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DMD50 Universal Satellite Modem
3.9
Reed-Solomon Codec
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 Figure 3-8 and Figure 3-9). 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.
3.9.3 Interleaving
Iinterleaving depths of 4, 8, or 12 R-S blocks are allowed. This allows burst errors to be spread
over multiple blocks in order to enhance the error correcting performance of the R-S Codec. For
Intelsat Network Modes, the interleaving depth is automatically set to 4 for QPSK or BPSK, or 8
for 8PSK. In Closed Network Mode, the interleaver depth can be manually set to 4 or 8, and in
DVB Network Mode, the interleaver depth is automatically set to 12.
MN-DMD50 Revision 4
Refer to Figure 3-8, Figure 3-9, and Table 3-1.
/N
. R-S is a block Codec where K data bytes are fed into the
b
o
Theory of Operation
3–15
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