Systems and methods for implementing lower layer triggered mobility (LTM) in next generation base stations

Abstract

The present disclosure describes a system and method performed by a centralized unit control plane (CU-CP) (110). The method may include identifying a target cell for a user equipment (UE) based on a lower layer triggered mobility LTM measurement received from the UE; performing admission control on the target cell by determining whether the number of UEs associated with the target cell is less than a threshold; generating an LTM configuration including an LTM candidate cell according to a determination that the number of UEs is less than a threshold, the LTM candidate cell including a target cell; and transmitting the generated LTM configuration to the UE in a radio resource control (RRC) reconfiguration message via a source distributed unit (DU) associated with the source cell for LTM.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to the field of wireless communication. In particular, the present disclosure relates to a system and method for implementing lower layer triggered mobility (LTM) in a next generation base station (gNB). BACKGROUND

[0002] Fifth generation (5G) New Radio (NR) is a wireless communication standard for the fifth generation mobile networks (5G). In 5G NR, a user equipment (UE) can move across different cells in a connected state, and this process of ensuring seamless connectivity and service continuity is referred to as mobility or handover. Different types of handovers are supported by the NR standard. A basic handover in NR includes three steps: handover preparation, handover execution, and handover completion. In the preparation phase, a base station (gNB) can configure the UE to report measurements, and based on the reported measurements or based on its own understanding of the network topology, the gNB sends a radio resource control (RRC) reconfiguration message to handover the UE from a source cell to another cell referred to as a target cell. During execution, the UE applies the target cell configuration and accesses the target cell, and during the completion step, the UE sends an RRC reconfiguration complete message. In an alternative approach, the handover is implemented by configuring the UE with execution conditions for triggering the handover, instead of a target cell configuration, and once the execution conditions are met, the UE can move to the target cell and send the RRC reconfiguration complete.

[0003] However, in the existing handover method referred to as layer 3 mobility, the gNB configures the UE with layer 3 measurements, and uses these measurements to prepare candidate / target cells, and subsequently sends a cell handover command to the UE. The sending of layer 3 messages by the UE results in considerable signaling overhead and latency issues.

[0004] Therefore, there is a need to overcome the above-mentioned limitations of the current technology, and provide a technique for implementing LTM in a gNB in an efficient manner. SUMMARY

[0005] SOLUTION TO THE PROBLEM Aspects of the disclosure address at least the above-mentioned problems and / or disadvantages and other advantages described below. Accordingly, one aspect of the disclosure describes systems and methods for implementing LTM in a gNB. In the disclosure, a method for implementing LTM in a gNB for intra-gNB LTM is described. Further, embodiments for implementing inter-DU and intra-DU LTM implementations are disclosed. Further, a number of steps related to the implementation of LTM in a centralized unit (CU) and a distributed unit (DU) are described. In step 1, a reference configuration at the CU is generated. In step 2, admission control at the CU and the DU is described. In step 3, early synchronization at the CU, source DU, and candidate DU is implemented. Further, timing advance management at the DU is implemented. The disclosure also describes interactions in various LTM scenarios at the CU, source DU, and candidate DU.

[0006] According to an aspect of the disclosure, a method for implementing lower layer triggered mobility (LTM) by a base station when performing a cell handover procedure of a user equipment (UE) from a source cell served by the base station to a target cell is disclosed. The method includes identifying a target cell for the UE for LTM based on LTM measurements received from the UE or based on one or more internal decisions, and thereafter, performing admission control for the target cell based on one or more factors. The method also includes generating an LTM configuration when the admission control is successful. The method further includes transmitting the generated LTM configuration to the UE in a radio resource control (RRC) reconfiguration message and to a source DU associated with the source cell. The method also includes facilitating early synchronization of the UE in the target cell based on an F1AP message received from a target DU associated with the target cell. The method further includes performing an LTM cell handover completion upon receiving an indication from the target DU indicating that access to the target cell is successful.

[0007] In an embodiment, the LTM configuration includes a full LTM candidate configuration; and at least one of an incomplete LTM candidate configuration and an LTM reference configuration. The LTM reference configuration is based on at least one of a cell group configuration associated with the source cell and a cell group configuration associated with the target cell and a layer 3 configuration associated with the CU. The layer 3 configuration includes a measurement configuration and a radio bearer configuration.

[0008] In an embodiment, the one or more factors include: a number of UEs associated with a neighboring cell that triggered the LTM, a number of RRC CONNECTED UEs in the target cell, a number of UEs associated with a neighboring cell configured with conditional handover. Performing admission control for the target cell includes performing one or more of: configuring the cell as an LTM candidate cell when the number of UEs associated with the cell that triggered the LTM is less than a first threshold; configuring the cell as an LTM candidate cell when a sum of the number of RRC CONNECTED UEs and the number of UEs associated with the cell that triggered the LTM is less than a second threshold; and configuring the cell as an LTM candidate cell when a sum of the number of RRC CONNECTED UEs and the number of UEs associated with the cell configured with conditional handover and the number of UEs associated with the cell that triggered the LTM is less than a third threshold.

[0009] In an embodiment, facilitating early synchronization of the UE in the target cell includes: receiving, by a centralized unit (CU) of the base station, an FlAP message including the LTM candidate cell identifier and the timing advance value from the target DU; and sending the LTM candidate cell identifier and the timing advance value to a source DU associated with the source cell.

[0010] In an embodiment, the method includes sending a physical downlink control channel (PDCCH) order to trigger or re-trigger the UE to perform early synchronization to the target cell when the timing advance value is not sent to the source DU by the target DU.

[0011] In an embodiment, the base station includes a centralized unit (CU) and one or more DUs for serving the UE. The CU includes a centralized unit control plane (CU-CP) and at least one centralized unit user plane (CU-UP), and the one or more DUs include a source DU and a target DU, the source DU and the target DU coupled to the at least one CU-UP for serving the UE.

[0012] In an embodiment, the method further includes the following operations: receiving LTM measurements from the UE. The LTM measurements include Layer 1 measurements, which include information associated with at least one of Reference Signal Received Power (RSRP), Reference Signal Received Quality (RSRQ), and Signal-to-Interference-plus-Noise Ratio (SINR). The method further includes: based on determining that the received LTM measurements satisfy a first threshold, sending a Physical Downlink Control Channel (PDCCH) command including information about a target cell to trigger the UE to perform early synchronization on the target cell. The information about the target cell includes an LTM candidate cell index. The method further includes: based on determining that the received LTM measurements satisfy a second threshold, sending a MAC Control Element (MAC CE) including information about the target cell to trigger the UE to perform cell handover, wherein the information about the target cell includes an LTM candidate cell index.

[0013] In one embodiment, a timing advance (TA) timer is started based on whether early synchronization has occurred in the UE; and the TA timer is reset when an indication about the cell handover process is received.

[0014] In this embodiment, one or more internal decisions include one or more decisions made by the base station based on deployment maps or load balancing techniques or predictive data received from one or more machine learning (ML) models. An RRC reconfiguration message is sent to the source DU in the F1AP message.

[0015] According to one aspect of this disclosure, a base station is disclosed for implementing lower-layer triggered mobility (LTM) during a cell handover process of a user equipment (UE) from a source cell to a target cell served by the base station. The base station is configured to: identify a target cell for LTM for the UE based on LTM measurements received from the UE or based on one or more internal decisions. The base station is further configured to: perform admission control on the target cell based on one or more factors. The base station is further configured to: generate an LTM configuration when admission control is successful. The base station is further configured to: send the generated LTM configuration to the UE in a Radio Resource Control (RRC) reconfiguration message and to the source DU associated with the source cell. The base station is further configured to: facilitate early synchronization of the UE in the target cell based on an F1AP message received from the target DU associated with the target cell, and perform LTM cell handover completion upon receiving an indication from the target DU indicating successful access to the target cell. Attached Figure Description

[0016] The above aspects, features, and advantages of certain embodiments of this disclosure will become clearer from the following description taken in conjunction with the accompanying drawings.

[0017] Figure 1This is a block diagram illustrating the NG-RAN architecture according to this disclosure.

[0018] Figure 2 This is an exemplary sequence diagram illustrating the overall process of LTM according to this disclosure.

[0019] Figures 3A-3D This is a flowchart illustrating access control in an LTM according to this disclosure.

[0020] Figure 4 This is a sequence diagram showing the generated sequence diagram according to the LTM configuration of this disclosure.

[0021] Figures 5A-5F This is a sequence diagram illustrating the Early Synchronization Random Access Channel (RACH) in LTM according to this disclosure.

[0022] Figure 6 This is a sequence diagram illustrating the determination of the downlink / uplink bandwidth portion (DL / ULBWP) for LTM according to this disclosure.

[0023] Figure 7 This is a sequence diagram illustrating the RRC connection re-establishment / radio link failure (RLF) handling using LTM according to this disclosure.

[0024] Figure 8A This is a sequence diagram illustrating a scenario for processing F1 Application Protocol (F1AP) reset during LTM according to this disclosure.

[0025] Figure 8B This is a sequence diagram illustrating a scenario for processing E1AP reset during LTM according to the present disclosure.

[0026] Figure 9 This is a sequence diagram illustrating the handling of bearer modification failures during LTM according to the present disclosure.

[0027] Figure 10 This is a sequence diagram illustrating the first scenario of an early synchronized RACH according to this disclosure.

[0028] Figure 11 This is a sequence diagram illustrating the second scenario of an early synchronized RACH according to this disclosure.

[0029] Figure 12 This is a flowchart illustrating a method for implementing LTM during a cell handover process according to the present disclosure.

[0030] Figure 13 This is an exemplary sequence diagram illustrating the basic process of LTM according to this disclosure.

[0031] Figure 14 This is a sequence diagram illustrating the third scenario of early synchronous RACH according to this disclosure.

[0032] Figure 15 This is a sequence diagram illustrating the fourth scenario of early synchronous RACH according to this disclosure.

[0033] Figure 16 This is a sequence diagram illustrating the fifth scenario of early synchronous RACH according to this disclosure.

[0034] Figure 17 This is a sequence diagram illustrating a scenario for processing E1 reset during LTM according to the present disclosure.

[0035] Figure 18 This is a sequence diagram illustrating a scenario of the Early Synchronous Random Access Channel (RACH) in LTM according to this disclosure.

[0036] Figure 19 It is the functional configuration of the device according to this disclosure.

[0037] Throughout the accompanying drawings, it should be noted that the same reference numerals are used to depict the same or similar elements, features, and structures. Detailed Implementation

[0038] In the following detailed description of embodiments of this disclosure, reference is made to the accompanying drawings, which form a part of this disclosure, and specific embodiments in which this disclosure may be practiced are illustrated by way of illustration. These embodiments have been described in sufficient detail to enable those skilled in the art to practice this disclosure, and it should be understood that other embodiments may be utilized and changes may be made without departing from the scope of this disclosure. Therefore, the following description should not be considered limiting.

[0039] The terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that an arrangement, device, apparatus, system, or method that includes a list of components or steps includes not only those components or steps but may also include other components or steps not expressly listed or inherent to such arrangement, device, apparatus, system, or method. In other words, the presence of one or more elements preceding "comprising..." in an apparatus, system, or device does not exclude the presence of other elements or additional elements in the system, unless further constraints are imposed.

