Secure Spatial Modulation via Radio Frequency Mirrors

This paper addresses the security issues for a spatial modulation system. Here, a multi-antenna transmitter equipped with a single radio frequency (RF) chain attempts to communicate with a legitimate receiver secretly, and the secret messages are modulated into the index of an active transmit antenna and a baseband symbol. Multiple RF mirrors are placed near the transmit antennas to facilitate the secure transmission. Particularly, in each transmission, one symbol duration is equally divided into multiple sub-symbol periods. During each sub-symbol period, a specific mirror activation pattern is determined, and the sub-symbol is sent along with the weighted jamming signals. The weighting coefficients of the intentionally interfering signals are properly designed to defend against eavesdropping by severely degrading the wiretap channel. The proposed secure SM scheme has two main advantages. First, the dimensionality for jamming is significantly enhanced by deploying multiple RF mirrors at the transmitter. This would on one hand improve the secrecy performance, and on the other hand guarantee the proposed scheme to be immune to eavesdroppers with a large number of antennas. Second, the usage of RF mirrors and sub-symbol division approach provides massive degrees of freedom to preprocess the modulated symbol for further secrecy performance improvement. Numerical results show that the proposed secure SM scheme significantly outperforms the benchmark techniques in secrecy rate.