Radyne DMD2050, Operation Manual

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DMD2050 Universal Satellite Modem Theory of Operation
TM110 – Rev. 1.3 3-21
3.14.1 Reed-Solomon Operation in the DMD2050
When the Reed-Solomon Codec is enabled, data is fed to the R-S Encoding Section of the
DMD2050 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
3-16 and 3-17). 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.
3.14.2 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 DMD2050 automatically
sets the correct R-S code rate for IDR/IBS open network operation in accordance with the data
shown in Table 3-1. In Closed Net Mode, the DMD2050 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.
3.14.3 Interleaving
The DMD2050 allows for interleaving depths of 4 or 8 R-S Blocks. This allows burst errors to be
spread over 4 or 8 R-S blocks in order to enhance the error correcting performance of the R-S
Codec. For Intelsat Network Modes, the DMD2050 automatically sets the interleaving depth 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 DMD2050 automatically sets the
interleaver depth to 12.
Figure 3-16. Reed-Solomon Encoder Functional Block Diagram
Figure 3-17. Reed-Solomon Decoder Functional Block Diagram