WO2016119210A1

WO2016119210A1 - Dual connectivity and handover

Info

Publication number: WO2016119210A1
Application number: PCT/CN2015/071937
Authority: WO
Inventors: Yanji Zhang; Tsunehiko Chiba; Yang Liu; Benoist Pierre Sebire; Guillaume DECARREAU
Original assignee: Nokia Solutions And Networks Oy
Priority date: 2015-01-30
Filing date: 2015-01-30
Publication date: 2016-08-04

Abstract

When an inter master network node handover for a user apparatus in dual connectivity with a handover source node and a secondary node is requested, determining in a handover target node, whether or not the secondary node may remain to provide dual connectivity, and sending in a response to the handover an indication whether or not the secondary node may remain to provide dual connectivity with the handover target node.

Description

DUAL CONNECTIVITY AND HANDOVER

TECHNICAL FIELD

The invention relates to communications.

BACKGROUND

The following description of background art may include insights， discoveries， understandings or disclosures， or associations together with dis-closures not known to the relevant art prior to the present invention but provid-ed by the invention. Some such contributions of the invention may be specifi-cally pointed out below， whereas other such contributions of the invention will be apparent from their context.

In recent years， the phenomenal growth of mobile Internet services and proliferation of smart phones and tablets has increased a demand for mo-bile broadband services， and hence more data transmission capacity is re-quired. One possibility to increase a data transmission rate of a user apparatus is dual connectivity. The basic principle of the dual connectivity is that the user apparatus may consume radio resources provided by at least two different network nodes， each network node controlling one or more cells， one of the network nodes being a master network node controlling radio resources for the user apparatus.

BRIEF DESCRIPTION

According to an aspect， there is provided the subject matter of the independent claims. Embodiments are defined in the dependent claims.

BRIEF DESCRIPTION OF DRAWINGS

In the following， the invention will be described in greater detail by means of preferred embodiments with reference to the attached drawings， in which

Figure 1 shows simplified architecture of a system and block dia-grams of some apparatuses according to an exemplary embodiment；

Figures 2 to 6 are flow charts illustrating exemplary functionalities；

Figures 7 and 8 illustrate exemplary signaling； and

Figure 9 is a schematic block diagram of an exemplary apparatus.

DETAILED DESCRIPTION OF SOME EMBODIMENTS

The following embodiments are exemplary. Although the specifica-tion may refer to “an” ， “one” ， or “some” embodiment (s) in several locations， this does not necessarily mean that each such reference is to the same embodi-ment (s) ， or that the feature only applies to a single embodiment. Single fea-tures of different embodiments may also be combined to provide other embod-iments.

The present invention is applicable to any access network/system and apparatus that can be or are configured to support dual connectivity. Ex-amples of such access networks/systems include LTE (Long Term Evolution) access system， Worldwide Interoperability for Microwave Access (WiMAX) ， Wireless Local Area Network (WLAN) ， LTE Advanced (LTE-A) ， and beyond LTE-A， such as 4G (fourth generation) and 5G (fifth generation) ， and any fu-ture generation. The specifications of different systems and networks， espe-cially in wireless communication， develop rapidly. Such development may re-quire extra changes to an embodiment. Therefore， all words and expressions should be interpreted broadly and they are intended to illustrate， not to restrict， the embodiment. For example， future networks may utilize network functions virtualization (NFV) which is a network architecture concept that proposes vir-tualizing network node functions into “building blocks” or entities that may be operationally dynamically instantiated， connected or linked together to provide network services. A virtualized network function (VNF) may comprise one or more virtual machines that run computer program codes using standard or general type servers instead of customized hardware. In other words， the VNF concept proposes to consolidate many network equipment (apparatus， node) types onto standard servers whose hardware can run computer program codes implementing network functions， without a need for installation of new equip-ment. Cloud computing and/or data storage may also be utilized. In radio communications this may mean node operations to be carried out， at least partly， in a server， host or node operationally coupled to a remote radio head. It is also possible that node operations will be distributed amongst a plurality of servers， nodes or hosts. Another networking paradigm is software-defined networking (SDN) in which lower-level functionality is abstracted by decoupling data forwarding (data plane) from overlying control decisions， such as routing and resource allocations. This is achieved by means of one or more software-based SDN controllers that allow the underlying network to be programmable via the SDN controllers independent of underlying network hardware. Hence， it should be understood that the distribution of labor between core network oper-ations and base station operations and user apparatuses may differ from that of the LTE-A， or even be non-existent， and the below described base station functionality may be migrated to any corresponding abstraction or apparatus.

In the following， different embodiments will be described using， as an example of an access architecture to which the embodiments may be ap-plied， a radio access architecture LTE-A， without restricting the embodiments to such an architecture.

A general architecture of an exemplary system 100 is illustrated in Figure 1. Figure 1 is a simplified system architecture only showing some ele-ments and functional entities， all being logical units whose implementation may differ from what is shown. It is apparent to a person skilled in the art that the system may also comprise other functions and structures that are not illustrat-ed， for example connections to the core network/system.

The exemplary system 100 illustrated in Figure 1 comprises user apparatuses 110 (only one illustrated in Figure 1) and three or more network nodes 120， 120’ ， 130 (only three illustrated in Figure 1) each controlling and/or providing one or more cells in a radio access system 101， and a mobility man-agement entity 140.

The mobility management entity (MME) 140 represents a mobility anchor entity in a core network (not illustrated in Figure 1) that is involved in the bearer activation/deactivation processes， for example.

