## 4 Research Directions for System Designs and Optimization Concepts
### Main Technical Challenges in System Design  
Keyless-based PLS techniques are well established transmission strategies that can enhance the performance variation between the main transmission channel and wiretap channels. Unlike conventional transmission methods, the optimization objectives, the conditions of constrained optimization problems and the performance parameters that are associated with SINR-based security techniques are based on the characteristics of wireless channels and information theory secrecy metrics. For secure physical layer design and optimization schemes, the selection of suitable performance metrics is crucial. As illustrated in Figure 9, multi-dimensional security and resource management strategies typically contain secure resource allocation and signal processing methods for cooperative wireless networks. Resource allocation systems are employed by the network operator, which are shown in the top position; simultaneously, the signal processing schemes are operated by transmitters, which are positioned on the bottom. Multiple-antenna techniques have been widely used in wireless networks due to their spatial degrees of freedom, which are offered by multi-service networks. Although secure multi-antenna techniques are widely implemented  
Figure 8: Secure transmission strategies to improve the security and robustness of physical layer designs. in various design approaches, they can also be mathematically modeled as optimization problems to find the most favorable transmission solutions. This can be realized and optimized by using information-theoretic security metrics to design secure beamforming and precoding, appropriate antenna or relay node selection and resource allocation. Most of the problems in PLS are caused by non-convex optimization due to the characteristics of quadratic programming functions in the performance metrics. Many researchers have carried out extensive work on formulating and solving these optimization issues in order to obtain the maximum level of security [96]. By considering the complexity of these non-convex constraint functions, several signal processing techniques and optimization methods have been developed to solve the corresponding optimization problems. Although many optimization methods have been developed to cope with the non-convexity of the optimization problems in PLS schemes, there are still many challenges to solve within the existing schemes; specifically, the basic assumptions that are related to system design and channel coding models. Given the limited resources in wireless networks, such as energy and bandwidth, a common problem that is associated with resource management is the adequate exploitation of the resource constraints to attain the conditions of the information-theoretic security metrics. In a transmission system with limited resources, design and optimization schemes must be considered for the resource allocation to the various consumers who use the data over the network so as to achieve a good performance. Several existing works have discussed the fundamental resource management issues within multi-dimensional wireless networks, such as power allocation, subcarrier allocation and joint power and subcarrier allocation [97; 98; 99; 100; 101; 102; 103; 104; 105; 106]. The existing secure resource allocation methods are generally based on communication nodes with three terminals (i.e., the Alice-Bob-Eve model) and lack sufficient investigation into multi-user scenarios and heterogeneous networks. On the other hand, the existing design and optimization of signal processing schemes are commonly based on the parallel fading channel and AWGN, with few investigations on massive MIMO and millimeter wave channels. Therefore, the PLS is still a great potential prospect for 5G wireless communication systems to mitigate all  
Figure 9: An illustration of multi-dimensional security and resource management within a multi-service wireless network. of these challenges.