Devices, methods and apparatuses for recovery operation

Abstract

Embodiments of the present disclosure disclose devices, methods and apparatuses for recovery operation for failure of a terminal device based timing advance (TA) estimation. In an aspect, the terminal device determines, based on a criterion, failure of a timing advance (TA) estimation performed by the terminal device for a candidate cell. Based on determining the failure of the TA estimation, the terminal device transmits, to a network device, an indication of a recovery operation for the failure of the TA estimation. A recovery mechanism for the recovery of the failure of TA estimation can be proposed according to the present disclosure such that the terminal device can switch to the target cell timely even the TA estimation at the terminal device is failed.

Description

DEVICES, METHODS AND APPARATUSES FOR RECOVERYOPERATIONFIELD

[0001] Embodiments of the present disclosure generally relate to the field of communication, and in particular, to devices, methods, apparatuses and a computer readable storage medium for recovery operation, especially for failure of a terminal device based timing advance (TA) estimation.BACKGROUND

[0002] A communication network can be seen as a facility that enables communications between two or more communication devices, or provides communication devices access to a data network. A mobile or wireless communication network is one example of a communication network.

[0003] Such communication networks operate in according with standards such as those provided by 3GPP (Third Generation Partnership Project) or ETSI (European Telecommunications Standards Institute). Examples of standards are the so-called 5G (5th Generation) standards provided by 3 GPP. The 3 GPP has agreed that the terminal device can perform a timing advance estimation for candidate cells after configured by the network device.SUMMARY

[0004] In general, example embodiments of the present disclosure provide devices, methods, apparatuses and a computer readable storage medium for recovery operation for failure of a terminal device based timing advance (TA) estimation.

[0005] In a first aspect, there is provided a terminal device. The terminal device may comprise: at least one processor and at least one memory storing instructions that, when executed by the at least one processor, cause the terminal device at least to: determine, based on a criterion, failure of a timing advance (TA) estimation performed by the terminal device for a candidate cell; and based on determining the failure of the TA estimation, transmit, to a network device, an indication of a recovery operation for the failure of the TA estimation.

[0006] In a second aspect, there is provided a first network device. The network device may comprise: at least one processor and at least one memory storing instructions that,when executed by the at least one processor, cause the first network device at least to: receive, from a terminal device, an indication of a recovery operation for failure of a timing advance (TA) estimation performed by the terminal device for a candidate cell provided by a second network device; and transmit, to the second network device, an indication that the terminal device is to perform a random access procedure towards the candidate cell

[0007] In a third aspect, there is provided a method. The method may comprise: determining, based on a criterion, failure of a timing advance (TA) estimation performed by the terminal device for a candidate cell; and based on determining the failure of the TA estimation, transmitting, to a network device, an indication of a recovery operation for the failure of the TA estimation

[0008] In a fourth aspect, there is provided a method. The method may comprise: receiving, from a terminal device, an indication of a recovery operation for failure of a timing advance (TA) estimation performed by the terminal device for a candidate cell provided by a second network device; and transmitting, to the second network device, an indication that the terminal device is to perform a random access procedure towards the candidate cell.

[0009] In a fifth aspect, there is provided an apparatus. The apparatus may comprise: means for determining, based on a criterion, failure of a timing advance (TA) estimation performed by the terminal device for a candidate cell; and means for based on determining the failure of the TA estimation, transmitting, to a network device, an indication of a recovery operation for the failure of the TA estimation.

[0010] In a sixth aspect, there is provided an apparatus. The apparatus may comprise: means for means for receiving, from a terminal device, an indication of a recovery operation for failure of a timing advance (TA) estimation performed by the terminal device for a candidate cell provided by a second network device; and means for transmitting, to the second network device, an indication that the terminal device is to perform a random access procedure towards the candidate cell

[0011] In a seventh aspect, there is provided a non-transitory computer readable medium comprising program instructions for causing an apparatus to perform at least the method according to third or fourth aspect.

[0012] In an eighth aspect, there is provided a computer program comprising instructions, which, when executed by an apparatus, cause the apparatus at least to perform at least the method according to third or fourth aspect.

[0013] In a ninth aspect, there is provided a terminal device. The terminal device may comprise: determining circuitry for determining, based on a criterion, failure of a timing advance (TA) estimation performed by the terminal device for a candidate cell; and transmitting circuitry for based on determining the failure of the TA estimation, transmitting, to a network device, an indication of a recovery operation for the failure of the TA estimation.

[0014] In a tenth aspect, there is provided a network device. The network device may comprise: receiving circuitry for receiving, from a terminal device, an indication of a recovery operation for failure of a timing advance (TA) estimation performed by the terminal device for a candidate cell provided by a second network device; and transmitting circuitry for transmitting, to the second network device, an indication that the terminal device is to perform a random access procedure towards the candidate cell.

[0015] It is to be understood that the summary section is not intended to identify key or essential features of embodiments of the present disclosure, nor is it intended to be used to limit the scope of the present disclosure. Other features of the present disclosure will become easily comprehensible through the following description.BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Some example embodiments will now be described with reference to the accompanying drawings, in which:

[0017] Fig. 1 illustrates an example network environment in which example embodiments of the present disclosure may be implemented;

[0018] Fig. 2 illustrates an example signaling process for recovery operation for failure of TA estimation according to some embodiments of the present disclosure;

[0019] Fig. 3 illustrates an example signaling process for recovery operation of switching to the target cell after UE-based TA failure according to some embodiments of the present disclosure;

[0020] Fig. 4 illustrates an example flowchart of a method implemented at a terminal device according to example embodiments of the present disclosure;

[0021] Fig. 5 illustrates an example flowchart of a method implemented at a first network device according to example embodiments of the present disclosure;

[0022] Fig. 6 illustrates an example simplified block diagram of an apparatus that is suitable for implementing embodiments of the present disclosure; and

[0023] Fig.7 illustrates an example block diagram of an example computer readablemedium in accordance with some embodiments of the present disclosure.

[0024] Throughout the drawings, the same or similar reference numerals represent the same or similar element.DETAILED DESCRIPTION

[0025] Principles of the present disclosure will now be described with reference to some example embodiments. It is to be understood that these embodiments are described only for the purpose of illustration and help those skilled in the art to understand and implement the present disclosure, without suggesting any limitation as to the scope of the disclosure. The disclosure described herein may be implemented in various manners other than the ones described below.

[0026] In the following description and claims, unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skills in the art to which the present disclosure belongs.

[0027] References in the present disclosure to “one embodiment,” “an embodiment,” “an example embodiment,” and the like indicate that the embodiment described may include a particular feature, structure, or characteristic, but it is not necessary that every embodiment includes the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to affect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described.

[0028] It may be understood that although the terms “first” and “second” etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and similarly, a second element could be termed a first element, without departing from the scope of example embodiments. As used herein, the term “and / or” includes any and all combinations of one or more of the listed terms.

[0029] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood thatthe terms “comprises”, “comprising”, “has”, “having”, “includes” and / or “including”, when used herein, specify the presence of stated features, elements, and / or components etc., but do not preclude the presence or addition of one or more other features, elements, components and / or combinations thereof. As used herein, “at least one of the following: ” and “at least one of ” and similar wording, where the list of two or more elements are joined by “and” or “or”, mean at least any one of the elements, or at least any two or more of the elements, or at least all the elements.