The user apparatus (user equipment， UE) 110 illustrates one type of an apparatus configurable to support dual connectivity and to which resources on the air interface are allocated and assigned， and thus any feature described herein with user apparatus (user equipment) may be implemented with a cor-responding apparatus. The user apparatus 110 refers to a portable computing device that includes wireless mobile communication devices operating with or without a subscriber identification module (SIM) ， including， but not limited to， the following types of devices： mobile phone， smart-phone， personal digital assistant (PDA) ， handset， laptop computer， e-reading device， tablet， a weara-ble device communicating in a smart-phone manner， a self-driving vehicle.

In the example of Figure 1， three

network nodes

120， 120’ ， 130 each depicts an apparatus controlling one or more cells via which access is provided to the network the user apparatuses and the network nodes are con- nected to. In an LTE-A system， such a network node is an evolved node B (eNB) . The evolved

node B

120， 120’ ， 130 or any corresponding network appa-ratus controlling one or more cells， is a computing device configured to control the radio resources， and connected to the evolved packet core network， there-by providing the user equipment 110 a connection to the communication sys-rem. Typically， but not necessarily， the evolved node B comprises all radio-related functionalities of the communication whereby the evolved node B， for example， schedules transmissions by assigning certain uplink resources for the user equipment and informing the user equipment about transmission for-mats to be used.

In dual connectivity， a user apparatus is connected to a master evolved node B (MeNB) controlling a primary cell and comprising a radio re-source controlling unit for the user apparatus， and to a secondary evolved node B (SeNB) controlling a secondary cell. In the illustrated example， two evolved node Bs 120， 120’ are configured to perform one or more master evolved node B functionalities described below with an embodiment/example， and they may be configured to perform functionalities from different embodi-ments/examples. For this purpose， the evolved node B 120， 120’ comprises a dual connectivity handover unit (DC-HO-U) 121， 121’ configured to support inter-master eNB handover maintaining dual connectivity to the same second-ary evolved node B and memory 122， 122’ usable for temporarily buffering， for example. Further， in the illustrated example， one evolved node B 130 is con-figured to perform one or more secondary evolved node B functionalities de-scribed below with an embodiment/example， and it may be configured to per-form functionalities from different embodiments/examples. For this purpose， the evolved node B 130 comprises an enhanced dual connectivity unit (e-DC-U) 131 configured to support inter-master eNB handover maintaining dual con-nectivity to the same secondary evolved node B and memory 132 usable for temporarily buffering， for example. Naturally an evolved node B may comprise both the dual connectivity handover unit and the enhanced dual connectivity unit.

The evolved node B also provides the cells but the exemplary em-bodiments may be implemented with a solution having a separate controlling apparatus， and separate cell providing apparatuses controlled by a controlling apparatus. Further， the cells may be macro cells， micro cells and/or small cells.

Below different examples/embodiments are described assuming that a target master evolved node accepts at least the handover (HO) from a source master evolved node. If not， no new procedure is needed； a new target master evolved node may be determined， for example. Another assumption made is that the secondary evolved node accepts an addition request from the target master evolved node. Further， below and in the Figures， the source master evolved node is called/referred as “source node” or eNB1 or S-eNB， the target master evolved node is called/referred as “target node” or eNB2 or T-eNB， and the secondary evolved node B is called/referred as “secondary node” or SeNB. Further， in the below a handover command/handover com-pleted messages equalizes with radio reconfiguration messages， since a handover command， for example， includes at least implicitly a radio resource reconfiguration message since during a handover radio resources are recon-figured.

Figure 2 is a flow chart illustrating an exemplary functionality of an evolved node B acting as a master node (anchor node) currently serving the user apparatus， or more precisely， an exemplary functionality of a dual connec-tivity handover unit， or its sub-unit when the evolved node is a source node in an inter-master evolved node handover.

Referring to Figure 2， a handover (HO) for an user apparatus oper-ating in dual connectivity to a target node is triggered in step 201. Therefore a handover request is sent to the intended target node in step 202， the handover request containing information on a secondary node. The information on the secondary node may be measurement report information， received from the user apparatus， of one or more cells controlled by the secondary node and serving the user apparatus.

Once an acknowledgment to the handover request is received in step 203， it is checked， whether or not the acknowledgement indicates that the target node has successfully performed an addition preparation procedure with the secondary node. The indication may be a new information element， like “dual connectivity without secondary node change” ， or the indication may be a secondary cell group (SGC) configuration with the same secondary node.

If the acknowledgement contains the indication (step 204) ， a hando-ver command is sent in step 205 to the user apparatus， and a connection re-lease request is sent in step 206 to the secondary node B. The handover command instructs the user apparatus to perform radio resource reconfigura- tion. If the indication in the acknowledgement comprises the secondary cell group configuration， it is added to the handover command.

If the acknowledgment does not contain the indication (step 204) ， or otherwise indicates an unsuccessfully addition preparation or an indication that the secondary node is for some reason unsuitable， in the illustrated example the source node is configured to end in step 207 the dual connectivity， i.e. change the dual connectivity to a single connectivity. However， it should be appreciated that other alternatives， like continuing the handover to the target cell， including sending a handover command to the user apparatus， or not con-tinuing the handover to the target node and instead of starting the process to another target node， may be implemented in step 207 as well.

Figure 3 is a flow chart illustrating an exemplary functionality of an evolved node B acting as a target node， or more precisely， an exemplary func-tionality of a dual connectivity handover unit， or its sub-unit when the evolved node is a target node in an inter-master evolved node handover.

