GB2640225A - Monitoring for LTM - Google Patents

Abstract

An apparatus, e.g. User Equipment, UE 100, comprising means for receiving 206 from a network node, e.g. gNB 200/201, information relating to at least one condition for triggering a handover report, generating the handover report, said handover report comprising a type of a handover and the at least one condition for triggering the handover report, wherein the at least one condition is a missing valid timing advance information or a requirement for the apparatus to perform a random access procedure towards the network node, and providing an indication on the handover report availability to the network node. The information is comprised in a low layer triggered mobility information element or in a configuration message (an RRC configuration message). The type of handover is a lower layer triggered handover and the handover report is a successful handover report, SHR. Further, in step 208 of figure 2 the UE 100 sends a RRC reconfiguration complete message to the gNB. In step 210, the gNB provides LTM config and triggers early TA acquisition. In step 212, the gNB sends a cell switch command to the UE without a valid TA value. In step 214, a random access procedure is executed and the UE generates a SHR when RRC connection was successfully completed with the target PCell via RA procedure in step 214 and UE sends a RRCSetupCompletemessage to the gNB in step 216. In step 220, the UE generates a SHR.

Description

Monitoring for LTM

TECHNICAL FIELD

The present disclosure relates to New Radio, NR, mobility, and more specifically to mobility robustness optimization (MRO) for NR with respect to L1/L2 Triggered Mobility (LTM).

BACKGROUND

io Any discussion of the background art throughout the specification should in no way be considered as an admission that such art is widely known or forms pad of common general knowledge in the field.

A Layer 1 or Layer 2 (L1/L2) triggered mobility (LTM) procedure is provided for handover (HO) of a terminal device (e.g., User Equipment UE) from a source cell to a target cell. In the LTM procedure, a network device (e.g., gNB) receives L1 measurement report(s) from a UE, and on their basis the gNB changes UE's serving cell by a cell switch command signaled via a Medium Access Control Element (MAC CE). The cell switch command indicates an LTM candidate configuration that the gNB previously prepared and provided to the UE through Radio Resource Control (RRC) signaling. Then the UE switches to the target configuration according to the cell switch command. The LTM procedure may be used to reduce the mobility latency.

When configured by the network, it is possible to initiate Uplink (UL) Timing Advance (TA) acquisition procedure to one or multiple cells that are different from the current serving cell. For example, the network may request the UE to perform early TA acquisition of a candidate cell before a cell switch. The early TA acquisition is triggered by Physical Downlink Control Channel (PDCCH) order message sent to the UE, which in turn triggers the sending of RACH preamble to the candidate target cell. The gNB to which the candidate cell belongs calculates based on received RACH preamble the TA value and sends it to the gNB to which the serving cell belongs to. The serving cell provides the newly created TA value to the UE in the LTM cell switch command MAC CE when triggering LTM cell switch. The UE determines whether to access the target cell with the Random Access (RA) procedure depending on whether a valid TA value is provided in the cell switch command. For RACH-less LTM, the UE accesses the target cell via a s configured grant provided in the LTM candidate cell configuration and selects the configured grant occasion associated with the beam indicated in the cell switch command.

SUMMARY

to In accordance with a first aspect of the present disclosure, there is provided an apparatus comprising means for receiving from a network node information relating to at least one condition for triggering a handover report, generating the handover report which comprises a type of a handover and the at least one condition for triggering the handover report, the at least one condition being a missing valid timing advance information or a requirement for the apparatus to perform a random access procedure towards the network node and providing an indication on the handover report availability to the network node.

According to a further aspect, the information is comprised in a low layer triggered mobility information element.

According to a further aspect, the information is comprised in a configuration message.

According to a further aspect, the at least one condition is a missing valid timing advance information or a requirement for the apparatus to perform a random access procedure towards the network node.

According to a further aspect, the type of the handover is a lower layer triggered handover.

According to a further aspect, the handover report is a successful handover report.

According to a further aspect, the handover report is a radio link failure report.

