WO2025152127A1

WO2025152127A1 - Devices and methods for communication

Info

Publication number: WO2025152127A1
Application number: PCT/CN2024/073136
Authority: WO
Inventors: Da Wang; Gang Wang
Original assignee: NEC Corp
Current assignee: NEC Corp
Priority date: 2024-01-18
Filing date: 2024-01-18
Publication date: 2025-07-24
Anticipated expiration: 2026-07-18
Also published as: WO2025152127A8

Abstract

Embodiments of the present disclosure provide a solution for Layer 1 or Layer 2 (L1/L2) triggered mobility (LTM) cell switch. In the solution, a first network device transmits, to a second network device in a radio access network (RAN), a first Layer 1 or Layer 2 (L1/L2) triggered mobility (LTM) cell change notification associated with a LTM procedure for a cell switch from a source cell to a target cell. The first LTM cell change notification indicates an initiation of a LTM cell switch command to a terminal device. The first network device is in the RAN and associated with the source cell. The second network device is associated with a candidate cell for the cell switch.

Description

DEVICES AND METHODS FOR COMMUNICATION

FIELDS

Example embodiments of the present disclosure generally relate to the field of communication techniques and in particular, to devices and methods for cell switch execution of a Layer 1 or Layer 2 (L1/L2) triggered mobility (LTM) cell switch procedure.

BACKGROUND

A LTM procedure is a layer 1 or layer 2 triggered mobility procedure. Generally, a LTM procedure includes a network triggered LTM and/or a conditional LTM procedure. The network triggered LTM is a LTM procedure in which a next generation NodeB (gNB) receives L1 measurement report (s) from user equipment (UE) , and on their basis the gNB changes UE’s serving cell by a cell switch command signaled via a Medium Access Control (MAC) Control Element (CE) . The cell switch command indicates an LTM candidate cell configuration that the gNB previously prepared and provided to the UE through Radio Resource Control (RRC) signaling. Then the UE switches to the target cell according to the cell switch command. The conditional LTM procedure is a LTM procedure initiated when one or more LTM execution conditions are satisfied. The LTM procedure can be used to reduce the mobility latency.

SUMMARY

In a first aspect, there is provided a first network device comprising: a processor configured to cause the first network device to: transmit, to a second network device in a radio access network (RAN) , a first Layer 1 or Layer 2 (L1/L2) triggered mobility (LTM) cell change notification associated with a LTM procedure for a cell switch from a source cell to a target cell, the first LTM cell change notification indicating an initiation of a LTM cell switch command to a terminal device, the first network device being in the RAN and associated with the source cell, and the second network device being associated with a candidate cell for the cell switch.

In a second aspect, there is provided a second network device comprising: a processor configured to cause the second network device to: receive, from a first network device in a radio access network (RAN) , a first Layer 1 or Layer 2 (L1/L2) triggered mobility (LTM) cell change notification associated with a LTM procedure for a cell switch from a source cell to a target cell, the first LTM cell change notification indicating an initiation of a LTM cell switch command to a terminal device, the first network device being associated with the source cell, and the second network device being in the RAN and associated with a candidate cell for the cell switch.

In a third aspect, there is provided a terminal device comprising: a processor configured to cause the terminal device to: perform a Medium Access Control (MAC) reset operation without stopping a time alignment timer (TAT) associated with a candidate cell or a target cell in a Layer 1 or Layer 2 (L1/L2) triggered mobility (LTM) procedure for a cell switch from a source cell to the target cell, and without considering that the TAT as expired.

In a fourth aspect, there is provided a first network device comprising: a processor configured to cause the first network device to: receive, from a second network device in a radio access network (RAN) , first timing advance (TA) information associated with a candidate cell in a Layer 1 or Layer 2 (L1/L2) triggered mobility (LTM) procedure for a cell switch from a source cell to a target cell, a first network device in the RAN being associated with the source cell, and the second network device being associated with the candidate cell.

In a fifth aspect, there is provided a second network device comprising: a processor configured to cause the second network device to: transmit, to a first network device in a radio access network (RAN) , first timing advance (TA) information associated with a candidate cell in a Layer 1 or Layer 2 (L1/L2) triggered mobility (LTM) procedure for a cell switch from a source cell to a target cell, the first network device being associated with the source cell, and the second network device being in the RAN and associated with the candidate cell.

In a sixth aspect, there is provided a communication method performed by a first network device. The method comprises: transmitting, to a second network device in a radio access network (RAN) , a first Layer 1 or Layer 2 (L1/L2) triggered mobility (LTM) cell change notification associated with a LTM procedure for a cell switch from a source cell to a target cell, the first LTM cell change notification indicating an initiation of a LTM cell switch command to a terminal device, the first network device being in the RAN and associated with the source cell, and the second network device being associated with a candidate cell for the cell switch.

In a seventh aspect, there is provided a communication method performed by a second network device. The method comprises: receiving, from a first network device in a radio access network (RAN) , a first Layer 1 or Layer 2 (L1/L2) triggered mobility (LTM) cell change notification associated with a LTM procedure for a cell switch from a source cell to a target cell, the first LTM cell change notification indicating an initiation of a LTM cell switch command to a terminal device, the first network device being associated with the source cell, and the second network device being in the RAN and associated with a candidate cell for the cell switch.

In an eighth aspect, there is provided a communication method performed by a terminal device. The method comprises: performing a Medium Access Control (MAC) reset operation without stopping a TAT associated with a candidate cell or a target cell in a Layer 1 or Layer 2 (L1/L2) triggered mobility (LTM) procedure for a cell switch from a source cell to the target cell, and without considering that the TAT as expired.

In a ninth aspect, there is provided a communication method performed by a first network device. The method comprises: receiving, from a second network device in a radio access network (RAN) , first timing advance (TA) information associated with a candidate cell in a Layer 1 or Layer 2 (L1/L2) triggered mobility (LTM) procedure for a cell switch from a source cell to a target cell, a first network device in the RAN being associated with the source cell, and the second network device being associated with the candidate cell.

In a tenth aspect, there is provided a communication method performed by a second network device. The method comprises: transmitting, to a first network device in a radio access network (RAN) , first timing advance (TA) information associated with a candidate cell in a Layer 1 or Layer 2 (L1/L2) triggered mobility (LTM) procedure for a cell switch from a source cell to a target cell, the first network device being associated with the source cell, and the second network device being in the RAN and associated with the candidate cell.

In an eleventh aspect, there is provided a computer readable medium having instructions stored thereon, the instructions, when executed on at least one processor, causing the at least one processor to carry out the method according to the sixth, seventh, eighth, ninth, or tenth aspect.

Other features of the present disclosure will become easily comprehensible through the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

Through the more detailed description of some example embodiments of the present disclosure in the accompanying drawings, the above and other objects, features and advantages of the present disclosure will become more apparent, wherein:

FIG. 1A illustrates an example communication environment in which example embodiments of the present disclosure can be implemented;

FIG. 1B illustrates another example communication environment in which example embodiments of the present disclosure can be implemented;

FIG. 2 illustrates a signaling flow for an example communication process in accordance with some embodiments of the present disclosure;

FIG. 3 illustrates a signaling flow for transmitting a cell change notification in accordance with some embodiments of the present disclosure;

FIG. 4 illustrates a signaling flow for an example communication process in accordance with some embodiments of the present disclosure;

FIG. 5 illustrates a signaling flow for an example communication process of Timing Advance (TA) information notification in accordance with some embodiments of the present disclosure;

FIG. 6 illustrates a signaling flow for an example communication process in accordance with some embodiments of the present disclosure;

FIG. 7 illustrates a signaling flow for an example communication process of LTM cell switch in accordance with some embodiments of the present disclosure;

FIG. 8 illustrates a flowchart of a method implemented at a first network device according to some example embodiments of the present disclosure;

FIG. 9 illustrates a flowchart of a method implemented at a second network device according to some example embodiments of the present disclosure;

FIG. 10 illustrates a flowchart of a method implemented at a terminal device according to some example embodiments of the present disclosure;

FIG. 11 illustrates a flowchart of a method implemented at a first network device according to some example embodiments of the present disclosure;

FIG. 12 illustrates a flowchart of a method implemented at a second network device according to some example embodiments of the present disclosure; and

FIG. 13 illustrates a simplified block diagram of an apparatus that is suitable for implementing example embodiments of the present disclosure.

Throughout the drawings, the same or similar reference numerals represent the same or similar element.

DETAILED DESCRIPTION

Principle of the present disclosure will now be described with reference to some example embodiments. It is to be understood that these embodiments are described only for the purpose of illustration and help those skilled in the art to understand and implement the present disclosure, without suggesting any limitation as to the scope of the disclosure. Embodiments described herein can be implemented in various manners other than the ones described below.

In the following description and claims, unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skills in the art to which this disclosure belongs.

As used herein, the term ‘terminal device’ refers to any device having wireless or wired communication capabilities. Examples of the terminal device include, but not limited to, user equipment (UE) , personal computers, desktops, mobile phones, cellular phones, smart phones, personal digital assistants (PDAs) , portable computers, tablets, wearable devices, internet of things (IoT) devices, Ultra-reliable and Low Latency Communications (URLLC) devices, Internet of Everything (IoE) devices, machine type communication (MTC) devices, devices on vehicle for V2X communication where X means pedestrian, vehicle, or infrastructure/network, devices for Integrated Access and Backhaul (IAB) , Space borne vehicles or Air borne vehicles in Non-terrestrial networks (NTN) including Satellites and High Altitude Platforms (HAPs) encompassing Unmanned Aircraft Systems (UAS) , eXtended Reality (XR) devices including different types of realities such as Augmented Reality (AR) , Mixed Reality (MR) and Virtual Reality (VR) , the unmanned aerial vehicle (UAV) commonly known as a drone which is an aircraft without any human pilot, devices on high speed train (HST) , or image capture devices such as digital cameras, sensors, gaming devices, music storage and playback appliances, or Internet appliances enabling wireless or wired Internet access and browsing and the like. The ‘terminal device’ can further has ‘multicast/broadcast’ feature, to support public safety and mission critical, V2X applications, transparent IPv4/IPv6 multicast delivery, IPTV, smart TV, radio services, software delivery over wireless, group communications and IoT applications. It may also incorporate one or multiple Subscriber Identity Module (SIM) as known as Multi-SIM. The term “terminal device” can be used interchangeably with a UE, a mobile station, a subscriber station, a mobile terminal, a user terminal or a wireless device.

The term “network device” refers to a device which is capable of providing or hosting a cell or coverage where terminal devices can communicate. Examples of a network device include, but not limited to, a Node B (NodeB or NB) , an evolved NodeB (eNodeB or eNB) , a next generation NodeB (gNB) , a transmission reception point (TRP) , a remote radio unit (RRU) , a radio head (RH) , a remote radio head (RRH) , an IAB node, a low power node such as a femto node, a pico node, a reconfigurable intelligent surface (RIS) , and the like.

The terminal device or the network device may have Artificial intelligence (AI) or Machine learning capability. It generally includes a model which has been trained from numerous collected data for a specific function, and can be used to predict some information.

The terminal or the network device may work on several frequency ranges, e.g., FR1 (e.g., 450 MHz to 6000 MHz) , FR2 (e.g., 24.25GHz to 52.6GHz) , frequency band larger than 100 GHz as well as Tera Hertz (THz) . It can further work on licensed/unlicensed/shared spectrum. The terminal device may have more than one connection with the network devices under Multi-Radio Dual Connectivity (MR-DC) application scenario. The terminal device or the network device can work on full duplex, flexible duplex and cross division duplex modes.

The embodiments of the present disclosure may be performed in test equipment, e.g., signal generator, signal analyzer, spectrum analyzer, network analyzer, test terminal device, test network device, channel emulator. In some embodiments, the terminal device may be connected with a first network device and a second network device. One of the first network device and the second network device may be a master node and the other one may be a secondary node. The first network device and the second network device may use different radio access technologies (RATs) . In some embodiments, the first network device may be a first RAT device and the second network device may be a second RAT device. In some embodiments, the first RAT device is eNB and the second RAT device is gNB. Information related with different RATs may be transmitted to the terminal device from at least one of the first network device or the second network device. In some embodiments, first information may be transmitted to the terminal device from the first network device and second information may be transmitted to the terminal device from the second network device directly or via the first network device. In some embodiments, information related with configuration for the terminal device configured by the second network device may be transmitted from the second network device via the first network device. Information related with reconfiguration for the terminal device configured by the second network device may be transmitted to the terminal device from the second network device directly or via the first network device.

