  With the rapid development of Internet of Things (IoT), massive devices are
deployed, which poses severe challenges on access networks due to limited
communication resources. When massive users contend for access, the information
freshness gets worse caused by increasing collisions. It could be fatal for
information freshness sensing scenarios, such as remote monitoring systems or
self-driving systems, in which information freshness plays a critical part. In
this paper, by taking the Age of Information (AoI) as the primary performance
indicator, the information freshness using AMP-based grant-free scheme is
investigated and compared with grant-based scheme. Base on the analysis, a user
scheduling strategy with sleep threshold and forcing active threshold is
proposed to further reduce average AoI (AAoI). Numerical results reveal that
the AMP-based grant-free scheme can provide sufficient access capability with
less pilot resources, and it is robust to the fluctuation of the number of
active users. That ensures that the AMP-based grant-free scheme can keep the
AAoI at a low level. It is also shown that the proposed threshold strategy can
effectively improve the information freshness.
