WO2024159091A1

WO2024159091A1 - Enabling a fast cell change with low overhead signaling

Info

Publication number: WO2024159091A1
Application number: PCT/US2024/013099
Authority: WO
Inventors: Chih-Hsiang Wu
Original assignee: Google Llc
Priority date: 2023-01-27
Filing date: 2024-01-26
Publication date: 2024-08-02

Abstract

A distributed unit (DU) of a distributed radio access network (RAN) node can implement a method for managing lower layer triggered mobility protocol procedure(s). The method includes: (i) receiving, at the DU from a centralized unit (CU) of the distributed base station, a request for lower layer triggered mobility preparation for a user equipment; (ii) generating, at the DU, a reference lower layer triggered mobility configuration; and (iii) transmitting, from the DU to the CU, the reference lower layer triggered mobility configuration for the UE in response to the request.

Description

ENABLING A FAST CELL CHANGE WITH LOW OVERHEAD SIGNALING CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to and the benefit of the filing date of provisional U.S. Patent Application No. 63/482,018 entitled “LOWER LAYER TRIGGERED MOBILITY WITH LOW OVERHEAD SIGNALING,” filed on January 27, 2023. The entire contents of the provisional application are hereby expressly incorporated herein by reference.

FIELD OF THE DISCLOSURE

[0002] This disclosure relates to wireless communications and, more particularly, to enabling a fast serving cell change for a user equipment (UE) using a control signaling of a protocol layer lower than a radio resource control (RRC) protocol layer.

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] In telecommunication systems, the Packet Data Convergence Protocol (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 technical specification (TS) 36.323) and New Radio (NR) (see 3GPP 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, in some examples, the UE and a base station use SRBs to exchange RRC messages as well as non-access stratum (NAS) messages, and can use DRBs to transport data on a user plane.

[0005] Depending on the scenario, UEs use several types of SRBs and DRBs. When operating in dual connectivity (DC), the cells associated with the base station operating the master node (MN) define a master cell group (MCG), and the cells associated with the base station operating as the secondary node (SN) define the secondary cell group (SCG). SRB1 resources carry RRC messages, which in some cases include NAS messages over the dedicated control channel (DCCH), and SRB2 resources support RRC messages that include logged measurement information or NAS messages, also over the DCCH but with lower priority than SRB1 resources. More generally, SRB1 and SRB2 resources allow the UE and the MN to exchange RRC messages related to the MN and embed RRC messages related to the SN. The SRB1 and SRB 2 resources 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.

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

[0007] When the UE moves from the coverage area of one cell to another cell in a RAN, the RAN should configure the UE for a serving cell change. 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 the change of the serving cell (e.g.,

PCell or PSCell). In cases where the UE operates in carrier aggregation (CA) of at least one secondary cell (SCell) with the PCell or PSCell, the RAN releases 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, it is desirable to develop new mobility techniques to reduce latency and overhead for fast serving cell change (i.e., lower-layer triggered mobility (LTM)). In some scenarios, the RAN prepares multiple cells for LTM for the UE and, accordingly, transmits multiple LTM configurations, each corresponding to a respective cell. The multiple LTM configurations cause a significant signaling overhead to the UE and RAN. To reduce the signaling overhead, the RAN can generate each of the multiple LTM configurations as a delta configuration based on a serving configuration (i.e., the latest serving configuration) that the RAN and UE use to communicate with each other. In some such scenarios, after providing the multiple LTM configurations to the UE, the RAN updates the serving configuration, which, depending on the scenario, causes the RAN to update the multiple LTM configurations. In some cases, the RAN frequently updates the serving configuration, which, depending on the scenario, frequently updates the LTM configurations. Frequently updating the LTM configurations in turn can result in significant signaling overhead to the RAN and UE.

SUMMARY

[0008] An example embodiment of the techniques of this disclosure is a method implemented in a distributed unit (DU) of a distributed base station, the method comprising: receiving, at the DU from a centralized unit (CU) of the distributed base station, a request for lower layer triggered mobility preparation for a user equipment (UE); generating, at the DU, a reference lower layer triggered mobility configuration; and transmitting, from the DU to the CU, the reference lower layer triggered mobility configuration for the UE in response to the request.

[0009] Another example embodiment of these techniques is a method implemented in a CU of a distributed base station, the method comprising: transmitting, from the CU to a distributed unit (DU) of the distributed base station, a request for lower layer triggered mobility preparation; receiving, at the CU, a reference lower layer triggered mobility configuration in response to the request; and transmitting, from the CU to a user equipment (UE), a lower layer triggered mobility configuration based on the reference lower layer triggered mobility configuration. [0010] Another example embodiment of these techniques is a distributed base station comprising processing hardware and configured to implement the method above.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Fig. 1A is a block diagram of an example system in which a radio access network (RAN) and a user device can implement the techniques of this disclosure for managing conditional procedures related to a secondary node (SN);

[0012] 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;

[0013] Fig. 2A is a block diagram of an example protocol stack according to which the UE of Fig. 1A communicates with base stations;

[0014] 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;

[0015] Fig. 3 is a messaging diagram of an example scenario where a CU provides a reference lower layer triggered mobility configures to a DU for generating a configuration for a UE;

[0016] Fig. 4A is a messaging diagram of an example scenario similar to that of Fig. 3, but in which the UE communicates in DC with an MN and an SN;

[0017] Fig. 4B is a messaging diagram of an example scenario similar to that of Fig. 4A, but in which the CU provides the reference lower layer triggered mobility configuration to the UE via the MN;

[0018] Fig. 5A is a messaging diagram of an example scenario similar to that of Fig. 3, but in which the CU communicates with an M-DU in the MN and an S-DU in the SN;

[0019] Fig. 5B is a messaging diagram of an example scenario similar to that of Fig. 5A, but in which the CU provides the reference lower layer triggered mobility configuration to the UE via the M-DU;

[0020] Fig. 6 is a flow diagram depicting an example method, implemented in a DU, in which the DU receives a request for LTM preparation from the CU and generates a reference lower layer triggered mobility configuration; [0021] Fig. 7 is a flow diagram depicting an example method, implemented in a DU, in which the DU determines whether to generate an LTM configuration or a non-LTM configuration based on whether the CU requests LTM preparation;

[0022] Fig. 8 is a flow diagram depicting an example method, implemented in a DU, in which the DU determines whether to generate a reference lower layer triggered mobility configuration and an LTM configuration or just the LTM configuration based on whether the DU has configured a reference lower layer triggered mobility configuration;

[0023] Fig. 9 is a flow diagram depicting an example method, implemented in a CU, in which the CU communicates with a UE via a DU and receives a reference lower layer triggered mobility configuration from the DU;

[0024] Fig. 10A is a flow diagram depicting an example method, implemented in a DU, in which the DU determines whether to include a DU configuration in a first or second field/IE based on whether the DU configuration is a reference lower layer triggered mobility configuration;

[0025] Fig. 10B is a flow diagram depicting an example method, similar to that of Fig. 10A, but in which the DU makes the determination based on whether the DU configuration is a non-reference lower layer triggered mobility configuration;

[0026] Fig. 10C is a flow diagram depicting an example method, similar to that of Fig. 10A, but in which the DU makes the determination based on whether the DU configuration is a reference lower layer triggered mobility configuration, a non-reference lower layer triggered mobility configuration, or a non-LTM configuration;

[0027] Fig. 11 A is a flow diagram depicting an example method, implemented in a DU, in which the DU determines whether to include a reference lower layer triggered mobility configuration, a non-reference lower layer triggered mobility configuration, and/or a non- LTM configuration in a DU-to-CU message;

[0028] Fig. 1 IB is a flow diagram depicting an example method, similar to that of Fig. 11 A, but in which the DU makes the determination based on whether the DU configuration includes a non-reference lower layer triggered mobility configuration;

[0029] Fig. 11C is a flow diagram depicting an example method, similar to that of Fig.

11 A, but in which the DU makes the determination based on whether the DU configuration includes a reference lower layer triggered mobility configuration and/or a non-reference lower layer triggered mobility configuration; and

[0030] Fig. 1 ID is a flow diagram depicting an example method, similar to that of Fig. 11 A, but in which the DU makes the determination based on whether the DU configuration includes a non-LTM configuration.

DETAILED DESCRIPTION OF THE DRAWINGS

[0031] Fig. 1A depicts an example wireless communication system 100 in which communication devices can implement 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 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.

[0032] In various configurations of the wireless communication system 100, the base station 104 can be implemented as a master eNB (MeNB) or a master gNB (MgNB), and the base station 106 can be implemented 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 a SgNB, the UE 102 can be in EUTRA-NR DC (EN-DC) with the MeNB and the SgNB.

[0033] 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 a 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.

[0034] In the scenarios where 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 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 operate in single connectivity (SC) with the base station 106, after completing the handover. The base stations 104 and 106 in this case operate as a source MN (S-MN) and a target MN (T-MN), respectively.

[0035] 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 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. Among other components, the EPC 111 can include a Serving Gateway (SGW) 112, a Mobility Management Entity (MME) 114, and a Packet Data Network Gateway (PGW) 116. The SGW 112 is generally configured to transfer user-plane packets related to audio calls, video calls, Internet traffic, etc., and the MME 114 is configured to manage authentication, registration, paging, and other related functions. The PGW 116 provides connectivity from the UE to one or more external packet data networks (e.g., an Internet network and/or an Internet Protocol (IP) Multimedia Subsystem (IMS) network). The 5GC 160 includes a User Plane Function (UPF) 162 and an Access and Mobility Management (AMF) 164, and/or Session Management Function (SMF) 166. The UPF 162 is generally configured to transfer user-plane packets related to audio calls, video calls, Internet traffic, etc., the AMF 164 is configured to manage authentication, registration, paging, and other related functions, and the SMF 166 is configured to manage PDU sessions.

[0036] 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 of the 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 cells 124B and 124C, and the base station 106 can support additional cell(s) (not shown in Fig. 1A). The cells 124A, 124B and 124C 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, 124B and 124C via one or more transmit and receive points (TRPs). More particularly, when the UE 102 is in DC with the base station 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. [0037] In general, the wireless communication network 100 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 also can apply to other suitable radio access and/or core network technologies such as sixth generation (6G) radio access and/or 6G core network or 5G NR-6G DC.

[0038] With continued reference to Fig. 1A, the base station 104 is equipped with 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 with one or more user devices (e.g., UE 102) via one or more cells (e.g., the cell(s) 124A, 124B and/or 124C) 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 with the one or more user devices via one or more cells (e.g., the cell(s) 124A, 124B and/or 124C) and/or one or more TRPs. The processing hardware 130 in an example implementation includes a MAC controller 134 configured to perform MAC functions with one or more user devices. The MAC functions includes a random access (RA) procedure, managing UL timing advance for the one or more user devices, and/or communicating UL/DL MAC PDUs with the one or more user devices. The processing hardware 130 can further include an RRC controller 136 to implement procedures and messaging at the RRC sublayer of the protocol communication stack. For example, the RRC controller 132 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. The base station 106 can include processing hardware 140 that is similar to processing hardware 130. In particular, components 142, 144, and 146 can be similar to the components 132, 134, and 136, respectively.

[0039] The UE 102 is equipped with 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 with the base station 104 or 106 via one or more cells (e.g., the cell(s) 124A, 124B, 124C 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 with the base station 104 or 106 via one or more cells (e.g., the cell(s) 124A, 124B, 124C 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 includes 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. 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.

[0040] 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) and/or downlink (from a base station to the UE 102) direction.

[0041] Fig. IB depicts an example distributed implementation of a base station such as the base station 104 or 106. The base station in this implementation can include a centralized unit (CU) 172 and one or more distributed units (DUs) 174. The CU 172 is equipped with 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 one example, the CU 172 is equipped with the processing hardware 130. In another example, the CU 172 is equipped with 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. The DU 174 is also equipped with 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 in an example implementation includes a medium access control (MAC) controller 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 include further a physical layer controller configured to manage or control one or more physical layer operations or procedures.

[0042] 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 or a gNB (e.g., one or more of the base stations 104, 106).

[0043] 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.

[0044] 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.”

[0045] On a control plane, the EUTRA PDCP sublayer 208 and the NR PDCP sublayer 210 can provide signaling radio bearers (SRBs) or 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.

[0046] Fig. 2B illustrates, in a simplified manner, an example protocol stack 250, 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.

[0047] Next, several example scenarios in which the base station operating in the system of Fig. 1A transmits a configuration to the UE 102 and later activates a configuration for communication between the UE 102 and base station. Generally speaking, events in Figs. 3- 7B that are similar are labeled with similar reference numbers e.g., event 316 is similar to event 416 of Figs 4A and 4B, event 516 of Fig. 5A, event 517 of Fig. 5B), with differences discussed below where appropriate. With the exception of the differences shown in the figures and discussed below, any of the alternative 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.

[0048] Referring first to Fig. 3, in a scenario 300, the base station 104 includes a CU 172 and a DU 174 and the DU 174 operates the cell 124A. The UE 102 initially communicates 302 with the DU 174 on the cell 124A using a serving DU configuration, and communicates with the CU 172 via the DU 174 (e.g., using a serving CU configuration). 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). In other implementations, the UE 102 in communicates with the DU 174 on the cell 124A only. In some implementations, the UE 102 communicates with the DU 174 on the cell 124A and/or other cell(s) via one or multiple TRPs. In some implementations, the cell 124A is 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 is an SCell, and one of the other cell(s) is a PCell. In such cases, the rest of the cells include SCell(s) and/or additional cell(s) associated with the PCell or an SCell. In the following description, depending on the implementation, the base station 104 is the DU 174, the CU 172, or the DU 174 and CU 172.

[0049] In some implementations, in the event 302, the UE 102 transmits UL PDUs and/or UL control signals to the base station 104 on the cell 124A and/or other cell(s) via one or multiple TRPs. In some implementations, the UE 102 communicates UL PDUs and/or DL PDUs with the base station 104 via radio bearers which include SRBs and/or DRB(s). In further implementations, the base station 104 configures the radio bearers for 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, in further implementations the UE 102 receives 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, channel state information reference signal(s) (CSI-RS(s)), and/or tracking reference signal(s)). In some implementations, the base station 104 transmits the DCIs on physical downlink control channel(s) (PDCCH(s)) monitored by the UE 102, on the cell 124A and/or other cell(s) via one or multiple TRPs.

[0050] In some implementations, the serving DU configuration includes physical layer configuration parameters, MAC configuration parameters, and/or RLC configuration parameters. In some implementations, the DU 174 transmits the configuration parameters to the CU 172. The CU 172 generates one or more messages (e.g., RRC reconfiguration message(s)) including the configuration parameters and transmits the one or more messages to the UE 102 via the DU 174. In other implementations, the DU 174 transmits the configuration parameters to the UE 102 directly. In some implementations, the serving DU configuration is CellGroupConfig IE (e.g., defined in 3GPP TS 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 (e.g., defined in 3GPP TS 38.331) or includes configuration parameters in the MeasConfig IE and/or RadioBearerConfig IE. 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 channel state information (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 remaining portion of these configuration parameters from the base station 104.

[0051] 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 at least one measurement report includes Layer 1 (LI) measurement report(s) and/or Layer 3 (L3) measurement report(s) for at least one serving cell of the UE 102 and/or at least one nonserving cell. For each of the L3 measurement report(s), the DU 174 transmits 306 a DU-to- CU message including the L3 measurement report to the CU 172. In some implementations, the DU-to-CU message(s) of the event 306 is/are Fl application protocol (F1AP) message(s) (e.g., UL RRC Message Transfer message(s)). In some implementations, the DU 174 does not transmit or refrains from transmitting the LI measurement report(s) to the CU 172. The at least one serving cell includes the cell 124A and/or other cell(s), and the at least one nonserving cell includes the cell 124B and/or cell 124C. In some implementations, the serving DU configuration or the serving CU configuration includes at least one measurement configuration. In some implementations, the UE 102 receives one or more RRC messages (e.g., RRCReconfiguration message(s)) including the at least one measurement configuration from the CU 172 via the DU 174 in the event 302. In accordance with the at least one measurement configuration, the UE 102 performs measurements and transmits 304 the at least one measurement report to the DU 174. In some implementations, the at least one measurement configuration includes L3 measurement configuration(s) (e.g., MeasConfig IE(s)) and/or LI measurement configuration(s). In some implementations, the LI measurement configuration(s) (e.g., CSl-MeasConfig IE(s)) includes LI measurement resource configuration(s) and/or LI measurement reporting configuration(s). In further implementations, the LI measurement resource configuration(s) configure resources of reference signal(s) (e.g., CSLRS(s)) for the UE 102 to measure and obtain LI measurement results. For example, the LI measurement resource configuration(s) is/are CS1- ResourceConfig IE(s). In another example, the LI measurement reporting configuration(s) configure way(s) the UE 102 uses to transmit LI measurement results/reports. For example, the LI measurement report configuration(s) is/are CSl-ReportConfig IE(s). For example, the UE 102 transmits the L3 measurement report(s) to the CU 172 via the DU 174 in accordance with the L3 measurement configuration(s). The UE 102 transmits the LI measurement report(s) to the DU 174 in accordance with the LI measurement configuration(s) or LI measurement reporting configuration(s). In some implementations, the DU 174 does not transmit the LI measurement report(s) to the CU 172.

