Differential Space-Time Line Code Hopping Method for Improving Covert Secrecy Rate

This study considers covert and secure communications in which a transmitter, Alice, sends information to a legitimate receiver, Bob, using a differential space-time line code (Diff-STLC) scheme. A warden, Willie, attempts to detect Alice’s transmission based on signal strength. Upon successful detection, Willie becomes an eavesdropper (Eve) and starts to eavesdrop on the information. Willie and Eve are adversaries against covert and secure communications, respectively. To enhance the limited secrecy of a conventional Diff-STLC system, we propose a code-hopping strategy by randomly switching between two Diff-STLC structures. As a result, only Bob, aware of the hopping pattern, can decode the signals. To quantify both covertness and secrecy, we define a covert secrecy rate (CSR) as the difference between the achievable rates at Bob and Eve. Using the detection probability of Willie and the bit error rates of Eve and Bob, we derive an analytical lower bound for CSR. Both analytical and Monte Carlo results confirm that the proposed Diff-STLC hopping method significantly enhances the performance of CSR, thus improving the overall covertness and secrecy in communications.