Mobile devices, access network nodes, and methods

Abstract

This disclosure relates to a method performed by user equipment (UE), the method comprising: receiving instructions for a first set of one or more candidate target cells for a lower layer mobility procedure from an access network node providing source cells; and determining a second set of candidate target cells for the lower layer mobility procedure, wherein the second set of cells is a subset of the first set of cells, and the cells in the second set of cells are prioritized to be selected as target cells.

Description

Technical Field

[0001] The present disclosure relates to a communication system.

Background Art

[0002] The present disclosure is particularly, but not limited to, related to wireless communication systems and devices operating according to 3rd Generation Partnership Project (3GPP (registered trademark)) standards or equivalent standards or derived standards (including LTE-Advanced, next generation, 5G networks, future generations, and subsequent generations). The present disclosure is particularly, but not limited to, related to lower-layer triggered mobility (LTM), radio link monitoring (RLM), and radio link failure (RLF) in "New Radio" systems and similar systems (also referred to as "next generation" systems).

[0003] Recent developments in 3GPP standards are referred to as Long-Term Evolution (LTE) of the Evolved Packet Core (EPC) network and the Evolved UMTS Terrestrial Radio Access Network (E-UTRAN), and are generally also referred to as "4G". Furthermore, the terms "5G" and "New Radio" (NR) refer to evolved communication technologies that are expected to support various applications and services. Various details of 5G networks are described, for example, in the "NGMN 5G White Paper" V1.0 by the Next Generation Mobile Networks (NGMN) Alliance, which is available from https: / / www.ngmn.org / 5g-white-paper.html. 3GPP plans to support 5G through the so-called 3GPP Next Generation (NextGen) radio access network (RAN) and 3GPP NextGen core network.

[0004] In the 3GPP standard, a NodeB (or eNB for LTE, gNB for 5G) is a radio access network (RAN) node (or simply an "access node," "access network node," or "base station") through which communication equipment (user equipment, i.e., "UE") connects to the core network and communicates with other communication equipment and remote servers. For simplicity, in this application, such access nodes are collectively referred to as RAN nodes, base stations, or access network nodes.

[0005] UEs can communicate using Special Cells (SpCells), such as the primary serving cell (PCell) of a Master Cell Group (MCG). If UE3 is configured to communicate using a secondary cell group (SCG), UE3 can also communicate via the primary SCG cell (PSCell). UE3 may also be provided with indications of one or more candidate target cells to which UE3 can switch. If a radio link failure (RLF) is detected in the primary serving cell, the UE may initiate a radio resource control (RRC) connection re-establishment procedure, which may include a random access procedure. [Prior art documents] [Non-patent literature]

[0006] [Non-Patent Document 1] The Next Generation Mobile Networks (NGMN) Alliance's "NGMN 5G White Paper" V1.0 is available from https: / / www.ngmn.org / 5g-white-paper.html. [Overview of the project] [Problems that the invention aims to solve]

[0007] Historically, mobility between different cells in cellular communications has relied on higher-layer communications such as Layer 3 (e.g., L3 or radio resource control (RRC) layer) signaling. More recently, with the aim of improving mobility, there has been consideration of developing and supporting Layer 1 (e.g., L1 or physical (PHY) layer) and / or Layer 2 (e.g., L2 or media access control (MAC) layer) centric mobility (also known as L1 / L2 centric mobility) rather than higher-layer (e.g., RRC layer) mobility. Such L1 / L2 centric mobility (also known as L1 / L2 triggered mobility or "LTM") has the potential to improve the mobility of devices operating in both sub-7GHz and mm-wave bands by supporting, for example, reduced handover delays and improved robustness.

[0008] However, when the UE communicates via SpCell and provides instructions for one or more candidate target cells of the LTM, improved methods and apparatus are needed for handling RLM and RLF. For example, methods and apparatus are needed for more efficient and reliable switching to communication via candidate cells. More generally, improved apparatus and methods are needed for handling RLF and lower layer triggered mobility. [Means for solving the problem]

[0009] In one embodiment, the Disclosure provides a method performed by user equipment (UE) which includes receiving instructions for a first set of one or more candidate target cells for a lower layer mobility procedure from an access network node providing source cells, and determining a second set of candidate target cells for the lower layer mobility procedure, wherein the second set of cells is a subset of the first set of cells, and the cells in the second set of cells are prioritized to be selected as target cells.

[0010] The method may further include obtaining timing advance information for communication using the cell for at least one of the candidate cells in the first set, and deciding to include the candidate cell from which the timing advance information is obtained in the second set of cells.

[0011] Acquiring the timing advance information may include receiving the timing advance information from the access network node via the source cell. The timing advance information may also be received from the access network node in the configuration information transmitted by the source cell.

[0012] The method may further include determining whether the timing advance information is valid for a candidate cell from which the timing advance information has been acquired, based on an associated timer; removing the candidate cell from the second set if it is determined that the timing advance information for the candidate cell is not valid; and keeping the candidate cell in the second set if it is determined that the timing advance information for the candidate cell is valid.

[0013] The method may include determining that the timing advance information for the candidate cell is not valid when the timer expires.

[0014] The method may further include receiving one or more communication resources from the access network node or another access network node providing a cell from the first set of cells for use in obtaining the timing advance information, and obtaining the timing advance information using the one or more communication resources.

[0015] The aforementioned one or more communication resources may include one or more physical random access channel (PRACH) resources.

[0016] The method may further include sending a request for the one or more communication resources to be used in obtaining the timing advance information to the access network node.

[0017] The second set of cells may include at least one cell for which valid timing advance information is available in the UE, and the second set of cells may include at least one cell for which valid timing advance information is not available in the UE, and the method may further include determining a ranking or priority for selecting the cell in the second set of cells as a target cell based on whether valid timing advance information is available for the cell.

[0018] The method may further include receiving from the access network node an instruction for the maximum number of cells for which the UE maintains the corresponding timing advance information.

[0019] The method may further include sending an instruction to the access network node for the maximum number of cells for which the UE maintains the corresponding timing advance information.

[0020] The method may further include receiving, from the access network node, an indication of the identifier of one or more of the candidate target cells for the UE to obtain corresponding timing advance information, and obtaining the timing advance information for the indicated cell.

[0021] The method may further include determining to obtain timing advance information for one or more cells of the set of the first cells, and the UE determines whether to obtain timing advance information for a cell based on at least one measurement of the transmission of the cell.

[0022] The UE may determine whether to obtain the timing advance information for a cell based on whether a random access channel (RACH) resource of the cell for obtaining the timing advance information is available at the UE.

[0023] The method may further include transmitting, to the access network node, an indication of the cell included in the set of the second candidate cells.

[0024] The method may include transmitting, to the access network node, an indication of the cell included in the set of the second candidate cells after adding or deleting a cell from the set of the second cells. [[ID=IS]]

[0025] The method may further include performing one or more measurements of the transmission of at least one cell of the set of the second cells, and determining a ranking or priority for selecting the cell as a target cell in the set of the second cells based on the measurements.

[0026] The measurement may further include at least one measurement of reference signal received power (RSRP), reference signal received quality (RSRQ), or RSRP and signal to noise interference ratio (RSRP-SINR).

[0027] The method may further include receiving, from the access network node, measurement configuration information regarding the one or more measurements, and performing the one or more measurements based on the measurement configuration information.

[0028] The method may further include determining that radio link failure (RLF) has occurred in the source cell, and performing the lower layer mobility procedure after determining that the RLF has occurred.

[0029] Determining that the RLF has occurred may include performing a radio link monitoring (RLM) procedure.

[0030] The RLM procedure may include performing a measurement of transmission of the source cell.

[0031] The RLM procedure may further include performing a measurement of transmission of at least one cell of the set of the second cells.

[0032] The RLM procedure includes a first RLM process for monitoring the source cell and one or more second RLM processes for monitoring cells of the set of the second cells.

[0033] If the UE determines that RLF has occurred in both the source cell and the cell of the set of the second cells monitored using the second RLM process, the UE may determine that the RLF has occurred.

[0034] The RLM procedure may include a joint RLM process for monitoring the source cell and for monitoring a cell in a second set of cells.

[0035] The method may further include determining that a failure has occurred in the handover procedure for the handover of the UE from the source cell, and, after determining that a failure has occurred in the handover procedure, performing the lower-layer mobility procedure.

[0036] The aforementioned lower-layer procedure may also be a layer 1 (L1) or layer 2 (L2) based mobility procedure.

[0037] The method may include determining that an RLF has occurred in the source cell, and determining to maintain the configuration for an RRC connection through the source cell for a first period of time, if the second set of cells includes at least one cell.

[0038] The source cell may be associated with a central unit of a base station, and the method may include deciding to maintain the settings for the RRC connection via the source cell for a first period of time, provided that the second set of cells includes at least one cell associated with the central unit.

[0039] The method may further include deciding to perform the lower-layer mobility procedure.

[0040] The lower-layer mobility procedure may include establishing or re-establishing a radio resource control (RRC) connection through a cell in the second set of cells.

[0041] The method may further include receiving an instruction from the access network node for the maximum number of cells to be included in the second set of cells.

[0042] The method may further include sending an instruction to the access network node regarding the maximum number of cells that the UE should include in the second set of cells.

[0043] In another aspect, the Disclosure provides a method performed by an access network node providing a source cell, the method comprising: transmitting instructions for a first set of one or more candidate target cells for a lower layer mobility procedure to user equipment (UE) in the source cell; and receiving instructions from the UE for a second set of candidate target cells for the lower layer mobility procedure, wherein the second set of cells is a subset of the first set of cells, and the cells in the second set of cells are prioritized to be selected as target cells for the cell lower layer mobility procedure.

[0044] The method may further include transmitting one or more communication resources to the UE to use for obtaining timing advance information for a cell in the second set of cells, the UE determining which cell is included in the second set of cells based on the timing advance information.

[0045] The aforementioned one or more communication resources may include one or more physical random access channel (PRACH) resources.

[0046] The method may further include receiving from the UE a request for the one or more communication resources for the UE to use to obtain the timing advance information.

[0047] The method may further include sending an instruction to the UE for the maximum number of cells for which the UE will maintain the corresponding timing advance information.

[0048] The method may further include receiving from the UE an instruction for the maximum number of cells for which the UE maintains the corresponding timing advance information.

[0049] The method may further include transmitting to the UE an instruction for the identifiers of one or more candidate target cells for the UE to obtain corresponding timing advance information.

[0050] The method may further include transmitting measurement setting information for one or more measurements performed by the UE for the transmission of at least one cell from the second set of cells to the UE.

[0051] The aforementioned lower-layer procedure may also be a layer 1 (L1) or layer 2 (L2) based mobility procedure.

[0052] The method may further include deciding to perform the lower-layer mobility procedure.

[0053] The method may further include sending an instruction to the UE for the maximum number of cells to be included in the second set of cells.

[0054] The method may further include receiving from the UE an instruction on the maximum number of cells the UE should include in the second set of cells.

[0055] In another embodiment, the Disclosure provides user equipment (UE) comprising means for receiving instructions for a first set of one or more candidate target cells for a lower layer mobility procedure from an access network node providing source cells, and means for determining a second set of candidate target cells for the lower layer mobility procedure, wherein the cells of the second set of cells are a subset of the first set of cells, and the cells in the second set of cells are prioritized to be selected as target cells.

[0056] In another aspect, the Disclosure provides an access network node configured to provide a source cell, the access network node comprising: means for transmitting instructions for a first set of one or more candidate target cells for a lower layer mobility procedure to user equipment (UE) in the source cell; and means for receiving instructions from the UE for a second set of candidate target cells for the lower layer mobility procedure, wherein the second set of cells is a subset of the first set of cells, and the cells in the second set of cells are prioritized to be selected as target cells for the lower layer mobility procedure.

[0057] In another aspect, the Disclosure provides a method performed by user equipment (UE) which includes receiving instructions for a first set of one or more candidate target cells for a lower layer mobility procedure from an access network node providing source cells, determining a second set of candidate target cells for the lower layer mobility procedure, wherein the second set of cells is a subset of the first set of cells, and selecting a cell as a target cell for the lower layer mobility procedure, wherein the UE prioritizes selecting the cell in the second set of cells as the target cell. [Brief explanation of the drawing]

[0058] Hereinafter, embodiments of this disclosure will be described as examples with reference to the attached drawings.

[0059] [Figure 1] Figure 1 provides a schematic overview of a mobile ("cellular" or "wireless") communication system. [Figure 2] Figure 2 shows a typical frame structure that may be used in the communication system shown in Figure 1. [Figure 3] Figure 3 is a schematic block diagram showing the main components of DU50, which can be used as part of RAN node 5 of the communication system 1 shown in Figure 1. [Figure 4] Figure 4 is a schematic block diagram showing the main components of CU60, which can be used as part of RAN node 5 of the communication system 1 shown in Figure 1. [Figure 5] Figure 5 shows the mobility procedure in which a handover occurs from the source base station to the target base station. [Figure 6] Figure 6 shows a random access (RA) procedure that can be executed in the system shown in Figure 1. [Figure 7] Figure 7 shows an example of intra-CU inter-DU mobility. [Figure 8] Figure 8 shows an exemplary method of inter-DU mobility. [Figure 9] Figure 9 shows the inter-cell inter-DU method. [Figure 10] Figure 10 shows the L1 mobility method of base station triggers, including measurement report filtering. [Figure 11] Figure 11 shows how radio link recovery occurs. [Figure 12] Figure 12 shows a method for determining radio link failure. [Figure 13] Figure 13 shows an example where UE3 communicates with base station 5 via SpCell and determines that an RLF has occurred. [Figure 14] Figure 14 shows an example where UE3 is provided with a set of fast recovery cells. [Figure 15] Figure 15 shows an example where UE3 stores FRSC and RLF occurs in the serving cell. [Figure 16] Figure 16 shows an example where an access network node providing a source cell / serving cell communicates with an access network node providing candidate cells for LTM and retrieves one or more PRACH resources requested by the UE. [Figure 17] Figure 17 shows a further example of a fast recovery cell set. [Figure 18] Figure 18 is a schematic block diagram showing the main components of the UE of the communication system in Figure 1. [Figure 19] Figure 19 is a schematic block diagram showing the main components of the base station in the communication system shown in Figure 1. [Figure 20] Figure 20 is a schematic block diagram showing the main components of the core network nodes or functions of the communication system in Figure 1. [Modes for carrying out the invention]

[0060] overview Here, referring to Figures 1 and 2, we will explain an exemplary communication system from a general perspective, purely as an example.

