# Multi-Layered Satellite Communications Systems for Ultra-High Availability and Resilience
## 6 Future Research Directions and MLSS Technology Roadmap  
In this section, we provide a brief summary of potential research topics as well as a roadmap for the MLSS technology. MLSS architectural issues. While the first MLSS implementations have included only two vendor-dependent satellite layers, such as GEO and LEO, an apparent development in MLSS architectural design is towards being able to handle more complicated structures including VLEO, highly-elliptical orbit (HEO), high-altitude platform (HAP), and low-altitude platform (LAP). The SDN and open radio access network (O-RAN) concepts, which have been considered for the terrestrial domain for a while, can be expected to be incorporated into the space segment in the future as well, to assist in the flexible and demand-based vendor-independent multilayer architectural designs [48]. Similarly, the advanced design of more fluid core networks oriented towards the 6G requirements [49; 50] would provide an additional benefit to the NTN deployments in terms of space capabilities and performance. MLSS can leverage these advances through the deep integration of ISLs and ILLs as part of the inter-node connectivity with guaranteed QoS and mobility support, further optimizing the system towards controlled end-to-end delay and jitter, as well as for extensive system reliability. MLSS spectrum allocation issues. Considering long-term research directions, dynamic spectrum management (DSM) remains a research topic for the future, requiring updates for the 6G era. There are multiple topics to be advanced, such as defining the most suitable frequency bands for systems and links and the development of spectrum-sharing mechanisms to manage the complexity of a dynamic and mobile 3D network. Flexible resource management with high granularity might require AI-based solutions. When a massive number of devices are involved in 6G networks and require spectrum assignment, AI-enabled spectrum management can intelligently support a massive number of connections and diverse services. New types of challenges will arise in network controller design due to the inclusion of non-terrestrial base stations (BSs) since both the serving and interfering BSs can move at the same time in the 3D plane. A careful analysis between dynamic, AI-based, very fast resource management and more controlled, database-assisted, dynamic spectrum management approaches that can guarantee QoS for all sharing parties is needed. MLSS link establishment issues. Satellite networks are very actively studied in 3 GPP standardization. However, the 5G New Radio (NR) technology and Narrowband IoT (NB-IoT) technology were originally developed for terrestrial systems and only later adapted for satellite links. When we move towards 6G networks, multi-layer technology is considered from the beginning, and NTN-native design for all the required links can be conducted better, with improved performance. The joint optimization of intersatellite links and the location and number of Earth stations is an interesting research direction. From the link perspective, it is essential to study the use of free space optical (FSO) links not only for ISLs and ILLs but also for space-to-ground links. In addition, the network should tolerate some disruptions for reliable EO and IoT data delivery, and, therefore, disruption-tolerant networking (DTN) needs to be included in the link design of future MLSS networks. MLSS terminal issues. The RF parts of the terminal form one of the most significant bottlenecks to enabling cost-efficient MLSS solutions, and, therefore, deserve further attention in the future, especially for the higher frequencies. Similarly to terrestrial networks, recent full duplexing techniques and flat panel multi-element arrays can open up more efficient terminals for MLSS scenarios. The complexity of user terminals can also be partly addressed by increasing the complexity of the satellite domain.