Choice Of Fec - Advantech AMT-73 Installation And Operation Manual

Therefore, to increase the 64kSymbol acquisition range to +/-30 kHz the rsweep command could be
set to 30,000 Hz and the rssize command set to 14kHz.
It is recommended NOT to use the Spectral Analysis (FFT assisted acquisition)
above 3MSymbols. This will significantly slow signal acquisition.
RSPANL (the command which enables or disables FFT assistance) is set to auto by default; in this
case the FFT is only enabled when the bit rate is less than 1.5Mbps.

Choice of FEC

3.2.2
The AMT-70 modem will always support a traditional basic DVB-S (RSV) coding scheme with BPSK
and QPSK modulation – either Intelsat IESS-308/9, or DVB-S (or both). Traditionally Intelsat systems
have been used for telecommunications data (multiple 64kbps streams and T1/E1 traffic) while DVB-S
has been used for MPEG digital video broadcasting. However, DVB systems are increasingly also
being used to transmit data both in MPEG format and as raw data.
DVB FEC systems always concatenate RS and Viterbi coding whereas the Intelsat systems support
RS as an option. Operation without RS can be important when transmitting low-latency services such
as voice traffic: removing the RS decoding overhead.
With BPSK and QPSK modulation, DVB-S supports ½, 2/3, ¾, 5/6 and 7/8 inner code rates with a RS
(204,188) outer code. Intelsat supports ½, ¾ and 7/8 inner code rates with various outer RS code rates
including RS (126,112) and RS (219,201). The Intelsat system without RS codes offers a few 100 data
bits of latency. With RS coding, an interleaver of depth 4 or 8 can be chosen to trade off latency and
performance. The DVB system has approximately 32kbits of latency (code-rate dependent).
The modem can transmit RS-530 or HSSI data using either DVB-S or Intelsat. DVB systems out-
perform (by approximately 1dB) Intelsat systems because they include a better interleaver between the
constituent Viterbi and RS codes.
The AMT-70 Modem also supports higher order modulation systems, such as 8PSK and 16QAM.
These systems are typically used when the link budget permits since, in combination with appropriate
FEC, they permit higher spectral efficiency and hence reduce the required satellite bandwidth.
8PSK modulation is currently more popular than 16QAM as it does not require high-linearity amplifiers
and transponders. The AMT-70 modulator can be enabled to support IESS-310 8PSK systems (a
PTCM 2/3 code, with an RS (219,201) outer code) giving about 2 bits/s/Hz. It can also support DVB-
DSNG (PTCM 2/3, 5/6 and 8/9 rates, with the RS (204,188) code) and eTPC rates 0.75,0.79, 0.87 and
0.92. Turbo modes are about 1dB better than the roughly equivalent DVB-DSNG systems. The Intelsat
system is about 1.5dB inferior to the DVB system, due to differences in the interleaving structure.
The AMT-70 Modem also supports 16QAM in two coding modes: DVB-DSNG and eTPC. DVB-DSNG
16QAM supports ¾ and 7/8 rates with the RS (204,188) outer code. 16QAM with eTPC can usefully
operate in 4 rates: 0.75, 0.78, 0.87 and 0.92.
Important Note: The DVB system can be configured to transmit both MPEG-2 and non MPEG-2 data.
When using DVB modes to transmit non MPEG-2 data it is necessary to turn DVB REMOVE MARK
ON in the demodulator (command RRMVMK ON). This is required, since DVB FEC utilizes the MPEG
framing structure, which must therefore be imposed by the modulator (and similarly removed by the
demodulator). There is a performance overhead associated with the sync byte, but it is only 0.5% of
data rate.
AMT-70/AMT-73/AMT-75 Installation and Operation
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