  In order to bolster future wireless networks, there has been a great deal of
interest in non-terrestrial networks, especially aerial platform stations
including the high altitude platform station (HAPS) and uncrewed aerial
vehicles (UAV). These platforms can integrate advanced technologies such as
reconfigurable intelligent surfaces (RIS) and non-orthogonal multiple access
(NOMA). In this regard, this paper proposes a multi-layer network architecture
to improve the performance of conventional HAPS super-macro base station
(HAPS-SMBS)-assisted UAV. The architecture includes a HAPS-SMBS, UAVs equipped
with active transmissive RIS, and ground Internet of things devices. We also
consider multiple-input single-output (MISO) technology, by employing multiple
antennas at the HAPS-SMBS and a single antenna at the Internet of things
devices. Additionally, we consider NOMA as the multiple access technology as
well as the existence of hardware impairments as a practical limitation. In
particular, we compare the proposed system model with three different
scenarios: HAPS-SMBS-assisted UAV that are equipped with active transmissive
RIS and supported by single-input single-output system, HAPS-SMBS-assisted UAV
that are equipped with amplify-and-forward relaying, and HAPS-SMBS-assisted
UAV-equipped with passive transmissive RIS. Sum rate and energy efficiency are
used as performance metrics, and the findings demonstrate that, in comparison
to all benchmarks, the proposed system yields higher performance gain.
Moreover, the hardware impairment limits the system performance at high
transmit power levels.
