# 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  
The fundamental framework for fountain-coding based transmissions [71; 72; 73] is described as below. The source file to be transmitted is first divided into packets with equal length, termed as information packets. Then, the transmitter encodes the information packets to produce the coded packets, and persistently sprays the coded packets towards the receiver. Here, each coded packet is the bit-by-bit XOR of several distinct source packets. Upon the reception of a coded packet, the receiver tries to decode by using some iterative decoding algorithms such as belief propagation [71; 73]. As the iterative decoding proceeds, more and more information packets are recovered. Once the entire file is reconstructed, the receiver will send a feedback signal to inform the transmitter to stop producing new coded packets. Some well known fountain codes proposed so far include LT code [71], Raptor code [74], and Reed-Solomon (RS) code [75]. The essential point of exploiting fountain codes to enhance security is to expedite the decoding process at the legitimate receiver, such that the wiretapper cannot accumulate enough packets for the reconstruction of the entire source file. To be more specific, we assume the number of information packets that constitute the source data block to be \(K\). Once the receiver has successfully received at least \(K\) independent coded packets, the entire file can be recovered [76]. This characteristic of fountain codes implies that, in wiretap channels, the data delivery from source to destination is secured if the destination can accumulate the \(K\) independent coded packets before the eavesdropper does. To achieve this goal, the unique features of the legitimate transmission, such as the error pattern, the CSI, and the data content characteristics, should be fully taken advantage of. Meanwhile, we should also assure that, this information, even known by the eavesdropper, cannot offer her any benefit. The first work exploiting fountain codes for wireless security can be found in [77], where a truncated-channel-inversion based power control policy was developed to ensure a constant SNR at the legitimate receiver and a randomly varying SNR at the eavesdropper. In this manner, with high probability, the legitimate receiver can accumulate enough coded packets before the eavesdropper does, thus realizing transmission secrecy. The intercept probability of the fountain-coding aided secure transmission scheme was analyzed in [78], where an optimization model was also developed to minimize the intercept probability under delay and reliability constraints. Following similar ideas as [77], Ref. [79] applied random linear network coding (RLNC) [80; 81] to realize secure layered video delivery [82], and devised a framework to prevent the eavesdropper from intercepting the enhancement layer video data. While [77; 78; 79] concentrated on a simple wiretap channel model with only three nodes, there are also several works studying fountain-coded secure transmission in cooperative relaying networks. In [34], we combined fountain coding and cooperative jamming to prevent information leakage in dual-hop decode-and-forward (DF) relay systems, where the constellation rotation technique was exploited to greatly deteriorate the received signal quality at the eavesdropper. Very recently, [PERSON]. [83] extended the work of [34] to multi-relay networks, where relay-jammer selection was utilized to harvest the opportunistic relaying gain. Common to the existing works [77; 78; 79; 80; 81; 82; 83] is that all of them focus on how to exploit channel fading and physical-layer techniques to achieve a higher reception rate at the legitimate receiver. However, none of these papers investigates how to construct fountain code from the secrecy perspective. Differently from these works, [PERSON]. [84] proposed a feedback-based mechanism to dynamically adjust the fountain encoder such that the decoding rate at the legitimate receiver can be improved.