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：本文研究了工业物联网（IIoT）系统中的信息新鲜度问题，其中每个物联网设备对共同目标进行部分观测，并将信息更新传输至中央接收器以恢复完整观测。我们采用采集年龄（AoC）性能作为信息新鲜度的度量指标。与传统信息年龄（AoI）指标不同，瞬时采集年龄仅在共同观测的所有协作数据包均被成功接收时才会降低。因此，在物联网设备间有效分配无线时频资源以实现中央接收器处的低平均采集年龄至关重要。本文考虑了三种多接入方案：无重传的时分多址（TDMA）、带重传的时分多址以及频分多址（FDMA）。首先，我们的理论分析表明，在平均采集年龄方面，带重传的TDMA优于其他两种方案。随后，我们基于这三种方案在软件无线电平台上实现了信息更新系统。实验结果表明，考虑实际中的媒体接入控制（MAC）开销，在高信噪比（SNR）区域，FDMA实现的平均采集年龄低于带或不带重传的TDMA。相比之下，带重传的TDMA在宽信噪比范围内提供稳定且相对较低的平均采集年龄，这有利于IIoT应用。总体而言，我们对IIoT信息更新系统中的采集年龄进行了理论与实验相结合的研究。