Patent Description:
In Release <NUM> of 3GPP specifications, an inter-radio access technology (inter-RAT) handover to E-UTRA New Radio (NR) Dual Connectivity (EN-DC) is not supported. For example, a source NR network device (such as, a next generation NodeB, also referred to as "gNB") can only first hand over a terminal device (such as, user equipment) from the gNB to a target Long Term Evolution (LTE) network device (such as, an Evolved NodeB, also referred to as "eNodeB" or "eNB") and then configure EN-DC by adding a secondary network device after the handover. It is desirable to introduce a direct inter-RAT handover from NR to EN-DC, which can reduce signaling overhead and ensure a consistent high data rate for enhanced mobile broadband (eMBB) services. <NPL> discloses, in a handover to dual connectivity, performing a random access procedure with a master target node and a secondary target node in parallel. <NPL> discloses, when a random access procedure to a target master node fails but a random access procedure to a target secondary node is successful, delaying dropping the connection to the target secondary node until the user equipment is connected to a new primary cell.

In general, example embodiments of the present disclosure provide methods, apparatuses and computer readable media for a handover to dual connectivity. Optional features are given in the dependent claims.

Through the more detailed description of some example embodiments of the present disclosure in the accompanying drawings, the above and other objects, features and advantages of the present disclosure will become more apparent, wherein:.

Further, when a particular feature, structure, or characteristic is described in connection with an example embodiment, it is submitted that it is within the knowledge of one skilled in the art to affect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described.

As used herein, the term "communication network" refers to a network following any suitable communication standards, such as Long Term Evolution (LTE), LTE-Advanced (LTE-A), Wideband Code Division Multiple Access (WCDMA), High-Speed Packet Access (HSPA), Narrow Band Internet of Things (NB-IoT), New Radio (NR) and so on. Furthermore, the communications between a terminal device and a network device in the communication network may be performed according to any suitable generation communication protocols, including, but not limited to, the first generation (<NUM>), the second generation (<NUM>), <NUM>, <NUM>, the third generation (<NUM>), the fourth generation (<NUM>), <NUM>, the future fifth generation (<NUM>) communication protocols, and/or any other protocols either currently known or to be developed in the future. Embodiments of the present disclosure may be applied in various communication systems. Given the rapid development in communications, there will of course also be future type communication technologies and systems with which the present disclosure may be embodied. It should not be seen as limiting the scope of the present disclosure to only the aforementioned system.

As used herein, the term "network device" refers to a node in a communication network via which a terminal device accesses the network and receives services therefrom. The network device may refer to a base station (BS) or an access point (AP), for example, a node B (NodeB or NB), an evolved NodeB (eNodeB or eNB), a NR NB (also referred to as a gNB), a Remote Radio Unit (RRU), a radio header (RH), a remote radio head (RRH), a relay, a low power node such as a femto, a pico, and so forth, depending on the applied terminology and technology. In the following description, the terms "network device", "BS", and "node" may be used interchangeably.

Although functionalities described herein can be performed, in various example embodiments, in a fixed and/or a wireless network node may, in other example embodiments, functionalities may be implemented in a user equipment apparatus (such as a cell phone or tablet computer or laptop computer or desktop computer or mobile IOT device or fixed IOT device). This user equipment apparatus can, for example, be furnished with corresponding capabilities as described in connection with the fixed and/or the wireless network node(s), as appropriate. The user equipment apparatus may be the user equipment and/or or a control device, such as a chipset or processor, configured to control the user equipment when installed therein. Examples of such functionalities include the bootstrapping server function and/or the home subscriber server, which may be implemented in the user equipment apparatus by providing the user equipment apparatus with software configured to cause the user equipment apparatus to perform from the point of view of these functions/nodes.

As described above, in Release <NUM> (Rel-<NUM>) of 3GPP specifications, an inter-RAT handover (HO) to EN-DC is not supported. For example, a source gNB can only first hand over a UE from the gNB to a target eNB and then configure EN-DC by adding a secondary network device after the handover. It is desirable to introduce a direct inter-RAT handover from NR to EN-DC, which can reduce signaling overhead and ensure a consistent high data rate for eMBB services.

In the 3GPP specification TS <NUM> (<NUM> eNB/gNB to Master Node change), in the procedure of a handover from an eNB to EN-DC, the current signaling requires that UE must complete the handover to a target master node (MN) first before completion of access to a secondary node (SN). This is performed by the following two steps: the target MN first receives an RRC connection reconfiguration complete message (to complete the handover), then the target MN informs the target SN of the handover success via an X2 message (a SgNB Reconfiguration Complete message) to add the target SN. In addition, the specification is not clear whether the UE can start access to the SN in parallel.

For a one-step handover to EN-DC, the target is to reduce the dual connectivity (DC) configuration latency and quickly recover the service data rate after the handover procedure. Some existing solution proposes performing random access to both the target MN and the target SN in parallel for fast SN activation in the HO to EN-DC. This solution confirms the problem that quick/parallel access is needed, but at the same time in case that the HO to Master Cell Group (MCG) fails, reverting user plane back (discarding the delivered downlink/uplink packets) is difficult in UE implementation, which wastes radio resources and power and thus should be avoided. The biggest issue in this solution is that it only considers the MCG failure case. There is no discussion about the case in which the SN can communication to the UE via signaling radio bearer <NUM> (SRB3) without existence of SRB1 and SRB2. Therefore, this solution is not a complete and workable solution.

