SOURCE ACCESS NODE, TARGET ACCESS NODE AND METHODS FOR ENHANCED HANDOVER

Abstract

Embodiments herein relate to, e.g., a method performed by a source access node relating to handover of a UE. The source access node sends to a target access node, an initial handover preparation message with a first explicit indicator for the target access node to request an enhanced Make-Before-Break Handover. The source access node receives an handover preparation response message from the target access node, with a second explicit indicator accepting or rejecting the requested enhanced Make-Before-Break Handover. Further, the source access node selects a possible fallback mechanism, upon reception of the handover preparation response message from the target access node indicating rejection or no support of enhanced Make-Before-Break Handover.

Claims

1. A method performed by a source access node relating to handover of a user equipment, UE, the method comprising: sending to a target access node, an initial handover preparation message for the target access node to request an enhanced Make-Before-Break Handover; receiving a handover preparation response message from the target access node accepting or rejecting the requested enhanced Make-Before-Break Handover; and selecting a possible fallback mechanism, upon reception of the handover preparation response message from the target access node.

2. The method according to claim 1, further comprising: notifying the target access node of a desired fallback in case of failure or reject of the requested enhanced Make-Before-Break Handover.

3. The method according to claim 1, further comprising: learning, from the handover preparation response message, a capability of the target access node related to enhanced Make-Before-Break handover.

4. The method according to claim 3, wherein the capability of the target access node is learned based on successful responses or failure responses of requested enhanced Make-Before-Break handovers.

5. The method according to claim 2, wherein selecting the possible fallback mechanism takes into account at least one of the desired fallback and the learned capability of the target access node.

6. The method according to claim 2, wherein the source access node notifies the target access node of the desired fallback, and wherein a first explicit indicator for Enhanced Make-Before-Break handover and an indicator for the desired fallback mechanism are combined in a single Information Element in the initial Handover Preparation message.

7. The method according to claim 6, wherein the first explicit indication is added as an additional information in the initial handover preparation message send from source to target.

8. The method according to claim 1, wherein the possible fallback mechanism comprises one or more of the following: a fallback to legacy handover; a fallback to release-14 Make-Before-Break handover; and a rejection of the handover request.

9. The method according to claim 1, further comprising: receiving a notification from the target access node of a selected possible fallback mechanism in case of rejection of the enhanced Make-Before-Break Handover request.

10. A method performed by a target access node relating to handover of a user equipment, UE, from a source access node to the target access node, the method comprising: receiving an initial handover preparation message with a first explicit indicator from the source access node requesting an enhanced Make-Before-Break Handover; and responding to the initial handover preparation message sent by the source access node, with a handover preparation response message accepting or rejecting the enhanced Make-Before-Break Handover request.

11. The method according to claim 10, further comprising: selecting a possible fallback mechanism in case of one of failure and rejection of the enhanced Make-Before-Break Handover; and notifying the source access node of the selected possible fallback mechanism in case of one of failure and rejection of the enhanced Make-Before-Break Handover request.

12. The method according to claim 11, responding with a handover preparation response message to the source access node rejecting the enhanced Make Before Break Handover request and an indicator of selected possible fallback method.

13. The method according to claim 10, wherein responding comprises inserting an explicit enhanced Make-Before-Break Handover indicator in the RRC HandoverCommand message transferred to the source access node.

14. The method according to claim 10, further comprising: receiving a notification from the source access node of a desired fallback in case of one of failure and rejection of the requested enhanced Make-Before-Break Handover.

15. A source access node for handling handover of a user equipment, UE, the source access node being configured to: send to a target access node, an initial handover preparation message for the target access node to request an enhanced Make-Before-Break Handover; receive a handover preparation response message from the target access node, with a second explicit indicator accepting or rejecting the requested enhanced Make-Before-Break Handover; select a possible fallback mechanism, upon reception of the handover preparation response message from the target access node.

16. The source access node according to claim 15, wherein the source access node is further configured to: notify the target access node of a desired fallback in case of one of failure and rejection of the requested enhanced Make-Before-Break Handover.

17. The source access node according to claim 15, wherein the source access node is further configured to: learn, from the handover preparation response message, a capability of the target access node related to enhanced Make-Before-Break handover.

18. The source access node according to claim 17, wherein the capability of the target access node is learned based on successful responses or failure responses of requested enhanced Make-Before-Break handovers.

19. The source access node according to claim 16, wherein the source access node is configured to select the possible fallback mechanism taking into account at least one of the desired fallback and the learned capability of the target access node.

20. The source access node according to claim 16, wherein the source access node is configured to notify the target access node of the desired fallback, and wherein the first explicit indicator for Enhanced Make-Before-Break handover and an indicator for the desired fallback mechanism is combined in a single Information Element in the initial Handover Preparation message.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0079] Examples of embodiments herein are described in more detail with reference to attached drawings in which:

[0080] FIG. 1 illustrates a wireless communication network according to prior art;

[0081] FIG. 2 illustrates handover signalling in 5G/NR;

[0082] FIG. 3 illustrates Make-before-break signalling LTE;

[0083] FIG. 4 illustrates a proposed Enhanced MBB procedure to support 0 ms HO interruption time in LTE Rel-16;

[0084] FIG. 5A illustrates a wireless communication network in which embodiments herein may be implemented;

[0085] FIG. 5B is a flow chart illustrating a method performed in a source access node according to one embodiment herein;

[0086] FIG. 5C is a flow chart illustrating a method performed in a source access node according to one embodiment herein;

[0087] FIG. 6A is a flow chart illustrating a method performed in a target access node according to another embodiment herein;

[0088] FIG. 6B is a flow chart illustrating a method performed in a target access node according to another embodiment herein;

[0089] FIG. 7A is a schematic block diagram illustrating one embodiment of a source access node; and

[0090] FIG. 7B is a schematic block diagram illustrating one embodiment of a target access node.

DETAILED DESCRIPTION

[0091] Intra-RAT (within same RAT), inter-RAT (between different RAT), NG-RAN, E-UTRAN and further disclaimers:

[0092] Most of the network and UE actions defined in embodiments herein are described as being performed in NG-RAN or E-UTRAN. In all these different flavors, the source access node and the target access node for which enhanced Make-Before-Break (eMBB) may be prepared may each be:

[0093] An E-UTRAN node, i.e. an eNodeB; An NG-RAN node, i.e. a gNodeB supporting NR; or a ng-eNodeB supporting LTE.

[0094] Then, the inter-node procedures described in embodiments herein may be between two eNodeBs, two gNodeBs, two ng-eNodeBs or any two RAN nodes from the same RAT or different RATs. Hence, they may be implemented in the XnAP protocol, in the case of NG-RAN nodes connected to 5GC, or X2AP protocol or both.