[0052] In some implementations, the LI measurement configuration(s) are new RRC IE(s) (e.g., defined in 3GPP TS 38.331 vl8.0.0 and/or later version) for a lower layer triggered mobility (LTM). In some implementations, the LI measurement resource configuration(s) are new RRC IE(s) (e.g., defined in 3GPP TS 38.331 vl8.0.0 and/or later version) for the LTM. In some implementations, the LI measurement reporting configuration(s) are new RRC IE(s) (e.g., defined in 3GPP TS 38.331 vl8.0.0 and/or later version) for the LTM. In some implementations, each of the LI measurement reporting configuration(s) includes a trigger event configuration configuring a trigger event to trigger the UE 102 to transmit an LI measurement report. If the UE 102 detects the trigger event, the UE 102 transmits an LI measurement report to the DU 174.

[0053] In some implementations, (each of) the LI measurement report(s) includes at least one LI measurement result. In some implementations, the at least LI measurement result includes at least one LI -reference signal received power (Ll-RSRP) value and/or at least one LI- Signal to Interference Noise Ratio (Ll-SINR) value. For each of the LI measurement report(s), the UE 102 transmits a PUCCH transmission including the LI measurement report to the DU 174, in some implementations. That is, the UE 102 transmits the each of the LI measurement report(s) on a PUCCH to the DU 174. In other implementations, for each of the LI measurement report(s), the UE 102 transmits a PUSCH transmission including the LI measurement report to the DU 174. That is, the UE 102 transmits each of the LI measurement report(s) on a PUSCH to the DU 174. In yet other implementations, the UE 102 transmits a portion of the LI measurement report(s) on PUCCH(s) and the rest of the LI measurement report(s) on physical UL shared channel(s) (PUSCH(s)) to the DU 174. That is, for each portion of the LI measurement report(s), the UE 102 transmits a PUCCH transmission including the LI measurement report to the DU 174, and for each of the remainder of the LI measurement report(s), the UE 102 transmits a PUSCH transmission including the LI measurement report to the DU 174. In some implementations, each of the LI measurement report(s) Is a part of the channel state information (CSI) (i.e., a CSI component) or CSI. In some implementations, the UE 102 includes other CSI component(s) in (each of) the PUCCH transmission(s) and/or PUSCH transmission(s) described above. In some implementations, the other CSI component(s) include a channel quality indicator (CQI), a Precoding Matrix Indicator (PMI), a CSLRS Resource Indicator (CRI), a Synchronization Signal (SS)/ Physical Broadcast Channel (PBCH) Resource Block Indicator (SSBRI), a Layer Indicator (LI), and/or a Rank Indicator (RI). In some implementations, the UE 102 does not transmit the LI measurement report(s) in the format of RRC message(s) to the DU 174.

[0054] In some implementations, each of the L3 measurement report(s) includes at least one L3 measurement result. In some implementations, the at least one L3 measurement result includes at least one RSRP (value) and/or at least one SINR (value). In some implementations, the UE 102 transmits each of the L3 measurement report(s) on a PUSCH to the CU 172 via the DU 174. In some implementations, each of the L3 measurement report(s) is an RRC message (e.g., MeasurementReport message). In some implementations, each of the L3 measurement configuration(s) includes a particular measurement identity (e.g., measld) and each of the L3 measurement report(s) includes a particular measurement identity in a particular L3 measurement configuration. In some implementations, when the CU 172 receives an L3 measurement report including a measurement identity and an L3 measurement result from the UE 102 via the DU 174, the CU 172 determines that the L3 measurement report is associated with an L3 measurement configuration identified by the measurement identity.

[0055] In some alternative implementations, for each of the at least one measurement report (e.g., LI measurement report(s)), the UE 102 transmits a MAC control element (CE), including the measurement report, to the DU 174 in the event 304. To transmit the MAC CE(s), the UE 102 generates one or more MAC PDUs, each including one or more of the MAC CE(s), to the DU 174 in the event 304.

[0056] In some implementations, the UE 102 performs measurements on one or more reference signals in accordance with the at least one measurement configuration. In some implementations, the one or more reference signals include one or more Synchronization Signal (SS)/ Physical Broadcast Channel (PBCH) Resource Blocks (SSBs) and/or one or more CSLRS s. The UE 102 obtains the at least one LI measurement result and/or at least one L3 measurement result from the measurements. The DU 174 transmits the one or more reference signals on the cell 124A and other cell(s) (e.g., the cell 124B, the cell 124C and/or cell(s) not shown in Fig. 1A).

[0057] After (e.g., in response to) receiving one or some of the at least one measurement report from the UE 102, the base station 104 (i.e., the CU 172 or DU 174) determines to prepare a first cell (e.g., the cell 124B) for LTM for the UE 102. In some implementations, the base station 104 determines to prepare the first cell for the UE 102 because the at least one measurement report indicates that the base station 104 can use the first cell 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 at least one measurement report indicates that the first cell qualifies to be a candidate cell that could be used for communication with the UE 102. In some implementations, if the L3 measurement report(s) indicates that signal strength and/or quality of the first cell is above a first predetermined threshold, is better than strength and/or quality of the cell 124A, and/or is better than strength and/or quality of the cell 124A by a first predetermined threshold, the CU 172 determines to prepare the first cell for the UE 102. In other implementations, if the LI measurement report(s) indicate that signal strength and/or quality of the first cell is above a first predetermined threshold, is better than signal strength and/or quality of the cell 124 A, and/or is better than signal strength and/or quality of the cell 124A by a first predetermined threshold, the DU 174 determines to prepare the first cell for the UE 102. Alternatively, the base station 104 determines to prepare the first cell for the UE 102 regardless of whether a measure report is received from the UE 102 or not.

[0058] In cases where the CU 172 determines to prepare the first cell for LTM, the CU 172 transmits 308 a first CU-to-DU message to the DU 174 to prepare the first cell for the UE 102. In some implementations, the CU 172 includes a cell identity (ID) of the first cell in the first CU-to-DU message to request the DU 174 to prepare the first cell for LTM for the UE 102. For example, the cell ID is 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 response to the first CU-to-DU message, the DU 174 generates a first LTM configuration (referred to herein after as LTM configuration 1) for the UE 102, which configures the first cell for LTM. The DU 174 then transmits 310 a first DU-to-CU message, including the LTM configuration 1, to the CU 172 in response to the first CU-to-DU message. In cases where the DU 174 determines to prepare the first cell, the DU 174 initiates transmission of the first DU-to-CU message to the CU 172 instead of in response to a CU-to-DU message received from the CU 172. In some implementations, the CU 172 includes an LTM cell list including the cell ID(s) 1 and/or 2, .. N, and includes the LTM cell list in the first CU-to-DU message. In some implementations, the LTM cell list functions as an indication that the first CU-to-DU message requests LTM preparation. Alternatively, in some implementations, the CU 172 includes, in the first CU-to-DU message, an indicator specifically indicating that the first CU- to-DU message requests LTM preparation.

[0059] In some implementations, the DU 174 includes, in the first DU-to-CU message, the cell ID of the first cell associated with the LTM configuration 1 to indicate that the LTM configuration 1 is configured for or associated with the first cell. The CU 172 identifies that the LTM configuration 1 is configured for or associated with the first cell. In some scenarios and implementations, the CU 172 includes additional cell ID(s) (e.g., cell ID(s) 2, ..., N) in the first CU-to-DU message to prepare additional cell(s) (e.g., cell(s) 2, ..., N) for LTM for the UE 102, and the DU 174 includes additional LTM configuration(s) (e.g., LTM configuration(s) 2, ..., N), each configuring a particular cell of the additional cell(s), as described below. In such cases, the DU 174 includes, in the first DU-to-CU message, the additional cell ID(s) respectively associated with the additional LTM configuration(s) to indicate which LTM configuration is associated with which cell (ID). The cell(s) 1 and/or 2, ..., N are candidate cell(s).

[0060] In some implementations, the DU 174 generates a reference LTM configuration, generates the LTM configuration(s) 1 and/or 2, ..., N (i.e., non-reference LTM configuration(s)) based on the reference LTM configuration, and includes the reference LTM configuration in the first DU-to-CU message. In some implementations, the DU 174 includes, in the reference LTM configuration, common configuration parameters which are common for (communication on any of) the cell(s) 1 and/or 2, ..., N. Thus, the DU 174 does not need to include the common configuration parameters in the LTM configuration(s) 1 and/or 2, ..., N, reducing the signaling overhead for configuring LTM. In some implementations, the DU 174 includes, in the LTM configuration(s) 1 and/or 2..., N, cellspecific configuration parameters which include different values for (communication on) the cell(s) 1 and/or 2, ..., N.

[0061] In some implementations, the reference LTM configuration includes physical layer configuration parameters, MAC configuration parameters, and/or RLC configuration parameters. In some implementations, the reference LTM configuration is CellGroupConfig IE (e.g., defined in 3GPP TS 38.331). In other implementations, the reference LTM configuration includes configuration parameters in the CellGroupConfig IE. In some implementations, the reference LTM configuration includes a CSl-MeasConfig IE or configuration parameters for channel state information (CSI) measurement and/or reporting.

[0062] When the LTM configuration(s) 1 and/or 2, ..., N are generated as delta configuration(s) based on the reference LTM configuration, the LTM configuration(s) 1 and/or 2, .. ,,N have no dependency with the serving DU configuration. In some implementations, after configuring the LTM configuration(s) 1 and/or 2, ... , N for the UE 102, the DU 174 updates the serving DU configuration without updating the LTM configuration(s), reducing the signaling overhead.

[0063] In some implementations, the reference LTM configuration is different from the serving DU configuration. In some implementations, a portion of the reference LTM configuration is the same as a portion of the serving DU configuration, and the rest of the reference LTM configuration is different from the rest of the serving DU configuration. In other implementations, the reference LTM configuration is the same as the serving DU configuration.

[0064] After receiving the first DU-to-CU message, the CU 172 generates an RRC reconfiguration message (e.g., an RRCReconfiguration message), including the LTM configuration 1, and transmits 316 a second CU-to-DU message including the RRC reconfiguration message to the DU 174. In some implementations, the CU 172 includes the reference LTM configuration in the RRC reconfiguration message 316. In other implementations, the CU 172 does not include a reference LTM configuration in the RRC reconfiguration message 316. In some implementations, if the CU 172 transmits the reference LTM configuration to the UE 102 during the event 302, the CU 174 does not include the reference LTM configuration in the RRC reconfiguration message 316. In other implementations, if the CU 172 receives the reference LTM configuration from the DU 174, the CU 172 includes the LTM configuration in the RRC reconfiguration message 316.

Otherwise, if the CU 172 does not receive a reference LTM configuration from the DU 174, the CU 172 does not include the reference LTM configuration in the RRC reconfiguration message 316.

[0065] After receiving the RRC reconfiguration message 316, the DU 174 transmits 318 the RRC reconfiguration message to the UE 102. In response, the UE 102 transmits 320 an RRC reconfiguration complete message (e.g., an RRCReconfigurationComplete message) to the DU 174, which in turn transmits 322 a second DU-to-CU message including the RRC reconfiguration complete message to the CU 172. In some implementations, the CU 172 performs security protection (e.g., integrity protection and/or encryption) on the RRC reconfiguration message. For example, the CU 172 generates a message authentication code for integrity (MAC-I) for the RRC reconfiguration message, encrypts the RRC reconfiguration message and the MAC-I to obtain an encrypted RRC reconfiguration message and an encrypted MAC-I, and transmits a PDCP PDU including the encrypted RRC reconfiguration message and encrypted MAC-I to the UE 102 via the DU 174 in the events 316 and 318. When the UE 102 receives the PDCP PDU from the CU 172 via the DU 174 (i.e., events 316 and 318), the UE 102 decrypts the encrypted RRC reconfiguration and encrypted MAC-I to obtain the RRC reconfiguration message and MAC-I and verifies whether the MAC-I is valid. If the UE 102 verifies the MAC-I is invalid, the UE 102 discards or ignores the RRC reconfiguration message. In some implementations, the UE 102 performs an RRC connection reestablishment procedure in response to the invalid MAC-I. Otherwise, in further implementations, if the UE 102 verifies the MAC-I is valid, the UE 102 processes the RRC reconfiguration. The UE 102 refrains from applying (i.e., executing) the LTM configuration 1 until receiving a configuration activation command activating the LTM configuration 1 (e.g., the event 330).

[0066] The events 308 (optional) and 310 are collectively referred to in Fig. 3 as an LTM preparation procedure 390. The events 316, 318, 320, 322 are collectively referred to in Fig.

3 as an LTM configuration delivery procedure 394.

[0067] In some implementations, the DU 174 transmits the reference LTM configuration to the UE 102 in procedures similar to the procedures 390 and 392 before receiving the first CU-to-DU message. In such cases, the DU 174 does not include the reference LTM configuration in the first DU-to-CU message.

[0068] In some implementations where the CU 172 performs the multiple LTM preparation procedures 390, the DU 174 includes the reference LTM configuration in the first DU-to-CU message in the first LTM preparation procedure of the LTM preparation procedures 390 and does not include the reference LTM configuration in the rest of the LTM preparation procedures 390. [0069] 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 or UE Context Modification Required message. In some cases including the UE Context Modification Required message, the CU 172 transmits a UE Context Modification Confirm message to the DU 174 in response to UE Context Modification Required message. In some implementations, the second CU-to-DU message is a DE RRC Message Transfer message. In other implementations, the second CU-to-DU message is a UE Context Modification Request message, and the DU 174 transmits a second DU-to-CU message (e.g., UE Context Modification Response message) to the CU 172 in response to the second CU-to-DU message.

[0070] In some implementations, the CU 172 includes the LTM configuration 1 in a first container (e.g., a field/IE) and includes the first container in the RRC reconfiguration message of the events 316 and 318. In such cases, the CU 172 generates the first container. The first container is to indicate to the UE 102 not to apply the LTM configuration 1 immediately. In some scenarios or implementations, the UE 102 receives an RRC reconfiguration message (e.g., the RRC reconfiguration message of the event 318) including a configuration (e.g., the LTM configuration 1). If the configuration is included in the first container, the UE 102 refrains from immediately applying the configuration. Otherwise, in further implementations, if the configuration is not included in the first container, the UE 102 applies the configuration immediately. In some implementations, the first container is a first addition or modification list (e.g., Itm-ConfigToAddModList field, ETM- ConfigToAddModList IE, Itm-CandidateConfigToAddModList field, or LTM- CandidateConfigToAddModEist IE). The CU 172 includes the LTM configuration 1 in a first element (referred to herein after as element 1) of the first addition or modification list. For example, the element 1 is an addition or modification IE (Itm-ConfigToAddMod field, ETM- ConfigToAddMod IE, Itm-CandidateConfigToAddMod field, or LTM- CandidateConfigToAddMod IE). In some implementations, when the UE 102 receives the first addition or modification list, the UE 102 stores the first addition or modification list (e.g., in a variable in the random access memory (RAM)). In other alternative implementations, the DU 174 generates the first container and includes the first container in the first DU-to-CU message. In yet other alternative implementations, the DU 174 generates the element 1 and includes the element 1 in the first DU-to-CU message.

The CU assigns an ID for the LTM configuration 1 [0071] In some implementations, the CU 172 includes, in the RRC reconfiguration message, a first LTM ID (referred to herein after as ID 1) for identifying the LTM configuration 1 or the element 1. In some implementations, the CU 172 includes the ID 1 in the first container or element 1. In some implementations, the CU 172 assigns the ID 1. In other implementations, the CU 172 receives the ID 1 from the DU 174 in the first DU-to-CU message, as described below.

[0072] In some implementations where the CU 172 assigns or generates the ID 1, the CU 172 transmits the ID 1 to the DU 174, and the DU 174 associates the ID 1 with the LTM configuration 1. In some implementations, in the first CU-to-DU message, the CU 172 includes the ID 1 and indicates that the ID 1 is associated with the LTM configuration 1. In other implementations, after receiving the first DU-to-CU message, the CU 172 transmits 312 a third CU-to-DU message including the ID 1 to the DU 174 instead of including the ID 1 in the first CU-to-DU message. In some implementations, in the third CU-to-DU message, the CU 172 includes the LTM configuration 1 and the ID 1. The CU 172 further indicates the association between the ID 1 and LTM configuration 1. Thus, the DU 174 directly associates the ID 1 with the LTM configuration 1. In other implementations, in the third CU-to-DU message, the CU 172 includes the cell ID 1 and the ID 1 (i.e., the first LTM ID), and indicates the association between the cell ID 1 and the ID 1. Thus, in some such implementations, the DU 174 associates the ID 1 with the LTM configuration 1 based on the association between the cell ID 1 and the ID 1 and the association between the cell ID 1 and the LTM configuration 1. In yet other implementations, in the third CU-to-DU message, the CU 172 includes the LTM configuration 1, the cell ID 1, and the ID 1, and indicates the association between the ID 1, LTM configuration 1, and the cell ID 1. In some implementations, the DU 174 transmits 314 a third DU-to-CU message to the CU 172 in response to the third CU-to-DU message. In some implementations, the third CU-to-DU message and third DU-to-CU message are a UE Context Modification Request message and UE Context Modification Response message. The events 312 (optional) and 314 (optional) are collectively referred to in Fig. 3 as an LTM ID assignment procedure 392. In other implementations, the CU 172 includes the ID 1, the cell ID 1, and/or the LTM configuration 1 in the second CU-to-DU message, as described above. Thus, the CU 172 can omit the third CU-to-DU message.

[0073] In some implementations where the CU 172 includes the ID 1 in the first CU-to-DU message, the DU 174 includes the ID 1 in the LTM configuration 1, first container or element 1. Alternatively, the DU 174 does not include the ID 1 in the LTM configuration 1, first container and/or element 1.

