Satellite constellations have become a promising solution for future
Terabits/s connectivity. However, conventional radio frequency (RF)
communication links are unable to achieve such high throughput due to
the limited spectrum available. Free-space optical (FSO) communication
has recently acquired a growing interest in the satellite communication
(Satcom) community as a potential solution. Nonetheless, FSO
communication is sensitive to the adverse effects of beam scintillation,
beam-wander-induced pointing errors, free-space loss, and weather
conditions. To mitigate these effects, we present a new space-air-ground
(SAG) FSO transmission system with a strategically deployed
high-altitude platform relay. It can effectively remedy the effect of
atmospheric impairments on FSO transmission, particularly at high zenith
angles. Then, we integrate the proposed SAG-FSO link and traditional
space-ground FSO/RF links to further improve system performance and
reliability. We carry out a thorough performance analysis of the
resulting hybrid SAG-FSO/RF communication systems. Tractable analytical
expressions for the symbol error and outage probabilities are derived.
The numerical results highlight the significant potential of the
proposed, highly innovative Satcom systems over existing solutions.
Furthermore, Monte-Carlo simulations are performed to validate the
accuracy of the analytical results.