Age-Optimal Rate Control Transport Protocol for Cohesive Clustered Satellite Systems

Cohesive clustered satellite (CCS) system integrates diverse payloads from multiple formation-flying low Earth orbit (LEO) satellites to provide pervasive intelligent services to ground user equipment (UE), which is regarded as the critical component of satellite-integrated Internet. Considering the CCS system aggregates information at a backbone satellite and connect to the ground station via the satellite-integrated Internet, achieving timely transmission within this bottleneck link becomes crucial. In this paper, we propose an age-optimal rate control (ARC) protocol in transport layer, where each satellite in the CCS system independently and iteratively adjusts its transmit rate (TR) according to the feedback queuing delay at the backbone satellite to approach its age-optimal rate (AR), aimed at minimizing the average age of information (AAoI) of CCS system. Specifically, we derive the upper bound AAoI by modeling an $M/M/1$ queue at backbone satellite, and formulate an AAoI minimization problem by utilizing the upper bound AAoI and solve it to obtain the AR. Then, each satellite can calculate its expected rate (ER) by calculating the queuing delay via the timestamp of Acknowledgements (ACK) from backbone satellite, which is significantly larger than AR. Further, we design an Exponential then Additive Increase Additive Decrease (EAIAD) algorithm to control the convergence position and oscillation of TR, thereby avoiding the significantly increase in AAoI resulting from TR exceeds AR. Finally, we conduct extensive simulations to demonstrate that our ARC protocol achieves minimum AAoI compared to the state-of-the-art transport protocols.