[0095] In other words, the discussions regarding the inter-node procedures and messages may be any of the following: [0096] Inter-node intra-RAT intra-system, such as gNodeBs over Xn; [0097] Inter-node intra-RAT intra-system, such as ng-eNodeBs over Xn; [0098] Inter-node intra-RAT intra-system, such as eNodeBs over X2; [0099] Inter-node inter-RAT intra-system, such as ng-eNodeBs and gNodeBs over Xn; [0100] Inter-node inter-RAT inter-system, such as E-UTRAN and NG-RAN, i.e. gNodeBs/en-eNodeBs and eNodeBs over NG and S1.

[0101] In addition, the procedures also describe solutions involving messages between RAN nodes and core network nodes over NG and S1 interface and between core network nodes from different systems, i.e. between EPC and 5GC, over the N26 interface.

[0102] A wireless communication system according to embodiments herein is illustrated in FIG. 5A. The wireless communication system may comprise one or more radio access network (RAN), where a radio access network 100 is shown with a user equipment (UE) 102, which communicates with one or multiple access nodes 103-104, using radio connections 107-108. The access nodes 103-104 are connected to a core network (CN) node 106. The access nodes 103-104 are part of the radio access network 100. A source access node 103 and a target access node 104. Embodiments herein relate to eMBB handover of the UE 102 from the source access node 103 to the target access node 104.

[0103] The method actions performed by the source access node 103 for handling HO of the UE 102 in the communication network 1 according to embodiments will now be described with reference to a flowchart depicted in FIG. 5B. The actions do not have to be taken in the order stated below, but may be taken in any suitable order. Actions performed in some embodiments are marked with dashed boxes.

[0104] Action 501. The source access node 103 sends to the target access node 104, the initial handover preparation message, e.g. a handover request, with a first explicit indicator for the target access node 104 to request eMBB handover. The first explicit indication may be added as an additional information in the initial handover preparation message sent from the source access node 103 to the target access node 104. The first explicit indicator may be included in an radio resource control (RRC) context signaled from the source access node 103 to the target access node 104. The first explicit indicator may be added in a handover request message.

[0105] Action 502. The source access node 103 then receives a handover preparation response message from the target access node 104, with a second explicit indicator accepting or rejecting the requested eMBB handover.

[0106] Action 503. The source access node 103 may learn, from the handover preparation response message, a capability of the target access node 104 related to enhanced Make-Before-Break handover. The capability of the target access node 104 may be learned based on successful responses or failure responses of requested enhanced Make-Before-Break handovers.

[0107] Action 504. The source access node 103 may further notify the target access node 104 of a desired fallback in case of failure or reject of the requested enhanced Make-Before-Break Handover. It should be noted that the first explicit indicator for Enhanced Make-Before-Break handover and an indicator for the desired fallback mechanism may be combined in a single Information Element in the initial Handover Preparation message in action 501.

[0108] Action 505. The source access node 103 may receive a notification from the target access node 104 of a selected possible fallback mechanism in case of rejection of the enhanced Make-Before-Break Handover request.

[0109] Action 506. The source access node 103 then selects a possible fallback mechanism, upon reception of the handover preparation response message from the target access node 104 indicating rejection or no support of eMBB Handover. The source access node 103 may select the possible fallback mechanism by taking the desired fallback into account and/or the learned capability of the target access node 104. E.g. the source access node 103 may take the desired fallback into account, which desired fallback may be based on the learned capability of the target access node 104. The possible and/or the desired fallback may comprise one or more of the following: a fallback to legacy handover; a fallback to release-14 MBB handover; and a rejection of the handover request. The source access node 103 may select the possible fallback mechanism by taking the notification from the target access node 104 of a selected possible fallback into account. Thus, embodiments herein may disclose that in response to receiving the Handover Preparation response message, the source access node 103 may take a decision, based on the Handover Preparation response message, about the possible fallback method upon reception of the target access node 104 rejecting eMBB.

[0110] Methods according to embodiments herein for enhanced handover will be described in detail in the following.

I. Source Access node 103 Signalling in Initial Handover Preparation Message:

[0111] According to an exemplified embodiment herein, a method performed in the source access node 103 will be described with reference to FIG. 5C. The method comprises the following actions or steps, which actions may be performed in any suitable order.

[0112] Action 511. The source access node 103 sends the initial Handover Preparation message with the first explicit indicator, e.g. Enhanced Make-Before-Break Indicator, for the target access node 104 to request an enhanced Make-Before-Break Handover.

[0113] In one implementation the first explicit indicator may be added as an additional information in the initial Handover Preparation message sent from source to target.

[0114] In another implementation the first explicit indicator may be included in the RRC Context signaled from source to target.

[0115] An example of a possible implementation of the first alternative above, taking XnAP (3GPP TS 38.423 v15.0.0) HANDOVER REQUEST message as baseline is shown below indicated by the underlined text:

9.1.1.1 Handover Request

[0116] This message is sent by the source NG-RAN node to the target NG-RAN node to request the preparation of resources for a handover.
Direction: source NG-RAN node.fwdarw.target NG-RAN node

TABLE-US-00007 IE type and IE/Group Name Presence Range reference Semantics description Message Type M 9.2.3.1 Source NG-RAN node UE M NG-RAN Allocated at the source NG- XnAP ID reference node UE RAN node XnAP ID 9.2.3.16 Cause M 9.2.3.2 Target Cell Global ID M 9.2.3.25 Includes either an E-UTRA CGI or an NR CGI GUAMI M 9.2.3.24 UE Context Information 1 >NG-C UE associated M AMF UE Allocated at the AMF on the Signalling reference NGAP ID source NG-C connection. 9.2.3.26 >Signalling TNL association M CP This IE indicates the AMF's IP address at source NG-C Transport address of the SCTP side Layer association used at the source Information NG-C interface instance. 9.2.3.31 >UE Security Capabilities M 9.2.3.49 >AS Security Information M 9.2.3.50 >Index to RAT/Frequency O 9.2.3.23 Selection Priority >UE Aggregate Maximum M 9.2.3.17 Bit Rate >PDU Session Resources 1 9.2.1.1 Similar to NG-C signalling, To Be Setup List containing UL tunnel information per PDU Session Resource; and in addition, the source side QoS flow DRB mapping >RRC Context M OCTET Either includes the STRING HandoverPreparationInformation message as defined in subclause 10.2.2. of TS 36.331 [14], if the target NG-RAN node is an ng-eNB, or the HandoverPreparationInformation message as defined in subclause 11.2.2 of TS 38.331 [10], if the target NG-RAN node is a gNB. >Location Reporting O 9.2.3.47 Includes the necessary Information parameters for location reporting. >Mobility Restriction List O 9.2.3.53 Trace Activation O 9.2.3.55 Masked IMEISV O 9.2.3.32 UE History Information M 9.2.3.64 UE Context Reference at O the S-NG-RAN node >Global NG-RAN Node ID M 9.2.2.3 >S-NG-RAN node UE M NG-RAN XnAP ID node UE XnAP ID 9.2.3.16 Enhanced Make-Before- O ENUMERATED Break Indicator (eMBB required, ... )

[0117] In one embodiment, the source access node 103 may notify the target access node 104 of a desired fallback in case the target access node 104 rejects the enhanced Make-Before-Break Handover (eMBB) request.