[0040] The terms "at least one" and "one or more" are used interchangeably throughout the specification. The terms "multiple" and "various" are used interchangeably throughout the specification. The terms "distributed unit," "distributed unit entity," and "DU" are used interchangeably throughout the specification. The terms "central unit control plane," "CU-CP," and "CU-CP entity" are used interchangeably throughout the specification. The terms "central unit user plane," "CU-UP," and "CU-UP entity" are used interchangeably throughout the specification. The terms "L1 / L2 triggered mobility" and "LTM" are used interchangeably throughout the specification. The terms "timing advance" and "TA" are used interchangeably throughout the specification. The terms "early sync" and "early synchronization" are used interchangeably throughout the specification. The terms "cell handover," "serving cell change," "mobility," and "handover" are used interchangeably throughout the specification. The terms "candidate gNB-DU" and "target gNB-DU" are used interchangeably throughout the specification. The terms "candidate cell" and "target cell" are used interchangeably throughout the specification. It should be understood that the interchangeable terms disclosed in the foregoing paragraphs may be repeated throughout this disclosure. However, this should not be construed in any way as limiting the scope of this disclosure.

[0041] Figure 1This is a block diagram 100 illustrating an NG-RAN architecture or communication system (also referred to as the "System") according to the present disclosure. The NG-RAN architecture includes at least one next-generation base station (gNB) 106 connected to a 5G core network 104 via an NG interface. In one embodiment, multiple gNBs can be interconnected via an Xn interface. Furthermore, gNB 106 comprises a control / centralized unit (CU) 108 and one or more distributed units (DUs) 114, 116. In one configuration, CU 108 is configured to serve DUs 114, 116, and DUs 114, 116 are configured to serve one or more UEs 122 via one or more cells. CU 108 and DUs 114, 116 are connected via an F1 interface. Furthermore, CU 108 includes a centralized unit control plane (CU-CP) 110 and one or more centralized unit user planes (CU-UP) 112 that respectively handle the control plane and user plane processing of CU 108. One or more CU-UPs may include at least one source CU-UP and a target CU-UP. CU-CP 110 is communicatively coupled to each CU-UP 112 via an E1 interface. CU-CP 110 is communicatively coupled to each DU 114, 116 via an F1-C interface. Each DU 114, 116 may be communicatively coupled to each CU-UP via an F1-U interface. CU-CP 110 hosts the Packet Data Convergence Protocol (PDCP) and Radio Resource Control (RRC) layers, while the DUs host the Radio Link Control (RLC) / Media Access Control (MAC) and Physical (PHY) layers. DU 114, which serves UE 122 and is located in source cell 118, may be referred to as source DU 114, and DU 116, which serves UE 122 and is located in target cell 120, may be referred to as target DU 114 or candidate DU 114. UE 122 may be communicatively coupled to at least one DU 114 or 116, which provides services to UE 122 via a fronthaul network depending on the cell in which UE 122 resides. In embodiments, the fronthaul network may include, but is not limited to, a private network and / or the Internet. CU, DU, CU-UP, and CU-CP may be logical entities; that is, they may be co-located in the same hardware and may be distinguishable by software. Embodiments of CU, DU, CU-UP, and CU-CP may represent embodiments of any node or entity performing the functions of CU, DU, CU-UP, and CU-CP as described in the 3GPP specifications.

[0042] like Figure 1As shown, the architecture of gNB 106 illustrates that DU 114 and DU 116 have corresponding cell coverage areas defined by cells 118 and 120, respectively. Inter-gNB-DU mobility can be defined as cell handover of UE 122 between cells 118 and 120 of different gNBs DU 114 and 116. In another embodiment, intra-gNB-DU mobility can be defined as cell handover of UE 122 from one cell 118 to another cell 120 within the same DU 114.

[0043] When UE 122 moves from one cell 118 to another cell 120 (i.e., from the source cell to the target cell), a serving cell change operation is triggered. In L3 mobility, serving cell changes are performed by reconfiguration triggered by RRC signaling, which results in longer latency, greater overhead, and longer downtime. Utilizing 3GPP Release 18, a new mechanism and process based on L1 / L2 inter-cell mobility was introduced to reduce mobility latency. L1 / L2 triggered mobility (LTM) enables serving cell change via L1 / L2 signaling. LTM is fundamentally different from conventional Layer 3 mobility. Here, UE 122 can dynamically perform a cell handover procedure to cell 120. In LTM, enabling serving cell change via L1 / L2 signaling minimizes latency, overhead, and downtime. Furthermore, gNB 106 can configure multiple candidate cells 120 for UE 122 to allow for rapid application of configurations for candidate cells 120. Therefore, LTM is essentially triggered based on L1 measurements rather than L3 measurements. In addition, LTM is performed without resetting lower layers such as MAC to avoid data loss and further reduce additional latency caused in data recovery.

[0044] LTM is the process by which the gNB 106 receives L1 measurement reports from the UE 122. Based on the L1 measurements, the gNB 106 changes the serving cell 120 of the UE 122 via MAC CE. The gNB 106 can also change the UE's serving cell via MAC CE based on its internal algorithms or factors, such as its internal understanding of the deployment map, load balancing, or predictions from its AI model. These factors are also referred to as internal decisions that help the gNB 106 identify candidate / target cells. The gNB 106 prepares one or more candidate cells 120 and provides the candidate cell configurations to the UE 122 via an RRC reconfiguration message. Then, the gNB 106 selects one of the candidate configurations as the target configuration for LTM, triggering an LTM cell handover. Candidate cell configurations can only be added, modified, and released by the network via RRC signaling.

[0045] LTM supports mobility within gNB-DU, within gNB-CU, and between gNB-DU. Figure 1A scenario describing mobility between gNB-DUs is presented, where mobility occurs between source DU 114 and target DU 116.

[0046] Figure 2 This is an exemplary sequence diagram 200 illustrating the overall process of LTM according to this disclosure.

[0047] It should be noted that Figure 2 The procedures disclosed in steps 0-21 (in the preceding paragraphs) provide for the exchange of signaling and information between various entities of gNB 106, such as source DU 202, target DU 204, CU-CP 206, source CU-UP 208, target CU-UP 210, and Access Management Function (AMF) 212. Source DU 202 is... Figure 1 The same source DU 114 is shown. Target DU 204 is the same. Figure 1 The target shown is the same as DU 116. CU-CP 206 is the same. Figure 1 The CU-CP 110 shown is the same. Figure 1 One or more CU-UP 112 can be with Figure 2 The source CU-UP 208 and the target CU-UP 210 are the same.

[0048] Typically, the LTM process can be divided into four phases: 1) LTM preparation, 2) early synchronization, 3) LTM execution, and 4) LTM completion. First, UE 122 sends a measurement report to gNB 106 (specifically, to the source DU 202 of gNB). In one embodiment, the measurement report is an L1 reference signal received power (RSRP) measurement, which may or may not be filtered. As disclosed herein… Figure 2As shown, in step 0, source DU 202 is configured to send the measurement report received from UE 122 to CU-CP 206. Specifically, source DU 202 configures the measurement process and UE report of UE 122 according to the measurement configuration. In step 1, CU-CP 206 makes an LTM configuration decision based on the measurement report, that is, CU-CP 206 decides to configure the UE for LTM of UE 122 based on the UE report. Based on the LTM configuration decision, in steps 2 and 3, CU-CP 206 sends a bearer context setting request to target CU-UP 210, and then CU-CP 206 receives the corresponding response from target CU-UP 210. In steps 4 and 5, CU-CP 206 sends a UE context setting request to target DU 204, and then CU-CP 206 receives the corresponding response from target DU 204. At step 6, CU-CP 206 can provide the UE with an LTM candidate configuration, i.e., configure the LTM candidate cell via an RRC reconfiguration message for target cell 120. In an embodiment, CU-CP 206 can further release or modify the candidate configuration. In another embodiment, even after moving to candidate cell 120 via LTM, UE 122 can store the LTM configurations of other candidate cells. At step 6, CU-CP 206 can also provide the UE via source DU 202 with a configuration for performing LTM measurements for different candidate frequencies and candidate cells and reporting based on the performed LTM measurements.

[0049] During the standard LTM candidate preparation phase, gNB 106 sends an RRC reconfiguration message to UE 122 to initiate LTM; that is, gNB 106 configures one or more LTM candidate target cells. Furthermore, UE 122 stores the configuration of the LTM candidate target cells and sends an RRC reconfiguration complete message to gNB 106. Step 7 describes the transmission of the RRC reconfiguration complete message from source DU 202 to CU-CP 206.

[0050] After making the LTM decision for cell handover at step 8, source DU 202 instructs UE 122 to hand over to the LTM candidate cell. UE 122 hands over to the LTM candidate target cell 120. Source DU 202 may notify CU-CP 206 of the LTM cell change notification. Based on the LTM cell decision, in steps 10 and 11, a bearer context modification request is sent from CU-CP 206 to source CU-UP 208, and then CU-CP 206 receives the corresponding response from source CU-UP 208. In steps 12 and 13, a bearer context modification request is sent from CU-CP 206 to target CU-UP 210, and then CU-CP 206 receives the corresponding response from target CU-UP 210. In step 14, data is forwarded from source CU-UP 208 to target DU 204 via target CU-UP 210. At step 15, target DU 204 detects UE 122, and at step 16, target DU 204 sends an access success notification to CU-CP 206. At step 17, a path update procedure is performed towards core network 104, and subsequently, at step 18, AMF 212 sends an end-of-path packet to source CU-UP 208, and at step 19, a new path is created. Finally, at steps 20 and 21, a bearer context release request is sent from CU-CP 206 to source CU-UP 208, and then CU-CP 206 receives the corresponding response from source CU-UP 208.

[0051] In this embodiment, CU-CP 206 will initiate LTM cell handover by being notified by source DU 202 (in... Figure 2 Receiving messages in step 9) and successfully completing LTM cell handover (e.g.) Figure 2 Any non-access stratum (NAS) messages received between steps 16 (such as successful access reception or RRC reconfiguration completion reception) are buffered and sent to the new primary cell (PCell) after successful LTM completion.

[0052] Figures 3A-3D This is a flowchart illustrating access control in an LTM according to this disclosure.

[0053] Before initiating a cell handover process, gNB 106 must ensure that the target cell can be configured for LTM. Admission control is a key step in gNB during UE 122 setup for target cell 120. Therefore, admission control directly depends on the source 118 and the total number of such UEs in the candidate or target cells 120.

[0054] Figure 3AA flowchart 301A illustrates a first scenario of admission control in LTM performed by CU 108 (and specifically by CU-CP 110 or 206). To ensure admission control, CU 108 maintains a list of all UEs configured as LTM candidate cells. Furthermore, CU 108 maintains a threshold for the number of UEs that can be configured as LTM candidate cells in each candidate cell. At step 301A-1, CU 108 receives a measurement report or determines to perform LTM configuration based on one or more internal factors. Thereafter, at step 301A-2, CU 108 determines whether the number of UEs that have triggered LTM is equal to or greater than the threshold. If the condition is met, at step 301A-3, CU 108 does not configure the cell as an LTM candidate cell for the UE. If the number of UEs that have triggered LTM is less than the threshold, at step 301A-4, CU 108 configures the cell as an LTM candidate cell for the UE.

