Study on feasibility of SATCOM for railway communication

Final Report
GEO / S-band
Criterion ID Criterion Evaluation Justification
CRT-TECH-4a
Transfer delay for the proposed satellite system shall be Low.
Note: It refers to the end-to-end (one-way) delay due to the
satellite channel. Therefore, it shall be measured from satellite
transmitter to satellite receiver.
4a: Single-hop required
NC
Since this solution is based on S-MIM communication standard, in the return link
the E-SSA random multiple access providing a low data rate is used (about 5
kbps). This low data rate makes the transmission time (Tx) not negligible in front of
the delay due to the use of a GEO satellite (of around 250 ms).
In addition, the reception process of asynchronous bursts sent requires an
iterative processing that can be even greater than the transmission time.
All these issues make the transfer delay in this system around the 500 ms.
It is also worth to mention that in the forward link (where the DVB-SH protocol is
used), channel impairments are counteracted by means of coding combined with a
long time interleaver at the physical or link layer level, increasing this way the
delay on forward link, as well.
CRT-TECH-4b 4b: Double-hop required NC
CRT-TECH-5
Delay jitter of the proposed satcom system shall be Low.
Note: Since this criterion refers to the one-way satellite
channel jitter, it shall be measured from satellite transmitter to
satellite receiver.
C (Low)
Since double hop transactions represents the double transmission delay that a
single-hop transaction, it does not comply either.
There exist mechanisms to compensate the delay time variations between packets
at IP level. Therefore, it is assumed that IP-based satcom solutions are capable to
comply with a Low delay jitter requirement.
In this case (GEO satcom solution), and considering that the lower the orbit the
lower the jitter, this criterion is market as a "C (Low)".
CRT-TECH-6
Network registration delay for the proposed satcom system
shall be Normal (with similar values of current GSM-R system).
C
The registration process of the GEO/S-band system can be designed in order to
assure a "normal" value. In fact, and taking into account the pure random access
method on return link, the network registration could be considered short.
Doc.Nº: SRAIL-FNR-010-IND
Edit./Rev.: 1/1
Date: 16/02/2017
Page 250 of 285