[0061] Figure 1 schematically shows a mobile ("cellular" or "wireless") communication system 1 to which embodiments of the present invention can be applied.

[0062] In communication system 1, user equipment (UE) 3-1, 3-2, 3-3 (such as mobile phones and / or other mobile devices) can communicate with each other via (radio) access network ((R)AN) nodes 5 (base stations 5, RAN equipment 5) operating according to one or more compatible radio access technologies (RATs). In the illustrated example, the (R)AN node 5 includes an NR / 5G base station 5 or "gNB" 5 operating one or more associated cells 9. Communication via base station 5 is typically routed via a core network 7 (such as a 5G core network or evolved packet core network (EPC)).

[0063] As those skilled in the art will understand, Figure 1 shows three UE3s and one base station 5 for illustrative purposes, but when the system is implemented, other base stations 5 and UE3s are usually included as well.

[0064] Each base station 5 controls one or more associated cells 9 directly or indirectly through one or more other nodes (such as home base stations, relays, remote radio heads, and distributed units). It will be understood that base stations 5 may be configured to support 4G, 5G, 6G, and / or other 3GPP or non-3GPP communication protocols.

[0065] The UE3 and its serving base station 5 are connected via a suitable radio interface (e.g., a so-called "Uu" interface). Adjacent base stations 5 may be connected to each other via suitable inter-base station interfaces (e.g., so-called "X2" interfaces, "Xn" interfaces, etc.).

[0066] The core network 7 includes multiple logical nodes (or “functions”) to support communication in the communication system 1. In this example, the core network 7 includes a control plane function (CPF) 10 and one or more user plane functions (UPFs) 11. The CPF 10 includes one or more Access and Mobility Management Functions (AMFs) 10-1, one or more Session Management Functions (SMFs), and several other functions 10-n.

[0067] Base station 5 is connected to the core network nodes via appropriate interfaces (or "reference points"), such as an N2 reference point for control signaling communication between base station 5 and AMF10-1, and an N3 reference point for user data communication between base station 5 and each UPF11. Each UE3 is connected to AMF10-1 via a logical non-access stratum (NAS) connection through an N1 reference point (equivalent to the S1 reference point in LTE). It will be understood that N1 communication is routed transparently through base station 5.

[0068] One or more UPF11s are connected to an external data network (such as an IP network like the Internet) via a reference point N6 for user data communication.

[0069] The AMF10-1 performs mobility management-related functions, maintains NAS signaling connections with each UE3, and manages UE registration. The AMF10-1 is also responsible for paging management. The SMF10-2 provides session management functions (forming part of the LTE MME functionality) and integrates several control plane functions (provided by the LTE serving gateway and packet data network gateway). The SMF10-2 assigns an IP address to each UE3.

[0070] The base station 5 of communication system 1 is configured to operate at least one cell 9 on an associated TDD carrier operating in a non-paired spectrum. It will also be understood that base station 5 may operate at least one cell 9 on an associated FDD carrier operating in a paired spectrum.

[0071] Furthermore, base station 5 is configured to transmit control information and user data, and multiple physical signals, via multiple downlink (DL) physical channels, and UE3 is configured to receive these. DL physical channels correspond to resource elements (REs) that transmit information originating from higher layers, and DL physical signals correspond to REs used in the physical layer that do not transmit information originating from higher layers.

[0072] Physical channels may include, for example, a physical downlink shared channel (PDSCH), a physical broadcast channel (PBCH), and a physical downlink control channel (PDCCH). The PDSCH transmits data that shares the PDSCH's capacity based on time and frequency. The PDSCH can transmit various data items, including, for example, user data, UE-specific upper-layer control messages mapped from higher channels, system information blocks (SIBs), and paging. The PDCCH transmits downlink control information (DCI) to support various functions, including, for example, scheduling downlink transmission on the PDSCH and uplink data transmission on the physical uplink shared channel (PUSCH). The PBCH provides the UE3 with a Master Information Block (MIB). The PBCH also works in conjunction with the PDCCH to support time and frequency synchronization and assist in cell acquisition, selection, and re-selection. UE3 may receive a Synchronization Signal Block (SSB), and may assume that the reception opportunities of the PBCH, primary synchronization signal (PSS), and secondary synchronization signal (SSS) are consecutive symbols forming an SS / PBCH block. Base station 5 may transmit multiple synchronization signal (SS) blocks corresponding to different DL beams. The total number of SS blocks may be limited, for example, to a duration of 5 ms as an SS burst. The period of SSB transmission may be indicated to the UE using any appropriate signaling (for example, per serving cell using ssb-periodicityServingCell). The period value of the SSB may be, for example, 20 ms or more. In the initial cell selection, UE3 may be configured to assume that SS bursts occur with a 2-frame period.UE3 may also be provided with instructions indicating which SSBs should be sent within a 5ms period (for example, using ssb-PositionsInBurst).

[0073] DL physical signals may include, for example, a reference signal (RS) and a synchronization signal (SS). The reference signal (sometimes known as a pilot signal) is a signal known to both the UE3 and the base station 5 and has a predefined specific waveform. The reference signal may include, for example, a cell-specific reference signal, a UE-specific reference signal (UE-RS), a downlink demodulation signal (DMRS), and a channel state information reference signal (CSI-RS).

[0074] Similarly, UE3 is configured to transmit control information and user data via multiple uplink (UL) physical channels corresponding to REs that transmit information originating from higher layers, and to transmit UL physical signals used in the physical layer that do not transmit information originating from higher layers, and base station 5 is configured to receive these. The physical channels may include, for example, PUSCH, physical uplink control channel (PUCCH), and / or physical random-access channel (PRACH). The UL physical signals may include, for example, demodulation reference signal (DMRS) for UL control / data signals, and / or sounding reference signal (SRS) used for UL channel measurement.

[0075] When UE3 first establishes a radio resource control (RRC) connection with base station 5 via cell 9, it registers with the appropriate core network node (e.g., AMF, MME). UE3 is in a so-called RRC connected state, and the associated UE context is maintained by the network. When UE3 is in a so-called RRC idle state, or RRC inactive state, UE3 selects an appropriate cell to camp in, allowing the network to determine UE3's approximate location (though not necessarily at the cell level).

[0076] Base station 5 may also be a base station 5 divided into one or more distributed units (DUs) 50 and a central unit (CU) 60, where the CU 60 typically performs high-level functions and communication with the next-generation core, and the DU 50 performs low-level functions and communication with nearby UE3s (i.e., within the cell operated by base station 5) via the radio interface. This type of base station 5 is sometimes called a “distributed” base station 5 or gNB5. A distributed gNB5 includes the following functional units: gNB Central Unit (gNB-CU): A logical node that hosts the Radio Resource Control (RRC), Service Data Adaptation Protocol (SDAP), and Packet Data Convergence Protocol (PDCP) layers of the gNB (or the RRC and PDCP layers of the en-gNB), and controls the operation of one or more gNB-DUs. The gNB-CU terminates the so-called F1 interface connected to the gNB-DUs. gNB Distributed Unit (gNB-DU): A logical node that hosts the Radio Link Control (RLC), Medium Access Control (MAC), and Physical (PHY) layers of a gNB or en-gNB, and whose operation is partially controlled by a gNB-CU. A single gNB-DU supports one or more cells. A single cell is supported by only one gNB-DU. A gNB-DU terminates the F1 interface connected to the gNB-CU. gNB-CU-Control Plane (gNB-CU-CP): A logical node that hosts the control plane portion of the RRC and PDCP protocols for the gNB-CU for en-gNB or gNB. gNB-CU-CP terminates the E1 interface connected to gNB-CU-UP and the F1-C (F1 control plan) interface connected to gNB-DU. gNB-CU-User Plane (gNB-CU-UP): This is a logical node that hosts the user plane portion of the PDCP protocol for the gNB-CU for en-gNB, and the user plane portions of the PDCP protocol and SDAP protocol for the gNB-CU for gNB. gNB-CU-UP terminates the E1 interface connected to gNB-CU-CP and the F1-U (F1 user plane) interface connected to gNB-DU.

[0077] When a distributed base station or a similar control plane-user plane (CP-UP) partition is employed, it will be understood that the control plane entity and the user plane entity may each include associated transceiver circuits, antennas, network interfaces, controllers, memory, operating systems, and communication control modules. If base station 5 is composed of distributed base stations, the network interfaces also include E1 and F1 interfaces (F1-C in the case of the control plane and F1-U in the case of the user plane) for communicating signals between the various functions of the distributed base station.

[0078] Frame structure Figure 2 shows a typical frame structure that may be used in communication system 1, where base station 5 and UE3 of communication system 1 communicate with each other using resources organized into frames with a time domain length of 10 ms. Each frame contains 10 subframes of equal size, each with a length of 1 ms. Each subframe is divided into one or more slots containing 14 orthogonal frequency-division multiplexing (OFDM) symbols of equal length.

[0079] As shown in Figure 2, communication system 1 supports several different numerologies (subcarrier spacing (SCS), slot length, and OFDM symbol length). Specifically, each numerology is identified by the parameter μ, where μ=0 represents 15kHz (corresponding to LTE SCS). Currently, the SCS for other values ​​of μ can be practically derived by scaling up from μ=0 to a power of 2 (i.e., SCS = 15 × 2μkHz). The relationship between the parameter μ and SCS(Δf) is shown in Table 1. [Table 1]

[0080] (R)AN node DU Figure 3 is a schematic block diagram showing the main components of DU50, which may be used as part of (R)AN node 5 of the communication system 1 shown in Figure 1. As shown, DU50 includes a transceiver circuit 451 for transmitting signals to and receiving signals from a communication device (such as UE3) via a radio unit (RU) and associated DU-RU interface 453, and a transceiver circuit 451 for transmitting signals to and receiving signals from CU60 of (R)AN node 5 via a CU interface 454 (including an F1 interface which may be divided into F1-U interface and F1-C interface for user plane signaling and control plane signaling, respectively).

[0081] The DU50 includes a controller 457 that controls the operation of the DU50. The controller 457 is connected to memory 459. Software may be pre-installed in memory 459 and / or downloaded, for example, via communication network 1 or from a removable data storage device (RMD). In this example, the controller 457 is configured to control the overall operation of the DU50 by program instructions or software instructions stored in memory 459.

[0082] As shown in the figure, these software instructions include, among other things, the operating system 461, the communication control module 463, the F1 module 465, the DU-RU module 468, the DU management module 472, the UE profile management module 473, and the mobility module 475.

[0083] The communication control module 463 is operable to control communication between DU50 and one or more RUs (i.e., between DU50 and UE3) and between DU50 and CU60. The communication control module 463 is configured to have overall control over the reception of signals corresponding to uplink communication from UE3 and to handle the transmission of downlink communication to UE3.

[0084] The F1 module 465 is responsible for the proper processing of signals received from or sent to the CU60 via one or more CU (e.g., F1) interfaces 454. These signals may be divided into user plane signals received from or sent to the CU-UP portion of the CU60 via the F1-U interface, and control plane signals received from or sent to the CU-CP portion of the CU60 via the F1-C interface.

[0085] The DU-RU module 468 is responsible for the proper processing of signals received from or sent to an RU via one or more RU (e.g., DU-RU) interfaces 453.

[0086] The DU management module 472 is responsible for managing the overall operation of the DU50 and the overall performance of the tasks required of the DU50. These tasks include generating and sending appropriate messages using the appropriate signaling application protocol, depending on the division of functions between the RU, DU50, and CU60, such as interpreting received MAC signaling and generating MAC signaling for transmission. The DU management module 472 may, if necessary, control the overall operation of the DU50 in one of the methods described below.

[0087] The UE profile management module 473 is responsible for performing functions related to the UE profile, which include (where applicable) receiving and storing the UE profile or associated assistance / preference information from UE3 or other locations in the network, (where applicable) determining appropriate mobility-specific settings based on the UE profile / assistance / preference information for implementation on UE3 and / or RAN equipment, and / or (where applicable) providing configuration information for appropriately configuring the UE with mobility-based settings. The UE profile management module 473 may, for example, store historical mobility information of UE3 (e.g., past movements of UE3 between different communication cells in the network). Depending on the implementation, it will be understood that the gNB-DU may not have to implement at least some of these functions.

[0088] The mobility module 475 is responsible for controlling one or more mobility procedures of the UE3. For example, the mobility module 475 may be configured to perform one or more measurements of the UE3's mobility or to select a candidate cell for a handover, according to one of the methods described below.

[0089] CU Figure 4 is a schematic block diagram showing the main components of the CU60 RAN equipment of the communication system 1 shown in Figure 1. As shown, the CU60 includes transceiver circuits 551 for transmitting signals to and receiving signals from the DU50 via one or more DU interfaces 554 (including an F1 interface which may be divided into F1-U interfaces and F1-C interfaces for user plane signaling and control plane signaling, respectively), and transceiver circuits 551 for transmitting signals to and receiving signals from the functions of the core network 7 via one or more core network interfaces 555 (including, for example, N2 interfaces and N3 interfaces).

[0090] CU60 includes a controller 557 that controls the operation of CU60. The controller 557 is connected to memory 559. Software may be pre-installed in memory 559 and / or downloaded, for example, via communication network 1 or from a removable data storage device (RMD). In this example, the controller 557 is configured to control the overall operation of CU60 by program instructions or software instructions stored in memory 559.

[0091] As shown in the figure, these software instructions include the operating system 561, communication control module 563, F1 module 565, E1 module 566, N2 module 568, N3 module 569, CU-UP management module 571, CU-CP management module 572, UE profile management module 573, and mobility module 575, among others. The function of mobility module 575 is the same as that described above with reference to Figure 3.