[0030] As used in this application, the term “circuitry” may refer to one or more or all of the following:(a) hardware-only circuit implementations (such as implementations in only analog and / or digital circuitry) and(b) combinations of hardware circuits and software, such as (as applicable):(i) a combination of analog and / or digital hardware circuit(s) with software / firmware and(ii) any portions of hardware processor(s) with software (including digital signal processor(s)), software, and memory(ies) that work together to cause an apparatus, such as a mobile phone or server, to perform various functions) and(c) hardware circuit(s) and or processor(s), such as a microprocessor(s) or a portion of a microprocessor s) that requires software (e.g., firmware) for operation, but the software may not be present when it is not needed for operation.

[0031] This definition of circuitry applies to all uses of this 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 portion of a hardware circuit or processor and its (or their) accompanying software and / or firmware. The term circuitry also covers, for example and if applicable to the particular claim element, a baseband integrated circuit or processor integrated circuit for a mobile device or a similar integrated circuit in server, a cellular network device, or other computing or network device.

[0032] As used herein, the term “communication network” refers to a network following any suitable communication standards, such as long term evolution (LTE), LTE-advanced (LTE-A), wideband code division multiple access (WCDMA), high-speed packet access (HSPA), narrow band Internet of things (NB-IoT) and so on. Furthermore, thecommunications between a terminal device and a network device in the communication network may be performed according to any suitable generation communication protocols, including, but not limited to, the third generation (3G), the fourth generation (4G), 4.5G, the fifth generation (5G) communication protocols, and / or beyond. Embodiments of the present disclosure may be applied in various communication systems. Given the rapid development in communications, there will of course also be future type communication technologies and systems with which the present disclosure may be embodied. It should not be seen as limiting the scope of the present disclosure to only the aforementioned system.

[0033] As used herein, the term “network device” refers to a node in a communication network via which a terminal device accesses the network and receives services therefrom. The network device may refer to a base station (BS) or an access point (AP), for example, a node B (NodeB or NB), an evolved NodeB (eNodeB or eNB), a NR NB (also referred to as a gNB), a remote radio unit (RRU), a radio header (RH), a remote radio head (RRH), a relay, a low power node such as a femto, a pico, and so forth, depending on the applied terminology and technology.

[0034] The term “terminal device” refers to any end device that may be capable of wireless communication. By way of example rather than limitation, a terminal device may also be referred to as a communication device, user equipment (UE), a subscriber station (SS), a portable subscriber station, a mobile station (MS), or an access terminal (AT). The terminal device may include, but not limited to, a mobile phone, a cellular phone, a smart phone, voice over IP (VoIP) phones, wireless local loop phones, a tablet, a wearable terminal device, a personal digital assistant (PDA), portable computers, desktop computer, image capture terminal devices such as digital cameras, gaming terminal devices, music storage and playback appliances, vehicle-mounted wireless terminal devices, wireless endpoints, mobile stations, laptop-embedded equipment (LEE), laptop-mounted equipment (LME), USB dongles, smart devices, wireless customerpremises equipment (CPE), an Internet of things (loT) device, a watch or other wearable, a head-mounted display (HMD), a vehicle, a drone, a medical device and applications (e.g., remote surgery), an industrial device and applications (e.g., a robot and / or other wireless devices operating in an industrial and / or an automated processing chain contexts), a consumer electronics device, a device operating on commercial and / or industrial wireless networks, and the like. In the following description, the terms “terminal device”, “communication device”, “terminal”, “user equipment” and “UE” may be usedinterchangeably.

[0035] When a user equipment (UE) moves from one cell to another cell, at some point a serving cell change needs to be performed. In some cases, the serving cell change is done by explicit radio resource control (RRC) reconfiguration signaling to trigger the synchronization of target cell based on L3 measurements report. It leads to longer latency, larger overhead, and longer interruption time than beam level mobility. Therefore, in 3 GPP Release 18, a new work item on further new radio (NR) mobility enhancements, named as layer 1 (LI) / layer 2 (L2)-triggered mobility (LTM), was approved to enable a serving cell change via L1 / L2 signaling, in order to reduce the latency, overhead and interruption time.

[0036] Further, in release 18, the 3GPP has also agreed the UE-based timing advance (TA) estimation mechanism. That is to say, 3GPP agrees to allow gNB to configure UE to perform UE based TA estimation if UE supports such capability. The RAN2 assumes that the exact time the UE performs TAmeasurement is up to UE implementation, and it is no need to specify in RAN2 text specification. That is to say, the exact time to start performing UE based TA estimation is up to UE implementation.

[0037] The procedure may be performed as follows, at the LTM cell switch, UE uses TA from the network if it is provided (target TA or TA=0 or TA=same as source). If the TA from the network is not provided and the UE is configured for UE based TA, then UE based TA is used. If the UE does not have or cannot derive the TA for target cell, the cell switch uses random access channel (RACH). That is to say, the TA provided by the network has the highest priority to be used. If this TA is not given, UE will use UE based TA if it is obtained correctly, if not, RACH procedure will be executed.

[0038] Regardless whether the UE is configured for UE based TA, the UE follows PDCCH-order, including requests to do RACH towards candidate cells, for which the UE could derive the TA by itself. That is to say, even if UE is configured to perform UE based TA, UE shall still follow any actions that are triggered by gNB. Further, regardless whether the UE has performed RACH towards candidate cell, the UE will follow configuration for UE based TA, if configured. That is to say, if UE has performed RACH towards the candidate cells, UE will follow the configuration to obtain TA if UE based TA is configured.

[0039] However, there are still many issues that could result in inefficient LTM. For example, the UE may be configured to perform UE-based TA for up to 8 candidate cell, however, it is not clear whether or not the UE could obtain the TA for all configured cellsin time in order not to trigger RACH after cell switch. Further, UE may fail to obtain a valid TA estimation for a candidate cell due to frequent varied channel model. Without this valid TAfor the selected target candidate cell, UE will resort to RACH after the media access control control element (MAC-CE) cell switch command, and this again will prolong the handover delay. Furthermore, gNB configures the UE based TA Estimation. However, if UE fails to estimate TA or TA is not valid, source cell is not aware of it, neither target cell. Assuming that UE has acquired the TA, in the case of RACH-less dynamic grant based cell switch, source cell, at the same time sending MAC-CE cell switch, will trigger target cell for PDCCH scheduling to UE, having the idea that UE has TA. However, the UE actually does not have the TA since it fails to estimate the TA, upon receiving MAC-CE cell switch from the source cell, UE would send PRACH on the first available opportunity. Until target gNB receive the PRACH message, target gNB will send PDCCH scheduling to UE.

[0040] In view of the above, some embodiments of the present disclosure propose a solution for recovery operation for failure of a terminal device based timing advance (TA) estimation. In some embodiments, the terminal device determines, based on a criterion, failure of a timing advance (TA) estimation performed by the terminal device for a candidate cell; and based on determining the failure of the TA estimation, transmits, to a network device, an indication of a recovery operation for the failure of the TA estimation. A recovery mechanism for the recovery of the failure of TA estimation can be proposed according to the present disclosure such that the terminal device can access to the target cell in time after the failure of TA estimation at the terminal device side.

[0041] For illustrative purposes, principle and example embodiments of the present disclosure for recovery operation for failure of timing advance (TA) estimation will be described below with reference to Figs. 1-7. However, it is to be noted that these embodiments are given to enable the skilled in the art to understand inventive concepts of the present disclosure and implement the solution as proposed herein, and not intended to limit scope of the present application in any way.

