Enhanced Layer 1-Layer 2 (L1 / L2) Triggered Mobility (LTM) Handover (HO) Procedure

Abstract

A Layer 1-Layer 2 (L1 / L2) triggered mobility (LTM) handover (HO) procedure and method is described. The gNB-centralized unit user plane (gNB-CU-CP) transmits a radio resource control (RRC) message with an LTM candidate cell configuration to the LTM candidate gNB-DU. Based on the RRC message, a Layer (L1) measurement report is transmitted to the LTM candidate gNB-DU. In response, the LTM candidate gNB-DU determines whether the radio conditions of the LTM candidate gNB-DU satisfy predetermined resource reservation criteria. Based on the satisfaction of the predetermined resource reservation criteria, the LTM candidate gNB-DU initiates reservation of radio resources in the LTM candidate cell for LTM cell switch. The LTM candidate gNB-DU transmits the updated LTM candidate cell configuration to the gNB-CU-CP. The gNB-CU-CP transmits the updated radio resource configuration to the UE via the serving gNB-DU.

Description

[Technical Field]

[0001] This description relates to a Layer 1-Layer 2 (L1 / L2) triggered mobility (LTM) handover (HO) procedure and its use. [Background technology]

[0002] Handover (mobility) is the process of transferring an ongoing communication session of a user equipment (UE) from one cell (i.e., base station or gNodeB (gNB)) to another cell in a connected state. The main motivation behind handover is to ensure seamless connectivity and continuity of service for a user, especially while the user is moving. Mobility can be classified into two types: beam-level mobility and cell-level mobility.

[0003] Beam-level mobility does not rely on explicit radio resource control (RRC) signaling to be triggered. Triggering can occur within a cell or between cells (i.e., Inter-Cell Beam Management (ICBM)). The gNB provides the UE with measurement configurations to trigger channel and interference measurements and reporting. Beam-level mobility is then handled at lower layers by Physical Layer (PHY) and Medium-Access Control (MAC) layer control signaling, and the UE does not rely on explicit RRC signaling to change to the target beam.

[0004] In handover types up to 3rd Generation Partnership Project (3GPP®) Release 17, serving cell changes are triggered by Layer 3 (L3) measurements and are achieved using RRC signaling (i.e., reconfiguration with synchronization information elements) for Primary Cell (PCell) and Primary Secondary Cell (PSCell) changes. Higher layer (e.g., RRC or Packet Data Convergence Protocol (PDCP)) reconfiguration and / or lower layer (e.g., MAC and / or PHY) reset are used, resulting in longer latency, more overhead, and longer interruption times than beam-level mobility.

[0005] Release 18 introduced Layer 1 / Layer 2 (L1) / L2 Triggered Mobility (LTM) to enable serving cell change via L1 / L2 signaling while maintaining higher layer configuration and / or minimizing lower layer configuration changes. LTM helps reduce latency, overhead, and disruption during handover. LTM supports intra-Distributed Unit (DU) and intra-Centralized Unit (CU)-to-DU mobility. During LTM, the user plane continues in the target cell whenever possible (e.g., within the DU) without resetting to avoid data loss and further delays in data recovery. Also, security updates are not performed during intra-NG-RAN node LTM.

[0006] In L1 / L2 triggered mobility (LTM), the gNB receives L1 measurement reports from the UE, and based on them, the gNB changes the UE's serving cell via MAC CE. The gNB prepares one or more candidate cells and provides the candidate cell configurations to the UE via RRC messages. A target cell is the candidate cell selected for LTM cell switching. In this specification, the terms candidate cell (c) and target cell are used interchangeably. Then, an LTM cell switch is triggered by selecting one of the candidate configurations as the target configuration for LTM by the gNB. Candidate cell configurations can be added, modified, and released by the network via RRC signaling.

[0007] Nevertheless, the duration between LTM handover (HO) preparation and execution is significant. In addition, not all target candidate cell configurations result in an LTM serving cell change. Therefore, resource reservation (including the Physical Random Access Channel (PRACH) preamble) is an issue.

[0008] In legacy baseline NR L3 mobility, the UE acquires a timing advance (TA) toward the target cell and completes uplink (UL) synchronization via a RACH procedure. Contention-free random access (CFRA) involves a four-step RACH process, while contention-based random access (CBRA) involves a two-step RACH process. The UE can complete the RACH procedure in two steps (Msg1+Msg2 or MsgA+MsgB), which is faster than CBRA in a four-step RACH (Msg1-Msg4). However, whether using CFRA in a four-step RACH or CBRA in a two-step RACH, the UE still waits for an available PRACH opportunity, transmits a preamble, and waits for a RAR. The time for the RACH procedure is approximately 10-20 milliseconds.

[0009] To reduce the interruption time during handover, RACH-less solutions have been investigated. However, RACH-less HO may not be feasible, for example, in response to the timing advance (TA) obtained from the candidate / target cell being out of date or not valid. If the TA of the candidate / target cell cannot be obtained, RACH-less HO is also not possible.

[0010] The second problem involves data transfer, which currently is slow and involves significant overhead, for example, data transfer to multiple target cells is expensive in terms of processing and buffer occupancy. Summary of the Invention [Means for solving the problem]

[0011] In at least some embodiments, a method for preparing and performing a Layer 1-Layer 2 (L1 / L2) triggered mobility (LTM) handover (HO) procedure includes receiving an L1 measurement report at one or more LTM candidate gNB distributed units (gNB-DUs) directly from a user equipment (UE) or from a serving gNB-DU, the L1 measurement report being based on one or more radio resource control (RRC) messages with LTM candidate cell configurations of the one or more LTM candidate gNB-DUs previously transmitted to the UE via a serving cell; determining, by the one or more LTM candidate gNB-DUs in response to receiving the L1 measurement report, whether radio conditions of one of the one or more LTM candidate gNB-DUs satisfy predetermined resource reservation criteria; and The method includes initiating, by one or more LTM candidate gNB-DUs, reservation of partial or full radio resources in one of the one or more LTM candidate cells for an LTM cell switch by the UE from a serving cell to one of the one or more LTM candidate cells based on the radio conditions of the M candidate gNB-DUs satisfying predetermined resource reservation criteria or based on the UE performing uplink (UL) synchronization by sending a random access channel (RACH) request to the one or more LTM candidate gNB-DUs to obtain timing advance, and transmitting an updated radio resource configuration in the candidate / target cell having partial or full radio resources of the one or more LTM candidate cells to the UE via the serving gNB-DU.

[0012] In at least one embodiment, a Next Generation Radio Access Network (NG-RAN) node includes a candidate distributed unit (DU), the candidate DU including: a memory storing computer-readable instructions; and a processor coupled to the memory, the processor executing the computer-readable instructions to receive Layer 1 (L1) measurement reports at one or more Layer 1 Layer 2 (L1 / L2) triggered mobility (LTM) candidate gNB-distributed units (gNB-DUs) directly from a user equipment (UE) or from a serving gNB-DU, the L1 measurement reports being based on one or more Radio Resource Control (RRC) messages with LTM candidate cell configurations of the one or more LTM candidate gNB-DUs previously transmitted to the UE via a serving cell, and in response to receiving the L1 measurement reports, determine whether a radio resource state of one of the one or more LTM candidate gNB-DUs satisfies a predetermined resource reservation criterion. and based on the radio conditions of one of the one or more LTM candidate gNB-DUs satisfying a predetermined resource reservation criterion or based on the UE performing uplink (UL) synchronization by sending a random access channel (RACH) request to the one or more candidate gNB-DUs to obtain timing advances of the one or more LTM candidate cells, initiate reservation of partial or full radio resources in one of the one or more LTM candidate cells using the one or more LTM candidate gNB-DUs for an LTM cell switch by the UE from the serving cell to one of the one or more LTM candidate cells, and transmit an updated radio resource configuration in the one or more candidate cells having the partial or full radio resources of the one or more LTM candidate cells to the UE via the serving gNB-DU.

[0013] In at least one embodiment, a non-transitory computer-readable medium having stored thereon computer-readable instructions that, when executed by a processor, include operations of receiving a Layer 1 (L1) measurement report at one or more LTM candidate gNB distributed units (gNB-DUs) directly from a user equipment (UE) or from a serving gNB-DU, where the L1 measurement report is based on one or more radio resource control (RRC) messages with LTM candidate cell configurations of the one or more LTM candidate gNB-DUs previously transmitted to the UE via the serving cell; determining, by the one or more LTM candidate gNB-DUs in response to receiving the L1 measurement report, whether radio conditions of one of the one or more LTM candidate gNB-DUs satisfy predetermined resource reservation criteria; and The processor performs the following operations: based on the radio conditions of the LTM candidate gNB-DU satisfying predetermined resource reservation criteria or based on the UE performing uplink (UL) synchronization by sending a random access channel (RACH) request to the one or more LTM candidate gNB-DUs to obtain timing advance, initiate reservation of partial or full radio resources in one of the one or more LTM candidate cells by the one or more LTM candidate gNB-DUs for an LTM cell switch by the UE from a serving cell to one of the one or more LTM candidate cells; and transmit an updated radio resource configuration in the one or more candidate cells having partial or full radio resources of the one or more LTM candidate cells to the UE via the serving gNB-DU.

[0014] In at least one embodiment, a method for preparing and performing a lower layer triggered mobility (LTM) handover (HO) procedure includes, at a candidate gNB distributed unit (gNB-DU), receiving a Layer 1 (L1) measurement report in one or more LTM candidate cells; determining, by the candidate gNB-DU, whether radio conditions of one of the one or more LTM candidate cells satisfy a predetermined resource reservation criterion based on the L1 measurement report; determining, based on the radio conditions of one of the one or more LTM candidate cells satisfying the predetermined resource reservation criterion or based on the UE performing uplink (UL) synchronization by transmitting a random access channel (RACH) request to the one or more candidate gNB-DUs to obtain timing advance uplink (UL) synchronization, reserved partial or full radio resources in one of the one or more LTM candidate cells for an LTM cell switch by the UE to one of the one or more LTM candidate cells; and transmitting, by the candidate gNB-DU, an updated LTM candidate cell configuration corresponding to the reserved partial or full radio resources to a gNB-centralized unit control plane (gNB-CU-CP).

[0015] In at least one embodiment, a Next Generation Radio Access Network (NG-RAN) node comprises a candidate gNB distributed unit (gNB-DU), the gNB-DU including: a memory storing computer readable instructions; and a processor coupled to the memory, the processor configured to execute the computer readable instructions on the memory storing the computer readable instructions, and the processor coupled to the memory, the processor executing the computer readable instructions to perform operations of receiving Layer 1 (L1) measurement reports at one or more LTM candidate cells; determining, based on the L1 measurement reports, whether radio conditions of one of the one or more LTM candidate cells meet predetermined resource reservation criteria; and The gNB-CU-DU is configured to perform an operation of determining reserved partial or full radio resources in one of the one or more LTM candidate cells for an LTM cell switch by the UE to one of the one or more LTM candidate cells based on radio conditions of one of the candidate cells satisfying a predetermined resource reservation criterion or based on the UE performing uplink (UL) synchronization by transmitting a random access channel (RACH) request to the one or more candidate gNB-DUs to obtain timing advance uplink (UL) synchronization, and an operation of transmitting an updated LTM candidate cell configuration corresponding to the reserved partial or full radio resources to a gNB-centralized unit control plane (gNB-CU-CP).

[0016] In at least one embodiment, a non-transitory computer-readable medium having stored thereon computer-readable instructions that, when executed by a processor, include operations of receiving, at a candidate gNB distributed unit (gNB-DU), a Layer 1 (L1) measurement report for one or more LTM candidate cells; determining, by the candidate gNB-DU, whether radio conditions of one of the one or more LTM candidate cells meet a predetermined resource reservation criterion based on the L1 measurement report; and determining, based on the radio conditions of one of the one or more LTM candidate cells meeting the predetermined resource reservation criterion or timing advance up. The method causes the processor to perform the following operations: based on the UE performing uplink (UL) synchronization by transmitting a random access channel (RACH) request to one or more candidate gNB-DUs to obtain uplink (UL) synchronization, determining reserved partial or full radio resources in one of the one or more LTM candidate cells for an LTM cell switch by the UE to one of the one or more LTM candidate cells; and transmitting, by the candidate gNB-DU, an updated LTM candidate cell configuration corresponding to the reserved partial or full radio resources to a gNB-centralized unit control plane (gNB-CU-CP).

[0017] In at least one embodiment, a method for preparing and performing a Lower Layer Triggered Mobility (LTM) Handover (HO) procedure includes: transmitting, by a gNB Centralized Unit Control Plane (gNB-CU-CP), either directly to a user equipment or via a serving gNB Distributed Unit (gNB-DU), one or more Radio Resource Control (RRC) messages with LTM candidate cell configurations of one or more LTM candidate gNB-DUs; transmitting, by the user equipment or the serving gNB-DU, a Layer 1 (L1) measurement report to the one or more LTM candidate gNB-DUs based on the one or more RRC messages; receiving, at the one or more LTM candidate gNB-DUs, the L1 measurement report; determining, by the one or more LTM candidate gNB-DUs in response to receiving the L1 measurement report, whether radio conditions of one of the one or more LTM candidate gNB-DUs satisfy predetermined resource reservation criteria; The method includes initiating, by one or more LTM candidate gNB-DUs, reservation of partial or full radio resources in one of the one or more LTM candidate cells for an LTM cell switch by the UE from a serving cell to one of the one or more LTM candidate cells, based on the radio conditions of the NB-DU satisfying a predetermined resource reservation criterion or based on the UE performing uplink (UL) synchronization by sending a random access channel (RACH) request to the one or more candidate gNB-DUs to obtain timing advance uplink (UL) synchronization; transmitting, by the one or more LTM candidate gNB-DUs, an updated LTM candidate cell configuration corresponding to the partial or full radio resources to the gNB-CU-CP; and transmitting, by the gNB-CU-CP, the updated radio resource configuration having the partial or full radio resources of the one or more LTM candidate cells to the UE via the serving gNB-DU.

