## 5 Application Scenarios
The work in [210] focuses on the role of satellites in 5G networks, highlighting that enhanced mobile broadband (eMBB), for which user data rates and spectrum efficiency are crucial, and massive Machine Type Communications (mMTC) are to be considered as common scenarios, in which the satellite plays a role as backhaul to interconnect separate parts of the same 5G network. In the case of mMTC, the ability to support a multitude of connections is fundamental, distributed over time and frequency, each exchanging few data packets. Additionally, satellite systems may strongly support delay-tolerant services requiring high reliability and high availability [210]. The work in [212] highlights that the terrestrial infrastructure, in its current state, may be insufficient to guarantee 5G Key Performance Indicators (KPIs) in some scenarios, for instance, in providing ubiquitous coverage, or in the case of infrastructure unavailability, thus requiring the use of aerospace solutions to increase both the resilience and the availability of the network, in turn improving the Quality of Experience (QoE) perceived by users. This is particularly true for IoT scenarios [198; 212; 213] in which both resilience and network availability may be key requirements, as in the case of smart grids [200]. The different roles and equipment envisaged for IoT devices in 5G scenarios are analyzed in [212] when considering the joint use of satellites, UAVs, and ground nodes, proposing UAVs to act as 5G User Equipment (UE), as base stations (5G-gNBs), or as transparent relay nodes. Satellites, especially LEO ones, can act as 5G-gNBs or as relays depending on the payload (regenerative or transparent, respectively). The case of future 6G networks is considered in [211], emphasizing that the SAGIN network paradigm will become even more central in upcoming developments, and underlining how the combination of Artificial Intelligence (AI) and Software Defined Networking (SDN)/Network Functions Virtualization (NFV) will enable zero-touch orchestration, optimization, and management of networks. The upcoming 6G standard, still in its definition phase, expands the service classes foreseen in 5G. According to [19], the ones to be added are the so-called Mobile Broadband  
Figure 9: Common application scenarios by leveraging SAGINs and key services offered by the network infrastructure at different layers of the SAGIN. Reliable Low Latency Communication (MBRLLC), the massive Ultra-Reliable Low Latency Communications (mURLLC), and what the authors dub as Human-Centric Services (HCS), and multi-purpose control, localization, sensing, and energy services. The latter two are classes comprising a vast group of applications, such as multisensory extend reality or even wireless Brain-Computer Interactions (BCI). Some services and some scenarios will require on-demand capacity to be deployed, or on-demand coverage in poorly covered or too busy areas. Because of this, the use of LAPs or HAPs according to the area and to the requirements is seen as a necessity to support the ground infrastructure when and where needed. Due to the integration of ground and airborne networks, communications must be supported in 3D space, accounting for the additional degrees of freedom because of the different heights of LAPs and HAPs, if not even satellites. Such a complex interplay has the potential to support existing services and open to new ones. For instance, the paradigm of autonomous driving is attracting increasing attention all around the world, and for it to be a reality in every corner of the world, satellite access is likely crucial, providing access to the network in poorly covered areas (see the case of rural ones), real-time maps updates and additional services, such as safety-related ones [18; 26; 50]. Furthermore, the idea of smart cities strongly relies on 3D communications, with much potential for UAVs to provide coverage extension services, on-demand bandwidth, monitoring services, and mobile crowdsensing [26], among others.