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DMD1050 Satellite Modem Board
3.11 Ethernet Data Interface
The modem supports dual port 10/100 Base T Interface. When selected, the Tx Clock Source
will default to SCTE and the Clock Polarity will default to Normal. In addition, the Buffer Clock
will default to RxSat and the Buffer Clock Polarity will default to Normal. Refer to Appendix C
for interface set up and supporting features.
3.12 Reed-Solomon Codec
Refer to Figure 3-10, Figure 3-11 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.
3.12.1 Reed-Solomon Operation in the DMD1050
When the Reed-Solomon Codec is enabled, data is fed to the R-S Encoding Section of the
DMD1050 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
3-10 and Figure 3-11). 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.
3.12.2 Reed-Solomon 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 DMD1050 allows the following N and K setting: (126, 112), (219, 201), (194, 178),
(225, 205). Table 3-1 reflects IBS/IDR compliant modes.
Custom Reed Solomon rate are available as an option.
MN-DMD1050
Revision 9
/N
. R-S is a block Codec where K data bytes are fed into the
b
o
Theory of Operation
3–20
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