Radio link failure in cellular communication networks

Abstract

According to an example aspect of the present disclosure, there is provided an apparatus comprising at least one processing core and at least one memory storing instructions that, when executed by the at least one processing core, cause the apparatus at least to receive from a first serving node a request to perform a random access channel, RACH, -less handover from a source cell to a first target cell, determine a radio link failure, RLF, after performing an attempt for the RACH-less handover to the first target cell, determine a first timing advance, TA, value used by the apparatus for the RACH-less handover, receive, after determining that the RACH-less handover of the apparatus to the first target cell failed, from a second node serving a second target cell, a second TA value to be used by the apparatus to set up a connection to the second target cell, set up the connection to the second target cell using the second TA value and transmit to the second target cell a report comprising information on the first TA value and the second TA value.

Description

RADIO LINK FAILURE IN CELLULAR COMMUNICATION NETWORKSFIELD

[0001] Various example embodiments relate in general to cellular communication networks and more specifically, to a Radio Link Failure, RLF, in such networks.BACKGROUND

[0002] Radio Link Failures, RLFs, may occur at least in cellular communication networks, e.g., if a handover of a User Equipment, UE, fails. Handling of such issues is very important in various cellular communication networks, such as, in cellular communication networks operating according to 5G radio access technology. 5G radio access technology may also be referred to as New Radio, NR, access technology. 3rd Generation Partnership Project, 3GPP, develops standards for 5G / NR and for future radio access technologies.SUMMARY

[0003] According to some aspects, there is provided the subject-matter of the independent claims. Some example embodiments are defined in the dependent claims.

[0004] The scope of protection sought for various example embodiments of the disclosure is set out by the independent claims. The example embodiments and features, if any, described in this specification that do not fall under the scope of the independent claims are to be interpreted as examples useful for understanding various example embodiments of the disclosure.

[0005] According to an aspect of the present disclosure, there is provided an apparatus comprising at least one processing core and at least one memory storing instructions that, when executed by the at least one processing core, cause the apparatus at least to receive from a first serving node a request to perform a random access channel, RACH, -less handover from a source cell to a first target cell, determine a radio link failure, RLF, after performing an attempt for the RACH-less handover to the first target cell, determine a first timing advance, TA, value used by the apparatus for the RACH-less handover, receive, after determining that the RACH-less handover of the apparatus to the first target cell failed, froma second node serving a second target cell, a second TA value to be used by the apparatus to set up a connection to the second target cell, set up the connection to the second target cell using the second TA value and transmit to the second target cell a report comprising information on the first TA value and the second TA value. The apparatus may be a user equipment or a control device configured to control the functioning thereof, when installed therein. Example embodiments of the aspect may comprise at least one feature from the following bulleted list or any combination of the following features:• wherein said information on the first TA value and the second TA value comprises an indication about the first TA value and an indication about the second TA value;• wherein said information about the first TA value and the second TA value comprises information on a difference between the first TA value and the second TA value;• wherein the report comprises information indicating that the RACH-less handover was attempted by the apparatus before the RLF occurred;• wherein the report comprises information indicating that the RACH-less handover failed due to a difference between the first TA value and the second TA value;• wherein the at least one processing core and the at least one memory further cause the apparatus at least to determine to include said information on the first TA value and the second TA value to the report when the first TA value and the second TA value differ by a factor;• wherein the at least one processing core and the at least one memory further cause the apparatus at least to determine to include an indication about an expiry of the first TA value to the report when the first TA value and the second TA value differ by a factor;• wherein the report comprises at least one of an indication of a time when a TA acquisition of the first TA value was instructed by the first serving node providing the source cell, an indication of a time since a TA acquisition of the first TA value was instructed by the first serving node providing the source cell, an indication of a time when a handover command comprising the first TA value was received by the apparatus from the first serving node providing the source cell or an indication of a time since a handover commandcomprising the first TA value was received by the apparatus from the first serving node providing the source cell;• wherein said setting up the connection to the second target cell comprises a re-establishment or lower-layer triggered, LTM, recovery;• wherein the first and second target cells are the same, or the first and second target cells are different;• wherein the at least one processing core and the at least one memory further cause the apparatus at least to determine, after setting up a connection to the second target cell, that the first target cell and the second target cell are the same and determine to include said information on the first TA value and the second TA value to the report;• wherein the at least one processing core and the at least one memory further cause the apparatus at least to include said information on the first TA value and the second TA value to the report without determining whether the first target cell and the second target cell are the same;• wherein the report is a radio link failure report.

[0006] According to an aspect of the present disclosure, there is provided an apparatus comprising at least one processing core and at least one memory storing instructions that, when executed by the at least one processing core, cause the apparatus at least to receive a first timing advance, TA, value, wherein the first TA value was used by a user equipment for a random access channel, RACH, -less handover from a source cell to a first target cell, receive a second TA value, wherein the second TA value was used by the user equipment to set up a connection to a second target cell determine that the RACH-less handover of the user equipment to the first target cell failed due to a difference between the first TA value and the second TA value and perform radio link failure, RLF, root cause analysis according to the difference. The apparatus may be a device configured to run an MRO / SON algorithm.

[0007] According to an aspect, there is provided a first method comprising, receiving from a first serving node a request to perform a random access channel, RACH, -less handover from a source cell to a first target cell, determining a radio link failure, RLF, after performing an attempt for the RACH-less handover to the first target cell, determining a first timing advance, TA, value used by the apparatus for the RACH-less handover, receiving, after determining that the RACH-less handover of the apparatus to the first target cell failed, from a second node serving a second target cell, a second TA value to be used by theapparatus to set up a connection to the second target cell, setting up the connection to the second target cell using the second TA value and transmitting to the second target cell a report comprising information on the first TA value and the second TA value. The method may be performed by a user equipment or a control device configured to control the functioning thereof, when installed therein.

[0008] According to an aspect, there is provided a second method comprising, receiving a first timing advance, TA, value, wherein the first TA value was used by a user equipment for a random access channel, RACH, -less handover from a source cell to a first target cell, receiving a second TA value, wherein the second TA value was used by the user equipment to set up a connection to a second target cell, determining that the RACH-less handover of the user equipment to the first target cell failed due to a difference between the first TA value and the second TA value and performing radio link failure, RLF, root cause analysis according to the difference. The method may be performed by a wireless network node or a control device configured to control the functioning thereof, when installed therein.

[0009] According to an aspect of the present disclosure, there is provided an apparatus comprising means for receiving from a first serving node a request to perform a random access channel, RACH, -less handover from a source cell to a first target cell, means for determining a radio link failure, RLF, after performing an attempt for the RACH-less handover to the first target cell, means for determining a first timing advance, TA, value used by the apparatus for the RACH-less handover, means for receiving, after determining that the RACH-less handover of the apparatus to the first target cell failed, from a second node serving a second target cell, a second TA value to be used by the apparatus to set up a connection to the second target cell, means for setting up the connection to the second target cell using the second TA value and means for transmitting to the second target cell a report comprising information on the first TA value and the second TA value. The apparatus of the aspect may be a user equipment or a control device configured to control the functioning thereof, when installed therein.