According to a further aspect, there is provided a method comprising the steps of receiving, from a network node, information relating to at least one condition for triggering a handover report; generating the handover report, said handover report s comprising a type of the handover and the at least one condition for triggering the handover report wherein the at least one condition is a missing valid timing advance information or a requirement for the apparatus to perform a random access procedure towards the network; and providing an indication on the handover report availability to the network node.

io According to a further aspect, the at least one condition is a missing valid timing advance information or a requirement for the apparatus to perform a random access procedure towards the network node.

According to a further aspect, there is provided a computer program comprising instructions for causing the apparatus to perform the steps of receiving, from a network node, information relating to at least one condition for triggering a handover report; generating the handover report, said handover report comprising a type of the handover and the at least one condition for triggering the handover report wherein the at least one condition is a missing valid timing advance information or a requirement for the apparatus to perform a random access procedure towards the network; and providing an indication on the handover report availability to the network node.

BRIEF DESCRIPTION OF THE DRAWINGS

Example embodiments of the disclosure will now be described, by way of example only, with reference to the accompanying drawings in which: Fig.1 illustrates a simplified diagram according to an example embodiment Fig. 2 illustrates an example of a signaling diagram for a cell switch procedure Fig. 3 illustrates an example flowchart according to an embodiment Fig. 4 illustrates a simplified block diagram of a device according to an example embodiment

DESCRIPTION OF EXAMPLE EMBODIMENTS

In the following, different exemplifying embodiments will be described using, as an example of a communication network to which examples of embodiments may be applied, a communication network architecture based on 3GPP standards for a communication network, such as a 5G/NR, without restricting the embodiments to such an architecture. It is apparent for a person skilled in the art that the embodiments may also be applied to other types of communication networks where mobile communication principles are integrated with a D2D (device-to-device) or V2X (vehicle to everything) configuration, such as SL (side link), Wi-Fi, worldwide interoperability for microwave access (WiMAX), Bluetooth®, personal communications services (PCS), wideband code division multiple access (WCDMA), systems using ultra-wideband (UWB) technology, mobile ad-hoc networks (MANETs), wired access, etc. Furthermore, without loss of generality, the description of some examples of embodiments is related to a mobile communication network, but principles of the disclosure can be extended and applied to any other type of communication network, such as a wired communication network.

The following examples and embodiments are to be understood only as illustrative examples. Although the specification may refer to "an", "one", or "some" example(s) or embodiment(s) in several locations, this does not necessarily mean that each such reference is related to the same example(s) or embodiment(s), or that the feature only applies to a single example or embodiment. Single features of different embodiments may also be combined to provide other embodiments. Furthermore, terms like "comprising" and "including" should be understood as not limiting the described embodiments to consist of only those features that have been mentioned; such examples and embodiments may also contain features, structures, units, modules, etc., that have not been specifically mentioned.

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

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.

A basic system architecture of a (tele)communication network including a mobile communication system where some examples of embodiments are applicable may include an architecture of one or more communication networks including wireless access network subsystem(s) and core network(s). Such an architecture may include one or more communication network control elements or functions, access network elements, radio access network elements, access service network gateways or base transceiver stations, such as a base station (BS), an access point (AP), a NodeB (NB), an eNB or a gNB, a distributed unit (DU) or a centralized/central unit (CU), which controls a respective coverage area or cell(s) and with which one or more communication stations such as communication elements or functions, like user devices or terminal devices, like a user equipment (UE), or another device having a similar function, such as a modem chipset, a chip, a module etc., which can also be part of a station, an element, a function or an application capable of conducting a communication, such as a UE, an element or function usable in a machine-to-machine communication architecture, or attached as a separate element to such an element, function or application capable of conducting a communication, or the like, are capable to communicate via one or more channels via one or more communication beams for transmitting several types of data in a plurality of access domains. Furthermore, core network elements or network functions, such as gateway network elements/functions, mobility management entities, a mobile switching center, servers, databases and the like may be included.

The following description may provide further details of alternatives, modifications and variances. A gNB comprises e.g., a node providing NR user plane and control plane protocol terminations towards the UE, and connected via the NG interface to the 5GC, e.g., according to 3GPP TS 38.300 V16.6.0 (2021-06) section 3.2 incorporated by reference.

