## 3 Airborne Segment
### Spectral Efficiency Improvement  
In future wireless communication generations, airborne communications are expected to play a prominent role in the delivery of next-generation services. The UAVs acting as flying platforms can provide a reliable aerial access link to different ground or satellite users in different scenarios such as temporary ground stations disruption, hotspot areas or large public venues--scenarios in which many users strain the available wireless resources [114]. Therefore, efficient wireless communication technologies are essentially for serving multiple users and ensure the desired QoS. The Multiple-Input-Multiple-Output (MIMO) technique represents a possible wireless technology strategy that can improve network performance by exploiting both the Diversity Gain (DG) and the Multiplexing Gain (MG) [4]. A MIMO system with the corresponding MIMO channel matrix \(H\), offers \(K\) = _rank_ (\(HH^{*}\)) parallel Single-Input-Single-Output (SISO) subchannels with different gain where it is possible to send different streams of data. In general, two scenarios are possible: absence of knowledge of the Channel State Information (CSI) at the transmitter and knowledge of the CSI at the transmitter (CSIT). In the former case, the transmitted power is evenly split into each Kth subchannel. Conversely, in case of CSIT, it is possible to obtain an optimum power allocation for each \(K\)th subchannel (water filling technique) according to the eigenvalues (\(\lambda_{i}\)) of the MIMO channel matrix \(H\) in order to optimize the spectral efficiency. However, in practice, perfect CSIT is not possible due to errors on channel estimation and feedback delay [115]. MIMO performance in case of feedback delay and channel estimation errors have been analyzed in [116; 117; 118]. For the airborne layer, the CSIT concern turns out to be even more pronounced due to the high altitudes and the movements of LAPs and HAPs flying platforms [47]. An interesting solution for the CSIT mitigation has been proposed in [119] where a TB relay and an effective interference alignment scheme for maximizing the spectral efficiency of HAPs-ground stations communications were presented. Moreover, a virtual MIMO (V-MIMO) system, realized by connecting multiple HAPs, has been proposed in [120]. Another attractive technology that could be exploited by aerial platforms for improving both Spectral Efficiency (SE) and Energy Efficiency (EE) is represented by massive  
Figure 4: Example of hybrid network topology by involving terrestrial, airborne and satellite segment. MIMO technology [6; 7; 8; 9] capable of serving multiple users simultaneously in the same time-frequency resource through smart array antennas with multibeam radiation pattern [6; 7; 8; 9]. Specifically, in [121; 122], the authors reported some examples of massive MIMO applied to HAPs, whereas the potential of massive MIMO systems for communication with UAV-based LAPs are illustrated in [123; 124; 125]. In addition of the aforementioned wireless communication techniques, Full-Duplex (FD) technology represents a promising solution to meet the tremendous increasing system requirements as well as a viable alternative in addressing spectrum scarcity [126]. More in detail, an FD wireless terminal is capable of transmitting and receiving simultaneously in the same frequency band by allowing, theoretically, to double the SE with respect to conventional Half-Duplex (HD) systems [127]. However, one of the biggest impediments of FD communication that leads to undermining hypothetical SE doubling is the presence of wireless interference. Due to the simultaneous uplink and downlink wireless communications, it is possible to generate interference to adjacent users or BSs and, at the same time, receive interference from them [128; 129]. Figure 5 shows an example of both HD and FD wireless communication.