Superposition Modulation for Physical Layer Security in Water-to-Air Visible Light Communication Systems

In this paper, we propose a physical layer security (PLS) scheme based on superposition modulation for water-to-air (W2A) visible light communication (VLC). We characterize the high correlation between W2A and air-to-water (A2W) link gains from ray tracing simulation and experiments. The proposed scheme selects the modulation order and superposition parameter based on the high correlation between the W2A and A2W link gains. The scheme performance is studied by exploring its immunity against different types of attacks. Based on the link gains from real measurements, it is shown that under random attack Eve's SER exceeds 0.77 when Eve's signal-to-noise ratio (SNR) varies from 5 dB to 30 dB, and under correlated attack Eve's SER decreases only for a high correlation coefficient when Eve is close enough to Bob, which is unrealistic for eavesdropping. In addition, the entropy of potential set under post-processing attack is derived. For intelligent attack, two unsupervised algorithms, namely K-means clustering and expectation maximization (EM) algorithm, are adopted for modulation identification. The distributions of Eve' SER under all four attacks are investigated based on the measured link gains in the laboratory environment, which demonstrates that intelligent attack with K-means clustering possesses the highest eavesdropping capability. However, more numerical results show that for intelligent attack with K-means clustering under wavy water surface, the proportion of the case where Eve has a higher SER than Bob is lower than 0.5 only when Eve lies between Alice and Bob with small horizontal distance, which is unrealistic for eavesdropping operation.