A gNB Central Unit (gNB-CU) comprises e.g., a logical node hosting e.g., RRC, 15 SDAP and PDCP protocols of the gNB or RRC and PDCP protocols of the en-gNB that controls the operation of one or more gNB-DUs. The gNB-CU terminates the Fl interface connected with the gNB-DU.

A gNB Distributed Unit (gNB-DU) comprises e.g., a logical node hosting e.g., RLC, MAC and PHY layers of the gNB or en-gNB, and its operation is partly controlled by the gNB-CU. One gNB-DU supports one or multiple cells. One cell is supported by only one gNB-DU. The gNB-DU terminates the Fl interface connected with the gNBCU.

A gNB-CU-Control Plane (gNB-CU-CP) comprises e.g., a logical node hosting e.g., 25 the RRC and the control plane part of the PDCP protocol of the gNB-CU for an engNB or a gNB. The gNB-CU-CP terminates the El interface connected with the gNB-CU-UP and the Fl-C interface connected with the gNB-DU.

A gNB-CU-User Plane (gNB-CU-UP) comprises e.g., a logical node hosting e.g., 30 the user plane part of the PDCP protocol of the gNB-CU for an en-gNB, and the user plane part of the PDCP protocol and the SDAP protocol of the gNB-CU for a gNB. The gNB-CU-UP terminates the El interface connected with the gNB-CU-CP and the Fl -U interface connected with the gNB-DU, e.g., according to 3GPP TS 38.401 V16.6.0 (2021-07) section 3.1 incorporated by reference.

Different functional splits between the central and distributed unit are possible, e.g., called options: Option 1 (1A-like split): * The function split in this option is similar to the 1A architecture in DC. RRC is in the central unit. PDCP, RLC, MAC, physical layer and RF are in the distributed unit.

Option 2 (3C-like split): * The function split in this option is similar to the 3C architecture in DC. RRC and PDCP are in the central unit. RLC, MAC, physical layer and RF are in the distributed unit.

Option 3 (intra RLC split): * Low RLC (partial function of RLC), MAC, physical layer and RF are in the distributed unit. PDCP and high RLC (the other partial function of RLC) are in the central unit.

Option 4 (RLC-MAC split): * MAC, physical layer and RF are in the distributed unit. PDCP and RLC are in the central unit.

Or else, e.g., according to 3GPP TR 38.801 V14.0.0 (2017-03) section 11 incorporated by reference.

A gNB supports different protocol layers, e.g., Layer 1 (L1) -physical layer. The layer 2 (L2) of NR is split into the following sublayers: Medium Access Control (MAC), Radio Link Control (RLC), Packet Data Convergence Protocol (PDCP) and Service Data Adaptation Protocol (SDAP), where e.g.: * The physical layer offers to the MAC sublayer transport channels; * The MAC sublayer offers to the RLC sublayer logical channels; * The RLC sublayer offers to the PDCP sublayer RLC channels; * The PDCP sublayer offers to the SDAP sublayer radio bearers; * The SDAP sublayer offers to 5GC QoS flows; * Comp. refers to header compression and Segm. To segmentation; * Control channels include (BCCH, PCCH).

Layer 3 (L3) includes e.g., Radio Resource Control (RRC), e.g., according to 3GPP TS 38.300 V16.6.0 (2021-06) section 6 incorporated by reference.

A RAN (Radio Access Network) node or network node like e.g. a gNB, base station, gNB CU or gNB DU or parts thereof may be implemented using e.g. an apparatus with at least one processor and/or at least one memory (with computer-readable instructions (computer program)) configured to support and/or provision and/or io process CU and/or DU related functionality and/or features, and/or at least one protocol (sub-)layer of a RAN (Radio Access Network), e.g. layer 2 and/or layer 3.