[0118] In one embodiment, the source access node 103 may take a decision about the desired fallback mechanism if the target access node 104 rejects the enhanced Make-Before-Break request. This decision may be based on e.g. measurements, quality of service (QoS), UE capabilities, network capabilities, subscription, etc. An example of the possible fallback methods are listed below: [0119] Fallback to legacy handover; [0120] Fallback to release-14 MBB handover; [0121] Reject the Handover Request.

[0122] In another embodiment, the source access node 103 may inform the target access node 104 of the desired fallback mechanism, via e.g. additional information in the initial Handover Preparation message.

[0123] In another embodiment, the indicator for Enhanced Make-Before-Break i.e. the first explicit indicator, and the indicator for desired fallback mechanism may be combined in a single Information Element in the initial Handover Preparation message.

[0124] An example of a possible implementation of source access node 103 informing the target access node 104, in an IE Enhanced Make-Before-Break Information, of desired fallback mechanism together with the acceptance of the Enhanced Make-Before-Break request, taking XnAP (3GPP TS 38.423 v.15.0.0) HANDOVER REQUEST message as baseline, is shown below indicated by the underlined text:

9.1.1.1 Handover Request

[0125] This message is sent by the source NG-RAN node to the target NG-RAN node to request the preparation of resources for a handover.
Direction: source NG-RAN node.fwdarw.target NG-RAN node.

TABLE-US-00008 IE type and IE/Group Name Presence Range reference Semantics description Message Type M 9.2.3.1 Source NG-RAN node UE M NG-RAN Allocated at the source NG- XnAP ID reference node UE RAN node XnAP ID 9.2.3.16 Cause M 9.2.3.2 Target Cell Global ID M 9.2.3.25 Includes either an E-UTRA CGI or an NR CGI GUAMI M 9.2.3.24 UE Context Information 1 >NG-C UE associated M AMF UE Allocated at the AMF on the Signalling reference NGAP ID source NG-C connection. 9.2.3.26 >Signalling TNL association M CP This IE indicates the AMF's IP address at source NG-C Transport address of the SCTP side Layer association used at the source Information NG-C interface instance. 9.2.3.31 >UE Security Capabilities M 9.2.3.49 >AS Security Information M 9.2.3.50 >Index to RAT/Frequency O 9.2.3.23 Selection Priority >UE Aggregate Maximum M 9.2.3.17 Bit Rate >PDU Session Resources 1 9.2.1.1 Similar to NG-C signalling, To Be Setup List containing UL tunnel information per PDU Session Resource; and in addition, the source side QoS flow DRB mapping >RRC Context M OCTET Either includes the STRING HandoverPreparationInformation message as defined in subclause 10.2.2. of TS 36.331 [14], if the target NG-RAN node is an ng-eNB, or the HandoverPreparationInformation message as defined in subclause 11.2.2 of TS 38.331 [10], if the target NG-RAN node is a gNB. >Location Reporting O 9.2.3.47 Includes the necessary Information parameters for location reporting. >Mobility Restriction List O 9.2.3.53 Trace Activation O 9.2.3.55 Masked IMEISV O 9.2.3.32 UE History Information M 9.2.3.64 UE Context Reference at O the S-NG-RAN node >Global NG-RAN Node ID M 9.2.2.3 >S-NG-RAN node UE M NG-RAN XnAP ID node UE XnAP ID 9.2.3.16 Enhanced Make-Before- O Break Information > Enhanced Make-Before- M ENUMERATED Break Information (eMBB required, ...) > Desired fallback method O ENUMERATED (legacy HO, rel14 MBB, reject, ...)

[0126] Action 512. The source access node 103 receives and processes a Handover Preparation response message from the target access node 104, with an explicit indicator, the second explicit indicator, accepting or rejecting the enhanced Make-Before-Break Handover request.

[0127] Action 513. In one embodiment, the source access node 103 may take the decision about the possible fallback method as described in Action 506, after receiving the target access node response to the initial Handover Preparation message. In that case the source access node 103 may make a choice between:

Accepting the Fallback Method Suggested by the Target Access Node 104;

[0128] Cancelling the handover procedure, e.g. using HANDOVER CANCEL message, and starting a new Handover procedure, with the same node, e.g. without eMBB indicator or with a different node.

[0129] The method actions performed by the target access node 104 handling handover of the UE from the source access node 103 to the target access node 104 according to embodiments herein will now be described with reference to a flowchart depicted in FIG. 6A. The actions do not have to be taken in the order stated below, but may be taken in any suitable order. Actions performed in some embodiments are marked with dashed boxes.

[0130] Action 601. The target access node 104 receives the initial handover preparation message with the first explicit indicator from the source access node 103 requesting an enhanced Make-Before-Break Handover.

[0131] Action 602. The target access node 104 may receive the notification from the source access node 103 of a desired fallback in case of failure or reject of the requested enhanced Make-Before-Break Handover.

[0132] Action 603. The target access node 104 may further select the possible fallback mechanism in case of failure or rejection of the enhanced Make-Before-Break Handover. E.g. selection of fallback solution in the target access node 104 may be based on the desired fallback included in the handover preparation message.

[0133] Action 604. The target access node 104 may then notify the source access node 103 of the selected possible fallback mechanism in case of failure or rejection of the enhanced Make-Before-Break Handover request.

[0134] Action 605. The target access node 104 responds to the initial handover preparation message sent by the source access node 103, with the handover preparation response message with the second explicit indicator accepting or rejecting the enhanced Make-Before-Break Handover request. The target access node 104 may respond with a handover preparation response message to the source access node 103 with the second indicator rejecting the enhanced Make Before Break Handover request and an indicator of selected possible fallback method. The target access node 104 may respond by inserting an explicit enhanced Make-Before-Break Handover indicator in the RRC HandoverCommand message transferred to the source access node 103. The target access node 104 may select the possible fallback mechanism by taking the received notification from the source access node 103 of the desired fallback into account.

II. Handover Preparation Response from the Target Access Node 104.

[0135] According to one exemplified embodiment herein, a method performed in the target access node 104 will be described with reference to FIG. 6B. The method comprises the following actions or steps, which actions may be performed in any suitable order.

[0136] Action 611. The target access node 104 receives and processes the initial Handover Preparation message with the explicit indicator, i.e. the first explicit indicator, from the source access node 103 requesting an enhanced Make-Before-Break Handover.

[0137] Action 612. The target access node 104 may then take a decision about the enhanced Make-Before-Break request, e.g. accept, fallback or reject. This decision may be based on e.g. QoS, network capabilities, available resources, configuration, etc. The target access node's decision may be as follows: [0138] Enhanced Make-Before-Break handover accepted; [0139] Fallback to legacy handover; [0140] Fallback to rel-14 Make-Before-Break handover; [0141] Handover rejected.

[0142] Action 613. If the target access node 104 accepts the eMBB handover, it sends Handover Preparation Response message to the source access node 103 with the second explicit indicator accepting the enhanced Make Before Break Handover request.

