Security and Reliability Tradeoff of UAV Relays Assisted Cognitive Transmissions With Hardware Impairments

In this article, we analyze the security and reliability tradeoff (SRT) performance of a unmanned aerial vehicle (UAV) relays assisted cognitive network consisting of a cognitive source (S), multiple cognitive UAV relays (Rs), a cognitive destination (D) in the face of an eavesdropper (E). We consider a practical scenario where mutual interference exists between the primary users and cognitive users. In order to protect the wireless transmission against the eavesdropping attack, we propose two UAV relay selection schemes, namely, noninterference aware relay selection (NIARS) scheme and interference aware relay selection (IARS) scheme. Specifically, in the NIARS scheme, a relay having the best instantaneous (R-D) link (spanning from Rs to D) is selected to forward message. By contrast, a relay maximizing the main channel capacity is selected, which relies not only on the channel state informations (CSIs) of main links but also on the CSIs of the interference links. For the purpose of comparison, we present the round robin (RR) scheme as a baseline. To evaluate the SRT performance, we derive the closed-form intercept probability (IP) and outage probability (OP) expressions for RR, NIARS, and IARS schemes under the impact of hardware impairments (HIs) over Nakagami- $m$ fading channels. Numerical results show that the proposed IARS and NIARS schemes perform better than RR scheme in terms of SRT performance. Additionally, the HI at D has a negative impact on SRT performance, while HI at E has a positive impact on SRT performance.