Covert Communication Over Gaussian Multiple-Access Channels With Feedback

We consider the covert communication over the two-user Gaussian multiple access channel (GMAC) with causal feedback. It is shown that any causal feedback does not enlarge the covert capacity region and the time-division scheme is optimal for a class of channels, which includes the case where the warden observes what the legitimate receiver receives. For the proof, we derive an outer bound on the covert capacity region of general GMACs with noiseless causal feedback, which implies that the square-root law holds for arbitrary channel gains. Furthermore, the extension to multi-user case is presented.