Referring to Figure 3， once a handover request containing infor-mation on a secondary node is received in step 301 from a source node， the target node checks in step 302， whether or not the secondary node is ac-ceptable to be a secondary node. If it is， an addition request is sent in step 303 to the secondary node to trigger secondary node addition preparation， and an acknowledgement from the secondary node is received in step 304， the acknowledgement containing secondary cell group configuration for the dual connectivity. After that the target node sends in step 305 to the source node an acknowledgement to the handover request indicating that the same secondary node will be used. In the illustrated example the acknowledgement contains as the indication the information element indicating successful secondary node addition preparation procedure.

Further， the target node starts in step 306 to buffer downlink (DL) data received from the source node and targeted to the user apparatus for which the handover procedure has been triggered.

Meanwhile the user apparatus performs synchronization with the target node and obtains access to corresponding cells to complete the hando-ver. Once information that the handover has been completed is received in step 307 from the mobile station， the target node starts in step 308 scheduling of the user apparatus and forwards in step 309 the buffered data， if there is buffered data， to the user apparatus.

Further， in the illustrated example， to continue the dual connectivity with the secondary node， the target node sends in step 310 a radio resource reconfiguration request to the user apparatus. The radio resource reconfigura-tion requests of step 310 contains a new radio resource configuration for the secondary cell group according the secondary cell group configuration re-ceived in step 304 in the acknowledgement.

Then information that the radio resource reconfiguration has been completed is received in step 311 from the user apparatus. Therefore， the tar-get node stops in step 312 the data scheduling for secondary cell group bear-er， and instructs in step 313 the secondary node to stop downlink data forward-ing for the secondary cell group bearer (if data forwarding for the secondary cell group bearer has previously taken place by the secondary node) . Further， a path switch request is sent in step 314 to the mobility management entity to inform the mobility management entity that the user apparatus has changed cell. In the illustrated example status transfer information is sent in step 315 to the secondary node to convey packet data convergence protocol sequence number (PDCP SN) . It should be noted that sending， from the target node to the secondary node， an instruction to stop data forwarding for secondary cell group bearer may be omitted in case the secondary node has been instructed not to start downlink data forwarding， or if no forwarded downlink data is re-ceived.

Once path request acknowledgement confirming the path switch is received in step 316 from the mobility management entity， a message instruct-ing the source node to release user apparatus context is sent in step 317， and the handover procedure with dual connectivity to the same secondary node has been performed.

If the secondary node is not acceptable to be a secondary node (step 302) ， an acknowledgement to the handover request is sent in step 318， the acknowledgement indicating that the same secondary node will not be used. The indication may be an empty information element.

Figure 4 is a flow chart illustrating an exemplary functionality of an evolved node B acting as a secondary node， or more precisely， an exemplary functionality of an enhanced dual connectivity unit， or its sub-unit when the evolved node is in an inter-master evolved node handover a secondary node that should remain to be used by the user apparatus undergoing the handover.

Referring to Figure 4， while providing dual connectivity to a user ap- paratus as a secondary node with a first master evolved node (source node) ， a secondary node addition request relating to the same user apparatus is re-ceived in step 401 from a second master evolved node (target node) and an acknowledgement is sent in step 401 to the addition request.

Then a release request， for example an SeNB release request， from the first master evolved node is received in step 402. Therefore the secondary node sends in step 403 a status transfer message to the first master evolved node to convey uplink packet data convergence protocol sequence number (PDCP SN) receiver status and downlink PDCP SN transmitter status of radio access bearers (E-RAB) for which PDCP status preservation applies.

In the illustrated example， the secondary node stops in step 404 da-ta forwarding to the user apparatus and starts to forward downlink data to the first master evolved node. The data forwarding is an optional operation， and the solution may be implemented so that data forwarding is always implement-ed， never implemented， or the data forwarding may be implemented to depend on an instruction received. For example， the SeNB release request message received in step 402 may comprise an indication not to start the data forward-ing to the source node. Accordingly， the secondary node may start to buffer the downlink data instead of forwarding it.

Once an indication to stop downlink data forwarding to the first mas-ter evolved node is received in step 405 from the second master evolved node， the secondary node stops in step 406 the data forwarding for secondary cell group bearer and starts in step 406 scheduling for downlink data to the user apparatus. The indication to stop downlink data forwarding may be received in a secondary node reconfiguration complete message. Naturally， in implemen-rations/situations in which the data forwarding is not implemented/started， it will not be stopped (i.e. step of stopping the data forwarding may be skipped) .

Further， in the illustrated example the secondary node receives in step 407 sequence number status transfer information from the second master evolved node， the transfer information containing PDCP SN. However， step 407 may be omitted.

Finally the secondary node receives in step 408 an end marker from the first master evolved node confirming that the path has been changed and the dual connectivity is now via the secondary node and the second master evolved node. Thus the communication with the UE may take place over the secondary cell group (SCG) bearer and a master cell group (MCG) bearer es- tablished for example by the process described above with Figure 3， or via a split bearer.

Figure 5 is a flow chart illustrating another exemplary functionality of an evolved node B acting as a target node， or more precisely， an exemplary functionality of a dual connectivity handover unit， or its sub-unit when the evolved node is a target node in an inter-master evolved node handover.

