9.3
SATELLITE COMMUNICATIONS
229
Table 9.6
QPSK Characteristics and Transmission Parameters for IDR Carriers
Parameter
Requirement
1. Information rate (IR)
64 kbps to 44.736 Mbit/s
2. Overhead data rate for carriers with IR
1.544 Mbps
96 kbps
3. Forward error correction encoding
Rate 3/4 convolutional encoding/Viterbi
decoding
4. Energy dispersal (scrambling)
As per ITU-R5.524-4
5. Modulation
Four-phase coherent PSK
6. Ambiguity resolution
Combination of differential encoding
(180
) and FEC (90
)
7. Clock recovery
Clock timing must be recovered from
the received data stream
8. Minimum carrier bandwidth (allocated)
0.7 R Hz of [0.933 (IR
+ Overhead)]
9. Noise bandwidth (and occupied bandwidth)
0.6 R Hz or [0.8 (IR
+ Overhead)]
10. E
b
/N
0
at BER (Rate 3/4 FEC)
10
-3
10
-7
10
-8
a. Modems back-to-back
5.3 dB
8.3 dB
8.8 dB
b. Through satellite channel
5.7 dB
8.7 dB
9.2 dB
11. C/T at nominal operating point
-219.9 + 10 log
10
(IR + OH), dBW/K
12. C/N in noise bandwidth at nominal operating point
(BER
10
-7
)
9.7 dB
13. Nominal bit error rate at operating point
1
× 10
-7
14. C/T at threshold (BER
= 1 × 10
-3
)
-222.9 + 10 log
10
(IR + OH), dBW/K
15. C/N in noise bandwidth at threshold
(BER
= 1 × 10
-3
)
6.7 dB
16. Threshold bit error rate
1
× 10
-3
Notes: IR is the information rate in bits per second. R is the transmission rate in bits per second and equals
(IR + OH)
times 4/3 for carriers employing rate 3/4 FEC. The allocated bandwidth will be equal to 0.7 times the transmission rate,
rounded up to the next highest odd integer multiple of 22.5 kHz increment (for information rates less than or equal to
10 Mbps) or 125-kHz increment (for information rates greater than 10 Mbps). Rate 3/4 FEC is mandatory for all IDR
carriers. OH
= overhead.
Source: IESS-308, Rev. 7, Ref. 5. Courtesy of INTELSAT.
are given in Table 9.6. These parameters are for the intermediate data rate (IDR) digital
carrier system. All IDR carriers are required to use at least
R = 3/4, where R is the code
rate.
16
Reference 3, Chapter 4, provides a detailed description of various FEC channel
coding schemes.
The occupied satellite bandwidth unit for IDR carriers is approximately equal to 0.6
times the transmission rate. The transmission rate is defined as the coded symbol rate.
To provide guardbands between adjacent carriers on the same transponder, the nominal
satellite bandwidth unit is 0.7 times the transmission rate.
9.3.8
Very-Small-Aperture Terminal (VSAT) Networks
9.3.8.1
Rationale
. VSATs are defined by their antenna aperture (diameter of the
parabolic dish), which can vary from 0.5 m (1.6 ft) to 2.5 m (8.125 ft). Such apertures
are considerably smaller than conventional earth stations. A VSAT network consists of
one comparatively large hub earth terminal and remote VSAT terminals. Some networks
in the United States have more than 5000 outlying VSAT terminals (a large drugstore
chain). Many such networks exist.
16
R = (information bit rate)/(coded symbol rate). When R = 3/4 and the information rate is 1.544 Mbps, the
coded symbol rate is 4/3 that value, or 2,058,666 symbols a second. FEC coding simply adds redundant bits
in a systematic manner such that errors may be corrected by the distant-end decoder.