[0055] Figure 3B A flowchart 301B illustrates a second scenario of admission control in LTM performed by CU 108 (and specifically by CU-CP 110 or 206). During admission control, CU 108 considers the number of UEs already in candidate cell 120 (i.e., the number of RRC_CONNECTED UEs in the cell) together with the number of UEs as LTM candidate cells. At step 301B-1, CU 108 receives a measurement report or determines to perform LTM configuration based on one or more internal factors. Subsequently, at step 301B-2, CU 108 determines whether the total number of UEs (RRC_CONNECTED UEs) in the cell and the total number of LTM candidate cells are greater than (or alternatively equal to) a threshold. If the condition is met, at step 301B-3, CU 108 may not configure the cell as an LTM candidate cell; otherwise, at step 301B-4, CU 108 configures the cell as an LTM candidate cell.

[0056] Figure 3CA flowchart 301C is shown illustrating a third scenario of admission control in LTM performed by CU 108 (and specifically by CU-CP 110 or 206). During admission control, CU 108 considers the number of UEs already in candidate cell 120 (i.e., the number of RRC_CONNECTED UEs in the cell), the number of candidate UEs configured for conditional handover, and the number of UEs serving as LTM candidate cells. At step 301C-1, CU 108 receives a measurement report or determines to perform LTM configuration based on one or more internal factors. Subsequently, at step 301C-2, CU 108 determines whether the total number of UEs in the cell (RRC_CONNECTED UEs), the number of UEs configured with the same cell as the candidate cell used for conditional handover, and the total number of UEs configured with the same LTM candidate cell are higher than (or alternatively equal to) a threshold. If the conditions are met, at step 301C-3, CU 108 may not configure the cell as an LTM candidate cell; otherwise, at step 301C-4, CU 108 configures the cell as an LTM candidate cell.

[0057] Figure 3D A flowchart 302A illustrating admission control for target / candidate DU 204 in LTM is shown. In an embodiment, target / candidate DU 204 maintains a list of all UEs configured as LTM candidate cells. In an embodiment, candidate DU 204 maintains a threshold for the number of UEs that can be configured as LTM candidate cells in each candidate cell. Furthermore, CU-CP 206 sends an F1AP UE context setting request (for intra-DU LTM) or an F1AP UE context modification request (for inter-DU LTM) for configuring LTM candidate cells. At step 302A-1, candidate DU 204 receives the request to configure LTM candidate cells, and then at step 302A-2 determines whether the number of UEs that triggered LTM is equal to or greater than the threshold. If the condition is met, then at step 302A-3, target / candidate DU 204 does not configure the cell for UE 122 as an LTM candidate cell and sends back a failure message for the F1AP UE context setting procedure or the F1AP UE context modification procedure. If it can accept LTM candidate cells, then in step 302A-4, the target / candidate DU 204 sends a success response for the F1AP UE context setting procedure or the F1AP UE context modification procedure. In an embodiment, when sending the success response, the target / candidate DU 204 may include RACH configuration for early synchronization in the F1AP UE context setting response or the F1AP UE context modification response.

[0058] Figure 4 This illustrates sequence diagram 400 generated according to the LTM configuration of this disclosure. It should be noted that...Figure 4 The procedures disclosed in steps 0-7 (in the preceding paragraphs) provide for the exchange of signaling and information between the source DU 202, the target DU 204, the CU-CP 206, the UE 122 (or UE-RRC), and the Operation, Administration and Maintenance (OAM) node 402.

[0059] Once a target cell is identified as capable of providing service to the UE, CU-CP 206 generates a reference configuration. Additionally, LTM candidate configurations are provided to configure LTM candidate cells. The candidate cell configuration can be provided as an incremental (delta) configuration on top of the reference configuration, thus forming a complete candidate cell configuration. The reference configuration is managed separately, and UE 122 stores it as a separate configuration. The reference configuration can be empty. When UE 122 receives the candidate cell configuration before receiving an LTM cell handover command, the complete candidate configuration is applied. In one embodiment, when UE 122 receives the candidate cell configuration after receiving an LTM cell handover command, UE 122 is configured to apply the complete candidate configuration. The complete candidate cell configuration is applied and replaces the current UE configuration at the time of reconfiguration execution. Although the reconfiguration process performs a replacement, it does not necessarily reset the MAC, RLC, or PDCP layers.

[0060] CU-CP 206 generates a reference configuration by including the cell group configuration (e.g., CellGroupConfig IE in NR) received from the current serving cell (i.e., from the current DU (source DU) 202) during one of the F1AP UE context settings or F1AP UE context modification procedures, and the Layer 3 configuration (such as measurement configuration, radio bearer configuration, etc.) generated by CU-CP 206. This in Figure 4 Steps 0 and 1 are shown. At step 0, CU-CP 206 receives an F1AP UE context setting / modification response including cell group configuration from source DU 202. At step 1, CU-CP 206 receives an F1AP UE context setting / modification response including cell group configuration from target DU 204.

[0061] CU-CP 206 also generates Layer 3 configurations such as measurement configurations and radio bearer configurations, and constructs reference configurations. This in Figure 4 Step 2 is illustrated. For example, CU-CP 206 receives L1 / L2 parameters from OAM 402 for generating cell group configuration. Subsequently, CU-CP 206 sends the generated reference configuration to source DU 202 via the F1AP interface. Figure 4 In step 4A), the generated reference configuration is sent to UE 122 in the RRC reconfiguration message. This is inFigure 4 Step 3 is shown. In response to the RRC reconfiguration message, CU-CP 206 can receive an RRC reconfiguration complete message from UE 122. This is shown in... Figure 4 Step 4 is shown.

[0062] In another embodiment, CU-CP 206 generates a complete configuration for the LTM candidate cell by including L1 / L2 parameters received from OAM 402. In another embodiment, candidate DU 204 sends a CellGroupConfig to CU-CP 206 in an F1AP UE context setting response or F1AP UE context modification response to generate the complete configuration. CU-CP 206 generates and includes an L3 configuration. If the L3 configuration is a complete LTM configuration, CU-CP 206 includes signaling radio bearers 1 and 2 (SRB1) and (SRB2), at least one data radio bearer (DRB), and a multicast radio bearer (MRB) in the complete LTM candidate configuration, but excludes the security configuration. Figure 4 (Steps 4-7). If the CU-CP identifies that the candidate cell belongs to another base station, a security configuration can be included.

[0063] If the LTM reference configuration IE is included within the reference configuration IE in the LTM information setting IE included in the UE context setting request message, then the source DU 202 (if supported) considers it for generating the lower-level LTM configuration. If a request for a lower-level configuration IE set to "true" is included within the reference configuration IE in the LTM information modification IE included in the UE context modification request message, then the source DU 202 (if supported) includes the CellGroupConfig IE in the UE context modification response message to provide the lower-level configuration for CU-CP 206 to generate the LTM reference configuration.

[0064] In one embodiment, CU-CP 206 releases the reference configuration and all LTM candidate cell configurations before sending the Xn handover (HO) request. In another embodiment, upon receiving the Xn HO request, target DU 204 instructs UE 122 to release the reference configuration and all LTM candidate cell configurations.

[0065] Figures 5A-5F This is a sequence diagram illustrating the Early Synchronization Random Access Channel (RACH) in LTM according to this disclosure.

[0066] After receiving the configuration information, during the early synchronization phase, UE 122 can perform downlink (DL) synchronization and timing advance (TA) acquisition with candidate target cell 120 before receiving the LTM cell handover command. In one configuration, DL synchronization for candidate cell 120 is supported at least based on the synchronization signal block (SSB) before the cell handover command. In another configuration, TA acquisition for candidate cell 120 is supported at least based on the random access channel (RACH) of the physical downlink control channel (PDCCH) command, where the PDCCH command is triggered only by the source cell 118.

[0067] During the LTM execution phase, UE 122 performs L1 measurements on the configured LTM candidate target cells and sends lower-layer measurements or measurement reports to gNB 106. The lower-layer measurements or measurement reports are carried on the L1 or MAC layer. In other words, UE 122 performs L1 measurements on source cell 118 and candidate cell 120, and reports the L1 measurements to source DU 202 via CSI reports. In response, source DU 202 may send a MAC CE requesting UE 122 to hand over to another cell that is an LTM candidate cell (e.g., LTM MAC CE or LTM cell handover MAC CE). Afterwards, UE 122 may perform random access during LTM cell handover, or the cell handover may be RACH-less.

[0068] Specifically, gNB 106 decides to perform an LTM cell handover to target cell 120 and sends a MAC CE that triggers the LTM cell handover by including the candidate configuration index of target cell 120. UE 122 then switches to the configuration of LTM candidate target cell 120.

[0069] UE 122 performs a random access procedure toward target cell 120, and during the LTM completion phase, UE 122 only indicates successful completion of the LTM cell handover toward target cell 120. In one configuration, an uplink signal or message after UE 122 has already handed over to target cell 120 is used to indicate successful completion of the LTM cell handover.

[0070] In one embodiment, UE 122 may be requested to perform random access to candidate cell 120 before cell handover, so that the network can calculate the timing advance before cell handover and notify UE 122 via a random access response or within a MAC CE sent for cell handover.

[0071] The gNB 106 can request / command UE 122 to perform random access to candidate cell 120 before cell handover, allowing the gNB 106 to calculate timing advance before handover and notify UE 122 via a random access response or within a MAC CE sent for cell handover. When initiating a random access procedure, UE 122 selects a random access resource set and initializes the following parameters for the random access procedure according to the values ​​configured by the RRC for the selected random access resource set: the RACH preamble is one such random access resource. The gNB 106 can configure UE 122 to perform random access (referred to as early TA, early synchronization TA, or TA for early synchronization) to one or more LTM candidate cells for receiving timing advance (TA) before performing cell handover. Random access for timing advance reception performed on LTM candidate cells can be referred to as random access for early TA. The gNB 106 sends a Physical Downlink Control Channel (PDCCH) command to initiate RACH for TA measurement of candidate cells. UE 122 receives the PDCCH command from serving cell 118. The random access procedure for LTM candidate cells can be initiated solely by a PDCCH command. Upon receiving the PDCCH command, UE 122 initiates a RACH for TA measurements of one or more candidate cells.

[0072] exist Figure 5A At step 0, UE 122 sends a RACH request to target DU 204. In other words, UE 122 sends a RACH preamble to the candidate cell and receives the timing advance (TA) value from the candidate cell. Figure 5A In step 1, the TA value is sent from target DU 204 to CU-CP 206 via an F1AP message. Figure 5A In step 2, the updated TA value and target cell ID are sent from CU-CP 206 to source DU 202 via an F1AP message. TAs for candidate cells can also be received from source cell 118. TAs can be received in a random access response, or they can be received via a MAC CE. gNB 106 can include the TA in the cell handover command. If source DU 202 instructs UE 122 to retransmit the RACH used for early TA, UE 122 retransmits the RACH used for early TA. gNB 106 can also send a PDCCH command to retransmit the RACH used for TA measurement (also known as the RACH used for early synchronization).

[0073] In other words, at step 1, when UE 122 performs random access for early synchronization in an LTM candidate cell, candidate DU 204 notifies CU-CP 206 of the timing advance via an F1AP message. Candidate DU 204 includes the LTM candidate cell identifier and the calculated timing advance in the F1AP message. At step 2, CU-CP 206 also notifies source DU 202 of the received candidate cell identifier and the corresponding timing advance via an F1AP message.

