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Chapter 10. FORWARD ERROR
10.1 Introduction
The CDM-750 Advanced High-Speed Trunking Modem operates with error correction based
upon the DVB standards:
•
DVB-S2 – QPSK, 8PSK, 16APSK and 32APSK with concatenated Low Density Parity
Code (LDPC) and Bose-Chaudhuri-Hocquenghem (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.
10.2 LDPC and BCH
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.
The error correcting capability of LDPC is improved by using large block sizes. Large block sizes
can increase latency in low bit rate applications (typically less than 2Mbps). In one-way broadcast
applications this is not a drawback. Links with LDPC normally operate at multi-megabit data
rates where latency effects are minimal. The standard block size for LDPC is 64,800 bits, and for
lower data rate applications there is a short frame block at 16,800 bits that suffers only a small
error correcting loss (0.2 to 0.5 dB) compared to the standard block.
CORRECTION
10–1
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