Additionally, during the Rel-<NUM> mobility enhancement work, the so-called Dual Active Protocol Stack (DAPS) handover is being specified. In that concept, UE may be communicating with both source and target cells at the same time, and if the target cell fails while the source cell communication is still ongoing, the UE can fall back to the source cell without re-establishment. However, in DAPS handover, the UE can establish a connection with a single target node anytime, but cannot establish connections to two target nodes simultaneously.

Embodiments of the present disclosure provide a solution for a handover to dual connectivity. According to embodiments of the present disclosure, a UE receives, from a source node, a connection reconfiguration message comprising configurations about a target MN and a target SN to be connected with the UE. The UE performs random access to the target MN and the target SN in parallel. If the UE successfully connects to the target SN but fails to connect to the target MN, the UE will reconnect to the target MN. In addition, the target SN indicates to the target MN a result (such as, success or failure) of the random access. For the target MN, different actions can be selected based on whether SRB3 exists between the target SN and the UE. As such, embodiments of the present disclosure can reduce signaling overhead and increase the success rate of the handover to dual connectivity.

<FIG> illustrates an example communication network <NUM> in which example embodiments of the present disclosure can be implemented. The communication network <NUM> includes a terminal device <NUM> and network devices <NUM>, <NUM> and <NUM>. Each of the network devices <NUM>, <NUM> and <NUM> can provide one or more cells. As shown in <FIG>, for example, the network device <NUM> may provide a cell <NUM>, the network device <NUM> may provide a cell <NUM> and the network device <NUM> may provide a cell <NUM>. It is to be understood that the number of network devices, terminal devices and/or cells is given for the purpose of illustration without suggesting any limitation to the scope of the present disclosure. The communication network <NUM> may include any suitable number of network devices, terminal devices and/or cells adapted for implementing implementations of the present disclosure.

In some example embodiments, some or all of the network devices <NUM>, <NUM> and <NUM> may use a same RAT, for example, LTE, NR or so on. Alternatively, in some example embodiments, the network devices <NUM>, <NUM> and <NUM> may use different RATs. For example, the network device <NUM> may be a gNB, while the network devices <NUM> and <NUM> are both eNBs. For another example, the network devices <NUM>, <NUM> and <NUM> may be all gNBs. For another example, the network devices <NUM> and the network device <NUM> are gNBs, while the network device <NUM> is an eNB. It is to be understood that the scope of the present disclosure is not limit in this aspect.

Communications in the communication system <NUM> may be implemented according to any proper communication protocol(s), comprising, but not limited to, cellular communication protocols of the first generation (<NUM>), the second generation (<NUM>), the third generation (<NUM>), the fourth generation (<NUM>) and the fifth generation (<NUM>) and on the like, wireless local network communication protocols such as Institute for Electrical and Electronics Engineers (IEEE) <NUM> and the like, and/or any other protocols currently known or to be developed in the future. Moreover, the communication may utilize any proper wireless communication technology, comprising but not limited to: Code Divided Multiple Address (CDMA), Frequency Divided Multiple Address (FDMA), Time Divided Multiple Address (TDMA), Frequency Divided Duplexer (FDD), Time Divided Duplexer (TDD), Multiple-Input Multiple-Output (MIMO), Orthogonal Frequency Divided Multiple Access (OFDMA) and/or any other technologies currently known or to be developed in the future.

As shown in <FIG>, initially, the terminal device <NUM> connects to the network device <NUM>, which is also referred to a "source network device" or "source node" in the following. The source node <NUM> initiates a handover to dual connectivity by transmitting a handover request to the network device <NUM> acting as a target master network device. The network device <NUM> may be also referred to as a "target master network device" or "target master node (T-MN)" in the following. To provide the terminal device <NUM> with dual connectivity, the T-MN <NUM> may select the network device <NUM> as a target secondary network device and transmit a request (for example, a SgNB addition request) for adding the network device <NUM> as the target secondary network device. The network device <NUM> may be also referred to as a "target secondary network device" or "target secondary node (T-SN)" in the following. The T-SN <NUM> may feedback an acknowledgement (for example, a SgNB addition request acknowledgement) comprising a configuration about Secondary Cell Group (SCG) to the T-MN <NUM>. The T-MN <NUM> may transmit, to the source node <NUM>, a handover request acknowledgement comprising configurations about both the T-MN <NUM> and the T-SN <NUM>. The source node <NUM> may transmit a connection reconfiguration message (for example, an RRC connection reconfiguration message) comprising the configurations about both the T-MN <NUM> and the T-SN <NUM> to the terminal device <NUM>.

In some example embodiments, the terminal device <NUM> performs random access to the T-MN <NUM> and the T-SN <NUM> in parallel based on the configurations received from the source node <NUM>. Regarding the T-SN <NUM>, it may transmit, to the T-MN <NUM>, a result of the random access, such as, via an X2/Xn interface. For example, the result may indicate whether the random access to the T-SN <NUM> performed by the terminal device <NUM> succeeds or fails.

