With the expansive deployment of ground base stations, low Earth orbit (LEO) satellites, and aerial platforms such as unmanned aerial vehicles (UAVs) and high altitude platforms (HAPs), the concept of space-air-ground integrated network (SAGIN) has emerged as a promising architecture for future 6G wireless systems. In general, SAGIN aims to amalgamate terrestrial nodes, aerial platforms, and satellites to enhance global coverage and ensure seamless connectivity. Moreover, beyond mere communication functionality, computing capability is increasingly recognized as a critical attribute of sixth generation (6G) networks. To address this, integrated communication and computing have recently been advocated as a viable approach. Additionally, to overcome the technical challenges of complicated systems such as high mobility, unbalanced payloads, limited resources, and various demands in communication and computing among different network segments, various solutions have been introduced recently. Consequently, this paper offers a comprehensive survey of the technological advances in communication and computing within SAGIN for 6G, including the system architecture, network characteristics, general communication, and computing technologies. Subsequently, we summarize the pivotal technologies of SAGIN-enabled 6G, including the physical layer, medium access control (MAC) layer, and network layer. Finally, we explore the technical challenges and future trends in this field.

Today, the worldwide economy and society are already taking advantage of the first deployments of 5G networks. However, the challenges that will be posed by 2030 networks on the infrastructure, technologies, and performance requirements call for a further leap in mobile communications. To this aim, evolutions of the 5G concepts in 5G-Advanced, and rethinking of future Radio Access Technologies, i.e., 6G which is expected to be the next revolution in the mobile ecosystem, are already being addressed by the scientific and industrial communities. In this framework, as demonstrated by 3GPP Rel. 17 specifications, Non-Terrestrial Networks (NTN) are expected to play a vital role in near and long term mobile communications, based on the joint optimisation of the terrestrial and non-terrestrial components in a unified 3D architecture. In this paper, we extensively discuss the 3GPP normative activities for 5G (Rel. 17), 5G-Advanced (Rel. 18 onward), and 6G (Rel. 20+) in terms of enabling features, architectures and interfaces, and protocols. Then, we explore the evolution of the 5G services towards 5G-Advanced and 6G NTN applications. Finally, we discuss the evolution to 6G network infrastructures with respect to the main performance requirements.

The next generation of terrestrial radio communications, so-called fifth generation (5G) New Radio (NR), beyond the traditional bands below 6 GHz, has been also specified to operate over millimeter waves (mmWaves), in the so-called Frequency Range 2 (FR2). Such frequency bands have been since decades the ‘natural habitat’ for fixed satellite services (FSS). In this new landscape, this paper preliminary investigates the feasibility of non-geostationary orbit (NGSO) satellites directly accessing NR-enabled User Equipment (UE) in mmWaves, from a regulatory, UE characteristics, space segment, link budget and system point of view. It also identifies future R&D needs in this area.

Non-orthogonal multiple access (NOMA) technologies are considered key technologies for terrestrial 5G massive machine-type communications (mMTC) applications. It is less known that NOMA techniques were pioneered about ten years ago in the satellite domain to match the growing demand for mMTC services. This paper presents the key features of the first NOMA-based satellite network, presenting not only the underlying technical solutions and measured performance but also the related deployment over the Eutelsat satellite fleet. In particular, we describe the specific ground segment developments for the user terminals and the gateway station. It is shown that the developed solution, based on an Enhanced Spread ALOHA random access technique, achieves an unprecedented throughput, scalability and service cost and is well matched to several mMTC satellite use cases. The ongoing R&D lines covering both the ground segment capabilities enhancement and the extension to satellite on-board packet demodulation are also outlined. These pioneering NOMA satellite technology developments and in-the-field deployments open up the possibility of developing and exploiting 5G mMTC satellite- and terrestrial-based systems in a synergic and interoperable architecture.