ID 1. In some implementations, upon receiving the Handover Request message, the CU 172 determines or selects the cell(s) 1, ..., M from the cell(s) 1, ..., N as LTM candidate cell(s). M is a positive integer and M < N. In other implementations, CU 172 prepares the cell(s) 1, ..., N for LTM as requested in the Handover Request message. The CU 172 performs LTM preparation procedure(s) 2, ..., M with the DU 174 to prepare the cell(s) 2, ..., M as LTM candidate cell(s) for the UE 102, respectively. The LTM preparation procedure(s) 2, ..., M are similar to the procedure 590 that the CU 172 performs with the DU 174 to prepare the cell 1. The CU 172 obtains the LTM candidate configuration(s) 2, ..., M for the cell(s) 2, ..., M as a result of the LTM preparation procedure(s) 2, ..., M respectively, similar to obtaining the LTM candidate configuration 1. The C-BS 106A may generate the LTM candidate configuration(s) 2, ..., M as complete configuration(s) or generate delta configuration(s) based on the LTM reference configuration, as described for the LTM candidate configuration 1. The CU 172 includes the LTM candidate configuration(s) 2, ..., M in the Handover Request Acknowledge message. In some implementations, the CU 172, or the S-BS 104A assigns LTM ID(s) 2, ..., M to identify the LTM preparation procedure(s) 2, ..., M respectively, as described for the LTM ID 1. When the CU 172 assigns the LTM ID(s) 1, ..., M, the CU 172 includes the LTM ID(s) 1, ..., M with the LTM candidate configuration(s) 1, ..., M, respectively in the Handover Request Acknowledge message, as described with reference to the LTM ID 1 and the LTM candidate configuration 1.

[0151] In some implementations, the CU 172 obtains early synchronization information 2, ..., M for the cell(s) 2, ..., M, respectively, as described with reference to the early synchronization information 1. The CU 172 includes the early synchronization information 2, ..., M in the Handover Request Acknowledge message. In some implementations, the CU 172, or the S-BS 104A obtains CSI resource configuration(s) 2, ..., M for the cell(s) 2, ..., M, respectively, as described with reference to the CSI resource configuration 1. When the CU 172 obtains the CSI resource configuration(s) 2, ..., M, the CU 172 includes the CSI resource configuration(s) 2, ..., M in the Handover Request Acknowledge message.

[0152] In some implementations, the CU 172, or the S-BS 104A obtains LTM SSB configuration(s) 2, ..., M for the cell(s) 2, ..., M, respectively, as described with reference to the LTM SSB configuration 1. When the CU 172 obtains the LTM SSB configuration(s) 2, ..., M, the CU 172 includes the LTM SSB configuration(s) 2, ..., M in the Handover Request Acknowledge message. In some implementations, the CU 172, or the S-BS 104A obtains PCI(s) 2, ..., M for the cell(s) 2, ..., M, respectively, as described for the PCI 1. When the CU 172 obtains the PCI(s) 2, ..., M, the CU 172 includes the PCI(s) 2, ..., M in the Handover Request Acknowledge message.

[0153] In some implementations, the CU 172 includes a list of {the cell ID 1, the LTM ID 1 (if obtained or optional), the LTM candidate configuration 1, the CSI resource configuration 1 (if obtained or optional), the TCI state configuration 1 (if obtained or optional), the early synchronization information 1 (if obtained or optional), the LTM SSB configuration 1 (if obtained or optional), PCI 1 (if obtained or optional)}, , {the cell ID

M, the LTM ID M (if obtained or optional), the LTM candidate configuration M, the CSI resource configuration M (if obtained or optional), the TCI state configuration M (if obtained or optional), the early synchronization information M (if obtained or optional), the LTM SSB configuration M (if obtained or optional), the PCI M (if obtained or optional)} in the Handover Request Acknowledge message.

[0154] In some implementations, the CU 172 obtains PDCCH order information 2, ..., M for the cell(s) 2, ..., M, respectively, as described for the PDCCH order information 1. In one implementation, the CU 172 includes the PDCCH order information 2, ..., M in the Handover Request Acknowledge message. In another implementation, the CU 172 includes the PDCCH order information 2, ..., M in the early synchronization information 2, ..., M, respectively. The S-BS 104A may obtain RACH configuration 2, TCI state configuration(s) 2 and/or the PDCCH order information 2, ...., RACH configuration M, TCI state configuration(s) M and/or the PDCCH order information M from the early synchronization information 2, ..., M, respectively or from the Handover Request Acknowledge message.

[0155] In some implementations, the S-BS 104A may perform the LTM configuration delivery procedure 2, ..., M with the UE 102 to transmit a list of the {LTM ID 2, the LTM candidate configuration 2, the CSI resource configuration 2 (if obtained), the TCI state configuration 2 (if obtained), the RACH configuration 2 (if obtained), the LTM SSB configuration 2 (if obtained), the PCI 2 (if obtained)}, ..., {the LTM ID M, the LTM candidate configuration M, the CSI resource configuration M, the TCI state configuration(s) M (if obtained), the RACH configuration M (if obtained), the LTM SSB configuration M (if obtained), the PCI M (if obtained)} to the UE 102, respectively. Each of the LTM configuration delivery procedure 2, ..., M is similar to the procedures 394, and/or 494. In other implementations, the S-BS 104A includes the list in the first RRC reconfiguration message.

[0156] In some implementations, the S-BS 104A may include measurement result(s) 1, ..., N for the cell(s) 1, ..., N respectively in the Handover Request message. The S-BS 104A receives the measurement result(s) from the UE 102. The C-BS 106A may select or determine the cell(s) 1, ...M, based on the measurement result(s) 1, ..., N. In other implementations, the C-BS 106A may select or determine the cell(s) 1, ...M, based on one or more other factors (e.g., capacity or load of the cell(s) 1, ..., N and/or a maximum number of LTM candidate cell(s)). In one implementation, the maximum number of LTM candidate cell(s) is included in the Handover Request message. In another implementation, the maximum number of the LTM candidate cell(s) is a predetermined number. [0157] Next, several example methods are discussed with reference to Figs. 6A-17. Descriptions provided with reference to Figs. 3-5 can apply to Figs. 6A-16. Generally speaking, similar events in Figs. 6A-20 are labeled with similar reference numbers that share two least significant digits, with differences discussed where appropriate. For example, event 302 is similar to event 602. With the exception of differences show in the figures and discussed below, any of the other implementations discussed with respect to a particular event (e.g., for messaging and processing) may apply to events labeled with similar reference numbers in other figures.

[0158] Figs. 6A-6D are messaging sequence diagrams of inter-CU LTM scenarios in which a UE is in DC. Referring first to Fig. 6A, in a scenario 600A, the base station 104B operates as an MN, the base station 106A operates as a candidate SN (C-SN) and the base station 104A operates as a serving or source SN (S-SN). The C-SN 106A includes a CU 172 and a DU 174. The MN 104B may include a CU and a DU (not shown in Fig. 6A), similar to the BS 104A in Fig. 3. The S-SN 104A may include a CU and a DU (not shown in Fig. 6A), similar to the C-SN 106A. The scenario 600A is similar to the scenario 500, except that the scenario 600A is an SN scenario in which a UE communicates with an MN and an SN and the scenario 500 is an MN scenario.

[0159] Initially, the UE 102 in DC communicates 602 with the MN 104B and with S-SN 104A. In some implementations, the UE 102 communicates with the DU of the S-SN 104A on cell 124A using a serving DU configuration and communicates with the CU of the S-SN 104A via the DU of the S-SN 104A using a serving CU configuration, similar to the event 302. In some implementations, the UE 102 in DC can communicate 602 UL PDUs and/or DL PDUs with the MN 104B and/or S-SN 104A via radio bearers which can include SRBs and/or DRB(s). The MN 104B and/or the S-SN 104A can configure the radio bearers to the UE 102. The UE 102 in DC communicates 602 UL PDUs and/or DL PDUs with the S-SN 104A on an SCG (i.e., SCG radio resources) that the S-SN 104A configures for communication with the UE 102. The UE 102 in DC communicates UL PDUs and/or DL PDUs with the MN 104B on an MCG (i.e., MCG radio resources) in accordance with an MN configuration (i.e., MCG configuration). In some implementations, the serving DU configuration is an SN configuration (i.e., SCG configuration). In the MN configuration, the MN 104B configures the MCG which includes at least one serving cell (e.g., the cell 124B and/or other cell(s)) operated by the MN 104B. In the serving DU configuration, the S-SN 104A configures the SCG which includes at least one serving cell (e.g., the cell 124A and/or other cell(s)) operated by the S-SN 104A. In some implementations, the MN configuration includes multiple configuration parameters, and the UE 102 receives the configuration parameters in one or more RRC messages from the MN 104B. As described with reference to Fig. 3, the serving DU configuration includes multiple configuration parameters. In some implementations, the UE 102 receives these configuration parameters in one or more RRC messages from the S-SN 104A, e.g., via the MN 104B and/or on an SRB (e.g., SRB3) that the MN 104B or the S-SN 104A configures to exchange RRC messages between the UE 102 and the S-SN 104A.

