Comtech EF Data DMD-2050E - Manual (Page 98)

DMD2050E Universal Satellite Modem Theory of Operation
MN-DMD2050E Revision 2 3–56
3.12.2 TPC and LDPC Summary
FOR AGAINST
Exceptionally good BER performance – significant improvement compared with every other
FEC method in use today.
Nothing!
Most modes have no pronounced threshold effect – fails gracefully.
Exceptional bandwidth efficiency .
Coding gain independent of data rate (in this implementation).
Low decoding delay for TPC.
Easy field upgrade in DMD2050E.
3.13 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.
See also:
Figure 3-20. Reed-Solomon Encoder Functional Block Diagram
Figure 3-21. Reed-Solomon Decoder Functional Block Diagram
Table 3-1. Spectral Efficiency using DoubleTalk Carrier-in-Carrier
3.13.1 Reed-Solomon Operation in the DMD2050E
When the Reed-Solomon Codec is enabled, data is fed to the R-S Encoding Section of the
DMD2050E 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 (see Figure 3-20
and Figure 3-21). Data is then sent to the modulator where it is convolutionally encoded,
modulated and transmitted to the satellite.