  This paper considers a cooperative status update system with a source aiming
to send randomly generated status updates to a designated destination as timely
as possible with the help of a relay. We adopt a recently proposed concept, Age
of Information (AoI), to characterize the timeliness of the status updates. We
propose an age-oriented opportunistic relaying (AoR) protocol to reduce the AoI
of the considered system. Specifically, the relay opportunistically replaces
the source to retransmit the successfully received status updates that have not
been correctly delivered to the destination, but the retransmission of the
relay can be preempted by the arrival of a new status update at the source. By
carefully analyzing the evolution of AoI, we derive a closed-form expression of
the average AoI for the proposed AoR protocol. We further minimize the average
AoI by optimizing the generation probability of the status updates at the
source. Simulation results validate our theoretical analysis and demonstrate
that the average AoI performance of the proposed AoR protocol is superior to
that of the non-cooperative system.
