## 3 Airborne Segment
### Network Topology  
LAPs and HAPs can be deployed in wireless communication networks with different topologies according to the mission needs within which they act mainly as aerial relays or aerial BSs to support wireless communication [45]. In the former case, the flying platforms profitably collaborate with ground BSs or the satellite layer by offering an alternative reliable link by forwarding the incoming data to the recipient. This mode of operation is particularly helpful in emergency situations such as military operations and disaster rescue [49]. Conversely, in the latter case, they play as aerial BS by providing a wide wireless connectivity between ground users and the core network in the absence of terrestrial network or temporary ground station malfunction or maintenance. Moreover, due to their rapid deployment, the airborne segment can help in quickly deploying communication networks after natural disasters such as floods and earthquakes [93; 112]. Furthermore, by using HAPs, it is possible to establish a consistent connection between terrestrial users or LAP and satellites constellations, such as CubeSats [73; 74] LEO satellites constellations. In addition to the aforementioned UAV applications, data gathering represents another promising use case. By exploiting their versatility and flexibility, they can collect and monitor data from different wireless sensor networks deployed to sense the environment easily and in a cost-effective way.  
A possible network partition is represented by non-hybrid or hybrid topology [45]. In the former scheme, illustrated in Figure 3, the flying platforms (i.e., LAPs or HAPS) can work as a BS transceiver or be part of a mesh network of the airborne layer [113], providing a communication link between end users and a core network.  
In more details, each UAV, equipped with multiple antenna arrays, is capable of establishing a directional communication link with the different users distributed on the coverage area as well as to provide a wireless communication link with its neighboring flying platforms, hence realizing a flying mesh network capable of improving the overall system performance. This network topology scenario appears to be mainly dedicated to rural zones devoid of terrestrial infrastructures.  
In a hybrid topology, as shown in Figure 4, the flying platforms can be fruitfully integrated into an air-ground or satellite-air-ground communication network. They can work both as aerial relays and aerial BS to help the whole wireless infrastructure in offering communication services. This communication scheme, crucial for achieving both ubiquitous and seamless connectivity, appears to be the most relevant for future communication systems [43].  
Figure 3: Example of non-hybrid network topology by involving the airborne segment only.  
In general, it is conceivable to think that the overall wireless communication system could be composed of different smaller wireless networks organized with a dissimilar topology. For this reason, the topology management system represents a challenging task to tackle in order to reach superior systems performance as well as to guarantee the desired QoS in future applicative scenarios. In this framework, AI and ML technology will represent a fundamental resource within the network management and automation as well as meet the reconfigurability demand [29].