A method of managing conditional handover in a mobile network, a method of inter-cell mobility in a mobile network, a user equipment, a radio access network entity and computer software configured to perform the methods

Abstract

A method of managing conditional handover in a mobile network is provided. The method comprises receiving, at a user equipment (UE) served by a first node, conditional handover configuration data for one or more nodes, the one or more nodes comprising a second node. The method further comprises starting a timer in response to an event during handover from the first node to the second node. The method further comprises, while the timer is running, preventing handover to the first node from being initiated from the second node to the first node.

Description

[0001] A method of managing conditional handover in a mobile network, a method of intercell mobility in a mobile network, a user equipment, a Radio Access Network entity and computer software configured to perform the methods

[0002] Field of the invention

[0003] The present invention relates to methods for avoiding repeated unnecessary handover in conditional handover and L1 / L2 based mobility in wireless communication systems.

[0004] Glossary

[0005] The following acronyms are used in this document:

[0006] UE - User Equipment

[0007] 3GPP - 3rd Generation Partnership Project

[0008] LTE - Long Term Evolution (4G)

[0009] NR - New Radio (5G)

[0010] CHO - Conditional Handover

[0011] CPAC - Conditional PSCell Addition / Change

[0012] SCG - Secondary Cell Group

[0013] PCell - Primary Cell

[0014] SCell - Secondary Cell

[0015] PSCell - Primary and Secondary Cell

[0016] RRC - Radio Resource Control gNB - gNodeB

[0017] MN - Master Node

[0018] MeNB - Master eNodeB

[0019] MgNB - Master gNodeB

[0020] SN - Secondary Node

[0021] SgNB - Secondary gNodeB

[0022] MAC - Medium Access Control

[0023] MAC CE - MAC Control Element

[0024] RLC - Radio Link Control

[0025] RLC AM - RLC Acknowledge Mode

[0026] LTM - L1 / L2 Triggered Mobility

[0027] PDCP - Packet Data Convergence Protocol DU - Distributed Unit

[0028] CU - Central Unit

[0029] CA - Carrier Aggregation

[0030] S1AP - S1 Application Protocol

[0031] X2AP - X2 Application Protocol

[0032] UDP - User Datagram Protocol

[0033] GPRS - General Packet Radio Service

[0034] GTP - GPRS Tunneling Protocol

[0035] SDAP - Service Data Adaption Protocol

[0036] PHY - Physical Layer

[0037] CBRA - Contention-Based Random Access

[0038] CFRA - Contention-Free random Access

[0039] TA - Timing Advance

[0040] TTL - Time To Live

[0041] RACH - Random Access procedure

[0042] DL / UL - Downlink / Uplink

[0043] RF - Radio Frequency

[0044] Background

[0045] In a mobile telecommunications network, standards, such as the 3GPP 5G New Radio (NR) standard, provide enhancements for multiple connections between a user equipment, UE, and one or more base stations in the network, particularly for high frequency transmissions (e.g., frequency range FR2, which encompasses approximately 6GHz and above, preferably 24.25 GHz to 71 .0 GHz). Some further enhancements in 5G NR include improving inter-cell mobility, which is a procedure that ensures that a UE is able to handover from one wireless cell to another wireless cell, whenever the UE detects an adjacent wireless cell with higher signal quality.

[0046] Support of UE mobility is an important procedure in mobile / wireless communication networks. As the UE moves physically in a geographic area served by the mobile communication system, the UE may cross cell boundaries. As a result, the serving base station should be updated in order for the UE to be served by the best cell, with the best radio quality link. There have been many enhancements to mobility procedures in ongoing 3GPP releases, as the standards have developed over time. One such enhancement provides methods for cell mobility to be triggered at lower levels of the protocol stack (L1 / L2 based mobility). Another improvement allows the use of preconfigured / pre-prepared cells for conditional handover. Both are currently discussed in 3GPP as a way of further enhancing the handover performance.

[0047] However, these proposed enhancements also give rise to new problems and challenges for UE mobility. Both these handover enhancements enable the UE to make decisions relating handover when the triggering criteria are met. Even though the network is responsible for the configuration and handover execution, the handover initiation is performed by the UE. In some scenarios, the UE may be in a situation where it sees more than one good quality cell fulfilling the handover trigger criteria. Therefore, due to small fluctuations in radio conditions over short periods of time, pre-configured conditional handover can result in repeated handover back-and-forth between two or more cells (an issue referred to as “ping- pong”). Frequent unnecessary handover reduces overall system performance and increases handover interruption to the UE.

[0048] Summary

[0049] A method of managing conditional handover in a mobile network is provided. The method comprises receiving, at a user equipment (UE) served by a first node, conditional handover configuration data for one or more nodes, the one or more nodes comprising a second node. The method further comprises starting a timer in response to an event during handover from the first node to the second node. The method further comprises, while the timer is running, preventing handover to the first node from being initiated from the second node to the first node.

[0050] The one or more nodes may further comprise the first node. In other words, the UE may receive conditional handover configuration data for both the first node and the second node. Optionally, the UE may also receive conditional handover configuration data for other potential target nodes.

[0051] The first node may be a source node prior to the handover (the handover in response to which the timer is started). Prior to handover, the second node may be a target node. Once handover is completed, the second node becomes the source node and the first node becomes a potential target node. However, handover to the first node is suppressed while the timer is running, even if one of the criteria are met.

