# Wireless Sensor Network Aided Assembly Line Monitoring According to Expectations of Industry 4.0
## 2 Materials and Methods
### Localization Technologies  
Localization technologies are either based on satellite technology or on network structures. The localization technologies in closed spaces, such as in assembling facilities, require the use of network structures. Usually, these technologies require the determination of the distance between the nodes. To determine the position of the moving node, the distance between the moving node and static nodes, which are usually at fixed positions(anchor nodes or access points), is determined. To be able to determine the distance between the nodes, different methods can be used:  
* Methods based on Time Difference of Arrival [24]
* Methods based on the Angle of Arrival [24]
* Methods based on Received Signal Strength Indicator (RSSI) [25; 26]  
As opposed to the first three methods, which do not have the specified medium for localization, the RSSI method uses wireless technology. RSSI localization is based on using the strength of the signal between two nodes for position determination. The RSSI method is not limited by a specific wireless communication standard. Therefore, WiFi (WLAN), Bluetooth, and ZigBee wireless technologies can be used as well. A huge advantage of RSSI method is the fact that no additional hardware is required for the localization. Wireless nodes use radio modules for wireless communication, which can be also used for RSSI value determination for neighboring nodes. This simple structure enables for low energy consumption, small dimensions, and low costs.  
Fingerprint localization method uses RSSI values obtained by wireless communication between sensor nodes [27]. Mostly, it is used for closed space localization by using WSN. The method requires fixed-anchor nodes. Fingerprint localization is performed in two steps: offline and online [28].  
During offline determination, the so-called RSSI map is formed for the chosen space and environment. The RSSI map is determined by recording the RSSI values between the mobile and anchor nodes, i.e., anchors, which are positioned at known fixed positions. The number and location of the anchors is determined based on the characteristics of the space and surroundings and the precision requirements. The mobile node is moved from one known position to another known position and the RSSI value is recorded at all positions (Figure 4). The known positions are usually set in a matrix structure. The resolution of the matrix, i.e., the distribution density of the positions is higher for higher accuracy requirements.  
The recorded RSSI values are stored together with the position coordinates regarding to all anchors. The mobile nod sends a separate signal to all anchors. Based on the reply, the RSSI value is determined. The determined values are sent to a base node that is connected to the computer on which the data is stored.  
During online localization, the mobile node is at an unknown position, which has to be determined. The mobile node sends signals to anchor nodes (Figure 5).  
Figure 4: Offline phase of the fingerprint localization.  
Based on the reply from the anchors, the RSSI value is determined and sent to the base node, which transfers the data to the computer. Using localization algorithms, the unknown position is determined based on stored data from offline localization and data from online localization.