Methods and related apparatus for re-establishment

Abstract

There is disclosed a method comprising: receiving, at UE apparatus, a fast re-establishment indication being an indication that the UE apparatus, instead of completing handover to candidate cell apparatus, can request Random Access Channel-less, RACH-less, re-establishment with the candidate cell apparatus; and based on receiving of the fast re-establishment indication, requesting RACH-less re-establishment with the candidate cell apparatus. The method may further comprise determining that handover to the candidate cell apparatus is likely to fail, wherein the requesting of RACH-less re-establishment with the candidate cell apparatus is based on both the determination that handover to the candidate cell apparatus is likely to fail and the receiving of the fast re-establishment indication. Also discloses is apparatus, a computer program and a non-transitory computer readable medium for the same.

Description

[0001] METHODS AND RELATED APPARATUS FOR RE-ESTABLISHMENT

[0002] Field of the Invention

[0003] Various example embodiments relate to methods and apparatus for cell handover and reestablishment and, in particular, methods for re-establishment between user equipment, UE, and a candidate cell for handover.

[0004] Background to the Invention

[0005] In 5G, security key handling in Xn handover is similar to intra-gNB handover but with a necessary handover completion phase including a path switch update. During this path switch update, an Access and Mobility Management Function (AMF) will provided a fresh Network Handover Key (NH) to a candidate / target gNB and UE, facilitating encrypted communication between the candidate / targetgNBand the UE. In case of inter-CU Lower-layer Triggered Mobility (LTM) handover, UE needs to derive keys based on a new Network Cell Counter (NCC) value which is typically expected to be provided by serving cell apparatus. Failure to provide NH or NCC can necessitate reestablishment instead of handover.

[0006] Summary of the Invention

[0007] In accordance with the present invention, there is provided methods of networking devices and corresponding network devices, computer programs and non-transitory computer readable media as claimed in the accompanying claims.

[0008] According to a first example embodiment, a method is provided comprising: receiving, at UE apparatus, a fast re-establishment indication being an indication that the UE apparatus, instead of completing handover to candidate cell apparatus, can request Random Access Channel-less, RACH-less, re-establishment with the candidate cell apparatus; and based on receiving of the fast reestablishment indication, requesting RACH-less re-establishment with the candidate cell apparatus. The method may further comprise: determining that handover to the candidate cell apparatus is likely to fail, wherein the requesting of RACH-less re-establishment with the candidate cell apparatus is based on both the determination that handover to the candidate cell apparatus is likely to fail and the receiving of the fast re-establishment indication.

[0009] The method may further comprise: determining that a Next-Hop Chaining Counter, NCC, has not been provided cell for handover to the candidate cell apparatus, wherein the determination that handover to the candidate cell apparatus is likely to fail is based on the determining that an NCC has not been provided.The fast re-establishment indication may be received from source cell apparatus. Where this is the case, the fast re-establishment indication may be received from the source cell apparatus in an RRC Reconfiguration message for executing handover to the candidate cell apparatus.

[0010] The requesting of RACH-less re-establishment may be done over the configured grant for the candidate cell apparatus.

[0011] The method may further comprise: completing RACH-less re-establishment with the UE apparatus. The requesting and / or completing of RACH-less re-establishment with the candidate cell apparatus may be done using information provided for handover to the candidate cell apparatus.

[0012] The information provided for handover to the candidate cell apparatus may include timing advance, TA, information and / or measurements taken between the UE apparatus to the candidate cell apparatus.

[0013] According to a second embodiment, a method is provided comprising: transmitting, to candidate cell apparatus and / or UE apparatus, a fast re-establishment indication being an indication that the UE apparatus, instead of completing handover to the candidate cell apparatus, can request RACH-less, re-establishment with the candidate cell apparatus.

[0014] The method may further comprises: determining to transmit the fast re-establishment indication based on performance requirements of the UE apparatus.

[0015] The fast re-establishment indication may be transmitted to the UE apparatus in an RRC Reconfiguration message for handover to the candidate cell apparatus. Also, the fast reestablishment indication may be transmitted to the candidate cell apparatus in a Handover, HO, Request for the UE apparatus to handover to the candidate cell apparatus.

[0016] According to a third embodiment, a method is provided comprising: receiving, at candidate cell apparatus from source cell apparatus, a fast re-establishment indication being an indication that UE apparatus, instead of completing handover to the candidate cell apparatus, can request RACH less re-establishment with the candidate cell apparatus.

[0017] The method may further comprise: receiving a RACH-less, re-establishment request from the UE apparatus. Where this is the case, the RACH-less, re-establishment request may be received over the configured grant for the candidate cell apparatus.

