  Age-of-information (AoI) is a metric quantifying information freshness at the
receiver. Since AoI combines packet generation frequency, packet loss, and
delay into a single metric, it has received a lot of research attention as an
interface between communication network and application. In this work, we apply
AoI to the problem of wireless scheduling for multi-loop networked control
systems (NCS), i.e., feedback control loops closed over a shared wireless
network. We model the scheduling problem as a Markov decision process (MDP)
with AoI as its observable states and derive a relation of control system error
and AoI. We further derive a stationary scheduling policy to minimize control
error over an infinite horizon. We show that our scheduler outperforms the
state-of-the-art scheduling policies for NCS. To the best of our knowledge,
this is the first work proposing an AoI-based wireless scheduling policy that
minimizes the control error over an infinite horizon for multi-loop NCS.
