# Secure Transmission in mmWave Wiretap Channels: On Sector Guard Zone and Blockages
Using AN to enhance the reliability of legitimate links and interfere with eavesdropping links, so as to enlarge the gap between legitimate links and eavesdropping links to improve security. For different network applications, References [17; 18] proposed the secrecy enhancement by using secrecy protected zone and AN, and discussed the relationship among protected zone, the transmission power and AN. In [19], the secrecy guard zone protocol was studied for achieving the secure transmission in an underlay cognitive radio network. However, all the aforementioned works on secrecy guard zone are only considered in the conventional microwave networks; they can not be directly applied and need to be re-evaluated in an mmWave system because of the unique characteristics of the mmWave communication system. PLS in mmWave systems has attracted interest with enthusiasm [20; 21; 22; 23]. The characteristics of mmWave communication system, such as larger bandwidth, large antenna arrays, directionality and short range transmission, could provide stronger PLS for mmWave system. Using analog beamforming in the mmWave base station, the secrecy throughput was analyzed from the perspectives of delay-tolerant and delay-limited transmissions in [24]. Considering the characteristics of mmWave cellular networks, [PERSON] [25] studied the secrecy performance of the noise-limited and the AN-assisted mmWave networks under the stochastic geometry framework. [PERSON] [26] examined the impact of AN on the secrecy rate; it was shown that power allocation between the information signal and AN need to be carefully determined for secrecy performance enhancement. A discrete angular domain channel model considering spatial discernibility path was proposed in [27], and three secure transmission schemes were investigated by depending on whether there was a common path between the destination and the eavesdropper. Although many insightful conclusions have been drawn in [25; 27; 28], the effects of blockages and the information leakage problem of the side lobe are not considered, but they are assumed to be ignored. In fact, blockages have different effects on communications in different environments, and side lobe may also lead to information leakage. On the other hand, directional beamforming is an important technique for mmWave systems because it provides array gains which overcome the huge path loss and acquire adequate link margins [4]. For mathematical tractability [29], when the simple maximum signal-to-noise ratio (SNR) beam steering is assumed, it is meaningful to approximate the actual array pattern with the sector pattern, where the directional gains of the main lobe and the side lobe are constant. The approximation of the sector mode makes it possible to describe the complex beamforming mode. Nevertheless, the locations of eavesdroppers in mmwave wiretap channels are randomly distributed, thus they may be located in signal beams and then could intercept confidential information. However, for a mmWave wiretap network, comprehensive secrecy performance analysis has not been provided under a sector secrecy guard zone, which motivates our work. In this paper, we investigated secrecy performance under the Nakagami fading channel in a mmWave wiretap network. In order to improve the secrecy performance of mmWave wiretap network, a secrecy guard zone is introduced around the transmitter, in which eavesdroppers are not allowed to roam. It is assumed that the eavesdroppers can be detected, provided that they enter secrecy guard zone. Considering a more practical mmWave communication scenario, the effects of blockage are taken into account such that links are either line-of-sight (LOS) or non-line-of-sight (NLOS). In our prior conference paper [30], we discussed how to use sector guard zone in mmWave networks. Based on our previous work, assuming the transmitter is capable of detecting the existence of eavesdroppers in the finite guard zone, an adaptive transmission scheme is adopted for secrecy transmission.