[0018] The method may further comprise: completing RACH-less re-establishment with the UE apparatus using information provided for handover to the candidate cell apparatus. Where this is the case, the information provided for handover to the candidate cell apparatus may include timing advance, TA, information and / or measurements taken between the UE apparatus to the candidate cell apparatus. Also, the method may further comprise: transmitting a path switch request to Access & Mobility Management Function, AMF, apparatus. In response, the method may further comprise: receiving from the AMF apparatus, a new NCC; and transmitting the new NCC to the UE apparatus (including by using a Medium Access Control, MAC, Control Element, CE, from a DU, Distributed Unit, of the candidate cell apparatus).According to a fourth example embodiment, apparatus is provided comprising: at least one processor; and at least one memory storing instructions that, when executed by the at least one processor, causes the apparatus to perform a method as previously described.

[0019] According to a fifth example embodiment, a computer program may be provided comprising instructions which, when executed by an apparatus, cause the apparatus to perform a method as previously described.

[0020] According to a sixth example embodiment, a non-transitory computer readable medium may be provided comprising program instructions that, when executed by an apparatus, cause the apparatus to perform a method a method as previously described.

[0021] According to a seventh example embodiment, apparatus is provided comprising: means fort receiving, at UE apparatus, a fast re-establishment indication being an indication that the UE apparatus, instead of completing handover to candidate cell apparatus, can request Random Access Channel-less, RACH-less, re-establishment with the candidate cell apparatus; and means for, based on receiving of the fast re-establishment indication, requesting RACH-less re-establishment with the candidate cell apparatus.

[0022] The apparatus method may further comprise: means for determining that handover to the candidate cell apparatus is likely to fail, wherein the requesting of RACH-less re-establishment with the candidate cell apparatus is based on both the determination that handover to the candidate cell apparatus is likely to fail and the receiving of the fast re-establishment indication.

[0023] The apparatus may further comprise: means for determining that a Next-Hop Chaining Counter, NCC, has not been provided cell for handover to the candidate cell apparatus, wherein the determination that handover to the candidate cell apparatus is likely to fail is based on the determining that an NCC has not been provided.

[0024] The fast re-establishment indication may be received from source cell apparatus. Where this is the case, the fast re-establishment indication may be received from the source cell apparatus in an RRC Reconfiguration message for executing handover to the candidate cell apparatus.

[0025] The requesting of RACH-less re-establishment may be done over the configured grant for the candidate cell apparatus.

[0026] The apparatus may further comprise: means for completing RACH-less re-establishment with the UE apparatus.

[0027] The requesting and / or completing of RACH-less re-establishment with the candidate cell apparatus may be done using information provided for handover to the candidate cell apparatus.

[0028] The information provided for handover to the candidate cell apparatus may include timing advance, TA, information and / or measurements taken between the UE apparatus to the candidate cell apparatus.

[0029] According to a eighth embodiment, apparatus is provided comprising: means for transmitting, to candidate cell apparatus and / or UE apparatus, a fast re-establishment indication being an indicationthat the UE apparatus, instead of completing handover to the candidate cell apparatus, can request RACH-less, re-establishment with the candidate cell apparatus.

[0030] The method may further comprises: determining to transmit the fast re-establishment indication based on performance requirements of the UE apparatus.

[0031] The fast re-establishment indication may be transmitted to the UE apparatus in an RRC Reconfiguration message for handover to the candidate cell apparatus. Also, the fast reestablishment indication may be transmitted to the candidate cell apparatus in a Handover, HO, Request for the UE apparatus to handover to the candidate cell apparatus.

[0032] According to a ninth embodiment, apparatus is provided comprising: means for receiving, at candidate cell apparatus from source cell apparatus, a fast re-establishment indication being an indication that UE apparatus, instead of completing handover to the candidate cell apparatus, can request RACH less re-establishment with the candidate cell apparatus.

[0033] The apparatus may further comprise: means receiving a RACH-less, re-establishment request from the UE apparatus. Where this is the case, the RACH-less, re-establishment request may be received over the configured grant for the candidate cell apparatus.

[0034] The apparatus may further comprise: means for completing RACH-less re-establishment with the UE apparatus using information provided for handover to the candidate cell apparatus. Where this is the case, the information provided for handover to the candidate cell apparatus may include timing advance, TA, information and / or measurements taken between the UE apparatus to the candidate cell apparatus. Also, the method may further comprise: transmitting a path switch request to Access & Mobility Management Function, AMF, apparatus. In response, the method may further comprise: receiving from the AMF apparatus, a new NCC; and transmitting the new NCC to the UE apparatus (including by using a Medium Access Control, MAC, Control Element, CE, from a DU, Distributed Unit, of the candidate cell apparatus).

[0035] Brief Description of the Drawings.

[0036] Example embodiments will now be described with reference to the accompanying figures in which:

[0037] Fig. 1 is a message sequence diagram illustrating conventional security key handing in Xn handover;

[0038] Figs. 2 and 3 are message sequence diagrams illustrating scenarios with a previously target / candidate cell, now a current source cell after a first cell switch, and when cell switch execution conditions are met for a second cell switch;

[0039] Figs. 4A and 4B are message sequence diagrams illustrating a similar scenario to that of figs.

