###### Abstract  
A review of technological solutions and advances in the framework of a Vertical Heterogeneous Network (VHetNet) integrating satellite, airborne and terrestrial networks is presented. The disruptive features and challenges offered by a fruitful cooperation among these segments within a ubiquitous and seamless wireless connectivity are described. The available technologies and the key research directions for achieving global wireless coverage by considering all these layers are thoroughly discussed. Emphasis is placed on the available antenna systems in satellite, airborne and ground layers by highlighting strengths and weakness and by providing some interesting trends in research. A summary of the most suitable applicative scenarios for future 6G wireless communications are finally illustrated. space-air-ground communication network; 5G; 6G; millimeter waves; massive MIMO; CubeSat; satellite internet access; Internet of Things; UAV; LAP; HAP; antenna; phased array +
Footnote †: journal: Sensors  
0000-0001-5895-5888]C.M. The deployment of 5G wireless communication infrastructure started in various parts of the world around 2019 [17]. While network installation and distribution are underway, researchers started to investigate next sixth generation (6G) wireless communication networks [17; 18; 19]. The ubiquitous and seamless wireless connectivity, one of the many 5G goals, cannot be satisfied by exploiting only terrestrial infrastructures. Terrestrial BSs cannot be deployed in off-grid or inaccessible areas such as rural zones, deserts, oceans and, in general, in harsh and remote environments. Thereby, the integration of Unmanned Aerial Vehicle (UAV)-assisted wireless communications into 5G systems has attracted tremendous interest in the last few years [20; 21; 22; 23; 24; 25]. Although the research on 6G is still in its infant stage [26], it is envisioned that the concept of anytime and anywhere network access undergoes breakthroughs with the advent of next wireless communication generation with the fruitful integration of space, air and ground networks in the framework of a Vertical Heterogeneous Network (VHetNet) [27; 28]. To this end, it will be necessary to consider, and to manage, the coexistence of different wireless connectivity platforms from ground segment to space segment composed of dissimilar software and hardware architectures, network topologies and communications protocols. Artificial Intelligent (AI) and Machine learning (ML) technology will play an increasingly crucial role within network management and automation and to meet the reconfigurability demand [29]. Figure 1 shows a sketch of a VHetNet scenario by considering some satellite, airborne and terrestrial communication networks, vital features for ubiquitous and seamless purposes. As schematically shown, the overall network comprises three main layers: space, air and ground segments. While both the terrestrial and space segments are well-established telecommunication connectivity services, they face a variety of respective drawbacks and challenges. Thereby, to solve or partially mitigate these problems, the air communication layer will play an important complementary role for future wireless communication systems in providing universal and favorable access to the global network with the required Quality of Services (QoS) [30]. Figure 1: Example of a VHetNet scenario by considering some space, air and ground network components as envisioned in 6G wireless communications. In general, the air segment turns out to be essentially based on UAVs, also known as drones or atmospheric satellites, especially for wireless communication missions. Owing to their autonomy, flexibility, versatility, and of contained CAPital EXpedinture (CAPEX) and OPERating EXpedinture (OPEX), UAVs are becoming a more and more appealing option [31; 32]. However, in general, depending on the mission applications and goals, these flying platforms may be manned as well [33].