# A Review of Physical Layer Security Techniques for Internet of Things: Challenges and Solutions
## 4 Promising PLS Solutions for IoT
### Fountain-Coding Based Secure Transmission
In our recent work [85], we developed a fountain-coding aided transmission scheme exploiting outage prediction and limited feedback, which has a very low implementation complexity and is thus applicable to future IoT. In what follows, we would like to present a brief introduction of this scheme. As illustrated in Figure 3, we consider a wireless sensor network consisting of many sensor nodes, a central control unit [PERSON], and a passive eavesdropper [PERSON]. The sensor node [PERSON] wants to securely deliver a confidential file (the sensed data) to [PERSON]. To achieve this goal, [PERSON] first splits its file into \(K\) source packets denoted by \((\mathbf{u}_{1},\mathbf{u}_{2},\dots,\mathbf{u}_{K})\). Then, fountain coding is employed to encode these packets into a potentially infinite number of fountain packets \((\mathbf{v}_{1},\mathbf{v}_{2},\dots)\). Finally, these fountain packets are further encoded by capacity-achieving code at the physical layer to produce the transmitted packets \((\mathbf{p}_{1},\mathbf{p}_{2},\dots)\). To better illustrate our idea, we focus on two consecutive transmission slots, say slot \(t-1\) and slot \(t\). Within time-slot \(t-1\), upon the reception of the transmitted packet \(\mathbf{p}_{t-1}\), [PERSON] predicts the conditional outage probability (COP) of slot \(t\) based on the current channel state and the specific channel model. If the predicted COP is lower than a predetermined outage threshold, the fountain coding strategy in slot \(t\) will be given by  
\[\mathbf{v}_{t}=\mathbf{u}_{d,1}\oplus\mathbf{u}_{d,2}\oplus...\oplus\mathbf{u }_{d,k}\oplus\mathbf{u}_{n,p}, \tag{7}\]  
where \(\mathbf{u}_{d,1},\dots,\mathbf{u}_{d,k}\) represent all the source packets that have been recovered by [PERSON] until slot \(t-1\), and \(\mathbf{u}_{n,p}\) is a randomly-chosen source packet that has not been recovered by [PERSON] yet. The rationale behind this policy is as follows. If the predicted COP is lower than the threshold, a good channel quality is expected for the legitimate link in the next slot. The coding strategy in Label (7) ensures the instant recovery of a new source packet once the next-slot transmission is successful. On the contrary, if the predicted COP is higher than the threshold, implying that the transmission over the legitimate link will be in failure with high probability, the fountain coding strategy in slot \(t\) should be  
\[\mathbf{v}_{t}=\mathbf{u}_{1}\oplus\mathbf{u}_{2}\oplus...\oplus\mathbf{u}_{K}, \tag{8}\]  
which is the XOR of all the source packets. In this manner, the eavesdropper is prevented from extracting a new source packet from its received signal. With the proposed strategy, the fountain encoder matches well with the time-varying channel conditions of the legitimate link such that the receiver can achieve a much higher decoding rate for the source packets compared to the eavesdropper, thus guaranteeing transmission secrecy. The complexity of the fountain coding based secure transmission scheme mainly includes two parts. First, fountain encoding/decoding is required at the application layer. Second, channel feedback is needed from [PERSON] to [PERSON] to dynamically adjust the fountain encoder structure. However, the fountain encoding/decoding can be realized by using several mature algorithms with linear time complexity. Besides that, only two bits are needed to be fed back. One bit is used to inform [PERSON] about the outage prediction result, and the other bit is used to indicate the decoding status (success or failure) of the currently-received coded packet. Therefore, the additional overhead of the proposed scheme is trivial, which can be affordable for most IoT applications.