Enabling early synchronization for inter-central unit lower layer triggered mobility

Abstract

Methods and apparatuses are described herein for enabling early synchronization for inter-central unit lower layer triggered mobility (LTM). One method implemented in a candidate base station can include receiving, from a source base station (S-BS), a handover request message including at least one of (i) a lower layer triggered mobility (LTM) indication for a candidate cell associated with the C-BS or (ii) a request for early synchronization information, for a user equipment (UE) communicating with the S-BS. The method can further include generating the early synchronization information in response to the request. The method can further include transmitting the early synchronization information to the source base station. Other methods and apparatuses are described.

Description

ENABLING EARLY SYNCHRONIZATION FOR INTER-CENTRAL UNIT LOWER LAYER TRIGGERED MOBILITYCROSS-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 / 563,273 entitled “Enabling Early Synchronization for InterCentral Unit Lower Layer Triggered Mobility,” filed on March 8, 2024. The entire content of the provisional application is hereby expressly incorporated herein by reference.FIELD OF THE DISCLOSURE

[0002] This disclosure relates to wireless communications and, more particularly, to enabling early synchronization for inter-central unit (inter-CU) lower layer triggered mobility (LTM).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 RAN nodes (e.g., base stations or components of a distributed base station), interconnected by a backhaul. Multi-Radio Dual Connectivity (MR-DC) refers to connectivity scenarios in which these multiple RAN nodes support different radio access technologies (RATs). 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 another 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, at some point a serving cell change has to be performed for the UE. To perform the serving cell change, the RAN configures 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. Thus, 3GPP has introduced.

[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) measurementresults 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 serving DU operates the LTM candidate, the LTM cell switch is an intra-CU intra- DU LTM cell switch. If a candidate DU (C-DU) of the DU(s) operates the LTM candidate cell, 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.SUMMARY

[0009] An example embodiment of these techniques is a method implemented in a candidate base station that includes a central unit (CU) and a distributed unit (DU). The method includes receiving, from a source base station (S-BS), a handover request message including at least one of (i) a lower layer triggered mobility (LTM) indication for a candidate cell associated with the C-BS or (ii) a request for early synchronization information, for a user equipment (UE) communicating with the S-BS; generating the early synchronization information in response to the request; and transmitting, to the S-BS, the early synchronization information.

[0010] An example embodiment of these techniques is a method implemented in a source base station that includes a central unit (CU) and a distributed unit (DU). The method includes transmitting, to a candidate base station (C-BS), a handover request message including at least one of (i) a lower layer triggered mobility (LTM) indication for a candidate cell associated with the C-BS or (ii) a request for early synchronization information, for a user equipment (UE) communicating with the S-BS; receiving, from the C-BS, early synchronization information; and transmitting, to the UE, the early synchronization information.

[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 device (UE) can implement the techniques of this disclosure;

[0013] Fig. IB is a block diagram of an example base station including a centralized unit (CU) and a distributed unit (DU) that can operate in the system of Fig. 1 A;

[0014] Fig. 2A is a block diagram of an example protocol stack according to which the UE of Fig. 1A communicates with base stations;

[0015] 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;

[0016] Fig. 3 is a message sequence diagram of an example intra-DU LTM scenario;

[0017] Fig. 4 is a message sequence diagram of an example inter-DU LTM scenario;

[0018] Fig. 5 is a message sequence diagram of an example inter-CU LTM scenario;

[0019] Fig. 6 is a flow diagram of an example method for generating early synchronization information in candidate base station;

[0020] Fig. 7 is a flow diagram of an example method for generating early synchronization information in central unit (CU) of a candidate base station;

[0021] Fig. 8A is a flow diagram of an example method for generating early synchronization information in candidate base station, in view of whether the UE supports early TA acquisition;

[0022] Fig. 8B is a flow diagram of an example method for generating early synchronization information in candidate base station, in view of whether the handover request message includes an early synchronization information request;

[0023] Fig. 9A is a flow diagram of an example method for generating early synchronization information in CU of a candidate base station, in view of whether the UE supports early TA acquisition;

[0024] Fig. 9B is a flow diagram of an example method for generating early synchronization information in CU of a candidate base station, in view of whether the handover request message includes an early synchronization information request;

[0025] Fig. 10A is a flow diagram of an example method for generating early synchronization information in CU of a candidate base station using information from the DU, in view of whether the UE supports early TA acquisition;

[0026] Fig. 10B is a flow diagram of an example method for generating early synchronization information in CU of a candidate base station using information from the DU, in view of whether the handover request message includes an early synchronization information request;

[0027] Fig. 1 lis a flow diagram of a first example method for obtaining early synchronization information in a source base station;

[0028] Fig. 12 is a flow diagram of a second example method for obtaining early synchronization information in a source base station;

[0029] Fig. 13 is a flow diagram of a third example method for obtaining early synchronization information in a source base station;

[0030] Fig. 14 is a flow diagram of a first example method for obtaining early synchronization information in a source base station;

[0031] Fig. 15 is a flow diagram of an example method for generating and providing early synchronization information in a candidate base station; and

[0032] Fig. 16 is a flow diagram of an example method for requesting and receiving early synchronization from a candidate base station.DETAILED DESCRIPTION OF THE DRAWINGS

[0033] A source base station or a candidate base station can implement the techniques of this disclosure to perform early synchronization in an LTM cell scenario.

[0034] 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) 104, a base station 106 and a core network (CN) 110. The base station 104 and 106 operate in a radio access network (RAN) 105. The UE 102 initially connects to the base station 104. In some scenarios, the base station 104 can perform an SN addition to configure the UE 102 to operate in dual connectivity (DC) with the base station 104 and the base station 106. The base stations 104 and 106 operate as an MN and an SN for the UE 102, respectively.

[0035] In various configurations of the wireless communication system 100, the base station 104 can operate as a master eNB (MeNB) or a master gNB (MgNB), and the base station 106 can operate as a secondary gNB (SgNB). The UE 102 can communicate with the base station 104 and the base station 106 via the same RAT such as EUTRA or NR, or different RATs. When the base station 104 is an MeNB and the base station 106 is an SgNB, the UE 102 can be in EUTRA-NR DC (EN-DC) with the MeNB and the SgNB.

[0036] In some cases, an MeNB or an SeNB is implemented as an ng-eNB rather than an eNB. When the base station 104 is a Master ng-eNB (Mng-eNB) and the base station 106 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 104 is an MgNB and the base station 106 is an SgNB, the UE 102 may be in NR-NR DC (NR-DC) with the MgNB and the SgNB. When the base station 104 is an MgNB and the base station 106 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.

