  This paper overviews point-to-point (P2P) links for integrated
satellite-aerial networks, which are envisioned to be among the key enablers of
the sixth-generation (6G) of wireless networks vision. The paper first outlines
the unique characteristics of such integrated large-scale complex networks,
often denoted by spatial networks, and focuses on two particular space-air
infrastructures, namely, satellites networks and high-altitude platforms
(HAPs). The paper then classifies the connecting P2P communications links as
satellite-to-satellite links at the same layer (SSLL), satellite-to-satellite
links at different layers (SSLD), and HAP-to-HAP links (HHL). The paper
overviews each layer of such spatial networks separately, and highlights the
possible natures of the connecting links (i.e., radio-frequency or free-space
optics) with a dedicated overview to the existing link-budget results. The
paper, afterwards, presents the prospective merit of realizing such an
integrated satellite-HAP network towards providing broadband services in
under-served and remote areas. Finally, the paper sheds light on several future
research directions in the context of spatial networks, namely large-scale
network optimization, intelligent offloading, smart platforms, energy
efficiency, multiple access schemes, and distributed spatial networks.
