# A Review of Physical Layer Security Techniques for Internet of Things: Challenges and Solutions
###### Abstract  
With the uninterrupted revolution of communications technologies and the great-leap-forward development of emerging applications, the ubiquitous deployment of Internet of Things (IoT) is imperative to accommodate constantly growing user demands and market scales. Communication security is critically important for the operations of IoT. Among the communication security provisioning techniques, physical layer security (PLS), which can provide unbreakable, provable, and quantifiable secrecy from an information-theoretical point of view, has drawn considerable attention from both the academia and the industries. However, the unique features of IoT, such as low-cost, wide-range coverage, massive connection, and diversified services, impose great challenges for the PLS protocol design in IoT. In this article, we present a comprehensive review of the PLS techniques toward IoT applications. The basic principle of PLS is first briefly introduced, followed by the survey of the existing PLS techniques. Afterwards, the characteristics of IoT are identified, based on which the challenges faced by PLS protocol design are summarized. Then, three newly-proposed PLS solutions are highlighted, which match the features of IoT well and are expected to be applied in the near future. Finally, we conclude the paper and point out some further research directions. Internet of Things; physical layer security; anti-eavesdropping; noise aggregation; constellation rotation; fountain code +
Footnote †: journal:  
0000-0001-4500-3899-  
the cryptographic technologies at higher layers of the protocol stack. Although the cryptographic approach is a popular method that is widely used for wired networks (e.g., computer networks) and infrastructure-based wireless networks (e.g., cellular networks), it is not fully suited to future IoT, which will be elaborated on in the following. First, IoT is composed of a large number of low-cost devices. The IoT devices are typically equipped with limited storage memory and powered with batteries, which in turn yield very limited capabilities in terms of computing and communications. As a result, complicated cryptographic protocols and sophisticated encryption/decryption algorithms are prohibited from being used. Second, IoT is a large-scale network supporting massive connections. As indicated by 3 GPP TR 45.820 Technical Specification [8], future IoT applications need to accommodate millions of IoT devices within a cellular cell. Moreover, although the processing capabilities as well as the communications range of individual IoT devices (e.g., sensors) are rather limited, the network as a whole should satisfy the wide-range coverage requirement such that the local sensed data can be delivered to the remote control center for further processing. To meet this demand, the network transmission protocols have to incorporate many new features such as multi-hop routing, cooperative relaying, dynamic access, etc. This makes the IoT highly heterogenous and dynamic. In such a network setting, it is extremely difficult to manage and distribute the secret keys. Third, due to the various application scenarios ranging from environmental monitoring and industrial control to telemedicine and inter-vehicular communications, IoT is expected to support diversified services. Different services have totally different requirements of QoS and security levels. For example, online payment calls for a much higher security level than the ordinary web browsing service does. However, encryption-based methods only provide \"binary\" security levels. The transmitted information is perfectly secured if the key can be securely exchanged and fully intercepted otherwise. Thus, service-oriented and user-centric security guarantee can not be achieved.