Method and device for managing a radio link failure

Abstract

The disclosure describes a method and device for handling a radio link failure (RLF). The method comprises detecting one or more trigger conditions that indicate a likelihood of occurrence of an RLF and transmitting a request for obtaining lower layers triggered mobility (LTM) configuration associated with one or more target cells, to a network entity. The method further comprises receiving the LTM configuration for each of the one or more target cells and selecting a target cell for performing an LTM handover based on the LTM configuration of each of the one or more target cells. The target cell meets a cell selection criterion. The method further comprises performing the LTM handover based on the selected target cell.

Description

METHOD AND DEVICE FOR MANAGING A RADIO LINK FAILURE

[0001] The disclosure generally relates to field of wireless communication, and more particularly relates to a method and device for managing a radio link failure (RLF).

[0002] A radio link failure (RLF) that is a failure that occurs when a radio link between a user equipment (UE) and a base station is lost. This may occur due to various reasons such as interference, coverage issues, handover failures, or equipment malfunction etc. Typically, when an RLF occurs, the UE needs to declare the RLF to the base station, which triggers a series of actions to recover the lost connection. One of such action is performing a handover to a neighbouring cell with better coverage in order to re-establish the lost connection. The UE attempts to re-establish the lost connection by performing a cell selection by which a best cell among the neighbouring cells is selected. The selection is performed based on measuring the signal strength of the neighbouring cells and selecting the cell with a strongest signal.

[0003] Therefore, as soon as the UE experiences lower layer failures, a series of actions are triggered before the lost connection may be again re-established. On some occasions, apart from a cell selection procedure, a re-establishment procedure may also be initiated. These series of actions consume a considerable amount of time. Typically, the time required to perform a cell selection and re-establishment procedure ranges from around 2 seconds to 5 seconds. During this time, the service to the user is disrupted, thereby greatly affecting user experience.

[0004] The information disclosed in this background of the disclosure section is only for enhancement of understanding of the general background of the disclosure and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.

[0005] Additional aspects will be set forth in part in the description which follows and, in part, will be apparent from the description, or may be learned by practice of the presented embodiments.

[0006] According to an example embodiment of the disclosure, a method performed by a user equipment (UE) for handling a radio link failure (RLF) is described. The method comprises detecting one or more trigger conditions that indicate a likelihood of occurrence of an RLF. The method further comprises transmitting a request for obtaining lower layers triggered mobility (LTM) configuration associated with one or more target cells, to a network entity, upon detecting the one or more trigger conditions. The method further comprises receiving the LTM configuration for each of the one or more target cells from the network entity, in response to the request. The method further comprises selecting a target cell for performing an LTM handover based on the LTM configuration of each of the one or more target cells. The target cell meets a cell selection criterion. The method further comprises performing the LTM handover based on the selected target cell.

[0007] According to an example embodiment of, a user equipment (UE) for handling a radio link failure (RLF) is described. The UE comprises memory storing instructions and at least one processor. The instructions, when executed by the at least one processor (208) individually or collectively, cause the UE to detect one or more trigger conditions that indicate a likelihood of occurrence of an RLF. The at least one processor is configured to transmit a request for obtaining lower layers triggered mobility (LTM) configuration associated with one or more target cells, to a network entity, upon detecting the one or more trigger conditions. The instructions, when executed by the at least one processor (208) individually or collectively, cause the UE to receive the LTM configuration for each of the one or more target cells from the network entity, in response to the request. The instructions, when executed by the at least one processor (208) individually or collectively, cause the UE to select a target cell for performing an LTM handover based on the LTM configuration of each of the one or more target cells. The target cell meets a cell selection criterion. The instructions, when executed by the at least one processor (208) individually or collectively, cause the UE to perform the LTM handover based on the selected target cell.

[0008] According to an example embodiment of the disclosure, a non-transitory computer-readable storage medium storing one or more programs comprising instructions. The instructions, when executed by the processor individually or collectively, cause a user equipment to detect one or more trigger conditions that indicate a likelihood of occurrence of an RLF, transmit, to a network entity, a request for obtaining lower layers triggered mobility (LTM) configuration associated with one or more target cells upon detecting the one or more trigger conditions, receive, from the network entity, the LTM configuration for each of the one or more target cells in response to the request, select a target cell for performing an LTM handover based on the LTM configuration of each of the one or more target cells, wherein the target cell meets a cell selection criterion, and perform the LTM handover based on the selected target cell.

[0009] To further clarify the advantages and features of the disclosure, a more particular description of various example embodiments illustrated in the appended drawings is provided. It is appreciated that these drawings depict example embodiments and are therefore not to be considered limiting its scope. The disclosure will be described and explained with additional specificity and detail with reference to the accompanying drawings.

[0010] The above and other features, aspects, and advantages of certain embodiments of the disclosure will be more apparent from the following detailed description, taken in conjunction with the accompanying drawings in which like characters represent like parts throughout the drawings, an in which:

[0011] FIG.　1a shows an exemplary environment 100-1 depicting a radio link failure (RLF) at a user equipment (UE).

[0012] FIG.　1b shows a flow diagram 100-2 of the exemplary environment 100-1 depicting a UE experiencing an RLF during a lower layer triggered mobility (LTM) handover.

[0013] FIG.　2 depicts a block diagram of a user equipment (UE), in accordance with the embodiments of the disclosure.

[0014] FIG. 3 shows an exemplary flowchart 300 depicting operations for efficiently managing an RLF, in accordance with the embodiments of the disclosure.

[0015] FIG. 4 shows a flow diagram 400 depicting efficient management of an RLF, in accordance with the embodiments of the disclosure.

[0016] FIG. 5a shows a flowchart 500-1 depicting operations for detecting out-of-sync indications and declaring an RLF based on the detection.

[0017] FIG. 5b shows an exemplary illustration 500-2 depicting the function of timer T310 in declaring an RLF.

[0018] FIG. 5c shows an exemplary flowchart 500-3 depicting operations for detecting out-of-sync trigger conditions for indicating a likelihood of occurrence of an RLF, in accordance with the embodiments of the disclosure.

[0019] FIG. 6a shows a flowchart 600-1 depicting operations for detecting handover failure and declaring an RLF based on the detection.

[0020] FIG. 6b shows a flow diagram 600-2 depicting a handover failure scenario.

[0021] FIG. 6c shows an exemplary flowchart 600-3 depicting operations for detecting a handover failure trigger condition for indicating a likelihood of occurrence of an RLF, in accordance with the embodiments of the disclosure.

[0022] FIG. 7a shows a flowchart 700-1 depicting operations for detecting radio link control (RLC) retransmission failure and declaring an RLF based on the detection.

[0023] FIG. 7b shows an exemplary flowchart 700-2 depicting operations for detecting RLC retransmission failure trigger conditions for indicating a likelihood of occurrence of an RLF, in accordance with the embodiments of the disclosure.

[0024] FIG. 8a shows a flow diagram 800-1 depicting failure of a random access channel (RACH) procedure.

[0025] FIG. 8b shows a flow chart 800-2 depicting operations for detecting message failures during a RACH procedure.

[0026] FIG. 8c shows an exemplary flowchart 800-3 depicting operations for detecting RACH failure trigger condition for indicating a likelihood of occurrence of an RLF, in accordance with the embodiments of the disclosure.

[0027] FIG. 9 shows a block diagram 900 for predicting the likelihood of occurrence of an RLF using a pre-trained machine learning (ML) model, in accordance with the embodiments of the disclosure.