[0037] In the scenarios in which the UE 102 hands over from the base station 104 to the base station 106, the base stations 104 and 106 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 104 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 and the additional base station, or the UE can operate in single connectivity (SC) with the base station 106, after completing the handover. In the latter case, the base stations 104 and 106 in this case operate as a source MN (S-MN) and as a target MN (T-MN), respectively.

[0038] 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. The CN 110 can support other cores (not shown). The base station 104 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 104 and 106 can support an X2 or Xn interface.

[0039] As illustrated in Fig. 1A, the base station 104 supports cell 124A, and the base station 106 supports a cell 126. The cells 124A and 126 can partially overlap, so that the UE 102 can communicate in DC with the base station 104 and the base station 106, where one ofthe base stations 104 and 106 is an MN and the other is an SN. The base station 104 can support additional cell(s) such as cell 124B, and the base station 106 can support additional cell(s) (not shown in Fig. 1A). The cells 124A andl24B can partially overlap, so that the UE 102 can communicate in carrier aggregation (CA) with the base station 104. The base station 104 can operate the cells 124A andl24B via one or more transmit and receive points (TRPs). More particularly, when the UE 102 is in DC with the base station 104 and the base station 106, one of the base stations 104 and 106 operates as an MeNB, an Mng-eNB or an MgNB, and the other operates as an SgNB or an Sng-eNB.

[0040] 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. 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 can also 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.

[0041] With continued reference to Fig. 1A, the base station 104 operates 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 signal on physical downlink (DL) channels and DL reference signals from 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 signal 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 124A) and / or one or more TRPs. In an example implementation, the processing hardware 130 includes a MAC controller 134 configured to perform MAC functions with one or more user devices. The MAC functions include performing 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 functions as described below. The processing hardware 130 can further include an RRC controller 136 to implement procedures andmessaging at the RRC sublayer of the protocol communication stack. For 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 104 operates as an MN relative to an SN or as an SN relative to an MN. When the base station is a distributed base station, a CU LTM Controller 137 A can operate in a CU, and a DU LTM controller 137B can operate in a DU (see Fig. IB). The base station 106 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.

[0042] 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 signal on physical DL channels and / or DL reference signals from the base station 104 or 106 via one or more cells (e.g., the cell(s) 124A and / or 126) 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 104 or 106 via one or more cells (e.g., the cell(s) 124A, 124B, and / or 126) 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 104 or 106. For example, the MAC functions include performing 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 104 or 106. 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.

[0043] 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 104 or the SN 106. 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) direction and / or in the downlink (from a base station to the UE 102) direction. UEs can use several types of SRBs and DRBs. 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 measurementinformation 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 SRB2 can 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.

[0044] Fig. IB depicts an example distributed or disaggregated implementation of any one or more of the base stations shown in Fig. 1A (e.g., base station 104 or base station 106). In this implementation, the base station 104, 106 includes a central unit (CU) 172 and one or more distributed units (DUs) 174. The CU 172 includes the 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. The processing hardware 140 in an example implementation includes an SN RRC controller 142 configured to manage or control one or more RRC configurations and / or RRC procedures when the base station 106 operates as an SN.

[0045] Each of the DU(s) 174A includes processing hardware 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. In some examples, the processing hardware includes a medium access control (MAC) controller (e.g., MAC controller 134, 144) 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 106 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.

[0046] 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).

[0047] 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.

[0048] 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 104, 106).

[0049] 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.

[0050] 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.”

[0051] 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.

[0052] 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 104 or 106 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.

[0053] Next, with reference to Figs. 305, 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.

[0054] Referring first to Fig. 3, in a scenario 300, the base station 104 includes a CU 172 and DU 174 and the DU 174 operates the cell 124A. The UE 102 initially communicates 302with the DU 174 on a serving cell (e.g., the cell 124A), using a serving DU configuration, and the UE 102 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) (e.g., cell 124D 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 in 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 cells(s) is a PCell. In such cases, the additional cell(s) includes SCell(s) and / or additional cell(s) associated with the PCell or an SCell. In the following description, the base station 104 can include the DU 174, the CU 172 or both the DU 174 and CU 172.

[0055] In the event 302, the UE 102 can transmit UL PDUs and / or UL control signals to the base station 104 on the cell 124A and / or on 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 104 via radio bearers which can include SRBs and / or DRB(s). The base station 104 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 104 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 104 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.

[0056] In some implementations, the serving DU configuration includes physical layer configuration parameters, MAC configuration parameters, and / or RLC configurationparameters. In some implementations, the serving DU configuration includes at least one first non-LTM 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 DRBs. 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 104, and the UE receives the remaining portion of these configuration parameters from the base station 104.

[0057] 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 aserving 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).

[0058] While communicating with the base station 104, 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). 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).

[0059] 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 104 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 104 to communicate with the UE 102. In some implementations, the base station 104 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 strength and / or the quality of the serving cell (e.g., cell 124A), and / or is better than the strength and / or the quality of the serving cell by a first predetermined threshold, the CU 172 determines toprepare 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.

[0060] In response to determining to prepare the first cell for LTM, the CU 172 transmits 385 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 that the DU 174 prepare the first cell for LTM for the UE 102. In one 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.

[0061] 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.

[0062] The events 385 and 310 are collectively referred to in Eig. 3 as an LTM preparation procedure 390.

[0063] 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.

[0064] 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 LTMreference 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., merely to augment of the LTM reference DU configuration.

[0065] 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.

[0066] 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.

[0067] 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.

[0068] 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 rest 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.

[0069] In some implementations, the LTM reference DU configuration is different from the serving DU configuration. In some implementations, a portion of the LTM reference DUconfiguration 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.

[0070] 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.

[0071] 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.

[0072] 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 does so to indicate that the CSI resource configuration is / are associated with the first cell. In another implementation, the CU 172 does so 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 CSIresource 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 1 in the first CU-to-DU message.

[0073] 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.

[0074] 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.

[0075] 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.

[0076] 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.

[0077] If the first DU-to-CU message includes the LTM reference DU configuration, theCU 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 LTMreference 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, similarly 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, similarly 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.

[0078] 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.

[0079] 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.

[0080] 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, similarly 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, similarly to the events 320 and 322.

[0081] 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 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 early UL synchronization with an LTM candidate cell. Otherwise, if the CU 172 determines that the UE 102 does not support 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 earlysynchronization information to the UE 102 via the DU 174, similarly 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, similarly to the events 320 and 322.

[0082] 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 in 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.

[0083] 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.

[0084] 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.