[0040] 2 and 3 but employing a 'fast reestablishment' indicator and RACH-less, re-establishment according to the present invention';

[0041] Fig. 5 illustrates an example embodiment of a UE method;Fig. 6 illustrates an example embodiment of a source cell method;

[0042] Fig. 7 illustrates an example embodiment of a candidate / target cell method; and

[0043] Fig. 8 is a simplified block diagram illustrating a device that is suitable for implementing example embodiments of the present disclosure.

[0044] Detailed Description of Embodiments

[0045] The principle of the present disclosure will now be described with reference to some example embodiments. It is to be understood that these example 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 can be implemented in various manners other than the ones described below. The terminology used herein to describe embodiments is not intended to limit the scope. The articles "a," "an," and "the" are singular in that they have a single referent, however the use of the singular form in the present document should not preclude the presence of more than one referent. In other words, elements referred to in the singular can number one or more, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises," "comprising," "includes," and / or "including," when used herein, specify the presence of stated features, items, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, items, steps, operations, elements, components, and / or groups thereof. Unless otherwise defined, all terms (including technical and scientific terms) used herein are to be interpreted as is customary in the art. It will be further understood that terms in common usage should also be interpreted as is customary in the relevant art and not in an idealized or overly formal sense unless expressly so defined herein.

[0046] 5G Security architecture (e.g. ETSI TS 133.501 V16.3.0, section 6.9.2) contemplate security key handling during Intra-gNB handover, Xn handover (where source and destination gNBs are interconnected) and N2-handover (where an Access and Mobility Management Function (AMF) manages the handover process). In case of intra-gNB handover, Release-18 assumes no key update. However, for inter-gNB scenarios, a key update is needed. In Release-19, for the key derivation, if a fresh Network Handover Key (NH) value exists in the gNB, vertical key derivation is used. In Release-18 both horizontal or vertical keys can be used by the gNB.

[0047] The security key handling in Xn handover is similar to intra-gNB handover but with a handover completion phase including a path switch update. During the path switch update, an AMF always provides a fresh NH value to the target gNB. In case of inter-CU Lower-layer Triggered Mobility (LTM) handover, UE needs to derive keys based on a new Network Cell Counter (NCC) value which is typically expected to be provided by the serving cell.

[0048] Fig. 1 illustrates conventional security key handing in Xn handover from gNBl to gNB2. Notably, ignoring that which is entirely conventional and not relevant to the present invention:Step 1: serving gNBl stores a fresh NH and NCC to be used in a following handover;

[0049] Step 5: having decided to handover to gNB2, gNBl calculates key KgNB*. If NCC is incremented with respect to the current key or a fresh NH value exists, this is done with respect to the current key with vertical key generation; otherwise horizontal key generation is used.

[0050] Steps 6-9: Key KgNB* is provided to gNB2 and a fresh NCC provided to gNBl (via a handover request acknowledgement) and to UE (via RRC Reconfiguration).

[0051] Step 10: With a fresh NCC, the UE calculates calculates key KgNB*. If NCC is incremented with respect to the current key, this is done with respect to the current key with vertical key generation; otherwise horizontal key generation is used.

[0052] Steps 11-13: gNBl and the UE having KgNB* enables RACH and encoded communication between gNBl and the UE.

[0053] Steps 14-17: gNB2 makes a path switch request prompting AMF to increment NCC and provide a fresh NH value to the gNB2 which are stored for future handover.

[0054] As appreciated by the present inventors, if a serving cell is unable to deliver security key KgNB* on time, a UE cannot encrypt the first uplink message with that key. As a result, it will not be possible to complete handover successfully. The present invention seeks to address this critical issue including where Conditional LTM (CLTM) is configured for the inter-CU candidate cell(s) where UEs can experience issues in terms of the availability of the updated NCC value.

[0055] Consider the scenario illustrated in fig. 2, where a new / updated NCC is unable to be delivered to the UE by a previously target / candidate cell which is now a current source cell after a first cell switch, and when cell switch execution conditions are met for a second subsequent CLTM cell switch. Notably in fig. 2:

[0056] Steps 3-19: Candidate cells 1 and 2 (established by Central Unit 1 and Distributed Unit 1, and Central Unit 2 and Distributed Unit 2 respectively) are identified as candidate cells for handover in a conventional manner.

[0057] Steps 20-24: Conditions for handover to Candidate cells 1 are met, and handover effected whereby Candidate Cell 1 becomes the serving cell.

[0058] Steps 25-34: A first problem arises if Candidate cell 2 has not provided an updated NCC value to Candidate cell 2 (now the source cell), as shown in Step 27. In Step 28 , the UE finds that it has not received the new NCC value from the Candidate cell 1 (now the source cell) gNB. Therefore, the UE can not generate the vertical key and encrypt the first UL message (RRC Reconfiguration Complete) which the UE is supposed to send to the Candidate Cell 2 during handover. Inevitably, in step 29, the UE enters RLF and triggers a Reestablishment procedure with Candidate Cell 2, causing interruption.Moreover, consider the similar scenario illustrated in fig. 3 where Candidate Cell 2 has not received an the updated NCC value from Candidate Cell 1 (the current source cell after a first cell switch). Notably in fig. 3:

[0059] Steps 31-32: Candidate Cell-1 instigates a path switch and obtains the updated NCC value (NCC*). However, in the meanwhile, the UE has already met the CLTM execution conditions for Candidate Cell-2 but does not have the updated NCC* value. Therefore, the UE can not derive the security key for inter-CU CLTM cell switch. Again, inevitably, the UE enters RLF and triggers a Re-establishment procedure, causing interruption.