[0092] The communication control module 563 is operable to control communication between the CU60 and one or more DU50s (i.e., between the CU60 and the UE3), and between the CU60 and the core network 7. The communication control module 563 is configured to have overall control over the reception of signals corresponding to uplink communication from the UE3 and to control the transmission of downlink communication.

[0093] The F1 module 565 is responsible for the proper processing of signals received from or transmitted to the DU 50 via one or more DU (e.g., F1) interfaces 554. These signals include user plane signals received by or transmitted to the CU-UP portion of the CU 60 via the F1-U interface, and control plane signals received by or transmitted to the CU-CP portion of the CU 60 via the F1-C interface.

[0094] The E1 module 566 is responsible for the proper processing of signals transmitted between the CU-UP portion and the CU-CP portion of the CU60 via the corresponding internal CU (e.g., E1) interface.

[0095] The N2 module 568 is responsible for the proper processing of signals received from or transmitted to the AMF10-1 via one or more corresponding core network (e.g., N2) interfaces 555.

[0096] The N3 module 569 is responsible for the appropriate processing of signals received from or transmitted to one or more core network user plane functions via one or more corresponding core network (e.g., N3) interfaces 555.

[0097] The CU-UP management module 571 is responsible for the overall operation of the CU-UP portion of the CU60 and for managing the overall performance of the tasks required for CU-UP.

[0098] The CU-CP management module 572 is responsible for the overall operation of the CU-CP portion of the CU60 and for managing the overall performance of tasks required by the CU-CP. These tasks include generating and sending appropriate messages using the appropriate signaling application protocol, such as interpreting received RRC signaling and generating RRC signaling for transmission, depending on the functional division between the RU, DU50, and CU60.

[0099] The UE profile management module 573 is responsible for performing functions related to the UE (mobility) profile, which (where applicable) includes receiving and storing the UE profile or associated assistance / preference information from UE3 or other locations in the network, determining appropriate mobility-specific settings based on the UE profile / assistance / preference information for implementation in UE3 and / or RAN equipment 5, and / or providing configuration information for properly configuring the UE with mobility-based settings. The UE profile management module 573 may also store historical mobility information of UE3 (e.g., past movements of UE3 between different communication cells in the network). Depending on the implementation, it will be understood that CU60 may not have to implement at least some of these functions.

[0100] System information and SIB It will be understood that transmissions in cell 9 of base station 5 may include one or more broadcast transmissions, one or more unicast transmissions for reception by UE3, and / or one or more multicast transmissions for reception by a group of UE3. System information (SI) transmitted within the cell may include "minimum SI" (MSI) and "other SI" (OSI). OSI may be broadcast on demand, for example, using a downlink shared channel (DL-SCH). OSI may be broadcast in response to a request from a UE3 that is in a radio resource control (RRC) idle or RRC inactive state. OSI may also be requested by a UE3 that is in an RRC connected state, for example, via one or more dedicated RRC transmissions.

[0101] The SI may include information that enables UE3 to complete cell selection (e.g., to configure settings), information that enables UE3 to complete cell re-selection procedures, or information that enables UE3 to receive one or more paging messages transmitted within a cell. The SI may be broadcast using a Master Information Block (MIB) and one or more System Information Blocks (SIBs).

[0102] The MSI includes the MIB and system information block 1 (SIB1). The MIB includes information that UE3 uses to receive SIB1, such as the subcarrier spacing of SIB1. The MIB provides information corresponding to the Control Resource Set (CORESET) and Search Space. SIB1 is sometimes referred to as the “remaining MSI” (RMSI). SIB1 may be transmitted in a dedicated RRC message, and other SIBs (e.g., SIB2-SIB9) may be transmitted using one or more other appropriate RRC transmissions (e.g., another dedicated RRC message). The MIB and SIB1 may provide UE3 with scheduling information instructions for receiving and decoding other SIBs such as SIB2-SIB9, and may provide information that UE3 uses to receive one or more paging messages. The OSI may include SIB2-SIB9 transmitted using DL-SCH in the SI message, for example. The mapping between SIB2-SIB9 and the corresponding SI message may be provided to UE3 by the base station 5. MIB and SIB1-SIB9 are described in more detail, for example, in 3GPP TS38.331. SIB2 provides information on intra-frequency, inter-frequency, and inter-system cell reselection. SIB3 provides cell-specific information on intra-frequency cell reselection. SIB4 provides information on inter-frequency cell reselection. SIB5 provides information on inter-system cell reselection for 4G (LTE). SIB6 and SIB7 provide information on earthquake and tsunami warning systems (ETWS). SIB8 provides information on commercial mobile alert service (CMAS) notifications, for example, for sending warning text messages to UE3.SIB9 includes information regarding coordinated universal time (UTC), global positioning system (GPS) time (e.g., for GPS initialization), and local time.

[0103] SIBs may be broadcast periodically (for example, according to a predetermined periodic pattern) or provided "on demand" upon request from, for example, UE3. For example, MIBs may be transmitted with a period of 80ms and repeated within 80ms, while SIB1 may be transmitted with a period of 160ms and a variable transmission repetition period of 160ms or less (for example, 20ms). SIB1 can be used to indicate to UE3 which SIBs are transmitted periodically and which SIBs are available on demand upon request from UE3. UE3 may be configured to request on-demand SIBs using message 1 (MSG1), also called an MSG1-based on-demand SI request, or message 3 (MSG3), also called an MSG3-based on-demand SI request.

[0104] A physical broadcast channel (PBCH) can be used to broadcast MIBs. Base station 5 may transmit a PBCH with a synchronization signal (SS) (e.g., a primary synchronization signal (PSS) and a secondary synchronization signal (SSS)) in an SS / PBCH block. An SS / PBCH block may comprise four orthogonal frequency-division multiplexed (OFDM) symbols mapped to the PSS, SSS, and PBCH associated with a demodulation reference signal (DM-RS). In the frequency domain, an SS / PBCH block consists of 240 consecutive subcarriers. If UE3 is in an RRC connection state, base station 5 may provide UE3 with instructions for the resources to be used for the SS / PBCH, for example, using dedicated signaling. SIB1 may be transmitted using a physical downlink shared channel (PDSCH). OSI may similarly be transmitted using a PDSCH, for example. If one or more beamformed transmissions are transmitted within a cell provided by base station 5, some of the SIs (e.g., some of the SIBs) may be transmitted using only a specific beam or using only a specific transmission / reception point (TRP).

[0105] UE Mobility Figure 5 outlines a possible mobility procedure performed in the type of communication system 1 shown in Figure 1. In this example, a handover of UE3 from source base station 5 to target base station 5 is performed.

[0106] In the optional step S501, UE3 performs a measurement. The measurement may be a measurement of the signal transmitted from source base station 5 or a measurement of the signal transmitted from target base station 5. The measurement may also be a measurement of signal strength, which can be used as part of UE3's decision to hand over from source base station 5 to target base station 5. In the optional step S502, UE3 sends a measurement report to source base station 5 providing instructions on the results of the measurement. The measurement report may be sent from UE3 to source base station 5 in an RRC message. In this example, source base station 5 uses the information provided in the measurement report to decide whether UE3 should hand over to target base station 5. However, it will be understood that the decision to perform a handover to target base station 5 may alternatively (or additionally) be based on a measurement performed at source base station 5 or target base station 5. Alternatively, UE3's decision to perform a handover may be based on factors other than signal measurement, such as the congestion level of the cell operated by source base station 5.

[0107] In step S503, the source base station 5 sends a handover request to the target base station 5, requesting a handover of the UE3 from the source base station 5 to the target base station 5. The handover request may include, for example, the identity of the source base station 5, the cause value of the handover, the identity of the target cell, context information of the UE3 (e.g., the maximum bitrate of the UE3, or the security capabilities of the UE3), and historical information of the UE. If the handover was triggered by a measurement report received by the source base station 5 in step S502, the cause value may indicate, for example, that a handover is desirable for radio reasons. Alternatively, if the handover was triggered to reduce the load on the source base station 5, the cause value may indicate that the handover is to reduce the load on the serving cell. The handover request message may also include instructions from the AMF10-1 providing service to the UE3.

[0108] In step S504, the target base station 5 sends a handover request acknowledgment message (also called a “handover request acknowledgement” message). The handover request acknowledgement message contains handover configuration information to be forwarded to the UE3. The handover request acknowledgement message also contains configuration information to enable the source base station 5 to begin forwarding user plane data for the UE3 to the target base station 5.

[0109] The transmissions in steps S503 and S504 may be performed via the Xn interface between the source base station 5 and the target base station 5 (therefore, the handover procedure in this example is also called an Xn-based handover procedure). Steps S501 to S504 are also called the "handover preparation phase".

[0110] In step S505, the source base station 5 transmits handover configuration information to the UE3. The configuration information for handover may be, for example, the RRC configuration transmitted in an RRC configuration message or an RRC reconfiguration message. In step S506, the UE3 applies the received handover configuration and transmits an instruction to the target base station 5 indicating that the handover configuration is complete. The message transmitted in step S505 may be, for example, an RRC Reconfiguration Complete message. Steps S505 and S506 are also called the "handover execution phase".

[0111] Following the handover execution phase, UE3 can operate to send uplink transmissions (e.g., uplink data) to target base station 5 and receive downlink transmissions (e.g., downlink data) from target base station 5.

[0112] It will be understood that the mobility methods and handover procedures for UE3 are not limited to the example shown in Figure 5. For example, UE3 may be configured to perform a conditional handover (CHO) that determines whether or not to perform a handover to a candidate cell of UE3 based on one or more execution conditions. It will also be understood that a handover may be performed in which DU50 changes but CU60 remains the same (inter-DU intra-CU handover), in which both DU50 and CU60 change (inter-DU inter-CU handover), or between two cells operated by the same DU50.

[0113] Random access Figure 6 shows a random access (RA) procedure that can be performed in the system of Figure 1. The RA procedure can be used, for example, for initial access by UE3 in RRC idle mode, or for the transition from RRC inactive mode to RRC connected mode. The RA procedure may also be used for initial access to the target base station 5 during the handover of UE3 from the source base station to the target base station (for example, the handover procedure described above, see Figure 5).

[0114] In step S601, UE3 transmits a random access preamble to base station 5. In this example, UE3 selects a random access preamble to transmit from a group of random access preambles shared with other UE3s. The transmission in step S601 is also called message 1 (MSG1) and is transmitted using PRACH.

[0115] In step S602, base station 5 transmits a random access response to UE3. The transmission in step S602 is also called message 2 (MSG2). The random access response indicates the time and / or frequency resources (e.g., resource blocks and / or symbols) that UE3 will use for subsequent transmissions to base station 5. The random access response may also include further information that UE3 will use for communication with base station 5, such as a timing advance (TA) value.

[0116] In step S603, UE3 sends a transmission to base station 5 using the specified time and / or frequency resources. The transmission in step S603 is also called message 3 (MSG3). The transmission in step S603 may be a layer 2 (L2) or layer 3 (L3) message. The transmission in step S603 may include, for example, an RRC setup request, an RRC resume request, an RRC reestablishment request, or an RRC reconfiguration complete message.

[0117] If two UE3s select and transmit the same random access preamble in step S601, and receive and decode MSG2 transmitted from base station 5 in step S602, the two UEs may transmit MSG3 using the same time and / or frequency resources. This situation is also called "contention" or "collision". To resolve this contention, in step S604, base station 5 sends a content resolution message to UE3. The transmission in step S604 is also called message 4 (MSG4). MSG4 indicates to UE3 whether the MSG3 transmitted from UE3 in step S603 was received and successfully decoded by the base station. If base station 5 decodes an MSG3 transmitted by another UE3 that is competing with UE3, or if interference occurs between the MSG3s transmitted by the two UE3s, the MSG3 transmitted in step S603 may not have been received or successfully decoded by base station 5. If MSG3 transmitted by UE3 is not decoded by base station 5 (UE3 can determine this if it did not receive MSG4 from base station 5), UE3 returns to step S601 of this method and transmits another MSG1 to base station 5 (for example, after selecting a different random access preamble).

[0118] The procedure shown in Figure 6 is an example of a contention-based RA procedure, in which UE3 selects a random access preamble from a group of preambles that other UE3s can also use (therefore, if two UE3s select the same random access preamble, a conflict may occur). Alternatively, base station 5 may send a random access preamble assignment to UE3 before UE3 sends MSG1 to base station 5, in which case the RA procedure is contention-free (it is not necessary to perform the conflict resolution in step S604). The random access preamble assignment may be sent to UE3 using an RRC message or layer 1 (L1) signaling (for example, using DCI carried in PDCCH). In the method shown in Figure 5, in step S505, a random access preamble assignment for communication with target base station 5 may be sent to UE3.

[0119] MSG1 and / or MSG3 may be used by UE3 to request an on-demand SI from base station 5.

[0120] Lower-layer mobility procedures When a UE3 moves from one cell to another, a change in the serving cell may be required. This serving cell change is triggered by layer 3 (L3) measurements and can be achieved using radio resource control (RRC) signaling. However, this process involves resetting layer 1 (L1) and layer 2 (L2), resulting in increased latency, increased overhead, and longer downtime. In the case of inter-cell mobility, the UE may need to perform reconfiguration and downlink / uplink (DL / UL) synchronization to the target cell. To achieve a more efficient handover, a lower-layer (L1 or L2) based handover may be used. The UE3 and base station can be configured to perform an intra-CU LTM procedure, allowing the UE3 to switch between pre-configured candidate lower-layer triggered mobility (LTM) cells relatively quickly (e.g., without requiring RRC reconfiguration) based on the contents of the lower-layer (L1 and / or L2) measurement report. Therefore, when UE3 moves around pre-configured candidate LTM cells, it can perform fast cell switching without requiring RRC reconfiguration.

[0121] Conditional Handover (CHO) A Conditional Handover (CHO) is a handover performed by the UE3 when one or more handover execution conditions are met. When the UE3 receives the CHO setting (from the network, e.g., base station 5), it begins evaluating one or more execution conditions, and when the handover is performed, it stops evaluating one or more execution conditions. The execution conditions may be based on measurements of reference signal received power (RSRP), reference signal received quality (RSRQ), and RSRP and signal to noise interference ratio (RSRP-SINR), which are performed by the UE3. In the case of layer 1 / layer 2 (L1 / L2) mobility, the handover is initiated based on the L1 / L2 measurement results.