[0052] While the timer is running, handover back to the first node is prevented. In this way, the proposed method is able to suppress the problem of “ping-pong”, where a UE performs handover back-and-forth in quick successions between two nodes or amongst a group of nodes. Handover back to a previous source node is permitted, only once a predetermined time since that node was last the serving node has elapsed. The length of time that is acceptable between occasions that the node can be the source node may be configured according to network requirements. By setting a longer period, the overall number of handovers may be reduced and network resources may be conserved. By setting a shorter period, the UE is more likely to handover back to a previous source node, if radio conditions for that node improve. This can improve UE performance by always seeking out the best connection. The period should not be set too short because this can result in ping- pong, which negatively affects both radio resource usage (due to increased numbers of handovers) and UE performance (due to the short-term impact to performance caused by the handover process).

[0053] The timer may be implemented in various ways. The timer may be a countdown timer of fixed duration. After the timer has been started, it may be in a running state while it counts down to zero. Once it reaches zero, it may be in an expired state. Alternatively, the timer may be a count up timer starting from zero and counting up to a pre-set duration, at which point the timer expires. As another alternative, a timer may be implemented by storing a time at which the timer started. When the timer is checked, the time elapsed may be determined by comparing a current time to the time at which the timer started. If the elapsed time is less than a threshold then handover may be prevented (analogous to a situation where the timer is still running). If the elapsed time is greater than a threshold then handover may be permitted (analogous to a situation where the timer is expired).

[0054] The conditional handover configuration data for the one or more nodes may comprise one or more handover criteria. The handover criteria may be specific to each node or may apply to each of the nodes. The method may further comprise measuring signal data for each of the one or more nodes and evaluating the one or more handover criteria in respect of the one or more nodes, based on the measured signal data.

[0055] The measured signal data may comprise one or more of: Received Signal Strength Indicator (RSSI); Reference Signal Received Power (RSRP); Reference Signal Received Quality (RSRQ); Received Signal Code Power (RSCP); Received Signal Level (RxLev); Received Signal Quality (RxQual);Signal to Noise Ratio (SNR); Signal-to-interference-plus- noise Ratio (SINR); Signal-to-noise-plus-interference Ratio (SNIR); SNR per bit (Eb / No); Downlink carrier-to-interference ratio (Ec / lo); Downlink carrier-to-noise ratio (Ec / No); and the like.

[0056] The criteria may comprise one or more of: the measured signal data for the respective node is better than a threshold; the measured signal data for the respective node is better than the source node; the measured signal data for the respective node is better than the source node by at least an offset; and the measured signal data for the respective node is better than a threshold and the measured signal data for the source node is worse than another threshold.

[0057] The method may further comprise periodically measuring signal data and evaluating the one or more handover criteria in respect of the one or more nodes.

[0058] Preventing handover to the first node from being initiated may comprise not measuring signal data for the first node while the timer is running. Alternatively, preventing handover to the first node from being initiated may comprise measuring signal data for the first node but not evaluating the handover criteria in respect of the first node while the timer is running. Alternatively, preventing handover to the first node from being initiated may comprise measuring signal data for the first node and evaluating the handover criteria in respect of the first node but suppressing initiation of handover to the first node while the timer is running, even if one of the handover criteria is met in respect of the first node.

[0059] The method may further comprise determining that one of the handover criteria is met in respect of the second node and executing handover to the second node in response. Executing handover to the second node may comprise: sending a message to initiate handover; initiating a random access procedure with the second node; and completing the random access procedure successfully.

[0060] The message to initiate handover may be sent to the first node. Alternatively, in the case of Dual Connectivity, the message to initiate handover may be sent to a master node.

[0061] The message to initiate handover may be a message to initiate reconfiguration of a connection from the first node to the second node (such as a RRC Reconfiguration Complete message).

[0062] The timer may be started in response to one of the following events during handover: sending the message to initiate handover (e.g., a RRC Reconfiguration Complete message to the first node or master node); initiation of the random access procedure; and successful completion of the random access procedure.

[0063] The method may further comprise, while the timer is running, determining that one of the handover criteria is met in respect of a third node of the one or more nodes, and initiating handover to the third node. Alternatively, the method may further comprise determining that the timer has expired, determining that one of the handover criteria is met in respect of the first node, and initiating handover to the first node.

[0064] The UE may be connected to a master node and a secondary node in a Dual Connectivity mode of operation, and wherein the one or more nodes are secondary nodes.

[0065] The conditional handover configuration data for the first node and the second node may be based on reference cell configuration data.

[0066] Conditional configurations (CHO / CPAC) can be stored and reused to save radio resources and avoid delays in re-configuration between potential cells. Conditional cell configuration may be provided based on a reference cell configuration, rather than based on the serving cell configuration. A method of inter-cell mobility in a mobile network is also provided (a method performed by a UE). The method comprises receiving configuration data for one or more target cells. The method further comprises receiving instructions to hand over from a source cell to a first target cell of the one or more target cells. The method further comprises starting a timer in response to an event during handover from the source cell to the first target cell. The method further comprises initiating handover to the first target cell, so that the source cell becomes a previous source cell and also becomes a target cell of the one or more target cells, and wherein the first target cell becomes the source cell. The method further comprises, while the timer is running, sending signal measurement data to a Radio Access Network, RAN, entity, the signal measurement data comprising signal measurement data for each of the one or more target cells, except the previous source cell (signal measurement data for the previous source cell being excluded).

