Comtech EF Data SNM-1010L - Manual (Page 177)

SNM-1010L Data/Control Modem Revision 0
Theory of Operation MN/SNM1010L.OM
ambiguities of the incoming data. When the decoder generates a lock signal, the
demodulator stops sweeping. If the signal level is within the operating range of the
demodulator, the modem declares lock.
Notes:
1. After the demodulator is locked, BER and frequency offset monitoring are
available via the remote interface.
2. For QPSK applications, the demodulator has the ability to lock to a carrier that
has gone through a spectral inversion.
3. For BPSK Viterbi mode, the bit ordering can be changed so that the modem is
compatible to either format.
6.5 Turbo Product Codec (Hardware Option)
6.5.1 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 industry wide, 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 and higher-order modulation TPC
modes the demod and decoder can remain synchronized 2 – 3 dB below the Viterbi/Reed-
Solomon or TCM cases
.
With this release of this modem, Comtech EF Data now provides the best Forward Error
Correction technology currently available, offering a broad range of TPC code rates,
combined with the entire range of modulation types, BPSK, QPSK, OQPSK, and 8PSK.
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