Introduction; Turbo Product Codec - Comtech EF Data CDM-700 Installation And Operation Manual

Chapter 7. FORWARD ERROR
7.1

Introduction

As standard, the CDM-700 Modem is equipped with two internal expansion slots to house
Forward Error Correction (FEC) encoders/decoders. A Turbo Product Coding (TPC) codec card
with encoder and decoder is currently available that operates over the modem's full range of data
and symbol rates.
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The TPC Codec provides data rate capability up to 155 Mbps and operates at Rates 3/4,
and 7/8.
For high data rate applications, TPC offers Eb/No performance superior to Viterbi or concatenated
Viterbi and Reed Solomon coding, and the later technology is not offered.
7.2

Turbo Product Codec

Turbo coding, an FEC technique developed within the last few years, delivers significant
performance improvements compared to more traditional techniques. Two general classes of Turbo
Codes have been developed: Turbo Convolutional Codes (TCC), and Turbo Product Codes (TPC),
a block coding technique. Comtech EF Data has chosen to implement an FEC codec based on TPC.
A TPC is a 2- or 3-dimensional array of block codes. Encoding is relatively straightforward, but
decoding is a very complex process requiring multiple iterations of processing for maximum
performance to be achieved.
Unlike the popular method of concatenating an R-S Codec with a primary FEC codec, TPC is an
entirely standalone method. It does not require the complex interleaving/de-interleaving of the R-S
approach – consequently, decoding delays are significantly reduced. Furthermore, the traditional
concatenated R-S schemes exhibit a very pronounced threshold effect – a small reduction in Eb/No
can result in total loss of demod and decoder synchronization. TPC does not suffer from this
problem – the demodulator and decoder remain synchronized down to the point where the output
error rate becomes unusable. This is considered to be a particularly advantageous characteristic in a
fading environment. Typically, in QPSK, 8-PSK and 16-QAM TPC modes the demod and decoder
can remain synchronized 2 – 3 dB below the Viterbi/R-S or TCM cases.
CORRECTION OPTIONS
7–1