[0010] According to an aspect of the present disclosure, there is provided an apparatus comprising means for receiving a first timing advance, TA, value, wherein the first TA value was used by a user equipment for a random access channel, RACH, -less handover from a source cell to a first target cell, means for receiving a second TA value, wherein the second TA value was used by the user equipment to set up a connection to a second target cell,means for determining that the RACH-less handover of the user equipment to the first target cell failed due to a difference between the first TA value and the second TA value and means for performing radio link failure, RLF, root cause analysis according to the difference. The apparatus may be a device configured to run an MRO / SON algorithm.

[0011] According to an aspect of the present disclosure, there is provided a computer program comprising instructions which, when the program is executed by an apparatus, cause the apparatus to carry out receiving from a first serving node a request to perform a random access channel, RACH, -less handover from a source cell to a first target cell, determining a radio link failure, RLF, after performing an attempt for the RACH-less handover to the first target cell, determining a first timing advance, TA, value used by the apparatus for the RACH-less handover, receiving, after determining that the RACH-less handover of the apparatus to the first target cell failed, from a second node serving a second target cell, a second TA value to be used by the apparatus to set up a connection to the second target cell, setting up the connection to the second target cell using the second TA value and transmitting to the second target cell a report comprising information on the first TA value and the second TA value.

[0012] According to an aspect of the present disclosure, there is provided a computer program comprising instructions which, when the program is executed by an apparatus, cause the apparatus to carry out receiving a first timing advance, TA, value, wherein the first TA value was used by a user equipment for a random access channel, RACH, -less handover from a source cell to a first target cell, receiving a second TA value, wherein the second TA value was used by the user equipment to set up a connection to a second target cell, determining that the RACH-less handover of the user equipment to the first target cell failed due to a difference between the first TA value and the second TA value and performing radio link failure, RLF, root cause analysis according to the difference.

[0013] According to an aspect of the present disclosure, there is provided a non- transitory computer readable medium having stored thereon a set of computer readable instructions that, when executed by at least one processor, cause an apparatus to at least perform receiving from a first serving node a request to perform a random access channel, RACH, -less handover from a source cell to a first target cell, determining a radio link failure, RLF, after performing an attempt for the RACH-less handover to the first target cell, determining a first timing advance, TA, value used by the apparatus for the RACH-lesshandover, receiving, after determining that the RACH-less handover of the apparatus to the first target cell failed, from a second node serving a second target cell, a second TA value to be used by the apparatus to set up a connection to the second target cell, setting up the connection to the second target cell using the second TA value and transmitting to the second target cell a report comprising information on the first TA value and the second TA value.

[0014] According to an aspect of the present disclosure, there is provided a non- transitory computer readable medium having stored thereon a set of computer readable instructions that, when executed by at least one processor, cause an apparatus to at least perform receiving a first timing advance, TA, value, wherein the first TA value was used by a user equipment for a random access channel, RACH, -less handover from a source cell to a first target cell, receiving a second TA value, wherein the second TA value was used by the user equipment to set up a connection to a second target cell, determining that the RACH- less handover of the user equipment to the first target cell failed due to a difference between the first TA value and the second TA value and performing radio link failure, RLF, root cause analysis according to the difference.BRIEF DESCRIPTION OF THE DRAWINGS

[0015] FIG. 1 illustrates an example of a network scenario in accordance with at least some example embodiments;

[0016] FIG. 2 illustrates a signaling diagram in accordance with at least some example embodiments;

[0017] FIG. 3 illustrates an example apparatus capable of supporting at least some example embodiments;

[0018] FIG. 4 illustrates a flow graph of a first method in accordance with at least some example embodiments; and

[0019] FIG. 5 illustrates a flow graph of a second method in accordance with at least some example embodiments.EXAMPLE EMBODIMENTS

[0020] Embodiments of the present disclosure provide enhancements related to RadioLink Failures, RLFs, in cellular communication networks. More specifically, embodiments of the present disclosure enable enhanced reporting of an RLF that has occurred during a Random Access Channel, RACH, -less handover of a User Equipment, UE. After determining that the RACH-less handover to a first target cell failed, i.e., the RLF took place, the UE may set up a connection to a second target cell. The UE may then transmit an enhanced report comprising information on a first Timing Advance, TA, value and a second TA value, wherein the first TA value was used by the UE for the RACH-less handover and the second TA value was used to set up the connection to the second target cell. Based on the enhanced report, the network may perform root cause analysis of the RLF and optimize operation of the network accordingly, thereby enabling better performance in the future.

[0021] FIG. 1 illustrates an example of a network scenario in accordance with at least some example embodiments. According to the example scenario of FIG. 1, there may be a beam-based wireless communication system, which comprises UE 110, wireless network nodes 120, 130 and core network 140. In some example embodiments, wireless network nodes 120, 130 may be Distributed Units, DUs, and in such a scenario a Central Unit, CU, may be located in the same Radio Access Network, RAN, as DUs 120, 130.

[0022] UE 110 may be first connected to wireless network node 120 via air interface 125 using beams, either simultaneously or one at a time. Wireless network node 120 may be a source node for a handover, e.g., a source gNB. Wireless network node 120 may be referred to as a first serving node of UE 110 before the handover. Wireless network node 130 may be a target node for the handover, e.g., a target gNB, and referred to as a second node serving UE 110 after the handover. In some example embodiments, one CU may control both DUs 120, 130, e g., in case of an intra-gNB handover. Alternatively, DUs 120, 130 may be controlled by different CUs, e g., in case of an inter-gNB handover.

[0023] It may be anticipated that at some point in time UE 110 would move from a coverage area of source wireless network node 120 to a coverage area of target wireless network node 130 and connect to target wireless network node 130 via air interface 135 using beams, either simultaneously or one at a time. One cell of wireless network node 120 may be a source cell for the handover and one cell of wireless network node 130 may be atarget cell for the handover. The source cell may be a serving cell of UE 110 before the handover and the target cell may be the serving cell of UE 110 after the handover.

[0024] UE 110 may comprise, for example, a smartphone, a cellular phone, a Machine-to-Machine, M2M, node, Machine-Type Communications, MTC, node, an Internet of Things, loT, node, a car telemetry unit, a laptop computer, a tablet computer or, indeed, any kind of suitable wireless terminal.

