  Near-space information networks (NSINs) composed of high-altitude platforms
(HAPs) and high- and low-altitude unmanned aerial vehicles (UAVs) are a new
regime for providing quick, robust, and cost-efficient sensing and
communication services. Precipitated by innovations and breakthroughs in
manufacturing, materials, communications, electronics, and control techniques,
NSINs have been envisioned as an essential component of the emerging
sixth-generation of mobile communication systems. This article reveals some
critical issues needing to be tackled in NSINs through conducting experiments
and discusses the latest advances in NSINs in the research areas of channel
modeling, networking, and transmission from a forward-looking, comparative, and
technical evolutionary perspective. In this article, we highlight the
characteristics of NSINs and present the promising use cases of NSINs. The
impact of airborne platforms' unstable movements on the phase delays of onboard
antenna arrays with diverse structures is mathematically analyzed. The recent
advances in HAP channel modeling are elaborated on, along with the significant
differences between HAP and UAV channel modeling. A comprehensive review of the
networking techniques of NSINs in network deployment, handoff management, and
network management aspects is provided. Besides, the promising techniques and
communication protocols of the physical (PHY) layer, medium access control
(MAC) layer, network layer, and transport layer of NSINs for achieving
efficient transmission over NSINs are reviewed, and we have conducted
experiments with practical NSINs to verify the performance of some techniques.
Finally, we outline some open issues and promising directions for NSINs
deserved for future study and discuss the corresponding challenges.