[0160] While communicating with the UE 102 in DC with the MN 104B, the S-SN 104A can perform 696 an intra-CU LTM configuration procedure with the UE 102, similar to the procedures 396 and/or 496. While communicating with the UE 102 in DC with the MN 104B and S-SN 104A, the S-SN 104A can perform 680 an intra-CU LTM procedure with the UE 102, similar to the procedures 380 and/or 480. In the procedure 696 or 680, the CU of the S- SN 104A may transmit one or more RRC reconfiguration messages, each including one or more LTM candidate configurations, to the UE 102 via the SRB 3, similar to the procedure 394 or 494. Alternatively, the CU of the S-SN 104A may transmit one or more RRC reconfiguration messages, each including one or more LTM candidate configurations, to the UE 102 via the MN 104B. In response to each of the RRC reconfiguration message(s), the UE 102 transmits an RRC reconfiguration complete message to the S-SN 104A via the MN 104B.

[0161] While communicating in DC with the MN 104B and S-SN 104A, the UE 102 can transmit 604A at least one measurement report to the S-SN 104A directly, e.g., via the SRB3, similar to the events 304/306, the events 404/406, and the event 504. Alternatively, the UE 102 can transmit the measurement report(s) to the MN 104B. The MN 104B generates at least one SN message including the measurement report(s) and transmits the SN message(s) to the CU 172. In one implementation, the SN message(s) include RRC Transfer message(s) and/or SN Modification Request message(s). The S-SN 104A (e.g., the CU of the S-SN 104A) generates a measurement configuration to configure the UE 102 to transmit the measurement report(s) and transmits the measurement report to the UE 102 directly (e.g., via the S-DU of the S-SN 104A) or via the MN 104B.

[0162] After (e.g., in response to) receiving one or some of the measurement report(s) from the UE 102, the S-SN 104A (e.g., the CU of the S-SN 104A) determines to prepare a first cell (e.g., the cell 126A) as an LTM candidate cell for the UE 102. In response to the determination, the S-SN 104A generates an SN Required message (a first SN Required message) including a first cell ID (e.g., cell ID 1) of the first cell (e.g., cell 1) and/or a target SN ID indicating the C-SN 106A. In some implementations, the SN Required message includes an LTM indicator indicating the SN Required message concerns LTM for the first cell ID. The S-SN 104A transmits 603 the SN Required message to the MN 104B. After (e.g., in response to) receiving the SN Required message, the MN 104B transmits 605 an SN Request message (a first SN Request message) including the first cell ID to the CU 172 to request preparing the first cell as an LTM candidate cell for the UE 102. In response to the SN Request message, the CU 172 performs an LTM preparation procedure 690 with the DU 174 to prepare the first cell as an LTM candidate cell for the UE 102, similar to the procedure 390, 490 or 590. In the procedure 690, the CU 172 transmits a first CU-to-DU message including the first cell ID to the DU 174 to request preparing the first cell, similar to the event 308. In response, the CU 172 may receive a first DU-to-CU message including an LTM DU configuration 1 from the DU 174, similar to the event 310. The CU 172 generates a first LTM candidate configuration (e.g., LTM candidate configuration 1). In response to the SN Request message, the CU 172 transmits 607 an SN Request Acknowledge message (e.g., a first SN Request Acknowledge message) including the first LTM candidate configuration to the MN 104B. In some implementations, the CU 172 includes the first cell ID in the SN Request Acknowledge message to indicate that the first LTM candidate configuration is provided for or associated with the first cell (e.g., an ID or first cell ID). The MN 104B transmits 609A an SN Confirm message (e.g., a first SN Confirm message) including the first LTM candidate configuration to the S-SN 104A. In some implementations, the MN 104B includes the first cell ID in the SN Confirm message to indicate that the first LTM candidate configuration is provided for or associated with the first cell (ID).

[0163] The events 603, 605, 690, 607, and 609A are collectively referred to in Eig. 6A as an SN-initiated inter-SN LTM preparation procedure 698A.

[0164] In some implementations, the SN Required message and the SN Confirm message are an SN Change Required message and an SN Change Confirm message, respectively. In other implementations, the SN Required message and the SN Confirm message are an SN Modification Required message and an SN Modification Confirm message, respectively. In some implementations, the SN Request message and the SN Request Acknowledge message are an SN Addition Request message and an SN Addition Request Acknowledge message, respectively. In other implementations, the SN Request message and the SN Request Acknowledge message are an SN Modification Request message and an SN Modification Request Acknowledge message, respectively.

[0165] In some implementations, the SN Required message includes a DU ID of the S-DU of the S-SN 104A. In such cases, the MN 104B may include the DU ID in the SN Request message and the CU 172 may include the DU ID in the first CU-to-DU message. In other implementations, the SN Required message includes a first BS ID of the S-SN 104A. In some implementations, the MN 104B may include the first BS ID in the SN Request message. When receiving the first BS ID, the CU 172 may include the first BS ID in the first CU-to- DU message. In some implementations, the MN 104B may include a second BS ID of the MN 104B in the SN Request message. When receiving the second BS ID, the CU 172 may include the second BS ID in the first CU-to-DU message. In some implementations, the first BS ID and the second BS ID are gNB IDs.

[0166] In some implementations, the CU 172 requests an LTM reference DU configuration in the procedure 690, as described with reference to Figs. 3, 4 and 5. In other implementations, the CU 172 does not request an LTM reference DU configuration in the procedure 690. In some implementations, the DU 174 transmits an LTM reference DU configuration to the CU 172 in the procedure 690, as described with reference to Figs. 3, 4 and 5. In other implementations, the DU 174 does not transmit an LTM reference DU configuration to the CU 172 in the procedure 690. In some implementations, the S-SN 104A (e.g., the CU of the S-SN 104A) may obtain an LTM reference configuration, as described with reference to Figs. 3 and 4. In other implementations, the S-SN 104A may receive an LTM reference configuration from the MN 106B (not shown in Fig. 6A) in another inter-SN LTM preparation procedure as described above and below. If the S-SN 104A obtains an LTM reference configuration, the S-SN 104A may include the LTM reference configuration (S-SN generated LTM reference configuration) in the SN Required message. In turn, the MN 104B includes the LTM reference configuration in the SN Request message. Alternatively, the S- SN 104A determines to request or cause the C-SN 106A to provide a complete LTM candidate configuration so that the S-SN 104A does not transmit the LTM reference configuration to the C-SN 106A. If the S-SN 104A does not obtain an LTM reference configuration, the S-SN 104A does not include an LTM reference configuration in the SN Required message. If the SN Required message includes an LTM reference configuration, the MN 104B includes the LTM reference configuration in the SN Request message. Otherwise, if the SN Required message does not include an LTM reference configuration, the MN 104B may not include the LTM reference configuration in the SN Request message.

[0167] If the SN Request message includes an LTM reference configuration as described above, the CU 172 may include the LTM reference configuration in the first CU-to-DU message. Alternatively, the CU 172 may extract an LTM reference DU configuration from the LTM reference configuration and includes the LTM reference DU configuration in the first CU-to-DU message. Otherwise, if the SN Request message does not include an LTM reference configuration, in some embodiments the CU 172 receives an LTM reference DU configuration from the DU 174 as described with reference to Fig. 3. In other embodiments, the CU 172 does not receive an LTM reference DU configuration from the DU 174. If the CU 172 receives an LTM reference DU configuration (e.g., in the first DU-to-CU message), the CU 172 generates an LTM reference configuration (C-SN generated LTM reference configuration) including the LTM reference DU configuration. The CU 172 may include an LTM reference CU configuration (candidate CU (C-CU) generated LTM reference CU configuration). Otherwise, if the CU 172 does not receive an LTM reference DU configuration from the DU 174 as described with reference to Fig. 3, the CU 172 does not generate an LTM reference configuration. Alternatively, the CU 172 generates an LTM reference configuration (C-SN generated LTM reference configuration) only including a C- CU generated LTM reference CU configuration. When the CU 172 generates an LTM reference configuration (C-SN generated LTM reference configuration), the CU 172 includes the C-SN generated LTM reference configuration in the SN Request Acknowledge message.

[0168] In some implementations, the S-SN 104A can be configured similarly to the S-BS 104A as described earlier herein with reference to Fig. 5 except that some configurations are provided in SN messages rather than in Handover messages. In some implementations, the S- SN 104A receives the CSI resource configuration and/or the LTM SSB configuration from the CU 172. For example, the CU 172 includes the CSI resource configuration and/or the LTM SSB configuration in the SN Request Acknowledge message. In this case, the MN 104B may include the CSI resource configuration and/or the LTM SSB configuration in the SN Confirm message. In some implementations, the CU 172 includes a PCI of the first cell in the SN Request Acknowledge message. In this case, the MN 104B may include the PCI of the first cell in the SN Confirm message. To prepare the first cell as a candidate LTM cell for the UE 102, the CU of the S-SN 104A performs an LTM CSI report configuration and/or LTM ID configuration procedure (not shown in Fig. 6A) with an S-DU of the S-SN 104A, similar to the procedure 392 or 492. In the LTM CSI report configuration and/or LTM ID configuration procedure, the CU of the S-SN 104A transmits the CSI resource configuration and/or the LTM SSB configuration to the S-DU of the S-SN 104A. In response, the CU of the S-SN 104A receives one or more CSI report configurations for the UE 102 from the S-DU of the S-SN 104A. In some implementations, the S-SN 104A receives a second serving DU configuration including the CSI report configuration(s).

