# A Review of Physical Layer Security Techniques for Internet of Things: Challenges and Solutions
## 5 Conclusions and Future Directions  
In this article, a comprehensive review of the physical layer security techniques in the Internet of Things was presented. The features as well as security requirements of IoT were first discussed. After that, the basic principle of physical layer security was introduced, and several representative IoT-oriented physical-layer-security technical solutions were summarized. Finally, we analyzed the challenges faced by IoT secure transmission protocol design, and introduced three emerging PLS solutions that can well address these challenges.  
Although the research on physical layer security has generated a large body of literature, with the work ranging from fundamental information-theoretic analysis to practical PLS strategy design, it is still challenging to develop PLS schemes that satisfy the multi-dimensional requirements of future IoT. Some issues that are worthy of further studies are listed as follows:  
1. The PLS scheme design to combat active attacks. Until now, the majority of PLS solutions focus on the anti-eavesdropping techniques. However, eavesdropping is just a simple and passive attack form. In future IoT, there will be various forms of active malicious attacks, e.g., message modification, information disclosure, pilot spoofing, jamming, masquerade attack, etc. It remains an open problem as to how to exploit the PLS techniques to deal with these attacks. Cross-layer design may be a promising method to address this issue.
2. The new metric for performance evaluation. In PLS research, the widely adopted performance metrics include the achievable secrecy rate and the secrecy outage probability. These metrics are proposed from an information-theoretical point of view. In future IoT, the heterogeneity of the devices and the services cause the diversity in user demands. This in turn calls for the proposal of new metric for evaluating the performance of PLS schemes. The new metric should take the multi-dimensional user requirements, e.g., secrecy, delay, throughput, packet loss rate, etc., into consideration, and give a comprehensive evaluation of the developed schemes.
3. The use of PLS techniques to new systems and scenarios. Current studies are mainly concerned about the wireless sensor networks as an application scenario. In future IoT, several new systems are emerging, and many new application scenarios will be considered, e.g., the backscatter systems. Thus, innovative research works are needed, including new models, new analytical tools, etc.  
[PERSON] conceived the plan of the paper and prepared Sections 1-3, 4.2-4.4 and 5 of the manuscript. [PERSON] prepared Section 4.1 of the manuscript.  
This research was funded by the National Natural Science Foundation of China (NSFC) under Grant No. 61671369 and 61431011, the Open Research Fund of the State Key Laboratory of Integrated Services Networks, Xidian University under Grant No. ISN18-02, the National Science and Technology Major Project of China under Grant No. 2016 ZX03001012-004, and the Key Research and Development Program of Shaanxi Province under Grant No. 2018 KW-019.  
The authors declare no conflict of interest.