Covertness and Timeliness of Data Collection in UAV-Aided Wireless-Powered IoT

In this work, we aim to maximize the timeliness of data collection subject to a covertness constraint in unmanned aerial vehicle (UAV)-aided Internet of Things (IoT) networks, where a UAV periodically conducts wireless power transfer (WPT) to charge an energy-constrained IoT device and then the IoT device opportunistically sends its collected data to the UAV. To this end, we first derive a lower bound on the covertness constraint and an analytical expression for Age of Information (AoI) to characterize timeliness. Then, we jointly optimize the UAV's transmit power for WPT, the WPT duration, and the data transmission duration by considering two practical scenarios. For the fixed total duration scenario, our analytical optimal solutions indicate that the total harvested energy at the IoT device is independent of the covertness constraint, although both the UAV's transmit power and the WPT duration are significantly affected by the covertness constraint. With the optimized total duration scenario, we prove that the UAV's optimal transmit power is always attained at its maximum value regardless of the existence of the covertness constraint, but the WPT and data transmission durations are sensitive to the required covertness. Overall, there exists a nontrivial tradeoff between the timeliness and covertness for data collection in the considered system, which is determined by the WPT design. Furthermore, our numerical results show that the optimal prior probability of the IoT device's opportunistic transmission is generally not 0.5 for the fixed total duration, but it is indeed 0.5 for the optimized total duration.