Handling of cell change from target relay UE during indirect path switching of sidelink based U2N relay

1 . An apparatus comprising:

at least one processor; and

at least one non-transitory memory including computer program code storing instructions that, when executed by the at least one processor, cause the apparatus at least to perform:

sending, by a remote user equipment of a source cell, towards a network node of the source cell a measurement report comprising a candidate relay list,

wherein the candidate relay list comprises at least one of a serving cell identification or camping cell identification for at least one candidate relay user equipment for an indirect path switching from the source cell to a target cell associated with a candidate relay user equipment;

detecting a change of a serving cell or camping cell of the candidate relay user equipment from the target cell to another cell based on receiving from the candidate relay user equipment at least one message that indicates at least one of serving cell identification or camping cell identification for the candidate relay user equipment is different than as reported to the network node in the measurement report; and

based on the detecting, performing one of sending an updated measurement report towards the network node triggering renewing a path switch decision or initiating a radio resource control connection re-establishment procedure with the source cell or the target cell,

wherein the apparatus is further caused to perform:

establishing a PC5 direct connection with the candidate relay user equipment being configured from the network node; and

during establishment of the PC5 direct connection, receiving from the candidate relay user equipment system information comprising System Information Block Type 1 (SIB1) including a cell identification of a current serving cell or camping cell of the candidate relay user equipment used to identify a cell change of the candidate relay user equipment,

wherein the at least one message comprises the system information from the candidate relay user equipment received by the remote user equipment during establishment of the PC5 direct connection with the candidate relay user equipment.

2 . The apparatus of claim 1 , wherein the apparatus further caused to perform:

identifying a path switch decision to use the candidate relay user equipment of the candidate relay list being configured from the network node for the path switch, wherein the identifying is based on receiving a radio resource control reconfiguration message from the network node to configure the remote user equipment for path switching via the candidate relay user equipment.

3 . The apparatus of claim 2 , wherein the apparatus further caused to perform:

initiating a radio resource control connection re-establishment procedure with the source cell or the target cell, wherein the initiating is based on that the change of the serving cell or camping cell of the candidate relay user equipment detected one of after identifying the path switch decision or after receiving the radio resource control reconfiguration message from the network node to configure the remote user equipment for path switching via the candidate relay user equipment.

4 . The apparatus of claim 1 , wherein the candidate relay user equipment is in an idle or inactive state, and wherein the candidate relay user equipment in the idle or inactive state has performed cell reselection from the source cell to the target cell causing the cell change of the candidate relay user equipment with a different at least one of serving cell identification or camping cell identification for the candidate relay user equipment.

5 . The apparatus of claim 1 , wherein the updated measurement report comprises an updated candidate relay list indicating the different at least one of a serving cell identification or camping cell identification for the candidate relay user equipment.

6 . The apparatus of claim 1 , wherein the updated measurement report is triggered based on the remote user equipment not yet receiving a radio resource control reconfiguration message from the network node to configure the remote user equipment for path switching via the candidate relay user equipment.

7 . The apparatus of claim 1 , wherein the candidate relay list further comprises a relay user equipment identifier, a serving cell identification of the relay user equipment, and sidelink measurement quantity information including Sidelink Discovery-Reference Signal Received Power (SD-RSRP).

8 . The apparatus of claim 1 , wherein the remote user equipment filters relay user equipment candidates according to relay selection criteria before reporting and reports only relay user equipment candidates that fulfill higher layer criteria.

9 . The apparatus of claim 1 , wherein the remote user equipment is configured to trigger another measurement report when it receives a relay discovery message from a candidate relay user equipment and the relay discovery message indicates the relay user equipment's serving/camping cell identification is different from the one reported in the candidate relay list, and wherein the measurement report is triggered when the remote user equipment has not yet received a radio resource control reconfiguration message for path switching.

10 . The apparatus of claim 3 , wherein, if a cell identification in a newly received relay discovery message or identified from system information forwarded during establishment of the PC5 direct connection is not the same as a target cell identification indicated in a radio resource control reconfiguration message, the remote user equipment is triggered to initiate the radio resource control connection re-establishment procedure instead of responding with a radio resource control reconfiguration complete message.

