Mocomtech CDM-QX - Manual (Page 168)

CDM-Qx/QxL Multi-Channel Satellite Modem with DoubleTalk™ Carrier-in-Carrier® Revision 7
Forward Error Correction Options MN/CDMQX.IOM
7–2
By choosing various coding rates (Rate 1/2, 3/4, or 7/8) the user can trade off coding gain for
bandwidth compression. Rate 1/2 coding gives the best improvement in error rate, but doubles the
transmitted data rate, and hence 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 Modem 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 6-1. Viterbi Decoding Summary
FOR AGAINST
Good BER performance – very useful coding gain. Higher coding gain possible with other methods
Almost universally used, with de facto standards for
constraint length and coding polynomials.
Shortest decoding delay (~200 bits) of any FEC
scheme – good for coded voice, VOIP, etc.
Short constraint length produces small error bursts –
good for coded voice.
No pronounced threshold effect – fails gracefully.
Coding gain independent of data rate.
7.3 Reed-Solomon Outer Codec
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 requirement to produce a given BER. Factors such as rain-fade
margin – particularly at Ku-band – are extremely important, and reducing link
EIRP can seriously degrade the availability of such a link
.
The concatenation of an outer Reed-Solomon (Reed-Solomon) Codec with Viterbi decoder first
became popular when it was introduced by Intelsat in the early 1990's. It permits significant
improvements in error performance without significant bandwidth expansion. The coding
overhead added by the Reed-Solomon outer Codec is typically around 10%, which translates to a
0.4 dB power penalty for a given link. Reed-Solomon codes are block codes (as opposed to
Viterbi and Sequential, which are convolutional), and in order to be processed correctly the data
must be framed and de-framed.
Additionally, Reed-Solomon codes are limited in how well they can correct errors that occur in
bursts. This, unfortunately, is the nature of the uncorrected errors from Viterbi decoders, which
produce clusters of errors that are multiples of half the constraint length. For this reason, the data
must be interleaved following Reed-Solomon encoding, and is then de-interleaved prior to
decoding. This ensures that a single burst of errors leaving the Viterbi or Sequential decoder is
spread out over a number of interleaving frames, so errors entering the Reed-Solomon decoder do
not exceed its capacity to correct those errors. In the case of the CDM-Qx/QxL, different Reed-
Solomon code rates are used, according to the mode of operation.