As used herein, the singular forms ‘a’ , ‘an’ and ‘the’ are intended to include the plural forms as well, unless the context clearly indicates otherwise. The term ‘includes’ and its variants are to be read as open terms that mean ‘includes, but is not limited to. ’ The term ‘based on’ is to be read as ‘at least in part based on. ’ The term ‘one embodiment’ and ‘an embodiment’ are to be read as ‘at least one embodiment. ’ The term ‘another embodiment’ is to be read as ‘at least one other embodiment. ’ The terms ‘first, ’ ‘second, ’ and the like may refer to different or same objects. Other definitions, explicit and implicit, may be included below.

In some examples, values, procedures, or apparatus are referred to as ‘best, ’ ‘lowest, ’ ‘highest, ’ ‘minimum, ’ ‘maximum, ’ or the like. It will be appreciated that such descriptions are intended to indicate that a selection among many used functional alternatives can be made, and such selections need not be better, smaller, higher, or otherwise preferable to other selections.

As used herein, the term “resource, ” “transmission resource, ” “uplink resource, ” or “downlink resource” may refer to any resource for performing a communication, such as a resource in time domain, a resource in frequency domain, a resource in space domain, a resource in code domain, or any other resource enabling a communication, and the like. In the following, unless explicitly stated, a resource in both frequency domain and time domain will be used as an example of a transmission resource for describing some example embodiments of the present disclosure. It is noted that example embodiments of the present disclosure are equally applicable to other resources in other domains.

As used herein, unless stated explicitly, performing a step “in response to A” does not indicate that the step is performed immediately after “A” occurs and one or more intervening steps may be included.

Principles and implementations of the present disclosure will be described in detail below with reference to the figures.

FIG. 1A illustrates a schematic diagram of an example communication environment 100A in which example embodiments of the present disclosure can be implemented. In the communication environment 100A, a plurality of communication devices, including a first network device 110, a second network device 120 and a terminal device 130, can communicate with each other. The first network device 110 may include one or more units or devices, which may, for example, communicate with each other. Likewise, the first network device 110 may include one or more units or devices, the second network device 120 may include one or more units or devices, which may, for example, communicate with each other. The units or devices may include, for instance, a centralized unit (CU) , for example, a gNB-CU, at least one distributed unit (DU) , for example, a gNB-DU, or other suitable units or devices.

The gNB-CU may be a logical node hosting RRC, Service Data Adaptation Protocol (SDAP) and Packet Data Convergence Protocol (PDCP) protocols of the gNB or RRC and PDCP protocols of the En-gNB that controls the operation of one or more gNB-DUs. The gNB-CU may terminate the F1 interface connected with the gNB-DU.

The gNB-DU may be a logical node hosting Radio Link Control (RLC) , MAC and Physical layer (PHY) layers of the gNB or en-gNB, and its operation is partly controlled by gNB-CU. One gNB-DU supports one or multiple cells. One cell is supported by only one gNB-DU. The gNB-DU may terminate the F1 interface connected with the gNB-CU.

In the communication environment 100A, the terminal device 130 may be a UE, and the first network device 110 and second network device 120 may be network devices (e.g., base stations) , one is serving the UE and the other one is to serve the UE. For example, in a LTM procedure for a cell switch from the source cell to a target cell, the first network device 110 may be associated with the source cell which is currently serving the terminal device 130, and the second network device 120 may be associated with a candidate cell, which may be selected as the target cell.

FIG. 1B illustrates a schematic diagram of an example communication environment 100B in which example embodiments of the present disclosure can be implemented. Similar to the communication environment 100A, the communication environment 100B may include a plurality of communication devices, for example, but not limited to, the first network device 110 and the second network device 120 and the terminal device 130. Both the first network device 110 and the second network device 120 may have a CU-DU split architecture.

In contrast to the communication environment 100A, in embodiments discussed with reference to FIG. 1B, more details about the first network device 110 and the second network device 120 are provided.

Specifically, as shown in FIG. 1B, the first network device 110 includes a CU (also referred to as a “first CU” ) 111 and a DU (also referred to as a “first DU” ) 112, and the second network device 120 includes a CU (also referred to as a “second CU” ) 121 and a DU (also referred to as a “second DU” ) 122. The first CU 111 and the second CU 121 may communicate with each other, e.g., via an Xn interface. In some embodiments, for example, the first network device and the second network device may communicate with each other, e.g., via the Xn interface. The first CU 111 and the first DU 112 may communicate with each other, e.g., via an F1 interface. Likewise, the second CU 121 and the second DU 122 may communicate with each other, e.g., via an F1 interface.

In some embodiments where the first network device 110 is serving the UE, the source cell in a LTM procedure may be the cell managed by the first network device 110. That is, the first network device 110 is associated with the source cell. In such cases, the first CU 111 may be referred to as a source CU 111, the first DU 112 may be referred to as a source DU 112 It is to be understood that although FIG. 1B shows merely one DU, i.e., the first DU 112, in the first network device 110, it is just an example, rather than suggest any limitation. In embodiments of the present disclosure, the first network device 110 may include one or more DUs other than the first DU 112.

In the LTM procedure, there may be one or more candidate cells for the cell switch and the target cell is one of these candidate cells. One candidate cell may be associated or managed by one network device (e.g., gNB) . In the embodiments of FIGS. 1A and 1B, the second network device 120 is associated with a candidate cell for the LTM procedure. In such cases, the second CU 121 may be referred to as a candidate CU 121, the second DU 122 may be referred to as a candidate DU 122. It is to be understood that although FIG. 1B shows merely one DU, i.e., the second DU 122, is included in the second network device 120, it is just an example, rather than suggest any limitation. In embodiments of the present disclosure, the second network device 120 may include more than one DU.

In some example embodiments, the first network device 110 may be discussed as a first Next Generation Radio Access Network (NG-RAN) and the second network device 120 may be discussed as a second NG-RAN which is different from the first NG-RAN. In some example embodiments, the first network device 110 may be referred to as source NG-RAN. The second network device 120 may be referred to as candidate NG-RAN, or sometimes, target NG-RAN. A NG-RAN node may include, for example, a gNB or an NG-eNB.

The gNB may be a node providing NR user plane and control plane protocol terminations towards UE, and connected via a NG interface to the 5GC. The NG-eNB may be a node providing E-UTRA user plane and control plane protocol terminations towards the UE, and connected via the NG interface to the 5GC.

It is to be understood that the number of devices and their connections shown in FIG. 1A or FIG. 1B are only for the purpose of illustration without suggesting any limitation. The communication environment 100A or 100B may include any suitable number of devices configured to implementing example embodiments of the present disclosure. It is noted that although illustrated as a network device, the first network device 110 and second network device 120 may be other devices than network devices. Although illustrated as a terminal device, the terminal device 130 may be another device than a terminal device.

In the following, for the purpose of illustration, some example embodiments are described with the first network device 110 and the second network device 120 operating as a base station and the terminal device 130 operating as a UE. However, in some example embodiments, operations described in connection with a terminal device may be implemented at a network device or other device, and operations described in connection with a network device may be implemented at a terminal device or other device.

In some example embodiments, a link from the first network device 110 or the second network device 120 to the terminal device 130 is referred to as a downlink (DL) , while a link from the terminal device 130 to the first network device 110 or the second network device 120 is referred to as an uplink (UL) . In DL, the first network device 110 or the second network device 120 is a transmitting (TX) device (or a transmitter) and the terminal device 130 is a receiving (RX) device (or a receiver) . In UL, the terminal device 130 is a TX device (or a transmitter) and the first network device 110 or the second network device 120 is a RX device (or a receiver) .

The communications in the communication environment 100A or 100B may conform to any suitable standards including, but not limited to, Global System for Mobile Communications (GSM) , Long Term Evolution (LTE) , LTE-Evolution, LTE-Advanced (LTE-A) , New Radio (NR) , Wideband Code Division Multiple Access (WCDMA) , Code Division Multiple Access (CDMA) , GSM EDGE Radio Access Network (GERAN) , Machine Type Communication (MTC) and the like. The embodiments of the present disclosure may be performed according to any generation communication protocols either currently known or to be developed in the future. Examples of the communication protocols include, but not limited to, the first generation (1G) , the second generation (2G) , 2.5G, 2.75G, the third generation (3G) , the fourth generation (4G) , 4.5G, the fifth generation (5G) communication protocols, 5.5G, 5G-Advanced networks, or the sixth generation (6G) networks.

In embodiments of the present disclosure, a LTM procedure is a cell switch procedure that may include, for example, the network triggered LTM procedure via MAC CE based on L1 measurements and the conditional LTM procedure which is initiated when corresponding condition (s) are satisfied.

Depending on the availability of a valid timing advance (TA) value, the UE may perform either a random access channel (RACH) -less LTM or RACH-based LTM cell switch. A RACH-less LTM may be an LTM cell switch procedure where UE skips the random access procedure. If the TA value is provided in the cell switch command, the UE applies the TA value as instructed by the network. In the case where UE-based TA measurement is configured, but no TA value is provided in the cell switch command, the UE applies the TA value by itself if available. Meanwhile, the UE performs RACH-less LTM cell switch upon receiving the cell switch command or when conditions for LTM cell switch is satisfied. If no valid TA value is available, the UE performs RACH-based LTM cell switch.

A subsequent LTM may be a LTM cell switch procedure between candidate cells without RRC reconfiguration by the network in between.

In embodiments of the present disclosure which are discussed below, the LTM procedure may be either a network triggered LTM procedure or a conditional LTM procedure, or other suitable LTM procedure. The network triggered LTM procedure may be triggered by a LTM command MAC CE. The conditional LTM procedure may be executed in response to that at least one LTM execution condition is met.

More particular, the conditional LTM procedure may be also discussed as a conditional handover or a conditional cell switch, which is a handover procedure that is executed only when execution condition (s) are met.

More details about the LTM cell switch execution and LTM cell switch command will be provided as follows.

1. LTM cell switch execution

Upon the indication by lower layers that an LTM cell switch procedure is triggered, or upon performing LTM cell switch following cell selection performed while timer T311 was running, the UE shall:

● release/clear all current dedicated radio configuration associated with the cell group for which the LTM cell switch procedure is triggered except for the following: the logicalChannelIdentity and logicalChannelIdentityExt of RLC bearers configured in RLC-BearerConfig and the associated RLC entities, their state variables, buffers, and timers; the UE variables VarLTM-Config, VarLTM-ServingCellNoResetID, and VarLTM-ServingCellUE-MeasuredTA-ID.

If the LTM cell switch is triggered on the MCG:

- the MCG C-RNTI;

- the AS security configurations associated with the master key;

- for each SRB/DRB in current UE configuration which is using the master key:

- keep the associated PDCP and SDAP entities, their state variables, buffers and timers;

- release all fields related to the SRB/DRB configuration except for srb-Identity and drb-Identity;

else, if the LTM cell switch is triggered on the SCG:

- the AS security configurations associated with the secondary key;

- for each SRB/DRB in current UE configuration which is using the secondary key:

● release/clear all current common radio configuration associated with the cell group for which the LTM cell switch procedure is triggered;

● use the default values for timers T310, T311 and constants N310, N311 associate to cell group for which the LTM cell switch procedure is triggered;

● if the value of field ltm-NoResetID contained within the LTM-Candidate IE in VarLTM-Config indicated by lower layers or for the selected cell is not equal to the value of ltm-ServingCellNoResetID within VarLTM-ServingCellNoResetID:

for each logicalChannelId and logicalChannelIdExt that is part of the current UE configuration for the cell group for which the LTM cell switch procedure is triggered: after the end of this procedure, re-establish the corresponding RLC entity, after applying the LTM configuration in ltm-CandidateConfig within LTM-Candidate IE in VarLTM-Config;

for each drb-Identity value that is part of the current UE configuration: if this DRB is an AM DRB: after the end of this procedure, trigger the PDCP entity of this DRB to perform data recovery, after applying the LTM configuration in ltm-CandidateConfig within LTM-Candidate IE in VarLTM-Config;

replace the value of ltm-ServingCellNoResetID in VarLTM-ServingCellNoResetID with the value of ltm-NoResetID in the LTM-Candidate in VarLTM-Config indicated by lower layers or for the selected cell.

2. LTM cell switch command

The network may instruct the UE to perform LTM cell switch procedure by sending the LTM Cell Switch Command MAC CE. The MAC entity shall: if the MAC entity receives an LTM Cell Switch Command MAC CE on a Serving Cell:

● indicate to upper layers that the LTM cell switch procedure is triggered and the Target Configuration ID included in the MAC CE;

● if the MAC reset operation is performed, as requested by upper layers:

if Timing Advance Command value (hexa-decimal) is not set as FFF:

● process the received Timing Advance Command;

● consider the RACH-less LTM cell switch to be ongoing;

● if the MAC entity is associated with SCG: indicate to upper layers to skip the Random Access procedure for this LTM cell switch.

else if the Timing Advance measurement is configured and the UE has successfully measured the Timing Advance for the indicated LTM target:

● process the measured Timing Advance;

● consider the RACH-less LTM cell switch to be ongoing.

if TCI state information is included:

● consider the SSB corresponding to the indicated TCI state as the one used for configured uplink grant selection for the initial uplink transmission towards the candidate cell for RACH-less LTM cell switch;

● indicate to lower layers the information regarding the TCI state information included in the LTM Cell Switch Command MAC CE.

