Reed-Solomon Codec; Reed-Solomon Operation; Reed-Solomon Code Rate - Radyne DMD20 Installation And Operation Manual

DMD20/20LBST Universal Satellite Modem
The DMD20 & 20LBST supports Radyne HDLC and Comtech HDLC modes,
offering compatibility with the SLM5650A Bridge Interface.
IMPORTANT
3.9

Reed-Solomon Codec

Refer to Figure 3-8, Figure 3-9, and Table 3-1.
IMPORTANT
Utilizing a Reed-Solomon (R-S) Outer Codec concatenated with a Convolutional Inner Codec is
an effective way to produce very low error rates even for poor signal-to-noise ratios while
requiring only a small increase in transmission bandwidth. Typically, concatenating an R-S
Codec requires an increase in transmission bandwidth of only 9 – 12% while producing a greater
than 2 dB improvement in E
encoder which adds 2t = (N – K) check bytes to produce an N byte R-S block. The R-S decoder
can then correct up to "t" erred bytes in the block.

3.9.1 Reed-Solomon Operation

When the Reed-Solomon Codec is enabled, data is fed to the R-S Encoding Section where it is
scrambled, formed into blocks, R-S encoded, and interleaved. Unique words are added so that
the blocks can be reformed in the Receiving Modem (Refer to Figures 3-14 and 3-15). Data is
then sent to the modulator where it is convolutionally encoded, modulated and transmitted to the
satellite.
When the signal is received and demodulated by the Receiving Modem, it is fed to a Viterbi
Decoder for the first layer of error correction. After error correction is performed by the Viterbi
Decoder, the unique words are located and the data is deinterleaved and reformed into blocks.
The R-S Decoder then corrects the leftover errors in each block. The data is then descrambled
and output from the R-S Section.

3.9.2 Reed-Solomon Code Rate

The R-S Code Rate is defined by (N, K) where N is the total R-S block size in bytes - data +
check bytes - and K is the number of data bytes input into the R-S Encoder. The transmission
rate expansion required by the R-S Codec is then defined by N/K. The modem automatically sets
the correct R-S code rate for IDR/IBS open network operation in accordance with the data shown
in Table 3-1. The modem allows the following N and K setting: (126, 112), (219, 201), (194, 178),
(225, 205).
Variable Reed-Solomon rates are available on the optional AS/5167 Super Card. Refer to
Appendix A for further information.
MN-DMD20-20LBST Revision 14
/N . R-S is a block Codec where K data bytes are fed into the
b o
Theory of Operation
3–15