Opportunities and Limitations of Space-Air-Ground Integrated Network in 6G Systems

Diminishment of dead zones, global coverage of the Earth and access to the Internet anywhere and anytime are expected with the development of the next-generation wireless networks. The latest technologies such as cloud computing, artificial intelligence and machine learning, augmented and virtual reality, the Internet of Things (IoT) and billions of connected devices are revolutionising the expansion of capabilities of wireless communication systems. 6G technology promises a reliable high-speed and almost instantaneous connection for any subscriber, and it allows us to create a personalised environment for each user using smart devices and artificial intelligence. The integration of airborne and spaceborne platforms into 6G Terrestrial Network (TN) will provide coverage across land, sea and sky, and connection whether users walk, drive or fly and supply services even if one of the services cannot be provided. In this study, we analyze the concept of developing a three-dimensional coverage of 6G communication services, where a Non-Terrestrial Network (NTN) is listed as a new network segment. The integration of NTN will lead to new research questions and an extension of 6G NTN application scenarios. Possible application scenarios, challenges and key technologies of the unified 6G NTN communication link are considered. In particular, we discuss (i) the effects of atmosphere, altitude, and speed of NTN platforms on signal quality; (ii) the architecture design supporting user-centric scalable Cell-Free Massive Multiple-Input Multiple-Output (CFmMIMO) for NTN, with the proposal of a 3D model of scalable user-centric cell-free massive MIMO with multiple interconnected Central Processing Units (CPUs); and (iii) briefly discuss the potential use of Reconfigurable Intelligent Surfaces (RIS) and Artificial Intelligence (AI).