Adaptive Covert Communications in Time-Varying Environments with Multi-Slot Covertness Constraints

Given the time-varying radio environments, legitimate users need to conduct covert communications over multiple time slots with different radio propagation conditions, while a warden performs joint signal detection based on the observations collected during these slots. Unfortunately, existing studies mainly deal with the design of covert communication scheme within a single slot where the radio environment remains unchanged. These scheme might suffer performance degradation when legitimate users make transmission decisions over multiple slots with different propagation conditions and a joint covertness constraint over these slots is imposed. In view of this challenge, we investigate the design of covert communication schemes in time-varying radio environments under a multi-slot covertness constraint. Given the coupling between the transmission decisions in different slots, we formulate the schematic design as a Markov decision process where the multi-slot covertness constraint is characterized with the concept of conditional value at risk to address the non-cumulative growth brought by the unknown eavesdropping channel. Unlike existing works, our scheme allows the legitimate users to adapt their transmission decisions to the current propagation condition and covertness margin. Extensive simulations demonstrate that our scheme can facilitate efficient covert data transmission under a multi-slot covertness constraint.