[0122] The following describes an example of a CHO. A "CHO ​​candidate cell" is a candidate cell for a CHO and has a corresponding CHO configuration. The CHO configuration includes the configurations of one or more CHO candidate cells generated by the candidate base station 5 and one or more execution conditions generated by the source base station 5.

[0123] The execution conditions may include one or two trigger conditions, which may also be called CHO events.

[0124] Similar to intra-NR RAN handover, in intra-NR RAN CHO, the preparation and execution phases of the conditional handover procedure can be performed without the involvement of the core network; that is, preparation messages are exchanged directly between base stations 5. The release of resources at the source base station during the conditional handover completion phase is triggered by the target base station 5.

[0125] In the CHO scheme, source base station 5 may decide that CHO should be used. Source base station 5 may request CHO from one or more candidate cells belonging to one or more candidate base stations 5. Subsequently, CHO request messages may be sent to each candidate cell.

[0126] Candidate base station 5 sends a CHO response to source base station 5 that includes the configuration of one or more CHO candidate cells. The CHO response message may be sent to each candidate cell.

[0127] Source base station 5 may send an RRC Reconfiguration message to UE3 containing the settings for one or more CHO candidate cells and one or more CHO execution conditions.

[0128] UE3 may send an RRC Reconfiguration Complete message to source base station 5.

[0129] If early data forwarding is applied, source base station 5 may send an early status transfer message.

[0130] After receiving the CHO configuration, UE3 maintains its connection with the source base station and begins evaluating the CHO execution conditions for one or more candidate cells. If at least one CHO candidate cell meets the corresponding CHO execution conditions, UE3 completes the RRC handover procedure by detaching from the source base station, applying the stored corresponding configuration to the selected candidate cell, synchronizing with that candidate cell, and sending an RRC Reconfiguration Complete message to target base station 5. After the handover procedure is successfully completed, UE3 releases the stored CHO configuration.

[0131] The target base station 5 sends a handover success message to the source base station 5 to notify it that UE3 has successfully accessed the target cell. In response, the source base station 5 sends a sequence number status transfer message (e.g., SN STATUS TRANSFER).

[0132] Subsequently, source base station 5 may cancel the CHO for UE3 by sending a handover cancel message to any other signaling connections or other candidate target base stations, if any.

[0133] Conditional configurations for conditional handovers may be provided as "delta configurations" for the serving cell configuration. In other words, the parameters and settings of the conditional configuration can be shown by indicating the difference between the conditional configuration and the serving cell configuration.

[0134] UE3 may be configured to indicate to another entity in network 1 that UE3 supports conditional handover, for example, by sending a signal containing instructions in a conditional handover field or information element.

[0135] The CHO candidate cell list can be used to indicate a list of candidate target cells for conditional handover. Candidate target cells for CHO are also called CHO candidates. For example, up to eight candidate cells associated with conditional handover execution conditions may be configured in UE3. The number of execution conditions may be two (or one, or three or more execution conditions may be used). UE3 executes CHO towards the selected target cell when the conditions are met by applying the corresponding conditional reconfigurations. This improves mobility robustness because CHO settings can be sent before the quality of the serving cell deteriorates, and UE3 can avoid mobility failures due to HO command failures.

[0136] intra-CU inter-DU (inter-CU) mobility Figure 7 shows an example of intra-CU inter-DU mobility.

[0137] In this case, the current serving cell and the candidate cell share the same CU. Because the source cell and target cell are served by different DUs, the radio link control (RLC) layer is re-established and the medium access control (MAC) layer is reset.

[0138] Figure 8 illustrates an exemplary method of inter-DU mobility. It shows the procedure for L1 / L2-based inter-cell mobility from source DU50a to target DU50b. As shown in the figure, this method includes a pre-configuration stage, an early-synchronization stage, and a cell switch stage, which are described later.

[0139] Before the LTM procedure is triggered, UE3 communicates user data via source DU50a and its associated CU60.

[0140] Pre-configuration The UE3, source DU50a, and associated CU60 perform L3 measurement control and reporting procedures in a pre-configured state. In this procedure, the UE3 typically sends an L3 (e.g., RRC) measurement report (e.g., a "MeasurementReport" message) to the source DU50a, which includes measurement results for one or more cells (e.g., measurement results for the serving cell and / or reference signals in one or more adjacent cells). The measurement results may include, for example, (but are not limited to) L3 filtered measurement results for the beam and / or cells. The source DU50a then sends an appropriate message (e.g., a UL RRC Message Transfer message as shown in Figure 8) to transmit the received measurement report to the CU60.

[0141] In steps 1 and 2, UE3 sends a layer 3 (L3) measurement report to source DU50a based on the measurement configurations. The measurement report is then forwarded to CU60.

[0142] In steps 3-8, CU60 determines a candidate set for UE3, sends a preparation request to target DU50b, and receives a corresponding acknowledgment from target DU50b. Subsequently, CU60 sends an RRC reconfiguration to UE3 and receives a corresponding RRC reconfiguration complete message via source DU50a.

[0143] In other words, CU60 sends one or more messages (e.g., a UE Context Setup Request, as shown in step 3 of Figure 8) requesting the setup of a context for UE3 to one or more candidate DUs containing the (candidate) target cells. This message is, in effect, a request for LTM setup at the receiving DU. If the candidate DU accepts the request for LTM setup for one or more (candidate) target cells, the candidate DU responds to CU60 with an appropriate response message (e.g., a UE Context Setup Response, as shown in step 4 of Figure 8) containing the generated lower layer RRC configuration for one or more accepted target candidate cells. It will be understood that at this stage, one or more UE context setup procedures may be used. In the illustrated procedure, target DU50b performs LTM candidate cell preparation and responds with a response message containing generated lower-layer RRC settings for one or more accepted target candidate cells of target DU50b.

[0144] CU60 sends an appropriate message to source DU50b, which includes the generated RRC reconfiguration message containing the L1 / L2 triggered mobility configuration. This message may be other messages, such as a UE Context Modification Request or a DL RRC Message Transfer message as shown in step 5 of Figure 8. Source DU50a forwards the received RRC reconfiguration message to UE3 in step 6 of Figure 8. The RRC reconfiguration message includes LTM candidate cell configurations, which include the corresponding cell Radio Network Temporary Identifiers (C-RNTI). UE3 responds with an RRC reconfiguration complete message in step 7 of Figure 8.

[0145] In step 8 of Figure 8, source DU50a forwards an RRC reconfiguration completion message to CU60 using an appropriate message. This message may be any other message, such as a UE Context Modification Response message or the UL RRC Message Transfer message shown in Figure 8.

[0146] Early synchronization In optional steps 9-11, UE3 performs L1 measurements and reports of reference signals (e.g., SSB or CSI-RS shown in Figure 10) corresponding to inter-cell beams, based on configurations from the network. Based on the L1 measurement report, network 1 may activate a transmission configuration information (TCI) state in which the physical cell ID (PCI) is quasi-co-located (QCL-ed) with a cell different from the serving cell. UE3 then performs synchronization (DL and optionally UL) of these cells.

[0147] Cell switch Following LTM pre-configuration and early-sync, UE3 and base station 5 execute the LTM cell switch procedure shown in Figure 8 to switch to the target DU50b's cell. Once UE3 accesses the new cell of target DU50b and target DU50b detects the access, target DU50b can notify CU60 of the successful access. Subsequently, UE3 can communicate user data via target DU50b and the associated CU60.

[0148] In steps 12-13 of Figure 8, DU50 may indicate the target cell and beam (TCI state) based on further L1 reports. UE3 applies the target cell configurations. In step 14, if a timing advance (TA) is unavailable, UE3 may perform a random access channel (RACH) procedure for the indicated target cell. In steps 15 and 16, UE3 receives the PDCCH from the target cell using the new TCI state.

[0149] In step 12 of Figure 8, UE3 may send a lower layer measurement report (including, for example, one or more L1 / L2 measurement results) to source DU50a (for example, for a service cell and / or one or more target / candidate cells).

[0150] Next, source DU50a decides to perform an LTM to switch to a candidate target cell. In other words, source DU50a makes an LTM handover decision. It will be understood that DU50a may also notify other nodes of the LTM cell switch decision. In step 13 of Figure 8, source DU50a sends an LTM cell switch command to UE3. It will be understood that the decision to switch to a specific candidate target cell (and the notification of the LTM cell switch decision to other nodes) may be made after source DU50a sends the LTM cell switch command to UE3 in step 13 (for example, using a MAC control element (CE)).

[0151] This allows UE3 to detach from the current source DU50a cell (and synchronize to the target cell of target DU50b if necessary). Then, in step 14, UE3 may perform a random access channel (RACH) based initial access procedure or a RACH-less initial access procedure with respect to target DU50b.

[0152] Furthermore, source DU50a may notify CU60 (for example, via an F1 interface using the F1 application protocol (F1AP)) about initiating LTM / sending LTM commands to UE3. This notification may be sent in parallel with (or after) UE3 detaching from the current source DU50a cell / synchronizing to the target cell of target DU50b. Target DU50b can detect UE access and notify CU60 of successful access. Subsequently, UE3 can communicate user data via target DU50b and associated CU60 (as shown in Figure 8).

[0153] As those skilled in the art will understand, in the case of inter-DU LTM, the release of resources in the source cell (and any prepared cell) in source DU50a can be achieved (if possible) in an appropriate manner.

[0154] The detailed steps of the procedure described with reference to Figure 8 will be understood to be illustrative and provided for the purpose of illustrating how the procedure is performed. As is known to those skilled in the art, there are various variations of this procedure, particularly the LTM pre-configuration and / or LTM cell switch portions.

[0155] Inter-cell inter-DU (inter-cell DU) Figure 9 shows the inter-cell inter-DU method. Step 1 involves performing a UE context setup / modification (for example, the CU sends a UE context / setup modification request message to the target DU). Step 2 involves performing RRC Reconfiguration (handover preparation). Step 3 involves performing DL synchronization. In step 4, source and target cell L1 measurement reports (SSB-RSRP or SSB-SINR) are sent from UE3 to source DU50a. In step 5, a determination is made as to whether the HO conditions are met, and the best cell / beam for HO is identified. In step 6, the physical downlink control channel (PDCCH) for handover to the target cell (which may include the target cell index, beam index, or TCI status) is transmitted from source DU50a to UE3. Step 7 includes UL Synchronization (which may include sending timing advance information as described below) and an optional RACH procedure.

[0156] Base station triggered L1 mobility Figure 10 shows the L1 mobility method of base station triggers, including measurement report filtering. Steps 1-4 in Figure 10 correspond to steps 1-4 in Figure 9. In step 5, an L1 measurement report reconfiguration (which may include one or more filtering parameters) is sent from CU60 to source DU50a. In step 5.1, L1 measurement report filtering is performed on source DU50a. Step 5.2 in Figure 10 corresponds to Step 5 in Figure 9. Steps 6 and 7 in Figure 10 correspond to steps 6 and 7 in Figure 9.

[0157] Handover Failure (HOF) As part of the handover procedure, UE3 may start a timer (e.g., T304) when it receives a handover command (i.e., reconfigurationWithSync) contained in an RRC Reconfiguration message from base station 5. UE3 attempts to access the target cell in accordance with the handover command. However, if UE3 is unable to successfully access the target cell before the timer expires, UE3 may experience a handover failure. As a fallback to address a handover failure, if the timer expires, UE3 attempts to restore network connectivity by performing a cell reselection and initiating an RRC re-establishment procedure to the reselected cell.

[0158] Radio Link Monitoring (RLM) and Radio Link Failure (RLF) UE3 may be configured to perform one or more radio link monitoring (RLM) procedures to monitor the radio link of communication over the primary serving cell (PCell) of the master cell group (MCG). If a secondary cell group (SCG) is configured in UE3, UE3 may use the RLM procedure for communication over the primary SCG cell (PSCell).

[0159] UE3 uses the physical layer to perform measurements for RLM. UE3 may perform measurements to monitor the status of serving cells and may also perform measurements for candidate cells for lower-layer triggered mobility. Measurement results may be passed to both the MAC and RRC layers of UE3. Radio link failure (RLF) can be detected using measurements and the RRC layer. In other words, the RRC layer evaluates the conditions for RLF based on measurements performed by UE3. If it is determined that an RLF has occurred, the corresponding RLF procedure is triggered, and RRC re-establishment is triggered. Configuration information regarding beam failure and beam failure recovery parameters may also be passed from the RRC layer to the MAC layer. Configuration information regarding UE3 measurements may also be passed from the RRC layer to the physical layer, for example, by providing a set of radio link monitoring reference signal resources (RLM-RS). The RLM-RS may include one or more SS / PBCH blocks (SSBs) and / or one or more channel state information reference signals (CSI-RS).

[0160] RLF can occur, for example, due to congestion within the cell of base station 5, or due to changes in radio conditions (e.g., bad weather, obstacles between UE3 and base station 5). In response to RLF detection, UE3 may stop transmitting one or more uplink transmissions (for example, within 40ms after RLF detection) to avoid uplink interference.

[0161] UE3 is configured to generate a first indication (also called an Out-of-sync indication) if the radio link quality of all monitored reference signals in the cell is worse than a first threshold quality (e.g., corresponding to the block error rate (BLER)). Similarly, UE3 is configured to generate a second indication (also called an In-sync indication) if the radio link quality of at least one monitored reference signal in the cell is better than a second threshold quality. Out-of-sync and In-sync indications are forwarded to the RRC layer. The RRC layer uses these indications to determine whether an RLF has occurred. When the RRC layer receives a predetermined number of Out-of-sync indications, the RLF timer is started. This timer is also called "T310," and the predetermined number of Out-of-sync indications is also called "N310." When the RRC layer receives a predetermined number of In-sync indications, the RLF timer is stopped. The predetermined number of In-sync indications is also called "N311." If the RLF timer expires before the RRC layer receives a predetermined number of in-sync instructions, UE3 determines that an RLF has occurred. The value of the RLF timer, the predetermined number of out-of-sync instructions, and the predetermined number of in-sync instructions may be set by the network (for example, transmitted from base station 5 to UE3). If UE3 measures that the quality of the reference signal received is between a first threshold quality and a second threshold quality, UE3 may not generate any out-of-sync or in-sync instructions during a particular measurement and evaluation period.