Regarding the T-MN <NUM>, in some example embodiments, if the terminal device <NUM> successfully accesses the T-MN <NUM>, the T-MN <NUM> may perform normal actions to complete the handover procedure, as in the legacy solutions. If the T-MN <NUM> receives from the T-SN <NUM> an indication that the random access to the T-SN <NUM> performed by the terminal device <NUM> succeeds, the T-SN <NUM> may treat the handover as partial success and relative UE contexts may be established. If the T-MN <NUM> receives from the T-SN <NUM> an indication that the random access to the T-SN <NUM> performed by the terminal device <NUM> succeeds but the terminal device <NUM> fails to access the T-MN <NUM> (for example, the T304 timer of the T-MN <NUM> expires), different actions can be selected based on whether SRB3 exists between the T-SN <NUM> and the terminal device <NUM>.

In some example embodiments, if SRB3 is established between the T-SN <NUM> and the terminal device <NUM> and if the T-MN <NUM> receives an indication of the existence of SRB3, the T-MN <NUM> may determine that the handover fails and transmit a request for releasing the target secondary node to the T-SN <NUM>. Then the T-SN <NUM> may transmit a connection release message (such as, an RRC release message) to the terminal device <NUM> via SRB3. In some example embodiments, if no SRB3 exists between the T-SN <NUM> and the terminal device <NUM> or if the T-MN <NUM> does not receive an indication of the existence of SRB3, the T-SN <NUM> may maintain the connection with the terminal device <NUM> and the T-MN <NUM> may wait for a connection reestablishment message (such as, an RRC reestablishment message) from the terminal device <NUM>. To facilitate this, in some example embodiments, upon reception of the SgNB addition request acknowledgement from the T-SN <NUM>, the T-MN <NUM> may initiate a timer with a longer expiration time to monitor the connection reestablishment message from the terminal device <NUM>.

Regarding the terminal device <NUM>, in some example embodiments, if the terminal device <NUM> successfully accesses the T-SN <NUM> but fails to access the T-MN <NUM> (for example, the T304 timer of the T-MN <NUM> expires), the terminal device <NUM> tries to reconnect to the T-MN <NUM> based on the configuration about the T-MN <NUM> received via the connection reconfiguration message. The reconnection to the T-MN <NUM> may be performed in several ways.

In some example embodiments, after timeout of the T304 timer, the terminal device <NUM> may initiate a new timer with an expiration time of N*t304, where t304 represents the original expiration time of the T304 timer and N > <NUM>. Once the new timer is initiated, the terminal device <NUM> may try to reconnect to the T-MN <NUM> in each time period (for example, one or more times of t304) of the new timer. If the terminal device <NUM> has tried to reconnect to the T-MN <NUM> for N times but all fails, that is, the new timer expires, the terminal device <NUM> may fall back to the source node <NUM> configured with a configuration to do cell selection. In some example embodiments, the expiration time of the new timer may be configured to the terminal device <NUM> by the T-MN <NUM> in the handover request acknowledgement including information element(s) indicating values of t304 and N, or indicating the value of t304 and a new expiration time (for example, equal to N*t304) for the new timer. Alternatively, in some example embodiments, the expiration time of the new timer may be configured to the terminal device <NUM> by the T-MN <NUM> in a reconfiguration message (for example, an RRC reconfiguration message) transmitted from the T-SN <NUM> via SRB3 (if it exists). For example, the reconfiguration message may include information element(s) indicating values of t304 and N, or indicating the value of t304 and a new expiration tome (for example, equal to N*t304) for the new timer.

Alternatively, in some example embodiments, after timeout of the T304 timer, the terminal device <NUM> may perform cell selection. Upon selecting the cell <NUM> provided by the T-MN <NUM> as a suitable cell, the terminal device <NUM> may transmit a connection reestablishment request (for example, an RRC connection reestablishment request) to the T-MN <NUM> by using Cell-Radio Network Temporary Identifier (C-RNTI) and a physical cell identity allocated to the T-MN <NUM>. Otherwise, the terminal device <NUM> may fall back to the source node <NUM>. In some example embodiments, if the terminal device <NUM> receives a connection release message (such as, an RRC release message) from the T-SN <NUM> via SRB3 (if configured) during the re-connection to the T-MN <NUM>, the terminal device <NUM> may fall back to the source node <NUM> after looking for any possible cells.

Regarding the source node <NUM>, in some example embodiments, all resources allocated to the terminal device <NUM> may be remained until a UE context release message is received from the T-MN <NUM> or the terminal device <NUM> successfully reconnects to the source node <NUM>. After the terminal device <NUM> successfully reconnects to the source node <NUM>, the source node <NUM> may inform the T-MN <NUM> of the handover failure. Regarding the source node <NUM> and two target nodes <NUM> and <NUM>, early data forwarding during the handover and forwarding data retrieved from the target node to the source node <NUM> when the handover failure occurs can be perform in a similar way as the legacy solutions.

<FIG> illustrates a schematic diagram of interactions <NUM> between devices according to according to some example embodiments of the present disclosure. The interactions <NUM> may be implemented at any suitable devices. Only for the purpose of illustration, in the following, the interactions <NUM> will be described with reference to the UE <NUM>, the T-MN <NUM>, the T-SN <NUM> and the source node <NUM>.

