# Secure Transmission in mmWave Wiretap Channels: On Sector Guard Zone and Blockages
## 5 Numerical Results  
In this section, some representative simulation results are presented to verify our theoretical analysis, and characterize the secrecy performance of the mmWave wiretap network. A set of closed-form expressions are derived in an adaptive transmission scheme to analyze the effects for different system parameters. We assume that the noise power is \(\sigma_{D}^{2}=\sigma_{E}^{2}=-70\) dBm, and the LOS probability function is \(P_{L}\left(r\right)=e^{-\beta r}\) with \(\frac{1}{\beta}=141.4\). According to [45], we focus on the carrier frequenciey of 28 GHz and 73 GHz. The Nakagami fading parameters of the LOS (NLOS) link are \(N_{L}=3\left(N_{N}=2\right)\), the parameters of path-loss model are \(\beta_{L}=61.4\) dB, \(\alpha_{\mathrm{L}}=2,\ \beta_{N}=72\) dB, \(\alpha_{\mathrm{N}}=2.92\) and \(\beta_{L}=69.8\) dB, \(\alpha_{\mathrm{L}}=2,\ \beta_{N}=82.7\) dB, \(\alpha_{\mathrm{N}}=2.69\), \(C_{L}=10^{-\frac{\beta_{L}}{10}}\) and \(C_{N}=10^{-\frac{\beta_{N}}{10}}\) can be regarded as path-loss intercepts on the reference distance of LOS and NLOS links. Figure 2 presents the effects of the \(p_{co}\) and \(p_{so}\) versus the transmit power in different frequency bands, namely 28 GHz and 73 GHz. Obviously, with the increasing of \(P\), the reliability performance of the legitimate receiver increases due to the decrease of the COP for a given power allocation factor, while the secrecy performance would decline. When the power increases to a certain value, the SNR received by the eavesdropper is close to a fixed value from Equation (6), the SOP remains unchanged and the COP of the legitimate receiver is close to zero. This can be explained as follows: on the one hand, although the eavesdropper is in the sector guard zone, the system can still guarantee a secure link to a legitimate receiver by transmitting AN to confuse the eavesdropper. On the other hand, it is because \(P\) has different effects on the COP and SOP in the case of LOS and NLOS. In addition, when \(P\) is large, the difference of SNR between legitimate link and eavesdropping link tends to be constant. It means that the reliability of the system can be improved effectively by increasing the power of the system. Again, we obtain an important observation that secrecy transmission at 28 GHz is better than that at 73 GHz in a low transmit power region. Figure 3 presents the effects of the SOP, \(p_{so}\), and the COP, \(p_{co}\), versus the eavesdropper density \(\lambda_{E}\) with the different frequency. As \(\lambda_{E}\) increases, we see that the \(p_{so}\) keeps increasing and the \(p_{co}\) remains constant for given a power \(P\). In particular, compared with the 73 GHz band, the 28 GHz band reduces the \(p_{co}\) and improves the reliability of the system, but at the same time increases the \(p_{so}\) and reduces the secrecy performance of the system. These observations can help the system designer to select different frequency bands according to the actual performance requirements. For example, when the actual system requires high reliability, it is suitable to select the 28 GHz band. Figure 4 presents the effects of the \(\eta\) versus the transmit power with the different sector secrecy guard zone radius \(r\) and frequency. We see that there exists an optimal \(P\) for maximizing the secrecy throughput at the considered mmWave frequencies. At a low transmission power region, the secrecy throughput of 28 GHz is better, and the same result can be achieved at 73 GHz when the transmission power becomes sufficiently large. The reason is that, in the case of low transmission power regime, mmWave link at lower mmWave frequencies experiences lower path loss and has stronger signal strength, thus achieving better performance. However, in the high transmission power regime, due to the high path loss at higher mmWave frequencies, the interference received by the legitimate user becomes lower, and the signal strength of the eavesdropper is also reduced at higher mmWave frequencies.