[0067] The source cell may be referred to as a first cell and the first target cell may be referred to as a second cell. The method may therefore be written as: receiving configuration data for one or more target cells; receiving instructions to handover from a first cell to a second cell, wherein the second cell is one of the one or more target cells; starting a timer in response to an event during handover from the first cell to the second cell; initiating handover to the second cell, so that the first cell becomes a target cell of the one or more target cells; while the timer is running, sending signal measurement data to a Radio Access Network, RAN, entity, the signal measurement data comprising signal measurement data for each of the one or more target cells, except the first cell (signal measurement data for the first cell being excluded).

[0068] The method may further comprise receiving configuration data for the previous source cell. The configuration data for the previous source cell may be received from a first RAN entity. The RAN entity to which signal measurement data is sent may be a second RAN entity. The second RAN entity may be the same as the first RAN entity or may be different. In other words, the LTM supports intra-gNB as well as inter-gNB mobility. The method may further comprise sending signal measurement data to the first RAN entity (before handover), the signal measurement data comprising signal measurement data for each of the one or more target cells.

[0069] The method may further comprise determining that the timer has expired and sending signal measurement data to the RAN entity, the signal measurement data comprising signal measurement data for each of the one or more target cells, including the previous source cell.

[0070] The handover instructions may be layer 1 or layer 2 signalling instructions.

[0071] The instructions may be Medium Access Control, MAC, Control Element, MAC CE, instructions.

[0072] The signal measurement data may be layer 1 (PHY) signal measurement data. The signal measurement data may be Channel State Information Reference Signal (CSI-RS).

[0073] After initiating handover to the first target cell, the method may further comprise executing the handover, so that the target cell becomes the source cell.

[0074] A method of inter-cell mobility in a mobile network is also provided (a method performed by a RAN entity). The method comprises sending, to a user equipment, UE, configuration data for one or more target cells. The method further comprises sending, to the UE, instructions to handover from a source cell to the first target cell. The method further comprises starting a timer in response to an event during handover from the source cell to the target cell. The method further comprises receiving confirmation that handover to the first target cell has been executed, so that the source cell becomes a previous source cell and also becomes a target cell of the one or more target cells, and wherein the first target cell becomes the source cell. The method further comprises receiving signal measurement data from the UE, the signal measurement data comprising signal measurement data for each of the one or more target cells. The method further comprises, while the timer is running, not sending instructions to the UE to handover to the previous source cell.

[0075] The source cell may be referred to as a first cell and the first target cell may be referred to as a second cell. The method may therefore be written as: sending, to a user equipment, UE, configuration data for one or more target cells; sending, to the UE, instructions to handover from a first cell to a second cell, wherein the second cell is selected from the one or more target cells; starting a timer in response to an event during handover from the first cell to the second cell; receiving confirmation that handover to the second cell has been executed, so that the first cell becomes a target cell of the one or more target cells; receiving signal measurement data from the UE, the signal measurement data comprising signal measurement data for each of the one or more target cells; while the timer is running, not sending instructions to the UE to handover to the first cell.

[0076] The method may further comprise (prior to the handover) receiving signal measurement data from the UE, the signal measurement data comprising signal measurement data for each of the one or more target cells. The method may further comprise determining that signal measurement data for a first target cell of the one or more target cells meets a handover condition. The instructions to handover from the source cell to the first target cell may be sent to the UE in response to determining that the signal measurement data for the first target cell meets the handover condition.

[0077] The method may further comprise determining that signal measurement data for the previous source cell meets a handover condition, and not sending instructions to the UE to handover to the previous source cell while the timer is still running.

[0078] The method may further comprise determining that the timer has expired, determining that signal measurement data for the previous source cell meets a handover condition, and sending instructions to the UE to handover to the previous source cell.

[0079] A User Equipment, UE, configured to perform a method described above is also provided.

[0080] A Radio Access Network, RAN, entity, configured to perform a method described above is also provided.

[0081] Computer software comprising instructions that, when executed on a processor, cause the processor to perform a method described above is also provided. Brief description of the drawings

[0082] Figure 1 illustrates a mobile network according to a specific example.

[0083] Figure 2 illustrates an example of a typical Dual connectivity scenario.

[0084] Figure 3 illustrates a communication flow diagram showing the operation of the UE in an example scenario.

[0085] Figure 4 illustrates an example CU-DU architecture.

[0086] Figure 5 illustrates the overall procedure for LTM.

[0087] Figure 6 illustrates a signalling procedure for LTM, according to a specific example.

[0088] Detailed Description

[0089] As illustrated in Figure 1 , a mobile network 100 according to a specific example comprises a core network 110, a radio access network (RAN) 120, and a user equipment (UE) 130.

[0090] The RAN 120 may operate in accordance with defined standards to communicate with the UE 130. For example, the RAN 120 may operate according to 3rd Generation Partnership Project (3GPP) New Radio (NR) specifications (also known as 5G). Alternatively, the RAN 120 may operate a combination of 5G NR and Evolved Universal Terrestrial Radio Access Network (eUTRAN) standards (also known as LTE or 4G). Hybrid RAN may also be referred to as next-generation RAN, or NG-RAN.

[0091] The RAN 120 includes a plurality of base stations 122, 124. A base station (also referred to as a node) is a network element in a RAN configured to transmit and receive data signals in one or more cells to and from one or more UEs.

