Age-Energy Region in Wireless Powered Communication Networks

This paper explores the age-energy region in a wireless powered communication network (WPCN), where an Internet of things (IoT) device works in a “harvest-then-transmit” protocol, in which the IoT device first harvests energy from radio-frequency (RF) signals transmitted by a hybrid access point (HAP) via wireless power transfer (WPT), and then transmits real-time update using the harvested energy to the HAP. For such a system, achievable age-energy region and age-energy function are defined as metrics to measure the trade-off between the age of information (AoI) and the storable energy at the IoT device. Moreover, the optimal transmission policy is designed by solving an AoI minimization problem formulated in terms of the storable energy at the IoT device, which also characterizes the boundaries of the system's age-energy region. As the problem is non-convex, it is first transformed into equivalent ones, and then solved with the closed-form solution. Numerical results show that the transmit power at the HAP and the distance between the HAP and the IoT device have greater effects on the storable energy than the system AoI, and the data size of the update packet has a greater effect on the system AoI than the storable energy. The minimal signal-to-noise-ratio (SNR) threshold has a bilateral influence on the age-energy region, while the impact of the maximal achievable transmit power of the IoT device on the age-energy region is not obvious.