Referring to Figure 5， once a handover request containing infor-mation on a secondary node is received in step 501 from a source node， it is checked in step 502， whether or not the secondary node is acceptable to be a secondary node. If it is， an addition request is sent in step 503 to the second-ary node to trigger secondary node addition preparation， and an acknowl-edgement from the secondary node is received in step 504， the acknowledge-ment containing secondary cell group configuration for the dual connectivity. After that the target node sends in step 505 to the source node an acknowl-edgement to the handover request indicating that the same secondary node will be used. In the illustrated example the acknowledgement contains as the indication the secondary cell group information received in step 504.

Further， the target node starts in step 506 to buffer downlink (DL) data received from the source node and targeted to the user apparatus for which the handover procedure has been triggered.

Meanwhile the user apparatus performs synchronization with the secondary node and the target and obtains access to corresponding cells to complete the handover. Once information that the handover has been com-pleted is received in step 507 from the mobile station， the target node starts in step 508 scheduling of the user apparatus and forwards in step 509 the buff-ered data， if there is buffered data， to the user apparatus.

Then the secondary node is informed that the user apparatus has completed the reconfiguration by sending in step 510 to the secondary node a radio resource control reconfiguration complete message. Further， a path switch request is sent in step 511 to the mobility management entity to inform the mobility management entity that the user apparatus has changed cell.

Once path request acknowledgement confirming the path switch is received in step 512 from the mobility management entity， a message instruct-ing the source node to release user apparatus context is sent in step 513， and the handover procedure with dual connectivity to the same secondary node has been performed.

If the secondary node is not acceptable to be a secondary node (step 502) ， an acknowledgement to the handover request is sent in step 514， the acknowledgement indicating that the same secondary node will not be used. The indication may be an empty information element.

Figure 6 is a flow chart illustrating another exemplary functionality of an evolved node B acting as a secondary node， or more precisely， an exem-plary functionality of an enhanced dual connectivity unit， or its sub-unit when the evolved node is in an inter-master evolved node handover a secondary node that should remain to be used by the user apparatus undergoing the handover.

Referring to Figure 6， while providing dual connectivity to a user ap-paratus as a secondary node with a first master evolved node (source node) ， a secondary node addition request relating to the same user apparatus is re-ceived in step 601 from a second master evolved node (target node) and an acknowledgement is sent in step 601 to the addition request.

Then a release request from the first master evolved node is re-ceived in step 602. The release request may comprise an indication not to start forwarding SCG bearer data but to buffer it instead.

Therefore the secondary node starts in step 603 to buffer the down-link data for a secondary cell group bearer and stops in step 603 scheduling of the user apparatus. Further， the secondary node stops in step 604 data for-warding to the user apparatus and instead starts in step 604 to forward down-link data to the first master evolved node if there is a split bearer.

Once information that the reconfiguration has been completed is re-ceived in step 605 from the second master evolved node， the secondary node stops in step 606 the data forwarding for the split bearer and starts in step 606 scheduling for downlink data to the user apparatus.

Finally the secondary node receives in step 607 an end marker from the first master evolved node confirming that the path has been changed and the dual connectivity is now via the secondary node and the second master evolved node. Thus the communication with the UE may take place over the secondary cell group (SCG) bearer and a master cell group (MCG) bearer es-tablished for example by the process described above with Figure 5， or via a split bearer.

As can be seen from the above， in an implementation based on ex-amples illustrated in Figure 2， Figure 3 and Figure 4， the user apparatus re- ceives separate commands that both trigger radio resource reconfiguration： one is received from the source node (step 205) ， and one from the target node (step 310) . However， in an implementation based on examples illustrated in Figure 2， Figure 5 and Figure 6， the user apparatus receives one command that triggers the radio resource reconfiguration： the message received from the source node (205) . Since the message received from the source node con-rained both mobility information and the secondary cell group configuration， the user apparatus acknowledges completion of the radio resource reconfigu-ration to the target node. Therefore the RRC Configuration Reconfiguration message is not needed to be sent at all from the target node.

Figures 7 and 8 are signalling charts illustrating exemplary signal-ling， both having as a starting situation that the user apparatus UE is in dual connectivity mode with a secondary node SeNB and with a first master evolved node eNB1. Further， in the examples it is assumed that that the secondary node is configured to buffer secondary cell group bearer data and to forward other possible data. This causes less load over the X2 interface between eNB1 and eNB2. However， it should be appreciated that the secondary node may be configured to forward also the secondary cell group bearer data until an in-struction to stop data forwarding is received.

Referring to Figure 7， UE sends measurement report 7-1 to eNB1. The measurement report may comprise measurement information related to a cell controlled by SeNB and serving UE. Based on the measurement report and radio resource management information， eNB1 decides in point 7-2 to per-form a handover to a second master evolved node eNB2 while maintaining the same SeNB. Therefore eNB1 sends in message 7-3 a handover request con-taining measurement report or corresponding information， including information on SeNB， to eNB2.

In response to the handover request containing the information on SeNB， eNB2 detects in point 7-4 that the handover is a dual connectivity handover targeting to maintain SeNB. Therefore， SeNB addition procedure is performed by message 7-5a ( “SeNB addition request” ) from eNB2 to SeNB and message 7-5b ( “SeNB addition acknowledge” ) from SeNB to eNB2， mes-sage 7-5b from SeNB providing secondary cell group configuration. Then eNB2 acknowledges the handover request by sending message 7-6 that con-tains an information element “DC without SeNB change” .

In response to the handover request acknowledgement 7-6 contain- ing the information element， eNB1 detects in point 7-7 that the dual connectivi-ty inter-master-eNBs with the same SeNB will succeed. Owing to the reception of this information element， eNB1 (source eNB) omits sending a UE Context Release message towards the secondary eNB (SeNB) after the successful handover to eNB2.