[0092] Handover methods in a 5G network are described in 3GPP TS 23.502, which is herein incorporated by reference. As described in section 4.9.1 handover may be triggered due to radio conditions, load balancing or specific service requirements. Enhancements to the handover procedure are described in:

[0093] • 3GPP TS 38.300, which describes conditional handover (CHO) methods in section 9.2.3.4; and

[0094] • 3GPP TS 37.340, which describes conditional PSCell addition / change (CPAC) methods in section 10.

[0095] Both are herein incorporated by reference. In CHO and CPAC, the UE is provided with a list of potential target cells for the mobility support. The UE is also provided with a condition where the UE performs the mobility procedure when the condition is met. After the mobility procedure is executed, the list of configurations for CHO / CPAC is released by the UE. The new cell where the UE has moved to provide a list of potential target cell configurations to the UE based on the current cell configuration.

[0096] The present invention relates to conditional handover and provides enhancements to existing conditional handover methods. In an existing conditional handover method, a UE is provided with target cell configuration for one or more potential target cells for the handover procedure. The configuration data also comprises one or more trigger criteria for each target cell. When a trigger criterion is met (e.g., the target cell quality is better than serving cell quality by a threshold amount), the UE accesses the target cell by performing a Random-Access procedure (RACH), in order to time synchronise to the target cell. If the Random-Access procedure is not successful, the UE falls back to the serving cell. If the random access to the target cell is successful, the UE attaches to the target cell and detaches from the serving cell.

[0097] For some scenarios, such as the system operating in very high frequency (FR2), the UE could cross many cells in a short period of time. A small cell size may be deployed in very high frequency to cater for fast attenuation of signal strength in high frequency. In such a scenario, release and reconfiguration of CHO / CPAC configurations is not optimal and could even miss a chance of serving the UE in a good quality cell. The release and reconfiguration require some time. While the reconfiguration is completed, the UE may even have passed a good quality small cell without connecting to it. Moreover, release and reconfiguration of CHO / CPAC requires signalling over the radio link resulting in radio resource usage. A further enhancement to the CHO procedure is proposed to improve behaviour and performance. In an enhanced method, conditional configurations (CHO / CPAC) can be stored and reused, the radio resources can be saved and reduced the possible delay in re-configuration of a list of potential cells. Conditional cell configuration may be provided based on a reference cell configuration to address the problem described above, due to the need for release and reconfiguration based on the serving cell configuration.

[0098] However, this enhancement introduces a potential issue. Pre-configured / pre-prepared cells for conditional handover enable the UE to make fast decisions on handover, when the triggering criteria are met. In some scenarios, the UE may see more than one good quality cell fulfilling the handover trigger criteria. Therefore, due to small fluctuations in radio conditions over short periods of time, pre-configured conditional handover can result in repeated handover back-and-forth between two or more cells (an issue referred to as “ping- pong”). Frequent unnecessary handover reduces overall system performance and increases handover interruption to the UE.

[0099] Figure 2 illustrates an example of a typical Dual connectivity scenario, in which a PSCell (SgNB) handover is performed. In this example, a conditional SCG configuration is computed based on a reference SCG configuration. The reference SCG configuration is provided to the UE at the RRC connection to the source PCell. In scenarios where Conditional PScell Change applies, it is assumed that the PCell does not change. Thus, the security information for the candidate PSCells are computed based on the security key of the PCell.

[0100] As illustrated in Figure 2, UE-3 is operating in Dual Connectivity mode. While UE-3 is moving towards SgNB2, UE-3 changes the SCG from SgNB1 to SgNB2 without changing the MeNB. The example above relates to the MeNB-SgNB scenario. However, the same principle applies for the MgNB-SgNB scenario in New Radio (NR).

[0101] While UE-3 is at the cell boarder of SgNB1 and SgNB2, UE-3 may observe that SgNB2 is better quality. While it moves to SgNB2, UE-3 may observe that SgNB1 is better quality. This can lead to “ping-pong” between SgNB1 and SgNB2. To avoid this issue, a new timer is used in this invention. When UE-3 executes conditional handover towards SgNB2, the UE-3 starts a new timer Tx. While the timer Tx is running, UE-3 refrains from handing back to SgNB1 , even though the channel quality of SgNB1 fulfils the conditional handover criteria. Figure 3 illustrates a communication flow diagram showing the operation of the UE in the above example scenario. MN is the master node MeNB, S-SN is the source secondary node SgNB1 (“source” because this is the serving node prior to the first handover), T-SN is the target secondary node SgNB2 (“target” because it is the serving node following the first handover).

[0102] Steps 301-307 are the same as those performed in existing methods (e.g., those described in 3GPP TS 37.340). The master node provides conditional handover configurations and criteria to the UE:

[0103] 301 : SgNB Change Required

[0104] 302A: SgNB Addition Request

[0105] 302B: SgNB Addition Request

[0106] 303A: SgNB Addition Request Acknowledge

[0107] 303B: SgNB Addition Request Acknowledge

[0108] 304: RRC Reconfiguration (may contain:

[0109] S-SN RRC Reconfiguration;

[0110] MN RRC Reconfiguration; and / or

[0111] T-SN RRC Reconfiguration)

[0112] 305: RRC Reconfiguration Complete (may contain S-SN RRC Reconfiguration Complete) 306: SgNB Change Confirm

[0113] The UE monitors the radio conditions, when a conditional configuration execution criterion is met, the UE initiates handover to the corresponding SgNB at step 308:

[0114] 308: RRC Reconfiguration Complete (may contain S-SN RRC Reconfiguration Complete)

[0115] At step 308A, upon transmission of RRC Reconfiguration Complete message in step 308, the UE starts timer Tx. While the timer Tx is running, the UE does not consider S-SN as a candidate for the conditional handover, even though it may satisfy the criteria for conditional handover trigger.

