## 2 Literature Review
### Elements and Characteristics of a Digital Twin  
As stated, a DT is composed by some elements. A list of components for DTs in construction industry is provided as follows:  
* A physical asset and its virtual counterpart, and data connecting them [23];
* Platform to visualize and manage sensor data, e.g., data and virtual model visualization, analysis, and simulation, which is a key aspect for real-time remote monitoring [23]. The platform should return insights, alerts, or predictions regarding the physical object, thus supporting the decision-making process for the definition of O&M objectives and plans [40; 58];
* An acquisition layer such as an IoT system [40; 46; 53; 59], since sensing is a vital component of a DT [60; 61; 62; 63], allowing for continuous monitoring of the physical asset. The virtual component enriched with real-time data regarding the real object represents a dynamic digital replica of the physical asset [46; 53];
* BIM model as a starting point, especially as regards the geometrical virtual replica of the building, allowing for the evolution and information enrichment of the BIM model itself [20; 23];
* Artificial intelligence (AI) tools to analyze data and provide predictions, simulations, and data analytics [20; 46]. In addition, some characteristics are fundamental for the correct definition of a DT:
* Synchronization between physical and virtual component [40], with data flowing at least in one direction allowing for analyses, control, and simulation on the virtual model [20; 46; 64]. Any change in the monitored characteristics or conditions of the asset is detected and, through data flow, is reflected in the virtual counterpart [20; 21];* Bidirectional communication between physical and virtual part, either with or without humans in the loop, defining a Passive DT or an Active DT, respectively [57]. The knowledge regarding the asset provided by the virtual part results in either human intervention of direct actuation in the real asset [21];
* A DT represents specific and selected aspects of the physical asset, i.e., the subjects of monitoring, simulating, and analyzing, so it does not represent an exact duplication of reality [57];
* Data or status visualization capabilities in order to support the monitoring and decision-making processes by the actors that are in charge of the asset O&M phase [46]. As previously specified, one of the main characteristics of a DT is the direct connection between physical and virtual entity, with the concept of twinning as alignment and reciprocity between the two components [19]. Therefore, a fundamental aspect for the definition of a DT is the data collection process, i.e., data quantity, quality, and granularity, on which depends the correct detection of changes of the real object over time, and thus the correspondence of the digital object to the real one and its continuous evolution through the building lifecycle [20; 21; 22]. A first fundamental step is the selection of sensor types that are most suitable for each specific application [23]. In addition, the spatial distribution of sensor network, i.e., the spatial distribution of sensors in the indoor spaces, is another theme that should be faced [24]. Furthermore, in order to allow a correct detection and collection of data, the IoT sensor system should be properly set and calibrated [25; 26] to ensure the quality of data collected. In fact, the output of an analysis strongly depends on the data that are used as input for the system or algorithm; therefore, to obtain satisfactory results, data quality is essential [22]. Nonetheless, existing studies tend to focus on different phases and aspects of the DT definition and creation, while IoT sensor system definition is a less investigated aspect, almost taken for granted [23]. As stated above, a fundamental preliminary step is a detailed analysis of sensor types in order to identify the most suitable ones for the research objectives.