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Content for  TS 23.256  Word version:  19.0.0

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5.13 Multiple USS serving different geographical areas  5.13.1 USS changeover  5.13.2 Procedure for USS changeover during a UAV flight 
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5.13  Multiple USS serving different geographical areas |R19|  p. 77

5.13.1  USS changeover  p. 77

In cases when a UAV moves over geographical areas served by different USSs, a 3GPP network may assist the UAV UE to perform a changeover from one USS to another. The following describes the interactions in order to perform the changeover:
  • USS can serve the UAV within its geographical area, and each USS may have an overlap with a geographical area primally served by another USS, i.e. USS' areas may not be strictly separated. A geographical area of a USS does not necessary identically match to tracking areas or cells of a 3GPP network. However, TAI(s) or cell IDs can be used by NEF to translate/map this information and vice versa whenever required.
  • Each USS is aware of its neighbouring USSs, border area(s) that it has with its neighbour USSs and possible border-crossing points for UAV.
  • USSs can communicate with each other using means outside the scope of the 3GPP specification.
  • USS can trigger a changeover by determining that a UAV is approaching a border its service and requesting an NEF/UAS NF to assist with the changeover.
  • USS can determine whether a changeover to another USS needs to take place. The determination can be based, for instance, on a notification from the network and/or information about a UAV's next point in the flight path in a pre-flight planning request. If the changeover is required, a serving USS is able to determine a possible USS (one or more) that can serve as a target USS and can trigger communication with them to determines candidate USS border-crossing point(s) for the UAV.
  • Alternatively, the determination can be based on a trigger from a UAV by sending, for example, a pre-flight planning request with a next point in the flight path outside an area of authority/responsibility of a serving USS.
To determine a changeover needs to take place and collecting the required information from all suitable target USSs, a serving USS requests the core network to assist with a changeover. The request goes to the NEF/UAS NF and includes the information received from the suitable target USSs and any additional information the NEF might need to perform this task (e.g. addresses of the suitable target USSs, requirements on the flight path, candidate flight path(s), accuracy level of predictions relevant to the flight monitoring). Based on the information in the request, the NEF/UAS NF determines the relevant NFs and specific service operations it needs to invoke to collect the required information for UAV flight planning/monitoring to determine with a defined likelihood that the UAV will leave the service area of one USS and moves into area of another USS (e.g. NWDAF analytics service for Movement Behaviour analytics, GMLC service for Ranging/Sidelink Positioning location, AMF for UAV's presence in bordering cells/TAs, AMF service for UAV's deviation for the expected/assigned trajectory). The serving USS also subscribes to get notified when it is time to make the changeover.
Once the serving USS detects that the UAV has left or is about to leave its service area (for instance, based on UAV's presence in a border TA/cell), the serving USS may determine to trigger USS changeover to a target USS to serve the UAV. The currently serving USS communicates with the selected target USS and passes the target USS the following information in order to establish communication towards UAV and 5GC:
  • which exposure services and notification are of relevance for the UAV;
  • UAV's identifiers;
  • any other information required by the target USS or by the UAV itself to establish the connection and perform authorization (e.g. a token containing information about the serving USS, the UAV, the target USS, and some other parameters such as issue date, validity data, or expire date).
The serving USS also informs the UAV about the changeover to the new target USS and may instruct the UAV (directly or via NEF/UAS NF) to reconnect and authorize with the target USS as well as trigger the exposure services towards 5GC NFs similar to what the previous serving USS had before the changeover. Once the changeover is complete and the UAV enters a TA/cell in the area served by a new USS, the new serving USS informs the old serving USS about the completion.
In order to continue with the changeover procedure, 3GPP system expects the new serving USS to respond to the notification in such a way confirming to the network about the completion of the changeover for the UAV. A UUAA authenticated context shall be established between the UAV and the new USS before the UAV interact with the new USS.
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5.13.2  Procedure for USS changeover during a UAV flight  p. 78

Procedure for USS changeover in cases when a UAV moves over geographical areas served by different USS is shown in Figure 5.13.2-1.
Reproduction of 3GPP TS 23.256, Fig. 5.13.2-1: Procedure for UAV changeover from one USS to another serving different geographical areas
Figure 5.13.2-1: Procedure for UAV changeover from one USS to another serving different geographical areas
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Step 1.
The UAV establishes a PDU Session for communication with a serving USS (shown as USS 1 in Figure 5.13.2-1) as described in clause 5.2.3.
Step 2a.
