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.

翻译：我们研究了中继辅助协作通信系统的信息年龄（AoI），其中源节点借助中继节点尽可能及时地向目的节点发送状态更新数据包。对于无中继的时隙系统，先前的研究表明，源节点应在每个时隙生成并发送新数据包至目的节点，以最小化平均AoI，无论目的节点在前一时隙是否成功解码该数据包。然而，当引入专用中继时，中继能否改善AoI性能仍需深入研究。具体而言，源节点的数据包生成与传输策略需与中继精心协同设计。根据源节点与中继是否允许同时传输，我们研究了两种中继辅助方案：时分多址（TDMA）与非正交多址（NOMA）方案。推导其理论平均AoI的关键挑战在于，不同时隙中目的节点成功接收更新数据包的概率不同。我们利用马尔可夫链对各方案建模，推导出相应的闭式平均AoI。有趣的是，理论分析表明，仅当源-目的链路信噪比低于-2dB时，中继辅助方案在平均AoI上才能优于无中继方案。此外，通过对比中继辅助方案的优劣，仿真结果显示TDMA方案能耗更低，而NOMA方案通常能实现更低的平均AoI。