[0143] The target access node 104 may inform the source access node 103 of its decision concerning the enhanced Make-Before-Break request via e.g. additional information in the Handover Preparation response message.

[0144] For successful or partial success, e.g. when fallback mechanism is used, an example of a possible implementation taking XnAP (3GPP TS 38.423 v.15.0.0) HANDOVER REQUEST ACKNOWLEDGE message as baseline, adding an IE Enhanced Make-Before-Break indicator as indicated with underlined text:

9.1.1.2 Handover Request Acknowledge

[0145] This message is sent by the target NG-RAN node to inform the source NG-RAN node about the prepared resources at the target.
Direction: target NG-RAN node.fwdarw.source NG-RAN node.

TABLE-US-00009 IE type and IE/Group Name Presence Range reference Semantics description Message Type M 9.2.3.1 Source NG-RAN node UE M NG-RAN node Allocated at the source NG-RAN XnAP ID UE XnAP ID node 9.2.3.16 Target NG-RAN node UE M NG-RAN node Allocated at the target NG-RAN XnAP ID UE XnAP ID node 9.2.3.16 PDU Session Resources M 9.2.1.2 Admitted List PDU Session Resources Not O 9.2.1.3 Admitted List Target NG-RAN node To M OCTET Either includes the Source NG-RAN node STRING HandoverCommand message as Transparent Container defined in subclause 10.2.2 of TS 36.331 [14], if the target NG-RAN node is an ng-eNB, or the HandoverCommand message as defined in subclause 11.2.2 of TS 38.331 [10], if the target NG-RAN node is a gNB. UE Context Kept Indicator O 9.2.3.68 Criticality Diagnostics O 9.2.3.3 Enhanced Make-Before- O ENUMERATED Break indicator (eMBB accepted, fallback to legacy HO, fallback to rel14 MBB)

[0146] Action 614. If the target access node 104 rejects the eMBB handover, the target access node 104 may select a possible fallback method.

[0147] For handover failure, e.g. when the target access node 104 rejects the HO and no fallback method is used, an example of a possible implementation may be taking XnAP (3GPP TS 38.423 v.15.0.0) HANDOVER PREPARATION FAILURE message as baseline and using a new cause value eMBB rejected as indicated with underlined text:

9.1.1.3 Handover Preparation Failure

[0148] This message is sent by the target NG-RAN node to inform the source NG-RAN node that the Handover Preparation has failed.
Direction: target NG-RAN node.fwdarw.source NG-RAN node.

TABLE-US-00010 IE/Group IE type and Semantics Name Presence Range reference description Message Type M 9.2.3.1 Source NG-RAN node UE M NG-RAN Allocated at the XnAP ID node UE source NG-RAN XnAP ID node 9.2.3.16 Cause M 9.2.3.2 Criticality Diagnostics O 9.2.3.3

9.2.3.2 Cause

[0149] The purpose of the Cause IE is to indicate the reason for a particular event for the XnAP protocol.

TABLE-US-00011 Semantics IE/Group Name Presence Range IE Type and Reference Description CHOICE Cause M Group >Radio Network Layer >>Radio M ENUMERATED Network ( Layer Cause Cell not Available, Handover Desirable for Radio Reasons, Handover Target not Allowed, Invalid AMF Set ID, No Radio Resources Available in Target Cell, Partial Handover, Reduce Load in Serving Cell, Resource Optimisation Handover, Time Critical Handover, TXn.sub.RELOCoverall Expiry, TXn.sub.RELOCprep Expiry, Unknown GUAMI ID, Unknown Local NG-RAN node UE XnAP ID, Inconsistent Remote NG-RAN node UE XnAP ID, Encryption And/Or Integrity Protection Algorithms Not Supported, Protection Algorithms Not Supported, Multiple PDU Session ID Instances, Unknown PDU Session ID, Unknown QoS Flow ID, Multiple QoS Flow ID Instances, Switch Off Ongoing, Not supported 5QI value, TXn.sub.DCoverall Expiry, TXn.sub.DCprep Expiry, Action Desirable for Radio Reasons, Reduce Load, Resource Optimisation, Time Critical action, Target not Allowed, No Radio Resources Available, Invalid QoS combination, Encryption Algorithms Not Supported, Procedure cancelled, RRM purpose, Improve User Bit Rate, User Inactivity, Radio Connection With UE Lost, Failure in the Radio Interface Procedure, Bearer Option not Supported, UP integrity protection not possible, UP confidentiality protection not possible, Resources not available for the slice(s), UE Maximum integrity protected data rate reason, CP Integrity Protection Failure, UP Integrity Protection Failure, Slice not supported by NG-RAN, MN Mobility, SN Mobility, Count reaches max value, Unknown Old NG-RAN node UE XnAP ID, PDCP Overload, DRB ID not available, eMBB rejected, Unspecified, . . . ) >Transport Layer >>Transport M ENUMERATED Layer Cause (Transport Resource Unavailable, Unspecified, . . . ) >Protocol >>Protocol M ENUMERATED Cause (Transfer Syntax Error, Abstract Syntax Error (Reject), Abstract Syntax Error (Ignore and Notify), Message not Compatible with Receiver State, Semantic Error, Abstract Syntax Error (Falsely Constructed Message), Unspecified, . . . ) >Misc >>Miscellaneous M ENUMERATED Cause (Control Processing Overload, Hardware Failure, O&M Intervention, Not enough User Plane Processing Resources, Unspecified, . . . )

[0150] The meaning of the different cause values is specified in the following table. In general, not supported cause values indicate that the related capability is missing. On the other hand, not available cause values indicate that the related capability is present, but insufficient resources were available to perform the requested action.