The UAV requests the pre-flight planning service from the serving USS, if needed. The request message includes an identifier of the UAV (e.g. GPSI, CAA-Level UAV ID), information of the starting and next point for the flight, requirements on the flight route (e.g. on time, shortest, highest, the farthest from a no-transmit zone) and may include candidate flight path(s) if available.
Step 2b.
The serving USS determines the need for the changeover to another USS based on the information about UAV's next point in the flight path or notification from the core network (e.g. UAV location reporting events from AMF/GMLC/LMF when serving USS has subscribed to such events).
Step 3.
The serving USS determines neighbouring USSs (one or more) that can serve as a target USS and triggers communication with all suitable target USSs (USS 2 in Figure 5.13.2-1) to determine candidate border-crossing point(s) for the UAV.
The serving USS in cooperation with the UTM derives any other information (e.g. acceptable deviations from the assigned flight plan/route) that can be used by 5GC to assist with the USS changeover and/or to provide flight planning assistance information.
Step 4.
The USS invokes a Nnef_UAVFlightAssistance_Get request to the NEF/UAS NF to collect from the core network information for the changeover. Inside the request, the serving USS includes the information derived in step 3 (e.g. indication about USS changeover, list of addresses for suitable target USS, a list of candidate border-crossing point(s), acceptable deviations for flight plan/route), UAV's identifier (e.g. GPSI), and other parameters such as requirements on the flight path, candidate flight path(s), accuracy level of predictions relevant to the flight planning.
Step 5.
When the NEF/UAS NF receives the Nnef_UAVFlightAssistance_Get request from the serving USS with the indication about the USS changeover, the NEF/UAS NF translates/maps, if required, the parameters included in the USS request to 3GPP identifiers. For instance, the NEF determines a cell ID/tracking area identifier (TAI) of the cell/TA where the UAV-requested starting and next point in the flight path are located; similarly, if the request includes an indication that UAV will cross the USS-border and/or a list of the candidate border-crossing point(s) in form of geographical coordinates, the NEF maps them to a list of border cell IDs/TAIs.
The NEF determines the relevant NFs and specific service operations it needs to invoke to collect the required information for the serving USS for the purpose of UAV flight planning (e.g. NWDAF analytics service for Movement Behaviour analytics, GMLC service for Ranging/Sidelink Positioning location, AMF for UAV's presence in bordering TAs/cells, AMF service for UAV's deviation for the expected/assigned trajectory, UDM service for expected UE behaviour parameter provision).
Step 6.
The NEF/UAS NF invokes service operations towards the identified NFs as described in Steps 8-12 of the procedure for the NEF-assisted pre-flight planning, see clause 5.12.
Step 7.
If the NEF/UAS NF receives a list of candidate border-crossing points, the NEF/UAS NF identifies the AMFs serving NG-RAN nodes in all identified border TA(s)/cell(s); for that the NEF/UAS NF uses the Nnrf_NFDiscovery service from NRF. Once the AMF(s) information is retrieved, the NEF/UAS NF invokes an Namf_EventExposure_Subscribe request to subscribe to the UE/UAV's presence in area of interest wherein the area of interest is set to cell ID(s)/TAI(s) of the border cells(s)/TA(s).
Step 8.
The NEF/UAS NF responds to the Nnef_UAVFlightAssistance_Get request with the collected information from the 5GC NFs for UAV's flight path between the starting point and all candidate USS border-crossing point(s).
Step 9.
Based on the received information, the serving USS selects the target USS from the list of pre-selected suitable USSs (step 3) and starts communication to request the target USS (USS 2 in Figure 5.13.2-1) to prepare for the USS changeover and, if required, to perform the flight planning for the UAV from the border-crossing point(s) to the next point in the flight path that is located in the geographical area served by the target USS.
Step 10.
The target USS (USS 2) performs Steps 4 - 8 to plan the UAV flight across its geographical area towards the UAV's next point in the flight path.
Step 11.
The target USS (USS 2) provides the serving USS (USS 1) information about UAV's flight path(s) from a border cell(s)/TA(s) to the next point in the flight path (e.g. primary flight path, secondary/alternative flight path etc.).
Step 12.
If the UAV requests a pre-mission flight planning in step 2a, the serving USS responds to this request with the planned flight routes and time schedule for the entire flight from the starting point to the next point in the flight path.
Step 13.
The serving USS sends a Nnef_UAVFlightAssistance_Create request to the NEF/UAS NF with the information about the planned UAV flight path(s), see clause 4.4.1.1.3.3. In this request, the serving USS may also include additional flight path information for each of the segment of the paths (served by USS 1 and USS 2), for instance, UAV's speed, flight height/altitude and/or time schedule for crossing/spending at each of the TAs/cells.
