Age of Information and Energy Harvesting Tradeoff for Joint Packet Coding in Downlink IoT Networks

The paper investigates the information freshness and energy harvesting (EH) in downlink Internet of Things (IoT) networks. Information freshness is measured by Age of Information (AoI). We consider a scenario where an access point periodically sends short packets to N independent IoT devices. All the devices are equipped with capacitors to store energy through wireless power transfer (WPT) when the devices receive signals conveying unwanted packets. Conventionally, short packets for multiple devices are jointly coded into one larger packet to improve reliability. However, whether short packets should be jointly coded to reduce average AoI and improve EH performance has not been well investigated. On the one hand, a larger number of packets jointly coded decreases the packet error rate (PER), which may reduce the average AoI because the time to successfully receive the next update may be shorter. On the other hand, more packets jointly coded cause less time for WPT, as the devices spend most of their time receiving update packets, resulting in poor EH performance. Therefore, we investigate the tradeoff between AoI and EH by examining the number of packets to be jointly coded. Closed-form AoI and EH formulas are derived. Numerical results show that there exist optimal numbers of jointly coded packets that can achieve both high information freshness and high EH at the same time.