[0060] In accordance with the present invention, when a UE is configured for handover including CLTM inter-CU handover and thus early Timing Advance (TA) and signal measurements have been obtained for candidate cell(s), if that the UE can not obtain a new NCC value from the source gNB, the UE will trigger RRC Re-establishment towards the inter-CU target candidate cell without waiting for Radio Link Failure. At this moment, the UE, contrary to conventional Re-establishment procedure, doesn't release information which would have facilitated handover such as early TA information. As such, if the target candidate cell has a new NCC value, it can be forwarded to the UE via a Re-establishment message or via legacy procedure using MAC CE. Once the new NCC is receive at the UE, the UE can complete the cell switch procedure by sending an RRC Reconfiguration Complete message which is now encrypted by the new key KgNB* using the new NCC value it obtained from the target either via Re-establishment message or MAC CE.

[0061] An advantage of this Re-establishment procedure over a legacy RRC Re-establishment procedure is that it can be much faster. This is because the early TA and / or signal measurements obtained during the UL synchronization process are not released and can be used to facilitate RACH-less Reestablishment. Also, although described in the context of CLTM, this principle may be applicable to other mobility procedures like non-conditional LTM.Certain network behaviour can facilitate this new procedure:

[0062] ■ A source cell including a source-CU while preparing for the inter-CU or intra-CU CLTM handover can flag a 'Fast Re-establishment' indication. For example, within an RRC Reconfiguration message to a UE, or in a Handover Request to a candidate-CU(s). The Fast Re-establishment indication can be also delivered to the UE via CLTM MAC CE by the serving source-DU along with the early TA or cell switch command (CSC MAC CE) if the cell switch is triggered by the network.

[0063] ■ A candidate-CU upon receiving the Fast Re-establishment indication can determine that a corresponding candidate-DU might receive a Re-establishment Request over the Configured Grant from the UE that has determined CLTM execution conditions have been met, but that the UE does not have the updated NCC value to generate the new key (KgNB*). In other words, a candidate cell can be forewarned of the possibility of receiving a RACH-less reestablishment request and prepare accordingly.

[0064] ■ A candidate-CU upon receiving the Fast Re-establishment indication can forward a new NCC value (if available) to the candidate-DU. Alternatively, a candidate-CU may include the Fast Re-establishment indication inside the candidate configuration (LTM-CandidateConfig-rl8) corresponding to the LTM-Candidate IE and send it to the source-CU over a HO Request Acknowledge.

[0065] ■ If a new NCC value is not available to a candidate-CU, it can trigger a Path Switch Request to the AMF and obtain an updated NCC and then share the NCC value to the candidate-DU.

[0066] ■ A candidate-DU may encapsulate the new NCC value along with the Re-establishment message. Alternatively, the candidate-DU may send the new NCC value separately over MAC CE. Note, an NCC value cannot necessarily be included in an RRC Reconfiguration message. Certain UE behaviour can also facilitate this new procedure:

[0067] ■ A UE upon receiving the Fast Re-establishment indication over an RRC Reconfiguration message may determine that no updated NCC value is available at the UE for the vertical key generation. I.e. that although the execution conditions for handover to a given CLTM candidate cell may be met, that handover is destined to fail. Furthermore, a UE may determine that an early TA for the corresponding CLTM candidate cell is available and thus the UE can trigger a RACH-less Re-establishment Request towards the candidate-CU over the Configured Grant (which is made available to the UE using the RRC Reconfiguration message). In this case, the UE can use the early TA it received during the early UL sync. As such, at this moment, the UE does not release the stored TA value for the selected candidate cell.

[0068] ■ The UE may transmit an the RRC Reestablishment Request message to a candidate cell including a reestablishment cause value indicating that the reestablishment cause is the unavailability of the NCC value, e.g., reestablishmentCause=unavailableNCC. As such, the UE can then receive the updated NCC either via a Re-establishment message or via MAC CE.Fig. 4A is a message sequence diagram illustrating a scenario with a previously target / candidate cell, which is now a current source cell after a first cell switch, when cell switch execution conditions are met for a second cell switch, and employing a 'fast reestablishment' indicator according to the present invention'. Notably:

[0069] Step 1: Source-CU obtains the new NCC and NH from the AMF

[0070] Step 2: Source-CU derives the new key (KgNB) based on the received NCC.

[0071] Step 3-4: the UE shares the L3 measurement report to the Source-CU via Source-DU. Step-5: Source-CU determines to prepare inter-CU LTM candidates.

