Mocomtech CDM-570L Manual De Usuario
CDM-570/570L Satellite Modem with Optional IP Module
Revision 4
Forward Error Correction Options
MN/CDM570L.IOM
7.2 Viterbi
The combination of convolutional coding and Viterbi decoding has become an almost
universal standard for satellite communications. The CDM-570/570L complies with the
Intelsat IESS 308/309 standards for Viterbi decoding with a constraint length of seven.
This is a de facto standard, even in a closed network environment, which means almost
guaranteed inter-operability with other manufacturer’s equipment. It provides very
useful levels of coding gain, and its short decoding delay and error-burst characteristics
make it particularly suitable for low data rate coded voice applications. It has a short
constraint length, fixed at 7, for all code rates. (The constraint length is defined as the
number of output symbols from the encoder that are affected by a single input bit.) By
choosing various coding rates (Rate 1/2, 3/4 or 7/8) the user can trade off coding gain for
bandwidth expansion. Rate 1/2 coding gives the best improvement in error rate, but
doubles the transmitted data rate, and doubles the occupied bandwidth of the signal. Rate
7/8 coding, at the other extreme, provides the most modest improvement in performance,
but only expands the transmitted bandwidth by 14%. A major advantage of the Viterbi
decoding method is that the performance is independent of data rate, and does not display
a pronounced threshold effect (i.e., does not fail rapidly below a certain value of Eb/No).
Note that in BPSK mode, the CDM-570/570L only permits a coding rate of 1/2. Because
the method of convolutional coding used with Viterbi, the encoder does not preserve the
original data intact, and is called non-systematic.
universal standard for satellite communications. The CDM-570/570L complies with the
Intelsat IESS 308/309 standards for Viterbi decoding with a constraint length of seven.
This is a de facto standard, even in a closed network environment, which means almost
guaranteed inter-operability with other manufacturer’s equipment. It provides very
useful levels of coding gain, and its short decoding delay and error-burst characteristics
make it particularly suitable for low data rate coded voice applications. It has a short
constraint length, fixed at 7, for all code rates. (The constraint length is defined as the
number of output symbols from the encoder that are affected by a single input bit.) By
choosing various coding rates (Rate 1/2, 3/4 or 7/8) the user can trade off coding gain for
bandwidth expansion. Rate 1/2 coding gives the best improvement in error rate, but
doubles the transmitted data rate, and doubles the occupied bandwidth of the signal. Rate
7/8 coding, at the other extreme, provides the most modest improvement in performance,
but only expands the transmitted bandwidth by 14%. A major advantage of the Viterbi
decoding method is that the performance is independent of data rate, and does not display
a pronounced threshold effect (i.e., does not fail rapidly below a certain value of Eb/No).
Note that in BPSK mode, the CDM-570/570L only permits a coding rate of 1/2. Because
the method of convolutional coding used with Viterbi, the encoder does not preserve the
original data intact, and is called non-systematic.
Table 7-1. Viterbi Decoding Summary
FOR
AGAINST
Good BER performance - very useful coding gain.
Higher coding gain possible
with other methods
with other methods
Almost universally used, with de facto standards for
constraint length and coding polynomials
constraint length and coding polynomials
Shortest decoding delay (~100 bits) of any FEC
scheme - good for coded voice, VOIP, etc.
scheme - good for coded voice, VOIP, etc.
Short constraint length produces small error bursts -
good for coded voice.
good for coded voice.
No pronounced threshold effect - fails gracefully.
Coding gain independent of data rate.
7.3
Reed-Solomon Outer Codec (Hardware Option)
IMPORTANT
It cannot be emphasized strongly enough that the purpose of the concatenated Reed-
Solomon is to dramatically improve the BER performance of a link under given
noise conditions. It should NOT be considered as a method to reduce the link EIRP
to the point where rain-fade margin, particularly at Ku-band, is no longer required.
Solomon is to dramatically improve the BER performance of a link under given
noise conditions. It should NOT be considered as a method to reduce the link EIRP
to the point where rain-fade margin, particularly at Ku-band, is no longer required.
The concatenation of an outer Reed-Solomon Codec with Viterbi decoder first became
popular when Intelsat introduced it in the early 1990s. It permits significant
7–2