Comtech EF Data DMD1050TS, Installation And Operation Manual

Page: 55 / 316

DMD1050TS Satellite Modem Board
Revision 1
Theory of Operation 3–23 MN-DMD1050TS
3.12 Reed-Solomon (R-S) Codec
See Figure 3-11, Figure 3-12 and Table 3-2.
Using a 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 Eb/No. R-S is a block Codec, where
K data bytes are fed into the 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.
3.12.1 R-S Operation in the DMD1050TS
When the R-S Codec is enabled, data is fed to the R-S Encoding Section of the DMD1050TS,
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 (See Figure 3-11 and Figure 3-12).
Data is then sent to the modulator, where it is encoded convolutionally, 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 done 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.
3.12.2 R-S Code Rate
The Standard 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. In Closed Net
Mode, the DMD1050TS allows these N and K settings: (126, 112), (219, 201), (194, 178), (225,
205). Table 3-2 shows IBS/IDR compliant modes. Custom R-S rates are available as an option.
3.12.3 Interleaving
The DMD1050TS allows for interleaving depths of 4 or 8 R-S Blocks, so burst errors are spread
over 4 or 8 R-S blocks to enhance the error correction of the R-S Codec. In Closed Network
Mode, set the interleaver depth to 4 or 8 manually. In DVB Network Mode, the DMD1050TS sets
the interleaver depth to 12, automatically.
Figure 3-11. R-S Encoder Functional Block Diagram
Figure 3-12. R-S Decoder Functional Block Diagram