TABLE-US-00012 Radio Network Layer cause Meaning Cell not Available The concerned cell is not available. Handover Desirable for Radio The reason for requesting handover is radio related. Reasons Handover Target not Allowed Handover to the indicated target cell is not allowed for the UE in question. Invalid AMF Set ID The target NG-RAN node doesn't belong to the same AMF Set of the source NG-RAN node, i.e. NG handovers should be attempted instead. No Radio Resources Available in The target cell doesn't have sufficient radio resources Target Cell available. Partial Handover Provides a reason for the handover cancellation. The target NG-RAN node did not admit all PDU Sessions included in the HANDOVER REQUEST and the source NG-RAN node estimated service continuity for the UE would be better by not proceeding with handover towards this particular target NG- RAN node. Reduce Load in Serving Cell Load in serving cell needs to be reduced. When applied to handover preparation, it indicates the handover is triggered due to load balancing. Resource Optimisation Handover The reason for requesting handover is to improve the load distribution with the neighbour cells. Time Critical Handover Handover is requested for time critical reason i.e. this cause value is reserved to represent all critical cases where the connection is likely to be dropped if handover is not performed. TXn.sub.RELOCoverall Expiry The reason for the action is expiry of timer TXn.sub.RELOCoverall. TXn.sub.RELOCprep Expiry Handover Preparation procedure is cancelled when timer TXn.sub.RELOCprep expires. Unknown GUAMI ID The target NG-RAN node belongs to the same AMF Set of the source NG-RAN node and recognizes the AMF Set ID. However, the GUAMI value is unknown to the target NG-RAN node. Unknown Local NG-RAN node UE The action failed because the receiving NG-RAN node does XnAP ID not recognise the local NG-RAN node UE XnAP ID. Inconsistent Remote NG-RAN The action failed because the receiving NG-RAN node node UE XnAP ID considers that the received remote NG-RAN node UE XnAP ID is inconsistent. Encryption And/Or Integrity The target NG-RAN node is unable to support any of the Protection Algorithms Not encryption and/or integrity protection algorithms supported by Supported the UE. Multiple PDU Session ID The action failed because multiple instances of the same PDU Instances Session had been provided to the NG-RAN node. Unknown PDU Session ID The action failed because the PDU Session ID is unknown in the NG-RAN node. Unknown QoS Flow ID The action failed because the QoS Flow ID is unknown in the NG-RAN node. Multiple QoS Flow ID Instances The action failed because multiple instances of the same QoS flow had been provided to the NG-RAN node. Switch Off Ongoing The reason for the action is an ongoing switch off i.e. the concerned cell will be switched off after offloading and not be available. It aides the receiving NG-RAN node in taking subsequent actions, e.g. selecting the target cell for subsequent handovers. Not supported 5QI value The action failed because the requested 5QI is not supported. TXnD.sub.Coverall Expiry The reason for the action is expiry of timer TXn.sub.DCoverall. TXn.sub.DCprep Expiry The reason for the action is expiry of timer TXn.sub.DCprep Action Desirable for Radio The reason for requesting the action is radio related. Reasons In the current version of this specification applicable for Dual Connectivity only. Reduce Load Load in the cell(group) served by the requesting node needs to be reduced. In the current version of this specification applicable for Dual Connectivity only. Resource Optimisation The reason for requesting this action is to improve the load distribution with the neighbour cells. In the current version of this specification applicable for Dual Connectivity only. Time Critical action The action is requested for time critical reason i.e. this cause value is reserved to represent all critical cases where radio resources are likely to be dropped if the requested action is not performed. In the current version of this specification applicable for Dual Connectivity only. Target not Allowed Requested action towards the indicated target cell is not allowed for the UE in question. In the current version of this specification applicable for Dual Connectivity only. No Radio Resources Available The cell(s) in the requested node don't have sufficient radio resources available. In the current version of this specification applicable for Dual Connectivity only. Invalid QoS combination The action was failed because of invalid QoS combination. In the current version of this specification applicable for Dual Connectivity only. Encryption Algorithms Not The requested NG-RAN node is unable to support any of the Supported encryption algorithms supported by the UE. In the current version of this specification applicable for Dual Connectivity only. Procedure cancelled The sending node cancelled the procedure due to other urgent actions to be performed. In the current version of this specification applicable for Dual Connectivity only. RRM purpose The procedure is initiated due to node internal RRM purposes. In the current version of this specification applicable for Dual Connectivity only. Improve User Bit Rate The reason for requesting this action is to improve the user bit rate. In the current version of this specification applicable for Dual Connectivity only. User Inactivity The action is requested due to user inactivity on all PDU Sessions. The action may be performed on several levels: - on UE Context level, if NG is requested to be released in order to optimise the radio resources; or S-NG-RAN node didn't see activity on the PDU session recently. - on PDU Session Resource or DRB or QoS flow level, e.g. if Activity Notification indicate lack of activity In the current version of this specification applicable for Dual Connectivity only. Radio Connection With UE Lost The action is requested due to losing the radio connection to the UE. In the current version of this specification applicable for Dual Connectivity only. Failure in the Radio Interface Radio interface procedure has failed. Procedure In the current version of this specification applicable for Dual Connectivity only. Bearer Option not Supported The requested bearer option is not supported by the sending node. In the current version of this specification applicable for Dual Connectivity only. UP integrity protection not The PDU session cannot be accepted according to the possible required user plane integrity protection policy. UP confidentiality protection not The PDU session cannot be accepted according to the possible required user plane confidentiality protection policy. Resources not available for the The requested resources are not available for the slice(s). slice(s) UE Maximum integrity protected The request is not accepted in order to comply with the data rate reason maximum data rate for integrity protection supported by the UE. CP Integrity Protection Failure The request is not accepted due to failed control plane integrity protection. UP Integrity Protection Failure The procedure is initiated because the SN (hosting node) detected an Integrity Protection failure in the UL PDU coming from the MN. Slice not supported by NG-RAN The PDU session cannot be accepted because the slice is not supported by the NG-RAN node. MN Mobility The procedure is initiated due to relocation of the M-NG-RAN node UE context. SN Mobility The procedure is initiated due to relocation of the S-NG-RAN node UE context. Count reaches max value, Indicates the PDCP COUNT for UL or DL reached the max value and the bearer may be released. Unknown Old NG-RAN node UE The action failed because the Old NG-RAN node UE XnAP ID XnAP ID or the S-NG-RAN node UE XnAP ID is unknown. PDCP Overload The procedure is initiated due to PDCP resource limitation. DRB ID not available The action failed because the M-NG-RAN node is not able to provide additional DRB IDs to the S-NG-RAN node. eMBB rejected The action failed because the enhanced Make-Before- Break request has been rejected. Unspecified Sent for radio network layer cause when none of the specified cause values applies.

TABLE-US-00013 Transport Layer cause Meaning Unspecified Sent when none of the above cause values applies but still the cause is Transport Network Layer related.

TABLE-US-00014 NAS cause Meaning Unspecified Sent when none of the above cause values applies but still the cause is NAS related.

TABLE-US-00015 Protocol cause Meaning Transfer Syntax The received message included a transfer Error syntax error. Abstract Syntax The received message included an abstract Error (Reject) syntax error and the concerning criticality indicated reject. Abstract Syntax The received message included an abstract syntax Error error and the concerning criticality indicated (Ignore And Notify) ignore and notify. Message Not The received message was not compatible with Compatible the receiver state. With Receiver State Semantic Error The received message included a semantic error. Abstract Syntax The received message contained IEs or IE Error groups in wrong order or with too many (Falsely occurrences. Constructed Message) Unspecified Sent when none of the above cause values applies but still the cause is Protocol related.

TABLE-US-00016 Miscellaneous cause Meaning Control Processing Overload NG-RAN node control processing overload. Hardware Failure NG-RAN node hardware failure. Not enough User Plane NG-RAN node has insufficient user plane Processing Resources processing resources available. O&M Intervention Operation and Maintenance intervention related to NG-RAN node equipment. Unspecified Sent when none of the above cause values applies and the cause is not related to any of the categories Radio Network Layer, Transport Network Layer or Protocol.

[0151] Action 615. In one embodiment, the target access node 104 sends Handover Preparation Response message to the source access node 103 with the second explicit indicator rejecting the enhanced Make Before Break Handover request and an indicator of selected possible fallback method.

