Age of Information for Short-Packet Covert Communication

In this letter, we develop a new framework to jointly characterize covertness and timeliness of short-packet communications, in which a new metric named covert age of information (CAoI) is first proposed and then a closed-form expression for the average CAoI is derived. Our examination explicitly reveals the tradeoff between communication covertness and timeliness affected by the block-length, transmit power and prior transmission probability. Multiple transmission designs are tackled in order to minimize the average CAoI subject to covertness constraint, where the resultant differences relative to the designs with traditional metrics (e.g., effective covert rate, AoI) as objective functions are clarified. Our examination demonstrates that the optimal block-length is not the largest one, which is optimal in delay-constrained covert communication without considering the communication timeliness, and the optimal prior transmission probability may not be 1/2, which has been widely assumed in the literature of covert communications.