[0085] 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 ContextModification 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.

[0086] 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 is 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.

[0087] 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.

[0088] 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.

[0089] 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 DUconfiguration 1 is a CellGroupConfig IE defined in 3GPP specification 38.331. In other implementations, the LTM DU configuration 1 includes configuration parameters in the CellGroupConfig IE.

[0090] 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 RSs, 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.

[0091] 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.

[0092] 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 configurations and / or one or more second CSI resource configuration that are included in the serving DU configuration in an event 302 and / or in 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).

[0093] After transmitting the RACH configuration to the UE 102 via the CU 172, the DU 174 may transmit 350 a PDCCH order to the UE 102 to command the UE 102 to transmit a RA preamble on the first cell. In response to the PDCCH order, the UE 102 transmits a RA preamble on the first cell. The DU 174 includes PDCCH order information in the PDCCH order. The PDCCH order information includes a RA preamble index, an 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, 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 a SS / PBCH Block Resource Indicator (SSBRI) corresponding to the SSB index. Thus, the DU 174 determines the SSB index based on the SSBRI.

[0094] 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 forearly 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.

[0095] 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, similarly to the event 324. The DU 174 determines to command that the UE 102 perform an LTM cell switch to the first cell based on the additional LI measurement report(s) and / or on 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 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 implementation, 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.

[0096] 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 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 a 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.

[0097] 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.

[0098] 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 a 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 implementations and / or on the RA configuration parameters, the RA procedure can be a four-step RA procedure or a 1two-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.

[0099] 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.

[0100] In some implementations, each of the TCI state configuration(s) include a TCI state ID. In some implementations, the DU 174 includes, in the LTM Cell Switch Command, a first TCI state ID indicating a first one of the TCI state configurations(s). The UE 102 identifies the first one of the TC state configurations(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.

[0101] 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 DUconfiguration(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 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 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 2, ..., N is similar to the procedure 394. In other implementations, the CU 172 includes the list in the first RRC reconfiguration message.

[0102] 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. 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.

[0103] In some implementations, an LTM ID is provided in different formats depending on which command or message provides the LTM ID. For example, the LTM ID may be provided differently in a PDCCH order, in an LTM Cell Switch Command, and in an RRCreconfiguration message. For 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. For 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.

[0104] The events 304, 306, 390, 392, 394, and 324 are collectively referred to in Fig. 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 Fig.3 as an intra-CU intra-DU LTM procedure 380.

[0105] Referring next to Fig. 4, in a scenario 400, the base station 104 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 first cell (e.g., cell 124B). 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 Fig. 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.

[0106] 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, similarly 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 396, the UE 102 does not perform an LTM cell switch. During the communication 402, the UE 102 transmits 404, 406 at least onemeasurement 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., cell 124B 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 (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 for Eig. 3. In some implementations, the C-DU 174B includes an LTM reference DU configuration in the first DU-to-CU message, as described for Eig. 3. In other implementations, the C-DU 174B does not include an LTM reference DU configuration in the first DU-to-CU message.

[0107] In some implementations, if the CU 172 receives an LTM reference DU configuration from the S-DU 174A as described with reference to Eig. 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 Eig. 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.

[0108] 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 {LTM ID 1, cell ID 1 } as a tuple in the second CU-to-DU message.

[0109] 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 for 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.

[0110] 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 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 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.

[0111] As described for Fig. 3, the CU 172 performs 494 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, 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 to the UE 102 in the first RRC reconfiguration message and / or other RRC reconfiguration message(s) transmitted to the UE 102, as described for Fig. 3. The CU 172 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 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 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.

[0112] In some implementations, the PDCCH order information includes a frequency domain resource assignment, a RA preamble index, an 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, the CU 172 includes {cell ID 1, PDCCH order information} as a tuple in the CU-to-DU message. In the case of 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 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. In the case of 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 a RA preamble to the C-DU174B 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, a 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 configurations(s).

[0113] In some implementations, the C-DU 174B determines the RA-RNTI based on a PRACH occasion in which the C-DU 174B receives 452 the RA preamble. 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 OLDM 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).

[0114] In response to determining to command the UE 102 to perform an LTM cell switch or transmitting 456 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 TCIstate ID in the CU-DU Cell Switch Notification message. The UE 102 and the C-DU 174B identify the first one of the TC state configurations(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.

[0115] 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, the CU 172 includes {cell ID 1, TCI state configuration(s)} as a tuple in the CU-to-DU message. In the case of the 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 configurations(s). In some implementations, the S-DU 174A determines the first TCI state configuration or the fist TCI state ID. The UE 102 identifies the one of the TCI state configurations(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.

[0116] 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 490 or 492, as described for Fig. 3.

[0117] 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 addition LTM preparation procedures are UE Context Modification procedures.

[0118] The events 404, 406, 490, 492, 494, 494, and 424 are collectively referred to in Fig. 4 as an intra-CU inter-DU LTM configuration procedure 496. The events 404, 406, 490, 492, 494, 494, 424, 450, 452, 454456, 458, 426, 428, 430, 432, 434, 436, 438, 440, 442, and 444 are collectively referred to in Fig. 4 as an intra-CU inter-DU LTM procedure 480.

[0119] Referring next to Fig. 5, in a scenario 500, the base station 104 operates as a serving or source base station (S-BS), and the base station 106 operates as a candidate base station (C-BS). The C-BS 106 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., interbase station scenario) while the scenarios 300 and 400 are intra-CU (i.e., intra-base station) scenarios. The S-BS 104 can include a CU and a DU (not shown in Fig. 5), similar to the base station 104 as shown in Figs. 3 and 4. Initially, the UE 102 communicates 502 with the S-BS 104 via serving cell(s) using a serving configuration. In some implementations, the S-BS 104 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 104 may perform 580 intra-CU LTM procedure(s) with the UE 102, similarly to the procedures 380 and / or 480.Alternatively, while the communicating 502 with the UE 102, the S-BS 104 may perform 596 intra-CU LTM configuration procedure(s) with the UE 102, similarly to the procedures 396 and / or 496.

[0120] While communicating with the S-BS 104, the UE 102 transmits 504 at least one measurement report to the S-BS 104. 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 126). The S-BS 104 determines to prepare a first cell (e.g., the cell 126) 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 126. The S-BS 104 identifies that the first cell is operated by the base station 106 based on the PCI, and the S-BS 104 determines that the first cell qualifies for LTM preparation based on the measurement result(s).