[0028] FIG. 10 shows a flow diagram 1000 depicting a scenario of a secondary cell group (SCG) failure.

[0029] FIG. 11 shows an exemplary flow diagram 1100 depicting operations of optimizing LTM handover by a UE, while in SCG, in accordance with the embodiments of the disclosure.

[0030] FIG. 12 shows an exemplary flow chart 1200 of a method of optimizing LTM by a UE, while in SCG, in accordance with the embodiments of the disclosure.

[0031] FIG.　13 shows an exemplary flow chart illustrating a method 1300 of efficiently managing an RLF, in accordance with some embodiments of the disclosure.

[0032] Further, skilled artisans will appreciate that elements in the drawings are illustrated for simplicity and may not have necessarily been drawn to scale. For example, the flow charts illustrate the method in terms of the most prominent steps / operations involved to help improve understanding of aspects of the disclosure. Furthermore, in terms of the construction of the device, one or more components of the device may have been represented in the drawings by conventional symbols, and the drawings may show only those specific details that are pertinent to understanding the embodiments of the disclosure so as not to obscure the drawings with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.

[0033] It should be understood at the outset that although illustrative implementations of the embodiments of the disclosure are illustrated below, the disclosure may be implemented using any number of techniques, whether currently known or in existence. The disclosure should in no way be limited to the illustrative implementations, drawings, and techniques illustrated below, including the design and implementation illustrated and described herein, but may be modified within the scope of the appended claims along with their full scope of equivalents.

[0034] The term "some" as used herein is defined as "none, or one, or more than one, or all." Accordingly, the terms "none," "one," "more than one," "more than one, but not all" or "all" would all fall under the definition of "some." The term "some embodiments" may refer to no embodiments, to one embodiment or to several embodiments or to all embodiments. Accordingly, the term "some embodiments" is defined as meaning "no embodiment, or one embodiment, or more than one embodiment, or all embodiments."

[0035] The terminology and structure employed herein is for describing, teaching, and illuminating some embodiments and their specific features and elements and does not limit, restrict, or reduce the spirit and scope of the claims or their equivalents.

[0036] More specifically, any terms used herein such as but not limited to "includes," "comprises," "has," "consists," and grammatical variants thereof do NOT specify an exact limitation or restriction and certainly do NOT exclude the possible addition of one or more features or elements, unless otherwise stated, and furthermore must NOT be taken to exclude the possible removal of one or more of the listed features and elements, unless otherwise stated with the limiting language "MUST comprise" or "NEEDS TO include."

[0037] Whether or not a certain feature or element was limited to being used only once, either way, it may still be referred to as "one or more features" or "one or more elements" or "at least one feature" or "at least one element." Furthermore, the use of the terms "one or more" or "at least one" feature or element does NOT preclude there being none of that feature or element, unless otherwise specified by limiting language such as "there NEEDS to be one or more . . ." or "one or more element is REQUIRED."

[0038] Unless otherwise defined, all terms, and especially any technical and / or scientific terms, used herein may be taken to have the same meaning as commonly understood by one having ordinary skill in the art.

[0039] It is to be understood that a singular form of a noun corresponding to an item may include one or more of the things, unless the relevant context clearly indicates otherwise. As used herein, each of such phrases as "A or B," "at least one of A and B," "at least one of A or B," "A, B, or C," "at least one of A, B, and C," and "at least one of A, B, or C," may include any one of, or all possible combinations of the items enumerated together in a corresponding one of the phrases. It is to be understood that if an element (e.g., a first element) is referred to, with or without the term "operatively" or "communicatively", as "coupled with," "coupled to," "connected with," or "connected to" another element (e.g., a second element), it means that the element may be coupled with the other element directly (e.g., wired), wirelessly, or via a third element.

[0040] The terms like "distributed unit", "distributed unit entity" and "DU" may be used interchangeably throughout the description. The terms like "L1 / L2 triggered mobility" and "LTM" may be used interchangeably throughout the description. The terms "cell switch", "serving cell change", "mobility" and "handover" may be used interchangeably throughout the description. The terms like "candidate DU" and "target DU" may be used interchangeably throughout the description. The terms like "candidate cell" and "target cell" may be used interchangeably throughout the description. It is to be appreciated that the interchangeable terms disclosed in foregoing paragraphs may be used repeatedly throughout the disclosure. However, the same shall not be construed limiting the scope of the disclosure in any sense.

[0041] FIG.　1a shows an exemplary environment 100 depicting a radio link failure (RLF) at a user equipment (UE).

[0042] As shown in FIG. 1a, an RLF occurs when a connection between a UE 102 and a base station 104 is lost. This occurs due to multiple reasons such as interference, coverage issues, handover failures, etc. When the RLF occurs, the UE 102 may try to re-establish the connection by either performing a cell selection procedure or on certain occasions a cell re-establishment procedure. As discussed in the background section, when the UE 102 experiences a lower layer failure, the UE 102 will wait to meet a criterion before declaring the RLF to the base station 104 or network. Upon declaration of the RLF, the UE 102 may perform the cell selection procedure by attempting to select a cell normally.

[0043] In case the selected cell is an already configured target cell in lower layers triggered mobility (LTM) or similar, the UE 102 may directly trigger the LTM procedure (or a conditional handover (CHO)) to switch to the target cell. In case the selected cell is not part of the target cell configuration, the UE 102 may trigger a re-establishment procedure.

[0044] This entire duration from the time when the UE 102 is waiting to meet a criterion before declaring the RLF till when performing the cell selection and / or cell re-establishment procedures to re-establish the connection usually takes a lot of time to complete. In other words, starting from the RLF until the connection is reset and the service is restored, takes approximately 2 to 5 seconds. In this time duration, the user may not get any service, as the UE 102 will be out-of-sync with the network. The disclosure focuses on providing uninterrupted service to the user thereby improving the quality of the service and network performance.

[0045] FIG.　1b shows a flow diagram 100-2 of the exemplary environment 100-1 depicting a UE experiencing an RLF during a lower layer triggered mobility (LTM) handover.

[0046] In fifth generation (5G) new radio (NR) standard, a handover is defined as a process through which UEs moves across different cells without disruption of service. A typical handover process may comprise of three phases: handover preparation, handover execution and handover completion. In the preparation phase, a base station (gNB) may configure the UE to report measurements and based on the reported measurements, the gNB sends radio resource control (RRC) reconfiguration message to the UE. During execution phase, a cell change operation is triggered and the UE applies a target cell configuration and accesses the target cell. During the completion phase, the UE sends RRC reconfiguration complete message.

[0047] FIG.　1b depicts one or more distributed units (DUs), e.g., a serving DU 106 and a target DU 108 of a next generation base station (gNB) 104 (of FIG. 1a). In an example embodiment, the serving DU 106 and the target DU 108 are configured to serve the UE 102 via one or more cells. The serving DU 106 serves the UE 102 when present in the source cell and the target DU 108 serves the UE 102 when present in the target cell 110. The UE 102 may be communicatively coupled to the serving DU 106 and target DU 108, which is serving it via a fronthaul network depending on the cell the UE 102 is stationed in. In an example embodiment, the fronthaul network which may comprise a private network, and / or the Internet, but not limited thereto.

[0048] In a typical LTM handover, as illustrated in operation 101 of FIG. 1b, the gNB 104 may prepare one or multiple candidate cells and provides the LTM candidate or target cell configurations to the UE 102. The gNB 104 may transmit a reconfiguration message including the LTM candidate cell configurations of the one or multiple candidate cells. As shown in FIG. 1b, the serving DU 106 may send this RRC configuration to UE 102 with target DU details (DU1 108 and DU2). The UE 102 may store these LTM candidate cell configurations (operation 1).