[0042] Fig. 1 illustrates an example network environment 100 in which example embodiments of the present disclosure may be implemented. The environment 100, which may be a part of a communication network, includes terminal devices and network devices.

[0043] As illustrated in Fig. 1, the communication network 100 may include a terminal device 110 (hereinafter may also be referred to as a user equipment (UE) 110). Thecommunication network 100 may further include a first network device 120, which can be a source distributed unit (DU) providing a plurality of cells, and one of them can be a serving cell (for example, cell 120-1) for serving the terminal device 110 now. The communication network 100 may further include a second network device 130, which can be a target DU providing a plurality of candidate cells, and one of the candidate cells may be a target cell (for example, cell 130-1) to be switched to by the terminal device 110. The communication network 100 may further include a third network device 140, which can be the center unit (CU) for managing the source DU and the target DU.

[0044] As shown in Fig. 1 when the UE 110 moves between different cells belonging to different DUs within a same CU. In short, this scenario is called as inter-DU mobility. As shown in Fig. 1, the serving cell 120-1 belongs to the first network device 120, and the target cell 130-1 belongs to the second network device 130, and the terminal device 110 moves from the cell 120-1 to the cell 130-1. It should be understood that each DU may have many cells, and only one cell is shown for illustration, for example, there are several candidate cells for the target DU, and only one cell as a target cell is shown for the purpose of illustration.

[0045] It is to be understood that the number of network devices and terminal devices is given only for the purpose of illustration without suggesting any limitations. The system 100 may include any suitable number of network devices and / or terminal devices adapted for implementing embodiments of the present disclosure. Although not shown, it would be appreciated that one or more terminal devices may be located in the environment 100.

[0046] Communications in the network environment 100 may be implemented according to any proper communication protocol(s), comprising, but not limited to, the third generation (3G), the fourth generation (4G), the fifth generation (5G) or beyond, wireless local network communication protocols such as institute for electrical and electronics engineers (IEEE) 802.11 and the like, and / or any other protocols currently known or to be developed in the future. Moreover, the communication may utilize any proper wireless communication technology, comprising but not limited to: multiple-input multiple-output (MIMO), orthogonal frequency division multiplexing (OFDM), time division multiplexing (TDM), frequency division multiplexing (FDM), code division multiplexing (CDM), Bluetooth, ZigBee, and machine type communication (MTC), enhanced mobile broadband (eMBB), massive machine type communication (mMTC),ultra-reliable low latency communication (URLLC), carrier aggregation (CA), dual connection (DC), and new radio unlicensed (NR-U) technologies.

[0047] Fig. 2 illustrates an example signaling process 200 for recovery operation for failure of timing advance (TA) estimation according to some embodiments of the present disclosure. For the purpose of discussion, the process 200 will be described with reference to Fig. 1. The process 200 may involve the terminal device 110, the first network device 120, and the second network device 130 as illustrated in Fig. 1. It would be appreciated that although the process 200 has been described in the communication environment 100 of Fig. 1, this process may be likewise applied to other communication scenarios with similar issues.

[0048] In the process 200, the terminal device 110 determines (205) failure of TA estimation performed by the terminal device 110 for a candidate cell provided by a second network device 130 (for example, a target DU). Then, based on determining the failure of the TA estimation, the terminal device 110 transmits (210), to a first network device 120 (for example, a source DU), an indication 201 of a recovery operation for the failure of the TA estimation. The first network device 120 receives (215) the indication 201 of a recovery operation for the failure of the TA estimation for the candidate cell. Then, the first network device 120 transmits (220) to the second network device 130 an indication 202 that the terminal device is to perform a random access procedure towards the candidate cell. Then, the second network device 130 receives (225) the indication 202.

[0049] In some embodiments, the terminal device 110 determines failure of TA estimation for a candidate cell provided by a second network device 130 based on a criterion. This criterion may be configured by the network device or determined by the terminal device itself. In some embodiments, the criterion comprises determining that a predetermined number of estimations has been performed by the terminal device; and determining that a resultant TA value for the candidate cell is not valid or not in a valid region. The criterion, for example, the predetermined number, can be configured by the network device or determined by the terminal device itself.

[0050] In some embodiments, the terminal device 110 may receive from the first network device 120 message comprising a predefined fallback resource (for example, CFRA resource) from the candidate cell for the failure of the TA estimation before determining the failure of the TA estimation. In some embodiments, the message, which comprises the predefined fallback resource from the candidate cell for the failure of theTA estimation, may be received by the terminal device 110 via an RRC message or via a MAC CE message, for example during the preparation phase of the access procedure. The transmission of the fallback resource from the candidate cell for the failure of the TA estimation will be described in details hereinafter with reference to Fig. 3. By predefining the fallback resource from the candidate cell for the failure of the TA estimation and indicating the resource in the RRC configuration and MAC CE message, it will allow the terminal device to have quick access to the resource instead of looking up the RRC configuration. From the network side, the target cell does not waste any dynamic grant or configured grant resources.

[0051] In some embodiments, the indication 201 of the recovery operation comprises a request to trigger a physical downlink control channel (PDCCH) order. That is to say, the terminal device 110 requests the first network device 120 to trigger the PDCCH order for ordering the terminal device 110 to perform a random access procedure towards the target cell. In some embodiments, upon receiving the indication 201, the first network device 120 may trigger the PDCCH order, and then transmit to the terminal device 110 the PDCCH order not configuring a specific resource for acquiring a TA value for the candidate cell. That is to say, by receiving the indication 201, the first network device 120 may know that the terminal device 110 fails to perform the TA estimation and then can trigger a PDCCH order, however, the PDCCH order may not configure a specific resource for acquiring a TA value for the candidate cell. In this way, for the purpose of immediate early TA acquisition before the cell switch, the terminal device 110 may use the predefined fallback resource from the candidate cell for the failure of the TA estimation which has been already received via the RRC message and MAC CE message. Therefore, before the cell switch command, the UE may acquire the TA from the network device early based on the predefined fallback resource from the candidate cell, and there is no need for looking up RRC configuration, and the TA value can be obtained early before the cell switch.

[0052] In some embodiments, the first network device 120 may trigger the PDCCH order when the RSRP measurement for the target cell reported by the terminal device 110 is below the threshold, for example, after the operation 314 and before the operation 315, which will be described with reference to Fig. 3 in details. If the RSRP measurement is below the threshold, the first network device 120 may know in advance before the actual estimation performed by the terminal device 110 that the terminal device 110 will be not able to perform the TA estimation. In this way, the first network device 120 may takeover the TA estimation task by triggering PDCCH order to determine the explicit TA value, and then may provide the explicit TA value to the terminal device 110 in the subsequent procedure, for example, in the cell switch command after deciding to switch to a target cell. That is to say, regardless of the indication from the terminal device 100 for requesting to trigger PDCCH order, the first network device 120 may trigger the PDCCH order itself when the RSRP measurement is below the threshold, and then transmit to the terminal device 110 the PDCCH order comprising the explicit TA value for the candidate cell.

[0053] In some embodiments, the indication 201 of the recovery operation may comprise an indication that the terminal device is to fall back to a random access procedure upon a cell switch towards the candidate cell. That is to say, if the TA estimation is failed, the terminal device 110 may indicate to the first network device 120 that it will fall back to a random access procedure upon receiving a cell switch command from the first network device 120. The first network device 120 may transmit a cell switch command after the first network device 120 decides that the serving cell has worse radio link or beam measurement than a target cell, regardless of reception of the indication from the terminal device 110 of falling back to random access procedure upon receiving a cell switch command. Therefore, the cell switch command may be transmitted even before the actual TA estimation performed at the terminal device 110 or after the actual TA estimation performed at the terminal device 110.

