# Secure On-Off Transmission in UAV Relay-Assisted mmWave Networks
## 4 Simulation Results  
In this section, some simulation results are provided to show the secrecy performance of UAV relay-assisted mmWave networks under both the on-off and non-on-off transmission schemes, and the impact of beam alignment error is discussed. Monte Carlo simulations are conducted in each figure to verify our numerical results. We consider that the uniform planar square antenna is adopted at each node, the antenna element number of the \(S\), UAV relays, \(D\) and eavesdroppers are denoted as \(N_{l}\), \(l\in\{S,R_{x},D,E_{e}\}\), and the main-lobe beamwidth, the antenna gain of the main lobe and side lobe of an \(N\) element antenna are given in Table 1. In addition, the mmWave communication operates at carrier frequency 28 GHz, and the simulation parameters are shown in Table 2.  
Figure 4 shows the TP versus the UAV relays' altitude \(H\) with different \(\mu\) and in both the on-off and non-on-off transmission schemes. First, under the on-off transmission scheme, we find that the TP first increases and then decreases as \(H\) becomes large. It can be explained that larger \(H\) causes higher LOS probability, but if the LOS probability is large enough, the TP is mainly affected by the path loss, which increases as \(H\) becomes large. Secondly, the TP increases by decreasing the transmission threshold \(\mu\). It demonstrates that we can adjust the TP by changing \(\mu\) under on-off transmission scheme. In addition, as \(C_{th}\) increases, no matter \(\mu\) equals to 4, 4.5, or 5, the TP decreases. The reason is that the selected UAV relay would be closer to \(S\) for achieving the decoding process when \(C_{th}\) increases, which causes that the path loss of second hop increases. \begin{table}
\begin{tabular}{c c} \hline Number of antenna elements & \(N\) \\ \hline Main-lobe Beamwidth \(\theta_{a}=\theta_{d}\) & \(\sqrt{\frac{N}{3}}\) \\ \hline Main-lobe Gain \(G_{M}\) & \(N\) \\ \hline Side-lobe Gain \(G_{m}\) & \(\frac{\sqrt{N}-\frac{S}{2\pi}N\sin\left(\frac{\alpha N}{3\sqrt{N}}\right)}{ \sqrt{N}-\frac{S}{2\pi}\sin\left(\frac{\alpha N}{3\sqrt{N}}\right)}\) \\ \hline \end{tabular}
\end{table} 
Table 1: 3D Antenna Pattern [29]. \begin{table}
\begin{tabular}{c c} \hline \hline
**Type** & **Values** \\ \hline Distance between \(S\) and \(D\) & 800 m \\ Radius of UAV relay area & \(Q=200\) m \\ Number of antenna elements & \(N_{S}=16\), \(N_{D}=N_{E_{e}}=4\) \\ Blockage parameters & \(a=9.6\), \(b=0.28\) \\ Path loss parameters & \(\alpha_{L}=2\), \(\zeta_{L}=61.4\) \\ Small-scale fading parameters & \(N_{L}=3\), \(N_{N}=2\) \\ Noise power [23] & \(N_{0}(\mathrm{dBm})=-174+10\lg(BW)+NF\) \\ Noise figure & \(NF=10\) dB \\ Bandwidth & \(\mathrm{BW}=1\) GHz \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Simulation Parameters. In Figure 5, we plot the SOP versus the transmit power \(P_{t}\) for different \(N_{R_{x}}\) and in both the on-off and non-on-off schemes. Observing from Figure 5, the SOP with \(N_{R_{x}}=16\) outperforms the SOP with \(N_{R_{x}}=9\) since the received SNR can be enhanced by adopting the antennas array with large amount of elements. Furthermore, the SOP can be improved in the on-off transmission scheme especially with large \(\mu\). This is because that the selected relay performs data transmission only when the channel quality of \(R_{0}\to D\) link beyond \(\mu\) in the on-off transmission scheme, and larger \(\mu\) means better channel from \(R_{0}\) to \(D\) when transmission occurs. It is worth mentioning that the SOP is not a monotonous function of \(P_{t}\) when \(\mu\) is large. It can be explained that the improvement of SOP is obvious with large \(\mu\) in the low \(P_{t}\) region due to the large gap between legitimate and eavesdropping channel capacities, but the eavesdroppers also benefit from larger \(P_{t}\), as such, the channel capacity gap becomes narrow and then approach a constant as \(P_{t}\) continue increasing.