[0116] 308A: SgNB Reconfiguration Complete

[0117] Steps 309 to 311 are in line with existing methods.

[0118] 309: Random Access Procedure

[0119] 310A: SN Status Transfer

[0120] 310B: SN Status Transfer After performing data forwarding, the UE context is released at the source SN at step 311 .

[0121] The UE stores the conditional configuration.

[0122] 311 : UE Context Release

[0123] The UE monitors for conditional configuration execution criteria. While the timer Tx is running, the UE does not consider S-SN as a candidate for the conditional handover, even though it may satisfy the criteria for conditional handover trigger. When the condition configuration criteria are met (a candidate cell quality becomes better than the configured threshold), the UE executes the conditional PSCell change procedure according to the stored conditional configurations. At step 312, the UE informs the network that the successful conditional SCG execution by transmitting SgNB Reconfiguration Complete message, including the SN RRC Reconfiguration Complete message for the target SN. 312: RRC Reconfiguration Complete (may contain S-SN RRC Reconfiguration Complete) 312A: SgNB Reconfiguration Complete

[0124] Upon transmission of RRC reconfiguration complete message in step 312, the UE starts timer Tx. While the timer Tx is running, the UE does not consider T-SN as a candidate for the conditional handover, even though it may satisfy the criteria for conditional handover trigger.

[0125] At step 313, the UE synchronizes to the target SN indicated in the RRC Connection Reconfiguration message in step 312.

[0126] 313: Random Access Procedure

[0127] At step 314, for SN terminated bearers using RLC AM, the source SN sends the SN Status Transfer, which the MN sends then to the target SN, if needed.

[0128] 314A: SN Status Transfer 314B: SN Status Transfer

[0129] At step 315, upon reception of the UE Context Release message, the source SN releases radio and C-plane related resources associated to the UE context. Any ongoing data forwarding may continue. If applicable, data forwarding from the source SN takes place. It may be initiated as early as the source SN receives the early data forwarding message from the MN. 315: UE Context Release

[0130] In the specific example described above, the timer is started in response to transmission of RRC reconfiguration complete message by the UE in step 308 (and step 312). In another example, the start of timer Tx could be performed at the initiation of Random-access procedure towards the selected SN (step 309 and step 313). In another example, the start of timer Tx could be performed at successful completion of the random-access procedure.

[0131] The above examples generally relate to handover triggered via protocols classed as layer 3 or higher. In 5G New Radio (NR) systems, inter-cell mobility may provide an alternative method that is triggered via layer 1 (i.e., the L1 or PHY layer) or layer 2 (i.e., the L2 or MAC layer) protocols and measurements. L1 / L2 Triggered Mobility (LTM) is a procedure in which a gNB receives L1 measurement reports from UEs and, on the basis of the measurements, the gNB changes UEs’ serving cell(s) through MAC CE.

[0132] The gNB prepares one or multiple candidate cells and provides the candidate cell configurations to the UE through a RRC message. Then, LTM cell switch is triggered by selecting one of the candidate configurations as a target configuration for LTM by the gNB (e.g., in response to the candidate cell meeting one of the handover conditions). The candidate cell configurations can be added, modified and released by the network, via RRC signalling.

[0133] Candidate cell configuration can be provided as delta configurations on top of a reference configuration. The reference configuration is managed separately, and a UE stores the reference configuration as a separate configuration.

[0134] In contrast to L3 handover methods, L1 / L2 mobility may enable communications on higher layer protocols to continue, without being disrupted by the cell switch. The user plane is continued whenever possible (e.g. intra-DU), without reset. This may avoid data loss and the additional delay of data recovery. Moreover, update of security configurations is not required in LTM.

[0135] The UE does not release other candidate cell configurations after LTM is triggered.

[0136] Therefore, subsequent LTM between candidates can be performed without requiring RRC reconfiguration. LTM supports both intra-gNB-DU and intra-gNB-CU, inter-gNB-DU mobility. LTM also supports inter-frequency mobility, including mobility to inter-frequency cell that is not a current serving cell. The following scenarios are supported:

[0137] PCell change in non-CA scenario;

[0138] PCell change without SCell change in CA scenario;

[0139] PCell change with SCell change(s) in CA scenario, including the following cases: a) The target PCell / target SCell(s) is not a current serving cell (CA-to-CA scenario with PCell change); b) The target PCell is a current SCell; and c) The target SCell is the current PCell; and

[0140] PSCell change in Dual Connectivity scenario (at least PSCell change, without MN involvement, i.e. intra-SN).

[0141] An illustration of a CU-DU architecture is shown in Figure 4. Cell switch trigger information is conveyed in a MAC CE, which contains at least a candidate configuration index. Cellspecific, radio bearer, and measurement configurations can be part of an LTM candidate cell configuration.

[0142] UE may perform CBRA or CFRA at cell switch. UE may also skip random access procedure if UE doesn’t need to acquire TA for the target cell during cell switch. RACH resources for CFRA are provided in RRC configuration.