11 . The apparatus of claim 1 , wherein, when the remote user equipment does not have enough time to identify the change of the camping cell of the relay user equipment and to send another or updated measurement report before a path switching decision, the remote user equipment requests PC5 connection establishment with the relay user equipment.

12 . The apparatus of claim 1 , wherein the change of the serving or camping cell of the relay user equipment occurs during a time period between the measurement report from the remote user equipment and PC5 direct connection establishment completed.

13 . The apparatus of claim 1 , wherein the source cell is served by a source gNB and the target cell is served by a target gNB different from the source gNB, and the indirect path switching is inter-gNB.

14 . A method, comprising:

sending, by a remote user equipment of a source cell, towards a network node of the source cell a measurement report comprising a candidate relay list,

wherein the candidate relay list comprises at least one of a serving cell identification or camping cell identification for at least one candidate relay user equipment for an indirect path switching from the source cell to a target cell associated with a candidate relay user equipment;

detecting a change of a serving cell or camping cell of the candidate relay user equipment from the target cell to a source cell based on receiving from the candidate relay user equipment at least one message that indicates at least one of serving cell identification or camping cell identification for the candidate relay user equipment is different than as reported to the network node in the measurement report; and

based on the detecting, performing one of sending an updated measurement report towards the network node triggering renewing a path switch decision or initiating a radio resource control connection re-establishment procedure with the source cell or the target cell,

wherein the method further comprises:

establishing a PC5 direct connection with the candidate relay user equipment being configured from the network node; and

during establishment of the PC5 direct connection, receiving from the candidate relay user equipment system information comprising System Information Block Type 1 (SIB1) including a cell identification of a current serving cell or camping cell of the candidate relay user equipment used to identify a cell change of the candidate relay user equipment,

wherein the at least one message comprises the system information from the candidate relay user equipment received by the remote user equipment during establishment of the PC5 direct connection with the candidate relay user equipment.

15 . An apparatus comprising:

at least one processor; and

at least one non-transitory memory including computer program code storing instructions that, when executed by the at least one processor, cause the apparatus at least to perform:

determining, by a candidate relay user equipment, a change of a serving cell or camping cell from a first cell to a second cell for the candidate relay user equipment; and

sending at least one message indicating at least a cell identification of the second cell based on the determining towards a remote user equipment served by a source cell of a network node and in need of an indirect path switch via the candidate relay user equipment,

wherein the sending causes an updated measurement report from the remote user equipment towards the network node triggering renewing a path switch decision or an initiated radio resource control connection re-establishment procedure from the remote user equipment with the source cell or the first cell,

wherein the apparatus is further caused to perform:

establishing a PC5 direct connection with the remote user equipment, as requested by the remote user equipment; and

during the direct connection establishment, sending towards the remote user equipment system information comprising System Information Block 1 (SIB1) including a cell identification of a current serving cell or camping cell of the candidate relay user equipment used to identify a cell change of the candidate relay user equipment,

wherein the at least one message comprises system information from the candidate relay user equipment sent during the remote user equipment establishment of the PC5 direct connection with the candidate relay user equipment,

wherein the candidate relay user equipment is configured to send the system information based on the at least one message for every PC5 direct connection establishment after every determined cell change within a configured time interval starting from a time instance the cell change is determined at the candidate relay user equipment,

wherein the candidate relay user equipment is configured to send one or more discovery messages as for the at least one message after every determined cell change within a configured time interval starting from a time instance the cell change is determined at the candidate relay user equipment,

wherein the sending is implemented by upper layer with cross layer interaction upon receiving the change or update of cell identification from access stratum layer at the relay user equipment,

wherein when the candidate relay user equipment selects a new cell or is handed over to a new cell, the access stratum layer of the candidate relay user equipment indicates to the upper layer comprising a non-access stratum layer or an application layer, a cell identification of the new cell and triggers the upper layer that is responsible for controlling and managing relay discovery message transmissions to send one or more additional relay discovery messages.

16 . The apparatus of claim 15 , wherein the candidate relay user equipment is in an idle or inactive state, and wherein the candidate relay user equipment in the idle or inactive state has performed cell reselection from the first cell to the second cell causing the cell change of the candidate relay user equipment with a different at least one of serving cell identification or camping cell identification for the candidate relay user equipment.