## 3 System Description
### Communication Architecture  
Our sensor platform, as depicted in Figure 2, is designed to be part of the slab track, inserted before its installation, so that it can be used in both installation and maintenance phases. The slab track therefore becomes an active element capable of monitoring and reporting information about the environment such as vibration performance, distance and inclination and is also able to assist operators in the installation phase. The number of monitored slab tracks is based on factors such as the cost of the monitoring system or the criticality of the railway section.  
Railway infrastructures cover really large areas, possibly in places where there is no Internet coverage or permanent electricity supply. The system proposed in this paper makes use of autonomous  
Figure 1: Hardware architecture.  
sensor nodes powered by batteries and solar panels. They wake up periodically to gather information. In order to deal with the connectivity/coverage issue the sensor nodes use trains as data mules. The information collected by the sensors, after being partly analyzed and reduced, is received by sink nodes placed on each of the passing trains. Finally, trains send the information to the remote control center.  
Most of the normal vibrations produced in the slab track are located in the 1-800 Hz range. In order to capture this acceleration data a high sampling rate is necessary. However, sensors are energy-limited devices and their communication speed is relatively low and energy consuming. In order to avoid transmitting a huge amount of data from each sensor to the train, each sensor locally processes the acceleration and obtains vibration frequency peaks in the frequency domain. The algorithm carried out locally by each sensor is as follows:  
1. Each sensor collects triaxial raw acceleration when a train is passing.
2. Acceleration data is transformed to the frequency domain using the FFT for each window.
3. The highest peaks in acceleration in the frequency domain are detected (in all windows).
4. The three biggest peaks (amplitude/frequency pair) that correspond to the main frequencies at which the slab track vibrates and their corresponding timestamps are sent to the next train that passes by and from the train to the control center (Figure 2).  
Additionally, periodic readings of temperature, inclination and distance between slab tracks are collected and stored. All these three magnitudes are measured with different sensors installed in the sensor nodes. To measure inclination dedicated inclinometers have been installed whereas to measure distance, ultrasonic sensors have been used. They are used to detect changes in the initial conditions of the deployment and therefore detect possible issues or abnormal situations. More details about the selection process and integration of these sensors are explained in [7].  
The communication architecture described allows us to monitor the structural health of the slab track. In addition, it can also be used to help in the installation phase (Figure 3). During the installation, sensors have to provide their initial measurements as they are needed as reference values for the analysis.  
Figure 2: Maintenance phase communication architecture.  
Sensors also have to give their unique identifiers to the application so they can be stored with their geographical position.