[0169] In some implementations, the CU 172 receives early synchronization information for the first cell and/or PDCCH order information from the DU 174, as described with reference to Fig. 5. The CU 172 includes the early synchronization information and/or the PDCCH order information in the SN Request Acknowledge message. The MN 104B includes the early synchronization information and/or the PDCCH order information in the SN Confirm message.

[0170] In some implementations, the CU 172 assigns an LTM ID (e.g., LTM ID 1) for identifying the first LTM candidate configuration and includes the LTM ID and/or an associated (or mapped) cell ID for the LTM ID in the SN Request Acknowledge message. The MN 104 includes the LTM ID and/or an associated (or mapped) cell ID for the LTM ID in the SN Confirm message. In other implementations, the S-SN 104A assigns an LTM ID (e.g., LTM ID 1) for identifying the first LTM candidate configuration. In yet other implementations, the MN 104B assigns an LTM ID (e.g., LTM ID 1) for identifying the first LTM candidate configuration. In such cases, the MN 104B may include the LTM ID in the SN Confirm message. After receiving or assigning the LTM ID, the CU of the S-SN 104A transmits the LTM ID and the first cell ID to the S-DU in an LTM ID configuration procedure, similar to the procedure 390, 392, or 492.

[0171] After (e.g., in response to) receiving the SN Confirm message, the S-SN 104A (e.g., the CU of the S-SN 104A) performs 694A an LTM configuration delivery procedure with the UE 102 to transmit the LTM ID and the LTM candidate configuration to the UE 102. In some implementations, the S-SN 104A transmits {LTM ID, the LTM candidate configuration] as a tuple in a first RRC reconfiguration message in the procedure 694A. Depending on the implementations, the S-SN 104A may include the LTM reference configuration, the CSI report configuration(s), the CSI resource configuration, the RACH configuration, the TCI state configuration(s), the LTM SSB configuration and/or the PCI of the first cell to the UE 102 in the first RRC reconfiguration message and/or other RRC reconfiguration message(s) transmitted to the UE 102, as described with reference to Figs. 3 and 4. The S-SN 104A includes the LTM ID in the first RRC reconfiguration or the other RRC reconfiguration message(s) to indicate the CSI resource configuration, the RACH configuration, the TCI state configuration(s), the LTM SSB configuration, and/or the PCI of the first cell are associated with the first cell. For example, the S-SN 104A includes the CSI resource configuration, the RACH configuration, the TCI state configuration(s), the LTM SSB configuration, and/or the PCI of the cell 1 in the tuple. In another example, the S-SN 104A includes {LTM ID 1, the CSI resource configuration, the RACH configuration, the TCI state configuration(s), the LTM SSB configuration, and/or the PCI of the cell 1 } as a tuple in the other RRC reconfiguration message(s). In response to each of the other RRC reconfiguration message(s), the UE 102 transmits an RRC reconfiguration complete message to the CU 172 via the S-DU 174A.

[0172] In some implementations, if the SN Required message includes the complete configuration indication to indicate that the LTM candidate configuration 1 is a complete configuration, the S-SN 104A includes, in the first RRC reconfiguration message, a complete configuration indication (e.g., an RRC field/IE) to indicate that the LTM candidate configuration 1 is a complete configuration. Otherwise, if the SN Required message does not include the complete configuration indication or includes the delta configuration indication to indicate that the LTM candidate configuration 1 is a delta configuration, the S-SN 104A excludes or does not include, in the first RRC reconfiguration message, the complete configuration indication (e.g., an RRC field/IE) to indicate that the LTM candidate configuration 1 is a delta configuration.

[0173] If the SN Confirm message includes the PDCCH order information (PDCCH order information 1), the S-SN 104A may transmit 650A a PDCCH order to the UE 102, based on the PDCCH order information. If the S-SN 104A is a distributed base station, the CU of the S-SN 104A may transmit the PDCCH order information to the S-DU of the S-SN 104A and the S-DU may transmit 650A the PDCCH order to the UE 102. For example, the S-SN 104A or the S-DU of the S-SN 104A transmits 650A a PDCCH order to the UE 102, including the PDCCH order information. In some implementations, the S-SN 104A or the S-DU may determine to transmit the PDCCH order, based on (e.g., in response to) the measurement report(s) 624A. In some implementations, the S-SN 104A or the S-DU may determine an SSB index included in the PDCCH order, based on LI measurement report(s) 624A, and/or the CSI resource configuration, the CSI report configuration and/or the LTM SSB configuration. In some implementations, the S-DU, or the S-SN 104A includes the LTM ID 1 in the PDCCH order to indicate the first cell. The UE 102 transmits 652 an RA preamble to the DU 174 on the first cell, using the RACH configuration and/or the PDCCH order information. The S-DU or the S-SN 104A derives a TA value based on the RA preamble. The DU 174 transmit 656 a DU-CU TA Information Transfer message including the TA value to the CU 172. The CU 172 transmits 657 a CU-CU TA Information Transfer message including the TA value to the MN 104B (e.g., the CU of the MN 104B). The MN 104B transmits 658 a CU-CU TA Information Transfer message including the TA value to the S-SN 104A. In some implementations, the DU 174 includes the cell ID 1, the RA preamble index, an RA-RNTI, and/or the DU ID of the S-DU of the S-SN 104A in the message 656. The DU 174 may additionally include the first BS ID and/or the second BS ID in the message 656. In some implementations, the CU 172 includes the cell ID 1, the RA preamble index, the RA-RNTI, and/or the DU ID of the S-DU in the message 657. In some implementations, the CU 172 may additionally include the first BS ID and/or the second BS ID in the message 657. The MN 104B then includes the cell ID 1, the RA preamble index, the RA-RNTI, and/or the DU ID of the S-DU in the message 658. In some implementations, the MN 104B may additionally include the first BS ID and/or the second BS ID in the message 658.

[0174] In some implementations, the CU 172 determines an address (e.g., an IP address) of the MN 104B or the CU of the MN 104B, based on the second BS ID. Thus, the CU 172 transmits the message 657 to the MN 104B in accordance with the address. In some implementations, the MN 104B determines an address (e.g., an IP address) of the S-SN 104A or the CU of S-SN 104A, based on the first BS ID. In other implementations, the MN 104B determines an address (e.g., an IP address) of the S-SN 104A or the CU of the S-SN 104A based on the DU ID of the S-DU of the S-SN 104A. With these implementations, the MN 104B sends 658 the CU-CU TA Information Transfer message to the S-SN 104A or the CU of the S-SN 104A in accordance with the address.

[0175] In response to determining to command the UE 102 to perform an LTM cell switch to the first cell, e.g., based on the LI measurement report(s) 624A and/or 654A, the S-DU or the S-SN 104A transmits 626A the LTM Cell Switch Command including the LTM ID 1 to the UE 102. In response to the LTM Cell Switch Command, the UE 102 may stop communication on the serving cell(s). In response to the LTM Cell Switch Command, the UE 102 accesses 632 the first cell and transmits 636A an RRC reconfiguration complete message to DU 174. The DU 174 in turn transmits 638 a DU-to-CU message including the RRC reconfiguration complete message to the CU 172. In some alternatively implementations, the UE 102 transmits the RRC reconfiguration complete message to the C-SN 106A via the MN 104B instead of via the DU 174. After receiving 638 the DU-to-CU message or the RRC reconfiguration complete message, the C-SN 106A communicates 640 with the UE 102 in DC with the MN 104B and the C-SN 106A in accordance with the first LTM candidate configuration and/or the LTM reference configuration. In some implementations, the UE 102 includes the LTM ID 1 in the RRC reconfiguration complete message 636A to indicate that the UE 102 applies the first LTM candidate configuration. In accordance with the LTM ID 1, the CU 172 identifies the first LTM candidate configuration and/or the LTM reference configuration. In other implementations, the CU 172 identifies the first LTM candidate configuration and/or the LTM reference configuration based on the first cell ID included in the Access Success message 634. In such implementations, the CU 172 maintains or stores association information between the first cell ID, and the first LTM candidate configuration and/or the LTM reference configuration. When the CU 172 receives the first cell ID in the Access Success message, the CU 172 identifies the first LTM candidate configuration and/or the LTM reference configuration in accordance with the first cell ID and the association information.

[0176] In some implementations, if the S-DU or the S-SN 104A receives a TA value as described above, the S-DU or the S-SN 104A may include the TA value in the LTM Cell Switch Command. In some implementations, the S-DU, or the S-SN 104A includes a first TCI state ID in the LTM Cell Switch Command. The first TCI state ID indicates a first one of the TCI state configuration(s).