[0152] In one embodiment, the target access node 104 not supporting eMBB may send an Handover Request Failure with an existing cause value, e.g. Abstract Syntax Error (Reject).

[0153] In one embodiment, the source access node 103 receives and may process the above Handover Preparation response message from the target access node 104, with the second explicit indicator accepting or rejecting the enhanced Make-Before-Break Handover request.

III. Target Access Node eMBB Capability Learning in the Source Access Node 103.

[0154] Referring to action 503 in FIG. 5B, in one embodiment, dependent of the embodiments described above, the source access node 103 may learn about the target access node eMBB capability according to the response from the target access node 104.

[0155] If the source access node 103 receives a successful response without any additional eMBB information, it may deduce that the target access node 104 does not support eMBB.

[0156] If the source access node 103 receives a successful response with additional eMBB information, e.g. eMBB accepted or desired fallback, it may deduce that the target access node 104 supports eMBB.

[0157] If the source access node 103 receives a failure response with additional eMBB information, e.g. cause value=eMBB not accepted, it may deduce that the target access node 104 supports eMBB.

[0158] This information may be stored in the source access node 103 in order to take a decision concerning a subsequent handover to the same target access node 104 for the same or a different UE.

IV. Enhanced Make-Before-Break Handover Indicator in the RRC Handover Command Being a Command Message Transferred to the Source Access Node 103

[0159] In one embodiment, in case the target access node 104 accepts the enhanced Make-Before-Request request from the source access node 103, the target access node 104 may insert the second indicator exemplified as an enhanced Make-Before-Break indicator in the RRC HandoverCommand message corresponding to the RRCConnectionReconfiguration message with an MobilityControlInfo IE in LTE and an RRCReconfiguration message with an reconfigurationWithSync IE in NR, to signal the UE that it should perform an enhanced Make-Before-Break handover.

[0160] An ASN.1 example for LTE is given below, where an enhancedMakeBeforeBreak-r16 IE (underlined) is added in the MobilityControlInfo contained in the RRCConnectionReconfiguration message for LTE:

TABLE-US-00017 MobilityControlInfo ::= SEQUENCE { targetPhysCellId PhysCellId, carrierFreq CarrierFreqEUTRA OPTIONAL, -- Cond HO-toEUTRA2 carrierBandwidth CarrierBandwidthEUTRA OPTIONAL, -- Cond HO-toEUTRA additionalSpectrumEmission AdditionalSpectrumEmission OPTIONAL, -- Cond HO-toEUTRA t304 ENUMERATED { ms50, ms100, ms150, ms200, ms500, ms1000, ms2000, ms10000-v1310}, newUE-Identity C-RNTI, radioResourceConfigCommon RadioResourceConfigCommon, rach-ConfigDedicated RACH-ConfigDedicated OPTIONAL, -- Need OP ..., [[ carrierFreq-v9e0 CarrierFreqEUTRA-v9e0 OPTIONAL -- Need ON ]], [[ drb-ContinueROHC-r11 ENUMERATED {true} OPTIONAL -- Cond HO ]], [[ mobilityControlInfoV2X-r14 MobilityControlInfoV2X-r14 OPTIONAL, -- Need ON handoverWithoutWT-Change-r14 ENUMERATED {keepLWA-Config, sendEndMarker} OPTIONAL,-- Cond HO makeBeforeBreak-r14 ENUMERATED {true} OPTIONAL, -- Need OR enhancedMakeBeforeBreak-r16 ENUMERATED {true} OPTIONAL, -- Need OR rach-Skip-r14 RACH-Skip-r14 OPTIONAL, -- Need OR sameSFN-Indication-r14 ENUMERATED {true} OPTIONAL -- Cond HO-SFNsynced ]], [[ mib-RepetitionStatus-r14 BOOLEAN OPTIONAL, -- Need OR OPTIONAL -- schedulingInfoSIB1-BR-r14 INTEGER (0..31) Cond HO-SFNsynced ]] }

[0161] An example for NR is given below, where an enhancedMakeBeforeBreak-r16 IE (underlined) is added in the reconfigurationWithSync IE included in the RRCReconfiguration message in NR:

TABLE-US-00018 ReconfigurationWithSync ::= SEQUENCE { spCellConfigCommon ServingCellConfigCommon OPTIONAL,-- Need M newUE-Identity RNTI-Value, t304 ENUMERATED {ms50, ms100, ms150, ms200, ms500, ms1000, ms2000, ms10000}, rach-ConfigDedicated CHOICE { uplink RACH-ConfigDedicated, supplementaryUplink RACH-ConfigDedicated } OPTIONAL,-- Need N enhancedMakeBeforeBreak-r16 ENUMERATED {true} OPTIONAL, -- Need OR ..., [[ smtc SSB-MTC OPTIONAL-- Need S ]] }
V. Fallback to Release-14 MBB when Target Access Node 104 does not Support eMBB.

[0162] In one embodiment the source access node 103 may combine the signaling method used for release-14 Make-Before-Break, i.e. adding makeBeforeBreak-r14 indicator, in RRC HandoverPreparation message, and the new indicator defined in action 501 i.e. the first explicit indicator. If the target access node 104 supports eMBB, and accepts the eMBB request, it will insert the RRC eMBB indicator in the RRC HandoverCommand message as defined in Action 605. If the target access node 104 supports eMBB, and rejects the eMBB request, it will insert the RRC release-14 MBB indicator in the RRC HandoverCommand message. If the target access node 104 does not support eMBB but supports release-14 MBB, it will insert the release-14 RRC MBB indicator in the RRC HandoverCommand message.

VI. Possible Source Access Node 103 and Target Access Node 104 Actions for Enhanced Make-Before-Break Preparation.

[0163] The embodiment herein is defining the possible source and target access nodes actions, related to network signaling, for the enhanced Make-Before-Break preparation. These actions are defined according to the different possible scenarios. The different possible scenarios are a combination of the following criteria/events: [0164] Target access node eMBB support and decision upon reception of an eMBB request from the source access node 103: [0165] I. Target access node 104 supports eMBB and accepts eMBB request; [0166] II. Target access node 104 supports eMBB and rejects eMBB request; [0167] III. Target access node 104 does not support eMBB.
Which node, i.e. source or target, takes the final decision on the fallback method to be used in case the target access node 104 rejects the eMBB request. [0168] A. Fallback method is decided by target access node 104. Source access node 103 can always cancel afterwards, if fallback method is not suitable; [0169] B. Fallback method is decided by the source access node 103 before the Handover Request; [0170] C. Fallback method is decided by the source access node 103 after an Handover Preparation Failure received from the target access node 104.
Fallback Method or other Source/Target Access Node Decisions: [0171] 1) Fallback to legacy Handover; [0172] 2) Fallback to release-14 Make-Before-Break; [0173] 3) Handover is rejected; [0174] 4) Handover is cancelled. [0175] By combining all the possible choices, the following behavior for source and target access nodes may be defined.