[0072] Step 6-7: Source-CU sends HO Request to the Candidate Cell-1 and Candidate Cell-2 under Target-CU-1 and Target-CU-2. The HO Request contains the new NCC value and may alsi contain the derived key KgNB*. In addition, the HO Request also contains the proposed Fast Re-establishment indication which indicates the candidate cell(s) that it may receive an early RRC Re-establishment Request from the UE which could be triggered by the UE due to the unavailability of the updated NCC value,

[0073] Step 8-9: Target CU-1 and Target CU-2 undertake admission control and send the UE Context Set Up Request to the corresponding DUs and obtains the UE Context Set Up Response. At this stage it is assumed that the involved DUs of the Candidate Cell-1 and Candidate Cell-2, decides to configure Conditional LTM at each of these candidate cells Step 10-11: Target-CU-1 and Target-CU-2 send the LTM HO Response which contains the CLTM execution conditions and Candidate Configuration for Candidate Cell-1 and Candidate Cell-2 respectively.

[0074] Step 12: Source CU sends the UE Context Modification Request to the Source-DU with a CSI- Resource Config request for LTM reporting configuration.

[0075] Step 13: Source-DU determines to configure Conditional LTM for the cell edge users.

[0076] Step 14: Source-DU responds to the Source-CU with UE Context Modification Response containing CSI Reporting Configuration which indicates the execution conditions for LTM for selected Candidate-IDs.

[0077] Step 15: Source-CU prepares the RRC Reconfiguration message which contains the candidate Configurations (i.e. for Candidate Cell-1 and Candidate Cell-2) along with the CLTM execution conditions and the proposed Fast Re-establishment Indication.

[0078] Step 16-17: Source-CU forwards the RRC Reconfiguration message to the UE via source-DU Step 18-19: The UE sends the RRC Reconfiguration Complete to the Source-CU via Source- DU to indicate the successful reception of the RRC reconfiguration message.

[0079] Step 20: The UE starts evaluating the CLTM conditions.

[0080] Step 21: The UE determines CLTM conditions are met for Candidate Cell-1.Step 22: The UE successfully executes CLTM handover towards Candidate Cell-1

[0081] Step 23: Candidate Cell-1 now becomes the serving cell.

[0082] Step 24: The UE obtains the Early Timing Advance (TA) for the Candidate Cell-2 (UL synchronization)

[0083] Scenario 1:

[0084] Step 25: The UE continues to evaluate the conditions for the second cell switch.

[0085] Step 26: The UE determines Candidate Cell-2 (located under Target CU-2) as the CLTM candidate.

[0086] Step 27: The UE determines that it does not have the updated NCC value to derive the vertical key (KgNB*) for the inter-CU cell switch.

[0087] Step 28: As per the newly proposed Fast Re-establishment Indication received within the RRC Reconfiguration message (Step-16 and Step-17), UE decides to trigger the Reestablishment Request using the configured grant without waiting for Radio Link Failure. Step 29: Unlike a convention Re-establishment procedure in which the UE releases the early TA, in this case, the UE doesn't releases the early TA acquired in Step-24 and the corresponding configuration for Candidate Cell-2.

[0088] Step 30: The UE transmits a RACH-less Re-establishment Request using the configured grant and the early TA to the Candidate Cell-2.

[0089] Step 31: Candidate Cell-2 (under Target CU-2) does not have the updated NCC* value. Step 32: As such, Candidate Cell-2 sends a Path Switch Request to the AMF.

[0090] Step 33: Candidate Cell-2 received the Path Switch Acknowledge containing the updated NCC* value using which Candidate Cell-2 derives the updated key KgNB* using the received NCC* value.

[0091] Step 34: Candidate Cell-2 may include the updated NCC value within the Re-establishment message and send it to the UE. Alternatively, the associated DU of the Candidate Cell-2 may forward the updated NCC value to the UE using MAC CE.

[0092] Fig. 4B is a message sequence diagram corresponding to step 1-24 of figure 4A, and with an alternative scenario where an updated NCC* is available at the Candidate cell-2 when it receives the Re-establishment Request from the UE over Configured Grant.

[0093] Scenario 2:

[0094] Step 25: Candidate Cell-1 sends Path Switch Request to the AMF.

[0095] Step 26: Candidate Cell-1 received the Path Switch Acknowledge containing the updated NCC.

[0096] Step 27: Candidate Cell-1 derives the updated key KgNB* using the received NCC value.Step 28: Candidate Cell-1 transmits the NCC value to the Candidate Cell-2 over the Xn interface.

[0097] Step 29: The UE continues to evaluate the conditions for the second cell switch.

[0098] Step 30: The UE determines Candidate Cell-2 (located under Target CU-2) as the CLTM candidate.

[0099] Step 31: The UE determines that it does not have the updated NCC value to derive the vertical key (KgNB*) for the inter-CU cell switch.

[0100] Step 32: As per the newly proposed Fast Re-establishment Indication received within the RRC Reconfiguration message (Step-16 and Step-17), the UE decides to trigger the Reestablishment Request using the Configured Grant instead of waiting till the Radio Link Failure.