The gNB CU and gNB DU parts may e.g., be co-located or physically separated. The gNB DU may even be split further, e.g., into two parts, e.g., one including processing equipment and one including an antenna. A Central Unit (CU) may also be called BBU/REC/RCC/C-RANN-RAN, 0-RAN, or part thereof. A Distributed Unit (DU) may also be called RRH/RRU/RE/RU, or part thereof. Hereinafter, in various example embodiments of the present disclosure, the CU-CP (or more generically, the CU) may also be referred to as a (first) network node that supports at least one of central unit control plane functionality or a layer 3 protocol of a radio access network; and similarly, the DU may be referred to as a (second) network node that supports at least one of distributed unit functionality or the layer 2 protocol of the radio access network.

A gNB-DU supports one or multiple cells, and could thus serve as e.g., a serving cell for a user equipment (UE). A user equipment (UE) may include a wireless or mobile device, an apparatus with a radio interface to interact with a RAN (Radio Access Network), a smartphone, an in-vehicle apparatus, an loT device, a M2M device, or else. Such UE or apparatus 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 certain operations, like e.g. RRC connection to the RAN. A UE is e.g., configured to generate a message (e.g., including a cell ID) to be transmitted via radio towards a RAN (e.g., to reach and communicate with a serving cell). A UE may generate and transmit and receive RRC messages containing one or more RRC PDUs (Packet Data Units).

The UE may have different states (e.g., according to 3GPP TS 38.331 V16.5.0 (2021-06) sections 42.1 and 4.4, incorporated by reference). A UE is e.g., either in RRC CONNECTED state or in RRC INACTIVE state when an RRC connection has been established.

In RRC CONNECTED state a UE may: to * store the AS context; * transfer unicast data to/from the UE; * monitor control channels associated with the shared data channel to determine if data is scheduled for the data channel; * provide channel quality and feedback information; * perform neighbouring cell measurements and measurement reporting.

The RRC protocol includes e.g. the following main functions: * RRC connection control; * measurement configuration and reporting; * establishment/modification/release of measurement configuration (e.g. intra-frequency, inter-frequency and inter-RAT measurements); * setup and release of measurement gaps; * measurement reporting.

The general functions and interconnections of the described elements and functions, which also depend on the actual network type, are known to those skilled in the art and described in corresponding specifications, so that a detailed description thereof may omitted herein for the sake of conciseness. However, it is to be noted that several additional network elements and signaling links may be employed for a communication to or from an element, function or application, like a communication endpoint, a communication network control element, such as a server, a gateway, a radio network controller, and other elements of the same or other communication networks besides those described in detail herein below.

A communication network architecture as being considered in examples of embodiments may also be able to communicate with other networks, such as a public switched telephone network or the Internet. The communication network may also be able to support the usage of cloud services for virtual network elements or functions thereof, wherein it is to be noted that the virtual network part of the telecommunication network can also be provided by non-cloud resources, e.g. an internal network or the like. It should be appreciated that network elements of an access system, of a core network etc., and/or respective functionalities may be implemented by using any node, host, server, access node or entity etc. being io suitable for such a usage. Generally, a network function can be implemented either as a network element on a dedicated hardware, as a software instance running on a dedicated hardware, or as a virtualized function instantiated on an appropriate platform, e.g., a cloud infrastructure.

Furthermore, a network element, such as communication elements, like a UE, a terminal device, control elements or functions, such as access network elements, like a base station / BS, a gNB, a radio network controller, a core network control element or function, such as a gateway element, or other network elements or functions, as described herein, and any other elements, functions or applications zo may be implemented by software, e.g., by a computer program product for a computer, and/or by hardware. For executing their respective processing, correspondingly used devices, nodes, functions or network elements may include several means, modules, units, components, etc. (not shown) which are required for control, processing and/or communication/signaling functionality. Such means, modules, units and components may include, for example, one or more processors or processor units including one or more processing portions for executing instructions and/or programs and/or for processing data, storage or memory units or means for storing instructions, programs and/or data, for serving as a work area of the processor or processing portion and the like (e.g. ROM, RAM, EEPROM, and the like), input or interface means for inputting data and instructions by software (e.g. floppy disc, CD-ROM, EEPROM, and the like), a user interface for providing monitor and manipulation possibilities to a user (e.g. a screen, a keyboard and the like), other interface or means for establishing links and/or connections under the control of the processor unit or portion (e.g. wired and wireless interface means, radio interface means including e.g. an antenna unit or the like, means for forming a radio communication pad etc.) and the like, wherein respective means forming an interface, such as a radio communication part, can be also located on a remote site (e.g. a radio head or a radio station etc.). It is to be noted that in the present specification processing portions should not be only considered to represent physical portions of one or more processors, but may also be considered as a logical division of the referred processing tasks performed by one or more processors. It should be appreciated that according to some examples, a so-called "liquid" or to flexible network concept may be employed where the operations and functionalities of a network element, a network function, or of another entity of the network, may be performed in different entities or functions, such as in a node, host or server, in a flexible manner. In other words, a "division of labor" between involved network elements, functions or entities may vary case by case.