[0074] In an alternative embodiment, upon receiving the random access preamble for early synchronization from UE 122 ( Figure 5B In step 0), candidate DU 204 notifies source DU 202 of candidate cell information (such as candidate cell index) and timing advance via the inter-DU interface. In other words, once UE 122 performs early synchronization for candidate cell 120, source DU 202 receives the timing advance notification from candidate DU 204 for the candidate cell from UE 122. Figure 5B Step 2 in the process.

[0075] In an embodiment, when a random access preamble for early synchronization is received from UE 122, candidate DU 204 notifies CU-CP 206 of candidate cell information (such as candidate cell index) and timing advance, and CU-CP 206 notifies source DU 202 via an F1AP message.

[0076] In an embodiment, candidate DU 204 may receive multiple random access preambles for early synchronization from the same UE 122 in the same candidate cell, but it notifies CU-CP 206 or source DU 202 only once, respectively, via F1AP message or inter-DU communication.

[0077] In an embodiment, if candidate DU 204 receives multiple random access preambles for early synchronization from the same UE 122 in the same candidate cell, and the timing advance value subsequently calculated differs from the timing advance value previously notified to CU-CP 206 or source DU 202, candidate DU 204 notifies CU-CP 206 or source DU 202 again by sending another F1AP message or inter-DU message.

[0078] In such Figure 5CIn the illustrated embodiment, after receiving a measurement report from UE 122, the reported value is compared with a first threshold, and a PDDCH command to trigger early synchronization is sent to UE 122 based on this comparison. Furthermore, if source DU 202 does not receive a timing advance from CU-CP 206 or target DU 204 within a certain time interval, source DU 202 repeats the PDDCH command. Source DU 202 sends a PDDCH command to UE 122 to perform early synchronization TA and indicates that the PDDCH command is for retransmission.

[0079] In an embodiment, if the source DU 202 does not receive a timing advance from the CU-CP 206 or the target DU 204 in any scenario, the source DU 202 repeats the PDCCH command.

[0080] In this embodiment, source DU 202 receives information from Operation, Administration and Maintenance (OAM) 402, CU-CP 206, or target DU 204 regarding whether UE 122 needs to repeat the RACH preamble for early access. Based on the received information, source DU 202 sends a PDCCH command to notify UE 122 whether to repeat the RACH preamble for early access.

[0081] like Figure 5D As shown, candidate DU 204 can receive multiple random access preambles for early synchronization from the same UE 122 in the same candidate cell within an interval, but candidate DU 204 can notify CU-CP 206 or source DU 202 only once via F1AP message or inter-DU communication. If candidate DU 204 receives random access preambles for early synchronization from the same UE 122 outside the interval, it can send F1AP messages or inter-DU communication to CU-CP 206 or source DU 202, including candidate cell information (such as candidate cell index) and timing advance value.

[0082] exist Figure 5E In step 502, once candidate DU 204 has identified the timing advance of candidate cell 120 based on the received random access preamble for early synchronization, candidate DU 204 starts a timer for the timing advance. After UE 122 performs a successful cell handover to the candidate cell, the timer continues to run, and once the timer expires, candidate DU 204 notifies UE 122 of the timing advance via MAC CE.

[0083] In an embodiment, if candidate DU 204 identifies that a cell handover has occurred on another cell (e.g., if it receives an indication from CU-CP 206 that a cell handover has occurred on another cell, or if it identifies that a cell handover has occurred on another candidate cell of the same DU, etc.), then candidate DU 204 resets the timer at steps 506 and 508.

[0084] In an embodiment, if candidate DU 204 identifies that a cell handover has occurred on another cell (e.g., if it receives an indication from CU-CP 206 that a cell handover has occurred on another cell, or if it identifies that a cell handover has occurred on another candidate cell of the same DU, etc.), then candidate DU 204 keeps the timer running, and if the timer expires while UE 122 is connected to another cell, then candidate DU 204 does not restart the timer.

[0085] In an embodiment, at step 510, if candidate DU 204 receives another random access preamble for early access and has identified a new timing advance value (and has notified CU-CP 206 or source DU 202 regarding the inter-DU situation), then at step 512, it restarts the timer.

[0086] In such Figure 5F In the illustrated embodiment, candidate DU 204 notifies CU-CP 206 whether it prefers repeating the random access preamble via an F1AP message (such as an F1AP UE context setting response, an F1AP UE context modification response, or any other F1AP message). Figure 5F Step 2).

[0087] In an alternative embodiment, candidate DU 204 notifies source DU 202 via inter-DU messages whether it prefers a repeated random access preamble.

[0088] Figure 6 This is a sequence diagram 600 illustrating the determination of the downlink / uplink bandwidth portion (DL / ULBWP) for LTM according to this disclosure.

[0089] In this embodiment, CU-CP 206 uses F1AP messages to notify source DU 202 of the downlink bandwidth portion (BWP) and uplink BWP of the target cell during cell handover.

[0090] In this embodiment, the source DU 202 includes DL and UL BWP in the MAC CE and sends them to UE 122 to trigger LTM cell handover.

[0091] Figure 7This is a sequence diagram 700 illustrating the RRC connection re-establishment / radio link failure (RLF) handling using LTM according to this disclosure.

[0092] UE 122 declares a radio link failure under at least one of the following conditions: a timer that was started when a measurement report with a configured timer was triggered while another radio problem timer was running expires; or a random access procedure fails; or an RLC fails.

[0093] In the case of LTM, for an RLF in the source cell, UE 122 selects a suitable cell, and if the selected cell is an LTM candidate cell, and if the network configures UE 122 to try LTM after an RLF, then UE 122 attempts to perform LTM based on RACH once; otherwise, it performs a reconstruction.

[0094] In an embodiment, when receiving an RRC reconstruction from UE 122 or an RLF indication from source DU 202, CU-CP206 clears the reference configuration and all candidate cell configurations for UE 122.

[0095] Specifically, such as Figure 7 As shown, after the source DU 202 makes an LTM decision, a cell change notification is sent from the source DU 202 to the CU-CP 206. Subsequently, the CU-CP 206 sends a bearer context modification request to both the source CU-UP 208 and the target CU-UP 210. In response, each of the source CU-UP 208 and the target CU-UP 210 then sends a bearer context modification response. Further, upon detecting an RRC rebuild or RLF, the CU-CP 206 sends an F1AP UE context modification request to the target DU 204 to remove the candidate cell, and in response, the target DU 204 sends an F1AP UE context modification response to the CU-CP 206. In this embodiment, the CU-CP 206 notifies the source DU 202 of the activation status of the target cell's SCell during cell handover. In this embodiment, the source DU 202 includes the activation status and information for identifying the SCell (such as the SCell identifier) ​​in the MAC CE sent to the UE 122 to trigger the LTM cell handover. If the MAC CE does not include an active state, the SCell's active state remains the same as before the cell handover. If the MAC CE does not include an active state, the SCell's active state is activated.

[0096] Figure 8A This is a sequence diagram illustrating a first scenario for processing F1 Application Protocol (F1AP) reset during LTM according to this disclosure.

[0097] In the event of a failure at target DU 204 (resulting in the loss of some or all transaction reference information), a RESET message is sent to CU-CP 206. An F1AP reset message is then sent on the F1-C interface between CU-CP 206 and target DU 204.

[0098] exist Figure 8A In the process, after the source DU 202 makes the LTM decision, the source DU 202 sends a cell change notification to CU-CP 206. Subsequently, CU-CP 206 sends a bearer context modification request to both the source CU-UP 208 and the target CU-UP 210. In response, each of the source CU-UP 208 and the target CU-UP 210 then sends a bearer context modification response. Upon receiving a RESET message, at CU-CP 206, CU-CP 206 sends an RRC connection reconfiguration to UE 122 to release all allocated resources associated with the UE on the F1 interface and remove the F1AP ID associated with the indicated UE. The RRC connection reconfiguration also helps release the corresponding DRB for UE 122. In addition, CU-CP 206 sends an F1AP UE context modification request to target DU 204 for removing the candidate cell, and in response, target DU 204 sends an F1AP UE context modification response to CU-CP 206, and then UE 122 sends an RRC connection reconfiguration complete message to CU-CP 206.

[0099] In an embodiment, when an F1AP reset message containing one or more LTM candidate cells is received from the target DU 204, the CU-CP 206 releases the LTM candidate configurations of all LTM candidate cells.

[0100] Figure 8B A sequence diagram illustrating a second scenario for processing E1AP reset during LTM is shown, according to some embodiments of this disclosure. The E1 interface is as follows: Figure 1 The point-to-point interface shown is between the CU-CP and gNB-CU-UP. This interface supports the exchange of signaling information between the endpoints.

[0101] exist Figure 8BIn the process, after the source DU 202 makes the LTM decision, the source DU 202 sends a cell change notification to CU-CP 206. Subsequently, CU-CP 206 sends a bearer context modification request to both the source CU-UP 208 and the target CU-UP 210. In response, each of the source CU-UP 208 and the target CU-UP 210 then sends a bearer context modification response. Upon receiving a RESET message, at CU-CP 206, CU-CP 206 sends an RRC connection reconfiguration to UE 122 to release all allocated resources associated with the UE on the E1 interface and remove the F1AP ID associated with the indicated UE. RRC connection reconfiguration helps release the corresponding DRB of the UE. In addition, CU-CP 206 sends an F1AP UE context modification request to target DU 204 for removing the candidate cell, and in response, target DU 204 sends an F1AP UE context modification response to CU-CP 206, and then UE 122 sends an RRC connection reconfiguration complete message to CU-CP 206.

[0102] In an embodiment, when an E1AP reset message for one or more LTM candidate cells is received from the source CU-UP 208, the CU-CP 206 releases the LTM candidate configuration for all LTM candidate cells.

[0103] In an alternative embodiment, CU-CP 206 selects a new CU-UP and sends an E1AP bearer context setting request to it. If CU-CP 206 receives a successful response for at least one E1AP bearer, CU-CP 206 may retain the LTM candidate cell configuration; otherwise, CU-CP 206 releases the LTM candidate configuration used for those LTM candidate cells.

[0104] Figure 9 Figure 900 illustrates a sequence diagram of handling bearer modification failures during LTM according to this disclosure.

[0105] exist Figure 9In this process, after source DU 202 makes an LTM decision, source DU 202 sends a cell change notification to CU-CP 206. Subsequently, CU-CP 206 sends a bearer context modification request to both source CU-UP 208 and target CU-UP 210. In response, each of source CU-UP 208 and target CU-UP 210 then sends a bearer context modification response. However, if the bearer context modification request message sent to target CU-UP 210 receives an unsuccessful response or if a timeout occurs, CU-CP 206 sends an RRC reconfiguration message to release the corresponding DRB. CU-CP 206 also sends an F1AP modification request to target DU 204 (for which it has completed a cell handover). In response, target DU 204 sends an F1AP UE context modification response to CU-CP 206, and then UE 122 sends an RRC connection reconfiguration message to CU-CP 206.

[0106] Figure 10 This is a sequence diagram 1000 illustrating the first scenario of early synchronous RACH according to this disclosure.

