Fountain-Coding-Based Secure Communications Exploiting Outage Prediction and Limited Feedback

This paper develops a fountain-coding-based scheme to secure communications over fading wiretap channels, in which the transmission between the legitimate nodes is overheard by a malicious eavesdropper. The transmitter first splits its message into K source packets and generates a potentially infinite number of fountain-coded packets, each of which is the XOR of distinct source packets chosen according to the channel feedback. Based on the characteristics of fountain-coded transmissions, a sufficient number of independent coded packets have to be successfully received to recover the original source message. Therefore, secrecy is guaranteed if the legitimate receiver can accumulate the required number of coded packets before the eavesdropper does. To realize this, we propose to exploit outage prediction and channel feedback to optimize the fountain encoding procedure. To be more specific, upon the reception of any packet, the receiver predicts the conditional outage probability of the next slot's transmission based on the current channel state and notifies the transmitter to adjust the structure of the fountain code accordingly. In this manner, the encoder design matches well with the time-varying channel conditions of the legitimate link such that the receiver can achieve a high accumulation rate for the fountain packets. Meanwhile, because of the mismatch between the fountain encoder and the eavesdropper's channel, it is extremely difficult for the eavesdropper to collect enough independent fountain-coded packets, and transmission secrecy is thus guaranteed. Simulation results demonstrate that the proposed scheme outperforms the existing candidate solutions in terms of the intercept probability, the decoding delay at the legitimate receiver, and the achievable throughput, as well as the quality-of-service violating probability.