Step 14.
The NEF/UAS NF may invoke additional services with 5GC NFs: with AMF to determine the UAV deviation from the expected UAV's flight trajectory (e.g. primary/secondary flight paths); with UDM to update the Expected UE Behaviour parameters via the parameter provision; with the NWDAF for the Movement Behaviour analytics of the UE/UAV to determine whether the UAV will likely leave the USS area and continue following the flight path or not.
The expected UAV's flight trajectory may possibly consist of multiple segments where each of the segments are identified by geographical areas that are served by a different USSs (e.g. the currently serving USS and the target USS for the changeover).
Step 14a.
The NEF/UAS NF may use the AMF event exposure service to get notified when the UAV does not follow (i.e. deviates from) the assigned flight plan. In this case, the NEF/UAS NF sends a Namf_EventExposure_Subscribe request to the serving AMF with the planned flight path information, which consists of a list of 3GPP locations (i.e. TAIs or cell IDs), a time schedule describing when the UE shall be present at these locations, height/altitude range and, optionally, acceptable deviations, e.g. the UAV not arriving on time.
Step 14b.
The NEF/UAS NF invokes the parameter provisioning service from the UDM (i.e. Nudm_ParameterProvision_Update, see clause 4.15.6.3 of TS 23.502) to provision the planned flight path information, which consists of a list of 3GPP locations (i.e. TAIs or cell IDs and, optionally, height/altitude information) together with the time schedule describing when the UE shall be present at these locations, optionally, acceptable deviations, e.g. UAV is not arriving on time or UAV flies below/above the assigned flight altitude.
Step 14c.
The NEF/UAS NF invokes a Nnwdaf_AnalyticsInfo_Request / Nnwdaf_AnalyticsSubscription_Subscribe service operation towards the NWDAF in order to obtain the Movement Behaviour analytics of the UE/UAV as described in clause 6.21 of TS 23.288.
Step 15.
If the NEF/UAS NF subscribes (in step 7) to UAV's presence in the borders cell(s)/TA(s)s, the AMF notifies the NEF/UAS NF once the UAV enters a border cell/TA of the serving USS about the event (i.e. UAV enters the Area of Interest). Similarly, if the NEF/UAS NF subscribes for event reporting when the UAV deviating from the expected/assigned trajectory and/or moving across different trajectory segments, the AMF notifies the NEF/UAS NF when either of the events are detected.
Step 16.
When the NEF/UAS NF receives a report from the AMF indicating the UAV is deviating the assigned flight plan, the NEF/UAS NF may invoke an Ngmlc_Location_ProvideLocation service request to a GMLC to get a accurate position of the UAV. In such case, the GMLC performs the 5G-MT-LR procedure to retrieve the accurate UAV location via AMF/LMF (as specified in TS 23.273) and provide the UAV location to the NEF/UAS NF in an Ngmlc_Location_ProvideLocation service response.
Step 17.
If the NEF/UAS NF determines that the UAV is leaving the geographical area served by the serving USS (USS 1), the NEF/UAS NF sends a Nnef_UAVFlightAssistance_Notify (as described in clause 4.4.1.1.3.5) request to the serving USS in which it includes information about which/when border-crossing point will be used by the UAV so that the USS timely triggers the changeover.
Step 18.
The serving USS communicates with the target USS to execute the changeover for the UAV; this communication and details are outside the scope of the 3GPP specification, however, it is expected that a serving USS pass the target USS the information on which exposure services/notifications are of relevance for the UAV (e.g. subscribed events and UAS NF address), UAV's identifiers (e.g. GPSI, CAA-Level UAV ID), and other information required by the target USS (or UAV itself) to establish the connection.
Step 19.
The AMF serving a border TA/cell on the target USS's side (USS 2) notifies a serving NEF/UAS NF about the UAV's presence in the border TA/cell, and the NEF/UAS NF invokes the Nnef_UAVFlightAssistance_Notify service operation to further notify the target USS.
Step 20.
The serving USS (USS 1) informs the UAV about the changeover to the new USS (USS 2) and, if required, may instruct the UAV to execute the UUAA procedure with the new USS (USS 2) as well as to trigger the exposure services towards 5GC NFs similar to what the previous USS had before the changeover.
Step 21.
After receiving the required information from the serving USS (the exact details are outside the scope of the 3GPP specification), the target USS responds to the Nnef_UAVFlightAssistance_Notify request from the NEF/UAS NF in such a way completing the changeover for the UAV.
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