As shown in <FIG>, the source node <NUM> may transmit <NUM> a handover request to the T-MN <NUM> to initiate a handover. In some example embodiments, according to respective RATs used by the source node <NUM>, the T-MN <NUM> and the T-SN <NUM>, the handover may be an inter-RAT or intra-RAT handover.

In response to the handover request from the source node <NUM>, the T-MN <NUM> may select the node <NUM> as the T-SN and transmit <NUM> a request (for example, a SgNB addition request) for adding the node <NUM> as the T-SN. The T-SN <NUM> may feedback <NUM> an acknowledgement (for example, a SgNB addition request acknowledgement) comprising a configuration about SCG to the T-MN <NUM>. Only for the purpose of illustration, it is assumed that the acknowledgement comprises no SRB3 configuration.

Upon reception of the acknowledgement, the T-MN <NUM> may initiate <NUM> a timer to monitor random access from the terminal device <NUM>. The T-MN <NUM> may transmit <NUM>, to the source node <NUM>, a handover request acknowledgement comprising configurations about both the T-MN <NUM> and the T-SN <NUM>. Since the T-MN <NUM> receives no SRB3 configuration from the T-SN <NUM>, the handover request acknowledgement may comprise no SRB3 configuration here. The source node <NUM> may transmit <NUM> a connection reconfiguration message (for example, an RRC connection reconfiguration message) comprising the configurations about both the T-MN <NUM> and the T-SN <NUM> to the terminal device <NUM>. The connection reconfiguration message may comprise no SRB3 configuration.

Upon reception of the configurations about both the T-MN <NUM> and the T-SN <NUM> via the connection reconfiguration message which includes no SRB3 configuration, the UE <NUM> may perform first random access 206A to the T-MN <NUM> and second random access 206B to the T-SN <NUM> in parallel. It is assumed that the second random access to the T-SN <NUM> succeeds <NUM> while no SRB3 is established. The T-SN <NUM> may transmit <NUM>, to the T-MN <NUM>, a result indicating the success of the second random access. In this case, since no SRB3 is established, the result may also comprise an indication that no SRB3 is established.

It is also assumed that the first random access to the T-MN <NUM> fails, and thus the UE <NUM> may reconnect <NUM> to the T-MN <NUM> based on the configuration indicated in the connection reconfiguration message. For example, the UE <NUM> may transmit a connection reestablishment message (for example, an RRC connection reestablishment message) to the T-MN <NUM>. In response to the connection reestablishment message being transmitted, the terminal device <NUM> may initiate <NUM> a timer to monitor a result of the reconnection. The T-MN <NUM> may wait for the connection reestablishment message from the UE <NUM> until its timer initiated at <NUM> expires. In response to success of the reconnection to the T-MN <NUM>, the T-MN <NUM> may transmit <NUM> a reconfiguration complete message (for example, a SgNB reconfiguration complete message) to the T-SN <NUM>. In response to success of the reconnection to the T-MN <NUM>, the UE <NUM> may transmit <NUM> a connection reconfiguration complete message to the T-MN <NUM> and the T-MN <NUM> may transmit <NUM> a UE context release message to the source node <NUM>.

In response to the handover request from the source node <NUM>, the T-MN <NUM> may select the node <NUM> as the T-SN and transmit <NUM> a request (for example, a SgNB addition request) for adding the node <NUM> as the T-SN. The T-SN <NUM> may feedback <NUM> an acknowledgement (for example, a SgNB addition request acknowledgement) comprising a configuration about SCG to the T-MN <NUM>. Only for the purpose of illustration, it is assumed that the acknowledgement comprises an SRB3 configuration.

Upon reception of the acknowledgement, the T-MN <NUM> may initiate <NUM> a timer to monitor random access from the terminal device <NUM>. The T-MN <NUM> may transmit <NUM>, to the source node <NUM>, a handover request acknowledgement comprising configurations about both the T-MN <NUM> and the T-SN <NUM>. Since the T-MN <NUM> receives the SRB3 configuration from the T-SN <NUM>, the T-MN <NUM> may forward the SRB3 configuration to the source node <NUM> via the handover request acknowledgement. The source node <NUM> may transmit <NUM> a connection reconfiguration message (for example, an RRC connection reconfiguration message) comprising the configurations about both the T-MN <NUM> and the T-SN <NUM> to the terminal device <NUM>. The connection reconfiguration message may also comprise the SRB3 configuration.

Upon reception of the configurations about both the T-MN <NUM> and the T-SN <NUM> via the connection reconfiguration message which includes no SRB3 configuration, the UE <NUM> may perform first random access 306A to the T-MN <NUM> and second random access 306B to the T-SN <NUM> in parallel. It is assumed that the second random access to the T-SN <NUM> succeeds <NUM> with SRB3 being established between the T-SN <NUM> and the UE <NUM> based on the SRB3 configuration. The T-SN <NUM> may transmit <NUM>, to the T-MN <NUM>, a result indicating the success of the second random access. In this case, since SRB3 has been established, the result may also comprise an indication that SRB3 has been established.