DU assigns an ID for the LTM configuration 1

[0074] In some alternative implementations, the DU 174 assigns the ID 1 identifying the LTM configuration 1. In some implementations, the DU 174 includes the ID 1 in the first DU-to-CU message. In some implementations, the CU 172 includes the ID 1 in the RRC reconfiguration message as described above. In other implementations, the DU 174 includes the ID 1 in the LTM configuration 1, first container, or element 1. Thus, the CU 172 does not include an ID identifying the LTM configuration 1 in the RRC reconfiguration message, first container and/or element 1.

[0075] In some implementations, neither the CU 172 nor the DU 174 assign an ID to identify the reference LTM configuration.

[0076] In some implementations, the LTM 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., RLC-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 configuration 1 is a CellGroupConfig IE (e.g., defined in 3GPP TS 38.331). In other implementations, the LTM configuration 1 includes configuration parameters in the CellGroupConfig IE.

[0077] In some implementations, the DU 174 includes a random access configuration in the LTM configuration 1. In other implementations, the DU 174 does not include a random access configuration in the LTM configuration 1. In some implementations, if the cell 124A and first cell are not synchronized, the DU 174 determines to include the random access configuration in the LTM configuration 1. Otherwise, if the cell 124A and first cell are synchronized, the DU 174 determines to not include the random access configuration in the LTM configuration 1. In other implementations, if the DU 174 determines that the UE 102 has not synchronized in UL with the first cell, the DU 174 determines to include the random access configuration in the LTM configuration 1. Otherwise, if the DU 174 determines that the UE 102 has synchronized in UL with the first cell, the DU 174 determines to not include the random access configuration in the LTM configuration 1. If the LTM configuration 1 includes the random access configuration, the UE 102 performs the random access procedure in the event 332 in accordance with the random access configuration, as described below. Otherwise, if the LTM configuration 1 does not include the random access configuration, the UE 102 skips or refrains from performing the random access procedure of the event 332 in response to the LTM configuration 1 excluding the random access configuration.

[0078] In some implementations, the DU 174 includes random access configuration parameters in the LTM configuration 1 and/or the reference LTM configuration regardless of whether the cell 124A and first cell are synchronized or not. The UE 102 performs the random access procedure in the event 332 in accordance with the random access configuration parameters, as described below.

[0079] In some implementations, if the cell 124A and first cell are synchronized, the DU 174 determines to include, in the LTM configuration 1, a first indication configuring the UE 102 not to perform a random access procedure on the first cell. Otherwise, if the cell 124A and first cell are not synchronized, the DU 174 determines to not include the first indication in the LTM configuration 1. In other implementations, if the DU 174 determines that the UE 102 has synchronized in UL with the first cell, the DU 174 determines to include the first indication in the LTM configuration 1. Otherwise, if the DU 174 determines that the UE 102 has not synchronized in UL with the first cell, the DU 174 determines to not include the first indication in the LTM configuration 1. If the LTM configuration 1 includes the first indication, the UE 102 skips or refrains from performing the random access procedure of the event 332 in accordance with or in response to the first indication. Otherwise, if the LTM configuration 1 does not include the first indication, the UE 102 performs the random access procedure in accordance with the random access configuration in the event 332, in response to the LTM configuration 1 excluding the first indication, as described below.

[0080] In some implementations, the DU 174 includes a reconfiguration with sync configuration (e.g., ReconfigurationWithSync IE) in the LTM configuration 1 or special cell configuration. In other implementations, the DU 174 does not include a reconfiguration with sync configuration (e.g., ReconfigurationWithSync IE) in the LTM configuration 1 or special cell configuration. In some implementations, if the cell 124A and first cell are not synchronized, the DU 174 determines to include the reconfiguration with sync configuration in the LTM configuration 1. Otherwise, if the cell 124A and first cell are synchronized, the DU 174 determines to not include the reconfiguration with sync configuration in the LTM configuration 1. In other implementations, if the DU 174 determines that the UE 102 has not synchronized in UL with the first cell, the DU 174 determines to include the reconfiguration with sync configuration in the LTM configuration 1. Otherwise, if the DU 174 determines that the UE 102 has synchronized in UL with the first cell, the DU 174 determines to not include the reconfiguration with sync configuration in the LTM configuration 1. In some implementations, if the LTM configuration 1 includes the reconfiguration with sync configuration, the UE 102 performs the random access procedure in the event 332 as described below, in response to or in accordance with the reconfiguration with sync configuration. Otherwise, if the LTM configuration 1 does not include the reconfiguration with sync configuration, the UE 102 skips or refrains from performing the random access procedure of the event 332. In some implementations, the DU 174 includes a cell ID (i.e., cell ID 1) of cell 1 (i.e., the first cell) in the LTM configuration 1. In some implementations, the cell ID 1 is a PCI. In another implementation, the cell ID 1 is a CGI. In some further implementations, the LTM configuration 1 includes a cell index 1 (e.g., a serving cell index or LTM cell index) indexing the cell ID 1 or the first cell (e.g., the cell index 1 is not a cell ID).

[0081] In some implementations, after (e.g., in response to) receiving one or some of the at least one measurement report of the event 304, the base station 104 (i.e., the CU 172 or DU 174) determines to prepare additional cell(s) (i.e., cell(s) 2, ..., N) of the base station 104 for LTM for the UE 102. In some implementations, the base station 104 determines to prepare the additional cell(s) for LTM for the UE 102 because the at least one measurement report indicates that the base station 104 can use the additional cell(s) to communicate with the UE 102. In some implementations, the additional cell(s) include the cell 124C and/or cell(s) other than the cells 124A, 124B, and 124C. In some implementations, if the L3 measurement report(s) indicate that signal strength and/or quality of a particular cell of the additional cell(s) is above a respective predetermined threshold and/or is better than the cell 124A, the CU 172 determines to prepare the particular cell for LTM for the UE 102. In other implementations, if the LI measurement report(s) indicate that signal strength and/or quality of a particular cell of the additional cell(s) is above a first predetermined threshold and/or is better than the cell 124A, the DU 174 determines to prepare the particular cell for LTM for the UE 102. In some implementations, the respective predetermined threshold(s) for the additional cells are different from the first predetermined threshold. In further implementations, the respective predetermined threshold(s) for the additional cell(s) are the same as the first predetermined threshold. In some implementations, the respective predetermined thresholds for the additional cells are the same or different. Alternatively, the base station 104 determines to prepare the additional cell(s) for the UE 102 regardless of whether a measurement report is received from the UE 102 or not.

[0082] In cases where the CU 172 determines to prepare the additional cell(s), the CU 172 initiates and performs at least one additional LTM preparation procedure with the DU 174 to prepare the additional cell(s) for LTM, where each of the LTM preparation procedure(s) is similar to the procedure 390. In cases where the DU 174 determines to prepare the additional cell(s), the DU 174 initiates and performs at least one additional LTM preparation procedure with the CU 172 to prepare the additional cell(s) for LTM, where each of the LTM preparation procedure(s) is similar to the procedure 390.

[0083] In some implementations, the CU 172 and DU 174 perform LTM preparation procedure(s) 2, ..., N to prepare the cell(s) 2, ..., N, respectively, similar to the procedure 390. In some implementations, the CU 172 includes the cell ID(s) 2, ..., N in CU-to-DU message(s) 2, ..., N in the LTM preparation procedure(s) 2, ..., N, respectively, similar to the first CU-to-DU message. In the LTM preparation procedure(s) 2, ..., N, the DU 174 generates LTM configuration(s) 2, ..., N configuring the cell(s) 2, ..., N and includes the LTM configuration(s) 2, ..., N in DU-to-CU message(s) 2, .., N, respectively, as described for the LTM configuration 1. In cases where the DU 174 receives the CU-to-DU message(s) 2, ..., N, the DU-to-CU message(s) 2, ..., N responds to the CU-to-DU message(s) 2, ..., N, respectively. “N” is an integer and larger than one. Lor example, “N” is 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13 14, 15, 16, etc. In another example, the maximum number of “N” is 4, 8, 16, 32, etc. Examples and implementations of the LTM configuration 1 can apply to the LTM configuration(s) 2, ..., N.

[0084] In other implementations, the CU 172 and DU 174 performs a single LTM preparation procedure (i.e., the LTM preparation procedure 390) to prepare the cell(s) 1, 2, ..., N. In such cases, the DU 174 includes the LTM configuration(s) 1, 2, ..., N for the cell(s) 1, 2, ..., N, respectively in the first DU-to-CU message. In some implementations, in the first DU-to-CU message, the DU 174 includes the cell ID(s) 1, 2, ..., N respectively associated with the LTM configuration(s) 1, 2, ..., N to indicate that the LTM configuration(s) 1, 2, .. N are configured for the cell ID(s) 1, 2, ..., N, respectively. In cases where the CU 172 determines to perform the LTM preparation procedure 390, the CU 172 includes the cell ID(s) 1, 2, ..., N in the first CU-to-DU message to request the DU 174 to prepare the cell(s) 1, 2, ..., N, respectively, for LTM.

[0085] In some implementations, after receiving the LTM configuration(s) 2, ..., N from the DU 174, the CU 172 includes the LTM configuration(s) 2, ..., N in the first container. In some implementations, the CU 172 includes the LTM configuration(s) 2, ..., N in element(s) 2, ..., N, respectively, and includes the element(s) 2, ..., N in the first container. In some implementations, the CU 172 includes, in the RRC reconfiguration message, LTM ID(s) (i.e., ID(s) 2, ..., N) for identifying the LTM configuration(s) 2, ..., N, respectively. In some implementations, the CU 172 includes the ID(s) 2, ..., N in the first container. For example, the CU 172 includes the ID(s) 2, ..., N and LTM configuration(s) 2, ..., N in the element(s) 2, ... , N in the first addition or modification list.

[0086] In some implementations, the CU 172 assigns the ID(s) 2, ..., N for the LTM configuration(s) 2, ..., N, respectively. In other implementations, the CU 172 receives the ID(s) 2, ..., N from the DU 174 in the first DU-to-CU message of the procedure 390. In yet other implementations, the CU 172 receives, from the DU 174, the ID(s) 2, ..., N in the DU- to-CU message(s) 2, ..., N of the LTM preparation procedure(s) 2, ..., N, respectively.

[0087] In some implementations, the CU 172 performs an LTM ID assignment procedure with the DU 174 for each of the LTM configuration(s) 2, ..., N, similar to the procedure 392. In other implementations, the CU 172 includes the ID(s) 2, ..., N and the LTM configuration(s) 2, ..., N in the third CU-to-DU message and indicates the association between the ID(s) 2, ..., N and the LTM configuration(s) 2, ..., N, respectively. Thus, in some implementations, the DU 174 associates the LTM configuration(s) 2, ..., N with the ID(s) 2, ..., N, respectively. In yet other implementations, the CU 172 includes the cell ID(s) 2, ..., N and the ID(s) 2, ..., N in the third CU-to-DU message and indicates the association between the cell ID(s) 2, ..., N and the ID(s) 2, ..., N, respectively. Thus, in further implementations, the DU 174 associates the LTM configuration(s) 2, ..., N with the ID(s) 2, ..., N, respectively, based on the association between the cell ID(s) 2, ..., N and the ID(s) 2, ..., N, and the association between the cell ID(s) 2, ..., N and the LTM configuration(s) 2, ..., N, respectively. In other implementations, the CU 172 includes the ID(s) 2, ..., N, the cell ID(s) 2, ..., N and/or the LTM configuration(s) 2, ..., N in the second CU-to-DU message as described above. Thus, the CU 172 can omit the third CU-to-DU message. In yet other implementations, the CU 172 includes the ID(s) 2, ..., N in the first CU-to-DU message and indicates that the ID(s) 2, ..., N are respectively associated with the cell ID(s) 2, ..., N. In some implementations, the DU 174 includes the ID(s) 2, ..., N in the LTM configuration(s) 2, ..., N. Thus, the CU 172 does not include the ID(s) 2, ..., N in the RRC reconfiguration message, first container, and/or element(s) 2, ..., N.

[0088] In some alternative implementations, the DU 174 assigns the ID(s) 2, ..., N. In some implementations, the DU 174 includes the ID(s) 2, ..., N in the first DU-to-CU message of the procedure 390. In yet other implementations, the DU 174 includes the ID(s) 2, ..., N in the DU-to-CU message(s) 2, ..., N of the LTM preparation procedure(s) 2, .. ,,N. In some implementations, the CU 172 includes the ID(s) 2, ..., N in the RRC reconfiguration message. In other implementations, the DU 174 includes the ID(s) 2, ..., N in the LTM configuration(s) 2, ..., N. Thus, the CU 172 does not include an ID (e.g., LTM ID) identifying each of the LTM configuration(s) 2, ..., N in the RRC reconfiguration message, first container, and/or element 1.

[0089] In some alternative implementations, the CU 172 generates a second container including the LTM configuration(s) 2, ..., N or element(s) 2, ..., N instead of using the first container. The CU 172 then transmits an additional RRC reconfiguration message, including the second container, to the UE 102 via the DU 174, similar to the events 316 and 318. In response, the UE 102 transmits an additional RRC reconfiguration complete message to the CU 172 via the DU 174, similar to the events 320 and 322. In some implementations, the second container is a second addition or modification list (e.g., Itm-ConfigToAddModList field, LTM-ConfigToAddModList IE, Itm-CandidateConfigToAddModList field, or LTM- CandidateConfigToAddModList IE), and each of the element(s) 2, ..., N is an addition or modification IE (e.g., Itm-ConfigToAddMod field, LTM-ConfigToAddMod l , Itm- CandidateConfigToAddMod field, or LTM-CandidateConfigToAddMod IE). In some implementations, when the UE 102 receives the second addition or modification list, the UE 102 stores the second addition or modification list together with the first addition or modification list (e.g., in a variable in the random access memory (RAM)).

[0090] In some implementations, the DU 174 includes cell ID(s) 2, ..., N in the LTM configuration(s) 2, ..., N to identify the cell(s) 2, ..., N, respectively. In some implementations, each of the cell ID(s) 2, ..., N is a PCI. In some further implementations, 1 the LTM configuration(s) 2, .. N includes (serving) cell index(es) 2 , .. N indexing the cell ID(s) 2, .. N or the cell(s) 2, .. N, respectively. In some implementations, the cell ID(s) 1, .. N in the LTM configuration(s) 1, .. N are different from the cell ID(s) 1, .. N in the CU-to-DU message(s) described above.

[0091] In some implementations, each of the LTM configuration(s) 1, ..., N includes physical configuration parameters, MAC configuration parameters, RLC configuration parameters, and/or LI measurement configuration(s). In some implementations, each of the LTM configuration(s) 1, ..., N is a CellGroupConfig IE (e.g., as defined in 3GPP TS 38.331). In other implementations, each of the LTM configuration(s) 1, ..., N include configuration parameters included in a CellGroupConfig IE (e.g., as defined in 3GPP TS 38.331). In some further implementations, the plurality of configuration parameters in each of the LTM configuration(s) include a particular special cell configuration (e.g., SpCellConfig IE) and/or one or more SCell configurations (e.g., SCellConfig IE(s)). In some implementations, the LTM configuration(s) 1, ..., N are CellGroupConfig IE(s) (e.g., defined in 3GPP TS 38.331). In other implementations, the LTM configuration(s) 1, ..., N include configuration parameters in the CellGroupConfig IE.

[0092] In some implementations, the CU 172 determines to release the LTM configuration M of the LTM configuration(s) 1, ..., N (or the element M of the element(s) 1, ..., M), where 1 < M < N. In response to the determination, the CU 172 transmits an RRC reconfiguration message to the UE 102 via the DU 174 to indicate to the UE 102 to release the LTM configuration M or element M. In some implementations, the CU 172 generates a release list including the ID (i.e., LTM ID) M for releasing the LTM configuration M or element M and includes the release list in the RRC reconfiguration message. In response to the RRC reconfiguration message, the UE 102 releases the LTM configuration M or element M and transmits an RRC reconfiguration complete message to the CU 172 via the DU 174. In response to the determination, the CU 172 transmits a CU-to-DU message to the DU 174 to indicate to the DU 174 to release the LTM configuration M. In some implementations, to indicate to the DU 174 to release the LTM configuration M, the CU 172 includes the cell ID M or the ID (i.e., LTM ID) M in a release indication (e.g., a field or IE) in the CU-to-DU message. In response, the DU 174 releases the LTM configuration M and transmits a DU-to- CU message to the CU 172. In some implementations, the CU-to-DU message and DU-to- CU message are a UE Context Modification Request message and a UE Context Modification Response message, respectively. [0093] In other implementations, the DU 174 determines to release the LTM configuration K. In response to the determination, the DU 174 transmits a DU-to-CU message to the CU 172 to release the LTM configuration K. In some implementations, to indicate that the LTM configuration K is released, the DU 174 includes the cell ID K or the ID (i.e., LTM ID) K in a release indication (e.g., a field or IE) in the DU-to-CU message. Further, 1 < K < N. After (e.g., in response to) receiving the DU-to-CU message, the CU 172 generates a release list including the ID (i.e., LTM ID) K to release the LTM configuration K or element K and transmits an RRC reconfiguration message including the release list to the UE 102 via the DU 174. In response, the UE 102 releases the LTM configuration K or element K and transmits an RRC reconfiguration complete message to the UE 102 via the DU 174. In some implementations, the CU 172 transmits a CU-to-DU message to the DU 174 in response to the DU-to-CU message. In some implementations, the DU-to-CU message and CU-to-DU message are a UE Context Modification Required message and a UE Context Modification Confirm message, respectively.