[0025] Air interface 125 between UE 110 and source wireless network node 120 may be configured in accordance with a first Radio Access Technology, RAT, which both UE 110 and source wireless network node 120 are configured to support. Air interface 135 between UE 110 and target wireless network node 130 may be configured in accordance with a second Radio Access Technology, RAT, which both UE 110 and target wireless network node 130 are configured to support. The first and second RATs may be the same and in such a case an intra-RAT handover may be performed. Alternatively, the first and second RATs might not be the same, i.e., an inter-RAT handover may be performed.

[0026] Examples of cellular RATs include Long Term Evolution, LTE, New Radio, NR, which may also be known as fifth generation, 5G, radio access technology, 6G radio access technology, and MulteFire. For example in the context of LTE, wireless network nodes 120, 130 may be referred to as eNBs while wireless network nodes 120, 130 may be referred to as gNB in the context of NR.

[0027] In some example embodiments, wireless network nodes 120, 130 may be referred to as a Transmission and Reception Point, TRPs, or control multiple TRPs that may be co-located or non-co-located. In any case, example embodiments of the present disclosure are not restricted to any particular wireless technology. Instead, example embodiments may be exploited in any beam-based wireless communication system, wherein enhanced reporting concerning RLFs would be beneficial.

[0028] Wireless network nodes 120, 130 may be connected, directly or via at least one intermediate node, with core network 140 via interface 145. Core network 140 may be, in turn, coupled with another network (not shown in FIG. 1), via which connectivity to further networks may be obtained, for example via a worldwide interconnection network. Wireless network nodes 120, 130 may be connected, directly or via at least one intermediate node,with core network 140 or with another core network. Wireless network nodes 120, 130 may be connected with each other directly as well.

[0029] If UE 110 moves from a coverage area of one cell of source wireless network node 120 to a coverage area of another cell, such as a cell of target wireless network node 130, a serving cell change needs to be performed at some point. The serving cell change may equivalently refer to a handover. The serving cell change may be triggered by Layer 3 measurements, e.g., based on a Radio Resource Control, RRC measurement report from UE. 110. The serving cell change may be done by downlink RRC signaling, i.e., by transmitting an RRC reconfiguration message with synchronization for change of a Primary Cell, PCell, and Primary Secondary Cell, PSCell. In addition, release and addition of Secondary Cells, SCells, may be performed when applicable. In such cases, a complete Layer 1 and / or Layer 2 reset would lead to longer latency, larger overhead, and longer interruption time than in case of beam switch mobility. The goal of Layer 1 and Layer 2 mobility enhancements is therefore to enable a serving cell change via Layer 1 and Layer 2 signaling, to reduce the latency, overhead and interruption time.

[0030] Self-Organizing Networks, SONs, may be exploited in various use cases, such as in use cases defined by 3rd Generation Partnership Project 3GPP in the standard specification TS 38.300. Each use case may aim at optimizing a different functionality (mobility, load balancing, energy efficiency, etc.), but they all share a similar way of working.

[0031] A device configured to run a SON algorithm may be for example a wireless network node, such as a gNB, or a network management node in core network 140. The device configured to run the SON algorithm may collect information, e.g., from both, UE 110 and wireless network node 120. The device configured to run the SON algorithm may collect information from UE 110 via at least one report, such as RLF report, Successful Handover Report, SHR, etc. The device configured to run the SON algorithm may also collect information from wireless network node 120, such as a UE context of UE 110. Via root cause analysis process, such information may be aggregated and use case specific Key Performance Indicators, KPIs, or counters may be created and updated. Once enough information is collected and considered as statistically relevant, the device configured to run the SON algorithm may determine which corrective actions need to be taken in order to improve network performance and user experience. Such corrective actions may then beimplemented by means of a configuration parameter re-adjustment, and their impact observed via the use case specific KPIs.

[0032] Improvements related to SON are required, e.g., to enable Mobility Robustness Algorithm, MRO, enhancements for mobility mechanisms. The MRO enhancements for mobility mechanisms may comprise for example Lower layer Triggered Mobility, LTM. More specifically, it would be beneficial to identify and specify necessary UE reporting to enhance the mobility parameter tuning. LTM may comprise Layer 1 and Layer 2 mobility.

[0033] At least one challenge is that source wireless network node 120, such as a source DU, may transmit to UE 110 a TA value in a cell switch command, for example in a Medium Access Control Control Element, MAC CE. The TA value may be a TA value of a cell of target wireless network node 130, i.e., a TA of a target cell served by a different DU. For this, source wireless network node 120 would need to trigger an early TA acquisition before a cell switch is triggered. The purpose of the early TA acquisition would be, e.g., to enable a RACH-less handover triggered by an LTM cell switch MAC-CE command and continue transmitting to a new target cell via a configured uplink grant, provided TA, etc. Such an operation would result in almost zero interruption time during the handover because UE 110 may skip the random access towards target wireless network node 130, if the TA value is acquired in advance. That is, UE 110 would be able to perform a RACH-less handover using the acquired TA.

[0034] Source wireless network node 120 may control the TA update for the envisaged cell switch by a criterion, such as a TA update criterion. The criterion may be used to ensure that UE 110 will get valid TA information along with the cell switch command. In some example embodiments, an invalid TA value may refer to a scenario, wherein the TA value was acquired correctly, but it is no longer usable due to an internal timer expiry of target wireless network node 130. Source wireless network node may assume 120 the TA value as valid in such a case, which is something based on implementation and could even be wrong from the actual usage perspective.

[0035] On the other hand, a wrong TA value may refer to a scenario, wherein UE 110 receives the TA value from source wireless network 120 in time but due to the movement of UE 110 or wrong calculation in target wireless network node 130 the TA value is not correct. In such a case, UE 110 would not be able to establish a connection with the prepared, target cell of target wireless network node 130 using the TA value. Embodiments of the presentdisclosure may be particularly beneficial for detecting and solving issues related to the wrong TA value.

[0036] An incorrect default setting of the TA update criterion for a specific cell pair may result in at least one of the following issues. As a first issue, frequently repeated TA update requests may be performed via Physical Downlink Control Channel, PDCCH, orders if the TA update criterion is too sensitive with respect to the used quantity. Too frequent updates would result if source wireless network node 120 would trigger a new TA acquisition to UE 110 assuming that the earlier acquired one is not valid anymore. Furthermore, acquisition of the TA value leads to interruptions at UE 110 for the currently connected cell of source wireless network node 120. Said interruptions may be caused by radio frequency tuning to the prepared cell of target wireless network node 130, for transmitting a Physical Random Access Channel, PRACH, preamble. Transmissions of PRACH preambles would also introduce extra signalling overhead and power consumption.

