Paradise P300 Change, Tx/Rx, Mod/Demod, REED-SOLOMON Menu, Introduction to REED-SOLOMON

6.6.12Change, 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

P300H P300 Series Modem Installation and Operating Handbook Page 68