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P300H P300 Series Modem Installation and Operating Handbook Page 68
6.6.12 Change, Tx/Rx, Mod/Demod, REED-SOLOMON Menu
This option is only available if the INTELSAT Reed-Solomon feature is available (standard P300-IBS and
above).
INTRODUCTION TO REED-SOLOMON
Reed-Solomon FEC is a powerful scheme `wrapped around` the normal inner FEC which may be used
to correct the remaining errors from the normal Turbo, Viterbi, Sequential, or TCM inner FEC. The code
rate is specified by three values `n`, `k`, and `t`. These have a fixed relationship of n - k = 2t, so `t` is simply
half the difference between `n` and `k`. A typical code rate may be specified such as (n,k,t) = (126,112,7).
RS 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 RS codec can correct
any bytes in error up to a value of `t`, so in the above example with t=7, up to seven bytes within the 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 which degrade gracefully Reed-Solomon fails
dramatically. Once the error correction threshold is passed (eg 8 errors in our example block) the codec
considers it uncorrectable and passes the block uncorrected, ie 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 are just below the threshold (and therefore fully correctable) and for some blocks
just above (and therefore uncorrectable). In practice this effect gives a RS satellite circuit a transition from
error free to unserviceable in about a 1dB change in Eb/No.
In addition to the (n,k,t) specification of the code rate, there is an additional parameter of interleaving depth.
Interleaving is used to `mix up` the blocks so that a burst of errors affects a few bytes from several blocks,
rather than a lot of bytes from a single block. Interleaving is usually of depth 4 or depth 8. The configuration
menus are described in the following paragraphs.
See also the applications section, para 8.4 "Choosing Optimum Custom RS N&k Values" on page 144