[0162] Figure 11 illustrates how radio link recovery occurs. As shown in Figure 11, the RRC layer receives a predetermined number (N310) of out-of-sync indications from the lower layers. This starts the RLF timer (T310). In this example, the RRC layer receives a predetermined number (N311) of in-sync indications before the RLF timer expires, so the UE3 determines that RLF has not occurred (i.e., radio link recovery has occurred).

[0163] Figure 12 illustrates how a radio link failure is determined. As shown in Figure 12, the RRC layer receives a predetermined number (N310) of out-of-sync indications from the lower layers. This starts the RLF timer (T310). In this example, the RRC layer received fewer than a predetermined number (N311) of in-sync indications before the RLF timer expired, so the UE3 determines that an RLF has occurred.

[0164] UE3 may be configured to determine that an RLF has occurred based on the number of retransmissions (e.g., RLC retransmissions) exceeding a threshold. Alternatively, UE3 may be configured to determine that an RLF has occurred based on the number of preamble transmissions (or retransmissions) during the RA procedure exceeding a threshold.

[0165] Following detection of an RLF in the primary serving cell, the UE may initiate an RRC connection re-establishment procedure (which may include a random access procedure). Following detection of an RLF in the PSCell, UE3 may provide an indication of the RLF failure to the corresponding RAN node via the MCG cell (for example, by sending SCG Failure Information).

[0166] If UE3 is in an RRC connection state, UE3 may perform RLM in the active bandwidth part (BWP) based on the reference signal (SSB / CSI-RS) and the signal quality threshold set by the network. SSB-based RLM is based on the SSB associated with the initial DL BWP and may be set for the initial DL BWP and DL BWPs including the SSB associated with the initial DL BWP. After the RLF is determined, UE3 may maintain the RRC connection state.

[0167] If an RLF is determined in the target cell for a handover, UE3 may select a suitable cell and initiate the RRC re-establishment procedure. If a suitable cell is not found within a certain time after the RLF is determined, UE3 enters the RRC idle state. In a conditional handover, if an RLF occurs in the source cell, UE3 may be configured to select a suitable target candidate cell and attempt to perform a CHO. Otherwise, it may perform the RRC re-establishment procedure.

[0168] Timing Advance (TA) UE3 may be provided with timing advance (TA) information, for example, as a "targetTA" information element, which refers to a timing adjustment indication that shows the value of the timing offset (NTA) between uplink and downlink radio frames for use by UE3 for the target timing advance group (TAG) (e.g., primary TAG (PTAG) in the case of a handover, primary secondary TAG (PSTAG) in the case of a secondary cell group (SCG) change). A TAG is a group of cells that share the same uplink transmission timing (e.g., a group of cells provided by the same RAN node). A time alignment timer (e.g., timeAlignmentTimer) may be set to define the maximum time between when UE3 receives TA information (e.g., TA commands) from base station 5 and when UE3 is considered synchronized for uplink transmissions within a cell. In other words, UE3 is considered synchronized for UL transmissions in a particular cell while the corresponding time alignment timer is operating. If the time alignment timer expires (because UE3 did not receive TA information from base station 5 during timer operation), UE3 can determine that it is no longer synchronized with the uplink transmission in the corresponding cell. Synchronization can be restored, for example, using the random access procedure described above, see Figure 6. It will be understood that in some cells, it may not be necessary to provide TA information to UE3 in order to achieve synchronization. For example, in small cells, the propagation delay of the transmission between UE3 and base station 5 may be negligibly small.

[0169] Each TAG may include at least one serving cell with a configured uplink, and the mapping between each serving cell and TAG may be configured by the RRC. For primary TAGs, UE3 may use a PCell as the timing reference, except for shared spectral channel access, where SCell may also be used, depending on the case. For secondary TAGs, UE3 may use one of the TAG's activated SCells as the timing reference cell.

[0170] Timing advance is used to control the UL transmit timing of UE3 (e.g., PUSCH and PUCCH) and improve the synchronization of communication between UE3 and base station 5. UE3s farther from base station 5 may be configured to use larger TA values ​​to compensate for the radio signal propagation delay between UE3 and base station 5. The TA value corresponds to the time difference between the start of the uplink radio frame transmitted by UE3 and the corresponding downlink radio frame received by UE3. The TA value may be set to be equal to (or approximately equal to) twice the propagation delay between UE3 and base station 5 plus an additional time offset (an additional time offset corresponding to the NTA). In other words, the TA value may be TA = (2 × propagation delay) + NTA × Tc, where the unit of NTA is Tc and equal to 1 / (480000 × 4096) seconds. The NTA value used by UE3 may be broadcast within the cell of base station 5 (e.g., using SIB1) or transmitted to UE3 using dedicated signaling. As UE3 moves around the cell, it will be understood that the propagation delay of signals transmitted between UE3 and base station 5 may be affected depending on whether UE3 is moving towards or away from base station 5, and therefore the TA may need to be updated. The TA value may be updated by sending a change in the TA value to UE3. For example, base station 5 may send an instruction to decrease the TA value by 17 μs. Alternatively, for example, the absolute value of the new TA value may be explicitly sent to UE3. Base station 5 may be configured to determine the new TA value based on the uplink transmission received from UE3. Timing advance updates can be signaled from base station 5 to UE3 using MAC CE commands.

[0171] RLF in SpCell UE3 may communicate using Special Cells (SpCells), such as the primary serving cell (PCell) of a Master Cell Group (MCG). If UE3 is configured to communicate using a secondary cell group (SCG), UE3 may communicate via the primary SCG cell (PSCell). An MCG is a group of serving cells associated with a master node. An SCG is a group of serving cells associated with a secondary node. The master node and secondary node may communicate via an Xn interface (e.g., an Xn-U interface and / or an Xn-C interface) provided between the nodes.

[0172] The following describes an example in which UE3 communicates with base station 5 via SpCell and determines that an RLF has occurred, referring to Figure 13.

[0173] In step S1301, UE3 is provided with one or more configurations for LTM candidate cells. Step 1301 corresponds, for example, to step 6 in Figure 8 and includes the corresponding cell Radio Network Temporary Identifiers (C-RNTI).

[0174] In step S1302, the RAN node providing the candidate LTM cell (a different base station 5, or a different DU50 of the same base station 5) prepares for access by UE3 (for example, in response to receiving a UE context setup request message in step 3 of Figure 8). It will be understood that step S1302 may be performed before step S1301.

[0175] In step 1303, UE3 performs an RLM on the serving cell's transmission and determines that an RLF has occurred between UE3 and the serving cell. For example, as illustrated with reference to Figure 12, UE3 may determine that an RLF has occurred if the number of in-sync indications received before the RLF timer expires is below a threshold. An RLF may occur, for example, when UE3 moves out of the coverage area of ​​the SpCell that UE3 is communicating with the RAN node.

[0176] However, UE3 may still be present within one or more coverage areas of candidate LTM cells.

[0177] In step S1304, UE3 declares an RLF. For example, UE3 may send an indication to the RAN node providing the serving cell that an RLF has occurred. In step S1305, the RRC re-establishment procedure is performed on the cell (in addition to the serving cell, for example, the PCell in the MCG).

[0178] The method in Figure 13 is explained with reference to RLF, but a similar procedure may be performed in the case of a handover failure (not shown in Figure 13). In the case of an LTM-based handover failure (HOF), the RRC re-establishment procedure is performed in the cell (in addition to the serving cell, e.g., the PCell in the MCG).

[0179] In some cases, the RRC re-establishment procedure may be performed on a cell not included in the LTM candidate cell set. However, this can increase delays in recovery from an RLF or HOF, and may even cause recovery from an RLF or HOF to fail. Furthermore, even when the RRC re-establishment procedure is performed on a cell in the LTM candidate cell set, the L3-based procedure may increase the time required for recovery from an RLF or HOF compared to the corresponding L1 / L2-based recovery. To mitigate these issues, UE3 may be configured to run a joint RLM process that monitors both the serving cell and the candidate LTM cell for RLFs. In another example, UE3 may be configured with a single RLM process to monitor the serving cell and additional RLM processes to monitor each candidate cell set for an LTM, and an RLF may be declared on either the serving cell or one or more candidate cells for an LTM. In yet another example, UE3 may have a single RLM process configured for the serving cell, and the RLM may not have to be configured for the candidate LTM cell (however, UE3 may attempt to switch to the candidate LTM cell if it determines that an RLF has occurred in communication via the serving cell). Therefore, advantageously, UE3 can decide not to declare an RLF and not initiate the RRC re-establishment procedure even if an RLF is detected in the serving cell, if one of the candidate LTM cells is available for communication. For example, UE3 may be configured to declare an RLF only if neither the serving cell nor the candidate LTM cell is available.

[0180] However, the inventors have found that considering the availability of LTM candidate cells can improve the reliability and efficiency of recovery from RLF (or response to HOF) in the serving cell. For example, as described below, considering which LTM candidate cell's latest timing advance parameters the UE3 can utilize can improve the reliability and efficiency of recovery from RLF or recovery from HOF to the target cell in the serving cell. Particularly advantageous methods and apparatus for improving the reliability and efficiency of recovery from RLF are described below.

[0181] Fast Recovery Cell Set (FRCS) Figure 14 shows an example where UE3 is provided with a set of fast recovery cells. In this example, UE3 is configured with a set of candidate cells for LTM. UE3 may receive configuration information for the candidate cells, for example, when sending an RRC Reconfiguration in step 6 of Figure 8.

[0182] In this example, the LTM candidate cells include cells 1 through 6. Conveniently, UE3 also provides a fast recovery cell set for LTM. In this example, cells 1, 4, and 5 are included in the fast recovery cell set. It will be understood that these cells can be identified within the LTM candidate cell set and the fast recovery cell set using any appropriate information to identify the cell (for example, by remembering the cell's corresponding ID). The additional cell set is called the “fast recovery” cell set. In this example, the fast recovery cell set is (but not necessarily) a subset of the LTM candidate cells. In this example, UE3 decides to include a cell in the fast recovery cell set based on whether UE3 remembers the cell’s valid timing advance information. For example, UE3 decides that UE3 remembers the valid timing advance information for cell 1, and therefore decides to include cell 1 in the fast recovery cell set. As described above, a time alignment timer (e.g., timeAlignmentTimer) can be configured to define the maximum time after UE3 receives TA information (e.g., TA commands) that it is considered synchronized for uplink transmissions in the corresponding cell. UE3 may be configured to include a cell in the fast recovery cell set if the corresponding time alignment timer has not expired. Conversely, UE3 may be configured to remove a cell from the fast recovery cell set if the corresponding time alignment timer has expired. In other words, if UE3 is synchronized for uplink transmissions in that cell, UE3 may be configured to add the cell to the fast recovery cell set (or keep the cell in the fast recovery cell set). Similarly, if UE3 is not synchronized for uplink transmissions in that cell, UE3 may be configured to remove the cell from the fast recovery cell set.

[0183] Following an RLF in the serving cell, or a HOF to the target cell, UE3 decides to establish a connection via one of the fast recovery cells. In other words, UE3 is configured to prioritize candidate cells included in the fast recovery cell set for recovery from an RLF in the serving cell, or a HOF to the target cell. Therefore, advantageously, UE3 prioritizes connecting to one of the fast recovery cells where UE3 stores valid timing advance information, thus reducing the risk of connection failure (or delay) to candidate cells. As will be discussed later, UE3 has the advantage of being able to re-establish the RRC connection via RACH-less access to one of the cells included in the fast recovery cell set (FRCS).

[0184] As described later, UE3 may provide an indication to the RAN node providing the serving cell that which cells are included in the fast recovery cell set (e.g., using UL MAC CE, dedicated MAC CE). For example, UE3 may decide to add a cell to the fast recovery cell set and then send an indication (e.g., L1, L2, or L3 indication) to the RAN node that the cell has been added. Alternatively, for example, UE3 may send an indication to the RAN node that UE3 has stored valid timing advance information for a particular cell, and the RAN node may decide to send an indication to UE3 that UE3 will add that cell to the fast recovery cell set (e.g., using MAC CE, dedicated MAC CE, or other appropriate L1, L2, or L3 transmission). Thus, advantageously, synchronization can be maintained between the fast recovery cell set stored in UE3 and the fast recovery cell set stored for UE3 at the RAN node providing the source / serving cell.

[0185] Cells in the fast recovery cell set may be ranked within the fast recovery cell set, in addition to prioritizing connectivity over cells in the LTM candidate cell set. For example, UE3 may perform RSRP, RSRQ, and / or RSRP-SINR measurements for each cell in the fast recovery cell set and prioritize connectivity to the cell with the best RSRP, RSRQ, and / or RSRP-SINR (UE3 may also prioritize cells measured to have better communication quality). UE3 may perform these measurements periodically. Alternatively, UE3 may perform the measurements under network control (for example, in response to receiving corresponding instructions and / or measurement configuration information (e.g., measurement gap) from base station 5).

[0186] RLF Recovery and FRCS Figure 15 shows an example where UE3 stores the FRSC and RLF occurs in the serving cell. Step S1501 corresponds to step S1301 in Figure 13, and UE3 is provided with one or more configurations for the LTM candidate cells.

[0187] Step S1502 corresponds to step S1302 in Figure 13, in which one or more RAN nodes providing candidate LTM cells (different base stations 5, or different DU50 of the same base station 5 providing source / serving cells) prepare for access by UE3 (for example, in response to receiving a UE context setup request message in step 3 in Figure 8). It will be understood that step S1502 may be performed before step S1501.