As mentioned above, the present disclosure generally relates to (LTM) procedure which is provided for handover of a terminal device (e.g., User Equipment UE) from a source cell to a target cell. Although a handover from one cell to another may be successful, there may still be outages during the handover, for example if the T310 timer may start but may not expire before the handover is complete. The Successful Handover Report (SHR) was introduced in Rel-17 to optimize the timing aspect in handovers.

SHR can be configured by both a source and a target node (different nodes configuring different parts of SHR) during handover (SHR configuration is within the handover command) and is fetched by the target node and forwarded back to the source node, if needed, using the UElnformationRequest/UElnformationResponse information elements. UE includes the availability of SHR to the network in each complete message sent in the RRC procedure, i.e., RRCReconfigurationComplete, RRCReestablishmentComplete, RRCSetupComplete, RRCResumeComplete messages, if it has available successful HO report to be reported. The UE may discard the SHR, by, for example, release the UE variable VarSuccHO-Report, 48 hours after the SHR is stored.

The content of SHR as currently defined in 3GPP contains: Source and target cell IDs of the handover, Location information, - Latest radio link measurements of all measurement IDs available at the time handover is executed for all handover types, - A cause that was the trigger for generating the SHR, i.e., t310-cause, t312-cause, t304-cause, - Latest radio measurement results of the candidate target cells in case of conditional HO, -Time elapsed between conditional HO execution towards target cell and corresponding latest configuration received for the selected target cell, - C-RNTI of target cell, RA-InformationCommon when T304 is above a threshold.

- In case of DAPS HO, an indication that the source cell failed previously to being release and the introduced interruption time are also logged.

The UE logs SHR, if SHR configuration is received previous to the execution of the handover and at least one of the following criteria is satisfied: -T310 timer of the UE exceeds the configured threshold by the source node, - T312 timer of the UE exceeds the configured threshold by the source node, - T304 timer of the UE exceeds the configured threshold by the target node.

According to the current SHR specification, the UE starts checking these criteria together with the HO execution which is initiated with the reception of the HO Command (RRC Connection Reconfiguration message with synchronization), and the report is finally logged after successful HO completion if at least one of the criteria is fulfilled. In LTM, the cell change is triggered on reception of Cell Switch Command (via MAC CE) and just continues transmitting to new cell, without a RACH procedure (RACH-less cell switch). For this reason, LTM is currently excluded from the SHR reporting. The existence and the performance of the LTM based handover are currently not reflected in the SHR.

More specifically, there may be the case where a MAC-Control Element (CE) command did not trigger a RACH-less cell switch since the UE may receive a MAC-CE without a valid TA value or information. This is currently not reflected in the SHR although information on whether the cell change occurred without a RACH procedure is crucial for LTM.

Further, there may be also the case where a cell change is triggered in a RACHless manner as the UE receives a MAC-CE including TA value or information. However, the first uplink transmission may not be successful leading to a number of to RLC-controlled retransmissions. If the number of these retransmissions remain below a threshold (e.g. a "rlc-MaxNumRetx" threshold) no Radio Link Failure is reported. In this case, the cell change is without RACH (RACH-less) and is considered to be successful, despite the radio link condition towards target cell not being optimal. Currently this situation may not be detected.

In another case, a UE may be configured to be able to perform different mobility procedures, including a legacy handover and also an LTM based handover. The situation may occur where the network may not be able to distinguish if the SHR it receives from the UE is based on legacy or LTM based handover.

