Existing redundant transmission mechanism defined in TS 23.501 can be used for the End-to-End C2 communication reliability with additional redundant information in C2 aviation payload and C2 authorization payload to allow UAV or UTM to synchronize information regarding the usage of redundant C2 connections via 5G system, because existing paring information within the aviation payload only includes one C2 connection. Following principles applies to establish redundant C2 connections for C2 communication reliability:

1. UAV and 5G system support the redundant transmission mechanism defined in clause 5.33.2 of TS 23.501 to be able to establish two redundant C2 connection with a UAV-C or UTM within one network.
2. The C2 redundant information is added into the C2 aviation payload (from UAV to UTM) and C2 authorization payload (from UTM to UAV) to exchange C2 redundant information between UAV and UTM. The redundant information can include the indication of establishment of redundant C2 connections. This C2 redundant information can be part of the C2 paring information associate with the UAV-C.
   It is up to UAV and UTM to decide which C2 PDU session is primary or secondary. 5G system is only aware of two C2 redundant connections being established and has no knowledge of the primary and secondary C2 connection.
3. The information exchange using C2 aviation payload and C2 authorization payload between UAV and UTM on the status change of C2 redundant connection can occur during C2 PDU connection establishment / modification phase, or UAV controller replacement phase.
   In the Figure A.1-1, the UE initiation PDU session establishment for 2 redundant PDU sessions for C2 communications with a UAV-C is specified.

Annex F in TS 23.501 specifies the mechanism that can be used to achieve redundant user plane paths. The similar mechanism can also be used to enhance the End-to-End C2 communication reliability in the scenarios where:

• a UAV has more than one SIM from a single operator and the UAV use them to set up redundant C2 connections to the UTM;
• a UAV is present in the area where RAN coverage allows the UAV to connect to multiple gNBs simultaneously. The selection of gNBs can be distinct from each other.

For a UAV with redundant connections to different gNBs to request the establishment of PDU Sessions that use independent RAN and CN network resources, the following per-conditions needs to be met:

• separate gNBs nodes are required to achieve user plane redundancy over the 3GPP system. It is, however, up to operator's deployment and configuration to ensure that separate gNBs are available and used;
• a UAV is able to establish two (or multiple) independent connections to two (or multiple) independent gNBs;
• RAN coverage is so that the UAV is able to connect to multiple gNBs located in the target area. To ensure that the UAV connects to different gNBs, the gNBs need to operate in such a way that allows the UAV to select and connect to different gNBs (e.g. gNB frequency allocation supports such configurations/ scenarios);
• deployment of the CN and UPF in particular supports configuration/scenarios with redundant user plane paths;
• the underlying transport network's topology is aligned with the RAN and UPF deployments and supports configurations/scenarios with redundant user plane paths;
• the physical network topology and geographical distribution of network functions also supports configuration/scenarios with redundant user plane paths to the extent deemed necessary by the operator;
• operation aspects of the redundant user plane paths (for instance, independent power supplies) are made independent to the extent deemed necessary by the operator.

Figure A.2-1 provides the architecture view. The UAV is connected to gNB1 and gNB2, respectively. Connection1 sets up a PDU Session via gNB1 to UPF1, while Connection2 sets up another PDU Session to UPF2 via gNB2. UPF1 and UPF2 interface with SMF1 and SMF2, respectively and UPF1 and UPF2 provide the UAV's connections to the same UTM.

To ensure that the UAV selects different gNBs, UAV's connections and cells of the gNBs are grouped into more than one group (so-called Reliability Group) and the UAV selects the cells from the same reliability group as the connection. Figure A.2-2 shows an example, where Connection 1 and the cells of gNB1 belong to Reliability Group A and Connection 2 and the cells of gNB2 belong to Reliability Group B.

To determine Reliability Groups for the UAV, the methods shown in Figure F-2 in Annex F of TS 23.501 can be used. A Reliability Group of each of the connections is represented using the existing parameters and sent from the AMF to the RAN during the RAN context establishment; thus, each of the gNBs knows Reliability Groups of the UAV's connections. Reliability groups of gNB cells are pre-configured via the O&M and gNBs can either learn the Reliability Groups of the neighbouring cells as the Xn connectivity is set up or also configured by the O&M. If the UAV leaves the cells of its currently connected Reliability Groups or if the quality of the connection drops below a certain threshold, the connections can be transferred to cells that belong to another Reliability Group. In such case, the gNB initiates the handover to cells in an appropriate Reliability Groups whenever such cells are available.