[0188] In step S1503, UE3 obtains timing advance information for one or more LTM candidate cells (cells indicated to UE3 in step S1501). For example, UE3 may communicate with one or more base stations 5 that provide LTM candidate cells to obtain the corresponding time offset (e.g., NTA as described above). UE3 may obtain TA information for a subset of LTM candidate cells (for example, because UE3 cannot obtain TA information for some cells). UE3 may obtain TA information for a subset of LTM candidate cells by receiving TA information in the configuration via serving cells without using or initiating a dedicated TA acquisition procedure. The method by which UE3 obtains TA information (for example, when TA information is not initially provided to UE3 via serving cells) will be described later.

[0189] In step S1504, UE3 generates or updates a fast recovery cell set. UE3 is configured to include LTM candidate cells in the fast recovery cell set if valid TA information for that cell is available in UE3. For example, UE3 may receive TA information for a cell and decide to include the cell in the fast recovery cell set if the corresponding time alignment timer has not expired. Alternatively, UE3 is configured to remove a cell from the fast recovery cell set if the corresponding time alignment timer has expired (and therefore the TA information available for that cell in UE3 is considered invalid).

[0190] In step S1505, UE3 exchanges information with the RAN node providing the serving cell about the fast recovery cell set. UE3 may provide the RAN node with explicit or implicit indications of the cells it has decided to include in the fast recovery cell set.

[0191] UE3 may be configured to repeat steps S1504 and S1505 (for example, periodically or based on a timer). For example, UE3 may be configured to repeat steps S1504 and S1505 when the TA information of a cell becomes invalid (when the corresponding time alignment timer expires).

[0192] Step S1506 corresponds to step S1304 in Figure 13, in which UE3 declares an RLF. For example, UE3 may send an indication to the RAN node providing the serving cell that an RLF has occurred. UE3 may determine that an RLF has occurred in the SpCell based on the RLF timer and the corresponding Out-of-sync and In-sync indications, as described above with reference to Figure 12. After UE3 has determined that an RLF has occurred in the serving cell, if UE3 has stored the LTM candidate cell configuration and there is at least one cell in the fast recovery cell set hosted by the same CU60, UE3 may be configured to maintain the RRC connection with the serving cell (for example, for the duration of the corresponding timer).

[0193] In step S1507, UE3 re-establishes an RRC connection through a cell in the fast recovery cell set. UE3 may select a cell in the fast recovery cell set based on the corresponding ranking or priority of the cells in the fast recovery cell set (for example, based on the RSRP, RSRQ, and / or RSRP-SINR measured as described above). If re-establishing the connection through the selected cell fails, UE3 may attempt to re-establish the RRC connection through the next highest priority or ranking cell in the fast recovery cell set. If UE3 cannot establish a connection to any cell in the fast recovery cell set (or, for example, if no cell is specified in the fast recovery cell set because UE3 does not remember valid TA information for any candidate LTM cell), UE3 may attempt to establish a connection to another cell in the LTM candidate cell set configured in step S1501 (for example, using a traditional procedure). UE3 may be configured to prioritize cells in the fast recovery cell set and / or cells in the LTM candidate cell set, as configured by the network (for example, via base station 5) during cell (re)selection. If UE3 selects a cell that is not included in the fast recovery cell set, the traditional RRC re-establishment procedure may be used. If UE3 selects a cell that is included in the LTM candidate cell set, UE3 may perform the autonomous LTM cell switch procedure to access that LTM candidate cell following the RLF.

[0194] In step S1507, the RLF recovery procedure during LTM preparation does not necessarily require the UE3 to be the result of a cell (re)selection procedure, as the UE3's decision to attempt to restore connectivity to the network via a cell from which valid TA information can be obtained (e.g., a cell in the fast recovery cell set). This is because the UE3 performs corresponding measurements of the cell (e.g., periodically) to maintain valid TA information for the cell, and therefore does not necessarily require the traditional cell (re)selection procedure to be performed following a connectivity failure (e.g., RLF).

[0195] Advantageously, because UE3 stores valid TA information for the target cells of the fast recovery cell set, RRC connection re-establishment can be done via RACH-less access to the cells of the fast recovery cell set. RACH-less access reduces data connection interruption time because it eliminates the need to perform random access when initially accessing the target cell, thereby reducing overall handover execution time.

[0196] The RRC connection recovery by UE3 may be an autonomous LTM cell switch, similar to a conditional handover cell switch. In other words, if UE3 has established a connection to a cell in the fast recovery cell set, it is not necessarily required to perform the full RRC re-establishment procedure following RLF or HOF, thus avoiding unnecessary interruptions to the procedure.

[0197] If there are no changes to CU60 during the LTM Cell Switch procedure, PDCP re-establishment is not necessarily required. However, PDCP data delivery from the UE to lower layers may be temporarily interrupted until a fast recovery to the target cell hosted by the same CU60 is successful. For fast recovery, UE3 can perform a configured grant-based UL transmission during this RACH-less LTM cell switch, which may include a C-RNTI MAC CE (along with a possible piggybacked BSR). The target cell of the target RAN node can identify UE3 via this C-RNTI MAC CE, update the UE context, and complete UE access. Alternatively, UE3 can send an RRC layer message (e.g., RRC Reconfiguration Complete or RRC Reestablishment) to the target base station 5 for the purpose of fast recovery announcement.

[0198] The method in Figure 15 was explained with reference to RLF, but a similar procedure can be applied to HOF recovery. That is, following HOF (instead of RLF in step S1506), UE3 may select a cell in the fast recovery set and attempt to connect to the cell selected in step S1507.

[0199] Acquisition and Maintenance of TA As illustrated with reference to step S1503 in Figure 15, UE3 is configured to obtain timing advance information for one or more LTM candidate cells (cells notified to UE3 in step S1501). For example, UE3 may communicate with one or more base stations 5 that provide LTM candidate cells and obtain the corresponding time offset (e.g., the NTA described above).

[0200] UE3 may be configured to request one or more PRACH resources to reacquire TA information after the expiration of the corresponding TA timer (e.g., time alignment timer). UE3 may be configured to remove the cell from the fast recovery cell set when the corresponding TA timer expires, but it is advantageous to add the cell back to the fast recovery cell set after reacquiring the TA information. Any appropriate uplink L1, L2, or L3 transmission may be used to request more PRACH resources to reacquire TA information. For example, a UL MAC CE (e.g., dedicated UL MAC CE) may be sent from UE3 to the RAN node providing the source / serving cell to request one or more PRACH resources to reacquire TA information.

[0201] Figure 16 shows an example where an access network node providing a source / serving cell communicates with an access network node providing a candidate cell for LTM and retrieves one or more PRACH resources requested by UE3 to obtain TA information.

[0202] In step S1601, UE3 sends a request for one or more PRACH resources to reacquire TA information after the expiration of the corresponding TA timer for the cell (e.g., a time alignment timer). Alternatively, this request may be for the initial acquisition of TA information for the cell. In step S1602, the access network node providing the source serving cell (e.g., DU50) sends a request for one or more corresponding PRACH resources to the access network node providing the candidate cell (e.g., DU50).

[0203] In step S1603a, the access network node providing the candidate cell sends the requested one or more PRACH resources to the source access network node. In step S1604, the access network node providing the source / serving cell forwards the requested one or more PRACH resources to the UE3. The source access network node may initiate a PDCCH order-based RACH with the UE3. Advantageously, the UE3 can obtain (or re-obtain) TA information for the candidate LTM cell and thus add the candidate LTM cell to the fast recovery cell list. Alternatively, as shown in step S1603b, the target access network node may send the requested PRACH resources directly to the UE3 instead of sending them via the source access network node.

[0204] In yet another example, instead of UE3 requesting one or more PRACH resources to retrieve / re-retrieve TA information from the access network node providing the source cell / serving cell, the source access network node may provide UE3 with one or more PRACH resources to retrieve TA information corresponding to the LTM candidate cell during the initial resource allocation procedure. For example, the PRACH resources may be provided to UE3 in step 6 of Figure 8, or in step S1501 of Figure 15.

[0205] Additional cells in high-speed recovery cell sets In the example above, the fast recovery cell set is described as containing only cells for which valid TA information is available in UE3, but is not necessarily limited to this. Alternatively, the fast recovery cell set may contain cells for which valid TA information is not available in UE3 (for example, UE3 is unable to obtain TA information for the cell, or UE3 has obtained TA information for the cell but the corresponding timer has expired). If the fast recovery cell set contains cells for which valid TA information is not available in UE3, these cells may be assigned / allocated by UE3. UE3 may be configured to attempt to access cells in the fast recovery cell set for which valid TA information is available, and if those access attempts fail, UE3 may attempt to access cells in the fast recovery cell set for which valid TA information is not available.

[0206] Figure 17 shows an example where a fast recovery cell set contains four cells, 1 through 4. Cells 3 and 1 are assigned the highest priority for selection by UE3 because valid TA information is available for them in UE3. Cells 4 and 2 are assigned a lower priority for selection by UE3 because valid TA information is not available for them. It will be understood that cells with valid TA information may be further ranked / prioritized based on measurements performed by UE3. For example, in this example, cell 3 is assigned a higher priority for selection by UE3 than cell 1, which may be based on the best RSRP, RSRQ, and / or RSRP-SINR measurements by UE3 for cells 1 and 3. Similarly, in this example, cell 4 is assigned a higher priority for selection by UE3 than cell 2, which may also be based on the best RSRP, RSRQ, and / or RSRP-SINR measurements by UE3 for cells 4 and 2.

[0207] Maximum number of fast recovery cells The maximum number of cells that UE3 maintains in the fast recovery cell set may be configurable by the network (for example, via corresponding signaling transmitted from the access network node providing source cells / serving cells to UE3). Source base station 5 can transmit an indication of the maximum number of cells to be included in the fast recovery cell set using any appropriate transmission, such as an RRC message (e.g., a dedicated RRC message, or another type of transmission containing dedicated information elements). The maximum number of cells that UE3 maintains in the fast recovery cell set may be, for example, 6 cells or 9 cells, but may be any other appropriate number of cells.

[0208] The maximum number of cells that UE3 can maintain within the fast recovery cell set may depend on the capabilities of UE3 (e.g., the memory capacity configured for UE3). UE3 may be configured to transmit capability information to base station 5 indicating the maximum number of cells that UE3 is configured to maintain within the fast recovery cell set.

[0209] The maximum number of cells that UE3 maintains (or attempts to maintain) valid TA information can also be set by the network (e.g., using RRC transmission, dedicated RRC transmission) and does not necessarily have to be the same as the maximum number of cells that can be maintained within a fast recovery cell set. Alternatively, the maximum number of cells that UE3 can maintain valid TA information may depend on the capabilities of UE3 (e.g., memory and / or communication capabilities), and the maximum number of cells that UE3 maintains valid TA information may be indicated from UE3 to the network by UE3 transmitting a corresponding indication to base station 5.

[0210] Early TA Acquisition As described above, UE3 may decide whether to include a cell in the fast recovery cell set based on whether valid TA information for that cell is available in UE3. To support RACH-less LTM, TA information for candidate cells can be obtained via a PDCCH ordered (e.g., requested or instructed) RACH, where the PDCCH order is sent via the source cell, indicating the candidate cell, and / or the candidate cell's RACH Occasion (RO) is sent to UE3 via DCI.

[0211] Alternatively or additionally, the network may configure a set of cells from which UE3 will acquire TA information, based on UE3's capabilities (e.g., UE3's memory or communication capabilities, or other appropriate UE capability information). UE3 may provide UE capability information to the network (e.g., via base station 5), and the network may determine LTM candidate cells from which UE3 will acquire TA information, based on the UE capability information.

[0212] In yet another alternative, UE3 may be configured to perform early TA acquisition on a subset of cells shown in the LTM candidate cell list (e.g., LTM candidate cells shown in step 6 of Figure 8, or step S1301 of Figure 13) based on one or more conditions or criteria. For example, UE3 may be configured to decide to acquire TA information for a cell if its signal intensity (based on measurements of other appropriate quantities such as RSRP, or RSRQ or RSRP-SINR) is greater than a threshold signal intensity. UE3 may also be configured to decide to acquire TA information for N cells that have the best / strongest RSR, RSRQ, or RSRP-SINR, where N may be the maximum number of cells that can be included in the fast recovery cell set described above (but may be less than the maximum).

[0213] UE3 may, alternatively or additionally, decide to retrieve TA information for a cell if a valid RACH resource is available for retrieving TA information.

[0214] RLM and Fast Recovery Cell Set As described above, UE3 may be configured to monitor both the serving / source cell and the candidate LTM target cell to detect RLF when performing RLM. The out-of-sync and in-sync instructions described above with reference to Figures 11 and 12 may be generated by UE3 based on UE3's monitoring of the serving / source cell and the candidate LTM target cell, and the count of the number of corresponding instructions may be the sum of instructions based on all monitored cells.

[0215] If UE3 is configured to run a joint RLM process that monitors both serving cells and cells in the fast recovery cell set, the count of out-of-sync instructions generated by UE3 may be based on all the cells being monitored. Conveniently, this avoids UE3 declaring an RLF if a sufficient number of in-sync instructions are generated based on measurements of cells in the fast recovery cell set, and the RLF timer (T310) is reset.

[0216] If UE3 is configured with a single RLM process for a serving cell and a single RLM process for each cell in a fast recovery cell set, the advantage is that UE3 can only declare an RLF if it is detected in all monitored cells. This is advantageous because it avoids UE3 declaring an RLF if it is detected only in the serving cell and not in at least one cell in the fast recovery cell set.

[0217] User equipment Figure 18 is a schematic block diagram showing the main components of UE3 as shown in Figure 1.

[0218] As shown in the figure, UE3 includes a transceiver circuit 310 capable of transmitting signals to and receiving signals from base station 5 via one or more antennas 330 (e.g., having one or more antenna elements). UE3 includes a controller 370 that controls the operation of UE3. The controller 370 is associated with memory 390 and connected to the transceiver circuit 310. Although not strictly necessary for the operation of UE3, UE3 may also include all the typical features of a traditional UE3 (e.g., a user interface 350 such as a touchscreen / keypad / microphone / speaker that enables direct user control and interaction with the user), which may be provided by hardware, software, and firmware, or any combination thereof, as needed. The software may be pre-installed in memory 390 and / or downloaded, for example, via a communication network or removable data storage device (RMD).

