# A Survey on Intelligent-Reflecting-Surface-Assisted UAV Communications
## 3 Overview of Previous Studies on IRS-Assisted UAV Communications
### Improvement of Security  
Security is an important issue of wireless communications systems due to the broadcasting nature of wireless signals. Although UAVs have many potentials, their capabilities raise concerns regarding system security at the same time. Previous studies showed that generating random radio channels enables UAVs to protect data transmission [78] and that with more IRS element antennas, a higher security can be achieved [79]. In this subsection, we introduce the research trends of the IRS-assisted UAV communications systems from the perspective of security. The secrecy rate, which is one of the most fundamental metrics to measure the security performance, is defined as the difference between the rate of the legitimate channel and the maximum rate of the eavesdropping channel [80]. Table 3 briefly summarizes the main features of the previous works relevant to the improvement of security.  
In [81; 82], the authors examined how to overcome obstacles through proper placement of UAV BSs and IRSs in the presence of authorized users and illegal eavesdroppers. The authors aimed to maximize the secrecy rate through the optimal placement and design of the beamforming weight matrix. The results were extended in [83] by additionally considering the power control of the UAV to maximize the average secrecy rate. Unlike other studies, Reference [81] analyzed the probability of LoS link generation depending on the existence of an IRS in UAV communication systems.  
In [84], a secure transmission problem was investigated in an IRS-assisted UAV mmWave communications system. The authors utilized a deep deterministic policy gradient (DDPG) framework to maximize the sum secrecy rate of all legitimate users and proposed a novel and effective paired DRL algorithm. The author of [85] considered the joint and channel-state-information (CSI)-robust design of the UAV trajectory, IRS passive beamforming, and legitimate transmit power to maximize the average worst-case  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Reference** & **Objective** & **Design Variables** & **Scenario** \\ \hline
[81] & Maximize the secrecy rate &
\begin{tabular}{c} UAV-BS and IRS positions, \\ UAV-BS and IRS beamforming \\ \end{tabular} & (a) \\ \hline
[82] & Maximize the secrecy rate &
\begin{tabular}{c} UAV location, \\ IRS phase shift \\ \end{tabular} & (e) \\ \hline
[83] & Maximize the average secrecy rate &
\begin{tabular}{c} IRS phase shift, \\ UAV trajectory, \\ transmit power of UAV \\ \end{tabular} & (a) \\ \hline
[84] & Maximize the sum secrecy rate of all legitimate users &
\begin{tabular}{c} UAV trajectory, \\ coefficients of the IRS elements, \\ UAV active beamforming \\ \end{tabular} & (a) \\ \hline
[85] & Maximize the worst-case secrecy rate &
\begin{tabular}{c} UAV trajectory, \\ IRS passive beamforming, \\ transmit power of the \\ legitimate transmitters \\ \end{tabular} & (a) \\ \hline \hline \end{tabular}
\end{table}
Table 3: Summary of previous works relevant to improvement of security.  
secrecy rate of the communications system. The authors showed that the use of IRS helps substantially improve the performance of the secrecy rate.