[0094] After receiving the RRC reconfiguration in the event 318 or transmitting the RRC reconfiguration complete message in the event 320, the UE 102 transmits 324 at least one measurement report to the DU 174, similar to the event 304. In some implementations, the DU 174 transmits 326 a DU-to-CU message, including the at least one measurement report, to the CU 172, similar to the event 306. In other implementations, the DU 174 does not transmit the at least one measurement report to the CU 172. In some implementations, the at least one measurement report of the event 324 includes LI measurement report(s) or L3 measurement repot(s), as described for the event 304. In some implementations, the UE 102 transmits 324 the at least one measurement report on PUCCH(s) and/or PUSCH(s) to the DU 174, similar to the event 304. In other implementations, the UE 102 transmits 324 at least one MAC CE including the at least one measurement report to the DU 174, similar to the event 304. In some implementations, the UE 102 does not transmit the LI measurement report(s) in format of RRC message(s) to the DU 174.

[0095] In some implementations, the UE 102 transmits 324 the at least one measurement report to the DU 174 in accordance with at least one measurement configuration. The at least one measurement configuration configures the UE 102 to perform measurements and report measurement results. The CU 172 transmits the at least one measurement configuration to the UE 102 via the DU 174. For example, the CU 172 transmits one or more RRC messages (e.g., RRCReconfiguration message(s)), including the at least one measurement configuration, to the UE 102 via the DU 174 in the event 302 and/or 316 and/or after the event 306 or 316. Depending on the implementation, the one or more RRC messages do or do not include the RRC reconfiguration message of the event 316. In accordance with the at least one measurement configuration, the UE 102 performs measurements on one or more reference signals. In some implementations, the one or more reference signals include one or more SSBs and/or one or more CSI-RSs. The UE 102 obtains the at least one LI measurement result and/or at least one L3 measurement result from the measurements and includes the at least one LI measurement result and/or at least one L3 measurement result in the at least measurement report of the event 324. The DU 174 transmits the one or more reference signals on the cell 124A, the cell 1, and/or the cell(s) 2, ..., N. Depending on the implementation, the one or more reference signals are CSI-RS(s) or SSB(s).

[0096] In some implementations, the at least one measurement configuration includes L3 measurement configuration(s) (e.g., MeasConfig IE(s)), as described for the event 304. In other implementations, the at least one measurement configuration includes LI measurement configuration(s), as described for the event 304. For example, the LI measurement configuration(s) are CSI-MeasConfig IE(s) (e.g., defined in 3GPP TS 38.331 vl8.0.0 and/or later versions). In some implementations, the LI measurement configuration(s) include measurement report configuration(s). The UE 102 transmits the LI measurement report(s) on PUCCH(s) or MAC CE(s) to the DU 174 in accordance with the measurement report configuration(s). The DU 174 receives the LI measurement report(s) on PUCCH(s) or MAC CE(s) in accordance with the measurement report configuration(s). In some implementations, the measurement report configuration(s) are CSl-ReportConfig IE(s). In other implementations, each of the measurement report configuration(s) is a new RRC IE. In some implementations, (each of) the measurement report configuration(s) configures periodically reporting and/or event-triggered reporting of the LI measurement result(s).

[0097] In yet other implementations, the at least one measurement configuration includes new-type measurement configuration(s) (e.g., LTM measurement configuration(s)). In some implementations, the new-type measurement configuration(s) are newly defined (e.g., in 3GPP TS v 18.0.0 and/or later versions). In some implementations, the new-type measurement configuration(s) include reference signal resource configuration(s) configuring resources where the DU 174 transmits reference signal(s). For example, the reference signal resource configuration(s) include CSLRS(s) and/or SSB(s). In some implementations, the reference signal resource configuration(s) are CSl-ResourceConfig IE(s). In another implementation, the new-type measurement configuration(s) include measurement report configuration(s), as described above. The UE 102 transmits the measurement report(s) on PUCCH(s) or MAC CE(s) to the DU 174 in accordance with the measurement report configuration(s). The DU 174 receives the measurement report(s) on PUCCH(s) or MAC CE(s) in accordance with the measurement report configuration(s). In some such cases, the measurement report(s) are LI measurement report(s) or new-type measurement report(s) (e.g., LTM measurement report(s)). In some implementations, the new-type measurement configuration includes configuration parameters newly defined (e.g., in a 3GPP TS v 18.0.0 and/or later versions).

[0098] After (e.g., in response to) receiving the at least one measurement report in the event 324, the DU 174 generates a first LTM command to activate the LTM configuration 1 (i.e., the first LTM command commands the UE 102 to apply the LTM configuration 1 or to perform a serving cell change to the cell 1). The DU 174 then transmits 330 the first LTM command to the UE 102. In some implementations, the DU 174 transmits the first LTM command on the cell 124A to the UE 102. In other implementations, the DU 174 transmits the first LTM command on the cell 124D to the UE 102. In some implementations, the DU 174 includes the ID 1 in the first LTM command to indicate the LTM configuration 1, and the UE 102 determines (e.g., identifies) the LTM configuration 1 or element 1 in accordance with the ID 1. In other implementations, the DU 174 includes the cell index 1 indexing the cell ID 1 in the first LTM command. The UE 102 determines (e.g., identifies) the LTM configuration 1 or element 1 based on the cell index 1. After determining the LTM configuration 1 or element 1, the UE 102 then applies the LTM configuration 1 in response to receiving the first LTM command.

[0099] In yet other implementations, the DU 174 includes a bit map in the first LTM command to activate the LTM configuration 1, instead of the ID 1 or cell index 1. The number of bits in the bit map is larger than or equal to “N”. In some implementations, bit 1, ..., N corresponds to the LTM configuration(s) 1, ..., N or the element(s) 1, ..., N, respectively, and the DU 174 sets a corresponding bit (e.g., bit 1) in the bit map to a first value to indicate the ID 1, the LTM configuration 1, or the element 1. Thus, in some such implementations, the UE 102 determines the ID 1, LTM configuration 1, or element 1 in accordance with the bit 1 set to the first value in the bit map. In further implementations, bit 0, ..., N-l corresponds to the LTM configuration s) 1, ..., N or the element (s) 1, ..., N, respectively, and the DU 174 sets a corresponding bit (e.g., bit 0) in the bit map to a first value to indicate the ID 1 or the LTM configuration 1. Thus, in some such implementations, the UE 102 determines the ID 1 or LTM configuration 1 in accordance with the bit 0 set to the first value in the bit map. In such implementations, the DU 174 sets the remaining bits in the bit map to a second value to indicate that the rest of the LTM configuration(s) 1, ..., N are not activated. In some implementations, the first value is one and the second value is zero. In other implementations, the first value is zero and the second value is one. Generally, depending on the implementation, if the DU 174 determines to activate the LTM configuration L, the DU 174 sets the corresponding bit (e.g., bit L or bit L-7) in the bit map to the first value and sets the remaining bits to the second value, where 1 <L <N.

[0100] In some implementations, the at least one measurement report (e.g., LI measurement report(s) or new-type measurement report(s)) of the event 324 includes at least one measurement result for the first cell, TRP(s) of the first cell or reference signal(s) transmitted on the first cell. In some implementations, the reference signal(s) are CSLRS(s) or SSB(s). The DU 174 determines to activate the LTM configuration 1 or transmit the first LTM command, based on the at least one measurement result. In some implementations, the DU 174 determines to activate the LTM configuration 1 because, when, or if the at least one measurement result is above a second predetermined threshold. In some implementations, the at least one measurement result includes Ll-RSRP value(s), Ll-RSRQ value(s) and/or Ll-SINR value(s). In other implementations, the at least one measurement result includes RSRP value(s), RSRQ value(s), and/or SINR value(s) for the new-type measurement report(s). In some implementations, the second predetermined threshold is different from the first predetermined threshold. In some implementations, the second predetermined threshold is larger than the first predetermined threshold. In this case, the at least one measurement result indicates that the first cell is suitable for communication with the UE 102. In another implementation, the second predetermined threshold is equal to the first predetermined threshold. In such cases, the at least one measurement result indicates that the first cell has been continuously above the second predetermined threshold or the first predetermined threshold. Such indicates that the first cell is suitable for communication with the UE 102. Thus, the DU 174 determines to activate the LTM configuration 1 in response to the signal strength or quality of the first cell being above the second predetermined threshold for the UE 102.

[0101] In some implementations, the at least one measurement report (e.g., L3 measurement report(s)) of the events 324 and 326 includes at least one measurement result for the first cell. The CU 172 determines to activate the LTM configuration 1 or transmit the first LTM command, because the at least one measurement result indicates that signal strength or quality of the first cell is above a second predetermined threshold. The second predetermined threshold is different from the first predetermined threshold. In some implementations, the second predetermined threshold is larger than the first predetermined threshold. In such an implementation, the at least one measurement report of the event 326 indicates that signal strength or quality of the first cell is suitable for communication with the UE 102. In further implementations, the second predetermined threshold is equal to the first predetermined threshold. In such an implementation, the at least one measurement report of the event 326 indicates that signal strength or quality of the first cell has been continuously above the second predetermined threshold or the first predetermined threshold, further indicating that the first cell is suitable for communication with the UE 102. Thus, the CU 172 determines to activate the LTM configuration 1 in response to the signal strength or quality of the first cell being above the second predetermined threshold. In response to the determination, the CU 172 transmits 328 a fourth CU-to-DU message to the DU 174 to activate the LTM configuration 1. In response to the fourth CU-to-DU message, the DU 174 transmits 330 the first LTM command to the UE 102 and, optionally, transmits a fourth DU- to-CU message to the CU 172. In some implementations, the CU 172 includes the cell index 1 in the fourth CU-to-DU message. Thus, in some such implementations, the DU 174 determines to activate the LTM configuration 1 in accordance with the cell index 1. In other implementations, the CU 172 includes the cell ID 1 in the fourth CU-to-DU message. Thus, the DU 174 determines to activate the LTM configuration 1 in accordance with the cell ID 1. In yet other implementations, the CU 172 includes the ID 1 in the fourth CU-to-DU message. Thus, in some such implementations, the DU 174 determines to activate the LTM configuration 1 in accordance with the ID 1. In some implementations, the fourth CU-to-DU message and fourth DU-to-CU message are a UE Context Modification Request message and a UE Context Modification Response message, respectively. In other implementations, the fourth CU-to-DU message and/or fourth DU-to-CU message are new interface messages (e.g., El application protocol (E1AP) messages (e.g., defined in 3GPP TS 38.473 vl8.0.0 and/or later versions)).

[0102] In some implementations, when or in response to determining to activate the LTM configuration 1 or transmit the first LTM command, the DU 174 transmits 329 to the CU 172 a DU-to-CU message indicating that LTM is (being) executed. In some implementations, the DU 174 includes the cell ID 1 or the ID 1 (i.e., LTM ID) in the DU-to-CU message 329 to indicate that the DU 174 is to activate the LTM configuration 1. Depending on the implementation, the DU transmits the DU-to-CU message 329 to the CU 172 before or after transmitting the LTM command 330.

[0103] In some implementations, the first LTM command is a MAC CE included in a MAC PDU that the UE 102 receives from the DU 174 in the event 330. In some implementations, the MAC CE is a new MAC CE (e.g., defined in 3GPP TS 38.321 vl8.0.0 and/or later versions). In some implementations, the DU 174 includes a subheader identifying the new MAC CE in the MAC PDU, and the UE 102 identifies the new MAC CE in the MAC PDU in accordance with the subheader. In some implementations, the subheader includes a logical channel ID or extended logical channel ID (e.g., defined in a 3GPP TS) to identify the new MAC CE. For example, the logical channel ID or extended logical channel ID are newly defined (e.g., in 3GPP TS 38.321 vl8.0.0 and/or later versions). In other implementations, the first LTM command is a DO that the UE 102 receives on a PDCCH from the DU 174 in the event 330. The DU 174 generates a CRC for the DO, scrambles the CRC with a first C-RNTI of the UE 102, and transmits the DCI and scrambled CRC on the PDCCH in the event 330. In some implementations, a format of the DCI is an existing DCI format (e.g., defined in a 3GPP TS (e.g., 3GPP TS 38.212)). In further implementations, the format of the DCI is a new DCI format (e.g., defined in a 3GPP TS (e.g., 3GPP TS 38.212 v 18.0.0 and/or later versions)).

[0104] In some implementations, the DU 174 does not perform security protection (e.g., integrity protection and/or encryption) on the first LTM command. This speeds up processing the first LTM command in the UE 102 because the UE 102 does not perform a security check (e.g., decryption and/or integrity check) on the first LTM command.

[0105] In some implementations, after receiving the first LTM command, the UE 102 transmits 331 an acknowledgement to the DU 174 on the cell 124A or cell 124D to indicate that the UE 102 receives the first LTM command. In some implementations, the acknowledgement is a HARQ ACK. In other implementations, the acknowledgement is a MAC CE. For example, the MAC CE is an existing MAC CE (e.g., defined in 3GPP TS 38.321). In another example, the MAC CE is a new MAC CE (e.g., defined in 3GPP TS 38.321 vl8.0.0 and/or later versions). In yet other implementations, the acknowledgement is a PUCCH transmission. [0106] In some implementations, the CU 172 transmits 316 the RRC reconfiguration message in response to the L3 measurement report 306 for the first cell. In some implementations, to configure the UE 102 to transmit the L3 measurement report 306, the CU 172 transmits a first RRC reconfiguration message, including the L3 measurement configuration (e.g., a MeasConfig IE), to the UE 102 before the event 306. In some implementations, the DU 174 transmits 330 the first LTM command in response to the LI measurement report(s) 324 for the first cell. In some implementations, to configure the UE 102 to transmit the LI or new-type measurement report(s) 324, the CU 172 transmits a second RRC reconfiguration message including the LI or new-type measurement configuration(s) to the UE 102. In some implementations, the first and second RRC reconfiguration messages are the same message (i.e., the same instance). In other implementations, the first and second RRC reconfiguration messages are different messages. In some implementations, the second RRC reconfiguration message is the RRC reconfiguration message of the event 316. In other implementations, the second RRC reconfiguration message is different from the RRC reconfiguration message of the event 316.

[0107] After (e.g., in response to) receiving the first LTM command, the UE 102 identifies the LTM configuration 1 in accordance with the ID 1 and applies the LTM configuration 1. In some implementations, the UE 102 performs 332 a random access procedure on the first cell with the DU 174 in response to applying the LTM configuration 1 or receiving the first LTM command. In some implementations, the UE 102 disconnects from the cell 124A after (e.g., in response to) receiving the first LTM command or after transmitting the acknowledgement. In other words, the UE 102 stops communicating on the cell 124A after (e.g., in response to) receiving 330 the first LTM command or transmitting 331 the acknowledgement. In such cases, the UE 102 performs 332 the random access procedure after disconnecting from the cell 124A. In some implementations, the UE 102 determines whether to perform the random access procedure in accordance with the LTM configuration 1. In some implementations, if the LTM configuration 1 configures the UE 102 to perform a random access procedure, the UE 102 performs the random access procedure in the event 332. For example, the LTM configuration 1 includes a reconfiguration with sync configuration (e.g., ReconfigurationWithSync IE) to configure the UE 102 to perform a random access procedure. Otherwise, if the LTM configuration 1 does not configure the UE 102 to perform a random access procedure or configures the UE 102 to skip a random access procedure, the UE 102 refrains from performing a random access procedure with the DU 174 upon receiving the first LTM command. In such cases, the UE 102 skips the event 316. For example, if the LTM configuration 1 excludes a reconfiguration with sync configuration, the LTM configuration 1 configures the UE 102 not to perform a random access procedure. In some implementations, the random access procedure is a four-step random access procedure. In other implementations, the random access procedure is a two-step random access procedure. In some implementations, the random access procedure is a contention-free random access procedure. In other implementations, the random access procedure is a contention-based random access procedure.

[0108] In cases where the UE 102 performs 332 the random access procedure, the UE 102 communicates 336 with the DU 174 on the first cell using the LTM configuration 1 and reference LTM configuration, and communicates with the CU 172 via the DU 174, after successfully completing the random access procedure. In such cases, the DU 174 communicates with the UE 102 on the first cell using the LTM configuration 1 in the event 332 and/or event 336. In some scenarios or implementations, the UE 102 communicates UL PDUs, DL PDUs, and/or physical layer signals (e.g., PUCCH transmissions and PDCCH transmissions) with the base station 104 in the event 336. In some implementations, the UE 102 determines that the UE 102 successfully completes the random access procedure when the UE 102 receives a contention resolution from the DU 174. In cases where the random access procedure is a four-step random access procedure, the UE 102 transmits a Message 3 including a UE identity to the DU 174 via the first cell in the random access procedure. In cases where the random access procedure is a two-step random access procedure, the UE 102 transmits a Message A including the UE identity to the DU 174 via the first cell in the random access procedure. In some implementations, if the LTM configuration 1 includes a second C-RNTI, the UE identity is the second C-RNTI of the UE 102. Otherwise, if the LTM configuration 1 does not include a C-RNTI, the UE identity is the first C-RNTI. In cases where the random access procedure is a contention free random access procedure, the UE 102 transmits the dedicated random access preamble to the DU 174 via the first cell. In such cases, the LTM configuration 1 includes the dedicated random access preamble.