[0049] At operation 102, the UE 102 may start measuring the LTM target cells along with the serving DU 106 and perform L1 measurements on the LTM candidate cell(s). In case any of the candidate LTM cells are meeting measurement reporting criteria, the UE 102 may send L1 report to the serving DU 106 or gNB 104.

[0050] However, in case at operation 103, the UE 102 may go out-of-sync with the gNB, due to a lower layer failure. The L1 report prepared by the UE 102 would not be sent to the serving DU 106 (operation 104 of FIG. 1b) as the uplink may be blocked. Thereafter, the UE 102 may declare the RLF after meeting RLF declaration criteria (operation 105) and initiate a cell selection procedure e.g., selection of a best cell available based on signal strength (operation 106 of FIG. 1b).

[0051] During the cell selection procedure, a suitable cell e.g., in present case target cell 110, may be selected from already configured LTM target cells (e.g., target DU1 108 or target DU2). The gNB 104 decides to execute cell switch to the target cell 110 and transmits a medium access control (MAC) control element (CE) triggering cell switch by including the candidate configuration index of the target cell 110. The UE 102 may switch to the configuration of the target cell 110.

[0052] In case the selected cell 110 is not part of the LTM configuration target cells, a re-establishment procedure may be performed. The UE 102 may perform a random access channel (RACH) procedure towards the target DU 1 108, if cell switch requires the performing of RACH procedure. The UE 102 may complete the LTM cell switch procedure by sending reconfiguration complete (RC) message to the target DU 1 108. For RACH-based LTM, the UE 102 may consider that LTM execution procedure is successfully completed when the RACH procedure is successfully completed. For RACH-less LTM, the UE 102 may consider that LTM execution procedure is successfully completed when the UE 102 determines that the network has successfully received its first uplink (UL) data.

[0053] It may be known in the art that LTM supports both intra-gNB-DU, intra-gNB-CU, and inter-gNB-DU mobility. The inter gNB-DU mobility may be defined as cell switch of the UE 102 between source cell and target cell 110 of different DUs, e.g., serving DU 106 and target DU1 108. The intra gNB-DU mobility may be defined as cell switch of the UE 102 from one cell to another cell of the same DU. The LTM also supports inter-frequency mobility, e.g., mobility to an inter-frequency cell that is not a current serving cell. The LTM procedure is applicable to scenarios of standalone, carrier aggregation (CA) and NR-dual connectivity with serving cell change within one cell group, intra-DU case and intra-CU, inter-DU case, for both intra-frequency and inter-frequency, and for both NR bands- FR1 and FR2.

[0054] FIG.　2 depicts a block diagram of a user equipment (UE), in accordance with various embodiments of the disclosure. The UE 200 may comprise a transmitter-receiver pair (202, 204) connected to one or more RF antennas 206. The UE 200 may implement the various embodiments consistent with the disclosure. The UE 200 may be configured to efficiently manage an RLF in a communication network. The UE 200 further may comprise at least one processor 208 (also referred to as central processing units or CPU) and a memory 210. The memory 210 may store instructions executable by the processor 208. The processor 208 may be also coupled to the one or more RF antennas 206. The processor 208 may comprise at least one data processor for executing program components for executing user or system-generated requests. In an example embodiment, the processor 208 may implement the various embodiments consistent with the disclosure. Furthermore, the multi-core processor 208 may include various processing circuitry and / or multiple processors. For example, as used herein, including the claims, the term "processor" may include various processing circuitry, including at least one processor, wherein one or more of at least one processor, individually and / or collectively in a distributed manner, may be configured to perform various functions described herein. As used herein, when "a processor", "at least one processor", and "one or more processors" are described as being configured to perform numerous functions, these terms cover situations, for example and without limitation, in which one processor performs some of recited functions and another processor(s) performs other of recited functions, and also situations in which a single processor may perform all recited functions. Additionally, the at least one processor may include a combination of processors performing various of the recited / disclosed functions, e.g., in a distributed manner. At least one processor may execute program instructions to achieve or perform various functions.

[0055] In an example embodiment, the processor 208 may be configured to detect one or more trigger conditions that indicate a likelihood of occurrence of an RLF at the UE 200. The processor 208 may be further configured to transmit a request for obtaining LTM configuration associated with one or more target cells, to a network entity upon detecting the one or more trigger conditions. In an example embodiment, the network entity may be a next generation base station (gNB) of a 5G network. One example of the network entity may be gNB 104 of FIG. 1a. In response to the request, the processor 208 may be further configured to receive the LTM configuration for each of the one or more target cells from the network entity 104. The processor 208 may be further configured to select, from the one or more target cells, a suitable target cell for performing an LTM handover based on the LTM configuration of each of the one or more target cells. The suitable target cell may meet a cell selection criterion. The processor 208 may be further configured to perform the LTM handover based on the selected target cell.

[0056] In an example embodiment, the processor 208 may include specialized processing units such as integrated system (bus) controllers, memory management control units, floating point units, graphics processing units, digital signal processing units, etc.

[0057] In an example embodiment of the disclosure, the processor 208 may be disposed in communication with one or more input / output (I / O) devices via I / O interface. The I / O interface may employ communication protocols / methods such as, without limitation, audio, analog, digital, monoaural, RCA, stereo, institute of electrical and electronics engineers (IEEE) -1394, serial bus, universal serial bus (USB), infrared, PS / 2, BNC, coaxial, component, composite, digital visual interface (DVI), high-definition multimedia interface (HDMI), radio rrequency (RF) antennas, S-Video, VGA, IEEE 802.n / b / g / n / x, Bluetooth, cellular (e.g., code-division multiple access (CDMA), high-speed packet access (HSPA+), global system for mobile communications (GSM), long-term evolution (LTE), WiMax, or the like), etc.

[0058] FIG. 3 shows an exemplary flowchart 300 depicting operations for efficiently managing an RLF, in accordance with the embodiments of the disclosure.

[0059] As discussed in sections corresponding to FIG. 1b, once an RLF is declared, the UE may perform a cell selection procedure. If the cell selected belongs to the LTM / CHO candidate cell, it may mean there are available LTM / CHO candidate cells in the area which meets the cell selection criterion. In such scenarios, it is not preferrable to wait for the UE to reach the failure point and try to perform cell selection and check if the cell selection belongs to LTM / CHO candidate cell and perform LTM procedure. This is because in such scenario, there may be unnecessary delay in providing proper service to the user that affects the quality of the service to the user and the network performance.

[0060] The disclosure attempts to detect the RLF in advance and apply the existing LTM configuration to minimize the time taken in re-establishing the connections and thereby avoids service interruption. The disclosure describes techniques to identify and predict the lower layer failures in advance and take corrective actions when the candidate cells for LTM are pre-configured, thereby facilitating a better quality of service (QoS).

[0061] The disclosure detects the RLF in prior by checking if a set of trigger conditions are met or not. The trigger conditions may indicate the likelihood of occurrence of the RLF. In case at least one of the trigger conditions is met, the existing LTM configuration of target cells is checked. However, in scenarios, where no LTM target cells are pre-configured, the possibilities of LTM target cells being configured is identified to minimize the delay in performing an LTM handover. Post identifying the possibility of LTM target cells, the UE may send measurement events indicating poor serving cell measurements to make the network configure the possible LTM candidate cells. Upon LTM configuration, the best cell out of all the configured candidate cells may be detected by checking if the cell is meeting a cell selection criterion or not. If the identified cell meets the cell selection criterion, the configuration may be applied and indicated to UE during LTM target cell configuration and thereafter the cell change is performed.