[0121] After (e.g., in response to) determining to prepare the first cell as an LTM candidate cell for the UE 102, the S-BS 104 (e.g., the CU of the S-BS 104) generates a HandoverRequest message including a first cell ID (i.e., cell ID 1) of the first cell (i.e., cell 1). The S- BS 104 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, similarly 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, similarly 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 104. 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).

[0122] The events 505, 590, 507 are collectively referred to in Fig. 5 as an inter-CU LTM preparation procedure (or called inter- MN LTM preparation procedure) 598.

[0123] In some implementations, the Handover Request message includes a DU ID of the S-DU of the S-BS 104. 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 104. When receiving the BS ID, the CU 172 may include the BS ID in the first CU-to- DU message. For example, the BS ID is a gNB ID.

[0124] In some implementations, the CU 172 requests an ETM reference DU configuration in the procedure 590, as described for Figs. 3 and 4. In other implementations, the CU 172 does not request an ETM reference DU configuration in the procedure 590. In some implementations, the DU 174 transmits an ETM reference DU configuration to the CU 172 in the procedure 590, as described for Figs. 3 and 4. In other implementations, the DU 174 does not transmit an ETM reference DU configuration to the CU 172 in the procedure 590. In some implementations, the S-BS 104 (e.g., the CU of the S-BS 104) may obtain an ETM reference configuration, as described for Figs. 3 and 4. In other implementations, the S-BS 104 may receive an LTM reference configuration from another BS (not shown in Fig. 5) inanother inter-CU LTM preparation procedure as described above and below. If the S-BS 104 obtains an LTM reference configuration, the S-BS 104 may include the LTM reference configuration (S-BS generated LTM reference configuration) in the Handover Request message. In some implementations, the S-BS 104 includes the LTM reference configuration in the inter-node RRC message HandoverPreparationlnformation 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 104 determines to request or cause the C-BS 106 to provide a complete LTM candidate configuration so that the S-BS 104 does not include the LTM reference configuration in the Handover Request message. If the S-BS 104 does not obtain an LTM reference configuration, the S-BS 104 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 for Fig. 3. Alternatively, the DU 174 may disregard the LTM reference (DU) configuration and generate an LTM DU configuration as a complete configuration, as described for Fig. 3.

[0125] 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 for 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 for 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) including only 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.

[0126] 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 new field / IE (e.g., LTM specific) to avoid 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 new field / IE (e.g., LTM specific) to avoid 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 two 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, and the BS-to-BS interface protocol field / IE can have the second value to indicate a delta configuration indication.

[0127] In some implementations, the S-BS 104 is preconfigured with a CSI resource configuration (e.g., (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 104 receives the CSI resource configuration and / or the LTM SSB configuration from an 0AM(Operations, Administration and Maintenance) node. In yet other implementations, the S-BS 104 receives the CSI resource configuration and / or the LTM SSB configuration from the CU172. 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 (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 104 performs an LTM CSI report configuration and / or LTM ID configuration procedure (not shown in Eig. 5) with an S-DU of the S-BS 104, similarly to the procedure 392. In the LTM CSI report configuration and / or LTM ID configuration procedure, the CU of the S-BS 104 transmits the CSI resource configuration and / or the LTM SSB configuration to the S-DU of the S-BS 104. In response, the CU of the S-BS 104 receives one or more CSI report configurations for the UE 102 from the S-DU of the S-BS 104. In some implementations, the CU of the S-BS 104 receives the CSI report configuration(s) in a second serving DU configuration from the S-DU.

[0128] To prepare the first cell as an LTM candidate cell for the UE 102, the CU 172 may receive early synchronization information (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 (RACH configuration 1) and / or at least one TCI state configuration (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.

[0129] 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 104 assigns an LTM ID (e.g., LTM ID 1) for identifying the first LTM candidate configuration.

[0130] After (e.g., in response to) receiving the Handover Request Acknowledge message, the S-BS 104 (e.g., the CU of the S-BS 104) transmits 518 a first RRC reconfigurationmessage to the UE 102, including { LTM ID 1, LTM candidate configuration 1 } as a tuple, similarly to the event 318. If the Handover Request Acknowledge message includes the LTM reference configuration, the S-BS 104 may include the LTM reference configuration in the first RRC reconfiguration message. The S-BS 104 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 received in the Handover Request Acknowledge message. Alternatively, the S-BS 104 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 104 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 104 in response to the first RRC reconfiguration message. The UE 102 transmits an additional RRC reconfiguration complete message to the S-BS 104 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 104 may include the second serving DU configuration in the first RRC reconfiguration message or in one of the additional RRC reconfiguration message(s).

[0131] 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 104 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 104 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.

[0132] If the Handover Request Acknowledge message includes the PDCCH order information (PDCCH order information 1), the S-BS 104 transmits 550 a PDCCH order based on the PDCCH order information. If the S-BS 104 is a distributed base station, the CU of the S-BS 104 may transmit the PDCCH order information to the S-DU of the S-BS 104. For example, the S-BS 104 or the S-DU of the S-BS 104 transmits 550 a PDCCH order to the UE 102, including the PDCCH order information. In some implementations, the S-BS 104 or the S-DU of the S-BS 104 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 104 includes the LTM ID 1 in the PDCCH order to indicate the first cell. The UE 102 transmits 552 a 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 104 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-SB 104 (e.g., the CU of the S-BS 104). In some implementations, the DU 174 includes the cell ID 1, the RA preamble index, a RA- RNTI, the DU ID of the S-DU of the S-BS 104, and / or the BS ID of the S-BS 104 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 104 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 104 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 104 to the S-DU of the S-BS 104. In some implementations, the CU of the S-BS 104 does not include the BS ID in the CU-DU TA Information Transfer message.

[0133] In some implementations, the CU 172 determines an address (e.g., an IP address) of the S-BS 104 or the CU of the S-BS 104, based on the BS ID of the S-BS 104. In other implementations, the CU 172 determines an address (e.g., an IP address) of the S-BS 104 or the CU of the S-BS 104, based on the DU ID of the S-DU of the S-BS 104. With these implementations, the CU 172 sends the CU-CU TA Information Transfer message to the S- BS 104 or the CU of the S-BS 104 in accordance with the address.

[0134] 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 104 transmits 526 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 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 106 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 includes the LTM ID 1 in the RRC reconfiguration complete message 536 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, 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.

[0135] In some implementations, if the S-DU or the S-BS 104 receives a TA value as described above, the S-DU or the S-BS 104 may include the TA value in the LTM Cell Switch Command. In some implementations, the S-DU or the S-BS 104 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 configurations(s).