[0018] Aspects of the present disclosure are best understood from the following detailed description when read in conjunction with the accompanying drawings. It should be noted that, according to standard industry practice, various features have not been drawn to scale. In fact, the dimensions of various features may be increased or decreased for clarity of illustration. [Brief explanation of the drawings]

[0019] [Figure 1] 1 illustrates a distributed radio access network (RAN) architecture according to at least one embodiment. [Figure 2] 1 illustrates the overall procedure in LTM according to at least one embodiment. [Figure 3] FIG. 1 is a flow diagram of delayed random access channel (RACH) resource reservation at a target distribution unit (DU) according to at least one embodiment. [Figure 4] FIG. 1 is a flow diagram of an optimized data transfer according to at least one embodiment. [Figure 5] 1 is a flowchart of a method for providing improvements for low-level triggered mobile (LTM) handover (HO) according to at least one embodiment. [Figure 6] FIG. 1 is a high-level functional block diagram of a processor-based system according to at least one embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0020] The embodiments described herein illustrate examples of implementing various features of the provided subject matter. To simplify the disclosure of the present invention, example components, values, operations, materials, arrangements, and the like are described below. Of course, these are examples and are not intended to be limiting. Other components, values, operations, materials, arrangements, and the like are contemplated. For example, the formation of a first feature above or on a second feature in the following description includes embodiments in which the first and second features are formed in direct contact with each other, and also includes embodiments in which an additional feature is formed between the first and second features such that the first and second features are not in direct contact with each other. Additionally, the present disclosure repeats reference numerals and / or letters in various examples. This repetition is for the purposes of brevity and clarity and does not dictate a relationship and / or configuration between the various embodiments described.

[0021] Additionally, spatially relative terms such as "beneath," "below," "lower," "above," and "upper" are used herein for ease of description to describe the relationship of one element or feature to another element(s) or feature(s), as illustrated in the figures. Spatially relative terms are intended to encompass different orientations of the device during use or operation in addition to the orientation shown in the figures. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly.

[0022] Terms such as "user equipment," "mobile station," "mobile," "mobile device," "subscriber station," "subscriber equipment," "access terminal," "terminal," "handset," and similar terms refer to wireless devices utilized by subscribers or users of wireless communication services to receive or transmit data, control, voice, video, sound, games, data streaming, or signaling streaming. The foregoing terms are used interchangeably in this specification and related drawings. Terms such as "access point," "base station," "Node B," "evolved Node B (eNode B)," next generation Node B (gNB), enhanced gNB (en-gNB), Home Node B (HNB), "Home Access Point (HAP)," and the like refer to components or devices of a wireless network that transmit and receive data, control, voice, video, sound, games, data streaming, or signaling streaming from a UE.

[0023] In at least one embodiment, a method for preparing and performing a Lower Layer Triggered Mobility (LTM) Handover (HO) procedure includes: transmitting, by a gNB Centralized Unit Control Plane (gNB-CU-CP), either directly to a user equipment or via a serving gNB Distributed Unit (gNB-DU), one or more Radio Resource Control (RRC) messages with LTM candidate cell configurations of one or more LTM candidate gNB-DUs; transmitting, by the user equipment or the serving gNB-DU, a Layer 1 (L1) measurement report to the one or more LTM candidate gNB-DUs based on the one or more RRC messages; receiving, at the one or more LTM candidate gNB-DUs, the L1 measurement report; determining, by the one or more LTM candidate gNB-DUs in response to receiving the L1 measurement report, whether radio conditions of one of the one or more LTM candidate gNB-DUs satisfy predetermined resource reservation criteria; The method includes initiating, by one or more LTM candidate gNB-DUs, reservation of partial or full radio resources in one of the one or more LTM candidate cells for an LTM cell switch by the UE from a serving cell to one of the one or more LTM candidate cells based on the condition satisfying a predetermined resource reservation criterion or based on the UE performing uplink (UL) synchronization by sending a random access channel (RACH) request to the one or more candidate gNB-DUs to obtain timing advance uplink (UL) synchronization; transmitting, by the one or more LTM candidate gNB-DUs, an updated LTM candidate cell configuration corresponding to the partial or full radio resources; and transmitting, by the gNB-CU-CP, to the UE via the serving gNB-DU, an updated radio resource configuration in the one or more LTM candidate cells having the partial or full radio resources of the one or more LTM candidate cells.

[0024] The embodiments described herein provide a method that provides one or more advantages: For example, configuration in LTM handover is performed in the gNB-CU-CP, but the LTM handover is executed autonomously by the gNB-DU without further interaction with higher layers; the lead time duration between preparation and execution of LTM HO is reduced, providing only target candidate cell configuration culminating in LTM serving cell change; the issue of slow data transfer is eliminated, and overhead in terms of processing and buffer occupancy is reduced.

[0025] FIG. 1 illustrates a distributed Radio Access Network (RAN) architecture 100 according to at least one embodiment.

[0026] 1, User Equipment 1 (UE1) 110, UE2 112, UE3 114, and UE4 116 access a mobile network via RAN 100. Radio access network 100 includes radio towers 120, 123, and 125. Radio towers 120, 123, and 125 are associated with Radio Units (RU) 1 122, RU2 124, and RU3 126, respectively.

[0027] RU 1 122, RU 2 124, and RU 3 126 process the digital front end (DFE) and part of the PHY layer, as well as digital beamforming functions. RU 1 122 and RU 2 124 are associated with gNB distributed unit (gNB-DU) 1 130, and RU 3 126 is associated with gNB-DU2 132. gNB-DU1 130 and gNB-DU2 132 are responsible for real-time Layer 1 and Layer 2 scheduling functions. For example, in 5G, Layer 1 is the physical layer, Layer 2 includes the media access control (MAC), radio link control (RLC), and packet data convergence protocol (PDCP) layers, and Layer 3 (network layer) is the radio resource control (RRC) layer. Layer 2 is the data link or protocol layer that specifies how data packets are encoded and decoded and how data is transferred between adjacent network nodes. Layer 3 is the network routing layer, which defines how data moves through the physical network.

[0028] gNB-DU1 130 is coupled to RU 1 122 via a fronthaul connection 150 and to RU 2 124 via a fronthaul connection 152. gNB-DU2 132 is coupled to RU 3 126 via a fronthaul connection 154. gNB-DU1 130 and gNB-DU2 132 perform the radio link control (RLC), MAC, and part of the physical (PHY) layer. Depending on the function splitting option, gNB-DU1 130 and gNB-DU2 132 contain a subset of eNB / gNB functions, and the operation of gNB-DU1 130 and gNB-DU2 132 is controlled by a centralized unit (CU) 140. gNB-CU 140 is responsible for the higher L2 and L3 layers in non-real time. The gNB-CU 140 server and associated software can be hosted on-site or in an edge cloud (data center or central office) depending on the transport availability and interface of the fronthaul connections 150, 152, 154. The gNB-CU 140 server and associated software can also be co-located in the gNB-DU1 130 or gNB-DU2 132 or hosted in a regional cloud data center.

[0029] The gNB-CU 140 handles the RRC layer and the PDCP layer. The gNB-CU 140 includes a gNB-CU-Control Plane (CU-CP) 142 and one or more gNB-CU-User Plane (CU-UP) 144. The gNB-CU-CP 142 is a logical node that hosts the RRC and control plane portion of the PDCP protocol of the gNB-CU 140 for the gNB 100. The gNB-CU-UP 144 is a logical node that hosts the user plane portion of the PDCP protocol of the gNB-CU 140 and the user plane portion of the PDCP protocol and the SDAP protocol of the gNB-CU 142. The gNB-DU1 130 and gNB-DU1 132 are connected to the gNB-CU-CP 142 via the F1-C 160, and the gNB-DU1 130 and gNB-DU1 132 are connected to one or more gNB-CU-UPs via the F1-U 162 interface. The split architecture allows 5G networks to utilize different distributions of protocol stacks between the gNB-CU 140 and the gNB-DU1 130 and gNB-DU2 132 depending on the network design and availability of midhaul connections 156. The gNB-CU-CP 142 is coupled to one or more gNB-CU-UPs 144 via one or more E1 interfaces 170.

[0030] In FIG. 1 , two connections are shown between the gNB-CU 140 and the gNB-DU1 130 and gNB-DU2 132. The gNB-CU 140 can provide additional connections to other gNB-DUs (not shown). A 5G gNB-CU 140 can implement, for example, 256 endpoints or gNB-DUs. The gNB-CU 140 supports gNB functions such as user data transfer, mobility control, RAN sharing (MORAN), positioning, and session management. However, one or more functions can be assigned to the gNB-DU1 130 and gNB-DU2 132. The gNB-CU 140 controls the operation of the gNB-DU1 130 and gNB-DU2 132 via the midhaul interface 156.

[0031] Layer 1 / Layer 2 (L1) / L2 Triggered Mobility (LTM) enables a serving cell change via L1 / L2 signaling while maintaining higher layer configuration and / or minimizing lower layer configuration changes. LTM helps reduce latency, overhead, and disruption time during handover. In at least one embodiment, gNB-DU1 130 is the serving gNB-DU for UE 3 114, and gNB-DU2 is a candidate gNB-DU for handover of UE 3 114.

[0032] FIG. 2 illustrates the overall procedure of an LTM 200 according to at least one embodiment.

[0033] 2 shows a UE 210 and a gNB 220. The gNB 220 includes one or more gNB-DUs 222 and a CU 224. The Mobility Enhancement 3rd Generation Partnership Project (3GPP) Release 18, RP-213565, specifies mechanisms and procedures for L1 / L2-based inter-cell mobility for mobility latency reduction. L1 / L2-based inter-cell mobility is applicable to standalone, carrier aggregation (CA), and new radio dual connectivity (NR-DC) cases with serving cell changes within a cell group (CG), intra-DU cases, and inter-DU cases within a CU (applicable to standalone and CA; no new Radio Access Network (RAN) interface is expected), and the intra-frequency and inter-frequency, and source and target cells in Frequency Range 1 (FR1) and FR2, can be synchronized or asynchronous.

[0034] As used herein, a candidate cell is a cell prepared for future execution of mobility or handover. A target cell is an LTM candidate cell selected for an LTM serving cell change or handover. Therefore, candidate cell (c) and target cell are used interchangeably, and according to the embodiments described herein, candidate cell and target cell are used as common terms. Also, handover and serving cell change are used interchangeably in the embodiments described herein. Therefore, LDM handover or LDM serving cell change have the same meaning as used herein. A serving gNB is a cell that the UE 210 is using for service and that the UE 210 has a radio link to the serving cell. The corresponding gNB-DU associated with that cell is referred to as the serving gNB-DU 222. Information on cells configured as LTM candidate cells is provided to the serving gNB-DU 222 and the UE 210.

[0035] In FIG. 2, the first stage is LTM preparation 230. The UE 210 is in RRC_Connected state 232. The UE 210 sends an L3 measurement report message 234 to the gNB 220. In FIG. 2, the CU 224 and the gNB-DU 222 are shown as part of the same gNB 220. The UE 210 is configured to report L3 measurements of candidate cells. The UE 210 continues to report these L3 measurements to the serving gNB-CU 224 on an eventual / periodic basis, and based on these L3 measurements, the gNB-CU 224 prepares one or more LTM candidate cells, and a candidate cell configuration is sent to the UE 210. The UE 210 is also configured to perform UL and DL synchronization with the candidate cells and periodically send an L1 measurement report 234 to the gNB-DU 222. Based on the received L1 measurement report 234, the gNB-DU 222 determines the LTM serving cell based on the time and threshold at which the UE 210 should be handed over to one of the other candidate cells, which then becomes the target cell. The candidate cell can be in the same gNB-DU 222 or in another gNB-DU (not shown) of the same gNB 220. However, since the LTM functionality is limited to one gNB in ​​3GPP Release 18, the candidate cell cannot be outside the gNB 220. For intra-gNB cells, L3 mobility procedures are used.

[0036] A maximum of eight candidate cells is described herein, which is the maximum number that can be configured per UE. However, that number can be increased in the future. Thus, the UE 210 can be configured with eight candidate cells, from which it can select one candidate cell as a target cell. The target cell is selected by the serving gNB-DU 222 based on event / periodic L1 measurement reports 234 received from the UE 210.

[0037] After receiving the L3 measurement report 234, the gNB 220 decides to use LTM and initiates LTM candidate preparation 236, e.g., the gNB 220 prepares one or more candidate cells, which includes the CU 224 sending an F1 message to the gNB-DU 222 and the gNB-DU 222 responding with the candidate cell configuration.

[0038] The gNB 220 sends an RRC reconfiguration message 237 to the UE 210 that includes the configuration of one or more LTM candidate target cells.

[0039] The UE 210 stores the configuration of the LTM candidate target cell 238 and sends an RRC reconfiguration complete message 239 to the gNB 220 to confirm that the RRC configuration is complete.

