5G private network adoption is gathering momentum due to their capacity to innovate providing connectivity to a large diversity of use cases and applications like industry 5.0, logistics, mining, energy, mission-critical or transport hubs like airports.  

The main advantages of private networks are: 

• Provide tailored coverage and throughput optimization due to the usage of dedicated spectrum and radio resources. 
• Low latency and data privacy thanks to the fact that the Core and application server are hosted locally, in consequence the data in not transmitted via internet.  

5G private networks are typically deployed on premises in isolated areas without public networks connectivity or very low signal strength, there are some scenarios where provide handover between public and private network is interesting to provide service continuity between public and private networks, for example:  

• Industry 4.0. A private network is deployed to provide connectivity to support the industrial process and applications. In addition, the workers may need to use their public SIM cards inside the factory to use their corporate applications and make and receive phone calls. 
• Transport hubs. Private networks in transport hubs like airports and trains stations are widely adapted because can provide tailored coverage for mission critical communications in areas where public networks are not available or are congested. Roaming between public and private networks can be an option to extend the public coverage inside the transport hubs using the private network avoiding MNOs to deploy additional infrastructure.  

In both cases is mandatory to provide seamless handover and service continuity without session interruption.  

The traditional approach for UE equipment handover is not adapted to public and private networks handovers because in some of cases there is an overlap between public and private network and, as a result, the UE equipment will keep attached to the public network although the network quality if the public network is poor, in consequence the user experience will be  degraded. 

An efficient handover mechanism is needed to facilitate the handover between public 5G and private networks providing service continuity, in this context, the Handover optimization techniques tested in 5GMED can be applied to this specific scenario: 

• UE roaming with AMF relocation: The N14 interface between public AMF, considered here as home AMF (h-AMF) and private AMF, considered as visited AMF (v-AMF) allows the v-AMF to fecht the UE context from the h-AMF eliminating the need of full attachment and registration in the private network. 

• Reducing failed attachments: Public and Private PLMN need to be configured as equivalent PLMN to reduce failed attachment. 
• UE roaming with handover: the public gNBs closed to the private gNB need to be configured as neighbour cells. Therefore, the UE will be instructed by the public network to scan the quality of the private network and network handover will be trigger when the handover threshold is crossed. 
• Early home network release: neighbour gNB in public and private network need to be configured to release the connection  once there is still a good signal level and the attachment to the private Network can be completed. 

The combination of the previous points will result in handover between public and private networks with interruption times around hundred milliseconds and the user experience will not be degraded enabling service continuity. These mechanisms are tested in 5GMED to enable seamless cross-border roaming for the different use cases presented in the project.