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Quantum and Evolution Series Installation and Operating Handbook
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RS mode. This controls whether the Reed-Solomon outer-FEC encoder is active.
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Advanced.This submenu allows custom values of n, k and interleaver depth to be set.
Reed-Solomon FEC is a powerful scheme wrapped around the inner FEC that can be used
to correct the remaining errors from the Viterbi and TCM inner FEC. The code rate is
specified by three values, namely, n, k, and t. These have a fixed relationship of n - k = 2t,
making t half the difference between n and k. A typical code rate is specified as (n, k, t) =
(126, 112, 7).
Reed-Solomon error correction works on codewords (blocks) of bytes, where n is the length
of the block of which there are k bytes of original data and 2t bytes of error correction
information. The Reed-Solomon codec can correct any bytes in error up to a value of t bytes.
In the above example with t=7, up to seven bytes within 126 may be corrected no matter
how many individual bits per byte are in error. However, when the error rate exceeds the
correctable level, unlike other FEC schemes that degrade gracefully, Reed-Solomon fails
catastrophically. Once the error correction threshold is passed then the codec considers it
uncorrectable and passes the entire block uncorrected, i.e. the error rate goes from 0 to no
error correction in one step. There is a small transitional area where due to the distribution of
the errors, the errors in some blocks may be just below the threshold (and therefore fully
correctable) and some blocks may be just above (and therefore uncorrectable). In practice,
this effect gives a Reed-Solomon satellite circuit a transition from error free to unserviceable
in approximately 1dB of change in Eb/No.
In addition to the (n, k, t) specification of the code rate, there is a parameter called
interleaving depth, which controls the number of Reed-Solomon codewords that are
interleaved. Interleaving is used to mix up the blocks such that a burst of channel errors is
distributed over a number of codewords, rather than affecting a lot of bytes from a single
block, thus improving the resilience to error bursts. Interleaving is usually of depth 4 or 8.
Reed-Solomon outer FEC must never be used in the absence of an inner FEC scheme.
Section 8.8 describes how to choose optimum custom values for n and k.
Edit-Tx-FEC-Outer-RS Mode Screen
RS FEC: [Normal]
Options:
Off
Normal
1:Off
2:Normal
3:Other
7-56
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