Apparatus, method and computer program for addressing radio link failure conditions

Abstract

An apparatus comprising: means for receiving a message from a network node on a source cell, the message indicating one or more low-layer mobility candidate cells; means for determining a failure of the source cell; and means for attempting, in response to the determination of the failure, completion of the low-layer mobility of the target cell selected from one of the one or more candidate cells.

Description

Technical Field

[0001] The present disclosure relates to apparatus, methods and computer programs, and particularly, but not exclusively, to apparatus, methods and computer programs for resolving radio link failure conditions. Background Art

[0002] A communication system may be viewed as a facility that enables a communication session between two or more entities by providing a carrier wave between the various entities involved in the communication path, such as communication devices, base stations, and / or other nodes.

[0003] The communication system may be a wireless communication system. Examples of wireless systems include public land mobile networks (PLMNs) operating based on radio standards (such as those provided by 3GPP), satellite-based communication systems, and various wireless local area networks, such as wireless local area networks (WLANs). Wireless systems can typically be divided into cells and are therefore often referred to as cellular systems.

[0004] Communication systems and associated equipment typically operate according to a given standard or regulation that sets out what the various entities associated with the system are allowed to do and how this should be achieved. Communication protocols and / or parameters that should be used for the connection are also typically defined. Summary of the Invention

[0005] According to one aspect, an apparatus is provided that includes: means for receiving a message from a network node on a source cell, the message indicating one or more low-layer mobility candidate cells; means for determining a failure of the source cell; and means for attempting completion of low-layer mobility to a target cell selected from one of the one or more candidate cells in response to the determination of failure.

[0006] The failure may include a radio link failure.

[0007] In the absence of a path switch command from the network, the means for attempting to access one or more of the one or more candidate cells may attempt to perform completion of low layer mobility.

[0008] The message may include configuration information for one or more candidate cells among the one or more candidate cells.

[0009] The apparatus may comprise means for causing a measurement report to be sent to the network, and receiving the message in response to the measurement report.

[0010] Low-layer mobility may include L1 / L2 triggered mobility.

[0011] The apparatus may include means for synchronizing with one or more of the one or more candidate cells.

[0012] The apparatus may include means for obtaining timing advance information for one or more candidate cells of the one or more candidate cells.

[0013] The apparatus may include means for determining, in response to a failure to determine, that one or more of the one or more candidate cells satisfies at least one threshold or at least one condition.

[0014] The at least one threshold may include a measurement power threshold or a quality threshold.

[0015] The at least one condition may include that one or more of the one or more candidate cells have been reported to the access node in a measurement report.

[0016] The apparatus may include means for performing a random access procedure on a target cell.

[0017] The apparatus may comprise means for causing information to be sent to the network node, the information indicating that low layer mobility to the target cell has been completed.

[0018] The message to be sent to the network node may indicate that the low layer mobility response to the target cell has failed.

[0019] The apparatus may be a communication device or may be provided in a communication device.

[0020] According to another aspect, an apparatus is provided, comprising: at least one processor and at least one memory including computer code for one or more programs, the at least one memory and the computer code being configured to, with the at least one processor, cause the apparatus to at least: receive a message from a network node on a source cell indicating one or more low-layer mobility candidate cells; determine a failure of the source cell; and, in response to the determination of failure, attempt completion of low-layer mobility of a target cell selected from the one or more candidate cells.

[0021] The failure may include a radio link failure.

[0022] In the absence of a path switch command from the network, an attempt to access one or more candidate target cells is performed.

[0023] The message may include configuration information for one or more candidate cells among the one or more candidate cells.

[0024] The apparatus may be caused to cause a measurement report to be sent to the network, and to receive said message in response to said measurement report.

[0025] Low-layer mobility may include L1 / L2 triggered mobility.

[0026] The apparatus may be caused to synchronize with one or more of the one or more candidate cells.

[0027] The apparatus may be caused to acquire timing advance information for one or more candidate cells of the one or more candidate cells.

[0028] The apparatus may be caused to: in response to determining a failure, determine that one or more of the one or more candidate cells satisfies at least one threshold or at least one condition.

[0029] The at least one threshold may include a measurement power threshold or a quality threshold.

[0030] The at least one condition may include that one or more of the one or more candidate cells have been reported to the access node in a measurement report.

[0031] The apparatus may be caused to perform a random access procedure on the target cell.

[0032] The apparatus may be caused to cause a message to be sent to the network node, the message indicating that low layer mobility to the target cell has been completed.

[0033] The message to be sent to the network node may indicate that the low layer mobility response to the target cell has failed.

[0034] The apparatus may be a communication device or may be provided in a communication device.

[0035] According to another aspect, a method is provided, comprising: receiving a message from a network node on a source cell, the message indicating one or more low-layer mobility candidate cells; determining a failure of the source cell; and in response to the determination of failure, attempting completion of low-layer mobility to a target cell selected from the one or more candidate cells.

[0036] The failure may include a radio link failure.

[0037] In the absence of a path switch command from the network, an attempt to access one or more candidate target cells is performed.

[0038] The message may include configuration information for one or more candidate cells among the one or more candidate cells.

[0039] The method may comprise causing a measurement report to be sent to a network, and receiving the message in response to the measurement report.

[0040] Low-layer mobility may include L1 / L2 triggered mobility.

[0041] The method may include synchronizing with one or more of the one or more candidate cells.

[0042] The method may include obtaining timing advance information for one or more candidate cells of the one or more candidate cells.

[0043] The method may include, in response to determining a failure, determining that one or more of the one or more candidate cells satisfies at least one threshold or at least one condition.

[0044] The at least one threshold may include a measurement power threshold or a quality threshold.

[0045] The at least one condition may include that one or more of the one or more candidate cells have been reported to the access node in a measurement report.

[0046] The method may include performing a random access procedure on the target cell.

[0047] The method may comprise causing a message to be sent to the network node, the message indicating that low layer mobility to the target cell has been completed.

[0048] The message to be sent to the network node may indicate that the low layer mobility response to the target cell has failed.

[0049] The method may be performed by an apparatus, which may be a communication device or may be provided in a communication device.

[0050] According to another aspect, an apparatus is provided, comprising: means for causing a message to be sent to a communication device connected to a source cell, the message indicating one or more low-layer mobility candidate cells; and means for receiving a message from the communication device, the message indicating completion of low-layer mobility to a target cell selected from the one or more candidate cells, the completion being a response to a failure of the source cell.