Regarding Maintenance of Uplink Time Alignment, when an LTM Cell Switch Command MAC CE including a Timing Advance Command is received, the Timing Advance Command for the PTAG is applied and the timeAlignmentTimer associated with the PTAG is started or restarted. When an LTM Cell Switch Command MAC CE is received and the UE has successfully measured the Timing Advance, the measured Timing Advance for the PTAG is applied and the timeAlignmentTimer associated with the PTAG is started or restarted.

To obtain early TA information associated to the candidate cell and transmit it to the UE, the source CU can utilize various methods. One approach is to rely on the UE’s measurement reports, which contain TA measurements for neighboring cells. On one hand, the target CU needs to receive the LTM cell change information through signaling messages exchanged. On the other hand, the source CU needs to obtain TA information from neighboring cells.

Embodiments of the present disclosure propose a solution for LTM cell switch. In this solution, the first network device 110 transmits, to a second network device 120 in a radio access network (RAN) , a first LTM cell change notification associated with a LTM procedure for a cell switch from a source cell to a target cell. In other words, a first LTM cell change notification associated with a LTM procedure for a cell switch is transmitted from a source NG-RAN to a target NG-RAN. The first LTM cell change notification indicates an initiation of a LTM cell switch command to a terminal device 130. The first network device 110 is in the RAN and associated with the source cell, and the second network device 120 is associated with a candidate cell for the cell switch. In this way, inter-CU LTM cell switch is supported.

The network triggered LTM procedure is a cell switch procedure that the network triggers via MAC CE based on L1 measurements. For the conditional LTM procedure, the UE evaluates the condition (s) and triggers the LTM cell switch procedure if the condition (s) are fulfilled, instead of triggered by MAC CE. The LTM cell switch procedure may contain the network triggered LTM and conditional LTM procedure. To ensure success of the LTM cell switch, the example embodiments of the present disclosure further propose a new solution. In this solution, the terminal device 130 performs a MAC reset operation without stopping a TAT associated with a candidate cell or a target cell in a LTM procedure for a cell switch from a source cell to the target cell, and without considering that the TAT as expired.

Furthermore, the example embodiments of the present disclosure further propose a new solution for obtaining TA information. In this solution, the first network device 110 receives, from a second network device 120 in a RAN, first TA information associated with a candidate cell in a LTM procedure for a cell switch from a source cell to a target cell. The first network device 110 in the RAN is associated with the source cell, and the second network device 120 is associated with the candidate cell. In this way, the terminal device 130 or network devices can get the TA value related to candidate cell upon or before LTM cell switch execution for inter-CU case.

In embodiments of the present disclosure which are discussed below, the first network device 110 may include a first CU and the second network device may include a second CU other than the first CU. Alternatively, the first network device 110 may include a first Next Generation Radio Access Network (NG-RAN) and the second network device 120 may include a second NG-RAN other than the first NG-RAN. In other words, the first network device 110 and the second network device 120 may communicate via an Xn interface.

Example embodiments of the present disclosure will be described in detail below with reference to the accompanying drawings.

Reference is made to FIG. 2, which illustrates a signaling flow 200 of LTM cell change notification transmission in accordance with some embodiments of the present disclosure. For the purposes of discussion, the signaling flow 200 will be discussed with reference to FIG. 1A, for example, by using the first network device 110 and the second network device 120.

In the signaling flow 200, the first network device 110 transmits (202) , to the second network device 120, a notification associated with a LTM cell change for a cell switch from a source cell to a target cell. In some embodiments, the notification may be referred to as a LTM cell change notification (also referred to as a first LTM cell change notification for purpose of discussion) . It should be understood that the “LTM cell change notification” is just an example for the above notification, rather than suggesting any limitation. In embodiments of the present disclosure, the above notification may have different names or may be implemented as other suitable indication, message or signaling. These variations of the above notification are all within the scope of the present disclosure.

The second network device 120 receives (204) the first LTM cell change notification accordingly. The first network device 110 is in a RAN and associated with the source cell, and the second network device is in the RAN and associated with a candidate cell for the cell switch. In some cases, there may be a plurality of candidate cells for the cell switch. The target cell is one of the candidate cells, which corresponds to the successful cell switch.

For example, the LTM cell change notification procedure is used to enable the source gNB-DU to inform the gNB-CU about the initiation of the LTM command to the UE. This procedure is also used to transfer the selected TCI state from the source gNB-DU to the gNB-CU and from the gNB-CU to the target gNB-DU. For another example, the LTM cell change notification procedure is used to enable the source gNB-DU to inform the source gNB-CU about the initiation of the LTM command to the UE. The source gNB-CU notifies the target gNB-CU about the initiation of the LTM command to the UE. The target gNB-CU notifies the target DU about the initiation of the LTM command to the UE. This procedure is also used to transfer the selected TCI state from the source gNB-DU to the source gNB-CU and/or from the source gNB-CU to the target gNB-CU and/or from the target gNB-CU to the target gNB-DU. The procedure uses UE-associated signalling.

In some example embodiments, the first network device 110 and the second network device 120 may communicate via an Xn interface.

The first LTM cell change notification may indicate an initiation of a LTM cell switch command to a terminal device 130. In other words, the first LTM cell change notification may enable the first network device 110 to inform the second network device 120 about the initiation of the LTM cell switch command to the terminal device 130.

In some example embodiments, the first network device 110 may comprise a first NG-RAN and the second network device 120 may comprise a second NG-RAN other than the first NG-RAN. A NG-RAN node may be referred to as either a gNB or an NG-eNB. In some example embodiments, the first network device 110 may be referred to as source NG-RAN. The second network device 120 may be referred to as target NG-RAN, or sometimes, candidate NG-RAN. For example, the LTM cell change notification procedure is used to enable the source NG-RAN to inform the target NG-RAN about the initiation of the LTM command to the UE.

In view of the above, the LTM cell change notification message is introduced to enable the source NG-RAN node to inform the candidate NG-RAN node about the initiation of the LTM cell switch command to the UE. Specifically, in some embodiments, the LTM cell change notification message is transmitted form the first CU or the source CU (e.g., source gNB-CU) 111 to the second CU or the candidate CU (e.g., candidate/target gNB-CU) 121.

In some example embodiments, the first LTM cell change notification may include an identification of the target cell (e.g., represented as Cell ID) and/or an identification of Transmission Configuration Indicator (TCI) state (e.g., represented as TCI State ID) .

Alternatively, or in addition, the first LTM cell change notification may include a first identification of a terminal device 130 over an Xn interface within the first network device 110 and/or a second identification of a terminal device 130 over an Xn interface within the second network device 120. The first identification may be an information element (IE) represented as Source NG-RAN node UE XnAP ID. The second identification may be another IE represented as Target NG-RAN node UE XnAP ID.

Alternatively, or in addition, the first LTM cell change notification may include a third identification of a terminal device 130 over a F1 interface within a CU and/or a fourth identification of a terminal device 130 over a F1 interface within a DU. The third identification may be an IE represented as gNB-CU UE F1AP ID. The fourth identification may be another IE represented as gNB-DU UE F1AP ID. Alternatively, the third identification may be an IE represented as source gNB-CU UE F1AP ID. The fourth identification may be another IE represented as source gNB-DU UE F1AP ID.

In some implementations, the LTM procedure is initiated by the first network device 110. Upon reception of the LTM cell change notification message, the second network device 120, e.g., the target gNB-CU 121, may consider that an LTM command was sent to the terminal device (e.g., UE) 130, where the target cell is indicated by the included NR CGI IE. Upon reception of the LTM cell change notification message, the target gNB-CU 121 send the LTM cell change notification to the target gNB-DU 122, the target gNB-DU 122 may consider that the information indicated the selected TCI state. In some example embodiments, the first network device 110 may be referred to as source NG-RAN. The second network device 120 may be referred to as target NG-RAN.

As another option, a new Xn message may introduced to carry the LTM cell change notification. For example, this new Xn message is sent by the source NG-RAN or gNB-CU to inform the target NG-RAN or target gNB-CU about the initiation of the LTM cell switch command to the UE. The message may be further used to transfer the selected beam from the source gNB-CU to the target gNB-CU, and from the target gNB-CU to the target gNB-DU. The procedure may use UE-associated signalling. The direction of message transmission is between the source NG-RAN and the target NG-RAN.

Table 1 shows an example of an Xn message carrying the LTM cell change notification. As shown in Table 1, the IEs of Message Type and Cell ID are mandatory, and the IEs of gNB-CU UE F1AP ID, gNB-DU UE F1AP ID, TCI State ID, Source NG-RAN node UE XnAP ID and Target NG-RAN node UE XnAP ID or NG-RAN node UE XnAP ID are optional. This message may be sent by the gNB-CU to inform the Source gNB-CU about LTM cell change notification. That is, this message is transmitted on an Xn interface between two CUs (i.e., inter-CU) .

Table 1

For example, the gNB-CU UE F1AP ID may be allocated so as to uniquely identify the UE over the F1 interface within a gNB-CU. When a gNB-DU receives a gNB-CU UE F1AP ID, it may store it for the duration of the UE-associated logical F1-connection for this UE. The gNB-CU UE F1AP ID may be unique within the gNB-CU logical node. The gNB-DU UE F1AP ID may be allocated so as to uniquely identify the UE over the F1 interface within a gNB-DU. When a gNB-CU receives a gNB-DU UE F1AP ID, it may store it for the duration of the UE-associated logical F1-connection for this UE. The gNB-DU UE F1AP ID may be unique within the gNB-DU logical node.

In some embodiments, one or more new IEs may be introduced to a F1 message carrying the LTM cell change notification, to enable the source CU/NG-RAN to know which CU/NG-RAN to send LTM cell change notification to. For example, the message is sent by the source gNB-DU to inform the source gNB-CU about the LTM cell change notification, and/or the message is sent by the target CU to inform the target gNB-DU about the LTM cell change notification. That is, the message is transmitted on a F1 interface between a CU and a DU. The direction of message transmission may be from the gNB-DU to the gNB-CU or from the gNB-CU to the gNB-DU.

Table 2 shows an example of a F1 message carrying the LTM cell change notification. As shown in Table 2, the message includes Source NG-RAN node UE XnAP ID and Target NG-RAN node UE XnAP ID or NG-RAN node UE XnAP ID which are optional.

Table 2

In some example embodiments, with reference to FIG. 1B, before the transmission of the first LTM cell change notification, another LTM cell change notification (also referred to as a second LTM cell change notification for purpose of discussion) may be transmitted from the first DU 112 to the first CU 111. The second LTM cell change notification may indicate the initiation of the LTM cell switch command to the terminal device 130. The first LTM cell change notification may be transmitted based on the second LTM cell change notification. In some example embodiments, the second LTM cell change notification may be carried in the F1 message shown in Table 2, as well as other suitable F1 messages.

In some example embodiments, with reference to FIG. 1B, after receiving the first LTM cell change notification, the second CU 121 may transmit, to the second DU 122, another LTM cell change notification (also referred to as a third LTM cell change notification for purpose of discussion) to indicate the initiation of the LTM cell switch command to the terminal device 130 based on the first LTM cell change notification. In some example embodiments, the third LTM cell change notification may be carried in the F1 message shown in Table 2, as well as other suitable F1 messages.

In some example embodiments, similar to the first LTM cell change notification, the second LTM cell change notification may comprise one of more of the following information: the identification of the target cell, the identification of TCI state, the first identification of a terminal device 130 over an Xn interface within the first network device 110, the second identification of a terminal device 130 over an Xn interface within the second network device 120, the third identification of a terminal device 130 over a F1 interface within a CU of the first network device 110, or the fourth identification of a terminal device 130 over a F1 interface within a DU of the first network device 110.

Alternatively or in addition, the third LTM cell change notification may also include similar information as the first LTM cell change notification or the second LTM cell change notification discussed above. More details related to the transmission of the LTM cell change notification will be provided with reference to FIG. 3 below.

In some embodiments, the first network device 110 may receive, from the second network device 120, first timing advance (TA) information associated with the candidate cell. In the case where the first network device 110 comprises the first CU 111 and the first DU 112 and the second network device 120 comprises the second CU 121 and the second DU 122, the second DU 122 may transmit TA information (also referred to as second TA information for purpose of discussion) to the second CU 121, and the second CU 121 may transmit the first TA information to the first CU 111 based on the second TA information. After receiving the first TA information, the first CU 111 may transmit further TA information (also referred to as third TA information for purpose of discussion) to the first DU 112 based on the first TA information.