[0040] Early synchronization 240 can be performed using downlink (DL) synchronization and timing advance (TA) acquisition 242 with the candidate cell and uplink (UL) synchronization and TA acquisition 244 with the candidate cell while the UE 210 is still connected to the current serving cell. DL synchronization 242 with the candidate cell before the cell switch command is supported based on at least SSB. TA acquisition with the candidate cell before the LTM cell switch command is supported based on at least PDCCH order RACH, where the PDCCH order is triggered by the source cell.

[0041] Subsequently, an LTM cell switch execution 250 is performed. The UE 210 performs L1 measurements on the configured LTM candidate target cell and transmits a lower layer (L1) measurement report 252 to the gNB 220. The L1 measurement report 252 is provided to the gNB-DU 222. To achieve CFRA, a PRACH preamble is reserved for the target cell. There is discussion in RAN1 regarding the transmission of UE / event-triggered L1 measurement reports 252 to one or more candidate cells. Within the 3GPP Technical Specification Group Radio Access Network (TSG RAN), RAN Working Group 1 (WG1) (RAN1) is responsible for developing specifications covering Evolved UMTS Terrestrial Radio Access (UTRA), 5G New Radio (NR), etc. RAN1 is responsible for specifying the physical layer of the air interface for the UE, Evolved UTRAN, Next Generation Radio Access Network (NG-RAN), etc. The L1 measurement report 252 identifies a report destination, indicating whether the report is transmitted to the serving cell or to one or more candidate cells.

[0042] The UE 210's transmission to the candidate cell is inherently slightly more complicated because the UE 210 is not connected to the candidate gNB-DU and requires an UL grant from the candidate gNB-DU, which is readily provided to the UE 210 by the serving cell 220 based on a scheduling request message from the UE 210. For this reason, RAN1 has made this agreement regarding whether or not to report L1 measurement reports 252 to the candidate gNB-DU.

[0043] The L1 measurement report 252 for 3rd Generation Partnership Project (3GPP) Release 18 LTM is transmitted to the serving cell 220 unless the UE 210 is in 3GPP Release 17 Inter-Cell Beam Management (ICBM) operation. Beam-level mobility does not rely on explicit RRC signaling to be triggered. Beam-level mobility can be intra-cell or inter-cell, the latter being referred to as Inter-Cell Beam Management (ICBM). In ICBM operation, UE-dedicated signals / channels are transmitted / received via non-serving cells. In this case, the L1 measurement report 252 can be transmitted to non-serving cells or candidate cells.

[0044] In response to the LTM candidate cell not being an ICBM non-serving cell, the network provides neighbor cell service to the UE 210 to report to the LTM candidate cell. For example, the UE serving cell 220 is cell 1, and cell 2 is one of the candidate cells. Because serving cell 1 has no available resources, the network lends cell 2's resources to cell 1. In such a case, the UE 210 can send an L1 measurement report to cell 2 even though serving cell 220 is still cell 1.

[0045] In this case, no inter-cell beam management is assumed, but e.g., cell 1 is the serving cell and cell 2 is the candidate cell. Reporting to the LTM candidate in response to cell 1 not being an ICBM non-serving cell is based on inter-cell or gNB-DU coordination to make measurements available in the serving cell and reserving UL resources in the candidate cell for the report to be transmitted by UE 210. However, the lead time, which is the duration between preparation and execution of the LTM HO, is significant.

[0046] The gNB 220 then makes an LTM decision 254 to select one of the candidate configurations in the target configuration for LTM and initiates an LTM cell switch to the target cell by sending a MAC CE message 256 including an LTM cell switch command that includes the candidate configuration index of the target cell.

[0047] After receiving the MAC CE 256, the UE 210 detaches from the serving cell and switches to the LTM candidate / target cell configuration by applying the target cell configuration 258. The candidate cell configuration can be added, changed, or released by the network via RRC signaling.

[0048] An optional RACH procedure 260 is performed (as shown by the dotted line). For example, the RACH procedure is used in response to the TA not being available. If the target cell TA is available, a RACH-less HO procedure can be performed. Therefore, the LTM HO procedure cannot use the RACH procedure 260. During HO, the UE 210 waits for a PRACH opportunity and performs RACH operations 260 to synchronize with the UL of the target cell. For example, the TA of the target cell may be different from the TA of the serving cell. During the RACH procedure, the UE obtains the TA of the UE 210 in the target cell. The TA is UE-specific and may be different for different UEs within a cell.

[0049] Once the LTM execution 250 is complete and the UE 210 has successfully completed the LTM cell switch to the target cell, the LTM completion stage 270 begins. An RRC reconfiguration complete message 272 is sent from the UE 210 to the gNB 220. The RRC reconfiguration complete message 272 indicates that the previously sent RRC reconfiguration has been successfully applied. Subsequent LTMs are performed by repeating early synchronization 240, LTM execution 250, and LTM completion 270 without releasing other candidates after LTM completion.

[0050] However, the lead time, e.g., the duration between preparing and executing an LTM HO, can be significant. Second, not all target candidate cell configurations result in an LTM serving cell change. This means that even if there are eight cells prepared for the UE 210, the UE may not move to all eight cells. Therefore, resource reservation, including the PRACH preamble, will be an issue in the legacy baseline and our L3 mobility.

[0051] In legacy baseline NR L3 mobility, the UE 210 acquires a timing advance (TA) toward the target cell and completes UL synchronization via a RACH procedure. Contention-free random access (CFRA) involves a four-step RACH process, while contention-based random access (CBRA) involves a two-step RACH process. The UE 210 can complete the RACH procedure in two steps (Msg1+Msg2 or MsgA+MsgB), which is faster than the CBRA in a four-step RACH (Msg1-Msg4). However, whether using CFRA in a four-step RACH or CBRA in a two-step RACH, the UE 210 waits for an available period, such as a PRACH opportunity, before transmitting a preamble and waiting for a random access response (RAR). The MAC layer of the gNB 220 generates the RAR in response to the random access preamble transmitted by the UE 210. The RACH procedure takes approximately 10 to 20 milliseconds.

[0052] The RACH-less solution is used to reduce interruption time during handover. Two RACH-less solutions are agreed upon in response to the target cell's TA being equal to 0, and in response to the target cell having the same TA as the source cell. Therefore, TA is used to ensure that cells operate properly without causing interference. If neighboring cells have the same timing advance, interference occurs, and the cell generates more noise than signal.

[0053] Therefore, the timing advances within the cells are different, and the UE 210 acquires the timing advance of the target cell to perform RACH-less HO.

[0054] However, a first problem exists because RACH-less HO may not be possible in response to the TA obtained from the candidate / target cell expiring or not being valid: in response to not being able to obtain the TA of the candidate / target cell, RACH-less HO is not possible.

[0055] The second problem involves data forwarding. Currently, data forwarding is slow and involves significant overhead. Data forwarding to multiple candidate / target cells, e.g., eight cells, is expensive in terms of processing and buffer occupancy. Data forwarding to eight cells is not optimal because the eight cells are candidate cells, and data is checked to see if it is successfully transmitted and then dropped. Another problem related to the serving gNB-DU triggering data forwarding from the gNB-CU-CP means that the serving gNB-DU needs to be aware of the topology of the association between the gNB-CU-UP and the gNB-DU. This is necessary for the gNB-CU-UP to determine whether it can perform data forwarding to a candidate gNB-DU.

[0056] FIG. 3 is a flow diagram 300 of delayed random access channel (RACH) resource reservation in a target distribution unit (DU) according to at least one embodiment.

[0057] In FIG. 3, a user equipment (UE) 310 is configured with a Layer 1-Layer 2 (L1 / L2) triggered mobility (LTM) 320 with one or more candidate / target cells 314 with one or more gNB-DUs. RACH resource reservation includes resources used by the UE to perform contention-free RACH access (PRACH resources for UE-dedicated RACH, RACH preamble ID as network side). Radio resource control (RRC) measurements (L3) 322 are sent by the UE 310 to a gNB-CU-Control Plane (gNB-CU-CP) 316. The gNB-CU-CP 316 uses the RRC measurements (L3) 322 to determine a target gNB-DU for preparing a contention-based random access (CBRA) target cell configuration and a configuration for the UE 310 to report L1 measurements to the target cell 314 324.

[0058] During LTM target cell preparation, the target gNB-DU 314 may not reserve a RACH preamble in the candidate / target cell configuration, or the RACH preamble assigned during candidate / target cell preparation may not be valid during LTM cell switch execution. Therefore, when the candidate / target cell configuration is being transmitted to the UE 310, the target gNB-DU 314 may not perform RACH resource reservation. Other RRM resources may also be scarce and not reserved during candidate / target cell preparation, as they are blocked for the entire duration of the "lead time," i.e., the duration between LTM cell switch preparation and execution. This may be indicated using a new information element (IE) by the gNB-CU-CP 316 in the UE Context Setup Request message 326 and the UE Context Setup Response message 328, along with the CBRA CellGroupConfiguration over the F1 interface.

[0059] The gNB-CU-CP 316 sends an F1:DL RRC message transfer 330 to the serving gNB-DU 312. The serving gNB-DU 312 sends an RRC reconfiguration message 332 to the UE 332. The RRC connection reconfiguration procedure is used to establish, modify, or release radio bearers. When the gNB-CU-CP 316 is sending or performing this target cell preparation, or requesting target cell preparation, the gNB-CU-CP 316 indicates that RACH resource reservation for this cell will not be performed.

[0060] Based on the RRC reconfiguration message 332, the UE 310 transmits an event-based L1 measurement report 334 sent to the target gNB-DU 314. The L1 measurement report 336 can be sent directly from the UE 310 to the target gNB-DU 314, for example, in response to UL resources being reserved in the target cell or in response to a multiplexed transmit and receive point (mTRP) transmission configured for the UE 310. Alternatively, the UE 310 can send the event-based L1 measurement report 334 to the serving gNB-DU 312, which sends the event-based L1 measurement report 338 to the CU 316, which forwards the event-based L1 measurement report 338 to the candidate / target gNB-DU 314.

[0061] The L1 measurement report 334 / 336, 336 of the UE 310 may include a preconfigured L1 RSRP threshold. Based on the UE 310's event-based L1 measurement report 334 / 338, 336 being received at the target gNB-DU 314 along with the preconfigured L1 RSRP threshold, the candidate / target gNB-DU 314 determines whether the radio conditions of one of the one or more LTM candidate gNB-DUs meet a predetermined resource reservation criterion, such as a predetermined Reference Signal Receive Power (RSRP) threshold 340. RSRP is the average received power of an RS resource element. The candidate / target gNB-DU 314 also reserves 340 RACH resources, including a RACH preamble, corresponding to the UE 310's best beam / beam group. Here, the solution described in the RAN1 agreement is used when event-based L1 measurement reports 336 are also provided to the target gNB-DU 314 or forwarded 338 to the target gNB-DU 314.

[0062] Conversely, if a RACH preamble is assigned to the UE 310 during candidate / target cell preparation, this may be indicated by the candidate / target gNB-DU 314 to the gNB-CU 316, and by the gNB-CU 316 to the serving gNB-DU 312 in an F1 message. The serving gNB-DU 312 may decide whether to issue a PDCCH order to the UE 310 for uplink (UL) synchronization with the candidate cell and obtain the timing advance (TA) of the candidate cell. The serving gNB-DU 312 may set RRM criteria (e.g., RSRP thresholds) for determining when to issue a PDCCH order to the UE 310. Since the UE 310 may be considered to be very close to performing an LTM cell switch to the candidate cell, the UE 310 may consider performing uplink (UL) synchronization, i.e., sending a random access channel (RACH) request message to obtain the TA of the candidate cell, as a trigger for the target gNB-DU 314 to reserve the remaining RRM resources.

[0063] The UE 310 may undergo mobility within the serving gNB-DU 314 after LTM target gNB-DU 312 preparation is performed, which may change the best beam / beam group serving the UE 310 in the target gNB-DU 314. This results in two problems: a) suboptimal resource reservation with respect to RACH resources, and b) unnecessary blocking of resources for a longer duration in case of lead time. Therefore, a pre-configured L1 threshold used as resource reservation criteria is shown in the gNB-DUs 312, 314 and the gNB-CU-CP 316. The pre-configured L1 threshold can also be the basis for the LTM handover criteria from the serving gNB-DU 312 to the target gNB-DU 314.

[0064] Based on the radio conditions of one or more LTM candidate gNB-DUs 314 meeting predetermined resource reservation criteria according to the L1 measurement reports, or based on the UE performing a RACH procedure to acquire a candidate cell TA, the one or more LTM candidate gNB-DUs 314 initiate reservation of partial or full radio resources in one of the one or more LTM candidate cells for an LTM cell switch of the UE 310 from the serving gNB-DU 312 to one of the one or more LTM candidate cells, i.e., the candidate / target gNB-DU 314. The target gNB-DU 314 allocates RACH resources, including a RACH preamble, to the UE 310 according to the best beam / beam group for the UE 310 based on the received L1 measurement reports 336, 338. Because the RACH resource allocation is closer to the change in serving cell 312 and is performed based on the radio conditions (based on a predetermined RSRP threshold), the allocation of RACH resources by the target gNB-DU 314 is more appropriate and the resource reservation is of shorter duration. Any other RRM resources that may not have been reserved during the preparation of the candidate cell can be reserved here.

[0065] The RACH resources and other RRM resources reserved at the target gNB-DU 314, 336 are sent from the target gNB-DU 314 to the gNB-CU-CP 316 using a UE Context Modification Request message 342 over the F1 interface, or using any other similar procedure. The gNB-CU-CP 316 responds with a UE Context Modification Acknowledgement (ACK) message 344.