[0051] The apparatus may include means for determining failure of the source cell and, in response to starting a timer, delaying removal of resources associated with one or more candidate cells to allow completion of low layer mobility to the target cell prior to removal of the resources.

[0052] The timer may be started without causing a path switch command to be sent to the communication device.

[0053] The failure may include a radio link failure.

[0054] The message to be sent to the communication device may include configuration information for one or more of the one or more candidate cells.

[0055] The apparatus may comprise means for receiving a measurement report from the communication device and the message being caused to be sent to the communication device in response to the measurement report.

[0056] Low-layer mobility may include L1 / L2 triggered mobility.

[0057] The message from the communications device may indicate that the low layer mobility response to the target cell has failed.

[0058] The apparatus may be in or provided by the access node.

[0059] According to another aspect, a method is provided, comprising causing a message to be sent to a communication device connected to a source cell, the message indicating one or more low-layer mobility candidate cells; and receiving a message from the communication device indicating completion of low-layer mobility to a target cell selected from one of the one or more candidate cells, the completion being a response to a failure of the source cell.

[0060] The method may include determining failure of a source cell and, in response to starting a timer, delaying removal of resources associated with one or more candidate cells to allow completion of low layer mobility to a target cell prior to removal of the resources.

[0061] The timer may be started without causing a path switch command to be sent to the communication device.

[0062] The failure may include a radio link failure.

[0063] The message to be sent to the communication device may include configuration information for one or more of the one or more candidate cells.

[0064] The method may comprise receiving a measurement report from a communications device, and in response to said measurement report said message being caused to be sent to the communications device.

[0065] Low-layer mobility may include L1 / L2 triggered mobility.

[0066] The message from the communications device may indicate that the low layer mobility response to the target cell has failed.

[0067] The method may be performed by an apparatus, and the apparatus may be in an access node or provided by the access node.

[0068] According to another aspect, an apparatus is provided, comprising: at least one processor and at least one memory, the at least one memory including computer code for one or more programs, the at least one memory and the computer code being configured to, together with the at least one processor, cause the apparatus to at least: cause a message to be sent to a communication device connected to a source cell, the message indicating one or more low-layer mobility candidate cells; and receive a message from the communication device, the message indicating: completion of low-layer mobility of a target cell selected from one of the one or more candidate cells, the completion being a response to a failure of the source cell.

[0069] The apparatus may be caused to determine a failure of a source cell and, in response to starting a timer, delay removal of resources associated with one or more candidate cells to allow completion of low-layer mobility to a target cell prior to removal of the resources. The timer may be started without causing a path switch command to be sent to the communication device.

[0070] The failure may include a radio link failure.

[0071] The message to be sent to the communication device may include configuration information for one or more of the one or more candidate cells.

[0072] The apparatus may be caused to receive a measurement report from the communications device and, in response to the measurement report, the message is caused to be sent to the communications device.

[0073] Low-layer mobility may include L1 / L2 triggered mobility.

[0074] The message from the communications device may indicate that the low layer mobility response of the target cell has failed.

[0075] The apparatus may be in or provided by the access node.

[0076] According to another aspect, there is provided a computer program comprising instructions which, when executed by an apparatus, cause the apparatus to perform any one of the methods previously set forth.

[0077] According to another aspect, there is provided a computer program comprising instructions which, when executed, cause any one of the methods previously set out to be performed.

[0078] According to an aspect, there is provided a computer program comprising computer executable code causing any of the methods previously set out to be performed.

[0079] According to one aspect, a computer-readable medium is provided, the medium including program instructions stored thereon for executing at least one of the above methods.

[0080] According to one aspect, a non-transitory computer-readable medium is provided, comprising program instructions, which, when executed by an apparatus, cause the apparatus to perform any one of the methods previously set forth.

[0081] According to one aspect, a non-transitory computer-readable medium is provided, comprising program instructions which, when executed, cause any one of the methods previously set forth to be performed.

[0082] According to one aspect, a non-volatile tangible memory medium is provided, the non-volatile tangible memory medium comprising program instructions stored thereon for executing at least one of the above methods.

[0083] In the above, many different aspects have been described. It should be understood that additional aspects can be provided by combining any two or more of the above aspects. BRIEF DESCRIPTION OF THE DRAWINGS

[0084] Embodiments will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0085] Figure 1 An example system architecture is shown;

[0086] Figure 2 A schematic diagram illustrating an example 5G core network and radio access portion;

[0087] Figure 3 A schematic diagram of an example apparatus is shown;

[0088] Figure 4 shows a schematic diagram of a communication device;

[0089] Figure 5 A signalling diagram is shown;

[0090] Figure 6 A first method of some embodiments is shown; and

[0091] Figure 7 A second method of some embodiments is shown;

[0092] Figure 8 A third method of some embodiments is illustrated; and

[0093] Figure 9 A fourth method of some embodiments is shown. DETAILED DESCRIPTION

[0094] The following description provides exemplary descriptions of some embodiments. Although this specification may refer to "one," "an," or "some" examples or embodiments in several places in the text, this does not necessarily mean that the same example (s) of the embodiment is referenced each time, or that a particular feature applies only to a single example or embodiment. Individual features of different examples and embodiments may also be combined to provide other embodiments.

[0095] A wireless communication system provides wireless communication to devices connected thereto. Typically, access points such as base stations are provided to enable communication. In the following, different scenarios will be described using the 3GPP 5G radio access architecture as an example of an access architecture.

[0096] However, the embodiments are not necessarily limited to this architecture. Some examples of other options for suitable systems are Universal Mobile Telecommunications System (UMTS) Radio Access Network (UTRAN or E-UTRAN), Long Term Evolution (LTE), LTE-A (LTE-Advanced), Wireless Local Area Network (WLAN or Wi-Fi), Worldwide Interoperability for Microwave Access (WiMAX), Personal communication devices (PCS), Wideband Code Division Multiple Access (WCDMA), systems using Ultra-Wideband (UMB) technology, sensor networks, Mobile Ad Hoc Networks (MANET), Cellular Internet of Things (IoT) RAN, and Internet Protocol Multimedia Subsystem (IMS) or any combination and further developments thereof.

[0097] Figure 1 An example of a simplified system architecture is depicted showing only some elements and functional entities which are logical units, the implementation of which may differ from what is shown. Figure 1 The connections shown in are logical connections; the actual physical connections may be different. It is obvious to those skilled in the art that a system typically includes, in addition to Figure 1 Other functions and structures than those shown in FIG.