[0219] In this example, the controller 370 is configured to control the overall operation of the UE3 by program instructions or software instructions stored in memory 390. As shown in the figure, these software instructions include, among other things, the operating system 410, the communications control module 430, the RLM module 450, and the TA acquisition module 470.

[0220] The communication control module 430 is operable to control communication between the UE3 and one or more serving base stations 5 (and other communication devices connected to base station 5, such as other UEs and / or core network nodes). The communication control module 430 is configured for the overall processing of uplink communication over relevant uplink channels (e.g., physical uplink control channel (PUCCH), random access channel (RACH), and / or physical uplink shared channel (PUSCH)), including both dynamic signaling and semi-static signaling (e.g., SRS). The communication control module 430 is also configured for the overall processing of receiving downlink communication over relevant downlink channels (e.g., physical downlink control channel (PDCCH) and / or physical downlink shared channel (PDSCH)), including both dynamic signaling and semi-static signaling (e.g., CSI-RS). The communication control module 430 is responsible for, for example, determining where to monitor downlink control information (such as the locations of CSS / USS, CORESET, and associated PDCCH candidates to be monitored), determining the resources (including interleaved resources and resources subject to frequency hopping) that the UE3 will use to transmit / receive UL / DL communications, managing frequency hopping on the UE side, determining how to configure slots / symbols (e.g., for UL, DL, or SBFD communications), determining which one or more bandwidth portions are configured for the UE3, determining how uplink transmissions should be encoded, and appropriately applying SBFD-specific communication settings. The RLM module 450 may be configured to control communications according to any of the RLM methods described above (e.g., for monitoring cells in a SpCell or fast recovery cell set). The TA acquisition module may be configured to acquire TA information for cells according to any of the methods described above.

[0221] base station Figure 19 is a schematic block diagram showing the main components of base station 5 of the communication system 1 shown in Figure 1. As shown, base station 5 comprises a transceiver circuit 510 for transmitting signals to and receiving signals from communication devices (such as UE3) via one or more antennas 530 (e.g., a single or multi-panel antenna array / large antenna), and a core network interface 550 (e.g., including N2, N3, and other reference points / interfaces) for transmitting signals to and receiving signals from network nodes in the core network 7. Although not shown, base station 5 may be connected to other base stations via appropriate interfaces (e.g., the so-called "Xn" interface in NR). Base station 5 comprises a controller 570 that controls the operation of base station 5. Controller 570 is associated with memory 590. Software may be pre-installed in memory 590 and / or downloaded via the communication network 1 or from a removable data storage device (RMD), etc. In this example, the controller 570 is configured to control the overall operation of the base station 5 by program instructions or software instructions stored in the memory 590.

[0222] As shown in the figure, these software instructions include, among other things, the operating system 610 and the communication control module 630.

[0223] The communication control module 630 is operable to control communication between the base station 5 and the UE3 and other network entities connected to the base station 5. The communication control module 630 is configured to comprehensively control the reception and decoding of uplink communications over relevant uplink channels (e.g., physical uplink control channel (PUCCH), random-access channel (RACH), and / or physical uplink shared channel (PUSCH)), including both dynamic and semi-static signaling (e.g., SRS). The communication control module 630 is also configured to comprehensively handle the transmission of downlink communications over relevant downlink channels (e.g., physical downlink control channel (PDCCH) and / or physical downlink shared channel (PDSCH)), including both dynamic and semi-static signaling (e.g., CSI-RS). The communication control module 630 is responsible for managing full-duplex communications (e.g., SBFD), including, if necessary, separating UL communications and DL communications via different physical antenna elements. The communication control module 630 is responsible for, for example, determining the configuration locations for UE3 to monitor downlink control information (such as the locations of CSS / USS, CORESET, and associated PDCCH candidates to be monitored), determining the resources (including interleaved resources and resources subject to frequency hopping) to be scheduled for transmission / reception of UL / DL communications by the UE, managing frequency hopping on the base station side, appropriately configuring slots / symbols (e.g., for UL, DL, or SBFD communications), configuring one or more bandwidth portions of UE3, and providing relevant configuration signaling to UE3. The communication control module 630 may be configured to control communications in any of the above ways (e.g., to send requested PRACH resources to UE3, or to provide UE3 with configuration information for fast recovery cell sets, such as the maximum number of cells in a fast recovery cell set).

[0224] Core network nodes / functions Figure 20 is a block diagram showing the main components of a core network node or function such as AMF, CPF, UPF, SMF, and OAM. As shown in the diagram, the core network function includes a transceiver circuit 710 that can operate to transmit signals to and receive signals from other nodes (including UE3, base station 5, and other core network nodes) via the network interface 720. The controller 730 controls the operation of the core network function according to software stored in memory 740. The software may be pre-installed in memory 740 and / or downloaded via communication system 1 or, for example, from a removable data storage device (RMD). The software includes an operating system 750 and a communication control module 760, etc.

[0225] The communication control module 760 is responsible for the core network functions and signaling processing (generation / transmission / reception) between the UE3, base station 5, and other core network nodes. The communication control module 630 may be configured to perform communication control according to any of the methods described above.

[0226] Revisions and alternatives As those skilled in the art will understand, the embodiments described above can be modified and substituted in several ways, while still benefiting from the disclosures described herein.

[0227] For example, for clarity, while terminology specific to cellular communication generations (such as 2G, 3G, 4G, 5G, and 6G) ​​may be used to refer to certain communication entities, the technical features described for a particular entity are not limited to devices of that specific communication generation. It will be understood that these technical features can be implemented in any functionally equivalent communication entity, regardless of the differences in terminology used to refer to them.

[0228] In the above description, the UE and base station are described as having several separate functional components or modules for the sake of ease of understanding. These modules may be provided in this way in certain applications, for example, when an existing system is modified to implement the disclosure. However, in other applications, such as systems designed from the outset with the features of the present invention in mind, these modules may be integrated into the entire operating system or code, and therefore these modules may not be identifiable as separate entities.

[0229] In the embodiments described above, several software modules have been explained. As those skilled in the art will understand, these software modules may be provided in compiled or uncompiled form, and may be supplied as signals over a computer network or via a recording medium. Furthermore, some or all of the functions performed by these software modules may be performed using one or more dedicated hardware circuits. However, to update the functions of a base station or UE, the use of software modules is preferred in order to facilitate their updates.

[0230] Each controller may include any suitable form of processing circuitry, including (but not limited to) one or more hardware-implemented computer processors, microprocessors, central processing units (CPUs), arithmetic logic units (ALUs), input / output (IO) circuits, internal memory / cache (programs and / or data), processing registers, communication buses (such as control buses, data buses and / or address buses), direct memory access (DMA) functions, hardware or software-implemented counters, pointers, and / or timers. Various other modifications will be obvious to those skilled in the art and will not be described in further detail here.

[0231] A base station may consist of a "distributed" base station comprising a central unit ("CU") and one or more individual distributed units (DUs).

[0232] In this disclosure, user equipment (or "UE," "mobile station," "mobile device," or "wireless device") is an entity connected to a network via a wireless interface.

[0233] It should be noted that this disclosure is not limited to dedicated communication devices, but may also be applied to any device having communication functions as described later.

[0234] The terms “User Equipment” or “UE,” “Mobile Station,” “Mobile Device,” and “Wireless Device” (as used in 3GPP) are generally intended to be synonymous with each other and include standalone mobile stations such as terminals, cell phones, smartphones, tablets, cellular IoT devices, IoT devices, and machines. It will be understood that the terms “Mobile Station” and “Mobile Device” also include devices that remain stationary for extended periods.

[0235] UE may include, for example, equipment or machinery for production or manufacturing and / or energy-related machinery, such as (boilers, engines, turbines, solar panels, wind turbines, hydroelectric generators, thermal power generators, nuclear power generators, batteries, nuclear systems and / or related equipment, heavy electrical machinery, pumps including vacuum pumps, compressors, fans, blowers, hydraulic equipment, pneumatic equipment, metalworking machinery, manipulators, robots and / or their application systems, tools, molds, rolls, conveying equipment, lifting equipment, material handling equipment, textile machinery, sewing equipment, printing and / or related machinery, paper processing machinery, chemical machinery, mining and / or construction machinery and / or related equipment, machinery and / or equipment for agriculture, forestry and / or fishing, safety and / or environmental protection equipment, tractors, precision bearings, chains, gears, power transmission equipment, lubrication equipment, valves, pipe fittings and / or application systems of any of the aforementioned equipment or machinery).

[0236] UE may be, for example, transportation equipment (railway cars, automobiles, motorcycles, bicycles, trains, buses, carts, rickshaws, ships and other water vehicles, aircraft, rockets, satellites, drones, balloons, etc.). Alternatively, UE may be information and communication equipment (for example, computers and related equipment, communications and related equipment, electronic components, etc.).

[0237] UE may include, for example, refrigeration machines, refrigeration machine applications, commercial and / or service industry equipment, vending machines, automated service machines, office equipment or machinery, consumer electronics and electronic devices (such as audio equipment, video equipment, loudspeakers, radios, televisions, microwave ovens, rice cookers, coffee makers, dishwashers, washing machines, dryers, electronic fans or related equipment, vacuum cleaners, etc.).

[0238] UE may also be an electrical application system or device, such as an X-ray system, particle accelerator, radioisotope equipment, acoustic equipment, electromagnetic application equipment, or electronic power application equipment.

[0239] UE may include, for example, electronic lamps, lighting fixtures, measuring instruments, analytical instruments, testing equipment, or measuring or sensing equipment (such as smoke detectors, motion alarm sensors, motion sensors, wireless tags, etc.), wristwatches or clocks, laboratory equipment, optical devices, medical equipment and / or systems, weapons, tableware, hand tools, etc.

[0240] The UE may be, for example, a personal digital assistant or related device with wireless capabilities (such as a wireless card or module designed to be attached to or inserted into another electronic device, e.g., a personal computer, an electrical measuring instrument).

[0241] The UE may also be part of a device or system that uses various wired and / or wireless communication technologies to provide the applications, services, and solutions described below with respect to the Internet of Things (IoT).

[0242] An Internet of Things device (or "things") is equipped with appropriate electronics, software, sensors, network connectivity, etc., and can collect and exchange data between these devices and with other communication devices. An IoT device may include automated equipment that follows software instructions stored in internal memory. An IoT device may operate without requiring human supervision or operation. An IoT device may also remain stationary or inactive for extended periods. An IoT device may be implemented as part of (generally) stationary equipment. An IoT device may be incorporated into non-stationary equipment (such as a vehicle) or attached to animals or people being monitored / tracked.

[0243] It will be understood that IoT technology can be implemented in any communication device that can connect to a communication network and send / receive data, regardless of whether such communication devices are controlled by human input or by software instructions stored in memory.

[0244] IoT devices will be understood to also be called Machine-Type Communication (MTC) devices or Machine-to-Machine (M2M) devices. It will be understood that a UE can support one or more IoT or MTC applications. Some examples of MTC applications are shown in the table below. This list is not exhaustive and is intended to illustrate some examples of machine-type communication applications. [Table 2]

[0245] Applications, services, and solutions may include Mobile Virtual Network Operator (MVNO) services, emergency radio communication systems, Private Branch eXchange (PBX) systems, PHS / digital cordless telephone systems, Point of Sale (POS) systems, advertise calling systems, Multimedia Broadcast and Multicast Service (MBMS), Vehicle to Everything (V2X) systems, train radio systems, location-related services, disaster / emergency radio communication services, community services, video streaming services, femtocell application services, Voice over LTE (VoLTE) services, billing services, wireless on-demand services, roaming services, activity monitoring services, carrier / network selection services, function restriction services, Proof of Concept (PoC) services, personal information management services, and ad-hoc network / delay-tolerant networking (DTN) services.

[0246] Furthermore, the aforementioned UE categories are merely examples of applications of the technical concepts and embodiments described herein. Of course, these technical concepts and embodiments are not limited to the aforementioned UEs, and various modifications are possible.

[0247] Since various other modifications will be obvious to those skilled in the art, they will not be described in further detail here.