[0054] In some embodiments, the indication 201, comprising comprise an indication that the terminal device is to fall back to a random access procedure upon a cell switch towards the candidate cell, is transmitted in a hybrid automatic repeat request (HARQ) acknowledgement of any MAC CE message from the network device, for example, a MAC CE message for transmission configuration indication (TCI) state activation command (for example, the operation 316 of Fig. 3) or a MAC CE message for cell switch (for example, the operation 321 of Fig. 3).

[0055] In some embodiments, the cell switch command may not comprise a TA value for the candidate cell and a specific resource for random access procedure. In some embodiments, the cell switch command may comprise a TA value for the candidate cell and a specific resource for random access procedure. It should be understood that he TA value may be determined by the network, if the terminal device based TA estimation is failed.

[0056] If the cell switch command comprises a specific resource for the random accessprocedure, when the terminal device 110 performs random access procedure towards the candidate cell, the random access procedure will be performed based on the specific resource for the random access procedure. That is to say, the specific resource for the random access procedure will have the highest priority during the random access procedure.

[0057] In some embodiments, the terminal device 110 may indicate the failure of the TA estimation to the first network device 120. In some embodiments, the terminal device 110 may not indicate the failure of the TA estimation to the first network device 120. The time for indicating TA failure to the network device 120 may be immediately after the terminal device 110 determines the failure of TA estimation, and it can be indicated to the network device even before the terminal device 100 transmits the indication 201 of recovery operation to the network device.

[0058] If the cell switch command does not comprise a TA value for the candidate cell and a specific resource for random access procedure and the failure of the TA estimation has been indicated to the first network device 120 already, when the terminal device 110 performs random access procedure towards the candidate cell, the random access procedure will be performed based on any resource for random access procedure configured via an RRC message. This any resource for random access procedure configured via an RRC message is not specific to the failure of TA estimation, and also can be resource for beam failure and the like, and in other words, this resource can be any general resource. In this way, any resource for random access procedure configured via an RRC message will have lower priority during the random access procedure than the specific resource for the random access procedure in the cell switch command. That is to say, if UE based TA fails and if the UE indicates TA failure to the NW and the cell switch command does not have any TA and CFRA resources, the UE can use any RRC configured CFRA resources.

[0059] If the cell switch command does not comprise a TA value for the candidate cell and a specific resource for random access procedure and the failure of the TA estimation is not indicated to the first network device 120, when the terminal device 110 performs random access procedure towards the candidate cell, the random access procedure will be performed based on predefined fallback resource for the failure of the TA estimation already configured via the RRC message or MAC CE message. That is to say, if UE based TA fails and if UE does not indicate a failure to the NW and the cell switch command does not have any TA and CFRA resources, the UE can use the RRC or MACCE configured CFRA resources for UE based TA failure. The RRC or MAC CE configured CFRA resources for UE based TA failure have lower priority than the specific resource for the random access procedure in the cell switch command.

[0060] By the process 200, a criterion for determining the failure of TA estimation can be provided, and the failure of TA estimation can be determined by the terminal device based on the criterion. Then, upon determining the failure of TA estimation, the terminal device may transmit an indication of a recovery operation for the failure of TA estimation to the network device. In this way, a recovery mechanism for the recovery of the failure of TA estimation can be proposed such that the terminal device can switch for accessing to the target cell in time even the TA estimation at the terminal device side is failed. By reporting the indication of recovery operation, there may be no need for any dynamic grant or configured grant resources at the network side.

[0061] Fig. 3 illustrates an example signaling process for recovery of switching to the target cell after UE-based TA failure according to some embodiments of the present disclosure. It should be understood that the UE 340 may be example of the terminal device 110 of Fig. 1, and the source DU 350 may be an example of the first network devicel20 of Fig. 1, and the target DU 360 may be an example of the second network device 130 of Fig. 1, and the CU 370 may be an example of the third network device 140 of Fig. 1.

[0062] As shown in Fig. 3, at 301, the UE 340 sends L3 measurement report containing the cell quality measurements of serving and neighboring cells to the source DU 350. At 302, the source DU 350 sends the UL RRC message transfer comprising the L3 measurement report to the CU 370. The UE 340 can be configured by the serving cell (in this embodiment, the source DU 350) to send measurement report early when it still has a good connection to the serving cell. Using the reported cell quality measurements, the CU 370 can identify a potential set of candidate target cells (for example, the target DU 360 is one of the candidate cells) to which the UE can be handed over to which may belong to the same DU or different one (for example, the target DU 360), and as shown in this embodiment, the source DU 350 is different from the target DU 360.

[0063] At 303, the CU 370 may perform LTM candidate preparation to decide about the preparation of new cells. At 304, the CU 370 requests the preparation of a candidate target cell(s) controlled by target DU 360 by sending UE Context Setup Request message to the target DU 360. At 305, the target DU 360 provides the configuration of the UE in UE Context Setup Response message containing a container from the DU to the CU.At 306 and 307, the CU 370 requests UE context modification from the source DU 350, and the source DU 350 responds to it.

[0064] Having received the UE configurations for the candidate target cell(s), at 308, the CU 370 generates an RRC Reconfiguration that is to be sent to the UE 340. Among other information, the RRC Reconfiguration message contains measurement reporting configuration for L1 / L2 handover, and configuration of the prepared candidate cell(s) which the UE needs to execute when it receives a MAC CE command from the source DU 350 to change the serving cell (perform handover).

[0065] At 309, the CU 370 provides the source DU 350 about the RRC configuration message. At 310, the source DU 350 forwards the RRC reconfiguration message to the UE 340, and configures the UE 340 to perform UE based TA estimation with an IE containing the fallback CFRA resources for TA failure, which is a first option for indicating the fallback CFRA resource from the candidate cells for TA failure to the UE 340, and this is shown as option 1 in Fig. 3.

[0066] At 311, the UE 340 informs the source DU 350 RRCReconfigurationComplete . At 312, the source DU 350 sends the UL RRC transfer to the CU 370. Then, at 313, the UE 340 performs the corresponding DL sync. Then the UE 340 performs LI measurement, and at 314, the UE 340 sends the LI measurement report to the source DU 350. At 315, the source DU 350 determines that there is a target candidate cell having a better radio link or beam measurement than the serving cell, e.g., Ll-RSRP of target beam measurement > Ll-RSRP of serving beam measurement + Offset for an amount of time e.g., Time -to-Trigger (TTT).

[0067] At 316, optionally, after knowing the best cell for the target DU 360, the source DU 350 sends transmission configuration indication (TCI) state activation command (which is a MAC CE message) to the UE 340, to allow the UE 340 to activate the beam to be used for the target cell via a MAC-CE. In this MAC-CE, the fallback CFRA resources from the candidate cells for TA failure may be indicated to the UE. This is the second option to indicate the fallback CFRA resources from the candidate cells for TA failure to the UE 340, and this is shown as option 2 in Fig. 3.

