# Multi-Layered Satellite Communications Systems for Ultra-High Availability and Resilience
## 3 MLSS Technology Assessment: Benefits and Challenges
Furthermore, the Doppler effect pre- and post-compensation, as well as handover and mobility management for high-speed LEO layers, were identified as requiring special attention [37]. Satellites move rapidly relative to the location of satellite communications equipment on Earth, and this movement is observed as a Doppler shift in the signal frequency. Pre-compensation for the Doppler effect can be performed at the satellite for the beam center of the satellite [38], and the frequency offset inside the beam can be handled by the protocols used. If the end-user device is equipped with a global navigation satellite system (GNSS) receiver and it knows the satellite ephemeris at a given time it can predict and calculate the required compensation values. In their initial studies, 3 GPP defined several scenarios of two-layer satellite-terrestrial access, including direct satellite access and several versions of indirect access types [39]. MLSS with multiple satellite layers had not been extensively studied within 3 GPP until \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Ref.** & **Objective** & **Environment** & **Observed MLSS Gains** \\ \hline
[30,31] & GEO-LEO data relaying & Space & Improves global connectivity of LEO satellites to ground stations \\ \hline
[32] & GEO-LEO automated orbit switching & Space & Throughput was doubled from 50 Mbps to 100 Mbps \\ \hline
[33] & GEO-LEO simultaneous connection & Space & Multilink connection enables more efficient interlayer handovers \\ \hline
[34] & MEO-LEO routing to avoid traffic congestion & Simulation & For high load, packet delay was reduced from 150 ms to 100 ms \\ \hline
[25] & GEO-MEO-LEO routing to reduce e2e delay & Simulation & For high load, 3-layer outperforms both 2-layer and 1-layer const. \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Assessment of selected MLSS technology setups. Release 18, although an example case was considered during the Release 16 studies, where the combination of GEO and LEO with interlayer link was considered as a potential solution for UEs in unserved areas. Introducing more satellite layers to the network, the following technical benefits over single-layer satellite network are potentially obtained:  
* Network reliability, coverage, and throughput are increased: more potential paths with diversified characteristics to allow for a more stable link quality and congestion avoidance; more intelligent traffic-aware routing protocols can be used. * Resource efficiency can be improved: unpopulated areas can be ruled in with resource management (frequency and energy) more efficiently. * Control efficiency can be improved: hierarchical structures can be applied to reduce control overhead and delay. * Service continuity can be improved: more handover options exist for different dual mobility models. * Cost efficiency of infrastructure can be improved: the total number of satellites can be potentially reduced per service with multiple layers optimized for the same objectives; similarly, the number of ground stations can be potentially reduced. However, we observe that the above benefits do not come for free, facing, for example, the following challenges, which could be mitigated via the potential rough solution enablers suggested at the end of each bullet point:  
* Achievable MLSS gains are contingent: While the extra costs for adding more layers to existing ones are there, the corresponding gains for adding more layers are not always realized and depend on a number of things such as user traffic load distribution and traffic delay requirements, routing distance and routing direction, etc. The gain does not necessarily come solely from adding different orbit altitudes but also different satellite densities that can be achieved by just increasing the sublayer density. Adding more intelligence and learning ability to network status is expected to maximize the gains, where attainable, if the system allows higher computational complexity for control decisions ([PERSON] et al., 2018).