[0037] As a second issue, if the TA update criterion is configured as too sloppy, i.e., too loose, in relation to the cell switch criterion, the TA value may be wrong and a first uplink transmission of UE 110 after a RACH-less cell switch fails with an RLF. Opposite to the first issue, in case of the second issue the timer may be too long and the TA value may be assumed as valid on the network side, even though the TA value would be wrong on a side of UE 110. Regarding the first issue, frequent TA acquisitions should be avoided, as TA acquisitions themselves create interruptions to the connections of UE 110 with the currently serving cell of source wireless network node 120. Also, each RACH attempt would cost power drainage of UE 110 and create additional signalling load on the network side. The first issue may not be as crucial as the second issue, but the first issue is in any case relevant in terms of optimization of an early synchronization procedure. The second issue may result in an RLF and then cause even longer interruptions due to contention-based random access synchronization in uplink, compared to a Layer 3 -based handover. There is therefore a need to improve the Layer 3 -based handover, by using the LTM.

[0038] If the RACH-less cell change fails, and UE 110 would re-establish a connection to a new cell, such as a cell of target wireless network node 130, it would be beneficial if a report of UE 110 would comprise information identifying why the RACH- less cell change failed because of an invalid TA value. For example, the report may be an RLF report and comprise information about the root cause of the RLF. The RLF may beidentified as an uplink radio issue caused by an expiry of a tinier, such as an expiry of timer T304, which may be used to monitor timing of the RACH cell. Alternatively, the RLF may be identified as an uplink radio issue by passing the RLF criterion, such as MaxNumberofRLCRetrans for uplink transmission issues, but in such a case it may be unclear that the RLF was caused by a wrong TA value.

[0039] Example embodiments of the present disclosure therefore provide enhancements for detecting and mitigating cell change failures caused by a wrong TA value. The failures may be detected, e.g., by using an MRO / SON algorithm. The wrong TA value may be a value that has been transmitted to UE 110 from source wireless network node 120, to be used in LTM for a prepared target cell, such as a cell of target wireless network node 130.

[0040] A first TA value, for performing a RACH-less handover to a cell of target wireless network node 130, may be acquired by completing a TA acquisition procedure to target wireless network node 130. The first TA value may be transmitted from target wireless network node 130 to source wireless network node 120, possibly via a CU, and source wireless network node 120 may further transmit the first TA value to UE 110, e.g., in a MAC-CE. After receiving a cell switch command comprising the first TA value, UE 110 may use the first TA value when trying to accesses a first target cell, such as a prepared target cell of target wireless network node 130 without RACH, i.e., when performing a RACH-less handover to the first target cell. However, the RACH-less handover to the first target cell may fail, e.g., either due to T304 expiry or if a maximum number of Radio Link Control, RLC, retransmission count has been reached.

[0041] After such a failure, UE 110 may set up a connection with a second target cell. For example, UE 110 may perform a re-establishment of a connection to the second target cell or LTM recovery to the second target cell, e.g., if the second target cell selected after the RACH-less failure is prepared for LTM and UE 110 is explicitly configured to perform LTM recovery. The first and second target cells may, or might not, be the same.

[0042] If the first and second target cell are the same and UE 110 attempts to setup a connection to the same cell as UE 110 tried previously for the RACH-less handover, UE 110 may perform a RACH procedure during a re-establishment procedure and receive a second TA value from target wireless network node 130. UE 110 may use the second TA in the RACH procedure after the RACH-less handover has failed. A root cause for the failedRACH-less handover may be that the first TA value was wrong or invalid (too early TA acquisition), or too early cell switch.

[0043] To enable an MRO / SON algorithm to be able to differentiate between the root causes, UE 110 may transmit a report concerning the RLF, such as an RLF report. The report may comprise at least one of the following:• Last Layer 1 and / or Layer 3 measurements, which have been transmitted / made before UE 110 has received a cell switch command, and last Layer 1 and / or Layer 3 measurements, which have been transmitted / made before a re-establishment of a connection. For example, a device configured to run an MRO / SON algorithm may apply it for TA adjustment if there is no big difference between the measurements made for the same target cell before the cell switch command and the reestablishment. The reason would be that the wrong or invalid first TA value might not be the cause of the handover failure problem in case, e.g., UE 110 moves with high velocity;• velocity and location of UE 110, e.g., for time instances when the first TA value and the cell switch command were received, e.g., in a MAC-CE;• information indicating that a RACH-less approach was carried out prior to the RLF, i.e., information indicating that a RACH-less handover to a first target cell, e.g., to a cell of target wireless network node 120, failed;• the first TA value, used by UE 110 used for the RACH-less handover to the first target cell; or• the second TA value used by UE 110 to set up a connection to a second target cell, e.g., to be used in successful re-establishment of a connection.

[0044] Since the timing between TA acquisition and a cell switch command may be very important for a properly working LTM, i.e.., RACH-less and seamless cell change, the report may comprise at least one of the following:• an indication a time when a TA acquisition was instructed. For example, the report may comprise the indication of a time when a TA acquisition of the first TA was instructed by a first serving node providing the source, i.e., source wireless network node 120. In such a case, the report may comprise a timestamp about when the TA acquisition was instructed, e.g., when UE 110 received a PDCCH order from source wireless network node 120. That is, the report may comprise a timestamp about whenan early TA acquisition was triggered by the network, as observed from the side of UE 110;• an indication of a time since a TA acquisition was instructed. For example, the report may comprise the indication of a time since a TA acquisition of the first TA value was instructed by the first serving node, until a time when UE 110 created the report In such a case, the report may comprise an indication of a time difference between a time of creation of the report at UE 110 and when UE 110 received a PDCCH order from source wireless network node 120;• an indication of a time when a handover command comprising the first TA value, e.g., in a MAC-CE command, was received by UE 110 from the first serving node. For example, the report may comprise a timestamp about when the handover command was transmitted / received from source wireless network node 120. UE 110 has such information since practically the time at which source wireless network node 120 transmits the first TA value, possibly in a MAC CE, may be almost the same as the time at which UE 110 receives the first TA value; or• an indication of a time since a handover command comprising the first TA value, e.g., in a MAC-CE command, was received by UE 110 from the first serving node, until creation of the report by UE 110. For example, the report may comprise an indication of a time difference creation of the report at UE 110 and when source wireless network node 120 transmitted the MAC-CE with the first TA value.

[0045] In some example embodiments, the first target cell for the RACH-less handover may be the same as the second target cell for the connection setup (re-established or recovered cell). UE 110 may incorporate the first and second TA values to the report, and possibly to the other indications above, only if the first target cell for the RACH-less handover as the second target cell for the connection setup (re-established or recovered cell) That is, UE 110 might not incorporate the first and second TA values if the target cell is not the same.

[0046] In some example embodiments, UE 110 may determine to include the first and second TA values to the report, and possibly to the other indications above, if the first and second TA values differ by a factor, e.g., absolute difference between two TA values. The factor may be hardcoded or configured to UE 110, e g., together with the LTM configuration.

[0047] In some example embodiments, UE 110 may determine to include an indication about an expiry of the first TA value to the report when the first TA value and the second TA value differ by the factor, e.g., absolute difference between two TA values. For example, UE 110 may include an “TA expired” flag in case the first and the second TA values differ by the factor. The factor may be hardcoded or configured to UE 110, e.g., together with the LTM configuration.

