Comtech EF Data SDM-300A - Manual (Page 243)

SDM-300A Satellite Modem Revision 6
Forward Error Correction (Options) MN/SDM300A.IOM
9–7
9.6 Turbo Product Codec (Hardware Option)
9.6.1 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 RS approach, and consequently, decoding
delays are significantly reduced. Furthermore, the traditional concatenated RS 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 mode the Demod and decoder can
remain synchronized 2 – 3 dB below the Viterbi/Reed-Solomon or TCM cases.
With this revision of the SDM-300A, Comtech EF Data now provides the best Forward
Error Correction technology currently available, offering a very broad range of TPC code
rates, combined with the entire range of modulation types.
9.6.2 End-to-End Processing Delay
In many cases, FEC methods that provide increased coding gain do so at the expense of
increased processing delay. However, with TPC, this increase in delay is very modest.
Table 9-1 shows, for the SDM-300A, the processing delays for the major FEC types,
including the three TPC modes: