Query Age of Information: Optimizing AoI at the Right Time.

This paper is eligible for the Jack Keil Wolf ISIT Student Paper Award. We study a communication system in which a source node is to send status updates over a channel to a remote destination. The update packets will be received error-free, however the channel imposes independent and identically distributed transmission delay on each transmission. The destination node will utilize the upcoming update packets at certain instants that are referred to as query instants. The Query Age of Information (QAoI) at the destination is defined to be the time-average Age of Information (AoI) measured at query instants. We define the problem where the receiver decides when to send requests to "pull" data from the source, to minimize the QAoI, as the pull-or-wait (PoW) problem. We contrast the solution of the PoW problem with that of the update-or-wait (UoW) problem, where the source decides when to generate updates to minimize the time-average AoI. We show that when the query instants occur according to a Poisson process, the solution of the PoW problem is equivalent to that of the UoW problem; however, under periodic query arrivals, the optimal QAoI is always less than or equal to the time average AoI under the same power constraint. We believe this motivates using QAoI as an objective instead of plain AoI in many applications requiring time-sensitive updates.