COBHAM Sailor 250 FleetBroadband, Training Manual, Page: 28 / 270

18
1
Chapter 1
Inmarsat Network
1.7.1 Forward direction Bearer types
Beam Forward carrier
type
FEC blocks
per 80 msec
frame
Normal
satellite EIRP
/carrier dBW
Band-
width
KHz
Modulation
scheme
Symbol
rate kSym/
sec
Bit rate
kb/sec
Global F80T0.25Q1B 1 25.5 QPSK 8.4 8.4 4.5
Regional F80T1Q4B 4 29.2 QPSK 33.6 40 21.6 to
55.6 (link
adaptation)
42.8 to
109.2 (link
adaptation
F80T1X4B 4 29.2 16-QAM 33.6 30.4
Narrow F80T4.5X8B 8 40.5 16-QAM 151.2 41.2 113.6
Figure 1-26: Forward direction Bearer types
Forward carrier type C/No
F80T0.25Q1B
41.5

48.5 dBHz
F80T1Q4B
46.5

54.0 dBHz
F80T1X4B
51.0

59.5 dBHz
F80T4.5X8B 57.0

65.5 dBHz
Figure 1-27: Forward carrier type related to C/No
The BGAN network air interface offers a range of forward direction
bearers. For each type of beam (global, regional, narrow) Inmarsat
has selected appropriate forward direction bearers on the basis
of the satellite EIRP, FleetBroadband Terminal receive sensitivity
and required data rate. A forward direction bearer is a carrier
transmitting a continuous stream of 80 ms data frames at constant
power. Each of the 80 ms frames consists of one, four or eight
blocks of data (“FEC blocks”) each separately turbo-coded. Each
80 ms frame is preceded by a unique-word pattern. Pilot symbols
are interspersed throughout the frame to assist the FleetBroadband
Terminal with channel estimation and synchronisation.
A single forward bearer type has been chosen for use in the
global beam. This bearer has a bandwidth of 10.5 kHz, uses
QPSK modulation (2 bits per symbol) and has a bit rate of 4.5
kbps. (F80T0.25Q) There are two choices of forward bearer in
the regional beam. The Class 14 FleetBroadband Terminal cannot
decode the higher data rate 16 QAM (4 bits per symbol) bearer
because the antenna is too small, however the QPSK bearer is
inefficient for the FleetBroadband Terminal classes with larger
antennas – hence we retain the option to transmit both bearer
types in the regional beams with the Class 14 FleetBroadband
Terminals tuned to the QPSK bearer with a bit rate of 26.1-55.6
kbps, and the Class 8 and Class 9 FleetBroadband Terminals
tuned to the 16 QAM bearer with a bit rate of 42.8-109.2 kbps
(F80T1Q and F80T1X).
A single forward bearer type has been chosen for use in the narrow
beam. This bearer has a bandwidth of 189 kHz and uses 16 QAM
modulation with a bit rate of 200-492 kbps (F80T4.5X).