[0068] This option 2 may be used when the fallback CFRA resources of option 1 has not been indicated to the UE 340 at 310 via the RRC reconfiguration message. That is to say, if the fallback CFRA resources from the candidate cells for TA failure has been already indicated to the UE 340 at 310, there is no necessity to provide this fallback CFRA resource again at 316, and the fallback CFRA resources from the candidate cells for TAfailure for the first option and the second option may be identical.

[0069] At 317, this is marked as a time point, when the UE 340 starts to perform TA estimation towards the cell that gNB activates TCI state. At 318, it may define the condition to declare UE based TA estimation failure. For example, after x time of attempts, the TA value is not valid or in a valid region, and it may indicate the UE based TA estimation failure. The number x of attempts to acquire TA of target cell before declaring failure is provided in RRC message as common LTM configuration parameter or can be determined by the UE itself.

[0070] At 319, upon detecting UE based TA not valid, the UE 340 may indicate to the source DU 350 how to recover the switching to the target cell or indicate the recovery operation for the TA estimation failure. For the first type of indication (option 3, UE requests gNB to trigger PDCCH order as show in Fig. 3), the UE may request the source DU 350 to trigger PDCCH order. For the second type of indication (option 4 of Fig. 3), the UE 340 may send the indication that it will fall back to RACH upon cell switch to the source DU 350 in the HARQ response of any MAC-CE, for example, the HARQ-ACK at 319 for the MAC CE at 316, or the HARQ-ACK at 322 for the MAC CE at 321.

[0071] By receiving the first type of indication (option 3), at 320, the source DU 350 may trigger PDCCH order TA. By informing the source DU 350 to trigger PDCCH order, the source DU 350 may know the UE based TA failure and know that the UE 340 fails to perform the UE-based TA estimation. In the subsequent downlink communication, the source DU 350 may not configure any resource for acquiring the TA, and the UE 340 may use the predefined fallback CFRA in the RRC configuration or MAC-CE for immediate early TA acquisition before the cell switch, that is before the cell switch command at 321.

[0072] In some embodiments (not shown), by receiving the LI measurement report at 314, the source DU 350 may also trigger PDCCH order TA to take over the TA estimation task, when the RSRP of a measurement goes below a certain threshold, because this might indicate the potential possibility of UE based TA failure. Although triggering PDCCH order is performed at 320 in Fig. 3, the triggering PDCCH order can be automatically performed by the source DU 350 just after receiving the LI measurement report at 314. By taking over the TA estimation task, the source DU 350 may determine the valid TA value to be used by the UE 340, and this valid TA value can be sent to the UE 340 when the UE 340 requests the source DU 350 to provide it subsequently, for example, in a cell switch command.

[0073] Since the source DU 350 has already decided to change the serving cell at 315, the source DU 350 sends a MAC Control Element (MAC CE) or a LI message at 321 to trigger the cell change to the target candidate cell (one of the cells in 318, for which the UE performs TA estimation). If there is no triggering PDCCH order, the cell switch command will still be sent. The gNB can send the cell switch command any time after it makes the decision at 315. The flow chart of Fig. 3 is intended to illustrate that when UE detect the failure based on the configuration setting, it will perform the operation 319 if gNB has not sent any cell switch command. If the detection of failure is shortly before the cell switch command, and UE has not yet triggered the operation 319, and gNB has sent the cell switch command, then there will be no operation 319, and only the operation 321 and the operation 322. Also, UE can detect the failure after the cell switch command, this means, the operation 318 can happen after the cell switch command, then UE will perform the operation 322 directly by indicating option 4 in the HARQ-ACK of the MAC CE cell switch command, and there is no option 3 and the operation 319 and the operation 320.

[0074] At 322, in the case that the cell-switch-command does not include TA value (assuming valid UE based TA) and the UE 340 also does not have valid TA, the UE 340 may report RACH fallback first to the source DU 350 before executing the cell switch command, for example, as an indication in the HARQ-ACK of the cell switch command at 322.

[0075] At 323, the source DU 350 informs the target DU 360 that RACH is expected from the UE 340. At 324 and 325, the UE 340 performs RACH towards the target DU 360. During the RACH procedure, the first priority would be using the CFRA resources indicated in the cell switch command at 321. If no such resource is given in the cell switch command at 321, the UE 340 can use any RRC configured CFRA resources, or can use the configured fallback CFRA for UE based TA failure defined in option 1 or option 2. It should be understood that any RRC configured CFRA resources can be general resources that are not configured resource specific to the UE-based TA failure, it may also include the resource for beam failure and the like. However, the configured fallback CFRA in option 1 and option 2 are specific to the UE based TA failure.

[0076] That is to say, if UE based TA fails and if the UE indicates TA failure to the NW (for example, the source DU 350) and the cell switch command does not have any TA and CFRA resources, the UE can use any RRC configured CFRA resources. Further, if UE based TA fails and if UE does not indicate a failure to the NW and the cell switchcommand does not have any TA and CFRA resources, the UE can use the RRC configured CFRA resources for UE based TA failure or the CFRA resource for UE based TA failure indicated in MAC CE.

[0077] At 326, the target DU 360 provides to the CU 370 the RRC Reconfiguration Complete which indicates to the latter that the UE is now served from the target DU, and at 327, the target DU 360 may indicate the CU 370 the access notification. At 328 and 329, the UE context is released from the source DU 350. At 330, a path switch is performed to the new serving DU 360.

[0078] Some example implementation options in TS 38.321 may be modified as follows in view of some embodiments of the present disclosure:If UE based TA fails, UE shall use the fallback CFRA in the configuration or MAC-CE for immedidate early TA acquisition before the cell switch.If UE based TA fails and if the UE indicates TA failure to the NW and the cell switch command does not have any TA and CFRA resources, the UE can use any RRC configured CFRA resources.If UE based TA fails and if UE does not indicate a failure to the NW and the cell switch command does not have any TA and CFRA resources, the UE can use the RRC or MAC CE configured CFRA resources for UE based TA failure.

[0079] Fig. 4 shows a flowchart of an example method 400 implemented at a terminal device (for example, the terminal device 110) in accordance with some embodiments of the present disclosure. For the purpose of discussion, the method 400 will be described from the perspective of the terminal device 110 with reference to Fig. 1.

[0080] At block 410, the terminal device 110 determines, based on a criterion, failure of a timing advance (TA) estimation performed by the terminal device for a candidate cell. At block 420, the terminal device 110 based on determining the failure of the TA estimation, transmits, to a network device, an indication of a recovery operation for the failure of the TA estimation.

[0081] In some embodiments, the terminal device 110 also receives, from the network device, message comprising a predefined fallback resource from the candidate cell for the failure of the TA estimation before determining the failure of the TA estimation. In some embodiments, the message is received via a radio resource configuration (RRC) message; or a media access control control element (MAC CE) message.

[0082] In some embodiments, the indication of the recovery operation comprises a request to trigger a physical downlink control channel (PDCCH) order. In someembodiments, the indication of the recovery operation comprises an indication that the terminal device is to fall back to a random access procedure upon a cell switch towards the candidate cell. In some embodiments, the indication is transmitted in a hybrid automatic repeat request (HARQ) acknowledgement of a MAC CE message from the network device, and the MAC CE message comprises at least one of the following: a MAC CE message for transmission configuration indication (TCI) state activation command; or a MACE CE message for cell switch.