[0048] Said information transmitted with the report may be used by SON and / or MRO to re-adjust responsible parameters in a device configured to run an MRO / SON algorithm, such as in source wireless network node 120, and serve as an input to perform long-term optimizations to achieve better handover decisions. For example, a device configured to run an MRO / SON algorithm during a root cause analysis of the RLF may perform at least one of the following based on the received information:• Use LTM MRO KPIs or counters, such as a counter for RACH-less cell switch failed due wrong TA and / or RACH-less cell switch to wrong cell. When the device configured to run an MRO / SON algorithm receives the report indicating a wrong TA, the device may increase the counter. The counters may be then used by the device in long term MRO analysis, which would then optimize handover decisions.• The second KPI above, i.e., RACH-less cell switch to a wrong cell, may be used by the device configured to use the MRO / SON algorithm to determine via long-term optimizations to not trigger a cell switch to the first target cell, to which the RACH- less handover failed. Instead, the device may trigger a cell switch to the second target cell, to which UE 110 setup the connection, i.e., the cell that UE 110 has re- estab lished / recovered. In this case, some measurements reported by UE 110 might not be used by the device configured to run the MRO / SON algorithm. In some example embodiments, UE 110 might not enhance the report with any information in case the first target cell is not equal to the second target cell (re- estab lished / recovered cell). In some example embodiments, UE 110 may enhance the report with said information, even though the information might not be used by the device configured to run the MRO / SON algorithm, thereby giving full control to the network and requiring minimum processing by UE 110. Such KPIs may be collected per boundary (per neighbor cell and / or per UE type and or per service type). The KPIs may be collected under further dedicated conditions, e.g., if the difference between the first TA value and the second TA value is larger than a threshold.

[0049] In some example embodiments, the first target cell for the RACH-less handover might not be the same as the second target cell for the connection setup (reestablished or recovered cell). In such a case, a root cause of the failure may be LTM to a wrong cell. Measurements and reported information elements may be used to identify such a failure. Alternatively, UE 110 might not enhance the report with said information.

[0050] In some example embodiments, UE 110 may include said information to the report without comparing whether the first target cell is the same as the second target cell. In such a case, a root cause analysis may be used to determine that the first target cell is different than the second target cell. Another root cause may be identified, namely a RACH- less cell switch command to the wrong cell. The device configured to run the MRO / SON algorithm may check out the report and determine whether the second target cell (re- established / recovered cell) is equal to the first target cell (to the cell that LTM procedure instructed UE 110 to connect to. If both target cells are the same, then said information for MRO optimizations would be valid. Otherwise, below KPIs might not be touched by the effects of the report.

[0051] A device configured to run an MRO / SON algorithm may receive the first TA value and the second TA value, possibly from UE 110 via at least one other network entity, such as target wireless network node 130. The device may further determine that that the RACH-less handover of the user equipment to the first target cell failed due to a difference between the first TA value and the second TA value and perform RLF root cause analysis according to the difference.

[0052] In some example embodiments, the device may determine, based on the difference, that the RACH-less handover of the user equipment to the first target cell failed due to the use of a wrong TA or determine, based on the difference, that the RACH-less handover of the user equipment to the first target cell failed due to a cell switch to a wrong cell. For example, the device may determine above-mentioned KPIs and / or counters, such as RACH-less cell switch failed due wrong TA and / or RACH-less cell switch to a wrong cell based on the difference.

[0053] FIG. 2 illustrates a signaling diagram in accordance with at least some example embodiments. On the vertical axes are disposed, from the left to the right, UE 110 and source wireless network node 120 and target wireless network node 130 of FIG. 1, in addition to CU 150 Time advances from the top towards the bottom. In FIG 2, source DU and targetDU are used as examples of source wireless network node 120 and target wireless network node 130, respectively. Source DU 120 may be the first serving node ofUE 110, providing the source cell. Target DU 130 may be a second node, serving a first target cell and a second target cell in the example of FIG. 2.

[0054] At step 202, UE 110 may be in a connected state with a cell of source DU 120. For example, UE 110 may be in RRC_CONNECTED state with the cell (Cell 0) of source DU 120. At step 204, LTM preparation / configuration may be performed for UE 110, for a cell (Cell 1) of target DU 130. LTM preparation / configuration may be made for the cell of target DU 130 (Cell 1) that is controlled by target DU 130 under CU 150, i.e., the same CU that controls source DU 120. In some example embodiments, operation may be enhanced similarly to inter-CU LTM switches and FIG. 2 is merely an example of a scenario, wherein source DU 120 and target DU 130 are not controlled by the same CU 150. The LTM configuration may be transmitted from CU 150 to UE 110.

[0055] At step 206, UE 110 may transmit a measurement report to source DU 120. The measurement report may be for example a LI measurement report. After receiving the measurement report, source DU 120 may determine that a TA acquisition trigger condition is met for the cell (Cell 1) of target DU 130. At step 208, target DU 130 may, based on said determination, transmit a TA acquisition command to UE 110, for the cell (Cell 1) of target DU 130.

[0056] At step 210, UE 110 may transmit a PRACH to target DU 130. Based on the received PRACH preamble, target DU 130 may determine a first TA value to be used for RACH-less handover to the cell (Cell 1) of target DU 130, i.e., to a first target cell. At step 212, target DU 130 may transmit a Random Access Response, RAR, message comprising the first TA value to CU 150 and CU 150 may forward the RAR message to source DU 120. Target DU 130 may thus provide the RAR message comprising the TA value, to be used by UE 110, to source DU 120 via CU 150. Source DU 120 may then store the first TA value to its memory.

[0057] At step 214, UE 110 may transmit another measurement report to source DU 120, such as another LI measurement report. After receiving said another measurement report, source DU 120 may decide that UE 110 needs to perform a serving cell change to the cell (Cell 1) of target DU 130, from the cell (Cell 0) of source DU 120. That is, based on the measurement report source DU 120 may decide to perform LTM handover of UE 110 to thecell (Cell 1) of target DU. At step 216, source DU 120 may trigger a cell change of UE 110, e.g., by transmitting a request to perform a RACH-less handover from a source cell (a cell of a source DU 120) to a first target cell (cell of a target DU 130). The request may be a MAC-CE command comprising the first TA value to UE 110, to trigger a cell change of UE 110.

[0058] At step 218, UE 110 may try to perform the RACH-less handover to the cell (Cell 1) of target DU 130, but the handover may fail. Thus, UE 110 may determine that the RACH-less handover of UE 110 to the first target cell (the cell (Cell 1) of the target DU 130)) has failed, e.g., if UE 110 has not detected an acknowledgement message from target DU 130 after transmitting a reconfiguration complete message, such as an RRCreconfigurationcomplete. UE 110 may hence determine an RLF after performing an attempt for the RACH-less handover to the first target cell . After that, UE 110 may determine the first TA value used by UE 110 for the RACH-less handover.

