# Secure On-Off Transmission in UAV Relay-Assisted mmWave Networks
## 1 Introduction
### Related Work and Motivation  
Cooperative relay has been identified as an effective technique to improve the system performance of mmWave networks, and has attracted considerable attention recently [12; 13; 14]. To be specific, in [12], the coverage probability of the mmWave networks aided by relay, in which the base stations and relays are modeled under the stochastic geometry framework, has been investigated in both the best path and best relay selection protocols. Deploying the directional antennas at relays helps improve the reliability in the mmWave networks, the maximum achievable rate of the mmWave relay networks has been examined by considering the beamwidth of directional antennas in [13]. In addition, [PERSON] et al. [14] has demonstrated the effects of co-channel interference on outage performance in the mmWave relay networks. Nevertheless, all above mentioned works focus on the mmWave communication networks on the ground. At present, due to the flexible deployment and the supply of line-of-sight (LOS) links for UAV, operating the UAVs as relays has emerged as a promising way for performance enhancement in wireless communication networks [15; 16; 17]. Specifically, considering an UAV relay-assisted network, the authors in [15] have studied the channel model and examined the system performance in terms of average rate, outage probability, and bit error rate. In [16], the authors have investigated the conditions of establishing multi-hop single link or multiple dual-hop links by using UAV relays for the purpose of achieving better outage performance. In addition, [PERSON] et al. [17] has demonstrated the effect of the altitude of UAV relay on the reliability performance in the UAV relay-assisted communication networks, then they has explored the optimal altitude of UAV relay for minimum outage probability. However, these works only investigate the performance of UAV relay-assisted networks without secrecy consideration. Physical-layer security can protect information transmission with lower computation complexity in UAV relay networks [18; 19; 20]. Considering an UAV relay system, where multiple relays and eavesdroppers exist in the air, the secrecy outage probability (SOP) has been examined in [18]. Meantime, in [19], the average secrecy rate of the UAV relay networks has been studied. Involving the mmWave bands, the related literature is small. In our previous work [20], we have considered an UAV-enabled mmWave communication system, in which a source transmits the signals to a destination via a determined UAV relay, physical-layer security has been studied by using the 3D directional antenna pattern. It is worth mentioning that the on-off transmission [21; 22], as an effective measure to guarantee the transmission quality and further enhance physical-layer security, still has not been investigated in UAV relay networks. On the other hand, the existing works of mmWave communication networks mainly considered that the perfect beam alignment occurs at the legitimate nodes. However, due to the practical limitations, e.g., the limited computational capacity and mobility of transmitters and receivers, the beam alignment error is inevitable in practical mmWave communication scenarios. In [23], the average rate and coverage performance has been examined in the mmWave cellular networks with beam alignment errors, which are modeled as independent Gaussian distributions. Later, [PERSON] et al. [24] has adopted flat-top model to describe the beam alignment error of directional antenna, and the coverage probability of mmWave cellular networks has been studied. Please note that these works have not clarified that how the beam alignment errors affect the secrecy performance in UAV relay-assisted mmWave networks. In summary, contrasting to the previous works, the secure communication in the UAV relay-assisted mmWave networks is still an open issue. First, with the aid of UAV relay in the mmWave networks, how to design the on-off transmission scheme to further enhance the physical-layer security is a challenging work.