In some example embodiments, the first network device 110 may be referred to as source NG-RAN. The second network device 120 may be referred to as target NG-RAN, or sometimes, candidate NG-RAN. For example, the first TA notification procedure is used to enable the candidate NG-RAN to inform the source NG-RAN about TA information related to candidate cell.

In some embodiments, the first TA information may include various information, for example, but not limited to, an identification of the candidate cell, a TA value, a first identification of a terminal device over an Xn interface within the first network device, a second identification of a terminal device over an Xn interface within the second network device, a global identification of the first network device, or an identification of the first network device in the RAN, an identification of a DU of the first network device and/or second network device, and/or TA related UE information, and/or the like.

In some embodiments, the second TA information or third TA information may include, but not limited to, an identification of the candidate cell, a TA value, a first identification of a terminal device over an Xn interface within the first network device, a second identification of a terminal device over an Xn interface within the second network device, a global identification of the first network device, or an identification of the first network device in the RAN, an identification of a DU of the first network device and/or second network device, TA related UE information, and/or the like. In some embodiments, the second TA information, and/or the third TA information may include the same information as the first TA information.

In some embodiments, the LTM procedure is a conditional LTM procedure, that is, the LTM procedure will be executed in response to that at least one LTM execution condition is met. In such cases, the first network device 110 may receive, from the second network device 120, a handover success message indicating that the terminal device has successfully accessed the target cell. In some example embodiments, the target CU detects that the terminal device 130 (also referred to as UE) is accessing the target cell, and the target CU sends the indication to notify the target DU about the UE access. The target DU starts transmission and/or reception with the terminal device 130. In some example embodiments, the target CU sends the handover success message to the source CU. After receiving the handover success, the source CU sends an indication to indicate the source DU. Upon receiving the indication from the source CU, the source DU may stop the transmission and/or reception with the terminal device 130.

In addition to the above, in some embodiments, the terminal device 130 may inform its own capability for the LTM procedure to the first network device 110. In such cases, the first network device 110 may receive, from the terminal device 130, various information about the capability of the terminal device 130 (also referred to as capability information of the terminal device) .

In some implementations, the capability information may include, for example, at least one of, but not limited to, a capability indicating that the terminal device supports a conditional LTM procedure, a capability indicating that the terminal device supports a conditional RACH-less LTM procedure, a capability indicating that the terminal device supports UE-based candidate cell TA maintains, or a capability indicating the number of terminal devices that support early TA acquire or TA maintain of candidate cell, or a capability indicating that the number of TATs maintained by the terminal device for candidate cells.

Reference is now made to FIG. 3, which illustrates a signaling flow 300 of an example communication process of LTM cell change notification transmission in accordance with some embodiments of the present disclosure. The embodiments illustrated with FIG. 3 are implementations of those embodiments discussed with FIG. 2. Specifically, FIG. 3 shows a source CU 311 which is an example of the first CU 111, and a source DU 312 which is an example of the first DU 112. FIG. 3 also shows a candidate CU 321 which is an example of the second CU 121, and a candidate DU 322 which is an example of the second DU 122. For the purposes of discussion, the signaling flow 300 will be discussed with reference to FIG. 1B.

In the signaling flow 300, the source DU 312 transmits (301) a LTM cell change notification, i.e., the second LTM cell change notification discussed above, to the associated source CU 311 via a F1 interface. The source CU 311 receives (302) the second LTM cell change notification. Then the source CU 311 transmits (303) the first LTM cell change notification via an Xn interface to the candidate or target CU 321. The candidate or target CU 321 receives the first LTM cell change notification and transmits (305) another LTM cell change notification (the third LTM cell change notification) to the corresponding candidate DU 322 via a F1 interface. Accordingly, the candidate DU 322 receives (306) the third LTM cell change notification. In this way, the inter-CU LTM cell change notification transmission is supported.

In summary, for inter-CU LTM cell switch, upon LTM cell switch is triggered, the source NG-RAN may send the LTM cell change notification to the target NG-RAN. In another world, the source DU may send the LTM cell change notification to the source CU, and the source CU may transfer it to the target DU through the target CU. By introducing an Xn signaling or message carrying the LTM cell change notification, the LTM cell change notification can be transmitted between different CUs or NG-RANs.

FIG. 4 illustrates a signaling flow 400 of transmission of TA information notification in accordance with some embodiments of the present disclosure. For the purposes of discussion, the signaling flow 400 will be discussed with reference to FIG. 1A, for example, by using the first network device 110 and the second network device 120.

In the signaling flow 400, the second network device 120 transmits (402) first timing advance (TA) information associated with a candidate cell to the first network device 110 in a LTM procedure for a cell switch from a source cell to a target cell. The source cell is associated with a first network device 110, and the candidate cell is associated with the second network device 120. The first network device 110 receives (404) the first TA information from the second network device 120.

In some embodiments, the first TA information may include various information, for example, but not limited to, an identification of the candidate cell, a TA value, a first identification of a terminal device over an Xn interface within the first network device, a second identification of a terminal device over an Xn interface within the second network device, a global identification of the first network device, or an identification of the first network device in the RAN, an identification of a DU of the first network device and/or second network device, TA related UE information, and/or the like. It is to be understood that the above are just examples for items in the first TA information, rather than suggest any limitations. In other embodiments of the present disclosure, the first TA information may include other suitable item (s) .

In some example embodiments, the first network device 110 may be referred to as source NG-RAN. The second network device 120 may be referred to as target NG-RAN, or sometimes, candidate NG-RAN. For example, the first TA notification procedure is used to enable the candidate NG-RAN to inform the source NG-RAN about TA information related to the candidate cell.

In some example embodiments, with reference to FIG. 1B, the second TA information may be transmitted from the second DU 122 to the second CU 121. The first TA information may be transmitted from the second CU 121 to the first CU 111 based on the second TA information. The first TA information may be received by the first CU 111, and the third TA information may be transmitted from the first CU 111 to the first DU 112 based on the first information.

In some embodiments, the second TA information or third TA information may include, but not limited to, an identification of the candidate cell, a TA value, a first identification of a terminal device over an Xn interface within the first network device, a second identification of a terminal device over an Xn interface within the second network device, a global identification of the first network device, or an identification of the first network device in the RAN, an identification of a DU of the first network device and/or second network device, and/or TA related UE information, and/or the like. In some embodiments, the second TA information and/or the third TA information may include the same information as the first TA information.

As shown in Fig. 4, by introducing an TA information related signalling or message over the Xn interface, the TA information can be transmitted from the candidate CUs or NG-RAN to the source CU or NG-RAN. Furthermore, the source CU or NG-RAN may send the TA related the candidate cells to the terminal device 130. Therefore, The terminal device 130 or the source NG-RAN or the source CU or the source DU may get the TA value related to the candidate cell before LTM cell switch execution for inter-CU case.

Reference is now made to FIG. 5, which illustrates a signaling flow 500 of an example communication process of Timing Advance (TA) information notification in accordance with some embodiments of the present disclosure. The embodiments illustrated with FIG. 5 are implementations of those embodiments discussed with FIG. 4. In FIG. 5, a UE 530 is a specific example of the terminal device 130. For the purposes of discussion, the signaling flow 500 will be discussed with reference to FIG. 1B.

In the signaling flow 500, the UE 530 transmits (502) a preamble for early TA acquisition and the candidate DU 322 receives (504) the preamble. The candidate DU determines the TA related to the corresponding candidate cell. The candidate DU 322 transmits (506) a TA information notify message including the second TA information to the corresponding candidate CU 321. The candidate CU 321 receives (508) the TA information notify message and transmits (510) TA information transfer message including the first TA information to the source CU 311 associated with the UE 530. The source CU 311 then receives (512) the TA information transfer message from the candidate CU 321 and transmits (514) a further TA information notify message including the third TA information to the source DU 312 associated with the UE 530. The source DU 312 receives (516) the TA information notify message from the source CU 311. The source CU transmits (518) a LTM command to the UE 530. The TA related to the LTM candidate cell (s) or target cell may include in LTM command, for example, the TA command included in LTM command MAC CE. The UE 530 then receives (520) the LTM command. The UE determine the TA related to the LTM candidate cell (s) or target cell based on the TA command in LTM command. In this way, the inter-CU TA information transfer is achieved.

It is to be understood that the “TA information transfer message” is just an example for a message that transfers the TA information between NG-RAN nodes, for example, transmitting the TA information related to the candidate cell associated with the candidate NG-RAN to the source NG-RAN. The specific name for this message does not suggest any limitation. In other embodiments of the present disclosure, the TA information transfer message may have different names or may be implemented in other suitable forms.

In some example embodiments, the first TA information, the second TA information, or the third TA information may each include one or more of the following: an identification of the candidate cell, a TA value, a first identification of a terminal device 130 over an Xn interface within the first network device 110, a global identification of the first network device 110, or an identification of the first network device 110 in the RAN, or an identification of a DU of the first network device 110.

The first identification (e.g., represented as source global gNB ID) may indicate which terminal device 130 the TA information belongs to. The global identification of the first network device 110, or an identification of the first network device 110 in the RAN (e.g., represented as RAN Node ID) , or an identification of a DU of the first network device 110 (e.g., represented as source gNB-DU ID) may be referred to as a gNB related identification which may be used to determine the source CU and/or a source DU to which the source CU is to send the TA information.

As shown in FIG. 5, a TA information transfer message (between CUs) and a TA information notify message (between CU and DU) are introduced to carry the TA information. The TA information transfer message is transmitted from the candidate CU 321 based on the TA information notify message received from the candidate DU 322. Upon reception of the TA information notify message, the candidate CU 321 may consider that the received TA information (the second TA information) is the TA information from a candidate cell that is indicated by the included candidate cell ID (with IE type of NR CGI) .

Table 3 shows an example of the TA information transfer message over the Xn interface. The TA information transfer message may be referred to the first TA information. As shown in Table 3, the TA information transfer message may include IEs, such as source NG-RAN node UE XnAP ID and gNB related ID (i.e., source gNB ID and/or source gNB-DU ID) , where the gNB related ID is optional, gNB related ID may be the source gNB ID and/or source gNB-DU ID. For example, the source gNB-DU ID uniquely identifies the gNB-DU at least within a gNB-CU, the source NG-RAN node UE XnAP ID uniquely identifies a UE over the Xn interface within the NG-RAN node, or this message is sent by the candidate gNB-CU to inform the source gNB-CU about TA information. The message transmission may be transmitted from the candidate CU 321 to the source CU 311.

Table 3

In some cases, new IEs may be introduced to the TA information notify message to enable the candidate CU 321 to know which CU to send TA information to. For example, the TA information notify message may be sent by the candidate DU 322 to inform the candidate CU 321 about the TA information. This message may also be sent by the candidate CU 321 to inform the source CU 311 about the TA information. The message may be transmitted from a gNB-DU to a gNB-CU or from a gNB-CU to a gNB-DU.

Table 4 shows an example of the TA information notify message over the F1 interface. The TA Notify message may be referred to the second TA information or the third TA information. As shown in Table 4, the new IEs (indicated in bold) may include source global gNB ID/RAN Node ID and Source NG-RAN node UE XnAP ID, where both are optional. For example, the source gNB-DU ID uniquely identifies the gNB-DU at least within a source gNB-CU, and the source NG-RAN node UE XnAP ID uniquely identifies a UE over the Xn interface within the source NG-RAN node.

Table 4

In summary, for inter-CU LTM cell switch, the source CU or source NG-RAN may obtain the early TA information related to the candidate cell in order to transmit it to the UE. The TA information related message via the Xn interface is introduced to transmit the TA value between the CUs or NG-RANs. In this way, the UE or source NG-RAN or CU or DU can get the TA value related to candidate cell before LTM cell switch execution for inter-CU case.

Moreover, the LTM cell switch procedure may further include other steps. The following will be described in detail.

In some example embodiments, the LTM procedure is a conditional LTM procedure that is executed in response to that at least one LTM execution condition is met. The first network device 110 may further receive, from the second network device 120, a handover success message indicating that the terminal device has successfully accessed the target cell.

Reference is now made to FIG. 6, which illustrates a signaling flow 600 for a MAC reset operation in accordance with some embodiments of the present disclosure. For the purposes of discussion, the signaling flow 600 will be discussed with reference to FIG. 1A, for example, by using the terminal device 130.

As shown in FIG. 6, the terminal device 130 performs (606) a MAC reset operation without stopping a Time Alignment Timer (TAT) associated with a candidate cell or a target cell in a LTM procedure for a cell switch from a source cell to the target cell, and without considering that the TAT as expired.

Details about an example of the MAC reset operation are provided in Table 5 below. It is to be understood that Table 5 just shows an example of the MAC reset operation for illustration, rather than suggesting any limitation to the MAC reset operation.