The exemplary process continues by the eNB1 sending message 7-8， for example a handover command including the mobility control information or RRC Connection Reconfiguration message generated by eNB2 and includ-ing mobility control information to UE， and to SeNB in message 7-9 an SeNB release request with an additional information indicating that the release re-quest relates to an inter-master-handover maintaining the same secondary node. The additional information may be an instruction not to forward second-ary cell bearer data， or it may be the same information element “DC without SeNB” change， that was received in message 7-6 . Hence， there are no re-strictions what the additional information， as long as SeNB is configured to in-terpret it correctly. In the examples herein， it is assumed that the additional information instructs to buffer. However， it should be appreciated that the addi-tional information may provide other kind of instructions how to handle down-link data.

Although not illustrated in Figure 7， UE stops， in response to receiv-ing message 7-8， uplink transmissions to SeNB and eNB1.

In response to receiving SeNB release request 7-9， SeNB stops， in point 7-10， UE scheduling， i.e. providing user data to the UE. Further， SeNB may start to forward data for a split bearer and/or master cell group bearer (il-lustrated by dashed line messages 7-12) . Further， in response to the additional information， SeNB starts， in point 7-10， to buffer data for the secondary cell group bearer， as explained above. In case data split bearer data and/or master cell group data is forwarded in messages 7-11 from SeNB via eNB1 to eNB2， eNB2 buffers in point 7-12 the forwarded data.

Meanwhile UE performs (not illustrated in Figure 7) synchronisation to eNB2 via random access channel (RACH) ， using information received in message 7-8. When UE has successfully accessed the target cell provided by eNB2， UE sends message 7-13， for example RRC Connection Reconfiguration Complete or handover complete， to confirm the handover. The buffered data， if any exists， can then be forwarded (message 7-11’ ) to UE.

In response to message 7-13 and to continue dual connectivity with SeNB， eNB2 sends message 7-14， for example RRC Connection Reconfigura-tion， to UE， message 7-14 including a new radio resource configuration of the secondary cell group.

In response to message 7-14， UE start to apply the received new radio resource configuration and acknowledges this by sending message 7-15， for example RRC Connection Reconfiguration Complete， to eNB2.

In response to message 7-15， eNB2 stops in point 7-16 secondary cell group scheduling， informs SeNB that UE has completed the hando-ver/reconfiguration procedure by sending message 7-17 to SeNB， for example an SeNB Reconfiguration Complete message. Message 7-17 may indi-cate/instruct the SeNB to stop the data forwarding of the SCG bearer， if any such forwarding started in step 7-11. In the illustrated example， eNB2 further informs the mobility management entity MME that UE has changed cell by sending path switch request 7-19.

In response to message 7-17， SeNB starts in point 7-18 scheduling UE， and， as said above， stops in point 7-18 forwarding split bearer and/or mas-ter cell group bearer data towards eNB1 if the data forwarding has taken place in step 7-12.

MME confirms the path switch (message 7-19) by sending an end mark in message 7-20 to eNB1 which forwards the end mark to SeNB. Then the downlink data 7-21 via SeNB and the downlink data 7-21’ via eNB2 may be forwarded over dual connectivity to UE. Further， as in conventional handovers， MME acknowledges the path request to eNB2 by message 7-22， which then sends UE context release message 7-23 to eNB1. Message 7-23 informs eNB1 that the handover was successful and triggers release of resources in eNB1.

As is evident from the above， there is no need to send an UE con-text request message towards SeNB， since the dual connectivity part via SeNB is maintained， and there is no need to release UE context in SeNB， and re-establish it.

Referring to Figure 8， UE sends measurement report 8-1 to eNB1. Based on the measurement report and radio resource management infor-mation， eNB1 decides in point 8-2 to perform a handover to a second master evolved node eNB2 while maintaining the same SeNB. Therefore eNB1 sends handover request 8-3 containing measurement report or corresponding infor-mation， including information on SeNB to eNB2.

In response to handover request 8-3 containing the information on SeNB， eNB2 detects in point 8-4 that the handover is a dual connectivity handover targeting to maintain SeNB. Therefore SeNB addition procedure is performed by message 8-5a ( “SeNB addition request” ) from eNB2 to SeNB and message 8-5b ( “SeNB addition acknowledge” ) from SeNB， the message from SeNB providing secondary cell group configuration. Then eNB2 acknowl-edges the handover request by sending message 8-6 that contains the sec-ondary cell group configuration that was coordinated with SeNB by exchanging messages 8-5a and 8-5b. Owing to the reception of the secondary cell group configuration， eNB1 (source eNB) omits sending a UE Context Release mes-sage towards the secondary eNB (SeNB) after the successful handover to tar-get MeNB.

In response to the handover request acknowledgement 8-6 contain-ing the secondary cell group configuration， eNB1 detects in point 8-7 that the dual connectivity inter-master-eNBs with the same SeNB will succeed， sends in message 8-8 a handover command triggering a radio resource reconfigura-tion， or an RRC Connection Reconfiguration with secondary cell group configu-ration information to UE， and to SeNB in message 8-9 SeNB release request with an additional information indicating that the release request relates to an inter-master-handover maintaining the same secondary node. The additional information may， as explained above， be an instruction not to forward second-ary cell bearer data. Also in this example it is assumed that the additional in-formation instructs to buffer. However， it should be appreciated that the addi-tional information may provide other kind of instructions how to handle down-link data.

