# A Review of Physical Layer Security Techniques for Internet of Things: Challenges and Solutions
## 4 Promising PLS Solutions for IoT
### Noise Aggregation and Self-Encryption  
The key idea of physical layer security is to exploit the randomness of wireless channels to degrade the received signal quality at the eavesdropper. One popular method to achieve this target is to inject the AN into the null space of the legitimate channel. However, the use of AN results in an additional power consumption, which is not acceptable for battery-powered IoT devices. To address this issue, we proposed a novel method termed as noise aggregation [63], which works as follows: prior to data transmissions, each packet to be sent is assigned a positive number corresponding to the packet index. The odd-numbered packets and the even-numbered packets are transmitted within the odd and even slots, respectively. During data transmissions, the odd-numbered packets are directly sent out, and the even-numbered data packets are XORed with odd-numbered data packets that have been successfully decoded by the legitimate receiver. Due to the independence between the legitimate link and the eavesdropping link, the decoding of the odd-numbered data packets at the eavesdropper might be incorrect. As a result, the \"decoding noise\" from the odd slots will propagate to the even slots, and aggregate with the channel noise to worsen the detection performance of even-numbered packets at the eavesdropper. This is the so-called noise aggregation effect, which utilizes the natural noises rather than artificial noises to realize transmission security. The principle of the noise aggregation method can be illustrated in more detail by using Figure 1. For the ease of exposition, we assume that the transmissions from Alice to Bob are slotted, and each slot is of the same length. The wireless channel of the Alice-Bob link as well as that of the Alice-Eve link is modeled as a binary symmetric channel (BSC). It is further supposed that the channel fading state remains unchanged within any slot, and varies independently from slot to slot. As illustrated by Figure 1, we consider two consecutive transmission slots. Without loss of generality, it is supposed that the channel quality of the legitimate link (Alice-Bob) is better than that of the eavesdropping link (Alice-Eve) within slot 1, while in slot 2, the channel quality of the legitimate link (Alice-Bob) is worse than that of the eavesdropping link (Alice-Eve). The detailed transmission procedure is described as follows. 1. Within the first slot, the legitimate channel is advantageous over the eavesdropping channel. Hence, there is no need to perform the one time pad encryption. Instead, transmission secrecy of the source message is guaranteed by using the wiretap coding developed by information theoreticians. Assume the codeword transmitted from [PERSON] is \(X_{1}\). Then, the received codewords  
Figure 1: Illustration of the noise aggregation method. at Bob and Eve will be \(X_{1}\oplus W_{1}\) (the upper left part of Figure 1) and \(X_{1}\oplus Z_{1}\) (the lower left part of Figure 1), respectively, where \(W_{1}\) and \(Z_{1}\) are the corresponding channel noises, respectively. 2. By exploiting the channel feedback from [PERSON] to [PERSON], [PERSON] can construct the capacity-achieving code to ensure that \(X_{1}\) can be successfully decoded by [PERSON]. Note that the eavesdropping channel is degraded compared to the legitimate channel during the 1 st slot. Therefore, the decoding of \(X_{1}\) at [PERSON] is in failure. This motivates us that we can utilize \(X_{1}\) as the key to encrypt the transmitted signal in slot 2. Assume the source data to be transmitted within slot 2 is \(X_{2}\). Then, the encrypted data is \(X_{2}\oplus X_{1}\). At the end of this slot, the received codewords at [PERSON] and [PERSON] are expressed by \(X_{2}\oplus X_{1}\oplus W_{2}\) (the upper right part of Figure 1) and \(X_{2}\oplus X_{1}\oplus Z_{2}\) (the lower right part of Figure 1), respectively. 3. Upon the reception of the signal \(X_{2}\oplus X_{1}\oplus W_{2}\), [PERSON] performs channel decoding to recover \(X_{2}\).