It is also assumed that the first random access to the T-MN <NUM> fails, and thus the UE <NUM> may reconnect <NUM> to the T-MN <NUM> based on the configuration indicated in the connection reconfiguration message. For example, the UE <NUM> may transmit a connection reestablishment message (for example, an RRC connection reestablishment message) to the T-MN <NUM>. In response to the connection reestablishment message being transmitted, the terminal device <NUM> may initiate <NUM> a timer to monitor a result of the reconnection. Since SRB3 exists, the T-MN <NUM> may instruct <NUM> the T-SN <NUM> to release the UE <NUM>. For example, the T-MN <NUM> may transmit a release request (for example, an SgNB release request) to the T-SN <NUM>. The T-SN <NUM> may then transmit a connection release message (for example, an RRC connection release message) to the UE <NUM> via SRB3. In response to the connection release message from the T-SN <NUM>, the UE <NUM> reconnects <NUM> to the source node <NUM>.

Upon reception of the configurations about both the T-MN <NUM> and the T-SN <NUM> via the connection reconfiguration message which includes no SRB3 configuration, the UE <NUM> may perform first random access 406A to the T-MN <NUM> and second random access 406B to the T-SN <NUM> in parallel. It is assumed that the second random access to the T-SN <NUM> succeeds <NUM> while no SRB3 is established. The T-SN <NUM> may transmit <NUM>, to the T-MN <NUM>, a result indicating the success of the second random access. In this case, since no SRB3 is established, the result may also comprise an indication that no SRB3 is established.

It is also assumed that the first random access to the T-MN <NUM> fails, and thus the UE <NUM> may reconnect <NUM> to the T-MN <NUM> based on the configuration indicated in the connection reconfiguration message. For example, the UE <NUM> may transmit a connection reestablishment message (for example, an RRC connection reestablishment message) to the T-MN <NUM>. The T-MN <NUM> may wait <NUM> for the connection reestablishment message from the UE <NUM> until its timer initiated at <NUM> expires. It is assumed here the timer of the T-MN <NUM> expires and thus the reconnection to the T-MN <NUM> fails. In response to failure of the reconnection to the T-MN <NUM>, the UE <NUM> may reconnect <NUM> to the source node <NUM>. The source node <NUM> may transmit <NUM> an indication of the handover failure to the T-MN <NUM>.

In view of the above, it can be seen that embodiments of the present disclosure support random access to the T-MN and the T-SN in parallel in a direct inter-RAT or intra-RAT handover to dual connectivity. Embodiments of the present disclosure can reduce signaling overhead and increase the success rate of the handover to dual connectivity by enabling the UE to reconnect to the T-MN while maintaining the connection with the T-SN.

<FIG> shows a flowchart of an example method <NUM> for a handover to dual connectivity in accordance with some example embodiments of the present disclosure. The method <NUM> can be implemented at the terminal device <NUM> as shown in <FIG>. For the purpose of discussion, the method <NUM> will be described from the perspective of the terminal device <NUM> with reference to <FIG>. In the following, the terminal device <NUM> is also referred to as a "first device <NUM>", the target master network device <NUM> is also referred to as a "second device <NUM>", the target secondary network device <NUM> is also referred to as a "third device <NUM>" and the source network device <NUM> is also referred to as a "fourth device <NUM>". It is to be understood that the method <NUM> may include additional blocks not shown and/or may omit some shown blocks, and the scope of the present disclosure is not limited in this regard.

At block <NUM>, the first device <NUM> receives a connection reconfiguration message from the fourth device <NUM>. The connection reconfiguration message may comprise a first configuration about the second device <NUM> and a second configuration about the third device <NUM>.

At block <NUM>, the first device <NUM> performs first random access to the second device <NUM> based on the first configuration and second random access to the third device <NUM> based on the second configuration to connect to the second device <NUM> and the third device <NUM>.

At block <NUM>, in response to failure of the first random access and success of the second random access, the first device <NUM> reconnects to the second device <NUM> based on the first configuration while keeping connected to the third device <NUM>.

In some example embodiments, the connection reconfiguration message comprises a configuration about a signaling radio bearer between the third device <NUM> and the first device <NUM>. In some example embodiments, in response to receiving a connection release message from the third device <NUM> via the signaling radio bearer, the first device <NUM> disconnects from the third device <NUM>, and reconnects to the fourth device <NUM> or connects to a fifth device discovered in cell selection initiated by the first device <NUM>.

In some example embodiments, the first device <NUM> may reconnect to the second device <NUM> by transmitting a connection reestablishment message to the second device <NUM>; or performing third random access to the second device <NUM> based on the first configuration.

In some example embodiments, the first device <NUM> determines whether the reconnection to the second device <NUM> fails. In accordance with a determination that the reconnection to the second device <NUM> fails, the first device <NUM> reconnects to the fourth device <NUM> or connects to a fifth device discovered in cell selection initiated by the first device <NUM>.

In some example embodiments, the first device <NUM> determines whether the reconnection to the second device <NUM> by: initiating a timer to monitor success of the reconnection to the second device <NUM>; and in response to timeout of the timer, determining that the reconnection to the second device <NUM> fails.

In some example embodiments, the first device <NUM> receives a configuration about the timer via one of the following: the connection reconfiguration message; or a reconfiguration message transmitted from the third device via a signaling radio bearer.

In some example embodiments, the first device <NUM> comprises a terminal device, the second device <NUM> comprises a target master network device, the third device <NUM> comprises a target secondary network device, and the fourth device <NUM> comprises a source network device which initiates a handover of the terminal device to the target master network device and the target secondary network device.