Although not illustrated in Figure 8， UE stops uplink transmissions to SeNB and eNB1.

In response to receiving SeNB release request 8-9， SeNB stops， in point 8-10， UE scheduling， i.e. providing user data to the UE. Further， SeNB may start to forward data for a split bearer and/or master cell group bearer (il-lustrated by dashed line messages 8-11) ， . Further， in response to the addition-al information， SeNB starts， in point 8-10， to buffer data for the secondary cell group bearer， as explained above. As said， split bearer data and/or master cell group data may be forwarded in messages 8-11 from SeNB via eNB1 to eNB2， eNB2 buffers in point 8-12 the forwarded downlink data.

Meanwhile UE performs (not illustrated in Figure 8) synchronisation to eNB2 and SeNB via random access channel (RACH) respectively， using information received in message 8-8. When UE has successfully accessed the target cell provided by eNB2， UE sends in message 8-13 a handover complete or RRC Connection Reconfiguration Complete to confirm the handover and SeNB addition. The buffered data， if any exists， can then be forwarded (mes-sage 8-11’ ) to UE.

In response to message 8-13， eNB2 informs SeNB that UE has completed the handover/reconfiguration procedure by sending reconfiguration complete message 8-14 to SeNB and informs the mobility management entity MME that UE has changed cell by sending path switch request 8-16.

In response to message 8-14， SeNB starts in point 8-15 scheduling UE.

MME confirms the path switch (message 8-16) by sending an end mark in message 8-15 to eNB1 which forwards the end mark to SeNB. Then the downlink data 8-18’ may be forwarded over dual connectivity to UE. Fur-ther， as in conventional handovers， MME acknowledges the path request to eNB2 by message 8-19， which then sends UE context release message 8-20 to eNB1. Message 8-20 informs eNB1 that the handover was successful and triggers release of resources in eNB1.

As is evident from the above， that there is no need to send an UE context request message towards SeNB， since the dual connectivity part via SeNB is maintained， and there is no need to release UE context in SeNB， and to re-establish it.

The steps/points， messages and related functions described above in Figures 2 to 8 are in no absolute chronological order， and some of the steps/points may be performed simultaneously or in an order differing from the given one. Other functions can also be executed between the steps/points or within the steps/points， and other messages sent. Some of the steps/points/messages or part of the steps/points/messages can also be left out or replaced by a corresponding step/point/message or part of the step/point/message. For example， a target master evolved node may be con-figured to accept a handover request from a source master evolved node with-out checking whether or not the secondary node is acceptable. In other words， step 302 and/or step 502 may be omitted. Another example is that the addi- tional information sent in message 7-9 and 8-9 to the secondary node is sent in a separate message.

The techniques described herein may be implemented by various means so that an apparatus/network node implementing one or more functions of a corresponding apparatus/network node described with an embodi-ment/example/implementation comprises not only prior art means， but also specific means for implementing the one or more functions of a corresponding apparatus described with an embodiment and it may comprise separate means for each separate function， or means may be configured to perform two or more functions. For example， the dual connectivity handover unit and/or the enhanced dual connectivity unit and/or algorithms may be software and/or software-hardware and/or hardware and/or firmware components (recorded indelibly on a medium such as read-only-memory or embodied in hard-wired computer circuitry) or combinations thereof. Software codes may be stored in any suitable， processor/computer-readable data storage medium (s) or memory unit (s) or article (s) of manufacture and executed by one or more proces-sors/computers， hardware (one or more apparatuses) ， firmware (one or more apparatuses) ， software (one or more modules) ， or combinations thereof. For a firmware or software， implementation can be through modules (e.g.， proce-dures， functions， and so on) that perform the functions described herein. Soft-ware codes may be stored in any suitable， processor/computer-readable data storage medium (s) or memory unit (s) or article (s) of manufacture and executed by one or more processors/computers.

Figure 9 is a simplified block diagram illustrating some units for an apparatus 900 configured to be a network apparatus (network node) compris-ing the dual connectivity decision unit， or corresponding functionality. In the illustrated example， the apparatus comprises an interface (IF) 901 for receiving and transmitting information over the radio access network， a processor 902 configured to implement at least the dual connectivity handover unit functionali-ty and/or the enhanced dual connectivity unit functionality or a corresponding sub-unit functionality， described herein， with corresponding algorithms 903， and memory 904 usable for storing a program code required for the dual con-nectivity handover unit and/or the enhanced dual connectivity unit， or a corre-sponding sub-unit， and the algorithms. The memory 904 is also usable for stor-ing other possible information， like information on configurations， and for buff-ering.

In other words， an apparatus configured to provide the network ap-paratus (network node) ， or an apparatus configured to provide one or more corresponding functionalities， is a computing device that may be any apparatus or device or equipment or node configured to perform one or more of corre-sponding apparatus functionalities described with an embodi-ment/example/implementation， and it may be configured to perform functionali-ties from different embodiments/examples/implementations. The dual connec-tivity handover unit and/or the enhanced dual connectivity unit， or a corre-sponding sub-unit， as well as other units， described above with an appa-ratus/network node may be separate units， even located in another physical apparatus， the distributed physical apparatuses forming one logical apparatus providing the functionality， or integrated to another unit in the same apparatus.