[0101] Step 33: Unlike convention Re-establishment procedure where the UE releases the Early Timing Advance, in this case, the UE doesn't releases the early TA acquired in Step-24 and the corresponding configuration for Candidate Cell-2.

[0102] Step 34: The UE transmits a RACH-less Re-establishment Request using the configured grant and the early TA to the Candidate Cell-2.

[0103] Step 35: Candidate Cell-2 sends the NCC value to the UE within the Re-establishment message. Alternatively, the associated DU of the Candidate Cell-2 may forward the updated NCC value to the UE using MAC CE.

[0104] In summary, the present invention provides for rapid, RACH-less Re-establishment in anticipation of a failed handover and RLF.

[0105] In so far implementation in contemporary telecoms standards are concerned, a new field in LTM-Config IE (ltm-FastRestablishment-Enabled-rl9) would prompt the UE to trigger RACH-less Reestablishment in accordance with the present invention. The IE LTM-Config is used to provide LTM configurations:

[0106] - ASN1 START

[0107] - TAG-LTM-CONFIG-START

[0108] LTM-Config-rl8 : : = SEQUENCE {

[0109] ltm-ReferenceConfiguration-rl8 SetupRelease {ReferenceConfiguration-rl8} OPTIONAL, - Need M

[0110] ltm-CandidateToReleaseList-rl8 SEQUENCE (SIZE (l..maxNrofLTM-Configs-rl8)) OF LTM-CandidateId-rl8 OPTIONAL, - Need N

[0111] ltm-CandidateToAddModList-rl8 SEQUENCE (SIZE (l..maxNrofLTM-Configs-rl8)) OF LTM-Candidate-rl8 OPTIONAL, - Need N

[0112] ltm-ServingCellNoResetID-rl8 INTEGER (l..maxNrofLTM-Configs-plusl-rl8) OPTIONAL, - Need N

[0113] ltm-CSI-ResourceConfigToAddModList-rl8 SEQUENCE (SIZE (l..maxNrofLTM-CSI- ResourceConfigurations-rl8)) OF LTM-CSI-ResourceConfig-rl8

[0114] OPTIONAL, - Need N

[0115] ltm-FastRestablishment-Enabled-rl9 ENUMERATED {true}

[0116] OPTIONAL, - Need NOPTIONAL, - Need N

[0117] ltm-CSI-ResourceConfigToReleaseList-rl8 SEQUENCE (SIZE (l..maxNrofLTM-CSI- ResourceConfigurations-rl8)) OF LTM-CSI-ResourceConfigId-rl8

[0118] OPTIONAL, - Need N

[0119] attemptLTM-Switch-rl8 ENUMERATED {true} OPTIONAL, - Cond LTM-MCG

[0120] ltm-ServingCellUE-MeasuredTA-ID-rl8 INTEGER (l..maxNrofLTM-Configs-plusl-rl8) OPTIONAL, - Need N

[0121] }

[0122] - TAG-LTM-CONFIG-STOP

[0123] - ASN1STOP

[0124] Alternatively, introduction of a new field in LTM-Candidate IE (ltm-FastRestablishment-Enabled-rl9) which would trigger UE to trigger RACH-less Re-establishment in accordance with the present invention and be candidate cell specific. The IE LTM-Candidate concerns a LTM candidate configuration to add or modify:

[0125] - ASN1 START

[0126] - TAG-LTM-CANDIDATE-START

[0127] LTM-Candidate-rl8 ::= SEQUENCE {

[0128] ltm-CandidateId-rl8 LTM-CandidateId-rl8,

[0129] ltm-CandidatePCI-rl8 PhysCellld OPTIONAL, - - Need M

[0130] ltm-SSB-Config-rl8 LTM-SSB-Config-rl8 OPTIONAL, - Need M

[0131] ltm-CandidateConfig-rl8 OCTET STRING (CONTAINING RRCReconfigu ration) OPTIONAL, - Need M

[0132] ltm-ConfigComplete-rl8 ENUMERATED {true} OPTIONAL, - Need R

[0133] ltm-EarlyUL-SyncConfig-rl8 OCTET STRING (CONTAINING EarlyUL-SyncConfig-rl8) OPTIONAL, - Need R

[0134] ltm-EarlyUL-SyncConfigSUL-rl8 OCTET STRING (CONTAINING EarlyUL-SyncConfig-rl8) OPTIONAL, - Need R

[0135] ltm-TCI-Info-rl8 LTM-TCI-Info-rl8 OPTIONAL, - - Need M

[0136] ltm-NoResetID-rl8 INTEGER (l..maxNrofLTM-Configs-plusl-rl8) OPTIONAL, - Need N

[0137] ltm-FastRestablishment-Enabled-rl9 ENUMERATED {true}

[0138] OPTIONAL, - Need R

[0139] OPTIONAL, - Need M

[0140] ltm-UE-MeasuredTA-ID-rl8 INTEGER (l..maxNrofLTM-Configs-plusl-rl8) OPTIONAL, - Need M