Table 5

In some example embodiments, the MAC reset operation may be performed (660) in response to that a LTM command is received or a LTM execution condition is satisfied. For example, upon receiving the LTM command, the terminal device 130 performs the MAC reset operation.

Still referring to FIG. 6, in some example embodiments, the terminal device 130 may perform the LTM procedure by applying a configuration of the target cell and perform a corresponding L2 behavior. For example, a procedure of LTM cell switch access may be performed. This LTM cell switch access procedure may involve both the terminal device 130 and the second network device 120.

In some example embodiments, the LTM procedure may comprise a network triggered LTM procedure, which is triggered by a LTM command MAC CE. Alternatively, or in addition, the LTM procedure may comprise a conditional LTM procedure that is executed in response to that at least one LTM execution condition is met. The LTM procedure may be applied to the L1 or L2 layer which is different from the L3 layer.

For RACH-less LTM, the terminal device 130 may maintain the TA for the LTM candidate cell. In this case, the timeAilgnmentTimer (TAT) for candidate cell may start when the terminal device 130 receives the TA command for the candidate cell. When the LTM, for example, the RACH-less conditional LTM, is triggered, the terminal device 130 performs the MAC reset operation, and the MAC entity may consider all timeAlignmentTimers as expired. When the timeAlignmentTimer expires, the MAC shall flush all HARQ buffers for all Serving Cells, clear any configured downlink assignments and configured uplink grants, etc. This may result the RACH-less conditional LTM failure. In addition, in some cases, the conditional LTM may not use the DG for the first Physical Uplink Shared Channel (PUSCH) transmission, so the confirmation of successful completion of the conditional LTM is different from the LTM triggered by the network.

To at least partially solve the above issues, in some example embodiments, during the RACH-less conditional LTM, in response to that an uplink grant has been received on a Physical Downlink Control Channel (PDCCH) for a MAC entity's Cell-Radio Network Temporary Identifier (C-RNTI) after a first PUSCH transmission to the target cell, and the uplink grant is for a new transmission regardless of a Hybrid Automatic Repeat Request (HARQ) process, the cell switch is considered to be successfully completed. In this way, it ensures success of the LTM cell switch.

In some embodiments, if a reset of the MAC entity is performed due to the LTM execution, the MAC entity will not stop TAT for the LTM candidate cell during the MAC reset operation. The MAC entity will not consider the timeAlignmentTimer (s) associated with the LTM candidate/target cell as expired during the RACH-less LTM. For example, in the case of RACH-less conditional LTM, if a reset of the MAC entity is performed, the MAC entity will not stop TAT for the conditional LTM candidate cell (s) during the MAC reset operation. The MAC entity will not consider the timeAlignmentTimer (s) associated with the LTM candidate/target cell as expired during the RACH-less conditional LTM.

In other words, when the MAC reset operation is performed, the terminal device 130 may stop all timers (if running) , expect for the timeAlignmentTimer (s) associated with the LTM candidate or target during the RACH-less conditional LTM. The MAC entity will consider the timeAlignmentTimers, inactivePosSRS-TimeAlignmentTimer, and cg-SDT-TimeAlignmentTimer (if configured) as expired and perform the corresponding actions, expect for the timeAlignmentTimer (s) associated with the LTM candidate during the RACH-less conditional LTM.

During an ongoing conditional RACH-less LTM, if an uplink grant is received on the PDCCH for the MAC entity’s C-RNTI after the first PUSCH transmission to the serving cell, and the uplink grant is for a new transmission, regardless of the HARQ process, the terminal device 130 (MAC entity) will consider the LTM cell switch to be successfully completed and indicate it to upper layers.

In some embodiments, if an uplink grant for this Serving Cell has been received on the PDCCH for the MAC entity's C-RNTI or Temporary C-RNTI, if there is an ongoing conditional RACH-less LTM cell switch, and if the uplink grant has been received on the PDCCH for the MAC entity's C-RNTI after the first PUSCH transmission to the Serving Cell, optionally and if the uplink grant is for a new transmission, the the terminal device 130 (e.g., UE) considers the LTM cell switch to be successfully completed and indicate it to upper layers.

Details about the uplink grant reception are provided in Table 6 below. It is to be understood that Table 6 just shows an example of the uplink grant reception for illustration, rather than suggesting any limitation.

Table 6

In some example embodiments, the terminal device 130 may transmit (602) , to a first network device 110 in a radio access network (RAN) , capability information of the terminal device 130 for the LTM procedure. The first network device 110 may be associated with the source cell in the LTM procedure. For example, the capability information may be reported by the UE via RRC signalling (e.g., via UECapabilityInformation) . Accordingly, the first network device 110 may receive (604) the capability information.

The capability information may include, for example, but not limited to, a capability indicating that the terminal device supports a conditional LTM procedure, a capability indicating that the terminal device supports a conditional RACH-less LTM procedure, a capability indicating that the terminal device supports UE-based candidate cell TA maintains, or a capability indicating the number of terminal devices that support early TA acquire or TA maintain of candidate cell, or a capability indicating that the number of TATs maintained by the terminal device for candidate cells.

A whole picture of the LTM procedure as discussed above will be described in conjunction with FIG. 7.

FIG. 7 illustrates a signaling flow 700 of a LTM process in accordance with some embodiments of the present disclosure. For the purposes of discussion, the signaling flow 700 will be discussed with reference to FIG. 1B. Specifically, a source CU 751 in FIG. 7 corresponds to the first CU 111 and a source DU 752 corresponds to the first DU 112 of the first network device 110. Furthermore, a candidate CU 761 corresponds to the second CU 121 and a candidate DU 762 corresponds to the second DU 122 of the second network device 120. FIG. 7 also shows a UE 770, which corresponds to the terminal device 130 in FIG. 1B.

In the signaling flow 700, the source CU 751 sends (702) a Handover Request message to the candidate CU 761. In case of the LTM process (also referred to as LTM Handover) . In case of network triggered LTM cell switch, the target gNB (e.g. target gNB-CU) is regarded as a candidate gNB (e.g., candidate gNB-CU) which is only accessed by the UE when the LTM cell change notification is received. In case of the conditional LTM cell switch, the target gNB is regarded as a candidate gNB which is only accessed by the terminal device 130 when the conditional LTM condition (s) are fulfilled.

A UE context Setup procedure is performed (704) between the candidate CU 761 and the candidate DU 762. The candidate CU 761 sends (706) a Handover Request Acknowledge message to the source CU 762.

The source CU 751 sends (708) a DL RRC Message Transfer message to the source DU 752, which includes the generated RRCReconfiguration message with the LTM configuration. The source DU 752 sends (710) the received RRCReconfiguration message to the UE 770.

The UE 770 sends (712) an RRCReconfigurationComplete message to the source DU 752. The source DU 752 sends (714) the RRCReconfigurationComplete message to the source CU 751 via an UL RRC Message Transfer message.

Optionally, early TA acquisition is performed. As shown in FIG. 7, the UE 770 transmits an early TA acquisition message (716) to the candidate DU 762, so as to acquire the early TA information.

The candidate DU 762 transmits (718) a TA information notify message to the candidate CU 761. The TA information notify message may indicate the TA information, the associated CFRA resource information, the candidate cell ID, and/or the source gNB related ID. This TA information notify message is a F1 message, that is, a messaged on the F1 interface. In some embodiments, the TA information or the TA information notify message may be transmitted in a similar way as discussed with reference to FIG. 4 and/or FIG. 5.

The candidate CU 761 sends (720) the TA information transfer message indicating the TA information to the source CU 762. The TA information transfer message is a message on the Xn interface between the candidate CU 761 and the source CU 762 or between the candidate NG-RAN and the source NG-RAN.

The TA information transfer message may be the one discussed with reference to FIG. 5, for example, the TA information transfer message shown in Table 3. It is to be understood that the “TA information transfer message” is just an example for a message that transfers the TA information between NG-RAN nodes, for example, transmitting the TA information related to the candidate cell associated with the candidate NG-RAN to the source NG-RAN. The specific name for this message does not suggest any limitation. In other embodiments of the present disclosure, the TA information transfer message may have different names or may be implemented in other suitable forms.

The source CU 751 sends (722) another TA information notify message indicating the TA information to the source DU 112. This TA information notify message is a F1 message, that is, a messaged on the F1 interface.

The UE 130 sends (724) a L1 measurement report to the source DU 752. The L1 measurement report at least indicates the L1 measurement result. Then, the source DU 752 sends (726) a LTM command to the UE 130.

The source DU 752 sends (728) a LTM cell change notification message to the source CU 751. This LTM cell change notification message enables the source DU 752 to inform the source CU 751 about the initiation of the L1/L2 triggered mobility command to the UE 770.

The source CU 751 sends (730) the LTM cell change notification message to the candidate CU 761 to indicate the initiation of the LTM command to the UE 130 including the target cell ID and the TCI state ID. This process may refer to that discussed in embodiments of FIGS. 2 and 3 and thus its details are not repeated here.

In some embodiments, it should be noted that the above actions 726-730 are only performed for the network triggered LTM. For the conditional LTM, the UE 770 evaluates the conditions and if the condition is met, the UE 770 triggers the LTM execution.

The candidate CU 761 sends (732) the LTM cell change notification message (including the target cell ID and the TCI state ID) to the candidate DU 762.

Upon the UE 770 receives the LTM command or the conditional LTM condition is met, the UE 770 performs (734) a LTM cell switch access procedure. In this procedure, the UE 770 may perform a MAC reset operation and access the candidate DU 762 (in such case, the candidate DU 762 becomes the target DU) . The target cell configuration is applied and the corresponding L2 behavior is performed.

In some embodiments, the L2 behavior may refer to L2 reset. The L2 behavior including the RLC and PDCP behavior, or the L2 behavior including MAC, RLC and PDCP behavior. In some embodiments, during the LTM cell switch access procedure the UE 770 determines whether to perform RLC and PDCP reestablishment based on the specific behavior, which will be described below.

As to the L2 reset for inter-CU LTM, there may be, for example, full L2 reset including MAC reset, RLC re-establish and PDCP re-establish, or full L2 reset including RLC re-establish and PDCP re-establish. Full L2 reset is also named complete L2 reset.

The first information and the second information may be introduced for determining the L2 reset behavior or determining whether perform the full L2 reset.

The first information is the full L2 reset indication related to each candidate cell. This first information is used by the UE 770 to determine whether a full L2 reset should be performed when LTM cell switch procedure is triggered. Or the first information is used to by the UE 770 to determine whether the LTM cell switch procedure is triggered towards an LTM candidate cell is inter-CU LTM cell switch.

In an option, a complete/full L2 reset group is introduced, and the number of groups is any integer from 1 to the maximum number of LTM candidate cells plus 1. Each group related to a group ID (e.g., ltm-FullResetID) and contains at least one LTM candidate cell. In this case, the first information for each candidate is the group ID related to this candidate cell.

In another option, the first information (e.g. field ltm-FullResetID) is introduced in each LTM candidate cell configuration.

The second information, e.g., field ltm-ServingCellFullResetID, is located in LTM configuration or Serving cell configuration. This second information is used by the UE to determine on whether complete/full L2 reset should be performed when an LTM cell switch procedure is triggered towards an LTM candidate cell. This second information is mandatory present in the first RRCReconfiguration message which include LTM configuration with at least one LTM candidate configuration. Otherwise, the information is absent, Need M.

There may be new UE variable (s) for the LTM procedure, e.g., VarLTM-ServingCellFullResetID. The variable is used to store the serving cell ID based on which the UE determines whether a full L2 reset is needed or not upon an LTM cell switch procedure.

Upon LTM cell switch execution, the UE shall release or clear all current dedicated radio configuration associated with the cell group for which the LTM cell switch procedure is triggered except for the VarLTM-ServingCellFullResetID.

Upon LTM cell switch execution, if the value of first information (e.g., field ltm-FullResetID) related to candidate LTM cell indicated by lower layers or for the selected cell while T311 is running is not equal to the value of second information (e.g., field ltm-ServingCellFullResetID) within UE variable (e.g., VarLTM-ServingCellFullResetID) , the UE may:

- for each logicalChannelId and logicalChannelIdExt that is part of the current UE configuration for the cell group for which the LTM cell switch procedure is triggered, re-establish the corresponding RLC entity (optionally, after applying the LTM configuration) ;

- for each drb-Identity value that is part of the current UE configuration, re-establish the PDCP entity of this DRB. Also, for SRB;

- replace the value of second information (e.g., field ltm-ServingCellFullResetID) in UE variable (e.g., VarLTM-ServingCellFullResetID) with the value of first information (e.g., ltm-FullResetID) in the LTM candidate cell configuration indicated by lower layers or for the selected cell while T311 is running;

- replace the value of ltm-ServingCellNoResetID in VarLTM-ServingCellNolResetID with the value of ltm-noResetID in the LTM candidate cell configuration indicated by lower layers or for the selected cell.