[0059] At step 220, UE 110 may receive, after determining that the RACH-less handover of UE 110 to the first target cell failed, from a second node (i.e., DU 130) serving a second target cell, a second TA value to be used by UE 110 to set up a connection to the second target cell. That is, UE 110 may receive the second TA value from target DU 130 serving the second target cell.

[0060] UE 110 may set up, after determining that the RACH-less handover of UE 110 to the first target cell (the cell (Cell 1) of the target DU 130)) has failed, a connection to a second target cell using the second TA value. In the example of FIG. 2, the first target cell is the same as the second target cell and hence, UE 110 may set up the connection with the cell (Cell 1) of target DU 130. UE 110 may thus perform re-establishment or LTM recovery to Cell 1 after picking Cell 1 as the best cell based on measurements. Setting up the connection may comprise a re-establishment of a connection or LTM recovery. The reestablishment may be an RRC re-establishment.

[0061] For example, setting up the connection may comprise transmitting, by UE 110, a PRACH preamble (MSG1) to target DU 130. Target DU 130 may respond by transmitting to target DU 130 a RAR (MSG2) comprising the second TA value, to be used by UE 110 to set up a connection to the second target cell, i.e., to the cell (Cell 1) of target DU 130 in the example of FIG. 2. In some example embodiments, the second target cell may be some other cell though, i.e., not the same as the first target cell (the cell (Cell 1) of target DU). Afterreceiving the RAR, UE 110 may respond by transmitting to target DU 130 on the PUSCH (MSG3) using an initial grant provided in the RAR with the second TA value. Target DU 130 may respond by transmitting to UE 110 a contention resolution message (MSG4). Thus, UE 110 may setup the connection to the second target cell using the second TA value.

[0062] At step 222, as the first target cell for the RACH-less handover and the second target cell for the connection setup (re-establishment or LTM recovery) are the same, UE 110 may prepare a report concerning the RLE The report may be an RLF report. The report may comprise information on the first TA value and the second TA value. For example, said information on the first TA value and the second TA value may comprise an indication about the first TA value and an indication about the second TA value. The indication may comprise flag(s) or actual value(s). Alternatively, or in addition, said information about the first TA value and the second TA value may comprise information on a relationship between the first TA value and the second TA value, such as a difference between the first TA value and the second TA value.

[0063] In some example embodiments, UE 110 may determine a Physical Cell Identifier, PCI, of each cell it tries to access every time. LTM recovery may be only allowed to a target cell to which the RACH-less attempt failed.

[0064] At step 224, UE 110 may be in a connected state with the cell of target DU 130, i.e., with the second target cell. For example, UE 110 may be in RRC_CONNECTED state with the cell (Cell 1) of target DU 130. At step 226, UE 110 may transmit the report to target DU 130 and thus, a retrieval of the report may be performed, such as RLF report retrieval. At step 228, target DU 130 may transmit the report to CU 150 and CU 150 may forward the report to source DU 120.

[0065] Source DU 120 may then modify MRO KPIs based on the received report. For example, source DU 120 may increase a counter, such as “RACK less cell switch failed due to a wrong TA counter”, if source DU 120 found out that the problem is based on the wrong TA.

[0066] Alternatively, or in addition, based on long term optimizations of MRO, TA acquisition may be triggered by source DU 120 in an optimal point in time, to not lead to a wrong TA usage in future LTM processes. For example, after the MRO KPI on wrong TA in LTM passes a threshold, MRO algorithm may force a wireless network node to not useearly TA acquisition method, e.g., in case the reported first TA value and the second TA value were all the time significantly differing by a factor. Another example may be performing more frequent TA acquisitions, due to TA being considered invalid earlier, e.g., in case the first TA value and the second TA value are not differing significantly and measurements of UE 110 show similar results (e.g., indicating low velocity). Frequency of TA acquisitions may be based on the reported “Time since / when TA acquisition was performed and MAC-CE was received”. Another alternative may be to assume a received TA to be valid for a shorter duration of time, so that in some cases, a TA is not provided to the UE in MAC-CE as the TA is not valid.

[0067] Example embodiments of the present disclosure therefore enable reducing, by an MRO / SON algorithm, cell switch failures caused by wrong timing advance value provided to 110 UE in LTM for the prepared target cells.

[0068] FIG. 3 illustrates an example apparatus capable of supporting at least some example embodiments. Illustrated is device 300, which may comprise, for example, UE 110 or a device configured to run an MRO / SON algorithm, or a control device configured to control the functioning thereof, possibly when installed therein. Comprised in device 300 is processor 310, which may comprise, for example, a single- or multi-core processor wherein a single-core processor comprises one processing core and a multi-core processor comprises more than one processing core. Processor 310 may comprise, in general, a control device. Processor 310 may comprise more than one processor. Processor 310 may be a control device. Processor 310 may comprise at least one application-specific integrated circuit, ASIC. Processor 310 may comprise at least one field-programmable gate array, FPGA. Processor 310 may be means for performing method steps in device 300. Processor 310 may be configured, at least in part by computer instructions, to perform actions.

[0069] A processor may comprise circuitry, or be constituted as circuitry or circuitries, the circuitry or circuitries being configured to perform phases of methods in accordance with example embodiments described herein. As used in this application, the term “circuitry” may refer to one or more or all of the following: (a) hardware-only circuit implementations, such as implementations in only analog and / or digital circuitry, and (b) combinations of hardware circuits and software, such as, as applicable: (i) a combination of analog and / or digital hardware circuit(s) with software / firmware and (ii) any portions of hardware processor(s) with software (including digital signal processor(s)), software, and memory(ies) that worktogether to cause an apparatus, such as a mobile phone or server, to perform various functions) and (c) hardware circuit(s) and or processor(s), such as a microprocessor(s) or a portion of a microprocessor(s), that requires software (e g., firmware) for operation, but the software may not be present when it is not needed for operation.

[0070] 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.

[0071] Device 300 may comprise memory 320. Memory 320 may comprise randomaccess memory and / or permanent memory. Memory 320 may comprise at least one RAM chip. Memory 320 may comprise solid-state, magnetic, optical and / or holographic memory, for example. Memory 320 may be at least in part accessible to processor 310. Memory 320 may be at least in part comprised in processor 310. Memory 320 may be means for storing information. Memory 320 may comprise computer instructions that processor 310 is configured to execute. When computer instructions configured to cause processor 310 to perform certain actions are stored in memory 320, and device 300 overall is configured to run under the direction of processor 310 using computer instructions from memory 320, processor 310 and / or its at least one processing core may be considered to be configured to perform said certain actions. Memory 320 may be at least in part comprised in processor 310. Memory 320 may be at least in part external to device 300 but accessible to device 300.