[0143] The overall procedure for LTM is shown in Figure 5. Subsequent LTM is done by repeating the early synchronization, LTM execution, and LTM completion steps, without releasing other candidates after each LTM completion.

[0144] Figure 6 illustrates a signalling procedure for LTM according to a specific example.

[0145] The signalling procedure for LTM in one specific example is as follows:

[0146] At step 601 , the UE is placed in RRC_CONNECTED mode.

[0147] At step 602, the UE sends a Measurement Report message to the gNB. At step 603, the gNB decides to use LTM and initiates LTM candidate preparation.

[0148] At step 604, the gNB transmits an RRC Reconfiguration message to the UE including the configuration of one or multiple LTM candidate target cells.

[0149] At step 605, the UE stores the configuration of LTM candidate target cell(s) and transmits a RRC Reconfiguration Complete message to the gNB.

[0150] At step 606, the UE performs DL synchronization and optional TA acquisition with candidate target cell(s), before receiving the LTM cell switch command.

[0151] At step 607, the UE performs L1 measurements on the configured LTM candidate target cell(s), and transmits lower-layer measurement reports to the gNB.

[0152] At step 608, the gNB decides to execute LTM cell switch to a target cell.

[0153] At step 609, the gNB transmits a MAC CE triggering LTM cell switch by including the candidate configuration index of the target cell.

[0154] At step 610. the UE detaches from the source cell and switches to the configuration of the LTM candidate target cell.

[0155] At step 611 , the UE performs random access procedure towards the target cell, if TA is not available.

[0156] At step 612, the UE indicates successful completion of the LTM cell switch towards the target cell.

[0157] An uplink signal or message after the UE has switched to the target cell is used to indicate successful completion of the LTM cell switch.

[0158] During LTM, the UE may perform a partial or full MAC reset during cell switch. To determine whether to reset L2 (MAC), there are two possible options:

[0159] 1) The UE determines whether the switch is intra-DU or inter-DU and follows a corresponding different rule or configuration. The rule or configuration determines whether or not to perform a MAC reset. Determination of whether the switch is intra-DU or inter-DU could be based on configuration (e.g., of a DU ID, cell group ID, and the like).

[0160] 2) The UE receives command to reset or not reset by MAC CE.

[0161] In either case, the UE re-establishes RLC and performs data recovery with PDCP. For UE processing, the following (not exhaustive) is assumed to be performed after receiving the cell switch command:

[0162] MAC / RLC reset (when configured)

[0163] RF retuning (e.g., needed for inter-frequency), baseband retuning

[0164] Since the UE does not release other candidate cell configurations after LTM is triggered, subsequent LTM between candidates can be performed without requiring RRC reconfiguration. As the subsequent LTM is performed without L3 involvement (without RRC reconfiguration), this can result in ping-pong between candidate cells.

[0165] To avoid or mitigate ping-pong in cell switch, the present invention proposes to start a timer during cell switch from a source cell to a target cell and suppress handover back to previous source cell while the timer is running. This may be implemented on the network side or on the UE side.

[0166] On the network side, when making the decision to send cell switch MAC CE, the RAN entity may take into account the previously serving cells and supresses cell switch instructions as long as a timer is active.

[0167] On the UE side, measurement reports for the previous source cell may be withheld as long at the timer is active. This is beneficial as the measurements are not reported unnecessarily, thus saving radio resources.

[0168] Any of the methods described herein may be implemented as a computer program. The computer program may be configured to control a RAT entity (e.g., a network node) and / or UE to perform any method according to the disclosure. A RAT entity (e.g., a network node) of a cellular network and / or a UE may also be provided, configured to operate in accordance with certain methods disclosed herein. For example, the RAT entity may include a processor and at least one communication interface, particularly comprising one or both of a transmitter and receiver. A UE may also be provided, configured to operate in accordance with certain methods disclosed herein. The UE may likewise include a processor and at least one communication interface, particularly comprising one or both of a transmitter and receiver.

[0169] Although specific embodiments have now been described, the skilled person will understand that various modifications and variations are possible. For example, whilst the disclosure is described in relation to existing network architecture, it will be understood that changes to the architecture (and / or nomenclature) are possible, but the present disclosure may still be applicable in this case. Also, combinations of any specific features shown with reference to one embodiment or with reference to multiple embodiments are also provided, even if that combination has not been explicitly detailed herein.

[0170] A base station may be referred to as a base transceiver station (BTS), a radio base station, a radio transceiver, a transceiver function, a basic service set (BSS), an extended service set (ESS), an access point (AP), a Node B (NB), an eNode B (eNB), a gNode B (gNB), a transmission and reception point (TRP), or some other suitable terminology, depending on the protocol, standard, context or technology. In some examples, a base station may include two or more transceivers that may or may not be collocated. Each transceiver may communicate on the same or different carrier frequency within the same or different frequency band.

[0171] Where this application refers to a server or network entity, for instance, this may actually be a pair of servers, or network entities (primary and failover), for redundancy.