Fig.1 illustrates a simplified diagram according to an example embodiment. In 102 the UE receives a cell switch command from the network (e.g.the gNB). The cell switch command may be considered to be a handover command. In the specific case of this example, the cell switch command does not contain a valid timing advance (TA) value or any other information relating to TA. The UE 100 then triggers the creation of a SHR. In this case, the SHR trigger criterion is a missing valid TA which should ensure that the UE is in-sync with target cell and UE has to fall back to RACH-based procedure 106. Once the RACH based procedure has been finalized the UE 100 responds to the network with a RRC reconfiguration message.

Fig.2 illustrates an example of a signaling diagram for a cell switch procedure when SHR is generated including the LTM-with-RACH-cause which serves two dislocated transmission points (TRP) where source PCell is linked TRP1 and target candidate PCell to TRP2. This procedure is used for the case when the UE moves within the same gNB-DU during NR operation from source PCell at TRP1 to target PCell at TRP2 by means of LTM. In this scenario, a valid TA value of the selected target candidate PCell is not provided within the Cell Switch Command message. This s leads to RACH fallback when the UE initiates the access to the target PCell via RACH procedure.

In step 202, the UE is connected with an RRC connection to a Primary cell (PCell). In step 206 the gNB sends an RRC reconfiguration message to the UE. This message includes information which relates to at least one condition for triggering a handover report which may follow a LTM. Triggering the handover report may be different for actually generating the handover report and the triggering may be considered as the reason for generating the handover report. In an embodiment this information which relates to at least one condition for triggering the handover report may be contained in an information element, e.g. LTM-FailureReporting. This information element may contain a flag, called for example LTM-with-RACH-r19, for checking if the LTM was executed with (flag set to true, T) or without (flag set to false, F) a RACH. Further, the information element may contain a threshold which is compared with a number of RLC retransmissions that a first uplink transmission needs until it is successfully completed. This threshold may be defined as an rlc-Retx_percentage_thres and it may be measured as a percentage P. In step 208 the UE sends a RRC reconfiguration complete message to the gNB.

In step 210, the gNB provides LTM config and triggers early TA acquisition. As part of this process, the following actions are executed: The UE sends a MeasurementReport message (L3 measurement result) to the gNB-DU containing measurements of neighbouring cells. The gNB-DU sends an UL RRC MESSAGE TRANSFER message conveying the received MeasurementReport message to the gNB-CU. The gNB-CU determines to initiate LTM configuration. The gNB-CU sends a UE CONTEXT MODIFICATION REQUEST message to the gNB-DU containing one target candidate cell ID, the LTM configuration ID of the candidate cell, and LTM configuration ID mapping list, the CSI resource configuration. The gNB-CU requests PRACH resources from the gNB-DU. The gNB-CU may request the gNB-DU to provide the lower layer configuration for the purpose of generating the reference configuration. If the gNB-DU accepts the request of LTM configuration, it responds with a UE CONTEXT MODIFICATION RESPONSE message including the generated lower layer RRC configurations (e.g., TCI state configuration, RACH configuration, and the CSI report configuration) for the accepted target candidate cell. The gNB-CU sends a UE CONTEXT MODIFICATION REQUEST message to the source gNB-DU and to the target gNB-DUincluding the collected TCI state configurations and the gNB-CU may send the CSI report configuration for all the accepted target candidate cells. The source gNB-DU responds with a UE CONTEXT MODIFICATION RESPONSE message which includes an updated lower layer configuration, e.g., containing the CSI report configuration of the source cell.

The gNB-CU sends a DL RRC MESSAGE TRANSFER message to the gNB-DU, which includes the generated RRCReconfiguration message with the LTM configuration. The gNB-DU forwards the received RRCReconfiguration message to the UE. The UE responds to the gNB-DU with an RRCReconfigurationComplete message. The gNB-DU forwards the RRCReconfigurationComplete message to the gNB-CU via an UL RRC MESSAGE TRANSFER message. The UE sends the L1 measurement result to the gNB-DU and the gNB-DU decides to execute LTM.

