  6th generation (6G) communication research is currently focusing on
non-terrestrial networks (NTNs) to promote ubiquitous and ultra-high-capacity
global connectivity. Specifically, multi-layered hierarchical networks, i.e.,
the orchestration among different aerial/space platforms, including Low and
High Altitude Platforms (LAPs and HAPs), and satellites co-operating at
different altitudes, currently represent one the most attractive technological
options to solve coverage and latency constraints associated with the NTN
paradigm. However, there are still several issues to be resolved for proper
network design. In this work, we evaluate the performance of different
multi-layered non-terrestrial configurations, and provide guidelines on the
optimal working point(s) for which it is possible to achieve a good compromise
between improved system flexibility and network performance, with respect to a
baseline standalone deployment.
