Turbo Products Codec (Hardware Option); Table 4-3. 8Psk/Tcm Coding Summary - Comtech EF Data SLM-7650 Installation And Operation Manual

SLM-7650 Satellite Modem
Theory of Operation
The detractions of the concatenated R-S approach apply here also, along with more
stringent requirements for phase noise and group delay distortion – the natural
consequences of the higher-order modulation.
Exceptionally bandwidth efficient
compared to QPSK
4.3.3.4 Turbo Product Codec (Hardware Option)
4.3.3.4.1 Introduction
Turbo coding is an FEC technique developed within the last few years, which 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 Turbo Product Code 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 a Reed Solomon codec with a primary FEC
codec, Turbo Product Coding is an entirely stand-alone method. It does not require the
complex interleaving/de-interleaving of the R-S approach, and 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
demod 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 CEVD/R-S or TCM cases

Table 4-3. 8PSK/TCM Coding Summary

FOR
4–7
AGAINST
Needs concatenated RS outer codec to give
acceptable coding gain performance
Demod acquisition threshold much higher than for
QPSK
8PSK is more sensitive to phase noise and group
delay distortion than QPSK
Revision 4
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