In step 212, the gNB sends a cell switch command to the UE without a valid TA value. In an embodiment, a lack of valid TA value may be the same as a non existing invalid TA value, i.e. there is no TA value included in the cell switch command. The cell switch command may be in a MAC layer Control Element. Without this TA value, or with a TA value which is invalid, the UE cannot adjust its UL transmission timing. Following this, in step 214, a random access procedure is executed and the UE generates a SHR when RRC connection was successfully completed with the target PCell via RA procedure in step 214 and UE sends a RRCSetupCompletemessage to the gNB in step 216. In step 218, the gNB-DU forwards the RRCSetupComplete message to the gNB-CU via an UL RRC MESSAGE TRANSFER message. In step 220, the UE generates a SHR. The SHR may be considered or also called a handover report. The SHR may contain an indication of the type of the handover that was executed, which may be for example a lower layer triggered handover. This type of lower layer triggered handover may be referred to as LTM type of handover. A low layer triggered handover may be an intra-gNB-DU handover, inter gNB-DU handover within the same gNB-CU-UP or inter gNB-DU handover with change of gNB-CU-UP.

Further, the SHR may comprise or indicate a condition based on which the SHR was triggered. This condition may be the missing valid TA information mentioned above. The condition may additionally or alternatively be a requirement for the UE to perform a RACH towards the gNB. In this case, the flag LTM-with-RACH-r19 is set to true, T. Finally in step 222, the gNB may retrieve the SHR from the UE. The retrieval may be executed as a result of an indication from the UE to the gNB that the SHR has been generated and/or made available.

Fig. 3 illustrates an example flowchart according to an embodiment. In step 300 the UE receives from a network node information which relates to one or more conditions for triggering a handover report. The UE may be referred to as an apparatus and the network node may be a gNB. The information may be comprised in a low layer triggered mobility information element. This may be named as LTM-FailureReporting-r19.

In step 310, the UE generates the handover report after it has received a cell switch command without indicating a valid TA value. The handover report may be a SHR and it may contain an indication of the handover type and an indication on the zo condition or conditions which triggered the handover report. These conditions may include the missing valid TA information or a requirement for the UE to perform a random access procedure towards the network node. The random access procedure may also be towards a different network node than the one from which the information relating to the conditions for triggering the handover report was received.

In step 320, the UE may provide an indication to the network node that the handover report is available. This indication does not mean that the UE actively sends the handover report to the network node. Instead the network node may retrieve the 30 handover report for example according to procedures defined in 3GPP TS 37.340.

An example of the existence of the information element LTM-FailureReporting-r19 mentioned above may be in the following 3GPP TS 38.331 specification containing a number of information elements.

Moreover, the content of SHR may additionally include the following elements: - Source and target cell Ds of the handover, - Location information, - Latest radio link measurements of all measurement IDs available at the time handover is executed for all handover types, -A cause that was the trigger for generating the SHR, i.e., t310-cause, t312-cause, t304-cause, LTM-with-RACH-cause, LTM-target-access-cause Latest radio measurement results of the candidate target cells in case of conditional HO, - Time elapsed between conditional HO execution towards target cell and corresponding latest configuration received for the selected target cell, - C-RNTI of target cell, - RA-InformationCommon when T304 is above a threshold.

- In case of DAPS HO, an indication that the source cell failed previously to being release and the introduced interruption time are also logged -HO_type ENUMERATED {Itm, spare 1, spare 2} [-t=.3 FIG. 4 illustrates a simplified block diagram of a device 400 that is suitable for implementing some example embodiments of the present disclosure. The device 400 may be provided to implement the communication device, for example, the user equipment 100, or the network node 200 or 201 as shown in FIG. 2. As shown, the device 400 includes one or more processors 420, one or more memories 430 coupled to the processor 420, and one or more communication modules 410 coupled to the processor 420.

The communication module 410 is for bidirectional communications. The communication module 410 has at least one antenna to facilitate communication. The communication interface may represent any interface that is necessary for communication with other network elements.