[0176] Target access node 104 supports eMBB and accepts eMBB request: [0177] I. Target access node 104 adds the first explicit indicator e.g. the eMBB indicator in the RRC HandoverCommand message and adds an optional IE the second explicit indicator e.g. eMBB accepted, to the Handover Request Acknowledge, i.e. the target access node 104 may explicitly signal that eMBB have been accepted so the source access node 103 knows without having to check RRC HandoverCommand message. It may be needed to differentiate an eMBB accepted from an eMBB not supported event in some cases. Source access node 103 can learn that target access node 104 supports eMBB.

[0178] Target access node 104 supports eMBB and rejects eMBB request: [0179] II.A.1. Target access node 104 does not add the eMBB indicator in the RRC HandoverCommand message and adds an optional IE e.g. fallback to legacy HO, to the Handover Request Acknowledge. The source access node 103 may learn that target access node 104 supports eMBB. [0180] II.A.2. Target access node 104 adds the rel-14 MBB indicator in the RRC HandoverCommand message and adds an optional IE e.g. fallback to rel-14 MBB, to the Handover Request Acknowledge. Source access node 103 can learn that target access node 104 supports eMBB. It is assumed that a source access node 103 supporting eMBB is also supporting rel-14 MBB. [0181] II.A.3. Target access node 104 may send legacy the Handover Preparation Failure message with a new cause value e.g. eMBB rejected. Source access node 103 may then learn that target access node 104 supports eMBB. Source access node 103 may try another Handover Request to the same node without eMBB indicator. Or try another target access node with or without eMBB. [0182] II.A.4. N/A [0183] II.B.1 Target access node 104 does not add the eMBB indicator in the RRC HandoverCommand message and adds an optional IE e.g. fallback to legacy HO to the Handover Request Acknowledge. Source access node 103 can learn that the target access node 104 supports eMBB. [0184] II.B.2. Target access node 104 adds the rel-14 MBB indicator in the RRC HandoverCommand message and adds an optional IE e.g. fallback to rel-14 MBB, to the Handover Request Acknowledge. Source access node 103 may learn that target supports eMBB. [0185] II.B.3. Target access node 104 sends an Handover Preparation Failure message with a new cause value e.g. eMBB rejected. Source access node 103 may learn that target access node 104 supports eMBB. Source access node 13 can try another target access node with or without eMBB. [0186] II.B.4. Not needed. In that case source access node 103 should use actions described in II.B.3. [0187] II.C.1. Target access node 104 sends an Handover Preparation Failure message with a new cause value e.g. eMBB rejected. Source access node 103 may learn that target access node 104 supports eMBB. Source access node 103 will send a new Handover Request without eMBB indicator. [0188] II.C.2. Target access node 104 sends an Handover Preparation Failure message with a new cause value e.g. eMBB rejected,. Source access node 103 may learn that target access node 104 supports eMBB. Source access node 103 will send a new Handover Request without rel-14 MBB indicator. Target access node 104 should be able to accept as rel-14 MBB does not need any specific action from target access node 104. [0189] II.C.3. Target access node 104 sends an Handover Preparation Failure message with a new cause value e.g. eMBB rejected. Source access node 103 may learn that target access node 104 supports eMBB. Source access node 103 may try another target access node with or without eMBB. [0190] II.C.4. N/A [0191] Target access node 104 does not support eMBB. [0192] III.A.1. Target access node 104 does not add the eMBB indicator in the RRC HandoverCommand message and does not add an optional IE to the Handover Request Acknowledge message. Source access node 103 may learn that target access node 104 does not support eMBB. [0193] III.A.2. N/A unless the eMBB indicator in RRC is similar to the rel-14 MBB indicator. In that case the target access node 104 adds the rel-14 MBB indicator in the HO Command and does not add an optional IE to the HO Request Ack. Source access node 103 may learn that target does not support eMBB. If this use-case needs to be covered, signaling from source could combine 1 and 2 to cover cases when target supports eMBB. [0194] III.A.3. N/A [0195] III.A.4. N/A [0196] III.B. N/A [0197] III.C.1. Similar to III.A.1. This is the default actions from a target access node 104 not supporting eMBB. [0198] III.C.2. Target access node 104 does not add the eMBB indicator in the RRC HandoverCommand message and does not add an optional IE to the Handover Request Acknowledge message. Source access node 103 may learn that target access node 104 does not support eMBB. Target access node 104 cancels the HO and sends a new Handover Request with rel-14 MBB indicator. [0199] III.C.3. N/A [0200] III.C.4. Target access node 104 does not add the eMBB indicator in the RRC HandoverCommand message eMBB. Source/target access node cancels the HO and can try another target access node with or without eMBB.

[0201] One alternative to the actions described above for a node not supporting eMBB is to send an Handover Request Failure with an existing cause value e.g. Abstract Syntax Error (Reject).

[0202] To perform the method in the source access node 103, the source access node 103 may comprise modules as shown in FIG. 7A. The source access node 103 may comprise a receiving module 710, a transmitting module 720, a determining module 730, a processing module 740, a memory 750 etc. The receiving module 710, transmitting module 720, determining module 730 and processing module 740 may be combined as one module, shown as processor 760.

[0203] The method according to embodiments herein may be implemented through one or more processors, such as the processor 760 in the source access node 103 together with computer program code for performing the functions and actions of the embodiments herein. The program code mentioned above may also be provided as a computer program product, for instance in the form of a data carrier 780 carrying computer program code 770, as shown in FIG. 7A, for performing the embodiments herein when being loaded into the source access node 103. One such carrier may be in the form of a CD ROM disc. It is however feasible with other data carriers such as a memory stick. The computer program code may furthermore be provided as pure program code on a server or a cloud and downloaded to the source access node 103.

[0204] The memory 750 in the source access node 103 may comprise one or more memory units and may be arranged to be used to store received information, measurements, data, configurations and applications to perform the method herein when being executed in the source access node 103.

[0205] The source access node 103, the processor 760 or the transmitting module 720 is configured to send to the target access node 104, the initial handover preparation message with the first explicit indicator for the target access node 104 to request an enhanced Make-Before-Break Handover.

[0206] The source access node 103, the processor 760 or the receiving module 710 is configured to receive the handover preparation response message from the target access node 104, with the second explicit indicator accepting or rejecting the requested enhanced Make-Before-Break Handover.

[0207] The source access node 103, the processor 760 or the determining module 730 is configured to select the possible fallback mechanism, upon reception of the handover preparation response message from the target access node 104 indicating rejection or no support of enhanced Make-Before-Break Handover.

[0208] The source access node 103, the processor 760 or the processing module 740 may be configured to notify the target access node 104 of the desired fallback in case of failure or reject of the requested enhanced Make-Before-Break Handover.

[0209] The source access node 103, the processor 760 or the processing module 740 may be configured to learn, from the handover preparation response message, the capability of the target access node 104 related to enhanced Make-Before-Break handover. The capability of the target access node 104 may be learned based on successful responses or failure responses of requested enhanced Make-Before-Break handovers. The source access node 103, the processor 760 or the determining module 730 may be configured to select the possible fallback mechanism taking the desired fallback into account and/or the learned capability of the target access node 104.