In some embodiments, for each LTM candidate cell configuration, for example each inter-CU LTM candidate cell configuration, it should include both ltm-NoResetID and first information (ltm-FullResetID) . Due to the intra-CU and inter-CU LTM can be co-existence, it may ensure that the UE can determine the L2 behavior during subsequent LTM, both ltm-NoResetID and ltm-FullResetID is needed.

Upon execution of the LTM cell switch, if the value of the first information (e.g., field ltm-FullResetID) related to candidate LTM cell indicated by lower layers or for the selected cell while T311 is running is equal to the value of second information (e.g., field ltm-ServingCellFullResetID) within UE variable (e.g., VarLTM-ServingCellFullResetID) , the UE may determine whether the value of field ltm-NoResetID related to the candidate cell ndicated by lower layers or for the selected cell is equal to the ltm-ServingCellNoResetID of the current serving cell. It may be referred to the LTM cell switch execution description before. Still referring to FIG. 7, the candidate DU 762 sends (736) the access success message (including target cell ID) to the target CU. In some example embodiments where the target CU detected that the UE access the target cell, the target CU may send the indication to notify the target DU about the UE access. The target DU starts the transmission or reception with the UE 770.

In case of conditional LTM cell switch, the target CU sends (738) a handover success message to the source CU 751 to inform that the UE 770 has successfully accessed the target cell. In some example embodiments, after receiving the handover success, the source CU sends the indication to indicate the source DU. Upon receiving the indication from the source CU, the source DU stop the transmission or reception with the UE 770.

In case of conditional LTM cell switch, a stop data transmission procedure is performed (740) to instruct the UE 770 to stop data transmission. For example, the procedure may be an F1 UE Context Modification procedure.

FIG. 8 illustrates a flowchart of a communication method 800 implemented at a first network device in accordance with some embodiments of the present disclosure. For the purpose of discussion, the method 800 will be described from the perspective of the first network device 110 in FIG. 1A.

At block 810, the first network device 110 transmits, to a second network device in a radio access network (RAN) , a first Layer 1 or Layer 2 (L1/L2) triggered mobility (LTM) cell change notification associated with a LTM procedure for a cell switch from a source cell to a target cell. The first LTM cell change notification indicates an initiation of a LTM cell switch command to a terminal device, the first network device is in the RAN and associated with the source cell, and the second network device is associated with a candidate cell for the cell switch.

In some example embodiments, the first network device 110 comprises a first centralized unit (CU) and a first distributed unit (DU) , and the second network device comprises a second CU and a second DU, a second LTM cell change notification is transmitted from the first DU to the first CU, the second LTM cell change notification indicating the initiation of the LTM cell switch command to the terminal device, and wherein the first LTM cell change notification is transmitted based on the second LTM cell change notification.

In some example embodiments, the first LTM cell change notification is received by the second CU, and a third LTM cell change notification indicating the initiation of the LTM cell switch command to the terminal device is transmitted from the second CU to the second DU based on the first LTM cell change notification.

In some example embodiments, at least one of the first LTM cell change notification, the second LTM cell change notification or the third LTM cell change notification comprises one of more of the following: an identification of the target cell, an identification of Transmission Configuration Indicator (TCI) state, a first identification of a terminal device over an Xn interface within the first network device, a second identification of a terminal device over an Xn interface within the second network device, a third identification of a terminal device over a F1 interface within a centralized unit (CU) of the first network device, or a fourth identification of a terminal device over a F1 interface within a distributed unit (DU) of the first network device.

In some example embodiments, the first network device 110 receives, from the second network device, first timing advance (TA) information associated with the candidate cell.

In some example embodiments, the first network device 110 comprises a first centralized unit (CU) and a first distributed unit (DU) , and the second network device comprises a second CU and a second DU, second TA information is transmitted from the second DU to the second CU, and wherein the first TA information is transmitted based on the second TA information.

In some example embodiments, the first TA information is received by the first CU, and third TA information is transmitted from the first CU to the first DU based on the first TA information.

In some example embodiments, at least one of the first TA information, the second TA information, or the third TA information comprises one or more of the following: an identification of the candidate cell, a TA value, a first identification of a terminal device over an Xn interface within the first network device, a global identification of the first network device, or an identification of the first network device in the RAN, or an identification of a distributed unit (DU) of the first network device.

In some example embodiments, the LTM procedure is a conditional LTM procedure that is executed in response to that at least one LTM execution condition is met, and the first network device is further caused to: receive, from the second network device, a handover success message indicating that the terminal device has successfully accessed the target cell.

In some example embodiments, the first network device 110 receives, from the terminal device, capability information of the terminal device for the LTM procedure, wherein the capability information comprises at least one of: a capability indicating that the terminal device supports a conditional LTM procedure, a capability indicating that the terminal device supports a conditional random access channel (RACH) -less LTM procedure, a capability indicating that the terminal device supports user equipment (UE) -based candidate cell TA maintains, or a capability indicating the number of terminal devices that support early TA acquire or TA maintain of candidate cell, or indicating that the number of TA timers (TATs) maintained by the terminal device for candidate cells.

In some example embodiments, the LTM procedure comprises at least one of: a network triggered LTM procedure, which is triggered by a LTM command MAC CE, or a conditional LTM procedure that is executed in response to that at least one LTM execution condition is met.

In some example embodiments, the first network device 110 comprises a first centralized unit (CU) and the second network device comprises a second CU other than the first CU, or wherein the first network device comprises a first Next Generation Radio Access Network (NG-RAN) and the second network device comprises a second NG-RAN other than the first NG-RAN.

In some example embodiments, the first network device 110 and the second network device 120 communicate via an Xn interface.

FIG. 9 illustrates a flowchart of a communication method 900 implemented at a second network device in accordance with some embodiments of the present disclosure. For the purpose of discussion, the method 900 will be described from the perspective of the second network device 120 in FIG. 1A.

At block 910, the second network device 120 receives, from a first network device in a radio access network (RAN) , a first Layer 1 or Layer 2 (L1/L2) triggered mobility (LTM) cell change notification associated with a LTM procedure for a cell switch from a source cell to a target cell. The first LTM cell change notification indicates an initiation of a LTM cell switch command to a terminal device, the first network device is associated with the source cell, and the second network device is in the RAN and associated with a candidate cell for the cell switch.

In some example embodiments, the first network device comprises a first centralized unit (CU) and a first distributed unit (DU) , and the second network device comprises a second CU and a second DU, a second LTM cell change notification is transmitted from the first DU to the first CU, the second LTM cell change notification indicating the initiation of the LTM cell switch command to the terminal device, and wherein the first LTM cell change notification is transmitted based on the second LTM cell change notification.

In some example embodiments, the second network device transmits, to the first network device, first timing advance (TA) information associated with the candidate cell.

In some example embodiments, the first network device comprises a first centralized unit (CU) and a first distributed unit (DU) , and the second network device comprises a second CU and a second DU, second TA information is transmitted from the second DU to the second CU, and wherein the first TA information is transmitted based on the second TA information.

In some example embodiments, the LTM procedure is a conditional LTM procedure that is executed in response to that at least one LTM execution condition is met, and the second network device is further caused to: transmit, to the first network device, a handover success message indicating that the terminal device has successfully accessed the target cell.

In some example embodiments, the handover success message is transmitted in response to an access success message received from a distributed unit (DU) of the second network device, the access success message indicating that the terminal device has successfully accessed the target cell.

In some example embodiments, the first network device comprises a first centralized unit (CU) and the second network device comprises a second CU other than the first CU, or wherein the first network device comprises a first Next Generation Radio Access Network (NG-RAN) and the second network device comprises a second NG-RAN other than the first NG-RAN.

In some example embodiments, the first network device and the second network device communicate via an Xn interface.

FIG. 10 illustrates a flowchart of a communication method 1000 implemented at a terminal device in accordance with some embodiments of the present disclosure. For the purpose of discussion, the method 1000 will be described from the perspective of the terminal device 130 in FIG. 1A.

At block 1010, the terminal device 130 performs a Medium Access Control (MAC) reset operation without stopping a TAT associated with a candidate cell or a target cell in a Layer 1 or Layer 2 (L1/L2) triggered mobility (LTM) procedure for a cell switch from a source cell to the target cell, and without considering that the TAT as expired.

In some example embodiments, the MAC reset operation is performed in response to that a LTM command is received or a LTM execution condition is satisfied.

In some example embodiments, the terminal device performs the LTM procedure by apply a configuration of the target cell and perform a corresponding L2 behavior.

In some example embodiments, during the RACH-less conditional LTM, in response to that an uplink grant has been received on a Physical Downlink Control Channel (PDCCH) for a MAC entity's Cell-Radio Network Temporary Identifier (C-RNTI) after a first Physical Uplink Shared Channel (PUSCH) transmission to the target cell, and the uplink grant is for a new transmission regardless of a Hybrid Automatic Repeat Request (HARQ) process, the cell switch is considered to be successfully completed.

In some example embodiments, the terminal device transmits, to a first network device in a radio access network (RAN) , capability information of the terminal device for the LTM procedure, the first network device being associated with the source cell in the LTM procedure, wherein the capability information comprises at least one of: a capability indicating that the terminal device supports a conditional LTM procedure, a capability indicating that the terminal device supports a conditional random access channel (RACH) -less LTM procedure, a capability indicating that the terminal device supports user equipment (UE) -based candidate cell timing advance (TA) maintains, or a capability indicating the number of terminal devices that support early TA acquire or TA maintain of candidate cell, or indicating that the number of TA timers (TATs) maintained by the terminal device for candidate cells.

FIG. 11 illustrates a flowchart of a communication method 1100 implemented at a first network device in accordance with some embodiments of the present disclosure. For the purpose of discussion, the method 1100 will be described from the perspective of the first network device 110 in FIG. 1A.

At block 1110, the first network device 110 receives, from a second network device in a radio access network (RAN) , first timing advance (TA) information associated with a candidate cell in a Layer 1 or Layer 2 (L1/L2) triggered mobility (LTM) procedure for a cell switch from a source cell to a target cell. A first network device in the RAN is associated with the source cell, and the second network device is associated with the candidate cell.

FIG. 12 illustrates a flowchart of a communication method 1200 implemented at a second network device in accordance with some embodiments of the present disclosure. For the purpose of discussion, the method 1200 will be described from the perspective of the second network device 120 in FIG. 1A.

At block 1210, the second network device 120 transmits, to a first network device in a radio access network (RAN) , first timing advance (TA) information associated with a candidate cell in a Layer 1 or Layer 2 (L1/L2) triggered mobility (LTM) procedure for a cell switch from a source cell to a target cell, the first network device being associated with the source cell, and the second network device being in the RAN and associated with the candidate cell.

FIG. 13 is a simplified block diagram of a device 1300 that is suitable for implementing embodiments of the present disclosure. The device 1300 can be considered as a further example implementation of any of the devices as shown in FIG. 1A. Accordingly, the device 1300 can be implemented at or as at least a part of the first network device 110, the second network device 120 or the terminal device 130.

As shown, the device 1300 includes a processor 1310, a memory 1320 coupled to the processor 1310, a suitable transceiver 1340 coupled to the processor 1310, and a communication interface coupled to the transceiver 1340. The memory 1320 stores at least a part of a program 1330. The transceiver 1340 may be for bidirectional communications or a unidirectional communication based on requirements. The transceiver 1340 may include at least one of a transmitter 1342 and a receiver 1344. The transmitter 1342 and the receiver 1344 may be functional modules or physical entities. The transceiver 1340 has at least one antenna to facilitate communication, though in practice an Access Node mentioned in this application may have several ones. The communication interface may represent any interface that is necessary for communication with other network elements, such as X2/Xn interface for bidirectional communications between eNBs/gNBs, S1/NG interface for communication between a Mobility Management Entity (MME) /Access and Mobility Management Function (AMF) /SGW/UPF and the eNB/gNB, Un interface for communication between the eNB/gNB and a relay node (RN) , or Uu interface for communication between the eNB/gNB and a terminal device.

The program 1330 is assumed to include program instructions that, when executed by the associated processor 1310, enable the device 1300 to operate in accordance with the embodiments of the present disclosure, as discussed herein with reference to FIGS. 1 to 12. The embodiments herein may be implemented by computer software executable by the processor 1310 of the device 1300, or by hardware, or by a combination of software and hardware. The processor 1310 may be configured to implement various embodiments of the present disclosure. Furthermore, a combination of the processor 1310 and memory 1320 may form processing means 1350 adapted to implement various embodiments of the present disclosure.