[0172] Whilst the above methods are described in relation to a 5G network, these methods, techniques, apparatuses, and systems may be applied to a variety of wireless multiple access systems. Examples of the multiple access systems include a code division multiple access (CDMA) system, a frequency division multiple access (FDMA) system, a time division multiple access (TDMA) system, an orthogonal frequency division multiple access (OFDMA) system, a single carrier frequency division multiple access (SC-FDMA) system, and a multicarrier frequency division multiple access (MC-FDMA) system. CDMA may be embodied through radio technology such as universal terrestrial radio access (UTRA) or CDMA2000. TDMA may be embodied through radio technology such as global system for mobile communications (GSM), general packet radio service (CPRS), or enhanced data rates for GSM evolution (EDGE). OFDMA may be embodied through radio technology such as institute of electrical and electronics engineers (IEEE) 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802.20, or evolved UTRA (E-UTRA). UTRA is a part of a universal mobile telecommunications system (UMTS). 3rd generation partnership project (3GPP) long term evolution (LTE) is a part of evolved UMTS (E-UMTS) using E-UTRA. 3GPP LTE employs OFDMA in DL and SC-FDMA in UL. LTE-advanced (LTE-A) is an evolved version of 3GPP LTE. For convenience of description, it is assumed that the present invention is applied to 3GPP LTE / LTE-A. However, the technical features of the present invention are not limited thereto. For example, although the following detailed description is given based on a mobile communication system corresponding to a 3GPP NR system, aspects of the present invention that are not specific to 3GPP NR are applicable to other mobile communication systems.

[0173] In the present invention, a user equipment (UE) may be a fixed or mobile device. Examples of the UE include various devices that transmit and receive user data and / or various kinds of control information to and from a base station (BS). The UE may be referred to as a terminal equipment (TE), a mobile station (MS), a mobile terminal (MT), a user terminal (UT), a subscriber station (SS), a wireless device, a personal digital assistant (PDA), a wireless modem, a handheld device, etc. In addition, in the present invention, a BS generally refers to a fixed station that performs communication with a UE and / or another BS, and exchanges various kinds of data and control information with the UE and another BS. The BS may be referred to as an advanced base station (ABS), a node-B (NB), an evolved node-B (eNB), a base transceiver system (BTS), an access point (AP), a processing server (PS), etc. In describing the present invention, a BS will be referred to as an gNB.

[0174] In the present invention, a node refers to a fixed point capable of transmitting / receiving a radio signal through communication with a UE. Various types of gNBs may be used as nodes irrespective of the terms thereof. For example, a BS, a node B (NB), an e-node B (eNB), a g-node B (gNB), a relay, a repeater, etc. may be a node.

[0175] In the present invention, a cell refers to a prescribed geographical area to which one or more nodes provide a communication service. Accordingly, in the present invention, communicating with a specific cell may mean communicating with an gNB or a node which provides a communication service to the specific cell. Furthermore, channel status / quality of a specific cell refers to channel status / quality of a channel or communication link formed between an gNB or node which provides a communication service to the specific cell and a UE. The UE may measure DL channel state received from a specific node using cellspecific reference signal(s) (CRS(s)) transmitted on a CRS resource and / or channel state information reference signal(s) (CSI-RS(s)) transmitted on a CSI-RS resource, allocated by antenna port(s) of the specific node to the specific node. Meanwhile, a 3GPP system uses the concept of a cell in order to manage radio resources and a cell associated with the radio resources is distinguished from a cell of a geographic region.

[0176] The examples may be carried out on any suitable data processing device, such as a personal computer, laptop, mobile telephone, server, virtual machine, and the like. The above description of the systems and methods has been simplified for purposes of discussion, and is intended to provide a specific example to illustrate the invention. Different types of systems and methods may be used, as will be appreciated by the skilled person. It will be appreciated that the boundaries between logic blocks are merely illustrative and that alternative embodiments may merge logic blocks or elements, or may impose an alternate decomposition of functionality upon various logic blocks or elements.

[0177] It will be appreciated that the above-mentioned functionality may be implemented as one or more corresponding modules as hardware and / or software. For example, the above- mentioned functionality may be implemented as one or more software components for execution by a processor of the system. Alternatively, the above-mentioned functionality may be implemented as hardware, such as on one or more field-programmable-gate-arrays (FPGAs), and / or one or more application-specific-integrated-circuits (ASICs), and / or one or more digital-signal-processors (DSPs), and / or other hardware arrangements. Method steps implemented in flowcharts contained herein, or as described above, may each be implemented by corresponding respective modules. Moreover, multiple method steps implemented in flowcharts contained herein, or as described above, may be implemented together by a single module.

[0178] Examples may be implemented by computer software or a “computer program”. A storage medium and a transmission medium carrying the computer software are also provided. The computer software may comprise one or more instructions, or code, that, when executed by a computer, causes the methods described to be performed. Computer software may be a sequence of instructions designed for execution on a computer system, and may include a subroutine, a function, a procedure, a module, an object method, an object implementation, an executable application, an applet, a servlet, source code, object code, a shared library, a dynamic linked library, and / or other sequences of instructions designed for execution on a computer system. The storage medium may be a magnetic disc (such as a hard drive or a floppy disc), an optical disc (such as a CD-ROM, a DVD-ROM or a BluRay disc), or a memory (such as a ROM, a RAM, EEPROM, EPROM, Flash memory or a portable / removable memory device), etc. The transmission medium may be a communications signal, a data broadcast, a communications link between two or more computers, etc.