[0177] In response to determining to command the UE 102 to perform the LTM cell switch or transmitting 626A the LTM Cell Switch Command, the S-DU of the S-SN 104A transmits a DU-CU Cell Switch Notification message to the CU of the S-SN 104A to indicate that the UE 102 performs an LTM cell switch to the first cell. In response to receiving the DU-CU Cell Switch Notification message, the CU of the S-SN 104A transmits 627A a CU-CU Cell Switch Notification message to the MN 104B to indicate that the UE 102 performs or is performing an LTM cell switch to the first cell. In response, the MN 104B transmits 628 a CU-CU Cell Switch Notification message to the CU 172 to indicate that the UE 102 performs or is performing an LTM cell switch to the first cell. In response, the CU 172 transmits 630 a CU-DU Cell Switch Notification message to the DU 174 to indicate that the UE 102 performs or is performing an LTM cell switch to the first cell. In some implementations, the S-SN 104A includes the first TCI state ID in the CU-CU Cell Switch Notification message 627A. In such cases, the MN 104B includes the first TCI state ID in the CU-CU Cell Switch Notification message 628. The CU 172 then includes the first TCI state ID in the CU-DU Cell Switch Notification message. The UE 102 and the DU 174 identify the first one of the TCI state configuration(s) based on the first TCI state ID and the UE 102 and the DU 174 apply the first TCI state configuration to communicate UL transmissions and/or DL transmissions in the events 632, 636A and/or 640. In other implementations, when separate TCI states are used, implementations can be similar for the DL TCI State and the UL TCI state as described earlier herein with reference to Fig. 5 except that an MN and SN communicate CU-to-CU messages. . The UE 102 applies the first TCI state configuration and the second TCI state configuration to communicate DL transmissions and UL transmissions, respectively, with the DU 174 in the events 632, 636A, and/or 640. The DU 174 identifies the first TCI state configuration and the second the TCI state configuration based on the first TCI state ID and second TCI state ID, respectively. The DU 174 applies the first and/or the second TCI state configuration to communicate DL transmissions and/or UL transmissions, respectively, with the UE 102 in the events 632, 636A and/or 640.

[0178] In some implementations, after (e.g., in response to) determining to command the UE 102 to perform the LTM cell switch or transmitting 626A the LTM Cell Switch Command, the S-SN 104A transmits 631 one or more Early Status Transfer messages to the MN 104B, each including a DL COUNT value or a DISCARD DL COUNT value for a DRB over which the UE 102 and the S-SN 104A communicate 602 data with each other. The MN 104B transmits one or more Early Status Transfer messages to CU 172, each including the DL COUNT value or the DISCARD DL COUNT value for the DRB over which the UE 102 and the S-SN 104A communicate 602 data with each other.

[0179] In some implementations, after receiving 634 the Access Success message or receiving 638 the DU-to-CU message or the RRC reconfiguration complete message, the CU 172 transmits 639 an LTM Success message to the MN 104B (e.g., to the CU of the MN 104B) to indicate that the LTM cell switch is completed successfully. In some implementations, the LTM Success message is an SN Modification Required message or a BS-to-BS Access Success message. In some implementations, the CU 172 includes the first cell ID in the LTM Success message. In other implementations, the CU 172 does not transmit a BS-to-BS message to the MN 104B (e.g., the CU of the MN 104B) to indicate that the LTM cell switch is completed successfully. In some implementations, after (e.g., in response to) receiving the LTM Success message or receiving the RRC reconfiguration complete message from the UE 102, the MN 104B may transmit an SN message (as shown event 645 in Fig. 6C) to the S-SN 104A. The SN message may indicate that the LTM cell switch is completed successfully, that the S-SN 104A is to suspend communication with the UE 102, or that the S-SN 104A is to release a UE context of the UE 102 and resources configured for the UE 102. The MN 104B may include the first cell ID in the SN message. In some implementations, the SN message is an SN Modification Request message, an SN Release Request message, an SN Access Success message or the like.

[0180] In some implementations, after (e.g., in response to) determining to command the UE 102 to perform the LTM cell switch, transmitting 626A the LTM Cell Switch Command, receiving the DU-CU Cell Switch Notification message from the S-DU, or receiving the SN message, the S-SN 104A (e.g., the CU of the S-SN 104A) transmits 641 an SN Status Transfer message to the MN 104B, including a DL COUNT value and/or a UL COUNT value for a/the DRB over which the UE 102 and the S-SN 104A communicate 602 data with each other. After (e.g., in response to) receiving the SN Status Transfer message 641, the MN 104B transmits 642 an SN Status Transfer message to the CU 172, including the DL COUNT value and/or the UL COUNT value.

[0181] In some implementations, the S-SN 104A (e.g., the CU of the S-SN 104A) may prepare additional cell(s) (i.e., cell(s) 2, ..., N) as LTM candidate cell(s) for the UE 102 with the MN 104B and one or more C-SNs (including the C-SN 106A), before or after transmitting the LTM Cell Switch Command or during, before or after the procedure 698A, as described above in a manner similar to that described with reference to Fig. 3 except that LTM preparation procedure(s) may be inter-SN and similar to procedure 698A. As described above, the MN 104B, S-SN 104A or C-SN(s) assign LTM ID(s) 2, ..., N to identify the LTM candidate configuration(s) 2, ..., N, respectively. Each of the LTM configuration delivery procedure(s) 2, ..., N is similar to the procedures 394, and/or 494. In other implementations, the S-SN 104A includes the tuples in the first RRC reconfiguration message.

[0182] In some implementations, an SN Required message and an SN Confirm message in each of the inter-SN LTM preparation procedure(s) 2, ..., N comprise an SN Modification Required message and an SN Modification Confirm message, respectively. In other implementations, an SN Required message and an SN Confirm message in each of the inter- SN LTM preparation procedure(s) 2, ..., N comprise an SN Change Required message and an SN Change Confirm message, respectively. In some implementations, an SN Request message and an SN Request Acknowledge message in each of the inter-SN LTM preparation procedure(s) 2, .. N comprise an SN Addition Request message and an SN Addition Request Acknowledge message, respectively. In other implementations, the SN Request message and the SN Request Acknowledge message in each of the inter-SN LTM preparation procedure(s) 2, ..., N comprise an SN Modification Request message and an SN Modification Request Acknowledge message, respectively. In some implementations, the information provided in the SN Confirm message (e.g., LTM candidate configuration, LTM reference configuration, first cell ID, or LTM ID) are alternatively provided in an SN Modification Request message from the MN 104B to the S-SN 104A (i.e., similar to the event 609B which will be described with respect to the scenario 600B in Fig. 6B below).

[0183] In other implementations, the S-SN 104A performs the procedure 698A with the CU 172 to prepare (one or more of) the cell 1 and cell(s) 2, ..., N as LTM candidate cell(s) for the UE 102. The cell(s) 2, ..., N are identified by cell ID(s) 2, ..., N, respectively, and operated by the DU 174 and/or other DU(s) of the C-SN 106A. In such implementations, the S-SN 104A includes the cell ID(s) 2, ..., N in the SN Required message 603 for LTM and the MN 104B includes the cell ID(s) 2, ..., N in the SN Request message, as described for the cell ID 1 above. In some implementations, upon receiving the SN Request message, the CU 172 determines or selects the cell(s) 1, ..., M from the cell(s) 1, ..., N as LTM candidate cell(s). M is a positive integer and M < N. In other implementations, CU 172 prepares the cell(s) 1, ..., N for LTM as requested in the SN Request message. The CU 172 performs LTM preparation procedure(s) 2, ..., M with the DU 174 to prepare the cell(s) 2, ..., M as LTM candidate cell(s) for the UE 102, respectively. The LTM preparation procedure(s) 2, ..., M are similar to the procedure 690 that the CU 172 performs with the DU 174 to prepare the cell 1. The CU 172 obtains the LTM candidate configuration(s) 2, ..., M for the cell(s) 2, ..., M as a result of the LTM preparation procedure(s) 2, ..., M respectively, similar to obtaining the LTM candidate configuration 1. The C-SN 106 A may generate the LTM candidate configuration(s) 2, ..., M as complete configuration(s) or the C-SN 106A may generate delta configuration(s) based on the LTM reference configuration, as described for the LTM candidate configuration 1. The CU 172 includes the LTM candidate configuration(s) 2, ..., M in the SN Request Acknowledge message. In some implementations, the CU 172, the MN 104B or the S-SN 104A assigns LTM ID(s) 2, ..., M to identify the LTM preparation procedure(s) 2, ..., M respectively, as described for the LTM ID 1 and the LTM candidate configuration 1. When the CU 172 assigns the LTM ID(s) 1, ..., M, the CU 172 includes the LTM ID(s) 1, ..., M with the LTM candidate configuration(s) 1, ..., M, respectively in the SN Request Acknowledge message, as described for the LTM ID 1 and the LTM candidate configuration 1.

[0184] In some implementations, the CU 172 obtains early synchronization information 2, ..., M for the cell(s) 2, ..., M, respectively, as described for the early synchronization information 1. The CU 172 includes the early synchronization information 2, ..., M in the SN Request Acknowledge message. In some implementations, the CU 172, or the S-SN 104A obtains CSI resource configuration(s) 2, ..., M for the cell(s) 2, ..., M, respectively, as described for the CSI resource configuration 1. When the CU 172 obtains the CSI resource configuration(s) 2, ..., M, the CU 172 includes the CSI resource configuration(s) 2, ..., M in the SN Request Acknowledge message.

[0185] In some implementations, the CU 172, or the S-SN 104A obtains LTM SSB configuration(s) 2, ..., M for the cell(s) 2, ..., M, respectively, as described for the LTM SSB configuration 1. When the CU 172 obtains the LTM SSB configuration(s) 2, ..., M, the CU 172 includes the LTM SSB configuration(s) 2, ..., M in the SN Request Acknowledge message. In some implementations, the CU 172, or the S-SN 104A obtains PCI(s) 2, ..., M for the cell(s) 2, ..., M, respectively, as described for the PCI 1. When the CU 172 obtains the PCI(s) 2, ..., M, the CU 172 includes the PCI(s) 2, ..., M in the SN Request Acknowledge message.

