Age of collection with non-orthogonal multiple access: A theoretical-plus-experimental study

Abstract

This paper examines the concept of information freshness in a collaborative surveillance scenario utilizing non-orthogonal multiple access (NOMA). In this scenario, each monitoring device observes a portion of a common target and reports the latest status to a common access point (AP), which then recovers the complete observations. We use the age of collection (AoC) as a metric for information freshness. Unlike the conventional age of information (AoI) metric, the instantaneous AoC decreases only when the AP receives all partial updates from different devices (i.e., successfully receives a “joint” update). Consequently, existing AoI-oriented analyses on NOMA cannot be easily generalized to AoC due to the different update rules between AoC and AoI. Conventional NOMA schemes typically employ multiuser decoding (MUD) techniques to decode update messages from different devices, referred to here as the MUD-only NOMA system. However, MUD techniques are less effective when the signal-to-noise ratios (SNRs) of different NOMA users are nearly balanced. Therefore, we consider network-coded multiple access (NCMA), an advanced NOMA scheme that integrates MUD with physical-layer network coding (PNC). PNC is a technique that transforms wireless mutual interference into useful network-coded information, working well even when the SNRs of different users are similar. Experimental results using software-defined radios indicate that NCMA is a practical solution for achieving low average AoC under varying channel conditions. The combination of MUD and PNC enables the AP to receive joint update messages much faster, significantly reducing the average AoC of the system.