Timely Information Update With Nonorthogonal Multiple Access

This article studies information freshness in information update systems with nonorthogonal multiple access (NOMA). Information freshness is characterized by age of information (AoI), defined as the time elapsed since the generation of the last successfully received update. Conventional orthogonal multiple access (OMA) systems, say time-division multiple access (TDMA) systems, lead to high average AoI when a large number of users take turns to transmit their latest samples to a common receiver over a wireless medium. In contrast to OMA, NOMA allows multiple users to transmit simultaneously. Although NOMA could lead to higher packet error rates (PER) due to the wireless interference among users, we show that higher PERs do not always lead to a higher average AoI. Specifically, our experiments on software-defined radio indicate that NOMA with conventional multiuser decoding (MUD) techniques leads to higher PERs but lower average AoI than OMA does in the high SNR regime. Furthermore, to improve the AoI performance in the medium SNR regime, we combine MUD with physical-layer network coding (PNC), a technique that turns wireless interference into useful network-coding information. PNC works well even when the SNRs of different NOMA users do not differ much. This article is the first attempt to apply PNC to information update systems. Experiments show that the combined use of PNC and MUD reduces the average AoI significantly in a practical network setting. Overall, PNC-enabled NOMA is a promising solution to information update systems.