Comtech EF Data OM20 - Manual (Page 54)

OM20 Universal Outdoor Modem OM20 Theory of Operation
MN-OM20– Revision 5 4–16
4.9 Reed-Solomon Codec
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
b
/N
o
. 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. Refer to Figures 4-8, 4-9 and Table 4-1
4.9.1 Reed-Solomon Operation in the OM20
When the Reed-Solomon Codec is enabled, data is fed to the R-S Encoding Section of the OM20
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. 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 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.
4.9.2 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).
4.9.3 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