# Secure Transmission in mmWave Wiretap Channels: On Sector Guard Zone and Blockages
## 5 Numerical Results
In addition, we observe that the secrecy throughput of \(r=50\) m is always superior to that of \(r=20\) m. The reason is that there does not exist an eavesdropper within the sector secrecy guard zone when transmitting, and the secrecy performance becomes better with \(r\) increasing. Again, for achieving the same secrecy throughput, the transmit power required at 28 GHz is lower than 73 GHz. Figure 5 presents the central angle \(\theta_{S}\) of sector secrecy guard zone on the secrecy throughput. It is obvious that the secrecy throughput of the system decreases with the central angle \(\theta_{S}\) of the sector secrecy guard zone when the transmit power and the power allocation factor are sufficiently large. Specifically, under the same conditions, the performance of 73 GHz is superior to that of 28 GHz, which is mainly due to the difference path loss [45]. In addition, in the larger central angle region, the smaller radius is better than the larger radius due to fact that the larger sector secrecy guard zone may contain more eavesdroppers, which is detrimental to the secrecy performance. Figure 6 presents the effects of transmit power allocation factor on the secrecy throughput with the different frequency. We note that there exists an optimal transmit power allocation factor \(\mu\) to maximize the secrecy throughput. When the \(\mu\) is very small, it means that almost all power is allocated to AN, and the secrecy throughput is very small. With the increase of the power allocated to the information signal, that is, the increase of the power allocation factor, the secrecy throughput increases gradually. However, when the power allocation factor increases to a certain value, the secrecy throughput starts to accelerate drop, the reason is that when the power allocation factor is increased to a certain value, the power used to AN decreases, which increases the possibility that an eavesdropper can intercept information, and the security cannot be guaranteed. Therefore, more power should be allocated to the AN to interfere with the eavesdropper, which demonstrates that it is very important to set the power allocation of the AN and the information signal properly. In addition, when the power allocated to AN is reduced to a certain value, the attenuation of \(\eta\) in the wider main lobe is faster, which is because more eavesdroppers may be located in the wider sector secrecy guard zone. Again, for the same circumstance, when the power allocation factor is increased to a certain value, the secrecy throughput reaches the maximum value, and the performance of 73 GHz is better than that of 28 GHz with the further increase of power allocation factor for a given \(r\). Figure 7 presents the effects of the \(\eta\) versus the eavesdropper density \(\lambda_{E}\) with the different central angle \(\theta_{S}\). We see that, when increasing of \(\lambda_{E}\), the secrecy throughput declines. This can be explained by when \(\lambda_{E}\) is low, the eavesdroppers located in \(E\in\Phi_{1}\) and \(E\in\Phi_{2}\) are indeed harmful for secrecy. However, as \(\lambda_{E}\) grows large, the secrecy throughput increases. This is because, in this case, the eavesdropper will be in the sector secrecy guard zone, the transmitter emits AN to interfere with the eavesdropper, and the secrecy performance will be improved, which shows that AN can improve the secrecy throughput of the system. If \(\lambda_{E}\) further increases, the secrecy throughput starts to decrease; the reason is that, as the number of eavesdroppers in the sector secrecy guard zone increases, the wiretapping capability of eavesdroppers increases, which deteriorates the secrecy performance. Obviously, when eavesdroppers exist in the sector secure region, transmitting AN is effective, but there is an appropriate \(\lambda_{E}\), which makes the secrecy throughput reach the maximum. This shows that increasing the density of eavesdroppers not always deteriorates the secrecy performance. In this case, the large sector secrecy guard zone is superior to the small one.