Wireless covert channel is an emerging covert communication technique which conceals the very existence of secret information in wireless signal including GSM, CDMA, and LTE. The secret message bits are always modulated into artificial noise superposed with cover signal, which is then demodulated with the shared codebook at the receiver. In this paper, we first extend the traditional KS test and regularity test in covert timing channel detection into wireless covert channel, which can be used to reveal the very existence of secret data in wireless covert channel from the aspect of multiorder statistics. In order to improve the undetectability, a wireless covert channel for OFDM-based communication system based on constellation shaping modulation is proposed, which generates additional constellation points around the standard points in normal constellations. The carrier signal is then modulated with the dirty constellation and the secret message bits are represented by the selection mode of the additional constellation points; shaping modulation is employed to keep the distribution of constellation errors unchanged. Experimental results show that the proposed wireless covert channel scheme can resist various statistical detections. The communication reliability under typical interference is also proved.

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To conceal the very existence of communication, the noise-based wireless covert channel modulates secret messages into artificial noise, which is added to the normal wireless signal. Although the state-of-the-art work based on constellation modulation has made the composite and legitimate signal undistinguishable, there exists an imperfection on reliability due to the dense distribution of covert constellation points. In this study, the authors design a wireless covert channel based on dither analog chaotic code to improve the reliability without damaging the undetectability. The dither analog chaotic code (DACC) plays the role as the error correcting code. In the modulation, the analog variables converted from secret messages are encode into joint codewords by chaotic mapping and dither derivation of DACC. The joint codewords are mapped to artificial noise later. Simulation results show that the proposed scheme can achieve better reliability than the state-of-the-art scheme while maintaining the similar performance on undetectability.