[0136] In response to determining to command the UE 102 to perform the LTM cell switch or subsequently to transmitting 526 the LTM Cell Switch Command, the S-DU of the S-BS 104 transmits a DU-CU Cell Switch Notification message to the CU of the S-BS 104 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 104 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 530a 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 104 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 TC 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.

[0137] 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 104 (e.g., the CU of the S-BS 104) 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 104 communicate 502 data with each other. In some implementations, after receiving 534 the Access Success message or 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 104 (e.g., the CU of the S-BS 104) 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 104 (e.g., the CU of the S-BS 104) to indicate that the LTM cell switch is completed successfully.

[0138] 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 104 (e.g., the CU of the S-BS 104) 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 104 communicate 502 data with each other.

[0139] In some implementations, after (e.g., in response to) receiving 534 the AccessSuccess 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 UEContext Release message to the S-BS 104. In response to the UE Context Release message, the S-BS 104 releases a UE context of the UE 102.

[0140] In some implementations, the S-BS 104 (e.g., the CU of the S-BS 104) 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 106. N is an integer and larger than 1. For example, the S-BS 104 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 104 receives LTM candidate configuration(s) 2, ..., N configuring the cell(s) 2, ..., N for LTM, respectively. As described above, the S-BS 104 or the C-BS 106 assigns LTM ID(s) 2, ..., N to identify the LTM candidate configuration(s) 2, ..., N, respectively. The S-BS 104 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 104 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 104 may obtain RACH configuration 2, ..., N for the cell(s) 2, ..., N respectively, as described for the RACH configuration 1. The S-BS 104 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 104 may obtain LTM SSB configuration 2, ..., N for the cell(s) 2, ..., N, respectively, as described for LTM SSB configuration 1. The S-BS 104 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 104 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 or 494. In other implementations, the S-BS 104 includes the list in the first RRC reconfiguration message.

[0141] In other implementations, the S-BS 104 performs the procedure 598 with the CU 172 to prepare one or more of the cells(s) 1, ..., N as LTM candidate cell(s) for the UE 102. In such implementations, the S-BS 104 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, similarly to the procedure in which the CU 172 obtains the LTM candidate configuration 1. The C-BS 106 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 104 assigns LTM ID(s) 2, ..., M to identify the LTM preparation procedure(s) 2, ..., M respectively, as described for the LTM ID 1. In the case 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.

[0142] 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 104 obtains CSI resource configuration(s) 2, ..., M for the cell(s) 2, ..., M, respectively, as described for the CSI resource configuration 1. In the case that 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.

[0143] In some implementations, the CU 172 or the S-BS 104 obtains LTM SSB configuration(s) 2, .. M for the cell(s) 2, .. M, respectively, as described with reference tothe LTM SSB configuration 1. In the case that 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 104 obtains PCI(s) 2, .. M for the cell(s) 2, .. M, respectively, as described with reference to the PCI 1. In the case that the CU 172 obtains the PCI(s) 2, ..., M, the CU 172 includes the PCI(s) 2, ..., M in the Handover Request Acknowledge message.

[0144] 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.

[0145] 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 104 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.

[0146] In some implementations, the S-BS 104 may perform 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 (ifobtained)} 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 104 includes the list in the first RRC reconfiguration message.

[0147] In some implementations, the S-BS 104 may include measurement result(s) 1, ..., N for the cell(s) 1, ..., N respectively in the Handover Request message. The S-BS 104 receives the measurement result(s) from the UE 102. The C-BS 106 may select or determine the cell(s) 1, ...M, based on the measurement result(s) 1, ..., N. In other implementations, the C-BS 106 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 LTM candidate cell(s) is a predetermined number.

[0148] Next, several example methods, which a RAN node (e.g., a base station 104, 106 (Fig. 1A), a DU 174 (Fig. IB) or a CU 172 (Fig. IB)) can implement for inter-CU LTM, are discussed with reference to Figs. 6 A- 13. Generally speaking, similar events in Figs. 6A-15 are labeled with similar reference numbers that share two least significant digits, with differences discussed where appropriate.

[0149] Fig. 6 illustrates an example method 600, which a C-BS (e.g., the C-BS 106 in Fig. 5) can implement. The method 600 begins at block 605, where the C-BS receives a Handover Request message from an S-BS (e.g., the S-BS 104 in Fig. 5), including an early sync information request to request early sync information. The flow proceeds to blocks 660 and / or 662. At block 660, the C-BS generates a RACH configuration in response to the early sync information request. At block 662, the C-BS generates one or more TCI state configurations in response to the early sync information request. At block 607, the C-BS transmits a Handover Request Acknowledge message to the S-BS, including the RACH configuration and / or the TCI state configuration(s). At block 652, the C-BS may receive a RA preamble from a UE via a first cell in accordance with the RACH configuration. At block 655, the C-BS may derive a timing advance value based on the random-access preamble. At block 658, the C-BS may transmit a first BS-to-BS message including the timing advance value to the S-BS. At block 627, the C-BS may receive a second BS-to-BS message from the S-BS, notifying that the S-BS initiates transmission of an LTM Cell Switch Command to the UE for an LTM cell switch to the first cell. At block 632, the C-BS may detect that the UEaccesses the first cell. At block 639, the C-BS may transmit an LTM Success message to the S-BS. At block 640, the C-BS may communicate with the UE. In some implementations, the C-BS may communicate with the UE via the first cell in accordance with the TCI state configuration(s).

[0150] In some implementations, the first BS generates PDCCH order information and includes the PDCCH order information in the Handover Request Acknowledge message. The C-BS may receive the RA preamble from the UE via a first cell in accordance with the RACH configuration and the PDCCH order information. In some implementations, the first BS-to-BS is a CU-to-CU interface protocol message (e.g., CU-CU TA Information Transfer message). In some implementations, the second BS-to-BS message is a CU-to-CU interface protocol message (e.g., CU-CU Cell Switch Notification message). For example, the CU-to- CU interface protocol is a XnAP.

[0151] Fig. 7 illustrates an example method 700 similar to the method 600, which can be implemented by a CU of a C-BS (e.g., the C-BS 106 of Fig. 5). The method 700 begins at block 705 performed by the CU. At block 785, the CU transmits a CU-to-DU message to a DU, including an early sync information request to request early sync information (e.g., event 385, 390, 312, 392, 490 or 590). At block 710, the CU receives a DU-to-CU message from the DU, including a RACH configuration and / or one or more TCI state configurations (e.g., event 310, 390, 314, 392, 490, or 590). The CU performs block 707 and may perform blocks 764, 658, 730, 734, 639, and / or 740. At block 764, the CU receives a DU-CU TA Information Transfer message including the timing advance value from the DU. At block 730, the CU transmits a CU-DU Cell Switch Notification message to the DU, notifying that the DU initiates transmission of an LTM Cell Switch Command to the UE for an LTM cell switch to the first cell. At block 734, the CU receives an Access Success message from the DU. At block 740, the CU communicates with the UE via the DU and the first cell.

