  We investigate the age of information (AoI) of a relay-assisted cooperative
communication system, where a source node sends status update packets to the
destination node as timely as possible with the aid of a relay node. For
time-slotted systems without relaying, prior works have shown that the source
should generate and send a new packet to the destination every time slot to
minimize the average AoI, regardless of whether the destination has
successfully decoded the packet in the previous slot. However, when a dedicated
relay is involved, whether the relay can improve the AoI performance requires
an in-depth study. In particular, the packet generation and transmission
strategy of the source should be carefully designed to cooperate with the
relay. Depending on whether the source and the relay are allowed to transmit
simultaneously, two relay-assisted schemes are investigated: time division
multiple access (TDMA) and non-orthogonal multiple access (NOMA) schemes. A key
challenge in deriving their theoretical average AoI is that the destination has
different probabilities of successfully receiving an update packet in different
time slots. We model each scheme using a Markov chain to derive the
corresponding closed-form average AoI. Interestingly, our theoretical analysis
indicates that the relay-assisted schemes can only outperform the non-relay
scheme in average AoI when the signal-to-noise ratio of the source-destination
link is below -2dB. Furthermore, comparing the merits of relay-assisted
schemes, simulation results show that the TDMA scheme has a lower energy
consumption, while the NOMA counterpart typically achieves a lower average AoI.
