# Multi-Layered Satellite Communications Systems for Ultra-High Availability and Resilience
## 4 System Model: MLSS Concept  
We propose a mission definition for a highly resilient, highly available, and multi-purpose MLSS that adopts at least one GEO satellite layer to be deployed as the first stage of the system, to be later augmented by one or more NGSO satellite layers, with preference for LEO or very-low earth orbit (VLEO) due to low-latency links. Extension to a three-layer structure including, e.g., MEO is possible. The basic concept is depicted in Figure 1 covering GEO and LEO layers. For the user link, the proposed system shall operate in one or multiple frequency bands, with at least one mandatory sub-6 GHz frequency band to satisfy user terminal size, weight, power consumption and cost (SWaPC), and antenna constraints (e.g., omnidirectional antennas). The system shall be able to serve handheld and mass IoT use cases and satisfy service availability requirements (such as rain fade resiliency), and an optional high-bandwidth frequency band operating above 10 GHz to satisfy high-throughput broadband use case requirements. A multi-band implementation is considered strongly beneficial but does not have to employ multi-band payloads, by instead splitting band support by layer. The system shall utilize NTN air interfaces and protocols of the 3 GPP at least for the sub-6 GHz operation of the system to support UE including IoT, smartphones, and portable customer-premises equipment (CPE). The adoption of the 3 GPP air interface across all layers and frequency bands of the system is considered strongly beneficial to maximize the ground infrastructure's commonality, seamless mobility, and interoperability. The aim is also to leverage economies of scale and the larger mobile ecosystem, especially for user terminals. We do not consider the capability of each terminal to be able to connect to all the layers, or even multiple layers of the system to be a baseline requirement for all terminals and use cases. However, for those user terminals and use cases where high availability and reliability are key requirements, this is considered strongly beneficial, if not in some cases required, to fully realize the MLSS strengths in high availability. In such cases, air interface commonality, antenna, and RF frontend commonality become even stronger considerations, and network infrastructure commonality or tight integration is considered a prerequisite. Figure 1: Multi-layered satellite system concept consisting of GEO and LEO satellites. Users are shown to be aerial and ground-based vehicles and devices including ships. The GEO satellite provides coverage for the whole area while LEO system coverages are pointed to specific locations. In other cases, it could also be possible to combine terminals with different air interfaces, antennas, and RF frontends at the network and application layer using an Edge Connectivity and Application manager. Even in such cases, tight network infrastructure integration and some levels of commonality are considered a requirement. The ability of the GSO system to relay and backhaul (see Figure 2) data traffic from lower NGSO layers of the system is considered a requirement to fully realize the technical, service, cost, and timeline benefits of the native MLSS design in comparison to multiple disjointed single-layer systems. This capability enables the removal or significant relaxation of the requirement for inter-satellite links between satellites, as well as for a large number of ground stations for the NGSO layers, by allowing traffic served by NGSO layers to be landed via GSO, resulting in fewer large gateways (GWs). Furthermore, it enables the capability to reuse NGSO layers as fallback relays and/or temporary landing gateways for satellites of the GSO layer(s) in case of GSO GW unreachability (see Figure 3). Therefore, the support for at least low-bandwidth LEO-GSO links is a baseline requirement to provide LEO-to-GSO backhaul of low-data rate, consumer messaging, IoT, and  
Figure 3: GEO to GEO and GEO to Ground relay via LEO.