[0152] In some implementations, the DU-to-CU message includes PDCCH order information, and the CU includes the PDCCH order information in the Handover Request Acknowledge message.

[0153] Examples and implementations described for Fig. 6 can apply to Fig. 7.

[0154] Fig. 8A illustrates an example method 800A, which can be implemented by a C-BS (e.g., the C-BS 106 of Fig. 5). The method 800A begins at block 805, where the C-BS receives a Handover Request message from an S-BS, requesting to prepare a first cell as anLTM candidate cell for a UE. At block 859, the C-BS determines whether the UE supports early TA acquisition. If the UE supports early TA acquisition (i.e., “Yes” branch of block 859), the flow proceeds to block 860. At block 860, the C-BS generates a RACH configuration for the UE. At block 862, the C-BS may generate one or more TCI state configurations for the UE. Otherwise, if the UE does not support the early TA acquisition (i.e., “No” branch of block 859), the flow proceeds to block 891. At block 891, the C-BS generates an LTM candidate configuration for the UE, where the LTM candidate configuration configures the first cell for LTM. The flow proceeds to block 891 from block 862 as well as block 859. At block 807, the C-BS transmits a Handover Request Acknowledge message to the second BS, including the RACH configuration, the TCI state configuration(s), and / or the LTM candidate configuration.

[0155] In some implementations, if the UE supports early TA acquisition, the C-BS generates PDCCH order information and includes the PDCCH order information in the Handover Request Acknowledge message. The S-BS can include PDCCH order information in a PDCCH order to be transmitted to the UE (e.g., event 550). Otherwise, if the UE does not support the early TA acquisition, the C-BS does not include (the) PDCCH order information in the Handover Request Acknowledge message.

[0156] In some implementations, the Handover Request message includes a UE capability IE of the UE. The C-BS determines whether the UE supports early TA acquisition in accordance with the UE capability. For example, the UE capability is a UE-NR-Capability IE. If the UE capability IE include a first capability indicating support of early TA acquisition, the C-BS determines that the UE supports the early TA acquisition. Otherwise, if the UE capability IE does not include the first capability, the C-BS determines that the UE does not support the early TA acquisition.

[0157] Fig. 8B is a flow diagram of an example method 800B similar to the method 800A, except that the method 800B includes block 861 instead of block 859. At block 861, the C-BS determines whether the Handover Request message includes an early sync information request. If the Handover Request message includes the early sync information request (i.e., “Yes” branch of block 861), the flow proceeds to block 860. Otherwise, if the Handover Request message does not include the early sync information request (i.e., “No” branch of block 861), the flow proceeds to block 891.

[0158] Fig. 9A illustrates an example method 900A similar to the methods 700 and 800A, which can be implemented by a CU of a C-BS (e.g., the C-BS 106 of Fig. 5). The method 900A begins at block 905, which is performed by the CU. At block 959, the CU determines whether the UE supports early TA acquisition. If the UE supports early TA acquisition (i.e., “Yes” branch of block 959), the flow proceeds to blocks 785 and 710. The flow proceeds to block 963 from blocks 785 and 710. Otherwise, if the UE does not support the early TA acquisition (i.e., “No” branch of block 959), the flow skips blocks 785 and 710 and proceeds to block 963. At block 963, the CU transmits a CU-to-DU message to a DU to request preparing the first cell as an LTM candidate cell for the UE. At block 965, the CU receives a DU-to-CU message from the DU, including an LTM DU configuration for the UE. At block 991, 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 907 from block 991.

[0159] Fig. 9B is a flow diagram of an example method 900B similar to the methods 900A, 800B and 700, except that the method 900B includes block 961 instead of block 959. If the Handover Request message includes an early sync information request (i.e., “Yes” branch of block 961), the flow proceeds to blocks 785, 710 and 963. Otherwise, if the Handover Request message does not include the early sync information request (i.e., “No” branch of block 961), the flow skips blocks 785 and 710 and proceeds to block 963.

[0160] Fig. 10A illustrates an example method 1000A similar to the methods 900A, which can be implemented by a CU of a first base station (BS) (e.g., the C-BS 106 of Fig. 5). The method 1000A begins at block 1005 performed by the CU. At block 1071, 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. At block 1059, the CU determines whether the UE supports early TA acquisition. If the CU determines that the UE supports the early TA acquisition (i.e., “Yes” branch of block 1059), the flow proceeds to block 1072. At block 1072, the CU includes an early sync information request in the CU-to-DU message to request early sync information. The flow proceeds to block 1008 from block 1072. Otherwise, if the UE does not support the early TA acquisition (i.e., “No” branch of block 1059), the flow skips block 1072 and proceeds to block 1008. At block 1008, the CU transmits the CU-to-DU message to a DU. The flow proceeds to blocks 1010 and 1007 from block 1008.

[0161] In some implementations, the CU includes an LTM indicator in the CU-to-DU message. In such cases, the DU-to-CU message includes an LTM DU configuration. In other implementations, the CU does not include an LTM indicator in the CU-to-DU message. In such cases, the DU-to-CU message may not include an LTM DU configuration.

[0162] Examples and implementations described for Fig. 6-9B can apply to Fig. 10A.

[0163] Fig. 10B is a flow diagram of an example method 1000B similar to the method 1000A, except that method 1000B includes block 1061 instead of block 1059. At block 1061, the CU determines whether the Handover Request message includes an early sync information request. If the Handover Request message includes an early sync information request (i.e., “Yes” branch of block 1061), the flow proceeds to block 1072. Otherwise, if the Handover Request message does not include the early sync information request (i.e., “No” branch of block 1061), the flow proceeds to block 1008.

[0164] Fig. 11 illustrates an example method 1100, which can be implemented by an S-BS (e.g., the S-BS 104 in Fig. 5). The method 1100 begins at block 1107, where the S-BS receives a RACH configuration, one or more TCI state configurations, and / or PDCCH order information for a UE from a C-BS. At block 1118, the S-BS transmits the RACH configuration and / or the TCI state configuration(s) to the UE. At block 1150, the S-BS may transmit a PDCCH order to the UE based on the PDCCH order information, where the PDCCH order commands the UE to transmit a RA preamble on a first cell operated by the second BS. At block 1158, the S-BS may receive a first BS-to-BS message including a timing advance value from the C-BS. At block 1126, the S-BS may transmit an ETM Cell Switch Command to the UE, commanding the UE to perform an LTM cell switch to the first cell. At block 1127, the S-BS transmits a second BS-to-BS message to the C-BS, notifying that the S- BS initiates transmission of an LTM Cell Switch Command to the UE for an LTM cell switch to the first cell. At block 1139, the S-BS may receive an LTM Success message from the second BS, indicating the UE successfully connects to the second BS via the first cell.

