# Industry 4.0 and Lean Six Sigma Integration: A Systematic Review of Barriers and Enablers
## 2 Theoretical Background  
### Industry 4.0 Technology  
Since its introduction in 2011, multiple authors have attempted to conceptualise Industry 4.0 [17]. However, to date, there has been no consensus on its definition. This fact makes it challenging to conduct academic studies on the topic [18] and has given rise to the incorrect assumption that I4.0 covers almost every technology. To shed light on this issue, we begin by reviewing some of the most cited articles that provide descriptions of I4.0. We then analyse these descriptions and identify a set of technologies that enable their implementation. Table 1 summarises the descriptions of I4.0, listed chronologically, to show how this notion has been shaped over the years. From a pure technology perspective, these descriptions show that, at a fundamental level, I4.0 is characterised by the confluence of the concepts of the Internet of Things (IoT) and cyber-physical systems (CPSs) [5]. The IoT focuses on the interconnectivity of physical objects, allowing their communication and collaboration [19]. In this regard, wireless communication technologies can be seen as enablers of this concept. Cyber-physical systems are integrations of physical objects, cloud technology, and algorithms, which allow the analysis and control of physical processes [20]. In this regard, technologies that allow data collection from physical objects, such as radio-frequency identification (RFID) tags and sensors, can be seen as enablers of such integrations (i.e., CPSs). A similar argument can be made in favour of technologies that facilitate data storage from physical objects (in the cloud) and technologies that allow data analysis. Examples of the latter include (but are not limited to) Big Data and Analytics (BDA), machine learning, artificial intelligence, simulation, and digital twins [21]. Furthermore, the control dimension of the CPSs means that the \"computations\" can affect the physical objects being monitored (and vice versa) [22], leading to a need for actuators as part of such cyber-physical systems. From the perspective of deployment, according to [5], I4.0 may be implemented in organisations through a combination of horizontal and vertical integrations, as well as the integration of engineering from beginning to end along the full value chain. Horizontal integration relates to the cooperation of information technology (IT) systems at different stages of the value chain, as well as their cooperation with the IT systems of other companies' value chains. Such cooperation entails security issues related to data and information sharing [23]. In this regard, technologies that enable secure data sharing, such as blockchain, are usually considered enablers of I4.0. In contrast, vertical integration relates to the hierarchical integration of IT systems, which requires the actuator and sensor signals to be fully integrated digitally across all levels, all the way up to the ERP level [5]. In this regard, systems for supervisory control and data acquisition (SCADA), manufacturing execution systems (MES), and enterprise resource planning modules (ERP modules) can be considered as part of the baseline technological infrastructure for I4.0 implementation. From a design perspective, in [19], I4.0 was characterised using the principles of interconnection, information transparency, decentralised decisions, and technical assistance. According to these authors, the latter characteristic is based on the notion that, in the \"Smart Factory\", the staff needs (1) technologies aggregating and displaying data for decision-making, such as smartphones and other wearables (e.g., augmented/virtual reality glasses) and (2) technologies automating time-consuming or safety-critical activities, such as robots. The literature also shows that, despite not being technologies themselves, the IoT and CPSs are recurrently considered as such.