[0107] CU-CP 206 notifies source DU 202 of a list of LTM candidate cells, including candidate cell identifiers and PCI (physical cell identifiers). Source DU 202 receives LTM measurements (including L1 measurements of one of RSRP, RSRQ, and SINR) from UE 122.

[0108] Source DU 202 maintains a first threshold for LTM measurements used for early synchronization. If the reported value of RSRP (or RSRQ or SINR) is greater than the first threshold, source DU 202 instructs UE 122 to perform early synchronization on the candidate cell by sending a PDCCH command including information about the candidate cell. In some embodiments, when UE 122 reports both source cell and candidate cell measurements, and if the reported value of RSRP (or RSRQ or SINR) of candidate cell 120 is greater than the first threshold for candidate cell 120 and the reported value of RSRP (or RSRQ or SINR) of source cell 118 is less than the first threshold for candidate cell 120, then source DU 202 may instruct UE 122 to perform early synchronization on the candidate cell by sending a PDCCH command including information about the candidate cell. Alternatively, when UE 122 reports both source cell and candidate cell measurements, and if the reported value of the RSRP (or RSRQ or SINR) of candidate cell 120 is greater than the reported value of the RSRP (or RSRQ or SINR) of the source cell by a first offset value, then source DU 202 may instruct UE 122 to perform early synchronization with candidate cell 120 by sending a PDCCH command that includes information about candidate cell 120. The instructed information may be an LTM candidate cell index. Alternatively, source DU 202 may instruct UE 122 to perform early synchronization with candidate cell 120 by sending a PDCCH command that includes information about the candidate cell, based on its internal measurements of UE 122 (UL L1 measurements) or its information about the topology.

[0109] Source DU 202 includes the candidate cell SSB with the highest L1 measurement (maximum RSRP value, maximum RSRQ value, or maximum SINR value) as a candidate cell SSB in the PDCCH command.

[0110] Figure 11 This is sequence diagram 1100 showing the second scenario of early synchronous RACH according to this disclosure.

[0111] In one embodiment, DU is source DU 202. CU-CP 206 notifies source DU 202 of a list of LTM candidate cells including candidate cell identifiers and PCIs. Source DU 202 receives LTM measurements from UE 122 (L1 measurements include one of Reference Signal Received Power (RSRP), Reference Signal Received Quality (RSRQ), Signal-to-Interference-plus-Noise Ratio (SINR), etc.).

[0112] In this embodiment, source DU 202 maintains a second threshold for LTM measurements. If the reported value of RSRP (or RSRQ or SINR, etc.) is greater than the second threshold, source DU 202 instructs UE 122 to perform a cell handover by sending a MAC CE (LTM MAC CE or cell handover MAC CE) that includes information about the candidate cells. The instruction information may be an LTM candidate cell index.

[0113] In some embodiments, when UE 122 reports both source cell and candidate cell measurements, and if the reported value of RSRP (or RSRQ or SINR) of candidate cell 120 is greater than a second threshold for candidate cell 120 and the reported value of RSRP (or RSRQ or SINR) of source cell 118 is less than the second threshold for candidate cell 120, then source DU 202 may instruct UE 122 to perform LTM cell handover. Alternatively, if UE 122 reports both source cell and candidate cell measurements, and if the reported value of RSRP (or RSRQ or SINR) of candidate cell is greater than a second offset value than the reported value of RSRP (or RSRQ or SINR) of source cell, then source DU 202 may instruct UE 122 to perform LTM cell handover.

[0114] In this embodiment, source DU 202 selects the UL / SUL (Supplemental UL) for performing random access based on received LTM measurements. For example, if the RSRP value (or RSRQ or SINR value, whichever is considered) for cell handover is higher in the SUL, source DU 202 includes the SUL in the MAC CE sent to trigger cell handover. Similarly, if the RSRP value (or RSRQ or SINR value, whichever is considered) for cell handover is higher in the UL, source DU 202 includes the UL in the MAC CE sent to trigger cell handover.

[0115] In one embodiment, the first threshold and the second threshold are two different thresholds. In an alternative embodiment, the first threshold and the second threshold have a common threshold.

[0116] In one embodiment, the first offset and the second offset have two different offset values. In an alternative embodiment, the first offset and the second offset have a common threshold.

[0117] Figure 12 This is a flowchart 1200 illustrating a method for implementing LTM executed by base station 106 during a cell handover process according to the present disclosure.

[0118] In one embodiment of this disclosure, a method 1200 for implementing lower-layer triggered mobility (LTM) is disclosed during a cell handover process of a user equipment (UE) from a source cell 118 served by a base station (gNB) 106 to a target cell 120. The gNB 106 includes a centralized unit (CU) 108 and one or more distributed units (DUs) for serving the UE 122. The one or more DUs include a source DU 202 and a target DU 204.

[0119] Method 1200 includes: at block 1202, identifying a target cell 120 for LTM of UE 122 by base station 106 (specifically, by CU 108) based on measurements received from UE 122 or based on one or more internal decisions, and thereafter at block 1204, performing admission control on the target cell 120 by base station 106 (specifically, by CU 108) based on one or more factors.

[0120] In one embodiment, one or more factors include: the number of UEs associated with neighboring cells that have triggered LTM, the number of RRC_CONNECTED UEs in target cell 120, and the number of UEs associated with neighboring cells that have configured conditional handover.

[0121] Performing admission control on target cell 120 includes performing one or more of the following: configuring the cell as an LTM candidate cell when the number of UEs associated with the cell that triggered LTM is less than a first threshold; configuring the cell as an LTM candidate cell when the sum of the number of RRC_CONNECTED UEs and the number of UEs associated with the cell that triggered LTM is less than a second threshold; and configuring the cell as an LTM candidate cell when the sum of the number of RRC_CONNECTED UEs, the number of UEs associated with the cell that has conditional handover configured, and the number of UEs associated with the cell that triggered LTM is less than a third threshold.

[0122] Method 1200 further includes, at block 1206, generating an LTM configuration by base station 106 (specifically, by CU 108) upon successful admission control. The LTM configuration includes at least one of: a complete LTM candidate configuration; and an incomplete LTM candidate configuration and an LTM reference configuration. The LTM reference configuration is based on at least one of a cell group configuration associated with source cell 118 and a cell group configuration associated with target cell 120, and a Layer 3 configuration associated with the CU.

[0123] Method 1200 further includes: at block 1208, base station 106 (specifically, CU 108) sends the generated LTM configuration to UE 122 in a Radio Resource Control (RRC) reconfiguration message and sends the generated LTM configuration to source DU 202 associated with source cell 118. An RRC reconfiguration message is sent to source DU 202 in an F1 Application Protocol (F1AP) message.

[0124] Method 1200 further includes: at block 1210, base station 106 (specifically, CU 108) facilitates early synchronization of UE 122 in target cell 120 based on F1AP messages received from target DU 204 associated with target cell 120.

[0125] In one embodiment, the method includes: when the target DU 204 does not transmit a timing advance value to the source DU 202, the source DU 202 sends a command to trigger or re-trigger the UE 122 to perform early synchronization on the target cell 120. In another embodiment, based on whether early synchronization has already occurred in the UE 122 as identified by the target DU 204, the target DU 204 starts a timing advance (TA) timer; and upon receiving an indication regarding the cell handover process, the target DU 204 resets the TA timer.

[0126] In one embodiment, facilitating early synchronization of UE 122 in target cell 120 includes: receiving an F1AP message including an LTM candidate cell identifier and a timing advance value from target DU 204; and sending the LTM candidate cell identifier and timing advance value to source DU 202 associated with source cell 118. In another embodiment, facilitating early synchronization of UE 122 in candidate cells includes: receiving information including an LTM candidate cell identifier and a timing advance value from source DU. Source DU 202 receives this information from target DU 204 via an inter-DU interface.

[0127] Method 1200 further includes: at block 1212, the base station 106 (specifically, the CU 108) performs an LTM cell handover completion upon receiving an indication from the target DU 204 indicating successful access to the target cell 120. In another embodiment, the layer 3 configuration includes: a measurement configuration and a radio bearer configuration.

[0128] In one embodiment, method 1200 further includes the following operation performed by a source DU 202 associated with source cell 118: receiving LTM measurements from the UE. The LTM measurements include Layer 1 measurements, which include information associated with at least one of Reference Signal Received Power (RSRP), Reference Signal Received Quality (RSRQ), and Signal-to-Interference-plus-Noise Ratio (SINR). Based on whether the received LTM measurements meet a first threshold, source DU 202 also executes a Physical Downlink Control Channel (PDCCH) command including information about target cell 120 to trigger UE 122 to perform early synchronization with target cell 120. The information about target cell 120 includes an LTM candidate cell index.

[0129] In some embodiments, when UE 122 reports both source cell and candidate cell measurements, and if the reported value of RSRP (or RSRQ or SINR) of candidate cell 120 is greater than a first threshold for candidate cell 120 and the reported value of RSRP (or RSRQ or SINR) of source cell 118 is less than the first threshold for candidate cell 120, then source DU 202 may instruct UE 122 to perform early synchronization on candidate cell 120 by sending a PDCCH command including information about candidate cell 120. Alternatively, when UE reports both source cell and candidate cell measurements, and if the reported value of RSRP (or RSRQ or SINR) of candidate cell 120 is greater than the reported value of RSRP (or RSRQ or SINR) of source cell 120 by a first offset value, then source DU 202 may instruct UE 122 to perform early synchronization on candidate cell 120 by sending a PDCCH command including information about candidate cell 120. The inferred information may be an LTM candidate cell index.

[0130] Alternatively, source DU 202 may instruct UE 122 to perform early synchronization with candidate cell 120 by sending a PDCCH command that includes information about the candidate cell, based on its internal measurements of UE 122 (UL L1 measurements) or its information about the topology.

[0131] In this embodiment, the source DU 202 maintains a second threshold for LTM measurements. Furthermore, the source DU 202 sends a MAC control element (MAC CE) including information about the target cell 120 based on whether the received LTM measurements meet the second threshold to trigger the UE 122 to perform a cell handover, wherein the information about the target cell 120 includes an LTM candidate cell index.

[0132] In an embodiment, if the reported value of RSRP (or RSRQ or SINR, etc.) is greater than a second threshold, the source DU202 instructs UE122 to perform a cell handover by sending a MAC CE (LTM MAC CE or cell handover MAC CE) including information about the candidate cell. The instructing information may be an LTM candidate cell index.

[0133] In some embodiments, when UE 122 reports both source cell and candidate cell measurements, and if the reported value of RSRP (or RSRQ or SINR) of candidate cell 120 is greater than a second threshold for candidate cell 120 and the reported value of RSRP (or RSRQ or SINR) of source cell 118 is less than the second threshold for candidate cell 120, then source DU 202 may instruct UE 122 to perform LTM cell handover. Alternatively, if UE 122 reports both source cell and candidate cell measurements, and if the reported value of RSRP (or RSRQ or SINR) of candidate cell is greater than a second offset value than the reported value of RSRP (or RSRQ or SINR) of source cell, then source DU 202 may instruct UE 122 to perform LTM cell handover.

[0134] In one embodiment, the first threshold and the second threshold are two different thresholds. In an alternative embodiment, the first threshold and the second threshold have a common threshold.

[0135] In one embodiment, the first offset and the second offset have two different offset values. In an alternative embodiment, the first offset and the second offset have a common threshold.