[0165] In some implementations, the S-BS transmits at least one Handover Request message to the C-BS, including an early sync information request to request early sync information (e.g., event 505). In response, the S-BS at block 1107 receives, from the C-BS, at least one Handover Request Acknowledge message that includes the RACH configuration, the TCI state configuration(s) and / or the PDCCH order information.

[0166] Examples and implementations described for Fig. 6 can apply to Fig. 11.

[0167] Fig. 12 illustrates an example method 1200 similar to the method 1100, which can be implemented by a CU of an S-BS (e.g., the S-BS 104 of Fig. 5). The method 1200 begins at block 1207 performed by the CU. The flow proceeds to block 1212 from block 1207. At block 1212, the CU transmits a CU-to-DU message to a DU, including the PDCCH order information and / or the TCI state configuration(s). The CU performs block 1218, and the CU may perform blocks 1158, 1258, 1228, 1127, and / or 1139. At block 1258, the CU transmits a CU-DU TA Information Transfer message including the timing advance value to the DU, after receiving the first BS-to-BS message at block 1158. At block 1228, the CU receives a DU-CU Cell Switch Notification message from the DU, notifying that the DU initiates transmission of an LTM Cell Switch Command to the UE for an LTM cell switch to the first cell. After receiving the DU-CU Cell Switch Notification message, the CU transmit the second BS-to-BS message.

[0168] In some implementations, the CU transmits a Handover Request message to the C- BS (e.g., event 505) and receives a Handover Request Acknowledge message from the C-BS in response (e.g., event 507). The Handover Request Acknowledge message includes the PDCCH order information, the RACH configuration, and / or the TCI state configuration(s). In some implementations, the CU includes, in the Handover Request message, an early sync information request to request early sync information. In other implementations, the CU does not include the early sync information request in the Handover Request message.

[0169] In some implementations, the CU includes an LTM indicator in the Handover Request message. In such cases, the Handover Request Acknowledge may include an LTM candidate configuration. In other implementations, the CU does not include the LTM indicator in the Handover Request message. In such cases, the Handover Request Acknowledge may not include an LTM candidate configuration.

[0170] Examples and implementations described for Eigs. 6 and 11 can apply to Eig. 12.

[0171] Eig. 13 illustrates an example method 1300, which can be implemented by an S-BS (e.g., the S-BS 104 in Fig. 5). The method 1300 begins at block 1374, where the S-BS determines to prepare a first cell as an LTM candidate cell for a UE. At block 1376, the S-BS includes a cell ID of the first cell in a Handover Request message for LTM. At block 1359, the S-BS determines whether the UE supports early TA acquisition. If the UE supports early TA acquisition (i.e., “Yes” branch of block 1359), the flow proceeds to block 1378. At block 1378, the S-BS includes an early sync information request in the Handover Request message.The flow proceeds to block 1305 from block 1378. Otherwise, if the UE does not support the early TA acquisition (i.e., “No” branch of block 1359), the flow skips block 1378 and proceeds to block 1305. At block 1305, the S-BS transmits the Handover Request message to a C-BS. At block 1307, the S-BS receives a Handover Request Acknowledge message from the C-BS, including a RACH configuration, one or more TCI state configurations and / or PDCCH order information.

[0172] Fig. 14 illustrates an example method 1400, which can be implemented by an S-BS (e.g., the S-BS 104 in Fig. 5). The method 1400 begins at block 1457, where the S-BS receives a TA value for a UE from a C-BS. At block 1423, the S-BS determines to initiate an ETM cell switch to a cell for the UE. The cell is operated by the C-BS. At block 1482, the S- BS determines whether the TA value is valid. If the TA value is valid (i.e., “Yes” branch of block 1482), the S-BS includes 1425 the TA value in an LTM Cell Switch Command. Otherwise, if the TA value is invalid (e.g., “No’ branch of block 1482), the flow skips block 1425 and proceeds to block 1426. In other words, if the TA valid is invalid, the S-BS refrains from including the TA value in the LTM Cell Switch Command. At block 1426, the S-BS transmits the LTM Cell Switch Command to the UE.

[0173] In some implementations, after receiving the TA value, the S-BS receives one or more (new) LI measurement reports (e.g., CSI report(s)) from the UE. The S-BS determines whether the TA value is valid based on the LI measurement report(s). For example, if measurement result(s) in the LI measurement report(s) is below or above a predetermined threshold, the S-BS determines that the TA value is invalid. Otherwise, the S-BS determines that the TA value is valid. In another example, the S-BS 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-BS determines that the TA value is in valid. Otherwise, the S-BS determines that the TA value is valid. If the S-BS determines that TA value is valid based on the LI measurement report(s), the S-BS performs includes the TA value in the LTM Cell Switch Command. Otherwise, if the S-BS determines that TA value is invalid based on the LI measurement report(s), the S-BS refrains from including TA value in the LTM Cell Switch Command.

[0174] Examples and implementations described for Figs. 6-12 can apply to Figs. 13-16.

[0175] Fig. 15 is a flow diagram of an example method 1500, which a C-BS (e.g., the C- BS 106 in Fig. 5) can implement, to generate and provide early synchronization information. The method 1500 begins at block 1505, with the C-BS receiving, from a source base station (S-BS), a handover request message including at least one of (i) a lower layer triggered mobility (LTM) indication for a candidate cell associated with the C-BS or (ii) a request for early synchronization information, for a user equipment (UE) communicating with the S-BS (e.g.., event 505, 605, 705, 805, 905, or 1005).

[0176] The method 1500 continues with block 1560 with the C-BS generating the early synchronization information in response to the request (e.g., event 660 or 662). Generating of the early synchronization information can include generating a RACH configuration, and / or a TCI state indicator. Generating of the early synchronization information can include transmitting, from a CU of the C-BS to a DU of the C-BS, a request for the early synchronization information. The method 1500 can include receiving the early synchronization information from the DU. Generating of the early synchronization information can be in response to determining that the UE supports early TA acquisition. Generating of the early synchronization information can be in response to determining that handover request message contains an IE that includes the request for early synchronization information.

