Age-Energy Tradeoff of Polar-Coded HARQ-CC in Space-Air-Ground Integrated Network

Age of Information (AoI) is a fundamental metric for mission-critical applications in Space-Air-Ground Integrated Network (SAGIN). Given the practical energy constraints of satellite systems and Internet of Things (IoT) devices, effective Energy Cost (EC) control assumes paramount importance in the realm of system design. This paper studies the age-energy balanced Hybrid Automatic Repeat reQuest with Chase Combining (HARQ-CC) protocol for polar-coded status update systems in SAGIN. We establish a practical polar-coded status update system in the SAGIN and introduce an update cost metric according to the decision-maker's preference to measure the age-energy performance. Based on the cost performance analysis of the polar-coded HARQ-CC, we propose an improved HARQ-CC strategy, which minimizes the cost by optimizing the information set and code length of polar-coded HARQ-CC. Considering the long-distance and complicated environment in SAGIN, we further develop a threshold-based fast HARQ-CC to reduce the end-to-end latency and EC. By exploiting the SNR-cumulative effect in HARQ-CC, the proposed fast scheme adaptively skips the unnecessary decoding and feedback operations to cope with the SNR-varying environment. Simulations verify the superiority of the improved HARQ-CC and threshold-based fast HARQ-CC schemes compared to the traditional HARQ-CC in terms of the update cost.