[0109] The DU 174 identifies or determines that the UE 102 connects to the first cell upon receiving the UE identity or the dedicated preamble from the UE 102 in the random access procedure 332. [0110] In cases where the UE 102 skips the random access procedure, the UE 102 directly communicates 336 with the DU 174 on the first cell in accordance with the LTM configuration 1 and communicates with the CU 172 via the DU 174, after (e.g., in response to) receiving the first LTM command. For example, the UE 102 directly communicates UL PDUs, DL PDUs, and/or physical layer signals (e.g., PUCCH transmissions and PDCCH transmissions) with the base station 104 in the event 336. In some implementations, the DU 174 includes, in the LTM configuration 1, configuration parameters configuring resources for the UE 102 to transmit the at least one PUCCH or PUSCH transmission, and the UE 102 transmits the at least one PUCCH or PUSCH transmission on the resources, using the configuration parameters, to indicate that the UE 102 connects to the first cell. In other implementations, the DU 174 transmits, to the UE 102, at least one DCI on a PDCCH on the first cell to command the UE 102 to transmit the at least one PUCCH or PUSCH transmission, after transmitting the first LTM command. The at least one DCI configures resources for the UE 102 to transmit the at least one PUCCH or PUSCH transmission, and the UE 102 transmits the at least one PUCCH or PUSCH transmission on the resources. The DU 174 identifies or determines that the UE 102 connects to the first cell upon receiving the PUCCH or PUSCH transmission. The DU 174 identifies or determines that the UE 102 connects to the first cell upon receiving the PUCCH or PUSCH transmission on the resources configured in the LTM configuration 1 or the at least one DCI.

[0111] In cases where the UE 102 receives the reference LTM configuration as described above, the UE 102 communicates 336 with the DU 174 on the first cell in accordance with the LTM configuration 1 and at least a portion of the reference LTM configuration. In other words, the UE 102 communicates 336 with the DU 174 in accordance with configuration parameters in the LTM configuration 1 and the reference LTM configuration. Similarly, the DU 174 communicates 336 with the UE 102 on the first cell in accordance with the LTM configuration 1 and at least a portion of the reference LTM configuration. In other words, the DU 174 communicates 336 with the UE 102 in accordance with configuration parameters in the LTM configuration 1 and the reference LTM configuration.

[0112] In some implementations, the UE 102 transmits an RRC message (e.g., RRC reconfiguration complete message) to the CU 172 via the DU 174 and the first cell to indicate that the UE 102 applies the LTM configuration 1. In some implementations where the UE 102 performs the random access procedure 332, the UE 102 includes the RRC message in the Message 3 or Message A. Alternatively, the UE 102 transmits the RRC message after completing the random access procedure. In cases where the UE 102 skips the random access procedure 332, the UE 102 includes the RRC message in a PUSCH transmission of the at least one PUSCH transmission. In some implementations, if the UE 102 maintains communication on the cell 124A with the base station 104 (i.e., the UE 102 does not disconnect from the cell 124A), the UE 102 transmits the RRC message to the base station 104 via the cell 124A. When the DU 174 receives the RRC message, the DU 174 transmits the RRC message to the CU 172.

[0113] In other implementations, the UE 102 refrains from transmitting the RRC message to the base station 104 in response to applying the LTM configuration 1 or receiving the first LTM command. In some such cases, the UE 102 includes or transmits data in the Message 3, Message A or PUSCH transmission as described above. In some implementations, the UE 102 generates a MAC PDU and/or an RLC PDU, including the data, and transmits or includes the MAC PDU and/or RLC PDU in the PUSCH transmission. For example, depending on the implementation, the data is a PDCP PDU, an SDAP PDU, an LTE Positioning Protocol (LPP) PDU, an RRC PDU, and/or a NAS PDU. The RRC PDU includes a UL-DCCH-Message excluding an RRC reconfiguration complete message. The NAS PDU includes a Mobility Management (MM) message or a Session Management (SM) message. Depending on the implementation, the MM message is a 5G MM message or a 6G MM message, and the SM message is a 5G SM message or a 6G SM message. When the DU 174 receives the data, the DU 174 transmits the data to the CU 172.

[0114] In some implementations, when the DU 174 determines that the UE 102 successfully connects to the first cell in the event 332 or 336, the DU 174 transmits 334 a DU-to-CU message (e.g., Access Success message) to the CU 172 (e.g., a CP of the CU 172). In some implementations, the DU 174 includes the cell ID 1 of the first cell in the DU-to-CU message of the event 334. In some implementations, the cell ID is a PCI or a CGI. Thus, the CU 172 determines that the UE 102 connects to the first cell upon receiving the DU-to-CU message of the event 334. In some implementations, when the DU 174 determines that the UE 102 successfully connect to the first cell in the event 332 or 336, the DU 174 transmits a DL Data Delivery Status message or frame to the CU 172 (e.g., a UP of the CU 172).

[0115] In some implementations, when determining that the UE 102 connects to the first cell, transmitting 330 the first LTM command, or receiving 331 the acknowledgement, the DU 174 stops communicating with the UE 102 on the cell 124A and/or releases resources of the cell 124 A configured for the UE 102.

[0116] In some implementations, the DU 174 generates some or all of the LTM configuration 1 and/or LTM configuration(s) 2, N as full configuration(s) to replace the serving DU configuration. If the LTM configuration 1 is a full configuration, the UE 102 and DU 174 communicate 336 with each other in accordance with the LTM configuration 1 instead of the serving DU configuration. In some implementations, the DU 174 includes an indication that the LTM configuration l is a full configuration in the LTM configuration 1. In some implementations, in each of the LTM configuration(s) 2, ..., N, the DU 174 includes an indication to indicate that the corresponding DU configuration is a full configuration. In some implementations, each of the indication(s) in the LTM configuration(s) 1, ..., N is a field or IE (i.e., the same field or IE). In other implementations, the CU 172 includes, in the RRC reconfiguration message of the events 316, 318, a single indication that the LTM configuration(s) 1 and/or 2, ..., N is/are full configuration(s). In some cases, for the second container, the CU 172 includes, in the additional RRC reconfiguration message, a single indication that the LTM configuration(s) 2, ..., N is/are full configuration(s). In yet other implementations, the CU 172 includes, in the first container, a single indication that the LTM configuration(s) 1 and/or 2, ..., N is/are full configuration(s). In yet other implementations, for each of the LTM configuration(s) 2, ..., N, the CU 172 includes, in the first container, a particular indication that the corresponding LTM configuration is a full configuration. In some cases, for the second container, the CU 172 includes, in the second container, a single indication that the LTM configuration(s) 2, ..., N is/are full configuration(s). In yet other implementations, the CU 172 includes, in the element 1, an indication that the LTM configuration 1 is a full configuration. In some implementations, in each of the element(s) 2, ..., N, the CU 172 includes an indication that the corresponding LTM configuration is a full configuration. In some implementations, the UE 102 determines that the LTM configuration 1 and/or LTM configuration(s) 2, ..., N are full configuration(s) based on the indication(s) above. In some implementations, each of the indication(s) above is different from a fullConfig field (e.g., as defined in the current 3GPP TS). In some implementations, each of the indication(s) above is a fullConfig field (e.g., as defined in the current 3GPP TS). In cases where the LTM configuration 1 is a full configuration, the UE 102 in the event 336 does not apply the reference LTM configuration if received from the base station 104 (e.g., in the RRC reconfiguration message 318). In some such cases, the DU 174 does not include a reference LTM configuration in the first DU-to-CU message 310.

[0117] In other implementations, the DU 174 generates the LTM configuration 1 and/or LTM configuration(s) 2, ..., N as delta configuration(s) that augment at least a portion of the reference LTM configuration. In other words, the DU 174 generates the LTM configuration(s) 1, .. ,N based on the reference LTM configuration. For example, if the LTM configuration 1 is a delta configuration, the UE 102 and DU 174 augment at least the portion of the reference LTM configuration with the LTM configuration 1. Thus, the UE 102 and DU 174 communicate 336 with each other in accordance with the LTM configuration 1 and unaugmented portion of the reference LTM configuration. In some implementations, the LTM configuration(s) 1 and/or 2..., N, first container, second container, or element(s) 1, ..., N exclude indication(s) indicating that the LTM configuration(s) 1 and/or 2..., N are full configuration(s) to indicate that the LTM configuration(s) 1 and/or 2, ..., N are delta configuration(s). In some implementations, the UE 102 determines that each of the LTM configuration(s) 1 and/or 2, ..., N is a delta configuration based on the indication being excluded in the LTM configuration(s) 1 and/or 2, ..., N, first container, second container, or element(s) 1 and/or 2, ..., N.

[0118] In some implementations, if the UE 102 does not receive a reference LTM configuration for the LTM configuration 1 and/or the LTM configuration(s) 2, ..., N, the UE 102 determines that the LTM configuration 1, and/or the LTM configuration(s) 2, ..., N are full configuration(s). Correspondingly, if the DU 174 does not obtain a reference LTM configuration for the UE 102 (i.e., the DU 174 does not generate a reference LTM configuration for the UE 102 and/or receive a reference LTM configuration for the UE 102 from the CU 174), the DU 174 generates the LTM configuration 1, and/or the LTM configuration(s) 2, ..., N as full configuration(s).

[0119] In other implementations, if the UE 102 does not receive a reference LTM configuration for the LTM configuration 1 and/or the LTM configuration(s) 2, ..., N, the UE 102 determines that the LTM configuration 1 and/or the LTM configuration(s) 2, ..., N are delta configuration(s) to augment the serving DU configuration. In such cases, the UE 102 communicates 336 with the DU 174 in accordance with the LTM configuration 1 and at least a portion of the serving DU configuration not augmented by LTM configuration 1. Correspondingly, if the DU 174 does not obtain a reference LTM configuration for the UE 102 (i.e., the DU 174 does not generate a reference LTM configuration for the UE 102 and/or receive a reference LTM configuration for the UE 102 from the CU 174), the DU 174 generates the LTM configuration 1 and/or the LTM configuration(s) 2, .. N as delta configuration(s) to augment the serving DU configuration. In such cases, the DU 174 communicates 336 with the UE 102 in accordance with the LTM configuration 1 and the at least a portion of the serving DU configuration.

[0120] In some implementations, the UE 102 uses a UE MAC entity (e.g., MAC 204B) to communicate with a DU MAC entity (e.g., MAC 204B) of the DU 174 (e.g., the events 302, 304, 318, 320, 324, 330, and/or 331). In some implementations, the UE 102 resets the UE MAC entity, after or in response to receiving the first LTM command and before performing 332 the random access procedure or communicating 336 with the DU 174 via the first cell. In some implementations, the DU 174 resets the DU MAC entity after (e.g., in response to) transmitting the first LTM command, receiving the acknowledgement 331, or determining that the UE 102 connects to the first cell.

[0121] In some implementations, when the UE 102 resets the UE MAC entity, the UE 102 performs at least one of the following actions for the UE MAC entity (i.e., UE MAC reset or full UE MAC reset): (i) initialize Bj for configured logical channel(s) to zero; (ii) stop one or more timers; (iii) consider timeAlignmentTimeris) as expired if the UE 102 is configured to perform the random access procedure (e.g., the event 332) in the configuration (e.g., the configuration 1); (iv) set new data indicator(s) (NDI(s)) for UL HARQ process(es) to value 0; (v) set NDI(s) for HARQ process ID(s) to value 0 for monitoring PDCCH in Sidelink resource allocation mode 1; (vi) flush Msg3 buffer; (vii) flush MSGA buffer; (viii) cancel, if any, triggered Scheduling Request procedure; (ix) cancel, if any, triggered Buffer Status Reporting procedure; (x) cancel, if any, triggered Power Headroom Reporting procedure; (xi) cancel, if any, triggered consistent LBT failure; (xii) cancel, if any, triggered BFR; (xiii) cancel, if any, triggered Sidelink Buffer Status Reporting procedure; (xiv) cancel, if any, triggered Pre-emptive Buffer Status Reporting procedure; (xv) cancel, if any, triggered Timing Advance Reporting procedure; (xvi) cancel, if any, triggered Recommended bit rate query procedure; (xvii) cancel, if any, triggered configured uplink grant confirmation; (xviii) cancel, if any, triggered configured sidelink grant confirmation; (xix) cancel, if any, triggered Desired Guard Symbol query; (xx) cancel, if any, triggered Positioning Measurement Gap Activation/Deactivation Request procedure; (xxi) flush soft buffers for DL HARQ process(es); (xxii) for each of the DL HARQ process(es), consider the next received transmission for a TB as the very first transmission; (xxiii) release, if any, Temporary C- RNTI; and/or (xiv) reset one or more counters (e.g., BFI_COUNTERs and/or LBT_COUNTERs).

[0122] In some implementations, when the DU 174 resets the DU MAC entity, the DU 174 performs at least one of the following actions for the DU MAC entity (i.e., DU MAC reset or full DU MAC reset): (i) stop one or more timers; (ii) consider limeAlignmenlTimer( ) that the DU 174 starts and/or maintains for the UE 102 as expired if the UE 102 is configured to perform the random access procedure (e.g., the event 332) in the configuration (e.g., the configuration 1); (iii) set NDI(s) for DL HARQ process(es) to value 0; (iv) flush soft buffers for UL HARQ process(es); (v) for each of the UL HARQ process(es), consider the next received transmission for a TB as the very first transmission; and/or (vi) reset one or more counters (e.g., BFI_COUNTERs and/or LBT_COUNTERs).

[0123] Depending on implementations, the UE 102 determines to partially or fully reset the UE MAC entity. In some implementations, when the UE 102 resets the UE MAC entity as described above, the UE 102 fully resets the UE MAC entity (i.e., a full UE MAC reset). In the full UE MAC reset, the UE 102 performs some or all of the actions described above. In other implementations, when the UE 102 resets the UE MAC entity as described above, the UE 102 partially resets the UE MAC entity (i.e., a partial UE MAC reset). In the partial UE MAC reset, the UE 102 performs a subset or portion of the some or all of the actions in the full UE MAC reset.

[0124] In some implementations, the partial UE MAC reset includes at least one of the following actions: (i) consider timeAlignmentTimeris) of the UE 102 as expired if the UE 102 is configured to perform the random access procedure (e.g., the event 332) in the configuration (e.g., the configuration 1); (ii) flush Msg3 buffer; (iii) flush MSGA buffer; (iv) release, if any, Temporary C-RNTI; and/or (v) reset one or more counters (e.g., BFI_COUNTERs and/or LBT_COUNTERs).

[0125] In some implementations, the partial UE MAC reset further includes at least one of the following actions: (i) cancel, if any, triggered Scheduling Request procedure; (ii) cancel, if any, triggered Buffer Status Reporting procedure; (iii) cancel, if any, triggered Power Headroom Reporting procedure; (iv) cancel, if any, triggered consistent LBT failure; (v) cancel, if any, triggered BFR; (vi) cancel, if any, triggered Sidelink Buffer Status Reporting procedure; (vii) cancel, if any, triggered Pre-emptive Buffer Status Reporting procedure; (viii) cancel, if any, triggered Timing Advance Reporting procedure; (ix) cancel, if any, triggered Recommended bit rate query procedure; (x) cancel, if any, triggered configured uplink grant confirmation; (xi) cancel, if any, triggered configured sidelink grant confirmation; (xii) cancel, if any, triggered Desired Guard Symbol query; and/or (xiii) cancel, if any, triggered Positioning Measurement Gap Activation/Deactivation Request procedure.

[0126] In some implementations, the partial UE MAC reset further includes at least one of the following actions: (i) stop a first portion of the one or more timers and retain the rest of the one or more timers; (ii) set new data indicator(s) (NDI(s)) for UL HARQ process(es) to value 0; (iii) set NDI(s) for HARQ process ID(s) to value 0 for monitoring PDCCH in Sidelink resource allocation mode 1; (iv) flush soft buffers for DL HARQ process(es); and/or (v) for each of the DL HARQ process(es), consider the next received transmission for a TB as the very first transmission;

[0127] Depending on implementations, the DU 174 determines to partially or fully reset the DU MAC entity. In some implementations, when the DU 174 resets the DU MAC entity as described above, the DU 174 fully resets the DU MAC entity (i.e., a full DU MAC reset). In the full DU MAC reset, the DU 174 performs some or all of the actions described above. In other implementations, when the DU 174 resets the DU MAC entity as described above, the DU 174 partially resets the DU MAC entity (i.e., a partial DU MAC reset). In the partial DU MAC reset, the DU 174 performs a subset or portion of the some or all of the actions in the full DU MAC reset.

[0128] In some implementations, the partial DU MAC reset includes at least one of the following actions in the partial MAC reset: (i) consider limeAlignmenlTimer( ) that the DU 174 starts and/or maintains for the UE 102 as expired if the UE 102 is configured to perform the random access procedure (e.g., the event 332) in the configuration (e.g., the configuration 1) and/or (ii) reset one or more counters (e.g., BFI_COUNTERs and/or LBT_COUNTERs).