[0066] The gNB-CU-CP 316 sends a downlink (DL) RRC message 350 to the serving gNB-DU 312 over the F1 interface. The DL RRC message 350 includes updated radio resource configurations of one or more LTM candidate / target cells 314 with reserved partial or full radio resources of the candidate / target cells. For example, the F1:DL RRC message 350 may include the LTM target cell configuration (CFRA preamble) and target cell identification as F1 protocol information. The information provided in the DL RRC message transfer message 350 is also used by the serving gNB-DU 312. Deferred resource reservation is applicable not only to RACH resources but also to other RRM resources such as UL PUCCH configuration.

[0067] The serving gNB-DU 312 sends an RRC reconfiguration message 352 to the UE 310. The RRC reconfiguration message 352 provides the UE with the LTM target cell configuration, including the CFRA preamble.

[0068] The UE 310 sends an intra-frequency / inter-frequency L1 measurement report 360 to the serving gNB-DU 312. The UE 310 also sends an intra-frequency / inter-frequency L1 measurement report 362 to the target gNB-DU 314. Based on the radio conditions of the target gNB-DU 314 being met, i.e., the radio conditions are greater than a predetermined RSRP threshold, an LTM cell switch is made 370 from the serving gNB-DU 312 to the prepared target gNB-DU 314.

[0069] The serving gNB-DU 312 sends a MAC CE command 372 (serving cell change command) to the UE 310. The serving gNB-DU 312 sends a serving cell change notification (Cell ID) 374 to the gNB-CU-CP 316. The UE 310 performs a RACH operation 380 using the RACH preamble provided to the target gNB-DU 314. The UE 310 then sends an RRC reconfiguration acknowledgement (ACK) 382 to the gNB-CU-CP 316.

[0070] FIG. 4 is a flow diagram 400 of optimized data transfer according to at least one embodiment.

[0071] In FIG. 4, the UE 410 is configured 420 with LTM with one or more target cells 414 with one or more gNB-DUs. The UE 410 is connected with a serving gNB-DU 412. Also shown in FIG. 4 are an operating gNB-CU-CP 416 and an operating gNB-CU-UP 418. Data transfers initiated before the LTM serving gNB-DU 412 changes, e.g., after the target cell 414 is ready, are early data transfers and cause significant overhead in terms of processing load and memory usage at the target gNB-DU 414. Data transfers initiated after an LTM serving cell change notification from the serving gNB-DU 412 to the gNB-CU-UP 418 (towards the target gNB-DU 414) are delayed data transfers and may be suboptimal due to the additional latency.

[0072] The UE 410 sends an RRC measurement report (L3) 422 to the gNB-CU-CP 416. The gNB-CU-CP 416 sends a UE context setup request 424 to the target gNB-DU 414. The target gNB-DU 414 responds with a UE context setup response with the cell group configuration of the CBRA 426 sent to the gNB-CU-CP 416.

[0073] The gNB-CU-CP 416 configures a UE L1 measurement report 428 to be sent by the UE 410 to the target cell 414. The gNB-CU-CP 416 sends a DL RRC message transfer 430 to the serving gNB-DU 412 over the F1 interface. The DL RRC message transfer 430 includes the LTM target cell configuration and multicast L1 measurements. The serving gNB-DU 412 sends an RRC reconfiguration message 432 to the UE 410 to provide the UE 410 with the LTM target cell configuration and multicast L1 measurements. The RRC connection reconfiguration procedure is used to establish, modify, or release radio bearers.

[0074] The gNB-CU-CP 416 sends a modify bearer context request 440 to the gNB-CU-UP 418 over the E1 interface. The modify bearer context request 440 is used to establish a bearer context in the gNB-CU-UP 418. The modify bearer context request 440 includes the F1-U Tunnel Endpoint Identifier (TEID) of the LTM target gNB-DU 414. The gNB-CU-UP 418 sends an E1 modify bearer context response message 442 to the gNB-CU-CP 418, including the F1-U UL TEID and transport layer address assigned by the gNB-CU-UP 416.

[0075] The UE 410 initiates transmission of an intra-frequency measurement report 450 with the serving gNB-DU 412. The UE 410 also transmits an intra-frequency measurement report 452 to the target gNB-DU 414. The target gNB-DU 414 determines 454 whether the L1 measurements reported by the UE 310 are greater than a predetermined resource reservation criterion, e.g., a predetermined RSRP threshold. In response to the L1 measurements reported by the UE 310 being greater than the predetermined resource reservation criterion, the target gNB-DU 414 indicates to the gNB-CU-UP 418 to initiate data forwarding. At least one embodiment shown in FIG. 4 uses optimized early data forwarding based on the received event-based L1 measurement report and a pre-configured L1 RSRP threshold, where the candidate / target gNB-DU 414 triggers a control PDU 460 or signaling message over F1 / E1 towards the gNB-CU-UP (PDCP host) 418 to initiate data forwarding to the target gNB-DU 414. A control PDU is a data packet intended to contain control information. A control PDU is a data PDU that can be transmitted from the target gNB-DU 414 to the gNB-CU-UP 418 via the F1-U interface. Control signaling messages, i.e., control plane messages, are sent by the target gNB-DU 414 to the gNB-CU-CP 416 via the F1 interface and are further forwarded to the gNB-CU-UP 418 via the E1 interface, as there is no direct control plane interface between the target gNB-DU 414 and the gNB-CU-UP 418. A pre-configured L1 threshold can be derived based on the LTM HO criteria in the serving gNB-DU 412. This can be indicated to the target gNB-DU 414 by the gNB-CU-CP 416.

[0076] Therefore, data transfer 462 is performed just before the serving cell is changed based on radio conditions. Therefore, data transfer 462 is initiated close to the LTM handover, using a pre-configured L1 threshold as a benchmark indicating that UE 410 is reporting target gNB-DU 414 at an acceptable level, so that UE 410 is most likely to come to this cell. Data transfer 462 is then performed, avoiding overhead. Therefore, buffer space is not wasted at target gNB-DU 414, and data transfer 462 is not delayed. Another advantage is that because data transfer 462 is initiated based on radio conditions, data transfer 462 is not performed for candidate gNB-DUs; only target gNB-DUs 414 that satisfy the radio conditions receive the transferred data. Even in this case, there is a possibility that handover will not occur and the UE will remain in the serving gNB-DU 412, or that UE 410 can move to another cell. However, because data transfer 462 is based on radio conditions and not solely on configuration, the possibilities are limited or reduced.

[0077] Optimized early data transfer 462 can also be initiated by the serving gNB-DU 412 based on the received L1 measurements 450. The serving gNB-DU 412 must be informed of the association between the target gNB-DUs and their corresponding gNB-CU-UPs 418 (e.g., addresses). This starts to become more complex during a gNB-CU-UP 418 relocation scenario. In accordance with at least one embodiment described herein, such complexity is avoided.

[0078] In response to the target gNB-DU 414 determining that the radio conditions of one of the one or more LTM candidate gNB-DUs fall below a predetermined resource reservation criterion, the target gNB-DU 414 sends a control PDU 460 to the gNB-CU-UP 418 instructing the gNB-CU-UP 418 to stop forwarding data to one of the one or more LTM candidate gNB-DUs 414.

[0079] An intra-frequency measurement report 470 is transmitted from the UE 410 to the serving gNB-DU 412. An intra-frequency measurement report 472 is also transmitted from the UE 410 to the target gNB-DU 414. The serving gNB-DU 412 determines 474 whether the radio conditions of the target cell 414 are met. In response to the radio conditions of the target cell 414 being met, an LTM cell switch is performed to a preparing target cell, such as the target gNB-DU 414.

[0080] The serving gNB-DU 412 sends a MAC CE 480 with a serving cell change command to the UE 410. The serving gNB-DU 412 also sends a serving cell change notification message 482 to the gNB-CU-CP 416. The serving cell change notification message 482 includes the cell ID of the target gNB-DU 414.

[0081] The UE 410 then sends an RRC reconfiguration acknowledgement (ACK) 392 to the gNB-CU-CP 416.

[0082] In response to the L1 measurement report being sent directly to the target gNB-DU 414, the target gNB-DU 414 can be configured to reserve specific radio resources for reception of the L1 measurement report 472 received from the UE 410. Such radio resources can be configured semi-statically or on-demand, and can be UE-dedicated resources or non-UE-dedicated resources.

[0083] Another applicable scenario is in response to the UE 410 being configured with a multiple transmit / receive point (mTRP) transmission and the non-serving cell of the mTRP transmission being the LTM target gNB-DU 414. Because the UE 410 has radio links to the serving cell gNB-DU 412 and the non-serving cell, the target gNB-DU 414, in the mTRP configuration, the event-based L1 measurement report 452 can be sent directly to the target gNB-DU 414. However, the data forwarding 462 according to at least one embodiment is not intended for ICBM (Inter-Cell Beam Management) scenarios where RACH cannot be performed.

[0084] Although delayed resource reservation is specifically proposed for the CFRA preamble in FIGS. 3-4, delayed resource reservation according to at least one embodiment is applicable to any other resources. For example, other resources that are used or have a shortage in the target gNB-DU 414 are applicable and can therefore be reserved in the delayed resource reservation. The gNB-centralized unit user plane (gNB-CU-CP) transmits a radio resource control (RRC) message with the LTM candidate cell configuration of the LTM candidate gNB-DU. Based on the RRC message, a layer (L1) measurement report is transmitted to the LTM candidate gNB-DU. In response, the LTM candidate gNB-DU determines whether the radio conditions of the LTM candidate gNB-DU satisfy predetermined resource reservation criteria. Based on the satisfaction of the predetermined resource reservation criteria, the LTM candidate gNB-DU initiates reservation of radio resources in the LTM candidate cell for the LTM cell switch. The LTM candidate gNB-DU transmits the updated LTM candidate cell configuration to the gNB-CU-CP. The gNB-CU-CP transmits the updated radio resource configuration to the UE via the serving gNB-DU.

[0085] FIG. 5 is a flowchart 500 of a method for providing enhancements for low-level triggered mobile (LTM) handover (HO) in accordance with at least one embodiment.

[0086] In Figure 5, the process starts at S502, where the UE is configured with Layer 1-Layer 2 (L1 / L2) triggered mobility (LTM) with one or more candidate / target cells (S510). Referring to Figure 3, the user equipment (UE) 310 is configured with Layer 1-Layer 2 (L1 / L2) triggered mobility (LTM) 320 with one or more candidate / target cells 314 with one or more gNB-DUs. The RACH resource reservation includes resources used by the UE to perform contention-free RACH access (PRACH resources for the UE dedicated RACH as the network side, RACH preamble ID).

[0087] In the gNB-CU-control plane (gNB-CU-CP), radio resource control (RRC) measurements (L3) are received from the UE (S514). Referring to Figure 3, the radio resource control (RRC) measurements (L3) 322 are sent by the UE 310 to the gNB-CU-control plane (gNB-CU-CP) 316.

[0088] The gNB-CU-CP determines a candidate / target cell configuration and a UE configuration for the UE to report L1 measurements to the target cell based on the RRC measurements (L3) (S518). Referring to Figure 3, the gNB-CU-CP 316 uses the RRC measurements (L3) 322 to determine a target gNB-DU and a configuration for the UE 310 to report L1 measurements to the target cell 314 324 to prepare a condition-based random access (CBRA) target cell configuration.

[0089] A UE context setup request message is sent by the gNB-CU-CP to the target cell (S522). Referring to FIG. 3, the target gNB-DU 314 may not reserve a RACH preamble in the candidate / target cell configuration when preparing the LTM target cell, or the RACH preamble assigned during candidate / target cell preparation may not be valid during LTM cell switch execution. Therefore, when the candidate / target cell configuration is sent to the UE 310, the target gNB-DU 314 may not perform RACH resource reservation. Other RRM resources may also be scarce and not reserved during candidate / target cell preparation because they are blocked for the entire duration of the "lead time," i.e., the duration between preparation and execution of the LTM cell switch. This may be indicated using a new information element (IE) by the gNB-CU-CP 316 in the UE context setup request message 326 and the UE context setup response message 328, along with the CBRA CellGroupConfiguration over the F1 interface.

[0090] A context setup response message with CellGroupConfiguration for CBRA is received at the gNB-CU-CP from the target cell (S526). Referring to FIG. 3, in response to the candidate / target cell configuration being sent to the UE 310, the target gNB-DU 314 may not perform RACH resource reservation. Other RRM resources may also be scarce and not be reserved during candidate / target cell preparation, as they are blocked for the entire duration of the "lead time," i.e., the duration between preparation and execution of the LTM cell switch. This may be indicated using a new information element (IE) by the gNB-CU-CP 316 in the UE context setup request message 326 and the UE context setup response message 328, along with the CellGroupConfiguration for CBRA over the F1 interface.