[0062] As illustrated in FIG. 3, at operation 302, different trigger points and conditions are checked for the likelihood of occurrence of an RLF. In the forthcoming sections related to FIGs. 5a to 9, each of these trigger points or conditions are discussed in detail. In the disclosure, the UE triggers LTM upon meeting of at least one of the following trigger conditions: expiry of RLF timer, e.g., T310, maximum radio link control (RLC) UL transmission, handover failure and RACH failure.

[0063] At operation 304, when the trigger points are met or predicted, the UE may check the LTM configuration a thereafter the possibility of LTM being configured. At operation 306, the possibility of LTM configuration may be checked using the prior or pre-stored configuration from the network. With the introduction of LTM (or CHO), UEs may be provided with the target cells configurations in prior, this is obtained using a history configuration that is pre-stored with the network. If there is a possibility of configuration, at operation 308, the UE may send measurement report to network to make the network configure LTM candidate cells. At operation 310, UE may check whether LTM is configured after sending the measurement report to network. In case the LTM is configured at operation 304 and operation 310, at operation 314, the best cell out of the configured candidate cell is identified. Upon checking whether any of the LTM candidate cells are meeting the cell selection criterion (at operation 316), the configuration of candidate cells may be applied in order to perform cell change (at operation 318). In case the LTM is not configured at operation 306 and operation 310, at operation 312, the UE may wait for the normal procedure to complete. The techniques of the disclosure aim to optimize an LTM handover by detecting an RLF prior to its declaration using the above-mentioned procedure.

[0064] FIG. 4 shows a flow diagram 400 depicting efficient management of an RLF, in accordance with the embodiments of the disclosure.

[0065] FIG. 4 illustrates a UE 402 and a CU (control unit) 404 and one or more DUs, e.g., a serving DU 406 and a target DU 408. Here, the UE 402 is similar in structure and function to the UE 200 as shown in FIG. 2. In an example embodiment, the serving DU 406 and the target DU 408 may be configured to serve the UE 402 via one or more cells. The serving DU 406 may serve the UE 402 present in the source cell and the target DU 408 serves the UE 402 present in the target cell. The UE 402 may be communicatively coupled to at least the serving DU 406 and target DU 408, which is serving it via a fronthaul network depending on the cell the UE 402 is stationed in. In an example embodiment, the UE 402 or the processor 208 may implement the embodiments as disclosed in the forthcoming paragraphs.

[0066] When the UE 402 moves from one cell to another cell, e.g., a source cell to a target cell, a serving cell change operation or handover may be triggered. An LTM handover may be triggered based on L1 measurements. In FIG. 4, the LTM handover may be optimized by detecting the likelihood of occurrence of an RLF upon detection of one or more trigger conditions (as illustrated in operation 401). At operation 403, considering the scenario, that an LTM is already configured, the cell selection procedure may be initiated. It may be noted that an LTM candidate cell configuration may be added, modified, and released by network. Further the network stores previous LTM configurations and may be utilized in checking whether the LTM is pre-configured. At operation 403, the best cell may be identified and checked for meeting a specified cell selection criterion. In case the selected cell meets the cell selection criterion, the pre-stored LTM configuration may be applied to the selected cell and a cell change operation is performed (operation 405). Further at operation 407, the UE 402 may send a RRC reconfiguration complete message to the target DU 108.

[0067] Considering the scenario, that LTM is not configured, at operation 409, a measurement report may sent to a central unit (CU) 404 of the gNB and at operation 411, an LTM configuration may be transmitted from the CU 404 to the UE 402. Thereafter at operation 413, the best cell may be identified and checked for meeting a specified cell selection criterion. In case the selected cell meets the cell selection criterion, the received LTM configuration may be applied to the selected cell and a cell change operation is performed (operation 415). Further at operation 417, the UE 402 may send a RRC reconfiguration complete message to the target DU 408.

[0068] FIG. 5a shows a flowchart 500-1 depicting operations for detecting out-of-sync indications and declaring an RLF based on the detection.

[0069] In a typical scenario, when there is a physical layer problem, the UE 402 may detect initiation of counter N310 (a specified counter configured by the network) for consecutive out-of-sync indications and starts a T310 timer. If the T310 timer expires, an RLF is declared. At any point, if the UE 402 gets an in-sync packet, the timer T310 is reset, and the number of in-sync indications are counted using an N311 counter. Upon expiry of T310 timer without receiving any in-sync indications, the RLF is declared.

[0070] As illustrated in FIG. 5a, at operation 501 and operation 502, N310 is initiated, when an "out-of-sync" indication is detected. At operation 503, a T310 timer is started, when N310 is hit. At operation 504, if any "in-sync" is received, while T310 is running, N311 is incremented (operation 505). If no "in-sync" is received, and expiry of T310 occurs (at operation 508), RLF is declared (operation 509). At operation 506, if N311 is hit or incremented, connection is restored (operation 507).

[0071] Whenever the physical layer constantly indicates poor link quality (e.g., low SNR, high block error rate) and the UE 402 continuously receives out-of-sync indications from the physical layer, triggering of an RLF takes place. As per 3GPP technical specification 38.331, whenever the UE 402 starts detecting out-of-sync, it immediately starts a counter called N310 and starts counting the number of times out-of-sync is detected. N310 is a constant that define maximum number of "out-of-sync" detected from lower layers.

[0072] Upon receiving N310 consecutive "out-of-sync" indications from lower layers while neither T300, T301, T304, T311, nor T316 is running, a timer T310 is started. N311 is a constant that defines the maximum number of "in-sync" indicated from lower layers. The value of this constant is typically set to 1. Upon receiving N311 consecutive "in-sync" indications from lower layers while T310 is running, the UE 402 shall stop timer T310. However, if the timer T310 is not stopped, and if T310 expires before N311 was hit, the UE 402 declares RLF. After the RLF occurred, the UE 402 performs a cell selection. Once a cell selection is completed, UE 402 initiates RRC re-establishment process and after the completion of RRC re-establishment, the UE 402 may get back to normal service.

[0073] FIG. 5b shows an exemplary illustration 500-2 depicting the function of timer T310 in declaring an RLF.

[0074] To detect the RLF, the UE 402 may use "Out-of-sync" indications in combination with N310, T310 and N311. These out-of-sync indications are calculated using block error rate (BLER), where BLERoutcorresponds to a quality level (Qout) that indicates unreliable radio link e.g., a physical downlink control channel (PDCCH) reception is not reliable and BLERincorresponds to a quality level (Qin) where the radio link is reliable.

[0075] Considering the example where max BLERoutis set to 10% and N310 is 3, N311 is 2 and T310 is 1sec. Once the current DL BLER is 11% or more, out-of-sync indications may start, based on N310 value. Here the value is 3. Thus, once it reaches max value, T310 timer may start for 1 sec, in between if an in-sync indication Qinis received, N311 count may start. In case, T310 expires before N311 value reaches to its configured value (in this case 2) T310 may expire and RLF may be declared.

[0076] FIG. 5c shows an exemplary flowchart 500-3 depicting operations for detecting out-of-sync indications for indicating a likelihood of occurrence of an RLF, in accordance with the embodiments of the disclosure.