[0129] In some implementations, when the partial DU MAC reset includes at least one of the following actions for the MAC entity (i.e., DU MAC reset): (i) stop a first portion of the one or more timers and retain the rest of the one or more timers; (ii) set NDI(s) for DL HARQ process(es) to value 0; (iii) flush soft buffers for UL HARQ process(es); (iv) for each of the UL HARQ process(es), consider the next received transmission for a TB as the very first transmission; and/or (v) reset one or more counters (e.g., BFI_COUNTERs and/or LBT_COUNTERs). [0130] In other implementations, the UE 102 refrains from resetting the UE MAC entity in response to receiving the first LTM command. Similarly, the DU 174 refrains from resetting the DU MAC entity after (e.g., in response to) transmitting the first LTM command, receiving the acknowledgement 331 or determining that the UE 102 connects to the first cell. In other words, the UE 102 communicates with the DU 174 on the first cell using the UE MAC entity (not reset). Similarly, the DU 174 communicates with the UE 102 using the DU MAC entity (not reset) on the first cell during or after the random access procedure 332 or after determining that the UE 102 connects to the first cell.

[0131] In some implementations, the UE 102 uses at least one UE RLC entity (e.g., RLC 206B) to communicate RLC PDUs with at least one DU RLC entity (e.g., RLC 206B) of the DU 174 (e.g., the events 302, 304, 318, 320, 324, 330 and/or 331). In some implementations, the UE 102 reestablishes some or all of the at least one UE RLC entity, after or in response to receiving the first LTM command and before performing 332 the random access procedure or communicating 336 with the DU 174 via the first cell. In some implementations, the DU 174 reestablishes some or all of the at least one DU RLC entity after (e.g., in response to) transmitting the first LTM command, receiving the acknowledgement 331 or determining that the UE 102 connects to the first cell.

[0132] In some implementations, the LTM configuration 1 does or does not include one or more RLC reestablishment indications (e.g., reestablishRLC field(s)) configuring the UE 102 to reestablish some or all of the at least one UE RLC entity. If the LTM configuration 1 includes the an RLC reestablishment indication configuring the UE 102 to reestablish a first UE RLC entity of the at least one UE RLC entity that the UE 102 uses to communicate RLC PDU(s) with the DU 174, the UE 102 reestablishes the first UE RLC entity in response to the RLC reestablishment indication and the first LTM command. In some implementations, the UE 102 reestablishes the first UE RLC entity before performing 332 the random access procedure or communicating 336 with the DU 174 via the first cell. In other implementations, the UE 102 reestablishes the first UE RLC entity while or after performing 332 the random access procedure. Otherwise if the LTM configuration 1 does not include the RLC reestablishment indication, the UE 102 refrains from reestablishing the first UE RLC entity in response to the first LTM command.

[0133] In some implementations, when the UE 102 reestablishes the first UE RLC entity, the UE 102 performs at least one of the following actions for the first UE RLC entity: (i) discard RLC SDU(s), RLC SDU segment(s), and RLC PDU(s), if any; (ii) stop and reset timer(s), if running; and/or (iii) reset state variables to initial values. In some implementations, the state variables and timer(s) are pre-defined (e.g., in 3GPP TS 38.322).

[0134] Otherwise, if the LTM configuration 1 does not include the RLC reestablishment indication for the first UE RLC entity, the UE 102 refrains from reestablishing the first UE RLC entity upon or when receiving the first LTM command. In other words, the UE 102 refrains from preforming the actions for reestablishing the first UE RLC entity of the UE 102 upon or when receiving the first LTM command. In some implementations, if the LTM configuration 1 or element 1 does not include the RLC reestablishment indication and includes an indication that the configuration 1 is a full configuration, the UE 102 reestablishes the first UE RLC entity of the UE 102 upon or when receiving the first LTM command. Otherwise, if the LTM configuration 1 or element 1 does not include the RLC reestablishment indication and the indication indicating that the configuration 1 is a full configuration, the UE 102 refrains from reestablishing the first UE RLC entity upon or when receiving the first LTM command.

[0135] Similarly, the DU 174 reestablishes some or all of at least one DU RLC entity (e.g., NR RLC 206B) that the DU 174 uses to communicate with the at least one UE RLC entity of the UE 102 (e.g., the events 302, 304, 318, 320, 324, 330, and/or 331) in response to the RLC reestablishment indication. In some implementations, the DU 174 reestablishes a first DU RLC entity of the at least one DU RLC entity after transmitting the first LTM command, receiving an acknowledgement for the first LTM command from the UE 102, or determining that the UE 102 connects to the first cell. In some implementations, the acknowledgement is a HARQ ACK. In other implementations, the acknowledgement is a MAC CE. In yet other implementations, the acknowledgement is a PUCCH transmission. In some implementations, when the base station 104 reestablishes the first DU RLC entity, the DU 174 performs at least one of the following actions for the first DU RLC entity: (i) discard RLC SDU(s), RLC SDU segment(s), and RLC PDU(s), if any; (ii) stop and reset timer(s), if running; and/or (iii) reset state variables to initial values. In some implementations, the state variables and timer(s) are pre-defined (e.g., in 3GPP TS 38.322).

[0136] In other implementations, the UE 102 refrains from reestablishing some or all of the at least one UE RLC entity in response to receiving the first LTM command. Similarly, the DU 174 refrains from reestablishing some or more of the at least one DU RLC entity after (e.g., in response to) transmitting the first LTM command, receiving the acknowledgement 331 or determining that the UE 102 connects to the first cell. In other words, the UE 102 communicates with the DU 174 on the first cell using the some or all of the at least one UE RLC entity (not reestablished). For example, the some or all of the at least one UE RLC entity includes the first UE RLC entity and/or a second UE RLC entity. Similarly, the DU 174 communicates with the UE 102 using the some or all of the at least one DU RLC entity (not reestablished) on the first cell during or after the random access procedure 332 or after determining that the UE 102 connects to the first cell. For example, the some or all of the at least one DU RLC entity includes the first DU RLC entity and/or a second DU RLC entity.

[0137] In some implementations, the UE 102 uses at least one UE PDCP entity (e.g., PDCP 210) to communicate UL PDCP PDUs and/or DL PDCP PDUs with at least one CU PDCP entity (e.g., PDCP 210) of the CU 172 in the event 302. In some implementations, the UE 102 performs a PDCP recovery procedure for some or all of the at least one UE PDCP entity, after or in response to receiving the first LTM command. For example, the UE 102 performs a PDCP recovery procedure for a first UE PDCP entity of the at least one UE PDCP entity, after or in response to receiving the first LTM command. Depending on the implementation, in the PDCP recovery procedure, the UE 102 does or does not reestablish the first UE PDCP entity. In some implementations, after or in response to performing the PDCP recovery procedure, the UE 102 retransmits at least a portion of the UL PDCP PDUs to the CU 172 via the DU 174 and the first cell in the event 336. Similarly, the CU 172 performs a PDCP recovery procedure for some or all of the at least one CU PDCP entity after or in response to transmitting the first LTM command. For example, the CU 172 performs a PDCP recovery procedure for a first CU PDCP entity of the at least one CU PDCP entity, after or in response to transmitting the first LTM command. In some implementations, the CU 172 performs the PDCP recovery procedure for the first CU PDCP entity in response to receiving the DU-to-CU message 329 or 334. In other implementations, the CU 172 performs the PDCP recovery procedure for the first CU PDCP entity in response to receiving the DL Data Delivery Status message. Depending on the implementation, in the PDCP recovery procedure, the CU 172 does or does not reestablish the first CU PDCP entity. In some implementations, after or in response to performing the PDCP recovery procedure, the CU 172 retransmits at least a portion of the DL PDCP PDUs to the UE 102 via the DU 174 and the first cell in the event 336. [0138] In other implementations, the UE 102 refrains from reestablishing some or all of the at least one UE PDCP entity in response to receiving the first LTM command. For example, the some or all of the at least one UE PDCP entity includes the first UE PDCP entity and/or a second UE PDCP entity. Similarly, the CU 172 refrains from reestablishing some or more of the at least one CU PDCP entity, after (e.g., in response to) receiving the DU-to-CU message 329 or 340 or after (e.g., in response to) receiving the DL Data Delivery Status message. In other words, the UE 102 communicates with the CU 172 via the DU 174 and the first cell using the some or all of the at least one UE PDCP entity (not reestablished). For example, the some or all of the at least one UE PDCP entity includes the first UE PDCP entity and/or a second UE PDCP entity. Similarly, the CU 172 communicates with the UE 102 using the some or all of the at least one CU PDCP entity (not reestablished) via the DU 174 and the first cell. For example, the some or all of the at least one CU PDCP entity includes the first CU PDCP entity and/or a second CU PDCP entity.

[0139] In some implementations, after determining that the UE 102 connects to the first cell, the CU 172 transmits 338 a CU-to-DU message (e.g., a UE Context Modification Request message) to the DU 174 to indicate to the DU 174 to stop communicating with the UE 102 and/or to release or suspend resources, of the cell 124A, configured for the UE 102. In some implementations, in response, the DU 174 stops communicating on the cell 124A with the UE 102 and/or releases or suspends resources, of the cell 124A, configured for the UE 102, and transmits 340 a DU-to-CU message (e.g., a UE Context Modification Response message) to the CU-172. The events 338 (optional) and 340 (optional) are collectively referred to in Fig. 3 as a resource release procedure 396.

[0140] Depending on the implementation, after or while communicating with the DU 174 on the first cell, events 344, 346, 348, 350, 351, 352, 354, and/or 356 occur, similar to the events 324, 326, 328, 330, 331, 332, 334, and/or 336, respectively. The UE 102 transmits 344 at least one measurement report to the DU 174. The at least one measurement report includes at least one measurement result for a second cell (i.e., the cell 2). The at least one measurement result indicates that the second cell is suitable for communication with UE 102 and/or the first cell is not suitable for communication with the UE 102. After (e.g., in response to) receiving the at least one measurement report, the DU 174 determines to activate the LTM configuration 2 and generates a second LTM command to activate the LTM configuration 2 (i.e., the second LTM command commands the UE 102 to apply the LTM configuration 2). The DU 174 then transmits 350 the second LTM command to the UE on the first cell to the UE 102.

[0141] In some implementations, when or in response to determining to activate the LTM configuration 2 or transmit the second LTM command, the DU 174 transmits 349 to the CU 172 a DU-to-CU message indicating LTM (being) executed. In some implementations, the DU 174 includes the cell ID 2 or the ID 2 (i.e., LTM ID) in the DU-to-CU message 349 to indicate that the DU 174 is to activate the LTM configuration 2. In some implementations, the DU transmits the DU-to-CU message 349 to the CU 172 before or after transmitting the LTM command 350.

[0142] The descriptions for the events 324, 326, 328, 330, 331, 332, 334, and/or 336 can be applied to the events 344, 346, 348, 350, 351, 352, 354, and/or 356 with simple changes. For example, “cell 124A”, “first LTM command”, “first cell”, “ID 1”, and/or “LTM configuration 1” are replaced with “first cell”, “second LTM command”, “second cell”, “ID 2”, and/or “LTM configuration 2”, respectively.

[0143] The events 344, 346, 348, 350, 351, 352, 354 are collectively referred to in Fig. 3 as an LTM execution procedure 398. The events 304, 306, 390, 392, 394, 324, 326, 328, 329, 330, 331, 332, 334, 336, 396, 398, 356 are collectively referred to in Fig. 3 as an LTM configuration and/or activation procedure 380.

[0144] Referring next to Fig. 4A, in a scenario 400A, the base station 106 operates as an MN, and the base station 104 operates as an SN. The SN 104 includes a CU 172 and a DU 174. The scenario 400A is similar to the scenario 300, except that the scenario 400A is a DC scenario and the scenario 300 is a single connectivity (SC) scenario. The MN 106 can include a CU and a DU similar to the base station 104 of Fig. 3.

[0145] Initially, the UE 102 in DC communicates with the MN 106 and with SN 104. In the event 402, the UE 102 communicates with the DU 174 on cell 124A using a serving DU configuration and communicates with the CU 172 via the DU 174 using a serving CU configuration, similar to the event 302. In some alternative implementations, the UE 102 does not communicate with the CU 172 via the DU 174 in the event 302. In some implementations, the UE 102 in DC communicates 402 UL PDUs and/or DL PDUs with the MN 106 and/or SN 104 via radio bearers which include SRBs and/or DRB(s). In some implementations, the MN 106 and/or the SN 104 configures the radio bearers to the UE 102. The UE 102 in DC communicates 402 UL PDUs and/or DL PDUs with the SN 104 on an SCG (i.e., SCG radio resources) that the SN 104 configures for communication with the UE 102. The UE 102 in DC communicates UL PDUs and/or DL PDUs with the MN 106 on an MCG (i.e., MCG radio resources) in accordance with an MN configuration (i.e., MCG configuration). In some implementations, the first configuration is an SN configuration (i.e., SCG configuration). In the MN configuration, the MN 106 configures the MCG which includes at least one serving cell (e.g., the cell 126 and/or other cell(s)) operated by the MN 106. In the first configuration, the SN 106 A configures the SCG which includes at least one serving cell (e.g., the cell 124A and/or other cell(s)) operated by the SN 104. In some implementations, the MN configuration includes multiple configuration parameters, and the UE 102 receives the configuration parameters in one or more RRC messages from the MN 106. As described for Fig. 3, the serving DU configuration includes multiple configuration parameters. In some implementations, the UE 102 receives these configuration parameters in one or more RRC messages from the SN 104 (e.g., via the MN 106 and/or on an SRB e.g., SRB3) that the MN 106 or SN 104 configures to exchange RRC messages between the UE 102 and the SN 104).

[0146] In some implementations, while the UE 102 communicates in DC with the MN 106 and SN 104, the MN 106 performs 480 an LTM configuration and/or activation procedure with the UE 102, similar to the procedures 380. In some implementations, while communicating in DC with the MN 106 and SN 104, the UE 102 transmits the at least one measurement report to the CU 172 via the DU 174 and cell 124A in the events 404 and 406, similar to the events 304 and 306, respectively. In other implementations, while communicating in DC with the MN 106 and SN 104, the UE 102 transmits 405 at least one measurement report to the MN 106 via the cell 126. The MN 106 in turn transmits 407 the at least one measurement report to the CU 172. In some implementations, the MN 106 generates at least one SN message including the at least one measurement report and transmits the at least one SN message to the CU 172 in the event 407. In some implementations, the at least one SN message include RRC Transfer message(s) and/or SN Modification Request message(s).

[0147] After (e.g., in response to) receiving the at least one measurement report or while the SN 104 communicates with the UE 102, the SN 104 determines to prepare the first cell for the UE 102, as described for Fig. 3. The events 490, 492, 494, 424, 426, 428, 429, 430, 431, 432, 434, 436, 496, 498, and 456 are similar to the events 390, 392, 394, 324, 326, 328, 329, 330, 331, 332, 334, 336, 396, 398, and 356, respectively. After receiving the first LTM command 430, transmitting the acknowledgement 431, or determining that the UE 102 successfully connects to the first cell 432 or 436, the UE 102 operating in DC with the MN 106 and SN 104 communicates 436 with the DU 174 on the first cell in accordance with the LTM configuration 1 and communicates 436 with the CU 172 via the DU 174, similar to the event 336. Later in time, in some implementations, the DU 174 and/or CU 172 performs the LTM execution procedure 498 with the UE 102 to command the UE 102 to perform a cell change from the first cell to the second cell, similar to the procedure 398. As a result of the procedure 498, the UE 102 operating in DC with the MN 106 and SN 104 communicates 456 with the DU 174 on the second cell in accordance with the LTM configuration 2 and communicates 456 with the CU 172 via the DU 174, similar to the event 356.

[0148] The events 404, 406, 405, 407, 490, 492, 494, 494, 424, 426, 428, 429, 430, 431, 432, 434, 436, 496, and 498 are collectively referred to in Fig. 4A as an LTM configuration and/or activation procedure 481.

[0149] Referring next to Fig. 4B, a scenario 400B is generally similar to the scenario 400A, except that the SN 104 transmits 417, 419 the RRC reconfiguration message to the UE 102 via the MN 106 and receives 421, 423 the RRC reconfiguration complete message from the UE 102 via the MN 106. The RRC reconfiguration message 417, 419 is similar to the RRC reconfiguration message 316, 318. The RRC reconfiguration complete message 421, 423 is similar to the RRC reconfiguration message 320, 322. In some implementations, the SN 104 generates a first SN message (e.g., SN Modification Required message, SN Modification Required message, or RRC Transfer message) including the RRC reconfiguration message and transmits the first SN message to the MN 106 in the event 417. The MN 106 generates an MN RRC message including the RRC reconfiguration message and transmits 419 the MN RRC message to the UE 102. In response, the UE 102 generates an MN RRC response message including the RRC reconfiguration complete message and transmits 421 the MN RRC response message to the MN 106. In some implementations, the MN 106 generates a second SN message (e.g., SN Reconfiguration Complete message or RRC Transfer message) including the RRC reconfiguration complete message and transmits the second SN message to the SN 104 in the event 423. In some implementations, the MN RRC message and MN RRC response message are an RRC reconfiguration message and an RRC reconfiguration complete message, respectively. [0150] The events 404, 406, 405, 407, 490, 492, 494, 417, 419, 421, 423, 424, 426, 428, 430, 431, 432, 434, 436, 496, 498 are collectively referred to in Fig. 4B as an LTM configuration and/or activation procedure 482.

[0151] Referring next to Fig. 5A, in a scenario 500A, the base station 104 operates as an MN and an SN, similar to the scenarios 300-400B. The base station 104 includes a CU 172, a master DU (M-DU) 174A and a secondary DU (S-DU) 174B. The CU 172 operates with the M-DU 174A as an MN, similar to the base station 104 in the Fig. 3 or the MN 106 in Figs. 4A-4B, and the CU 172 operates with the S-DU 174B as an SN, similar to the SN 104 in Figs. 4A-4B.

