Space information network (SIN) is a heterogeneous network that has multiple platforms including GEO/MEO/LEO satellites, stratosphere balloons, unmanned aerial vehicles in the backbone, and terrestrial terminals (e.g., ground stations) functioning as access nodes. The appearance of SIN realizes efficient resource sharing and utilization of distributed information within a large-scale space. Due to the diversity of network nodes, the condition of communication links in SINs is very complicated. Specifically, there are six types of SIN links:inter-satellite link, inter-aircraft link, terrestrial link, satellite-aircraft link, satellite-ground link, and aircraft-ground link. In this paper, we extensively investigate the recent advances in the field of communication links in the SIN, and analyze the research findings, especially the studies on radio frequency (RF), free space optics (FSO) and FSO/RF hybrid communication in the SIN. The analysis demonstrates that, compared with the traditional RF communication, FSO technology has high bandwidth, large capacity, and negligibly small requirement of spectral examination, at the additional cost of narrow spread, slow motion tracking, and vulnerability to bad weather. We further show that FSO is most suitable for inter-satellite links, while for the other links over atmosphere influence or rapid positioning, the FSO/RF hybrid communication has more obvious advantages. Since there have been minor efforts on the FSO/RF hybrid communication, we propose an adaptive switching mechanism for FSO/RF hybrid communication based on big data prediction.

space information network  /  radio frequency  /  free space optics  /  hybrid communication