[0077] As illustrated in FIG. 5c, at operation 501 and operation 502, N310 is initiated, when an "out-of-sync" indication is detected. At operation 502, upon getting N310 indication, at operation 510, the UE 402 may check for suitable candidate LTM cells. In case such pre-configured LTM cell is found, at operation 511, the T310 timer is stopped, and the new configuration is applied to the target cell and and LTM handover is performed at operation 512. One of the trigger conditions of the disclosure is getting N310 indication. In an example embodiment, one of the trigger conditions is detecting high BLER and a specified number of out-of-sync indications from lower layer. In case at operation 510, the suitable candidate LTM cells are not found, the T310 timer is started at operation 503. At operation 504, if any "in-sync" is received, while T310 is running, N311 starts incrementing at operation 505. If no "in-sync" is received, and expiry of T310 occurs at operation 508, RLF is declared at operation 509. At operation 506, if N311 is hit, connection is restored at operation 507.

[0078] FIG. 6a shows a flowchart 600-1 depicting operations for detecting handover failure and declaring an RLF based on the detection.

[0079] During a baseline handover scenario, the UE 402 may keep on measuring the target candidate cells, where the UE 402 may camp for better coverage based on measurement object configured by the network. The baseline handover is also referred as L3 handover, as it uses layer 3 (L3) signaling. Once the handover event is hit, the RRC decides the handover from a source cell to the target cell and it releases all its context from source cell and attaches to the target cell. If it is unsuccessful in doing so, handover failure is declared and thereby radio link failure is declared.

[0080] In the case of an RLF due to a handover failure, the UE 402 may not receive the necessary handover parameters from the target base station or may not be able to establish a connection with the target base station, resulting in a loss of radio link. This occurs due to various reasons such as weak signal strength, interference, or network congestion.

[0081] At operation 601, the UE 402 may receive a handover command message in RRC connection re-configuration. At operation 602, once the UE 402 gets the handover command, the timer T304 is initiated with the timer values as indicated in the mobilityControlInfo (operation 602 and operation 603). During the RACH procedure (operation 604), if the T304 timer expires, an RLF is declared (operation 607). At operation 605 upon completion of RACH procedure, the T304 timer is stopped, and successful handover is declared (operation 606).

[0082] FIG. 6b shows a flow diagram 600-2 depicting a handover failure scenario.

[0083] At operation 611, a measurement report (MR) is sent from a UE 610 to a serving base station 620. At operation 612, a handover request is sent to a target base station 630 and in turn the target base station 630 sends a handover request acknowledgment (ACK) to the serving base station 620 (operation 613). At operation 614, a RRC reconfiguration message comprising the context of handover is updated to the UE 610. Accordingly at operation 615, a secondary node (SN) status transfer is sent from the serving base station 620 to the target base station 630. The purpose of the SN status transfer procedure is to transfer an uplink packet data convergence protocol (PDCP) SN and hyper frame number (HFN) receiver status and the downlink PDCP SN and HFN transmitter status from serving base station 620 to the target base station 630. Thereafter, a handover cancel message is sent from the target base station 630 to the serving base station 620 (operation 616). The handover cancel procedure is used to enable the serving base station 620 to cancel an ongoing handover preparation or an already prepared handover. On detection of an unsuccessful handover, an RLF is declared.

[0084] FIG. 6c shows an exemplary flowchart 600-3 depicting operations for detecting a handover failure trigger condition for indicating a likelihood of occurrence of an RLF, in accordance with the embodiments of the disclosure.

[0085] In operation 621 and operation 622, the RRC sends a handover command with mobility control information including timer values of T304. At operation 623, on receiving the handover command, the T304 timer starts. At operation 624 and operation 625, if the UE 402 successfully attaches to the target cell and the T304 timer is stopped, a successful handover is declared (operation 626). In operation 4, if the UE 402 is not able to attach to the target cell and the T304 timer expires, a handover failure message is sent followed by an RLF (operation 627).

[0086] The timer T304 governs RRC reconfiguration procedure for the handover. In other words, if the UE does not complete the handover before the T304 timer expires, the UE reverts back to the associated configuration and initiates an RRC connection reestablishment procedure. The T304 timer may be initiated with a value such as 50. 100, 150. 200, 500. 1000 or 2000 ms. The UE starts the T304 timer upon receiving the RRC Reconfiguration message for the mobility control information- mobilityControlInfo. If T304 expires before the UE MAC successfully completes the random-access procedure on the target cell, the UE initiates the RRC connection re-establishment.

[0087] In the disclosure, at operation 608, the UE 402 checks for suitable candidate LTM cells before making a maximum number of RACH re-attempts (MaxR). In other words, the UE 402 checks for suitable candidate LTM cells upon reaching a specified number of RACH re-attempts where the specified number of RACH re-attempts is less than the maximum number of RACH re-attempts (MaxR) configured by the network. At operation 609, a new configuration is applied to the target cell and and the LTM handover is performed. One of the trigger conditions of the disclosure is meeting a specified number of RACH re-attempts.

[0088] FIG. 7a shows a flowchart 700-1 depicting operations for detecting RLC retransmission failure and declaring an RLF based on the detection.

[0089] The uplink (UL) radio link control (RLC) retransmissions refer to the retransmission of UL RLC protocol data units (PDUs) from the UE 402 to the network. When the network receives a PDU from the UE 402, it sends an ACK or negative acknowledgment (NACK) to indicate whether the PDU was successfully received or not. If NACK is received, the UE 402 needs to retransmit the PDU.

[0090] UL RLC retransmissions occurs due to various reasons such as channel interference, transmission errors, congestion in the network, or UE movement. The number of UL RLC retransmissions impacts the overall system performance and results in increased latency and reduced throughput. After repeated NACK, the UE 402 may have to keep on re-transmitting the PDUs. An RLF is typically triggered by UE 402 when a maximum number of UL retransmissions are reached.

[0091] In operation 701, the multiple NACKs are received from network and in operation 702, the UE 402 re-transmits PDUs. In operation 703, the maximum threshold (Xmax) is crossed for re-transmitting the PDUs and an RLF is declared. If the number of retransmission reaches the maximum allowed attempts, the UE 402 triggers RLF in operation 704.

[0092] FIG. 7b shows an exemplary flowchart 700-2 depicting operations for detecting RLC retransmission failure trigger conditions for indicating a likelihood of occurrence of an RLF, in accordance with the embodiments of the disclosure.

[0093] In operation 711, the multiple NACKs are received from network and in operation 712, UE 402 re-transmits PDUs. In operation 713, a pre-configured threshold (X) is checked, if the pre-configured threshold is reached, at operation 715, the UE 402 checks for suitable candidate LTM cells. At operation 716, the new configuration of the target cell is applied after the suitable candidate cell is found. If the maximum threshold is reached, the UE 402 triggers RLF (operation 714).

[0094] The checking for suitable candidate cells (operation 715) is implemented when UE starts re-transmitting PDU but before maximum re-transmit threshold (Xmax) is reached. That is, the LTM handover may be performed before reaching the maximum re-transmit threshold Xmax. Considering Xmax=10, at a pre-configured value X=5, the UE may check for suitable candidate LTM cells, and perform LTM handover. One of the trigger conditions of the disclosure is meeting a specified number of re-transmit attempts but before reaching the maximum re-transmit threshold.

[0095] FIG. 8a shows a flow diagram 800-1 depicting failure of a RACH procedure.