The apparatus configured to provide the network apparatus (net-work node) ， or an apparatus configured to provide one or more corresponding functionalities may generally include a processor， controller， control unit， micro-controller， or the like connected to a memory and to various interfaces of the apparatus. Generally the processor is a central processing unit， but the pro-cessor may be an additional operation processor. Each or some or one of the units/sub-units and/or algorithms described herein may be configured as a computer or a processor， or a microprocessor， such as a single-chip computer element， or as a chipset， including at least a memory for providing storage ar-ea used for arithmetic operation and an operation processor for executing the arithmetic operation. Each or some or one of the units/sub-units and/or algo-rithms described above may comprise one or more computer processors， ap-plication-specific integrated circuits (ASIC) ， digital signal processors (DSP) ， digital signal processing devices (DSPD) ， programmable logic devices (PLD) ， field-programmable gate arrays (FPGA) ， and/or other hardware components that have been programmed in such a way to carry out one or more functions of one or more embodiments/implementations/examples. In other words， each or some or one of the units/sub-units and/or the algorithms described above may be an element that comprises one or more arithmetic logic units， a num-ber of special registers and control circuits.

Further， the apparatus configured to provide the network apparatus (network node) ， or an apparatus configured to provide one or more corre-sponding functionalities may generally include volatile and/or non-volatile memory， for example EEPROM， ROM， PROM， RAM， DRAM， SRAM， double floating-gate field effect transistor， firmware， programmable logic， etc. and typi-cally store content， data， or the like. The memory or memories may be of any type (different from each other) ， have any possible storage structure and， if required， being managed by any database management system. The memory may also store computer program code such as software applications (for ex-ample， for one or more of the units/algorithms) or operating systems， infor-mation， data， content， or the like for the processor to perform steps associated with operation of the apparatus in accordance with examples/embodiments. The memory， or part of it， may be， for example， random access memory， a hard drive， or other fixed data memory or storage device implemented within the processor/apparatus or external to the processor/apparatus in which case it can be communicatively coupled to the processor/network node via various means as is known in the art. An example of an external memory includes a removable memory detachably connected to the apparatus.

The apparatus configured to provide the network apparatus (net-work node) ， or an apparatus configured to provide one or more corresponding functionalities may generally comprise different interface units， such as one or more receiving units and one or more sending units. The receiving unit and the transmitting unit each provides an interface in an apparatus， the interface in-cluding a transmitter and/or a receiver or any other means for receiving and/or transmitting information， and performing necessary functions so that the infor-mation， etc. can be received and/or sent. The receiving and sending units may comprise a set of antennas， the number of which is not limited to any particular number.

Further， the apparatus may comprise other units.

It will be obvious to a person skilled in the art that， as the technology advances， the inventive concept can be implemented in various ways. The in-vention and its embodiments are not limited to the examples described above but may vary within the scope of the claims.

Claims

A method comprising：

receiving in an unit in a target network node a handover request from a source network node， the handover request indicating that a user appa-ratus to be handed over is in dual connectivity with the source network node as a master node and a third network node as a secondary node；

determining， by the unit in the target network node， whether or not the third network node may remain as the secondary node also for the target network node as a master node； and

causing， by the unit in the target network node， the target network node to send to the handover request an acknowledgement with an indication indicating whether or not the third network node may remain as the secondary node also for the target network node.
A method as claimed in claim 1， further comprising， in response to the third network node remaining as the secondary node：

triggering a secondary node addition procedure between the target network node and the third network node.
A method as claimed in claim 2， wherein the indication comprises secondary cell group configuration determined during the secondary node ad-dition procedure.
A method as claimed in claim 1 or 2， wherein the indication is an information element conveying the information.
A method as claimed in claim 34， further comprising， in response to the third network remaining as the secondary node：

causing， by the unit in the target network node， in response to re-ceiving in the target network node from the user apparatus a message indicat-ing that the handover of the user apparatus to target network node has been completed， the target network node to send a radio resource configuration in-struction to the user apparatus.
A method as claimed in any preceding claim， further comprising，

in response to the third network node remaining as the secondary node：

causing， by the unit in the target network node， the target network node to buffer downlink data targeted to the user apparatus until a message indicating that the handover of the user apparatus to target network node has been completed is received from the user apparatus； and

causing， by the unit in the target network node， the target network node to forward buffered data to the user apparatus in response to the mes-sage.
A method as claimed in claim 6， further comprising：

causing， by the unit in the target network node， the target network node to instruct the third network node to stop forwarding downlink secondary cell group bearer data a targeted to the user apparatus in response to the message.
A method as claimed in any preceding claim， further comprising， in response to the third network node remaining as the secondary node：

causing， in response to receiving in the target network node infor-mation that the user apparatus has completed radio resource reconfiguration， by the unit in the target network node， the target network node to send to the source network node an instruction to release context of the user apparatus.
A method comprising

receiving from a target network node in an unit in a source network node an acknowledgement to a handover request relating to a user apparatus to be handed over from the source network node to the target network node， the user apparatus being in dual connectivity with the source network node as a master node and a third network node as a secondary node；

determining， by the unit in the source network node， whether or not the acknowledgement contains an indication indicating that the third network node remains as the secondary node also for the target network node as a master node； and

in response to the indication indicating that the third network node remains as the secondary node， the method further comprises：

causing， by the unit in the source network node， the source network node to send to the user apparatus a command for radio resource reconfigura-tion； and

causing， by the unit in the source network node， the source network to send to the third network node an instruction to release the source network node.
A method as claimed in claim 9， wherein the indication is an in-formation element conveying the information.
A method as claimed in claim 9， wherein the indication is sec-ondary cell group configuration， and in response to the indication indicating that the third network node remains as the secondary node， the method further comprises：