The memory 1320 may be of any type suitable to the local technical network and may be implemented using any suitable data storage technology, such as a non-transitory computer readable storage medium, semiconductor based memory devices, magnetic memory devices and systems, optical memory devices and systems, fixed memory and removable memory, as non-limiting examples. While only one memory 1320 is shown in the device 1300, there may be several physically distinct memory modules in the device 1300. The processor 1310 may be of any type suitable to the local technical network, and may include one or more of general purpose computers, special purpose computers, microprocessors, digital signal processors (DSPs) and processors based on multicore processor architecture, as non-limiting examples. The device 1300 may have multiple processors, such as an application specific integrated circuit chip that is slaved in time to a clock which synchronizes the main processor.

According to embodiments of the present disclosure, a first network device comprising a circuitry is provided. The circuitry is configured to: transmit, to a second network device in a radio access network (RAN) , a first Layer 1 or Layer 2 (L1/L2) triggered mobility (LTM) cell change notification associated with a LTM procedure for a cell switch from a source cell to a target cell, the first LTM cell change notification indicating an initiation of a LTM cell switch command to a terminal device, the first network device being in the RAN and associated with the source cell, and the second network device being associated with a candidate cell for the cell switch. According to embodiments of the present disclosure, the circuitry may be configured to perform any method implemented by the first network device as discussed above.

According to embodiments of the present disclosure, a second network device comprising a circuitry is provided. The circuitry is configured to: receive, from a first network device in a radio access network (RAN) , a first Layer 1 or Layer 2 (L1/L2) triggered mobility (LTM) cell change notification associated with a LTM procedure for a cell switch from a source cell to a target cell, the first LTM cell change notification indicating an initiation of a LTM cell switch command to a terminal device, the first network device being associated with the source cell, and the second network device being in the RAN and associated with a candidate cell for the cell switch. According to embodiments of the present disclosure, the circuitry may be configured to perform any method implemented by the second network device as discussed above.

According to embodiments of the present disclosure, a terminal device comprising a circuitry is provided. The circuitry is configured to: perform a Medium Access Control (MAC) reset operation without stopping a TAT associated with a candidate cell or a target cell in a Layer 1 or Layer 2 (L1/L2) triggered mobility (LTM) procedure for a cell switch from a source cell to the target cell, and without considering that the TAT as expired. According to embodiments of the present disclosure, the circuitry may be configured to perform any method implemented by the terminal device as discussed above.

According to embodiments of the present disclosure, a first network device comprising a circuitry is provided. The circuitry is configured to: receive, from a second network device in a radio access network (RAN) , first timing advance (TA) information associated with a candidate cell in a Layer 1 or Layer 2 (L1/L2) triggered mobility (LTM) procedure for a cell switch from a source cell to a target cell, a first network device in the RAN being associated with the source cell, and the second network device being associated with the candidate cell. According to embodiments of the present disclosure, the circuitry may be configured to perform any method implemented by the first network device as discussed above.

According to embodiments of the present disclosure, a second network device comprising a circuitry is provided. The circuitry is configured to: transmit, to a first network device in a radio access network (RAN) , first timing advance (TA) information associated with a candidate cell in a Layer 1 or Layer 2 (L1/L2) triggered mobility (LTM) procedure for a cell switch from a source cell to a target cell, the first network device being associated with the source cell, and the second network device being in the RAN and associated with the candidate cell. According to embodiments of the present disclosure, the circuitry may be configured to perform any method implemented by the second network device as discussed above.

The term “circuitry” used herein may refer to hardware circuits and/or combinations of hardware circuits and software. For example, the circuitry may be a combination of analog and/or digital hardware circuits with software/firmware. As a further example, the circuitry may be any portions of hardware processors with software including digital signal processor (s) , software, and memory (ies) that work together to cause an apparatus, such as a terminal device or a network device, to perform various functions. In a still further example, the circuitry may be hardware circuits and or processors, such as a microprocessor or a portion of a microprocessor, that requires software/firmware for operation, but the software may not be present when it is not needed for operation. As used herein, the term circuitry also covers an implementation of merely a hardware circuit or processor (s) or a portion of a hardware circuit or processor (s) and its (or their) accompanying software and/or firmware.

According to embodiments of the present disclosure, a first network apparatus is provided. The first network apparatus comprises means for transmitting, to a second network device in a radio access network (RAN) , a first Layer 1 or Layer 2 (L1/L2) triggered mobility (LTM) cell change notification associated with a LTM procedure for a cell switch from a source cell to a target cell, the first LTM cell change notification indicating an initiation of a LTM cell switch command to a terminal device, the first network device being in the RAN and associated with the source cell, and the second network device being associated with a candidate cell for the cell switch. In some embodiments, the first apparatus may comprise means for performing the respective operations of the method 800. In some example embodiments, the first apparatus may further comprise means for performing other operations in some example embodiments of the method 800. The means may be implemented in any suitable form. For example, the means may be implemented in a circuitry or software module.

According to embodiments of the present disclosure, a second network apparatus is provided. The second network apparatus comprises means for receiving, from a first network device in a radio access network (RAN) , a first Layer 1 or Layer 2 (L1/L2) triggered mobility (LTM) cell change notification associated with a LTM procedure for a cell switch from a source cell to a target cell, the first LTM cell change notification indicating an initiation of a LTM cell switch command to a terminal device, the first network device being associated with the source cell, and the second network device being in the RAN and associated with a candidate cell for the cell switch. In some embodiments, the second apparatus may comprise means for performing the respective operations of the method 900. In some example embodiments, the second apparatus may further comprise means for performing other operations in some example embodiments of the method 900. The means may be implemented in any suitable form. For example, the means may be implemented in a circuitry or software module.

According to embodiments of the present disclosure, a terminal apparatus is provided. The terminal apparatus comprises means for performing a Medium Access Control (MAC) reset operation without stopping a TAT associated with a candidate cell or a target cell in a Layer 1 or Layer 2 (L1/L2) triggered mobility (LTM) procedure for a cell switch from a source cell to the target cell, and without considering that the TAT as expired. In some embodiments, the third apparatus may comprise means for performing the respective operations of the method 1000. In some example embodiments, the third apparatus may further comprise means for performing other operations in some example embodiments of the method 1000. The means may be implemented in any suitable form. For example, the means may be implemented in a circuitry or software module.

According to embodiments of the present disclosure, a first network apparatus is provided. The first network apparatus comprises means for receiving, from a second network device in a radio access network (RAN) , first timing advance (TA) information associated with a candidate cell in a Layer 1 or Layer 2 (L1/L2) triggered mobility (LTM) procedure for a cell switch from a source cell to a target cell, a first network device in the RAN being associated with the source cell, and the second network device being associated with the candidate cell. In some embodiments, the fourth apparatus may comprise means for performing the respective operations of the method 1100. In some example embodiments, the fourth apparatus may further comprise means for performing other operations in some example embodiments of the method 1100. The means may be implemented in any suitable form. For example, the means may be implemented in a circuitry or software module.

According to embodiments of the present disclosure, a second network apparatus is provided. The second network apparatus comprises means for transmitting, to a first network device in a radio access network (RAN) , first timing advance (TA) information associated with a candidate cell in a Layer 1 or Layer 2 (L1/L2) triggered mobility (LTM) procedure for a cell switch from a source cell to a target cell, the first network device being associated with the source cell, and the second network device being in the RAN and associated with the candidate cell. In some embodiments, the fifth apparatus may comprise means for performing the respective operations of the method 1200. In some example embodiments, the fifth apparatus may further comprise means for performing other operations in some example embodiments of the method 1200. The means may be implemented in any suitable form. For example, the means may be implemented in a circuitry or software module.

In summary, embodiments of the present disclosure provide the following aspects.

In an aspect, it is proposed a first network device comprising: a processor configured to cause the first network device to: transmit, to a second network device in a radio access network (RAN) , a first Layer 1 or Layer 2 (L1/L2) triggered mobility (LTM) cell change notification associated with a LTM procedure for a cell switch from a source cell to a target cell, the first LTM cell change notification indicating an initiation of a LTM cell switch command to a terminal device, the first network device being in the RAN and associated with the source cell, and the second network device being associated with a candidate cell for the cell switch.

In some embodiments, the first network device comprises a first centralized unit (CU) and a first distributed unit (DU) , and the second network device comprises a second CU and a second DU, a second LTM cell change notification is transmitted from the first DU to the first CU, the second LTM cell change notification indicating the initiation of the LTM cell switch command to the terminal device, and wherein the first LTM cell change notification is transmitted based on the second LTM cell change notification.

In some embodiments, the first LTM cell change notification is received by the second CU, and a third LTM cell change notification indicating the initiation of the LTM cell switch command to the terminal device is transmitted from the second CU to the second DU based on the first LTM cell change notification.

In some embodiments, at least one of the first LTM cell change notification, the second LTM cell change notification or the third LTM cell change notification comprises one of more of the following: an identification of the target cell, an identification of Transmission Configuration Indicator (TCI) state, a first identification of a terminal device over an Xn interface within the first network device, a second identification of a terminal device over an Xn interface within the second network device, a third identification of a terminal device over a F1 interface within a centralized unit (CU) of the first network device, or a fourth identification of a terminal device over a F1 interface within a distributed unit (DU) of the first network device.

In some embodiments, the first network device is further caused to: receive, from the second network device, first timing advance (TA) information associated with the candidate cell.

In some embodiments, the first network device comprises a first centralized unit (CU) and a first distributed unit (DU) , and the second network device comprises a second CU and a second DU, second TA information is transmitted from the second DU to the second CU, and wherein the first TA information is transmitted based on the second TA information.

In some embodiments, the first TA information is received by the first CU, and third TA information is transmitted from the first CU to the first DU based on the first TA information.

In some embodiments, at least one of the first TA information, the second TA information, or the third TA information comprises one or more of the following: an identification of the candidate cell, a TA value, a first identification of a terminal device over an Xn interface within the first network device, a global identification of the first network device, or an identification of the first network device in the RAN, or an identification of a distributed unit (DU) of the first network device.

In some embodiments, the LTM procedure is a conditional LTM procedure that is executed in response to that at least one LTM execution condition is met, and the first network device is further caused to: receive, from the second network device, a handover success message indicating that the terminal device has successfully accessed the target cell.

In some embodiments, the first network device is further caused to: receive, from the terminal device, capability information of the terminal device for the LTM procedure, wherein the capability information comprises at least one of: a capability indicating that the terminal device supports a conditional LTM procedure, a capability indicating that the terminal device supports a conditional random access channel (RACH) -less LTM procedure, a capability indicating that the terminal device supports user equipment (UE) -based candidate cell TA maintains, or a capability indicating the number of terminal devices that support early TA acquire or TA maintain of candidate cell, or indicating that the number of TA timers (TATs) maintained by the terminal device for candidate cells.

In some embodiments, the LTM procedure comprises at least one of: a network triggered LTM procedure, which is triggered by a LTM command MAC CE, or a conditional LTM procedure that is executed in response to that at least one LTM execution condition is met.

In some embodiments, the first network device comprises a first centralized unit (CU) and the second network device comprises a second CU other than the first CU, or wherein the first network device comprises a first Next Generation Radio Access Network (NG-RAN) and the second network device comprises a second NG-RAN other than the first NG-RAN.

In some embodiments, the first network device and the second network device communicate via an Xn interface.

In an aspect, it is proposed a second network device comprising: a processor configured to cause the second network device to: receive, from a first network device in a radio access network (RAN) , a first Layer 1 or Layer 2 (L1/L2) triggered mobility (LTM) cell change notification associated with a LTM procedure for a cell switch from a source cell to a target cell, the first LTM cell change notification indicating an initiation of a LTM cell switch command to a terminal device, the first network device being associated with the source cell, and the second network device being in the RAN and associated with a candidate cell for the cell switch.

In some embodiments, the second network device is further caused to: transmit, to the first network device, first timing advance (TA) information associated with the candidate cell.

In some embodiments, the LTM procedure is a conditional LTM procedure that is executed in response to that at least one LTM execution condition is met, and the second network device is further caused to: transmit, to the first network device, a handover success message indicating that the terminal device has successfully accessed the target cell.

In some embodiments, the handover success message is transmitted in response to an access success message received from a distributed unit (DU) of the second network device, the access success message indicating that the terminal device has successfully accessed the target cell.

In an aspect, it is proposed a terminal device comprising: a processor configured to cause the terminal device to: perform a Medium Access Control (MAC) reset operation without stopping a TAT associated with a candidate cell or a target cell in a Layer 1 or Layer 2 (L1/L2) triggered mobility (LTM) procedure for a cell switch from a source cell to the target cell, and without considering that the TAT as expired.

In some embodiments, the MAC reset operation is performed in response to that a LTM command is received or a LTM execution condition is satisfied.