[0098] However, the embodiments are not limited to the systems given as examples, but a person skilled in the art may apply the solutions to other communication systems provided with the necessary characteristics.

[0099] Figure 1 The example of FIG. 1 shows a portion of an exemplary radio access network.

[0100] Figure 1Devices 100 and 102 are shown. Devices 100 and 102 are configured to wirelessly connect to a node 104 over one or more communication channels. Node 104 is also connected to a core network 106. In one example, node 104 can be an access node, such as an (e / g)NodeB serving device in a base station or cell. In one example, node 104 can be a non-3GPP access node. The physical link from the device to the (e / g)NodeB is referred to as an uplink or reverse link, and the physical link from the (e / g)NodeB to the device is referred to as a downlink or forward link. It should be understood that the (e / g)NodeB or its functionality can be implemented using any entity such as a node, host, server, or access node suitable for such use.

[0101] A communication system typically includes more than one (e / g)NodeB, in which case the (e / g)NodeBs may also be configured to communicate with each other over wired or wireless links designed for this purpose. These links may be used for signaling purposes. An (e / g)NodeB is a computing device configured to control the radio resources of the communication system to which it is coupled. A NodeB may also be referred to as a base station, an access point, or any other type of interface device, including a relay station capable of operating in a wireless environment. The (e / g)NodeB includes or is coupled to a transceiver. From the transceiver of the (e / g)NodeB, a connection is provided to an antenna unit, which establishes a bidirectional radio link to the device. The antenna unit may include multiple antennas or antenna elements. The (e / g)NodeB is also connected to the core network 106 (CN). Depending on the deployed technology, the (e / g)NodeB is connected to a serving network gateway and a packet data network gateway (S-GW and P-GW) or a user plane function (UPF) (for routing and forwarding user data packets and for providing connectivity of the device to one or more external packet data networks), and to a mobility management entity (MME) or an access mobility management function (AMF) (for controlling access and mobility of devices).

[0102] Exemplary embodiments of an apparatus are a subscriber unit, user equipment, user equipment (UE), terminal equipment, mobile station, mobile device, and the like.

[0103] Device generally refers to a mobile or stationary device (e.g., a portable or non-portable computing device), including wireless mobile communication devices that operate with or without a Universal Subscriber Identity Module (USIM), including but not limited to the following types of devices: mobile phones, smartphones, personal digital assistants (PDAs), cell phones, devices using wireless modems (such as alarm or measurement devices), laptop and / or touch screen computers, tablets, game consoles, notebooks, wireless interfaces or other wireless interface devices (e.g., USB dongles), and multimedia devices. It should be understood that a device can also be almost exclusively an uplink-only device, an example of which is a camera or video camera that uploads images or video clips to a network. A device can be a machine type communication (MTC) device, an Internet of Things (IoT) type communication device, or a cellular Internet of Things (CIoT) device, or any combination of these devices. A device can be a device capable of operating in an Internet of Things (IoT) network, which is a scenario in which objects are provided with the ability to transmit data over a network without human-to-human or human-to-computer interaction, such as is used in smart grids and connected vehicles. A device can utilize the cloud. In some applications, the device may comprise a user-portable device with a radio portion (such as a watch, headphones, or glasses), and the computation is performed in the cloud.

[0104] The device illustrates a type of apparatus to which resources on the air interface are allocated and assigned, and thus any features described herein using a device can be implemented using a corresponding device, such as a relay node. An example of such a relay node is a layer 3 relay (self-backhaul relay) toward a base station. The device (or in some embodiments, a layer 3 relay node) is configured to perform one or more user equipment functions of a user equipment function.

[0105] Additionally, although the apparatus has been depicted as a single entity, different units, processors and / or memory units (in Figure 1 Not all are shown in the figure) can be implemented.

[0106] The communication system can also communicate with other networks 112 (such as a public switched telephone network, a VoIP network, the Internet, or a private network), or utilize services provided by them. The communication network can also be capable of supporting the use of cloud services, for example, at least part of the core network operations can be performed as a cloud service (this is in the Figure 1 (depicted by “cloud” 114 in FIG. 1 ). The communication system may also include a central control entity or the like, providing facilities for networks of different operators to collaborate, for example, in spectrum sharing.

[0107] Edge cloud technology can be introduced into the radio access network (RAN) by leveraging network function virtualization (NFV) and software defined networking (SDN). Using edge cloud technology can mean that access node operations are at least partially performed in a server, host or node that is operably coupled to a remote power head or base station that includes a radio portion. Node operations may also be distributed among multiple servers, nodes or hosts. The application of cloud RAN architecture enables RAN real-time functions to be performed at or near the remote antenna site (for example, in the distributed unit DU 108 of the access point) and non-real-time functions to be performed in a centralized manner (for example, in the centralized unit CU 110 of the access point).

[0108] It will be apparent to those skilled in the art that the depicted system is only an example of a portion of a radio access system, and in practice, the system may include multiple (e / g)NodeBs, a device may have access to multiple radio cells, and the system may also include other devices, such as physical layer relay nodes or other network elements. In a geographical area of ​​a radio communication system, multiple different types of radio cells as well as multiple radio cells may be provided. A radio cell may be a macro cell (or umbrella cell), which is a large cell typically having a diameter of up to tens of kilometers, or a smaller cell, such as a micro cell, a femto cell, or a pico cell. Figure 1 A (e / g)NodeB may provide any kind of these cells.A cellular radio system may be implemented as a multi-layer network comprising several kinds of cells, and therefore multiple (e / g)NodeBs are required to provide this network structure.

[0109] In the following examples, the communication device is referred to as UE. However, it should be understood that the communication device may be any suitable communication device.

[0110] Figure 2 A schematic representation of a 5G system (5GS) is shown. The 5GS may include terminals, a (radio) access network ((R)AN), a 5G core network (5GC), one or more application functions (AFs), and one or more data networks (DNs). The 5G(R)AN may include one or more gNodeB (gNB) distributed unit functions connected to one or more gNodeB (gNB) centralized unit functions. The 5GC may include an access management function (AMF), a session management function (SMF), an authentication server function (AUSF), a user data management (UDM), a user plane function (UPF), and / or a network exposure function (NEF).