[0072] Device 300 may comprise a transmitter 330. Device 300 may comprise a receiver 340. Transmitter 330 and receiver 340 may be configured to transmit and receive, respectively, information in accordance with at least one cellular or non-cellular standard. Transmitter 330 may comprise more than one transmitter. Receiver 340 may comprise more than one receiver. Transmitter 330 and / or receiver 340 may be configured to operate in accordance with Global System for Mobile communication, GSM, Wideband Code Division Multiple Access, WCDMA, Long Term Evolution, LTE, and / or 5G / NR standards, for example.

[0073] Device 300 may comprise a Near-Field Communication, NFC, transceiver 350. NFC transceiver 350 may support at least one NFC technology, such as Bluetooth, Wibree or similar technologies.

[0074] Device 300 may comprise User Interface, UI, 360. UI 360 may comprise at least one of a display, a keyboard, a touchscreen, a vibrator arranged to signal to a user by causing device 300 to vibrate, a speaker and a microphone. A user may be able to operate device 300 via UI 360, for example to accept incoming telephone calls, to originate telephone calls or video calls, to browse the Internet, to manage digital files stored in memory 320 or on a cloud accessible via transmitter 330 and receiver 340, or via NFC transceiver 350, and / or to play games.

[0075] Device 300 may comprise or be arranged to accept a user identity module 370. User identity module 370 may comprise, for example, a Subscriber Identity Module, SIM, card installable in device 300. A user identity module 370 may comprise information identifying a subscription of a user of device 300. A user identity module 370 may comprise cryptographic information usable to verify the identity of a user of device 300 and / or to facilitate encryption of communicated information and billing of the user of device 300 for communication effected via device 300.

[0076] Processor 310 may be furnished with a transmitter arranged to output information from processor 310, via electrical leads internal to device 300, to other devices comprised in device 300. Such a transmitter may comprise a serial bus transmitter arranged to, for example, output information via at least one electrical lead to memory 320 for storage therein. Alternatively to a serial bus, the transmitter may comprise a parallel bus transmitter. Likewise processor 310 may comprise a receiver arranged to receive information in processor 310, via electrical leads internal to device 300, from other devices comprised in device 300. Such a receiver may comprise a serial bus receiver arranged to, for example, receive information via at least one electrical lead from receiver 340 for processing in processor 310. Alternatively to a serial bus, the receiver may comprise a parallel bus receiver.

[0077] Device 300 may comprise further devices not illustrated in FIG. 3. For example, where device 300 comprises a smartphone, it may comprise at least one digital camera. Some devices 300 may comprise a back-facing camera and a front-facing camera, wherein the back-facing camera may be intended for digital photography and the front-facing camera for video telephony. Device 300 may comprise a fingerprint sensor arranged to authenticate, at least in part, a user of device 300. In some example embodiments, device 300 lacks at least one device described above. For example, some devices 300 may lack an NFC transceiver 350 and / or user identity module 370.

[0078] Processor 310, memory 320, transmitter 330, receiver 340, NFC transceiver 350, UI 360 and / or user identity module 370 may be interconnected by electrical leads internal to device 300 in a multitude of different ways. For example, each of the aforementioned devices may be separately connected to a master bus internal to device 300, to allow for the devices to exchange information. However, as the skilled person will appreciate, this is only one example and depending on the example embodiment various ways of interconnecting at least two of the aforementioned devices may be selected without departing from the scope of the example embodiments.

[0079] FIG. 4 is a flow graph of a first method in accordance with at least some example embodiments. The phases of the illustrated first method may be performed by UE 110 or by a control device configured to control the functioning thereof, when installed therein.

[0080] The first method may comprise, 410, receiving from a first serving node a request to perform a random access channel, RACH, -less handover from a source cell to a first target cell. The first method may also comprise, at step 420, determining a radio link failure, RLF, after performing an attempt for the RACH-less handover to the first target cell. In addition, the first method may comprise, at step 430, determining a first timing advance, TA, value used by the apparatus for the RACH-less handover. At step 440, the first method may comprise receiving, after determining that the RACH-less handover of the apparatus to the first target cell failed, from a second node serving a second target cell, a second TA value to be used by the apparatus to set up a connection to the second target cell. At step 450, the first method may comprise setting up the connection to the second target cell using the second TA value. Finally, the first method may comprise, at step 460, transmitting to the second target cell a report comprising information on the first TA value and the second TA value.

[0081] FIG. 5 is a flow graph of a second method in accordance with at least some example embodiments. The phases of the illustrated first method may be performed by adevice configured to run an MRO / SON algorithm or by a control device configured to control the functioning thereof, when installed therein.

[0082] The second method may comprise, at step 510, receiving a first timing advance, TA, value, wherein the first TA value was used by a user equipment for a random access channel, RACH, -less handover from a source cell to a first target cell. At step 520, the second method may comprise receiving a second TA value, wherein the second TA value was used by the user equipment to set up a connection to a second target cell. At step 530, the second method may comprise determining that the RACH-less handover of the user equipment to the first target cell failed due to a difference between the first TA value and the second TA value. Finally, the second method may comprise, at step 540, performing radio link failure, RLF, root cause analysis according to the difference.

[0083] It is to be understood that the example embodiments disclosed are not limited to the particular structures, process steps, or materials disclosed herein, but are extended to equivalents thereof as would be recognized by those ordinarily skilled in the relevant arts. It should also be understood that terminology employed herein is used for the purpose of describing particular example embodiments only and is not intended to be limiting.

[0084] Reference throughout this specification to one example embodiment or an example embodiment means that a particular feature, structure, or characteristic described in connection with the example embodiment is included in at least one example embodiment. Thus, appearances of the phrases “in one example embodiment” or “in an example embodiment” in various places throughout this specification are not necessarily all referring to the same example embodiment. Where reference is made to a numerical value using a term such as, for example, about or substantially, the exact numerical value is also disclosed.

[0085] As used herein, a plurality of items, structural elements, compositional elements, and / or materials may be presented in a common list for convenience. However, these lists should be construed as though each member of the list is individually identified as a separate and unique member. Thus, no individual member of such list should be construed as a de facto equivalent of any other member of the same list solely based on their presentation in a common group without indications to the contrary In addition, various example embodiments and examples may be referred to herein along with alternatives for the various components thereof. It is understood that such example embodiments, examples,and alternatives are not to be construed as de facto equivalents of one another, but are to be considered as separate and autonomous representations.

[0086] In an example embodiment, an apparatus, such as, for example, UE 110 or a device configured to run an MRO / SON algorithm, may comprise means for carrying out the example embodiments described above and any combination thereof.

[0087] In an example embodiment, a computer program may be configured to cause a method in accordance with the example embodiments described above and any combination thereof. In an example embodiment, a computer program product, embodied on a non-transitoiy computer readable medium, may be configured to control a processor to perform a process comprising the example embodiments described above and any combination thereof.