[0152] In the scenario 500A, the UE 102 initially communicates 502 in DC with the M-DU 174A and S-DU 174B and communicates 502 with the CU 172 via the M-DU 174A and S- DU 174B. In the event 502, the UE 102 communicates with the S-DU 174B on cell 124A using a serving DU configuration and communicates with the CU 172 via the S-DU 174B using a serving CU configuration, similar to the event 302. Events 504 and 506 are similar to the events 304 and 306. In some implementations, the UE 102 transmits 505 at least one measurement report to the M-DU 174A, similar to the event 304. The M-DU 174A in turn transmits 507 at least one DU-to-CU message including the at least one measurement report to the CU 172, similar to the event 306. In some implementations, while the UE 102 communicates in DC with the M-DU 174A and S-DU 174B, the CU 172 performs 580 an LTM configuration and/or activation procedure with the UE 102 via the M-DU 174A, similar to the procedure 380.

[0153] Referring next to Fig. 5B, a scenario 500B similar to the scenarios 300-400B and 500A, except that that the CU 172 transmits 517, 519 the RRC reconfiguration message to the UE 102 via the M-DU 174A and receives 521, 523 the RRC reconfiguration complete message from the UE 102 via the M-DU 174A. Events 517, 519, 521, and 523 are similar to events 417, 419, 421, and 423, respectively.

[0154] Next, several example methods, that a RAN node such as a DU or a CU can implement to enable LTM, are discussed with reference to Figs. 6-1 ID. Examples and implementations described for Figs. 3-5B can apply to Figs. 6-1 ID.

[0155] Fig. 6 illustrates an example method 600, which a DU (e.g., the DU 174 of the base station 104 or 106) can implement, for configuring LTM for a UE (e.g., the UE 102) and a CU (e.g., the CU 172 of the base station 104 or 106). [0156] The method 600 begins at block 602, where the DU communicates with the UE (e.g., events 302, 402, 502, 380, 480, 580). At block 604, the DU receives a first CU-to-DU message requesting LTM preparation for the UE from the CU (e.g., events 308, 390, 380, 480, 490, 580, 590). At block 606, the DU generates a reference LTM configuration (e.g., events 310, 390, 380, 480, 490, 580, 590). At block 608, the DU generates at least one first LTM configuration (i.e., non-reference LTM configuration) based on the reference LTM configuration (e.g., events 310, 390, 380, 480, 490, 580, 590). At block 610, the DU transmits a first DU-to-CU message, including the reference LTM configuration and at least one first LTM configuration to the CU (e.g., events 310, 390, 380, 480, 490, 580, 590). At block 612, the DU transmits the reference LTM configuration and at least one first LTM configuration to the UE (e.g., events 318, 394, 494, 594). In some alternative implementations, the DU transmits the reference LTM configuration and at least one first LTM configuration to the UE 102 via an MN or the CU (e.g., events 490, 417, 419, 590, 517, 519).

[0157] At block 614, the DU receives a second CU-to-DU message requesting LTM preparation for the UE from the CU (e.g., events 310, 390, 380, 480, 490, 580, 590). At block 616, the DU generates at least one second LTM configuration based on the reference LTM configuration (e.g., events 310, 390, 380, 480, 490, 580, 590). At block 618, the DU transmits a second DU-to-CU message, including the at least one second LTM configuration, to the CU (e.g., events 310, 390, 380, 480, 490, 580, 590). At block 620, the DU transmits the at least one second LTM configuration (i.e., non-reference LTM configuration) to the UE (e.g., events 318, 394, 494, 594). In some alternative implementations, the DU transmits the at least one second LTM configuration to the UE 102 via an MN or the CU (e.g., events 490, 417, 419, 590, 517, 519). In some implementations, the DU refrains from including the reference LTM configuration in the second DU-to-CU message, which prevents the CU from transmitting the reference LTM configuration to the UE. Thus, the UE, DU, and CU can save power by refraining from transmitting the reference LTM configuration.

[0158] In some implementations, the DU communicates with the UE using a first serving DU configuration, before and/or when receiving the first CU-to-DU message. In some implementations, the DU receives a first RRC message, including the reference LTM configuration and at least one first LTM configuration, from the CU and, at block 612, transmits the first RRC message to the UE using the first serving DU configuration. In some implementations, the DU communicates with the UE using the first serving DU configuration, before and/or when receiving the second CU-to-DU message. In such cases, the DU receives a second RRC message including the at least one second LTM configuration from the CU and, at block 620, transmits the second RRC message to the UE using the first serving DU configuration. In other implementations, the DU communicates with the UE using a second serving DU configuration, before and/or when receiving the second CU-to- DU message. In such cases, the DU receives a second RRC message including the at least one second LTM configuration from the CU and, at block 620, transmits the second RRC message to the UE using the second serving DU configuration. The second serving DU configuration includes configuration parameters different from the first serving DU configuration. Depending on the implementation, the second RRC message does not include the reference LTM configuration.

[0159] In some implementations, the first RRC message and second RRC message are RRC reconfiguration messages. In some implementations, the first serving DU configuration and second serving DU configuration are cell group configurations (e.g., CellGroupConfig IES). In other implementations, the first serving DU configuration and second serving DU configuration include configuration parameters in a CellGroupConfig IE (e.g., as defined in 3GPP TS 38.331).

[0160] In some implementations, the first CU-to-DU message includes an indication requesting LTM preparation, and the DU determines that the first CU-to-DU message requests LTM preparation based on the indication. In some implementations, the second CU- to-DU message includes an indication requesting LTM preparation, and the DU determines that the second CU-to-DU message requests LTM preparation based on the indication. In some implementations, the indication in the first CU-to-DU message and the indication in the second CU-to-DU message are each a new IE (e.g., defined in 3GPP TS 38.473 vl8.0.0 and/or later version). Lor example, the indication is an LTM cell list (e.g., a field/IE) including one or more cell IDs, each identifying a particular cell for LTM. In another example, the indication is a field/IE including the LTM cell list. In yet another example, the indication is an indicator specifically indicating that the first CU-to-DU message requests LTM preparation.

[0161] In some implementations, the first CU-to-DU message and first DU-to-CU message are a UE Context Modification Request message and UE Context Modification Response message, respectively. In some implementations, the second CU-to-DU message and second DU-to-CU message are a UE Context Modification Request message and UE Context Modification Response message, respectively.

[0162] Fig. 7 illustrates an example method 700, which a DU (e.g., the DU 174 of the base station 104 or 106) can implement, for configuring LTM for a UE (e.g., the UE 102) and a CU (e.g., the CU 172 of the base station 104 or 106).

[0163] The method 700 begins at block 702, where the DU communicates with the UE (e.g., events 302, 402, 502, 380, 480, 580). At block 704, the DU receives a CU-to-DU message for the UE from the CU (e.g., events 308, 390, 380, 480, 490, 580, 590). At block 706, the DU determines whether the CU-to-DU message requests LTM preparation. If the DU determines that the CU-to-DU message requests LTM preparation at block 706, the flow proceeds to block 708, where the DU generates a reference LTM configuration and includes the reference LTM configuration in a DU-to-CU message. At block 710, the DU generates at least one LTM configuration based on the reference LTM configuration and includes the at least one LTM configuration in the DU-to-CU message. At block 712, the DU transmits the DU-to-CU message to the CU (e.g., events 310, 308, 390, 380, 480, 490, 580, 590). At block 714, the DU transmits the reference LTM configuration and/or the at least one LTM configuration (i.e., non-reference LTM configuration) to the UE (e.g., events 318, 394, 494, 594).

[0164] Otherwise, if the DU determines that the CU-to-DU message does not request LTM preparation at block 706, the flow proceeds to block 716, where the DU generates a non- LTM configuration. At block 718, the DU transmits a DU-to-CU message, including the non-LTM configuration, to the CU. At block 720, the DU transmits the non-LTM configuration to the UE (e.g., events 318, 394, 494, 594). In some alternative implementations, the DU transmits the non-LTM configuration to the UE 102 via an MN or the CU (e.g., events 490, 417, 419, 590, 517, 519). After transmitting the non-LTM configuration to the UE, the DU communicates with the UE using the non-LTM configuration without transmitting an LTM command to the UE.

[0165] In some implementations, the DU determines whether the CU-to-DU message requests LTM preparation based on whether the CU-to-DU message includes an indication. If the CU-to-DU message includes the indication, the DU determines that the CU-to-DU message requests LTM preparation. Otherwise, if the CU-to-DU message does not include the indication, the DU determines that the CU-to-DU message does not request LTM preparation. For example, the indication is an LTM cell list (e.g., a field/IE) including one or more cell IDs, each identifying a particular cell for LTM. In another example, the indication is a field/IE including the LTM cell list. In yet another example, the indication is an indicator specifically indicating that the first CU-to-DU message requests LTM preparation.

[0166] In some implementations, the DU at block 702 communicates with the UE using a serving DU configuration. In some implementations, the DU at block 716 generates the non- LTM configuration as a delta configuration to augment the serving DU configuration.

[0167] Fig. 8 illustrates an example method 800, which a DU (e.g., the DU 174 of the base station 104 or 106) can implement, for configuring LTM for a UE (e.g., the UE 102) and a CU (e.g., the CU 172 of the base station 104 or 106).

[0168] The method 800 begins at block 802, where the DU communicates with the UE (e.g., events 302, 402, 502, 380, 480, 580). At block 804, the DU receives a CU-to-DU message to request LTM preparation for the UE from the CU (e.g., events 308, 390, 380, 480, 490, 580, 590). At block 806, the DU determines whether the DU has configured a reference LTM configuration for the UE (i.e., before receiving the CU-to-DU message). If the DU determines that the DU has not configured a reference LTM configuration for the UE at block 806, the flow proceeds to block 807, where the DU generates a reference LTM configuration. At block 808, the DU generates at least one LTM configuration based on the reference LTM configuration. At block 810, the DU transmits a DU-to-CU message, including the reference LTM configuration and the at least one LTM configuration, to the CU (e.g., events 310, 308, 390, 380, 480, 490, 580, 590). At block 812, the DU transmits the reference LTM configuration and the at least one LTM configuration (i.e., non-reference LTM configuration) to the UE (e.g., events 318, 394, 494, 594).

[0169] Otherwise, if the DU determines that the DU has not configured a reference LTM configuration for the UE at block 806, the flow proceeds to block 809, where the DU generates at least one LTM configuration (i.e., non-reference LTM configuration) based on the reference LTM configuration. At block 814, the DU transmits a DU-to-CU message, including the at least one LTM configuration, to the CU. In some such cases, the DU refrains from including the reference LTM configuration in the DU-to-CU message. At block 816, the DU transmits the at least one LTM configuration to the UE (e.g., events 318, 394, 494, 594). In some alternative implementations, the DU transmits the at least one LTM configuration to the UE 102 via an MN or the CU (e.g., events 490, 417, 419, 590, 517, 519). In some implementations, the DU refrains from including the reference LTM configuration in the DU-to-CU message of block 812, which prevents the CU from transmitting the reference LTM configuration to the UE. Thus, the UE, DU and CU can save power by refraining from transmitting the reference LTM configuration.

[0170] Examples and implementations described for Figs. 6 can apply to Figs. 7 and 8.

[0171] Fig. 9 illustrates a method 900, which a CU (e.g., the CU 172 of the base station 104 or 106) can implement, for configuring LTM for a UE (e.g., the UE 102).

[0172] The method 900 begins at block 902, where the CU communicates with a UE via a DU (e.g., events 302, 402, 502, 380, 480, 580). At block 904, the CU transmits a first CU-to- DU message, requesting at least one LTM configuration for the UE, to the DU (e.g., events 308, 390, 380, 480, 490, 580, 590). At block 906, the CU receives, from the DU, a first DU- to-CU message including a first reference LTM configuration and at least one first LTM DU configuration (i.e., non-reference LTM configuration) from the DU (e.g., events 310, 390, 380, 480, 490, 580, 590). At block 908, the CU transmits the reference LTM configuration and at least one first LTM configuration to the UE via the DU (e.g., events 318, 394, 494, 594). In some alternative implementations, the CU transmits the reference LTM configuration and at least one first LTM configuration to the UE via an MN or another DU (e.g., events 490, 417, 419, 590, 517, 519).

[0173] At block 910, the CU transmits a second CU-to-DU message requesting at least one LTM configuration for the UE to the DU (e.g., events 308, 390, 380, 480, 490, 580, 590). At block 912, the CU receives, from the DU, a second DU-to-CU message, including at least one second LTM DU configuration (i.e., non-reference LTM configuration), from the DU (e.g., events 310, 390, 380, 480, 490, 580, 590). At block 914, the CU transmits the at least one second LTM configuration to the UE via the DU (e.g., events 318, 394, 494, 594). In some alternative implementations, the CU transmits the at least one second LTM configuration to the UE via an MN or another DU (e.g., events 490, 417, 419, 590, 517, 519).

[0174] In some implementations, the CU in Fig. 9 is the CU described in Fig. 6, and examples and implementations described for Figs. 6 can apply to Fig. 9.

[0175] In some implementations, the reference LTM configuration, non-reference LTM configuration, and non-LTM configuration are configured in the same format (i.e., the DU configuration or CellGroupConfig RRC IE). When the DU transmits a DU configuration to a CU, the CU does not know that the received DU configuration is a reference LTM configuration, non-reference LTM configuration, or a non-LTM configuration. Figs. 10A- 1 ID illustrate example methods to indicate that a DU configuration is a reference LTM configuration, non-reference LTM configuration, or a non-LTM configuration. Thus, the CU can determine that a received DU configuration is a reference LTM configuration, non- reference LTM configuration, or a non-LTM configuration accordingly.

[0176] Fig. 10A illustrates an example method 1000A, which a DU (e.g., the DU 174 of the base station 104 or 106) can implement, for configuring LTM for a UE (e.g., the UE 102) and a CU (e.g., the CU 172 of the base station 104 or 106).

[0177] The method 1000A begins at block 1002, where the DU communicates with the UE (e.g., events 302, 402, 502, 380, 480, 580). At block 1004, the DU determines to transmit a DU configuration for the UE. At block 1006, the DU determines whether the DU configuration is a reference LTM configuration. If the DU determines that the DU configuration is a reference LTM configuration at block 1006 (i.e., the DU generates the DU configuration as a reference LTM configuration), the flow proceeds to block 1008, where the DU includes the reference LTM configuration in a first field/IE in a DU-to-CU message (e.g., events 310, 308, 390, 380, 480, 490, 580, 590). Otherwise, if the DU determines that the DU configuration is not a reference LTM configuration at block 1006 (i.e., the DU generates the DU configuration not as a reference LTM configuration), the flow proceeds to block 1010. At block 1010, the DU includes the DU configuration in a second field/IE in the DU-to-CU message (e.g., events 310, 308, 390, 380, 480, 490, 580, 590). The flow proceeds to block 1012 from block 1008 as well as block 1010. At block 1012, the DU transmits the DU-to-CU message to the CU (e.g., events 310, 308, 390, 380, 480, 490, 580, 590). At block 1014, the DU transmits the DU configuration to the UE (e.g., events 318, 394, 494, 594).

Alternatively, the DU transmits the DU configuration to the UE via an MN or the CU (e.g., events 490, 417, 419, 590, 517, 519).

[0178] In some implementations, the first field/IE and second field/IE are Fl application protocol (AP) fields/IEs. In some implementations, the first field/IE is a new F1AP field/IE (e.g., defined in 3GPP TS 38.473 vl8.0.0 and/or later version).

[0179] In some implementations where the DU configuration is not a reference LTM configuration, the DU at block 1006 determines that the DU configuration is a non-LTM configuration. In such cases, the DU includes the non-LTM configuration in the second field/IE at block 1010. In some implementations, the second field/IE is a DU to CU RRC Information F1AP IE (e.g., defined in 3GPP TS 38.473). In other implementations, the second field/IE is a CellGroupConfig F1AP field or CellGroupConfig F1AP IE (e.g., defined in 3GPP TS 38.473).

[0180] In some implementations where the DU configuration is not a reference LTM configuration, the DU at block 1006 determines that the DU configuration is a non-reference LTM configuration. In such cases, the DU includes the non-reference LTM configuration in the second field/IE at block 1010. In some implementations, the second field/IE is a DU to CU RRC Information F1AP IE (e.g., defined in 3GPP TS 38.473). In other implementations, the second field/IE is a CellGroupConfig F1AP field or CellGroupConfig F1AP IE (e.g., defined in 3GPP TS 38.473). In some implementations, the DU includes an indication (e.g., a F1AP field/IE) in the DU to CU RRC Information IE to indicate that the DU configuration is a non-reference LTM configuration. In yet other implementations, the second field/IE is a new F1AP field/IE (e.g., defined in 3GPP TS 38.473 vl8.0.0 and/or later version). In some alternative implementations where the DU configuration is not a reference LTM configuration, the DU at block 1006 determines that the DU configuration is a non-reference LTM configuration. In such cases, the DU includes the non-reference LTM configuration in the first field/IE at block 1008.

[0181] In some implementations, the DU configuration is a CellGroupConfig RRC IE (e.g., defined in 3GPP TS 38.331).

[0182] Fig. 10B illustrates an example method 1000B similar to the method 1000A, except that the method 1000B includes block 1005 instead of block 1006. At block 1005, the DU determines whether the DU configuration is a non-reference LTM configuration. If the DU determines that the DU configuration is a non-reference LTM configuration at block 1005 (i.e., the DU generates the DU configuration as a non-reference LTM configuration), the flow proceeds to block 1008. Otherwise, if the DU determines that the DU configuration is not a non-reference LTM configuration at block 1005 (i.e., the DU generates the DU configuration not as a non-reference LTM configuration), the flow proceeds to block 1010.

