  Non-terrestrial networks have been attracting much interest from the industry
and academia. Satellites and high altitude platform station (HAPS) systems are
expected to be the key enablers of next-generation wireless networks. In this
paper, we introduce a novel downlink satellite communication (SatCom) model
where free-space optical (FSO) communication is adopted between a satellite and
a HAPS node. A hybrid FSO/radio-frequency (RF) transmission model is used
between the HAPS node and ground station (GS). In the first phase of
transmission, the satellite selects the HAPS node that provides the highest
signal-to-noise ratio (SNR). In the second phase, the selected HAPS decodes and
forwards the signal to the GS. To evaluate the performance of the proposed
system, outage probability expressions are derived for exponentiated Weibull
(EW) and shadowed-Rician fading models while considering the atmospheric
turbulence, stratospheric attenuation, and attenuation due to scattering, path
loss, and pointing errors. Additionally, asymptotic analysis is carried out and
diversity gain is provided. Furthermore, the impact of aperture averaging
technique, temperature, and wind speed are investigated. We also provide some
important guidelines that can be helpful for the design of practical HAPS-aided
SatCom. Finally, the results show that the use of HAPS improves the system
performance and that the proposed model performs better than all other existing
models.