<FIG> shows a flowchart of an example method <NUM> for a handover to dual connectivity in accordance with some example embodiments of the present disclosure. The method <NUM> can be implemented at the target master network device <NUM> as shown in <FIG>. For the purpose of discussion, the method <NUM> will be described from the perspective of the target master network device <NUM> with reference to <FIG>. In the following, the terminal device <NUM> is also referred to as the "first device <NUM>", the target master network device <NUM> is also referred to as the "second device <NUM>", the target secondary network device <NUM> is also referred to as the "third device <NUM>" and the source network device <NUM> is also referred to as the "fourth device <NUM>". It is to be understood that the method <NUM> may include additional blocks not shown and/or may omit some shown blocks, and the scope of the present disclosure is not limited in this regard.

At block <NUM>, the second device <NUM> transmits, to the fourth device <NUM> connected with the first device <NUM>, a connection reconfiguration message comprising configurations about the second device <NUM> and a third device <NUM> to be connected with the first device <NUM>, to enable the first device <NUM> to perform first random access to the second device <NUM> and second random access to the third device <NUM>.

In some example embodiments, prior to transmitting the connection reconfiguration message to the fourth device <NUM>, the second device <NUM> transmits, to the third device <NUM>, a request for adding the third device <NUM> to provide the first device <NUM> with dual connectivity. The second device <NUM> receives, from the third device <NUM>, an acknowledgement comprising the second configuration.

In some example embodiments, the connection reconfiguration message comprises a configuration about a first timer to be used by the first device <NUM> to monitor the reconnection to the second device <NUM>.

At block <NUM>, the second device <NUM> determines whether the first random access and the second random access succeed.

In some example embodiments, the second device <NUM> determines whether the first random access succeeds by: in response to receiving the acknowledgement from the third device <NUM>, initiate a second timer to monitor success of the first random access; and in response to timeout of the timer, determining that the first random access fails.

In some example embodiments, a first expiration time of the first timer is the same as a second expiration time of the second timer. Alternatively, in some example embodiments, the first expiration time is different from the second expiration time. For example, the first expiration time may be shorter or longer than the second expiration time.

In some example embodiments, the second device <NUM> determines whether the second random access succeeds by: obtaining a result of the second random access from the third device <NUM>, the result indicating whether the second random access succeeds or fails.

In some example embodiments, the acknowledgement comprises no configuration about a signaling radio bearer between the third device <NUM> and the first device <NUM>, and the result comprises an indication that no signaling radio bearer is established.

In some example embodiments, the acknowledgement comprises a configuration about a signaling radio bearer to be established between the third device <NUM> and the first device <NUM>, and the result comprises an indication that the signaling radio bearer has been established based on the configuration.

At block <NUM>, in accordance with a determination that the first random access fails and the second random access successes, the second device <NUM> causes the first device <NUM> to reconnect to the second device <NUM>.

In some example embodiments, the second device <NUM> further transmits, to the third device <NUM>, a request for releasing a connection between the third device <NUM> and the first device <NUM>, such that the third device <NUM> transmits a connection release message to the first device <NUM> via the signaling radio bearer.

In some example embodiments, the second device <NUM> further transmits, to the third device <NUM>, a request for reconfiguring the first timer, such that the third device <NUM> transmits a reconfiguration message for reconfiguring the first timer to the first device <NUM> via the signaling radio bearer.

<FIG> shows a flowchart of an example method <NUM> for a handover to dual connectivity in accordance with some example embodiments of the present disclosure. The method <NUM> can be implemented at the target secondary network device <NUM> as shown in <FIG>. For the purpose of discussion, the method <NUM> will be described from the perspective of the target secondary network device <NUM> with reference to <FIG>. In the following, the terminal device <NUM> is also referred to as the "first device <NUM>", the target master network device <NUM> is also referred to as the "second device <NUM>", the target secondary network device <NUM> is also referred to as the "third device <NUM>" and the source network device <NUM> is also referred to as the "fourth device <NUM>". It is to be understood that the method <NUM> may include additional blocks not shown and/or may omit some shown blocks, and the scope of the present disclosure is not limited in this regard.

At block <NUM>, the third device <NUM> receives, from the second device <NUM>, a request for adding the third device <NUM> to provide the first device <NUM> with dual connectivity.

At block <NUM>, the third device <NUM> transmits an acknowledgement comprising a second configuration about the third device <NUM> to the second device <NUM>, to enable the first device <NUM> to perform random access to the third device <NUM> based on the second configuration.

At block <NUM>, the third device <NUM> determines whether the random access succeeds or fails.

At block <NUM>, the third device <NUM> transmits a result of the determination to the second device <NUM>.

In some example embodiments, the acknowledgement comprises a configuration about a signaling radio bearer to be established between the third device <NUM> and the first device <NUM>. In response to success of the random access, the third device <NUM> establishes the signaling radio bearer between the third device <NUM> and the first device <NUM> based on the configuration.

In some example embodiments, the third device <NUM> transmits the result to the second device <NUM> by: in response to the signaling radio bearer being established between the third device <NUM> and the first device <NUM>, transmitting the result comprising an indication that the signaling radio bearer has been established to the second device <NUM>.