adding， by the unit in the source network node， to the command the secondary cell group configuration before causing the command to be sent.
A method as claimed in claim 9， 10， or 11， further comprising， in response to the indication indicating that the third network node remains as the secondary node：

adding， by the unit in the source network node， to the instruction to release the source network node an additional information indicating that the instruction relates to a handover between two master network nodes maintain-ing the secondary node before causing the instruction to be sent to the third network node.
A method as claimed in claim 9， 10， 11 or 12， further comprising， in a response to the indication indicating that the third network node remains as the secondary node， deciding not to transmit to the third network node a message releasing a user apparatus context in the third network node.
A method comprising：

serving， by a network node， a user apparatus as a secondary node in dual connectivity of the user apparatus；

receiving， in the network node， from a second network node that is a master node in the dual connectivity of the user apparatus， an instruction to release the second network node，

detecting， by an unit in the network node， that the instruction con-tains an indication indicating that the release relates to an inter-master node handover that maintains the network node as the secondary node；

handling， in response to the indication， data to be forwarded over a secondary cell group bearer， in a specific manner.
A method as claimed in claim 14， wherein the handling in a specific manner comprises buffering the data to be forwarded over a second-ary cell group bearer.
A method as claimed in claim 14 or 13， wherein the instruction defines the specific manner.
A network apparatus， comprising：

at least one processor， and

at least one memory for storing instructions to be executed by the processor， wherein

the at least one memory and the instructions are configured to， with the at least one processor， cause the apparatus at least to perform：

determining， in response to a handover request from a source net-work apparatus indicating that a user apparatus to be handed over to the net-work apparatus is in dual connectivity with the source network apparatus as a master node and a third network apparatus as a secondary node， whether or not the third network apparatus may remain as the secondary node also for the network apparatus node as a master node； and

causing sending to the handover request an acknowledgement with an indication indicating whether or not the third network apparatus may remain as the secondary node also for the target network node.
An apparatus as claimed in claim 17， wherein

the at least one memory and the instructions are configured to， with the at least one processor， further cause， in response to the third network ap-paratus remaining as the secondary node， the apparatus at least to trigger a secondary node addition procedure between the apparatus and the third net-work apparatus.
An apparatus as claimed in claim 18， wherein

the at least one memory and the instructions are configured to， with the at least one processor， further cause， in response to the third network ap-paratus remaining as the secondary node， the apparatus at least to perform：

using as the indication secondary cell group configuration deter-mined during the secondary node addition procedure.
An apparatus as claimed in claim 17 or 18， wherein

the at least one memory and the instructions are configured to， with the at least one processor， further cause， in response to the third network ap-paratus remaining as the secondary node， the apparatus at least to perform：

using an information element as the indication；

sending， in response to receiving from the user apparatus a mes-sage indicating that the handover of the user apparatus to apparatus has been completed， a radio resource configuration instruction to the user apparatus.
A network apparatus， comprising：

at least one processor， and

at least one memory for storing instructions to be executed by the processor， wherein

the at least one memory and the instructions are configured to， with the at least one processor， cause the apparatus at least to perform：

sending， in response to a handover decision to hand over a user apparatus， to a target network apparatus a handover request informing that the user apparatus is in dual connectivity with the network apparatus as a master node and with a third network apparatus as a secondary node；

determining， in response to receiving from the target network appa-ratus an acknowledgement to a handover request， whether or not the acknowledgement contains an indication indicating that the third network appa-ratus remains as the secondary node also for the target network apparatus as a master node；

sending， in response to the indication indicating that the third net-work apparatus remains as the secondary， to the user apparatus a command for radio resource reconfiguration； and

sending， in response to the indication indicating that the third net-work apparatus remains as the secondary to the third network apparatus an instruction to release the network apparatus.
An apparatus as claimed in claim 21， wherein

the at least one memory and the instructions are configured to， with the at least one processor， further cause， in response to the third network ap-paratus remaining as the secondarynode， the apparatus at least to perform：

adding， in response to the acknowledgement containing a second-ary cell group configuration， to the command the secondary cell group configu-ration before sending the command.
An apparatus as claimed in claim 21 or 22， wherein

the at least one memory and the instructions are configured to， with the at least one processor， further cause， in response to the third network ap-paratus remaining as the secondary node， the apparatus at least to perform：

adding to the instruction to release the network apparatus additional information indicating that the instruction relates to a handover between two master network nodes maintaining the secondary node before sending the in-struction.
A network apparatus， comprising：

at least one processor， and

at least one memory for storing instructions to be executed by the processor， wherein

the at least one memory and the instructions are configured to， with the at least one processor， cause the apparatus at least to perform：

detecting， in response to receiving from a second network appa-ratus that is a master node in a dual connectivity for which the network appa-ratus is a secondary node， an instruction to release the second network appa-ratus， that the instruction contains an indication indicating that the release re-lates to an inter-master node handover that maintains the network apparatus as the secondary node；

buffering， in response to the indication， data to be forwarded over a secondary cell group bearer， unless the instruction indicates another specific manner.
An apparatus comprising means for implementing a method as claimed in any of claims 1 to 8 and/or means for implementing a method as claimed in any of claims 8 to 13， and/or means for implementing a method as claimed in claims 14， 15 or 16.
A computer program product comprising program instructions configuring an apparatus to perform any of the steps of a method as claimed in any one of claims 1 to 16 when the computer program is run.
A computer program product embodied on a distribution medi-um readable by a computer and comprising program instructions which， when loaded into an apparatus， execute the method according to any of claims 1 to 16.