[0091] An RRC message for the UE to provide L1 measurement reports to the serving cell and candidate / target cells is sent by the gNB-CU-CP to the UE and the serving cell (S530). Referring to FIG. 3, the gNB-CU-CP 316 sends an F1:DL RRC message transfer 330 to the serving gNB-DU 312. The serving gNB-DU 312 sends an RRC reconfiguration message 332 to the UE 332. The RRC connection reconfiguration procedure is used to establish, modify, or release radio bearers. When the gNB-CU-CP 316 is sending or performing this target cell preparation or requesting target cell preparation, the gNB-CU-CP 316 indicates that RACH resource reservation for this cell will not be performed. 4, after the gNB-CU-CP 416 sends the DL RRC message 430 to the serving cell 412 and the UE 410, the gNB-CU-CP 416 may also send a modify bearer context request 440 to the gNB-CU-UP 418 via the E1 interface. The modify bearer context request 440 is used to establish a bearer context in the gNB-CU-UP 418. The modify bearer context request 440 includes the F1-U Tunnel Endpoint Identifier (TEID) of the LTM target gNB-DU 414. The gNB-CU-UP 418 sends an E1 modify bearer context response message 442 to the gNB-CU-CP 418, including the F1-U UL TEID and transport layer address assigned by the gNB-CU-UP 416.

[0092] Based on the RRC message, an L1 measurement report is transmitted from the UE directly or via the serving cell to the candidate cell (S534). Referring to FIG. 3, the UE 310 transmits an event-based L1 measurement report 334 transmitted to the target gNB-DU 314 based on the RRC reconfiguration message 332. The L1 measurement report 336 can be transmitted directly from the UE 310 to the target gNB-DU 314, for example, in response to UL resources being reserved in the target cell or in response to a multiplexed transmit and receive point (mTRP) transmission configured for the UE 310. Alternatively, the UE 310 can transmit the event-based L1 measurement report 334 to the serving gNB-DU 312, which transmits the event-based L1 measurement report 338 to the CU 316, which forwards the event-based L1 measurement report 338 to the candidate / target gNB-DU 314.

[0093] In response to receiving the L1 measurement report, the LTM candidate cell determines whether the radio conditions of one of the LTM candidate cells meet a predetermined resource reservation criterion (S538). Referring to FIG. 3, the L1 measurement report 334 / 336, 336 of the UE 310 may include a preconfigured L1 RSRP threshold. Based on the UE 310's event-based L1 measurement report 334 / 338, 336 being received at the target gNB-DU 314 along with the preconfigured L1 RSRP threshold, the candidate / target gNB-DU 314 determines whether the radio conditions of one of the one or more LTM candidate gNB-DUs meet a predetermined resource reservation criterion, for example, a predetermined reference signal received power (RSRP) threshold 340. RSRP is the average received power of an RS resource element. The candidate / target gNB-DU 314 also reserves a RACH resource 340, including a RACH preamble, corresponding to the UE 310's best beam / beam group. Here, the solution described in the RAN1 agreement is used when event-based L1 measurement reports 336 are also provided to the target gNB-DU 314 or forwarded 338 to the target gNB-DU 314.

[0094] Based on the radio conditions of the LTM candidate cell satisfying the predetermined resource reservation criteria, an updated LTM candidate cell configuration is sent by the LTM candidate cell to the gNB-CU-CP for the UE to perform an LTM cell switch from the serving cell to the LTM target cell (S542). Referring to FIG. 3, based on the radio conditions of one or more LTM candidate gNB-DUs 314 satisfying the predetermined resource reservation criteria via the L1 measurement report, or based on the UE performing a RACH procedure to obtain a candidate cell TA, the one or more LTM candidate gNB-DUs 314 start reserving partial or full radio resources in one of the one or more LTM candidate cells for the UE 310's LTM cell switch from the serving gNB-DU 312 to one of the one or more LTM candidate cells, i.e., the candidate / target gNB-DU 314. The target gNB-DU 314 allocates RACH resources, including a RACH preamble, to the UE 310 according to the UE's 310 best beam / beam group based on the received L1 measurement reports 336, 338. Because RACH resource allocation is performed closer to changes in the serving cell 312 and based on radio conditions (based on a predetermined RSRP threshold), the allocation of RACH resources by the target gNB-DU 314 is more appropriate and the resource reservation is of shorter duration. Any other RRM resources that may not have been reserved during candidate cell preparation can now be reserved. The RACH resources and other RRM resources reserved by the target gNB-DU 314, 336 are transmitted from the target gNB-DU 314 to the gNB-CU-CP 316 using a UE Context Modification Request message 342 over the F1 interface, or any other similar procedure. The gNB-CU-CP 316 responds with a UE Context Modification Acknowledgement (ACK) message 344. Referring to FIG. 4, in response to the L1 measurements reported by the UE 310 being greater than a predetermined resource reservation criterion, the target gNB-DU 414 indicates to the gNB-CU-UP 418 to initiate data transfer.At least one embodiment shown in FIG. 4 uses optimized early data forwarding based on received event-based L1 measurement reports and pre-configured L1 RSRP thresholds, where the candidate / target gNB-DU 414 triggers a control PDU 460 or signaling message over F1 / E1 towards the gNB-CU-UP (PDCP host) 418 to initiate data forwarding to the target gNB-DU 414. A control PDU is a data packet intended to contain control information. A control PDU is a data PDU that can be transmitted from the target gNB-DU 414 to the gNB-CU-UP 418 over the F1-U interface. The control signaling message, i.e., the control plane message, is transmitted by the target gNB-DU 414 to the gNB-CU-CP 416 over F1 and is further forwarded to the gNB-CU-UP 418 over the E1 interface because there is no direct control plane interface between the target gNB-DU 414 and the gNB-CU-UP 418. The pre-configured L1 threshold may be derived based on an LTM HO criterion in the serving gNB-DU 412, which may be indicated to the target gNB-DU 414 by the gNB-CU-CP 416. In response to the target gNB-DU 414 determining that the radio conditions of one of the one or more LTM candidate gNB-DUs fall below a predetermined resource reservation criterion, the target gNB-DU 414 sends a control PDU 460 to the gNB-CU-UP 418 instructing the gNB-CU-UP 418 to stop forwarding data to one of the one or more LTM candidate gNB-DUs 414.

[0095] The gNB-CU-CP transmits an updated radio resource configuration with reserved partial or full radio resources of one or more LTM candidate cells to the UE via the serving cell (S546). Referring to FIG. 3, the gNB-CU-CP 316 transmits a downlink (DL) RRC message 350 to the serving gNB-DU 312 via the F1 interface. The DL RRC message 350 includes the updated radio resource configuration of the candidate / target cell with reserved partial or full radio resources of one or more LTM candidate / target cells 314. For example, the F1:DL RRC message 350 may include the LTM target cell configuration (CFRA preamble) and target cell identification as F1 protocol information. The information provided in the DL RRC message transfer message 350 is also used by the serving gNB-DU 312. Delayed resource reservation is applicable not only to RACH resources but also to other RRM resources such as UL PUCCH configuration. The serving gNB-DU 312 sends an RRC reconfiguration message 352 to the UE 310. The RRC reconfiguration message 352 provides the UE with the LTM target cell configuration, including the CFRA preamble.

[0096] Using the updated radio resource configuration, an LTM cell switch from the serving cell to the LTM candidate cell is performed (S550). Referring to FIG. 3, the UE 310 sends an intra-frequency / inter-frequency L1 measurement report 360 to the serving gNB-DU 312. The UE 310 also sends an intra-frequency / inter-frequency L1 measurement report 362 to the target gNB-DU 314. Based on the radio conditions of the target gNB-DU 314 being satisfied, i.e., the radio conditions are greater than a predetermined RSRP threshold, an LTM cell switch is performed 370 from the serving gNB-DU 312 to the prepared target gNB-DU 314. The serving gNB-DU 312 sends a MAC CE command 372 (serving cell change command) to the UE 310. The serving gNB-DU 312 sends a serving cell change notification (Cell ID) 374 to the gNB-CU-CP 316. The UE 310 performs a RACH operation 380 using the RACH preamble provided to the target gNB-DU 314. The UE 310 then sends an RRC reconfiguration acknowledgement (ACK) 382 to the gNB-CU-CP 316.

[0097] The process then ends S560.

[0098] In at least one embodiment, a method for preparing and performing a Lower Layer Triggered Mobility (LTM) Handover (HO) procedure includes: sending, by a gNB Centralized Unit Control Plane (gNB-CU-CP), either directly to a user equipment or via a serving gNB Distributed Unit (gNB-DU), one or more Radio Resource Control (RRC) messages with LTM candidate cell configurations of one or more LTM candidate gNB-DUs; sending, by the user equipment or the serving gNB-DU, a Layer 1 (L1) measurement report to the one or more LTM candidate gNB-DUs based on the one or more RRC messages; receiving, at the one or more LTM candidate gNB-DUs, the L1 measurement report; determining, by the one or more LTM candidate gNB-DUs in response to receiving the L1 measurement report, whether radio conditions of one of the one or more LTM candidate gNB-DUs satisfy predetermined resource reservation criteria; -Initiating, by one or more LTM candidate gNB-DUs, reservation of partial or full radio resources in one of the one or more LTM candidate cells for an LTM cell switch by the UE from a serving cell to one of the one or more LTM candidate cells, based on the radio conditions of the DU satisfying predetermined resource reservation criteria or based on the UE performing uplink (UL) synchronization by sending a random access channel (RACH) request to the one or more candidate gNB-DUs to obtain timing advance; transmitting, by the one or more LTM candidate gNB-DUs, an updated LTM candidate cell configuration corresponding to the partial or full radio resources; and transmitting, by the gNB-CU-CP, to the UE via the serving gNB-DU, an updated radio resource configuration in the one or more LTM candidate cells having the partial or full radio resources of the one or more LTM candidate cells.

[0099] FIG. 6 is a high-level functional block diagram of a processor-based system 600 according to at least one embodiment.

[0100] In at least one embodiment, processing circuit 600 enhances a lower layer mobility (LTM) handover (HO) procedure. Processing circuit 600 implements the enhancements to the LTM HO procedure using processor 602. Processing circuit 600 also includes a non-transitory computer-readable storage medium 604 used to implement the enhancements to the LTM HO procedure. Among other things, non-transitory computer-readable storage medium 604 is encoded with, i.e., stored with, instructions 606, i.e., computer program code, that are executed by processor 602, causing processor 602 to perform operations to enhance the LTM HO procedure. Execution of instructions 606 by processor 602 represents (at least in part) an application that implements at least a portion of a method (hereinafter referred to as a process and / or method) described herein, according to one or more embodiments.

[0101] The processor 602 is electrically coupled to a non-transitory computer-readable storage medium 604 via a bus 608. The processor 602 is electrically coupled to an input / output (I / O) interface 610 by the bus 608. A network interface 612 is also electrically connected to the processor 602 via the bus 608. The network interface 612 is connected to a network 614, thereby connecting the processor 602 and the non-transitory computer-readable storage medium 604 to external elements via the network 614. The processor 602 is configured to execute instructions 606 encoded in the non-transitory computer-readable storage medium 604 to enable the processing circuit 600 to perform at least portions of processes and / or methods. In one or more embodiments, the processor 602 is a central processing unit (CPU), a multiprocessor, a distributed processing system, an application-specific integrated circuit (ASIC), and / or other suitable processing unit.

[0102] Processing circuit 600 includes an I / O interface 610. I / O interface 610 is coupled to external circuitry. In one or more embodiments, I / O interface 610 includes a keyboard, keypad, mouse, trackball, trackpad, touchscreen, and / or cursor direction keys for communicating information and commands to processor 602.

[0103] Processing circuit 600 also includes a network interface 612 coupled to processor 602. Network interface 612 enables processing circuit 600 to communicate with a network 614 to which one or more other computer systems are connected. Network interface 612 includes a wireless network interface such as Bluetooth, Wi-Fi, Worldwide Interoperability for Microwave Access (WiMAX), General Packet Radio Service (GPRS), or Wideband Code Division Multiple Access (WCDMA), or a wired network interface such as Ethernet, Universal Serial Bus (USB), or Institute of Electrical and Electronics Engineers (IEEE) 864.

[0104] The processing circuit 600 is configured to receive information via an I / O interface 610. The information received via the I / O interface 610 includes one or more of instructions, data, design rules, a library of cells, and / or other parameters for processing by the processor 602. The information is transferred to the processor 602 via the bus 608. The processing circuit 600 is configured to receive information related to a user interface (UI) 620 via the I / O interface 610. The information is stored in the non-transitory computer-readable storage medium 604 as the UI 620. The UI 620 can process network data 622.

[0105] In one or more embodiments, one or more non-transitory computer-readable storage media 604 that store instructions 606 (in compressed or uncompressed format) that can be used to program a computer, processor, or other electronic device to perform the processes or methods described herein. The one or more non-transitory computer-readable storage media 604 include one or more of an electronic storage medium, a magnetic storage medium, an optical storage medium, a quantum storage medium, etc.

[0106] For example, the non-transitory computer-readable storage medium 604 may include, but is not limited to, a hard drive, a floppy diskette, an optical disk, a read-only memory (ROM), a random-access memory (RAM), an erasable programmable ROM (EPROM), an electrically erasable programmable ROM (EEPROM), a flash memory, a magnetic or optical card, a solid-state memory device, or any other type of physical medium suitable for storing electronic instructions. In one or more embodiments that use an optical disk, the one or more non-transitory computer-readable storage media 604 includes a compact disk read-only memory (CD-ROM), a compact disk read / write (CD-R / W), and / or a digital video disk (DVD).

[0107] In one or more embodiments, the non-transitory computer-readable storage medium 604 stores instructions 606 configured to cause the processor 602 to execute at least a portion of a process and / or method for improving the LTM HO procedure. In one or more embodiments, the non-transitory computer-readable storage medium 604 also stores information, such as algorithms, that facilitate the execution of at least a portion of the process and / or method for improving the LTM HO procedure.

