Study on feasibility of SATCOM for railway communication

Final Report
T up is the propagation delay from ground to the overhead satellite
T down is the propagation time from the satellite to the ground
It is worth mentioning that previous formula includes the ‖T rx ‖ because the processing of the reception of an asynchronous CDMA burst based on E-SSA
cannot be considered a negligible value. In fact, it can be assumed (refer to [RD-23]) that the reception process can take in mean a period of time of 1.5
times de transmission time (i.e. T rx = 1.5*T tx ).
In order to provide an example, following data is assumed:
Data packet: 120 bytes
Transmission in return link: 10 kbps
Since the delay study is performed with the most restrictive channel, it will be made with the return channel.
Then, the end-to-end transmission packet delay in return link is:
T tx = packet_size / throughput = 120 bytes / 10000 bps = 96 ms
T rx = 1.5*T tx = 144 ms
T up = T down = sat_distance / speed = 36000 km / 300000 km/sec = 120 ms
This result shows that in this case, the low throughput provided by the ANTARES CS and the non-negligible processing time at the reception, duplicates
the transmission delay regarding those solutions providing higher throughput. On the contrary, the random access provided by ANTARES has the
advantage of sending the information when it is available without the need of requesting for resources.
Doc.Nº: SRAIL-FNR-010-IND
Edit./Rev.: 1/1
Date: 16/02/2017
Page 243 of 285