[0111] Figure 3An example of an apparatus 200 is illustrated. The apparatus may be provided in a communication device or may be a communication device. The apparatus may be provided in a DU or CU or may be a DU or CU. The apparatus may include at least one memory. The at least one memory may include at least one random access memory (RAM) 211a and / or at least one read-only memory (ROM) 211b. The apparatus may include at least one processor 212, 213. The apparatus may include an input / output interface 214. The at least one processor 212, 213 may be coupled to the RAM 211a and the ROM 211b. The at least one processor 212, 213 may be configured to execute appropriate software code 215. The software code 215 may, for example, enable execution of one or more steps to perform one or more aspects of the present invention. The software code 215 may be stored in the ROM 211b or other memory.

[0112] Figure 4 An example of a communication device 300 is shown, such as in Figure 1 The communication device 300 may be provided by any device capable of sending and receiving radio signals. Non-limiting examples have been discussed previously. For example, the communication device 300 may provide communication for carrying data for communication. The communication may be one or more of the following: voice, email, text message, multimedia, data, machine data, etc.

[0113] The communication device 300 may receive signals over the air or radio interface 307 via suitable means for receiving, and may transmit signals over the air or radio interface 307 via suitable means for transmitting radio signals. Figure 4 In FIG, the transceiver arrangement is schematically represented by block 306. The transceiver arrangement 306 may be provided, for example, by means of a radio part and an associated antenna arrangement. The antenna arrangement may be arranged internally or externally to the mobile device.

[0114] The communication device 300 may be provided with at least one processor 301 and at least one memory. The at least one memory may be at least one ROM 302a and / or at least one RAM 302b. The communication device may include other possible components 303 for use in software and hardware-assisted execution of the tasks it is designed to perform, including controlling access to and communication with access systems and other communication devices. The at least one processor 301 is coupled to the RAM 302a and the ROM 211b. The at least one processor 302 may be configured to execute appropriate software code 308. The software code 308 may, for example, enable execution of one or more aspects of the present invention. The software code 308 may be stored in the ROM 302b or other suitable memory.

[0115] The processor, memory and other related control devices may be provided on an appropriate circuit board and / or in a chipset. This feature is indicated by reference numeral 304. The device may optionally have user equipment such as a keyboard 305, a touch-sensitive screen or touch-sensitive pad, a combination thereof, or the like. Depending on the type of device, one or more of a display, a speaker, a microphone may optionally be provided.

[0116] As a user equipment (UE) moves through a wireless communication system, it may move through cells comprising radio coverage areas supported by one or more network access nodes. The ability for a UE to maintain effective communication with the wireless communication system while moving through the radio coverage areas is generally referred to as mobility. Layer 1 or Layer 2, sometimes referred to as low-layer triggered mobility, is increasingly being considered to reduce the latency, overhead, and interruption time associated with UEs moving between different cells.

[0117] Some embodiments relate to low layer triggered mobility (LTM) for a user equipment moving through a communication network having multiple network nodes supporting radio coverage in multiple cells. Each network node supports radio coverage in one or more cells, and the user equipment will be handed off between cells as it moves.

[0118] Low Layer Triggered Mobility (LTM), also denoted as L1 / 2 inter-cell mobility, is an upcoming goal to enhance mobility and can be used in new radio networks. According to the paradigm, the decision on cell change can be based on Layer 1 measurements (physical layer) and can be made at the L2 MAC (medium access control) layer.

[0119] In a network including a gNB that may have a central unit and one or more distributed units, decisions regarding cell changes may be made in a distributed unit or node (DU).

[0120] When a UE moves from the coverage area of ​​one cell to the coverage area of ​​another cell, at some point a serving cell change needs to be performed. In the absence of low-layer mobility, the serving cell change is triggered by L3 measurements and is done for the serving cell change with synchronized reconfiguration triggered by RRC signaling. The serving cell can be a PCell (primary cell), a PSCell (primary secondary cell), or other serving cells. All cases involve a complete L2 (and L1) reset, which can result in longer latency, greater overhead, and longer interruption time compared to beam switching mobility. The goal of L1 / L2 mobility enhancement is to enable serving cell changes via L1 / L2 signaling, with the aim of reducing latency, overhead, and interruption time.

[0121] In some embodiments, the network provides configuration information to the UE, where the configuration information includes configuration information for one or more LTM candidate target cells.

[0122] In some embodiments, the UE stores a configuration of LTM candidate target cell(s).

[0123] Then, before receiving the LTM cell handover command from the network, the UE may perform DL (downlink) synchronization and TA (timing advance) acquisition with one or more candidate cells from the list, so that once the command is received, the UE is ready to complete the handover to the selected one of the multiple target cells.

[0124] When a UE moves from the coverage area of ​​one cell to the coverage area of ​​another, a serving cell change needs to be performed at some point. Currently, serving cell changes are triggered by L3 (Layer 3) measurements and are performed by RRC (Radio Resource Control) signaling with synchronization reconfiguration. This can be used for changes in PCell (Primary Cell), changes in PSCell (Primary Secondary Cell), or the release or addition of SCell (Secondary Cell).

[0125] This involves a complete L2 (Layer 2) and L1 (Layer 1) reset. This may result in longer latency, greater overhead and / or longer outage time compared to beam switching mobility.

[0126] L1 layer 1 is a physical layer, L2 layer 2 is a MAC (Medium Access Control) layer, and L3 layer 3 is an RRC layer.

[0127] One purpose of L1 / L2 mobility enhancement may be to enable serving cell change via L1 / L2 signaling. This may reduce latency, overhead and / or interruption time.

[0128] The network NW may provide a configuration to the UE, which includes one or more LTM candidate target cells. The UE stores the configuration of the LTM candidate target cells. Before receiving the LTM cell handover command, the UE may perform DL synchronization and TA acquisition with one or more target cells.

[0129] Some embodiments may address the issue of a source cell failure that may occur before a cell handover command is received. The failure condition may be a radio link failure (RLF), a reconfiguration failure, an integrity check failure, or other failure conditions. While some embodiments are described with respect to RLF, it should be understood that the failure condition may be any other suitable failure condition. Currently, the UE will need to initiate a reestablishment process, in which cell selection is performed to find an appropriate cell.

[0130] In some embodiments, in case of RLF failure before a cell handover command is received, the UE selects one of the LTM candidate target cells. The LTM candidate target cell may be considered ready, ie, the UE context is available.

[0131] In contrast, a method in which the RRC re-establishment procedure for a cell change is performed may be more time-consuming than attempting to use one of the LTM candidate target cells.

[0132] It should be noted that if the RRC re-establishment procedure results in the UE selecting a cell that is not prepared, ie, UE context is not available, then the acquisition of the UE context for that cell may result in further delays.

