## 4 Research Directions for System Designs and Optimization Concepts  
Mainstream studies on PLS system design can be generally summarized by two main approaches. The first is related to secrecy features, which particularly focuses on the characterization of security capacity and eavesdropping or the more general trade-off between attainable secrecy capacity and confidentiality equivocation based on the concepts of information-theoretic fundamentals. The second approach is related to secure system design, which mainly focuses on the design and optimization of secure transmission strategies through the use of signal processing techniques [37; 69; 94; 95]. Many conventional technologies in physical layer security, without the consideration of secrecy communication, can be reconstructed for secure data transmission under the fundamental framework of PLS. To realize the basic optimization problems and performance metrics of PLS, the main issues and research directions are expected to include three candidates for secure design strategies, as illustrated in Figure 8. These research topics are signal processing techniques, secure resource allocation and secure antenna selection/cooperative nodes. Improvements in secret communication within PLS can be supported by these research topics [30]. The signal processing techniques utilize secure precoding and beamforming to achieve the design of efficient secure communication strategies. The secure precoding and beamforming designs fully utilize the characteristics of multi-node and \begin{table}
\begin{tabular}{c c c} \hline \hline
**Metric Types** & **Definition** & **Optimization Problems** \\ \hline \multirow{2}{*}{Secrecy capacity} & The maximum (upper bound) & \multirow{2}{*}{\begin{tabular}{c} Transmission effectiveness of \\ secure communication \\ strategies. \\ \end{tabular} } \\  & of the secrecy rate [72]. & \\ \hline \multirow{4}{*}{Secrecy rate} & The transmission rate that can & \multirow{2}{*}{\begin{tabular}{c} Transmission effectiveness of \\ secure communication \\ strategies. \\ \end{tabular} } \\  & be genuinely supported by the & & \\ \cline{1-1}  & main transmission channel & & \\ \cline{1-1}  & but not decoded on the & & \\ \cline{1-1}  & eavesdropper channel [85]. & \\ \hline \multirow{4}{*}{Secrecy outage probability (SOP)} & The probability that the actual & \multirow{2}{*}{\begin{tabular}{c} Reliability and security of \\ communication systems. \\ \end{tabular} } \\  & or targeted transmission rate & & \\ \cline{1-1}  & is greater than the & & \\ \cline{1-1}  & instantaneous secrecy capacity & & \\ \cline{1-1}  & [86; 87; 88]. & \\ \hline \multirow{4}{*}{Quality of service (QoS)} & The performance & \multirow{2}{*}{\
\begin{tabular}{c} QoS and security of \\ transmission systems. \\ \end{tabular} } \\  & improvement of secure & & \\ \cline{1-1}  & transmission strategies, which & & \\ \cline{1-1}  & includes the SINR-based, & & \\ \cline{1-1}  & PER-based and BER-based & & \\ \cline{1-1}  & metrics [27; 31; 89]. & & \\ \hline \multirow{4}{*}{Power/energy consumption} & The minimum power & \multirow{4}{*}{
\begin{tabular}{c} Resource consumption costs \\ for secrecy performance. \\ \end{tabular} } \\  & consumption that is needed to & \\ \cline{1-1}  & ensure secure QoS & & \\ \cline{1-1}  & requirements for different & & \\ \cline{1-1}  & services [90; 91; 92; 93]. & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Summary of performance metrics for PHY security. multi-antenna scenarios, which may form virtual or massive MIMO networks. Resource allocation, which is usually adopted within conventional communication systems, includes the allocation of power and subcarriers. It mainly focuses on resource management systems that utilize multi-faceted wireless resources, including power, time slots and frequency. Cooperative nodes or secure antenna selection, such as jammer selection, relay node selection and user selection, which are widely used in multiple node scenarios, have been fully explored as promising methods to improve the design of PLS.