[0183] In some implementations where the DU configuration is not a non-reference LTM configuration, the DU at block 1005 determines that the DU configuration is a non-LTM configuration. In such cases, examples and implementations described for Fig 10A can be applied. [0184] In some implementations where the DU configuration is not a non-reference LTM configuration, the DU at block 1006 determines that the DU configuration is a reference LTM configuration. In such cases, the DU includes the reference LTM configuration in the second field/IE at block 1010. In such cases, examples and implementations for the second field/IE as described for Fig. 6A can be applied. In some implementations, the DU includes an indication (e.g., a F1AP field/IE) in the DU to CU RRC Information IE to indicate that the DU configuration is a reference LTM configuration.

[0185] Fig. 10C illustrates an example method 1000C similar to the method 1000A, except that the method 1000C includes blocks 1007 and 1013 instead of block 1006. At block 1007, the DU determines whether the DU configuration is a reference LTM configuration, a nonreference LTM configuration, or non-LTM configuration. If the DU determines that the DU configuration is a reference LTM configuration at block 1007 (i.e., the DU generates the DU configuration as a reference LTM configuration), the flow proceeds to block 1008.

Otherwise, if the DU determines that the DU configuration is a non-LTM configuration (i.e., the DU generates the DU configuration as a non-LTM configuration), the flow proceeds to block 1010. Otherwise, if the DU determines that the DU configuration is a non-reference LTM configuration (i.e., the DU generates the DU configuration as a non-reference LTM configuration), the flow proceeds to block 1013. At block 1013, the DU includes the DU configuration in a third field/IE in the DU-to-CU message. The flow proceeds to block 1012 from block 1013.

[0186] In some implementations, the first field/IE, second field/IE, and third field/IE are F1AP fields/IEs. In some implementations, the first field/IE is a new F1AP field/IE (e.g., defined in 3GPP TS 38.473 vl8.0.0 and/or later version).

[0187] In some implementations, the second field/IE is a DU to CU RRC Information F1AP IE (e.g., defined in 3GPP TS 38.473), and the DU includes an indication (e.g., a F1AP field/IE) in the DU to CU RRC Information IE to indicate that the DU configuration is a non- reference LTM configuration. In other implementations, the second field/IE is a CellGroupConfig F1AP field or CellGroupConfig F1AP IE (e.g., defined in 3GPP TS 38.473). In some implementations, the third field/IE is a new F1AP field/IE (e.g., defined in 3GPP TS 38.473 V18.0.0 and/or later version). [0188] Fig. 11A illustrates an example method 1100A, which a DU (e.g., the DU 174 of the base station 104 or 106) can implement, for configuring LTM for a UE (e.g., the UE 102) and a CU (e.g., the CU 172 of the base station 104 or 106).

[0189] The method 1100A begins at block 1102, where the DU communicates with the UE (e.g., events 302, 402, 502, 380, 480, 580). At block 1104, the DU determines to transmit at least one DU configuration for the UE. At block 1106, the DU determines whether the at least one DU configuration includes a reference LTM configuration. If the DU determines that the at least one DU configuration includes a reference LTM configuration at block 1106, the flow proceeds to block 1108 A, where the DU includes the reference LTM configuration in a first field/IE in a DU-to-CU message (e.g., events 310, 308, 390, 380, 480, 490, 580, 590). Otherwise, if the DU determines that the DU configuration does not include a reference LTM configuration at block 1106, the flow proceeds to block 1110A. At block 1110A, the DU determines whether the at least one DU configuration includes a non-LTM configuration and/or non-reference LTM configuration(s). If the DU determines that the at least one DU configuration includes a non-LTM configuration and/or non-reference LTM configuration(s) at block 1110A, the flow proceeds to block 1112A, where the DU includes the non-LTM configuration and/or non-reference LTM configuration(s) in a second field/IE in the DU-to-CU message (e.g., events 310, 308, 390, 380, 480, 490, 580, 590). The flow proceeds to block 1114 from block 1112A. Otherwise, if the DU determines that the at least one DU configuration neither includes the non-LTM configuration nor the non-reference LTM configuration(s) at block 1110A, the flow proceeds to block 1114. At block 1114, the DU transmits the DU-to-CU message to the CU (e.g., events 310, 308, 390, 380, 480, 490, 580, 590). At block 1116, the DU transmits the at least one DU configuration to the UE (e.g., events 318, 394, 494, 594). Alternatively, the DU transmits the at least one DU configuration to the UE via an MN or the CU (e.g., events 490, 417, 419, 590, 517, 519).

[0190] Eig. 1 IB illustrates an example method 1100B similar to the method 1100A, except that the method 1100B includes blocks 1105, 1108B, 1110B, and 1112B instead of blocks 1106, 1108A, 1110A, and 1112A. At block 1105, the DU determines whether the at least one DU configuration includes a non-reference LTM configuration. If the DU determines that the at least one DU configuration includes non-reference LTM configuration(s) at block 1105, the flow proceeds to block 1108B. At block 1108B, the DU includes the non-reference LTM configuration(s) in a first field/IE in a DU-to-CU message. The flow then proceeds to block 1110B from block 1108B. At block 1110B, the DU determines whether the at least one DU configuration includes a non-LTM configuration and/or reference LTM configuration(s). If the DU determines that the at least one DU configuration includes a non-LTM configuration and/or reference LTM configuration(s) at block 1110B, the flow proceeds to block 1112B. Otherwise, if the DU determines that the at least one DU configuration neither includes a non-LTM configuration nor a reference LTM configuration at block 1110B, the flow proceeds to block 1114. Otherwise, if the DU determines that the at least one DU configuration does not include a non-reference LTM configuration at block 1105, the flow proceeds to block 1112B. At block 1112B, the DU includes the non-LTM configuration and/or reference LTM configuration(s) in a second field/IE in the DU-to-CU message. The flow proceeds to block 1114 from block 1112B.

[0191] Fig. 11C illustrates an example method 1100C similar to the method 1100A, except that the method 1100C includes blocks 1107, 1108C, 1110C, and 1112C instead of blocks 1106, 1108A, 1110A, and 1112A. At block 1107, the DU determines whether the at least one DU configuration includes a reference LTM configuration and/or non-reference LTM configuration(s). If the DU determines that the at least one DU configuration includes a reference LTM configuration and/or non-reference LTM configuration(s) at block 1107, the flow proceeds to block 1108C. At block 1108C, the DU includes the reference LTM configuration and/or the non-reference LTM configuration(s) in a first field/IE in a DU-to- CU message. The flow then proceeds to block 1110C from block 1108C. At block 1110C, the DU determines whether the at least one DU configuration includes a non-LTM configuration. If the DU determines that the at least one DU configuration includes a non- LTM configuration at block 1110C, the flow proceeds to block 1112C. Otherwise, if the DU determines that the at least one DU configuration does not include a non-LTM configuration at block 1110C, the flow proceeds to block 1114. Otherwise, if the DU determines that the at least one DU configuration neither includes a reference LTM configuration nor a non- reference LTM configuration at block 1107, the flow proceeds to block 1112C. At block 1112C, the DU includes the non-LTM configuration in a second field/IE in the DU-to-CU message. The flow proceeds to block 1114 from block 1112C.

[0192] Fig. 1 ID illustrates an example method 1100D similar to the methods 1100A, 1100B and 1100C, except that the method 1100D includes block 1113. If the DU determines that the at least one DU configuration includes a reference LTM configuration at block 1106, the flow proceeds to block 1108A. The flow then proceeds to block 1110C from block 1108A. Otherwise, if the DU determines that the at least one DU configuration does not include a reference LTM configuration at block 1106, the flow proceeds to block 1110C. If the DU determines that the at least one DU configuration does not include a non-LTM configuration at block 1110C, the flow proceeds to block 1105. If the DU determines that the DU includes a non-LTM configuration, the flow proceeds to block 1112C. At block 1105, the DU determines whether the at least one DU configuration includes non-reference LTM configuration(s). If so, the flow proceeds to block 1113, where the DU includes the nonreference LTM configuration(s) in a third field/IE in the DU-to-CU message. The flow then proceeds to block 1114 from block 1113. Otherwise, if the DU determines that the DU does not include a non-reference LTM configuration, the flow proceeds to block 1114.

[0193] Examples and implementations described for Figs. 10A-10C can apply to Figs. 11A-11D.

[0194] The following list of examples reflects a variety of the embodiments explicitly contemplated by the present disclosure:

[0195] Example 1. A method implemented in a distributed unit (DU) of a distributed base station, the method comprising: receiving, at the DU from a centralized unit (CU) of the distributed base station, a request for lower layer triggered mobility preparation for a user equipment (UE); generating, at the DU, a reference lower layer triggered mobility configuration; and transmitting, from the DU to the CU, the reference lower layer triggered mobility configuration for the UE in response to the request.

[0196] Example 2. The method of example 1, wherein the request is a first request for lower layer triggered mobility preparation, further comprising: receiving, at the DU, a second request for lower layer triggered mobility preparation; and transmitting, from the DU to the CU, a non-reference lower layer triggered mobility configuration in response to the request.

[0197] Example 3. The method of example 2, further comprising: generating, at the DU, the non-reference lower layer triggered mobility configuration.

[0198] Example 4. The method of example 3, wherein: generating the non-reference lower layer triggered mobility configuration is based on the reference lower layer triggered mobility configuration.

[0199] Example 5. The method of any one of examples 2-4, wherein: the second request includes a lower layer triggered mobility indicator to request lower layer triggered mobility preparation. [0200] Example 6. The method of any one of examples 2-5, wherein: the second request includes a lower layer triggered mobility identifier to associate with a lower layer triggered mobility configuration.

[0201] Example 7. The method of example 6, further comprising: associating, at the DU, the identifier with the non-reference lower layer triggered mobility configuration.

[0202] Example 8. The method of any one of examples 2-7, further comprising: receiving, at the DU, a third request including a lower layer triggered mobility identifier to associate with a lower layer triggered mobility configuration.

[0203] Example 9. The method of example 8, further comprising: associating, at the DU, the identifier with the reference lower layer triggered mobility configuration.

[0204] Example 10. The method of any one of examples 2-9, wherein: the second request includes the reference lower layer triggered mobility configuration.

[0205] Example 11. The method of any one of the preceding examples, wherein: the request includes a lower layer triggered mobility indicator to request lower layer triggered mobility preparation.

[0206] Example 12. A method implemented in a centralized unit (CU) of a distributed base station, the method comprising: transmitting, from the CU to a distributed unit (DU) of the distributed base station, a request for lower layer triggered mobility preparation; receiving, at the CU, a reference lower layer triggered mobility configuration in response to the request; and transmitting, from the CU to a user equipment (UE), a lower layer triggered mobility configuration based on the reference lower layer triggered mobility configuration.

[0207] Example 13. The method of example 12, wherein the request is a first request for lower layer triggered mobility preparation, further comprising: transmitting, from the CU to the DU, a second request for lower layer triggered mobility preparation; and receiving, at the CU, a non-reference lower layer triggered mobility configuration in response to the request.

[0208] Example 14. The method of example 13, further comprising: transmitting, from the CU to the UE, the non-reference lower layer triggered mobility configuration.

[0209] Example 15. The method of example 13 or 14, wherein: the second request includes a lower layer triggered mobility indicator to request lower layer triggered mobility preparation. [0210] Example 16. The method of any one of examples 13-15, wherein: the second request includes a lower layer triggered mobility identifier to associate with a lower layer triggered mobility configuration.

[0211] Example 17. The method of example 16, further comprising: transmitting, from the CU to the UE, the lower layer triggered mobility identifier to the UE.

[0212] Example 18. The method of any one of examples 13-17, further comprising: transmitting, from the CU to the DU, a third request including a lower layer triggered mobility identifier to associate with a lower layer triggered mobility configuration.

[0213] Example 19. The method of example 18, further comprising: transmitting, from the CU to the UE, the lower layer triggered mobility identifier to the UE.

[0214] Example 20. The method of any one of examples 13-19, wherein: the second request includes the reference lower layer triggered mobility configuration.

[0215] Example 21. The method of any one of examples 12-20, wherein: the request includes a lower layer triggered mobility indicator to request lower layer triggered mobility preparation.

[0216] Example 22. The method of any one of examples 12-21, further comprising: refraining, at the CU, from assigning an identifier to the reference lower layer triggered mobility configuration.

[0217] Example 23. The method of example 1, further comprising: transmitting, from the DU to the UE, the non-reference LTM configuration; and transmitting, from the DU to the UE, an LTM command to initiate a cell change according to the non-reference LTM configuration.

[0218] Example 24. The method of example 2, wherein: the reference LTM configuration is transmitted in a same message as the non-reference LTM configuration.

[0219] Example 25. The method of example 12, wherein: the reference LTM configuration is received in a same message as the non-reference LTM configuration.

[0220] Example 26. The method of example 24 or 25, wherein the same message is a UE context modification response.

[0221] Example 27. A distributed base station comprising processing hardware and configured to implement a method according to any one of the preceding examples. [0222] The following description may be applied to the description above.

[0223] 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. In some implementations, “message” is used and can be replaced by “information element (IE)”, and vice versa. In some implementations, “IE” is used and can be replaced by “field”, and vice versa. In some implementations, “configuration” can be replaced by “configurations” or “configuration parameters”, and vice versa. In some implementations, the “LTM command” can be replaced by “serving cell change command”, “Layer 1/Layer 2 switching command”, “lower layer switching command” or “lower layer serving cell change command”. In some implementations, “some” means “one or more”. In some implementations, “at least one” means “one or more”. In some implementations, the “DU configuration” can be replaced by “cell group configuration”.

[0224] A user device in which the techniques of this disclosure can be implemented (e.g., the UE 102) can be any suitable device capable of wireless communications such as a smartphone, a tablet computer, a laptop computer, a mobile gaming console, a point-of-sale (POS) terminal, a health monitoring device, a drone, a camera, a media- streaming dongle or another personal media device, a wearable device such as a smartwatch, a wireless hotspot, a femtocell, or a broadband router. Further, the user device in some cases may be embedded in an electronic system such as the head unit of a vehicle or an advanced driver assistance system (ADAS). Still further, the user device can operate as an internet-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.

[0225] 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.

[0226] 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.

[0227] Upon reading this disclosure, those of skill in the art will appreciate still additional and alternative structural and functional designs for handling mobility between base stations through the principles disclosed herein. Thus, while particular embodiments and applications have been illustrated and described, it is to be understood that the disclosed embodiments are not limited to the precise construction and components disclosed herein. Various modifications, changes and variations, which will be apparent to those of ordinary skill in the art, may be made in the arrangement, operation and details of the method and apparatus disclosed herein without departing from the spirit and scope defined in the appended claims.

Claims

What is claimed is:

1. A method implemented in a distributed unit (DU) of a distributed base station, the method comprising: receiving, at the DU from a centralized unit (CU) of the distributed base station, a request for lower layer triggered mobility (LTM) preparation for a user equipment (UE); generating, at the DU, a reference LTM configuration; and transmitting, from the DU to the CU, the reference LTM configuration for the UE in response to the request.

2. The method of claim 1, wherein the request is a first request for LTM preparation, further comprising: receiving, at the DU, a second request for LTM preparation; and transmitting, from the DU to the CU, a non-reference LTM configuration in response to the second request.

3. The method of claim 1, wherein: the reference LTM configuration is transmitted in a same message as a non-reference LTM configuration based on the reference LTM configuration.

4. The method of claim 3, wherein the same message is a UE context modification response message.

5. The method of any one of claims 2-4, further comprising: transmitting, from the DU to the UE, the non-reference LTM configuration; and transmitting, from the DU to the UE, an LTM command to initiate a cell change according to the non-reference LTM configuration.

6. The method of any one of claims 2-4, further comprising: generating, at the DU, the non-reference LTM configuration based on the reference LTM configuration.

7. The method of any one of claims 2-6, wherein: at least one of the first request or the second request includes an LTM indicator to request LTM preparation.

8. The method of any one of claims 2-7, wherein: at least one of the first request or the second request includes an LTM identifier to associate with an LTM configuration.

9. A method implemented in a centralized unit (CU) of a distributed base station, the method comprising: transmitting, from the CU to a distributed unit (DU) of the distributed base station, a request for lower layer triggered mobility (LTM) preparation; receiving, at the CU, a reference LTM configuration in response to the request; and transmitting, to the DU for a user equipment (UE), an LTM configuration based on the reference LTM configuration.

10. The method of claim 9, wherein the request is a first request for LTM preparation, further comprising: transmitting, from the CU to the DU, a second request for LTM preparation; and receiving, at the CU, a non-reference LTM configuration in response to the second request; wherein the LTM configuration based on the reference LTM configuration is the nonreference LTM configuration.

11. The method of claim 9, wherein: the reference LTM configuration is received in a same message as a non-reference LTM configuration based on the reference LTM configuration.

12. The method of claim 11, wherein the same message is a UE context modification response message.

13. The method of claim 9, wherein: the LTM configuration based on the reference LTM configuration is the reference LTM configuration.

14. The method of any one of claims 9-13, further comprising: refraining, at the CU, from assigning an identifier to the reference LTM configuration.

15. A distributed base station comprising processing hardware and configured to implement a method according to any one of the preceding claims.