Covert Communications: A Comprehensive Survey

Information security has always been a critical issue in wireless networks. Apart from other secure techniques, covert communication emerges as a potential solution to security for wireless networks owing to its high-security level. In covert communication networks, the transmitter hides the transmitted signals into environmental or artificial noise by introducing randomness to avoid detection at the warden. By eliminating the existence of transmitted signals at the warden, information security can be preserved more solidly than other secure transmission techniques, i.e., without noticing the existence. Due to the promising security protection, covert communication has been successfully utilized in tremendous wireless communication scenarios. However, fundamental challenges in its practical implementation still exist, e.g., the effectiveness of randomness utilization, the low signal-to-interference-plus-noise ratio at legitimate users, etc. In this survey, we demonstrate a comprehensive review concentrating on the applications, solutions, and future challenges of covert communications. Specifically, the covert principle and research categories are first introduced. Then, the applications in the networks with different topologies and the effective covert techniques in the existing literature are reviewed. We also discuss the potential implementation of covert communications in future networks and the open challenges.