Introduction; Viterbi And Reed Solomon; Ldpc And Bch - Comtech EF Data CDM-710 Installation And Operation Manual

Chapter 7. FORWARD ERROR
7.1

Introduction

The CDM-710 Broadcast Satellite Modem operates with error correction base upon the DVB
standards:
•
DVB-S – QPSK with concatenated Viterbi and Reed Solomon.
•
DVB-DSNG – 8-PSK and 16-QAM with concatenated Viterbi and Reed Solomon.
•
DVB-S2 – QPSK, 8-PSK, 16-APSK and 32-APSK with concatenated Low Density
Parity Code (LDPC) and Bose-Chaudhuri-Hocquenghem (BCH).
DVB-S and DVB-DSNG anchor one the most widely adopted modulation and coding schemes
deployed today and are universally employed for satellite broadcast and related applications
today. Now DVB-S2 has defined a new generation of performance that boosts throughput by
about 30% over the same transponders using a new type coding that exceeds the capability of
concatenated Viterbi and Reed Solomon coding.
7.2

Viterbi and Reed Solomon

The concatenated Viterbi and Reed Solomon coding technique produces significant improvement
over Viterbi decoding alone. Simplistically, a Reed Solomon block decoder follows the
convolutional Viterbi decoder to further enhance error correction. Errors exiting the Viterbi
decoder tend to occur in clusters or errors bursts. The Reed Solomon decoder works well
correcting burst errors so the combination delivers improved performance. To further improve the
error correcting capability, an interleaver is placed between the two schemes to spread the errors
so fewer occur in a given block.
7.3

LDPC and BCH

LDPC and BCH is also concatenated 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.
CORRECTION OPTIONS
7–1