[0136] Figure 13 Figure 1300 is an exemplary sequence diagram illustrating the basic flow of LTM according to this disclosure. The 5G New Radio (NR) physical layer, or layer 1, serves as the basis for seamless communication by converting data into radio signals and vice versa. Layer 2 of the NR is divided into the following sublayers: Media Access Control (MAC), Radio Link Control (RLC), Packet Data Convergence Protocol (PDCP), and Service Data Adaptation Protocol (SDAP). The physical layer provides transport channels to the MAC sublayer. The MAC sublayer provides logical channels to the RLC sublayer. The RLC sublayer provides RLC channels to the PDCP sublayer. The PDCP sublayer provides radio bearers to the SDAP sublayer. The RRC layer, or layer 3, broadcasts basic network information to both the Non-Access Stratum (NAS) and the Access Stratum (AS) for efficient communication.

[0137] Layer 3 mobility requires reconfiguration of the upper layers (e.g., RRC or PDCP) and / or reset of the lower layers (e.g., MAC and / or PHY), while in L1 / L2 mobility, the configuration of the upper layers is preserved and changes to the configuration of the lower layers are minimized.

[0138] Figure 13 The LTM at each layer is shown. At the RRC layer, in Figure 13 In step 1, an RRC reconfiguration including the candidate LTM configuration is sent from gNB 106 to UE 122. The RRC layer of gNB 106 can be referred to as 1302. Figure 13 At step 2, an RRC reconfiguration completion indication is sent from UE 122 to gNB 106. The RRC layer of UE 122 can be referred to as 1310. At layers 1 and 2, L1 measurements are performed at UE 122 (step 3) and then sent to source DU 202. Layers 1 and 2 of UE 122 are referred to as 1308, and layers 1 and 2 of source DU 202 are referred to as 1306. Furthermore, at step 5, an LTM is triggered at UE 122 based on the L1 measurement. At step 6, the received LTM configuration is applied by UE 122, and then at step 7, an RRC reconfiguration completion indication is sent from UE 122 to gNB 106.

[0139] Figure 14 It is shown that, according to this disclosure, it is similar to Figure 5C , Figures 10-11 The sequence diagram of the third scene of the early synchronous RACH is shown in Figure 1400.

[0140] Similar to Figure 5C The steps shown are in Figure 14 In step 1, the measurement value reported by UE 122 is compared with a first threshold, and a PDDCH command to trigger early synchronization is sent to UE 122 based on this comparison (step 2). In step 3, UE 122 sends a RACH request to the target DU 204. In step 4, the timing advance value is notified to CU-CP 206 using an F1AP UE message. In an alternative embodiment, in step 5, the timing advance value is notified to the source DU 202 using the inter-DU interface. In step 6, the timing advance and target cell ID are updated and sent to an F1AP message. Furthermore, in... Figure 14 In the process, the measured value reported by UE 122 will also be compared with the second threshold (step 7), such as Figure 11 The process is as follows. Then, at step 8, an LTM cell handover command is sent to UE122, and at step 9, the UE performs the cell handover.

[0141] Figure 15 It is shown that, according to this disclosure, it is similar to Figure 14 The sequence diagram of the fourth scene of the early synchronous RACH is shown in Figure 1500.

[0142] Similar to Figure 14 The steps shown are inFigure 15 In step 1, the measurement value reported by UE 122 is compared with a first threshold, and based on this comparison, a PDDCH command to trigger early synchronization is sent to UE 122 (step 2). However, as... Figure 15 As shown, in step 3, the RACH request sent by UE 122 may not be successfully received by target DU 204. Therefore, in step 4, after the timer expires, the PDDCH command used to trigger early synchronization is retransmitted to UE 122. In step 5, the RACH request is retransmitted by UE 122. Figure 15 Steps 6 to 11 and Figure 14 Steps 4 through 9 are the same.

[0143] Figure 16 It is shown that, according to this disclosure, it is similar to Figure 15 The sequence diagram for the fifth scene of the early synchronized RACH is shown in Figure 1500. Similar to... Figure 15 The steps shown are in Figure 16 In step 1, the measurement value reported by UE 122 is compared with a first threshold, and based on this comparison, a PDDCH command to trigger early synchronization is sent to UE 122 (step 2). However, as... Figure 16 As shown, in step 3, the RACH request sent by UE 122 may not have been successfully received by target DU 204. Figure 15 In this embodiment, the PDDCH command that triggers early synchronization is retransmitted to UE 122. However, in Figure 16 In the process, although the RACH request is successfully received, the measurement value reported by UE 122 is directly compared with the second threshold (step 4). Subsequently, at step 5, an LTM cell handover command is sent to UE 122, and at step 9, the UE performs a cell handover and then executes the RACH request to receive the timing advance value.

[0144] Figure 17 It is shown that, according to this disclosure, it is similar to Figures 8A-8B A sequence diagram for scenarios used to handle E1 resets during LTM. Figure 17 The steps are similar to Figure 8B The steps are shown. However, Figure 17The process consists of additional steps 4 through 7 and steps 16 through 17. At step 4, an F1AP UE context modification request including the LTM configuration is sent to source DU 202, and at step 5, source DU 202 sends an F1AP UE context modification response. At step 6, an F1AP UE context setting request including the LTM configuration is sent to target DU 204, and at step 7, target DU 204 sends an F1AP UE context setting response. Furthermore, at step 16, CU-CP 206 sends an F1AP UE context release command to target DU 204, and in response, target DU 204 sends an F1AP UE context release complete message to CU-CP 206.

[0145] Figure 18 It is shown that, according to this disclosure, it is similar to Figure 5C The diagram shows a sequence of scenarios for the Early Synchronous Random Access Channel (RACH) in LTM. Figure 18 Steps 1 to 4 and Figure 5C Steps 1 through 4 are the same. However, as shown in Figure 5, target DU 204 failed to notify CU-CP 206 of the timing advance value. Therefore, source DU 202 did not receive the timing advance value (step 8) and retransmitted the PDDCH command to trigger early synchronization to UE 122.

[0146] Figure 19 This refers to the functional configuration of the device according to this disclosure. Device 1900 (or electronic device) may be base station 106 or a component of base station 106. For example, device 1900 may be CU 108 of base station 106. For example, device 1900 may be DU 114 or 116 of base station 106. For example, device 1900 may be CU-CP 110 of base station 106. For example, device 1900 may be CU-UP 112 of base station 106. Device 1900 may be UE 122.

[0147] refer to Figure 19 Device 1900 may include at least one transceiver 1910, at least one memory 1920, and at least one processor 1930. The type and / or number of components included in device 1900 are not limited to... Figure 19 Those shown. For example, device 1900 may only include those shown. Figure 19 Some of the components shown.

[0148] Transceiver 1910 can perform functions for transmitting and receiving signals via a wireless channel. For example, transceiver 1910 can perform conversion functions between baseband signals and bit strings according to the system's physical layer specifications. For example, during data transmission, transceiver 1910 generates complex symbols by encoding and modulating the transmitted bit strings. For example, during data reception, transceiver 1910 recovers the received bit strings by demodulating and decoding the baseband signals. Furthermore, transceiver 1910 can upconvert baseband signals to radio frequency (RF) band signals for transmission via an antenna, and downconvert RF band signals received via the antenna back to baseband signals.

[0149] For this purpose, transceiver 1910 may include transmit filters, receive filters, amplifiers, mixers, oscillators, digital-to-analog converters (DACs), analog-to-digital converters (ADCs), etc. For example, transceiver 1910 may include multiple transmit / receive paths. For example, transceiver 1910 may include at least one antenna array having multiple antenna elements. In terms of hardware, transceiver 1910 may include digital units and analog units, wherein the analog units may include multiple sub-units according to operating power, operating frequency, etc.

[0150] Transceiver 1910 transmits and receives signals as described above. Therefore, transceiver 1910 may be referred to as a "transmitter unit," a "receiver unit," or a "transceiver unit." Furthermore, throughout the following description, transmission and reception performed via wireless channels, backhaul networks, optical fibers, Ethernet, and other wired paths are used to include the meaning of the processes performed by transceiver 1910 as described above. According to embodiments, transceiver 1910 can provide an interface for communicating with other nodes in the network. In other words, transceiver 1910 can convert bit strings sent from device 1900 to another node (such as, for example, another access node, another base station, a higher node, the core network, etc.) into physical signals, and convert physical signals received from another node into bit strings.

[0151] Memory 1920 can store data, such as, for example, basic programs, application programs, and setup information for operating device 1900. Memory 1920 can store various types of data used by at least one component (e.g., transceiver 1910 or processor 1930). Data may include, for example, software and input or output data for instructions associated therewith. Memory 1920 may include volatile memory, non-volatile memory, or a combination of volatile and non-volatile memory. Furthermore, memory 1920 can provide data stored therein upon request from processor 1930.

[0152] Processor 1930 can control the overall operation of device 1900. For example, processor 1930 records data in memory 1920 or reads data from memory 1920. For example, processor 1930 sends and receives signals via transceiver 1910. Although Figure 19 A processor is shown, but embodiments of this disclosure are not limited thereto. Device 1900 may include at least one processor to perform the example embodiments. Processor 1930 may be referred to as a control unit or control device and may include processing circuitry. According to embodiments, processor 1930 may control device 1900 to perform at least one of the operations or methods according to the example embodiments.

[0153] The processor 1930 of device 1900 may include various processing circuitry and / or multiple processors. For example, the term "processor" as used herein (including the claims) may include various processing circuitry comprising at least one processor, and one or more of the at least one processor may be configured to individually and / or collectively perform the various functions described below in the distributed scheme. When "a processor," "at least one processor," and "one or more processors" are described as being configured to perform the various functions used below, these terms are not limited to examples and include cases where one processor performs a portion of the referenced function and another processor performs another portion of the referenced function, and cases where one processor can perform all of the referenced functions. Additionally, for example, at least one processor may include a combination of processors performing the various functions listed / disclosed in the distributed scheme. At least one processor may execute program instructions to implement or perform various functions.

[0154] Although only a single element (e.g., transceiver 1910, memory 1920, processor 1930) is shown in the accompanying drawings, it is not excluded that the device 1900 according to embodiments of the present disclosure may include multiple elements. For example, device 1900 may include multiple transceivers. Device 1900 may include multiple memories. Device 1900 may include multiple processors.

[0155] According to an embodiment, a method can be executed by a base station to implement lower-layer triggered mobility (LTM) during a cell handover process of a user equipment (UE) from a source cell served by the base station to a target cell. The method may include: identifying a target cell for LTM for the UE based on LTM measurements received from the UE or based on one or more internal decisions. The method may include: performing admission control on the target cell based on one or more factors. The method may include: generating an LTM configuration when admission control is successful. The method may include: sending the generated LTM configuration to the UE in a Radio Resource Control (RRC) reconfiguration message and to a Source Distributed Unit (DU) associated with the source cell. The method may include: facilitating early synchronization of the UE in the target cell based on an F1 Application Protocol (F1AP) message received from the target DU associated with the target cell. The method may include: performing an LTM cell handover completion upon receiving an indication from the target DU indicating successful access to the target cell.

[0156] In an embodiment, the LTM configuration may include at least one of the following: a complete LTM candidate configuration; and a non-complete LTM candidate configuration and an LTM reference configuration. The LTM reference configuration may be based on at least one of a cell group configuration associated with the source cell and a cell group configuration associated with the target cell, as well as a Layer 3 configuration. The Layer 3 configuration may include a measurement configuration and a radio bearer configuration.

