  Age of information is a new network performance metric that captures the
freshness of information at end-users. This paper studies the age of
information from a scheduling perspective. To that end, we consider a wireless
broadcast network where a base-station (BS) is updating many users on random
information arrivals under a transmission capacity constraint. For the offline
case when the arrival statistics are known to the BS, we develop a structural
MDP scheduling algorithm and an index scheduling algorithm, leveraging Markov
decision process (MDP) techniques and the Whittle's methodology for restless
bandits. By exploring optimal structural results, we not only reduce the
computational complexity of the MDP-based algorithm, but also simplify deriving
a closed form of the Whittle index. Moreover, for the online case, we develop
an MDP-based online scheduling algorithm and an index-based online scheduling
algorithm. Both the structural MDP scheduling algorithm and the MDP-based
online scheduling algorithm asymptotically minimize the average age, while the
index scheduling algorithm minimizes the average age when the information
arrival rates for all users are the same. Finally, the algorithms are validated
via extensive numerical studies.
