The last few decades have witnessed an explosive growth of the Internet-of-Things (IoT) systems, which provide ubiquitous sensing and computing services. When adopted in industrial and manufacturing environments, IoT is referred to as the industrial IoT (IIoT), which has attracted increasing research attention. Energy efficiency is one of the most important research topics in green IIoT, as 1) the limited resource can significantly affect the lifetime of IIoT systems and 2) massive sensors, devices, machines keep consuming a considerable amount of energy, and increasing the carbon footprint. In this article, we present a comprehensive survey on energy-efficient communications and computation mechanisms in IIoT systems (such as smart grids). We categorize the existing works, review, discuss, and compare the works to explore their pros and cons. We also discuss the open issues and research challenges, considering the recent 5G communications and edge computing trends.

Low-power wireless networks (LPWNs) are of paramount importance for the pervasive deployment of Internet-of-Things (IoT). To deal with the lossy nature of LPWNs, opportunistic routing (OR) and multichannel communications (MC) have received significant research interests. In particular, coupling OR with MC has become an important way to further enhance the communication performance in LPWNs. However, as OR requires nodes in the same channel while MC separates nodes into different channels, the benefits of MC-OR combination are largely under-utilized. To address this problem, we investigate the challenging issue of establishing opportunistic routing in multichannel LPWNs. Different from the existing studies that separately assign channels and select forwarders, we propose a synergistic multichannel and opportunistic routing (SMOpp) approach, which jointly combines the benefits of both OR and MC by considering link correlation. SMOpp explicitly evaluates the routing opportunities of each channel/forwarder set and then employs a forwarder-initiated scheme to select the best combinations of senders, forwarders, and channels. The testbed evaluation shows that compared to the existing methods, SMOpp significantly improves the transmission efficiency for LPWNs.