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Description
PSM-500/500L/500LT SCPC Satellite Modem
1.2.8.0 Special Codec CT Modes
The wide range of FECs available today and the possibility of many different operating modes and
parameters for each can make compatibility with other brands of modems extremely difficult. To aid
in FEC compatibility with other brands of modems, the PSM-500 series offers special "CT" modes,
standing for "Competing Technology". These modes use the same parameter settings as popular
modems by other manufacturers.
Take note of the special CT modes available for Viterbi, Reed-Solomon and TPC modes below.
1.2.8.1 Viterbi, Trellis Code Modulation Codec
Viterbi has been the standard high performance FEC used in satellite communications for
approximately 10 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. So, for voice circuits or DAMA links
requiring fast acquisition times Viterbi may be the FEC of choice.
Viterbi is part of a class of FECs considered "convolutional", basically meaning folded as in the
redundancy bits are folded into the data bit stream. The important issue here is that it is not block
oriented and not framed. It is therefore the responsibility of the FEC decoder to determine which are
the proper data bits.
Trellis Code Modulation or "TCM" is a standard part of the M500 used with 8PSK at rate 2/3.
The Viterbi CT option mode follows that of competitive modems for 16QAM operation at rates ¾ 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.
1.2.8.2 Reed-Solomon Codec Capability
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. The PSM-500 type R-S Codec is capable of operating in multiple standard and
custom modes as shown in the table below.
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 below.
Page 1-18
PSM-500/500L/500LT - Rev. 0.91
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