[0141] }

[0142] LTM-SSB-Config-rl8 :: = SEQUENCE {

[0143] ssb-Frequency-rl8 ARFCN-ValueNR,

[0144] subcarrierSpacing-r!8 SubcarrierSpacing,ssb-Periodicity-rl8 ENUMERATED {ms5, mslO, ms20, ms40, ms80, msl60, spare2, sparel} OPTIONAL, -- Need S

[0145] ssb-PositionsIn Bu rst-rl 8 CHOICE {

[0146] shortBitmap BIT STRING (SIZE (4)),

[0147] mediumBitmap BIT STRING (SIZE (8)),

[0148] longBitmap BIT STRING (SIZE (64))

[0149] } OPTIONAL, - Need R ss-PBCH-BlockPower-rl8 INTEGER (-60..50) OPTIONAL, - Need R

[0150] }

[0151] - TAG-LTM-CANDIDATE-STOP

[0152] - ASN1STOP

[0153] Also, in respect of 3GPP TS 38.331, section 5.3.7.4 'Actions related to transmission of RRCReestablishmentRequest message' after:

[0154] The UE shall set the contents of RRCReestablishmentRequest message as follows:

[0155] 1> if the procedure was initiated due to radio link failure as specified in 5.3.10.3 or reconfiguration with sync failure as specified in 5.3.5.8.3; or

[0156] 1> if the procedure was initiated due to mobility from NR failure as specified in 5.4.3.5 and if voiceFallbacklndication is included in the MobilityFromNRCommand message:

[0157] 2> set the reestablishmentCellld in the VarRLF-Report to the global cell identity of the selected cell;

[0158] could be supplemented with:

[0159] 1> if the procedure was initiated due to unavailability of the updated NCC value at the UE when cell switch execution conditions are met (in case of conditional mobility for e,g. CLTM, CHO) or UE receives a network triggered cell switch command while UE has not yet declared radio link failure:

[0160] 2> UE shall use the Configured Grant (CG) and early TA if acquired earlier for a RACH-less delivery of RRCReestablishmentRequest message.

[0161] The present invention is described primarily in the context of existing 3GPP cellular architectures, however, it will be appreciated that the same principles might apply to other cellular architectures including as yet unspecified, future architectures. Other variations to the present invention will suggest themselves to those skilled in the art.

[0162] Fig. 5 illustrates a further example embodiment of method performed by UE comprising receiving (step 501) a fast re-establishment indication; determining (step 502) that handover to the candidate cell apparatus is likely to fail by virtue of no NCC having been provided for handover by the candidate cell apparatus; and requesting and completing (step 503), based on receiving of the fast reestablishment indication, RACH-less re-establishment with the candidate cell apparatus.Fig. 6 illustrates a further example embodiment of method performed by a source cell comprising determining (step 601) determining performance requirements of the UE; and transmitting (step 602), to candidate cell apparatus and / or the UE apparatus, a fast re-establishment indication. Fig. 7 illustrates a further example embodiment of method performed by a candidate cell comprising receiving (step 701), at candidate cell apparatus from source cell apparatus, a fast re-establishment indication; and receiving a request for and completing (step 702) RACH-less re-establishment with the UE instead of completing handover.

[0163] FIG. 8 is a simplified block diagram of a device 800 that is suitable for implementing example embodiments of the present disclosure. The device 800 can be implemented at or as a part of either UE apparatus or cell providing apparatus.

[0164] As shown, the device 800 includes a processor 810, a memory 820 coupled to the processor 810, a communication module 830 coupled to the processor 810, and a communication interface (not shown) coupled to the communication module 830. The memory 820 stores at least a program 840. The communication module 830 is for bidirectional communications, for example, via multiple antennas. The communication interface may represent any interface that is necessary for communication.

[0165] The program 840 is assumed to include program instructions that, when executed by the associated processor 810, enable the device 800 to operate in accordance with the example embodiments of the present disclosure, as discussed herein with reference to FIGS. 1-7. The example embodiments herein may be implemented by computer software executable by the processor 810 of the device 800, or by hardware, or by a combination of software and hardware. The processor 810 may be configured to implement various example embodiments of the present disclosure.

[0166] The memory 820 may be of any type suitable to the local technical network and may be implemented using any suitable data storage technology, such as a non-transitory computer readable storage medium, semiconductor based memory devices, magnetic memory devices and systems, optical memory devices and systems, fixed memory and removable memory, as non-limiting examples. While only one memory 820 is shown in the device 800, there may be several physically distinct memory modules in the device 800. The processor 810 may be of any type suitable to the local technical network, and may include one or more of general purpose computers, special purpose computers, microprocessors, digital signal processors (DSPs) and processors based on multicore processor architecture, as non-limiting examples. The device 800 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.

[0167] 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

[0168] (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

[0169] (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 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."

[0170] 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.

[0171] Generally, various example 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 example 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.

