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OM20 Universal Outdoor Modem
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 de-interleaved 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.
4.8.2 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 OM20 automatically sets
the correct R-S code rate for IDR/IBS open network operation in accordance with the data shown
in Table 4-1. In Closed Net Mode, the OM20 allows the following N and K setting: (126, 112),
(219, 201), (194, 178), (225, 205).
4.8.3 Interleaving
The OM20 allows for interleaving depths of 4, 8 or 12 R-S Blocks. 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 modem 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 unit automatically sets the interleaver depth to 12.
Figure 4-8. Reed-Solomon Encoder Functional Block Diagram
Figure 4-9 Reed-Solomon Decoder Functional Block Diagram
Type of
Data Rate
Service
(Kbps)
Small IDR
64
(With 16/15
128
O/H)
256
384
512
TM116 – Rev. 1.2
Table 4-1. Reed-Solomon Codes
R-S Code
Bandwidth
1
(n, k, t)
Expansion
[ (n/k) -1 ]
(126, 112, 7)
0.125
(126, 112, 7)
0.125
(126, 112, 7)
0.125
(126, 112, 7)
0.125
(126, 112, 7)
0.125
Theory of Operation
Interleaving
Maximum
Depth
R-S Codec
Delay (ms)
4
115
4
58
4
29
4
19
4
15
2
4-16
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