  This paper considers a cooperative Internet of Things (IoT) system with a
source aiming to transmit randomly generated status updates to a designated
destination as timely as possible under the help of a relay. We adopt a
recently proposed concept, the age of information (AoI), to characterize the
timeliness of the status updates. In the considered system, delivering the
status updates via the one-hop direct link will have a shorter transmission
time at the cost of incurring a higher error probability, while the delivery of
status updates through the two-hop relay link could be more reliable at the
cost of suffering longer transmission time. Thus, it is important to design the
relaying protocol of the considered system for optimizing the information
freshness. Considering the limited capabilities of IoT devices, we propose two
low-complexity age-oriented relaying (AoR) protocols, i.e., the
source-prioritized AoR (SP-AoR) protocol and the relay-prioritized AoR (RP-AoR)
protocol, to reduce the AoI of the considered system. By carefully analyzing
the evolution of the instantaneous AoI, we derive closed-form expressions of
the average AoI for both proposed AoR protocols. We further optimize the
generation probability of the status updates at the source in both protocols.
Simulation results validate our theoretical analysis, and demonstrate that the
two proposed protocols outperform each other under various system parameters.
Moreover, the protocol with better performance can achieve near-optimal
performance compared with the optimal scheduling policy attained by applying
the Markov decision process (MDP) tool.
