Age-Optimal NC-HARQ Protocol for Multi-hop Satellite-based Internet of Things

In satellite-based internet of things (S-IoT), a noted limitation is the non-trivial propagation delay due to the long distances. Hence, to support emergent real-time IoT applications, where information must be transmitted with short end-to-end latency, the traditional hybrid automatic repeat request (HARQ) strategies in terrestrial network are not fit anymore because the reliable feedback transmission has low efficiency in S-IoT. In this paper, we propose a network code HARQ (NC-HARQ) transmission protocol combined with the concept of information timeliness, i.e., age of information (AoI) to realize limited/no feedback multi-hop transmission in S-IoT. We consider a two-hop end-to-end transmission scenario in the S-IoT, and derive the closed form expressions for average AoI of our NC-HARQ protocol through establishing a four states Markov chain. Simulation results illustrate that the NC-HARQ protocol achieves lower average AoI compare with several state-of-the-art HARQ schemes.