# An Unsupervised Machine Learning Approach for UAV-Aided Offloading of 5G Cellular Networks
## 4 Performance Evaluation
### Comparative Evaluation of UDP in Terms of the Received Signal Strength (RSS)  
In Figure 4, we observe users' average downlink received signal strength (RSS) before and after the UDP process. Specifically, the MBS bar refers to users' average RSS when they are served from the MBSs with a value equal to \(-\)73.5 dBm. Also, with the optimal placement of 5 UAV-BSs through the UDP process, the average RSS from the UEs increases to \(-\)68.8 dBm, which is an expected value, as the available access points introduced to the existing network increases. Additionally, both UDP and KDP methods that employ machine learning algorithms outperform the PDPP and PDPR deployment scenarios under both LOS and NLOS channel conditions. This happens since the picocells are fixed placed at expected points of users' concentration. On the contrary, UAV-BSs are placed in actual concentration points because they consider users' locations in real-time, as well as the on-demand user traffic. Moreover, UDP outperforms KDP with average received signal strengths of the UEs, \(-\)68.8 dBm and \(-\)69.8 dBm, respectively. This is due to the nature of k-means and k-medoids algorithms. Specifically, k-medoids clustering is more robust to noise and outliers than k-means and ensures that the UAV-BSs will be adequately positioned, thus resulting in improved average RSS value of the UEs located inside the formed clusters. The robustness of UDP compared to KDP is confirmed in Figure 5, which shows the improvement of the total average received signal power from the UEs by integrating either more PBS or UAV-BSs. As the number of base stations increases, regardless of whether it is PBS or UAV-BS, the quality of the user's reception improves. This is reasonable as the integration of more PBSs or UAV-BSs decreases the average distance between the users and the available base stations. Furthermore, as it can be observed from Figure 5, UDP outperforms KDP at all configurations. Figure 3: Percentage of UEs offloaded from the MBSs for different number of UAV-BSs/PBSs. Comparative Evaluation of UDP in Terms of System Sum Rate, User Average throughput, and Spectral Efficiency  
In this section, simulation results are presented in terms of system sum rate, user average throughput, and spectral efficiency. We consider a 5G H-CRAN system operating under orthogonal frequency division multiplexing (OFDM), based on a standalone orthogonal multiple access (OMA) scheme [27]. Thus, the available spectrum is equally  
Figure 4: Average downlink RSS with 5 UAV-BSs/PBSs. Figure 5: Average downlink RSS for all UEs in the region of interest for different number of UAV-BSs/PBSs. distributed to the users of the system. In Figure 6, comparisons between the proposed and the compared methods are presented in terms of system sum rate. Before the deployment of either UAV-BSs or PBSs, MBS downlink sum rate is equal to 122 Mbps. We present the MBS downlink sum rate as the lower bound for the system sum rate. After the integration of UAV-BSs / PBSs, we observe that as the number of UAV-BSs / PBSs increases, the system sum-rate increases significantly. Notable system sum-rate gains can be harvested using the UDP and KDP methods, as compared to the PDPP and PDPR under both LOS and NLOS channel conditions, for all configurations. Furthermore, UDP behaves marginally better than KDP in terms of system sum rate, when the number of UAV-BSs is less than 4. For more than 4 UAV- BSs, UDP exceeds KDP in terms of system sum rate. The maximum difference of 28 Mbps between UDP and KDP is presented when the available UAV-BSs are 10. In this case, UDP achieves system sum-rate equal to 857 Mbps, while the KDP method achieves 829 Mbps. Moreover, as shown in Figure 7, the trend of the average throughput for each UE is the same as the trend shown for the system sum rate regarding the examined methods. The average throughput of each UE utilizing only MBSs is equal to 0.8 Mbps, while with the use of UDP and KDP methods it ranges from 1.3 to 5.7 Mbps and 1.2 to 5.5 Mbps, respectively.