[0186] In some implementations, the CU 172 includes a list, similar to the list provided in the Handover Request Acknowledge message 507 described earlier herein, in the SN Request Acknowledge message. The MN 104B include the list or a portion of the list in the SN Confirm message.

[0187] In some implementations, the CU 172 obtains PDCCH order information 2, ..., M for the cell(s) 2, ..., M, respectively, as described for the PDCCH order information 1. In one implementation, the CU 172 includes the PDCCH order information 2, ..., M in the SN Request Acknowledge message. In another implementation, the CU 172 includes the PDCCH order information 2, ..., M in the early synchronization information 2, ..., M, respectively. The MN 104B may include the PDCCH order information 2, ..., M in the SN Confirm message as described for the PDCCH order information 1. The S-SN 104A may obtain RACH configuration 2, TCI state configuration(s) 2 and/or the PDCCH order information 2, ...., RACH configuration M, TCI state configuration(s) M and/or the PDCCH order information M from the early synchronization information 2, .. M, respectively or from the SN Confirm message.

[0188] In some implementations, the S-SN 104A may perform LTM configuration delivery procedure(s) 2, .. M similar to the procedures 394, and/or 494. In other implementations, the S-SN 104A includes the tuples in the first RRC reconfiguration message.

[0189] In some implementations, the MN 104B may include measurement result(s) 1, ..., N for the cell(s) 1, ..., N respectively in the SN Request message similar to the Handover Request message described earlier herein.

[0190] The events 626A, 627A, 628, 630, 631, 632, 633, and 634 are collectively referred to in Fig. 6A as an LTM execution procedure 699.

[0191] Referring next to Fig. 6B, a scenario 600B is generally similar to the scenario 600A, except that the scenario 600B includes events 604B, 609B, and 611 instead of events 603 and 609A. In the scenario 600B, the MN 104B initiates 698B inter-SN LTM preparation procedure, while in the scenario 600A, the S-SN 104A initiates 698A inter-SN LTM preparation procedure. While communicating in DC with the MN 104B and S-SN 104A, the UE 102 can transmit 604B at least one measurement report to the MN 104B, e.g., via the SRB1, similar to the events 304/306, the events 404/406, and the event 504. If the MN 104B is a distributed base station, the MN 104B includes a CU and a DU. The CU of the MN 104B receives the measurement report(s) from the UE 102 via the DU of the MN 104B. The MN 104B (e.g., the CU of the MN 104B) generates a measurement configuration to configure the UE 102 to transmit the measurement report(s) and the MN 104B transmits the measurement configuration to the UE 102 (e.g., via the DU of the MN 104B).

[0192] After (e.g., in response to) receiving one or some of the measurement report(s) from the UE 102, the MN 104B (e.g., the CU of the MN 104B) determines to prepare a first cell (e.g., the cell 124B) as an LTM candidate for the UE 102. In response to the determination, the MN 104B generates an SN Request message (a first SN Request message) including a first cell ID (e.g., cell ID 1) of the first cell (e.g., cell 1) and transmits 605 the SN Request message to the C-SN 106A. In response to the SN Request message, the CU 172 decides to perform and performs the LTM preparation procedure 690 with the DU 174 and transmits 607 the SN Request Acknowledge message to the MN 104B, as described with reference to Fig. 6A. After (e.g., in response to) receiving the message 607, the MN 104B transmits 609B an SN Modification Request message to the S-SN 104A, including a first LTM candidate configuration (LTM candidate configuration 1), an LTM reference configuration, and/or the first cell ID, similar to the event 609A. If the SN Request Acknowledge message includes LTM related configurations such as the RACH configuration, the TCI state configuration(s), the CSI resource configuration, the LTM SSB configuration, and/or the PCI of the first cell for configuring the first cell as an LTM candidate cell, the MN 104B includes the LTM related configurations in the SN Modification Request message. In response to the SN Modification Request message, the S-SN 104A transmits 611 an SN Modification Request Acknowledge message to the MN 104B and performs 694 the LTM configuration delivery procedure with the UE 102 to transmit the first LTM candidate configuration and/or the LTM reference configuration to the UE 102. The events 605, 690, 607, 609B, and 611 are collectively referred to in Fig. 6B as an MN-initiated inter-SN LTM preparation procedure 698B.

[0193] As described with reference to Fig. 6A, the SN Request Acknowledge message 607 may include the PDCCH order information. If the SN Request Acknowledge message 607 includes the PDCCH order information, the MN 104B includes the PDCCH order information in the SN Modification Request message. If the SN Modification Request message includes the PDCCH order information, the S-SN 104A may transmit 650A a PDCCH order to the UE 102, based on the PDCCH order information, as described with reference to Fig. 6A. If the SN Modification Request message includes the TCI state configuration(s), the S-SN 104A may include a first TCI state ID in the LTM Cell Switch Command as described with reference to Fig. 6A.

[0194] In some implementations, the MN 104B (e.g., the CU of the MN 104B) may perform one or more LTM preparation procedures to prepare additional cell(s) (i.e., cell(s) 2, ..., N) as LTM candidate cell(s) for the UE 102 with the CU 172 or other C-SN(s. Each of the LTM preparation procedure is similar to the procedure 698B. The MN 104B may perform the LTM preparation procedure(s) before or after transmitting the LTM Cell Switch Command or during, before or after the procedure 698B.

[0195] In other implementations, the MN 104B performs the procedure 698B with the CU 172 to prepare one or more of the cell 1 and cell(s) 2, ..., N as LTM candidate cell(s) for the UE 102. N is an integer larger than 1. The cell(s) 2, ..., N are identified by cell ID(s) 2, ..., N, respectively and operated by the DU 174 and/or other DU(s) of the C-SN 106A. In such implementations, the MN 104B includes the cell ID(s) 2, ..., N in the SN Request message 605, as described with reference to the cell ID 1 above.

[0196] Referring next to Fig. 6C, a scenario 600C is generally similar to the scenarios 600A and 600B, except that the scenario 600C includes events 694C, 636C, and 645 instead of events 694A, 636A, and 638. In the scenario 600C, the MN 104B transmits, to the UE 102, the second serving DU configuration (including the CSI report configuration), the LTM ID, the first LTM candidate configuration, the LTM reference configuration, the CSI resource configuration, the LTM SSB configuration, the RACH configuration, the TCI state configuration(s), and/or the PCI of the first cell for configuring the first cell as an LTM candidate cell in one or more RRC reconfiguration messages in the LTM configuration delivery procedure 694C similar to the procedure 394 or 494. In some implementations, the MN 104B generates the RRC reconfiguration message(s) in the procedure 694C, while the S- SN 104A generates the RRC reconfiguration message(s) in the procedure 694A. In some implementations, the MN 104B assigns the LTM ID in the scenario 600C, while the S-SN 104A assigns the LTM ID in the scenarios 600A and 600B.

[0197] When the UE 102 performs 699 the LTM execution procedure, the UE 102 transmits 636C an RRC reconfiguration message to the MN 104B, similar to the event 636A. In some implementations, the UE 102 includes the LTM ID in the RRC reconfiguration complete message to indicate that the UE 102 applies the first LTM candidate configuration. After (e.g., in response to) receiving the RRC reconfiguration message, the MN 104B may transmit 645 the SN message to the S-SN 104A as described for Eig. 6A. In some implementations, the MN 104B may include a first cell ID of the first cell in the SN message. The MN 104B may determine the first cell ID based on the LTM ID received in the RRC reconfiguration complete message.

[0198] Referring next to Eig. 6D, a scenario 600D is generally similar to the scenarios 600A, 600B, and 600C, except that the scenario 600D includes events 624D, 650D, 654D, 626D, and 627D instead of events 624A, 650A, 654A, 626A, and 627A. The events 624D, 650D, 654D, 626D, and 627D are similar to the events 624A, 650A, 654A, 626A, and 627A, respectively, except that the MN 104B implements or is involved in the events 624D, 650D, 654D, 626D, and 627D while the S-SN 104A implements or is involved in the events 624A, 650A, 654A, 626A, and 627A. The event 627D is different from the event 627A in that in the event 627D, the MN 104B may transmit the CU-CU Cell Switch Notification message, which may include the first TCI State ID and/or the second TCI State ID, to the S-SN 104A. [0199] In some implementations, the MN 104B may generate the CSI report configuration(s) based on a CSI resource configuration, as described for S-SN 104A. The CSI resource configuration configures RS(s) transmitted on the first cell. The MN 104B transmits the CSI report configuration(s) to the UE 102 in the procedure 694C. In some implementations, the MN 104B is preconfigured with CSI resource configuration. In other implementations, the MN 104B receives the CSI resource configuration from an 0AM node. In yet other implementations, the MN 104B receives the CSI resource configuration from the CU 172 or the S-SN 104A.

[0200] Next, several example methods, which can operate in a RAN node (e.g., a base station, a DU or a CU) or a UE, for LTM, are discussed with reference to Figs. 7-17. Generally speaking, similar events described for Figs. 3-6D are labeled similarly in Figs. 7- 17 with similar reference numerals that share at least two significant digits, with differences discussed where appropriate. For example, event 302 is similar to events 402, 502, 602, etc.). With the exception of differences shown in the figures and discussed below, any of the other implementations discussed with respect to a particular event (e.g., for messaging and processing) may apply to events labeled with similar reference numbers in other figures.

