This paper investigates the information freshness of Industrial Internet of Things (IIoT) systems, where each IoT device makes a partial observation of a common target and transmits the information update to a central receiver to recover the complete observation. We consider the age of collection (AoC) performance as a measure of information freshness. Unlike the conventional age of information (AoI) metric, the instantaneous AoC decreases only when all cooperative packets for a common observation are successfully received. Hence, effectively allocating wireless time-frequency resources among IoT devices to achieve a low average AoC at the central receiver is paramount. Three multiple access schemes are considered in this paper: time-division multiple access (TDMA) without retransmission, TDMA with retransmission, and frequency-division multiple access (FDMA). First, our theoretical analysis indicates that TDMA with retransmission outperforms the other two schemes in terms of average AoC. Subsequently, we implement information update systems based on the three schemes on software-defined radios. Experimental results demonstrate that considering the medium access control (MAC) overhead in practice, FDMA achieves a lower average AoC than TDMA with or without retransmission in the high signal-to-noise ratio (SNR) regime. In contrast, TDMA with retransmission provides a stable and relatively low average AoC over a wide SNR range, which is favorable for IIoT applications. Overall, we present a theoretical-plus-experimental investigation of AoC in IIoT information update systems.

翻译：本文针对工业物联网系统的信息新鲜度进行了研究，其中每个物联网设备对一个共同目标进行部分观测，然后将信息更新传输给中央接收器以恢复完整观测。我们将采集时间（Age of Collection, AoC）指标作为信息新鲜度的衡量标准。与传统的信息时代（Age of Information，AoI）度量不同的是，仅当所有合作数据包都成功接收时，瞬时的AoC才会下降。因此，有效分配无线时频资源以在中央接收器处实现较低的平均AoC是至关重要的。本文考虑了三种多用户接入方案：无重传时分复用（TDMA）、有重传TDMA和频分复用（FDMA）。首先，我们的理论分析表明，TDMA重传方案在平均AoC方面优于另外两种方案。随后，我们在软件定义无线电上实现了基于三种方案的信息更新系统。实验结果表明，在实际中考虑媒体访问控制（MAC）开销，FDMA在高信噪比（SNR）范围内比TDMA（有或无重传）实现了更低的平均AoC。相比之下，TDMA重传方案在较宽的SNR范围内提供了稳定且相对较低的平均AoC，这对于工业物联网应用非常有利。总的来说，本文对IIoT信息更新系统中的AoC进行了理论与实验探究。