[0096] If a UE experiences difficulty accessing the RACH or if the RACH is congested, it may lead to RLF. This occurs due to various reasons such as interference, insufficient RACH resources, or collisions with other UEs. When the UE experiences an RLF due to RACH issues, it may try to re-establish the connection with the network by attempting to access the RACH again. If the RACH is still congested or the UE continues to experience difficulty accessing the RACH, it may trigger a re-establishment procedure to establish a new connection with the network.

[0097] The RACH occurs in two scenarios - two step procedure or four step procedure. Fig. 8a depicts the UE 402 (of FIG.4) and a target base station 802. In case of two step procedure, a preamble contained in Msg1 is sent and a response in the form of Msg2 is awaited. If there is any failure in sending Msg1 or receiving Msg2 by the base station 802, a RACH failure occurs. This occurs after maximum number of preamble transmissions are reached. Similarly in the four step procedure, if there is any failure in sending or receiving one or more messages (Msg1, Msg2, Msg3 and Msg4), a RACH failure occurs.

[0098] FIG. 8b shows a flow chart 800-2 depicting operations for detecting message failures during a RACH procedure.

[0099] As illustrated in FIG. 8b, at operation 811, RACH is triggered for two step RACH procedure. At operation 812, Msg1 is sent by the base station 802 (of FIG. 8a). At operation 813, if the UE 402 is unable to receive Msg1 from the base station 802, the base station 802 retransmits Msg1 with higher TX power. If the retransmission of Msg1 also fails, at operation 814, a RACH failure occurs. In case at operation 2, the UE 402 is able to receive Msg1 from the base station 802, Msg2 is sent by the UE 402. At operation 815, if the base station 802 receives the Msg2, the RACH procedure is successful. At operation 815, if Msg2 is not able to receive by the base station 802, the UE declares failure at Msg2 (operation 814) and the RACH is aborted resulting in RRC re-establishment.

[0100] The RACH Procedure fails when a maximum number of preamble transmission are reached. In case of two step RACH procedure, RACH fails at either non-reception of Msg1 by the UE 402 or non-reception of Msg2 by the base station 802.

[0101] FIG. 8c shows an exemplary flowchart 800-3 depicting operations for detecting RACH failure trigger condition for indicating a likelihood of occurrence of an RLF, in accordance with the embodiments of the disclosure.

[0102] The techniques of the disclosure are implemented after increasing the transmission power to send the RACH preamble. In other words, if the calculated Tx power is more than the maximum available Tx power, the UE may trigger the checking for the suitable LTM candidate cells and the configuration of the suitable cell if found is applied.

[0103] As illustrated in FIG. 8c, at operation 821, the RACH is triggered for two step RACH procedure. At operation 822, Msg1 is sent by the base station 802. At operation 823, if the UE 402 is unable to receive Msg1 from the base station 802, the base station 802 retransmits Msg1 with higher TX power. Here, the calculated Tx power is more than the maximum available Tx power configured by the network. If the retransmission of Msg1 with higher Tx power also fails, at operation 824, a RACH failure occurs. In case at operation 822, the UE 402 is able to receive Msg1 from the base station 802, Msg2 is sent by the UE 402. At operation 825, if the base station 802 receives the Msg2, the RACH procedure is successful. At operation 825 and operation 826, if Msg2 is not able to receive by the base station 802, the base station 802 retransmits Msg1 with higher TX power and checks for prepared candidate LTM cell (operation 827). If the candidate LTM cell is found, at operation 828 the new configuration of the target cell is applied. One of the trigger conditions of the disclosure when calculated Tx power is more than the maximum available power when sending Msg1.

[0104] FIG. 9 shows a block diagram 900 of a pre-trained machine learning (ML) model that predicts the likelihood of occurrence of an RLF, in accordance with the embodiments of the disclosure.

[0105] The RLF usually occurs due to various reasons but in most cases, the primary cause is poor signal conditions. Here, the poor signal conditions include decline in the following metrics: reference signal received power (RSRP), received signal strength indicator (RSSI), reference signal received quality (RSRQ), signal to noise (SNR) ratio, BLER and transmission power (Tx Power). However, declaring RLF for poor signal conditions, may cause poor user experience. This is because, the signal conditions may start to recover after declaring RLF. However, if no RLF is declared even when the signal conditions keep on declining and, it may result in poor user experience too. Thus, an ML model 902 of the disclosure is trained to detect continuous declining signal conditions and predict the likelihood of occurrence of an RLF. In an example embodiment, the ML model simultaneously monitors increasing signal conditions. In such case, even if a poor signal condition is identified, but other signal conditions are improving or not declining, the pre-trained ML model is trained to not predict the likelihood of occurrence of an RLF.

[0106] Considering an example, when the RSRP is declining for a specified period of time, the ML model 902 is trained to detect a probable RLF. However, in case a sudden increase in RSRP signal occurs, the model 902 may not detect a probable RLF. The ML model 902 is pre-trained to identify early RLF and thereby provides trigger points to the UE to check for suitable LTM candidate cells and perform LTM handover. One of the trigger conditions of the disclosure is when the pre-trained ML model 902 predicts a RLF by monitoring declining signal conditions.

[0107] FIG. 10 shows a flow diagram depicting a scenario of a secondary cell group (SCG) failure.

[0108] There are following two types of cell groups defined for dual connectivity by the 3GPP Specifications-master cell group (MCG) and secondary cell group (SCG). MCG is a group of serving cells associated with the master node (MN), comprising of the special cell (SpCell) which is known as the primary cell (PCell) and optionally one or more secondary cell (SCell). SCG is a group of serving cells associated with the secondary node (SN), comprising of a SpCell which is known as the primary sCell (PSCell) and optionally one or more SCells.

[0109] Typically, the secondary node change procedure is initiated either by the MN or SN. The procedure entails transferring a UE context from a source SN to a target SN for changing the SCG configuration in UE from one SN to another.

[0110] When a UE experiences lower layer failures in SCG, it may release the entire SCG and send SCG failure information to the network through MCG. This results in losing the entire SCG for the UE and thereby causing an impact on the user experience.

[0111] In 3GPP release-18, feature like LTM is introduced for SCG as well. In other words, the LTM candidate cells are configured for SCG handover as well. Even in such scenario, with the SCG failure, the UE may lose the entire secondary cell configuration.

[0112] As illustrated in FIG 10, a normal SCG Failure scenario is depicted. The network may comprise a UE MCG 1002, a UE SCG 1004, and a CU 1006. Here, the UEs 1002 and 1004 is similar in structure and function to the UE 402 as shown in FIG. 4. In operation 1001, an ongoing data transfer occurs over the MCG 1002 and the SCG 1004. In case the UE SCG 1004 experiences SCG Failure (operation1003), the UE MCG 1002 will send SCG failure information to the network (operation 1005). At operation 1007, the network may send new measurement configuration to add a new SCG. Further at operation 1009, the UE MCG 1002 may measure the cells and send measurement report to network. At operation 1011, upon receiving the measurement report, the network may configure the SCG again based on the UE's measurement report and seamless data transfer continues over UE MCG 1002 and UE SCG 1004 (operation 1013). In other words, when a lower layer failure happened in SCG, the UE MCG 1002 may send the SCG Failure information to the network and wait for SCG to be configured again and impacting user experience.

[0113] FIG. 11 shows an exemplary flow diagram depicting operations of optimizing LTM handover by a UE, while in SCG, in accordance with the embodiments of the disclosure.