[0108] Thus, in at least one embodiment, the processor 602 executes instructions 606 stored on one or more non-transitory computer-readable storage media 604 to implement enhancements to the LTM HO procedure. Network data 622 is processed via a user interface 620. The UE is configured for L1 / L2 triggered mobility (LTM) with LTM candidate cells. The non-transitory computer-readable storage medium 604 stores data for implementing enhancements to the LTM HO procedure. For example, the non-transitory computer-readable storage medium 604 stores L3 RRC measurements 630 from the UE. The UE context setup request and response 676 provides cell group configuration for CBRA 682. The user equipment (UE) then transmits a DL RRC message 662. The DL RRC message 662 provides RRC reconfiguration, such as an LTM target cell configuration 666 and multicast L1 measurements 668. Based on the RRC reconfiguration 664, the UE returns L1 measurements 632 to the serving cell and the target cell. RACH resources 634, including RACH preambles, are also maintained in the non-transitory computer-readable storage medium 604. Information identifying and relating to candidate / neighbor cells 636, target cell 638, and serving cell 640 is stored in the non-transitory computer-readable storage medium 604. An LTM configuration 642 is also maintained in the non-transitory computer-readable storage medium 604. L1 measurements 632 at the LTM candidate cell 636 are received from the UE. Based on the L1 measurements 632, whether radio conditions at the LTM candidate cell 636 satisfy predetermined resource reservation criteria 644, such as RSRP. In response to the L1 measurements 632 satisfying the predetermined resource reservation criteria 644, an updated radio resource configuration, such as an LTM target cell configuration 666, including a CBRA preamble, multicast L1 measurements 668, and a CFRA preamble 670, is determined, for example, using a UE context modification request / response 676 transmitted over the F1 interface.Further, in response to the L1 measurements 632 satisfying the predetermined resource reservation criteria 644, a reservation of partial or full radio resources in one of the one or more LTM candidate cells is initiated for an LTM cell switch by the UE from the serving cell to one of the one or more LTM candidate cells. The UE context modification request / response 676 includes a RACH preamble 678 and a beam group 680. The LTM target cell 638 is prepared without reserving RACH resources 634, including the RACH preamble. The user equipment (UE) is sent a DL RRC message 662. The second DL RRC message 662 provides an updated radio resource configuration. The second DL RRC message 662 includes an RRC reconfiguration, such as an LTM target cell configuration with a CBRA preamble 666. In at least one other embodiment, in response to the L1 measurements 632 satisfying the predetermined resource reservation criteria 644, a data transfer is initiated 648 using a control PDU / signaling message 646. The modify bearer context request 672 provides an LTM target DU tunnel endpoint identifier (TEID) 674. Data forwarding to the LTM target cell is stopped 650 based on the radio conditions of the LTM target cell falling below a predetermined resource reservation criterion 644. The predetermined resource reservation criterion 644 may also include a stop L1-RSRP threshold. After the UE is provided with an RRC reconfiguration 664, the UE provides updated L1 measurements 632 to the serving cell and the target cell. In response to the radio conditions for the target cell being met, a serving cell change is performed to the target cell. The serving cell sends a MAC CE 658 to the UE that includes a serving cell change command 660. The UE sends a serving cell change notification 654 that includes the cell ID 656. The RACH resources 634, including the RACH preamble of the LTM target cell 638, are provided to the UE before the serving cell change 660 occurs.Providing the UE with RACH resources 634 including a RACH preamble of the LTM target cell 638 before the serving cell change 660 change occurs may include allocating RACH resources 634 including a RACH preamble corresponding to a best cell beam configuration 652 in the UE that includes a best beam group 680. The display 690 includes a user interface 692 for presenting network data 694.

[0109] At least one embodiment of a method for preparing and performing a Lower Layer Triggered Mobility (LTM) Handover (HO) procedure includes: transmitting, by a gNB Centralized Unit Control Plane (gNB-CU-CP), either directly to a user equipment or via a serving gNB Distributed Unit (gNB-DU), one or more Radio Resource Control (RRC) messages with LTM candidate cell configurations of one or more LTM candidate gNB-DUs; transmitting, by the user equipment or the serving gNB-DU, a Layer 1 (L1) measurement report to the one or more LTM candidate gNB-DUs based on the one or more RRC messages; receiving, at the one or more LTM candidate gNB-DUs, the L1 measurement report; determining, by the one or more LTM candidate gNB-DUs in response to receiving the L1 measurement report, whether radio conditions of one of the one or more LTM candidate gNB-DUs satisfy predetermined resource reservation criteria; The method includes initiating, by one or more LTM candidate gNB-DUs, reservation of partial or full radio resources in one of the one or more LTM candidate cells for an LTM cell switch by the UE from a serving cell to one of the one or more LTM candidate cells, based on the radio conditions of the UE satisfying predetermined resource reservation criteria or based on the UE performing uplink (UL) synchronization by sending a random access channel (RACH) request to one or more candidate gNB-DUs to obtain timing advance; transmitting, by the one or more LTM candidate gNB-DUs, an updated LTM candidate cell configuration corresponding to the partial or full radio resources; and transmitting, by the gNB-CU-CP, to the UE via the serving gNB-DU, an updated radio resource configuration in the one or more LTM candidate cells having the partial or full radio resources of the one or more LTM candidate cells.

[0110] The embodiments described herein provide a method that provides one or more advantages: For example, configuration in LTM handover is performed in the gNB-CU-CP, but the LTM handover is executed autonomously by the gNB-DU without further interaction with higher layers; the lead time duration between preparation and execution of LTM HO is reduced, providing only target candidate cell configuration culminating in LTM serving cell change; the issue of slow data transfer is eliminated, and overhead in terms of processing and buffer occupancy is reduced.

[0111] One aspect of this description is a method [1] for preparing and performing a Layer 1-Layer 2 (L1 / L2) triggered mobility (LTM) handover (HO) procedure, comprising: receiving Layer 1 (L1) measurement reports from a user equipment (UE) or directly from a serving gNB-DU at one or more LTM candidate gNB-distributed units (gNB-DUs), the L1 measurement reports being based on one or more Radio Resource Control (RRC) messages with LTM candidate cell configurations of the one or more LTM candidate gNB-DUs previously sent to the UE via a serving cell;

[0112] determining, by the one or more LTM candidate gNB-DUs in response to receiving the L1 measurement report, whether radio conditions of one of the one or more LTM candidate gNB-DUs satisfy predetermined resource reservation criteria;

[0113] Initiating partial or full radio resource reservation by the one or more LTM candidate gNB-DUs in one of the one or more LTM candidate cells for an LTM cell switch by the UE from the serving cell to one of the one or more LTM candidate cells based on radio conditions of the one or more LTM candidate gNB-DUs satisfying predetermined resource reservation criteria or based on the UE performing uplink (UL) synchronization by sending a random access channel (RACH) request to the one or more LTM candidate gNB-DUs to obtain timing advance;

[0114] transmitting to the UE via the serving gNB-DU an updated radio resource configuration in one or more LTM candidate cells with reserved partial or full radio resources of the one or more LTM candidate cells; The present invention relates to a method [1] comprising:

[0115] The method of [1], wherein determining whether the radio conditions of one of the one or more LTM candidate gNB-DUs satisfy a predetermined resource reservation criterion further includes at least determining whether an L1 measurement value of one of the one or more LTM candidate cells is greater than a predetermined reference signal received power (RSRP) threshold.

[0116] The method of [1]-[2] further includes one or more candidate gNB-DUs initiating data transfer from a gNB-centralized unit user plane (gNB-CU-UP) to one or more LTM candidate gNB-DUs based on radio conditions of one of the one or more LTM candidate gNB-DUs reported by the UE that meet predetermined resource reservation criteria.

[0117] The method described in [1]-[3] further includes determining that radio conditions of one of the one or more LTM candidate gNB-DUs fall below a predetermined resource reservation criterion, and in response, stopping data transfer of data to one of the one or more LTM candidate gNB-DUs.

[0118] The method according to any one of [1] to [4], wherein initiating partial or complete radio resource reservation in one of the one or more LTM candidate cells for the UE by one or more LTM candidate gNB-DUs includes initiating reservation of at least random access channel (RACH) resources including a RACH preamble of one of the one or more LTM candidate cells for the UE before the LTM cell switches from the serving cell to one of the one or more LTM candidate cells.

[0119] The method described in [1]-[5], wherein initiating reservation of at least RACH resources of one of one or more LTM candidate cells of a UE by one or more LTM candidate gNB-DUs includes initiating reservation of at least RACH resources of one of one or more LTM candidate cells of the UE including a RACH preamble corresponding to the best beam or best beam group of one of the one or more LTM candidate cells in the UE based on an L1 measurement report.

[0120] The method described in [1]-[6], wherein initiating data transfer from the gNB-CU-UP to one or more LTM candidate gNB-DUs is triggered based on receiving a control protocol data unit (PDU) from one of the one or more LTM candidate gNB-DUs or receiving a signaling message from one of the one or more LTM candidate gNB-DUs by a gNB centralization unit (gNB-CU) via the F1 and E1 interfaces, respectively, in the gNB-CU-UP.

[0121] The method described in [1]-[7] further includes configuring the UE for L1 / L2 triggered mobility (LTM) with one or more LTM candidate cells before receiving an L1 measurement report, receiving Layer 3 (L3) RRC measurements from the UE at the gNB-CU-CP, and sending one or more RRC reconfiguration messages with LTM candidate cell configurations of the one or more LTM candidate cells to the UE via the serving gNB-DU.

[0122] One aspect of this description relates to a Next Generation Radio Access Network (NG-RAN) node [9] comprising a candidate distributed unit (DU), the candidate DU including: a memory storing computer-readable instructions; and a processor coupled to the memory, the processor executing the computer-readable instructions to perform an operation to receive Layer 1 (L1) measurement reports at one or more Layer 1-Layer 2 (L1 / L2) triggered mobility (LTM) candidate gNB-distributed units (gNB-DUs) directly from a user equipment (UE) or from a serving gNB-DU via a gNB-CU, the L1 measurement reports being based on one or more Radio Resource Control (RRC) messages with LTM candidate cell configurations of the one or more LTM candidate gNB-DUs previously transmitted to the UE via a serving cell; and in response to receiving the L1 measurement reports, determining that a radio resource state of one of the one or more LTM candidate gNB-DUs is in a predetermined resource state. and, based on the radio conditions of one of the one or more LTM candidate gNB-DUs satisfying a predetermined resource reservation criterion or based on the UE performing uplink (UL) synchronization by transmitting a random access channel (RACH) request to the one or more candidate gNB-DUs to obtain timing advance of the candidate cell, initiate, in the one or more LTM candidate gNB-DUs, reservation of partial or full radio resources in one of the one or more LTM candidate cells for an LTM cell switch by the UE from the serving cell to one of the one or more LTM candidate cells; and transmit, to the UE via the serving gNB-DU, an updated radio resource configuration in the one or more candidate cells having the partial or full radio resources of the one or more LTM candidate cells.

[0123] [9] A next generation radio access network (NG-RAN) node comprising a candidate distribution unit as described in [9], wherein the processor is further configured to determine whether radio conditions of one of the one or more LTM candidate gNB-DUs satisfy predetermined resource reservation criteria by at least determining whether an L1 measurement value of one of the one or more LTM candidate cells is greater than a predetermined reference signal received power (RSRP) threshold.

[0124] A next generation radio access network (NG-RAN) node comprising a candidate distribution unit as described in [9]-

[10] , wherein the processor is further configured to initiate data transfer to one of the one or more LTM candidate gNB-DUs based on radio conditions of the one of the one or more LTM candidate gNB-DUs reported by the UE that meet predetermined resource reservation criteria.

[0125] [9]-

[11] . A next generation radio access network (NG-RAN) node comprising a candidate distribution unit as described in [9]-

[11] , wherein the processor is further configured to determine that radio conditions of one of the one or more LTM candidate gNB-DUs fall below a predetermined resource reservation criterion, and in response, stop data forwarding of data to one of the one or more LTM candidate gNB-DUs.

[0126] The processor is further configured to initiate reservation of partial or full radio resources in one of the one or more LTM candidate cells for the UE in one or more LTM candidate gNB-DUs by initiating reservation of random access channel (RACH) resources including a RACH preamble of one of the one or more LTM candidate cells for the UE before the LTM cell switches from the serving cell to one of the one or more LTM candidate cells.

[0127] A next generation radio access network (NG-RAN) node comprising a candidate distribution unit as described in [9]-

[13] , wherein the processor is further configured to initiate reservation of at least RACH resources of one of one or more LTM candidate cells of the UE by initiating reservation of at least RACH resources of one of one or more LTM candidate cells of the UE based on an L1 measurement report, the RACH preamble corresponding to the best beam or best beam group of the one or more LTM candidate cells of the UE.

[0128] A next generation radio access network (NG-RAN) node comprising a candidate distribution unit as described in [9]-

[14] , wherein the processor is further configured to initiate data forwarding from a gNB-centralized unit user plane (gNB-CU-UP) to one or more LTM candidate gNB-DUs based on receiving a control protocol data unit (PDU) from one of the one or more LTM candidate gNB-DUs by a gNB centralized unit control plane (gNB-CU-CP) via the F1 and E1 interfaces, respectively, or receiving a signaling message from one of the one or more LTM candidate gNB-DUs.

[0129] A next generation radio access network (NG-RAN) node comprising a candidate distribution unit as described in [9]-

[15] , wherein the processor is further configured to configure the UE for L1 / L2 triggered mobility (LTM) with one or more LTM candidate cells before receiving the L1 measurement report, receive Layer 3 (L3) RRC measurements from the UE, and send one or more RRC reconfiguration messages to the UE with LTM candidate cell configurations of the one or more LTM candidate cells.