[0083] In some embodiments, a specific resource for acquiring a TA value for the candidate cell is not configured in the PDCCH order, the terminal device 110 further receives, from the network device, message comprising a predefined fallback resource from the candidate cell for the failure of the TA estimation before determining the failure of the TA estimation; and acquires, from the PDCCH order, the TA value for the candidate cell before cell switch by using the predefined fallback resource. In some embodiments, a TA value for the candidate cell is included in the PDCCH order, and the terminal device 110 further receives the TA value, from the network device, via the PDCCH order.

[0084] In some embodiments, the terminal device 110 may further indicate the failure of the TA estimation to the network device; receive, from the network device, a cell switch command comprising a specific resource for the random access procedure; and perform the random access procedure towards the candidate cell based on the specific resource.

[0085] In some embodiments, the terminal device 110 may further indicate the failure of the TA estimation to the network device; receive, from the network device, a cell switch command not comprising a TA value for the candidate cell and a specific resource for random access procedure; and perform the random access procedure towards the candidate cell based on any resource for random access procedure configured via an RRC message.

[0086] In some embodiments, the terminal device 110 may further receive, from the network device, a cell switch command not comprising a TA value for the candidate cell and a specific resource for random access procedure; and perform the random access procedure towards the candidate cell based on the predefined fallback resource for the failure of the TA estimation.

[0087] In some embodiments, the resource comprises contention-free random access (CFRA) resource. In some embodiments, the criterion comprises: determining that a predetermined number of estimations has been performed by the terminal device; and determining that a resultant TA value for the candidate cell is not valid or not in a validregion. In some embodiments, the criterion is configured by the network device or determined by the terminal device, and the predetermined number is provided in an RRC message or determined by the terminal device.

[0088] Fig. 5 shows a flowchart of an example method 500 implemented at a network device (for example, the first network device 120) in accordance with some embodiments of the present disclosure. For the purpose of discussion, the method 500 will be described from the perspective of first network device 120 with reference to Fig. 1.

[0089] At 510, the first network device 120 receives, from a terminal device, an indication of a recovery operation for failure of a timing advance (TA) estimation performed by the terminal device for a candidate cell provided by a second network device. At block 520, first network device 120 transmits, to the second network device, an indication that the terminal device is to perform a random access procedure towards the candidate cell.

[0090] In some embodiments, the first network device 120 may further transmit, to the terminal device, message comprising a predefined fallback resource from the candidate cell for the failure of the TA estimation before receiving the indication of the recovery operation for the failure of the TA estimation. In some embodiments, the message is transmitted via at least one of the following: a radio resource configuration (RRC) message; or a media access control control element (MAC CE) message.

[0091] In some embodiments, the indication of the recovery operation comprises a request to trigger a physical downlink control channel (PDCCH) order. In some embodiments, the indication of the recovery operation comprises an indication that the terminal device is to fall back to a random access procedure upon a cell switch towards the candidate cell.

[0092] In some embodiments, the indication is received in a hybrid automatic repeat request (HARQ) acknowledgement of a MAC CE message from the first network device, and the MAC CE message comprises at least one of the following: a MAC CE message for transmission configuration indication (TCI) state activation command; or a MACE CE message for the cell switch command.

[0093] In some embodiments, the first network device 120 may further trigger the PDCCH order upon reception of the indication; and transmit, to the terminal device, the PDCCH order not configuring a specific resource for acquiring a TA value for the candidate cell. In some embodiments, the first network device 120 may further trigger a PDCCH order upon reception of a reference signal power received power (RSRP)measurement below a threshold; and transmit, to the terminal device, the PDCCH order including a TA value for the candidate cell.

[0094] In some embodiments, the first network device 120 may further receive, from the terminal device, a further indication of the failure of the TA estimation; and transmit, to the terminal device, a cell switch command comprising a specific resource for the random access procedure towards the candidate cell.

[0095] In some embodiments, the first network device 120 may further receive, from the terminal device, a further indication of the failure of the TA estimation; and transmit, to the terminal device, a cell switch command not comprising a TA value for the candidate cell and a specific resource for the random access procedure towards the candidate cell. In some embodiments, the resource comprises contention-free random access (CFRA) resource.

[0096] In some embodiments, an apparatus capable of performing any of operations of the method 400 (for example, the terminal device 110) may include means for performing the respective steps of the method 400. The means may be implemented in any suitable form. For example, the means may be implemented in a circuitry or software module.

[0097] In some embodiments, the apparatus may include means for determining, based on a criterion, failure of a timing advance (TA) estimation performed by the terminal device for a candidate cell; and means for based on determining the failure of the TA estimation, transmitting, to a network device, an indication of a recovery operation for the failure of the TA estimation.

[0098] In some embodiments, the apparatus may further receive, from the network device, message comprising a predefined fallback resource from the candidate cell for the failure of the TA estimation before determining the failure of the TA estimation. In some embodiments, the message is received via at least one of the following: a radio resource configuration (RRC) message; or a media access control control element (MAC CE) message.

[0099] In some embodiments, the indication of the recovery operation comprises a request to trigger a physical downlink control channel (PDCCH) order. In some embodiments, the indication of the recovery operation comprises an indication that the terminal device is to fall back to a random access procedure upon a cell switch towards the candidate cell. In some embodiments, the indication is transmitted in a hybrid automatic repeat request (HARQ) acknowledgement of a MAC CE message from the network device, and the MAC CE message comprises at least one of the following: aMAC CE message for transmission configuration indication (TCI) state activation command; or a MACE CE message for cell switch.

[0100] In some embodiments, a specific resource for acquiring a TA value for the candidate cell is not configured in the PDCCH order, the apparatus may further receive, from the network device, message comprising a predefined fallback resource from the candidate cell for the failure of the TA estimation before determining the failure of the TA estimation; and acquire, from the PDCCH order, the TA value for the candidate cell before cell switch by using the predefined fallback resource.

[0101] In some embodiments, a TA value for the candidate cell is included in the PDCCH order, and the apparatus may further receive the TA value, from the network device, via the PDCCH order.

[0102] In some embodiments, the apparatus may further indicate the failure of the TA estimation to the network device; receive, from the network device, a cell switch command comprising a specific resource for the random access procedure; and perform the random access procedure towards the candidate cell based on the specific resource.

[0103] In some embodiments, the apparatus may further indicate the failure of the TA estimation to the network device; receive, from the network device, a cell switch command not comprising a TA value for the candidate cell and a specific resource for random access procedure; and perform the random access procedure towards the candidate cell based on any resource for random access procedure configured via an RRC message.

[0104] In some embodiments, the apparatus may further receive, from the network device, a cell switch command not comprising a TA value for the candidate cell and a specific resource for random access procedure; and perform the random access procedure towards the candidate cell based on the predefined fallback resource for the failure of the TA estimation.

[0105] In some embodiments, the resource comprises contention-free random access (CFRA) resource. In some embodiments, the criterion comprises: determining that a predetermined number of estimations has been performed by the terminal device; and determining that a resultant TA value for the candidate cell is not valid or not in a valid region. In some embodiments, the criterion is configured by the network device or determined by the terminal device, and the predetermined number is provided in an RRC message or determined by the terminal device.

[0106] In some embodiments, the apparatus further comprises means for performingother steps in some embodiments of the method 400. In some embodiments, the means comprises at least one processor and at least one memory including computer program code, the at least one memory and computer program code configured to, with the at least one processor, cause the performance of the apparatus.

[0107] In some embodiments, an apparatus capable of performing any of the method 500 (for example, the first network device 120) may include means for performing the respective steps of the method 500. The means may be implemented in any suitable form. For example, the means may be implemented in a circuitry or software module.

