## 3 Simulation Results  
In this section, we evaluate the performance of the proposed network of multiple UAV relays supporting the uplink mmWave NOMA system. In the considered system, UAV relays are located in different positions with different spatial directions with respect to the BS, and hybrid mmWave beamforming with MUSIC technique is adopted to derive the different DoAs in order to optimize the achievable uplink sum-rate. In detail, we simulate three different scenarios, i.e., an UL mmWave NOMA without a UAV network, a single UAV and a network of multiple UAVs supporting the UL mmWave NOMA. The cellular network has an area of \(300\times 300\) m\({}^{2}\) with the BS located in the center. A total of 32 users are randomly and uniformly distributed in this area. The BS is equipped with an ULA of \(N_{R}=64\) antennas and \(N_{RF}=4\) RF chains to simultaneously serve a number K of UAVs with \(K\leq N_{RF}\). The UAVs deployed in the cell have different DoAs at the BS uniformly distributed between \([-\pi/2,\,...\,\,\pi/2]\), and each UAV covers a pair of users who have poor connections with the BS due to a severe blockage condition. The UAV has \(N_{UR}=16\) receiving antennas which transmits towards the BS with one antenna. We choose to associate the considered blocked user pair not to the closest UAV in term of distance, but to the one offering the best communication performance, as especially in a very dense urban scenario, the nearest available UAV may be in NLOS visibility. An accurate DoA estimation at the BS can monitor the fluctuations of the UAV motion and consequently improve the performance of the overall uplink sum rate. The main parameters for an urban scenario are summarized in Table 1[20,27]. In Figure 3, the achievable sum rate is shown in terms of the varying signal-to-noise ratio (SNR) for the multi-UAV network with respect to a single UAV scheme in the case of an LOS environment. The multi-UAV scheme outperforms the single UAV scheme even for low SNR values, as the channels from users to each UAV are in LOS in these simulations realizations and consequently demand low power to transmit towards the UAV. In the same figure, the case of a multi-UAV scheme without DoA information acquisition is highlighted to validate the performance of the MUSIC method. Indeed, the DoA estimation error is an important problem due to the UAV mobility and dynamic channel variations. However, a more accurate DoA estimation occurs when the UAV is far from the BS[30], and in particular the use of the MUSIC technique allows both the DoA and received powers to be estimated more accurately with respect to other methods[19,24,31]. Figure 4 shows the achievable sum rate results with respect to SNR when multiple UAVs are considered for both the case of LOS and NLOS environments. The MUSIC method at the BS to acquire the DoA information of the UAVs is used. In these simulation results, the heights of UAVs are not considered too high, because even if the links have a higher probability to be in LOS, the impact of the increased distance between the pair of users and the serving UAV decreases the overall link budget, and the links can have a worse SNR. The results are compared with the performance of the UL NOMA mmWave system without the multi-UAV network, i.e, the users can communicate directly by NOMA mmWave link to the BS. The NOMA mmWave link to BS is considered only for evaluating the performance comparison as, practically, the user pair covered by each UAV has a severe blockage and cannot communicate with the BS. The achievable sum rate of the multiple-UAV network achieves a considerably better performance than the direct NOMA mmWave connection, as shown in Figure 4, providing a improvement of about 6 dB for high SNR values.