[0133] Figure 5 The flow chart of LTM (L1 / L2 triggered mobility) is shown in FIG. The whole process for LTM is shown in FIG. Figure 5 Figure 1 shows the signaling between the UE and the gNB. The gNB entities may include a CU entity and one or more DU entities. The DU may provide one or more downlink cells. The CU hosts radio functions such as RRC and PDCP (Packet Data Convergence Protocol), while the DU hosts the PHY (physical) layer, MAC (medium access control) layer, and RLC (radio link control) layer. Figure 5 The four stages of LTM are shown:

[0134] LTM preparation: The target cell can be prepared. This can be based on UE measurements. The measurements can be L3 measurements.

[0135] Early synchronization;

[0136] LTM execution: Based on the UE's L1 report, the network indicates to the UE which of the prepared cells should be handed over.

[0137] LTM completion: The UE completes the HO to the target cell in case of context release and path switching.

[0138] Subsequent LTMs are completed by repeating the early synchronization, LTM execution, and LTM completion steps without releasing other candidate cells after each LTM is completed.

[0139] The process used for LTM is as follows.

[0140] The LTM preparation phase will now be described.

[0141] As shown in Figure 1, the UE sends a measurement report message to the gNB.

[0142] The gNB decides to use LTM and initiates LTM candidate preparation.

[0143] As shown in Figure 2, the gNB sends an RRC reconfiguration message to the UE, which includes the configuration of one or more candidate target cells.

[0144] As shown in Figure 3, the UE stores the configuration of one or more LTM candidate target cells and sends an RRC reconfiguration complete message to the gNB.

[0145] The early synchronization phase will now be described.

[0146] As shown in Figure 4, the UE may perform DL (downlink) synchronization and / or TA (timing advance) acquisition with one or more of the candidate cells. This is before the UE receives the LTM cell handover command.

[0147] The synchronization for the candidate cell(s) before the cell handover command may be based at least on, for example, an SSB (Synchronization Signal Block).

[0148] TA acquisition of cell(s) prior to a cell handover command may be at least partially supported by a RACH (Random Access Channel) commanded by a PDCCH (Physical Downlink Control Channel), where the PDCCH command is triggered only by the source cell.

[0149] The LTM execution phase will now be described.

[0150] As shown in Figure 5, the UE performs L1 measurements on one or more configured LTM candidate target cells and sends a low layer measurement report to the gNB.

[0151] The low-layer measurement report may be carried on the L1 or L2 or MAC layer.

[0152] As shown in Figure 6, the gNB decides to perform LTM cell handover to the target cell and sends a MAC CE (control element) triggering LTM cell handover. This may include the candidate configuration index of the target cell.

[0153] The UE will leave the source cell and apply the configuration for the LTM candidate target cell.

[0154] As shown in 7, if the TA is unavailable, the UE performs a random access procedure toward the target cell.

[0155] The LTM completion phase is now described.

[0156] As shown in Figure 8, the UE indicates the successful completion of the LTM cell handover towards the target cell. After the UE has been handed over to the target cell, an uplink signal or message may be sent to the gNB to indicate that the UE has been handed over to the target cell.

[0157] Some embodiments allow the UE to attempt to access an LTM candidate target cell upon RLF (Radio Link Failure) in the serving cell.Before starting a new cell selection process, the UE attempts to access an LTM candidate target cell.

[0158] The LTM candidate target cell may be prepared. UE context may be available. This may be due to previous configuration within the network. A serving gNB and a target gNB may exist. A CU may exist. The target cell gNB may serve other cells to which the UE is connected.

[0159] The target cell can be prepared as discussed in the following examples. The CU can control the serving gNB-DU and the target gNB-DU. In some embodiments, the serving and target gNB-DUs can communicate without the CU. The CU can establish a relationship between the source DU and the target DU regarding the cells to be measured by the UE and candidate target cells for LTM. These candidate target cells can be configured at the UE prior to RLF via the serving DU.

[0160] For LTM operation of the UE in the target cell, resources have been allocated, prepared and configured in the target cell. This can be through the target DU, through the CU, or through both.

[0161] Based on measurement results that suggest which target cells may be candidates for LTM operation for the UE, the CU configures those candidate target cells at the UE. This can be done via RRC signaling via the serving gNB-DU. The CU prepares / configures the gNB-DUs that support LTM operation for those target cells for the UE. For example, the CU sends a UE context to the target gNB-DU, which includes information on the UE and its communication status with the network. This can trigger the preparation of radio resources in the target cell for LTM operation, as well as resource preparation for the user plane connection towards the CN via the CU (user plane part). When the UE attempts to access the target cell for LTM operation in the completion phase, the target cell is enabled to proceed without further delay caused by preparing the connection for the UE.

[0162] It should be understood that in other embodiments, the target cell may be prepared in different ways.

[0163] Some embodiments can introduce error tolerance into the cell change process. The serving cell and the UE detect RLF, but the UE is expected to "return" to the serving cell or to any of the configured candidate target cells. Since one or more target cells for LTM have already been configured, the UE attempts to access those target cells before starting the regular cell search process. This regular cell search process may even cause a longer delay in reestablishing connectivity to the network.

[0164] Some embodiments propose that before starting the cell selection process, the UE attempts to access an LTM candidate target cell upon RLF in the serving cell.

[0165] Due to previous configuration within the network (serving gNB-DU, target gNB-DU, CU), the LTM candidate cell may already be prepared, meaning the UE context is available. The gNB CU provides the configuration via the serving gNB-DU. The gNB provides the configuration to the UE via RRC signaling. The target gNB-DU is then the prepared entity and makes the UE context available when LTM is triggered.

[0166] The target DU that receives the UE signal to operate in LTM will notify the CU accordingly. This indicates to the CU that, despite notification of RLF by the serving cell, the CU should not immediately release UE content and resources. This will allow the target DU to continue the UE's LTM operation. Therefore, the target DU will be changed to the serving cell and serving DU triggered by the CU.

[0167] The embodiments may be applied where there is RLF in the source cell before any mobility commands have been received from the source cell to move the UE to the target cell.

[0168] When RLF occurs in the source cell, the UE needs to determine whether it is allowed to access the candidate target cell via LTM completion or random access procedure. This can be based on an evaluation of the candidate target cells. For example, the candidate cell with the highest signal power or quality is selected.

[0169] One or more network entities may allow completion of LTM for a UE. The network entity may be the target gNB-DU and / or the CU. This may be for a predetermined time period at the CU / DU. This may be allowed even if RLF has been detected and reported to the CU by the source gNB-DU.

