# Multi-Layered Satellite Communications Systems for Ultra-High Availability and Resilience
## 6 Future Research Directions and MLSS Technology Roadmap
The cost-efficiency comes largely from the marriage of the highly optimized implementation of modern baseband and RF functionalities, as well as the adaptations of air interface protocols that can support seamless multilayer access. It must have an integrated antenna for LEO and GEO and a single modern capable of switching between LEO and GEO processing characteristics. MLSS network management issues. SDN-based controllers are foreseen as one of the main approaches to achieving a feasible and efficient network management framework. Management decisions involve a sufficient amount of SDN controllers and their placement and roles in the overall MLSS network. The controller placement can also be performed dynamically, involving a specific migration cost. The optimal controller locations and the functional split of access and core network protocols are difficult to decide in a highly dynamic 3D network with different delay profiles, as terrestrial network protocols do not readily tolerate delay increases. It is important to incorporate the effect of the cost of unnecessary (e.g., ping-pong handovers) handovers between ISL and ILL and to minimize the overall cost as well as the trade-off between delay and coverage for different load conditions and heterogeneous traffic demands. Additionally, new approaches that can better handle the large memory requirements of routing protocols are needed for the MLSSs with large constellation sizes. Adding more intelligence and learning ability to the network status is expected to maximize the gains attainable if the system allows for higher computational complexity for control decisions. MLSS technology roadmap summary. Lastly, we present a concise summary of a roadmap of the MLSS technology by presenting a collection of some key past and expected milestones. In particular, we include some important past integration development steps of different satellite layers including conventional terrestrial/satellite layer integration development, subsequent MLSS testbeds, and anticipated MLSS system features in the future, for which more information for specific technologies is provided, e.g., in [24; 49; 50; 51]. The roadmap illustrating the development trends of MLSS is depicted in Figure 15. Some main observations of the roadmap include the following:  
* The first MLSS topology designs were started in 2000 [52]. * Subsequently, several communication methodologies for MLSS started to appear, e.g., for MLSS routing and interlayer links. * The 3 GPP started their NTN considerations in Release 15 (2017) and later also conceptualized some MLSS scenarios. * Various kinds of trials and products for satellite and terrestrial integration have been announced, such as the first dual-mode satellite and LTE products in 2017, NB-IoT NTN technologies in 2021, and the first direct-to-satellite voice calls with unmodified cellular handhelds in 2023. * Several MLSS communication trials have appeared between 2019-2023 (Inmarsat, Intelsat, Viasat, SES). * Terminals capable of connecting multiple orbits simultaneously have started to appear in 2022 (Isotropic, Kymeta). * Several other MLSS activities are expected within the next five years (Inmarsat, Lockheed Martin, Intelsat, IRIS2). * The key technology enablers of the MLSS that are expected to be realized in the future in real systems include, e.g., DSM, software-defined satellite networks with resource virtualization, O-RAN satellite technologies, machine learning-based MLSS controlling approaches, and extended constellation structures, e.g., with VLEO and HEO. * Clearly, the ISL and ILL link technologies play a significant role in MLSS system design. The heterogeneous choice between RF and optical transmissions remains an important design problem. This is affected by the satellite capabilities and the set of target user services which may also include Earth observation systems.