Comtech EF Data SNM-1001L - Manual (Page 178)

SNM-1001L Satellite Modem Revision 1
Forward Error Correction Options MN/SNM1001L.IOM
7–2
7.2 Coding
The SNM-1001L can have any of the following configurations:
• Basic SNM-1001L (Sequential or Viterbi Decoder)
• FAST options (Sequential or Viterbi Decoder)
• FAST options with Reed-Solomon hardware (Sequential or Viterbi Decoder)
• Turbo hardware.
7.3 Turbo Product Codec (Hardware Option)
7.3.1 Introduction
Comtech EF Data implements the FEC Codec based on TPC. A Turbo Product Code is a
two 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, or 8-PSK TPC modes the demod and decoder
can remain synchronized 2 – 3 dB below the Viterbi/Reed-Solomon or TCM cases
. Table
7-1 is a listing of all the available TPC modes and rates.
Table 7-1. Available TPC Modes
Code Rate/Modulation Data Rate Range
Rate 21/44 BPSK 4.8 kbps to 1193 Mbps
Rate 5/16 BPSK 4.8 kbps to 781.25 kbps
Rate 1/2 QPSK/Offfset QPSK 4.8 kbps to 2386 Mbps
Rate 3/4 QPSK/Offset QPSK 7.2 kbps to 3750 kbps
Rate 3/4 8-PSK 384 kbps to 5.0 Mbps