Reed-Solomon Codec; Reed-Solomon Operation In The Dmd20; Reed-Solomon Code Rate; Interleaving - Radyne DMD20 Installation And Operation Manual

Theory of Operation
5. Press <ENTER>. The modem has now copied the current Rx Settings to the Rx
Channel/Timeslot display ).
Note: It is not mandatory to assign timeslots in sequential order, although the lowest
timeslot must be entered in the lowest channel. For example: timeslots may be assigned
1 - 2, 2 - 5, etc. but not 1 - 5, 2 - 2.

3.14 Reed-Solomon Codec

Note: Refer to Figures 3-16, 3-17, and Table 3-1.
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.

3.14.1 Reed-Solomon Operation in the DMD20

When the Reed-Solomon Codec is enabled, data is fed to the R-S Encoding Section of the
DMD20 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 3-13 and
3-14). 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 DMD20 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 DMD20 allows the following N and K setting: (126, 112),
(219, 201), (194, 178), (225, 205).

3.14.3 Interleaving

The DMD20 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 Open Network Modes, the DMD20 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.
3-20
/N . R-S is a block Codec where K data bytes are fed into the
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DMD20 Universal Satellite Modem
TM103 - Rev. 1.1