## 2 System Description  
### UIC Cable Descrcription  
As described in [14], most trains running on European tracks are equipped with UIC cables, being the main core of the remote control and communication line. In particular, an 18-conductor flexible shielded cable (terminated by a plug) connects the stocks to one another and is connected (inside each stock) to a connection box (present at both ends of each vehicle). Inside the vehicle, the 18 conductors are divided into two different cables: 16-conductor and two-conductor cables, both shielded. Figure 1 shows the section of the 16-conductor cable with the cable numbering as explained in [14].  
The 16 conductors have different roles in the train control, and they are divided into functional groups: Each single group has one conductor serving as a reference and one or more conductors as signal line. A brief description of the role of the conductors is the following:  
1. Conductors 1 to 8 are by default dedicated to the transmission of voice signals and are characterized by AC signals at a frequency range 100-8000 Hz, and permanent DC signals characterized by a value between 18 and 33 V.
2. Conductors 9 to 16 are by default dedicated to the transmission of DC pulses (with amplitude between 15 and 33 V and 2-second duration) for doors opening and closing and for the light signals \"stop\" and \"go\", with conductor #12 as common ground.  
In addition, some users (German railways, DB, Austrian railways, OBB, French railways, SNFC, and Swiss railways, CFF) utilize conductors 9 to 12 for additional functions.
* 4. Conductors 9 to 12 are dedicated to the emergency brake and the electropneumatic actuators (DC pulses with value between 15 and 33 V, DB, OBB, CFF).
* 5. Conductors 9 and 10 can carry an AC signal at 1 kHz relating to the emergency brake information (DB, OBB, CFF).
* 6. Conductors 10 and 11 carry AC signals (around 100 kHz, 600-950 mV) relating to traction commands (DB).
* 7. Conductors 7, 9, 10 and 11 carry additional signals in case the train has to leave the station in the opposite direction of its arrival (6 V, frequency band 10-20 kHz (SNCF).
* 8. Conductors 7 and 8 are used for voice messages in case there are external loudspeakers (DC signal, between 18 and 33 V, SNCF).  
All the above conductors are good candidates to carry the broadband powerline channel, since standard OFDM (Orthogonal Frequency Division Multiplexing) protocols (such the widely diffused HomePlug AV) work in the frequency range 2-30 MHz and there is no overlapping in the frequency bands mentioned above.  
In [15; 16], the authors chose conductor #12 as a return, and one of the conductors #10 or #11 (dedicated to the switching on and off of the light signals \"stop\" and \"go\") because of the specific default application (\"stop\" and \"go\" signal) with the application to the Italian national railway system.  
Taking into account also the additional functions described above, the previous choice could still be used, but in this paper we investigate a different choice: the selected two-conductor line comprises conductors 5 and 6. This pair of conductors is dedicated to the switching of the speakers and is characterized by DC signals of high magnitude (15-33 V), which are not affected by the low-power OFDM signal in a high-frequency band.