[0114] FIG. 11 illustrates the UE MCG 1002, the UE SCG 1004, and the CU 1006 (of FIG.10) and one or more distributed units (DUs), e.g., a serving DU 1102 and a target DU 1104. Here, the DUs 1102 and 1106 are similar in structure and function to the DUs 406 and 408 as shown in FIG. 4. In an example embodiment, the DUs 1102 and 1104 are configured to serve the UE MCG 1002 via one or more cells. The UE MCG 1002 may be communicatively coupled to at least the DUs 1102 and 1104, which is serving via a fronthaul network depending on the cell the UE MCG 1002 is stationed in. In an example embodiment, the UEs 1002 and 1004 or the processor 208 implements the embodiments as disclosed in the forthcoming paragraphs.

[0115] To overcome the limitations of the existing techniques, in the disclosure when an SCG failure happens, the UE MCG 1002 may try to switch to the LTM configured cells in SCG instead of releasing SCG completely. Here, the network is not involved, and the implementation is completely UE-specific.

[0116] In the disclosure, when the UE SCG 1004 is configured by the network, a measurement report threshold is initially configured by the network as a criterion to add SCG and is suitably stored at the CU 1006. As illustrated in FIG. 11, at operation 1101, the LTM is configured and at operation 1103 seamless data transfer occurs between the UE MCG 1002 and UE SCG 1004. At operation 1105, when SCG failure happens, the LTM is checked for being configured or not (operation 1107). In case, it is determined that the LTM is not configured, the SCG failure information is sent to the network (CU 1006 and serving DU 1102). At operation 1109, in case the LTM target cells are configured, the current measurements of the candidate cells are checked with the measurement report threshold initially stored by the CU 1006. Further the best cell out of all candidate cells is found. At operation 1111, in case the measurement of the selected cell is better than the stored criterion, the LTM configuration of the selected cell is applied and consequently a cell switch is performed. At operation 1113, a RRC reconfiguration complete message is sent by the UE SCG 1004 to the Target DU 1104.

[0117] FIG. 12 shows an exemplary flow chart 1200 of a method of optimizing LTM by a UE, while in SCG, in accordance with the embodiments of the disclosure. As shown in FIG. 12, at operation 1202, the SCG is configured by the network and at operation 1204, a measurement report threshold is initially configured by the network as a criterion to add SCG and is suitably stored. At operation 1206, a SCG failure is detected and at operation 1208, the network may check if the LTM is configured or check for possibilities of LTM being configured. This is performed by checking the history of configuration, e.g., identifying LTM cells configured previously in the network. This ensures that all the possibilities of LTM being configured are checked in prior. If there is a possibility of LTM being configured, the UE 402 may send the measurement report to the network to make the UE 402 configure the LTM. If the LTM is configured with candidate cells at operation 1212, initially it is checked whether the candidate cells meet the basic cell selection criterion. In case the candidate cells meet the basic criterion, the measurements of the candidate cells are checked at operation 1212 and the best cell is selected among the candidate cells (at operation 1214). At operation 1216, the measurement of the best cell is checked with the pre-stored criterion. In an example embodiment of the disclosure, the pre-stored criterion is a B1 threshold value. In case the pre-stored criterion is met, at operation 1218, the UE 402 switches to the selected and LTM candidate cell.

[0118] In other words, in the disclosure, when the SCG Failure occurs, the UE 402 checks for the LTM configuration shared for SCG. In case the LTM configurations are available and if there are cells meeting the pre-stored B1 threshold criterion, the LTM configuration is applied, and the cell switch is implemented to continue a user session without losing the SCG. On of the trigger conditions of the disclosure is meeting the pre-stored B1 threshold criterion.

[0119] FIG.　13 shows an exemplary flow chart illustrating a method 1300 of efficiently managing an RLF, in accordance with some embodiments of the disclosure.

[0120] The method 1300 performed by a UE for efficiently handling a RLF may comprise at operation 1302, detecting one or more trigger conditions that indicate a likelihood of occurrence of an RLF. In an example embodiment, the detecting of the one or more trigger conditions may comprise detecting an out-of-sync indication when a BLER value exceeds a specified threshold value. Further, one or more trigger conditions further may comprise detecting that a retransmission count is likely to exceed a threshold count. The one or more trigger conditions further may comprise detecting that a RACH procedure is not likely to be completed on a particular target cell upon receiving a handover command. The one or more trigger conditions further may comprise detecting a retransmission failure of a RACH preamble of a specified message. The retransmission occurred at an increased transmission power (Tx Power). The one or more trigger conditions further may comprise detecting, using a pretrained machine learning (ML) model 902 associated with the UE, a continuous deterioration of one or more parameters associated with the signal quality.

[0121] In an example embodiment, determining of the retransmission count may comprise monitoring NACKs received from a network entity and retransmitting the PDUs upon receiving NACKs from the network entity. Thereafter, determining the retransmission count based on a number of retransmissions of the PDUs.

[0122] At operation 1304, the method 1300 further may comprise transmitting a request for obtaining LTM configuration associated with one or more target cells, to a network entity, upon detecting the one or more trigger conditions. In an example embodiment, the network entity is a next generation base station (gNB) of a 5G network. One example of the network entity is gNB 104 of FIG. 1a. In an example embodiment, the LTM configuration may be a pre-established LTM configuration stored previously by the network entity for possible target cells. In another embodiment, the LTM configuration may be a current LTM configuration configured based on a measurement report transmitted by the UE to the network entity, when the pre-established LTM configuration does not exist.

[0123] At operation 1306, the method 1300 further may comprise in response to the request, receiving the LTM configuration for each of the one or more target cells from the network entity.

[0124] At operation 1308, the method 1300 further may comprise selecting, from the one or more target cells, a suitable target cell for performing an LTM handover based on the LTM configuration of each of the one or more target cells. The suitable target cell meets a cell selection criterion.

[0125] At operation 1310, the method further may comprise performing the LTM handover based on the selected target cell.

[0126] In an example embodiment, the UE is associated with a MCG.

[0127] In an example embodiment, when the UE is associated with a SCG, the method 1300 further may comprise upon selection of the target cell, determining that a measurement report value associated with the selected target cell is greater than a pre-stored threshold value. The pre-stored threshold value is associated with an SCG addition criterion. The method 1300 further may comprise performing the LTM handover to the selected target cell, based on the determination.

[0128] As per existing RLF detection and recovery techniques, when there is a lower layer failure, the UE may perform cell selection. If the cell selected is a cell that belongs to the configured LTM candidate cells, the UE may perform LTM or if it is a cell outside the configured LTM candidate cells, the UE may perform re-establishment procedure. In the disclosure, instead of waiting for the declaration of the lower layer failure, the UE identifies certain trigger points of the failure and checks if there are suitable LTM candidate cells or not. In case they are already available, instead of waiting for the failure to happen and triggering a cell selection procedure, the UE directly performs the cell switch by applying the configuration of already available cell, thereby avoiding the failure and service interruption.

[0129] Furthermore, one or more computer-readable storage media may be utilized in implementing embodiments consistent with the disclosure. A computer-readable storage medium refers to any type of physical memory on which information or data readable by a processor may be stored. Thus, a computer-readable storage medium may store instructions for execution by one or more processors, including instructions for causing the processor(s) to perform steps / operations or stages consistent with the embodiments described herein. The term "computer-readable medium" should be understood to include tangible items and exclude carrier waves and transient signals, e.g., be non-transitory. Examples include random access memory (RAM), read-only memory (ROM), volatile memory, non-volatile memory, hard drives, compact　disc　read-only memory (CD ROMs), digital video disc　(DVDs), flash drives, disks, and any other known physical storage media.