The processor 420 may be of any type suitable to the local technical network and may include one or more of the following: general purpose computers, special purpose computers, microprocessors, digital signal processors (DSPs) and processors based on multicore processor architecture, as non-limiting examples. The device 400 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.

The memory 430 may include one or more non-volatile memories and one or more volatile memories. Examples of the non-volatile memories include, but are not limited to, a Read Only Memory (ROM) 434, an electrically programmable read only memory (EPROM), a flash memory, a hard disk, a compact disc (CD), a digital video disk (DVD), and other magnetic storage and/or optical storage. Examples of the volatile memories include, but are not limited to, a random access memory (RAM) 432 and other volatile memories that will not last in the power-down duration.

A computer program which may be stored in the ROM 434 may include computer executable instructions that are executed by the associated processor 420. The processor 420 may perform any suitable actions and processing by loading the program into the RAM 432.

The embodiments of the present disclosure may be implemented by means of the program so that the device 400 may perform any process of the disclosure as discussed with reference to FIG. 2. The embodiments of the present disclosure may also be implemented by hardware or by a combination of software and hardware.

In some example embodiments, the program may be tangibly contained in a computer readable medium which may be included in the device 400 (such as in the memory 430) or other storage devices that are accessible by the device 400. The device 400 may load the program from the computer readable medium to the RAM 432 for execution. The computer readable medium may include any types of tangible non-volatile storage, such as ROM, EPROM, a flash memory, a hard disk, CD, DVD, and the like.

io In some example embodiments, there is provided an apparatus comprising at least one processor and at least one memory storing instructions that when executed by the at least one processor cause the apparatus at least to receive from a network node information relating to at least one condition for triggering a handover report, generate the handover report, said handover report comprising a type of a handover and the at least one condition for triggering the handover report, wherein the at least one condition is a missing valid timing advance information or a requirement for the apparatus to perform a random access procedure towards the network node, and provide an indication on the handover report availability to the network node.

LIST OF ABBREVIATIONS

CE Control Element CHO Conditional Handover CP Control Plane CU Central Unit DU Distributed Unit gNB Next generation NodeB HO Handover KPI Key Performance Indicator LTM L1/L2 Triggered Mobility MAC Medium Access Control MRO Mobility Robustness Optimization PHY Physical Layer is RA Random Access RACH Random Access Channel RAN Radio Access Network RLC Radio Link Control RLF Radio Link Failure zo RRC Radio Resource Control SHR Successful Handover Report TA Timing Advance ICI Transmission Configuration Indicator

TS Technical Specification

UE User Equipment UP User Plane

Claims (10)

1. CLAIMS1. An apparatus comprising means for receiving from a network node information relating to at least one condition for triggering a handover report, generating the handover report, said handover report comprising a type of a handover and the at least one condition for triggering the handover report, wherein the at least one condition is a missing valid timing advance information or a requirement for the apparatus to perform a random access procedure towards the to network node, and providing an indication on the handover report availability to the network node.
2. 2. The apparatus according to claim 1, wherein the information is comprised in a low layer triggered mobility information element.
3. 3. The apparatus according to claim 1 or 2, wherein the information is comprised in a configuration message.
4. 4. The apparatus according to claim 3, wherein the configuration message is a radio resource control configuration message.
5. 5. The apparatus according to any preceding claim, wherein the at least one condition is a missing valid timing advance information or a requirement for the apparatus to perform a random access procedure towards the network node.
6. 6. The apparatus according to claim 1, wherein the type of the handover is a lower layer triggered handover.
7. 7. The apparatus according to claim 1, wherein the handover report is a successful handover report.
8. 8. A method comprising: receiving, from a network node, information relating to at least one condition for triggering a handover report; generating the handover report, said handover report comprising a type of the handover and the at least one condition for triggering the handover report wherein the at least one condition is a missing valid timing advance information or a requirement for the apparatus to perform a random access procedure towards the network node; and providing an indication on the handover report availability to the network node.
9. 9. The method according to claim 8, wherein the at least one condition is a missing valid timing advance information or a requirement for the apparatus to perform a random access procedure towards the network node.
10. 10. A computer program comprising instructions for causing an apparatus to perform the method according to claims 8-9.