[0201] Fig. 7 illustrates an example method 700, which can be implemented by a C-SN (e.g., the C-SN 106A in Figs. 6A-6D). The method 700 begins at block 705, where the C-SN receives an SN Request message from an MN, including an early synchronization information request to request early synchronization information. The flow proceeds to block 760 and/or 762. At block 760, the C-SN generates a RACH configuration in response to the early synchronization information request. At block 762, the C-SN generates one or more TCI state configuration(s) in response to the early synchronization information request. At block 707, the C-SN transmits an SN Request Acknowledge message to the MN, including the RACH configuration and/or the TCI state configuration(s). At block 752, the C-SN may receive a random-access preamble from a UE via a first cell in accordance with the RACH configuration. At block 755, the C-SN may derive a timing advance value based on the random-access preamble. At block 757, the C-SN may transmit a CU-to-CU message including the timing advance value to the MN. At block 728, the C-SN may receive a CU-to- CU message from the MN, indicating an initiation of an ETM cell switch to the first cell for the UE. At block 732, the C-SN may detect that the UE accesses the first cell. At block 739, the C-SN may transmit an ETM Success message to the MN, indicating that the UE successfully accesses the first cell and/or the C-SN. At block 740, the C-SN may communicate with the UE via the first cell in accordance with the TCI state configuration(s).

[0202] In some implementations, the C-SN generates PDCCH order information for the UE and includes the PDCCH order information in the SN Request Acknowledge message. The C-SN receives the random-access preamble in accordance with the RACH configuration and the PDCCH order information.

[0203] Fig. 8 illustrates an example method 800 similar to the method 700, which can be implemented by a CU of a C-SN (e.g., the C-SN 106A of Figs. 6A-6D). The method 800 begins at block 805. At block 808, the CU transmits a CU-to-DU message to a DU, including an early sync information request to request synchronization information (e.g., early synchronization information). At block 810, the CU receives a DU-to-CU message from the DU, including one or more of a RACH configuration and/or one or more TCI state configuration(s). The flow may proceed to blocks 807, 856, 757, 728, 834, 739, and 740. At block 856, the CU may receive a DU-CU TA Information message from the DU, including a timing advance value. At block 834, the CU may receive an Access Success message from the DU, indicating that the UE accesses the first cell.

[0204] In some implementations, the CU receives PDCCH order information for the UE and includes the PDCCH order information in the SN Request Acknowledge message. The CU transmits the PDCCH order information to an S-DU of the C-SN.

[0205] Fig. 9A illustrates an example method 900A, which can be implemented by a C-SN (e.g., the C-SN 106A of Figs. 6A-6D). The method 900A begins at block 905, where the C- SN receives an SN Request message from an MN, requesting to prepare a first cell for LTM. At block 959, the C-SN determines whether the UE supports early TA acquisition. If the UE supports the early TA acquisition (i.e., “Yes” branch of block 959), the flow proceeds to block 960. At block 960, the C-SN generates a RACH configuration for the UE. At block 962, the C-SN may generate one or more TCI state configuration(s) for the UE. The flow proceeds to block 991 from block 962 or 960. Otherwise, if the UE does not support the early TA acquisition (i.e., “No” branch of block 959), the flow skips blocks 960 and 962 and proceeds to block 991. At block 991, the C-SN generates an LTM candidate configuration for the UE, where the LTM candidate configuration configures the first cell for LTM. At block 907, the C-SN transmits an SN Request Acknowledge message to the MN, including the RACH configuration, the TCI state configuration(s) and/or the LTM candidate configuration. [0206] In some implementations, if the UE supports the early TA acquisition, the C-SN generates PDCCH order information and includes the PDCCH order information in the SN Request Acknowledge message. The PDCCH order information is for the MN or an S-SN to include in a PDCCH order to be transmitted to the UE (e.g., event 650A or 650D). Otherwise, if the UE does not support the early TA acquisition, the C-SN does not include (the) PDCCH order information in the SN Request Acknowledge message.

[0207] Fig. 9B is a flow diagram of an example method 900B similar to the method 900A, except that the method 900B includes block 961 instead of block 959. At block 961, the C- SN determines whether the SN Request message includes an early synchronization information request. If the SN Request message includes an early synchronization information request (e.g., “Yes” branch of block 961), the flow proceeds to block 960. Otherwise, if the SN Request message does not include an early synchronization information request (e.g., “No” branch of block 961), the flow skips blocks 960 and 962 and proceeds to block 991.

[0208] Fig. 10A illustrates an example method 1000A similar to the method 900A, which can be implemented by a CU of a C-SN (e.g., the C-SN 106A of Figs. 6A-6D). The method 1000A begins at block 1005. At block 1059, the CU determines whether the UE supports early TA acquisition. If the UE supports the early TA acquisition (i.e., “Yes” branch of block 1059), the flow proceeds to blocks 808, 810 and 1091. Otherwise, if the UE does not support the early TA acquisition (i.e., “No” branch of block 1059), the flow skips block 808 and 810 and proceeds to block 1091. The flow proceeds to blocks 1010, 1093 and 1007 from block 1091. At block 1093, the CU generates an LTM candidate configuration including the LTM DU configuration, where the LTM candidate configuration configures the first cell for LTM. The flow proceeds to block 1007.

[0209] Fig. 10B is a flow diagram of an example method 1000B similar to the methods 1000A, 900A, and 900B. At block 1061, the CU determines whether the SN Request message includes an early synchronization information request. If the SN Request message includes an early synchronization information request (i.e., “Yes” branch of block 1061), the flow proceeds to blocks 808, 810 and 1091. Otherwise, if the SN Request message does not include an early sync information request (i.e., “No” branch of block 1061), the flow skips block 808 and 810 and proceeds to block 1091. [0210] Fig. 11 A illustrates an example method 1100A similar to the methods 900A and

1000A, which can be implemented by a CU of C-SN (e.g., the C-SN 106A of Figs. 6A-6D). The method 1100A begins at block 1105. At block 1171, the CU includes a cell ID of the first cell in a CU-to-DU message to request preparing the first cell for LTM for the UE. The flow proceeds to block 1159. If the UE supports the early TA acquisition (i.e., “Yes” branch of block 1159), the flow proceeds to blocks 1172 and 1191. At block 1172, the CU includes an early synchronization information request in the CU-to-DU message to request early synchronization information. Otherwise, if the UE does not support the early TA acquisition (i.e., “No” branch of block 1159), the flow skips block 1172 and proceeds to block 1191. At block 1191, the CU transmits the CU-to-DU message to a DU. The flow proceeds to blocks 1110 and 1107 from block 1191.

[0211] Fig. 1 IB is a flow diagram of an example method 1100B similar to the methods 1000A, 900A, and 900B. If the SN Request message includes an early synchronization information request (i.e., “Yes” branch of block 1161), the flow proceeds to blocks 1172 and 1191. Otherwise, if the SN Request message does not include an early synchronization information request (e.g., “No” branch of block 1161), the flow skips block 1172 and proceeds to block 1191.

[0212] Fig. 12 illustrates an example method 1200, which can be implemented by an S-SN (e.g., the S-SN 104A of Figs. 6A-6D). The method 1200 begins at block 1209, where the S- SN receives a RACH configuration, one or more TCI state configurations, and/or PDCCH order information from an MN. At block 1218, the S-SN transmits the RACH configuration and/or the TCI state configuration(s) to a UE. At block 1250, the S-SN transmits a PDCCH order to the UE based on the PDCCH order information, where the PDCCH order commands the UE to transmit an RA preamble on a first cell operated by a C-SN. At block 1258, the S- SN receives a CU-to-CU message including a timing advance value from the MN. At block 1226, the S-SN transmits an LTM Cell Switch Command to the UE, commanding the UE to perform an LTM cell switch to the first cell. In some implementations, the S-SN includes the timing advance value in the LTM Cell Switch Command. At block 1227, the S-SN transmits a CU-to-CU message to the MN, indicating an initiation of the LTM cell switch to the first cell for the UE. At block 1239, the S-SN receives an LTM Success message from the MN, indicating the UE successfully connects to the second BS via the first cell.

[0213] In some implementations, the S-SN transmits at least one SN Required message to the MN, including an early synchronization information request to request early synchronization information (e.g., event 603). In response, the S-SN at block 1209 receives, from the MN, at least one SN Confirm message that includes the RACH configuration, the TCI state configuration(s) and/or the PDCCH order information.

[0214] Fig. 13 illustrates an example method 1300 similar to the method 1200, which can be implemented by a CU of an SN (e.g., the S-SN 104A of Figs. 6A-6D). The method 1300 begins at block 1309. At block 1312, the CU transmits a CU-to-DU message to the DU, including the PDCCH order information and/or the TCI state configuration(s). The flow proceeds to blocks 1318 and may proceeds to blocks 1258, 1227, and/or 1239. In some implementations, the CU receives the timing advance value from the DU and includes the timing advance value in the CU-to-CU message at block 1258. In some implementations, the CU receives a DU-CU Cell Switch Notification message from the DU, indicating the initiation of the LTM cell switch to the first cell for the UE and the UE transmits the CU-to- CU message to the MN at block 1227, in response.

[0215] In some implementations, the CU transmits an SN Required message to the MN (e.g., event 603) and receives an SN Confirm message from the MN in response (e.g., event 609 A or 609B). The SN Confirm message includes the PDCCH order information, the RACH configuration, and/or the TCI state configuration(s). In some implementations, the CU includes, in the SN Required message, an early synchronization information request to request early synchronization information. In other implementations, the CU does not include an early synchronization information request in the SN Required message.