In some example embodiments, in response to receiving a request for releasing a connection between the third device <NUM> and the first device <NUM> from the second device <NUM>, the third device <NUM> transmits a connection release message to the first device <NUM> via the signaling radio bearer.

In some example embodiments, the first device <NUM> is configured with a timer. In response to receiving a request for reconfiguring the timer of the first device <NUM> from the second device <NUM>, the third device <NUM> transmits a reconfiguration message for reconfiguring the timer of the first device <NUM> to the first device <NUM> via the signaling radio bearer.

In some example embodiments, an apparatus capable of performing the method <NUM> may comprise means for performing the respective steps of the method <NUM>. The means may be implemented in any suitable form. For example, the means may be implemented in a circuitry or software module.

In some example embodiments, the apparatus capable of performing the method <NUM> (for example, the terminal device <NUM>) comprises: means for receiving, at the first device, a connection reconfiguration message from a fourth device connected with first device, the connection reconfiguration message comprising a first configuration about a second device and a second configuration about a third device; means for performing first random access to the second device based on the first configuration and second random access to the third device based on the second configuration, to connect to the second device and the third device; and means for in response to failure of the first random access and success of the second random access, reconnecting to the second device based on the first configuration while keeping connected to the third device.

In some example embodiments, the connection reconfiguration message comprises a configuration about a signaling radio bearer between the third device and the first device and the apparatus capable of performing the method <NUM> further comprises: means for in response to receiving a connection release message from the third device via the signaling radio bearer, disconnecting from the third device; and means for reconnecting to the fourth device or connecting to a fifth device discovered in cell selection initiated by the first device.

In some example embodiments, the means for reconnecting to the second device comprises: means for transmitting a connection reestablishment message to the second device; or means for performing third random access to the second device based on the first configuration.

In some example embodiments, the apparatus capable of performing the method <NUM> further comprises: means for determining whether the reconnection to the second device fails; and means for in accordance with a determination that the reconnection to the second device fails, reconnecting to the fourth device or connecting to a fifth device discovered in cell selection initiated by the first device.

In some example embodiments, the means for determining whether the reconnection to the second device fails comprises: means for initiating a timer to monitor success of the reconnection to the second device; and means for in response to timeout of the timer, determining that the reconnection to the second device fails.

In some example embodiments, the apparatus capable of performing the method <NUM> further comprises: means for receiving a configuration about the timer via one of the following: the connection reconfiguration message; or a reconfiguration message transmitted from the third device via a signaling radio bearer.

In some example embodiments, the first device comprises a terminal device, the second device comprises a target master network device, the third device comprises a target secondary network device, and the fourth device comprises a source network device which initiates a handover of the terminal device to the target master network device and the target secondary network device.

In some example embodiments, the apparatus capable of performing the method <NUM> (for example, the target master network device <NUM>) comprises: means for transmitting, from a second device to a fourth device connected with a first device, a connection reconfiguration message comprising configurations about the second device and a third device to be connected with the first device, to enable the first device to perform first random access to the second device and second random access to the third device; means for determining whether the first random access and the second random access succeed; and means for in accordance with a determination that the first random access fails and the second random access succeeds, causing the first device to reconnect to the second device.

In some example embodiments, the connection reconfiguration message comprises a configuration about a first timer to be used by the first device to monitor the reconnection to the second device.

In some example embodiments, the apparatus capable of performing the method <NUM> further comprises: means for prior to transmitting the connection reconfiguration message to the fourth device, transmitting, to the third device, a request for adding the third device to provide the first device with dual connectivity; and means for receiving, from the third device, an acknowledgement comprising the second configuration.

In some example embodiments, the means for determining whether the first random access succeeds comprises: means for in response to receiving the acknowledgement from the third device, initiate a second timer to monitor success of the first random access; and means for in response to timeout of the timer, determining that the first random access fails.

In some example embodiments, the means for determining whether the second random access succeeds comprises: means for obtaining a result of the second random access from the third device, the result indicating whether the second random access succeeds or fails.

In some example embodiments, the acknowledgement comprises no configuration about a signaling radio bearer between the third device and the first device, and the result comprises an indication that no signaling radio bearer is established.

In some example embodiments, the acknowledgement comprises a configuration about a signaling radio bearer to be established between the third device and the first device, and the result comprises an indication that the signaling radio bearer has been established based on the configuration.

In some example embodiments, the apparatus capable of performing the method <NUM> further comprises: means for transmitting, to the third device, a request for releasing a connection between the third device and the first device, such that the third device transmits a connection release message to the first device via the signaling radio bearer.

In some example embodiments, the connection reconfiguration message comprises a configuration about a first timer to be used by the first device to monitor the reconnection to the second device, and the apparatus capable of performing the method <NUM> further comprises: means for transmitting, to the third device, a request for reconfiguring the first timer, such that the third device transmits a reconfiguration message for reconfiguring the first timer to the first device via the signaling radio bearer.

In some example embodiments, the apparatus capable of performing the method <NUM> (for example, the target secondary network device <NUM>) comprises: means for receiving, at a third device and from a second device, a request for adding the third device to provide a first device with dual connectivity; means for transmitting an acknowledgement comprising a second configuration about the third device to the second device, to enable the first device to perform random access to the third device based on the second configuration; means for determining whether the random access succeeds or fails; and means for transmitting a result of the determination to the second device.