[0210] The source access node 103, the processor 760 or the determining module 730 may be configured to notify the target access node 104 of the desired fallback, and wherein the first explicit indicator for Enhanced Make-Before-Break handover and an indicator for the desired fallback mechanism is combined in a single Information Element in the initial Handover Preparation message.

[0211] The source access node 103, the processor 760 or the transmitting module 720 may be configured to add the first explicit indication as an additional information in the initial handover preparation message.

[0212] The first explicit indication may be included in a RRC context signalled from the source access node to the target access node. The first explicit indication may be added in a handover request message.

[0213] The possible and/or the desired fallback may comprise one or more of the following: a fallback to legacy handover; a fallback to release-14 MBB handover; and a rejection of the handover request.

[0214] The source access node 103, the processor 760 or the receiving module 710 may be configured to receive the notification from the target access node 104 of a selected possible fallback mechanism in case of rejection of the enhanced Make-Before-Break Handover request.

[0215] The source access node 103, the processor 760 or the determining module 730 may be configured to select the possible fallback mechanism by taking the notification into account.

[0216] To perform the method in the target access node 104, the target access node 104 may comprise modules as shown in FIG. 7B. The target access node 104 may comprise a receiving module 711, a transmitting module 721, a determining module 731, a processing module 741, a memory 751 etc. The receiving module 711, transmitting module 721, determining module 731 and processing module 741 may be combined as one module, shown as processor 761.

[0217] The target access node 104, the processor 761 and/or the receiving module 711 is configured to receive the initial handover preparation message with the first explicit indicator from the source access node 103 requesting the enhanced Make-Before-Break Handover.

[0218] The target access node 104, the processor 761 and/or the transmitting module 721 is configured to respond to the initial handover preparation message sent by the source access node 103, with the handover preparation response message with the second explicit indicator accepting or rejecting the enhanced Make-Before-Break Handover request.

[0219] The target access node 104, the processor 761 and/or the determining module 731 may be configured to select the possible fallback mechanism in case of failure or rejection of the enhanced Make-Before-Break Handover.

[0220] The target access node 104, the processor 761 and/or the transmitting module 721 may be configured to notify the source access node 103 of the selected possible fallback mechanism in case of failure or rejection of the enhanced Make-Before-Break Handover request.

[0221] The target access node 104, the processor 761 and/or the transmitting module 721 may be configured to respond with the handover preparation response message to the source access node 103 with the second indicator rejecting the enhanced Make Before Break Handover request and the indicator of selected possible fallback method.

[0222] The target access node 104, the processor 761 and/or the transmitting module 721 may be configured to respond by inserting the explicit enhanced Make-Before-Break Handover indicator in the RRC HandoverCommand message transferred to the source access node 103.

[0223] The target access node 104, the processor 761 and/or the receiving module 711 may be configured to receive the notification from the source access node 103 of the desired fallback in case of failure or reject of the requested enhanced Make-Before-Break Handover. The target access node 104, the processor 761 and/or the determining module 731 may be configured to select the possible fallback mechanism taking the received notification into account.

[0224] The method according to embodiments herein may be implemented through one or more processors, such as the processor 761 in the target access node 104 together with computer program code for performing the functions and actions of the embodiments herein. The program code mentioned above may also be provided as a computer program product, for instance in the form of a data carrier 781 carrying computer program code 771, as shown in FIG. 7B, for performing the embodiments herein when being loaded into the target access node 104. One such carrier may be in the form of a CD ROM disc. It is however feasible with other data carriers such as a memory stick. The computer program code may furthermore be provided as pure program code on a server or a cloud and downloaded to the target access node 104.

[0225] The memory 751 in the target access node 104 may comprise one or more memory units and may be arranged to be used to store received information, measurements, data, configurations and applications to perform the method herein when being executed in the target access node 104.

[0226] Below are some embodiments described.

[0227] Embodiment 1: A method in a source access node to perform handover of a UE to a target access node comprising: [0228] sending an initial Handover Preparation message with an indicator for the target access node to request an enhanced Make-Before-Break Handover; [0229] notifying the target access node of the desired fallback in case of rejection of the enhanced Make-Before-Break Handover request; [0230] receiving a Handover Preparation response message from the target access node, with an indicator accepting or rejecting the enhanced Make-Before-Break Handover request; [0231] in response to receiving the Handover Preparation response message, taking a decision about the possible fallback method upon reception of the target access node rejecting enhanced Make-Before-Break Handover.

[0232] Embodiment 2: according to the Embodiment 1, wherein the source access node learns the target access node enhanced Make-Before-Break capability from the target response message to the enhanced Make-Before-Break Handover request.

Group B Embodiments

[0233] Embodiment 3: A method in a target access node to perform handover of a UE from a source access node comprising: [0234] receiving an initial Handover Preparation message with an indicator from the source access node requesting an enhanced Make-Before-Break Handover; [0235] responding to the initial Handover Preparation message sent by the source access node, with an indicator accepting or rejecting the enhanced Make-Before-Break Handover request; [0236] selecting a possible fallback method in case of rejection of the enhanced Make-Before-Break Handover; [0237] notifying the source access node of the selected fallback method in case of rejection of the enhanced Make-Before-Break Handover request; [0238] inserting an explicit enhanced Make-Before-Break Handover indicator in the RRC HandoverCommand message transferred to the source access node, in order to notify the UE.

[0239] In case the texts in FIGS. 2-4 are too small and blurry to read, references are given below: [0240] FIG. 2: From 3GPP TS 38.300 v15.2.0, FIG. 9.2.3.2.1-1 [0241] FIG. 3: From 3GPP TS 36.300 v14.8.0, FIG. 10.1.2.1.1-1 [0242] FIG. 4: From R2-1817396, Enhancements to Make-Before-Break, Ericsson, 3GPP TSG-RAN WG2 #104, Spokane, USA, 12th-16th Nov. 2018.
At least some of the following abbreviations may be used in this disclosure. If there is an inconsistency between abbreviations, preference should be given to how it is used above. If listed multiple times below, the first listing should be preferred over any subsequent listing(s).

TABLE-US-00019 Abbreviation Explanation 5GS 5G System 5GC 5G Core network AMF Access and Mobility Management Function CHO Conditional Handover C-RNTI Cell RNTI DL Downlink eNB Evolved Node B eMBB Enhanced Make-before-break E-UTRAN Evolved Universal Terrestrial Access Network EPC Evolved Packet Core network gNB 5G Node B HO Handover IE Information Element LTE Long-term Evolution MBB Make-before-break NCC Next Hop Chaining Counter NG-RAN Next Generation Radio Access Network NR New Radio PDCP Packet Data Convergence Protocol RA Random Access RAR Random Access Response RAT Radio Access Technology RNTI Radio Network Temporary Identifier RRC Radio Resource Control Rx Receive SDU Service Data Unit SN Sequence Number Tx Transmit UE User Equipment UL Uplink UPF User Plane Function