I. Introduction
The information freshness has become an increasingly important performance metric in this era of the Internet of Things (IoT). Various IoT services, such as remote monitoring and control, require the underlying information to be delivered as timely as possible [1], [2]. To quantify the information timeliness and freshness, the age of information (AoI) metric, defined as the time elapsed since the generation time of the latest received status update at the monitor, has been investigated in [3]–[7]. Early work (e.g., [4]–[10]) on the AoI focused on single-user systems, while recent work (e.g., [11]–[17]) shifted to multi-user systems, such as broadcast systems and multiuser uplink systems, where the AoI not only depends on the single-user behaviors but also depends on the interactions among different end nodes.