Near Space Communications: A New Regime in Space-Air-Ground Integrated Networks

Precipitated by the technological innovations in near-space platform stations (NSPSs), the near-space communication (NS-COM) network has emerged as an indispensable part of the next-generation space-air-ground integrated network (SAGIN) that facilitates ubiquitous coverage and broadband data transfer. This article aims to provide a comprehensive overview of NS-COM. First, we investigate the differences between NS-COM and the existing terrestrial cellular networks as well as satellite-based and unmanned-aerial-vehicle (UAV)-based communication networks, which is followed by a review of NS-COM development. Then we explore the unique characteristics of NS-COM regarding the platforms and the propagation environment of near space. The main issues of NS-COM are identified, resulting from the extremely long transmission distance, limitations of the communication payloads on NSPS, and the complex atmospheric constitution of near space. Then various application scenarios of NS-COM are discussed, where the special technical requirements are also revealed, from physical-layer aspects like transceiver design to upper-layer aspects like computational offloading and NSPS placement. Furthermore, we investigate the coexistence of NS-COM and ground networks by treating each other as interferers or collaborators. Finally, we list several potential technologies for NS-COM from the perspective of spectrum usage, and highlight their technical challenges for future research.