[0130] The terms "an embodiment", "embodiment", "embodiments", "the embodiment", "the embodiments", "one or more embodiments", "some embodiments", and "one embodiment" mean "one or more (but not all) embodiments of the disclosure(s)" unless expressly specified otherwise.

[0131] The terms "including", "comprising", "having" and variations thereof mean "including but not limited to", unless expressly specified otherwise.

[0132] The enumerated listing of items does not imply that any or all of the items are mutually exclusive, unless expressly specified otherwise. The terms "a", "an" and "the" mean "one or more", unless expressly specified otherwise.

[0133] A description of an example embodiment with several components in communication with each other does not imply that all such components are required. On the contrary, a variety of optional components are described to illustrate the wide variety of possible embodiments of the disclosure.

[0134] When a single device or article is described herein, it will be readily apparent that more than one device / article (whether or not they cooperate) may be used in place of a single device / article. Similarly, where more than one device or article is described herein (whether or not they cooperate), it will be readily apparent that a single device / article may be used in place of the more than one device or article, or a different number of devices / articles may be used instead of the shown number of devices or programs. The functionality and / or the features of a device may be alternatively embodied by one or more other devices which are not explicitly described as having such functionality / features. Thus, other embodiments of the disclosure need not include the device itself.

[0135] The illustrated operations of Figure 1a-13 shows certain events occurring in a certain order. In alternative embodiments, certain operations may be performed in a different order, modified, or removed. Moreover, steps / operations may be added to the above-described logic and still conform to the described embodiments. Further, operations described herein may occur sequentially or certain operations may be processed in parallel. Yet further, operations may be performed by a single processing unit or by distributed processing units.

[0136] Finally, the language used in the specification has been principally selected for readability and instructional purposes, and it may not have been selected to delineate or circumscribe the inventive subject matter. It is therefore intended that the scope of the disclosure be limited not by this detailed description, but rather by any claims that issue on an application based here on. Accordingly, the disclosure of the embodiments of the disclosure is intended to be illustrative, but not limiting, of the scope of the disclosure, which is set forth in the following claims.

[0137] While various aspects and embodiments have been disclosed herein, other aspects and embodiments will be apparent to those skilled in the art. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope being indicated by the following claims.

Claims

1.A method (1300) for handling a radio link failure (RLF) by a user equipment (UE), the method (1300) comprising:detecting (1302) one or more trigger conditions that indicate a likelihood of occurrence of an RLF;transmitting (1304), to a network entity, a request for obtaining lower layers triggered mobility (LTM) configuration associated with one or more target cells upon detecting the one or more trigger conditions;receiving (1306), from the network entity, the LTM configuration for each of the one or more target cells in response to the request;selecting (1308) a target cell for performing an LTM handover based on the LTM configuration of each of the one or more target cells, wherein the target cell meets a cell selection criterion; andperforming (1310) the LTM handover based on the selected target cell.2.The method as claimed in claim 1, wherein detecting the one or more trigger conditions comprises:detecting an out-of-sync indication based on a block error rate (BLER) value exceeding a specified threshold value;detecting that a retransmission count exceeds a threshold count value;detecting that a random access channel (RACH) procedure does not complete on a particular target cell upon receiving a handover command;detecting a retransmission failure of a RACH preamble of a specified message, wherein a retransmission of the RACH preamble occurred at an increased transmission power; anddetecting a continuous deterioration of one or more parameters associated with signal quality.3.The method as claimed in claim 1, wherein the LTM configuration comprises one of:a pre-established LTM configuration stored by the network entity for at least one target cell among the one or more target cells; anda current LTM configuration configured based on a measurement report transmitted by the UE to the network entity, when the pre-established LTM configuration does not exist.4.The method as claimed in claim 2, wherein determining the retransmission count comprises:monitoring negative acknowledgements　(NACKs) received from the network entity;retransmitting protocol data units (PDUs) upon receiving the NACKs from the network entity; anddetermining the retransmission count based on a number of retransmissions of the PDUs.5.The method as claimed in claim 1, wherein the UE is associated with a master cell group (MCG).6.The method as claimed in claim 1, wherein based on the UE being associated with a secondary cell group (SCG), the method comprising:upon selection of the target cell, determining that a measurement report value associated with the selected target cell is greater than a threshold value, wherein the threshold value is associated with an SCG addition criterion; andperforming the LTM handover to the selected target cell, based on the determination.7.A user equipment (UE) (200) for handling a radio link failure (RLF), the UE comprising:memory (210) storing instructions; andat least one processor (208),wherein the instructions, when executed by the at least one processor (208) individually or collectively, cause the UE to:detect one or more trigger conditions that indicate a likelihood of occurrence of an RLF;transmit, to a network entity, a request for obtaining lower layers triggered mobility (LTM) configuration associated with one or more target cells upon detecting the one or more trigger conditions;receive, from the network entity, the LTM configuration for each of the one or more target cells in response to the request;select a target cell for performing an LTM handover based on the LTM configuration of each of the one or more target cells, wherein the target cell meets a cell selection criterion; andperform the LTM handover based on the selected target cell.8.The UE as claimed in claim 7, wherein the instructions, when executed by the at least one processor (208) individually or collectively, cause the UE to detect the one or more trigger conditions by:detecting an out-of-sync indication based on a block error rate (BLER) value exceeding a specified threshold value;detecting that a retransmission count is likely to exceed a threshold value;detecting that a random access channel (RACH) procedure does not complete on a particular target cell upon receiving a handover command;detecting a retransmission failure of a RACH preamble of a specified message, wherein a retransmission of the RACH preamble occurred at an increased transmission power; anddetecting a continuous deterioration of one or more parameters associated with signal quality.9.The UE as claimed in claim 7, wherein the LTM configuration comprises one of:a pre-established LTM configuration by the network entity for at least one target cell among the one or more target cells; anda current LTM configuration configured based on a measurement report transmitted by the UE to the network entity, when the pre-established LTM configuration does not exist.10.The UE as claimed in claim 8, wherein the instructions, when executed by the at least one processor (208) individually or collectively, cause the UE to determine the retransmission count by:monitoring negative acknowledgements　(NACKs) received from the network entity;retransmitting protocol data units (PDUs) upon receiving the NACKs from the network entity; anddetermining the retransmission count based on a number of retransmissions of the PDUs.11.The UE as claimed in claim 7, wherein the UE is is associated with a master cell group (MCG).12.The UE as claimed in claim 7, wherein based on the UE being associated with a secondary cell group (SCG), the instructions, when executed by the at least one processor (208) individually or collectively, cause the UE to:upon selection of the target cell, determine that a measurement report value associated with the selected target cell is greater than a threshold value, wherein the threshold value is associated with an SCG addition criterion; andperform the LTM handover to the selected target cell, based on the determination.13.A non-transitory computer-readable storage medium storing one or more programs comprising instructions that, when executed by the processor individually or collectively, cause a user equipment (UE) to:detect one or more trigger conditions that indicate a likelihood of occurrence of an RLF;transmit, to a network entity, a request for obtaining lower layers triggered mobility (LTM) configuration associated with one or more target cells upon detecting the one or more trigger conditions;receive, from the network entity, the LTM configuration for each of the one or more target cells in response to the request;select a target cell for performing an LTM handover based on the LTM configuration of each of the one or more target cells, wherein the target cell meets a cell selection criterion; andperform the LTM handover based on the selected target cell.

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