# A Review of Physical Layer Security Techniques for Internet of Things: Challenges and Solutions
## 2 Literature Review of PLS Techniques for IoT
### PLS Schemes for IoT: State of the Art
#### 2.2.4 Cooperative Secrecy  
IoT is typically composed of massive physical objects such as sensors, controllers, and actuators. Although the processing capability of any single device is limited, the users' secrecy requirements can still be satisfied by harnessing the cooperation among these low-power devices. The key idea of cooperative secrecy is to let the friendly nodes serve as jammers to send artificial interference to degrade the signal reception at the eavesdropper. Ref. [44] developed cooperative jamming (CJ) strategies for amplify-and-forward (AF) and decode-and-forward (DF) systems, respectively, where the relay nodes independently transmit the weighted artificial noises to worsen the eavesdropper channel. Ref. [30] combined secure beamforming with cooperative jamming to enhance the physical layer security. In this work, an optimization problem was formulated to minimize the secrecy outage probability subject to the secrecy rate requirement.  
In addition to cooperative jamming, another popular cooperative secrecy approach is the secure relay selection technique. Ref. [45] developed a relay selection policy to select both the information forwarding helper and the friendly jammer, and introduced an adaptive mechanism to select the cooperation mode such that the secrecy outage probability can be minimized. [PERSON]. [46] proposed a low-complexity single-relay selection scheme, taking both the transmission reliability and security into consideration. In [47; 48], security-enhancing relay selection strategies were devised, where the impact of co-channel interference and the outdated CSI were investigated, respectively. Common to the works [45; 46; 47; 48] is that all of them assume that the eavesdropper is an external malicious node in addition to the legitimate parties. Different from these papers, [PERSON]. [49; 50] considered secure transmission in dual-hop relaying networks where the intermediate relay nodes are untrustworthy. It has been proven that a positive secrecy rate can be achieved for this system by enlisting the help from the destination who performs cooperative jamming. Inspired by the pioneering works [49; 50], plenty of papers have appeared that are dedicated to the PLS protocol design and performance analysis for untrusted relaying systems. In [51], the relay selection policy that maximizes the achievable secrecy rate was developed, and the scaling law of the network secrecy capacity was derived as well. In [52], the work of [51] was extended to a more realistic scenario where the information leakage during both the first phase and the second phase are considered. In [53], several relay selection policies were developed for successive AF relaying networks with untrusted nodes, where different complexity requirements were considered. The schemes described above are mainly applicable to the uplink transmission of IoT, where the sensor nodes send data to the central controller. For downlink transmission, [PERSON]. [54] developed a cooperative privacy preserving method to prevent information leakage across the users. Through CSI-based AN design and cooperative AN injection, the data confidentiality of the desired user can be protected. To motivate the users to participate in cooperation, a user-grouping based selection criterion was also devised. Very recently, [PERSON]. [55] proposed a constellation overlapping scheme to secure two-way untrusted relaying systems, where a truncation-channel-inversion based technique was introduced to make the signals from different terminal users experience the same equivalent channel, thereby realizing full constellation overlapping at the relay. Consequently, a high error floor is created, and data confidentiality is protected.