Comtech EF Data CDM-800 - Manual (Page 123)

B–1
Appendix B. FEC (FORWARD
ERROR CORRECTION)
B.1 Introduction
The method of FEC (Forward Error Correction) used by the CDM‐800 Gateway Router is based
upon the DVB‐S2 standard for QPSK, 8PSK, 16APSK and 32APSK with concatenated Low Density
Parity Code (LDPC) and Bose‐Chaudhuri‐Hocquenghem (BCH).
B.2 DVB-S2: LDPC and BCH
The DVB‐S2 specification has defined a new generation of performance that boosts throughput
by about 30% over DVB‐S using the same amount of bandwidth. This new type of coding and
modulation far exceeds the capability of concatenated Viterbi and Reed Solomon coding.
LDPC and BCH is also a concatenated error correction technique. LDPC is a very powerful coding
scheme with significant, Near‐Shannon Bound Performance. In some cases, as the carrier‐to‐noise
ratio increases, the LDPC error correction starts flaring toward an error floor so BCH error correction
follows LDPC and eliminates the flare for any practical range of error rates.
LDPC also functions differently than Viterbi decoding by using iterative decoding. In this process
the data initially corrected by the LDPC decoder is re‐encoded and run through the decoder
again to correct additional errors. Key to this is the soft decision output from the LDPC decoder
and a high‐speed processor operating at a rate much higher than the data rate. The LDPC
decoder runs the iterative process as many times as possible before corrected data is finally
output to make way for a new block of data entering the decoder. LDPC also uses interleaving to
spread the errors. In contrast, Viterbi error correction operates by passing data through the
convolutional error correction process using a single error correction pass.
B.2.1 Range of Data Rates
For the range of Data Rates, refer to Sect. 1.4 SUMMARY OF SPECIFICATIONS .
B.2.2 BER, QEF, Eb/No, Es/No Spectral Efficiency, and Occupied Bandwidth
Depending upon the operating mode, the DVB standard uses different modes of specifying
performance with a unit in IF Loop and Additive White Gaussian Noise (AWGN):