In some embodiments, the terminal device is further caused to: perform the LTM procedure by apply a configuration of the target cell and perform a corresponding L2 behavior.

In some embodiments, during the RACH-less conditional LTM, in response to that an uplink grant has been received on a Physical Downlink Control Channel (PDCCH) for a MAC entity's Cell-Radio Network Temporary Identifier (C-RNTI) after a first Physical Uplink Shared Channel (PUSCH) transmission to the target cell, and the uplink grant is for a new transmission regardless of a Hybrid Automatic Repeat Request (HARQ) process, the cell switch is considered to be successfully completed.

In some embodiments, the terminal device is further caused to: transmit, to a first network device in a radio access network (RAN) , capability information of the terminal device for the LTM procedure, the first network device being associated with the source cell in the LTM procedure, wherein the capability information comprises at least one of: a capability indicating that the terminal device supports a conditional LTM procedure, a capability indicating that the terminal device supports a conditional random access channel (RACH) -less LTM procedure, a capability indicating that the terminal device supports user equipment (UE) -based candidate cell timing advance (TA) maintains, or a capability indicating the number of terminal devices that support early TA acquire or TA maintain of candidate cell, or indicating that the number of TA timers (TATs) maintained by the terminal device for candidate cells.

In an aspect, it is proposed a first network device comprising: a processor configured to cause the first network device to: receive, from a second network device in a radio access network (RAN) , first timing advance (TA) information associated with a candidate cell in a Layer 1 or Layer 2 (L1/L2) triggered mobility (LTM) procedure for a cell switch from a source cell to a target cell, a first network device in the RAN being associated with the source cell, and the second network device being associated with the candidate cell.

In an aspect, it is proposed a second network device comprising: a processor configured to cause the second network device to: transmit, to a first network device in a radio access network (RAN) , first timing advance (TA) information associated with a candidate cell in a Layer 1 or Layer 2 (L1/L2) triggered mobility (LTM) procedure for a cell switch from a source cell to a target cell, the first network device being associated with the source cell, and the second network device being in the RAN and associated with the candidate cell.

In an aspect, a first network device comprises: at least one processor; and at least one memory coupled to the at least one processor and storing instructions thereon, the instructions, when executed by the at least one processor, causing the device to perform the method implemented by the first network device discussed above.

In an aspect, a second network device comprises: at least one processor; and at least one memory coupled to the at least one processor and storing instructions thereon, the instructions, when executed by the at least one processor, causing the device to perform the method implemented by the second network device discussed above.

In an aspect, a terminal device comprises: at least one processor; and at least one memory coupled to the at least one processor and storing instructions thereon, the instructions, when executed by the at least one processor, causing the device to perform the method implemented by the terminal device discussed above.

In an aspect, a computer readable medium having instructions stored thereon, the instructions, when executed on at least one processor, causing the at least one processor to perform the method implemented by the first network device discussed above.

In an aspect, a computer readable medium having instructions stored thereon, the instructions, when executed on at least one processor, causing the at least one processor to perform the method implemented by the second network device discussed above.

In an aspect, a computer readable medium having instructions stored thereon, the instructions, when executed on at least one processor, causing the at least one processor to perform the method implemented by the terminal device discussed above.

In an aspect, a computer program comprising instructions, the instructions, when executed on at least one processor, causing the at least one processor to perform the method implemented by the first network device discussed above.

In an aspect, a computer program comprising instructions, the instructions, when executed on at least one processor, causing the at least one processor to perform the method implemented by the second network device discussed above.

In an aspect, a computer program comprising instructions, the instructions, when executed on at least one processor, causing the at least one processor to perform the method implemented by the terminal device discussed above.

Generally, various embodiments of the present disclosure may be implemented in hardware or special purpose circuits, software, logic or any combination thereof. Some aspects may be implemented in hardware, while other aspects may be implemented in firmware or software which may be executed by a controller, microprocessor or other computing device. While various aspects of embodiments of the present disclosure are illustrated and described as block diagrams, flowcharts, or using some other pictorial representation, it will be appreciated that the blocks, apparatus, systems, techniques or methods described herein may be implemented in, as non-limiting examples, hardware, software, firmware, special purpose circuits or logic, general purpose hardware or controller or other computing devices, or some combination thereof.

The present disclosure also provides at least one computer program product tangibly stored on a non-transitory computer readable storage medium. The computer program product includes computer-executable instructions, such as those included in program modules, being executed in a device on a target real or virtual processor, to carry out the process or method as described above with reference to FIGS. 1 to 13. Generally, program modules include routines, programs, libraries, objects, classes, components, data structures, or the like that perform particular tasks or implement particular abstract data types. The functionality of the program modules may be combined or split between program modules as desired in various embodiments. Machine-executable instructions for program modules may be executed within a local or distributed device. In a distributed device, program modules may be located in both local and remote storage media.

Program code for carrying out methods of the present disclosure may be written in any combination of one or more programming languages. These program codes may be provided to a processor or controller of a general purpose computer, special purpose computer, or other programmable data processing apparatus, such that the program codes, when executed by the processor or controller, cause the functions/operations specified in the flowcharts and/or block diagrams to be implemented. The program code may execute entirely on a machine, partly on the machine, as a stand-alone software package, partly on the machine and partly on a remote machine or entirely on the remote machine or server.

The above program code may be embodied on a machine readable medium, which may be any tangible medium that may contain, or store a program for use by or in connection with an instruction execution system, apparatus, or device. The machine readable medium may be a machine readable signal medium or a machine readable storage medium. A machine readable medium may include but not limited to an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, or device, or any suitable combination of the foregoing. More specific examples of the machine readable storage medium would include an electrical connection having one or more wires, a portable computer diskette, a hard disk, a random access memory (RAM) , a read-only memory (ROM) , an erasable programmable read-only memory (EPROM or Flash memory) , an optical fiber, a portable compact disc read-only memory (CD-ROM) , an optical storage device, a magnetic storage device, or any suitable combination of the foregoing.

Further, while operations are depicted in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. In certain circumstances, multitasking and parallel processing may be advantageous. Likewise, while several specific implementation details are contained in the above discussions, these should not be construed as limitations on the scope of the present disclosure, but rather as descriptions of features that may be specific to particular embodiments. Certain features that are described in the context of separate embodiments may also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment may also be implemented in multiple embodiments separately or in any suitable sub-combination.

Although the present disclosure has been described in language specific to structural features and/or methodological acts, it is to be understood that the present disclosure defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims.

Claims

A first network device comprising:

a processor configured to cause the first network device to:

transmit, to a second network device in a radio access network (RAN) , a first Layer 1 or Layer 2 (L1/L2) triggered mobility (LTM) cell change notification associated with a LTM procedure for a cell switch from a source cell to a target cell, the first LTM cell change notification indicating an initiation of a LTM cell switch command to a terminal device, the first network device being in the RAN and associated with the source cell, and the second network device being associated with a candidate cell for the cell switch.
The device of claim 1, wherein the first network device comprises a first centralized unit (CU) and a first distributed unit (DU) , and the second network device comprises a second CU and a second DU, a second LTM cell change notification is transmitted from the first DU to the first CU, the second LTM cell change notification indicating the initiation of the LTM cell switch command to the terminal device, and

wherein the first LTM cell change notification is transmitted based on the second LTM cell change notification.
The device of claim 2, wherein the first LTM cell change notification is received by the second CU, and a third LTM cell change notification indicating the initiation of the LTM cell switch command to the terminal device is transmitted from the second CU to the second DU based on the first LTM cell change notification.
The device of any of claims 1 to 3, wherein at least one of the first LTM cell change notification, the second LTM cell change notification or the third LTM cell change notification comprises one of more of the following:

an identification of the target cell,

an identification of Transmission Configuration Indicator (TCI) state,

a first identification of a terminal device over an Xn interface within the first network device,

a second identification of a terminal device over an Xn interface within the second network device,

a third identification of a terminal device over a F1 interface within a centralized unit (CU) of the first network device, or

a fourth identification of a terminal device over a F1 interface within a distributed unit (DU) of the first network device.
The device of claim 1, wherein the LTM procedure is a conditional LTM procedure that is executed in response to that at least one LTM execution condition is met, and the first network device is further caused to:

receive, from the second network device, a handover success message indicating that the terminal device has successfully accessed the target cell.
The device of any of claims 1 to 5, wherein the first network device is further caused to:

receive, from the terminal device, capability information of the terminal device for the LTM procedure, wherein the capability information comprises at least one of:

a capability indicating that the terminal device supports a conditional LTM procedure,

a capability indicating that the terminal device supports a conditional random access channel (RACH) -less LTM procedure,

a capability indicating that the terminal device supports user equipment (UE) -based candidate cell TA maintains, or

a capability indicating the number of terminal devices that support early TA acquire or TA maintain of candidate cell, or indicating that the number of time alignment timers (TATs) maintained by the terminal device for candidate cells.
The device of any of claims 1 to 6, wherein the LTM procedure comprises at least one of:

a network triggered LTM procedure, which is triggered by a LTM command MAC CE, or

a conditional LTM procedure that is executed in response to that at least one LTM execution condition is met.
The device of claim 1, wherein the first network device comprises a first centralized unit (CU) and the second network device comprises a second CU other than the first CU, or

wherein the first network device comprises a first Next Generation Radio Access Network (NG-RAN) and the second network device comprises a second NG-RAN other than the first NG-RAN.
The device of any of claims 1 to 8, wherein the first network device and the second network device communicate via an Xn interface.
A terminal device comprising:

a processor configured to cause the terminal device to:

perform a Medium Access Control (MAC) reset operation without stopping a time alignment timer (TAT) associated with a candidate cell or a target cell in a Layer 1 or Layer 2 (L1/L2) triggered mobility (LTM) procedure for a cell switch from a source cell to the target cell, and without considering that the TAT as expired.
The device of claim 10, wherein the MAC reset operation is performed in response to that a LTM command is received or a LTM execution condition is satisfied.
The device of claim 10, wherein the terminal device is further caused to:

perform the LTM procedure by apply a configuration of the target cell and perform a corresponding L2 behavior.
The device of claim 10, wherein during the RACH-less conditional LTM, in response to that an uplink grant has been received on a Physical Downlink Control Channel (PDCCH) for a MAC entity's Cell-Radio Network Temporary Identifier (C-RNTI) after a first Physical Uplink Shared Channel (PUSCH) transmission to the target cell, and the uplink grant is for a new transmission regardless of a Hybrid Automatic Repeat Request (HARQ) process, the cell switch is considered to be successfully completed.
The device of any of claims 10 to 13, wherein the terminal device is further caused to:

transmit, to a first network device in a radio access network (RAN) , capability information of the terminal device for the LTM procedure, the first network device being associated with the source cell in the LTM procedure, wherein the capability information comprises at least one of:

a capability indicating that the terminal device supports a conditional LTM procedure,

a capability indicating that the terminal device supports a conditional random access channel (RACH) -less LTM procedure,

a capability indicating that the terminal device supports user equipment (UE) -based candidate cell timing advance (TA) maintains, or

a capability indicating the number of terminal devices that support early TA acquire or TA maintain of candidate cell, or indicating that the number of TATs maintained by the terminal device for candidate cells.
A second network device comprising:

a processor configured to cause the second network device to:

transmit, to a first network device in a radio access network (RAN) , first timing advance (TA) information associated with a candidate cell in a Layer 1 or Layer 2 (L1/L2) triggered mobility (LTM) procedure for a cell switch from a source cell to a target cell, the first network device being associated with the source cell, and the second network device being in the RAN and associated with the candidate cell.
The device of claim 15, wherein the first network device comprises a first centralized unit (CU) and a first distributed unit (DU) , and the second network device comprises a second CU and a second DU, second TA information is transmitted from the second DU to the second CU, and wherein the first TA information is transmitted based on the second TA information.
The device of claim 16, wherein the first TA information is received by the first CU, and third TA information is transmitted from the first CU to the first DU based on the first TA information.
The device of any of claims 15 to 17, wherein at least one of the first TA information, the second TA information, or the third TA information comprises one or more of the following:

an identification of the candidate cell,

a TA value,

a first identification of a terminal device over an Xn interface within the first network device,

a global identification of the first network device, or an identification of the first network device in the RAN, or

an identification of a distributed unit (DU) of the first network device.
The device of any of claims 15 to 18, wherein the LTM procedure comprises at least one of:

a network triggered LTM procedure, which is triggered by a LTM command MAC CE, or

a conditional LTM procedure that is executed in response to that at least one LTM execution condition is met.
The device of claim 19, wherein the first network device comprises a first centralized unit (CU) and the second network device comprises a second CU other than the first CU, or

wherein the first network device comprises a first Next Generation Radio Access Network (NG-RAN) and the second network device comprises a second NG-RAN other than the first NG-RAN.