[0108] In some embodiments, the apparatus may include means for receiving, from a terminal device, an indication of a recovery operation for failure of a timing advance (TA) estimation performed by the terminal device for a candidate cell provided by a second network device; and means for transmitting, to the second network device, an indication that the terminal device is to perform a random access procedure towards the candidate cell.

[0109] In some embodiments, the apparatus may further transmit, to the terminal device, message comprising a predefined fallback resource from the candidate cell for the failure of the TA estimation before receiving the indication of the recovery operation for the failure of the TA estimation. In some embodiments, the message is transmitted via at least one of the following: a radio resource configuration (RRC) message; or a media access control control element (MAC CE) message.

[0110] In some embodiments, the indication of the recovery operation comprises a request to trigger a physical downlink control channel (PDCCH) order. In some embodiments, the indication of the recovery operation comprises an indication that the terminal device is to fall back to a random access procedure upon a cell switch towards the candidate cell.

[0111] In some embodiments, the indication is received in a hybrid automatic repeat request (HARQ) acknowledgement of a MAC CE message from the first network device, and the MAC CE message comprises at least one of the following: a MAC CE message for transmission configuration indication (TCI) state activation command; or a MACE CE message for the cell switch command.

[0112] In some embodiments, the apparatus may further trigger the PDCCH order upon reception of the indication; and transmit, to the terminal device, the PDCCH order not configuring a specific resource for acquiring a TA value for the candidate cell. In some embodiments, the apparatus may further trigger a PDCCH order upon reception of areference signal power received power (RSRP) measurement below a threshold; and transmit, to the terminal device, the PDCCH order including a TA value for the candidate cell.

[0113] In some embodiments, the apparatus may further receive, from the terminal device, a further indication of the failure of the TA estimation; and transmit, to the terminal device, a cell switch command comprising a specific resource for the random access procedure towards the candidate cell.

[0114] In some embodiments, the apparatus may further receive, from the terminal device, a further indication of the failure of the TA estimation; and transmit, to the terminal device, a cell switch command not comprising a TA value for the candidate cell and a specific resource for the random access procedure towards the candidate cell. In some embodiments, the resource comprises contention-free random access (CFRA) resource.

[0115] In some embodiments, the apparatus further comprises means for performing other steps in some embodiments of the method 500. In some embodiments, the means comprises at least one processor and at least one memory including computer program code, the at least one memory and computer program code configured to, with the at least one processor, cause the performance of the apparatus.

[0116] FIG. 6 is a simplified block diagram of a device 600 that is suitable for implementing embodiments of the present disclosure. The device 600 may be provided to implement the communication device, for example the terminal device 110, the first to third network device 120, 130, and 140 as shown in Fig. 1. As shown, the device 600 includes one or more processors 610, one or more memories 620 coupled to the processor 610, and one or more communication modules 640 coupled to the processor 610.

[0117] The communication module 640 is for bidirectional communications. The communication module 640 has at least one antenna to facilitate communication. The communication interface may represent any interface that is necessary for communication with other network elements.

[0118] The processor 610 may be of any type suitable to the local technical network and may include one or more of the following: general purpose computers, special purpose computers, microprocessors, digital signal processors (DSPs) and processors based on multicore processor architecture, as non-limiting examples. The device 600 may have multiple processors, such as an application specific integrated circuit chip that is slaved in time to a clock which synchronizes the main processor.

[0119] The memory 620 may include one or more non-volatile memories and one or more volatile memories. Examples of the non-volatile memories include, but are not limited to, a read only memory (ROM) 624, an electrically programmable read only memory (EPROM), a flash memory, a hard disk, a compact disc (CD), a digital video disk (DVD), and other magnetic storage and / or optical storage. Examples of the volatile memories include, but are not limited to, a random access memory (RAM) 622 and other volatile memories that will not last in the power-down duration.

[0120] A computer program 630 includes computer executable instructions that are executed by the associated processor 610. The program 630 may be stored in the ROM 624. The processor 610 may perform any suitable actions and processing by loading the program 630 into the RAM 622.

[0121] The embodiments of the present disclosure may be implemented by means of the program so that the device 600 may perform any process of the disclosure as discussed with reference to Figs. 2, 3, 4 and 5. The embodiments of the present disclosure may also be implemented by hardware or by a combination of software and hardware.

[0122] In some embodiments, the program 630 may be tangibly contained in a computer readable medium which may be included in the device 600 (such as in the memory 620) or other storage devices that are accessible by the device 600. The device 600 may load the program 630 from the computer readable medium to the RAM 622 for execution. The computer readable medium may include any types of tangible non-volatile storage, such as ROM, EPROM, a flash memory, a hard disk, CD, DVD, and the like. Fig. 7 shows an example of the computer readable medium 700 in form of CD or DVD. The computer readable medium has the program 630 stored thereon.

[0123] Generally, various embodiments of the present disclosure may be implemented in hardware or special purpose circuits, software, logic or any combination thereof. Some aspects may be implemented in hardware, while other aspects may be implemented in firmware or software which may be executed by a controller, microprocessor or other computing device. While various aspects of embodiments of the present disclosure are illustrated and described as block diagrams, flowcharts, or using some other pictorial representations, it is to be understood that the block, apparatus, system, technique or method described herein may be implemented in, as non-limiting examples, hardware, software, firmware, special purpose circuits or logic, general purpose hardware or controller or other computing devices, or some combination thereof.

[0124] The present disclosure also provides at least one computer program producttangibly stored on a non-transitory computer readable storage medium. The computer program product includes computer-executable instructions, such as those included in program modules, being executed in a device on a target real or virtual processor, to carry out process 200, 300, the method 400 or 500 as described above with reference to Fig. 2, Fig. 3, Fig. 4, or Fig. 5. Generally, program modules include routines, programs, libraries, objects, classes, components, data structures, or the like that perform particular tasks or implement particular abstract data types. The functionality of the program modules may be combined or split between program modules as desired in various embodiments. Machine-executable instructions for program modules may be executed within a local or distributed device. In a distributed device, program modules may be located in both local and remote storage media.

[0125] Program code for carrying out methods of the present disclosure may be written in any combination of one or more programming languages. These program codes may be provided to a processor or controller of a general purpose computer, special purpose computer, or other programmable data processing apparatus, such that the program codes, when executed by the processor or controller, cause the functions / operations specified in the flowcharts and / or block diagrams to be implemented. The program code may execute entirely on a machine, partly on the machine, as a stand-alone software package, partly on the machine and partly on a remote machine or entirely on the remote machine or server.

[0126] In the context of the present disclosure, the computer program codes or related data may be carried by any suitable carrier to enable the device, apparatus or processor to perform various processes and operations as described above. Examples of the carrier include a signal, computer readable medium, and the like.

[0127] The computer readable medium may be a computer readable signal medium or a computer readable storage medium. A computer readable medium may include but not limited to an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, or device, or any suitable combination of the foregoing. More specific examples of the computer readable storage medium would include an electrical connection having one or more wires, a portable computer diskette, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), an optical fiber, a portable compact disc read-only memory (CD-ROM), an optical storage device, a magnetic storage device, or any suitable combination of the foregoing. The term “non-transitory,”as used herein, is a limitation of the medium itself (i.e., tangible, not a signal) as opposed to a limitation on data storage persistency (e.g., RAM vs. ROM).