[0170] The CU may delay removal of already allocated resources related to an ongoing procedure for a UE with RLF for a predetermined period of time.

[0171] In some embodiments, the UE may attempt LTM completion after RLF (Radio Link Failure) has occurred in the source cell (e.g., source SpCell / PCell / PSCell). SpCell is a "special cell" that is a general name for the function of calling PCell or PSCell in the MAC layer.

[0172] In some embodiments, RLF with the source cell occurs before the path switch command is received, i.e., before signaling reference 6. If RLF with the source cell occurs after the path switch command is issued, then the UE will switch to the target cell based on the handover command.

[0173] In some embodiments, RLF with the source cell following reception of an LTM candidate cell in signaling reference 2 may result in the UE attempting LTM completion using the candidate cell. Embodiments may be used after the UE has received candidate target cell configuration(s) and has (optionally) confirmed that configuration to the NW, where RLF would have occurred afterwards.

[0174] In some embodiments, after RLF occurs, the UE attempts to complete LTM on the configured LTM candidate target cell(s).

[0175] In some embodiments, after an RLF occurs, the UE attempts to complete LTM on the configured LTM candidate target cell(s). This may be done only if one or more of the LTM candidate target cells meets at least one threshold and / or one or more conditions. For example, the threshold may be a measurement power threshold and / or a quality threshold. For example, the condition may be that the measurement of the reporting time is met by the UE.

[0176] The condition may be that the candidate target cell(s) have been reported. That is, the UE has sent a measurement report to the gNB, which includes the candidate target cell(s). The measurement report may be triggered by a measurement event. The measurement report may include information on the candidate cell. The information on the candidate cell may include one or more of the following: cell ID, signal quality, signal strength, which measurement report event triggered the report, and / or the like.

[0177] In some embodiments, the at least one threshold comprises one or more of: (L1) RSRP (reference signal received power) threshold; RSRQ (reference signal received quality) threshold; or SINR (signal to interference and noise ratio) threshold.

[0178] In some embodiments, when RLF occurs on a configured LTM candidate target cell or cell set, the UE may attempt to complete LTM, and the UE measures and / or reports the highest (L1) RSRP for the configured LTM candidate target cell or cell set to the NW. This may be reported to the CU via the source DU.

[0179] In some embodiments, the UE attempts to complete LTM after RLF occurs on candidate target cell(s), where the UE has: performed DL synchronization; performed TA acquisition; and / or where the UE has a valid TA. This may be based on, for example, LTM configuration information).

[0180] In some embodiments, if the TA is unavailable, the UE attempts to complete LTM on the configured LTM candidate target cell(s) by performing a random access procedure towards the target cell.

[0181] In some embodiments, the UE attempts to complete LTM on the configured LTM candidate target cell(s) by indicating successful completion of the LTM cell handover towards the target cell. The indication may be an uplink signal or message.

[0182] In some embodiments, the UE indicates in the RRC Reconfiguration Complete message that LTM is performed due to a failure in the source cell or is performed without LTM commanded by the NW (ie, by the source cell).

[0183] The UE may indicate the source cell information in the RRC Reconfiguration Complete message. For example, the PCI (Physical Layer Cell Identifier) ​​may be used to identify the source cell. This may allow the NW to initiate, for example, an intra-NW path switching procedure and / or the like. The UE may identify itself via the C-RNTI (Cell Radio Network Temporary Identifier) ​​MAC CE in Msg3 of the RA procedure in the target cell. This may be accomplished, for example, as in a traditional HO.

[0184] In some embodiments, when the UE selects one of at least one LTM candidate target cells during cell selection after an RLF occurs, the UE initiates a new RRC procedure and / or sends a new RRC message to a network node. This is based on the fact that the target cell is prepared for the UE and has the UE configuration. The network node can be a gNB or a CU via a target DU.

[0185] For example, such an RRC message (e.g., an RRC request message) may include one or more of the following: the UE ID in the target cell configured in the LTM configuration (e.g., C-RNTI cell radio network temporary identifier); information of the source cell (e.g., PCI); and / or security information. For example, the security information may be a shortMAC-I (Message Authentication Code-Integrity) generated based on the target cell configuration information so that the target cell can verify the UE integrity without information from the source cell. The RRC message may be sent on SRB0 (Signaling Radio Bearer) or any other suitable SRB. The source cell information may be used to initiate a path switching procedure within the NW.

[0186] In one example, the NW configures the UE to attempt LTM after RLF.

[0187] In some embodiments, the NW will attempt LTM completion for the UE on one or more candidate cells after RLF.

[0188] In some embodiments, the NW may configure the UE to attempt LTM completion on candidate target cell(s) where the UE has performed DL synchronization and / or TA acquisition.

[0189] In some embodiments, the UE will indicate to the gNB that it is capable of supporting LTM completion in the event of an RLF failure. This may be provided by a capability indication. The gNB may use this information to prevent candidate cells for resource association from being immediately removed in the event of an RLF.

[0190] In some embodiments, the gNB will indicate to the UE that it should attempt LTM completion in the event of an RLF failure. This may be provided by an indicator. This may be provided during connection establishment. The UE may use this information to attempt LTM completion in the event of an RLF failure.

[0191] In some embodiments, there may be a message exchange between the UE and the gNB so that both the UE and the gNB are aware that LTM completion is not only supported but also implemented in the case of RLF failure behavior.

[0192] In some embodiments, RLF is declared by the UE when one or more of the following criteria are met:

[0193] expiry of the radio problem timer which starts after an indication of a radio problem from the physical layer (if the radio problem is recovered before the timer has expired, the UE stops the timer); or

[0194] a timer that was started upon triggering a measurement report for a measurement identity for which the timer has been configured expires while another radio problem timer is running; or

[0195] The random access procedure fails (e.g., the number of preamble transmissions reaches a configured threshold); or

[0196] RLC (Radio Link Control) failure (e.g., the number of RLC retransmissions reaches a configured threshold); or

[0197] Detection of persistent uplink LBT (Listen Before Talk) failure for operation with shared spectrum channel access; or

[0198] For IAB-MT (Integrated Access and Backhaul Mobile Termination), the reception of BH (Backhaul) RLF indication is received from its parent node. Backhaul is the connection between two cells.

[0199] In some embodiments, the UE is allowed to attempt LTM completion on a candidate target cell after RLF. This can minimize the delay in completing a successful LTM cell handover.