[0130] One aspect of this description relates to a non-transitory computer-readable medium

[17] having computer-readable instructions stored thereon, which, when executed by a processor, include operations of receiving Layer 1 (L1) measurement reports at one or more LTM candidate gNB distributed units (gNB-DUs) directly from a user equipment (UE) or from a serving gNB-DU, where the L1 measurement reports are based on one or more Radio Resource Control (RRC) messages with LTM candidate cell configurations of the one or more LTM candidate gNB-DUs previously transmitted to the UE via a serving cell; determining, by the one or more LTM candidate gNB-DUs in response to receiving the L1 measurement reports, whether radio conditions of one of the one or more LTM candidate gNB-DUs satisfy predetermined resource reservation criteria; and determining whether radio conditions of one of the one or more LTM candidate gNB-DUs satisfy predetermined resource reservation criteria. The processor performs operations including: initiating reservation of partial or full radio resources in one of the one or more LTM candidate cells by one or more LTM candidate gNB-DUs for an LTM cell switch by the UE from a serving cell to one of the one or more LTM candidate cells based on the radio conditions of the M candidate gNB-DUs satisfying predetermined resource reservation criteria or based on the UE performing uplink (UL) synchronization by sending a random access channel (RACH) request to the one or more LTM candidate gNB-DUs to obtain timing advance; and transmitting an updated radio resource configuration in the one or more candidate cells having partial or full radio resources of the one or more LTM candidate cells to the UE via the serving gNB-DU.

[0131] One aspect of this description relates to a method

[18] for preparing and performing a Layer 1-Layer 2 (L1 / L2) triggered mobility (LTM) handover (HO) procedure, the method comprising: receiving, at a candidate gNB distributed unit (gNB-DU), a Layer 1 (L1) measurement report in one or more LTM candidate cells; determining, by the candidate gNB-DU, based on the L1 measurement report, whether radio conditions of one of the one or more LTM candidate cells satisfy a predetermined resource reservation criterion; and determining, based on the radio conditions of one of the one or more LTM candidate cells satisfying the predetermined resource reservation criterion, whether a timing adjustment is performed. The method includes determining reserved partial or full radio resources in one of the one or more LTM candidate cells for an LTM cell switch to one of the one or more LTM candidate cells by the UE based on the UE performing uplink (UL) synchronization by transmitting a random access channel (RACH) request to the one or more candidate gNB-DUs to obtain an advance, and transmitting, by the candidate gNB-DU, an updated LTM candidate cell configuration corresponding to the reserved partial or full radio resources to a gNB-centralized unit control plane (gNB-CU-CP).

[0132] The method described in

[18] , wherein sending an updated LTM candidate cell configuration corresponding to the reserved partial or complete radio resources includes sending an F1:UE context modification request message to the gNB-CU-CP, receiving a UE context modification acknowledgement from the gNB-CU-CP, and sending an RRC reconfiguration message including the updated LTM candidate cell configuration from the gNB-CU-CP to the UE.

[0133] The method described in

[18] -

[19] further includes receiving data from a gNB-centralized unit user plane (gNB-CU-UP) by the candidate gNB-DU based on the radio conditions of the candidate gNB-DU satisfying predetermined resource reservation criteria.

[0134] The method described in

[18] -

[20] further includes transmitting a control protocol data unit (PDU) or a signaling message to a gNB-centralized unit user plane (gNB-CU-UP).

[0135] The method described in

[18] -

[21] further includes transmitting a data forwarding stop instruction from the candidate gNB-DU to the gNB-centralized unit user plane (gNB-CU-UP) to stop forwarding data to the candidate gNB-DU in response to the radio conditions of the candidate gNB-DU falling below a predetermined resource reservation criterion.

[0136] The method according to any one of

[18] to

[22] , further comprising receiving, in the gNB-DU, at least a random access channel (RACH) resource including a RACH preamble included in an updated radio resource configuration transmitted to the UE.

[0137] The method described in

[18] -

[23] , wherein receiving at least a random access channel (RACH) resource including a RACH preamble in a gNB-DU includes receiving at least a random access channel (RACH) resource including a RACH preamble corresponding to the best beam or best beam group of the candidate gNB-DU based on an L1 measurement report.

[0138] One aspect of this description relates to a Next Generation Radio Access Network (NG-RAN) node

[25] comprising a candidate gNB distributed unit (gNB-DU), the gNB-DU comprising: a memory that stores computer-readable instructions; and a processor coupled to the memory, the processor configured to execute the computer-readable instructions to perform operations of receiving Layer 1 (L1) measurement reports at one or more LTM candidate cells; determining, based on the L1 measurement reports, whether radio conditions of one of the one or more LTM candidate cells satisfy a predetermined resource reservation criterion; determining, based on the radio conditions of one of the one or more LTM candidate cells satisfying the predetermined resource reservation criterion or based on the UE performing uplink (UL) synchronization by transmitting a random access channel (RACH) request to the one or more candidate cells to obtain timing advance, reserved partial or full radio resources at one of the one or more LTM candidate cells for an LTM cell switch by the UE to one of the one or more LTM candidate cells; and transmitting an updated LTM candidate cell configuration corresponding to the reserved partial or full radio resources to a gNB-centralized unit control plane (gNB-CU-CP).

[0139] The processor is further configured to: send an updated LTM candidate cell configuration corresponding to the reserved partial or full radio resources by sending an F1: UE context modification request message to the gNB-CU-CP and receiving a UE context modification acknowledgement from the gNB-CU-CP;

[25] A next generation radio access network (NG-RAN) node comprising a gNB-DU as described in

[25] .

[0140] The processor is further configured to receive data from a gNB-centralized unit user plane (gNB-CU-UP) based on the L1 measurement report indicating that the radio conditions of the candidate cell satisfy a predetermined resource reservation criterion.

[25] -

[26] A next generation radio access network (NG-RAN) node having a gNB-DU as described in

[25] -

[26] .

[0141] The processor is further configured to transmit a control protocol data unit (PDU) or a signaling message to a gNB-centralized unit user plane (gNB-CU-UP).

[25] -

[27] A next generation radio access network (NG-RAN) node having a gNB-DU as described in.

[0142]

[25] -

[28] . A next generation radio access network (NG-RAN) node having a gNB-DU as described in

[25] -

[28] , wherein the processor is further configured to send a data forwarding stop instruction to a gNB-centralized unit user plane (gNB-CU-UP) to stop forwarding data in response to radio conditions falling below a predetermined resource reservation criterion.

[0143] The processor is further configured to receive at least a random access channel (RACH) resource including a RACH preamble included in an updated radio resource configuration transmitted to the UE.

[25] -

[29] A next generation radio access network (NG-RAN) node having a gNB-DU as described in

[25] -

[29] .

[0144] A next generation radio access network (NG-RAN) node having a gNB-DU as described in

[25] -

[30] , wherein the processor is further configured to receive at least a random access channel (RACH) resource including a RACH preamble by receiving at least a random access channel (RACH) resource including a RACH preamble corresponding to the best beam or the best beam group based on an L1 measurement report.

[0145] One aspect of this description relates to a non-transitory computer-readable medium

[32] having computer-readable instructions stored thereon, the computer-readable instructions, when executed by a processor, comprising: operations, at a candidate gNB distributed unit (gNB-DU), of receiving a Layer 1 (L1) measurement report at one or more LTM candidate cells; determining, by the candidate gNB-DU, whether radio conditions of one of the one or more LTM candidate cells meet a predetermined resource reservation criterion based on the L1 measurement report; and determining, based on the radio conditions of one of the one or more LTM candidate cells meeting the predetermined resource reservation criterion, whether the radio conditions of one of the one or more LTM candidate cells meet the predetermined resource reservation criterion or whether the timing advertisement is received. and transmitting, by the candidate gNB-DU, an updated LTM candidate cell configuration corresponding to the reserved partial or full radio resources to a gNB-centralized unit control plane (gNB-CU-CP).

[0146] One aspect of this description relates to a method

[33] for preparing and performing a Layer 1-Layer 2 (L1 / L2) triggered mobility (LTM) handover (HO) procedure, the method comprising: transmitting, by a gNB centralized unit control plane (gNB-CU-CP), either directly to a user equipment or via a serving gNB distributed unit (gNB-DU), one or more radio resource control (RRC) messages with LTM candidate cell configurations of one or more LTM candidate gNB-DUs; transmitting, by the user equipment or the serving gNB-DU, a Layer 1 (L1) measurement report, an L1 measurement report, to the one or more LTM candidate gNB-DUs based on the one or more RRC messages; receiving, in the one or more LTM candidate gNB-DUs, the L1 measurement report; and determining, by the one or more LTM candidate gNB-DUs in response to receiving the L1 measurement report, whether radio conditions of one of the one or more LTM candidate gNB-DUs satisfy a predetermined resource reservation criterion. The method includes initiating, by the one or more LTM candidate gNB-DUs, reservation of partial or full radio resources in one of the one or more LTM candidate cells for an LTM cell switch by the UE from a serving cell to one of the one or more LTM candidate cells, based on the radio conditions of the one or more LTM candidate gNB-DUs satisfying predetermined resource reservation criteria or based on the UE performing uplink (UL) synchronization by sending a random access channel (RACH) request to the one or more candidate gNB-DUs to obtain timing advance; transmitting, by the one or more LTM candidate gNB-DUs, an updated LTM candidate cell configuration corresponding to the partial or full radio resources to the gNB-CU-CP; and transmitting, by the gNB-CU-CP, an updated radio resource configuration having the partial or full radio resources of the one or more LTM candidate cells to the UE via the serving gNB-DU.

[0147] Separate instances of these programs may be running on or distributed on any number of separate computer systems. Thus, although particular steps are described as being performed by particular devices, software programs, processes, or entities, this is not necessarily the case. Various alternative implementations will be appreciated by those skilled in the art.

[0148] Moreover, those skilled in the art will readily recognize that the foregoing techniques can be utilized in a variety of devices, environments, and contexts. Although the embodiments have been described in language specific to structural features or methodological acts, the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described. Rather, the specific features and acts are disclosed as example forms of implementing the claims.

Claims

1. 1. A method for preparing and performing a Layer 1-Layer 2 (L1 / L2) triggered mobility (LTM) handover (HO) procedure, comprising: Receiving Layer 1 (L1) measurement reports at one or more LTM candidate gNB distributed units (gNB-DUs) directly from a user equipment (UE) or from a serving gNB-DU, the L1 measurement reports being based on one or more radio resource control (RRC) messages with LTM candidate cell configurations of the one or more LTM candidate gNB-DUs previously sent to the UE via a serving cell; In response to receiving the L1 measurement report, determining, by the one or more LTM candidate gNB-DUs, whether radio conditions of one of the one or more LTM candidate gNB-DUs satisfy predetermined resource reservation criteria; Initiating partial or complete radio resource reservation by the one or more LTM candidate gNB-DUs for an LTM cell switch by the UE from the serving cell to one of the one or more LTM candidate cells based on the radio conditions of the one or more LTM candidate gNB-DUs satisfying the predetermined resource reservation criteria or based on the UE performing uplink (UL) synchronization by sending a random access channel (RACH) request to the one or more LTM candidate gNB-DUs to obtain timing advance; Transmitting an updated radio resource configuration in the one or more LTM candidate cells having the reserved partial or complete radio resources of the one or more LTM candidate cells to the UE via the serving gNB-DU; A method comprising:

2. The method of claim 1, wherein determining whether the radio conditions of the one of the one or more LTM candidate gNB-DUs satisfy the predetermined resource reservation criteria further comprises at least determining whether an L1 measurement value of the one of the one or more LTM candidate cells is greater than a predetermined reference signal received power (RSRP) threshold.

3. 2. The method of claim 1, further comprising the one or more candidate gNB-DUs initiating data transfer from a gNB-centralized unit user plane (gNB-CU-UP) to the one or more LTM candidate gNB-DUs based on radio conditions of the one of the one or more LTM candidate gNB-DUs reported by the UE that meet the predetermined resource reservation criteria.

4. The method of claim 3, further comprising determining that the radio conditions of the one of the one or more LTM candidate gNB-DUs fall below the predetermined resource reservation criterion, and in response thereto, stopping the data transfer of data to the one of the one or more LTM candidate gNB-DUs.

5. 4. The method of claim 3, wherein initiating the reservation of the partial or complete radio resources in the one of the one or more LTM candidate cells of the UE by the one or more LTM candidate gNB-DUs includes initiating reservation of at least a random access channel (RACH) resource including a RACH preamble of the one of the one or more LTM candidate cells of the UE before the LTM cell switches from the serving cell to the one of the one or more LTM candidate cells.

6. The method of claim 5, wherein initiating reservation of at least the RACH resources of one of the one or more LTM candidate cells of the UE by the one or more LTM candidate gNB-DUs includes initiating reservation of at least the RACH resources of one of the one or more LTM candidate cells of the UE including the RACH preamble corresponding to the best beam or best beam group of the one of the one or more LTM candidate cells at the UE based on the L1 measurement report.

7. The method of claim 3, wherein initiating the data transfer from the gNB-CU-UP to the one or more LTM candidate gNB-DUs is triggered at the gNB-CU-UP based on receiving a control protocol data unit (PDU) from one of the one or more LTM candidate gNB-DUs or receiving a signaling message from one of the one or more LTM candidate gNB-DUs by a gNB centralization unit (gNB-CU) via F1 and E1 interfaces, respectively.