[0157] In an embodiment, one or more factors may include: the number of UEs associated with neighboring cells that triggered LTM, the number of RRC_CONNECTED UEs in the target cell, and the number of UEs associated with neighboring cells configured with conditional handover. Performing admission control on the target cell may include performing one or more of the following: configuring the target cell as an LTM candidate cell when the number of UEs associated with the cell that triggered LTM is less than a first threshold; configuring the cell as an LTM candidate cell when the sum of the number of RRC_CONNECTED UEs and the number of UEs associated with the cell that triggered LTM is less than a second threshold; and configuring the cell as an LTM candidate cell when the sum of the number of RRC_CONNECTED UEs, the number of UEs associated with the cell configured with conditional handover, and the number of UEs associated with the cell that triggered LTM is less than a third threshold.

[0158] In an embodiment, facilitating early synchronization of the UE in the target cell may include: receiving an F1AP message including an LTM candidate cell identifier and a timing advance value from the target DU by a centralized unit (CU) of the base station. Facilitating early synchronization of the UE in the target cell may also include: sending the LTM candidate cell identifier and the timing advance value to the source DU associated with the source cell.

[0159] In an embodiment, the method may further include: when the target DU does not transmit a timing advance value to the source DU, sending a physical downlink control channel (PDCCH) command to trigger or re-trigger the UE to perform early synchronization on the target cell.

[0160] In an embodiment, the base station may include a centralized unit (CU) and one or more DUs for providing services to the UE. The CU may include a centralized unit control plane (CU-CP) and at least one centralized unit user plane (CU-UP). The one or more DUs may include a source DU and a target DU coupled to at least one CU-UP for providing services to the UE.

[0161] In an embodiment, the method may further include: receiving LTM measurements from the UE, wherein the LTM measurements include Layer 1 measurements, the Layer 1 measurements including information associated with at least one of Reference Signal Received Power (RSRP), Reference Signal Received Quality (RSRQ), and Signal-to-Interference-plus-Noise Ratio (SINR). The method may further include: based on determining that the received LTM measurements satisfy a first threshold, sending a Physical Downlink Control Channel (PDCCH) command including information about a target cell to trigger the UE to perform early synchronization on the target cell, wherein the information about the target cell includes an LTM candidate cell index. The method may further include: based on determining that the received LTM measurements satisfy a second threshold, sending a MAC Control Element (MAC CE) including information about the target cell to trigger the UE to perform cell handover, wherein the information about the target cell includes an LTM candidate cell index.

[0162] In an embodiment, the method may further include: starting a timing advance (TA) timer based on whether early synchronization has occurred in the UE. The method may further include: resetting the TA timer upon receiving an indication regarding a cell handover process.

[0163] In an embodiment, one or more internal decisions may include one or more decisions made by the base station based on deployment graphs or load balancing techniques or predictive data received from one or more machine learning (ML) models, wherein an RRC reconfiguration message is sent to the source DU in an F1AP message.

[0164] According to an embodiment, a base station for implementing lower-layer triggered mobility (LTM) during a cell handover process of a user equipment (UE) from a source cell to a target cell served by a base station can be configured to identify the target cell for LTM for the UE based on LTM measurements received from the UE or based on one or more internal decisions. The base station can be configured to perform admission control on the target cell based on one or more factors. The base station can be configured to generate an LTM configuration upon successful admission control. The base station can be configured to send the generated LTM configuration to the UE in a Radio Resource Control (RRC) reconfiguration message and to the source distributed element (DU) associated with the source cell. The base station can be configured to facilitate early synchronization of the UE in the target cell based on F1 Application Protocol (F1AP) messages received from the target DU associated with the target cell. The base station can be configured to perform LTM cell handover completion upon receiving an indication from the target DU indicating successful access to the target cell.

[0165] In an embodiment, the LTM configuration may include at least one of the following: a complete LTM candidate configuration; and a non-complete LTM candidate configuration and an LTM reference configuration. The LTM reference configuration may be based on at least one of a cell group configuration associated with the source cell and a cell group configuration associated with the target cell, as well as a Layer 3 configuration, wherein the Layer 3 configuration includes a measurement configuration and a radio bearer configuration.

[0166] In an embodiment, one or more factors may include: the number of UEs associated with neighboring cells that triggered LTM, the number of RRC_CONNECTED UEs in the target cell, and the number of UEs associated with the target cell configured with conditional handover. To perform admission control on the target cell, the base station may be adapted to configure the target cell when the number of UEs associated with the target cell that triggered LTM is less than a first threshold. To perform admission control on the target cell, the base station may be adapted to configure the target cell when the sum of the number of RRC_CONNECTED UEs and the number of UEs associated with the target cell that triggered LTM is less than a second threshold. To perform admission control on the target cell, the base station may be adapted to configure the target cell when the sum of the number of RRC_CONNECTED UEs, the number of UEs associated with the target cell configured with conditional handover, and the number of UEs associated with the target cell that triggered LTM is less than a third threshold.

[0167] In this embodiment, to facilitate early synchronization, the base station can be configured to receive an F1AP message including an LTM target cell identifier and a timing advance value from the target DU via a centralized unit (CU) of the base station. To further facilitate early synchronization, the base station can also be configured to send the LTM target cell identifier and timing advance value to the source DU associated with the source cell.

[0168] In this embodiment, the base station can also be configured to send a physical downlink control channel (PDCCH) command to trigger or re-trigger the UE to perform early synchronization with the target cell when the target DU does not transmit a timing advance value to the source DU.

[0169] In an embodiment, the base station may include a centralized unit (CU) for providing services to a UE and one or more centralized unit users (DUs). The CU may include a centralized unit control plane (CU-CP) and at least one centralized unit user plane (CU-UP). The one or more DUs may include a source DU and a target DU, which are coupled to at least one CU-UP for providing services to the UE.

[0170] In an embodiment, the base station may further be configured to receive LTM measurements from the UE, wherein the LTM measurements include Layer 1 measurements, which include information associated with at least one of Reference Signal Received Power (RSRP), Reference Signal Received Quality (RSRQ), and Signal-to-Interference-plus-Noise Ratio (SINR). The base station may also be configured to: based on determining that the received LTM measurements satisfy a first threshold, transmit a Physical Downlink Control Channel (PDCCH) command including information about the target cell to trigger the UE to perform early synchronization on the target cell, wherein the information about the target cell includes an LTM candidate cell index. The base station may further be configured to: based on determining that the received LTM measurements satisfy a second threshold, transmit a MAC Control Element (MAC CE) including information about the target cell to trigger the UE to perform a cell handover procedure, wherein the information about the target cell includes an LTM candidate cell index.

[0171] In this embodiment, the base station may also be configured to initiate a timing advance (TA) timer based on whether early synchronization has occurred in the UE. The base station may also be configured to reset the TA timer upon receiving an indication regarding a cell handover process.

[0172] In an embodiment, one or more internal decisions may include: one or more decisions made by the base station based on deployment maps or load balancing techniques or predictive data received from one or more machine learning (ML) models, wherein an RRC reconfiguration message is sent to the source DU in an F1AP message.

[0173] According to an embodiment, a method performed by a centralized cell control plane (CU-CP) may include: identifying a target cell for LTM (Lapse Measure) for a user equipment (UE) based on lower-layer triggered mobility LTM measurements received from a UE. The method may include: performing admission control on the target cell by determining whether the number of UEs associated with the target cell is less than a threshold. The method may include: generating an LTM configuration including LTM candidate cells, which include the target cell, based on the determination that the number of UEs is less than the threshold. The method may include: sending the generated LTM configuration to the UE in a Radio Resource Control (RRC) reconfiguration message via a source distributed element (DU) associated with the source cell for LTM.

[0174] In an embodiment, the method may further include: facilitating early synchronization of the UE in the target cell based on an F1 Application Protocol (F1AP) message received from a target DU associated with the target cell. The method may further include: performing LTM cell handover completion by receiving an indication of successful access to the target cell from the target DU.

[0175] In an embodiment, the LTM configuration may include at least one of an LTM candidate configuration or an LTM reference configuration for configuring LTM candidate cells. The LTM reference configuration may be based on at least one of a cell group configuration associated with the source cell and a cell group configuration associated with the target cell, as well as a Layer 3 configuration. The Layer 3 configuration may include a measurement configuration and a radio bearer configuration.

[0176] In an embodiment, the number of UEs associated with the target cell may include one or more of the following: the number of UEs that have triggered LTM, the number of RRC_CONNECTED UEs in the target cell, and the number of UEs configured for conditional handover. Performing admission control on the target cell may include performing one of the following: configuring the target cell as an LTM candidate cell when the number of UEs that have triggered LTM is less than a first threshold; configuring the target cell as an LTM candidate cell when the sum of the number of RRC_CONNECTED UEs and the number of UEs that have triggered LTM is less than a second threshold; and configuring the target cell as an LTM candidate cell when the sum of the number of RRC_CONNECTED UEs, the number of UEs configured for conditional handover, and the number of UEs that have triggered LTM is less than a third threshold.

[0177] In an embodiment, facilitating early synchronization of the UE in the target cell may include receiving an F1AP message from the target DU that includes an LTM candidate cell identifier and a timing advance value. Facilitating early synchronization of the UE in the target cell may also include sending the LTM candidate cell identifier and the timing advance value to the source DU associated with the source cell.

[0178] In an embodiment, the method may further include: sending a physical downlink control channel (PDCCH) command to trigger or re-trigger the UE to perform early synchronization with the target cell if the timing advance value is not sent from the target DU to the source DU.

[0179] In an embodiment, the CU may include a CU-CP and at least one centralized unit user plane (CU-UP). The CU may include a CU-CP, and at least one CU-UP is connected to one or more DUs for providing services to the UE. The one or more DUs may include a source DU and a target DU, which are coupled to at least one CU-UP for providing services to the UE.

[0180] In an embodiment, the method may further include: receiving LTM measurements from the UE, wherein the LTM measurements include Layer 1 measurements, the Layer 1 measurements including information associated with at least one of Reference Signal Received Power (RSRP), Reference Signal Received Quality (RSRQ), or Signal-to-Interference-plus-Noise Ratio (SINR). The method may further include: based on determining that the received LTM measurements are greater than a first threshold, sending a Physical Downlink Control Channel (PDCCH) command including information about a target cell to trigger the UE to perform early synchronization on the target cell. The information about the target cell may include an LTM candidate cell index. The method may further include: based on determining that the received LTM measurements are greater than a second threshold, sending a MAC Control Element (MAC CE) including information about the target cell to trigger the UE to perform cell handover, wherein the information about the target cell includes an LTM candidate cell index.

[0181] In an embodiment, the method may further include: starting a timing advance (TA) timer based on determining whether early synchronization has occurred in the UE. The method may further include: resetting the TA timer upon receiving an indication regarding a cell handover process.

[0182] In this embodiment, the target cell can also be identified based on one or more internal decisions, including decisions based on deployment maps or load balancing techniques or predictive data received from one or more machine learning (ML) models. An RRC reconfiguration message can be sent to the source DU in an F1AP message.