[0200] refer to Figure 6 , which illustrates a method of some embodiments. The method may be performed by an apparatus. The apparatus may be a UE or provided in a UE.

[0201] As shown in A1, the apparatus is configured so that a measurement report is sent to the gNB. This can be sent to the CU via the source DU.

[0202] As shown in A2, the device receives a candidate cell configuration for a mobility procedure. The candidate cell configuration may be received from the gNB. In particular, the candidate cell configuration may be received from the CU via the source DU.

[0203] As indicated by A3 , the apparatus is configured such that synchronization is performed with respect to one or more of the candidate cells.

[0204] As indicated by A4 , the apparatus is configured to determine that there has been a radio link failure with respect to the connection with the source cell.

[0205] As shown in A5, the apparatus is configured to attempt to complete the mobility procedure with one or more of the candidate cells.The apparatus may be configured to attempt to complete the mobility procedure with respect to the candidate cells for which synchronization has been performed.

[0206] As indicated by A7, if the process has been successfully completed using one of the candidate cells, then the candidate cell becomes the new source cell for the device.

[0207] As with reference to A6, a determination is made as to whether the mobility procedure has been successfully completed using one of the candidate cells.

[0208] If the process is not successful, then the apparatus is configured to initiate a cell selection process.

[0209] refer to Figure 7 , which illustrates another method of some embodiments. The method may be performed by an apparatus.

[0210] The apparatus may be in or be a gNB. In some embodiments, the apparatus may be in or be a CU.

[0211] As shown in B1, the device may receive a measurement report from the UE.

[0212] As shown in B2, the apparatus is configured to determine a candidate cell configuration for a mobility procedure. This is based on a measurement report received from the UE. The apparatus is configured so that the candidate cell configuration for the mobility procedure is sent to the UE.

[0213] As shown in B3, the apparatus is configured to receive radio link failure information from the source cell, which indicates that a radio link failure has occurred between the source cell and the UE.

[0214] As shown in B4, the device is configured to start a timer.

[0215] As shown in B5, the apparatus is configured to determine whether the UE has managed to complete the mobility procedure using one or more of the candidate cells before the expiration of the timer. It should be understood that in some embodiments, in response to receiving a message from the UE before the expiration of the timer, the apparatus will determine that the UE has managed to complete the mobility procedure using one or more candidate cells.

[0216] As shown in B6, if the UE has not managed to complete the mobility procedure before the expiry of the timer, then the resources associated with one or more of the candidate cells are removed. In other words, the UE context established with respect to the candidate cells is removed.

[0217] As indicated by B7 7 , if the UE has managed to complete the mobility procedure before the expiry of the timer, then the UE will use the candidate cell for which the mobility procedure was completed.

[0218] refer to Figure 8 , which illustrates the methods of some embodiments.

[0219] The method may be performed by an apparatus. The apparatus may be in a communication device or may be a communication device.

[0220] The apparatus may comprise suitable circuitry for providing the method.

[0221] Alternatively or additionally, the apparatus may include: at least one processor and at least one memory storing instructions, which when executed by the at least one processor cause the apparatus to at least provide the following method.

[0222] Alternatively or additionally, the device may be such as one involving Figure 3 or Figure 4 discussed.

[0223] The methods may be provided by computer program code or computer executable instructions.

[0224] As indicated by C1 , the method may include receiving a message from a network node on a source cell, the message indicating one or more low-layer mobility candidate cells.

[0225] As indicated by C2, the method may include: determining a failure of the source cell.

[0226] As indicated at C3 , the method may include, in response to a determination of failure, attempting completion of low layer mobility to a target cell selected from one of the one or more candidate cells.

[0227] It should be understood that Figure 8 The method outlined in can be modified to include any of the features previously described.

[0228] refer to Figure 9 , which illustrates the methods of some embodiments.

[0229] The method may be performed by an apparatus, which may be in an access node or be an access node.

[0230] The apparatus may comprise suitable circuitry for providing the method.

[0231] Alternatively or additionally, the apparatus may include: at least one processor and at least one memory storing instructions, which when executed by the at least one processor cause the apparatus to at least provide the following method.

[0232] Alternatively or additionally, the device may be such as Figure 3 discussed.

[0233] The methods may be provided by computer program code or computer executable instructions.

[0234] As indicated at D1 , the method may include causing a message to be sent to a communication device connected to a source cell, the message indicating one or more low-layer mobility candidate cells.

[0235] As indicated at D2, the method may include receiving a message from the communication device indicating completion of low layer mobility of a target cell selected from one of the one or more candidate cells, the completion being a response to a failure of the source cell.

[0236] It should be understood that Figure 9 The method outlined in can be modified to include any of the features previously described.

[0237] The computer program code may be downloaded and stored in one or more memories of the relevant apparatus or device.

[0238] Thus, although certain embodiments are described above by way of example with reference to certain example architectures for wireless networks, technologies, and standards, the embodiments may be applied to any other suitable form of communication system than those illustrated and described herein.

[0239] It is also noted herein that while the above describes example embodiments, there are several variations and modifications which may be made to the disclosed solution without departing from the scope of the present invention.

[0240] As used herein, “at least one of: ” and “at least one of ” and similar expressions, where a list of two or more elements is connected by “and” or “or”, mean any one of the elements, or at least any two or more of the elements, or at least all of the elements.

[0241] In general, various embodiments may be implemented in hardware or dedicated circuitry, software, logic, or any combination thereof. Some aspects of the present disclosure may be implemented in hardware, while other aspects may be implemented in firmware or software, which may be executed by a controller, microprocessor, or other computing device, although the present disclosure is not limited thereto. Although various aspects of the present disclosure may be illustrated and described as block diagrams, flow charts, or using some other graphical representation, it is well understood that the blocks, devices, systems, techniques, or methods described herein may be implemented, as non-limiting examples, in hardware, software, firmware, dedicated circuitry or logic, general-purpose hardware or a controller or other computing device, or some combination thereof.

[0242] As used in this application, the term "circuitry" may refer to one or more or all of the following:

[0243] (a) hardware circuit implementations only (such as implementations in analog and / or digital circuitry only) and

[0244] (b) a combination of hardware circuitry and software such as (where applicable):

[0245] (i) a combination of analog and / or digital hardware circuit(s) with software / firmware, and

[0246] (ii) any portion of a hardware processor(s) with software (including digital signal processor(s), software and memory(s) that work together to enable a device (such as a mobile phone or server) to perform various functions); and

[0247] (c) Hardware circuit(s) and or processor(s), such as microprocessor(s) or portion(s) of microprocessor(s), that require software (e.g., firmware) for operation, but in which case the software may be absent when not required for operation.

