## 2 Background  
Bandwidth scarcity is one of the technical challenges to be addressed in future HTS systems. The entire Ka frequency band was allocated to the user link to prevent the bandwidth limitation, moving the feeder link to EHF (or FSO) with spatial diversity of the gateways to combat the channel impairments that may randomly appear due to atmospheric conditions [4]. Various approaches to gateway redundancy were proposed to prevent the need for doubling the infrastructure required for the ground segments. These approaches assumed that all gateways are interconnected by high-speed terrestrial links, and the traffic of a given user can be redirected to another gateway. These approaches, denoted as _Smart Gateway Diversity_--SGD [5; 6] use Network Control Centers to make the routing decision of serving each user beam by, 1. carriers from all (N+0 site diversity), 2. a subset of the gateways using \(P\) redundant gateways (N+P site diversity) so that the traffic of a gateway experiencing an outage can be routed through an alternative gateway, or 3. the concurrent operations of all gateways, including the redundant ones (++N+P site diversity).  
While most of the works about SDN/NFV integration focus on traffic steering towards VNF instances that implement network functions, only limited works exist for implementing an SDN controller as a VNF. A systematic literature review [7] lists possible controller functions implemented as VNF in existing works. An SDN controller that functions as a traffic load balancer was implemented as a VNF [8]. The controller was implemented as a virtual network service to deploy additional controllers based on the network traffic to improve the overall network performance. An SDN controller that load balances traffic between two snort (intrusion prevention systems) VNF instances based on a control-theoretic approach was proposed [9]. A deep learning-based traffic classifier was implemented as a VNF directs an SDN controller to route traffic with application awareness [10]. An elastic routing service implemented as a Ryu controller is deployed as a VNF to load balance the network traffic across dynamically provisioned switches in a framework called UNIFY ESCAPE [11].  
On the other hand, a few works have explored the application of Software Defined Networking, Virtualization, and Network Function Virtualization to satellite networks. For example, [PERSON] et al. [12] suggested likely broadband communication scenarios for these technologies, including an inter-hub handover with site diversity case, enhancements for virtual network operator (VNO) services, and the integration of satellite and terrestrial networks. In another work, [PERSON] et al. [13] emphasized the use of NFV to ensure the competitiveness of the satellite communications sector. Several approaches are certainly possible for this integration. [PERSON] et al. [14] focused on the use of SDN/NFV to orchestrate the control, forwarding, access, service, and management planes operating at different orbits of the space segment. A system architecture for the Internet of Space Things/CubeSats (IoST) was proposed based on SDN and NFV to improve network resource utilization and simplify network management [15]. Prior works such as the one by [PERSON] et al. [16] have formalized the optimization problem to be handled by the VNF. However, these approaches require global knowledge of the link states.