  High altitude platform stations (HAPS) have recently emerged as a new key
stratospheric player in non-terrestrial networks (NTN) alongside satellites and
low-altitude platforms. In this paper, we present the main communication links
between HAPS and other NTN platforms, their advantages, and their challenges.
Then, prospective network architectures in which HAPS plays an indispensable
role in the future NTNs are presented such as ad-hoc, cell-free, and integrated
access and backhaul. To showcase the importance of HAPS in the NTN, we provide
comprehensive performance insights when using HAPS in the prospective
architectures with the most suitable communication link. The insights show the
HAPS' ability to interconnect the NTN nexus as well as their versatility by
incorporating different metrics into the analysis such as routing latency,
energy efficiency, coverage probability, and channel capacity. Depending on the
architecture, HAPS will play different roles in NTN, such as a UAV network
center, satellite relay, and ground network extension. Finally, the performance
gain provided by HAPS usage in NTN is further highlighted by comparing the
results when no HAPS are used.

Networking and Internet Architecture;Signal Processing