[0248] For example, all or part of the embodiments disclosed above may also be described as follows, but are not limited to these. (Note 1) A method performed by user equipment (UE), Receiving instructions for a first set of one or more candidate target cells for a lower-layer mobility procedure from an access network node providing source cells, Determining a second set of candidate target cells for the lower-layer mobility procedure, wherein the second set of cells is a subset of the first set of cells, The cells in the second set of cells are prioritized to be selected as the target cells. method. (Note 2) For at least one of the candidate cells in the first set, obtaining timing advance information for communication using the cell, Deciding to include the candidate cell from which the timing advance information is obtained in the second set of cells, The method described in Appendix 1, further including the method described in Appendix 1. (Note 3) For the candidate cell from which the timing advance information was obtained, it is determined whether the timing advance information is valid based on the associated timer. If it is determined that the timing advance information for the candidate cell is not valid, the candidate cell is removed from the second set. If it is determined that the timing advance information for the candidate cell is valid, the candidate cell is kept in the second set. The method described in Appendix 2, further including the method described in Appendix 2. (Note 4) This includes determining that the timing advance information for the candidate cell is no longer valid when the timer expires. The method described in Appendix 3. (Note 5) Receiving one or more communication resources for use in acquiring the timing advance information from the access network node or another access network node that provides a cell from the first set of cells, Acquiring the timing advance information using one or more of the aforementioned communication resources, The method described in any of the appendices 2 to 4, further including the method described in any of the appendices 2 to 4. (Note 6) The aforementioned one or more communication resources include one or more physical random access channel (PRACH) resources. The method described in Appendix 5. (Note 7) Further includes sending a request for one or more communication resources to be used in obtaining the timing advance information to the access network node, The method described in Appendix 5 or 6. (Note 8) The second set of cells includes at least one cell in which valid timing advance information is available in the UE, and the second set of cells includes at least one cell in which valid timing advance information is not available in the UE. The further includes determining a ranking or priority for selecting the cell in the second set of cells as a target cell, based on whether valid timing advance information for the cell is available. The method described in any of the appendices 2 to 7. (Note 9) The UE further includes receiving instructions from the access network node for the maximum number of cells for which it maintains the corresponding timing advance information. The method according to any one of Appendices 2 to 8. (Appendix 10) Further including transmitting an indication of the maximum number of cells for maintaining the corresponding timing advance information by the UE to the access network node, The method according to any one of Appendices 2 to 8. (Appendix 11) Receiving an indication of the identifier of one or more of the candidate target cells for the UE to obtain the corresponding timing advance information from the access network node, Obtaining the timing advance information for the indicated cell, Further including the method according to any one of Appendices 2 to 10. (Appendix 12) Further including determining to obtain the timing advance information for one or more cells in the set of the first cells, The UE determines whether to obtain the timing advance information for a cell based on at least one measurement of the transmission of the cell. The method according to any one of Appendices 2 to 10. (Appendix 13) The UE determines whether to obtain the timing advance information for a cell based on whether the random access channel (RACH) resource of the cell for obtaining the timing advance information is available at the UE. The method according to Appendix 12. (Appendix 14) Further including transmitting an indication of the cell included in the set of the second candidate cells to the access network node. The method according to any of the above Appendices. (Appendix 15) Including transmitting an indication of the cell included in the set of the second candidate cells to the access network node after adding or deleting a cell from the set of the second cells. The method described in Appendix 14. (Note 16) Performing one or more measurements of transmission of at least one cell from the second set of cells, Based on the measurement, determine a ranking or priority for selecting the cell in the second set of cells as a target cell, The method described in any of the preceding appendices, further comprising: (Note 17) The measurement includes at least one of the following measurements: reference signal received power (RSRP), reference signal received quality (RSRQ), or RSRP and signal-to-noise interference ratio (RSRP-SINR). The method described in Appendix 16. (Note 18) Receiving measurement setting information for one or more of the aforementioned measurements from the access network node, Performing one or more measurements based on the aforementioned measurement setting information, The method described in Appendix 16 or 17, further including the method described in Appendix 16 or 17. (Note 19) It is determined that a radio link failure (RLF) occurred in the aforementioned source cell, After determining that the aforementioned RLF has occurred, the lower layer mobility procedure is performed, The method described in any of the preceding appendices, further comprising: (Note 20) Determining that the aforementioned RLF has occurred includes performing the radio link monitoring (RLM) procedure. The method described in Appendix 19. (Note 21) The RLM procedure includes performing a measurement of the source cell's transmission, The method described in Appendix 20. (Note 22) The RLM procedure further includes performing a transmission measurement of at least one cell from the second set of cells. The method described in Appendix 21. (Note 23) The RLM procedure includes a first RLM process for monitoring the source cell and one or more second RLM processes for monitoring a cell in the second set of cells. The method described in Appendix 22. (Note 24) If the UE determines that an RLF has occurred in both the source cell and the cell in the second set of cells monitored using the second RLM process, the UE determines that an RLF has occurred. The method described in Appendix 23. (Note 25) The RLM procedure includes a joint RLM process for monitoring the source cell and for monitoring a cell in the second set of cells. The method described in Appendix 22. (Note 26) Determining that a failure occurred in the handover procedure for the handover of the UE from the source cell, After determining that the handover procedure has failed, the lower-layer mobility procedure is executed. The method described in any of the appendices 1 to 18, further including the method described in any of the appendices 1 to 18. (Note 27) The aforementioned lower-layer procedure is a layer 1 (L1) or layer 2 (L2) based mobility procedure. The method described in any of the above appendices. (Note 28) Determining that an RLF occurred in the aforementioned source cell, If the second set of cells includes at least one cell, it is decided to maintain the settings for RRC connection via the source cell for a first period of time. A method according to any of the above appendices, including: (Note 29) The source cell is associated with a central unit of a base station, including determining to maintain the configuration for the RRC connection via the source cell during the first period when the set of the second cells includes at least one cell associated with the central unit, The method according to Appendix 28. (Appendix 30) further including determining to execute the lower layer mobility procedure, The method according to any of the above Appendices. (Appendix 31) The lower layer mobility procedure includes establishing or re - establishing a radio resource control (RRC) connection via a cell among the set of the second cells, The method according to any of the above Appendices. (Appendix 32) further including receiving an indication of a maximum number of cells included in the set of the second cells from the access network node, The method according to any of the above Appendices. (Appendix 33) further including transmitting an indication of a maximum number of cells that the UE includes in the set of the second cells to the access network node, The method according to any of Appendices 1 to 31. (Appendix 34) A method performed by an access network node providing a source cell, including: transmitting an indication of a first set of one or more candidate target cells for a lower layer mobility procedure to a user equipment (UE) within the source cell; and receiving an indication of a second set of candidate target cells for the lower layer mobility procedure from the UE, wherein the set of the second cells is a subset of the set of the first cells, The cells in the second set of cells are prioritized to be selected as target cells for the lower layer mobility procedure. method. (Note 35) The UE further includes transmitting one or more communication resources to the UE to use to obtain timing advance information for a cell in the first set of cells, The UE determines the cells included in the second set of cells based on the timing advance information. The method described in Appendix 34. (Note 36) The aforementioned one or more communication resources include one or more physical random access channel (PRACH) resources. The method described in Appendix 35. (Note 37) The further includes receiving from the UE a request for one or more communication resources for the UE to use to obtain the timing advance information, The method described in Appendix 35 or 36. (Note 38) The UE further includes sending an instruction to the UE for the maximum number of cells for which the UE maintains the corresponding timing advance information. The method described in any of the appendices 34 to 37. (Note 39) The UE further includes receiving from the UE an instruction for the maximum number of cells for which the UE maintains the corresponding timing advance information. The method described in any of the appendices 34 to 37. (Note 40) The UE further includes transmitting instructions for the identifiers of one or more candidate target cells for the UE to obtain corresponding timing advance information. The method described in any of the appendices 34 to 39. (Note 41) Further including transmitting measurement setting information for one or more measurements performed by the UE for the transmission of at least one cell from the second set of cells, The method described in any of the appendices 34 to 40. (Note 42) The aforementioned lower-layer procedure is a layer 1 (L1) or layer 2 (L2) based mobility procedure. The method described in any of the appendices 34 to 41. (Note 43) Further including deciding to perform the lower-layer mobility procedure, The method described in any of the appendices 34 to 42. (Note 44) Further including sending an instruction to the UE for the maximum number of cells included in the second set of cells, The method described in any of the appendices 34 to 43. (Note 45) The UE further includes receiving instructions from the UE regarding the maximum number of cells it should include in the second set of cells. The method described in any of the appendices 34 to 43. (Note 46) User equipment (UE), Means for receiving instructions for a first set of one or more candidate target cells for a lower-layer mobility procedure from an access network node providing source cells, Means for determining a second set of candidate target cells for the lower layer mobility procedure, wherein the second set of cells is a subset of the first set of cells, The cells in the second set of cells are prioritized to be selected as the target cells. UE. (Note 47) An access network node configured to provide source cells, Means for transmitting instructions for a first set of one or more candidate target cells for a lower-layer mobility procedure to user equipment (UE) in the source cell, Means for receiving instructions from the UE for a second set of candidate target cells for the lower layer mobility procedure, wherein the second set of cells is a subset of the first set of cells, The cells in the second set of cells are prioritized to be selected as target cells for the lower layer mobility procedure. Network node.

[0249] This application claims priority based on UK Patent Application No. 2305106.3, filed on April 5, 2023, the disclosures of which are incorporated herein by reference in their entirety. [Explanation of Symbols]

[0250] 1. Communication System 3. User equipment 5 base station 7 Core Network 9 cells 10 CONTROL PLANE FUNCTION 11 USER PLANE FUNCTION 20 External data network 50 Distributed Units (DU) 60 Central Unit (CU) 451 Transceiver Circuit 453 RU interface 454 CU interface 457 Controller 459 memory 461 Operating Systems 463 Communication control module 465 F1 module 468 DU-RU Module 472 DU Management Module 473 UE Profile Management Module 475 Mobility Module 551 Transceiver Circuit 554 DU Interface 555 CU Interface 557 Controller 559 memory 561 Operating Systems 563 Communication control module 565 F1 module 566 E1 module 568 N2 module 569 N3 module 571 CU-UP Management Module 572 CU-CP Management Module 573 UE Profile Management Module 575 Mobility Module 310 Transceiver Circuit 330 Antenna 350 User Interfaces 370 Controller 390 memory 410 Operating Systems 430 Communication control module 450 RLM Modules 470 TA Acquisition Module 510 Transceiver Circuit 530 Antenna 550 core network interfaces 570 Controller 590 memory 610 Operating Systems 630 Communication control module 710 Transceiver Circuit 720 Network Interfaces 730 Controller 740 memory 750 Operating Systems 760 Communication Control Module

Claims

1. A method performed by user equipment (UE), Establishing a Radio Resource Control (RRC) connection with the access network node via the source cell, This includes, if at least one cell of one or more candidate target cell sets is available, performing cell reselection of a target cell from the set of one or more candidate target cells for a lower-layer mobility procedure without using a random access procedure, The UE has timing advance information for each cell in the set of one or more candidate target cells. method.

2. The re-selection of the target cell is performed by autonomous Lower Layer Triggered Mobility (LTM) cell switching. The method according to claim 1.

3. The aforementioned one or more candidate target cells include a further set, The UE does not possess timing advance information for any of the cells in the further set of the one or more candidate target cells. The set of one or more candidate target cells is preferred over any further set of one or more candidate target cells in order to be re-selected as a target cell. The method according to claim 1 or 2.

4. Further includes maintaining the set of one or more candidate target cells in synchronization with the source cell, The method according to any one of claims 1 to 3.

5. The aforementioned synchronization is performed using the Media Access Control Element (MAC CE). The method according to claim 4.

6. If at least one of the aforementioned timing advance information becomes invalid, Deleting each cell corresponding to at least one of the invalidated timing advance information from the set of one or more candidate target cells, or Request a physical random access channel (PRACH) resource to reacquire the valid timing advance information for each cell corresponding to at least one of the aforementioned timing advance information, The method according to any one of claims 1 to 5, further comprising:

7. The PRACH resource is allocated by either the source cell or the target cell based on the request for the PRACH resource. The method according to claim 6.

8. The aforementioned request is performed using the Media Access Control Element (MAC CE). The method according to claim 6 or 7.

9. Further including reporting to the source cell that each of the cells corresponding to at least one of the invalidated timing advance information is removed from the set of one or more candidate target cells, The method according to claim 6.

10. The process further includes determining whether each of the aforementioned timing advance pieces of information is invalid based on the time alignment timer. The method according to any one of claims 6 to 9.

11. Performing measurements on each cell in the set of one or more candidate target cells, The method further includes ranking the cells within the set of one or more candidate target cells based on the results of the measurement, The cell reselection is performed based on the ranking of the cells. The method according to any one of claims 1 to 10.

12. To monitor the out-of-sync state of the aforementioned source cell, Monitoring for cell desynchronization in the set of a specified number of source cells and one or more target cells, Monitoring all out-of-sync cells in the set of the source cell and the one or more target cells, The further includes determining a wireless link failure by at least one of the following: The method according to any one of claims 1 to 11.

13. Performing the cell reselection of the target cell without using the random access procedure is performed when both the source cell and the target cell are operated by the same central unit of the access network node. The method according to any one of claims 1 to 12.

14. The method further includes receiving the respective timing advance information from each of the cells in the set of one or more candidate target cells using a physical downlink control channel (PDCCH)-ordered random access channel (RACH), The PDCCH order is transmitted from the source cell, The PDCCH order indicates the RACH opportunity for each cell in the set of one or more candidate cells, and / or for each cell in the set of one or more candidate cells. The method according to any one of claims 1 to 13.

15. Based on the capabilities of the UE, the system further includes receiving the respective timing advance information from each of the cells in the set of the one or more candidate target cells. The method according to any one of claims 1 to 13.

16. The process further includes receiving the respective timing advance information from a specific cell in the set of one or more candidate target cells. The aforementioned specific cell is The signal strength of each of the aforementioned specific cells is greater than the threshold, and effective resources are available for receiving the respective timing advance information, or The signal intensity of each of the aforementioned specific cells is within the range of N best cells. Satisfying at least one of the following: The method according to any one of claims 1 to 13.

17. The set of one or more target cells, and / or The respective timing advance information that the UE can maintain, The further includes receiving information from the access network node indicating the maximum number, The method according to any one of claims 1 to 16.

18. The set of one or more target cells that the UE can maintain, and / or The respective timing advance information that the UE can maintain, Further includes transmitting UE capability information indicating the maximum number to the access network node, The method according to any one of claims 1 to 17.

19. If any of the set of one or more target cells is unavailable, or if the UE selects a target cell from a further set of one or more target cells, the random access procedure further includes performing a cell reselection of another cell other than the set of one or more target cells. The method according to any one of claims 1 to 18.

20. A method performed by an access network node, Establishing a Radio Resource Control (RRC) connection with user equipment (UE) via a source cell, This includes, if at least one cell of one or more candidate target cell sets is available, performing cell reselection of a target cell from the set of one or more candidate target cells for a lower-layer mobility procedure without using a random access procedure, The UE has timing advance information for each cell in the set of one or more candidate target cells. method.

21. User equipment (UE), A means for establishing a Radio Resource Control (RRC) connection with an access network node via a source cell, The system includes means for performing cell reselection of a target cell from the set of one or more candidate target cells for a lower-layer mobility procedure, without using a random access procedure, when at least one cell of the set of one or more candidate target cells is available, The UE has timing advance information for each cell in the set of one or more candidate target cells. UE.

22. A means for establishing a Radio Resource Control (RRC) connection with user equipment (UE) via a source cell, The system includes means for performing cell reselection of a target cell from the set of one or more candidate target cells for a lower-layer mobility procedure, without using a random access procedure, when at least one cell of the set of one or more candidate target cells is available, The UE has timing advance information for each cell in the set of one or more candidate target cells. Access network node.

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