[0088] In an example embodiment, an apparatus, such as, for example, UE 110 or a device configured to run an MRO / SON algorithm, may comprise at least one processor, and at least one memory including computer program code, wherein the at least one memory and the computer program code are configured to, with the at least one processor, cause the apparatus at least to perform the example embodiments described above and any combination thereof.

[0089] Furthermore, the described features, structures, or characteristics may be combined in any suitable manner in one or more example embodiments. In the preceding description, numerous specific details are provided, such as examples of lengths, widths, shapes, etc., to provide a thorough understanding of example embodiments of the disclosure. One skilled in the relevant art will recognize, however, that the disclosure can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the disclosure.

[0090] While the forgoing examples are illustrative of the principles of the example embodiments in one or more particular applications, it will be apparent to those of ordinary skill in the art that numerous modifications in form, usage and details of implementation can be made without the exercise of inventive faculty, and without departing from the principles and concepts of the disclosure. Accordingly, it is not intended that the disclosure be limited, except as by the claims set forth below.

[0091] The verbs “to comprise” and “to include” are used in this document as open limitations that neither exclude nor require the existence of also un-recited features. The features recited in depending claims are mutually freely combinable unless otherwise explicitly stated. Furthermore, it is to be understood that the use of "a" or "an", that is, a singular form, throughout this document does not exclude a plurality.INDUSTRIAL APPLICABILITY

[0092] At least some example embodiments find industrial application in cellular communication networks, for example in 3 GPP networks, enhanced reporting concerning RLFs would be beneficial.ACRONYMS LIST3GPP 3rd Generation Partnership ProjectBS Base StationCU Central UnitDU Distributed UnitGSM Global System for Mobile communicationKPI Key Performance IndicatorIAB Integrated Access and Backhaul loT Internet of ThingsLTE Long-Term EvolutionLTM Lower-layer Triggered MobilityMAC CE Medium Access Control Control ElementM2M Machine-to-MachineMTC Machine-Type CommunicationsMRO Mobility Robustness AlgortihmNEC Near-Field CommunicationNR New RadioPCell Primary CellPCI Physical Cell IdentifierPDCCH Physical Downlink Control ChannelPRACH Physical Random Access ChannelPSCell Primary Secondary CellRACH Random Access ChannelRAN Radio Access NetworkRAT Radio Access TechnologyRLC Radio Link ControlRLF Radio Link FailureRRC Radio Resource ControlScell Secondary CellSHR Successful Handover ReportSON Self-Organizing NetworkTA Timing AdvanceTRP Transmission and Reception PointUE User EquipmentUI User InterfaceWCDMA Wideband Code Division Multiple AccessWiMAX Worldwide Interoperability for Microwave AccessWLAN Wireless Local Area NetworkREFERENCE SIGNS LIST

Claims

CLAIMS:

1. An apparatus comprising at least one processing core and at least one memory storing instructions that, when executed by the at least one processing core, cause the apparatus at least to:- receive from a first serving node a request to perform a random access channel, RACH, -less handover from a source cell to a first target cell;- determine a radio link failure, RLF, after performing an attempt for the RACH-less handover to the first target cell;- determine a first timing advance, TA, value used by the apparatus for the RACH-less handover;- receive, after determining that the RACH-less handover of the apparatus to the first target cell failed, from a second node serving a second target cell, a second TA value to be used by the apparatus to set up a connection to the second target cell;- set up the connection to the second target cell using the second TA value; and- transmit to the second target cell a report comprising information on the first TA value and the second TA value.

2. An apparatus according to claim 1, wherein said information on the first TA value and the second TA value comprises an indication about the first TA value and an indication about the second TA value.

3. An apparatus according to claim 1 or claim 2, wherein said information about the first TA value and the second TA value comprises information on a difference between the first TA value and the second TA value.

4. An apparatus according to any of the preceding claims, wherein the report comprises information indicating that the RACH-less handover was attempted by the apparatus before the RLF occurred.

5. An apparatus according to any of the preceding claims, wherein the report comprises information indicating that the RACH-less handover failed due to a difference between the first TA value and the second TA value.

6. An apparatus according to any of the preceding claims, wherein the at least one processing core and the at least one memory further cause the apparatus at least to:- determine to include said information on the first TA value and the second TA value to the report when the first TA value and the second TA value differ by a factor.

7. An apparatus according to any of the preceding claims, wherein the at least one processing core and the at least one memory further cause the apparatus at least to:- determine to include an indication about an expiry of the first TA value to the report when the first TA value and the second TA value differ by a factor.

8. An apparatus according to any of the preceding claims, wherein the report comprises at least one of:- an indication of a time when a TA acquisition of the first TA value was instructed by the first serving node providing the source cell;- an indication of a time since a TA acquisition of the first TA value was instructed by the first serving node providing the source cell;- an indication of a time when a handover command comprising the first TA value was received by the apparatus from the first serving node providing the source cell; or- an indication of a time since a handover command comprising the first TA value was received by the apparatus from the first serving node providing the source cell.

9. An apparatus according to any of the preceding claims, wherein said setting up the connection to the second target cell comprises a re-establishment or lower-layer triggered, LTM, recovery.

10. An apparatus according to any of the preceding claims, wherein the first and second target cells are the same, or the first and second target cells are different11. An apparatus according to any of the preceding claims, wherein the at least one processing core and the at least one memory further cause the apparatus at least to:- determine, after setting up a connection to the second target cell, that the first target cell and the second target cell are the same; anddetermine to include said information on the first TA value and the second TA value to the report.

12. An apparatus according to any of claims 1 to 10, wherein the at least one processing core and the at least one memory further cause the apparatus at least to:- include said information on the first TA value and the second TA value to the report without determining whether the first target cell and the second target cell are the same.

13. An apparatus according to any of the preceding claims, wherein the report is a radio link failure report.

14. An apparatus comprising at least one processing core and at least one memory storing instructions that, when executed by the at least one processing core, cause the apparatus at least to:- receive a first timing advance, TA, value, wherein the first TA value was used by a user equipment for a random access channel, RACH, -less handover from a source cell to a first target cell;- receive a second TA value, wherein the second TA value was used by the user equipment to set up a connection to a second target cell;- determine that the RACH-less handover of the user equipment to the first target cell failed due to a difference between the first TA value and the second TA value; and- perform radio link failure, RLF, root cause analysis according to the difference.

15. An apparatus according to claim 14, wherein the at least one processing core and the at least one memory further cause the apparatus at least on of:- determine, based on the difference, that the RACH-less handover of the user equipment to the first target cell failed due to the use of a wrong TA; or- determine, based on the difference, that the RACH-less handover of the user equipment to the first target cell failed due to a cell switch to a wrong cell.

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