[0179] Each feature disclosed in this specification, unless stated otherwise, may be replaced by alternative features serving the same, equivalent or similar purpose. Thus, unless stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

[0180] As used herein, including in the claims, unless the context indicates otherwise, singular forms of the terms herein are to be construed as including the plural form and vice versa. For instance, unless the context indicates otherwise, a singular reference herein including in the claims, such as "a" or "an" (such as a UE, a node, a network entity, a RAN entity, or a cell) means "one or more” (for instance one or more UE, one or more nodes, one or more network entities, one or more RAN entities, or one or more cells). Throughout the description and claims of this disclosure, the words "comprise", "including", "having" and "contain" and variations of the words, for example "comprising" and "comprises" or similar, mean "including", and are not intended to (and do not) exclude other components.

[0181] The use of any and all examples, or exemplary language ("for instance", "such as", "for example" and like language) provided herein, is intended merely to better illustrate the invention and does not indicate a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any nonclaimed element as essential to the practice of the invention.

[0182] Any steps described in this specification may be performed in any order or simultaneously unless stated or the context requires otherwise. Moreover, where a step is described as being performed after a step, this does not preclude intervening steps being performed. All of the aspects and / or features disclosed in this specification may be combined in any combination, except combinations where at least some of such features and / or steps are mutually exclusive. As described herein, there may be particular combinations of aspects that are of further benefit, such the aspects of determining a set of compensation parameters and applying a set of compensation parameters to measurements. In particular, the preferred features of the invention are applicable to all aspects of the invention and may be used in any combination. Likewise, features described in non- essential combinations may be used separately (not in combination). A method of manufacturing and / or operating any of the devices disclosed herein is also provided. The method may comprise steps of providing each of the features disclosed and / or configuring or using the respective feature for its stated function.

Claims

CLAIMS:1 . A method of managing conditional handover in a mobile network, the method comprising: receiving, at a user equipment, UE, served by a first node, conditional handover configuration data for one or more nodes, the one or more nodes comprising a second node; starting a timer in response to an event during handover from the first node to the second node; while the timer is running, preventing handover to the first node from being initiated.

2. The method of claim 1 , wherein the conditional handover configuration data for the one or more nodes comprises one or more handover criteria, wherein the method further comprises measuring signal data for each of the one or more nodes and evaluating the one or more handover criteria in respect of the one or more nodes, based on the measured signal data.

3. The method of claim 2, wherein preventing handover to the first node from being initiated comprises one of: while the timer is running, not measuring signal data for the first node; while the timer is running, measuring signal data for the first node but not evaluating the handover criteria in respect of the first node; and while the timer is running, measuring signal data for the first node and evaluating the handover criteria in respect of the first node but suppressing initiation of handover to the first node, even if one of the handover criteria is met in respect of the first node.

4. The method of claim 2 or claim 3, wherein the method further comprises determining that one of the handover criteria is met in respect of the second node, and executing handover to the second node in response.

5. The method of claim 4, wherein executing handover to the second node comprises: sending a message to initiate handover; initiating a random access procedure with the second node; and completing the random access procedure successfully.

6. The method of claim 5, wherein the timer is started in response to one of the following events during handover: sending the message to initiate handover; initiation of the random access procedure; or successful completion of the random access procedure.

7. The method of any of claims 2 to 6, further comprising: while the timer is running, determining that one of the handover criteria is met in respect of a third node of the one or more nodes, and initiating handover to the third node; OR determining that the timer has expired, determining that one of the handover criteria is met in respect of the first node, and initiating handover to the first node.

8. The method of any preceding claim: wherein the UE is connected to a master node and a secondary node in a Dual Connectivity mode of operation, and wherein the one or more nodes are secondary nodes; and / or wherein the conditional handover configuration data for the first node and the second node are based on reference cell configuration data.

9. A method of inter-cell mobility in a mobile network, the method comprising: receiving configuration data for one or more target cells; receiving instructions to handover from a source cell to a first target cell of the one or more target cells; starting a timer in response to an event during handover from the source cell to the first target cell; initiating handover to the first target cell, so that the source cell becomes a previous source cell and also becomes a target cell of the one or more target cells, and wherein the first target cell becomes the source cell; while the timer is running, sending signal measurement data to a Radio Access Network, RAN, entity, the signal measurement data comprising signal measurement data for each of the one or more target cells, except the previous source cell.

10. The method of claim 9, wherein: the handover instructions are layer 1 or layer 2 signalling instructions; orthe instructions are Medium Access Control, MAC, Control Element, MAC CE, instructions.

11. A method of inter-cell mobility in a mobile network, the method comprising: sending, to a user equipment, UE, configuration data for one or more target cells; sending, to the UE, instructions to handover from a source cell to a first target cell of the one or more target cells; starting a timer in response to an event during handover from the source cell to the target cell; receiving confirmation that handover to the first target cell has been executed, so that the source cell becomes a previous source cell and also becomes a target cell of the one or more target cells, and wherein the first target cell becomes the source cell; receiving signal measurement data from the UE, the signal measurement data comprising signal measurement data for each of the one or more target cells; while the timer is running, not sending instructions to the UE to handover to the previous source cell.

12. The method of claim 11 , further comprising one or more of: determining that signal measurement data for the previous source cell meets a handover condition, and not sending instructions to the UE to handover to the previous source cell while the timer is still running; and determining that the timer has expired, determining that signal measurement data for the previous source cell meets a handover condition, and sending instructions to the UE to handover to the previous source cell.

13. A User Equipment, UE, configured to perform the method of any of claims 1 to 10.

14. A Radio Access Network, RAN, entity, configured to perform the method of claim 11 or claim 12.

15. Computer software comprising instructions that, when executed on a processor, cause the processor to perform the method of any of claims 1 to 12.

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