[0248] This definition of circuitry applies to all uses of the term in this application, including in any claims. As a further example, as used in this application, the term circuitry also covers an implementation of merely a hardware circuit or processor (or multiple processors) or a portion of a hardware circuit or processor and its (or their) accompanying software and / or firmware. For example, if applicable to a particular claim element, the term circuitry also covers an integrated circuit such as a baseband integrated circuit or a processor integrated circuit for a mobile device, or a similar integrated circuit in a server, cellular network device, or other computing or network device.

[0249] The embodiments of the present disclosure may be implemented by computer software executable by a data processor of a mobile device (such as in a processor entity), or by hardware, or by a combination of software and hardware. Computer software, also referred to as program products, including software routines, applets and / or macros, may be stored in any device-readable data storage medium, and they include program instructions to perform specific tasks. A computer program product may include one or more computer executable components that are configured to execute an embodiment when the program is running. One or more computer executable components may be at least one software code or a portion thereof.

[0250] Furthermore, in this regard, it should be noted that any block of the logic flow in the figure may represent program steps, or interconnected logic circuits, blocks and functions, or a combination of program steps and logic circuits, blocks and functions. The software may be stored on physical media such as memory chips or memory blocks implemented within a processor, magnetic media (such as hard disks or floppy disks), and optical media (such as, for example, DVDs and their data variants, CDs). Physical media are non-transitory media.

[0251] As used herein, the term "non-transitory" is a restriction on the medium itself (ie, tangible, not a signal), not on the persistence of the data storage (eg, RAM versus ROM).

[0252] The memory may be of any type suitable for the local technical environment and may be implemented using any suitable data storage technology, such as semiconductor-based memory devices, magnetic memory devices and systems, optical memory devices and systems, fixed memory, and removable memory. The data processor may be of any type suitable for the local technical environment and may include, by way of non-limiting example, one or more of the following: a general-purpose computer, a special-purpose computer, a microprocessor, a digital signal processor (DSP), an application-specific integrated circuit (ASIC), an FPGA, a gate-level circuit, and a processor based on a multi-core processor architecture.

[0253] Embodiments of the present disclosure may be practiced in various components, such as integrated circuit modules. The design of integrated circuits is generally a highly automated process. Complex and powerful software tools are available to convert a logic-level design into a semiconductor circuit design ready to be etched and formed on a semiconductor substrate.

[0254] The foregoing description has provided a comprehensive and informative description of exemplary embodiments of the present disclosure by way of non-limiting examples. However, various modifications and adaptations may become apparent to those skilled in the relevant art in view of the foregoing description when read in conjunction with the accompanying drawings and the appended claims. In fact, there are further embodiments comprising a combination of one or more embodiments with any of the other embodiments previously discussed. The scope of protection sought by some embodiments of the present disclosure is set forth in the claims. The embodiments and features (if any) described in this specification that do not fall within the scope of the claims are to be construed as examples useful for understanding the various embodiments of the present disclosure. It should be noted that in related applications such as divisional applications or continuation applications, different claims with different claim scopes may be pursued.

Claims

1. A device comprising: means for receiving a message from a network node on a source cell, the message indicating one or more low-layer mobility candidate cells; means for determining failure of said source cell; as well as means for attempting completion of low layer mobility to a target cell selected from one of one or more candidate cells in response to said determination of said failure.

2. The apparatus of claim 1 , wherein the failure comprises: Radio link failure.

3. The apparatus of claim 1 or 2, wherein the means for attempting to access one or more of the one or more candidate target cells attempts to perform low layer mobility completion in the absence of a path switch command from the network.

4. An apparatus according to any preceding claim, wherein the message comprises: Configuration information for the one or more candidate cells among the one or more candidate cells.

5. An apparatus according to any preceding claim, comprising: Means for causing a measurement report to be sent to the network, and receiving the message in response to the measurement report.

6. The apparatus of any preceding claim, wherein the low-level mobility comprises: L1 / L2 triggered mobility.

7. An apparatus according to any preceding claim, comprising: Means for synchronizing with one or more of the one or more candidate cells.

8. An apparatus according to any preceding claim, comprising: Means for obtaining timing advance information for one or more of the one or more candidate cells.

9. An apparatus according to any preceding claim, comprising: Means for determining, in response to determining the failure, that one or more of the one or more candidate cells satisfy at least one threshold or at least one condition.

10. The apparatus of claim 9, wherein the at least one threshold comprises: Measure power threshold or quality threshold.

11. The apparatus according to claim 9 or 10, wherein the at least one condition comprises: One or more of the one or more candidate cells have been reported to the access node in a measurement report.

12. An apparatus according to any preceding claim, comprising: A component for performing a random access procedure on the target cell.

13. An apparatus according to any preceding claim, comprising: Means for causing a message to be sent to the network node, the message indicating that the low layer mobility to the target cell has been completed.

14. An apparatus according to any preceding claim, wherein the message to be sent to the network node indicates that the low layer mobility to the target cell is responsive to the failure.

15. An apparatus comprising: means for causing a message to be sent to a communications device connected to a source cell, the message indicating one or more low-layer mobility candidate cells; Means for receiving a message from a communications device indicating completion of low layer mobility to a target cell selected from one of one or more candidate cells, the completion being a response to a failure of the source cell.

16. The apparatus according to claim 15, comprising: Means for determining failure of the source cell and, in response to starting a timer, delaying removal of resources associated with the one or more candidate cells to allow completion of low layer mobility to a target cell prior to said removal of the resources.

17. The apparatus according to any one of claims 15 or 16, wherein the timer is started without being caused to send a path switch command to the communication device.

18. The apparatus of claim 15, 16 or 17, wherein the failure comprises: Radio link failure.

19. A method comprising: receiving a message from a network node on a source cell, the message indicating one or more low-layer mobility candidate cells; determining a failure of the source cell; as well as Responsive to the determination of the failure, completion of low layer mobility is attempted to a target cell selected from one or more candidate cells.

20. A method comprising: causing a message to be sent to a communications device connected to a source cell, the message indicating one or more low-layer mobility candidate cells; as well as A message is received from a communications device indicating completion of low layer mobility to a target cell selected from one of one or more candidate cells, the completion being in response to a failure of the source cell.

21. A computer program comprising computer executable instructions which, when executed, cause the method according to claim 19 or 20 to be performed.

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