Comtech EF Data ODM-840 - Manual (Page 171)

B–1
Appendix B. FORWARD ERROR
CORRECTION (FEC)
B.1 FEC Overview
The method of Forward Error Correction (FEC) used by the ODM/R‐840 Outdoor Remote Router
depends on the direction of signal processing in use:
• On the receive (Rx) side, the ODM/R‐840 operates with error correction based upon the
DVB‐S2 standard for QPSK, 8PSK, 16APSK and 32APSK with concatenated Low Density
Parity Code (LDPC) and Bose‐Chaudhuri‐Hocquenghem (BCH).
• On the transmit (Tx) side of the ODM/R‐840, a new family of short‐block LDPC codes
called VersaFEC ® are used. A patents‐pending technology wholly owned and developed
by Comtech EF Data and CEFD sister division Comtech AHA Corp. (the VersaFEC name is a
trademark registered to Comtech AHA), VersaFEC is ideal for lower data rates that
demand the shortest possible latency.
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.