Abstract : This paper investigates the information freshness of relay-aided status updating in coordinated direct and relay transmission (CDRT) for the first time. Information freshness is measured by age of information (AoI), defined as the time elapsed since the generation of the last successfully received update packet. In CDRT, one sensor communicates directly with the receiver while another sensor requires the assistance of a decodeand-forward relay. Conventional CDRT is achieved via time division multiple access (TDMA) or non-orthogonal multiple access (NOMA). Interestingly, our experiments on software-defined radios (SDR) indicate that in error-prone wireless networks, TDMA-CDRT and NOMA-CDRT are not superior to each other in terms of peak AoI in all signal-to-noise ratio (SNR) regimes. Specifically, the direct sensor prefers NOMA-CDRT while the relay-aided sensor prefers TDMA-CDRT, leading to a dilemma in optimizing the peak AoI of the system. To this end, we put forth a hybrid CDRT scheme that combines TDMA and NOMA. The SDR experimental results show that the hybrid CDRT scheme not only balances the peak AoI of the direct and relay-aided sensors, but also achieves better information freshness for the whole CDRT system.
Index terms : Age of information (AoI), coordinated direct and relay transmission (CDRT), Internet of things (IoT), nonorthogonal multiple access (NOMA).