[0172] The present disclosure also provides at least one computer program product tangibly stored on a non-transitory computer readable storage medium. The computer program product includes computer-executable instructions, such as those included in program modules, being executed in a device on a target real or virtual processor, to carry out the methods of figures 5 and 6. 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 example 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.

[0173] 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-alonesoftware package, partly on the machine and partly on a remote machine or entirely on the remote machine or server.

[0174] 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 media.

[0175] 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), Digital Versatile Disc (DVD), an optical storage device, a magnetic storage device, or any suitable combination of the foregoing.

[0176] The present invention is described primarily in the context of a 5G Security architecture, however, it will be appreciated that the same principles might apply to other cellular architectures including as yet unspecified, future architectures including 6G. Other variations to the present invention will suggest themselves to those skilled in the art.

Claims

CLAIMS1. A method comprising:receiving, at UE apparatus, a fast re-establishment indication being an indication that the UE apparatus, instead of completing handover to candidate cell apparatus, can request Random Access Channel-less, RACH-less, re-establishment with the candidate cell apparatus; andbased on receiving of the fast re-establishment indication, requesting RACH-less reestablishment with the candidate cell apparatus.

2. A method according to claim 1, further comprising:determining that handover to the candidate cell apparatus is likely to fail, wherein the requesting of RACH-less re-establishment with the candidate cell apparatus is based on both the determination that handover to the candidate cell apparatus is likely to fail and the receiving of the fast re-establishment indication.

3. A method according to claim 2, further comprising:determining that a Next-Hop Chaining Counter, NCC, has not been provided cell for handover to the candidate cell apparatus,wherein the determination that handover to the candidate cell apparatus is likely to fail is based on the determining that an NCC has not been provided.

4. A method according to any preceding claim, wherein the fast re-establishment indication is received from source cell apparatus.

5. A method according to claim 4, wherein the fast re-establishment indication is received from the source cell apparatus in an RRC Reconfiguration message for executing handover to the candidate cell apparatus. (Annexture-1)6. A method according to any preceding claim, wherein the requesting of RACH-less reestablishment is done over the configured grant for the candidate cell apparatus.

7. A method according to any preceding claim, further comprising:completing RACH-less re-establishment with the candidate cell apparatus.

8. A method according to any preceding claim, wherein the requesting and / or completing of RACH-less re-establishment with the candidate cell apparatus is done using information provided for handover to the candidate cell apparatus.

9. A method according to claim 8 wherein the information provided for handover to the candidate cell apparatus includes timing advance, TA, information and / or measurements taken between the UE apparatus to the candidate cell apparatus.

10. A method comprising:transmitting, to candidate cell apparatus and / or UE apparatus, a fast reestablishment indication being an indication that the UE apparatus, instead of completing handover to the candidate cell apparatus, can request RACH-less, re-establishment with the candidate cell apparatus.

11. A method according to claim 10 further comprising:determining to transmit the fast re-establishment indication based on performance requirements of the UE apparatus.

12. A method according to claim 10 or claim 11, wherein the fast re-establishment indication is transmitted to the UE apparatus in an RRC Reconfiguration message for handover to the candidate cell apparatus.

13. A method according to claim 11 or claim 12, wherein the fast re-establishment indication is transmitted to the candidate cell apparatus in a Handover, HO, Request for the UE apparatus to handover to the candidate cell apparatus.

14. A method comprising:receiving, at candidate cell apparatus from source cell apparatus, a fast reestablishment indication being an indication that UE apparatus, instead of completing handover to the candidate cell apparatus, can request RACH-less re-establishment with the candidate cell apparatus.

15. A method according to claim 14, further comprising:receiving a RACH-less, re-establishment request from the UE apparatus.

16. A method according to claim 15, wherein the RACH-less, re-establishment request is received over the configured grant for the candidate cell apparatus.

17. A method according to claim 15 or claim 16, further comprising:completing RACH-less re-establishment with the UE apparatus using information provided for handover to the candidate cell apparatus.

18. A method according to claim 17 wherein the information provided for handover to the candidate cell apparatus includes timing advance, TA, information and / or measurements taken between the UE apparatus to the candidate cell apparatus19. A method according to any of claims 17 and 18, further comprising:transmitting a path switch request to Access & Mobility Management Function, AMF, apparatus.

20. A method according to claim 19, further comprising:receiving from the AMF apparatus, a new NCC; andtransmitting the new NCC to the UE apparatus.

21. A method according to claim 20, wherein the new NCC is transmitted to the UE apparatus using a Medium Access Control, MAC, Control Element, CE, from a DU, Distributed Unit, of the candidate cell apparatus.

22. Apparatus comprising:at least one processor; andat least one memory storing instructions that, when executed by the at least one processor, causes the apparatus to perform a method according to any of the preceding claims.

23. A computer program comprising instructions which, when executed by an apparatus, cause the apparatus to perform a method according to any of claims 1 to 21.

24. A non-transitory computer readable medium comprising program instructions that, when executed by an apparatus, cause the apparatus to perform a method according to any of claims 1 to 21.20

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