[0128] Further, while operations are depicted in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. In certain circumstances, multitasking and parallel processing may be advantageous. Likewise, while several specific implementation details are contained in the above discussions, these should not be construed as limitations on the scope of the present disclosure, but rather as descriptions of features that may be specific to particular embodiments. Certain features that are described in the context of separate embodiments may also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment may also be implemented in multiple embodiments separately or in any suitable subcombination.

[0129] Although the present disclosure has been described in languages specific to structural features and / or methodological acts, it is to be understood that the present disclosure defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims.

Claims

WHAT IS CLAIMED IS:

1. A terminal device comprising: at least one processor; and at least one memory storing instructions that, when executed by the at least one processor, cause the terminal device at least to: determine, based on a criterion, failure of a timing advance (TA) estimation performed by the terminal device for a candidate cell; and based on determining the failure of the TA estimation, transmit, to a network device, an indication of a recovery operation for the failure of the TA estimation.

2. The terminal device of claim 1, wherein the terminal device is further caused to: receive, from the network device, message comprising a predefined fallback resource from the candidate cell for the failure of the TA estimation before determining the failure of the TA estimation.

3. The terminal device of claim 1, wherein the indication of the recovery operation comprises a request to trigger a physical downlink control channel (PDCCH) order.

4. The terminal device of claim 1, wherein the indication of the recovery operation comprises an indication that the terminal device is to fall back to a random access procedure upon a cell switch towards the candidate cell.

5. The terminal device of claim 2, wherein the message is received via at least one of the following: a radio resource configuration (RRC) message; or a media access control control element (MAC CE) message.

6. The terminal device of claim 4, wherein the indication is transmitted in a hybrid automatic repeat request (HARQ) acknowledgement of a MAC CE message from the network device, and the MAC CE message comprises at least one of the following:a MAC CE message for transmission configuration indication (TCI) state activation command; or a MACE CE message for cell switch.

7. The terminal device of claim 3, wherein a specific resource for acquiring a TA value for the candidate cell is not configured in the PDCCH order, the terminal device is further caused to: receive, from the network device, message comprising a predefined fallback resource from the candidate cell for the failure of the TA estimation before determining the failure of the TA estimation; and acquire, from the PDCCH order, the TA value for the candidate cell before cell switch by using the predefined fallback resource.

8. The terminal device of claim 3, wherein a TA value for the candidate cell is included in the PDCCH order, and the terminal device is further caused to: receive the TA value, from the network device, via the PDCCH order.

9. The terminal device of any of claims 1 to 8, wherein the terminal device is further caused to: indicate the failure of the TA estimation to the network device; receive, from the network device, a cell switch command comprising a specific resource for the random access procedure; and perform the random access procedure towards the candidate cell based on the specific resource.

10. The terminal device of any of claims 1 to 8, wherein the terminal device is further caused to: indicate the failure of the TA estimation to the network device; receive, from the network device, a cell switch command not comprising a TA value for the candidate cell and a specific resource for random access procedure; and perform the random access procedure towards the candidate cell based on any resource for random access procedure configured via an RRC message.

11. The terminal device of any of claims 2 to 8, wherein the terminal device is further caused to: receive, from the network device, a cell switch command not comprising a TA value for the candidate cell and a specific resource for random access procedure; and perform the random access procedure towards the candidate cell based on the predefined fallback resource for the failure of the TA estimation.

12. The terminal device of any of claims 1 to 11, wherein the resource comprises contention-free random access (CFRA) resource.

13. The terminal device of any of claims 1 to 12, wherein the criterion comprises: determining that a predetermined number of estimations has been performed by the terminal device; and determining that a resultant TA value for the candidate cell is not valid or not in a valid region.

14. The terminal device of claim 13, wherein the criterion is configured by the network device or determined by the terminal device, and the predetermined number is provided in an RRC message or determined by the terminal device.

15. A first network device comprising: at least one processor; and at least one memory storing instructions that, when executed by the at least one processor, cause the first network device at least to: receive, from a terminal device, an indication of a recovery operation for failure of a timing advance (TA) estimation performed by the terminal device for a candidate cell provided by a second network device; and transmit, to the second network device, an indication that the terminal device is to perform a random access procedure towards the candidate cell.

16. The first network device of claim 15, wherein the first network device is further caused to: transmit, to the terminal device, message comprising a predefined fallback resource from the candidate cell for the failure of the TA estimation before receiving theindication of the recovery operation for the failure of the TA estimation.

17. The first network device of claim 16, wherein the message is transmitted via at least one of the following: a radio resource configuration (RRC) message; or a media access control control element (MAC CE) message.

18. The first network device of any of claims 15 to 17, wherein the indication of the recovery operation comprises a request to trigger a physical downlink control channel (PDCCH) order.

19. The first network device of any of claims 15 to 17, wherein the indication of the recovery operation comprises an indication that the terminal device is to fall back to a random access procedure upon a cell switch towards the candidate cell.

20. The first network device of claim 19, wherein the indication is received in a hybrid automatic repeat request (HARQ) acknowledgement of a MAC CE message from the first network device, and the MAC CE message comprises at least one of the following: a MAC CE message for transmission configuration indication (TCI) state activation command; or a MACE CE message for the cell switch command.

21. The first network device of claim 18, wherein the first network device is further caused to: trigger the PDCCH order upon reception of the indication; and transmit, to the terminal device, the PDCCH order not configuring a specific resource for acquiring a TA value for the candidate cell.

22. The first network device of claim 15, wherein the first network device is further caused to: trigger a PDCCH order upon reception of a reference signal power received power (RSRP) measurement below a threshold; and transmit, to the terminal device, the PDCCH order including a TA value for thecandidate cell.

23. The first network device of any of claims 15 to 22, wherein the first network device is further caused to: receive, from the terminal device, a further indication of the failure of the TA estimation; and transmit, to the terminal device, a cell switch command comprising a specific resource for the random access procedure towards the candidate cell.

24. The first network device of any of claims 15 to 22, wherein the first network device is further caused to: receive, from the terminal device, a further indication of the failure of the TA estimation; and transmit, to the terminal device, a cell switch command not comprising a TA value for the candidate cell and a specific resource for the random access procedure towards the candidate cell.

25. The first network device of any of claims 15 to 24, wherein the resource comprises contention-free random access (CFRA) resource.

26. A method at a terminal device, comprising: determining, based on a criterion, failure of a timing advance (TA) estimation performed by the terminal device for a candidate cell; and based on determining the failure of the TA estimation, transmitting, to a network device, an indication of a recovery operation for the failure of the TA estimation27. A method at a first network device, comprising: receiving, from a terminal device, an indication of a recovery operation for failure of a timing advance (TA) estimation performed by the terminal device for a candidate cell provided by a second network device; and transmitting, to the second network device, an indication that the terminal device is to perform a random access procedure towards the candidate cell.

28. An apparatus, comprising:means for determining, based on a criterion, failure of a timing advance (TA) estimation performed by the terminal device for a candidate cell; and means for based on determining the failure of the TA estimation, transmitting, to a network device, an indication of a recovery operation for the failure of the TA estimation.

29. An apparatus, comprising: means for receiving, from a terminal device, an indication of a recovery operation for failure of a timing advance (TA) estimation performed by the terminal device for a candidate cell provided by a second network device; and means for transmitting, to the second network device, an indication that the terminal device is to perform a random access procedure towards the candidate cell.

30. A non-transitory computer readable medium comprising program instructions for causing an apparatus to perform at least the method of claim 26 or 27.

Images