[0183] According to an embodiment, a centralized unit control plane (CU-CP) may include a memory storing instructions. The CU-CP may include at least one processor. When executed individually or jointly by at least one processor, the instructions may cause the CU-CP to: identify a target cell for LTM for a user equipment (UE) based on lower-layer triggered mobility LTM measurements received from the UE. When executed individually or jointly by at least one processor, the instructions may cause the CU-CP to: perform admission control on the target cell by determining whether the number of UEs associated with the target cell is less than a threshold. When executed individually or jointly by at least one processor, the instructions may cause the CU-CP to: generate an LTM configuration including LTM candidate cells, which includes the target cell, based on the determination that the number of UEs is less than the threshold. When executed individually or jointly by at least one processor, the instructions may cause the CU-CP to: transmit the generated LTM configuration to the UE in a Radio Resource Control (RRC) reconfiguration message via a Source Distributed Unit (DU) associated with the source cell for LTM.

[0184] In an embodiment, when executed by at least one processor individually or jointly, the instruction can cause the CU-CP to facilitate early synchronization of the UE in the target cell based on an F1 Application Protocol (F1AP) message received from the target DU associated with the target cell. When executed by at least one processor individually or jointly, the instruction can also cause the CU-CP to perform LTM cell handover completion upon receiving an indication from the target DU indicating successful access to the target cell.

[0185] In an embodiment, the LTM configuration may include at least one of an LTM candidate configuration or an LTM reference configuration for configuring LTM candidate cells. The LTM reference configuration may be based on at least one of a cell group configuration associated with the source cell and a cell group configuration associated with the target cell, as well as a Layer 3 configuration. The Layer 3 configuration may include a measurement configuration and a radio bearer configuration.

[0186] In an embodiment, the number of UEs associated with the target cell may include one or more of the following: the number of UEs that have triggered LTM, the number of RRC_CONNECTED UEs in the target cell, and the number of UEs configured for conditional handover. When executed individually or jointly by at least one processor, the instruction, used by the CU-CP to perform admission control on the target cell, performs one of the following: when the number of UEs that have triggered LTM is less than a first threshold, configure the target cell as an LTM candidate cell; when the sum of the number of RRC_CONNECTED UEs and the number of UEs that have triggered LTM is less than a second threshold, configure the target cell as an LTM candidate cell; and when the sum of the number of RRC_CONNECTED UEs, the number of UEs configured for conditional handover, and the number of UEs that have triggered LTM is less than a third threshold, configure the target cell as an LTM candidate cell.

[0187] According to various embodiments, a non-transitory computer-readable medium may store one or more programs including instructions that, when executed individually or jointly by at least one processor of a centralized unit control plane (CU-CP), cause the CU-CP to: identify a target cell for LTM (Lapse Measure) for a user equipment (UE) based on lower-layer triggered mobility LTM measurements received from a UE. When executed individually or jointly by at least one processor, the instructions may cause the CU-CP to: perform admission control on the target cell by determining whether the number of UEs associated with the target cell is less than a threshold. When executed individually or jointly by at least one processor, the instructions may cause the CU-CP to: generate an LTM configuration including LTM candidate cells, which includes the target cell, based on the determination that the number of UEs is less than the threshold. When executed individually or jointly by at least one processor, the instructions may cause the CU-CP to: transmit the generated LTM configuration to the UE in a Radio Resource Control (RRC) reconfiguration message via a Source Distributed Unit (DU) associated with a source cell for LTM.

[0188] The embodiments disclosed herein can be implemented by at least one software program that runs on at least one hardware device and performs network management functions to control elements. Elements can be at least one of a hardware device or a combination of a hardware device and a software module. The foregoing description of the specific embodiments will so fully reveal the general nature of the embodiments herein that others can readily modify and / or adapt such embodiments for various applications by applying present knowledge without departing from the general concept, and therefore, such adaptations and modifications should and are intended to be understood as being within the meaning and scope of equivalent forms of the disclosed embodiments. It should be understood that the wording or terminology used herein is for descriptive purposes and not for limitation. Therefore, although embodiments herein have been described according to at least one embodiment, those skilled in the art will recognize that the embodiments herein can be practiced with modifications within the spirit and scope of the embodiments described herein.

Claims

1. A method performed by a centralized unit control plane, CU-CP (110), the method comprising: identifying, based on lower layer triggered mobility, LTM, measurements received from a user equipment, UE (122), a target cell (120) for LTM of the UE (122); performing admission control for the target cell (120) by determining whether a number of UEs associated with the target cell (120) is less than a threshold value; generating, in accordance with a determination that the number of UEs is less than the threshold value, an LTM configuration comprising LTM candidate cells, the LTM candidate cells comprising the target cell (120); and sending, via a source distributed unit, DU (202) associated with a source cell (118) for the LTM, the generated LTM configuration to the UE (122) in a radio resource control, RRC, reconfiguration message.

2. The method of claim 1, the method further comprising: facilitating (1210) early synchronization of the UE (122) in the target cell (120) based on a F1 application protocol, F1AP, message received from a target DU (204) associated with the target cell (120); and performing (1212) LTM cell handover completion by receiving an indication from the target DU (204) indicating that access to the target cell (120) was successful. the LTM configuration comprises at least one of:

3. The method of claim 1, wherein, an LTM candidate configuration configuring the LTM candidate cells; or an LTM reference configuration, wherein the LTM reference configuration is based on at least one of a cell group configuration associated with the source cell and a cell group configuration associated with the target cell and a layer 3 configuration, and wherein the layer 3 configuration comprises a measurement configuration and a radio bearer configuration. the number of UEs associated with the target cell comprises one or more of:

4. The method of claim 1, wherein, a number of UEs for which LTM is triggered, a number of RRC CONNECTED UEs in the target cell, and a number of UEs configured with conditional handover, wherein performing the admission control for the target cell comprises performing one of: configuring the target cell as the LTM candidate cell when the number of UEs for which LTM is triggered is less than a first threshold value; configuring the target cell as the LTM candidate cell when a sum of the number of RRC CONNECTED UEs and the number of UEs for which LTM is triggered is less than a second threshold value; and configuring the target cell as the LTM candidate cell when a sum of the number of RRC CONNECTED UEs and the number of UEs configured with conditional handover and the number of UEs for which LTM is triggered is less than a third threshold value. facilitating early synchronization of the UE in the target cell comprises:

5. The method of claim 2, wherein, receiving, from the target DU, the F1AP message comprising an LTM candidate cell identifier and a timing advance value; and ​ sending the LTM candidate cell identifier and the timing advance value to the source DU associated with the source cell.

6. The method of claim 5, further comprising: sending a physical downlink control channel (PDCCH) order to trigger or retrigger the UE to perform early synchronization to the target cell in case the timing advance value is not sent from the target DU to the source DU.

7. The method of claim 1, wherein, a CU comprising the CU-CP and at least one centralized unit user plane (CU-UP), wherein the CU comprising the CU-CP and the at least one CU-UP is connected to one or more DUs for serving the UE, and wherein the one or more DUs comprise the source DU and the target DU, the source DU and the target DU being coupled to the at least one CU-UP for serving the UE.

8. The method of claim 1, further comprising: receiving the LTM measurements from the UE, wherein the LTM measurements comprise layer 1 measurements comprising information associated with at least one of reference signal received power (RSRP), reference signal received quality (RSRQ), or signal to interference plus noise ratio (SINR); based on determining that the received LTM measurements are greater than a first threshold, sending a physical downlink control channel (PDCCH) order comprising information about the target cell to trigger the UE to perform early synchronization to the target cell, wherein the information about the target cell comprises an LTM candidate cell index; and based on determining that the received LTM measurements are greater than a second threshold, sending a MAC control element (MAC CE) comprising information about the target cell to trigger the UE to perform a cell handover, wherein the information about the target cell comprises an LTM candidate cell index.

9. The method of claim 1, further comprising: starting a timing advance (TA) timer based on determining whether early synchronization has occurred in the UE; and resetting the TA timer upon receiving an indication about a cell handover procedure.

10. The method of claim 1, wherein, the target cell is further identified based on one or more internal decisions comprising one or more decisions based on a deployment map or load balancing techniques or prediction data received from one or more machine learning (ML) models, and wherein the RRC reconfiguration message is sent to the source DU in an FlAP message.

11. A centralized unit control plane (CU-CP) (110), the CU-CP comprising: a memory having instructions stored therein; at least one processor, wherein the instructions, when executed by the at least one processor individually or collectively, cause the CU-CP (110) to: identify a target cell (120) for lower layer triggered mobility (LTM) of a user equipment (UE) (122) based on LTM measurements received from the UE (122). perform admission control for the target cell (120) by determining whether a number of UEs associated with the target cell (120) is less than a threshold value; generate, in accordance with a determination that the number of UEs is less than the threshold value, an LTM configuration comprising an LTM candidate cell, the LTM candidate cell comprising the target cell (120); and send, via a source distributed unit, DU (202) associated with a source cell (118) for the LTM, the generated LTM configuration to the UE (122) in a radio resource control, RRC, reconfiguration message.

12. The base station of claim 11, wherein, the instructions, when executed by the at least one processor individually or collectively, cause the CU-CP (110) to: facilitate early synchronization of the UE (122) in the target cell (120) based on an F1 application protocol, F1AP, message received from a target DU (204) associated with the target cell (120); and perform LTM cell handover completion by receiving, from the target DU (204), an indication indicating that access to the target cell (120) is successful.

13. The base station of claim 11, wherein, the LTM configuration comprises at least one of: an LTM candidate configuration configuring the LTM candidate cell; or an LTM reference configuration, wherein the LTM reference configuration is based on at least one of a cell group configuration associated with the source cell and a cell group configuration associated with the target cell and a layer 3 configuration, and wherein the layer 3 configuration comprises a measurement configuration and a radio bearer configuration.

14. The base station of claim 11, wherein, the number of UEs associated with the target cell comprises one or more of: a number of UEs for which LTM is triggered, a number of RRC CONNECTED UEs in the target cell, and a number of UEs configured with conditional handover, wherein the instructions, when executed by the at least one processor individually or collectively, cause the CU-CP (110) performing the admission control for the target cell to perform one of: configure the target cell as the LTM candidate cell when the number of UEs for which LTM is triggered is less than a first threshold value; configure the target cell as the LTM candidate cell when a sum of the number of RRC CONNECTED UEs and the number of UEs for which LTM is triggered is less than a second threshold value; and configure the target cell as the LTM candidate cell when a sum of the number of RRC CONNECTED UEs and the number of UEs configured with conditional handover and the number of UEs for which LTM is triggered is less than a third threshold value.

15. A non-transitory computer-readable medium storing one or more programs comprising instructions, which when executed by at least one processor of a centralized unit control plane, CU-CP (110), individually or collectively, cause the CU-CP (110) to: identify, based on lower layer triggered mobility (LTM) measurements received from a user equipment (UE) (122), a target cell (120) for LTM of the UE (122); perform admission control for the target cell (120) by determining whether a number of UEs associated with the target cell (120) is less than a threshold value; generate, in accordance with a determination that the number of UEs is less than the threshold value, an LTM configuration including LTM candidate cells, the LTM candidate cells including the target cell (120); and send, via a source distributed unit (DU) (202) associated with a source cell (118) for the LTM, the generated LTM configuration to the UE (122) in a radio resource control (RRC) reconfiguration message.

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