# A Review of Physical Layer Security Techniques for Internet of Things: Challenges and Solutions
## Challenges of the PLS Protocol Design for Internet of Things  
As mentioned previously, the research of PLS techniques has formed a large body of literature, ranging from fundamental information-theoretic studies to practical PLS protocol design. However, how to design PLS strategies that match the unique features of IoT well is still an open problem. The majority of the existing PLS techniques have the following drawbacks that prohibit their direct applications to IoT. First, IoT devices are featured by \"low-cost\", which means that these devices usually have very limited storage memory and processing capabilities. Moreover, the IoT devices are generally powered with batteries, which imposes significant energy constraints. The low cost and low power consumption features require that the PLS strategies must be highly energy-efficient, and can be implemented with very low complexity. However, the majority of the existing PLS schemes realize security with the cost of additional energy consumption or increased hardware complexity. For example, the AN based PLSapproaches rely on the injection of artificial noise signals, which yields additional power consumption; the secure beamforming/precoding methods are typically based on the multi-antenna structure at the transmitter, which is infeasible considering the size and cost of IoT devices; the cooperation based PLS schemes require the existence of friendly jammers that send jamming signals, which also complicates the protocol design and increases the power consumption. From a practical point of view, the IoT-oriented PLS protocols have to take into account the resource constraints at the IoT devices, and make a trade-off among security, complexity, and energy consumption as well. Second, from the networking perspective, IoT is expected to support wide-range transmission. For instance, in wireless sensor networks which is an embodiment of IoT, the local sensed data collected by sensors has to be delivered to the remote control center for further processing. However, the single-hop transmission distance in IoT is rather limited due to the low power of IoT devices. Therefore, to address the wide-range coverage requirement, the network transmission protocols have to incorporate many new features such as multi-hop routing, cooperative relaying, cognitive transmission, etc. This in turn requires the PLS strategies to be \"smart\" enough to adapt to complicated network environments. For example, the IoT devices have to cooperate with many untrusted immediate nodes. Although these untrusted nodes may not be malicious entities, it is possible that they are non-authenticated and with lower security clearance than the IoT devices. How can we exploit the untrusted nodes to assist information delivery while keeping the data content confidential to them? How does the number of untrusted relays impact the network security performance? These issues are not fully understood yet and more in-depth research is needed. Third, IoT is targeted at providing massive connection capability which can accommodate millions of devices per square kilometer to exchange information. Moreover, the IoT devices, produced by different corporations, are heterogeneous nodes with totally different service types, traffic patterns, and transmission modes. These features make IoT a large-scale heterogenous network, for which the scalability issue is a primary concern. However, the existing PLS schemes are mainly developed for small-scale networks, for which only the link-level performance metrics are cared about, e.g., secrecy rate, information intercept probability, etc. Several fundamental problems for secure transmission in large-scale networks are not handled well. For example, how does the system secrecy rate scale with the number of nodes in the network? How can we translate the massive connection capability of the network into a powerful anti-eavesdropping resource? To answer these questions, new mathematical tools need to be developed, and innovative networking transmission protocols should be devised.