Antenna Coding and Rate Optimization for Covert Wireless Communications

The covert communication technology has emerged as a novel method for network authentication, copyright protection, and providing the evidence of cybercrimes. However, how to design the covert communication scheme in the physical layer of wireless networks and how to optimize the data rate for the covert communication channels are very challenging. In this article, we propose a wireless covert communication system (CCS), where the transmit antennas are selected and coded to generate a covert codebook. According to the covert codebook, the antennas can be dynamically combined to transmit different covert messages. In addition, we adopt a modulation scheme, named covert quadrature amplitude modulation (QAM), to modulate the covert messages, where the precoding method is designed to deviate the constellations for covert information bits from those for the public information bits. Furthermore, we derive the closed-form expressions of capacity and bit error ratio (BER) for the proposed CCS. To maximize the covert data rate of the CCS, we formulate an optimization problem of the covert data rate and solve the problem to find the optimal precoding matrix. To reduce the covert information leakage, artificial noise is introduced to the system to jam the communication between the transmitting and watching nodes. We design a beamforming scheme to maximize the secure rate for the CCS, where the leakage of covert information can be minimized while the covert communication is not influenced. Simulation results show that the proposed CCS can significantly improve the covert data rate and reduce the covert BER in comparison with the traditional CCSs.