Covert communication with a spectrum sharing relay in the finite blocklength regime

In this paper, we investigate the feasibility and performance of the covert communication with a spectrum sharing relay in the finite blocklength regime. Specifically, the relay opportunistically forwards the source's messages to the primary receiver or conveys the covert messages to its own receiver via the sharing spectrum, while the warden attempts to detect the transmission. First, we derive a lower bound on the covertness constraint, and the analytical expressions of both the primary average effective covert throughput (AECT) and sum AECT are presented by considering the overall decoding error performance. Then, we formulate two optimization problems to maximize the primary and sum AECT respectively by optimizing the blocklength and the transmit power at the source and the relay. Our examinations show that there exists an optimal blocklength to maximize the primary and sum AECT. Besides, it is revealed that, to maximize the primary AECT, the optimal transmit power of each hop increases as its channel quality deteriorates. Furthermore, in the optimization for maximizing the sum AECT, the optimal transmit power at the source equals to zero when the channel quality from relay to the secondary receiver is not weaker than that from relay to the primary receiver.