[0177] The method 1500 continues with block 1507 with the C-BS transmitting, to the S- BS, the early synchronization information (e.g., event 507, 607, 707, 807, 907, 1007). The method 1500 can further include receiving, from the UE in a candidate cell of the C-BS, a random-access preamble in accordance with the RACH configuration, (e.g., event 552, 652). The method 1500 can further include deriving a timing advance (TA) value based on the random-access preamble (e.g., event 556 or 655). The method 1500 can further include transmitting the TA value to the S-BS (e.g., event 558 or 658).

[0178] The method 1500 can further include receiving, from the S-BS, an indication that the S-BS has initiated a lower layer triggered mobility (LTM) switch of the UE to the candidate cell (e.g., event 527). The method 1500 can further include determining that the UE has accessed the candidate cell and transmitting, to the S-BS, an LTM success indication (e.g., event 534, 539, 632, or 639).

[0179] Fig. 16 is a flow diagram of an example method 1600 for requesting and receiving early synchronization from a candidate base station. An S-BS (e.g., the S-BS 104 in Fig. 5) can implement method 1600.

[0180] The method 1600 begins with block 1605 with the S-BS transmitting, to a C-BS, a handover request message including at least one of (i) a lower layer triggered mobility (LTM) indication for a candidate cell associated with the C-BS or (ii) a request for early synchronization information, for a user equipment (UE) communicating with the S-BS (e.g., event 505 or 605). Transmitting of the request for early synchronization information can be in response to determining that the UE supports early TA acquisition (e.g., event 1359).

[0181] The method 1600 continues with block 1607 with the S-BS receiving, from the C- BS, early synchronization information (e.g., event 507 or 607). The early synchronization information can include one of a RACH configuration or a TCI state configuration. When the early synchronization information includes a TA value, the method 1600 can further include, in response to determining that the TA value is valid, transmitting the TA value to the UE in an LTM cell switch command (e.g., the events of Fig. 14).

[0182] The method 1600 continues with block 1618 with the S-BS transmitting, to the UE, the early synchronization information (e.g., event 518).

[0183] The following list of examples reflects a variety of the embodiments explicitly contemplated by the present disclosure.

[0184] Example 1. A method implemented in a candidate base station, the method comprising: receiving, from a source base station, a request for early synchronization information for the candidate base station, for a user equipment (UE) communicating with the source base station; generating the early synchronization information in response to the request; and transmitting the early synchronization information to the source base station.

[0185] Example 2. The method of example 1, wherein the generating the early synchronization information incudes generating a random-access channel (RACH) configuration.

[0186] Example 3. The method of example 1 or 2, wherein the generating the early synchronization information incudes generating a transmission configuration indicator (TCI) state configuration.

[0187] Example 4. The method of example 2, further comprising: receiving, from the UE in a candidate cell of the candidate base station, a random-access preamble in accordance with the RACH configuration.

[0188] Example 5. The method of example 4, further comprising: deriving a timing advance (TA) value based on the random-access preamble; and transmitting the TA value to the source base station.

[0189] Example 6. The method of example 4 or 5, further comprising: receiving, from the source base station, an indication that the source base station has initiated a lower layer triggered mobility (LTM) switch of the UE to the candidate cell.

[0190] Example 7. The method of example 6, further comprising: determining that the UE has accessed the candidate cell; and transmitting, to the source base station, an LTM success indication.

[0191] Example 8. The method of any of the preceding examples, wherein: the request from the source base station includes a handover request, and the early synchronization information is transmitted in a handover request acknowledge message.

[0192] Example 9. The method of any of the preceding examples, wherein the generating the early synchronization information includes: transmitting, from a central unit (CU) of the candidate base station to a distributed unit (DU) of the candidate base station, a request for the early synchronization information; and receiving the early synchronization information from the DU.

[0193] Example 10. The method of any of the preceding examples, wherein the generating of the early synchronization information is in response to determining that the UE supports early TA acquisition.

[0194] Example 1 l.The method of example 1, wherein: the request for early synchronization information is a handover request message requesting that the candidate base station prepare a candidate cell for an LTM switch.

[0195] Example 12. The method of example 1, wherein: the request for early synchronization information is included in a message requesting that the candidate base station prepare a candidate cell for an LTM switch.

[0196] Example 13. A base station comprising a transceiver and processing hardware, the base station configured to implement a method of any of the preceding examples.

[0197] 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.

[0198] 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 “DU configuration” can be replaced by “cell group configuration.” 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.” “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, “include” can be replaced by “comprise.” In some implementations, “exclude” can be replaced by “refrain from including.”

[0199] 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 asmartphone, 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.

[0200] 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.

[0201] When implemented in software, the techniques can be provided as part of the operating system, a library used by multiple applications, a particular software application, etc. The software can be executed by one or more general-purpose processors or one or more special-purpose processors.

[0202] 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. Variousmodifications, 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

What is claimed is:

1. A method implemented in a candidate base station (C-BS), the method comprising: receiving, from a source base station (S-BS), a handover request message including at least one of (i) a lower layer triggered mobility (LTM) indication for a candidate cell associated with the C-BS or (ii) a request for early synchronization information, for a user equipment (UE) communicating with the S-BS; generating the early synchronization information in response to the request; and transmitting, to the S-BS, the early synchronization information.

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

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

4. The method of claim 2, further comprising: receiving, from the UE in a candidate cell of the C-BS, 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 S-BS.

6. The method of claim 4 or 5, further comprising: receiving, from the S-BS, an indication that the S-BS 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 S-BS, 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-BS to a distributed unit (DU) of the C- BS, 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 handover request message contains an information element (IE) that includes the request for early synchronization information.

11. A method implemented in a source base station (S-BS), the method comprising: transmitting, to a candidate base station (C-BS), a handover request message including at least one of (i) a lower layer triggered mobility (LTM) indication for a candidate cell associated with the C-BS or (ii) a request for early synchronization information, for a user equipment (UE) communicating with the S-BS; receiving, from the C-BS, early synchronization information; and transmitting, to the UE, the early synchronization information.

12. The method of claim 11, wherein the early synchronization information includes one of a random-access channel (RACH) configuration or a transmission configuration indicator (TCI) state configuration.

13. The method of claim 11 or 12, wherein the transmitting of the request for early synchronization information is in response to determining that the UE supports early timing advance (TA) acquisition.

14. The method of claim 11, wherein the early synchronization information includes a timing advance (TA) value; the method further comprising:in response to determining that the TA value is valid, transmitting the TA value to the UE in an LTM cell switch command.

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

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