M7/M7L/M7LT Modular Satellite Modem Operation
M7/M7L/M7LT - Rev. 0.05 1-23
allow entering either just the 3 digit FEC options or the full Modulation, FEC Type, Option,
Code rate and Reed-Solomon mode (MTOCR) in a single entry.
Additional table notes:
TPC 4k and 16k restrictions apply to that line and Code Rate only
TPC16k operates up to up to 51.66Mbps depending on the Feature Set and modulation
The Viterbi CT modes, Rate 3/4 & 7/8, 16QAM are only for Comtech modem compatibility as they
only operate in this mode with R-S at 220, 200, depth of 4. R-S is auto-enabled
TPC Advanced modes are Datum Systems proprietary implementations that require the TPC16k
option only for the highlighted lines. They offer superior performance to CT modes
1.6.1. Viterbi, Trellis Code Modulation Codec
Viterbi has been the standard high performance FEC used in satellite communications for many
years. It has only been in the past few years that new technologies have emerged which provide
more coding gain with reasonable implementations. Viterbi still maintains the advantage of fairly high
coding gain with very low latency.
Trellis Code Modulation or “TCM” is a standard part of the modem used with 8PSK at rate 2/3.
The Viterbi CT option mode allows interoperability with competitive modems for 16QAM operation at
rates 3/4 and 7/8. The CT mode at this setting defaults to selecting Reed-Solomon CT mode at n, k
and depth values of 220, 200, depth of 4 and inverts the data. This is a closed circuit mode that does
not meet normal IESS standards, but is necessary in order to link to those modems.
1.6.2. FlexLDPC FEC
The addition of the FlexLDPC FEC allows replacement of the standard Viterbi FEC selectively for
transmit and receive paths. The performance improvement achieved by FlexLDPC is the highest and
most flexible of any specialized FEC technology to date, outperforming TPC across all modes. For
example, the FlexLDPC at rate 1/2 is capable of operating at a sustained Eb/No of only 1.5 dB with
an error rate less than 10
-9
1.6.3. Reed-Solomon Codec (RS)
Reed-Solomon Codec places a second Forward Error Correction (FEC) process outside of and in
series with the existing Viterbi FEC. The two FECs are thus considered "Concatenated". In addition,
the data between the two FECs is "interleaved" which effectively reduces the possibility of multiple
consecutive errored block symbols, thus improving the Reed-Solomon Codec performance.
The performance improvement achieved by this combination is significant. For example, the BER vs.
Eb/No performance of concatenated Viterbi rate 3/4 coding with R-S is better than Viterbi rate 1/2
alone and it uses less bandwidth than the Viterbi rate 1/2 alone.
Reed-Solomon is a block oriented code, meaning that data is framed into fixed size blocks and
processed in a specific way. A full block must be received before processing can begin, thus adding
to the latency.
Reed-Solomon uses framing which allows the use of a synchronous scrambler resulting in slightly
improved performance relative to the self-Synchronized scrambler normally used.
A simplified block diagram of the Reed-Solomon Codec is shown in the Figure 1-19.
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