In some example embodiments, the acknowledgement comprises a configuration about a signaling radio bearer to be established between the third device and the first device. The apparatus capable of performing the method <NUM> further comprises: means for in response to success of the random access, establishing the signaling radio bearer between the third device and the first device based on the configuration.

In some example embodiments, the means for transmitting the result to the second device comprises: means for in response to the signaling radio bearer being established between the third device and the first device, transmitting the result comprising an indication that the signaling radio bearer has been established to the second device.

In some example embodiments, the apparatus capable of performing the method <NUM> further comprises: means for in response to receiving a request for releasing a connection between the third device and the first device from the second device, transmitting a connection release message to the first device via the signaling radio bearer.

In some example embodiments, the first device is configured with a timer, and the apparatus capable of performing the method <NUM> further comprises: means for in response to receiving a request for reconfiguring the timer of the first device from the second device, transmitting a reconfiguration message for reconfiguring the timer of the first device to the first device via the signaling radio bearer.

<FIG> is a simplified block diagram of a device <NUM> that is suitable for implementing embodiments of the present disclosure. For example, the terminal device <NUM>, the target master network device <NUM>, the target secondary network device <NUM> and/or the source network device <NUM> as shown in <FIG> can be implemented by the device <NUM>. As shown, the device <NUM> includes one or more processors <NUM>, one or more memories <NUM> coupled to the processor <NUM>, and one or more communication modules <NUM> coupled to the processor <NUM>.

It should be appreciated that future networks may utilize network functions virtualization (NFV) which is a network architecture concept that proposes virtualizing network node functions into "building blocks" or entities that may be operationally connected or linked together to provide services. A virtualized network function (VNF) may comprise one or more virtual machines running computer program codes using standard or general type servers instead of customized hardware. Cloud computing or data storage may also be utilized. In radio communications, this may mean node operations to be carried out, at least partly, in a central/centralized unit, CU, (e.g. server, host or node) operationally coupled to distributed unit, DU, (e.g. a radio head/node). It should also be understood that the distribution of labour between core network operations and base station operations may vary depending on implementation.

In an embodiment, the server may generate a virtual network through which the server communicates with the distributed unit. In general, virtual networking may involve a process of combining hardware and software network resources and network functionality into a single, software-based administrative entity, a virtual network. Such virtual network may provide flexible distribution of operations between the server and the radio head/node. In practice, any digital signal processing task may be performed in either the CU or the DU and the boundary where the responsibility is shifted between the CU and the DU may be selected according to implementation.

Therefore, in an embodiment, a CU-DU architecture is implemented. In such case the device <NUM> may be comprised in a central unit (e.g. a control unit, an edge cloud server, a server) operatively coupled (e.g. via a wireless or wired network) to a distributed unit (e.g. a remote radio head/node). That is, the central unit (e.g. an edge cloud server) and the distributed unit may be stand-alone apparatuses communicating with each other via a radio path or via a wired connection. Alternatively, they may be in a same entity communicating via a wired connection, etc. The edge cloud or edge cloud server may serve a plurality of distributed units or a radio access networks. In an embodiment, at least some of the described processes may be performed by the central unit. In another embodiment, the device <NUM> may be instead comprised in the distributed unit, and at least some of the described processes may be performed by the distributed unit.

In an embodiment, the execution of at least some of the functionalities of the device <NUM> may be shared between two physically separate devices (DU and CU) forming one operational entity. Therefore, the apparatus may be seen to depict the operational entity comprising one or more physically separate devices for executing at least some of the described processes. In an embodiment, such CU-DU architecture may provide flexible distribution of operations between the CU and the DU. In practice, any digital signal processing task may be performed in either the CU or the DU and the boundary where the responsibility is shifted between the CU and the DU may be selected according to implementation. In an embodiment, the device <NUM> controls the execution of the processes, regardless of the location of the apparatus and regardless of where the processes/functions are carried out.

The present disclosure also provides at least one computer program product tangibly stored on a non-transitory computer readable storage medium. The computer program product includes computer-executable instructions, such as those included in program modules, being executed in a device on a target real or virtual processor, to carry out the method <NUM> as described above with reference to <FIG>, the method <NUM> as described above with reference to <FIG> and/or the method <NUM> as described above with reference to <FIG>. Generally, program modules include routines, programs, libraries, objects, classes, components, data structures, or the like that perform particular tasks or implement particular abstract data types. The functionality of the program modules may be combined or split between program modules as desired in various embodiments. Machine-executable instructions for program modules may be executed within a local or distributed device. In a distributed device, program modules may be located in both local and remote storage media.

Claim 1:
A method comprising:
receiving, at a user equipment, a connection reconfiguration message for a handover to dual connectivity from a source node connected with the user equipment, the connection reconfiguration message comprising a first configuration about a target master node and a second configuration about a target secondary node;
performing, in parallel, first random access to the target master node based on the first configuration and second random access to the target secondary node based on the second configuration, to connect to the target master node and the target secondary node;
in response to failure of the first random access and success of the second random access, reconnecting to the target master node based on the first configuration while keeping connected to the target secondary node;
determining whether the reconnection to the target master node fails; and
in accordance with a determination that the reconnection to the target master node fails, reconnecting to the source node or connecting to another target node discovered in cell selection initiated by the user equipment.