The space-air-ground integrated network (SAGIN) represents a major breakthrough in the advancement of Beyond 5G/6G, seamlessly combining terrestrial network (TN) and nonterrestrial networks (NTN) into a unified multi-layered system by creating a more cohesive and efficient communication infrastructure. However, the high mobility of NTN platforms creates significant challenges in managing user transitions across these interconnected layers. This article offers a concise overview of the unique demands of mobility management (MM) in SAGIN and the architectural developments required to meet the needs of various service categories, including enhanced mobile broadband (eMBB), massive machine type Communications (mMTC), and ultra reliable low latency Communications (URLLC). Additionally, the article details specific strategies for vertical handovers in SAGIN, as described in the literature, with a focus on the roles of software defined network (SDN), network function virtualization (NFV), and network slicing (NS).

Integrating terrestrial (TN) and non-terrestrial (NTN) networks into 3D user-centric cell-free massive multiple-input multiple-output (UC-CFmMIMO) architectures poses significant challenges, including extreme Doppler shifts, spatially diverse delay spreads and dynamic topology changes. This paper provides the first comprehensive comparison of four waveforms — Orthogonal Time-Frequency-Space (OTFS), Affine Frequency-Division-Multiplexing (AFDM), Orthogonal-Delay-Doppler-Multiplexing (ODDM) and Orthogonal-Frequency-Division-Multiplexing (OFDM) — across three deployment scenarios: ground-based UC-CFmMIMO, NTN-dominant UC-CFmMIMO and 3D UC-CFmMIMO. By integrating 3GPP TN (TR 38.901) and NTN (TR 38.811) channel models within a unified framework, we demonstrate the superiority of AFDM in NTN scenarios and the robustness of OTFS in unified space-air-ground networks. ODDM’s staggered pulse shaping reduces ground-level intersymbol interference and OFDM remains optimal for low-mobility deployments. We discuss the need for adaptive waveform switching to balance Doppler/delay robustness, latency and complexity, offering actionable guidelines for 6G system designers.

3D Network convergence across ground, air and space wireless communications holds immense promise, offering disruptive use cases and addressing the challenges of ubiquitous connectivity. Next-generation mobile networks may need to transcend the current ground-focused paradigm and fully embracing aerial and spaceborne communications becomes essential. The unified 3D network requires adaptations at the radio as well as at the network level. This paper presents an analysis of the dynamic placement of Radio Access Network (RAN) functions, exploring the specific challenges associated with the links between Distributed Units (DUs) and Radio Units (RUs), as well as between DUs and Centralized Units (CUs), within the context of 3D unified network. The dynamic placement of RAN functions introduces significant complexities, including latency management, synchronization, resource allocation, etc. The DU-RU link faces challenges such as high bandwidth requirements, real-time processing needs and efficient beamforming techniques to maintain robust connectivity. Meanwhile, the DU-CU link must address issues related to mobility, handover processes and maintaining low-latency communication despite the physical separation of network components. By analyzing these challenges, this paper provides insights into the necessary technological advancements and strategic approaches required to optimize the dynamic placement of RAN functions, ensuring the seamless operation and high performance of 6G networks.

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).