[0216] Fig. 14 illustrates an example method 1400, which can be implemented by an MN (e.g., the MN 104B in Figs. 6A-6D or a CU of the MN 104B). The method 1400 begins at block 1402, where the MN communicates with a UE operating in DC with the MN and a S- SN. At block 1499, the MN receives a RACH configuration, one or more TCI state configurations, and/or PDCCH order information from a C-SN. The flow proceeds to blocks 1418. The flow proceeds to block 1450 or 1409. At block 1409, the MN transmits the PDCCH order information to an S-SN. The flow proceeds to block 1457 from block 145 0 and block 1409. At block 1457, the MN receives a CU-to-CU message including a timing advance value from a C-SN. At block 1427, the MN receives a CU-to-CU message from the C-SN, indicating an initiation of an LTM cell switch to the first cell for the UE. At block 1439, the MN transmits an LTM Success message to the S-SN, indicating that the UE successfully connects to the second BS via the first cell. [0217] In some implementations, the MN transmits at least one SN Request message to the C-SN, including an early synchronization information request to request early synchronization information (e.g., event 605). In response, the MN at block 1407 receives, from the C-SN, at least one SN Request Acknowledge message that includes the RACH configuration, the TCI state configuration(s) and/or the PDCCH order information.

[0218] Fig. 15 illustrates an example method 1500 similar to the method 1400, which can be implemented by a CU of an MN (e.g., the MN 104B in Figs. 6A-6D). As with others of Figs. 7-14, events similarly labeled with same least significant digits can include similar operations. For example, the method 1500 begins at blocks 1502 (which can be similar to event 1402, 602, etc.). The method 1500 continues with operation 1507, with the CU of the MN receiving a RACH configuration, one or more TCI state configuration(s), PDCCH order information, etc. from a C-SN. The method 1500 can continue with operation 1512 with the MN CU transmitting a CU-to-DU message to a DU, including the PDCCH order information, TCI state configuration(s) and/or other early synchronization information. The method can continue with operation 1518 with transmitting the RACH configuration, TRCI state configuration(s) etc. to a UE . The flow may proceed to blocks 1409, 1457, 1427, and/or 1439.

[0219] Fig. 16 illustrates an example method 1600, which can be implemented by an S-SN (e.g., the S-SN 104A in Figs. 6A-6D). The method 1600 begins at block 1674, where the S- SN determines to prepare a first cell for LTM. At block 1676, the S-SN includes a cell ID of the first cell in an SN Required message for LTM. At block 1659, the S-SN determines whether the UE supports early TA acquisition. If the UE supports the early TA acquisition (i.e., “Yes” branch of block 1659), the flow proceeds to block 1678. At block 1678, the S-SN includes an early synchronization information request in the SN Required message. Otherwise, if the UE does not support the early TA acquisition (i.e., “No” branch of block 1659), the flow proceeds to block 1605. At block 1605, the S-SN transmits the SN Required message to an MN. The flow proceeds to block 1605 from block 1678 as well as block 1659. At block 1607, the S-SN receives an SN Confirm message from the MN. In some implementations, the SN Confirm message includes a RACH configuration, one or more TCI state configurations and/or PDCCH order information.

[0220] Fig. 17 illustrates an example method 1700, which can be implemented by an S-SN (e.g., the S-SN 104A in Figs. 6A-6D). The method 1700 begins at block 1757, where the S- SN receives a TA value for a UE from an MN. At block 1723, the S-SN determines to initiate an LTM cell switch to a cell for the UE. The cell is operated by a C-SN. At block 1782, the S-SN determines whether the TA value is valid. If the TA value is valid (i.e., “Yes” branch of block 1782), the S-SN includes the TA value in an LTM Cell Switch Command. Otherwise, if the TA value is invalid (e.g., “No’ branch of block 1782), the flow skips block 1725 and proceeds to block 1726. In other words, if the TA valid is invalid, the S-SN refrains from including the TA value in the LTM Cell Switch Command. At block 1726, the S-SN transmits the LTM Cell Switch Command to the UE.

[0221] In some implementations, after receiving the TA value, the S-SN receives one or more (new) LI measurement reports (e.g., CSI report(s)) from the UE. The S-SN determines whether the TA value is valid based on the LI measurement report(s). Lor example, if measurement result(s) in the LI measurement report(s) is below or above a predetermined threshold, the S-SN determines that the TA value is invalid. Otherwise, the S-SN determines that the TA value is valid. In another example, the S-SN received one or more old LI measurement reports (e.g., CSI report(s)) from the UE before receiving the new LI measurement report(s). If a difference between measurement result(s) in the new LI measurement report(s) and measurement result(s) in the old LI measurement report(s) is above a predetermined threshold, the S-SN determines that the TA value is in valid. Otherwise, the S-SN determines that the TA value is valid. If the S-SN determines that TA value is valid based on the LI measurement report(s), the S-SN performs includes the TA value in the LTM Cell Switch Command. Otherwise, if the S-SN determines that TA value is invalid based on the LI measurement report(s), the S-SN refrains from including TA value in the LTM Cell Switch Command.

[0222] Eig. 18 illustrates an example method 1800, which a base station can implement to support C-SN functionality. At block 1805, the C-SN receives, from an MN, an SN request including at least one of (i) an LTM indication for a candidate cell associated with the C-SN or (ii) a request for early synchronization information for the candidate cell (see, e.g., event 605 and blocks 705, 805, 905, 1005, and 1105). At block 1807, the C-SN transmits, to the MN and in response to the SN request, an SN request acknowledgement including the early synchronization information (see, e.g., event 707 and blocks 707, 807, 907, 1007, and 1107).

[0223] Finally, Fig. 19 illustrates an example method 1900, which a base station can implement to support S-SN functionality. At block 1975, the S-SN determines a candidate cell for LTM of a UE that communicates in DC with an MN and the S-SN, where the candidate cell is associated with a C-SN (see, e.g., block 1674). At block 1905, the S-SN transmits, to the MN, an SN required message including a request for early synchronization information for the candidate cell (see, e.g., event 605 and blocks 705, 805, 905, 1005, and 1105).

[0224] The following list of examples reflects a variety of the embodiments explicitly contemplated by the present disclosure.

[0225] Example 1. A method implemented in a secondary node (SN) which, with a master node (MN), provides dual connectivity to a user equipment (UE), the method comprising: receiving, from the MN, a handover request to configure a candidate cell associated with the SN for lower layer triggered mobility (LTM), the handover request including a request for synchronization information associated with the candidate cell; and transmitting, to the MN and in response to the handover request, the synchronization information.

[0226] Example 2. The method of example 1, further comprising: in response to the request, transmitting, to a distributed unit (DU) associated with the SN, a request to prepare the candidate cell for LTM.

[0227] Example 3. The method of example 2, further comprising transmitting a request for synchronization information to the DU.

[0228] Example 4. The method of example 3, further comprising receiving at least one of a random-access channel (RACH) configuration or a transmission configuration indicator (TCI) state configuration in response to the request for synchronization information.

[0229] Example 5. The method of example 4, further comprising: generating random access control information in response to the request; and transmitting the random-access control information in the response.

[0230] Example 6. The method of example 5, further comprising: receiving, at the DU, a random-access preamble from a user equipment (UE).

[0231] Example 7. The method of example 6, wherein: the random-access preamble is in accordance with the RACH configuration.

[0232] Example 8. The method of example 6, further comprising: generating a timing advance (TA) value based on the random-access preamble.

[0233] Example 9. The method of example 8, further comprising: transmitting the TA value to the MN. [0234] Example 10. The method of example 4, further comprising: generating a transmission configuration indicator (TCI) state configuration in response to the request.

[0235] Example 11. The method of example 10 further comprising: generating a first TCI state configuration for uplink communications and a second TCI state configuration for downlink communications.

[0236] Example 12. The method of example 11, wherein: the first TCI state configuration and the second TCI state configuration are received from the DU in response to the request for synchronization information.

[0237] Example 13. The method of example 1, wherein: the handover request comprises an SN Request message.

[0238] Example 14. The method of example 13, wherein: the SN Request message comprises an SN Addition Request.

[0239] Example 15. The method of example 1, further comprising: including physical downlink control channel (PDCCH) information in the response if a connected user equipment (UE) supports early timing advance (TA) acquisition.

[0240] Example 16. A method implemented in a secondary node (SN) which provides, with a master node (MN), dual connectivity (DC) to a user equipment (UE), the method comprising: receiving, from the MN, synchronization information for a candidate cell associated with a candidate SN (C-SN; and transmitting the synchronization information and a lower layer triggered mobility (LTM) Cell Switch Command to the UE to command the UE to communicate with the candidate cell using the synchronization information.

[0241] Example 17. The method of example 16, wherein the synchronization information includes a random-access channel (RACH) configuration.

[0242] Example 18. The method of example 16, wherein the synchronization information includes one or more transmission configuration indicator (TCI) state configurations.

[0243] Example 19. The method of example 18, further comprising transmitting a cell switch notification that the UE is performing a cell switch to the candidate cell, the cell switch notification including a TCI state identifier to identify a TCI state configuration used by the UE.

[0244] Example 20. The method of example 16, wherein the synchronization information includes physical downlink control channel (PDCCH) order information. [0245] Example 21. The method of example 16, further comprising: transmitting a PDCCH order to the UE to trigger the UE to provide a random access (RA) preamble to the C-SN.

