# Multi-Layered Satellite Communications Systems for Ultra-High Availability and Resilience
## 3 MLSS Technology Assessment: Benefits and Challenges  
The majority of the related literature concerns the conventional terrestrial-satellite integration that has started already in 2G communication systems and has now accelerated in 5G systems. While in 2G systems, the integration target was quite modest to support some simple dual operations between satellite and terrestrial systems, the integration target in 5G and 6G is to make satellite links more comprehensively compatible with that of the terrestrial system and to identify ways to tolerate the distinctive features of satellite links [9; 22; 24; 25]. The MLSS has been typically used either to (i) enable more efficient hierarchical control architecture or (ii) improve the data distribution efficiency of the overall system by exploiting the conflicting features of different orbits. The first category aims at reducing the control complexity and overheads (cf. [26]) while the latter category aims at enabling the delay optimization by utilizing tradeoffs introduced by different propagation and multihop delays in each path (cf. [27]). In general, the higher layers can control lower layers more efficiently with better visibility to network states and fewer hops for distant traffic. The most important benefit and system-level decision of deploying an MLSS architecture is to optimally combine the strengths of different layers that include a larger coverage area as one moves up by each layer of altitude and a shorter delay as one moves down by each layer of altitude. The former factor dealing with coverage is more related to the operators' incentive to optimize the network performance and is particularly important for non-homogeneous traffic distributions where higher altitude constellations have a better ability to balance loads to provide fair and efficient utilization of network resources. The latter factor dealing with the delay affects the user's quality of experience more directly and is more suitable for real-time applications. Often, only propagation delay has been the main concern when analyzing satellite networks. However, as the constellation density increases and more traffic is delivered via satellites, the risk of local network congestion increases with additional queueing delays. When looking at different layers separately, the well-known trade-off is the aforementioned coverage-delay tradeoff: increasing the layer altitude leads to better coverage per satellite but also a larger propagation delay. However, there are a number of other influencing attributes, including some non-technical aspects. The long-lasting debate between the GEO, MEO, and LEO layers is still expected to continue. For instance, it is claimed that GEO still has the best cost-per-bit performance when compared with MEO and LEO for the delay-tolerant services, and emerging micro-GEOs further provide improvements [28], while MEO beats both GEO and LEO when looking jointly at both coverage and energy efficiency [29]. As the fair comparison in practice is rather difficult, the validity of these claims should be put to the test in different system conditions. Some interesting MLSS activities are listed in Table 1.  
The introduction of support for NTN access by the 3 GPP specifications, starting from Release 17, is an important milestone that will likely significantly impact both the satellite and terrestrial ecosystems to an unprecedented degree. The first target of the NTN effort was to address the compatibility issues between 5G and satellite, while in the future the integration is foreseen to be deeper, to further boost the potential gains of using different layers [35]. The focus of 3 GPP integration work has been on taking the terrestrial NR specification and seeing how the NTN paradigms should be incorporated with the least number of changes [36]. The majority of changes have included protocol modifications to tolerate longer propagation delays and timing relationships within a physical layer, medium access control, and radio link control.