Age of Information Variation of LEO Satellite-Terrestrial Uplink Transmissions

Age of Information (AoI) is a critical metric that can be used to assess the freshness of information in highly dynamic low-earth orbit (LEO) terrestrial-satellite communications. In most existing research on the AoI of wireless transmissions, the packet transmission rate rarely changes. However, over a given period of time and without perfect transmit power control, the transmission rate of LEO terrestrial-satellite links cannot remain constant. This is due to the time-varying path loss fading experienced by the transmitted signal over the LEO terrestrial-satellite links, as the LEO satellites can travel at speeds exceeding Mach 10 and complete 12-16 Earth orbits per day. Inspired by this observation, this paper investigates the effects of the swift motion of LEO satellites on the freshness of information in LEO terrestrial-satellite uplink transmissions. At the same time, the Slotted Aloha protocol is considered to manage the process of multiple ground nodes accessing the LEO satellites. The closed-form expression for the average AoI of the LEO terrestrial-satellite uplink transmissions is explicitly derived for the single ground node scenario. Furthermore, a model for analyzing the average AoI in the multi-ground node scenario is proposed accordingly. Finally, numerical results are presented to validate the proposed analysis models of the average AoI and to investigate the AoI variation in the target LEO terrestrial-satellite transmission systems.