[0246] Example 22. The method of example 16, wherein the synchronization information is received in an SN Confirm message.

[0247] Example 23. The method of example 22, wherein the synchronization information is received in an SN Change Confirm message.

[0248] Example 24. A method implemented in a master node (MN) which, with a secondary node (SN), provides dual connectivity to a user equipment (UE), the method comprising: transmitting, to the SN, a handover request to configure a candidate cell associated with the SN for lower layer triggered mobility (LTM), the handover request including a request for synchronization information associated with the candidate cell; and receiving, from the SN and in response to the request, the synchronization information.

[0249] Example 25. The method of claim 24, wherein: the synchronization information includes at least one of a random-access channel (RACH) configuration, a transmission configuration indicator (TCI) state configuration or physical downlink control channel (PDCCH) order information.

[0250] Example 26. The method of example 24, further comprising: receiving a timing advance (TA) value from the SN.

[0251] Example 27. The method of example 24, wherein: the handover request comprises an SN Request message.

[0252] Example 28. The method of example 27, wherein: the SN Request message comprises an SN Addition Request.

[0253] The following description may be applied to the description above.

[0254] In some implementations, the SN Required message and the SN Confirm message are an SN Change Required message and an SN Change Confirm message, respectively. In other implementations, the SN Required message and the SN Confirm message are an SN Modification Required message and an SN Modification Confirm message, respectively. In some implementations, the SN Request message and the SN Request Acknowledge message are an SN Addition Request message and an SN Addition Request Acknowledge message, respectively. In other implementations, the SN Request message and the SN Request Acknowledge message are an SN Modification Request message and an SN Modification Request Acknowledge message, respectively.

[0255] Generally speaking, description for one of the above figures can apply to another of the above figures. Examples, implementations and methods described above can be combined, if there is no conflict. An event or block described above can be optional or omitted. For example, an event or block with dashed lines in the figures can be optional. The description described from the perspective of the receiving node also applies to the sending node. For example, a description that a receiving node (e.g., DU) receives a message from a sending node (e.g., CU) may be replaced by the sending node sending a message to the receiving node. Similarly, a description that a receiving node (e.g., CU) receives a message from a sending node (e.g., DU) may be replaced by the sending node sending a message to the receiving node.

[0256] In some implementations, “message” is used and can be replaced by “information element (IE),” and vice versa. In some implementations, “IE” is used and can be replaced by “field,” and vice versa. In some implementations, “configuration” can be replaced by “configurations” or “configuration parameters,” and vice versa. In some implementations, the “LTM command” can be replaced by “serving cell change command,” “Layer 1/Layer 2 LTM Cell Switch Command”, “lower layer switching command” or “lower layer serving cell change command”. In some implementations, “some” means “one or more.” In some implementations, “at least one” means “one or more.” In some implementations, the “DU configuration” can be replaced by “cell group configuration.” In some implementations, the “cell index” can be replaced with “serving cell index,” “LTM cell index”, “special cell (SpCell) index”, “PCell index” or “PSCell index”. In some implementations, the “serving” can be replaced by “source.” In some implementations, the “measurement report” can be replaced by “measurement result(s)” or “CSI report.” In some implementations, the “early TA acquisition” can be replaced by “early UL timing synchronization” or “early UL synchronization.” In some implementations, the “early TA acquisition on a/the candidate cell” can be replaced by “early UL timing synchronization with a/the candidate cell” or “early UL synchronization with a/the candidate cell.” In some implementations, “include” can be replaced by “comprise.” In some implementations, “exclude” can be replaced by “refrain from including.” In some implementations, “Handover Request” and “Handover Request Acknowledge” described above are for illustration of the invention and can be replaced by messages with general names. For example, “Handover Request” and “Handover Request Acknowledge” can be replaced by a first CU-to-CU message and a second CU-to-CU message, respectively. In another example, “Handover Request” and “Handover Request Acknowledge” can be replaced by a first interface message and a second interface message, respectively. In yet another example, “Handover Request” and “Handover Request Acknowledge” can be replaced by a first BS-to-BS message and a second BS-to-BS message, respectively. In some implementations, “SN Request,” “SN Confirm”, “SN Request” and “SN Request Acknowledge” described above are for illustration of the invention and can be replaced by messages with general names. For example, the “SN Required message” can be replaced by an SN-to-MN message. In another example, the “SN Confirm message” can be replaced by an MN-to-SN message.

[0257] A user device in which the techniques of this disclosure can be implemented (e.g., the UE 102) can be any suitable device capable of wireless communications such as a smartphone, a tablet computer, a laptop computer, a mobile gaming console, a point-of-sale (POS) terminal, a health monitoring device, a drone, a camera, a media- streaming dongle or another personal media device, a wearable device such as a smartwatch, a wireless hotspot, a femtocell, or a broadband router. Further, the user device in some cases may be embedded in an electronic system such as the head unit of a vehicle or an advanced driver assistance system (ADAS). Still further, the user device can operate as an intemet-of-things (loT) device or a mobile-internet device (MID). Depending on the type, the user device can include one or more general-purpose processors, a computer-readable memory, a user interface, one or more network interfaces, one or more sensors, etc.

[0258] Certain embodiments are described in this disclosure as including logic or a number of components or modules. Modules may can be software modules (e.g., code, or machine- readable instructions stored on non-transitory machine-readable medium) or hardware modules. A hardware module is a tangible unit capable of performing certain operations and may be configured or arranged in a certain manner. A hardware module can comprise dedicated circuitry or logic that is permanently configured (e.g., as a special-purpose processor, such as a field programmable gate array (FPGA) or an application-specific integrated circuit (ASIC), a digital signal processor (DSP), etc.) to perform certain operations. A hardware module may also comprise programmable logic or circuitry (e.g., as encompassed within a general-purpose processor or other programmable processor) that is temporarily configured by software to perform certain operations. The decision to implement a hardware module in dedicated and permanently configured circuitry, or in temporarily configured circuitry (e.g., configured by software) may be driven by cost and time considerations.

[0259] When implemented in software, the techniques can be provided as part of the operating system, a library used by multiple applications, a particularhandover software application, etc. The software can be executed by one or more general-purpose processors or one or more special-purpose processors.

[0260] Upon reading this disclosure, those of skill in the art will appreciate still additional and alternative structural and functional designs for handling mobility between base stations through the principles disclosed herein. Thus, while particular embodiments and applications have been illustrated and described, it is to be understood that the disclosed embodiments are not limited to the precise construction and components disclosed herein. Various modifications, changes, and variations, which will be apparent to those of ordinary skill in the art, may be made in the arrangement, operation and details of the method and apparatus disclosed herein without departing from the spirit and scope defined in the appended claims.

Claims

The claims:

1. A method implemented in a candidate secondary node (C-SN), the method comprising: receiving, from a master node (MN) that provides, with a source secondary node (S- SN), dual connectivity (DC) to a user equipment (UE), an SN request including at least one of (i) a lower layer triggered mobility (LTM) indication for a candidate cell associated with the C-SN or (ii) a request for early synchronization information for the candidate cell; and transmitting, to the MN and in response to the SN request, an SN request acknowledgement including the early synchronization information.

2. The method of claim 1, wherein the generating of the early synchronization information includes generating a random access channel (RACH) configuration.

3. The method of claim 1 or 2, wherein the generating of the early synchronization information incudes generating a transmission configuration indicator (TCI) state configuration.

4. The method of claim 2, further comprising: receiving, from the UE in the candidate cell, a random access preamble in accordance with the RACH configuration.

5. The method of claim 4, further comprising: deriving a timing advance (TA) value based on the random access preamble; and transmitting the TA value to the MN.

6. The method of claim 4 or 5, further comprising: receiving, from the MN, an indication that the C-SN has initiated a lower layer triggered mobility (LTM) switch of the UE to the candidate cell.

7. The method of claim 6, further comprising: determining that the UE has accessed the candidate cell; and transmitting, to the MN, an LTM success indication.

8. The method of any of the preceding claims, wherein the generating of the early synchronization information includes: transmitting, from a central unit (CU) of the C-SN to a distributed unit (DU) of the C- SN, a request for the early synchronization information; and receiving the early synchronization information from the DU.

9. The method of any of the preceding claims, wherein the generating of the early synchronization information is in response to determining that the UE supports early TA acquisition.

10. The method of any of the preceding claims, wherein the generating of the early synchronization information is in response to determining that the SN request contains an information element (IE) that includes the request for early synchronization information.

11. A method implemented in a source secondary node (S-SN), the method comprising: determining a candidate cell for lower layer triggered mobility (LTM) of a user equipment (UE) that communicates in dual connectivity (DC) with a master node (MN) and the S-SN, the candidate cell being associated with a candidate secondary node (C-SN); and transmitting, to the MN, an SN required message including a request for early synchronization information for the candidate cell.

12. The method of claim 11, further comprising: including the request for early synchronization information in the SN required message in response to determining that the UE supports early synchronization.

13. The method of claim 11 or 12, further comprising: receiving, from the MN, a timing advance (TA) value; and transmitting, to the UE, the TA value in an LTM cell switch command.

14. The method of claim 13, further comprising: including the TA value in the LTM cell switch command in response to determining that the TA value is valid.

15. A base station comprising a transceiver and processing hardware, the base station configured to implement a method of any of the preceding claims.