Peak Age of Collection in Coordinated Direct and Relay Transmission with Physical-Layer Network Coding

This paper investigates the information freshness of joint status updates, quantified by the age of collection (AoC), in uplink coordinated direct and relay transmission (CDRT). In an uplink CDRT setup, a direct sensor communicates directly with the destination, while a relay-aided sensor relies on a decode-and-forward relay. The update packet of each sensor contains partial information about a common observation target. The AoC measures the time elapsed since the generation of the latest set of update packets received at the destination. Hence, unlike the age of information (AoI) metric, the AoC decreases only when all update packets from multiple sources for a common observation are collected (i.e., a successful joint update). When simultaneous transmissions from the direct and relay-aided sensors cause packet collisions at the relay, conventional multiuser decoding (MUD) is usually used to decode native packets explicitly from the superimposed signals. Nevertheless, MUD does not work well when the signal-to-noise ratios (SNRs) of different sensors are (nearly) equal. To this end, this paper puts forth a physical-layer network coding (PNC)-aided CDRT scheme for joint information updating, utilizing both MUD and PNC decoders. The PNC decoder decodes superimposed signals into network-coded packets, particularly effective when the SNRs of different sensors are close. We design an automatic repeat request (ARQ) protocol tailored to low AoC and study how network-coded packets can be utilized to reduce the AoC of uplink CDRT, where the closed-form peak AoC formula is derived. We evaluate the PNC-aided CDRT scheme using software-defined radios. Experimental results show that our PNC-aided CDRT scheme significantly reduces the average peak AoC under various SNR conditions.