8. before receiving the L1 measurement report, configuring the UE for L1 / L2 triggered mobility (LTM) with the one or more LTM candidate cells; Receiving Layer 3 (L3) RRC measurements from the UE in a gNB-centralized unit user plane (gNB-CU-UP); Sending one or more RRC reconfiguration messages with LTM candidate cell configurations of the one or more LTM candidate cells to the UE via the serving gNB-DU; The method of claim 1 further comprising:

9. A Next Generation Radio Access Network (NG-RAN) node comprising a candidate distribution unit (DU), the candidate DU comprising: a memory storing computer readable instructions; a processor coupled to the memory, the processor executing the computer-readable instructions to: An operation of receiving a Layer 1 (L1) measurement report at one or more Layer 1-Layer 2 (L1 / L2) triggered mobility (LTM) candidate gNB-distribution units (gNB-DUs) directly from a user equipment (UE) or from a serving gNB-DU via a gNB-CU, wherein the L1 measurement report is based on one or more Radio Resource Control (RRC) messages with LTM candidate cell configurations of the one or more LTM candidate gNB-DUs previously transmitted to the UE via the serving cell; In response to receiving the L1 measurement report, determining whether radio conditions of one of the one or more LTM candidate gNB-DUs meet predetermined resource reservation criteria; an operation of initiating partial or complete radio resource reservation in one of one or more LTM candidate cells in the one or more LTM candidate gNB-DUs for an LTM cell switch by the UE from the serving cell to the one of the one or more LTM candidate cells, based on the radio conditions of the one of the one or more LTM candidate gNB-DUs satisfying the predetermined resource reservation criteria or based on the UE performing uplink (UL) synchronization by sending a random access channel (RACH) request to the one or more candidate gNB-DUs to obtain timing advance of the candidate cell; An operation of transmitting an updated radio resource configuration in the one or more candidate cells having the partial or complete radio resources of the one or more LTM candidate cells to the UE via the serving gNB-DU; The NG-RAN node is configured to:

10. 10. The NG-RAN node of claim 9, wherein the processor is configured to determine whether the radio conditions of the one of the one or more LTM candidate gNB-DUs satisfy the predetermined resource reservation criteria by at least determining whether an L1 measurement value of the one of the one or more LTM candidate cells is greater than a predetermined reference signal received power (RSRP) threshold.

11. The NG-RAN node of claim 9, wherein the processor is configured to initiate data transfer to one of the one or more LTM candidate gNB-DUs based on the radio conditions of the one of the one or more LTM candidate gNB-DUs reported by the UE that meet the predetermined resource reservation criteria.

12. The NG-RAN node of claim 11, wherein the processor is further configured to determine that the radio conditions of the one of the one or more LTM candidate gNB-DUs fall below the predetermined resource reservation criterion, and accordingly stop the data forwarding of data to the one of the one or more LTM candidate gNB-DUs.

13. 12. The NG-RAN node of claim 11, wherein the processor is configured to initiate the reservation of the partial or complete radio resources in the one of the one or more LTM candidate cells for the UE in the one or more LTM candidate gNB-DUs by initiating reservation of a random access channel (RACH) resource including a RACH preamble for the one of the one or more LTM candidate cells for the UE before the LTM cell switches from the serving cell to the one of the one or more LTM candidate cells.

14. The NG-RAN node of claim 13, wherein the processor is configured to initiate reservation of at least a RACH resource of one of the one or more LTM candidate cells of the UE by initiating reservation of at least a RACH resource of the one of the one or more LTM candidate cells of the UE, including the RACH preamble corresponding to the best beam or best beam group of the one or more LTM candidate cells at the UE, based on the L1 measurement report, in the one or more LTM candidate gNB-DUs.

15. The NG-RAN node of claim 11, wherein the processor is configured to initiate the data forwarding from a gNB-concentration unit user plane (gNB-CU-UP) to the one or more LTM candidate gNB-DUs based on receiving a control protocol data unit (PDU) from the one or more LTM candidate gNB-DUs or receiving a signaling message from the one or more LTM candidate gNB-DUs by a gNB centralization unit control plane (gNB-CU-CP) via F1 and E1 interfaces, respectively.

16. The processor: before receiving the L1 measurement report, configuring the UE for L1 / L2 triggered mobility (LTM) with the one or more LTM candidate cells; receiving Layer 3 (L3) RRC measurements from the UE; and sending one or more RRC reconfiguration messages to the UE with LTM candidate cell configurations of the one or more LTM candidate cells; The NG-RAN node of claim 9, further configured to:

17. A non-transitory computer-readable medium having computer-readable instructions stored thereon, the computer-readable instructions, when executed by a processor, receiving Layer 1 (L1) measurement reports at one or more LTM candidate gNB distributed units (gNB-DUs) directly from a user equipment (UE) or from a serving gNB-DU, the L1 measurement reports being based on one or more radio resource control (RRC) messages with LTM candidate cell configurations of the one or more LTM candidate gNB-DUs previously transmitted to the UE via a serving cell; In response to receiving the L1 measurement report, determining, by the one or more LTM candidate gNB-DUs, whether radio conditions of one of the one or more LTM candidate gNB-DUs satisfy predetermined resource reservation criteria; An operation of initiating, by the one or more LTM candidate gNB-DUs, reservation of partial or complete radio resources in one of one or more LTM candidate cells for an LTM cell switch by the UE from the serving cell to the one of the one or more LTM candidate cells, based on the radio conditions of the one or more LTM candidate gNB-DUs satisfying the predetermined resource reservation criteria or based on the UE performing uplink (UL) synchronization by sending a random access channel (RACH) request to the one or more candidate gNB-DUs to obtain timing advance; An operation of transmitting an updated radio resource configuration in the one or more candidate cells having the partial or complete radio resources of the one or more LTM candidate cells to the UE via the serving gNB-DU; A non-transitory computer-readable medium that causes the processor to perform operations including:

18. 1. A method for preparing and performing a Lower Layer Triggered Mobility (LTM) Handover (HO) procedure, comprising: At a candidate gNB distributed unit (gNB-DU), receiving Layer 1 (L1) measurement reports in one or more LTM candidate cells; determining, by the candidate gNB-DU, based on the L1 measurement report, whether radio conditions of one of the one or more LTM candidate cells satisfy predetermined resource reservation criteria; determining reserved partial or complete radio resources in the one of the one or more LTM candidate cells for an LTM cell switch to the one of the one or more LTM candidate cells by the UE based on the radio conditions of the one of the one or more LTM candidate cells satisfying the predetermined resource reservation criteria or based on the UE performing uplink (UL) synchronization by sending a random access channel (RACH) request to the one or more candidate gNB-DUs to obtain timing advance; sending, by the candidate gNB-DU, an updated LTM candidate cell configuration corresponding to the reserved partial or complete radio resources to a gNB-centralized unit control plane (gNB-CU-CP); A method comprising:

19. transmitting the updated LTM candidate cell configuration corresponding to the reserved partial or complete radio resources, F1: Sending a UE context modification request message to the gNB-CU-CP; receiving, from the gNB-CU-CP, a UE context modification confirmation response from the gNB-CU-CP; Sending an RRC reconfiguration message including the updated LTM candidate cell configuration from the gNB-CU-CP to the UE; 20. The method of claim 18, comprising:

20. The method of claim 18, further comprising receiving, by the candidate gNB-DU, data from a gNB-centralized unit user plane (gNB-CU-UP) based on the radio conditions of the candidate gNB-DU satisfying the predetermined resource reservation criteria.

21. The method of claim 18, further comprising transmitting a control protocol data unit (PDU) or a signaling message to a gNB-centralized unit user plane (gNB-CU-UP).

22. The method of claim 18, further comprising: in response to the radio conditions of the candidate gNB-DU falling below the predetermined resource reservation criterion, transmitting a data forwarding stop instruction from the candidate gNB-DU to a gNB-concentration unit user plane (gNB-CU-UP) to stop forwarding data to the candidate gNB-DU.

23. The method of claim 18, further comprising receiving, in the gNB-DU, at least a random access channel (RACH) resource including a RACH preamble included in an updated radio resource configuration transmitted to the UE.

24. The method of claim 23, wherein receiving at least the random access channel (RACH) resource including a RACH preamble in the gNB-DU includes receiving at least the random access channel (RACH) resource including the RACH preamble corresponding to the best beam or best beam group of the candidate gNB-DU based on the L1 measurement report.

25. A next generation radio access network (NG-RAN) node comprising a candidate gNB distributed unit (gNB-DU), the gNB-DU comprising: a memory storing computer readable instructions; a processor coupled to the memory; wherein the processor executes the computer-readable instructions to receiving a Layer 1 (L1) measurement report in one or more LTM candidate cells; determining, based on the L1 measurement report, whether radio conditions of one of the one or more LTM candidate cells satisfy a predetermined resource reservation criterion; determining reserved partial or complete radio resources in the one of the one or more LTM candidate cells for an LTM cell switch to the one of the one or more LTM candidate cells by the UE based on the radio conditions of the one of the one or more LTM candidate cells satisfying the predetermined resource reservation criterion or based on the UE performing uplink (UL) synchronization by transmitting a random access channel (RACH) request to the one or more candidate cells to obtain timing advance; sending an updated LTM candidate cell configuration corresponding to the reserved partial or complete radio resources to a gNB-Centralized Unit Control Plane (gNB-CU-CP); The NG-RAN node is configured to:

26. The NG-RAN node of claim 25, wherein the processor is further configured to transmit the updated LTM candidate cell configuration corresponding to the reserved partial or complete radio resources by sending an F1: UE context modification request message to the gNB-CU-CP and receiving a UE context modification acknowledgement from the gNB-CU-CP.

27. The NG-RAN node of claim 25, wherein the processor is further configured to receive data from a gNB-centralized unit user plane (gNB-CU-UP) based on the L1 measurement report indicating that the radio conditions of the candidate cell satisfy the predetermined resource reservation criterion.

28. The NG-RAN node of claim 25, wherein the processor is further configured to send a control protocol data unit (PDU) or a signaling message to a gNB-centralized unit user plane (gNB-CU-UP).

29. 26. The NG-RAN node of claim 25, wherein the processor is further configured to send a data forwarding stop instruction to a gNB-centralized unit user plane (gNB-CU-UP) to stop forwarding data in response to the radio conditions falling below the predetermined resource reservation criterion.

30. 26. The NG-RAN node of claim 25, wherein the processor is further configured to receive at least a Random Access Channel (RACH) resource including a RACH preamble included in an updated radio resource configuration transmitted to the UE.

31. The NG-RAN node of claim 25, wherein the processor is further configured to receive at least the random access channel (RACH) resource including the RACH preamble by receiving the at least the random access channel (RACH) resource including the RACH preamble corresponding to the best beam or best beam group based on the L1 measurement report.

32. A non-transitory computer-readable medium having computer-readable instructions stored thereon, the computer-readable instructions, when executed by a processor, receiving, at a candidate gNB distributed unit (gNB-DU), Layer 1 (L1) measurement reports for one or more LTM candidate cells; determining, by the candidate gNB-DU, whether radio conditions of one of the one or more LTM candidate cells satisfy predetermined resource reservation criteria based on the L1 measurement report; determining reserved partial or complete radio resources in the one of the one or more LTM candidate cells for an LTM cell switch to the one of the one or more LTM candidate cells by the UE based on the radio conditions of the one of the one or more LTM candidate cells satisfying the predetermined resource reservation criterion or based on the UE performing uplink (UL) synchronization by transmitting a random access channel (RACH) request to the one or more candidate cells to obtain timing advance; sending, by the candidate gNB-DU, an updated LTM candidate cell configuration corresponding to the reserved partial or complete radio resources to a gNB-Centralized Unit Control Plane (gNB-CU-CP); A non-transitory computer-readable medium that causes the processor to perform operations including:

33. 1. A method for preparing and performing a Lower Layer Triggered Mobility (LTM) Handover (HO) procedure, comprising: Sending one or more radio resource control (RRC) messages with LTM candidate cell configurations of one or more LTM candidate gNB-DUs to a user equipment directly or via a serving gNB-distributed unit (gNB-DU) by a gNB-centralized unit control plane (gNB-CU-CP); Based on the one or more RRC messages, sending a Layer 1 (L1) measurement report, the L1 measurement report, by the user equipment or the serving gNB-DU to the one or more LTM candidate gNB-DUs; Receiving the L1 measurement report at the one or more LTM candidate gNB-DUs; In response to receiving the L1 measurement report, determining, by the one or more LTM candidate gNB-DUs, whether radio conditions of one of the one or more LTM candidate gNB-DUs satisfy predetermined resource reservation criteria; Initiating partial or complete radio resource reservation by the one or more LTM candidate gNB-DUs for an LTM cell switch by the UE from the serving cell to one of the one or more LTM candidate cells based on the radio conditions of the one or more LTM candidate gNB-DUs satisfying the predetermined resource reservation criteria or based on the UE performing uplink (UL) synchronization by sending a random access channel (RACH) request to the one or more candidate gNB-DUs to obtain timing advance; transmitting, by the one or more LTM candidate gNB-DUs, an updated LTM candidate cell configuration corresponding to the partial or complete radio resources to the gNB-CU-CP; The gNB-CU-CP transmits the updated radio resource configuration having the partial or complete radio resources of the one or more LTM candidate cells to the UE via the serving gNB-DU; A method comprising:

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