# A Review of Physical Layer Security Techniques for Internet of Things: Challenges and Solutions
## 4 Promising PLS Solutions for IoT
### Summary of the IoT-Oriented PLS Solutions  
Compared with the classical PLS strategies, the schemes described in Section 4 are better suited to the Internet of Things, the reasons of which are summarized as follows. First, the schemes developed in Section 4 are energy efficient, thus satisfying the low energy consumption requirements of IoT. To be specific, the noise aggregation approach utilizes the intrinsic noise in wireless channels, rather than the artificial noise, to degrade the eavesdropper's received signal quality, and guarantees transmission secrecy without the cost of additional power. The constellation rotation method creates an irreducible error floor at the untrusted relay via the optimized design of signal constellation, thus avoiding the excessive consumption of energy in generating the artificial noise. The fountain-coding aided strategy realizes secrecy by exploiting the characteristics of fountain coded transmission that the entire file can be recovered only if a sufficient number of coded packets can be accumulated. This method does not require any additional energy to guarantee secrecy. Second, the three schemes described in Section 4 enjoy very low implementation complexity, which matches the low-cost feature of IoT devices well. As stated previously, noise aggregation is essentially a self-encryption technique that utilizes the previously transmitted messages to encrypt new messages. Only a simple XOR operation is required at the transmitter, which is affordable by the IoT devices. In the constellation rotation scheme, the constellation rotation operation incurs negligible overhead. Moreover, the rotation angles only depend on the adopted modulation formats and can be calculated offline prior to data transmissions, yielding trivial computational load as well. The overheads involved in the fountain coding based scheme mainly include the fountain encoding/decoding operations and channel feedback from Bob  
Figure 3: Fountain coding based secure transmission system. to Alice. However, the fountain encoding/decoding algorithm has a linear time complexity, and the amount of feedback within any slot is only two bits. Thus, the fountain-coding based scheme is also attractive for IoT applications. Third, from the perspective of IoT services, the schemes discussed in Section 4 are also more competitive compared to the classical PLS solutions. In particular, by using the noise aggregation approach, the message-prioritization based unequal secrecy protection can be realized, which is of practical significance to the video surveillance applications. In the constellation rotation scheme, the power ratio of the information-bearing signal to the artificial noise in the composite signals (4) and (5) can be adaptively adjusted, thereby providing a flexible trade-off between the reliability and security performances. Finally, in the fountain-coding based secure transmission scheme, the outage threshold can be optimized such that various aspects of the system QoS requirements can be balanced, including secrecy, throughput, as well as transmission delay. Although the emerging PLS techniques described in Section 4 have notable advantages, there are also some disadvantages associated with them. Briefly speaking, all of the three strategies require channel feedback or information exchange between the legitimate entities, which causes a slight degradation of the rete performance of the legitimate system. Therefore, these schemes are not suitable to real-time applications where the data delivery has stringent delay requirements. However, IoT services are typically with very low data rates. Thus, the disadvantages of the schemes do not hamper their adoption in future IoT. In Table 1, we summarize the advantages and disadvantages of all the PLS techniques discussed in Sections 2 and 4, highlighting the energy consumption and implementation complexity aspects. We also show the potential application scenarios to which each candidate PLS solution applies.