Study on feasibility of SATCOM for railway communication

Final Report
Main advantages of OFDM are related with its ability to cope with severe channel conditions, such as:
- Spectrum efficiency, using orthogonal overlapping subcarriers
- Immunity to selective fading due to multipath
- Resilience to interference, since interference appearing in a channel will not affect all
the sub-channels.
- Etc
The main disadvantage of OFDM is the high PAPR (Peak to Average Power Ratio) that may occur.
Therefore, DVB-SH is an interesting standard since its hybrid satellite/terrestrial solution eases the
implementation of gap fillers. But as weak points, the few number of modulations and coders to fight
against channel impairments and the use of its high flexible channel interleaver (that offers time
diversity from about one hundred milliseconds to several seconds), which helps to signal recovery at
receiver side but introducing a big delay not allowed by critical applications.
Another weak point of DVB-SH is the absence of commercial solutions (i.e. equipments), and even of
satellite, since in April 2009 the W2A satellite was launched after Inmarsat and Solaris Mobile (a joint
venture between Eutelsat and Astra) were awarded each a 2×15 MHz portion of the S band by the
European Commission.
6.4.4 S-MIM
S-MIM (S-band Mobile Interactive Multimedia) is a radio interface standardized by ETSI. It is
especially designed to provide messaging services over S-band GEO satellites.
Since S-band satellites are planned for the mobile broadcast of multimedia content to handhelds or
vehicle-mounted mobile devices, the main goal of this standard is to get low power (low-cost)
terminals and high bandwidth efficiency.
The forward link defined by S-MIM uses the DVB-SH as a broadcast radio interface. For the return
link, S-MIM system provides 3 sets of service segments:
- SS1) Broadcast and interactive services
- SS2) Data acquisition services
- SS3) Real time (emergencies) services
The return link air interface is based on 2 non-exclusive options, depending on the service required:
- Asynchronous access using spread spectrum ALOHA (SSA), similar to ANTARES.
First deployment was carried out in European project called SAFETRIP.
- Synchronous access using quasi-synchronous code multiple access (QS-CDMA).
Despite this standard is assuming S-band payload of a GEO satellite, non-GEO satellites are also
compatible provided that Doppler compensation mechanisms are put in place. Therefore, other
frequency bands such as L-, C- or Ku/Ka- bands might also be considered.
6.4.5 ANTARES CS
The European Space Agency (ESA), in coordination with SESAR Joint Undertaking and the European
Organization for the Safety of Air Navigation (Eurocontrol), initiated the Iris programme to develop a
satellite-based communication system and to standardise a new communication solution specifically
designed for the provision of safety aeronautical Air-Ground communications in accordance with the
future SESAR Air Traffic Management (ATM) concept. The new Communication Standard (CS) was
Doc.Nº: SRAIL-FNR-010-IND
Edit./Rev.: 1/1
Date: 16/02/2017
Page 101 of 285