Study on feasibility of SATCOM for railway communication

Final Report
GAP FILLERS
Figure 11 also shows the gap fillers (GF), which are the gateways (based on SATCOM terminals) in
charge of resending the information received from satellite to an area without satellite visibility (and
vice-versa), i.e. without coverage (like for example in tunnels). There are several types of gap fillers,
and they can be grouped mainly in transparent and non-transparent:
- Transparent gap-fillers re-send the same satellite signal to the area without coverage. It is the
best option if the satellite system has been designed to operate in a multipath environment
(such as CDMA or OFDM), since the GF can be simply an amplifier of the satellite signal. It
offers scarce flexibility since the whole signal on the satellite segment is re-transmitted without
waveform optimization, but if the satellite signal is good enough to be retransmitted in urban
areas/tunnels this solution is simpler and implies lower cost.
- However, non-transparent gap-fillers changes satellite signal to another (terrestrial)
technology. This type of GF is generally used when the signal transmitted by the satellite has
not been designed to operate in a multipath environment, such as TDM based. Nontransparent
GF are more complex than transparent ones, but there is flexibility in the choice of
the tunnel repeater technology and resource allocation, including the possibility to use a
system based on OFDM modulation in order to overcome multipath. This option also implies
the use of two receivers at user side (one for satellite and another one for the GF).
It is interesting to highlight that GF final coverage range will depend on the type of gap filler
(transparent/non-transparent) and also on the final equipment selected and their features.
SATCOM TOPOLOGIES
Two main network topologies are typical in SATCOM networks: star and mesh. It is shown in the next
figure:
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HUB
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a) Star b) Mesh
Figure 12: Satellite topologies
Doc.Nº: SRAIL-FNR-010-IND
Edit./Rev.: 1/1
Date: 16/02/2017
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