## 3 Airborne Segment
### Airborne Antennas Technologies
However, the benefit of adopting a triangular lattice in a massive MIMO scenario by providing a superior angular resolution as a function of the antenna beam steering is presented in [151; 152; 15]. An alternative approach using a triangular lattice of beams has also demonstrated interesting performance in array design with beam-switching operation [153]. A Ka-band phased array for HAPs application composed of open-ended substrate-integrated square waveguides and a 4-channel beamformer circuit produced by Anokiwave was described in [154]. A relevant mmWave beam steering 8 \(\times\) 8 array design solution operating from 26.5 to 31 GHz for 5G BSs based on gap waveguide technology is presented in [155]. Low loss feeding, high gain and exceptional thermal handling are guaranteed by an all-metal multi-layer assembly. Advances in 3D-printed technology and manufacturing processes make Dielectric Resonator Antenna (DRA) technology another attractive solution for the development of commercial array antennas at mmWave [156]. For instance, reference [157]  
Figure 7: Example of cells configuration in the \(uv\) plane: (**a**) single layer with \(n\) = 8 cells and (**b**) two layers with \(n\) = 20 cells. presents an 8 \(\times\) 8 array based on DRA fed by a slot antenna operating within a 5G wireless communications mmWave frequency band. An extensive overview of available antenna array technologies for mmWave communications is reported in [158]. With the purpose to reduce both cost and power consumption, key factors for future wireless communications, unconventional arrays designing such as sparse arrays [159; 160], thinned arrays [161] and subarrays techniques [162; 163] will represent a noteworthy airborne-array-designing technique in the future. However, achieving the same Equivalent Isotropic Radiated Power (EIRP) of a classic array--namely, each radiating element arranged on a regular and periodic lattice equipped with a Transmit/Receive Module (TRM) able to control both amplitude and phase of the signal--requires that the unconventional arrays' Power Amplifiers (PAs) have to provide a higher output power. This aspect introduces new challenges at the system level due to a greater tendency of PA nonlinearities that can affect the Error Vector Module (EVM) or the Adjacent Channel Power Ratio (ACPR), namely the modulation error of the signal with respect to the reference constellation and the users interference operating in the adjacent channels [164]. To overcome this issue, some linearization techniques such as the Digital Predistortion (DPD) [165] can be successfully adopted in order to maintain transceiver linearity as compliant with the systems' requirements. Another crucial aspect that phased array designers must face is calibration [166; 167], which allows one to balance some manufacturing errors and electronic inaccuracies (e.g., TRM amplitude and phase unbalance) capable of approaching the array theoretical radiative performance such as gain and side lobe level reduction. Some alterations of both the amplitude and phase of array elements feeding inevitably degrade the beamforming quality and hence the link data rate. However, array calibration represents one of the main array costs, and hence, its usage must be accurately assessed by making a sort of tradeoff between the desired performance and overall cost [168]. For instance, within the framework of 5G, many phased arrays without the calibration procedure have been proposed [168; 169; 170; 171] with the purpose to drastically reduce their cost by highlighting acceptable array performance degradation. Some phased array calibrations methods are described in [172; 173]. Despite the advantages of mmWave communications, such as larger spectrum, the adoption of large phased arrays for both UAVs and user mobility makes the antenna beam alignment between transmitter (TX) and receiver (RX) a challenging task to be tackled to guarantee the link robustness and hence satisfy the expected QoS [113].